Study of the Structure, Energetics, and Vibrational Properties of Small Ammonia Clusters (NH₃)_n (n = 2−5) Using Correlated ab Initio Methods

Abstract

Equilibrium geometries, interaction energies, and harmonic frequencies of (NH3)n isomers (n = 2-5) have been computed using correlated calculations (MP2) in conjunction with Dunning's aug-cc-pVXZ (X = D, T, Q) basis sets and the Counterpoise procedure. Whenever available, literature values for the binding energy and geometry of dimers and trimers agree well with our data. Low lying isomers for (NH3)4 and (NH3)5 have been found to have similar binding energies (roughly 16 and 20 kcal/mol for the tetramer and pentamer, respectively), perhaps suggesting the presence of a very smooth energy landscape. Using BSSE corrected forces or freezing the monomer structure to its gas phase geometry have been found to have only a weak impact on the energetic and structural properties of the clusters. The effect of zero-point energy (ZPE) on the relative stability of the clusters has been estimated using harmonic frequencies. The latter also highlighted the presence of vibrational fingerprints for the presence of double acceptor ammonia molecules. Many-body effects for (NH3)n isomers (n = 2-4) have been investigated to explore the possibility of building a pairwise interaction model for ammonia. In the frame of the work presented, we have found the 3-body effect to account for 10-15% of the total interaction energy, whereas the 4-body effects may be neglected as first approximation.

From the (NH3)2–5 clusters to liquid ammonia: Molecular dynamics simulations using the NVE and NpT ensembles

“…2 indicates that all the geometries attained along the trajectory have similar values of the potential energy. These results are in agreement with the existence of small barriers interconnecting equivalent isomers [57].…”

“…In such structure all the N atoms tend to be placed at the same distance from the center of mass of the aggregate. A boat configuration has been also proposed as the most stable one by Janeiro and Mella [57] at the MP2-aug-cc-pVTZ level of calculation, which also found a planar structure, almost degenerate with the boat one, with sign and magnitude of their small energy difference dependent on the calculation level.…”

A Molecular Perspective on Lithium–Ammonia Solutions

“…For example, we calculated that a hydrogen-bonded tetramer (found to be the most stable isomer of (NH 3 ) 4 in Ref. [94]) is 11.4 kcal mol À1 more stable than four isolated ammonia molecules (gas phase). This is comparable with the 9.9 kcal mol À1 that comes from the reorientation of the ammonia dipoles in e À @(NH 3 ) 4 ("H-in" vs. "H-out").…”

Determining the dissociation threshold of ammonia trimers from action spectroscopy of small clusters

“…For example, they potentially make the individual hydrogen bonds in an oligomer stronger than the single hydrogen bond in the dimer. There is a large set of calculations on the binding energy and structure of the ammonia dimer, [1][2][3][4][5][6][7][8][9][10][11] but relatively few of them include zero-point effects that allow comparison with experimental dissociation energies. [7][8][9][10] There are fewer ab initio calculations on the structure of the trimer, [7][8][9][10][11] four of which include zero-point energy terms and predict the energy, E trimer , required to remove one NH 3 unit from the trimer, [7][8][9][10] (NH 3 ) 3 → (NH 3 ) 2 + NH 3 .…”