Molecules stretching vibrations

Figure Bl.25.12 illustrates the two scattering modes for a hypothetical adsorption system consisting of an atom on a metal [3]. The stretch vibration of the atom perpendicular to the surface is accompanied by a change m dipole moment the bending mode parallel to the surface is not. As explained above, the EELS spectrum of electrons scattered in the specular direction detects only the dipole-active vibration. The more isotropically scattered electrons, however, undergo impact scattering and excite both vibrational modes. Note that the comparison of EELS spectra recorded in specular and off-specular direction yields infomiation about the orientation of an adsorbed molecule.

Figure B2.5.18 compares this inter molecular selectivity with intra molecular or mode selectivity. In an IR plus UV, two-photon process, it is possible to break either of the two bonds selectively in the same ITOD molecule. Depending on whether the OFI or the OD stretching vibration is excited, the products are either IT -t OD or FIO + D [24]- hr large molecules, mirmnolecular selectivity competes with fast miramolecular (i.e. unimolecular) vibrational energy redistribution (IVR) processes, which destroy the selectivity. In laser experiments with D-difluorobutane [82], it was estimated that, in spite of frequency selective excitation of the...

Van der Waals complexes can be observed spectroscopically by a variety of different teclmiques, including microwave, infrared and ultraviolet/visible spectroscopy. Their existence is perhaps the simplest and most direct demonstration that there are attractive forces between stable molecules. Indeed the spectroscopic properties of Van der Waals complexes provide one of the most detailed sources of infonnation available on intennolecular forces, especially in the region around the potential minimum. The measured rotational constants of Van der Waals complexes provide infonnation on intennolecular distances and orientations, and the frequencies of bending and stretching vibrations provide infonnation on how easily the complex can be distorted from its equilibrium confonnation. In favourable cases, the whole of the potential well can be mapped out from spectroscopic data. 

Most infrared spectroscopy of complexes is carried out in tire mid-infrared, which is tire region in which tire monomers usually absorb infrared radiation. Van der Waals complexes can absorb mid-infrared radiation eitlier witli or without simultaneous excitation of intennolecular bending and stretching vibrations. The mid-infrared bands tliat contain tire most infonnation about intennolecular forces are combination bands, in which tire intennolecular vibrations are excited. Such spectra map out tire vibrational and rotational energy levels associated witli monomers in excited vibrational states and, tluis, provide infonnation on interaction potentials involving excited monomers, which may be slightly different from Arose for ground-state molecules. 

Once prepared in S q witli well defined energy E, donor molecules will begin to collide witli batli molecules B at a rate detennined by tire batli-gas pressure. A typical process of tliis type is tire collision between a CgFg molecule witli approximately 5 eV (40 000 cm or 460 kJ mor ) of internal vibrational energy and a CO2 molecule in its ground vibrationless state 00 0 to produce CO2 in tire first asymmetric stretch vibrational level 00 1 [11,12 and 13]. This collision results in tire loss of approximately AE= 2349 cnA of internal energy from tire CgFg,... 

When the above analysis is applied to a diatomic species such as HCl, only k = 0 is present since the only vibration present in such a molecule is the bond stretching vibration, which has a symmetry. Moreover, the rotational functions are spherical harmonics (which can be viewed as D l, m, K (Q,< >,X) functions with K = 0), so the K and K quantum numbers are identically zero. As a result, the product of 3-j symbols... 

The above figure shows part of the infrared absorption spectrum of HCN gas. The molecule has a CH stretching vibration, a bending vibration, and a CN stretching vibration. 

Other general circumstances in which normal vibrations tend to be localized in a particular group of atoms arise when there is a chain of atoms in which the force constant between two of them is very different from those between other atoms in the chain. For example, in the molecule HC=C—CH=CH2 the force constants in the C—C, C=C and C=C bonds are quite dissimilar. It follows that the stretchings of the bonds are not strongly coupled and that each stretching vibration wavenumber is typical of the C—C, C=C or C=C group. 

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. 

C-nmr data have been recorded and assigned for a great number of hydantoin derivatives (24). As in the case of H-nmr, useful correlations between chemical shifts and electronic parameters have been found. For example, Hammett constants of substituents in the aromatic portion of the molecule correlate weU to chemical shifts of C-5 and C-a in 5-arylmethylenehydantoins (23). Comparison between C-nmr spectra of hydantoins and those of their conjugate bases has been used for the calculation of their piC values (12,25). N-nmr spectra of hydantoins and their thio analogues have been studied (26). The N -nmr chemical shifts show a linear correlation with the frequencies of the N—H stretching vibrations in the infrared spectra. 

Solvation effects on the molecular vibrations of 128 were studied by SCRF methods and by supermolecular approaches of 128 with one water molecule [97JPC(B) 10923, 98JPC(A)6010]. Correlations between the N—H (uracil) and O—H (water) bond elongations and the corresponding frequency shifts of the stretching vibrations are reported as... 

Because an applied field in the y direction Ev can induce a dipole M with a component in the x direction Mx as well as the component in the y direction My, it is necessary that we specify the components of the polarizability tensor by two subscripts (Fig. 3). If the bond A—B of a diatomic molecule stretches during a vibrational mode, Mx and Mv will vary and therefore the corresponding polarizability tensor components will vary. 

The antisymmetric stretching vibration. The molecule loses its original symmetry during the vibration. At the two extrema of the vibration the shapes of the molecule will be identical. Because the molecular polarizability is essentially the summation of all bond polarizabilities and is independent of direction along the internuclear axis, it will have identical values at the extrema. Consequently, the vibration is Raman inactive. 

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... 

Now consider a molecular stretching vibration that alternately elongates and compresses axial (parallel to z, let s say) and equatorial bonds as outlined in Fig. 7-3. Imagine an extreme vibration of this kind that eventually distorts an octahedral molecule so as to gradually remove two trans ligands (again, let this direction be... 

---