Infrared spectroscopy stretching motions

Infrared, Raman, microwave, and double resonance techniques turn out to offer nicely complementary tools, which usually can and have to be complemented by quantum chemical calculations. In both experiment and theory, progress over the last 10 years has been enormous. The relationship between theory and experiment is symbiotic, as the elementary systems represent benchmarks for rigorous quantum treatments of clear-cut observables. Even the simplest cases such as methanol dimer still present challenges, which can only be met by high-level electron correlation and nuclear motion approaches in many dimensions. On the experimental side, infrared spectroscopy is most powerful for the O—H stretching dynamics, whereas double resonance techniques offer selectivity and Raman scattering profits from other selection rules. A few challenges for accurate theoretical treatments in this field are listed in Table I. 

Vibrational spectroscopy has been used in the past as an indicator of protein structural motifs. Most of the work utilized IR spectroscopy (see, for example, Refs. 118-128), but Raman spectroscopy has also been demonstrated to be extremely useful (129,130). Amide modes are vibrational eigenmodes localized on the peptide backbone, whose frequencies and intensities are related to the structure of the protein. The protein secondary structures must be the main factors determining the force fields and hence the spectra of the amide bands. In particular the amide I band (1600-1700 cm-1), which mainly involves the C=0-stretching motion of the peptide backbone, is ideal for infrared spectroscopy since it has an large transition dipole moment and is spectrally isolated... 

Infrared spectroscopy identifies the kinds of functional groups in a compound. Bonds vibrate with stretching and bending motions. Each stretching and bending vibration occurs with a characteristic frequency. It takes more energy to stretch a bond than to bend it. When a compound is bom-... 

To the extent that we can trust the harmonic approximation, each level of vibrational excitation (each increment in vibrational quantum number v) costs one vibrational constant in energy. As Table 8.2 shows, the vibrational constants of typical stretching motions place vibrational excitation energies in the infrared region of the spectrum. We can measure vibrational transitions that occur by absorption or emission or by scattering. Section 6.3 introduced the concept of Raman scattering, which in principle can be applied to the spectroscopy of any degree of freedom, but which is most commonly used for spectroscopy of vibrational states. 

CONTEXT Carbon monoxide, a toxic byproduct of incomplete combustion, may be detected in the atmosphere using infrared absorption spectroscopy, which stimulates the C — O stretching motion. This transition is allowed because CO is a polar molecule, and stretching the bond changes the dipole moment (i. e., the dipole derivative is not zero, Section 8.4). 

Vibrational spectroscopy can help us escape from this predicament due to the exquisite sensitivity of vibrational frequencies, particularly of the OH stretch, to local molecular environments. Thus, very roughly, one can think of the infrared or Raman spectrum of liquid water as reflecting the distribution of vibrational frequencies sampled by the ensemble of molecules, which reflects the distribution of local molecular environments. This picture is oversimplified, in part as a result of the phenomenon of motional narrowing The vibrational frequencies fluctuate in time (as local molecular environments rearrange), which causes the line shape to be narrower than the distribution of frequencies [3]. Thus in principle, in addition to information about liquid structure, one can obtain information about molecular dynamics from vibrational line shapes. In practice, however, it is often hard to extract this information. Recent and important advances in ultrafast vibrational spectroscopy provide much more useful methods for probing dynamic frequency fluctuations, a process often referred to as spectral diffusion. Ultrafast vibrational spectroscopy of water has also been used to probe molecular rotation and vibrational energy relaxation. The latter process, while fundamental and important, will not be discussed in this chapter, but instead will be covered in a separate review [4],... 

In the context of the heat capacity, the term "vibrational motions" should be interpreted in the most general manner possible, to encompass all possible modes of motion of atoms or groups of atoms in a macromolecule. Such motions include bond stretching, bond bending and "rocking" motions, torsional oscillations, the "flipping" of a structural unit from one equilibrium position to another, and large-scale cooperative motions. These internal motions of atoms in a material are most directly studied by vibrational (infrared and Raman) spectroscopy [23]. 

In the infrared spectmm of free carbon dioxide, the asymmetric stretching frequency is observed at 2349 cm-1 (gas) and 2342 cm-1 (solid). The infrared absorption belonging to the bending motion of the molecule is found at 667 cm-1 (gas). Symmetric stretching of free CO2 can be detected only by Raman spectroscopy... 

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