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

Abstract

Szwnmary.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 R X (symmetry C,) the lone pair band is split into two components, separated by a gap A . 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 bctween R and X. (c) Notwithstanding (b), the gap A 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 a t higher potential sharpens up.Consider a halide molecule RX (X = halogen atom) in which the bond R-X coincides with an n-fold axis of the system (n 3 3). Photoejection RX + hv ---+ RX++ e of an electron e from a n-type "lone pair" orbital of X yields the radical cation RX+ in a doublet spin state. Because of spin-orbit coupling this term is split into two levels, 2173fz and 21Tlfz, which differ in energy by 1)Part 25 of 'Applications of Photoelectron Spectroscopy'. Part 24: [l].

An experimental and theoretical study of the valence shell photoelectron spectrum of bromochlorofluoromethane

“…The atomic spin-orbit coupling parameters are 0.073 and 0.305 eV for Cl and Br, respectively [57], thus it might be anticipated that the splitting between the essentially Br lone-pair orbitals in CHFClBr would be greater than that between the essentially Cl lone-pair orbitals, as is commonly observed in halogen containing molecules [20,21,23,24,26].…”

The He(I) photoelectron spectra of the propenoyl halides

“…It has been demonstrated that this magnitude of splitting also occurs in alkyl bromides of C, symmetry and is insensitive to the degree of conjugation of n;, with other a" orbitals (19). This behaviour has been explained with the use of a simple orbital model which showed that two opposing trends, increased splitting with increased conjugation of n;, and increased spin-orbit splitting with increased localization on the halogen atom, compensated each other (20). A similar situation is believed to hold in the case of propenoyl bromide.…”

Oberflächen-Reaktionen, 9 Dehydrohalogenierungen von Halogenkohlenwasserstoffen an Raney-Nickel / Surface Reactions, 9 Dehydrohalogenation of Halohydrocarbons at Raney Nickel

“…9: 10,1 eV; vgl. [27]) nicht überlappt. Eine Optim ie rung der Reaktionsbedingungen mit Hilfe PE-spektroskopischer Echtzeit-Gasanalytik [12] könnte zu sätzlich anhand der Spin/Bahn-gekoppelten HBr-Ionisierungsnadeln bei 11,70 eV und 12,03 eV [16] Die zum Vergleich zusätzlich untersuchte Thermolyse von Bromcyclobutan im leeren Q uarzrohr bei IO-2 mbar zeigt bei etwa 770 K Ringöffnung zu 4-Brombuten-1 und oberhalb 800 K beginnende H Br-A bspaltung zu Butadien (vgl.…”