The near ultra-violet spectrum of tropolone vapour and its relevance to the molecular structure

Abstract

The two most intense bands of the 370 nm electronic band system of tropolone have been rotationally analysed. They are separated by 18·93 cm-1 and it has been shown that the high wavenumber band is the 0--0- (H1 1) transition in what is almost certainly the internal hydrogen-bonding vibration v H : the low wavenumber band is the 0+-0+ (00 0) transition. A rotational contour analysis of both bands shows that there is an intensity alternation in K a″ such that the ratio K a″ even : odd is 10 : 6 in the 0+-0+ band and 6 : 10 in the 0--0- band. The intensity alternation, the nearly equal intensities of the 0+-0+ and 0--0- bands, the separation of these two bands and the anharmonic behaviour of v H show that the separation of the 0+ and 0- levels is small in the ground electronic state (probably less than 50 cm-1) and is 18·93 cm-1 larger in the excited electronic state. The 0+-0+ and 0--0- bands are both type B showing that the electronic transition is à 1 B 2-X 1 A 1 and therefore π*-π rather than π*-n. The π*-n transition is probably shifted to high wavenumber by the internal hydrogen-bonding.