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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Robert G. Bergmana), K. Peter C. Vollhardt3)4) and Arthur J. Ashe 1115) (6. X. 75) Summavy. The He1 photoelectron spectra of mono-, di-, and triacetylenes are presented. In these compounds the two-centre n-orbitals of the ethynyl groups conjugate with the n-orbitals of double bonds or benzene moieties, or with the Walsh orbitals of three-membered ring systems. Assuming the validity of Koopmans' approximation, the observed encrgies of the radical cation states reached by electron ejection from n-orbitals can be rationalized in terms of a simple LCBO-MO model in those cases, where the molecule is planar. The corresponding numerical results for the ionization encrgies are in excellent agreement with experiment, if the three parameters of the model are properly calibrated. In contrast, the bands assigned to ejection from in plane n-orbitals are shifted to lower energies by ca. 0.5 eV with respect to the expectation values derived from the above model, due to 'through-bond' interaction with lower lying a-orbitals.Extensive o/n mixing occurs in the non planar compounds for all orbitals. The assignments of the spectra of diethynylmethane, 1,4-hexadiyne, 1,2-diethynylethane and of cis-and tvansdiethynylcyclopropane are backed by semiempirical SCF calculations. The spectra of the cis and trans isomers of diethynylethyleneoxide and diethynylethylenesulfide are discussed by comparison with the corresponding hydrocarbons and with oxirane and thiirane respectively.Finally, the following topics are considered in detail: (a) The effect of spin orbit coupling on the spectrum of 1-iodo-1-butyne-3-ene; (b) the effect of the essentially free internal rotation in divinylacetylene on the band shapes of its photoelectron spectrum and (c) the relationship between the conjugative properties of eth ylenic n-orbitals and of the Walsh-orbitals of cyclopropane. To gain further understanding about the nature of these interactions and the ground state electronic structure of some acetylenic substrates an investigation of the photoelectron spectra of a series of o/n-conj ugated acetylenes was undertaken.

Die PE-Spektren der Benzocycloalkcnc 3 (n) mit n = 3 -8 wurden aufgenommen. Will man den EinfluR der Alkyleinheit --[CHz],, --2 auf die Orbitalcnergien E (bl ( T ) ) und E (a2(x)) mittels eines Orbitalmodclls erklaren, wclches im Rahmen der Huckelschen Niiherung nur induktive und hyperkorijugative Parameter zulaRt, so findet man, daR dcr Methylengruppe CH2 in 3 (3) ein negativer, der Athylengruppe .-[CH:], in 3 (4) eio verschwindender indukliver Effekt zuzuordnen ist. Dieses Ergebnis verlangt, daR die Destabilisierung des ohersten besetzten ?r-Orbitals bl (rr) in 3 (3) und 3 (4) auf hyperkonjugative Wechselwirkungen zuruckgefuhrt wird. Das oberste bcsetzte a-Orbital der Kohlcnwasserstoffe 3 (11) ist fur n = 3 und 4 vor allem in den CC-Bindungcn, fur n 2 5 in den CH-Bindungen der Cycloalken-Einheit lokalisicrt.The PE. spectra of benzocycloalkcnes 3 (n) with n = 3 to 8 have been recorded. It is shown that an attempt to interpret the influence of the alkyl moiety ~ [CH2Il1_ on the x-orbital energies c (bl (x)) and E (ar(x)) by the simple inductive/hyperconjugative Hiickel molecular orbital model of alkyl groups demands that the methylene group CH2 in 3 (3) and the ethylene group ~ [CH2I1-in 3 (4) be assigned negative and zero inductive effects, respectively. This in turn leads to the interpretation that the destabilization of the highest occupied x-orbitals bI(?r) in 3 (3) and 3 (4) is due to hyperconjugation. The highest occupied a-orbitals of the hydrocarbons 3 (n) are localized mainly in the CC-bonds of the cycloalkene part if n =.-3 or 4 and in the CH-bonds if n 2 5. 1) 46. Mitteil. der Relhe Anwendungen der Photoelektronen-Spektroskopie; 45. Mittcil.: M . J. Goldstein, E. Heilbronner, V. Hornutzg und S. Notowshy, Helv. Chim. Acta, in1 Ilruck.

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