Howard Maskill

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.

Rates and mechanisms of the thermal solvolytic decomposition of arenediazonium ions

Arenediazonium tetrafluoroborates have been prepared and the kinetics of solvolysis have been investigated in water, trifluoroethanol, water–trifluoroethanol mixtures, hexafluoropropan-2-ol, trifluoroacetic acid, and ethanol by a UV method. A heterolytic mechanism involving short-lived aryl cations leads to products derived from nucleophilic capture of the aryl cations by solvent or a solute. Ionic solutes in aqueous trifluoroethanol and trifluoromethoxybenzene in trifluoroethanol have no kinetic effect and neither does replacement of the tetrafluoroborate counter-ion by chloride in trifluoroethanol. Rate constants for any one compound are not very solvent dependent, the reactions generally being characterised by high enthalpies of activation and appreciably positive entropies of activation. Compounds with 4-Cl, 4-F, 4-NO2, and 4-MeO substituents proved too unreactive for kinetic studies, but for different reasons. In ethanol, a radical reaction with characteristically different activation parameters competes with the heterolytic path and leads to hydrodediazoniation (reduction) by hydrogen atom abstraction from the CH2 group of ethanol.

Dediazoniation reactions of arenediazonium ions under solvolytic conditions: fluoride anion abstraction from trifluoroethanol and α-hydrogen atom abstraction from ethanol

Arenediazonium salts decompose thermally and photochemically in trifluoroethanol to yield trifluoroethyl ethers and (in part by fluoride abstraction from the solvent) fluoroarenes; the less reactive compounds in trifluoroethanol decompose readily in ethanol to give arenes in a radical reaction involving abstraction of the α-hydrogen from the ethanol.

Designing aryl cations for direct observation in solution: ab initio MO calculations of UV spectra

A detailed study of the vertical UV/visible spectra of the phenyl cation and some derivatives has been undertaken to identify an aryl cation which, when produced by dediazoniation, will be observable by laser flash photolysis. Calculations on the phenyl cation itself confirmed that it has no strongly allowed transition in the desired region of the spectrum. It was found that the primary effect of heteroatom substitution external to the ring upon the spectrum arose from the alteration of existing orbital energies. Particularly important was the effect of the heterosubstituent on the energy of the empty σ orbital, located on the dehydro carbon. Also important was the polarisation of π orbitals by the substituent and, in some cases, the configurational mixing of states of lower symmetry. Several externally substituted systems showed promise in terms of being potentially observable; however, in the cases where the transitions were in the right region of the spectrum, the intensities were rather modest and, when transitions were calculated to possess large intensities, they were not usually in the most convenient UV spectral range. Internal heteroatom substitution leading to heterocyclic aryl cations was also investigated. As well as the alteration of orbital energies, polarisation of the π space, and configurational mixing, this type of substitution perturbed the spectrum of the phenyl cation in an additional way: new transitions involving the excitation of non-bonding electrons appeared. In the majority of cases, however, the heterocyclic cations did not exhibit particularly strong transitions in the desired spectral region; an exception was the pyrimidin-5-yl cation which was calculated to possess a particularly promising electronic state (1-1A1) but its diazonium ion precursor is synthetically inaccessible. Externally substituted pyrimidin-5-yl cations were calculated to show a persistently strong n–σ transition in the 270–290 nm wavelength range. In the 2-amino-4-hydroxypyrimidin-5-yl cation, the intensity of this transition was particularly high, and the precursor arenediazonium ion appears to be synthetically accessible.

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.

The mechanism of deamination of methoxy substituted tritylammonium ions in methanolic aqueous acid

In methanolic aqueous acid, methoxy-substituted tritylammonium ions undergo heterolysis to give an equilibrium mixture of the substituted trityl cation, the corresponding alcohol (and methyl ether), and the ammonium cation via an SN1 mechanism; the detection of a reaction channel first order in hydronium ions may implicate substituted trityl cation–ammonia (ion–molecule) pairs as reactive intermediates.

Secondary deuterium kinetic isotope effects in the solvolysis of 4-t-butylcyclohexyl and bicyclo[3.2.1]octan-3-yl toluene-p-sulphonates. New results using 50% aqueous ethanol, acetic acid, and 97% aqueous hexafluoropropan-2-ol

Secondary α-deuterium kinetic isotope effects for the solvolysis of cis- and trans-4-t-butylcyclohexyl toluene-p-sulphonates in 50% aqueous ethanol (1.200 ∓ 0.007 and 1.16 ∓ 0.01 respectively, 44.8°), acetic acid (1.172 ∓ 0.004 and 1.13 ∓ 0.01 respectively, 79.6°), and 97% aqueous hexafluoropropan-2-ol (1.232 ∓ 0.013, 40.0° and 1.175 ∓ 0.010, 56.2°, respectively) and for the solvolysis of endo- and exo-bicyclo[3.2.1]octan-3-yl toluene-p-sulphonates in 97% aqueous hexafluoropropan-2-ol (1.246 ∓ 0.009 and 1.234 ∓ 0.004 respectively, 25.2°) have been measured. Mechanisms for these reactions which involve ion-pair intermediates are proposed. Even in 97% aqueous hexafluropropan-2-ol, intimate ion-pairs from trans-4-t-butylcyclohexyl toluene-p-sulphonate undergo further reaction more rapidly than internal return. The other three compounds investigated in this highly ionizing solvent of low nucleophilicity are believed to react, as anticipated, through reversibly formed intimate ion-pairs.

Molar absorptivity and pKR+ of the 4,4′-dimethoxytrityl carbenium ion in methanolic water, and its equilibrium with chloride ion

The 4,4′-dimethoxytrityl carbenium ion has been generated in 80 : 20 (v/v) water–methanol containing perchloric acid in a rapid reaction from 4,4′-dimethoxytrityl alcohol and in a slower reaction from 4,4′-dimethoxytritylamine. The pKR+(–0.95; KR+= 8.98) and molar absorptivity (IµR+= 297 000) of the cation have been measured directly under equilibrium conditions in this highly aqueous medium at ionic strength = 1.0 mol dm–3 by a convenient spectrophotometric method at 25 °C. In addition, the pKR+ has been measured at other temperatures, which allows the determination of the standard enthalpy (ΔH⊖=–11.4 kJ mol–1) and entropy (ΔS⊖=–20 J K–1 mol–1) of the reaction of the cation with the solvent, and at other ionic strengths. The equilibrium constant for the formation of 4,4′-dimethoxytrityl chloride from the cation and chloride has also been measured in the same medium (KRCI= 8.20 at 25.0 °C, ionic strength = 1.0 mol dm–3) by generating the cation from the alcohol and perchloric acid in the presence of 0.10 mol dm–1 sodium chloride.

The α-deuterium secondary kinetic isotope effect upon the hydrolysis of 2-(p-nitrophenoxy)tetrahydropyran and its relationship to values for the solvolysis of secondary alkyl arenesulfonates and the enzyme-catalysed hydrolysis of acetals

The α-deuterium secondary kinetic isotope effect for the uncatalysed hydrolysis of 2-(p-nitrophenoxy)tetrahydropyran is 1.17 in water (46°C), a result which is very similar to values for the solvolysis of simple secondary alkyl arenesulfonates which proceed with rate-limiting ionization and appreciably higher than results for enzyme-catalysed hydrolysis of other acetals.

Deamination of endo- and exo-bicyclo[3.2.1]octan-3-ylamines and their derivatives

Bicyclo[3.2.1] octan-3-ylamines have been deaminated in acetic acid by nitrous acid and via their N-phenyltriazenes; their ethyl N-nitrosocarbamates have also been solvolysed in ethanol. The exo-isomers give mainly unrearranged substitution, some elimination, and very little rearrangement. The unrearranged substitution, is derived from both the solvent (the external nucleophile, either acetic acid or ethanol) and the internal nucleophile liberated in the deaminative fragmentation step (water from the nitrous acid reaction, aniline from the triazene, and ethyl carbonate from the nitrosocarbamate). It is of predominantly retained configuration in all three reactions with the solvent, and, as demonstrated in the nitrosocarbamate solvolysis, with the internal nucleophile. The endo-isomers give mainly elimination, some unrearranged substitution, and appreciable rearrangement. The solvent-derived unrearranged substitution is with predominant inversion of configuration in all three reactions whereas that from the internal nucleophile, established by the nitrosocarbamate solvolysis, is predominantly with retention. Rearrangement from both endo- and exo-compounds is best explained in terms of hydride shift from the first formed carbonium ions (in nitrogen-separated complex ion-pairs with hydrogen-bonded anions) produced in the deaminative fragmentation. This gives rearranged classical bicyclo[3.2.1]octan-2-yl carbonium ions which, in competition with nucleophilic capture and proton loss, undergo further stepwise rearrangemnt to a common unsymmetrical non-classical carbonium ion. The non-classical cation and its classical precursors (which, from exo- and endo- substrates, differ in the location of the counter-anion) give rise to substitution products derived from solvent and the internal nucleophile. The non-classical carbonium ion also gives some tricyclo[3.2.1.02,7] octane. The high yields of internal substituion products from both endo- and exo-compounds rule out long lived intermediates such as diazonium ions.