Isomerization reactions of RSNO (R=H, C_nH_2n+1 n ≤ 4)

We have applied various theoretical methods to gain detailed insights into the isomers as well as the transition states (TSs) along the corresponding reaction pathways for RSNO (R=H, C_nH_2n+1 n ≤ 4). On the basis of G2 and G2MP2 results, the relative order of stability for R=H is estimated to be trans-HSNO > cis-HSNO > HNSO > cis-HONS ≈ trans-HONS, while it is cis-CH₃SNO ≈ trans-CH₃SNO > CH₃NSO > trans-CH₃ONS > cis-CH₃ONS for R=CH₃. A similar trend is also obtained from the B3P86 method with considerably less computing effort if the nearly isoenergetic isomers cis-HONS and trans-HONS are ignored. Based on the results of B3P86, cis-RSNO is more stable than trans-RSNO when R=H is replaced by alkyl groups except for R=t-Bu. Natural bond orbital analyses allow us to explore whether the high reactivity of S-nitrosothiols is due to the strong negative hyperconjugation (n _πO ↔ σ*_N-S). The mesomeric effect of S-nitrosothiols, although non-negligible, does not cause the breakage of N-O bond due to the compensation of columbic attraction between N and O.