Computational predictions regarding ultrafast bond breakage and conformational changes in aliphatic chloro-amines

In a series of TD-DFT calculations we have investigated the homolytic cleavage of N-Cl and C-Cl bonds in protonated and neutral aliphatic amines on the ground- and excited-state potential energy surfaces. Our focus is to address the possibility of ultrafast bond breakage and the further processes that the resulting N and C based radicals can undergo. The S₁ n → σ^* states of the N-chloro-amines are repulsive whereas the S₁ states of the N-(chloroalkyl)-amines are bound Rydberg states. On the S₁ states of the N-chloro-amines the possibility therefore exists for ultrafast formation of N-based radicals. This state is conveniently located for a femtosecond resolved pump-probe study at 266 nm. An analysis of the conformers of the resulting radicals shows that - except in the N-protonated δ-alkyl radical - there is no driving force to induce a conformational change in the radicals. The driving force in the N-protonated δ-alkyl radical is most likely an interaction between the N-H bond and the radical site that has been shown to be important in amine radical cations [S. Hammerum, C.B. Nielsen, J. Phys. Chem. A 109 (2005) 12046]. The calculations could be taken to indicate that it is likely for a conformational change to take place in simple aliphatic amines; in this case all of the lowest excited states are Rydberg states and the excited-state amine therefore resembles a ground state radical cation in which the conformational change is believed to occur.