V. V. Nechaev

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.

Theoretical interpretation of the vibrational spectrum of bicyclo[1.1.0]butane in terms of an ab initio anharmonic model

The anharmonic vibrational IR and Raman spectra of the bicyclo[1.1.0]butane molecule have been calculated in the range of up to 4000 cm−1 using a numerical and analytical realization of the van Vleck second-order operator perturbation theory. Cubic and quartic force constants in normal coordinates, as well as cubic surfaces of the dipole moment and polarizability, have been found by numerical differentiation of the corresponding first and second derivatives calculated by the MP2/cc-pVTZ quantum-mechanical method. In order to increase the prediction accuracy of vibrational transitions, corresponding harmonic frequencies have been obtained by the CCSD(T)/cc-pVTZ high-precision quantum mechanical method. The anharmonic intensities of the IR and Raman spectra have been calculated using canonical transformations of the operators of the dipole moment and polarizability expanded into a Taylor series around the equilibrium configuration. The assignment of experimental vibrational bands in the IR and Raman spectra has been analyzed. It has been shown that the anharmonic calculation based on the above-described procedure of combining more exact harmonic frequencies with the anharmonic force field obtained with a more economical method makes possible the reliable interpretation of the majority of spectral bands, including Fermi and Darling-Dennison resonances.

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.

Density Functional Study of the Structure and Infrared Spectrum of Ethyl Chlorophyllide (A)

A DFT/B3LYP calculation using the 6-31G(d) basis set was performed to obtain the structure, normal vibration frequencies, and absolute IR intensities of ethyl chlorophyllide (A). The calculated structural parameters agree well with the experimental X-ray diffraction data. The role of saturated substituents in stabilization of radical ion states of chlorophyll is discussed. The effective force field of ethyl chlorophyllide (A) was obtained in independent and dependent internal coordinates. The IR vibrational spectrum is modeled. The experimental IR spectrum of chlorophyll (A) is interpreted on the basis of the calculation.

Structure of the excited biacetyl molecule and analysis of its absorption and fluorescence spectra

A structural-dynamical model of biacetyl and biacetyl-d6 molecules in the excited electronic state is derived on the basis of semiempirical correlations and the method of hybrid atomic orbitals. The vibrational structure of absorption and fluorescence spectra is calculated in satisfactory agreement with experimental data. Interpretation of the lines is corrected. The effect of variations in the molecule valence angles, caused by electronic excitation, on the intensity of vibrational components is ascertained. A possibility of calculating relatively small structural and spectral variations resulting from methyl substitution in the sequence of glyoxal, methylglyoxal, and biacetyl molecules is demonstrated.

Calculation of normal vibrations of adenine and its deuterium-substituted analogs

The vibrational spectra of adenine and its deuterium-substituted analogs are calculated in the valence force field approximation. The frequencies and forms of normal vibrations of the molecule at 1700–300 cm-1 are interpreted.

Hydrogen bond effect on the structure and vibrational spectra of complementary pairs of nucleic acid bases. III. Guanine-cytosine

Vibrational spectra of the isolated complementary guanine-cytosine pair are calculated at the B3LYP/6-311++G(d,p) level. The hydrogen bond effect on the structure, frequency positions, and intensities of normal vibrations of the pair are analyzed in comparison with the spectra of isolated guanine and cytosine molecules. Characteristic spectral features of the formation of the complementary guanine-cytosine base pair are revealed.

Experimental and theoretical study of the IR spectrum of 4-amyloxy-4′-cyanobiphenyl

The IR absorption spectrum of polycrystalline 4-amyloxy-4′-cyanobiphenyl (5OCB) is measured in a KBr pellet over the range 400–4000 cm−1. The structure of the molecule and the frequencies and intensities of the bands in the spectrum are calculated in the approximation of the B3LYP hybrid density functional with the 6-31G(d) and 6-31+G(d) basis sets. With the method of linear scaling of frequencies, 39 bands of the experimental IR spectrum are assigned. On the basis of calculations for related compounds, the vibrations belonging to the substituents and the biphenyl fragment are ascertained. It is shown that the IR absorption spectra of polycrystalline 4-hydroxypropyl-4′-cyanobiphenyl and 5OCB are almost identical and differ by the occurrence of three bands associated with vibrations localized on the oxyamyl radical.

Experimental and theoretical study of the IR spectrum and barriers to internal rotation of 4-hydroxypropyl-4′-cyanobiphenyl

The IR absorption spectrum of polycrystalline 4-hydroxypropyl-4′-cyanobiphenyl is measured in a KBr pellet over the range 400–4000 cm−1. The structure of the molecule and the frequencies and the intensities of the bands in the spectrum are calculated in the approximation of the B3LYP hybrid density functional with the 6-31 G(d) and 6-31+G(d) basis sets. The normal vibrations are reliably assigned by the method of linear scaling of frequencies, which made it possible to reproduce the experimental IR spectrum with high accuracy. The barriers to internal rotation of phenyl rings and three barriers for the hydroxypropyl radical are calculated.

CALCULATION OF THE VIBRONIC SPECTRA OF ADENINE

The vibrational structure of the absorption spectra of the first two π-π* singlet transitions of adenine is calculated in the Franck-Condon approximation including Herzberg-Teller interactions. The effect of excitation-induced changes in molecular angles on the intensities of the vibrational components is estimated. Structural models of the adenine molecule in the excited states are constructed. The theoretical and absorption spectra of the first π-π* transition are compared. The results of the electronic structure calculations of adenine by different CNDO/S methods are discussed.