3986
`
`J. Am. Chem. Soc. 1992, 114, 3986-3988
`empty p orbitals with the occupied s orbitals, i.e., hybridization.
`A ligand field of Dy symmetry is shown to induce strong bonding
`between two Be atoms which are weakly bound in the Be2 mol-
`ecule.
`
`orbitals. Upon bonding to the two CO molecules, the Be 2s orbital
`mixes with its low-lying 2p orbitals to form sp2 hybrid orbitals,
`two of which are used to form bonds with the carbon lone pair
`orbital of the CO, while the third is occupied by one of the un-
`paired electrons. The second unpaired electron occupies a b,
`orbital perpendicular to the plane of the molecule, which is a Be
`2p type orbital that is involved in a ir back-bonding interaction
`with the CO ir* orbital.
`The 3B) state of Be(CO)2 is analogous to the lowest 3B, state
`of methylene on the basis of the nature and type of the highest
`occupied orbitals. Formation of Be2(CO)4 from two Be(CO)2
`molecules in their 3B, states results in a Be-Be double bond. An
`analysis of the two highest occupied orbitals of (CO)2BeBe(CO)2
`shows significant bonding between the two Be atoms. The highest
`occupied (b3u) orbital is bonding between the Be atoms, with
`each Be also involved in back-bonding with the t* orbitals of
`the CO. The second highest occupied orbital (ag) is a a type
`bonding orbital. The triplet state arising from the one-electron
`excitation to the * orbital of (CO)2BeBe(CO)2 is 16.2 kcal below
`the singlet state at the HF level of theory. The inclusion of electron
`the order of the states with the 'A,
`correlation effects reverses
`below the triplet state by 10.6 kcal at the MP2 level of theory.
`The binding energy of (CO)2BeBe(CO)2 is computed with
`respect to dissociation to two Be('S) and four CO molecules as
`well as to two ^ Be(CO)2 fragments. The computed binding
`energies are given in Table I. Even at the HF level, the interaction
`energy between the two Be(CO)2 fragments in (CO)2BeBe(CO)2
`is 15.5 kcal. Note that the Be2 molecule shows no binding at this
`level of theory.11
`*1The complete fragmentation of (CO)2BeBe-
`(CO)2 into two Be atoms and four CO molecules requires 36.0
`kcal at the HF level of theory. At the MP4(SDQ) level, the
`strength of the Be-Be bond in this molecule is 50.0 kcal, and the
`complete fragmentation requires 68.3 kcal. The contributions of
`electron correlation to the binding energy of both Be(CO)2 and
`(CO)2BeBe(CO)2 are large, indicating the need to reoptimize the
`geometry of these molecules to include electron correlation ef-
`fects.10 This trend is consistent with other studies of CO binding
`to metal atoms.12
`At the HF level of theory the BeBe distance in (CO)2BeBe-
`(CO)2 is only 1.938 Á, which is considerably shorter than the
`experimental bond distance of 2.45 Á in Be2,13 the shortest Be-Be
`distance of 2.226 Á in Be metal,14 and the Be-Be bond length
`of 2.124 Á in HBeBeH at the same level of theory.15
`The
`computed harmonic vibrational frequency for the Be-Be stretch
`for this molecule of 942 cm"1 is more than 4 times larger than
`the experimental value of the vibrational frequency of Be2 at 223
`cm"1 and is also larger than the Be-Be stretching frequency of
`645 cm"1 in HBeBeH at the same level of theory. The barrier
`for rotation about the Be-Be bond, computed at the HF geometry
`of the ground state by the GVB method16 correlating only the pair
`of electrons in the bond, is 6.70 kcal. The computed properties
`of the Be-Be bond, bond distance, bond energy, vibrational
`stretching frequency, and barrier to rotation, all lend support to
`the idea of a double bond between the Be atoms in this molecule.
`The nature of the bonding and the stability of (CO)2BeBe(CO)2
`demonstrate that it is indeed possible to form strong double bonds
`between Be atoms. The bonding in this molecule where two
`formally s2 closed subshells interact arises from the mixing of the
`
`(10) A detailed account of the electron correlation effect on the geometries
`of the singlet and triplet states, binding energy, and the singlet-triplet energy
`separation will be published separately.
`(11) Harrison, R. J.; Handy, N. C. Chem. Phys. Lett. 1986,123, 321 and
`references therein.
`(12) (a) Balaji, V.; Sunil, K. K.; Jordan, K. D. Chem. Phys. Lett. 1987,
`(b) Blomberg, M. R. A.; Brandemark, U. B.; Johansson, J.; Si-
`136, 309.
`eghbahn, P. E. M.; Wennerberg, J. J. Chem. Phys. 1988, 88, 2344.
`(c)
`Blomberg, M. R. A.; Brandemark, U. B.; Siegbahn, P. E. M.; Wennerberg,
`J.; Bauschlicher, C. W. J. Am. Chem. Soc. 1988, 110, 6650.
`(13) Bondybey, V. E. Chem. Phys. Lett. 1984, 109, 436.
`(14) Donahue, J. Structure of Elements; John Wiley: New York, 1974.
`(15) Sana, M.; Leroy, G. Theor. Chem. Acta 1990, 77, 383.
`Ill
`In Modern Theoretical
`(16) Bobrowitz, F. W.; Goddard, W. A.,
`Chemistry: Methods of Electronic Structure Theory; Scheafer, H. F., Ill,
`Ed.; Plenum: New York, 1977; Vol. 3.
`
`It is a pleasure to acknowledge Profs. K. D.
`Acknowledgment.
`Jordan, L. C. Allen, and J. M. Schulman and Dr. V. Balaji for
`many insightful discussions and Ms. D. M. Corsi for critical
`comments on the manuscript.
`
`Proton Affinities of the 20 Common -Amino Acids*
`Greg S. Gorman, J. Paul Speir, Cheryl A. Turner, and
`I. Jonathan Amster*
`Department of Chemistry, University of Georgia
`Athens, Georgia 30602
`Received December 27, 1991
`We report the first measurement of the gas-phase basicities
`(-AG of protonation) of all of the 20 common -amino acids, from
`which we can derive their proton affinities (- /7 of protonation).
`The basicities are determined by observing the occurrence
`or
`of reaction 1, and its reverse reaction, in which
`nonoccurrence
`A represents an amino acid, B a reference base, and AH"1" and
`BH+ their respective protonated forms. The observation of proton
`A + BH+ — AH"1" + B
`(1)
`transfer between the protonated molecule of a base and an amino
`acid implies a negative free energy for reaction 1. The reverse
`reaction is observed if the free energy of reaction 1 is positive.
`The -amino acids are low-volatility compounds and thus are
`difficult subjects for the type of gas-phase equilibrium experiments
`generally utilized to measure gas-phase basicity or proton affinity.1
`To date, values of the proton affinity of only 6 of the 20 common
` -amino acids have been published.2·3 Meot-Ner et al. used
`high-pressure mass spectrometry to measure the gas-phase basicity
`of six amino acids (glycine, alanine, valine, leucine, phenylalanine,
`and proline), and they also used variable-temperature studies to
`directly measure the enthalpy of the proton-transfer reaction and
`thus the proton affinities of three of the amino acids.2 Locke and
`Mclver measured the gas-phase basicities of glycine and alanine
`with ion cyclotron resonance spectrometry.3 In these studies, for
`the cases where enthalpies were not measured directly, proton
`affinity values were obtained from the gas-phase basicity data by
`estimating the entropy of the reaction, which was found to be a
`small or negligible correction.
`In this study, we use Fourier transform ion cyclotron resonance
`spectrometry to observe the reaction of laser-desorbed, neutral
`amino acid molecules with a series of protonated reference bases,
`as in reaction 1. With this technique, called laser desorption/
`chemical ionization (LD/CI), low-volatility or nonvolatile com-
`pounds can be utilized as the neutral partner in studies of ion-
`molecule reactions.4 The reverse of reaction 1 is examined by
`forming a protonated molecule of an amino acid by matrix-assisted
`laser desorption of the amino acid,5 ****followed by reaction with a
`• Author to whom correspondence should be addressed.
`* Dedicated to Professor Fred W. McLafferty on the occasion of his re-
`tirement.
`(1) The experimental methods for making equilibrium gas-phase basicity
`measurements have been reviewed by Aue, D. H.; Bowers, . T. In Gas Phase
`Ion Chemistry: Bowers, . T., Ed.; Academic Press: New York, 1979; Vol.
`2, Chapter 9.
`(2) Meot-Ner, M.; Hunter, E. P.; Field, F. H. J. Am. Chem. Soc. 1979,
`101, 686-689.
`(3) Locke, M. J.; Mclver, R. T., Jr. J. Am. Chem. Soc. 1983, 105,
`4226-4232.
`(4) Amster, I. J.; Land, D. P.; Hemminger, J. C; Mclver, R. T., Jr. Anal.
`Chem. 1989,61, 184-186. Speir, J. P.; Gorman, G. S.; Cornett, D. S.; Amster,
`I. J. Anal. Chem. 1991, 63, 65-69. Speir, J. P.; Gorman, G. S.; Amster, I.
`J. In Laser Ablation, Mechanisms and Applications: Miller, J. C., Haglund,
`R. F., Eds.; Springer-Verlag: Berlin, 1991; pp 174-179.
`(5) Karas, M.; Hillenkamp, F. Anal. Chem. 1988, 60, 2299-2301. Beavis,
`R. C.; Chait, B. T. Rapid Commun. Mass Spectrom. 1989, 3, 432-435.
`Hillenkamp, F.; Karas, M.; Beavis, R. C; Chait, B. T. Anal. Chem. 1991, 63,
`1193A-1202A.
`
`0002-7863/92/1514-3986$03.00/0
`
`© 1992 American Chemical Society
`
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`

`Communications to the Editor
`
`J. Am. Chem. Soc., Vol. 114, No. 10, 1992
`
`3987
`
`Table I. Gas-Phase Basicities (GB) and Proton Affinities (PA) of
`the 20 a-Amino Acids (italics) and Reference Bases
`GB (350 K)“·6
`reference base
`amino acids
`200.7 ± 1.5
`acetophenone
`202.4 ± 3.2
`204.1 ± 1.5
`205.4 ± 2.8
`206.6 ± 1.5
`209.2 ± 1.5
`210.3 ± 2.6
`211.3 ± 1.5
`212.3 ± 2.5
`213.2 ± 1.5
`214.8 ± 3.1
`216.3 ± 1.5
`217.4 ± 2.6
`218.4 ± 1.5
`219.5 ± 2.6
`
`pyrrole
`
`2-fluoropyridine
`methylamine
`
`3-fluoropyridine
`
`ethylamine
`
`isopropylamine
`
`pyridine
`
`PA“C
`210.0 ± 1.5
`211.7 ± 3.2
`213.4 ± 1.5
`214.6 ± 2.7
`215.7 ± 1.5
`219.6 ± 1.5
`220.2 ± 2.1
`220.7 ± 1.5
`222.1 ± 2.9
`223.4 ± 1.5
`225.0 ± 3.1
`226.5 ± 1.5
`227.2 ± 2.2
`227.8 ± 1.5
`228.5 ± 2.2
`
`Gly
`
`Cys
`
`Ser, Asp
`Ala, Val
`Leu, lie
`
`Phe, Tyr, Asn
`
`Thr, Met, Gin,
`His
`
`Pro, Trp
`
`Glu
`
`Lys
`
`terf-butylamine
`
`trimethylamine
`
`diethylamine''
`
`di-n-propylamine1'
`triethylamine*'
`tri-n-butylamine‘,
`
`229.2 ± 1.5
`220.5 ± 1.5
`231.0 ± 3.3
`221.9 ± 2.9
`232.7 ± 1.5
`223.3 ± 1.5
`240.6 ± 1.9
`223.7 ± 1.9
`233.5 ± 1.5
`224.1 ± 1.5
`242.6 ± 3.4
`226.0 ± 3.4
`237.3 ± 1.5
`227.9 ± 1.5
`239.6 ± 1.5
`230.2 ± 1.5
`233.8 ± 1.5
`243.2
`>243.2
`>233.8
`Arg
`“All values are in kcal/mol. 6Relative GB values of the reference
`bases are from ref 7 and are adjusted for 350 K. The absolute GB
`values are based on a value of 198.2 kcal/mol for NH3 at 350 K. CPA
`values of the reference bases are from ref 7.
`* Relative GB and PA
`values are from ref 1 and are referenced to the values of GB and PA of
`trimethylamine from ref 7.
`
`neutral volatile base.6 Use of a series of reference bases with
`small differences in gas-phase basicity allows an amino acid to
`be bracketed within a narrow range. Most of the reference bases
`used in this study were chosen from a recently published ladder
`of proton affinities and gas-phase basicities.7 The study that
`produced the revised ladder of basicities eliminated systematic
`in the upper end of the proton affinity scale previously in
`errors
`use8 and yielded measurements in agreement with proton affinity
`values obtained from absolute determinations at various positions
`in the ladder.
`The gas-phase basicities and proton affinities measured for the
`20 -amino acids are listed in Table I, along with the literature
`values for the reference bases used in this study. For most of the
`amino acids, the entropy of reaction 1 can be calculated from
`changes in the rotational symmetry numbers ( ) of the reactants
`and products according to eq 2.9 The amino acids and reference
`AS,0i = R In [ +/ + ]
`(2)
`bases used in this study have low symmetry, yielding rotational
`entropy values of approximately ±2 cal/mol-K. At the temper-
`ature of the system in this study, 350 K,10 *the contribution of
`entropy to the free energy of reaction 1 is ±0.7 kcal/mol. The
`uncertainty in the assignment of the gas-phase basicities of the
`amino acids, determined by the spacings between the gas-phase
`
`(6) For formation of protonated molecules by matrix-assisted laser de-
`sorption, amino acids were mixed 1:1000 with sinapinic acid and desorbed with
`the 248-nm line of an excimer laser. The desorbed ions were thermalized by
`20-100 collisions with argon, admitted through a pulsed valve, prior to their
`reaction with the reference bases.
`(7) Meot-Ner, M.; Sieck, W. J. Am. Chem. Soc. 1991, 113, 4448-4460.
`(8) Lias, S. G.; Liebman, J. F.; Levin, R. D. J. Phys. Chem. Ref. Data
`1984, 13, 695-808.
`(9) Kebarle, P. Anna. Rev. Phys. Chem. 1977, 28, 445-476.
`(10) The cell temperature was determined by measuring the reaction rate
`of Ar+ with Na, suggested as a “thermometer” reaction by Bartmess, J. E. In
`Structure / Reactivity and Thermochemistry of Ions·, Ausloos, P., Lias, S. G.,
`Eds.; Reidel: Dordrecht, The Netherlands, 1987; pp 367-371. The kinetic
`energy dependence of this reaction has been reported by Dotan, I.; Lindinger,
`W. J. Chem. Phys. 1982, 76, 4972-4977.
`
`Table II. A Comparison of the Proton Affinity Values of Six
`a-Amino Acids with Published Values"
`
`Locke and
`Mclver**
`210.3
`213.1
`
`Meot-Nerc
`current study6
`204.0-207.4
`208.2
`glycine
`alanine
`211.5-214.0
`212.2
`211.5-214.0
`valine
`213.9
`leucine
`214.0-216.5
`214.5
`216.5-218.1
`215.1
`phenylalanine
`218.8-222.1
`218.4
`proline
`“All values are in kcal/mol. 6Values from this study have been ad-
`justed to match the proton affinity ladder from ref 9. CPA values are
`from ref 2 and are based on the proton affinity ladder of ref 9.
`8 PA
`values are from ref 3 and are adjusted to match the proton affinity
`ladder from ref 9.
`
`basicities of the reference bases, is ±2.5 kcal/mol on average. The
`contribution of entropy, caused by changes in rotational symmetry,
`to the free energy of reaction 1
`is much smaller than the un-
`certainty in the assignment of the free energy and can be neglected,
`so that AH = AG for reaction 1. This assumption has been verified
`in a previous study of amino acid proton affinities.2 A notable
`exception to this simplification occurs when an intramolecular
`hydrogen-bonded cyclization occurs, as has been observed for
` , -diamines.11 Lysine (2-carboxy-1,5-pentanediamine) fulfills
`the geometric requirement for such a cyclization. The formation
`of the hydrogen bond in cyclized diamines causes a substantial
`increase in their gas-phase basicities and proton affinities relative
`to monoamines of similar polarizability. The difference in gas-
`phase basicity between lysine and the structurally similar mo-
`noamine, leucine (2-carboxyisopentylamine), 11.2 ± 3.2 kcal/mol,
`is approximately the same as that between 1,5-pentanediamine
`and n-pentylamine, 10.7 ± 1.5 kcal/mol,1 suggesting the intra-
`molecular cyclization of protonated lysine. The entropy of cy-
`clization for lysine should be approximately equal
`to that of
`-20.5 cal/mol-K).1 The enthalpy
`1,5-pentanediamine (AScyc =
`of protonation of lysine via reaction 1 is equal to the free energy
`-7.2 kcal/mol at 350 K. The proton affinity value
`plus TAS =
`of lysine in Table I has been adjusted to account for cyclization
`of the protonated molecule.12
`The difference in gas-phase basicities between glutamic acid
`and aspartic acid (
`= 13 kcal/mol) is much greater than for
`the corresponding amides, glutamine and asparagine (
`= 2
`kcal/mol). The structures of these amino acids differ by one
`methylene group in the side chain, and so only a small difference
`in gas-phase basicity is expected, as is observed for the amides.
`The anomalously high gas-phase basicity of glutamic acid suggests
`cyclization of its protonated form. The proton affinity assignment
`has been adjusted to account for the entropy of cyclization ( 56 0
`-21.5 cal/mol-K, as for 4-hydroxybutanamine,1 TAScyc = -7.5
`Interestingly, the gas-phase basicity of glutamine does
`kcal/mol).
`not suggest cyclization of that amino acid.
`Arginine is the most basic of the amino acids, presumably
`because of the strongly basic guanido group present in its side
`It is more basic than any of the reference bases that are
`chain.
`used in this study. As a result, we can only set a lower limit on
`the gas-phase basicity of this amino acid, 234 kcal/mol, and on
`the proton affinity, 243 kcal/mol. Although arginine has the
`
`=
`
`(11) Aue, D. H.; Webb, J. M.; Bowers, . T. J. Am. Chem. Soc. 1973,
`95, 2699-2701. Yamdagni, R.; Kebarle, P. J. Am. Chem. Soc. 1973, 95,
`3504-3510. Meot-Ner, M.; Hamlet, P.; Hunter, E. P.; Field, F. H. J. Am.
`Chem. Soc. 1980, 102, 6395-6399.
`(12) The adjustment of the proton affinity values is valid only if endo-
`thermic but exoergic reactions are rapid compared to the ion-molecule col-
`lision rate, as has been observed in high-pressure mass spectrometry mea-
`surements; see ref 11, Meot-Ner et al. There is controversy as to whether
`endothermic processes can be driven by entropy in low-pressure experiments,
`as has been discussed by Henchman, M. In Structure j Reactivity and Ther-
`mochemistry of Ions· Ausloos, P„ Lias, S. G., Eds.; Reidel: Dordrecht, The
`Netherlands, 1987; pp 381-399. Although ions are detected under low-
`pressure, collisionless conditions (1CT9 Torr) in the experiments reported here,
`the ion-molecule reactions occur under multiple collision conditions, for which
`the reaction entropy must be considered. For example, protonated amino acid
`molecules undergo 20-100 collisions with a reference base.
`
`
`
`-3987-
`
`
`
`
`

`

`kH
`
`3988
`
`J. Am. Chem. Soc. 1992, 114, 3988-3989
`appropriate geometry to form an intramolecular hydrogen bond,
`there is insufficient data on the gas-phase basicities of guanidine
`compounds to determine whether cyclization occurs for this amino
`acid on the basis of the measurements reported here.
`Table II compares prior measurements of the proton affinities
`of six amino acids with the measurements reported here. To
`facilitate the comparison, the proton affinity values in the table
`are referenced to the proton affinity ladder used in the earliest
`study.2 We find excellent agreement with Meot-Ner’s mea-
`surements of the proton affinities of alanine, valine, leucine, and
`proline and close agreement for glycine and phenylalanine.2
`Likewise, we find excellent agreement with the proton affinity
`value of alanine reported by Locke and Mclver.3 Our value for
`the proton affinity of glycine is lower than, but similar to, that
`measured by Locke and Mclver. The comparisons show no
`systematic differences between ours and prior measurements. Our
`relative ordering of the proton affinities of the amino acids agrees
`with that proposed by Bojensen with a few notable exceptions.13
`Bojensen places histidine as the second most basic amino acid,
`while we find it sixth on our
`In Bojensen’s study, glutamine
`list.
`is more basic than glutamic acid, opposite to our
`finding. We
`find alanine, methionine, and threonine to be more basic than does
`Bojensen. While the exact reason for the differences between the
`two measurements is not known, it should be emphasized that the
`measurements reported here are based on well-established ion-
`molecule techniques and theory. Bojensen relates ion abundances
`in a metastable ion mass spectrum to thermodynamic properties
`of the product ions, a technique proposed by Cooks and co-workers
`for measuring relative proton affinities of closely related, mono-
`functional compounds.14 The discrepancies between our mea-
`surements and those of Bojensen may be a limitation of Cook’s
`method, which has never been tested with structurally diverse,
`polyfunctional molecules such as the amino acids.
`Acknowledgment. This work was supported by the National
`Science Foundation (CHE-9024922). Acknowledgement is made
`to the donors of the Petroleum Research Fund, administered by
`the American Chemical Society, for partial support of this work.
`
`~
`
`Table I. Rate Constants0 for the Reaction of Muonium Atoms with
`Aromatic N-Heterocyclic Solutes in Water at ~295 K. Comparison
`with Published Data on ‘H4
`WC
`solute
`*m/*h
`fcH/N
`33 ± 3
`benzene
`5.5
`(1.5)'
`9
`3.7
`58 ± 4
`pyridine
`7.4
`7.84
`7.8
`12
`50 ± 3
`2.7'
`12.5
`pyridazine
`1.4
`18.5
`37 ± 2
`0.46
`40.2
`pyrimidine
`0.927
`9
`77 ± 5
`3.0*
`25.7
`pyrazine
`19
`1.5
`"All k values are in units of 10s dm3 mol'1 s'1; kM/C and fcH/N are
`See footnotes c to g for pH dependence.
`described in the text.
`' Calculated as kH per C atom. 4The 7.8 refers to nat-
`4 Reference 8.
`ural pH. At pH ~1, however, fcH =
`1.7 X 108 M'1 s'1 for pyridine
`where >99% of the solutes were protonated; so protonation decreases
`kH by ~4.5-fold. This contrasts with Mu, where kM increased from 58
`to 74 (X108) on changing from pH ~7 to pH 1.2, for pyridine. Thus
`kM/kH increases from 7.4 at pH ~7 to 44 at pH ~1.
`'kH measured
`1, where ~99% of the solutes have one of their N atoms
`at pH ~
`protonated; so the number of unprotonated N atoms is half that rele-
`fkH measured at pH ~1, where ~65% of the
`to the fcM data.
`vant
`solutes have one of their N atoms protonated.
`* kH measured at pH
`1, where ~30% of the solutes have one of their N atoms protonated.
`study it shows a different type of reaction.
`Recently we found that Mu produces the free radical which
`arises from addition into the aromatic ring of pyrazine at its C
`atoms.4 Now we report the rate constants measured for that
`reaction and Mu’s reaction with other N-heterocyclic compounds.
`Table I gives our observed rate constants (kM) for reaction of Mu
`with benzene, pyridine, and the 1,2-, 1,3-, and 1,4-diazines. These
`data were obtained by measuring, the chemical decay rate of
`muonium using the muon-spin-rotation technique5 on millimolar
`aqueous solutions of these compounds at natural pH. They are
`compared in Table I with the published data for reaction of ‘H
`atoms with the same solutes under similar conditions. Since Mu
`adds to a ring C,4 whereas
`attaches at the more electronegative
`N atoms,6 these data have been normalized in columns 3 and 5
`of Table I by dividing fcM by the number of C sites on the solutes
`and kH by the number of N sites.
`Mu is seen to react 3.7 times faster than with benzene. This
`is not inconsistent with Mu’s 3-fold-higher mean thermal velocity
`stemming from its one-ninth atomic mass:
`a kinetic isotope effect
`expected for diffusion-limited reactions.3 The overall rate constants
`ratio, kM/kH, then increases to 7.4 due to the presence of one ring
`N, and up to 40 with two N atoms. This ratio rises partly by ku
`increasing and partly by kH decreasing.
`The enhancement of kM by ring nitrogens implies that Mu is
`“nucleophilic”, because N draws electron density from the C atoms
`where Mu reacts. Preliminary results even suggest that there is
`a direct proportionality between -log (fcM) and the Hückel mo-
`lecular orbital localization energy on each C.7 By contrast, the
`presence of the ring N of pyridine seems to switch H’s reaction
`site from C to N (as seen through ESR data6). This results in
`a minor overall decrease in kH (but an increase per atom where
`reaction takes place). The presence of a second N atom then
`reduces the rate further. Also,
`has already been shown to
`display a negative Hammett p parameter,1·2 and the differences
`between the diazines have been interpreted as consistent with this
`electrophilic character.1
`information to be gleaned from the pH
`There is additional
`dependence of these rate constants arising from protonation of
`the solute. For pyridine, which is the only N-heterocycle for which
`data are available, kM/kH jumps from 7.4 at pH ~7 to 44 at pH
`1. Almost all of this jump (see footnotes to Table I) emerges
`from a decrease in kH. This consequently corroborates the view
`that reaction of H, but not Mu, occurs on the N atom.
`
`(13) Bojensen, G. J. Am. Chem. Soc. 1987, 109, 5557-5558.
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`
`Different Reaction Paths Taken by Hydrogen Isotopes
`Zhennan Wu, John M. Stadlbauer,7 8and David C. Walker*
`Chemistry Department and TRIUMF
`University of British Columbia
`Vancouver, V6T 1Y6 Canada
`Received September 6, 1991
`Kinetic isotope effects usually reveal differences in degree but
`not in kind. However, in studying the chemistry of muonium
`(hydrogen’s light isotope with a positive muon as nucleus) we find
`it to behave as a nucleophile in its reactions with aromatic N-
`heterocyclic solutes in water. This contrasts with the electrophilic
`character of .1,2 Muonium (Mu) has one-ninth the atomic mass
`of but virtually the same reduced mass, ionization energy, size,
`etc. as all hydrogen isotopes. Hitherto Mu has shown kinetic and
`spectroscopic isotopic effects which are understandable in terms
`of its different atomic mass from that of ;3 but in the present
`
`’ Permanent address: Chemistry Department, Hood College, Frederick,
`MD 21701.
`(1) (a) Neta, P.; Schuler, R. H. J Am. Chem. Soc. 1972, 94, 1056.
`(b)
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`(3) (a) Walker, D. C. Muon and Muonium Chemistry·, Cambridge
`(b) Walker, D. C. Can. J. Chem. 1990,
`University Press: Cambridge, 1983.
`86, 1719.
`
`~
`
`(4) Wu, Z.; Barnabas, . V.; Stadlbauer, J. M.; Venkateswaran, K.;
`Porter, G. B.; Walker, D. C. J. Am. Chem. Soc. 1991, 113, 9096.
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`(7) Z. Wu et al., preliminary unpublished data.
`(8) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. Natl.
`Stand. Ref. Data Ser. (U.S., Natl. Bur. Stand.) 1988, NSRDS-NBS.
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`0002-7863/92/1514-3988S03.00/0
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`© 1992 American Chemical Society
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