Cyclic Polyynes: Generation, Characterization, and Reactions - Polyynes: Synthesis, Properties, and Applications - page 105

FIGURE 6.6 Molecular orbital (MO) functions of the highest-occupied MO
(HOMO) and lowest-unoccupied MO (LUMO) levels in the in-plane p and out-of-
plane p systems of monocyclic C 20 , a member of the 4m carbon rings. In each panel,
the bond between the atoms 1 and 2 corresponds to the single bond (longer bond)
and the alternating bond lengths follow. The LUMO function has amplitudes on the
single bonds while the HOMO function on the triple bonds.

softening towards the acetylenic structure [35c]. The schematic drawing in

Figure 6.7 represents the distortion of the potential energy surface of a

monocyclic carbon ring along the normal coordinate that induces the ace-
tylenic structure. Owing to the first- and second-order effects, the distortion
is introduced to give the identical double minima corresponding to the
acetylenic form. Depending on the degree of the electron correlation consi-
dered, the size of cyclic C 4m þ 2 at which the cumulenic form changes into
acetylenic one largely scatters from m ¼ 3tom>10 [35].

6.2.4 P HOTOELECTRON S PECTROSCOPY

Ultraviolet photoelectron spectroscopy (UPS) of anionic C n provides
information on the electron affinity (EA), and electronic and vibrational
structures of the corresponding neutral species. The size dependence of EAs
and the vibrational fingerprints are useful to distinguish isomers when
combined with mass spectrometry. In 1988 Yang et al. reported the first
experimental indication of the presence of monocyclic carbon clusters
C n in laser vaporization of graphite in helium gas [17]. For clusters C n