How reliable are ab initio calculations? Experimental and theoretical investigation of the structure and conformation of chlorocarbonyl isocyanate, ClC(O)NCO

Abstract

A gas electron diffraction study of ClC(O)NCO results in a mixture of trans ( anti) and cis ( syn) isomers in a ratio of ca. 3:1 (Δ G = 0.7(3)kcal mol −1), and confirms the interpretation of vibrational spectra. The following geometric parameters ( r a, values with 3σ uncertainties) have been derived for the trans structure CO (isocyanate) = 1.139(16)Å, CO (carbonyl) = 1.201(16) Å, NC = 1.218(23) Å, N-C = 1.384(6) Å, C-Cl = 1.757(5) Å, C-NC = 127.1(16)°, N-CO = 124.8(15)°, N-C-Cl = 115.8(8)° and NCO = 173.4(23)°. Calculated (HF/6-31G ∗) differences between geometric parameters of cis and trans isomers have been incorporated in the experimental structure determination. These ab initio calculations reproduce the experimental trans structure reasonably well, but the relative stability of the two isomers is at variance with the experiment. Calculations with different basis sets (4-31G ∗, 6-31G ∗ and D95 ∗) and at various levels of theory (HF, MP2 and MP4SDTQ) predict the cis form to be more stable by ΔE = 0.40 to 1.58 kcal mol −1, whereas experiments demonstrate the trans isomer to be more stable.