The rotational spectra of single molecular eigenstates of 2-fluoroethanol: Measurement of the conformational isomerization rate at 2980 cm−1

Abstract

The rotational spectroscopy of single molecular eigenstates has been used to measure the conformational isomerization rate in 2-fluoroethanol. Eigenstates in the asymmetric –CH2(F) stretch spectrum of the Gg′ conformer near 2980 cm−1 are prepared with an infrared laser. These eigenstates are approximately 2000 cm−1 above the barrier to Gg′−Tt conformational isomerization. The rotational spectrum is measured using an infrared-microwave double-resonance technique based on the Autler–Townes splitting of states in a strong microwave field. This technique does not require saturation of the infrared preparation step. Two types of rotational transitions are observed. These are assigned to rotational transitions from vibrational states with Tt conformation (near 15.8 GHz) and to “isomerization states” (near 17.1 GHz) where the torsional wave functions are above the isomerization barrier. The isomerization kinetics are obtained from the linewidth of the ensemble eigenstate rotational spectrum. The lifetime for the Tt conformer is 2.7 ns. The isomerization states relax at approximately twice the rate of the Tt states (1.5 ns lifetime). This result is consistent with a kinetics model where the isomerization proceeds by “over-the-barrier” pathways. Both lifetimes are longer than the bright-state IVR lifetime (275 ps) indicating that the intramolecular dynamics occur on two distinct time scales. The isomerization rate for the Tt states is three orders-of-magnitude slower than predicted by a simple RRKM rate expression.

Molecular Self‐Recognition: Rotational Spectra of the Dimeric 2‐Fluoroethanol Conformers

“…Based on the MP2/6-311 + + GA C H T U N G T R E N N U N G (d,p) calculations, the G + gÀ/GÀg + pair is more stable by 7 kJ mol À1 compared to the next higher energy conformer. This is consistent with the previous experimental and theoretical studies, [13,23,24] showing that the G + gÀ/GÀg + pair is by far the dominant conformations.…”

“…The rotational spectrum of the dominating G + gÀ/GÀg + conformations of the FE monomer was reported previously. [23,24,26] The known FE transitions were Table 1. Calculated raw (D e ) and ZPE-corrected (D 0 ) dissociation energies, rotational constants, and the magnitudes of the electric dipole moment components of the six most stable hydrogen bonded dimeric FE conformers at the MP2/6-311…”

Vibrational Energy Flow: A State Space Approach

“…Nonexponential decays implicating multiple timescales have also been observed in methanol and benzene [108], although the larger dilution factors lead to a reduced timescale over which the IVR can be followed. Infrared-microwave double resonance experiments initiated in hydrogenic modes, but capable of revealing dynamics among bath states with more backbone character, have also shown discrepancies between the initial and "bath" timescales for IVR [109]. Figure 5 compares infrared and double resonance spectra (the latter of of one conformer) of 2-fluoroethanol, clearly illustrating the discrepancy in line widths and hence lifetimes (0.311s for initial dephasing, > 2 ns for "bath" dephasing corresponding to isomerization).…”

“…Interior states are less accessible because of unfavorable Franck-Condon factors. Experimental studies thus primarily probe edge-to-interior evolution, although double-resonance methods [40,109] and SEP spectra probing low-frequency modes [37,86] allow a look deeper into the interior and skeletal IVR regions of state space.…”

“…For edge states, this timescale can become even longer because the transition from edge to interior states can instead control the dynamics. This is the reason why rotational double-resonance measurements initiated in overtones, although capable of distinguishing edge and interior timescales [109], cannot yet probe the fact that the interior evolution in fact has no timescale; instead, one has to look at the deviations of lineshapes from Lorentzians and of survival probabilities from exponentials measured in overtone or preferably pump-dump combination band experiments [22,37]. In the future, as statistics improve so more than a linewidth can be assigned to rotational double resonance spectra, and experiments are initiated in states closer to the interior of state space, vibration-rotation double-resonance measurements are expected to become another excellent probe of multiscale IVR among bath modes.…”