Long Range Interactions Between Dipole Molecules

The first-order van der Waals forces between symmetrical top molecules which carry a dipole along their figure axis are calculated. Their strength is compared with that of other interactions, and found to be greater than London's second-order effect for linear dipoles over the main part of the range of the latter. A simple preliminary theory of the effect of these forces on the width of spectral lines in bands of polyatomic molecules is given, and it is shown that they lead to results of the correct order of magnitude.