Microwave spectra, molecular structure and theoretical calculation of two isotopic species of acetyl isocyanate CD3C(O)NCO and 13CH3C(O)NCO

The microwave spectra of two isotopic species of acetyl isocyanate, 13CH3C(O)NCO and CD3C(O)NCO, were observed in order to determine the ro structure and confirmation of the molecular conformation. These isotopic species were prepared by reacting acetyl-2-13C-chloride or acetyl-d3 chloride with sliver cyanate. The rotational spectra of A-level in 26.5-60.0 GHz region have been observed by Stark-modulated microwave spectrometer. Some absorption lines in E-level were observed in 13CH3C(O)NCO. The rotational constants in the ground vibrational state were determined to be A = 10654.8(18), B = 2177.32(2), and C = 1827.65(2) MHz for 13CH3C(O)NCO, and A = 9713.90(6), B = 2042.04(2), and C = 1722.78(2) MHz for CD3C(O)NCO, respectively. The values of ΔI (= Ic − Ia − Ib) of the 13C species (−3.024(13) uÅ2) and the d3 species (−6.163(3) uÅ2) indicate that the molecule has Cs symmetry. The rs coordinates of the carbon atom in the methyl group were determined to be |a| = 2.183(3), |b| = 0.706(9), and |c| = 0.080(87) Å. The determined coordinates were in agreement with those calculated for the cis form, in which the carbonyl group is eclipsed by the NCO group. The six structural parameters of the cis form were adjusted by fitting to the observed rotational constants. The observed rotational constants of the cis form were in better agreement with those calculated using the QCISD/6-31G (d, p) level rather than those calculated using the MP2/6-31G (d, p) level. The barrier of internal rotation of the methyl group was determined as 4.283(16) kJ mol−1 in 13CH3C(O)NCO. The structural tendencies and the relationship between ∠RNC and 14N quadrupole coupling constants (χcc) were discussed.