alpha,omega-diaminoalkanes as models for bases that dicoordinate the proton: An evaluation of the kinetic method for estimating their proton affinities

Citation
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
Citations number
50
Categorie Soggetti
Physical Chemistry/Chemical Physics
Journal title
JOURNAL OF PHYSICAL CHEMISTRY A
ISSN journal
10895639 → ACNP
Volume
103
Issue
43
Year of publication
1999
Pages
8700 - 8705
Database
ISI
SICI code
1089-5639(19991028)103:43<8700:AAMFBT>2.0.ZU;2-S
Abstract
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.