Equilibria between the tautomers of heterocyclic azoles like 1 , 2 , and 4 have been studied many times. We here show that the application of density functional methods in combination with matrix IR spectroscopy is a useful tool for determining which protomer is preferred in rare gas matrices and thus also can be assumed to dominate in the gas phase. Photolysis of tetrazole (4) in cryogenic matrices allows the IR-spectroscopic identification of a new CHzNz isomer, the long soughtafter nitrilimine HCNNH (6). Flash pyrolysis of 4 also yielded nitrilimine. Upon irradiation this species is converted to a second, previously unknown "isomer", an HCN/NH complex 13. Nitrilimine (6) can also be generated by photolyzing 1,2,3-(1) and 1,2,4-triazole (2) in Ar matrices. The complex photochemistry of all three heterocyclic precursor compounds was unveiled. Our findings were supported by isotopic substitution experiments and by high-level a b initio calculations. Moreover, the IR bands of iminocyanide HNCN were tentatively assigned. This radical has up to now not been observed in a matrix.Recently, we reported on the results of matrix isolation studies of the decomposition of 1,2,3-triazole (1) and 1,2,4-triazole (2)['1. Both the photolysis of 1 and the flash pyrolysis of 2 caused the loss of a nitrogen molecule. While in the IR spectra of the photolysis products of 1 only the two strongest absorptions of the previously unknown hydrogen cyanide N-methylide HCNCH2 (3) could be found, the thermolysis of 2 turned out to be more efficient for the structural elucidation of 3. Five fundamental vibrations and two overtones of this interesting intermediate were observed, their positions being in accordance with ab initio calculations. Mono-, di-, and trideuterated 2 were synthesized and upon pyrolysis yielded the anticipated isotopically substituted products. In continuation of these investigations we also studied tetrazole (4). In addition, we intensified our efforts to clarify the complex photochemistry of 1,2,3-triazole (1) and examined the photolysis of 1,2,4-triazole (2). The outcome of these experiments was then substantiated by photolysis studies with cyanamide, diazomethane, and hydrogen cyanide. Structure of TetrazoleAlthough tetrazoles have been known for over a centuryf2], there is still considerable and growing interest in this field[3-51. Recent developments include the patented use of tetrazole in air bag@] and pharmaceutical applications [']. For any study of the decomposition of tetrazole the equilibrium between the three conceivable tautomers 4a-c is of vital importance. While in the solid state the 1H form 4b is clearly preferred18] the situation in solution is less clear-cut [9]. Depending on the nature of the solvents and the experimental method used, either lH- ['l or 2H-tetrazole~[~~] appear to be favored. In the gas phase only 4a was detected by microwave spectroscopy ["], but the presence of 4b could not be ruled out. Mass spectrometric investigations indicated that the vapor consists of a mixture of both arom...

Flash pyrolysis of a gas mixture containing iodine, oxygen and argon yields the hitherto unknown iodine superoxide, which can be identified by its UV absorption (λ max= 254 nm) after trapping the pyrolysate at 12 K. Irradiation converts iodine superoxide into iodine dioxide, identified by its IR and UV spectra.

Die Blitzpyrolyse eines Gasgemischs von Brom, Sauerstoff und Argon liefert Bromsuperoxid. Dieses kann nach Abschrecken des Pyrolysats auf 12 K IR‐ und UV‐spektroskopisch identifiziert werden. Matrixphotolyse gestattet die Umwandlung von Bromsuperoxid in Bromdioxid. Letzteres kann durch geeignete Wahl der Wellenlänge wieder in Bromsuperoxid zurückverwandelt werden. Bereits bei Raumtemperatur bildet sich in einem Br2/O2/Ar‐Gasgemisch eine nachweisbare Menge an Bromsuperoxid.

The complex photochemistry of matrix-isolated dihaloform-produces the hydrogen-bonded complexes HalNCO/HHal oximes 8a, 8b, and 8c, which are transformed upon irradi-12a-c', which can be regarded as intermediates on the phoation into the new hydrogen-bonded complexes HalCN... tochemical pathway to the final product isocyanic acid HOHal 10a-c', identified by their IR spectra, has been un-HNCO ( l ) , and are tentatively identified by their IR absorpveiled. An earlier assignment of the experimental IR absorp-tions. Our findings are supported by deuteration experiments tions to isofulminic acid HONC (4) must be withdrawn. A and theoretical calculations using the density functional mecompetitive reaction to the formation of complexes 10a-c'

The C 2 BrN potential-energy surface was explored by means of density functional calculations (B3LYP/6-311 G*). Six open-chain and three cyclic structures were found as minima. Photolysis of matrix-isolated (Ar, 12 K) cyanogen azide N 3 CN (1) in the presence of BrCN (4) led, in addition to the expected nitrenes NCN (2) and CNN (3), to the bromine-containing singlet isocyanocarbene BrCNC (11 s). The formerly unknown 11 s was identified and characterized by comparison of the experimental and calculated IR absorptions. Our findings were substantiated by isotopic substitution experiments. Only a tentative assignment could be made for the questionable singlet bromocyanocarbene BrCCN (6 s). In similar matrix-isolation experiments with cocondensed 1 and ClCN (5), the chlorine analogue singlet chloroisocyanocarbene ClCNC (7 s) was also identified by IR spectroscopy.