Rotationally Resolved Electronic Spectra of 2- and 3-Methylanisole in the Gas Phase:  A Study of Methyl Group Internal Rotation

Abstract

Rotationally resolved fluorescence excitation spectra of several torsional bands in the S1 <-- S0 electronic spectra of 2-methylanisole (2MA) and 3-methylanisole (3MA) have been recorded in the collision-free environment of a molecular beam. Some of the bands can be fit with rigid rotor Hamiltonians; others exhibit perturbations produced by the coupling between the internal rotation of the methyl group and the overall rotation of the entire molecule. Analyses of these data show that 2MA and 3MA both have planar heavy-atom structures; 2MA has trans-disposed methyl and methoxy groups, whereas 3MA has both cis- and trans-disposed substituents. The preferred orientations (staggered or eclipsed) in two of the conformers and the internal rotation barriers of the methyl groups in all three conformers change when they are excited by light. Additionally, the values of the barriers opposing their motion depend on the relative positions of the substituent groups, in both electronic states. In contrast, no torsional motions of the attached methoxy groups were detected. Possible reasons for these behaviors are discussed.

Methyl Internal Rotation in the Microwave Spectrum of o‐Methyl Anisole

“…In 2006 Alvarez‐Valtierra et al. determined the rotational constants A =2.4900(1) GHz, B =1.5589(1) GHz, and C =0.9707(1) GHz, as well as a V 3 potential of 345 cm −1 for OMA by fluorescence spectroscopy . The rotational constants are not as accurate as in the present work, but they are in agreement with those deduced from our fit.…”

“…The barrier height of the ring methyl group has only been determined by Alvarez‐Valtierra et al. with fluorescence spectroscopy, where the authors found for the electronic ground state a V 3 potential of 345 cm −1 …”

“…The barrier height of the ring methyl group has only been determinedb yA lvarez-Valtierra et al with fluorescence spectroscopy,w here the authors found for the electronic ground state a V 3 potentialof3 45 cm À1 . [8] The structures of substituted anisoles are also interesting: Anisole itself has ap lanar heavy atom frame [9] like almost all other related compounds, such as phenetole, [10] p-fluoroanisole, [11] and p-anisaldehyde. [12] However,n onplanar structures cannotbeexcluded, because, for example, Lister et al reported on as econd isomer with the methylsulfanyl (CH 3 -S) group in an early perpendicular orientation for p-fluorothioanisole.…”

Full Conformational Landscape of 3‐Methoxyphenol Revealed by Room Temperature mm‐wave Rotational Spectroscopy Supported by Quantum Chemical Calculations

“…and with the anisole molecule which presents inertial defects very close to 3‐MP . This last point seems to indicate that the phenol group does not impact the planarity of the 3‐MP molecule contrary to the methyl internal rotor in methylanisole where the inertial defects are significantly larger compared to methoxyphenol ,…”

High Resolution Electronic Spectroscopy of o- and m-Toluidine in the Gas Phase. Barrier Height Determinations for the Methyl Group Torsional Motions

“…6 However, substitution of a second ring hydrogen atom with a methoxy group changes this situation dramatically. A recent study of 2-and 3-methylanisole 7 showed that the methyl group internal motions are significantly more rigid in one of the electronic states and less rigid in the other, with the corresponding barriers depending on the positions of the two groups relative to one another.…”