Ethanol 2003 overtone

H. L. Fang and R. L. Swofford, Molecular conformers in gas phase ethanol A temperature study of vibrational overtones. Chem. Phys. Lett. 105, 5 11 (1984). 

Figure 23 A proposal for dephasing in ethanol by solvent-assisted intramolecular vibrational redistribution (IVR). The yym-methyl stretch is initially excited, but rapidly equilibrates with one or more modes within kT (the ayym-methyl stretch and/or CH bend overtones). Dephasing occurs with this rapid equilibration time Tivr- However, significant population remains in the sym-methyl stretch after equilibration. Relaxation from this group of state to lower states causes the final relaxation of the population to zero, which is measured as Tj in energy relaxation experiments. (Adapted from Ref. 7.)...

A nonlinear molecule with n atoms generally has 3n — 6 fundamental vibrational modes. Water (3 atoms) has 3(3) -6 = 3 fundamental modes, as shown in the preceding figure. Methanol has 3(6) - 6 = 12 fundamental modes, and ethanol has 3(9) - 6 = 21 fundamental modes. We also observe combinations and multiples (overtones) of these simple fundamental vibrational modes. As you can see, the number of absorptions in an infrared spectrum can be quite large, even for simple molecules. 

In the region of NIR FT Raman spectroscopy, at F = 5000...10000 cm , substances with X-H bonds (X = any element) show overtones and combinations of the normal frequencies. They may have considerable intensity as demonstrated by Fig. 3.5-3 with the NIR absorption spectra of liquid H2O, D2O, ethanol, and cyclohexane. The linear decadic absorption coefficient a of water at a Raman shift of about 2500 cm is of the order of 10. The transmission of a layer of d = 1 cm is given by ... 

P. Tuomikoski. Suomen Kemistilehti 21B, 59-6 1 (1948). IR solvent effect on second overtone of ethanol in acetone, CHCla, CeHe, etc. 

In another study (13), a semicarbazide coating was prepared by dissolving 100 mg of semicarbazide hydrochloride in 10 ml of 1 1 acetone/ethanol, followed by the addition of 5 ml of 6 M ammonia solution. The clear solution was allowed to stand to ensure neutralization of the hydrochloric acid before application to the crystal electrodes. The coated 9 MHz crystal was driven at its third overtone of 27 MHz. The detector had a sensitivity of 12.4 Hz jjI and a linear response in the concentration range 5.50 tjg 1. the highest response was obtained in a dry nitrogen stream at a flow rate of 90 ml min, and decreased to about 33% in a wet air stream (58% relative humidity) at a flow rate of 200 ml min". The relative sensitivities to the potential intererences studied (chloroform, acetaldehyde, ethanol, acetone, n-octanol, ethyl-butyrate and n-hexylacetate) ranged from 0.000000 to 0.000029 compared to a relative response of one to acetoin. 

Figure 3. Changes in the normalized third overtone resonance frequency, (black line), and dissipation, AD (gray line), during adsorption of PLL(20)-g[3.5]-PEG(5) onto a Si02 sputter-coated surface for the HEPES—methanol system (polymer injection at arrow number 2). Before injection of the polymer the baseline of the Si02-coated quartz crystal was measured in methanol and subsequently in HEPES buffer solution. The exchange of methanol for HEPES buffer solution is indicated by the arrow number 1. The measurement chamber was rinsed with polymer-free aqueous HEPES buffer solution 30 min after polymer injection (arrow number 3). Subsequently, the aqueous HEPES buffer solution was replaced by methanol, and the resonance frequency fo and the dissipation factor D were measured again (arrow number 4). The reproducibility of the A/o and KD shifts upon solvent changes was tested by replacing methanol by aqueous HEPES buffer solution (arrow number 5). This measurement protocol for the methanol solvent system was repeated and applied to the other two solvent systems, ethanol and 2-propanol.

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... 

Due to the difference in acidity of phenol relative to simple aliphatic alcohols, its spectra in different solvents are quite different from those of methanol and ethanol. In solvents of low hydrogenbonding capability, phenol shows both free and bonded OH first overtones because the solvent cannot bond the phenol completely. In solvents that are more capable of hydrogen bonding, phenol behaves like the alcohols, except that the sequence is accelerated for example, the spectrum of phenol in N, A-dimethylformamide appears similar to the spectrum of ethanol in pyridine. 

Following Overton s early observation of the direct relationship between the efficacy of an alcohol and oil/water partition coefficient of that alcohol, most research concerning the mechanism of action of ethanol and similar anesthetics has focused on the interior of neuronal membranes and model membrane systems. The lipid perturbation hypothesis simply states that the effects of alcohols result from changes in the fluidity of the interior of the membrane. The observed changes in membrane order, however, are usually small or occur at nonphysiological concentrations of alcohol. ... 

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