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

Controversy exists on whether the second hydration shell of the aqueous chromium +3 cation is observable by XAFS. The problem is aggravated by strong first shell multiple scattering contributions competing with the second hydration shell signal. By finding ab initio values for nearly all free parameters in the theory, we greatly reduce the number of parameters to be fit, thus allowing an unambiguous resolution of this controversy. Quantum chemistry calculations yielded a parameterized force field model which was used in classical molecular dynamics simulations to calculate all the multiple scattering Debye-Waller factors. The self-consistent FEFF8 code fixes Eo to within 1 eV. The predicted spectrum is in good agreement with experiment. Fitted distances for the first and second hydration shell are 2.0008 +/- 0.0068 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 signal at the position of the second shell.