Symmetric stretching vibrations

The most widely known molecular movements are stretching vibrations (symmetrical and asymmetrical) and bending vibrations of angular deformation (Figure 10.6). Alternately, in the region of the spectrum below 1500cm the absorption bands are numerous and differ for each compound. These are deformation vibrations of both the bonds and the skeleton and are difficult to assign with accuracy. 

The stretching vibrational modes of methylene group. The two drawings show stretching vibrations — symmetric (on the left) and asymmetric on the right. 

For microporous solids, the bands from framework vibrations depend on both the structure type and the composition. For zeolites, substitution of framework aluminium results in broadening of these bands and a shift to lower wavenumber, which is attributable to the Al-O bonds being weaker than Si-O bonds. As a result, and once the variation of frequency with composition of a certain framework stretching vibration (symmetric or asymmetric... 

Infrared IR spectroscopy is quite useful in identifying carboxylic acid derivatives The, carbonyl stretching vibration is very strong and its position is sensitive to the nature of IKT the carbonyl group In general electron donation from the substituent decreases the double bond character of the bond between carbon and oxygen and decreases the stretch mg frequency Two distinct absorptions are observed for the symmetric and antisym metric stretching vibrations of the anhydride function... 

The symmetric and antisymmetric stretching vibrations of methylamine can be viewed on Learning By Modeling... 

Normal modes of vibration, with their corresponding normal coordinates, are very satisfactory in describing the low-lying vibrational levels, usually those with u = 1 or 2, which can be investigated by traditional infrared absorption or Raman spectroscopy. For certain types of vibration, particularly stretching vibrations involving more than one symmetrically equivalent terminal atom, this description becomes less satisfactory as v increases. 

Consider the CFI stretching vibrations of benzene, for example. Since there are six identical C—FI bonds there are six CFI stretching vibrations. These belong to various symmetry species but only one, V2, illustrated in Figure 6.39, is totally symmetric 0. ... 

Figure 6.39 The totally symmetric CH stretching vibration V2 of benzene...

Figure 9.24 shows part of the laser Stark spectrum of the bent triatomic molecule FNO obtained with a CO infrared laser operating at 1837.430 cm All the transitions shown are Stark components of the rotational line of the Ig vibrational transition, where Vj is the N-F stretching vibration. The rotational symbolism is that for a symmetric rotor (to which FNO approximates) for which q implies that AA = 0, P implies that A/ = — 1 and the numbers indicate that K" = 7 and J" = 8 (see Section 6.2.4.2). In an electric field each J level is split into (J + 1) components (see Section 5.2.3), each specified by its value of Mj. The selection mle when the radiation is polarized perpendicular to the field (as here) is AMj = 1. Eight of the resulting Stark components are shown. 

If the proton is not equidistant between A and B, it will undergo some movement in the symmetric stretching vibration. Isotopic substitution will, therefore, result in a change in transition state vibrational frequency, with the result that there will be a zero-point energy difference in the transition state. This will reduce the kinetic isotope effect below its maximal possible value. For this type of reaction, therefore, should be a maximum when the proton is midway between A and B in the transition state and should decrease as H lies closer to A or to B. 

The symmetric stretching vibration. During this vibration the ellipsoid breathes (i.e. expands and contracts) at the frequency of the vibration (Fig. 4). The dimensions of the ellipsoid obviously change during the vibration and consequently the vibration is Raman active. 

An alternative way to view these changes in polarizability is illustrated in Fig. 5. During the symmetric stretching vibration (curve 1), the polariz-... 

Infrared activity of vibrations is readily deduced. The symmetric stretching vibration has no associated dipole moment change during the vibration and is, therefore, infrared inactive. The asymmetric stretching vibration has an associated dipole moment which fluctuates with the frequency of the vibration. The vibration is, therefore, infrared active. 

Each normal mode of vibration can be described by a normal coordinate Qi which is a linear combination of nuclear displacement coordinates of the molecule. For the symmetric stretching vibration vi of C02, the normal coordinate is of the form... 

The 520 nm absorption of S4 has also been observed in the spectrum of certain red-colored ultramarine samples [21] the assignment of this band to the C2V isomer of 4is supported by the simultaneously observed Raman line at 678 cm which represents the symmetrical stretching vibration of the terminal SS bonds of this molecule (see Table 1 below). 

In a KI matrix the electronic absorption maximum of 82 - is observed at 400 nm, and the 88 stretching vibration by a Raman line at 594 cm k 83 shows a Raman line at 546 cm and an infrared absorption at 585 cm which were assigned to the symmetric and antisymmetric stretching vibrations, respectively. The bromides and iodides of Na, K, and Rb have also been used to trap 82 - but the wavenumbers of the 88 stretching vibration differ by as much as 18 cm- from the value in KI. The anion S3- has been trapped in the chlorides, bromides and iodides of Na, K, and Rb [120]. While the disulfide monoanion usually occupies a single anion vacancy [116, 122], the trisulfide radical anion prefers a trivacancy (one cation and two halide anions missing) [119]. 

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