Identity Proton-Transfer Reactions from C−H, N−H, and O−H Acids. An ab Initio, DFT, and CPCM-B3LYP Aqueous Solvent Model Study

Abstract

Identity proton-transfer reactions between 21 acids, Y-X-H, and their conjugate bases, (-)X-Y, were studied according to the reaction scheme, Y-X-H + (-)X-Y --> (Y-X-H...(-)X-Y)(cx) --> [Y-X...H...X-Y](ts) --> (Y-X..H-X-Y)(cx) --> Y-X(-) + H-X-Y, where cx indicates an ion-molecule complex and ts indicates the proton-transfer transition state. All species were optimized at the MP2/6-311+G level, and these geometries were used for single-point calculations by other methods: coupled-cluster, DFT (gas phase), and a polarizable continuum aqueous solvent model (COSMO). All methods gave enthalpies of deprotonation which correlate well with experimental measurements of deltaH(ACID) (gas) or pK(a) (aq). Calculated gas-phase enthalpies of deprotonation (deltaH(ACID)) and enthalpies of activation (deltaH(#)) are poorly correlated except for small, carefully selected sets. This result stands in contrast to the many aqueous phase Brönsted correlations of kinetic and equilibrium acid strength. On the other hand, gas-phase enthalpies of complexation and deltaH(#) are well correlated, indicating that factors which stabilize the transition state are at work in the bimolecular ion-molecule complex although to a smaller degree. We infer that intermoiety electrostatic and other interactions, similar within the complex and the transition state, but absent in the separated reactants (products), cause the lack of correlation between deltaH(ACID) and the other two quantities. Such differences are strongly attenuated in water because reactants and products do interact with polar/polarizable matter (the solvent) if not with each other. Charge distributions (NPA) were computed, allowing calculation of Bernasconi's "transition state imbalance parameter". Such measures provide intuitively satisfactory trends, but only if the reaction termini, X, are kept the same. As X is made more electronegative, the magnitude of the apparent imbalance increases, a result of greater negative charge on X in the transition state. This result gives additional support for the importance of the ion-triplet structure, [YX(-)...H(+)...(-)XY], to the stability of the transition state. Additional qualitative support for this conclusion is provided by the inverse relationship between the activation barrier and the charge on the in-flight hydrogen in the transition state, and by the dominance of polar over resonance substituent effects on the stability of the transition state. Calculations also show that the "nitroalkane anomaly", well established in solution, does not exist in the gas phase. The COSMO model partly reproduces this anomaly and performs adequately except when strong, specific intermolecular forces such as hydrogen bonding between solvent and anions are important.

Absolute Rate Calculations: Atom and Proton Transfers in Hydrogen‐Bonded Systems

“…The valence bond picture of such transition states reveals that the height of the barrier depends on the energy of the structure Y − ⋅⋅H + ⋅⋅X − , and that its energy is stabilized by the electrostatic interaction between the central proton and the X and Y groups 23. Ab initio calculations also support the stabilization of the transition state by an increase in the charge on the in‐flight hydrogen in the transition state, or equivalently, as reaction termini X and Y become more electronegative 25…”

The Role of Carbonate and Sulfite Additives in Propylene Carbonate-Based Electrolytes on the Formation of SEI Layers at Graphitic Li-Ion Battery Anodes

“…The solvent effect is included in both optimization and single point calculations through the polarized continuum using the conductor polarizable continuum model (CPCM) [57][58][59][60][61] with tetrahydrofuran (THF) dielectric. Density functional theory calculations are performed with hybrid parameter B3LYP as implemented in Gaussian 09W.…”

Effect of Allylic Groups on SN2 Reactivity

“…To do this, we use acidities as measured by the calculated Δ H ACID values for the R–CH 3 compounds shown in Table 7, taking advantage of a previously established relationship between calculated (MP2/6-311+G**) Δ H ACID values 28 and gas-phase substituent constants. 29 The acids considered here are two singlet carbenes and 6-methylfulvene.…”

“…By this measure, the electron-withdrawing group (EWG) order is CH ≥ CCH 3 > fulvenyl. We can then estimate the resonance portion of the electron withdrawing effects in the form of the gas phase resonance substituent constant, σ R- 29 using an established three-parameter correlation obtained for Δ H ACID values obtained in earlier work, 28 given here as eq 1, and in which the resonance effect, ρ R – = 143.2, is seen to be dominant.…”