RAMAN AND INFRARED SPECTRA, CONFORMATIONAL STABILITY, BARRIERS TO INTERNAL ROTATION, AND AB INITIO CALCULATIONS OF ACETYL ISOCYANATE

Abstract

The Raman (3100 to $10 cm^{-1}$) and infrared (3100 to $30 cm^{-1}$) spectra of acetyl isocyanate, $CH_{3}C(O)NCO$, and the $d_{3}$-isotopomer have been recorded for the gases and solids. Additionally, the Raman spectra of the liquids were recorded and qualitative depolarization values were obtained. The observed bands are assigned on the basis of the more stable cis (syn) conformer (isocyanate group cis to the carbonyl bond) and a less stable trans (anti) conformer in the vapor state. The $\Delta H$ of the gas is estimated to be $433 cm^{-1} (1.24 kcal/mol)$ from the relative intensity of the conformer pair at 800 and $763 cm^{-1}$. From the temperature dependence of the Raman spectrum of the liquid, two conformer pairs have been used to determine experimentally a $\Delta H$ value of $370 \pm 60 cm^{-1} (1.06 \pm 0.17$ kcal/mol) but now with the trans conformer the more stable form. In the annealed crystalline solid, only the trans conformer remains. The fundamental asymmetric torsion of the cis conformer has been observed at $79 cm^{-1}$ with two excited states falling to lower frequency. Utilizing these data and the $\Delta H$ for the gas the potential function governing the conformer interchange is estimated. A complete vibrational assignment is proposed for both conformers based on infrared band contours, Raman depolarization data, group frequencies, and normal coordinate calculations. The experimental conformational stability, barriers to internal rotation, and fundamental vibrational frequencies are compared with those obtained from ab initio Hartree-Fock gradient calculations employing the $RHF/6-31G^{\ast}$ basis set and to the corresponding quantities obtained for some similar molecules.