On the assignment of the fundamental vibration frequencies in O 3 , F 2 O, Cl 2 O, NO, and N 3 -

Abstract

In the preceding paper our object was to investigate critically the application of simplified types of force fields to the correlation of the vibration frequencies of triatomis molecules. Starting from the better known molecules and using them as test cases, we have been able to estimate the probable errors in the results obtained by applying the same type of field to the less well-known molecules. On the whole we have found a remarkable regularity in that isoelectronic molecules tend to have the same geometrical shape, and that the values of the force constants associated with particular deformations lie within quite strict limits. There are, however, a few cases in which this regularity is completely negatived on applying the existing assignment of the fundamental frequencies of the molecules, and for some of these we suggested re-assignments which bring them more into line with the others. The purpose of the present paper is to justify such reassignments in relation to the experimental data. We hope to show that the assignments which we have proposed in the foregoing paper provide an interpretation at least as good as, if not better than, the earlier ones. We also suggest here experiments which might be made to discriminate between the different assignments in as simple and definite a way as possible. There have been several investigations of the infra-red absorption spectrum of ozone, and one unsuccessful attempt to obtain its Raman spectrum. The most recent work is that of Hettner, Pohlman, and Schumacher, and it is of extreme importance as they show that one absorption band which all previous investigators had taken to be due to ozone (viz., at 7·6 μ) is due to nitrogen pentoxide. The results of Hettner, Pohlman, and Schumacher have been summarized in Table I, although refernce should be made to their original paper for details which would take too much space for reproduction here. The interpretation of these results which Hettner, Pohlman, and Schumacher favour is that the fundamental frequencies are the three most intense bands in the spectrum, viz., 710 cm -1 , 1037 cm -1 , and 2105 cm -1 . These they assign as v 3 , v 2 , and v 1 respectively. The notation to describe the frequencies is the conventional numerical one used and illustrated in the preceding paper. Such an assignment can only mean that the force required to alter the distance between the two basal atoms of the O 3 molecule is very much greater than that required to make a similar alteration in the distance between the apical atom and either of the basal ones. In fact, using a central force field, Hettner, pohlman, and Schumacher conclude that the ozone molecule has the form of a very sharp isosceles triangle with the apical angle close to 40°. They calculate the force constant between the basal atoms to be 20·8 x 10 5 dynes/cm and that between each of the other pairs to be 3·86 x 10 5 dynes/cm. The smallness of the latter, they say, accounts for the ease with which the ozone molecule parts with an atom of oxygen in oxidizing processes.