Matrix isolation IR spectra

Infrared spectroscopy is by far the most popular tool for the inverstigation of matrix-isolated species. By virtue of the suppression of most rotations in sohd matrices, IR spectra recorded under these conditions typically show patterns of very narrow peaks, compared to spectra obtained under normal laboratory conditions (solution, Nujol, or KBr pellets), where bands due to different vibrations often overlap to the extent that they cannot be separated. As a consequence, matrix isolation IR spectra are—at least potentially—are a very rich source of information on the species under investigation. Whether and how all this information can be used depends on the ability to assign the spectra, a subject to which we will return below. 

Figure 6. Matrix isolation IR spectra (left) and MS spectra (right) for selected unknowns in charbroiled chicken (cf. Figure 5). Top Rt 28.821 min., C8-dienal. Middle Rt 28.986 min., mixture of 7-hexalactone and phenylacetaldehyde. Bottom Rt 29.207 min., 2-hydroxybenzaldehyde.

Case Studies Gas and Matrix Isolated IR Spectra of the Vinyl Radical... 

An important step in the process of establishing the existence of silenes and the nature of their bonding was the direct observation of their matrix-isolated IR spectra, accomplished independently by three research groups in the same year97-100. The matrix isolation spectrum of the parent silene was reported subsequently101. A detailed account of the matrix-isolation work is given in Reference 2. 

Fig. 1.36. Matrix isolation IR spectra of Cr(CO)e in N2 (a) andAr (b) matrices. The bottom spectra were obtained by computer simulation [ 127].

Matrix-isolation IR spectra have been obtained for the HOOH.CO adduct, including vOH 3547.7 cm- and 800H 1286.8 cm" (Ar matrix data). Similar data were reported for the 1 1 complexes H2O2.NX3 (X = H or Me) and H20.H202. The Raman spectrum of [Mc20CF3] included bands from... 

There is FTIR evidence for the formation of Ge2C. Matrix-isolation IR spectra of Me2Ge=CH2 contain vGe=C at 818.8/847.3 cm (coupled with the... 

A comprehensive review of matrix-isolation studies of organic photochemistry is not possible in a work of this type, because it would run to many hundreds — possibly thousands — of references. Fortunately, two useful bibhographies have been pubhshed the first covers the literature from the invention of the technique up to about 1985, while the second extends this period to about 1997. These bibliographies are not free of omissions and errors, nor will they be found in every library, but both contain formula and author indexes and are valuable sources of information on matrix-isolation research. An earlier bibliography, which covered IR spectra at sub-ambient temperatures, including matrix-isolation IR spectra. and an associated review of IR spectra of reactive species trapped in matrices give useful entry points to the literature up to about 1968. Also, various aspects of matrix studies in organic photochemistry have been reviewed. ... 

A matrix isolation IR study of cyclic siladienes was more successful (Khabashesku et al., 1992). At first, unstable l-silacyclopenta-2,4-diene [128] was generated by vacuum pyrolysis (800°C 10 -10 Torr) of 5-silaspiro[4.4]nona-2,7-diene [129] or pyrolysis and photolysis (A = 248 nm) of l,l-diazido-l-silacyclopenta-2,4-diene [130] it has been studied by UV and IR spectroscopy in an argon matrix at 12 K. The UV band at Amax = 278 nm and nine IR bands (including two sp Si-H stretching vibrations at 2175 and 2144 cm ) have been recorded in matrix spectra of [128]. Reversible photochemical interconversion of [128] with silacy-... 

As mentioned above, it is often advantageous to look at difference spectra, and this is especially true in matrix isolation IR spectroscopy, because, due to the... 

The matrix isolation technique allows one to study the IR spectra of free radicals and other reactive species. Here, one cools a mixture of a compound of interest and a gas such as Ar or N2 to a temperature of a few degrees Kelvin. Photolysis of the frozen sample produces free radicals trapped in an inert matrix their IR spectra can then be observed. Species studied include CH3, CF2, HCO, SiF2. [See L. Andrews, Ann. Rev. Phys. Chem., 22, 109 (1971).]... 

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-... 

IR spectroscopy is not confined to stable substances. In recent years, matrix isolation IR spectroscopy has become important in the investigation of short-lived, unstable molecular species. A gas containing such highly-reactive molecules - produced by photolysis of a reaction mixture, or in a high-temperature furnace - is suddenly cooled by contact with an inert solid (e.g. argon at c. 40 K). The matrix-isolated molecules are protected by the low temperature from unimolecular decomposition, and - by sheer isolation, if the dilution is sufficient - from bimolecular processes such as dimerisation or disproportionation. For example, the photolysis of Mn(CO)5H by a laser produces the otherwise unstable Mn(CO)5 and Mn(CO)4H molecules whose IR spectra can be measured in an argon matrix. Because of the low temperature, the lack of inter-molecular interactions and the rigidity with which the molecules are trapped in the matrix, such spectra are often very well resolved, better than can be achieved by conventional methods. Thus matrix isolation spectroscopy is widely used in the study of stable species, in preference to conventional techniques. 

A high-resolution matrix-isolation IR study of 13C02 in the v2 and v3 regions shows that (in a neon matrix) v2 is split (649.37, 648.73 cm-1), while v3 shows as a singlet at 2282.15 cm-1.344 The phase behaviour of C02 was followed by in situ Raman spectroscopy up to 67 GPa and 1660 K.345 A high-resolution IR study has been made of the (30°1) band of C02 (6230-6250 cm-1).346 IR spectra and DFT calculations were used to probe the effects on C02 vibrations of adsorption on SWCNT.347 IR spectra were also used to study C02 trapped in SWCNT bundles, via behaviour of the vas mode near 2330 cm-1.348... 

Fig. 2.19. Matrix isolation IR (a) and computer simulation spectra (b) of GeCU. Vertical lines in (b) show the five-peak chlorine isotope pattern of GeCh.

Figure 1 MI/FT-IR spectra (fingerprint region) of (A) menthol, (B) neomenthol, (C) isomenthol and (D) neoisomenthol, 25 ng each. Reproduced with permission of the Society for Applied Spectroscopy from Coleman III WM and Gordon BM (1989) Examinations of the matrix isolation infrared spectra of organic compounds Part XIII. Applied Spectroscopy4Z 303.

In solution the keto forms dominate. In matrix-isolation IR observation of C=0 stretch, frequencies points to keto forms but too many lines are present in the spectra to be able to exclude other tautomers as well [28]. The same problem manifests itself in resonance-enhanced multiphoton ionization (REMPI or R2PI) spectroscopy, where a dense spectrum is observed that potentially consists of contributions of multiple tautomers [29], These contributions can be separated by UV-UV hole-burning, revealing the presence of four different isomers in the gas-phase jet-cooled conditions of these experiments [30]. Three of these tautomer-selective REMPI spectra appear in Figure 7.3. Further identification is possible by IR-UV hole burning and by tautomeric blocking with methyl derivatives [30-32]. The spectra of species A (with the red-most origin at... 

Mn(II) tetraphenylporphyrin [Mn(II)TPP] binds dioxygen at low temperatures, forming the 1 1 adduct [84]. EPR spectra have signals indicative of a S=3/2 ground state, which has been interpreted as arising from a Mn(IV)-peroxo complex. Moreover, the presence of six-line hyperfine features supports the assignment of a monomeric complex. Matrix isolation IR experiments at 15 K found vibrational bands at 983, 958 and 933 cm when Mn(II)TPP was co-condensed with a 1 2 1 mixture of 02, and 02. The bands at 983 and 933 cm corre-... 

In the numerous studies of the IR-spectra of the matrix-isolated reactants (see, e.g., Frei and Pimentele [1983]) there have been found a number of chemical conversions. Among them, from the point of view of the present review, the reaction NO with O3 [Lucas and Pimentele 1979],... 

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