Rotational Spectra and Structures of Phenyl Isocyanate and Phenyl Isothiocyanate

The pure rotational spectra of phenyl isocyanate (PhNCO) and phenyl isothiocyanate (PhNCS) were investigated using Fourier transform microwave spectroscopy in the range from 4 to 26 GHz. For each molecule, rotational transitions due to the parent species and nine minor isotopologues including seven ¹³C, one ¹⁵N, and one ¹⁸O/³⁴S have been observed in natural abundance. The r_m⁽¹⁾ geometries were derived from the resulting sets of rotational constants and are consistent with the equilibrium structures (r_e) from ab initio calculations performed at the MP2/aug-cc-pVTZ level. NBO and Townes-Dailey analyses were conducted to better understand the electronic structure and geometry of each compound. In the case of PhNCS, the nitrogen atom displays more sp-like character resulting in shorter C-N bonds and a larger CNC angle relative to those of PhNCO.