Symmetrical stretching methyl

FIG. 10 Vibrational sum frequency spectrum of saturated monolayers of dilauroyl- (DLPQ, dimyristoyl- (DMPC), dipalmitoyl- (DPPC), and distearoyl-phosphatidylcholine (DSPC) at the D2O-CCI4 interface at ambient temperature in the region of the methylene and methyl symmetrical stretches. (From Ref 139, copyright American Chemical Society.)... 

From studies of Langmuir films (insoluble surfactants) and films adsorbed to solid substrates, alkyl chains are known to be well ordered. For soluble surfactants at oU/water, however, the picture is much different. Several studies from this laboratory have demonstrated these differences [6,37-39]. Figure 2.6 shows the ssp spectra for sodium do-decyl sulfate (SDS) at the CCI4/D2O interface at monolayer coverage (squares) and at extremely low surface coverage (circles) with the spectra normalized to the methyl symmetric stretch [6]. 

Figure 2.10 shows the methylene and methyl symmetric stretch region of the spectrum for each headgroup before (solid squares) and after (solid triangles) the exposure of the monolayer to halothane. For each spectrum shown, there is a small increase in the overall intensity when halothane is present. This increase occurs across the entire spectrum for each of the phospholipids smdied. We believe that the small change in intensity... 

Fig. 6 Intensity ratios of the CH3 S5nninetric stretch to the CH2 synunetric stretch for DLPC, DMPC, DPPC, and DSPC versus bulk aqueous phospholipid concentration. Lines are shows as guides for the eye. (Insert) SF spectra of DLPC and DSPC at saturated mono-layers coverage under ambient conditions (T = 22 °C). Spectra contain both the methylene and methyl symmetric stretches (CH2 ss at 2850 cm and CH3 ss at 2872 cm ). Solid lines represent fits to the spectra using Voight profiles for the different bands...

Model chromatographic interfaces were examined using sum-frequency generation spectroscopy (SFG) in order to study the effect of various solvents on the structure and conformation of the stationary phase. Monolayers formed from mixed alkyltrichlorosilanes on fused silica were examined in contact with air, acetonitrile, isooctane, and water. Results show that monolayers of 100% Cig composition show little disorder in the alkyl chains for all the solvents examined, though a very distinct solvent shift is apparent in the methyl symmetric stretch. When mixed composition (Cis and Ci) monolayers were examined, significant disorder was induced in the alkyl chains for all the solvents. However, the largest change occurred when the monolayer was in contact with water. These results support a collapsed structure model for the mixed monolayer in contact with water. 

The SF spectra obtained with the same polarization combination (ssp) and of the same surface compositions in contact with water are shown in Figure 3B. Comparing Figures 3A and 3B, one can see that the water causes a much different structural response in the stationary phase. We see the appearance of the methylene symmetric stretch, as expected, but we see a much larger change in the methyl symmetric stretch intensity than in air, indicating that a larger amount of chain disorder is induced by... 

A unique trend emerges as we examine acetonitrile and isooctane in contact with the same monolayers. Figures 4A and 4B show these spectra. By determining the ratio of the intensities of the methyl symmetric stretch (ICHsSS) to the methylene symmetric stretch (ICH2SS) th ssp polarization, we can get a semi-quantitative comparison of the amounts of gauche defects in the alkyl chains. The values for this ratio for 100% Cis and partial Cig coverages are shown in the first two rows of Table... 

Figure 6 SF intensity ratios for A methyl symmetric stretch to methylene symmetric stretch intensities for ssp polarization for various solvents, B methyl symmetric stretch for ssp to the methyl symmetric stretch for sps polarization for (A) 100% C is and ( ) partial Qs coverages.

As seen from the data in Table 7-4 (abstracted from frequencies provided in Appendix A7) symmetric methyl group CH stretching frequencies change with substitution. The smallest value is for methylamine (chosen as the reference compound) and the largest is for methyl chloride. (Ethane has been excluded from this comparison as the symmetric stretch here involves all six hydrogens.)... 

Figure 6.3 Effects of hydrocarbon chain modifications on melting points of similar-sized cuticular lipids. When lipids melt, the absorption frequency of C-H symmetric stretching vibrations increases from -2849 cm1 to -2854 cm4. From right to left, compounds are (chemical change relative to n-alkane, molecular mass in daltons) filled circles, n-dotnacontane (no change, 450) open circles, palmitic acid myristyl ester (wax ester, 452) filled triangles, 13-methylhentriacontane (methyl-branched alkane, 450) open triangles, (Z)-13-tritriacontene (double bond, 462) filled squares, 9,13-dimethylhentriacontane (2 methyl branches, 464) open squares, oleic acid oleyl ester (2 double bonds and an ester link, 532). Data from Gibbs and Pomonis (1995) and Patel el al. (2001).

FIGURE 2.10. Representative spectra of dipalmitoyl phospholipid monolayers at the CCU/D2O interface under the ssp polarization scheme showing the methyl and methylene symmetric stretch (a) DPPC, (b) DPPE, (c) DPPG, (d) DPPS. Spectra of the monolayers are shown in solid squares. Spectra of the monolayers with halothane are shown with solid triangles. The lines ate fits to the data. From Ref. [62]. 

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