Triplet carbene, structure

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. 

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... 

Its formation can be observed after short irradiation with a wavelength of A. = 313 nm in argon at 10 K. It is possible to use the comparison of the experimental IR spectrum with the calculated spectra for the singlet and triplet carbene to determine the singlet multiplicity of the isolated molecule. Its structure cannot easily be described by normal valence formulas. The calculated... 

Carbenes with triplet ground states are, of course, paramagnetic. Thus they exhibit characteristic epr spectra which can be used to confirm their presence, to provide some data on their structure, and, at least in principle, to reveal AGST in favorable cases. The first epr spectrum of a randomly oriented triplet carbene was observed for DPM (Murray et al., 1962). In subsequent years spectra of many carbenes have been recorded (Table 3). These spectra are commonly interpreted by fitting the results to a spin Hamiltonian H (3), to give the zero field parameters D and E (Stevens, 1952). In the usual... 

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

EPR Spectroscopy. The structure of triplet carbenes is unequivocally characterized by EPR zero-field splitting (ZFS) parameters, which are analyzed by D and E values. In a simple model, the ZFS parameters D and E for a triplet depend on the distance between electrons with parallel spins as given by... 

However, be aware that the ratio of E/D is not always a reliable guide to the structure of triplet carbenes. For example, diphenylcarbene and fluorenylidene have very different geometries but almost exactly the same D and E values.If one really wants to learn about the geometry, one must label the carbene carbon with and measure the hyperfine coupling constants. 

Like UV-vis spectra, IR spectra do not give concrete evidence for the multiplicity of carbene in question and, hence, comparison with EPR spectra is highly desired. However, IR spectroscopic data encode a lot of structural information and can be analyzed with the help of computational methods, thus aiding in the identification of the observed species. Since geometries of the singlet and triplet carbenes are usually different, their IR spectra are expected to be different. One can often assign a multiplicity of the carbene if one can calculate theoretical... 

---