Andreas Bach

Potential Energy Surface for (Retro-)Cyclopropanation: Metathesis with a Cationic Gold Complex

The gas-phase cyclopropanation and apparent metathesis reactivity of ligand-supported gold arylidenes with electron-rich olefins is explained by quantum-chemical calculations. A deep potential minimum corresponding to a metal-bound cyclopropane adduct is in agreement with the measured absolute energies of the cyclopropanation and metathesis channels and is also consistent with previously reported electronic effects of arylidenes and supporting phosphorus ylid ligands on the product ratios. In the gas phase, the rate-determining step for the cyclopropanation is dissociation of the Lewis-acidic metal fragment, whereas the metathesis pathway features several rate-limiting transition states that are close in energy to the final product dissociation and hence contribute to the overall reaction rate. Importantly, the presented potential energy surface also accounts for the recently reported gold-catalyzed solution-phase retro-cyclopropanation reactivity.

Transmetalation Supported by a PtIICuI Bond

Passing Me over: platinum-to-copper methyl transfer is observed upon collision-induced dissociation (CID) of the cations formed by the interaction of the [(R(3)P)Cu](+) fragment with [(dmpe)PtMe(2)] (R=Me, Ph, Cy, tBu; dmpe=bis(dimethylphosphino)ethane; see X-ray crystal structure of coordination spheres for R=tBu). The thermochemistry of these processes for R=Me is investigated by mass-spectrometric methods.

Gas-Phase Energetics of Reductive Elimination from a Palladium(II) N-Heterocyclic Carbene Complex

Energy-resolved collision-induced dissociation experiments using tandem mass spectrometry are reported for an phenylpalladium N-heterocyclic carbene (NHC) complex. Reductive elimination of an NHC ligand as a phenylimidazolium ion involves a barrier of 30.9(14) kcal mol(-1), whereas competitive ligand dissociation requires 47.1(17) kcal mol(-1). The resulting three-coordinate palladium complex readily undergoes reductive C-C coupling to give the phenylimidazolium pi complex, for which the binding energy was determined to be 38.9(10) kcal mol(-1). Density functional calculations at the M06-L//BP86/TZP level of theory are in very good agreement with experiment. In combination with RRKM modeling, these results suggest that the rate-determining step for the direct reductive elimination process switches from the C-C coupling step to the fragmentation of the resulting sigma complex at low activation energy.

Water-chain clusters: vibronic spectra of 7-hydroxyquinoline·(H2O)n, n=1–4

Abstract Mass- and isomer-selected S 1 ← S 0 resonant two-photon ionization spectra of the series of supersonically cooled 7-hydroxyquinoline·(H 2 O) n , n =1–4 clusters are reported. These represent a series of hydrogen-bonded “water-chain” clusters, strung between the –O–H group and the N atom of the aromatic molecule. The vibronic spectra of n =2 and 4 clusters are strongly overlapped with the n =3 spectrum, and are identified for the first time; all exhibit large red-shifts (≈2000 cm −1 ). The n =2 and 3 clusters form solely water-chain clusters, while n =4 exists as water-chain clusters and as three other isomers, as shown by UV/UV laser-holeburning.

Ground- and excited state proton transfer and tautomerization in 7-hydroxyquinoline.(NH3)n clusters: Spectroscopic and time resolved investigations

Mass-selected S1↔S0 two color resonant two photon ionization (2C-R2PI) spectra, fluorescence spectra and fluorescence decay times are measured for supersonically cooled 7-hydroxyquinoline (7HQ)⋅(NH3)n clusters with n=4–10. For n=4, the S1←S0 2C-R2PI spectrum shows a 20 cm−1 broad electronic origin at 27 746 cm−1, followed by an intermolecular vibrational progression with band widths that increase up to ≈45 cm−1. In contrast, the 2C-R2PI spectra of the mixed 7HQ⋅(NH3)3H2O and 7HQ⋅(NH3)2(H2O)2 clusters exhibit narrow bands of 1–2 cm−1 width. The large band widths of 7HQ⋅(NH3)4 are due to a fast (k>1012 s−1) excited state process which is blocked when replacing one or more NH3 molecules by H2O in the cluster. For the n=5–10 clusters, the 2C-R2PI spectra display two broad absorption bands peaking at 25 000 and 27 000 cm−1. The latter is characteristic of the 7-quinolinate (7Q−) anion, implying that ground state proton transfer from 7HQ to the ammonia cluster occurs for n⩾5. Excitation at 27 000 cm−1 leads to ...

Nonstatistical effects in the dissociation of ethyl radical: Finding order in chaos

How does one identify order in complex dynamical systems? A Born-Oppenheimer molecular dynamics simulation of the dissociation of ethyl radical, C(2)H(5), produces an ensemble of classical trajectories which are decomposed in the time-frequency domain using wavelets. A time-dependent scalar metric, the normalized instantaneous orbital complexity, is constructed and shown to correlate not only to the more conventional Lyapunov exponents but also to the dissociation time for an individual trajectory. The analysis of the ensemble of trajectories confirms that the long-lived trajectories are associated with a low degree of ergodicity. While the analysis of molecular dissociation dynamics is the narrow focus of the present work, the method is more general for discovery and identification of ordered regimes within large sets of chaotic data.

Proton transfer in 7-hydroxyquinoline⋅(NH3)n solvent clusters

Mass- and isomer-selected S1←S0 resonant two-photon ionization and S1→S0 fluorescence spectra were obtained for the supersonically cooled 7-hydroxyquinoline⋅(NH3)n clusters with n=2–16. For n=2 and 3, the absorption and emission spectra exhibit discrete and narrow bands, characteristic of nonreactive hydrogen bonded ammonia-chain clusters. For n⩾4, the S1←S0 R2PI spectra are completely broadened, with an onset at ≈355 nm. For n=4–7, a weak fluorescence emission is observed, Stokes-shifted by ≈185 nm, with a maximum at ≈540 nm, which shows discrete structure on a broad background. From comparison to fluorescence emission observed in bulk solution, we conclude that S1 state enol→keto tautomerization occurs. For the n⩾7 or 8 clusters, the fluorescence emission spectra become completely unstructured and shift to the blue, peaking at 435–450 nm. This emission indicates the occurrence of either S1-state proton transfer to the ammonia solvent cluster and formation of the 7-HQ anion, or of the ground-state intrac...

Quasiperiodic trajectories in the unimolecular dissociation of ethyl radicals by time-frequency analysis.

Direct classical trajectory calculations for ethyl radical, C2H5, at the HCTH147@6-31 +G**/6-31G** level of theory support the experimental observation that the dissociation of highly excited ethyl radicals to ethylene and and a hydrogen atom can occur much more slowly than predicted by statistical rate theories. Only 78% of the trajectories of ethyl radicals prepared in a microcanonical ensemble with 120-kcal/mol excitation energy above the zero-point energy and zero total angular momentum dissociate to form C2H4 + H. The remaining hot ground-state ethyl radicals have a lifetime of >>2 ps, during which a time-frequency analysis finds them trapped for extended periods of time in long-lived quasiperiodic trajectories.

H atom transfer along an ammonia chain: Tunneling and mode selectivity in 7-hydroxyquinoline⋅(NH3)3

Excitation of the 7-hydroxyquinoline(NH(3))(3) [7HQ(NH(3))(3)] cluster to the S(1) (1)pi pi(*) state results in an O-H-->NH(3) hydrogen atom transfer (HAT) reaction. In order to investigate the entrance channel, the vibronic S(1)<-->S(0) spectra of the 7HQ.(NH(3))(3) and the d(2)-7DQ.(ND(3))(3) clusters have been studied by resonant two-photon ionization, UV-UV depletion and fluorescence techniques, and by ab initio calculations for the ground and excited states. For both isotopomers, the low-frequency part of the S(1)<--S(0) spectra is dominated by ammonia-wire deformation and stretching vibrations. Excitation of overtones or combinations of these modes above a threshold of 200-250 cm(-1) for 7HQ.(NH(3))(3) accelerates the HAT reaction by an order of magnitude or more. The d(2)-7DQ.(ND(3))(3) cluster exhibits a more gradual threshold from 300 to 650 cm(-1). For both isotopomers, intermolecular vibrational states above the threshold exhibit faster HAT rates than the intramolecular vibrations. The reactivity, isotope effects, and mode selectivity are interpreted in terms of H atom tunneling through a barrier along the O-H-->NH(3) coordinate. The barrier results from a conical intersection of the optically excited (1)pi pi(*) state with an optically dark (1)pi sigma(*) state. Excitation of the ammonia-wire stretching modes decreases both the quinoline-O-H...NH(3) distance and the energetic separation between the (1)pi pi(*) and (1)pi sigma(*) states, thereby increasing the H atom tunneling rate. The intramolecular vibrations change the H bond distance and modulate the (1)pi pi(*)<-->(1)pi sigma(*) interaction to a much smaller extent.

Water-chain clusters: Vibronic spectra of 7-hydroxyquinoline⋅(H2O)2

Mass- and isomer-selected S1←S0 resonant two-photon ionization and S1→S0 fluorescence spectra were obtained for the supersonically cooled 7-hydroxyquinoline⋅(H2O)2 cluster. UV/UV-holeburning measurements show that >98% of the spectrum is due to a single “water-chain” cluster isomer, although two different tautomers (7-keto- and 7-hydroxyquinoline), two different rotamers (cis- and trans-hydroxy), and two torsional conformers of the chain are possible. Ab initio calculations of structures and vibrations of five different tautomers/ rotamers/ conformers of this cluster are reported. These predict that the cis-7-hydroxyquinoline⋅(H2O)2 “up/down” water-chain form is the most stable cluster. The experimentally observed S0 and S1 state vibrational frequencies agree well with those calculated for this isomer. We find no evidence for either the trans-rotamer or the keto tautomer clusters. S1←S0 excitation leads to contraction of all three hydrogen-bonds along the hydrogen-bonded water chain, inducing intermolecul...