Near infrared spectral investigations of hydration of 18-crown-6 in HDO-D2O solutions at different temperatures

Near infrared spectra of solution of 6.4 M HDO in D2O have been obtained at 15, 25, 30, and 35 degreesC. It was observed that the bands of HDO in D2O occur at 1416, 1525, 1556, and 1666 nm, which are in good agreement with the similar data reported earlier by Worley and Klotz. The calculations of enthalpy change for hydrogen bond fortnation (DeltaHdegrees) yielded the value of -2.5 +/- 0.4 kcal mol(-1), which is in excellent agreement with the value reported by Walrafen. Similar spectra were recorded for 1 and 2 m 18-crown-6 (18C6) dissolved in the solution of HDO in D2O at different temperatures. The band positions remain unchanged, however, the variation of intensity as a function of concentration of 18C6 and temperature clearly indicate that 18C6 acts as a structure making solute. The structural temperature and DeltaHdegrees values have been obtained for the 18C6 solutions. These results are explained on the basis of the stabilization of 18C6 in the D-3d conformation through hydrogen bonding of HDO molecules [doubly hydrogen bonded, i.e. bridging, and singly hydrogen bonded] to the oxygen atoms of 18C6 molecules. Slightly different DeltaHdegrees values obtained can be attributed to clathrate like structure at 1 m 18C6 concentration while at 2 m 18C6 concentration it is postulated that the hydrophobic interactions are contributing additionally. (C) 2002 Elsevier Science B.V. All rights reserved.