Infrared rotation-vibration spectra, band

The spectroscopic analysis and study of molecules is related to atomic spectroscopy in that spectral line positions provide information about the molecular structure. However, the non destructive method of transmission spectroscopy is much more prevalent for molecular species allowing the determination of characteristic spectral information not only for gas phase but also liquid and solid phase substances. The majority of rotation vibration absorption bands of molecules occur in the infrared region of the spectrum. 

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... 

Both absorption and emission may be observed in each region of the spectrum, but in practice only absorption spectra are studied extensively. Three techniques are important for analytical purposes visible and ultraviolet spectrometry (electronic), infrared spectrometry (vibrational) and nuclear magnetic resonance spectrometry (nuclear spin). The characteristic spectra associated with each of these techniques differ appreciably in their complexity and intensity. Changes in electronic energy are accompanied by simultaneous transitions between vibrational and rotational levels and result in broadband spectra. Vibrational spectra have somewhat broadened bands because of simultaneous changes in rotational energy, whilst nuclear magnetic resonance spectra are characterized by narrow bands. 

The absorption bands in the ultraviolet and visible part of the spectrum correspond to changes in the energy of the electrons but simultaneously in the vibrational and rotational energy of the molecule. In this way a system of bands is produced in the gaseous state. In the liquid state there is nothing of the rotational fine structure to be seen, and usually little or nothing of the vibrational structure, as a result of the interaction with the molecules of the solvent. With aromatic compounds in non-polar solvents such as hexane and carbon tetrachloride the vibrational structure is, however, still clearly visible in the ultraviolet absorption spectrum. This vibrational structure is mainly determined by the vibrations of the excited state, which therefore do not occur in the infrared and Raman spectrum of the normal molecule. 

Infrared spectroscopy has broad appHcations for sensitive molecular speciation. Infrared frequencies depend on the masses of the atoms involved in the various vibrational motions, and on the force constants and geometry of the bonds connecting them band shapes are determined by the rotational stmcture and hence by the molecular symmetry and moments of inertia. The rovibrational spectmm of a gas thus provides direct molecular stmctural information, resulting in very high specificity. The vibrational spectrum of any molecule is unique, except for those of optical isomers. Every molecule, except homonudear diatomics such as O2, N2, and the halogens, has at least one vibrational absorption in the infrared. Several texts treat infrared instrumentation and techniques (22,36—38) and their appHcations (39—42). 

In a heteronuclear molecule such as HCl, the centers of positive and negative charge do not coincide, and the molecule has a permanent dipole moment. As this molecule vibrates, the displacement of the centers of charge varies and the magnitude of the dipole moment changes. The corresponding vibration-rotation band appears in the infrared. Rotation of the HCl molecule will produce an oscillation of the component of the dipole moment along a specified axis hence, HCl has a pure rotational spectrum in the far infrared. 

Molecules do not have a pure vibrational spectrum because the selection rules require a change in the vibrational state of the molecule to be accompanied by a change in the rotational state as well. As a result, in the infrared region of the spectrum there are vibration-rotation bands each band consists of several closely spaced lines. The appearance of a band can be simply interpreted by supposing that the vibrational and rotational energies of the molecule are additive. For simplicity we consider a diatomic molecule the... 

The interpretation of the vibrational spectrum, and in particular the assignment of gaseous-phase infrared or Raman frequencies to the different symmetry species, is sometimes facilitated by consideration of the shape of the envelope of the band, which is determined by the rotational energy levels and selection rules. In this appendix a brief resume of the rotational theory will be developed. 

Mention has already been made of the effects of vibrational hot bands on the vibrational/rotational (infrared or Raman) spectrum. However, there are significant effects of the thermal distribution of molecules among rotational states. If one considers the Raman (lyH intensity of absorption (due to rotational transition) in the wings of a vibrational fundamental band then figure 11 and equation 1.2 show that the only difference between the AJ = +2 and AJ = -2 sides of the band is a factor... 

While infrared and Raman spectrum both involve vibrational and rotational energy levels, they are not duplicates of each other but rather complement each other (see Fig. 1.31). This is because the intensity of the spectral band depends on how effectively the photon energy is transferred to the molecule and the mechanism for photon energy transfer differs in the two techniques. This will be shown below. 

The vibrational-rotational states of diatomic molecules are probed in spectroscopic experiments using radiofrequency or microwave radiation (low energy, pure rotational spectroscopy) or infrared radiahon (vibrahonal spectroscopy). The former requires a permanent dipole moment for a transition to take place and a change in the rotational quantum number of A/ = 1 (A/ = -1 in emission). The latter requires that the dipole moment change in the course of the vibrational motion and that An = 1 (An = -1 in emission) and A/ = 1 (except for diatomic radicals where A/ can also be 0). These selechon rules lead to a pattern of lines in the high-resolution vibrational spectrum, and the lines make up a vibrational band. 

The distinction between in-plane A symmetry) and out-of-plane (A" symmetry) vibrations resulted from the study of the polarization of the diffusion lines and of the rotational fine structure of the vibration-rotation bands in the infrared spectrum of thiazole vapor. 

This general behaviour is characteristic of type A, B and C bands and is further illustrated in Figure 6.34. This shows part of the infrared spectrum of fluorobenzene, a prolate asymmetric rotor. The bands at about 1156 cm, 1067 cm and 893 cm are type A, B and C bands, respectively. They show less resolved rotational stmcture than those of ethylene. The reason for this is that the molecule is much larger, resulting in far greater congestion of rotational transitions. Nevertheless, it is clear that observation of such rotational contours, and the consequent identification of the direction of the vibrational transition moment, is very useful in fhe assignmenf of vibrational modes. 

As in the infrared spectrum, overtone bands with Ac > 1 are possible, but have much weaker intensity and are usually not observed.) The A/= -2, 0, and +2 branches of a vibration-rotation Raman band are called O, Q, and S branches, respectively, in an extension of the P, Q, R notation used in infrared spectra. 

---