GAS-PHASE PROTONATION OF HNO - A GAUSSIAN-1 ABINITIO MO STUDY OF THE STRUCTURE, STABILITY, AND UNIMOLECULAR INTERCONVERSION PROCESSES OF VARIOUS [H2,N,O]+ ISOMERS

Ab initio post-SCF calculations employing the Gaussian-1 level of theory have been performed on the structure, the stability, and the interconversion processes of various isomers of protonated HNO. The most stable form is obtained by protonation at the nitrogen atom, having a proton affinity (PA) of 166.0 +/- 2 kcal mol-1. By protonation at the oxygen atom of HNO, two stable isomers are obtained, i.e., the trans- and cis-HNOH+ ions (2 and 3, respectively), the former being more stable by ca. 7 +/- 2 kcal mol-1. A proton affinity of 148.4 +/- 2 kcal mol-1 is computed in this case. The structure and stability of the NOH2+ ion were also investigated, and its most stable electronic state was found to correspond to the 3A2 state 8. The 1A1 state 9 is higher in energy by 23.8 kcal mol-1. The interconversion processes of various [H-2,N,O]+ isomers have been studied at the Gaussian-1 level of theory. The relevant transition structures have been located, and the following conclusions can be drawn: The H2NO+ --> HNOH+ isomerization process 1 --> 2 is predicted to have a barrier of 65.5 kcal mol-1 with respect to the most stable isomer. Several cis --> trans isomerization paths of the HNOH+ isomers are also characterized as high-energy processes; the energetically most favorable transition structure for the interconversion trans-HNOH+ --> cis-HNOH+ corresponds to 5; this barrier is 38 kcal mol-1 higher in energy than 2. Isomerization via 6 is even more energy demanding by at least 20 kcal mol-1. A 90 kcal mol-1 energy barrier is computed for the isomerization of 2 to the least stable N-OH2+ 1A1 ion 9 proceeding via transition structure 10. On the triplet potential surface the degenerate isomerization of N-OH2+ (7) via an inversion process involving the 3A2 state 8 is with 1.4 kcal mol-1 nearly barrier-free. In addition, the triplet form N-OH2+ (7) is 24 kcal mol-1 more stable than the singlet electromer 1A1 of N-OH2+ (9).