END-CHAIN CONFORMATIONS OF N-ALKANE MOLECULES IN UREA INCLUSION-COMPOUNDS - A RAMAN-SCATTERING STUDY

Urea inclusion compounds containing n-alkane (CnH2n+2 with n = 8 and 1 9) guest molecules were investigated by means of Raman spectroscopy. V ibrational modes due to the urea confirm the structural identity of th e inclusion compounds, and lattice modes of the urea host structure we re assigned tentatively. Investigations of the vibrational properties of the CnH2n+2 guest molecules were focused on the longitudinal accord ion mode (LAM-1) and the methyl rocking r(CH3) vibrations. These modes were studied as a function of (i) the length (n) of the guest molecul e, (ii) temperature and (iii) pressure, The vibrational bands characte ristic of the end-gauche gt- and tg-defects were therefore identified. From these results, trends in the relative amounts of gauche gt- end- groups for the various guest molecules were assessed in both the low- and high-temperature structural phases of the compounds and found to b e constant and of the order of ca, 5%. It was also shown that the amou nt of n-alkane chains distorted at their ends depends markedly on the hydrostatic pressure. These results are discussed and compared with li terature data.