How does the intramolecular proton transfer change on the partial change of the saturation characteristics in 8-hydroxyquinoline? A detailed computational study

Context

Intramolecular proton transfer (IPT) has been investigated in 8-hydroxyquinoline (8-HQ) and one of its derivatives, 5,6-dihydroquinolin-8-ol (8-DQ). Computations identify the global minima as the enol (E) in S₀ and the proton transferred keto (K^*) form in S₁. The barrier for IPT process reduces on excitation.The pathway was investigated by altering IPT coordinate and intrinsic reaction coordinates (IRC). The frontier molecular orbital analysis shows the shift in electron distribution to the pyridine ring upon excitation. The OH stretching vibration shows a red shift upon E → E^* excitation, confirming the possibility of excited state intramolecular proton transfer (ESIPT). These molecules adhere to the maximum hardness principle (MHP) and the minimum electrophilicity principle (MEP). Natural bond orbital (NBO) analysis reveals the onset of hyperconjugative interactions involving the lone pair of N atom in S₁. Ring and bond critical points are identified. A mixed covalent and noncovalent character of the N₁···H₁₂ bonds signifies the onset of ESIPT. DFT computations with M06-2X and B3LYP functionals show some marked differences, particularly in terms of hydrogen bond lengths and charges, demanding experimental results to ascertain the choice of functionals.

Methods

DFT (B3LYP, M06-2X, and ωB97X-D functionals) and ab initio methods (MP2) are used in S₀ whereas CIS and TDDFT in S₁. 6–311 +  + G(d, p) and aug-cc-pVDZ are the corresponding basis sets used. The computations involve the Gaussian 09 program, which include NBO analysis. PED and QTAIM analyses are performed through the Veda 4 and Multiwfn programs.