Rotamers, matrix isolation

The third mechanism of isomerization, photoinduced rearrangements of radical cations, has been pursued in a variety of systems. Matrix isolated radical cations have been noted to undergo some rigorous reorganizations as well as subtle ones. For example, the ring opening of cyclohexadiene to hexatriene radical cation and the interconversion of its different rotamers have been achieved by irradiation with UV or visible light [173-174]. 

Amino-oxo and amino-hydroxy forms of 9-methylguanine have been identified in approximately equal amounts by IR studies in an argon matrix at 12 K. The amino-hydroxy tautomer occurs as two rotamers. The tautomer ratio is sensitive to UV <91 mi 711-05). The same authors have similarly studied matrix-isolated guanine, 2-dimethylamino-6-hydroxypurine and some related compounds. The estimated molecular equilibrium constants in nickel matrixes were ca. 3.6 for guanine and 5.9 for 9-methylguanine <9UST(156)29>. 

In the gase phase, the infrared bands are broad (50 cm ), due to the rotational structure, overlapping vibrations, and hot transitions. In the solid state, the rotational motions are quenched, but due to intermolecular (hydrogen bond) and correlation field interactions, the band positions are shifted and the bands are even broader. The infrared absorptions of matrix-isolated molecules are close to the gas-phase frequencies and exhibit a sharp line-like character (half-widths 0.1 to 2 cm ). Hence the spectra of matrix-isolated molecules are less complicated, and, in comparison to gas phase or solid state spectra, the sensitivity and selectivity of detection increase by a factor of about 10 to 100. Closely spaced vibrations attributed to mixtures of similar molecules, such as conformers, rotamers, molecular complexes, or isotopic species, e.g., H C104 and H CI04, are easily distinguished. 

In subsequent argon matrix isolation studies, similar bands were found when hexatriene or cyclohexadiene are ionized , and eventually, five of the six possible rotamers of hexatriene radical cation were identified by selective, wavelength-specific in ter conversions . Similar results were later obtained for octatetraene , where six of the twenty possible rotamers are formed on ionization in argon (Figure 31) which could be interconverted and identified by selective photolysis . Interestingly, in the case of the butadiene radical cation the s-cis rotamer could not be detected, even if the diene radical cation was formed from the cyclobutene radical cation . In contrast, in a recent resonance Raman study, some weak bands were detected and assigned to the s-cA-butadiene radical cation which might have escaped detection in the earlier ESR and EA experiments . ... 

One distinctive feature of polyene radical cations (especially of the long ones) is their great photosensitivity. For example, exposure of a sample of matrix isolated all-fra s octatetraene radical cation to diffuse daylight leads, within about an hour, to a new pho-tostationary equilibrium containing at least six rotamers. It is by virtue of this sensitivity, which is in part due to the very large absorptivities of the second EA bands, that a highly selective rotamer interconversion can be achieved, using a very narrow bandwidth . 

A very good example of the enormous benefits that DFT computations of IR spectra have brought to matrix-isolation studies giving the technique a veritable new lease of life in the study of organic reactive intermediates - has been provided by a study of the photolysis of a-pyrone (32) and its 4,6-dimethyl derivative (Breda et al.. Chapter 6). The photochemistry of a-pyrone was the subject of some of the earliest matrix-isolation studies of organic species, but the use of DFT computations has now allowed a virtually complete identification of the individual rotamers of the ring opened aldehyde-ketene (33)-(36). 

The photochemistry of a-pyrone (57a) (Scheme 10) was the subject of some of the earliest matrix-isolation studies of organic species. This system still has some interest for further study, however, especially on account of the success that DFT computations have had in assisting with the assignments of matrix IR spectra. Recently, the matrix photolyses of both a-pyrone and its 4,6-di-methyl derivative (57b) have been investigated with the aid of DFT calculations. When (57a) is irradiated (A > 285 nm) in Ar or Kr matrices, rapid formation of rotamers, (58a) and (59a), of the Z isomer of the ring-opened... 

Vibrational assignment of the rotational isomers (rotamers) of methyl nitrite has been accomplished by means of a photochemical effect vibrational and electronic spectra were measured for the vapour phase and matrix-isolated samples of H3CONO and DsCONO. ... 

A combined ab initio and variable-temperature Raman spectroscopic study provides strong evidence for the presence of three rotamers, gauche, ortho and anti, in the liquid state of the unstrained disilane CF3SiMe2SiMe2Cp3, a phenomenon which has so far been only experimentally proven for the carbon chain n-C4Fio. We are confident to obtain further proof for the existence of three conformers for CF3SiMe2SiMc2CF3 by matrix-isolation IR spectroscopy. 

When -2,5-dimethyl-l,3,5-hexatriene (16) was isolated in Ar matrices, it was found to exist almost exclusively as the tEt rotamer, but UV irradiation led to the formation of thermodynamically less stable rotamers (Scheme 4). The various rotamers could be identified by comparison of their experimental and computed IR transitions. When the Z triene (17) was isolated in Ar matrices, it existed predominantly as the cZt rotamer, but it could also be generated in the cZc form by matrix photolysis of 1,4-dimethyl-cyclohexa-1,3-diene (18). In contrast to the E isomer, the Z isomer underwent photo-induced double bond isomerization, generating rotamers of the E form. It should be noted that matrix isolation provides... 

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