Ethylenes Raman-active frequency

Normalization, 6 Normal modes, 240-244 of benzene, 438-439 of boron trifluoride, 281, 290 of carbon dioxide, 242, 248, 262, 265 of ethylene, 291 and group frequencies, 266-268 IR active, 457 Raman active, 457 and symmetry, 246-249,430-439 of water, 431-437 Normal operator, 108 Nuclear g factor, 3 24 Nuclear magnetic moments, 323-325 Nuclear magnetic resonance, 129-130, 323-366... 

In their now classic study of the effect of surface forces on adsorbed molecules, Sheppard and Yates (26) found that some of the Raman-active vibrations of methane, ethylene, and hydrogen appeared in the infrared when these materials were adsorbed on silica. The frequency shifts for the molecule on going from the gas phase to the adsorbed phase were all rather small, indicating that no chemical change in the species was brought about by the adsorption. 

High level HF/SCF calculations on ethylene using analytic derivative methods have been reported by Frisch et al. [341]. Their results are presented in Table 10.4. Vibrational frequencies and infrared intensities are also given. Interestingly, even at this high level of SCF theory the frequencies for vio(B]u) and vi](B2g) are predicted with reversed order. There are, however, no difficulties in assigning these bands for the correct normal mode on the basis of symmetry properties of die respective normal modes and the fact that B2g mode is Raman active wdiile Biu -infrared active. Both infrared and Raman intensities are relatively well predicted at the SCF large basis set calculations. 

When IR and Raman spectroscopic techniques are used in combination, the resuits are much greater than with the use of either technique individually. The combined use of IR and Raman spectroscopy extracts most of the obtainable information (silent, or optically inactive, modes and extremely weak modes are not detected). The complementary nature of the IR and Raman data has important practical applications. This complementary nature arises from the differences in selection rules governing the vibrational energy levels. For molecules with a center of symmetry (there are identical atoms on either side of the center of symmetry), no vibrational frequencies are common to the IR and Raman spectra. This principle is called the mutual exclusion principle. Although symmetry might be considered important for low-molecular-weight substances like ethylene and benzene (both of which have a center of symmetry), polymers are not usually expected to have a center of symmetry. Polyethylene has a center of symmetry, and the observed IR and Raman lines do not coincide in frequency (see Fig. 5.1). Theory predicts that eight modes for polyethylene are active in the Raman while only five in the infrared. 

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