PE spectra

Figure 9.50 Processes involved in obtaining (a) an ultraviolet photoelectron spectrum, (b) a zero kinetic energy photoelectron (ZEKE-PE) spectrum by a one-photon process and (c) a ZEKE-PE spectrum by a two-photon process in which the first photon is resonant with an excited electronic state of the molecule...

H-Dibenz[6,/]azepine, 10,11-dihydro-acylation, 7, 511 alkylation, 7, 511 amination, 7, 512 lithiation, 7, 528 PE spectrum, 7, 502 pharmacological properties, 7, 546 reactions... 

Photoelectron spectroscopy (PES) has been applied to determine the structure of 1-aza- and 1,4,7-triazatricy-clo[5.2.1.04,10]decane 37 and 40 <1997JMT(392)21>. The PES spectrum of ATQ shows four composite bands in the region 7-17 eV. A first band peaked at 7.80 eV is attributed to the NLPO (nitrogen lone-pair orbital). A second prominent broad band system, extending from 10.5 to 13.0 eV is associated with photoionizations from the cr-orbital manifold. The third composite band is produced by two photoemissions. The second band may be attributed to emissions arising from a sequence of seven near-lying MOs. 

In TATCD the uppermost band consists of two components. Due to symmetry restrictions in these NLPOs of TATCD produce only two bands. The second complex band system of ATQ associated with cr-ionizations is displaced toward higher binding energy and is slightly more resolved in the PES spectrum of TATCD. 

Fragalia and co-workers have reported the details of the He(I) and He(II) excited photoelectron spectra of Cp2Ti(CO)2 and concluded that evidence exists for significant backbonding between the Ti 3d orbitals and empty carbonyl v orbitals. Further, there is no evidence of important overlap between Ti and Cp orbitals. A small electrostatic perturbation of the Cp ligands is caused by the titanium atom (85). Bohm has described an elaborate study of the low energy PE spectrum of Cp2Ti(CO)2 (1) by means of semiempirical MO calculations (86). 

As a real example we show in Figure 2 the PE spectrum of 1,1-divinylcyclopropane (46 in Table 1), taken from the considerable number of diene and polyene PE spectra published by R. Gleiter and his coworkers. In the second column of the insert (5) are listed the / values in eV corresponding to the first bands of 46. 

In principle, refined and relatively reliable quantum-theoretical methods are available for the calculation of the energy change associated with the process of equation 2. They take into account the changes in geometry, in electron distribution and in electron correlation which accompany the transition M(1 fio) — M+ (2 P/-), and also vibronic interactions between the radical cation states. Such sophisticated treatments yield not only reliable predictions for the different ionization energies 7 , 77 or 7 , but also rather precise Franck-Condon envelopes for the individual bands in the PE spectrum. However, the computational expenditure of these methods still limits their application to smaller molecules. We shall mention them later in connection with examples where such treatments are required. 

The simplest model of this kind—and admittedly a rather naive one—is to choose two-centre 7r-orbitals 7r(t as basis functions. The prototype for the jc is the -orbital of ethene. Because ejection of an electron from this orbital yields the low-energy band, at /[ = 10.5 eV, in the ethene PE spectrum shown in Figure 4, A = —I = —10.5 eV is... 

Assuming that the jr-system of the (2Z,4Z) isomer 26 is flat, r = 0°, equation 41 yields the twist angles r indicated in the last line of display 42 for the other two isomers. A further example of strong steric prohibition of coplanarity is provided by 2,3-di-t-butylbuta-l,3-diene 3537, whose PE spectrum yields a gap AIT =0.3 eV, corresponding to a twist angle r = 80°. 

The disproof of the naive expectation that the PE spectrum of a (non-planar) hydrocarbon containing z non-conjugated double bonds would show z 7r-bands, where each one corresponds to the removal of an electron from only one of the z localized two-centre Tr-orbitals, was one of the earliest successes of PE spectroscopy. In particular, if in a... 

Contrary to what is suggested by an independent electron treatment, the energy gap A/v = l — I between the ionization energies corresponding to the first two bands in the PE spectrum of a molecule is not equal to the difference AE between the first two... 

FIGURE 30. Potential energy curves for a neutral molecule M, and its radical cation M in the ground and first excited state (equilibrium distances with respect to an arbitrary coordinate q along which the three geometries differ). Note the shift in the M+ /(M+ ) energy difference AE on going from e/eq of M (AE = A/v from the PE spectrum of M) to qeq of M (AE corresponds to /-IM ,X from the EA spectrum of M+")... 

We mentioned in Section III.A that one of the unique features of radical ion optical spectroscopy is that it allows one to measure excited-state energies of a molecule at two different geometries, namely that of the neutral species (If in PE spectra) and that of the relaxed radical cation (Xmax of the EA bands). In many cases this feature is of little relevance because either the geometry changes upon ionization are too small to lead to noticeable effects (e.g. in aromatic hydrocarbons), or because such effects are obscured, due to the invisibility of the states in one or other of the two experiments (i.e. strong cr-ionizations in the PE spectrum) or because of the near-cancellation of opposing effects (as in the case of linear conjugated polyene radical cations). 

The relative intensities of the bands, i.e. the band-area ratios, are very meaningful for the interpretation of a PE spectrum since they are proportional to the relative probabilities of ionization. The absolute value of the area of a spectral band depends, among other factors to be discussed shortly, also on the density of the target, which is quite difficult to measure, so that usually the spectral intensities are given in arbitrary units. For the purpose of the analysis of the electronic structure of a molecule, the intensity ratio between the different bands is sufficient to give valuable indications. 

In the cyclophane 1, although the overlap between the n-system (2p) and the bridging cr-bonds (2s2p) is most effective, these orbital energy levels match worst, the first ionization potentials being 9.25 eV for benzene and 12.1 eV for ethane. As a result, the HOMOs are the almost pure it MOs with the b2g and b3g combinations. Both the PE spectrum and theoretical calculation demonstrate the degeneracy of the two HOMO levels. The absorption bands are attributed to the 17-17 transitions associated with the HOMOs. 

---