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The belgian chemist Paul Havrez (1838–1875) proposed two highly symmetrical, space-filling models for the benzene molecule, only a few months after Kekule's seminal proposal of a cyclic benzene structure. One of these models is characterized by the revolutionary assumption of equal 3/2 bonds between all neighbouring carbon atoms, in sharp contrast to Kekule's formula with alternating double and single bonds. With Considerable hind-sight, Havrez's model could be considered as a forerunner of the benzene structure with delocalized π bonds.

Vermutlich findet sich in irgendeiner Klosterkapelle der Steiermark jenes Votivfenster, welches einer der Autoren, frei aus dem Gedächtnis, in Abbildung 1 nachgezeichnet hat. Es zeigt Josef Loschmidt, der Christophorus-gleich den kleinen Friedrich August Kekulé zum Ufer der Erkenntnis tragt. Die Darstellung beruht auf einer alpenländischen Legende, derzufolge Loschmidt die cyclische Verkettung der sechs Kohlenstoffatome im Benzol-Molekül 1 bereits 1861 - und somit fünf Jahre vor Kekulé - in seiner Formel 2 (sie trägt im Original [1, 2] die Nummer 185) vorweggenommen hat (vergleiche die Veröffentlichungen [3] bis [9]).

Controlled ozonolysis of 2,3,5,6-tetramethylidene-7-oxabicyclo[2.2.1]heptane (1) afforded 3,5,6-trimethylidene-7-oxabicyclo[2.2.1]heptan-2-one (2). Ozonolysis of 2 gave a 1:1 mixture of 3,5-dimethylidene-7-oxa-bicyclo[2.2.1]heptane-2,6-dione (3) and 3,6-dimethylidene-7-oxabicyclo[2.2.1]heptane-2,5-dione (4). The He(Iα) photoelectron (PE) spectra of 2 and 4 have been recorded. Comparison with the PE data of related systems, and with the result of ab initio STO-3G calculations, confirm the existence of significant through-bond interactions between the oxygen lone-pair orbitals n(CO) of the carbonyl functions and n(O) of the O(7) ether bridge.

The He(Iα) photoelectron (PE) spectra of tris(perfluorocyclobuta)benzene 4(F)1 , tris(perfluorocyclopenta)benzene 5(F)2 , tetrakis(perfluorocyclobuta)cyclooctatetraene 6(F)3 , and of tetrakis(perfluorocyclopenta)cyclooctatetraene 7(F)4 are reported. A tentative assignment of the PE spectra is derived by empirical correlation with those of relevant reference compounds. The results suggest that 6(F) retains the D4h-conformation in the gas phase, i.e. A conformation with a planar cyclooctatetraene ring, as observed in the crystal. All four compounds exhibit a sharp increase of their first ionization energies, relative to the corresponding parent hydrocarbons, due to the perfluoro effect.

The near degeneracy of the two lowest ionization energies of Dewar benzene can be understood in terms of closely competitive ‘through-bond’ and ‘through-space’ interaction. Empirical, semiempirical, and open-shell ab initio procedures converge to require mutually consistent symmetry assignments.

The ionization energies of four [1,2,4,5]tetrazinodi(heteroarenes) of C2h Symmetry have been measured by PE spectroscopy. It is found that the first ionization energy is very low (6.2 eV to 6.4 eV) and rather independent of the size and/or topology of the molecules. This is explained, by comparison with the results of a simple MO treatment, as being due to strong localization of the HOMO (of Au symmetry) on the [1,2,4,5]tetrazino moiety of the molecules. The PE-spectroscopic data are shown to be in agreement with the UV/VIS-and electrochemical-oxidation data published previously.

The He(Iα) photoelectron spectra and the ionization energies of symmetrically substituted di-n-alkyl-diacetylenes R-(CC)2-R (with R CH3, C2H5, n-C3H7, n-C4H9) are presented. The effect of the alkyl substitutents is that the two acetylenic ionization energies, Iv,1 and Iv,2, shift by the same amount, i.e. their difference Iv,2 – Iv,1 remains constant (2.45 ± 0.05 eV). Between 12.5 eV and 17 eV the band system in the photoelectron spectrum of R-(CC)2-R is superimposable with that in the spectrum of the corresponding alkane, RH, with the exception of a uniformly small shift of all the bands to higher ionization energy.

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 He (Iα) photoelectron spectra of the four unsaturated oxides 3,4-dihydropyran (6), γ-pyran (7), 2, 3-dihydro-1, 4-dioxin (9) and 1, 4-dioxin (10) are reported and analysed. Band assignments are based on ab-initio calculations, using the STO-3G basis set. The proposed orbital sequences (with reference to the coordinate systems given in Table 1) are, for the top three orbitals: 6, π, nσ, nπ; 7, 3b1(π), 1a2(π), 11a1(σ); 9, 11b(π), 12a(σ), 11a(π); 10, 2b3u(π), 1b1g(π), 6ag(σ). Finally the (almost) localized π-orbitals have been computed by the Foster-Boys localization procedure.

The He(II) photoelectron spectra of phosphabenzene 2 and arsabenzene 3 yield additional ionization energies in the region 17 to 20 eV. The relative intensity changes of the first three bands, compared to their relative intensities in the He(I) spectra, provide support for the previously proposed sequence of states ˜X = 2B1, ˜A = 2A2, ˜B = 2A1. This and the angular dependence of the photoelectron band intensities in the spectra of 2 and 3 is in excellent agreement with the theoretical predictions made by von Niessen, Diercksen & Cederbaum [9].

The photoelectron spectra (PE.) of ten cyclophanes (7 to 14, 16, 18) have been assigned on the basis of a simple molecular orbital model proposed recently for the cyclophanes 2 (1,4) to 6. It is shown that the agreement between calculated and observed band positions provides strong evidence for the validity of the model.

The He(Ia) photoelectron (PE) spectra of the (E,E)-, (E,Z)- and (Z,Z)-isomers of the title compound have been recorded to obtain information about their conformation in the gas phase. For a valid correlation with the PE data of other dienes it is necessary to take the potentials V(φ) for internal rotation and the corresponding conformer population densities P (φ) into account, as well as the rather complicated way wy in which the π−1 ionization energy gap ΔI(φ) depends on the direct π-orbital interaction and the long-range ‘through-space’ interaction between the semi-localized methyl-group orbitals and the π-orbitals. These factors being taken into account, the mean twist angles, φ, compatible with the PE-spectroscopic results are φ (E,E) ≈ O° ± 30°, φ (E,Z) ≈ 80° or 110° within ± 15°, and φ (Z,Z) ≈ 85° to 105°. These results are in rough agreement with electron diffraction data by Traetteberg [15], other spectroscopic results and, for the (E, E)- and (Z,Z)-isomers, internal rotation potentials V(φ) previously calculated by Roth [17]. On the other hand the potential V (φ) proposed for the (E,Z)-isomer does not seem to be compatible with our findings.

The n ionization energies I and the gas-phase basicities GB of CH3-, Cl-, or CN-substituted quinuclidines have been measured by PE and ICR spectroscopy. The dependence of the shifts ΔI and ΔGB (relative to the values of the parent molecule) allow conclusions about the charge dispersal accompanying the n ionization or the protonation of quinuclidine in the gas phase. The agreement with the results of a minimal basis set abinitio calculation is excellent. Comparison of the solution pKa values with either I or GB reveals that 2-substituted quinuclidines exhibit sizeable solvent-induced proximity effects, i.e. that the corresponding quinuclidinium ions are more acidic in solution than expected on the basis of the gas-phase basicities. This agrees with earlier results concerning 2-substituted pyridines.

The lowest states of dibenzo-p-quinodimethane radical cation are 2B1u (7.95 eV); 2B2g (8.66 eV); [2B3g, 2Au, unresolved] (9.1 eV); [2B1u, 2B2g, unresolved] (10.7 eV). This confirms the assignment proposed by Koenig et al. [1] for the states of p-quinodimethane radical cation.

The higher homologues of n-alkanes H(CH2)NH with N > ∽ 13 yield photoelectron spectra in which the C2s-bands merge to form a double humped, unresolved C2s-band system in the interval of I=15 to 25 eV [1]. It is shown that with the help of an equivalent bond orbital model one can derive a closed formula, which gives the individual C2s-band positions I=−ej in function of N and j with sufficient accuracy, assuming the validity of Koopmans' approximation. The calculated I values (j=1 to N) folded with an appropriate shape function for the individual C2s-bands reproduce the observed Franck-Condon envelope of the C2s-band system within narrow limits of error. However, a comparison of the observed total width of the C2s-band system with the computed one, indicate that for large n-alkanes (N≥ ∽ 13), the simplification which consists of taking into account only the interaction matrix elements between vicinal bond orbitals [2], is no longer a satisfactory one.

The PE spectroscopic data for a series of [2.2]paracyclophanes with olefinic and/or aromatic bridges, and of a few related molecules are reported, together with tentative assignments based on simple MO models.

The σ-orbital manifold of cubane 1, as suggested by its PE spectrum, is divided into two sets separated by a 3 eV gap extending from ∼ -10.5 eV to ∼ -13.5 eV. Halogen substituents with np AO basis energies falling into this gap (e.g. Cl or Br) will, therefore, hyperconjugate appreciably with both sets. Interaction with the lower-lying set will lead to the usual destabilization (‘normal’ hyperconjugation), whereas interaction with the set above will necessarily lead to a ‘stabilization’ (‘inverted’ hyperconjugation). As a result the lone-pair ionization energies of Cl or Br substituted cubanes (derived from PE spectra) are much larger than naively expected for an alkyl halide containing as much as 8 C-atoms. In particular no significant shift of the e lone-pair bands in the PE spectra of 1,4-dichloro- and 1,4-dibromocubane can be detected with respect to the first ionization energies of the free atoms Cl and Br, or of HCl and HBr.

The photoelectron (PE.) spectrum of the title compound has been assigned by comparison with the PE. spectrum of cubane (2), aided by ab initio STO-3G calculations using localized molecular orbitals. On the basis of the information available to date, the most satisfactory orbital sequence, Koopmans theorem implied, is, in descending order of energy: band system : (2e″2, 3e′2 2e″1, 3e′1); band system : 3a′1 (2e′2, 2a″2); band : 2e′1.(Sequence of orbitals in parenthesis uncertain).

Equivalent bond orbital (EBO) calculations by a procedure based on recalibrated, localized bond orbitals derived from an ab initio model, suggest that the two frontier ‘ribbon’ orbitals φHOMO, φHOMO-1 of polycyclic hydrocarbons consisting of all-trans-connected six-membered rings are well separated in energy from the remaining manifold of σ-orbitals. Assuming the validity of Koopmans' theorem, a PE-spectroscopic investigation of perhydroanthracenes, 2-methylperhydrophenalene and perhydroperylene have shown that this is indeed the case. According to the theoretical treatment these ribbon orbitals are evenly delocalized over the whole of the molecular frame and they conserve the characteristic phase relationship postulated by Hoffmann et al. [16] for cyclohexane moieties. Their behaviour under the successive introduction of centres of unsaturation, i.e., of double bonds is discussed.

The He(Iα) photoelectron (PE.) spectra of a series of substituted 1,2-dithietes have been recorded and assigned with respect to the orbital sequence derived from an STO-3G model calculation and by correlation with the PE. spectra of related compounds. The results provide additional support for the presence of a closed, four-membered ring moiety in all the 1,2-dithietes investigated. In all cases the two highest occupied molecular orbitals are b2(π)= HOMO, a2(π), with exception of 3,4-bis(trifluoromethyl)-1,2-dithiete where the sequence b2(π), a2(π) or a2(π), b2(π) is uncertain.

13C-NMR. and He (Iα) photoelectron spectra of alkyl phenyl sulfides, sulfoxides and sulfones have been used to probe how their conformations depend on the size of the alkyl groups R. The results are interpreted to indicate that in the sulfides the S, R-bond is twisted out of the planar conformation with increasing size of the alkyl group, whereas in the sulfoxides and sulfones the preferred conformation with the S, R-bond perpendicular to the phenyl group plane seems to be independent of the size of R. These conclusions are in agreement with previous work on the alkyl phenyl sulfides and provide strong support for previous conjectures concerning the preferred conformations of alkyl phenyl sulfoxides and sulfones.

The He(Iα) PE spectra of cyclopropabenzene, cyclopropa[b]naphthalene, cyclopropa[b]anthracene, their 1,1-difluoro derivatives and of some of their substituted derivatives have been recorded, analyzed and assigned by correlation with the PE spectra of the parent molecules benzene, naphthalene and anthracene. It is shown that the PE spectroscopic data yield no information about the presence or absence of a Mills-Nixon effect in the molecules investigated. The inductive effect of the bridging CH2 group of the cyclopropeno ring is close to zero, whereas that of the CF2 group leads to positive ionization energy shifts in good agreement with previous experience. The hyperconjugative interaction of the local pseudo-π orbital of the CH2 group yields a significant destabilization of the π-orbitals of B1 symmetry.

The photoelectron (PE.) spectra of thirteen methyl-substituted [2.2]paracyclophanes have been recorded and analyzed, to assess the influence of methyl-substitution on their ionization energies. It is shown that this influence is qualitatively and quantitatively the same as for benzenes and other π-systems. Comparison with the previous results obtained for the [2r]cyclophanes (r=2 to 6) strongly suggests that the hyperconjugative model for alkyl-group/π-system interactions is more appropriate than the inductive one.

Monomolecular fragmentations of butadiyne- and 1,3-pentadiyne radical cations in their electronically excited states have been investigated by use of the photoelectron-photoion coincidence technique. Breakdown curves for the molecular and various fragment ions have been recorded over the ionization energy range of 9 to 17 eV. These breakdown curves reveal the extent of the competition between the radiative relaxation (ion fluorescence) and the non-radiative internal conversion followed by fragmentation for the decay of the first excited electronic states. Extensive hydrogen scrambling and/or ionic-carbon-skeleton isomerization takes place prior to fragmentation, as revealed by the breakdown curves obtained for the site specifically deuteriated 1,3-pentadiynes.

The He(Iα) PE spectra of the two pairs of 1(Ph)-3 and 2–4 have been recorded and assigned by comparison with spectra of related molecules, or by using simple MO arguments. This analysis shows that the PE spectra are in agreement with the assumption that the twist angles observed in the crystalline state are more or less conserved in the gas phase. The observed first adiabatic ionization energies I allow the calculation of the molar reaction enthalpy ΔrH which accompanies the enolization of the radical cations of the title compounds. It is found that ΔrH ≈ –−70 kJ mol−1 is roughly the same for both keto/enol pairs, in qualitative and quantitative agreement with expectation. These results complement and correct ionization energies determined previously by mass spectrometric methods and conclusions based on them.

The photoelectron spectra of the hydrocarbons 1 to 4 have been recorded and interpreted by qualitative correlation with the spectra of their subsystems norbornadiene, quadricyclane, allene and cyclopropane. It has been shown that the interactions of the symmetry adapted orbitals of the subsystems are governed not only by their symmetry behaviour with respect to D2d in 1 and 2, or C2ν in 3 and 4, but also by the local symmetry at the points of attachment of the subsystems.

The He(Iα) and He(IIα) spectra of tetrafluorobutatriene 3(F) have been recorded for comparison with those of butatriene 3(H). Ab initio double-zeta basis self-consistent field (SCF) and configuration interaction calculations on butatriene show that, contrary to previous assignment, no shake-up band is expected to appear in the 9–10 eV energy range of the photoelectron spectrum. Further, such SCF calculations on tetrafluorobutatriene support the use of the perfluoro effect in assigning the purely π orbital ionizations. It is argued that 3(F) is a key compound for the study of the perfluoro effect. This is supported by a qualitative comparison of its photoelectron-spectroscopic results with those of other perfluoro systems.

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 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.

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.

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!).

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.

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.)

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.