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
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Creator |
SINHA, RK
PRADHAN, B SINGH, BP KUNDU, T BISWAS, P CHAKRABORTY, T |
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Subject |
internal-rotation barriers
bond orbital analysis supersonic jet hyperconjugation mechanism fluorescence spectroscopy aromatic-molecules p-fluorotoluene states acetaldehyde isomers |
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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
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Publisher |
AMER INST PHYSICS
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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 |
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Type |
Article
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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 |
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Language |
en
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