Hydrogen bonds, charge-assisted

Scheme 15. Contribution of two factors in strengthening of hydrogen bonds (charge and polarization) in the proton-transfer compounds charge and polarization-assisted hydrogen bonds...

The hydrogen bond in a neutral phosphoramidate is of the type polarization-assisted hydrogen bond whereas, in the proton-transfer and phosphate compounds two factors help to strength of hydrogen bond polarization-assisted and charge-assisted. 

It is interesting to note in the latter connection that nucleophilic substitution transition states in which there apparently is not strong delocalization of pi electron density into the substituent tend to fall into the <7 type (cf. reactions 18 and 19 of Table VIII). In set 18, there are two ortho nitro groups which apparently take up much of the pi charge (thus it is unavailable to X), whereas in set 19, the positive piperidinium center may cause (perhaps with assistance from the NH hydrogen bonding permitted by the aprotic solvent) the... 

Fig. la-d Schematic representation of the relationship between neutral, internally charge-assisted and externally charge-assisted hydrogen bonds... 

An alternative, widely used, approach to charge assisted networks is based on the exploitation of direct acid-base reactions. There are, broadly speaking, essentially two different means to obtain charge-assisted interactions, which depend on whether the network is constructed of ions of the same charge (homoionic hydrogen bonded networks) or of ions of opposite charge (hetero-ionic networks). These two limiting situations are shown in Fig. lc,d. The utilization of... 

Fig. 12 An example of guanidinium-sulfonate superstructures. The fundamental interaction responsible for both robustness and flexibility is the charge-assisted (+)N-H---0( ) hydrogen bonding between the guanidinium cations and the sulfonate anions, which can be varied in shape and length [44]...

Kovalchuk A, Bricks JL, Reck G et al (2004) A charge transfer-type fluorescent molecular sensor that lights up in the visible upon hydrogen bond-assisted complexation of anions. Chem Commun 1946-1947... 

In the first report on the beryllinm dihydrogen-bonded complex, pnblished in 1996 [5], the H-Be-H- H-N(H3)+ system was located at an energy minimnm of the potential energy surface calcnlated at the MP2/6-311-H-G level. By analogy with classical hydrogen bonds, this dihydrogen complex can be said to be charge-assisted. 

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