Semi-Experimental (r_s/r_e) Structures for the Heavy Atom Backbones of Two Moderately Large Molecules Obtained from Microwave Spectroscopy and Quantum Chemical Calculations

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Microwave

Scientific paper

From recent microwave investigations of 1-methyl-4-piperidone and tropinone ground state rotational constants are available for the equatorial conformers of the normal species and the isotopologues with single substitution of all the heavy atoms. Vibration-rotation constants (alphas) were computed with Gaussian 03 (G03) for the B3LYP/cc-pVTZ model and used to convert ground state rotational constants into equilibrium rotational constants. Using the Kraitchman equations (RS method), the equilibrium (RE) Cartesian coordinates were determined for all the heavy atoms in the principal axis framework. Equilibrium bond lengths and bond angles are compared with those computed with the B3LYP/cc-pVTZ model. We have compared the ground state rotational constants computed with G03, after scaling by factors based on the normal species, with observed values. The agreement is within 0.1% for the full set of constants (0.04% for methyl-piperidone and 0.1% for tropinone). This agreement between experiment and theory is so good that it seems possible to use calculated ground state rotational constants in place of experimental ones for determining RS/RE structures for organic molecules of this size.
L. Evangelisti, A. Lesarri, M. Jahn, E. Cocinero, W. Caminati, J.-U. Grabow J. Phys. Chem. A, submitted.
E. J. Concinero, A. Lesarri, P. Ecija, J.-U. Grabow, J. A. Fernandez, F. Castano PCCP 12, 6076-6083 (2010).
} ground state rotational constants are available for the equatorial conformers of the normal species and the isotopologues with single substitution of all the heavy atoms. Vibration-rotation constants (alphas) were computed with Gaussian 03 (G03) for the B3LYP/cc-pVTZ model and used to convert ground state rotational constants into equilibrium rotational constants. Using the Kraitchman equations (RS method), the equilibrium (RE) Cartesian coordinates were determined for all the heavy atoms in the principal axis framework. Equilibrium bond lengths and bond angles are compared with those computed with the B3LYP/cc-pVTZ model. We have compared the ground state rotational constants computed with G03, after scaling by factors based on the normal species, with observed values. The agreement is within 0.1% for the full set of constants (0.04% for methyl-piperidone and 0.1% for tropinone). This agreement between experiment and theory is so good that it seems possible to use calculated ground state rotational constants in place of experimental ones for determining RS/RE structures for organic molecules of this size.

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