Molecule combination bands

The CN- problem deserves reinvestigation, not only to establish dynamically the involvement of the named solvent molecule combination bands and to confirm in this context the Coulomb force dominance for the VET, but also to explore the possible role in the VET of any counter ions in the CN- first solvation shell at higher concentrations of the solute (17). The latter issue, which is in principle ubiquitous in ionic solute VET studies, has yet to receive any theoretical attention. Some improvement is... 

In addition to the bands centered on the fundamental frequencies, other bands appear in the spectra of polyatomic molecules. We have mentioned overtone bands in the spectrum of diatomic molecules due to violation of the selection rule, Ap = +1, that is permitted because of anharmonicity. But in polyatomic molecules, combination bands also appear. For example, in the case of water if the absorbed quantum splits to raise from 0 to 1 and V2 from 0- 1, there will be a vibration-rotation band centered on the combination frequency, + V2 This process is relatively less probable than the absorbtion of a single quantum at either fundamental frequency, so the intensity of the band is relatively weak. Nonetheless, combination bands appear with sufficient intensity to be an important feature of the infrared spectra of polyatomic molecules. Even in the case of a simple molecule like water, there are a large number of prominent bands, several of which are listed in Table 25.2. 

Infrared Spectroscopy. The infrared spectroscopy of adsorbates has been studied for many years, especially for chemisorbed species (see Section XVIII-2C). In the case of physisorption, where the molecule remains intact, one is interested in how the molecular symmetry is altered on adsorption. Perhaps the conceptually simplest case is that of H2 on NaCl(lOO). Being homo-polar, Ha by itself has no allowed vibrational absorption (except for some weak collision-induced transitions) but when adsorbed, the reduced symmetry allows a vibrational spectrum to be observed. Fig. XVII-16 shows the infrared spectrum at 30 K for various degrees of monolayer coverage [96] (the adsorption is Langmuirian with half-coverage at about 10 atm). The bands labeled sf are for transitions of H2 on a smooth face and are from the 7 = 0 and J = 1 rotational states Q /fR) is assigned as a combination band. The bands labeled... 

Most infrared spectroscopy of complexes is carried out in tire mid-infrared, which is tire region in which tire monomers usually absorb infrared radiation. Van der Waals complexes can absorb mid-infrared radiation eitlier witli or without simultaneous excitation of intennolecular bending and stretching vibrations. The mid-infrared bands tliat contain tire most infonnation about intennolecular forces are combination bands, in which tire intennolecular vibrations are excited. Such spectra map out tire vibrational and rotational energy levels associated witli monomers in excited vibrational states and, tluis, provide infonnation on interaction potentials involving excited monomers, which may be slightly different from Arose for ground-state molecules. 

Figure 6.27 shows fhe f Sg infrared combination band of acefylene, where Vj is fhe symmefric CFI sfrefching vibration and Vj fhe cis bending vibration, as an example of a 77 — Zg band of a linear molecule. Nofe fhaf fhe P branch sfarts wifh P(2), rafher fhan / (f) as if would in a Z-Z fype of fransifion, and fhaf fhere is an intensify alternation of 1 3 for J"... 

The number of fundamental vibrational modes of a molecule is equal to the number of degrees of vibrational freedom. For a nonlinear molecule of N atoms, 3N - 6 degrees of vibrational freedom exist. Hence, 3N - 6 fundamental vibrational modes. Six degrees of freedom are subtracted from a nonlinear molecule since (1) three coordinates are required to locate the molecule in space, and (2) an additional three coordinates are required to describe the orientation of the molecule based upon the three coordinates defining the position of the molecule in space. For a linear molecule, 3N - 5 fundamental vibrational modes are possible since only two degrees of rotational freedom exist. Thus, in a total vibrational analysis of a molecule by complementary IR and Raman techniques, 31V - 6 or 3N - 5 vibrational frequencies should be observed. It must be kept in mind that the fundamental modes of vibration of a molecule are described as transitions from one vibration state (energy level) to another (n = 1 in Eq. (2), Fig. 2). Sometimes, additional vibrational frequencies are detected in an IR and/or Raman spectrum. These additional absorption bands are due to forbidden transitions that occur and are described in the section on near-IR theory. Additionally, not all vibrational bands may be observed since some fundamental vibrations may be too weak to observe or give rise to overtone and/or combination bands (discussed later in the chapter). 

In 1990 we showed that ethenedithione (115) is a stable molecule under matrix conditions. It can be prepared by photolysis of the matrix-isolated precursors 113, 114, and 116.143 Different pathways to 115 have been found by Wentrup et al.144 The matrix IR spectrum of 115 shows one absorption corresponding to the only IR active stretching mode. The IR active bending vibration is expected to appear in the for us unobservable far infrared region. The position of both IR inactive stretching vibrations were derived from two observed combination bands. The IR spectra allow no decision about the multiplicity of 115, since calculations show, that the equilibrium geometries of both states are almost identical. Recent calculations121 145 favor the triplet state. 

To make the spectrum more complicated, Combination Bands also exist. These are simply two or more bands that are physically adjacent or nearby on a molecule that add or subtract their energies (in the frequency domain) to produce a new or separate band. For example, the molecule S02, according to the formula for allowed bands,... 

Better determination of overtone and combination bands of familiar molecules and the spectroscopic characterization of new species [radicals (Shida, 1991 Bemath, 1990), ions (Miller and Bondybey, 1983 Leach, 1980), and van der Waals molecules in particular (Nesbitt, 1988 Saykally, 1989 Hutson, 1990 Heaven, 1992)] continue to receive wide attention (Figure 0.3). 

However, in polyatomic molecules, transitions to excited states involving two vibrational modes at once (combination bands) are also weakly allowed, and are also affected by the anharmonicity of the potential. The role of combination bands in the NIR can be significant. As has been noted, the only functional groups likely to contribute to the NIR spectrum directly as overtone absorptions are those containing C-H, N-H, O-H or similar functionalities. However, in combination with these hydride bond overtone vibrations, contributions from other, lower frequency fundamental bands such as C=0 and C=C can be involved as overtone-combination bands. The effect may not be dramatic in the rather broad and overcrowded NIR absorption spectrum, but it can still be evident and useful in quantitative analysis. 

The rule of mutual exclusion states that for a molecule with a center of symmetry, a given vibrational transition cannot appear in both the IR and Raman spectra. (For the proof, see Chapter 9.) Some fundamentals may be both IR and Raman inactive their frequencies can often be determined from IR or Raman combination bands. 

Because of the many normal modes and the presence of overtone and combination bands, hot bands, and impurity bands, which may overlap one another, the IR spectra of medium-sized and large molecules are complex and may be difficult to assign. Incorrect IR vibrational assignments have, unfortunately, been quite common. 

The infrared spectrum therefore consists of a number of absorption bands arising from infrared active fundamental vibrations however, even a cursory inspection of an i.r. spectrum reveals a greater number of absorptions than can be accounted for on this basis. This is because of the presence of combination bands, overtone bands and difference bands. The first arises when absorption by a molecule results in the excitation of two vibrations simultaneously, say vl5 and v2, and the combination band appears at a frequency of -I- v2 an overtone band corresponds to a multiple (2v, 3v, etc.) of the frequency of a particular absorption band. A difference band arises when absorption of radiation converts a first excited state into a second excited state. These bands are frequently of lower intensity than the fundamental absorption bands but their presence, particularly the overtone bands, can be of diagnostic value for confirming the presence of a particular bonding system. 

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). 

Figure 3. Overtone and combination band spectrum oj ethylene and water adsorbed on Mn"A zeolite. The ethylene bands lie close to the gaseous (V -f- vn), (vt + vs), 2vlu and 2vs vibrational modes, indicating that the ethylene molecule has retained its chemical composition and structural integrity (-, 1) MnA 4- ethylene (-------------------,2) MnA hydrated CtHt (g) bands.

DRS has been applied both to the analysis of vibrational spectra of surface species in the fundamental, overtone, and combination band regions,and to the determination of time correlation motion of adsorbed molecules by Fourier inversion of the spectra onto... 

Note that to first order this is simply the sum of the fundamental frequencies, after allowing for anharmonicity. This is an oversimplification, because, in fact, combination bands consist of transitions involving simultaneous excitation of two or more normal modes of a polyatomic molecule, and therefore mixing of vibrational states occurs and... 

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