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Investigation of force constants of molecules become less accurate as the number of atoms in the molecule increases. This is due to several obvious difficulties such as obtaining exact formulæ for the normal frequencies of a system with many vibrational degrees of freedom, and assigning correctly observed fundamental frequencies. There is, however, a less obvious source of error in dealing with molecules whose equilibrium configuration in one of strain. The force constant of any link measures the curvature of the nuclear separation-potential energy curve for the link in question at its equilibrium separation. This equilibrium separation corresponds to the minimum in this curve for diatomic and non-closed triatomic molecules. But if the link be under strain the equilibrium separation does not correspond to this minimum, and the curvature measured by the force constant may be very different from that for the same link joining the same atoms in a diatomic molecule. Hence force constants deduced from observed vibrational frequencies of a molecule whose bonds are under strain cannot be compared with those of similar bonds of diatomic molecules unless they are corrected for deviation from the minimum of the potential energy curve. Boron trifluoride and trichloride are molecules likely to yield information regarding the effect of strain on observed force constants; they are stable gases at room temperature, and the chloride is readily prepared and purified. Further, quantum mechanics predicts that they should have a plane configuration, and allocation of fundamental frequencies should be possible with some degree of certainty.

proceedings of the royal society of london. series a, mathematical and physical sciences

Investigations in the infra-red region of the spectrum - XI—The absorption spectrum and molecular structure of boron trichloride, and the effect of strain on plane groups of the type XY <sub>3</sub>

Investigations in the infra-red region of the spectrum - XI—The absorption spectrum and molecular structure of boron trichloride, and the effect of strain on plane groups of the type XY 3

Investigations in the infra-red region of the spectrum - XI—The absorption spectrum and molecular structure of boron trichloride, and the effect of strain on plane groups of the type XY ₃