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 and t3C NMR as well as X-ray diffraction studies of two selectively deuterated 2-methylhexadecanes in UIC and TIC are presented. An analysis of the derived T~ 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 motionaI contributions have to be considered, namely, chain rotation, chain wobbling, trans-gauche isomerization and methyl group rotation. 2-Methylhexadecane in UIC exists in ah almost all-trans conformation (gauche amount not more than 5%) and undergoes fast chain rotation (6-site jump process, activation energy E A = 16.7 kJ/mol). The analysis of the 2H NMR spectra of 2-methylhexadecane-l,l',2-d 7 in urea proves that the branched chain end exists in ah eclipsed conformation. The T~ data of 2-methylhexadecane-3-d 2 in thiourea can be reproduced by an overall rotation (E A = 9.8 kJ/mol) a n d a trans-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, the 2H NMR spectra of 2-methylhexadecane-l,l',2-d 7 in TIC can be only explained by the existence of an eclipsed conformation at the branched chain end.

Slow Magnetic Relaxation Triggered by a Structural Phase Transition in Long-Chain-Alkylated Cobalt(II) Single-Ion Magnets

“…31 ppm) and anti conformations of the alkyl chains (ca. 34 ppm). − When the temperature was lowered, the disappearance of the gauche form with concomitant increase of the anti form was observed over the temperature range of 275–250 K. The above results thus clearly indicate the conformational isomerization of the alkyl chains in the vicinity of the spin transition temperature for 2 . DSC measurements for 3 show the peak attributed to the structural phase transition at 289 K in the cooling process; this phase transition temperature is higher than that for 2 (Figure S13).…”

Network Phases with Multiple-Junction Geometries at the Gyroid–Diamond Transition

“…As illustrated in Figure S25, the planar junctions provide more space above and below the plane of the junctions for the lateral chains. Thus, the formation of planar junctions is beneficial to reducing the packing frustration arising in the Fmmm phase at low temperature, which is due to the parallel alignment of larger fractions of the all-trans chain segments at lower temperature. , Also, such alignment provides an additional stabilization of less curved interfaces.…”

“…Growing chain length enlarges the effective volume of these chains, which tends to increase the curvature. However, a growing length of the n -alkyl chains also allows an increasing length of linear all-trans chain segments in the alkyl chains, , which favors their parallel alignment and thus tends to decrease the curvature, and this effect competes with the aforementioned steric effect. In a nutshell, the observed order of phase transformation from DD via P 6 3 / m to DG with growing chain length is in line with a decreasing curvature due to chain stiffening.…”

“…Thus, the formation of planar junctions is beneficial to reducing the packing frustration arising in the Fmmm phase at low temperature, which is due to the parallel alignment of larger fractions of the all-trans chain segments at lower temperature. 57,58 Also, such alignment provides an additional stabilization of less curved interfaces. Upon increasing the lateral chain length further, in 6/14 and 6/16, another birefringent mesophase was found (Figure 6b).…”