N4 Triplet Potential Energy Surface

Computations using CAS(12,12) indicated that 8 dissociates via a transition state of C2 symmetry (9TS) [30], The zero-point corrected activation barrier was shown to be 8.5 kcal/mol at the CAS(12,12)/cc-pVTZ level. 

A second closed structured triplet state N4 molecule (10, C2v 3/ i) was also found to be a true minimum in the CAS(12,12) calculations [30]. However, this structure is more than 48 kcal/mol higher in energy than 1. In addition, it has a near zero barrier towards dissociation. Thus, 10 cannot be considered a potential intermediate for synthesis of a stable N4 molecule. 

The second open-chain N4 triplet is of Cs symmetry in the 3A state (13). This structure is 7.9 kcal/mol higher in energy at 0 K than 12 at the CCSD(T)/c-pVTZ//CCSD(T)/DZP level. However, in contrast to 12, 13 was found to be stable towards dissociation at the CAS(12,12) level. In addition, Korkin et al. 

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Fig. 2. Relative energies in kcal/mol at 0 K for stationary points on the N4 singlet potential energy surface. The energies are best estimates based on CCSD(T), MR-CI and CAS-SCF calculations reported in Ref [17, 19, 26, 27]. Note that the spin-forbidden dissociation pathway that passes via the minimum crossing point 7S-T is included. This pathway is discussed in the section (2.2) on the N4 triplet potential energy surface.. ...

Fig. 3. Optimized geometries of stationary points on the N4 triplet potential energy surface. Note that 6 is not a stationary point at higher levels of theory, and 7S-T is the minimum crossing point between the 1A and the " surfaces.

Fig. 2. Relative energies in kcal/mol at 0 K for stationary points on the N4 triplet potential energy surface. The energies are best estimates based on CCSD(T), MR-CI and CAS-SCF calculations reported in Ref. [30].

Tetrahedral N4 is expected to dissociate into two N2 molecules, but this reaction is forbidden by orbital symmetry. Dunn and Morokuma [33] characterized a transition state for the exothermic dissociation of tetrahedral N4 into two N2 and estimated the activation barrier to be 63 kcal/mol at the CASSCF(12e,12o) level, which indicates that N4 is a metastable species with significant kinetic stability. The calculated potential energy surface of N4 suggests that the low-lying triplet state might cross with the singlet surface (Fig. 3), which could reduce the activation energy barrier to about 30 kcal/mol [29,31,32],... 

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