Basicity Blue-shift

Nicolet, P. and Laurence, C. (1983) Spectroscopic scales of basicity. Blue shifts of the visible transition of iodine by complex formation with substituted pyridines. J. Chim. Phys. Phys.-Chim. Biol, 80, 677-680. 

Using the ROKS method a blue shift of 0.23 eV (experimental value 0.21 eV [94]) is calculated for the case in which only acetone itself is included in the QM region. Addition of the first solvation shell has only a tiny effect (a shift of 0.03-0.04 eV) indicating that the solvent shift is basically converged with respect to... 

This equation shows that if the emission band of the basic form is located at higher wavelengths than that of the acidic form, the excited-state pK is lower than the ground-state pK, and AH is a stronger acid than AH. Conversely, if the emission band of the basic form is blue-shifted with respect to the emission band of the acidic form, the acid in the excited state is weaker than in the ground state. 

The concept of polarity covers all types of solute-solvent interactions (including hydrogen bonding). Therefore, polarity cannot be characterized by a single parameter. Erroneous interpretation may arise from misunderstandings of basic phenomena. For example, a polarity-dependent probe does not unequivocally indicate a hydrophobic environment whenever a blue-shift of the fluorescence spectrum is observed. It should be emphasized again that solvent (or microenvironment) relaxation should be completed during the lifetime of the excited state for a correct interpretation of the shift in the fluorescence spectrum in terms of polarity. 

Using various amines added to the ammonia bath (in most cases with added hydrazine), sphalerite ZnS fihns were obtained with a crystal size of ca. 3 nm [ 118]. Rutherford Backscattering Spectroscopy (RBS) analyses showed that there was about twice as much Zn in the fihns as S. (More basic solution and more hydrazine gave more stoichiometric films). Extended X-ray Absorption Fine Structure (EX-AFS) and Fourier Transform Infra-red (EUR) spectroscopy showed that the fihns did not have Zn-0 groups but rather Zn-OH ones [122] and that there is probably a mixture of ZnS and unreacted Zn(OH)2, quite likely as a ZnS shell around a Zn(OH)2 core. Optical spectra gave a bandgap of ca. 3.85 eV, considerably blue-shifted from the bulk value of 3.6 eV, as expected from such small crystals. 

The blue-shift results from the chemical environment inside the pores of MCM-41, which is dominated by residual silanol and, especially, non reacted amino groups, and thus more basic compared to that in solution. The interaction of the dye molecules with the host is further confirmed by a broadening of the absorption bands of anchored dyes (e.g. rhodamine B sulfonylchloride, Figure 4) in comparison to the main bands of the free chromophores in solution. 

Other Papers.—Various iron species prepared by the vacuum pyrrolysis of acetyl-ferrocene-furfural resins at 400°C have been studied by Mossbauer spectroscopy. These consist of an amorphous glass-like carbon matrix containing free iron atoms, Fe+ ions, iron clusters, superparamagnetic iron, and ferromagnetic iron.333 The effect of pressure of up to 50kbar on the absorption spectra of five iron(m), two iron(n) and one mixed valence compound has been studied. In six of the compounds, but not in basic ferric acetate or soluble Prussian Blue, the observed pressure-induced bands were assigned to d-d transitions of converted iron(n) for the ferric compounds and to spin-forbidden d-d bands for the ferrous compounds. The charge-transfer band from iron(n) to iron(m) in soluble Prussian Blue showed a blue shift at pressures up to 7.2 kbar.334... 

The large red shifts observed in dichloromethane and benzene could not be exclusively accounted for by TCA-induced changes in the polarizability of the medium. The authors explained the effect by suggesting that the Coulombic interaction with a negative ion close to the PRSB is associated with a red shift rather than with the previously proposed (123,128-132,146) blue shift. This argument is difficult to accept since it leaves unexplained the basic spectral difference between PRSB in vacuum (theoretical) and in a nonpolar solution. Moreover, it is based on the... 

We may now interpret the blue shift of the n —w transition. Pre-i sumably the excitation of a nonbonding electron localized on a basic functional group to an antibonding pi orbital shifts electron density away from the basic group. This reduces its base strength, and results in a weaker H bond in the excited state. The red shift of a tt —>- tt transition in a base can be attributed to electron redistribution with... 

A shift in A,max to shorter wavelength is called a hypsochromic effect, or blue shift, and usually occurs when compounds with a basic auxochrome ionise and the lone pair is no longer able to interact with the electrons of the chromophore. Hypsochromic effects can also be seen when spectra are run in different solvents or at elevated temperatures. Spectral shifts of this type can be used to identify drugs that contain an aromatic amine functional group, e.g. the local anaesthetic benzocaine (see Figure 7.9). 

The current list of blue-shifting hydrogen bonds is quite impressive (see Table 3 which collects most of them, basically those which in some sense have become classical) and it considerably grows each year (interestingly, among more than 1000 articles published in Journal of Molecular Structure between 1982 and 1996, only nine were referred to the C-H- -B bonding, see Ref [50]). 

COjEt R"" = COPh, = COPh R = Bu", R = H, R = COPh Z = As, R = Ph, R2 = H, R3 = COPh or COjMe MejSCHCOPh CsHjNCHCOPh), in contrast to the dinuclear complexes [Hg2(Ylide)2Cl4] formed with HgCl2. It was found that v(CO) for the complexes exhibits a blue shift relative to the free ylide, the values approaching those of completely protonated onium salts, indicative of coordination via the methine C atom. Similar thiocyanato-complexes showed both N and S bonding modes with N co-ordination favoured by complexes of the least basic ylides. 

Section 6.4.2 discussed that the donor-acceptor transition in I2—Lewis base complexes is modified depending on the extent of the donor interaction with the I2 LUMO. The blue shift (to higher energy) in the d-n/ 9o- transition upon I2 complexation has also been correlated to Lewis base strength. As shown in Figure 6.10, this transition increases in energy as the base strength increases. Table 6.11 lists blue shifts induced by selected bases, used to assess Lewis basicity. 

For tabulated infrared shifts of ICN, I2, and ICl as well as TtTj 9cr blue shifts C. Laurence and J.-F. Gal, Lewis Basicity and Affinity Scale Data and Measurement, John Wiley and Sons, United Kingdom, 2010, pp. 286-306. 

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