Ab initio molecular orbital theory and Monte Carlo method with molecular mechanics are used to calculate the hydration numbers and solvent configurations around 1-adamantyl chloride in 324 water molecules. In the highest probability region for distribution of water molecules, a water molecule forms two hydrogen bonds with Cl and the adamantane skeleton; in the second highest probability region, a water molecule is located at the rear side. The CH center dot center dot center dot O interaction in the rear side is enhanced by the Cl substitution. The water orientation in the rear side of 1-adamantyl chloride is different from that around a hydrophobic adamantane molecule. (C) 2011 Elsevier B.V. All rights reserved.