Exploring electric field induced structural evolution of water clusters, (H2O)n [n = 9-20]: density functional...

Response of neutral water clusters (H(2)O)(n), n = 9-20, to external uniform dipolar static electric fields is studied for some lowest-energy conformers for each "n" within an energy band of about 9 kcal mol(-1) of their field-free counterparts. We perform density functional theory computations with B3LYP∕6-311++G(2d,2p) model chemistry. Increasing the electric field destabilizes and distorts a cluster by elongating, hence weakening its hydrogen bonds, culminating into a catastrophic structural breakdown beyond a specific threshold field-strength. The electric field induced conformational transitions to extended structures stretched along the field direction to lower-energy configurations that appear as local minima on their potential energy surface are presented. It is observed that a typical structural transition of this type is always accompanied by an abrupt increase in the electric dipole moment of the cluster over and above its smooth increment with increasing applied field; the increase being phenomenal during breakdown. Interestingly, the HOMO-LUMO energy gap for a given conformer is found to diminish with increasing field strength, abruptly approaching zero at structural breakdown. In essence, the structural evolution traced through hydrogen-bond networks of the clusters reveals multiple enhancements in size by "opening up" of three-dimensional morphologies to form net-like structures with less number of hydrogen bonds. These clusters exhibit greater structural complexity than that encountered in the relatively small clusters reported previously.