The unimolecular chemistry of protonated glycine and the proton affinity of glycine: a computational model

The potential energy hypersurface of protonated glycine, GH⁺, has been investigated. The calculated G2(MP2) value for the proton affinity (PA) of glycine, PA _calc=895kJ mol⁻¹, is in good agreement with the experimental value which has been estimated to lie in the range 864kJ mol⁻¹ < PA _exp <891kJ mol⁻¹. Ab initio quantum chemical calculations of relevant parts of the potential energy surface of GH⁺ give a reaction model which is consistent with the observed mass spectrometric fragmentation pattern. The lowest energy unimolecular reactions of GH⁺ are two distinct processes: (1) loss of CO, which has a substantial barrier for the reverse reaction, and (2) loss of CO plus H₂O, which has no barrier for the reverse reaction. Received: 15 November 1996 / Accepted: 6 May 1997     

The structure of protonated disilene

The open and cyclic forms of Si₂ H₅⁺ have been calculated at the SCF level and with inclusion of electron correlation energy. Our final results indicate that both structures are about equally stable. The proton affinity of disilene is calculated as 207 kcal/mol.     

The structure of protonated diamines and polyamines

The reduced mobilities in air, at 200C, of six isomeric C₇H₁₈N₂ protonated diamines, two triamines (caldine and spermidine), and two tetramines (thermine and spermine) were measured by ion mobility spectrometric (IMS) techniques. The results indicated that all these polyamines undergo proton-induced cyclization, with the proton forming a bridge between two amino groups. It appears as if the favored configuration of the protonated polyamines involves a six- or seven-membered ring rather than a bridge between the terminal amino groups. It is believed that in the tetramines the cyclic structure is formed between the two central, more basic, secondary amine sites.     

Effect of the structure and substituents on the proton affinity of immunoactive azasteroid molecules

The effect of the structure and substitution on the proton affinity of immunoactive 8-azasteroid molecules was studied by semi-empirical methods of the quantum chemistry. It was shown that the proton affinity of these molecules is determined by the carbonyl oxygen atoms. The methoxy substitutents on the aromatic ring of the steroid have a weak effect on the proton affinity of the molecules in the ground state, whereas the direction of the effect for excited molecules depends on the orbital nature of the excited state. The substitution of the NH group or oxygen in the 16-position causes a substantial increase in the proton-acceptor power. The reasons for these changes are discussed.     

The mobility and ion structure of protonated aminoazoles

The reduced mobility of protonated pyrazole derivatives was measured by ion mobility spectrometry (IMS) in air, nitrogen, and carbon dioxide, at temperatures between 150 and 250°C. It was found that the mobility of protonated 5-amino-1-phenylpyrazole was higher than that of its 3- and 4-isomers. This was attributed to the fact that in the 5-isomer the preferred site of protonation is on the endocyclic nitrogen, which leads to delocalization of the ionic charge, and thus to a diminished interaction with the drift gas molecules. On the other hand, protonated 5-amino-1-methylpyrazole has a slightly lower mobility than its isomers, which is indicative of a different protonation mechanism.     

The gas-phase basicity and proton affinity of 1,3,5-cycloheptatriene--energetics,structure and interconversion of dihydrotropylium ions

The hitherto unknown gas-phase basicity and proton affinity of 1,3,5-cycloheptatriene (CHT) have been determined by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Several independent techniques were used in order to exclude ambiguities due to proton-induced isomerisation of the conjugate cyclic C(7)H(9)(+) ions, [CHT + H](+). The gas-phase basicity obtained by the thermokinetic method, GB(CHT) = 799 +/- 4 kJ mol(-1), was found to be identical, within the limits of experimental error, with the values measured by the equilibrium method starting with protonated reference bases, and with the values resulting from the measurements of the individual forward and reverse rate constants, when corrections were made for the isomerised fraction of the C(7)H(9)(+) population. The experimentally determined gas-phase basicity leads to the proton affinity of cycloheptatriene, PA(CHT) = 833 +/- 4 kJ mol(-1), and the heat of formation of the cyclo-C(7)H(9)(+) ion, deltaH(f)(0)([CHT + H](+)) = 884 +/- 4 kJ mol(-1). Ab initio calculations are in agreement with these experimental values if the 1,2-dihydrotropylium tautomer, [CHT + H((1))](+), generated by protonation of CHT at C-1, is assumed to be the conjugate acid, resulting in PA(CHT) = 825 +/- 2 kJ mol(-1) and deltaH(f)(0)(300)([CHT + H((1))](+)) = 892 +/- 2 kJ mol(-1). However, the calculations indicate that protonation of cycloheptatriene at C-2 gives rise to transannular C-C bond formation, generating protonated norcaradiene [NCD + H](+), a valence tautomer being 19 kJ mol(-1) more stable than [CHT + H((1))](+). The 1,4-dihydrotropylium ion, [CHT + H((3))](+), generated by protonation of CHT at C-3, is 17 kJ mol(-1) less stable than [CHT + H((1))](+). The bicyclic isomer [NCD + H](+) is separated by relatively high barriers, 70 and 66 kJ mol(-1) from the monocyclic isomers, [CHT + H((1))](+) and [CHT + H((3))](+), respectively. Therefore, the initially formed 1,2-dihydrotropylium ion [CHT + H((1))](+) does not rearrange to the bicyclic isomer [NCD + H](+) under mild protonation conditions.     

Split-valence basis sets for describing the structure of the methylsilane molecule

I. V. Grebenshchikov Institute of the Chemistry of Silicates. Translated from Zhurnal Strukturnoi Khimii, Vol. 29, No. 6, pp. 128–130, November–December, 1988.     

On the proton affinity of pteridine

All-valence-electron and perturbation calculations suggest that pteridine may preferably be protonated at the pyrazine moiety. Correlation between these results and some experimental data is presented.     

On the structure of protonated methane

Results of a Fourier transform-ion cyclotron resonance study are reported concerning the reactivity of protonated perdeuteromethane and deuteronated methane, generated under varying pressure conditions in an external chemical ionization ion source, toward ammonia. The competition between proton and deuteron transfer from both protonated perdeuteromethane and deuteronated methane to ammonia exhibits chemically distinguishable hydrogens. The chemical behavior of protonated methane appears to be compatible with the theoretically predicted stable structure with C_S symmetry, involving a three-center two-electron bond associating two hydrogens and the carbon atom. Interconversion of this structure due to exchange between one of these hydrogens and one of the three remaining hydrogens appears to be a fast process that is induced by interactions with the chemical ionization gas.     

The electronic structure of protonated forms of azaindoles

The Pariser-Parr-Pople (PPP) method has been applied to protonated 4-, 5-, 6-, and 7-azaindoles, and the results are compared with published ones [1] for indole and azaindoles. These show that protonation and solvation at the N in the six-membered ring can play a major part in the chemical behavior of the azaindoles. The differences between the compounds are due to induction rather than to differences in the structure of the -system. The method of restricted configuration interaction (RCI) has been used to calculate the energies of the lowest - transitions.We are indebted to L. N. Yakhontov and M. Ya. Uritskaya for numerous discussions.     

Theoretical proton affinity and fluoride affinity of nerve agent VX

Proton affinity and fluoride affinity of nerve agent VX at all of its possible sites were calculated at the RI-MP2/cc-pVTZ//B3LYP/6-31G* and RI-MP2/aug-cc-pVTZ//B3LYP/6-31+G* levels, respectively. The protonation leads to various unique structures, with H(+) attached to oxygen, nitrogen, and sulfur atoms; among which the nitrogen site possesses the highest proton affinity of -ΔE ～ 251 kcal/mol, suggesting that this is likely to be the major product. In addition some H(2), CH(4) dissociation as well as destruction channels have been found, among which the CH(4) + [Et-O-P(═O)(Me)-S-(CH(2))(2)-N(+)(iPr)═CHMe] product and the destruction product forming Et-O-P(═O)(Me)-SMe + CH(2)═N(+)(iPr)(2) are only 9 kcal/mol less stable than the most stable N-protonated product. For fluoridization, the S-P destruction channel to give Et-O-P(═O)(Me)(F) + [S-(CH(2))(2)-N-(iPr)(2)](-) is energetically the most favorable, with a fluoride affinity of -ΔE ～ 44 kcal. Various F(-) ion-molecule complexes are also found, with the one having F(-) interacting with two hydrogen atoms in different alkyl groups to be only 9 kcal/mol higher than the above destruction product. These results suggest VX behaves quite differently from surrogate systems.     

A coarse-graining approach for the proton complex in protonated aluminosilicates

We have developed a computational framework for the adsorption of linear alkanes in protonated aluminosilicates. These zeolites contain trace amounts of water that form hydrated proton complexes. The presence of hydrated protons makes the simulations at the fully atomistic level difficult. Instead of constructing an elaborate and complex model, we show that an approach based on a coarse-graining of the proton-complex accurately describes the available experimental isotherms, Henry coefficients, heats of adsorption, and oxygen-proton distances. Our approach is supported by MP2 quantum mechanical simulations. The model gives remarkably good agreement with experimental data beyond the initial calibration set.     

A novel mechanism of proton transfer in protonated peptides

The study presents quantum-chemical calculations on proton transfer in protonated N-acetylglycyl-N1-methylglycinamide (AGA) as a short oligopeptide model. All calculations employ the B3LYP functional and the 6-31++G** basis set. Two different mechanisms of proton transfer are discussed. The rate-determining step of the first mechanism exhibits an energy barrier of about 17.7 kcal mol-1, and it is represented by an isomerization of the proton around the double bond of the carbonyl group. The second mechanism is based on the large conformational flexibility of AGA, where all carbonyl oxygens cooperate. The rate-determining step of this mechanism exhibits an energy barrier of only 8.3 kcal mol-1.     

SiH+5 and the proton affinity of monosilane

Tandem mass spectrometric studies show that SiH⁺₅ is formed in bimolecular reactions of SiH₄ and NH⁺₂, C₂H⁺₃, C₂H⁺₆ and C₃H⁺₈ ions. The dependence of the reaction cross sections on ion energy indicates the formation of SiH⁺₅ from NH⁺₂, C₂H⁺₃, and C₂H⁺₆ to be exothermic reactions, while formation from C₃H⁺₈ is endothermic. Using known thermochemical data, these facts permit the assignment of 150 and 156 kcal/mole to the lower and upper limits of the proton affinity of monosilane.     

Role of 2‐oxo and 2‐thioxo modifications on the proton affinity of histidine and fragmentation reactions of protonated histidine

A combination of electrospray ionisation (ESI), multistage and high‐resolution mass spectrometry experiments was used to compare the gas‐phase chemistry of the amino acids histidine (1), 2‐oxo‐histidine (2), and 2‐thioxo‐histidine (3). Collision‐induced dissociation (CID) of all three different proton‐bound heterodimers of these amino acids led to the relative gas‐phase proton affinity order of: histidine >2‐thioxo‐histidine >2‐oxo‐histidine. Density functional theory (DFT) calculations confirm this order, with the lower proton affinities of the oxidised histidine derivatives arising from their ability to adopt the more stable keto/thioketo tautomeric forms. All protonated amino acids predominately fragment via the combined loss of H₂O and CO to yield a₁ ions. Protonated 2 and 3 also undergo other small molecule losses including NH₃ and the imine HN=CHCO₂H. The observed differences in the fragmentation pathways are rationalised through DFT calculations, which reveal that while modification of histidine via the introduction of the oxygen atom in 2 or the sulfur atom in 3 does not affect the barriers against the loss of H₂O+CO, barriers against the losses of NH₃ and HN=CHCO₂H are lowered relative to protonated histidine. Copyright © 2010 John Wiley & Sons, Ltd.     

The structure of the protonated forms of 1-hydrazinophthalazine and phthalazone hydrazone derivatives

Phthalazone derivatives Ia–h are protonated at the exocyclic nitrogen and phthalazine derivatives IIa–c at the N-2 of the ring. The cations of hydrazones Va–e have the cyclic structures VIa–e.