Non-hydrogen bonded system

B. Solvent Selectivity Effects—Non-Hydrogen-Bonding Systems... 

Finally we conclude that the physical mechanism responsible for ESIPT reactions in systems with and without intramolecular hydrogen bonds is basically different. The novel PIDA mechanism proposed in Ref.[36] for ESIPT in non-hydrogen bonded systems has to wait for experimental verification by direct observation of atomic hydrogen migration after optical excitation of suitable molecular systems. 

The photodecomposition and thermodecomposition of nitromethane have been extensively studied as model systems in combustion, explosion and atmosphere pollution processes[l]. On another hand, nitromethane was selected as a model solvent in experiments aimed at examining non hydrogen-bonded solvent effects in a general acid-base theory of organic molecules [2.3]. This selection is based on the electronic and structural characteristics of nitromethane that has a high dielectric constant, and at the same time cannot form hydrogen bonds with solute molecules. 

As is obvious from the table, Tc is almost doubled upon deuteration. These isotope effects are one of the largest observed in any solid state system. The question arises about isotope effects in non-hydrogen-bonded ferro- and antiferroelectrics. As already mentioned in the Introduction, within a mean-field scheme and in a purely ionic model it was predicted that these systems should not exhibit any isotope effect in the classical limit, which has been verified experimentally. Correspondingly, there was not much effort to look for these effects here. However, using a nonlinear shell-model representation it was predicted that in the quantum limit an isotope effect should... 

Professor Coulson defines the electrostatic forces to be those between the proton and the field mainly of the non-bonding electrons without change in the other bonds of the hydrogen-bonded systems. This is an understandable simplification arising from mathematical requirements. However, the only true distinction between the purely electrostatic conception of the hydrogen bond and... 

Scheme 5. Stabilization of dinuclear species in aqueous solution by intramolecular hydrogen-bond formation (VI) is well established. Stabilization of the mononuclear species in aqueous solution by intermolecular hydrogen bonds (IV) may be important in some systems. Interconversion between mononuclear and dinuclear species may occur via non-hydrogen-bonded and hydrogen-bonded transition states, respectively, as schematically shown in II and V. Dashed lines denote hydrogen bonds dotted lines denote bond making and bond breaking.

The results of studies on the primary NMR isotope effect performed with a set of protonated DMAN-s are illustrated in Fig. 19.4 which presents the relationships between the isotope effect and the XH chemical shift of non-deuterated compounds. The experimental points (full circles) come mainly from paper [23] and a few of them from [31]. The results collected for DMAN-s are compared with the corresponding relationship for OHO hydrogen-bonded systems [32]. The majority of the experimental points are in the region of A<5 0.6-0.7 ppm and r> 17.3— 18.8 ppm, i.e., close to data for non-substituted DMAN H+, for which 8= 18.65 ppm and A<5 = 0.66 ppm. One should mention here that asymmetry of the cation does not markedly affect either the (5 or A8 values. This is most probably due to similar electron density distributions on both nitrogen atoms and, consequently, similar populations of the two potential energy minima. Therefore, in our case, we can omit the contribution of the equilibrium effect on A8 value. It seems that this effect is below 0.1 ppm. 

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