Origin of methyl torsional barrier in 1-methyl-2-(1H)-pyridone

Abstract

The laser induced fluorescence excitation and single vibronic excitation dispersed fluorescence spectra have been studied for supersonic jet cooled 1-methyl-2͑1h͒-pyridone. The methyl torsional bands and some low frequency vibrational transitions were assigned for both ground and excited states. The torsional parameters V 3 = 244 cm −1 and V 6 =15 cm −1 for the ground state and V 3 = 164 cm −1 and V 6 =40 cm −1 for the excited state were obtained. To get the insight into the methyl torsional barrier, ab initio calculations were performed and compared with the experimental results. Origin of potential barrier was traced by partitioning the barrier energy into changes in bondantibond interaction, structural, and steric energies accompanying methyl rotation using natural bond orbital analysis. The role of local interactions in ascertaining the barrier potential reveals that its nature cannot be understood without considering the molecular flexing. The hyperconjugation between CH * and ring * observed in lowest unoccupied molecular orbital ͑LUMO͒ stabilizes the methyl group conformer that undergoes a 60°rotation in the excited state with respect to that of the ground state, and it is the change in LUMO that plays important role in the excited state barrier formation.

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

“…The spectral features resemble more the 2-pyridone 22 than the 1MPY excitation spectrum. Unlike 2e 145 127 142 124 3a 2 145 127 142 124 3a 1 264 245 271 234 4e 267 247 272 237 5e 351 348 379 325 6a 2 376 363 392 345 6a 1 405 418 451 386 1MPY, 17 here the O 0 0 transition is found to be very intense and the spectrum is less congested. An equally strong band is seen at 86 cm −1 from the S 0 → S 1 ͑O 0 0 ͒ transition.…”

“…2͑a͒. In our earlier work, 17 the observed band at 29 822.3 cm −1 was assigned as S 0 → S 1 ͑O 0 0 ͒ of the -* transition. The other bands at 86 and at 120 cm −1 relative to this O 0 0 band were assigned as the methyl torsional transitions to 2e and 3a 1 levels, respectively, from the ground zero point energy state, and the band at 172 cm −1 was assigned to the 34 0 1 ring deformation vibrational mode in the excited state.…”

“…Assignments of these observed bands in the FE spectrum are listed in Table III. 17 to calculate the FC factor. The shift of the potential between the excited state and the ground state for both the progressions has been taken to be 60°as observed previously.…”

“…Features of the IR spectrum of 1-MPI were found to exhibit a similarity with that of 1MPY. 16 Recently, Pradhan et al 17 studied 1-methyl-2͑1H͒-pyridone ͑1MPY͒ in supersonic jet cooled molecular beam by using fluorescence excitation ͑FE͒ and dispersed fluorescence ͑DF͒ spectroscopies along with quantum chemical calculations and successfully assigned the observed bands in the ground electronic state and a few bands in the excitation spectrum based on observed DF spectra upon excitation of these bands.…”

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

“…To explain this behavior, Nakai and Kawai 31 and Nakai and Kawamura 32 have pointed out a new type of interaction, namely, the * -* hyperconjugation using a molecular orbital theory. By considering such an interaction they were able to successfully interpret the variation of the barrier magnitude and the methyl group conformation for substituted toluenes, 31 naphthalenes, 32 and azabenzenes 33,34 in the excited state. While a conventional hyperconjugation appears between an occupied bond orbital or lone pair and an unoccupied antibond orbital, the * -* hyperconjungation appears between an unoccupied ring * orbital and an unoccupied CH * orbital.…”