XAFS Debye-Waller factors in aqueous Cr+3 from molecular dynamics

Controversy exists on whether the second hydration shell of the aqueous chr omium +3 cation is observable by XAFS. The problem is aggravated by strong first shell multiple scattering contributions competing with the second hyd ration shell signal. By finding ab initio values for nearly all free parame ters in the theory, we greatly reduce the number of parameters to be fit, t hus allowing an unambiguous resolution of this controversy. Quantum chemist ry calculations yielded a parameterized force field model which was used in classical molecular dynamics simulations to calculate all the multiple sca ttering Debye-Waller factors. The self-consistent FEFF8 code fixes Eo to wi thin 1 eV. The predicted spectrum is in good agreement with experiment. Fit ted distances for the first and second hydration shell are 2.0008 +/- 0.006 8 Angstrom and 3.914 +/- 0.096 Angstrom, respectively. The second shell is shown to be responsible for about 1/3 of the XAFS Fourier transformed signa l at the position of the second shell.