  A long-standing controversy concerning the heat of formation of methylenimine
has been addressed by means of the W2 (Weizmann-2) ab initio thermochemical
approach. Our best calculated values, DeltaHf298(CH2NH)=21.1+/-0.5 kcal/mol and
DeltaHf298(CH2NH2+)=179.4+/-0.5 kcal/mol, are in good agreement with the most
recent measurements but carry a much smaller uncertainty. As a by-product, we
obtain the first-ever accurate anharmonic force field for methylenimine: upon
consideration of the appropriate resonances, the experimental gas-phase band
origins are all reproduced to better than 10 cm-1. Consideration of the
difference between a fully anharmonic zero-point vibrational energy and
B3LYP/cc-pVTZ harmonic frequencies scaled by 0.985 suggests that the
calculation of anharmonic zero-point vibrational energies can generally be
dispensed with, even in benchmark work, for rigid molecules.

Chemical Physics;Computational Physics