Carbenes electronic states

Nevertheless, a more traditional approach to the stabilization of carbenes and the investigation of their spectral properties deals with the direct generation of carbenes in low-temperature matrices, e.g. by the photolysis of diazo-compounds or ketenes. The method allows stabilization of carbenes in their ground electronic state, prevents intramolecular isomerization and also facilitates direct spectroscopic monitoring of their chemical transformations in low-temperature matrices. 

Studies have shown that carbene reactivity toward a wide variety of substrates is dramatically affected by the nature and multiplicity of the electronic state. - Similarly, the structure, electronic state, thermochemical stability, and reaction kinetics of both singlet and triplet carbenes can be significantly affected by the R-substituents. If R provides steric hindrance, the carbene center can be shielded to slow down inter-molecular reactions (kinetic stabilization). Additionally, bulky and/or geometrically... 

The nitrogen analogs of carbenes are called nitrenes. As with carbenes, both singlet and triplet electronic states are possible. 

It has become common to classify all molecular compounds, which fulfill the above characteristics, as carbene analogs 9,13>. As a consequence, compounds of divalent silicon, germanium, tin, and lead may be regarded as carbene-like and are therefore called silylenes, germylenes, stannylenes, and plumbylenes. In contrast to carbenes they have one property in common the energetically most favorable electronic state is the singlet 1a2 found by experiments and calculations 9). 

It should be clear by the definition given so far that the carbene-analogous state is limited to molecular species. The oligomer of EX2 (EX2)n is, of course, much more stable than EX2 in every respect. It should nevertheless be noted that also the oxidation number does not change in going from the monomer to the polymer the chemical, structural, and electronic properties of these species are completely different. 

It is well known that double bonds have a different effect on the singlet-triplet energy separation and thus on the reactivity of the carbene than alkyl groups. The description of the electronic structure of such carbenes is rendered more difficult by the fact that several low-lying electronic states are possible. For... 

After filling of the lbi, 2bi, and la2 bonding it MOs, the last two electrons must be distributed between the ai, 3bi, and 2a2 orbitals. As depicted in Fig. 8, the carbene hybrid orbital (ai) is lower than but close in energy to the 3bi and 2a2 it MOs. Five electronic states are thus worthy of consideration 1 Ai, 2, Bi, 3A2, and 3Bi. The electron configurations of these states and the CASSCF optimized geometry of each are shown in Fig. 9.57... 

A defining feature for carbenes is the existence of two non-bonding orbitals on one carbon atom. There are two electrons to distribute among these two orbitals and their placement defines the electronic state of the molecule. A simple representation showing the electron occupancy of the non-bonding orbitals is displayed in Fig. 1. The orbital perpendicular to the... 

The chemical and physical properties of DMFL contrast most sharply with those of FL. The geometrical features, in particular the carbene-carbon bond angle, of these two carbenes are expected to be identical. The most important difference between DMFL and FL is that the spin multiplicities of the lowest electronic states appear to have been inverted. The experiments indicate that the ground-state of DMFL is the singlet. This conclusion is outlined in the reactions shown in Scheme 4. 

Despite that it might initially be generated in a different electronic state and/or conformation, the CH2 " fragment ion can be expected to decompose further in a way identical to the molecular ion of carbene ... 

Electronic States and Structure of Triplet Carbenes 2.1. Calculations... 

In this section the electronic states of carbenes will be briefly discussed. As an example, some semiempirical calculations are presented for methylene, in order to gain a better understanding of the electronic states of carbenes. 

Fig. 2. Energies of various electronic states of CH2 Generalization from Methylene to Other Carbenes...

One important feature of carbenes is the presence of two nonbonding electrons and two available orbitals, which are nominally located on the carbon atom. In bent carbenes, the two orbitals have different energies and are often denoted as a and -ir, with a being the in-plane orbital and -ir the out-of-plane orbital. Within this simple picture, four electronic states can be envisioned, which are depicted in Fig. la. Singlets and S3 have the two electrons in the same orbital (a and -IT, respectively) and are often characterized as closed-shell singlets. On the other hand, singlet S2 has the same open-shell electronic occupation (arr) as the triplet (T) state. 

As mentioned in Section II.A, several possible electronic states can arise from the presence of two diradical sites (carbenes and/or nitrenes) in a molecule. When... 

Figure 3 Valence-bond depictions of electronic states arising from the coupling of two reactive centers (carbene, nitrene) g via the 1,4-phenylene linker. 5 ...

Figure 11 shows some of the geometrical parameters (computed at the MCSCF level of theory) for the five electronic states of 33. The local geometries of the carbene and nitrene subunits (bond angle at the carbene center, C—bond length of the carbene and the bond lengths between the diradical centers and the benzene ring) of the quintet state ( A ) are very similar to that of triplet phenylcar-bene and phenynitrene computed at the same level of theory (Fig. 12). The situation is less clear for the other two A states ( A and A ), but the geometry of...

The nitrogen analogs of carbenes are called nitrenes. As with carbenes, both singlet and triplet electronic states are possible. The triplet state is usually the ground state for simple structures, but either species can be involved in reactions. The most common... 

Given these conditions, the influence of the substituents on the carbene ground-state multiphcity can be easily analyzed in terms of steric and electronic effects. 

Although carbenes have two electronic states of different stability, it is not always the ground-state multiplicity that is involved in the reaction. What are then factors that control the reaction pattern of carbenes ... 

Effects of electronic factors on AGst have been examined more systematically for a series of cyclic aromatic carbenes incorporated into a presumably planar ring of five or six atoms. The carbene bond angle in those examples is not expected to change much and the diversity of chemical behavior must then be primarily associated with electronic changes. The parameter AGst of each carbene has been estimated from the rate constants for reactions of the electronic states and are summarized in Table 9.6. ... 

Much of the interest in carbene chemistry derives from the complexity induced by the presence of two electronic states, singlet and triplet. In this respect, it is crucial... 

While carbenes are found with both singlet and triplet ground electronic states, only singlet silylenes and germylenes are well documented. 

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