3t bands

FIGURE 4.3 The hand structure of nickel. Note the shape of the 3t/band. 

Many substituted azobenzenes belong to the azobenzene type, as well. Substitution may shift the n 3t band from 440 nm (azobenzene), to 465 nm (hexamethylazobenzene), to 480 nm (2,2 -dimethyl-4,6,4 ,6 -tetra-tert.butylazobenzene), and even to 520 nm (hexaphenylazobenzene). Hydrocarbon, halogen, nitro, carboxy, acetyl, hydroxy, m-amino and even 2,2 4,4, 6,6 hexaphenyl substitution influences the (7t,7i )-(n,7t ) state energy gap, but not so much as to shift the molecule into the aminoazobenzene group. In the context of this book, it is important that the long-chain and polymeric molecules containing azobenzene units coupled by means of hydroxy and carboxy substituents are of the azobenzene type. 

FIG. 6.21 Observed tc-tc band maximunri and absorbance at the maximum for the 7C-tc band of aggregated (323 nm) and nonaggregated (348 nm) trans-chromophores, as well as the absorbance at the n-3T band (characteristic for the cis-isomer) as a function of the irradiation time (at X == 360 nm) of a LBK film of polymer 38 (reproduced with permission from reference 93). 

The copper(I) ion, electronic stmcture [Ar]3t/ , is diamagnetic and colorless. Certain compounds such as cuprous oxide [1317-39-1] or cuprous sulfide [22205-45 ] are iatensely colored, however, because of metal-to-ligand charge-transfer bands. Copper(I) is isoelectronic with ziac(II) and has similar stereochemistry. The preferred configuration is tetrahedral. Liaear and trigonal planar stmctures are not uncommon, ia part because the stereochemistry about the metal is determined by steric as well as electronic requirements of the ligands (see Coordination compounds). 

For ruthenium and the following elements of the 4d series the free-ion f values become in excess of 1000 cm-1, and apart from the exceptions mentioned previously, the relative intensities of the spin-forbidden bands are found to be significantly larger than for the 3d elements. For the intra-subshell (t g) transitions 3T g XT%g and 3T g 1Eg some 2%... 

However, an alternative approach is possible since the 32.0 kK. band was described by Hugill and Peacock as being very strong thus suggesting that a charge-transfer assignment is a possibility. The 23.0 kK. peak could then be attributed to the 3T g 3T2g transition and the narrow... 

In the low energy region the measurements of Alien et al. went down only to 4 kK. but hexachlorobutadiene mull data indicated the presence of a further band at 3.0—3.4 kK.. This and the 5.6 kK. band were therefore readily identified as transitions within the spin-orbit split ground state manifold — the 3 kK. absorption as 3Tig (A) - 3i g (A) and the 5.6 kK. peak as 3Tlg (A) -+3T g (A, A). Allen et al. were unable to resolve the broad band at 30 kK. but it may be noted that six formally... 

Equation (12) has at least JV — 1 real roots. The corresponding wave fiuictions are nonlocalized, and the energies lie in the range defined by Equation (10). This is the normal band of crystal states. The remaining two roots may both be real, and in this case they also lie in the normal crystal band, and the system has only nonlocalized states. On the other hand, one or both of the remaining roots may have values of 0 of the form if or 3T + if with f real and positive. The corresponding wave functions... 

The other doubledeckers containing the metals La, Pr, Nd, Sm -Lu have rather similar spectra that have less well distinct bands in the visible region. The extinction at 573 nm is much less than in A, Peaks occur at about 540 and 670 nm. The latter absorption is typical for 3T-cation radicals (10), Indeed, the composition Ln(0EP)2 can only be accomodated with the normal trivalent state of nearly all of the lanthanoid ions if one assumes that the... 

By inspection of the energy level diagrams it is possible to see directly what sort of spectrum the ion should have in the given environment. For example, it can be seen from Figure 9.3 that a d2 ion in an octahedral complex, say [V(H20)J3+, should have three spin-allowed transitions, from the 37, ground state to the upper states 37, 3T, and 3A2. Experimentally, two absorption bands have been found at —17,000 and —24,000 cm-1, and these may be assigned to the T — 37 and 3T - 3T transitions if A0 is taken as —21,500... 

Some Molv complexes of the type MoCl4-2L, including L = Cl, show rather intense bands at 2OOO0 and 26000 cm"1 which have tentatively been assigned as 37 2g<-3rlg and 3T g(P) -3T g in octahedral symmetry. The Mo(CN)84 ion shows rising absorption from 18000 (s 1) to 38000 cm-1 (e 1000) with structure which has been assigned to the four transitions between the cf-orbitals split by the dodecahedral stereochemistry.142... 

For 3T, a broad A band was observed that becomes spectrally narrower within the first picosecond. The maximum of A1 lies at X = 600 nm. Additionally, small positive values of AD at very small delay times, indicating a further transient absorption A0, were found. The transient behavior of 4T showed that the maximum of A was found at about X = 770 nm. It was found that the whole absorption band A decays simultaneously and single exponentially with x = 530 ps. Positive AD values due to an absorption A0 seem to appear weak at zero decay. 

When evaluating the energy differences, one should take the 0-phonon transitions of 1S and 3T into account and not the maximum of the band envelope. 

Finally, it is noted that the splitting of the middle band in the [Ni(H20)6]2+ spectrum arises from spin-orbit interaction between the 3T S(F) and 1Eg states (cf. Fig. 8.5.9). These two states are close in energy at the A0 value generated by six H20 molecules. But they are far apart at the stronger field of three en molecules. As a result, no splitting is observed in the spectrum of [Ni(en)3]2+. 

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