THE STRUCTURES OF LINC, NANC, AND KNC - POTENTIAL-ENERGY SURFACE FOR THE ORBITING MOTION OF THE METAL CATION AROUND THE CN GROUP

Sets of XNC (X = Li, Na, and K) structures have been investigated usin g a variety of theoretical methods and basis sets. Two linear and a br idged structure correspond to an energy minimum for LiCN. For NaCN and KCN, the linear isocyanide is a minimum at most levels of theory but becomes a second-order stationary point when correlated levels and lar ge basis sets are used. Two transition structures are involved in the interconversion of the bridged form and each linear isomer. The isomer ization of the bridged minimum to the linear XCN involves a substantia l barrier in each case, so that the satellite motion of the metal is f acile at one end but not at the other. Calculations with large basis s ets and with electron correlation, fourth-order Moller-Plesset theory for KNC, and quadratic configuration interaction [QCISD(T)] for LiNC a nd NaNC give qualitative and quantitative agreement with this experime nt. The sodium and potassium cyanide prefer bridged forms, in contrast to the linear isocyanide LiNC. (C) 1994 by John Wiley and Sons, Inc.