Kinetics and Thermodynamics of Reactions Involving Criegee Intermediates: An Assessment of Density Functional Theory and Ab Initio Methods Through Comparison with CCSDT(Q)/CBS Data

Reactions involving Criegee intermediates (CIs, R₁R₂COO) are important in atmospheric ozonolysis models. In recent years, density functional theory (DFT) and CCSD(T)-based ab initio methods are increasingly being used for modeling reaction profiles involving CIs. We obtain highly accurate CCSDT(Q)/CBS reaction energies and barrier heights for ring-closing reactions involving atmospherically important CIs (R₁/R₂ = H, Me, OH, OMe, F, CN, cyclopropene, ethylene, acetaldehyde, and acrolein). We use this benchmark data to evaluate the performance of DFT, double-hybrid DFT (DHDFT), and ab initio methods for the kinetics and thermodynamics of these reactions. We find that reaction energies are more challenging for approximate theoretical procedures than barrier heights. Overall, taking both reaction energies and barrier heights into account, only one of the 58 considered DFT methods (the meta-GGA MN12-L) attains near chemical accuracy, with root-mean-square deviations (RMSDs) of 3.5 (barrier heights) and 4.7 (reaction energies) kJ mol⁻¹. Therefore, MN12-L is recommended for investigations where CCSD(T)-based methods are not computationally feasible. For reaction barrier heights performance does not strictly follow Jacob's Ladder, for example, DHDFT methods do not perform better than conventional DFT methods. Of the ab initio methods, the cost-effective CCSD(T)/CBS(MP2) approach gives the best performance for both reaction energies and barrier heights, with RMSDs of 1.7 and 1.4 kJ mol⁻¹, respectively. All the considered Gaussian-n methods show good performance with RMSDs below the threshold of chemical accuracy for both reaction energies and barrier heights, where G4(MP2) shows the best overall performance with RMSDs of 2.9 and 1.5 kJ mol⁻¹, respectively. © 2019 Wiley Periodicals, Inc.     

BDE261: a comprehensive set of high-level theoretical bond dissociation enthalpies

We have used the high-level W1w protocol to compile a comprehensive collection of 261 bond dissociation enthalpies (BDEs) for bonds connecting hydrogen, first-row and second-row p-block elements. Together they cover 45 bond types, and we term this the BDE261 set. We have used these benchmark values to assess the performance of computationally less demanding theoretical procedures, including density functional theory (DFT), double-hybrid DFT (DHDFT), and high-level composite procedures. We find that the M06-2X (DFT), ROB2-PLYP and DuT-D3 (DHDFT), and G3X(MP2)-RAD and G4(MP2)-6X (composite) procedures yield absolute BDEs with satisfactory to excellent accuracy. Overall, we recommend G4(MP2)-6X as an accurate and relatively cost-effective procedure for the direct computation of BDEs. One important finding is that the deviations for DFT and (especially) DHDFT procedures are often quite systematic. This allows an alternative approach to obtaining accurate absolute BDEs, namely, to evaluate accurate relative BDEs (RBDEs) using a computationally less demanding procedure, and to use these RBDEs in combination with appropriate and accurate reference BDEs to give accurate absolute BDEs. We recommend DuT-D3 for this purpose. For a still less computationally demanding approach, we introduce the deviation from additivity of the RBDE (DARBDE), and demonstrate that the combination of lower-level DARBDEs for larger systems and higher-level (W1w) reference RBDEs and BDEs for small systems can be utilized to obtain improved RBDEs for multiply substituted systems at low cost.     

Assessment of theoretical methods for the calculation of methyl cation affinities

The methyl cation affinity (MCA; 298 K) of a variety of neutral and anionic bases has been examined computationally with a wide variety of theoretical methods. These include high-level composite procedures such as W1, G3, G3B3, and G2, conventional ab initio methods such as CCSD(T) and MP2, as well as a selection of density functional theory (DFT) methods. Experimental results for a variety of small model systems are well reproduced with practically all these methods, and the performance of DFT based methods are far superior in comparison to their MP2 analogs for these small models. For larger model, systems including motifs frequently encountered in organocatalysts, the performance deteriorates somewhat for DFT methods, while it improves significantly for MP2, rendering the former methods unreliable for common organic bases. Thus, MP2 calculations performed in combination with basis sets such as 6-31+G(2d, p) or larger, appear to offer a practical and reliable approach to compute MCAs of organic bases.     

o-Quinone methide as alkylating agent of nitrogen, oxygen, and sulfur nucleophiles. The role of H-bonding and solvent effects on the reactivity through a DFT computational study

The reactivity of the alkylating agent o-quinone methide (o-QM) toward NH(3), H(2)O, and H(2)S, prototypes of nitrogen-, oxygen-, and sulfur-centered nucleophiles, has been studied by quantum chemical methods in the frame of DF theory (B3LYP) in reactions modeling its reactivity in water with biological nucleophiles. The computational analysis explores the reaction of NH(3), H(2)O, and H(2)S with o-QM, both free and H-bonded to a discrete water molecule, with the aim to rationalize the specific and general effect of the solvent on o-QM reactivity. Optimizations of stationary points were done at the B3LYP level using several basis sets [6-31G(d), 6-311+G(d,p), adding d and f functions to the S atom, 6-311+G(d,p),S(2df), and AUG-cc-pVTZ]. The activation energies calculated for the addition reactions were found to be reduced by the assistance of a water molecule, which makes easier the proton-transfer process in these alkylation reactions by at least 12.9, 10.5, and 6.0 kcal mol(-1) [at the B3LYP/AUG-cc-pVTZ//B3LYP/6-311+G(d,p) level], for ammonia, water, and hydrogen sulfide, respectively. A proper comparison of an uncatalyzed with a water-catalyzed reaction mechanism has been made on the basis of activation Gibbs free energies. In gas-phase alkylation of ammonia and water by o-QM, reactions assisted by an additional water molecule H-bonded to o-QM (water-catalyzed mechanism) are favored over their uncatalyzed counterparts by 5.6 and 4.0 kcal mol(-1) [at the B3LYP/6-311+G(d,p) level], respectively. In contrast, the hydrogen sulfide alkylation reaction in the gas phase shows a slight preference for a direct alkylation without water assistance, even though the free energy difference (DeltaDeltaG(#)) between the two reaction mechanisms is very small (by 1.0 kcal mol(-1) at the B3LYP/6-311+G(d,p),S(2df) level of theory). The bulk solvent effect, evaluated by the C-PCM model, significantly modifies the relative importance of the uncatalyzed and water-assisted alkylation mechanism by o-QM in comparison to the case in the gas phase. Unexpectedly, the uncatalyzed mechanism becomes highly favored over the catalyzed one in the alkylation reaction of ammonia (by 7.0 kcal mol(-1)) and hydrogen sulfide (by 4.0 kcal mol(-1)). In contrast, activation induced by water complexation still plays an important role in the o-QM hydration reaction in water as solvent.     

Pristine Graphene as a Racemization Catalyst for Axially Chiral BINOL

Despite versatile applications of functionalized graphene in catalysis, applications of pure, unfunctionalized graphene in catalysis are in their infancy. This work uses both computational and experimental approaches to show that single-layer graphene can efficiently catalyze the racemization of axially chiral BINOL in solution. Using double-hybrid density functional theory (DHDFT) we calculate the uncatalyzed and catalyzed Gibbs free reaction barrier heights in a number of representative solvents of varying polarity: benzene, diphenyl ether, dimethylformamide (DMF), and water. These calculations show that (i) graphene can achieve significant catalytic efficiencies (▵▵G^≠_cat) varying between 47.2 (in diphenyl ether) and 60.7 (in DMF) kJ mol⁻¹. An energy decomposition analysis reveals that this catalytic activity is driven by electrostatic and dispersion interactions. Based on these computational results, we explore the graphene-catalyzed racemization of axially chiral BINOL experimentally and show that single-layer graphene can efficiently catalyze this process. Whilst the uncatalyzed racemization requires high temperatures of over 200 °C, a pristine single-layer graphene catalyst makes it accessible at 60 °C.     

Accurate reaction barrier heights of pericyclic reactions: Surprisingly large deviations for the CBS‐QB3 composite method and their consequences in DFT benchmark studies

Accurate barrier heights are obtained for the 26 pericyclic reactions in the BHPERI dataset by means of the high‐level Wn‐F12 thermochemical protocols. Very often, the complete basis set (CBS)‐type composite methods are used in similar situations, but herein it is shown that they in fact result in surprisingly large errors with root mean square deviations (RMSDs) of about 2.5 kcal mol^?1. In comparison, other composite methods, particularly G4‐type and estimated coupled cluster with singles, doubles, and quasiperturbative triple excitations [CCSD(T)/CBS] approaches, show deviations well below the chemical‐accuracy threshold of 1 kcal mol^?1. With the exception of SCS‐MP2 and the herein newly introduced MP3.5 approach, all other tested Møller‐Plesset perturbative procedures give poor performance with RMSDs of up to 8.0 kcal mol^?1. The finding that CBS‐type methods fail for barrier heights of these reactions is unexpected and it is particularly troublesome given that they are often used to obtain reference values for benchmark studies. Significant differences are identified in the interpretation and final ranking of density functional theory (DFT) methods when using the original CBS‐QB3 rather than the new Wn‐F12 reference values for BHPERI. In particular, it is observed that the more accurate Wn‐F12 benchmark results in lower statistical errors for those methods that are generally considered to be robust and accurate. Two examples are the PW6B95‐D3(BJ) hybrid‐meta‐general‐gradient approximation and the PWPB95‐D3(BJ) double‐hybrid functionals, which result in the lowest RMSDs of the entire DFT study (1.3 and 1.0 kcal mol^?1, respectively). These results indicate that CBS‐QB3 should be applied with caution in computational modeling and benchmark studies involving related systems. © 2015 Wiley Periodicals, Inc.     

W4‐17: A diverse and high‐confidence dataset of atomization energies for benchmarking high‐level electronic structure methods

Atomization reactions are among the most challenging tests for electronic structure methods. We use the first‐principles Weizmann‐4 (W4) computational thermochemistry protocol to generate the W4‐17 dataset of 200 total atomization energies (TAEs) with 3σ confidence intervals of 1 kJ mol⁻¹. W4‐17 is an extension of the earlier W4‐11 dataset; it includes first‐ and second‐row molecules and radicals with up to eight non‐hydrogen atoms. These cover a broad spectrum of bonding situations and multireference character, and as such are an excellent benchmark for the parameterization and validation of highly accurate ab initio methods (e.g., CCSD(T) composite procedures) and double‐hybrid density functional theory (DHDFT) methods. The W4‐17 dataset contains two subsets (i) a non‐multireference subset of 183 systems characterized by dynamical or moderate nondynamical correlation effects (denoted W4‐17‐nonMR) and (ii) a highly multireference subset of 17 systems (W4‐17‐MR). We use these databases to evaluate the performance of a wide range of CCSD(T) composite procedures (e.g., G4, G4(MP2), G4(MP2)‐6X, ROG4(MP2)‐6X, CBS‐QB3, ROCBS‐QB3, CBS‐APNO, ccCA‐PS3, W1, W2, W1‐F12, W2‐F12, W1X‐1, and W2X) and DHDFT methods (e.g., B2‐PLYP, B2GP‐PLYP, B2K‐PLYP, DSD‐BLYP, DSD‐PBEP86, PWPB95, ωB97X‐2(LP), and ωB97X‐2(TQZ)). © 2017 Wiley Periodicals, Inc.     

Highly accurate first-principles benchmark data sets for the parametrization and validation of density functional and other approximate methods. Derivation of a robust, generally applicable, double-hybrid functional for thermochemistry and thermochemical kinetics

We present a number of near-exact, nonrelativistic, Born-Oppenheimer reference data sets for the parametrization of more approximate methods (such as DFT functionals). The data were obtained by means of the W4 ab initio computational thermochemistry protocol, which has a 95% confidence interval well below 1 kJ/mol. Our data sets include W4-08, which are total atomization energies of over 100 small molecules that cover varying degrees of nondynamical correlations, and DBH24-W4, which are W4 theory values for Truhlar's set of 24 representative barrier heights. The usual procedure of comparing calculated DFT values with experimental atomization energies is hampered by comparatively large experimental uncertainties in many experimental values and compounds errors due to deficiencies in the DFT functional with those resulting from neglect of relativity and finite nuclear mass. Comparison with accurate, explicitly nonrelativistic, ab initio data avoids these issues. We then proceed to explore the performance of B2x-PLYP-type double hybrid functionals for atomization energies and barrier heights. We find that the optimum hybrids for hydrogen-transfer reactions, heavy-atoms transfers, nucleophilic substitutions, and unimolecular and recombination reactions are quite different from one another: out of these subsets, the heavy-atom transfer reactions are by far the most sensitive to the percentages of Hartree-Fock-type exchange y and MP2-type correlation x in an (x, y) double hybrid. The (42,72) hybrid B2K-PLYP, as reported in a preliminary communication, represents the best compromise between thermochemistry and hydrogen-transfer barriers, while also yielding excellent performance for nucleophilic substitutions. By optimizing for best overall performance on both thermochemistry and the DBH24-W4 data set, however, we find a new (36,65) hybrid which we term B2GP-PLYP. At a slight expense in performance for hydrogen-transfer barrier heights and nucleophilic substitutions, we obtain substantially better performance for the other reaction types. Although both B2K-PLYP and B2GP-PLYP are capable of 2 kcal/mol quality thermochemistry, B2GP-PLYP appears to be the more robust toward nondynamical correlation and strongly polar character. We additionally find that double-hybrid functionals display excellent performance for such problems as hydrogen bonding, prototype late transition metal reactions, pericyclic reactions, prototype cumulene-polyacetylene system, and weak interactions.     

Benchmark study of DFT functionals for late-transition-metal reactions

The performance of a wide variety of DFT exchange-correlation functionals for a number of late-transition-metal reaction profiles has been considered. Benchmark ab-initio reference data for the prototype reactions Pd + H2, Pd + CH4, Pd + C2H6 (both C-C and C-H activation), and Pd + CH3Cl are presented, while ab-initio data of lesser quality were obtained for the catalytic hydrogenation of acetone and for the low-oxidation-state and high-oxidation-state mechanisms of the Heck reaction. "Kinetics" functionals such as mPW1K, PWB6K, BB1K, and BMK clearly perform more poorly for late-transition-metal reactions than for main-group reactions, as well as compared to general-purpose functionals. There is no single "best functional" for late-transition-metal reactions, but rather a cluster of several functionals (PBE0, B1B95, PW6B95, and TPSS25B95) that perform about equally well; if main-group thermochemical performance is additionally considered, then B1B95 and PW6B95 emerge as the best performers. TPSS25B95 and TPSS33B95 offer attractive performance compromises if weak interactions and main-group barrier heights, respectively, are also important. In the ab-initio calculations, basis set superposition errors (BSSE) can be greatly reduced by ensuring that the metal spd shell has sufficient radial flexibility in the high-exponent range. Optimal HF percentages in hybrid functionals depend on the class of systems considered, increasing from anions to neutrals to cations to main-group barrier heights; transition-metal barrier heights represent an intermediate situation. The use of meta-GGA correlation functionals appears to be quite beneficial.     

Comparative theoretical study of transition structures,barrier heights,and reaction energies for the intramolecular tautomerization in acetaldehyde/vinyl alcohol and acetaldimine/vinylamine systems

The transition structures associated with the possible intramolecular tautomerization for acetaldehyde/vinyl alcohol and acetaldimine/vinylamine systems as models of keto/enol and imine/enamine interconversion processes, respectively, were characterized. The relative stabilities of the tautomers and the associated barrier heights were calculated. Ab initio analytical gradients and second derivatives at the HF level of theory and 3-21G, 6-31G, 6-31G**, 6-31++G**, and 6-311++G** basis-set, DFT (BP86/6-311++G** and BLYP/6-311++G**), and semiempirical (AM1 and PM3) procedures were used to identify the stationary points. Correlation effects were estimated using the perturbational approach at MP2/6-31G**, MP2/6-311++G**, and MP2/6-311++G (3df,2p) levels. The geometry, electronic structure, harmonic vibrational frequencies, and transition vector associated with the transition structures as well as the relative stabilities of different isomers and barrier heights were analyzed. The dependence of these properties upon theoretical methods is analyzed and discussed. The transition structures are four-membered rings and the corresponding transition vectors are associated to collective fluctuations. The 1,3 intramolecular hydrogen migration is much more advanced than are the hybridization changes on donor and acceptor centers at the transition structure. The corresponding barrier heights can be related to the change of bond orders and acid/base properties of these centers. A comparison of the results obtained with different methods renders that the nature of the transition structure seems to be a rather robust entity. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66 : 9–24, 1998     

Theoretical studies of proton-transfer reactions of 2-hydroxypyridine--(H2O)n (n = 0-2) in the ground and excited states

The potential energy profiles for proton-transfer reactions of 2-hydroxypyridine and its complexes with water were determined by MP2, CASSCF and MR-CI calculations with the 6-31G** basis set. The tautomerization reaction between 2-hydroxypyridine (2HP) and 2-pyridone (2PY) does not take place at room temperature because of a barrier of approximately 35 kcal/mol for the ground-state pathway. The water-catalyzed enol-keto tautomerization reactions in the ground state proceed easily through the concerted proton transfer, especially for the two-water complex. The S1 tautomerization between the 2HP and 2PY monomers has a barrier of 18.4 kcal/mol, which is reduced to 5.6 kcal/mol for the one-water complex and 6.4 kcal/mol for the two-water complex. The results reported here predict that the photoinduced tautomerization reaction between the enol and keto forms involves a cyclic transition state having one or two water molecules as a bridge.     

Double-hybrid functionals for thermochemical kinetics

We propose two new double-hybrid functionals, denoted B2K-PLYP and mPW2K-PLYP, which yield thermochemical performance comparable to existing double-hybrid functionals but offer superior performance for barrier heights of various kinds. We show that the new functionals yield excellent performance for all of the following: (a) main-group thermochemistry; (b) main-group thermochemical kinetics; (c) late transition metal reactions. In addition, B2K-PLYP performs well for weak interactions.     

Ab initio and density function theory computational studies of the CH4 + H → CH3 + H2 reaction

The activation barrier for the CH₄ + H → CH₃ + H₂ reaction was evaluated with traditional ab initio and Density Functional Theory (DFT) methods. None of the applied ab initio and DFT methods was able to reproduce the experimental activation barrier of 11.0-12.0 kcal/mol. All ab initio methods (HF, MP2, MP3, MP4, QCISD, QCISD(T), G1, G2, and G2MP2) overestimated the activation energy. The best results were obtained with the G2 and G2MP2 ab initio computational approaches. The zero-point corrected energy was 14.4 kcal mol⁻¹. Some of the exchange DFT methods (HFB) computed energies which were similar to the highly accurate ab initio methods, while the B3LYP hybrid DFT methods underestimated the activation barrier by 3 kcal mol⁻¹. Gradient-corrected DFT methods underestimated the barrier even more. The gradient-corrected DFT method that incorporated the PW91 correlational functional even generated a negative reaction barrier. The suitability of some computational methods for accurately predicting the potential energy surface for this hydrogen radical abstraction reaction was discussed.     

Catalytic effect of molecular iodine in Diels–Alder reaction: a density functional theory study

We explore here the feasibility of employing molecular iodine as Lewis acid catalyst for Diels–Alder (DA) reaction using conceptual density functional theory (DFT) based reactivity indices and transition state analysis at the DFT (B3LYP)/6-31G(d) level of theory. Catalytic effect of iodine is probed using reactivity indices considering six different substituents for the diene at the 2-position and five different substituents at the 1-position of the olefinic dienophile. Comparison of HOMO diene–LUMO dienophile gap between the catalyzed and uncatalyzed processes confirms catalytic effect of iodine in DA reaction. Mechanistic details of both the uncatalyzed and the iodine catalyzed processes is achieved through transition state analysis for four possible stereoisomeric reactive channels with respect to isoprene–acrolein model reaction. A significant cutback in activation barrier is observed in presence of iodine. Influence of iodine on regioselectivity of the reaction and asynchronicity of bond formation is analyzed using local version of the HSAB principle and philicity index.     

The MC‐DFT approach including the SCS‐MP2 energies to the new minnesota‐type functionals

We have applied the multicoefficient density functional theory (MC‐DFT) to four recent Minnesota functionals, including M06‐2X, M08‐HX, M11, and MN12‐SX on the performance of thermochemical kinetics. The results indicated that the accuracy can be improved significantly using more than one basis set. We further included the SCS‐MP2 energies into MC‐DFT, and the resulting mean unsigned errors (MUEs) decreased by approximately 0.3 kcal/mol for the most accurate basis set combinations. The M06‐2X functional with the simple [6–311+G(d,p)/6–311+G(2d,2p)] combination gave the best performance/cost ratios for the MC‐DFT and MC‐SCS‐MP2|MC‐DFT methods with MUE of 1.58 and 1.22 kcal/mol, respectively. © 2014 Wiley Periodicals, Inc.     

Ab initio calculations on the barrier height for the hydrogen addition to ethylene and formaldehyde. The importance of spin projection

Heats of reaction and barrier heights have been computed for H + CH₂CH₂ → C₂H₅, H + CH₂O → CH₃O, and H + CH₂O → CH₂OH using unrestricted Hartree-Fock and Møller–Plesset perturbation theory up to fourth order (with and without spin annihilation), using single-reference configuration interaction, and using multiconfiguration self-consistent field methods with 3-21G, 6-31G(d), 6-31G(d,p), and 6-311G(d,p) basis sets. The barrier height in all three reactions appears to be relatively insensitive to the basis sets, but the heats of reaction are affected by p-type polarization functions on hydrogen. Computation of the harmonic vibrational frequencies and infrared intensities with two sets of polarization functions on heavy atoms [6-31G(2d)] improves the agreement with experiment. The experimental barrier height for H + C₂H₄ (2.04 ± 0.08 kcal/mol) is overestimated by 7?9 kcal/mol at the MP2, MP3, and MP4 levels. MCSCF and CISD calculations lower the barrier height by approximately 4 kcal/mol relative to the MP4 calculations but are still almost 4 kcal/mol too high compared to experiment. Annihilation of the largest spin contaminant lowers the MP4SDTQ computed barrier height by 8?9 kcal/mol. For the hydrogen addition to formaldehyde, the same trends are observed. The overestimation of the barrier height with Møller-Plesset perdicted barrier heights for H + C₂H₄ → C₂H₅, H + CH₂O → CH₃O, and H + CH₂O → CH₂OH at the MP4SDTQ /6-31G(d) after spin annihilation are respectively 1.8, 4.6, and 10.5 kcal/mol.     

DFT studies of the phosphinidene derivative HPNaF and its insertion reaction with R–H(R=F,OH, NH<Subscript>2</Subscript>, CH<Subscript>3</Subscript>)

The formations of the phosphinidene derivative HPNaF and its insertion reactions with R–H (R=F, OH, NH₂, CH₃) have been systematically investigated employing the density functional theory (DFT), such as the B3LYP and MPW1PW91 methods. A comparison with the results of MP2 calculations shows that MPW1PW91 underestimates the barrier heights for the four reactions considered. Similarly, the same is also true for the B3LYP method depending on the selected reactions, but by much less than MPW1PW91, where the barrier heights of the four reactions are 25.2, 85.7, 119.0, and 142.4 kJ/mol at the B3LYP/6-311+G* level of theory, respectively. All the mechanisms of the four reactions are identical to each other, i.e., an intermediate has been located during the insertion reaction. Then, the intermediate could dissociate to substituted phosphinidane(H₂RP) and NaF with a barrier corresponding to their respective dissociation energies. Correspondingly, the reaction energies for the four reactions are −92.2, −68.1, −57.2, and −44.3 kJ/mol at the B3LYP/6-311+G* level of theory, respectively, where both the B3LYP and MPW1PW91 methods underestimate the reaction energies compared with the MP2 results. The linear correlations between the calculated barrier heights and the reaction energies have also been observed. As a result, the relative reactivity among the four insertion reactions should be as follows: H–F > H–OH > H–NH₂ > H–CH₃.     

Computational studies on the BF3-catalyzed cycloaddition of furan with methyl vinyl ketone: a new look at Lewis acid catalysis

Transition structures for both uncatalyzed and BF3-catalyzed Diels-Alder reactions involving furan and methyl vinyl ketone have been determined at the hybrid DFT (B3LYP/6-31G*) level of theory. The transition structures are predicted to be relatively concerted and highly asynchronous in all cases. A subsequent bond-order analysis has been carried out at the MP2/6-31G*//B3LYP/6-31G*. The role of the Lewis acid and the origin of the endo selectivity have been discussed in terms of the nature and number of interactions present in the four possible transition structures. The partition of the potential energy barrier has also been used to estimate the contributions of the pure deformation energy and the differential interaction between the reaction partners on passing from the ground state to the saddle point. This analysis reveals that the major influence arises from the heterodiene-dienophile interaction instead of that corresponding to a BF3-dienophile interaction.