Solid-state NMR Studies of Functional Group Recognition in Channel Inclusion Compound Formation

Abstract

Solid-state ¹³C CP-MAS NMR spectra of urea inclusion compounds of methyl undecanoate (MUDA) show a significant bias in the relative alignment of guest molecules in the channels. In these spectra, asymmetric doublets are observed for C₁₁ and methoxy carbons, while bands from internal carbons, including the carbonyl carbon, are unsplit. Band doubling is caused by differences in nearest neighbors, as shown by comparison with spectra of symmetric analogs, such as dimethyl sebacate/urea. During crystal formation of MUDA/urea from methanol, head to tail alignment is favored over head to head (plus tail to tail) alignment by a factor of 1.27±0.06, as shown by simulation of C₁₁ resonances for spectra taken with a wide variety of cross polarization contact times. Adsorption of the ester group onto the growing face of the inclusion compound crystal probably gives rise to the observed bias.