Dynamic properties of malonic acid in solution from spin relaxation of a 13C labeled methylene group

Abstract

The motional properties of malonic acid in dimethylsulfoxide solution have been studied using NMR spin relaxation methods. In particular, the spin coupled ¹³C resonance of the central methylene of the malonic acid molecule is observed using nonselective pulse techniques. Analysis of recovery curves, which includes consideration of both autocorrelation and cross‐correlation spectral densities, allows extraction of the three principle diffusion tensor elements as well as determination of random field contributions arising from intermolecular interactions. We find that the diffusion of malonic acid can be described with an axially symmetric diffusion tensor D_⊥ = 2.0×10⁹ s⁻¹ and D_∥ = 1.9×10¹⁰ s⁻¹. This result is intermediate between the predictions of stick and slip boundary conditions. Experiments conducted in deuterated solvents allow evaluation of the diffusion tensors with minimal interference from intermolecular effects. Use of protonated solvents introduces substantial intermolecular effects. However, these effects can be separated and an essentially identical diffusion tensor measured by concentration of experimental measurements in the early time protions of the relaxation time course. The fact that only a single nucleus is observed in these experiments and the fact that only nonselective pulses are required make these techniques particularly useful in extension to more complex systems.