INVESTIGATING WEAKER HYDROGEN BONDS: THE CASE OF FROZEN, QUANTUM AND T >0 K AMMONIA CLUSTERS

Aggregates of hydrogen bonded molecules constitute ideal model systems for investigating the energetics and mechanism underlying the formation of hydrogen bonds (HB) networks since they represent intermediate systems between molecular species and bulk with a manageable level of complexity. Compared to (H2O)n and (HF)n clusters, ammonia aggregates have been less investigated despite their interest for interpreting many ammonia properties, with the theoretical and spectroscopic experimental analysis so far agreeing only on their complicate and fluxional nature, especially when n ⩾ 5. We thus discuss the results of our theoretical effort in studying the properties of pure and doped ammonia clusters (NH3)n (n=2-20), with a view to foster a better analysis of experimental results and investigating possible materials for hydrogen or energy storage. An introduction on the theoretical approached employed during the investigation is given, touching the construction of next-generation interaction models [1,2] and more efficient quantum simulation methods [3,4,5]. Fundamental data as global minimum structures, density of states, as well as binding (BE) and evaporation energies (EE) versus n are then discussed to explore the basic properties of aggregates [1]. An investigation mimicking the helium droplets pick-up technique via a “molecule by molecule” building of aggregates was also carried out to verify the possibility of kinetic control on the formation of isomeric species [6]. All the conclusions obtained were put under scrutiny simulating ammonia clusters up to n=11 with rigid-body path integral Monte Carlo to include both quantum and thermal effects [3]. The simulations results clearly evidence the fluxionality and mixed-isomeric nature of a few aggregates, thus providing a better ground for the interpretation of spectroscopic results in molecular beams. The simulations also evidenced that a) isomers are separated by barriers similar to the EE’s (6-8 kcal/mol), thus making the calculation of average properties demanding at all but the highest T employed, and b) quantum effects substantially modify the relative BE/EE landscape as function of n eroding at least 40% of the classical EE [3]. Quantum and thermal fluctuations were separated using a novel diffusion Monte Carlo (DMC) scheme for clusters up to n=6, showing that their fluxionality is entirely of quantum origin [4]. We also addressed the question of preferential dwelling of isotopically substituted ammonia molecules and relative preference for H-bonds involving D or T rather than H. Finally, H2 sorption energies on and inside (NH3)n were obtained exploiting higher order DMC methods for highly quantum rigid body rotators as an initial step toward the exploration of the phase diagram of H2-(NH3) clathrates or the stability of H2-(NH3) sludges [7]. Our data indicates that H2 preferentially adsorbs on rhomboidal motifs with four dangling N-H and that clathrates should be metastable under a wide range of conditions if formed.