Acid–base equilibrium dynamics in methanol and dimethyl sulfoxide probed by two-dimensional infrared spectroscopy

다국어 초록 (Multilingual Abstract)

<P>Two-dimensional infrared (2DIR) spectroscopy, which has been proven to be an excellent experimental method for studying thermally-driven chemical processes, was successfully used to investigate the acid dissociation equilibrium of HN<SUB>3</SUB> in methanol (CH<SUB>3</SUB>OH) and dimethyl sulfoxide (DMSO) for the first time. Our 2DIR experimental results indicate that the acid–base equilibrium occurs on picosecond timescales in CH<SUB>3</SUB>OH but that it occurs on much longer timescales in DMSO. Our results imply that the different timescales of the acid–base equilibrium originate from different proton transfer mechanisms between the acidic (HN<SUB>3</SUB>) and basic (N<SUB>3</SUB><SUP>−</SUP>) species in CH<SUB>3</SUB>OH and DMSO. In CH<SUB>3</SUB>OH, the acid–base equilibrium is assisted by the surrounding CH<SUB>3</SUB>OH molecules which can directly donate H<SUP>+</SUP> to N<SUB>3</SUB><SUP>−</SUP> and accept H<SUP>+</SUP> from HN<SUB>3</SUB> and the proton migrates through the hydrogen-bonded chain of CH<SUB>3</SUB>OH. On the other hand, the acid–base equilibrium in DMSO occurs through the mutual diffusion of HN<SUB>3</SUB> and N<SUB>3</SUB><SUP>−</SUP> or direct proton transfer. Our 2DIR experimental results corroborate different proton transfer mechanisms in the acid–base equilibrium in protic (CH<SUB>3</SUB>OH) and aprotic (DMSO) solvents.</P>

<P>Graphic Abstract</P><P>Two-dimensional infrared (2DIR) spectroscopy was successfully used to investigate the acid dissociation equilibrium of HN<SUB>3</SUB> in methanol (CH<SUB>3</SUB>OH) and dimethyl sulfoxide (DMSO).
<IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c5cp02368h'>
</P>