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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 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.

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 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 photoelectron spectrum of cyanogen fluoride (FCN) has been recorded, using He(I) and He(II) radiation. The spectrum can be unambiguously assigned through a correlation with the known photoelectron-spectroscopic data of the other cyanogen halides XCN, X = Cl, Br, I. The observed electronic states of the radical cation FCN+ are: A = 2Π32, 12; A = 2Σ+; B = 2Π32, 12; C = 2Σ+.

The 30.4 nm He(II) photoelectron spectra of fluorine(F2), of acetylene and diacetylene and their fluorosubstituted derivatives have been recorded. The use of He(II) radiation allowed the confirmation of the conjectured B∼2Σ+g state of the fluorine radical cation at 21.1 eV. The spectra of the fluoroacetylenes and -diacetylenes are characterized by a small number of distinct bands. The assignment is readily obtained on qualitative considerations only. We have examined these spectra with special attention to the qualitative rules known as “perfluoro-effect”. In addition, the experimental ionization energies have been compared to calculations performed with the following methods: many-body Green's functions, STO-3G, HAM/3 and CNDO/S.

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.

Several fluorinated acetylenes and nitriles have been isolated from the reaction mixture obtained from Tesla excited fluorinated aromatic compounds. Differences in product distributions using Tesla or radio frequency discharges are discussed.

The emission spectra of the cations of cyanodiacetylene, methylcyanodiacetylene and ethylcyanodiacetylene have been detected using low energy electron beam excitation. The emission band systems lie in the 550-670 nm wavelength region and are assigned to the A2Π→text-decoration:overlineX2Π, A2E→text-decoration:overlineX2E and A2A″→text-decoration:overlineX2A″ electronic transitions of these three cations, respectively. This follows from the photoelectron spectra of these compounds, which have also been obtained, and from their assignment. The vibrational frequencies (± 10 cm–1) of four or five of the totally symmetric fundamentals of the three cyanodiacetylene cations in the text-decoration:overlineX(π–1) states have been inferred from the vibrational analyses of the emission spectra. These frequencies are supplemented by values (± 160 cm–1) obtained for the A(π–1) states from the photoelectron spectra. The lifetimes of the title cations in the lowest vibrational level of their A states have been measured as 15 ± 2, 8 ± 2 and ⩽ 6 ns, respectively.

The results of the search for the A → X radiative relaxation of haloethylene cations in the gaseous phase are reported. Only in the case of cis-1,2-difloroethylene cation was an emission spectrum detected. It is identified as the A 2 A 1 → X 2 B 1 band system on the basis of photoelectron spectroscopic measurements. An assignment of the emission bands yields the vibrational frequencies of four of the A 1 fundamentals (under C 2 , symmetry) for the X state and one for the A state. Well resolved Ne(I) photoelectron spectra of cis- and trans-1,2-difluoroethylene are presented, from which some vibrational frequencies for these cations in the X and A states are also obtained. The lifetimes of cis-1,2-difluoroethylene cation in the lowest vibrational levels of the A 2 A 1 state have been measured. The decay of this cation is unusual as these levels are depleted both by, radiative, and pathways leading to fragment ions (C 2 HF + ). The lack of detectable emissions with other fluoro-, chloro- and bromo-ethylene cations is discussed and the likely symmetries of the A states are proposed.

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.

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.

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

HeIα excited photoelectron spectra of pentatetraene and the inferred ionization energies are reported. The first band has a characteristic Franck-Condon envelope similar to the first photoelectron bands of allene and butatriene. The four bands found below 16 eV ionization energy have been assigned to the X2E, A2E, B2E and C2B2 states of the radical cation of pentatetraene by comparison with STO-3G and SPINDO calculations on the cumulene series. The correlation scheme includes the 2s shell ionization energies of ethylene, allene and butatriene. The π-orbital ionization trends of the cumulenes are discussed in the framework of localized orbitals calculated with the STO-3G basis set.

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.

Experimental valence ionisation energies of 143 hydrocarbons, CnHm, have been determined from their He (IIα) (40.80 eV) and He (Iα) (21.22 eV) excited photoelectron spectra. Ionization energies, usually up to 26 eV are given in tabular form for 108 hydrocarbons with n≤6, together with their (tentative) assignment. The ionization energies up to approximately 25 eV, of 35 selected hydrocarbons with 7≤n≤10, are presented by means of their He (IIα) photoelectron spectra.

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.

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.