Imidogen

There followed, after the initial study of imidogen (42), papers on boron carbide (43), nitric oxide and its ions (44), as well as fluoronitride and its cation (45). Results from these calculations are still useful since they offer mutually consistent representations of potential curves for a considerable number of states. This successful campaign was furthermore beneficial to the development at Aarhus since several Danes were given the opportunity to participate in the work and to learn to appreciate Yngve s way of scientific quest. 

D. M. Silver, K. Ruedenberg, and E. L. Mehler, Electron correlation and separated pair approximation in diatomic molecules. 3. Imidogen. J. Chem. Phys. 52(3), 1206—1227 (1970). 

Compounds containing neutral, monovalent nitrogen atoms are known as nitrenes. The parent structure, NH, is also called imidogen. Because most stable compounds of neutral nitrogen have a valence of 3, it is no surprise that nitrenes typically are very short lived, reactive intermediates. A short history of nitrenes has been presented by Lwowski who points out that they were first proposed by Tiemann in 1891 as transient intermediates in the Lossen rearrangement. 

The nitrene nitrogen atom of imidogen is sp hybridized. Both the NH bond and the sigma lone pair of electrons use nitrogen orbitals that are rich in 2s character. 

In triplet imidogen, there are two electrons with parallel spins, singly occupying pure Px and orbitals. This state is designated as by spectroscopists. 

Because of the high symmetry of the NH molecule, the singlet state of imidogen cannot be classihed as either open or closed shell. The closed-shell and open-shell singlets have exactly the same energy and form the two components of a doubly degenerate state. °... 

The imidogen anion (NH ) can ionize to form either the singlet or triplet nitrene, thus, the difference in the kinetic energies of the photoelectrons leading to NH and NH is just the singlet-triplet splitting of NH. 

Imidogen can be generated in the gas phase by photolysis or by pyrolysis of hydrazoic acid (HN3) although other precursors find occasional use. Exposure of HN3 to either 248 or 266-nm light generates NH almost exclusively in its lowest singlet state with a quantum yield near unity. 

Singlet NH inserts into the CH bonds of hydrocarbons, much like singlet methylene (see Chapter 7 in this volume). Triplet NH abstracts hydrogen atoms from hydrocarbons to form aminyl (NHp radicals and alkyl radicals in the same manner as triplet methylene, in spite of the fact that the reactions of CH2 are exothermic, whereas some reactions of NH are endothermic, depending on the alkane Absolute rate constants for many of these processes have been measured in the gas phase. However, the gas-phase chemistry of methylene is much more developed than that of imidogen. ... 

Hydrazoic acid is a reagent in the well-known Schmidt reactions in solution, but imidogen is probably not involved in these transformations. Systematic mechanistic studies of the photochemistry of HN3 in solution and the chemistry of NH with hydrocarbons, particularly cis- and trans- alkenes as per the detailed studies of CH2, have not been performed. 

Gas-Phase Spectroscopy. The 336-nm absorption band of triplet imidogen was first observed by Eder in 1892 and has been the subject of numerous subsequent studies. The singlet state of NH absorbs at 324 nm. The CASSCF (6,5)/ CASPT2 level of theory predicts transitions at 323 and 293 nm for singlet and triplet imidogen, respectively. In each spin state, an electron is promoted from the sp hybrid lone pair to a singly occupied 2p orbital as shown below for NH. 

Singlet and triplet imidogen have not yet been detected in solution by LEP methods. Future effort should be expended in this direction and it seems safe to predict that such work will be reported in the near future. 

Although singlet and triplet imidogen have similar absorption spectra, singlet and triplet methylene do not. In fact, most carbenes have rather poor chromo-phores for UV-vis detection by LEP and must be visualized by trapping with pyridine to form ylides. The exceptions are arylcarbenes, which have n n transitions localized on the aromatic n system. " ... 

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. 

The electron paramagnetic resonance (EPR) spectrum of triplet imidogen immobilized in a cryogenic matrix has not yet been observed. One negative attempt was reported in argon, krypton, and xenon matrices in 1967. It would seem to be worthwhile for a new attempt in a xenon matrix. 

Although AEst of phenylnitrene is small in comparison to that of imidogen, it is still very large compared to phenylcarbene. Thus, while singlet and triplet phenylcarbene interconvert rapidly, singlet to triplet intersystem crossing of phen-... 

The ground state of PH has triplet multiplicity, as does imidogen, but the singlet-triplet splitting of the phosphorus compound is only 22 kcal/mol, " ... 

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