Average Dipole Orientation Research Articles

Threshold photoelectron photoion coincidence spectroscopy and selected ion flow tube cation-molecule reaction studies of cyclic-C4F8

Using tunable vacuum-UV radiation from a synchrotron, the threshold photoelectron and threshold photoelectron photoion coincidence (TPEPICO) spectra of cyclic-C4F8 in the range 11-25 eV have been recorded. The parent ion is observed very weakly at threshold, 11.60 eV, and is most likely to have cyclic geometry. Ion yield curves and branching ratios have been determined for five fragments. Above threshold, the first ion observed is C3F5+, at slightly higher energy C2F4+, then successively CF+, CF2+ and CF3+ are formed. The dominant ions are C3F5+ and C2F4+, with the data suggesting the presence of a barrier in the exit channel to production of C3F5+ whilst no barrier to production of C2F4+. In complementary experiments, the product branching ratios and rate coefficients have been measured in a selected ion flow tube (SIFT) at 298 K for the bimolecular reactions of cyclic-C4F8 with a large number of atomic and small molecular cations. Below the energy where charge transfer becomes energetically allowed, only one of the ions, CF2+, reacts. Above this energy, all but one of the remaining ions react. Experimental rate coefficients are consistently greater than the collisional values calculated from modified average dipole orientation theory. The inclusion of an additional ion-quadrupole interaction has allowed better agreement to be achieved. With the exception of N+, a comparison of the fragment ion branching ratios from the TPEPICO and SIFT data suggest that long-range charge transfer is the dominate mechanism for reactions of ions with recombination energy between 12.9 and 15.8 eV. For all other ions, either short-range charge transfer or a chemical reaction, involving cleavage and making of new bond(s), is the dominant mechanism.

Rate estimates for collisions of ionic clusters with neutral reactant molecules

Measured reaction rate constants of ionic clusters often exceed the collision rate calculated with the widely used Average Dipole Orientation (ADO) theory. Accurate trajectory calculations, however, are in most cases not possible since detailed information on the cluster geometry and the interaction potential with the neutral is not readily available. Two simple and universally applicable models to estimate these collision rates are presented here. In both models, the cluster and the neutral reaction partner are treated as hard spheres, and the charge is treated as a point charge. The attraction between the point charge and the neutral molecule is described with the interaction potential from ADO theory. For the Hard Sphere Average dipole orientation (HSA) collision rate, the charge is located in the center of the cluster, while in the Surface Charge Capture (SCC) model, the charge is drawn to the cluster surface by the attractive interaction with the neutral collision partner. Both models lead to significantly increased collision rate constants, in comparison with Langevin or ADO capture rates and purely geometric hard-sphere collisions. For weakly polarizable clusters like (H 2O) n −, the newly derived HSA rate constants lie close to experimentally measured rate constants. For the reactions of metal clusters with CO, the SCC model is in surprisingly good agreement with experiment. Although both models are far from being exact theories, they seem valuable for the interpretation of gas phase reactivity experiments of ionic clusters.

Negative Ion Chemistry of Ozone in the Gas Phase

A novel ozone source was developed to study the negative ion chemistry of ozone in the gas phase. Rate constants and product ion branching fractions are reported for 17 negative ion−molecule reactions involving ozone (O3). This is the most comprehensive set of O3 reactions with negative ions to date. The reactions proceed primarily through charge transfer and O atom transfer. The reaction rate constants for O-, O2-, and OH- are large and approximately equal to the thermal energy capture rate constant given by the Su-Chesnavich equation based on average dipole orientation theory. The negative ions NO2-, CO4-, SF6-, and PO2- are somewhat less reactive, reacting at approximately 20−50% of the thermal capture rate. The hydrofluorocarbon ions CF3- and C2F5- react at 80% of the thermal capture rate, and F- is the major product ion formed. NO3-, CO3-, PO3-, CF3O-, F-, Cl-, and Br- are found to be unreactive with rate constants < 5 × 10-12 cm3 s-1, which is the present detection limit of our apparatus using this ozone source. The I- ion was observed to cluster with O3 to form IO3- with a rate constant of approximately 1 × 10-11 cm3 s-1, which is a factor of 2 above our detection limit, and no other product channels were observed. All of the anions listed above showed no reactivity, k < 5 × 10-13 cm3 s-1, with O2.

Gas-Phase Condensation Reactions of SixOyHz- Oxyanions with H2O

Water was reacted with gas-phase oxyanions having the general composition SixOyHz- that were formed and isolated in an ion trap-secondary ion mass spectrometer (IT-SIMS). The radical SiO2•- reacted slowly with H2O to abstract HO•, forming SiO3H-, at a rate of 8 × 10-13 cm3 molecule-1 s-1, corresponding to an efficiency of about 0.03% compared with the theoretical collision rate constant (average dipole orientation). The product ion SiO3H- underwent a consecutive condensation reaction with H2O to form SiO4H3- at a rate that was approximately 0.4−0.7% efficient. SiO4H3- did not undergo further reaction with water. The multiple reaction pathways by which radical SiO3•- reacted with H2O were kinetically modeled using a stochastic approach. SiO3•- reacted with water by three parallel reaction pathways: (1) abstraction of a radical H• to form SiO3H-, which then reacted with a second H2O to form SiO4H3-; (2) abstraction of a radical OH• to form SiO4H-, which further reacted by consecutive H• abstractions to for...

Kinetics of solvent addition on electrosprayed ions in an electrospray source and in a quadrupole ion trap

Benzylpyridinium cations readily fragment in the electrospray source by loss of pyridine to give benzyl cations (M−79). The full-scan spectra obtained with some instruments also show, in addition, an m/z (M−38) peak corresponding to the addition of acetonitrile, being present in the solvent mixture, on the benzyl cations. Here we report that the addition reaction can occur in the source region of electrospray mass spectrometry instruments, and in a quadrupole ion trap. The kinetics of acetonitrile addition was monitored in an ion trap, acetonitrile being provided by leakage from the source, through the heated capillary. For benzyl ions with different substituents, the addition kinetics has been found positively correlated with the Brown parameter σ+ of the benzyl radical, and therefore with the effective charge density on the α-carbon atom of the benzyl ion. This is consistent with the Langevin or average-dipole-orientation (ADO) theory of ion–molecule reaction kinetics.

Field-dipole orientation mechanism and higher order modes in atom guides

In cavity quantum electrodynamics (CQED), cavity size, dipole position and dipole orientation are the main factors controlling cavity effects, for example, suppression and enhancement of spontaneous emission. However, the dynamical effects of dipole orientation in CQED have, to date, remained largely unexplored, with most treatments simply concentrating on two (or three) orthogonal directions to illustrate the variations of CQED effects with dipole orientation. No mechanism which determines the evolution of the dipole orientation has been put forward to describe typical situations where atoms move in the field of an excited cavity mode. We emphasize here that in the presence of a cavity mode, the average dipole orientation is automatically determined at every point to be parallel to the direction of the electric field vector of the cavity mode. Besides giving rise to a single value for the spontaneous emission rate, this mechanism is shown to have important consequences for the dynamics of atoms in atom guides. In particular, it determines the average trapping potential distributions and the average radiation forces which guide the atoms along a cylindrical cavity. The effects of the field-dipole orientation mechanism are illustrated with reference to typical situations involving sodium atoms in sub-micron cylindrical guides. The role of a higher order cavity mode of the cylinder in the dynamics is highlighted in terms of its influence on the rotational and vibrational motions in such guides.

Kinetics of the reaction of SF6- with O3

The reaction of SF6− with O3 (1) has been studied at room temperature using a recently constructed flowing-afterglow apparatus. The measured rate constant is k1 = (1.7 ± 0.5) × 10−10 cm3 molecule−1 s−1, i.e. below the value calculated by the average dipole orientation theory and in disagreement with previous experimental data. Possible systematic errors have been rejected on the basis of kinetic results obtained in the same apparatus for the reactions of SF6− with NO2 (2) and Cl2 (3), and for the reactions of O3 with O2− (4) and CO4− (5). The rate constants measured for these reactions are in agreement with previous determinations: k2 = (1.5 ± 0.7), k3 = (1.1 ± 0.3), k4 = (6.3 ± 2.0) and k5 = (3.4 ± 1.0), in units of 10−10 cm3 molecule−1 s−1.

Reactions of O2+ with OH-isoprene adduct isomers: Exothermicity, product ions, and rate constants

The ion-molecule reaction C5H8OH+O2+→C5H8OH++O2 has been investigated in order to develop detection scheme for the OH-isoprene adduct radicals using the chemical ionization mass spectrometry (CIMS) method. Ab initio molecular orbital calculations have been employed to determine the structures and energies of the product ions from the charge transfer reactions of O2+ with the OH-isoprene adduct radicals. Geometry optimizations of the OH-isoprene adduct cations were performed with density function theory (DFT)-B3LYP calculations and the energies were computed with the single-point calculation using the coupled-cluster theory with single and double excitations including perturbative corrections for the triple excitations [CCSD(T)]. At the CCSD(T)/6-311G** level of theory with corrected zero-point-energy (ZPE), the ionization potentials of the four OH-isoprene adduct isomers with OH additions to C1- to C4-positions (isomers I to IV) are 6.9, 5.6, 6.0, and 7.1 eV, respectively. The ion-molecule collision rate constants between O2+ and the OH-isoprene adduct radicals were estimated using the average-dipole-orientation (ADO) theory, along with our calculated the permanent dipole moment and polarizability for the OH-isoprene adduct isomers. We also report direct measurements of the ion-molecule reaction rate constant using the CIMS method, which are in good agreement with the predicted values using the ADO theory.

Influence of ion activation and thermalization effects on reaction rate constants in a quadrupole ion trap mass spectrometer

The insertion of a ‘cooling’ time period following ionization in a quadruple ion trap mass spectrometer, so as to cool ion spatial and kinetic energy distributions, also presents an opportunity for ion-molecule reactions to occur. We report here on the observation of such reactions between the m/ z 91 and m/ z 92 fragment ions from butylbenzene and parent molecules. The measured rate constants for these reactions are relatively low and do not influence unduly the estimation of precursor ion internal energy from the intensity ratio of these fragment ions. The values of these reaction rate constants can be changed by variation of experimental parameters, they can be enhanced by increasing the helium pressure (leading to kinetic energy thermalization) and by decreasing the reactant ion q z , trapping parameter. Both an increase and decrease of the rate constant was observed for the charge exchange reactions of an ion of the m/ z 78 benzene molecular ion with butylbenzene and of the m/ z 134 butylbenzene molecular ion with p-butylaniline as the cooling period duration was varied. The effect of reaction exothermicity on the magnitude of the rate constant for a series of charge exchange reactions of the m/ z 134 butylbenzene molecular ion with substituted butylbenzenes has been explored. The results have been rationalized on the basis of the average dipole orientation treatment of the ion-dipole complex theory. The quasi-equilibrium theory has been used to account for the influence of ion internal energy on the reaction rate constant.

Gas-phase ion-molecule reactions in ammonia-methylsilane mixtures studied by ion trapping

The gas-phase reactivity of ammonia-methylsilane mixtures has been studied by ion trap mass spectrometry. The mechanisms of ion-molecule reactions have been determined, and their rate constants measured and compared with the corresponding collisional rate constants calculated according to the average dipole orientation theory. The SiCH n + ( n = 2−5) and SiH n + ( n = 0−3) primary ions react with methylsilane according to the previosuly reported self-condensation processes. These ions also give the SiNH n + ( n = 2−4) and SiCNH n + ( n = 5, 6) ionic species in different reactions with NH 3. Among them, SiNH 4 + and SiCNH 6 +, which are the most abundant NSi-containing ions, react further with ammonia to form SiN 2H n + ( n = 5, 6) and SiCH 2H 9 + respectively. The most frequent pathways of silicon-containing ions reacting with ammonia correspond to elimination of H 2, or H, or adduct formation. Chain propagation proceeds mainly through ions of general formula Si x C y H n + in reactions with NH 3, while ions containing only silicon, nitrogen and hydrogen give species, such as NH 4 + and SiN 2H n + ( ifn = 5, 6), which do not react further.

Density functional theory of chemical reactivity indices in some ion?molecule reaction systems

Three isoelectronic reactions, proton transfer (PT), hydrogen abstraction (HA), and electron transfer (ET), of NH+3 with NH,3 H2O, and HF have been studied using ab initio molecular orbital calculations. For the reaction of NH+3 + H2O, the energy of the transition state (TS) is higher than that of the reactants. This is consistent with the experimental observation that the rate constant is less than the average dipole orientation (ADO) rate constant. It seems reasonable that the reaction rate for the reaction NH+3 + H2O would hardly depend on the v2 mode of NH+3 at least for low-lying excited states (Eint≤ 0.714 eV) of the v2 mode, because the v2 mode contributes mainly to the normal mode orthogonal to the reaction coordinate at the TS. This is consistent with experimental observation. A similar prediction can be made for the NH+3 + HF reaction. The electron-transfer processes for the HA reactions have been examined in terms of the intrinsic reaction coordinate (IRC). The order of reactivity with NH+3 is NH3 > H2O > HF. It is found that the degree of the electron transfer and the reactivity are correlated with the absolute hardness (η) of NH3, H2O, and HF. This is in accord with the softness as the chemical reactivity index in the density functional theory. © 1996 John Wiley & Sons, Inc.

Density functional theory of chemical reactivity indices in some ion—molecule reaction systems

Three isoelectronic reactions, proton transfer (PT), hydrogen abstraction (HA), and electron transfer (ET), of NH+3 with NH,3 H2O, and HF have been studied using ab initio molecular orbital calculations. For the reaction of NH+3 + H2O, the energy of the transition state (TS) is higher than that of the reactants. This is consistent with the experimental observation that the rate constant is less than the average dipole orientation (ADO) rate constant. It seems reasonable that the reaction rate for the reaction NH+3 + H2O would hardly depend on the v2 mode of NH+3 at least for low-lying excited states (Eint≤ 0.714 eV) of the v2 mode, because the v2 mode contributes mainly to the normal mode orthogonal to the reaction coordinate at the TS. This is consistent with experimental observation. A similar prediction can be made for the NH+3 + HF reaction. The electron-transfer processes for the HA reactions have been examined in terms of the intrinsic reaction coordinate (IRC). The order of reactivity with NH+3 is NH3 > H2O > HF. It is found that the degree of the electron transfer and the reactivity are correlated with the absolute hardness (η) of NH3, H2O, and HF. This is in accord with the softness as the chemical reactivity index in the density functional theory. © 1996 John Wiley & Sons, Inc.

Theoretical Study of Landau−Zener Electronic Transmission Factor for Outer-Sphere Electron Transfer Reactions in Solution

On the basis of the original Landau−Zener formalism, this paper presents an accurate scheme for determining the theoretical values of the electronic transmission factor in terms of the experimental vibrational spectroscopic and thermodynamic data of the complex ions in solution. The quantity of central importance in this regard is the slope of the potential energy surface, and it is obtained from the accurate Morse function and the modified harmonic oscillator potential function determined from the experimental vibrational spectroscopic data and thermodynamic data. Also, the slopes are obtained using the classical improved average dipole orientation method. The various activation parameters are obtained in terms of a new improved self-exchange activation model. The coupling matrix elements are determined from adiabatic and nonadiabatic activation energies which are obtained from the new activation model and experiments, and are compared with the ab initio values in recent literatures. Theoretical results of electronic transmission factors of electron transfer reactions obtained using the values of slopes from the accurate potential functions (the Morse function and the modified harmonic oscillator potential) as well as the harmonic oscillator potential and the classical ion−dipole capture force field potential are found to be in close agreement with the experimental values of electronic transmission factor obtained from experimental data of the rate constants. These theoretical values of electronic transmission factors, as well as those from experimental data of rate constants, are found to be less than unity. These results indicate that outer-sphere electron transfer reactions in solution involving aquo complexes of the transition metal ions studied in this work are nonadiabatic in nature. The further extension of this theoretical scheme is also discussed.

Ion–molecule reactions of ArN+2 with simple aliphatic hydrocarbons at thermal energy

The product ion distributions and rate constants are determined for ion–molecule reactions of ArN+2 with C2Hn (n=2,4,6) and C3Hn (n=6,8) by using a thermal ion–beam apparatus. Although charge-transfer channels leading to parent ions and/or fragment ions are found, no displacement reaction leading to ArCmH+n and N2CmH+n is detected. A comparison of the product ion distributions with breakdown patterns of the parent ions suggests that fragment ions, formed through cleavage of C–H and/or C–C bonds, are produced via near-resonant ionic states in the 13.1–13.4 eV range. The branching ratios of parent ions for C2H4 (68%) and C3H6 (20%) are larger than those for C2H6 (5%) and C3H8 (5%). The large branching ratios of the parent ions for the unsaturated hydrocarbons are explained as due to a strong interaction of a vacant orbital of ArN+2 with the highest occupied πC=C orbital of the unsaturated hydrocarbons which induces nonresonant charge transfer. The total rate constant for C2H2 is 6.8×10−10 cm3 s−1, while those for C2Hn (n=4,6) and C3Hn (n=6,8) are in the range (8.5–9.8)×10−10 cm3 s−1. The former and the latter values correspond to 69% and 77%–90% of the calculated values from Langevin or average dipole orientation (ADO) theory. The smaller kobs/kcalc ratio for C2H2 is attributed to the lack of near-resonant ionic states with favorable Franck–Condon factors for ionization.

Interaction between the ion dipole and the ion-induced dipole in reactions of the polar ion ArH+3

The reactions of the highly polar ion ArH+3 with the nonpolar neutrals Xe, Kr, CH4, CO, NO, O2, and N2 and with the polar neutrals SO2, H2O, NH3, CH3I, and C2H5I have been investigated in a selected ion flow drift tube experiment at near thermal conditions. The rate coefficients obtained have typically 15%–35% larger values than the collisional limiting values kL and kADO obtained by the Langevin and ADO (average dipole orientation) theory, respectively. These data are explained on the basis of the interaction between the dipole of ArH+3 and the ion-induced dipole and the permanent dipole, respectively, of the neutral reactants. Rate coefficients for the reactions of H+3 with the above neutrals were also obtained for comparison, as well as the zero field mobility of ArH+3 in helium being (18.5±1.5) cm2/V s. The ion KrD+3 (KrH+3) was detected unambiguously for the first time. Its dissociation energy is larger than 0.29 eV which is the lower limit of the dissociation energy of ArH+3.

Dissociative charge-transfer reactions of Ar+ with fluoromethanes at thermal energy

A flowing-afterglow apparatus coupled with a low pressure chamber has been used to measure product ion distributions and rate constants in the charge-transfer reactions of Ar+ with CHnF4−n (n=1–3) at thermal energy. Near-resonant dissociative charge transfer followed by loss of H and/or F are major product channels observed. In the Ar+/CH3F reaction, CHF+ and CH2+ ions, which are either absent or very weak in He(i) photoionization, are also found as minor products. The product ion distributions suggest that the Ar+/CHnF4−n (n=1–3) reactions proceed dominantly through near-resonant charge transfer. The total rate constants are 1.7±0.6, 1.9±0.7, and 2.0±0.6×10−9 cm3 s−1 for CH3F, CH2F2, and CHF3, respectively. These values are in reasonable agreement with those predicted from the average dipole orientation (ADO) theory, being independent of the existence of ionic states with favorable Franck–Condon factors for ionization.

Thermal energy reactions of H +3 and H 3O + with a series of small organic molecules

A flowing afterglow has been used to determine the rate coefficients and ion product distributions for the reactions of H + 3 and H 3O + with the small organic molecules CH 3OH,CH 3CHO, C 2H 5OH, (CH 3) 2O, (CH 3) 2 CO and (C 2H 5) 2O at 300 K. The rate coefficients for all the reactions are close to the average dipole orientation collisional values, indicating that the reactions have unit efficiency. However, the products for the H + 3 and H 3O + reactions are very different. The H 3O + reactions proceed almost totally by non-dissociative proton transfer, as dictated by reaction energetics. The H + 3 reactions are much more exothermic and non-dissociative proton transfer is not the only process, the reactions being dominated by many dissociative processes leading to the production of ions such as CH + 3, HCO +, H 3CO +, C 2H + 5 and C 2H 5O +. The data are compared with previous measurements where possible, and the mechanisms of the reactions are discussed.

New mechanism for the vibrational mode-specific proton-transfer reaction NH 3+ (ν) + NH 3 → NH 2 + NH 4+

A new mechanism has been presented for the vibrational mode-specific depression of the proton-transfer reaction NH 3 + (ν) + NH 3 → NH 2 + NH 4 +. We adopt the standpoint of the ADO (average dipole orientation) theory, proposed by Su and Bowers (J. Chem. Phys. 58 (1973) 3027), and append a new long-range interaction created by a vibration-induced dipole to the ADO theory. A “reaction zone” concept is crucial in this approach; outside of the zone the long-range intermolecular interactions play a key role. A qualitatively good agreement is observed for the experimental result of the title reaction obtained by Chupka and Russel (J. Chem. Phys. 48 (1968) 1527).

Force constant of transition metal ion–ligand bonds: molecular-orbital calculations

Values of force constants for transition-metal-ion–ligand bonds calculated using the intermediate neglect of differential overlap. INDO/2-MO, method are given. The results obtained from the INDO/2-MO method agree well with those obtained from vibrational spectroscopic data. Values of force constants have also been computed using the classical improved average dipole orientation (IADO) method and these data are found to be in good agreement with those obtained from vibration spectroscopic data. The agreement of the force constants calculated using the INDO/2-MO method with those obtained from the IADO method further supports the validity of the latter.

Electronic transmission coefficient for outer-sphere electron transfer reactions in solution: A Landau–Zener formalism

The Landau–Zener formulation and literature values of electronic transition matrix were utilized to compute the theoretical values of electronic transmission coefficient of several outer-sphere electron transfer reactions in solution. The slopes of the energy surfaces that are needed for the Landau–Zener equation were obtained using both intermediate neglect of differential overlaps (INDO/2) molecular orbital and classical improved average dipole orientation (IADO) methods. Theoretical results of electronic transmission coefficient of electron transfer reaction obtained using values of slopes from INDO/2-MO (molecular orbital) as well as IADO methods are found in close agreement with the quasiexperimental values of electronic transmission coefficient obtained from experimental data of rate constant. These theoretical values of electronic transmission coefficient, as well as those from experimental values of rate constant are found to be less than unity. These results indicate that outer-sphere electron transfer reactions in solution involving aquo and amine complexes of the transition metal ions studied in this work are nonadiabatic in nature. Theoretical results of electronic transmission coefficient obtained using values of slopes from the classical improved average dipole orientation (IADO) method is found in close agreement with those obtained from the quantum chemical INDO/2-MO method and also with those from experimental values of rate constant and hence justifies the validity of the use of the former.