A GAUSSIAN-1 TYPE AB-INITIO MO STUDY OF THE [CH2NO](-ENERGY SURFACE -THEORETICAL SUPPORT FOR THE EXISTENCE OF 3 EXPERIMENTALLY DISTINGUISHABLE ISOMERS IN THE GAS-PHASE() POTENTIAL)

The potential energy surface (PES) of [CH2NO](+) ions with an NCO conn ectivity has been explored by GAUSSIAN-1 type calculations. Six isomer s having an NCO backbone are found and characterized as minima on the PES; three of them are separated by isomerization barriers reasonably large enough to allow an experimental verification. The favored proton ation site of neutral isocyanic acid (HNCO) is the nitrogen atom (prot on affinity, calculated = 171.4 kcal/mol; experimental = 173 +/- 1 kca l/mol) leading to the most stable isomer H2NCO+ (1). For the protonati on of NCOH at the nitrogen atom we predict a proton affinity for the c yanic acid of 179.7 kcal/mol. The resulting ion HNCOH+(2) is 15.9 kcal /mol higher in energy than 1. The third stable isomer 5 having the NCO connectivity can be formally viewed as a complex between CN and the w ater radical cation; 5 is 72.9 kcal/mol less stable than H2NCO+ (1). I n addition, two high-energy isomers HN-C(HO+ (3) and HO-C(H)N+ (4) hav e been found, but according to the shape of the PES, they are predicte d not to be accessible experimentally. The calculated thermochemical d ata are in excellect agreement with experimentally available values. T he calculations are extended to the unimolecular fragmentation channel s, which should be observed in mass-spectrometric experiments. The ana lysis of the results suggests that the hydrogen-atom elimination proce ss is favored over all other dissociation channels, followed by loss o f NH2. and NH.