Proton transfer along hydrogen bonds

Recently, new experimental techniques have been added to the existing methods of obtaining information about the potential energy function of a proton in a hydrogen bond X - H- Y. Four types of potential function are 

The location of protons in intermolecularly hydrogen-bonded carboxylate-carboxylic acid complexes, for example sodium hydrogen bis(acetate) [1], 

In the hydrogen bis(quinuclidone) cation [2] the N H N distance is short (263.5 pm) compared with the van der Waals contact distance (ca 300 pm) and the proton is placed centrally (Roziere et al., 1982). However, the N H- N hydrogen bond in hydrogen bis(9-ethylguanine) [3] (bond length 263.7 pm) is thought to be asymmetric (Mandel and Marsh, 1975). 

Infra-red and ultra-violet spectroscopy has been widely used for investigating the structure of intermolecularly hydrogen-bonded complexes in the solid state (Novak, 1974) and in solution (Zundel, 1976, 1978 Clements et al., 1971a,b,c Pawlak et al., 1984). By analysing the infra-red spectra of equimolar liquid mixtures of amines with formic or acetic acid, the relative importance of structures [10] and [11] was estimated (Lindemann and Zundel, 1977). It was proposed that [10] and [11] make equal contributions to the observed structure of the complex when the p/C,-value of the carboxylic acid is approximately two units lower than that of the protonated amine. 

Hence for this complex the proton is held roughly equally between the carboxylate and amine bases in a double-minimum potential well. 

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Tunnelling spectroscopy is unique to observing quantum nonlinear dynamics in crystals. Evidence for proton transfer along hydrogen bonds is another outstanding contribution of INS. It is another manifestation of the decoupling of proton dynamics from the crystal lattice. The quantum nature of proton transfer dynamics even at room temperature is quite unforeseen and contrasts with mechanisms based on semiclassical diffusion jumps. 

The transfer of a proton between an acidic and a basic group within the same molecule is often more complex than the process shown in (1). The proton may be transferred along hydrogen-bonded solvent molecules between the acidic and basic groups if these are too remote to permit formation of an intramolecular hydrogen bond. Alternatively, two inter-molecular proton transfers with an external acid or base may be necessary. Tautomerisation of oxygen and nitrogen acids and bases (3) will be described in Section 6. The reactions are usually quite rapid and fast reaction... 

A severe consequence of the overestimation of the intermolecular distance at the Hartree-Fock level of theory is evident in the potential curve for proton transfer. At the longer distances, Hartree-Fock potential curves often exhibit two distinct minima, corresponding to traditional A-H- -B hydrogen bonds and to hydrogen-bonded A - - - H-B ion pairs. Because early studies which examined proton transfer in hydrogen-bonded complexes between second-period elements A and B were carried out at the Hartree-Fock level, models for proton transfer were developed which include double minima along the proton transfer coordinate. However, more recent calculations at the MP2 level indicate that only a single... 

In aqueous solutions we see enhanced mobility and conductivity of the hydrogen ions, which is caused by additional proton transfer along chains of water molecules linked by hydrogen bonds (see Section 7.2.4). Solutions with nonaqueous, proton-containing solvents (e.g., in ammonia) sometimes also exhibit enhanced hydrogen... 

To conclude our description of techniques, the use of nanosecond and picosecond spectroscopy which has been applied to excited state intramolecular proton transfer (ESIPT) will be mentioned briefly (Beens et al., 1965 Huppert et al., 1981 Hilinski and Rentzepis, 1983). A large number of inter-and intramolecular proton transfers have been studied using these methods (Ireland and Wyatt, 1976) but in the case of processes which are thought to involve simple proton transfer along an intramolecular hydrogen bond it is usually only possible to estimate a lower limit for the rate coefficient. 

HsO ) in one region of an aqueous solution to produce a hydronium ion at a distant site. Note that the proton released locally from the initial HsO remains in its vicinity, and is not the same as the proton forming the hydronium ion at the distant site. For this reason, the ionic mobility appears to be much greater than would be expected on the basis of diffusion alone. Facilitated proton transfer along rigidly and accurately positioned hydrogen bonds could be of fundamental importance in enzyme catalysis. See Water... 

Commonly, intermolecular proton transfer along a hydrogen bond, B, can be represented as the formation of a new hydrogen-bonded complex, C ... 

The process in eq. (3.3) is very close to proton transfer along the lines of that in classical hydrogen bonds. By analogy, it has been suggested that proton-hydride bonding interactions, +H, precede the protonation of tran-... 

As we have already noted, classical hydrogen bonds can be thermodynamically very strong but at the same time, easy to transform, due to fast proton transfer along a strong hydrogen bond. Such a dualism can also be seen in the dihydrogen bonding Y-H- H-X,... 

By proton inventory, a technique that determines whether acid and base groups act simultaneously, we found that hydrolysis of 36 by artificial enzyme 44 involves two protons moving in the transition state [130]. Thus, ImH+ of 46 is hydrogen bonded to a phosphate oxyanion of bound substrate 36 water hydrogen bonded to the Im then attacks the phosphorus, and as the O-P bond forms the ImH+ proton transfers (along with the water proton) to produce the phosphorane monoanion 47. This then goes on to the cleaved product in later catalyzed steps before there is time for pseudo-rotation. These general conclusions have been described and summarized in several publications [131-137]. 

A special type of pseudo-polymorphism is that related to the proton transfer along an X-H Y interaction. The motion may not be associated with a phase transition, but may well imply the transformation of a molecular crystal into a molecular salt. Wilson [34] has discussed, on the basis of an elegant neutron diffraction study, the migration of the proton along an O-H O bond in a co-crystal urea-phosphoric acid (1 1) whereby the proton migrates towards the mid-point of the hydrogen bond as the temperature is increased, becoming essentially centred at T > 300 K. Wiechert and Mootz [35] isolated two crystalline materials composed of pyridine and formic acid of different composition. In the 1 1 co-crystal the formic acid molecule retains its proton and transfer to the basic N-atom on the... 

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