Magnetic circular dichroism studies of matrix-isolated metal atoms

Results obtained from work on Ni, Pd, Al and Ga isolated in rare gas matrices are used to illustrate the type of information that can be obtained from MCD experiments.It is possible to identify different species in the same matrix through their temperature dependence. The interaction between guest and host can be seen by the quenching of the orbital angular momentum for Al and Ga. The signed nature of the MCD spectra makes it possible to confirm that the assignment of Ni atom bands is constant when the host gas is changed. The extra sensitivity of MCD allows the detection of a long-lived excited state species of Pd, which cannot be seen in the absorption spectrum.     

MCD spectroscopy and magnetization studies of matrix-isolated aluminium atoms

The electronic absorption spectra and magnetic circular dichroism (MCD) spectra of matrix-isolated aluminium atoms have been studied with particular reference to the 3p4s and 3p3d transitions. The g values of the isolated atoms also have been measured via the MCD magnetization technique. It is found that, in all matrices, the orbital angular momentum of the atom is heavily quenched giving g values very near 2.0. A consistent analysis of this phenomenon and of the spectra has been developed using a model in which the surrounding noble gas atoms exert an electrostatic field upon the aluminium atom and also enter into molecular orbital formation with it. This interpretation leads to the conclusion that the spin-orbit coupling of the optical electron is negative in both ground and excited states for Al/Kr and Al/Xe, but in the excited state only for Al/Ar. These results confirm and extend the findings of earlier EPR measurements.     

Crystal field effects in the optical spectra of matrix-isolated palladium atoms

A Slater-Condon calculation including a crystal field perturbation has been performed for Pd atoms. The optical spectrum of Pd in Ne is reproduced very well with this calculation, assuming a repulsive crystal field of O_h symmetry.     

Photoinduced ethane formation from reaction of ethene with matrix-isolated Ti, V, or Nb atoms

The reactions of matrix-isolated Ti, V, or Nb atoms with ethene (C(2)H(4)) have been studied by FTIR absorption spectroscopy. Under conditions where the ethene dimer forms, metal atoms react with the ethene dimer to yield matrix-isolated ethane (C(2)H(6)) and methane. Under lower ethene concentration conditions ( approximately 1:70 ethene/Ar), hydridic intermediates of the types HMC(2)H(3) and H(2)MC(2)H(2) are also observed, and the relative yield of hydrocarbons is diminished. Reactions of these metals with perdeuterioethene, and equimolar mixtures of C(2)H(4) and C(2)D(4), yield products that are consistent with the production of ethane via a metal atom reaction involving at least two C(2)H(4) molecules. The absence of any other observed products suggests the mechanism also involves production of small, highly symmetric species such as molecular hydrogen and metal carbides. Evidence is presented suggesting that ethane production from the ethene dimer is a general photochemical process for the reaction of excited-state transition-metal atoms with ethene at high concentrations of ethene.     

The electronic and magnetic circular dichroism spectra of matrix-isolated scandium atoms and the scandium dimer

The electronic and magnetic circular dichroism (MCD) spectra of scandium atoms isolated in argon, krypton and xenon matrices have been measured and the bands assigned. Some aspects of the assignment present problems and the resulting matrix shifts are rather irregular. Magnetization studies of the above systems are also reported and the data show that there are particularly strong guest—host interactions in the case of Sc/Xe. Furthermore, they suggest that there is significant guest—host interaction in the ground electronic state. Computer simulation of the magnetization curves and the MCD spectra, using a crystal field model, enables some tentative suggestions concerning the nature of the matrix sites to be made. All sites show an axial character. MCD bands of a scandium dimer have been observed. The form and magnetization properties of one band support a ⁵Σ ground state for the molecule.     

Photoemission from solid neon

The photoelectric yield of solid Ne has been studied for photon energies between 15 and 30 eV. Strong photoemission from the valence band sets in at 21.4 ± 0.1 eV. The value of the electron affinity derived from this is discussed. A surface resonance for Ne atoms adsorbed on gold is observed at 17.1 eV.     

Chemical synthesis with metal atoms: the reaction of chromium and nickel atoms with styrene

The preparation of C₈H₈Cr(CO)₃ (or C₈H₈Cr(PF₃)₃) and polystyrenechromium tricarbonyl (or polystyrenechromium tris(trifluorophosphine) from chromium atoms, styrene and carbon monoxide (or trifluorophosphine) is described; nickel atoms and styrene produce tristyrenenickel, which reacts with α,α′-bipyridyl to yield bipyridylstyrenenickel.     

Photoemission from small organic crystals into gases

Electrons emitted from micron-sized positively charged crystals in relatively dense gases in the presence of a levitating external field can escape recapture. The observed photoemission yield depends on the electron kinetic energy, the gas pressure, the particle size and state of charge, and the magnitude of the external field. Data and a theory are presented to describe the probability of escape. For organic crystals, photoemission due to a binuclear process, can exhibit various light intensity dependences. This may be accomplished by varying the triplet exciton concentration in the crystal. A theory to account for this effect using the concept of excitonic detrapping at recombination centers is presented.     

Photoemission from tin and lead cluster anions

Photoelectrons from mass-identified jet-cooled tin and lead cluster anions (Sn _n ^? , Pb _n ^? ) are detached by ultraviolet laser light (hν=3.68 eV). The photoelectron energy spectra give the detachment energies of ground state cluster anions (electron affinities) as well as excitation energies of neutral clusters in the geometry of the anions. The energy spectra for Sn _n ^? are dominated by flat thresholds with ann-dependence similar to that of other group IV clusters. In contrast, for Pb _n ^? we find pronounced narrow lines close to threshold, generally followed by a 0.3–1.4 eV gap which indicates closed-shell behaviour of Pb _n ^? for nearly alln.     

Oxidative dehydrogenation of diaminomaleonitrile coordinated to the nickel and platinum atoms

The reactions of diaminomaleonitrile H₂NC(CN)=C(CN)NH₂ (1) with the nine-nuclear carboxylate nickel(II) complex Ni₉(HOOCCMe₃)₄(μ₄-OH)₃(μ_n-OOCCMe₃)₁₂ (under an inert atmosphere or in air) and with K₂[PtX₄] (in air) afforded the bis-chelate mononuclear complexes M[HNC(CN)C(CN)NH]₂ (M=Ni (2) and Pt (3), respectively). The structural features of compounds 1, 2, and 3, which were determined by X-ray diffraction analysis, are discussed.     

Modeling the spectra of matrix-isolated molecules

Modeling the properties of molecules isolated in low-temperature inert matrices is discussed in terms of cluster approaches. The analysis uses molecular dynamic and quantum chemical methods. Particular attention is paid to spectral features resulting from the matrix environment effects. Translated from Zhurnal Struktumoi Khimii, Vol. 38, No. 2, pp. 256–262, March–April, 1997.     

Vibrational circular dichroism of matrix-isolated molecules

Vibrational circular dichroism (VCD) spectra are reported for (+), (?) and (±) α-pinene, and (?) ß-pinine, isolated in argon matrices at ≈ 18 K. These are the first observations of VCD of matrix-isolated molecules. Spectra are limited to the CH stretching region (2800–3100 cm^?1). In all cases, the VCD spectra are substantially more structured than the corresponding room-temperature spectra, as a result of much narrower linewidths. Further, VCD magnitudes are greater due to decreased cancellation of overlapping transitions with oppositely signed VCD. The largest anisotropy ratios observed are >5 × 10^?4 and are larger than any reported for room-temperature solutions of organic molecules. This technique will permit substantially more definitive evaluations of theoretical calculations of VCD.     

Photorotamerization of matrix-isolated acrylic acid revisited

In this study, the conformational preferences and photochemistry of acrylic acid (AA, CH(2)=CHCOOH) monomer isolated in cryogenic argon and krypton matrices were interpreted, based on results of quantum chemical calculations. Natural bond orbital analysis allowed to shed light on the main electronic effects determining the relative stability of the conformers of the molecule in the ground electronic state. The conformational isomerization taking place upon UV-irradiation of the matrix-isolated compound (λ ～ 243 nm) was explained, based on theoretical complete active space self-consistent field/complete active space with second order perturbation theory (CASSCF/CASPT2) and time-dependent density functional theory (TD-DFT) results, allowing to rationalize the nearly equal populations of the two lowest energy conformers of the molecule observed in the photostationary state. Besides, details of the infrared spectra of the compound were reinterpreted based on the calculated spectra for the two most stable conformers of the molecule. In particular, the assignments for the out-of-plane A" symmetry vibrations were revised.     

ESR-spectrum of silver atoms stabilized in gamma-irradiated aluminium silicate modified by nickel

ESR technique was used to study the effect of conditions chosen to prepare samples to obtain and stabilize silver atoms forming under the action of⁶⁰Co -irradiation at 77 K in aluminium silicate modified by nickel and containing silver ions. The use of¹⁰⁹Ag isotope made it possible to detect two types of silver atoms Ag_I and Ag_II in -irradiated aluminium silicate modified by nickel and containing silver ions introduced by various techniques. The parameters of the ESR-spectrum of the silver particles under study have been determined and their thermal stability has been investigated.     

The MCD spectrum of matrix-isolated osmium tetroxide

The MCD spectrum of OsO₄ in argon and nitrogen matrices at ≈ 20 K has been measured and is remarkably similar to the first band of MnO₄⁻ in KIO₄ at 4.2 K. From this it is concluded that there are two electronic origins in the region 30 000–39 000 cm⁻¹ and the spectrum of OsO₄ is reassigned accordingly.     

Optical spectra of size-selected matrix-isolated silicon clusters

Size selected silicon clusters have been isolated in rare gas matrices and studied by optical absorption spectroscopy. The clusters were produced in a pulsed laser vaporization source, size selected with a quadrupole mass spectrometer and deposited at low energies into a cocondensed krypton matrix held at T<20 K. A comparison of the optical spectra of ten atom wide bands (Si₂₅-Si₃₅, Si₃₅-Si₄₅ and Si₄₅-Si₅₅) shows the general size evolution of the optical properties. Single cluster sizes have also been isolated and show somewhat sharper spectra than the bands. The measured spectra show similarities to spectra calculated using Mie theory and bulk optical constants. Cluster-cluster agglomeration was studied by evaporating the inert gas matrix. The results suggest that the clusters agglomerate into larger particles even under the mildest "soft landing" conditions.     

Raman,spectra of matrix-isolated lead molecules

Raman spectra of lead molecules in low-temperature rare-gas matrices show that dimers and larger clusters are isolated. Besides 7 “normal” Raman bands, two strong resonance progressions are found with frequencies of 108.5 and 118.5 cm^?1 in Xe and 111 and 119 cm^?1 in Kr. The 110 cm^?1 peak is assigned to Pb₂, close to the frequency for the X-O⁺_g state of gaseous Pb₂.