Spectra, "tilting parallel

The external reflection of infrared radiation can be used to characterize the thickness and orientation of adsorbates on metal surfaces. Buontempo and Rice [153-155] have recently extended this technique to molecules at dielectric surfaces, including Langmuir monolayers at the air-water interface. Analysis of the dichroic ratio, the ratio of reflectivity parallel to the plane of incidence (p-polarization) to that perpendicular to it (.r-polarization) allows evaluation of the molecular orientation in terms of a tilt angle and rotation around the backbone [153]. An example of the p-polarized reflection spectrum for stearyl alcohol is shown in Fig. IV-13. Unfortunately, quantitative analysis of the experimental measurements of the antisymmetric CH2 stretch for heneicosanol [153,155] stearly alcohol [154] and tetracosanoic [156] monolayers is made difflcult by the scatter in the IR peak heights. 

In homonuclear 2D /-resolved spectra, couplings are present during <2 in heteronuclear 2D /-resolved spectra, they are removed by broad-band decoupling. This has the multiplets in homonuclear 2D /-resolved spectra appearing on the diagonal, and not parallel with F. If the spectra are plotted with the same Hz/cm scale in both dimensions, then the multiplets will be tilted by 45° (Fig. 5.20). So if the data are presented in the absolute-value mode and projected on the chemical shift (F2) axis, the normal, fully coupled ID spectrum will be obtained. To make the spectra more readable, a tilt correction is carried out with the computer (Fig. 5.21) so that Fi contains only /information and F contains only 8 information. Projection... 

The tilted configuration of butane was not considered in the analysis of the monolayer vibrational spectrum in Sec. II.C.2. Normal mode calculations are now being performed (20) with this orientation to see if the fit to the observed spectrum can be improved. The tilting of the molecule may be related to the inconsistency encountered in the plane-parallel model in which different atom-substrate force constants had to be introduced for the co-planar CH- and CH, hydrogens. In the tilted configuration this bottom layer of hydrogens is split into two separate levels of atoms. This difference in height above the surface may provide a physical basis for two different force constants. 

It is noted in Fig. 4 that the intensities of the bands at 2222, 1547, and 1529 cm-1 due to in-plane TCNQ modes are much stronger in the RA spectrum than in the transmission spectrum. Therefore, it seems that the TCNQ plane is nearly perpendicular to the substrate surface. Transition moments of the two C=C stretching bands at 1547 and 1531cm-1 are perpendicular to the molecular axis of the TCNQ chromophore [20]). It should be noted that both have comparable intensities in the transmission and RA spectra (Fig. 4). Therefore, it may be concluded that the molecular axis of the TCNQ chromophore is neither parallel nor perpendicular to the surface, being in an intermediate direction. The intensities of CH2 antisymmetric and symmetric stretching bands are also comparable between the two spectra, suggesting that the alkyl chain is tilted considerably with respect to the surface normal. 

In the 1970s, we were all impressed by the ability of the proton 2D /-resolved technique to provide (after tilting of the twice-Fourier-transformed data matrix) a proton-decoupled proton spectrum comprised of a singlet at each of the 1H chemical shifts, and resolved /-multiplets by taking slices parallel to the F1 axis. These /-multiplets displayed all of the coupling constants for a resonance (no chemical shift) and were remarkable in that they were sharper than the spin multiplets in ID H NMR spectra, due to the refocusing effect of the 180° pulse in the /-resolved sequence. 

Figure 9 Absorption spectrum (a) and electrochromism (b) of [Ru(bpy)3] dissolved in a thin polymer film at 77 K. Absorption and electrochromism spectra are taken through semitransparent conducting electrodes. Electrochromism is usually measured with the externally applied electric field parallel to the propagation direction koflight( = 0°). Experiments can also be made with the film tilted away from the normal to the propagation direction (Drawn from data in Oh and Boxer )...

Fig. 6.10. Tilting effects of doubly oriented films showing settings for observing (A) parallel tilting spectrum, and (B) perpendicular tilting spectrum. The vectors are E.V., the electrical vector (perpendicular to slit) M, the transition moment vector parallel to chain axis Mj, the vector perpendicular to doubly oriented film surface and M3, the vector perpendicular to chain axis and parallel to film surface. (Marchessault et al., 1960.)...

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