Laura A. Philips

High‐resolution infrared spectroscopy of 2‐fluoroethanol in a molecular beam

The high‐resolution infrared spectrum of 2‐fluoroethanol (2FE) in a molecular beam was obtained in the region of 2990–2977 cm−1. This spectral region contains the asymmetric CH stretch of the fluorinated carbon. Excitation of the CH stretch has previously been observed to photochemically isomerize 2FE from the Gg’ to the Tt conformation. The high‐resolution spectrum of this transition provides the information necessary to quantitatively evaluate the amount of vibrational mode‐coupling between the asymmetric CH stretch and the torsional reactive coordinate. Minimal amounts of vibrational mode coupling were observed in the spectrum which is consistent with the slow photoisomerization rate. The correlation of the amount of mode coupling and the isomerization rate supports the conclusion that mode‐selective vibrational coupling plays an important role in the photochemical dynamics. It is further suggested that the strong intramolecular attractive interactions limit the magnitude of the vibrational mode coupling.

High Resolution Infrared Spectroscopy of Pyrazine and Naphthalene in a Molecular Beam

The high resolution infrared spectrum of pyrazine and naphthalene were measured in a molecular beam in the vicinity of the C–H stretching transition. The rotational structure in the spectrum of pyrazine from 3065–3073 cm−1 reveals that the C–H stretch is coupled to one other vibrational mode in the molecule. The mode coupling is manifested in the spectrum as two overlapping vibrational bands. Each of these two bands are well modeled by an asymmetric top/rigid rotor Hamiltonian. The lack of any angular momentum dependence on the coupling indicates that the vibrations are coupled by an anharmonic mechanism. The magnitude of the coupling matrix element was determined to be 0.36 cm−1. The rotational structure in the spectrum of naphthalene from 3063–3067 cm−1 reveals that except for several local perturbations, the spectrum is well modeled by an asymmetric top/rigid rotor Hamiltonian. The local perturbations include transitions that are split into doublets as well as transitions that have been shifted from th...

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.

Vibrationally Induced Rotational Axis Switching: A Novel Mechanism for Vibrational Mode-Coupling

High resolution IR spectra of small‐ to medium‐sized molecules such as 2‐fluoroethanol (2FE) show that the effective density of coupled states is often greater than that obtained by a direct count of vibrational states. A novel mechanism for rotation–vibration interaction, vibrationally induced rotational axis switching (VIRAS), is proposed as a possible explanation for these discrepancies. VIRAS has its origin in centrifugal distortion, and is physically distinct from Coriolis coupling. In the case of 2FE, we explicitly treat the coupling of overall rotation with large‐amplitude internal rotation about the C–C bond. Assuming a uniform coupling of all dark vibration–torsion states to the bright state, we predict a density of coupled states in good agreement with that observed in the C–H stretching region at 2980 cm−1.

High-resolution infrared spectroscopy of formamide and deuterated formamide in a molecular beam

High-resolution infrared spectra of formamide and deuterated formamide (DCONH2) were collected by molecular-beam optothermal spectroscopy. The spectrum of formamide was found to be perturbed such that each rotational transition was split into two peaks. The splitting was absent in the spectrum of deuterated formamide. The large splitting in the spectrum of formamide (~0.7 cm−1) prevented an accurate deconvolution of the spectrum into zeroth-order bright and dark states. A list of the possible coupling modes is presented. The coupling, however, is not well represented as a simple coupling of two rigid rotors.

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...

Data analysis for rotationally resolved spectra: A simulated annealing approach

A new method for the analysis of rotationally resolved spectra is presented. The method employs a simulated annealing algorithm with two modifications. First, the standard simulated annealing process is extended to take advantage of parallel computing. Second, rather than using a continuous random search of the parameter space, at distinct intervals new optimization processes are started at points in parameter space that have promising χ2 values. Together these modifications make more efficient use of computer time than a standard simulated annealing approach. The technique is applied to the analysis of simulated data as well as real high-resolution experimental spectra to demonstrate the effectiveness of the parallel simulated annealing algorithm.

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 32u2009625 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.