Reorientation and vibrational energy relaxation of pseudohalide ions confined in reverse micelle water pools

Abstract

Reorientation and vibrational energy relaxation times have been measured by ultrafast transient polarizationIR spectroscopy for the antisymmetric stretching band in the 2000–2200 cm−1 region of pseudohalide ions, N 3 − , NCO − , and NCS − , confined in nanosize water pools of reverse micelles (RMs). The RMs are composed of nonionic nonylphenyl poly-oxyethylene surfactant in cyclohexane. Vibrational energy relaxation times (T 1 ) of the NCO − and NCS − ions are about three times longer in small RMs compared to bulk water, similar to our earlier results on the N 3 − ion [J. Chem. Phys. 118, 7074 (2003)]. The longer T 1 times are attributed to the reduced interaction between the ions and water molecules due to confinement effects and hydration of the surfactant headgroups by water molecules. Reorientation times (T R ) of the N 3 − and NCS − ions are found to be more than one order of magnitude longer in small RMs than in bulk H 2 O . The observed longer T R times support the notion that water molecules inside RMs have very restricted mobility. As the water pool becomes larger with increasing water content, the interior water is less affected by the interface and bulk water properties reappear, leading to shorter T 1 and T R times.