Enhancement of Through-Bond Coupling by Bond-Angle Distortion: Pyrazine

The cyclobutapyrazine derivative, endo-3,4-pyrazinotricyclo[4.2.1.02,5]nonane (3), has been synthesized and subjected to a spectroscopic investigation. An inequivalence of the two faces of the pyrazine π system is revealed by the measured relative polarization directions of the 1nπ* and 1ππ* electronic transitions and is ascribed to a delocalization of the pyrazine orbitals into the low-symmetry saturated polycyclic system. A striking red shift of the 1nπ* transition in 3 relative to 2,3-dimethylpyrazine and a similar shift of the n+ionization energy to lower values are ascribed to an enhanced through-bond coupling of the two nitrogen lone-pair orbitals due to distortion of bond angles at carbons 2 and 3 and to the resulting enhancement of the p character of the σ bond orbital through which the coupling occurs. © 1982, American Chemical Society. All rights reserved.