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I know that at extremely low temperatures (mK and lower), Helium can form diatomic molecules. Do the other noble gasses also form molecules at extremely low temperatures?

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    $\begingroup$ You might have a hard time to find them named as “diatomic molecules”, unless they are specifically called “van der Waals molecules” or “London molecules”… But searching for “neon (or argon, or …) dimer binding energy (or binding curve)” turns up plenty of both theoretical and experimental papers on the topic. This one is available in full text and includes a good number of references. $\endgroup$ – F'x May 24 '12 at 22:21
  • $\begingroup$ PS: I won't have time to write a full answer, but I encourage anyone who wishes to pick up on the paper above and write a full answer based on it and references therein… $\endgroup$ – F'x May 24 '12 at 22:22
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Yeah, dimers of noble gas atoms are stable due to dispersion aka London forces. More generally, neutral, non-polar molecules can still form very weak bound states arising entirely from the induction and interaction of instantaneous dipole moments (that is, fluctuations in the electron density around an atom arising from the motion of electrons). My understanding is that physicists would explain this interaction as a special case of the Casimir-Polder interaction. This interaction means that even noble gases are nonideal.

Correctly predicting the negative interaction energy of a noble gas dimer is a pathological test of dispersion-corrected DFT functionals, as F'x's reference1 and the references therein indicate, and many methods fail qualitatively. I recall that Zhao and Truhlar used noble gas dimers to test the efficacy of their various metahybrid functionals2,3. One of their papers2 makes reference to a table of binding energies and equilibrium distances derived4 by Wang and Ogilvy, giving for instance a $\ce{Ne\cdots{}Ne}$ binding energy of 0.35 kJ mol-1 (that's tiny) and less than half that for $\ce{He\cdots{}Ne}$. The $\ce{Ne\cdots{}Ne}$ equilibrium distance is given as 309 pm, which is extreme. As the paper5 you refer to indicates, however, the interaction energy for the $\ce{^4He\cdots{}^4He}$ dimer is much, much smaller than either of these and has an equilibrium distance of 5.2 nm (!!!), which is something else entirely.


(1) Ruzsinszky, A.; Perdew, J.P.; Csonka, G.I.; Binding Energy Curves from Nonempirical Density Functionals II. van der Waals Bonds in Rare-Gas and Alkaline-Earth Diatomics; J. Phys. Chem. A; 2005, 109, pp. 11015-11021

(2) Zhao, Y.; Truhlar, D.G.; Hybrid Meta Density Functional Theory Methods for Thermochemistry, Thermochemical Kinetics, and Noncovalent Interactions: The MPW1B95 and MPWB1K Models and Comparative Assessments for Hydrogen Bonding and van der Waals Interactions; J. Phys. Chem. A; 2004, 108, 6908-6918

(3) Zhao, Y.; Truhlar, D.G.; Comparative DFT Study of van der Waals Complexes: Rare-Gas Dimers, Alkaline-Earth Dimers, Zinc Dimer, and Zinc-Rare-Gas Dimers; J. Phys. Chem. A; 2006, 110, pp. 5121-5129

(4) Ogilvie, J.F.; Wang, F.Y.H.; Potential-energy functions of diatomic molecules of the noble gases: II. Unlike nuclear species; J. Mol. Struct.; 1993, 291, pp. 313-322

(5) Grisenti, R.E.; Schöllkopf, W.; Toennies, J.P.; Hegerfeldt, G.C.; Köhler, T.; Stoll, M.; Determination of the Bond Length and Binding Energy of the Helium Dimer by Diffraction from a Transmission Grating; Phys. Rev. Lett.; 2000, 85, pp. 2284-2287

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