Probing the Structure, Pseudorotation, and Radial Vibrations of Cyclopentane by Femtosecond Rotational Raman Coherence Spectroscopy

Abstract

The 10 nJ pulses from a Kerr-lens mode-locked Ti:sapphire oscillator (Mai Tai, SpectraPhysics) are amplified in a Ti:sapphire multipass chirped-pulse amplifier system (ODIN DQC, Quantronix), which is pumped by a 14 W pulsed frequency- The output of the fs laser system is reduced to a laser pulse energy between 75 − 120 µJ and split into three equally intense pump, dump and probe beams, which are aligned in a forward BOXCARS degenerate four-wave mixing (DFWM) arrangement, and focused by an f = 1000 mm achromatic lens into the center of either a 1.0 m long stainless-steel gas-cell or into the supersonic jet vacuum chamber. The probe beam runs over a retroreflector that is mounted on a high-precision 1000 mm long delay stage, housed in a vacuum tank with 800 nm AR coated windows that is evacuated to < 10 −3 mbar. The time-delay of the probe pulse is by measuring the retroreflector displacement with a two-axis He-Ne laser interferometric system (SP2000 D; SIOS GmbH) with an accuracy of ±30 nm. After the output window of gas-cell or jet vacuum chamber, the three input beams are blocked by a mask, while the DFWM signal beam is collimated, spatially filtered and detected by a thermoelectrically cooled GaAs photomultiplier (H7422-50, Hamamatsu). The signals are recorded with a 1 GHz/8-bit oscilloscope at a sweep rate of 8 GHz/s. The experiment is controlled and the data acquired by a PC using LabView.

Quasistructural molecules

“…As a consequence, molecules for which pseudorotation has been established do not possess a static molecular structure but the structures are determined by dynamic effects, which is another characteristic of quasistructural molecular systems. Converting the puckering/twisting model Hamiltonian corresponding to the Pitzer model to polar coordinates enables the clear visualization of the pseudorotation of cyclopentane: interconversion of the 10 energetically equivalent but permutationally distinct twist minima (of C 2 point‐group symmetry) through 10 bent conformations (of C s point‐group symmetry), with diminishing barriers. Thus, while the equilibrium structure of cyclopentane is a polar asymmetric top, the “true” structure of the molecule becomes a nonpolar symmetric top .…”

Intermolecular dissociation energies of dispersively bound 1-naphthol⋅cycloalkane complexes

“…The dissociation energies differ by only 0.3 kJ/mol. Because the barrier to pseudorotation is practically zero in free cyclopentane, 41 it is conceivable that pseudorotational interconversion between these two isomers might occur in the 1NpOH·cyclopentane complex even at the low vibrational temperature in the supersonic beam expansion (T vib ∼ 5 − 10 K). However, the treatment of these dynamics is beyond the scope of this work; here we discuss the complex in terms of a static structure.…”

Revealing pseudorotation and ring-opening reactions in colloidal organic molecules

“…In molecular cyclopentane, these conformations are thought to interconvert continuously. This process is known as pseudorotation, and has been suggested as early as the 40s, but so far has only been confirmed by indirect spectroscopic evidence 21 – 24 . Likewise, we find that in colloidal cyclopentane, envelope and twist conformations interconvert continuously.…”