TY - JOUR
T1 - Anisotropic displacement parameters for H atoms using an ONIOM approach
AU - Whitten, A.E.
AU - Spackman, Mark
PY - 2006
Y1 - 2006
N2 - X-ray diffraction data cannot provide anisotropic displacement parameters ( ADPs) for H atoms, a major outstanding problem in charge-density analysis of molecular crystals. Although neutron diffraction experiments are the preferred source of this information, for a variety of reasons they are possible only for a minority of materials of interest. To date, approximate procedures combine rigid-body analysis of the molecular heavy-atom skeleton, based on ADPs derived from the X-ray data, with estimates of internal motion provided by spectroscopic data, analyses of neutron diffraction data on related compounds, or ab initio calculations on isolated molecules. Building on these efforts, an improved methodology is presented, incorporating information on internal vibrational motion from ab initio cluster calculations using the ONIOM approach implemented in GAUSSIAN03. The method is tested by comparing model H-atom ADPs with reference values, largely from neutron diffraction experiments, for a variety of molecular crystals: benzene, 1-methyluracil, alpha-glycine, xylitol and 2-methyl-4-nitroaniline. The results are impressive and, as the method is based on widely available software, and is in principle widely applicable, it offers considerable promise in future charge-density studies of molecular crystals.
AB - X-ray diffraction data cannot provide anisotropic displacement parameters ( ADPs) for H atoms, a major outstanding problem in charge-density analysis of molecular crystals. Although neutron diffraction experiments are the preferred source of this information, for a variety of reasons they are possible only for a minority of materials of interest. To date, approximate procedures combine rigid-body analysis of the molecular heavy-atom skeleton, based on ADPs derived from the X-ray data, with estimates of internal motion provided by spectroscopic data, analyses of neutron diffraction data on related compounds, or ab initio calculations on isolated molecules. Building on these efforts, an improved methodology is presented, incorporating information on internal vibrational motion from ab initio cluster calculations using the ONIOM approach implemented in GAUSSIAN03. The method is tested by comparing model H-atom ADPs with reference values, largely from neutron diffraction experiments, for a variety of molecular crystals: benzene, 1-methyluracil, alpha-glycine, xylitol and 2-methyl-4-nitroaniline. The results are impressive and, as the method is based on widely available software, and is in principle widely applicable, it offers considerable promise in future charge-density studies of molecular crystals.
U2 - 10.1107/S0108768106020787
DO - 10.1107/S0108768106020787
M3 - Article
C2 - 16983168
SN - 0108-7681
VL - B62
SP - 875
EP - 888
JO - Acta Crystallographica Section B: Structural Science
JF - Acta Crystallographica Section B: Structural Science
IS - 5
ER -