Far-infrared spectra of ring compounds. XIII. The spectrum and structure of 1,4-dioxacyclohexadiene-2,5

The far‐infrared absorption spectrum of gaseous 1,4‐dioxacyclohexadiene‐2,5 in the range 230–20 cm⁻¹ consists of a regular series of Q‐branches decreasing in intensity toward high frequency. They have been assigned to various transitions of the B_2u ring bending mode and from them the potential function V(x) of a one‐dimensional Hamiltonian has been evaluated: $\mathrm{V(x)\hspace{0.17em}}{\mathrm{in\; cm}}^{\text{−1}}{\text{= 0.2949× 10}}^4\hspace{0.17em}{\mathrm{x}}^2{\text{+0.2930× 10}}^6{x}^4$ . in which the variable x is the usual ring bending coordinate. This expression of V(x) results from a fixed reduced mass of 99.64 amu. When the reduced mass is allowed to vary with x, higher‐order kinetic energy terms are introduced into the Hamiltonian and a slightly different potential function is obtained: V(x) in ${\mathrm{cm}}^{\text{−1}}{\text{= 0.8251× 10}}^3{x}^2{\text{+0.4702× 10}}^6{x}^4$ . The latter treatment gives somewhat better agreement between calculated and observed Q‐branch frequencies. Results of a normal coordinate calculation for the 24 vibrations of dioxadiene are also presented.