Hydrogen Bonding between Water and Tetrahydrofuran Relevant to Clathrate Formation.

Tetrahydrofuran (THF) is well-known as a clathrate former as well as a promoter for gas hydrate formation. This work examines interactions between water and tetrahydrofuran via the effect on water's vibrational spectrum. Due to water's large oscillator strength in the hydrogen-bonded region, interactions are diagnosed by isolating small clusters in a transparent medium (carbon tetrachloride in this study). A weak THF/water hydrogen bond is reflected by a 3450 cm(-1) OH-donor vibration (blue shifted from the water/water hydrogen bond) and a 3685 cm(-1) nonbonded OH stretch (blue shifted 22 cm(-1) from the decoupled OH stretch in this medium). Increasing the THF concentration results in another 20 cm(-1) blue shift of the OH-donor stretch. Additional THF does not complex with free water but rather joins with existing THF/water structures to form a cluster enriched in THF. These results complement previous work examining THF vibrations in clathrate hydrates. Together, they generate a picture in which water mediates between THF pairs--mediation that affects vibrational frequencies of both species. In addition to a frequency shift, water's hydrogen-bonded resonance gains oscillator strength due to its mediating configuration.