4804
Bonding Contribution of Sulfur d Orbitals in Thiophene. An Extension of the Self-consistent Field Molecular Orbital Method' Michael J. Bielefeld' and Donald D. Fitts Coritrihution f r o m tile Department of Chemistr), and the Laboratory f o r Research ihe Sinic fure of M a f fer, Unicersiij. of Pennsylc-ania, Philadelphia, Pciinsy lcania. Receiged March 18,1966
oil
Abstract: The 3d,, and 3d,, atomic orbitals on sulfur are included in the linear combination of atomic orbitals in a semiempirical self-consistent field molecular orbital study of thiophene. The extension of the SCF MO method to include more than one atomic orbital per atomic site is accomplished by a point-charge model for the evaluation of two-center repulsion integrais. A comparison of the SCF molecular orbitals with and without the inclusion of these higher atomic orbitals shows that the d orbitals participate in the R-electronic structure of thiophene to only a small extent, but that their pxticipation alrects the calculated electronic properties to a great extent.
In
a study which pioneered the extension of the Huckel niolecular orbital (MOj theory:] to include heterocyclic systems, Wheland and Pauling4 applied the MO method to the thiophene molecule for the first time. Their treatment of thiophene implicitly assumed that the sulfur atom in the molecule contributes only a 3 p I atomic orbital (AO) to the 7r electronic MO wave functions when thc latter are written as a linear combination of atomic orbitals (LCAO). Schoniaker and Pauling' later suggested that resonance forms contributing to the structure of thiophene include those in which the sulfur atom violates 'the octet rule and the M shcll is evpanded to hold ten electrons: i.e.. the 3d orbitals on t h e sulfur atom are partially occupied in the ground statc of the molecule. Lon gue t - H i ggi n s in c orp o ra teii this idea in a H iickel M O calculation on thiopliene by eniploying 3pZ-3d,,3d,: hybrid atomic orbitals on the sulfur atom. This approach with sonic variatioiis has bccii further applied t o thiophene a n d other sulfiir-cniit;iining molecules by many :iuthor s.'; A controversy sliil exists as to whether d-orbital participation should bc included i n MO calculations on sulfur-containing Iicterocyclic molecules. Although the inclusion of the d orbitals on sulfur has proved .' authorsq-12 believe that useful in some c ~ s c ' s , ~many fi
( I ) This stuilj \\:is tupportcd in part h q the Advanced Resc'rrch Projects Agciicq uiidcr Contract SD-69; thij u o r k rcprsscnts part of n disscrtiitioii i n Chemistry submitted by M. J. Bielefcld in partial fulfillmsiit of the rcquirernents for the P1i.D. dcgi-ce from the University of Pcnnsyl\.;it1 iii
thiophene and related molecules may be successfully treated without their explicit consideration. Moreover, some authors feel that d-oribital participation has never been proved and state that calculations based on the Longuet-Higgins model are fallacious. l 3 Both of these approaches to the MO study of thiophene are somewhat unsatisfactory in that they bias the calculation either by omitting the d orbitals entirely or by including their participation arbitrarily. I n an effort to overcome this difficulty, we present in this article an MO calculation on thiophene i n which the possibility of d-orbital participation is admitted by including the 3d,, and 3d,,? orbitals on the sulfur atom in the basis set of atomic orbitals in the LCAO approximation. Since the energy of an electron in a 4p orbital of atomic sulfur is comparable to that for an electron in a 3d orbital, we also include the 4p, A 0 on sulfur in the basis set for thiophene. For purposes of comparison, we make three separate LCAO MO calculations on thiophene. The first considers only the 2p, orbitals on the carbon atoms and the 3p, orbital on sulfur. The second includes two additional orbitals: the 3d,, and 3d,, atomic orbitals on the sulfur atom. The third calculation adds the 4p2 atomic orbital on sulfur to the basis set in the LCAO approximation. Hereafter, we refer to these three choices for the basis set as the five-orbital model, the seven-orbital model, and the eight-orbital model, respectively. Thus, the molecular orbitals @ Lfor the 7r electrons of thiophcne are written in the form
( 2 ) Nutioncil Science Foundation Predoctoral Fellow, 1962-1964. (3) E. Hiickcl, 2. Phj.sik, 70, 204 (1931). (4) G . W. Wheland and L. Pauling, J. A m . Chem. Soc., 57, 2086
(1935).
(5) V. Schomaker and L. Pauling, ibid., 61, 1769 (1939). (6) H . C. Longuet-Higgins, Trans. Furudflj, SOC.,45, 173 (1949). , 52 (1954); J . de (7) J . Metzgcr and F. Rufflcr, J . Chin7. P h j . ~ . 51, Hecr, J . Ani. Chem. SOC.,76, 4802 (1954); L. Melander, Arkiti Kenzi, 8, 361 (1955); I
