A Determination of the Bond Dissociation Energy (D0(H−SH)): Threshold Ion-Pair Production Spectroscopy (TIPPS) of a Triatomic Molecule

Unraveling the Rich Fragmentation Dynamics Associated with S-H Bond Fission Following Photoexcitation of H2S at Wavelengths ∼129.1 nm

H2S is being detected in the atmospheres of ever more interstellar bodies, and photolysis is an important mechanism by which it is processed. Here, we report H Rydberg atom time-of-flight measurements following the excitation of H2S molecules to selected rotational (JKaKc') levels of the 1B1 Rydberg state associated with the strong absorption feature at wavelengths of λ ∼ 129.1 nm. Analysis of the total kinetic energy release spectra derived from these data reveals that all levels predissociate to yield H atoms in conjunction with both SH(A) and SH(X) partners and that the primary SH(A)/SH(X) product branching ratio increases steeply with ⟨Jb2⟩, the square of the rotational angular momentum about the b-inertial axis in the excited state. These products arise via competing homogeneous (vibronic) and heterogeneous (Coriolis-induced) predissociation pathways that involve coupling to dissociative potential energy surfaces (PES(s)) of, respectively, 1A″ and 1A' symmetries. The present data also show H + SH(A) product formation when exciting the JKaKc' = 000 and 111 levels, for which ⟨Jb2⟩ = 0 and Coriolis coupling to the 1A' PES(s) is symmetry forbidden, implying the operation of another, hitherto unrecognized, route to forming H + SH(A) products following excitation of H2S at energies above ∼9 eV. These data can be expected to stimulate future ab initio molecular dynamic studies that test, refine, and define the currently inferred predissociation pathways available to photoexcited H2S molecules.

The highly exothermic hydrogen abstraction reaction H2Te + OH → H2O + TeH: comparison with analogous reactions for H2Se and H2S

The "gold standard" CCSD(T) method is adopted along with the correlation consistent basis sets up to aug-cc-pV5Z-PP to study the mechanism of the hydrogen abstraction reaction H₂Te + OH. The predicted geometries and vibrational frequencies for reactants and products are in good agreement with the available experimental results. With the ZPVE corrections, the transition state in the favorable pathway of this reaction energetically lies 1.2 kcal mol^-1 below the reactants, which is lower than the analogous relative energies for the H₂Se + OH reaction (-0.7 kcal mol^-1), the H₂S + OH reaction (+0.8 kcal mol^-1) and the H₂O + OH reaction (+9.0 kcal mol^-1). Accordingly, the exothermic reaction energies for these related reactions are predicted to be 47.8 (H₂Te), 37.7 (H₂Se), 27.1 (H₂S), and 0.0 (H₂O) kcal mol^-1, respectively. Geometrically, the low-lying reactant complexes for H₂Te + OH and H₂Se + OH are two-center three-electron hemibonded structures, whereas those for H₂S + OH and H₂O + OH are hydrogen-bonded. With ZPVE and spin-orbit coupling corrections, the relative energies for the reactant complex, transition state, product complex, and the products for the H₂Te + OH reaction are estimated to be -13.1, -1.0, -52.0, and -52.6 kcal mol^-1, respectively. Finally, twenty-eight DFT functionals have been tested systematically to assess their ability in describing the potential energy surface of the H₂Te + OH reaction. The best of these functionals for the corresponding energtics are -9.9, -1.4, -46.4, and -45.4 kcal mol^-1 (MPWB1K), or -13.1, -2.4, -57.1, and -54.6 kcal mol^-1 (M06-2X), respectively.

Rotational and nuclear-spin level dependent photodissociation dynamics of H2S

The detailed features of molecular photochemistry are key to understanding chemical processes enabled by non-adiabatic transitions between potential energy surfaces. But even in a small molecule like hydrogen sulphide (H₂S), the influence of non-adiabatic transitions is not yet well understood. Here we report high resolution translational spectroscopy measurements of the H and S(¹D) photoproducts formed following excitation of H₂S to selected quantum levels of a Rydberg state with ¹B₁ electronic symmetry at wavelengths λ ~ 139.1 nm, revealing rich photofragmentation dynamics. Analysis reveals formation of SH(X), SH(A), S(³P) and H₂ co-fragments, and in the diatomic products, inverted internal state population distributions. These nuclear dynamics are rationalised in terms of vibronic and rotational dependent predissociations, with relative probabilities depending on the parent quantum level. The study suggests likely formation routes for the S atoms attributed to solar photolysis of H₂S in the coma of comets like C/1995 O1 and C/2014 Q2.

The photodissociation dynamics of small molecules in the vacuum ultraviolet range can have key implications for astrochemical modelling, but revealing such dynamical details is a challenging task. Here the authors, combining high resolution experimental techniques, provide a detailed description of the fragmentation dynamics of selected rotational levels of a predissociated Rydberg state of H₂S.

Ultraviolet photolysis of H2S and its implications for SH radical production in the interstellar medium

Hydrogen sulfide radicals in the ground state, SH(X), and hydrogen disulfide molecules, H₂S, are both detected in the interstellar medium, but the returned SH(X)/H₂S abundance ratios imply a depletion of the former relative to that predicted by current models (which assume that photon absorption by H₂S at energies below the ionization limit results in H + SH photoproducts). Here we report that translational spectroscopy measurements of the H atoms and S(¹D) atoms formed by photolysis of jet-cooled H₂S molecules at many wavelengths in the range 122 ≤ λ ≤155 nm offer a rationale for this apparent depletion; the quantum yield for forming SH(X) products, Γ, decreases from unity (at the longest excitation wavelengths) to zero at short wavelengths. Convoluting the wavelength dependences of Γ, the H₂S parent absorption and the interstellar radiation field implies that only ~26% of photoexcitation events result in SH(X) products. The findings suggest a need to revise the relevant astrochemical models.

Sulfur is abundant in the Universe, but the observed abundance ratio of SH to H₂S doesn’t agree with astrochemical models. The authors measure product state-resolved translational energy spectra of photoproducts in a jet-cooled H₂S beam as a function of wavelength, showing that SH yield is lower than assumed in the models.

Dipolar dissociation dynamics in electron collisions with carbon monoxide

Dipolar dissociation processes in the electron collisions with carbon monoxide have been studied using time of flight (TOF) mass spectroscopy in combination with the highly differential velocity slice imaging (VSI) technique. By probing ion-pair states, both positive and/or negative ions may be detected. The ion yield curve of negative ions provides the threshold energy for the ion-pair production. On the other hand, the kinetic energy distributions and angular distributions of the fragment anion provide detailed dynamics of the dipolar dissociation process. Two ion-pair states have been identified based on angular distribution measurements using the VSI technique.

Double-arm three-dimensional ion imaging apparatus for the study of ion pair channels in resonance enhanced multiphoton ionization

We present a novel experimental configuration for the full quantitative characterization of the multichannel resonance enhanced multiphoton ionization (REMPI) of small molecules in cases when the ion-pair dissociation channel is important. For this purpose, a double-arm time-of-flight mass spectrometer with three-dimensional (3D) ion imaging detectors at both arms is constructed. The REMPI of HCl molecules is used to examine the constructed setup. The apparatus allows us to perform simultaneous measurements of the 3D velocity vector distributions of positive (H(+), HCl(+), and Cl(+)) and negative (Cl(-)) photoions. The characterization consists of the determination of "two-photon absorption cross sections" for the process HCl(X)+2hν → HCl*, one-photon absorption cross sections for subsequent processes HCl* + hν → HCl*, and the probability of the subsequent non-adiabatic transition HCl* → HCl(B) → H(+) + Cl(-), which leads to ionic pairs. All these data should be obtained from the analysis of the dependencies of the number of ions on the laser energy. The full characterization of the laser beam and the knowledge of the ion detection probability are necessary parts of the analysis. Detailed knowledge of losses of produced ions in the mass spectrometer before detection requires understanding and characterization of such processes like electron emission from metallic grids under ion bombardment or charge transfer between positive ions and the metal surface of the grids, like Cl(+) + (grid) → Cl(-). These important phenomena from surface science are rarely discussed in the imaging literature, and here, we try to compensate for this shortcoming.

The divide-and-conquer second-order proton propagator method based on nuclear orbital plus molecular orbital theory for the efficient computation of proton binding energies

An efficient computational method to evaluate the binding energies of many protons in large systems was developed.

Density functional theory study of hydrogenation of S to H2S on Pt–Pd alloy surfaces

In this work, the adsorption of S-containing species (S, HS, and H₂S) and the hydrogenation of S on the Pt–Pd alloy were investigated by using the periodic density functional theory (DFT).

Hylleraas hydride binding energy: diatomic electron affinities

Theoretical adiabatic electron affinities are often considered inaccurate because they are referenced to only a single value. Ground state electron affinities for all the main group elements and homonuclear diatomics were identified recently using the normalized binding energy of the hydrogen atom: [0.75420375(3)/2 = 0.37710187(1) eV]. Here we revisit experimental values and extend the identifications to diatomics in the G2-1 set. We assign new ground state electron affinities: (eV) Cl2, 3.2(2); Br2, 2.87(14); CH, 2.1(2); H2, 0.6 ; NH, 1.1, SiH, 1.90. Anion Morse potentials are calculated for H2 and N2 from positive electron affinities and for hyperfine superoxide states for the first time.

Ion-pair dissociation dynamics of H2S in the photon energy range 15.26-15.55 eV

The H(+) velocity map images from the ion-pair dissociation of H(2)S + hν → SH(-)(X(1)Σ(+), υ = 0, 1) + H(+) have been measured at the excitation energies 15.259, 15.395, and 15.547 eV, respectively. The experimental results show that most of the available energies are transformed into the translational energies. The angular distributions of the fragments SH(-)(X(1)Σ(+), υ = 0) indicate that the dissociation occurs via pure parallel transition with limiting anisotropy parameter of +2. Because the ion-pair dissociation usually occurs via the predissociation of Rydberg states, this suggests that the ion cores of the excited Rydberg states have linear geometries. The geometries and electronic structures of the linear H(2)S(+) have been calculated employing the quantum chemistry calculation method at the CASPT2/avqz level. The electronic structures for the ion-pair states have been calculated at the CASSCF/avtz level, which indicates that the equilibrium geometries of the ion-pair states have bent geometries.

Radical stability and its role in synthesis and catalysis

The thermodynamic stability of carbon-centered radicals may be defined in quantitative terms using the hydrogen transfer reaction shown in . The stability values obtained in this way for substituted systems may be understood as the stabilizing or destabilizing influence of substituents on the neighboring radical center. This approach can be easily adapted to oxygen- or sulfur-centered radicals as expressed in eqn (b). [Formula: see text] The stability values obtained in this way do not only serve as a quantitative basis for the discussion of substituent effects, but also allow for quantitative estimates of reaction energies for hydrogen transfer reactions. These occur as key steps in a multitude of synthetically useful radical-chain processes in apolar solution, in enzyme-mediated non-chain processes in biological systems, and in the oxidative degradation of a variety of biomolecules. The review will highlight the usefulness of radical stability values for the rationalization of successful (and not so successful) synthetic radical reactions as well as the potential design of new radical reactions.

Structure of the Vinyldiazomethyl Anion and Energetic Comparison to the Cyclic Isomers

The 351.1 nm photoelectron spectrum of the vinyldiazomethyl anion has been measured. The ion is generated through the reaction of the allyl anion with N(2)O in helium buffer gas in a flowing afterglow source. The spectrum exhibits the vibronic structure of the vinyldiazomethyl radical in its electronic ground state as well as in the first excited state. Electronic structure calculations have been performed for these molecules at the B3LYP/6-311++G(d,p) level of theory. A Franck-Condon simulation of the X (2)A'' state portion of the spectrum has been carried out using the geometries and normal modes of the anion and radical obtained from these calculations. The simulation unambiguously shows that the ions predominantly have an E conformation. The electron affinity (EA) of the radical has been determined to be 1.864 +/- 0.007 eV. Vibrational frequencies of 185 +/- 10 and 415 +/- 20 cm(-1) observed in the spectrum have been identified as in-plane CCN bending and CCC bending modes, respectively, for the X (2)A'' state. The spectrum for the A (2)A' state is broad and structureless, reflecting large geometry differences between the anion and the radical, particularly in the CCN angle, as well as vibronic coupling with the X (2)A'' state. The DFT calculations have also been used to better understand the mechanism of the allyl anion reaction with N(2)O. Collision-induced dissociation of the structural isomer of the vinyldiazomethyl anion, the 1-pyrazolide ion, has been examined, and energetics of the structural isomers is discussed.

Proton affinities of maingroup-element hydrides and noble gases: Trends across the periodic table, structural effects, and DFT validation

We have carried out an extensive exploration of the gas-phase basicity of archetypal neutral bases across the periodic system using the generalized gradient approximation (GGA) of the density functional theory (DFT) at BP86/QZ4P//BP86/TZ2P. First, we validate DFT as a reliable tool for computing proton affinities and related thermochemical quantities: BP86/QZ4P//BP86/TZ2P is shown to yield a mean absolute deviation of 2.0 kcal/mol for the proton affinity at 298 K with respect to experiment, and 1.2 kcal/mol with high-level ab initio benchmark data. The main purpose of this work is to provide the proton affinities (and corresponding entropies) at 298 K of the neutral bases constituted by all maingroup-element hydrides of groups 15-17 and the noble gases, that is, group 18, and periods 1-6. We have also studied the effect of step-wise methylation of the protophilic center of the second- and third-period bases.

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.

Thermochemical Studies of Pyrazolide†

The 351.1 nm photoelectron spectrum of 1-pyrazolide anion has been measured. The 1-pyrazolide ion is produced by hydroxide (HO(-)) deprotonation of pyrazole in a flowing afterglow ion source. The electron affinity (EA) of the 1-pyrazolyl radical has been determined to be 2.938 +/- 0.005 eV. The angular dependence of the photoelectrons indicates near-degeneracy of low-lying states of 1-pyrazolyl. The vibronic feature of the spectrum suggests significant nonadiabatic effects in these electronic states. The gas phase acidity of pyrazole has been determined using a flowing afterglow-selected ion flow tube; Delta(acid)G(298) = 346.4 +/- 0.3 kcal mol(-1) and Delta(acid)H(298) = 353.6 +/- 0.4 kcal mol(-1). The N-H bond dissociation energy (BDE) of pyrazole is derived to be D(0)(pyrazole, N-H) = 106.4 +/- 0.4 kcal mol(-1) from the EA and the acidity using a thermochemical cycle. In addition to 1-pyrazolide, the photoelectron spectrum demonstrates that HO(-) deprotonates pyrazole at the C5 position to generate a minor amount of 5-pyrazolide anion. The photoelectron spectrum of 5-pyrazolide has been successfully reproduced by a Franck-Condon (FC) simulation based on the optimized geometries and the normal modes obtained from B3LYP/6-311++G(d,p) electronic structure calculations. The EA of the 5-pyrazolyl radical is 2.104 +/- 0.005 eV. The spectrum exhibits an extensive vibrational progression for an in-plane CCN bending mode, which indicates a substantial difference in the CCN angle between the electronic ground states of 5-pyrazolide and 5-pyrazolyl. Fundamental vibrational frequencies of 890 +/- 15, 1110 +/- 35, and 1345 +/- 30 cm(-1) have been assigned for the in-plane CCN bending mode and two in-plane bond-stretching modes, respectively, of X (2)A' 5-pyrazolyl. The physical properties of the pyrazole system are compared to the isoelectronic systems, pyrrole and imidazole.

ION PAIR DISSOCIATION: Spectroscopy and Dynamics

Ion pair dissociation processes may be studied using coherent vacuum ultraviolet laser sources in a manner entirely analogous to photoelectron spectroscopy, albeit with the anion playing the role of a heavy electron. If the excitation energy is above the dissociation energy and the kinetic energy of the fragment is measured using ion imaging, this approach is termed ion pair imaging spectroscopy (IPIS) and is related to conventional photoelectron spectroscopy. If the excitation energy is just below the dissociation energy and pulsed-field dissociation is employed, this approach is analogous to mass analyzed threshold ionization (MATI) spectroscopy and is termed threshold ion pair production spectroscopy (TIPPS). These approaches provide a novel means of investigating ion thermochemistry and spectroscopy and superexcited state decay dynamics at high resolution.

Threshold ion-pair production spectroscopy of HCN

The spectroscopic technique of threshold ion-pair production spectroscopy (TIPPS) has been applied to the triatomic molecule HCN. We have recorded the total ion-pair yield and TIPP spectra for the HCN-->H(+) + CN(-) process using coherent vacuum ultraviolet excitation. From the simulation of our high-resolution TIPP spectrum we have precisely measured the HCN ion-pair threshold E(IP) (0) to be 122 244 +/- 4 cm(-1). This value could be used to determine the bond dissociation energy D(0)(H-CN) to unprecedented accuracy. Our fitting result also showed that rotationally excited instead of cold CN(-) fragment is favored as the ion-pair dissociation product in the threshold region.

Photoelectron Spectra and Ion Chemistry of Imidazolide†

The 351.1 nm photoelectron spectrum of imidazolide anion has been measured. The electron affinity (EA) of the imidazolyl radical is determined to be 2.613 +/- 0.006 eV. Vibrational frequencies of 955 +/- 15 and 1365 +/- 20 cm(-1) are observed in the spectrum of the (2)B1 ground state of the imidazolyl radical. The main features in the spectrum are well-reproduced by Franck-Condon simulation based on the optimized geometries and the normal modes obtained at the B3LYP/6-311++G(d,p) level of density functional theory. The two vibrational frequencies are assigned to totally symmetric modes with C-C and N-C stretching motions. Overtone peaks of an in-plane nontotally symmetric mode are observed in the spectrum and attributed to Fermi resonance. Also observed is the photoelectron spectrum of the anion formed by deprotonation of imidazole at the C5 position. The EA of the corresponding radical, 5-imidazolyl, is 1.992 +/- 0.010 eV. The gas phase acidity of imidazole has been determined using a flowing afterglow-selected ion tube; delta(acid)G298 = 342.6 +/- 0.4 and delta(acid)H298 = 349.7 +/- 0.5 kcal mol(-1). From the EA of imidazolyl radical and gas phase acidity of imidazole, the bond dissociation energy for the N-H bond in imidazole is determined to be 95.1 +/- 0.5 kcal mol(-1). These thermodynamic parameters for imidazole and imidazolyl radical are compared with those for pyrrole and pyrrolyl radical, and the effects of the additional N atom in the five-membered ring are discussed.

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.

Ultraviolet photodissociation dynamics of the SH radical

Ultraviolet (UV) photodissociation dynamics of jet-cooled SH radical (in X 2pi(3/2), nu"=0-2) is studied in the photolysis wavelength region of 216-232 nm using high-n Rydberg atom time-of-flight technique. In this wavelength region, anisotropy beta parameter of the H-atom product is approximately -1, and spin-orbit branching fractions of the S(3P(J)) product are close to S(3P2):S(3P1):S(3P0)=0.51:0.36:0.13. The UV photolysis of SH is via a direct dissociation and is initiated on the repulsive 2sigma- potential-energy curve in the Franck-Condon region after the perpendicular transition 2sigma(-)-X 2pi. The S(3P(J)) product fine-structure state distribution approaches that in the sudden limit dissociation on the single repulsive 2sigma- state, but it is also affected by the nonadiabatic couplings among the repulsive 4sigma-, 2sigma-, and 4pi states, which redistribute the photodissociation flux from the initially excited 2sigma- state to the 4sigma- and 4pi states. The bond dissociation energy D0(S-H)=29,245+/-25 cm(-1) is obtained.

Gas-Phase Interaction of H2S with O2: A Kinetic and Quantum Chemistry Study of the Potential Energy Surface

Quantum chemical calculations were carried out to study the interaction of hydrogen sulfide with molecular oxygen in the gas phase. The basic mechanism, the rates of reaction, and the potential energy surface were calculated. Isomers and transition states that connect the reactants with intermediates and products of reaction were identified using the G2 method and B3LYP/6-311+G(3df,2p) functional. Hydrogen abstraction to form HO2 + SH is the dominant product channel and proceeds through a loose transition state well-described at the level of calculation employed. The temperature dependence of the rate coefficient in the range 300-3000 K has been determined on the basis of the ab initio potential energy surface and with variational transition-state theory. The reaction is 169.5 kJ mol(-1) endothermic at 0 K with a rate constant given by 2.77 x 10(5) T(2.76) exp(-19 222/T) cm3 mol(-1) s(-1) and should proceed slowly under atmospheric thermal conditions, but it offers a route to the initiation of H2S combustion at relatively low temperatures.

Determination of the internal state distribution of the SD product from the S(1D)+D2 reaction

The S(1D)+D2-->SD+D reaction has been studied through a photolysis-probe experiment in a cell. S(1D) reagent was prepared by 193 nm photolysis of CS2, and the SD(X 2Pi) product was detected by laser fluorescence excitation. The nascent rotational/fine-structure state distribution of the SD(X 2Pi) product was determined. This reaction, previously studied theoretically and in a crossed molecular beam experiment, is known to proceed through formation and decay of a long-lived collision complex involving the deep well in the H2S ground electronic state. The determined SD rotational state distribution in the v=0 vibrational level was found to be approximately statistical, with a small preference for formation of the F1 (Omega=3/2) fine-structure manifold over F2 (Omega=1/2). The branching into the Lambda doublet levels was also investigated, and essentially equal populations of levels of A' and A" symmetry were found. The present results are compared with previous investigations of this reaction and the analogous O(1D)+D2 reaction.