The hydrogen-bond collective dynamics in liquid methanol

The relatively simple molecular structure of hydrogen-bonded (HB) systems is often belied by their exceptionally complex thermodynamic and microscopic behaviour. For this reason, after a thorough experimental, computational and theoretical scrutiny, the dynamics of molecules in HB systems still eludes a comprehensive understanding. Aiming at shedding some insight into this topic, we jointly used neutron Brillouin scattering and molecular dynamics simulations to probe the dynamics of a prototypical hydrogen-bonded alcohol, liquid methanol.

The comparison with the most thoroughly investigated HB system, liquid water, pinpoints common behaviours of their THz microscopic dynamics, thereby providing additional information on the role of HB dynamics in these two systems. Figure 1 shows how the MD simulation data (blue curve) faithfully reproduce the experimental data Iexp(Q, E) once the multiple scattering (green curve) and instrumental resolution are taken into account, and emphasizes the overall accuracy of both measurements and calculations.
The dispersion curves obtained by analyzing the center-of-mass dynamic structure factor and the longitudinal and transverse currents are shown in Fig. 2 portray a dynamic behaviour of methanol much richer than what so far known, and prompts us to establish striking analogies with the features of liquid and supercooled water. In particular, based on the strong differences between the structural properties of the two systems, our results suggest that the assignment of some dynamical properties to the tetrahedral character of water structure should be questioned.
A pictorial representation of the coupling between the exchanged wavevector Q and the longitudinal and transverse waves propagating in water (tetrahedral) and methanol (non tetrahedral) is given in Fig. 3.

Caption Fig.1: Fit of neutron Brillouin data obtained for liquid deuterated methanol at T = 298 K at Q= 0.4 and 10 nm-1. The red line is the result of a linear combination of the simulation (blue dots connected by a solid line) and multiple scattering (green dashes) data to the experimental spectra.

Caption Fig.2: Frequencies of longitudinal (open symbols) and transverse (closed symbols) modes for liquid deuterated methanol at T=298K. The black line is the linear dispersion corresponding to the adiabatic sound speed of cs = 1100 m/s while the dashed line corresponds to an apparent propagation velocity of ~2750 m/s. 

Caption Fig.3: Schematic representation of longitudinal and transverse modes contributing to the THz spectrum of liquid water and methanol.

Reference:  S. Bellissima, M. A. Gonzalez, U. Bafile, A. Cunsolo, F. Formisano, S. De Panfilis, and E. Guarini, “Switching of  hydrogen-bond-driven excitation modes in liquid methanol”, Sci. Rep. 7, Article number: 10057 (2017).