Definitive heat of formation of methylenimine, CH₂NH, and of methylenimmonium ion, CH₂NH₂⁺, by means of W2 theory

Abstract

Abstract 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, Δ H ° f ,298 (CH 2 NH) = 21.1±0.5 kcal/mol and Δ H ° f ,298 (CH 2 NH 2 + ) = 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. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1297–1305, 2001