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

Abstract

The laser induced fluorescence excitation and dispersed fluorescence spectra of three nitrogen heterocyclic molecules 1-methyl-2͑1H͒pyridone ͑1MPY͒, 1-methyl-2͑1H͒pyridinimine ͑1MPI͒, and 3-methyl-2͑1H͒pyridone ͑3MPY͒ 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 * -* 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.

A Computational Study on the Mechanism of Intramolecular Oxo−Hydroxy Phototautomerism Driven by Repulsive πσ* State

“…As it was shown by the experimental studies of the fluorescence excitation spectra, recorded for jet-cooled 2PD/2HP system, the 0−0 line in the S 0 →S 1 transition appears at 29831 cm −1 (335.2 nm) for the oxo form and at 36136 cm −1 (276.7 nm) for the hydroxy form . The above assignment of the 0−0 transitions to the oxo and hydroxy forms, respectively, was proved by a measurement of the fluorescence excitation spectrum of 1-methyl-2(1 H )-pyridinone, where the 0−0 line was found at 29822 cm −1 (335.3 nm) . Similarly, for the 4PMD/4HPM system, the 0−0 transitions were observed at 30528 cm −1 (327.6 nm) for the oxo form and at 35337 cm −1 (283.0 nm) for the hydroxy form .…”

“…19 The above assignment of the 0-0 transitions to the oxo and hydroxy forms, respectively, was proved by a measurement of the fluorescence excitation spectrum of 1-methyl-2(1H)pyridinone, where the 0-0 line was found at 29822 cm -1 (335.3 nm). 20 Similarly, for the 4PMD/4HPM system, the 0-0 transitions were observed at 30528 cm -1 (327.6 nm) for the oxo form and at 35337 cm -1 (283.0 nm) for the hydroxy form. 21 Hence, the energies of S 1 states in the oxo substrates of the phototautomeric reactions are lower than the energies of S 1 states in the hydroxy photoproducts.…”

Excitation Energy-Dependent Decay Dynamics of the S1 State of N-Methyl-2-pyridone

“…Kundu and co-workers investigated the laser-induced fluorescence excitation and dispersed fluorescence spectra of NMP under supersonic jet-cooled molecular beam conditions. , The experimental 0–0 band origin of the S 1 ← S 0 electronic transition (π* ← π) was found to be at 29,822.3 cm –1 (335.3 nm) for NMP . The internal rotational levels of the methyl group and some of the low-frequency vibrations in the S 1 excited state were assigned and the spectral analysis revealed considerable mixing of the methyl torsion with the low-frequency vibrational modes such as ring out-of-plane in the excited state . No vibronic band was found to be active beyond 400 cm –1 from the S 1 (0 0 0 ) ← S 0 electronic origin transition.…”

New torsion potential expression for molecules without rotational symmetry

“…Although experimental techniques to determine the barrier heights of torsion potential profile have been well established, 16,27 it is still difficult to experimentally determine the details of torsion potential profile. In this work, the torsion potential profiles were generated in the following a͒ procedure: firstly, the generation of the C-C bond rotational potential profile of CH 3 CH 3 and torsion potential profile of a single H-C-C-H dihedral ͑for clarity, we will use "rotational potential" for the rotational potential of the whole molecule and "torsion potential" for the rotational potential of a single dihedral͒, subsequently, the generation of the C-C bond rotational potential profile of CH 3 CH 2 X and torsion potential profile of a single H-C-C-X dihedral, where X = F, Cl, Br and I, and finally the generation of the C-C bond rotational potential profile of CH 2 XCH 2 Y and torsion potential profile of a single X-C-C-Y dihedral, where X or Y represents F, Cl, Br, or I.…”