Electronic structure, of carbenes

This class of ion-radicals is characterized by the localization of an unpaired electron at the atom bearing a free (valence) electron pair. Although their applicability in organic synthesis remains an open question, the preparative methods and electron structure of carbene ion-radicals attract some attention of the researchers. Probably, it is an initial step to a new chapter in organic ion-radical chemistry. 

Simple considerations of the electronic structure of carbenes indicate that, of the six valence electrons associated with the divalent carbon, four are taken up by the two covalent bonds and two are non-bonding. The divalent carbon atom has two orbitals available to accommodate... 

In the form in which it has so far been applied to the study of carbenes, EPR spectroscopy is unable to investigate the hyperfine interactions of the unpaired spins with the constituent atomic nuclei because of the broad lines which are observed. However, the technique of electron nuclear double resonance ( endor ) promises to permit such investigations to be made, so providing even more detailed information about the electronic structure of carbenes (Hutchison, 1967). 

In November I submitted a manuscript [19] to the Journal of the American Chemical Society titled Electronic Structure of Carbenes I, CH2, CHF, and CF2 in which I calculated a CH2 angle of 132.5°. I was certain of the CH2 angle and began to study the effect of substituents on the geometry and singlet-triplet splitting in carbenes. 

The Walsh diagram provides a satisfying analysis of the electronic structure of carbenes, and the essential features of the system can be summarized by the simple representations shown in the margin. The triplet has two electrons in two very different orbitals, what we have called p and o-(out). We might expect its reactivity to be similar to that of radicals, and indeed this is the case. The singlet state is quite different. It contains a lone pair of electrons in an MO [a(out)] that is reminiscent of an sp hybrid. It also contains an empty p orbital, just like a simple carbenium ion. Its reactivity patterns should be quite different from radicals, and we will see that they are (Chapter 10). 

From these discussions, it is clear that one can strongly influence the electronic state and the reactivity of the carbenes by appropriately designing a substitution pattern. In its turn, the electronic structure of carbenes will strongly impact... 

Fischer-type carbene complexes, generally characterized by the formula (CO)5M=C(X)R (M=Cr, Mo, W X=7r-donor substitutent, R=alkyl, aryl or unsaturated alkenyl and alkynyl), have been known now for about 40 years. They have been widely used in synthetic reactions [37,51-58] and show a very good reactivity especially in cycloaddition reactions [59-64]. As described above, Fischer-type carbene complexes are characterized by a formal metal-carbon double bond to a low-valent transition metal which is usually stabilized by 7r-acceptor substituents such as CO, PPh3 or Cp. The electronic structure of the metal-carbene bond is of great interest because it determines the reactivity of the complex [65-68]. Several theoretical studies have addressed this problem by means of semiempirical [69-73], Hartree-Fock (HF) [74-79] and post-HF [80-83] calculations and lately also by density functional theory (DFT) calculations [67, 84-94]. Often these studies also compared Fischer-type and... 

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... 

The matrix IR spectra of la and several isotopomers (cU-la, l80-la) reveal details of the electronic structure of the carbene.23 In particular the red-shift of the C=0 stretching vibration (compared to p-benzoquinone) below 1500 cm-1 indicates a substantial contribution of the phenoxyl/phenyl resonance structure to the wave function of la. The C2V symmetry of the carbene was experimentally revealed by measuring the IR dichroism of partially oriented samples of matrix-isolated la. The orientation of la in an argon matrix was achieved by irradiation with linearly polarized light. 

Despite the undeniable synthetic value of the benzannulation reaction of aryl and alkenyl Fischer carbene complexes, the details of its mechanism at the molecular level remain to be ascertained. Indeed, although a relatively large number of theoretical studies have been directed to the study of the molecular and electronic structure of Fischer carbene complexes [22], few studies have been devoted to the analysis of the reaction mechanisms of processes involving this kind of complexes [23-30]. The aim of this work is to present a summary of our theoretical research on the reaction mechanism of the Dotz reaction between ethyne and vinyl-substituted hydroxycarbene species to yield p-hydroxyphenol. 

Spectra of carbenes are very useful sources of information on the structure of the free carbenes, e,g. the R—C—R angle, or the multiplicity of their lowest state. However, these data were mostly obtained under conditions different from those in solution, where chemical reactions normally occur. The spectra are usually recorded either in matrices at low temperatures, say at 4 or 77 °K, or in the gas phase. Only very few investigations of that type have been carried out in solution. The most important spectroscopic technique used in the investigations of carbenes is ESR. Other spectroscopic methods, such as flash photolysis which produces electronic spectra of carbenes, and infrared and lately CIDNP spectroscopy have been successfully employed. 

Carbocations have similar electronic structures to carbenes. The P-protonated derivative of phosphinine should also be similar to 23. Indeed, while investigating the proton affinity of 3. the most preferred protonation site was phosphorus and not carbon, whereby the cyclic jt system would be interrupted. ... 

The divalent carbon(O) atom in L C L has two lone electron pairs which makes CL2 a particular class of ligands which may bind as a bidentate Lewis base to one and to two monodentate Lewis acids. The nature of the ligand L determines whether a divalent carbon atom behaves as a carbone or as a carbene. The newly gained insight into the electronic structure of carbones opens a large field for theoretical and experimental research. 

The aromatic stabilization of the planar bond configuration may qualitatively be interpreted in terms of a model of the electronic structure of pseudo-olefines (87JA5303), in which the planar structure (249) is viewed as resulting from the interaction between two analogs of carbene XH2 in the triplet state, whereas the frans-pyramidalized structure (250) results from that between two carbene-type units XH2 in the singlet state. 

The electronic structure of the carbene center of an imidazole-2-ylidene can be approximated as a singlet carbene, where the carbene carbon atom is close to a sp -hybridization as shown in Scheme 3. 

The electronic structure of these carbenes was investigated by early theoretical studies [36,40-48] to elucidate the reasons for the surprising stability, which came to different conclusions concerning the importance of the sta-bihzing effect of the /r-delocalization. While early studies predicted that the C-N 7r-interaction does not play a major role [33], others found that the pM population at the carbene carbon atom is 30% higher for the unsaturated case, indicating that cychc delocalization is clearly enhanced in the unsaturated carbene [48]. 

Triplet Carbene Structure and ZFS Parameters. The ZFS parameters are thus shown to provide information on the molecular and electronic structures of triplet carbenes. We will see next how the parameters change systematically by examining a series of triplet carbenes. 

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