out-of-plane bending coordinate in molecular geometry

Given by

$Δ θ i − j k l = Δ z j r e i j sin ⁡ Φ k i l$

where the numbering of the atoms is given in the diagram. $Φ k i l$ denotes

the angle between the bonds $i k$ and $i l$ , $Δ z j$ the perpendicular distance of the atom $j$ from the instantaneous plane $i k l$ and $r e i j$ the equilibrium length of the bond $i j$ .

Source:

PAC, 1978, 50, 1707 (Definition and symbolism of molecular force constants) on page 1710