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Abstract

Isotopically mixed methyl groups CDxH3−x with zero averaged deuteron/hydrogen scattering length 0=〈a〉=xaD+(3−x)aH are expected to be invisible in a neutron diffraction experiment. We find, indeed, in the scattering length density of aspirin-CDxH3−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 3φ periodicity at 10 K. f(φ) is the basis for the determination of the hindrance potential with cos(φ) as the leading term.