Frequency local environment

Adsorption enthalpies and vibrational frequencies of small molecules adsorbed on cation sites in zeolites are often related to acidity (either Bronsted or Lewis acidity of H+ and alkali metal cations, respectively) of particular sites. It is now well accepted that the local environment of the cation (the way it is coordinated with the framework oxygen atoms) affects both, vibrational dynamics and adsorption enthalpies of adsorbed molecules. Only recently it has been demonstrated that in addition to the interaction of one end of the molecule with the cation (effect from the bottom) also the interaction of the other end of the molecule with a second cation or with the zeolite framework (effect from the top) has a substantial effect on vibrational frequencies of the adsorbed molecule [1,2]. The effect from bottom mainly reflects the coordination of the metal cation with the framework - the tighter is the cation-framework coordination the lower is the ability of that cation to bind molecules and the smaller is the effect on the vibrational frequencies of adsorbed molecules. This effect is most prominent for Li+ cations [3-6], In this contribution we focus on the discussion of the effect from top. The interaction of acetonitrile (AN) and carbon monoxide with sodium exchanged zeolites Na-A (Si/AM) andNa-FER (Si/Al= 8.5 and 27) is investigated. 

The idea of a transition between two energy levels suggests that the transition will occur at only one precise frequency as a sharp spike in the absorption or emission spectrum. This is not the case and, in fact, the transitions have an intrinsic width and shape containing information about the local environment of the atoms. The line profile of an atomic transition has contributions from three effects ... 

We conclude that it is likely that the OH stretching frequency depends on both 00 separation and hydrogen bond donor angle, and that neither the VRB or KHLP correlations is complete. For the present we accept the VRB association of spectral components with local environments as a conjecture which must be tested against other experimental data. 

Figure 6. Li MAS NMR spectrum of the layered compound Li2MnOs acquired at a MAS frequency, Vr, of 35 kHz. Spinning sidebands are marked with asterisks. The local environment in the Mn +/Li+ layers that gives rise to the isotropic resonance at 1500 ppm is shown. Spin density may be transferred to the 2s orbital of Li via the interaction with (b) a half-filled t2g orbital and (c) an empty d/ Mn orbital to produce the hyperfine shifts seen in the spectrum of Li2MnOs. The large arrows represent the magnetic moments of the electrons in the t2g and p orbitals, while the smaller arrows indicate the sign of the spin density that is transferred to the Li 2s and transition-metal d orbitals.

Figure 12. Li MAS NMR spectra of Li[Li(i-2A)/3Mn(2- r)/3-NiJ02, with A-= 1/10, 1/3, and 1/2. The resonances corresponding to local environments Li(OMn)6 and Li-(OMn)4(OLl2) found in Li2Mn03 are marked. The Li(OMn)6 local environment is shown on the right. The frequencies of the major resonances are indicated asterisks indicate spinning sidebands.

Both the V and Li NMR spectra show multiple vanadium and lithium local environments for the as-synthesized material x = 0.15), and the spectra cannot be explained by using a simple model based on the number of crystallographically distinct vanadium sites. On Li-ion intercalation, the V resonances sharpen and shift to higher frequencies (Figure 15) three sharp resonances along with two broader resonances are clearly resolved for the samples prepared at potentials of 3.4 and 3.0 V (x = 0.3 and 0.5, respectively). This behavior is consistent with solid—solution behavior in this potential range and is ascribed to the presence of localized defects at X close to 0 and electron delocalization for 1 > x > 0.05. Three lithium sites were observed in the Li... 

These results mean that the surface ions are located on the silica surface in at least three different local environments (in approximate agreement with the picture illustrated in Fig. 37). The frequencies of the D(CO) stretching modes (all higher than that of the CO gas) are similar to those observed for CO on ZnO thus, electrostatic and a-type overlap forces are mainly involved. Most likely, the electrostatic interaction prevails in the complexes absorbing at 2200 cnT1 (as expected for an ion carrying a charge of +3). 

The principle of the experiment is illustrated by Fig. 12. On the left-hand side the absorption band of an inhomogeneously broadened transition, e.g., the OH-stretching band in different H-bonded local environments, is depicted schematically. Structures with different OH-O bond angles and/or 0-0 bond lengths show up in the spectrum with different OH frequencies linear bonds and shorter bond length correspond to larger red shifts... 

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