Fundamental Vibrational Frequencies Small Molecules

Other systems consisting of molecules other than H2 have similar rotovibrational spectra. However, the various rotational lines cannot usually be resolved, owing to the smallness of the rotational constants B and the typically very diffuse induced lines. One example, the spectrum of compressed oxygen, was shown above, Fig. 1.1. It consists basically of three branches, the Q, S, and O branch. The latter two are fairly well modeled by the envelope of the rotational stick spectra, similar to that shown in Fig. 3.20, but shifted by the fundamental vibration frequency. 

The above discussion has outlined the theoretical approach to the determination of the fundamental vibration frequencies of a molecule. The practical solution of the problem as formulated above presents, however, certain more or less serious difficulties. For example, the completely general potential function of equation (4) is generally not usable even for small molecules, because it contains more independent constants than can be determined from the experimental data. However, by making certain assumptions about the nature of the force field in the molecule, the number of constants can be reduced. One assumption which often works quite well in practice is that of a valence force field [Herzberg 76)]. This assumes that contributions to the potential energy... 

FUNDAMENTAL VIBRATIONAL FREQUENCIES OF SMALL MOLECULES (continued)... 

Force Constants for Bond Stretching Fundamental Vibrational Frequencies of Small Molecules Spectroscopic Constants of Diatomic Molecules Infrared Correlation Charts... 

When a compound is irradiated with monochromatic radiation, most of the radiation is transmitted unchanged, but a small portion is scattered. If the scattered radiation is passed into a spectrometer, we detect a strong Rayleigh line at the unmodified frequency of radiation used to excite the sample. In addition, the scattered radiation also contains frequencies arrayed above and below the frequency of the Rayleigh line. The differences between the Rayleigh line and these weaker Raman line frequencies correspond to the vibrational frequencies present in the molecules of the sample. For example, we may obtain a Raman line at 1640 cm-1 on either side of the Rayleigh line, and the sample thus possesses a vibrational mode of this frequency. The frequencies of molecular vibrations are typically 1012—1014 Hz. A more convenient unit, which is proportional to frequency, is wavenumber (cm-1), since fundamental vibrational modes lie between 4000 and 50 cm-1. 

In the IR absorption spectrum of ACN (Fig. 5) there is a small relatively sharp transition at 3150 cm-1. This transition has previously been assigned as a combination of C-C stretch (918 cm-1) and C N(2253 cm-1) stretch (84). Since the combination transition overlaps the higher energy tail of the C-H stretch fundamental, pumping at this frequency is expected to produce vibrationally excited molecules where all three vibrations interact. The excitations may be viewed as weakly interacting independent vibrations (47). The weak interaction is assured since the anharmonicity is relatively small (<0.3% of the vibrational frequency). Any coherent vibrational states produced by the pump pulse will lose coherence very rapidly, since the dephasing time constant (T2 0.5 ps) (51) is much faster than the VER lifetime. 

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