Why does o-fluorine substitution raise the barrier to ring expansion of phenylnitrene?

In an attempt to understand the dramatic difference in reactivity between phenylnitrene (1a) and pentafluorophenylnitrene (1b), the ring expansion reactions of several fluorinated arylnitrenes have been studied computationally at the CASPT2N/cc-pVDZ//CASSCF(8,8)/6-31G* + ZPE level of theory. The nitrenes considered include 2,6-difluorophenylnitrene (1c), 3,5-difluorophenylnitrene (1d), 4-fluorophenylnitrene (1e), and 2-fluorophenylnitrene (1f). In all cases ring expansion of the open-shell singlet nitrenes (1) to the ketenimines (3) was calculated to occur via the same two-step mechanism as that predicted for the parent system (1a), with the first step-cyclization to the azirines (2)-being rate-determining. The calculated barrier of 13.4 kcal/mol for cyclization of 1c is 3.5-4.5 kcal/mol higher than the corresponding barriers for nitrenes 1e and 1f, and for 1a. For nitrene 1f, the calculated barrier of 13.0 kcal/mol for cyclization toward fluorine is 3 kcal/mol higher than the barrier for cyclization away from fluorine. These calculated differences in barrier heights are consistent with known experimental data for nitrenes 1a-f and are attributed largely to steric repulsion in the transition state when the nitrogen atom cyclizes toward fluorine. This steric explanation is based on an analysis of the optimized geometries of the transition states and is supported by calculations on the cyclization reactions of 2-chlorophenylnitrene (1h) and 2-methylphenylnitrene (1j).