AB INITIO SEARCH FOR THE EQUILIBRIUM STRUCTURE OF THE AMMONIA DIMER
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Date
1993
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Ohio State University
Abstract
The equilibrium structure of the $NH_{3}$ dimer is investigated using large basis sets, $6-311+G(3d, 2p)$ and $[7s5p3d, 4s1p]$ extended with bond functions, at the second-order M\o ller-Plesset perturbation approximation (MP2) and higher levels. The counterpoise method for corrections of the basis set superposition error is fully applied in the determination of intermolecular energies and the optimization of dimer structures. The stabilities of two possible equilibrium structures, one containing a nearly linear hydrogen bond with C, symmetry and the other a cyclic configuration with $C_{2h}$ symmetry, are examined. In a basis without bond functions, the C, structure is found more stable. As bond functions are added, however, the $C_{2h}$ structure becomes more stable. The stability of the $C_{2h}$ structure relative to the $C_{s}$ is is retained at the higher levels up to the complete forth order (MP4SDTQ). The minimum energy path connecting the two equilibrium $C_{5}$, structures via the $C_{2h}$ structure is calculated. The resulting potential curves are extraordinarily flat in a broad region of the potential path around the $C_{2h}$ structure but rise steeply upon approaching the $C_{5}$, structure containing a nearly linear hydrogen bond. The exact equilibrium structure for the $NH_{3}$ dimer is difficult to ascertain, owing to the flatness of the lowest energy potential path representing widely different structures. The $C_{s}$ structure with a nearly linear hydrogen bond is, however, unlikely to be the equilibrium structure. The expected equilibrium structure from the present study is cyclic in form and close to the $C_{2h}$ structure.
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Author Institution: Department of Chemistry, Harvard University