Normal coordinate analysis of disilenes and diphosphenes. The upsilon(Si=Si) and upsilon(P=P) stretching vibrations

Normal coordinate analysis (NCA) of some disilene and diphosphene molecules has been performed and compared with NCA of corresponding alkenes. For various models of disilenes containing the C2Si=SiC2 moiety, NCA shows that Si=Si and Si-C stretching coordinates are heavily mixed, their in-phase combination resulting in a normal mode in the region 450-550 cm(-1), while their out-of-phase combination is in the region 600-750 cm(-1). In contrast, the results of NCA for diphosphenes show that the vibration at approximate to 600 cm(-1) is well localized, being predominantly nu P=P stretch ( approximate to 90% in PED). (C) 1997 Elsevier Science B.V.