The Heterolytic Dissociation of Neutral and Protonated Nitrous Acid

S-Nitrosation of Aminothiones

Nitrosation reactions span a diverse range of applications, from biochemistry to industrially important processes. This study examines nitrosation of aminothiones in acidic solutions and re-evaluates currently accepted diffusion limits and the true nature of the nitrosating agent for nitrous acid initiated reactions. Experimental measurements from stopped-flow UV/vis spectrophotometry afforded derivation of equilibrium constants and reaction enthalpies. Apparent Keq corresponds to 559-382 M(-2) for thioacetamide (TA, 15-25 °C) and 12600-5590 M(-2) for thiourea (TU, 15-35 °C), whereas the reaction enthalpies amount to -27.10 ± 0.05 kJ for TA and -29.30 ± 0.05 kJ for TU. Theoretical calculations via a thermochemical cycle agree well with reaction free energies from experiments, with errors of -2-4 kJ using solvation method SMD in conjunction with hybrid meta exchange-correlation functional M05-2X and high-accuracy multistep method CBS-QB3 for gas-phase calculations. The kinetic rates increase with acidity at activation energies of 54.9 (TA) and 66.1 kJ·mol(-1) (TU) for the same temperature range, confirming activation-controlled reactions. At pH 1 and below, the main decomposition pathway for the S-nitroso species leads to formation of nitric oxide.

Nitrosation Chemistry of Pyrroline, 2-Imidazoline, and 2-Oxazoline: Theoretical Curtin−Hammett Analysis of Retro-Ene and Solvent-Assisted C−X Cleavage Reactions of α-Hydroxy-N-Nitrosamines

The results are presented of a theoretical study of the nitrosation chemistry of pyrroline 1 (X = CH2), imidazoline 2 (X = NH), and 2-oxazoline 3 (X = O). Imines 1-3 are converted to the alpha-hydroxy-N-nitrosamines 7-9 via the N-nitrosoiminium ions 4-6. The NN-cis isomers of 7-9 may undergo retro-ene reactions to the delta-oxoalkyl diazotic acids 10-12. With the opportunity for microsolvation, C-X cleavage becomes possible for 8 and 9 and leads to the formation of N-(2-aminoethyl)- and N-(2-hydroxyethyl)-N-nitrosoformamides 15 and 16, respectively. The NN-isomerization barriers are comparable to the barriers for the ring-opening reactions, and the consideration of two Curtin-Hammett scenarios is required: CH-I for the NN-trans-rotamers of 7-9 to undergo C-X cleavage or NN-isomerization and CH-II for the NN-cis-rotamers to undergo C-X cleavage, C-N cleavage, or NN-isomerization. We determined all stereoisomers of the substrates, the products, and of all transition states structures for the retro-ene reactions of 7-9, the C-X cleavages of microsolvated 8 and 9, and the NN-isomerizations of 8 and 9. The potential energy surfaces were explored at the B3LYP/6-31G level, and the results are discussed with emphasis on the comparison of the kinetics and thermodynamics of C-N versus C-X cleavage. The study shows all decompositions to be very fast with activation barriers below 21 kcal.mol(-1), and the comparative analysis predicts that the chemical toxicologies of 1 and 3 should be similar and remarkably different from that of 2.