Thermoneutral isotope-exchange reactions of cations in the gas phase

Abstract

364 1 but the differences are greatly attenuated. This observation might be attributed to a substantial amount of diffuse character in these negative ion states which is greatly diminished in condensed phase^.'^,'^ Summary. The key findings of the present investigation are (a) that significant through-space methyl-?r interactions exist in the +' * and G4* negative ion states of 4 and 5, (b) the this interaction in fi4*, and probably also in &*, primarily involves the hydrogen 1s (T) rather than the carbon p (~) orbital of the methyl group, and (c) that conformations with interior out-of-plane methyl hydrogens make a significant (but as yet unspecified) contribution to the distribution of conformations in 4. Similar considerations also apply to 5. We believe that the electron-transmission technique may prove to be quite powerful in probing certain conformational questions. This will serve to greatly expand its already recognized importance in providing electron affinities and attachment energies of complex molecules. Acknowledgment.We thank the National Science Foundation (Grant No. CHE77-14930) for support of this work and the Gillette Research Foundation for a fellowship to J.C.G. must be directed toward Cz of the diene system (Le., must be an interior methyl H). This would result in a through-space interaction of the C3 P(T) orbital and, more importantly, the C2 p(?r) orbital with a hydrogen 1s orbital (or orbitals) which would account for the observed destabilization of +4*. The major interaction is illustrated in Figure 6 for conformation 4b, although there are clearly an infinite number of conformations with interior out-of-plane methyl hydrogens which would account for this observation. (The occurrence of such conformations can be rationalized on the basis of an interaction between the eclipsed methyl hydrogen and H2 in conformation 4a.) A similar through-space interaction accounts for the relative stabilization of J.s* in 4 and 5 compared to 2 and 3, respectively. Indeed the agreement of the relative trans-cis shifts of both q3* and q4* with the foregoing through-space interaction is an important factor in support of this analysis.The long-wavelength electronic-absorption band for the negative ions of 1, 3, and 4, prepared by y-irradiation in methyltetrahydrofuran glasses at -196 "C, has been attributed to a transition between IC3* and +4*.14 The energies of these transitions (2.18, 2.16, and 2.20 eV, respectively) are in the same relative order as are the values of Abstract Rate constants have been measured for reactions of the type AD2+ + MH -MD + ADH+, where AD: is CD' CND' , CD3CDOD+, (CD3COCD3)D+, or (C2D5)20D+ and the MH molecules are alcohols, acids, mercaptans, H2S, AsH3, PH3, or aromatic molecules. Rate constants are also presented for the reactions ArHD+ + D20 -ArDD+ + HDO, where k H D + is a deuteronated aromatic molecule and ArDD+ is the same species with a D atom incorporated on the ring. In all but two cases, the competing deuteron transfer is sufficiently endothermic that it cannot be observ...

Coexisting stable conformations of gaseous protein ions.

“…This H/2H exchange requires a low difference (<=25 kcal/mol) between the proton affinity (PA) of the reacting molecule and that of water (31,32). For amino acid side chains, aryl and 0-linked H atoms have PA values that are 20-30 kcal/mol above that of water, while N-linked H atoms have PA values >40 kcal/mol higher (33); the latter sites should be more favorable for H/2H exchange in solution.…”

Protonated Heme

“…H/D exchange: H/D-exchange reactions are a useful probe of ionic structures and provide a valuable tool for the study of both positively and negatively charged species. [24] The efficiencies for the isotope-exchange reaction are found to correlate with the energetics of formation of the association complex between the ion and the exchange reagent, at least within a wide range of relatively simple species. [25] Renewed interest in H/D-exchange reactions is due to their use as a tool to provide information on highly complex species, such as protonated peptides and proteins.…”

H/D exchange kinetics: Experimental evidence for formation of different b fragment ion conformers/isomers during the gas-phase peptide sequencing

“…where (Im i /⌺Im i ) is the normalized intensity of the precursor ion, k is the reaction rate constant, [M] is the concentration of neutral species (ND 3 ), t is the reaction time, and C is a constant [46]. The normalized ion intensity value for each species (e.g., …”