Relative proton transfer abilities of acids and alcohols in gas phase and solutions

The present paper applies a method, based on SCF CNDO MO charge densities and assumption of the validity of the virial theorem, for calculation of relative proton affinities of alcohols, and substituted and unsubstituted carboxylic acids in the gas phase. The paper also chalks out a path for calculation of their relative acidities in solution phase by utilising a solvation energy equation and binding energy data in the gas phase. The results obtained by the present method are mostly in agreement with ion cyclotron resonance mass spectrometric experimental studies. The method has also been applied to cover the cases of amines in the gas phase.     

Hydrogen bonding and dissociation effects on the gas phase proton transfer reactions of ozone

Recently, the proton affinity (PA) of ozone was experimentally determined by Cacace and Speranza [Science (1994) 265: 208] using a bracketing technique that involved the proton transfer (PT) reactions: O₃H⁺+B⇒O₃+BH⁺; for different Br?nsted bases B. These authors showed that the simple collision model is not adequate to describe PT. We now present a theoretical model reflecting this bracketing procedure by explicitly introducing H-bonding complexing, dissociation and PT contributions, to discuss the kinetic model that assumes that PT occurs through one elementary step. The methods used include semiempirical density functional theory and ab initio Hartree-Fock methods. The procedure is gauged by using estimated PA of ozone obtained from deprotonation reactions including the Br?nsted bases BNH₃, H₂O, HOCl, SO₂, CH₃F and Kr. The PA-obtained range was from 145.3 to 160.3 kcal/mol, in fair agreement with the experimental value of 148.0±3 kcal/mol. The model seems to be fairly independent of the reference bases used to evaluate the PA. H-bonding effects appear to be a determining factor to explain collision efficiencies. Received: 5 August 1997 / Accepted: 25 September 1997     

Double proton transfer and one-electron oxidation behavior in double H-bonded glycinamide-glycine complex in the gas phase

The behaviors of double proton transfer (DPT) occurring in a representative glycinamide-glycine complex have been investigated employing the B3LYP/6-311++G** level of theory. Thermodynamic and especially kinetic parameters, such as tautomerization energy, equilibrium constant, and barrier heights have been discussed, respectively. The relevant quantities involved in the DPT process including geometrical changes, interaction energies, and deformation energies have also been studied. Analogous to that of tautomeric process assisted with a formic acid molecule, the participation of a glycine molecule favors the proceeding of the proton transfer (PT) for glycinamide compared with that without mediator-assisted case. The DPT process proceeds with a concerted mechanism rather than a stepwise one because no zwitterionic complexes have been located during the DPT process. The barrier heights are 12.14 and 0.83 kcal/mol for the forward and reverse directions, respectively. However, both of them have been reduced by 3.10 and 2.66 kcal/mol to 9.04 and -1.83 kcal/mol with further inclusion of zero-point vibrational energy (ZPVE) corrections, where the disappearance of the reverse barrier height implies that the reverse reaction should proceed with barrierless spontaneously, analogous to those of DPTs occurring between glycinamide and formic acid (or formamide). Additionally, the oxidation process for the double H-bonded glycinamide-glycine complex has also been investigated. The oxidated product is characterized by a distonic radical cation due to the fact that one-electron oxidation takes place on glycine fragment and a proton has been transferred from glycine to glycinamide fragment spontaneously. As a result, the vertical and adiabatic ionization potentials for the neutral complex have been determined to be about 8.71 and 7.85 eV, respectively, where both of them have been reduced by about 0.54 (1.11) and 0.75 (1.13) eV relative to those of isolated glycinamide (glycine) due to the formation of the intermolecular H-bond.     

Slow proton exchange in water in the gas phase

To study proton exchange in water the technique of quantitative measurements of water content at very low concentrations of water in solutions or at low vapor pressure in the gas phase (for water content of 1–10 μg/ml) has been developed. Under these conditions the rate of proton exchange slows down significantly. In particular, in mixtures of H₂O and D₂O vapors under the pressure of a 10–17 mm Hg proton life-time during the exchange process reaches the values of 10–30 min. The lifetime was measured using the gradual slow growth of the signal intensity of the mixed isotopomer HDO.     

Humidity independent mass spectrometry for gas phase chemical analysis via ambient proton transfer reaction

In this work, a humidity independent mass spectrometric method was developed for rapid analysis of gas phase chemicals. This method is based upon ambient proton transfer reaction between gas phase chemicals and charged water droplets, in a reaction chamber with nearly saturate humidity under atmospheric pressure. The humidity independent nature enables direct and rapid analysis of raw gas phase samples, avoiding time- and sample-consuming sample pretreatments in conventional mass spectrometry methods to control sample humidity. Acetone, benzene, toluene, ethylbenzene and meta-xylene were used to evaluate the analytical performance of present method. The limits of detection for benzene, toluene, ethylbenzene and meta-xylene are in the range of ∼0.1 to ∼0.3 ppbV; that of benzene is well below the present European Union permissible exposure limit for benzene vapor (5 μg m⁻³, ∼1.44 ppbV), with linear ranges of approximately two orders of magnitude. The majority of the homemade device contains a stainless steel tube as reaction chamber and an ultrasonic humidifier as the source of charged water droplets, which makes this cheap device easy to assemble and facile to operate. In addition, potential application of this method was illustrated by the real time identification of raw gas phase chemicals released from plants at different physiological stages.     

Excited state proton transfer in guanine in the gas phase and in water solution: a theoretical study

Theoretical investigations were performed to study the phenomena of ground and electronic excited state proton transfer in the isolated and monohydrated forms of guanine. Ground and transition state geometries were optimized at both the B3LYP/6-311++G(d,p) and HF/6-311G(d,p) levels. The geometries of tautomers including those of transition states corresponding to the proton transfer from the keto to the enol form of guanine were also optimized in the lowest singlet pipi* excited state using the configuration interaction singles (CIS) method and the 6-311G(d,p) basis set. The time-dependent density function theory method augmented with the B3LYP functional (TD-B3LYP) and the 6-311++G(d,p) basis set was used to compute vertical transition energies using the B3LYP/6-311++G(d,p) geometries. The TD-B3LYP/6-311++G(d,p) calculations were also performed using the CIS/6-311G(d,p) geometries to predict the adiabatic transition energies of different tautomers and the excited state proton transfer barrier heights of guanine tautomerization. The effect of the bulk aqueous environment was considered using the polarizable continuum model (PCM). The harmonic vibrational frequency calculations were performed to ascertain the nature of potential energy surfaces. The excited state geometries including that of transition states were found to be largely nonplanar. The nonplanar fragment was mostly localized in the six-membered ring. Geometries of the hydrated transition states in the ground and lowest singlet pipi* excited states were found to be zwitterionic in which the water molecule is in the form of hydronium cation (H3O(+)) and guanine is in the anionic form, except for the N9H form in the excited state where water molecule is in the hydroxyl anionic form (OH(-)) and the guanine is in the cationic form. It was found that proton transfer is characterized by a high barrier height both in the gas phase and in the bulk water solution. The explicit inclusion of a water molecule in the proton transfer reaction path reduces the barrier height drastically. The excited state barrier height was generally found to be increased as compared to that in the ground state. On the basis of the current theoretical calculation it appears that the singlet electronic excitation of guanine may not facilitate the excited state proton transfer corresponding to the tautomerization of the keto to the enol form.     

Thermochemical data relative to the complex formation in gas phase derived from computational chemistry

The structure and thermodynamics of the gaseous 1:1 addition compound of zirconium tetrachloride (ZrCl₄) with phosphorus oxychloride (POCl₃) have been determined using density functional theory calculations; the temperature-dependent values of the entropy of this molecule in the gaseous state have been derived from a structural and a vibrational analysis of this complex. The formation of gaseous ZrCl₄ · POCl₃ from gaseous ZrCl₄ and POCl₃ has then been thermodynamically characterized. © 1997 John Wiley & Sons, Inc.     

Energetics and dynamics of electron transfer and proton transfer in dissociation of metal(III)(salen)-peptide complexes in the gas phase

Time- and collision energy-resolved surface-induced dissociation (SID) of ternary complexes of Co(III)(salen)+, Fe(III)(salen)+, and Mn(III)(salen)+ with several angiotensin peptide analogues was studied using a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) specially equipped to perform SID experiments. Time-resolved fragmentation efficiency curves (TFECs) were modeled using an RRKM-based approach developed in our laboratory. The approach utilizes a very flexible analytical expression for the internal energy deposition function that is capable of reproducing both single-collision and multiple-collision activation in the gas phase and excitation by collisions with a surface. The energetics and dynamics of competing dissociation pathways obtained from the modeling provides important insight on the competition between proton transfer, electron transfer, loss of neutral peptide ligand, and other processes that determine gas-phase fragmentation of these model systems. Similar fragmentation behavior was obtained for various Co(III)(salen)-peptide systems of different angiotensin analogues. In contrast, dissociation pathways and relative stabilities of the complexes changed dramatically when cobalt was replaced with trivalent iron or manganese. We demonstrate that the electron-transfer efficiency is correlated with redox properties of the metal(III)(salen) complexes (Co > Fe > Mn), while differences in the types of fragments formed from the complexes reflect differences in the modes of binding between the metal-salen complex and the peptide ligand. RRKM modeling of time- and collision-energy-resolved SID data suggests that the competition between proton transfer and electron transfer during dissociation of Co(III)(salen)-peptide complexes is mainly determined by differences in entropy effects while the energetics of these two pathways are very similar.     

Hydrogen transfer between sulfuric acid and hydroxyl radical in the gas phase: competition among hydrogen atom transfer, proton-coupled electron-transfer, and double proton transfer

In an attempt to assess the potential role of the hydroxyl radical in the atmospheric degradation of sulfuric acid, the hydrogen transfer between H2SO4 and HO* in the gas phase has been investigated by means of DFT and quantum-mechanical electronic-structure calculations, as well as classical transition state theory computations. The first step of the H2SO4 + HO* reaction is the barrierless formation of a prereactive hydrogen-bonded complex (Cr1) lying 8.1 kcal mol(-1) below the sum of the (298 K) enthalpies of the reactants. After forming Cr1, a single hydrogen transfer from H2SO4 to HO* and a degenerate double hydrogen-exchange between H2SO4 and HO* may occur. The single hydrogen transfer, yielding HSO4* and H2O, can take place through three different transition structures, the two lowest energy ones (TS1 and TS2) corresponding to a proton-coupled electron-transfer mechanism, whereas the higher energy one (TS3) is associated with a hydrogen atom transfer mechanism. The double hydrogen-exchange, affording products identical to reactants, takes place through a transition structure (TS4) involving a double proton-transfer mechanism and is predicted to be the dominant pathway. A rate constant of 1.50 x 10(-14) cm(3) molecule(-1) s(-1) at 298 K is obtained for the overall reaction H2SO4 + HO*. The single hydrogen transfer through TS1, TS2, and TS3 contributes to the overall rate constant at 298 K with a 43.4%. It is concluded that the single hydrogen transfer from H2SO4 to HO* yielding HSO4* and H2O might well be a significant sink for gaseous sulfuric acid in the atmosphere.     

Reactions of atomic metal anions in the gas phase: competition between electron transfer, proton abstraction and bond activation

Bare metal anions K(-), Rb(-), Cs(-), Fe(-), Co(-), Ni(-), Cu(-), and Ag(-), generated by electrospray ionization of the corresponding oxalate or tricarballylate solutions, were allowed to react with methyl and ethyl chloride, methyl bromide, nitromethane, and acetonitrile in the collision hexapole of a triple-quadrupole mass spectrometer. Observed reactions include (a) the formation of halide, nitride, and cyanide anions, which was shown to be likely due to the insertion of the metal into the C-X, C-N, and C-C bonds, (b) transfer of H(+) from the organic molecule, which is demonstrated to most likely be due to the simple transfer of a proton to form neutral metal hydride, and (c) in the case of nitromethane, direct electron transfer to form the nitromethane radical anion. Interestingly, Co(-) was the only metal anion to transfer an electron to acetonitrile. Differences in the reactions are related to the differences in electron affinity of the metals and the Δ(acid)H° of the metals and organic substrates. Density functional theory calculations at the B3-LYP/6-311++G(3df,2p)//B3-LYP/6-31+G(d) level of theory shed light on the relative energetics of these processes and the mechanisms by which they take place.     

A study of light-induced proton transfer from gas phase (radical) cations to reference bases. Bracketing of proton transfer from excited ions and associated reaction kinetics

By use of Fourier transform ion cyclotron resonance, it is shown that protonated naphthalene when excited with laser light of 488 nm is more reactive in proton transfer to reference bases than in its ground state. The excitation leads to reaction with bases for which proton transfer in the ground state is endothermic up to a detected maximum of 60 kJ/mol. For indene radical cations excited at 514.5 nm, it is shown that the rate constant for proton transfer to 3-pentanone is either about 10 or about 100 times lower than the rate constant for relaxation by collisions with 3-pentanone. From the energy deposited in the ions, 0.5-0.6 eV is available for proton transfer to a base which seems reasonable when taking into account a complete randomization of the initially deposited energy.     

Thermochemical studies of benzoylnitrene radical anion: the N-H bond dissociation energy in benzamide in the gas phase

The thermochemical properties of benzoylnitrene radical anion, C6H5CON-, were determined by using a combination of energy-resolved collision-induced dissociation (CID) and proton affinity bracketing. Benzoylnitrene radical anion dissociates upon CID to give NCO- and phenyl radical with a dissociation enthalpy of 0.85 +/- 0.09 eV, which is used to derive an enthalpy of formation of 33 +/- 9 kJ/mol for the nitrene radical anion. Bracketing studies with the anion indicate a proton affinity of 1453 +/- 10 kJ/mol, indicating that the acidity of benzamidyl radical, C6H5CONH, is between those of benzamide and benzoic acid. Combining the measurements gives an enthalpy of formation for benzamidyl radical of 110 +/- 14 kJ/mol and a homolytic N-H bond dissociation energy in benzamide of 429 +/- 14 kJ/mol. Additional thermochemical properties obtained include the electron affinity of benzamidyl radical, the hydrogen atom affinity of benzoylnitrene radical anion, and the oxygen anion affinity of benzonitrile.     

Evidence for long-range glycosyl transfer reactions in the gas phase

A long-range glycosyl transfer reaction was observed in the collision-induced dissociation Fourier transform (CID FT) mass spectra of benzylamine-labeled and 9-aminofluorene-labeled lacto-N-fucopentaose I (LNFP I) and lacto-N-difucohexaose I (LNDFH I). The transfer reaction was observed for the protonated molecules but not for the sodiated molecules. The long-range glycosyl transfer reaction involved preferentially one of the two L-fucose units in labeled LNDFH I. CID experiments with labeled LNFP I and labeled LNFP II determined the fucose with the greatest propensity for migration. Further experiments were performed to determine the final destination of the migrating fucose. Molecular modeling supported the experiments and reaction mechanisms are proposed.     

Theoretical investigation of the proton affinities of benzazoles in the gas phase and in solution

The ground‐state equilibrium geometries of benzothiazole, benzoxazole, and benzimidazole were optimized at the density functional theory (DFT)/6‐31G** level of theory. Proton affinities on each of the possible sites in the studied series of compounds have been calculated at the DFT/6‐31G**/6‐311++G** level. The results indicate clearly that N‐site protonation is strongly favored over X‐site protonation (X = NH, O, S) for the series studied. Correlation of the computed proton affinities to the energy (E_HOMO) of the highest occupied MO in the gas phase and in solution has been explored and discussed. A comprehensive investigation of the effect of solvent on the process of protonation of the studied compounds has been performed. Different dielectric continuum models (i.e., Onsager, PCM, and IPCM) have been tested; their performance and range of applicability are reported and discussed. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Isotope effects in dissociation reactions of proton bound amine dimers in the gas phase

Kinetic and thermodynamic isotope effects on the unimolecular dissociation of proton bound dimers were studied in the gas phase using mass spectrometry techniques. In addition proton transfer reactions were investigated using equilibrium techniques in conjunction with a theoretical study. Normal isotope effects were observed for all of the amine systems studied. The effect of label position, extent of labeling, size and structure of the proton bound dimers have been discussed with respect to (i) the kinetic and thermodynamic isotope effect on the dissociation reaction, (ii) the kinetic energy release on the dissociation reaction, (iii) the thermodynamic isotope effect on the proton exchange reaction between the labeled and unlabeled amines, and (iv) the effective temperatures and the excess energies of the metastable proton bound dimers. Other compound classes (CH₃OH, (CH₃)₂O, CH₃CN and (CH₃)₂CO) were studied and discussed in the same way, though not as thoroughly. All the systems show normal isotope effects, except for the proton bound dimer of CH₃CN and CD₃CN, which showed an inverse isotope effect.