The theoretical description of the (ππ*) excited states of ethylene

✍ Scribed by T.H. Dunning Jr.; W.J. Hunt; W.A. Goddard III

• Publisher: Elsevier Science
• Year: 1969
• Tongue: English
• Weight: 452 KB
• Volume: 4
• Category: Article
• ISSN: 0009-2614
• DOI: 10.1016/0009-2614(69)80081-3

✦ Synopsis

It is shown that the excited singlet ($8 o --) state (v) of planar ethylene in both the Llarkee-Fdck zn& z-electron configuration interaction descriptions is quite diffuse. The extent of the F-orbital in the ii-direction as measured by (22) is 42 a$ in the V state as compared to 2.7 a; in the corresponding triplet state (T). In addition, we find that the Hartree-Fock vertical excitation energy for the V state is 7.42 eV as compared to the experimental value of 7.6 eV. The excited states of ethylene which are of most interest both experimentally and theoretically are the singlet and triplet states arising from the (air*) configuration.

If the molecule lies in the xy-plane with f along the CC bond axis, these states are of B3u symmetry and have been designated V(lB3,)

and T(3B3u) by MuIliken. The (n)2 ground state is denoted as N(IAlg).

The experimental resu1t.s have been interpreted as involving a vertical excitation energy of 4.6 eV for the T state [l] and 7.6 eV for the V state [2]. Previous theoretical calculations [3 -61 (Hartree-Fock) for planar ethylene with the ground state geometry have given = 3.4 eV as the excitation energy (AE) for the T state and 9.3 -12.0 eV for the V state. The low AE for the T state is understandable since there should be about 1 eV less correlation energy in the triplet state than in the ground state. However, these results imply that the V state has 1.7 eV name correlation energy than the ground state. One would normally expect the correlation

energy for an open-shell state to be less than a closed-shell state of similar electronic structure, so there have been a number of attempts to determine the origin of the extra correlation in the V state [6]. We shall see below that previous SCF calculations on the V state used inadequate basis sets and that the use of a more flexible set leads toAE(V) = 7.42 eV and, thus, 7 Partially supported by a grant (GP-6965