Complexation-induced frequency shifts

The complexation-induced frequency shifts for ArnDF/HF allow the characterization of the small shifts in monomer vibrational energy upon the incremental addition of solvent atoms. For large values of n, the... 

The binding of a ligand to a paramagnetic lanthanide ion induces frequency shifts in the NMR spectrum (LIS) of the ligand relative to that observed for the corresponding diamagnetic complex. The LIS has two contributions, the Fermi contact (6c) and the dipolar or pseudocontact shifts (5pc) ... 

Some new lanthanide porphyrin complexes of general formula Ln(iii)TAP(/i-diketonate), where TAP = tetraarylporphine, have been prepared (468,469) and their shift reagent capabilities assessed. The complexes of the early lanthanides (Pr, Nd, Sm, Eu) induce very small shifts whereas the complexes of Tb, Dy, and Ho produce quite large low frequency shifts in y-picoline. Analogous shift reagents based on silicon, germanium, low-spin iron, (517, 518) and cobalt (519) have been reported. 

Assume that the sample does not consist of planar layers, but instead of a sand pile, a froth, an AFM tip, an assembly of spheres or vesicles, a cell culture, a droplet, or any other kind of heterogeneous material. There are many interesting samples of this kind. The frequency shift induced by such objects can be estimated from the average ratio of stress and speed at the crystal-sample interface. The latter is the load impedance of the sample. The concept of the load impedance tremendously broadens the range of applicability of the QCM. The load impedance is the conceptual link between the QCM and complex samples. If we want to predict the frequency shift induced by a complex sample, we must ask for the average stress-speed ratio. If this ratio can be estimated in one way or another, a quantitative analysis of the experimental QCM data is in reach. Otherwise, the analysis must remain qualitative. 

Nematic liquid crystals (LCs) are a classical example of complex fluids. If we trust the small-load approximation as well as the matured theory of nema-todynamics [76], we must be able to predict the frequency shift induced by nematic LCs. The theory of nematic LCs in contact with the QCM has been worked out in detail by people who did not know about the QCM as a tool to probe these phenomena. These authors performed ultrasonic reflectometry. As we know from Sect. 5, the results of these studies can be transported to the QCM in a straightforward way by just using Eq. 39. 

According to our calculations, C2H2 adsorption on MgO is qualitatively different from methane adsorption [121]. CH4 was shown not to bind at sites of the regular MgO(OOl) surface. Therefore, the adsorption-induced red shift of IR frequencies of CH4 relative to the free molecule was assigned to adsorption complexes at low-coordinated edge and comer sites. However, adsorption of C2H2 at MgO(OOl) terraces is bound. In MgO samples of low surface area, adsorption sites on regular (001) terraces likely dominate the number of populated... 

For vibrational properties, solute—solvent short-range interactions not only can induce a shift in the frequency but they can also modify the normal modes. This effect can be taken into account only by including some specific solvent molecules within the QM portion of the system. This supermolec-ular approach, however, introduces some additional aspects that make the analysis more complex. First of aU, the selection of the number and the position of the solvent molecules to be included in the QM part is not unequivocal moreover, as now the solvent enters as a QM component, we cannot easily dissect the response of the solute from that of the solvent molecules. 

Figure 9.12 shows the crystal structure of [Fe(pyrazine) Pt(CN)4 ] [13]. This complex shows a thermally induced spin-crossover transition (Tct = 284 K, T l = 308 K) with a thermal hysteresis of 24 K, which was observed by means of magnetic susceptibility measurement and Raman spectroscopy. The spin-crossover transition has been confirmed by Fe Mossbauer spectroscopy [13]. The Mossbauer spectrum at 300 K in the cooling mode consists of a single doublet with quadrupole splitting (QS) of 1. 159(5) mm s and isomer shift (IS) of 1.047(3) mm s whose values are typical of the HS state ( T2g, S = 2) of Fe(ll). At 80 K, a new doublet with quadrupole splitting of 0.306(4) mm s and isomer shift of 0.439(2) mm s whose values are typical of the LS state ( A g, 5 = 0) of Fe(ll). The photoinduced spin conversion between the LS and HS states around room temperature has been confirmed by means of Raman spectroscopy within the thermal hysteresis loop of spin-crossover transition, which is shown in Fig. 9.13 [13]. In this complex, the frequency of...

Table 14 reports the shifts in the frequencies of each molecule as a result of formation of the complex. Both Vj and V3 are red-shifted by several wave numbers whereas a small positive shift occurs in V2. The near insensitivity of these frequencies to perturbations induced by formation of the H-bond is consistent with prior experimental measurements [176]. Certainly the largest change occurs in the stretch of the proton donor. Somasundram et al. calculate a red shift of some 260 cm, somewhat smaller than the experimental measurement of 353 cm" [177]. Latajka and Scheiner s calculated shift for HCl of 105 cm" is similarly smaller than an experimental value of 216 cm" [178]. Whereas the experimental shift for H2O HF corresponds to a gas-phase system, the latter value was measured in N2 matrix which might explain some of the discrepancy. A primary point of comparison is that all of the frequency shifts are smaller in the H2O HCl complex, consonant with the weaker binding. 

The entropy difference A5tot between the HS and the LS states of an iron(II) SCO complex is the driving force for thermally induced spin transition [97], About one quarter of AStot is due to the multiplicity of the HS state, whereas the remaining three quarters are due to a shift of vibrational frequencies upon SCO. The part that arises from the spin multiplicity can easily be calculated. However, the vibrational contribution AS ib is less readily accessible, either experimentally or theoretically, because the vibrational spectrum of a SCO complex, such as [Fe(phen)2(NCS)2] (with 147 normal modes for the free molecule) is rather complex. Therefore, a reasonably complete assignment of modes can be achieved only by a combination of complementary spectroscopic techniques in conjunction with appropriate calculations. 

The behavior of the Raman spectrum under stress of the stretching vibration of the B—H complex has been reported recently by Stutzmann and Herrero (1988a,b) and by Herrero and Stutzmann (1988a,b). Spectra measured at 100 K are shown in Fig. 18 for several values of [100] stress. The dependence of the mode frequency on [100] and [112] stress is shown in Fig. 19. There were stress induced splittings observed for [100], [112], and [110] stress directions. For the [111] stress direction the line broadened for low stresses but did not split. Further, the stress-split component that shifts upward in frequency as the stress is increased decreases in intensity. 

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