THEORETICAL-STUDY OF THE ELECTRONIC-STRUCTURE AND HYPERPOLARIZABILITIES OF DONOR-ACCEPTOR CUMULENES AND A COMPARISON WITH THE CORRESPONDINGPOLYENES AND POLYYNES

The structures of an extended series of cumulenes containing a donor a nd acceptor group situated at either end of the chain have been calcul ated using the AM1 method and the 3-21G basis set. The dipole moments increase sharply with increasing chain length and show little sign of saturation even at long chain length. The hyperpolarizabilities of the se structures calculated using a sum-over-states method also increase with increasing chain length to give very large values at extended cha in length. Related calculations on the donor-acceptor polyenes show th at the starting structure adopted for the subsequent calculation of bo th the dipole moment and hyperpolarizability has a large effect on the result, particularly for long chain systems. At short chain lengths o f up to eight carbons, the polyenes are predicted to have larger hyper polarizabilities than-the cumulenes, but the position reverses with fu rther chain extension; both have greater values than those calculated for the corresponding polyynes.