EPR SPECTROSCOPY AND MNDO CALCULATIONS FOR THE INTERPRETATION OF THE H-4(+) CENTER DETECTED IN OXYNITRIDE GLASSES

gamma-irradiated oxynitride glass was investigated by EPR spectroscopy at the X-band. A paramagnetic centre was detected at high (10 dB) mic rowave power. The high power spectrum was attributed to the H-4(+) cen tre. The signal of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) in eth anol solution at 116 K was also detected. The simulation of both spect ra yielded the hfs spectroscopic parameters A(x)=101 . 41, A(y)=5 . 60 and A(x)=5 . 60 MHz. The linewidths of the signal were 5 G in the gla ss and 14 G in TEMPO. The ratio of the g value strain distributions we re 1200, 1, and 1500 for Delta(x), Delta(y), and Delta(z), respectivel y. The hfs constants allowed the calculation of the spin densities of the 2s(N), and 2p(N), orbitals. These spin densities were compared wit h the spin densities of several centres calculated using a MNDO progra m. This comparison suggested that both centres are due to (=R(R=Si,C)) (2)N-O structure. This new model for the H-4(+) centre is confirmed b y the comparison between hfs parameters and line shape of the two cent res' signals. Furthermore there is able experimental and theoretical e vidence for the explanation of the paramagnetic centre obtained in nit roxide spin labels such as TEMPO that have attributed this state to (= R(R=Si,C))(2)N-O. Linewidth differences ave due to the separate spin lattice relaxation time constants affected by the lattice vibrations o f the host environment. The g strain is greater for the H-4(+) centre due to the random Si-N-Si bond angles while the C-N-C bond angle in th e TEMPO molecule are move restricted in the well defined environment.