C. Cameron Miller

High Resolution Infrared Spectroscopy of Cyclobutane: A Study of Vibrational Mode Coupling Involving Large Amplitude, Low Frequency Modes

The high resolution IR spectrum of cyclobutane in a supersonic molecular beam was obtained for the region of 2981 to 2991 cm−1. The spectrum reveals four overlapping bands suggestive of vibrational mode coupling in the C–H stretching region. Ground state combination differences demonstrate that these bands originate from two different ground states, the symmetric and asymmetric ring puckering states. Evidence of vibrational mode coupling is present in all four bands. The coupling depends on both J and the symmetry of the puckering state. A model coupling scheme involving two qualitatively different types of couplings is developed to explain the observed spectrum. Symmetry restrictions and the interaction between molecular rotation and ring puckering qualitatively accounts for the dramatically different coupling behavior between the two ring puckering states.

High resolution spectroscopy of 1,2‐difluoroethane in a molecular beam: A case study of vibrational mode‐coupling

The high resolution infrared spectrum of 1,2‐difluoroethane (DFE) in a molecular beam has been obtained over the 2978–2996 cm−1 spectral region. This region corresponds to the symmetric combination of asymmetric C–H stretches in DFE. Observed rotational fine structure indicates that this C–H stretch is undergoing vibrational mode coupling to a single dark mode. The dark mode is split by approximately 19 cm−1 due to tunneling between the two identical gauche conformers. The mechanism of the coupling is largely anharmonic with a minor component of B/C plane Coriolis coupling. Effects of centrifugal distortion along the molecular A‐axis are also observed. Analysis of the fine structure identifies the dark state as being composed of C–C torsion, CCF bend, and CH2 rock. Coupling between the C–H stretches and the C–C torsion is of particular interest because DFE has been observed to undergo vibrationally induced isomerization from the gauche to trans conformer upon excitation of the C–H stretch.

ROTATIONAL SPECTRUM OF A DARK STATE IN 2-FLUOROETHANOL USING MICROWAVE/RADIO-FREQUENCY-INFRARED MULTIPLE RESONANCE

Microwave/radio‐frequency‐infrared multiple resonance has been used with an electric‐resonance optothermal spectrometer to characterize a weak 21.6 MHz perturbation in the infrared spectrum of the ν14 C–O stretching vibration of 2‐fluoroethanol. The infrared spectrum of 2‐fluoroethanol was recorded at a resolution of ∼2 MHz using a tunable microwave‐sideband CO2 laser. The spectrum is fit by an asymmetric‐rotor Hamiltonian to a precision of 0.6 MHz, except for the transitions to the 413 upper state which are split into doublets by an interaction between the 413 level and a rotational level of a nearby background, or dark, vibrational state. Microwave/radio‐frequency‐infrared double and triple resonance reveals that the 413 level of the C–O stretching vibration is interacting with the 431 level of the dark state. The rotational constants determined for the dark state allow us to assign the perturbing state to the ν18+4ν21 combination vibration of the lowest energy conformer, where ν18 is the CCO bending vi...

Linking structure and vibrational mode coupling using high‐resolution infrared spectroscopy: A comparison of gauche and trans 1‐chloro‐2‐fluoroethane

The high‐resolution infrared spectrum of 1‐chloro‐2‐fluoroethane in a molecular beam was collected over the 2975–2994 cm−1 spectral region. The spectral region of 2975–2981 cm−1 contains a symmetric C–H stretching vibrational band of the gauche conformer containing the 35Cl isotope. The spectral region of 2985–2994 cm−1 contains three vibrational bands of the trans conformer. Two of the three bands are assigned as an antisymmetric C–H stretch of each of the two different chlorine isotopes. The third band is assigned as a symmetric C–H stretch of the 35Cl isotope. The gauche conformer of 1‐chloro‐2‐fluoroethane showed doublet patterns similar to those previously observed in 1,2‐difluoroethane. The model for 1,2‐difluoroethane is further refined in the present work. These refinements suggest that the coupling dark state in 1,2‐difluoroethane is composed of 1 quantum C–H bend, 1 quantum C–C stretch, and 12 quanta of torsion. For 1‐chloro‐2‐fluoroethane the dark state could not be identified due to a small da...

OPTOTHERMAL DETECTION OF NONRADIATIVE RELAXATION CHANNELS IN ELECTRONICALLY EXCITED MOLECULES

Optothermal detection has been used to observe nonradiative relaxation channels in aniline, p‐bromoaniline, and trans‐stilbene. p‐Bromoaniline has no detectable fluorescence due to a heavy atom effect which increases the rate of intersystem crossing to the triplet state. An optothermal spectrum of p‐bromoaniline was observed with the origin at 32 625 cm−1. For trans‐stilbene, the differences between the laser excitation spectrum and the optothermal spectrum of the S1 state clearly show the onset of isomerization at ∼1250 cm−1 above the origin. Absolute quantum yields of fluorescence, Franck–Condon factors, nonradiative rates, and radiative rates have been obtained for a series of vibronic transitions. For low energy vibrational states, there is good agreement between the current study and previous work. For vibrational energies above the barrier of isomerization, predicted quantum yields do not agree with our experimental results.