Vibrational spectral diffusion of azide in water.
Vibrational spectral diffusion denotes the time-dependent fluctuations of a solute's vibrational frequencies due to local environmental dynamics. Vibrational line shapes are weakly sensitive to spectral diffusion, whereas three-pulse vibrational echoes are much more sensitive. We report here on theoretical studies of spectral diffusion of the asymmetric stretch of the azide anion in heavy water. We run a classical molecular dynamics simulation of rigid azide in rigid water, and at every time step we calculate the azide's anharmonic asymmetric stretch frequency using an optimized quantum mechanics/molecular mechanics method developed earlier. This generates a frequency trajectory, which we use to calculate the absorption line shape and integrated three-pulse echo intensity. Our results for both the line width and the integrated echo intensity are in excellent agreement with experiment. Our calculated frequency time-correlation function is in excellent agreement with experiment for long times (greater than 250 fs) but differs considerably from experiment at short times; our theoretical correlation function has a very pronounced oscillation, presumably due to intermolecular azide-water hydrogen-bond stretching dynamics.
Li
Shuzhou
S
Theoretical Chemistry Institute and Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706, USA.
Schmidt
J R
JR
Piryatinski
A
A
Lawrence
C P
CP
Skinner
J L
JL
D000838:Anions / Q000737:chemistry; D001386:Azides / Q000737:chemistry*; D004058:Diffusion; D006860:Hydrogen Bonding; D012996:Solutions; D017550:Spectroscopy, Fourier Transform Infrared; D014732:Vibration; D014867:Water
D000838:Anions; D001386:Azides; D012996:Solutions; D014867:Water