Singlet covalent

Clearly, the three Z CSFs retain purely covalent R —> °° character even in the heteronuclear case. The Z, lZ, and JZ (all three of which can undergo Cl mixing now) possess one covalent and two ionic components of the form sxa Sy(3 + sya sx(3, sxa sx(3, and sya sy(3. The three singlet CSFs therefore can be combined to produce a singlet covalent product function sxa Sy(3 + sya sx(3 as well as both X + Y 2 and X + Y ionic product wavefunctions... 

It is perhaps worth remarking that had we chosen, instead of the standard basis, the Rumer basis of spin functions, then the five VB singlet covalent functions are just the two well-known Kekule Structures and the three Dewar structures. 

The UIIF wnive fimction can also apply to singlet molecules. F sn-ally, the results are the same as for the faster RHF method. That is, electron s prefer to pair, with an alpha electron sh arin g a m olecu lar space orbital with a beta electron. L se the L lIF method for singlet states only to avoid potential energy discontinuities when a covalent bond Is broken and electron s can impair (see Bond Breaking on page 46). 

The classical VB wave function, on the other hand, is build from the atomic fragments by coupling the unpaired electrons to form a bond. In the H2 case, the two electrons are coupled into a singlet pair, properly antisymmetrized. The simplest VB description, known as a Heitler-London (HL) function, includes only the two covalent terms in the HF wave function. 

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... 

Oxidative repair is not a unique feature of our Rh(III) complexes. We also demonstrated efficient long-range repair using a covalently tethered naphthalene diimide intercalator (li /0 1.9 V vs NHE) [151]. An intercalated ethidium derivative was ineffective at dimer repair, consistent with the fact that the reduction potential of Et is significantly below the potential of the dimer. Thymine dimer repair by a series of anthraquinone derivatives was also evaluated [151]. Despite the fact that the excited triplets are of sufficient potential to oxidize the thymine dimer ( 3 -/0 1.9 V vs NHE), the anthraquinone derivatives were unable to effect repair [152]. We attribute the lack of repair by these anthraquinone derivatives to their particularly short-lived singlet states anthraquinone derivatives that do not rapidly interconvert to the excited triplet state are indeed effective at thymine dimer repair [151]. These observations suggest that interaction of the dimer with the singlet state may be essential for repair. 

In connection with Chichibabin s hydrocarbon it is appropriate to mention that there is little or no resonance between states of different multiplicity.4 Thus the singlet-triplet transition represented by the covalent and diradical structures of the hydrocarbon is a true equilibrium and not an example of resonance. 

It should be noted that the existence of different centers Is also found in covalently-linked dimer 11. Moreover, the analysis of all data obtained for dimers points clearly towards the efficient transfer of the excited singlet state energy from two centers of compound 1 to two acceptor centers of compound 2 in dimers (14,30). Increase In the porphyrin concentration by 300-700 times (say, for compounds 1 and 2) does not cause additional changes In electronic spectra as against diluted solutions. If the results obtained from temperature experiments (Fig. 2b) and measurements of fluorescence lifetimes In different bands are taken Into account, one may conclude that the additional centers observed In... 

If the sulfur atom released from parathion, as proposed in Figure 2, is in its singlet state, it would be a highly reactive electrophile that would bind readily to nucleophiles near the site of its release. The thiono-sulfur atom of parathion has been found to be covalently bound to tissue macromolecules following adminis-... 

---