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.     

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.     

Photoemission from matrix-isolated nickel atoms

UV photoelectron spectra from Ni atoms isolated in a xenon matrix at 9 K have been measured for the first time using Hell (40 8 eV). Two peaks have been found at 2.8 and 4.6 eV below E_F, which are interpreted as being due to different d⁸s¹ final states of the Ni ion. including a relaxation shift of 2.0 eV. The existence of atomic Ni species in the matrix has been verified by in situ optical absorption spectra.     

MCD studies of biphenyls

The magnetic circular dichroism (MCD) spectra of biphenyl derivatives with electron donating substituents at the C₄ and/or C₄′ positions of biphenyl were measured at room temperature in the wavelength regions of 200 to 350 nm. The biphenyls studied exhibited Faraday B terms caused by a magnetic coupling of the non-degenerate electronic states. A careful inspection of the MCD spectra has enabled us to find hidden bands which were buried in the intense, so-called “conjugation band” of biphenyl, resulting in the preferable assignments of the bands.     

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.     

Luminescence spectroscopy of matrix-isolated z 6P state atomic manganese

The relaxation of electronically excited atomic manganese isolated in solid rare gas matrices is observed from recorded emission spectra, to be strongly site specific. z 6P state excitation of Mn atoms isolated in the red absorption site in Ar and Kr produces narrow a 4D and a 6D state emissions while blue-site excitation produces z 6P state fluorescence and broadened a 4D and a 6D emissions. MnXe exhibits only a single thermally stable site whose emission at 620 nm is similar to the broad a 6D bands produced with blue-site excitation in Ar and Kr. Thus in Ar(Kr), excitation of the red site at 393 (400) nm produces narrow line emissions at 427.5 (427.8) and 590 (585.7) nm. From their spectral positions, linewidths, and long decay times, these emission bands are assigned to the a 4D72 and a 6D92 states, respectively. Excitation of the blue site at 380 (385.5) nm produces broad emission at 413 (416) nm which, because of its nanosecond radiative lifetime, is assigned to resonance z 6P --> a 6S fluorescence. Emission bands at 438 (440) and 625 (626.8) nm, also produced with blue-site excitation, are broader than their red-site equivalents at 427.5 and 590 nm (427.8 and 585.7 nm in Kr) but from their millisecond and microsecond decay times are assigned to the a 4D and a 6D states. The line features observed in high resolution scans of the red-site emission at 427.5 and 427.8 nm in MnAr and MnKr, respectively, have been analyzed with the W(p) optical line shape function and identified as resolved phonon structure originating from very weak (S=0.4) electron-phonon coupling. The presence of considerable hot-phonon emission (even in 12 K spectra) and the existence of crystal field splittings of 35 and 45 cm(-1) on the excited a 4D72 level in Ar and Kr matrices have been identified in W(p) line shape fits. The measured matrix lifetimes for the narrow red-site a 6D state emissions (0.29 and 0.65 ms) in Ar and Kr are much shorter than the calculated (3 s) gas phase value. With the lifetime of the metastable a 6D92 state shortened by four orders of magnitude in the solid rare gases, it is clear that the probability of the "forbidden" a 6D --> a 6S atomic transition is greatly enhanced in the solid state. A novel feature identified in the present work is the large width and shifted 4D and 6D emissions produced for Mn atoms isolated in the blue sites of Ar and Kr. In contrast, these states produce narrow, unshifted (gas-phase-like) 4D and 6D state emissions from the red site.     

Electron energy-loss vibronic spectroscopy of matrix-isolated benzene and multilayer benzene films

Electronic transitions near the surface of benzene multilayer films (50–100 A) and in Xe matrices containing 2% benzene have been studied by high-resolution electron energy-loss spectroscopy at primary energies between 5 and 14 eV. The lowest three transitions exhibit vibrational structure that can be ascribed to the ³E_1u³B_1u³B_2u free-molecule configurations of C₆H₆.     

NMR, CD and MCD studies of vanadate-nucleoside complexes

Complex formation between tetraoxovanadate(V) and each of the nucleosides adenosine, guanosine, cytidine and uridine has been studied in a constant salt medium at pH 7. 13C- and 51V NMR studies show that only complexes with the formula V2L2 (V = vanadate, L = nucleoside) are formed, and their formation constants have been determined. They have 51V NMR resonances around -523 ppm relative to VOCl3 and they exhibit no CD in the spectral region of the charge-transfer transitions. MCD spectra were also measured, and all experiments are in accord with a molecular structure composed by two edge-sharing VO6 octahedra forming an O4V(mu-O)2VO4 skeleton with each of the nucleoside ligands bridging the two vanadium centres through the ribose 2',3'-oxygens, which are the oxygens outside the V2O6 plane. Admixture of imidazole-HCl buffer at pH 7 gives rise to additional complexes of 1:1 stoichiometry. They have been characterized by 51V NMR and CD, and their formation constants are reported. Vanadate(V) and the deoxynucleosides deoxyadenosine, deoxyguanosine, deoxycytidine and thymidine form very weak complexes which cannot be detected by 51V NMR or CD under conditions for which vanadate and the nucleosides form complexes.     

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.     

Electronic spectral studies of molybdenyl complexes. 2. MCD spectroscopy of [MoOS4]- centers

Magnetic circular dichroism (MCD) and absorption spectroscopies have been used to probe the electronic structure of [PPh4][MoO(p-SC6H4X)4] (X = H, Cl, OMe) and [PPh4][MoO(edt)2] complexes (edt = ethane-1,2-dithiolate). The results of density functional calculations (DFT) on [MoO(SMe)4]- and [MoO(edt)2]- model complexes were used to provide a framework for the interpretation of the spectra. Our analysis shows that the lowest energy transitions in [MoVOS4] chromophores (S4 = sulfur donor ligand) result from S-->Mo charge transfer transitions from S valence orbitals that lie close to the ligand field manifold. The energies of these transitions are strongly dependent on the orientation of the S lone-pair orbitals with respect to the Mo atom that is determined by the geometry of the ligand backbone. Thus, the lowest energy transition in the MCD spectrum of [PPh4][MoO(p-SC6H4X)4] (X = H) occurs at 14,800 cm-1, while that in [PPh4][MoO(edt)2] occurs at 11,900 cm-1. The identification of three bands in the absorption spectrum of [PPh4][MoO(edt)2] arising from LMCT from S pseudo-sigma combinations to the singly occupied Mo 4d orbital in the xy plane suggests that there is considerable covalency in the ground-state electronic structures of [MoOS4] complexes. DFT calculations on [MoO(SMe)4]- reveal that the singly occupied HOMO is 53% Mo 4dxy and 35% S p for the equilibrium C4 geometry. For [MoO(edt)2]- the steric constraints imposed by the edt ligands result in the S pi orbitals being of similar energy to the Mo 4d manifold. Significant S pseudo-sigma and pi donation may also weaken the Mo identical to O bond in [MoOS4] centers, a requirement for facile active site regeneration in the catalytic cycle of the DMSO reductases. The strong dependence of the energies of the bands in the absorption and MCD spectra of [PPh4][MoO(p-SC6H4X)4] (X = H, Cl, OMe) and [PPh4][MoO(edt)2] on the ligand geometry suggests that the structural features of the active sites of the DMSO reductases may result in an electronic structure that is optimized for facile oxygen atom transfer.     

Immersion of hydrogen atoms in aluminium clusters

The immersion of a H atom at the centre of Al _N clusters with 8≦N≦21 has been studied using the density functional formalism and a pseudopotential averaged about the cluster centre. The immersion energy is negative forN≦12, due to the fact that the corresponding pure Al clusters contain a central vacancy, which shows a strong affinity for the H atom. For larger sizes the immersion energy is positive (with a few exceptions), since the H atom must displace a central Al atom to the cluster surface, and it shows non negligible size oscillations.     

IR studies of NO and CO adsorbed on matrix-isolated silver clusters

The interaction of CO and NO on matrix-isolated silver clusters (D ≈ 100 A) causes a red-shift of the vibrational frequency. Compared to matrix-i     

UV photoelectron spectroscopy of matrix-isolated and condensed CO and N2

UV photoelectron spectra of matrix-isolated and condensed CO and N₂ have been measured. The emission from the 4σ, 1π and 5σ orbitals show similar relaxation shifts of about 1 eV. This shift is 0.3 eV larger for the species isolated in xenon matrices. The width of the peaks is smaller by 0.3 eV in the Xe matrix in comparison with the pure solids but remains larger than those measured for the gas phase.     

Photoelectron spectroscopy of chemisorbed atoms

We outline a theory of UV and higher-energy photoemission spectroscopy of chemisorbed atoms, that aims at the accurate calculation of inner electron binding energies and photoabsorption cross sections by including solid state and localized relativistic and correlation effects. It is based on an “atom on (in) solids” approach where one first extracts a surface potential and then uses it in a coupled Hartree–Fock theory to obtain self-consistently the shifts and splittings of atomic levels. A first application of this theoretical program has been carried out on Na on the Al(100) system, by calculating from first principles the binding energies of the Na 1s and 2s electrons. For a coverage of 1.23 × 10¹⁴ adatoms/cm² we find BE (1s) = 1075.2 eV and BE (2s) = 66.2 eV. Also, the Na 2p orbitals are found to split in the cylindrical symmetry by about 0.2 eV.     

Low temperature IR spectroscopy and photochemistry of matrix-isolated α-pyridil

α-Pyridil [(C₆H₄NO)₂] has been isolated in low temperature argon and xenon matrices and studied by FTIR spectroscopy, supported by DFT(B3LYP)/6–311++G(d,p) calculations. Calculations predicted the existence of three different conformers exhibiting skewed conformations around the intercarbonyl bond and the two C₅H₄NC(O) fragments nearly planar. The two higher energy forms, TCG and CCSk were estimated theoretically to be, respectively, 21.0 and 35.1 kJ mol⁻¹ higher in energy than the most stable form, TTG. In consonance with the relatively high energies predicted by the calculations for the two less stable conformers of α-pyridil, only the most stable conformer was found spectroscopically to be present in the studied matrices. Infrared spectra obtained for the neat low temperature amorphous and crystalline states reveals that the TTG conformer is also the sole conformer present in these phases. UV irradiation (λ > 235 nm) of matrix-isolated α-pyridil led to its isomerization into unusual molecular species bearing Hückel-type pyridine (aza-benzvalene) rings.     

A study of aluminium monofluoride and aluminium trifluoride by high-temperature photoelectron spectroscopy

The Hel photoelectron spectrum of gaseous AIF(X¹Σ⁺) has been recorded and the first three cationic states have been assigned with the aid of PNO/CEPA calculations. The first band shows vibrational structure and analysis of the component separations and relative intensitives leads to values of ω_c = 1040 ± 40 cm^?1 and r_c = 1.59 ± 0.01 Å in the AIF⁺ (X²Σ⁺) state; the corresponding theoretical values are 960 cm^?1 and 1.60 Å respectively. The first adiabatic ionization potential, 9.73 ± 0.01 eV, allows a determination of the quantum defect, δ, in a number of previously observed Rydberg states of AIF. The Hel photoelectron spectrum of gaseous AIF₃ has also been obtained. It is assigned on the basis of ab initio molecular orbital calculations and comparison with the corresponding BF₃ spectrum.     

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.     

Collision spectroscopy with aligned and oriented atoms

Electron capture processes in the H⁺?Na(3s) and H⁺?Na(3p) collisions are experimentally investigated in the 0.3–3 keV energy range using a crossed beam experiment. The excited Na(3p) target is produced with a well-defined alignment using laser pumping. The time of flight technique enables the identification of all the H(n)+Na⁺ channels populated in the collision. Total cross section ratios σ_3p (n=2)/σ_3s (n=2),σ_3p (n=3)/σ_3s (n=2) and σ_3s (n=3)/σ_3s (n=2) for the production of H(n=2) and H(n=3) are measured in the H⁺?Na (3s) and H⁺?Na (3p) collisions. They reveal a strong dominance of the production of H(n=2) in the H⁺?Na(3p) collision, especially for energies below 1 keV.