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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]).

Hexahydropyrimidines N,N-bridged by a chain of n methylene groups (1,(n+2)-diazabicyclo[n.3.1]alkanes) adopt out,out (axial,axial) structures for n=2, 3, and 4. When n=5, the photoelectron spectrum shows evidence of the presence of some of the out, in (axial,equatorial) isomer in the gas phase, although none can be found in solution. When n=6, the compound is apparently entirely out,in in the gas phase but exists as a mixture of out,out and out,in conformers in solution. For n=1, only the diamond lattice out,in isomer can be detected in solution. These experimental data are correlated with force field (MM2) calculations; multiple minimum search methods have been used to locate all low-energy conformations

In π systems with localized double bonds, the switch from one localization pattern to another symmetry-equivalent one, and the corresponding changes in electron distribution are often considered to proceed via a delocalized transition state of higher symmetry. We present a different mechanism, according to which the double-bond localization and the concomitant charge distributions can fluctuate without change in energy and without passing through a delocalized, higher-symmetry transition state. This result is obtained within a simple, independent-electron (HMO-type) treatment.

In π systems with localized double bonds, the switch from one localization pattern to another symmetry-equivalent one, and the corresponding changes in electron distribution are often considered to proceed via a delocalized transition state of higher symmetry. We present a different mechanism, according to which the double-bond localization and the concomitant charge distributions can fluctuate without change in energy and without passing through a delocalized, higher-symmetry transition state. This result is obtained within a simple, independent-electron (HMO-type) treatment.

1 Introduction 2 Photoelectron (PE) Spectroscopy 3 Excited States of Polyene Radical Cations by Other Methods 4 Acknowledgements Keywords: diene and polyene radical cation electronic structure; photoelectron (PE) spectroscopy; PE spectra of dienes and polyenes; standard Huckel treatment; deviations from planarity; excited states of polyene radical cations by other methods; Koopmans and non-Koopmans states; linear conjugated polyenes

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 low-energy part of the He(Iα) or He(IIα) PE spectrum of a hypothetical molecule M (shown in Fig. 1) consists of four bands, ① to ④, which — for simplicity — we assume not to overlap. The bands ① and ② exhibit resolved vibrational fine structure, whereas the fine structure of ③ and ④ cannot be resolved because of line broadening, of random noise and/or the resolution of the recording, the latter being typically of the order of 20 meV.

It is shown that the ab initio STO-3G treatment applied to simple saturated linear, branched and cyclic hydrocarbons, assuming standard geometries, yields orbital energies ϵ for their canonical orbital φj which correlate perfectly with the observed C2s ionization energies I, if Koopmans' approximation is accepted. Applying the Foster-Boys localization procedure to these canonical orbitals φj leads to localized orbitals λμ and their corresponding Hartree-Fock matrix Fλ = (Fλ,μv). An examination of the matrix elements Fλ,μv, i.e. of the self-energies Aμ = Fλ,μμ of the localized CC- and CH-orbitals λμ and of the cross terms Fλ, μv (μ ≠ v) between them, leads to the conclusion that a satisfactory approximation should be obtained by setting Aμ = A for all μ, Fλ,μv = B if λμ and λv are vicinal and neglecting all other cross terms. The resulting model is nothing but the well-known equivalent bond orbital model of Lennard-Jones & Hall, which however can now be calibrated using the known C2s-ionization energies of hydrocarbons. Due to the discrete structure and the wider range (∽ 8 eV) of the C2s band systems in the photoelectron spectra of these molecules this leads to a more satisfactory parametrization than using the narrower and badly resolved C2p band system. Comparison of calculated band positions using the calibrated model with observed C2s-band ionization energies for a series of hydrocarbons reveals that the simple equivalent bond orbital model is better than one might have expected.

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.

Note: Univ lausanne,inst chim organ,2 rue barre,ch-1005 lausanne,switzerland. univ basel,inst phys chem,ch-4056 basel,switzerland. Reference LGSA-ARTICLE-1987-009doi:10.1002/hlca.19870700612 Record created on 2005-11-09, modified on 2017-05-12

The electronic structure of the title compound (1) has been investigated using He(Iα) PE spectroscopy and assuming the validity of Koopmans' theorem. It is found that: (a) in analogy to the situation prevailing in the cyclophanes, hyperconjugation of the out-of-phase linear combination of the π-orbitals with the ethano-bridge σ orbitals is symmetry-forbidden; (b) in agreement with the ideas developed in Part I of this series [1], the strong local B 1u -type bending of the ethylene moieties does not affect their basis energy significantly; and (c) the “through-space” coupling of the two double-bond π orbitals is of the same order as the benzene π-orbital coupling in [2 n ] cyclophanes, even if the net contribution to the repulsion of the two double bonds is probably small. Because of the near degeneracy of the HOMO and HOMO — 1 of 1 , the ground state of the radical cation 1 + cannot be determined uniquely: it is either 2 A g or 2 B 1u .

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.

Recent work concerning the photoelectron spectra of unsubstituted and substituted acetylenes and polyacetylenes is reviewed. It is shown that these systems are ideal cases for the investigation of some of the principles which govern photoelectron spectra of organic compounds.

Photoelectron spectra excited by He(lα), and in some cases also by He(IIα), radiation of the following twelve acetylenic derivatives are reported: CH3CCX, CF3CCX, NCCCX with X = Cl, Br, I and NCCCX with X = F, CH3, CF3. The ionization energies leading to the doublet states of their radical cations accessible by such photoionization processes have been obtained and interpreted. Vibrational frequencies of some of the totally symmetric modes in the cationic states of these molecules have been inferred from the fine structure discernible on the photoelectron bands. Trends in the ionization energies are pointed out and the models used previously to assign the photoelectron spectra of such species are summarized. Theoretical calculations of the ionization energies of the molecules RCN with R = H, F, CH3, CF3, CN and RCCCN with R = H, F, CH3, CN both in Koopmans' approximation and with a many-body Green's function method permit a complete interpretation of their photoelectron spectra. For the majority of the RCN and RCCCN molecules Koopmans' approximation fails to predict both the correct ordering of ionic states as well as trends in the ionization energies, whereas, the Green's function calculations are in good agreement with experiment in both respects.

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).

A tentative assignment is proposed for the He (Ia) photoelectron (PE.) spectra of the title compounds. It is based on the assumption that the first four π-bands occupy the same position in both spectra, as suggested by a qualitative correlation with the PE. spectrum of [24](1,2,4,5)cyclophane and by a previously proposed molecular orbital model. If this is accepted, then the bands due to electron ejection from the lone-pair orbitals are split by roughly 0.8 eV in the case of the 4, 13-diaza isomer, whereas no split can be detected in the case of the 4, 16-diaza isomer.

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.

‘Face-to-Face’ Benzo-anellated Homologous Hypostrophenes. Syntheses, X-Ray-Structure Analyses and PE Spectra ‘Face-to-face’ Benzo-anellated homologous hypostrophenes (series M), of interest as substrates for [6 + 6]-photocycloaddition reactions, have been synthesized. From X-ray structural analyses of 13b and 13c shortest CC distances of 2.80(2.76)/2.71 A and interorbital angles (ω) of 129° (128°)/130° between the benzene rings have been determined. The PE spectra are discussed in the context of transannular π,π interactions.

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.

A PE-spectroscopic study of exo- and endo-2-norbornyl iodides suggests that the relative ability of the 2-norbornyl group to stabilize an electron deficiency on a substituent X (e.g. I) in exo- or endo-position depends on the location of the positive charge. There is no difference if the positive hole is strongly localized on on the substituent X (e.g. the 5p−1 state of the title compounds). On the other hand, our results indicate that teh positive hole semi-localized in an exo-CX bond is better stabilized by the 2-norbornyl group than a semi-localized, positive hole in an endo-CX bond.

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.