Extended tunnelling states in the benzoic acid crystal: Infrared and Raman spectra of the OH and OD stretching modes

Abstract

We compare Raman and infrared spectra of the nuOH/OD modes in benzoic acid crystal powders at 7 K. The extremely sharp Raman bands contrast to the broad infrared profiles and suggest adiabatic separation of hydrogen (deuterium) dynamics from the crystal lattice. There is no evidence of any proton-proton coupling term. The assignment scheme is consistent with a quasisymmetric double-minimum potential, largely temperature independent. Tunnel splitting is a major band shaping mechanism, in addition to anharmonic coupling with lattice modes. The proton/deuteron dynamics are rationalized with nonlocal pseudoparticles and extended states. We propose a symmetry-related damping mechanism to account for the broad infrared profiles, as opposed to the sharp Raman bands. We assign spectral features to distinct interconversion mechanisms based on either pseudoparticle transfer or adiabatic pairwise transfer. We establish close contacts with theoretical models based on first-principles calculations.

Unusual behavior of benzoic acid at low temperature: Raman spectroscopic study

“…[9][10][11][12][13][14][15]. However, at T = 0 K the dimers are ordered in a unique configuration L ( Figure 1) throughout the crystal.…”

Evidence of macroscopically entangled protons in a mixed isotope crystal of KHpD1−pCO3

“…By contrast, vibrational spectra are consistent with nonlocal observables represented by normal coordinates. There is no evidence of any symmetry breaking due to local disorder and the density-of-states remains discrete at any temperature [9,10,11].…”

Neutron scattering studies of K3H(SO4)2 and K3D(SO4)2: The particle-in-a-box model for the quantum phase transition

“…However, this is clearly excluded by the crystal symmetry and vibrational spectra. 4 Quasi-symmetric double wells have been reported for centrosymmetric dimers of the hydrogencarbonate family (MHCO 3 with M = K, Rb, Cs), 5,6 or benzoic acid, 7,8 for which R OO ≈ 2.60Å, but these hydrogen bonds are not symmetric.…”