State-to-state thermal rate coefficients for reactions of all H(3)(+) + H(2) isotopic variants are derived and compared to new experimental data. The theoretical data are also sought for astrochemical modeling of cold environments (<50 K). The rates are calculated on the basis of a microcanonical approach using the Langevin model and the conservation laws of mass, energy, angular momentum, and nuclear spin. Full scrambling of all five nuclei during the collision is assumed for the calculations and alternatively partial dynamical restrictions are considered. The ergodic principle of the collision is employed in two limiting cases, neglecting (weak ergodic limit) or accounting for explicit degeneracies of the reaction mechanisms (strong ergodic limit). The resulting sets of rate coefficients are shown to be consistent with the detailed balance and thermodynamical equilibrium constants. Rate coefficients, k(T), for the deuteration chain of H(3)(+) with HD as well as H(2)D(+)/H(3)(+) equilibrium ratios have been measured in a variable temperature 22-pole ion trap. In particular, the D(2)H(+) + HD --> D(3)(+) + H(2) rate coefficient indicates a change in reaction mechanism when going to higher temperatures. The good overall agreement between experiment and theory encourages the use of the theoretical predictions for astrophysical modeling.

Context. The high degree of deuteration observed in some prestellar cores depends on the ortho-to-para H 2 ratio through the H + 3 fractionation. Aims. We want to constrain the ortho/para H 2 ratio across the L183 prestellar core. This is required to correctly describe the deuteration amplification phenomenon in depleted cores such as L183 and to relate the total (ortho+para) H 2 D + abundance to the sole ortho-H 2 D + column density measurement. Methods. To constrain this ortho/para H 2 ratio and derive its profile, we make use of the N 2 D + /N 2 H + ratio and of the ortho-H 2 D + observations performed across the prestellar core. We use two simple chemical models limited to an almost totally depleted core description. New dissociative recombination and trihydrogen cation-dihydrogen reaction rates (including all isotopologues) are presented in this paper and included in our models. Results. We estimate the H 2 D + ortho/para ratio in the L183 cloud, and constrain the H 2 ortho/para ratio: we show that it varies across the prestellar core by at least an order of magnitude, being still very high (≈0.1) in most of the cloud. Our time-dependent model indicates that the prestellar core is presumably older than 1.5−2 × 10 5 years but that it may not be much older. We also show that it has reached its present density only recently and that its contraction from a uniform density cloud can be constrained. Conclusions. A proper understanding of deuteration chemistry cannot be attained without taking into account the whole ortho/para family of molecular hydrogen and trihydrogen cation isotopologues as their relations are of utmost importance in the global scheme. Tracing the ortho/para H 2 ratio should also place useful constraints on the dynamical evolution of prestellar cores.

For decades, protonated methane, CH(5)(+), has provided new surprises and challenges for both experimentalists and theoreticians. This is because of the correlated large-amplitude motion of its five protons around the carbon nucleus, which leads to so-called hydrogen scrambling and causes a fluxional molecular structure. Here, the infrared spectra of all its H/D isotopologues have been measured using the 'Laser Induced Reactions' technique. Their shapes are found to be extremely dissimilar and depend strongly on the level of deuteration (only CD(5)(+) is similar to CH(5)(+)). All the spectra can be reproduced and assigned based on ab initio quantum simulations. The occupation of the topologically different sites by protons and deuterons is found to be strongly non-combinatorial and thus non-classical. This purely quantum-statistical effect implies a breaking of the classical symmetry of the site occupations induced by zero-point fluctuations, and this phenomenon is key to understanding the spectral changes studied here.

Context. The depletion of heavy elements in cold cores of interstellar molecular clouds can lead to a situation where deuterated forms of H + 3 are the most useful spectroscopic probes of the physical conditions. Aims. The aim is to predict the observability of the rotational lines of H 2 D + and D 2 H + from prestellar cores. Methods. Recently derived rate coefficients for the H + 3 + H 2 isotopic system were applied to the "complete depletion" reaction scheme to calculate abundance profiles in hydrostatic core models. The ground-state lines of H 2 D + (o) (372 GHz) and D 2 H + (p) (692 GHz) arising from these cores were simulated. The excitation of the rotational levels of these molecules was approximated by using the state-to-state coefficients for collisions with H 2 . We also predicted line profiles from cores with a power-law density distribution advocated in some previous studies. Results. The new rate coefficients introduce some changes to the complete depletion model, but do not alter the general tendencies. One of the modifications with respect to the previous results is the increase of the D + 3 abundance at the cost of other isotopologues. Furthermore, the present model predicts a lower H 2 D + (o/p) ratio, and a slightly higher D 2 H + (p/o) ratio in very cold, dense cores, as compared with previous modelling results. These nuclear spin ratios affect the detectability of the submm lines of H 2 D + (o) and D 2 H + (p). The previously detected H 2 D + and D 2 H + lines towards the core I16293E, and the H 2 D + line observed towards Oph D can be reproduced using the present excitation model and the physical models suggested in the original papers.

The method of laser induced reaction is used to obtain high-resolution IR spectra of H2D+ and D2H+ in collision with n-H2 at a nominal temperature of 17 K. For this purpose three cw-laser systems have been coupled to a 22-pole ion trap apparatus, two commercial diode laser systems in the ranges of 6100-6600 cm(-1) and 6760-7300 cm(-1), respectively, and a high-power optical parametric oscillator tunable in the range of 2600-3200 cm(-1). In total, 27 new overtone and combination transitions have been detected for H2D+ and D2H+, as well as a weak line in the nu1 vibrational band of H2D+ (2(20)<--1(01)) at 3164.118 cm(-1). The line positions are compared to high accuracy ab initio calculations, showing small but mode-dependent differences, being largest for three vibrational quanta in the nu2 symmetric bending of H2D+. Within the experimental accuracy, the relative values of the ab initio predicted Einstein B coefficients are confirmed.