Skeletal modes

Ion Symm. type Skeletal modes H modes Total Select, rules... 

The porphyrin skeletal mode frequencies are all slightly lower both for the triplet and for the ground state. Table II lists the triplet downshifts, l-7cm , along with the shifts observed for radical cations (13a) and anions (13b) for MgTPP and ZnTPP reported by Itoh and coworkers. Since the triplet excited states are formed by... 

The spectra presented in Figures 1-3 demonstrate that high quality, transient resonance Raman spectra can be obtained for Ni(OEP) and Ni(PP) solutions using Soret excitation. These spectra can be interpreted on a molecular level by comparison with the extensive theoretical and experimental data base that exists for ground state nickel porphyrin species (8-16 and refs, therein). The coordination state of nickel porphyrins can easily be detetmined from the resonance Raman spectrum of the sample (10.12). Several modes in the Raman spectrum of porphyrins are quite sensitive to the state of axial ligation (10.12). In particular, the marker lines V4, 11 2> 3 10 (porphyrin skeletal mode designations follow those of Abe et al., (I2a). The designation... 

Several calculational and descriptive models of ROA have been applied to the interpretation of the 3-methylcyclohexanone low-frequency ROA spectrum (11, 75-77). The main difliculty in understanding the source of ROA couplets in general has been the lack of a complete, accurate description of the normal modes that give rise to specific ROA features. A study carried out recently in our laboratory (77) used the Raman spectra of specifically deuterated isotopo-mers of 3-methylcyclohexanone and the APT model of ROA (43) as a basis for generating a more accurate force field for the skeletal modes. From the com-... 

Different types of chemical reactions involve different types of vibrational modes, e.g. dissociation reactions may be controlled by stretching vibrations, isomerizations by skeletal modes, and so on. The argument that infrared quanta are relatively energy-poor and infrared transitions generally have low absorption cross sections, especially if multiphoton excitation is required, limits the choice of suitable molecular transitions. With respect to these constraints the type of reaction chosen and described below was dissociation, involving molecules with maximal transition dipole moments, comparatively weak bonds to be broken, and vibrational excitation in the mid-infrared spectral range. 

The double proton transfer of [2,2 -Bipyridyl]-3,3 -diol is investigated by UV-visible pump-probe spectroscopy with 30 fs time resolution. We find characteristic wavepacket motions for both the concerted double proton transfer and the sequential proton transfer that occur in parallel. The coherent excitation of an optically inactive, antisymmetric bending vibration is observed demonstrating that the reactive process itself and not only the optical excitation drives the vibrational motions. We show by the absence of a deuterium isotope effect that the ESIPT dynamics is entirely determined by the skeletal modes and that it should not be described by tunneling of the proton. 

We observe the coherent excitation of an optically inactive mode proving that the reactive process itself and not only the optical excitation drives the observed vibrational motions. Further we demonstrate that during the ESIPT the proton is adiabatically shifted from one site to the other and tunneling of the proton is not rate determining. The dynamics is entirely controlled by the skeletal modes. Interestingly, this is quite similar to ground state proton transfer of HC1, where the fluctuations of the water environment enable the adiabatic process [8]. 

In cases where q = 4, e. g., when the positions of substituents on the chain increase the true identity period to four elements, we can show that two other skeletal modes can become potentially active. This of course assumes that the substituents can be neglected and the previous treatment used, an unlikely assumption. Nevertheless the results may serve as crude guides to the location of other possible skeletal frequencies for such polymers. Application of equations (34)—(37) shows that these modes are expected at... 

In addition, an out-of-plane skeletal mode can be active whose frequency is given by [Pitzer (774)]... 

Summarizing, we may say that the normal vibration analysis permits a good understanding of the skeletal modes (and hydrogen modes, as we shall see later) of the polyethylene chain. Extension of such calculations to other polymers is needed. 

It has been suggested [Tadokoro, Seki, and Nitta (218) Tadokoro (215b)] that this band arises from a symmetric C—C stretching mode, which would be consistent with its polarization parallel to the zig-zag chain, but this assignment presents difficulties too. In particular, it places such a skeletal mode at what seems to be much too high a frequency, and the skeletal vibration involved, viz., i>+(0), is already adequately assigned to the band at 915 cm-1. 

While it cannot be said that all of the assignments in the spectrum of PVdC are satisfactorily established, it would seem that the majority are. The important ones from the standpoint of the chain structure are the v (CC12) modes, and the present evidence on their assignment supports the correctness of the Fuller structure. A study of a partially deuterated polymer might be of further assistance in this respect. What is also clearly needed is a more detailed study of the skeletal modes to be expected from the proposed chain structure, since these clearly contribute significantly to the spectrum. Finally, knowledge of the crystal structure might permit more to be said on the nature of interchain interactions and their effect on the spectrum. 

Kirkwood, J. G. The skeletal modes of vibration of long chain molecules. J. chem. Phys. 7, 506—509 (1939). 

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