Quantum-mechanical calculation of the intensity distribution in resonance Raman spectra of cytosine

A quantum-mechanical calculation of the relative intensities of lines in resonance Raman (RR) spectra of cytosine excited by laser radiation at 266, 218, and 200 nm was performed in different approximations of the vibronic theory. Both the Herzberg-Teller effect and the contribution from electronic states located close to the resonance state are shown to play a significant role in determining the relative intensities of lines. A satisfactory agreement between the calculated results and experimental data is obtained. The specific features of the intensity distribution in the RR spectra of cytosine are compared with those in the spectra of the previously studied thymine and uracil, which have a similar structure and also belong to the simplest nucleic acid bases.     

Quantum-mechanical calculation of the intensity distribution in resonance Raman spectra of guanine-cytosine pair

The method of quantum-mechanical calculation of the relative line intensities in the resonance Raman (RR) spectra of polyatomic molecules, which was previously applied to the analysis of the spectra of individual cyclic molecules and makes it possible to take into account the Herzberg-Teller and Duschinsky effects, as well as the frequency effect, is applied for the first time to the calculation of the spectra of a pair guanine-cytosine. Satisfactory agreement between the calculated results and the available experimental data is obtained. The particular features of the intensity distribution in the RR spectra of the guanine-cytosine pair excited by laser radiation at 266, 240, 218, and 200 nm are analyzed. The RR spectra of the guanine-cytosine pair are compared with the spectra of the individual guanine and cytosine molecules excited by the laser radiation at the same wavelengths.     

Quantum-mechanical calculation of the intensity distribution in resonance Raman spectra of thiosubstituted derivatives of uracil

A quantum-mechanical calculation of the intensity distribution in the resonance Raman (RR) spectra of 2-thiouracil and 4-thiouracil is carried out for exciting laser radiation at 300, 257, and 248 nm. It is shown that, for satisfactory agreement between the calculated results and the experimental data, it is necessary to take into account in the calculations of the relative intensities of lines the Herzberg-Teller effect and the contribution from excited electronic states adjacent to the resonance state. The general and specific features of the intensity distribution in the RR spectra of uracil and its thiosubstituted derivatives are compared and discussed.     

Quantum-mechanical calculation of the intensity distribution in the resonance Raman spectrum of benzonitrile

A direct quantum-mechanical calculation of the resonance Raman spectrum of a benzonitrile molecule upon excitation with laser radiation at a wavelength of 228.7 nm is performed in the Herzberg-Teller approximation with allowance made for the Duschinsky effect. The results of the calculation are in reasonable agreement with the available experimental data. The intensity distribution in the calculated resonance Raman spectrum of the benzonitrile molecule is compared with the intensity distributions in the spectra of benzene, methyl-substituted benzenes, and halogenated benzenes. It is revealed that the intensity distributions in the resonance Raman spectra of these compounds are characterized by a number of common features.     

Quantum-mechanical analysis of the intensity distribution in spectra of resonant Raman scattering spectra of aqueous solutions of tyrosine

Quantum-mechanical calculations of the intensity distribution in the resonant Raman scattering spectra of aqueous solutions of tyrosine excited by laser radiation with wavelengths of 244, 229, 218, 200, and 193 nm, as well as in the nonresonant Raman scattering spectrum excited at a wavelength of 488 nm, are performed. Satisfactory agreement is achieved between the calculation results and the experimental data. It is shown that the changes in the intensity distribution observed in the spectra with a change in the excitation wavelength from 244 to 193 nm correlate with the determined changes in the contribution made by excited electronic states into the scattering tensor components. It is noted that it is necessary to take into account the Herzberg–Teller effect and that the number of excited electronic states taken into account considerably affects the calculated relative intensities of lines. The possibility of existence of several tyrosine conformers in aqueous solution at room temperature is shown.     

Quantum-mechanical analysis of resonance Raman scattering spectra of the uracil molecule

A direct quantum-mechanical calculation of the relative intensities of lines in the resonance Raman scattering (RRS) spectra of uracil is performed by a method developed earlier by the authors on the basis of the adiabatic model in the Herzberg-Teller approximation [1]. It is shown that the main regularities in the intensity distribution of spectra can be explained only by taking into account the vibronic mixing of electronic states and the contribution to the scattering tensor components from the excited electronic states adjacent to the resonance state. The calculated results are in satisfactory agreement with the results of experimental studies of the RRS spectra of uracil excited by laser radiation at 266, 240, 218, and 200 nm.     

Quantum-mechanical calculation of the intensity distributions in spectra of resonance hyper-Raman scattering and two-photon absorption of indole and skatole

We have performed a quantum-mechanical calculation of the relative intensities of lines in the spectra of resonance hyper-Raman scattering and two-photon absorption of isolated indole, indole in an aqueous solution, isolated skatole, and skatole-water complex. The effects of hydrogen bonds and intermolecular interaction on the spectra have been considered. Particular features of the intensity distributions in the spectra of indole and skatole have been compared.     

Quantum-mechanical description of the intensity distribution in two-photon absorption spectra of dihalogenated benzenes

Using molecules of dihalogenated benzenes as an example, the verification of an earlier proposed method of direct quantum-mechanical calculation of the relative intensities of lines in the two-photon absorption spectra of polyatomic molecules is continued. The method of calculation is based on the adiabatic model in the context of the Herzberg-Teller approximation. A comparative analysis of the intensity distribution in the two-photon absorption spectra of dihalogenated and monosubstituted benzenes is carried out, and the vibrational characteristics of these molecules in the lowest singlet excited electronic states are determined. A satisfactory agreement with experimental values, along with the analysis of the possibilities of the method, confirms the expediency of its use for gaining information about the intensity distribution in two-photon absorption spectra of polyatomic molecules.     

Quantum-mechanical calculations of the intensity distribution in spectra of adenine tautomers: II. Two-photon absorption spectra

We have performed quantum-mechanical calculations of the two-photon absorption spectra of three tautomeric forms of adenine in the gas phase and of four forms in the aqueous solution. Based on a comparison of the results of these calculations with experimental data, the occurrence of three structures, Ade-N₉,N₁H⁺, Ade-N₉H (am), and Ade-N₇H(am), in aqueous solution of adenine has been confirmed.     

Quantum-mechanical calculations of the intensity distribution in spectra of adenine tautomers: I. Spectra of resonant hyper-Raman scattering

The spectra of resonant hyper-Raman scattering of five tautomeric forms of adenine have been theoretically determined for the first time based on quantum-mechanical calculations in the Herzberg-Teller approximation. The occurrence of three structures, Ade-N₉,N₁H⁺, Ade-N₉H(am), and Ade-N₇H(am), in an aqueous solution of adenine has been established.     

Relaxation effects in resonance Raman spectra

A density-matrix theory of resonance scattering spectroscopy is presented to characterize the relaxation processes which may be studied in these spectra. The distinction between Resonance Raman and Fluorescence emerges clearly from this treatment. It is shown that novel properties of the rotational and vibrational relaxation may be obtained from the lineshape of the scattered light. A new effect is discovered in the Resonance Raman excitation profile which, in principle, could be used to study the relaxation rate between specific vibronic levels of an excited electronic state.     

First-principles calculation of nuclear resonance vibrational spectra

We present a ‘first-principles’ methodology for the calculation of the parameters that are required for the simulation of nuclear resonance vibrational spectra (NRVS) of molecular systems. Formulae are given for the intensities of vibrational transitions corresponding to the so-called single- and double-phonon contributions to the NRVS signal. The method is also valid for those vibrations that are not in the high-frequency/low-temperature limit. We have rigorously treated the issue of orientational averaging of the Lamb–Mössbauer factor and the effect of the neglect of its anisotropy on the calculated NRVS pattern. Normal mode composition factors are determined in a compact form as appropriate components of an orthogonal matrix that diagonalizes the Hessian matrix. The method is illustrated by simulating the NRVS spectra and the partial vibrational density of states of [FeO(H₂O)5]²⁺ on the basis of vibrational frequencies and normal mode composition factors obtained from density functional theory (DFT) calculations.     

Atypical intensity distribution in the Raman spectrum of 9,10-anthraquinone

The structure, the frequencies of the normal vibrations, and the absolute intensities of the bands in the IR and Raman spectra of 9,10-anthraquinone and its four symmetrical isotopomers are calculated in terms of the DFT/B3LYP method with the 6-31G(d) basis set. The effective harmonic force field of 9,10-anthraquinone is found by the Pulay method. A technique for directly obtaining the effective force fields without using experimental data on the frequencies of fundamental vibrations is proposed. An atypical intensity distribution in the Raman spectrum of 9,10-anthraquinone between two totally symmetric A _g and two nontotally symmetric B _3g vibrations is found. A new interpretation of these four experimentally observed vibrational Raman bands is proposed.     

Quantum mechanical analysis of resonance Raman spectra of thymine

A direct quantum mechanical calculation of the relative intensities of lines in resonance Raman (RR) spectra of thymine was performed. The method of calculation is based on the adiabatic model in the Herzberg-Teller approximation. It is shown that the basic features of the intensity distribution in the spectra can be explained only by taking into account the vibronic mixing of electronic states and the contribution to the components of the scattering tensor from excited electronic states located close to the resonance state. The calculated results agree satisfactorily with experimental RR spectra of thymine excited by laser radiation at 266, 240, 218, and 200 nm. A comparative analysis of the intensity distribution in the RR spectra of thymine and uracil is carried out.     

杨氏双干涉的光强分布计算

数值计算了杨氏双孔干涉的光强分布,与傍轴近假条件下的结果作了比较,并以图形的方式显示了干涉条纹的形状。     

圆孔衍射光强分布的数值计算

利用数值积分的方法,计算了由点光源经圆孔衍射形成的光强分布,分别得到夫琅禾费及菲涅耳衍射图样,宽定量分析了满足夫琅禾费衍射的条件。