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

The rotational spectroscopy of single molecular eigenstates has been used t o measure the conformational isomerization rate in 2-fluoroethanol. Eigenst ates in the asymmetric -CH2(F) stretch spectrum of the Gg' conformer near 2 980 cm(-1) are prepared with an infrared laser. These eigenstates are appro ximately 2000 cm(-1) above the barrier to Gg'-Tt conformational isomerizati on. The rotational spectrum is measured using an infrared-microwave double- resonance technique based on the Autler-Townes splitting of states in a str ong microwave field. This technique does not require saturation of the infr ared preparation step. Two types of rotational transitions are observed. Th ese 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. Th e isomerization kinetics are obtained from the linewidth of the ensemble ei genstate rotational spectrum. The lifetime for the Tt conformer is 2.7 ns. The isomerization states relax at approximately twice the rate of the Tt st ates (1.5 ns lifetime). This result is consistent with a kinetics model whe re the isomerization proceeds by "over-the-barrier'' pathways. Both lifetim es are longer than the bright-state IVR lifetime (275 ps) indicating that t he intramolecular dynamics occur on two distinct time scales. The isomeriza tion rate for the Tt states is three orders-of-magnitude slower than predic ted by a simple RRKM rate expression. (C) 1999 American Institute of Physic s. [S0021-9606(99)00304-9].