Dissociation kinetics of energy-selected Cp(2)Mn(+) ions studied by threshold photoelectron-photoion coincidence spectroscopy

Threshold photoelectron-photoion coincidence spectroscopy has been used to investigate the dissociation kinetics of the manganocene ion, Cp(2)Mn(+) (Cp = eta(5)-cyclopentadienyl). The Cp loss reaction was found to be extremely slow over a large ion internal energy range. By simulating the measured asymmetric time-of-flight peak shapes and breakdown diagram, the 0 K thermochemical dissociation limit for CpMn(+) production was determined to be 9.55 +/- 0.15 eV. A CpMn(+)-Cp bond energy of 3.43 eV was obtained by combining this CpMn(+) + Cp dissociation limit with the Cp(2)Mn adiabatic ionization energy of 6.12 +/- 0.07 eV. Combining the measured onset with known heats of formation of Cp and Mn(+), the Cp-Mn(+) bond energy was determined to be 3.38 +/- 0.15 eV. These results lead to 298 K heats of formation of Cp(2)Mn(+) and CpMn(+) of 863 +/- 7 and 935 +/- 16 kJ/mol, respectively. Finally, by combining these results with a previous measurement of the CpMn(CO)(3) --> CpMn(+) + 3CO + e(-) dissociation limit, we arrive at a new value for Delta(f)H degrees (298K)(CpMn(CO)(3)) of -424 +/- 17 kJ/mol.     

The dissociation kinetics of energy-selected CpMn(CO)(3)(+) ions studied by threshold photoelectron-photoion coincidence spectroscopy

Threshold photoelectron-photoion coincidence spectroscopy has been used to investigate the dissociation kinetics of the cyclopentadienyl manganese tricarbonyl ion, CpMn(CO)(3)(+). The ionization energy of CpMn(CO)(3) was measured from the threshold photoelectron spectrum to be 7.69 +/- 0.02 eV. The dissociation of the CpMn(CO)(3)(+) ion proceeds by the sequential loss of three CO molecules. The first and third CO loss reactions were observed to be slow (lifetimes in the microsecond range). By simulating the resulting asymmetric time-of-flight peak shapes and breakdown diagram, 0 K onsets for three product ions were determined to be 8.80 +/- 0.04, 9.43 +/- 0.04, and 10.51 +/- 0.06 eV, respectively. Combined with the adiabatic ionization energy, the three successive Mn-CO bond energies in the CpMn(CO)(3)(+) were found to be alternating with values of 1.11 +/- 0.04, 0.63 +/- 0.04, and 1.08 +/- 0.06 eV, respectively. Using a scaled theoretical Cp-Mn(+) bond energy of 3.10 +/- 0.10 eV and the combined results from theory and experiment, the 298 K gas-phase heat of formation of CpMn(CO)(3) is suggested to be -419 +/- 15 kJ/mol. Based on this value, the 298 K heats of formation of CpMn(CO)(3)(+), CpMn(CO)(2)(+), CpMnCO(+), and CpMn(+) are 325 +/- 15, 546 +/- 15, 719 +/- 15, and 938 +/- 15 kJ/mol, respectively. By scaling theoretical calculated neutral bond energies with the experimental information derived in this study, the successive Mn-CO bond energies were estimated to be 1.88, 1.10, and 1.03 eV, respectively, while the Cp-Mn bond energy was found to be 2.16 eV. Comparison between the quantum chemical calculations and experimental values shows that the loss of CO groups follows the lowest energy adiabatic path, in which electronic spin on the metal center is not conserved.     

Dissociative photoionization of methyl chloride studied with threshold photoelectron-photoion coincidence velocity imaging

Utilizing threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging, dissociation of state-selected CH(3)Cl(+) ions was investigated in the excitation energy range of 11.0-18.5 eV. TPEPICO time-of-flight mass spectra and three-dimensional time-sliced velocity images of CH(3)(+) dissociated from CH(3)Cl(+)(A(2)A(1) and B(2)E) ions were recorded. CH(3)(+) was kept as the most dominant fragment ion in the present energy range, while the branching ratio of CH(2)Cl(+) fragment was very low. For dissociation of CH(3)Cl(+)(A(2)A(1)) ions, a series of homocentric rings was clearly observed in the CH(3)(+) image, which was assigned as the excitation of umbrella vibration of CH(3)(+) ions. Moreover, a dependence of anisotropic parameters on the vibrational states of CH(3)(+)(1(1)A') provided a direct experimental evidence of a shallow potential well along the C-Cl bond rupture. For CH(3)Cl(+)(B(2)E) ions, total kinetic energy released distribution for CH(3)(+) fragmentation showed a near Maxwell-Boltzmann profile, indicating that the Cl-loss pathway from the B(2)E state was statistical predissociation. With the aid of calculated Cl-loss potential energy curves of CH(3)Cl(+), CH(3)(+) formation from CH(3)Cl(+)(A(2)A(1)) ions was a rapid direct fragmentation, while CH(3)Cl(+)(B(2)E) ions statistically dissociated to CH(3)(+) + Cl via internal conversion to the high vibrational states of X(2)E.     

Dissociation processes of Kr2(+) and Kr3(+) studied by threshold photoelectron-photoion coincidence measurements

A time-of-flight (TOF) ion mass spectrum in coincidence with threshold photoelectrons was measured in the photon energy region between the first and second dissociation limits of Kr2(+) to examine the decay processes of the Kr2(+) II(1/2u) state. The measured TOF spectrum reveals that Kr+ fragment ions are produced through dissociation of the repulsive I(1/2g) state, which can be formed by the decay process of the II(1/2u) state accompanied with emission of photons. The potential-energy curve of the I(1/2g) state is deduced with detailed analysis of the observed TOF spectrum, in which the radiative lifetime of the II(1/2u) state was also derived to be 2.5 micros. Additionally, evidence of the dissociation process of Kr3(+) ions was obtained in the same photon energy region, where the dominant channel is Kr3(+) --> Kr2(+) + Kr.     

The decay of 1-chloropropyne cation studied by photoelectron-photoion coincidence spectroscopy

The decay of internal energy selected 1-chloropropyne cations is investigated using the fixed wavelength (He-Iα) photoelectron-photoion coincidence technique. The breakdown curves of the molecular ion and the C₃H₂Cl⁺, C₃HCl⁺, CCl⁺, C₃H⁺₃, C₃H⁺₃, C₃H⁺ fragment ions are reported. For 1-chloropropyne cations initially formed in their $\text{A}\text{?}$²E state it is found that four fragmentation channels compete with a non-dissociative relaxation pathway. The average kinetic energies released on formation of C₃H⁺₃ and C₃H⁺₃ are deduced from the time-of-flight distributions of these fragment ions measured at different internal energies of the molecular ion. The coincidence data are supplemented by electron impact appearance energies. The obtained decay pattern of 1-chloropropyne cation is compared with the breakdown diagrams reported for the C₃H⁺₄ isomers, i.e. allene-, propyne- and cyclopropene cations.     

Understanding the complex dissociation dynamics of energy selected dichloroethylene ions: neutral isomerization energies and heats of formation by imaging photoelectron-photoion coincidence

The dissociative photoionization of 1,1-C(2)H(2)Cl(2), (E)-1,2-C(2)H(2)Cl(2), and (Z)-1,2-C(2)H(2)Cl(2) has been investigated at high energy and mass resolution using the imaging photoelectron photoion coincidence instrument at the Swiss Light Source. The asymmetric Cl-atom loss ion time-of-flight distributions were fitted to obtain the dissociation rates in the 10(3) s(-1) < k < 10(7) s(-1) range as a function of the ion internal energy. The results, supported by ab initio calculations, show that all three ions dissociate to the same C(2v) symmetry ClC═CH(2)(+) product ion. The 0 K onset energies thus establish the relative heats of formation of the neutral isomers, that is, the isomerization energies. The experimental rate constants, k(E), as well as ab initio calculations indicate an early isomerization transition state and no overall reverse barrier to dissociation. The major high energy channels are the parallel HCl loss and the sequential ClC═CH(2)(+) → HCCH(+) + Cl process, the latter in competition with a ClC═CH(2)(+) → ClCCH(+) + H reaction. A parallel C(2)H(2)Cl(2)(+) → C(2)HCl(2)(+) + H channel also weakly asserts itself. The 0 K onset energy for the sequential Cl loss reaction suggests no barrier to the production of the most stable acetylene ion product; thus the sequential Cl-atom loss is preceded by a ClC═CH(2)(+) → HC(Cl)CH(+) reorganization step with a barrier lower than that of the second Cl-atom loss. The breakdown diagram corresponding to this sequential dissociation reveals the internal energy distribution of the first C(2)H(2)Cl(+) daughter ion, which is determined by the kinetic energy release in the first, Cl loss reaction at high excess energies. At low kinetic energy release, this distribution corresponds to the predicted two translational degrees of freedom, whereas at higher energies, the excess energy partitioning is characteristic of only one translational degree of freedom. New Δ(f)H(o)(298K) of 3.7, 2.5, and 0.2 ± 1.75 kJ mol(-1) are proposed for 1,1-C(2)H(2)Cl(2), (E)-1,2-C(2)H(2)Cl(2), and (Z)-1,2-C(2)H(2)Cl(2), respectively, and the proton affinity of ClCCH is found to be 708.6 ± 2.5 kJ mol(-1).     

Heats of formation of HCCl3, HCCl2Br, HCClBr2, HCBr3, and their fragment ions studied by threshold photoelectron photoion coincidence

The dissociative photoionization onsets for Cl and Br loss reactions were measured for HCCl3, HCCl2Br, HCClBr2, and HCBr3 by threshold photoelectron photoion coincidence (TPEPICO) in order to establish the heats of formation of the mixed halides as well as the following fragment ions: HCCl2(+), HCClBr(+), HCBr2(+). The first zero Kelvin onsets were measured with a precision of 10 meV. The second onsets, which are in competition with the lower energy onsets, were established with a precision of 60 meV. Because both the chloroform and bromoform have relatively well established heats of formation, these measurements provide a route for establishing the heats of formation of the mixed halomethanes within uncertainties of less than 5 kJ mol(-1).     

The dichotomy of methyl loss from 2-methylalkane molecular ions

The kinetic energy release for methyl loss from the molecular ion of 2-methylpentane increases c. twofold over the time-range 1–15 μs. This result from a mixture of secondary and tertiary pentyl cations. The appearance energy showed that at threshold (and long times) only tertiary ions are formed. The molecular ion of 2-methylhexane produces tertiary hexyl ions by loss of methyl at threshold, but in the μs time-frame only secondary hexyl ions appear to be formed, because the kinetic energy release is small and independent of the observation time.     

Heats of formation of the propionyl ion and radical and 2,3-pentanedione by threshold photoelectron photoion coincidence spectroscopy

The dissociative photoionization onsets for the formation of the propionyl ion (C(2)H(5)CO(+)) and the acetyl ion (CH(3)CO(+)) were measured from energy selected butanone and 2,3-pentanedione ions using the technique of threshold photoelectron photoion coincidence (TPEPICO) spectroscopy. Ion time-of-flight (TOF) mass spectra recorded as a function of the ion internal energy permitted the construction of breakdown diagrams, which are the fractional abundances of ions as a function of the photon energy. The fitting of these diagrams with the statistical theory of unimolecular decay permitted the extraction of the 0 K dissociation limits of the first and second dissociation channels. This procedure was tested using the known energetics of the higher energy dissociation channel in butanone that produced the acetyl ion and the ethyl radical. By combining the measured dissociative photoionization onsets with the well-established heats of formation of CH(3)(*), CH(3)CO(+), CH(3)CO(*), and butanone, the 298 K heats of formation, Delta(f)H degrees (298K), of the propionyl ion and radical were determined to be 618.6 +/- 1.4 and -31.7 +/- 3.4 kJ/mol, respectively, and Delta(f)H degrees (298K)[2,3-pentanedione] was determined to be -343.7 +/- 2.5 kJ/mol. This is the first experimentally determined value for the heat of formation for 2,3-pentanedione. Ab initio calculations at the Weizmann-1 (W1) level of theory predict Delta(f)H degrees (298K) values for the propionyl ion and radical of 617.9 and -33.3 kJ/mol, respectively, in excellent agreement with the measured values.     

Dissociation of energy-selected c-C2H4S+ in a region 10.6-11.8 eV: threshold photoelectron-photoion coincidence experiments and quantum-chemical calculations

Dissociation of energy-selected c-C2H4S+ was investigated in a region of 10.6-11.8 eV with a threshold photoelectron-photoion coincidence technique and a synchrotron as a source of vacuum ultraviolet radiation. Branching ratios and average releases of kinetic energy in channels of formation of c-C2H4S+, CH3CS+, and HCS+ were obtained from well-resolved time-of-flight peaks in coincidence mass spectra. Measured average releases of kinetic energy for channel CH3CS+ + H of least energy are substantial and much greater than calculated with quasiequilibrium theory; in contrast, small releases of kinetic energy near the appearance onset for channel HCS+ + CH3 agree satisfactorily with statistical calculations. Calculations of molecular electronic structures and energetics of c-C2H4S+ and C2H3S+ isomers and various fragments and transition states were also performed with Gaussian 3 method to establish dissociation mechanisms. A predicted dissociation energy of 11.05 eV for c-C2H4S --> HCS+ + CH3 agrees with a linearly extrapolated threshold at 10.99+/-0.04 eV and a predicted dissociation mechanism that c-C2H4S+ isomerizes to CH3CHS+ before dissociating to HCS+ + CH3 supports the experimental results. The large releases of kinetic energy for channel CH3CS+ + H might result from a dissociation mechanism according to which c-C2H4S+ isomerizes to a local minimum CH3CSH+ and then dissociates through a transition state to form CH3CS+ + H.     

The formation of complexes by tetraethylresorcarene with planar guest ions studied by electrospray ionisation Fourier transform ion cyclotron resonance mass spectrometry

Resorcarenes have become a popular subject of study in the field of supramolecular chemistry. In this work the formation of host-guest complexes between a synthetic macrocyclic host, tetraethylresorcarene, and various eligible planar guests, was studied by mass spectrometric methods. The size and nature of the guest ion strongly influenced the complex formation. Collision-induced dissociation experiments revealed the fragmentation patterns of the resorcarene skeleton and the differences in fragmentation induced by the guest ions.     

Pulsed field ionization-photoelectron photoion coincidence spectroscopy with synchrotron radiation: the heat of formation of the C2H5+ ion

Pulsed field ionization photoelectron (PFI-PE) spectroscopy combined with ion coincidence detection has been used with multi-bunch synchrotron radiation at the Advance Light Source (ALS) to energy select ions and to measure their breakdown diagram. The resolution for ion state selection achieved with Ar+ (2P3/2, 1/2) employing this PFI-PE-photoion coincidence apparatus is 0.6 meV (full width at half maximum). The production of C2H5+ from C2H5Br was investigated near the dissociative photoionization limit with this pulsed field ionization-threshold photoelectron photoion coincidence (PFI-PEPICO) scheme. Although the PFI-PE spectra of C2H5Br, C2H5I, and benzene show that the production of ions in the Franck-Condon gap regions is quite low, the selectivity for PFI-PE detection and the suppression of prompt electrons is such that we can detect 1 PFI-PE out of 25,000 total electrons s-1. The derived C2H5+ heat of formation from the analysis of the C2H5Br+ breakdown diagram and a critical analysis of other results is 900.5 +/- 2.0 kJ mol-1 at 298 K, or 913.2 +/- 2.0 kJ mol-1 at 0 K. This leads to an ethylene proton affinity at 298 K of 682.0 kJ mol-1. The measured IE of C2H5Br is 10.307 eV.     

Dissociation dynamics and thermochemistry of tin species, (CH3)4Sn and (CH3)6Sn2, by threshold photoelectron-photoion coincidence spectroscopy

The dissociative photoionization of tetramethyltin (Me?Sn) and hexamethylditin (Me?Sn?) has been investigated by threshold photoelectron-photoion coincidence (TPEPICO). Ions are energy-selected, and their 0 K dissociation onsets are measured by monitoring the mass spectra as a function of ion internal energy. Me?Sn(+) dissociates rapidly by methyl loss, with a 0 K onset of E? = 9.382 ± 0.020 eV. The hexamethylditin ion dissociates slowly on the time scale of the experiment (i.e., during the 40 μs flight time to the detector) so that dissociation rate constants are measured as a function of the ion energy. RRKM and the simplified statistical adiabatic channel model (SSACM) are used to extrapolate the measured rate constants for methyl and Me?Sn(?) loss to their 0 K dissociation onsets, which were found to be 8.986 ± 0.050 and 9.153 ± 0.075 eV, respectively. Updated values for the heats of formation of the neutral Me?Sn and Me?Sn? are used to derive the following 298.15 K gas-phase standard heats of formation, in kJ·mol?1: Δ(f)H(m)(o)(Me?Sn(+),g) = 746.3 ± 2.9; Δ(f)H(m)(o)(Me?Sn?(+),g) = 705.1 ± 7.5; Δ(f)H(m)(o)(Me?Sn(?),g) = 116.6 ± 9.7; Δ(f)H(m)(o)(Me?Sn,g) = 123.0 ± 16.5; Δ(f)H(m)(o)(MeSn(+),g) = 877.8 ± 16.4. These energetic values also lead to the following 298.15 K bond dissociation enthalpies, in kJ·mol?1: BDE(Me?Sn-Me) = 284.1 ± 9.9; BDE(Me?Sn-SnMe?) = 252.6 ± 14.8.     

Methylcyclopropane ion formation by elimination of water from n-butanol and 2-methylpropanol ions

C₄H₈⁺˙ ions generated from ionized n-butanol (a) and from ionized 2-methylpropanol (b) were characterized by reaction of the daughter ions with ammonia in a Fourier transform mass spectrometer. Hydrogen transfers occur in a in the ratio five-membered ring:six-membered ring = 1:3.2. However, both hydrogen-transfers are followed by the formation of ionized methylcyclopropane. Ionized methylcyclopropane is also produced on elimination of water from b.     

Thermochemistry and dissociative photoionization of Si(CH3)4, BrSi(CH3)3, ISi(CH3)3, and Si2(CH3)6 studied by threshold photoelectron-photoion coincidence spectroscopy

Threshold photoelectron-photoion coincidence spectroscopy (TPEPICO) has been used to study the dissociation kinetics and thermochemistry of Me(4)Si, Me(6)Si(2), and Me(3)SiX, (X = Br, I) molecules. Accurate 0 K dissociative photoionization onsets for these species have been measured from the breakdown diagram and the ion time-of-flight distribution, both of them analyzed and simulated in terms of the statistical RRKM theory and DFT calculations. The average enthalpy of formation of trimethylsilyl ion, Delta fH(o)298K(Me(3)Si(+)) = 617.3 +/- 2.3 kJ/mol, has been determined from the measured onsets for methyl loss (10.243 +/- 0.010 eV) from Me(4)Si, and Br and I loss from Me(3)SiBr (10.624 +/- 0.010 eV) and Me(3)SiI (9.773 +/- 0.015 eV), respectively. The methyl loss onsets for the trimethyl halo silanes lead to Delta fH(o)298K(Me(2)SiBr(+)) = 590.3 +/- 4.4 kJ/mol and Delta fH(o)298K(Me(5)Si(2)(+)) = 487.6 +/- 6.2 kJ/mol. The dissociative photoionization of Me(3)SiSiMe(3) proceeds by a very slow Si-Si bond breaking step, whose rate constants were measured as a function of the ion internal energy. Extrapolation of this rate constant to the dissociation limit leads to the 0 K dissociation onset (9.670 +/- 0.030 eV). This onset, along with the previously determined trimethylsilyl ion energy, leads to an enthalpy of formation of the trimethylsilyl radical, Delta fH(o)298K(Me(3)Si(*)) = 14.0 +/- 6.6 kJ/mol. In combination with other experimental values, we propose a more accurate average value for Delta fH(o)298K(Me(3)Si(*)) of 14.8 +/- 2.0 kJ/mol. Finally, the bond dissociation enthalpies (DeltaH(298K)) Si-H, Si-C, Si-X (X=Cl, Br, I) and Si-Si are derived and discussed in this study.     

The heat effects of formation of Co2+ ion complexes with carboxylic acids

The heat effects of complex formation between cobalt(II) ions and malonic, maleic, and succinic acids were measured on an isothermic-shell calorimeter at 298.15 K and several ionic strength values against the background of NaNO₃. The standard thermodynamic characteristics of complex formation in aqueous solution were calculated.     

The nature of the micellar stern region as studied by reaction kinetics. 2

The nature of rate-retarding effects of cationic micelles on the water-catalyzed hydrolyses of a series of para-substituted 1-benzoyl-1,2,4-triazoles (1a-f) and 1-benzoyl-3-phenyl-1,2,4-triazole (2) has been studied using kinetic methods. A comparison is drawn between medium effects in the micellar Stern region and in model solutions for the micellar Stern region. Simple model solutions involving concentrated aqueous solutions of a small ionic molecule resembling the surfactant headgroup, as reported before,(1) were improved. New model solutions for alkyltrimethylammonium bromide micelles contain both tetramethylammonium bromide (TMAB), mimicking micellar headgroups, and 1-propanol, mimicking hydrophobic tails. The rate-retarding effect of micelles on the hydrolysis of 1a-f and 2 is caused by the high concentration of headgroups as well as by hydrophobic tails in the Stern region where 1a-f and 2 bind to the micelle. Individual contributions of these interactions are quantified. Rate-retarding effects found for different probes, with different sensitivities for interactions as they occur when the probe binds to the micellar Stern region, as well as the micellar Stern region's micropolarity as reported by the E(T)(30) probe, are satisfactorily reproduced by new model solutions containing both TMAB and 1-propanol.     

Thermal decomposition of methyl 2-azidopropionate studied by UV photoelectron spectroscopy and matrix isolation IR spectroscopy: heterocyclic intermediate vs imine formation

Methyl 2-azidopropionate (N(3)CH(3)CHCOOCH(3), M2AP) has been synthesized and characterized by different spectroscopic methods, and the thermal decomposition of this molecule has been investigated by matrix isolation infrared (IR) spectroscopy and ultraviolet photoelectron spectroscopy (UVPES). Computational methods have been employed in the spectral simulation of both UVPES and matrix IR spectra and in the rationalization of the thermal decomposition results. M2AP presents a HOMO vertical ionization energy (VIE) of 9.60 ± 0.03 eV and contributions from all four lowest-energy conformations of this molecule are detected in the gas phase. Its thermal decomposition starts at ca. 400 °C and is complete at ca. 650 °C, yielding N(2), CO, CO(2), CH(3)CN, and CH(3)OH as the final decomposition products. Methyl formate (MF) and CH(4) are also found during the pyrolysis process. Analysis of the potential energy surface of the decomposition of M2AP indicates that M2AP decomposes preferentially into the corresponding imine (M2IP), through a 1,2-H shift synchronous with the N(2) elimination (Type 1 mechanism), requiring an activation energy of 160.8 kJ/mol. The imine further decomposes via two competitive routes: one accounting for CO, CH(3)OH, and CH(3)CN (ΔE(G3) = 260.2 kJ/mol) and another leading to CO(2), CH(4), and CH(3)CN (ΔE(G3) = 268.6 kJ/mol). A heterocyclic intermediate (Type 2 mechanism)-4-Me-5-oxazolidone-can also be formed from M2AP via H transfer from the remote O-CH(3) group, together with the N(2) elimination (ΔE(G3) = 260.2 kJ/mol). Finally, a third pathway which accounts for the formation of MF through an M2AP isomer is envisioned.     

1-Phenylethenol: The enol form of acetophenone. Preparation,ionization energy and the heat of formation in the gas phase

The enol form of acetophenone was generated in the gas phase and its ionization energy was determined as 8.01 ± 0.03 eV. The heat of formation of the enol was assessed as −46 ± 6 kJ.mol⁻¹. The enol is destabilized against acetophenone by 41 kJ.mol⁻¹.