Proton transfer,

Transfer of a P-proton from the propagating carbocation is the most important chain-breaking reaction. It occurs readily because much of the positive charge of the cationic propagating center resides not on carbon, but on the P-hydrogens because of hyperconjugation. Monomer, counterion or any other basic species in the reaction mixture can abstract a P-proton. Chain transfer to monomer involves transfer of a P-proton to monomer with the formation of terminal unsaturation in the polymer. 

There are two different types of P-protons, and two different unsaturated end groups are possible for isobutylene as well as some other monomers such as indene and a-methylstyr-ene. The relative amounts of the two end groups depend on the counterion, identity of the propagating center, and other reaction conditions. Only one type of unsaturated end group (internal) is possible for other monomers such as styrene, ethyl vinyl ether, and coumarone. 

It should be noted that the kinetic chain is not terminated by this reaction since a new propagating species is regenerated. Many polymer molecules are usually produced for each initiator-coinitiator species present. Chain transfer to monomer is on much more 

Chain transfer to monomer is the principal reaction that limits polymer molecular weight for most monomers, especially at reaction temperatures higher than about 20°C. Since chain transfer to monomer generally has a higher activation energy than propagation, it is usually suppressed by working at lower reaction temperatures. 

Another type of chain transfer to monomer reaction is that involving hydride ion transfer from monomer to the propagating center [Kennedy and Squires, 1967]. 

Proton transfer could certainly be another full chapter in this book. With applications ranging from photosynthesis to fuel cells this is one of the most important elementary reactions and as such was and is intensively investigated. This section does not pretend to provide any coverage of this process, and is included here mainly as a reminder that this important reaction should be on the mind of a researcher in condensed phase chemical dynamics. It is also of interest to point out an interesting 

What makes this reaction conceptually special is that there is no simple answer to this question. Rather, proton transfer should probably be described with respect to two coordinates The solvent reorganization energy that constituted the reaction coordinate of electron transfer reactions and the proton position between its two sites. On this two-dimensional free energy surface one coordinate (proton position) is quantum. The other (solvent reorganization) is essentially classical. This combination of higher dimensionality and mixed quantum and classical dynamics, together with the availability of an additional observable—the kinetic isotope effect associated with the reaction, make proton transfer a unique process.  

Here we present the derivation of the expression (16.97) that relates the coupling between two nonadiabatic electronic states a and b to the optical transition dipole between the corresponding adiabatic states 1 and 2, as described in Section 16.10. 

The eigenstates Tfr and if2 of this Hamiltonian are, by definition, the adiabatic states, which are exact states in the Bom-Oppenheimer approximation. They are 

Further reading K. D. Kreuer, Proton conductivity Materials and applications, Chem. Mater. 8, 610 (1996) K. Ando and J. T. Hynes, Adv. Chem. Phys. 110, 381 (1999) Philip M. Kiefer and J. T. Hynes, Sol. St. Ionics, 168, 219 (2004). 

Proton transfer mechanisms rely on several, generally slow, dynamical events. They are much studied at ambient temperatures in the case of biological samples but high temperatures are needed to obtain significant conduction from crystalline solids. These studies often rely on quasielastic neutron scattering techniques that are not discussed in this book, but see [47]. However, at low temperatures INS spectroscopy can provide direct information on the potential surface that controls the fast step, where the hydrogen atom transfers from the donor well to the acceptor well. 

Proton transfer occurs in the crystaUine addition complexes of orthophosphoric acid with hydrazine. 

Both hydrazinium salts contain a three-dimensional network of H bonds formed from O-H-O linkages between the 02P(0H)j anions and N-H-O linkages from the N2HJ or N2H6+ cations to the same anions. 

Internal proton transfer takes place in crystalline acid monoesters (13.18) and most amine derivatives which form addition complexes with orthophosphoric acid should be correctly assigned a zwitterion formulation (13.19). 

R NH2 H3PO4 R NHJH2P04 Some typical salts of biochemical interest are 

0-Phosphoserine N-Phosphoarginine Urea phosphate Spermine phosphate Spermidine phosphate Putrescinium phosphate 

2 Other Contributions to Thermodynamics of Association Proton Transfer 

Some of the largest rate enhancements due to solvent change have been observed for proton transfer processes. For example, the rate of the methoxide-catalyzed racemi-zation of 2-methyl-3-phenylpropionitrile, PhCHjCH(CH3)CN, is increased by a factor of 10 on going from methanol to DMSO (Cram et ah, 1960). Racemization processes generally involve rate-determining proton abstraction followed by fast reaction with a solvent to yield a planar carbanion. Fast protonation of this carbanion results in the formation of the two enantiomers in equal proportion. 

Isotopic hydrogen exchange follows a similar mechanism, as shown in Equations 6.19 and 6.20 for the case of a protium-containing substrate undergoing exchange with 

The isotopic exchange between molecular hydrogen (dihydrogen) and a hydroxyUc solvent under base catalysis poses a rather intriguing mechanistic problem. The two main mechanisms that had been proposed (Symons and Buncel, 1972) involve ratedetermining proton transfer (Eqs. 6.22 and 6.23) or formation of an addition complex (Eqs. 6.24 and 6.25)  

The rate constant for exchange increases by ca. 10 on changing the medium composition from purely aqueous to 99.5 mole % DMSO at 65°. It is significant that this increase in rate is considerably less than observed in many other reactions for 

TABLE 6.7 Enthalpies of IVansfer (kcalMol for Reactants and for the IVansition State of the D -HO Exchange Process in the DMSO-H O System  

El (top), methane Cl (middle) and isobutane Cl (bottom) mass spectra of butyl methacrylate. The ionization techniques (El vs Cl) and the reagent gases (methane vs isobutane) influence the amount of fragmentation and the prominence of the protonated molecular ions detected at 143 Th. 

The acidity of excited molecules often differs substantially from that in the ground state and the ionization of excited acids may occur adiabatically (Section 2.3), yielding the 

In order to place the transition energies /i(l o(AH) and /i(l (I(A ), which are determined from the absorption spectra of AH and A, on the free energy scale, we ignore volume and entropy effects, that is, we assume that the changes in volume and entropy associated with ionization in the ground state, Aionym(AH) and Aion5 m(AH), are similar to those in the excited state (Equation 5.9). 

Case Study 5.1 Mechanistic photochemistry - adiabatic proton transfer reactions of 2-naphthol and 4-hydroxyacetophenone 

The wavenumbers of the 0 0 transitions for the acid, AH, and the base, A, estimated from the intersection points of the corresponding absorption and fluorescence spectra, are Vo o(AH) = 2.95 pm 1 and Vo o(A ) = 2.65 pm By inserting these data into Equation 5.10 we obtain pA a = 3.2 for the acidity constant of 10 in the excited singlet state. Thus the acidity of 10 is predicted to increase by over six orders of magnitude upon excitation to the lowest singlet state. The prediction was tested by a fluorescence titration The relative fluorescence intensity at the emission 

The sharply increased acidity of phenols in the excited state can be used to lower the pH of aqueous solutions by a pulsed light source within nanoseconds (photoacid). However, the equilibrium is rapidly re-established in the ground state by diffusion-controlled recombination of the released protons with the basic phenolates. 

In 1923, a theory for acids and bases was developed by two Danish chanists, Johannes Br0nsted and Niels Bjerrum, and independently by the English chanist Martin Lowry. The chemical equilibrium between adds and bases is given by 

HA and BH are acids and B and A are bases. HCl is a strong acid in aqueous solution and NH3 is a base. Protons cannot exist in a free state in a water solution, but are bonded to the water molecules. Thus, in an aqueous solution. Equation 9.1 has to include a water molecule 

FIGURE 9.1 Hydrogen bond (dashed line) between NH and H2O (a) and between NH3 and HjO (b), as obtained in an ah initio calculation. In (a), all remaining NH bonds are 1.009 A. 

In work that has been ongoing since the late 1960s, Pizer and co-workers extensively examined the physical properties of trigonal boron acid - ligand complexation. From initial studies it was observed that the overall reaction proceeded with a change in geometry at boron, from trigonal to tetrahedral, on complexation. 

The observations were rationalised on the basis of the leaving group expelled and the minimisation of charge repulsion. In the case where a fully protonated ligand was complexed, the more acidic proton would be transferred from the ligand to the hydroxyl on boron to allow the elimination of water. One 

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Still another type of adsorption system is that in which either a proton transfer occurs between the adsorbent site and the adsorbate or a Lewis acid-base type of reaction occurs. An important group of solids having acid sites is that of the various silica-aluminas, widely used as cracking catalysts. The sites center on surface aluminum ions but could be either proton donor (Brpnsted acid) or Lewis acid in type. The type of site can be distinguished by infrared spectroscopy, since an adsorbed base, such as ammonia or pyridine, should be either in the ammonium or pyridinium ion form or in coordinated form. The type of data obtainable is illustrated in Fig. XVIII-20, which shows a portion of the infrared spectrum of pyridine adsorbed on a Mo(IV)-Al203 catalyst. In the presence of some surface water both Lewis and Brpnsted types of adsorbed pyridine are seen, as marked in the figure. Thus the features at 1450 and 1620 cm are attributed to pyridine bound to Lewis acid sites, while those at 1540... 

Several processes are unique to ions. A common reaction type in which no chemical rearrangement occurs but rather an electron is transferred to a positive ion or from a negative ion is tenued charge transfer or electron transfer. Proton transfer is also conunon in both positive and negative ion reactions. Many proton- and electron-transfer reactions occur at or near the collision rate [72]. A reaction pertaining only to negative ions is associative detaclunent [73, 74],... 

After some straightforward manipulations of A3.8.22. the PI-QTST estimate of the proton transfer rate constant can be shown to be given by 48... 

Figure A3.8.3 Quantum activation free energy curves calculated for the model A-H-A proton transfer reaction described 45. The frill line is for the classical limit of the proton transfer solute in isolation, while the other curves are for different fully quantized cases. The rigid curves were calculated by keeping the A-A distance fixed. An important feature here is the direct effect of the solvent activation process on both the solvated rigid and flexible solute curves. Another feature is the effect of a fluctuating A-A distance which both lowers the activation free energy and reduces the influence of the solvent. The latter feature enliances the rate by a factor of 20 over the rigid case.

Lobaugh J and Voth G A 1994 A path integral study of electronic polarization and nonlinear coupling effects in condensed phase proton transfer reactions J. Chem. Phys. 100 3039... 

Kuipers E W, Vardi A, Danon A and Amirav A 1991 Surface-molecule proton transfer—a demonstration of the Eley-Ridel mechanism Phys.Rev. Lett. 66 116... 

Marzocchi M P, Mantini A R, Casu M and Smulevich G 1997 Intramolecular hydrogen bonding and excited state proton transfer in hydroxyanthraquinones as studied by electronic spectra, resonance Raman scattering, and transform analysis J. Chem. Phys. 108 1-16... 

Chudoba C, Riedle E, Pfeiffer M and Elsaesser T 1996 Vibrational coherence in ultrafast excited-state proton transfer Cham. Phys. Lett. 263 622-8... 

The phenomenon of intemiolecular exchange is very common. The loss of couplings to hydroxyl protons in all but the very purest etiianol samples was observed at a very early stage. Proton transfer reactions are still probably the most carellilly studied [14] class of intemiolecular exchange. 

Viappiani C, Bonetti G, Carcelli M, Ferrari F and Sternieri A 1998 Study of proton transfer processes in... 

Barbara P F, Walker G C and Smith T P 1992 Vibrational modes and the dynamic solvent effect in electron and proton transfer Science 256 975-81... 

Covalent bonding, in all the cases so far quoted, produces molecules not ions, and enables us to explain the inability of the compounds formed to conduct electricity. Covalently bonded groups of atoms can, however, also be ions. When ammonia and hydrogen chloride are brought together in the gaseous state proton transfer occurs as follows ... 

The other halides dissociate at lower temperatures and, if put into water, all are decomposed, the proton transferring to water which is a better electron pair donor ... 

Fig. 1. The rate-determining step in the neutral hydrolysis of paramethoxy-phenyl dichloroacetate. In the reactant state (a) a water molecule is in proximity of the carbonyl carbon after concerted proton transfer to a second water molecule and electron redistribution, a tetrahedral intermediate (b) is formed.

Berendsen, H.J.C., Mavri, J. Simulating proton transfer processes Quantum dynamics embedded in a classical environment. In Theoretical Treatments of Hydrogen Bonding, D. Hadzi, ed., Wiley, New York (1997) 119-141. 

Drukker, K., Hammes-Schiffer, S. An analytical derivation of MC-SCF vibrational wave functions for the quantum dynamical simulation of multiple proton transfer reactions Initial application to protonated water chains. J. Chem. Phys. 107 (1997) 363-374. 

Bala, P., Lesyng, B., McCammon, J.A. Application of quantum-classical and quantum-stochastic molecular dynamics simulations for proton transfer processes. Chem. Phys. 180 (1994) 271-285. 

Mavri, J., Berendsen, H.J.C., Van Gunsteren, W.F. Influence of solvent on intramolecular proton transfer in hydrogen malonate. Molecular dynamics study of tunneling by density matrix evolution and nonequilibrium solvation. J. Phys. Chem. 97 (1993) 13469-13476. 

Hammes-Schiffer, S., Tully, J.C. Proton transfer in solution Molecular dynamics with quantum transitions. J. Chem. Phys. 101 (1994) 4657 667. 

Van der Spoel,D., Berendsen, H.J.C. Determination of proton transfer rate constants using ab initio, molecular dynamics and density matrix evolution calculations. Pacific Symposium on Biocomputing, World Scientific, Singapore (1996) 1-14. 

Edsall, J. T. George Scatchard, John G. Kirkwood, and the electrical interactions of amino acids and proteins. Trends Biochem. Sci. 7 (1982) 414-416. Eigen, M. Proton transfer, acid-base catalysis, and enzymatic hydrolysis. Angew. Chem. Int. Ed. Engl. 3 (1964) 1-19. 

This is followed by proton transfer to give the intermediate (IV). 

Turning the argument around reactions that do not involve proton transfer steps will only experience a significant effect of the Lewis acids if a direct interaction exists between catalyst and reactant. The conventional Diels-Alder reaction is a representative of this class of reactions. As long as monodentate reactants are used, the effects of Lewis acids on this reaction do not exceed the magnitude expected for simple salt effects, i.e. there are no indications for a direct interaction between Lewis-acid and substrate. 

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