Association forms of NO in sodium ion-exchanged A-type zeolite: Temperature-dependent Q-band EPR spectra

Nitrogen monoxide (NO) introduced into sodium ion-exchanged A-type (Na-A) zeolite was studied by temperature- and NO pressure-dependent EPR measurements at Q-band in the temperature range between 5 and 140 K. The experimental spectra were successfully utilized to determine accurate values of the g and A(N-14) tensors of NO monoradical and the g tensors and zero field splitting (ZFS) parameters, D and E, of the NO-NO biradical. Two different EPR spectra corresponding to low and high temperature forms of the NO monoradical were observed when the NO pressure was low (P-NO less than or equal to 3.1 kPa). Based on the g and A tensors, the former was attributed to a rigid form and the latter to a rotational form of the monoradical. For higher NO pressure, the biradical became dominant. The structure of the biradical was discussed in terms of the D and g tensors combined with the spectral line shape simulation method.