Chain molecules, stretching vibrations

Other general circumstances in which normal vibrations tend to be localized in a particular group of atoms arise when there is a chain of atoms in which the force constant between two of them is very different from those between other atoms in the chain. For example, in the molecule HC=C—CH=CH2 the force constants in the C—C, C=C and C=C bonds are quite dissimilar. It follows that the stretchings of the bonds are not strongly coupled and that each stretching vibration wavenumber is typical of the C—C, C=C or C=C group. 

Sea snake short-chain toxins have a molecular weight of only 6,800. The small size with four disulfide bonds makes these toxins very compact and stable molecules. Therefore, when the Pelamis platurus toxin is subjected to heat treatment at 100 C and subsequent cooling, it does not change its conformation substantially. Amide I and ni bands and S-S stretching vibration did not change by heat treatment. 

Chapter 3 is devoted to dipole dispersion laws for collective excitations on various planar lattices. For several orientationally inequivalent molecules in the unit cell of a two-dimensional lattice, a corresponding number of colective excitation bands arise and hence Davydov-split spectral lines are observed. Constructing the theory for these phenomena, we exemplify it by simple chain-like orientational structures on planar lattices and by the system CO2/NaCl(100). The latter is characterized by Davydov-split asymmetric stretching vibrations and two bending modes. An analytical theoretical analysis of vibrational frequencies and integrated absorptions for six spectral lines observed in the spectrum of this system provides an excellent agreement between calculated and measured data. 

Monolayer Films at the A/W Interface. Previous studies of phospholipid monolayers at gas-liquid interfaces have shown that it is possible to follow the first order thermodynamic phase transition of these monolayer films using the infrared reflectance techniques described in this manuscript (see e.g. ref. 6 and references cited therein). For long chain hydrocarbon molecules, it has been demonstrated that the frequencies of the antisymmetric and symmetric CH2 stretching vibrations are conformation-sensitive, and may be empirically correlated with the order (i.e. the trans-gauche character) of the hydrocarbon chains (9-11). 

In Figure 7 a comparison is made of the frequency of the CHj antisymmetric stretching vibration as a function of molecular area for DPPC monolayer films at the A/W and A/Ge interfaces. As described above, the frequency of (his vibration is related to the overall macromolecular conformation of the lipid hydrocarbon chains. For the condensed phase monolayer (-40-45 A2 molecule 1), the measured frequency of the transferred monolayer film is virtually the same as that of the in-situ monolayer at the same molecular area, indicating a highly ordered acyl chain, predominately all-trans in character. For LE films as well as films transferred in the LE-LC phase transition region, however, the measured frequency appears independent (within experimental uncertainty) of the surface pressure, or molecular area, at which the film was transferred. The hydrocarbon chains of these films are more disordered than those of the condensed phase transferred films. However, no such easy comparison can be made to the in-situ monolayers at comparable molecular areas. For the LE monolayers (> ca. 70 A2 molecule 1), the transferred monolayers are more ordered than the in-situ film. In the LE-LC phase transition region ( 55-70 A2 molecule 1), the opposite behavior occurs. 

This model, however, is only valid for chain molecules at zero Kelvin. When 7V 0 the chain parts possess thermal energy vibration causes them to move in random directions, which always results in a contraction of the stretched chain. The chain tends to a state of higher probability, and eventually reaches a fully unoriented random conformation, as described in 2.4 ( random walk conformation ). To... 

Stretching vibrations of cyclic molecules and molecular chains... 

Also using IRRAS, Mendelsohn and co-workers have studied monolayers of phospholipids with deuterated acryl chains. In such systems C-H and C-D stretching vibrations can be monitored simultaneously. This permits, for example, observation of individual components in a mixed lipid monolayer or conformational analysis of different parts of the acryl chains. Measurements on mono-layers consisting of tail-end deuterated DPPC molecules showed that the chains posses more conformational order adjacent to the head group than at their tails. ... 

NOTES Although the intensity of the absorption band caused by the C=C stretching vibration is variable, it is strongest when the alkyne group is monosubstituted. When this group is disubstituted in open chain compounds, the intensity of the C=C stretching vibration band diminishes as its position in the molecule tends to establish a pseudo center of symmetry. In some instances this band is too weak to be detected and, thus, its absence in the spectrum does not, necessarily, establish proof of the absence of this linkage. 

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