NoteTaking_C4: Computational Organic Chemistry

CHAPTER 2: FUNDAMENTALS OF ORGANIC CHEMISTRY (1)

1. Bond Dissociation Enthalpy (BDE)

AB → A⋅ + B⋅
BDE=E(A⋅) + E(B⋅) - E(A-B)

HF significantly underestimated the BDE.
MP2 is better by taking into account some correlation energy.
B3LYP can work quite well for some types of bond cleavages, but fails completely for some molecules. B3LYP may be acceptable for small molecules but its error increases with the size of the molecule. Be cautious when using B3LYP!
The composite methods are the best and are always chosen as the benchmark method. But the computational  expense becomes unaffordable for large molecules.

Case 1: BDF of R-X with different α-substitution (J. Phys. Chem. A, 2005, 109, 7558)
BDE of the C-H bond and C-C decreases with increasing α-substitution. The opposite trend is observed for the C-F bond and C-OH bonds. And there is a turn over for C-OMe (first increases then decreases). B3LYP predicted the correct trend of decreasing C-H and C-C BDEs with increasing α-substitution, and increasing C-F BDEs with increasing α-substitution. But B3LYP fails to predict the trend of C-OH, and the turn over in C-OMe is too easy.

α-substitution increases the electron-donating capability of R.   When α-substitution increases, the R-X bond becomes more polar. R-X can be considered as a highbrid of three resonant structures: R:X ↔ R^- X⁺ ↔ R⁺X^-.  When X=F and OH, the ionic structure R⁺X^- becomes very important, and a more polared R-X bond will have high strength. When X=H or C, the stability of alkyl radical dominates the trend in BDEs.  When X=O, the two trends work in opposite directions.

Case2: BDE of Oximes

R₁R₂C=NOH → R₁R₂C=NO⋅+ H⋅

DFT and CBS-QB3 help to calirfy the controversy of the BDE of oximes.

2. Acidity 

AH → A^- + H⁺

DPE: DeProtonation Energy
The effect of diffuse functions is very important to get accruate DPE.
The role of diffuse functions is to preperly describe the tail behavior of orbitals, especially in electron rich systems where electron-electron repulsion leads to a more extensive distribution than usual.
MP2/6-31+G(d) is a reasonable accurate method.

B3LYP:

" For a set of 45 acids, the average absolute error for the B3LYP/6-31+G(d)-predicted DPEs is 4.7 kcal/mol. Improvement of the basis set leads to smaller errors. With the B3LYP/6-311+G(2d,p) method, even the DPE of acetic acid is predicted with an error less than 2.5 kcal/mol. For the set of 45 acids, the average error is only 2.1 kcal mol21 with the B3LYP/6-311++G(3df,2pd) level. "

B3PW91 is also a good functional.