Distinct Hydrogen Bonding Dynamics Underlies the Microheterogeneity in DMF-Water Mixtures

The hydrogen bonding interaction between the amide functional group and water is fundamental to understanding the liquid-liquid heterogeneity in biological systems. Herein, the structure and dynamics of the N,N-dimethylformamide (DMF)-water mixtures have been investigated by linear and nonlinear IR spectroscopies, using the hydroxyl stretch and extrinsic probe of thiocyanate as local vibrational reporters. According to vibrational relaxation dynamics measurements, the orientational dynamics of water is not directly tied to those of DMF molecules. Wobbling-in-a-cone analysis demonstrates that the water molecules have varying degrees of angular restriction depending on their composition due to the formation of specific water-DMF networks. Because of the preferential solvation by DMF molecules, the rotational dynamics of the extrinsic probe is slowed significantly, and its rotational time constants are correlated to the change of solution viscosity. The unique structural dynamics observed in the DMF-water mixtures is expected to provide important insights into the underlying mechanism of microscopic heterogeneity in binary mixtures.

Structural Dynamics of Short Ligands on the Surface of ZnSe Semiconductor Nanocrystals

ZnSe semiconductor nanocrystals (NCs) with a size comparable to their Bohr radius are synthesized, and the native capping agents with long hydrocarbon tails are replaced with short thiocyanate (SCN) ligands through a ligand exchange method. The structural dynamics of SCN ligands on the surface of ZnSe NCs in solution is investigated by ultrafast infrared spectroscopy. Vibrational population relaxation of SCN ligands is accelerated due to the specific interaction with the positively charged sites on the surface of NCs. The orientational anisotropy of the bound SCN ligands decayed at a rate much faster than that in the control solution containing Zn²⁺ cations. From the wobbling-in-the-cone model analysis, we found that the SCN ligand undergoes wobbling orientational diffusion with a relatively large cone semiangle on the surface of ZnSe NCs, and the overall orientational diffusion of bound SCN is found to be strongly dependent on the size of ZnSe NCs.

Exploring the Structure and Complexation Dynamics of Azide Anion Recognition by Calix[4]pyrroles in Solution

The structure and anion recognition dynamics between calix[4]pyrroles and azide (N₃^-) anions in the form of its TBA⁺ and Na⁺ salts were investigated in dimethyl sulfoxide solutions by Fourier transform infrared (FTIR) spectroscopy and ultrafast IR spectroscopy. Vibrational energy redistribution of the N₃^- anion in the complex is accelerated through hydrogen bonding interactions with the N-H proton of the receptor. Rotational dynamics of the bound N₃^- is greatly restricted, demonstrating a distinct countercation effect. The detailed binding modes of N₃^- with the receptor were further evaluated by the density functional theoretical (DFT) calculations and nuclear magnetic resonance (NMR) spectroscopy. All of these measurements support the notion that the calix[4]pyrroles are capable of capturing the azide anion in solution. However, the calix[4]pyrroles may not necessarily undergo a conformational change to a cone-like geometry when they bind to the azide anion in the solution.

Cationic Effects on the Structural Dynamics of the Metal Ion-Crown Ether Complexes Investigated by Ultrafast Infrared Spectroscopy

It is usually believed that the binding affinity and selectivity of an alkali metal ion with crown ether are defined by the size matching model. However, the underlying mechanism of the specific host-guest interactions and the structural dynamics of the metal ions confined in the cavity of the crown ethers in the solutions are still not clear. In this report, a series of alkali thiocyanate salts (XSCN; X = Li, Na, K, and Cs) complexed with 18-crown-6 (a typical crown ether) in the chloroform solutions were studied by the polarization-selective infrared pump-probe spectroscopy and the ultrafast two-dimensional infrared (2D IR) spectroscopy. The SCN^- counteranions were employed as the local vibrational probe to reveal the specific host-guest interactions in the crown ether complexes. The rotational dynamics and spectral diffusion of SCN^- vibration were both measured by ultrafast IR spectroscopy, and it was found that the metal cations hosted by the crown ethers can have a pronounced effect on the rotational dynamics of the counteranions. The reorientational time constants of the SCN^- vibration in the complexation follow the order Li⁺ > Na⁺ > K⁺ ≃ Cs⁺. More importantly, the spectral diffusion dynamics of SCN^-, which quantifies the decay of the correlation of the frequency fluctuations in the complexation, was also affected by the metal ions but showed a different order of cationic effect. A detailed analysis of the 2D IR data showed that the spectral diffusion of SCN^- counteranion clearly decayed with two different time scales in the complex of 18-crown-6 with K⁺. The 3-4-fold slowdown in spectral diffusion indicated that the fluctuation of SCN^- vibrational transition frequency was strongly affected by the K⁺ cation due to the geometric constraint imposed by the crown ether. The results should help the researchers to unravel the specific host-guest interactions and further reveal the origination of the binding selectivity of crown ether for metal cations in the condensed phases from the perspective of structural dynamics.

Structural and dynamical changes observed when transitioning from an ionic liquid to a deep eutectic solvent

The microscopic molecular structure and dynamics of a new deep eutectic solvent (DES) composed of an ionic liquid (1-hexyl-3-methylimidazolium chloride) and an amide (trifluoroacetamide) at various molar ratios were investigated using linear and non-linear infrared spectroscopy with a vibrational probe. The use of the ionic liquid allows us to investigate the changes that the system undergoes with the addition of the amide or, equivalently, the changes from an ionic liquid to a DES. Our studies revealed that the vibrational probe in the DES senses a very similar local environment irrespective of the cation chemical structure. In addition, the amide also appears to perceive the same molecular environment. The concentration dependence studies also showed that the amide changes from being isolated from other amides in the ionic liquid environment to an environment where the amide-amide interactions are favored. In the case of the vibrational probe, the addition of the amide produced significant changes in the slow dynamics associated with the making and breaking of the ionic cages but did not affect the rattling-in-cage motions perceived by it. Furthermore, the concentration dependence of slow dynamics showed two regimes which are linked to the changes in the overall structure of the solution. These observations are interpreted in the context of a nanoscopic heterogeneous environment in the DES which, according to the observed dynamical regimes, appears at very large concentrations of the amide (molar ratio of greater than 1:1) since for lower amide molar ratios, the amide appears to be not segregated from the ionic liquid. This proposed molecular picture is supported by small angle x-ray scattering experiments.

Photoelectron Spectroscopy and Theoretical Study on Monosolvated Cyanate Analogue Clusters ECX-·Sol (ECX- = NCSe-, AsCSe-, and AsCS-; Sol = H2O, CH3CN)

Six monosolvated cyanate analogue clusters ECX^-·Sol (ECX^- = NCSe^-, AsCSe^-, and AsCS^-; Sol = H₂O and CH₃CN) were investigated using negative ion photoelectron spectroscopy (NIPES). NIPES experiments show that these clusters possess similar spectra overall compared to their respective isolated ECX^- anions but shift to higher electron binding energy with CH₃CN solvent, stabilizing the excess electrons slightly more than H₂O. For the ECX^-·H₂O series, vertical detachment energies and their increments relative to the bare species are measured to be 3.700/0.370, 3.085/0.415, and 3.085/0.430 eV for NCSe^-, AsCSe^- and AsCS^-, respectively, while the corresponding values in the ECX^-·CH₃CN series are 3.835/0.505, 3.145/0.475, and 3.135/0.480 eV. Ab initio electronic structure calculations indicate that the excess charges were located at the terminal N and Se atoms in NCSe^- and migrated to the central C atom in AsCSe^- and AsCS^-. For NCSe^-, the solvation is driven by the interactions with the two negatively charged terminal ends, while for AsCSe^- and AsCS^-, the solvation revolves around the interactions with the central C atom, where all the excess negative charge is concentrated. Two nearly degenerate isomers for NCSe^-·H₂O are identified, one forming a single strong N···H-O hydrogen bond (HB) and the other featuring a bidentate HB with two hydroxyl H atoms pointing to N and Se ends. In contrast, the negative central C atom in AsCSe^-/AsCS^- allows the formation of a bifurcated HB with H₂O. Similar effects are observed for the acetonitrile case, in which the three H atoms of the methyl group interact with the two negatively charged terminal ends in NCSe^-, while preferring to bind to the central negative carbon atom in AsCSe^-/AsCS^-. The different binding motifs derived in this work may suggest different solvation properties in NCSe^- versus AsCSe^-/AsCS^- with the former anion leading to asymmetric solvation at the N end of the solute, while the latter species creates more "isotropic" solvation around the central C equatorial plane.

An ultrafast vibrational study of dynamical heterogeneity in the protic ionic liquid ethyl-ammonium nitrate. I. Room temperature dynamics

Using ultrafast two-dimensional infrared spectroscopy (2D-IR), a vibrational probe (thiocyanate, SCN^-) was used to investigate the hydrogen bonding network of the protic ionic liquid ethyl-ammonium nitrate (EAN) in comparison to H₂O. The 2D-IR experiments were performed in both parallel (⟨ZZZZ⟩) and perpendicular (⟨ZZXX⟩) polarizations at room temperature. In EAN, the non-Gaussian lineshape in the FTIR spectrum of SCN^- suggests two sub-ensembles. Vibrational relaxation rates extracted from the 2D-IR spectra provide evidence of the dynamical differences between the two sub-ensembles. We support the interpretation of two sub-ensembles with response function simulations of two overlapping bands with different vibrational relaxation rates and, otherwise, similar dynamics. The measured rates for spectral diffusion depend on polarization, indicating reorientation-induced spectral diffusion (RISD). A model of restricted molecular rotation (wobbling in a cone) fully describes the observed spectral diffusion in EAN. In H₂O, both RISD and structural spectral diffusion contribute with similar timescales. This complete characterization of the dynamics at room temperature provides the basis for the temperature-dependent measurements in Paper II of this series.

Specific Host-Guest Interactions in the Crown Ether Complexes with K+ and NH4+ Revealed from the Vibrational Relaxation Dynamics of the Counteranion

The specific host-guest interactions in the corresponding complexes of K⁺ and NH₄⁺ with typical crown ethers were investigated by using FTIR and ultrafast IR spectroscopies. The counteranions, i.e., SCN^-, were employed as a local vibrational probe to report the structural dynamics of the complexation. It was found that the vibrational relaxation dynamics of the SCN^- was strongly affected by the cations confined in the cavities of the crown ethers. The time constant of the vibrational population decay of SCN^- in the complex of NH₄⁺ with the 18-crown-6 was determined to be 6 ± 2 ps, which is ∼30 times faster than that in the complex of K⁺ with the crown ethers. Control experiments showed that the vibrational population decay of SCN^- depended on the size of the cavities of the crown ethers. A theoretical calculation further indicated that the nitrogen atom of SCN^- showed preferential coordination to the K⁺ ions hosted by the crown ethers, while the NH₄⁺ can form hydrogen bonds with the oxygen atoms in the studied crown ethers. The geometric constraints formed in the complex of crown ethers can cause a specific interaction between the NH₄⁺ and SCN^-, which can facilitate the intermolecular vibrational energy redistribution of the SCN^-.

Temperature-dependent study of Fermi resonance of CH3CN and CH3CN---Li+ complex in CH3CN-LiClO4 mixture by Raman spectroscopy

The Raman spectra of acetonitrile-LiClO₄ mixture solution have been measured in the temperature range 20 to -196 °C at ambient pressure. Detailed Raman spectroscopy analysis revealed that, in acetonitrile-LiClO₄ mixture solution, the liquid CH₃CN transformed into solid phase β at approximately -50 °C, and then into solid phase α at approximately -60 °C. Besides, the Fermi resonance parameters of CH₃CN and CH₃CN---Li⁺ complex at different temperatures were calculated by using the Bertran's equations, respectively. It was found that the Fermi resonance coefficient W of CH₃CN---Li⁺ complex was not sensitive to the variation of temperature from 20 to -45 °C. In the case of CH₃CN, however, the Fermi resonance coefficient W decreased from the temperature of 20 to -196 °C during which a sudden increase was observed at the temperature of -50 °C coinciding with the temperature of phase transition from liquid to solid phase β. Finally, the temperature induced precipitation behavior of LiClO₄ and the structural evolution of CH₃CN on the Fermi resonance have been analyzed.

Vibrational Spectroscopy of N3- in the Gas and Condensed Phase

Azido-derivatized amino acids are potentially useful, positionally resolved spectroscopic probes for studying the structural dynamics of proteins and macromolecules in solution. To this end, a computational model for the vibrational modes of N₃^- based on accurate electronic structure calculations and a reproducing kernel Hilbert space representation of the potential energy surface for the internal degrees of freedom is developed. Fully dimensional quantum bound state calculations yield the antisymmetric stretch vibration at 1974 cm^-1 compared with 1986 cm^-1 from experiment. This mode shifts by 64 cm^-1 (from the frequency distribution) and 74 cm^-1 (from the IR line shape) to the blue, respectively, compared with 61 cm^-1 from experiment for N₃^- in water. The decay time of the frequency fluctuation correlation function is 1.1 ps, which is in good agreement with experiment (1.2-1.3 ps) and the full width at half maximum of the asymmetric stretch in solution is 18.5 cm^-1 compared with 25.2 cm^-1 from experiment. A computationally more efficient analysis based on instantaneous normal modes is shown to provide comparable, albeit somewhat less quantitative results compared to solving the three-dimensional Schrödinger equation for the fundamental vibrations.

Effect of Hydrogen Bonds on the Vibrational Relaxation and Orientational Relaxation Dynamics of HN3 and N3(-) in Solutions

Hydrogen bonds (H-bonds) play an important role in determining the structures and dynamics of molecular systems. In this work, we investigated the effect of H-bonds on the vibrational population relaxation and orientational relaxation dynamics of HN3 and N3(-) in methanol (CH3OH) and N,N-dimethyl sulfoxide (DMSO) using polarization-controlled infrared pump-probe spectroscopy and quantum chemical calculations. Our detailed analysis of experimental and computational results reveals that both vibrational population relaxation and orientational relaxation dynamics of HN3 and N3(-) in CH3OH and DMSO are substantially dependent on the strength of the H-bonds between the probing solute and its surrounding solvent. Especially in the case of N3(-) in CH3OH, the vibrational population relaxation of N3(-) is found to occur by a direct intermolecular vibrational energy transfer to CH3OH due to large vibrational coupling strength. The orientational relaxation dynamics of HN3 and N3(-), which are well fit by a biexponential function, are analyzed by the wobbling-in-a-cone model and extended Debye-Stokes-Einstein equation. Depending on the intermolecular interactions, the slow overall orientational relaxation occurs under slip, stick, and superstick boundary conditions. For HN3 and N3(-) in CH3OH and DMSO, the vibrational population relaxation becomes faster but the orientational relaxation becomes slower as the H-bond strength is increased. Our current results imply that H-bonds have significant effects on the vibrational population relaxation and orientational relaxation dynamics of a small solute whose size is comparable to the size of the solvent.

Two-dimensional ultrafast vibrational spectroscopy of azides in ionic liquids reveals solute-specific solvation

The stereochemistry and the reaction rates of bimolecular nucleophilic substitution reactions involving azides in ionic liquids are governed by solute-solvent interactions. Two-dimensional ultrafast vibrational spectroscopy (2D-IR) shows that the picosecond dynamics of inorganic azides are substantially slower than organic azides in a series of homologous imidazolium ionic liquids. In water, both organic and inorganic azides spectrally diffuse with a ∼2 ps time constant. In the aprotic solvent tetrahydrofuran, both kinds of azides spectrally diffuse on a timescale >5 ps. In ionic liquids, like 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), organic azides spectrally diffuse with a 2-4 ps time constant, and inorganic azides spectrally diffuse with a >40 ps time constant. Such a striking difference suggests that neutral (organic) and charged (inorganic) azides are incorporated in the ionic liquids with different solvation structures.

Femtosecond 2DIR spectroscopy of the nitrile stretching vibration of thiocyanate anions in liquid-to-supercritical heavy water. Spectral diffusion and libration-induced hydrogen-bond dynamics

Femtosecond two-dimensional infrared (2DIR) spectroscopy was carried out to study the dynamics of vibrational spectral diffusion of the nitrile stretching vibration of thiocyanate.

Effect of solvent polarity on the vibrational dephasing dynamics of the nitrosyl stretch in an Fe(II) complex revealed by 2D IR spectroscopy

The vibrational dephasing dynamics of the nitrosyl stretching vibration (ν(NO)) in sodium nitroprusside (SNP, Na2[Fe(CN)5NO]·2H2O) are investigated using two-dimensional infrared (2D IR) spectroscopy. The ν(NO) in SNP acts as a model system for the nitrosyl ligand found in metalloproteins which play an important role in the transportation and detection of nitric oxide (NO) in biological systems. We perform a 2D IR line shape study of the ν(NO) in the following solvents: water, deuterium oxide, methanol, ethanol, ethylene glycol, formamide, and dimethyl sulfoxide. The frequency of the ν(NO) exhibits a large vibrational solvatochromic shift of 52 cm(-1), ranging from 1884 cm(-1) in dimethyl sulfoxide to 1936 cm(-1) in water. The vibrational anharmonicity of the ν(NO) varies from 21 to 28 cm(-1) in the solvents used in this study. The frequency-frequency correlation functions (FFCFs) of the ν(NO) in SNP in each of the seven solvents are obtained by fitting the experimentally obtained 2D IR spectra using nonlinear response theory. The fits to the 2D IR line shape reveal that the spectral diffusion time scale of the ν(NO) in SNP varies from 0.8 to 4 ps and is negatively correlated with the empirical solvent polarity scales. We compare our results with the experimentally determined FFCFs of other charged vibrational probes in polar solvents and in the active sites of heme proteins. Our results suggest that the vibrational dephasing dynamics of the ν(NO) in SNP reflect the fluctuations of the nonhomogeneous electric field created by the polar solvents around the nitrosyl and cyanide ligands. The solute solvent interactions occurring at the trans-CN ligand are sensed through the π-back-bonding network along the Fe-NO bond in SNP.