Ammonia extinction coefficients

Reversible bleaching experiments, in which the R-band decays to form the V-band, give a ratio of AZ>85o/AZ>65o = 0.92 at —78° C. Assuming that this is a dimer - monomer conversion, we thus obtain R-band = (4.9 X 104) (2) (0.92) = (9.0 it 2) X 104, for the molar extinction coefficient of the dimer at its absorption peak, at — 78 ° C. Table II summarizes these values and corresponding exinction coefficients in ammonia. 

Liquid-Ammonia Solutions—Spectrum and Photolysis of Potassium Amide. Figure 6 shows the spectrum of a bleached solution of potassium in ammonia, at —48°C. Only a single peak due to the amide is seen at 335 m/z before the ammonia cutoff. The molar decadic extinction coefficient, as determined by titration, is e = (1.0 =fc 0.2) X 104 at —48 ° C. and 335 m/z. (A careful Beer s Law check on the 335 m/z band has not yet been made.) The temperature coefficient of the peak is —20 cm. 1 deg. 1 (3), which is close to the value given above for the longwave band in decomposed ethylamine solution. 

The progress of the reaction was followed by monitoring the decrease in formaldehyde concentration with time. Previous studies used the hydroxylamine hydrochloride method of analysis (5 -55), but this was avoided in the current study as it requires tedious pH titrations. Instead, a colorimetric method was used that was first developed by Nash (55), involving formation of 3,5-diacetyl-1,4-dihydrolutidine, by reaction of formaldehyde with ammonia and acetyl acetone at neutral pH. The cyclic product absorbs at 412 nm with a molar extinction coefficient of 8,000 (55). Other colorimetric methods cannot be used as they all involve very strongly acidic or basic media (55), which would force the phenol-formaldehyde reaction to completion. 

G-values of ammonia were measured after reduction of amino-acids and peptides (7, 9), and of some proteins such as histones that contain relatively low amount of sulfur residues and histidines (26 ). They may reach the G(e aq) value. The high reactivity of e aq toward peptidic bonds is also responsible for the progression of the rate constant with the number of carbonyl groups in small peptides (table 1). The deaminated radical was observed by pulse radiolysis (absorption maximum around 430 run, extinction coefficient at this wavelength ca. 1100 mol 1 cm (3). 

There are several probe molecules for which infrared spectroscopy can differentiate between adsorption on Bronsted and Lewis acid sites and even estimate the amounts adsorbed. Pyridine is the most widely used because it gives well-resolved bands when protonated by Bronsted acid sites (e.g., 1540 and 1640 cm ) or when coordinated to Lewis acid sites (1450 and 1620 cm ). The values of extinction coefficients are available in the literature [121] for these bands, which makes possible semiquantitative measurements, separately, of Lewis and Bronsted sites. Ammonia, with a smaller kinetic diameter that enables it to reach more easily the acid sites in smaller pores, can also be used to distinguish betwen Bronsted and Lewis acid sites however, the use of ammonia is less reliable, mainly because the resulting IR bands overlap each other [122]. Another base that can distinguish between Bronsted and Lewis acid sites is quinoline because its size is greater than that of pyridine quinoline can also be used to differentiate between acid sites at the external surface and those in pores smaller than its kinetic diameter (6 A). Bronsted sites can be selectively measured with IR methods by using substituted pyridines as probe molecules [123]. 

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