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1. Hyperconjugation effect (Gauche effect)
* Paul’s comment on the alkane torsion parameters
He found dramatically different torsion parameter signs for F-C-C-F and O-C-C-O;
Test calculation showed that F-C-C-F and O-C-C-O didn’t result in the gauche (60 degree) conformation as lowest energy.
>> with a negative k value of three-fold component, which seems counter-intuitive, with all the other factors, for example 1-4 non-bonded interactions and other torsion energy term, it still reproduce QM torsion profiles and gauche conformation.
(1) Parameters before and after the optimization
>> O-C-C-O
t5 | [#8X2:1]-[#6X4:2]-[#6X4:3]-[#8X2:4] | |
initial | 0.144*(1+cos(3* theta)) + 1.175*(1+cos(2*theta)) | |
v1.1.0 | -0.3014444642399*(1+cos(3* theta)) + 1.12459107565*(1+cos(2*theta)) | |
v1.2.0 | -0.04029961117620*(1+cos(3*theta)) + 0.5796570519069*(1+cos(2*theta)) | |
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positive k vs negative k of three-fold torsion component:
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(2) MM vs QM torsion profiles
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(3) QM vs. MM optimized geometry
FDA set
hexafluoroethane