Anthony J. H. M. Meijer

Flux analysis for calculating reaction probabilities with real wave packets

Abstract A recent approach to obtain scattering probabilities involves propagating the real part of a wave packet and analysis in the asymptotic product channel. We show that real wave packet propagation can also be used with analysis methods based on flux through a surface. We also show the relation between asymptotic product analysis and flux-based methods. Flux analysis with real wave packets is illustrated with an application to D + H 2 → DH + H in three dimensions.

Formation of molecular hydrogen on a graphite surface via an Eley-Rideal mechanism

Abstract The associative desorption of hydrogen atoms on graphite via an Eley–Rideal mechanism is studied theoretically. Time-independent quantum reactive scattering calculations are performed on a potential energy surface calculated using the generalised gradient approximation of density functional theory. The absence of a barrier to reaction leads to a reaction probability which is close to one, even for low collision energies. The vibrational product distribution of H 2 shows a peak in the ( v =2) vibrational state with appreciable population of higher states. The significance of these results for chemistry in the interstellar medium is discussed.

Time-dependent quantum mechanical calculations on H+O2 for total angular momentum J>0 II: On the importance of Coriolis coupling

The H+O2→OH+O reaction has been studied for total angular momentum J>0 with a time-dependent wave packet method using the Coriolis coupled method of Goldfield and Gray [E. M. Goldfield and S. K. Gray, Comp. Phys. Commun. 98, 1 (1996)] on parallel computers. Helicity conserving (HC) and coupled channel (CC) calculations were performed for J=1, J=2, J=5, and J=10 using two different embeddings for the body fixed coordinate system to investigate the importance of Coriolis coupling for this reactive system. If the H–O2 distance is taken to be the z axis of the coordinate system, we find poor agreement between the HC and the CC calculations for J>2. When the O2 bond is taken to be the z axis, we find good agreement between the CC and HC calculations at low J. For higher J the agreement gets progressively worse, especially at higher energies. We can explain these results using a classical model from a previous paper on H+O2 [A. J. H. M. Meijer and E. M. Goldfield, J. Chem. Phys. 108, 5404 (1998)].

TIME-DEPENDENT QUANTUM MECHANICAL CALCULATIONS ON H+O2 FOR TOTAL ANGULAR MOMENTUM J>0

The H+O2→OH+O reaction has been studied with a time-dependent wave packet method for total angular momentum J=0, 1, 2, and 5, using the Coriolis coupled method [E. M. Goldfield and S. K. Gray, Comp. Phys. Commun. 98, 1 (1996)] on parallel computers. We find that at higher energies the total reaction probability decreases by a factor of 2 in going from a J=0 calculation to a J=1 calculation. The effect for higher J with respect to J=1 is less dramatic. We investigated the decrease in reaction probability for J>0 by examining the different initial conditions with respect to Ω, the projection of J onto the body-fixed z axis for the J>0 calculations. We conclude that the reaction probability is a strong function of Ω. If Ω=0 for J>0, collision geometries are accessible that lead to an enhanced reaction probability.

Toward control of electron transfer in donor-acceptor molecules by bond-specific infrared excitation

Electron transfer (ET) from donor to acceptor is often mediated by nuclear-electronic (vibronic) interactions in molecular bridges. Using an ultrafast electronic-vibrational-vibrational pulse-sequence, we demonstrate how the outcome of light-induced ET can be radically altered by mode-specific infrared (IR) excitation of vibrations that are coupled to the ET pathway. Picosecond narrow-band IR excitation of high-frequency bridge vibrations in an electronically excited covalent trans-acetylide platinum(II) donor-bridge-acceptor system in solution alters both the dynamics and the yields of competing ET pathways, completely switching a charge separation pathway off. These results offer a step toward quantum control of chemical reactivity by IR excitation. Vibrational excitation can modulate electron transfer probabilities in real time. Big impact from a well-placed shake Since the advent of ultrashort laser pulses, chemists have sought to steer reaction trajectories in real time by setting particular molecular vibrations in motion. Using this approach, Delor et al. have demonstrated a markedly clear-cut influence on electron transfer probabilities along the axis of a platinum complex. The complex comprised donor and acceptor fragments—which respectively give and take electrons upon ultraviolet excitation—bridged together by triply bonded carbon chains linked to the metal center. By selectively stimulating the carbon triple-bond stretch vibration with an infrared pulse, the authors could induce substantial changes in the observed electron transfer pathways between the fragments. Science, this issue p. 1492

Time-dependent quantum mechanical calculations on H+O2 for total angular momentum J>0. III. Total cross sections

The H+O2→OH+O reaction has been studied with a time-dependent wave packet method for total angular momentum J=15, 20, 25, 35. This work is a continuation of previous studies for J⩽10. The calculations were performed combining a real wave packet method with the Coriolis coupled method on parallel computers. We find that for most energies there is a monotonic decrease of reaction probability with increasing J. Nevertheless, due to the 2J+1 degeneracy, higher angular momentum states contribute significantly to the total reaction cross section. A smoothing/interpolation/extrapolation scheme is employed to compute total reaction cross sections. These cross sections are compared with quasiclassical results on the same potential energy surface, and the most recent experimental cross sections. Comparisons with quasiclassical results show the significance of zero-point energy constraints. The quantum mechanical theoretical cross sections are smaller than the experimental ones everywhere, suggesting that a more acc...

On the mechanism of vibrational control of light-induced charge transfer in donor–bridge–acceptor assemblies

Nuclear-electronic (vibronic) coupling is increasingly recognized as a mechanism of major importance in controlling the light-induced function of molecular systems. It was recently shown that infrared light excitation of intramolecular vibrations can radically change the efficiency of electron transfer, a fundamental chemical process. We now extend and generalize the understanding of this phenomenon by probing and perturbing vibronic coupling in several molecules in solution. In the experiments an ultrafast electronic-vibrational pulse sequence is applied to a range of donor-bridge-acceptor Pt(II) trans-acetylide assemblies, for which infrared excitation of selected bridge vibrations during ultraviolet-initiated charge separation alters the yields of light-induced product states. The experiments, augmented by quantum chemical calculations, reveal a complex combination of vibronic mechanisms responsible for the observed changes in electron transfer rates and pathways. The study raises new fundamental questions about the function of vibrational processes immediately following charge transfer photoexcitation, and highlights the molecular features necessary for external vibronic control of excited-state processes.

Exciton localization in disordered poly(3-hexylthiophene)

Singlet exciton localization in conformationally disordered poly(3-hexylthiophene) (P3HT) is investigated via configuration interaction (singles) calculations of the Pariser-Parr-Pople model. The P3HT structures are generated by molecular dynamics simulations. The lowest-lying excitons are spatially localized, space filling, and nonoverlapping. These define spectroscopic segments or chromophores. The strong conformational disorder in P3HT causes breaks in the pi-conjugation. Depending on the relative values of the disorder-induced localization length and the distances between the pi-conjugation breaks, these breaks sometimes serve to pin the low-lying localized excitons. The exciton confinement also causes a local spectrum of low-lying exciton states. Coulomb-induced intra- or interchain interactions between spectroscopic segments in close spatial proximity can delocalize an exciton across these segments, in principle causing phase coherent transition dipole moments.

Diastereoselective Cycloadditions and Transformations of N-Alkyl and N-Aryl Maleimides with Chiral 9-Anthrylethanol Derivatives

Thermal Diels-Alder reactions of chiral 9-methoxyethyl and 9-hydroxyethyl anthracene have been investigated both experimentally and computationally with a range of N-substituted maleimides. Whereas cycloadditions with 9-methoxyethyl anthracene proceeded with almost complete diastereselectivity, those with 1-anthracene-9-yl-ethanol resulted in essentially no diastereoselectivity. Subsequent regio- and stereoselective transformations with reducing agents and carbon nucleophiles demonstrated the synthetic utility of this methodology, which was applied to the enantioselective synthesis of pyrrolo[2,1-a]isoquinolines and an attempted synthesis of the alkaloid crispine A. Computational studies supported the proposed hypotheses for the stereoselectivity observed in the transformations described.

Dinitrogen Release from Arylpentazole: A Picosecond Time-Resolved Infrared, Spectroelectrochemical, and DFT Computational Study

p-(Dimethylamino)phenyl pentazole, DMAP-N5 (DMAP = Me2N-C6H4), was characterized by picosecond transient infrared spectroscopy and infrared spectroelectrochemistry. Femtosecond laser excitation at 310 or 330 nm produces the DMAP-N5 (S1) excited state, part of which returns to the ground state (τ = 82 ± 4 ps), while DMAP-N and DMAP-N3 (S0) are generated as double and single N2-loss photoproducts with η ≈ 0.14. The lifetime of DMAP-N5 (S1) is temperature and solvent dependent. [DMAP-N3](+) is produced from DMAP-N5 in a quasireversible, one-electron oxidation process (E1/2 = +0.67 V). Control experiments with DMAP-N3 support the findings. DFT B3LYP/6-311G** calculations were used to identify DMAP-N5 (S1), DMAP-N3(+), and DMAP-N in the infrared spectra. Both DMAP-N5 (S1) and [DMAP-N5](+) have a weakened N5 ring structure.