Low-temperature matrix isolation

An extensive series of low-temperature matrix isolation experiments has failed to provide any evidence of oxirene formation, either by diazo ketone photolysis (82CB2192) or by attempted photo-retro-cycloaddition (82CB2202). 

Either UV-VIS or IR spectroscopy can be combined with the technique of matrix isolation to detect and identify highly unstable intermediates. In this method, the intomediate is trapped in a solid inert matrix, usually one of the inert gases, at very low temperatures. Because each molecule is surrounded by inert gas atoms, there is no possiblity for intermolecular reactions and the rates of intramolecular reactions are slowed by the low temperature. Matrix isolation is a very useful method for characterizing intermediates in photochemical reactions. The method can also be used for gas-phase reactions which can be conducted in such a way that the intermediates can be rapidly condensed into the matrix. 

The results of low-temperature matrix-isolation studies with 6 [41a] are quite consistent with the photochemical formation of cyclo-Cif, via 1,2-diketene intermediates [59] and subsequent loss of six CO molecules. When 6 was irradiated at A > 338 nm in a glass of 1,2-dichloroethane at 15 K, the strong cyclobut-3-ene-1,2-dione C=0 band at 1792 cm in the FT-IR spectrum disappeared quickly and a strong new band at 2115 cm appeared, which was assigned to 1,2-diketene substructures. Irradiation at A > 280 nm led to a gradual decrease in the intensity of the ketene absorption at 2115 cm and to the appearance of a weak new band at 2138 cm which was assigned to the CO molecules extruded photo-chemically from the 1,2-diketene intermediates. Attempts to isolate cyclo-Cig preparatively by flash vacuum pyrolysis of 6 or low-temperature photolysis of 6 in 2-methyltetrahydrofuran in NMR tubes at liquid-nitrogen temperature have not been successful. 

The photochemical dissociation of Me2Ge from 7,7-dimethyl-l,4,5,6-tetraphenyl-2,3-benzo-7-germanorbomadiene (14) has been studied by flash photolysis, low-temperature matrix isolation and CIDNP 3H NMR techniques30. The results suggest that a biradical (15) is formed as an intermediate species in the photoreaction. The biradical is initially formed in the singlet state, which undergoes conversion to the triplet state before irreversible decomposition to form Me2Ge and tetraphenylnaphthalene (TPN) (reaction 19). 

Y. Koizumi, S. Seki, A. Acharya, A. Saeki, and S. Tagawa, Delocalization of positive and negative charge carriers on oligo- and poly-fluorenes studied by low-temperature matrix isolation technique, Chem. Lett., 33 1290-1291, 2004. 

Time-resolved laser experiments make possible the detection of species with short lifetimes [1-6]. Usually, such experiments are carried out under normal conditions and a quick enough snapshot can capture the intermediate in action. A different philosophy is to prolong the lifetime of the reactive species by generating it in an inert environment at low temperature. Matrix isolation... 

Ab initio calculations show [83JA(105) 1760] that the D6/l structure of hexazine corresponds to a very shallow minimum on the PES. Hexazine has been detected experimentally by means of the low-temperature matrix isolation (80AG745) but, according to [83JMS(105)351], this result still requires verification. 

Here, the different identified coordination modes of C02 with transition and nontransition metals are described, together with trends along the Periodic Table, and theoretical contributions to the understanding of bonding in these systems through three types of study (i) low-temperature matrix isolation spectroscopy of electron-deficient metal/C02 moieties (ii) theoretical studies of reactions of metals with C02 and (iii) the synthesis of stable complexes. 

The reaction of CH2 with C02 was first postulated by Kistiakowsky and Sauer (13) as taking place via an a-lactone intermediate. The occurrence of this reaction was subsequently demonstrated by Milligan and Jacox (14) in low temperature matrices. These low temperature matrix isolation experiments, however, could not determine definitely the structure of the CH2 C02 intermediate. The result of our laser absorption experiment shows that the CO is vibrationally excited up to v — 4 with a distribution close to the one predicted by a statistical model assuming the existence of a long-lived CH2 C02 complex. This calculation, however, is insensitive to the structure of the complex assumed. Since the ground state triplet CH2 is known to be less reactive and kinetically behaves like CHj (15,16), which does not react readily with C02, the singlet A CH2 is assumed to be involved in the reaction. 

The existence of BH3 has been established by many groups90 however, structural information is only available from theoretical calculations91 and from a low-temperature matrix isolation study of pyrolysis products of BH3C092. The molecule is assumed to possess ideal D3h symmetry with a bond distance of 1.32 A91. ... 

The chemistry of krypton is considerably more extensive than that of argon nevertheless, it is stiU rather restricted. A series of krypton species containing Ki H, KiMI , and RtMI)l bonds were recently formed in low-temperature matrix isolation experiments by Rasanen and coworkers 10). The only hulk compounds of krypton that have been isolated are KrF2 (a very powerful fluorinating agent) and a series of cationic species derived from it, e.g., [KrF]+[SbF6] 11). All of these compoimds contain krypton in the +2 oxidation state. 

IR and UV/Vis [65a], mass spectrometric [65b], photoelectron [65c], microwave [65d], as well as low-temperature matrix-isolation IR spectroscopic measurements [65e] reveal that 2- and 4-hydroxypyridine (as well as 2- and 4-mercaptopyridine [65f]) exist in the gas phase and in inert matrices (N2, Ar) under equilibrium conditions mainly in the lactim (hydroxy or mercapto) form, in contrast to the situation in solution. While in nonpolar solvents such as cyclohexane and chloroform both tautomers exist in comparable amounts, the tautomeric equilibrium is shifted entirely in favour of the lactam (0x0 or thioxo) form in polar solvents such as water, as well as in the crystalUne state [66, 67, 141-145, 251-255], Supercritical-fluid 1,1-difluoroethane can be used to adjust the tautomeric constant Ki = [(llb)]l[(lla)] iso thermally over a continuum from gas-phase values to those measured in polar solvents, simply by increasing the pressure [254]. The gas-phase and solution equilibrium constants of 2- and 4-hydroxypyridine are given in Table 4-4. 

The photodecarboxylation of alkoxyacetic acids in the presence of HgO and iodine has been studied.Karauchi et al. have studied the photodecarboxylation of the Schilf base (117). Low-temperature matrix isolation techniques have been used to trap and study the pyranone (118) obtained by the decarboxylation of the dilactone (119). Photodecarboxylation of the carbamates (120) in acetonitrile provides a simple route to the azabicyclobutanes (121). ... 

The interesting pyranone (1) has been obtained by photodecarboxylation of (2) using low-temperature matrix isolation techniques (Bleasdale et al.). Bart-nik et al. have described a simple photodecarboxylation route to azabicyc-lobutanes such as (3). Attention is drawn to a new photochemical synthesis of p-lactones (Aoyama et al.) and -lactams (see ref. 50 in Part III, Chapter 2). Skinner and Weedon have reported an interesting method for photodeconjugation of a,p-unsaturated esters e.g., (4) (5). Polar solvents are required,... 

Mascetti and Tranquille25 prepared C02 adducts by low-temperature matrix isolation techniques. Recently, matrix isolation studies of Ni atoms with C02 were done the f—C,O mode was preferred in C02 matrices. These results, and studies of isotopically labeled species, suggest that Ni(N2)(C02) and Ni(N2)2(C02) were formed.26... 

As mentioned earlier, conformational isomerization about the formal single bonds of polyene systems is facile in the ground state, where it occurs with activation barriers on the order of 2-4 kcalmol in acyclic systems". The process also occurs in acyclic dienes upon direct excitation, as was shown by SquiUacote and coworkers using low temperature matrix isolation techniques, at temperatures where thermal conformational reequilibration is suppressed (10-20 K) " . Thus, direct irradiation of tzaMi-l,3-butadiene in an argon matrix at 15 K results in the efficient formation of the cis -conformer, distinguishable from the irani-conformer by its distinct UV absorption and infrared... 

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