MOLECULAR-ORIGIN OF THE TEMPERATURE-DEPENDENT NMR-SPECTRUM OF 1-1 CROWN ETHER MACROCYCLES CONTAINING A 2,2'-BIPYRIDYL SUBUNIT

A strongly temperature-dependent NMR spectrum has been reported for certain cyclic molecules prepared from 2, 2ʹ-bipyridyl and polyoxyethylene oligomers (x-mers). A study of the conformational characteristics of this class of cyclic molecules was undertaken to attempt an identification of the molecular origin of the temperature-dependent NMR. The polyoxyethylene chain in an acyclic analogue is assumed to behave according to the rotational isomeric state model developed by Mark and Flory for unperturbed polyoxyethylene. Monte Carlo calculations were performed using a priori and conditional probabilities deduced from the rotational isomeric state model. Calculations predict that cyclization is impossible for x = 2, barely possible for x = 3, and most readily achieved with x = 5-7, in reasonable accord with experiment. Many polyoxyethylene chain conformations are consistent with cyclization when x = 6. Cyclization is achieved with little change in probabilities for occupancy of trans and gauche states by bonds in the polyoxyethylene chain of this molecule. Calculations were repeated using a priori and conditional probabilities appropriate for temperatures of - 125 to 125 °C. The polyoxyethylene chain conformation at low temperature can most easily achieve cyclization if the rotational state about the bond between the aromatic rings tends toward cis. In these conformations the polyoxyethylene chain occupies a position aw ay from the face of the aromatic system. As the temperature increases, cyclization also becomes possible if the rotational state about this bond tends toward trans. Part of the polyoxyethylene moiety is then drawn across the face of the aromatic system. The temperature-dependent NMR is a consequence of thermal alteration in the distribution of polyoxyethylene chain conformations consistent with cyclization. © 1979, American Chemical Society. All rights reserved.