Close Non-Bonded Contacts: Effects on Structure and Conformational Reactions

Tulane University, New Orleans LA

Investigators

Abstract

In this NSF project, funded by the Chemical Structure, Dynamic & Mechanism B Program of the Chemistry Division, Professor Robert Pascal, Jr. of the Department of Chemistry at Tulane University prepares new compounds that contain interacting functional groups in very close, but non-bonded contact and/or interacting molecular surfaces whose extent of contact varies with conformation. Some of these molecules are likely to exhibit "world record" shortening of bond distances and non-bonded contact distances. Many of these compounds have highly unusual shapes that are essentially unknown in the natural world. The research is fundamental in nature and will improve our understanding of chemical bonding. These compounds may also possess unique chiroptical and photophysical properties that would make them candidates for chiral catalysts, chiral sensors, or organic electronic materials. They also provide stringent tests of the accuracy of computational methods that are widely used in the chemical and biological sciences. The research is ideal for the training of laboratory chemists at all levels, including students from underrepresented groups in science. In,in-cyclophanes, complex polyphenyl aromatic molecules, and polycyclic aromatic structures containing strongly interacting surfaces share a common feature: conventional and dispersion-corrected density functional theory (DFT) calculations describe them quite differently, but which of these methods better agrees with experiment? (1) The proposed in,in-cyclophanes contain functional groups pressed very close together along the central axes of the molecules, but the two types of DFT calculations give significantly different interatomic distances. (2) The proposed polyphenyl aromatics have chiral lowest-energy conformations, but the two types of DFT calculations give very different barriers for racemization. (3) The proposed molecules with interacting aromatic polycycles may adopt distinct conformations that either maximize or minimize intramolecular van der Waals interactions, but the two types of DFT calculations give very different equilibrium constants for their interconversion. X-ray crystallography, dynamic NMR spectroscopy, chromatographic resolution on chiral supports, and NMR determination of equilibrium constants are being employed to judge the accuracy of the various computational methods. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.