Conformational and vibrational properties of α,ω-dihalogenoalkane/urea inclusion compounds: a Raman scattering investigation

Abstract

Raman spectroscopic investigations of urea inclusion compounds containing α,ω-dihalogenoalkane [X(CH₂)_nX; n= 8 for X = Cl; n= 7–11 for X = Br; n= 8 for X = I] guest molecules are reported. In these inclusion compounds, the urea molecules form a tunnel structure within which the guest molecules are located. Vibrational modes due to the urea confirm the structural identity of the inclusion compounds, and lattice modes of the urea host structure are assigned tentatively. Investigations of the vibrational properties of the X(CH₂)_nX guest molecules included within this host structure have focused on the longitudinal acoustic mode (LAM-1) and the C—X stretching vibrations. Bands in the Raman spectrum due to the ν(C—X) mode have been studied as a function of: (i) the length (n) of the guest molecule; (ii) the identity of the terminal substituent X; (iii) temperature and (iv) pressure. From these results, trends in the relative amounts of gauche and trans end-groups for the X(CH₂)_nX guest molecules in their urea inclusion compounds have been assessed.