An ab initio molecular orbital study of the structures and energies of neutral and charged bimolecular complexes of NH₃ with the hydrides AH_n (A = N, O, F, P, S, and Cl)

Abstract

Hartree-Fock 6-31G(d) structures for the neutral, positive ion, and negative ion bimolecular complexes of NH3 with the first-and second-row hydrides AH, (AH, = NH3,0Hz, FH, PH3, SH2, and ClH) have been determined. All of the stable neutral complexes except (NH3)2, the positive ion complexes with NH3 as the proton acceptor, and the negative ion complexes containing first-row anions exhibit conventional hydrogen bonded structures with essentially linear hydrogen bonds and directed lone pairs of electrons. The positive ion complex NH,' . . . OH2 has the dipole moment vector of H 2 0 instead of a lone pair directed along the intermolecular line, while the complexes of NH,' with SH2, FH, and C1H have structures intermediate between the lone-pair directed and dipole directed forms. The negative ion complexes containing second-row anions have nonlinear hydrogen bonds. The addition of diffuse functions on nonhydrogen atoms to the valence double-split plus polarization 6-31G(d,p) basis set usually decreases the computed stabilization energies of these complexes. Splitting d polarization functions usually destabilizes these complexes, whereas splitting p polarization functions either has no effect or leads to stabilization. The overall effect of augmenting the 6-31G(d,p) basis set with diffuse functions on nonhydrogen atoms and two sets of polarization functions is to lower computed stabilization energies. Electron correlation stabilizes all of these complexes. The second-order MZller-Plesset correlation term is the largest term and always has a stabilizing effect, whereas the third and fourth-order terms are smaller and often of opposite sign. The recommended level of theory for computing the stabilization energies of these complexes is MP2/6-31+G(2d, 2p), although MP2/6-31+G(d,p) is appropriate for the negative ion complexes.

Density functional studies on hydrogen‐bonded complexes

“…The complex built from the ammonia and the ammonium ion belongs to the strong hydrogenbonded systems with an experimental value of -24.8 kcal/mol for the binding enthalpy reported in [35]. In [34], a value of -25.9 kcal/mol was obtained at the M P~ level with a 6-3 1 +c(2d, 2 p ) basis set, taking into account the zero-point correction obtained from vibrational frequencies calculated with the Hartree-Fock method using a 6-31G** basis set. This compares reasonably well with the values obtained with the nonlocal potentials corrected for the BSSE.…”

“…N distance is 2.62 A, whereas the nonlocal potentials yield values of 2.70 and 2.81 A, respectively. The corresponding distance found in [34] at the Hartree-Fock level was 2.816 A.…”

“…The results are listed in Tables V and VI, respectively. For comparison, structural parameters obtained with the Hartree-Fock method using a 6-31~** basis set [34] are included in Table V. The hydrogen-bond energy obtained with this basis set [34] and the experimental values of the binding enthalpy [35] are also shown in Table VI.…”

“…Similar results have been published in [36], which found an The intermolecular interaction energy, A E (in kcal/mol), has been calculated with and without correction for BSSE. aCalculated for the structure optimized at Hartree-Fock level with the 6-31~** basis set [34]. b [35].…”

Structures, binding energies, and thermodynamic functions of NH4+, NH3•+, and their H2O complexes

“…Experimental and theoretical investigations of gas phase ion-solvent clusters for solvents such as water have been of interest for many years (1)(2)(3)(4)(5)(6). Mass spectrometric investigations provide accurate thermodynamic data on the binding of solvent molecules, but do not as a rule lead to explicit structural information (1,2).…”