Raman spectroscopy of vibrational and rotational relaxation of acetonitrile molecules dissolved in ionic liquids

Vibrational and rotational relaxation processes of acetonitrile (AN) molecules as molecular probes dissolved in the imidazolium-based room-temperature ionic liquid (RTIL) 1-decyl-3-methyl-imidazolium bromide have been studied by Raman spectroscopy technique (the band shape analysis using different polarizations of the incident and scattered light). It has been shown that the vibrational relaxation processes are quickened in the RTIL surrounding. The corresponding vibrational correlation times decrease from about 3 ps in the neat AN to 1.5-2 ps in AN solution in RTIL (10 and 20 wt % of AN). The effect of RTIL environment on the rotational dynamics of AN, namely, on the tumbling motion of dipole moments, is rather small. However, the slow-down of AN reorientations in RTIL was distinguished, namely, the rotational correlation times increase from 1.0-1.5 ps (neat AN) to 1.6-1.9 ps (AN/ RTIL solutions). The results have been discussed in terms of various molecular and media contributions to these processes (H-bonding, highly viscous media effect, intermode exchange of vibrational energy, etc). The demolition of structural effects (short-range order and mesoscopic structures in the nano-scale) in 1-decyl-3-methyl-imidazolium bromide by the mutual penetration of AN and RTIL species is suspected.