THEORETICAL PREDICTION OF THE VIBRATIONAL-SPECTRUM OF FLUORENE AND PLANARIZED POLY(P-PHENYLENE)

The a priori predicted vibrational spectra of fluorene and newly synth esized methylene bridged planarized poly(p-phenylene)(PPP) are present ed. The calculated vibrational frequencies of fluorene are compared to those obtained experimentally to resolve differences between earlier experiments. As a result, some reassignments are made. The vibrational spectra of planarized PPP are extrapolated using oligomers as a start ing point within the scaled quantum mechanical oligomer force field (S QMOFF) method.; The basis get used is split valence double-zeta qualit y 3-21G. The scaling factors are transferred from benzene and cyclopen tadiene as fixed parameters in the polymer calculations. Planarized PP P is predicted to have an inter-ring stretching frequency of 1338 cm(- 1), an upward shift as compared to that of PPP (about 1280 cm(-1) expe rimental value). This shift is largely attributed to the increase in t he rigidity of the polymer backbone due to the presence of the bridgin g group. This frequency shift is also indicative of a higher degree of conjugation due to the planar conformation of methylene bridged PPP. The predicted Raman intensity ratio of the A, modes is compared to tha t predicted by the effective conjugation coordinate (ECC) theory. Beca use of the presence of the bridging methylene group, the predictive po wer of the ECC theory is limited. The predicted IR spectrum shows a st rong peak at 844 cm(-1), a characteristic C-H out-of-plane bending fre quency of a 1,2,4,5-tetrasubstituted phenyl ring. Besides the three ne w bands originating from the CH2 bridge, we predict two more strong sk eletal bands, at 1440 and 1351 cm(-1), in the IR spectrum of planarize d PPP.