Proton transfer, transition state structure

The dimers of phosphinic acid derivatives are one of the strongest HB complexes found in gas phase [134—137]. The chirality of the phosphinic acids relies on the presence of two nonidentical substituents on the phosphorus atom and the position of the hydrogen bonded to one of the two oxygens linked to the phosphorus. The chiral recognition in the minimum and proton transfer transition state structures of fifteen pairs of chiral phosphinic acid dimers (Scheme 3.24) has been carried out using DFT and MP2 methods, up to MP2/6-311++G(3df,2p) level [30]. 

In all these cases, ion-pair transition structures have been characterized, and the intimate nature of bonding has been discussed using the electron properties arising from ELF analysis. In particular, charge separations of 0.48e, 0.42e and 0.18e can be deduced from the basin structure and populations for the oxygen to oxygen, sulfur to oxygen and sulfur to sulfur proton-transfer transition states, respectively. 

Because a relates the sensitivity to structural changes that the proton-transfer process exhibits to that exhibited by dissociation of the acid, it is frequently assumed that the value of a can be used as an indicator of transition-state structure. The closer a approaches unity, the greater is the degree of proton transfer in the transition state. There are limits to the generality of this interpretaton, however. ... 

In fact, the barrier for proton transfer in the maleate anion appears to lie below the zero-point vibrational energy level (W. M. Westler, private communication). Thus, vibrationally averaged properties of the maleate anion will correspond to a symmetrically bridged Cjv transition-state structure rather than to either of the asymmetrically bridged equilibrium structures in Fig. 5.22. For present purposes this interesting feature of the potential surface can be ignored. 

The reaction was found to be first order with respect to amines and acrylamides and no base catalysis was observed. The reaction is believed to occur in a single step in which the addition of amine to Cp of acrylamide and proton transfer from amine to Ca of acrylamide take place concurrently with a four-membered cyclic transition-state structure. The Hammett (px) and Brpnsted (/3X) coefficients are rather small, suggesting an early transition state (TS). The sign and magnitude of the cross-interaction constant, pxy(= —0.26), is comparable to those found in the bond formation processes in the. S n2 and addition reactions. The normal kinetic isotope effect ( h/ d > 1.0) and relatively low A and large negative Avalues are also consistent with the mechanism proposed.192... 

The pattern of substituent effects on the magnitude of the isotope effects suggests a wide variation in transition state structure with changing structure of both the carbene and alcohol (Bethell, Howard and Newall, unpublished). At its limits, the range of structures could embrace both ylid and proton-transfer mechanisms. 

The preceding discussion has suggested that a (or 0) may be considered a measure of transition state structure even if the expected reactivity-selectivity relationship is not observed. There is, however, strong evidence to suggest that the Br nsted coefficient does not always reflect the degree of proton transfer in the transition state. 

The clearest example of the danger in using a as a measure of transition state structure is illustrated in the work of Bordwell et al. (1969, 1970, 1975). In the rate-equilibrium relationship for the deprotonation of a series of nitroalkanes the unprecedented Br nsted slopes of 1 61 for l-aryl-2-nitropropanes and 1-37 for 1-arylnitro-ethanes were obtained. The simple exposition of the mechanistic significance of a disallows values greater than 1. This, coupled with the fact that the transition state for the proton transfer is not product-like (as established by alternative criteria) indicates at best that, in at least some cases, a does not reflect the selectivity of a particular reaction. Several attempts to rationalize these anomalous results have been made. 

There are additional factors which may invalidate the use of a as a measure of transition state structure. Murdoch (1972) has demonstrated that for multi-step reactions, even when proton transfer is rate-determining, the value of a obtained may be greatly influenced... 

In conclusion, it is apparent that the use of the Br nsted coefficient as a measure of selectivity and hence of transition state structure appears to be based on extensive experimental data. However, the many cases where this use of the Br nsted coefficient is invalid suggest that considerable caution be used in drawing mechanistic conclusions from such data. The limitations on the mechanistic significance of a require further clarification, but the first steps in defining them appear to have been taken. The influence of change in the intrinsic barrier and variable intermolecular interactions in the transition state, both of which will result in a breakdown of the rate-equilibrium relationship, as well as internal return appear to be some of the key parameters which determine the magnitude of the Br nsted coefficient in addition to the degree of proton transfer. 

High-quality ab initio calculations of various cluster models, were in favor of the view that water is only physisorbed on the zeolite and found that the proton-transferred hydronium ion structure is a transition state between two minima corresponding to the water H-bonded to the acid site (cf. Fig. 5). 

The foregoing results indicate that it is difficult to obtain information about transition state structure from the size of the Bronsted exponent in the ionization of nitroparaffins. The current view is that proton transfer is about half-complete when the transition state is reached [76(b), 106]. It is difficult to reconcile the results for nitroparaffins with the hope that the Bronsted exponent may in general give an indication of transition state structure and there is a general tendency to treat nitroparaffins as exceptional and still use this concept in other cases. 

Fig. 10. Variation in transition state structure for a proton transfer reaction with the standard free energy change (AGj) of the reaction.

This topic has been reviewed in detail [205] and we will restrict ourselves to the question as to what predictions may be made about transition state structure (for example, the degree of proton transfer) from the size of the primary isotope effect. A theoretical treatment bearing on this point has been presented which predicts that large primary isotope effects should be observed for a proton transfer reaction occurring through a symmetrical transition state (123) in which the force constants... 

It is clear from the experimental results that the magnitude of the primary isotope effect in proton transfer to and from carbon provides a useful guide to transition state structure. It is also clear that reactions which apparently proceed through a rate-limiting proton transfer to or from carbon may not always show large primary isotope effects. Small isotope effects will be observed when the transition state is strongly product-like or reactant-like (Sect. 4.3). 

---