A MOLECULAR-ORBITAL STUDY OF BONDING AND FREQUENCY-SHIFTS IN FREE ANDSOLVATED LITHIUM THIOCYANATE AGGREGATES

The MNDO method has been used to determine the stable structures of li thium thiocyanate aggregates. Two classes of aggregates have been foun d: (a) N-bonded (LiNCS)p (1 less-than-or-equal-to p less-than-or-equal -to 4) (i.e. linear LiNCS), the rhombic dimer, the triangular trimer a nd a cubane-like tetramer; (b) (LiSCN)p aggregates (1 less-than-or-equ al-to p less-than-or-equal-to 3) with lithium bridges between S and C. The structures of class (a) are in agreement with those experimentall y determined in solution; in this case unusual negative frequency shif ts of nu(CN) due to mu2 or mu3 nitrogen coordination have been reprodu ced by our vibrational frequency calculations. IR intensity variations are also in qualitative agreement with experiment. The lithium solvat ion has been simulated by adding water molecules in the first solvatio n shell of Li in order to complete its coordination number to 4. This model gives a good account of aggregation energies in solution and of nu(CN) frequency variations.