Gregory F. Metha

Electronic transitions of cobalt carbide, CoC, near 750 nm: A good example of case (bβS) hyperfine coupling

The laser induced fluorescence spectrum of jet‐cooled CoC near 750 nm has been measured at high resolution following the reaction of laser‐ablated cobalt atoms with methane. The X2Σ+ ground state of CoC is an unusually good example of Hund’s case (bβS) coupling. Since Co has a nuclear spin I=7/2, each rotational level is split by the Fermi contact interaction into G=3 and G=4 components, where G=I+S; the splitting for N=0 is more than 0.5 cm−1. The X2Σ+ state begins to uncouple toward case (bβJ) with increasing rotation. Transitions to various 2Π excited states occur in the region 13 000–14 500 cm−1; the most prominent of these (for which high resolution spectra have been recorded) lie at 13 079 cm−1 (2Π3/2) and 13 343 cm−1 (2Π1/2). The (bβS) coupling in the ground state produces some unexpected hyperfine intensity patterns, which have been studied in detail. A very low‐lying 2Δi state, whose Ω=5/2 and Ω=3/2 components lie at 221 and 1173 cm−1, has been identified. Laser excitation of the 2Π3/2–2Δ5/2 tran...

HCO (N,Ka,Kc,J) distributions from near-threshold photolysis of H2CO (J,Ka,Kc)

The dynamics of the reaction H2CO+hν(λ≈330 nm)→H+HCO have been studied following excitation of formaldehyde into the A(1A2) state, just above the dissociation threshold of the X(1A1) state. Formaldehyde was excited via specific J, Ka, Kc rotational states and the ensuing rotational distribution of HCO measured by fully resolving N, Ka, Kc, and J=N±S of the fragment. When only the N and Ka quantum numbers of both formaldehyde and the formyl radical are considered, the distributions are generally modeled well by phase space theory (PST). Within ≈10 cm−1 of the threshold, however, the PST predictions consistently exceed the experimental populations. This was accounted for by the inclusion of a centrifugal barrier in the PST model. The attractive part of the effective centrifugal potential was modeled by a dipole-induced dipole plus dispersion interaction. The barrier is weak and long range (>5 A). Resolution of Kc in the reaction, in both parent and product, gave large deviations from the PST model. The HCO...

Chemically synthesised atomically precise gold clusters deposited and activated on titania. Part II

Synchrotron XPS was used to investigate a series of chemically synthesised, atomically precise gold clusters Au(n)(PPh3)y (n = 8, 9 and 101, y depending on the cluster size) immobilized on anatase (titania) nanoparticles. Effects of post-deposition treatments were investigated by comparison of untreated samples with analogues that have been heat treated at 200 °C in O2, or in O2 followed by H2 atmosphere. XPS data shows that the phosphine ligands are oxidised upon heat treatment in O2. From the position of the Au 4f(7/2) peak it can be concluded that the clusters partially agglomerate immediately upon deposition. Heating in oxygen, and subsequently in hydrogen, leads to further agglomeration of the gold clusters. It is found that the pre-treatment plays a crucial role in the removal of ligands and agglomeration of the clusters.

The Optical Spectrum of a Large Isolated Polycyclic Aromatic Hydrocarbon: Hexa-peri-hexabenzocoronene, C42H18

The first optical spectrum of an isolated polycyclic aromatic hydrocarbon large enough to survive the photophysical conditions of the interstellar medium is reported. Vibronic bands of the first electronic transition of the all-benzenoid polycyclic aromatic hydrocarbon hexa-peri-hexabenzocoronene were observed in the 4080-4530 A range by resonant 2-color 2-photon ionization spectroscopy. The strongest feature at 4264 A is estimated to have an oscillator strength of f = 1.4 × 10−3, placing an upper limit on the interstellar abundance of this polycyclic aromatic hydrocarbon at 4 × 1012 cm−2, accounting for a maximum of ~0.02% of interstellar carbon. This study opens up the possibility to rigorously test neutral polycyclic aromatic hydrocarbons as carriers of the diffuse interstellar bands in the near future.

Kick: constraining a stochastic search procedure with molecular fragments.

An extension of the Kick program developed by Bera et al. (J Phys Chem A 2006, 110, 4287) is described in which chemically sensible molecular fragments are used in an automated stochastic search algorithm. This results in a vastly reduced region of the potential energy surface which can be explored very quickly. We present use of this modified algorithm to the search for low‐lying isomers, and we present candidates for the global energy minimum, for a range of chemical systems. We highlight the usefulness of this procedure for exploring reactions of molecules with transition metal clusters and to the microsolvation of a small dipeptide.

Determination of the Molecular Structure of Diacetylene from High-Resolution FTIR Spectroscopy

The high-resolution infrared absorption spectra of eight2H or13C substituted isotopomers of diacetylene have been recorded, and the bands corresponding to thev4 fundamental andv6 combination of the major isotopomer have been analyzed using a Loomis-Wood-type program. Effective ground-state rotational constants have been obtained from ground-state combination differences. A number ofr0,rs,rmς, and (rmς)corr structures have been calculated from the available data and are compared to those obtained by ab initio methods. The (rmς)corr structure, which is a reliable near-equilibrium structure of diacetylene, isrC−H=106.131(13) pm;rC−C=137.081(16) pm;rC-C=120.964(14). (rmς)corr structures of the related molecules cyanogen, cyanoacetylene, and cyanodiacetylene have been calculated, and near-equilibrium structures of triacetylene and dicyanoacetylene have been predicted.

Factors influencing the catalytic oxidation of benzyl alcohol using supported phosphine-capped gold nanoparticles

Two phosphine-stabilised gold clusters, Au101(PPh3)21Cl5 and Au9(PPh3)8(NO3)3, were deposited and activated on anatase TiO2 and fumed SiO2. These catalysts showed an almost complete oxidation of benzyl alcohol (>90%) within 3 hours at 80 °C and 3 bar O2 in methanol with a high substrate-to-metal molar ratio of 5800 and turn-over frequency of 0.65 s−1. Factors influencing catalytic activity were investigated, including metal–support interaction, effects of heat treatments, chemical composition of gold clusters, the size of gold nanoparticles and catalytic conditions. It was found that the anions present in gold clusters play a role in determining the catalytic activity in this reaction, with NO3− diminishing the catalytic activity. High catalytic activity was attributed to the formation of large gold nanoparticles (>2 nm) that coincides with partial removal of ligands which occurs during heat treatment and catalysis. Selectivity towards the formation of methyl benzoate can be tuned by selection of the reaction temperature. The catalysts were characterised using transmission electron microscopy, UV-vis diffuse reflectance spectroscopy and X-ray photoelectron spectroscopy.

Reactivity of Oxygen Deficient Cerium Oxide Clusters with Small Gaseous Molecules

Oxygen deficient cerium oxide cluster ions, Ce(n)O(m)(+) (n = 2-10, m = 1-2n) were prepared in the gas phase by laser ablation of a cerium oxide rod. The reactivity of the cluster ions was investigated using mass spectrometry, finding that oxygen deficient clusters are able to extract oxygen atoms from CO, CO2, NO, N2O, and O2 in the gas phase. The oxygen transfer reaction is explained in terms of the energy balance between the bond dissociation energy of an oxygen containing molecule and the oxygen affinity of the oxygen-deficient cerium oxide clusters, which is supported by DFT calculations. The reverse reaction, i.e., formation of the oxygen deficient cluster ions from the stoichiometric ones was also examined. It was found that intensive heating of the stoichiometric clusters results in formation of oxygen deficient clusters via Ce(n)O(2n)(+) → Ce(n)O(2n-2)(+) + O2, which was found to occur at different temperatures depending on cluster size, n.

Generation of transient species by laser induced pyrolysis. The high resolution fourier transform infrared spectrum of NCN

Abstract An experimental system has been developed for obtaining high resolution FTIR absorption spectra of short lived species and radicals generated by the technique of infrared laser powered pyrolysis. The high resolution spectrum of the free radical NCN generated by infrared laser powered pyrolysis has been recorded at a resolution of 0.01 cm −1 . Analysis of the 3 0 1 fundamental and 2 1 1 3 0 1 hot band of the 3 Σ g − ground electronic state has resulted in the following parameters for NCN: B 0 = 0.3972469(45) cm −1 , D 0 = 1.71(19) × 10 −7 cm −1 , α 3 = 4.3 × 10 −3 cm −1 , α 2 = −5.544 × 10 −3 cm −1 , q 2 = 1.094 × 10 −3 cm −1 .

Phosphine-stabilised Au9 clusters interacting with titania and silica surfaces: The first evidence for the density of states signature of the support-immobilised cluster

Chemically made, atomically precise phosphine-stabilized clusters Au9(PPh3)8(NO3)3 were deposited on titania and silica from solutions at various concentrations and the samples heated under vacuum to remove the ligands. Metastable induced electron spectroscopy was used to determine the density of states at the surface, and X-ray photoelectron spectroscopy for analysing the composition of the surface. It was found for the Au9 cluster deposited on titania that the ligands react with the titania substrate. Based on analysis using the singular value decomposition algorithm, the series of MIE spectra can be described as a linear combination of 3 base spectra that are assigned to the spectra of the substrate, the phosphine ligands on the substrate, and the Au clusters anchored to titania after removal of the ligands. On silica, the Au clusters show significant agglomeration after heat treatment and no interaction of the ligands with the substrate can be identified.