Estimation of the conformation of 2-hydroxy-4,6-dimethylbenzophenone and 2,4,6-trimethoxybenzophenone from electronic absorption spectra and PPP LCI ca

The twisting effect of benzene rings on the electronic spectra of derivatives of benzophenone (BPh), 2-hydroxy-4,6-dimethylbenzophenone (2-OH-4,6-diCH₃BPh) and 2,4,6-trimethoxybenzophenone (2,4,6-triOCH₃BPh) has been interpreted by the PPP method. The effect of structure, protonation, ionization and interaction with proton-acceptor solvent on the twisting of aromatic rings is discussed. The following twist angles of substituted ring (θ_I) and unsubstituted ring (θ_II) in 2-OH-4,6-diCH₃BPh were found: neutral form in cyclohexane θ_I = 39°–48°, θ_II = ?30°–(?45°); neutral form in ethanol θ_I = 66°–72°, θ_II = ?30°–0°; protonized form θ_I = 64°–6°, θ_II = ?30°–0°; ionized form θ_I = 66°–74°, θ_II = ?30°–0°. In the neutral form of 2,4,6-triOCH₃PBh the substituted ring is twisted by angle θ_I ~ 70° while in the protonated form the unsubstituted ring is twisted by angle θ_II ~ 60° and the substituted one is coplanar with the CO group.