We present a theoretical study of the structure, local curvature angles and reactivity of cap-ended (7,0), defective and non-defective carbon nanotubes. We find that the most reactive sites are the atoms that form part of the caps even when the Stone-Wales defect is present. Each carbon in the carbon nanotube is located at the top of a pyramidal structure with three walls of 5-, 6- or 7-carbon rings. Among the carbons making up the caps, the most reactive sites are the top pyramidal atoms between two 5-carbon rings and one 6-carbon ring and each 5-carbon ring has attached another 5-carbon ring. The least reactive sites are the top pyramidal atoms between three 6-carbon rings. The activity of each pyramidal structure is strongly correlated to its local curvature angle. The interaction of the carbon nanotubes with one water molecule confirms the location of the most active site and we find that molecular water can form stable complexes with the nanotubes. There is an energy barrier for water to adsorb molecularly. The dissociation of water produces a hydroxyl group and a hydrogen atom united each to two adjacent carbon atoms. The dissociation process of water on carbon nanotubes is energetically favorable starting from the molecularly adsorbed state and less favorable starting from the isolated molecules.

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