First Principles Examination of the Acetylene−Water Clusters, HCCH−(H<INF>2</INF>O)<INF>x</INF><INF></INF>, x = 2, 3, and 4

The acetylene−water (A−W) interactions have been investigated by examining the van der Waals clusters AW<INF>x</INF><INF></INF>, x = 2, 3, and 4, at the second order (MP2) perturbation theory using the correlation-consistent basis sets, aug-cc-pVnZ, n = D (AW<INF>2</INF>, AW<INF>3</INF>, and AW<INF>4</INF>), T (AW<INF>2</INF>). We located 4 minima (m) and 2 saddle points (sp), 10 m and 3 sp, and 30 m and 3 sp on the potential energy surfaces of the AW<INF>2</INF>, AW<INF>3</INF>, and AW<INF>4</INF> clusters, respectively. We report the fully optimized geometries and interaction energies ΔE<INF>e</INF>, including corrections for basis set superposition error, ΔE<INF>e</INF>(BSSE), as well as zero-point energies, ΔE<INF>0</INF>(BSSE), for the various stationary points. The global minima of the AW<INF>2</INF> and AW<INF>3</INF> clusters are cyclic configurations in which the acetylene molecule inserts into the water hydrogen bonding network. The corresponding interaction energies ΔE<INF>e</INF>(BSSE)[ΔE<INF>0</INF>(BSSE)] are AW<INF>2</INF>, −10.37 [−6.70] kcal/mol (MP2/aug-cc-pVTZ) and AW<INF>3</INF>, −17.80 [−11.46] kcal/mol (MP2/aug-cc-pVDZ). The global minimum of AW<INF>4</INF> corresponds to a van der Waals complex between a cyclic water tetramer W<INF>4</INF> and A with an interaction energy of −28.01 [−18.67] kcal/mol (MP2/aug-cc-pVDZ). The 4 and 10 local minima for the x = 2 and 3 clusters span an energy range of 4.3 and 6.1 kcal/mol above the respective global minima. For AW<INF>4</INF>, the energy range for the 30 minima is 14.1 kcal/mol; however, the first 28 lie within 8.4 kcal/mol above the global minimum. The analysis of the many-body interaction energy terms suggests that the global and low-lying ring networks are stabilized by the maximization of the many-body (mainly the 3-body) terms, whereas the higher lying minima are mainly described by 2-body interactions.