Conformational stability of allylbenzene: A combined study by dispersed fluorescence spectroscopy and quantum chemistry calculation

Abstract

Two conformational isomers of allylbenzene are identified in a supersonic free jet expansion by use of laser-induced fluorescence excitation and dispersed fluorescence spectroscopy. With the aid of the predictions of ab initio quantum chemistry calculations at the MP2 level for a series of extended basis sets [6-311+G(d,p), 6-311++G(d,p), and cc-pVTZ], the major species of the electronic spectrum is shown to be an eclipsed conformer in which the allyl group is oriented perpendicular to the plane of the benzene ring and a terminal hydrogen atom of the ethylene moiety is poised nearly above the aromatic π electrons. The minor species is identified as an internal rotational isomer that is obtained by rotating the ethylene group about the Cα–Cβ bond by 120° from the eclipsed configuration. This predicted order of conformational preference is reversed for calculations at relatively low levels of theory: MP2/6-31G(d,p), HF/6-311++G(d,p), HF/6-31G(d,p), and B3LYP/6-31G(d,p). The relative intensities of the vibronically induced nontotally symmetric and totally symmetric transitions are significantly different in the electronic spectra of the two conformers.

Gas-phase conformations and exciton couplings in 5,6,11,12-tetrahydrodibenzo[a,e]cyclooctene

“…Whereas, the C conformer is predicted to be the most stable at the HF and DFT levels of theory. The difference between the MP2 and HF/DFT results can be attributed to the fact that the MP2 method better accounts for the non-bonded intramolecular dispersive interactions between the p electron clouds of the aromatic rings [13]. These interactions are particularly important in the TB conformer, where the aromatic rings are closer and partially overlapping.…”

“…These observations indicate that only two THDC conformers with electronic origins at 36956.3 and 37147.9 cm -1 are present in the gas phase. The two electronic origins can be assigned by comparison with the previously analyzed electronic spectra of allylbenzene [13] and alkylbenzenes [17]. The electronic origins of the compact gauche conformers of alkylbenzenes and allylbenzene (in which the substituent group points toward the aromatic chromophores) are red-shifted relative to the electronic origins of the anti conformers.…”

“…Therefore, the TB conformer predominates in the gas and liquid phases. The higher stability of the TB conformer can be attributed to non-bonded intramolecular dispersive interactions between the p electron clouds of the aromatic rings [13].…”

Singlet-Singlet Electronic Excitation Energy Transfer in a Bichromophore Cooled in a Supersonic Jet

“…In [2,3], the bichromophore molecule is considered as a single system with intramolecular radiationless energy transfer. Recently, works have been carried out [4,5] dealing with the study of the transfer of electronic energy in aromatic bichromophores cooled in supersonic jets.In the present work, for the first time in investigations of electronic excitation energy transfer in bichromophore molecules cooled in a supersonic jet we have used fluorescence polarization. The object of investigation was the heterocyclic bichromophore molecule consisting of 2,5-diphenyloxazole (PPO) and 1-(5 ′ -phenyl-1 ′ ,3 ′ -oxazole-2 ′ -yl)-4-(5 ′ -phenyl-1 ′ ,3 ′ ,4 ′ -oxadiazole-2 ′ -yl)-benzene (POPDP) -a molecule that has not been studied before.…”

“…In [2,3], the bichromophore molecule is considered as a single system with intramolecular radiationless energy transfer. Recently, works have been carried out [4,5] dealing with the study of the transfer of electronic energy in aromatic bichromophores cooled in supersonic jets.…”