Professor Terry B. McMahon,
Department of Chemistry
University of Waterloo
B.Sc.(Honours), University of Alberta, 1969 Ph.D., California Institute of Technology, 1973 PDF, University of Alberta (Paul Kebarle), 1973-74
Research in the McMahon Laboratories
My research program is directed toward the investigation of structure, energetics and reaction dynamics of gaseous ions. Most recently, the majority of our work has focussed on cluster ions. To carry out this research we use high pressure mass spectrometry (HPMS) and Fourier transform ion cyclotron resonance (FTICR) spectrometers. HPMS is uniquely suited to the investigation of energetics of gaseous ion-molecule equilibria, particularly equilibria associated with ion-molecule clustering and exchange reactions. Using this technique we have amassed a considerable database for the energetics as well as kinetics of a wide variety of cluster species. Our FTICR spectrometer is equipped with a high pressure ion source also. Using this instrument we can generate cluster species, transfer them to the FTICR cell under UHV conditions and then study their subsequent bimolecular or unimolecular chemistry. It was with this technique that we were the first to demonstrate that ambient black body radiation can act to induce unimolecular dissociation of weakly bound cluster species. This technique has since been widely adopted by others, particularly as a means of probing structure and energetics in large ions derived from biopolymers. We can also create ions within the ICR cell by electron bombardment. Under very low pressure conditions nascent ion-molecule complexes are found to stabilize by emission of infrared photons, a process termed radiative association. Using a combination of our experimentally determined rate constants for complex formation, computational chemistry and statistical modeling we can obtain binding energies for the ion-molecule complexes.
I collaborate with four independent research groups in France, either at Ecole Polytechnique (EP) or Université Paris XI, Orsay (O). With Henri Audier (EP) we perform both FTICR and HPMS experiments at Waterloo to complement experiments and calculations done at EP in the areas of solvated distonic ions and three-body ion-neutral complexes. With Guy Bouchoux (EP) we perform HPMS experiments on the basicities of bifunctional molecules in conjunction his ab initio calculations. With Gilles Ohanessian (O) we perform HPMS experiments at Waterloo to complement his ab initio calculations on metal ion-ligand interactions. With the group of Christophe Jouvet (Laboratorie de Photophysique Moleculaire, Orsay), I have recently initiated a new collaboration on synchrotron experiments at LURE using threshold ion/electron coincidence measurements to probe the appearance energies of ions derived from van der Waals clusters. Using this method, we have recently determined the transition state energy for a catalysed isomerization from a normal to a distonic radical cation. This is an experimental ‘first'. We have also determined thresholds for appearance of a number ions derived from significant rearrangement processes within ionised van der Waals clusters. In addition, with Professor Manfred Schlosser of the University of Lausanne (Switzerland) we have begun an investigation of the energetics of gaseous benzyne anions with strongly electron withdrawing substituents.
Recent Publications(2001-present) (full publication list)
145. T.D. Fridgen and T.B. McMahon, Enthalpy Barriers for Assymetric SN2 Alkyl Cation Transfer Reactions Between Neutral and Protonated Alcohols, J. Phys. Chem. A. (submitted for publication, August 2002, #JP026818J).
B. Bogdanov and T.B. McMahon, Structures, Thermochemistry, and Infrared
Spectra of Chloride Ion-Fluorinated Acetone Complexes and Neutral Fluorinated
Acetones in the Gas Phase: Experiment and Theory, Int. J. Mass Spectrom. 219,
T.D. Fridgen and T.B. McMahon, Experimental Determination of
Activation Energies for Gas Phase Ethyl and n-Propyl Transfer Reactions. J.
Phys. Chem. A. (JP013874W, ASAP March 30th, 2002, set for publication Oct.
140. G. van der Rest, J. Chamot-Rooke, P. Mourgues, T.B. McMahon and H.E. Audier, Ter-Body Intermediates in the Gas Phase: Reaction of Ionized Enols with tert-Butanol. J. Am. Soc. Mass Spectrom. 12, 938-947 (2001).
139. P. Mourgues, J. Chamot-Rooke, G. van der Rest, H. Nedev, H.E. Audier and T.B. McMahon, Catalyzed Keto-Enol Tautomerism of Ionized Acetone: An FTICR Study of Proton Transport Isomerization. Int. J. Mass Spectrom., 210/211, 429-446 (2001).
138. T.D. Fridgen, J.D. Keller and T.B. McMahon, Direct Experimental Determination of Energy Barriers for Methyl Cation Transfer in the Reactions of Methanol with Protonated Methanol, Protonated Acetonitrile and Protonated Acetaldehyde: A Low Pressure FTICR Study. J. Phys. Chem. A. 105, 3816-3824 (2001).
137. B. Bogdanov, H.J.S. Lee and T.B. McMahon, The Influence of Fluorine Substitution on the Structure and Thermochemistry of Chloride Ion – Ether Complexes in the Gas Phase, Int. J. Mass Spectrom., 210/211, 387-402 (2001).
136. Reaction of Protonated Dimethyl Ether with Dimethyl Ether: Temperature and Isotope Effect on the Side Reaction Producing Trimethyl Oxonium Ion and Methanol, T.D. Fridgen and T.B. McMahon, J. Am. Chem. Soc. 123, 3980-3985 (2001).
135. A Fourier Transform Ion Cyclotron Resonance Mass Spectrometric Study of the Temperature and Isotope Effects on the Kinetics of Low-Pressure Association Reactions of Protonated Dimethyl Ether with Dimethyl Ether, T.D. Fridgen and T.B. McMahon, J. Phys. Chem. A., 105, 1011-1019 (2001)