Vibrational spectra cyclohexane

We commence our discussion with a consideration of the vibrational spectrum of the much-studied cyclohexane molecule when adsorbed on various metal surfaces. The majority of published papers have been concerned with adsorption on metal single-crystal faces. 

In DMSO the spectrum consisting of an asymmetric broad band, extended towards the blue wing of the maximum, is correctly reproduced by the simulation (O Fig. 39-25 right). This proves the reliability of the calculated vibrational progression hidden within the band. However, similar to the case of cyclohexane, the spectrum is found to be red-shifted by 400 cm The authors ascribe the latter to the effect of dynamic solvent fluctuations which are much more pronounced in polar solvents. Finally, the DMSO cyclohexane solvatochromic shift is estimated to be only 150 cm smaller than its experimental counterpart. 

Figure 5 A CARS vibrational spectrum produced by monitoring the output beam intensity (at C04) while wavelength scanning an OPO (see Figure 4(B)). This spectrum shows Raman-active peaks from benzene (b), oxygen (0), nitrogen (n), and cyclohexane (c) covering a range from 681 cm- (Aqpo = 552 nm) to 3098 cm- (Iqpo = 637 nm). Zero frequency shift corresponds tolopo = 532 nm.

Figure 9.17 UV-visible spectrum of benzene (6 x 10 4 mol dm 3) in cyclohexane at 298 K, showing vibrational fine structure...

Figure 9.17 shows a spectrum of benzene (IV) in cyclohexane, which clearly shows small peaklets superimposed on a broader band (or envelope). These peaklets are called vibrational fine structure. In benzene, they are caused by excitation from v" = 0 to v = 1, v = 2, etc. Consideration of Figures 9.15 and 9.16 suggests that excitation from v" = 0 to v = 1 requires less energy than from v" = 0 to v = 2, so the excitation v" = 0 -> v = 1 occurs on the right-hand side of the figure, i.e. relates to processes of lower energy. 

Another explanation for their resonance Raman results could be a change in the zwitterionic nature of the merocyanine isomers in the different solvents which may result in changes in the Raman transition probabilities, or the spectral changes could be due to solvent shifts of the absorption spectrum, resulting in a change in the relative contribution of the different vibrational modes to each resonance Raman spectrum. We note that in the same article, the authors report the transient absorption spectra of the merocyanine forms, which clearly show that the BIPS spectrum in cyclohexane has more discrete vibrational modes than are observed in the polar solvents, which show more spectral broadening. Al-... 

Tamai and Masuhara [26] also worked on NOSH, but in 1-butanol. They could examine femtosecond dynamics for the C—O bond breaking and formation of a primary photo-product X, which formed within 1 psec and had a broad absorption with peaks at 450 and 700 nm. The spectrum of X then evolved, forming a broad merocyanine-type spectrum, which itself evolved with time to form the usual merocyanine spectrum in that solvent after less than 400 psec. The spectral broadening was said to be either due to the formation of a vibrationally hot ground state or to an equilibration between isomeric forms because the spectrum that formed at early times was similar to the spectrum usually obtained in cyclohexane. Tamai s spectra are shown in Fig. 3. 

The ultraviolet spectrum of pyrazine in cyclohexane shows maxima at 260 nm (corresponding to a tt-tt transition) and 328 nm (corresponding to a n-7r transition) in each case with vibrational fine structure the coefficients of molecular extinction are 5600 and 1040, respectively.78,79 Substitution of halogen has a bathochromic effect on the ultraviolet spectrum of pyrazine.80 A useful index of the ultraviolet and visible spectra of pyrazine derivatives is available for the period from 1955 to 1963.81 The far-ultraviolet spectrum of pyrazine... 

The solvent often exerts a profound influence on the quality and shape of the spectrum. For example, many aromatic chromophores display vibrational fine structure in non-polar solvents, whereas in more polar solvents this fine structure is absent due to solute-solvent interaction effects. A classic case is phenol and related compounds which have different spectra in cyclohexane and in neutral aqueous solution. In aqueous solutions, the pH exerts a profound effect on ionisable chromophores due to the differing extent of conjugation in the ionised and the non-ionised chromophore. In phenolic compounds, for example, addition of alkali to two pH units above the pKa leads to the classical red or bathochromic shift to longer wavelength, a loss of any fine structure, and an increase in molar absorptivity (hyper chromic... 

The sensitivity of lattice modes to structural changes is illustrated by the recent study of Mueller and Connor [25] on the effects of cyclohexane adsorption on the structure of MFI zeolites. The adsorption of molecules such as paraxylene and benzene into MFI zeolites causes a structural transition from monoclinic to orthorhombic symmetry, which has been characterized by X-ray powder diffraction and 29 si NMR spectroscopy [26]. Cyclohexane has a slightly larger kinetic diameter than benzene or paraxylene (0.60 nm compared with 0.585nm), but does not cause the same structural transition. Cyclohexane adsorption does however affect the zeolite lattice mode vibrational frequencies. Figure 7 shows spectra taken from reference 25 before and after (upper spectrum) adsorption of cyclohexane in a low aluminium MFI zeolite. 

A solvent for ultraviolet/visible spectroscopy must be transparent in the region of the spectrum where the solute absorbs and should dissolve a sufficient quantity of the sample to give a well-defined analyte spectrum. In addition, we must consider possible interactions of the solvent with the absorbing species. For example, polar solvents, such as water, alcohols, esters, and ketones, tend to obliterate vibration spectra and should thus be avoided to preserve spectral detail. Nonpolar solvents, such as cyclohexane, often provide spectra that more closely approach that of a gas (compare, for example, the three spectra in Figure 24-14). In addition, the polarity of the solvent often influences the position of absorption maxima. For qualitative analysis, it is therefore important to compare analyte spectra with spectra of known compounds measured in the same solvent. 

A solution of Pd(PPhj)4 (767 mg, 0.665 mmol), Af-phenyldiphenylketenimine (2.93 g, 10.9 mmol) and (l-methylethylidene)cyclopropane (2.03 g, 35.6 mmol) in anhyd toluene (30 mL) was placed in a 200-mL stainless steel autoclave. The autoclave was pressurized with 4 MPa of Nj at rt and then heated to 123 °C for 30 h with stirring. After the reaction was complete (most easily monitored by the disappearance of the ketenimine stretching vibration in the IR spectrum), the mixture was filtered through a pad of alumina (EtjO) in order to remove the solid elemental palladium. After removal of the readily volatile components (bp up to 30 C/10" Torr), the brown, vi.scous residue was purified by medium-pressure chromatography (silica gel, CHjClj/cyclohexane 1 1). Under these conditions, most of the impurities were washed off, while the cycloadduct remained on top of the column and was finally eluted with pure Et20 yield 3.3 g (93%) brown needles 98% pure (as determined by GC) mp 148°C (toluene). 

The ultraviolet absorption spectrum of quinoxaline in cyclohexane shows bands with vibrational fine structure at 340 (log e 2.84), 312 (log e 3.81), and 232 nm (log e 4.51) which are attributed to n—tt and 77-77 transitions. In ethanol the vibrational fine structure disappears and the less intense n-77 band appears as a shoulder on the long-wave 77- 77 band. However in methanol and in water the n—tt band is obscured by the more intense 77-77 band. The weak n-77 bands in the ultraviolet spectra of 6-chloro- and 6-bromoquinoxaline ° and certain... 

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