13) Good Curie law behavior was observed for most samples down to liquid helium temperature. Although some showed deviations near 10°K, this was related to the prior history of the sample.

Product distributions have been studied for photolysis of azobisisobutyronitrile (AIBN) in fluid solution, in glassy benzyl benzoate, in frozen mixtures, and in two pure crystalline modifications. Azobis-3-cyano-3pentane (AGP) was similarly studied in benzene solution and in the pure crystal. For AGP the fluid and crystalline products were similar with a ketenimine predominating and somewhat less tetraethylsuccinodinitrile, but for AIBN the crystalline products (95% disproportionation, 5% tetramethylsuccinodinitrile (TMSN)) differed dramatically from those in solution (5% disproportionation, 40% TMSN, 55% ketenimine). A number of alternatives to explaining the product distribution for AIBN crystals in terms of a very specific influence of the ordered environment on the behavior of the intermediate radical pair are considered and excluded. The transition between the crystalline phases of AIBN was studied thermochemically (AH monoclinic -triclinic, 0.32 ± 0.04 and 0.26 ± 0.04 kcal/mol for AIBN and AIBN-i/^, respectively). The cage effect in crystalline AIBN is greater than 91 %.

We wish to report evidence that the tunnel effect makes an important contribution to hydrogen isotope effects in proton transfers but does not control the form of the dependence of the isotope effect on the strength of the attacking base.

No abstract

Lattice parameters were determined for normal and perdeuterated azobisisobutyronitrile (AIBN) in PI and P2,/c modifications and showed significant variation with isotopic substitution. Three dimensional intensity data were collected by diffractometer for three of these crystals and for a crystal of azobis-3-cyano-3-pentane (ACP) and all were refined to conventional R's of 0.05-0.09. The refinement of ACP was in space group Cmca (Z = 4) and gave a long C-C bond, a wide C-C-C angle, and unrealistic thermal parameters suggesting disorder with respect to the mirror plane which was confirmed by the observation of diffuse scattering. Otherwise bond lengths and angles are consistent among the determinations and with those of other azoalkanes. The nitrile and azo groups are nearly eclipsed to give planar, S-shaped N=CCN=NCCe=N backbones for molecules in these crystals. The backbones are arranged into sheets containing a herringbone pattern of nitrile groups. The photochemistry of crystalline AIBN and ACP is discussed in terms of these packings, and it is concluded that dipolar c 6.201( 1) 6.204(1,1) -0.003 a 71.33(1) 71.31(1,0) 0.02 ß 77.95(2) 77.98(2,2) -0.03 79.19(2) 79.25(2,1) -0.06 Vol 248.9 248.4 0.