Ab initio study of the reactions between a series of substituted singlet nitrenium ions and water

Highly correlated ab initio molecular orbital calculations have been used t o study the energetics and mechanisms governing the reactions of a series o f substituted singlet nitrenium ions ((1)[R-N-H](+), R = H, Cl, F, CN, and Me) with water in gas phase and in solution. It was found that, in gas phas e and in solution, the nitrenium ions react in a barrierless fashion to pro duce an intermediate [HNR-OH2](+), which in turn undergoes a 1,2-hydrogen s hift to produce the corresponding protonated hydroxylamine [RNH2OH](+). Res ults obtained at the QCISD(T)/6-311++G**//QCISD/6-311++G** indicate that in the gas phase, the intermediates are located between 27.7 and 339.6 kJ/mol below the reactants depending upon the substituent on the nitrenium ion. T he transition states are located between 93.1 and 141.9 kJ/mol above the in termediates depending upon the substituent in question. Electrostatic inter actions with the solvent increases the intrinsic barrier of the process, as it lowers the relative energy of the intermediates (and reactants) with re spect to the transition state. It is also found that the reaction of water with the methyl nitrenium ion inhibits the decomposition of this species in gas phase. The effects of the substituent, the solvent, and the levels of theory employed on the reaction paths are discussed in detail. In addition, the reaction of singlet nitrenium ion with two water molecules is also stu died and the results are discussed in the context of the importance of the explicit treatment of the solvent in the determination of the energetics an d mechanisms of these processes.