[Show abstract][Hide abstract] ABSTRACT: High-level ab initio molecular orbital calculations at the G2(+) level of theory have been carried out on the identity front-side nucleophilic substitution reactions with retention of configuration, X- + CH3X, for X = F, Cl, Br, and I. Overall gas-phase barrier heights do not show a strong variation with halogen atom and are calculated to be 184.5 (X = F), 193.8 (X = Cl), 178.9 (X = Br), and 171.4 kJ mol-1 (X = I), substantially higher than the corresponding barriers for back-side attack (−8.0 for X = F, 11.5 for X = Cl, 5.8 for X = Br, and 6.5 kJ mol-1 for X = I). The difference between the overall barrier for back-side attack and front-side attack is smallest for X = I (164.9 kJ mol-1). Central barrier heights for front-side attack decrease in the following order: 241.0 (X = F), 237.8 (X = Cl), 220.0 (X = Br), and 207.4 kJ mol-1 (X = I). The minimum energy pathways for both back-side and front-side SN2 reactions are found to involve the same ion−molecule complex (X-···H3CX), with the front-side pathway becoming feasible at higher energies. Indeed, our results suggest that the chloride exchange in CH3Cl, which has been found in gas-phase experiments at high energies, may be the first example of a front-side SN2 reaction with retention of configuration at saturated carbon. Analysis of our computational data in terms of frontier orbital theory suggests that elongation of the bond between the central atom and X could be a significant factor in decreasing the unfavorable nature of the front-side SN2 reaction with retention of configuration in going from X = F to X = I. Ion−molecule complexes CH3−X···X-, which may be pre-reaction complexes in direct collinear halophilic attack, were found for X = Br and I but not for X = F and Cl. The calculated complexation energies (ΔHcomp) for halophilic complexes are considerably smaller (7.3 and 19.4 kJ mol-1 for X = Br and I, respectively) than those for the corresponding pre-reaction complexes for SN2 attack at carbon (41.1 and 36.0 kJ mol-1 for X = Br and I, respectively). Nucleophilic substitution reactions at the halogen atom in CH3X (X = F−I) (halophilic reactions) are highly endothermic and appear to represent an unlikely mechanistic pathway for identity halide exchange.
Full-text · Article · Nov 1996 · Journal of the American Chemical Society
[Show abstract][Hide abstract] ABSTRACT: The aromatic stabilization energy of the cyclopropenyl cation (CH)3+ is assessed with G2 theory by calculating its homodesmotic stabilization energy (247.3 kJ mol-1) and by comparing the ionization energies of the cyclopropenyl radical (6.06 eV) and the cyclopropyl radical (8.24 eV). These data indicate substantial stabilization of the two π-electron system in what is considered the archetypal aromatic cation. The calculated enthalpy of formation of the cyclopropenyl cation is 1074.0 kJ mol-1 and agrees with the experimental estimate of 1075 kJ mol-1. The small stabilization energy of the cyclopropenyl radical (37.4 kJ mol-1) suggests that this radical should not be classified as aromatic, in contrast to earlier suggestions. Our G2-calculated enthalpy of formation of the cyclopropenyl radical (ΔHf 298 = 487.4 kJ mol-1) and its ionization energy are different from experimental estimates and suggest that the experimental values may need to be revised. The most stable structure for the cyclopropenyl anion is a nonplanar Cs singlet structure containing a strongly pyramidalized carbon. The open-chain isomers of (CH)3- as well as the nonplanar triplet cyclic structures are all found to be higher in energy. The nonplanar C2 “allylic-type” cyclic structure of (CH)3- is 8.9 kJ mol-1 higher energy than the cyclic Cs structure and corresponds to a first-order saddle point. While the G2 stabilization energy of the cyclopropenyl anion estimated using the energy of the homodesmotic reaction cyclopropenyl anion + cyclopropane → cyclopropene + cyclopropyl anion is negative (−17.3 kJ mol-1), its absolute value is substantially less than the corresponding stabilization energy calculated for cyclobutadiene (−129.6 kJ mol-1). A comparison of the G2-calculated gas-phase acidities of cyclopropene (1755.4 kJ mol-1) and cyclopropane (1737.1 kJ mol-1) also suggests the antiaromatic destabilization energy of the cyclopropenyl anion to be small. However, the electron affinity of the cyclopropenyl radical is found to be negative (−0.18 eV), indicating that the cyclopropenyl anion is not bound in the gas phase.
No preview · Article · Nov 1996 · The Journal of Physical Chemistry
[Show abstract][Hide abstract] ABSTRACT: Ab initio molecular orbital calculations and transition-state theory have been used to study reactions between ethyl radical and ethylene that model chain transfer in the polymerization of ethylene, and the results rationalized with the aid of the curve-crossing model. It is found that the endothermic transfer of hydrogen from ethylene (abstraction) is kinetically favored over the thermoneutral transfer of hydrogen to ethylene (transfer). A possible explanation lies in the fact that only one bond needs to be broken in the abstraction reaction, in contrast to the two bonds that need to be broken in the transfer reaction.
No preview · Article · Oct 1996 · The Journal of Physical Chemistry
[Show abstract][Hide abstract] ABSTRACT: The performance of the B3-LYP variant of density functional theory when used in conjunction with the 6-31G(d) and 6-311 + G(3df, 2p) basis sets in describing the prototypical gas-phase SN2 reactions of Cl− + CH3Cl and CH3Br has been examined in detail. Reasonable values of the complexation energies (ΔHcomp) for the ion-molecule complexes formed in these reactions are obtained. However, the overall (ΔHovr#) and central (ΔHcent#) barriers for these reactions calculated using the B3-LYP functional are significantly underestimated when compared with G2(+) or experimental results. This implies that the B3-LYP energies for the Cl(H3C)Cl− (D3h) and Cl(H3C)Br− (C3v) transition structures are relatively too low. The B3-LYP errors appear to be systematic, with similar errors being found for corresponding quantities for the two reactions examined.
No preview · Article · Oct 1996 · Chemical Physics Letters
[Show abstract][Hide abstract] ABSTRACT: High-level ab initio molecular orbital calculations at the G2(+) level of theory have been carried out for the six non-identity nucleophilic substitution reactions, Y- + CH3X → YCH3 + X-, for Y, X = F, Cl, Br, and I. Central barrier heights (ΔHcent) for reaction in the exothermic direction vary from 0.8 kJ mol-1 for Y = F, X = I up to 39.5 kJ mol-1 for Y = Cl, X = Br (at 0 K), and are in most cases significantly lower than those for the set of identity SN2 reactions X- + CH3X → XCH3 + X- (X = F−I). Overall barriers (ΔHovr) for reaction in the exothermic direction are all negative (varying from −68.9 kJ mol-1 for Y = F, X = I to −2.3 kJ mol-1 for Y = Br, X = I), in contrast to the overall barriers for the identity reactions where only the value for X = F is negative. Complexation enthalpies (ΔHcomp) of the ion−molecule complexes Y-···CH3X vary from 30.4 kJ mol-1 for Y = F, X = I to 69.6 kJ mol-1 for Y = I, X = F (at 298 K), in good agreement with experimental and earlier computational studies. Complexation enthalpies in the reaction series Y- + CH3X (Y = F−I, X = F, Cl, Br, I) are found to exhibit good linear correlations with halogen electronegativity. Both the central barriers and the overall barriers show good linear correlations with reaction exothermicity, indicating a rate−equilibrium relationship in the Y- + CH3X reaction set. The data for the central barriers show good agreement with the predictions of the Marcus equation, though modifications of the Marcus equation that consider overall barriers are found to be less satisfactory. Further interesting features of the non-identity reaction set are the good correlations between the central barriers and the geometric looseness (%L), geometric asymmetry (%AS), charge asymmetry (Δq(X−Y)), and bond asymmetry (ΔWBI) of the transition structures.
No preview · Article · Jul 1996 · Journal of the American Chemical Society
[Show abstract][Hide abstract] ABSTRACT: The acidities, proton affinities, ionization energies, dissociation energies, and heats of formation of the hypohalous and hydrohalic acids have been calculated at the G2 level of theory. Where reliable experimental data are available, our results are generally in good agreement but in other cases our predictions serve to fill important gaps. The calculated gas-phase acidities of the hypohalous acids (1507.9 (HOF), 1490.0 (HOCl), 1490.6 (HOBr), and 1487.0 (HOI) kJ mol-1 at 298 K) agree well with available experimental data and are close to one another (lying within a range of 20.9 kJ mol-1), showing that the nature of the halogen has relatively little impact on their acidity. In contrast, the ΔHacid values for the hydrohalic acids HX increase by 232.9 kJ mol-1 in going from HF to HI. Hypohalous acids are more acidic than water. In addition, hypofluorous acid is a slightly stronger acid than HF. However, other hypohalous acids are weaker than the hydrohalic acids HX (X = Cl−I). The calculated proton affinities at oxygen (HOX → H2OX+: 565.9 (F), 641.9 (Cl), 678.0 (Br), and 724.7 (I) kJ mol-1 at 298 K) and at the halogen (HOX → HOXH+: 488.7 (F), 581.5 (Cl), 601.0 (Br), and 642.3 (I) kJ mol-1 at 298 K) are larger than PA(HX) values (484.0 (F), 561.5 (Cl), 584.8 (Br), and 626.0 (I) kJ mol-1 at 298 K) for all the halogens. The HOXH+ structures are higher in energy than the O-protonated forms, H2OX+. The ionization energy (IE) values for HOX decrease from HOF (12.71 eV) to HOI (9.89 eV) in a manner parallel to that found for the IE values for HX (X = F−I). The IE(HOX) values are all smaller than the corresponding IE(HX) values, but the IE difference decreases substantially in going from F to I. The G2 heats of formation for the hypohalous acids (−88.3 (HOF), −76.0 (HOCl), −58.3 (HOBr), and −48.9 (HOI) kJ mol-1 at 298 K) show good agreement with available experimental values.
No preview · Article · Feb 1996 · The Journal of Physical Chemistry
[Show abstract][Hide abstract] ABSTRACT: The recent suggestion, based on gas-phase experimental data, that the most stable isomer of protonated benzene has a face-protonated π-complex structure is not supported by our detailed computations which indicate that the π-complex is a second-order saddle point on the potential energy surface, lying 199 kJ mo–1 higher in energy than the well-established C2vσ-protonated structure.
No preview · Article · Nov 1995 · Journal of the Chemical Society Chemical Communications
[Show abstract][Hide abstract] ABSTRACT: High-level ab initio molecular orbital calculations at the G2(+) level of theory have been carried out for the identity nucleophilic substitution reactions at saturated nitrogen, X- + NH2X→XNH2 + X-, for X = F, Cl, Br, and I, and the results compared with data for the analogous reactions at saturated carbon, X- + CH3X→XCH3 + X-. Central barriers ΔH‡cent for substitution at nitrogen are found to lie within a relatively narrow range, decreasing in the following order: Cl (58.5 kJ mol-1) ≥ F (58.2 kJ mol-1) > Br (46.9 kJ mol-1) ≥ I (39.1 kJ mol-1). They are surprisingly similar to those for substitution at carbon, the barriers at nitrogen being slightly higher than the corresponding barriers at carbon for X = F and Cl and slightly lower for X = Br and I. The overall barriers relative to the reactants (ΔH‡ovr) are negative for all halogens: -55.8 (F), -9.3 (Cl), -13.7 (Br), and -10.9 kJ mol-1 (I), in contrast to the analogous reactions at carbon where the overall barrier is negative only for X = F. This suggests that nucleophilic substitution is likely to be more facile at nitrogen than at carbon. Stabilization energies of the ion-molecule complexes (ΔHcomp) decrease in the order F (114.0 kJ mol-1) > Cl (67.8 kJ mol-1) > Br (58.4 kJ mol-1) > I (50.0 kJ mol-1) and are found to correlate well with halogen electronegativities.
No preview · Article · Sep 1995 · Journal of the American Chemical Society
[Show abstract][Hide abstract] ABSTRACT: Basis sets have been developed for carrying out G2 calculations on bromine‐ and iodine‐containing molecules using all‐electron (AE) calculations and quasirelativistic energy‐adjusted spin–orbit‐averaged seven‐valence–electron effective core potentials (ECPs). Our recommended procedure for calculating G2[ECP] energies for such systems involves the standard G2 steps introduced by Pople and co‐workers, together with the following modifications: (i) second‐order Møller–Plesset (MP2) geometry optimizations use polarized split‐valence [31,31,1] basis sets for bromine and iodine together with 6‐31G(d) for first‐ and second‐row atoms; (ii) single‐point higher‐level energies are calculated for these geometries using our new supplemented bromine and iodine valence basis sets along with supplemented 6‐311G and McLean–Chandler 6‐311G bases for first‐ and second‐row atoms, respectively; and (iii) first‐order spin–orbit corrections are explicitly taken into account. An assessment of the results obtained using such a procedure is presented. The results are also compared with corresponding all‐electron calculations. We find that the G2[ECP] calculations give results which are generally comparable in accuracy to those of the G2[AE] calculations but which involve considerably lower computational cost. They are therefore potentially useful for larger bromine‐ and iodine‐containing molecules for which G2[AE] calculations would not be feasible.
No preview · Article · Aug 1995 · The Journal of Chemical Physics
[Show abstract][Hide abstract] ABSTRACT: The thermochemical properties of cyclobutadiene and tetrahedrane have been calculated using high-level MO calculations at the G2 level of theory. Enthalpies of formation of cyclobutadiene and tetrahedrane at 298 K are calculated to be 426 +/- 4 and 535 +/- 4 kJ mol(-1), respectively. Antiaromatic destabilization of cyclobutadiene and the strain energy of tetrahedrane have been estimated to be 170 +/- 7 and 571 +/- 4 kJ mol(-1) (at 298 K), respectively. The value of the antiaromatic destabilization of cyclobutadiene (42.5 kJ mol(-1)) per pi electron is larger than the aromatic stabilization of benzene per pi electron (15.4 kJ mol(-1)).
Preview · Article · May 1995 · The Journal of Physical Chemistry
[Show abstract][Hide abstract] ABSTRACT: High-level ab initio calculations and variable-temperature proton-transfer equilibrium constant measurements have been used to obtain new thermochemical data for protonated halogenomethanes (CH(3)XH(+), X = F, Cl, Pr, and I) and protonated diazomethane (CH3NN+). Proton affinities of CH(3)X and CH2NN and methyl cation affinities of HX and N-2 have been derived. The theoretical and experimental results are in good agreement with one another but in several cases are in conflict with currently accepted experimental proton and methyl cation affinities. Experimental and theoretical methyl cation affinities are presented for a variety of molecules, leading to the proposal of a new methyl cation affinity scale.
No preview · Article · Dec 1994 · The Journal of Physical Chemistry
[Show abstract][Hide abstract] ABSTRACT: High-level ab initio calculations at the QCISD/6-311G** + ZPVE level have been carried out to study the addition reactions of CH3., CH2OH., and CH2CN. radicals to the substituted alkenes CH2=CHX (X = H, NH2, F, Cl, CHO, and CN) and the results analyzed with the aid of the curve-crossing model. We find that the reactivity of CH3. is primarily governed by enthalpy effects, whereas both enthalpy and polar effects are important for the reactions of CH2OH. and CH2CN.. There is no general barrier height-enthalpy correlation for the latter two radicals because of the presence in some cases of polar effects that stabilize the transition states without a corresponding stabilization of the products. The polar effects are not sufficient, however, to significantly shift the location of the transition states, so a general structure-enthalpy correlation is observed.
No preview · Article · Jul 1994 · Journal of the American Chemical Society
[Show abstract][Hide abstract] ABSTRACT: Ab initio calculations at the QCISD(T)/6–311G** level have been carried out to study the addition of the methyl radical to a series of substituted alkenes, and the results analyzed with the aid of the curve‐crossing model. It is found that (a) reaction exothermicity is the main factor that dominates reactivity, (b) polar contributions to the transition states are generally small and of minor energetic consequences, and (c) the general observation that π‐electron‐accepting substituents in the alkene enhance reactivity is a secondary correlation that is a consequence of the effect of these substituents on reaction exothermicity. There is no evidence for the prevalent view that the methyl radical is generally nucleophilic towards alkenes.
No preview · Article · Jan 1993 · Israel Journal of Chemistry
[Show abstract][Hide abstract] ABSTRACT: The curve crossing model was applied to a series of hydrogen abstraction reactions from a family of alkanes, RH (R = methyl, ethyl, isopropyl, tert-butyl) by alkyl, hydrogen and chlorine radicals. The analysis was based on quantitative data obtained from an ab initio MO study. Schematic reaction profiles for the reaction of RH with alkyl and hydrogen radicals are built up from just two configurations: reactant, DA, and product D3* A. For the Cl atom reaction, however, a significant contribution of D+ A−, a charge-transfer configuration, is also shown to be present. A simple explanation for differences in the intrinsic barrier for the identity radical abstraction reaction based on the initial gap size between DA and D3* A configurations is provided. The influence of the D+ A− configuration on the nature of the transition state of the Cl atom reaction and its intrinsic barrier is described. It is the D+ A− configuration that is responsible for the polar character often observed in radical abstraction and addition reactions.
No preview · Article · Mar 1991 · Journal of Physical Organic Chemistry
[Show abstract][Hide abstract] ABSTRACT: Application of the curve crossing model to singlet carbene addition to alkenes reveals that for electrophilic carbenes the reaction barrier is generated from the avoided crossing of DA, 3D*3A* and D+A− configurations whereas for nucleophilic carbenes the third configuration is D−A+.
No preview · Article · Dec 1990 · Tetrahedron Letters
[Show abstract][Hide abstract] ABSTRACT: The curve-crossing model is applied to the problem of barrier heights for nucleophilic attack on cation radicals, RH•+, and cations, R+. It is shown that the barrier height depends on the ionization potential of the nucleophile, the electron affinity of the cation, and, for cation radicals, also on the singlet-triplet energy gap of the corresponding neutral molecule, RH. It is shown that in general, cation radicals are likely to be less reactive than cations (of the same acceptor ability) toward nucleophilic attack, because the product configuration for cation radicals is doubly excited (D+ 3*A-), whereas that for regular cations is singly excited (D+A-). A semiquantitative analysis is presented that shows that those cases where cation radicals are likely to react rapidly with nucleophiles can be predicted in a straightforward manner.
No preview · Article · Jun 1989 · Journal of the American Chemical Society
[Show abstract][Hide abstract] ABSTRACT: The configuration mixing (CM) model is applied to the migratory insertion of carbon monoxide into a transition metalcarbon bond. A qualitative reaction profile for the reaction is constructed using an energy plot of the three electronic configurations, which describe the reactant, the product, and the five-coordinate intermediate. The model provides a simple picture of reactivity trends in these systems, as well as indicating the way in which a mechanistic spectrum, encompassing both step-wise and concerted pathways, is generated. The model analyzes the effect of modifications of the migrating group, the acceptor group, and the entering ligand. Migratory insertion assisted by either prior oxidation or reduction of the starting metal complex is also considered. The conclusions are supported by experimental and computational data.
No preview · Article · Feb 1988 · Journal of Organometallic Chemistry