Molecular orbital study of the structure and interactions of ylidene rhodanines

Abstract

Semiempirical (with the AM1 Hamiltonian) and ab initio (mainly with the 6-31G* and LANL2DZ + + (d,p) basis sets) molecular oribital calculations show the predominant tautomer of gas phase methylidene rhodanine is 2-thioxo--4-thiazolidinone in agreement with earlier work on other types of rhodanines. Inclusion of solvation effects in the AM 1 calculations corroborates with this tautomer is also preferred in aqueous solution. Energy-optimized bond lengths and angles show good agreement with those for a crystalline benzylidene rhodanine. The geometry of a hydrogen bonded dimer matches closely the crystalline state arrangement. The two N--H 9 9 9 O hydrogen bonds that form in the dimer provide a stabilization energy of about -10 kcal/mol. The interactions of methylidene rhodanine with a calcium ion are modeled with basis sets as large as LANL2DZ+ + (d,p). The preferred binding site is near the carbonyl oxygen for the neutral rhodanine and near the nitrogen and thione sulfur in a bidentate arrangement for the rhodanine anion. Implications for the interaction of benzylidene rhodanines with various possible protein and metalloprotein receptors are discussed.

Theoretical study of the dimerization of rhodanine in various tautomeric forms

“…The hydrogen bonding energy in this dimer is -9.58 kcal/mol. The hydrogen bonding stabilization energy calculated by the RHF/6-31G* method for the analogous dimer of methylidene rhodanine by Loncharich et al [13] is -10.3 kcal/mol, which is close to our value.…”

“…The enol and thioenol tautomers can each form two rotamers, while a total of eleven tautomers and rotamers exist taking account of the possible dissociation of rhodanine at the methylene group CH bond [17]. Thus, we might expect the existence of a wide variety of dimeric complexes, which may be formed in crystals, solutions, and the gas phase.Loncharich et al [13] have carried out an X-ray diffraction structural analysis and experimentally identified some of the dimers predicted in our earlier work. This offered the prospect of identifying other dimeric complexes.…”

“…Loncharich et al [13] have carried out an X-ray diffraction structural analysis and experimentally identified some of the dimers predicted in our earlier work. This offered the prospect of identifying other dimeric complexes.…”

“…The X-ray diffraction structural data [13,14] indicate that rhodanine and its derivatives in the crystalline state exist just as these dimers. The hydrogen bonding energy in this dimer is -9.58 kcal/mol.…”

“…The rhodanine group in dye compositions acts as an electron density acceptor. Rhodanine also forms strong colored coordination compounds with many heavy metal ions such as Pd 2+ , Pt 2+ [8], Ag + , Au + [9], Cu + [10], Ni 2+ , Co 2+ [11], Cu 2+ , Hg 2+ , Au 3+ , and Ru 3+ [12] what is used for their spectrophotometric determination.Rhodanine may exist in ketone, enol, and thioenol tautomeric forms [13][14][15][16][17] due to the presence of ketone and thioketone functional groups in a molecule and an active hydrogen atom of the NH bond (with dissociation constant K d = 10 -8 in water [13,14]). The enol and thioenol tautomers can each form two rotamers, while a total of eleven tautomers and rotamers exist taking account of the possible dissociation of rhodanine at the methylene group CH bond [17].…”