Spectrophotometric, Fourier transform infrared spectroscopic and theoretical studies of the charge–transfer complexes between methyldopa [(S)-2 amino-3-(3,4-dihydroxyphenyl)-2-methyl propanoic acid] and the acceptors (chloranilic acid, o-chloranil and dichlorodicyanobenzoquinone) in acetonitrile and their thermodynamic properties

Abstract

Methyldopa is a much used antihypertensive drug. It is the subject matter of study mostly for the determination and estimation of methyldopa in pharmaceutical properties. These considerations led us to study the charge–transfer interactions between methyldopa, a centrally acting antihypertensive agent of limited use with the known acceptors like o-chloranil (o-ClN), chloranilic acid (ClA) and dichlorodicyanobenzoquinone (DDQ). Methyldopa (MDP) formed beautifully colored complexes (having absorption maxima at 581nm and 368nm; 519nm; 583.5nm, 547nm and 346nm, respectively) with the acceptors mentioned before. The physico-chemical properties of the complexes were studied using UV–visible spectrophotometry and FTIR measurements. The composition, the accurate association constants and thermodynamics of the complexes were determined spectrophotometrically. Attempts were made to interpret the thermodynamics of complexes in terms of IDV, EAV and hνCT. Solid CT complexes between MDP+o-ClN, MDP+ClA and MDP+DDQ were prepared and FTIR spectra of the complexes were studied. The energies hνCT of the charge–transfer complexes and vertical ionization potential IDV of methyldopa were compared with the theoretical values of hνCT obtained from HOMO and LUMO of the donors and acceptors calculated using Density Function Theory utilizing different basis sets. The agreement between the results can be regarded to be reasonable. Oscillator strengths and dipole strengths of the complexes were determined theoretically and experimentally and the limitations of the calculations were outlined.