Structures and Other Properties of Polyynes and their Isomers: Theoretical and Experimental Results - Polyynes: Synthesis, Properties, and Applications - page 370

hydrogen atoms bonded to the central carbon atom also has two imaginary
frequencies in the singlet ground state but none in the triplet ground state.
The six possible constructs produce only four molecules on the potential
energy surface of C 3 H 2 .
There has been much debate about the symmetry of 3O1(t). Is its ground-
state symmetry D 1 h or C 2 ? A consensus has emerged that the ground-state
symmetry is ''M-shaped C 2 '' (or ''W-shaped'' when one rotates the printed
page 180 ). Seburg and McMahon [51] have commented emphatically: ''The
conventional wisdom that 'triplet acetylenic carbenes are linear' is thus
incorrect in the case of the parent system'' (in this case C 3 H 2 ). However,
later in this section we shall present evidence which suggests a fluxional
molecule involving both D 1 h and C 2 geometries.
Absolute values of SCF energies vary somewhat depending on the
computational method used, hence will not be reported here. However, the
energy difference between two isomers should be less method-dependent.
SCF values from various sources including our
own data. Qualitatively, there is agreement among all results that isomer
3C1(s) has the lowest energy on the C 3 H 2 potential surface and that 3O1(t),
3O2(s), and 3O3(t) are the next lowest in that order. The table also reports
zero point vibrational energies (ZPVE), and dipole moments.

Table 17.5 reports these E

17.3.2 V IBRATIONAL D ATA

Fundamental vibrational frequencies were measured or calculated by many

investigators. We begin by reporting the data for 3O1(t) in Table 17.6 .

Entries from the literature are exact copies of data in references. When more
than one set of values is entered for a given reference, the results had been
obtained by various computational methods. The reader is referred to the
original publications for details [7,51,60]. Here we report only the data for
the configurations of lowest energies.

We now turn to the vibrational frequencies of 3O2(s), listed in Table
17.7 . The vibrational spectrum of this isomer is obviously dominated by the

CC stretches near 2000 cm 1 . Overall, the computed frequencies, especially
the more recent data, are within a few percent of the experimentally
observed values.
Next we report in the vibrational frequencies of isomer
3O3(t). Only few results are available because this isomer has not been
intensely studied. Hehre et al. [52] and Mebel et al. [65] have discussed the
isomer but have not calculated its vibrational frequencies. There are
significant differences between the results of Ochsenfeld et al. [46] and ours.
We find that the CC stretch/scissor frequency is nearer 1300 cm 1 than
2000 cm 1 . Whereas the spectrum of Ochsenfeld et al. [46] is dominated
by the intensity of the line at

Table 17.8

650 cm 1 , we find that the strongest line

750 cm 1 is at and that five other lines have intensities greater than

10 km mol 1 .