Neutron thermal motion correction

Hg(CN)2 in the solid state has a structure (I42d neutron diffraction), completely different from that of Cd(CN)2 Almost-linear molecules (r(Hg—C) 201.9, r(C—N) 116.0pm (corrected for thermal motion) a(C—Hg—C) 175.0°) are arranged such that four secondary bonds N" Hg (274.2 pm) yield the often-occurring 2 + 4 coordination around Hg.103 Analysis of the 199Hg MAS NMR spectrum of Hg(CN)2 has yielded the chemical shift and shielding tensor parameters.104... 

In neutron diffraction structure analyses, the correction for thermal motion is much more complex for hydrogen atoms than for nonbydrogen atoms. It has long been recognized that vibrational thermal motion causes an apparent reduction in interatomic distances relative to those for the atoms at rest, and the methods for thermal motion analysis and geometry correction are well developed [190—192]. 

Fig. 3.2. Thermal ellipsoids (at 99% probability) for 1,2,4-triazole by neutron diffraction at 15 K illustrating the relative thermal motion of hydrogen and nonhydrogen atoms. That of the hydrogen bonded H(l) is only slightly less than that of H(3) and H(5), and the corrections of the X-H bond lengths are +0.005 A for N-H versus +0.006 for the C-H bonds at 15 K [199]...

Various models are reported in the literature for correcting the bond lengths and angles of H2O molecules in solid hydrates for thermal motion and anharmonicity. However, as recently shown , both positive and negative terms exist, which partially compensate for each other. Therefore, data obtained by neutron diffraction do not show systematic errors larger than 3 pm and 2°, respectively, and, hence, the differences between the average water molecule in crystalline hydrates and that in the gas phase discussed above should be real. ... 

WILLIAMS pointed out that even in the case of neutron diffraction, hydrogen atom positions may not be well determined because of large thermal motion. In p-CH3C6H4NH3 (HF2), the apparent methyl C-H distances range from 0.92 A to 0.96 A these short values are artifacts caused by large-amplitude torsional motion of the methyl group. Furthermore, it is very difficult to correct reliably for such effects. 

The dHH can be corrected for thermal motion,120 but difficulties in accounting for the internal vibrational motion of the H atoms can lead to an overestimation of the correction factor and large uncertainties in Jhh- For example, the corrected c/im for Mo(CO)(dppe)2(H2) is estimated to be 0.85-0.88 A. A summary of neutron diffraction data for dihydrogen complexes, including some that have been corrected for thermal motion, is provided by Koetzle.121... 

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