The weak hydrogen bonding structures of protonated triethylamine in aqueous solution were investigated by Raman spectroscopy as a function of hydrostatic pressure. The formation of a weak hydrogen bond was directly evidenced by a low-frequency shift of the hydrogen-bonded C-H stretches in the protonated triethylamine moiety. For pure triethylamine, an increase in pressure leads to a blue frequency shift of the C-H bands. Nevertheless, the high pressure study of the C-H bands for a dilute protonated triethylamine/D2O solution yielded an unusual nonmonotonic pressure dependence. This result indicates charge-enhanced C-H center dot center dot center dot O hydrogen-bond formation at high pressure. (c) 2006 Elsevier B.V. All rights reserved.