 
Summary: Chemical Physics 61 (19Sl) 189204
NorthHolland Publishing Company
THE ROTATING ROD MODEL:
OPACETY, EXCITATION, DEFLECTION AND ANGULAR DISTREBUTION
FUNC'FIONS FROM COLLINEAR REACTION PRO&ABILH?TIES*
Noam AGMON**`f
Department of Chemistry, Harvard Uniuersity, 12 Oxford Street, Cambridge, MA 02138, USA
Received 13 April 1981
A "rotating md" model for predominantly collinear atomdiatom collisions is presented. In this model it is assumed
that the forces acting during the collision align the three atoms. The reaction probability at a given impact parameter is
related to the 1D reaction probability by subtracting the triatomic rotational and bending energies from the total energy.
An additional "sudden" approximation is used to derive a relation of deflection angles to impact parameters, resulting in
an expression for angular distributions. Com@ison is made with quantal and classical trajectory calculations for HtHz
and DfHz and with trajectory calculations for Cl+ HD. A previously proposed analytical expression for ihe number of
states along the reaction coordinate which gives a reasonable apprcximation for the collinear microcanonical reaction
probability; is used for comparison with experiment. In this approximation all reaction attributes follow from the
diatomic spectroscopic constants and the "intrinsic barrier", which is common to the reaction series. Comparison is made
with angular distributions and excitation funcdon for D +H,. A simple, anaIytic expression derived for thermal rate
constants is compared with experimental Arrhenius plots for DiHz and HtDz and with kinetic isotope effects for
Cl + HD.
