Juan Crugeiras

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.

Kinetics and thermodynamics of ionization of 4,4′-dimethoxytrityl alcohol in acetonitrile–aqueous acids: determination of enthalpies and entropies of activation and of reaction

We have studied the equilibration shown in eqn. (2) of 4,4′-dimethoxytrityl alcohol in aqueous perchloric, nitric, and hydrochloric acids containing 20% acetonitrile at different temperatures using stopped-flow kinetics techniques. The observed overall pseudo first-order rate constant for equilibration, kobs, decreases with increasing electrolyte concentrations at constant hydronium ion concentration; kobs, has been resolved into forward and reverse components using the equilibrium UV absorbance and the temperature-independent molar absorptivity of the 4,4′-dimethoxytrityl carbenium ion. The forward reaction (rate constant kf) is first order in both the alcohol and the acid; the reverse reaction (rate constant kr) is pseudo first order with respect to the carbocation. At constant hydronium ion concentration, the forward rate constant increases with the concentration of electrolyte whereas the reverse rate constant decreases. Quantitative effects for perchlorate, nitrate, and chloride are different. Results are accommodated by a mechanism which involves pre-equilibrium protonation of the alcohol, heterolysis of the protonated alcohol to give a 4,4′-dimethoxytrityl carbenium ion–water ion–molecule pair, then conversion of this into the dissociated carbenium ion in equilibrium with ion pairs. There are additional parallel reaction channels from the protonated alcohol and electrolyte anions. Some of the elementary rate constants have been evaluated, and enthalpies and entropies of activation have been determined. Correspondingly, some equilibrium constants of the proposed mechanism have been evaluated, and associated enthalpies and entropies of reaction have been determined.

Kinetics of electrophilic bromine transfer from N-bromosuccinimide to amines and amino acids

The kinetics of the reactions of glycine, sarcosine, 2-aminoisobutyric acid, proline and pyrrolidine with N-bromosuccinimide (NBS) have been studied and it has been found that the rates of formation of the N-bromoamino products are proportional to the NBS and amino substrate concentrations; that they decrease linearly with increasing H+ concentration in the –log [H+] interval 8.50 to 4.00; and that they are independent of ionic strength and the concentration and nature of the buffer. The second order reaction rate constant increases with the basicity of the amino substrate, ranging from 2.7 × 106 dm3 mol–1 s–1 for the formation of N-bromoglycine to 4.74 × 106 dm3 mol–1 s–1 for the formation of N-bromopyrrolidine (I= 0.1 mol dm–3). The activation enthalpies measured for these reactions are small and typical of fast processes while the activation entropies are large and negative and indicative of a highly solvated transition state.On the basis of these observations, we propose that the N-bromo compounds form via a concerted reaction in which the unprotonated amino group of the substrate attacks the NBS bromine, the overall result being ‘Br+’ transfer from the nitrogen atom of NBS to the nitrogen atom of the substrate.