Hydrogen bond acidic Fourier transform infrared

This unit describes the attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopic method (AOCS, 1999a AOAC International, 2000), a novel method for measuring the total amount of fat with isolated trans double bonds. It is applicable to natural fats (ruminant fats) and processed fats and oils (partially hydrogenated fats and oils or refined vegetable oils) consisting of long-chain fatty acid methyl esters or triacylglycerols with trans levels >5%, as percent of total fat (AOAC International, 2000). 

Using diffuse reflectance Fourier transform infrared spectroscopy, Adhikari et al. studied the binding of oleic acid (88), triacylglycerol (89), and phosphatidylcholine (90) on silica gel at room temperature. Their interpretation was that oleic acid and triacylglycerols bind to silica surface hydroxyls via hydrogen bonding interactions between the carboxylate and ester carbonyls of these molecules, respectively. In contrast, phospholipids hydrogen bond to silica surface hydroxyls via the phosphate... 

Oximes and oxime ethers exist as a mixture of E and Z isomers with a relatively low difference of AG° and a moderate energy barrier to isomerization (<10 kcal mol-1) [28]. They show some similarities with imines and may interconvert at room temperature, spontaneously, by an acid- or base-catalyzed isomerization involving a nitronium ion, and photochemically [29,30]. Oxime ethers have been employed as amide surrogates in peptides where they display a marked Z-E isomerism which is mainly controlled by the formation of H-bonds, which stabilize a given isomer. As an example, the structure of pseudopeptide 6 was investigated by Fourier transform infrared spectroscopy (FTIR) and NMR spectroscopy which both showed that Z-6 is folded in a /Mike conformation by a strong bifurcate hydrogen bond whereas the E isomer adopts an extended conformation (Fig. 13.5) [31]. 

Kohler and Kirkland noted that for chromatographic column packings analyzed by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and chromatographic behavior, silica appears to possess associated and isolated surface silanol groups [ I9. The associated silanol (possessing weak-acid characteristics) is identified with a silanol hydrogen-bonded to its nearest neigh-... 

Variable-temperature diffuse reflectance infrared Fourier transform spectroscopy was used in conjunction with pyridine desorption studies to assess the acidity of a siliceous surface. An amorphous, porous silica substrate was investigated. The results contribute to an understanding of the acidic strength and the distribution of acidic sites on this material. A hydrogen-bonding interaction was observed between pyridine and the surface. Isothermal rate constants and an activation energy for the desorption process are reported and can be used as direct measures of surface site acidity. 

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