Polyynes: Possible Bulk Synthesis and Chemical Properties - Polyynes: Synthesis, Properties, and Applications - page 431

FIGURE 18.8 Electronic absorption spectra recorded by a diode-array detector of
the HPLC showing two dienes and a triene (R t ¼ 1.743min) present in the reduced
mixture of polyynes.

Table 18.1 are molecules that are not end capped with hydrogen atoms but

capped by other adventitious groups which are essentially alkyl groups in
hexane. Furthermore, the electric arc in hexane gives also polycyclic
aromatic compounds (PAHs) as secondary products [27]. Some of the
compounds in Table 18.1 without a number in the first column may belong
to the class of PAHs molecules.
In Table 18.1 are hence reported the complete set of polyynes identified
(from 4 to 16 carbon atoms), the alkyl-capped analogs and other secon-
dary products. As stated, the partial hydrogenation changes the absorp-
tion spectra completely. The spectra of the main components detected
by the chromatographic analysis of the ene-ynes mixture are shown in

Figure 18.8- 18.10 with their retention time. In Figure 18.8 are shown the

spectra of the three lower molecular weight compounds having relatively
short retention time corresponding to polyynes with C 4 (R t ¼

1.556 and

1.649min) and C 6 carbon atoms (R t ¼ 1.743min). Judging from their

spectra, also in comparison to the spectra of their polyyne counterpart (see
Table 18.1), we may conclude that we are dealing with simple polyenes
(dienes for the C 4 and trienes for the C 6 ) in this specific case. In fact the
spectrum of vinylacetylene (the C 4 ene-yne) is rather peculiar [28] and does
not correspond to the spectra reported in Figure 18.8. The spectrum of the
C 6 compound (having R t ¼ 1.743) is clearly the spectrum of a higher homo-

log of that at R t ¼ 1.649 min. In fact it is characterized by a maximum at 225

and two shoulders at 253 and 265 nm while the preceding compound has a
maximum at about 200 nm and two shoulders at 235 and 245 nm (note