Hydrogen bond formation, effect

Solvent effects also depend on the ground-state structure of the substrate and on the transition-state structure, as is shown below. Here let us merely note that A-heterocyclic compounds tend to form a hydrogen bond with hydroxylic solvents even in the ground state. Hydrogen-bond formation in this case is a change in the direction of quaternization of the aza group, as demonstrated by spectral evidence. Therefore, it is undoubtedly a rate-enhancing interaction. 

After 19 hours, no reaction between the zinc chelate 2 and benzaldehyde can be detected at 20 °C. However, 10 mol % of the zinc chelate effectively catalyzes theenantioselective addition of diethylzinc to aromatic aldehydes. The predominant formation of the S-configurated products, effected by this conformationally unambiguous catalyst, can be explained by a six-mem-bered cyclic transition state assembly17. The fact that the zinc chelate formed from ligand M is an equally effective catalyst clearly demonstrates that activation of the aldehyde moiety does not occur as a consequence of hydrogen bond formation between the ammonium proton of the pyrrolidine unit and the aldehydic oxygen. 

One of the most popular applications of molecular rotors is the quantitative determination of solvent viscosity (for some examples, see references [18, 23-27] and Sect. 5). Viscosity refers to a bulk property, but molecular rotors change their behavior under the influence of the solvent on the molecular scale. Most commonly, the diffusivity of a fluorophore is related to bulk viscosity through the Debye-Stokes-Einstein relationship where the diffusion constant D is inversely proportional to bulk viscosity rj. Established techniques such as fluorescent recovery after photobleaching (FRAP) and fluorescence anisotropy build on the diffusivity of a fluorophore. However, the relationship between diffusivity on a molecular scale and bulk viscosity is always an approximation, because it does not consider molecular-scale effects such as size differences between fluorophore and solvent, electrostatic interactions, hydrogen bond formation, or a possible anisotropy of the environment. Nonetheless, approaches exist to resolve this conflict between bulk viscosity and apparent microviscosity at the molecular scale. Forster and Hoffmann examined some triphenylamine dyes with TICT characteristics. These dyes are characterized by radiationless relaxation from the TICT state. Forster and Hoffmann found a power-law relationship between quantum yield and solvent viscosity both analytically and experimentally [28]. For a quantitative derivation of the power-law relationship, Forster and Hoffmann define the solvent s microfriction k by applying the Debye-Stokes-Einstein diffusion model (2)... 

Since the stationary phase is basically nonpolar in RPC, it is not expected to effect solute retention by ionic attraction, hydrogen bonding, formation of charge transfer complexes, or by any of the other strong non-covalent interactions familiar to the chemist. The only attractive force between the stationary phase and the eluite would seem to be van der Waals forces. However, these farces also act between the eluite and the mobile phase so that the net effect is generally not sufficien to account for the strong retention often observed with nonpolar compotinds in RPC. 

MD simulations have provided a unique molecular description of cholesterol-phospholipid interactions [31]. Atomistic simulations have succeeded in reproducing the condensing effect of cholesterol on phospholipid bilayers [32-34], With atomistic detail, many properties can be determined, such as the effect of cholesterol on lipid chain ordering or on hydrogen bond formation. Other simulations have focused on the interaction of cholesterol and SM [35-37], Aittoniemi et al. [38] showed that hydrogen bonding alone cannot explain the preferential interaction between cholesterol and SM compared to cholesterol and POPC. 

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