Infrared-inactive

Michaels C A, Mullin A S, Park J, Chou J Z and Flynn G W 1998 The collisional deactivation of highly vibrationally excited pyrazine by a bath of carbon dioxide excitation of the infrared inactive (10°0), (02°0), and (02 0) bath vibrational modes J. Chem. Phys. 108 2744-55... 

Some vibrations which are both Raman and infrared inactive may be allowed in the hyper Raman effect. Indeed, the occasional appearance of such vibrations in Raman spectra in a condensed phase has sometimes been attributed to an effect involving the hyperpolarizability. 

Infrared activity of vibrations is readily deduced. The symmetric stretching vibration has no associated dipole moment change during the vibration and is, therefore, infrared inactive. The asymmetric stretching vibration has an associated dipole moment which fluctuates with the frequency of the vibration. The vibration is, therefore, infrared active. 

It is important to appreciate that Raman shifts are, in theory, independent of the wavelength of the incident beam, and only depend on the nature of the sample, although other factors (such as the absorbance of the sample) might make some frequencies more useful than others in certain circumstances. For many materials, the Raman and infrared spectra can often contain the same information, but there are a significant number of cases, in which infrared inactive vibrational modes are important, where the Raman spectrum contains complementary information. One big advantage of Raman spectroscopy is that water is not Raman active, and is, therefore, transparent in Raman spectra (unlike in infrared spectroscopy, where water absorption often dominates the spectrum). This means that aqueous samples can be investigated by Raman spectroscopy. 

If the vibration does not produce a modulation of the dipole moment (e.g., as with the symmetric stretch vibration of the CO2 molecule), its infrared intensity vanishes because (3 l/3R i) = 0. One says that such transitions are infrared "inactive". 

C-H stretching motion in CH4 do not induce a dipole moment, and are thus infrared inactive non-totally-symmetric vibrations can also be inactive if they induce no dipole moment. 

The selection rule for Raman spectroscopy requires a change in the induced dipole moment or polarizability of the molecule, and so it is a complementary technique to infrared which requires a change in the permanent dipole moment. For molecules having a center of inversion, all Raman-active bands are infrared inactive and vice versa. As the symmetry of the molecule is lowered, the coincidences between Raman-active and infrared-... 

It is a remarkable fact that the translational transitions of virtually all supermolecules are infrared active - even if the individual molecules are not. The only exceptions are supermolecules that possess a symmetry which is inconsistent with the existence of a dipole moment. Pairs of like atoms, e.g., He-He, have inversion symmetry, implying a zero dipole moment and, hence, infrared inactivity. But dissimilar atomic pairs, e.g., He-Ar, or randomly oriented molecular pairs, e.g., H2-H2, generally lack such symmetry. As a consequence, more or less significant collision-induced dipoles exist for the duration of the interaction which generate the well known collision-induced spectra. 

Supermolecular spectra may also be of the electronic or rotovibrational type. This book deals with the rotovibrational types, which should perhaps be called rovibro-translational spectra to express the significant involvement of translational transitions of supermolecular systems. Even if the molecules by themselves are infrared inactive, the translational motion will generally be infrared active. Supermolecular electronic spectra exist but are not as universal as the rotovibrational induced spectra. Collision-induced electronic spectra will be briefly considered in Chapter 7. 

Spectroscopic techniques have been applied most successfully to the study of individual atoms and molecules in the traditional spectroscopies. The same techniques can also be applied to investigate intermolecular interactions. Obviously, if the individual molecules of the gas are infrared inactive, induced spectra may be studied most readily, without interference from allowed spectra. While conventional spectroscopy generally emphasizes the measurement of frequency and energy levels, collision-induced spectroscopy aims mainly for the measurement of intensity and line shape to provide information on intermolecular interactions (multipole moments, range of exchange forces), intermolecular dynamics (time correlation functions), and optical bulk properties. 

In Welsh s first study [128], the shape of the absorption profile was found to be independent of the density of the gas if normalized by density squared, a(v)/n2, at nearly all frequencies v. Pair spectra show no variation with density other than the density squared relationship - up to the point where ternary (or higher-order) spectra affect the measurements (if monomers are infrared inactive in the spectral band considered). 

Above we have looked at the translational spectra of binary systems. These are obtainable experimentally at sufficiently low densities, especially when absorption of the infrared inactive gases is studied. An induced spectrum may be considered to be of a binary nature if the integrated intensity varies as density squared. In that case, the shape of the densities-normalized absorption coefficient, cc/q Q2, is invariant, regardless of the densities employed. ... 

A translational line like the one seen above in rare gas mixtures is relatively weak but discernible in pure hydrogen at low frequencies (<230 cm-1), Fig. 3.10. However, if a(v)/[l —exp (—hcv/kT)] is plotted instead of a(v), the line at zero frequency is prominent, Fig. 3.11 the 6o(l) line that corresponds to an orientational transition of ortho-H2. Other absorption lines are prominent, Fig. 3.10. Especially at low temperatures, strong but diffuse So(0) and So(l) lines appear near the rotational transition frequencies at 354 and 587 cm-1, respectively. These rotational transitions of H2 are, of course, well known from Raman studies and correspond to J = 0 -> 2 and J = 1 — 3 transitions J designates the rotational quantum number. These transitions are infrared inactive in the isolated molecule. At higher temperatures, rotational lines So(J) with J > 1 are also discernible these may be seen more clearly in mixtures of hydrogen with the heavier rare gases, see for example Fig. 3.14 below. 

In Chapter 5 the absorption spectra of complexes of interacting atoms were considered. If some or all of the interacting members of a complex are molecular, additional degrees of freedom exist and may be excited in the presence of radiation. As a result, besides the translational profiles discussed in Chapter 5, new spectral bands appear at the rotovibrational transition frequencies of the molecules involved, and at sums and differences of such frequencies - even if the non-interacting molecules are infrared inactive. The theory of absorption by small complexes involving molecules is considered in the present Chapter. 

Several types of collision-induced light scattering spectra are known. We have already mentioned the depolarized translational spectra of rare gas pairs and bigger complexes which arise from the anisotropy of the diatom polarizability. Contrary to the infrared inactivity of like pairs, e.g., Ar-Ar like pairs are Raman active. Furthermore, polarized translational spectra... 

For C02, it is clear that vx leaves the dipole moment unchanged, whereas v2 and v3 give a change in dipole moment. We say that vx is infrared inactive, whereas v2 and v3 are infrared active. Table 6.2 lists some of the IR bands of gaseous C02. The superscript on v2 will be explained in Section 6.8. Since v2 is rather low, some of the bands are hot bands, that is, bands for which the initial level is not (000). 

Polymers, with their highly stereoregular structures, are frequently of sufficiently high symmetry for infrared spectroscopy to give only an incomplete picture of the vibrational characteristics of the compounds. In some, as many as half of the fundamental modes are infrared inactive. These non-absorbing modes can frequently be observed in the Raman effect (e.g. polyethylene where mutual exclusion" applies and at least eight modes are Raman active and infrared silent ). 

Polyethylene has been studied spectroscopically in greater detail than any other polymer. This is primarily a result of its (supposedly) simple structure and the hope that its simple spectrum could be understood in detail. Yet as simple as this structure and spectrum are, a satisfactory analysis had not been made until relatively recently, and even then significant problems of interpretation still remained. The main reason for this is that this polymer in fact generally contains structures other than the simple planar zig-zag implied by (CH2CH2) there are not only impurities of various kinds that differ chemically from the above, but the polymer always contains some amorphous material. In the latter portion of the material the chain no longer assumes an extended planar zig-zag conformation, and as we have noted earlier, such ro-tationally isomeric forms of a molecule usually have different spectra. Furthermore, the molecule has a center of symmetry, which as we have seen implies that some modes will be infrared inactive but Raman active, so that until Raman spectra became available recently it was difficult to be certain of the interpretation of some aspects of the spectrum. As a result of this work, and of detailed studies on the spectra of n-paraffins, it now seems possible to present a quite detailed assignment of bands in the vibrational spectrum of polyethylene. 

The out-of-phase mode would be infrared inactive because there would be no net change of dipole moment perpendicular to the surface. When a... 

The infrared spectrum of physically adsorbed methane had a band at 2899 cm.-1 which is not present in the equivalent path-length of either the gas or the liquid. This corresponds to the symmetrical C—H stretching vibration which produces a Raman band at 2916 cm.-1. The spectrum of adsorbed ethylene shows an extra weak shoulder at 3010 cm.-1 which was assumed to be the normally infrared inactive v vibration in which all four hydrogens are vibrating in phase and which produces a Raman band at 3019 cm.-1. 

Here, fix, ny and /rz are the x, y and z components of the dipole moment at the electronic ground state, respectively, i/v and l/v, are vibrational wavefunctions where v and v" are the vibrational quantum numbers before and after the transition, respectively. Qa is the normal coordinate of the normal vibration, a. If one of these integrals is nonzero, this vibration is infrared-active. If all three integrals are zero, it is infrared-inactive. 

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