Low temperature X-ray molecular structure, tautomerism and spectral properties of 2,3-dihydroxyquinoxaline

The solid state molecular structure of 2,3-dihydroxyquinoxaline (DHQ) has been studied using X-ray single crystallography. The equilibrium geometry of six possible DHQ isomers have been calculated using the B3LYP/6-311++G(d,p) method in order to predict the most stable gas-phase isomer. The effect of solvent polarity on the relative stability of these isomers is assessed at the same level of theory using PCM. In agreement with the experimental results, it was found that keto form, DHQ1, is the most stable isomer both in the gaseous state and solution. The scaled harmonic vibrational frequencies are in good agreement with the experimental data. The effect of solvent on the vibrational frequencies of the DHQ1 tautomer showed a bathochromic shift for the υ(CO) and υ(NH) bands. These shifts increase with the polarity of the solvent. The electronic transitions of the most stable isomer DHQ1 were calculated using TDDFT and the NMR chemical shifts were calculated using the GIAO method. The NLO properties are predicted to be greater than urea by a factor of five. Two stable dimer forms of DHQ were calculated at same level of theory. Dimer A is more stable by 13.38 kcal mol−1 than dimer B. The former is stabilized by stronger NH⋯O H-bonds compared to the weaker CH⋯O interactions in the latter. The effect of these H-bonding interactions on the molecular structure and vibrational spectra of these compounds are predicted.