Definitive heat of formation of methylenimine, CH2=NH, and of methylenimmonium ion, CH2NH2+, by means of W2 theory

A long-standing controversy concerning the heat of formation of methylenimine has been addressed by means of the W2 (Weizmann-2) thermochemical approach. Our best calculated values, DeltaH(f,298)degrees (CH2NH) = 21.1 +/- 0.5 kcal/mol and DeltaH(f,298)degrees (CH2NH2+) = 179.4 +/- 0.5 kcal/mol, are in good agreement with the most recent measurements but carry a much smaller uncertainty. As a byproduct, 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. (C) 2001 John Wiley & Sons, Inc.