Coupled Large Amplitude Motions: A Case Study of the Dimethylbenzaldehyde Isomers

Abstract

The microwave spectra of the 3,4- (syn and anti), 2,5- (syn), and 3,5-dimethylbenzaldehyde (DMBA) molecules have been recorded for the first time in the 2-26.5 GHz frequency range, using the high resolution COBRA-FTMW spectrometer in Hannover. The experimental assignments and fits are supplemented by ab initio quantum chemical calculations of the conformational energy landscape and dipole moment components. The analysis of the spectra of the four observed isomers, including spectroscopic constants and large amplitude motion parameters, are presented in this paper. The DMBA isomers belong to a series of similar molecules obtained formally by adding one or more methyl group(s) at the aromatic ring. These molecules serve as prototype systems for the development of the theoretical model of asymmetric top molecules having C(s) symmetry while containing in addition two nonequivalent methyl tops (C(3v)), exhibiting different barrier heights and coupling terms. Thus, the DMBA isomers represent good species for testing the recently written two-top internal rotors BELGI program.

The effects of two internal rotations in the microwave spectrum of ethyl methyl ketone

“…This is similar to the case of syn-2,5 dimethyl-benzaldehyde, 11 an isomer of DMBA which also has one low and one high barrier to internal rotation. The internal rotation parameters related to the high barrier are not so well determined, since the internal rotation splittings used for their determination are small.…”

“…Then, a third molecule with three isomers, 3,4-(syn and anti), 2,5-(syn), and 3,5-dimethylbenzaldehyde (DMBA), which explored various barrier heights, was also studied. 11 In the present study, results from the BELGI-Cs-2Tops code will be compared with those obtained from the XIAM code.…”

Laboratory microwave, millimeter wave and far-infrared spectra of dimethyl sulfide

“…As explained in (Tudorie et al 2011), terms in this Hamiltonian can be constructed by taking symmetry-allowed and Hermitian products of a rotational factor (chosen from operators of the form J The BELGI-C s -2Tops program has been applied for fitting high resolution torsion-rotational spectra of molecules with: (i) two nonequivalent methyl rotors; (ii) two different three-fold barriers and (iii) a plane of symmetry at equilibrium, e.g. methyl acetate (Tudorie et al 2011), methyl propionate (Nguyen et al 2012), three isomers of dimethyl benzaldehyde (Tudorie et al 2013) and ethyl methyl ketone (Nguyen et al 2014). In the case of DMS, we modified the code slightly so that all internal rotor parameters corresponding to one methyl group, referred as top 1, possess the same magnitudes as those of the other methyl top, referred as top 2.…”

High-Resolution Fourier-Transform Microwave Spectroscopy of Methyl- And Dimethylnapthalenes

“…In contrast, the methyl substituent of 2-MN causes splitting in the rotational spectrum, and the barrier was determined to be much lower, 2.717 kJ mol −1 , in close agreement with the previously measured value of 2.8 kJ mol −1 (Tan et al 1991b). 1,2-DMN is an interesting prototype of molecules that have two methyl substituents at adjacent carbon atoms of an aromatic ring, analogous to 3,4-dimethylbenzaldehyde, which has previously been studied in detail (Tudorie et al 2013). The methyl substituents at both the 1 and 2 positions of 1,2-DMN cause splittings in the rotational spectrum, and the barriers were determined to be 3.670 and 5.238 kJ mol −1 , respectively.…”