D spectra

On absorption of an energy A.cb one of the t2g electrons will be promoted into the Cg set, as on the right side of Fig. 3-8. As the Cg set is now full, no further electronic promotions are possible so that this corresponds to the one and only excited state of the octahedral d configuration. We thus observe a single absorption band in the d-d spectrum. The excitation is equivalent to the transfer of the hole... 

We note that three spin-allowed electronic transitions should be observed in the d-d spectrum in each case. We have, thus, arrived at the same point established in Section 3.5. This time, however, we have used the so-called weak-field approach. Recall that the adjectives strong-field and weak-field refer to the magnitude of the crystal-field effect compared with the interelectron repulsion energies represented by the Coulomb term in the crystal-field Hamiltonian,... 

The figure below abstracts just the spin-triplet part of the Tanabe-Sugano diagram in the previous box. Suppose we have recorded the electronic d-d spectrum of [V(H20)6] and identified two out of the three possible spin-allowed (triplet-triplet) bands at energies 17,200 cm and 25,600 cm ... 

Finally, we must remember that just as a d-d spectrum is not properly described at the strong-field limit - that is, without recognition of interelectron repulsion and the Coulomb operator - neither is a full account of the energies or number of charge-transfer bands provided by the present discussion. Just as a configuration... 

Many biologically important sugars are derivatives having a chromophoric group that absorbs within the range of commercial instrumentation. Not only is the c.d. spectrum of such molecules easier to measure, but the interpretation of the spectrum is simplified, because only the chromophore is involved. Many laboratories have concentrated on the c.d. of such monomers and their polymers, and the results will be discussed. 

Because so many different chromophores are involved in the c.d. spectrum of a monosaccharide, fundamental interpretation of the spectrum is difficult. On the other hand, similarities among the various difference-spectra having identical conformations in near neighboring groups suggest that a catalog... 

Fresh solutions of pustulan, which are presumably in a random-coil conformation, have a positive c.d. at 180 nm (see Fig. 8). The c.d. spectrum... 

Lewis and Johnson compared the c.d. spectra of amylose and cyclomaltohexaose, and showed that amylose is helical in aqueous solution. Cyclomaltohexaose is chromophorically equivalent to amylose, and it is known to assume a pseudohelix having zero pitch, and thus, no helical chirality. The conformation of amylose is clearly different from that of cyclomaltohexaose, as their c.d. spectra are very different (see Fig. 9). The difference in conformation was considered to be a matter of helical chirality. To confirm this, these workers measured the c.d. spectrum of an amylose-1-butanol complex presumed to have the V-form of helical conformation with the 1-butanol complexed in the channel of the helix. The c.d. spectrum of the complex is identical to that of amylose in aqueous solution. 

The c.d. spectrum of a film of this polymer is shown in Fig. 14. It is similar, but opposite in sign, to the c.d. spectrum of agarose, with the short-wavelength band somewhat red-shifted. 

However, workers do not agree as to the shape of the c.d. spectrum for these sugars at shorter wavelengths, as Fig. 15 demonstrates. The correct spectrum still remains an open question, but the intense c.d. band expected at 190 nm for the amide mr c.d. bands are of opposite sign for the two anomers and nearly cancel in the equilibrium mixture. Thus, differences in the anomeric mixtures could explain differences in the c.d. spectra. The amide irir c.d. band is obvious for the anomeric mixture from 2-acetamido-... 

The 3-O-methyl derivative of methyl 2-acetamido-2-deoxy-/3-D-glucopyranoside has the same c.d. spectrum in water and in fluorinated alcohols. This confirms that solvent binding to the 3-hydroxyl group is important in determining the orientation of the amide relative to the 3-substituent. 

Buffington and Stevens measured the c.d. of 2-acetamido-2-deoxy-D-glucose as a film cast from HFIP. The spectrum is considerably more intense than that observed by Dickinson and coworkers for a solution in HFIP, but shows the same general features shifted somewhat towards the red. This vacuum-u.v. c.d. spectrum (see Fig. 18) has, at 218 nm, an intense, positive band due to the mr, an intense negative band due to the amide tttt at 200 nm, and a shoulder at 180 nm, but no other significant features down to 145 nm. 

The c.d. spectra of chitin, both in HFIP solution and as a film cast from HFIP, are shown in Fig. 21. Chitin gels have a c.d. spectrum similar to... 

The carboxyl chromophore is axial for the a anomer and equatorial for the p anomer. The sugar was studied as the carboxylate anion as it has a (low) piC of 2.6, and the compound is degraded in acidic solution. The c.d. spectrum of this compound contains contributions from the carboxylate n-jr at 217 nm, the amide n-tr at 210 nm, and the amide 7T7r at 190 nm. Apparently, all of these bands are positive, giving rise to a c.d. spectrum (see Fig. 29) having " a maximum at 199 nm and a shoulder at 210 nm. The c.d. spectra of a number of derivatives confirmed these assignments. 

The c.d. spectra are also available for a few oligosaccharides that contain N-acetylneuraminic acid. " These spectra depend on the intersaccharide linkages and the state of ionization of the carboxyl group, but no systematic scheme has yet been set up to derive configurational information from the c.d. spectra. Of particular interest is the c.d. spectrum of beef ganglioside, which fully differentiates the amide mr, amide tttt, and carboxylate nir c.d. bands. 

Listowsky and coworkers showed that the c.d. of this sugar derivative is due entirely to lactic acid, and confirmed that this chromophore is in the D configuration for muramic acid. N-Acetylmuramic acid, in which the amino group is replaced by an amido group at C-2, has a c.d. spectrum that is roughly a linear combination of the lactic acid in muramic acid and the amide in 2-acetamido-2-deoxy-D-glucose. This indicates that the amide chromophore and the lactic acid chromophore in N-acetylmuramic acid behave independently. 

Figure 1. In Situ MOssbauer Spectra. A) Spectrum collected while the sample (sample 1) was at room temperature In flowing helium gas. The sample had been previously treated for 3 hours In water-gas shift synthesis gas at 613 K B) Spectrum collected while the sample was at 613 K In flowing water-gas shift synthesis gas C) Spectrum collected while the sample was at room temperature In flowing helium gas. The sample had been previously treated for 25 hours In water-gas shift synthesis gas at 613 K D) Spectrum collected at room temperature In flowing helium gas. The sample had been previously treated for 65 hours In flowing water-gas shift synthesis gas at 613 K.

In contrast to 4, 2-acetylpyridine iV-methylthiosemicarbazone, 3a, formed yellow-green, paramagnetic, octahedral [Ni(3a)2A2] complexes with nickel(II) chloride and bromide. The neutral form of the ligand was proposed to be NN eoordinated [180]. Brown, paramagnetic [Ni(3a-H)2] was prepared from nickel(II) acetate with NNS coordination, and its d-d spectrum and ligand field... 

A complex containing a mixture of ligands will have a d—d spectrum at energies close to the mean of the spectra of the complexes with only one type of ligand (17). 

In this example, both the -CH2- and the -CH3 would be excellent targets for irradiation and we would recommend making use of both of them. A brief inspection of the 1-D spectrum (Spectrum 8.3) is enough to confirm that the compound does have both substituents on one of the rings as four protons can easily be observed as one continuous spin system (8.13, 7.85, 7.6 and 7.48 ppm) whilst the remaining... 

Second, the resolution achieved in a 2-D experiment, particularly in the carbon domain is nowhere near as good as that in a 1-D spectrum. You might remember that we recommended a typical data matrix size of 2 k (proton) x 256 (carbon). There are two persuasive reasons for limiting the size of the data matrix you acquire - the time taken to acquire it and the shear size of the thing when you have acquired it This data is generally artificially enhanced by linear prediction and zero-filling, but even so, this is at best equivalent to 2 k in the carbon domain. This is in stark contrast to the 32 or even 64 k of data points that a 1-D 13C would typically be acquired into. For this reason, it is quite possible to encounter molecules with carbons that have very close chemical shifts which do not resolve in the 2-D spectra but will resolve in the 1-D spectrum. So the 1-D experiment still has its place. 

If you do manage to get everything right, NMR offers excellent quantisation results. What s more is that it is free if you have acquired a 1-D spectrum. Note that you can use this approach to quantify other nuclei - it works just as well for 19F. Note that it won t work very well for 13C because we normally acquire 13C data with NOE enhancement from the protons so the signals are not quantitative. (It is possible to collect carbon data in a quantitative way but it is not something that we would normally do...). 

Fig. 2.5. Measurement of pKas of serotonin by target factor analysis (TFA). (A) 3-D spectrum produced by serotonin in pH gradient experiment (equivalent to A matrix). (B) Molar absorptivity of three serotonin species (equivalent to E matrix). (C) Distribution of species (equivalent to C matrix). In this graph the three sets of data points denote the three...

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