G2(+) investigation on the alpha-effect in the SN2 reactions at saturated carbon

As a continuing theoretical study on the alpha-effect in the S(N)2 reactions at saturated carbon centers, 28 gas-phase reactions have been examined computationally by using the high-level G2(+) method. The reactions include: Nu(-)+CH(3)X-->CH(3)Nu+X(-) (X=F and Cl; Nu(-)=HO(-), HS(-), CH(3)O(-), Cl(-), Br(-), HOO(-), HSO(-), FO(-), ClO(-), BrO(-), NH(2)O(-), and HC(==O)OO(-)). It was found that all alpha-nucleophiles examined exhibit downward deviations from the correlation line between the overall barriers and proton affinities for normal nucleophiles, indicating the existence of the alpha-effect in the gas phase. The transition states (TS) for the alpha-nucleophiles are characterized by less advanced C--X bond cleavages than the normal nucleophiles, leading to smaller deformation energies and overall barriers. The size of the alpha-effect is related to the electron density on the alpha-atom, and increases when the position of alpha-atom is changed from left to right and from bottom to top in the periodic table. The reaction with CH(3)F exhibits a larger alpha-effect than that with CH(3)Cl, which can be explained by a later TS and a more positively charged methyl group at the TS for CH(3)F, [NuCH(3)F](- not equal). Thus, a higher electron density on the alpha-atom and a more positive methyl moiety at the TS result in a larger alpha-effect.     

Nucleophilic substitution at the imidoyl carbon atom: intermediate mechanistic and reactivity behavior between carbonyl and vinyl carbon substitution.

Gas-phase nucleophilic substitution reactions at the imidoyl carbon have been investigated using chloride exchanges, Cl- + RY=CHCl right harpoon over left harpoon RY=CHCl + Cl- with Y = N and R = F, H or CH3, at the MP2, B3LYP and G2(+) levels using the MP2/6-311+G geometries. The results are compared with those for the vinyl (Y = CH) and carbonyl (Y = O) carbon substitution. The mechanism and reactivity of substitution at the imidoyl carbon are intermediate between those of carbonyl (SNpi) and vinyl carbon (SNsigma) substitution, which is directly related to the electronegativity of Y, CH < N < O. The prediction of competitive SNsigma with SNpi path for the imidoyl chloride is consistent with the S(N)1-like mechanism proposed for reactions in solution. The important factors in favor of an in-plane concerted SN2 (SNsigma) over an out-of-plane pi-attack (SNpi) path are (i) lower proximate sigma-sigma* charge-transfer energies (DeltaECT), (ii) stronger electrostatic stabilization (DeltaENCT), and (iii) larger lobe size on C(alpha) for the sigma*- than pi*-LUMO despite the higher sigma* than pi* level. The electron correlation energy effects at the MP2 level are overestimated for the relatively delocalized structure (S(N)pi TS) but are underestimated for the localized structure (SNsigma TS) so that the MP2 energies lead to a wrong prediction of preferred reaction path for the vinyl chloride. The DFT at the B3LYP level predicts correct reaction pathways but overestimates the electron correlation effects.     

Nucleophilic substitution at silicon (SN2@Si) via a central reaction barrier

It is textbook knowledge that nucleophilic substitution at carbon (SN2@C) proceeds via a central reaction barrier which disappears in the corresponding nucleophilic substitution reaction at silicon (SN2@Si). Here, we address the question why the central barrier disappears from SN2@C to SN2@Si despite the fact that these processes are isostructural and isoelectronic. To this end, we have explored and analyzed the potential energy surfaces (PES) of various Cl-+CR3Cl (R=H, CH3) and Cl-+SiR3Cl model reactions (R=H, CH3, C2H5, and OCH3). Our results show that the nature of the SN2 reaction barrier is in essence steric, but that it can be modulated by electronic factors. Thus, simply by increasing the steric demand of the substituents R around the silicon atom, the SN2@Si mechanism changes from its regular single-well PES (with a stable intermediate transition complex, TC), via a triple-well PES (with a pre- and a post-TS before and after the central TC), to a double-well PES (with a TS; R=OCH3), which is normally encountered for SN2@C reactions.     

Gas phase reactions of NH2Cl with anionic nucleophiles: Nucleophilic substitution at neutral nitrogen

Reactions of chloramine, NH2Cl, with HO-, RO- (R = CH3, CH3CH2, CH3CH2CH2, C6H5CH2, CF3CH2), F- , HS- , and Cl- have been studied in the gas phase using the selected ion flow tube technique. Nucleophilic substitution (S(N)2) at nitrogen to form Cl- has been observed for all the nucleophiles. The reactions are faster than the corresponding S(N)2 reactions of methyl chloride; the chloramine reactions take place at nearly every collision when the reaction is exothermic. The thermoneutral identity S(N)2 reaction of NH2Cl with Cl-, which occurs approximately once in every 100 collisions, is more than two orders of magnitude faster than the analogous reaction of CH3Cl. The significantly enhanced S(N)2 reactivity of NH2Cl is consistent with a previous theoretical prediction that the barrier height for the S(N)2 identity reaction at nitrogen is negative relative to the energy of the reactants, whereas this barrier height for reaction at carbon is positive. Competitive proton abstraction to form NHCl- has also been observed with more highly basic anions (HO-, CH3O-, and CH3CH2O-), and this is the major reaction channel for HO- and CH3O-. Acidity bracketing determines the heat of deprotonation of NH2Cl as 374.4 +/- 3.0 kcal mol(-1).     

Does α‐effect exist in E2 reactions? A G2(+) investigation

The gas-phase base-induced bimolecular elimination (E2) reactions at saturated carbon with 13 bases, B(-) + CH3CH2Cl --> BH + CH2=CH2 + Cl(-) (B = HO, CH3O, CH3CH2O, FCH2CH2O, ClCH2CH2O, Cl, Br, FO, ClO, BrO, HOO, HSO, and H2NO), were investigated with the high-level G2(+) theory. It was found that all alpha-bases with adjacent lone pair electrons examined exhibited downward deviations from the correlation line between the overall barriers and proton affinities for the normal bases without adjacent lone pair electrons, indicating the existence of the alpha-effect in the gas phase E2 reactions. The sizes of the alpha-effect for the E2 reaction, DeltaH(alpha)(E2), span a smaller range if the alpha-atoms are on the same column in the periodic table, in contrast to the corresponding S(N)2 reactions, where the DeltaH(alpha)(S(N)2) values significantly decrease from an upper to a lower column. The origin of the alpha-effects in E2 reactions can be interpreted by the favorable orbital interaction between the lone-pair electrons and positively charged anti-bonding orbital. It is worth noticing that the neighboring electron-rich pi lobe instead of lone pair electrons could also cause the alpha-effect in E2 reaction.     

The alpha-effect in gas-phase SN2 reactions revisited

This paper re-examines gas-phase S(N)2 reactions at saturated carbon for model reactions Nu(-) + CH(3)Cl --> CH(3)Nu + Cl(-) (Nu(-) = HO(-), MeO(-), NH(2)(-), HS(-), Cl(-), Br(-), I(-), HOO(-), MeOO(-), HSS(-), and NH(2)NH(-)) using the G2(+) theory. The calculated results show that the alpha-effect does exist in the gas-phase S(N)2 reaction at the sp(3) carbon, contrary to the currently accepted notion of the absence of the alpha-effect in the gas phase.     

The alpha-effect in reactions of sp-hybridized carbon atom: Michael-type reactions of 1-aryl-2-propyn-1-ones with primary amines

[reaction: see text] Second-order rate constants (kN) have been measured for the Michael-type reaction of 1-(X-substituted phenyl)-2-propyn-1-ones (2a-f) with a series of primary amines in H2O at 25.0 +/- 0.1 degree C. A linear Br?nsted-type plot with a small beta(nuc) value (beta(nuc) = 0.30) has been obtained for the reactions of 1-phenyl-2-propyn-1-one (2c) with non-alpha-nucleophile amines. Hydrazine is more reactive than other primary amines of similar basicity (e.g., glycylglycine and glycine ethyl ester) and results in a positive deviation from the linear Br?nsted-type plot. The reactions of 2a-f with hydrazine exhibit a linear Hammett plot, while those with non-alpha-nucleophile amines give linear Yukawa-Tsuno plots, indicating that the electronic nature of the substituent X does not affect the reaction mechanism. The alpha-effect increases as the substituent X in the phenyl ring of 2a-f becomes a stronger electron-donating group. However, the magnitude of the alpha-effect for the reactions of 2a-f is small (e.g., kN(hydrazine)/kN(glycylglycine) = 4.6-13) regardless of the electronic nature of the substituent X. The small beta(nuc) has been suggested to be responsible for the small alpha-effect. A solvent kinetic isotope effect (e.g., kN(H2O)/kN(D2O) = 1.86) was observed for the reaction with hydrazine but absent for the reactions with non-alpha-nucleophile amines. The reactions with hydrazine and other primary amines have been suggested to proceed through a five-membered intramolecular H-bonding structure VI and a six-membered intermolecular H-bonding structure VII, respectively. The transition state modeled on VI can account for the substituent dependent alpha-effect and the difference in the solvent kinetic isotope effect exhibited by the reactions with hydrazine and other primary amines. It has been proposed that the beta(nuc) value is more important than the hybridization type of the reaction site to determine the magnitude of the alpha-effect.     

Hydrolysis of New Transplatin Analogue Containing One Aliphatic and One Planar Heterocyclic Amine Ligand: A Density Functional Theory Study

Herein we give a theoretical study of the hydrolysis processes of a novel anticancer drug trans-PtCl2(3-pico)(ipa) (3-pico=3-methylpyridine, ipa=isopropylamine). Two different models, model 1 relative to isolated reactant/product (R/P, wherein R=platinum complex+H2O, P=platinum complex+Cl-) and model 2 relative to reactant complex/product complex (RC/PC, wherein RC=(platinum complex)(H2O),PC=(platinum complex)(Cl-) are employed and the geometric structures are optimized at the B3LYP level of DFT method. It is found that the processes of the reactions follow the established theory for ligand substitution in square planar complexes; the geometries of the transition states (TS) agree with the previous related work and all of the reactions are endothermic. The effects originating from the inclusion of the attacking water/released chloride into the second coordination shell of platinum in RC/PC play an important role in the thermodynamic and kinetic profiles of the reactions, that is, the barrier heights of the reactions of model 2 are increased by ~6.3 and ~23.8 kJ/mol for step1 and step2 respectively, and the endother-micity is considerably decreased by ~420.5 and ~771.2 kJ/mol compared to model 1 in the gas phase. Theconsideration of the bulk solvation effects increase the barrier heights for both steps of model 1 by ~27.6 and ~6.7 kJ/mol respectively, whereas it reduces the barrier heights by ~7.9 and ~29.3 kJ/mol for model 2.The reaction energies are all decreased, especially for model 1, indicating more stable complexes solvated in the bulk aqueous solution than in the gas phase. Additionally, to get an accurate energy picture of the title complex, the relative free energies derived from the DFT-SCRF (density functional theory self-consistent field) calculations are compared with the relative total energies. The results are that activation energies rise for the first hydrolysis and fall for the second hydrolysis for all the systems, and for all the systems,the barrier height of the second hydrolysis is always higher than that of the first step. The rate constants indicate that transplatin analogue is kinetically comparable to cisplatin and its analogue in the hydrolysis process     

Coordination chemistry of trivalent lanthanide and actinide ions in dilute and concentrated chloride solutions

We have used EXAFS spectroscopy to investigate the inner sphere coordination of trivalent lanthanide (Ln) and actinide (An) ions in aqueous solutions as a function of increasing chloride concentration. At low chloride concentration, the hydration numbers and corresponding Ln,An-O bond lengths are as follows: La3+, N = 9.2, R = 2.54 A; Ce3+, N = 9.3, R = 2.52 A; Nd3+, N = 9.5, R = 2.49 A; Eu3+, N = 9.3, R = 2.43 A; Yb3+, N = 8.7, R = 2.32 A; Y3+, N = 9.7, R = 2.36 A; Am3+, N = 10.3, R = 2.48 A; Cm3+, N = 10.2, R = 2.45 A. In ca. 14 M LiCl, the early Ln3+ ions (La, Ce, Nd, and Eu) show inner sphere Cl- complexation along with a loss of H2O. The average chloride coordination numbers and Ln-Cl bond lengths are as follows: La3+, N = 2.1, R = 2.92 A; Ce3+, N = 1.8, R = 2.89 A; Nd3+, N = 1.9, R = 2.85 A; Eu3+, N = 1.1, R = 2.81 A. The extent of Cl- ion complexation decreases going across the Ln3+ series to the point where Yb3+ shows no Cl- complexation and no loss of coordinated water molecules. The actinide ions, Am3+ and Cm3+, show the same structural effects as the early Ln3+ ions, i.e., Cl- ion replacement of the H2O at high chloride thermodynamic activities. The Clion coordination numbers and An-Cl bond lengths are: Am3+, N = 1.8, R = 2.81 A; Cm3+, N = 2.4, R = 2.76 A. When combined with results reported previously for Pu3+ which showed no significant chloride complexation in 12 M LiCl, these results suggest that the extent of chloride complexation is increasing across the An3+ series. The origin of the differences in chloride complex formation between the Ln3+ and An3+ ions and the relevance to earlier work is discussed.     

Solvent effects on the S(N)2 reaction: Application of the density functional theory-based effective fragment potential method

The performance of the density functional theory (DFT)-based effective fragment potential (EFP) method is assessed using the S(N)2 reaction: Cl- + nH2O + CH3Br = CH3Cl + Br- + nH2O. The effect of the systematic addition of water molecules on the structures and relative energies of all species involved in the reaction has been studied. The EFP1 method is compared with second-order perturbation theory (MP2) and DFT results for n = 1, 2, and 3, and EFP1 results are also presented for four water molecules. The incremental hydration effects on the barrier height are the same for all methods. However, only full MP2 or MP2 with EFP1 solvent molecules are able to provide an accurate treatment of the transition state (TS) and hence the central barriers. Full DFT and DFT with EFP1 solvent molecules both predict central barriers that are too small. The results illustrate that the EFP1-based DFT method gives reliable results when combined with an accurate quantum mechanical (QM) method, so it may be used as an efficient alternative to fully QM methods in the treatment of larger microsolvated systems.     

A theoretical study of the iminoxyl/oxime self-exchange reaction. A five-center, cyclic proton-coupled electron transfer

In solution, the self-exchange reactions for oxygen-centered pi-radicals, e.g., PhO. + PhOH <==>PhOH + PhO., are known to occur with low activation enthalpies (E(a) approximately equal to 2 kcal/mol). For the PhO./PhOH couple and, we conclude, for other O-centered pi-radicals, exchange occurs by proton-coupled electron transfer (PCET) with the proton transferred between oxygen electron pairs while the electron migrates between oxygen orbitals orthogonal to the -O- - -H- - -O- transition state plane (Mayer et al. J. Am. Chem. Soc. 2002, 123, 11142). Iminoxyls, R(2)C=NO., are sigma-radicals with substantial spin density on the nitrogen. The R(2)C=NO./R(2)C=NOH self-exchange has a significant E(a) (Mendenhall et al. J. Am. Chem. Soc. 1973, 95, 627). For this exchange, DFT calculations have revealed a counterintuitive cisoid transition state in which the seven atoms, >C=NO- - -H- - -ON=C<, lie in a plane (R = H, Me) or, for steric reasons, two planes twisted at 45.2 degrees (R = Me(3)C). The planar transition state has the two N-O dipoles close to each other and pointing in the same direction and an O- - -H- - -O angle of 165.4 degrees . A transoid transition state for R = H lies 3.4 kcal/mol higher in energy than the cisoid despite a more favorable arrangement of the dipoles and a near linear O- - -H- - -O. It is concluded that iminoxyl/oxime self-exchange reactions occur by a five-center, cyclic PCET mechanism with the proton being transferred between electron pairs on the oxygens and the electron migrating between in-plane orbitals on the two nitrogens (R(N-N) = 2.65 A). The calculated E(a) values (8.8-9.9 kcal/mol) are in satisfactory agreement with the limited experimental data.     

Solvent effect on the alpha-effect: ground-state versus transition-state effects; a combined calorimetric and kinetic investigation

In a study of the solvent effect on the alpha-effect, second-order rate constants (kNu-) have been determined spectrophotometrically for reactions of a series of substituted phenyl acetates with butan-2,3-dione monoximate (Ox-, alpha-nucleophile) and p-chlorophenoxide (p-ClPhO-, reference nucleophile) in DMSO-H2O (DMSO = dimethyl sulfoxide) mixtures of varying compositions at 25.0 +/- 0.1 degrees C. The magnitude of the alpha-effect, kOx-/kp-ClPhO-, increases as the DMSO content in the medium increases up to 40-50 mol %, reaching 500, one of the largest alpha-effect values, and then decreases on further addition of DMSO, resulting in a bell-shaped alpha-effect profile regardless of the nature of the substrates. The magnitude of the alpha-effect is found to be significantly dependent on the substrates (or, more quantitatively, on beta(nuc)). Thus, beta(nuc) is an important predictor of the magnitude of the alpha-effect. The bell-shaped alpha-effect profile found in the present system is attributed to the differential change in the sensitivity of the medium effect on the Ox- and p-ClPhO- systems but not due to a change in the reaction mechanism or to a drastic change in the basicity of the two nucleophiles on addition of DMSO to the medium. Through application of calorimetric measurements of ground-state solvation combined with the diagnostic beta(nuc) values, it is shown that the transition-state effect is more dominant than the ground-state effect as the origin of the alpha-effect in the present system.     

Ethyl anomaly in the nucleophilic substitution at a series of β-methylated alkyl RCH2Z and carbonyl RCOZ carbon centers for R = Me, Et, i-Pr, t-Bu, and Z = LG

We have carried out DFT studies to explore the cause of anomalously fast reaction rates of ethyl group (R = Et) in the gas-phase S(N)2 reactions of RCH(2)Cl+Cl(-) and RCH(CN)Cl+Cl(-), and also for those in the cationic forms of RCH(2)(+) and RCH(CN)(+) with R = Me, Et, i-Pr, and t-Bu. The TS stabilization by hyperconjugative donor-acceptor vicinal charge transfers (CTs) from R to the major NBOs at the reaction center carbon in the S(N)2 TSs were estimated using natural bond orbital (NBO) analyses. In all cases the hyperconjugative CT stabilization increases in the order R = t-Bu < i-Pr < Me < Et in agreement with the experimental as well as theoretical rate orders, exhibiting an ethyl anomaly. We have also determined the reorganization energies and hyperconjugative CTs from R to the two major NBOs, C-O(-) and C-N(+), in the tetrahedral intermediate formed with five water molecules, T(5w), by transformation of sp(2) to sp(3) centers in the reactions of RC(═O)OC(6)H(5) with NH(3). The reorganization energy is the lowest and CT stabilization is the strongest with R = Et in line with the fastest experimental rate. We conclude that C-H is a better donor than C-C bond orbital and hyperconjugative vicinal σ chain extension leads to a stronger CT stabilization in the TS. The stronger CT stabilization for R = Et rather than Me is achieved by enhanced hyperconjugative CT to the reaction center in the TS as a result of narrower energy gap and greater overlap brought about by long-range orbital mixing as the C-H σ-chain is extended from n = 2 for Me to n = 3 for Et. We find that CT properties of the all-trans vicinal hyprconjugative C-H σ-chains are closely analogous to the corresponding conjugative polyene π-chains although skeletal patterns of bridge bonds are different and the stabilization energy gained by extension of the σ-chain is much weaker than that gained by the π-chain.     

RCH=CH2与过氧甲酸反应的量子化学研究

本文用MINDO/3方法研究了烯烃RCH=CH2(R=H, CH3, CHO和NO2)与过氧甲酸反应的机理。研究结果表明, RCH=CH2与过氧甲酸反应是亲电反应, 在加热条件下较容易进行。乙烯与过氧甲酸反应的过渡态具有局部对称结构; 若R为取代基时, 这种对称性不复存在, 对于R为给电子基, 过氧基的氧偏向与取代基相连的乙烯碳原子, R为吸电子基, 过氧基氧偏向乙烯的另一碳原子; 取代基的给、吸电子能力越强, 过渡态偏离对称结构越显著, 活化势垒降低或升高也越大。     

Solvent deuterium kinetic isotope effects for the methanolyses of neutral C=O, P=O and P=S esters catalyzed by a triazacyclododecane : Zn(2+)-methoxide complex

The methanolyses of several organophosphate/phosphonate/phosphorothioate esters (O,O-diethyl O-(4-nitrophenyl) phosphate, paraoxon, ; O,O-diethyl S-(3,5-dichlorophenyl) phosphorothioate, ; O-ethyl O-(2-nitro-4-chlorophenyl) methylphosphonate, ; O,O-dimethyl O-(3-methyl-4-nitrophenyl) phosphorothioate, fenitrothion, ; O-ethyl S-(3,5-dichlorophenyl) methylphosphonothioate ) and a carboxylate ester (p-nitrophenyl acetate, ) catalyzed by methoxide and the Zn(2+)((-)OCH(3)) complex of 1,5,9-triazacyclododecane ( : Zn(2+)((-)OCH(3))) were studied in methanol and d(1)-methanol at 25 degrees C. In the case of the methoxide reactions inverse skie's were observed for the series with values ranging from 2 to 1.1, except for where the k(D)/k(H) = 0.90 +/- 0.02. The inverse k(D)/k(H) values are consistent with a direct nucleophilic methoxide attack involving desolvation of the nucleophile with varying extents of resolvation of the TS. With the : Zn(2+)((-)OCH(3)) complex all the skie values are k(D)/k(H) = 1.0 +/- 0.1 except for where the value is 0.79 +/- 0.06. Arguments are presented that the fractionation factors associated with complex : Zn(2+)((-)OCH(3)) are indistinguishable from unity. The skie's for all the complex-catalyzed methanolyses are interpreted as being consistent with an intramolecular nucleophilic attack of the Zn(2+)-coordinated methoxide within a pre-equilibrium metal : substrate complex.     

Theoretical Study on Mechanism of Reaction of OH with HO2NO2

The reaction of HO2NO2 (peroxynitric acid, PNA) with OH was studied by the hybrid density functional B3LYP and CBS-QB3 methods. Based on the calculated potential energy surface, five reaction channels, H2O+NO2+O2, HOOH+NO3, NO2+HO3H, HO2+HONO2 and HO2+HOONO, were examined in detail. The major reaction channel is PNA+OH→M1→TS1→H2O+NO2+O2. Taking a pre-equilibrium approximation and using the CBS-QB3 energies, the theoretical rate constant of this channel was calculated as 1.13×10-12 cm3/(molecule s) at 300 K, in agreement with the experimental result. Comparison between reactions of HOONO2+OH and HONO2+OH was carried out. For HOR+OH reactions, the total rate constants increase from R=NO2 to R=ONO2, which is consistent with experimental measurements.     

Substitution reactions of H2GeFBeF with RH (R = F,OH, NH2): A theoretical study

A combined density functional and ab initio quantum chemical study of the substitution reactions of the germylenoid H₂GeFBeF with RH (R = F, OH, NH₂) compounds was carried out. The geometries of all the stationary points of the reactions were optimized using the DFT B3LYP method and then the QCISD method was used to calculate the single-point energies. The theoretical calculations indicated that along the potential energy surface, there were one transition state (TS) and one intermediate (IM) which connected the reactants and the products. The three substitution reactions of H₂GeFBeF with RH are compared with the addition reactions of H₂Ge with RH. And based on the calculated results we concluded that the substitution reactions of H₂GeFBeF + RH involve two steps. One is dissociation onto H₂Ge + BeF₂, and the other is the addition reaction of H₂Ge with RH.     

第五、六族元素的有机化合物在有机合成中应用的研究 LII: 含氟烯醚的合成

含氟烯醚可作为亲偶极体进行偶极环加成反应. 某些含氟烯醚在亲核试剂作用下可作为烷基化试剂. 本文报导了Wittig反应通过芳氧.     

HNCO与CX（X=F，Cl，Br）自由基反应机理的密度泛函理论研究

用量子化学密度泛函理论的B3LYP方法,在6-31+G~*水平上按BERNY能量梯度解 析全参数优化了HNCO与CX（X=F,Cl,Br）反应势能面上各驻点的几何构型,通过 振动频率分析确认了中间体和过渡态,内禀反应坐标（IRC）对反应物、中间体、 过渡态和产物的相关性予以证实,对各驻点进行了零点能校正（ZPE）在此基础上 计算了反应能垒。研究结果表明,与HNCO和其它小分子自由基反应不同,HNCO与 CX自由基反应首先发生分子间H原子迁移,随后N与CX的C（1）原子相互靠近成键并 生成较稳定的中间体,再发生N-C（2）键的断裂,完成N向C（1）上的迁移并进一 步解离为产物。反应按反应物→TS1→IM→TS2→产物通道进行。反应为放热反应。     

On gas phase alpha-effects. 1. The gas-phase manifestation and potential SET character

The possibility of a gas-phase alpha-effect has been explored for the methyl transfer from methyl formate to hydroxide, hydroperoxide, and ethoxide by computing barrier heights at the HF/6-311++G(2df,2p) level of theory. The alpha-nucleophile (hydroperoxide) is found to have a lower barrier than the gas-phase-acidity-matched normal nucleophile (ethoxide) by 3.6 kcal/mol, offering evidence for a gas phase alpha-effect. A Shi-Boyd analysis for these reactions indicates that there is more single-electron-transfer character in the hydroperoxide transition state than for either hydroxide or ethoxide, further bolstering the existence of a gas-phase alpha-effect and the appropriateness of the Hoz model for the alpha-effect.