Less Symmetrical Chains

1 3-Methylglutaric Acid A somewhat less symmetrical series of open-chain compounds can be obtained by placing a substituent on the center carbon atom of the chain (see Fig. 4.37, structures f, g, h, and i). For example, consider 3-methylglutaric acid  

Because of the 3-substituent, there is no plane of symmetry through the chain in the plane of the paper, and, as discussed in Section 4.12.1, the protons in each methylene group are not interchangeable and are thus diastereotopes. 

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Information on the conformational state of the hydrocarbon chains and their orientation has been obtained from external infrared reflection absorption spectroscopy (IRRAS). The first systematic IRRAS studies on phospholipid Langmuir monolayers were reported by Dluhy et al ) (see, for instance fig. 3.62). For DPPC monolayers in the LE phase the positions of the conformation-sensitive symmetric and anti-symmetric C-H stretching bands in the IRRAS spectra were found to be at the same positions as for bilayer systems of DPPC above the Kralft temperature. In the LC phase the frequencies of these bands indicate that the hydrocarbon chains of the lipid molecules are in the all-trans ) conformation (i.e. zig-zag) and analysis of polarized IRRAS spectra show that their average tilt is ca 35° relative to the monolayer normal. This is in reasonable agreement with the tilt angle of 30° obtciined from X-ray diffraction on DPPC monolayers (30°). 

Moreover, Fig. 2 does not contradict the well-known Murrel-Laidler theorem [35,36] which forbids taking the locally symmetric intermediate for the transition state because for the reason of symmetry at least two independent paths exist for its isomerization, Le., for the iiKertion of the monomer into the polymer chain. In other words, the bifurcation of the reaction coordinate proceeds in the locally symmetric intermediate. Nevertheless, this is forbidden for the transition state by the Murrel-Laidler theorem which asserts that the matrix of force constants in the transition state has a single negative value, i.e., that the transition state corresponds to a single reaction coordinate. 

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