STRUCTURE, CONFORMATION, AND MOTIONS OF POLYTETRAHYDROFURAN (PTHF) INTHE HEXAGONAL FORM OF THE PTHF-UREA INCLUSION COMPOUND

Combination of differential scanning calorimetry, x-ray diffraction, F ourier transform infrared, and C-13 nuclear magnetic resonance observa tions made on the crystalline inclusion compound (IC) formed between p olytetrahydrofuran (PTHF) and urea (U), together with their comparison to identical observations performed on bulk semicrystalline samples o f PTHF, have permitted an analysis of the conformations, motions, and environments available to PTHF chains in both solid-state phases. The isolated PTHF chains occupying the narrow channels of the PTHF-U-IC ar e highly extended, though small rotational deviations averaging 24 deg rees from the nearly all trans, planar zig zag conformation of bulk cr ystalline PTHF chains produce some significant differences in their be haviors. PTHF chains in PTHF-U-IC possess much greater mobility than b ulk crystalline PTHF chains as evidenced by C-13 spin lattice relaxati on times, T-1, 50 times shorter (1.5 s) than observed for bulk crystal line PTHF chains (75 s). FTIR observations are consistent with very li ttle specific interaction between guest PTHF chains and host urea matr ix molecules and result in similar spectra for bulk and IC PTHF, excep t for the presence of the CH, rocking vibration band at 745 cm(-1) obs erved for bulk PTHF. The absence of this band in the IC PTHF can be un derstood by considering the symmetry of the all trans, planar zig zag conformation of bulk crystalline PTHF chains, which prevents the CH, r ocking mode from coupling with skeletal stretching and bending modes a s occurs in the nonplanar, helical PTHF chains in PTHF-U-IC. (C) 1995 John Wiley and Sons, Inc.