Pulsed High-pressure Mass Research Articles

Investigations of the clustering reactions of protonated amino acid esters by high pressure mass spectrometry and quantum chemical calculations

The equilibria involved in the formation of the proton-bound dimers of the three simplest amino acid methyl esters (glycine, alanine and valine methyl esters) were characterized by means of pulsed ionization high pressure mass spectrometry (PHPMS) experiments and density functional theory (DFT) calculations. Our results would indicate that, within the temperature range employed in these experiments, the most stable proton-bound dimer conformers are formed in the case of glycine and alanine whereas a more entropically favoured isomer would dominate in the case of valine. Various possible isomers of each of the proton-bound dimer species have been investigated computationally each exhibiting a combination of inter- and intramolecular hydrogen bonding. A system of nomenclature for these various species is proposed. The possibility of structures exhibiting 'salt-bridge’ interactions have also been explored, recognizing that such structures would necessarily result from highly energetic structural rearrangements.

The Proton Affinity Scale, and Effects of Ion Structure and Solvation

AbstractFor Abstract see ChemInform Abstract in Full Text.

The proton affinity scale, and effects of ion structure and solvation

In the last three decades, several extensive thermochemical ladders were constructed that connect molecules over a wide range of gas-phase basicities (GBs, i.e., −Δ G° protonation) and proton affinities (PAs, i.e., −Δ H° protonation). The data include GB ladders from low pressure ion cyclotron resonance (ICR), GB and PA ladders from pulsed high pressure mass spectrometry (PHPMS), absolute reference PAs from spectroscopic measurements, and ab initio calculations. Comparison amongst the ladders identifies some systematic expansions or contractions (mostly <10%), mainly due to uncertainties in temperature measurement. After global adjustments for these effects and anchoring to accurate local standards, all the data are consistent, including the directly measured PA ladders from variable temperature PHPMS measurements. Bracketing experiments and association thermochemistry provide further independent verification of the GB and PA scales. The derived entropies of protonation ( S p) from PHPMS measurements and theoretical calculations are structurally reasonable and mutually consistent, which supports the entropy results and the consistencies of the GB and PA scales. Special structural effects such as internal hydrogen bonds in polyfunctional and biomolecules ions can also be quantified. The overall data suggest that the GB and PA scales, spanning a range of 120 kcal mol −1 for 38 reference compounds, have been established within ±0.8 kcal mol −1, and the S p values within ±1.5 cal mol −1 K −1, which may be the limits of accuracy of current methods. The gas-phase data, used in thermochemical cycles, allows calculating the solvation energies of ions. Molecular and solvation effects can be separated, and the latter decomposed into continuum and hydrogen bonding terms. Altogether, the gas-phase data allows decomposing the energetics of organic acid–base chemistry into structurally sensible and quantitatively understandable factors.

Gas-phase acidities and sites of deprotonation of 2-ketones and structures of the corresponding enolates

The gas-phase acidities of seven different 2-alkanone molecules have been investigated using pulsed ionization high pressure mass spectrometry (HPMS). From the temperature dependence of the equilibrium constant for proton-exchange reactions, the enthalpy and entropy changes for these reactions were determined. These experimental values agreed well with the G3(MP2) calculated values. From the thermochemistry of the proton-exchange reactions a gas-phase acidity scale, relative to acetone, was constructed for 2-ketones up to and including 2-decanone. Furthermore, an absolute gas-phase acidity scale is presented which is anchored to the gas-phase acidity value for acetone determined by Bartmess et al. [J. Am. Chem. Soc. 101 (1979) 6046]. The entropy changes associated with the proton-exchange reactions were all found to be very close to zero which suggests that there is no intramolecular solvation occurring in any of the enolate ions over the temperature range studied. The G3(MP2) calculations predict that the primary and secondary enolates of butanone, formed by deprotonation at either C1 or C3, respectively, have nearly identical basicities. However, as the 2-ketone chain length increases, the secondary enolate becomes more stable, with respect to the primary enolate. These theoretical predictions are in agreement with the stability ordering for enolates (2°≥1°⪢3°) found experimentally by Chyall et al. [J. Am. Chem. Soc. 116 (1994) 8681].

Effects of oxygen and water on the resonance electron capture reactions of low electron affinity compounds

The reactions of the molecular anions of several low electron affinity (EA) compounds, including anthracene, quinazoline, benzophenone, quinoxaline, and azulene, with oxygen and water have been studied by pulsed high pressure mass spectrometry (PHPMS). It is shown that in the simultaneous presence of oxygen and water, these molecular anions, M −, are rapidly destroyed and the ion, O 2 −(H 2O), is rapidly formed. It is shown that the high rate with which this transition occurs can not be explained by the simplest model envisioned that is based on well-known ion molecule reactions. These results can be explained, however, by inclusion into the model of a previously uncharacterized reaction between the molecular ion–oxygen complex, MO 2 −, and water. The results reported here explain why the molecular anions of compounds that have lower EA’s than that of azulene are not readily observed in electron capture ion sources of 1 atm buffer gas pressure. In addition, it is shown that the reactions characterized here lead to a state of chemical equilibrium between the M − and O 2 −(H 2O) ions within the PHPMS ion source from which the EA values of the low-EA compounds can be determined. By this method the electron affinities of anthracene, quinazoline, benzophenone, and quinoxaline are found to be 0.54, 0.56, 0.61, and 0.68 eV, respectively.

Effect of buffer gas alterations on the thermal electron attachment and detachment reactions of azulene by pulsed high pressure mass spectrometry

A principal motivation for the present study is to determine the ion source conditions required for achievement of the high pressure limit (HPL) of kinetic behavior for the resonance electron capture (REC) reaction of azulene (Az), Az + e → Az −. This goal is accomplished here by measuring rate constants for the reverse process, thermal electron detachment by molecular anions of azulene, Az − → Az + e, by pulsed high pressure mass spectrometry by using a variety of buffer gases, methane, argon, nitrogen, and helium, over a range of pressures, from 1 to 6 Torr, over a range of temperatures, from 150 to 200 °C. From these measurements, it is shown that the ion source conditions commonly used in electron capture mass spectrometry for the trace analysis of REC-active molecules would not be sufficient for achievement of the HPL of the REC reaction of azulene and, therefore, would likely result in significantly reduced sensitivity to this compound. The problem highlighted here for the case of azulene is undoubtedly shared by many other REC-active compounds. The resolution of this problem is expected to require accommodation of several relevant factors shown here to be important in the case of azulene, including the choice of buffer gas, pressure, and ion source temperature.

Ionic Hydrogen Bonds in Bioenergetics. 4. Interaction Energies of Acetylcholine with Aromatic and Polar Molecules

The binding energies of the quaternary ions (CH3)4N+ and acetylcholine (ACh) to neutral molecules have been measured by pulsed high-pressure mass spectrometry and calculated ab initio, to model interactions in the acetylcholine receptor channel. Binding energies to C6H6 and C6H5CH3 are similar to those to H2O (33−42 kJ/mol (8−10 kcal/mol)), but are weaker than those to polar organic ligands such as CH3CO2CH3 (50−63 kJ/mol (12−15 kcal/mol)) and to amides (up to 84 kJ/mol (20 kcal/mol)). These data suggest that aromatic residues that line the groove leading to the ACh receptor site may provide stabilization comparable to water, and therefore allow entry from the aqueous environment, yet do not bind ACh as strongly as polar protein groups, and therefore allow transit, without trapping, to the receptor site. Four of the five distinct ACh conformers located computationally are stabilized by internal C−H···O hydrogen bonds involving the quaternary ammonium group, which is supported by the thermochemistry of the protonated analogue, CH3CO2CH2CH2N(CH3)2H+, and by the measured bonding energy between models of the ACh end groups, (CH3)4N+ and CH3CO2CH3. Each conformer forms a number of stable complexes with water or benzene. Several possible roles for an ACh conformational change upon entry into the channel are discussed, including partial compensation for the loss of bulk solvation. An additional role for the aromatic environment is also suggested, namely lowering the energy barrier to the formation of the active all-trans ACh rotamer required at the receptor site.

The Carbon Lone Pair as Electron Donor. Ionic Hydrogen Bonds in Isocyanides

Hydrogen bond dissociation energies (ΔH°D) in protonated dimer ions containing isocyanides were measured by pulsed high-pressure mass spectrometry, and the interactions were analyzed by ab initio calculations. Strong bonding (80−105 kJ/mol (19−25 kcal/mol)) is observed when the carbon lone pair is the electron donor, i.e. in the complexes of isocyanides with protonated amines and protonated isocyanides (R3NH+···CNR and RNCH+···CNR complexes). The bonding is weaker (60−90 kJ/mol (14−21 kcal/mol)) in the complexes of oxygen bases with protonated isocyanides, i.e. in RNCH+···O-type complexes. Inverse linear correlations between ΔH°D and the proton affinity difference of the components show slopes of −0.22 for R3NH+···CNR- and −0.25 for RNCH+···O-type complexes. The intercepts yield intrinsic bond strengths (ΔPA = 0) of 107.7 (25.7 kcal/mol) and 100.0 kJ/mol (23.9 kcal/mol), respectively. Geometry optimizations were carried out at four calculational levels, the largest of which is MP2/6-31+G(d,p). Single-point energies were obtained with increasingly flexible basis sets up to cc-pVTZ+. Trends in dissociation energies within the cyanide and isocyanide series of complexes and between the two series of complexes hold for every basis set considered. Calculated and experimental ΔH°D values agree within the standard uncertainty of ±6 kJ/mol (1.5 kcal/mol) for only four of the nine complexes for which experimental data are available. The hydrogen bonding properties of sp-type carbon vs nitrogen lone pairs are illustrated by comparing analogous isocyanide and cyanide complexes. The relative importance of the electrostatic and delocalization components of the dissociation energy is different for the two sets of complexes, with delocalization effects being more important for the isocyanides.

Pulsed high pressure mass spectrometry with near-viscous flow ion sampling

Rate constants for the reaction, F − + CH 3I → CH 3F + I −, are determined at 150°C by pulsed electron beam high pressure mass spectrometry (PHPMS) under conditions where the ions within the source are sampled by near-viscous flow through ion-exit apertures of relatively large diameter. It is shown that the rate constants thereby determined are of high accuracy and are not subject to a common source of measurement error that is intrinsic to PHPMS measurements made under the molecular or near-molecular flow conditions that are generally used. The success of ion sampling by near-viscous flow demonstrated here also suggests that the PHPMS technique can be successfully extended to the study of ion/molecule reaction rates at significantly greater buffer gas pressures than previously considered feasible within the conventional view of the method.

Complexing of the Ammonium Ion by Polyethers. Comparative Complexing Thermochemistry of Ammonium, Hydronium, and Alkali Cations

The binding energies of NH4+ to polyethers, and for comparison, to acetone molecules, were measured by pulsed high-pressure mass spectrometry. The binding energies in the polydentate complexes increase with increasing ligand size and number of available oxygen groups. However, the binding energies are smaller than in complexes with free Me2CO molecules, reflecting the geometrical constraints in the polydentate ligands. The binding energy of NH4+ in various polydentate complexes is similar to that of K+ and smaller than that of H3O+ by 105 ± 12 kJ/mol (25 ± 3 kcal/mol). Based on the H3O+ to crown ether binding energies, these relations can be used to estimate the binding energy of NH4+ and K+ to 15-crown-5 of 248 ± 12 kJ/mol (59 ± 3 kcal/mol) and to 18-crown-6 of 296 ± 12 kJ/mol (71 ± 3 kcal/mol). The order of binding energies of oxygen-containing ligands to H3O+ > Na+ > NH4+ ≈ K+ is reproduced by ab initio calculations on complexes with MeOCH2CH2OMe. The amount of calculated charge transfer to the ligands follows the order of binding energies. The difference between the binding energies of H3O+ and NH4+ to oxygen-containing ligands in the gas-phase complexes is similar to the condensed-phase aqueous heats of solvation of these ions.

Concentration enrichment in the ion source of a pulsed electron beam high pressure mass spectrometer

Concentration enrichment in the ion source of a pulsed electron beam high pressure mass spectrometer

Concentration enrichment in the ion source of a pulsed electron beam high pressure mass spectrometer

The magnitude of concentration enrichment of methyl iodide in methane, helium, and argon buffer gases within the ion source of a pulsed e-beam high pressure mass spectrometer (PHPMS) is characterized here by a theoretical model and by kinetic measurements of the gas phase ion/molecule reaction, F − + CH 3I → CH 3F + I −, at 150°C. A relatively simple PHPMS ion source is used in which the magnitude of enrichment is related only to the ventilation of individual molecules out of the ion source by passage through its ion exit and electron entrance slits. It is shown that significant enrichment does occur under typical operating conditions of this source and that the magnitude of enrichment is strongly dependent on the choice of ion source pressure. These observations are explained in terms of the flow conditions thought to exist in the ion source. Recommendations for improving the accuracy of future kinetic and equilibrium measurements by the PHPMS are provided.

The Ionic Hydrogen Bond. 5. Polydentate and Solvent-Bridged Structures. Complexing of the Proton and the Hydronium Ion by Polyethers

The thermochemistry associated with protonated complexes containing one or two glyme (McOCH{sub 2}CH{sub 2}-OMe, (G1)), diglyme (Me(OCH{sub 2}CH{sub 2}){sub 2}OMe, (G2)), or triglyme (Me(OCH{sub 2}CH{sub 2}){sub 3}OMe (G3)) molecules and 0-3 H{sub 2}O molecules was measured by pulsed high-pressure mass spectrometry. Comparison of polyether, crown ether, and acetone complexes with H{sup +} and H{sub 3}O{sup +} shows increasing binding energies with increasing flexibility in the ligands. For example, in protonated clusters containing ligands with a total of four polar groups, the proton is bonded by a total energy of (kJ/mol (kcal/mol)): four Me{sub 2}CO molecules, 1044.8 (249.7): two G1 molecules, 987.4 (236.0); one G3 molecule, 962.3 (230.0); 12-crown-4, 941.4 (225.0). Stabilization of the proton by dipoles of the free ether groups contributes significantly; for example, 17 and 54 kJ/mol (4 and 13 kcal/mol) in the binding energy within (G1){sub 2}H{sup +} and (G2){sub 2}H{sup +} dimers, respectively. The thermochemistry of H{sub 3}O{sup +} binding indicates bidentate complexes with one each of G2, G3, and 15-crown-5 molecules and two G1 molecules, with binding energies of 310-352 kJ/mol (74-84 kcal/mol). In these complexes the second OH{sup +}.O bond contributes up to 113 kJ/mol (27 kcal/mol). 34 refs., 11 figs., 3 tabs.

Intermolecular forces in organic clusters

The effects of alkyl groups on the binding energies of cluster ions were investigated. The data were obtained by variable-temperature pulsed high-pressure mass spectrometric equilibrium measurements on protonated clusters A{sub n}H{sup +} (A = MeOH, n-PrOH, n-AmOH, MeCOOH, MeNH{sub 2}, n-PrNH{sub 2}, and Me{sub 2}NH). Inner shell binding energies are not affected by alkylation, suggesting pure hydrogen-bonding interactions with the core ions. Shell-filling effects are also not affected. Thus, going to the second shell causes an equal drop of 17 {plus_minus} 4 kJ/mol (4 {plus_minus} 1 kcal/mol) in all the observed clusters, despite the variation in the number of hydrogen-bonding sites and charge densities at the core ions. This suggests opposing effects of intramolecular and intermolecular solvation at the shell-filling step. However, the alkyl groups do affect the outer shells. The bonding energies in the outer shells of the alcohol clusters approach the respective bulk condensation energies within {plus_minus} 6 kJ/mol ({plus_minus}1.5 kcal/mol.) This suggests liquidlike hydrophobic alkyl-alkyl interactions in the outer shells which may contribute 12-25 kJ/mol (3-6 kcal/mol) to the bonding energies. 70 refs., 12 figs., 2 tabs.

A pulsed electron beam, variable temperature, high pressure mass spectrometric re-evaluation of the proton affinity difference between 2-methylpropene and ammonia

A new constructed pulsed electron beam high pressure mass spectrometer which incorporates a reverse geometry, BE, sector mass spectrometer has been used to examine proton transfer equilibria as a function of temperature for a series of bases intermediate in proton affinity between 2-methylpropene and ammonia. The data obtained support the recent conclusion of Meot-Ner and Sieck (J. Am. Chem. Soc., 113 (1991) 448) that the proton affinity of ammonia should be revised upward by some 4 kcal mol −1. Examination of clustering equilibria at high temperatures for strongly bound proton bound dimer species establishes that temperatures are accurately measured in the new apparatus. Thermochemial data obtained for the association of NH + 4 with 2-methylpropene suggest that the proton affinity of t-butylamine must also be revised upward by nearly 8 kcal mol −1. Use of the capability of the reverse geometry configuration of the mass spectrometer to examine collision-induced dissociation of mass-selected ions establishes that the cluster of NH + 4 with 2-methylpropene has the same structure as protonated t-butylamine. Consideration of the kinetics of three-body association reactions reveals that temporal profiles in high pressure mass spectrometry experiment may have to be examined for up to 10 ms to ensure that true thermal equilibrium is established in clustering reactions.

Electron thermalisation in a pulsed e-beam high pressure mass spectrometer

A pulsed e-beam high pressure mass spectrometer (HPMS) is shown to be well-suited to the measurement of electron thermalisation and thermal electron capture (EC) processes. By measurement of the intensity of the negative ions observed as a function of time after the e-beam pulse, it is possible to verify that EC reactions, alone, are responsible for the formation of the negative ions. It is also demonstrated that most of the electrons within the ion source can be brought into thermal equilibrium with the buffer gas prior to their attachment to an EC-active compound. Of five different buffer gases studied, methane and hydrogen are shown to be particularly effective in thermalising electrons. Additional experiments indicate that less control of electron energy and/or EC processes is achieved with use of the continuous e-beam mode of the HPMS. However, good agreement was obtained between the continuous and pulsed modes of the HPMS when hydrogen was used as the buffer gas.

On the structure of Al(acetone) +2

Initial results from direct clustering measurements of enthalpies and entropies of binding of acetone to Al + using a high pressure mass spectrometer led to the hypothesis that ligandligand bonding was involved in Al(acetone) + 2. In this study, however, collision-induced dissociation, ligand-displacement reactions, and infrared multiphoton photodissociation of the mixed species Al(acetone)(acetone- d 6) + in a Fourier transform mass spectrometer show that the two acetone ligands in Al(acetone) + 2 must be equivalent. Ab initio calculations support the conclusion that there are no ligandligand bonds, but that polarisation of the Al + leads to both acetones binding on the same side. The resulting ligandligand interaction explains the difference in the observed entropy for the Al(acetone) + system and those of other simple association reactions. Finally, direct clustering results from a newly constructed pulsed laser ionisation high pressure mass spectrometer are in excellent agreement with the Fourier transform mass spectrometry and ab initio findings.

Solvation of halide ions with CH 3OH in the gas phase

The gas-phase solvation reactions, X −(CH 3OH) n−1 + CH 3OH ⇌ X − (CH 3OH) n , for X −  F −, Br −, and I −, were studied using a pulsed electron beam high pressure mass spectrometer. The −Δ G° n−1, n values are in the order F − > Cl − > Br − > I −. However, the −Δ H° n−1, n values for Br − and I − decrease more gradually with n than those for F − and Cl − and they follow the order F − > Br − > Cl − with n ≥ 6. The first solvation shells of X − (CH 3OH) n for X −  F −, Cl −, and Br − seem to be completed with six CH 3OH ligands. The −Δ H° n−1, n values for I − are almost independent of n and the completion of the first solvation shell does not take place up until n = 8. Owing to crowding in the inner shell, the −Δ S° n−1, n values for I − increase steadily with n. The differences between the cumulative thermochemical values, Δ G° 0, n , Δ H° 0, n , and TΔ S° 0, n , do not approach the differential thermochemical values for solution.

Equilibrium studies by pulsed high pressure mass spectrometry: a calibration, and some pitfalls and solutions

The accuracy of pulsed high pressure mass spectrometry (PHPMS) thermochemical measurements was assessed using a ladder of charge transfer equilibria involving aromatic molecules. The ladder was constructed using Δ H° values from variable temperature measurements. For the 14 compounds in the ladder, the average agreement of the equilibrium results with spectroscopic ionisation potentials (IPs) is ± 0.6 kcal mol −1. Over the span of the ladder, from 1,2,3,5-tetramethylbenzene to C 6F 6, the thermochemical ladder yields a ΔIP of 40.7 kcal mol −1, compared with the spectroscopic value of 42.3 kcal mol −1 i.e., the equilibrium values cover the span of 40 kcal mol −1 with an accuracy of ±5%. The sources of error in these studies are evaluated, and some common pitfalls in PHPMS studies are discussed. These include: steady states that appear as equilibria; isomerisation; pyrolysis of ions and neutrals; slow reactions and non-reactive systems; reaction outside the ion source; vapor pressure problems; and mass coincidences. Tests and solutions for these complications are also discussed.

Entropy-driven proton-transfer reactions

The relation between kinetics and thermochemistry in fast reactions is examined, including reactions with substantial entropy changes. Rate constants for such reactions, in the range of (0.02-3.0) {times} 10{sup {minus}9} cm{sup 2}s{sup {minus}1}, were measured by pulsed high-pressure mass spectrometry. The following relations were observed: (1) The reaction efficiency in either direction is controlled uniquely and completely by the overall reaction free energy change. Specifically, the efficiency r is determined by the equilibrium constant according to r = K/(1+K). (2) The sum of reaction efficiencies in the forward (exergonic) and reverse (endergonic) directions is near unity (r{sub f}+r{sub r}{approx equal} 1). These relations are observed in anionic and cationic systems, in reactions with {Delta}H{degree} up to 12 kcal/mol and with {Delta}S{degree} up to 15 cal/(mol K). Consistent with (1), reactions that are endothermic up to 7 kcal/mol can nevertheless proceed near the collision rate, when positive entropy changes make the reactions exergonic. The entropy changes are effective regardless of their structural origin. Relations analogous to (1) and (2) are also derived for reactions with multiple channels that proceed without significant barriers through a common intermediate.