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Some molecular orbital computations of the internal rotational barrier heights in benzaldehyde and its 4-fluoro, 4-cyano and 4-hydroxy derivatives

1993; Elsevier BV; Volume: 281; Issue: 2-3 Linguagem: Inglês

10.1016/0166-1280(93)87083-p

1872-7999

Ted Schaefer, Rudy Sebastian, Frank E. Hruska,

Molecular spectroscopy and chirality

Some molecular orbital calculations on the title molecules imply that correlation-gradient computations will be necessary if the internal barriers are to agree with those deduced from torsional frequencies. For example, the energy difference between the planar and perpendicular conformers of benzaldehyde is 34.7 kJ mol−1 for an MP2/6-311G∗//6-31G∗(5D) computation. A calculation of the difference of the zero point energies of the two conformers, using somewhat less flexible basis sets, suggests that the enthalpy difference of the two conformers will not be substantially less than 32.5 kJ mol−1. The barrier height deduced from torsional frequencies is 19.3 kJ mol−1. Computations of the torsional frequencies, using STO-3G and 6-31G bases, suggests that the ensuing two-fold barriers are substantially lower than the computed energy and enthalpy differences of the two conformers for all the compounds. Comparisons are made with the measured barriers in condensed media. The computed dipole moments of the planar and perpendicular conformers imply that the solvent effects on the internal barriers differ in sign among the compounds. For 4-hydroxybenzaldehyde this sign depends on the orientation of the hydroxyl group relative to the CO bond. The need for certain new measurements and computations is emphasized.