Combination bands origination

Near-infrared reflectance analysis is particularly well suited to chemical sensing because it operates on "as is" samples and yet has chemical specificity. The absorptions observed originate from vibrations of a relatively few chemical groups whose overtones and combination bands appear in the near infrared region. 

The original positions of the fundamental and the overtone/combination bands can be calculated from the spectra using the formula (1) given by J. Overend (37), were v, and v2 are the observed band frequencies and p is the intensity ratio of the bands. 

Acetonitrile (ACN) possesses some unique properties, such as a high dielectric constant (35.95) and the solubilization of many inorganic and organic materials . It is actually one of the few simple aprotic solvents miscible in water at any ratio. X-ray diffraction studies of pure acetonitrile revealed that ACN molecules do not strongly interact with themselves and are only weakly associated via dipole-dipole interaction . The IR spectrum of pure acetonitrile includes two major bands placed at 2257 and 2295 cm . The former, called V2, originates from the C=N stretching mode while the latter is a combination band composed of the CCH bend V3 and C—C stretch V4 modes. ... 

Figure 10.11 The absorption spectrum of Ar-Cl2 to the v = 10 level of the B state. The strong peak at -177.28 cm (relative to the origin of the B state) is the zero-order bright state at V = 10. The resonance at -176.69 cm corresponds to a combination band consisting of v(Cl2) = 9 and the 36 van der Waals vibrational level. This strong mixing of states is attributed to IVR. Taken with permission from Halberstadt et al. (1992).

According to the selection rule for the harmonic oscillator, any transitions corresponding to An = 1 are allowed (Sec. I-2). Under ordinary conditions, however, only the fundamentah that originate in the transition from u = 0 to u = 1 in the electronic ground state can be observed because of the Maxwell-Boltzmann distribution law. In addition to the selection rule for the harmonic oscillator, another restriction results from the symmetry of the molecule (Sec. 1-9). Thus the number of allowed transitions in polyatomic molecules is greatly reduced. The overtones and combination bands of these fundamentals are forbidden by the selection rule of the harmonic oscillator. However, they are weakly observed in the spectrum because of the anharmonicity of the vibration... 

Shriver and co-workers located the antisymmetric and symmetric stretching vibrations of the W-H-W bric%e at 1683 and 900cm , respectively. However, the latter splits into four bands at 960, 869, 832, and 702 cm . Although the origin of this splitting is not clear, the possibility of Fermi resonance with an overtone or a combination band involving the (WC) or 5(WCO) was ruled out based on CO-C 0 isotopic shifts. ... 

In some aromatic acid chlorides one may observe another rather strong band, often on the lower-frequency side of the C=0 band, which makes the C=0 appear as a doublet. This band, which appears in the spectrum of benzoyl chloride (Fig. 2.56) at about 1730 cm is probably a Fermi resonance band originating from an interaction of the C=0 vibration, with an overtone of a strong band for 1-C stretch often appearing in the range from 900 to 800 cm . When a fundamental vibration couples with an overtone or combination band, the coupled vibration is called Fermi resonance. The Fermi resonance band may also appear on the higher-frequency side of the C=0 in many aromatic acid chlorides. This type of interaction can lead to splitting in other carbonyl compounds, as weU. 

Deo et al. observed a weak combination band, Vi -1- V3, at 2470 cm and the more intense asymmetric stretching band with V3=1330 cm" shifted by A (V1-1-V3) = -42 and Av3=-31 cm with respect to the gas phase wavenumbers (Vi -h V3 = 2512, V3 = 1361 cm )> respectively. No further band appeared, in particular no V2 and no overtone band 2Vi. On admission of H2S to the S02-loaded Y-zeolite sample, a band around 6h2o=1650 cm developed originating from the deformation mode of H2O, which had formed according to Eq. (29). 

The PE spectrum of nitrosomethane shows three distinct ionization regions a separate band at low energy (IPa = 9.15, IPy = 9.76 eV ) followed by two intense and broad composite bands with maxima at 13.9 and 16.3 eV. The assignment of the first band as arising from n (10a ) is unambiguous, whereas to the second band n (2a") and n+ (9a ) contribute, and the third band originates from ji (la") and perhaps a a ionization. The two 7T MOs of MeNO are the in-phase and the out-of-phase combinations of tiN=q and a pseudo n orbital of the methyl group (ttmc)-... 

Complications in evaluation of IR spectra are the overlapping of individual bands and the appearance of additional bands, e. g. overtone and combination bands, which may be caused by anharmonicity of some vibrations. In the NIR region, aU bands are overtone or combination bands. They are always weaker in intensity than the corresponding fundamental bands. Originally considered as a drawback, the weak intensity of the NIR bands turned out to be the background for the large success of NIR spectroscopy in process analysis. 

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