Substituent effects on the structures and energies of isocyanates and imines by ab initio molecular orbital calculations

Abstract

The energies and geometries of substituted isocyanates RNCO and imines RNCH₂ have been obtained by ab initio MP2/6–31 G*//MP2/6-31 G* calculations. For imines the stabilization energies SE =ΔE for the isodesmic exchange reaction of the substituent with alkenes show there is a very large energetic preference for electropositive substituents to be substituted on the imine compared with the alkene, with a linear correlation of SE with group electronegativities χ_BE. For isocyanates the SE values derived by comparison with either alkenes or imines give excellent correlations with χ_BE values. The geometries of both imines and isocyanates bearing strongly electropositive substituents are linear, and this is attributed to charge repulsions and a preference for sp hybridization at nitrogen when this atom is bonded to an electropositive group. The much higher dependence of substituent stabilization SE of isocyanates compared with other cumulenes is attributed to the greater polar character of the isocyanates, and a linear relationship between the HOMO coefficient at the substituted atom of the cumulene and the slope of the dependence of SE on χ_BE was found. Atomic charges calculated for these substrates by the natural bond orbital (NBO) method give distinctly better correlations with χ_BEvalues than do charges calculated by the Mulliken method. Calculated and experimental isocyanate IR asymmetric stretching frequencies are in reasonable agreement, but the effects of substituents on the isocyanate frequencies differ from those for ketenes.