Solvent effects on vibrational phase relaxation and frequency shifts in isobutylene

Abstract

The Raman line shapes and frequencies of the ν4 asymmetric (C=CH2) stretching mode at 1657 cm−1 and the ν9 symmetric (C–CH3) stretching mode at 805 cm−1 (both of symmetry A1) have been measured in dilute solutions of isobutylene in several polar and nonpolar solvents. The frequency shifts in the different solutions can be interpreted in terms of repulsion, dispersion, induction, and dipolar forces between molecular bonds. The induction and dipolar forces play only a minor role. The blue shift due to the repulsion forces and the red shift due to the attractive dispersion forces are nearly equal for ν9, while the dispersion forces are much more important for ν4. Analogous behavior has already been described in our earlier work dealing with pure compressed isobutylene. The frequency shift and the linewidth are connected via the first and second spectral moments, and the order of magnitude of the width can be predicted from the frequency shifts. However, quantitative calculations are more difficult for linewidths than for shifts. Qualitatively, the linewidth behavior is consistent with the result for the vibrational frequencies concerning the relative importance of the repulsive and attractive energies.

Shift in the Raman symmetric stretching band of N2, CO2, and CH4 as a function of temperature, pressure, and density

“…The variation in Raman peak position with PT reflects interactions between molecules. As noted above, dominance of attractive forces leads to a shift in the Raman peak position to lower wave numbers whereas dominance of repulsive forces leads to a shift in the Raman peak position to higher wave numbers . The reason for this is that dominance of repulsive forces contracts the bond length slightly whereas dominance of attractive forces expands the bond length slightly .…”

“…Raman peak positions of fluids have been shown to vary as functions of pressure, temperature, and density . Shifts in the Raman peak position of fluids with changing conditions have been related to the attractive and repulsive forces between molecules: attractive forces cause Raman peak position to shift to lower wave number, whereas repulsive forces cause Raman peak position to shift to higher wave number, and the overall direction of peak shift with changing conditions implies dominance of either attractive or repulsive forces …”

Elucidating the Vibrational Fingerprint of the Flexible Metal–Organic Framework MIL-53(Al) Using a Combined Experimental/Computational Approach

“… 20 This weakens the forces between neighboring atoms, yielding lower covalent and ionic bond strengths. 62 , 63 …”

Kinetic Isotope Effects in Asymmetric Reactions

“…There are, however, some points of conceptual contact with the 13 C KIE measurements reported herein and studies upon vibrational frequency shifts observed as a function of pressure. Frequencies corresponding largely to CC single‐bond stretches are almost exclusively blueshifted upon the application of pressure 51. 52 However, heavy‐atom bonds with significant dipoles, such as the CF bond in CFCl 3 , the CC bond in CH 2 CCl 2 , and the CN bond in CH 3 CN are redshifted over a significant portion of the pressure range explored.…”