Microwave spectrum of torsionally excited acetic acid

The microwave spectrum corresponding to the first excited state of the methyl torsion in acetic acid has been identified by means of microwave-microwave double resonance. Although the A-E splittings are extremely large, a reasonable fit has been obtained for the v = 0 and v = 1 states simultaneously by using a Hamiltonian which allows for geometry relaxation upon internal rotation. Barrier parameters are V₃ = 169.90 ± 0.06 cm^?1 and V₆ = ?6.74 ± 0.02 cm^?1. An interpretation of the parameters describing nonrigidity is given in terms of a model with two relaxing bond angles, which is qualitatively supported by ab initio calculations.     

Intermolecular hydrogen bonds in acetic acid and its solutions

The band at 896 cm⁻¹ in the Raman spectrum of acetic acid consists of at least two lines (873 and 896 cm⁻¹) whose intensities and depolarization factors differ strongly. A temperature increase leads to an increase in the intensity of the 873-cm⁻¹ line as compared to the intensity of the 896-cm⁻¹ line. Dilution of the acid with acetonitrile leads to a similar but much more pronounced effect. In solutions with CCl₄ and water, the relative intensity of the low-frequency line decreases; in aqueous solutions, the width of the 896-cm⁻¹ line passes through a maximum with 0.4 mol. fractions of the acid (for this concentration, the line width is larger by a factor of 2.5 than in a pure liquid), while in the solutions with CCl₄ it decreases smoothly (almost twofold with strong dilution: 0.05 mol. fractions). Experimental data are in agreement with the assumption that the 873- and 896-cm⁻¹ lines refer to the vibrations of the same atoms in a molecule and to associates with a free and an H-bonded atom of the oxygen of a C=O group, respectively. The difference in the frequencies and depolarization factors of the lines causes the differences in the distribution of an electron cloud in the molecule. To whom correspondence should be addressed. Alisher Navoi Samarkand State University, 15, Universitetskaya Str., Samarkand, 703004, Uzbekistan. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 4, pp. 467–470, July–August, 1999.     

Lattice vibrational spectrum of diamond

The lattic dynamics of covalent crystals are discussed with reference to known models. The dispersion curves of diamond have been computed on the basis of the shell model of Cochran applicable to covalent crystals. Parameters have been determined using elastic constants and dispersion curves along the Δ and A directions from the neutron spectrometric data of Warren et al. In general there is good agreement between the calculated curves and experiment. The average error is about 4.8%. Although the calculated curves seem to be a definite improvement over the curves calculated by Smith there appears to be a certain discrepancy between theory and experiment. Plausible causes of this discrepancy are pointed out.     

The vibrational spectrum of fluorobenzene

The high resolution infrared spectra of three fluorobenzene isotopes were obtained and interpreted. Vibrational assignments of most bands were made and values for most of the fundamental vibrations were obtained. Accurate values for the infrared inactive a₂ vibrations were also obtained. Several major modifications of literature values for the fundamentals were made and a Fermi resonance interaction identified.     

The millimeter and submillimeter spectrum of CN in its first four vibrational states

The rotational absorption frequencies of 65 new lines in the millimeter and submillimeter region of the spectrum have been measured for the CN radical in its ground electronic state. These measurements were made in a low pressure glow discharge of methane and nitrogen and include 25 lines from the v = 2 and v = 3 vibrational states, in addition to 40 lines from v = 0 and v = 1. The Dunham constants, as well as the spin-rotation and hyperfine constants of these four vibrational states, were calculated by means of a global nonlinear least squares fit of these data.     

Hydration of acetic acid and acetate ion in water studied by 1D-RISM theory

Structural and thermodynamic aspects of the hydration of acetic acid (CH₃COOH) and acetate ion (CH₃COO⁻) were studied with the 1D-RISM integral equation method. It was found that the average number of water molecules in the hydrophobic hydration shell of the CH₃ group is 8.9 for acetic acid and 10 for acetate. The average number of H-bonds formed by the COOH group is 2.5, whereas that of COO⁻ is 6, indicating that deprotonation of acetic acid leads to increased H-bonding of water molecules with the carboxylate moiety. This step involves significant reorientation of water molecules surrounding the carboxylic group. The hydration free energies and the aqueous ionization constant calculated with the set of semi-empirical corrections are in reasonable agreement with available experimental results.     

The privileged spectrum of cnoidal ion holes and its extension by imperfect ion trapping

The fundamental properties of nonlinear ion hole modes propagating in current-driven collisionless plasmas are derived. Making use of Schamel's alternative method their spatial structure $?(x)$ and phase velocities $u_0$ are analyzed and found to depend crucially on the used trapped ion distribution $f_{it}$. A regular $f_{it}$ represents a continuous spectrum, which is called privileged or perfect since it yields a definite $u_0$ and appears most realistic. A singular $f_{it}$, on the other hand, involving jumps and moderate slope singularities at the separatrix, does reveal further classes of hole equilibria at the cost, however, of a well-defined $u_0$. This explains why Bernstein, Greene, Kruskal (BGK)-solutions of the Vlasov–Poisson system, exhibiting a strong slope singularity of their derived trapped particle distribution, can principally not provide definite $u_{0}s$. The nonlinear dispersion relation (or $u_0$) of privileged ion holes, on the other hand, is equivalent with that of cnoidal electron holes, i.e. in addition to the ordinary ion acoustic branch there exists a correspondence to the “Langmuir” branch and to the multiple “slow electron acoustic” branches, reflecting different trapping scenarios.     

Interpretation of the vibrational spectra of polycrystalline adenine

The infrared and Raman spectra of the tetramer of the adenine N₉H are calculated and analyzed. The vibrational spectra of polycrystalline adenine are interpreted. It is demonstrated that the method for calculating the vibrational spectra of molecular complexes formed by hydrogen bonds can be used for interpreting the vibrational spectra of polyatomic molecules in the solid state.     

Interpretation of vibrational IR spectrum of uracil using anharmonic calculation of frequencies and intensities in second-order perturbation theory

The anharmonic frequencies of fundamental vibrations, overtones, and combination vibrations, as well as the intensities of absorption bands in the IR spectrum of uracil, are calculated. The anharmonic quartic force field and the third-order dipole moment surface calculated by the DFT quantum-mechanical method (B3LYP/6-31+G(d,p)) are taken as the initial parameters. The anharmonic frequencies and intensities of vibrations are determined using the second-order perturbation theory in the form of contact transformations. Multiple Fermi resonances and polyads are determined by the diagonalization of a small interaction matrix of vibrations of different types (fundamental, combination, and overtone frequencies). The total experimental IR spectrum of matrix-isolated uracil is interpreted. It is shown that the used method of calculating anharmonic frequencies and intensities can form a basis for anharmonic calculations of vibrations of moderate molecules.     

Raman scattering and the low-frequency vibrational spectrum of glasses

We present a theory of low-frequency Raman scattering in glasses, based on the concept that light couples to the elastic strains via spatially fluctuating elasto-optic (Pockels) constants. We show that the Raman intensity is not proportional to the vibrational density of states (as was widely believed), but to a convolution of Pockels constant correlation functions with the dynamic strain susceptibilities of the glass. Using the dynamic susceptibilities of a system with fluctuating elastic constants we are able for the first time to describe the Raman intensity and the anomalous vibration spectrum of a glass on the same footing. Good agreement between the theory and experiment for the Raman spectrum, the density of states, and the specific heat is demonstrated at the example of glassy As(2)S(3).     

Computer calculation of the vibrational spectrum of nanoparticles

A new method for calculating the vibrational spectrum of nanoparticles is proposed. The method is based on a molecular-dynamics simulation of the oscillations of the center of mass and of individual atoms and subsequent Fourier analysis of the obtained time series. It is shown by way of a concrete example that, depending on the dimensions of the nanocrystallite, the calculated spectrum of intrinsic vibrations can consist of one or more dominant harmonics. Correspondence of this method to the open resonator model and to calculations of longitudinal intrinsic vibrations of a rod in the theory of elasticity is demonstrated. Fiz. Tverd. Tela (St. Petersburg) 39, 2062–2064 (November 1997)