Structure and stability of the nitric acid–ammonia complex in the gas phase and in water

Abstract

The nitric acid–ammonia system is studied by high level ab initio calculations. The equilibrium structure, vibrational frequencies, and binding energy of the system in the gas phase are calculated at the second-order Mo/ller–Plesset perturbation level with the extended basis set 6-311++G(d,p). The potential energy surface along the proton transfer pathway is investigated by calculations at the same level of theory, and the effect of water as a solvent on the structure and stability of the system is investigated using self-consistent reaction field theory. It is found that the equilibrium structure contains a strong hydrogen bond with nitric acid acting as the hydrogen bond donor and ammonia as the acceptor. The binding energy is calculated to be D0=12.25 kcal/mol (De=14.26 kcal/mol), which is about three times greater than the binding energy for the water dimer. The OH stretching frequency of nitric acid in the hydrogen-bonded complex is found to be red shifted by over 800 cm−1, with an enhancement of over an order of magnitude in the infrared intensity from the isolated nitric acid molecule. The structure of ammonium nitrate corresponding to the product of a proton transfer reaction is found to be highly unstable on the potential energy surface. The most energetically favorable gas phase reaction of nitric acid and ammonia in the absence of a solvent results in proton exchange, not proton transfer from acid to base. The structure and stability of the system change drastically in the water solvent medium. In the water solvent, the hydrogen-bonded structure is no longer stable and the system exists as an ammonium nitrate ion pair resulting from the completed transfer of a proton from nitric acid to ammonia. On the basis of these results, we conclude that it is unlikely to form gaseous ammonium nitrate from nitric acid and ammonia, and that the formation of particulate ammonium nitrate most likely involves a heterogeneous mechanism.

Theoretical Study on Proton Transfer and Energetics in Ammonium Nitrate (NH4NO3)n Cluster System

“…The DFT method gives good agreement with the MP2 calculations for structural parameters in ammonium nitrate monomer and dimer [12]. The basis set superposition error (BSSE) was previously estimated to be small (less than 0.4 kcal/mol) in monomer and dimer systems when using the 6-311++G(d, p) basis [11], [12]. Contracted atomic charges from natural bond orbital (NBO) analysis were used to calculate classical Coulomb force for electric field intensities and electrostatic interaction [17].…”

“…1(a) and Fig. 1(b) [11], [12]. The occurrence of proton transfer from nitric acid to ammonia is expected to overcome this destabilization by increasing the stability of the intermolecular interaction.…”

Density functional theory characterization of the structure and gas-phase, mid-infrared absorption spectrum of 2-azido-N,N-dimethylethanamine (DMAZ)

“…A larger basis set [6-311++G(d,p)] [12,13] with the G98-defined "ultrafine" grid was later employed to characterize the conformers identified in the initial survey. The 6-311++G(d,p) basis set was chosen based on its use in ammonia-nitric acidwater reaction studies published by Tao and coworkers [14,15], their results expected to provide benchmarks for planned hypergol-nitric acid reaction studies. The use of the ultrafine grid resolved the issue with the size of rotational/ translational mode eigenvalues appearing in the initial survey, an issue not rectified by the use of a larger basis set [6-311+G(2d,2p)] alone.…”

Formation of semivolatile inorganic aerosols in the Mexico City Metropolitan Area during the MILAGRO campaign

“…The water content of aerosols is calculated using the ZSR relationship (Stokes and Robinson, 1966). The addition of the soluble crustal elements, especially magnesium, increases the water content in the coarse mode (under the same RH) which eventually shifts the reversible reaction of NH 3 with HNO 3 towards the aerosol phase producing more ammonium nitrate (Nguyen et al, 1997;Finlayson-Pitts and Pitts, 2000). Appreciable amounts of aerosol water (hence ammonium nitrate) is present even at moderate RH, given that some of the mineral salts deliquesce at low to moderate RH (e.g., 33 % for MgCl 2 and 54 % for Mg(NO 3 ) 2 at 298 K).…”