Isotopic orientational order in acetyl salicylic acid

Abstract

Isotopically mixed methyl groups CD x H 3− x with zero averaged deuteron/hydrogen scattering length 0=〈 a〉= xa D+(3− x) a H are expected to be invisible in a neutron diffraction experiment. We find, indeed, in the scattering length density of aspirin-CD x H 3− x , reconstructed by maximum-entropy methods, at room temperature only three very week minima. At 10 K, however, one positive and two negative extrema are visible: unique evidence for orientational isotopic order. From a combination of 1-d-Fourier and algebraic methods we deconvolute 〈 a〉 and derive the orientational distribution function f( φ) which has three equivalent maxima/minima at 300 K and loses this 3 φ periodicity at 10 K. f( φ) is the basis for the determination of the hindrance potential with cos( φ) as the leading term.