UPSC Combined Geo-Scientist, Mains · 2019 · Mains Paper I

Q1(g). Small dipole moment of CO

Question

The dipole moment of CO is merely 0.12 D although electronegativity difference between C and O is large. Why?

Opposing contributions to the molecular charge distribution nearly cancel; electronegativity difference alone does not predict the net CO dipole.

Oxygen tends to attract bonding electron density, favouring a contribution with O negative and C positive.
The full molecular-orbital distribution, including the strongly carbon-localised highest occupied σ orbital, contributes in the opposite direction.
Their near cancellation leaves only a small dipole, experimentally with the carbon end slightly negative.

The full model answer

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CoversSmall dipole moment of COMolecular polarity