The effects of symmetry and rigidity on non-adiabatic dynamics in tertiary amines: a time-resolved photoelectron velocity-map imaging study of the cage-amine ABCO.

Liv B Klein,Ruth A Livingstone,Thorbjørn J Morsing,Dave Townsend,Theis I Sølling

doi:10.1039/c5cp07910a

Abstract

The non-adiabatic relaxation dynamics of the tertiary cage-amine azabicyclo[2.2.2]octane (ABCO, also known as quinuclidine) have been investigated following 3p Rydberg excitation at 201 nm using femtosecond time-resolved photoelectron imaging (TRPEI). The aim of the study was to investigate the influence of the rigid and symmetric cage structure found in ABCO on the general non-adiabatic relaxation processes commonly seen in other tertiary aliphatic amines (TAAs). Our data is compared with TRPEI results very recently obtained for several structurally less rigid TAA systems [J. O. F. Thompson et al., Chem. Sci., 2016, 7, 1826-1839] and helps to confirm many of the previously reported findings. The experimental results for ABCO in the short-time (<1 ps) regime strongly support earlier conclusions suggesting that planarization about the N-atom is not a prerequisite for efficient 3p-3s internal conversion. Additionally, individual photoelectron peaks within our ABCO data show no temporal shifts in energy. As confirmed by our supporting quantum mechanical calculations, this demonstrates that neither internal conversion within the 3p manifold or significant conformational re-organization are possible in the ABCO system. This result therefore lends strong additional support to the active presence of such dynamical effects in other, less conformationally restricted TAA species, where photoelectron peak shifts are commonly observed. Finally, the extremely long (>1 ns) 3s Rydberg state lifetime seen in ABCO (relative to other TAA systems at similar excitation energies) serves to illustrate the large influence of symmetry and conformational rigidity on intramolecular vibrational redistribution processes previously implicated in mediating this aspect of the overall relaxation dynamics.

Highlights

We recently reported a time-resolved photoelectron imaging (TRPEI) study of N,N-dimethylisopropylamine (DMIPA), N,N-dimethylpropylamine (DMPA) and N-methylpyrrolidine (Mpyr).[1]
It should be noted that the time axis is plotted on a mixed linear-logarithmic scale and the energy axis is plotted in terms of Rydberg state binding energy to allow for ease of comparison with several previous tertiary aliphatic amines (TAAs) photoelectron studies.[1,26,27]
The mono-conformeric nature of ABCO, and the fact that the optically bright 3p state is the lowest-lying within the 3p manifold removes the two factors thought to give rise to observed temporal energy shifts of the peaks in the photoelectron spectra of more flexible TAAs.[1,28,29,30]

Summary

Introduction

We recently reported a time-resolved photoelectron imaging (TRPEI) study of N,N-dimethylisopropylamine (DMIPA), N,N-dimethylpropylamine (DMPA) and N-methylpyrrolidine (Mpyr).[1]. In the case of Mpyr, initial excitation predominantly populates the lowest member of the 3p Rydberg manifold, which undergoes internal conversion directly to the 3s state with an exponential decay time t1 = 400 Æ 30 fs This is subsequently followed by conformational relaxation on the 3s potential surface (t2 = 640 Æ 50 fs), as revealed by a distinct energy shift in the 3s photoelectron peak with time. We note that the predominantly Rydberg character exhibited by the low-lying excited states of TAA systems potentially offers an extremely useful ‘‘workbench’’ for using photoionization to systematically study the general interplay between molecular structure, nonadiabatic dynamics and chemical function in great detail. This is a consequence of Rydberg state energies exhibiting high sensitivity to ion core geometry (the so-called ‘‘Rydberg fingerprint’’)[2]

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Conclusion