Vibrational couplings and energy transfer pathways of water's bending mode

A new rigid non-polarizable model for semi-heavy water: TIP4P/2005-SHW

In molecular dynamics (MD) simulations of water, rigid non-polarizable models are still valuable and widely used because of their low computational cost and excellent performance. Most of such models have been for light water (H2O) for a long time, and a few have recently been reported for heavy water (D2O). The specific models of D2O are needed because of its notably slower dynamics and stronger hydrogen bond than H2O. To our knowledge, no models of semi-heavy water (HOD) were made, although HOD is the most abundant and, therefore, most important chemical species in isotopically diluted water (IDW) prepared by mixing equal amounts of H2O and D2O. For precise MD simulations of IDW, a specific model of HOD is definitely needed. Here, we report the development of a new rigid non-polarizable model of HOD, TIP4P/2005-SHW, on the basis of the most popular H2O model, TIP4P/2005, and its D2O counterpart, TIP4P/2005-HW. We show the details of the development and demonstrate the high reproducibility of TIP4P/2005-SHW in terms of the density, the temperature of maximum density, the viscosity, and the diffusion coefficient.

Probing the Electric Field Response of a Water Molecule Confined in Small Carbon Nanocages: A Density Functional Theory Investigation

We consider a water molecule under tight confinement in the small-sized fullerenes (C28 ${_{28} }$ , C30 ${_{30} }$ , C32 ${_{32} }$ ) within the density functional theory (DFT) calculations with suitable exchange-correlation functionals. Such nanoscopic molecular cages provide an ideal setup to study their characteristic properties not present in the condensed phase. The water molecule entirely loses its feature of typical water when it is confined in small fullerenes of size equal to C30 ${_{30} }$ or smaller, in which the asymmetric O-H stretching vibration occurs at a lower wavenumber than the symmetric stretching. We study the response of the confined water molecule to the applied electric fields in terms of change in geometrical parameters, NMR spin-spin coupling constants, dipole moment, HOMO-LUMO (HL) gap, and vibrational frequency shift. The electric field shielding property of small-sized fullerene cages is explored and found to be strongly correlated with the HL gap. Since the electric field modulates the gap to decrease generally, shielding efficiency varies with field strength, thereby making large fields better shielded than small fields for the small penetration factor at large fields. The results that hold significance for technological applications are discussed.

Dissecting the hydrogen bond network of water: Charge transfer and nuclear quantum effects

The molecular structure of water is dynamic, with intermolecular hydrogen (H) bond interactions being modified by both electronic charge transfer and nuclear quantum effects (NQEs). Electronic charge transfer and NQEs potentially change under acidic or basic conditions, but such details have not been measured. In this work, we developed correlated vibrational spectroscopy, a symmetry-based method that separates interacting from noninteracting molecules in self- and cross-correlation spectra, giving access to previously inaccessible information. We found that hydroxide (OH-) donated ~8% more negative charge to the H bond network of water, and hydronium (H3O+) accepted ~4% less negative charge from the H bond network of water. Deuterium oxide (D2O) had ~9% more H bonds compared with water (H2O), and acidic solutions displayed more dominant NQEs than basic ones.

Ultraviolet Resonant Raman Scattering of Electrolyte Solutions

Saltwater stands as the most prevalent liquid on Earth. Consequently, substantial interest has been directed toward its characterization, both as an independent system and as a solvent for complex structures such as biomacromolecules. In the last few decades, special emphasis was placed on the investigation of the hydration properties of ions for the fundamental role they play in numerous chemical processes. In this study, we employed multi-wavelength Raman spectroscopy to examine the hydration shell surrounding bromide ions in solutions of simple electrolytes, specifically lithium bromide, potassium bromide, and cesium bromide, at two different concentrations. Cation-induced differences among electrolytes were observed in connection to their tendency to form ion pairs. An increased sensitivity to reveal the structure of the first hydration shell was evidenced when employing ultraviolet excitation in the 228-266 nm range, under resonance conditions with the charge transfer transition to the solvent peaked at about 200 nm. Other than a significant increase in the Raman cross-section for the OH stretching band when shifting from pure water to the solution, a larger enhancement for the Raman signal of the H-O-H bending mode over the stretching vibration was observed. Thus, the bending band plays a crucial role in monitoring the H-bond structure of water around the anions related to the charge distribution within the first hydration shell of anions, being an effective probe of hydration phenomena.

In vivo assessment of topically applied silver nanoparticles on entire cornea: comprehensive FTIR study

Silver nanoparticles (AgNPs) have gained attention in medicine for their potent antibacterial, antiviral, and anti-inflammatory properties. The use of silver nanoparticles in ophthalmic solutions raises concerns regarding potential toxicity of nanoparticles to ocular tissues, such as the cornea, conjunctiva, and retina, which necessitates further toxicity assessments aiding in the development of safer ophthalmic solutions. This study investigates the impact of AgNPs on corneal tissue using ophthalmic investigations, Fourier transform infrared (FTIR) spectroscopy, and chemometric analyses. Three concentrations of AgNPs (0.48 µg/mL, 7.2 µg/mL, and 15.5 µg/mL) were topically applied twice daily for 10 days, synthesized biologically by reducing silver nitrate with almond kernels water extract. Corneas, obtained by cutting 2-3 mm below the ora serrata, were analyzed with FTIR spectroscopy and subjected to chemometric analyses. Results reveal AgNPs' influence on constituents with OH and NH groups, affecting corneal lipids and reducing the lipid saturation index. AgNPs alter both bulk and interfacial water, leading to changes in corneal hydration thus modifying corneal physico-chemical properties. The influence extends to the water environment around proteins and lipids, releasing bound water from phospholipids and disrupting hydrogen bonding networks around proteins. In conclusion, the applied AgNPs concentrations can be linked to dry eye onset.

Temperature-dependent vibrational energy relaxation of hydrogen-bonded and free OD groups at the air-water interface

Water interfaces play a crucial role in regulating interactions and energy flow. Vibrational sum-frequency generation (vSFG) spectroscopy provides structural and dynamic information on water molecules at interfaces. It has revealed, for instance, the presence of the hydrogen-bonded and free OH groups at the air-water interface. Here, using temperature-dependent, time-resolved vSFG, we focus on the vibrational energy relaxation dynamics of interfacial heavy water (D2O). We reveal that while the relaxation timescale for hydrogen-bonded OD stretch modes is temperature-independent, the lifetime of the free OD stretch mode decreases with increasing temperature. Our data, supported by simulations, suggest that both intramolecular energy transfer and rotational reorientation mechanisms jointly contribute to the energy relaxation process of the free OD, with temperature influencing these mechanisms differently.

Single-Molecule Spectral Fluctuation Originates from the Variation in Dipole Orientation Connected to Accessible Vibrational Modes

Fluctuation in fluorescence emission of an immobilized single molecule is typically ascribed to the chromophore's intrinsic structural conformations and the influence of local environmental factors. Despite extensive research since its initial observation, a direct connection between these spectral fluctuations and the rearrangement of emission dipole orientations has remained elusive. Here, we elucidate this fundamental molecular behavior and its underlying mechanisms by employing unique single-molecule multidimensional tracking to simultaneously monitor both the emission spectrum and the three-dimensional dipole orientation of individual fluorophores. We present compelling evidence demonstrating a correlation between spectral fluctuations and dipolar rearrangements at room temperature. Our observations reveal that variations in the radiative relaxation probabilities among different vibronic emission bands, coupled with the interaction of associated vibrational modes, drive these spectral fluctuations. We identify significant out-of-plane dipole reorientations during pronounced spectral fluctuations, commonly known as spectral jumps, which primarily arise from transitions between dominant vibrational modes. Furthermore, we emphasize the potential for constructing vibrational spectra and optical nanoscopy with vibrational specificity, leveraging the vibronic emissions from single emitters.

FTIR determination of the degree of molar substitution for hydroxypropyl chitosan

We developed and validated a novel Fourier transform infrared (FTIR) method to determine the degree of molar substitution (MS) for hydroxypropyl chitosan (HPCS) using nuclear magnetic resonance (1H NMR) as a reference, and investigated the factors influencing the MS assay. Through extensive screening of integration methods for candidate bands in the FTIR spectrum of HPCS using 20 HPCS samples with degrees of acetylation (DA) ranging from 0.003 to 0.139, we found that when using band area at 2970 cm-1 as a probe integral, the MS values obtained via the 1H NMR method exhibited linear correlations (R2 > 0.98) with at least 16 integral ratios derived from their FTIR spectra. The optimal reference bands with high reliability are located at 3440 cm-1 and 1415 cm-1, with R2 exceeding 0.99 and a MS range of 0.17-1.92. The band at 2875 cm-1 is less affected by the trace moisture present in HPCS samples than the others. The results of the method validation demonstrated a mean recovery of 98.9 ± 2.8 % and an RSD below 10 %, suggesting a simple, robust, and highly accurate and precise method. This method could be extendable for the determination of the MS of insoluble HPCS derivatives and other hydroxypropylated polysaccharides.

Modeling Vibrational Sum Frequency Generation Spectra of Interfacial Water on a Gold Surface: The Role of the Fermi Resonance

Studying the hydrogen bonding structure of H2O at the metal-water interface is a highly complex yet fascinating endeavor. The intricate interactions and diverse orientations of water molecules on metal surfaces with varying potentials pose a significant challenge in elucidating the coupling between O-H stretching and H-O-H bending modes. In this study, we employed DFT-MD simulation to explore how the orientation of interfacial water molecules changes with the applied potential on the Au(111) surface. Based on the surface-specific velocity-velocity correlation function (ssVVCF) formula, we calculated vibrational sum frequency generation (VSFG) spectra for the O-H stretches. We found that three assigned peaks (∼3300, ∼3450, and 3650 cm-1) shifted toward lower frequencies as the potential moved toward more negative values. Our results align remarkably well with experimental Raman spectroscopy data. Notably, our VSFG analysis revealed a significant change in the VSFG spectra of the hydrogen-bonded O-H groups (∼3300 cm-1), switching from a negative to a positive sign with decreasing potential. This alteration suggests a substantial change in the orientation of these low-frequency O-H groups owing to their increased interactions with the Au surface. In contrast, the orientations of both the high-frequency O-H groups (∼3450 cm-1) and the dangling O-H groups (∼3650 cm-1) remained unaffected by the applied potentials. Furthermore, our analysis of the decomposed vibrational density of states (VDOS) for the H-O-H bending mode uncovered the coupling between the H-O-H bending and O-H stretching vibrations, known as the Fermi resonance. Our work suggests that the H-O-H bending vibration becomes restricted when water molecules transition from the ″one-H-down″ to the ″two-H-down″ conformation, leading to a redshift in the O-H stretching vibration through the Fermi resonance. By constructing the VSFG and decomposed VDOS spectra, we gained valuable insights into the structural changes that Raman spectra alone cannot fully interpret. Specifically, our analysis revealed the critical role of the Fermi resonance effect in shaping the spectroscopic signature of interfacial water molecules on the Au(111) surface.

Superdiffusive Rotation of Interfacial Water on Noble Metal Surface

Interfacial water on a metal surface acts as an active layer through the reorientation of water, thereby facilitating the energy transfer and chemical reaction across the metal surface in various physicochemical and industrial processes. However, how this active interfacial water collectively behaves on flat noble metal substrates remains largely unknown due to the experimental limitation in capturing librational vibrational motion of interfacial water and prohibitive computational costs at the first-principles level. Herein, by implementing a machine-learning approach to train neural network potentials, we enable performing advanced molecular dynamics simulations with ab initio accuracy at a nanosecond scale to map the distinct rotational motion of water molecules on a metal surface at room temperature. The vibrational density of states of the interfacial water with two-layer profiles reveals that the rotation and vibration of water within the strong adsorption layer on the metal surface behave as if the water molecules in the bulk ice, wherein the O-H stretching frequency is well consistent with the experimental results. Unexpectedly, the water molecules within the adjacent weak adsorption layer exhibit superdiffusive rotation, contrary to the conventional diffusive rotation of bulk water, while the vibrational motion maintains the characteristic of bulk water. The mechanism underlying this abnormal superdiffusive rotation is attributed to the translation-rotation decoupling of water, in which the translation is restrained by the strong hydrogen bonding within the bilayer interfacial water, whereas the rotation is accelerated freely by the asymmetric water environment. This superdiffusive rotation dynamics may elucidate the experimentally observed large fluctuation of the potential of zero charge on Pt and thereby the conventional Helmholtz layer model revised by including the contribution of interfacial water orientation. The surprising superdiffusive rotation of vicinal water next to noble metals will shed new light on the physicochemical processes and the activity of water molecules near metal electrodes or catalysts.

Direct observation of photoinduced sequential spin transition in a halogen-bonded hybrid system by complementary ultrafast optical and electron probes

A detailed understanding of the ultrafast dynamics of halogen-bonded materials is desired for designing supramolecular materials and tuning various electronic properties by external stimuli. Here, a prototypical halogen-bonded multifunctional material containing spin crossover (SCO) cations and paramagnetic radical anions is studied as a model system of photo-switchable SCO hybrid systems using ultrafast electron diffraction and two complementary optical spectroscopic techniques. Our results reveal a sequential dynamics from SCO to radical dimer softening, uncovering a key transient intermediate state. In combination with quantum chemistry calculations, we demonstrate the presence of halogen bonds in the low- and high-temperature phases and propose their role during the photoinduced sequential dynamics, underscoring the significance of exploring ultrafast dynamics. Our research highlights the promising utility of halogen bonds in finely tuning functional properties across diverse photoactive multifunctional materials.

Structure and Ultrafast X-ray Diffraction of the Hydrated Metaphosphate

We study the pathway of metaphosphate hydration when a metaphosphate anion is dissolved in liquid water with an explicit water model. For this purpose, we propose a sequential Monte Carlo algorithm incorporated with the ab initio quantum mechanics/molecular mechanics (QM/MM) method, which can reduce the amount of ab initio QM/MM sampling while retaining the accuracy of the simulation. We demonstrate the numerical calculation of the standard enthalpy change for the successive addition reaction PO3-·2H2O + H2O ⇌ PO3-·3H2O in the liquid phase, which helps to clarify the hydration pathway of the metaphosphate. With the obtained hydrated structure of the metaphosphate anion, we perform ab initio calculations for its relaxation dynamics upon vibrational excitation and characterize the energy transfer process in solution with simulated ultrafast X-ray diffraction signals, which can be experimentally implemented with X-ray free-electron lasers.

Harmonic and anharmonic studies on THz spectra of two vanillin polymorphs

Polymorphism commonly exists in organic molecular crystals. The fingerprint features in low-frequency vibrational range are important information reflecting different intermolecular interactions of polymorphs. Interpreting these features is very helpful to understand vibrational property of polymorphs and reveal the thermodynamic stability. In this work, the low-frequency vibrations of form I and II of vanillin are investigated using terahertz time-domain spectroscopy. Static DFT calculation and ab initio molecular dynamics (AIMD) are employed to interpret their low-frequency vibrations of both forms in harmonic and anharmonic ways, respectively. Their low-frequency vibration characteristics in harmonic calculations are discussed, and anharmonic mode couplings between OH bond stretch and the stretching and bending motion of hydrogen bonds are uncovered. Moreover, the thermodynamic energies including electronic potential energy and vibrational/kinetic energy arising from nuclear motions are calculated. The result reveals that the stability order of the two forms is mainly dependent on their electric potential energy difference.

Exceptions to Fourier's Law at the Macroscale

The usual basis to analyze heat transfer within materials is the equation formulated 200 years ago, Fourier's law, which is identical mathematically to the mass diffusion equation, Fick's law. Revisiting this assumption regarding heat transport within translucent materials, performing the experiments in vacuum to avoid air convection, we compare the model predictions to infrared-based measurements with nearly mK temperature resolution. After heat pulses, we find macroscale non-Gaussian tails in the surface temperature profile. At steady state, we find macroscale anomalous hot spots when the sample is topographically rough, and this is validated by using two additional independent methods to measure surface temperature. These discrepancies from Fourier's law for translucent materials suggest that internal radiation whose mean-free-path is millimeters interacts with defects to produce small heat sources that by secondary emission afford an additional, non-local mode of heat transport. For these polymer and inorganic glass materials, this suggests unique strategies of heat management design.

Hyper-Raman spectroscopy of biomolecules

The hyper-Raman scattering (HRS) spectra of biologically significant molecules (D-glucose, L-alanine, L-arabinose, L-tartaric acid) in aqueous solutions are reported. The HRS spectra were measured using a picosecond laser at 532 nm operating at a MHz repetition rate. High signal to noise spectra were collected with a commercial spectrometer and CCD without resonant, nanoparticle, or surface enhancement. The HRS peak frequencies, relative intensities, band assignments, and depolarization ratios are examined. By comparing HRS to Raman scattering (RS) and infrared absorption spectra we verify that the IR-active vibrational modes of the target molecules are observed in HRS spectra but come with substantially different peak intensities. The HRS of the biomolecules as well as water, dimethyl sulfoxide, methanol, and ethanol were deposited into a data repository to support the development of theoretical descriptions of HRS for these molecules. Depositing the spectra in a repository also supports future dual detection RS, HRS microscopes which permit simultaneous high-spatial-resolution vibrational spectroscopy of IR-active and Raman-active vibrational modes.

Bulklike Vibrational Coupling of Surface Water Revealed by Sum-Frequency Generation Spectroscopy

Vibrational coupling between interfacial water molecules is important for energy dissipation after on-water chemistry, yet intensely debated. Here, we quantify the interfacial vibrational coupling strength through the linewidth of surface-specific vibrational spectra of the water's O─H (O─D) stretch region for neat H_{2}O/D_{2}O and their isotopic mixtures. The local-field-effect-corrected experimental SFG spectra reveal that the vibrational coupling between hydrogen-bonded interfacial water O─H groups is comparable to that in bulk water, despite the effective density reduction at the interface.

Simulating the isotropic Raman spectra of O-H stretching mode in liquid H2O based on a machine learning potential: the influence of vibrational couplings

In this study, we trained a deep potential (DP) for H2O, an accurate machine learning (ML) potential. We performed molecular dynamics (MD) simulations of liquid water using the DP model (or DeePMD simulations). Our results showed that the DP model exhibits DFT-level accuracy, and the DeePMD simulation is a promising approach for modeling the structural properties of liquid water. Based on the DeePMD simulation trajectories, we calculated the isotropic Raman spectra of the O-H stretching mode using the surface-specific velocity-velocity correlation function (ssVVCF), showing that the DeePMD/ssVVCF approach can correctly capture the bimodal characteristics of the experimental Raman spectra, with one peak located near 3400 cm-1 and the other near 3250 cm-1. The success of the DeePMD/ssVVCF approach should be credited to (1) the DFT-level accuracy of the DP model for H2O, (2) the ssVVCF formulation considering the coupling between vibrational modes, and (3) non-Condon effects. Furthermore, the DeePMD simulations revealed that the anharmonic interactions between the coupled water molecules in the first and second hydration shells should play an essential role in the strong mixing of the H-O-H bending mode and the O-H stretching mode, leading to the delocalization of the O-H stretching band. In particular, increasing the strength of hydrogen bonds would enhance the bend-stretch coupling, leading to the red-shifting of the O-H vibrational spectra and the increase in the intensity of the shoulder around 3250 cm-1.

Excitation of NH Stretching Modes in Aromatic Molecules: o-Toluidine and α-Methylbenzylamine

Selectively excited o-toluidine and α-methylbenzylamine have been studied with quasi-classical trajectory procedures to determine the extent and timescales of intramolecular energy flow. The initial excitation is in the stretching mode of the para-CH bond, and its flow is initiated by interaction with an argon atom. Energy flow to the NH stretching mode is the dominant relaxation pathway, and its effectiveness is enhanced strongly by the methyl-NH interaction. Energy flow characteristics in both molecules are similar, but the flow is more effective in o-toluidine than in α-methylbenzylamine because the methyl group bonded to the benzene ring exerts stronger perturbation on the energy-flow pathway than the group bonded to the side chain. The relaxation of the initially excited CH completes on a timescale of several picoseconds, but the main part of energy flow to the NH occurs on a subpicosecond scale. In o-toluidine, carbon-carbon overtone modes lead to ring-CC bonds gaining and transporting more energy than high-frequency CH bonds, but they all gain far less energy than the NH stretching mode.

Physical state of water controls friction of gabbro-built faults

Earthquakes often occur along faults in the presence of hot, pressurized water. Here we exploit a new experimental device to study friction in gabbro faults with water in vapor, liquid and supercritical states (water temperature and pressure up to 400 °C and 30 MPa, respectively). The experimental faults are sheared over slip velocities from 1 μm/s to 100 mm/s and slip distances up to 3 m (seismic deformation conditions). Here, we show with water in the vapor state, fault friction decreases with increasing slip distance and velocity. However, when water is in the liquid or supercritical state, friction decreases with slip distance, regardless of slip velocity. We propose that the formation of weak minerals, the chemical bonding properties of water and (elasto)hydrodynamic lubrication may explain the weakening behavior of the experimental faults. In nature, the transition of water from liquid or supercritical to vapor state can cause an abrupt increase in fault friction that can stop or delay the nucleation phase of an earthquake.

Is Unified Understanding of Vibrational Coupling of Water Possible? Hyper-Raman Measurement and Machine Learning Spectra

The impact of the vibrational coupling of the OH stretch mode on the spectra differs significantly between IR and Raman spectra of water. Unified understanding of the vibrational couplings is not yet achieved. By using a different class of vibrational spectroscopy, hyper-Raman (HR) spectroscopy, together with machine-learning-assisted HR spectra calculation, we examine the impact of the vibrational couplings of water through the comparison of isotopically diluted H₂O and pure H₂O. We found that the isotopic dilution reduces the HR bandwidths, but the impact of the vibrational coupling is smaller than in the IR and parallel-polarized Raman. Machine learning HR spectra indicate that the intermolecular coupling plays a major role in broadening the bandwidth, while the intramolecular coupling is negligibly small, which is consistent with the IR and Raman spectra. Our result clearly demonstrates a limited impact of the intramolecular vibration, independent of the selection rules of vibrational spectroscopies.

THz-PCR Based on Resonant Coupling between Middle Infrared and DNA Carbonyl Vibrations

The carbonyl groups of deoxyribonucleotide can resonantly couple with 53 THz middle infrared, which can highly transmit water without ionization-based damage to DNA molecules. Herein, we predict that vibrational coupling with THz irradiation could lower down the hybridization landscape of nucleic acids and thus affect DNA replication. Using polymerase chain reaction (PCR) as a measure, we found that THz shining can reduce the denature temperature of DNA duplexes by about 3 °C, which allows one to conduct PCR at lower temperature, facilitating long-time amplification reaction without losing enzymatic fidelity, i.e., normal PCR should be carried out at denaturing temperature ∼4 °C higher than the melting temperature (T_m), but THz-PCR only requires temperature ∼1 °C higher than T_m due to the nonthermal effect of THz shining. Moreover, the melting time can also be shortened to 1/5 due to the enhanced vibration coupling with 53 THz irradiation. We proposed THz-PCR as an innovated DNA amplification technique with ultrahigh specificity and sensitivity and also successfully demonstrated its advantages in forensic detections.

Brushite mineralised Scots pine (Pinus sylvestris L.) sapwood - revealing mineral crystallization within a wood matrix by in situ XRD

Dicalcium phosphate dihydrate (CaHPO4·2H2O, DCPD, brushite) crystals were synthesised within Scots pine sapwood via a wet-chemistry route from aqueous solutions of Ca(CH3COO)2 and NH4H2PO4 salts. SEM/EDS analysis was used to assess the saturation of the wood cell lumina and cell wall as well as morphological features and elemental composition of the co-precipitated mineral. Brushite mineral crystallization and crystallite growth within the wood matrix was studied by in situ XRD. The chemical composition of the mineral before and after the dissolution was evaluated using FTIR spectroscopy. The overall impact of brushite on the thermal behaviour of wood was studied by TGA/DSC and TGA/DTA/MS analysis under oxidative and pyrolytic conditions. Bending and compression strength perpendicular and parallel to the fibre directions as well as bending strengths in longitudinal and transverse directions of the mineralised wood were also evaluated. Results indicate the viability of the wet-chemistry processing route for wood reinforcement with crystalline calcium phosphate (CaP)-based minerals, and imply a potential in producing hybrid bio-based materials that could be attractive in the construction sector as an environmentally friendly building material.

Influence of a Methyl Group on the Unidirectional Flow of Vibrational Energy in an Adenine-Thymine Base Pair

The role of a methyl group in intramolecular vibrational energy redistribution (IVR) of the hydrogen-bonded adenine-thymine base pair has been studied using classical dynamics procedures. Energy transferred to the doorway bond thymine-NH from the vibrationally excited H₂O(v) efficiently redistributes among various bonds of the base pair through vibration-to-vibration coupling, depositing a large fraction of the available energy in the terminal bond adenine-NH. On the other hand, the extent of energy flow in the reverse direction from the excited adenine-NH to thymine-NH is insignificant, indicating IVR in adenine-thymine resulting from the intermolecular interaction with a vibrationally excited H₂O molecule, is direction-specific. The unidirectional flow is due to the coupling of stretch-torsion vibrations of a methyl group with conjugated bonds on the thymine ring, when the methyl rotor is present and is adjacent to the vibrationally excited thymine-NH. The insignificance of energy flow from the terminal-to-terminal bond in the reverse direction is attributed to the absence of a methyl group on the adenine moiety, even though the molecule has many CC and CN bonds coupled to their neighbors.

Physical properties and biological effects of ceramic materials emitting infrared radiation for pain, muscular activity, and musculoskeletal conditions

BACKGROUND

Up to 33% of the general population worldwide suffer musculoskeletal conditions, with low back pain being the single leading cause of disability globally. Multimodal therapeutic options are available to relieve the pain associated with muscular disorders, including physical, complementary, and pharmacological therapies. However, existing interventions are not disease modifying and have several limitations.

METHOD

Literature review.

RESULTS

In this context, the use of nonthermal infrared light delivered via patches, fabrics, and garments containing infrared-emitting bioceramic minerals have been investigated. Positive effects on muscular cells, muscular recovery, and reduced inflammation and pain have been reported both in preclinical and clinical studies. There are several hypotheses on how infrared may contribute to musculoskeletal pain relief, however, the full mechanism of action remains unclear. This article provides an overview of the physical characteristics of infrared radiation and its biological effects, focusing on those that could potentially explain the mechanism of action responsible for the relief of musculoskeletal pain.

CONCLUSIONS

Based on the current evidence, the following pathways have been considered: upregulation of endothelial nitric oxide synthase, increase in nitric oxide bioavailability, anti-inflammatory effects, and reduction in oxidative stress.

Dephasing Dynamics across Different Local Vibrational Modes and Crystalline Environments

The perturbed free induction decay (PFID) observed in ultrafast infrared spectroscopy was used to unveil the rates at which different vibrational modes of the same atomic-scale defect can interact with their environment. The N_{3}VH^{0} defect in diamond provided a model system, allowing a comparison of stretch and bend vibrational modes within different crystal lattice environments. The observed bend mode (first overtone) exhibited dephasing times T_{2}=2.8(1)  ps, while the fundamental stretch mode had surprisingly faster dynamics T_{2}<1.7  ps driven by its more direct perturbation of the crystal lattice, with increased phonon coupling. Further, at high defect concentrations the stretch mode's dephasing rate was enhanced. The ability to reliably measure T_{2} via PFID provides vital insights into how vibrational systems interact with their local environment.

Hydrogen-Bond Configurations of Hydration Water around Glycerol Investigated by HOH Bending and OH Stretching Analysis

Toward a comprehensive understanding of the mechanism of glycerol as a moisturizer, studies on the hydrogen-bond (HB) structure of hydration water, which is known to be disordered by glycerol, are insufficient. To this aim, we evaluated the HB configurations based on the HOH bending and OH stretching spectra of the hydration water from those of glycerol/water mixtures by subtracting the contributions of bulk water and glycerol using dielectric relaxation spectroscopy. Analysis of the HOH bending band showed that hydration water-donating HBs lose the intermolecular bending coupling with increasing glycerol by replacing the water-water HBs with water-glycerol HBs. The OH stretching band provided more detailed insight into the HB configuration, indicating that the double-donor double-acceptor and double-donor single-acceptor configurations in bulk water change to a predominantly double-donor single-acceptor configuration in hydration water around glycerol. The formation of more donor HBs than acceptor HBs may be due to the steric constrains by glycerol and/or differences in the partial charge on the oxygen atom between water and glycerol.

Theoretical Sum Frequency Generation Spectra of Protein Amide with Surface-Specific Velocity-Velocity Correlation Functions

Vibrational sum frequency generation (vSFG) spectroscopy is widely used to probe the protein structure at interfaces. Because protein vSFG spectra are complex, they can only provide detailed structural information if combined with computer simulations of protein molecular dynamics and spectra calculations. We show how vSFG spectra can be accurately modeled using a surface-specific velocity-velocity scheme based on ab initio normal modes. Our calculated vSFG spectra show excellent agreement with the experimental sum frequency spectrum of LTα14 peptide and provide insight into the origin of the characteristic α-helical amide I peak. Analysis indicates that the peak shape can be explained largely by two effects: (1) the uncoupled response of amide groups located on opposite sides of the α-helix will have different orientations with respect to the interface and therefore different local environments affecting the local mode vibrations and (2) vibrational splitting from nearest neighbor coupling evaluated as inter-residue vibrational correlation. The conclusion is consistent with frequency mapping techniques with an empirically based ensemble of peptide structures, thus showing how time correlation approaches and frequency mapping techniques can give independent yet complementary molecular descriptions of protein vSFG. These models reveal the sensitive relationship between protein structure and their amide I response, allowing exploitation of the complicated molecular vibrations and their interference to derive the structures of proteins under native conditions at interfaces.

Intramolecular vibrational energy redistribution in nucleobases: Excitation of NH stretching vibrations in adenine–uracil + H2O

Redistribution of vibrational energy in the adenine–uracil base pair is studied when the base pair undergoes an intermolecular interaction with an overtone-bending vibration excited H2O(2[Formula: see text]bend) molecule. Energy transfer is calculated using the structural information obtained from density functional theory in the solution of the equations of motion. Intermolecular vibrational energy transfer (VET) from H2O(2[Formula: see text]bend) to the uracil–NH stretching mode is efficient and rapidly followed by intramolecular vibrational energy redistribution (IVR) resulting from coupling between vibrational modes. An important pathway is IVR carrying energy to the NH-stretching mode of the adenine moiety in a subpicosecond scale, the energy build-up being sigmoidal, when H2O interacts with the uracil–NH bond. The majority of intermolecular hydrogen bonds between the base pair and H2O are weakened but unbroken during the ultrafast energy redistribution period. Lifetimes of intermolecular HB are on the order of 0.5 ps. The efficiency of IVR in the base pair is due to near-resonance between coupled CC and CN vibrations. The resonance also exists between the frequencies of H2O bend and NH stretch, thus facilitating VET. When H2O interacts with the NH bond at the adenine end of the base pair, energy flow in the reverse direction to the uracil–NH stretch is negligible, the unidirectionality discussed in terms of the effects of uracil CH stretches. The energy distributed in the CH bonds is found to be significant. The IVR process is found to be nearly temperature independent between 200 and 400 K.

Water does not dance as ions sing: A new approach in elucidation of ion-invariant water fluctuations

Aqueous solutions of salts composed from monovalent ions are explored using temperature-dependent FT-IR spectroscopy in transmission. Water combination band, being extremely sensitive to the network of hydrogen bonds due to the contribution of water librations (ρ_LH₂O), is analyzed in uni- and multivariate fashion. Univariate analysis of the combination band maximum (ν_max) reveals that perturbation of water hydrogen bond network by ions is primary driven by electrostatic interactions between water and ions. Using multivariate curve resolution with alternating least squares and evolving factor analysis this band is separated into two components that represent low- and high-density water. The observed asymmetry in their behavior is interpreted in terms of fluctuations of a hydrogen bond network of two water components. The significance of the found phenomenon is unambiguously confirmed by performing analogous analysis in the spectral range that contains partial signature of water linear bending (δHOH) and is free from ρ_LH₂O, in which the asymmetry is absent. Additionally, we show that this phenomenon, namely ion-invariant behavior of water fluctuations, persists even in the regime of high ionic strengths. Although ions indeed participate in shaping of water hydrogen bond network, this straightforward approach shows that its temperature-dependent fluctuations are ion-independent.

Time-Dependent Friction Effects on Vibrational Infrared Frequencies and Line Shapes of Liquid Water

From ab initio simulations of liquid water, the time-dependent friction functions and time-averaged nonlinear effective bond potentials for the OH stretch and HOH bend vibrations are extracted. The obtained friction exhibits not only adiabatic contributions at and below the vibrational time scales but also much slower nonadiabatic contributions, reflecting homogeneous and inhomogeneous line broadening mechanisms, respectively. Intermolecular interactions in liquid water soften both stretch and bend potentials compared to the gas phase, which by itself would lead to a red-shift of the corresponding vibrational bands. In contrast, nonadiabatic friction contributions cause a spectral blue shift. For the stretch mode, the potential effect dominates, and thus, a significant red shift when going from gas to the liquid phase results. For the bend mode, potential and nonadiabatic friction effects are of comparable magnitude, so that a slight blue shift results, in agreement with well-known but puzzling experimental findings. The observed line broadening is shown to be roughly equally caused by adiabatic and nonadiabatic friction contributions for both the stretch and bend modes in liquid water. Thus, the quantitative analysis of the time-dependent friction that acts on vibrational modes in liquids advances the understanding of infrared vibrational frequencies and line shapes.

Ultrafast Rotational and Translational Energy Relaxation in Neat Liquids

The excess energy flow pathways during rotational and translational relaxation induced by rotational or translational excitation of a single molecule of and within each of four different neat liquids (H₂O, MeOH, CCl₄, and CH₄) are studied using classical molecular dynamics simulations and energy flux analysis. For all four liquids, the relaxation processes for both types of excitation are ultrafast, but the energy flow is significantly faster for the polar, hydrogen-bonded (H-bonded) liquids H₂O and MeOH. Whereas the majority of the initial excess energy is transferred into hindered rotations (librations) for rotational excitation in the H-bonded liquids, an almost equal efficiency for transfer to translational and rotational motions is observed in the nonpolar, non-H-bonded liquids CCl₄ and CH₄. For translational excitation, transfer to translational motions dominates for all liquids. In general, the energy flows are quite local; i.e., more than 70% of the energy flows directly to the first solvent shell molecules, reaching almost 100% for CCl₄ and CH₄. Finally, the determined validity of linear response theory for these nonequilibrium relaxation processes is quite solvent-dependent, with the deviation from linear response most marked for rotational excitation and for the nonpolar liquids.

Hydration and Hydrogen Bond Order of Octadecanoic Acid and Octadecanol Films on Water at 21 and 1 °C

The temperature-dependent hydration structure of long-chain fatty acids and alcohols at air-water interfaces has great significance in the fundamental interactions underlying ice nucleation in the atmosphere. We present an integrated theoretical and experimental study of the temperature-dependent vibrational structure and electric field character of the immediate hydration shells of fatty alcohol and acid headgroups. We use a combination of surface-sensitive infrared reflection-absorption spectroscopy (IRRAS), surface potentiometry, and ab initio molecular dynamics simulations to elucidate detailed molecular structures of the octadecanoic acid and octadecanol (stearic acid and stearyl alcohol) headgroup hydration shells at room temperature and near freezing. In experiments, the alcohol at high surface concentration exhibits the largest surface potential; yet we observe a strengthening of the hydrogen-bonding for the solvating water molecules near freezing for both the alcohol and the fatty acid IRRAS experiments. Results reveal that the hydration shells for both compounds screen their polar headgroup dipole moments reducing the surface potential at low surface coverages; at higher surface coverage, the polar headgroups become dehydrated, which reduces the screening, correlating to higher observed surface potential values. Lowering the temperature promotes tighter chain packing and an increase in surface potential. IRRAS reveals that the intra- and intermolecular vibrational coupling mechanisms are highly sensitive to changes in temperature. We find that intramolecular coupling dominates the vibrational relaxation pathways for interfacial water determined by comparing the H₂O and the HOD spectra. Using ab initio molecular dynamics (AIMD) calculations on cluster systems of propanol + 6H₂O and propionic acid + 10H₂O, a spectral decomposition scheme was used to correlate the OH stretching motion with the IRRAS spectral features, revealing the effects of intra- and intermolecular coupling on the spectra. Spectra calculated with AIMD reproduce the red shift and increase in intensity observed in experimental spectra corresponding to the OH stretching region of the first solvation shell. These findings suggest that intra- and intermolecular vibrational couplings strongly impact the OH stretching region at fatty acid and fatty alcohol water interfaces. Overall, results are consistent with ice templating behavior for both the fatty acid and the alcohol, yet the surface potential signature is strongest for the fatty alcohol. These findings develop a better understanding of the complex surface potential and spectral signatures involved in ice templating.

Liquid Structure of a Water-in-Salt Electrolyte with a Remarkably Asymmetric Anion

Water-in-salt systems, i.e., super-concentrated aqueous electrolytes, such as lithium bis(trifluoromethanesulfonyl)imide (21 mol/kgwater), have been recently discovered to exhibit unexpectedly large electrochemical windows and high lithium transference numbers, thus paving the way to safe and sustainable charge storage devices. The peculiar transport features in these electrolytes are influenced by their intrinsically nanoseparated morphology, stemming from the anion hydrophobic nature and manifesting as nanosegregation between anions and water domains. The underlying mechanism behind this structure-dynamics correlation is, however, still a matter of strong debate. Here, we enhance the apolar nature of the anions, exploring the properties of the aqueous electrolytes of lithium salts with a strongly asymmetric anion, namely, (trifluoromethylsulfonyl)(nonafluorobutylsulfonyl) imide. Using a synergy of experimental and computational tools, we detect a remarkable level of structural heterogeneity at a mesoscopic level between anion-rich and water-rich domains. Such a ubiquitous sponge-like, bicontinuous morphology develops across the whole concentration range, evolving from large fluorinated globules at high dilution to a percolating fluorous matrix intercalated by water nanowires at super-concentrated regimes. Even at extremely concentrated conditions, a large population of fully hydrated lithium ions, with no anion coordination, is detected. One can then derive that the concomitant coexistence of (i) a mesoscopically segregated structure and (ii) fully hydrated lithium clusters disentangled from anion coordination enables the peculiar lithium diffusion features that characterize water-in-salt systems.

Energy relaxation dynamics of hydrogen-bonded OH vibration conjugated with free OH bond at an air/water interface

Vibrational energy relaxation dynamics of the excited hydrogen-bonded (H-bonded) OH conjugated with free OH (OD) at an air/water (for both pure water and isotopically diluted water) interface are elucidated via non-equilibrium ab initio molecular dynamics (NE-AIMD) simulations. The calculated results are compared with those of the excited H-bonded OH in bulk liquid water reported previously. In the case of pure water, the relaxation timescale (vibrational lifetime) of the excited H-bonded OH at the interface is T₁ = 0.13 ps, which is slightly larger than that in the bulk (T₁ = 0.11 ps). Conversely, in the case of isotopically diluted water, the relaxation timescale of T₁ = 0.74 ps in the bulk decreases to T₁ = 0.26 ps at the interface, suggesting that the relaxation dynamics of the H-bonded OH are strongly dependent on the surrounding H-bond environments particularly for the isotopically diluted conditions. The relaxation paths and their rates are estimated by introducing certain constraints on the vibrational modes except for the target path in the NE-AIMD simulation to decompose the total energy relaxation rate into contributions to possible relaxation pathways. It is found that the main relaxation pathway in the case of pure water is due to intermolecular OH⋯OH vibrational coupling, which is similar to the relaxation in the bulk. In the case of isotopically diluted water, the main pathway is due to intramolecular stretch and bend couplings, which show more efficient relaxation than in the bulk because of strong H-bonding interactions specific to the air/water interface.

Interfacial Water Structure of Binary Liquid Mixtures Reflects Nonideal Behavior

The evaporation of molecules from water-organic solute binary mixtures is key for both atmospheric and industrial processes such as aerosol formation and distillation. Deviations from ideal evaporation energetics can be assigned to intermolecular interactions in solution, yet evaporation occurs from the interface, and the poorly understood interfacial, rather than the bulk, structure of binary mixtures affects evaporation kinetics. Here we determine the interfacial structure of nonideal binary mixtures of water with methanol, ethanol, and formic acid, by combining surface-specific vibrational spectroscopy with molecular dynamics simulations. We find that the free, dangling OH groups at the interfaces of these differently behaving nonideal mixtures are essentially indistinguishable. In contrast, the ordering of hydrogen-bonded interfacial water molecules differs substantially at these three interfaces. Specifically, the interfacial water molecules become more disordered (ordered) in mixtures with methanol and ethanol (formic acid), showing higher (lower) vapor pressure than that predicted by Raoult's law.

Thermophysical properties of water using reactive force fields

The widescale importance and rich phenomenology of water continue to motivate the development of computational models. ReaxFF force fields incorporate many characteristics desirable for modeling aqueous systems: molecular flexibility, polarization, and chemical reactivity (bond formation and breaking). However, their ability to model the general properties of water has not been evaluated in detail. We present comprehensive benchmarks of the thermophysical properties of water for two ReaxFF models, the water-2017 and CHON-2017_weak force fields. These include structural, electrostatic, vibrational, thermodynamic, coexistence, and transport properties at ambient conditions (300 K and 0.997 g cm^-3) and along the standard pressure (1 bar) isobar. Overall, CHON-2017_weak predicts more accurate thermophysical properties than the water-2017 force field. Based on our results, we recommend potential avenues for improvement: the dipole moment to quadrupole moment ratio, the self-diffusion coefficient, especially for water-2017, and the gas phase vibrational frequencies with the aim to improve the vibrational properties of liquid water.

Disentangling Sum-Frequency Generation Spectra of the Water Bending Mode at Charged Aqueous Interfaces

The origin of the sum-frequency generation (SFG) signal of the water bending mode has been controversially debated in the past decade. Unveiling the origin of the signal is essential, because different assignments lead to different views on the molecular structure of interfacial water. Here, we combine collinear heterodyne-detected SFG spectroscopy at the water-charged lipid interfaces with systematic variation of the salt concentration. The results show that the bending mode response is of a dipolar, rather than a quadrupolar, nature and allows us to disentangle the response of water in the Stern and the diffuse layers. While the diffuse layer response is identical for the oppositely charged surfaces, the Stern layer responses reflect interfacial hydrogen bonding. Our findings thus corroborate that the water bending mode signal is a suitable probe for the structure of interfacial water.

Ab initio molecular dynamics study on energy relaxation path of hydrogen-bonded OH vibration in bulk water

The vibrational energy relaxation paths of hydrogen-bonded (H-bonded) OH excited in pure water and in isotopically diluted (deuterated) water are elucidated via non-equilibrium ab initio molecular dynamics (NE-AIMD) simulations. The present study extends the previous NE-AIMD simulation for the energy relaxation of an excited free OH vibration at an air/water interface [T. Ishiyama, J. Chem. Phys. 154, 104708 (2021)] to the energy relaxation of an excited H-bonded OH vibration in bulk water. The present simulation shows that the excited OH vibration in pure water dissipates its energy on a timescale of 0.1 ps, whereas that in deuterated water relaxes on a timescale of 0.7 ps, consistent with the experimental observations. To decompose these relaxation energies into the components due to intramolecular and intermolecular couplings, constraints are introduced on the vibrational modes except for the target path in the NE-AIMD simulation. In the case of pure water, 80% of the total relaxation is attributed to the pathway due to the resonant intermolecular OH⋯OH stretch coupling, and the remaining 17% and 3% are attributed to intramolecular couplings with the bend overtone and with the conjugate OH stretch, respectively. This result strongly supports a significant role for the Förster transfer mechanism of pure water due to the intermolecular dipole-dipole interactions. In the case of deuterated water, on the other hand, 36% of the total relaxation is due to the intermolecular stretch coupling, and all the remaining 64% arises from coupling with the intramolecular bend overtone.