alpha,omega-diaminoalkanes as models for bases that dicoordinate the proton: An evaluation of the kinetic method for estimating their proton affinities
Z. Wang et al., alpha,omega-diaminoalkanes as models for bases that dicoordinate the proton: An evaluation of the kinetic method for estimating their proton affinities, J PHYS CH A, 103(43), 1999, pp. 8700-8705
The effectiveness of the kinetic method for estimating the proton affinitie
s of bases that di-coordinate the proton is evaluated using alpha,omega-dia
minoalkanes as model bases. The proton affinities of these diamines have pr
eviously been examined using the equilibrium method and critically evaluate
d. Calculations using density functional theory at the B3LYP/6-31++G(d,p) l
evel confirm that protonated alpha,omega-diaminoalkanes have cyclic structu
res with the proton covalently bound to one of the amino nitrogen atoms and
hydrogen-bended to the other. Furthermore, this cyclic structure persists
in the protonated heterodimer ion between an alpha,omega-diaminoalkane and
ammonia (the model reference base); binding of the two bases takes place vi
a a second hydrogen bond between the RNH3+ and ammonia. Measuring the proto
n affinities under several collision energies and extrapolating to zero col
lision energy yields proton affinities that are smaller than the reference
values by -2.8 kcal/mol, on average. Application of the Fenselau correction
gives proton affinities that differ from the reference values by +/-1.0 kc
al/mol. These results indicate that the kinetic method is effective for est
imating the proton affinities of molecules that tend to have more than one
potential protonation site. Application of this method is particularly suit
ed to biological molecules, such as peptides, where application of the equi
librium method is impossible due to low sample volatility.