Ab initio Quantum-Mechanical Study of the Stability of Cyclic α-Acetoxy-N-Nitrosamines: Amine N→no Dative Bonding in α-Hydroxy-N-Nitrosamines versus N→Carbocation Dative Bonding in N-Nitrosiminium Ions

The dissociations of α-hydroxy- N -nitrosopyrrolidine, 1 , and α-hydroxy- N -nitrosopiperidine, 2 , to form the nitrosiminium ions, 3 and 4 , respectively, have been studied as models for the S _N 1 process of the corresponding α-acetoxy- N -nitrosamines. In excellent agreement with experiments, the ionization of the pyrrolidine derivative is found to be more endothermic at ab initio levels up to MP4(SDTQ)/6-31 G*//RHF/6-31 G* and including corrections for thermal motions. This finding is explained with more efficient N→NO π-dative bonding in 1 compared to 2 while ring size effects in 3 and 4 are rather small. The activation barriers to NN-rotation provide quantitative measures of the strengths of the N→NO π-dative bonding in 1-4 and the transition-state structures 1-TS-4-TS for the rotation about the NN bonds in 1-4 were determined. Electron density analyses (NBO) and electrostatic field analysis (CHELPG, MKS) were performed to assess the extent of N→NO π-dative bonding in α-hydroxy- N -nitrosamines and the electronic relaxation associated with the competition between amine N→NO and N→carbocation π-dative bonding in N -nitrosiminium ions. The comparative analysis of the structural and electronic relaxation associated with ionization in the presence ( 1-4 ) or absence ( 1-TS-4-TS ) of the possibility for N→NO π-dative bonding demonstrate in a compelling fashion that σ-polarizations are responsible for most of the electron density relaxation.