Adamantane Matrix Research Articles

Infrared Spectra of Monohydrogenated Aniline, ortho- and para-HC6H5NH2, Generated in Solid para-Hydrogen.

The isomers of monohydrogenated aniline (HC6H5NH2) are regarded as important intermediates in reduction reactions of aniline, but their spectral identification has been limited to electron paramagnetic resonance in an adamantane matrix. We report here infrared (IR) spectra of two least-energy isomers of HC6H5NH2, produced on electron bombardment during the deposition of a matrix of aniline and para-hydrogen at 3.2 K. The intensities of IR lines of HC6H5NH2 increased during maintenance of the electron-bombarded matrix in darkness for a prolonged period because of the neutralization of protonated aniline, H+C6H5NH2, by trapped electrons and further reactions between aniline and the unreacted hydrogen atoms that were produced during electron bombardment. The observed lines were grouped according to their behaviors on secondary photolysis with light at 520, 465, and 375 nm. On comparison of experimental spectra with quantum chemically predicted spectra for four possible isomers of HC6H5NH2, lines in one group were assigned to the most stable ortho-HC6H5NH2 and those in the other group were assigned to the secondmost stable para-HC6H5NH2. Their photolytic behaviors at varied wavelengths are consistent with predicted ultraviolet absorption bands. The mechanisms of formation of these isomers are discussed according to semiquantitative analysis.

Electron Paramagnetic Resonance Spectroscopic Evidence for the Interaction of HAlOH with Water Molecules

The complex HAlOH:(H(2)O) has been detected by matrix-isolation IR spectroscopy. This complex was speculated to be the species responsible for the chemiluminescent glow associated with the explosion of trimethyl aluminum or aluminum grenades in the upper atomosphere. Theoretical studies suggest that HAlOH:(H(2)O)(n) is a critical precursor in the formation of H(2) in the reaction of Al with liquid water. In our study, Al atoms were reacted with mixtures of D(2)O/He or H(2)(17)O/He in an adamantane matrix in a metal-atom reactor, known as a rotating cryostat, maintained at 77 K and at <10(-6) Torr. In addition to DAlOD and HAlOH, which formed from the reaction of Al atoms with adventitious water, EPR analysis of the Al-D(2)O/He reaction mixture from 77 to 290 K showed that HAlOH:(D(2)O) and DAlOD:(D(2)O) formed. The experimental nuclear hyperfine interactions (hfis) for these species were in close agreement with those calculated using the B3LYP density functional method and the 6-311+G(2df,p) basis set. The effect of complexation is to lower the Al hfi of HAlOH and DAlOD by ca. 8%, the H hfi of HAlOH by ca. 28%, and the D hfi of DAlOD by ca. 35%.

Activation of C–O and C–C Bonds and Formation of Novel HAlOH-Ether Complexes: An EPR Study of the Reaction of Ground-State Al Atoms with Methylethyl Ether and Diethyl Ether

Reaction mixtures, containing Al atoms and methylethyl ether (MEE) or diethyl ether (DEE) in an adamantane matrix, were prepared with the aid of a metal-atom reactor known as a rotating cryostat. The EPR spectra of the resulting products were recorded from 77-260 K, at 10 K intervals. Al atoms were found to insert into methyl-O, ethyl-O, and C-C bonds to form CH(3)AlOCH(2)CH(3), CH(3)OAlCH(2)CH(3), and CH(3)OCH(2)AlCH(3), respectively, in the case of MEE while DEE produced CH(3)CH(2)AlOCH(2)CH(3) and CH(3)AlCH(2)OCH(2)CH(3), respectively. From the intensity of the transition lines attributed to the Al atom C-O insertion products of MEE, insertion into the methyl-O bond is preferred. The Al hyperfine interaction (hfi) extracted from the EPR spectra of the C-O insertion products was greater than that of the C-C insertion products, that is, 5.4% greater for the DEE system and 7% greater for the MEE system. The increase in Al hfi is thought to arise from the increased electron-withdrawing ability of the substituents bonded to Al. Besides HAlOH, resulting from the reaction of Al atoms with adventitious water, novel mixed HAlOH:MEE and HAlOH:DEE complexes were identified with the aid of isotopic studies involving H(2)(17)O and D(2)O. The Al and H hfi of HAlOH were found to decrease upon complex formation. These findings are consistent with the nuclear hfi calculated using a density functional theory (DFT) method with close agreement between theory and experiment occurring at the B3LYP level using a 6-311+G(2df,p) basis set.

Activation of C–Cl by ground-state aluminum atoms: an EPR and DFT investigation

The reaction of ground-state Al atoms with dichloromethane (CH(2)Cl(2)) in an adamantane matrix at 77 K yielded two mononuclear Al species. The magnetic parameters, extracted from the axial EPR spectrum of Species A/A' (g(1) = 2.0037, g(2) = g(3) = 2.0030, a(Al,1) = 1307 MHz, a(Al,2) = a(Al,3) = 1273 MHz, a(35Cl) = 34 MHz and a(37Cl) = 28 MHz) were assigned to the Al-atom insertion product, ClCH(2)AlCl. Density functional theory (DFT) calculations of the values of the Al and Cl hyperfine interaction (hfi) of the Cl(1)-Cl(2)gauche conformer were in close agreement with the experimental values of ClCH(2)AlCl. The second species, B/B', had identical magnetic parameters to those of ClCH(2)AlCl with the exception that the Al hfi was 15% smaller. Coordination of a ligand, possessing a lone pair of electrons, to the Al atom of the insertion product, [ClCH(2)AlCl]:X, could cause the a(Al) to decrease by 15%. Alternatively, it is possible that the Cl(1)-Cl(2) anti conformer of ClCH(2)AlCl is also isolated in the matrix. Support for the spectral assignments is given by calculation of the nuclear hfi of [ClCH(2)AlCl]:H(2)O and the Cl(1)-Cl(2) anti conformer of ClCH(2)AlCl using a DFT method. The potential energy hypersurface for an Al atom approaching CH(2)Cl(2), calculated at the B3LYP level, suggests that Al atom abstraction of Cl forming AlCl and CH(2)Cl is favoured in the gas phase. When produced in a matrix, the close proximity of AlCl and CH(2)Cl could account for the formation of ClCH(2)AlCl. EPR evidence was also found for the formation of the CHCl(2) radical.

Electron Paramagnetic Resonance Study of the Mononuclear Al Species Formed in the Reaction of Al Atoms and Cyclopropylamine at 77 K in an Adamantane Matrix

Ground-state Al atoms have been reacted with cyclopropylamine (CpNH2) in an adamantane matrix at 77 K. The four paramagnetic species generated in the reaction have been studied by electron paramagnetic resonance (EPR) spectroscopy and their magnetic parameters extracted from the EPR spectra. The isotropic Al hyperfine interactions (hfis) for these radicals vary from 801 to 962 MHz and are consistent with values previously reported for structurally similar Al-centered radicals. The major species formed in the reaction has a highly anisotropic a tensor indicative of an Al-centered radical with a substantial amount of the unpaired spin density residing in the Al 3p orbital. In addition, there is EPR evidence that the insertion products, CpNH(AlH) and CpAlNH2, are formed in the reaction, suggesting that Al atoms can activate the N-H and C-N bonds of amines. It has been shown that a molecule of CpNH2 coordinates to CpNH(AlH), causing the Al, N and H hfi of the N-H insertion product to decrease. The values of the nuclear hfis calculated for the insertion products, using a density functional theory (DFT) method, are comparable to the experimental values.

ESR studies of rhodium atoms deposited from thermal sources into hydrocarbon matrices at 77 K

Thermally generated rhodium atoms have been trapped in an adamantane matrix at 77 K and the resulting X- and K-band ESR spectra investigated over a range of temperatures. All three g-shifts of the observed orthorhombic spectrum are well resolved and the spectrum is assigned to neutral Rh atoms trapped in asymmetric matrix sites. The spectra were well simulated with the following parameters: g1 = 2.761, g2 = 2.572, g3 = 1.940 and A1 = 305 MHz, A2 = 284 MHz, A3 = 244 MHz. These are used to estimate spin density contributions of 0.55 (4d), 0.22 (5s) and 0.23 (5p) toward the ground state electronic configuration of the trapped atoms.

EPR spectroscopic study of the reaction of ground-state aluminium atoms with acyclic alcohols in a rotating cryostat

Ground-state aluminium atoms have been reacted with methanol, ethanol and 2-methylpropan-2-ol, in an adamantane matrix, in a rotating cryostat at 77 K. We found no electron paramagnetic resonance (EPR) evidence under these experimental conditions for aluminium atom insertion into the C–C, C–H or O–H bonds of these alcohols. There are, however, several mononuclear aluminium species formed with large aluminium hyperfine interactions (hfi) (1206–1537 MHz). A comparison of the magnetic parameters with those determined for similar systems suggests that in the aluminium–methanol reaction, Al(OCH3)2 is the major product. In addition, our results suggest that HOAlOCH3 and Al(OH)2 are present in the reaction mixture. We have calculated the aluminium hfi for these species using density functional theory (DFT) methods and the results are in close agreement with the experimental values. The aluminium–ethanol reaction gives Al(OCH2CH3)2 and HOAlOCH2CH3 while only HOAlOC(CH3)3 is observed from reaction of aluminium atoms with 2-methylpropan-2-ol.

Matrix Effect on Hydrogen Atom Tunneling from Alkane to Free Deuterium Atoms in Cryogenic Solid

ESR study on alkyl radicals generated in deuterated organic matrixes at 77 K by hydrogen atom tunneling from alkane molecules to free deuterium atoms has been carried out for elucidating control factors for the tunneling. For small alkane molecules, the tunneling rate is determined by the height of the potential energy barrier for the tunneling. For larger molecules in glassy matrixes, the rate decreases with increasing number and length of alkyl chains bonded to a carbon atom to be hydrogen abstracted. The tunneling rate from antepenultimate tertiary carbon is much slower than that from penultimate secondary carbon, even the potential energy barrier is lower. This abnormal effect is explained as being due to the steric hindrance by matrix molecules to the deformation of alkane molecules during the reaction. The alkyl chains surrounded by matrix molecules prevent the deformation of the chemical bonds of the carbon atom from the initial sp3 to the final sp2 configuration, which may cause the increase of the thickness of the potential energy barrier and thereby decrease of the tunneling rate. Free deuterium atoms in a crystalline adamantane matrix selectively abstract hydrogen atoms from the antepenultimate tertiary carbon, probably due to less steric hindrance to the deformation.

EPR Spectrum of NaCO

Reaction of ground state Na atoms with 13CO in an adamantane matrix in a rotating cryostat at 77 K gives a mononuclear monoligand complex whose EPR spectrum consists of an isotropic quartet of doublets with a(Na) = 889 MHz, a(13C) = 21.8 MHz, and g = 2.0015. This suggests that a Na atom can interact with one CO to generate NaCO with a singly occupied molecular orbital that is made up of Na 3s and C 2s orbitals with ca. 96% unpaired spin population in the Na 3s orbital.

Low pressure pyrolysis of hexamethyldisilane: Electron spin resonance identification of radical intermediates

Radicals produced in the pyrolysis of hexamethyldisilane at pressures below 1.5 Torr have been trapped in an adamantane matrix at 77 K and characterised by electron spin resonance spectroscopy. At 1 Torr, results were consistent with mechanisms proposed from gas-phase kinetic studies. At lower pressures the results suggest surface decomposition predominates, and they reveal a surprising feature of the mechanism that would not have been detected by the methods used in the earlier pyrolysis studies.

The first ENDOR spectrum of naked metal clusters: 7Li 3, 6Li 7Li 2 and 7Li 3(H 2O) x in an adamantane matrix

The electron double resonance (ENDOR) spectra of naked and hydrated neutral homonuclear metal clusters are reported. 7Li 3, 6Li 7Li 2, and 7Li 3(H 2O) x clusters have been prepared in an adamantane matrix using the rotating cryostat variant of the matrix isolation technique. The centre of the ENDOR doublet for the major cluster, 7Li 3, occurs at 46.15 MHz giving an isotropic hyperfine interaction (hfi) of 92.30 MHz which is close to that of 92.70 MHz determined previously from the EPR spectrum of Li 3. Weaker ENDOR transitions from 6Li 7Li 2 give hfis of 37.1 MHz for 6Li and 51.84 MHz for 7Li. These are consistent with the values for 7Li 3 if account is taken of the statistical factor of two favouring the location of the single 6Li nucleus in 6Li 7Li 2 at the terminal positions of the obtuse angled structures in the minima of the ‘Mexican hat’ hypersurface of the cluster. A weaker doublet, centred at 36.59 MHz is assigned to a hydrated cluster complex 7Li 3(H 2O) x . The reduction in the hfi value compared with that in the neutral cluster 7Li 3 indicates a 20% transfer of unpaired electron spin density to the coordinated water molecules. A complex series of doublets centred at the proton frequency (14.31 MHz) indicate the presence of a number of hydrated Li 3 clusters. These first observations of the ENDOR transitions of neutral trimeric metal clusters suggest that it may be possible to observe the spectra of other, larger and more complex, clusters.

Preparation of small copper particles of high catalytic activity using a rotating cryostat

The rotating cryostat technique was used to trap small naked copper clusters, Cun, n=3–7, and the copper carbonyls Cun(CO)m, n=1–2, m=1–6 in adamantane at 77 K. These clusters and carbonyls were then released from the adamantane for incorporation into inert supports (Al2O3 and SiO2) at low temperatures (110 K) by slowly dissolving the host adamantane matrix into an isopentane containing a slurry of the support. Highly active catalysts for CO oxidation and CO/H2 conversion to alkanes, alkenes and alcohols and with virtually unit dispersion were produced in this way. Efficient catalytic materials were also produced by directly implanting copper atoms into previously evaporated and condensed sodium chloride layers. In this system the copper was embedded in nanocrystals of the halide of <8 nm diameter.

The chemistry and the geometric and electronic structures of small naked metal clusters prepared using a rotating cryostat and studied by electron paramagnetic resonance

A rotating cryostat has been used to generate several new naked metal clusters whose structures and reactions have been studied by EPR spectroscopy. Most alkali-metal trimers adopt an obtuse triangular geometry with a 2B2 ground state in C2v symmetry. A new acute Na3 cluster was formed by interaction with water, Na3(H2O)x where x 1. The first EPR spectrum of a mixed alkali-metal trimer, Na2Li, has been recorded. The fluxionality of both Na3 and Li3 in an adamantane matrix is lost and a static form results with the Li located at the apical position. The first ENDOR (electron nuclear double resonance) spectrum of a naked metal cluster, Li3, has been recorded. It gives more accurate paramagnetic parameters and shows conclusively that the cluster is a fluxional, pseudo-rotating Jahn–Teller molecule. The ENDOR spectra of Li3(H2O)x complexes were also observed. A new coinage metal trimer, Au2Ag, has been characterised by EPR spectroscopy and has the silver in the apical position in a static obtuse structure. Reactions of the alkali-metal and coinage-metal trimers with simple molecules such as water and ethene resulted in a change in geometry from an obtuse or fluxional molecule to an acute static entity. The flexible nature of clusters is discussed in the context of modelling the chemical events occurring at the active metallic sites of heterogeneous catalysts.

The ESR spectrum and structure of the mixed alkali metal cluster Na 2K 5 prepared using a rotating cryostat

The ESR spectrum of the mixed alkali metal cluster, Na 2K 5, in an adamantane matrix prepared using a rotating cryostat has been observed. Its isotropic ESR parameters are g = 1.996, a(2Na) = 12.9 mT and a(5K) ≈ 0.035 mT, indicating a SOMO with most of the unpaired spin density (82%) in the 3s orbitals of two sodium atoms. Like its congeners, Na 7, K 7 and Li 7, the mixed cluster has a pentagonal bipyramid structure (D 5h symmetry) with a 2A″ 2 ground state. The sodium atoms occupy the two apical positions and the five potassium atoms lie in the pentagonal basal plane.

EPR spectrum of an acute-angled Na3 cluster in an adamantane matrix

The isotropic EPR spectrum of an acute-angled Na3 cluster associated with water in an adamantane matrix has the EPR parameters g= 2.001, aNa(1)= 17.96 mT and aNa(2)= 1.25 mT.

The EPR spectrum and structure of the mixed alkali metal trimer cluster Na2Li prepared using a rotating cryostat

The EPR spectrum of the mixed alkali metal cluster Na2Li in an adamantane matrix has the isotropic parameters g= 2.001, a(2 23Na)= 12.66 mT and a(7Li)= 0.35 mT indicating a static obtuse-angled structure with an apical lithium atom in C2v symmetry with a 2B2 ground state.

Electron paramagnetic resonance spectroscopic study of the gallium trimer, Ga3

The 69Ga3 cluster has been prepared and trapped in an adamantane matrix at 77 K using a rotating cryostat. It has been examined by EPR spectroscopy and has three magnetically equivalent 69Ga nuclei. Attempts to simulate the powder spectrum recorded at 4 K gave ‖a⊥(69)‖=240±3 MHz, g⊥=1.9651±0.002, and g∥=1.9568±0.0025. Spectra were less sensitive to the magnitude of ‖a∥(69)‖ but a value of 120±10 MHz appeared to give the best fit. There were no other fine structure lines in the spectrum suggesting that the electronic ground state for this group 13 trimer is probably 2A1′ although a 4A2 ground state cannot be completely excluded.

Characterization of the intermediates formed in the reaction of Al atoms with H2O, H2S and H2Se by EPR spectroscopy

Ground-state Al atoms have been reacted with H2O, D2O, H2S, D2S and H2Se in an adamantane matrix in a rotating cryostat at 77 K. EPR spectra demonstrate that Al atoms are inserted into these Group 16 hydrides and deuterides to give the thermally unstable species, HAlOH, DAlOD, HAlSH, DAlSD and HAlSeH. These species have large almost isotropic Al and H hyperfine interactions (hfi) of 911–984 and 134–286 MHz, respectively, and g factors of ca. 1.999. In addition, HAlSH and HAlSeH have a small H hfi of 14 and 20 MHz, respectively. Theoretical calculations show that these novel aluminium complexes have bent geometries with most of the unpaired spin located on the HAl function. While the reactions with H2O and D2O form primarily the insertion product, H2S and H2Se also produce a number of mononuclear aluminium species tentatively assigned to Al(XH)2 and Al(XH)2(H2X)n, where X = S or Se and n= 1 or 2. The Al hfi for these complexes range from 830 to 1218 MHz. Finally, in the case of the reaction of Al atoms with H2S, the tight ion pair Al+HSSH–, with the parameters aAl= 21 MHz and aH(2)= 21 MHz, is generated. The results obtained from the Al atom–D2S reaction support this assignment. No evidence has been obtained for the reaction of ground-state Ga atoms with H2O to give HGaOH. H2S, on the other hand, gives Ga+HSSH– and there is tentative evidence for the formation of HGaSH and Ga(H2S)4.

Rearrangements of Free Radicals, XIII[1]. Thermal and Photochemical Rearrangements of Cyclic C8H9 Radicals in Adamantane Matrix

AbstractThe ESR spectra observed after X‐irradiation of bicyclo‐[5.1.0]octa‐2,5‐diene (homotropylidene) (12) in a [D16]adamantane matrix at 210 K have been identified to be mainly due to the cyclooctatrienyl radical (8), formed by thermal ring opening of the initial bicyclo[5.1.0]octa‐2,5‐diene‐4‐yl (homotropylidenyl) radical (7). The same spectrum has also been observed in X‐irradiation of bromocyclooctatriene and a mixture of bromocyclooctatriene and 7‐bromobicyclo[4.2.0]cycloocta‐2,4‐diene in a [D16]adamantane matrix. In all cases, UV irradiation of the matrix caused an irreversible transformation of radical 8 (and probably 7) into the bicyclo[3.3.0]octa‐2,6‐diene‐4‐yl radical (17). Tricyclo[3.3.0.02,4]oct‐6‐en‐7‐yl radical (19), generated by X‐irradiation of tricyclo[3.3.0.02,4]oct‐2‐ene (20) in adamantane, is thermally stable up to 370 K, but also undergoes a facile rearrangement to the radical 17 on UV irradiation. This process is reasonably explained to occur stepwise via radicals 7 and 8. The postulated reaction paths and the spectral assignments are supported by semiempirical (AM1, PM3), abinitio (UHF/3‐21G*), and molecular mechanics (MM2ERW) calculations, which are in accord with the finding that 8 is energetically more stable than 7. The quantum mechanical calculations predict that a degenerate sigmatropic circumambulation of the cyclopropane ring in radical 7 should favorably compete with its ring opening.

The observation of pseudorotating lithium and potassium clusters Li5 and K7 in an adamantane matrix

The pseudorotating clusters, Li5 and K7, have been identified in an adamantane matrix by EPR spectroscopy. These new clusters were synthesised by depositing the parent alkali metal atoms at 77 K in the hydrocarbon matrix followed by careful annealing to temperatures above 200 K.