Hydroxyl first overtone

A hydroxyl first overtone peak may also be split due to interactions with the electrons of phenyl rings or halogens, as in benzyl alcohol, for example. 

FIGURE 8.6 Hydroxyl first overtone peak decrease and amide NH peak increase during a reaction. (From Dittmar, K. and Siesler, H.W, Fresenius, J. Anal. Chem., 362, 111, 1998. With permission.)... 

As mentioned above, HOSi(OA)3 may be taken as the simplest cluster model of the terminal hydroxyl group in silicas. Indeed, even with this cluster CNDO/BW provided a quite satisfactory description of the lower part of the curve representing potential energy as a function of the OH stretching vibration coordinate ROH (Fig. 2) (48,49). The respective experimental curve was plotted by Kazansky et al. (49) based on the analysis of the fundamental frequency vOH and the first overtone of the characteristic OH stretching vibration in terms of the Morse potential function. The frequencies of the second and third overtones were also determined in that work, and it was shown that the Morse potential reproduced well the potential curve within a rather wide range of ROH. 

The middle IR could be used to look at the C-0 ester and ether bands and the secondary hydroxyl in the 1000 to 1500 cm"l region. However, more useful were the results from the near IR in which the epoxy C-H, C=0, and 0-H bands can be compared to the first overtone of the 3.5. M c-H band which appears at 1.66... 

Moisture and hydroxyl number are important parameters, which are determined by measuring either the first overtone at 6890 cm or the combination band at 5180 cm . A few details about chemical structure are accessible by interpretation of these bands. Changes in hydrogen bonding lead to changes in the band shape and band location. Difference spectra or second derivatives must be calculated in order to detect minor chemical interactions of OH with other molecular species in the sample. The number of double bonds is another important parameter to describe the properties of fats and oils, e. g. their degree of unsaturation. 

Overtones appear in the spectra because of the anharmonicity of the O-H bond. It is important to consider that the Vq2(OH) bands appear always at a wavenumber lower than twice that of the fundamental hydroxyl stretching mode. The difference is used to calculate the anharmonicity of the O-H oscillator according to Equation (2.1) and also the harmonic frequencies according to Equation (2.2). In principle, accurate calculations should be based also on the next overtones but it was reported that, for OH oscillators, calculations using the first overtone always give correct results (613). In the absence of any other factors, the anharmonicity... 

A detailed analysis of the combination bands of hydroxyl groups on sifica (3748 cm ) has been provided by Bumeau and Carteret (613). They ranked the different combination modes by their intensity in relation to the intensity of the first overtone. The only combination band with an intensity comparable to the intensity of the first overtone (40—50%) is that of the (v(OH)+6(OH)) combination mode. The SiOH bending mode is coupled with Si—O—Si stretching vibrations, thus inducing two surface modes with bending character, at 760 and 835 cm As a result, the combination mode is spfit into two components, at 4516 and 4582 cm. For the... 

The first overtone of a free hydroxyl group in dilute CCI4 solution or a low-density gas is at about 7090 cm (1410 nm). This peak is at different positions for primary, secondary, and tertiary alcohols, as seen in Figure 5.1. Primary and secondary butanols can be split into doublets by rotational isomerization. The splits are better seen in Figure 5.2, in the second derivation spectra of the same spectral region. Maeda et al. observed an additional peak in the first overtone region when they subtracted the spectrum at a lower temperature from that at a higher one. They felt that temperature effects further separated species that were weakly bonded to the carbon tetrachloride solvent and a terminal free OH of a self-associated species. 

In hydrogen-bonded alcohols, there is a broad peak in the 1460-1600 nm region (6850 cm -6240 cm ), which has been generally attributed to the first overtone of the hydroxyl. This bonded OH peak appears to be broader as a first overtone than its fundamental, as was observed in a series of spectra taken at different temperatures. On the other hand, the nonbonded OH first overtone is stronger relative to the bonded species than it is in the fundamental, and can be readily detected even when not visible in the fundamental region. Figure 5.3 illustrates the differences between the bonded and nonbonded hydroxyls of methanol. The broad bonded OH is indistinct in the neat spectrum, whereas the nonbonded OH in a dilute carbon tetrachloride solution is very sharp and strong. 

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... 

Carbohydrates in general may have a free OH-stretch absorption near 6940 cm (1440 nm). This band has been reported in crystalline sucrose, for example, and has been assigned specifically to the C4 hydroxyl within a crystalline matrix. Trott et al. discuss four different OH first overtone bands in carbohydrates in different solvent systems, using a monomer (glucose) and its polymer (glycogen) as models. 

The first overtone of the nonbonded hydroxyl peak of a hydroperoxide in dilute solution is far enough removed from that of acids and alcohols that it can be used for quantitative analysis. The peak is at about 6850 cm (1460 nm) as compared to 7100 cm (1410 nm) for alcohols. The spectrum of cumene hydroperoxide shown in Figure 5.7 shows a splitting of the hydroxyl... 

In the mid-infrared region, the amine NH-stretch absorption is weak relative to primary alcohols, about 1-2 1/mol-cm compared to 50-100 1/mol-cm. However, the intensity of the first overtone of aliphatic amines is of the same order of magnitude as the fundamental. For example, n-butyl amine s first overtone has an absorptivity of 0.61/mol-cm compared to 2.41/mol-cm for the fundamental. The overtones of amines and hydroxyls are of approximately the same magnitude, and it may be easier to detect an amine in the presence of alcohols in the near-infrared than the mid-infrared. 

The first overtone of P-H stretching was found at 5288 cm (1891 nm) in a number of organo-phosphorus compounds. It is slightly more intense than the S-H absorption, having a molar absorptivity of about 0.24 1/mol-cm. It is described as being more diffuse and less sharp, however. The POH group is observed in phosphorothioic acids. The absorption is significantly shifted relative to the hydroxyl in alcohols, as it is in mid-infrared. The near-infrared (NIR) peak appears at about 5241 cm (1908 nm). 

Detailed near-infrared spectra of PET exposed to different relative humidities indicated three different subbands of the first overtone of water at 7080 cm 7010 cm and 6810 cm The comparison with the water spectmm of bulk water suggested that most of the water is only weakly bonded with PET (89). The analysis of difference spectra of dry nylon and nylon exposed to different humidities, indicated that there were distinct populations of hydrogen-bonded water in it (90). Recently, Musto et al. (91) investigated the nature of molecular interactions of water in epoxy resins by means of near-infrared spectroscopy as proposed by Eukuda et al. (89,90). They found three subbands at 7076 cm 6820 cm and 6535 cm evidencing two kinds of water adsorbed in the polymer (mobile water localized in micro vide and water molecules firmly bonded to the network). However, hydroxyl groups of epoxy may complicate the analysis of water content in polymers because they absorb also in the same overtone region as water. 

What are the wavelengths of the first and second overtone bands of the hydroxyl absorption band at 3673 cm" of vitreous silica (watch the units)... 

The ionic hydroxyl group in aqueous solutions has its own specific absorption bands with first and second overtone bands at 7040 cm (1421 nm) and 1034 cnr (967 nm), respectively. - Also, a broad band centered at about 11(X) nm has been attributed to the binding of two water molecules to the hydroxide ion. As the hydroxide concentration is increased above 5 molal, the water peaks at 6900 cm (1450 nm) and 1025 cm (976 nm) decrease, and the hydroxide ion peaks at 1421 and 967 nm become prominent. This effect is complicated, however, and involves water activity and the ability of the solutes to confine the bulk solvent within hydration spheres. KOH solutions do not behave the same as NaOH solutions, and the effects of the cation on the water absorption need to be accounted for to obtain good measurements of hydroxide. Figure 6.7 illustrates the prominent ionic hydroxyl peak near 7040 cm (1421 nm). It is especially noticeable in the 50% solution. 

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