Molecules infrared inactive

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". 

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. 

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. 

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 relevant vibrations for this review are the N=N and C-N (Ph-N) stretching vibrations and, perhaps, torsional vibrations around the C-N bond. The E-azobenzene molecule has a center of inversion, and therefore the N=N vibration is infrared-inactive, but Raman-active, and has been found to be at 1442 cm". By IR spectroscopy, Kiibler et al. located the symmetric C-N stretching vibration at 1223 cm" in E- and at 866 cm in Z-azobenzene. The N=N vibration in Z-azobenzene is at 1511 cm" (in KBr pellets). These numbers are confirmed by newer work Biswas and Umapathy report 1439 and 1142 cm for the N=N and C-N vibrations (in CCE), and Fujino and Tahara found nearly identical results (1440 cm" and 1142 cm ). A thorough vibrational analysis of the E-isomer is given by Amstrong et al. The vibrations in the (n,7t ) excited state are very similar 1428 cm" and 1130 cm"h... 

Begun et al. (2) report the infrared (gas phase) and Raman (liquid phase) spectra and correlate the observations by means of normal coordinate calculations for the similar molecules BrF, IF, 5 XeOF. The fundamental frequencies are taken from Begun et al. (2) except for Ug which was beyond the range of their Infrared measurements. The value Wg = 245 cm" for the gas has been observed by McDowell and Asprey (2) and is confirmed by 237 cm" found in the Raman spectra of the liquid. Raman values from the liquid were used for the infrared inactive fundamentals and Vg. One Raman active frequency, persumably Vg, is not observed in any of the four molecules. The value Vg = 281 cm" was obtained from the normal coordinate calculation. Previous Raman studies were reported by Stephenson and Jones (4). 

The formal requirement for these transitions to occur is that the molecular vibration should produce a change in dipole moment. Thus, in a molecule such as carbon dioxide, the symmetrical stretching vibration (vj) will be infrared inactive and the bending (V2) and asymmetric stretch (V3) modes will be active. The fundamental frequency at which a particular vibration will occur is given by the classical formula for a diatomic harmonic oscillator ... 

Other sources of data have generally been of much less use for anhar-monic force field studies than high-resolution infrared and microwave measurements. Solid-state infrared measurements using the matrix-isolation technique have recently provided useful vibrational data for some species inaccessible to gas-phase measurements, e.g., the cyanate (OCN )14 ls and nitrate (NO3)16 anions. Gas-phase Raman measurements of vibration/rotation bands are very difficult to perform with adequate resolution, and Raman data have mainly been utilized for locating some of the infrared-inactive fundamentals in symmetrical molecules such as C02, CS2,17 C2H2,18 CH4,19 20 C. 21"23 High-resolu-... 

Dimethyl-2-butene, in contrast, is a symmetrical molecule, so its C = C bond has no dipole moment. When the bond stretches, it still has no dipole moment. Since stretching is not accompanied by a change in dipole moment, no absorption band is observed. The vibration is infrared inactive. 2,3-Dimethyl-2-heptene experiences a very small change in dipole moment when its C = C bond stretches, so only an extremely weak absorption band (if any) will be detected for the stretching vibration of the bond. 

Molecules like 02H take the nonplanar Q structure (twisted about the 0-0 bond by ca. 90"), whereas N2F2 and [N203] exist in two forms /mm-planar (C2/,) and cis-planar (C21.). Figure 11-7 shows the six normal modes of vibration for the C2, and C2 structures. The selection rules for these two structures are different only in the vibration, which is infrared inactive and Raman depolarized in the planar model but infrared active and Raman polarized in the nonplanar model. 

The relevant vibrations for this review are the N=N and C-N (Ph-N) stretching vibrations and, perhaps, torsional vibrations around the C-N bond. The E-azobenzene molecule has a center of inversion, and therefore the N=N vibration is infrared-inactive, but Raman-active, and has been found to be at 1442 By IR spectroscopy, Kiibler et al. located the symmetric... 

If the integral is zero, then no light absorption can occur. This relation is called selection rule. The dipole moment /u. may be a permanent dipole moment or an induced dipole moment, which is caused by oscillation. The asymmetrical stretch vibration and the bending vibration of CO2 are examples for oscillations that induce a dipole moment. Molecules like N2 and O2 have a zero dipole moment. These molecules are called infrared inactive. 

Raman spectroscopy yields analogous information and is complementary to infrared spectroscopy. At least for molecules with a center of symmetry, vibrations that are infrared inactive are generally Raman active, and vice... 

Several molecules are inactive in the infrared spectral part (Sherman Hsu,... 

---