Adamantane Matrices Research Articles

ChemInform Abstract: Reactions of Yttrium Atoms in Hydrocarbon Matrices: Electron Paramagnetic Resonance Evidence for Matrix-Isolated YO and Yn (n = 5 or 7).

Deposits given by condensing resistively heated yttrium metal into cyclohexane and adamantane matrices under high vacuum and at 77 K on a rotating cryostat have been examined by EPR spectroscopy. The principle paramagnetic products under these conditions are YO in at least two distinct trapping sites and yttrium clusters, Yn, where n= 5 or 7. Analysis of the yttrium hyperfine interaction for YO indicates that the unpaired electron has 60% metal 5s character with the remainder of the unpaired spin in oxygen, matrix and metal p and 4d orbitals. An yttrium cluster, Yn, is produced in adamantane that is tumbling freely or pseudorotating above 205 K and has ca. 10% unpaired spin in the Y 5s orbital with the rest pressumed to be in metal 4d orbitals. A different Y cluster is formed in cyclohexane that could be a rigid pentemer or septemer with a trigonal bipyamidal or pentagonal bipyramidal structure. It is tentatively suggested that Y5 is formed in adamantane and that Y7 is formed in cyclohexane.

An ESR study of the reaction of silver atoms with B2F4 under matrix isolation conditions

Diboron tetrafluoride has been found by ESR to complex with silver atoms when the species are deposited together in hydrocarbon matrices at 77 K in a rotating cryostat. The major feature in the ESR spectra of the deposit in adamantane and cyclohexane matrices (Ag ∶ B2F4 ∶ matrix ratio of ∼1 ∶ 5 ∶ 500) was an almost isotropic set of two doublets of 13 (or possibly 15)-line multiplets. The two sets of doublets arise from the almost equally abundant 107Ag and 109Ag isotopes with I = 1/2. The multiplets on each silver line indicate hyperfine interaction with either four equivalent 11B nuclei (I = 3/2) or two equivalent 11B nuclei and four equivalent 19F nuclei. Multiplets from the less abundant 10B isotope (19.8%, I = 3) were not resolvable. The almost isotropic multiplets are best simulated as axial spectra with |A∥|(11B) = 17.5 MHz and |A⊥|(11B) = 14.1 MHz and Aiso(107Ag) = 1371.0 MHz. The complex is either [Ag(B2F4)2] or [Ag(B2F4)]. Assuming that A∥ and A⊥ are both positive, the total spin density is calculated to be 0.85 in [Ag(B2F4)2] and 0.80 in [Ag(B2F4)]. The most likely structures together with density function calculations on these two possibilities are discussed.

Reactions of yttrium atoms in hydrocarbon matrices: electron paramagnetic resonance evidence for matrix-isolated YO and Y n (n= 5 or 7)

Deposits given by condensing resistively heated yttrium metal into cyclohexane and adamantane matrices under high vacuum and at 77 K on a rotating cryostat have been examined by EPR spectroscopy. The principle paramagnetic products under these conditions are YO in at least two distinct trapping sites and yttrium clusters, Yn, where n= 5 or 7. Analysis of the yttrium hyperfine interaction for YO indicates that the unpaired electron has 60% metal 5s character with the remainder of the unpaired spin in oxygen, matrix and metal p and 4d orbitals. An yttrium cluster, Yn, is produced in adamantane that is tumbling freely or pseudorotating above 205 K and has ca. 10% unpaired spin in the Y 5s orbital with the rest pressumed to be in metal 4d orbitals. A different Y cluster is formed in cyclohexane that could be a rigid pentemer or septemer with a trigonal bipyamidal or pentagonal bipyramidal structure. It is tentatively suggested that Y5 is formed in adamantane and that Y7 is formed in cyclohexane.

The ferrocenylmethyl radical

Ferrocenylmethyl, dideuterio(ferrocenyl)methyl and 1′-methylferrocenylmethyl radicals have been prepared by γ-irradiation of the parent ferrocene derivatives in adamantane matrices. Ferrocenyl-methyl shows a triplet splitting (14.7 G) in its EPR spectrum attributed to the two α-protons; the couplings from the ring protons are buried in the linewidth. The high g value of 2.0200 indicates involvement of the Fe orbitals in the delocalized radical.

Electron paramagnetic resonance study of silacyclohexyl and 1-methylsilacyclohexyl radicals

Silacyclohexyl and 1-methylsilacyclohexyl radicals have been studied in adamantane matrices from –86° to room temperature. The preferred conformation for silacyclohexyl and the exclusive conformation for 1-methylsilacyclohexyl is the chair form with the SOMO on silicon equatorial. Long-range couplings to the equatorial γ-C—H protons are observed. For silacyclohexyl, a small proportion of the conformer with the SOMO axial is observed at –86°, giving rise to a substantial coupling to the axial β-C—H protons. At room temperature, equilibration of the two conformers is relatively fast, giving rise to averaged spectra. From the EPR spectra of silacyclopentyl and silacyclohexyl radicals, the dependence of β-C—H coupling constants on the dihedral angle with the SOMO on silicon is shown to have a double-maximum form with a maximum value at 180° and a smaller maximum at 0°.

Substituent effects in cycloheptatrienyl radicals: An ESR spectroscopic study

AbstractESR spectral parameters of methoxy‐, hydroxy‐ and cyano‐substituted cycloheptatrienyl radicals are reported. The radicals were obtained in adamantane matrices by the rearrangement of substituted bicyclo[3.2.0]heptadienyl radicals. The hyperfine splitting parameters are interpreted in terms of the effect of donor and acceptor substituents on the degeneracy of the relevant singly occupied ψs or ψA ABMO of the cycloheptatrienyl radical. In addition, an attempt was made to study the deuterium isotope effect on the degenerate MOs at low temperatures in a [2H16]adamantane matrix.

Strained-ring silicon-centred radicals: silacyclobut-1-yl and related radicals

Silacyclobut-1-yl, 1-methylsilacyclobut-1-yl and 3-methylsilacyclobut-1-yl radicals have been prepared by γ-irradiation of the parent silane in adamantane matrices. Electron paramagnetic resonance studies show that the radicals have non-planar rings; the SOMO on silicon is pseudo-equatorial. There is a substantial long range coupling of 16.4 G to the pseudo-equatorial γ-proton in the silacyclobut-1-yl radical and a smaller δ-coupling is shown to the pseudo-equatorial methyl protons in the 3-methylsilacyclobut-1-yl radical. In fluid solution at −40°C t-butoxy radicals react with silacyclobutane by an S H2 reaction at silicon to give 4-t-butoxy-4-silabut-1-yl rdicals, rather than by the expected hydrogen abstraction to give silacyclobut-1-yl radicals.

Structure of radicals produced by γ-radiolysis. Part 4. Adamantane matrices doped with various 5-membered heterocyclic molecules

The effects of γ-adiolysis at room temperature on adamantane and adamantane-d16 matrices doped with 5-membered heterocyclic molecules has been examined by X-band electron spin resonance (esr) spectrometry. Radical structures formed from heterocyclic solute molecules are discussed and tentative assignments made for 2-methyl-, 3-methyl-, and 2,5-dimethylthiophene, 2-methyl-, and 2,5-dimethylfuran. Discussion of possible radical structures derived from furan, tetrahydrofuran, 2-methyl- and 3-methyltetrahydrofuran, 2,5-dihydrofuran, and 4,5-dihydro-2-methylfuran, is included, but unambiguous assignments of structure cannot be made for these compounds from the esr data obtained. Other molecules examined included thiophene, pyrrole, and 1-methyl- and 1,2,5-trimethylpyrrole. For these latter molecules, experimentally obtained first derivative X-band esr spectra were too poorly resolved to allow rational discussion of possible structures. It was noted that perdeuterated adamantane matrices provided superior resolution for esr spectra derived from radicals with a delocalized spin center such as allyl or allenyl species.

Radiolysis of biochemical systems using adamantane as an isolation matrix.

SummaryThe use of adamantane matrices for investigating radiolysis processes in biochemically relevant systems is described. Data are given for radiolyses of D-(-)-ribose, D-2-deoxyribose, thymine, and cytosine, and possible radicals formed are discussed. Experimental details and data are given for radiolyses conducted in vacuo, under various gaseous atmospheres (CO, CO2, O2, air, N2O, and SO2), and for samples irradiated in vacuo and then exposed to various atmospheres to investigate the possible binding of small molecules to radical sites.

Structure of Radicals Produced by γ-Radiolysis. Part 2. Radicals Derived from 5-Membered Alicyclic Molecules

Data are presented for the results of γ-radiolysis on cyclopentane, methyl cyclopentane, cyclopentene, and cyclopentadiene, held as solutes in adamantane matrices. Data from this particular matrix isolation technique are in accord with liquid phase studies for cyclopentane and cyclopentene, and indicate that tertiary hydrogen loss occurs in methyl cyclopentane, and that cyclopentadiene yields at least three radicals. Two of the radicals derived from cyclopentadiene have been identified as the C5H5•entity with five equivalent protons, and the [Formula: see text] allylic-type radical formed by hydrogen atom addition to molecular C5H6.