Ab initio search for the equilibrium structure of the ammonia dimer

A Tug of War between the Self- and Cross-associating Hydrogen Bonds in Neutral Ammonia-Water Clusters: Energetic Insights by Molecular Tailoring Approach

In the present work, the energies of various types of individual HBs observed in neutral (NH₃ )_m (H₂ O)_n , (m+n=2 to 7) clusters were estimated using the molecular tailoring approach (MTA)-based method. The calculated individual HB energies suggest that the O-H…N HBs are the strongest (1.21 to 12.49 kcal mol^-1 ). The next ones are the O-H…O (3.97 to 9.30 kcal mol^-1 ) HBs. The strengths of N-H…N (1.09 to 5.29 kcal mol^-1 ) and N-H…O (2.85 to 5.56 kcal mol^-1 ) HBs are the weakest. The HB energies in dimers also follow this rank ordering. However, the HB energies in dimers are much smaller than those obtained by the MTA-based method due to the loss in cooperativity contribution in the dimers. Thus, the calculated cooperativity contributions, for different types of HBs, fall in the range 0.64 to 5.73 kcal mol^-1 . We wish to emphasize based on the energetic rank ordering obtained by the MTA-based method that the O-H of water is a better HB donor than the N-H of ammonia. The reasons for the observed energetic rank ordering are two folds: (i) intrinsically stronger O-H…N HBs than the O-H…O ones as revealed by dimer energies and (ii) the higher cooperativity contribution in the former than the later ones. Indeed, the MTA-based method is useful in providing the missing energetic rank ordering of various type of HBs in neutral (NH₃ )_m (H₂ O)_n clusters, in the literature.

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.

A full-dimensional ab initio potential energy and dipole moment surfaces for (NH3)2

A full-dimensional ab initio potential energy surface (PES) and dipole moment surface (DMS) for the ammonia dimer (NH₃)₂ are reported. The database of the PES consists of 27 736 ab initio energy points and all of these points were calculated at the UCCSD(T)-F12a/AVTZ level. The PES was fitted by using the fundamental invariant neural network (FI-NN) method that satisfies the permutational symmetry of identical atoms, and the root mean square fitting error for the PES is very small as low as 0.562 meV. The geometries for the (NH₃)₂ DMS are the same as those used for the PES and are calculated at the XYG3/AVTZ level. This PES can describe a variety of internal floppy motions, including all kinds of vibrational modes no matter intermolecular or intramolecular. The CCSD(T)-PES can dissociate correctly to two NH₃ monomers, with D_e = 1135.55 cm^-1 (13.58 kJ/mol) which agrees accurately with the 13.5 ± 0.3 kJ/mol predicted by previous work.

Nonadiabatic dynamics of floppy hydrogen bonded complexes: the case of the ionized ammonia dimer

Non-adiabatic dynamics of a floppy hydrogen bonded ammonia dimer was studied by ab initio molecular dynamics simulations.

Structures and relative stabilities of ammonia clusters at different temperatures: DFT vs. ab initio

The global minimum energy structures of (NH₃)_n=2–10are pointed out for the first time at a given temperature.

Structures, Energies and Electronic Properties of Interaction of Hydrogen Halides with Pentamer Water Cluster

Molecular orbital and density functional theory calculations have been performed to study the interaction of hydrogen halides with hydrogen bonded cluster of water pentamer, i.e. HX·(H2O)5 (where X = F, Cl, Br). Second order Møller–Plesset perturbation theory of ab initio method, B3LYP level of theory of DFT method, implementing 6-311++G** basis set have been used for the calculations. The strength of hydrogen bonds have been studied using the interaction energies of the water molecules, which were calculated after eliminating the basis set superposition error (BSSE) by Boys Counterpoise (CP) correction method and the stability of this cluster with hydrogen halides has been studied using the dipole moments, charge transfer and the topological properties. The chemical hardness and chemical potential have been calculated for the above cluster with hydrogen halides and tested the principle of maximum hardness.

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.

Pulsed-field ionization electron spectroscopy and conformation of copper-diammonia

Copper-diammonia, Cu(NH3)2, and its deuterated species, Cu(ND3)2, are produced in supersonic molecular beams and studied by pulsed-field ionization zero electron kinetic energy photoelectron spectroscopy and ab initio calculations. Structural isomers with a copper atom binding to an ammonia dimer or two ammonia molecules are obtained by the calculations. By comparing the experimental measurements to the theoretical calculations, the neutral and ionic forms of copper-diammonia are determined to be in a doubly bound linear conformation in their ground electronic states. The adiabatic ionization potentials of Cu(NH3)2 and Cu(ND3)2 are measured as 29,532 (5) and 29,313 (5) cm(-1), respectively. The metal-ligand symmetric stretching frequencies are measured to be 436 cm(-1) for Cu+-(NH3)2 and 398 cm(-1) for Cu+-(ND3)2, and the metal-ligand bending frequencies 75,139 cm(-1) for CuCu+-(NH3)2 and 70125 cm(-1) for CuCu+-(ND3)2. Moreover, the dissociation energy of Cu(NH3)2-->CuNH3+NH3 is determined to be 11(3) kcal mol(-1) through a thermodynamic relationship.

On the Performance of Bond Functions and Basis Set Extrapolation Techniques in High-Accuracy Calculations of Interatomic Potentials. A Helium Dimer Study

Helium dimer interaction energies, Eint, obtained recently using the Gaussian geminal implementation of the coupled cluster doubles (CCD) and singles and doubles (CCSD) theory, were employed to evaluate the performance of conventional orbital calculations applying the correlation-consistent polarized valence X-tuple zeta (cc-pVXZ) bases, with X ranging from 4 to 7, and very large sets of bond functions. We found that while the bond functions improve dramatically the convergence of the doubles and triples contribution to the interaction energy, these functions are inefficient or even counterproductive in predicting the effect of the single excitations and the small contribution beyond the CCSD(T) (CCSD model with noniterative account of triple excitations) level of electronic structure theory. We also found that bond functions are very effective in extrapolation techniques. Using simple two-point extrapolations based on the single-power laws X-2 and X-3 for the basis set truncation error, the Gaussian geminal CCSD result for Eint, equal to -9.150 ± 0.001 K at the equilibrium interatomic distance of R = 5.6 bohr, could be reproduced with an error of 2-3 mK. Linear extrapolation of the functional dependence of the CCSD energy on the value of the second-order Møller-Plesset energy and the use of the known accurate value of the latter leads to an even smaller error. Using these extrapolation techniques with basis sets up to doubly augmented septuple-zeta quality and containing large sets of bond functions, we estimated the contribution of triple excitations within the CCSD(T) model to be -1.535 ± 0.002 K, with the error bars reflecting the spread of extrapolated results. The contribution beyond the CCSD(T) model, estimated from full configuration interaction (FCI) calculations with up to 255 orbitals, amounts to -0.323 ± 0.005 K. Combining the Gaussian geminal value of the CCSD energy with the orbital estimations of the CCSD(T) and FCI contributions, we found that Eint = -11.008 ± 0.008 K. This value is consistent with recent high-level orbital computations (van Mourik T., Dunning T. H.: J. Chem. Phys. 1999, 111, 9246; Klopper W.: J. Chem. Phys. 2001, 115, 761) but has substantially tighter error bounds. It differs somewhat, however, from the value of -10.98 ± 0.02 K obtained recently from the "exact" quantum Monte Carlo calculations (Anderson J. B.: J. Chem. Phys. 2001, 115, 4546).