Ab initio prediction of extremely large first hyperpolarizability of polyphosphaacetylene and polyphosphasilyne

Calculations at MP2/6-31G(d) level predict a very large first hyperpolarizability ( β) for two phosphorus-containing oligomers, namely, polyphosphaacetylene, – ( P CH ) n – and polyphosphasilyne, – ( P SiH ) n – , i.e., 212 777 and 90 811 a.u., respectively, at n = 16. These values are 5.3 and 2.3 times of that for the nitrogen analogue of polyphosphaacetylene, that is, polymethineimine. In striking contrast, polyborophosphene, – ( PH BH ) n – , is predicted to have extremely small β values, with a β/ n value approaching zero at large n. The causes for the relative β values in these four series are discussed, in light of the bonding structure, the π bond strength, the size of the nuclei, and the degree of delocalization.