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Context. The high degree of deuteration observed in some prestellar coresdepends on the ortho-to-para H2 ratio through the H3+ fractionation. Aims. Wewant to constrain the ortho/para H2 ratio across the L183 prestellar core. Thisis mandatory to correctly describe the deuter- ation amplification phenomenonin depleted cores such as L183 and to relate the total (ortho+para) H2D+abundance to the sole ortho-H2D+ column density measurement. Methods. Toconstrain this ortho/para H2 ratio and derive its profile, we make use of theN2D+ /N2H+ ratio and of the ortho-H2D+ observations performed across theprestellar core. We use two simple chemical models limited to an almost totallydepleted core description. New dissociative recombination and trihydrogencation-dihydrogen reaction rates (including all isotopologues) are presented inthis paper and included in our models. Results. We estimate the H2D+ ortho/pararatio in the L183 cloud, and constrain the H2 ortho/para ratio : we show thatit is varying across the prestellar core by at least an order of magnitudebeing still very high (~0.1) in most of the cloud. Our time-dependent modelindicates that the prestellar core is presumably older than 1.5-2 x 10^5 yearsbut that it may not be much older. We also show that it has reached its presentdensity only recently and that its contraction from a uniform density cloud canbe constrained. Conclusions. A proper understanding of deuteration chemistrycannot be attained without taking into account the whole ortho/para family ofmolecular hydrogen and trihydrogen cation isotopologues as their relations areof utmost importance in the global scheme. Tracing the ortho/para H2 ratioshould also give useful constraints on the dynamical evolution of prestellarcores.

Chemical modeling of L183 (= L134N) : an estimate of the ortho/para H2 ratio