The intersecting-state model: a link between molecular spectroscopy and chemical reactivity

Nucleophilicities and Nucleofugalities of Thio- and Selenoethers

Rate constants for the reactions of dialkyl chalcogenides with laser flash photolytically generated benzhydrylium ions have been measured photometrically to integrate them into the comprehensive benzhydrylium‐based nucleophilicity scale. Combining these rate constants with the previously reported equilibrium constants for the same reactions provided the corresponding Marcus intrinsic barriers and made it possible to quantify the leaving group abilities (nucleofugalities) of dialkyl sulfides and dimethyl selenide. Due to the low intrinsic barriers, dialkyl chalcogenides are fairly strong nucleophiles (comparable to pyridine and N‐methylimidazole) as well as good nucleofuges; this makes them useful group‐transfer reagents.

Hydrogen abstraction of carbon/phosphorus-containing radicals in photoassisted polymerization

Free-radical-promoted photopolymerization has successfully improved the curing performance in cationic photopolymerization and is now employed in promoted autoxidation.

Farewell to the HSAB treatment of ambident reactivity

The concept of hard and soft acids and bases (HSAB) proved to be useful for rationalizing stability constants of metal complexes. Its application to organic reactions, particularly ambident reactivity, has led to exotic blossoms. By attempting to rationalize all the observed regioselectivities by favorable soft-soft and hard-hard as well as unfavorable hard-soft interactions, older treatments of ambident reactivity, which correctly differentiated between thermodynamic and kinetic control as well as between different coordination states of ionic substrates, have been replaced. By ignoring conflicting experimental results and even referring to untraceable experimental data, the HSAB treatment of ambident reactivity has gained undeserved popularity. In this Review we demonstrate that the HSAB as well as the related Klopman-Salem model do not even correctly predict the behavior of the prototypes of ambident nucleophiles and, therefore, are rather misleading instead of useful guides. An alternative treatment of ambident reactivity based on Marcus theory will be presented.

The rates of S(N)2 reactions and their relation to molecular and solvent properties

The energy barriers of symmetrical methyl exchanges in the gas phase have been calculated with the reaction path of the intersecting/interacting-state model (ISM). Reactive bond lengths increase down a column of the Periodic Table and compensate for the decrease in the force constants, which explains the near constancy of the intrinsic barriers in the following series of nucleophiles: F(-) approximately Cl(-) approximately Br(-) approximately I(-). This compensation is absent along the rows of the Periodic Table and the trend in the reactivity is dominated by the increase in the electrophilicity index of the nucleophile in the series C<N<O<F. Solvent effects have been quantitatively incorporated into the ISM model through a correlation between electrophilicity and the solvent acceptor number. This correlation is transferable between nucleophiles and solvents and allows the methyl transfer rate constants in solution to be calculated with remarkable simplicity and accuracy. The relationship between the S(N)2 and electron-transfer mechanisms is clarified and it is shown that smaller solvent static effects should be expected for electron transfer in the absence of a thermodynamic driving force.

Nucleophilicity-periodic trends and connection to basicity

The potential energy profiles of 18 identity S(N)2 reactions have been estimated by using G2-type quantum-chemical calculations. The reactions are: X- + CH3-X --> X-CH3 + X- and XH + CH3-XH+ --> +HX-CH3 + XH (X = NH2, OH, F, PH2, SH, Cl, AsH2, SeH, Br). Despite the charge difference, the barrier heights and the geometrical requirements upon going from the reactant to the transition structure are surprisingly similar for X- and XH. The barrier heights decrease on going from left to right in the periodic table, and increasing ionization energy (of X- and XH) is correlated with decreasing barrier. The observed trends are explained in terms of substrates with stronger electrostatic character giving rise to lower energetic barriers due to decreased electron repulsion in the transition structure. On the basis of this study, the relationship between the kinetic concept of nucleophilicity and the thermodynamic concept of basicity has been analyzed and clarified. Since the trends in intrinsic nucleophilicity (only defined for identity reactions) and basicity are opposite, overall nucleophilicity (defined for any reaction) will be determined by the relative contribution of the two factors. Only for strongly exothermic reactions will basicity and nucleophilicity be matching.

Strain energy release and intrinsic barriers in internal nucleophilic reactions

This paper reports computational data for the energetics of internal attacks, both in ring-opening reactions (eq 3) where strain energy is released and in model, strain-free systems (eq 4). A comparison is drawn with the corresponding bimolecular processes. The exothermicity of three-membered ring-opening reactions is significantly larger than that of the four-membered ring systems. However, using the Marcus equation, it is shown that the higher reactivity of the three-membered rings is intrinsic to the system and does not stem only from a higher thermodynamic driving force. The intrinsic barriers for the strain-free reactions are shown to be dominated by the position of the nucleophilic and nucleofugic atoms in the periodic table, as in the bimolecular SN2 reactions, although a pi rather than a sigma bond is formed in these reactions.

Absolute Rate Calculations. Proton Transfers in Solution

The reaction path of the intersecting-state model is used in transition-state theory with the semiclassical correction for tunneling (ISM/scTST) to calculate the rates of proton-transfer reactions from hydrogen-bond energies, reaction energies, electrophilicity indices, bond lengths, and vibration frequencies of the reactive bonds. ISM/scTST calculations do not involve adjustable parameters. The calculated proton-transfer rates are within 1 order of magnitude of the experimental ones at room temperature, and cover very diverse systems, such as deprotonations of nitroalkanes, ketones, HCN, carboxylic acids, and excited naphthols. The calculated temperature dependencies and kinetic isotope effects are also in good agreement with the experimental data. These calculations elucidate the roles of the reaction energy, electrophilicity, structural parameters, hydrogen bonds, tunneling, and solvent in the reactivity of acids and bases. The efficiency of the method makes it possible to run absolute rate calculations through the Internet.

Electron Transfer in Supercritical Carbon Dioxide: Ultraexothermic Charge Recombination at the End of the “Inverted Region”

Charge-recombination rates in contact radical-ion pairs, formed between aromatic hydrocarbons and nitriles in supercritical CO(2) and heptane, decrease with the exothermicity of the reactions until they reach -70 kcal mol(-1), but from there on an increase is observed. The first decrease in rate is typical of the "inverted region" of electron-transfer reactions. The change to an increase in the rate for ultra-exothermic electron transfer indicates a new free-energy relationship. We show that the resulting "double-inverted region" is not due to a change in mechanism. It is an intrinsic property of electron-transfer reactions, and it is due to the increase of the reorganisation energy with the reaction exothermicity.

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

We calculate energy barriers of atom- and proton-transfer reactions in hydrogen-bonded complexes in the gas phase. Our calculations do not involve adjustable parameters and are based on bond-dissociation energies, ionization potentials, electron affinities, bond lengths, and vibration frequencies of the reactive bonds. The calculated barriers are in agreement with experimental data and high-level ab initio calculations. We relate the height of the barrier with the molecular properties of the reactants and complexes. The structure of complexes with strong hydrogen bonds approaches that of the transition state, and substantially reduces the barrier height. We calculate the hydrogen-abstraction rates in H-bonded systems using the transition-state theory with the semiclassical correction for tunneling, and show that they are in excellent agreement with the experimental data. H-bonding leads to an increase in tunneling corrections at room temperature.

Calculation of triplet-triplet energy transfer rates from emission and absorption spectra. The quenching of hemicarcerated triplet biacetyl by aromatic hydrocarbons

The kinetics of triplet-triplet (T-T) energy transfer have been analysed with a view to linking theories of chemical reactions (involving the rupture and formation of bonds) with theories of processes, such as electron transfer or energy transfer, which preserve chemical bonding. As for the latter, our analysis does not support the claim that, of the two rival expressions for T-T energy transfer, both rooted in the golden rule, only one is applicable to electron transfer or T-T transfer. Though the two expressions do reflect different standpoints, the distinction is eroded by the assumption of a delta-function distribution for the vibrational spectrum. It is shown that theories of chemical reactions also furnish estimates of Franck-Condon factors; rates of chemical reactions and chemical processes are both related to the properties (strengths and lengths) of the reactive bonds, but differ in the mode of energy dissipation. The relationship between the rates of reactions and processes presents new possibilities for a unified view of chemical reactivity.

Absolute rate calculations for atom abstractions by radicals: energetic, structural and electronic factors

We calculate transition-state energies of atom-transfer reactions from reaction energies, electrophilicity indices, bond lengths, and vibration frequencies of the reactive bonds. Our calculations do not involve adjustable parameters and uncover new patterns of reactivity. The generality of our model is demonstrated comparing the vibrationally adiabatic barriers obtained for 100 hydrogen-atom transfers with the corresponding experimental activation energies, after correction for the heat capacities of reactants and transition state. The rates of half of these reactions are calculated using the Transition-State Theory with the vibrationally adiabatic path of the Intersecting-State Model and the semiclassical correction for tunneling (ISM/scTST). The calculated rates are within an order of magnitude of the experimental ones at room temperature. The temperature dependencies and kinetic isotope effects of selected systems are also in good agreement with the available experimental data. Our model elucidates the roles of the reaction energy, electrophilicity, structural parameters, and tunneling in the reactivity of these systems and can be applied to make quantitative predictions for new systems.