Internal conversion mediated by specific nuclear motions: The nitrogen inversion in amines

To investigate the influence of amine structure on the internal conversion from S₂ to S₁, three aliphatic amines containing the same number of degrees of freedom, but with different degree of N-substitution, were investigated with femtosecond time-resolved mass spectrometry. As N-substituents lower the excitation energies, and the excitation in all cases is by a 200 nm photon, the S₁ density-of-states in the Franck-Condon region will be high for the more N-alkylated amine. This, according to standard models, should lead to faster internal conversion. The experimental results are in contrast to this, indicating non-ergodic behavior and a coupling of the states by very few molecular vibrations. This was further investigated by substituting differing numbers of hydrogen atoms with deuterium in two amines, one primary and one tertiary, thus freezing the degrees of freedom while altering the N-substituent mass. In both case, deuteration resulted in a significantly longer lifetime of the initially excited state, confirming the importance of the N-inversion motion in the description of the dynamics of internal conversion.