Hydrated spectrum

The bands due to water vapour are no longer useful in the analysis of the spectra. They are consequently eliminated by this subtraction procedure and only the spectrum of the dried sample D is kept, together with the hydration spectrum Hy-D-0.73 W. Subtraction of the water vapour features causes no degradation of the signal-to-noise ratio, as, in opposition to ATR spectra where features due to bulk HgO molecules are predominant, they have small intensities (they are nearly all free HgO molecules that do not establish H-bonds). The intensities of the bands of this hydration spectrum are proportional to their sensitivity... 

Figure 11.2 IR spectra of HA, in its dried state (D) when in equilibrium with the surrounding atmosphere with no water vapour (p/pQ = 0), and partially hydrated when in equilibrium with the surrounding atmosphere having an hygrometry p/pQ = 0.51 (Hy). The hydration spectrum" labelled Hy-D-0.73 W is equal to the subtraction to spectrum Hy of spectrum D and spectrum Wmultiplied by a coefficient equal to 0.73, which ensures the disappearance of the narrow bands due to water vapour. These bands are shown in spectrum W, which is the spectrum of the cell without sample but with an atmosphere with calibrated water vapour pressure p. ...

P correlation of two inorganic hydrated phosphates, (a) bnishite and (b) a bone, are shown [48]. In both cases two phosphorus sites that completely overlap in the 1D MAS spectrum are clearly visible in the 2D spectrum. 

The UV spectrum of quinazoline is less simple bands at 220, 267 and 311 all represent -rr - -rr transitions and a characteristic inflexion of low intensity at 330 nm represents the TT transition (63PMH(2)l). Quinazoline as cation (17) is 3,4-hydrated covalently, so... 

The salts of some enamines crystallize as hydrates. In such cases it is possible that they are derived from either the tautomeric carbinolamine or the amino ketone forms. Amino ketone salts (93) ( = 5, 11) can serve as examples. The proton resonance spectra of 93 show that these salts exist in the open-chain forms in trifluoroacetic acid solution, rather than in the ring-closed forms (94, n = 5, 11). The spectrum of the 6-methylamino-l-phenylhexanone cation shows a multiplet at about 2.15 ppm for phenyl, a triplet for the N-methyl centered at 7.0 ppm and overlapped by signals for the methylene protons at about 8.2 ppm. The spectrum of 93 ( = 11) was similar. These assignments were confirmed by determination of the spectrum in deuterium oxide. Here the N-methyl group of 93 showed a sharp singlet at about 7.4 ppm since the splitting in —NDjMe was much reduced from that of the undeuterated compound. 

The stabilizing influence in the hydrated cation is the amidinium resonance. If a solution of the cation is neutralized, a short-lived hydrated neutral molecule (4) (half-life 9 sec at pH 10) is obtained with an ultraviolet spectrum similar to that of the hydrated cation but shifted to longer wavelengths (5 m/ ). Supporting evidence can be derived from the anhydrous nature of the cation of 4-nitroiso-quinoline (pK 1.35), in which the nitro group has a similar electronic influence to that of the ring nitrogen atom N-I in quinazoline and where amidinium resonance is not possible. 

The structure of the unstable hydrated neutral molecule (4) was deduced from the similarity of its ultraviolet spectrum with that of tlie pseudo base (5), derived from (6) of known structure. This... 

As yet no quinazoline has been discovered which has any appreciable amount (say, 1%) of hydrated species in the neutral molecule,but several quinazolines were shown to contain a mixture of anhydrous and hydrated species in the cations. Anhydrous neutral molecules and anhydrous cations have an ultraviolet absorption spectrum of the general type C (Fig. 3) and hydrated cations, the type... 

A. Cations, on the other hand, which are a mixture of hydrated and anhydrous species have an intermediate spectrum of the type B B. The long-wavelength absorption band B is due to the anhydrous cation, and the short-wavelength absorption band B to the hydrated... 

The hydrated cation of quinazoline in dilute acid solution becomes dehydrated when the acidity of the solution is progressively increased. At Ho —4.3, the solution consists predominantly of the anhydrous cation with some anhydrous dication ( 7%). The ultraviolet spectrum of the anhydrous cation is similar to that of the neutral molecule (there is a small bathochromic shift) and it is also similar to that of quinazoline in anhydrous dichloroacetic acid. When the acid strength is further increased to Ho —9.4, the quinazoline dication is formed (pKa —5.5). 

The spectra of an organic compound in various solvents differ only in small detail so long as no serious interaction takes place between solute and solvent. Thus the spectrum of a substance in an aprotic solvent (e.g. cyclohexane) should be almost the same as that in water. When addition of water occurs across a C=N bond, the spectrum of the hydrate in water can be vastly different from the spectrum of the anhydrous substance in cyclohexane, and this test has been used on several occasions determine whether or not a neutral species... 

Another use for this solvent is exemplified by 1,4,5,8-tetraazanaph-thalene, the anhydrous species of which has a predicted i Ka value of — 2.7 (the observed pA in water is + 2.51). The spectrum obtained in anhydrous dichloroacetic acid is almost identical with that of the predominantly anhydrous neutral species determined in water, but quite different from the spectrum measured in dilute aqueous acid. Moreover, addition of water to the anhydrous dichloroacetic acid solution of this base caused the fine structure present in the spectrum of the neutral species to disappear and the band due to the hydrated cation (i.e. the spectrum obtained in water at pH 0.5) to appear. Addition of water to dichloroacetic acid solutions has been used to show that the cations of 3- and 8-nitro-l,6-naphthyridine20 are hydrated in aqueous acid at pH 0.5. 

Thus, a methyl group placed at the site of hydration decreases the proportion of the hydrated species and, hence, shifts both the ultraviolet spectra (cf. Fig. 2A and B) and the ionization constant of the substance towards normality. A valuable means for locating the site of hydration, therefore, is to introduce a methyl group in various likely places until the anomalous spectrum is lost and the spectrum of the predominantly anhydrous species restored. The effect of such a methyl group on the pjfiT value is also revealing because a decrease in the amount of the hydrated species causes a decrease in the p value,... 

Anhydrous quinazoline hydrochloride absorbs one molecule of water readily, and. the product is difficult to dehydrate completely even in a high vacuum at 60°. Infrared spectral data suggest that this water is covalently bound because of (o) the absence of several bands in the spectrum of the hydrate which are present in the spectrum of the anhydrous hydrochloride and (6) the presence of extra bands at 1474 and 1240 cm that have been attributed to C— H and O— H bending vibrations of the — CHOH group. 

The anomalous behavior of quinazoline was first discovered by Albert et who made the surprising observation that 4-methyl-quinazoline 2.5) was a weaker base than quinazoline (pA 3.5). Mason then observed that the ultraviolet spectrum of the quinazoline cation was abnormal but that the spectrum of 4-methylquin-azoline was normal (see Fig. 2). These anomalies led to the suggestion that water adds covalently to the cation of quinazoline to give 12 (R = H). The occurrence and position of hydration were confirmed by a detailed study of the ultraviolet and infrared spectra of the anhydrous and hydrated hydrochlorides and by mild oxidation of the cation to 4(3 )-quinazolinone. Using the rapid-reaction technique (the continuous-flow method), the spectrum of the unstable... 

---