Pyridines—continued hydrogen-bonding

Figures 2.a-c show the pyridine adsorption results. Bronsted acidity is manifested by the bands at 1440-1445,1630-1640 and 1530-1550 cm . Bands at 1600-1630 cm are assigned to pyridine bonded to Lewis acid sites. Certain bands such as the 1440-1460 and 1480-1490 cm can be due to hydrogen-bonded, protonated or Lewis-coordinated pyridine species. Under continuous nitrogen purging, spectra labeled as "A" in Figures 2a-c represent saturation of the surface at room temperature (90 25 unol pyridine/g found in all three tungsta catalysts) and "F" show the baseline due to the dry catalyst. We cannot entirely rule out the possibility of some extent of weakly bound pyridine at room temperature. Nevertheless, the pyridine DRIFTS experiments show the presence of Brpnsted acidity, which is expected to be the result of water of reduction that did not desorb upon purging at the reduction temperature. It is noted that, regardless of the presence of Pt, the intensity of the DRIFTS signals due to pyridine are...

The same effect is observed for the substituted pyridyl-pyrazole and -imidazole systems. While 2-(pyrazol-l-yl)pyridine 24 gives a low spin iron(II) complex a continuous spin transition is observed centred just above room temperature in solid salts of [Fe (31)3]2+ and just below in solution [39]. Spin crossover occurs in the [Fe N6]2+ derivative of 2-(pyridin-2-yl)benzimidazole 32 (Dq(Ni2+)=1050 cm"1) but not in that of the 6-methyl-pyridyl system 33 (Dq(Ni2+)=1000 cm"1). Although the transition in salts of [Fe 323]2+ is strongly influenced by the nature of the anion and the extent of hydration, suggesting an influence of hydrogen-bonding, in all instances it is continuous [40]. 

Strictly speaking, we should consider nicotine a bifunctional species and so use the analysis in J. S. Chickos, D. G. Hesse and J. F. Liebman, J. Org. Chem., 54, 5250 (1989) to estimate enthalpy of vaporization. However, in that the enthalpy of combustion data are old and the two substituents (tertiary amine and pyridine) are rather far apart and not involved in hydrogen bonding, we continue with our simplified analysis. 

There are potentially a large number of supramolecular motifs that can be designed into a molecule or polymer to allow access to supramolecular liquid crystals. Figure 4 shows just a selection of some hydrogen bond motifs that have been utilized in the early development of supramolecular liquid crystals. As we shall see, some of these motifs, particularly the benzoic acid/pyridine and amide motifs, have continued to be popular choices in the design of new SLCPs. 

---