FAR-INFRARED SPECTRA AND RING-PUCKERING POTENTIAL-ENERGY FUNCTION OF 4H-PYRAN - CONFORMATIONS AND BONDING OF 1,4-CYCLOHEXADIENE AND ITS OXYGEN ANALOGS
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The far-infrared, mid-infrared, and Raman spectra of 4H-pyran have been recorded and analyzed. The far-infrared gas-phase spectrum of this molecule shows a series of 12 bands between 115 and 128 cm1 arising from the ring-puckering vibration. The kinetic energy (reciprocal reduced mass) expansion was calculated for this mode and utilized in the determination of the potential energy function, which was found to be V (cm1) = (1.21 105)x4 + (2.24 104)x2, where is the ring-puckering coordinate in angstroms. This shows the ring to be planar. The corresponding potential energy functions for the analogous 1,4-cyclohexadiene and 1,4-dioxin are respectively nearly harmonic and nearly quartic. Molecular mechanics (MM3) calculations satisfactorily reproduce the 1,4-cyclohexadiene potential function but underestimate the torsional forces of the C=C-O-C system in the oxygen analogues. In fact, 1,4-dioxin is incorrectly predicted to have a boat configuration. It is shown that weak interactions between the C=C bonds and the nonbonded oxygen orbitals account for the increases in the torsional forces and the resulting potential functions. 1993, American Chemical Society. All rights reserved.