The Ring-Closing Reaction of Cyclopentadiene Probed with Ultrafast X-ray Scattering.

The dynamics of cyclopentadiene (CP) following optical excitation at 243 nm was investigated by time-resolved pump-probe X-ray scattering using 16.2 keV X-rays at the Linac Coherent Light Source (LCLS). We present the first ultrafast structural evidence that the reaction leads directly to the formation of bicyclo[2.1.0]pentene (BP), a strained molecule with three- and four-membered rings. The bicyclic compound decays via a thermal backreaction to the vibrationally hot CP with a time constant of 21 ± 3 ps. A minor channel leads to ring-opened structures on a subpicosecond time scale.

Infrared spectra and fragmentation dynamics of isotopologue-selective mixed-ligand complexes.

Isolated mixed-ligand complexes provide tractable model systems in which to study competitive and cooperative binding effects as well as controlled energy flow. Here, we report spectroscopic and isotopologue-selective infrared photofragmentation dynamics of mixed gas-phase Au(12/13CO)n(N2O)m+ complexes. The rich infrared action spectra, which are reproduced well using simulations of calculated lowest energy structures, clarify previous ambiguities in the assignment of vibrational bands, especially accidental coincidence of CO and N2O bands. The fragmentation dynamics exhibit the same unexpected behaviour as reported previously in which, once CO loss channels are energetically accessible, these dominate the fragmentation branching ratios, despite the much lower binding energy of N2O. We have investigated the dynamics computationally by considering anharmonic couplings between a relevant subset of normal modes involving both ligand stretch and intermolecular modes. Discrepancies between correlated and uncorrelated model fit to the ab initio potential energy curves are quantified using a Boltzmann sampled root mean squared deviation providing insight into efficiency of vibrational energy transfer between high frequency ligand stretches and the softer intermolecular modes which break during fragmentation.

An Infrared Study of Gas-Phase Metal Nitrosyl Ion-Molecule Complexes.

We present a combined experimental and quantum chemical study of gas-phase group 9 metal nitrosyl complexes, M(NO)n+ (M = Co, Rh, Ir). Experimental infrared photodissociation spectra of mass-selected ion-molecule complexes are presented in the region 1600 cm-1 to 2000 cm-1 which includes the NO stretch. These are interpreted by comparison with the simulated spectra of energetically low-lying structures calculated using density functional theory. A mix of linear and nonlinear ligand binding is observed, often within the same complex, and clear evidence of coordination shell closing is observed at n = 4 for Co(NO)n+ and Ir(NO)n+. Calculations of Rh(NO)n+ complexes suggest additional low-lying five-coordinate structures. In all cases, once a second coordination shell is occupied, new spectral features appear which are assigned to (NO)2 dimer moieties. Further evidence of such motifs comes from differences in the spectra recorded in the dissociation channels corresponding to single and double ligand loss.

An infrared study of CO2 activation by holmium ions, Ho+ and HoO.

We report a combined experimental and computational study of carbon dioxide activation at gas-phase Ho+ and HoO+ centres. Infrared action spectra of Ho(CO2)n+ and [HoO(CO2)n]+ ion-molecule complexes have been recorded in the spectral region 1700-2400 cm-1 and assigned by comparison with simulated spectra of energetically low-lying structures determined by density functional theory. Little by way of activation is observed in Ho(CO2)n+ complexes with CO2 binding end-on to the Ho+ ion. By contrast, all [HoO(CO2)n]+ complexes n ≥ 3 show unambiguous evidence for formation of a carbonate radical anion moiety, . The signature of this structure, a new vibrational band observed around 1840 cm-1 for n = 3, continues to red-shift monotonically with each successive CO2 ligand binding with net charge transfer from the ligand rather than the metal centre.

Spectroscopy and Infrared Photofragmentation Dynamics of Mixed Ligand Ion-Molecule Complexes: Au(CO)x(N2O)y.

We report a combined experimental and computational study of the structure and fragmentation dynamics of mixed ligand gas-phase ion-molecule complexes. Specifically, we have studied the infrared spectroscopy and vibrationally induced photofragmentation dynamics of mass-selected Au(CO)x(N2O)y+ complexes. The structures can be understood on the basis of local CO and N2O chromophores in different solvation shells with CO found preferentially in the core. Rich fragmentation dynamics are observed as a function of complex composition and the vibrational mode excited. The dynamics are characterized in terms of branching ratios for different ligand loss channels in light of calculated internal energy distributions. Intramolecular vibrational redistribution appears to be rapid, and dissociation is observed into all energetically accessible channels with little or no evidence for preferential breaking of the weakest intermolecular interactions.

Atomic Cluster Au_{10}

We report an intense broadband midinfrared absorption band in the Au_{10}^{+} cluster in a region in which only molecular vibrations would normally be expected. Observed in the infrared multiple photon dissociation spectra of Au_{10}Ar^{+}, Au_{10}(N_{2}O)^{+}, and Au_{10}(OCS)^{+}, the smooth feature stretches 700-3400 cm^{-1} (λ=14-2.9 µm). Calculations confirm unusually low-energy allowed electronic excitations consistent with the observed spectra. In Au_{10}(OCS)^{+}, IR absorption throughout the band drives OCS decomposition resulting in CO loss, providing an alternative method of bond activation or breaking.

Infrared action spectroscopy of nitrous oxide on cationic gold and cobalt clusters.

Understanding the catalytic decomposition of nitrous oxide on finely divided transition metals is an important environmental issue. In this study, we present the results of a combined infrared action spectroscopy and quantum chemical investigation of molecular N2O binding to isolated Aun+ (n ≤ 7) and Con+ (n ≤ 5) clusters. Infrared multiple-photon dissociation spectra have been recorded in the regions of both the N[double bond, length as m-dash]O (1000-1400 cm-1) and N[double bond, length as m-dash]N (2100-2450 cm-1) stretching modes of nitrous oxide. In the case of Aun+ clusters only the ground electronic state plays a role, while the involvement of energetically low-lying excited states in binding to the Con+ clusters cannot be ruled out. There is a clear preference for N-binding to clusters of both metals but some O-bound isomers are observed in the case of smaller Con(N2O)+ clusters.

Free electron laser infrared action spectroscopy of nitrous oxide binding to platinum clusters, Ptn(N2O).

Infrared multiple-photon dissociation spectroscopy has been applied to study Ptn(N2O)+ (n = 1-8) clusters which represent entrance-channel complexes on the reactive potential energy surface for nitrous oxide decomposition on platinum. Comparison of spectra recorded in the spectral region 950 cm-1 to 2400 cm-1 with those simulated for energetically low-lying structures from density functional theory shows a clear preference for molecular binding via the terminal N atom, though evidence of O-binding is observed for some cluster sizes. Enhanced reactivity of Ptn+n≥ 6 clusters towards N2O is reflected in the calculated reactive potential energy surfaces and, uniquely in the size range studied, Pt6(N2O)+ proved impossible to form in significant number density even with cryogenic cooling of the cluster source. Infrared-driven N2O decomposition, resulting in the formation of cluster oxides, PtnO+, is observed following vibrational excitation of several Ptn(N2O)+ complexes.

Infrared Study of OCS Binding and Size-Selective Reactivity with Gold Clusters, Aun+ (n = 1-10).

OCS binding to and reactivity with isolated gold cluster cations, Aun+ (n = 1-10), has been studied by infrared multiple photon dissociation (IR-MPD) spectroscopy in conjunction with quantum chemical calculations. The distribution of complexes AunSx(OCS)m+ formed reflects the relative reactivity of different cluster sizes with OCS, under the multiple collision conditions of our ablation source. The IR-MPD spectra of Aun(OCS)+ (n = 3-10) clusters are interpreted in terms of either µ1 or µ2 S binding motifs. Analysis of the fragmentation products following infrared excitation of parent Aun(OCS)+ clusters reveals strongly size-selective (odd-even) branching ratios for OCS and CO loss, respectively. CO loss signifies infrared-driven OCS decomposition on the cluster surface and is observed to occur predominantly on even n clusters (i.e., those with odd electron counts). The experimental data, including fragmentation branching ratios, are consistent with calculated potential energy landscapes, in which the initial species trapped are molecularly bound entrance channel complexes, rather than global minimum inserted structures. Attempts to generate Rhn(OCS)+ and Ptn(OCS)+ equivalents failed; only sulfide reaction products were observed in the mass spectrum, even after cooling the cluster source to -100 °C.

IR Signature of Size-Selective CO2 Activation on Small Platinum Cluster Anions, Ptn - (n=4-7).

Infrared multiple photon dissociation spectroscopy (IR-MPD) has been employed to determine the nature of CO2 binding to size-selected platinum cluster anions, Ptn - (n=4-7). Interpreted in conjunction with density functional theory simulations, the results illustrate that the degree of CO2 activation can be controlled by the size of the metal cluster, with dissociative activation observed on all clusters n≥5. Of potential practical significance, in terms of the use of CO2 as a useful C1 feedstock, CO2 is observed molecularly-bound, but highly activated, on the Pt4 - cluster. It is trapped behind a barrier on the reactive potential energy surface which prevents dissociation.

Infrared Spectroscopy of Au+(CH4) n Complexes and Vibrationally-Enhanced C-H Activation Reactions.

A combined spectroscopic and computational study of gas-phase Au+(CH4) n (n = 3-8) complexes reveals a strongly-bound linear Au+(CH4)2 core structure to which up to four additional ligands bind in a secondary coordination shell. Infrared resonance-enhanced photodissociation spectroscopy in the region of the CH4 a 1 and t 2 fundamental transitions reveals essentially free internal rotation of the core ligands about the H4C-Au+-CH4 axis, with sharp spectral features assigned by comparison with spectral simulations based on density functional theory. In separate experiments, vibrationally-enhanced dehydrogenation is observed when the t 2 vibrational normal mode in methane is excited prior to complexation. Clear infrared-induced enhancement is observed in the mass spectrum for peaks corresponding 4u below the mass of the Au+(CH4) n=2,3 complexes corresponding, presumably, to the loss of two H2 molecules.