PM-IRRAS spectra

Figure 9. PM-IRRAS spectra for Ag electrode in 0.03 M azide in 0.1 M Na01O4. These are obtained by taking the difference of the spectra taken at the specified potential and at -0.95 V (Ag/AgCl). (Reprinted with permission from ref. 50. Copyright 1988 American Institute of Physics.)...

In Figure 1 we show the PM-IRRAS spectra for a Pt electrode exposed to saturated C0/H S0 solutions which contain various concentrations of different organic nitriles. For comparison, we have also included a spectrum recorded in saturated CO/H SO with no added nitrile. The adsorption step was accomplished by pulling the electrode back into the bulk solution and cycling the potential from 0.55 V(SHE) up to 1.15 V, down to 0.0 V, and back to 0.55 V. The spectra were recorded after re-positioning the electrode against the cell window while the potential was held at 0.55 V. 

Figure 1. PM-IRRAS spectra of CO adsorbed on Pt in 0.5 M H S0, at 0.55 V(SHE) in the presence of different added nitrile compounds.

Under the optimum incidence angle, the direction and the intensity of the bands on PM-IRRAS spectra are governed by a surface selection rule that depends on the orientation of the corresponding transition moment a transition moment... 

The applications of PM-IRRAS also include fatty acids, phospholipids, and protein conformations. Desbat and co-workers reported on the variation of the dissociation of a Langmuir monolayer of arachidic acid at the air-water interface as a function of the subphase pH and for several cations (Cd2+, Ca2 +, Mg2 +, and Na+) with the help of the PM-IRRAS method [92]. Fig. 14 shows the PM-IRRAS spectra of Langmuir monolayer of deuterated arachidic acid in the presence of CdCb as a function of the subphase pH. At low subphase pH (pH = 3.5), the spectrum only presents absorption bands related to the acid form, i.e., the C = O stretching vibration (v(C = O)) and the OH bending (<5(0-H)) located at 1720 and 1270 cm respectively. The frequency position of the v(C = O) is characteristic of a hydrogen-bonded carbonyl group. As the subphase pH is increased, the arachidic acid is progressively deprotonated to... 

Fig. 14. Normalized PM-IRRAS spectra of a monolayer of deuterated arachidic acid spread onto a water subphase containing 3.5 x 10-3M CdCl2 as a function of the subphase pH in the headgroups vibration range (a) and in the alkyl chains vibration range (b). Taken from Ref. [92] with permission from American Chemical Society.

Since PM-IRRAS is insensitive to the strong IR absorption of water vapor, it has proved to be an efficient way to study the conformation and orientation of protein molecules because only important bands arising from the monolayer are observed [72,97-103], The first in situ study of the protein conformation by PM-IRRAS technique was reported by Dziri et al. [97]. The vibrational spectrum of acetylcholinesterase (AChE) at the air-water interface in its free form and bound to either its substrate or organophosphorus (OP) inhibitor was measured. PM-IRRAS spectra collected during compression of the AChE... 

Fig. 15. PM-IRRAS spectra of l-DSPC monolayer hydrolysis by PLA2 with the enzyme concentration in the subphase of 173ng/mL. The surface pressures were kept at different initial surface pressure for the hydrolysis reaction, (a) % — 40 mN/ m and (b) n — 10 mN/m for 180min, respectively, and then the monolayer was compressed to % — 40 mN/m for the PM-IRRAS measurements, (c) % — 0.5 mN/m for the hydrolysis reaction of 180 min, and then the monolayer was compressed to % — 10 mN/m for the PM-IRRAS measurements. Taken from Ref. [94] with permission from American Chemical Society.

Fig. 17. Effect of the orientation of a helical structure on the position of the amide I band adapted from Cornut et al. [98]. Variation of the amide I band wavenumber maximum on the PM-IRRAS spectra as a function of the 6 tilt angle of the a-helix is shown. 9 is the tilt angle between the helical axis and the normal to the interface. Taken from Ref. [107] with permission from American Chemical Society.

Fig. 18.5. Comparison of the polarisation modulated (PM-IRRAS) spectra of linear COad, derived from adsorption of CO and of methanol from solution onto a Pt surface from, respectively, CO(sat)/0.5 M H2S04 and 1 M MeOH/O.5 M H2S04 at 0.4 V for 15 min. The polarisation modulation was conducted at 80 Hz by using a rotating polariser, and spectra are referred to 0.8 V. The total adsorbate coverage of 0.86 is common to the two spectra.

Fig. 34. PM-IRRAS spectra of adsorbed CN on a polycrystalline Pt electrode in KCN +1 M NaClO solutions at -0.5 V vs. SHE. The CN concentrations are indicated. Note the formation of bridge-bonded CN at low CN concentrations. (After [120]). Reprinted by permission of Elsevier Science.

Fig. 4.2 Comparison of bulk and thin-film PM-IRRAS spectra of Diblock 2.

Fig. 4.3 Comparison of pure PCL and Diblock 2 thin-films PM-IRRAS spectra.

Polarization-modulation infrared reflection-absorption spectroscopy (PM-IR-RAS) spectra were recorded with a Bruker ITS 66/S Fourier transform infrared spectrometer equipped with a PMA 37 polarization modulation module and a ititrogen-cooled MCT detector. The infrared beam was first p-polarized with a ZnSe wire grid polarizer (Specac) before passing through a photoelastic modulator (Hinds Instruments, PEM-90), which modulated at a frequency of 74 kHz. A lock-in ampHfier (Stanford model SR-830) was used to obtain the PM-IRRAS spectra. The half-wave retardation frequency was set at 4000 cm . The PM-IR-RAS spectra were recorded as S= R -Rs)/(R +Rg). A total of 250 scans at a resolution of 4 cm were collected for each measurement at an angle of incidence of 82.5° with respect to the normal to the sample surface. 

In order to calculate A.I/ I) from the measured PM IRRAS spectra, one has to determine functions J2 and Jq in an independent experiment. A reliable method to measure the PEM response functions was described by Buffeteau et al. [69]. Below we describe a similar method that we adapted with minor changes to use for electrochemical systems [81]. The spectroelectrochemical cell is replaced by the dielectric total external reflection mirror (a Cap2 equilateral prism can be used for this purpose). The second polarizer is inserted just after the PEM and set to admit p-polarized light (identical setting to that of the first polarizer). The PEM is turned off and the reference spectrum is acquired. This spectrum gives the intensity of the p-polarized light Ip (cal), which passes through the whole optical bench. 

The last step in processing the experimental PM IRRAS spectra involves the subtraction of the background. This background arises from the slowly varying broad-band absorbance of infrared radiation by the aqueous electrolyte. In order to remove the background, a procedure similar to that published by Earner et al. [66] was developed by Zamlynny [36]. The baseline is created from the experimental data points using spline interpolation. Successful interpolation requires knowledge of the exact positions of the absorption bands and a little experience. 

Fig. 9.31 Spline interpolation procedures for background corrections of PM IRRAS spectra, (a) A raw PM IRRAS spectrum of a DM PC bilayer on a Au(lll) surface at the electrode potential E=-0.2 V. (b) A spline (dashed line) for the first rough baseline correction, open circles are data points used...

Fig. 9.33 (a) CH stretching region of PM iRRAS spectra of a DM PC biiayer on an Au(lll) eiectrode in 0.1 M NaF/D20 soiution at potentiais indicated in the figure. The top trace piots spectrum caicuiated for 5.5-nm thick fiim of randomiy oriented DM PC moiecuies using opticai constants of DMPC for vesicies dispersion in D2O. 

Fig. 5 PM-IRRAS spectra of linearly adsorbed CO at a Pt electrode immersed in 1 M HCIO4 saturated with CO. The electrode potential was ...

Figure 7.54. Normalized PM-IRRAS spectra of acetylcholinesterase (AChE) monolayer at AW interface at different surface pressures. AChE solution was spread at zero surface pressure. PM-IRRAS spectra recorded on Nicolet 740 spectrometer equipped with MCT detector using setup shown in Fig. 4.52. Modulation frequency was set up at 1666 cm and 400 scans were collected for each spectrum. Angle of incidence was 75°. Reprinted, by permission, from L. Dziri, B. Desbat, and R. M. Leblanc, J. Am. Chem. Soc. 121, 9618 (1999), p. 9620, Fig. 1. Copyright 1999 American Chemical Society.

The complexation of PABA with nucleotides was further studied using polarization modulated infrared reflection absorption spectroscopy (PM-IRRAS). PM-IRRAS spectra of PABA films exhibit all the characteristic vibrations of polyaniline and boronic acid (Figure 3.27) [93, 94]. After complexation with NAD+ (Figure 3.27, b) and NADH (Figure 3.27, c), the disappearance of the free B-OH group vibration at 986 cm and increase in the intensity of the asymmetric B-O bond vibration at 1330 cm indicate the formation of boronate ester. The new vibrations at 1080 and 1470 cm have been attributed to ribose and adenine moieties, respectively [153]. The vibrations at 1218 (NAD+), 1245... 

Figure 3.27 PM-IRRAS spectra of PABA film (a) reacted with 10 jlM (b) NAD+ and (c) NADH in pH 7.4 PBS. (Reprinted with permission from Chemistry of Materials, 17, 2918. Copyright (2005) American Chemical Society.)...

Figure 5. PM-IRRAS spectra in the C—H stretching region of the series of phosphate SAMs. The number of carbon atoms ( ) in the alkyl chain of the phosphates is indicated alongside the spectra.

F. 3 PM-IRRA spectra obtained from the three alkanethiol SAM films... 

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