LINEAR AND NONLINEAR POLARIZABILITIES OF POLYDIACETYLENE AND POLYBUTATRIENE CHAINS - AN AB-INITIO COUPLED HARTREE-FOCK INVESTIGATION

Ab initio Hartree-Fock 6-31G longitudinal polarizabilities and second hyperpolarizabilities of polydiacetylene and polybutatriene oligomers are computed and, then, extrapolated to the infinite polymer limit. Bo th the electronic and vibrational components are obtained with the lat ter evaluated in the double harmonic approximation. For static fields the electronic hyperpolarizability of the butatrienic form is almost 5 0 times larger than the acetylenic form; the ratio of the vibrational to the electronic component is 0.86 for the latter and 3.3 for the for mer. Thus, the two components are sensitive to the difference in bond length alternation (BLA) in different ways. Depending upon the particu lar nonlinear process it is shown that the vibrational hyperpolarizabi lity may remain large even at optical frequencies. Three intense Raman -active k=0 modes dominate the vibrational component. At the level of theory used here these modes occur in the frequency range 1000-2500 cm (-1) and they consist of changes in the BLA as well as hydrogen waggin g motions. For polydiacetylene, there is a 1:1 relation with the corre sponding vibrations that make the most important hyperpolarizability c ontributions in polyacetylene and polyyne. (C) 1997 American Institute of Physics.