Allowed Spin Multiplicity of Transition States

 We have been trying to calculate transition state energies and
 geometries for reactions that can be be summarized as
 AB   +   CD -----> { A---B---C---D}  ---->  AC  +  BD.
 AB is a known singlet,  CD is a known triplet,   AC is a known
 singlet and BD  is a triplet.    {A--B--C---D} is the T.S.  and the
 reaction is not photochemical.
 My questions are:
 1.  What is/are the allowed spin(s) of the transition state?  Are
 there general rules that govern the choice(s)?
 2.  Given the spin multiplicities of the reactants, are there general
 rules which fix the spins of the individual products?
 Any information would be greatly appreciated!
 Joe Capitani