Stretching force constants as descriptors of energy and geometry of F···HF hydrogen bonds.

In this work applicability of proton donor group stretching vibration force constants ks and intermolecular stretching force constants kσ for evaluations of hydrogen bond strength and geometry are discussed. For a set of 30 complexes with F···HF hydrogen bonds in a wide range 0.5-48 kcal/mol by means of quantum chemical calculations equilibrium geometries, complexation energies, vibrational frequencies and corresponding force constants were calculated (MP2/aug-cc-pVTZ). It is shown, that properties of a hydrogen bond are more strictly correlated with the values of force constants than with vibrational frequencies. Easy-to-use equations for estimations of hydrogen bond energy ∆E and geometry (rFH, rFF) based on ks and kσ values are proposed.

Lone pairs mapping by Laplacian of 3 He NMR chemical shift.

Results of approbation of a new quantum mechanical approach of lone pairs (LPs) visualization, its optimization and testing on a range of model molecules are presented. The main idea of proposed methodology is using 3 He atom as a probe for investigating electronic shells of species with LPs. As model objects, we consider "classical" examples of hydrogen cyanide, methanimine, ammonia, phosphine, formaldehyde, water, and hydrogen sulfide. It is shown that LPs can be visualized by means of 3D maps of Laplacian of 3 He chemical shift ∇2 δHe . NMR calculations could be performed using level of theory as low as B3LYP/6-31G, allowing for the reduction of computational time without significant loss of quality. Advantages of our approach are discussed in comparison with usual methods of lone pairs visualization (electron localization function, molecular electrostatic potential).

Infrared Studies of the Symmetry Changes of the 28SiH4 Molecule in Low-Temperature Matrixes. Fundamental, Combination, and Overtone Transitions.

Infrared spectra of 28SiH4 in argon and nitrogen matrixes at low temperature 6.5-20 K in the region of overtone and combination transitions were recorded for the first time. Additionally, the high-resolution spectra were obtained in the fundamental region. The frequencies and the relative intensities of all bands were determined. The set of experimental data suggests that the symmetry of molecules studied in the matrixes is different from the symmetry of the free molecules because of an interaction with the environment. The symmetry of 28SiH4 changes from Td to C3v on transition from the gas phase to a nitrogen matrix and to D2d on transition to an argon matrix. A modeling of SiH4 molecule force fields explains the experimental data as a change of a force constant of the selected SiH bond in the case of SiH4 in the nitrogen matrix or force constants of two opposite angles in the case of SiH4 in the argon matrix. In spite of small values of these changes, they result in noticeable spectroscopic effects: the band splitting and appearance of new bands in matrix spectra compared with spectra of free SiH4. The interpretation of transitions in the fundamental and combination regions was performed.

Infrared spectra and structures of SiH4 and GeH4 dimers in low-temperature nitrogen matrixes.

IR absorption spectra of monoisotopic silane (28)SiH4 and germane (76)GeH4 are studied in nitrogen matrixes at T = 11 K. It is shown that the absorption spectra of silane and germane are similar in the regions of the ν3 stretching and ν4 bending vibrations. A significant influence of a nitrogen matrix on the absorption spectra of XH4 molecules was established. The bands in the ν3 region of silane and germane in a nitrogen matrix are blue-shifted. The spectra of monomer molecules of SiH4 and GeH4 contain a triplet in the stretching region and a doublet in the bending region, which is explained by the change in the molecular symmetry of an XH4 molecule from T(d) to C3 on passage from the gas phase to solid nitrogen. The calculations of the matrix structure were made by the Monte Carlo method with the ensemble of particles containing the SiH4 molecule and 863 N2 molecules. The calculations predicted a set of equilibrium structures with the C3 symmetry. The structure of the absorption bands of the dimer is analyzed in the ν3 stretching and ν4 bending regions. The intensity of these absorption bands increases quadratically with concentration. The contribution of resonance dipole-dipole interactions in the formation of the absorption bands of (XH4)2 dimers in the ν3 stretching and ν4 bending regions is discussed.

Calculation of structural and spectroscopic parameters of trans-DONO and the NH3···trans-DONO complex. Comparison with analogous parameters of trans-HONO and NH3···trans-HONO.

The frequencies and absolute intensities for transitions between vibrational states of trans-DONO and NH3···trans-DONO are calculated using the approach earlier tested in calculations of trans-HONO and NH3···trans-HONO. The spectroscopic parameters were obtained in the harmonic approximation and from variational solutions of anharmonic Schrödinger equations in one to four dimensions with accurate potential energy and dipole moment surfaces. The calculated frequencies of trans-DONO are in good agreement with the experimental data. Our intensity values for trans-DONO and the frequencies and intensities for NH3···trans-DONO can be useful in interpretation of future experiments. Comparison of the transition frequencies and intensities and the averaged values of O-H (O-D) and N···O distances calculated for trans-DONO and its complex with ammonia with analogous parameters of trans-HONO and NH3···trans-HONO provides information about diverse isotope effects. For example, on H/D substitution the fundamental intensity for the ν2(N═O) mode of the lighter complex increases by a factor of 2.7, and the ν1 transition goes out of resonance with the 2ν3 overtone.

Anharmonic calculation of the structure, vibrational frequencies and intensities of the NH3···trans-HONO complex.

The equilibrium geometry of the NH3···trans-HONO complex and the harmonic vibrational frequencies and intensities are calculated in the MP2/aug-cc-pVTZ approximation with the basis set superposition error taken into account. Effects of anharmonic interactions on spectroscopic parameters are studied by solving vibrational Schrödinger equations in 1-4 dimensions using the variational method. Anharmonic vibrational equations are formulated in the space of normal coordinates of the complex. Detailed analysis is performed for the H-bond stretching vibration and internal vibrations of the trans-HONO isomer in the complex. The intermode anharmonicity and anharmonic coupling between two, three, and four vibrational modes are studied on the basis of correct ab initio potential energy surfaces calculated in the above approximation. The combinations of normal modes of the complex most strongly coupled to one another are examined. The calculated frequencies and intensities of vibrational bands are compared with the experimental data on the NH3···trans-HONO complex in an argon matrix and results of earlier calculations of monomeric HONO. In this calculation the strong resonance between the first excited state of the OH stretching vibration and the doubly excited state of the NOH bending vibration of trans-HONO isomer in the complex is thoroughly studied by solving vibrational equations in two and four dimensions.

Influence of resonance interactions and matrix environment on the spectra of SF(6) dimers in low-temperature nitrogen matrixes. Theory and experiment.

The IR absorption spectra of (SF(6))(2) dimers were studied in N(2) matrixes at 11 K. Absorption bands due to SF(6) monomers and to (SF(6))(2) dimers have been identified. As a result of the resonance dipole-dipole interaction between two SF(6) subunits, the band of the triply degenerate vibration nu(3) is split into two components nu(X),(Y) and nu(Z), where Z is the axis connecting the two sulfur atoms. The main distinction between the spectra of (SF(6))(2) dimers recorded here compared to spectra in the gas phase is the splitting of the nu(X),(Y) component. A model that takes into account the influence of the matrix on the spectra of dimers is developed. The model makes it possible to successively (i) calculate the resonance spectrum of an isolated dimer in terms of the model of local modes including the resonance interactions, (ii) determine with the help of the Monte Carlo method the structure of a matrix consisting of 864 N(2) molecules and a rigid (SF(6))(2) dimer, and (iii) take into account the interactions of local dipole moments of a dimer with host particles in the approximation of dipole-induced dipole and dipole-quadrupole interactions. The calculated spectra sufficiently well reproduce the main characteristics of the experimental spectra in N(2) matrixes, in particular, the decrease of the resonance splitting upon transition from the gas phase to a matrix and the splitting of nu(X,Y) component in the nitrogen matrix.