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. \(\mathit{\Phi} _{kil}\) denotes
the @[email protected] between the bonds \(ik\) and \(il\), \(\Delta z_{j}\) the perpendicular distance of the atom \(j\) from the instantaneous plane \(ikl\) and \(r_{eij}\) the equilibrium length of the bond \(ij\).
PAC, 1978, 50, 1707. (Definition and Symbolism of Molecular Force Constants) on page 1710 [Terms] [Paper]