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Direct Estimate of the Strength of Conjugation and Hyperconjugation by the Energy Decomposition Analysis Method
Author(s) -
Fernández Israel,
Frenking Gernot
Publication year - 2006
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200501405
Subject(s) - hyperconjugation , chemistry , conjugated system , moiety , acceptor , stereochemistry , decomposition , molecule , photochemistry , crystallography , computational chemistry , organic chemistry , physics , condensed matter physics , polymer
The intrinsic strength of π 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 H 2 CCH(HCCH) n CHCH 2 ( n =1–3) show that the strength of π conjugation for each CC moiety is higher than in trans ‐1,3‐butadiene. The absolute values for the conjugation between SiSi π bonds are around two‐thirds of the conjugation between CC bonds but the relative contributions of Δ E π to Δ E orb in the all‐silicon systems are higher than in the carbon compounds. The π conjugation between CC and CO or CNH bonds in H 2 CCHC(H)O and H 2 CCHC(H)NH is comparable to the strength of the conjugation between CC bonds. The π conjugation in H 2 CCHC(R)O decreases when R=Me, OH, and NH 2 while it increases when R=halogen. The hyperconjugation in ethane is around a quarter as strong as the π conjugation in ethyne. Very strong hyperconjugation is found in the central CC bonds in cubylcubane and tetrahedranyltetrahedrane. The hyperconjugation in substituted ethanes X 3 CCY 3 (X,Y=Me, SiH 3 , F, Cl) is stronger than in the parent compound particularly when X,Y=SiH 3 and Cl. The hyperconjugation in donor–acceptor‐substituted ethanes may be very strong; the largest Δ E π value was calculated for (SiH 3 ) 3 CCCl 3 in which the hyperconjugation is stronger than the conjugation in ethene. The breakdown of the hyperconjugation in X 3 CCY 3 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 π interactions between two CC double bonds, one CC double bond and CH 3 or CMe 3 substituents, and between two CH 3 or CMe 3 groups, which are separated by one CC single bond, are in a ratio of 4:2:1. Very strong hyperconjugation is found in HCCC(SiH 3 ) 3 and HCCCCl 3 . 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 MeC(R)O is half as strong as the conjugation in H 2 CCHC(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 σ interactions and electrostatic forces may compensate for the hyperconjugative effect.