Rotation spectra, infrared Raman

If the resolving capacity of the instruments is ideal then vibrational-rotational absorption and Raman spectra make it possible in principle to divide and study separately vibrational and orientational relaxation of molecules in gases and liquids. First one transforms the observed spectrum of infrared absorption FIR and that of Raman scattering FR into spectral functions... 

Not all vibrations and rotations are infrared-active. If there is no change in dipole moment, then there is no oscillating electric field in the motion, and there is no mechanism by which absorption of electromagnetic radiation can take place. An oscillation, or vibration, about a center of symmetry, therefore, will not be observed in the infrared spectrum (absorption) but can be observed in the Raman spectrum (scattering). 

The Raman spectrum of gases can now also be recorded with Fourier-Transform Raman spectrometers with near infrared excitation (Dyer and Hendra, 1992). Fig. 4.3-19 shows a survey spectrum of air obtained in 4 hours of sampling time (Bruker, 1993). The region of the rotational spectrum is presented on an expanded scale in Fig. 4.3-20, it can be compared with Fig. 4.3-18. The intensities of the lines below about 80 cm are weakened by the Rayleigh line suppression filter and the resolution is limited to 1 cm", mainly by the laser used for excitation. 

Many early infrared and Raman papers have reported studies on polar molecules which subsequently have been reexamined in the microwave region. In most of these cases, the microwave woik is clearly superior and the infrared results have not been included in these tables. In some cases, however, the addition of even relatively low precision optical data, when combined with microwave data, will lead to improved structural estimates. For example, frequently the Aq (or Co) rotational constant of a symmetric top can be obtained either from pertutbation-induced transitions in the infrared spectrum or from suitable combinations of transitions in a fundamental band, a combination band and a hot band, or else by the analysis of a perpendicular band in the Raman spectrum. It is not possible to obtain this rotational constant in the pure rotational spectrum of a symmetric top molecule, and therefore combining the optical and microwave data leads to much improvement in determining the positions of the off-axis atoms of such molecules. 

Since the coefficients (dp/dq)o are very small, one needs large incident intensities to observe hyper-Raman scattering. Similar to second-harmonic generation (Vol. 1, Sect. 5.8), hyper-Rayleigh scattering is forbidden for molecules with a center of inversion. The hyper-Raman effect obeys selection rules that differ from those of the linear Raman effect. It is therefore very attractive to molecular spectroscopists since molecular vibrations can be observed in the hyper-Raman spectrum that are forbidden for infrared as well as for linear Raman transitions. For example, spherical molecules such as CH4 have no pure rotational Raman spectrum but a hyper-Raman spectrum, which was found by Maker [357]. A general theory for rotational and rotational-vibrational hyper-Raman scattering has been worked out by Altmann and Strey [358]. 

Figure 6.7 Rotational transitions accompanying a vibrational transition in (a) an infrared spectrum and (b) a Raman spectrum of a diatomic molecule...

The dipole and polarization selection rules of microwave and infrared spectroscopy place a restriction on the utility of these techniques in the study of molecular structure. However, there are complementary techniques that can be used to obtain rotational and vibrational spectrum for many other molecules as well. The most useful is Raman spectroscopy. 

Vibrational spectroscopy can help us escape from this predicament due to the exquisite sensitivity of vibrational frequencies, particularly of the OH stretch, to local molecular environments. Thus, very roughly, one can think of the infrared or Raman spectrum of liquid water as reflecting the distribution of vibrational frequencies sampled by the ensemble of molecules, which reflects the distribution of local molecular environments. This picture is oversimplified, in part as a result of the phenomenon of motional narrowing The vibrational frequencies fluctuate in time (as local molecular environments rearrange), which causes the line shape to be narrower than the distribution of frequencies [3]. Thus in principle, in addition to information about liquid structure, one can obtain information about molecular dynamics from vibrational line shapes. In practice, however, it is often hard to extract this information. Recent and important advances in ultrafast vibrational spectroscopy provide much more useful methods for probing dynamic frequency fluctuations, a process often referred to as spectral diffusion. Ultrafast vibrational spectroscopy of water has also been used to probe molecular rotation and vibrational energy relaxation. The latter process, while fundamental and important, will not be discussed in this chapter, but instead will be covered in a separate review [4],... 

Rotational Raman spectroscopy is a powerful tool to determine the structures of molecules. In particular, besides electron diffraction, it is the only method that can probe molecules that exhibit no electric dipole moment for which microwave or infrared data do not exist. Although rotational constants can be extracted from vibrational spectra via combination differences or by known correction factors of deuterated species the method is the only one that yields directly the rotational constant B0. However for cyclopropane, the rotational microwave spectrum, recording the weak AK=3 transitions could be measured by Brupacher [20],... 

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. 

The Raman spectra are quicker and easier to determine than the infrared absorption spectra because ordinary optical equipment can be used, but frequently they are more difficult to interpret. The quantum restrictions in the two phenomena, particularly for symmetrical molecules, are not always the same, because the Raman spectrum involves an intermediate excited state of the molecule. For this reason, it is desirable to have the data of both Raman and infrared absorption spectra in order to determine completely the rotational and rotational-vibrational energy levels in the molecule. The Raman spectrum can be obtained in some solutions where direct absorption measurements are impossible because the solvent is opaque in the infrared. Aqueous solutions offer a good example of such a case. 

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. 

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