Spectroscopic studies on methyl torsional behavior in 1-methy 1-2(1H)-pyridone, 1-methyl-2(1H)-pyridinimine, and 3-methyl-2(1 H)-pyridone. I. Excited state

Rajeev K. Sinha, B. Pradhan, S. Wategaonkar, Bhanu P. Singh, T. Kundu

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Abstract

The laser induced fluorescence excitation and dispersed fluorescence spectra of three nitrogen heterocyclic molecules 1-methyl-2(1H)pyridone (1MPY), 1-methyl-2(1H)pyridinimine (1MPI), and 3-methyl-2(1H)pyridone (3MPY) have been studied under supersonic jet cooled condition. The methyl torsional and some low frequency vibrational transitions in the fluorescence excitation spectrum were assigned for 1MPY. These new assignments modify the potential parameters to the methyl torsion reported earlier. Some striking similarities exist between the torsional and vibrational transitions in the fluorescence excitation spectra of 1MPY and 1MPI. Apart from pure torsional transitions, a progression of vibration-torsion combination bands was observed for both these molecules. The excitation spectrum of 3MPY resembles the spectrum of its parent molecule, 2-pyridone. The barrier height of the methyl torsion in the excited state of 3MPY is highest amongst all these molecules, whereas the barrier in 1MPI is higher than that of 1MPY. To get an insight into the methyl torsional barrier for these molecules, results of the ab initio calculations were compared with the experimental results. It was found that the conformation of the methyl group undergoes a 60° rotation in the excited state in all these molecules with respect to their ground state conformation. This phase shift of the excited state potential is attributed to the π*-σ* hyperconjugation between the out-of-plane hydrogen of the methyl group and the molecular frame. It has been inferred that the change in lowest unoccupied molecular orbital energy plays the dominant role in the excited state barrier formation.

Original languageEnglish
Article number114312
JournalJournal of Chemical Physics
Volume126
Issue number11
DOIs
Publication statusPublished - 08-04-2007

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Pyridones
Excited states
Molecules
Fluorescence
Torsional stress
excitation
torsion
molecules
Conformations
fluorescence
Vibrational spectra
Molecular orbitals
Phase shift
Ground state
Hydrogen
Nitrogen
progressions
laser induced fluorescence
molecular orbitals
phase shift

All Science Journal Classification (ASJC) codes

  • Physics and Astronomy(all)
  • Physical and Theoretical Chemistry

Cite this

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title = "Spectroscopic studies on methyl torsional behavior in 1-methy 1-2(1H)-pyridone, 1-methyl-2(1H)-pyridinimine, and 3-methyl-2(1 H)-pyridone. I. Excited state",
abstract = "The laser induced fluorescence excitation and dispersed fluorescence spectra of three nitrogen heterocyclic molecules 1-methyl-2(1H)pyridone (1MPY), 1-methyl-2(1H)pyridinimine (1MPI), and 3-methyl-2(1H)pyridone (3MPY) have been studied under supersonic jet cooled condition. The methyl torsional and some low frequency vibrational transitions in the fluorescence excitation spectrum were assigned for 1MPY. These new assignments modify the potential parameters to the methyl torsion reported earlier. Some striking similarities exist between the torsional and vibrational transitions in the fluorescence excitation spectra of 1MPY and 1MPI. Apart from pure torsional transitions, a progression of vibration-torsion combination bands was observed for both these molecules. The excitation spectrum of 3MPY resembles the spectrum of its parent molecule, 2-pyridone. The barrier height of the methyl torsion in the excited state of 3MPY is highest amongst all these molecules, whereas the barrier in 1MPI is higher than that of 1MPY. To get an insight into the methyl torsional barrier for these molecules, results of the ab initio calculations were compared with the experimental results. It was found that the conformation of the methyl group undergoes a 60° rotation in the excited state in all these molecules with respect to their ground state conformation. This phase shift of the excited state potential is attributed to the π*-σ* hyperconjugation between the out-of-plane hydrogen of the methyl group and the molecular frame. It has been inferred that the change in lowest unoccupied molecular orbital energy plays the dominant role in the excited state barrier formation.",
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Spectroscopic studies on methyl torsional behavior in 1-methy 1-2(1H)-pyridone, 1-methyl-2(1H)-pyridinimine, and 3-methyl-2(1 H)-pyridone. I. Excited state. / Sinha, Rajeev K.; Pradhan, B.; Wategaonkar, S.; Singh, Bhanu P.; Kundu, T.

In: Journal of Chemical Physics, Vol. 126, No. 11, 114312, 08.04.2007.

Research output: Contribution to journalArticle

TY - JOUR

T1 - Spectroscopic studies on methyl torsional behavior in 1-methy 1-2(1H)-pyridone, 1-methyl-2(1H)-pyridinimine, and 3-methyl-2(1 H)-pyridone. I. Excited state

AU - Sinha, Rajeev K.

AU - Pradhan, B.

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AU - Singh, Bhanu P.

AU - Kundu, T.

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N2 - The laser induced fluorescence excitation and dispersed fluorescence spectra of three nitrogen heterocyclic molecules 1-methyl-2(1H)pyridone (1MPY), 1-methyl-2(1H)pyridinimine (1MPI), and 3-methyl-2(1H)pyridone (3MPY) have been studied under supersonic jet cooled condition. The methyl torsional and some low frequency vibrational transitions in the fluorescence excitation spectrum were assigned for 1MPY. These new assignments modify the potential parameters to the methyl torsion reported earlier. Some striking similarities exist between the torsional and vibrational transitions in the fluorescence excitation spectra of 1MPY and 1MPI. Apart from pure torsional transitions, a progression of vibration-torsion combination bands was observed for both these molecules. The excitation spectrum of 3MPY resembles the spectrum of its parent molecule, 2-pyridone. The barrier height of the methyl torsion in the excited state of 3MPY is highest amongst all these molecules, whereas the barrier in 1MPI is higher than that of 1MPY. To get an insight into the methyl torsional barrier for these molecules, results of the ab initio calculations were compared with the experimental results. It was found that the conformation of the methyl group undergoes a 60° rotation in the excited state in all these molecules with respect to their ground state conformation. This phase shift of the excited state potential is attributed to the π*-σ* hyperconjugation between the out-of-plane hydrogen of the methyl group and the molecular frame. It has been inferred that the change in lowest unoccupied molecular orbital energy plays the dominant role in the excited state barrier formation.

AB - The laser induced fluorescence excitation and dispersed fluorescence spectra of three nitrogen heterocyclic molecules 1-methyl-2(1H)pyridone (1MPY), 1-methyl-2(1H)pyridinimine (1MPI), and 3-methyl-2(1H)pyridone (3MPY) have been studied under supersonic jet cooled condition. The methyl torsional and some low frequency vibrational transitions in the fluorescence excitation spectrum were assigned for 1MPY. These new assignments modify the potential parameters to the methyl torsion reported earlier. Some striking similarities exist between the torsional and vibrational transitions in the fluorescence excitation spectra of 1MPY and 1MPI. Apart from pure torsional transitions, a progression of vibration-torsion combination bands was observed for both these molecules. The excitation spectrum of 3MPY resembles the spectrum of its parent molecule, 2-pyridone. The barrier height of the methyl torsion in the excited state of 3MPY is highest amongst all these molecules, whereas the barrier in 1MPI is higher than that of 1MPY. To get an insight into the methyl torsional barrier for these molecules, results of the ab initio calculations were compared with the experimental results. It was found that the conformation of the methyl group undergoes a 60° rotation in the excited state in all these molecules with respect to their ground state conformation. This phase shift of the excited state potential is attributed to the π*-σ* hyperconjugation between the out-of-plane hydrogen of the methyl group and the molecular frame. It has been inferred that the change in lowest unoccupied molecular orbital energy plays the dominant role in the excited state barrier formation.

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