A deuterium NMR study of guest molecular dynamics of acetone in two organic inclusion compounds

Abstract

Deuterium nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation times (T1) are used to investigate the dynamics of the guest molecule, acetone, in tris(5-acetyl-3-thienyl)methane (TATM) and cyclotriveratrylene (CTV) inclusion compounds. 13C CPMAS powder NMR spectra were obtained for each clathrate, to verify inclusion. In acetone: TATM, the guest molecule is undergoing twofold reorientation about the CO bond, exchanging the two methyl groups. An activation energy of 20 (± 1.4) kJ/mol, for the two-site jump motion, was found, independently, from deuterium NMR spectra an T1 measurements. Acetone in CTV performs the same type of motion as acetone in TATM. Activation energies of 25.0 (± 3.2) kJ/mol and 24.1 (± 0.5) kJ/mol were determined using the same two techniques. both inclusion compounds, the rate of methyl rotation within the acetone molecule is greater than 108 Hz even at the lowest temperature measured (84 K). Analytical expressions for the spin-lattice relaxation time (T1), for a twofold jump, were derived. Calculated values of the effective quadrupolar coupling constant and T1min for the guests agree very well with the experimental data. The 84 K spectrum of acetone:TATM unexpectedly shows some asymmetry, the origin of which is discussed. Finally, these two clathrates are compared to the recently examined acetone: tri-ortho-thymotide inclusion compound. Key words: inclusion compounds, deuterium NMR, solid state NMR spectroscopy, molecular dynamics.