Proton donors, hydrogen-bonded complexes

Sulphoxides and sulphones form hydrogen bonds with proton donors and association complexes with metallic salts by using either the oxygen atom or the sulphur atom in their S—O linkages. The capability for the formation of hydrogen bonds or complexes depends... 

The concept of hydrogen bonding is constantly evolving from classical hydrogen bonds to nonclassical (or nonconventional) hydrogen-bonded complexes. Here, on the basis of new experimental and theoretical data and new approaches to this problem, the nature of a proton-donor component and a proton-acceptor site is reformulated completely. In addition, experimental criteria that have been used successfully earlier for the detection of hydrogen bonds are also changed. 

Obtained for hydrogen-bonded complexes X-H- B (X-H is a proton donor site) at the MP2/6-311-H-G level. 

There are, however, examples indicating that in ion molecule reactions between a protonated species (AH+) and benzene (B), two isomeric forms of the intermediate complex may exist (AH+)(B) and (A)(BH+) [74,286]. In the cases of water [287] and propene [74], quantum chemical calculations clearly indicate that the former corresponds to a n complex where A-H acts as a hydrogen bond donor towards the centre of the benzene ring, while the latter is a hydrogen bonded complex between the benzenium ion and A. In neither case has a barrier been located, but is probably rather low in both cases. The role of the n complex has still not been clarified, since direct downhill routes from the reactants to the a complex exist. It has been pointed out that n complex formation between a pro electrophile and the substrate may be important in solution and in biological systems for molecular recognition purposes. In such cases the proelectrophile is activated to form the actual electrophile subsequent to n complexation, thereupon giving rise to the a complex. This has been shown by quantum chemistry to provide a reasonable scenario for the reaction between HF and benzene, in which BF3 is ultimately required to promote ion formation of the HF/benzene tt complex [288]. 

The system 2-naphthylamine/triethylamine forms proton donor/acceptor interaction, which was investigated in the excited state by measuring time-resolved fluorescence spectra. While the similar 2-naphthol/triethylamine system affords the ion pair interaction, via the hydrogen bond complex, the 2-naphthylamine/triethylamine system presents hydrogen bond interaction which shows a low-temperature absorption with 7max = 370 nm, and Amax = 370 nm in the fluorescence spectrum148. 

X-H is a proton donor, particularly if the X-H bond is significantly stretched in a complex with a proton-shared hydrogen bond ... 

Figure 8 presents a plot of [(Jc - Jm)/Jm] versus [(Rc - Rm)/Rm] x P2, where P2 is the square of the Pauling electronegativity, for 16 different X-H proton donors and 64 hydrogen-bonded complexes with these donors. There are two remarkable features of this plot. The first is that a linear relationship exists between these variables, with a correlation coefficient of 0.97. This relationship is dramatic, given that the raw data... 

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