Tautomerism, Raman, infrared and ultraviolet-visible spectra, vibrational assignments, MP2 and B3LYP calculations of dienol 3,4-dihydroxypyridine, keto-enol 3-hydroxypyridin-4-one and keto-enol dimer

Raman (3500-100 cm^-1) and infrared (4000-200 cm^-1) spectra of 3,4-dihydroxypyridine (3,4-DHP) have been recorded in the solid phase. In addition, the UV spectrum (350-190 nm) of 3,4-DHP was measured in ethanol solution. Thirteen structures were initially proposed for 3,4-DHP as a result of keto-enol tautomerism and rotation(s) of hydroxyl group(s) around the CO bond. The conformational energies have been calculated with the methods of MP2, MP2(full) and B3LYP/DFT utilizing a variety of basis sets up to 6-311++G(d,p). Moreover, TD-DFT/B3LYP/6-311+G(d,p) computations of dienol (DHP) and keto-enol (HPO) tautomers were used to predict the electronic absorption spectra in ethanol solution utilizing a PCM. The theoretical results were compiled with infrared and Raman spectral data, favoring a mixture of dienol 3,4-dihydroxypyridine (structure 2) and keto-enol 3-hydroxypyridin-4-one (structure 9) in the solid phase. However, the keto-enol HPO tautomer is favored in solutions in agreement with the observed/calculated UV spectra. Moreover, mass spectral analysis indicates the presence of equimolar proportions of 3,4-DHP monomer and its dimer. Aided by DFT/B3LYP and ab intio/MP2(full) frequency calculations at 6-31G(d) basis set and the simulated vibrational spectra of dienol DHP and keto-enol HPO mixture, a complete vibrational assignment of the observed infrared and Raman bands has been proposed supported by normal coordinate analysis and potential energy distributions (PEDs). The results reported herein are compared with similar structural analogues whenever appropriate.