Hydrogen bonding origins

As observed above, the solid state differs from the liquid state of solutions, under the usual experimental conditions, but some new ideas for an approach to hydrogen bonding originate from crystal analysis28. An example of this is offered by the fact that the hydrogen bonds observed in crystal structures are rarely linear. Bifurcated bonds (5,... 

The topology of water clusters observed under supercritical conditions can also be easily analyzed. In this analysis, we do not make any distinction between donor and acceptor types of hydrogen bonds originating at a particular water molecule. Thus, every molecule is depicted as a structureless dot in Figure 16, where all observable topological types of 3-, 4-, and 5-mers are shown along with their relative abundances (normalized as a fraction of molecules participating in clusters of certain type) under several supercritical thermodynamic conditions listed below in Tables 4 and 5. 

Next, we consider one pair of it electrons and one pair of cj elections. The cj electrons may originate from a CH or from a CC bond. Let us consider the loop enclosed by the three anchors formed when the electron pair comes from a C-H bond. There are only three possible pairing options. The hydrogen-atom originally bonded to carbon atom 1, is shifted in one product to carbon atom 2,... 

Morokuma K 1977. Why Do Molecules Interact The Origin of Electron Donor-Acceptor Complexes, Hydrogen Bonding, and Proton Affinity. Accounts of Chemical Research 10 294-300. 

Appreciating the beneficial influences of water and Lewis acids on the Diels-Alder reaction and understanding their origin, one may ask what would be the result of a combination of these two effects. If they would be additive, huge accelerations can be envisaged. But may one really expect this How does water influence the Lewis-acid catalysed reaction, and what is the influence of the Lewis acid on the enforced hydrophobic interaction and the hydrogen bonding effect These are the questions that are addressed in this chapter. 

In the case of the retro Diels-Alder reaction, the nature of the activated complex plays a key role. In the activation process of this transformation, the reaction centre undergoes changes, mainly in the electron distributions, that cause a lowering of the chemical potential of the surrounding water molecules. Most likely, the latter is a consequence of an increased interaction between the reaction centre and the water molecules. Since the enforced hydrophobic effect is entropic in origin, this implies that the orientational constraints of the water molecules in the hydrophobic hydration shell are relieved in the activation process. Hence, it almost seems as if in the activated complex, the hydrocarbon part of the reaction centre is involved in hydrogen bonding interactions. Note that the... 

The picture of the process of substitution by the nitronium ion emerging from the facts discussed above is that of a two-stage process, the first step in which is rate-determining and which leads to a relatively stable intermediate. In the second step, which is relatively fast, the proton is lost. The transition state leading to the relatively stable intermediate is so constructed that in it the carbon-hydrogen bond which is finally broken is but little changed from its original condition. 

Until surface contact, the force between molecules is always one of attraction, although this attraction has different origins in different systems. London forces, dipole-dipole attractions, acid-base interactions, and hydrogen bonds are some of the types of attraction we have in mind. In the foregoing list, London forces are universal and also the weakest of the attractions listed. The interactions increase in strength and also in specificity in the order listed. 

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