Intrinsic Reaction Coordinate - Converting To Reaction Coordinate with Units of Distance
Intrinsic reaction coordinate (IRC) is not some internal coordinate of a molecular system (such as, bond length, bond angle, etc.), rather it is a curvilinear coordinate that describes the intrinsic reaction path (IRP), which is the reaction path along the direction of the gradient. Only for some simple reactions one can visually (and approximately) identify IRC as one of the internal coordinates, but in general that is not possible.