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Ab initio Hartree-Fock and second-order Moller-Plesset perturbation theory (MP2) calculations have been performed on the cis tautomer of porphyrin, an intermediate in the process of N-H tautomerism of free base porphyrin. According to our best estimate (MP2), cis-porphyrin is higher in energy than the trans tautomer by about -7.6 kcaymol. The structure of cis-porphyrin exhibits no significant nonplanarity and differs from that of trans-porphyrin mainly in the internal bond angles of the central C12N4 ring.It is now generally agreed that the N-H tautomerism of free base porphyrins occurs in a stepwise manner, proceeding via transient cis-porphyrin intermediates which quickly tunnel to the transThe proton migration is also believed to be strongly coupled to deformation of the porphyrin skeleton. The complexity of the process has so far precluded a thorough understanding of its dynamics. In particular, knowledge about the portion of the potential energy surface that includes the metastable cis tautomer would be valuable. The following are some of the relevant questions: How does a cis configuration of the central protons affect the skeletal geometry of porphyrin? Does the proximity of central protons in cis-porphyrin result in significant nonplanarity of the skeleton? Is there any significant electronic difference between the cis and trans tautomers of porphyrin? What is the relative stability of the cis and trans tautomers?A number of semiempirical theoretical studies on the molecular geometry of free base porphyrin, using such Hamiltonians as Q C F F / P~,~ spin-restricted AMl,5 etc., have predicted a Cz, symmetric structure for the ground state of trans-porphyrin, with the Cz axis coinciding with the N-I+ -.H-N vector. Spinunrestricted AM1 optimizations5 did lead to a Dzh symmetric geometry of porphyrin but predicted (unrealistic) nonzero a and / 3 spin populations on alternate carbon atoms of the free base porphyrin molecule. Even at the ab initio spin-restricted Hartree-Fock level, geometry optimizations of trans-porphyrin result in unrealistic CzV structures with alternating single and double These problems are due to a lack of inclusion of electron correlation effects in the methods employed. Second-order Moller-Plesset perturbation theory and density functional theory result in D2h symmetric structures of free base porphyrin, in agreement with experiment.8 This rather long history of confusion in theoretical studies on the structure of porphyrin prompted us to choose fairly high-quality methods and basis sets in this work.gThe structures of cis-and trans-porphyrin were optimized with the density functional program DMOL'O using the local exchange-correlation potential due to von Barth and Hedin' ' and a numerical double-c plus polarization ("DNP") basis set. Only a C2 (as opposed to CzV) symmetry constraint was employed to allow for possible out-of-plane distortions of the macrocycle in cis-porphyrin. A DZh constraint was used for trans-porphyrin.6 Ab initio Hartree-Fock (HF) and secondorder Mgller-Plesset pe...

In the present work large-scale contracted CI calculations have been performed in order to investigate the bonding situation in Fe(C0)5. Near degeneracy correlation effects are shown to be mainly responsible for the failure of the Hartree-Fock model to correctly reproduce the metal-to-ligand distances (axial and equatorial) and the corresponding force constants. In addition, some aspects concerning the computational treatment of large transition-metal systems are discussed.

The fully halogenated benzenes C6F6, C6C16, C6Br6, and C6I6 have been studied in a series of all-electron ab initio Hartree-Fock calculations using basis sets of double-¿"quality or better. Geometry optimizations have been performed with the aim of resolving controversies among experimental results with regard to the planarity of the heavier molecules in the series. The compounds are all found to be planar, and the present results therefore reject the notion of geometrical distortions due to steric overcrowding in these compounds.

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