POLYMERIZATION OF MOLECULAR (YELLOW) ARSENIC

DTA studies were performed to investigate the polymerization of molecu lar (yellow) arsenic (y-As), in the dark and also under illumination/i rradiation. The semiempirical quantum chemical CNDO/BW method for calc ulating the dimerization reaction pathway of two As-4 molecules was ap plied, and showed that the stability of y-As decreases, due to the for mation of stable fragments of the polymeric phase (a-As). Using the pr inciple of orbital symmetry conservation, it was shown that polymeriza tion takes place most favourably from a staggered ''face-to-face'' con figuration of As-4 molecules. Dark polymerization proceeds under activ ation. For a result of the initial stage of polymerization, CNDO/BW si mulation predicts the formation of an As-8 dimer. The molecular cluste r As-8 of D-2h symmetry forms a six-membered chair-shaped ring which c onstitutes one of the dominating units in the polymeric phase (a-As).