5502 Figure 1. PhotFelectron spectrum of 1,4-~yclohexadiene, obtained using the 304-A helium line with interpretation according to a SPINDOil calculation. For the ?r orbitals the symmetry is indicated. lo ' l1 carbon. For the carbon atom USs = -48.289 eV and Slater exponent = 1.925. In the resonance integrals H p v = S f i v [ I p A + IvBlf(RAB) the factors f have different values for different types of interaction: for ls/ls, J(R) = 0.13647; for ls/2s, f(R) = 0.17832; for ls/2pu,f(R) = 0.35100; for 2s/2s, f ( R ) = 0.20187 + 0.09500/R2; for 2s/2pu, f ( R ) = 0.27625 + 0.13000/R2; for 2pu/2pu,f(R) = 0.47000 + 0.24000/R2 + 100 exp(-5R); for 2p~/2p7r, f(R) = 0.40375 + 0.19000/R2. The parameters were chosen to give reasonable agreement in the case of benzene (with interpretation according to Jonsson and Lind-h0lm4), methane, ethane, ethylene, and acetylene.To demonstrate the usefulness of the new procedure we will in this communication apply it to two molecules: 1,4-cyclohexadiene and trans-l,3-butadiene.In the SPINDO calculation of cyclohexadiene the "planar" geometry discussed by Oberhammer and Bauers was used (cJ ref 6,7). The SPINDO orbital energies are compared in Figure 1 with the photoelectron spectrum, observed in our photoelectron spectrometer.8 Its low-energy part agrees well with an earlier study.g It can be seen that the distribution of the orbital energies corresponds well to the maxima of the curve. This supports our method and indicates that also the form of the molecular orbitals is obtained approximately correctly from the SPINDO calculation.The most interesting feature of the molecular orbitals of 1,4-cyclohexadiene is that the u orbitals are nearly identical with the corresponding orbitals in benzene. Also the orbital energies are nearly the same as in benzene, although, of course, the degenerate benzene orbitals are split. The introduction of two extra hydrogens in cyclohexadiene does not influence the uorbital system much.The P orbitals of 1,4-cyclohexadiene have no similarity with those of benzene. Owing to the two extra hydrogens the cyclohexadiene has four P orbitals. The two highest are in-phase @ and out-of-phase @ combinations of the ethylene T orbitals and the two lowest are similar combinations of the CH2 orbitals, @ and 0. That the in-phase combination @ is highest depends upon the destabilizing influence of the two CH, orbitals. This result is in good agreement with earlier (4) B. 0. Jonsson and E. Lindholm, Ark. Fys., 39,65 (1969). (5) H. Oberhammer and S . H. Bauer, J . Amer. Chem. SOC., 91, 10 (6) G. Dalinga and L. H. Toneman, 2L 22 20 18 16 1L 12 10 8 CV Figure 2. Photoelectron spectrum of butadiene16 compared with orbital energies from SPINDO/l and from a n ab initio c a l c~l a t i o n .~~ The band at 22.4 eV was observed by Brundle and Robin.21treatments of interaction through bonds10-12 in this molecule.In the SPINDO calculation of butadiene the geometry given by Haugen and Traettebergl3>l4 and Kuchitsu, et a1.,15 was used. The SPINDO orbital energies are compared in Figure 2 w...