Hydrogen Atom Abstraction Reactions Research Articles

DFT investigation of hydrogen atom-abstraction reactions of NHC-boranes by various carbon-centered radicals: barriers and correlation analyses.

In this study, we employed a quantum-mechanical computational method to investigate the hydrogen-atom abstraction reactions of two nitrogen heterocyclic carbene boranes (NHC-boranes), NHC-BH3 and NHC-BH2CN, by a series of carbon-centered radicals bearing various substituents. We explored the degree of correlation of the activation and free energy barriers to their components. Furthermore, we also investigated the effects of the radical and substituent sizes, nucleophilicity/electrophilicity indices, and the spin density distribution of the radical reactants on the three fundamental barriers and the thermal contribution of the reaction energy to the kinetic barrier. Using the generated data, we assessed the abilities of the various radical reactants to abstract the hydrogen atom from NHC-boranes. Further, we performed a similar analysis after dividing those radical reactants into four groups, which were classified based on the dominant factor affecting their electronic density distribution, which involves the inductive effect, conjugation, hyperconjugation, and the feedback of lone-pair electrons. The results and conclusions of this investigation not only provide insight into the relationships between some of the key kinetic and thermodynamic parameters, which is useful for understanding the dynamics of such hydrogen-abstraction reactions, but also provide information for selecting suitable radical reactants for further experimental investigations.

Mechanism and Kinetics of Diuron Oxidation Initiated by Hydroxyl Radical: Hydrogen and Chlorine Atom Abstraction Reactions.

Diuron is a herbicide that has been classified as an environmental pollutant because of its harmful effects on living beings and environment. In the present work, the OH-initiated oxidation reaction of diuron is investigated by performing electronic structure calculations based on density functional theory (DFT) methods, M06-2X, ωB97X-D, and MPWB1K using the 6-31G(d,p) basis set. The CBS-QB3 method is used to validate the results obtained from the DFT methods. All possible initial hydrogen and chlorine atom abstraction reaction pathways involved in the oxidation of diuron were studied, and the favorable reaction pathways were found by analyzing the potential energy surface and thermochemistry of the reactions. The results obtained from the present work show that hydrogen atom abstraction from methyl and amine groups of diuron are energetically favorable, which leads to the formation of diuron radical intermediate and water molecule. The rate constant is calculated for most favorable reaction pathways by using canonical variation transition state theory (CVT) with small curvature tunnelling (SCT) correction over the temperature range 298-1000 K. The atmospheric lifetime of diuron is found to be around 15 days. The subsequent reaction of most favorable diuron radical intermediate with other atmospheric reactive species, such as O2, H2O, HO2, and NOx (x = 1, 2) radicals are studied. The time-dependent DFT calculation is performed to study the photolysis of diuron and favorable diuron radical intermediates. This study provides thermochemical and kinetic data for the oxidation of diuron initiated by OH radical through H atom abstraction reaction.

Rapid Acceleration of Hydrogen Atom Abstraction Reactions of OH at Very Low Temperatures through Weakly Bound Complexes and Tunneling.

A generally accepted principle of chemical kinetics is that a reaction will be very slow at low temperatures if there is an activation barrier on the potential energy surface to form products. However, this Account shows that the reverse is true for gas-phase hydrogen abstraction reactions of the hydroxyl radical, OH, with organic molecules with which it can form a weakly bound (5-30 kJ mol-1) hydrogen-bonded complex. For hydrogen atom abstraction reactions of OH with volatile organic compounds (VOCs) containing alcohol, ether, carbonyl, and ester functional groups, the reaction accelerates rapidly at very low temperatures, with rate coefficients, k, that can be up to a 1000 times faster than those at room temperature, despite the barrier to products. The OH radical is a crucial intermediate in Earth's atmosphere, combustion processes, and the chemistry of the interstellar medium, where temperatures can reach as low as 10 K, so this behavior has very important implications for gas-phase chemistry in space. The key point is that at low temperatures the lifetime of the OH-VOC complex against re-dissociation back to reactants becomes much longer, and hence the probability of quantum mechanical tunneling under the reaction barrier to form products becomes much higher. These observations were made possible by using Laval nozzles to generate uniform supersonic flows at extremely low temperatures so that condensation of the reagents at reactor walls is avoided. In this Account, the use of laser flash-photolysis combined with laser-induced fluorescence spectroscopy within Laval flows is described to study the unusual kinetics of this type of reaction at temperatures down to 21 K and demonstrate the rapid upturn in the rate coefficient. For the reaction of OH with CH3OH, further evidence for the precomplex and tunneling mechanism comes from observation of the CH3O reaction product at very low temperatures, despite it being formed over the higher barrier to reaction. The experimental observations are supported by theoretical calculations using the MESMER master equation package to calculate k( T) and product yields as a function of temperature and which make use of potential energy surfaces determined using ab initio methods. The CH3O product is formed over a narrower barrier with a larger imaginary frequency and is calculated to be the sole product at very low temperatures. The kinetics of the OH reaction with CH3OH were measured to be independent of pressure, consistent with a tunneling mechanism rather than any collisional stabilization of the prereactive complex. In this Account, we collate available kinetic data and show that this newly discovered mechanism for H atom transfer reactions appears to be generally applicable for reactions of OH with organic molecules containing oxygenated functional groups, which have been observed in space by radio-astronomy. Rather than being ignored for a range of interstellar environments, these OH reactions are now being included in chemical networks in space and have been shown to significantly influence the abundance of OH, the organic molecules themselves, and reaction products and provide novel routes to forming even more complex functional groups, for example, precursors to prebiotic molecules.

Alkyl hydrogen atom abstraction reactions of the CN radical with ethanol.

We present a study of the abstraction of alkyl hydrogen atoms from the β and α positions of ethanol by the CN radical in solution using the Empirical Valence Bond (EVB) method. We have built separate 2 × 2 EVB models for the Hβ and Hα reactions, where the atom transfer is parameterized using ab initio calculations. The intra- and intermolecular potentials of the reactant and product molecules were modelled with the General AMBER Force Field, with some modifications. We have carried out the dynamics in water and chloroform, which are solvents of contrasting polarity. We have computed the potential of mean force for both abstractions in each of the solvents. They are found to have a small and early barrier along the reaction coordinate with a large energy release. Analyzing the solvent structure around the reaction system, we have found two solvents to have little effect on either reaction. Simulating the dynamics from the transition state, we also study the fate of the energies in the HCN vibrational modes. The HCN molecule is born vibrationally hot in the CH stretch in both reactions and additionally in the HCN bends for the Hα abstraction reaction. In the early stage of the dynamics, we find that the CN stretch mode gains energy at the expense of the energy in CH stretch mode.

Theoretical and kinetic study of the hydrogen atom abstraction reactions of unsaturated C6 methyl esters with hydroxyl radical

This work reports a systematic ab initio and chemical kinetic study of the rate constants for hydrogen atom abstraction reactions by hydroxyl radical (OH) on typical isomers of unsaturated C6 methyl esters at the CBS/QB3 level of theory. The high-pressure limit rate constants at different reaction sites for all the methyl esters in the temperature range from 500 to 2000K are calculated via transition-state theory with the Wigner method for quantum tunneling effect and fitted to the modified three parameters Arrhenius expression using least-squares regression. Further, a branching ratio analysis for each reaction site has been performed.

A cobalt(ii) iminoiodane complex and its scandium adduct: mechanistic promiscuity in hydrogen atom abstraction reactions.

In addition to oxometal [M(n+)[double bond, length as m-dash]O] and imidometal [M(n+)[double bond, length as m-dash]NR] units, transient metal-iodosylarene [M((n-2)+)-O[double bond, length as m-dash]IPh] and metal-iminoiodane [M((n-2)+)-N(R)[double bond, length as m-dash]IPh] adducts are often invoked as a possible "second oxidant" responsible for the oxo and imido group transfer reactivity. Although a few metal-iodosylarene adducts have been recently isolated and/or spectroscopically characterized, metal-iminoiodane adducts have remained elusive. Herein, we provide UV-Vis, EPR, NMR, XAS and DFT evidence supporting the formation of a metal-iminoiodane complex 2 and its scandium adduct 2-Sc. 2 and 2-Sc are reactive toward substrates in the hydrogen-atom and nitrene transfer reactions, which confirm their potential as active oxidants in metal-catalyzed oxidative transformations. Oxidation of para-substituted 2,6-di-tert-butylphenols by 2 and 2-Sc can occur by both coupled and uncoupled proton and electron transfer mechanisms; the exact mechanism depends on the nature of the para substituent.

Reduced-Dimensionality Semiclassical Transition State Theory: Application to Hydrogen Atom Abstraction and Exchange Reactions of Hydrocarbons

Quantum mechanical methods for calculating rate constants are often intractable for reactions involving many atoms. Semiclassical transition state theory (SCTST) offers computational advantages over these methods but nonetheless scales exponentially with the number of degrees of freedom (DOFs) of the system. Here we present a method with more favorable scaling, reduced-dimensionality SCTST (RD SCTST), that treats only a subset of DOFs of the system explicitly. We apply it to three H abstraction and exchange reactions for which two-dimensional potential energy surfaces (PESs) have previously been constructed and evaluated using RD quantum scattering calculations. We differentiated these PESs to calculate harmonic frequencies and anharmonic constants, which were then used to calculate cumulative reaction probabilities and rate constants by RD SCTST. This method yielded rate constants in good agreement with quantum scattering results. Notably, it performed well for a heavy-light-heavy reaction, even though it does not explicitly account for corner-cutting effects. Recent extensions to SCTST that improve its treatment of deep tunneling were also evaluated within the reduced-dimensionality framework. The success of RD SCTST in this study suggests its potential applicability to larger systems.

A Theoretical Study of the Kinetics of the Hydrogen Atom Abstraction Reactions from Cyclopropane by H, O (3P), and Cl (2P3/2) Atoms and OH Radicals

ABSTRACTThe rate constants of the H‐abstraction reactions from cyclopropane by H, O (3P), Cl (2P3/2), and OH radicals have been calculated over the temperature range of 250−2500 K using two different levels of theory. Calculations of optimized geometrical parameters and vibrational frequencies are performed using the MP2 method combined with the cc‐pVTZ basis set and the 6–311++G(d,p) basis set. Single‐point energy calculations have been carried out with the highly correlated ab initio coupled cluster method in the space of single, double, and triple (perturbatively) electron excitations CCSD(T) using either the cc‐pVTZ, aug‐cc‐pVTZ, and aug‐cc‐pVQZ basis sets or the 6–311++G(3df,3pd) basis set. The CCSD(T) calculated potential energies have been extrapolated to the complete basis limit (CBS) limit. The Full Configuration Interaction (FCI) energies have been also estimated using the continued‐fraction approximation as proposed by Goodson (J. Chem. Phys., 2002, 116, 6948–6956). Canonical transition‐state theory combined with an Eckart tunneling correction has been used to predict the rate constants as a function of temperature using two kinetic models (direct abstraction or complex mechanism) at two levels of theory (CCSD(T)‐cf/CBS//MP2/cc‐pVTZ and CCSD(T)‐cf/6–311++G(3df,3pd)//MP2/6–311++G(d,p)). The calculated kinetic parameters are in reasonable agreement with their literature counterparts for all reactions. In the light of these trends, the use of the Pople‐style basis sets for studying the reactivity of other systems such as larger cycloalkanes or halogenated cycloalkanes is recommended because the 6–311++G(3df,3pd) basis set is less time consuming than the aug‐cc‐pVQZ basis set. Based on our calculations performed at the CCSD(T)‐cf/CBS//MP2/cc‐pVTZ level of theory, the standard enthalpy of formation at 298 K for the cyclopropyl radical has been reassessed and its value is (290.5 ± 1.6) kJ mol−1.

Theoretical and kinetic study of the hydrogen atom abstraction reactions of ethyl esters with hydrogen radicals

Ab initio and chemical kinetic study of the hydrogen abstraction reactions by the hydrogen radical on ethyl formate, ethyl acetate, ethyl propanoate, and ethyl butanoate have been performed at the CCSD(T)/CBS//B3LYP/6-311G(d, p) level of theory. High-pressure limit rate constants at temperatures from 300 to 2500K have been calculated for all of the reaction channels using transition state theory with Eckart tunneling corrections, and the data are fitted to the modified three parameters Arrhenius expression using least-squares regression. A branching ratio analysis for each reaction site has also been investigated for all of the ethyl esters.

Theoretical study of the rate constants for the hydrogen atom abstraction reactions of esters with (•)OH radicals.

A systematic investigation of the rate constants for hydrogen atom abstraction reactions by hydroxyl radicals on esters has been performed. The geometry optimizations and frequency calculations were obtained using the second-order Møller-Plesset method with the 6-311G(d,p) basis set. The same method was also used in order to determine the dihedral angle potential for each individual hindered rotor in each reactant and transition state. Intrinsic reaction coordinate calculations were used in order to connect each transition state to the corresponding local minimum. For the reactions of methyl ethanoate with an (•)OH radical, the relative electronic energies were calculated using the G3 and the CCSD(T)/cc-pVXZ (where X = D, T, and Q) methods, which were extrapolated to the complete basis set (CBS) limit. The electronic energies obtained using the G3 method were then benchmarked against the CBS results and were found to be within 1 kcal mol(-1) of one another. The high-pressure limit rate constants for every reaction channel were calculated by conventional transition-state theory, with an asymmetric Eckart tunneling correction, using the energies obtained with the G3 method. We report the individual, average, and total rate constants in the temperature range from 500 to 2200 K. Our calculated results are within a factor of 2 for methyl ethanoate and between 40% to 50% for methyl propanoate and methyl butanoate when compared to previously reported experimental data.

Hydrogen atom abstraction reactions independent of C–H bond dissociation energies of organic substrates in water: significance of oxidant–substrate adduct formation

Adduct formation between Ru(iv)–oxo complexes and substrates with hydrogen bonding affords condensed transition states for substrate oxidations in water.

Theoretical and Kinetic Study of the Hydrogen Atom Abstraction Reactions of Esters with HȮ2 Radicals

This work details an ab initio and chemical kinetic study of the hydrogen atom abstraction reactions by the hydroperoxyl radical (HȮ2) on the following esters: methyl ethanoate, methyl propanoate, methyl butanoate, methyl pentanoate, methyl isobutyrate, ethyl ethanoate, propyl ethanoate, and isopropyl ethanoate. Geometry optimizations and frequency calculations of all of the species involved, as well as the hindrance potential descriptions for reactants and transition states, have been performed with the Møller-Plesset (MP2) method using the 6-311G(d,p) basis set. A validation of all of the connections between transition states and local minima was performed by intrinsic reaction coordinate calculations. Electronic energies for all of the species are reported at the CCSD(T)/cc-pVTZ level of theory in kcal mol(-1) with the zero-point energy corrections. The CCSD(T)/CBS (extrapolated from CCSD(T)/cc-pVXZ, in which X = D, T, Q) was used for the reactions of methyl ethanoate + HȮ2 radicals as a benchmark in the electronic energy calculations. High-pressure limit rate constants, in the temperature range 500-2000 K, have been calculated for all of the reaction channels using conventional transition state theory with asymmetric Eckart tunneling corrections. The 1-D hindered rotor approximation has been used for the low frequency torsional modes in both reactants and transition states. The calculated individual and total rate constants are reported for all of the reaction channels in each reaction system. A branching ratio analysis for each reaction site has also been investigated for all of the esters studied in this work.

Kinetics of the Hydrogen Atom Abstraction Reactions from 1-Butanol by Hydroxyl Radical: Theory Matches Experiment and More

In the present work, we study the H atom abstraction reactions by hydroxyl radical at all five sites of 1-butanol. Multistructural variational transition state theory (MS-VTST) was employed to estimate the five thermal rate constants. MS-VTST utilizes a multifaceted dividing surface that accounts for the multiple conformational structures of the transition state, and we also include all the structures of the reactant molecule. The vibrational frequencies and minimum energy paths (MEPs) were computed using the M08-HX/MG3S electronic structure method. The required potential energy surfaces were obtained implicitly by direct dynamics employing interpolated variational transition state theory with mapping (IVTST-M) using a variational reaction path algorithm. The M08-HX/MG3S electronic model chemistry was then used to calculate multistructural torsional anharmonicity factors to complete the MS-VTST rate constant calculations. The results indicate that torsional anharmonicity is very important at higher temperatures, and neglecting it would lead to errors of 26 and 32 at 1000 and 1500 K, respectively. Our results for the sums of the site-specific rate constants agree very well with the experimental values of Hanson and co-workers at 896-1269 K and with the experimental results of Campbell et al. at 292 K, but slightly less well with the experiments of Wallington et al., Nelson et al., and Yujing and Mellouki at 253-372 K; nevertheless, the calculated rates are within a factor of 1.61 of all experimental values at all temperatures. This gives us confidence in the site-specific values, which are currently inaccessible to experiment.

Theoretical analysis of hydrogen atom abstraction reactions by radicals within the framework of generalized Polanyi-Semenov relationship taking into account the formation of pre- and post-reactive complexes

Density functional theory was used to calculate the intrinsic reaction coordinate of hydrogen atom abstraction from a number of organic molecules of different classes by C7F15 radical. These reactions involve the formation of stable pre- and post-reactive complexes with binding energies comparable to the activation barriers and reaction energies. An analysis of the results obtained using the dimensionless reaction coordinate showed that the generalized Polanyi-Semenov relationship Ea = A + 0.5ΔH + ΔH2/(2W) is fulfilled. For primary and secondary C-H bonds of esters and ketones, it reproduces the calculated activation energies with an error of at most 1 kcal mol−1 provided A = 8.5 kcal mol−1 and W = 43 kcal mol−1. The accuracy of the generalized Polanyi-Semenov relationship decreases when the enthalpy difference between the pre- and post-reactive complexes is used as the ΔH value because, as a rule, the structures of these complexes are not directly related to the structure of the transition state.

Learning about Regiochemistry from a Hydrogen-Atom Abstraction Reaction in Water

An experiment has been developed in which the hydrogen-atom abstraction and the coupling of propionitrile, using Fenton’s reagent, are investigated. Students learn about the regiochemistry of radical formation, the stereochemistry of product formation, and the interpretation of GC–MS data, in a safe reaction that can be easily completed in one laboratory session.

Hydrogen Atom Abstraction Reactions from Tertiary Amines by Benzyloxyl and Cumyloxyl Radicals: Influence of Structure on the Rate-Determining Formation of a Hydrogen-Bonded Prereaction Complex

A time-resolved kinetic study on the hydrogen atom abstraction reactions from a series of tertiary amines by the cumyloxyl (CumO(•)) and benzyloxyl (BnO(•)) radicals was carried out. With the sterically hindered triisobutylamine, comparable hydrogen atom abstraction rate constants (k(H)) were measured for the two radicals (k(H)(BnO(•))/k(H)(CumO(•)) = 2.8), and the reactions were described as direct hydrogen atom abstractions. With the other amines, increases in k(H)(BnO(•))/k(H)(CumO(•)) ratios of 13 to 2027 times were observed. k(H) approaches the diffusion limit in the reactions between BnO(•) and unhindered cyclic and bicyiclic amines, whereas a decrease in reactivity is observed with acyclic amines and with the hindered cyclic amine 1,2,2,6,6-pentamethylpiperidine. These results provide additional support to our hypothesis that the reaction proceeds through the rate-determining formation of a C-H/N hydrogen-bonded prereaction complex between the benzyloxyl α-C-H and the nitrogen lone pair wherein hydrogen atom abstraction occurs, and demonstrate the important role of amine structure on the overall reaction mechanism. Additional mechanistic information in support of this picture is obtained from the study of the reactions of the amines with a deuterated benzyloxyl radical (PhCD(2)O(•), BnO(•)-d(2)) and the 3,5-di-tert-butylbenzyloxyl radical.

Hydrogen atom abstraction reactions of the sugar moiety of 2′‐deoxyguanosine with an OH radical: A quantum chemical study

AbstractMechanisms of hydrogen atom abstraction reactions of the sugar moiety of 2′‐deoxyguanosine with an OH radical were investigated using the B3LYP and BHandHLYP functionals of density functional theory and the second order Møller–Plesset Perturbation (MP2) theory in gas phase and aqueous media. The 6‐31+G* and AUG‐cc‐pVDZ basis sets were used. Gibbs free barrier energies and rate constants of the reactions in aqueous media suggest that an OH radical would abstract the hydrogen atoms of the sugar moiety of 2′‐deoxyguanosine in the following order of preference: H5′ ≈ H5″ > H3′ > H4′ > H1′ ≈ H2′ > H2″, the rate constant for H5′ abstraction being 103–105 times greater than that for H2″ at the different levels of theory. Relative stabilities of the different deoxyribose radicals are also discussed. The most and least favored hydrogen abstraction reactions found here are in agreement with experimental observation. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

Chromium(v)-oxo and chromium(iii)-superoxo complexes bearing a macrocyclic TMC ligand in hydrogen atom abstraction reactions

A Cr(V)-oxo complex bearing a macrocyclic TMC ligand, [CrV(TMC)(O)(OCH3)]2+, was synthesized, isolated, and characterized by various physicochemical methods, including UV-vis, ESI-MS, resonance Raman, EPR and X-ray analysis. The reactivity of the Cr(V)-oxo complex was investigated in C–H and O–H bond activation reactions. The reactivity of a Cr(III)-superoxo complex, [CrIII(TMC)(O2)(Cl)]+, was investigated in O–H bond activation reactions as well. By comparing reactivities of the Cr(III)-superoxo and Cr(V)-oxo complexes under the identical reaction conditions, we were able to demonstrate that the Cr(III)-superoxo complex is more reactive than the Cr(V)-oxo complex in the activation of C–H and O–H bonds. The present results provide strong evidence that under certain circumstances, metal-superoxo species can be an alternative oxidant for high-valent metal-oxo complexes in oxygenation reactions.

Influence of hydrogen bonding on hydrogen-atom abstraction reactions of dehydropyridinium cations in the gas phase.

The reactions of several substituted, positively charged dehydropyridinium cations with cyclohexane, methanol, and tetrahydrofuran have been examined in a Fourier-transform ion cyclotron resonance mass spectrometer. All of the charged monoradicals react with the neutral reagents exclusively via hydrogen atom abstraction. For cyclohexane, there is a good correlation between the reaction efficiencies and the calculated electron affinities at the radical sites; that is, the greater the electron affinity of the charged monoradical at the radical site, the faster the reaction. The reaction efficiencies with methanol and tetrahydrofuran, however, do not correlate with the calculated electron affinities. Density functional theory (DFT) calculations indicate that for these reagents a stabilizing hydrogen bonding interaction exists in the hydrogen atom abstraction transition states for some of the charged monoradicals but not for others. At both the MPW1K and G3MP2B3 levels of theory, there is a good correlation between the calculated activation enthalpies and the observed reaction efficiencies, although the G3MP2B3 method provides a slightly better correlation than the MPW1K method. The extent of enhancement in the reaction efficiencies caused by the hydrogen bonding interactions parallels the calculated hydrogen bond lengths in the transition states.

Correlation of hydrogen-atom abstraction reaction efficiencies for aryl radicals with their vertical electron affinities and the vertical ionization energies of the hydrogen-atom donors.

The factors that control the reactivities of aryl radicals toward hydrogen-atom donors were studied by using a dual-cell Fourier-transform ion cyclotron resonance mass spectrometer. Hydrogen-atom abstraction reaction efficiencies for two substrates, cyclohexane and isopropyl alcohol, were measured for 23 structurally different, positively charged aryl radicals, which included dehydrobenzenes, dehydronaphthalenes, dehydropyridines, and dehydro(iso)quinolines. A logarithmic correlation was found between the hydrogen-atom abstraction reaction efficiencies and the (calculated) vertical electron affinities (EA) of the aryl radicals. Transition state energies calculated for the reaction of three of the aryl radicals with isopropyl alcohol were found to correlate linearly with their (calculated) EAs. No correlation was found between the hydrogen-atom abstraction reaction efficiencies and the (calculated) enthalpy changes for the reactions. Measurement of the reaction efficiencies for the reactions of 15 different hydrogen-atom donors with two selected aryl radicals revealed a logarithmic correlation between the hydrogen-atom abstraction reaction efficiencies and the vertical ionization energies (IE) of the hydrogen-atom donors, but not the lowest homolytic X-H (X = heavy atom) bond dissociation energies of the hydrogen-atom donors. Examination of the hydrogen-atom abstraction reactions of 29 different aryl radicals and 18 different hydrogen-atom donors showed that the reaction efficiency increases (logarithmically) as the difference between the IE of the hydrogen-atom donor and the EA of the aryl radical decreases. This dependence is likely to result from the increasing polarization, and concomitant stabilization, of the transition state. Thus, the hydrogen-atom abstraction reaction efficiency for an aryl radical can be "tuned" by structural changes that influence either the vertical EA of the aryl radical or the vertical IE of the hydrogen atom donor.