Electronic assignment as1A′(ππ*)-1A′ of the 2980 Å system of 2-aminopyridine by rotational band contour analysis

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Abstract
In 2-aminopyridine the electronic origin band of the 2980 å electronic system, the longest wavelength system which has been observed, is shown to be a type B-type A hybrid band containing 55 ± 10 per cent of type A character. It follows that the electronic transition is 1 A′-1 A′ which implies a π*-π and not a π*-n electron promotion. The degree of type A character is interpreted as indicating a swing of the transition moment by 10 ± 10° from the b axis of pyridine (the in-plane axis perpendicular to C 2). Since it is expected that the first π*-π transition in pyridine has its transition moment polarized along the b axis the amino-group does not seem to perturb the pyridine transition strongly in this respect. The changes of rotational constants from the ground to the excited state in 2-aminopyridine are very similar to those in aniline and probably reflect similar changes in geometry, namely a substantial contraction of the C-NH2 bond, an opening of the internal ring angle adjacent to the substituent and a benzene-like expansion of the ring. The origin of the 0-0 band is at 33471·1 ± 0·1 cm-1.

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