Potentials. Photoelectron Spectrum

The first adiabatic ionization potential was measured by photoionization mass spectrometry as 13.11 0.01 [1] and by photoelectron (PE) spectroscopy as 13.11 0.05 [2,3] and 13.13 [4]. The first vertical ionization potential was obtained from PE spectroscopy as 13.25 [2,3] and 13.26 [4, 5] and from electron impact mass spectrometry as 13.7 0.2 [6]. A more recent electron impact value is 13.12 0.05 [28]. The tabulated Ej = 13.7 0.2 [7, 8] was used [9] fora discussion of the effect of F substituents on the ionization potentials of molecules. Another electron impact value Ej = 13.54 0.03 [7] was taken from [10]. The average Ej = 13.6 was used [11,12] for a comparison of the electronic structures of the fluorides and hydrides of first-row elements. For calculated values, see the following discussion of the PE spectrum and Section 3.4.5.2.1, p. 10, on the orbital energies. 

A vibrational structure was observed for the first peak in the PE spectrum. The wavenumber 1010 cm of the vibrational progression was assigned to the totally symmetric 0-F stretching vibration Vi of OFJ. The lower wavenumber 928 cm of OFg (see p. 28) is in accordance with the antibonding character of the outermost 2bi orbital [2]. vi =1032 40 cm was observed by others [4, 5]. 

Higher ionization potentials due to the above mentioned PE spectra follow  

The unequivocal assignment of the PE peaks to the MOs on the basis of theoretical ionization potentials is still not possible, as is shown in the following table. The experimental vertical ionization potentials (in eV) of Brundle et al. [2,3] are listed in the heading. Calculated ionization potentials are reported in the remarks (in the order of the MOs listed in the respective row) together with the respective calculation procedures used  

For previous attempts to interpret the PE spectrum on the basis of Koopmans theorem, see [2 to 5]. 

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In this method, photons of an energy well in excess of the ionization potential are directed onto a molecule. The photoelectron spectrum which results allows assessment of the energies of filled orbitals in the molecule, and thus provides a characterization of a molecule. Comparisons between photoelectron spectra of related compounds give structural information, for example, on the tautomeric structure of a compound by comparison of its spectrum with those of models of each of the fixed forms. 

Figure 7. The photoelectron spectrum of formaldehyde with the data on adiabatic ionization potentials. The fourth potential can be determined only at higher resolutions. [From (103) by permission of D. W. Turner and the publishing house].

Photoelectron and X-ray Spectroscopy. - The photoelectron spectrum of the n2 phosphaalkene (50) was similar to that of the corresponding imine. Its first ionisation potential was at 9.69... 

The photoelectron spectrum of selenophene vapor down to 1350 A has been studied. By analogy with the other heterocyclic derivatives, Rydberg-type transitions occur, leading to the first ionization potential of the molecule.23... 

Energy Levels for Hole Injection. For the hole conductor TPD (6), measurements are available from different groups that allow a direct comparison of different experimental setups. The ionization potential that corresponds to the HOMO level under the assumptions mentioned above was measured by photoelectron spectroscopy to be 5.34 eV [230]. Anderson et al. [231] identified the onset of the photoelectron spectrum with the ionization potential and the first peak with the HOMO energy, and reported separate values of 5.38 and 5.73 eV, respectively. The cyclovoltammetric data reveal a first oxidation wave at 0.34 V vs. Fc/Fc+ in acetonitrile [232], and 0.48 V vs. Ag/0.01 Ag+ in dichloro-methane [102], respectively. The oxidation proceeds by two successive one-electron oxidations, the second one being located at 0.47 V vs. Fc/Fc+. 

Fig. 9a, b. A portion of a photoelectron spectrum (idealized) showing (a) the identification of adiabatic and vertical ionization potentials with resolved (1) and unresolved (2) vibrational structures, (b) the identification of a higher adiabatic ionization potential with a break . 

An upsurge of interest in the N-methylborazines in the early 1970 s was coupled with a convenient method of synthesis and purification for these compounds The photoelectron spectrum of N-trimethylborazine has been reported. Table 6 summarizes the theoretical and experimental data comparing the location of the molecular orbitals of N-trimethylborazine with those of borazine. The HOMO is predicted and observed to be an e" (w) orbital as in borazine The methyl substitution on nitrogen destabilizes the e" and the a2 jr-orbitals, but does not signiBcantly effect the e (a) orbital. The result is a lowering of the ionization potential for electrons in the two TT-orbitals. This effect, predicted in the dieoretical calculations, was also verified experimentally. 

Figure 1. Potential energy diagram for cyclopropane in its ground state ( A/) and the two lowest states of its radical cation ( A, and 82). The vertical transition yields two bands (Jahn-Teller split) in the photoelectron spectrum.

The photoelectron spectrum is frequently discussed in terms of Koopmans theorem, which states that the ionization potentials (IPs) are approximately related to the energies of the canonical orbital found in molecular orbital calculations.106. The relationship is approximate because two factors are neglected the change in the correlation energy, and the reorganization energy, which is a consequence of the movement of electrons in response to the formation of a cation. The two quantities are approximately equal and opposite. 

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