Quintets

Figures 1.9a and b demonstrate the effeet of proton broadband deeoupling in the C NMR speetrum of a mixture of ethanol and hexadeuterioethanol. The C//j and CH2 signals of ethanol appear as intense singlets upon proton broadband deeoupling while the CD3 and CD2 resonanees of the deuteriated eompound still display their septet and quintet fine strueture deuterium nuelei are not affected by H decoupling because their Larmor frequeneies are far removed from those of protons further, the nuelear spin quantum number of deuterium is 7/3=/ in keeping with the general multiplieity rule (2nx Ix+ 1, Seetion 1.4), triplets, quintets and septets are observed for CD, CD2 and CD3 groups, respeetively. The relative intensities in these multiplets do not follow Paseal s triangle (1 1 1 triplet for CD 1 3 4 3 1 quintet for CD2 1 3 6 7 6 3 1 septet for CD3).

The structural fragments B - J converge to 3,4,5-trimethyl-5,6-dihydronaphtho[2,3-/r]furan, K. Whether this is the 5(5)-or 5(7 )-enantiomer (as shown) cannot be decided conclusively from the NMR measurements. It is clear, however, that the 5-CiT proton at = 3.36 is split into a pseudo-quintet with 7.1 Hz, this is only possible if one of the 6-CH2 protons (at = 2.63) forms a dihedral angle of about 90° with the 5-CiT proton so that Jhh 0 Hz. 

The EPR spectra of the related 1,2,4,6,3,5-thiatriazadiphosphinyl radicals (3.20) reveal a distinctly different electronic structure.The observed spectrum consists of a quintet of triplets consistent with coupling of the unpaired electron with two equivalent nitrogen atoms and two equivalent phosphorus atoms [Fig. 3.4(a)]. This interpretation was confirmed by the observation that the quintet collapses to a 1 2 1 triplet when the nitrogen atoms in the ring are 99% N-enriched [Fig 3.4(b)]. Thus the spin delocalization does not extend to the unique nitrogen atom in the phosphorus-containing system 3.20. 

The number of excited determinants thus grows factorially with the size of the basis set. Many of these excited determinants will of course have different spin multiplicity (triplet, quintet etc. states for a singlet HF determinant), and can therefore be left out in the calculation. Generating only the singlet CSFs, the number of configurations at each excitation level is shown in Table 4.1. 

The amount of spin contamination is given by the expectation value of die operator, (S ). The theoretical value for a pure spin state is S S + 1), i.e. 0 for a singlet (Sz = 0), 0.75 for a doublet (S = 1/2), 2.00 for a triplet (S = 1) etc. A UHF singlet wave function will contain some amounts of triplet, quintet etc. states, increasing the (S ) value from its theoretical value of zero for a pure spin state. Similarly, a UHF doublet wave function will contain some amounts of quartet, sextet etc. states. Usually the contribution from the next higher spin state from the desired is... 

The spectrum of radical 101 appears as a quintet (1 2 3 2 1) caused by the hyperfine interaction (HFI) with two equivalent nitroxide nitrogen nuclei (<2n = 0.74 mT), each line of the quintet being additionally split due to hyperfine... 

For a threefold degenerate MO occupied by three electrons, a configuration of a triradical having a quartet multiplicity is required hy Hund s rule. In organic chemistry, systems of this type occur rarely but with transition element complexes, quartets, quintets, and sextets, they are common. 

With these assignments at hand the analysis of the hyperfine shifts became possible. An Fe(III) in tetrahedral structures of iron-sulfur proteins has a high-spin electronic structure, with negligible magnetic anisotropy. The hyperfine shifts of the protons influenced by the Fe(III) are essentially Fermi contact in origin 21, 22). An Fe(II), on the other hand, has four unpaired electrons and there may be some magnetic anisotropy, giving rise to pseudo-contact shifts. In addition, there is a quintet state at a few hundred cm which may complicate the analysis of hyperfine shifts, but the main contribution to hyperfine shifts is still from the contact shifts 21, 22). 

Figure 15. Calculated potential energy surface and geometries of intermediates of the V" + CO2 reaction. The energy of the lowest energy state for the quintet (solid hnes) and triplet (dotted lines) stationary points are shown. Energies are calculated at the CCSD(T)/6-311+G(3df) level, at the B3LYP/6-311+G(d) geometry and include zero-point energy at the B3LYP/6-311+G(d) level.

Consider the simple model complex [FeO(NH3)5] in its lowest triplet and quintet states (Fig. 5.9). 

The shape of these orbitals does not change strongly between the lowest triplet state ( 2g) with the configuration. .. b2) 3e) (2b f and the lowest quintet state ( Aig) with the configuration. .. (Ifi2) 3e) (2fii) ... 

The anionic Si atom is nearly tetrahedral, being bonded to the two Si atoms of substituents and two Li atoms. The resonance of this Si anionic center in the Si NMR spectrum of 55 Li2 was observed at a very high field (-292.0ppm) as a quintet due to coupling with the 2 equivalent Li atoms (/ = 1) with a coupling constant of 15.0Hz. This chemical shift is largely shifted upheld compared with that of (MesSOsSiLi (-189.4ppm), ° because of the sharply increased electron density... 

Recently, Switzer and co-workers have further extended the multi-stranded motifs for nucleic acids with the formation of a quintet assembly with oligonucleotides containing 2,deoxy-iso-guanosine (74). To support the quintet, metal ions larger than those appropriate for quartet stabilization were required, and Cs+ ions were found to best meet this requirement. From modeling studies, a structure in which a central Cs+ interacts with ten 02 iG atoms at a distance of 3.5 A was proposed. 

Meadows, J. Elizabethan quintet. London Longmans, Green, 1956. 304p. 

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