PNNL

Abstract

Weak C-H…O hydrogen bonding has been recognized to play a major role in biological molecular structures and functions. A newly developed low-temperature photoelectron spectroscopy apparatus is used here to study the C-H…O hydrogen bonding between unactivated alkanes and the carboxylate functional group. We observed that gaseous linear carboxylates, CH3(CH2)nCO2-, assume folded structures at low temperatures due to weak C-H…O hydrogen bonding between the terminal CH3 and CO2- groups for n=5. Temperature-dependent studies showed that the folding transition depends on both the temperature and the aliphatic chain length. Theoretical calculations revealed that for n = 3-8, the folded conformations are more stable than the linear structures, but C-H…O hydrogen bonding only forms for species with n=5 due to steric constraint in the smaller species. One C-H…O hydrogen-bond is formed in the n = 5 and 6 species, whereas two C-H…O hydrogen-bonds are formed for n = 7 and 8. Comparison of the photoelectron spectral shifts for the folded relative to the linear conformations yielded lower limits for the strength of the C-H…O hydrogen-bonds in CH3(CH2)nCO2-, ranging from 1.2 kcal/mol for n = 5 to 4.4 kcal/mol for n = 8.