The ground and first excited torsional states of methyl carbamate

A global fit within experimental accuracy of microwave and millimeter-wave transitions in the ground and first excited torsional states of methyl carbamate (H2NC(O)OCH3) is presented. The data set consisting of 995 vt = 0 and 731 vt = 1 transition frequencies combines 1544 new measurements from Kharkov with previously published vt = 0 microwave lines. In this study the so-called “rho axis method” that treats simultaneously both A and E species of the ground and first excited torsional states is applied to the methyl carbamate data set for the first time. The final fit requires only 32 parameters to achieve a unitless weighted standard deviation for the whole fit of 0.89 for a total of 1726 transitions with rotational quantum numbers up to J ⩽ 20 and Ka ⩽ 10. The barrier to internal rotation of the methyl group obtained in this study, V3 = 359.141(24) cm−1, is in good agreement with previously published values but more accurate.