Ab initio studies on vibrational spectra of XSO2NCO (X = F,Cl): harmonic force fields and frequency assignments

The harmonic vibrational force fields and the IR spectrum of XSO₂NCO (X= F, C1) molecules have been studied usingab initio HF/SCF method with the 6-31G’ basis set. Theab initio harmonic force fields are scaled empirically using the scaled quantum mechanical (SQM) method of Pulay. A set of scale factors are optimized by the least-squares fitting to the experimental frequencies of FSO₂NCO and then are transferred to CISO₂NCO to give ana priori prediction of its fundamental frequencies. The average deviations between the theoretical frequencies and the experimental values for FSO₂NCO and C1SO₂NCO are 3 and 5 cm^-1, respectively. The assignments of the fundamentals for these two molecules are also made atcording to the potential energy distributions and theab initio IR intensities Project supported by the National Natural Science Foundation of China (Grant No. 29673029)     

氟磺酸氯分子振动光谱的从头算研究

采用从头算HF方法以6-31G^*基组研究了对ClOSO₂F分子的几何结构、振动谐性力场和红外光谱.理论力场由Pulay的标度量子力学方法进行标度,算得的振动频率与实验值比较,平均偏差为6.0cm^-1.根据振动频率的势能分布和从头算红外光谱强度值对此分子的振动基频进行了理论归属.     

氟磺酸氟振动光谱的从头算

采用从头算HF／SCF方法以6-31G基组研究了FOSO2F分子的几何结构、振动谐性力场和红外光谱强度．理论力场由Pulay的标度量子力学方法进行标度,计算得到的振动频率与实验值比较平均偏差为6．3cm-1．根据振动频率的势能分布和从头算红外光谱强度值对此分子的振动基频进行了理论归属．     

Ab Initio Calculations on Nitramide and Its Methyl Derivatives( Ⅱ )——The Harmonic Force Fields and Vibrational Spectra of Nitramide and Its Isotopic Derivatives

The harmonic force field and the vibrational spectrum of nitramide were calculated by using the ab initio gradient program TEXAS at the Hartree-Fock level with a 4-21G basis set. The directly computed theoretical harmonic force field was scaled by using empirical scale factors which are transferred from other molecules and provided an a priori prediction of fundamental frequencies and intensities. The average deviations between predicted vibrational frequencies of nitramide and experimental IR spectrum in an argon matrix are 63 cm-1 for symmetric vibrations and 41 cm-1 for antisymmetric modes. A new set of scale factors was optimized in this paper. These scale factors reduced the average deviations to 2. 3 cm-1 for symmetric modes and 0. 8 cm-1 for antisymmetric ones. The vibrational spectra of three isotopic derivatives of nitramide were predicted by using the force field resulted from the optimized set of scale factors, which are in good agreement with their experimental data in an argon matrix.     

γ-吡啶甲酸及其同位素衍生物的谐性力场和振动光谱的从头计算研究

本文采用TEXAS分析梯度法从头计算程序, 以STO-4-21G基组计算了γ-吡啶甲酸的谐性力场和振动光谱。直接理论计算的谐性力场经由相关分子转移来的校正因子校正后, 得到的振动基频的预测值和固体样品红外光谱实验值之间的平均偏差为20cm^-^1(面内振动23cm^-^1, 面外振动11cm^-^1)。用这组校正因子得到的力场预测了γ-吡啶甲酸的两个同位素衍生物(-C^1^8O~2H和-C^1^6O~2D)的振动光谱, 所得同位素位移值与实验数据符合良好。对平面内振动的个别校正因子依据实验光谱进行了优化, 平面内振动的平均偏差降为15cm^-^1, 总的偏差为14cm^-^1。对预测中的偏差和某些基频的指认进行了讨论。     

Theoretical studies of force fields and IR spectra of isocytosine

The optimized geometries, complete harmonic force fields, and infrared intensities of isocytosine tautomers, amino‐hydroxy and amino‐oxoN(1)H, were calculated at the ab initio Hartree–Fock level using the 6‐31G* basis set. The theoretical force fields were scaled by empirical scale factors, which were determined by fitting to the IR spectrum of the amino‐oxo form and were then transferred to the amino‐hydroxy form. The average deviations between experimental and computed frequencies are 7.6 cm⁻¹ for amino‐oxo and 9.5 cm⁻¹ for amino‐hydroxy, respectively. The assignments of the fundamental frequencies and the transferability of the force constant scale factors are also presented. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 53–60, 1999     

Vibrational spectra and ab initio analysis of tert-butyl, trimethylsilyl, and trimethylgermyl derivatives of 3,3-dimethyl cyclopropene V. 3,3-dimethyl-1-(trimethylgermyl)cyclopropene

3,3-dimethyl-1-(trimethylgermyl)cyclopropene (I) was synthesised using a standard procedure. The IR and Raman spectra of I in the liquid phase were measured. The molecular geometry of I was optimised completely at the HF/6-31G* level. The HF/6-31G*//HF/6-31G* force field was calculated and scaled using the set of scale factors transferred from those determined previously for scaling the theoretical force fields of 3,3-dimethylbutene-1 and 1-methyl-, 1,2-dimethyl-, and 3,3-dimethylcyclopropene. The assignments of the observed vibrational bands were performed using the theoretical frequencies calculated from the scaled HF/6-31G*//HF/6-31G* force field and the ab initio values of the IR intensities, Raman cross-sections and depolarisation ratios. The theoretical spectra are given. The completely optimised structural parameters of I and its vibrational frequencies are compared with corresponding data of related molecules.     

Vibrational spectroscopic study of pyrrole and its deuterated derivatives: comparison of the quality of the applicability of the DFT/Becke3P86 and the DFT/Becke3LYP functionals

The aim of our work is a reassignment of the vibrational spectra of pyrrole based on high-level quantum-chemical calculations. This is a continuation of our earlier works on one side of pyrrolydine and N-methylpyrrolidine and on the other side of five-membered ring parent molecules with two-ring nitrogens.

Infrared and Raman spectra are reported for the isotopic species of pyrrole, 1-deuteropyrrole and pentadeuteropyrrole.

The molecular structure and the harmonic force field were calculated applying the ab initio density functional theory (DFT) level with both the Becke3P86 and the Becke3LYP functionals with the 6-311G(d,p) basis set. The force fields were fitted to the experimental fundamentals using in both cases eight scale factors. Though some scaled frequencies show larger deviations from the experimental ones, the percentage deviations of the calculated frequencies from the experimental ones are less than 1.0% for pyrrole and less than 1.2% for the deuterated derivatives in the case of both applied functionals.     

Systematic study of vibrational frequencies calculated with the self-consistent charge density functional tight-binding method

We present a detailed study of harmonic vibrational frequencies obtained with the self-consistent charge density functional tight-binding (SCC-DFTB) method. Our testing set comprises 66 molecules and 1304 distinct vibrational modes. Harmonic vibrational frequencies are computed using an efficient analytical algorithm developed and coded by the authors. The obtained results are compared to experiment and to other theoretical findings. Scaling factor for the SCC-DFTB method, determined by minimization of mean absolute deviation of scaled frequencies, is found to be 0.9933. The accuracy of the scaled SCC-DFTB frequencies is noticeably better than for other semiempirical methods (including standard DFTB method) and approximately twice worse than for other well established scaled ab initio quantum chemistry methods (e.g., HF, BLYP, B3LYP). Mean absolute deviation for the scaled SCC-DFTB frequencies is 56 cm(-1), while standard deviation is 82 cm(-1), and maximal absolute deviation is as large as 529 cm(-1). Using SCC-DFTB allows for substantial time savings; computational time is reduced from hours to seconds when compared to standard ab initio techniques.     

硝胺及其甲基衍生物的从头计算研究 3: 甲硝胺及其同位素衍生物的谐性力场和振动光谱

用TEXAS从头计算程序,取STO-4-21G基组,计算了甲硝胺的谐性力场和振动光谱.直接理论计算的谐性力场经由其他分子转移来的经验校正因子校正后,提供了甲硝胺振动基频的预测.预测值和甲硝胺分子在气相中的振动光谱实验值之间的平均偏差为31cm^-1.为了获得更合适的气相甲硝胺振动力场和预测它的同位素衍生物的振动光谱,我们优化了一组新的校正因子,使理论值和实验值的平均偏差减为8.9cm^-1.用这组校正因子得到的力场预测了三个同位素衍生物的振动光谱,其同位素位移的理论预测值和实验值符合良好.     

Vibrational frequencies and assignments for some isotopomers of urcail using a scaled ab initio force field

Vibrational frequencies and IR band intensities for 18 isotopomers of uracil, including deuterated ¹⁵N and ¹⁸O species, have been calculated using the scaled ab initio force field of Ref. 1. The results obtained are compared with available experimental data, and a number of refinements in former assignments are proposed. The good agreement between the calculated and experimental frequencies confirms the reliability of the scaled quantum mechanical-force field.     

Effects of hydration on scale factors for ab initio force constants

Experimentally measured vibrational frequencies from the polar groups of peptides in aqueous solutions do not agree with frequencies calculated from scaled quantum mechanical force fields (SQMFF) using differential scale factors developed for molecules in the vapor phase. Measured stretching frequencies for carbonyl groups are more than 50 wavenumbers lower than the calculated values. On the other hand, frequencies for non-polar groups calculated using these scale factors are relatively accurate. Our goal is to develop a SQMFF that yields accurate calculated frequencies for peptides in aqueous solutions. To this end, we have calculated scale factors for ab initio force constants for formic acid, acetic acid, and acetone using a least squares fit of calculated and experimental frequencies. We compare these scale factors with changes observed in the ab initio force constants calculated for these molecules at various states of hydration. These force constants are calculated using fully optimized geometries for these hydrated molecules using the 4-31G basis. We present a comparison of the experimental and calculated frequencies, along with their potential energy distributions, for both vapor and aqueous phases. The results indicate that scale factors can simulate the effects of solvation on molecular force constants to yield accurate scaled ab initio force fields.     

Calculation of Chlorin IR Spectrum by Density Functional Theory

The structure, frequencies of normal vibrations, and absolute intensities of bands in the IR spectra of chlorin and four of its symmetric isotopomers were calculated using DFT/B3LYP with the 6-31G(d) basis set. The force field was scaled by the Pulay method in independent and dependent natural coordinates. A method for obtaining effective force fields without using experimental data for the frequencies of fundamental vibrations was proposed. It has been demonstrated that most vibrations of the porphyrin macrocycle have characteristic frequencies upon hydrogenation of the pyrrolenine ring and only 12 vibrations differed significantly. The IR spectra of chlorin and its isotopomers were modelled. Frequencies were assigned and normal vibrations were interpreted for the examined molecules.     

Ab initio and DFT studies of the molecular structures and vibrational spectra of succinonitrile

The Molecular structure, conformational stability and vibrational frequencies of succinonitrile NCCH2CH2CN have been investigated with ab initio and density functional theory (DFT) methods implementing the standard 6-311++G* basis set. The potential energy surfaces (PES) have been explored at DFT-B3LYP, HF and MP2 levels of theory. In agreements with previous experimental results, the molecule was predicted to exist in equilibrium mixture of trans and gauche conforms with the trans form being slightly lower in energy. The vibrational frequencies and the corresponding vibrational assignments of succinonitrile in both C2h and C2 symmetry were examined theoretically and the calculated Infrared and Raman spectra of the molecule were plotted. Observed frequencies for normal modes were compared with those calculated from normal mode coordinate analysis carried out on the basis of ab initio and DFT force fields using the standard 6-311++G* basis set of the theoretical optimized geometry. Theoretical IR intensities and Raman activities are reported.     

Fluorosulfonic acid and chlorosulfonic acid: possible candidates for OH-stretching overtone-induced photodissociation

We have calculated the stationary points and internal reaction coordinate pathway for the dissociation of fluorosulfonic acid (FSO3H) and chlorosulfonic acid (ClSO3H). These sulfonic acids dissociate to sulfur trioxide and hydrogen fluoride and chloride, respectively. We have calculated the frequencies and intensities of the OH-stretching transitions of FSO3H and ClSO3H with an anharmonic oscillator local mode model. We find that excitation of the fourth and third OH-stretching overtones provide adequate energy for photodissociation of FSO3H and ClSO3H, respectively. We propose that experimental detection of the products of OH-stretching overtone-induced photodissociation of FSO3H and ClSO3H would be easier than the sulfuric acid (H2SO4) equivalent. The photodissociation of H2SO4 is thought to be important in the stratosphere. The FSO3H and ClSO3H experiment could be used in proxy to support the recently proposed OH-stretching overtone-induced photodissociation mechanism of H2SO4.     

Analysis of vibrational spectra of L-alanylglycine based on density functional theory calculations

FT Raman and IR spectra of the crystallized biologically active molecule, L-alanylglycine (L-Ala-Gly) have been recorded and analyzed. The equilibrium geometry, bonding features and harmonic vibrational frequencies of L-Ala-Gly have been investigated with the help of B3LYP density functional theory (DFT) method. The calculated molecular geometry has been compared with the experimental data. The assignments of the vibrational spectra have been carried out with the help of normal coordinate analysis (NCA) following the scaled quantum mechanical force field methodology (SQMFF). The optimized geometry shows the non-planarity of the peptide group of the molecule. Potential energy surface (PES) scan studies has also been carried out by ab initio calculations with B3LYP/6-311+G** basis set. The red shifting of NH3+ stretching wavenumber indicates the formation of N-H...O hydrogen bonding. The change in electron density (ED) in the sigma* antibonding orbitals and E2 energies have been calculated by natural bond orbital analysis (NBO) using DFT method. The NBO analysis confirms the occurrence of strong intermolecular hydrogen bonding in the molecule.     

Scaled quantum mechanical force field for glycine in basic solution

We obtain scale factors for three glycinate-nH₂O ab initio force fields, using the 4–31G basis set, that can be used in building a scaled quantum mechanical force field for alanine and, subsequently, for peptides in aqueous solutions. Force constants from the fully optimized glycinate-nH₂O supermolecules were scaled by using experimentally determined vibrational frequencies of glycine in water at pH 13. Similar calculations were performed for methylamine and acetate. Scale factors for the stretching modes of acetate are within 2% of the related scale factors for glycinate. The scale factor for the NH₂ scissor mode in methylamine is also in agreement with that of glycinate. Changes in the scale factors as a function of the number of hydrating water molecules were also similar between glycinate and acetate. Amine groups showed relatively small changes. Scale factors for glycinate with no hydrating molecules were extrapolated from the supermolecule results, since the optimized structure of isolated glycinate obtained with the 4–31G basis set yielded one imaginary frequency. Good agreements between calculated and experimental frequencies for glycinate, acetate, and methyl amine were obtained for each set of scale factors. Scaling appears to compensate for the systematic effects of hydration on force constants, making it possible to obtain reliable frequency predictions for amino acids in water without resorting to expensive super-molecule calculations.     

Vibration Frequencies of Mg3Al2Si3O12 Pyrope. An ab initio study with the CRYSTAL code

The vibrational spectrum of Mg(3)Al(2)Si(3)O(12) pyrope is calculated at the Gamma point by using the periodic ab initio CRYSTAL program that adopts an all-electron Gaussian-type basis set and the B3LYP Hamiltonian. The full set of frequencies (17 IR active, 25 RAMAN active, 55 silent modes) is calculated. The effect of the basis set and of the computational parameters on the calculated frequencies is discussed. It is shown that the mean absolute difference with respect to the experimental IR and RAMAN data is as small as 6 and 8 cm(-1), respectively. The IR and RAMAN modes are fully characterized by various tools such as isotopic substitution, direct inspection of the eigenvectors, and graphical representation. The present calculation permits to clarify some of the assignment and interpretation problems raised by experiment and previous simulations with force fields.     

Interpretation of the vibrational spectra of matrix-isolated uracil from scaled ab initio quantum mechanical force fields

The vibrational spectrum of uracil trapped in an argon matrix has been interpreted based on ab initio Hartree–Fock SCF calculations with a split-valence 4?21 basis set. The directly computed theoretical general valence force field was scaled with empirical scale factors in order to correct for the systematic errors originating in the limitation of the theoretical model. Scale factors transferred from related molecules provided a priori prediction of fundamental frequencies and intensities, permitting several corrections to be proposed for earlier assignments. Using the observed spectrum with the few altered assignments, a new set of scale factors was optimized to give the best force field available from combined consideration of the experimental and the theoretical data. For unknown reasons, the out-of-plane force field predicted a spectrum agreeing slightly less well with experiment than did the in-plane force field. However, the overall agreement between theory and experiment provided additional support for the assumptions involved in the method. The computed force fields were compared with others available from previous work. The comparison demonstrated the importance of expanding the energy surface around the true energy minimum and of using a proper scaling procedure. Previous scaled CNDO /2 calculations were found to be surprisingly good despite the large corrections required and the fact that they were made at an incorrect geometry.