My doubt, is dihedral values reduces conjugation in organic molecules?. if organic molecule is planar or twisted nature what is the dihedral angle of planar or twisted ? which one dihedral angle values highly affect the conjugation length. Please give me some answers, Definitions.
angle means the relationship with two adjacent bonds,
dihedral angle means the relationship with three adjacent bonds.
Hi. In a twisted molecule P orbitals are not parallel and this reduces the conjugation. Even two planner isomers with different dihedral angels (like cis stilbene and trans stilbene) have different conjugations.
The axes of the p orbitals should be parallel (φ = 0) for optimal conjugation Cmax, ; if the axes are at right angles (φ = 90) there is no conjugation. If there is some other angle the wave amplitude that is delocalised is Cmax.cosφ. The delocalisation energy would be proportional to cos^2(φ). However, that is a first order theoretical argument and there is a little more to it than that, as a sequence of double bonds conjugate better than a sequence of triple bond, even though the triple bonds offer double the overlap possibilities.
Steric hindrance deviates a ring double bond in a methyl-substituted cyclohexene ring out of an adjacent polyene chain reducing conjugation to ca. 20%, e.g. in carotenoids. To obtain Neff, the effective number of conjugated double bonds, a deconjugation factor of 0.8 must be subtracted from the nominal number of double bonds N, thus Neff = N – 0.8.
Braude, E. A.; Jones, E. R. H.; Koch, H. P.; Richardson, R. W.; Sondheimer, F.; Toogood, J. B., Studies in light absorption. Pat VI. Steric inhibition of resonance in natural and synthetic derivatives of cyclohexene. J. Chem. Soc. 1949, 1890-1897
Hirayama, K., Absorption spectra and chemical structures 1. Conjugated polyenes and p-polyphenyls. J. Am. Chem. Soc. 1955, 77 (2), 373-379
Hirayama, K., Absorption spectra and chemical structure 2. Solvent effect. J. Am. Chem. Soc. 1955, 77 (2), 379-381
Martinez, A., Zeeshan, M., Zaidi, A., Sliwka, H.R., Naqvi, K.R., Partali, V., On fininitenes – Reliable calculation of lamda ∞ and molecular modeling of lemniscate structured carotenoids. Comput. Theor. Chem. 2018, 1125, 133-141
dihedral values should reduce conjugation in organic molecules. I agree with the answer from Prof. Miller that the axes of the p orbitals should be parallel (φ = 0) for optimal conjugation Cmax, ; if the axes are at right angles (φ = 90) there is no conjugation.
If the axes of p-orbitals are parallel optimal conjugation is observed. Dihedral angle of 180o (parallel , φ = 0) has a s-trans configuration, causing the structure to be planar. Out of plane rotations of the conjugated end cycles results in a loss of conjugation. This shortens the conjugation length. If axes are at 90o (right angle, φ = 90) there is no conjugation. A dihedral angle of 0o has a s-cis conformation.
The reason for the loss of conjugation is that to get delocalization the wave must have a common axis. The cos term arises through considering components on the two axes. The reason why there must be a common axis is that if there were not, the wave axis has to turn a corner, and to turn a corner it has to refract. To refract, it has to change refractive index, i.e. velocity of the particle, but here is where matter waves differ from light waves - if the velocity of the particle changes, so does the energy and hence the frequency of the wave. And a specific wave can have only one frequency to be a single solution to the Schrödinger equation. Therefore, the wave has to resolve into components and only the parallel components can delocalize.
Strictly speaking, that explanation only really applies to my Guidance Wave interpretation of quantum mechanics because that interpretation alone requires a physical wave with a specific frequency assigned to the particle energy. However, the explanation explains what we see.
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