Vibrational spectra ethane

In the case of the ethane (45) and butane (46) monolayers adsorbed on graphite, it has been possible to analyze the neutron diffraction patterns using all three Euler angles of the molecule as orientational parameters. Here we limit discussion to the butane monolayer which we have taken as a model system and whose vibrational spectrum was discussed in Sec. II. 

The first task is to get reasonable starting values for these parameters. The undeformed (ideal) bond distance Tq can be estimated from the average of known experimental bond distances of relatively undistorted compounds. Experimental bond lengths are usually elongated by 1-10% due to steric strain (e.g., mutual repulsion of the substituents - remember the picture of ethane above). A first approximation to the force constant kb can be calculated from the fundamental vibration frequency v, taken from the vibrational spectrum of a representative compound, for example, [Co(NH3)b] [Eq. (17.31)]. 

Figure 4 illustrates the infrared spectrum for a sample of PPE. The absorptions of the peaks at 3.4, 6.9 and 7.3 pm were assigned to C-H stretch and C-H bending frequencies in CH2 and CH3 (33). These absorptions are proportional to the surface density of deposited ethane (16). However, the absorptions at photons near 10 pm are attributable to OH deformations and CO stretchings of alcoholic groups and vibrations of alkyl ketones (22). They also indicate the existence of branches in unsaturated chain (33). 

The first ionization potential of ethane was measured by photoionization techniques more than 30 years ago [56-62] and it was found to be between 11.4 and 11.65 eV. Some indication of vibrational fine structure was found by Chupka and Berkowitz [62]. The classic Hel photoelectron spectrum of ethane was recorded by liirner s group (Baker et al. [25, 57] it is reproduced on Figure 3. 

The new results by Buenker et al. [87] constitute a challenge to the interpretation of the ethane spectrum what became traditional in the last 30 years. It clearly indicates the need for more theoretical work with an even more extended basis set and a computation of the potential surfaces for all the relevant vibrational motions, implying a closer look at the consequences of the Jahn-Teller effect in the degenerate states as well as possible vibronic couplings between close-lying excited states. Equally needed are experimental works carried out at even higher resolution than has hitherto been possible more information on the vibrational and rotational structures are badly needed. This concerns not only the absorption spectra but also the electron-impact and photoelectron spectra. 

Moreover, it is remarkable that at least four bands in the photoelectron spectrum exhibit vibrational fine structure. Thus, the ion possesses as many stable excited states. Only one of them, the first one is due to ionization from a n-orbital. (This makes the fine structure observed in both the photoelectron and electronic spectra of ethane less surprising.) Vy, U2. and V3 are Raman active, but v is both Raman and infrared inactive and its frequency had to be determined by indirect methods (ref. 96). 

Figure 8 illustrates how sharp atomic lines are. In this figure the lead spectrum is plotted with the molecular absorption spectrum of ethanal. The lead spectrum is a sharp line, whereas ethanal has a broad absorption band with fine structure. This is a consequence of the different energy levels (Figure 9). Free atoms cannot by their nature have vibrational or rotational fine structures in their energy levels like the molecules do.

The vibrational fine structure of the PE bands in the spectrum of ethane has been analysed by Rabalais and Katrib . 

D Bermejo, J Santos, P Cancio, JM Fernandez, S Montero. Vibrational-torsional coupling. High-resolution stimulated Raman spectrum of the V3 band of ethane ( C2H6). J Chem Phys 97 7055-7063, 1992. 

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