Orientational relaxation dynamics in aqueous ionic solution: Polarization-selective two-dimensional infrared study of angular jump-exchange dynamics in aqueous 6M NaClO4

Abstract

The dynamics of hydrogen bond (H-bond) formation and dissociation depend intimately on the dynamics of water rotation. We have used polarization resolved ultrafast two-dimensional infrared (2DIR) spectroscopy to investigate the rotational dynamics of deuterated hydroxyl groups (OD) in a solution of 6M NaClO(4) dissolved in isotopically mixed water. Aqueous 6M NaClO(4) has two peaks in the OD stretching region, one associated with hydroxyl groups that donate a H-bond to another water molecule (OD(W)) and one associated with hydroxyl groups that donate a H-bond to a perchlorate anion (OD(P)). Two-dimensional IR spectroscopy temporally resolves the equilibrium inter conversion of these spectrally distinct H-bond configurations, while polarization-selective 2DIR allows us to access the orientational motions associated with this chemical exchange. We have developed a general jump-exchange kinetic theory to model angular jumps associated with chemical exchange events. We use this to model polarization-selective 2DIR spectra and pump-probe anisotropy measurements. We determine the H-bond exchange induced jump angle to be 49 ± 5° and the H-bond exchange rate to be 6 ± 1 ps. Additionally, the separation of the 2DIR signal into contributions that have or have not undergone H-bond exchange allows us to directly determine the orientational dynamics of the OD(W) and the OD(P) configurations without contributions from the exchanged population. This proves to be important because the orientational relaxation dynamics of the populations that have undergone a H-bond exchange differ significantly from the populations that remain in one H-bond configuration. We have determined the slow orientational relaxation time constant to be 6.0 ± 1 ps for the OD(W) configuration and 8.3 ± 1 ps for the OD(P) configuration. We conclude from these measurements that the orientational dynamics of hydroxyl groups in distinct H-bond configurations do differ, but not significantly.