Anharmonic vibration of n-paraffin molecules in urea adducts

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Abstract
X-ray integrated intensities of meridional Bragg reflections due to paraffin molecules in urea adducts are measured from 173 to 351 K. The B factor in the Debye–Waller factor, exp[−2B(T)(sin θ/λ)2], becomes extraordinarily large with increasing temperature (11.4 nm2 at 351 K for the second-order meridional reflection) and depends on the reflection index l for the meridional reflections; this l dependence cannot be explained by the harmonic approximation. The experimental results are analyzed with a model where a paraffin molecule in urea adducts vibrates in an anharmonic potential field with cubic and quartic terms in addition to a harmonic one. The origin of the anharmonic potential field is discussed.