The Microwave Spectrum of 4-Methylisothiazole

Abstract

The microwave spectrum of 4-methylisothiazole was assigned by means of a pulsed molecular beam microwave Fourier transform spectrometer. Analysis of the spectrum yielded the rotational constants A = 7370.642 (

14N Nuclear quadrupole coupling and methyl internal rotation in the microwave spectrum of 2-methylpyrrole

“…Some results obtained for 2MP are compared with those of other five-membered heterocycles in Figure 3. The V3 potential varies in a wide range from 34 cm 1 for 2-methylthiazole ( 6) [1], 106 cm 1 for 4-methylisothiazole ( 8) [2], 252 cm 1 for 2-methyloxazole ( 7) [21] to 280 cm 1 for 2MP (5).…”

“…Many investigations have been carried out on such aromatic systems, especially heterocyclic five-membered rings, to determine the methyl barrier heights, i.a. 2-methylthiazole (34.9 cm 1 ) [1], 4-methylisothiazole (105.8 cm 1 ) [2], 2,5dimethylthiophene (248.0 cm 1 ) [3], 2-acetyl-5-methylfuran (369.8 cm 1 for the trans conformer and 356.5 cm 1 for the cis conformer) [4], and 2,5-dimethylfuran (439.2 cm 1 ) [5]. From those studies it is obvious that the torsional potentials of methyl groups attached to planar aromatic rings vary in a wide range in both, shape and height.…”

“…a All parameters refer to the rho axis system and cannot be directly compared to those referring to the principal axis system. Pa, Pb,2 , where F = ħ 2 /2rIα.…”

The microwave spectrum of 2-methylthiazole: 14N nuclear quadrupole coupling and methyl internal rotation

“…The program BELGI has been extended in its hyperfine versions BELGI-Cs-hyperfine and BELGI-C 1 -hyperfine, which can treat molecules with one internal rotor and one weakly coupling nucleus like 14 N. Its predictive power has been proven for several molecules containing one methyl rotor and a nitrogen nucleus such as Ntert-butylacetamide (Cs), 11 N-ethylacetamide (C 1 ), 11 3-nitrotoluene (Cs), 12 and 2-methylpyrrole (Cs). 13 Monomethyl derivatives of aromatic heterocyclic five-membered rings containing a nitrogen nucleus are ideally suited to test the Hamiltonian model because the V 3 potential value of the methyl group covers a wide range from low barriers like those in 2-methylthiazole (34.9 cm −1 ) 14 and 4-methylisothiazole (105.8 cm −1 ) 15 to intermediate barriers such as in 4-and 5-methylthiazole (357.6 cm −1 and 332.0 cm −1 , respectively), 16,17 as well as in 2-, 4-, and 5-methyloxazole (252.0 cm −1 , 428.3 cm −1 , and 478.2 cm −1 , respectively). 18 The first molecule mentioned above, 2-methylthiazole (2MTA), has been studied by Grabow et al in the frequency range from 7 GHz to 13 GHz.…”

The Microwave Spectrum of m-Tolunitrile: Methyl Internal Rotation and 14N Nuclear Quadrupole Coupling

“…This is due to the coupling of internal methyl and overall rotation yielding first-order contributions from off-diagonal elements of the tensor for lines with E symmetry. Although this effect has been described earlier (10), it has been exploited only in a few cases when analyzing the nuclear quadrupole hyperfine structures of rotational lines of molecules with internal rotation of methyl groups (11)(12)(13).…”