Alkanes spectra

Before proceeding, it is worthwhile to check that the assumption that the -alkanes are good models for polyethylene is valid. Fig. 10.9 shows a comparison of the INS spectrum of -alkanes of increasing chain length with that of polyethylene. The rapid convergence of the n-alkane spectra to a very close resemblance of that of polyethylene is apparent. 

Figure 2. Comparison of the loss-corrected Auger spectrum of polyethylene with the spectra of gas-phase methane, neopentane and n-pentane. The polyethylene spectrum is similar in lineshape and energy to those of the gas phase alkanes. The v-S eV shift of features in the spectrum relative to corresponding features in the alkane spectra, emphasized by the vertical solid and slanting dashed lines, is due to the dielectric response of the solid (see Ref. 3).

Some classes of compounds are so prone to fragmentation that the molecular ion peak IS very weak The base peak m most unbranched alkanes for example is m/z 43 which IS followed by peaks of decreasing intensity at m/z values of 57 71 85 and so on These peaks correspond to cleavage of each possible carbon-carbon bond m the mol ecule This pattern is evident m the mass spectrum of decane depicted m Figure 13 42 The points of cleavage are indicated m the following diagram... 

The IR spectrum of an alkane is fairly uninformative because no functional groups are present and all absorptions are due to C-H and C-C bonds. Alkane C-H bonds show a strong absorption from 2850 to 2960 cm-1, and saturated C—C bonds show a number of bands in the 800 to 1300 cm-1 range. 

Hydrogens on carbon next to aromatic rings also show distinctive absorptions in the NMR spectrum. Benzylic protons normally absorb downfield from other alkane protons in the region from 2.3 to 3.0 5. 

Carbon atoms of an aromatic ring absorb in the range 110 to 140 8 in the 13C NMR spectrum, as indicated by the examples in Figure 15.16. These resonances are easily distinguished from those of alkane carbons but occur in the same range as alkene carbons. Thus, the presence of l3C absorptions at 110 to 140 8 does not in itself establish the presence of an aromatic ring. Confirming evidence from infrared, ultraviolet, or 1H NMR is needed. 

Carbon atoms bonded to electron-withdrawing -OH groups are deshielded and absorb at a lower field in the 1,it NMR spectrum than do typical alkane carbons. Most alcohol carbon absorptions tall in the range 50 to 80 S, as the following data illustrate for cyclohexanol ... 

Hydrogens on carbon next to an ether oxygen are shifted downfield from the normal alkane resonance and show U-f NMR absorptions in the region 3.4 to 4.5 8. This downfield shift is clearly seen in the spectrum of dipropyl ether shown in Figure 18.4. 

Carbons next to amine nitrogens are slightly deshielded in the NMR spectrum and absorb about 20 ppm downfield from where they would absorb in an alkane of similar structure. In N-methylcyclohexylamine, for example, the... 

Infrared radiation, electromagnetic spectrum and, 419, 422 energy of. 422 frequencies of, 422 wavelengths of, 422 Infrared spectroscopy, 422-431 acid anhydrides, 822-823 acid chlorides, 822-823 alcohols. 428, 632-633 aldehydes, 428. 730-731 alkanes, 426-427 alkenes, 427 alkynes, 427 amides. 822-823 amines, 428, 952 ammonium salts, 952-953 aromatic compound, 427-428, 534 bond stretching in, 422... 

These calculations have been conducted on the basis of RHF optimized geometries, considering the 6-31G basis set for the n-alkane compounds (11), and the 6-31G basis set for the polyacene series (12). In both cases, the basis set contention has been checked by comparison with more thorough investigations on small compounds, such as ADC[3] calculations (11a) on n-butane based on the 6-31IG, 6-31G and 6-31G basis, or the MRSDCI ionization spectrum of ethylene as obtained by Murray and Davidson (33) using a 196-CGTO basis set. 

Figure 1.10 Aldehydic regions of 400 MHz H-NMR spectra of unheated (a) and repeatedly used samples (b) of culinary frying oil obtained from a fast-food/take-away establishment. A total of 0.30 ml of each sample was diluted to a final volume of 0.90 ml with C HCls. High levels of n-alkanals, trans-2-alkenals and alka-2,4-dienals are detectable in spectrum (b).

The reductive activation reaction of the 13C-labeled pyrrolo[ 1,2-a]indole shown in Scheme 7.14 was carried out in methanol and a 13C-NMR spectrum was obtained for the crude organic extract. This 13C-NMR spectrum, shown in Fig. 7.14, reveals the presence of starting material as well as products with 13C-labeled alkene and alkane centers. We confirmed the 13C assignments shown... 

The formation of lipid components in an aqueous phase at temperatures from 370 to 620 K was studied by Rushdie and Simoneit (2001), who heated aqueous solutions of oxalic acid in a steel vessel for 2 days the yield of oxidized compounds reached a maximum (5.5% based on oxalic acid) between 420 and 520 K. A broad spectrum of compounds was obtained, from n-alkanes to the corresponding alcohols, aldehydes and ketones. At higher temperatures, i.e., above 520-570 K, cracking reactions competed with the synthetic reactions. 

The index of refraction is a measure of the ability of the alkane to bend (refract) light rays. The values reported are for light of the D line of the sodium spectrum ( D). 

Hentz and Kenney-Wallace (1972, 1974) made a detailed study of esin 25 neat alcohols and three alkane solutions in 1-hexadecanol at 30° using a 5-ns electron pulse. Most data were new, but in some cases they confirmed earlier observations (Dorfman, 1965 Baxendale and Wardman, 1971). The authors found the spectrum fully developed at the end of the pulse, with no spectral change thereafter. The spectra are all broad, asymmetric, and structureless,... 

The majority of marine isonitriles are sesquiterpenes with the molecular formula, C16H25N. Often cyclic, these are alkanes or alkenes possessing only a single isocyano-related functional group. In the mass spectrum, they exhibit a molecular ion at w/z 231, or an intense fragment ion at m/z 204, indicative of M+-HCN. Some are crystalline (see Table 2). With few exceptions most of the isothiocyano and formamido analogs are minor noncrystalline metabolites (see Table 3). 

Unlike methane and the other alkanes, aromatic hydrocarbons have absorptions in the UV part of the spectrum, and thus may be detected through UV spectrometry using silica fibers. This scheme is useful for "aromatic" water pollutants such as toluenes and xylenes with their absorption bands between 250 and 300 nm. Similarly, nitrate anion can be monitored (albeit with low sensitivity) in water via its UV absorption at 250 nm. 

The chemistry of all of these molecules is fascinating but, concentrating on the origins of life, a detailed look at the organic species is appropriate to see what molecules are present and how they might have been formed. The only alkane detected directly in the ISM is methane but this is due to the problem of detection. All alkanes are non-polar and so do not have a pure rotation spectrum. However, there is one allowed vibration of methane that is infrared active and with the low moment of inertia of methane the vibration-rotation spectrum can be observed and a rotational progression identifies the molecule with confidence. 

Of the alkenes (Figure 5.5) only ethene has been detected and of the aromatics only benzene has been seen unambiguously surprisingly propene has not been seen despite its well-understood microwave spectrum. Of interest to the origins of life is the onset of polymerisation in HCN to produce cyanopolyynes. These molecules could provide a backbone for the formation of information-propagating molecules required for self-replication. The survival of these species in a planetary atmosphere depends on the planet oxidation would be rapid in the atmosphere of today s Earth but what of the early Earth or somewhere altogether more alkane-based such as Titan ... 

Fig. 10 Estimation of the tilt angle for an alkane between gold electrodes, determined by fitting the computed IETS spectrum with the experiment (panel b below). Result is a 40 degree tilt angle perpendicular to the plane of the carbon chain, as illustrated in the lighter shade structure in the sketch (b) above. Sketch (a) above and panel (a) below refer to the alkane tilted in the plane of the carbon chain. The structures in sketch (a) do not fit so well an those in (b), suggesting the methyl group position shown in (b) above. From [107], Reproduced by permission of the PCCP Owner Societies...

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