Infrared active bond molecular vibrations

We can readily illustrate the effect of molecular symmetry on molecular dipole moments, and thus on infrared active modes of vibration, by considering the linear molecule CO2. The two C—O bond distances are equal (116 pm) and the molecule is readily identified as being symmetrical strictly, CO2 possesses symmetry. As a consequence of its symmetry, CO2 is non-polar. Although both the asymmetric stretch and the bend (Figure 3.11) give rise to a change in dipole moment (generated transiently as the vibration occurs), the symmetric stretch does not. Thus, only two fundamental absorptions are observed in the IR spectrum of CO2. 

The Infrared Region 515 12-4 Molecular Vibrations 516 12-5 IR-Active and IR-lnactive Vibrations 518 12-6 Measurement of the IR Spectrum 519 12-7 Infrared Spectroscopy of Hydrocarbons 522 12-8 Characteristic Absorptions of Alcohols and Amines 527 12-9 Characteristic Absorptions of Carbonyl Compounds 528 12-10 Characteristic Absorptions of C—N Bonds 533 12-11 Simplified Summary of IR Stretching Frequencies 535 12-12 Reading and Interpreting IR Spectra (Solved Problems) 537 12-13 Introduction to Mass Spectrometry 541 12-14 Determination of the Molecular Formula by Mass Spectrometry 545... 

Symmetry concepts can be extremely useful in chemistry. By analyzing the symmetry of molecules, we can predict infrared spectra, describe the types of orbitals used in bonding, predict optical activity, interpret electronic spectra, and study a number of additional molecular properties. In this chapter, we first define symmetry very specifically in terms of five fundamental symmetry operations. We then describe how molecules can be classified on the basis of the types of symmetry they possess. We conclude with examples of how symmetry can be used to predict optical activity of molecules and to determine the number and types of infrared-active stretching vibrations. 

The prototypical van der Waals molecule infrared photodissociation experiment was originally described by Klemperer in 1974 ( ) He proposed that infrared active constituents of weakly bonded clusters formed in molecular beams be excited with a laser. Since typical vibrational quanta are larger than typical van der Waals bond energies, subsequent intramolecular vibrational energy redistribution would lead to fragmentation that could be detected as beam loss with a mass spectrometer. 

Infrared spectroscopy probes the characteristic vibrational bands of chemical groups as the atoms move with respect to one another in response to an oscillating electromagnetic field of the appropriate frequency. The normal modes of a particular molecule or group depend on the molecular structure, interatomic forces (bond strengths) and masses of the atoms concerned, but they will only be IR active if the vibrational mode involves a change in dipole moment. 

The complexity of Raman spectra for polymers is reduced as with infrared spectra because vibrations of the same type superimpose. In addition, as with infrared spectroscopy, selection rules aid in determining which molecular vibrations are active. However, the criterion for Raman aetivity is a change in bond polarizability with molecular vibration or rotation in contrast to the infrared criterion of a change in dipole moment (Figure 6.6). This means that, for molecules such as carbon dioxide that show both a change in dipole moment and a change in polarizability,... 

Not all molecular vibrations can be detected. For a dbration to be infrared active, it must produce a net change in the dipole moment of the molecule, which means that symmetrical vibrations are either weak or in dsible in the IR region of the electromagnetic spectrum. For example, ethylene shows no IR signal for the carbon-carbon double-bond stretch. However, Raman spectroscopy, which is related to IR spectroscopy, does detect symmetrical dbrations, and shows a signal at 1623 cm for ethylene. Signals in IR and Raman are usually broad and actually extend over several wavenumbers. As a result the signal is often referred to as a band. 

There are multiple types of molecular vibrations that absorb at unique wavelengths or frequencies of near-infrared energy depending upon the bond type. Several normal (or normal mode) types of molecular vibrations active within the NIR region are illustrated in the following figures. Each of these types of vibrations has a unique frequency where absorption occurs. The location of these frequencies and the associated molecular structures (spectra-structure correlations) are the purpose of this book. 

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