Midinfrared and Quantum-Chemical Study of the Structure, Conformation, and Isomerization of the Unstable CH3CH2OCN Molecule

Abstract

Gaseous ethyl cyanate, CH 3 CH 2 OCN, has been generated from the gas/solid reaction of O-ethyl thiocarbamate with mercury oxide and characterized in the gas phase by infrared spectroscopy for the first time. Experimental data indicate the presence of two conformers in the gas phase, the gauche (synclinal) and the trans (antiperiplanar) form. The molecular geometries and energetics of the possible conformers are obtained from DFT calculations at the B3LYP level and from ab initio calculations at the MP2, MP3, MP4, QCISD, and CCSD(T) levels of theory. The assignment of the gas-phase infrared spectrum is assisted by normal coordinate calculations based on the scaled computed force field of the two conformers. The kinetic instability of CH 3 -CH 2 OCN toward isomerization is studied at the B3LYP level, in a vacuum and in solutions. Solvent effects are modeled using the polarized continuum model (PCM). Calculations show that the isomerization is not a unimolecular process at ambient temperatures, and bimolecular processes are responsible for the instability. In polar solvents, the OCN - anion plays a key role in the isomerization, being an effective catalyst for the cyanate-isocyanate rearrangement.