Benzene vibrational modes

Figure Bl.6.10 Energy-loss spectrum of 3.5 eV electrons specularly reflected from benzene absorbed on the rheniiun(l 11) surface [H]. Excitation of C-H vibrational modes appears at 100, 140 and 372 meV. Only modes with a changing electric dipole perpendicular to the surface are allowed for excitation in specular reflection. The great intensity of the out-of-plane C-H bending mode at 100 meV confimis that the plane of the molecule is parallel to the metal surface. Transitions at 43, 68 and 176 meV are associated with Rli-C and C-C vibrations.

Let us eonsider the vibrational motions of benzene. To eonsider all of the vibrational modes of benzene we should attaeh a set of displaeement veetors in the x, y, and z direetions to eaeh atom in the moleeule (giving 36 veetors in all), and evaluate how these transform under the symmetry operations of D6h- For this problem, however, let s only inquire about the C-H stretehing vibrations. 

Using the information provided above, whieh of the C-H vibrational modes of benzene will be infrared-aetive, and how will the transitions be polarized How many C-H vibrations will you observe in the infrared speetrum of benzene ... 

The first study belongs to Brown and Rogers", who measured the IR spectra of methyl-and ethyllithium as Nujol mulls, aud of ethyllithium in benzene solution, and assigned the bands observed at 880 and 925 cm to the C—Li stretching vibrational modes of the monomeric and polymeric species, respectively. 

The lowest excited states of benzene are actually of B2u and Bi symmetry, and they can be reached directly by neither one-photon nor two-photon purely electronic transitions (the 0-0 band at energy Eq-o. the origin of the transition, is absent from the spectra). However, excitation into these states can be obtained through vibronic coupling (VC), if a vibrational mode of an appropriate symmetry is coupled to the electronic transition. The IPA or 2PA spectra can then show a series of narrow peaks shifted from the 0-0 band... 

Fig. 3 IPA (dashed line) and 2PA (solid line) spectra of liquid benzene. The 2PA spectrum was obtained by the thermal lensing method. The units on the energy scale are kilokaysers (1 kK = 1000 cm ) and refer to the total energy of the transition. 0-0 is the origin of the Lb band, 18, 14, and 1 are indices for the vibrational modes (the superscript and subscript are the quantum numbers for the mode in the excited state and ground state, respectively). Reproduced with permission from [33]. 1986, American Chemical Society...

In 1995, we decided to stop further research on this topic because it seemed a philosophical idea that engages primarily theoreticians with little or no impact on experimental chemistry. That this was a premature decision became obvious at the end of 1995, when we learned from Haas and Zilberg31 about the frequency exaltation of the bond alternating vibrational mode in the l B2U excited state of benzene. This created a link of the purely theoretical notion to an experimental probe. The synthesis of Siegel s distorted benzene32,33 demonstrated that a ground state cyclohexatriene has been finally achieved and that this may be a case where 7r-dis-tortivity is at work. This was followed by a demon-... 

One might have thought that the state- and mode-specificity of the frequency exaltation is dictated by the symmetry of the states and vibrational mode. Scheme 35 shows, however, that benzene derivatives where the DSh symmetry is clearly absent exhibit this frequency upshift of Kekule mode.3 234 235 Moreover, in all these systems, no other state and no other mode... 

The vibrational spectrum of benzene around 1000 cnf has also been measured. IQ. Benzene was physisorbed on a cooled copper substrate in the vacuum chamber. Figure 19 shows the transmission for several thicknesses of benzene and a prism separation of 3 cm. The thickness was determined from the measured transmission in transparent regions using Eg. (7). The solid curves were calculated from Eqs. (5) and (6) using optical constants for benzene obtained from an ordinary transmission experiment.il The benzene film was assumed to be isotropic. Of the two absorption lines seen, one belongs to an in-plane vibrational mode, and one to an out-of-plane vibration. Since the electric field of the SEW is primarily perpendicular to the surface, the benzene molecules are clearly not all parallel or all perpendicular to the copper surface. Also it should be noted that the frequencies are the same (within the experimental resolution) as those of solid benzene22 and of nearly the same width. These features indicate that the benzene interacts only weakly with the copper surface, as would be expected for physisorbed molecules. 

Once the possibility of distortion is raised, an immediate next question is the direction of this distortion, considered as a vibrational mode of the undistorted system. A simple model for the directional aspect of 7r-distortivity was proposed in a tutorial context for benzene itself by Heilbronner [9]. It has an easy generalisation, which turns out to predict the most likely pattern of distortion of many systems by pencil-and-paper construction of the Heilbronner vectors or Heilbronner modes [13]. The present paper reviews some symmetry- and graph-theoretical aspects... 

Due to these large deviations the direct application of the SQMF method to the B3 molecule may lead to incorrect assignment of the vibrational modes. Therefore we performed a preliminary SQMF calculation on the benzene molecule, where the normal modes are well ascribed [1] and discriminated by symmetry. The extracted scale factors correspond to the set of symmetry-adapted internal coordinates, as introduced by Wilson [1], The same scale factors were then applied to the B3 molecule, which was considered as a system constructed by three benzene molecules. The single C-N bonds in B3 were treated with the same scale factors as the single C-H bonds in the benzene molecule. The scale factors corresponding to the N-H bonds were initially set equal to 1. The as-obtained scaled force-field, after minor adjustment of the scaling factors, was employed to calculate the normal frequencies of B3. Fig. 2(b) shows the corresponding pattern of the calculated scaled frequencies. It can be seen that there... 

Figure 7.6c shows that the vibrational mode b2u interchanges the Kekule structures K and Kr. Since the excited state is made only from these Kekule structures, then by analogy to benzene, one will expect that the frequency of this mode will be larger in the B2u excited state than in the Ag ground state, which is indeed what is observed by two-photon spectroscopy (16,24). Another analogy... 

L. Goodman, M. J. Berman, A. G. Ozkabak, J. Chem. Phys. 90, 2544 (1989). The Benzene Ground State Potential Surface. III. Analysis of b2u Vibrational Mode Anharmonicity Through Two-Photon Intensity. 

The 20 vibrational modes of benzene are pictorially illustrated in Fig. 7.3.16. Also shown are the observed frequencies. In this figure, i (CH) and i (CC) represent C-H and C-C stretching modes, respectively, while 8 and n denote in-plane and out-of-plane bending modes, respectively. For each E mode, only one component is shown. 

Vibrational modes and their observed frequencies (in cm-1) of benzene. 

Li et al. [47] fabricated SAMs of ruthenium phthalocyanine (RuPc) on a silver substrate precoated with an SAM of 4-mercaptopyridine (PySH) or l,4-bis[2-(4-pyridyl)ethenyl]-benzene (BPENB). SERS spectroscopy was used to explore the structure and orientation of the self-assembled films, and they successfully observed Raman bands due to vibrational modes of the pigment molecules in the composite films in the SERS spectra. 

The active vibrational modes for benzene phosphorescence. From ref. [83]. 

---