HYDROGEN BOND INTERACTION BETWEEN METHYLENE BLUE AND WATER

ABSTRACT: Density-functional theory calculations using B3LYP/6-31+G(d) are performed on the hydrogen bond interaction between methylene blue (MB+) and water to analyze the structure, binding energy and change in spectroscopic properties. The vibrational frequency observed in the region of 1200 cm−1 is found to present the larger shift (+10 cm−1), and corresponds to the asymmetric in-plane twist mode of C—C bond of the central ring. The binding energy between MB+ and water is calculated for three different isomers giving values varying between 3.1 and 5.1 kcal/mol, after correcting for basis-set superposition error. The binding energy is also calculated using Hartree–Fock and other density-functionals, such as B3P86/6-31+G(d) and B3PW91/6-31+G(d). In the most stable isomer considered, the water plays the role of the proton acceptor and the oxygen atom makes multiple hydrogen bonds with MB+. The stability of this isomer is also influenced by other electrostatic interaction between MB+ and water. The influence of the complexation on the characteristic visible absorption band of MB+ is analyzed using INDO/CIS. Overall, the results indicate that several hydrogen bonds can be formed and some present multiple bonds. However, they all seem to be numerically unimportant fo