The Potential Energy Surfaces of CN and CP Dimers

There is yet another linear isomer CCNN (4d). This can be conceived as a codimer of C2 and N2 monomers, held together by a donor-acceptor bond between the 3ag LUMO of C2 and the doubly occupied 3og of N2. The central 

C—N bond distance in 4d is 1.267 A. A more detailed discussion of this bond has been given by Scheller, Cederbaum, and Tarantelli.115 CCNN is rather high in energy, in fact even higher than the least stable CN dimer 4c (although by only 3.2 kcal/mol). Apparently, the CN dimers, especially NCCN, are thermodynamically stable with respect to dissociation. 

Another point concerns the inertness or kinetic stability with respect to unimolecular rearrangements. We have investigated the transformations of 4a to 4b, and of 4b to 4c via transition states in which the respective CN groups 

The tendency to distort, away from linearity, increases even further when both CP radicals bind via phosphorus as in CPPC (5c), which is a fourth-order 

How do these fragment orbitals interact in the dimers We recall from our earlier discussion that in a symmetry, the SOMOs on the two monomers provide the two-center electron-pair (2c-2e) bond by forming the (aSOMO + aSOMO )2 configuration (7). An important feature inherent to the SOMO is its rather low energy compared to completely unoccupied orbitals such as the 

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