Motional behavior of "asperlin" in solution. A 13C spin-lattice relaxation study

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Abstract
13C nuclear magnetic resonance spin-lattice relaxation times (T1) have been used to probe the motional behavior of 5-acetoxy-5,6-dihydro-6-(1,2-epoxypropyl)-2-pyrone ("asperlin") in dimethyl sulfoxide solution. This molecule offers structural features suited to a study of internal motions, i.e., epoxypropyl and methyl internal motions superimposed on an anisotropic overall reorientation. The rigidity of the pyrone ring and its semiplanar conformation result in an overall ellipsoidal shape, and hence the rotational dynamics of asperlin are adequately approximated by the diffusion of a prolate ellipsoid with the major axis passing through the C(2).sbd.H(2) bond. The description of the internal motion of the epoxypropyl ring is strongly model dependent. Furthermore, the relaxation data for the oxirane ring carbons do not uniquely define a dynamic model. Due to similarities in the activation energies of the overall and internal motions, based on temperature-dependent measurements, it has not been feasible to interpret the relaxation data by a single type of motion. Internal rotation of the epoxymethyl substitutent is rationalized by applying the stochastic diffusion model of multiple internal rotations.