Spin, unpaired

Divalent carbon species called carbenes are capable of fleeting existence. For example, methylene, CH2, is the simplest carbene. The two unshared electrons in methylene can be either spin-paired in a single orbital or unpaired in different orbitals. Predict the type of hybridization you expect carbon to adopt in singlet (spin-paired) methylene and triplet (spin-unpaired) methylene. Draw a picture of each, and identify the valence orbitals on carbon. 

F has a small high spin unpaired electrons present... 

Transition-metal and rare-earth atoms that contain partially occupied d or f valence subshells also give rise to spectral tine structure, often with very complicated multiplet splitting [2,27,28]. The spin-unpaired valence d or f electrons can undergo spin-orbit coupling with the unpaired core electron (remaining in the orbital from which the photoelectron was removed), producing multiple non-degenerate final states manifested by broad photoelectron peaks [2,27]. 

The cations in these compounds are Fe and/or Fe". In iron oxides, Fe " is always in the high spin (unpaired d electrons) state. As Fe with five d electrons has no crystal field stabilization energy (CFSE see Chap. 6), regardless of whether it is octa-hedrally or tetrahedrally coordinated, there is little preference for one or the other type of site. For Fe , on the other hand, CFSE is higher for octahedral than for tetrahedral coordination, so the octahedral coordination is favoured. 

Naturally, the product of two-electron reduction of the para derivative can be depicted (Scheme 3-54) as an anionic diborataquinoid system. In contrast with the para-dib-orataquinoid dianion, an anionic meta-quinoid system is impossible. Indeed, the meta-sub-stituted isomer depicted in Scheme 3-54 has been characterized as a spin-unpaired triplet species with boron-centered spins (Rajca et al. 1995). This dianion diradical can be viewed as two stable borane anion radicals linked with a ferromagnetic coupling unit, i.e., 1,3-phenylene see Chapter 1. 

Mocala, K., Navrotsky, A. Sherman, D. M. (1992) High-temperature heat capacity of C03O4 spinel Thermally induced spin-unpairing transition. Phys. Chem. Minerals, 19, 88—95. 

According to the Pauli Exclusion Principle, two electrons in the same orbital must have opposite spins. Thus, the two electrons of triplet (spin-unpaired) methylene must occupy different orbitals. In triplet methylene, sp-hybridized carbon forms one bond to each of two hydrogens. Each of the two unpaired electrons occupies a p orbital. In singlet (spin-paired) methylene the two electrons can occupy the same orbital because they have opposite spins. Including the two C-H bonds, there are a total of three occupied orbitals. We predict sp2 hybridization and planar geometry for singlet methylene. 

The term spin (unpaired electron) is commonly used to refer to any species, e.g., free radical, paramagnetic ion, which gives an ESR spectrum. 

The origin of the conduction mechanism has been a source of controversy ever since conducting polymers were first discovered. At first, doping was assumed to simply remove electrons from the top of the valence band (oxidation) or add electrons to the bottom of the conduction band (reduction). This model associates charge carriers with free spins (unpaired electrons). However, the measured conductivity in doped polyacetylene (and other conducting polymers such as polyphenylene and polypyrrole) is r greater than what can be accounted for on the basis of free spin alone. 

The essential condition for ferromagnetic behaviour is the presence of magnetic dipoles in a substance. This is set by atomic structure. At least one of the ferrite cations must have one or more unpaired electrons. The spinning unpaired electron will impart to the ion a magnetic moment. This condition is complied with in the group of transition elements (Mn, Fe, Co, Ni, Cu, etc.). 

B2. The six electrons should give the electron configuration (ox)2(a in the basis of Fig. 3-14b. The two electrons in the % orbital should have heir spins unpaired in accord with Hund s first rule, which applies to the filing of MO s as well as to atomic orbitals. On the other hand, according o Fig. 3-14a, the configuration would be (ffi)2(cr2)2(ff3)2, with no unpaired dectrons. Experiment shows that B2 has two unpaired electrons in a % irhital thus, Fig. 3-14b is still correct at B2. The bond order is 1 (one net... 

Magnetic hyperfine splitting is seen at nominally diamagnetic Au in ferromagnetic alloys, presumably as a result of conduction-electron polarisation. The 6i-contribution to the conduction band becomes partially spin-unpaired by interaction with the magnetic atoms. The lines are usually only partly resolved, and the first magnetic data were not analysed successfully... 

Positron annihilation lifetime spectra of MCM-41 and zeolite Y were measured in vacuum and air. For MCM-41 a very long positron lifetime component could be observed not only in vacuum, but also in air, while for zeolite Y it can only be observed in vacuum with a weak intensity. These peculiar positron annihilation characteristics were explained by air quenching mechanism of o-Ps annihilation in MCM-41. For comparison, positron annihilation lifetime spectra of MCM-41 were also measured in different O2 and N2 level. The results show that oxygen is more effective in quenching than nitrogen because of its spin unpaired electron. 

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