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 3-pulse vibrational echoes are much more sensitive. Theor. studies are reported of spectral diffusion of the asym. stretch of N3- in D2O. A classical mol. dynamics simulation is run of rigid N3- in rigid H2O, and at every time step the N3-'s anharmonic asym. stretch frequency was calcd. using an optimized quantum mechanics/mol. mechanics method developed earlier. This generates a frequency trajectory, which was used to calc. the absorption line shape and integrated 3-pulse echo intensity. The results for both the line width and the integrated echo intensity are in agreement with expt. The calcd. frequency time-correlation function is in agreement with expt. for long times (>250 fs) but differs considerably from expt. at short times; the theor. correlation function has a very pronounced oscillation, presumably due to intermol. N3--H2O H bond stretching dynamics. [on SciFinder (R)]