Hydrogen Atom Affinity Research Articles

Selective and Functional-Group-Tolerant Photoalkylation of Imines by Energy-Transfer Photocatalysis.

Basic amines show broad bioactivity and remain a promising source of new medicines. The direct photoalkylation of imines offers a promising strategy for complex amines. However, the lack of efficient imine photoreactivity hinders this reaction and remains a fundamental limitation in organic photochemistry. We report an efficient photoalkylation of imines that provides primary amines directly without protecting or leaving groups. The transformation effects C-H addition across N-H imines under energy-transfer photocatalysis by a ketone. Our method is distinguished from organometallic, metal-catalyzed, and photoredox approaches to imine alkylation by its lack of protecting groups and its broad scope, which includes unactivated alkanes, protic substrates, basic amines, heterocycles, and ketone imines. We highlight this scope through the condensation and alkylation of two pharmaceutical ketones, providing complex amines succinctly. Our mechanistic analysis supports a three-step process, involving hydrogen-atom transfer to an imine triplet excited state, intersystem crossing, and radical recombination, with photocatalytic enhancement through energy transfer. We further show that N-H imines are more photoreactive than N-substituted imines, a distinction partially explained by sterics and side reactions. To fully explain this distinction, we introduce the thermodynamic parameter excited-state hydrogen-atom affinity, which is highly effective at predicting the photoreactivity of imines.

Thermodynamics Evaluation of Selective Hydride Reduction for α,β-Unsaturated Carbonyl Compounds

The selective reduction of α,β-unsaturated carbonyl compounds is one of the core reactions and also a difficult task for organic synthesis. We have been attempting to study the thermodynamic data of these compounds to create a theoretical basis for organic synthesis and computational chemistry. By electrochemical measurement method and titration calorimetry, in acetonitrile at 298 K, the hydride affinity of two types of unsaturated bonds in α,β-unsaturated carbonyl compounds, their single-electron reduction potential, and the single-electron reduction potential of the corresponding radical intermediate are determined. Their hydrogen atom affinity, along with the hydrogen atom affinity and proton affinity of the corresponding radical anion, is also derived separately based on thermodynamic cycles. The above data are used to establish the corresponding "Molecule ID Card" (Molecule identity card) and analyze the reduction mechanism of unsaturated carbonyl compounds. Primarily, the mixture of any carbonyl hydride ions and Ac-tempo+ will stimulate hydride transfer process and create corresponding α,β-unsaturated carbonyl compounds and Ac-tempoH from a thermodynamic point of view.

Nanoparticle O-H Bond Dissociation Free Energies from Equilibrium Measurements of Cerium Oxide Colloids.

A novel equilibrium strategy for measuring the hydrogen atom affinity of colloidal metal oxide nanoparticles is presented. Reactions between oleate-capped cerium oxide nanoparticle colloids (nanoceria) and organic proton-coupled electron transfer (PCET) reagents are used as a model system. Nanoceria redox changes, or hydrogen loadings, and overall reaction stoichiometries were followed by both 1H NMR and X-ray absorption near-edge spectroscopies. These investigations revealed that, in many cases, reactions between nanoceria and PCET reagents reach equilibrium states with good mass balance. Each equilibrium state is a direct measure of the bond strength, or bond dissociation free energy (BDFE), between nanoceria and hydrogen. Further studies, including those with larger nanoceria, indicated that the relevant bond is a surface O-H. Thus, we have measured surface O-H BDFEs for nanoceria-the first experimental BDFEs for any nanoscale metal oxide. Remarkably, the measured CeO-H BDFEs span 13 kcal mol-1 (0.56 eV) with changes in the average redox state of the nanoceria colloid. Possible chemical models for this strong dependence are discussed. We propose that the tunability of ceria BDFEs may be important in explaining its effectiveness in catalysis. More generally, metal oxide BDFEs have been used as predictors of catalyst efficacy that, traditionally, have only been accessible by computational methods. These results provide important experimental benchmarks for metal oxide BDFEs and demonstrate that the concepts of molecular bond strength thermochemistry can be applied to nanoscale materials.

One electron less or one proton more: how do they differ?

From the NIST website and the literature, we have collected the Ionisation Energies (IE) of 3,052 and the Proton Affinities (PA) of 1,670 compounds. For 614 of these, both the IE and PA are known; this enables a study of the relationships between these quantities for a wide variety of molecules. From the IE and PA values, the hydrogen atom affinities (HA) of molecular ions M•+ may also be assessed. The PA may be equated to the heterolytic bond energy of [MH]+ and HA to the homolytic bond energy. Plots of PA versus IE for these substances show (in agreement with earlier studies) that, for many families of molecules, the slope of the ensuing line is less negative than −1, i.e. changes in the PA are significantly less than the concomitant opposite changes in IE. At one extreme (high PA, low IE) are the metals, their oxides and hydroxides, which show a slope of close to −1, at the other extreme (low PA, high IE) are the hydrogen halides, methyl halides and noble gases, which show a slope of ca. −0.3; other molecular categories show intermediate behaviour. One consequence of a slope less negative than −1 is that the changes in ionic enthalpies of the protonated species more closely follow the changes in the enthalpies of the neutral molecules compared with changes in the ion enthalpies of the corresponding radical cations. This is consistent with findings from ab initio calculations from the literature that the incoming proton, once attached to the molecule, may retain a significant amount of its charge. These collected data allow a comparison of the thermodynamic stability of protonated molecules in terms of their homolytic or heterolytic bond cleavages. Protonated nitriles are particularly stable by virtue of the very large hydrogen atom affinities of their radical cations.

Tautomerization of Some Methylacenes and the Role of Reverse Radical Disproportionation.

The thermokinetics for the tautomerization of a series of methylenedihydroacenes to the corresponding methylacenes (toluene to 6-methylpentacene) have been investigated by means of CBS-QB3 calculations. Only for 6-methylpentacene does the methylenedihydro form predominate at room temperature. The obtained equilibrium ratios are consistent with various theoretical methods, but the agreement with the scarce experimental data is only qualitative. The noncatalyzed thermal tautomerization of the methylenedihydroacene in an inert solvent may proceed by means of a reverse radical disproportionation reaction (RRD) as the rate-determining step. The benzylic BDE(C-H)s and the hydrogen atom affinities (HA) of the tautomers have been used to calculate the reaction enthalpy, ΔRRDH. It appears that the Ea,RRD is substantially higher than ΔRRDH. This implies that the opposite reaction (and the tautomer forming step), a radical-radical disproportionation (RD), is an activated process. This is an often ignored or overlooked kinetic feature. The consequence is that although the RRD reaction may be kinetically feasible at elevated temperatures, the products are not the tautomers but rather dimers stemming from radical-radical recombination reactions, with p-isotoluene as a clear exception. It is further shown that the RRD self-reaction of phenalene is too slow at 298 K, despite claims to the contrary.

Investigation of the mechanism of electron capture and electron transfer dissociation of peptides with a covalently attached free radical hydrogen atom scavenger

The mechanisms of electron capture and electron transfer dissociation (ECD and ETD) are investigated by covalently attaching a free-radical hydrogen atom scavenger to a peptide. The 2,2,6,6-tetramethylpiperidin-l-oxyl (TEMPO) radical was chosen as the scavenger due to its high hydrogen atom affinity (ca. 280kJ/mol) and low electron affinity (ca. 0.45eV), and was derivatized to the model peptide, FQXTEMPOEEQQQTEDELQDK. The XTEMPO residue represents a cysteinyl residue derivatized with an acetamido-TEMPO group. The acetamide group without TEMPO was also examined as a control. The gas phase proton affinity (882kJ/mol) of TEMPO is similar to backbone amide carbonyls (889kJ/mol), minimizing perturbation to internal solvation and sites of protonation of the derivatized peptides. Collision-induced dissociation (CID) of the TEMPO-tagged peptide dication generated stable odd-electron b and y type ions without indication of any TEMPO radical induced fragmentation initiated by hydrogen abstraction. The type and abundance of fragment ions observed in the CID spectra of the TEMPO and acetamide tagged peptides are very similar. However, ECD of the TEMPO-labeled peptide dication yielded no backbone cleavage. We propose that a labile hydrogen atom in the charge reduced radical ions is scavenged by the TEMPO radical moiety, resulting in inhibition of NCα backbone cleavage processes. Supplemental activation after electron attachment (ETcaD) and CID of the charge-reduced precursor ion generated by electron transfer of the TEMPO-tagged peptide dication produced a series of b+H (bH) and y+H (yH) ions along with some c ions having suppressed intensities, consistent with stable OH bond formation at the TEMPO group. In summary, the results indicate that ECD and ETD backbone cleavage processes are inhibited by scavenging of a labile hydrogen atom by the localized TEMPO radical moiety. This observation supports the conjecture that ECD and ETD processes involve long-lived intermediates formed by electron capture/transfer in which a labile hydrogen atom is present and plays a key role with low energy processes leading to c and z ion formation. Ab initio and density functional calculations are performed to support our conclusion, which depends most importantly on the proton affinity, electron affinity and hydrogen atom affinity of the TEMPO moiety.

Determination and Comparison of Thermodynamic Driving Forces of Elementary Steps for the Reductions of Alkynes and the Corresponding Alkenes in Acetonitrile

In this work, five substituted 1-phenyl-2-benzensulfonyl ethynes and the corresponding five substituted 1-phenyl-2-benzensulfonyl ethenes were designed and synthesized as representatives of the polar alkynes and the polar alkenes. Thermodynamic driving forces of eight elementary steps for reductions of the substituted ethynes and ethenes to the corresponding alkenes and alkanes in acetonitrile were determined. The differences of chemical properties between the alkynes and the alkenes as well as their various derived reaction intermediates were quantitatively examined or compared according to the determined thermodynamic driving forces of the eight elementary steps. The relative C–C π-bond heterolytic and homolytic dissociation energies of the alkynes and alkenes in acetonitrile were estimated according to the difference of the hydride affinities and hydrogen atom affinities of the related chemical species. The relative effective charges on the active center atom of the alkynes and the alkenes as well as t...

Stability of Flavin Semiquinones in the Gas Phase: The Electron Affinity, Proton Affinity, and Hydrogen Atom Affinity of Lumiflavin

Examination of electron transfer and proton transfer reactions of lumiflavin and proton transfer reactions of the lumiflavin radical anion by Fourier transform ion cyclotron resonance mass spectrometry is described. From the equilibrium constant determined for electron transfer between 1,4-naphthoquinone and lumiflavin the electron affinity of lumiflavin is deduced to be 1.86 ± 0.1 eV. Measurements of the rate constants and efficiencies for proton transfer reactions indicate that the proton affinity of the lumiflavin radical anion is between that of difluoroacetate (331.0 kcal/mol) and p-formyl-phenoxide (333.0 kcal/mol). Combining the electron affinity of lumiflavin with the proton affinity of the lumiflavin radical anion gives a lumiflavin hydrogen atom affinity of 59.7 ± 2.2 kcal/mol. The ΔG298 deduced from these results for adding an H atom to gas phase lumiflavin, 52.1 ± 2.2 kcal/mol, is in good agreement with ΔG298 for adding an H atom to aqueous lumiflavin from electrochemical measurements in the literature, 51.0 kcal/mol, and that from M06-L density functional calculations in the literature, 51.2 kcal/mol, suggesting little, if any, solvent effect on the H atom addition. The proton affinity of lumiflavin deduced from the equilibrium constant for the proton transfer reaction between lumiflavin and 2-picoline is 227.3 ± 2.0 kcal mol(-1). Density functional theory calculations on isomers of protonated lumiflavin provide a basis for assigning the most probable site of protonation as position 1 on the isoalloxazine ring and for estimating the ionization potentials of lumiflavin neutral radicals.

Reaction Pathway and Free Energy Profile for Conversion of π-Conjugation Modes in Porphyrin Isomer

Porphycene is a structural isomer of porphyrin with 18π-conjugated aromatic character. Porphycene modified with trifluoromethyl (CF(3)) groups in the periphery of the framework readily affords the isolable 20π-conjugated antiaromatic form through a reaction with a proton-donating reductant. The 20π-conjugated form can be characterized by not only a variety of spectroscopies in solutions but also X-ray crystallography. This paper focuses on the free energy profile in the conversion of the 18π-conjugated porphycene into the 20π-conjugated form. From the results of kinetics, electrochemical measurements, and acid/base titrations, the 20π-conjugated CF(3) porphycene is formed by a concerted proton-electron transfer (CPET) from a hydroquinone reagent to the 18π-conjugated form. The hydrogen-atom affinity of the 18π-conjugated CF(3) porphycene (for two hydrogen atoms) was calculated to be -490 kJ mol(-1), indicating that the N-H bonds in the 20π-conjugated form are rather easily cleaved. This reflects the antiaromatic characteristics of the 20π-conjugated porphycene. We propose that the kinetic and thermochemical analysis using redox potentials and pK(a) data is applicable for determining the reaction pathway in conversion of aromatic/antiaromatic mode of π-conjugated macrocycles as well as popular investigations for oxidations of organic molecules.

Thermal Activation of Methane and Ethene by Bare MO.+ (M=Ge, Sn, and Pb): A Combined Theoretical/Experimental Study

The thermal ion/molecule reactions (IMRs) of the Group 14 metal oxide radical cations MO(·+) (M=Ge, Sn, Pb) with methane and ethene were investigated. For the MO(·+)/CH(4) couples abstraction of a hydrogen atom to form MOH(+) and a methyl radical constitutes the sole channel. The nearly barrier-free process, combined with a large exothermicity as revealed by density functional theory (DFT) calculations, suggests a fast and efficient reaction in agreement with the experiment. For the IMR of MO(·+) with ethene, two competitive channels exist: hydrogen-atom abstraction (HAA) from and oxygen-atom transfer (OAT) to the organic substrate. The HAA channel, yielding C(2)H(3·) and MOH(+) predominates for the GeO(·+)/ethene system, while for SnO(·+) and PbO(·+) the major reaction observed corresponds to the OAT producing M(+) and C(2)H(4)O. The DFT-derived potential-energy surfaces are consistent with the experimental findings. The behavior of the metal oxide cations towards ethene can be explained in terms of the bond dissociation energies (BDEs) of MO(+)-H and M(+)-O, which define the hydrogen-atom affinity of MO(+) and the oxophilicity of M(+), respectively. Since the differences among the BDEs(MO(+)-H) are rather small and the hydrogen-atom affinities of the three radical cations MO(·+) exceed the BDE(CH(3)H) and BDE(C(2)H(3)-H), hydrogen-atom abstraction is possible thermochemically. In contrast, the BDEs(M(+)-O) vary quite substantially; consequently, the OAT channel becomes energetically less favorable for GeO(·+) which exhibits the highest oxophilicity among these three group 14 metal ions.

Thermal Activation of Methane by Diatomic Metal Oxide Radical Cations: PbO+⋅ as One of the Missing Pieces

Missing in action: Based on the high computed hydrogen atom affinities and the oxygen-centered radical characters, group IVA metal monoxide radical cations are predicted to be the missing pieces in the thermal activation of methane, and combined experimental and computational studies show that the group IVA metal oxide PbO+⋅ is indeed capable of abstracting a hydrogen atom from methane at room temperature.

Mass-spectrometric and computational study of tryptophan radicals (Trp + H)˙ produced by collisional electron transfer to protonated tryptophan in the gas phase

Hydrogen atom adducts to tryptophan were generated for the first time in the gas phase by collisional electron transfer to protonated tryptophan at 7170 eV kinetic energy. The radicals showed fast dissociations by C(α)-C(β) bond cleavage and cross-ring cleavages occurring on the 7.3 μs time scale. The mechanism of the C(α)-C(β) bond cleavage was explained by ab initio computational analysis of the radical potential energy surface. This showed spontaneous isomerization of the primary tryptophan radical by ammonium hydrogen atom migration to the carboxyl group. The stable intermediate formed by the isomerization can undergo radical-induced scission of the C(α)-C(β) bond in competition with H-atom migrations to the C-2 and C-4 positions of the indole ring. RRKM calculations of unimolecular rate constants on the B3-ROMP2/6-311++G(2d,p) potential energy surface indicated that the C(α)-C(β) bond cleavage was the fastest unimolecular reaction of the radical intermediates within the range of internal energies acquired upon electron transfer. We also report an updated G2(MP2) proton affinity of tryptophan (PA = 946 kJ mol(-1)) and hydrogen atom affinities of the tryptophan indole ring of relevance to electron-based peptide dissociations.

Host–Guest Hydrogen Atom Transfer Induced by Electron Capture

1,n-Alkanediammonium cations in noncovalent complexes with two dibenzo-18-crown-6-ether (DBCE) ligands undergo an unusual intramolecular tandem hydrogen atom and proton transfer to the crown ether ligand upon charge reduction by electron capture. Deuterium labeling established that both migrating hydrogens originated from the ammonium groups. The double hydrogen transfer was found to depend on the length of the alkane chain connecting the ammonium groups. Ab initio calculations provided structures for select alkanediammonium.dibenzo-18-crown-6-ether complexes and dissociation products. This first observation of an intra-complex hydrogen transfer is explained by the unusual electronic properties of the complexes and the substantial hydrogen atom affinity of the aromatic rings in the crown ligand.

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.

Electron Capture in Spin-Trap Capped Peptides. An Experimental Example of Ergodic Dissociation in Peptide Cation-Radicals

Electron capture dissociation was studied with tetradecapeptides and pentadecapeptides that were capped at N-termini with a 2-(4'-carboxypyrid-2'-yl)-4-carboxamide group (pepy), e.g., pepy-AEQLLQEEQLLQEL-NH(2), pepy-AQEFGEQGQKALKQL-NH(2), and pepy-AQEGSEQAQKFFKQL-NH(2). Doubly and triply protonated peptide cations underwent efficient electron capture in the ion-cyclotron resonance cell to yield charge-reduced species. However, the electron capture was not accompanied by backbone dissociations. When the peptide ions were preheated by absorption of infrared photons close to the dissociation threshold, subsequent electron capture triggered ion dissociations near the remote C-terminus forming mainly (b(11-14) + 1)(+)* fragment ions that were analogous to those produced by infrared multiphoton dissociation alone. Ab initio calculations indicated that the N-1 and N-1' positions in the pepy moiety had topical gas-phase basicities (GB = 923 kJ mol(-1)) that were greater than those of backbone amide groups. Hence, pepy was a likely protonation site in the doubly and triply charged ions. Electron capture in the protonated pepy moiety produced the ground electronic state of the charge-reduced cation-radical with a topical recombination energy, RE = 5.43-5.46 eV, which was greater than that of protonated peptide residues. The hydrogen atom in the charge-reduced pepy moiety was bound by >160 kJ mol(-1), which exceeded the hydrogen atom affinity of the backbone amide groups (21-41 kJ mol(-1)). Thus, the pepy moiety functioned as a stable electron and hydrogen atom trap that did not trigger radical-type dissociations in the peptide backbone that are typical of ECD. Instead, the internal energy gained by electron capture was redistributed over the peptide moiety, and when combined with additional IR excitation, induced proton-driven ion dissociations which occurred at sites that were remote from the site of electron capture. This example of a spin-remote fragmentation provided the first clear-cut experimental example of an ergodic dissociation upon ECD.

Alpha,2-, alpha,3-, and alpha,4-dehydrophenol radical anions: formation, reactivity, and energetics leading to the heats of formation of alpha,2-, alpha,3-, and alpha,4-oxocyclohexadienylidene.

We have regiospecifically generated the alpha,2-, alpha,3-, and alpha,4-dehydrophenoxide anions by collisional activation of o-, m-, and p-nitrobenzoate. The alpha,2 and alpha,4 isomers also were synthesized by reacting o-benzyne radical anion with carbon dioxide and electron ionization of p-diazophenol. All three dehydrophenol radical anions were differentiated from each other and identified by probing their chemical reactivity with several reagents. Each isomer was converted to phenoxide and its corresponding quinone as well. Thermochemical measurements were carried out on all three radical anions and their hydrogen-atom affinities, proton affinities, and electron binding energies are reported. These measured quantities are combined in thermodynamic cycles to derive the heats of formation of each of the radical anions and their corresponding carbenes (i.e., alpha,2-, alpha,3-, and alpha,4-dehydrophenol). These results are compared to MCQDPT2, G3, G2+(MP2), and B3LYP calculations and experimental data for appropriate reference compounds.

Energetics of Uracil Cation Radical and Anion Radical Ion−Molecule Reactions in the Gas Phase

The uracil cation radical was calculated to exist predominantly as the 1,3-dioxo tautomer 1•+, similar to the most stable tautomer of neutral uracil (1). The enol forms of 1•+ were found to be 10−173 kJ mol-1 less stable than 1•+ and should not be significantly populated at 298 K thermal equilibrium. Cation radical 1•+ is a moderately strong gas-phase acid of topical acidities ΔHacid = 829, 921, 916, and 879 kJ mol-1 for the H-1, H-3, H-5, and H-6 protons, respectively. Ion 1•+ is capable of exothermic protonation of adenine, guanine, and cytosine, and of the arginine, lysine, histidine, and tryptophan amino acid residues in proteins. The hydrogen atom affinities of 1•+ were −ΔHrxn = 432, 371, and 360 kJ mol-1 for H-atom additions to O-4, O-2, and C-5, respectively. 1•+ was calculated to exothermically abstract the thiol hydrogen atom from CH3SH, the hydroxyl hydrogen from phenol, and an α-hydrogen atom from glycine N-methylamide. Uracil radicals formed by deprotonation of 1•+ were calculated to have large hydrogen atom affinities that should allow for exothermic abstraction of H-atoms from thiol groups, phenolic hydroxyls, and amino acid backbone α-methylene and methine groups. Protonation by a uracil cation radical followed by hydrogen atom abstraction can propagate radiation damage from the initial ionization site. In contrast to the highly reactive uracil cation radicals and radicals, the weakly bound uracil anion radical (1•-) was predicted to be much less reactive in the gas phase. Ion−molecule reactions of 1•- by proton and hydrogen atom abstractions from thiols, phenol, and α-positions of amino acids were calculated to be endothermic and thus very slow in the gas phase. 1•- can selectively deprotonate carboxylic groups as calculated for the reaction with glycine.

Electron Capture Dissociation of Gaseous Multiply-Charged Proteins Is Favored at Disulfide Bonds and Other Sites of High Hydrogen Atom Affinity

Disulfide bonds in gaseous multiply-protonated proteins are preferentially cleaved in the mass spectrometer by low-energy electrons, in sharp contrast to excitation of the ions by photons or low-energy collisions. For S−S cyclized proteins, capture of one electron can break both an S−S bond and a backbone bond in the same ring, or even both disulfide bonds holding two peptide chains together (e.g., insulin), enhancing the sequence information obtainable by tandem mass spectrometry on proteins in trace amounts. Electron capture at uncharged S−S is unlikely; cleavage appears to be due to the high S−S affinity for H• atoms, consistent with a similar favorability found for tryptophan residues. RRKM calculations indicate that H• capture dissociation of backbone bonds in multiply-charged proteins represents nonergodic behavior, as proposed for the original direct mechanism of electron capture dissociation.

Simple determination of relative hydrogen atom affinities by supercritical fluid chromatography/atmospheric pressure chemical ionization mass spectrometry

AbstractCoupled supercritical fluid chromatography and atmospheric pressure chemical ionization mass spectrometry was applied to estimate hydrogen atom affinities. The intensities of the M+ and MH+ peaks in the mass spectra were measured at various drift voltages (collision energies). The ratios of the two peaks were plotted against the drift voltages to obtain threshold drift voltages. The threshold drift voltages of polycyclic aromatic hydrocarbons were plotted and found to be proportional to the values of reported hydrogen atom affinities. The value for acenaphthylene was estimated to be 292.9 kJ mol−1 from this relationship.

Proton transfer from a fullerene dication: bracketing the gas-phase acidity of C60H.bul.2+

Several gas-phase ion-molecule reactions of the hydrogenated fullerene dication C[sub 60]H[sup [center dot]2+] (produced by the reaction of C[sub 60][sup [center dot]3+] + C[sub 3]H[sub 8]) have been studied using a SIFT apparatus at 0.35 [+-] 0.01 Torr and 294 [+-] 2 K. Proton transfer from C[sub 60]H[sup [center dot]2+] to the neutrals CH[sub 2]CHCN, CH[sub 3]OH, CH[sub 3]NO[sub 2], HCOOH, CH[sub 2](CN)[sub 2], H[sub 2]CO, and HCN has been observed but was not detected in reactions of C[sub 60]H[sup [center dot]2+] with several other neutrals. The occurrence of efficient proton transfer to neutrals having a gas-phase basicity (GB) value > 170 kcal mol[sup [minus]1] and failure to proton transfer with neutrals having GB < 163 kcal mol[sup [minus]1] is used to bracket the gas-phase acidity of GA(C[sub 60]H[sup [center dot]2+]) = 124 [+-] 8 kcal mol[sup [minus]1], when consideration is taken of the effects of Coulombic repulsion between the product ions. The reactivity of C[sub 60]H[sup [center dot]2+] is compared to that of C[sub 60][sup 2[center dot]] and thermochemical data are derived for [delta]H[degrees][sub f] (C[sub 60]H[sup [center dot]2+]), IE(C[sub 60]H[sup +]), the hydrogen-atom affinity of C[sub 60][sup 2+], and the hydride affinity of C[sub 60][sup [center dot]3+]. 47more » refs., 2 figs., 1 tab.« less