Determining the vibrational pattern via overtone cold spectra: C-H methyl stretches of propyne

Two-Dimensional Spectroscopy Isolates Infrared Rovibrational Patterns

The rovibrational spectra of freely rotating gas phase molecules are often plagued by spectral congestion due to the high density of rotational peaks. The congestion is especially severe at higher infrared frequencies due to the large numbers of overlapping overtones and combination bands that form polyads. As a result, rovibrational peaks in the near-infrared region of the spectrum are seldom assigned. This work describes how two-dimensional (2D) rovibrational spectroscopy can use the coupling between vibrational modes to isolate rovibrational bands that would otherwise remain overlapped and congested. Multidimensional spectroscopic techniques that make use of the large number of cross-peaks that form rich 2D rovibrational patterns are explored. Propyne is used to demonstrate 2D methods for identifying the frequencies and symmetries of coupled vibrations and for assigning rotational quantum numbers, even in regions that are heavily congested.

Overtone spectroscopy of v(C=O) stretching vibration of hexafluoroacetone: Experimental and ab initio determination of peak positions, absolute intensities, and band shapes

Infrared overtone spectra of the ν(C = O) stretching vibration (ν₁) of gaseous hexafluoroacetone ((CF₃)₂C = O, HFA) were recorded in the spectral range of 7450-3300 cm^-1 with a resolution of 0.1 cm^-1. Experimental absolute IR intensities and vibrational band centers of the overtones 2ν₁, 3ν₁, 4ν₁ of HFA were measured and compared with their ab initio counterparts, calculated by the second-order canonical vibrational perturbation theory (CVPT2). A hybrid potential energy surface (PES) was evaluated using the quantum-mechanical models MP2/cc-pVQZ for harmonic and MP2/cc-pVTZ for anharmonic parts. Cubic surfaces of dipole moment components were determined using the MP2/cc-pVTZ model. The predicted IR intensities for the first and second overtones reproduced the experimental values with a discrepancy of 6% and 9%, respectively. A weak Fermi resonance between the first overtone 2ν₁ and combination tone ν₁ + ν₂ + ν₈ was predicted. The appropriate model was employed for simulating the bands studied using: (i) the asymmetric-top vibration-rotational structure of the ν₁ mode, (ii) the inhomogeneous band structure due to contributions of the vibrational states populated at the room temperature, and (iii) the homogeneous broadening of each vibration-rotation transition due to the intramolecular vibrational redistribution (IVR). The rotational and vibrational anharmonic constants were taken from the ab initio calculations, whereas the IVR data were obtained from the experimental data of Chekalin et al. (JPCA 2014, 118:955) with a time resolution of ≈100 fs. A good level of agreement of the predicted shapes of bands studied with experiment is achieved.

Vibrational dynamics of pyrrole via frequency-domain spectroscopy

The N-H stretch overtones of pyrrole, a key constituent of biologic building blocks, were studied by room temperature photoacoustic and jet-cooled action spectroscopies to unravel their intramolecular dynamics. Contrary to "isolated" states excited with two and three N-H stretch quanta, the one with four quanta shows strong accidental resonances with two other states involving three quanta of N-H stretch and one quantum of C-H stretch. The inhomogeneously reduced features in the action spectra provide the means for getting insight into the intramolecular interactions and the factors controlling energy flow within pyrrole. The time dependence of the survival probability of the 4ν(1) N-H stretch, deduced from the vibrational Hamiltonian, shows an initial decay in ~0.3 ps with ensuing quantum beats from the N-H-C-H resonance and their decay with a time constant of about 5 ps as a result of weaker coupling to bath states.

Vibrational overtone spectroscopy and intramolecular dynamics of ethene

The first through fourth C-H stretching overtone regions of ethene were measured by photoacoustic spectroscopy of room-temperature molecules and action spectroscopy of jet-cooled molecules. The rotational cooling led to improved resolution in the action spectra, turning these spectra into key players in determining the multiple band appearance in each region, their types, and origins. These manifolds arise from strong couplings of the C-H stretches to doorway states and were analyzed in terms of a simplified joint local-mode/normal-mode (LM)/(NM) model and an equivalent NM model, accounting for principal resonances. The diagonalization of the LM/NM and NM vibrational Hamiltonians and the least-square fittings revealed model parameters, enabling assignment of A- and B-type bands. These bands behave differently through the V = 2-4 manifolds, showing coupling to doorway states for the former but not for the latter. The energy flow out of the fourth C-H overtone is governed by the interaction with bath states due to the increase in the density of states.

Vibrational structure and methyl C-H dynamics in propyne

Our previous study [J. Chem. Phys. 122, 224316 (2005)] presented the photoacoustic and action spectra of the V=2, 3, 4, and 5 manifolds of the C-H methyl stretching vibrations of propyne and their analysis in terms of a simplified joint local mode/normal mode model. In the current paper the C-H transition intensities were calculated using B3LYP6-311++G(d,p) level of theory to obtain the dipole moment functions. The diagonalization of the vibrational Hamiltonian revealed new model parameters obtained by least square fitting of the eigenvalues to the action spectra band origins, while examining the correspondence between the calculated intensities and simulated band areas. The newly derived parameters predict well the band positions and the observed intensities, allowing new assignment of the features. The derived Hamiltonian was also used to obtain the overall temporal behavior of the C-H stretches as a result of the Fermi couplings and interactions with the bath states. These results indicate that any specificity attained by suitable excitation of the methyl C-H stretches is lost on picosecond time scale, primarily due to strong interactions with doorway states in the lower overtone and coupling with bath states in the region of the higher ones.

Overtone spectroscopy of C-H ethyl stretches of 1-butyne

Room-temperature photoacoustic (PA) spectra and jet-cooled action spectra of the first to third overtone regions of the ethyl C-H stretches in vapor phase 1-butyne, CH3CH2C[Triple Bond]C-H, were measured. Both the PA and action spectra exhibit a complex multiple peak structure being better resolved and more pronounced in the latter, due to inhomogeneous structure reduction. The observed manifolds were analyzed in terms of a simplified joint local-/normal-mode (LM/NM) model accounting for two types of C-H stretches (methyl and methylene) and for Fermi resonances between stretches and deformations. The retrieved parameters, used for calculation of the eigenstates, come from the best-fit parameters based on the diagonalization of the vibrational Hamiltonian in the LM/NM basis. The parameters were obtained by comparing the eigenvalues and the sum of the squares of the expansion coefficients of the eigenvectors of the C-H stretches of methyl and methylene to the action spectra peak positions and intensities, respectively. This approximate model vibrational Hamiltonian is proposed to explain most observed spectral features, corresponding to C-H stretch bands and to combinations of C-H stretches and deformations, indicating the importance of the Fermi resonance. The model was also applied to calculate the dynamics of the C-H stretching modes resulting from coupling with the deformations, implying rapid initial state decay on subpicosecond time scale. Decays of several picoseconds were found for complete transfer of probability from the initially prepared state of methylene and methyl to the counterpart LM states.