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

Abstract

The microwave spectrum of o-methyl anisole (2-methoxytoluene), CH OC H CH has been measured by using a pulsed molecular jet Fourier transform microwave spectrometer operating in the frequency range 2-26.5 GHz. Conformational analysis using quantum chemical calculations at the MP2/6-311++G(d,p) level of theory yields only one stable conformer with a C structure, which was assigned in the experimental spectrum. A-E splittings due to the internal rotation of the ring methyl group could be resolved and the barrier to internal rotation was determined to be 444.05(41) cm . The experimentally deduced molecular parameters such as rotational and centrifugal distortion constants as well as the torsional barrier of the ring methyl group are in agreement with the calculated values.

Interplay Between Microwave Spectroscopy and X‐ray Diffraction: The Molecular Structure and Large Amplitude Motions of 2,3‐Dimethylanisole

“…[9] In almost all of our recent investigations, the MP2/6-311 + +G(d,p) level was applied. Variation of the methods and basis sets in our previous studies on OMA [3] and p-methyl anisole (PMA) [15] confirms that the structural parameters do not change significantly and the rotational constants are calculated with sufficient accuracy at the B3LYP/6-311 + + G(d,p) level. It is well-known that the MP2/6-311 + +G(d,p) level of theory often yields imaginary frequency for stable planar ring systems, which has been reported for benzene and arenes.…”

“…Using these Fourier coefficients, the potential energy curve was drawn as depicted in Figure 1. Similar to the case of OMA, [3] sterical hindrance prevents the existence of a second minimum at d = 1808 because a methyl group is present at the o-position. Similar to the case of OMA, [3] sterical hindrance prevents the existence of a second minimum at d = 1808 because a methyl group is present at the o-position.…”

“…[3] In MMA, the methyl group at the mposition is further away from the methoxy group, which creates a symmetric local environment near the methyl group and decreases the ring methyl barrier height by one order of magnitude (36.6342(84) and 55.7693(90) cm À1 for the trans and the cis conformer, respectively). The sterical hindrance between the methoxy and the o-methyl group is the main reason for this intermediate value of V 3 , which has been confirmed by the results obtained for a number of osubstituted toluenes.…”

“…A potential curve was calculated, where the corresponding dihedral angle d = ff(C 6 ÀC 1 ÀO 14 ÀC 15 ) was varied in a grid of 108, while all other geometry parameters were optimized. [10][11][12] However, for some previously studied molecules containing aromatic rings such as OMA, [3] 2,5-dimethylthiophene, [6] and phenetole, [13] harmonic frequency calculations yielded one imaginary vibrational mode, which is a bending vibration of the phenyl ring. [9] In almost all of our recent investigations, the MP2/6-311 + +G(d,p) level was applied.…”

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 ,…”