Proton delocalization and tunneling in terephthalic acid: Raman spectroscopic study

Raman spectra of terephthalic acid (TPA), regular TPA(H) and deuterated TPA(D), were recorded in the temperature range of 5–300 K. It is shown that temperature dependences of vibration frequencies provide both the data on proton tunneling in tautomeric bonds and the data on phonon-assisted proton jumps. At the same time, the quantum delocalization of proton (deuteron), i.e. penetration of proton (deuteron) density distribution function into the neighboring potential well, affects the temperature dependence of the frequency of translational vibrations O⋅⋅⋅O and is observed at T ≤ 180 K in TPA(H) and at T ≤ 170 K in TPA(D). Proton tunneling through the barrier affects the temperature dependence of the frequency of C[dbnd]O stretchings in carboxyl groups and is observed at T ≤ 100 K. No coordinated proton (deuteron) jumps over the potential barrier with a rate capable to change notably the vibration frequency of C[dbnd]O bonds was observed in TPA over the entire temperature range.