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Triple bonds take on a very different shape in heavier atoms where relativistic quantum effects are at play, researchers in the US have found. The researchers say their spectroscopic study on the triple-bonded CBi\u2013 anion shows how molecular orbitals mix together, as predicted by theory, but also reveals unexpected differences that raise important questions about bonding in heavier atoms.<\/p>\n
The conventional understanding of chemical bonding relies on the notion that valence electron orbitals on bonding atoms overlap with each other to form the bonds between the atoms. The atomic s or p orbitals combine in a linear manner to form \u03c3 and \u03c0 orbitals, for example, creating an energy ladder that fills up as extra electrons are added. In this picture, a triple bond is made up of a \u03c3 orbital and two \u03c0 orbitals, each occupied by a pair of electrons with opposite spin. But this only works when the angular momentum of the orbital \u2013 which controls its shape \u2013 and electron spin are treated separately. When electrons are moving at close to the speed of light, this model breaks down because those two quantities mix \u2013 a phenomenon known as spin\u2013orbit coupling \u2013 and \u03c3 and \u03c0 orbitals with the same total angular momentum can combine with each other. This becomes increasingly relevant in the heavier elements where orbitals contract under the influence of the greater nuclear charge and so their electrons move faster.<\/p>\n
Molecular orbitals get mixed up<\/p>\n
Physical chemist Lai-Sheng Wang<\/a> and his group at Brown University used cryogenically cooled, angle-resolved photoelectron spectroscopy to study the shape of the electronic orbitals in CBi\u2013.1 By irradiating the anion with photons and then measuring the electrons that are subsequently ejected, they reconstructed the average electron density in the molecule to create images of its molecular orbitals. As theory predicts, they saw some mixing of the \u03c3 and \u03c0 orbitals, but Wang\u2019s student Deniz Kahraman was surprised to find that the two \u03c0 orbitals \u2013 which he expected to appear identical \u2013 looked completely different from each other. \u2018Deniz said he couldn\u2019t find any evidence in the literature that this was known,\u2019 recalls Wang. \u2018I said: \u201cIf nobody has observed this \u2026 it\u2019s really important because it\u2019s a fundamental question\u201d.\u2019 Further experimental work with another member of the group, Jie Hui, confirmed what Kahraman had observed. \u2018At this point, I decided I didn\u2019t know what we were talking about,\u2019 says Wang.<\/p>\n<\/p>\n