Kinetic Energy Release Distributions in the Dissociation of Energy-Selected Fluoroethene and 1,1-Difluoroethene Ions

Acetone cation chemistry under varying electron density conditions: from unimolecular decomposition to dissociative recombination processes

The chemistry of ionized acetone:Ar mixtures under varied ionizing electron density conditions has been studied using matrix-isolation techniques. Gaseous acetone diluted in excess argon gas was subjected to electron bombardment with 300 eV electrons at currents between 20 and 200 μA. Linear wire 'pin' and metal 'plate' electron collector geometries were employed, allowing a wide range of electron density conditions to be explored. The products of subsequent reaction processes were matrix isolated and analyzed by Fourier transform infrared absorption spectroscopy. Products included methane, ketene, 1-propen-2-ol (the enol isomer of acetone), CO, HCO, ethane, ethane, acetylene and CCCO. Product absolute and relative yields varied with acetone number density, the choice of anode geometry and the rate of electron bombardment. The overall chemistry observed is rationalized in terms of mechanistic steps involving unimolecular cation decomposition, ion-molecule reactions, radical-radical reactions and dissociative recombination processes.

Electron-bombardment matrix isolation and PEPICO studies as complementary techniques in ion chemistry: an FTIR study of the decomposition of the vinyl fluoride cation

FTIR spectra have been obtained for matrices formed following electron bombardment of gas mixtures containing varying amounts of vinyl fluoride (VF) in Ar (1:400 to 1:25,600; VF/Ar). The major matrix-isolated products are a π-complex of HF/C(2)H(2) , fluoroacetylene (HC≡CF) and two isomers of C(2)H(2)F(•). These products correspond well with the products of photoionization of VF near 15.8 eV. These observations support the dominant mechanism of ionization in the EB-MI environment as charge transfer of the substrate molecule to Ar(•+). Some differences are noted between the observed EB-MI products and the results from PEPICO studies, primarily in that the EB-MI products are observed as neutralized forms. The close correlation in EB-MI and photoionization results allows the EB-MI technique to be utilized as an ion structural analysis tool in complement to PEPICO studies, and allows the use of PEPICO studies to help predict and elucidate high-pressure chemistry mechanisms through EB-MI studies. The differences in the EB-MI results and ions observed using the PEPICO technique are rationalized in terms of the differences in the experimental techniques. Using VF as the test system, reagent partial pressure conditions that best complement PEPICO studies are determined. Although the major results are observed for all VF partial pressures, dilute samples give rise to further ionization of the primary products, and more concentrated samples give rise to radical-radical reaction chemistry. As a result, a nominal range of 1:3200 (VF/Ar) is demonstrated to provide the best correlation with the gas-phase PEPICO measurements.

Ground and Excited State Dissociation Dynamics of Ionized 1,1-Difluoroethene

The kinetic energy release distributions (KERDs) for the fluorine atom loss from the 1,1-difluoroethene cation have been recorded with two spectrometers in two different energy ranges. A first experiment uses dissociative photoionization with the He(I) and Ne(I) resonance lines, providing the ions with a broad internal energy range, up to 7 eV above the dissociation threshold. The second experiment samples the metastable range, and the average ion internal energy is limited to about 0.2 eV above the threshold. In both energy domains, KERDs are found to be bimodal. Each component has been analyzed by the maximum entropy method. The narrow, low kinetic energy components display for both experiments the characteristics of a statistical, simple bond cleavage reaction: constraint equal to the square root of the fragment kinetic energy and ergodicity index higher than 90%. Furthermore, this component is satisfactorily accounted for in the metastable time scale by the orbiting transition state theory. Potential energy surfaces corresponding to the five lowest electronic states of the dissociating 1,1-C2H2F2+ ion have been investigated by ab initio calculations at various levels. The equilibrium geometry of these states, their dissociation energies, and their vibrational wavenumbers have been calculated, and a few conical intersections between these surfaces have been identified. It comes out that the ionic ground state X2B1 is adiabatically correlated with the lowest dissociation asymptote. Its potential energy curve increases in a monotonic way along the reaction coordinate, giving rise to the narrow KERD component. Two states embedded in the third photoelectron band (B2A1 at 15.95 eV and C2B2 at 16.17 eV) also correlate with the lowest asymptote at 14.24 eV. We suggest that their repulsive behavior along the reaction coordinate be responsible for the KERD high kinetic energy contribution.

A Multireference Configuration Interaction Investigation of the Excited-State Energy Surfaces of Fluoroethylene (C2H3F)

Multireference configuration interaction with singles and doubles (MR-CISD) calculations has been performed for the optimization of conical intersections and stationary points on the fluoroethylene excited-state energy surfaces. For the planar ground state geometry, the vertical spectrum including 3s and 3p Rydberg states was calculated. From this geometry, a rigid torsion around the CC bond strongly reduces the energy gap between S0 and S1 states. Furthermore, a search for the minimum of the crossing seam shows that there exists a conical intersection close to the twisted structure and two additional ones for cis and trans pyramidalized structures. These three intersections are connected by the same seam. We have shown that the Hula-Twist process is an alternative way to the direct CC twisting in order to reach this part of the seam. Other conical intersections were also located in the CH3CF and CH2FCH, H-migration, and C(3v) structures. The photodynamics of the system is discussed based on topological features of these intersections.

Frequency selective heteronuclear dipolar recoupling in rotating solids: accurate (13)C-(15)N distance measurements in uniformly (13)C,(15)N-labeled peptides

We describe a magic-angle spinning NMR experiment for selective (13)C-(15)N distance measurements in uniformly (13)C,(15)N-labeled solids, where multiple (13)C-(15)N and (13)C-(13)C interactions complicate the accurate measurement of structurally interesting, weak (13)C-(15)N dipolar couplings. The new experiment, termed FSR (frequency selective REDOR), combines the REDOR pulse sequence with a frequency selective spin-echo to recouple a single (13)C-(15)N dipolar interaction in a multiple spin system. Concurrently the remaining (13)C-(15)N dipolar couplings and all (13)C-(13)C scalar couplings to the selected (13)C are suppressed. The (13)C-(15)N coupling of interest is extracted by a least-squares fit of the experimentally observed modulation of the (13)C spin-echo intensity to the analytical expression describing the dipolar dephasing in an isolated heteronuclear spin pair under conventional REDOR. The experiment is demonstrated in three uniformly (13)C,(15)N-labeled model systems: asparagine, N-acetyl-L-Val-L-Leu and N-formyl-L-Met-L-Leu-L-Phe; in N-formyl-[U-(13)C,(15)N]L-Met-L-Leu-L-Phe we have determined a total of 16 internuclear distances in the 2.5-6 A range.