Singlet state phenylnitrene

A. Electronic Structures of the Lowest Singlet States of Phenylcarbene and Phenylnitrene... 

Figure 2. Orbital occupancies for the nonbonding electrons and CASSCF(8,8)/6-31G optimized geometries of the lowest triplet and singlet states of phenylnitrene (1 b).61 Bond lengths in angstroms, and bond angles in degrees.

Our calculations on the ring expansion of the lowest singlet state of phenylnitrene ( A2-lb) to azacycloheptatetraene (3b) predict a two-step mechanism that is analogous to that for the rearrangement of la to 3a and which involves the bicyclic azirine intermediate 2b.61 The CASPT2 energetics are depicted in Fig. 5, and the CASSCF optimized geometries of the stationary points are shown in Fig. 6. 

The apparent absence of large electronic effects on the cyclization reactions of derivatives of phenylnitrene has been attributed to the nature of the wave function for the lowest singlet state, which, as discussed earlier, resembles a... 

The triplet state of NH is 36 kcal/mol lower in energy than the singlet state. The triplet state is favored because, on the average, electrons with parallel spin spend less time in proximity with each other than electrons with antiparallel spin. Consequently, the Coulombic electron-electron repulsion in the triplet state is less severe than in the singlet state (cf. Borden, Chapter 22 in this volume.) As we will see in vinyl- and phenylnitrene, delocalization of an unpaired electron by conjugation dramatically stabihzes the singlet relative to the triplet states of nitrenes. 

Figure 22.4. Schematic depiction of the electronic structures of the lowest singlet states of phenylcarbene (la) and phenylnitrene (lb).

Secondly, Leyva et al found that the solution phase photochemistry of phenylazide (PA) was temperature dependent. Photolysis of PA in the presence of diethylamine at ambient temperature yields azepine 2, first prepared by Doering and Odum. Lowering the temperature suppresses the yield of 2 and encomages the formation of azo compound. Thus, high temperatures favor reactions of singlet state intermediates, whilst low temperatures favor reactions associated with triplet phenylnitrene. 

Calculations of PN are more challenging than that of PN because it is, of course, an excited state of phenylnitrene. The first two electronically excited singlet states of PN are both of Ai symmetry and are calculated to be at 1610 and 765 nm. Neither of these transitions have been detected, since both of these states have zero oscillator strength due to symmetry considerations, and they lie outside the wavelength range accessible to our spectrometer. ... 

Figure 8.3 Configuration cartoons for the ground-state triplet and first three singlet states of phenylnitrene. Note that the cartoon for SI glosses over its two-determinantal character...

In certain favorable instances, one can coax the SCF equations to converge to different determinants of the same electronic state symmetry. For instance, phenylnitrenes have two different closed-shell singlet states, as re-illustrated in Figure 14.3 (cf. Section 8.5.3),... 

Many of the same considerations affecting these vinylidene examples arise in comparing the relative energies of the electronic states of phenylnitrene (Figure 14.3). In this system, there are many different theoretical data available to compare to experiment, which itself is available for the lowest two singlet states. Results from ASCF calculations at the HF and DFT levels of theory are listed in Table 14.1, as are results from many additional levels that will be discussed at appropriate points later in the chapter. 

It appears that carbonylnitrenes insert at a detectable rate only in their singlet states, while phenylnitrene and cyanonitrene can insert in both their singlet and triplet states. Also, activated C—bonds (such as aromatic C—bonds or those next to ether linkages) may react with triplet carbonylnitrenes . ... 

Figure 5.11 Ground state triplet (left) and lowest singlet state (right) configurations of phenylnitrene...

ESR parameters for triplet carbenes15 and nitrenes16 have been summarized, and it has been shown that phenylnitrene is produced predominantly (87-88%) in the singlet state by direct photolysis of phenyl azide in low-temperature matrices.17 The first spectroscopic observation of a singlet nitrene has been reported nanosecond-laser photolysis of 1-azidopyrene gives the S0 nitrene (Amax 450 nm) which has a lifetime of 22 nsec at room temperature (in benzene) and 34 nsec at 77 K in rigid solution. At room temperature it decays to the triplet ground state (Tj, Amax 415 nm) with a rate constant of about 4.4 x 107 sec. Tt is formed directly by biacetyl sensitized photolysis of the azide. The lifetime of the excited triplet (T2) was about 7 nsec. T dimerizes to azopyrene.18... 

Table 5.10 Spin-Orbit Coupling Constants for the First Three Singlet States of Four Para-Substituted Phenylnitrenes in...

The A st is much lower in 29a (15kcal/mol) ° than in NH (36kcal/mol) " or CH3N (31.2kcal/mol) because the C=C substituent allows the n electron in the A" state to become localized in a region of space that is disjoint from the region of space that is occupied by the C7 electron. As will be discussed in Section 11.6 for the same reason, the lowest singlet state in phenylnitrene is also the open-shell Aa state of 29a. 

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