Stretching anti symmetric

The IR spectra of silicon oxides, in the framework region mode, is dominated by a strong absorption around 1000 cm due the anti-symmetric stretching of the Si - 0 - Si unit (Raman inactive mode) and by a less intense absorption around 800cm due the symmetric stretching of the Si-O-Si unit (Raman active mode). In the transparency window between these two modes, the IR spectra of TS-1 shows an additional absorption band located at 960 cm ... 

Strong bands due to the anti-symmetrical and symmetrical stretching vibrations of the COO- grouping. 

Stretching vibrations are due to stretching and contracting bonds with no change in bond angles. This stretching can be symmetric or anti symmetric. In symmetric it is in the same direction. In anti symmetric stretching, one atom approaches the central atom while the other departs from it. 

This molecule has two different sets of symmetry axes. There is a C2 axis passing through the carbon atom at right angles to the bond direction and there is an oo fold axix (C ) passing through the bond axis itself. The names symmetric stretch and anti symmetric stretch are self evident. The symmetric stretch produces no dipole which remains zero. So this vibration is not infra red active. 

Figure 7.6 Overview of the CN and CO stretching frequencies observed for the several states of the A. vinosum [NiFe] hydrogenase.The symmetric and anti-symmetric stretching frequencies of the CN groups and of the stretching frequency of the CO molecule from the Fe(CN)2(CO) moiety in the active site are shown.The band of externally added CO is indicated in grey.

Diketones, syn-trans-open chains 1730-1710 plane as the C=0. Anti-symmetrical stretching... 

The vibrational spectrum of 4-pyridine-carboxylic acid on alumina in Fig. 4d is equivalent to an infrared or Raman spectrum and can provide a great deal of information about the structure and bonding characteristics of the molecular layer on the oxide surface. For example, the absence of the characteristic > q mode at 1680 cm 1 and the presence of the symmetric and anti-symmetric O-C-O stretching frequencies at 1380 and 1550 cm indicate that 4-pyridine-carboxylic acid loses a proton and bonds to the aluminum oxide as a carboxylate ion. 

Excitation of symmetric and anti-symmetric stretch motion... 

Manz and Romelt (1981). Rm and 7 hi are the two I-H bond distances. The heavy point marks the saddle point and the shaded area indicates schematically the Franck-Condon region in the photodetachment experiment. The arrow along the symmetric stretch coordinate (f Hi = -Rih) illustrates the early motion of the wavepacket and the two heavy arrows manifest dissociation into the two identical product channels, (b) The same PES as in (a) but represented in terms of hyperspherical coordinates (p, i9) defined in (7.33). The horizontal and the vertical arrows illustrate symmetric and anti-symmetric stretch motions, respectively, as indicated by the two insets. 

Fig. 7.19. Photodetachment spectrum for IHI the wavelength of the excitation laser is 266 nm. u s denotes the number of quanta in the anti-symmetric stretch mode in the transition region. The widths of the peaks do not represent the true lifetimes of the meta-stable states. Adapted from Neumark (1990)...

The autocorrelation function S(t) shown in Figure 7.20 reflects these two types of motion the wide oscillations with period Tss represent the slow symmetric stretch vibration and the rapid oscillations with period Tas represent the fast anti-symmetric stretch vibration. Because the two exit channels are so exceedingly narrow leakage into the product channels is rather weak. Although no barrier hinders the dissociation the system is trapped in the inner region for a long time. 

Fig. 7.20. Autocorrelation function for the dissociation of EHI. The wide oscillation reflects the symmetric stretch motion of the two iodine atoms with respect to the stationary hydrogen atom with period Tss whereas the fast oscillations manifest the anti-symmetric stretch motion of hydrogen between the two iodine atoms with period Tas. Adapted from Engel (1991a).

Fig. 8.2. Typical potential energy surface for a symmetric triatomic molecule ABA. The potential energy surface of H2O in the first excited electronic state for a fixed bending angle has a similar overall shape. The two thin arrows illustrate the symmetric and the anti-symmetric stretch coordinates usually employed to characterize the bound motion in the electronic ground state. The two heavy arrows indicate the dissociation path of the major part of the wavepacket or a swarm of classical trajectories originating in the FC region which is represented by the shaded circle. Reproduced from Schinke, Weide, Heumann, and Engel (1991).

The period of the anti-symmetric stretch periodic trajectory does not correspond, however, to any of the three recurrences we see in Figure 8.4. This is not at all surprising in order to come back to the FC region, which in this case is considerably displaced from the anti-symmetric stretch orbit, the trajectory must necessarily couple to the symmetric stretch mode. If we were to launch the wavepacket at the outer slope of the saddle point, the anti-symmetric stretch periodic orbit would support recurrences by itself without coupling to the symmetric stretch mode. An example is the dissociation of IHI discussed in Section 7.6.2. 

A time-independent adiabatic approximation, based on the local separability of symmetric and anti-symmetric stretch motion in the region of the saddle point, provides a complementary picture (Pack 1976). Within the adiabatic limit the eigenenergies of the symmetric stretch motion on top of the potential ridge are defined through the one-dimensional... 

Fig. 13.3. Left-hand side Contour plots of the modulus square of the lowest four anti-symmetric eigenfunctions of H2O in the electronic ground state, multiplied with the X — A transition dipole function. They have a node on the symmetric stretch line. The assignment rnn ) is based on the local mode expansion (13.5)...

---