Solvents are an indispensable part of the chemist's toolbox, providing the medium for almost all of organic chemistry to take place. Trifluoroethanol (TFE) is commonly used as a co-solvent with water for protein folding studies. However, when mixed with water, TFE displays some unusual behaviour, such as the formation of small self-aggregates, leading to incomplete mixing of the two solvents at the microscopic scale. We do not yet know the precise structure of these aggregates, but they are expected to strongly influence the behaviour of these co-solvents and their solutes. Now, writing in Angewandte Chemie International Edition, Yunjie Xu and co-workers have used rotational spectroscopy, coupled with first-principles calculations, to shed light on the intermolecular interactions governing the behaviour of TFE on its first steps towards aggregation in the condensed phase.
The TFE molecule can exist in three conformations: gauche(+), gauche(−) and trans. “One particular phenomenon that drew our attention is that the trans configuration of TFE appears to be unstable when isolated in the gas phase,” explains Xu, “whereas the trans:gauche ratio increases to 40:60 in the liquid state.” In order to understand this change, the researchers posed a simple question: what is the minimum cluster size in which the trans conformation of TFE emerges? Addressing the problem directly, Xu and her team produced different-sized TFE clusters in a pulsed molecular expansion and analysed them using chirped pulse microwave spectroscopy, a highly sensitive technique that allowed them to probe the individual conformations of each cluster with unprecedented precision.
Surprisingly, in clusters of only three molecules (trimers), they found that the dominant intermolecular configuration already contained one trans-TFE subunit. This is in contrast to earlier work on 2-fluoroethanol (FE): although FE can display similar hydrogen bonding interactions to TFE, each FE molecule within a trimer preferentially adopts a gauche conformation.
In order to disentangle the intermolecular contributions that stabilize the TFE clusters, Xu and her team carried out quantum mechanical calculations and an electron density analysis of the non-covalent interactions. They found that the all-gauche configuration, as in FE, would be the most stable in the TFE trimer if hydrogen bonding were the only intermolecular interaction involved. Instead, they discovered that additional attractive weak F···F halogen bonds were decisive in preferentially stabilizing the trans conformation of a TFE subunit in the trimer.
http://www.nature.com/articles/s41570-017-0033
The TFE molecule can exist in three conformations: gauche(+), gauche(−) and trans. “One particular phenomenon that drew our attention is that the trans configuration of TFE appears to be unstable when isolated in the gas phase,” explains Xu, “whereas the trans:gauche ratio increases to 40:60 in the liquid state.” In order to understand this change, the researchers posed a simple question: what is the minimum cluster size in which the trans conformation of TFE emerges? Addressing the problem directly, Xu and her team produced different-sized TFE clusters in a pulsed molecular expansion and analysed them using chirped pulse microwave spectroscopy, a highly sensitive technique that allowed them to probe the individual conformations of each cluster with unprecedented precision.
Surprisingly, in clusters of only three molecules (trimers), they found that the dominant intermolecular configuration already contained one trans-TFE subunit. This is in contrast to earlier work on 2-fluoroethanol (FE): although FE can display similar hydrogen bonding interactions to TFE, each FE molecule within a trimer preferentially adopts a gauche conformation.
In order to disentangle the intermolecular contributions that stabilize the TFE clusters, Xu and her team carried out quantum mechanical calculations and an electron density analysis of the non-covalent interactions. They found that the all-gauche configuration, as in FE, would be the most stable in the TFE trimer if hydrogen bonding were the only intermolecular interaction involved. Instead, they discovered that additional attractive weak F···F halogen bonds were decisive in preferentially stabilizing the trans conformation of a TFE subunit in the trimer.
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