Molecular vibrations resonance Raman spectroscopy

Raman spectroscopy is primarily useful as a diagnostic, inasmuch as the vibrational Raman spectrum is directly related to molecular structure and bonding. The major development since 1965 in spontaneous, c.w. Raman spectroscopy has been the observation and exploitation by chemists of the resonance Raman effect. This advance, pioneered in chemical applications by Long and Loehr (15a) and by Spiro and Strekas (15b), overcomes the inherently feeble nature of normal (nonresonant) Raman scattering and allows observation of Raman spectra of dilute chemical systems. Because the observation of the resonance effect requires selection of a laser wavelength at or near an electronic transition of the sample, developments in resonance Raman spectroscopy have closely paralleled the increasing availability of widely tunable and line-selectable lasers. 

Both Xe2 and I2 have a vibrational frequency roughly half that of molecular iodine, consistent with the (molecular-orbital based) idea of ahalf bond for the ionic species versus a full bond for molecular iodine. Many other ionic species may be expected to be trapped in matrices and studied in the future by resonance Raman spectroscopy. 

In normal Raman spectroscopy, the exciting frequency lies in the region where the compound has no electronic absorption band. In resonance Raman spectroscopy, the exciting frequency falls within the electronic band (Sec. 1.2). In the gaseous phase, this tends to cause resonance fluorescence since the rotational-vibrational levels are discrete. In the liquid and solid states, however, these levels are no longer discrete because of molecular collisions and/or intermolecular interactions. If such a broad vibronic bands is excited, it tends to give resonance Raman rather than resonance fluorescence spectra [101,102]. 

In essence, two of these three electrons form a n bond, on top of the existing (j bond, and the remaining electron is place into an antibonding n orbital. The sulfur-sulfur bond thus assumes a partial 7c-character 1112,113]. As a direct consequence, the rotation becomes restricted and, particularly in cyclic disulfides, molecular structures may flatten. Even the possible formation of cis- and rran -isomers of (MeSSMe) could recently be verified through identification of two distinct vibrational stretching frequencies evaluated from time-resolved resonance Raman spectroscopy measurements. These findings were also supported by corresponding calculations [118, 119, 146]. 

Resonance Raman spectroscopy (RRS) leads to increased selectivity in Raman spectral measurements. The Raman spectrum of individual components in a complex mixture can be selectively enhanced by a judicious choice of laser wavelength. Only the Raman bands of the chromophore which is in resonance at the wavelength of excitation are significantly enhanced. Raman bands of non-absorbing species are not enhanced and do not interfere with those of the chromophore. Clearly, resonance Raman is a very sensitive analytical tool capable of providing detailed molecular vibrational information. 

Another experimental method to probe the Franck-Condon excited state is resonance Raman spectroscopy. Recall that Raman scattering is an inelastic process entailing a transition to a virtual or real molecular state (or sum of states), and a nominally instantaneous return to the ground state, but in a higher or lower vibrational state. In resonance Raman spectroscopy the incident radiation nearly coincides with the frequency of the electronic transition of the sample. Only a few vibrational modes contribute to the scattering of the... 

Time-Resolved Resonance Raman Spectroscopy (TR S) is a technique used to get structural, kinetic, and molecular interaction data from chemical and biological systems by recording resonance Raman spectra in a short time span. Using TR S, a transient molecular species can be analyzed by (i) monitoring the frequency of vibrational... 

Silicone elastomers are covalently crosslinked networks with an effective infinite molecular weight and are, as such, insoluble and intractable. Consequently the interrogative spectroscopic and imaging methodologies that may be employed to probe such a network structure are almost wholly confined to the solid state. Surface vibrational spectroscopies attenuation total reflectance Fourier transform infrared (ATR-FTIR) and resonance Raman spectroscopy... 

This spectrum is called a Raman spectrum and corresponds to the vibrational or rotational changes in the molecule. The selection rules for Raman activity are different from those for i.r. activity and the two types of spectroscopy are complementary in the study of molecular structure. Modern Raman spectrometers use lasers for excitation. In the resonance Raman effect excitation at a frequency corresponding to electronic absorption causes great enhancement of the Raman spectrum. 

---