Water-induced folding of 1,7-diammoniumheptane.

Effects of hydration on the gaseous structures of diprotonated 1,7-diaminoheptane and protonated heptylamine are investigated by infrared photodissociation (IRPD) spectroscopy and computational chemistry. IRPD spectra in the hydrogen bond stretching region (2800-3900 cm(-1)) indicate that 1,7-diammoniumheptane is linear and that hydration occurs predominantly by alternate solvation of the two protonated amine groups for clusters with up to 10 water molecules. The relative intensities of bonded versus free hydroxyl (OH) stretches are greater in the spectra of 1,7-diammoniumheptane with more than 12 water molecules attached than the corresponding reference spectra of heptylammonium. This indicates that in the larger clusters, 1,7-diammoniumheptane adopts a more folded conformation in which the two protonated amine groups are solvated by a single water nanodrop. These results are supported by molecular dynamics simulations which show more hydrogen bonds in representative folded structures of hydrated 1,7-diammoniumheptane versus those with linear structures. These results indicate that the increase in Coulomb energy as a result of bringing the two positive charges closer together in the folded structures is compensated for by the additional hydrogen bonds that are possible when a single nanodrop solvates both protonated amine groups.