Quantum-chemical calculations of the structure,vibrational spectra,and torsional and inversion potentials of methylcarbamate

We present results of ab initio and DFT calculations of the structure, potential functions of the methyl group internal rotation and the amino group inversion, and vibrational frequencies and intensities in IR and Raman spectra of methylcarbamate. The calculations were carried out using different basis sets in the HF, MP2, and DFT/B3LYP approximations. The influence of both the basis set size and the allowance for electronic correlation on peculiarities of the structure of the amino group in methylcarbamate has been analyzed. It is shown that the B3LYP/6-311++G(2d, p) and B3LYP/cc-pVDZ calculations reproduce highly accurately experimental geometric parameters of methylcarbamate. Parameters of torsional and inversion potentials and characteristics of vibrational spectra calculated in different approximations show satisfactory agreement with experimental values. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 3, pp. 349–357, May–June, 2009.     

Calculation of the IR Spectrum of Ethyl Chlorophyllide a by the Method of Density Functional

The structure, frequencies of normal vibrations, and the absolute IR intensities of ethyl chlorophyllide a have been calculated by the DFT/B3LYP/6-31G(d) density functional method. The force constants have been scaled by the Pulay method. The force field of ethyl chlorophyllide a has been obtained in independent and dependent natural coordinates. The vibrational IR spectrum of ethyl chlorophyllide a has been modeled. The experimental IR spectrum of chlorophyll a has been interpreted on the basis of the calculation performed.     

Raman spectra of alkyl-substituted azaoxa[8]circulenes: DFT calculation and experiment

Raman lines in spectra of recently synthesized azaoxa[8]circulenes have been assigned in detail on the basis of obtained experimental data and B3LYP/6-31G(d) calculations. Particular features of the structure of three azaoxa[8]circulenes have been considered based on data of X-ray structural analysis and DFT calculations. Regularities in changes of the geometric parameters have been found for benzene, furan, pyrrole, and naphthalene rings that occur upon formation of the macroring in comparison with free molecules. Raman spectra of macrorings have been shown to contain a characteristic set of lines the frequencies of which differ from those of analogous lines in Raman spectra of benzene, pyrrole, furan, and naphthalene. Results of quantum-chemical calculations agree well with experimental data with respect to line frequencies and intensities due to recalculation of Raman activities of normal vibrations into intensities of Raman lines.     

Structural Dynamic Models and Vibrational Spectra of Nitrobenzene and Nitropyridines

Using the DFT/B3LYP method with the basis sets 6-31G*(**), we analyzed the vibrational spectra of nitrobenzene and nitropyridines for free molecules and their solutions in water, ethanol, and benzene. The influence of a solvent on the behavior of the characteristic spectral parameters of nitrobenzene is revealed, and the assignment of vibrations for nitropyridines is given. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 4, pp. 450–453, July–August, 2005.     

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.     

Quantum chemical analysis of vibrational spectra of methylphenylcarbamate

We present results of ab initio and DFT calculations of the structure, potential function of internal rotation of the methyl group, and vibrational frequencies and intensities in IR and Raman spectra of methylphenylcarbamate. The calculations were carried out in different basis sets in the HF, MP2, and DFT/B3LYP approximations with partial force field scaling. The influence of the phenyl substituent on structural and spectral characteristics of the urethane group has been analyzed. Calculated characteristics of vibrational spectra show satisfactory agreement with experimental values.     

Vibrational investigation of 1-cyclopentylpiperazine: A combined experimental and theoretical study

FT-IR and Raman spectra of 1-cyclopentylpiperazine(1cppp)have been experimentally examined in the region of 4000–200cm-1.The optimized geometric parameters,conformational equilibria,normal mode frequencies and corresponding vibrational assignments of 1cppp(C9H18N2)are theoretically examined by means of B3LYP hybrid density functional theory(DFT)method together with 6-31++G(d,p)basis set.On the basis of potential energy distribution(PED)reliable vibrational assignments have been made and the thermodynamics functions,highest occupied and lowest unoccupied molecular orbitals(HOMO and LUMO)of 1cppp have been predicted.Calculations are employed for four different conformations in C1 and Cs point groups of 1cppp in gas phase.Comparison between the experimental and theoretical results indicates that B3LYP method is able to provide satisfactory results for predicting vibrational frequencies and the structural parameters,vibrational frequencies and assignments.Furthermore,C1(equatorial-axial)point group has been found as the most stable conformer of 1cppp.     

Calculation of Plane Vibration Frequencies of the Porphin Molecule

By the DFT/B3LYP method with a 6-31G^** basis set the frequencies of normal vibrations of porphin and its five derivatives have been calculated. Scaling of the force constants for plane vibrations in independent natural coordinates has been carried out. Symmetry coordinates have been introduced and a force field for plane vibrations of the porphin molecule in independent symmetry coordinates has been obtained. Based on an analysis of special matrices and the potential energy distribution, complete matching of the plane vibration frequencies of porphin and its four isotopomers has been performed.     

Calculation of Normal Vibrations of Chlorin by the Method of Density Functionals

Using the DFT/B3LYP method with a 6-31G(d) basis set, the structure, normal vibration frequencies, and the absolute band intensities in the IR spectra of the chlorin molecule and its four symmetric isotopomers have been calculated. Scaling of the force field by the Pulay method in independent and natural coordinates has been carried out. A method for obtaining effective force fields without using experimental data on the fundamental vibration frequencies is proposed. By comparing the vibration modes and constructing special matrices, complete assignment of the fundamental frequencies of porphin and chlorin has been carried out. It has been shown that the majority of porphin macroring vibrations upon pyrrolenine ring hydrogenation are frequency-characteristic and only 12 vibrations change considerably. A frequency correlation with regard for the mode transition between chlorin and all its isotopomers under consideration has been established. Comparative analysis of the force fields of porphin and chlorin in dependent natural coordinates has revealed the unique nonlocal character of the change in force constants of the macroring upon hydrogenation of one pyrrolenine ring. Modeling of the IR spectra of chlorin and its isotopomers has been performed. Assignment and interpretation of the normal vibrations of the molecules under consideration have been carried out.     

硝酸丙酯结构、振动频率和热力学性质的密度泛函理论研究

用密度泛函理论(DFT)研究硝酸丙酯化合物的分子结构、振动光谱和热力学等基本性质.取BLYP、B3LYP方法和6-31G*、6-31G**、6-311G*、6-311G**基组,对硝酸丙酯分子的几何构型进行全优化计算并分析其电子结构性质.和考虑了二级相关能校正的MP2/6-311G*计算结果比较表明,B3LYP/6-31G*是研究许多较大体系化合物卓有成效和颇有前途的方法.在B3LYP/6-31G*的水平上对优化后的结构进行了正则振动频率分析,用因子0.95校正后的振动光谱和实验结果比较,符合较好.进一步