Nitrenes matrix isolation spectroscopy

These studies reveal a general problem in matrix isolation spectroscopy, that different species have very different sensitivities to different spectroscopic methods. EPR spectroscopy is a very sensitive tool for the detection of triplet phenyl nitrene but is totally blind towards a dehydroazepine. The dehydroazepine has a distinctive ketenimine chromophore enabling facile IR detection but no such characteristic vibration exists for triplet phenyl nitrene. Furthermore the molar absorptivities of the molecules of interest are not known thus it is impossible to quantify accurately the yield of a given species produced in the matrix. Thus Chapman s work [24,79] clearly demonstrated the formation of triplet phenyl nitrene and of dehydroazepine and the absence of benzazirine, but it did not reveal the ratio of nitrene to dehydroazepine present in the matrix, nor did it indicate which species is initially formed in the matrix. 

Photochemical properties of azido derivatives of six-member aza-heterocycles (pyridine, pyrimidine, triazine, quinoline, acridine) are discussed. Data on the stmcture of the reaction products formed under photolysis of azides in different conditions (solvent, temperature, additives), and also data on the matrix isolation spectroscopy of heterocyclic nitrenes, including high-spin nitrenes, produced by low-temperature photolysis of the corresponding azides are shortly examined. 

Tomioka, H., Ichikawa, N., and Komatsu, K., Photochemistry of 2-(methoxycarbonyl)phenyl azide studied by matrix-isolation spectroscopy. A new sHppery energy surface for phenyl nitrene, /. Am. Chem. Soc., 115, 8621,1993. 

For species of higher than singlet multiplicity, matrix isolation coupled with ESR spectroscopy is often used for detection and characterization purposes. This is how the first examples of exotic organic compounds with quintet ground states were obtained. Since then, this kind of spectroscopy has been closely linked to the developments in the area of carbenes and nitrenes. 

Matrix isolated NH reacts with NO, an excellent nitrene trap, to form trans-HNNO, which was characterized by infrared (IR) spectroscopy. Triplet imidogen also reacts with oxygen in matrices. 

No transient absorption >350 nm is detected upon LFP of 1-naphthylazide. A band with absorption maxima at 370 nm is formed with a time constant of 2.8 ps after the laser pulse. The carrier of the 370-nm absorption reacts over >100 ps to form azonaphthalene. The carrier of the 370-nm absorption is identified as triplet 1-naphthylnitrene that has previously been characterized as a persistent species at 77 K by UV-vis (A,nmx = 367 nm) and EPR spectroscopy. Azirine 43, detected by TRIR spectroscopy must not absorb significantly >350 nm, a fact that was established later by the matrix isolation studies of Wentrup s and Rally s groups. Assuming a rapidly equilibrating mixture of azirine and nitrene, and given that kisc = 1 X 10 s (determined by Tsao by LFP at 77 K and assumed to have the same value at 298 then K = [singlet nitrene]/[azirine 43] = 0.038 at 298 K. 

The thermal decomposition of 2-azidoacetic acid (N3CH2CO2H) in the vapour phase has been shown, by photoelectron and matrix isolation infrared spectroscopy, to involve simultaneous formation of CO2 and methanimine (CH2NH) with concerted ejection of N2.52 No evidence was found for formation of intermediate nitrene (NCH2CO2H) or the imine (HNCHCO2H) to which it could be converted by 1,2-hydrogen shift. 

The photolysis of benzoyl azide (PhCON3) has been investigated by means of matrix isolation, time-resolved IR spectroscopy, and computations at various levels of theory.In argon at 12 K, formation of two species was observed. One of these was phenyl isocyanate (PhCNO), while the other had an IR spectrum consistent with the predicted spectrum of the reactive singlet species PhCON, which appears to have a structure intermediate between that of a carbonyl nitrene and an oxazirene. Analogous observations were also made for 4-acetyl-benzoyl azide. Time-resolved IR spectroscopy on the nanosecond time scale provided additional evidence for the singlet ground state of benzoylnitrene. 

The formation of highly strained bridgehead imines was observed on the irradiation of a series of matrix-isolated bridgehead azides.The photochemistry of matrix-isolated 1-azidonorbornane (6) was studied using monochromatic irradiation IR, UV, and ESR spectroscopy and trapping with methanol and CO. The azide photochemistry was very complicated, and the formation of two types of imines (7 and 8) and triplet nitrene 9 were observed. 

The photochemistry of HN3 in the argon and nitrogen matrices was studied for the first time by J. Pimentel et al. and the IR spectra of triplet nitrene NH and radical NH2 were recorded. A series of studies of matrix isolated NH (in ground triplet and excited singlet A states) and its deutero-substitute analogue (ND) were later performed using UV and luminescence spectroscopy. " The spectroscopy and relaxation of the lowest excited singlet state of NH / ND ( A) were studied in detail in Ne, Ar, Kr and Xe matrices... 

A number of trifluoromethyl-substituted 2-pyridyl azides, e.g., 97, have been matrix-isolated and photolyzed in Ar at 12-18 Like other 2-pyridyl azides, 97 exists in equilibrium with the tetrazole valence tautomer 98, but sublimation for the purpose of matrix isolation usually causes significant ring opening to the azido forms. In any case, the tetrazole forms as weU as the azides eliminate Nj when photolyzed. All the 2-pyridyl azides or tetrazoles investigated underwent conversion into the corresponding l,3-diazacyclohepta-l,2,4,6-tetraenes, e.g., 99 from 97, and these ring expanded products, which are cycHc carbodiimides, had v(N=C=N)35 bands at about 2000 cm" in their IR spectra. In the case of 97, the triplet nitrene was also observed by both IR and EPR spectroscopy, and it was seen to give 99 on further photolysis. 

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