Linear Free Energy Correlations Research Articles

Intramolecular Charge Transfer with 1‐Naphthanilides and 2‐Naphthanilides

AbstractA series of 1‐naphthanilides (1) and 2‐naphthanilides (2) with varied substituents at the para‐ or meta‐position of anilino phenyl ring were prepared and their absorption and fluorescence spectra in a nonpolar solvent cyclohexane were investigated. An abnormal long wavelength emission assigned to the charge transfer (CT) state was found for all of the prepared naphthanilides in cyclohexane. A linear free energy correlation between the CT emission energies and the Hammett constants of the substituent was found within series 1 and 2. The value of the linear slope with 1 (0.42 eV) was higher than that with 2 (0.32 eV) being close to that of the substituted benzanilides 3 (0.31 eV). The higher slope value suggested higher charge separation extent in the CT state of 1 than that of 2. It was found that the corresponding linear slope of anilino‐substituted benzanilides remained unchanged when para‐, meta‐, ortho‐, or ortho, ortho‐methyls were introduced into the anilino moiety, which ruled out the possible contribution of the difference in the steric effect and the electron accepting ability of the naphthoyl acceptor in 1 and 2. Compared with the early reported N‐substituted‐benzoyl‐aminonaphthalene derivatives 4 and 5, it was considered that 1‐naphthoyl enhanced the charge transfer in 1 and the proximity of its 1La and 1Lb states was suggested to be responsible. It was shown that 1‐ and/or 2‐substituted naphthalene cores acting as either electron acceptor (naphthoyl) or electron donor (aminonaphthalene) were different in not only electron accepting (donating) ability but also shaping the charge transfer pathway.

The Kinetics of the Reactions of C2 (a3Πu) with Alcohols

The reactions of C2 (a 3pi(u)) radicals with a series of alcohols have been studied at about 6.5 Torr total pressure and room temperature using the pulsed laser photolysis/laser-induced fluorescence technique. The relative concentration of C2 (a 3pi(u)) radicals, which are generated via the photolysis of C2Cl4 with the focused output from the fourth harmonic of a Nd:YAG laser (266 nm), was monitored by laser-induced fluorescence (LIF) in the (0, 0) band of the C2 (d 3pi(g)<--a 3pi(u)) transition at 516.5 nm. Under pseudo-first-order conditions, we measured the time evolution of C2 (a 3pi(u)) and determined the rate constants for reactions of C2 (a 3pi(u)) with alcohols. The rate constants increase linearly with the number of C atoms in the alcohols. All of them are larger than those for reactions of C2 (a 3pi(u)) with alkanes (C1-C5). Based on the bond dissociation energy and linear free energy correlations, we believe the reactions of C2 (a 3pi(u)) with alcohols proceed via the mechanism of hydrogen abstraction. The experimental results show that the H-atom on the C-H bonds is activated at the presence of the OH substituent group in the alcohol molecule. The theoretical calculations for the reaction of C2 (a 3pi(u)) with methanol also support these hypotheses.

Linear Free Energy Correlations for Enzymatic Base Flipping: How Do Damaged Base Pairs Facilitate Specific Recognition?

Linear Free Energy Correlations for Enzymatic Base Flipping: How Do Damaged Base Pairs Facilitate Specific Recognition?

Linear free energy correlations for enzymatic base flipping: how do damaged base pairs facilitate specific recognition?

To efficiently maintain their genomic integrity, DNA repair glycosylases must exhibit high catalytic specificity for their cognate damaged bases using an extrahelical recognition mechanism. One possible contribution to specificity is the weak base pairing and inherent instability of damaged sites which may lead to increased extrahelicity of the damaged base and enhanced recognition of these sites. This model predicts that the binding affinity of the enzyme should increase as the thermodynamic stability of the lesion base pair decreases, because less work is required to extrude the base into its active site. We have tested this hypothesis with uracil DNA glycosylase (UDG) by constructing a series of DNA duplexes containing a single uracil (U) opposite a variety of bases (X) that formed from zero to three hydrogen bonds with U. Linear free energy (LFE) relationships were observed that correlated UDG binding affinity with the entropy and enthalpy of duplex melting, and the dynamic accessibility of the damaged site to chemical oxidation. These LFEs indicate that the increased conformational freedom of the damaged site brought about by enthalpic destabilization of the base pair promotes the formation of extrahelical states that enhance specific recognition by as much as 3000-fold. However, given the small stability differences between normal base pairs and U.A or U.G base pairs, relative base pair stability contributes little to the >10(6)-fold discrimination of UDG for uracil sites in cellular DNA. In contrast, the intrinsic instability of other more egregious DNA lesions may contribute significantly to the specificity of other DNA repair enzymes that bind to extrahelical bases.

The Transition State for Carboxylic Acid Deprotonation on Cu(100)

The kinetics of acid deprotonation on the Cu(100) surface have been studied using four different fluorinated carboxylic acids (CF2HCO2H, CF3CO2H, CF2HCF2CO2H, and CF3CF2CO2H). All four acids adsorb molecularly on Cu(100) at 90 K but deprotonate during heating to form carboxylates on the surface at temperatures below 300 K. Temperature-programmed reaction spectroscopy and X-ray photoemission spectroscopy were used to verify that the acids deprotonate on the Cu(100) surface. Work function measurements were used to study the deprotonation kinetics during heating and to estimate the activation energy barriers ( ) to deprotonation. The nature of the transition state to acid deprotonation on Cu(100) was probed by analysis of linear free energy relationships (LFER) or correlations of with the acid substituent constants, σF. The field reaction constant, ρF, or the slope of the LFER was found to be ρF = −52 ± 13 kJ/mol in the limit of zero coverage and ρF = −19 ± 2 kJ/mol at a coverage of 1/2 ML. These values of ρF can be compared to the value of ρF = −104 kJ/mol for acid deprotonation in the gas phase. This comparison suggests that the transition state for acid deprotonation on Cu(100) must be anionic with respect to the reactant (RCO2H(ad) ↔ [RCO2δ-···Hδ+]‡ → ).

Using Linear Free Energy Relationship to Predict the Stability Constants of Aqueous Complexes of Metal-Organic Ligands

AbstractThe Sverjensky-Molling linear free energy relationship was originally developed to correlate the Gibbs free energies of formation of an isostrutural family of solid phases to the thermodynamic properties of aqueous cations. In this paper, we demonstrate that the similar relationship also exists between metal complexes and simple metal cations in aqueous solutions. We extend the Sverjensky-Molling relationship to predict the Gibbs free energies of formation or dissociation constants for a family of metal complexes with a given complexing ligand. The discrepancies between the predicted and experimental data are generally less than 1.5 kcal/mol (or one log unit for stability constants). The use of this linear free energy correlation can significantly enhance our ability to predict the speciation, mobility, and toxicity of heavy metals in natural environments. According the obtained results, Gibbs free energies of formation of cations (δG0f, Mn+) can be used as an indicator for the hardness/softness of a metal cation (acid). The higher negative value of a metal cation, the harder acid it will be. It is logical to postulate that the coefficienta*MLcharacterizes the softness of a complexing ligand (base).

Linear free energy correlation analysis on the electronic effects of Rh(II) carbene O-H insertion.

The relative rate constants for the Rh(II)-catalyzed insertion of diazoacetone into the O-H bond have been measured through intermolecular competitions. The kinetic data were subjected to Hammett correlation analysis, and mechanistic implication of the results with respect to a stepwise vs a concerted O-H insertion pathway is discussed.

Substituent effects in the binding of bis(4-fluorobenzyl)ammonium ions by dianilino[24]crown-8

A series of para-substituted dianilino[24]crown-8 (DA24C8) macrocycles were synthesized and their ability to form host–guest complexes with bis(4-fluorobenzyl)ammonium ions (DFA +) were investigated. Although these crown ethers contain weakly hydrogen bonding aniline motifs, they do bind DFA + in CDCl 3/CD 3NO 2 solution, presumably in a pseudorotaxane-like manner. A plot of the values of the relative binding strengths (log[ K a(R)/ K a(H)]) versus the Hammett substituent constants σ + of the groups at the para-position of the aniline units suggests that a linear free energy correlation exists for this self-assembly process. The strength of the binding between the crown ether and the thread-like ion can be fine-tuned over a narrow range by judicious choice of the substituting groups.

Kinetics of CCN Radical Reactions with a Series of Normal Alkanes

The reactions of CCN(X̃2Πi) radicals with normal alkanes have been studied at about 10 Torr total pressure and room temperature using the pulse laser photolysis/laser-induced fluorescence technique. CCN (X̃2Πi) radicals are generated by the fourth harmonic of a Nd:YAG laser photolysis of Cl3CCN at 266 nm. The relative concentration of CCN (X̃2Πi) radicals was monitored in the (0−0) band of the CCN (Ã2Δ ← X̃2Πi) transition at 470.9 nm by laser induced fluorescence (LIF). From the analysis of the relative concentration−time behavior of CCN (X̃2Πi) under pseudo-first-order conditions, the rate constants for the reaction of CCN (X̃2Πi) with a series of normal alkanes (C1−C8) were determined for the first time. The new data establish that the gas-phase reactivity of small alkanes (C1−C8) towards the CCN radicals follows the linear free energy relationship typical of hydrogen abstraction. A comparison with the corresponding reactions of CN and OH radicals with a series of normal alkanes, leads us to suggest that the reaction of CCN (X̃2Πi) with small normal alkanes proceeds via the mechanism of hydrogen abstraction. The plausibility of the suggested reaction mechanism is strengthened by bond dissociation energy (BDE) correlations and linear free energy correlations.

Distinguishing rate-limiting electron versus H-atom transfers in Cu2O2-mediated oxidative N-dealkylations: application of inter- versus intramolecular kinetic isotope effects.

Copper-dioxygen adducts are important biological oxidants. To gain a better understanding of the underlying chemistries of such species, we report on a series of Cu2II-O2 complexes, [{CuII(MePY2)R'}2(O2)](B(C6F5)4)2 (1R') (where (MePY2)R' is a 4-pyridyl substituted bis[2-(2-(4-R'-pyridyl)ethyl]methylamine; R' = H, MeO, Me2N; Zhang, C. X.; et al. J. Am. Chem. Soc. 2003, 125, 634-635), which readily oxidize exogenous substrates. In this study, we explore the mechanism by which 1R' facilitates the oxidative N-dealkylation of para-substituted N,N-dimethylanilines (R-DMA; R = MeO, Me, H, CN). In the case of 1H, the linear free-energy correlation plot (rho = -2.1) and intramolecular deuterium kinetic isotope effect (KIEintra, using p-R-(C6H4)-N(CH3)(CD3)) profile suggest that R-DMA oxidation occurs through rate-limiting electron transfer (ET). This mechanism was further enforced by comparison of KIEintra versus the intermolecular KIE (KIEinter, using p-R-(C6H4)-N(CH3)2 versus p-R-(C6H4)-N(CD3)2). It was found that KIEinter < KIEintra, suggesting an ET process. In the case of both 1MeO and 1Me2N, the KIEintra profile and linear free-energy correlation plots (rho = -0.49 and -0.99 for 1Me2N and 1MeO with especially poor fitting for the latter) are inconclusive in distinguishing between a rate-limiting ET or hydrogen atom transfer (HAT) pathway. Comparisons of KIEinter versus KIEintra demonstrate a switch in mechanism from ET to HAT for 1Me2N and 1MeO oxidation of R-DMA as R-DMA is made less reducing. In the case of 1Me2N, MeO-DMA and Me-DMA are oxidized via a rate-limiting ET (KIEinter < KIEintra), while H-DMA and CN-DMA are oxidized through a HAT pathway (KIEinter approximately KIEintra). For 1MeO, oxidation occurs through an ET pathway for MeO-, Me-, and H-DMA (KIEinter < KIEintra), while CN-DMA is oxidized though a HAT process (KIEinter approximately KIEintra). Copper complex attributes, which may contribute to the mechanistic observations, are suggested.

A history of the international group for correlation analysis in chemistry

AbstractThe author has been Secretary of the International Group for Correlation Analysis in Chemistry (formerly Organic Chemistry) since it was founded in 1982. A series of conferences in the USA, which began with a Symposium on Linear Free Energy Correlations at Durham, NC, in 1964, eventually stimulated the development of a similar series in Europe, starting with a conference on Correlation Analysis in Organic Chemistry at Assisi, Italy, in 1979. This conference established a continuing organization in a provisional form and, at the second conference in Hull, UK, in 1982, this became the International Group for Correlation Analysis in Organic Chemistry. The conferences have continued ever since at intervals of between two and three years at venues in Europe or (on two occasions) in Asia, and now total nine. The Group (which dropped the ‘Organic’ from its title in 1991) has been an Associated Organization of IUPAC since 1990. The article describes the development of the Group as a society during twenty years and its various activities, including the conferences and a substantial Newsletter, produced at intervals of six months to a year for 24 issues. Copyright © 2003 John Wiley &amp; Sons, Ltd.

Fragmentation reactions of protonated peptides containing phenylalanine: a linear free energy correlation in the fragmentation of H–Gly–Xxx–Phe–OH

The fragmentation reactions of a variety of protonated tri- and tetra-peptides containing phenylalanine have been examined using metastable ion studies and energy-resolved collision-induced dissociation studies. For peptides with the sequence H–Gly–Xxx–Phe–OH (Xxx=Gly,Ala,Val,Leu,Phe) the major primary fragmentation of MH + involves cleavage of the C-terminus amide bond to form either the b 2 ion or the y 1″ ion. For metastable ion fragmentation it is found that log([b 2]/[ y 1″]) increases linearly with the increase in the gas phase basicity of H–Xxx–OH. This linear free energy correlation is in contrast to the lack of such a correlation in the fragmentation of protonated H–Gly–Xxx–Gly–OH [J. Mass Spectrom. 30 (1995) 290]. When Phe is the central residue in tripeptides, the major primary fragmentation reaction involves formation of the b 2 ion which fragments further to the a 2 ion; at higher internal energies the a 2 ion fragments to give the phenylalanine immonium ion which becomes the dominant fragment. When Phe is in the N-terminus position, as in Phe–Gly–Gly–OH, the phenylalanine immonium ion is the dominant fragment and is formed, in part, directly by fragmentation of MH +. The fragmentation of the tetrapeptides H–Gly–Gly–Phe–Leu–OH, H–Phe–Gly–Gly–Phe–OH and H–Val–Ala–Ala–Phe–OH are more complex but show a substantial directional effect of the phenylalanine residue(s).

Intrinsic (Gas Phase) Thermodynamic Stability of 2-Adamantyl Cation. Its Bearing on the Solvolysis Rates of 2-Adamantyl Derivatives

The standard enthalpy of formation of gaseous 2-adamantyl chloride(2-Ad-Cl) was determined by calorimetric techniques. The standard Gibbs energy change for the chloride anion exchange between 1-adamantyl (1-Ad+) and 2-adamantyl (2-Ad+) cations in the gas phase was obtained by Fourier transform ion cyclotron resonance spectroscopy (FT ICR). Theoretical calculations at the G2(MP2) level were performed on these and other relevant species. This and data from the literature provided three highly consistent independent estimates of the relative stabilities of 2-Ad+ and 1-Ad+. This difference in gas-phase stability was compared to the differential structural effects on the rates of solvolysis of the corresponding chlorides and tosylates, and it was shown that the thermodynamic stability of the secondary cation is the leading factor determining the solvolytic reactivity of the precursors in the absence of solvent effects. Thus, under these conditions, the previously established linear free energy correlation between carbenium ion stability and solvolytic reactivity of bridgehead derivatives applies also to secondary derivatives.

Prediction of trace metal partitioning between minerals and aqueous solutions: a linear free energy correlation approach

Trace metal partitioning between authigenic minerals and aqueous solutions is of great interest to both geochemical and environmental communities. In this paper, we have developed a linear free energy correlation model that correlates metal partition coefficients with metal cation properties: −2.303 RT log K d + ΔG f,M Z+ 0 = a ∗ M H X Δ G n,M Z+ 0 + β ∗ M H X r M Z+ + b ∗ M H X where a ∗ M H X , β ∗ M H X , and b ∗ M H X are constants, which can be determined by a regression analysis. For isovalent metal partitioning, because of the constraint of K d = 1 for the host metal, this correlation can be also expressed as: −2.303 RT log K d = a ∗ M H X (Δ G n,M H Z+ 0 − Δ G n,M 0 ) + β ∗ M H X ( r M Z+ − r n,M H Z+ ) − (Δ G f,M Z+ 0 − Δ G 0 f,M H Z+ ). Host minerals from an isostructural family have the same linear free energy relationship, as long as the relationship is expressed as a function of the differences in cation properties between substituent and host metals. We have applied our model to both isovalent and non-isovalent metal partitioning in carbonate minerals. The model closely fits experimental data, demonstrating the robustness of the proposed linear free energy relationship. Using the model, we have predicted the partition coefficients of divalent and trivalent metals between various carbonate minerals and aqueous solutions. The differences between the predicted and experimental values are generally less than 1 logarithmic unit for divalent cations and less than 0.4 logarithmic unit for trivalent cations. Magnesite is predicted to have the largest partition coefficients among the carbonate minerals with a calcite structure and therefore, can be a good scavenger for toxic metals. The kinetic effect on metal partitioning can be described graphically by a “seesaw” line anchored at a host cation. To explain the rate dependence of partition coefficients, we have proposed a conceptual model that relates metal partitioning to surface adsorption. The conceptual model suggests that as the rate of host mineral precipitation increases, the ratio of substituent to host cation in a solid approaches what is in the adsorbed layer. The linear free energy correlation model developed in this paper provides a useful tool for systematizing the existing experimental data and for predicting unknown partition coefficients.

Application of a linear free energy correlation to actinide mononitrides

Application of a linear free energy correlation to actinide mononitrides

Metal-centered redox chemistry of substituted cobalt phthalocyanines adsorbed on graphite and correlations with MO calculations and Hammett parameters. Electrocatalytic reduction of a disulfide

We have studied the metal-centered redox chemistry of unsubstituted cobaltphthalocyanine (CoPc) and the cobalt complexes of the substituted ligands, cobaltoctaethylhexyloxyphthalocyanine (CoOEHPc), cobalttetramethoxyphthalocyanine (CoMeOPc), cobalttetraneopentoxyphthalocyanine (CoTNPPc), cobalttetraaminophthalocyanine (CoTAPc), cobalttetrasulfonatophthalocyanine (CoTSPc) and cobalthexadecafluorophthalocyanine (CoPcF 16) adsorbed on ordinary pyrolytic graphite (OPG). These complexes exhibit two distinct metal-centered reversible redox processes attributed to the Co(II)/Co(I) and Co(III)/Co(II) couples. The redox potentials of these couples are shifted according to the electron withdrawing or donating character of the groups located at the periphery of the phthalocyanine ligand, as seen by comparison to molecular orbital theoretical calculations. Correlations between the redox potential and the Hammett parameters of the substituents provide good linear correlations. The slopes of the linear free energy correlations give values of 0.41 and 0.49 V σ −1 for the Co(III)/Co(II) and Co(II)/Co(I) redox couples, respectively. We have also tested the electrocatalytic activity of these adsorbed phthalocyanines for the reduction of 2-hydroxyethyldisulfide. A plot of log i (current density at constant potential) versus the Co(II)/Co(I) redox potential gives a linear correlation where the rate of the reaction increases with the driving force of the catalyst, suggesting that that the reaction takes place via an outer-sphere mechanism.

New mechanistic insights into the reactivity of the R2 protein of E. coli ribonucleotide reductase (RNR)

Further to a linear free-energy correlation of cross-reaction rate constants k 12 for the reaction of eight organic radicals (OR), e.g. MV + , from methyl viologen, with cytochrome c(III), we consider here similar studies for the reduction of the R2 protein of Escherichia coli ribonucleotide reductase, which has Fe III 2 and Tyr redox components. The same two techniques of pulse radiolysis and stopped-flow were used. Cross-reaction rate constants (22 °C) at pH 7.0, I=0.100 M (NaCl), were determined for the reduction of active-R2 with the eight ORs, reduction potentials E 0 1 from −0.446 to +0.194 V. Samples of active-R2 have an Fe III 2 met-R2 component, which in the present studies was close to 40%. Concurrent reactions have to be taken into account for the five most reactive ORs, corresponding to reduction of the Fe III 2 of met-R2 and then of active-R2. Separate experiments on met-R2 reproduced the first of these rate constants, which on average is ∼66% larger than the second rate constant. A single Marcus free-energy plot of log k 12−0.5 log 10 f versus − E 0 1/0.059 describes all the data and the slope of 0.54 is in satisfactory agreement with the theoretical value of 0.50. Such behaviour is unexpected since the Tyr is a much stronger oxidant ( E 0∼1.0 V versus NHE) as compared to Fe III 2 ( E 0 close to zero). X-ray structures of the met- and red-R2 states have indicated that electroneutrality of the ∼10 Å buried active site is maintained. Proton transfer is therefore proposed as a rapid sequel to electron transfer. Other reactions considered are the much slower conventional time-range reductions of active-R2 with hydrazine and dithionite. For these reactions one and/or two-equivalent changes are possible. With both reductants, met-R2 reacts about four-fold faster than active-R2, and as with the ORs the less strongly oxidising Fe III 2 component is reduced before the Tyr .

Mechanistic Implications of a Linear Free-Energy Correlation of Rate Constants for the Reduction of Active- and Met-R2 Forms ofE. coliRibonucleotide Reductase with Eight Organic Radicals

Cross-reaction rate constants k12 (22 °C) at pH 7.0 have been determined for the reduction of FeIII2 and tyrosyl-radical-containing active-R2 from E. coli ribonucleotide reductase with eight organic radicals (OR), e.g., MV•+ from methyl viologen. The more reactive OR's were generated in situ using pulse radiolysis (PR) techniques, and other OR's were generated by prior reduction of the parent with dithionite, followed by stopped-flow (SF) studies. In both procedures it was necessary to include consideration of doubly-reduced parent forms. Values of k12 are in the range 109 to 104 M-1 s-1 and reduction potentials Eo1 for the OR vary from −0.446 to +0.194 V. Samples of E. coli active-R2 also have an FeIII2 met-R2 component (with no Tyr•), which in the present work was close to 40%. From separate experiments met-R2 gave similar k12 rate constants (on average 66% bigger) to those for active-R2, suggesting that reduction of the FeIII2 center is the common rate-limiting step. A single Marcus free-energy plot of...

Structural chemistry of polycyclic heteroaromatic compounds. Part 10: photoelectron spectra and electronic structures of 1-substituted 1 H-benzotriazoles

The electronic structures of 1 H-benzotriazole ( 1) and its 1-substituted derivatives 2– 20 have been investigated by ultraviolet photoelectron spectroscopy and quantum chemical methods. The conformational properties of some compounds were studied by semi-empirical and B3LYP calculations. For the alkenyl derivative 14 a twisted conformation was ascertained both from IP values and calculations, and in the amino derivative 20 the amino group is twisted by 90° which is reflected by the absence of n/π interactions. With only a few exceptions, the ionization potentials corresponding to the five occupied π MOs (π 1–π 5) and the two n(N) orbitals of the 1 H-benzotriazole unit (Bt 1) could be determined and assigned for 1– 20. Linear free energy correlations of the IPs related to these orbitals with inductive substituent parameters σ I and F were satisfactory except for IP(π 3). The energies of π and n MOs—except for π 3—are thus affected primarily by the substituent's electronegativity. The different behaviour of π 3 is caused by direct interactions with substituent orbitals. For some substituents for which no σ I or F parameters were known these could be determined from their IPs by correlation analysis.

Linear free energy correlations in mass spectrometry

Linear free energy correlations have played a major role in the development of physical organic chemistry and have led to many of the now widely used concepts of reactivity. This paper reviews the application of linear free energy correlations to the field of gas-phase ion chemistry. The results indicate that many of the concepts developed to systematize liquid-phase properties and reactivities are applicable to gas-phase ionic systems. Copyright © 1999 John Wiley & Sons, Ltd.