The potential energy function about the Cndash;C single bond for the ground state 1,3hyphen;butadiene has been derived fromabinitiocalculations at both the Hartreendash;Fock (HF) level with 6hyphen;31G, 6hyphen;31Gast;, and 6hyphen;311Gast;ast; basis sets and the secondhyphen;order Moslash;llerndash;Plesset perturbation (MP2) level with 6hyphen;31Gast; basis set with the complete geometry optimizations at each of 15 fixed CCCC dihedral angles; the total energies and optimized geometries for theshyphen;trans,gauche, andshyphen;cisconformers were also determined at MP2 level with 6hyphen;311Gast; basis set and the thirdhyphen;order Moslash;llerndash;Plesset perturbation (MP3) level with 6hyphen;31Gast; basis set. The second stable conformer of the butadiene is predicted to be agauchestructure from all the calculations with a CCCC dihedral angle between 35deg; and 40deg; and a barrier of 0.5ndash;1.0 kcal/mol to theshyphen;cistransition state, and the theoretical torsional potentials are in good agreement with the experimental potential function oftransndash;gauchendash;gauchecase derived by Durigetal.; by contrast, the theoretical torsional components differ significantly from the experimental results obtained from atransndash;cismodel. Vibrational frequencies and force field forshyphen;transandgaucheconformers of 1,3hyphen;butadiene are determined at the Hartreendash;Fock and MP2 levels with 6hyphen;31G, 6hyphen;31Gast;, 6hyphen;311G, and 6hyphen;311Gast; basis sets. The mean absolute percentage deviations of the calculated frequencies from the experimental values (not corrected for anharmonicity) are sim;10percnt;ndash;13percnt; and 3percnt;ndash;6percnt; for the Hartreendash;Fock and MP2 methods, respectively. The effects of polarization functions and electron correlation on the force fields are studied, and the additivity of correlation anddfunction effects are discussed. Comparisons are made with other force fields, including experimental and previousabinitioresults.
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