Partial spin pairing

One of the possibilities is to replace the evolution of the single-quantum states by the multiple-quantum ones during extended evolution periods. As theoretically predicted by Griffey and Redfield [34] and experimentally demonstrated by Grzesiek and Bax [35], keeping a spin pair in the state of multiple-quantum coherence (MQ) eliminates most of the dipolar contribution to the spin-spin relaxation. With the X-H (X is a heteronucleus) spin pairs, this is partially offset by a higher rate of proton cross-relaxation in the transverse plane with the remote H spins. Since the MQ coherences consist of transverse... 

Since an appreciable amount of the photoelimination of aliphatic ketones occurs from an excited singlet, the reported partial retention of optical activity in photocyclobutanol formation355,356 is easily rationalized. The reaction from a singlet could be either concerted or proceed via a spin-paired biradical which would cyclize extremely rapidly. 

Electron transport is impossible in closed shell systems (the moving electron would have to occupy an antibonding orbital) and in half-filled open shell systems (spin pairing would prohibit the transport of electrons). Only the existence of hole states in partially oxidized dithiolenes allows electrons to flow freely. 

By monitoring the H MAS spectra, it has been suggested that the loss of water from bone tissue is an irreversible process under air drying conditions.38 It has been shown that CP transfer within an isolated heteronuclear spin pair will exhibit a characteristic oscillation, from which the dipolar coupling constant can be accurately determined.197 The technique of LG-CPMAS can probe this oscillation pattern due to a dominating 31P-1H interaction in a dipolar coupled network, if any. However, such oscillation is seldom observed in 31P 1H CPMAS measurements of biominerals because of the damping effect of the homonu-clear dipole-dipole interaction. Thus, it is rather remarkable that the buildup curve of 31P 1H LG-CPMAS obtained for a sample of powdered femoral cortical bone, which is partially hydrated, shows a distinctive... 

The sum must be made over all spin pairs in the proton-rich solid. In the absence of large-amplitude molecular motion this Hamiltonian describes a line shape of width of up to 100 kHz. In the presence of molecular motion the angular part of Equation 13.1 becomes time-dependent, and the partial averaging of this term results in reduced linewidths. In polymers the geometry of main-chain motion is limited by the structure of the polymer chain, and is inherently anisotropic. As a general rule, as the measurement temperature is increased the motion tends to become more isotropic in nature as the free volume increases, and the extent of averaging of the dipolar Hamiltonian increases. This... 

Partial alignment introduces a residual dipolar coupling on a spin pair like the backbone bond. A spin pair in... 

The electron transfer ) character of the visible absorption spectra of iron complexes of conjugating imine ligands was first recognized by WiUiams (77). From the opposite effect of methyl substituents on max of ferrous (or cuprous), and of ferric tris-phen complexes, it was concluded that the excitation involves a partial electron transfer from a filled metal electron transfer (ET) transitions in spin-paired iron(II) tris-diimine complexes, was further elaborated by Jorgensen (78). 

We finally consider the spectral effects of partially replacing diiminc by other ligands. In spin-paired Ru(bipy)2L complexes, the main ET band shifts to smaller wavenumbers in the order ... 

The measurement and interpretation of RDCs between spatially proximate spin-pairs in biopolymers such as proteins, nucleic acids, and oligosaccharides in partially oriented environments has become a popular method for structure determination." All observed RDCs, D s contain an unknown contribution from the anisotropic part of coupling tensor, J in addition to the direct dipolar contribution, Bryce and Wasylishen" evaluated the influence of J on RDCs... 

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