Torsions of N-methylpyrrole and its cation

Strong Torsion-Vibration Interaction in N-Methylpyrrole Observed by Far-Infrared Spectroscopy

An interaction between methyl torsion and the low-lying out-of-plane methyl wag vibration has been observed in toluene, p-fluorotoluene, and m-fluorotoluene, contravening the traditional assumption used when analyzing spectra that methyl torsion can be treated independently of the small-amplitude vibrations. When a methyl group is attached to a planar frame, out-of-plane methyl wag vibrations always occur, and hence this type of interaction between methyl torsion and vibration is potentially extensive. To probe whether this coupling occurs beyond toluene and its derivatives, we have studied the far-infrared absorption band for the out-of-plane methyl wagging mode in N-methylpyrrole. The torsional sequence structure reveals a particularly strong torsion-vibration interaction. Spectral simulations yield a torsion-vibration coupling matrix element of 34.0 cm^-1, over twice the value for toluene. The large torsion-vibration coupling constant implies that there is a significant tilting of the methyl group out of plane. Quantum chemistry calculations reveal a much larger out-of-plane methyl tilt angle in N-methylpyrrole compared to toluene, qualitatively consistent with this expectation. This is the first nontoluene derivative for which this type of torsion-vibration interaction has been reported and shows that the effect extends beyond toluenes. When present, this interaction links small-amplitude vibrations to the methyl torsion, providing a mechanism to bring the increased density of states into play and accelerate the rate of intramolecular vibrational energy redistribution.

Comment on "Electronic, vibrational and torsional couplings in N-methylpyrrole: Ground, first excited and cation states" [J. Chem. Phys. 154, 224305 (2021)]

Two-color (1 + 1') zero-electron-kinetic-energy (ZEKE) and photoionization efficiency (PIE) spectra are reported via different levels in the S₁ ← S₀ (ùA₂←X̃¹A₁) one-photon transition of jet-cooled N-methylpyrrole. The laser radiation is produced using two dye lasers, one with an 1800 l/mm grating and one with 2400 l/mm. We report spectra where the excitation and ionization radiation are produced with both combinations of the dye lasers; these spectra differ markedly. This is attributed to Wood's anomalies with the 2400 l/mm grating: one aspect is a loss in light intensity over a range of wavelengths, attributed to a resonance anomaly. Another is the appearance of a "shadow" ZEKE spectrum and PIE curve at apparently higher ionization wavenumbers; under some conditions, a third ZEKE spectrum was observed-these latter observations arise from higher-order dispersion effects, likely caused by a Rayleigh anomaly. We comment on these observations and report more representative ZEKE and PIE spectra than those presented in a recent paper by our group [A. R. Davies, D. J. Kemp, and T. G. Wright, J. Chem. Phys. 154, 224305 (2021)] for four intermediate S₁ levels.

Electronic, vibrational, and torsional couplings in N-methylpyrrole: Ground, first excited, and cation states

The electronic spectrum associated with the S₁ ← S₀ (ùA₂←X̃¹A₁) one-photon transition of jet-cooled N-methylpyrrole is investigated using laser-induced fluorescence (LIF) and (1 + 1) resonance-enhanced multiphoton ionization (REMPI) spectroscopy; in addition, the (2 + 2) REMPI spectrum is considered. Assignment of the observed bands is achieved using a combination of dispersed fluorescence (DF), two-dimensional LIF (2D-LIF), zero-electron-kinetic energy (ZEKE) spectroscopy, and quantum chemical calculations. The spectroscopic studies project the levels of the S₁ state onto those of either the S₀ state, in DF and 2D-LIF spectroscopy, or the ground state cation (D₀ ⁺) state, in ZEKE spectroscopy. The assignments of the spectra provide information on the vibrational, vibration-torsion (vibtor), and torsional levels in those states and those of the S₁ levels. The spectra are indicative of vibronic (including torsional) interactions between the S₁ state and other excited electronic states, deduced both in terms of the vibrational activity observed and shifts from expected vibrational wavenumbers in the S₁ state, attributed to the resulting altered shape of the S₁ surface. Many of the ZEKE spectra are consistent with the largely Rydberg nature of the S₁ state near the Franck-Condon region; however, there is also some activity that is less straightforward to explain. Comments are made regarding the photodynamics of the S₁ state.