Intermolecular potentials for ammonia based on the test particle model and the coupled pair functional method

Ammonia dimer: extremely fluxional but still hydrogen bonded

In the 1980s, Nelson, Fraser, and Klemperer (NFK) published an experimentally derived structure of the ammonia dimer dramatically different from the structure determined computationally, which led these authors to the question "Does ammonia hydrogen bond?". This question has not yet been answered satisfactorily. To answer it, we have developed an ab initio potential energy surface (PES) for this dimer at the limits of the current computational capabilities and performed essentially exact six-dimensional calculations of the vibration-rotation-tunneling (VRT) spectra of NH₃-NH₃ and ND₃-ND₃, obtaining an unprecedented agreement with experimental spectra. In agreement with other recent electronic structure calculations, the global minimum on the PES is in a substantially bent hydrogen-bonded configuration. Since the bottom of the PES is exceptionally flat, the dimer is extremely fluxional and the probability of finding it in configurations that are not hydrogen bonded is high. Nevertheless, the probability of hydrogen-bonded configurations is large enough to consider the ammonia dimer to be hydrogen bonded. We also show that NFK's inference that the ammonia dimer is nearly rigid actually results from unusual cancellations between quantum effects that generate differences in spectra of different isotopologues.

Does ammonia hydrogen bond?

Spectroscopic characterizations of the stereochemistry of complexes of ammonia (NH(3)) have strongly confirmed some long-held ideas about the weak interactions of NH(3) while casting doubt on others. As expected, NH(3) is observed to be a nearly universal proton acceptor, accepting hydrogen bonds from even some of the weakest proton donors. Surprisingly, no evidence has been found to support the view that NH(3) acts as a proton donor through hydrogen bonding. A critical evaluation of the work that has been done to gather such evidence, as well as of earlier work involving condensed-phase observations, suggests that NH(3) might well be best described as a powerful hydrogen-bond acceptor with little propensity to donate hydrogen bonds.

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

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.

Structures and stability of salt-bridge in aqueous solution

Structures and stability of salt-bridges in aqueous solutions were investigated using a complex formed from the guanidinium (Gdm+) and formate (FmO-) ions as a model system. The Test-particle model (T-model) potentials to describe the interactions in the Gdm+-H2O, FmO(-)-H2O and Gdm+-FmO- complexes were constructed, tested and applied in molecular dynamics (MD) simulations of the aqueous solutions at 298 K. The three-dimensional structures and energetic of the hydrogen bond (H-bond) networks of water in the first hydration shells of the Gdm+ and FmO- ions, as well as the Gdm+-FmO- complex, were visualized and analyzed using various probability distribution (PD) maps. The structures of the average potential energy landscapes at the H-bond networks were employed to characterize the stability and dynamic behavior of water molecules in the first hydration shells of the solutes. It was observed that water molecules in the first hydration shell of the close-contact Gdm+-FmO- complex form associated H-bond networks, which introduce a net stabilization effect to the ion-pair, whereas those in the interstitial H-bond network destabilize and break the solvent-separated Gdm+-FmO- complex. The present results showed that, in order to provide complete insights into the structures and stability of ion-pairs in aqueous solutions, explicit water molecules have to be included in the model calculations.