Concluding remarks

Abstract

I shall not attempt to make a lengthy critical assessment of all that we have heard today. We have been given a wealth of new experimental results, and have been shown some stimulating approaches on the theoretical side. Yet it might be difficult to draw any clear conclusion other than that there is a need for much more experimental work if we are to disentangle the complex of intermolecular interactions of the different kinds. Among these interactions we have discussed dipole-dipole, London dispersion (induced dipole), and quadrupole effects, spin decoupling and repulsion overlap, ‘field’ effects and ion-pair perturbations. They are manifest in the changes produced in the electronic, vibrational, rotational or nuclear spin energy levels. A solvent perturber can cause blue or red shifts of the electronic spectra by displacement of either the lower or upper electronic level concerned in the transitions. Some progress has been made in the interpretation of such shifts. We have been told about the astonishing effects of perturbing molecules upon the intensities of certain electronic absorption bands, such as the influence of minute amounts of dissolved oxygen on the absorption of benzene, where the paramagnetic disturber appears to affect the singlet-triplet transition through spin-orbital decoupling