The Competition between Spin Orbit Coupling and Conjugation in Alkyl Halides and its Repercussion on their Photoelectron Spectra

A crude molecular orbital model for alkyl halides is proposed, which provides a semi-quantitative rationalization for the following experimental observations: (a) In the photoelectron spectra of alkyl halides RX (symmetry C₈) the lone pair band is split into two components, separated by a gap Δ. This gap is equal to the splitting associated with spin-orbit coupling in systems where X lies on a symmetry axis of order n ≥ 3. (b) The vibrational pattern of the two components indicates substantial conjugation between R and X. (c) Notwithstanding (b), the gap Δ is largely independent of the type of alkyl group R. (d) For strongly conjugating alkyl groups (e. g. R = cyclopropyl) the first component of the lone pair band (i. e. the one at lower ionization potential) broadens while the one at higher potential sharpens up.