Vibrational signatures of hydrogen bonding in the protonated ammonia clusters NH4+(NH3)1−4

Hydrogen bond network structures of protonated dimethylamine clusters H+(DMA)n (n = 3-7)

Infrared spectroscopy of protonated dimethylamine clusters, H+(DMA)n, (n = 3-7), and their Ar-tagged clusters was performed in the NH and CH stretching vibrational region to explore their hydrogen bond network structures. A stable isomer search and vibrational spectral simulations of the clusters were also carried out to support the interpretations of the observed spectra. Weakly hydrogen-bonded NH stretching vibrational bands, which are characteristic of cyclic structures of small-sized protonated clusters, are observed in the spectra of the Ar-tagged clusters of n ≥ 5, while only linear chain type structures are suggested for the Ar-tagged clusters of n = 3-4 and the bare clusters of all the sizes. These results demonstrate that the size and temperature dependence of the hydrogen bond network structures of the protonated dimethylamine clusters is analogous to that of protonated monohydric alcohol clusters.

Proton Sponge Zwitterions with Positively and Negatively Charged Hydrogen Bonds: Extremely Polar Organic Compounds and Interplay of [NHN]+ and [NHN]- Properties

Six previously unknown zwitterions with positively and negatively charged [NHN] hydrogen bonds were synthesized by acylation of 4,5-bis(dimethylamino)-1-tosylamino-8-aminonaphthalene with subsequent alkaline treatment of the resulting 8-acylamino derivatives. Using NMR and XRD measurements in conjunction with quantum chemical DFT/PBE1PBE/6-311++G(d,p) calculations, it was shown that the negatively charged [NHN]- bond in such compounds commonly differs from the [NHN]+ bond by significantly lower linearity, higher asymmetry, and moderate to strong paramagnetic shift of the chelated NH proton signal. Among other remarkable findings, the most important are (1) unusually high polarity (μ = 21-26 D) of the obtained zwitterions, (2) sharp difference in structures of the solid 1,8-bis(tosylated) zwitterion (BTZ) grown from MeCN or DMF, and (3) registration for one of the stereoisomers of BTZ with the record short [NHN]- hydrogen bridge (N···N = 2.510 Å) almost reaching the theoretical limit (2.50 Å) for the [NHN]+ hydrogen bond.

Anharmonic IR spectra of solvated ammonium and aminium ions: resemblance between water and bisulfate solvations

In this work, we analyze the vibrational spectra of ammonium, methylammonium, and dimethylammonium ions solvated by either water molecules or bisulfate anions using anharmonic vibrational algorithms. Rich and complicated spectral features in the 2700-3200 cm^-1 region of the experimental spectra of these clusters are attributed to originate from strong Fermi resonance between hydrogen-bonded NH stretching fundamentals and NH bending overtones. Additional weaker bands around 2500-2600 cm^-1 in solvated aminium ions are assigned to the combination tones involving the CH-NH (methyl-amino) rocking modes. Furthermore, the qualitative resemblance in band positions and spectral patterns between two-water-solvated and two-bisulfate-solvated cations suggest a common vibrational coupling scheme beneath the two seemingly different micro-solvation environments.

Infrared spectroscopy of the protonated HCl dimer and trimer

The protonated HCl dimer and trimer complexes were prepared by pulsed discharges in supersonic expansions of helium or argon doped with HCl and hydrogen. The ions were mass selected in a reflectron time-of-flight spectrometer and investigated with photodissociation spectroscopy in the IR and near-IR regions. Anharmonic vibrational frequencies were computed with VPT2 at the MP2/cc-pVTZ level of theory. The Cl-H stretching fundamentals and overtones were measured in addition to stretch-torsion combinations. VPT2 theory at this level confirms the proton-bound structure of the dimer complex and provides a reasonably good description of the anharmonic vibrations in this system. The trimer has a HCl-HClH⁺-ClH structure in which a central chloronium ion is solvated by two HCl molecules via hydrogen bonding. VPT2 reproduces anharmonic frequencies for this system, including several combinations involving core ion Cl-H stretches, but fails to describe the relative band intensities.

Molecular vibrational frequencies from analytic Hessian of constrained nuclear-electronic orbital density functional theory

Nuclear quantum effects are important in a variety of chemical and biological processes. The constrained nuclear-electronic orbital density functional theory (cNEO-DFT) has been developed to include nuclear quantum effects in energy surfaces. Herein, we develop the analytic Hessian for cNEO-DFT energy with respect to the change in nuclear (expectation) positions, which can be used to characterize stationary points on energy surfaces and compute molecular vibrational frequencies. This is achieved by constructing and solving the multicomponent cNEO coupled-perturbed Kohn-Sham (cNEO-CPKS) equations, which describe the response of electronic and nuclear orbitals to the displacement of nuclear (expectation) positions. With the analytic Hessian, the vibrational frequencies of a series of small molecules are calculated and compared to those from conventional DFT Hessian calculations as well as those from the vibrational second-order perturbation theory (VPT2). It is found that even with a harmonic treatment, cNEO-DFT significantly outperforms DFT and is comparable to DFT-VPT2 in the description of vibrational frequencies in regular polyatomic molecules. Furthermore, cNEO-DFT can reasonably describe the proton transfer modes in systems with a shared proton, whereas DFT-VPT2 often faces great challenges. Our results suggest the importance of nuclear quantum effects in molecular vibrations, and cNEO-DFT is an accurate and inexpensive method to describe molecular vibrations.

Evolution of the linker in microhydrated hydrogen dinitrate anions: From H+ to H5 O2. J Comput Chem 2021;42:1514-1525. [PMID: 33990989 DOI: 10.1002/jcc.26560] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Hydrogen dinitrate anion, HNO₃ (NO₃ ^- ), is a proton-bound dimer with a very strong hydrogen bond. By employing ab initio molecular dynamics (AIMD) method, we studied the effects of the proton transfer and the rotation of the nitrates on the vibrational profiles of HNO₃ (NO₃ ^- )(H₂ O)_n (n = 0-2). The AIMD results indicate that the structure of the n = 0 cluster is very flexible, even though its hydrogen bond is quite strong. Significant rotations around the hydrogen bond and frequent transfers of proton from HNO₃ to NO₃ ^- are observed in AIMD simulations. Dynamic changes are therefore an important factor in understanding the broadening of vibrational features. For n = 1, the extent of structural fluctuation increases further, as H₂ O could move around the anion while the HNO₃ (NO₃ ^- ) core also goes through structural changes. Its vibrational spectrum can be understood as a mixture of many isomers visited during AIMD simulations. By n = 2, the structure is stabilized around one isomer, with the linker between the two nitrates being H₅ O₂ ⁺ , rather than H⁺ . Due to strong hydrogen bonds between nitrates and water molecules, this H₅ O₂ ⁺ takes the extraordinary structure with the H⁺ localized on one H₂ O, rather than being shared. While this novel structure is stable during AIMD simulations, the dynamic fluctuations in hydrogen bond distances still produce significant broadening in its vibrational profile.

Demystifying the Diffuse Vibrational Spectrum of Aqueous Protons Through Cold Cluster Spectroscopy

The ease with which the pH is routinely determined for aqueous solutions masks the fact that the cationic product of Arrhenius acid dissolution, the hydrated proton, or H⁺(aq), is a remarkably complex species. Here, we review how results obtained over the past 30 years in the study of H⁺⋅(H₂O)_n cluster ions isolated in the gas phase shed light on the chemical nature of H⁺(aq). This effort has also revealed molecular-level aspects of the Grotthuss relay mechanism for positive-charge translocation in water. Recently developed methods involving cryogenic cooling in radiofrequency ion traps and the application of two-color, infrared-infrared (IR-IR) double-resonance spectroscopy have established a clear picture of how local hydrogen-bond topology drives the diverse spectral signatures of the excess proton. This information now enables a new generation of cluster studies designed to unravel the microscopic mechanics underlying the ultrafast relaxation dynamics displayed by H⁺(aq).

Anharmonic coupling behind vibrational spectra of solvated ammonium: lighting up overtone states by Fermi resonance through tuning solvation environments

Fascinating Fermi resonance bands emerge from anharmonic couplings between NH stretching fundamentals and bending overtones in ammonium-centered clusters.

Spectroscopy of Proton Coordination with Ethylenediamine

Protonated ethylenediamine monomer, dimer, and trimer were produced in the gas phase by an electrical discharge/supersonic expansion of argon seeded with ethylenediamine (C₂H₈N₂, en) vapor. Infrared spectra of these ions were measured in the region from 1000 to 4000 cm^-1 using laser photodissociation and argon tagging. Computations at the CBS-QB3 level were performed to explore possible isomers and understand the infrared spectra. The protonated monomer exhibits a gauche conformation and an intramolecular hydrogen bond. Its parallel shared proton vibration occurs as a broad band around 2785 cm^-1, despite the formally equivalent proton affinities of the two amino groups involved, which usually leads to low frequency bands. The barrier to intramolecular proton transfer is 2.2 kcal mol^-1 and does not vanish upon addition of the zero-point energy, unlike the related protonated ammonia dimer. The structure of the dimer is formed by chelation of the monomer's NH₃⁺ group, thereby localizing the excess proton and increasing the frequency of the intramolecular shared proton vibration to 3157 cm^-1. Other highly fluxional dimer structures with facile intermolecular proton transfer and concomitant structural reorganization were computed to lie within 2 kcal mol^-1 of the experimentally observed structure. The spectrum of the trimer is rather diffuse, and a clear assignment is not possible. However, an isomer with an intramolecular proton transfer like that of the monomer is most consistent with the experimental spectrum.

Intra-cavity proton bonding and anharmonicity in the anionophore cyclen

Proton bonding drives the supramolecular chemistry of a broad range of materials with polar moieties. Proton delocalization and electronic charge redistribution have a profound impact on the structure of proton-bound molecular frameworks, and pose fundamental challenges to quantum chemical modelling. This study provides insights into the structural and spectral signatures of the intramolecular proton bond formed in a benchmark polyazamacrocycle anionophore (cyclen, 1,4,7,10-tetraazacyclododecane). Infrared action spectroscopy is employed to characterize the macrocycle, isolated in protonated form. In its most stable configuration, protonated cyclen adopts an open arrangement of Cs symmetry with a particularly strong NHδ+N bond across the cavity. The quantum chemical analysis of the infrared spectrum reveals intrinsic difficulties for the accurate description of the vibrational modes of the system. The reconciliation of the computational predictions with experiment demands a careful anharmonic treatment of the proton motion, which exposes the limitations of current methods. Best results are obtained with the incorporation of anharmonicity only to the fundamental modes directly related to motions of the proton. However, the full anharmonic treatment of the system fails to describe correctly the vibrations related to the macrocycle backbone. The results should serve as motivation for new developments in the modelling of proton bonded systems.

Protons and Hydroxide Ions in Aqueous Systems

Understanding the structure and dynamics of water's constituent ions, proton and hydroxide, has been a subject of numerous experimental and theoretical studies over the last century. Besides their obvious importance in acid-base chemistry, these ions play an important role in numerous applications ranging from enzyme catalysis to environmental chemistry. Despite a long history of research, many fundamental issues regarding their properties continue to be an active area of research. Here, we provide a review of the experimental and theoretical advances made in the last several decades in understanding the structure, dynamics, and transport of the proton and hydroxide ions in different aqueous environments, ranging from water clusters to the bulk liquid and its interfaces with hydrophobic surfaces. The propensity of these ions to accumulate at hydrophobic surfaces has been a subject of intense debate, and we highlight the open issues and challenges in this area. Biological applications reviewed include proton transport along the hydration layer of various membranes and through channel proteins, problems that are at the core of cellular bioenergetics.

Persistence of dual free internal rotation in NH4(+)(H2O)·Hen=0-3 ion-molecule complexes: expanding the case for quantum delocalization in He tagging

To explore the extent of the molecular cation perturbation induced by complexation with He atoms required for the application of cryogenic ion vibrational predissociation (CIVP) spectroscopy, we compare the spectra of a bare NH4(+)(H2O) ion (obtained using infrared multiple photon dissociation (IRMPD)) with the one-photon CIVP spectra of the NH4(+)(H2O)·He1-3 clusters. Not only are the vibrational band origins minimally perturbed, but the rotational fine structures on the NH and OH asymmetric stretching vibrations, which arise from the free internal rotation of the -OH2 and -NH3 groups, also remain intact in the adducts. To establish the location and the quantum mechanical delocalization of the He atoms, we carried out diffusion Monte Carlo (DMC) calculations of the vibrational zero point wave function, which indicate that the barriers between the three equivalent minima for the He attachment are so small that the He atom wave function is delocalized over the entire -NH3 rotor, effectively restoring C3 symmetry for the embedded -NH3 group.

A unified diabatic description for electron transfer reactions, isomerization reactions, proton transfer reactions, and aromaticity

A way is found for describing general chemical reactions using diabatic multi-state and “twin-state” models. (Image adapted with permission from https://www.flickr.com/photos/cybaea/64638988/).

Infrared spectroscopy of large-sized neutral and protonated ammonia clusters

Size selective IR spectroscopy shows the nature of hydrogen bond networks in neutral and protonated ammonia clusters.

The Ionic Hydrogen/Deuterium Bonds between Diammoniumalkane Dications and Halide Anions

Halide-anion binding to 1,12-dodecanediammonium, tetramethyl-1,12-dodecanediammmonium, and tetramethyl-1,7-heptanediammonium has been investigated with infrared multiple-photon dissociation (IRMPD) spectroscopy in the 1000-2250 cm^-1 spectral region and with theory. Both charged ammonium groups in these diammonium compounds interact with the halide anion resulting in an ionic hydrogen bond (IHB) stretching frequency outside of the spectral frequency range that can be measured with the free-electron laser (FEL). This frequency is shifted into the spectral range upon exchanging all of the labile hydrogen atoms with deuterium atoms, thus making measurement of the ionic deuterium bond (IDB) stretching frequency possible. The IDB stretching frequency shifts to higher values with increasing halide-anion size, methylation of the ammonium groups, and alkane chain length, consistent with the halide-anion-deuterium bond strength decreasing with decreasing gas-phase basicity of the halide anion and the increasing gas-phase basicity of the ammonium groups. The IDB stretching frequency also depends on the alkane chain length owing to constraints on the angle of the bonds between the halide anion and the two ammonium groups. There are additional bands in the IDB stretching feature in the IRMPD spectra, which are attributed to Fermi resonances and arise from coupling with overtone or combination bands that can be identified from theory and depend on the halide-anion identity and alkane chain length.

Harmonic bath averaged Hamiltonian: an efficient tool to capture quantum effects of large systems

Starting from a reaction path Hamiltonian, a suitably reduced harmonic bath averaged Hamiltonian is derived by averaging over all the normal mode coordinates. Generalization of the harmonic bath averaged Hamiltonian to any dimensions are performed and the feasibility to use a linear reaction path/surface are investigated and discussed. By use of a harmonic bath averaged Hamiltonian, the tunneling splitting and proton transfer dynamics of malonaldehyde is briefly discussed and shows that the harmonic bath averaged Hamiltonian is an efficient tool to capture quantum effects in larger systems.

A generalized reactive force field for nonlinear hydrogen bonds: hydrogen dynamics and transfer in malonaldehyde

Using molecular dynamics (MD) simulations, the spectroscopy and dynamics of malonaldehyde is investigated. To this end, the recently proposed molecular mechanics with proton transfer (MMPT) potential is generalized to nonlinear hydrogen bonds. The calculated properties for malonaldehyde in both gas and condensed phases, including equilibrium geometries, infrared spectra, tunneling splittings, and hydrogen transfer rates, compare well with previous experimental and computational works. In particular, by using a harmonic bath averaged (HBA) Hamiltonian, which is based on a reaction path Hamiltonian, it is possible to estimate the tunneling splitting in an efficient manner. It is found that a zero point corrected barrier of 6.7 kcal/mol and effective masses of 1.234 (i.e., 23.4% larger than the mass of a physical H-atom) and 1.117 (for the physical D-atom) are consistent with the measured splittings of 21.6 and 2.9 cm(-1), respectively. The HBA Hamiltonian also yields a pair of hydrogen transfer fundamentals at 1573 and 1267 cm(-1), similar to results obtained with a reaction surface Hamiltonian on a MP2/6-31G(**) potential energy surface. This amounts to a substantial redshift of more than 1000 cm(-1) which can be rationalized by comparison with weakly (HCO(+): rare gas) and strongly (H(2)O-H(+)-OH(2)) proton-bound systems. Hydrogen transfer rates in vacuum and water were determined from the validated MMPT potential and it is found that the solvent enhances the rate by a factor of 5 at 300 K. The rates of 2.4/ns and 10/ns are commensurate with previous density functional tight binding ab initio MD studies.