Computational study of dynamic motions and possibility of crossing between surfaces in 3-hydroxy-5-(1H-pyrrol-2-yl)-2H-pyrrol-2-one

Potential energy surfaces (PESs) for tautomerism and two dynamic motions of 3-hydroxy-5-(1H-pyrrol-2-yl)-2H-pyrrol-2-one and its tautomer were calculated using density functional theory. Calculated energies confirm that T1 is 10.95 kJ mol^? 1 more stable than T2. Dynamic study of possible motions shows the high energy-level transition state at D8 = 90° for ring rotation and at D15 = 80° and 90° for rotation of OH bond, respectively, in T1 and T2. In addition, calculated rate constant for conversion of T1 to T2 (tautomerism) is 116 M^? 1 s^? 1, for relative rotations of rings in T1 and T2 are, respectively, 5.62 × 10^? 2 and 1.53 M^? 1 s^? 1 and for rotations of OH bond in T1 and T2 are, respectively, 3.56 × 10⁵ and 6.15 × 10⁴ M^? 1 s^? 1. In the next part of the study, orbital occupancies, natural bond orbitals (NBO) charges and hybridisation in relative rotation of rings and internal reaction coordinate (IRC) steps have been extracted to study the possibility of level crossing. These data show that PESs for IRC and ring rotation have different symmetries. So that these two potential curves cannot have effective non-adiabatic level crossing. However, the levels are weakly avoided or the possibility of level crossing is higher than related system because the energy difference between their barrier energies is not very high.