A theory-experiment conundrum for proton transfer.

For the past 60 years, the framework for understanding the kinetic behavior of proton transfer has been transition state theory. Found throughout textbooks, this theory, along with the Bell tunneling correction, serves as the standard model for the analysis of proton/hydrogen atom/hydride transfer. In comparison, a different theoretical model has recently emerged, one which proposes that the transition state occurs within the solvent coordinate, not the proton transfer coordinate, and proton transfer proceeds either adiabatically or nonadiabatically toward product formation. This Account discusses the central tenets of the new theoretical model of proton transfer, contrasts these with the standard transition state model, and presents a discrepancy that has arisen between our experimental studies on a nonadiabatic system and the current understanding of proton transfer. Transition state theory posits that in the proton transfer coordinate, the proton must surmount an electronic barrier prior to the formation of the product. This process is thermally activated, and the energy of activation is associated with the degree of bond making and bond breaking in the transition state. In the new model, the reaction path involves the initial fluctuation of the solvent, serving to bring the reactant state and the product state into resonance, at which time the proton is transferred either adiabatically or nonadiabatically to form the product. If this theory is correct, then all of the deductions derived from the standard model regarding the nature of the proton transfer process are called into question. For weakly hydrogen-bonded complexes, two sets of experiments are presented supporting the proposal that proton transfer occurs as a nonadiabatic process. In these studies, the correlation of rate constants to driving force reveals both a normal region and an inverted region for proton transfer. Yet, the experimentally observed kinetic behavior does not align with the recent theoretical formulation for nonadiabatic proton transfer, underscoring the gap in the collective understanding of proton transfer phenomena.

Picosecond kinetic study of the photoinduced homolysis of benzhydryl acetates: the nature of the conversion of radical pairs into ion pairs.

The conversion of benzhydryl acetate geminate radical pairs to contact ion pairs following photoinduced homolysis in solution is studied using picosecond pump-probe spectroscopy. The dynamics for the decay of the geminate radical pairs into contact ion pairs is modeled within a Marcus-like theory for nonadiabatic electron transfer. A second decay channel for the geminate radical pairs is diffusional separation to free radicals. The kinetics of this latter process reveals an energy of interaction between the two radicals in the geminate pair.

Nonadiabatic proton/deuteron transfer within the benzophenone-triethylamine triplet contact radical ion pair: exploration of the influence of structure upon reaction.

The dynamics of proton transfer within a variety of substituted benzophenone-triethylamine triplet contact radical ion pairs are examined in the solvents acetonitrile and dimethylformamide. The correlation of the proton-transfer rate constants with DeltaG reveals an inverted region. The kinetic deuterium isotope effects are also examined. The solvent and isotope dependence of the transfer processes are analyzed within the context of the Lee-Hynes model for nonadiabatic proton transfer. Theoretical analysis of the experimental data suggests that the reaction path for proton/deuteron transfer involves tunneling, and the origin of the inverted region is attributed to a curved tunneling path.

Dynamic processes leading to covalent bond formation for S(N)1 reactions.

The SN1 reaction mechanism is one of the most fundamental processes in organic chemistry. As such, it has been the subject of study for over 70 years with the purpose of seeking to understand the fundamental parameters that control reactivity. With recent advances in both electronic structure theory and condense-phase reaction dynamics theory as well as in experimental probes of these reactions on the femtosecond and picosecond time scale, we are beginning to gain new insights into the nature of these reactions.

Further evidence of an inverted region in proton transfer within the benzophenone/substituted aniline contact radical ion pairs; importance of vibrational reorganization energy.

The dynamics of proton transfer within the triplet contact radical ion pair of a variety of substituted benzophenones with N,N-diethylaniline, N,N-dimethyl-p-toluinide, and N,N-diallylaniline are examined in solvents of varying polarity. The correlation of the rate constants with driving force reveal both a normal region and an inverted region providing support for the nonadiabatic nature of proton transfer within these systems. The reorganization of both the solvent and the molecular framework are central in governing the overall reaction dynamics.

Dynamic nature of the transition state for the SN1 reaction mechanism of diphenylmethyl acetates.

Picosecond absorption spectroscopy is employed in the study of the reaction dynamics for the contact ion pairs produced upon the photolysis of a series of substituted diphenylmethyl acetates in the solvents acetonitrile, dimethyl sulfoxide, and 2,2,2-trifluoroethanol. From the temperature dependence of the rate constants, the activation parameters associated with covalent bond formation and diffusional separation to the solvent-separated ion pair are obtained. The activation parameters for bond formation are examined within the context of the Hynes theory for solvent dynamical effects on the passage through the transition state; deviations from the transition-state theory are found to be large. Factors that control nucleophilicity are discussed. Finally the validity of applying the Marcus equation to the SN1 reaction mechanism is addressed.

Orbital Symmetry Control in the Photochemistry of trans-2,3-Diphenyloxirane.

We report quantum yields following 266 nm photolysis of trans-2,3-diphenyloxirane (TDPO) for the formation of the trans-ylide (Scheme 1, Phi = 0.099 +/- 0.014), cis-2,3-diphenyloxirane (CDPO, Phi = 0.10 +/- 0.009), benzaldehyde (Phi = 0.47 +/- 0.04), and deoxybenzoin (Phi = 0.077 +/- 0.002). Photolysis of TDPO may lead to both orbital symmetry allowed and forbidden products, but the trans-ylide decays solely via an orbital symmetry predicted pathway to CDPO. Coupling our quantum yields to results of Das and co-workers,(1) we report an extinction coefficient for the trans-ylide of 9.4 x 10(4) M(-)(1) cm(-)(1) at 470 nm. We also report pseudo-first-order rate constants for the reaction between maleic anhydride and the trans-ylide in acetonitrile (k(Q) = 3.32 +/- 0.04 x 10(9) M(-)(1) s(-)(1)) and in cyclohexane (k(Q) = 5.36 +/- 0.07 x 10(9) M(-)(1) s(-)(1)) and between fumaronitrile and the trans-ylide in acetonitrile (k(Q) = 1.57 +/- 0.02 x 10(9) M(-)(1) s(-)(1)) and cyclohexane (k(Q) = 3.69 +/- 0.04 x 10(9) M(-)(1) s(-)(1)). We report the crystal structure of rac-(2R,3R,4R,5R)-3,4-dicyano-2,5-diphenyltetrahydrofuran, the sole product of 1,3-dipolarophilic addition between the trans-ylide and fumaronitrile in cyclohexane. A brief discussion of solvent and steric effects in 1,3-dipolarophilic additions is included.