Infrared-active molecules

A major advantage of infrared absorption spectroscopy derives from the characteristic fingerprints associated with infrared-active molecules. On the other hand, interferences from common atmospheric components such as C02 and HzO are significant, so that the sensitivity and detection limits that can be obtained are useful primarily for polluted urban air situations. For atmospheric work, long optical path lengths are needed. 

For a molecule to show infrared absorptions it must possess a specific feature, i.e. an electric dipole moment of the molecule must change during the vibration. This is the selection rule for infrared spectroscopy. Figure 1.4 illustrates an example of an infrared-active molecule, a heteronuclear diatomic molecule. The dipole moment of such a molecule changes as the bond expands and contracts. By comparison, an example of an infrared-inactive molecule is a homonuclear diatomic molecule because its dipole moment remains zero no matter how long the bond. 

FIGURE 1.12 Model of infrared active molecule as a vibrating dipole between two atoms. (From Workman, J., Interpretive spectroscopy for near-infrared, ApjpZ. Spectrosc. Revs., 31 (3) 251-320, 1996. With permission.)... 

A particular vibration will give an absorption peak in the IR spectrum only if the dipole moment of the molecule changes dunng the vibration Which vibration of carbon dioxide the sym metric stretch or the antisymmetric stretch is infrared active 2... 

In the case of H2O it is easy to see from the form of the normal modes, shown in Figure 4.15, that all the vibrations Vj, V2 and V3 involve a change of dipole moment and are infrared active, that is w=l-0 transitions in each vibration are allowed. The transitions may be labelled Ig, 2q and 3q according to a useful, but not universal, convention for polyatomic molecules in which N, refers to a transition with lower and upper state vibrational quantum numbers v" and v, respectively, in vibration N. 

Although we have been able to see on inspection which vibrational fundamentals of water and acetylene are infrared active, in general this is not the case. It is also not the case for vibrational overtone and combination tone transitions. To be able to obtain selection mles for all infrared vibrational transitions in any polyatomic molecule we must resort to symmetry arguments. 

At higher frequencies (above 200 cm ) the vibrational spectra for fullerenes and their cry.stalline solids are dominated by the intramolecular modes. Because of the high symmetry of the Cgo molecule (icosahedral point group Ih), there are only 46 distinct molecular mode frequencies corresponding to the 180 6 = 174 degrees of freedom for the isolated Cgo molecule, and of these only 4 are infrared-active (all with Ti symmetry) and 10 are Raman-active (2 with Ag symmetry and 8 with Hg symmetry). The remaining 32 eigcnfrequencies correspond to silent modes, i.e., they are not optically active in first order. 

The example of COj discussed previously, which has no vibrations which are active in both the Raman and infrared spectra, is an illustration of the Principle of Mutual Exclusion For a centrosymmetric molecule every Raman active vibration is inactive in the infrared and any infrared active vibration is inactive in the Raman spectrum. A centrosymmetric molecule is one which possesses a center of symmetry. A center of symmetry is a point in a molecule about which the atoms are arranged in conjugate pairs. That is, taking the center of inversion as the origin (0, 0, 0), for every atom positioned at (au, yi, z ) there will be an identical atom at (-a ,-, —y%, —z,). A square planar molecule XY4 has a center of symmetry at atom X, whereas a trigonal planar molecule XYS does not possess a center of symmetry. 

When one of the cartesian coordinates (i.e. x, y, or z) of a centrosymmetric molecule is inverted through the center of symmetry it is transformed into the negative of itself. On the other hand, a binary product of coordinates (i.e. xx, yy, zz, xz, yz, zx) does not change sign on inversion since each coordinate changes sign separately. Hence for a centrosymmetric molecule every vibration which is infrared active has different symmetry properties with respect to the center of symmetry than does any Raman active mode. Therefore, for a centrosymmetric molecule no single vibration can be active in both the Raman and infrared spectrum. 

Centrosymmetric molecules represent a limiting case as far as molecular symmetry is concerned. They are highly symmetric molecules. At the other extreme, molecules with very low symmetry should produce a set of Raman frequencies very similar to the observed set of infrared frequencies. Between these two extremes there are cases where some vibrations are both Raman and infrared active and others are active in Raman or infrared but not in both. Nitrate ion is an example of a molecule in this intermediate class. 

Vibrations of the symmetry class Ai are totally symmetrical, that means all symmetry elements are conserved during the vibrational motion of the atoms. Vibrations of type B are anti-symmetrical with respect to the principal axis. The species of symmetry E are symmetrical with respect to the two in-plane molecular C2 axes and, therefore, two-fold degenerate. In consequence, the free molecule should have 11 observable vibrations. From the character table of the point group 04a the activity of the vibrations is as follows modes of Ai, E2, and 3 symmetry are Raman active, modes of B2 and El are infrared active, and Bi modes are inactive in the free molecule therefore, the number of observable vibrations is reduced to 10. 

The frequencies of these vibrations generally decrease in the order v > 8 > y > x. Not all vibrations can be observed absorption of an IR photon occurs only if a dipole moment changes during the vibration. The intensity of the IR band is proportional to the change in dipole moment. Thus species with polar bonds (e.g. CO, NO and OH) exhibit strong IR bands, whereas molecules such as H2 and N2 are not infrared active at all. 

Many molecules are composed of functional groups (hat can rotate with respect to the rest of the molecule. The classical example is ethane, as the possibility of rotation of one methyl group against the other was recognized long ego. Because the torsional mode does not result in infrared activity, its frequency was estimated from thermodynamic data. 

There exist two geometrically different varieties of these sites, which are referred to as B5 sites because both can be made to accommodate a nitrogen molecule, which is then coordinated by five atoms. They occur at steps on the (100) and (111) planes, and particularly on (110), (311), and other high-index planes. A later paper by van Hardeveld and van Montfoort (10) contains additional evidence showing that the B5 sites are indeed responsible for the infrared-active form of nitrogen adsorption, and also that the number of B5 sites in the sample can be estimated with fair accuracy from the intensity of the 2200 cm-1 band. This means that infrared study of nitrogen adsorption can give valuable information about the structure of the surface of metal particles. 

DFT was used to calculate the heats of formation and infrared active vibrational frequencies of 12 furazan compounds (Figure 1). The absolute values of the heats of formation are unreliable but the trends with systematic variations of the bridge and terminal groups are reasonable. The assignments of the vibrational motions to IR frequencies based on a force field analysis are given to clarify the complex coupling in these molecules <2000MI247>. 

The chemistry of all of these molecules is fascinating but, concentrating on the origins of life, a detailed look at the organic species is appropriate to see what molecules are present and how they might have been formed. The only alkane detected directly in the ISM is methane but this is due to the problem of detection. All alkanes are non-polar and so do not have a pure rotation spectrum. However, there is one allowed vibration of methane that is infrared active and with the low moment of inertia of methane the vibration-rotation spectrum can be observed and a rotational progression identifies the molecule with confidence. 

In an effort to understand the mechanisms involved in formation of complex orientational structures of adsorbed molecules and to describe orientational, vibrational, and electronic excitations in systems of this kind, a new approach to solid surface theory has been developed which treats the properties of two-dimensional dipole systems.61,109,121 In adsorbed layers, dipole forces are the main contributors to lateral interactions both of dynamic dipole moments of vibrational or electronic molecular excitations and of static dipole moments (for polar molecules). In the previous chapter, we demonstrated that all the information on lateral interactions within a system is carried by the Fourier components of the dipole-dipole interaction tensors. In this chapter, we consider basic spectral parameters for two-dimensional lattice systems in which the unit cells contain several inequivalent molecules. As seen from Sec. 2.1, such structures are intrinsic in many systems of adsorbed molecules. For the Fourier components in question, the lattice-sublattice relations will be derived which enable, in particular, various parameters of orientational structures on a complex lattice to be expressed in terms of known characteristics of its Bravais sublattices. In the framework of such a treatment, the ground state of the system concerned as well as the infrared-active spectral frequencies of valence dipole vibrations will be elucidated. 

An electric dipole operator, of importance in electronic (visible and uv) and in vibrational spectroscopy (infrared) has the same symmetry properties as Ta. Magnetic dipoles, of importance in rotational (microwave), nmr (radio frequency) and epr (microwave) spectroscopies, have an operator with symmetry properties of Ra. Raman (visible) spectra relate to polarizability and the operator has the same symmetry properties as terms such as x2, xy, etc. In the study of optically active species, that cause helical movement of charge density, the important symmetry property of a helix to note, is that it corresponds to simultaneous translation and rotation. Optically active molecules must therefore have a symmetry such that Ta and Ra (a = x, y, z) transform as the same i.r. It only occurs for molecules with an alternating or improper rotation axis, Sn. 

Polymer films were produced by surface catalysis on clean Ni(100) and Ni(lll) single crystals in a standard UHV vacuum system H2.131. The surfaces were atomically clean as determined from low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Monomer was adsorbed on the nickel surfaces circa 150 K and reaction was induced by raising the temperature. Surface species were characterized by temperature programmed reaction (TPR), reflection infrared spectroscopy, and AES. Molecular orientations were inferred from the surface dipole selection rule of reflection infrared spectroscopy. The selection rule indicates that only molecular vibrations with a dynamic dipole normal to the surface will be infrared active [14.], thus for aromatic molecules the absence of a C=C stretch or a ring vibration mode indicates the ring must be parallel the surface. 

As the isoquinoline molecule reorients in the order listed above, the absorption of infrared radiation by the in-plane vibrational modes would be expected to increase, while that of the out-of-plane modes would be predicted to decrease (in accordance with the surface selection rule as described above). In the flat orientation there is no component of the dipole moment perpendicular to the surface for the in-plane modes, and under the surface selection rule these modes will not be able to absorb any of the incident radiation. However, as mentioned above, infrared active modes (and in some cases infrared forbidden transitions) can still be observed due to field-induced vibronic coupled infrared absorption (16-20). We have determined that this type of interaction is present in this particular system. 

Very large rate constants have been found for near resonant energy transfer between infrared active vibrations in CO2 Such near-resonant transitions and their dependence on temperature have also been studied for collisions between vibrationally excited CO2 and other polyatomic molecules as CH4, C2H4, SF et al. The deactivation cross-sections range from 0.28 for CH3F to 4.3 for SFs at room temperature, and decrease with increasing temperature. 

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