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According to Wikipedia, the helium hydride ion $\ce {HeH+}$ has a dipole moment of 2.26 or 2.84. What contributes to this large dipole moment in this molecular ion?

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  • $\begingroup$ "2.26 or 2.84" That's quite a large variation o.O Did you pick the two values from two different sources? $\endgroup$ May 12, 2018 at 2:12
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    $\begingroup$ @GaurangTandon For a particle so rare and obscure, that's OK. $\endgroup$ May 12, 2018 at 3:42
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    $\begingroup$ Think of it as a heteronuclear diatomic then a dipole is expected. Either the bond length is large or charge shift great or both. You will have to look into details of bonding to understand further. $\endgroup$
    – porphyrin
    May 12, 2018 at 7:29
  • $\begingroup$ en.wikipedia.org/wiki/Helium_hydride_ion The bond is 0.77A long (H2 0.74A), $\endgroup$
    – Karl
    May 12, 2018 at 8:17
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    $\begingroup$ @TanYongBoon I would surmise it is pretty large with on the positive charge localizing on the hydrogen side. I just thought it was worth knowing that I wouldn't put much faith in those particular values. For such a small molecule, a much more accurate calculation could be done on a normal computer in a matter of minutes. $\endgroup$
    – Tyberius
    May 13, 2018 at 2:41

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The dipole moment of an array of charges is defined as $$ \vec{\mu}=\sum_i \vec{r}_iq_i $$ where $\vec{r}_i$ is the position of charge $q_i$ in the molecule with respect to the center of mass. In a neutral molecule the sum of the charges is zero and therefore the terms in the sum partially cancel. For ions this is of course not the case and the dipole moment is much larger than for the corresponding neutral molecule (provided that the ion is not completely symmetric such as H$_2^+$).

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