Spectrum of Methane

Figure 8.17 A short, barely resolved, vibrational progression in the v- vibration of CHj in the carbon Is X-ray photoelecton spectrum of methane obtained with a monochromatized X-ray source. (Reproduced, with permission, from Gelius, U., Svensson, S., Siegbahn, H., Basilier, E., Faxalv, A. and Siegbahn, K., Chem. Phys. Lett, 28, 1, 1974)...

The photoelectron spectrum of methane shows two bands, at 23 and 14 eV, and not the single band we would expect from the equivalency of the four C—H bonds. The reason is that ordinary sp hybridization is not adequate to explain phenomena involving ionized molecules (such as the CH4" radical ion, which is left behind when an electron is ejected from methane). For these phenomena it is... 

The peak obtained in the spectrum is referred to as M+ peak and the intensity of this highest peak is called the base peak and it corresponds to the molecular weight of the substance and the intensities of all other peaks are expressed relative to the base peak. The base peak is caused by the fragment ion which is most stable and whose formation requires least energy. Mass spectrum of methane is given on page 268. 

In addition to this there are some additional peaks which is unusual in the typical organic compounds, The mass spectrum of methane shows m/e values of 14, 13, 12, 2 and 1. This is explained as due to the formation of cationic fragments as follows ... 

A simple example is the mass spectrum of methane, CH4 (Figure 10.18). A methane molecule breaks into the following fragments (in order of relative abundance) CH, CH, CH, CH+, H+, C+, and 13CH4. In the figure, each peak is identified as being due to one of these fragments. Notice that one of the peaks... 

See Servos, Physical Chemistry from Ostwald to Pauling, 128133, 265274 and especially on molecular spectroscopy and quantum chemistry, see Assmus, "Molecular Structure." Assmus notes the interest of Niels Bohr, H. A. Kramers, and Wolfgang Pauli in Dennison s Ph D. dissertation, "Molecular Structure and the Infrared Spectrum of Methane" in Alexi J. Assmus, "The Creation of Postdoctoral Education and the Siting of American Scientific Research," MS. 

In the mass spectrum of methane (Fig. 1.2), there is a tiny peak at m/z 17 that has not been mentioned in the introduction. As one can infer from Table 3.1 this should result from the content of natural carbon which belongs to the X-rl elements in our classification. 

The most intensive peaks in the El mass spectrum of methane are the molecular ion, m/z 16 and the fragment ion at m/z 15 (Fig. 6.1). Explicitly writing the electrons helps to understand the subsequent dissociations of CH4 to yield CH3, m/z 15, by H loss (oi) or by CH3 loss (02), respectively. In general, it is more convenient to write the molecular ion in one of the equivalent forms. The charge and radical state are then attached to the brackets (often abbreviated as 1) enclosing the molecule. 

Fig. 6.1. El mass spectrum of methane. Used by permission of NIST. NIST 2002.

Examples To rationalize the mass spectrum of methane, reactions 6.2-6.6 were proposed. They all obey the mle. You should check the mass spectra and fragmentation schemes throughout this chapter for additional examples of the nitrogen rule ... 

The El mass spectrum of methane has already been discussed (Chap. 6.1). Rising the partial pressure of methane from the standard value in El of about 10 Pa to 10 Pa significantly alters the resulting mass spectrum. [1] The molecular ion, CH/ , m/z 16, almost vanishes and a new species, CHs", is detected at m/z 17 instead. [16] In addition some ions at higher mass occur, the most prominent of which may be assigned as C2H5, m/z 29, [17,18] and CsHs", m/z 41 (Fig. 7.2). The positive ion Cl spectrum of methane can be explained as the result of competing and consecutive bimolecular reactions in the ion source [4,6,10]... 

The relative abundances of these product ions change dramatically as the ion source pressure increases from El conditions to 25 Pa. Above 100 Pa, the relative concentrations stabilize at the levels represented by the Cl spectrum of methane reagent gas (Pig. 7.3). [4,19] Portunately, the ion source pressure of some 10 Pa in Cl practice is in the plateau region of Pig. 7.3, thereby ensuring reproducible Cl conditions. The influence of the ion source temperature is more pronounced than in PI because the high collision rate rapidly effects a thermal equilibrium. 

Figure 6 ESR spectrum of methane hydrate irradiated by x-rays at 11 K. Methyl radicals and hydrogen atoms in synthetic methane hydrates (Takeya et al., 2004).,m...

Although the four bonds of methane are equivalent according to most physical and chemical methods of detection (for example, neither the nmr nor the ir spectrum of methane contains peaks that can be attributed to different kinds of C—H bonds), there is one physical technique that shows that the eight valence electrons of methane can be differentiated. In this tech-... 

If all the valence shell electrons of methane (i.e., all but the carbon Is electrons) were equivalent, the photoionization spectrum of methane would show only one peak. In fact, as seen in Figure 8.8, it shows two, which have energies and intensities appropriate to the A, and T2 MOs. 

T.X. Carroll, N. Berrah, J. Bozek, J. Hahne, E. Kukk, L.J. Saethre, T.D. Thomas, Carbon Is photoelectron spectrum of methane Vibrational excitation and core-hole lifetime, Phys. Rev. A 59 (1999) 3386. 

Most elements are composed of more than one isotope. In the case of carbon the relative abundances of the two major isotopes, l2C and l3C, are 100 to 1.1. In a sample of methane, then, for every 100 molecules composed of l2CH4 there are 1.1 molecules of l3CH4. Therefore, in the mass spectrum of methane the mlz 17 peak, which is due to 13CH4, has 1.1% of the intensity of the mlz 16 peak, which is due to 12CH4. Benzene has six carbons, each with a 1.1% chance of being l3C. Therefore, the M + 1 peak at mlz 79 should be approximately 6 X 1.1 = 6.6% of the M+ peak at 78 mlz, in good agreement with the observed value of 6.8%. The M + 2 peak, with 0.2% of the intensity of the Mt peak, is due primarily to molecules that contain two atoms of 13C. 

However, such a mechanism would require a minimum lifetime of CH4 of the order of a microsecond. This, in turn, would suggest that the absorption spectrum of methane should exhibit structure as in the case of acetylene. Further, the excited state should be subject to quenching by collision with an inert gas and the proportion of HD should decrease as the pressure increases. The available evidence is opposed to such a reaction.119 It is likely that HD is a result of either abstraction reactions of H and D atoms or association of H and D atoms. 

Spectrum of methane ionization plasma at 20 Pa. The relative intensities depend on the pressure in the source. 

The mass spectrum of methane ta relatively simple because few fraf mentations a re possible. As Figure 12.2a ahows, th4 base peak has miz 16. which corresponds to- the unbragmented methane cation radical, called the parent peak or the molecuJar ion mass spectrum also shows peaks at miz = 15 and 14, corresponding to cleavage of the molecular ion into CH, and CH2 fragments. 

The photoelectron spectrum of methane shows two bands,at 23 and 14 eV, and not the single band we would expect from the equivalency of the four C—H... 

F. D. Medina and W. B. Daniels. Low frequency Raman spectrum of methane at high densities. J. Chem. Phys., 66 2228-2229 (1977). 

W. A. Chupka (1 ) measured the photoionization spectrum of methane and from the onset of CHg production has reported AatH (CH3-H) = 103.244 0.12 kcal. This value was corrected for the thermal rotation energy of the fragment ions and is the value adopted here. 

After water vapor and C02, methane (CH4) is the third most important greenhouse gas. Each additional molecule of CH4 added to the atmosphere absorbs about 20 times as much long-wave infrared radiation as does a molecule of carbon dioxide. This occurs in part because some of the absorption spectrum of methane lies in windows in the carbon dioxide absorption spectrum (see Fig. 4-42) therefore, methane absorbs wavelengths that are not already being highly attenuated by carbon dioxide. Currently, the global concentration of methane in the atmosphere is approximately 1.7 ppm and is increasing at an annual rate of approximately 0.01 ppm per year (Table 4-14). The seasonal fluctuations shown in Fig. 4-44 may correspond to seasonal... 

The determination of further electron affinities is not an easy matter it is possible that the electronic equilibrium method could be extended to a few more elements, but at the temperatures involved, molecules and radicals would be decomposed. The only reasonable hope of estimating the electron affinities of radicals would seem to lie in a study of the appearance potential of negative ions, and the determination of their kinetic energies, although it must be borne in mind that a careful search of the mass spectrum of methane has failed to reveal the existence of a CH3(-)ion. [4]... 

There is experimental support for this MO pattern. The ESCA spectrum of methane is illustrated in Figure 1.16. It shows two peaks for valence electrons at 12.7 and 23.0 eV, in addition to the band for the core electron at 291.0 eV. Each band... 

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