CHARACTERISTIC VIBRATIONAL FREQUENCIES OF COMPOUNDS CONTAINING Si—O—Si, P—O—P, S—O—S, AND Cl—O—Cl BRIDGING GROUPS FORCE CONSTANTS AND BOND ORDERS FOR THE BRIDGE BONDS

Abstract

The assignment of characteristic frequencies for the X—O—X group in compounds containing Si—O—Si, P—O—P, S—O—S, and Cl—O—Cl bonds is discussed with reference to the vibrational spectra of disiloxanes, diphosphoryl, and disulphuryl compounds and dichlorine heptoxide. The stretching force constants of the X—O—X bridging bonds have been obtained by treating the molecules as simple triatomic species X 2 O • X—O bond orders and bond lengths have been calculated from the stretching force constants and the calculated and observed bond lengths are compared. The variation in the observed X—O—X angles is related to the bond orders and is discussed in terms of the electron-pair repulsion theory. It is shown that bond orders in the isoelectronic series Si 2 O 7 6− , P 2 O 7 4− , S 2 O 7 2− , and Cl 2 O 7 may be simply deduced from the empirical rule that the valency shell of a second-row element tends to be occupied by twelve electrons (the duodecet rule).