Ab initio SCF and DFT(B3LYP) calculations are performed with 6-311G** basis sets for obtaining insights into molecular internal rotations in HXNX (X = O,S), different vibrational modes in water and double proton transfer reaction in (HONO) 2 . While chemical reactivity is analyzed in terms of the profile of the global reactivity parameters, such as energy, chemical potential, hardness, polarizability, molecular valency and electrophilicity indices, the site selectivity is understood through the variations in local descriptors, such as the Fukui function and atomic valency. Principles of maximum hardness and molecular valency and the minimum polarizability principle are found to be valid in almost all cases. Rotational isomerization reactions can be better characterized by making use of the maximum hardness principle along with Hammond's postulate. Extremum points in electrophilicity during internal rotations, vibrations and chemical reaction can be located from those of chemical potential and hardness. The Fukui function and atomic valency show inverse behaviour in most cases.