Two water molecules can bind loosely with each other using weak hydrogen bonds to form the smallest molecular cluster. The water dimer has been studied extensively for probing the properties of hydrogen bonds that are so important to water-based life on earth – so much so that it has been called a theoretical guinea pig.
Replace oxygen in water with sulphur, its neighbour one floor below in the periodic table, and we should expect to get similar phenomena. Hydrogen sulphide too forms dimeric clusters.
There are, of course, differences between the behaviour of water and hydrogen sulphide. As per experimental results so far, H2O in ice interacts with four of its neighbours while frozen H2S interacts with twelve of its neighbours. This difference led to the speculation that water molecules have hydrogen bonds while H2S molecules have isotropic van der Waals interactions.
Fifteen years ago, E Arunan at the IISc Bangalore, felt uncomfortable with this view point. Around this time, Frank Lovas from the National Institute of Standards and Technology, Maryland, USA was looking at H2S dimer, using microwave spectroscopy. So he too, along with his student, attacked the problem. The results from both labs were ambiguous.
But, recently, experiments with a broadband microwave spectrometer at the University of Newcastle by Chris Medcraft and Nick Walker, threw up lines that appeared to be from H2S dimer. They contacted both Lovas and Arunan.
So, with the help of another PhD student, Arunan attacked the problem again. They used a pulsed nozzle FT microwave spectrometer. The technique is simple: mix a little hydrogen sulphide with a lot of helium and pressurise the gas mixture, let it out through a nozzle of less than one millimetre. While the gas expands, it cools and in the process, H2S dimers are formed. At that precise moment, give a microwave pulse of one microsecond. Record the results. Repeat the process 2000 times so that one gets adequate sample size to separate the signals from noise, riff from raff.
Thus, they detected the signal that showed that indeed, H2S, too, has hydrogen bonds. The bond angle between S-H and S was about 175 degrees and the hydrogen bond distance is found to be about 2.778 angstroms – less than the sum of van der Waals radii of the H and S atoms.
“In solids, the thermal energy along a coordinate that can break the hydrogen bond is greater than the barrier along that coordinate, leading to an average spherical shape for H2S”, says Arunan.
“That is not all. We could observe the signals only when we restrained the molecules by reducing the temperature significantly. The thermal motion was coming in the way of our experiments earlier”, smiles Arijit Das, his student.
“The four hydrogens in water dimer show tunnelling behaviour such that all of them can participate in the hydrogen bonds. We should expect that the case is similar with H2S also. We will be coming out with another paper on the subject soon”, says Pankaj Mandal from IISER Pune, a collaborator in the research.
Angew Chem Int Ed Engl. Sep 28 (2018) DOI: 10.1002/anie.201808162
Udham P K