The stabilization of unusual conformers in guest-host systems: Solid-state NMR investigations of 2-methylhexadecane in urea and thiourea

Abstract

Solid-state nuclear magnetic resonance (NMR) spectroscopy is employed for the first time on urea and thiourea inclusion compounds (UICs and TICs) containing branched alkyl chains. In the present work,2H and13C NMR as well as X-ray diffraction studies of two selectively deuterated 2-methylhexadecanes in UIC and TIC are presented. An analysis of the derivedT 1 data reveals significant differences between UICs and TICs, which can be attributed to differences in the motional features of the guest species. It is found that four different motional contributions have to be considered, namely, chain rotation, chain wobbling,trans-gauche isomerization and methyl group rotation. 2-Methylhexadecane in UIC exists in an almost all-trans conformation (gauche amount not more than 5%) and undergoes fast chain rotation (6-site jump process, activation energyE A=16.7 kJ/mol). The analysis of the2H NMR spectra of 2-methylhexadecane-1,1′,2-d7 in urea proves that the branched chain end exists in an eclipsed conformation. TheT 1 data of 2-methylhexadecane-3-d2 in thiourea can be reproduced by an overall rotation (E A=9.8 kJ/mol) and atrans-gauche isomerization with torsional jumps around the C-3-C-4 bond (E A=11.0 kJ/mol,gauche population=15%). As for the corresponding UIC, the2H NMR spectra of 2-methylhexadecane-1,1′,2-d7 in TIC can be only explained by the existence of an eclipsed conformation at the branched chain end.