Infrared spectroscopy polyatomic

Infrared spectroscopy, before its present expansion as a quantitative method, was always used as a semi-empirical method for structural analysis. This application stems from the extension of the above rules to organic polyatomic molecules. 

Raman Selection Rules. For polyatomic molecules a number of Stokes Raman bands are observed, each corresponding to an allowed transition between two vibrational energy levels of the molecule. (An allowed transition is one for which the intensity is not uniquely zero owing to symmetry.) As in the case of infrared spectroscopy (see Exp. 38), only the fundamental transitions (corresponding to frequencies v, V2, v, ...) are usually intense enough to be observed, although weak overtone and combination Raman bands are sometimes detected. For molecules with appreciable symmetry, some fundamental transitions may be absent in the Raman and/or infrared spectra. The essential requirement is that the transition moment F (whose square determines the intensity) be nonzero i.e.. 

Herzberg G (1945) Molecular spectra and molecular structure - II Infrared and Raman spectra of polyatomic molecules, van Nostrand Reinhold Comp, New York, p 196 Hair ML (1967) Infrared spectroscopy in surface chemistry. Marcel Dekker, New York, p 47 Hair ML (1967) Infrared spectroscopy in sm ace chemistry. Marcel Dekker, New York, p292... 

Electrolyte chemists turn to non-aqueous solvents to explore the effect of a changing, and usually low, dielectric constant on the degree of ion pair formation and to observe differences attributed to a changed solvation sphere, specific solvation or changed solvent structure. The stretching vibration of an ion pair — A may be detectable by infrared spectroscopy (as mentioned in sect. 4.10) but is undetectable by Raman spectroscopy. If is a polyatomic anion, however, new lines characteristic of a bound species may be detected. 

Herscher, L. W. in Applied Infrared Spectroscopy (D. N. Kendall, ed.). Reinhold, New York, 1966. Herzberg, G. Molecular Spectra and Molecular Structure Infrared and Raman Spectra of Polyatomic Molecules, D. Van Nostrand. New York, 1945. 

Table 1.1 Degrees of freedom for polyatomic molecules. From Stuart, B., Modem Infrared Spectroscopy, ACOL Series, Wiley, Chichester, UK, 1996. University of Greenwich, and reproduced by permission of the University of Greenwich...

There is little doubt that striking advances have been made in the area of polyatomic van der Waals molecules and small clusters. The increasing application of high resolution infrared spectroscopy has provided solutions to a number of problems. An intrinsic requirement of pure rotational spectroscopy, the existence of a (permanent) dipole moment, is removed allowing in particular the study of the non-polar complex. This is now firmly established by a number of well known examples[27]. 

As with a polyatomic ligand, when a free ion becomes covalently bound, there is a decrease in its structural symmetry. This results in the removal of the degeneracy of some vibrations, causing new bands to be observed in the infrared (and Raman) region. Hence, infrared spectroscopy may be u.sed to distinguish between ionic and covalent bonds in coordination complexes. 

In low-resolution infrared spectroscopy of diatomic molecules, the rotational fine structure is lost, and some feature in a spectrum assigned to be a fundamental transition is but a single peak. The frequency of that peak is taken to be the vibrational frequency in the absence of any more precise experiments, and that is the extent of the information obtained. This low-resolution information corresponds mostly with a nonrotating picture or else a rotationally averaged picture of the molecule s dynamics to the extent that we can analyze the data, we need only consider pure vibration. To understand the internal dynamics of polyatomic molecules, it is helpful to start with a low-resolution analysis. This means neglecting rotation, or presuming the molecules to be nonrotating. 

In the previous chapter, vibrational/rotational (i.e. infrared) spectroscopy of diatomic molecules was analyzed. The same analysis is now applied to polyatomic molecules. Polyatomic molecules have more than one bond resulting in additional vibrational degrees of freedom. Rotation of linear polyatomic molecules is mechanically equivalent to that of diatomic molecules however, the rotation of non-linear polyatomic molecules results in more than one degree of rotational freedom. The result of the additional vibrational and rotational degrees of freedom for polyatomic molecules is to complicate the vibrational/rotational spectra of polyatomic molecules relative to spectra of diatomic molecules. Though the spectra of polyatomic molecules are more complicated, many of the same features exist as in the spectra of diatomic molecules. As a result, a similar approach wiU be used in this chapter. The mechanics of a model system will be solved, determine the selection rules, and the features of a spectrum will be predicted. 

The selection rules for rotational transitions in linear polyatomic molecules are also the same as for diatomic molecules. The transition AJ is equal to 1 in infrared spectroscopy and -nl in purely rotational spectroscopy (i.e. microwave spectroscopy) but only if the molecule has a non-zero dipole moment (see Section 6.7). A rotational transition for H-C=C-C1 will be observed whereas for H-C=C-H no rotational transition will be observed due to its zero dipole moment. 

This book, originally published in 1950, is the first of a classic tliree-volume set on molecular spectroscopy. A rather complete discussion of diatomic electronic spectroscopy is presented. Volumes 11 (1945) and 111 (1967) discuss infrared and Raman spectroscopy and polyatomic electronic spectroscopy, respectively. 

Information about the structure of gas molecules haB been obtained by several methods. Spectroscopic studies in the infrared, visible, and ultraviolet regions have provided much information about the simplest molecules, especially diatomic molecules, and a few polyatomic molecules. Microwave spectroscopy and molecular-beam studies have yielded very accurate interatomic distances and other structural information about many molecules, including some of moderate complexity. Molecular properties determined by spectroscopic methods are given in the two books by G. Herzberg, Spectra of Diatomic Molecules, 1950. and Infrared and Raman Spectra, 1945, Van Nostrand Co., New York. The information obtained about molecules by microwave spectroscopy is summarised by C. H. Townes and A. L. Schawlow in their book Microwave Spectroscopy of Gases, McGraw-Hill Book Co., New York, 1955. 

Microwave spectrometer, 219-221 Microwave spectroscopy, 130, 219-231 compilations of results of, 231 dipole-moment measurements in, 225 experimental procedures in, 219-221 frequency measurements in, 220 and molecular structure, 221-225 and rotational barriers, 226-228 and vibrational frequencies, 225-226 Mid infrared, 261 MINDO method, 71,76 and force constants, 245 and ionization potentials, 318-319 Minimal basis set, 65 Minor, 14 Modal matrix, 106 Molecular orbitals for diatomics, 58 and group theory, 418-427 for polyatomics, 66... 

Herzberg, G. Molecular spectroscopy and molecular structure. II. Infrared and raman spectra of polyatomic molecules. New York, Cincinnati, Toronto, London, and Melbourne Van Nostrand Reinhold Company 1945... 

The principal reaction discussed above forms oxygen molecules in high vibrational levels of the ground state. This is chemi-excitation but is not chemiluminescence vibration-rotation transitions of homonuclear molecules are forbidden. For such cases electronic absorption spectroscopy is the required technique. For reactions in which a heteronuclear diatomic (or a polyatomic) molecule is excited these transitions are allowed. They are overtones of the molecular transitions that occur in the near infrared. These excited products emit spontaneously. The reactions are chemiluminescent, their emission spectra may be obtained and analyzed in order to deduce the detailed course of the reaction. 

Most fundamental rotation-vibration bands are located in the mid-infrared region from 4000 - 400 cm". A few vibrational bands appear in the far infrared where purely rotational spectra of light molecules with two or three atoms are also observed. This is in contrast to heavier polyatomic molecules the study of their rotational spectra is the domain of the microwave spectroscopist who employs different equipment, particularly, monochromatic tunable radiation sources. Rotational constants determined from IR-work are therefore usually less accurate than those obtained by microwave spectroscopy. 

Near-infrared absorption is therefore essentially due to combination and overtone modes of higher energy fundamentals, such as C-H, N-H, and O-H stretches, which appear as lower overtones and lower order combination modes. Since the NIR absorption of polyatomic molecules thus mainly reflects vibrational contributions from very few functional groups, NIR spectroscopy is less suitable for detailed qualitative analysis than IR, which shows all (active) fundamentals and the overtones and combination modes of low-energy vibrations. On the other hand, since the vibrational intensities of near-infrared bands are considerably lower than those of corresponding infrared bands, optical layers of reasonable size (millimeters, centimeters) may be transmitted in the NIR, even in the case of liquid samples, compared to the layers of pm size which are detected in the infrared. This has important consequences for the direct quantitative study of chemical reactions, chemical equilibria, and phase equilibria via NIR spectroscopy. 

Dlott, D.D. (2001) Vibrational energy redistribution in polyatomic liquids 3D infrared-Raman spectroscopy. Chem. Phys., 266, 149-166. 

Infrared radiation is only absorbed by the irradiated molecule at the appropriate frequency if the corresponding vibration results in a change in molecular dipole moment. This means that not all vibrational modes are infrared active. An analysis of which vibrational modes in a polyatomic molecule are active is based on group theory and the symmetry properties of the molecule. More details about this subject may be found in monographs devoted to spectroscopy [G4]. 

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