Microwave spectra, molecular structure and theoretical calculation of two isotopic species of acetyl isocyanate CD 3C(O)NCO and 13CH 3C(O)NCO

Abstract

The microwave spectra of two isotopic species of acetyl isocyanate, 13CH 3C(O)NCO and CD 3C(O)NCO, were observed in order to determine the r o structure and confirmation of the molecular conformation. These isotopic species were prepared by reacting acetyl-2- 13C-chloride or acetyl- d 3 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 13CH 3C(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 13CH 3C(O)NCO, and A = 9713.90(6), B = 2042.04(2), and C = 1722.78(2) MHz for CD 3C(O)NCO, respectively. The values of Δ I (= I c − I a − I b ) of the 13C species (−3.024(13) uÅ 2) and the d 3 species (−6.163(3) uÅ 2) indicate that the molecule has C s symmetry. The r s 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 13CH 3C(O)NCO. The structural tendencies and the relationship between ∠ RNC and 14N quadrupole coupling constants ( χ cc) were discussed.