Complexation-induced shift

Besides, information on intermolecular interactions has been derived in these studies from complexation-induced shifts (CIS). The chemical shift is an indicator for the shielding of a nucleus and thus for the electronic state of a specific proton. Since the electronic environment may change on complexation, CIS can be used to monitor where host-guest contacts may take place. If these interactions occur stereoselectively, the CIS will be different for the two guest enantiomers (AS distinct from 0) giving possibly some insight into the chiral recognition mechanism. 

The stability constants in Table 2 were calculated from saturation curves obtained by NMR titration under the assumption that the complexation-induced shift of the cation protons is independent on whether an anion is simultaneously bound or not. 

Fig. 2.4.5. Left Monte Carlo-optimized structure of 6 Ac-Orn-Ala-OMe in chloroform (1000 steps). Right Arrows indicate complexation-induced shifts in host and guest during the titration.

Equation (10.15) describes the complexation-induced shift (CIS) of an NMR signal. The systematic variation of 8 with changing values of r (= [GJo/[H]0) forms the basis of a general technique known as NMR titration for the determination of K values for complexation. 

The NMR chemical shift of the guest molecule was calculated using the GIAO-DFT method with B3LYP/6-311G(d,p) and compared to the chemical shift of carbon disulfide calculated at the same level. It was found that on the simple model, shift difference is in good agreement with the experimentally determined complex-induced shift. 

Fig. 22. The complexation induced shifts of the energies of the lowest excitations of nucleic acid bases in the dimers. Shifts are calculated with respect to the free molecule (guanine or cytosine in the guanine-cytosine complex and adenine or thymine in the adenine-thymine complex). For each monomer four excitations are given singlet-singlet A , singlet-singlet A , singlet-triplet A , and singlet-triplet A . Data taken from [Wesolowski, J. Am. Chem. Soc., 126, (2004) 11444].

Note Ring-current (anisotropy) effects 60-85% (orientation dependent). Linear electric field effects 15-40% (orientation dependent) complexation-induced shifts in ppm. 

A primary indication on how well the monomers have been chosen is to simply see whether they are capable of assisting solubilization of the template in the prepolymerization mixture. A small-scale solubility test may thus be a good way to initially screen the monomers for strong monomer-template interactions. Weak interactions may be revealed by complexation induced spectral changes (in NMR,UVor fluorescence spectra). The complexation induced shifts of the characteristic H-NMR signals of the template upon increasing monomer concentrations are often used to estimate the monomer-template association constants. Prior to this, however, knowledge about the stoichiometry of the monomer-template complexation and the tendency of the monomer and template to self-associate are required.The former can be obtained by means of a so-called Job s plot whereas the latter by a dilution experiment. 

This expectation is supported by the observations of the different trends of C-13 shift displacements in the high-resolution solid state C-13 NMR spectra of corresponding a-CD inclusion complexes[3]. In the solid-state, the a-CD complexation induced shifts are caused by transference of the guest molecule from the free state, surrounded by the same molecules, to the CD cavity. 

The trends of the a-CD complexation-induced shifts for the ptotonated carbons, i.e., C2 auid C3, of each guest compoiind, are also not always reproducible by the continuum solvent effect model. Since the hydrogen atoms bonded to these carbons ceui contact with a-CD s H-3 euid H-5 located inside the cavity in the complexed state, the steric shielding effect must also be considered for the explanation of the C-13 shifts of these carbons. 

Solid-state guest dynamics of dimeric capsules of tetra-tolyl urea caltx[4]arene filled with different aromatic guests such as benzene-d6, fluorobenzene-ds and 1,4-difluorobenzene were studied. Upon inclusion, all guest moieties revealed complexation-induced shifts varying from 2.8 to 5.1 ppm. All guest molecules were shown to undergo... 

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