Essentials of Computational Chemistry

350 9 CHARGE DISTRIBUTION AND SPECTROSCOPIC PROPERTIES
F
F F
I
F
∆
I
I
hn
F F
+ I
F
F
hn
∆
H
H
H H
F
F F
retro-Bergman
Bergman
cyclization
F
+ I
Figure 9.9 Bergman cycloaromatization reactions for hex-3-en-1,5-diyne and its perfluorinated
congener, as well as a photochemical reaction scheme for generating the perfluorinated diradical
from an iodinated precursor. What spectral features would be expected to be most diagnostic
of the different intermediates? What levels of theory would be appropriate for predicting these
spectral signatures? (Note that equilibrium arrows of unequal length indicate which species
predominates at equilibrium.)
Table 9.8 Experimental and computed IR spectra (cm −1 )forA,B,and
C, and ITFP, PFPB, and PFHED, respectively
A ITFP B PFPB C PFHED
693 693 677 576
834 834 690 591
942/956 954 925 911 680
1138 1145 1148 912 918
1188 1195 1117 1151 1072 1067
1259 1298 1154 1151
1352 1400 1360 1363
1428 1441 1407 1421 1398 1414
1472 1487 1502/1516 1499 1678 1707
1574 1585 1560 2337 2419
1610 2426
B3LYP/6-31G(d) in Table 9.3 would result in slightly improved agreement for A and C) as
would be suggested by the synthetic scheme in Figure 9.9. Intensity data were also used,
although those are not shown here; importantly, the ‘missing’ bands in the experimental IR
spectra are all predicted to be of very low intensity in the computed spectra. Interestingly,
both CASSCF and unrestricted B3LYP predict the singlet and triplet states of the diradical
to be essentially degenerate, leaving the question open as to which (if either) is the lower
in energy.
hn
H
H
F
F
H
H
F
F