Spectroscopic basicity measurement

Section 5.1 presents the fundamental method as the heart of the chapter— the statistical thermodynamics approach to hydrate phase equilibria. The basic statistical thermodynamic equations are developed, and relationships to measurable, macroscopic hydrate properties are given. The parameters for the method are determined from both macroscopic (e.g., temperature and pressure) and microscopic (spectroscopic, diffraction) measurements. A Gibbs free energy calculation algorithm is given for multicomponent, multiphase systems for comparison with the methods described in Chapter 4. Finally, Section 5.1 concludes with ab initio modifications to the method, along with an assessment of method accuracy. 

The 1,3,4-thiadiazole ring system, with three heteroatoms, does not exhibit tautomerism in its fully conjugated form. However, when certain substituents are present, tautomerism is possible. l,3,4-Thiadiazolin-2-ones (39 X = 0) and -2-thiones (39 X = S) exist in the oxo and thione forms, respectively, as shown by spectroscopic and LCAO-MO calculations. 2-Amino-l,3,4-thiadiazoles exist in the amino form in solution and in the solid state the Kt value is 10s as shown by basicity measurements. UV spectroscopy and LCAO-MO calculations show that the amino tautomer is also the main species when there is an alkoxy group in the 5-position (40 R1 = alkoxy, R2 = NH2), or if the exocyclic nitrogen atom is part of a hydrazone group (40 R2 = NHN=CR2). 

Thermodynamic parameters for the protonation of 2,3,5,6-tetramethylpyrazine 1,4-dioxide have been obtained from basicity measurements at 25,40,60,80, and 90° using a spectroscopic method (746). 

Acetonitrile, CH3CN, is an aprotic, weakly basic solvent with a moderate dielectric constant (e = 36.64) that allows separation of ion pairs to free ions. Moreover, it is transparent in the ultraviolet region, allowing the use of spectroscopic pK measurements. 

Spectroscopic methods like such as ir require finger printing vibration frequencies, which are specifically crystal- or amorphous-sensitive. The nmr method basically measures mobility of nuclei. Above the glass transition temperature of a semicrystalline sample, the amorphous part becomes mobile and the crystalline part is still rigid the nuclei in these two parts possess different mobilities and thus can be detected by nmr (for detailed information, see Ref 5). 

Figure C3.1.1. The basic elements of a time-resolved spectral measurement. A pump source perturbs tlie sample and initiates changes to be studied. Lasers, capacitive-discharge Joule heaters and rapid reagent mixers are some examples of pump sources. The probe and detector monitor spectroscopic changes associated with absorjDtion, fluorescence, Raman scattering or any otlier spectral approach tliat can distinguish the initial, intennediate and final...

Recently Stamhuis et al. (33) have determined the base strengths of morpholine, piperidine, and pyrrolidine enamines of isobutyraldehyde in aqueous solutions by kinetic, potentiometric, and spectroscopic methods at 25° and found that these enamines are 200-1000 times weaker bases than the secondary amines from which they are formed and 30-200 times less basic than the corresponding saturated tertiary enamines. The baseweakening effect has been attributed to the electron-withdrawing inductive effect of the double bond and the overlap of the electron pair on the nitrogen atom with the tt electrons of the double bond. It was pointed out that the kinetic protonation in the hydrolysis of these enamines occurs at the nitrogen atom, whereas the protonation under thermodynamic control takes place at the -carbon atom, which is, however, dependent upon the pH of the solution (84,85). The measurement of base strengths of enamines in chloroform solution show that they are 10-30 times weaker bases than the secondary amines from which they are derived (4,86). 

In general, physical methods have been used to study tautomerism more successfully than chemical methods, and, of the physical methods, those involving measurements of basicities and ultraviolet spectra are the most important, followed by those involving measurement of infrared and proton resonance spectra. An attempt is made here to delineate the scope and to indicate the advantages and disadvantages of the various methods. A short review by Mason of the application of spectroscopic methods appeared in 1955. Recently a set of reviews on the applications of physical methods to heterocyclic chemistry has appeared, which treats incidentally the determination of tautomeric structure. 

The main problem in Eas0 vs. correlations is that the two experimental quantities are as a rule measured in different laboratories with different techniques. In view of the sensitivity of both parameters to the surface state of the metal, their uncertainties can in principle result of the same order of magnitude as AX between two metals. On the other hand, it is rare that the same laboratory is equipped for measuring both single-crystal face is not followed by a check of its perfection by means of appropriate spectroscopic techniques. In these cases we actually have nominal single-crystal faces. This is probably the reason for the observation of some discrepancies between differently prepared samples with the same nominal surface structure. Fortunately, there have been a few cases in which both Ea=0 and 0 have been measured in the same laboratory these will be examined later. Such measurements have enabled the resolution of controversies that have long persisted because of the basic criticism of Eazm0 vs. 0 plots. 

The most basic method for the determination of the methylxanthines is ultraviolet (UV) spectroscopy. In fact, many of the HPLC detectors that will be mentioned use spectroscopic methods of detection. The sample must be totally dissolved and particle-free prior to final analysis. Samples containing more than one component can necessitate the use of extensive clean-up procedures, ajudicious choice of wavelength, the use of derivative spectroscopy, or some other mathematical manipulation to arrive at a final analytical measurement. A recent book by Wilson has a chapter on the analysis of foods using UV spectroscopy and can be used as a suitable reference for those interested in learning more about this topic.1... 

The identification of anionic polyfacrylic acid) sizes can be carried out by staining with a fluorescent cationic dye (Cl Basic Orange 14) followed by spectroscopic measurement of excitation wavelength and fluorescence emission [195,196]. Such methods can also be used (with Cl Basic Orange 14 or Cl Basic Red 1) to detect and estimate carboxymethylcellulose, poly(vinyl alcohol) and starch derivatives [197]. 

Byme L., Lau K.T., Diamond D., Monitoring of headspace total volatile basic nitrogen from selected fish species using reflectance spectroscopic measurements of pH sensitive films, Analyst 2002 127 (10) 1338-1341. 

A value for AGSX for FL can be obtained from analysis of the rate of reaction of the carbene with methyl alcohol within the spin-specific reaction framework identified above. Basically, the observed rate of reaction of 3FL with the alcohol is a measure of the amount of XFL in the equilibrium mixture. This gives (28) which links Kemeasured rate constant. The equilibrium constant in turn gives AGSX and, when combined with the picosecond spectroscopic results, and kTS (Table 8). 

Results and data gathered on mass spectroscopy of various indole alkaloids have been summarized by Hesse (320). The derivation of the characteristic fragments of indolo[2,3-a]quinolizidines has been interpreted by Gribble and Nelson (321), who investigated C-3, C-5, C-6, C-20, and C-21 deuterated derivatives of octahydroindolo[2,3-a]quinolizine (1). Kametani et al. have observed and proved, with labeled compounds, a methyl transfer from the ester function of reserpine derivatives to the basic nitrogen atom during mass-spectroscopic measurement (322). 

These basic ideas are applicable to all the measurements made using the spectroscopic methods discussed previously. 

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