Absorption shifts

There has been much discussion of the relative contributions of the no-bond and dative structures to the strength of the CT complex. For most CT complexes, even those exhibiting intense CT absorption bands, the dative contribution to the complex stability appears to be minor, and the interaction forces are predominantly the noncovalent ones. However, the readily observed absorption effect is an indication of the CT phenomenon. It should be noted, however, that electronic absorption shifts are possible, even likely, consequences of intermolecular interaetions of any type, and their characterization as CT bands must be based on the nature of the spectrum and the structures of the interaetants. This subject is dealt with in books on CT complexes. ... 

A hypsochromic shift of 20-50 cm is observed in the double-bond stretching region, when the enamines are converted to the corresponding iminium salts by the electrophilic addition of a proton at the /3-carbon atom. The shift is accompanied by an enhancement in the intensity of the band. Leonard and co-workers (68,71-74) have used this absorption shift as a diagnostic tool for the determination of the position of the double bond... 

The enamines in which the protonation at the -carbon atom is not allowed due to the lack of coplanarity, or, in other words, the lack of electronic overlap, do not exhibit this characteristic absorption shift. For instance in the case of neostrychnine (134) where the overlap is not permitted since this would involve the formation of a double bond at the bridgehead, there is no appreciable difference in the C—C stretching region of the free amine and its perchlorate salt they absorb at 1666 cm and 1665 cm , respectively (70). 

Plutonium(IV) polymer has been examined by infrared spectroscopy (26). One of the prominent features in the infrared spectrum of the polymer is an intense band in the OH stretching region at 3400 cm 1. Upon deuteration, this band shifts to 2400 cm 1. However, it could not be positively assigned to OH vibrations in the polymer due to absorption of water by the KBr pellet. In view of the broad band observed in this same region for I, it now seems likely that the bands observed previously for Pu(IV) polymer are actually due to OH in the polymer. Indeed, we have observed a similar shift in the sharp absorption of U(0H)2S0ir upon deuteration (28). This absorption shifts from 3500 cm 1 to 2600 cm 1. 

Both AG and AH were eshmated by direct experimental procedures. Data obtained using any type of estimation of those values, including spectroscopic absorption shifts and intensities or some other such parameters, were excluded from considerahon. 

The spectral absorptions shift to longer wavelengths as the solvent polarity increases. However, care must be taken to distinguish them from the spectral shifts due to deprotonation. 

In many other situations of donor-acceptor solutes in aprotic solvents, such as quaternary alkylammonium salts (R4NX), a UV absorption shift to higher wavelength has proved the occurrence of simple cation-anion ion... 

Lin and Frei (133), upon loading of aqueous H2602 into TS-1 and removal of the solvent by evacuation, detected a peroxidic 0-0 stretch absorption at 837 cm-1 and a broad band at 3400 cm-1 by infrared difference spectroscopy. The former absorption shifted to 793 cm-1 when aqueous H2802 was loaded in TS-1 instead of H2602 (Fig. 18). No bands were observed at 837 or 3400 cm-1 with the same loading of H202 on silicalite-1. 

Another interesting chiral chain end effect is exhibited by the helical polymer block co-polymer, poly(l,l-dimethyl-2,2-di-/z-hexylsilylene)- -poly(triphenylmethyl methacrylate), reported by Sanji and Sakurai (see Scheme 7) and prepared by the anionic polymerization of a masked disilene.333 The helical poly(triphenylmethyl methacrylate) block (PTrMA) is reported to induce a PSS of the same sign in the poly(di- -propylsilylene) block in THF below — 20 °C, and also in the solid state, by helicity transfer, as evidenced by the positive Cotton effect at 340 nm, coincident with a fairly narrow polysilane backbone UV absorption characteristic of an all-transoid-conformation. This phenomenon was termed helical programming. Above 20°C, the polysilane block loses its optical activity and the UV absorption shifts to 310 nm in a reversible, temperature-dependent effect, due to the disordering of the chain, as shown in Figure 45. 

The first pH indicators studied possessed the acid-base site (phenol, aniline, or carboxylic acid) as an integral part of the fluorophore. Structurally, in the most general sense, pH sensitivity is due to a reconfiguration of the fluorophorets re-electron system that occurs on protonation. Consequently, the acid and the base forms often show absorption shifts and also, when the two forms fluoresce, emission shifts or at least, when only one form emits, a pH-dependent fluorescence intensity. This class of compounds has been reviewed 112 and the best structures have to be designed according to the medium probed and the technique used. After a short consideration of physiological pH indicators we will describe the main photophysical processes sensible to protonation. 

The isolation of vincadioline (31) from C. roseus has been reported (96) only in the patent literature. The C-NMR data of 31 (76) (Table X) support the 3 -hydroxyvinblastine structure. Both C-2 and C-4 absorptions shifted downfield compared to the corresponding values of vinblastine (1). In addition, upfield shifts of C-1 and C-19 were observed as a result of the y-effect of the new oxygen atom at C-3. The resonance of C-19 shifted 4.8 ppm, a magnitude consistent with an axial hydroxyl at C-3. Such circumstances should lead to a nearly equal shielding effect at C-5, as is indeed observed. These data allow the assignment of the a relative steric position of the C-3 hydroxyl group. 

Mesoionic 4-amino-l,2,3,5-thiatriazoles constitute the only class of mesoionic 1,2,3,5-thiatriazoles known. They are prepared by the reaction of l-amino-l-methyl-3-phenylguanidine with approximately 2 equivalents of thionyl chloride with pyridine as solvent (88ACS(B)63>. They are obtained as the yellow 1 1 pyridine complexes (17). The dark-violet mesoionic 1,2,3,5-thiatriazole (18) was liberated on treatment with aqueous potassium carbonate (Scheme 3). The structure is established on the basis of elemental analysis and spectroscopic data. In particular, the IR spectrum is devoid of NH absorptions. Compound (18) exhibits a long-wavelength absorption at 463 nm in methanol. When mixed with an equivalent amount of pyridinium chloride, complex (17) is formed and the absorption shifts to 350 mn. The mesoionic thiatriazoles are sensitive towards mineral acids and aqueous base and although reaction takes place with 1,3-dipolarophiles such as dimethyl acetylene-dicarboxylate, a mixture of products were obtained which were not identified. 

A shoulder at 950 cm."1, which might well have been vinyl absorption shifted by oxygen and chlorine, disappeared during the first 30 minutes. 

As the sphere is flattened into a disk the position of maximum absorption shifts to longer wavelengths. For example, if c/a = 368/1390, it follows from Fig. 5.6 that Lx is about 0.19 and from (14.6) that Xs is 3040 A. This is an appreciable shift—over 800 A—but still short of the measured value 4100 A. Our analysis, however, implicitly assumed isolated spheroids, a condition that was not satisfied in the experiments. 

M. Wanko, M. Hoffmann, P. Strodel, A. Koslowski, W. Thiel, F. Neese, T. Frauenheim, and M. Elstner. Calculating absorption shifts for retinal proteins computational challenges, J. Phys. Chem. B, 109 3606-3615 (2005). 

Quinones, which have both carbonyl groups in the same ring, absorb in the 1690-1655 cm1 region. With extended conjugation, in which the carbonyl groups appear in different rings, the absorption shifts to the 1655-1635 cm-1 region. 

A third widely used procedure, introduced by Bradfords and modified by others, measures the binding of the dye Coomassie brilliant blue whose peak absorption shifts from 465 nm to 595 nm upon binding. The change occurs within two minutes and is stable. However, the color yield varies from one protein to another. 

Comparative studies with the 4-, 0-, and 6-mem bered ring homologs of ethylene sulfide have shown that the absorption shifts to longer wavelengths as the electron density on the sulfur increases. A correlation of ring size and electron distribution in the ring is the subject of an earlier study.104... 

The absorption at 277 nm for 2-butanone is an n — tt transition, and with 3-buten-2-one, this absorption shifts to longer wavelengths (324 nm). There is also an intense absorption band for 3-buten-2-one at 219 nm, which is a 77 —> 77 transition. With 2-butanone a corresponding absorption occurs at 185 nm, which is out of the range of the spectrometer used to take the spectra of Figure 9-20. 

The division of the substrates amongst those which increase the separation between the g values, e.g. aniline, and those which decrease the separation e. g. steroids is the same as the Type I and Type II division (32) of substrates based on absorption shifts (see above). 

Table III shows x values for other structurally regular substituted silane high polymers measured both at 1.064 and 1.907 /an. Examination of this data suggests relatively little difference between the polysilanes with nonplanar, yet regular structures and trans planar PDN6S which is included in the table for comparison. This result is a little surprising given that changes in backbone conformation can cause spectral absorption shifts of more than 60 nm.

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