Modeling of the Hydrogen Bond-Induced Frequency Shifts of the HOH and HOD Bending Modes of Water

The intramolecular bending mode of water is a possible useful probe of the hydrogen-bond situations in aqueous systems, but the behavior of its frequency and intensity should be further elucidated for better understanding on its nature and, hence, for its better utilization as a probe. Here, an analysis toward this goal is conducted by doing theoretical calculations on molecular clusters of normal isotopic and deuterated species of water and examining the correlations among the vibrational, structural, and electrostatic properties. It is shown that electrostatic interactions, particularly both of the in-plane components of the electric field along the OH bond and perpendicular to it, play a major role in controlling the hydrogen bond-induced shifts of the force constant, but additional factors, including the intermolecular structural and/or charge-transfer properties, are also important. Models of the hydrogen bond-induced shifts of the force constant are presented in a form that may be combined with classical molecular dynamics. With regard to the infrared intensity changes, it is shown on the basis of the electron density analysis that the intermolecular charge flux and polarization effect play an important role, depending on the angular characteristics of the hydrogen bond.

Singular value decomposition analysis of the electron density changes occurring upon electrostatic polarization of water

In-depth elucidation of how molecules are electrically polarized would be one key factor for understanding the properties of those molecules under various thermodynamic and/or spatial conditions. Here this problem is tackled for the case of hydrogen-bonded water by conducting singular value decomposition of the electron density changes that occur upon electrostatic polarization. It is shown that all those electron density changes are approximately described as linear combinations of ten orthonormal basis “vectors”. One main component is the interatomic charge transfer through each OH bond, while some others are characterized as the atomic dipolar polarizations, meaning that both of these components are important for the electrostatic polarization of water. The interaction parameters that reasonably well reproduce the induced dipole moments are derived, which indicate the extent of mixing of the two components in electrostatic polarization.

The main features of the electron density changes that occur upon electrostatic polarization of water are elucidated by conducting singular value decomposition analysis of those changes.

Hidden Halogen-Bonding Ability of Fluorine Manifesting in the Hydrogen-Bond Configurations of Hydrogen Fluoride

Elucidating how the intermolecular interactions of a covalently bonded fluorine atom are similar to and different from those of the other halogen atoms will be helpful for a better unified understanding of them. In the present study, the case of hydrogen fluoride is theoretically studied from this viewpoint by using the techniques of electron density analysis, molecular dynamics of liquid, and others. It is shown that the extra-point model, which locates an additional charge site on the line extended from (not within) the covalent bond and has been adopted for halogen-bonding systems as a key to the generation of proper stability and directionality, works well also in this case. A significantly bent hydrogen-bond configuration, which is characteristic of the intermolecular interactions of hydrogen fluoride, is reasonably well reproduced, meaning that it is a manifestation of the latent halogen-bonding ability, which is hidden by the strongly electronegative nature.

Role of Intermolecular Charge Fluxes in the Hydrogen-Bond-Induced Frequency Shifts of the OH Stretching Mode of Water

The relation between the vibrational properties and the electrostatic situations of the vibrating functional group is useful to predict vibrational spectroscopic features based on, for example, classical molecular dynamics of liquids or biomolecular systems, but to pursue its generality or the extent of applicability, it is required to understand the mechanisms giving rise to it. Here such an analysis is carried out for the OH stretching mode of water. By examining the correlations among various (structural, vibrational, and electrostatic) properties and by analyzing the spatial characteristics of the behavior of electrons occurring upon the vibration, it is shown that the dependence of the vibrational frequency and the dipole derivative of the OH stretching mode on the electric field is not of purely electrostatic origin, and the delocalized electronic motions occurring with this mode, called intermolecular charge fluxes, related to both the dipole first and second derivatives play important roles.

Dissecting the electric quadrupolar and polarization effects operating in halogen bonding through electron density analysis with a focus on bromine

The form of the electron density change (or difference) is usable as a kind of fingerprint of the electronic structural origin or mechanism that gives rise to intermolecular interactions. Here, this method is applied to halogen-bonding brominated systems to dissect the electric quadrupolar effect (arising from the anisotropic distribution of the valence electrons and intrinsic to the s²p_x ²p_y ²p_z electronic configuration) and the polarization effect (induced by a partial negative charge of the halogen-bond accepting atom). It is shown that a suitable location of the "extra point" for placing a partial positive charge to represent the former is crucial and is clearly found from the electron density difference from the spherically isotropic Br^- ion, while the latter consists of the dipolar polarization of the Br atom and the delocalized polarization of the whole molecule. A practical way for application to molecular dynamics simulations, etc., to represent these two factors is discussed.

Correlation of the partial charge-transfer and covalent nature of halogen bonding with the THz and IR spectral changes

Changes in the spectral intensities in the THz region are good probes for the non-electrostatic aspect of halogen bonding.

Water-like Behavior of Formamide: Jump Reorientation Probed by Extended Depolarized Light Scattering

Water is a strong self-associated liquid with peculiar properties that crucially depend on H-bonding. As regards its molecular dynamics, only recently has water reorientation been successfully described based on a jump mechanism, which is responsible for the overall H-bonding exchange. Here, using high-resolution broad-band depolarized light scattering, we have investigated the reorientational dynamics of formamide (FA) as a function of concentration from the neat liquid to diluted aqueous solutions. Our main findings indicate that in the diluted regime the water rearrangement can trigger the motion of FA solute molecules, which are forced to reorient at the same rate as water. This highlights an exceptional behavior of FA, which perfectly substitutes water within its network. Besides other fundamental implications connected with the relevance of FA, its water-like behavior provides rare experimental evidence of a solute whose dynamics is completely slaved to the solvent.