VIBRATIONAL ANALYSIS OF NEAR ULTRAVIOLET SPECTRUM OF 2,1,3-BENZOTHIADIAZOLE AND ANALOGOUS SPECTRA OF BENZOFURAZAN AND 2,1,3-BENZOSELENADIAZOLE

Following a rotational band contour analysis of type B and type A bands in the 3280 Å system of 2,1,3-benzothiadiazole [2] this system is here vibrationally analysed and shown to be electronically allowed 1B2-1A1. About one fifth of the intensity of the system is due to type A bands, lA1-1A1, in which b2 vibrations are active but the system is dominated by an a1 vibration of wavenumber 485·0 cm-1 which is tentatively assigned as a C5C4C9 angle bending vibration. This is consistent with the observed [2] contraction of the molecule about the long in-plane axis in the excited state and with the fact that the corresponding electronic systems in 2,1,3-benzoselenadiazole and benzofurazan are dominated by a vibration which also has a wavenumber of about 500 cm-1. Since the vibration is not affected by the change of atom in the five-membered ring it is probably a vibration of the six-membered ring. A rotational band contour analysis of the origin band of the 3560 Å system of 2,1,3-benzoselenadiazole shows that this is a type B band and that this system is also electronically allowed 1B2-1A1. At lower wavelength all three molecules have a stronger system which may be electronically allowed 1A1-1A1. However the only evidence for this assignment is (a) the intensity stealing from a 1A1-1A1 electronic transition in the 3280 Å system of 2,1,3-benzothiadiazole, and (b) the fact that the low wavelength system has the appearance of being electronically allowed showing a very strong band at the onset of the system. © 1969.