Tautomerization of Nucleobase Model Compounds: The 4-Pyridinol and 4(1H)-Pyridinone Monomers and Their Dimers

摘要

Nucleobases are important in many biochemical pathways, and one of their key features is that low-energy tautomerization processes can cause large changes in their chemical properties. Similar effects are also seen for photovoltaic molecules such as quinacridones, except that, in these systems, tautomerization is achieved through intermolecular proton transfer. An excellent model for both of these systems is the tautomerization between the 4-pyridinol and 4(1H)-pyridinone monomers and their dimers. Indeed, 4-pyridinol is known to be the most stable monomer in the gas phase, while chemically diverse 4(1H)-pyridinone is the most stable monomer both neat and in solution in polar solvents. We evaluate the energetics of gas-phase tautomerization of both monomers and dimers of these molecules using B3LYP, HCTH, SCF, MP2, MP4, QCISD, CCSD, and CCSD(T) methodologies. For the monomers, estimates of the CCSD(T)/aug-cc-pVTZ energies are obtained, while for the dimers, estimates of basis-set-superposition-error-corrected CCSD/aug-cc-pVDZ energies are obtained. Vibrational analyses are performed at the B3LYP/cc-pVDZ level to determine zero-point energy corrections and OH- and NH-stretch vibrational frequency changes. The hydrogen-bond energies show a clear preference for 4(1H)-pyridinone-containing dimers, and the dimer in which 4-pyridinol donates a hydrogen bond to 4(1H)-pyridinone is calculated to be only slightly higher in energy than the 4-pyridinol dimer.