Conformational Effects on Gas-Phase Acidities of Isomeric C3 and C5 Alkanols.

The competitive threshold collision-induced dissociation technique is used to examine conformational effects on the relative gas-phase acidities of selected alcohols. By use of HF and H2O as reference acids in a local thermochemical network to obtain absolute acidities, the measured 0 K gas-phase acidities for the propanol and pentanol isomers are Δacid H0(CH3CH2CH2O-H) = 1563.9 ± 2.9 kJ/mol, Δacid H0((CH3)2CHO-H) = 1568.2 ± 2.7 kJ/mol, Δacid H0(CH3(CH2)4O-H) = 1556.4 ± 2.9 kJ/mol, and Δacid H0((CH3)2CHCH2CH2O-H) = 1556.5 ± 3.0 kJ/mol. Conformational stabilization during deprotonation results in the observed acidity differences between isomers, which can be compared with the "intrinsic" acidity strength defined as deprotonation of the extended all- anti staggered conformations without relaxation. The intrinsic acidities for the propanol and pentanol isomers are 1567 and 1562 kJ/mol, respectively. The difference in intrinsic and observed acidity is largely due to the result of a twisted geometry of the alkoxide ion, stabilized by electrostatic interaction between the electronegative terminal O atom and a H atom on the γ-carbon. These interactions are primarily due to internal rotation about the Cα-Cß bonds for n-propoxide and the primary pentoxides.

Radical Thermometers, Thermochemistry, and Photoelectron Spectra: A Photoelectron Photoion Coincidence Spectroscopy Study of the Methyl Peroxy Radical.

We investigated the simplest alkylperoxy radical, CH3OO, formed by reacting photolytically generated CH3 radicals with O2, using the new combustion reactions followed by photoelectron photoion coincidence (CRF-PEPICO) apparatus at the Swiss Light Source. Modeling the experimental photoion mass-selected threshold photoelectron spectrum using Franck-Condon simulations including transitions to triplet and singlet cationic states yielded the adiabatic ionization energy of 10.265 ± 0.025 eV. Dissociative photoionization of CH3OO generates the CH3+ fragment ion at the appearance energy of 11.164 ± 0.010 eV. Combining these two values with ΔfH0K°(CH3) yields ΔfH0K°(CH3OO) = 22.06 ± 0.97 kJ mol-1, reducing the uncertainty of the previously determined value by a factor of 5. Statistical simulation of the CH3OO breakdown diagram provides a molecular thermometer of the free radical's internal temperature, which we measured to be 330 ± 30 K.

Anchoring the Gas-Phase Acidity Scale from Hydrogen Sulfide to Pyrrole. Experimental Bond Dissociation Energies of Nitromethane, Ethanethiol, and Cyclopentadiene.

A meta-analysis of experimental information from a variety of sources is combined with statistical thermodynamics calculations to refine the gas-phase acidity scale from hydrogen sulfide to pyrrole. The absolute acidities of hydrogen sulfide, methanethiol, and pyrrole are evaluated from literature R-H bond energies and radical electron affinities to anchor the scale. Relative acidities from proton-transfer equilibrium experiments are used in a local thermochemical network optimized by least-squares analysis to obtain absolute acidities of 14 additional acids in the region. Thermal enthalpy and entropy corrections are applied using molecular parameters from density functional theory, with explicit calculation of hindered rotor energy levels for torsional modes. The analysis reduces the uncertainties of the absolute acidities of the 14 acids to within ±1.2 to ±3.3 kJ/mol, expressed as estimates of the 95% confidence level. The experimental gas-phase acidities are compared with calculations, with generally good agreement. For nitromethane, ethanethiol, and cyclopentadiene, the refined acidities can be combined with electron affinities of the corresponding radicals from photoelectron spectroscopy to obtain improved values of the C-H or S-H bond dissociation energies, yielding D298(H-CH2NO2) = 423.5 ± 2.2 kJ mol(-1), D298(C2H5S-H) = 364.7 ± 2.2 kJ mol(-1), and D298(C5H5-H) = 347.4 ± 2.2 kJ mol(-1). These values represent the best-available experimental bond dissociation energies for these species.

Energy-resolved collision-induced dissociation of peroxyformate anion: enthalpies of formation of peroxyformic acid and peroxyformyl radical.

We measure the oxygen-oxygen bond dissociation energy of the peroxyformate anion (HCO(3)(-)) using energy-resolved collision-induced dissociation with a guided ion beam tandem mass spectrometer. The analysis of the dissociation process from HCO(3)(-) ((1)A') to HCO(2)(-) ((1)A(2)) + O((3)P) requires consideration of the singlet-triplet crossing along the reaction path and of the competing OH(-) and O(2)H(-) product channels. The measured oxygen-oxygen bond dissociation energy is D(0)(HCO(2)(-)-O) = 1.30 ± 0.13 eV (126 ± 12 kJ/mol). This threshold energy measurement is used in thermochemical cycles to derive the enthalpies of formation for peroxyformic acid, Δ(f)H(0)(HCO(3)H) = -287 ± 19 kJ/mol, and peroxyformyl radical, Δ(f)H(0)(HCO(3)(•)) = -98 ± 12 kJ/mol. These values are in good agreement with computational energies.

Optimization of a quadrupole ion storage trap as a source for time-of-flight mass spectrometry.

Designs of a quadrupole ion trap (QIT) as a source for time-of-flight (TOF) mass spectrometry are evaluated for mass resolution, ion trapping, and laser activation of trapped ions. Comparisons are made with the standard hyperbolic electrode ion trap geometry for TOF mass analysis in both linear and reflectron modes. A parallel-plate design for the QIT is found to give significantly improved TOF mass spectrometer performance. Effects of ion temperature, trapped ion cloud size, mass, and extraction field on mass resolution are investigated in detail by simulation of the TOF peak profiles. Mass resolution (m/Δm) values of several thousand are predicted even at room temperature with moderate extraction fields for the optimized design. The optimized design also allows larger radial ion collection size compared with the hyperbolic ion trap, without compromising the mass resolution. The proposed design of the QIT also improves the ion-laser interaction volume and photon collection efficiency for fluorescence measurements on trapped ions.

Photoelectron spectra of dihalomethyl anions: testing the limits of normal mode analysis.

We report the 364-nm negative ion photoelectron spectra of CHX(2)(-) and CDX(2)(-), where X = Cl, Br, and I. The pyramidal dihalomethyl anions undergo a large geometry change upon electron photodetachment to become nearly planar, resulting in multiple extended vibrational progressions in the photoelectron spectra. The normal mode analysis that successfully models photoelectron spectra when geometry changes are modest is unable to reproduce qualitatively the experimental data using physically reasonable parameters. Specifically, the harmonic normal mode analysis using Cartesian displacement coordinates results in much more C-H stretch excitation than is observed, leading to a simulated photoelectron spectrum that is much broader than that which is seen experimentally. A (2 + 1)-dimensional anharmonic coupled-mode analysis much better reproduces the observed vibrational structure. We obtain an estimate of the adiabatic electron affinity of each dihalomethyl radical studied. The electron affinity of CHCl(2) and CDCl(2) is 1.3(2) eV, of CHBr(2) and CDBr(2) is 1.9(2) eV, and of CHI(2) and CDI(2) is 1.9(2) eV. Analysis of the experimental spectra illustrates the limits of the conventional normal mode approach and shows the type of analysis required for substantial geometry changes when multiple modes are active upon photodetachment.

Pulsed ion extraction diagnostics in a quadrupole ion trap linear time-of-flight mass spectrometer.

Pulsed extraction techniques are investigated for a quadrupole ion trap (QIT) interfaced to a linear time-of-flight (TOF) mass analyzer. A nonfocusing short-pulse mode of operation is developed and characterized. The short-pulse mode creates a near-monoenergetic ion packet, which is useful for reaction kinetics experiments and for making diagnostic measurements of the ion cloud size in the trap. Monopolar and bipolar pulsing modes, with the voltage pulses applied to one or both QIT endcaps to extract the ions into the TOF region, are compared. Ion TOF peak distributions are characterized experimentally and by ion trajectory simulations. Also, first-order spatial (Wiley-McLaren) focusing of ions is characterized for the conventional long-pulse extraction mode. The nonparallel fields in the QIT, which serves as the first acceleration region in the linear-TOF mass spectrometer, are shown to degrade spatial focusing and mass resolution.

The photoelectron spectrum of CCl2-: the convergence of theory and experiment after a decade of debate.

We report new 351 nm negative ion photoelectron spectra of CCl(2)(-), CBr(2)(-), and CI(2)(-). This study was undertaken in an attempt to understand the major discrepancy between dihalocarbene (CX(2), X = Cl, Br, I) singlet-triplet splittings reported by our laboratory (R. L. Schwartz, G. E. Davico, T. M. Ramond, W. C. Lineberger, J. Phys. Chem. A., 1999, 103, 8213) and new theoretical values. Our recent experiments show that a dihalomethyl anion (CHX(2)(-)) contaminant in the dihalocarbene anion beam, previously considered insignificant, made a major contribution to the reported CX(2)(-) photoelectron spectra. Thus, the interpretations of the earlier CX(2)(-) spectra and the reported singlet-triplet splittings were incorrect. Replacing O(-) with OH(-) in the anion formation process yields a pure dihalomethyl anion, whose highly structured photoelectron spectrum can be subtracted from the contaminated spectrum, yielding a clean CX(2)(-) photoelectron spectrum. The new CCl(2)(-) photoelectron spectrum displays resolved vibronic transitions to the two lowest electronic states of CCl(2): X(1)A(1) and a(3)B(1). The electron affinity of X(1)A(1) CCl(2) is 1.593(6) eV. A large change in geometry between the anion and the neutral triplet state precludes the direct observation of the triplet origin. The energy difference between the X(1)A(1) and a(3)B(1) states of CCl(2) is estimated to be approximately 0.9(2) eV, consistent with high-level theoretical studies. While we confirm similar dihalomethyl anion contaminants in the earlier photoelectron spectra of CBr(2)(-) and CI(2)(-) and report new photoelectron spectra for these ions, the paucity of resolved features in the spectra provides limited additional thermochemical information.

Low-energy photoelectron imaging spectroscopy of nitromethane anions: Electron affinity, vibrational features, anisotropies, and the dipole-bound state.

We present low-energy velocity map photoelectron imaging results for nitromethane anions. The photoelectron spectrum is interpreted with the aid of ab initio theory and Franck-Condon factor calculations. We obtain a new value for the adiabatic electron affinity of nitromethane of (172+/-6) meV and observe the dipole-bound state of nitromethane. The photoelectron angular distributions of the observed features are discussed in the context of threshold laws for photodetachment.

Fluorescence and photodissociation of rhodamine 575 cations in a quadrupole ion trap.

The fluorescence and photodissociation of rhodamine 575 cations confined to a quadrupole ion trap are observed during laser irradiation at 488 nm. The kinetics of photodissociation is measured by time-dependent mass spectra and time-dependent fluorescence. The rhodamine ion signal and fluorescence decay are studied as functions of buffer gas pressure, laser fluence, and irradiation time. The decay rates of the ions in the mass spectra agree with decay rates of the fluorescence. Some of the fragment ions also fluoresce and further dissociate. The photodissociation rate is found to depend on the incident laser fluence and buffer gas pressure. The implications of rapid absorption/fluorescence cycling for photodissociation of dye-labeled biomolecular ions under continuous irradiation are discussed.

Statistical rate theory and kinetic energy-resolved ion chemistry: theory and applications.

Ion chemistry, first discovered 100 years ago, has profitably been coupled with statistical rate theories, developed about 80 years ago and refined since. In this overview, the application of statistical rate theory to the analysis of kinetic-energy-dependent collision-induced dissociation (CID) reactions is reviewed. This procedure accounts for and quantifies the kinetic shifts that are observed as systems increase in size. The statistical approach developed allows straightforward extension to systems undergoing competitive or sequential dissociations. Such methods can also be applied to the reverse of the CID process, association reactions, as well as to quantitative analysis of ligand exchange processes. Examples of each of these types of reactions are provided and the literature surveyed for successful applications of this statistical approach to provide quantitative thermochemical information. Such applications include metal-ligand complexes, metal clusters, proton-bound complexes, organic intermediates, biological systems, saturated organometallic complexes, and hydrated and solvated species.

Photodissociation and collisional cooling of rhodamine 575 cations in a quadrupole ion trap.

The photodissociation of rhodamine 575 cations held in a quadrupole ion trap is studied using 514 nm light as a function of buffer gas pressure, irradiation time, and laser fluence. The laser-induced photodissociation decays of rhodamine ions have lifetimes on the order of seconds for the range of pressures and powers investigated and exhibit strong nonlinear pressure dependence. Dissociation mechanisms are considered that involve the sequential absorption of multiple photons and several collisional deactivation steps.

Threshold collision-induced dissociation of hydrogen-bonded dimers of carboxylic acids.

Energy-resolved competitive collision-induced dissociation is used to investigate the proton-bound heterodimer anions of a series of carboxylic acids (formic, acetic, and benzoic acid) and nitrous acid with their conjugate bases. The dissociation reactions of the complexes [CH3COO.H.OOCH]-, [CH3COO.H.ONO]-, [HCOO.H. ONO]-, [C6H5COO.H.OOCH]-, and [C6H5COO.H.ONO]- are investigated using a guided ion beam tandem mass spectrometer. Cross sections of the two dissociation channels are measured as a function of the collision energy between the complex ions and xenon target gas. Apparent relative gas-phase acidities are found by modeling the cross sections near the dissociation thresholds using statistical rate theory. Internal inconsistencies are found in the resulting relative acidities. These deviations apparently result from the formation of higher-energy conformers of the acids within the complex ions induced by double hydrogen bonding, which impedes the kinetics of dissociation to ground-state product acid conformations.

Hydrogen atom transfer reactions of C2-, C4-, and C6-: bond dissociation energies of linear H-C2n- and H-C2n (n = 1, 2, 3).

The reactions of C2-, C4-, and C6- with D2O and ND3 and of C4- with CH3OH, CH4, and C2H6 have been investigated using guided ion beam tandem mass spectrometry. Hydrogen (or deuterium) atom transfer is the major product channel for each of the reactions. The reaction threshold energies for collisional activation are reported. Several of the reactions exhibit threshold energies in excess of the reaction endothermicity. Potential energy calculations using density functional theory show energy barriers for some of the reactions. Dynamic restrictions related to multiple wells along the reaction path may also contribute to elevated threshold energies. The results indicate that the reactions with D2O have the smallest excess threshold energies, which may therefore be used to derive lower limits on the C-H bond dissociation energies of the C2nH- and C2nH (n = 1-3) linear species. The experimental lower limits for the bond dissociation energies of the neutral radicals to linear products are D0(C2-H) >or= 460 +/- 15 kJ/mol, D0(C4-H) >or= 427 +/- 12 kJ/mol, and D0(C6-H) >or= 405 +/- 11 kJ/mol.

Gas-phase acidities and O-H bond dissociation enthalpies of phenol, 3-methylphenol, 2,4,6-trimethylphenol, and ethanoic acid.

Energy-resolved, competitive threshold collision-induced dissociation (TCID) methods are used to measure the gas-phase acidities of phenol, 3-methylphenol, 2,4,6-trimethylphenol, and ethanoic acid relative to hydrogen cyanide, hydrogen sulfide, and the hydroperoxyl radical using guided ion beam tandem mass spectrometry. The gas-phase acidities of Delta(acid)H298(C6H5OH) = 1456 +/- 4 kJ/mol, Delta(acid)H298(3-CH3C6H4OH) = 1457 +/- 5 kJ/mol, Delta(acid)H298(2,4,6-(CH3)3C6H2OH) = 1456 +/- 4 kJ/mol, and Delta(acid)H298(CH3COOH) = 1457 +/- 6 kJ/mol are determined. The O-H bond dissociation enthalpy of D298(C6H5O-H) = 361 +/- 4 kJ/mol is derived using the previously published experimental electron affinity for C6H5O, and thermochemical values for the other species are reported. A comparison of the new TCID values with both experimental and theoretical values from the literature is presented.

Collision-induced dissociation of HS-(HCN): unsymmetrical hydrogen bonding in a proton-bound dimer anion.

The energy-resolved competitive collision-induced dissociation of the proton-bound complex [HS.H.CN](-) is studied in a guided ion beam tandem mass spectrometer. H(2)S and HCN have nearly identical gas-phase acidities, and therefore, the HS(-) + HCN and the CN(-) + H(2)S product channels exhibit nearly the same threshold energies, as expected. However, the HS(-) + HCN channel has a cross section up to a factor of 50 larger than CN(-) + H(2)S at higher energies. The cross sections are modeled using RRKM theory and phase space theory. The complex dissociates to HS(-)+ HCN via a loose transition state, and it dissociates to CN(-) + H(2)S via a tight transition state. Theoretical calculations show that the proton-transfer potential energy surface has a single minimum and that the hydrogen bonding in the complex is strongly unsymmetrical, with an ion-molecule complex of the form HS(-)..HCN rather than CN(-)..H(2)S or an intermediate structure. The requirement for proton transfer before dissociation and curvature along the reaction path impedes the CN(-) + H(2)S product channel.

Photoelectron spectroscopy of phosphorus hydride anions.

Negative-ion photoelectron spectroscopy is applied to the PH-, PH2-, P2H-, P2H2-, and P2H3-molecular anions. Franck-Condon simulations of the photoelectron spectra are used to analyze the spectra and to identify various P2H(n)- species. The simulations employ density-functional theory calculations of molecular geometries and vibrational frequencies and normal modes, and coupled-cluster theory calculations of electron affinities. The following electron affinities are obtained: EA0(PH) = 1.027 +/- 0.006 eV, EA0(PH2) = 1.263 +/- 0.006 eV, and EA0(P2H) = 1.514 +/- 0.010 eV. A band is identified as a mixture of trans-HPPH- and cis-HPPH-. Although the trans and cis bands cannot be definitively assigned from experimental information, using theory as a guide we obtain EA0(trans-HPPH)= 1.00 +/- 0.01 eV and EA0(cis-HPPH) = 1.03 +/- 0.01 eV. A weak feature tentatively assigned to P2H3- has a vertical detachment energy of 1.74 eV. The derived gas-phase acidity of phosphine is delta(acid)G298(PH3) < or = 1509.7 +/- 2.1 kJ mo1(-1).

Threshold collision-induced dissociation of diatomic molecules: a case study of the energetics and dynamics of O2- collisions with Ar and Xe.

The energetics and dynamics of collision-induced dissociation of O2- with Ar and Xe targets are studied experimentally using guided ion-beam tandem mass spectrometry. The cross sections and the collision dynamics are modeled theoretically by classical trajectory calculations. Experimental apparent threshold energies are 2.1 and 1.1 eV in excess of the thermochemical O2- bond dissociation energy for argon and xenon, respectively. Classical trajectory calculations confirm the observed threshold behavior and the dependence of cross sections on the relative kinetic energy. Representative trajectories reveal that the bond dissociation takes place on a short time scale of about 50 fs in strong direct collisions. Collision-induced dissociation is found to be remarkably restricted to the perpendicular approach of ArXe to the molecular axis of O2-, while collinear collisions do not result in dissociation. The higher collisional energy-transfer efficiency of xenon compared with argon is attributed to both mass and polarizability effects.

Systematic and random errors in ion affinities and activation entropies from the extended kinetic method.

An evaluation of the extended kinetic method with full entropy analysis was conducted using RRKM theory to simulate data for collision-induced dissociation under single-collision conditions. A rigorous method for analyzing kinetic method data, orthogonal distance regression, is introduced and compared with previous methods in the literature. The results demonstrate that the use of the extended kinetic method is definitely superior to the standard kinetic method, but final ion affinities and activation entropies differ intrinsically from the correct values. Considering the effects of both systematic and random error in Monte Carlo simulations of the full entropy analysis, error distributions of +/-4 to +/-12 kJ mol(-1) for ion affinities and of +/-9 to +/-30 J mol(-1) K(-1) for activation entropy differences are found (+/-2 standard deviations of the sample populations). The systematic errors in ion affinities are larger for systems with large activation entropy differences. These uncertainties do not include any error in the absolute calibration of the reference ion affinity scale. We argue that application of an empirical correction factor is inadvisable.