Metal Atom Dynamics Research Articles

Advanced techniques for quantum-state specific reaction dynamics of gas phase metal atoms

One of the themes of modern molecular reaction dynamics is to characterize elementary chemical reactions from “quantum state to quantum state”, and the study of molecular reaction dynamics in excited states can help test the validity of modern chemical theories and provide methods to control chemical reactions. The subject of this review is to describe the recent experimental techniques used to study the reaction dynamics of metal atoms in the gas phase. Through these techniques, information such as the internal energy distribution and angular distribution of the nascent products or the three-dimensional stereodynamic reactivity can be obtained. In addition, by preparing metal atoms with specific excited electronic states or orbital arrangements, information about the reactivity of the electronic states enriches the relevant understanding of the electron transfer mechanism in metal reaction dynamics.

Hyperfine interactions and metal atom dynamics in a number of t-butyl ferrocene derivatives: The counter intuitive effects of ring substitution

Using temperature-dependent Mössbauer effect spectroscopy, the hyperfine interactions and metal atom dynamics of three structurally related 1,1′ bis tert. butyl ferrocenes have been examined in detail over the temperature range 90 < T < 300° K. It is remarkable that while ferrocene is a solid at room temperature (448° K), the replacement by two tert. butyl groups for the Cp ring protons lowers the melting point close to RT. Further ring substitution again raises the melting point above RT. As noted previously, the substitution of a tert. butyl group for a ring proton has little effect on the IS and QS parameters at 90° K. In contrast, the temperature-dependence of the IS reflects the covalency of the metal atom-ring interaction. The evaluation of the k2<x2> parameter, and its comparison to its value in the parent ferrocene shows that the vibrational amplitude of the metal atom is significantly smaller in the butyl compounds than in ferrocene. The metal atom motions are isotropic in the phenyl phosphine and bromine complexes, indicative of the absence of a Goldanskii-Karayagin effect in these two molecular solids.

Ferrocene and ferrocenium inclusion compounds with cucurbiturils: a study of metal atom dynamics probed by Mössbauer spectroscopy.

Temperature-dependent 57Fe Mössbauer effect (ME) spectroscopic studies were carried out on ferrocene (Fc), 1,1'-dimethylferrocene (1,1'(CH3)2Fc) and ferrocenium hexafluorophosphate (FcPF6) guest species in cucurbit[n]uril (n = 7, 8) inclusion complexes. The solid inclusion complexes were isolated by freeze-drying of dilute aqueous solutions and/or microwave-assisted precipitation from concentrated mixtures. The presence of genuine 1 : 1 (host : guest) inclusion complexes in the isolated solids was supported by liquid-state 1H and solid-state 13C{1H} MAS NMR, elemental and thermogravimetric analyses, powder X-ray diffraction, FTIR spectroscopy, and diffuse reflectance UV-Vis spectroscopy. The ME spectra of the complexes CB7·Fc and CB7·1,1'(CH3)2Fc consist of well-resolved doublets with hyperfine parameters (isomer shift and quadrupole splitting at 90 K) and temperature-dependent recoil-free fraction data that are very similar to those for the neat parent compounds, Fc and 1,1'(CH3)2Fc, suggesting that the organometallic guest molecules do not interact significantly with the host environment over the experimental temperature range. The ME spectra for CB7·FcPF6 and CB8·FcPF6 consist of a major broad line resonance attributed to a paramagnetic FeIII site. From the temperature-dependence of the recoil-free fraction it is evident that the charged guest species in these systems interact with the host environment significantly more strongly than was observed in the case of the neutral guest species, Fc and 1,1'(CH3)2Fc. Moreover, the ME data indicate that the vibrational amplitude of the ferrocenium guest molecule is significantly larger in the CB8 host molecule than in the CB7 homologue, as expected on the basis of the different cavity sizes.

Hyperfine Interactions and Metal Atom Dynamics in a Number of Stannyl Phosphide Compounds and the Detailed Crystal Structure of Triphenyltin Chloride Revisited

Temperature-dependent 119Sn Mossbauer effect (ME) spectroscopy has been used to elucidate the hyperfine parameters (isomer shift and quadrupole splitting) as well as the dynamics of the motion of tin in three triphenyltin phosphide complexes, namely P(SnPh3)3 (C54H45PSn3, 1), P7(SnPh3)3 (C54H45P7Sn3, 2), and [Na(benzo-15-crown-5)][P(SnPh3)2] (C50H50NaO5PSn2, 3), as well as in triphenyltin chloride (4). Although the isomer shifts are very similar for all four compounds, the quadrupole splitting is dependent upon their detailed structures. The metal atom vibrations are essentially isotropic with respect to the principal symmetry axis passing through the tin atom and the triphenyltin moiety. The crystal structure of triphenyltin chloride has been re-determined at four different temperatures, and the root-mean-square amplitudes-of-vibration of the metal atom have been evaluated from the crystallographic Ui,j values and compared with the corresponding data derived from the Mossbauer measurements. These metal atom dynamics have been compared with those of the stannyl phosphide complexes derived from ME data and related to their structural details. The sensitivity of the ME technique with respect to elucidating the metal atom dynamics is demonstrated in detail.

Metal–ligand bonding and metal atom dynamics in Fe–Fe and Ru–Fe triple-decker sandwich complexes

Three structurally related bimetallic triple-decker complexes of iron and ruthenium have been studied by temperature-dependent 57Fe Mössbauer Effect (ME) spectroscopy and their redox-behavior has been elucidated. The triple-decker compounds are [Cp′FeCp′FeCp′]BF4, [CpRuCp′FeCp′]PF6 and [Cp*RuCp′FeCp′]PF6 where Cp = C5H5, Cp′ = C5Me4H and Cp* = C5Me5. The metal atom (Fe) root-mean-square-vibrational-amplitude has been determined over the temperature range ∼90 < T < 300 K, and is found to be essentially isotropic in all three cases. The ME spectrum for [Cp′FeCp′FeCp′]BF4 is similar to the spectra for the two Ru–Fe compounds indicating that the substitution of Ru for Fe has only a minor effect on the hyperfine and associated parameters. Similarly, analysis of the metal atom dynamics for [CpRuCp′FeCp′]PF6 and [Cp*RuCp′FeCp′]PF6 shows that ring substitution has a negligible effect on the bonding parameters. A comparison of the three triple-decker compounds with the mono-iron diamagnetic compound Cp*FeCp* and the paramagnetic [Cp′FeCp′]BF4 compound is also detailed.

Metal atom dynamics of phthalocyanin tin complexes: SnPc and SnPc2

High purity samples of SnPc and SnPc2 (Pc=phthalocyanin, C32H16N82−) have been examined by temperature-dependent 119Sn Mössbauer effect (ME) spectroscopy in order to elucidate the metal atom dynamics over the temperature range 94<T<301K. Contrary to earlier reported data, the temperature-dependence of the recoil-free fraction, f, of the Sn atom is distinctly different for the two structures, and the root-mean-square-amplitude-of-vibration has been determined for the two Sn atom sites. The metal atom motion in SnPc is essentially isotropic over the above temperature range with respect to the symmetry axis involving the Pc ligand, the metal atom, and the lone pair of electrons.

Atomic Bonding between Metal and Graphene

To understand structural and chemical properties of metal–graphene composites, it is crucial to unveil the chemical bonding along the interface. We provide direct experimental evidence of atomic bonding between typical metal nano structures and graphene, agreeing well with density functional theory studies. Single Cr atoms are located in the valleys of a zigzag edge, and few-atom ensembles preferentially form atomic chains by self-assembly. Low migration barriers lead to rich dynamics of metal atoms and clusters under electron irradiation. We demonstrate no electron-instigated interaction between Cr clusters and pristine graphene, though Cr has been reported to be highly reactive to graphene. The metal-mediated etching is a dynamic effect between metal clusters and pre-existing defects. The resolved atomic configurations of typical nano metal structures on graphene offer insight into modeling and simulations on properties of metal-decorated graphene for both catalysis and future carbon-based electronics.

Metal atom dynamics of CpFe ligated to a concave π-bowl sumanene

Iron atom dynamics of CH 3CpFe ligated to a concave face in π-bowl sumanene was investigated using temperature-dependent Mössbauer effect (ME) spectroscopy.

Metal atom dynamics in a charge transfer complex

Temperature-dependent 57Fe Mössbauer spectroscopy over the interval 89 < T < 335 K has been used to study the detailed metal atom dynamics in the charge transfer complex (CT) decamethlferrocene-acenaphthenequinone. The quadrupole splitting, area ratio and recoil-free fraction parameters clearly reflect the phase transition (T pt) at 257 K. The root-mean-square-amplitude of vibrations of the metal atom in the CT complex have been compared to that determined earlier for decamethyl ferrocene. The vibrational amplitudes are isotropic below T pt but anisotropic above this temperature.

One electron oxidation of triferrocenylmethanol: synthesis, metal atom dynamics, electron delocalization, and the crystal structure of [Fc3COH]+ PF6−

The title compound 2 was prepared and its crystal structure was determined at 100 K. The neat solid was examined by temperature dependent (57)Fe Mössbauer effect (ME) spectroscopy over the interval 92 < T < 318 K, and evidences two diamagnetic Fe(II) sites and one paramagnetic Fe(III) site. The latter shows spin-lattice relaxation, but there is no evidence of electron delocalization among the three iron sites in the above temperature interval. The mean-square-amplitude-of-vibration of the diamagnetic iron site has been determined from the recoil-free fraction ME resonance, and compared to the neutral Fc(3)COH homologue (1). The ME dynamical data are in good agreement with the U(i,j) value at 100 K extracted from the crystallographic results. The ME parameters at 5 K have also been determined with the sample compound embedded in a paraffin wax matrix as well as pelletized with BN.

Migration of gold atoms in graphene ribbons: Role of the edges

The migration of gold atoms attached to single vacancies near the edges of graphene ribbons is studied using density-functional theory calculations. The stable position for a single gold atom is found to be on top of a vacancy, as in an infinite graphene sheet. An energy of 5 eV is needed for the Au atom to move through the vacancy to the other side of the sheet, but the Au atom can migrate in lateral direction together with the vacancy, with a migration barrier of about 2.2 eV. The sites near the edges of the graphene layer are energetically more favorable for gold-atom-vacancy pairs than sites in the middle of extended graphene layers. The migration barriers for different pathways show that it is easier for the gold atom to move toward the edge where it can be captured. When the gold atom reaches the edge, it can migrate along the edge with an energy barrier of only 1.4 eV. Our results explain recent experimental observations [Y. Gan et al., Small 4, 587 (2008)] and provide information on the dynamics of metal atoms on substitutional sites in graphene as well as on their agglomeration at defects and at edges of graphene ribbons.

Metal atom dynamics in organometallics: Resolving the dichotomy between Mössbauer and X-ray derived values

Temperature-dependent 57Fe and 119Sn Mössbauer spectroscopy (ME) has been employed to study the metal atom dynamics in a number of organometallic compounds. Such experiments can yield information concerning the vibrational amplitudes of the metal atom as well as the anisotropy (if any) of this motion as a function of temperature. The difference between the vibrational amplitudes derived from ME data and that derived from single crystal X-ray Ui,j data is considered in the light of the structural details of these compounds.

Metal atom dynamics in four triferrocenylmethane derivatives and the crystal structure of Fc3COH

The metal atom dynamics of four triferrocenylmethane derivatives have been elucidated, using temperature-dependent 57Fe Mössbauer effect (ME) spectroscopy. The hyperfine parameters (IS and QS) at 90 K are compared to each other and those of related ferrocenoids. The metal atom vibrational amplitudes as a function of temperature have been extracted from the ME recoil-free fraction data, and are compared to the X-ray U(i,j) values for two of the compounds. The single crystal structural data for triferrocenylmethanol have been determined at 173 and 295 K. The vibrational anisotropy of the metal atom for all of the compounds is negligibly small over the accessible temperature range.

Hyperfine Interactions and Metal Atom Dynamic Effects of Pentafluorophenyl Substituents on Ferrocene Complexes

Seven ferrocenyl complexes, in which one or more of the Cp ring protons have been substituted by strongly electron-withdrawing pentafluorophenyl groups, have been examined by temperature-dependent 57Fe Mössbauer spectroscopy (MES) to determine both the electronic and dynamic effects of such substitution. The mean-square-vibrational-amplitudes of the metal atom have been expressed in terms of the dimensionless parameter ₣z,T=k2<xave2>, where z=M for the Mössbauer data, z=X for the X-ray data, k is the wave vector of the gamma radiation, and <xave2> is the mean square amplitude of the metal atom vibration. The differences between the M and X values can be related to the effects of local (atomic) and non-local (molecular) contributions to the metal atom dynamics. The MES data also elucidate the vibrational anisotropy of the metal atom motion, which in all of the complexes examined is isotropic within the experimental error.

Bonding and Metal-Atom Dynamics in Two Unique Sn(I)−Sn(III) Complexes

Two recently reported tin complexes, which incorporate both Sn(I) and Sn(III) sites in their structure, have been examined by temperature-dependent 119Sn Mössbauer effect (ME) spectroscopy. The distinct hyperfine parameters of the two sites make it possible to assign relative occupancies of the two sites (at 90 K) on the basis of the recoil-free fraction data extracted from the ME spectra. The temperature dependencies of the recoil-free fractions, elucidated from the ME spectra, show that the metal-atom dynamics are very similar for the two sites in the temperature range 90<T<150 K.

Metal Atom Dynamics and Spin-Lattice Relaxation in Multilayer Sandwich Compounds

Temperature-dependent 57Fe Mossbauer spectroscopy has been used to elucidate the hyperfine parameters and dynamical behavior of the metal atom in several organo-iron complexes which have one or more η 5 P5 ring structures as ligated groups. The spin-lattice relaxation of the (paramagnetic) one-electron oxidation products occurs on a time scale fast compared to τ 1/2 (ME) at temperatures in the range 85 < T < 320 K.

Mössbauer Spectroscopy of Bucky Ferrocenes: Lattice Dynamics and Motional Anisotropy of the Metal Atom

Temperature-dependent (57)Fe Mössbauer effect spectroscopy has been used to elucidate the metal atom dynamics in three neutral and two cationic bucky ferrocenes. For the three diamagnetic complexes Fe(C(60)H(5))Cp (1), Fe(C(60)Me(5))Cp (2), and Fe(C(60)Ph(5))Cp (3), the metal atom motion is anisotropic and the temperature dependence of the mean-square amplitude of vibration of the metal atom at a number of temperatures is reported. The Mössbauer lattice temperatures have been determined and compared to the parent ferrocene (6). The synthesis and X-ray crystal structure of 3 have been determined at 153(2) K, and the (1)H and (13)C NMR spectra have been recorded. The cationic complexes derived from 2 and 3 show spin-lattice relaxation. The relaxation rate appears insensitive to the nearest-neighbor environment of the metal atom in this pair.

Bonding, motional anisotropy, and metal atom dynamics of iron in two ferrocene substituted boron ring compounds

Temperature-dependent 57Fe Mössbauer spectroscopy over the range 90≤ T≤360 K has been used to elucidate the hyperfine interactions and dynamical behavior of the iron atom in two ferrocenyl substituted six-membered ring compounds. The root-mean-square amplitude of vibration (rmsav) of the iron atom in ferrocenyl boroxine I extracted from the Mössbauer measurements is in good agreement with the average U ij values observed in X-ray diffraction experiments, but is in less good agreement with the X-ray data reported for ferrocenyl borazine II. Moreover, the Mössbauer data lead to the conclusion that with respect to the iron atom motion, the boroxine compound I is a softer lattice than II. Significant motional anisotropy of the iron atom at 294 K is observed in I in agreement with the room temperature X-ray data reported earlier.

Photophysics of metal atoms in rare-gas complexes, clusters and matrices

Experimental studies of electronic spectra and dynamics of metal atoms M of groups I, II and III of the periodic system interacting with rare-gas atoms G in 1:1 MG complexes, MGn clusters and M/G centres in matrices are reviewed. It is shown that the necess n ary condition for a good understanding of the properties of M/G solids is a detailed knowledge of the M-G interactions in diatomic MG molecules. In spite of their apparent simplicity, the electronic spectra of MG and M/G systems are complex because of a drastic difference between the M-G potentials in ground and excited states of M atoms.