Direct Estimate of the Conjugative and Hyperconjugative Stabilization in Diynes, Dienes, and Related Compounds

Bickelhaupt

Cossío

2009

Double group transfer (DGT) reactions, such as the bimolecular automerization of ethane plus ethene, are known to have high reaction barriers despite the fact that their cyclic transition states have a pronounced in-plane aromatic character, as indicated by NMR spectroscopic parameters. To arrive at a way of understanding this somewhat paradoxical and incompletely understood phenomenon of high-energy aromatic transition states, we have explored six archetypal DGT reactions using density functional theory (DFT) at the OLYP/TZ2P level. The main trends in reactivity are rationalized using the activation strain model of chemical reactivity. In this model, the shape of the reaction profile DeltaE(zeta) and the height of the overall reaction barrier DeltaE( not equal)=DeltaE(zeta=zeta(TS)) is interpreted in terms of the strain energy DeltaE(strain)(zeta) associated with deforming the reactants along the reaction coordinate zeta plus the interaction energy DeltaE(int)(zeta) between these deformed reactants: DeltaE(zeta)=DeltaE(strain)(zeta)+DeltaE(int)(zeta). We also use an alternative fragmentation and a valence bond model for analyzing the character of the transition states.

Section: Theoretical Methodsmentioning

confidence: 99%

Double Group Transfer Reactions: Role of Activation Strain and Aromaticity in Reaction Barriers

Bickelhaupt

Cossío

2009

“…[4] This is unfortunate since the original EDA provides precisely defined mathematical terms that make it possible to identify chemical concepts with numerical data in a consistent manner. [5] The central term of the EDA is the instantaneous interaction energy DE int between the fragments in the geometry of the species that is the subject of the analysis [Eq. (1)]:…”

Section: C···x]mentioning

confidence: 99%

Response to the Comment on “The Interplay between Steric and Electronic Effects in S_N2 Reactions”

Frenking

Uggerud

2010

Self Cite

We reply to the preceding Correspondence article in this issue by Zeist and Bickelhaupt, [1] in which the authors challenge the interpretation of the energy decomposition analysis (EDA) [2] that was applied by us in a recent study of the reaction course of S N 2 reactions.[3] Unlike previous work, in which the S N 2 reaction often is analyzed in terms of interactions between an attacking nucleophile and a substrate, we also considered the effect of the leaving group on the energy terms of the EDA along the reaction path. We think this is a particularly reasonable approach for an identity reaction such as X À + R 3 C À X, where in the transition structure [X···R 3 C···X] À the nucleophile and the leaving group are equivalent, and also since this is a heterolytic process. We found that the steric repulsion between the nucleophile and the substrate is overcompensated by the loss of steric repulsion between the substrate and the leaving group. This conclusion was reached by calculating the interactions between [X···X] 2À and CR 3 + along the least energy reaction path and by applying the EDA to the calculated energies.Zeist and Bickelhaupt used the same approach and obtained numerical results for the systems F À + R 3 CÀF (R= H, Me), which are essentially identical to our data. The curves of the EDA values that are shown in Figure 1 b in their Correspondence [1] support the conclusion that the steric repulsion, which is identified with the DE Pauli term of the EDA, decreases along the reaction path and that the DE Pauli value for the transition structure is smaller than for the reactants. The small hump in the curve for F À + Me 3 CÀ F, which was already observed by us in our previous study, [3] is interesting but not relevant for the present topic. Zeist and Bickelhaupt then introduce a new scenario where they consider a hypothetical system in which they suppress the geometric relaxation of the substrate when the nucleophile approaches. Not surprisingly, they observe a steep rise of the Pauli repulsion yielding a curve that exhibits an increase of the DE Pauli term along the reaction course (Figure 1 c in the Correspondence by Zeist and Bickelhaupt). The authors thus conclude that "increased steric congestion around the five-coordinate central atom causes the barrier in the S N 2 reaction".We do not consider this statement of Zeist and Bickelhaupt to be valid. The use of a hypothetical system where the geometry of the substrate is frozen implies by necessity that the Pauli repulsion term increases as the nucleophile approaches. This procedure is also questionable from a formal point of view since only electronic and not nuclear degrees of freedom are optimized. The behavior of this artificially constrained system is exactly opposite of what was found in the system for which the positions of the nuclei are allowed to adjust. In addition, the conclusion of Zeist and Bickelhaupt is based on a different interpretation of the EDA because energy terms in the EDA, which are not associated with steric repulsion, are ar...

“…Most of our previous work was carried out at the BP86/ TZ2P level of theory and may therefore be used to estimate the performance of the level of theory. [9][10][11][12] An auxiliary set of s, p, d, f, and g STOs was used to fit the molecular densities and to represent the Coulomb and exchange potentials accurately in each SCF cycle. [16] All structures were verified as minima on the potential-energy surface by calculating the Hessian matrices.…”

Section: Computational Detailsmentioning

confidence: 99%

“…Further details about EDA can be found in the literature. [8] In recent communications we reported on the energy decomposition analysis of the p interactions in 1,3-butadiene, 1,3-butadiyne, and related systems [11] and we compared the calculated strength of the p conjugation in meta-and parasubstituted benzylic cations and anions by using Hammett s parameters. [12] Herein we report on a systematic study of the strength of the p conjugation and hyperconjugation in a variety of acyclic molecules.…”

Section: Introductionmentioning

confidence: 99%

Direct Estimate of the Strength of Conjugation and Hyperconjugation by the Energy Decomposition Analysis Method

Frenking

2006

Self Cite

119

The intrinsic strength of pi interactions in conjugated and hyperconjugated molecules has been calculated using density functional theory by energy decomposition analysis (EDA) of the interaction energy between the conjugating fragments. The results of the EDA of the trans-polyenes H2C=CH-(HC=CH)n-CH=CH2 (n = 1-3) show that the strength of pi conjugation for each C=C moiety is higher than in trans-1,3-butadiene. The absolute values for the conjugation between Si=Si pi bonds are around two-thirds of the conjugation between C=C bonds but the relative contributions of DeltaE pi to DeltaE orb in the all-silicon systems are higher than in the carbon compounds. The pi conjugation between C=C and C=O or C=NH bonds in H2C=CH--C(H)=O and H2C=CH-C(H)=NH is comparable to the strength of the conjugation between C=C bonds. The pi conjugation in H2C=CH-C(R)=O decreases when R = Me, OH, and NH2 while it increases when R = halogen. The hyperconjugation in ethane is around a quarter as strong as the pi conjugation in ethyne. Very strong hyperconjugation is found in the central C-C bonds in cubylcubane and tetrahedranyltetrahedrane. The hyperconjugation in substituted ethanes X3C-CY3 (X,Y = Me, SiH3, F, Cl) is stronger than in the parent compound particularly when X,Y = SiH3 and Cl. The hyperconjugation in donor-acceptor-substituted ethanes may be very strong; the largest DeltaE pi value was calculated for (SiH3)3C-CCl3 in which the hyperconjugation is stronger than the conjugation in ethene. The breakdown of the hyperconjugation in X3C-CY3 shows that donation of the donor-substituted moiety to the acceptor group is as expected the most important contribution but the reverse interaction is not negligible. The relative strengths of the pi interactions between two C=C double bonds, one C=C double bond and CH3 or CMe3 substituents, and between two CH3 or CMe3 groups, which are separated by one C-C single bond, are in a ratio of 4:2:1. Very strong hyperconjugation is found in HC[triple bond]C-C(SiH3)3 and HC[triple bond]C-CCl3. The extra stabilization of alkenes and alkynes with central multiple bonds over their terminal isomers coming from hyperconjugation is bigger than the total energy difference between the isomeric species. The hyperconjugation in Me-C(R)=O is half as strong as the conjugation in H2C=CH-C(R)=O and shows the same trend for different substituents R. Bond energies and lengths should not be used as indicators of the strength of hyperconjugation because the effect of sigma interactions and electrostatic forces may compensate for the hyperconjugative effect.