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

Abstract

High-resolution pure rotational spectra of four alkylnaphthalenes were measured in the range of 6-15 GHz using a molecular-beam Fourier-transform microwave spectrometer. Both a-and b-type transitions were observed for 1-methylnaphthalene (1-MN), 1,2-dimethylnaphthalene (1,2-DMN), and 1,3-dimethylnaphthalene (1,3-DMN); only a-type transitions were observed for 2-methylnaphthalene (2-MN). Geometry optimization and vibrational analysis calculations at the B3LYP/6-311++G(d,p) level of theory aided in the assignments of the spectra and the characterization of the structures. Differences between the experimental and predicted rotational constants are small, and they can be attributed in part to low-lying out-of-plane vibrations, which distort the alkylnaphthalenes out of their equilibrium geometries. Splittings of rotational lines due to methyl internal rotation were observed in the spectra of 2-MN, 1,2-DMN, and 1,3-DMN, and allowed for the determination of the barriers to methyl internal rotation, which are compared to values from density functional theory calculations. All four species are moderately polar, so they are candidate species for detection by radio astronomy, by targeting the transition frequencies reported here.

Steric effects on two inequivalent methyl internal rotations of 3,4-dimethylfluorobenzene

“…methyl methacrylate [9], linalool (400 cm -1 ) [10], s-cis-acetovanillone (522 cm -1 ) [11], N,N-diethylacetamide (517 cm -1 ) [12], and phenylacetone (238 cm -1 ) [13]. Some two-top molecules like acetone [14], 2,5-dimethylfuran [15], 2-acetyl-5-methylfuran [16], and 1,2dimethylnapthalene [17] also belong to this class. The methyl torsions cause splittings up to several hundred MHz in the rotational spectrum, which can be sufficiently modeled with high accuracy by including the V3 terms of the potential function and the orientations of the internal rotors in the Hamiltonian model.…”

Microwave Spectrum of Two‐Top Molecule: 2‐Acetyl‐3‐Methylthiophene

“…However, the number of investigations decreases significantly if the number of internal rotors increases. While dozens of twotop molecules studied by high resolution spectroscopy can still be found in the literature, [32][33][34] only a handful of three-top molecules have been reported [35][36][37] and only one four-top molecule is found in an on-going study. [1,38] Such molecules with multiple methyl internal rotors exhibit complicated spectral features, depending on the number of rotors and their interactions.…”

Rotational spectroscopy of methyl benzoylformate and methyl mandelate: structure and internal dynamics of a model reactant and product of enantioselective reduction

“…32,33 Neon was used as backing gas at pressures between 1 and 2 atm; a heated coil of copper tubing was used to pre-heat the gas stream before it reached the sample chamber. At room temperature, MBF is a liquid, and MM is a solid; however, the vapour pressures of both species are very low.…”