Vibrational overtone spectra interpretation

Another complication arises in the interpretation of absorption spectra. If a molecule vibrates with pure harmonic motion and the dipole moment is a linear function of the displacement, then the absorption spectrum will consist of fundamental transitions only. If either of these conditions is not met, as is usually the case, the spectrum will contain overtones (multiples of the fundamental) and combination bands (sums and differences). Most of these overtones and combination bands occur in the near-infrared (0.8-2.0/un). 

Interpretable Laser Raman and Laser mass spectra have been obtained from certain other microstructures ( Ramsaysphaera ) (Fig. 32). The Laser mass spectra are characterized by CN and CNO ions (Fig. 33). Raman lines (Fig. 32) appear at 1360, 1600, 2720, 2960 cm-1 within the organic range of the spectrum. The strong line at 1360 cm-1 may be atrributed to a symmetric N—O vibration of the N02 group, the weaker line at 1600 cm-1 is characteristic of aromatic double bonds C = C. The first overtone of the 1360 cm-1 line is observed at 2720 cm-1. The spectrum has the features of a resonant Raman spectrum. It is very often obtained with this type of product in which a large delocalisation of electrons is possible. 

The difficulties in interpreting the vibrational spectrum of water are obvious. Because of the broad nature of the bands, the two stretching modes Vj and V3 overlap. In addition, overtones complicate the assignment of bands. Thus, the first overtone of the V2 bending mode, 2v2, lies close in frequency to the stretching modes Vj and V3. For these reasons, there are advantages in studying HOD, a molecule for which the vibrational frequencies are quite different. As a result,... 

In the IR spectrum of the gas at 60 to 400Torr, the rotational subbands of the three fundamental vibrations and of the overtone 2v2(see p. 150) of HOF and DOF we re observed [7]. The spectrum of matrix-isolated HOF was studied at HOF N2 ratios of -1 3000 and -1 20000. The fundamental vibrations (see p. 149) were observed as well as a small band at 1393 cm definite only in the more concentrated matrix and presumably due to the bending mode of an HOF molecule hydrogen-bonded to HF, a product of thermal decomposition of HOF [8]. The complex spectra in N2 and Ar matrices are also partly due to HF HOF [9]. IR spectra of solid films were studied at -195°C, and the intramolecular vibrations v, V2, V3, 2v2, and 2v3 of HOF, DOF, and H OF were found. Bands at 628 and 448 cm" for HOF were ascribed to a simple cyclic dimer. But infinite planar zigzag chains could not be ruled out [10], and this latter interpretation is supported by the Raman spectrum (see table p. 154). 

The vinyl group in polybutadienes has been studied more extensively than simple alkenes, and it has a similar spectrum with peaks at about 4484, 4597, and 4660 cm" Bands at 4717 and 4481 cm in polybutadiene have been assigned as second overtones of the symmetric and asymmetric bending vibrations of the CH2 of the uncoupled vinyl group, as interpreted by a local mode model. There are no corresponding fundamental vibrations for these peaks at 1469 and 1572 cm" though ... 

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