Spectrum 1.2,3.4-tetrazine

X4)2 (s-tetrazine dimer) electronic spectrum, 397tf... 

H-Azepine derivatives form a diene complex with tricarbonyliron, leaving uncomplexed the third of the double bonds. If the 3-position is substituted, two different such complexes are possible, and are in equilibrium, as seen in the NMR spectrum. An ester group in the 1-position of the complex can be removed by hydrolysis, to give an NH compound which, in contrast to the free 1/f-azepine, is stable. The 1-position can then be derivatized in the manner usual for amines (Scheme 22). The same tricarbonyliron complex can, by virtue of the uncomplexed 2,3-double bond, serve as the dienophile with 1,2,4,5-tetrazines. The uncomplexed N-ethoxycarbonylazepine also adds the tetrazine, but to the 5,6-double... 

The 13C NMR spectrum (75 MHz) is available only for pyrimido[ 1,2-3]-l,2,4,5-tetrazin-6-ones 8, which showed the characteristic carbonyl carbon resonance at 8 156.8ppm <2004JCM399>. 

Variable temperature 13C NMR spectroscopy has shown that the 1,2,3,4,5,6-hexamethyl-hexahydro-l,2,4,5-tetrazine (57) is the trans isomer with the two C-methyl groups equatorial and the N-methyl groups symmetrically diaxial and diequatorial. The observed dynamic 13C NMR effects are consistent with (57a) and (57b) as the major conformation and probably (57c) as the minor one. The first coalescence represents the freezing out of (57c) while the (57a) (57b) interconversion remains fast. The nitrogen inversion barrier was found to be 32.2 kJ moF1 <79JCS(P2)98l). The He(I) photoelectron spectrum of (57) confirmed the NMR data (80JCS(P2)9l). 

Figure 9.46 shows an example of a fluorescence excitation spectrum of hydrogen bonded dimers of. v-tctrazinc (1,2,4,5-tetraazabenzene). The pressure of s-tetrazine seeded into helium carrier gas at 4 atm pressure was about 0.001 atm. Expansion was through a 100 pm diameter nozzle. A high-resolution (0.005 cm-1) dye laser crossed the supersonic jet 5 mm downstream from the nozzle. 

The mass spectra of quinones often give rise to M -(-1 and M -(- 2 ions by addition of hydrogen and this has been considered to be due to reaction with water on the surfaces of a hot inlet system (Aplin and Pike, 1966). Similar results, which were temperature- but not pressure-dependent, have been obtained with a direct inlet system (Ukai et al., 1967 Dean and Houghton, 1968 Oliver and Rashman, 1968). The mass spectra of vinylchlorins (Budzikiewicz and Drewes, 1968) and of s-tetrazines (Yates et al., 1968) all showed M-f2 ions, whilst the strongest peaks in the mass spectrum of N-bromosuccinimide were due to succinimide (Bentley and Johnstone, 1968b). 

Since the initial demonstrations of the participation of substituted l,2,4,S-tetrazines ° and oxa-zoles ° ° in [4 + 2] cycloaddition reactions with alkene and alkyne dienophiles, the investigation and application of the Diels-Alder reactions of heteroaromatic systems possessing reactive azadienes have been pursued extensively. A number of general reviewshave treated the spectrum of heteroaromatic azadienes that participate in [4 + 2] cycloaddition reactions and many of the individual heteroaromatic systems have been reviewed separatcly. ° ° An extensive account was published recently and should be consulted for descriptions of the [4 + 2] cycloaddition reactions of the common heteroaromatic azadienes that have been observed to date. 

Detailed measurements of the s-tetrazine gas-phase spectrum were made. With these data, measurement of the absolute Stokes shift S(t) is possible. Because the Stokes shift is zero in the absence of solvent nuclear dynamics, the magnitude of the Stokes shift at the earliest times represents the amount of relaxation within the experimental time resolution. The steady-state absorption and fluorescence spectra were also measured to provide an independent value of the equilibrium Stokes shift S< With this data, the absolute solvation response function... 

Figure 21-14 shows visible spectra for 1,2,4,5-tetrazine that were obtained under three different conditions gas phase, liquid phase, and aqueous solution. Notice that in the gas phase, the individual tetrazine molecules are sufficiently separated from one another to vibrate and rotate freely, so many individual absorption peaks resulting from transitions among the various vibrational and rotational states appear in the spectrum. In the liquid state and in solution, however, tetrazine molecules are unable to rotate freely, so we see no fine structure in the spectrum. Furthermore, because frequent collisions and interactions between tetrazine and water molecules cause the vibrational levels to be modified energetically in an irregular way. the spectrum appears as a single broad peak. The trends shown in the spectra... 

Figure 24-14 Typical ultraviolet absorption spectra. The compound is 1,2,4,5-tetrazine. In (a), the spectrum is shown in the gas phase, where many lines due to electronic, vibrational, and rotational transitions are seen. In a nonpolar solvent (b), the electronic transitions can be observed, but the vibrational and rotational structure has been lost. In a polar solvent (c), the strong intermolecular forces have caused the electronic peaks to blend together to give only a single smooth absorption peak. (From S. F. Mason, J. Chem.

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