1.2.4- Oxadiazoles spectra

Figure 24 HPLC spectrum resulting from the incubation of the oxadiazole with monkey liver microsomes.

The chemiluminescence spectrum matches the fluorescence of both thianthrene and 2.5-diphenyl-1.3.4-oxadiazole (430 and 340 nm, respectively). 

The 170 NMR spectrum of 4,5-diphenyl-l,3,4-oxadiazolium-2-olate was reported earlier <1996CHEC-II(4)268>. Since then, no report on 170 NMR spectra of 1,3,4-oxadiazoles has been published. 

A number of other derivatives were tested in order to evaluate their biological activity. The spectrum of the activity is very broad. Oxadiazole derivatives were active inhibitors of several enzymes 150 <1996BML2693>, 150a... 

Derivatives of 5-alkyl-2-(l,3,4-oxadiazol-2-yl)thiophenes 168 were synthesized and their photochromic and fluorescent properties studied. A solution of the photochrome was subjected to irradiation over a wide range, including the lines of the mercury spectrum at 313, 365, 405, 436, 546, and 578 nm. It was discovered that the open form of compounds 168 showed strong fluorescence <2002CHE165>. 

The microwave spectrum of 1,2,4-oxadiazole has been reported <67NAT1301>. 

Nuclear magnetic resonance studies on meso-ionic l,2,4-triazol-3-ones (200) were used to examine their relationship to the alternative l,3,4-oxadiazol-2-imine structure (153). The effect of solvent polarity upon the ultraviolet spectrum of anhydro-3-hydroxy-1,4-diphenyl-1,2,4-triazolium hydroxide (200, R = = Ph, R = H) has been discussed... 

The NMR spectrum of unsubstituted oxadiazole consists of two peaks corresponding to the two different protons present in the 5 resp. 

Muchall et al. (98CC238) have recently investigated the gas-phase thermolysis of 2,5-dihydro-2,2-dimethoxy-2,5,5-trimethyl-l//-l,2,4-oxadiazole (75) by PE spectroscopy. Decomposition of 75 was induced by means of a continuous wave (CW) C02 laser as directed heat source at 26 W, which corresponds to a temperature of 500 50°C. When the PE spectra of acetone, tetramethoxyethene, and dimethyl oxalate were subtracted from the pyrolysis spectrum, a sim-ple spectrum remained that could be identified as that of dimethoxycarbene. Thermolysis in solution (94JA1161) had shown formation of tetramethoxyethene, and FVP experiments (92JA8751) gave dimethyl oxalate, both of which arise from the common precursor, dimethoxycarbene. Thermolysis of oxadiazolines similar to 75 in solution affords dialkoxycarbenes via an intermediate carbonyl ylide (94JOC5071). 

Unlike 1,2,4- and 1,3,4-oxadiazoles, the 1,2,4-triazole heterocycle has the possibility of existing in different tautomeric forms (Scheme 20). X-ray crystal analysis of the peptide-based triazole 63 showed only one tautomerJ102 Additionally, the 13C NMR spectrum for this compound indicated only one tautomeric form since the signals due to C3 and C5 appeared as sharp peaks. This is not the case for some other peptidergic triazoles (Table 5) where peaks corresponding to C3 and C5 were broad and of low intensity or not visible. Differences in tautomeric preferences have been explained by the ability of some triazole-containing peptides to form intermolecular hydrogen bonds and thus stabilize one tautomer over the other. 

Treatment of isothiocyanate 2 with nicotinoyl hydrazide in boiling 1,4-dioxane afforded 84 in 85% yield this cyclized on treatment with yellow mercury(II) oxide in ethanol67,69 to oxadiazole 85. The mass spectrum of, and H-n.m.r. data for, 85 were also reported.68 Interestingly, in this case, the cyclization of 84 occurred preferentially, with the formation of the 1,3,4-oxadiazole derivative 85, in contrast to literature70 detailing similar... 

Boulton and co-workers156 identified peaks in the mass spectrum of 3,5-bis(p-chlorophenyl)-l,2,4-oxadiazole 4-oxide corresponding to the fragments ArCNO+, ArCN+, and ArCO+ in addition to an intense parent peak. 

In the 15N NMR spectrum of the systems (29 X = O or S) the chemical shifts of the nitrogens at position 1 correspond reasonably well with the nitrogen signals in benzo[c]-l,2,5-oxadiazole ( —35.6 ppm) and benzo[c]-l,2,5-thiadiazole (49.1 ppm). The nitrogens at position 3 appear to be deshielded by the effect of nitrogen at position 4. In the thiadiazine rings the pyridine-like nitrogens at... 

Enough 1,2,4-oxadiazoles and their reduced derivatives have been made that there is general agreement on where to expect the C=N absorption in the IR spectrum, i.e. 1560-1590 cm-1. In the partially reduced (oxadiazoline) ring the A2 double bond appears at 1550-1565 cm-1 and the NH bands at 3320 and 3220-3250 cm-1. In an amino derivative, 5-amino-3-phenyl-l,2,4-oxadiazole, the C=N bond absorbs at higher frequency (1660 cm"1) (63PMH(2)229>. 

The ESR spectrum of the anion radical derived from 2,5-diphenyl-l,3,4-oxadiazole has been determined and the hyperfine splittings are based on MO calculations (80AHC(27)3i). In the same review, hyperfine splittings are given for the radicals produced on oxidation by lead oxide of 3-substituted oxadiazolidine-2,5-diones. ESR spectra of radical cations derived from 3,4-disubstituted 1,3,4-oxadiazolidines have been described (74JA2916). 

In a second investigation of the microwave spectrum of 1,3,4-thiadiazole, 14N quadrupole coupling and centrifugal distortion for lines with J = 5 and / 50, respectively, are determined. These are compared with results obtained from 1,3,4-oxadiazoles and pyridazine. Comprehensive tables showing microwave transitions, the microwave spectrum and the a and n population from 14N quadrupole coupling data for 1,3,4-thiadiazole are also given (71JST(9)163> (see also Sections 4.01.3.2, 4.01.4.2.2.(i) and (iii), and 4.01 Table 2). 

The aromatic character of 1,3,4-thiadiazole can be demonstrated with the aid of micro-wave spectroscopy. Using the differences between the measured bond lengths and covalent radii, aromaticity, as shown by 7r-electron delocalization, diminishes in the order 1,2,5-thiadiazoles > thiophene > 1,3,4-thiadiazole > 1,2,5-oxadiazole (66JSP(19)283). The micro-wave spectrum was further refined by later workers (7lJST(9)l63). 

Figure 26 Emission spectra (PL, EL) in PC at room temperature of 40 wt% TPD donor solution with a 40 wt% of PBD acceptor added. The photoluminescence (PL) spectrum excited at 360 nm, the electroluminescence (EL) spectra (I, II) originate from the recombination radiation in a 60 nm thick film, taken at two different voltages. Absorption (Abs) and PL spectra (excitation at 360 nm) of (75wt% TPD 25wt% PC) and (75wt% PBD 25wt% PC) spin-cast films are given for comparison. Molecular structures of the compounds used are given in the upper part of the figure TPD [N,Nf-diphenyl-A v/V/-bis(3-methylphenyl)-l,l -biphenyl-4,4 diamine PBD [2-(4-biphenyl)-5-(4- er .-butylphenyl)l,3,4-oxadiazole PC[bisphe-nol-A-polycarbonate]. Adapted from Ref. 112.

The 1,3,4-oxadiazole system has an electronic spectrum equivalent to that of benzene and the maxima of the oxadiazole derivatives are only slightly shifted hypsochromically compared with benzene.103 Thus, for example, diphenyl and 2-phenyl-l,3,4-oxadiazole absorb almost identically at 248 ra.fi and p-terphenyl and 2,5-diphenyl-l,3,4-oxadiazole absorb at 276 and 280 mfi, respectively.103 The oxadiazole system is effective in the conjugative transmission of the effects of substituents.103 An extension of the system of conjugated rt bonds... 

On being heated, the oxadiazoles (39, 40) both yielded phenyl isocyanate and benzonitrile, but only the mass spectrum of (39) showed strong peaks corresponding to these pyrolysis products at m/e 103, 119 (J. L. Cotter and Knight, 1966). 

NH)-l,2,3-oxadiazoles. The l4N spectra of sydnones show a sharp signal which is superimposed on a broad resonance (111). The sharp signal at about +100 ppm may be assigned to the N -R moiety (Table XX), since the line-width is typical for such structures and because the spectrum of 3,4-diphenylsydnone-2-15N indicates (112) that the N-2 shift is +42 ppm. 

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