out-of-plane bending coordinate

in molecular geometry
Given by \[\Delta \theta _{i- jkl}=\frac{\Delta z_{j}}{r_{eij}}\ \sin \mathit{\Phi }_{kil}\] where the numbering of the atoms is given in the diagram. Φ k i l denotes
the angle between the bonds ik and il, Δ z j the perpendicular distance of the atom j from the instantaneous plane ikl and r e i j the equilibrium length of the bond ij.
PAC, 1978, 50, 1707. 'Definition and Symbolism of Molecular Force Constants' on page 1710 (https://doi.org/10.1351/pac197850111707)