The vapor-phase infrared and Raman spectra of p-difluorobenzene (h4) and (d4)

Abstract

This new study of the vapor-phase infrared and Raman spectra of pdifluorobenzene (h,) and ( 4) includes assignments from the high-resolution electronic spectrum, from fluorescence spectra, and one assignment from the two photon electronic spectrum. A new set of frequencies and assignments has been made for the fundamentals of this important intermediate-sized molecule. The most important changes concern the assignments previously made for the lowfrequency a, mode (us), two members of the &, class (ui9 and YH)), and one very important change for the lowest frequency fundamental of the baU (out-of-plane) class (va), which has been observed directly in the far infrared. The results are all corroborated by the assignments of the perdeuterated analog where changes in previous assignments of Fermi resonance pairs have also been made. Some interesting differences between the Raman spectra of the vapor and the liquid perproto compounds have not been adequately explained in this study. 8 1985 Academic PRS. Inc.

Consistent assignment of the vibrations of symmetric and asymmetric para-disubstituted benzene molecules

“…The assignments in Ref. [15] largely confirm those made in earlier investigations, but several vibrational wavenumbers are reassigned and this work has been used as the key reference for the S 0 vibrational wavenumbers in later studies. Despite this, we believe several vibrational wavenumbers have been incorrectly assigned in Ref.…”

“…The first of which is the vibrational wavenumber assigned to D 24 (which is a b 2u symmetry mode, ν 19 in Mulliken numbering). This vibration was assigned to a wavenumber of 1437 cm -1 in previous work [23], however Zimmerman and Dunn suggested that this could be incorrect if that feature corresponded to one they saw at 1431 cm -1 with the wrong band profile; on this basis they assigned a wavenumber of 1633 cm -1 to this vibration [15]. However, based on calculated vibrational wavenumbers, and through comparison with the wavenumber of this vibration in other para-disubstituted molecules (vide infra), we believe that this assignment is incorrect, while the original assignment fits well with our calculated wavenumber and trends across the series.…”

“…Despite this, we believe several vibrational wavenumbers have been incorrectly assigned in Ref. [15]. The first of which is the vibrational wavenumber assigned to D 24 (which is a b 2u symmetry mode, ν 19 in Mulliken numbering).…”

“…As such these modes can be described with any of the three labelling schemes discussed herein. The Duschinsky matrix in Wilson modes  5 and  17b appear to have in and out-of-phase, out-of-plane motion of the ipso and para hydrogens and these combine together to form the D 15 and D 16 modes in pDFB there is also mixing in of the  10b motion , which has a similar out-of-plane C-H motion as  5 , but these two vibrations have a different phase with respect to the motion of the other C-H groups. It is notable that the low-wavenumber  11 vibration contributes to these vibrations ( Figure 3.…”

“…[15] was given as 3073 cm -1 , was given as 3091 cm -1 in Table 1 and the text of that work. We note that the value of 3091 cm -1 for this vibration is consistent with the corresponding value for pDClB, but not with pDBrB, which is more in line with the lower value -our calculated values are not able to differentiate between these.…”