方酸构象和振动光谱的从头算研究

在6-31G^*^*水平上对方酸(3, 4-二羟基-3-环丁烯-1, 2-二酮)三种构象异构体进行了SCF计算。结果表明ZZ型异构体最稳定, ZE型次之。用等键反应能量分析方酸的稳定性, 并与苯作比较, 讨论方酸的芳香性。在6-31G水平上计算了方酸三种构象的振动频率。     

A comparative ab initio study of the geometry and force field of thionformic acid with formic and thiolformic acids

The energy and force field for the planar cis and trans conformers of thionformic acid have been calculated using the 4–31 G basis set, augmented by a complete set of d-functions on the sulfur atom, with full geometry optimization. Extensive comparisons are made between the changes in geometry and selected force constants in going from cis- (chain) to the trans- (ring) structures of thionformic, thiolformic and formic acid. These changes are discussed in terms of a hydrogen bonding type of interaction in the O---HS, S---HO and O---HO structural units respectively. Of the thioacid conformers, the trans-thiol is found to be the most stable; the trans-thion and cis-thiol both about 10 kJ mol⁻¹ less stable; and the cis-thion the least stable by about 38 kJ mol⁻¹.     

The ab initio calculated molecular structures,force fields and vibrational frequencies of some organic azides

Double zeta basis molecular calculations were carried out on the hydrazoic acid and azidomethane molecules. The molecular structures were optimized by the gradient method and the force fields were obtained by numerical differentiation of the gradient vector. The computed harmonic force fields and vibrational frequencies were compared with experimental values.     

Vapor phase infrared spectroscopy and ab initio fundamental anharmonic frequencies of ammonia borane

The infrared absorption spectrum of ammonia borane vapor has been recorded between 3600 and 600 cm(-1). Of the eleven infrared active fundamental vibrational modes, seven modes of NH(3)(11)BH(3) and four modes of NH(3)(10)BH(3) were observed. The spectra were recorded with sufficient resolution to observe the rotational structure of the bands, which allowed for preliminary least-squares fitting of the band origins and rotational constants. First-principles electronic structure calculations were performed to obtain anharmonic band origins and their intensities. The band assignments are discussed in relation to other spectroscopic techniques that have been previously used to study this molecule. A semi-empirical estimate of the vapor pressure of ammonia borane at room temperature (22 °C) was made and found to be ~1 × 10(-4) Torr. The assignment of the measured modes was aided by the calculated anharmonic frequencies and their infrared intensities. The combination of the CCSD(T) harmonic frequencies with the B3LYP anharmonicities, obtained from second-order vibrational perturbation theory, was found to produce an overall best agreement with the measured band origins.     

An ab initio study on the geometries and vibrational frequencies of hydrazoic acid and methyl azide

The geometric parameters for hydrazoic acid and methyl azide were optimized at the HF/6-31G^** and MP2/6-31G^** levels and the vibrational frequencies of the compounds were calculated by use of these optimized geometries. The experimental frequencies are assigned on the basis of the calculated results. The effects of deutero-substitution and substitution of hydrogen in HN₃ by a methyl group are also discussed.     

Ab initio study of vibrational frequencies,force field and normal coordinate analysis of the cis- and trans-isomets of nitrosomethanol

In this paper, the equilibrium geometries of two isomers of the newly found compound — nitrosomethanol—have been optimized by ab initio SCF MO method with 3–21G basic set by gradient technique. And the second derivatives of potential energy (i. e. the force constant matrix elements) have been calculated analytically. Hence the entire force fields of the two isomers of nitrosomethanol have been obtained theoretically. The theoretical vibrational frequencies and the corresponding normal modes were obtained and compared with the experimental values, and the structures of two isomeric forms of nitrosomethanol are established.     

SiH2Cl2: ab initio anharmonic force field, dipole moments, and infrared vibrational transitions

The vibrational spectra of SiH2Cl2 have been recorded in the 1000-13,000 cm(-1) region, utilizing the Fourier-transform spectroscopy and Fourier-transform intracavity laser absorption spectroscopy. Totally 61 band centers and intensities are derived from the infrared spectra. An ab initio quartic force field is obtained by applying the second-order Moller-Plesset perturbation theory and correlation-consistent polarized valence triplet-zeta basis sets [J. Chem. Phys. 90, 1007 (1989); 98, 1358 (1993)]. Most observed bands are assigned by the vibration analysis based on the second-order perturbation theory. Reduced-dimensional ab initio dipole moment functions (two dimensional and three dimensional) have also been calculated to investigate the absolute band intensities of the SiH2 chromophore. The calculated values agree reasonably with the observed ones.     

Harmonic force constants of formic acid: ab initio results and the uniqueness problem of force fields derived from vibrational data

A complete set of harmonic force constants of formic acid (HCOOH) is calculated using a 6-31G basis set. It is used to determine a modified valence force field consistent with both ab initio results and vibrational data.     

An ab initio analysis of the electronic structure and harmonic frequencies of nickel porphyrin

Ab initio calculations are performed to understand the geometry, electronic structure, and vibrational frequencies of nickel porphyrin (NiP). Hartree-Fock (HF) and second-order perturbation (MP2) theories are applied with polarized basis sets. The calculated geometrical parameters are in very good agreement with the crystal structure determination. The electronic structure and bonding are analyzed in terms of complexation and correlation effects. Not unexpectedly, the HF depiction of the metal-porphyrin interaction is rather ionic while ligand σ donation is dominant at the MP2 level. Scaled HF frequencies of NiP and its isotopomers are in very good agreement with observed infrared and resonance Raman data. Received: 7 January 1997 / Accepted: 6 May 1997     

Molecular structure,vibrational frequencies and ionization potential of tin dihalides: An ab initio SCF and CI study

The geometrical parameters for SnX₂ (X = Cl, Br, I) and SnX₂⁺ have been optimized at the SCF and Cl levels using non-empirical pseudopotentials and a basis set of double-zeta plus polarization quality. For the neutral molecules the geometrical parameters are in agreement with the experimental values, the difference being of only 2%. Vibrational frequencies and ionization potentials are also in agreement with experiment.     

The formate anion: the harmonic and anharmonic force fields

Harmonic and anharmonic symmetry force constant matrices have been obtained for six isotopically substituted species of the formate anion. The valence force fields derived from the above matrices were used to recalculate the vibrational frequencies. These force fields are compared with others previously reported and obtained via other methods.     

Simulations of vibrational spectra from classical trajectories: calibration with ab initio force fields

An algorithm allowing simulating vibrational spectra from classical time-dependent trajectories was applied for infrared absorption, vibrational circular dichroism, Raman, and Raman optical activity of model harmonic systems. The implementation of the theory within the TINKER molecular dynamics (MD) program package was tested with ab initio harmonic force fields in order to determine the feasibility for more extended MD simulations. The results suggest that sufficiently accurate frequencies can be simulated with integration time steps shorter than about 0.5 fs. For a given integration time step, lower vibrational frequencies ( approximately 0-2000 cm(-1)) could be reproduced with a higher accuracy than higher-frequency vibrational modes (e.g., O-H and C-H stretching). In principle, the algorithm also provides correct intensities for ideal systems. In applied simulations, however, the intensity profiles are affected by an unrealistic energy distribution between normal modes and a slow energy relaxation. Additionally, the energy fluctuations may cause weakening of the intensities on average. For ab initio force fields, these obstacles could be overcome by an arbitrary normal mode energy correction. For general MD simulations, averaging of many shorter MD trajectories started with randomly distributed atomic velocities provided the best spectral shapes. alpha-pinene, D-gluconic acid, formaldehyde dimer, and the acetylprolineamide molecule were used in the tests.     

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.     

An ab initio study of ascorbic acid

The conformations of ascorbic acid and its singly ionized analog are found to agree with the X-ray structures. The calculations are discussed in terms of the known chemistry of ascorbic acid and its metabolites.     

A systematic ab initio study of the equilibrium geometry and vibrational wave numbers of bismuthine

The equilibrium structure and the harmonic and anharmonic force fields of BiH(3) are determined by high-level ab initio calculations using a variety of correlation treatments, basis sets, and pseudopotentials, partly in combination with core polarization potentials. Spin-orbit effects are included by a configuration interaction treatment. This systematic study serves to establish a reliable computational protocol for such calculations and, in particular, to minimize basis set superposition errors through an improved new basis set and/or counterpoise corrections. Using the recommended procedures, the best ab initio results for the equilibrium geometry and the fundamental vibrational wave numbers are in good agreement with the available experimental data, which further supports the recent spectroscopic identification of BiH(3). The ground-state total atomization energy of BiH(3) is predicted to be 153.1 kcal/mol.     

An ab initio study of pyruvic acid

The geometries and vibrational frequencies of two conformers of pyruvic acid have been obtained at the ab initio second order Möller-Plesset level of theory using the 6-311++G^** basis set. While the calculated geometries have been compared to the experimental microwave data, the vibrational frequencies have been assigned, using the experimental gas phase IR spectra of 13 isotopes of pyruvic acid by a recently developed scaling procedure (IRPROG). An attempt has been made to explain the stability of the eclipsed conformation over the staggered conformation of pyruvic acid by taking account of the molecular orbitals.     

An ab initio study of the structures and the harmonic and anharmonic stretching force constants of the cyanides HCN,LiCN, FCN,ClCN and isocyanides HNC,LiNC, FNC,ClNC

Equilibrium structures and both harmonic and anharmonic stretching force constants have been calculated ab initio for the four cyanides HCN, LiCN, FCN, and C1CN, and the four isocyanides HNC, LiNC, FNC, and ClNC, using several basis sets of about triple-zeta quality. The CN and NC bond lengths, as well as diagonal stretching force constants, are nearly the same throughout both series. Trends in the off-diagonal stretching force constants are discussed, and dipole moment values are correlated with the structural type, XCN and XNC, and the electronegativity of the ligand X.     

An ab initio structural and vibrational analysis of gauche,trans,trans- and gauche,cis,trans-hexa-1,3,5-trienes

The results of complete geometry optimizations of the high energy stable gauche,Trans,trans- and gauche,Cis,trans- rotamers of hexa-1,3,5-trienes are reported at the RHF/6-31G//RHF/B-31G level. The angles of rotation around one of the single C-C bonds are found to be 33.7° and 45.5°, respectively. The corresponding harmonic force fields of these molecules are also reported at this level and corrected using scale factors transferred from buta 1,3-diene. Aspecial scale factor was used for the central C=C double bond stretching coordinate to take into account vibronic coupling. The theoretical vibrational frequencies, calculated with the scaled quantum mechanical (SQM) force fields, allow a complete interpretation of the experimental vibrational spectra of these molecules.Preliminary results were reported at the Austin XII Symposium on Molecular Structure, Austin, TX, February 28 through March 3, 1988, S 18, p. 111 (USA) and at the XIXth European Congress on Molecular Spectroscopy, Dresden, September 4 through September 8, 1989, p. 226 (GDR).