Methyl group torsional dynamics from rotationally resolved electronic spectra. 1- and 2-methylnaphthalene

Abstract

Rotationally resolved fluorescence excitation spectra of three vibronic bands in the S1←S0 transitions of 1- and 2-methylnaphthalene (1 and 2MN) have been obtained. Each band exhibits perturbations that are produced by an interaction between the restricted torsional motion of the attached methyl group and the overall rotational motion of the entire molecule. A complete analysis of these effects yields values of the torsional barrier heights, the rotational constants, and the torsion–rotation perturbation coefficients of all vibronic levels that participate in the transitions. These values depend significantly on the position of the methyl group attachment, on the electronic state of the naphthalene chromophore, and on its vibrational state, as well. For example, V3 (the threefold torsional barrier) decreases from 809 cm−1 in 00 1MN to 128 cm−1 in 00 2MN. D (the largest first-order torsion–rotation perturbation term) increases from 0.03 MHz in 00 1MN to 406 MHz in 00 2MN, a change of more than 4 orders of magnitude. The V3 values of 00 and 8̄1 1MN are 563 and ≤ 373 cm−1, respectively. A full discussion of these dynamically relevant effects and their dependence upon both electronic and geometric factors is given.

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

“…The parameters of the lone methyl substituent of 1-MN could not be determined, because no splitting was observed in the spectrum. This observation is indicative of a Table 5 Measured Transition Frequencies of 2-methylnaphthalene high barrier to internal rotation, consistent with an earlier experimental study; the barrier has been determined to be 9.7 kJ mol −1 , based on the rotationally resolved electronic spectrum (Tan et al 1991b). 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).…”

“…This observation is indicative of a Table 5 Measured Transition Frequencies of 2-methylnaphthalene high barrier to internal rotation, consistent with an earlier experimental study; the barrier has been determined to be 9.7 kJ mol −1 , based on the rotationally resolved electronic spectrum (Tan et al 1991b). 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).…”

“…Furthermore, laboratory-generated carbon nanoparticles containing alkyl-substituted naphthalenes have been shown to have infrared emission features consistent with dust in the ISM (Duley & Hu 2012), so alkylnaphthalenes are candidate species for identification in the ISM. Although rotationally resolved electronic spectra for 1-methylnapthalene, 2-methylnaphthalene (Tan et al 1991b), and 2,3-dimethylnaphthalene (Tan et al 1991a) have been reported in the past, no pure rotational spectra have previously been measured for alkylnaphthalenes.…”

“…The calculations gave one minimum energy structure for each alkylnaphthalene (see Figure 1). For 1-MN and 2-MN, transitions were easily located using previously reported rotational constants, which were determined from the rotationally resolved electronic spectra of the species (Tan et al 1991b). For 1,2-and 1,3-DMN, automated frequency scans (consisting of 0.2 MHz frequency steps) were used to search for targeted a-and b-type transitions within 100 MHz intervals, centered at the predicted frequencies.…”

Dynamics of OH Production in the Reaction of O(1D2) with Cyclopropane

“…The coupling between methyl rotation and low-frequency ring modes is known to be important for the promotion of the vibrational state mixing. 22,23 Moreover, aliphatic methyl internal rotation in combination with another large-amplitude modes, may play a role in enhancing IVR even if, by itself, it is generally less effective than other types of molecular flexibility. For skeletal torsion as an IVR accelerator, the effects of centers of flexibility (COF) on IVR state mixing have been reported.…”