High-Pressure Mass Spectrometric (HPMS) experiments have been
carried out to probe the details of the
double minimum potential energy surface for gas-phase SN2
reactions. The well depths and entropy changes
associated
with the formation of entrance and exit channel electrostatic complexes
for the chloride and bromide adducts of
methyl, ethyl, isopropyl, and tert-butyl chlorides and
bromides have been determined from the temperature
dependence
of the equilibrium constants for adduct formation. In the cases of
“symmetric” complexes associated with identity
SN2 reactions, there is an increase in well depth as the
size and, therefore, polarizability of the alkyl group
increases.
Concomitant with this is an increase in the magnitude of the
negative entropy change for complex formation which
is the result of an increase in the frequency of the intermolecular
mode(s) of the complex arising from the increased
bond strength. The data for the unsymmetrical adducts for the
non-identity SN2 reactions show the same pattern
of
increasing well depth with increasing alkyl group size with the
chloride adducts of alkyl bromides being more strongly
bound than the bromide adducts of the corresponding alkyl chlorides.
Enthalpies and entropies associated with
transition state formation are determined from the temperature
dependence of the rate constant for the net halide
displacement reaction. These data show that the transition state
for the reaction of chloride ion with alkyl bromides
may lie below (CH3Br), near
(C2H5Br), or above
(i-C3H7Br,
t-C4H9Br) the energy of
separated reactants. These
three situations exhibit different changes in rate constant with
increasing temperature. In addition, the lifetime of
the transient, chemically activated intermediate formed between
chloride ion and methyl chloride has been determined
from the pressure dependence of the rate constant for formation of the
observable, collisionally stabilized electrostatic
adduct. The lifetime thus obtained is in excellent agreement with
trajectory calculations performed by Hase and
co-workers.
Travelling wave ion mobility mass spectrometry (TWIM-MS) with post-TWIM and pre-TWIM collision-induced dissociation (CID) experiments were used to form, separate and characterize protomers sampled directly from solutions or generated in the gas phase via CID. When in solution equilibria, these species were transferred to the gas phase via electrospray ionization, and then separated by TWIM-MS. CID performed after TWIM separation (post-TWIM) allowed the characterization of both protomers via structurally diagnostic fragments. Protonated aniline (1) sampled from solution was found to be constituted of a ca. 5:1 mixture of two gaseous protomers, that is, the N-protonated (1a) and ring protonated (1b) molecules, respectively. When dissociated, 1a nearly exclusively loses NH(3) , whereas 1b displays a much diverse set of fragments. When formed via CID, varying populations of 1a and 1b were detected. Two co-existing protomers of two isomeric porphyrins were also separated and characterized via post-TWIM CID. A deprotonated porphyrin sampled from a basic methanolic solution was found to be constituted predominantly of the protomer arising from deprotonation at the carboxyl group, which dissociates promptly by CO(2) loss, but a CID-resistant protomer arising from deprotonation at a porphyrinic ring NH was also detected and characterized. The doubly deprotonated porphyrin was found to be constituted predominantly of a single protomer arising from deprotonation of two carboxyl groups.
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