  First systematic spin probe ESR study of water freezing has been conducted
using TEMPOL and TEMPO as the probes. The spin probe signature of the water
freezing has been described in terms of the collapse of narrow triplet spectrum
into a single broad line. This spin probe signature of freezing has been
observed at an anomalously low temperature when a milimoler solution of TEMPOL
is slowly cooled from room temperature. A systematic observation has revealed a
spin probe concentration dependence of these freezing and respective melting
points. These results can be explained in terms of localization of spin probe
and liquid water, most probably in the interstices of ice grains, in an ice
matrix. The lowering of spin probe freezing point, along with the secondary
evidences, like spin probe concentration dependence of peak-to-peak width in
frozen limit signal, indicates a possible size dependence of these
localizations/entrapments with spin probe concentration. A weak concentration
dependence of spin probe assisted freezing and melting points, which has been
observed for TEMPO in comparison to TEMPOL, indicates different natures of
interactions with water of these two probes. This view is also supported by the
relaxation behavior of the two probes.
