Research Highlights

 

Chemical and Engineering News, weakly magazine published by the American Chemical Society, has a story on carbon bond in their 6 January 2014 issue

Arunan's group defines carbon bond now

-         Arunan, E.,  IPC


Prof. Arunan from the Department of Inorganic and Physical Chemistry led an international task group formed by IUPAC and redefined hydrogen bond in 2011, the international year of chemistry. They have been using a home-built pulsed nozzle Fourier transform microwave spectrometer to investigate weakly bound complexes that are bound by hydrogen bonding and van der Waals interactions. In a recent study on Ar-propargyl alcohol complex, they noted a structure in which Ar was directly 'bonded' to C of the CH2OH group. Similar interactions were found in Ar-CH3OH complex as well (http://dx.doi.org/10.1002/cphc.201200760). They have shown earlier that the tetrahedron face of CH4 is electron rich and could accept a hydrogen bond. They realized that replacing an H atom with an electronegative element like F or O, can make the tetrahedron face electron deficient. This can be seen by looking at the electrostatic potential of CH4 and CH3OH as shown below (red – electron rich and blue – electron deficient):

 

If a water molecule approaches CH3OH through the CH3 face, they realized, O of H2O could directly bond to the C of CH3OH and indeed it does as shown by the atoms in molecules theoretical analysis. Moreover, this interaction weakens the C-O bond in CH3OH leading to a red-shift of C-O stretching frequency. Such X-C•••Y interactions have very similar characteristics as observed in X-H•••Y hydrogen bonds which have been known for a century  now.  In the recent years,  halogen bonding  (Group 17), chalcogen bonding (group 16)

and pnicogen bonding (group 15) were identified and Arunan’s group has extended this to the most important element on earth, carbon. As the illustration below shows, carbon bonding has a lot of similarity with hydrogen bonding and halogen bonding. Hydrogen bonds are of moderate strength and can be broken and made under ambient conditions, helping life. Carbon bonds on the other hand are weak, and if they were not, much of what we know about life could not be. The uniqueness of hydrogen atom enabled identification of hydrogen bonding through macroscopic studies in the 20th century. Carbon bonding had to wait for the 21st century advances in microscopic investigations of intermolecular interactions. Organic chemists can identify carbon bonded structures as intermediates in SN2 reactions. See a blog at the Journal website:
http://blogs.rsc.org/cp/2013/06/13/exploring -the-carbon-bond/

 

Paper can be accessed at:  http://pubs.rsc.org/en/Results?SearchText=doi:c3cp51658j