Energetics, dynamics and infrared spectra of the DNA base-pair analogue 2-pyridone·2-hydroxypyridine

The energetics and infrared (IR) spectrum of the double proton transfer (DPT) in the hydrogen bonded 2-pyridone·2-hydroxypyridine (2PY·2HP) dimer are studied using quantum chemical and classical molecular dynamics methods and compared with recent experimental work. The experimental spectrum shows IR absorption between 2500 and 3000 cm−1 (with peak absorption at 2700 cm−1). High resolution spectroscopy of the S1 ← S0 origin of 2PY·2HP have revealed a tunneling splitting which was attributed to DPT. MP2 and density functional calculations predict DPT barriers between 8 and 8.5 kcal mol−1 in the electronic ground state. Two-dimensional potential energy surfaces and the minimum-energy reaction path (MEP) for double proton transfer are calculated: Near the minima, the MEP is dominated by the 2PY·2HP intermolecular stretch, whereas close to the transition state the MEP corresponds to almost purely biprotonic exchange motion. Classical MD trajectories are calculated using the self consistent charges–density functional tight binding method at six different internal energies. Using activated dynamics, classical IR spectra are calculated from the time-dependence of the dipole moment function. The first ≈500 fs of the dynamics show broad spectral features in the mid-IR between 2000–2800 cm−1, as found in the experiment. These early phases of the activated dynamics are associated with large-amplitude vibrations of the N–H and O–H protons and may lead to double proton transfer. After 500 fs, the broad IR features disappear from the 2000–2800 cm−1 range.