Pulsed Electron Beam High Pressure Research Articles

Lewis acid–base interactions of SiF 4 with molecular anions formed by electron capture reactions

Lewis acid–base interactions between SiF 4 and a wide range of molecular negative ions are reported here for the first time. The molecular anions include those formed by simple electron attachment to p-benzoquinone, benzophenone, nitrobenzene, and 21 substituted nitrobenzenes and also include the o- and p-nitrophenoxy anions. From measurements performed by pulsed electron-beam high pressure mass spectrometry, equilibrium constants and free energies for the association reactions, M − + SiF 4 ⇌ M −(SiF 4), at 150 °C are reported for each of the molecular anions, M −. It is shown that the strengths of these Lewis acid–base interactions of SiF 4 are much greater than ion–dipole interactions previously reported between these molecular anions and several common solvent molecules of relatively high dipole moment, including methanol, acetonitrile, dimethylformamide, and dimethlysulfoxide. The strengths of the Lewis acid–base interactions of SiF 4 with molecular anions show a strong inverse dependence on the electron affinity of the parent molecule and on the availability of a specific Lewis base site on the molecular anion that can be closely approached by the central Si atom of SiF 4. It is also shown that strong interactions between SiF 4 and the molecular anions derived from compounds of very low electron affinity can be gainfully used for the trace detection of such compounds by electron capture mass spectrometry.

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.

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.

Gas-phase solvation of NO+, O+2, N2O+, N2OH+, and H3O+ with N2O

Gas-phase clustering reactions of NO+, O+2, N2O+, N2OH+, and H3O+ with N2O were measured with a pulsed electron-beam high pressure mass spectrometer. The bond in NO+(N2O)n is found to be electrostatic, while those in O+2(N2O)1, N2O+(N2O)1, and N2OH+(N2O)1 have covalent character. The observed n dependence of −ΔH0n−1,n for the clustering reactions suggests that the cluster ions have the structures of the core ion plus ligand molecules, i.e., NO+(N2O)3(N2O)n−3, O+2N2O(N2O)n−1, (N2O)+2(N2O)n−1, H+N2O(N2O)1(N2O)n−1, and H3O+(N2O)3(N2O)n−3. The N2O molecule forms stronger bonds with NO+, O+2, and H3O+ ions than the isoelectronic CO2 molecule, indicating that N2O is a stronger nucleophilic reagent than CO2.

Bond strengths of the gas-phase cluster ions X − (CS 2) n (X = F, Cl, Br and I)

Thermodynamic quantities, Δ H 0 n –1, n , and Δ S 0 n –1, n , for the clustering reactions, X − (CS 2) n –1 + CS 2=X − (CS 2) n , X=F, Cl, Br and I, were measured with a pulsed electron-beam high pressure mass spectrometer. A large binding energy, 35.0 kcal/mol, for F − …CS 2 has been obtained, whereas those for other X − …CS 2 species are approximately 7–9 kcal/mol. Except the C 2v F − (CS 2) 1 geometry, linear X − (CS 2) 1 geometries are found, arising from the S δ+ −C δ −S δ+ electronic nature and an effective polarization of “soft” atoms.

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.

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.

Ion/molecule reactions in a mixture of Ar and H 2: high pressure mass spectrometry and quantum chemical calculations

The ion/molecule reactions taking place as the result of electron impact on a 100:1 mixture of Ar and H 2 have been stidied by pulsed electron beam high pressure mass spectrometry. It is shown that ArH + hydrogen bonds to Ar to give (ArHAr) + and to H 2 to give ArH + 3. The bond to H 2 is stronger by 20 ± 13 kJ mol −1. The experimental data agree well with results from high level ab initio quantum chemical calculation.

Proton affinities of the hydrogen, methyl, and trifluoromethyl halides from pulsed electron beam high pressure mass spectrometric equilibria measurements

Pulsed electron beam high pressure mass spectrometric techniques have been used to investigate proton transfer equilibria involving hydrogen and trifluoromethyl halides. Methyl cation transfer equilibria have also been used to determine methyl cation affinities of hydrogen halides. Proton affinities obtained are HCl = 137.5, HBr = 142.9, Hl = 150.9, CF3Cl = 139.0, CF3Br = 141.3, and CF3l = 150.4 kcal mol−1. Methyl cation affinities obtained are HF = 15 ± 2, HCl = 19.8, HBr = 21.3, and Hl = 31.1 kcal mol−1. The values of methyl cation affinity of hydrogen halides can in turn be used to calculate proton affinities of the methyl halides of CH3Cl = 160.8; CH3Br = 164.8; and CH3l = 171 kcal mol−1. Linear correlations are found between proton affinities and valence ionization potentials and between proton affinities and methyl cation affinities.

The formyl and isoformyl cations. A pulsed electron beam high pressure mass spectrometric study of the energetics of HCO+ and HOC+

Gas phase proton transfer equilibria involving compounds in the basicity regime near CO have been examined using pulsed electron beam high pressure mass spectrometric techniques. Using the well established proton affinity of C2H4 of 162.6 kcal mol−1 as a reference standard a new proton affinity for CO of 145.6 kcal mol−1 is established. The disagreement between this value and those obtained by photoionization mass spectrometric appearance potential measurements of HCO+ ions from compounds of the type HCOR is interpreted as being due to an activation energy barrier for the reverse process: HCO++R ̇→HCOR+. This barrier results from the significant geometry change involved in HCO+ formation. Correlations of proton affinity of O1S binding energies have been used to establish the proton affinity of the oxygen site of CO as 109 kcal mol−1. This result implies that HOC+ can be readily formed by reaction of H+3 with CO in interstellar clouds.

ChemInform Abstract: BRIDGING THE GAP. A CONTINUOUS SCALE OF GAS‐PHASE BASICITIES FROM METHANE TO WATER FROM PULSED ELECTRON BEAM HIGH PRESSURE MASS SPECTROMETRIC EQUILIBRIA MEASUREMENTS

Chemischer InformationsdienstVolume 16, Issue 36 Preparative Organic Chemistry ChemInform Abstract: BRIDGING THE GAP. A CONTINUOUS SCALE OF GAS-PHASE BASICITIES FROM METHANE TO WATER FROM PULSED ELECTRON BEAM HIGH PRESSURE MASS SPECTROMETRIC EQUILIBRIA MEASUREMENTS T. B. MCMAHON, T. B. MCMAHONSearch for more papers by this authorP. KEBARLE, P. KEBARLESearch for more papers by this author T. B. MCMAHON, T. B. MCMAHONSearch for more papers by this authorP. KEBARLE, P. KEBARLESearch for more papers by this author First published: September 10, 1985 https://doi.org/10.1002/chin.198536104AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume16, Issue36September 10, 1985 RelatedInformation

Bridging the gap. A continuous scale of gas-phase basicities from methane to water from pulsed electron beam high pressure mass spectrometric equilibria measurements

Bridging the gap. A continuous scale of gas-phase basicities from methane to water from pulsed electron beam high pressure mass spectrometric equilibria measurements

Electron affinities from electron transfer equilibria in the gas phase and the electron affinity of SO2

Measurements of gas phase electron transfer equilibria [Eq. (1)] A−+B=A+B− were performed with a pulsed electron beam high ion source pressure mass spectrometer. The equilibrium constants K1 were used to evaluate ΔG01=−RT ln K1. The ΔG01 were found to change little with temperature such that ΔG01≊ΔH01. Good agreement was observed between the present results and earlier measurements by Fukuda and McIver, which used ion cyclotron resonance and worked at much lower pressures. Absolute electron affinities were obtained by anchoring the relative electron affinities (ΔH1 values) to known absolute E.A.’s from the literature. The relative results agree well with differences predicted by absolute measurements. The resulting E.A. are in eV: CH3NO2, 0.49; CS2, 0.54; (C6H5)CO, 0.64; 2, 4, 6-trimethyl nitrobenzene 0.68; 2, 3-butanedione 0.71; 2, 3-dimethyl nitrobenzene, 0.83; 2-methyl nitrobenzene, 0.88; 3-methyl nitrobenzene, 0.94; nitrobenzene, 0.97; 1-fluoro-4-nitrobenzene, 1.06; SO2 1.11; 1-chloro-3-nitrobenzene, 1.23; maleic anhydride, 1.42; 1,3-dinitrobenzene, 1.6; 2, 5-dimethyl-p-benzoquinone, 1.72; methyl-p-benzoquinone, 1.78; p-benzoquinone, 1.86. The assignment of the present absolute values disagrees with that by Fukuda and McIver based on ICR bracketing experiments. It is suggested that the bracketing experiments were misleading due to presence of excitation in the ionic reactants used.

A comparison of methyl and phenyl substituent effects on the gas phase basicities of amines and phosphines

The proton affinities of phenyl phosphine and cyclohexylphosphine were measured by determining the equilibrium constants of proton transfer equilibria with a pulsed electron beam high ion source pressure mass spectrometer. These proton affinities combined with values for methyl and phenyl phosphines and the analogous amines provide an interesting comparison of the methyl and phenyl substituent effects on the basicities of phosphine and ammonia. Methyl substitution increases the basicity of both ammonia and phosphine; however, the increase is significantly larger for the phosphine. Phenyl substitution increases the basicity of ammonia and phosphine and the increase for phosphine is very much larger. Calculations at the STO-3G, 4-31G, STO-3G*, and 4-31G* (* with d orbitals) for PH3, MePH2, PhPH2, the protonated species, and the nitrogen analogues predict proton transfer reaction energies in good agreement with the experimental results. A shortening of the C—P bond is predicted for protonation of MePH2 and particularly PhPH2, while a lengthening of the C—N bond is predicted for the corresponding nitrogen compounds. The much stronger increase in proton affinity of the phosphines caused by phenyl substitution is due to the stabilization of the phenyl phosphonium ion by π donation from the phenyl group to the empty orbitals of phosphorus in the [Formula: see text] group; in contrast, in the amines, it is the free base aniline which is stabilized by conjugation of the nitrogen lone pair with the aromatic ring. This stabilization of the free base is less important in phenyl phosphine. The participating empty orbitals of phosphorus in the conjugation of phenyl with [Formula: see text] in phenyl phosphonium are mostly π* with some -πd participation. The stabilization of the aniline free base contributes considerably more than the conjugation in the phosphonium ion, to the phenyl substituent difference for the amines and phosphines. The factors involved in the bigger substituent effect of methyl in the phosphines are somewhat similar to those for phenyl: stabilization of the methyl amine by conjugation of the nitrogen lone pair with empty orbitals of CH3 and stabilization of the [Formula: see text] by hyperconjugation. An alternate description can be given in terms of hybridization changes.

Temperature dependent slow reactions of C2H6+ with C2H6.

The reaction of C2H6+ with C2H6 was studied in a pulsed electron beam high ion source pressure mass spectrometer. Ethane at variable pressures in the 16—150 mTorr range in ~4 Torr hydrogen was used in experiments covering the temperature range from −145 to 400°C. Reaction: C2H6++C2H6 was found to have a rate constant k=10-6.6 T-1.5 whose magnitude decreased with temperature. The reaction proceeds via a (C4H12+)* intermediate, which at low temperature can be stabilized and becomes the major product. A very slow decrease of C4H12+ with time was observed at low temperature which was completely accounted for by an appearance and an increase of the trimer ion C6H18+. The two isomeric structures for C4H12+ are proposed in order to interpret the slow clustering reactions of C4H12+ with C2H6.

Hydrogen bonding solvent effect on the basicity of primary amines CH3NH2, C2H5NH2, and CF3CH2NH2

The equilibria RNH3+(H2O)n−1 + H2O = RNH3+(H2O)n were measured for R = CH3, C2H5, and CF3CH2 from n = 1 to n = 3 with a pulsed electron beam high ion source pressure mass spectrometer. The proton and hydrate transfer equilibria CH3NH3+(H2O)n + C2H5NH2 = CH3NH2 + C2H5NH3+(H2O)n were measured for n = 0 to n = 3. These data allow the evaluation of ΔH0 and ΔG0 for the reactions: R0NH3+(H2O)n + RNH3+ = R0NH3+ + RNH3+(H2O)n. ΔH0 = δΔH00,n(RNH3+), ΔG = δΔG00,n(RNH3+). These data are compared with δΔE0,3 (STO-3G) evaluated by Hehre and Taft. In general good agreement is observed at n = 3. The δΔH00,3(RNH3+) ≈ δΔE0,3(RNH3+) are also found close to the ion hydration free energy difference in aqueous solutions.

Ion molecule reactions in ethane. Thermochemistry and structures of the intermediate complexes: C4H11+ and C4H10+ formed in the reactions of C2H5+ and C2H4+ with C2H6

The reactions of C2H5+ and C2H4+ with ethane were studied in a pulsed electron beam high ion source pressure mass spectrometer. Ethane at variable pressures in the 10–100 m Torr range in ~5 Torr hydrogen was used in experiments covering the temperature range −145 to 400 °C. Reaction [7]: C2H5+ + C2H6 = sec-C4H9+ + H2 was found to have a rate constant whose magnitude decreased with temperature: k7 = 10−5.12 T−2 (molecule−1 cm3 s−1). The reaction proceeds via a C4H11+ (b) intermediate, which at low temperature can be stabilized and becomes the major product. The rate constant for thermal decomposition of C4H11(b) by reaction [6t]: C4H11+ (b) = sec-C4H9+ + H2 could be measured. The activation energy was found to be E6t = 9.6 kcal/mol. From consideration of the above data and the known ΔH7, it was concluded that C4H11+ (b) has the structure[Formula: see text]Before dissociation to sec-C4H9+ + H2, this ion rearranges to[Formula: see text]The barrier for this rearrangement is ~9.6 kcal/mol.C2H4+ reacts with C2H6 to give C4H10+ (d) at low temperatures. At high temperatures C4H10+ (d) becomes an intermediate in the dissociation to sec-C3H7+ + H2. The formation of C4H10+ at low temperature has a rate constant whose magnitude decreases with temperature. The temperature dependence of the equilibrium constant K10 for the reaction [10]: C2H4+ + C2H6 = C4H10+ (d) could be determined. This led to ΔH10 = −15.3 kcal/mol. The rate constant for the high temperature reaction [11]: C2H4+ + C2H6 = sec-C3H7+ + H2 was k11 = 8.4 × 10−10 exp (−3.9/RT kcal/mol) (molecule−1 cm3 s−1). A potential energy diagram for the reaction system is proposed. C4H10+ (d) is probably a complex between C2H4+ and C2H6 held largely by ion induced dipole process. Reaction [11] probably proceeds via C4H10+ (d) → n-C4H10+ → sec-C3H7+ + H2. The barrier between C7H10+ (d) and n-C4H10+ is ~20 kcal/mol.

Gas phase ion equilibria: heats of formation of acylium ions RCO+ and protonated acids RC(OH)2+; gas phase catalysis of proton shift RC(OH)2+ → RCO(OH2)+

The equilibria [2]: [Formula: see text] for R = CH3, C2H5, and C6H5 were studied in a pulsed electron beam high ion source pressure mass spectrometer. van't Hoff plots led to ΔH2 values: (CH3), 24.6; (C2H5), 22.7; (C6H5), 21.9 kcal/mol. ΔHf(RC(OH)2+) were obtained from gas phase basicity ladders combined with the new ΔHf(t-butyl+) = 163 kcal/mol (Beauchamp). The ΔHf(RC(OH)2+) were: (CH3), 71.3; (C2H5), 63.6; (C6H5), 95.5 kcal/mol. Combination of ΔH2 with ΔHf(RC(OH)2+) leads to ΔHf(RCO+): (CH3), 153.7; (C2H5), 144; (C6H5), 174.6 kcal/mol. These results are in agreement with selected data from appearance potentials. The energies and structures of the participants in reaction [2] were calculated by MINDO/3 and STO-3G. MINDO/3 gave good agreement with ΔH2. The establishment of the equilibria [2] was unusually slow. A study of the kinetics revealed that k2f is approximately third order, unusually small, and has an unusually large negative temperature coefficient. Furthermore, reaction [2] was found to be catalyzed by RCOOH. An explanation of these observations is given by assuming that the proton shift RCO(OH2)+ → RC(OH)2+ has a large activation energy barrier in the gas phase. This barrier is removed by formation of a hydrogen bonded complex with RCOOH.

Stabilities of complexes (N2)nH+, and (O2)nH+ for n = 1 to 7 based on gas phase ion-equilibria measurements

The equilibria Bn−1H+ + B = BnH+ for B = N2, CO, and O2 were measured with a pulsed electron beam high ion source pressure mass spectrometer. Equilibria up to n = 7 could be observed. van't Hoff plots of the equilibrium constants lead to ΔGn−1,n0, ΔHn−1,n0, and ΔSn−1,n0. While the proton affinities increase in the order O2 < N2 < CO, the stabilities of the B2H+ towards dissociation to BH+ + B increase in the reverse order, i.e. CO < N2 < O2. The stabilities towards dissociation of B for BnH+ where n > 2 are much lower for all three compounds; however for N2 and CO the stability decreases only very slowly from n = 3 to n = 6, then there is a large fall off for n = 7. The (O2)nH+ clusters show large decrease of stabilities as n increases. The BnH+ (for n > 3) of CO are more stable than those of N2 or O2. The above experimental results can be partially explained with the help of results from molecular orbital STO-3G calculations for B, BH+, and B2H+ and general considerations. BH+ and B2H+ for CO and N2 are found to be linear while those for O2 are bent. The most stable O2H+ is a triplet, while (O2)2H+ is a quintuplet.