Low-frequency shift

For most purposes only the Stokes-shifted Raman spectmm, which results from molecules in the ground electronic and vibrational states being excited, is measured and reported. Anti-Stokes spectra arise from molecules in vibrational excited states returning to the ground state. The relative intensities of the Stokes and anti-Stokes bands are proportional to the relative populations of the ground and excited vibrational states. These proportions are temperature-dependent and foUow a Boltzmann distribution. At room temperature, the anti-Stokes Stokes intensity ratio decreases by a factor of 10 with each 480 cm from the exciting frequency. Because of the weakness of the anti-Stokes spectmm (except at low frequency shift), the most important use of this spectmm is for optical temperature measurement (qv) using the Boltzmann distribution function. 

In the present work low temperature adsoi ption of fluoroform and CO, were used to characterize surface basicity of silica, both pure and exposed to bases. It was found that adsorption of deuterated ammonia results in appearance of a new CH stretching vibration band of adsorbed CHF, with the position typical of strong basic sites, absent on the surface of pure silica. Low-frequency shift of mode of adsorbed CO, supports the conclusion about such basicity induced by the presence of H-bonded bases. 

Regitz has shown that the reaction of azaphosphole 38 with two equivalents of DEAD furnishes the zwitterionic 2 1 adduct 39 (Scheme 8) [52]. The extreme low frequency shift of the P NMR signal by more than 220 ppm in comparison to that of the azaphosphole confirms the formation of a betaine possessing a hexacoordinated phosphorus atom. 

The low-frequency shift and the broadening of the CO spectra at 0 ps suggest that the low-frequency modes of adsorbed CO, that is, stretching, frustrated rotation, and frustrated translation modes of Pt-CO, were thermally excited by pump pulses, as reported by Bonn et al. [82] Thus, it is concluded that the transient site migration of adsorbed CO on the Pt electrode surface was caused by a transient rise in the surface temperature of Pt induced by pump pulses. 

The obtained results are in agreement with our previous hypothesis [2-4] that absolute intensities and the distribution of relative intensities of IR bands in the spectra of adsorbed species are sensitive to the chemical activation of the corresponding bonds arising from polarization by adsorption sites. Hence, in addition to the low frequency shifts, intensites can be used as a criterion for chemical activation. Indeed, according to the fundamentals of IR spectroscopy, the intensities of IR stretching bands are proportional to the square of the dipole moment changes (dp) created by the stretching vibrations over the normal coordinates q of these vibrations [6] I °c [dp/d q]2. 

Figure 15 gives the superposition of RR (full line) and RY (dotted plot) spectral densities at 300 K. For the RR spectral density, the anharmonic coupling parameter and the direct damping parameter were taken as unity (a0 = 1, y0 = ffioo), in order to get a broadened lineshape involving reasonable half-width (a = 1 was used systematically, for instance, in Ref. 72). For the RY spectral density, the corresponding parameters were chosen aD = 1.29, y00 = 0.85angular frequency shift (the RY model fails to obtain the low-frequency shift predicted by the RR model) and a suitable adjustment in the intensities that are irrelevant in the RR and RY models. 

Observe that all the mechanisms—that is, the classical indirect mechanism and the two quantum ones—predict a satisfactory isotope effect when the proton of the H bond is substituted by deuterium All the damping mechanisms induce approximately a l/y/2 low-frequency shift of the first moment and a 1 / y/2 narrowing of the breadth, which is roughly in agreement with experiment. As a consequence, the isotope effect does not allow us to distinguish between the two damping mechanisms. 

A5 C(C1) = —29.8, (A5 C(C1) = —54.5) are observed and a relatively large /(C2H) coupling constant of 165.9 Hz is detected. This counter-intuitive low-frequency shift of the C NMR resonance of Cl and C2 as well as the large scalar CH coupling constant was rationalized for similar bishomoaromatic carbon cations like the 7-norbornenyl cation, 79, by the hypercoordinated nature of the vinylic C atoms and was put forward as spectroscopic evidence for bishomoaromaticity. " ... 

Fig. 11. Low frequency shift Raman Specfra in the (zy) scattering geometry for various temperatures in FeFj ss)...

The change from morphine to 3,6-diacetylmorphine is accompanied by a low frequency shift of C(7) (S 133-43 to 129-24) due to the y-effect of the acetoxyl group. C(10) appears at unusually low frequency (y-effects from C(8), C(16), and V-methyll in a number of morphine alkaloids with the exception of thebaine (cf. d 20-20 in morphine, 29-5 in thebaine) in which the gauche relationship between C(10) and C(8),... 

In the spectrum of the cancentrine model compound [171] one of the methoxyl group of protons absorbs at low frequency (5 3-38) compared with absorption for the other methoxyl protons at <5 3 85, 3-85, and 3 90. This low frequency signal was assigned to the 7-methoxyl protons. In the hydrogenated derivative the lowest frequency methoxyl proton absorption is at <5 3-65. The low frequency aromatic proton absorption ((56-59) in [171] was assigned to the 8-proton. Both low frequency shifts are attributed to the shielding effects of ring c. (127)... 

The 13C NMR spectra of a large number of epimeric N-alkylnoratropine derivatives have been studied and substituent effects suggested. The pair of compounds [256] and [257] show the expected low frequency shift of C(2) and C(4) in the axial TV-ethyl compound [257]. (168)... 

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