The C - H bond dissociation energies for naphthalene were determined using a negative ion thermochemical cycle involving the gas-phase acidity (Delta H (acid)) and electron affinity (EA) for both the alpha- and beta-positions. The gas-phase acidity of the naphthalene alpha- and beta-positions and the EAs of the alpha- and beta-naphthyl radicals were measured in the gas phase in a flowing after glow-triple quadrupole apparatus. A variation of the Cooks kinetic method was used to measure the EAs of the naphthyl radicals by collision-induced dissociation of the corresponding alpha- and beta-naphthylsulfinate adducts formed by reactions in the flow tube portion of the instrument. Calibration references included both pi and sigma radicals, and full entropy analysis was performed over a series of calibration curves measured at collision energies ranging from 3.5 to 8 eV (center-of-mass). The measured EAs are 33.0 +/- 1.4 and 31.4 +/- 1.0 kcal mol(-1) (1 kcal = 4.184 kJ) for the alpha- and beta-naphthyl radicals, respectively. The gas-phase acidities for naphthalene were measured by the DePuy silane cleavage method, which utilizes the relative abundances of aryldimethylsiloxides and trimethylsiloxide that result from competitive cleavages from a proposed penta coordinate hydroxysiliconate intermediate. The measured acidities are 394.0 +/- 5.0 and 397.6 +/- 4.8 kcal mol(-1) for the alpha- and beta- positions, respectively. The C - H bond dissociation energies calculated from the thermochemical cycle are 113.4 +/- 5.2 and 115.4 +/- 4.9 kcal mol(-1) for the alpha- and beta-positions, respectively. These energies are, to within experimental error, indistinguishable and are approximately the same as the first bond dissociation energy for benzene.

ADVERTISEMENT RETURN TO ISSUEPREVCommunicationNEXTThe Phenylcarbyne AnionRandal A. Seburg, Brian T. Hill, Rachel A. Jesinger, and Robert R. SquiresView Author Information Department of Chemistry, Purdue University West Lafayette, Indiana 47907 Cite this: J. Am. Chem. Soc. 1999, 121, 26, 6310–6311Publication Date (Web):June 16, 1999Publication History Received23 November 1998Published online16 June 1999Published inissue 1 July 1999https://doi.org/10.1021/ja984026qCopyright © 1999 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views175Altmetric-Citations17LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit Read OnlinePDF (39 KB) Get e-AlertsSUBJECTS:Anions,Chemical reactions,Ions,Isomerization,Reactivity Get e-Alerts

The gas-phase negative ion chemistry of a series of Lewis acid-base complexes [Me2SBH3 (1), Me3NBH3 (2), Me3PBH3 (3), Me2SBF3 (4), Me2OBF3, Et3NBH3, and Et2OBF3] was investigated with use of the flowing afterglow triple-quadrupole technique. Ab initio MO calculations using the G2(MP2) and CBS-4 models were carried out for 1-4 and related species. The gas-phase reaction between OH- and complex I produces a stable carbanion, MeS(BH3)CH2- (1a), that does not isomerize under thermal conditions at room temperature to either of the lower energy berate isomers MeSCH2BH3- (Ib) and CH3CH2SBH3- (Ic). The structure of la was determined by tandem mass spectrometry and by ion/molecule reactions. The barriers for rearrangement of la to Ib and to Ic were calculated to be 29.3, and 27.6 kcal/mol, respectively, at the G2-(MP2) level. The stable carbanions Me2N(BH3)CH2- (2a), Me2P(BH3)CH2- (3a), and MeS(BF3)CH2- (4a) were also generated by proton abstraction from the corresponding neutral complexes. The gas-phase acidities (Delta H-acid) of 1-3 were determined from bracketing experiments to be 372.5 +/- 2.0, 393.0 +/- 2.0, and 374.5 +/- 2.0 kcal/mol, respectively. Compared to their uncomplexed bases, the acidities of 1-3 are enhanced by 18-20 kcal/mol. The acidity enhancements were shown to be mainly due to the increased electron binding energies of the carbanions in the deprotonated complexes that result from electrostatic interactions with the strongly dipolar Lewis acid-base bonds. Enhanced reactivity of the Lewis acid-base complexes was also characterized. The complexes 1, 2, and Me2OBF3 undergo nucleophilic substitution at carbon with F- or NH2-, while no such reactions occur for the uncomplexed bases Me2S, Me3N, and Me2O. Similarly, facile beta-elimination reactions occur between F- or OH- and the ethylated complexes Et3NBH3 and Et2OBF3, while the uncomplexed species are unreactive.

The reactions of o-, m- and p-benzyne anions and the phenide ion with a series of neutral reagents are described. The m- and p-benzyne anions display similar behavior towards Bronsted acids, CS2, N2O, NO and O-2, which is analogous to that of phenide ion but clearly different from that of o-benzyne anion. The strongly basic and nucleophilic character of m- and p-benzyne anions dominates their reactivity, and radical-type reactions are generally not observed. Novel bifunctional reactions between m- and p-benzyne anions and both CS, and NO are observed in which two sequential S-atom abstractions and two NO additions, respectively, take place. (C) 1998 John Wiley & Sons, Ltd.

The formation of neutral [C2,H2,O2] has been investigated by tandem mass spectrometry in a sector instrument and by energy-resolved collision-induced dissociation in a flowing afterglow-triple quadrupole apparatus. The neutral species are generated by two different methods: (i) neutralization of the distonic anion radical •CH2COO− by collisional electron detachment and (ii) collision-induced loss of halides X− concomitant with formation of [C2,H2,O2] from α-haloacetate ions XCH2COO−. The tandem mass spectrometry results suggest that neutralization of •CH2COO− and high-energy collisional activation of α-haloacetate ions lead to a mixture of α-acetolactone, c-(CH2C(O)O), and the acetoxyl diradical, •CH2COO•. Low-energy collisions with α-chloroacetate ions in the triple quadrupole analyzer produce α-acetolactone exclusively at the dissociation threshold. From the dissociation threshold measured for the appearance of Cl− from ClCH2COO− the heat of formation of acetolactone is determined to be ΔHf,298 = −47.3 ± 4.7 kcal/mol.

Borane complexation at the heteroatom increases the gas-phase CH acidities of dimethyl sulfide, trimethylamine, and trimethylphosphane by up to 20 kcal mol−1. Deprotonation of the volatile Lewis acid–base complexes in the gas phase produces dipole-stabilized carbanions (see below) that do not rearrange to the more stable borate isomers. The complexation also enhances the reactivity of pendant methyl and ethyl groups toward substitution and elimination reactions.

The isomeric m- and p-benzyne anions have been generated in the gas phase in a flowing afterglow-triple quadrupole instrument from the reactions of molecular fluorine (F2) with m- and p-(trimethylsilyl)phenyl anions. The mechanism of the F2 reaction involves electron transfer from the (trimethylsilyl)phenyl anion to F2, followed by nucleophilic attack on the resulting (trimethylsilyl)phenyl radical by the nascent F- formed within the intermediate ion/molecule complex. The structures of o-, m-, and p-benzyne anion are unambiguously identified by a classical derivatization scheme wherein the ions are first converted to the corresponding o-, m-, and p-nitrobenzoates by stepwise addition of CO2 and NO2. These derivatives are then identified by forming proton-bound dimers of each ion with CHF2CO2H, and comparing the O2NC6H4CO2-/CHF2CO2- yield ratios obtained by collision-induced dissociation (CID) with those obtained from analogous experiments with the authentic nitrobenzoate ions. Estimates of the electron af...

A flowing afterglow/selected ion flow tube instrument has been used to measure the rates of reaction of amide ion with benzene and phenide ion with ammonia: C6H6 + NH2- reversible arrow C6H5- + NH3. The ratio of these rate constants gives a free energy change: Delta(reaction)G(300) = -3.58 +/- 0.06 kcal mol(-1). Use of the established gas-phase acidity of ammonia gives a value for Delta(acid)G(300)(C6H6) of 392.9 +/- 0.4 kcal mol(-1). From the computed value for Delta(acid)S(300)(C6H6) of 29.6 +/- 1.0 cal mol(-1) K-1, the enthalpy change, Delta(acid)H(300)(C6H6) = 401.7 +/- 0.5 kcal mol(-1), is derived. The enthalpy of deprotonation of benzene, the C-H bond dissociation energy, and the electron affinity of the phenyl radical are simply related to each other: Delta(acid)H(300)(C6H6) = DH300(C6H5-H) + IP(H) - EA(C6H5). Since earlier photoelectron experiments have provided a value for the electron affinity for the phenyl radical, EA-(C6H5) = 25.3 +/- 0.1 kcal mol(-1), the enthalpy of deprotonation can be used to extract a value for the C-H bond enthalpy of benzene at 300 K and the C-H bond energy at 0 K, D-0. These bond energies are used to compute the heats of formation of the phenyl radical at 0 and 300 K: DH300(C6H5-H) = 113.5 +/- 0.5 kcal mol(-1), Delta(f)H(300)(C6H5) = 81.2 +/- 0.6 kcal mol(-1); D-0(C6H5-H) = 112.0 +/- 0.6 kcal mol(-1), Delta(f)H(0)(C6H5) = 84.3 +/- 0.6 kcal mol(-1).

The bond energies in F-2(-) and HF2- have been determined using energy-resolved collision-induced dissociation in a flowing afterglow-triple quadrupole instrument. The quantity D-0(F-2(-)) is measured to be 27.9 +/- 1.6 kcal/mol. This can be used to derive EA(F-2) = 3.01 +/- 0.07 eV, which is in good agreement with literature values. The bond energy in bifluoride, DH298(FH-F-), is found to be 45.8 +/- 1.6 kcal/mol. This is significantly higher than the value determined from fluoride transfer equilibrium measurements (38.6 +/- 2.0 kcal/mol) but is in good agreement with the results from recent theoretical studies. Ab initio calculations of these quantities using the G2 method have been performed, which are found to be in excellent agreement with the experimental results.

An experimental method is presented for determining the regioselectivity of deprotonation of unsymmetrical ketones in the gas phase. Mixtures of tautomeric enolate ions were prepared in a flowing afterglow apparatus and then assayed through a reaction with n-butyl nitrite in the collision cell of a triple quadrupole mass analyzer. Enolate ions were also prepared regioselectively by desilylation of the corresponding trimethylsilyl enol ethers with fluoride ion. Rate coefficients for the methanol-catalyzed tautomerization of the regioisomers were measured and were used to derive the equilibrium ratio of the tautomers. For 2-butanone it was found that the equilibrium mixture of enolate ions consisted of 55% of the more substituted isomer. For 3-methyl-2-butanone and 2-methyl-3-pentanone the equilibrium mixture comprised greater than 95% of the less substituted isomer. Several different bases were used to prepare nonequilibrium mixtures of enolate ions. Strong bases deprotonate these ketones irreversibly and in a statistical fashion. Deprotonation with hindered bases altered the composition of regioisomers only slightly. Ab initio molecular orbital calculations were performed on 2-butanone, 3-methyl-2-butanone, and their corresponding enolate ions at the MP4SDQ/ 6-31+G(d)//HF/6-31+G(d) level of theory. For 2-butanone, the calculations predict that the Z secondary enolate and the primary enolate have equal stabilities (Delta E < 0.1 kcal/mol), while the E secondary enolate is 4.1 kcal/mol higher in energy than the Z enolate ion. For 3-methyl-2-butanone, the tertiary enolate ion is calculated to be 4.3 kcal/mol higher in energy than the primary enolate ion. The computed gas-phase acidities of the two ketones are in excellent agreement with the experimentally determined values.

The acetate radical anion, CH2CO2.-, has been generated in the gas phase at room temperature and its thermochemical properties and reactivity have been examined with use of a flowing afterglow-triple quadrupole instrument. This ion is formed in high yield from the reaction between F-2 and the enolate ions of either acetic acid or trimethylsilyl acetate. Collision-induced dissociation (CID) of CH2CO2.- occurs by loss of CO2, forming CH2.- with a measured threshold energy of 60.9 +/- 2.7 kcal/mol. The (oxygen) proton affinity of CH2CO2.- (Delta H-acid-[(CH2CO2)-C-.-H]) has been determined to be 347.0 +/- 1.1 kcal/mol from measurements of the relative yields of the carboxylate ion fragments resulting from CID of proton-bound dimer ions formed by termolecular association of CH2CO2.- with carboxylic acids with known gas-phase acidities (i.e., by the Cooks kinetic method). This result indicates that removal of a hydrogen atom from the alpha-carbon of acetic acid (Delta H-acid(CH3CO2H) = 348.6 +/- 2.9 kcal/mol) increases the acidity by 1.6 kcal/mol. These data are used to derive the 298 K heat of formation for acetate radical anion, Delta H-f,H-298(CH2CO2.-) = -78.2 +/- 2.7 kcal/mol, and the C-H bond dissociation energies D-298-[(O2CCH2)-O---H] = 93.7 +/- 4.0 kcal/mol and D-298[HO2CCH2-H] = 95.3 +/- 2.9 kcal/mol. The acetate radical anion undergoes gas-phase reactions with NO, SO2, and NO2 by CH2.- transfer, forming CH2NO-, CH2SO2.-, and CH2NO2-, respectively, and reacts with CH3SSCH3 by CH3S abstraction. Hydrogen atom transfer is shown to occur during the formation of cluster ions of CH2CO2.- with certain carboxylic acids.

Absolute heats of formation for alpha,2-, alpha,3-, and alpha,4-dehydrotoluene biradicals have been determined from the measured threshold energies for dissociation of chloride, bromide, and iodide ion from the corresponding o-, m-, and p-halobenzyl anions in the gas phase. The apparent heats of formation derived for the alpha,2- and alpha,4-dehydrotoluene biradicals exhibit a dependence upon the particular halide ion used for the threshold energy measurement (decreasing with increasing halide atomic number), while the final heat of formation obtained for the alpha,3-dehydrotoluene biradical is invariant with changes in the halide. The 298 K heats of formation derived from the iodobenzyl anion results for alpha,2-, alpha,3-, and alpha,4-dehydrotoluene are all found to be 103 +/- 3 kcal/mol. This value is in fair agreement with the predicted heats of formation for the ground state of each biradical obtained from MCSCF calculations (105-106 kcal/mol) and significantly lower than the value of 107.6 +/- 1.7 kcal/mol predicted by a simple bond energy additivity calculation. The MCSCF calculations indicate alpha,2- and alpha,4-dehydrotoluene to be ground-state triplet biradicals with open-shell singlets lying 7.4 and 8.1 kcal/mol higher in energy, respectively, while alpha,3-dehydrotoluene is found to be a ground-state singlet with the triplet lying 3.0 kcal/mol higher in energy. The halide ion dependence of the apparent heats of formation for the alpha,2- and alpha,4-dehydrotoluene biradicals is attributed to the spin-forbidden nature of the dissociation reactions that produce them. The intersystem crossing required to form ground-state triplet products from the halobenzyl anion precursors is associated with a reverse activation energy and/or a kinetic shift in the reaction onset due to slow unimolecular decomposition kinetics. Both effects would be expected to diminish with the heavier halides. In contrast, dissociation of a m-halobenzyl anion to produce alpha,3-dehydrotoluene is spin-allowed, so the reaction occurs at the true thermochemical limit.

The absolute heats of formation of 1,2-, 1,3-, and 1,4-dehydrobenzene (ortho-, meta-, and para-benzyne) have been determined from measurements of the threshold energies for collision-induced dissociation (CID) of ortho-, meta-, and para-chlorophenyl anions in a flowing afterglow-triple quadrupole apparatus. The 298 K heats of formation for ortho-, meta-, and para-benzyne derived in this manner are 106.6 +/- 3.0, 122.0 +/- 3.1, and 137.3 +/- 3.3 kcal/mol, respectively. The values for meta- and para-benzyne are higher than those reported previously (Wenthold, P. G.;Paulino, J. A.;Squires, R. R., J. Am. Chem. Sec. 1991, 113, 7414) but are in excellent agreement with recently reported MCSCF and CI calculations. Several control experiments are described which demonstrate that the earlier results for meta- and para-benzyne suffered from an acid-catalyzed isomerization of the reactant chlorophenyl anions in the flowing afterglow prior to CID threshold analysis. It is shown that CID threshold measurements with chlorophenyl-d(4) anions are necessary in order to obtain the correct dissociation energies for the isomerically pure species. The gas-phase acidities of the meta and para positions of chlorobenzene were determined at 298 K using the silane cleavage method to be 391.1 +/- 2.0 and 393.9 +/- 2.0 kcal/mol, respectively. The heats of formation for the benzynes determined in this study are used to derive the gas-phase acidity of the 2-position of meta-benzyne (365 +/- 3 kcal/mol) and the ortho, meta, and para C-H bond strengths in phenyl radical (79.9 +/- 3.1, 95.3 +/- 3.2, and 110.6 +/- 3.4 kcal/mol, respectively). The cycloaromatization of cis-3-hexene-1,5-diyne to para-benzyne (the Bergman cyclization) is calculated to be endothermic by 13 kcal/mol.

An experimental method is described for determining the kinetics of gas-phase ion/molecule reactions involving reactant and product ions with the same nominal or exact masses (i.e., isobaric ion/molecule reactions) using a flowing afterglow-triple quadrupole instrument. The method involves formation of specific monitor ions in the triple quadrupole analyzer by collision-induced dissociation, exothermic ion/molecule reactions or energy-resolved ion/molecule reactions for use in determining the kinetics of reaction of one component of an isobaric ion mixture in the flow tube. The measured depletion of these types of monitor ions is shown to provide a reliable means of determining the rate coefficient for the overall reaction taking place when a neutral reagent is added to the flow reactor.