Electron attachment to POCl3. III. Measurement and kinetic modeling of branching fractions

Electron attachment to POCl(3) was studied in the bath gas He over the pressure range 0.4-3.1 Torr and the temperature range 300-1210 K. Branching fractions of POCl(3)(-), POCl(2)(-), Cl(-), and Cl(2)(-) were measured. The results are analyzed by kinetic modeling, using electron attachment theory for the characterization of the nonthermal energy distribution of the excited POCl(3)(-?) anions formed and chemical activation-type unimolecular rate theory for the subsequent competition between collisional stabilization of POCl(3)(-?) and its dissociation to various dissociation products. Primary and secondary dissociations and∕or thermal dissociations of the anions are identified. The measured branching fractions are found to be consistent with the modeling results based on molecular parameters obtained from quantum-chemical calculations.     

Electron attachment to PSCl3

Electron attachment to PSCl3 was studied in 133-Pa pressure of helium gas at temperatures from 298-550 K. Measurements of rate constants and branching fractions were made in a flowing-afterglow Langmuir-probe (FALP) apparatus. These experiments yielded an electron attachment rate constant of 5.1 x 10(-8) cm3 s(-1) that was found not to change significantly in the 298-550 K temperature range. This rate constant represents an attachment efficiency of about 14%. Attachment in 133 Pa of He gas yielded only the dissociative ion products PSCl2- and Cl-. The FALP data suggest that there is an activation energy of about 17 meV for production of PSCl2-. Attachment to PSCl3 was also studied at high pressure (9-93 kPa) of N2 in an ion mobility mass spectrometer, at 298 K. In contrast to the low-pressure data, the parent anion product channel (PSCl3-) was observed (along with the dissociative channels), and increased in importance with N2 pressure. Gaussian-3 (G3) calculations were carried out for PSCl3 and PSCl2 neutrals and anions to aid in interpretation of the experimental results. The calculations indicate that the electron affinity EA(PSCl2) is slightly smaller than EA(Cl), which may account for the observed branching fractions for PSCl2- and Cl- in the low-pressure experiments. A natural population analysis was performed to obtain the charges associated with each atom in the molecules in order to estimate how the attached electron is distributed. Comparison is made between the present study of electron attachment to PSCl3 and our earlier work on attachment to POCl3, and G3 calculations are reported here for neutral and anionic POCl2 and POCl3. In contrast to PSCl2, the calculations imply that EA(POCl2) is slightly greater than EA(Cl). For both PSCl3 and POCl3, the calculations show that the dissociative electron attachment process is close to thermoneutral.     

Observation of dihalide elimination upon electron attachment to oxalyl chloride and oxalyl bromide, 300-550 K

Rate coefficients have been measured for electron attachment to oxalyl chloride [ClC(O)C(O)Cl] and oxalyl bromide [BrC(O)C(O)Br] in He gas at 133 Pa pressure over the temperature range of 300-550 K. With oxalyl chloride, the major ion product of attachment is Cl2(-) at all temperatures (66% at 300 K); its importance increases slightly as temperature increases. Two other product ions formed are Cl- (18% at 300 K) and the phosgene anion CCl2O- (16% at 300 K) and appear to arise from a common mechanism. With oxalyl bromide, the Br2(-) channel represents almost half of the ion product of attachment, independent of temperature. Br- accounts for the remainder. For oxalyl chloride, the attachment rate coefficient is small [(1.8 +/- 0.5) x 10(-8) cm3 s(-1) at 300 K], and increases with temperature. The attachment rate coefficient for oxalyl bromide [(1.3 +/- 0.4) x 10(-7) cm3 s(-1) at 300 K] is nearly collisional and increases only slightly with temperature. Stable parent anions C2Cl2O2(-) and C2Br2O2(-) and adduct anions Cl- (C2Cl2O2) and Br- (C2Br3O2) were observed but are not primary attachment products. G2 and G3 theories were applied to determine geometries of products and energetics of the electron attachment and ion-molecule reactions studied. Electron attachment to both oxalyl halide molecules leads to a shorter C-C bond and longer C-Cl bond in the anions formed. Trans and gauche conformers of the neutral and anionic oxalyl halide species have similar energies and are more stable than the cis conformer, which lies 100-200 meV higher in energy. For C2Cl2O2, C2Cl2O2(-), and C2Br2O2(-), the trans conformer is the most stable conformation. The calculations are ambiguous as to the oxalyl bromide geometry (trans or gauche), the result depending on the theoretical method and basis set. The cis conformers for C2Cl2O2 and C2Br2O2 are transition states. In contrast, the cis conformers of the anionic oxalyl halide molecules are stable, lying 131 meV above trans-C2Cl2O2(-) and 179 meV above trans-C2Br2O2(-). Chien et al. [J. Phys. Chem. A 103, 7918 (1999)] and Kim et al. [J. Chem. Phys. 122, 234313 (2005)] found that the potential energy surface for rotation about the C-C bond in C2Cl2O2 is "extremely flat." Our computational data indicate that the analogous torsional surfaces for C2Br2O2, C2Cl2O2(-), and C2Br2O2(-) are similarly flat. The electron affinity of oxalyl chloride, oxalyl bromide, and phosgene were calculated to be 1.91 eV (G3), and 2.00 eV (G2), and 1.17 eV (G3), respectively.     

An electrochemical study of PCl3 and POCl3 in the room temperature ionic liquid [C4mpyrr][N(Tf)2

Voltammetric studies of PCl3 and POCl3 have not been reported in the literature to date, probably due to the instability of these molecules in conventional aprotic solvents giving unstable and irreproducible results. From a previous study [Amigues et al. Chem. Commun. 2005, 1-4], it was found that ionic liquids have the ability to offer a uniquely stable solution phase environment for the study of these phosphorus compounds. Consequently, the electrochemistry of PCl3 and POCl3 has been studied by cyclic voltammetry on a gold microelectrode in the ionic liquid [C4mpyrr][N(Tf)2] (1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide). For both compounds, reduction and oxidation waves were observed and a tentative assignment of the waves is given. For PCl3, the reduction was thought to proceed via the following mechanism: PCl3 + e- <=> PCl3-, PCl3- <=> Cl- + P*Cl2, and Cl- + PCl3 <=> PCl4-. For POCl3, the suggested reduction mechanism was analogous to that of PCl3: POCl3 + e- <=> POCl3-, POCl3- <=> Cl- + P*OCl2, and Cl- + POCl3 <=> POCl4-. In both cases P*Cl2 and P*OCl2 are likely to engage in further reactions. Potential step microdisk chronoamperometry was carried out on the reductive waves of PCl3 and POCl3 to measure diffusion coefficients and number of electrons transferred. It was found that the diffusion of PCl3 was unusually slow (3.1 x 10(-12) m2 s(-1)): approximately 1 order of magnitude less than that for POCl3 (2.2 x 10(-11) m2 s(-1)). For both PCl3 and POCl3, a "split wave" was observed, with an overall electron count of 1. This observation is shown to be consistent with and to "fingerprint" the mechanisms proposed above.     

Reactions of POxCly- ions with H and H2 from 298 to 500 K

Rate constants and product branching ratios for POxCly- ions reacting with H and H2 were measured in a selected ion flow tube (SIFT) from 298 to 500 K. PO2Cl-, PO2Cl2-, POCl2-, and POCl3- were all unreactive with H2, having a rate constant with an upper limit of <5 x 10(-12) cm3 s(-1). PO2Cl2- did not react with H atoms either, having a similar rate constant limit of <5 x 10(-12) cm3 s(-1). The rate constants for PO2Cl-, POCl2-, and POCl3- reacting with H showed no temperature dependence over the limited range of 298-500 K and were approximately 10-20% of the collision rate constant. Cl abstraction by H to form HCl was the predominant product channel for PO2Cl-, POCl2-, and POCl3-, with a small amount of Cl- observed from POCl2- + H. Reactions of O2 and O3 with the POCl- products ions from the reaction of POCl2- + H were observed to yield predominantly PO3- and PO2-, respectively. POCl- reacted with O2 and O3 with rate constants of 8.9 +/- 1.1 x 10(-11) and 5.2 +/- 3.3 x 10(-10) cm3 s(-1), respectively. No associative electron detachment in the reactions with H atoms was observed with any of the reactant ions; however, detachment was observed with a PO- secondary product ion at high H atom concentrations. Results of new G3 theoretical calculations of optimized geometries and energies for the products observed are discussed.     

Dipole and Coulomb forces in electron capture dissociation and electron transfer dissociation mass spectroscopy

Ab initio electronic structure calculations were performed on a doubly charged polypeptide model H(+)-Lys(Ala)(19)-CO-CH(NH(2))-CH(2)-SS-CH(2)-(NH(2))CH-CO-(Ala)(19)-Lys-H(+) consisting of a C-terminal protonated Lys followed by a 19-Ala α-helix with a 20th Ala-like unit whose side chain is linked by a disulfide bond to a corresponding Ala-like unit connected to a second 19-Ala α-helix terminated by a second C-terminal-protonated Lys. The Coulomb potentials arising from the two charged Lys residues and dipole potentials arising from the two oppositely directed 72 D dipoles of the α-helices act to stabilize the SS bond's σ* orbital. The Coulomb potentials provide stabilization of 1 eV, while the two large dipoles generate an additional 4 eV. Such stabilization allows the SS σ* orbital to attach an electron and thereby generate disulfide bond cleavage products. Although calculations are performed only on SS bond cleavage, discussion of N-C(α) bond cleavage caused by electron attachment to amide π* orbitals is also presented. The magnitudes of the stabilization energies as well as the fact that they arise from Coulomb and dipole potentials are supported by results on a small model system consisting of a H(3)C-SS-CH(3) molecule with positive and negative fractional point charges to its left and right designed to represent (i) two positive charges ca. 32 ? distant (i.e., the two charged Lys sites of the peptide model) and (ii) two 72 D dipoles (i.e., the two α-helices). Earlier workers suggested that internal dipole forces in polypeptides could act to guide incoming free electrons (i.e., in electron capture dissociation (ECD)) toward the positive end of the dipole and thus affect the branching ratios for cleaving various bonds. Those workers argued that, because of the huge mass difference between an anion donor and a free electron, internal dipole forces would have a far smaller influence over the trajectory of a donor (i.e., in electron transfer dissociation (ETD)). The present findings suggest that, in addition to their effects on guiding electron or donor trajectories, dipole potentials (in combination with Coulomb potentials) also alter the energies of SS σ* and amide π* orbitals, which then affects the ability of these orbitals to bind an electron. Thus, both by trajectory-guiding and by orbital energy stabilization, Coulomb and dipole potentials can have significant influences on the branching ratios of ECD and ETC in which disulfide or N-C(α) bonds are cleaved.     

Phenol, chlorobenzene and chlorophenol isomers: resonant states and dissociative electron attachment

This paper reports a study of resonant dissociative electron attachment (DEA) to the phenol, chlorobenzene, p-, m-, and o-chlorophenol molecules. On the basis of spectroscopic and thermochemical approaches the resonant states of the molecular negative ions (NIs) and the structures of some dissociative decay products are assigned. In the electron energy range up to 3 eV, DEA processes are determined by the two 2[pi*]-shape resonances resulting mainly in formation of [M-H]- and/or Cl- ions. At higher electron energies the energy correlation between peaks in the negative ion effective yield curves and bands of UV spectra allowed identification of the core-excited resonances. The peculiarities of Cl- ion formation and the vibrational fine structure on the effective yield curves of the [M-H]- ions are discussed. The mass spectrometric procedures for measurement of relative cross sections for NI formation are described.     

Steric effects in electron transfer from potassium to pi-bonded oriented molecules CH3CN, CH3NC, and CCl3CN

Electron transfer from K atoms to oriented CH3CN, CH3NC, and CCl3CN is studied in crossed beams at energies near the threshold for forming an ion pair. For the methyl compounds, the dominant ions are K+ and CN-; the steric asymmetry is very small and energy-independent, characteristic of sideways attack with the electron apparently entering the pi*CN antibonding orbital. Migration of the electron to the sigma*CC orbital to break the C-C bond is greatly facilitated by interaction with the atomic donor. CH2CN- is formed in collisions preferring CH3-end attack, and the steric asymmetry becomes very large near threshold. CCl3CN mostly forms Cl- in collisions slightly favoring the CCl3 end with a small energy dependence with the electron apparently entering the sigma* LUMO. CN- is formed in much smaller yield with a slight preference for the CN end. The parent negative ion CCl3CN- is observed, and a lower limit for its electron affinity is estimated to be 0.3 eV. Fragment ions CCl2CN- and CClCN- are also observed with upper limits for the quantity bond dissociation energy - electron affinity (BDE - EA) estimated to be 0.6 and 1.0 eV, respectively.     

Empty level structure and dissociative electron attachment cross section in (bromoalkyl)benzenes

The gas-phase electron transmission (ET) and dissociative electron attachment (DEA) spectra are reported for the series of (bromoalkyl)benzenes C6H5(CH2)nBr (n = 0-3), where the bromine atom is directly bonded to a benzene ring or separated from it by 1-3 CH2 groups, and the dihalo derivative 1-Br-4-Cl-benzene. The relative DEA cross sections (essentially due to the Br- fragment) are reported, and the absolute cross sections are also evaluated. HF/6-31G and B3LYP/6-31G* calculations are employed to evaluate the virtual orbital energies (VOEs) for the optimized geometries of the neutral state molecules. The pi* VOEs, scaled with empirical equations, satisfactorily reproduce the corresponding experimental vertical electron attachment energies (VAEs). According to the calculated localization properties, the LUMO (as well as the singly occupied MO of the lowest lying anion state) of C6H5(CH2)3Br is largely localized on both the benzene ring and the C-Br bond, despite only a small pi*/sigma*C-Br interaction and in contrast to the chlorine analogue where the LUMO is predicted to possess essentially ring pi character. This would imply a less important role of intramolecular electron transfer in the bromo derivative for production of the halogen negative fragment through dissociation of the first resonant state. The VAEs calculated as the anion/neutral energy difference with the 6-31+G* basis set which includes diffuse functions are relatively close to the experimental values but do not parallel their sequence. In addition the SOMO of some compounds is not described as a valence MO with large pi* character but as a diffuse sigma* MO.     

Formation of SF5 in electron attachment to SF6; swarm and beam results reconciled

Based on the results of recent swarm experiments, it has been proposed that the increase in the cross section for SF₅⁻ formation observed at an electron energy, E_e of about 0.3 eV in electron beam studies of electron attachment to SF₆ is due to the combined (opposing) effects of the vibrational heating of the molecule by the attached electron, which enhances the dissociation of the nascent (SF₆⁻)^* ion, and the reduction of the cross section for capture (s-wave) of the electron by SF₆ with increasing E_e. Further, it has been shown that the dissociation reaction is endothermic by 0.12 eV, and that, contrary to previous suggestions, there is no potential barrier to this dissociation reaction. Now we have carried out electron beam studies of the SF₆ attachment reaction in Berlin at gas temperatures, T_g, over the range 300 to 920 K and in Innsbruck at T_g below 300 K. These studies have provided support for the above proposals concerning the appearance of the SF₅⁻ peak and for a reaction endothermicity of 0.12 eV. Thus these studies have clarified the doubts about the products of the SF₆ attachment reaction at low electron energy.     

Temporary anion states and dissociative electron attachment to isothiocyanates

The temporary anion states of isothiocyanates CH3CH2=C=S (and CH3CH2N=C=O for comparison), C6H5CH2N=C=S, and C6H5N=C=S are characterized experimentally in the gas phase for the first time by means of electron transmission spectroscopy (ETS). The measured vertical electron attachment energies (VAEs) are compared with the virtual orbital energies of the neutral-state molecules supplied by MP2 and B3LYP calculations with the 6-31G* basis set. The calculated energies, scaled with empirical equations, reproduce satisfactorily the experimental VAEs. The first VAE is also closely reproduced as the total energy difference between the anion and neutral states calculated at the B3LYP/6-31+G* level. Due to mixing between the ring and N=C=S pi-systems, C6H5N=C=S possesses the best electron-acceptor properties, and its lowest-lying anion state is largely localized at the benzene ring. The anion states with mainly pi*C=S and pi*N=C character lie at higher energy than the corresponding anion states of noncumulated pi-systems. However, the electron-acceptor properties of isothiocyanates are found to be notably larger than those of the corresponding oxygen analogues (isocyanates). The dissociative electron attachment (DEA) spectra show peaks close to zero energy and at 0.6 eV, essentially due to NCS- negative fragments. In spite of the energy proximity of the first anion state in phenyl isothiocyanate to the DEA peak, the zero-energy anion current in the benzyl derivative is about 1 order of magnitude larger.     

Temporary anion states and dissociative electron attachment in diphenyl disulfide

The temporary anion states of gas-phase diphenyl disulfide are characterized by means of electron transmission (ET) and dissociative electron attachment (DEA) spectroscopies. The measured energies of vertical electron attachment are compared to the virtual orbital energies of the neutral state molecule supplied by MP2 and B3LYP calculations with the 6-31G basis set. The calculated energies, scaled with empirical equations, reproduce satisfactorily the attachment energies measured in the ET spectrum. The first anion state of diphenyl disulfide is stable, thus escaping detection in ETS. The vertical and adiabatic electron affinities, evaluated with B3LYP/6-31+G calculations as the energy difference between the neutral and anion states, are predicted to be 0.37 and 1.38 eV, respectively. The anion current displayed in the DEA spectrum has a sharp and intense peak at zero energy, essentially due to the C6H5S- negative fragment. In agreement, according to the calculations, the localization properties of the first anion state are strongly S-S antibonding, and the energetic requirement for its dissociation along the S-S bond is fulfilled even at zero energy.     

Absolute rate coefficients and branching percentages for the reactions of POxCly- + N (4S(3/2)) and POxCly- + O (3P) at 298 K in a selected-ion flow tube instrument

The absolute rate coefficients and product ion branching percentages at 298 K for the reactions of several POxCly- species with atomic nitrogen (N (4S(3/2))) and atomic oxygen (O (3P)) have been determined in a selected-ion flow tube (SIFT) instrument. POxCly- ions are generated by electron impact on POCl3 in a high-pressure source. O atoms are generated by quantitative titration of N atoms with NO, where N atoms are produced by microwave discharge on N2. The experimental procedure allows for the determination of rate coefficients for the reaction of the reactant ion with N (4S(3/2)) and O (3P) as well as with N2 and NO. None of the ions react with N2 or NO, giving an upper limit to the rate coefficient of <5 x 10(-12) cm3 molecules(-1) s(-1). POCl3- and POCl2- do not react with N atoms, giving an upper limit to the rate coefficient of <1 x 10(-11) cm3 molecules(-1) s(-1). The major product ion for POCl3- and POCl2- reacting with O involves loss of Cl from the reactant ion, accounting for >85% of the products. PO2- is a minor product (相似文献   

Dissecting the proline effect: dissociations of proline radicals formed by electron transfer to protonated Pro-Gly and Gly-Pro dipeptides in the gas phase

We report a combined experimental and computational study of the proline effect in model dipeptides Pro-Gly and Gly-Pro. Gas-phase protonated peptide ions were discharged by glancing collisions with potassium or cesium atoms at 3 keV collision energies, and the peptide radical intermediates and their dissociation products were analyzed following collisional ionization to anions. The charge reversal (+CR-) mass spectra of (Pro-Gly + H)+(1a+) and (Gly-Pro + H)+ (2a+) showed dramatic differences and thus provided a sensitive probe of ion structure. Whereas 1a+ completely dissociated upon charge inversion, 2a+ gave a nondissociated anion as the most abundant product. Ab initio and density functional theory calculations provided structures and vertical recombination energies (REvert) for 1a+ and 2a+. The recombination energies, REvert = 3.07 and 3.36 eV for 1a+ and 2a+, respectively, were lower than the alkali metal ionization energies and indicated that the collisional electron transfer to the peptide ions was endoergic. Radical 1a* was found to exist in a very shallow local energy minimum, with transition state energies for loss and migration of H indicating very facile dissociation. In contrast, radical 2a* was calculated to spontaneously isomerize upon electron capture to a stable dihydroxycarbinyl isomer (2e*) that can undergo consecutive and competitive isomerizations by proline ring opening and intramolecular hydrogen atom transfers to yield stable radical isomers. Radical 2e* and its stable isomers were calculated to have substantial electron affinities and thus can form the stable anions that were observed in the +CR- mass spectra. The calculated TS energies and RRKM kinetic analysis indicated that peptide N-C alpha bond dissociations compete with pyrrolidine ring openings triggered by radical sites at both the N-terminal and C-terminal sides of the proline residue. Open-ring intermediates were found in which loss of an H atom was energetically preferred over backbone dissociations. This provided an explanation for the proline effect causing low incidence of electron capture dissociations of N-C alpha bonds adjacent to proline residues in tryptic peptides and also for some peculiar behavior of proline-containing protein cation-radicals.     

Bond dissociation energies and equilibrium structures of Cu+(MeOH)x, x = 1-6, in the gas phase: competition between solvation of the metal ion and hydrogen-bonding interactions

The solvation of Cu+ by methanol (MeOH) was studied via examination of the kinetic energy dependence of the collision-induced dissociation of Cu+(MeOH)x complexes, where x = 1-6, with Xe in a guided ion beam tandem mass spectrometer. In all cases, the primary and lowest-energy dissociation channel observed is the endothermic loss of a single MeOH molecule. The primary cross section thresholds are interpreted to yield 0 and 298 K bond dissociation energies (BDEs) after accounting for the effects of multiple ion-neutral collisions, kinetic and internal energy distributions of the reactants, and lifetimes for dissociation. Density functional theory calculations at the B3LYP/6-31G* level are performed to obtain model structures, vibrational frequencies, and rotational constants for the Cu+(MeOH)x complexes and their dissociation products. The relative stabilities of various conformations and theoretical BDEs are determined from single-point energy calculations at the B3LYP/6-311+G(2d,2p) level of theory using B3LYP/6-31G*-optimized geometries. The relative stabilities of the various conformations of the Cu+(MeOH)x complexes and the trends in the sequential BDEs are explained in terms of stabilization gained from sd hybridization, hydrogen-bonding interactions, electron donor-acceptor natural bond orbital stabilizing interactions, and destabilization arising from ligand-ligand repulsion.     

Intramolecular electron transfer in heterosubstituted benzene derivatives as probed by dissociative electron attachment

The electron transmission and dissociative electron attachment spectra of the 1-chloroalkyl benzene derivatives, C(6)H(5)(CH(2))(3)Cl and C(6)H(5)(CH(2))(4)Cl, and of the sulfur and silicon derivatives, C(6)H(5)SCH(2)Cl, C(6)H(5)Si(CH(3))(2)CH(2)Cl and C(6)H(5)CH(2)Si(CH(3))(2)CH(2)Cl, are presented for the first time. The relative Cl(-) fragment anion currents generated by electron attachment to the benzene pi* LUMO are measured in the series C(6)H(5)(CH(2))(n)Cl, with n = 1-4, and in the heteroatomic compounds. The Cl(-) yield reflects the rate of intramolecular electron transfer between the pi-system and the remote chlorine atom, which in turn depends on the extent of through-bond coupling between the localized pi* and sigma*(Cl-C) orbitals. In compounds C(6)H(5)(CH(2))(n)Cl the Cl(-) current rapidly decreases with increasing length of the saturated chain. This decrease is significantly attenuated when a carbon atom of the alkyl skeleton is replaced with a third-row heteroatom. This greater ability to promote through-bond coupling between the pi* and sigma*(Cl-C) orbitals is attributed to the sizably lower energy of the empty sigma*(S-C) and sigma*(Si-C) orbitals with respect to the sigma*(C-C) orbitals. In the sulfur derivative the increase of the Cl(-) current is larger than in the silicon analogue. In this case, however, other negative fragments are observed, due to dissociation of the S-C bonds.     

Low-energy electron attachment to SF6. II. Temperature and pressure dependences of dissociative attachment

Low-energy electron-molecule collisions, leading to dissociative attachment through metastable anionic states, are kinetically modeled within the framework of statistical unimolecular rate theory. The reaction e(-)+SF(6)-->SF(5)(-)+F is used as an illustrative example. The modeling is applied to new measurements of branching fractions for SF(5)(-) formation in the bath gas He between 360 and 670 K at 1 and 2 Torr, and between 490 and 620 K over the range of 0.3-9 Torr. The analysis of the data follows the previous kinetic modeling of the nondissociative electron attachment, e(-)+SF(6)-->SF(6)(-), from Part I of this series. Experimental results from the present work and the literature on branching fractions and total cross sections for anion formation as functions of electron energies, bath gas temperatures and pressures, as well as observation times are analyzed. The assumption of a participation of the electronic ground state of SF(6)(-) alone suffices to model the available experimental data. A value of the dissociation energy of SF(6)(-) into SF(5)(-)+F of E(0,dis)=1.61(+/-0.05) eV is determined, which may be compared to the electron affinity of SF(6), EA=1.20(+/-0.05) eV, such as derived in Part III of this series.     

Photochemistry of vinyl chloride physisorbed on Ag(111) through molecular anion formation induced by substrate electron attachment

Pulsed 266 and 355 nm ultraviolet laser irradiation of monolayer vinyl chloride physisorbed on Ag(111) results in molecular dissociation leading to C2H3 and Cl, much of which is adsorbed to the surface. On the basis of observations made on dissociation dependences on chlorine isotope and photon energy, it is deduced that upon excitation vinyl chloride forms a transient negative ion through a substrate mediated, vertical electron attachment mechanism. The anion either dissociates or relaxes through energy transfer to the neutral state causing the neutral molecule to desorb. The threshold for vertical attachment of substrate electron is estimated to be 0.8 eV below the vacuum level, in agreement with the experimentally observed wavelength dependence in photoinduced dissociation. Chemisorbed Cl on the Ag(111) surface inhibits the photodissociation process by increasing the substrate work function and consequently the energy threshold for electron vertical attachment. Upon heating the Ag(111) surface, adsorbed vinyl combines to produce 1,3-butadiene in a first order, diffusion limited, process with an activation energy of 10.4 kcal/mol.     

Violation of frozen shell approximation in dissociative electron capture by halogenated anthraquinones

A series of halogenated anthraquinone (AQ) derivatives has been studied by means of electron capture negative ion (NI) mass spectrometry (ECNI-MS). 1Cl-AQ and 2Br-AQ display dramatically steep positive temperature dependencies of Hal(-) ion abundance in the low electron energy region. Molecular NI intensity decreases rapidly with increasing temperature in the case of 1I-AQ. In the case of 2Br-AQ, a metastable NI peak (m/z 22.9) corresponding to the process BrAQ(-) --> Br(-) + AQ(0) was recorded. This means that the characteristic dissociation lifetime of the molecular NI Br-AQ(-) is at least approximately 25 micros at the energy approximately 0.67 eV in the low-temperature spectrum (T approximately 80 degrees C), and at the energy approximately 0.13 eV in the hot spectrum (T approximately 290 degrees C). Together with the observed temperature dependence of the 2Br-AQ curves of effective yield (CEY), this proves that this anion dissociates according to Coulson's model. The same halogen anion behavior is observed in the case of 1Cl-AQ. There are three consecutive stages in the process of molecular NI dissociation of Cl- and Br-substituted AQ, namely, electron capture into the empty pi-orbital by means of the shape resonance mechanism, followed by a radiationless transition into the ground electronic pi-state of the anion, as predicted by Compton in the case of the parabenzoquinone molecule, and, finally, a fluctuative dissociation of the molecular NI accompanied by the transition from the pi-term into the sigma-term, so-called predissociation. Calculations show reasonable agreement with the experimental data. In the case of 1I-AQ, an effect of inversion of empty levels in the process of electron capture by the molecule takes place, a violation of the so-called frozen shell approximation. The phenomenon found may be of significance not only in the case of ECNI-MS, but also in other experimental investigations using low-energy electron-molecule and ion-molecule collisions.     

Structures and energetics of the deprotonated adenine-uracil base pair, including proton-transferred systems

The B3LYP/DZP++ level of theory has been employed to investigate the structures and energetics of the deprotonated adenine-uracil base pairs, (AU-H)-. Formation of the lowest-energy structure, [A(N9)-U]- (which corresponds to deprotonation at the N9 atom of adenine), through electron attachment to the corresponding neutral is accompanied by proton transfer from the uracil N3 atom to the adenine N1 atom. The driving force for this proton transfer is a significant stabilization from the base pairing in the proton transferred form. Such proton transfer upon electron attachment is also observed for the [A(N6b)-U]- and [A(C2)-U]- anions. Electron attachment to the A-U(N3) radical causes strong lone pair repulsion between the adenine N1 and the uracil N3 atoms, driving the two bases apart. Similarly, lone pair repulsion in the anion A(N6a)-U causes the loss of coplanarity of the two base units. The computed adiabatic electron attachment energies for nine AU-H radicals range from 1.86 to 3.75 eV, implying that the corresponding (AU-H)- anions are strongly bound. Because of the large AEAs of the (AU-H) radicals, the C-H and N-H bond dissociation in the AU- base pair anions requires less energy than the neutral AU base pair. The computed C-H and N-H bond dissociation energies for the AU- anion (i.e., the AU base pair plus one electron) are in the range 1.0-3.2 eV, while those for neutral AU are 4.08 eV or higher.