Singlet structure

Both the ionic and the covalent structure of sphalerite, for instance, are singlet structures, with no unpaired electrons, so that either extreme or any intermediate is possible, and in such a case evidence from various properties of the particular substance must be considered to decide which extreme is more closely approached. On the other hand, in a crystal such as (NH FeFg or (7V774)3Z e(GW)s the lowest ionic state of the [FeXft] complex does not combine with the lowest covalent state, so that the transition from one extreme to the other is discontinuous. The actual state of the complex in the crystal can be determined from the multiplicity. With an ionic state, Fe+++ and -For ( C Nthe F or... 

The terminal SS bond (192.2 pm) of S3O is considerably shorter than the central bond (204.0 pm) and represents a double bond the same holds for the SO bond (147.1 pm). The angles SSO (112.1°) and SSS (111.6°) show the expected values. The trans-planar singlet structure is by AH°298=18 kj moG less stable at this level of theory. 

D).64 In these molecules, the empty p orbital on the carbene carbon can be part of the aromatic % system and delocalized over the entire ring. Currently available data indicate that the ground-state structure for both C and D is a singlet, but for D, the most advanced theoretical calculations indicate that the most stable singlet structure has an electronic configuration in which only one of the nonbonding electrons is in the n orbital.65... 

There has been a great deal of discussion on the question of whether methylene is best described as a radical (triplet) or nonradical (singlet) species and on the related question of whether methylene is linear or bent. The early theoretical work of Lennard-Jones86 indicated a singlet state, and Laidler and Casey82 postulated a bent singlet structure for CH2 with C—H bonds formed from p orbitals of the carbon atom, the... 

As for TiHT the ground state of the (H2)TiH+ complex is a triplet (section 5.1). The C3v singlet structure is thus very high in energy, 53.8 kcal/mol... 

The symmetrical molecule with three CH2 groups arranged trigonally about a carbon atom is interesting theoretically. It could have a singlet structure with two charges, both of which can be delocalized, but no neutral form can be drawn. Alternatively, it could be a tripletwith the two unpaired electrons equally delocalized over the three CH2... 

The isoelectronic complexes Re(NO)(PPh3)Cp 2(p-C=CC=Q (96/ Ph),249 Fe(dppe)Cp 2(p-C=CC=C) (107)258 270 and Ru(PPh3)(L)Cp 2 ( i-C=CC=C) (109)262 all feature fully occupied, closed-shell singlet structures for their HOMOs. The experimental structures are in accord with this description, and display bond lengths consistent with a poly-yne-like structure. These complexes, and closely related phosphine derivatives, display two (Re), three (Fe) or four (Ru) 1-e oxidation processes. 

Virtually every microtubule in a cell is a simple tube, a singlet microtubule, built from 13 protofilaments. In rare cases, singlet microtubules contain more or fewer protoflla-ments for example, certain microtubules in the neurons of nematode worms contain 11 or 15 protofilaments. In addition to the simple singlet structure, doublet or triplet microtubules are found in specialized structures such as cilia and flagella (doublet microtubules) and centrioles and basal bodies (triplet microtubules). Each doublet or triplet contains one complete 13-protofllament microtubule (A tubule) and one or two additional tubules (B and C) consisting of 10 protofilaments (Figure 20-4). 

A singlet structure for this carbene could provide a cyclopenta-dienylide-like ring with a delocalised sextet of electrons [85],... 

E.s.r. spectra of this cation indicated that it has a triplet ground state [261,263]. Electrochemical determination of the pKp+ value were hampered by a number of uncertainties but indicate that it is exceedingly low, and much lower than the value for the allyl cation [264]. Calculations of the ionic heats of formation of the cyclopentyl, cyclopentenyl and cyclopentadienyl cations from the measured ionisation potentials of the corresponding radicals and the heats of formation of these radicals show that whereas introduction of one double bond stabilises the cation, introduction of the second double bond destabilises the system [265]. E.s.r. spectra of the pentaphenyl [258,259] and pentachloro [259,266] derivatives show that the former has a ground state singlet structure but with a triplet state less than 1 kcal.mol" higher, while the latter has a triplet ground state [252,259]. 

Electronic spectra of samples of tetramethylcyclobutadiene prepared from different precursors differ among themselves, again indicating intermolecular interactions within the matrix [34]. Thus if pyridazine or phthalic derivatives are also present the products are coloured whereas when o-di(hydroxymethyl)benzene or 3,4-benzo-dihydrofuran are the coproducts the mixtures are colourless [34]. Cyclobutadiene and its alkyl derivatives either absorb at about 300-305 nm or have no bands above 300 nm, consistent with rectangular singlet structures [34-37]. 

There exists the possibility of an electron transfer from the tt system, leading to structure D, which would have two unpaired electrons. A molecular orbital calculation at the 4-3IG level has indicated that the singlet structure C is more stable than the triplet structure by about 20 kcal/mol, but it is highly distorted with the C-C-C bond angle at the divalent carbon calculated to be 145°." " Phenyl cations are formed by the thermal decomposition of aryldiazonium ions." The relatively high stability of arylidiazonium ions, even in comparison with primary aliphatic diazonium ions, reflects the instability of the phenyl cation. The cation is also extremely reactive. 

The DFT optimized singlet structures of Qgo, C240, C320, and C540 are depicted in O Fig. 16-4. These structures have been fully optimized at the all-electron level using DZVP basis sets in combination with the VWN functional. 

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