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Keywords:

  • benzyl alcohol;
  • benzyl fluoride;
  • microwave spectroscopy;
  • ab initio calculations;
  • molecular structure;
  • molecular conformations;
  • OH…π interactions

Abstract

The conformations of several benzyl compounds, C6H5CH2X, have been characterized by rotational spectra and theoretical structure calculations. We have observed the microwave spectrum of benzyl alcohol and its OD isotopomer at high resolution in a pulsed-jet Fourier transform microwave spectrometer. The spectrum is consistent with an asymmetric stable conformation characterized by a C[BOND]C[BOND]C[BOND]O dihedral angle of approximately 60°. The spectrum is strongly perturbed by tunneling interactions. The transitions are split into doublets consistent with tunneling interactions between two equivalent conformational minima. The observed splittings diminish upon deuterium substitution. These observations are similar to those previously reported by us for benzyl fluoride. However, the minimum energy conformation for benzyl fluoride has the C[BOND]C[BOND]F plane orthogonal to the plane of the phenyl ring. This conformation for benzyl fluoride is consistent with a model that minimizes steric repulsion between the fluorine of the [BOND]CH2F group and the closest hydrogens located on the phenyl ring. For benzyl alcohol, previous studies have suggested a weak attraction between the π electrons of the phenyl ring and the substituent [BOND]OH group as the explanation for the observed stable conformation. A theoretical analysis of the atomic charges in benzyl alcohol suggests another possible explanation of the observed structure. Atomic charges generated by fits to the electrostatic potential indicate a relatively strong dipole–dipole coupling between the [BOND]C[BOND]H group in the methylene side chain and the closest [BOND]C[BOND]H group in the phenyl ring. This results in a nearly planar orientation of the [BOND]C[BOND]H group in the methylene side chain with the phenyl ring, a conformation that yields the observed C[BOND]C[BOND]C[BOND]O dihedral angle. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004