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Are elemental compounds composed of different isotopes polarised? I'd expect any such effect would only really be noticeable for very light elements such as hydrogen.

I'm not sure which way the charge would be distributed and whether a deuterium protium pair would be more like deuterium protide or protium deuteride.

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  • $\begingroup$ yes! $\ce{C^1H^2H^3HC{^1H}_3}$ would be optically active. $\endgroup$ – MaxW Jan 21 '16 at 5:31
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When taken isotopes into account, properties of compounds change. The polarisation of the compound included. Your hunch is correct, this probably only matters for very light compounds, or more precise compounds where the mass difference between the isotopes is large. The most extreme case is probably $\ce{HT}$, which has roughly double the mass of $\ce{H2}$.

I found an article by M. Trefler and H. P. Gush on the electric dipole moment of $\ce{HD}$ which explains the presence of the dipole moment as follows:[1]

The center of positive charge oscillates along the axis of the molecule during a molecular vibration because the excursions of the proton are greater than those of the deuteron; since the electrons do not follow exactly the motion of the positive charge, a weak, vibrating dipole moment results.

There are more articles of the dipole moment of $\ce{HD}$, I simply did not have the time to go through all of them. I also think the above already makes the point you were interested in.[2]

A very thorough theory is presented by W. R. Thorson et. al.,[3] which essentially explains the dipole moment of HD as follows:

[...] ; it arises directly from asymmetry of the electronic Hamiltonian, provided that we cast. that Hamiltonian in an appropriate set of canonical coordinates. The model of this Hamiltonian as having a pair of nuclei with slightly different effective charges is particularly pleasing, since we can link that directly to the orbital-size and binding-energy effects which we know must be associated with the difference in electronic reduced masses for D and for H.

M. Trefler and H. P. Gush found the dipole moment to be very small, i.e. $\mathbf{p}=(5.85\pm0.17)\times 10^{-4}~\mathrm{D}$. Hence to speak of deuterium protide or protium deuteride is a stretch.

Since the effect is quite small in $\ce{HD}$ already, one can probably assume it will not have a significant effect on the polarisation in other than symmetric diatomic molecules.

However, with isotopes a few notable applications can be achieved. One of the prime examples is “chiral acetate”, (R)-deuterotritioacetic acid $\ce{HDTC-COOH}$, which helped elucidate biosynthetic transformations. For a quite comprehensive, educational article see [4].


References

  1. M. Trefler and H. P. Gush; Phys. Rev. Lett. 1968, 20, 703.
  2. W. R. Thorson, J. H. Choi, and S. K. Knudson; Phys. Rev. A 1985, 31, 22.
  3. (a) S. M. Blinder; J. Chem. Phys. 1960, 32, 105. (b) S. M. Blinder: The dipole moment of HD. http://hdl.handle.net/1811/7809 (c) J. Bradley Nelson and G. C. Tabisz; Phys. Rev. Lett. 1982, 48, 1393. (d) G. Herzberg; Nature 1950, 166, 563. (e) etc. see google scholar
  4. Addison Ault; J. Chem. Educ. 2003, 80 (3), 333.
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