Fermi resonance between the ν ₁ and 2ν ₅ ⁰ infrared bands of methyl chloride

Anharmonic resonances between the upper rovibrational levels of the and infrared bands of CH335Cl studied in a high-resolution FTIR spectrum from 4250 to 4600 cm-1

“…values given in table 1 are quite reasonable. In particular, the W6 coupling constant between vl + v~ 1 and 3vg 1 is quite close to 21/2 Wxss, where Wlss is the coupling constant between vl and 2v ~ , and is found to be equal to 35crn -1 [12]. The value of the band centre of vl + vs is of course completely different from the value reported in [2], since the Fermi resonance with 3v~ 1 was not taken into account in that study.…”

“…The additional perturber is likely to be the K'= 7 sublevel of 2v 3 + 3v6 t, which would be linked to the K' = 7 sublevel of v~ 1 + v~ 1 through a weak anharmonic 852 H. Najib et al resonance with a coupling constant of about 0.26crn -t. As the I'Q9 branch of v~ + v I also exhibits a crossing, we obtain, with the assumption that 2v3 + 3v6 x is again the perturber, for its band centre (2v3 + 3v6;1)~ ~ 4468cm -1 and A~ 1-2 cm-1, which is not unreasonable. Besides, we recall that the K" = 6 (AK = 0) sub-band of vl + v2 has already been detected [2,12]. In both of the latter papers it was explained that this sub-band takes its intensity from v2 + v4 through a weak Coriolis resonance between vl + v2 and v2 + v4; but, since an anharmonic resonance between v2 + v4 and vl + v5 is now clearly established, it seems more reasonable to suggest that the K" = 6 sub-band of Vl + v2 becomes observable through the strong x-y Coriolis interaction between v2 and Vs.…”

“…As four K sub-bands of the 3v~ 1 band have been identified in the low-wavenumber part of the spectrum around and below 4300cm-1 (see e.g. figure 3), it has been possible to take into account the Fermi resonance between vi + vs ~ 1 and 3v5 ~ 1, as has already been considered [2] as a consequence of the Fermi resonance between vl and 2v ~ [12]. The anharmonic resonance between vl + v~ and v2'+ yr. 1 is quite sensitive to K': in fact, the (Vl = 1, vs = 11, K') levels are below the (v4 = 1-x, v5 = 1-1, K') levels when K' is small, and become closer and closer with increasing K' until a level crossing occurs for K' = 14 (the Fermi resonance with 3v5 ~ 1 being taken into account in the calculation of the upper levels of v I + vs).…”

Anharmonic resonances between thev1+v±16,v3+v±15+ 2v06andv3+v∓15+ 2v±26vibrational bands of CH379Br

“…In methyl chloride, a Fermi coupling constant W~55 ~ 35cm -~ was found [1] between the v~ and 2v ~ parallel bands, through the study of the centre of several v~ + vz and 2v ~ + vz pairs of bands: for that, it was necessary to know already the xsz anharmonicity constant, and, of course, to assume that the W~55 coupling constant is the same for all these pairs of bands. Later on, Duncan set up a joint local and normal mode model which was used to perform least squares calculations over a number of band centres, generally taken from the literature, in order to obtain the value oflhe anharmonicity constants and of the W~55 coupling constant.…”

“…If we follow the method of reference[1] it would then be necessary to study triads like vl + vz, v3 + v~ + v6 ~1 + vz and 2v ~ + Vz (instead of pairs of bands), or even tetrads if the resonance between vl and 2vz (i.e. between vl + Vz and 2v2 + v~) is taken into account, as has been advised by Duncan.…”