Difference spectra, matrix isolation

UV-vis spectra of matrix-isolated intermediates are not so informative as matrix IR spectra. As a rule, an assignment of the UV spectrum to any intermediate follows after the identification of the latter by IR or esr spectroscopy. However, UV-vis spectra may sometimes be especially useful. It is well known, for example, that the energy of electronic transitions in singlet ground-state carbenes differs from that of the triplet species. In this way UV spectroscopy allows one to identify the ground state of the intermediate stabilized in the matrix in particular cases. This will be exemplified below. 

The third member, trimethylenemethane (3), had some relevance to our studies on carbenes, since besides methylene and its simply substituted derivatives trimethylenemethane 3 is one of the few molecules having a triplet ground state.22 Also the experience with 3 could be of help in order to deal with the singlet/triplet differentiation in matrix-isolated carbenes. We learned that, if the calculated IR spectra of the singlet and triplet molecule are sufficiently different, it might be possible to determine the multiplicity of the matrix-isolated species by comparison with the experimental IR spectrum. In this context it is also worth mentioning that we were able to measure the matrix IR spectrum of 3, but a special technique (irradiation in halogen-doped xenon matrices) had to be developed in order to achieve a concentration of 3 sufficient for its IR detection.23... 

In 1990 we showed that ethenedithione (115) is a stable molecule under matrix conditions. It can be prepared by photolysis of the matrix-isolated precursors 113, 114, and 116.143 Different pathways to 115 have been found by Wentrup et al.144 The matrix IR spectrum of 115 shows one absorption corresponding to the only IR active stretching mode. The IR active bending vibration is expected to appear in the for us unobservable far infrared region. The position of both IR inactive stretching vibrations were derived from two observed combination bands. The IR spectra allow no decision about the multiplicity of 115, since calculations show, that the equilibrium geometries of both states are almost identical. Recent calculations121 145 favor the triplet state. 

At low enough temperatures vibrational fine structure of aromatic chromophores may be well resolved, especially if they are embedded in a suitable matrix such as argon or N2, which is deposited on a transparent surface at 15 K. This matrix isolation spectroscopy77166 may reveal differences in spectra of conformers or, as in Fig. 23-16, of tautomers. In the latter example the IR spectra of the well-known amino-oxo and amino-hydroxy tautomers of cytosine can both be seen in the matrix isolation IR spectrum. Figure 23-16 is an IR spectrum, but at low temperatures electronic absorption spectra may display sharp vibrational structure. For example, aromatic hydrocarbons dissolved in n-heptane or n-octane and frozen often have absorption spectra, and therefore fluorescence excitation spectra, which often consist of very narrow lines. A laser can be tuned to excite only one line in the absorption spectrum. For example, in the spectrum of the carcinogen ll-methylbenz(a)anthrene in frozen octane three major transitions arise because there are three different environments for the molecule. Excitation of these lines separately yields distinctly different emission spectra.77 Likewise, in complex mixtures of different hydrocarbons emission can be excited from each one at will and can be used for estimation of amounts. Other related methods of energy-... 

The different techniques of flash photolysis are used to detect transient species, that is atoms, molecules and fragments of molecules which have very short lifetimes. These cannot be observed by usual experimental techniques which require rather long observation times. For example, the measurement of an absorption or fluorescence spectrum takes several seconds, and this is of course far too long in the case of transient species which exist only for fractions of a second. In some cases these transient species can be stabilized through inclusion in low-temperature rigid matrices, a process known as matrix isolation . 

Figure 6.8. Fluorescence excitation spectra of matrix isolated 9-deuteroxyphenalenone (lower) and methyl-9-deuteroxyphenalenone (upper) as examples of nearly symmetric and asymmetric double well potentials for hydrogen transfer, shown on the left. The suppression of hot band 01 is shown in a separate spectrum at 3.5 K. Due to asymmetry of the potential, the wave functions are linear combinations of the left and right well function with different amplitudes if/t = a2 + b2, ij/a = -b2 + a2, b/a = 0.22 and 0.80 in the ground and excited states of the methyl derivative. (From Barbara et al. [1989].)...

Figure 4.4-4 Difference IR spectrum of matrix-isolated pyrolysis products according to Eq. 4.4-4 in Ar at 10 K before and after UV photolysis for 10 minutes. Photolysis isomerizes silaethene (o) to methylsilylene (V). The bands marked by a dash are due to hexafluoroxylene which has not been completely eliminated. (Reproduced with permission from Chem. Ber. (1984) 117 2369).

It is always desirable to back up IR absorption spectroscopy with Raman measurements. The different selection rules for the two techniques means that, at least for symmetric species, it is often necessary to have data from both types of measurement to have a full picture of the vibrational spectrum. Raman spectroscopy has been used to study many matrix-isolated species although there are problems regarding intensity and photosensitivity. An excellent review exists on the subject that highlights both the applications and difficulties of the method. A molecule that has been well characterized by both IR and Raman spectroscopy is the matrix-isolated species Mo(C )s(N2) (15). Spectra for (15) are illustrated... 

Fig. 16 IR difference spectra of 3QC isolated in argon matrix matrix irradiated for 50 min at A>235 nm minus the matrix irradiated for 100 min at A>315 nm (top) matrix irradiated for 100 min at l>315nm minus the as-deposited matrix (middle), and simulated difference spectrum built from the B3LYP/6-311- - -G(d,p) calculated spectra for syn (bands pointing up) and anti (bands pointing down) 3QC conformers (bottom). " " Reprinted with permission from N. Kus, M. S. Henriques, J. A. Paixao, L. Lapinski and R. Fausto, J. Phys. Chem. A, 2014, 118, 8708. Copyright (2014) American Chemical Society.

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