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The HeI photoelectron spectra of mono-, di-, and triacetylenes are presented. In these compounds the two-centre π-orbitals of the ethynyl groups conjugate with the π-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 energies of the radical cation states reached by electron ejection from π-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 energies 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 π-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 σ-orbitals. Extensive σ/π 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 trans-diethynylcyclopropane 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 ethylenic π-orbitals and of the Walsh-orbitals of cyclopropane.

The scheme of ‘through-space’ and ‘through-bond’ interaction of (semi)localized orbitals, originally proposed by Hoffmann, is reexamined in terms of SCF many-electron treatments. It is shown that the two types of interaction can be characterized by examining the corresponding off-diagonal matrix elements of the Hartree-Fock matrices of the localized or the symmetry adapted localized orbitals and of the partially diagonalized Hartree-Fock matrices referring to ‘precanonical orbitals’. The procedure outlined is applied to three practical examples using the semiempirical many-electron treatments SPINDO, MINDO/2 and CNDO/2: a A reassessment of ‘through-space’ and ‘through-bond’ interaction in norbornadiene indicates, that the latter type of interaction is also of importance for the orbital based mainly on the antisymmetric combination of the localized x-orbitals. The differences in the predictions derived from the three models are critically examined. b The competition between ‘through-space’ and ‘through-bond’ interaction in the series of bicyclic dienes from norbornadiene to bicyclo[4.2.2]-dcca-7,9-diene and in cyclohexa-1,4-diene, i. e. their dependence on the dihedral angle UI is reexamined. It is found that the rationalization for the orbital crossing near ω = 130° deduccd from PE. spectroscopic data can not be as simple as originally suggested and that the relay’ orbitals responsible for ‘through-bond interaction affecting both the symmetric and the antisymmetric combination of the π-orbitals extend over the whole CC-σ-system of the six membered ring. c ‘Through-bond’ interaction of the two lone pair orbitals in 1,4-diazabicyclo[2.2.2]octane is found to be large for their symmetric and the antisymmetric linear combination. The analysis quoted, draws attention to some of the dangers involved in using semiempirical treatments for the interpretation of PE. data in terms of Koopmans′ theorem, without due caution.

The vertical ‘lone-pair’ ionization potentials Iv(n) of fifteen 4-substituted quinuclidines, of aza-adamantane and of aza-twistane have been determined by photoelectron spectroscopy. It is shown that the dependence of Iv(n) on the nature of the substituent S can be rationalized in terms of the contribution to the local potential at the ionization site (i.e. the nitrogen centre) by the dipole of S. Both the solution pK's and Iv(n) correlate with classical substituent parameters (e. g. σ* or F) and with each other. However, no conclusions can be drawn from such regressions concerning a close relationship between the electronic mechanisms responsible for the observed changes in pK or Iv(n).

The ionization energies JJ, of 1,6;8,13-alkanediylidene-[14]annulenes (2 to 5) and of dicyclohepta[cd,gh]pentalene (1) have been determined by photoelectron spectroscopy, using HeI radiation. The data are interpreted in terms of Koopmans' theorem (JJ = −eJ) on the basis of correlation diagrams and with the help of simple molecular orbital models. If the bridge is an ethane-, propane- or butane-diylidene group, the π-orbital sequence, in descending order of orbital energies, is (in C2v): b1, b2, a2, a1. The sequence is due to a complicated and not uniquely definable interplay of inductive, conjugative and homoconjugative effects. A detailed analysis of these effects suggests that the effective angle of twist between two consecutive basis-AOs 2pμ, 2pν of the peripheral π-system should be smaller than the twist angles θμν determined by X-ray analysis, i.e. that the pi;-ribbon adjusts elastically and is no longer locally orthogonal to the σ-frame. In the non-alternant hydrocarbon 1 of symmetry D2h, the sequence is 2b2g, 3b1u, 2b3g, 1au, 2b1u. The sequence 3b1u above 2b3g, i.e. the reverse of b2 above a1 in the bridged [14]annulenes, is explained as being due to the interaction of the semilocalized perimeter orbitals b1u and b3g with the bonding (π(B1u))and antibonding (π*(B3g)) orbital of the central double bond. In 2 the replacement of the two latter orbitals by the Walsh-orbitals of the cyclopropane moiety leads to the sequence b1, b2, a1, a2. From the data observed for 1 to 5 and for 1,6-methano-[10]annulene [11], a crude estimate for the orbital energies of the hypothetical all-cis D10h-[10]- and D14h-[14]annulenes can be derived.

The photoelectron spectra of 1,5-cyclooctadiyne (2) and of 1,6-dithiacyclodeca-3,8-diyne (3) have been recorded. The first four (2) or six (3) PE. bands have been assigned as follows; in increasing order of ionization potentials: The relative sequence and the positions of the PE. bands are explained in terms of through-bond and through-space interactions between the basis π-orbitals and σ-orbitals of appropriate symmetry behaviour. An analysis of the PE. spectroscopic data for cyclooctyne (1) and for (2) indicates that a cis-bend of the acetylene moiety by θ < 20° leads to a split in energy of the in-plane and out-of-plane basis π-orbitals which is smaller than ∽ 0.2 eV. This is in agreement with the predictions derived from semiempirical models (MINDO/2, SPINDO) and qualitative orbital arguments. However, it is shown by using orbital localization procedures, that the rationales underlying the two semiempirical models differ significantly.

The He(I) photoelectron spectra of seventeen tropones are presented. With the help of correlation diagrams, implemented by simple molecular orbital models, a self-consistent interpretation of the observed data can be derived. 1. (a) The orbital sequence in tropone (1) proposed by Bunzli, Frost and Weiler (J. Amer. Chem. Soc., 96 (1974) 1952) and by Muller, Schweig and Vermeer (Angew. Chem., 86 (1974) 275) is confirmed (assuming Koopmans' approximation). 2. (b) The heuristically useful scheme proposed by the latter authors for the separation of the “inductive” and “conjugative” effects of the carbonyl group is reexamined. It is seen that the quantitative parameters derived through it depend critically on the necessary ad hoc assumptions that have to be made for the hypothetical reference system. 3. (c) In the 2,7-polymethylene-4,5-benzotropones (8(n)), the change in the oxygen lone-pair orbital energy with decreasing size n of the alkyl moiety confirms previous deductions concerning the change in electronic structure with increasing out-of-plane deformation of the tropone system. The drift in π-orbital energies, as a function of n, points to an important homoconjugative through-space interaction between the localized two-centre π-orbitals πa and πc in 8(n) with n ⩽ 9. 4. (d) Both LCBO (Linear Combination of Bond Orbitals) and HMO-type models (including first order perturbations for bond alternation) yield equally respectable parametrizations for the tropones investigated. From this we conclude that photoelectron spectroscopy does not yield information concerning the “aromatic character” (?) of tropones.

Photoelectron spectra of butatriene (I), tetradeuterobutatriene (II), tetramethylbutatriene (III) and tetra- t -butyl- butatriene (IV) are reported and discussed. The four main bands of the spectra of I and II are assigned to ionization from the 1b 3g , 2b 3u , 1b 2u and 1b 2g molecular orbitals on the basis of molecular orbital calculations (ab initio with STO-3G and 4-31G bases and semi-empirical SPINDO) all of which lead to the electron configuration… (1b 2g ) 2 (1b 2u ) 2 (2b 3u ) 2 (1b 3g ) 2 . The first three bands of III and the first two of IV correspond to 2b 3g , 5b 3u , 2b 2u and 7b 3g , 12b 3u molecular orbitals respectively. The spectra of I and II also show an additional band which cannot be assigned to direct ionization of an occupied molecular orbital in the ground state configuration. it is suggested that this may arise from a cation state… (1b 2g ) 2 (1b 2u ) 2 (2b 3u ) 2 (2b 2u ) 2 B 2u . Such a state is correlated with the 2 B 3g cationic ground state by internal rotation but corresponds to a local maximum in the rotational potential. Some simple model calculations are presented of Franck-Condon factors for the internal motion corresponding to these states. These show a composite band of the general type observed. However, the observed vibrational spacings indicate that significant carbon skeletal as well as torsional changes take place on ionization.

From the known model for the dispersive interaction of a non-polar solute with a non-polar solvent the following predictions can be derived for the behaviour of the electronic spectrum of a polyene dissolved in binary mixed solvents: (a) With increasing concentration of the more strongly interacting solvent component the 1 A → 1 B o transition is shifted toward longer wavelengths (b) This bathochromic shift is accompanied by a decrease of the oscillator strength f M of the 1 A → 1 B o transition, (c) Both Δ v and the relative oscillator strength f ′ M / f M , are linear functions of the more concentration of the more strongly interacting solvent component expressed in volume per cent. (d) Both Δ v and f ′ M / f M , are in a first approximation independent of the number of conjugated double bonds of the polyene. Thesame predictions can be derived from a theoretical model due to Liptay which includes second order effects. These predictions have been verified using as model systems solutions of carotenoids (with 5 to 15 conjugated double bonds) in mixtures of hexane/carbon disulfide or cyclohexane/carbon disulfide.

The ‘observed’ π-orbital energies ϵv,j = −Iv,j derived from the vertical ionization potentials obtained by a photoelectron spectroscopic investigation of the acenes benzene A(1), naphthalene A(2), anthracene A(3), naphthacene A(4) and pentacene A(5) have been compared with π-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 π-electron model of the type proposed by Pople and by Pariser & Parr. In agreement with previous experience it is found that model (b) yields the most satisfactory parametrization of the experimental data.

The enthalpies of formation of 2,7-dimethyl-4,5-benzotropone (I), 2,7 - pentamethylene- 4,5 - benzotropone (III) and 2,7 - dodeca-methylene- 4,5-benzotropone (II) have been determined by combustion calorimetric methods. The loss of 36 kcal/moles in delocalisation energy for III relative to I and II is due to the steric inhibition of coplanarity of the 4,5-benzo-tropone-system in III.

The photoelectron spectra of the six propellanes 1 to 6 have been recorded. Those bands which correspond to ejection of an electron from a π-orbital have been assigned, using the concept of through bond and through space interaction. Homoconjugative interaction between the π-systems of the two butadiene moieties of 6 is of the same order as that of the π-orbitals in norbornadiene.

The photoelectron spectra of cis-cycloalkenes (with three to ten carbon atoms in the ring), of three isomeric, non-conjugated cyclodecadienes, and of cis, cis, -1, 3-cycloalkadienes (with five to eight carbon atoms in the ring) have been recorded. The essential features of their π-bands and of the first σ-bands are briefly discussed.

The preparation and the spectroscopic properties of 2,7-polymethylene-4, 5-benzotropones are described. The trends in these properties with decreasing number of methylene groups in the polymethylene ring are due to the steric inhibition of the benzotropylium resonance.

Photoelectron spectra of endo- and exo-cyclopropano-norbornene ( = endo- and exo-tricyclo[3.2.1.02.4]octa-6-ene) show that a significant homoconjugation exists between the π-orbital of the double bond and the symmetric Walsh-es-orbital of the cyclopropane ring in the exo-isomer, whereas the interaction is negligeable in the endo-derivative.

The photoelectron spectra of benzo[b]thiophene (2), benzo[c]thiophene (3), thieno[3,2-b] thiophene (4) and thieno[2,3-b]thiophene (5) together with 2,1,3-benzothiadiazol (6) and benzofurazan (7) have been recorded and the first three bands have been assigned to π-orbitals on the basis of band shapes and semiempirical calculations. The good agreement obtained between measured and calculated ionization potentials suggests that sulfur 3d orbital participation must be very small.

The semi-empirical configuration interaction treatment due to Pariser and Parr and to Pople has been modified by reducing the set of electronic interaction integrals to γ′11 and γ′12 and by limiting the number of configurations to 10. Huckel MOs are used as basis functions. The dependence of the resonance integral β′μv on the interatomic distance Rμv is simulated by making β′μv a quadratic function of the Huckel bond orders Pμv.

The competitive through-space and through-bond interaction of two symmetry-equivalent, semi-localized orbitals φa, φb, (= π or lone-pair orbitals) in a molecule M of Cs or higher symmetry can lead either to the natural order of the corresponding delocalized orbitals, i.e., ψ− ≈ (φa - φb)/√2 above ψ+ ≈ (φa - φb)/√2 or to the inverted order, i.e., ψ+ above ψ−. It is shown that one can discriminate between these two situations through an analysis of the PE spectrum of a molecule M′ which differs from M only by an additional semi-localized orbital φc of known symmetry behaviour. The method is exemplified by an investigation of the dependence of the n-orbital sequence in 3,6-bridged 1,4-cyclohexadienes on the dihedral angle between the two CCHCHC moieties.

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.

A comparative photoelectron spectroscopical investigation of the title compound (11) and its 3,4- and 7,8-dihydro derivatives (9 and 10) indicates that a considerable ‘through bond’ interaction exists between the π-orbitals in 11. The PE. spectra of the 3,4-diaza-analogue of 10 and 11, which contain a cis azo group in a four-membered ring, yield a splitting Δn (4-memb. ring) = 1.55–1.60 eV between the nitrogen lone-pair orbital energies. This value contrasts with those obtained for a three-membered ring analogue (3,3-dimethyldiazirine (5), Δn (3-memb. ring) = 3.55 eV) and for a five-membered ring analogue (2,3-diazanorbornene (7) Δn(5-memb. ring) = 3.10 eV). The sequence Δn (3-memb. ring) > Δn (4-memb. ring) Δn (5-memb. ring) is satisfyingly reproduced by MINDO/2- and EHT-calculations for the model systems with n = 3,4,5. A similar trend can be deduced from MINDO/2-calculations for cis-diimid where Δn becomes minimal for a NNH angle φ ≈ 100°, whereas Δn for the corresponding trans-structure goes through a maximum in this region. The experimental finding as well as the calculated results confirm the predictions made by Gimarc [15] who attributes the behaviour of Δn for cis-azo groups to a ‘through-bond’ interaction of the n+-orbital with a lowerlying NN σ-orbital; this interaction becomes maximal for NNR angles of the size present in a fourmembered ring, e.g. in 12 or 13.

The photoelectron spectra of bicyclo[3.2.2]nonane and its five dehydrogenated analogues are reported. Analysis of the π-bands reveals that the unsymmetrical diene 5 retains the ‘natural order’ of the π-orbitals present in the lower homologues, i. e. the in-phase below the out-of-phase combination of the basis π-orbitals πa and πc. The isomeric diene 4 is the first member of the symmetrical homologous series (I), which inverts this order, so that a1(π) (in phase!) lies above b2(π) (out-of-phase!).

Die Photoelektronen-Spektren des 2-Diazopropans (3) und des Diazocyclopentadiens (4) zeigen, das das oberste besetzte Molekulorbital in 3 und somit im Diazomethan (1) ein nicht-bindendes b2(π)-Orbital ist. The Orbital Sequence in Aliphatic Diazo Compounds The photoelectron spectra of 2-diazopropane (3) and diazocyclopentadiene (4) show that the highest occupied molecular orbital in 3 and thus in diazomethane (1) is the non-bonding orbital b2(π).

The photoelectron spectra of NSCl and NSF3 have been recorded. By comparison with the PE.-bands of NSF and semiempirical calculations the first bands of NSCl and NSF3 are assigned.

The ionization potentials of methyl-, t-butyl- and trimethylsilyl-substituted pyridines have been determined by high-resolution photoelectron spectroscopy. The results are discussed in terms of destabilizing inductive and stabilizing conjugative effects of the substituents, and have been parametrized on the basis of a HMO-type perturbation treatment. Our data are in accord with the idea that the topmost occupied orbital in pyridine is a ‘lone pair’ orbital.

It is shown that for H2+ and H2 an atomic orbital exponent ζ chosen from the condition\(\overline {\Delta T}\) = - †Eobs. gives calculated total energies which are insignificantly different from those obtained using a ζ obtained through a variational treatment. A scheme is proposed in which the atomic orbital exponents of π-electron molecular orbitals are taken to be a function of the orbital energies, such that these molecular orbitals qualitatively satisfy the virial theorem.

It is shown that the correlation of the π-ionization potentials of ethylene (1), butadiene (2) and trans-1,3,5-hexatriene (4) favours the orbital sequence π, π, σ in butadiene and π, π, σ, π in the hexatriene in excellent agreement with the results of SPINDO calculations. Within the experimental error the π-ionization potentials of cis-1,3,5-hexatriene (3) and trans-1,3,5-hexatriene (4) are the same. Methyl-substitution of 2 lowers the π-ionization potentials I1(π) and I2(π). For 1 and/or 4 substitution the difference I2(π)−I1(π) remains constant (≈ 2.5 eV). On the other hand 2 and/or 3 substitution leads to a smaller gap of I2(π)–I1(π) ≈ 1.6 to 2.0 eV without changing the mean π-ionization potential I(π) relative to the corresponding 1,4 substituted derivatives. This result is rationalized in terms of a through space interaction between double bond π-orbitals and non-bonded pseudo-π-orbitals of the substituting methyl groups. The reduced split I2(π)–I1(π) in cyclopentadiene is attributed to hyperconjugation across the methylene group.

In many textbooks attention is drawn to the close analogy that seems to exist between the “Electron-in-a-Box”-wave functions Ψn and their LCAO-MO counterparts ψJ (J = n) for the movement of an electron in a π-system. It is often implied that the “wave lengths” Λ of Ψn and λ of ψJ (J = n) which satisfy to a high degree the relation Λ=λ, have the same physical meaning. It is shown that this is not the case. Λ for a linear system (e.g. a one-dimensional “Electron-in-a-Box”-model) is directly connected with the momentum of the electron and therefore with its kinetic energy according to the deBroglie relation. However, there is no such simple relationship between A and the corresponding kinetic energy component in LCAO-MO's ψJ. (The necessary two-center kinetic energy integrals have been computed for 1s-type atomic orbitals.)

A method outlined previously [1] is used to show that in norbornadiene (3) the b2(π) orbital lies above a1(π), as predicted by theory. This indicates that in 3 through-space interaction between the two basis π-orbitals πa and πb is more important than through-bond interaction. Analysis of the PE.-spectra of 8-isopropylidene-tricyclo[3.2.1.02,4]-octane (13) and the corresponding octene (15) confirms that the π-orbital πc of the exocyclic double bond conjugates more strongly with the symmetric Walsh-orbital es of the cyclopropano moiety than with the π-orbital πa of a double bond in the same position.

In order to understand the origin of the surprising bathochromic shift produced by phenyl groups in bis peri -tetraphenylnaphthalene both experimental and theoretical methods were used. The geometry of the molecule was determined by means of X-ray diffraction. Absorption, fluorescence and phosphorescence spectra were also analyzed. The epr signal of the triplet state was studied in order to obtain an indication of the distribution of the spin density for that state. Finally electronic wave functions were constructed. It is shown that both electron delocalization and energy delocalization play important roles in the spectroscopic behaviour of bis peri -tetraphenylnaphthalene.

The experimental and theoretical basis of a recently proposed reassignment of the bands in the PE. spectra of pyridine, pyridazine, pyrimidine and pyrazine is discussed in detail. A characteristic feature of the derived orbital sequence is that it takes the ‘through-space’ and ‘through-bond’ interaction between the ‘lone pair’ basis orbitals explicitly into account. A simple parametrization of the orbital energies, based on HMO-type models for the π-orbitals and for the ‘lone pair’ linear combinations, yields excellent agreement with the observed band positions in the PE. spectra of s-triazine and s-tetrazine. Our new assignment is compared to those proposed previously.

Acenaphthylene (I) and fluoranthene (II) in their electronic ground state are often regarded as consisting of two weakly interacting subsystems (naphthylene/ethylene or naphthalene/ benzene), which conserve much of their individuality. This seems to suggest that the lower electronically excited states of I and II can also be linked in a simple fashion to the low energy L b , L a , B b , and B a states of naphthalene and of benzene in II and of naphthalene in I, according to either Simpson's “Independent System Approach” or the“ Molecules in Molecules” scheme proposed by Longuet-Higgins and Murrell. Contrary to expectation this approach fails even for the lowest excited states of I and II, because of the extensive mixing of high energy locally excited and charge transfer configurations. These involve molecular orbitals of the subsystems naphthalene/ethylene or naphthalene/benzene, which do not contribute to their low energy L b , L a , B b or B a states.

The preparation of pure 4,5-benzo-azulene is described and some of its spectroscopic properties are compared with those of its isomers 1,2-benzo-azulene and 5,6-benzo-azulene as well as with those of the parent compound azulene.

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.

The photoelectron-spectra of the six dihaloacetylenes XCCY with X, Y Cl, Br, I have been recorded. The first five bands of these spectra can be assigned (in order of increasing ionization potentials) to the states 2∏(u)(1), 2∏(g)(2), 2∏(u)(3), 2∑(g)+(4), 2∑(u)+(5) where the lower indices g, u are valid only for the centro-symmetric molecules I(X, X). The first three states correspond to π-ionization, the last two to σ-ionization of the axially symmetrical lone pairs. The 2∏-states split under the influence of spin-orbit-coupling into components 2∏3/2 and 2∏1/2, the former having the lower energy. The assignment of states and components to the individual bands is confirmed by the size of the split due to spin-orbit-coupling, the vibrational fine structure of the bands and by a simple parametrization of the ZDO. molecular orbital model assumed as a basis for the qualitative discussion.

The interaction between the two non-conjugated π-bonds in 1, 4-cyclohexadiene, norbornadiene and bicyclo[2.2.2]octadiene has been determined by photoelectron spectroscopy to be 1.0, 0.85 and 0.6 eV respectively.

The first of the two π-bands in the photoelectron spectrum of cis-cis-cis-1, 4, 7-cyclononatriene (I, symmetry C3v) shows a Jahn-Teller split. This is consistent with the prediction of molecular orbital theory that the top occupied orbitals of I are e(π) and a1(π) respectively. From the difference ϵ(e(π)) - ϵ(a1(π)) = 0.90 to 0.97 eV a value of β1,3 = −0.68 eV = 0.27 β (β = -2.5 eV) is obtained for the homoconjugative interaction of two π-orbitals in I. This value represents almost exclusively through-space interaction between the π-orbitals. Through-bond interaction (hyperconjugation) is a minor effect in I. A comparison of the photoelectron data of bicyclo [4.2.1] nonatriene with those of norbornene and cycloheptadiene shows that homoconjugation (homo-aromaticity) can only be detected by photoelectron spectroscopy if the interacting π-bonds (basis orbitals) are symmetry equivalent or have accidentally (almost) degenerate energies.

The photoelectron spectra of fulvene (II) and of 3,4-dimethylene-cyclobutene (III) have been recorded. The PE. bands are correlated, in order of increasing ionisation potentials, with the following orbitals: II: 1a2(π), 2b1(π), 7b2(π), 1b1(π); III: 2b1(ω), 1a2(π), 10a1(ω), 8b2(ω), 1b1(π). This assignment is based on a semi-quantitative perturbation MO-model and on the SCF-LCAO-MO calculations reported by Praud, Millie & Berthier [5] for benzene, II and III.

The photoelectron spectra (He I excitation) of trimethylsilylacetylene M(H) and of the four trimethylsilylhaloacetylenes M(X) with X = F, Cl, Br, I hav

Die PE-Spektren der Benzocycloalkene 3 (n) mit n = 3 –8 wurden aufgenommen. Will man den Einflus der Alkyleinheit –[CH2]n–2 auf die Orbitalenergien e(b1 (π)) und e(a2(π)) mittels eines Orbitalmodells erklaren, welches im Rahmen der Huckelschen Naherung nur induktive und hyperkonjugative Parameter zulast, so findet man, das der Methylengruppe CH2 in 3 (3) ein negativer, der Athylengruppe.– [CH2]2 in 3 (4) ein verschwindender induktiver Effekt zuzuordnen ist. Dieses Ergebnis verlangt, das die Destabilisierung des obersten besetzten π-Orbitals b1 (π) in 3 (3) und 3 (4) auf hyperkonjugative Wechselwirkungen zuruckgefuhrt wird. Das oberste besetzte σ-Orbital der Kohlenwasserstoffe 3 (n) ist fur n = 3 und 4 vor allem in den CC-Bindungen, fur n ≥ 5 in den CH-Bindungen der Cycloalken-Einheit lokalisiert. The Photoelectron Spectra of Benzocycloalkenes The PE. spectra of benzocycloalkenes 3 (n) with n = 3 to 8 have been recorded. It is shown that an attempt to interpret the influence of the alkyl moiety — [CH2]n−2 on the π-orbital energies e(b1 (π)) and e(a2(π)) by the simple inductive/hyperconjugative Huckel molecular orbital model of alkyl groups demands that the methylene group CH2 in 3 (3) and the ethylene group –[CH2]2- 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 π-orbitals b1(π) in 3 (3) and 3 (4) is due to hyperconjugation. The highest occupied σ-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 ≥ 5.

The four bands which have been observed in the photoelectron-spectra of the cyanogen halides CICN, BrCN and ICN (region from 10 to 20 eV) have been assigned to the following states: .

The PE. spectra of [2, 2]paracyclophane (1), 4-amino[2, 2]paracyclophane (2) and 1, 1, 2, 2, 9, 9, 10, 10-octafluoro[2, 2]paracyclophane (3) are presented. The bands corresponding to ejection of the photoelectron from the five highest occupied π-orbitals have been assigned. The ‘observed’ orbital energies (i.e. the negative ionization potentials) are discussed in terms of ‘through space’ and ‘through-bond’ interactions between the semi-localized π-orbitals (e1g) of the benzene moieties and the C, C-σ-orbitals of the ethylene bridges. The PE. spectrum of 3 shows that the fluorine-induced lowering of the C, C-σ-orbital energy effectively ‘turns-off’ the ‘through-bond’ interaction. The resulting pattern of the first four bands confirms the assignment given for 1. Finally the band shifts induced by an amino group in position 4 are again in agreement with this assignment. Attention is drawn to the phenomenon of ‘orbital switching’ as a consequence of substitution in loosely coupled systems such as 1.

The photoelectron spectrum of cyclooctatetraene (COT; 1) has been re-examined. Correlation with the spectra of the dihydro- (2, 3), tetrahydro- (4, 5, 6), hexahydro- (7) and octahydro- (8) derivatives confirms the orbital sequence originally proposed by Eland, namely 5a1 (π), 7e (π), 4b2 (π), σ. It is shown that the positions and the relative spacing of the π-orbitals are the result of a competition between through-space and through-bond interactions. The photoelectron spectroscopic results for the hydrogenated derivatives of COT yield information about their conformation; in particular about the twist angles between consecutive π-bonds. The spectrum of 1,5-cyclooctadiene (6) confirms that this molecule preferentially adopts the boat conformation. An empirical rule is proposed which yields estimates for the orbital energies of basis π-orbitals in monocyclic hydrocarbons.

Alkyl substitution of benzenoid hydrocarbons shifts their 1Lα band to longer wavelengths. A statistical analysis of the available data for naphthalene, phenanthrene and 1,2-benzanthracene indicates that simple Huckel theory does not yield as good a prediction for these shifts as has previously been assumed. It is shown that a significant improvement results if many electron wavefunctions are used and if mixing between the configurations Ψ 1 −1 and Ψ 2 −2 is introduced.

To obtain further information concerning the interaction between Walsh-orbitols of ‘conjugated’ cyclopropane rings, the photoelectron spectra of the following compounds have been recorded: bicyclo[4.1.0]heptane 1, cis- and trans-tricyclo[5.1.03, 5]octane 2, 3, diademane 4, trans-pentacyclo[3.3.2.02, 9.04, 10, 06, 8]decan 5 and bicyclo[4.1.0]heptene-2 6. The first bands in the PE.-spectra of these compounds have been assigned on the basis of a ZDO HMO-approximation. For 2 and 4 the value for resonance integral between linked 2p atomic orbitals of two adjacent eclipsed cyclopropane rings is found to be −1.73 eV.