Breaking the Structure of Liquid Hydrogenated Alcohols Using Perfluorinated tert-Butanol: A Multitechnique Approach (Infrared, Raman, and X-ray Scattering) Analyzed by DFT and Molecular Dynamics Calculations

The state of aggregation at room temperature of tert-butanol (TBH) and perfluoro tert-butanol (TBF) liquid mixtures is assessed by vibrational spectroscopy (Raman and infrared) and X-ray diffraction and analyzed using density functional theory (DFT) and molecular dynamics (MD) simulations. It is shown that larger clusters (mostly tetramers) of TBH are destroyed upon dilution with TBF. Small oligomers, monomers, and mainly heterodimers are present at the equimolar concentration. At variance with slightly interacting solvents, the signature of hetero-oligomers is shown by the appearance of a new broad band detected in the infrared region. The same spectral observation is detected for mixtures of other hydrogenated alcohols (methanol and 1-butanol). The new infrared feature is unaffected by dilution in a polar solvent (CDCl3) in a high-concentration domain, allowing us to assign it to the signature of small hetero-oligomers. MD simulations are used to assess the nature of the species present in the mixture (monomers and small hetero-oligomers) and to follow the evolution of their population upon the dilution. Combining MD simulations with DFT calculations, the infrared spectral profile is successfully analyzed in equimolecular mixtures. This study shows that TBF is a structure breaker of hydrogen-bonded alcohol networks and that the TBF (donor)–TBH (acceptor) heterodimer is the dominant species in an extended range of concentration, centered in the vicinity of the equimolar fraction.


Experimental conditions
The Raman spectra were measured with a resolution of 4 cm -1 in the spectral range 200 cm -1 to 3800 cm -1 on a Horiba Jobin-Yvon XploRA spectrometer using lasers diode operating at a wavelength of 785 nm and 638 nm in a back-scattering geometry. Typical spectra have been collected during 20 seconds and accumulated 60 times to improve the signal-to-noise ratio. In order to take accurate line positions the spectrometer was calibrated by recording different emission lines of a neon bulb.
The Infrared spectra were measured on a Bruker-Alpha FT-IR spectrometer with a 4 cm -1 resolution in the spectral range 400 cm -1 to 4000 cm -1 after collecting 64 scans. We use KBr windows for the absorption Specac Omni-cell and mylar or lead spacers of 6 or 25 micrometer, respectively. The path-lengths have been controlled on the empty cell using the standard method based upon interferences fringes.
X-ray diffraction measurements were performed using a variable geometry device equipped with a Max-Flux TM Optic grade multilayer monochromator for Cu Kα radiation and a gas curved counter INEL CPS 590. It allows obtaining patterns on a range of momentum transfer Q from 0.15 to 2 Ǻ -1 . Samples were contained in a capillary tube of 1 mm diameter. . Raman spectra of pure tert-butanol in the spectral domain of ν OH -stretching vibration: hydrogenated (black), tert-butanol-D9 (red).

Raman Spectra of the ν OH stretching of TBH corrected from Combination Bands
The Raman spectrum of the ν OH stretching vibration of TBH presents on its low frequency side two bands assigned to combinations vibrations of the CH 3 group. The former, observed S3 at about 3190 cm -1 , involves the symmetric stretching ν s of the methyl with the out of plane bending vibration τ and the latter, at about 3245 cm -1 , the asymmetric stretching vibration ν a with the τ bending. [1][2][3] The spectrum of TBH has been corrected from these combination bands using the spectrum of deuterated tert-butanol D9 which is free from these combination transitions (Figure S1 [TBH] molar concentration Figure S2. Integrated intensity of the infrared spectra of equimolar TBH -TBF mixtures diluted in CDCl 3 measured in the spectral domain 2300-3500 cm -1 versus the TBH molar concentration. Intensities are corrected from the C-H stretching contributions (domain 2800 cm -1 to 3050 cm -1 ).  Figure S3. Infrared spectra of TBH -TBF non equimolar mixtures diluted in CDCl 3 . The spectrum of the diluted equimolar mixture.is displayed for comparison (black).

Molecular models
All the intramolecular and dispersive interactions of TBH were modelled as in the original OPLS-AA paper. 4 The model for TBF was adapted from the forcefield of 2,2,2trifluoroethanol (CF 3 CH 2 OH) developed by the same research group, 5,6 by replacing each H atom of the CH 2 group with another CF 3 group; the single missing dihedral torsion function (F-C-C-C) was taken from the OPLS-AA model of perfluoroalkanes. 7 The electrostatic interactions of both molecules were modelled by assigning atom-centered partial charges, derived 8 from ab initio calculated electron density distributions using the CHELPG method. 9 The full set of atomic partial charges is shown in

Simulation details
The simulations were performed using the GROMACS 5.0.7 Software, 11 with systems consisting of either 300 or 3000 total molecules in cubic simulation boxes with periodic S5 boundary conditions in all directions. A cut-off distance of 14 Å was used for both nonbonded Lennard-Jones and electrostatic potentials, with the application of standard analytic tail corrections for the energy and pressure dispersion terms, and of the particle-mesh Ewald method to the electrostatic interactions beyond the cut-off. A time step of 2 fs was used, with all bonds involving hydrogen atoms constrained to their equilibrium distances using the LINCS algorithm.
All simulations started from random low-density configurations, to which a steepest gradient energy minimization procedure was applied to relax any unphysical high energy contacts between the molecules. A pre-equilibration simulation in the NpT ensemble was performed to each system until its density reached a constant value, using the Berendsen thermostat and barostat with coupling constants of 0.5 and 1 ps, respectively. The systems were then simulated in the NpT ensemble at 298.15 K and 1 bar for at least 2 ns, to determine the equilibrium volume, and the dimensions of the final configuration were rescaled to this value.
The simulation results shown were calculated from subsequent NVT trajectories of at least 6 ns, discarding the first 1 ns for equilibration. The Nosé-Hoover thermostat and Parrinello-Rahman barostat were used in the latter runs to control the temperature and pressure, with coupling constants of 0.5 ps and 10.0 ps, respectively. Table S2. Probability p'(zz) of finding an aggregate of a given type zz as a function of the TBH-TBF mixture composition, obtained from the molecular dynamics simulations.

DFT Calculations details
The DFT calculations were carried out using the program Gaussian16.C01 package. 12 Table S7. DFT scaled band centre frequency and infrared activity. The probability of found an aggregate in the equimolar mixture TBH-TBF was obtained from MD (Table S2.). The integrated intensity associated to each aggregate was calculated multiplying its IR activity by its probability.