Gliding on Ice in Search of Accurate and Cost-Effective Computational Methods for Astrochemistry on Grains: The Puzzling Case of the HCN Isomerization

Abstract

The isomerization of hydrogen cyanide to hydrogen isocyanide on icy grain surfaces is investigated by an accurate composite method (jun-Cheap) rooted in the coupled cluster ansatz and by density functional approaches. After benchmarking density functional predictions of both geometries and reaction energies against jun-Cheap results for the relatively small model system HCN···(H 2 O) 2 , the best performing DFT methods are selected. A large cluster containing 20 water molecules is then employed within a QM/QM′ approach to include a realistic environment mimicking the surface of icy grains. Our results indicate that four water molecules are directly involved in a proton relay mechanism, which strongly reduces the activation energy with respect to the direct hydrogen transfer occurring in the isolated molecule. Further extension of the size of the cluster up to 192 water molecules in the framework of a three-layer QM/QM′/MM model has a negligible effect on the energy barrier ruling the isomerization. Computation of reaction rates by the transition state theory indicates that on icy surfaces, the isomerization of HNC to HCN could occur quite easily even at low temperatures thanks to the reduced activation energy that can be effectively overcome by tunneling.

Reliable Gas Phase Reaction Rates at Affordable Cost by Means of the Parameter-Free junChS-F12 Model Chemistry

“…While the energy barrier is too high to allow effective isomerization in the interstellar medium (ISM), the rate constant of the reaction represents the reference value for studying catalytic effects by water molecules on icy grains. 86 In this case, the rate constants computed by different models are in good agreement in the whole temperature range among themselves and with previous computations. 87 The last reaction considered (NHT2) is the simplest heavy atom transfer.…”

Carbon Atom Condensation on NH3–H2O Ices. An Alternative Pathway to Interstellar Methanimine and Methylamine

“…We note that this is not generally the case, depending on each system under consideration. Finally, we also note that proton-transfer reaction do depend on the local environment, and a study similar to Baiano et al or Perrero et al for key proton-transfer reactions (i.e., neither barrierless nor having a high barrier) like reaction can be helpful to predict barriers.…”

“…Because other proton-transfer reactions studied in this work were found to be viable (see the next sections), we then determine that our activation energy for this reaction must be high enough to consider reaction not important in astrophysical environments. We note that these proton-transfer reactions depend on the local water arrangements ,, and that different barriers could be present at different binding sites. However, the value we obtained of the activation energy is much higher than the highest one found for 3 COH 2 → 3 HCOH (around 11.5 kJ mol –1 as a maximum), which prompts us to consider 3 CNH 3 as a more stable molecule on the surface.…”

Reliable Gas Phase Reaction Rates at Affordable Cost by Means of the Parameter-Free JunChS-F12 Model Chemistry

“…While the energy barrier is too high to allow effective isomerization in the interstellar medium (ISM), the rate constant of the reaction represents the reference value for studying catalytic effects by water molecules on icy grains. 91 In this case, the rate constants computed by different models are in good agreement in the whole temperature range among themselves and with previous computations. 92 The last reaction considered (NHT2) is the simplest heavy atom transfer.…”