1. Simplest Inorganic Double-Helix Structures
Alexander S. Ivanov1, Andrew J. Morris2, Chris J. Pickard2, Alexander I. Boldyrev1 1Department of Chemistry and Biochemistry, Utah State University (USA); 2Department of Physics and Astronomy, University College London (UK)
Introduction
Theoretical methods
Results - infinite double-helix
Results - bulk phase of LiP
We performed an unbiased quantum-chemical search for LixPx (x = 5 – 9) clusters using a Coalescence Kick program initially at the B3LYP/3-21G level of theory. All low-lying isomers found by this method were reoptimized with follow-up frequency calculations at the B3LYP level of theory using the G* basis set. The total energies of the lowest isomers of Li5P5, Li6P6, and Li7P7 stoichiometries were calculated at the CCSD(T)/CBS//B3LYP/6-311+G*.
The solid-state study was performed using the plane-wave
density functional theory code CASTEP and the PBE exchange-correlation functional, ultrasoft pseudopotentials,
and a Brillouin zone sampling finer than 2π x 0.05 Å-1. The structure was fully geometry-optimized and phonon calculations were performed to demonstrate its stability.
We also recovered the P21/c symmetric Li1P1 bulk phase comprising of packed double helices where adjacent helices are of opposite sense and 9.69 kcal mol-1 lower in energy than the infinite double-helix chain.
Results
A 2 x 2 supercell containing a P21/c symmetric Li1P1 bulk phase comprising of packed double helices.
When most people think of a double helix, they think of DNA structure — a now familiar image thanks to Watson and Crick’s landmark 1953 discovery of the double-stranded molecules of nucleic acids.1 Yet human fascination with the distinctive twisted ladder-shape stretches back through ancient history. Think of Giambologna’s ‘Rape of the Sabine Women’ and Momo’s spiral staircase in the Vatican Museum. The geometric structure of a double-helix plays a significant role in metabolism and evolution. While it appears in forms great and small in varied organisms, it is very rare in inorganic chemistry.
Here we report the theoretical prediction of the existence
of double-helix structures in the series of LixPx (x = 5 – 9)
clusters and in an infinite LiP chain and compare them with
bulk phases of LiP.2 The discovered double helices are rather simple species consisting of only two elements: lithium and phosphorus.
For the Li5P5 stoichiometry the double-helix structure is the second lowest isomer, which is 5.6 kcal mol-1 higher than the global minimum. For the Li6P6 stoichiometry, the helical structure is the third isomer and is 12.6 kcal mol-1 higher than the global minimum structure. Our search for the global minimum in the Li7P7 stoichiometry revealed that the double-helix structure is the global minimum with the second isomer containing a seven-membered ring similar to sulfur clusters. The double helices were also found to be the global minimum structures for both the Li8P8 and Li9P9 stoichiometries.
Conclusions
We theoretically predicted the simplest double-helix structures for small lithium-phosphorus clusters. Our results extended the family of double-helical solids and we hope that other inorganic double-helix structures which obey the Zintl rule may be found.
The LiP infinite double-helix chain was found to be stable. The structure comprises four functional units (FU) of Li1P1 per turn and is 6.00 kcal mol-1 lower in energy than Li9P9.
References
J. D. Watson, F. H. C. Crick, “Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid”, Nature 1953, 171, 737
A. S. Ivanov, A. J. Morris, K. V. Bozhenko, C. J. Pickard, A. I. Boldyrev, “Inorganic Double-Helix Structures of Unusually Simple Li-P Species”, Angew. Chem. Int. Ed., 2012, 51, 33, 8330
This article was highlighted in Chemical & Engineering News (Science & Technology Concentrates section)
S. Ritter, C&EN, Vol. 90 , Issue 33, p. 33
Acknowledgement
The theoretical work done at Utah State University was supported by the National Science Foundation (grant number: CHE ). The computational resource, the Uinta cluster supercomputer, was provided through the National Science Foundation under grant number CTS with matching funds provided by Utah State University.
A.J.M. and C.J.P. were funded by the Engineering and Physical Sciences Research Council (EPSRC) of the UK.
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