# Why does the helium hydride ion have such a large dipole moment?

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?

• "2.26 or 2.84" That's quite a large variation o.O Did you pick the two values from two different sources? – Gaurang Tandon May 12 '18 at 2:12
• @GaurangTandon For a particle so rare and obscure, that's OK. – Ivan Neretin May 12 '18 at 3:42
• 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. – porphyrin May 12 '18 at 7:29
• en.wikipedia.org/wiki/Helium_hydride_ion The bond is 0.77A long (H2 0.74A), – Karl May 12 '18 at 8:17
• @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. – Tyberius May 13 '18 at 2:41

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$_2^+$).