Ruifeng Liu

Density functional theory study of vibrational spectra. 1. Performance of several density functional methods in predicting vibrational frequencies

Abstract Harmonic vibrational frequencies of several small organic molecules which were used to validate the scaled quantum mechanical (SQM) force field procedure of Pulay et al. were calculated using six popular density functional (DFT) methods and compared with experimental results. The combination of Beckes exchange with either Lee-Yang-Parr (BLYP) or Perdews correlation functionals (BP86) reproduces the observed frequencies satisfactorily with deviations similar to those of the Hartree-Fock SQM methods. Three hybrid DFT methods are found to yield frequencies which were generally higher than the observed fundamental frequencies. When the calculated frequencies were compared with ‘experimental’ harmonic frequencies however, Beckes three-parameter hybrid method with Lee-Yang-Parr correlation functional is found to be slightly more accurate, especially for C-H stretching modes. The results indicate that BLYP calculation is a very promising approach for understanding the observed spectral features.

Density functional theory study of vibrational spectra, 6: assignment of fundamental vibrational frequencies of benzene isomers: Dewar benzene, benzvalene, trimethylenecyclopropane, prismane, and 3,4-dimethylenecyclobutene

Abstract The molecular structures and vibrational spectra of Dewar benzene, benzvalene, trimethylenecyclopropane, prismane and 3,4-dimethylenecyclobutene have been investigated by density functional theory using Beckes exchange with Lee—Yang—Parrs correlation functionals and the 6–31G∗ basis set. Both the calculated structural parameters and vibrational frequencies are in good agreement with available experimental data. On the basis of the agreement between the calculated and experimental results, assignments of fundamental vibrational frequencies of Dewar benzene, benzvalene, and trimethylenecyclopropane were examined and some reassignments are proposed. The calculations also predict prominent IR and Raman spectral features of prismane and 3,4-dimethylenecyclobutene, which can assist experimental identification of these compounds and the assignment of observed spectral features when they are available.

Toward understanding the vibrational spectra of BEDT-TTF, a scaled density functional force field approach

Abstract Density functional theory B3LYP and ab initio MP2 calculations were carried out to study the structures and vibrational spectra of bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) and the related compounds, 1,3-dithiole-2-thione (DTT), tetrathiafulvalene (TTF), and 4,5-ethylenedithio-1,3-dithiole-2-thione (EDT-DTT). It is found that B3LYP functional overestimates C-S bond lengths significantly and underestimates frequencies of modes involving C-S stretching accordingly. The errors in B3LYP force fields are shown to be satisfactorily corrected by scaled quantum mechanical force field procedure. After applying the scale factors derived from DTT, the scaled B3LYP force fields reproduce the observed frequencies of TTF, EDT-DTT, and BEDT-TTF satisfactorily, with a mean deviation between the calculated and observed frequencies of less than 10 cm −1 . On the basis of agreement between the calculated and observed frequencies, isotope shifts, as well as IR and Raman intensities, assignments of the fundamental vibrational modes of these molecules are given in terms of the true molecular symmetries of the equilibrium structures. This study shows that the scaled density functional force field procedure is a powerful approach for understanding the spectral features of large and low symmetry molecules.

Density functional theory study of vibrational spectra: 9. Structures and vibrational assignments of dicyanobenzenes

Abstract Density functional theory BLYP and ab initio HF calculations have been carried out to investigate the structures and vibrational spectra of dicyanobenzenes. The calculated results are in good agreement with reliable experimental data and indicate that the benzene rings of all three isomers are only slightly distorted by the two cyano groups. Vibrational frequencies calculated by BLYP/6–31G ∗ force fields agree very well with experimental results, with a mean deviation of about 14 cm −1 for non-CH stretching modes. On the basis of agreement between the calculated and observed results, assignments of the fundamental vibrational modes were examined and some reassignments were proposed. This study demonstrates that the density functional theory BLYP calculation is a powerful approach to understanding the vibrational spectra of organic compounds.

Density functional theory study of vibrational spectra. 4. Comparison of experimental and calculated frequencies of all-trans-1,3,5,7-octatetraene — The end of normal coordinate analysis?

Abstract Comparison of the observed and calculated vibrational frequencies of all-trans -octatetraene indicates that the density functional theory (DFT) using Beckes exchange and Lee-Yang-Parrs correlation functionals is as accurate as the Hartree-Fock (HF)-based scaled quantum mechanical force field approach in predicting fundamental vibrational frequencies. As the DFT calculation does not use any empirical parameters pertaining to the subject molecule and its computational cost scales more favorably than that of the HF theory, it is a more promising approach to molecular vibrational problems and should replace the empirical normal coordinate analysis for assisting vibrational assignments.

Density functional theory study of vibrational spectra. 3. Assignment of fundamental vibrational modes of quadricyclane

Density functional theory BLYP (Beckes exchange and Lee-Yang-Parrs correlation functionals), ab initio Hartree-Fock (HF) and hybrid DFT/HF B3LYP calculations were carried out to study the structure and vibrational spectra of quadricyclane. The BLYP/6-31G * and scaled HF/6-31G * frequencies correspond well with each other and with available experimental assignment of the fundamental vibrational modes. Based on conformity between the calculated and experimental results, a plausible assignment of two remaining a1 modes and all non-CH stretching a2, b1, and b2 modes is proposed.

THEORETICAL STUDY OF THE STRUCTURE AND VIBRATIONAL SPECTRUM OF 1,3-DITHIOLE-2-THIONE

Abstract Ab initio Hartree-Fock and density functional theory calculations were carried out to investigate the structure and vibrational spectrum of 1,3-dithiole-2-thione. All the calculations predicted a planar structure with C2v symmetry. Harmonic force field and vibrational mode calculations provided convincing theoretical evidence for reassignments of some fundamental vibrational modes. The reassignments are in line with the observed polarization data of Dyer et al. This study shows that the density functional theory is a powerful tool for understanding the vibrational spectra of organic molecules.

On assignment of fundamental vibrational modes of hypophosphite anion and its deuterated analogue

Abstract Results of ab initio and density functional theory calculations on the structure and vibrational frequencies of hypophosphite anion indicate earlier experimental assignments of the fundamental vibrational modes are correct while the recent reassignments of several modes proposed by Bickley et al. are inconsistent with the calculated results.

Reassignment of fundamental vibrational modes of cyclic S4N3 cation

MP2/6 31G* calculations were carried out to investigate the vibrational spectrum of cyclic S4N3+. The results indicate that previous assignments of several fundamental vibrational modes are in error. On the basis of the calculated results, reassignments of these modes are proposed.

Theoretical evidence for reassignment of two fundamental vibrational modes of tetrafluorooxirane-16O and −18O

Abstract Ab initio and density functional theory calculations confirm Craigs assignment of the fundamental vibrational modes of tetrafluorooxirane with the exception that assignments of the Cue5f8F stretching modes ν 9 ( b 1 ) and ν 13 ( b 2 ) should be exchanged. The calculated structural parameters are in good agreement with results of microwave studies except for the Cue5f8C bond length for which all the calculated results are slightly too long.