Methyl combination bands

The spectra of substituted thiazole derivatives, especially the methyl derivatives, can be used to determine harmonic and combination bands of the T(ch) modes 27(c.,h> T(c.hi + T[Pg.351]

Fig. 5. IINS spectrum (MARI, ISIS) at 20 K of activated AI2O3 + 1.12 mmol of CH3OH adsorbed at 300 K. The methyl rock (PCH3), deformation (5CH3), stretch (i CH3), and the rock + deformation combination band (PCH3 + 5CH3) are at 1165, 1454, 2997, and 2614 cm respectively.

FIGURE 2.1 Illustrates aliphatic C-H (as predominantly methyl C-H str.) from 2,2,4-dimethyl pentane (isooctane). The top spectrum (from left to right) shows the 4th overtone (5v), the 3rd overtone (4v), and combination bands. The center spectrum illustrates the 2nd overtone (3v) and combination bands. The bottom spectrum presents the 1st overtone (2v) and combination bands, and a portion of the fundamental. (From Workman, L,Appl. Spectmsc. Revs., 31(3), 277, 1996. With permission.)... 

FIGURE 2.9 Demonstration of the third overtone (4 v) harmonic and the second overtone (3 v) combination region, illustrating the relative intensities and band positions. The third overtone (4 v) near-infrared C-H stretching aromatic, methyl, and methylene band positions are shown. A region of second overtone (3 v) combination bands is also seen in this figure. 

FIGURE 2.10 Illustration of the second overtone (3 V) spectral region. Also observed are the first overtone (2 v) stretching and bending combination bands for methyl and methylene as labeled in the figure. 

Substitution on the benzene ring reduces the symmetry of the molecule, potentially allowing for additional vibrational peaks. In the case of alkyl aromatics, there is also the addition of the alkyl absorptions. Figure 4.2 compares the near-infrared spectra of benzene and toluene. The primary differences appear to be the addition of the methyl combination bands near 4300 cm (2300 nm) and the methyl first overtones near 5800 cm (1750 nm). There is also a noticeable peak at 3836 cm (2607 nm) that could be related to the sum of a C-H stretch and one of the ring-wagging vibrations involving a ring with five adjacent protons. 

The second overtone of the nonbonded OH stretch occurs at about 10,400 cm" (960 nm), and the third at about 13,500 cm (740 nm) for simple alcohols. The second overtone has also been used for a number of hydrogen-bonding studies. Variations in the structure of the alcohol result in splitting of the band and systematic shifts. Second overtones of the OH stretch appear to have less interference from CH combination bands than first overtones, and can therefore be more useful for thermodynamic studies. Additional overtones of the nonbonded hydroxyl stretch of alcohols, using gaseous ethanol as a model, are the fourth at 16,700 cm" (600 nm) and the fifth at 19,500 cm" (510 nm). Additional bonded hydroxyl bands include the OH-stretch second overtone at 9550 cm (1047 nm), a combination of the first overtone of the OH stretch and twice the methyl CH deformation at 9386 cm" (1065 nm), and a combination of the OH-stretch first overtone plus three times the CO stretch at 9720 cm" (1029 nm). i... 

Higher-wavenumber combination bands include one that combines the first overtone of the OH stretch and twice the methyl CH deformation at 9386 cm" (1065 nm), and a combination of the OH stretch first overtone plus three times the CO stretch at 9720 cm" (1029 nm). ... 

Asymmetric methyl (C-H) stretch (3v) 2nd overtone Asymmetric methylene (C-H) stretch (3v) 2nd overtone Methyl and methylene (C-H) conibination Asymmetric methyl (C-H) stretch (2v) 1st overtone Asymmetric methylene (C-H) stretch (2v) 1st overtone Symmetric methyl (C-H) stretch (2v) 1st overtone C-H bend (35) 2nd overtone C-H stretch and C-C stretching combination C-H combination band... 

H combination band, alcohols or water 2vO-Hand 8C-H methyl combination Alcohols as R-C-O-H... 

Composition of methyl acrylate (MA) and methyl methylacrylate (MMA) comonomer was analyzed in temperature-equilibrated a liquid cell (30°C). The MA concentration ranged from 10 to 18% and the MMA concentration ranged from 82 to 99%. The MA calibration used the following wavelengths 1728, 2312, and 2400 nm and the MMA calibration used 2192, 2280, and 2400 nm. Combination bands are affected by small molecular symmetry differences and thus are often used to separate materials with similar organic functional groups. 

NIR reflectance spectra of the six types of recyclable plastics collected for this study are shown in Figure 36.4. Bands seen are primarily those arising from overtones and combinations of C—H and O—H stretching and bending vibrations. Combination bands of C—H are found between 2100 and 2500 nm while overtones of aromatic, methyl, and methylene C—H stretches are found between 1600 and 1800 nm and between 1150 and 1230 nm. 

For example, let us consider the vibrational spectrum of methylacetylene, CH3C=CH. The 15 normal vibrations reduce to five Aj-labeled motions and five -labeled motions. Table 14.6 lists the 10 unique vibrational frequencies of methyl-acetylene. Also listed are other absorptions that are attributed to various overtones and combination bands. The nonideality of the molecule permits some of these combinations to appear with detectable intensity. 

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