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The α‐bromocyclohexanone ring: The relationship between its conformation and the ultraviolet and infrared absorption of the keto group
Author(s) -
Laing Michael
Publication year - 1991
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.610040705
Subject(s) - chemistry , infrared , van der waals force , infrared spectroscopy , halogen , ring (chemistry) , crystallography , ultraviolet , absorption spectroscopy , analytical chemistry (journal) , molecule , optics , organic chemistry , physics , alkyl
A series of diterpenoids whose C‐ring was an α‐bromocyclohexanone showed a discontinuity in the relationship between the BrCCO torsion angle and the Br…O contact distance at about 50°, below which the Br…O distance remained effectively constant at about 3.0 Å. This effect is caused by the non‐compressibility of the van der Waals radii of the Br and O atoms. Analysis of the parameters for all compounds in the Cambridge Data File containing an α‐bromocyclohexanone ring confirmed this phenomenon. Once the Br…O distance approaches 3.0 Å, the BrCC and CCO angles are forced open to accommodate the Br…O compression strain as the torsion angle is further reduced. An approximate value of the Br…O distance, d in Å, can be estimated for any torsion angle, ø, by the empirical equation d 2 = R – S cos ϕ + T cos 2 ϕ where R = 11.73, S = 3.62 and T = 0.75. There is a linear relationship between the Br…O separation and both the infrared vibration frequency v and the ultraviolet absorption wavelength δ max of the CO group: the greater the distance, the lower is the frequency v and longer the wavelength δ max . Thus measurement of the ultraviolet and infrared spectra of a compound containing an α‐bromocyclohexanone system can yield information about the Br…O distance and hence the conformation of the six‐membered ring. There are also systematic trends in the effect on the spectroscopic properties of the keto group that accompany changing the halogen from fluorine through to iodine. When the halogen is axial, its electronegativity has a maximum influence on the CO vibration frequency by a ‘through‐bond’ inductive process. When the halogen is equatorial, it exerts a steric compression on the non‐bonded electrons in the 2p orbital of the oxygen atom. This enhances hybridization to the sp 2 state and this simultaneously strengthens the CO bond and enlarges the n → π* energy gap. This ‘through‐space’ influence is simply a process of steric compression, dependent on the size of the halogen atom. It accounts well for all of the systematic trends in both infrared and ultraviolet frequencies of the CO group when the halogen is equatorial and eclipses the CO bond.