First principles examination of the acetylene-water clusters, HCCH-(H2O)x, x=2, 3, and 4

The acetylene-water (A-W) interactions have been investigated by examining the van der Waals clusters AW(x) = 2, 3, and 4, at the second order (MP2) perturbation theory using the correlation-consistent basis sets, aug-cc-pVnZ, n = D (AW(2), AW(3), and AW(4)), T(AW(2)). We located 4 minima (m) and 2 saddle points (sp), 10 m and 3 sp, and 30 in and 3 sp on the potential energy surfaces of the AW2, AW3, and AW4 clusters, respectively. We report the fully optimized geometries and interaction energies AE(e), including corrections for basis set superposition error, DeltaE(e)(BSSE), as well as zero-point energies, AE(0)(BSSE), for the various stationary points. The global minima of the AW2 and AW3 clusters are cyclic configurations in which the acetylene molecule inserts into the water hydrogen bonding network. The corresponding interaction energies DeltaE(e)(BSSE)[DeltaE(0)(BSSE)] are AW(2), -10.37 [-6.70] kcal/mol (MP2/aug-cc-pVTZ) and AW(3), -17.80 [-11.46] kcal/mol (MP2/aug-cc-pVDZ). The global minimum of AW4 corresponds to a van der Waals complex between a cyclic water tetramer W-4 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 AW4, the energy range for the 30 minima is 14.1 kcal/mol; however, the first 28 he 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.