The conformational preferences of the diastereomeric neurotransmitters (1R2S) ephedrine and (1S2S) pseudoephedrine have been studied in the gas phase, under free jet-expansion conditions, using ultraviolet spectroscopy (both R2PI and LIF) and infrared ion-dip and hole-burning spectroscopy in combination with ab initio calculation. This has led to the identification and assignment of two conformers in ephedrine and four in pseudoephedrine. Assignments have been made by comparing their experimental infrared and LIF spectra with ab initio vibrational frequencies and ultraviolet rotational band contours. The relative stabilities of the conformers are controlled by a delicate balance between intramolecular hydrogen bonding and dispersive interactions between the methyl groups of the side chain, both with each other and with the aromatic ring. The relative conformational stabilities calculated for ephedrine do not agree with the experimental results; two of its low-lying conformers were detected, but a third, lying at an intermediate energy, was not. The possibility of its collisional relaxation into the global minimum during the supersonic expansion was not supported by the ab initio calculations, which predict a substantial barrier along the minimum energy pathway. It is possible that the combination of a relatively weak transition moment and a lack of facile pathways for relaxation from higher lying structures into the “missing” conformer may play a role.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.