The Hydrogen Region

Alkanes Most compounds have an alkyl part and therefore have absorption bands in the 3000-2850 crrT1 region due to their CH bonds.The feature that distinguishes alkanes is the absence of bands for hydroxy groups, carbonyl groups, and so on.The spectrum of an alkane usually has many fewer absorptions than those of compounds with other functional groups. 

The bands for hydrogens bonded to sp2-hybridized carbons appear at wavenumbers just greater than 3000 cm-1. 

Alkenes Most alkenes have absorption bands in the 3100-3000 cm-1 region, due Co the CH bonds where the carbon is sp2 hybridized, and an absorption in the 1660-1640 cm-1 region due to the CC double bond. 

However, the CH absorption is absent if the double bond is cetrasubstituted. Furthermore, the CC double-bond absorption is often weak because the bond is not very polar and may be difficult to discern. 

---

Figure 3.5 Schematic cathodic voltammograms in the hydrogen region. (—) total current,...

The cyclic voltammogram of PtdlO) in 0.5 M sulfuric add is shown in Fig. 2-16. Hs drogen adsorption-desorption current consisted of a lax e peak at 150 mV and a shoulder at 200 mV. The oxidation begins at 800 mV. Unlike PtClll), the shape of the voltammogram in the hydrogen region did not change much after the reconstruction of the surface by a sweep to an anodic potential of 1550 mV. 

Figure 2—17 shows the voltammogram for PtClOO). At the hydrogen region, there are two peaks, one at 250 mV, the other at 350 mV. The oxidation of platinum starts at 850 mV. The peak at 350 mV disappeared after the electrode experienced the formation and reduction of Pt-OH. 

In the hydrogen region (50 - 350 mV) in the first cycle, the hydrogen adsorption-desorption ciirrents were depressed because the surface was covered with COad COad oxidized to CO2 in the anodic peak between 700 mV and 1000 mV. This peak overlapped with the platiniim oxidation whose voltammogram is shown as the dashed line. After this peak the voltammogram became identical with the voltammogram without CO. The h rdrogen adsorption-desorption peaks were fully recovered. This shows the COad completely oxidized and there was no CO in the liquid phase. 

The hydrogen region will only tolerate hydrogen and still give a reasonable rate. When other groups fall into this region, the stereospecificity decreases (vide infra). The aryl region will tolerate... 

Overall, then, the hydrogen region is very useful. We can establish the presence or absence of O—H and N—H groups, and we can identify four different types of C—H bonds. The exact functional group or groups that are present in the compound can be confirmed by examination of the other regions of the spectrum. 

Indicate the positions of the absorption bands and any other noteworthy features in the hydrogen region of the IR spectra of these compounds ... 

Let s try a problem. The IR spectrum of an unknown compound is shown in Figure 13.23. First, let s examine the hydrogen region. The absence of absorptions in the 3600 to 3100 cm-1 region indicates that the compound does not have any O—H or N—H groups. The bands in the 3100 to 3000 cm-1 region indicate the presence of... 

Amides The absorption for the carbonyl group of an amide appears in the region of 1690-1630 cm-1, lower wavenumbers than most other carbonyl bands. In addition, other relatively strong bands often appear at slightly lower wavenumbers. Amides derived from ammonia or primary amines have bands in the hydrogen region due to their NH bonds. 

For Pt(lll), the hydrogen region is comparatively little disturbed during methanol oxidation in sulphuric acid, strongly suggesting that methanol does not chemisorb on Pt(lll) in this electrolyte, at least on the time scale of the dynamic experiments carried out, but the methanol oxidation currents are very low, reaching a maximum of only c. 0.15 mAcm-2. 

Currents are significantly higher on Pt(lll) if the sulphuric acid is replaced by perchloric acid in the presence of methanol, the hydrogen region between 0.0 and 0.4 V is now significantly suppressed (by about 40%) and the butterfly at 0.8 V is absent, suggesting that in perchloric acid there much more rapid and complete chemisorption of methanol. 

Fig. 19. Plot of the coverage-induced band shift for CO adsorbed on Pt(lll) in 0.1 M HCIO4 in the double layer region at 0.1 V vs. SCE (upper eurve) and in the hydrogen region at -0.25 vs. SCE (lower curve). (Extracted from data of [53]). Reprinted by permission of Journal of Chemical Physics AIR...

Strong band broadening upon adsorption in the hydrogen region is observed for both Pt(lll) (Fig. 21) and Pt(llO) (Fig. 22) surfaces. On Pt(lll) much less band broadening is observed for CO/H2O than for CO/H co-adsorption. In the case of CO adsorbed on Pt(llO) in the double layer region, the band splitting makes it diffi-... 

Figure 3.16 Mid-IR spectra for resin, hardener and their mixture, (a) in the hydrogenic region and (h) in the fingerprint region. Reproduced with permission from St John (1993).

---