Isomers, matrix isolation

Nonmetal-atom, matrix-isolation spectroscopy has proved useful in structure and isomer determination of stable, metal carbonyls. Fe(CO)4(NO) was investigated (157) in low-temperature matrices with CO enrichment, and it was demonstrated that the IR spectrum is consistent with C v symmetry (trigonal bipyramid with an equatorial NO), in agreement with X-ray studies (55). The work resolves the dis-... 

By ab initio MO and density functional theoretical (DPT) calculations it has been shown that the branched isomers of the sulfanes are local minima on the particular potential energy hypersurface. In the case of disulfane the thiosulfoxide isomer H2S=S of Cg symmetry is by 138 kj mol less stable than the chain-like molecule of C2 symmetry at the QCISD(T)/6-31+G // MP2/6-31G level of theory at 0 K [49]. At the MP2/6-311G //MP2/6-3110 level the energy difference is 143 kJ mol" and the activation energy for the isomerization is 210 kJ mol at 0 K [50]. Somewhat smaller values (117/195 kJ mor ) have been calculated with the more elaborate CCSD(T)/ ANO-L method [50]. The high barrier of ca. 80 kJ mol" for the isomerization of the pyramidal H2S=S back to the screw-like disulfane structure means that the thiosulfoxide, once it has been formed, will not decompose in an unimolecular reaction at low temperature, e.g., in a matrix-isolation experiment. The transition state structure is characterized by a hydrogen atom bridging the two sulfur atoms. 

In a detailed study of the IR transitions of matrix-isolated 1-methylsilene and its isomer dimethylsilylene the structures of these species154 have been clearly defined. 

Vibrational spectra have been used to identify matrix-isolated chalcogen-nitrogen species, which are unstable under ambient conditions.35,36 In this way the species NS, SNS (and the less stable isomer NSS), NNS, NSe, SeNSe and [SeNSe]+ have been identified. The technique of 15N-enrichment has been used in several cases to distinguish S-N from S-S vibrations. 

S-37 (see above) it is also possible to prepare and to matrix-isolate the silicon species 124, 125, and 126, which again exist in a photoequilibrium. Our first entry to 1-silacyclopropenylidene (124) was the pulsed flash pyrolysis of 2-ethynyl-l,l,l-trimethyldisilane (123).71,72 Even though the structure of educt molecule 123 suggests formation of ethynylsilylene (125), the isolated product was 124. Obviously 125 had already thermally isomerized to the most stable isomer 124 before the products were condensed at 10 K. 

In 1986, Kesselmayer and Sheridan reported the matrix isolation and IR spectrum of chloro-methoxymethylene. They were able to show that two isomeric forms were present, trans (12) and cis (13). This is in line with Schaefer s " finding that hydroxymethylene should exist in cis- and frons-forms. While Kesselmayer and Sheridan were able to assign several of the bands in the matrix mixture to the cis- and trans-forms, they were unable to assign the majority of the bands in the IR spectrum they obtained to the specific isomer. In order to further confirm their isolation of these alkoxy carbenes, as well as to help in the interpretation of the observed IR spectrum, we undertook the calculation of the IR spectra of 12 and 13. ... 

A detailed assignment of the IR absorption frequencies of 1,2,4-trioxolane (1) in solid argon was achieved by comparison with the spectra of various isotopically substituted species <82JPC3154>. Similarly, for a series of substituted 1,2,4-trioxolanes, characteristic IR bands obtained via matrix isolation were assigned and compared with those of 1,2,4-trioxolane (1) (ethylene ozonide) (Table 7) <82JPC4548>. The spectra of cis- and trani-1,2,4-trioxolanes indicate that the cis isomer has characteristic absorptions in the range 820-855 cm with the trans isomer at 1320-1360 cm . 

Evidence for the formation of 34 (R = Ph) was provided by neutralization reionization mass spectrometry and more directly by the matrix isolation and spectroscopic investigations on 34 (R = Ph) in an argon matrix at 12 K. The UV spectrum of 34 (R = Ph) exhibits characteristic bands at X = 364, 386, 404, 420, 440, 470 and 502 nm, resembling those of the electronic spectrum of anthracene, but with the expected bathochromic shifts. If one irradiates into the maximum at X = 502 nm, all bands shown in the spectrum disappear completely within 5 minutes. The vanishing of these characteristic bands can again be explained by the photoisomerization of silaanthracene 34 (R = Ph) to the corresponding Dewar valence isomer. 

A relatively long known access to silylenes is the thermolysis of disilanes13, used for preparative purposes180 as well as for matrix-isolation studies of silylenes107. In our group this method was recently used in the generation of C2H2Si isomers. Trimethylsilane... 

The [n]cyclophanes are the archetypal small cyclophanes and, within this area, it is the [n]paracyclophanes (Fig. 1) that have received by far the most attention. A detailed computational study of [4]paracyclophane [1], which has only been prepared as a transient species via matrix isolation [2], was recently reported. An energy difference of 9 kcal mol between it and its Dewar benzene isomer was predicted. This paper also provides a comprehensive summary of the literature of the [n]paracylophanes. 

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