Associate Professor

Office: 510 Physical Sciences Laboratory Building
Phone: (419) 372-8439
Email: cable@bgsu.edu

Research Interests
Our research has focused on the effects of hydrogen bonding on the structures of conjugated solutes in order to better understand the nature of the hydrogen bonding interaction. We have employed electronic spectroscopy on jet-cooled isolated molecules and their clusters with hydrogen bonding solvents to determine molecular structures in both ground and excited electronic states. Initial studies have been undertaken on diphenylamines and diphenylethers, as representatives of a class of compounds having diphenyl substituted heteroatoms. Since the electron density at the heteroatom is sensitive to the degree of conjugation with the phenyl rings, the hydrogen bonding interaction can be modulated by the molecular conformation. 

Conformation of Isolated and Hydrogen Bonded Diphenylamine
A fairly extensive investigation of the conformation of diphenylamine and a number of its chemically and isotopically substituted derivatives, as shown below in both the ground and excited electronic states, has recently been completed. Substitutions which are asymmetric with respect to the two phenyl rings clearly show that in the ground state conformation the two phenyl rings are equivalent. This is consistent with a C₂ symmetry structure in which the two phenyl rings adopt equal torsional angles, 28°, and the central nitrogen is in a planar bonding environment. This contrasts with the singly phenyl substituted amine, aniline, in which the nitrogen is in a pyramidal environment as in alkyl substituted amines. In the excited state the nitrogen remains planar and the phenyl torsional angles decrease by 7°. 

Clusters of diphenylamine solvated by both water and ammonia have also been investigated. In the case of water, two modes of hydrogen bonding are possible depending on whether the amine acts as a donor or acceptor. Our studies clearly show that diphenylamine acts as a hydrogen bond donor to water and also, as would be expected, to ammonia. In the hydrogen bonded complexes, the phenyl rings adopt smaller torsional angles as a means to enhance the hydrogen bonding interaction. We also find that the hydrogen bond with water is nearly 2 kcal/mol more stable in the excited electronic state, indicative of the substantially increased acidity of diphenylamine following electronic excitation. 

Conformation of Diphenylether
Substitution of the nitrogen in diphenylamine for oxygen results in diphenylether which can only interact with hydrogen bonding solvents as an acceptor. Our initial investigations of the conformation of isolated diphenylether find a C₂ symmetry structure similar to that of diphenylamine but having substantially larger phenyl torsional angles. An interesting observation made from a singly deuterated derivative shows that the barrier to internal rotation for the two phenyl rings is very much lower than in diphenylamine. The effect of hydrogen bonding with the ether oxygen on both the equilibrium structure and rotational barrier are currently under investigation.