Ionization potential determination

Even the photoelectron spectroscopy of closed-shell molecules is valuable for the physical chemistry of radicals because a difference between the nth and the first adiabatic ionization potentials determines the excitation energy in a radical cation for a transition from the ground doublet state to the (n — 1) excited doublet state. 

Early attempts to generate and characterize m-benzyne (13) comprise the pyrolysis of 1,3-diiodobenzene (14) and the flash photolysis of benzenediazonium-3-carboxylate (15). " While in the former case only enediynes (16) were in accordance with the ionization potentials determined (pyrolysis temperature 960 °C), the results of the latter work remain unclear. A transient with m/z = 76 and a lifetime of 400 ps was observed, however, it was not possible to unambiguously identify this species as 13 due to the complexity of the product spectrum (Scheme 16.6). Indirect evidence for the existence of 13 stems from trapping experiments. ... 

The valence MO energies and Mulliken populations for H3PCH2 at various theory levels are shown in Table 6. The low HOMO energy is consistent with the first ionization potentials determined for substituted ylides which are in the range 6-7 eV (Section II.B.l.f). The Mulliken populations show that there is a build-up of electron density on carbon, also reflected in the integrated spatial electron population (ISEP) value of 9.2 at the 3-21G level 65, with a corresponding decrease in electron density of phosphorus. The semiquantitative approach of Jardine et a/.190 gives a similar result. 

The reactivity sequence furan > selenophene > thiophene > benzene has also been observed in the nucleophilic substitutions of the halogenonitro derivatives of these rings.21,22 This shows that the observed trend does not depend on the effectiveness of lone-pair conjugation of the heteroatoms NH, O, Se, and S and the 77-electron density at the carbon atoms. It is interesting to note that a good correlation is observed between molecular ionization potentials (determined from electron impact measurements) and reactivity data in electrophilic substitution, in that higher reactivities correspond to lower ionization potentials182 pyrrole furan < selenophene < thiophene benzene (see Table VII). This is expected in view of a... 

Interestingly, it has been observed that the ionization potentials (determined by the electron-impact technique) of several substituted five-membered heterocycles correlate with the substituent effects is larger than for benzene (p = —14.7) and larger than that measured for the most sensitive electrophilic substitution (bromination in acetic acid, which gives p = —10.0 for thiophene). As pointed out by Linda et al. [12], the sensitivity to substituent effects... 

Figure 2. Ionization potential determined by (a) photoionization [Refill], ( ) electron impact [Ref. 12,13] and, (O) theoretical data estimated from the HOMO energies given by AMI method vs. Tm of benzene (B), toluene (T), naphthalene (N), m-xylene (mX) (obtained from the second TPD peak), and mesitylene (TMB) (second TPD peak) for H-ZSM-11 zeolite.

Such UPS data offer a unique opportunity to compare experimental data and MO calculations on the nature of the molecular orbitals, including ground state and both singlet and triplet excited states. Although the ionization potentials determined from MO calculations were not exact, relative energies were in good agreement with the theoretical values (Sections III and VIII). 

Toroidal forms of carbon were predicted to be stable on the basis of molecular dynamics simulations using a Stillinger-Weber-type potential.To test these predictions, the stability of a C120 torus was compared with that of Ceo using ab initio self-consistent field calculations and ionization potentials determined by Koopman s theorem (Table 4). The C120 structure investigated hadDs symmetry, and appears to... 

The Cr-I system has been examined in detail and Crij (m.p. 867 °Q and Crlj (m.p. 857 were characterized by their X-ray diffraction patterns and eutectics identified at 831 °C (69 at % 1) and 113°C (ca. 100 at % The structure of Crl2 contains layers of elongated Crl octahedra in which four Cr—I bonds are of length 273 pm and two of length 323 pm. X-Ray photoelectron spectra of CrXj (X = F, Cl, or Br) have been reported and the metal 2p- and 3s-orbital ionization potentials determined. 

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