V.P. Gupta

Semi-empirical calculation and normal coordinate study of the conformation and electronic and vibrational spectra of acrolein

Abstract Detailed quantum chemical and normal coordinate calculations have been conducted on acrolein and its deutero derivatives to understand the structural and spectroscopic characteristics of its rotational isomers, in particular the cis isomer. The results of molecular orbital studies in the MNDO (using the modified AM1 Hamiltonian), CS-INDO and CNDO approximations on potential energy curves, rotational barrier heights, conformations and physico-chemical properties of the ground and first excited ( n π ∗ ) electronic states of acrolein are reported. Using multielectron configuration interaction based on the subroutine CIPSI, the electronic transitions have been explained. Generalized valence force fields have been obtained for the trans and cis conformers of acrolein and their monodeutero derivatives. A complete assignment of the in-plane and out-of-plane vibrational modes has been provided. It is inferred that while in the ground state the trans conformer of acrolein is more stable than the cis conformer. the reverse is the case in the first excited electronic state. In addition, a gauche conformer (Φ = 90°) may also exist in the excited state.

Conformational stability, optimized geometries, vibrational and electronic spectra of methacryloyl bromide in ground and excited electronic states

In order to understand conformational isomerism in methacryloyl bromide (MABR) in the ground (S(0)) and the first excited (S(1)) electronic states and to interpret the vibrational and electronic spectra of its conformers in the S(0) state, quantum mechanical calculations using Density Functional Theory (DFT) and RHF methods with extended basis sets 6-31G, 6-31G** and 6-311+G(d,p) have been conducted. In RHF calculations, electron correlation effects have been included at the Møller-Plesset MP2 level. It is inferred that in both the electronic states the molecule may exist in two isomeric forms-s-trans and s-cis; the former being more stable than the later by about 1.629 kcal mol(-1) in the S(0) state and by about 2.218 kcal mol(-1) in the S(1) state. Electronic transition tends to increase the s-trans/s-cis and s-cis/s-trans, rotational barriers from 7.059 kcal mol(-1) (2468.1 cm(-1)) and 5.428 kcal mol(-1) (1897.8 cm(-1)) in S(0) state to 23.594 kcal mol(-1) (8249.4 cm(-1)) and 21.376 kcal mol(-1) (7473.9 cm(-1)) in the S(1) state. Completely optimized geometries of the two conformers in S(0) state reveal that while there is no significant difference in their bond lengths, some of the bond angles associated with COBr group are appreciably different. Electronic excitation tends to change both the bond lengths and bond angles. Based on suitably scaled DFT and RHF results obtained from the use of 6-31G** and 6-311+G(d,p) basis sets, a complete assignment is provided to the fundamental vibrational bands of both the s-trans and s-cis conformers in terms of frequency, form and intensity of vibrations and potential distribution across the symmetry coordinates in the S(0) state and a comparison has been made with experimental assignments. A theoretical prediction of the electronic transitions in the near UV-region in the two conformers and their tentative assignment has been provided on the basis of CI level calculations using 6-31G basis set.