Carbon Chain Molecules in Cryogenic Matrices - Polyynes: Synthesis, Properties, and Applications - page 6

FIGURE 1.4 The UV-vis spectrum of a dense and heavily annealed carbon-bearing
matrix. During annealing, the absorptions gradually appear at longer and longer
wavelength and seem to belong to longer and longer odd n C n chains. The regular
spacing suggests that the species are linear up to possibly C 27 .

succession and must belong to successively larger species. The numbers
given in the upper spectrum show the size of the carbon molecule which
produces the feature. Most of the assignments are based on the already
mentioned work of Maier's group. One recognizes in the spectra that the
prominent peaks (with the exception of linear C 6 ) are from species with an
odd number of atoms, and that an almost constant increment in wavelength
exists in going from one odd carbon molecule to the next. Such a sequence in
wavelength is precisely what is expected for linear species. Figure 1.4, which
reproduces the spectrum of a heavily annealed matrix, shows the regularities
more clearly and suggests that the chains can grow quite large, reaching up
to 25 or 27 atoms. We thus conclude that the molecules with an odd number
of atoms are linear chains. With the exception of C 6 these odd chains
apparently dominate the spectrum, probably because their absorptions
(which are 1

1

u transitions) are more prominent compared to those
of their even-atomic partners. It should be remarked that all odd chains
have a closed-shell electronic structure in their ground state (i.e. are singlet
states) and that all even chains have just two electrons in a

g !

orbital which

can host four electrons (i.e. possess a triplet ground state).
The regularities in the odd chain spectra may be used to predict the
wavelength position of the

1 1

u transition of linear C 3 by extra-
polation of the available data. The prediction yields a wavelength of about

g !