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Origin of threefold symmetric torsional potential of methyl group in 4-methylstyrene

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Title Origin of threefold symmetric torsional potential of methyl group in 4-methylstyrene
 
Creator SINHA, RK
PRADHAN, B
SINGH, BP
KUNDU, T
BISWAS, P
CHAKRABORTY, T
 
Subject internal-rotation barriers
bond orbital analysis
supersonic jet
hyperconjugation mechanism
fluorescence spectroscopy
aromatic-molecules
p-fluorotoluene
states
acetaldehyde
isomers
 
Description To understand the effect of the para position vinyl group substitution in toluene on methyl torsion, we investigated 4-methylstyrene, a benchmark molecule with an extended pi conjugation. The assignment for a 33 cm(-1) band in the excitation spectrum to the 3a(2) torsional transition, in addition to the assignments suggested previously for the other bands in the excitation spectrum, leads to the model potentials for the ground as well as excited states with V-3(')=19.6 cm(-1), V-6(')=-16.4 cm(-1) and V-3(')=25.6 cm(-1), V-6(')=-30.1 cm(-1), respectively. These potentials reveal that both in ground and excited states, the methyl group conformations are staggered with a 60 degrees phase shift between them. MP2 ab initio calculations support the ground state conformations determined from experiments, whereas Hartree-Fock calculations fail to do so. The origin of the modified ground state potential has been investigated by partitioning the barrier energy using the natural bond orbital (NBO) theoretical framework. The NBO analysis shows that the local delocalization (bond-antibond hyperconjugation) interactions of the methyl group with the parent molecule is sixfold symmetric. The threefold symmetric potential, on the other hand, stems from the interaction of the vinyl group and the adjacent ring pi bond. The threefold symmetric structural energy arising predominantly from the pi electron contribution is the barrier forming term that overwhelms the antibarrier contribution of the delocalization energy. The observed 60 degrees phase shift of the excited state potential is attributed to the pi(*)-sigma(*) hyperconjugation between out of plane hydrogens of the methyl group and the benzene ring. (c) 2006
 
Publisher AMER INST PHYSICS
 
Date 2011-07-16T11:10:57Z
2011-12-26T12:49:44Z
2011-12-27T05:35:23Z
2011-07-16T11:10:57Z
2011-12-26T12:49:44Z
2011-12-27T05:35:23Z
2006
 
Type Article
 
Identifier JOURNAL OF CHEMICAL PHYSICS, 124(14), -
0021-9606
http://dx.doi.org/10.1063/1.2189233
http://dspace.library.iitb.ac.in/xmlui/handle/10054/4412
http://hdl.handle.net/10054/4412
 
Language en