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A series of 2,1,3-benzothiadiazole derivatives has been studied by means of electron capture negative ion mass spectrometry (ECNI-MS), photoelectron spectroscopy (PES), absorption spectroscopy and AM1 quantum chemical calculations. It has been shown that the low-energy H-atom detachment from molecular negative ions occurs due to excitation of the hindered internal rotation of an OH group. Interpretation of the observed resonant states of molecular negative ions has been facilitated by the AM1 calculations. All molecules under investigation produce long-lived molecular radical anions as the dominant channel of negative ion formation. # 1998 John Wiley & Sons, Ltd. 'Received 3 February 1998; Revised 16 March 1998; Accepted 18 March 1998 The search for correlation between the electronic structure and biological activity of molecules is one of the most important problems of modern biochemistry. 2,1,3-Benzothiazole derivatives demonstrate biological activity in a wide range:1,2 they can modulate the growth and development of plants and insects, and they possess anti-virus and radioprotective properties. Corresponding mechanisms relate to inhibition of DNA synthesis and to the effect on fermentation activity and on the permeability of cell membranes. Their electron structure, however, has not yet been investigated. A combination of photoelectron spectroscopy, absorption spectroscopy and ECNI-MS, gives complete information about ionization energies, transition energies, and excited state energies of neutral molecules and negative ions. [3][4][5] In addition, a combined application of these methods is helpful for interpretation of experimental data. 6,7 EXPERIMENTALNegative ion mass spectra were obtained using a modified 8 MI-1201 mass spectrometer under the following conditions: accelerating voltage 3.6 kV, electron trap current $1 mA, full width at half maximum (FWHM) of electron energy distribution~E 1/2 = 0.3 eV, E el varied in the range 0-10 eV. Vaporization temperatures of compounds under investigation are in the range 340-350 K. Photoelectron (PE) spectra were obtained using a PE spectrometer (ES-3201) with a hemispherical kinetic energy analyzer.