“…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.…”