Sulfur orbitals

Calculations of the UV spectra of the isomeric thieno-thiophenes " indicate that, as in the case of thiophene,3d and higher sulfur orbitals hardly affect the electronic spectra of sulfur-containing condensed heteroaromatics. In the case of sulfones of thienothiophene 2 and dithienothiophenes 5-10 in which the sulfur... 

Fig. 1. Occupancy of the 3sp2 (a, b, c) and 3p, (d) sulfur orbitals by electrons in various types of conjugated compounds. In cases A and B, c is a non-bonding atomic orbital (the orbitals are somewhat deformed for the sake of clarity).

The question of the relative stability of the 2-furyl and the 2-thienyl carbanions has been neatly answered (73JCS(P2)1912) by determining the product composition in the hydrolysis of the phosphonium salt (164). Here furan and thiophene are formed in the ratio 1 3, indicating the greater stability of the 2-thienyl carbanion 3d sulfur orbital participation is therefore strongly implicated. 

Additional information has been obtained from single crystal, polarized optical and ESR spectroscopic studies924 on poplar plastocyanin, which have allowed a correlation of the electronic structure of the blue copper active site with its geometric structure. In summary, the three dominant absorption bands at 13 350, 16 490 and 17 870 cm-1 were assigned to CysS- Cu (d 2-,2 charge-transfer transitions. The methionine makes only a small contribution, due to the long Cu—S(Met) bond (2.9 A) and the poor overlap of the methionine sulfur orbitals with the dx y orbital of copper. Histidine-Cu charge transfer contributes to the weaker absorptions at 21 390 and... 

Methoxybutadiene can be used to model the diene in a qualitative treatment. AMI calculations give very similar S and C LUMO coefficients for 0=CH—CH=S however, the difference is more pronounced in STO-3G and 3-21G calculations. Note that even with AMI calculations (C coefficient = —0.64 S coefficient = 0.65), a preference is still expected for attack at sulfur, as sulfur orbitals are much more diffuse than carbon orbitals (covalent radii 1.02 and 0.77 A respectively). 

Note The sulfur orbitals being diffuse (see the previous exercise), /3SS should be the largest of all resonance integrals. We therefore expect the major product to be in fact 17. Indeed, the reaction initially gives a mixture of 8 and 17, but NMR studies show that the latter disappears completely upon standing for 1 week at room temperature.12... 

Metal-Sulfur Orbital Overlap Populations in Mo(S2C2H2)3 ... 

The indices of chemical reactivity were also calculated72 using a model with participation in the conjugation of the d orbital of sulfur.82 In the calculation, the sulfur orbitals are replaced by two 2pz orbitals (representing the tt electron component of ethylene), linked together by a standard bond (p = 1), but attached to the other part of the molecule by weakened bonds (p..C8,. = 0.6). The other parameters were as parameters 2. 

The electronic structure of SO2 has been the subject of several theoretical studies which have provided a basis for interpretations of UV and photoelectron spectroscopic measurements These will not be reviewed in detail here since they all agree on those features which are important for a qualitative understanding of transition metal complex formation. The ground state configuration obtained from the extended Hiickel approach, for example, is (Iai) (lb2) (2ai) (lbi) (3ai) (2b2) (la2) (3b2) (4ai) (2bi) (4b2) (5ai) using 2 s, 2 p oxygen orbitals and 3 s, 3 p sulfur orbitals as a basis. The salient features from the M-SO2 bonding standpoint are the HOMO (4ai) and the LUMO (2bi), depicted in Fig. 2. These two orbitals can be described qualitatively as follows ... 

Pentaammine Ru(III) polarizes thiourea by complexing sulfur, and the complex [Ru(NH3)5 8C(NH2)2](S206)3/2Tf20 is isolated. The data are consistent with LMCT from a filled p r sulfur orbital to the vacant dxy r-orbital on Ru(III). ... 

Find the reducible representation for all the a bonds, reduce it to its irreducible representations, and determine the sulfur orbitals used in bonding for SOCI2. 

The sulfur orbitals used in u bonding are the 3px,3py, and 3p. The 35 could be involved, but the nonplanar shape of the molecule requires that all three p orbitals be used. 

The role of the sulfur -orbitals in bonding has been discussed for (10) and (11). If an appropriate... 

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