On the thermal expansion of molecules

Experimental results for a variety of molecules have shown that their bond lengths expand appreciably when the molecules are heated, as expected from the asymmetric Morse-like potentials characterizing the bonds. However, in a series of papers on structures determined by gas-phase electron diffraction, Giricheva et al. claimed that, for very hot MX3 molecules, effects of out-of-plane vibrations cancel the thermal expansion of the M-X bonds. This is incorrect. Although the computations to support their claim were correct as far as they went, the authors neglected the effects of asymmetric vibrational modes and centrifugal stretches of the bonds. In the present report, we show that quantum chemical computations for LaI₃ reveal the crucial roles played by the terms neglected by Giricheva et al., which terms are responsible for thermal bond stretches of approximately 0.023 Å at 1142 K. In addition, because the iodine atoms in LaI3 are further apart in the mean structure than the sum of their Pauling van der Waals radii, the geminal nonbonded interactions are attractive. This accounts for the fact that the Morse asymmetry constant for the symmetric stretch mode is smaller than that for the asymmetric stretch. It also helps to explain the very large amplitude of the out-of-plane puckering mode, which tends to decrease the La-I bond length during the puckering trajectory.