The trans/gauche conformational equilibrium and associated thermodynamic parameters of liquid 1,2-dibromoethane as studied by infrared electroabsorption spectroscopy

Revisiting the Conformational Isomerism of Dihaloethanes: A Hybrid Computational and Experimental Laboratory for the Undergraduate Curriculum

The conformational isomerism of disubstituted ethanes is a well-known concept that is part of every chemistry curriculum. Due to the species' simplicity, studying the (free) energy difference between the gauche and anti isomers has been the testing ground of experimental and computational techniques, such as Raman and IR spectroscopy, quantum chemistry, and atomistic simulations. While students normally receive formal training in spectroscopic techniques during their early undergraduate years, computational methods often receive less attention. In this work, we revisit the conformational isomerism of 1,2-dichloroethane and 1,2-dibromoethane and design a hybrid computational and experimental laboratory for our undergraduate chemistry curriculum with a focus on introducing computational techniques as a complementary research tool to experimentation. We show how commonly available Raman spectrometers and atomistic simulations performed on desktop computers can be combined to study the conformational isomerism of disubstituted ethanes while discussing the advantages and limitations of the different approaches.

Observation of the Pockels Effect in Ionic Liquids and Insights into the Length Scale of Potential-Induced Ordering

Ionic liquids (ILs) under electric fields play essential roles in the electrochemical utilization of ILs. Recently, long-range organization of ILs in the vicinity of charged (and even neutral) surfaces has been revealed, but experimental evidence for such an ordering is still limited and its spatial length scale remains controversial. Here, we use confocal Raman microspectroscopy to investigate the effect of an applied electric potential on the IL 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and its analogues in a space-resolved manner. Much to our surprise, the observed Raman difference spectra of the ILs obtained with and without an applied potential exhibit uniform intensity changes independent of vibrational modes of cations and anions, a finding in sharp contrast with the electric field effects on molecular liquids that we have previously observed. We interpret this unexpected finding in terms of the Pockels effect that occurs as a result of a potential-induced ordering of the IL near an IL-electrode interface. The refractive index changes due to the applied potential are estimated using the experimental Raman intensity changes. The results allow us to deduce that the length scale of the ordering in the ILs is tens to hundreds of nanometers, extending more than would be expected for the electrical double layer but not as far as a micrometer scale.

Tautomer Structures in Ketose-Aldose Transformation of 1,3-Dihydroxyacetone Studied by Infrared Electroabsorption Spectroscopy

The acyclic form of monosaccharides exists in a structural equilibrium, with aldose having the aldehyde group and ketose the ketone group (ketose-aldose equilibrium). A basic catalyst facilitates their transformation, which affects the chemical properties of the monosaccharide. In this study, we investigated the ketose-aldose transformation of 1,3-dihydroxyacetone (1,3-DHA), one of the simplest systems of the ketose-aldose equilibrium. We examined the effects of piperidine as the basic catalyst and used IR electroabsorption spectroscopy to study the responses to an external electric field. We analyzed the changes in IR absorption by considering the changes in the molecular orientation and number of molecules in response to the external electric field. The results of the analysis revealed the permanent dipole moment μ_P, an angle η between μ_P and μ_T (the transition moment of the molecular vibration), and the equilibrium constants. The ketose-aldose transformation of 1,3-DHA can be explained in terms of the equilibrium of three states. In the presence of piperidine, a five-state equilibrium was concluded. On the basis of the experimental data, we propose plausible models of dihydroxyacetone, E-enediols, Z-enediol, or glyceraldehyde for each state. The results of our structural analysis of these tautomers provide a detailed understanding of the ketose-aldose transformation of acyclic saccharides and the effects of the basic catalyst.