Thermal bond length shortening

The measured negative thermal expansion of high-quartz [2.36] and possibly also that of keatite (up to 300 °C) [2.37] are direct consequences of this bond-length shortening. In a-quartz the effect of bond-length shortening is masked by the tetrahedral rotation that causes a large positive expansion with temperature. 

The Li-F distances (185 pm) are about 7 pm shorter than the Li-N contacts (192pm). The lithium ion and the SiF2 group shorten the N(l)-Si(l)-bond length to 162.9 pm. LiF can be eliminated thermally, and silyl-bridged four-membered rings are isolated. Such compounds have recently been used as precursors for Si3N4 ceramics.1,16... 

High thermal parameters for C2 and C3 result in artificially shortened C—C bond lengths and distorted angles within the ligand see text for discussion. 

Finally, there is the thermal motion problan in X-ray crystallography." Molecules vibrate internally, and molecules in a aystal lattice also vibrate with what is referred to as the rigid-body motion. That is simply the whole molecule moving back and forth and undergoing rotational types of oscillation in the crystal lattice. (These are motions that correspond to translations and rotations in the isolated molecule that are converted into vibrations by the constraints imposed by the crystal lattice.) The molecule as a whole is quite heavy, so these vibrational frequencies tend to be quite low, and the vibrational amplitudes are large. The most serious part of this problem comes not from the translational motions but rather from the rotational oscillations of the molecule in the crystal lattice. The effect of these is to shorten the apparent bond lengths and compact the apparent size of the molecule. 

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