PCCP Blog

Elangannam Arunan and Devendra Mani of the Indian Institute of Science have investigated an interesting C···Y bond, a “carbon bond”, which occurs when one of the hydrogen atoms of methane is replaced by an electron withdrawing group.

Weak interactions are very important in molecules of life, such as water and DNA, in supramolecular chemistry and in crystal design and engineering. Traditionally, these interactions were classified as hydrogen bonding and van der Waals interactions, but in recent decades, other weak interactions, such as halogen bonds, chalcogen bonds and pnicogen bonds, have been investigated and classified. An interesting question is whether carbon atoms can also play a role in weak interactions, in addition to the more electronegative elements known to take part.

When one of the hydrogen atoms in methane is replaced with an electron withdrawing group, such as -OH or –F, the CH₃ tetrahedral face becomes a positive centre. Using NBO analysis and vibrational frequency data, Arunan and Mani showed that this positive centre could accept electron density from atoms like O in water, giving rise to a novel C···Y bond, which could be called a carbon bond.

Arunan says that given the abundance of alkyl groups in biological systems, such carbon bonding interactions could play a significant role in biology, which has yet to be recognised. “Hydrogen bonds are just sufficiently strong and can be broken and made under ambient conditions, helping life.  Carbon bonds are weak, and if they were not much of what we know about life could not be.”

For more details, read their article:

The X-C•••Y (X=O/F, Y=O/S/F/Cl/Br/N/P) ‘carbon bond’ and hydrophobic interactions
Devendra Mani and E Arunan
DOI: 10.1039/C3CP51658J

Hydrogen is the only element in the periodic table that is not truly part of a group, although it is often nominally assigned to group 1. All chemists are familiar with the concept of the hydrogen bond, but how many think of the halogen bond in the same light? How many are even aware of the halogen bond as a special entity, less still that the two interactions are to all intents and purposes the same thing?

In his recent paper, Grabowski uses theoretical techniques to show that both interactions are ruled by the same electrostatic mechanism. He also provides an excellent summary and comparison of the information currently known about the two interactions that indicates a clear progression in some bonding properties from hydrogen through to the heavy halogens.

He describes how the atomic volume of the halogen decreases as the positive charge on it increases, and that this effect is magnified by shortening the internuclear distance. This information accompanies the observation that the strength of the Lewis acid-base interaction increases with the increasing atomic number of the halogen involved, although with some exceptions, hydrogen bonds are generally stronger still.

This discovery clearly has important implications for our understanding of non-covalent molecular interactions, and our understanding of how best to classify hydrogen based on its bonding properties.

by Victoria Wilton

Read this HOT PCCP article today:

Hydrogen and halogen bonds are ruled by the same mechanisms
Sławomir J. Grabowski
DOI: 10.1039/C3CP50537E

The fact that modern life relies heavily on fossil fuels is a environmental and a political problem, which will become your problem come when our supplies of fossil fuels run out. Solutions that deal with our need for domestic and industrial energy demands are—more or less—readily available. Our main problem is how to find transportable energy materials to fuel our cars, planes and ships, when no oil-based fuels are available. In the group of Tejs Vegge they have focused on ammonia as a possible mobile energy material.

The main issue in generating next-generation fuels is to remove the energy cost associated with transforming electricity into chemical energy. The solution is catalysts. The manufacture of ammonia, through the Haber–Bosch process, is enabling planet Earth to sustain the 7 billion humans that inhabit it today. If the process were stopped a couple of billion humans would die. Research has made the process of making ammonia very efficient, and has revolutionized our understanding of heterogeneous catalysis. Even so, the Haber–Bosch process is a high-energy process, consuming approximately 2 % of our total energy production.

The paper titled ‘DFT based study of transition metal nano-clusters for electrochemical NH₃ production’ is focusing on finding a process where electricity can be used to generate ammonia in an electrochemical cell. That is, how to make ammonia efficiently in a small scale, low temperature process on site at/in wind turbines and solar power plants. Using computational chemistry several catalysts are screened.

by Dr Thomas Just Sørensen

If you want to learn more see the paper, which was published in PCCP:

DFT based study of transition metal nano-clusters for electrochemical NH₃ production
J. G. Howalt, T. Bligaard, J. Rossmeisl and T. Vegge
Phys. Chem. Chem. Phys., 2013, 15, 7785-7795
DOI: 10.1039/C3CP44641G