Spectra, "tilting

Fig. 3.27 2D spectrum (tilted) of peracetylated glucose from a homonuclear 2D J-Resolved experiment. 

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. 

Figure Bl.22.8. Sum-frequency generation (SFG) spectra in the C N stretching region from the air/aqueous acetonitrile interfaces of two solutions with different concentrations. The solid curve is the IR transmission spectrum of neat bulk CH CN, provided here for reference. The polar acetonitrile molecules adopt a specific orientation in the air/water interface with a tilt angle that changes with changing concentration, from 40° from the surface nonnal in dilute solutions (molar fractions less than 0.07) to 70° at higher concentrations. This change is manifested here by the shift in the C N stretching frequency seen by SFG [ ]. SFG is one of the very few teclnhques capable of probing liquid/gas, liquid/liquid, and even liquid/solid interfaces.

XPS can be used to determine the composition of a solid as a function of distance away from the surface and into the bulk of the solid. Such a depth profile can be constructed in two ways. One way in which a depth profile can be constructed is by using a beam of inert gas ions to sputter away material from the surface of the sample and to then record the XPS spectrum. If this procedure is repeated several times, a profile showing the composition of the material as a function of sputtering time and thus of depth into the sample can be constructed. Another way to construct a depth profile involves tilting the sample with respect to the X-ray beam. In Fig. 17A, the take-off angle or the angle between the sample surface and the direction of propagation of the ejected photoelectrons is 90 . In... 

The infra-red measurements were of two types, normal-film measurements with the sample sandwiched between KBr plates, and tilted-film experiments with the sample sandwiched between 45° prisms of KBr, in each case with layers of Nujol to provide optical matching. Whereas the 1616 cm 1 Raman line occurs in a region well clear of other lines so that it was satisfactory to measure peak intensities, the infra-red spectrum of PET shows many overlapping bands. Accurate assessment of absorption intensities therefore requires the computer separation of the spectrum into a set of overlapping peaks (shown to be Lorentzian in profile) and a linear background. The procedures adopted and the band assignments are discussed in detail by Hutchinson et al. 6). 

At the end of the 2D experiment, we will have acquired a set of N FIDs composed of quadrature data points, with N /2 points from channel A and points from channel B, acquired with sequential (alternate) sampling. How the data are processed is critical for a successful outcome. The data processing involves (a) dc (direct current) correction (performed automatically by the instrument software), (b) apodization (window multiplication) of the <2 time-domain data, (c) Fourier transformation and phase correction, (d) window multiplication of the t domain data and phase correction (unless it is a magnitude or a power-mode spectrum, in which case phase correction is not required), (e) complex Fourier transformation in Fu (f) coaddition of real and imaginary data (if phase-sensitive representation is required) to give a magnitude (M) or a power-mode (P) spectrum. Additional steps may be tilting, symmetrization, and calculation of projections. A schematic representation of the steps involved is presented in Fig. 3.5. 

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... 

Fig. 20. A few examples of the Doppler-selected TOF data are exemplified. The TOF spectra have been converted into velocity space and weighted by a term. For each spectrum, the VUV laser frequency is selected to slice through the Newton sphere near the center-of-mass, i.e. wcm- The cap marked on the top corresponds to the (v, j ) state of the co-product F1F for Ec = 1.18kcal/mol. Note the slight tilt of the dashed lines which act as a visual guide for quantum state assignments.

Information about the surface and interface structures in hexadecylamine-capped CdSe NC of 2 nm size has been obtained by a variety of 1H, 13C, 113Cd, and 77Se NMR techniques [342]. The 77Se CP-MAS-NMR spectrum showed five partially resolved peaks from surface or near-surface Se environments. It was possible to obtain 2D heteronuclear correlation (HETCOR) spectra between 1H and the other three nuclei despite the inherent sensitivity limitations (the 77Se- 3I-I HETCOR experiment required 504 h ). The latter experiment indicated that the methylene protons of the hexadecylamine chain interact with the surface Se atoms via a tilt of the chain toward the surface. The surface Se atoms were not seen to interact with thiophenol present, and it was suggested that thiophenol binds to a selenium vacancy at the surface. 

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...

Fig. 7.1. Tilted-band picture of WSL energy spectrum showing Wannier wave functions and localization length L = 4/3/T. Reprinted from Hacker and Obermair (1970) with permission from Springer.

Fig. 9. (a) Tilted 2H Q-COSY 2D map obtained for the ( )-EHA. The spectrum was symmetrized prior to the tilt procedure. In the FI and F2 dimensions are displayed the 2H— 1H ID spectrum (b) isotropic - FI spectrum of ( )-EHA recorded in CDC13 (c) expansion of the region of methyl groups. (Reproduced by permission of American Chemical Society.)... 

The scaling factor of 40 kHz was independently determined at — 60 °C from the powder spectrum of a random dispersion of a deuteriated PM sample59. This value was furthermore confirmed in the simulations of the 2H NMR tilt series for each of the three labeled methyl groups, according to the total spectral width and line-shape arguments discussed by Ulrich and Watts58. 

The analysis of a full tilt series of 2H NMR spectra not only allows the determination of the unique bond angle for a deuteriated methyl group, but also provides an internal check for the consistency of the spectral interpretation. In particular, simulations provide a means for the analysis of line-broadening effects, which arise from the sample mosaic spread as well as the intrinsic line width of the nuclear transition and instrumental factors. When line shapes are fitted to a full tilt series of spectra in a concerted manner and are also compared with the powder spectrum of an unoriented sample, the different contributions can be discerned. In that way an intrinsic line width of around 2 kHz is found for the spectra shown here, together with a mosaic spread between 8° and 10° for the three samples. 

FIGURE 48. Representative 2H NMR spectra (full lines) of dark-adapted bR (90 mg) containing deuteriated retinal, with line shape simulations (dashed lines) superimposed. Both the powder spectrum (a) from randomly oriented PM patches and the tilt series (b) over sample inclinations, a = 0, 45° and 90°, were recorded at — 60 °C (number of scans, 1.7 x 105, for a = 0°). Spectrum (c) was measured at 21 °C with a = 0° (number of scans, 3 x 105). Reprinted with permission from Reference 60. Copyright (1997) American Chemical Society... 

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