Hetero-intermolecular hydrogen bonding

When two or more molecules of the same type associate, so-called homo-intermolecular hydrogen bonds are formed (Fig. 2-4). The association of different molecules e.g. R—O—H- -NRj) results in hetero-intermolecular hydrogen bonds. The designations homo- and heteromolecular [192] as well as homo- and heteroconjugated hydrogen bond are also in use. A remarkable example of a competitive solvent-dependent equilibrium between homo- and hetero-intermolecular hydrogen-bond associated species has been found in solutions of 4-hydroxyacetophenone and 2-(2-hexyloxyethoxy)ethanol [319]. 

The formation of hetero-intermolecular hydrogen bonds has also been applied for the preparation of discotic complexes. Equimolar amounts of triazine and carboxylic acid were complexed to form doubly hydrogen bonded mesogen 47 [67], which display disordered hexagonal columnar mesophase. The transitional temperature of the resulting complex is lower than that of the triazine component itself. 

This chapter focuses on supramolecular hydrogen-bonded liquid crystals built by the formation of hetero-intermolecular hydrogen bonds. The H-bonded liquid crystals obtained by the association of identical molecules are described in other chapters. 

In 2002, Huang and Rawal found that the hetero Diels-Alder reaction of aminosiloxydienes with aldehydes was accelerated in alcoholic solvents [65], They subsequently elucidated that TADDOL (19) is an efficient chiral catalyst for the hetero-Diels-Alder reaction (Figure 10.17, Equation 10.33) [66]. The internal hydrogen bond in TADDOL observed in its crystal structure is expected to render the hydroxy proton more acidic, hence enabling it to participate better in intermolecular hydrogen bonding with the carbonyl group of the dienophile [67]. The Mukaiyama aldol reaction was also reported [68]. 

Semenov MA, Blyzniuk luN, Bolbukh TV, Shestopalova AV, Evstigneev MP, Maleev VY (2012) Intermolecular hydrogen bonds in hetero-complexes of biologically active aromatic molecules probed by the methods of vibrational spectroscopy. SpectrochimicaActaPartA Mol Biomol Spectrosc 95(2) 224-229... 

Representative example of preparation of such potential energy funetion, ealled now ab initio potential would be MCY potential for water-dimer by dementi et al. Later, similar potentials have been proposed for hetero-dimer such as water-methanol. In the ease of such hydrogen-bonded dimers, the intermolecular energy E2-2E, ean be fairly large value and determination of fitted parameters is successful. In the case of weak interaetion, ab initio calculation needs long computer time and optimization of parameter beeomes difficult. In spite of such a situation, some attempts are made for potential preparation, e.g., for benzene and carbon dioxide with limited success. 

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