Collect. Czech. Chem. Commun. 1993, 58, 1969-1977

Indication of Odd Number of Equivalent Atoms in an EPR Spectrum by the Fourier Transform

Oldřich Dračka

Department of Theoretical and Physical Chemistry, Masaryk University, 611 Brno, Czech Republic


A numerical criterion is proposed for the indication of odd number of equivalent atoms in an EPR spectrum. Physically, this "oddness" criterion is the relation for the content of negative and positive parts of the imaginary part of the EPR spectrum Fourier transform, provided that the spectrum x-coordinate start is shifted into the spectrum symmetry center. The effect of noise present in the spectrum is analysed and the best limit of Fourier transform noise effect minimalization is proposed. With even numbers of equivalent atoms in EPR spectrum only, the oddnes criterion value for the noiseless spectrum is zero and the deviation from this value is due to the effect of the noise. A criterion value greater than 0.3 indicates the presence of at least one well-resolved component with an odd number of equivalent atoms, with spin 1/2 in the spectrum. A value smaller than 0.3 indicates the absence of such component. However, if this value is greater than 1.0, the presence of either a well resolved component with a single atom with spin 1 or an ill-resolved component with an odd number of equivalent atoms with spin 1/2 is indicated. Application of the proposed criterion to noise-loaded synthetic EPR spectra is exhibited. The possibility of identifying methyl group splitting by a sharp decrease of the proposed criterion value after the spectrum contraction with the proper splitting constant is demonstrated.