Brightening and Locking a Weak and Floppy N−H Chromophore: The Case of Pyrrolidine

Abstract

The N-H stretching signature of the puckering equilibrium between equatorial and axial pyrrolidine is analyzed via FTIR and Raman spectroscopy in supersonic jets as a function of aggregation. Vibrational temperatures along the expansion axis can be extracted from the Raman spectra and allow for a localization of the compression shock waves. While the equatorial conformation is more stable in the ground state monomer, this preference is probably switched in the excited state with one N-H stretching quantum. Furthermore, the dominant dimer involves an axial donor and the trimer and tetramer structures seem to prefer uniform axial conformations. The IR intensity is boosted by up to 3 orders of magnitude upon aggregation, whereas the Raman scattering intensity shows only moderate hydrogen bond effects. B3LYP and MP2 calculations provide a reasonable description of the N-H vibrational dynamics under the influence of self-aggregation. In mixed dimers with pyrrole, pyrrolidine assumes the role of a hydrogen bond acceptor.

Solute–solvent interactions in cryosolutions: a study of halothane–ammonia complexes

“…8A are given the observed values of the complexation shift relative to its value in the monomer under the same circumstances, D compl n/n mon , as a function of temperature. In it, we have positioned the relative shift for the jet-experiments at the estimated temperature of 50(20) K. 31 It can be seen that the shift observed under equilibrium vapor phase conditions decreases quasi linearly with decreasing temperature. If this trend is extrapolated linearly, the jet-shift is predicted to occur at a meaningless negative temperature.…”

Femtisecond single-mole infrared spectroscopy of molecular clusters

“…54,89 IR intensity is also strongly dependent on the hybridization state, as well as on temperature. 88 A particularly interesting case is found for pyrrolidine, 90 with an almost negligible monomer NH stretch intensity and up to three orders of magnitude intensity gain in the hydrogen-bonded dimer mode. Another effect of hydrogen bonding in this case is to switch the energetical preference of the NH bond from equatorial to axial.…”

“…The combination of Raman and IR spectra helps us to assign tunneling splittings 73,199 and hot bands from other low frequency modes. 90 Furthermore, Raman spectra give direct access to symmetric low frequency hydrogen bond modes, 145 whose antisymmetric IR active counterparts are much more difficult to observe. 52…”

Two-dimensional Infrared Spectroscopy Reveals Better Insights of Structure and Dynamics of Protein

“…The vibrational frequency calculation within the harmonic approximation is advantageous; however, it generally has insufficient significance because most of the biologically relevant molecules are "floppy" and influenced by strong anharmonic effects [34,42]. Strong anharmonic effects are observed particularly in weakly bound molecular complexes, for example, hydrogen-bonded complexes with surrounding water [43][44][45][46][47].…”