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An isolated boron trifluoride molecule has trigonal planar molecular geometry and the D3h molecular point group. Its three equal fluorine contributions give $I_{\parallel }=3m_{\mathrm{F}}{R}^2$ and $I_{\perp }=3m_{\mathrm{F}}{R}^2/2$ in the rigid rotor approximation.

An oblate symmetric top has two equal smaller principal moments of inertia and a larger moment of inertia about its symmetry axis. A planar trigonal molecule, such as boron trifluoride, is an example. Its axial rotational constant is smaller than its transverse rotational constant.

In the printed geometry, one B–F bond lies along the in-plane axis and contributes zero. The other two fluorines have perpendicular distance $R\sin 60^{\circ }=\sqrt{3}R/2$; boron is again on the axis. ThereforeThe other in-plane principal moment is the same by threefold symmetry. Hence Boron trifluoride is an oblate symmetric top with $I_c>I_b$ and $\tilde{C}=\tilde{B}/2$.

The combined boron trifluoride series givesUsing $I_b=h/(8{\pi }^{2}c\tilde{B})$ with the wavenumber in inverse metres,With integer fluorine mass $m_{\mathrm{F}}=19u$ and $I_b=3m_{\mathrm{F}}{R}^2/2$,Using $4\tilde{B}$ for the observed spacing would incorrectly treat BF${}_3$ as a linear diatomic rotor and overestimate the bond length by $\sqrt{2}$.

A molecular geometry in which three attached chemical atoms and their central atom lie in one plane. The ideal symmetric arrangement has three $120^{\circ }$ angles and D3h molecular point group symmetry, as in an isolated boron trifluoride molecule.