Alex G. Harrison

University of Toronto, Toronto, Ontario, Canada

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Publications (199)583.03 Total impact

  • Benjamin J Bythell, Alex G Harrison
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    ABSTRACT: It is well-known that oxazolone b2 ions fragment extensively by elimination of CO to form a2 ions, which often fragment further to form a1 ions. Less well-known is that some oxazolone b2 ions may fragment directly to form a1 ions. The present study uses energy-resolved collision-induced dissociation experiments to explore the occurrence of the direct b2→a1 fragmentation reaction. The experimental results show that the direct b2→a1 reaction is generally observed when Gly is the C-terminal residue of the oxazolone. When the C-terminal residue is more complex, it is able to provide increased stability of the a2 product in the b2→a2 fragmentation pathway. Our computational studies of the relative critical reaction energies for the b2→a2 reaction compared with those for the b2→a1 reaction provide support that the critical reaction energies are similar for the two pathways when the C-terminal residue of the oxazolone is Gly. By contrast, when the nitrogen of the oxazolone ring in the b2 ion does not bear a hydrogen, as in the Ala-Sar and Tyr-Sar (Sar = N-methylglycine) oxazolone b2 ions, a1 ions are not formed but rather neutral imine elimination from the N-terminus of the b2 ion becomes a dominant fragmentation reaction. The M06-2X/6-31+G(d,p) density functional theory calculations are in general agreement with the experimental data for both types of reaction. In contrast, the B3LYP/6-31+G(d,p) model systematically underestimates the barriers of these SN2-like b2→a1 reaction. The difference between the two methods of barrier calculation are highly significant (P < 0.001) for the b2→a1 reaction, but only marginally significant (P = 0.05) for the b2→a2 reaction. The computations provide further evidence of the limitations of the B3LYP functional when describing SN2-like reactions.
    Journal of the American Society for Mass Spectrometry 03/2015; 26(5). DOI:10.1007/s13361-015-1080-7 · 3.19 Impact Factor
  • Alex G. Harrison
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    ABSTRACT: This retrospective paper briefly summarizes the very early work (pre 1930) on the kinetics of ion-molecule reactions but the main emphasis is on the extensive work in the 1950s and 1960s using single-source mass spectrometers which established the field of gas-phase ion-molecule reactions as an important field of study. More modern techniques such as ICR, ion traps and flow systems are briefly mentioned but are discussed in detail in other papers.
    International Journal of Mass Spectrometry 04/2014; 377. DOI:10.1016/j.ijms.2014.04.003 · 2.23 Impact Factor
  • Alex G Harrison
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    ABSTRACT: The effect of N-methylation on sequence scrambling in the fragmentation of b5 ions has been investigated by studying a variety of peptides containing sarcosine (N-methylglycine). The product ion mass spectra for the b5 ions derived from Sar-A-A-A-Y-A and Sar-A-A-Y-A-A show only minor signals for non-direct sequence ions the major fragmentation reactions occurring from the unrearranged structures. This is in contrast to the b5 ions where the Sar residue is replaced by Ala and sequence scrambling occurs. The b5 ion derived from Y-Sar-A-A-A-A shows a product ion mass spectrum essentially identical to the spectrum of the b5 ion derived from Sar-A-A-A-Y-A, indicating that in the former case macrocyclization has occurred but the macrocyclic form shows a strong preference to reopen to put the Sar residue in the N-terminal position. Similar results were obtained in the comparison of b5 ions derived from A-Sar-A-A-Y-A and Sar-A-A-Y-A-A. The product ion mass spectra of the MH(+) ions of Y-Sar-A-A-A-A and A-Sar-A-A-Y-A show substantial signals for non-direct sequence ions indicating that fragmentation of the MH(+) ions channels extensively through the respective b5 ions and further fragmentation of these species. Copyright © 2014 John Wiley & Sons, Ltd.
    Journal of Mass Spectrometry 02/2014; 49(2):161-7. DOI:10.1002/jms.3323 · 2.71 Impact Factor
  • Alex G Harrison
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    ABSTRACT: The fragmentation reactions of the MH(+) ions as well as the b7, a7, and a7* ions derived therefrom have been studied in detail for the octapeptides MAAAAAAA, AAMAAAAA, AAAAMAAA, and AAAAAAMA. Ionization was by electrospray using a QqToF mass spectrometer, which allowed a study of the evolution of the fragmentation channels as a function of the collision energy. Not surprisingly, the product ion mass spectra for the b7 ions are independent of the original precursor sequence, indicating macrocyclization and reopening to the same mixture of protonated oxazolones prior to fragmentation. The results show that this sequence scrambling results in a distinct preference to place the Met residue in the C-terminal position of the protonated oxazolones. The a7 and a7* ions also produce product ion mass spectra independent of the original peptide sequence. The results for the a7 ions indicate that fragmentation occurs primarily from an amide structure analogous to that observed for a4 ions (Bythell et al. in J Am Chem Soc 132:14766-14779, 2010). Clearly, the rearrangement reaction they have proposed applies equally well to an ions as large as a7. The major fragmentation modes of the MH(+) ions at low collision energies produce b7, b6, and b5 ions. As the collision energy is increased further fragmentation of these primary products produces, in part, non-direct sequence ions, which become prominent at lower m/z values, particularly for the peptides with the Met residue near the N-terminus.
    Journal of the American Society for Mass Spectrometry 08/2013; 24(10). DOI:10.1007/s13361-013-0706-x · 3.19 Impact Factor
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    Alex G Harrison, Cagdas Tasoglu, Talat Yalcin
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    ABSTRACT: The fragmentation reactions of the MH(+) ions of Leu-enkephalin amide and a variety of heptapeptide amides have been studied in detail as a function of collision energy using a QqToF beam type mass spectrometer. The initial fragmentation of the protonated amides involves primarily formation of bn ions, including significant loss of NH3 from the MH(+) ions. Further fragmentation of these bn ions occurs following macrocyclization/ring opening leading in many cases to bn ions with permuted sequences and, thus, to formation of non-direct sequence ions. The importance of these non-direct sequence ions increases markedly with increasing collision energy, making peptide sequence determination difficult, if not impossible, at higher collision energies.
    Journal of the American Society for Mass Spectrometry 08/2013; 24(10). DOI:10.1007/s13361-013-0707-9 · 3.19 Impact Factor
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    Arpád Somogyi, Alex G Harrison, Béla Paizs
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    ABSTRACT: Middle-sized b ( n ) (n ≥ 5) fragments of protonated peptides undergo selective complex formation with ammonia under experimental conditions typically used to probe hydrogen-deuterium exchange in Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Other usual peptide fragments like y, a, a*, etc., and small b ( n ) (n ≤ 4) fragments do not form stable ammonia adducts. We propose that complex formation of b ( n ) ions with ammonia is characteristic to macrocyclic isomers of these fragments. Experiments on a protonated cyclic peptide and N-terminal acetylated peptides fully support this hypothesis; the protonated cyclic peptide does form ammonia adducts while linear b ( n ) ions of acetylated peptides do not undergo complexation. Density functional theory (DFT) calculations on the proton-bound dimers of all-Ala b ( 4 ), b ( 5 ), and b ( 7 ) ions and ammonia indicate that the ionizing proton initially located on the peptide fragment transfers to ammonia upon adduct formation. The ammonium ion is then solvated by N(+)-H…O H-bonds; this stabilization is much stronger for macrocyclic b ( n ) isomers due to the stable cage-like structure formed and entropy effects. The present study demonstrates that gas-phase guest-host chemistry can be used to selectively probe structural features (i.e., macrocyclic or linear) of fragments of protonated peptides. Stable ammonia adducts of b ( 9 ), b ( 9 ) -A, and b ( 9 ) -2A of A(8)YA, and b ( 13 ) of A(20)YVFL are observed indicating that even these large b-type ions form macrocyclic structures.
    Journal of the American Society for Mass Spectrometry 09/2012; DOI:10.1007/s13361-012-0487-7 · 3.19 Impact Factor
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    Alex G Harrison
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    ABSTRACT: The doubly-protonated peptides Ala-Ala-Xaa-Ala-Ala-Ala-Arg show extensive loss of H(2)O when Xaa = Ser or Thr. Using quasi-MS(3) techniques the fragmentation reactions of the [M + 2H - H(2)O](+2) ions have been studied in detail. For both Ser and Thr, the [M + 2H - H(2)O](+2) ions show three primary fragmentation reactions, elimination of CH(3)CH=NH, elimination of one Ala residue, and elimination of two Ala residues, in all cases forming doubly-charged products. From a study of the further fragmentation of these products, it is concluded that elimination of two Ala residues results in formation of a three-membered aziridine ring by interaction with the adjacent amide function as H(2)O is lost. The elimination of one Ala residue results in formation of a five-membered oxazoline ring through interaction with the N-terminal adjacent carbonyl function as H(2)O is lost. The elimination of CH(3)CH=NH appears to involve formation of an eight-membered ring by interaction with the remote N-terminal carbonyl function as H(2)O is lost. However, this initial structure undergoes rearrangement through interaction with the adjacent C-terminal carbonyl function prior to further fragmentation. The [MH - H(2)O](+) ion of Ala-Ala-Ser-Ala-Ala-Ala also shows elimination of CH(3)CH=NH, one Ala residue and two Ala residues.
    Journal of the American Society for Mass Spectrometry 11/2011; 23(1):116-23. DOI:10.1007/s13361-011-0282-x · 3.19 Impact Factor
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    Alex G Harrison
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    ABSTRACT: A detailed study has been made of the b(5) and a(5) ions derived from the amides H-Ala-Ala-Ala-Ala-Pro-NH(2), H-Ala-Ala-Ala-Pro-Ala-NH(2), and H-Ala-Ala-Pro-Ala-Ala-NH(2). From quasi-MS(3) experiments it is shown that the product ion mass spectra of the three b(5) ions are essentially identical, indicating macrocyclization/reopening to produce a common mixture of intermediates prior to fragmentation. This is in agreement with numerous recent studies of sequence scrambling in b ions. By contrast, the product ion mass spectra for the a(5) ions show substantial differences, indicating significant differences in the mixture of structures undergoing fragmentation for these three species. The results are interpreted in terms of a mixture of classical substituted iminium ions as well as protonated C-terminal amides formed by cyclization/rearrangement as reported recently for a(4) ions (Bythell, Maître , Paizs, J . Am. Chem. Soc. 2010, 132, 14761-14779). Novel fragment ions observed upon fragmentation of the a(5) ions are protonated H-Pro-NH(2) and H-Pro-Ala-NH(2) which arise by fragmentation of the amides. The observation of these products provides strong experimental evidence for the cyclization/rearrangement reaction to form amides and shows that it also applies to a(5) ions.
    Journal of the American Society for Mass Spectrometry 09/2011; 23(4):594-601. DOI:10.1007/s13361-011-0232-7 · 3.19 Impact Factor
  • Alex G. Harrison
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    ABSTRACT: The fragmentation reactions of the doubly-protonated tryptic-type peptides APAAAAR, AAPAAAR, AAAPAAR and AAAAPAR have been studied using an electrospray/quadrupole/time-of-flight (QqTo F) mass spectrometer. The tendency to cleave amide bonds N-terminal to proline (P) is in competition with the tendency to cleave the second amide bond, counting from the N-terminus; the result of such competition depends on the position of the Pro residue. When the Pro residue is remote from the Arg (R) residue a strong proline effect is observed resulting in formation, to a large extent, of a doubly-charged y species and a neutral fragment, so-called asymmetric amide bond cleavage. By contrast, when the Pro residue approaches the C-terminal Arg residue the proline effect is reduced with respect to cleavage of the second amide bond; in both cases formation of singly-charged y and b ions, so-called symmetric bond cleavage, increases significantly in importance. The results are discussed in terms of relative energetics for symmetric and asymmetric bond cleavage as revealed by approximate proton affinities of the b species and the singly-charged y species.Graphical abstractView high quality image (65K)Research highlights▶ Tryptic-type peptides undergo symmetric or asymmetric amide bond cleavage.. ▶ Competition between cleavage N-terminal to Pro versus cleavage of second amide bond. ▶ Position of Pro residue has major effect on fragmentation modes.
    International Journal of Mass Spectrometry 09/2011; 306(2-3):182-186. DOI:10.1016/j.ijms.2010.10.015 · 2.23 Impact Factor
  • Alex. G. Harrison
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    ABSTRACT: The use of variable low energy collision-induced dissociation to probe the energy dependence of the fragmentation of gaseous anions and, thus, to probe the potential energy surfaces involved, is illustrated using R(CH3)2CO- ions as an example. The fragmentation of proton-bound cluster ions [RO--H--OR’]- is discussed and it is shown that the relative intensities of RO- and R’O- can be related to the relative gas phase acidities of the alcohols ROH and R’OH. The use of both high energy and low energy collisional methods in confirming the structures of gaseous anions and in differentiating isomeric neutral structures is illustrated. It is shown that, for small anions at least, low energy collisional activation provides more certain structural information than high energy collisional activation.
    07/2011: pages 289-313;
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    ABSTRACT: The product ion spectra of proline-containing peptides are commonly dominated by y(n) ions generated by cleavage at the N-terminal side of proline residues. This proline effect is investigated in the current work by collision-induced dissociation (CID) of protonated Ala-Ala-Xxx-Pro-Ala (Xxx includes Ala, Ser, Leu, Val, Phe, and Trp) in an electrospray/quadrupole/time-of-flight (QqTOF) mass spectrometer and by quantum chemical calculations on protonated Ala-Ala-Ala-Pro-Ala. The CID spectra of all investigated peptides show a dominant y(2) ion (Pro-Ala sequence). Our computational results show that the proline effect mainly arises from the particularly low threshold energy for the amide bond cleavage N-terminal to the proline residue, and from the high proton affinity of the proline-containing C-terminal fragment produced by this cleavage. These theoretical results are qualitatively supported by the experimentally observed y(2)/b(3) abundance ratios for protonated Ala-Ala-Xxx-Pro-Ala (Xxx = Ala, Ser, Leu, Val, Phe, and Trp). In the post-cleavage phase of fragmentation the N-terminal oxazolone fragment with the Ala-Ala-Xxx sequence and Pro-Ala compete for the ionizing proton for these peptides. As the proton affinity of the oxazolone fragment increases, the y(2)/b(3) abundance ratio decreases.
    Journal of the American Society for Mass Spectrometry 06/2011; 22(6):1032-9. DOI:10.1007/s13361-011-0092-1 · 3.19 Impact Factor
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    Alex G Harrison
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    ABSTRACT: The product ion mass spectra resulting from collisional activation of doubly-protonated tryptic-type peptides Ala-Ala-Xaa-Ala-Ala-Ala-Arg have been determined for Xaa = Ala(A), Ser(S), Val(V), Thr(T), Ile(I), Phe(F), Tyr(Y), Sar, Met(M), Trp(W), Pro(P), and Gln(Q). The major fragmentation reaction involves cleavage of the second amide bond (counting from the N-terminus) except for Xaa = Ser and Thr where elimination of H(2)O from the [M + 2H](+2) ion forms the base peak. In general, the extent of cleavage of the second amide bond shows little dependence on the identity of Xaa and little dependence on whether the bond cleavage involves symmetrical bond cleavage to form a y(5)/b(2) ion pair or asymmetrically to form y (5) (+2) and a neutral b(2) species. Notable exceptions to this generalization occur for Xaa equal to Pro or Sar. For Xaa = Pro only cleavage of the second amide bond is observed, consistent with a pronounced proline effect, i.e., cleavage N-terminal to Pro. When Xaa = Sar considerably enhanced cleavage of the second amide bond also is observed, suggesting that at least part of the proline effect relates to the tertiary nature of the amide nitrogen. In the competition between symmetric and asymmetric bond cleavage an attempt to establish a linear free energy correlation in relating ln(y(5)(+2)/y(5)) to PA(H-Xaa-OH) did not lead to a reasonable correlation although the trend of increasing y(5)(+2)/y(5) ratio with increasing proton affinity of H-Xaa-OH was clear. Proline showed a unique behavior in giving a much higher y(5)(+2)/y(5) ratio than any of the other residues studied.
    Journal of the American Society for Mass Spectrometry 05/2011; 22(5):906-11. DOI:10.1007/s13361-011-0091-2 · 3.19 Impact Factor
  • Eric J. Reiner, Alex. G. Harrison, Richard D. Bowen
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    ABSTRACT: The collision-induced dissociation (CID) mass spectra of the [MH]+ ions of a variety of C4 to C6 mono-, di-, and tri-alkyl amines have been determined at 8 keV collision energy and also as a function of collision energy over the range 5–100 eV (laboratory scale). The two major primary fragmentation pathways observed following either mode of activation are (i) production of an alkyl cation by expulsion of ammonia or an alkyl amine, and (ii) formation of a smaller protonated amine by loss of an olefin. In addition, alkane elimination from [MH]+, by a variety of pathways, is a common reaction for protonated dialkyl and trialkyl amines, especially in the 8 keV spectra. However, these alkane elimination reactions are of considerably less importance in the low energy CID spectra because they have high onset energies. The differences observed in the spectra produced by the two methods of activation are discussed in terms of the distributions of internal energies deposited in [MH]+ by the collision process. Keywords: protonated amines, collision-induced fragmentation, energy-resolved mass spectrometry.
    Canadian Journal of Chemistry 02/2011; 67(12):2081-2088. DOI:10.1139/v89-324 · 1.01 Impact Factor
  • Jinsong Ni, Alex. G. Harrison
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    ABSTRACT: The H/D exchange reactions occurring on collision of protonated alkylbenzenes with C2H5OD, CH3OD, and D2O at low collision energies in the quadrupole collision cell of a hybrid tandem mass spectrometer have been studied for 22 alkylbenzenes. The exchange reactions with D2O are much less efficient than the exchange reactions with C2H5OD or CH3OD; this difference is rationalized on energetics grounds. The H/D exchange of the protonated alkylbenzenes with C2H5OD or CH3OD leads, typically, to 40–70% deuterium incorporation in the protonated alkylbenzene and, for most cases, to significant ion signals corresponding to exchange of the added hydrogen and the aromatic hydrogens. Thus, the method is a feasible approach to counting the number of aromatic hydrogens in alkylbenzenes. The extent of exchange decreases with increasing size of the alkyl group in monoalkylbenzenes for reasons that are not readily understood. Keywords: protonated alkylbenzenes, H/D exchange, reactive collisions.
    Canadian Journal of Chemistry 02/2011; 73(11):1779-1784. DOI:10.1139/v95-219 · 1.01 Impact Factor
  • Alex G Harrison
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    ABSTRACT: The fragmentation reactions of a variety of protonated tripeptides containing tyrosine in the three possible positions have been studied by energy-resolved collision-induced dissociation mass spectrometry. The primary fragmentation reactions involve cleavage of the N-terminal and (or) C-terminal amide bond with the relative importance of the two cleavages depending strongly on the identity and position of the amino acid residues in the tripeptide. The results are interpreted in terms of the a1–y mechanism for cleavage of the N-terminal amide bond and the bx–yz mechanism for cleavage of the C-terminal amide bond and, indeed, provide support for these mechanisms. However, it appears likely that, for protonated H-Val-Tyr-Pro-OH, the neutral accompanying formation of the y1 (protonated proline) ion is a cyclic dipeptide (cyclo-Val-Tyr) rather than the oxazolone predicted by the bx–yz mechanism.Key words: tyrosine-containing peptides, fragmentation mechanisms, tandem mass spectrometry.
    Canadian Journal of Chemistry 02/2011; 83(11):1969-1977. DOI:10.1139/v05-206 · 1.01 Impact Factor
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    John V. Headley, Alex. G. Harrison
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    ABSTRACT: The proton transfer chemical ionization mass spectra of eleven C5H10O isomers have been obtained using H3+, N2H+, HCO+, and D3+ as reagent ions. The chemical ionization mass spectra in combination with isotopic labelling and metastable ion studies have made it possible to elucidate the major fragmentation reaction channels of the C5H11O+ ions formed and their dependence on precursor structure. From collision induced dissociation studies nine stable distinct C5H11O+ ion structures have been identified; protonated 3-methylbutanone and protonated 2,2-dimethylpropanal readily interconvert by a pinacolic – retro-pinacolic rearrangement.
    Canadian Journal of Chemistry 02/2011; 63(3):609-618. DOI:10.1139/v85-100 · 1.01 Impact Factor
  • Adam W. McMahon, Alex. G. Harrison
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    ABSTRACT: The unimolecular fragmentation reactions occurring on the metastable ion time scale have been determined for protonated alkyl formates and acetates and for the adducts of CD3+ and C2H5+ with these esters. The protonated esters, R1CO2RH+, fragment to form R+ + R1CO2H or, by internal hydrogen transfer, R1CO2H2+ + (R – H) with the relative yields depending on the identity of R1 and R. Using deuterium labelling it has been shown that the added proton interchanges with hydrogens of the R group prior to fragmentation. The methyl-d3-cationated esters, R1CO2R•CD3+, mostly fragment to give R1CO2CD3, H+ by elimination of (R – H), although R+ formation is observed in a few cases. The ethyl-cationated esters fragment primarily to give R1CO2C2H5•H+ by elimination of (R–H); specifically no elimination of C2H4 is observed. All of the results can be rationalized in terms of a step-wise mechanism of fragmentation involving initial formation of an ion–dipole complex of the alkyl ion R+ with a carbonyl compound. This complex may either fragment to form R+ or may proceed by way of an ion–dipole complex of a protonated carbonyl and an olefin to the protonated species plus (R – H). The results indicate that primary alkyl ions are not formed but rather that rearrangement to the more stable secondary or tertiary structure occurs. There is some evidence that s-butyl cations are not formed, but that these too rearrange to the t-butyl structure.
    Canadian Journal of Chemistry 02/2011; 66(9):2403-2409. DOI:10.1139/v88-378 · 1.01 Impact Factor
  • Nancy L. Bosma, Alex. G. Harrison
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    ABSTRACT: The unimolecular metastable ion and low-energy collision-induced fragmentation reactions of alkyl ammonium ions RnH4−nN+ (n = 2 to 4, R = n-C3H7 and n-C4H9) have been studied and breakdown graphs expressing the % fragment ion abundances as a function of the centre-of-mass collision energy have been established. The dialkyl and trialkyl ammonium ions fragment primarily by olefin ([R − H]) elimination or by formation of the alkyl ion R+ (with rearrangement to the more stable secondary structure). By contrast the tetraalkyl ammonium ions show both elimination of [R − H] and elimination of an alkane, which is C5H12 for the propyl compound C7H16 for the butyl compound. The results are interpreted in terms of competition between heterolytic bond cleavage to form a [R+---NH4−nRn−1] complex and homolytic bond cleavage to form a [Rn−1H4−nN+•---•R] complex. The former complex fragments either to form R+ or undergoes internal proton transfer to form Rn−1H4−nNH+ + [R − H], while the latter complex fragments by alkane elimination resulting from attack of R• on the α-CC bond of the alkyl group. For the propyl ammonium ions plausible potential energy profiles are established which show that the former complex is favoured for n = 2, 3 while the two complexes have similar energies for n = 4.
    Canadian Journal of Chemistry 02/2011; 72(11):2205-2211. DOI:10.1139/v94-281 · 1.01 Impact Factor
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    ABSTRACT: The proton-transfer chemical ionization mass spectra of the C3 to C5 monoalkyl amines as well as a number of di- and tri-alkyl amines have been determined using H3+ and (in some cases) HCO+ as protonating agent. The RNH3+ ions fragment to form alkyl ions R+ and eliminate alkenes to form NH4+. In addition, abundant immonium ions are observed in the CI mass spectra corresponding to elimination of alkane from RNH3+ or to direct alkide ion abstraction from RNH2; these ions serve to characterize the alkyl groups attached to the α-carbon atom of the amine. Although alkane elimination from RNH3+ is the thermochemically favoured reaction, only R+ and NH4+ are formed in decomposition of metastable RNH3+ ions. The potential energy profile for fragmentation of i-C3H7NH3+ has been calculated by abinitio molecular orbital methods. These calculations show that CH4 elimination has a large energy barrier additional to the reaction endothermicity while formation of NH4+ has only a small additional barrier and formation of C3H7+ has no barrier additional to the endothermicity. It is concluded that the immonium ions probably arise primarily by direct alkide ion abstraction reactions.
    Canadian Journal of Chemistry 02/2011; 64(8):1652-1660. DOI:10.1139/v86-272 · 1.01 Impact Factor
  • Roger S. Mercer, Alex G. Harrison
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    ABSTRACT: The collisionally activated dissociation reactions of the C2 to C5 alkoxide ions have been studied for collisons occurring at 8 keV kinetic energy and also over the range 5 to 100 eV kinetic energy. The alkoxide ions fragment by 1,2-elimination of H2 and/or an alkane. Thus, primary alkoxide ions fragment by elimination of H2 only, secondary alkoxide ions show elimination of H2 and alkane molecules, while tertiary alkoxide ions show elimination of alkanes only. In alkane elimination, loss of CH4 is much more facilie than loss of larger alkanes. For secondary alkoxide ions, where more than one elimination reaction occurs, the energy dependence of fragmentation has been explored over the collision energy range 5 to 100 eV. The results are interpreted in terms of a step-wise mechanism involving formation of an anion-carbonyl compound ion-dipole complex, followed by proton abstraction by the H− or alkyl anion leading to the final products. The relative importance of the reaction channels is determined by the relative stabilities of these ion-dipole complexes.
    Canadian Journal of Chemistry 02/2011; 66(11):2947-2953. DOI:10.1139/v88-455 · 1.01 Impact Factor