Abortive reaction leads to selective adsorbate rotation.

Electron-induced dissociation of a fluorocarbon adsorbate CF3 (ad) at 4.6 K is shown by Scanning Tunnelling Microscopy (STM) to form directed energetic F-atom 'projectiles' on Cu(110). The outcome of a collision between these directed projectiles and stationary co-adsorbed allyl 'target' molecules was found through STM to give rotational excitation of the target allyl, clockwise or anti-clockwise, depending on the chosen collision geometry. Molecular dynamics computation linked the collisional excitation of the allyl target to an 'abortive chemical reaction', in which the approach of the F-projectile stretched an H-C bond lifting the allyl above the surface, facilitating isomerization from 'Across' to 'Along' a Cu row.

Reversible 1D chain-reaction gives rise to an atomic-scale Newton's cradle.

An F-atom with ∼1 eV translational energy was aimed at a line of fluorocarbon adsorbates on Cu(110). Sequential 'knock-on' of F-atom products was observed by STM to propagate along the 1D fluorocarbon line. Hot F-atoms travelling along the line in six successive 'to-and-fro' cycles paralleled the rocking of a macroscopic Newton's cradle.

Direct Observation of Knock-on in Surface Reactions at Zero Impact Parameter.

Reaction dynamics examines molecular motions in reactive collisions. The aiming of reagents at one another has been achieved at selected miss distances (impact parameters, b) by using the corrugations on crystalline surfaces as collimator. Prior experimental work and ab initio calculation showed single atoms aimed at chemisorbed molecules with b = 0 gave knock-on of atomic reaction products through a linear transition state. Here we report a study of b = 0 collision between directed CF2 and stationary chemisorbed CF3. Experiments and ab initio calculations again show linear reaction with a linear transition state, despite the additional degrees of freedom for CF2. The directed motion of CF2 is conserved through this linear transition state. Conservation of directionality is evidenced experimentally by the observation of a knock-on chain reaction along a line of chemisorbed CF3.

Direct observation of knock-on reaction with umbrella inversion arising from zero-impact-parameter collision at a surface.

In Surface-Aligned-Reactions (SAR), the degrees of freedom of chemical reactions are restricted and therefore the reaction outcome is selected. Using the inherent corrugation of a Cu(110) substrate the adsorbate molecules can be positioned and aligned and the impact parameter, the collision miss-distance, can be chosen. Here, substitution reaction for a zero impact parameter collision gives an outcome which resembles the classic Newton's cradle in which an incident mass 'knocks-on' the same mass in the collision partner, here F + CF3 → (CF3)' + (F)' at a copper surface. The mechanism of knock-on was shown by Scanning Tunnelling Microscopy to involve reversal of the CF3 umbrella as in Walden inversion, with ejection of (F)' product along the continuation of the F-reagent direction of motion, in collinear reaction.

Contrasting Efficiency of Electron-Induced Reaction at Cu(110) in Aliphatic and Aromatic Bromides.

We report a comparative study of the electron-induced reaction of pentyl bromide (PeBr) and phenyl bromide (PhBr) on Cu(110) at 4.6 K, observed by scanning tunneling microscopy (STM). The induced dissociation of the intact adsorbed molecule for both reagents occurred at an energy of 2.0 eV, producing a hydrocarbon radical and a Br atom. Electron-induced C-Br bond dissociation was found to be a single-electron process for both reagents. The impulsive two-state (I2S) model was used to describe the Br atom recoil as due to molecular excitation to a repulsive anti-bonding state, in which recoil of the dissociation products occurred due to C·Br repulsion along the prior C-Br bond direction. The measured reaction yield was 3 orders of magnitude greater for PeBr, 2.0 × 10-7 reactive events per electron, than for PhBr with a yield of 1.7 × 10-10. The low yield of dissociation products from the aromatic PhBr was attributed to the presence of two additional anionic states below the 2.0 eV energy limit, absent for the aliphatic PeBr; these additional anionic states for PhBr could provide a pathway for electron transfer to the surface in the case of the aromatic, but not the aliphatic, anion. The consequent shorter lifetime of the repulsive aromatic anion of PhBr is consistent with the observation of shorter mean recoil distance (3.2 Å) of its Br dissociation product, as compared with the markedly longer recoil (8.7 Å) of Br observed from the anion of PeBr.

Electron-induced molecular dissociation at a surface leads to reactive collisions at selected impact parameters.

Crossed molecular beams of gases have provided definitive information concerning the dynamics of chemical reactions. The results have, however, of necessity been averaged over collisions with impact parameters ranging from zero to infinity, thus obscuring the effect of this important variable. Here we employ a method through which impact parameter averaging is suppressed in a surface reaction. We aim a highly collimated reactive 'projectile' molecule along a surface at a stationary adsorbed 'target' molecule, with both the projectile and target being observed by Scanning Tunnelling Microscopy (STM). The projectile was CF2 recoiling from electron-induced bond-breaking in chemisorbed CF3 on Cu(110) at 4.6 K. The collimation of the resulting CF2 'surface-molecular-beam' restricted it to a lateral spread of ±1° as a consequence of its interaction with the Cu rows below. This collimation was modelled by molecular dynamics simulations. In the experiments the recoiling CF2 projectile was aimed, successively, at impact parameters of b = 0 and +3.6 Å at a chemisorbed second CF2, b = +1.8 Å at a chemisorbed I-atom, or b = -4.0, b = -0.4 and b = +3.2 Å at a chemisorbed vinyl radical. The pattern of reactive and non-reactive scattering was determined by STM. These collimated surface-molecular-beams have the potential to aim molecular projectiles with selected impact parameters at the many target species identifiable at a surface by STM.

Approaching the forbidden fruit of reaction dynamics: Aiming reagent at selected impact parameters.

Collision geometry is central to reaction dynamics. An important variable in collision geometry is the miss-distance between molecules, known as the "impact parameter." This is averaged in gas-phase molecular beam studies. By aligning molecules on a surface prior to electron-induced dissociation, we select impact parameters in subsequent inelastic collisions. Surface-collimated "projectile" molecules, difluorocarbene (CF2), were aimed at stationary "target" molecules characterized by scanning tunneling microscopy (STM), with the observed scattering interpreted by computational molecular dynamics. Selection of impact parameters showed that head-on collisions favored bimolecular reaction, whereas glancing collisions led only to momentum transfer. These collimated projectiles could be aimed at the wide variety of adsorbed targets identifiable by STM, with the selected impact parameter assisting in the identification of the collision geometry required for reaction.

The localization-delocalization matrix and the electron-density-weighted connectivity matrix of a finite graphene nanoribbon reconstructed from kernel fragments.

Bader's quantum theory of atoms in molecules (QTAIM) and chemical graph theory, merged in the localization-delocalization matrices (LDMs) and the electron-density-weighted connectivity matrices (EDWCM), are shown to benefit in computational speed from the kernel energy method (KEM). The LDM and EDWCM quantum chemical graph matrices of a 66-atom C46H20 hydrogen-terminated armchair graphene nanoribbon, in 14 (2×7) rings of C2v symmetry, are accurately reconstructed from kernel fragments. (This includes the full sets of electron densities at 84 bond critical points and 19 ring critical points, and the full sets of 66 localization and 4290 delocalization indices (LIs and DIs).) The average absolute deviations between KEM and directly calculated atomic electron populations, obtained from the sum of the LIs and half of the DIs of an atom, are 0.0012 ± 0.0018 e(-) (∼0.02 ± 0.03%) for carbon atoms and 0.0007 ± 0.0003 e(-) (∼0.01 ± 0.01%) for hydrogen atoms. The integration errors in the total electron population (296 electrons) are +0.0003 e(-) for the direct calculation (+0.0001%) and +0.0022 e(-) for KEM (+0.0007%). The accuracy of the KEM matrix elements is, thus, probably of the order of magnitude of the combined precision of the electronic structure calculation and the atomic integrations. KEM appears capable of delivering not only the total energies with chemical accuracy (which is well documented) but also local and nonlocal properties accurately, including the DIs between the fragments (crossing fragmentation lines). Matrices of the intact ribbon, the kernels, the KEM-reconstructed ribbon, and errors are available as Supporting Information .

Primary retention following nuclear recoil in ß-decay: Proposed synthesis of a metastable rare gas oxide ((38)ArO4) from ((38)ClO4(-)) and the evolution of chemical bonding over the nuclear transmutation reaction path.

Argon tetroxide (ArO4) is the last member of the N=50 e(-) isoelectronic and isosteric series of ions: SiO4(4-), PO4(3-), SO4(2-), and ClO4(-). A high level computational study demonstrated that while ArO4 is kinetically stable it has a considerable positive enthalpy of formation (of ~298kcal/mol) (Lindh et al., 1999. J. Phys. Chem. A 103, pp. 8295-8302) confirming earlier predictions by Pyykkö (1990. Phys. Scr. 33, pp. 52-53). ArO4 can be expected to be difficult to synthesize by traditional chemistry due to its metastability and has not yet been synthesized at the time of writing. A computational investigation of the changes in the chemical bonding of chlorate (ClO4(-)) when the central chlorine atom undergoes a nuclear transmutation from the unstable artificial chlorine isotope (38)Cl to the stable rare argon isotope (38)Ar through ß-decay, hence potentially leading to the formation of ArO4, is reported. A mathematical model is presented that allows for the prediction of yields following the recoil of a nucleus upon ejecting a ß-electron. It is demonstrated that below a critical angle between the ejected ß-electron and that of the accompanying antineutrino their respective linear momentums can cancel to such an extent as imparting a recoil to the daughter atom insufficient for breaking the Ar-O bond. As a result, a primary retention yield of ~1% of ArO4 is predicted following the nuclear disintegration. The study is conducted at the quadratic configuration interaction with single and double excitations [QCISD/6-311+G(3df)] level of theory followed by an analysis of the electron density by the quantum theory of atoms in molecules (QTAIM). Crossed potential energy surfaces (PES) were used to construct a PES from the metastable ArO4 ground singlet state to the Ar-O bond dissociation product ArO3+O((3)P) from which the predicted barrier to dissociation is ca. 22kcal/mol and the exothermic reaction energy is ca. 28kcal/mol [(U)MP2/6-311+G(d)].