VIBRATIONAL-SPECTRA OF BUTATRIENE (C4H4) AND PENTATETRAENE (C5H4) - IS PENTATETRAENE BENT

Abstract

The vibrational spectra of butatriene (C4H4) and pentatetraene (C5H4) are predicted with use of the configuration interaction method and double-zeta (DZ) basis sets with/without polarization functions. The experimental assignment for most of the vibrations of C4H4 agrees with the present theoretical predictions for the linear structure, but some tentative Raman assignments (for CH2 rocking, wagging, and the b2g CCC bending) seem to be questionable since they differ from the best theoretical evaluations by as much as 60%. The vibrational spectrum of C5H4 is completely characterized for the first time, and it is found that there are large discrepancies between most of the experimentally assigned IR bands and the present theoretical frequencies obtained for the linear structure. It is also shown that the CCC bending frequencies in C4H4 and C5H4 are extremely sensitive to basis set and electron correlation method. Thus, the DZ, 6-31G, and triple-zeta (TZ) basis sets, when used in conjunction with methods explicitly incorporating the effects of electron correlation, give quite different predictions of the vibrational frequencies for CCC bending in these molecules. The second-order Moller-Plesent perturbation and the single and double configuration interaction methods are also significantly different in this respect. All theoretical methods except TZ MP2 and those involving the 6-31G basis set predict that the carbon skeleton of pentatetraene is linear. However, it is concluded that more experimental studies of C5H4 are necessary to characterize the structure for its ground electronic state.