II.Hydrogen bond switching dynamics:Multidimensional infrared spectroscopy of water.

摘要

We use multidimensional infrared spectroscopy of the OH stretch of HOD in D_2O to measure the interconversion of different hydrogen bonding environments.The OH stretching frequency distinguishes hydrogen bonded (HB) and non-hydrogen-bonded (NHB) configurations by their absorption on the low (red) and high (blue) sides of the line shape.Measured asymmetries in the two dimensional infrared OH line shapes are manifestations of the fundamentally different spectral relaxations of HB and NHB.HB oscillators exhibit coherent oscillations within the hydrogen-bonded free energy well before undergoing activated baffler crossing,resulting in the exchange of hydrogen bonded partners.Conversely,NHB oscillators rapidly return to HB frequencies within 150 fs.These results support a picture where NHB configurations are only visited transiently during large fluctuations about a hydrogen bond or during the switching of hydrogen bonding partners.The results are not consistent with the presence of entropically stabilized dangling hydrogen bonds or a conceptual picture of water as a mixture of environments with varying hydrogen bond strength separated by barriers >kT.