The 'observed' n-orbital energies cv,j = -Iv, j derived from the vertical ionization potentials obtained by a photoelectron spectroscopic investigation of the acenes benzene A ( l ) , naphthalene A(2), anthracene A(3), naphthacene A(4) and pentacene A(5) have been compared with n-orbital energies calculated by three different approximations : (a) the standard Huckel HMO model; (b) a first-order perturbation treatment, based on (a), that takes into account bond length changes which follow the ionization process; (c) a SCF n-electron model of the type proposcd by Pople and by Pariser & Purr. I n agreement with previous experience i t is found that model (b) yields the most satisfactory parametrization of the experimental data. l) 2) Part 37 of 'Applications of Photoelectron Spectroscopy'. Part 36: [l]Permanent Address :

Summary. The bands with I, < 13 eV in the photoclcctron spsctra of quinoline (IX), isoquinoline (X), cinnoline (XI), quinazoline (XII), and quinoxaline (XIII) h a w been reassigned in a way consistent with the assignment proposed for pyridine (11). the diazines (

The PE.‐spectra of triacetylene 1 (3), methyldiacetylene 2 (2), dimethylacetylene 3 (1) and dimethyldiacetylene 3 (2) have been recorded. The π‐ionization potentials of the unsubstituted acetylenes can be parametrized in terms of a LCBO‐model.

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].

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