AB-INITIO STUDIES ON THE MIXED HETERODIMERS OF AMMONIA AND HYDROGEN-CYANIDE

Ab initio molecular structure calculations have been used to examine t he association of NH3 with HCN to form a heterodimer, Because of the p otential importance of electron correlation in weakly associated compl exes, the structure and energy of the heterodimers and the associated monomers are reported at both Hartree-Fock and Moller-Plesset levels o f calculation. Other calculated properties including harmonic vibratio nal frequencies and rotational constants are also reported for each sy stem. Several stationary points on the potential surface were located and characterized. In the two most stable configurations, the monomer units are associated through a hydrogen bond with HCN acting as the hy drogen donor. The structure predicted to be the most stable has a line ar hydrogen bond and C3, symmetry in accordance with the experimentall y observed symmetric top spectrum. The stability of this structure is 5.28 kcal/mol (with MP2FULL/6-31 + G(3df, 2p) and 5.01 kcal/mol with c ounterpoise correction). The second structure having only C, symmetry is identified as a transition state. A third stable hydrogen bonded st ructure has HCN acting as the hydrogen acceptor and is the most weakly bound of the three.