Structural Features of Triethylammonium Acetate through Molecular Dynamics

Abstract

I have explored the structural features and the dynamics of triethylammonium acetate by means of semi-empirical (density functional tight binding, DFTB) molecular dynamics. I find that the results from the present simulations agree with recent experimental determinations with only few minor differences in the structural interpretation. A mixture of triethylamine and acetic acid does not form an ionic liquid, but gives rise to a very complex system where ionization is only a partial process affecting only few molecules (1 over 4 experimentally). I have also found that the few ionic couples are stable and remain mainly embedded inside the AcOH neutral moiety.

Keywords: ionic liquids; molecular dynamics; semi-empirical methods.

Figure 1

Calculated IR absorption spectra. Top panel: total IR spectra. Bottom panel: contribution from AcOH/AcO^– (black) and from TEA/TEAH⁺ (red).

Figure 2

Energy profile of the AcOH/TEA pair as a function of the AcO–H distance (see text for details). The black dots are the M062X/def2-SVP optimized energies. The red ones are M062X/def2-TZVP single point energies, and the blue ones are the density functional tight binding (DFTB) results. Left: scan without any further constraint except the O–H distance. Right: scan with an additional constraint that keeps the N–O distance fixed at its equilibrium value. To allow for comparison, the minimum energy in each set was set equal to zero. The structures on top correspond to the first and last geometries in each scan.

Figure 3

Solid lines: center-of-mass radial distribution functions between the components of the fluid regardless of their charge status as obtained from the 20:20 simulation. The dashed line was obtained from the I20:20 system to show where the pair distribution would peak in a highly ionized system.

Figure 4

Interatomic g(r) for the H-bonds in AcOH (left, black lines) and for AcOH–TEA interactions (right, red lines). In green, on the right, I have reported the N–N interactions between TEA molecules.

Figure 5

Top: N–H distances as a function of time for the four TEA molecules that acquire the proton from AcOH. Bottom left: the initial simulation cell where the protons attached to AcO are shown as white spheres. Bottom right: the simulation cell at the end of the production (after 300 ps) where four protons have migrated onto TEA (yellow spheres).

Figure 6

One of the AcOH→TEA proton transfers. The plot reports the O–H and N–H distances (red and blue, respectively). The superimposed snapshots show the geometric configurations of the involved molecules along the dynamics (around 75, 130, and 200 ps).

Figure 7

One of the double AcOH→AcOH proton transfers. The two plots report the four O–H distances involved, in red and black for acceptors and donors, respectively. The two snapshots below show the geometric configurations of the involved molecules before and after the transfer.

Figure 8

Radial distribution functions characterizing the immediate surroundings of two of the [TEAH⁺][AcO^–] ionic pairs in the 20:20 simulation.

Figure 9

N–H g(r) in the 8:2 simulation and relative volumetric integral (that is, the coordination number) for different sampling intervals. The black lines are obtained by sampling the entire trajectory (950 ps), the red one by sampling the second half (475 ps), and the blue one by sampling the last third (320 ps).

Figure 10

N–H distances in TEAH⁺ during the simulations initially prepared as a completely ionized system (I20:20). Distances were saved every 500 fs. The migrating protons are highlighted by different colors, while all the others are reported in gray.