Stretching perturbations, asymmetric

Note that the structural form of each compound is implied by the presence (T-shaped structure) or absence (distorted structure) of the perturbed asymmetric stretching mode at about 550 cm . 

The experimental spectra are interpreted by Tozer and Sosa as follows In the Na compound, the structure is of the form NaF...F2, and it exhibits an absorption due to the complex at 455 cm, with a 460 splitting (this mode is denoted (Oj). For the other two, T-shaped compounds, the two highest frequencies resemble perturbed forms of the symmetric and asymmetric F-F-F stretching modes that we saw in the F3 anion, which we denote (O2 and (O3. The Cs compound exhibits the asymmetric F3 stretching ((O3) at 550 cm", while the K structure exhibits this vibration at 549 cm" along with a weak absorption at 467 cm". The latter may represent a weakly-active symmetric stretch ((03). 

The high frequency shift in the asymmetric P—O" stretching frequency caused by adsorption on hydroxyapatite appears to be a perturbation of lattice bonds as a result of surface changes. The minimum specific surface necessary to cause a lattice shift by a particular adsorbate has not been ascertained. The difference in sensitivity between different preparations of hydroxyapatite is shown in Table IV. These differences are best explained, at present, by differences in surface groups resulting from minor differences in washing procedure. Rootare, Deitz, and Carpenter (10) discuss hydrolysis reactions of surface phosphate ions and the... 

Migchels et al. have evaluated the effect of interaction with a proton-donating water or methanol molecule upon the internal vibrational frequencies of a number of imines. These perturbations are reported in Table 3.61 at the SCF/6-31G level and indicate first that the red shift of the hydroxyl group of methanol is quite a bit larger than that of water. This discrepancy may be due to the symmetric nature of the water molecule which thoroughly mixes the two O—H stretches into a symmetric and asymmetric pair. The frequency chosen by the authors as a reference point for water is 4145 cm quite distinct from the 4032 cm of the purer O—H stretch in CH OH. Nonetheless, the large shifts in the methanol com-... 

In order to know whether the Pd ions or complexes are anchored to the zeolite framework or not, the IR framework vibrations of Pd-H-ZSM-5(0.49) were investigated (Figure 5). After activation under O2, a weak band at 930 cm" forms. Upon NO adsorption, the 930 cm band disappear while a new band appears at 980 cm". These bands are attributed to asymmetric internal stretching vibrations of T-O-T bonds (T = Si or Al) perturbed by Pd ions. The higher the perturbation, the lower the frequency. Therefore, the 930 cm band could be related to anchored Pd(II) ions or complexes formed upon decomposition of exchanged complexes, and the 980 cm band could be due to Pd(I) nitrosyl entities formed upon NO contact. Similar observations were found on Cu-ZSM-5 catalysts (34). 

Interestingly, four (compared to two in Fig. 3.6) O-H stretching bands were observed in the IR-PD spectrum of the H5OJ H2 complex ]49]. Based on a comparison with computed vibrational frequencies, it was concluded that the perturbation exerted by H2, which is bound by less than 4 kcal moh, is already sufficient to break the symmetry and stabilizes an asymmetric H20- -H30+ structure, for which four O-H bands are predicted. 

The LIF of polyatomic molecules opens possibilities to recognize perturbations both in excited and in ground electronic states. If the upper state is perturbed its wave function is a linear combination of the BO wave functions of the mutually perturbing levels. The admixture of the perturber wave function opens new channels for fluorescence into lower levels with different symmetries, which are forbidden for unperturbed transitions. The LIF spectrum of NO2 depicted in Fig. 1.56 is an example where the forbidden vibrational bands terminating on the vibrational levels (ui, U2, U3) with an odd number U3 of vibrational quanta in the asymmetric stretching vibrational mode in the electronic ground state are marked by an asterisk [182]. 

Two new bands — in-phase (ui) and out-of-phase (I ls) antisymmetric CH2 stretching vibrations of allyl radical have been obtained in the slit jet discharge spectrometer, as the sample spectra shown in the top panel of Fig. 5.18. The data have been successfully analyzed with a Watson asymmetric rotor Hamiltonian, yielding precise band origins and rotational constants for both bands. The high quality of least squares fits to ground state combination differences indicates that the rotational level structure in the lower state is well behaved, while the reduced quality of fits to the vibrational transitions, on the other hand, suggest the presence of Coriolis mediated rotational perturbations in the upper state. Due to sub-Doppler resolution (Ai/ 70 MHz) in the slit jet expansion. 

---