Cationic H-bonds

The cationic analogue of (HOH -OH) is (H20H -0H2) , in which two water molecules are held together by a fifth proton. The nature of the geometry in this complex, like the anion, has generated a good deal of theoretical study. Early studies at the SCF level, and with small basis sets, had indicated the complex contains a short, centrosymmetric H-bond 9 ° 3. 

This question has been probed by much higher levels of calculation recently and it appears that the previous lower-level work was largely correct. More precisely, the bottom of the proton transfer potential appears to be only very slightly asymmetric if at all the barrier, if it exists, is less than 0.5 kcal/mol. As in the (HOH OH) anion, the character of the potential alternates from single to double well upon small changes in level of theory, the former being favored by electron correlation. The calculations, employing CCSD(T) treat- 

The comparisons in bond strength between cation and anion are highlighted in Table 6.12 which illustrates that the aforementioned patterns are true at SCF as well as correlated levels . Correlation tends to add to the binding energy, particularly in the case of second-row atoms. MP2 appears to provide results close to MP4 in most cases. MP4 results are illustrated in Table 6.13 to nearly coincide with coupled cluster including triples . (FH-F) is apparently the strongest H-bond, with an interaction energy of some 44 kcal/mol. 

Many of the earlier calculations, up to 1987, have been compiled along with experimental H-bond enthalpies . Although somewhat dated, as most of the calculations are limited to SCF level with fairly small basis sets like 4-3IG, many of the trends are illustrative of what is seen at higher levels of theory. 

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Donor-Acceptor Complexes of Low-Coordinated Cationic H-Bonded Phosphorus Systems... 

N.m.r. spectra of xanthine, theobromine, theophylline, caffeine, and their conjugate acids have been analysed. The AT-ray crystal structure of (theobromine)2 H2I8 shows it to be a polyiodide salt with alternating cationic (H-bonded protonated theobromines) and anionic (Iti ions) layers. ... 

Cationic H-bonded metal complexes as multivalent templates... 

The crystallisation of Mn(Cl04)2 6H2O with 2,4-bpe or 4,4 -bpe in absence of NaNCS produces cationic H-bonded metal complexes, bearing as basic building unit a tetraaquocomplex [Mn(bpe)2(OH2)4] (where bpe can be 2,4-bpe or 4,4 -bpe molecule). Each metal complex displays two coordinated bpe in a trans configuration together with four coordinated water molecules around the metal ion. 

The cases of tin oxide hydrate and HPA remain. The well-determined structure of HPA, with a long anion-cation H bond and medium length... 

H bonds in the spatial water aggregates, favour the pure V (mss) process. For tin dioxide, its amphoteric behaviour towards water must lead to medium or long anion-cation H bonds and a partition of protons between anions and the water bed. The longer the anion-water bed H bonds, the more difficult is the proton transfer, and the remaining motion is Tj or, more probably, V. 

A fuzzier atom type participating in these descriptors has been defined that is pharmacologically relevant - the physicochemical type at near-neutral pH [24], which is one of the following seven binding property classes 1 = cation 2 = anion 3 = neutral hydrogen-bond donor 4 = neutral H-bond acceptor ... 

FIGURE 4 16 Hyper conjugation in ethyl cation Ethyl cation is stabilized by delocalization of the elec trons in the C—H bonds of the methyl group into the vacant 2p orbital of the posi tively charged carbon... 

The (I)-(III)-samples sorption ability investigation for cationic dyes microamounts has shown that for DG the maximum rate of extraction is within 70-90 % at pH 3. The isotherm of S-type proves the physical character of solution process and a seeming ionic exchange. Maximal rate of F extraction for all samples was 40-60 % at pH 8 due to electrostatic forces. The anionic dyes have more significant affinity to surface researching Al Oj-samples comparatively with cationic. The forms of obtained soi ption isotherms atpH have mixed character of H,F-type chemosorption mechanism of fonuation of a primary monolayer with the further bilayers formation due to H-bonds and hydrophobic interactions. The different values of pH p for sorbents and dyes confirm their multifunctional character and distinctions in the acid-base properties of adsoi ption centers. 

The predicted bond order for a given bond is listed at the intersection of the two atoms of interest in the bond orders table. The illustration at the left shows the predicted bond orders for this molecule (where 1.0 is a traditional single bond, 2.0 is a double bond, and so on). The C-H bonds all have predicted bond orders of about. 9, while the C-C bonds have predicted bond orders of about 1.4. The latter arc consistent with the known resonance structure for allyl cation. ... 

The preparation and structural characterization of the ions HX2 has been an important feature of such work/ As expected, these H-bonded ions are much less stable than Hp2 though crystalline salts of all three anions and of the mixed anions HXY (except HBrI ) have been isolated by use of large counter cations, typically Cs+ and NR4+ (R = Me, Et, Bu") — see pp. 1313-21, of ref. 23 for further details. Neutron and X-ray diffraction studies suggest that [C1-H - C1] can be either centrosymmetric or non-centrosymmetric depending on the crystalline environment. An example of the latter mode involves interatomic distances of 145 and 178 pm respectively and a bond angle of -168 (Cl- -Cl 321.2pm).( >... 

Figure 6.12 Stabilization of the ethyl carbocation, CH3CH2+, through hyperconjugation. Interaction of neighboring C H

I We could remove the hydrogen atom and both electrons (H ) from the C-H bond, leaving a cydopentadienyl cation. 

The protons come from the water molecules that hydrate these metal cations in solution (Fig. 10.19). The water molecules act as Lewis bases and share electrons with the metal cations. This partial loss of electrons weakens the O -H bonds and allows one or more hydrogen ions to be lost from the water molecules. Small, highly charged cations exert the greatest pull on the electrons and so form the most acidic solutions. 

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