Ana Rios

DFT and AIM study of the protonation of nitrous acid and the pKa of nitrous acidium ion.

The gas phase and aqueous thermochemistry, NMR chemical shifts, and the topology of chemical bonding of nitrous acid (HONO) and nitrous acidium ion (H(2)ONO(+)) have been investigated by ab initio methods using density functional theory. By the same methods, the dissociation of H(2)ONO(+) to give the nitrosonium ion (NO(+)) and water has also been investigated. We have used Beckes hybrid functional (B3LYP), and geometry optimizations were performed with the 6-311++G(d,p) basis set. In addition, highly accurate ab initio composite methods (G3 and CBS-Q) were used. Solvation energies were calculated using the conductor-like polarizable continuum model, CPCM, at the B3LYP/6-311++G(d,p) level of theory, with the UAKS cavity model. The pK(a) value of H(2)ONO(+) was calculated using two different schemes: the direct method and the proton exchange method. The calculated pK(a) values at different levels of theory range from -9.4 to -15.6, showing that H(2)ONO(+) is a strong acid (i.e., HONO is only a weak base). The equilibrium constant, K(R), for protonation of nitrous acid followed by dissociation to give NO(+) and H(2)O has also been calculated using the same methodologies. The pK(R) value calculated by the G3 and CBS-QB3 methods is in best (and satisfactory) agreement with experimental results, which allows us to narrow down the likely value of the pK(a) of H(2)ONO(+) to about -10, a value appreciably more acidic than literature values.

Kinetic and thermodynamic barriers to chlorine transfer between amines in aqueous solution.

Third-order rate constants for the acid-catalyzed reversible reaction of N-chlorotaurine with benzylamine and dimethylamine were determined in water at 25 degrees C and I = 0.5 (NaClO4). The reaction with benzylamine shows inverse solvent deuterium isotope effects of kH/kD=0.57 and 0.47 in the forward and reverse directions, respectively. These isotope effects, together with the absence of detectable general acid catalysis for this reaction, provide evidence for a stepwise mechanism involving fast equilibrium protonation of N-chlorotaurine followed by rate-determining chlorine transfer from the protonated chloramine to benzylamine. The observation of strong catalysis by general acids of the reaction of dimethylamine with N-chlorotaurine suggests a change to a concerted mechanism with proton and chlorine transfer occurring in a single step. This change in mechanism is enforced by the absence of a significant lifetime for protonated chlorotaurine in contact with this strongly nucleophilic amine. The kinetic and thermodynamic parameters for the reaction between protonated chlorotaurine and benzylamine are used to estimate a Marcus intrinsic reaction barrier of deltaG(0)+/+ =4.1 kcal/mol for chlorine transfer between amines. Comparison of this intrinsic barrier with those reported previously for bromine transfer between carbanions points to the existence of certain similarities between halogen and proton transfer reactions.

Oxidation of benzylamine by ClOH and N-chlorosuccinimide: a kinetic study †

The reaction of benzylamine with N-chlorosuccinimide involves addition of the amine to electrophilic chlorine to yield N-chlorobenzylamine and not hydride abstraction at the α-carbon of the amine by the oxidant as proposed in the literature.

Halogen transfer through halogen bonds in halogen-bound ammonia homodimers

Ab initio MP2(full)/aug-cc-pVTZ calculations have been carried out to investigate the halogen transfer between haloamines and ammonia. The results show that the formation of a halogen bond complex between ammonia and the protonated N-haloamine is a preliminary step in the halogen transfer process. The complexation energies, optimized geometries, topology of electron density and potential energy surfaces for halogen transfer in these complexes have been analysed. It has been found that halogen-bound ammonia homodimers ([H3NXNH3]+, with X = Cl, Br, I) formed by interaction between NH3 and H3NX+ are symmetric, and energetically more stable than the corresponding complexes formed between NH3 and H2NX. Calculated potential energy surfaces for the transfer of the central halogen atom between the two NH3 units in [H3NXNH3]+ show single well and double well potentials for short and large N-N distances, respectively. The particular case of fluorine complexes has also been analysed. The results provide an explanation for some of the experimental facts observed for halogen transfer reactions between amines in aqueous solution.