Spectroscopic, energetic and structural information obtained
by DFT and G3-type computational studies demonstrates that charged
proton donors can form moderately strong hydrogen bonds to simple
alkyl radicals. The presence of these bonds stabilizes the adducts
and modifies their structure, and gives rise to pronounced shifts of
IR stretching frequencies and to increased absorption intensities. The
hydrogen bond acceptor properties of alkyl radicals equal those of many
conventional acceptors, e.g., the bond length changes and IR red-shifts
suggest that tert-butyl radicals are slightly better acceptors than
formaldehyde molecules, while propyl radicals are as good as H2O. The
hydrogen bond strength appears to depend on the proton affinity of the
proton donor and on the ionization energy of the acceptor alkyl radical,
not on the donor-acceptor proton affinity difference, reflecting that the
charge-transfer aspects of hydrogen bonding are particularly conspicuous
when the acceptor polarity and basicity is low.
Udgivelsesdato: 2 juni 2009