Accurate Estimation of the One-Electron Reduction Potentials of Various Substituted Quinones in DMSO and CH₃CN

The one-electron reduction potentials of 116 important p- and o-quinones in DMSO and CH3CN were predicted for the first time by using the B3LYP/DZP++ method and the PCM cluster continuum model. The calculated gas-phase electron affinities and one-electron reduction potentials agree well with the available experimental observations, respectively. The study showed the one-electron reduction potentials of the 116 quinones range from −0.949 to 1.128 V in DMSO and from −0.904 to 0.971 V in CH3CN. The one-electron reduction potentials of p-quinones are generally smaller than those of the o-quinones by about 0.132 V. For quinones with aromatic properties, 2-substituted-1,4-naphthquinones have the largest one-electron reduction potentials, followed by substituted-1,4-anthraquinones and then by substituted-9,10-anthraquinones. The study also showed that the one-electron reduction potentials of quinones in DMSO are linearly dependent on the sum of the Hammett substituent parameters σp: ENHE(p-Q/p-Q• −) = 0.45Σσp − 0.194 (V) and ENHE(o-Q/o-Q• −) = 0.45Σσp − 0.059 (V). Combined with the hydride affinities of quinones in the former paper [ΔGH-A(p-Q) = −16.0Σσp − 70.5 (kcal/mol) and ΔGH-A(o-Q) = −16.2Σσp − 81.5 (kcal/mol)] and the one-electron reduction potentials of quinones estimated in this work, we obtained the homolytic bond dissociation energies of the hydroquinone anions (QH−) and found that these thermodynamic parameters also have linear correlations against the sum of the Hammett substituent parameters σp if only the substituents have no larger electrostatic inductive force and no large steric hindrance: BDE(p-QH−) = 5.05Σσp + 63.18 (kcal/mol) and BDE(o-QH−) = 5.33Σσp + 71.30 (kcal/mol). Knowledge about the redox potentials of the quinones should be of great value for the understanding of the nature of chemical reactions of quinones, the designing of new electronic materials of quinones, and the examining of biological activities of quinones

Hydride Affinity Scale of Various Substituted Arylcarbeniums in Acetonitrile

Combined with the integral equation formalism polarized continuum model (IEFPCM), the hydride affinities of 96 various acylcarbenium ions in the gas phase and CH3CN were estimated by using the B3LYP/6-31+G(d)//B3LYP/6-31+G(d), B3LYP/6-311++G(2df,2p)//B3LYP/6-31+G(d), and BLYP/6-311++G(2df,2p)//B3LYP/6-31+G(d) methods for the first time. The results show that the combination of the BLYP/6-311++G(2df,2p)//B3LYP/6-31+G(d) method and IEFPCM could successfully predict the hydride affinities of arylcarbeniums in MeCN with a precision of about 3 kcal/mol. On the basis of the calculated results from the BLYP method, it can be found that the hydride affinity scale of the 96 arylcarbeniums in MeCN ranges from −130.76 kcal/mol for NO2−PhCH+−CN to −63.02 kcal/mol for p-(Me)2N−PhCH+−N(Me)2, suggesting most of the arylcarbeniums are good hydride acceptors. Examination of the effect of the number of phenyl rings attached to the carbeniums on the hydride affinities shows that the increase of the hydride affinities takes place linearly with increasing number of benzene rings in the arylcarbeniums. Analyzing the effect of the substituents on the hydride affinities of arylcarbeniums indicates that electron-donating groups decrease the hydride affinities and electron-withdrawing groups show the opposite effect. The hydride affinities of arylcarbeniums are linearly dependent on the sum of the Hammett substituent parameters σp+. Inspection of the correlation of the solution-phase hydride affinities with gas-phase hydride affinities and aqueous-phase pKR+ values reveals a remarkably good correspondence of ΔGH−A(R+) with both the gas-phase relative hydride affinities only if the α substituents X have no large electron-donating or -withdrawing properties and the pKR+ values even though the media are dramatically different. The solution-phase hydride affinities also have a linear relationship with the electrophilicity parameter E, and this dependence can certainly serve as one of the most effective ways to estimate the new E values from ΔGH−A(R+) or vice versa. Combining the hydride affinities and the reduction potentials of the arylcarbeniums, we obtained the bond homolytic dissociation Gibbs free energy changes of the C−H bonds in the corresponding hydride adducts in acetonitrile, ΔGHD(RH), and found that the effects of the substituent on ΔGHD(RH) are very small. Simple thermodynamic analytic platforms for the three C−H cleavage modes were constructed. It is evident that the present work would be helpful in understanding the nature of the stabilities of the carbeniums and mechanisms of the hydride transfers between carbeniums and other hydride donors

Scales of Oxidation Potentials, p<i>K</i>_a, and BDE of Various Hydroquinones and Catechols in DMSO

The one-electron oxidation potentials [EoxNHE(H2Q)], pKa (pKa1 and pKa2) values, and bond dissociation energies (BDE1 and BDE2) of 118 important p- and o-dihydroquinones in DMSO were systematically predicted for the first time by using DFT method and the PCM cluster continuum model. The calculated results agree well with the available experimental determinations. The study shows that all the five thermodynamic parameters correlate well with the Hammett substituent parameters σp (for p-H2Q, EoxNHE(H2Q·+/H2Q) = 1.66Σσp + 0.54, pKa1 = −5.69Σσp + 16.54, pKa2 = −5.19Σσp + 23.91, BDE1 = 3.43Σσp + 82.29, BDE2 = 4.64Σσp + 67.70 and for o-H2Q, EoxNHE(H2Q·+/H2Q) = 1.85Σσp + 0.46, pKa1 = −5.53Σσp + 13.28, pKa2 = −5.24Σσp + 26.70, BDE1 = 3.54Σσp + 82.08, BDE2 = 3.82Σσp + 75.93), which hints that we can get these thermodynamic parameters as long as the structure of the hydroquinones were known. The comparisons of the calculated five thermodynamic parameters between p-hydroquinones and o-hydroquinones and the number of the phenyl ring effects on these thermodynamic parameters were also studied. At last, intramolecular hydrogen bond energies in hydroquinones at neutral, radical cation, radical, anion different state were systematically calculated and analyzed. Combined with the papers published in our group before, we will have a systematic thermodynamic picture of the transfer details between different kinds of quinones and corresponding hydroquinones, which strongly promote the fast development of the understanding and applications of quinones

Thermodynamic Diagnosis of the Properties and Mechanism of Dihydropyridine-Type Compounds as Hydride Source in Acetonitrile with “Molecule ID Card”

A series of 45 dihydropyridine-type organic compounds as hydride source were designed and synthesized. The thermodynamic driving forces (defined as enthalpy changes or redox potentials in this work) of the dihydropyridines to release hydride anions, hydrogen atoms (hydrogen for short), and electrons in acetonitrile, the thermodynamic driving forces of the radical cations of the dihydropyridines to release protons and hydrogens in acetonitrile, and the thermodynamic driving forces of the neutral pyridine-type radicals of the dihydropyridines to release electron in acetonitrile were determined by using titration calorimetry and electrochemical methods. The rates and activation parameters of hydride transfer from the dihydropyridines to acridinium perclorate, a well-known hydride acceptor, were determined by using UV−vis absorption spectroscopy technique. The relationship between the thermodynamic driving forces and kinetic rate of the hydride transfer was examined. Thermodynamic characteristic graph (TCG) of the dihydropyridines as an efficient “Molecule ID Card” was introduced. The TCG can be used to quantitatively diagnose or predict the characteristic chemical properties of the dihydropyridines and their various reaction intermediates. The mechanism of hydride transfer from the dihydropyridines to acridinium perclorate was diagnosed and elucidated by using the determined thermodynamic parameters and the activation parameters

Determination of Hydride Affinities of Various Aldehydes and Ketones in Acetonitrile

The hydride affinities of 21 typical aldehydes and ketones in acetonitrile were determined by using an experimental method, which is valuable for chemists choosing suitable reducing agents to reduce them. The focus of this paper is to introduce a very facile experimental method, which can be used to determine the hydride affinities of various carbonyl compounds in solution

Which Hydrogen Atom Is First Transferred in the NAD(P)H Model Hantzsch Ester Mediated Reactions via One-Step and Multistep Hydride Transfer?

Which Hydrogen Atom Is First Transferred in the NAD(P)H Model Hantzsch Ester Mediated Reactions via One-Step and Multistep Hydride Transfer

Mechanism and Driving Force of NO Transfer from <i>S</i>-Nitrosothiol to Cobalt(II) Porphyrin: A Detailed Thermodynamic and Kinetic Study

The thermodynamics and kinetics of NO transfer from S-nitrosotriphenylmethanethiol (Ph3CSNO) to a series of α,β,γ,δ-tetraphenylporphinatocobalt(II) derivatives [T(G)PPCoII], generating the nitrosyl cobalt atom center adducts [T(G)PPCoIINO], in benzonitrile were investigated using titration calorimetry and stopped-flow UV-vis spectrophotometry, respectively. The estimation of the energy change for each elementary step in the possible NO transfer pathways suggests that the most likely route is a concerted process of the homolytic S−NO bond dissociation and the formation of the Co−NO bond. The kinetic investigation on the NO transfer shows that the second-order rate constants at room temperature cover the range from 0.76 × 104 to 4.58 × 104 M-1 s-1, and the reaction rate was mainly governed by activation enthalpy. Hammett-type linear free-energy analysis indicates that the NO moiety in Ph3CSNO is a Lewis acid and the T(G)PPCoII is a Lewis base; the main driving force for the NO transfer is electrostatic charge attraction rather than the spin−spin coupling interaction. The effective charge distribution on the cobalt atom in the cobalt porphyrin at the various stages, the reactant [T(G)PPCoII], the transition-state, and the product [T(G)PPCoIINO], was estimated to show that the cobalt atom carries relative effective positive charges of 2.000 in the reactant [T(G)PPCoII], 2.350 in the transition state, and 2.503 in the product [T(G)PPCoIINO], which indicates that the concerted NO transfer from Ph3CSNO to T(G)PPCoII with the release of the Ph3CS• radical was actually performed by the initial negative charge (−0.350) transfer from T(G)PPCoII to Ph3CSNO to form the transition state and was followed by homolytic S−NO bond dissociation of Ph3CSNO with a further negative charge (−0.153) transfer from T(G)PPCoII to the NO group to form the final product T(G)PPCoIINO. It is evident that these important thermodynamic and kinetic results would be helpful in understanding the nature of the interaction between RSNO and metal porphyrins in both chemical and biochemical systems

Negative Kinetic Temperature Effect on the Hydride Transfer from NADH Analogue BNAH to the Radical Cation of <i>N</i>-Benzylphenothiazine in Acetonitrile

The reaction rates of 1-(p-substituted benzyl)-1,4-dihydronicotinamide (G-BNAH) with N-benzylphenothiazine radical cation (PTZ•+) in acetonitrile were determined. The results show that the reaction rates (kobs) decreased from 2.80 × 107 to 2.16 × 107 M-1 s-1 for G = H as the reaction temperature increased from 298 to 318 K. The activation enthalpies of the reactions were estimated according to Eyring equation to give negative values (−3.4 to −2.9 kcal/mol). Investigation of the reaction intermediate shows that the charge-transfer complex (CT-complex) between G-BNAH and PTZ•+ was formed in front of the hydride transfer from G-BNAH to PTZ•+. The formation enthalpy of the CT-complex was estimated by using the Benesi−Hildebrand equation to give the values from −6.4 to −6.0 kcal/mol when the substituent G in G-BNAH changes from CH3O to Br. Detailed thermodynamic analyses on each elementary step in the possible reaction pathways suggest that the hydride transfer from G-BNAH to PTZ•+ occurs by a concerted hydride transfer via a CT-complex. The effective charge distribution on the pyridine ring in G-BNAH at the various stagesthe reactant G-BNAH, the charge-transfer complex, the transition-state, and the product G-BNA+was estimated by using the method of Hammett-type linear free energy analysis, and the results show that the pyridine ring carries relative effective positive charges of 0.35 in the CT-complex and 0.45 in the transition state, respectively, which indicates that the concerted hydride transfer from G-BNAH to PTZ•+ was practically performed by the initial charge (−0.35) transfer from G-BNAH to PTZ•+ and then followed by the transfer of hydrogen atom with partial negative charge (−0.65). It is evident that the present work would be helpful in understanding the nature of the negative temperature effect, especially on the reaction of NADH coenzyme with the drug phenothiazine in vivo

Characteristic Activity Parameters of Electron Donors and Electron Acceptors

It is well-known that for an electron transfer reaction, the electron-donating ability of electron donors and the electron-accepting ability of electron acceptors can be quantitatively described by the oxidation potential of electron donors and the reduction potential of electron acceptors. However, for an electron transfer reaction, the electron-donating activity of electron donors and the electron-accepting activity of electron acceptors cannot be quantitatively described by a characteristic parameter of electron donors and a characteristic parameter of electron acceptors till now. In this paper, a characteristic activity parameter of electron donors and electron acceptors named as their thermo-kinetic parameter is proposed to quantify the electron-donating activity of electron donors and the electron-accepting activity of electron acceptors in electron transfer reactions. At the same time, the thermo-kinetic parameter values of 70 well-known electron donors and the corresponding 70 conjugated electron acceptors in acetonitrile at 298 K are determined. The activation free energies of 4900 typical electron transfer reactions in acetonitrile at 298 K are estimated according to the thermo-kinetic parameter values of 70 electron donors and 70 conjugated electron acceptors, and the estimated results have received good verification of the corresponding independent experimental measurements. The physical meaning of the thermo-kinetic parameter is examined. The relationship of the thermo-kinetic parameter with the corresponding redox potential as well as the relationship of the activation free energy with the corresponding thermodynamic driving force of electron transfer reactions is examined. The results show that the observed relationships between the thermo-kinetic parameters and the redox potentials as well as the observed relationships between the activation free energy and the thermodynamic driving force depend on the choice of electron donors and electron acceptors as well as the electron transfer reactions. The greatest contribution of this paper is to realize the symmetry and unification of kinetic equations and the corresponding thermodynamic equations of electron transfer reactions

Determination and Comparison of Thermodynamic Driving Forces of Elementary Steps for the Reductions of Alkynes and the Corresponding Alkenes in Acetonitrile

In this work, five substituted 1-phenyl-2-benzensulfonyl ethynes and the corresponding five substituted 1-phenyl-2-benzensulfonyl ethenes were designed and synthesized as representatives of the polar alkynes and the polar alkenes. Thermodynamic driving forces of eight elementary steps for reductions of the substituted ethynes and ethenes to the corresponding alkenes and alkanes in acetonitrile were determined. The differences of chemical properties between the alkynes and the alkenes as well as their various derived reaction intermediates were quantitatively examined or compared according to the determined thermodynamic driving forces of the eight elementary steps. The relative C–C π-bond heterolytic and homolytic dissociation energies of the alkynes and alkenes in acetonitrile were estimated according to the difference of the hydride affinities and hydrogen atom affinities of the related chemical species. The relative effective charges on the active center atom of the alkynes and the alkenes as well as their derived various reaction intermediates, which can be used to quantitatively measure the polarity of the corresponding chemical species, were estimated according to the Hammett substituent effects using the Hammett-type linear free energy relationships. Molecule ID Cards of the alkynes and the alkenes in acetonitrile were constructed from the determined thermodynamic driving forces of the eight elementary steps. The thermodynamic tendencies and detailed mechanisms for the reductions of the alkynes and alkenes by Hantzsch ester in acetonitrile were diagnosed according to the thermodynamic analytic platforms that were made of the Molecule ID Cards of the related reactants. It is clear that the results of this work are not only to provide good guidance for synthetic chemists to safely choose a suitable reducing agent for selective reductions of alkynes and alkenes and to rationally examine the reaction mechanisms but also to facilitate theoretical chemists to develop novel calculation methods to examine the chemistry of alkynes and alkenes