Authors: Pavel A. Korzhavyia, Inna L. Sorokab, Eyvaz I. Isaevc,d, Christina Liljae, and Börje Johanssona,f
Journal: Proceedings of the National Academy of Sciences of the United States of America
Affiliation: a) Departments of Materials Science and Engineering, and b) Applied Physical Chemistry, Royal Institute of Technology, Stockholm, Sweden; c) Department of Physics, Chemistry, and Biology, Linköping University, Linköping, Sweden; d) Department of Theoretical Physics, National University of Science and Technology “Moscow Institute of Steel and Alloys,” Moscow, Russia; e) Swedish Nuclear Fuel and Waste Management Company, Stockholm, Sweden; and f) Materials Theory, Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden. Edited by Ho-Kwang Mao, Carnegie Institution of Washington, Washington, DC.
Take-home Importance According to the Authors: Quantum mechanical calculations along with experimental studies suggested that CuH and CuOH are stable in the solid form, which may prove to be useful for the storage of hydrogen-based fuels with simple binary salts based on abundant copper.
Take-home Importance According to the Blogger: Chemists probably would not willingly search for CuOH unless there is some theoretical basis for its existence. Without changing the oxidation states, a readied reaction could be 2 CuOH –> Cu2O + H2O. Its existence has been speculated on, now the really interesting work begins with the compound’s isolation (for a synthetic chemist).
Tidbit from the Blogger: the DOI (digital object identifier) of the article is probably the most reliable way to reference to an article because the document keeps it before the final journal page assignment after acceptance, which I find this website to be extremely helpful in locating articles in interest. Although it is called the Organic Chemistry Reference Resolver, it really works better than Google in most of the times (or the publisher web pages, some rather horrendous in terms of navigation). Zhurakovskyi, you are my hero.
Consider compound of copper, with only oxygen and/or hydrogen, there are only a handful of simple salts that are stable for chemists to manipulate. Notably, copper oxides, both the cuprous (+1 charged copper) and cupric (+2 charged copper), are common reagents used for synthesis and catalysis (copper-based Ullman coupling usually includes cuprous salts such as iodide). Some HTC (high-temperature superconductor) also involves an oxide salt of copper and various other elements. The superconductivity of YBCO (yttrium barium copper oxide) for example involves the copper-oxygen chain in the structure. Hydroxides and hydrides of copper (Cu(OH)2 and CuH) are at most metastable, and CuOH is completely unknown.
So what would they be if they actually exists? And this is the question carried out in this work. Since they are rather unstable, we need a fast and accurate signature by spectroscopy. Of course, to generate the spectroscopic signature, we need first generate the structure and understand the nature of bonding. Although it sounds difficult of constructing something that has not been isolated, the tools are fairly straightforward. Since the compositions of the material in question consists only of ions (breaking OH– down to O2- and H+), we can analyze the packing of those small definite charged spheres. And considering Pauling’s coordination rule, that considering any charged species with its coordinated partial charges, the total is neutral. Namely, the rule Σ(zi / qi) = 0 holds, where z is the coordinate number and q is the formal charge.
Then, with first principle DFT (density functional theory) calculation and several assumptions (See details. I am not an expert on this subject, sorry. :P) , minimizing the energy gave a proposed structure for CuOH, as thus generating the phonon vibrational modes and crystal lattices to define a signature in terms of vibrational spectrum (probable by technique such as FT-IR) and diffraction pattern. Much of the study goes into a careful assignment of vibrational modes to molecular motions. And if you consider the formula carefully, the name for CuOH, cuprice, is a perfect reflection of the metastable nature of the material to decompose into water (the compound bears crystallographic similarity with ice) and Cu2O. Most importantly, with the free energy calculation, the authors reject the possibility of obtaining the CuOH phase from a reaction of water and Cu2O. So unless we were able to sustain an approximate 10kJ/mol energy difference, it will be rather difficult to see the emergence of cuprice any time soon.
Computational methods have evolved very quickly in recent years, as theories become more refined and the computing resources become more available. Although DFT is not perfect, it can still be a great tool that offers insights to access experimental results. In this case, the calculations on CuH match with previous experimental data. However, DFT can never confirm a result. We have come to a greater understanding of the interactions of atoms, but the ultimate truth of what molecules can be made and what cannot and what will take significant efforts to do so is a force of nature.
Still, it is pretty amazing of what DFT can do.