Effect of pressure on molecular conformations. III. Internal rotation angle in gauche‐1,2‐dichloroethane and gauche‐1,2‐dibromoethane

Trapping the Transition State in a [2,3]-Sigmatropic Rearrangement by Applying Pressure

Transition states are of central importance in chemistry. While they are, by definition, transient species, it has been shown before that it is possible to "trap" transition states by applying stretching forces. We here demonstrate that the task of transforming the transition state of a chemical reaction into a minimum on the potential energy surface can be achieved using hydrostatic pressure. We apply the computational extended hydrostatic compression force field (X-HCFF) approach to the educt of a [2,3]-sigmatropic rearrangement in both static and dynamic calculations and find that the five-membered cyclic transition state of this reaction becomes a minimum at pressures in the range between 100 and 150 GPa. Born-Oppenheimer molecular dynamics (BOMD) simulations suggest that slow decompression leads to a 70:30 mix of the product and the educt of the sigmatropic rearrangement. Our findings are discussed in terms of geometric parameters and electronic rearrangements throughout the reaction. To provide reference data for experimental investigations, we simulated the IR, Raman, and time-resolved UV/vis absorption spectra for the educt, transition state, and product. We speculate that the trapping of transition states by using pressure is generally possible if the transition state of a chemical reaction has a more condensed geometry than both the educt and the product, which paves the way for new ways of initiating chemical reactions.

Loose crystals engineered by mismatched halogen bonds in hexachloroethane

The shortest intermolecular contacts in the engineered loose crystal of hexachloroethane are longer than the sum of van der Waals radii, reached only at the pressure of 1.2 GPa.

High-pressure Induced Conformational and Phase Transformations of 1,2-Dichloroethane Probed by Raman Spectroscopy

1,2-Dichloroethane (DCE) was loaded into diamond anvil cells and compressed up to 30 GPa at room temperature. Pressure-induced transformations were probed using Raman spectroscopy. At pressures below 0.6 GPa, fluid DCE exists in two conformations, gauche and trans in equilibrium, which is shifted to gauche on compression. DCE transforms to a solid phase with exclusive trans conformation upon further compression. All the characteristic Raman shifts remain constant in fluid phase and move to higher frequencies in the solid phase with increasing pressure. At about 4-5 GPa, DCE transforms from a possible disordered phase into a crystalline phase as evidenced by the observation of several lattice modes and peak narrowing. At 8-9 GPa, dramatic changes in Raman patterns of DCE were observed. The splitting of the C-C-Cl bending mode at 325 cm-1, together with the observation of inactive internal mode at 684 cm-1 as well as new lattice modes indicates another pressure-induced phase transformation. All Raman modes exhibit significant changes in pressure dependence at the transformation pressure. The new phase remains crystalline, but likely with a lower symmetry. The observed transformations are reversible in the entire pressure region upon decompression.

Conformational Equilibrium of 1,2-Dichloroethane in Water: Comparison of PCM and RISM-SCF Methods

The RISM-SCF and polarizable continuum model (PCM) approaches have been applied to study the conformational equilibrium of 1,2-dichloroethane (DCE) in water. Both the electron correlation effect and basis sets play an important role in the relative energies of the gauche and trans conformers in gas and solution phases. Both PCM and RISM-MP2 methods resulted in a consistent trend with the previous experimental and theoretical studies that the population of the gauche conformer increases in going from the gas phase to the aqueous solution. However, the PCM treatment could not describe the solvent effect completely in that the sign of the relative free energy of the gauche and trans forms is opposite to the most recent experimental and theoretical data, while the RISM-MP2 gives the right sign in the free energy difference. We found that the larger excess chemical potential gain (by ca. -4.1 kcal/mol) for the gauche conformer is large enough to result in the gauche preference of DCE in water, though it has to compensate for more solute reorganization energy (approximately 1.6 kcal/mol) and overcome the energy difference (approximately 1.6 kcal/mol) in the gas phase. The radial distribution functions between DCE and the nearest water shows that the electrostatic repulsion between chlorine and oxygen atoms is higher in the trans conformer than in the gauche one, while the attractive interaction between chlorine and hydrogen of water is higher in the gauche conformer.