Double proton acceptor

It is usually energetically unfavorable for a molecule to act as a double proton acceptor as BH would in Fig. 5.1. For similar reasons, cooperativity is typically negative also when a molecule acts as double proton donor. Of course, even in the case of negative cooperativity, formation of the second H-bond is usually energetically favorable when compared to the complete absence of a second H-bond. That is, even though the CH -BH interaction above is weaker than it would be in the absence of the other proton donor, AH, this interaction energy is still negative, and so wiU form spontaneously. In other words, two H-bonds are always better than one (or usually so). 

Figure 5.6 Alternate geometries of the HCCH trimer. The central molecule acts as a double proton acceptor in (a) and as a double donor in (b).

The HjO HP-HF complex was reexamined more recently in conjunction with a comparison with H O H O -HF. The focus of this work was an exploration of the potential energy surface to identify all the local minima. In addition to the cyclic structure of Fig. 5.27, a bifurcated geometry, in which the water serves as double proton acceptor, was also located as a minimum of the PES of H O -HF—HF. In the complex containing a pair of water molecules and one HF, the only minimum located was of cyclic type. Both of these structures are illustrated in Fig. 5.28. 

Figure 12 The complex Au3-(H20)J with a nonconventional double proton-acceptor character of the gold atom Au3...

The other two hydroxyls in each POi, group of the chain link it with water or some other proton acceptor molecules in the solution. In a diluted PA there are less direct P0i,-H-P0h connections, with more water molecules inserted between POit groups, which means that dilution hinders the formation of long PA chains. In a more concentrated PA double hydrogen bonds between POi, groups appear (17) leaving less free hydroxyls for the side linkages of PA chain with other molecules in the solution. 

Especial points which emerge from these studies include (a) the almost complete absence of reactivity of the hydroxy-groups of simple carbohydrates in water, which is attributed to their powerful solvation by water preventing a close approach of any other solute and (b) the ability of ester groups to interact with proton-acceptors. The refractive index tests, examination of m.p. or b.p., and infrared spectra of certain mono- and poly-esters appear to be interpreted most simply by assuming the formation of weak CH bonds by ester groups under the activating influence of the adjacent (5—0 double bond. These bonds can account for certain properties of l 2-diesters and for the adsorption of proton-acceptor solutes by cellulose acetates. 

After the ESIPT the molecule exhibits a pronounced ringing in this mode. The 295 cm-1 mode is a symmetric in-plane stretching vibration (see Fig. 3). The corresponding contraction of the molecule reduces the donor acceptor distances in both chelate rings simultaneously and initiates the electronic configuration change of the concerted ESIPT. The concerted double proton transfer leads therefore to a ringing of the molecule in this second mode. 

Intermediate between the N atom of amines, which is involved in single bonds, and the triply bonded N in nitriles, lie the imines with their double bonds. Recent calculations have combined methyleneimine, and its methylated derivatives as proton acceptor, with water and with methanol as donor. The general arrangement of water with methyleneimine is illustrated in Fig. 2.20 where it is emphasized that the hydrogen of the water not participating in the H-bond prefers to lie outside of the molecular plane of H2C=NH. The same is true of the methyl group when water is replaced by methanol. 

For the sake of consistency of terminology, triads of molecules in which the central unit acts simultaneously as both proton donor and acceptor will be termed sequential to distinguish such configurations from those in which the central molecule acts as double proton donor or double acceptor. A perhaps more quantitative expression of cooperativity is referred to in the literature as nonadditivity. The latter term is commonly taken as the difference between the total interaction energy of an aggregation of molecules on one hand and the sum of all the pairwise interactions on the other. 

The geometric features of the binary complexes are listed in Table 5.28, along with the 1 2 complex in the last two rows. The expected contraction of the O—F distance as the second HF molecule is added is immediately apparent. This contraction amounts to 0.10 A at the SCF level, less than that in the HjN—HF- HF complex where the shrinkage was 0.13 A. Correlation has little influence on the H-bond reduction the MP2 contraction is 0.11 A. From the perspective of adding the proton acceptor molecule to the FIF dimer, R(F"F) is reduced by 0.12 A for both H2O and NH3, at the SCF level. The distinction between the latter two molecules is perhaps most clearly seen in the bond length of the inner HF molecule, ij. This bond stretches by 0.017 A when the outer HF molecule is added to HjN- HF, but by only 0.009 A if H N is replaced by HjO. Note, however, that when correlation is added, this stretching doubles. 

Perhaps another litmus test of the ability of the alkynic C—H group to donate a proton in a H-bond arises when a molecule of this type is paired with a hydrogen halide, HX. One then has two distinct possibilities. The X atom, although a weak proton acceptor by nature, can form a complex of the C—H—XH type. An alternative would have the XH acting as the proton donor, with the electron-rich alkyne triple bond acting as the acceptor. Experimental measurements indicate the latter is the more stable of the two alternatives. Indeed, a similar sort of geometry is adopted when HE approaches the n system of ethylene , even though the electron source in this double bond is less rich than in the triple bond of an alkyne. 

All azoaromaties, be they of the azobenzene, aminoazpbenzene, or donor/acceptor pseudo-stUbene type, experience considerable spectral changes on protonation or complexation. Orthp metallation has the same effect. Usually the n— n band is red-shifted, possibly due to the localized charge at the N-atom. By the same token, the (n,K ) state is shifted to higher energies. Minor band shifts and intensity changes mdicate double protonation of azobenzene. 

Besides E, elimination, in some cases, an Ei elimination mechanism can be followed, and the more stable olefin is formed. Instead of Hofmann s rule, Zaitsev s rule is obeyed (the double bond goes mainly toward the most highly substituted carbon). In some reactions the direction of elimination is determined by the need to minimize steric interactions, sometimes even when the steric hindrance appears only during the transition state. Also an E2 mechanism may be followed. It should be remembered that an E2 reaction requires a proton acceptor and occurs as follows ... 

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