Quantum proton transfer and interconversion in the benzoic acid crystal: vibrational spectra, mechanism and theory Original Research Article

The infrared and Raman spectra of powdered crystals at various temperatures from liquid helium to 300 K of fully hydrogenated benzoic acid (BA-h6), ring deuterated (BA-d5h) and OD (BA-h5d) analogues are presented. In the frequency range corresponding to internal modes there is no evidence for thermal equilibrium due to tautomerisation. Noticeable temperature effects are observed in Raman for a band at ∼80 cm−1. As the temperature increases, the band intensity decreases according to an Arrhenius law with activation energy of (54±6) cm−1. It is concluded that interconversion of protons occurs upon specific excitation of the lattice mode observed at νR′=55 cm−1. In the ground state all dimers have the same configuration throughout the crystal, whilst the excited phonon state is a coherent superposition of the two tautomers. An isotopic mixture containing 5% of OH and 95% of OD residues demonstrates the absence of dynamical correlation between protons. The OH (OD) stretching band profiles in the infrared are decomposed into two broad components whose shapes are rationalised with the strong coupling scheme for the ν OH and ν O⋯O coordinates. The splitting is attributed to proton tunnelling in the n=2 and 3 states of a quasi-symmetric double minimum potential. This assignment is confirmed with the 0→1 transition (the splitting of the ground state) observed with the inelastic neutron scattering technique at ν01=172 cm−1 for BA-d5h. The potential governing the quantum transfer dynamics of a single proton is thus totally determined from experimental data. The potential barrier is ∼5000 cm−1 and semiclassical jumping over the top is negligible. We propose a theory based on uncorrelated transfer of protons via thermally activated tunnelling to account for the interconversion dynamics. The analytical function for the interconversion rate versus temperature accounts for NMR T1 and quasi-elastic neutron scattering (QENS) measurements. Both the potential function for protons and the coherent superposition of tautomers survive at room temperature.