How Does a Dication Lose a Proton?

A detailed study of the mechanism by which a proton is lost from a dication reveals that such processes are more complicated than is often assumed. In many cases, a deprotonation reaction is best viewed as a two-stage process: Initially, the departing unit is better described as a hydrogen atom than as a proton and only later, at some point further along the decomposition pathway, does a spontaneous electron transfer take place to form the eventual products. Consequently, it is found that, contrary to conventional wisdom, restricted Hartree-Fock (RHF) theory does not necessarily offer a satisfactory theoretical treatment of such fragmentations. The circumstances under which it is appropriate to use R H F or UHF procedures (and the M0ller-Plesset perturbation theories based on these) are examined in light of a recent model for dication dissociation, and it is found that the A parameter of that model is a useful aid in choosing the theoretical formalism appropriate to a given dication. The chemistry of gas-phase dications has received considerable attention in recent years, from both theoreticians and experimentalists.' Such species are usually thermodynamically unstable with respect to dissociation into two monocations, but significant kinetic stability may result if sufficiently high barriers impede fragmentation. For this reason, the accurate assessment of such barriers is of paramount importance in the theoretical investigations of dications. One ubiquitous fragmentation route for dications is proton loss (eq 1). The observation that the transition structure for such AH2+ A+ + H" (1) reactions often occurs very late along the reaction path has recently been rationalized3 in terms of a in which the potential curve for the fragmentation is viewed as arising from an avoided crossing between an ion-ion repulsive state, which correlates with A+ + H+, and an ion-induced-dipole attractive state, which correlates with A2+ + H. This model may be used, for example, to show that if the second ionization energy of A is a little larger than 13.6 eV (the ionization energy of H) , a late-transition structure for proton loss may be anticipated. Further inspection of this model can give considerable insight into the dissociation process and reveals certain features that have previously been overlooked. In particular, if AH2+ is a closed-shell singlet species, it is conventionally assumedS that the proton loss may be treated within the framework of restricted (RHF), as opposed to unrestricted (UHF), Hartree-Fock theory. However, as we show in this paper, the choice between these alternatives is less straightforward than is normally realized, and indeed, for late-transition structures, RHF ought not be used. ( I ) For recent reviews, see: (a) Koch, W.; Maquin, F.; Stahl, D.; Schwarz, H. Chimia 1985, 39, 376. (b) Koch, W.; Schwarz, H. In StructurelReactiuity and Thermochemistry of Ions; Ausloos, P., Lias, S. G., Eds.; NATO AS1 Series; Reidel: Dordrecht, The Netherlands, 1987. (2) Dorman, F. H.; Morrison, J. D. J . Chem. Phys. 1961, 35, 575. (3) Gill, P. M. W.; Radom, L. Chem. Phys. Lett. 1987, 136, 294. (4) Gill, P. M. W.; Radom, L. Chem. Phys. Lett., in press. ( 5 ) For example: Schleyer, P. v. R. Adu. Mass Spectrom. 1985, 287. Table I. Calculated Bond Lengths (A) and Total Energies (Hartrees) of the Equilibrium and Transition Structures of AIH2+ and Corresponding Barriers for Deprotonation (kJ mol-')o r, E, rm Em barrier6 RHF 1.614 -241.56264 3.009 -241.511 15 135 RMP2 1.641 -241.595 18 3.118 -241.54900 121 RMP3' 1.653 -241.59439 3.169 -241.54543 129 RMP4' 1.660 -241.59729 3.142 -241.54939 126 RCISD"" 1.669 -241.59980 3.206 -241.55029 130 TCSCF' 1.671 -241.58999 3.415 -241.53045 156 UHF 1.645 -241.56284 3.589 -241.52068 111 UMP2 1.641 -241.595 18 3.309 -241.53953 146 UMP3' 1.653 -241.59439 3.276 -241.53397 159 UMP4' 1.660 -241.59729 3.250 -241.53572 162 UCISD'sd 1.669 -241.599 80 3.206 -241.550 23 130 "6-31G* basis set used throughout. bETs E,. 'Frozen-core approximation used. "Corresponds to full CI for the valence electrons. '4u and 5a molecular orbitals were active.