Hydrogen-bonded species

Fig. 6. Freeman and Lewis stabilized structure and the likely results of orr/to-polymerization. As polymerization consumes o-hydroxymethyl groups, the hydrogen-bonded species cannot fomi.

Polynuclear complexes with aminoalcohols and iminoalcohols as ligands O-bridged and hydrogen bonded species. J. A. Bertrand and P. G. Eller, Prog. Inorg. Chem., 1976, 21, 29-53 (54). 

The main difference between hydrogen bond and the halogen bond lies in the propensity of the hydrogen bond to be non-linear [28,29], when symmetry of the complex is appropriate (molecular point group Cs or Ci). In so far as complexes B- ClF are concerned, the nuclei Z Cl - F, where Z is the acceptor atom/centre in B, appear to be nearly collinear in all cases, while the nuclei Z- H - Cl in complexes B- HC1 of appropriate symmetry often show significant deviations from collinearity. This propensity for the hydrogen-bonded species B- HC1 to exhibit non-linear hydrogen bonds can be understood as follows. 

Structures for the hydrogen-bonded species [12], [13], and [14] have been suggested on the basis of their infra-red spectra in solution (Clements et al.,... 

Microwave rotational spectra of intramolecularly hydrogen-bonded species in the gas phase have been analysed to provide details of the symmetry of the species and thereby to determine whether the potential energy is best represented by a single- or double-minimum function. The spectrum of 6-hydroxy-2-formylfulvene is compatible with a structure of C2v symmetry in which the proton is centred in the hydrogen bond [15] or with a rapidly... 

The timescale of a microwave observation is ca 10 12s so that an average of the properties of the species in equilibrium (35) is obtained if the equilibrium occurs in a time shorter than this. The X-ray photoelectron spectra of intramolecularly hydrogen-bonded species in the gas phase have been studied in an attempt to obtain an instantaneous picture of the structure of these molecules. In this technique the ionisation of core electrons which occurs within 10 16s is observed. For malondialdehyde, 6-hydroxy-2-formyl-fulvene, 2-hydroxy-1,1,1,5,5,5-hexafluoropent-2-ene-4-one, 9-hydroxyphen-alenone [19], and tropolone [20], two peaks are observed in the Ou region of the photoelectron spectrum (Brown et al., 1979). If these molecules existed in the C2v form with a symmetrical hydrogen bond and equivalent oxygen... 

In the two examples of buffer catalysis of proton transfer from an intramolecularly hydrogen-bonded acid which have been discussed, it seems reasonably certain that the mechanism in Scheme 7 applies. The reactions are of the first order with respect to the catalyst B and it therefore follows that proton removal from the non-hydrogen-bonded species is rate-limiting k j > 2[B]- If this step consists of diffusion-controlled proton removal from a low concentration intermediate, the value k2 lx 109dm3 moP s-1 will apply for proton transfer to an amine. In the case of proton removal by hydroxide ion from 4-(3-nitrophenylazo)salicylate ion, the reaction was found to be of the first order in hydroxide ion up to the highest concentrations which could be studied (0.003 mol dm-3) with a rate... 

Bertrand, J. A. and Eller, P. G., Polynuclear Complexes with Aminoalcohols and Iminoalcohols as Ligands Oxygen-Bridged and Hydrogen-Bonded Species 21 29... 

Some para-substituted anilines in strongly hydrogen bonding solvents show two non-equivalently hydrogen bonded species with differently hybridized aniline nitrogens72 rehybridization of aromatic amino nitrogens depends on the OH acidity of the solvent molecule and the basicity of the substituted anilines. 14 shows three possible modes of interaction of p-aminoacetophenone and acohol, where the amino group is simultaneously both a proton donor and an acceptor. 

Removal of the proton from an intramolecular hydrogen bond by a base occurs in a two-step mechanism (a rapid equilibrium between H-bonded and non-H-bonded forms followed by base catalyzed proton removal from the non-H-bonded form) rather than by direct attack of the base on the intramolecularly hydrogen bonded species (6). 

The O—H N hydrogen bond in the phenylazoresorcinol monoanion is strong, giving a large and positive value of A[S( H) — 8( H)], characteristic of a double minimum potential (Hibbert and Phillips, 1989). Two examples of intermolecularly hydrogen-bonded species for which the isotope effect on the chemical shift has been measured are also given in Table 5. 

A large number of different techniques have been used to measure isotopic fractionation factors, and any method that can be used for measuring the equilibrium position of reaction (3) is appropriate. The discussion here will be limited to the more recent methods and those that have been used for studying hydrogen-bonded species. The discussion is further limited to fractionation factors for species in solution. Recent measurements (Larson and McMahon, 1986, 1987, 1988) of

hydrogen-bonded species in the... 

This predicted pattern of behaviour is shown in Fig. 10 for some chosen hydrogen-bonded species. The value of the bond length of F HF in the solid state (see Emsley, 1980, for a discussion) and the negative value (Gunnarsson et al 1976) of the isotope effect on the chemical shift of the... 

Fig. 10 Possible variation of isotopic fractionation factors with bond strength for hydrogen-bonded species. A,AH".

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