Time-dependent self-consistent field approach to infrared laser multiphoton excitation

A classical version of the time-dependent self-consistent field (TDSCF) approach is presented for the non-perturbative treatment of intense field multiphoton excitation (MPE) and dissociation (MPD) of polyatomic molecules. The method is applied to the study of infrared MPE of SO₂ It is found that besides being much more computationally efficient, the TDSCF method reproduces the fully classical trajectory results quantitatively. Furthermore, the essential frequency- and intensity-dependent dynamical excitation phenomena predicted by the TDSCF calculations accord with both the experimental and the quantum-mechanical results.     

Kinetics of Surface-Induced Dissociation of N(CH3) 4 + and N(CD3) 4 + Using Silicon Nanoparticle Assisted Laser Desorption/Ionization and Laser Desorption/Ionization

The implementation of surface-induced dissociation (SID) to study the fast dissociation kinetics (sub-microsecond dissociation) of peptides in a MALDI TOF instrument has been reported previously. Silicon nanoparticle assisted laser desorption/ionization (SPALDI) now allows the study of small molecule dissociation kinetics for ions formed with low initial source internal energy and without MALDI matrix interference. The dissociation kinetics of N(CH₃)₄⁺ and N(CD₃)₄⁺ were chosen for investigation because the dissociation mechanisms of N(CH₃)₄⁺ have been studied extensively, providing well-characterized systems to investigate by collision with a surface. With changes in laboratory collision energy, changes in fragmentation timescale and dominant fragment ions were observed, verifying that these ions dissociate via unimolecular decay. At lower collision energies, methyl radical (CH₃) loss with a sub-microsecond dissociation rate is dominant, but consecutive H loss after CH₃ loss becomes dominant at higher collision energies. These observations are consistent with the known dissociation pathways. The dissociation rate of CH₃ loss from N(CH₃)₄⁺ formed by SPALDI and dissociated by an SID lab collision energy of 15 eV corresponds to log k = 8.1, a value achieved by laser desorption ionization (LDI) and SID at 5 eV. The results obtained with SPALDI SID and LDI SID confirm that (1) the dissociation follows unimolecular decay as predicted by RRKM calculations; (2) the SPALDI process deposits less initial energy than LDI, which has advantages for kinetics studies; and (3) fluorinated self-assembled monolayers convert about 18% of laboratory collision energy into internal energy. SID TOF experiments combined with SPALDI and peak shape analysis enable the measurement of dissociation rates for fast dissociation of small molecules.     

Effect of hydrogen gas impurities on the hydrogen dissociation on iron surface

A small addition of oxygen to hydrogen gas is known to mitigate the hydrogen embrittlement (HE) of steels. As atomic hydrogen dissolution in steels is responsible for embrittlement, catalysis of molecular hydrogen dissociation by the steel surface is an essential step in the embrittlement process. The most probable role of oxygen in mitigating HE is to inhibit the reactions between molecular hydrogen and the steel surface. To elucidate the mechanism of such surface reaction of hydrogen with the steel in the presence of oxygen, hydrogen, and oxygen adsorption, dissociation, and coadsorption on the Fe(100) surface were investigated using density functional theory. The results show that traces of O₂ would successfully compete with H₂ for surface adsorption sites due to the grater attractive force acting on the O₂ molecule compared to H₂. The H₂ dissociation would be hindered on iron surfaces with predissociated oxygen. Prompted by the notable results for H₂ + O₂, other practical systems were considered, that is, H₂ + CO and CH₄. Calculations were performed for the CO chemisorption and H₂ dissociation on iron surface with predissociated CO, as well as, CH₄ surface dissociation. The results indicate that CO inhibition of H₂ dissociation proceeds via similar mechanism to O₂ induced inhibition, whereas CH₄ traces in the H₂ gas have no effect on H₂ dissociation. © 2014 Wiley Periodicals, Inc.     

Infrared multiphoton and collision-induced dissociation studies of some gaseous alkylamine ions

Collision-induced dissociation and infrared multiphoton dissociation of ions formed in di- and tri-ethylamine, di- and tri-n-propylamine, and di-isopropylamine were investigated by Fourier-transform ion-cyclotron resonance mass spectrometry. Molecular ions of all amines except di-n-propylamine produced similar fragment ions when subjected to either dissociation technique. The initial fragmentation involved C_αC_β bond cleavage, loss of an alkyl radical, and formation of an immonium ions. Subsequent fragmentations of the immonium ions produced by both dissociation mechanisms involved McLafferty-type rearrangements and loss of alkenes. The molecular ion of di-n-propylamine fragmented by a different mechanism when subjected to infrared irradiation. Protonated molecules of di- and tri-n-propylamine yielded C₃H₆ and an ammonium ion upon infrared multiphoton dissociation, while protonated molecules of the other amines did not dissociate when this technique was applied. In contrast, collision-induced dissociation produced fragmentation for all protonated molecules. Explanation of the different fragmentations observed for the two dissociation techniques is given in terms of a mechanism involving a tight transition state for protonated di- and tri-n-propylamine dissociation.     

Laser-induced dissociation of phenetole ions prepared by resonance-enhanced two-photon,two-color ionization

Laser-induced dissociation of phenetole ions has been carried out. The ions have been prepared in a well-defined excited state by resonance-enhanced two-photon, two-color ionization (R2PI/2C). Appearance energies of 2.431 and 2.82 eV have been determined for the first dissociation pathways leading to C₆H₆O⁺ and C₅H⁶⁺ ionic fragment. The dissociation spectrum as well as detailed dissociation rate constants versus internal energy have been obtained. The dissociation rates are compared with those expected from RRKM theory. A complex reaction scheme has been assumed in order to explain the observed discrepancy.     

Effects of hydroxyl group on H2 dissociation on graphene: A density functional theory study

Graphene-based materials are promising for hydrogen production and storage. In this work, using density functional theory calculations, we explored how a hydroxyl group influences H₂ dissociation on graphene. Presence of the hydroxyl group makes the binding of H atom with graphene stronger, as the binding energy of H atom with the hydroxyl-modified graphene is higher than that with the pristine graphene. The para-site is the most favorable site for H₂ dissociation on both the pristine and hydroxyl-modified graphene. The energy barrier of H₂ dissociation at para-site on the pristine graphene is 3.10 eV whereas that on the hydroxyl-modified graphene is 2.46 eV, indicating a more facile H₂ dissociation on the hydroxyl-modified graphene.     

Study of nitrogen isotope-selective two-stage IR + UV dissociation of ammonia molecules

The one-photon IR excitation and subsequent UV dissociation of ammonia molecules selective with respect to nitrogen isotopes were studied. The selectivity of vibrational excitation is achieved by tuning CO₂ laser radiation to resonance with ¹⁴NH₃ or ¹⁵NH₃ molecules. The dependences of the yield of dissociation for each isotopic component and the selectivity on the buffer gas (N₂, O₂, Ar) pressure, the partial pressure of ammonia, and the time of delay between IR and UV laser pulses were established. At low pressures (67–270 Pa) of the isotopic mixture with a ¹⁵N concentration of 4.8%, the dissociation selectivity for ¹⁵N was 17. The mechanisms responsible for the selectivity of IR + UV-initiated dissociation are discussed. The phenomenological model has been developed that takes into consideration the effect of the interisotopic V-V exchange and V-T relaxation on the formation of the yield and selectivity of the two-stage IR+UV dissociation of ammonia.     

Infrared multiphoton dissociation of tetramethoxygermane

Infrared multiphoton dissociation of Ge(OCH₃)₄ molecules by irradiation with a pulsed TEA CO₂ laser has been studied. The basic characteristics of the process have been experimentally investigated. The spectral characterization of the dissociation has been made. The product composition of the dissociation has been analyzed, and possible dissociation pathways have been proposed.     

Anharmonic Effect of N‐Propyl Peroxy Dissociation

The anharmonic and harmonic dissociation rate constants of alkylperoxy (RO₂) in different pathways, as well as those for the reactions of the n‐propyl peroxy radical, were calculated using the Rice–Ramsperger–Kassel–Marcus (RRKM) theory. When the temperature/total energy increased, the rate constants of the different pathways varied independently, causing changes in the dominating/leading products. Anharmonic rate constants were larger than harmonic rate constants, and their difference increased with increasing temperature/energy. Therefore, the anharmonic effect cannot be neglected. The rate‐determining steps of CH₃CH₂CH₂OO dissociation are discussed. Then the anharmonic effect was found clearly for CH₃CH₂CH₂OO dissociation, especially for the hydroperoxyalkyl radical (QOOH) dissociation. At low temperature, the rate constants were similar to those found from experiment, which indicated the RRKM theory was suitable for calculating the dissociation rates of RO₂ species.     

Prompt and Slow Electron‐Detachment‐Dissociation/Electron‐Photodetachment‐Dissociation of a 21‐Mer Peptide

Electron detachment dissociation (EDD) and electron photodetachment dissociation (EPD) are relatively new dissociation methods that involve electron detachment followed by radical‐driven dissociation from multiply deprotonated species. EDD yields prompt dissociation whereas only electron detachment is obtained by EPD; subsequent vibrational activation of the charge‐reduced radical anion is required to obtain the product ions. Herein, the fragmentation patterns that were obtained by EDD and by vibrational activation of the charge‐reduced radical anions that were produced through EDD or EPD (activated‐EDD and activated‐EPD) were compared. The observed differences were related to the dissociation kinetics and/or the contribution of electron‐induced dissociation (EID). Time‐resolved double‐resonance experiments were performed to measure the dissociation rate constants of the EDD product ions. Differences in the formation kinetics were revealed between the classical EDD/EPD ′a^._i/′′x_j complementary ions and some ′a^._i/c_i/′′′z^._j product ions, which were produced with slower dissociation rate constants, owing to the presence of specific neighbouring side chains. A new fragmentation pathway is proposed for the formation of the slow‐kinetics ′a^._i ions.     

The three-body dissociation dynamics of Cl2O at 248 and 193 nm

The photodissociation of Cl₂O has been studied at 248 and 193 nm using photofragment translational spectroscopy (PTS) experiments with tunable VUV photoionization detection. The sole products observed were Cl, O and ClO fragments. Based on the derived translational energy distributions for the ClO and Cl photofragments we conclude that at 248 nm 15% of Cl₂O excitation results in three-body dissociation. At 193 nm no Cl₂ fragments are observed and we conclude that the oxygen atoms arise solely from three-body dissociation. Dissociation geometries derived from forward convolution fitting suggest two qualitatively distinct three-body dissociation pathways: asymmetric concerted dissociation and symmetric concerted dissociation in agreement with recent theoretical predictions.     

Infrared photolysis of C2F4S2: a comparison of multiphoton dissociation models

Infrared multiphoton dissociation of the planar ring compound

has been shown to yield two CF₂S molecules. The threshold for dissociation using the R(16) transition of the 9.6 μm CO₂ laser is 0.12 J/cm². At fluences above 1 J/cm³ dissociation is accompanied by the emission of light in the 300–750 nm region. The fractional dissociation per pulse and the average energy deposition have been measured as functions of laser fluence, laser wavelength, C₂F₄S₂ pressure, and added argon pressure. The variations of the fractional dissociation with fluence and absorbed energy are compared to the predictions of two models. The first model is based on the assumption of a Planck distribution for the dissociating C₂F₄S₂ molecules, while the second is based on a rate equation approach to the dissociation. Both models predict that the dissociating parent compound has a rather broad vibrational distribution.     

Metastable Atom-Activated Dissociation Mass Spectrometry of Phosphorylated and Sulfonated Peptides in Negative Ion Mode

The dissociation behavior of phosphorylated and sulfonated peptide anions was explored using metastable atom-activated dissociation mass spectrometry (MAD-MS) and collision-induced dissociation (CID). A beam of high kinetic energy helium (He) metastable atoms was exposed to isolated phosphorylated and sulfonated peptides in the 3– and 2– charge states. Unlike CID, where phosphate losses are dominant, the major dissociation channels observed using MAD were C_α – C peptide backbone cleavages and neutral losses of CO₂, H₂O, and [CO₂ + H₂O] from the charge reduced (oxidized) product ion, consistent with an electron detachment dissociation (EDD) mechanism such as Penning ionization. Regardless of charge state or modification, MAD provides ample backbone cleavages with little modification loss, which allows for unambiguous PTM site determination. The relative abundance of certain fragment ions in MAD is also demonstrated to be somewhat sensitive to the number and location of deprotonation sites, with backbone cleavage somewhat favored adjacent to deprotonated sites like aspartic acid residues. MAD provides a complementary dissociation technique to CID, ECD, ETD, and EDD for peptide sequencing and modification identification. MAD offers the unique ability to analyze highly acidic peptides that contain few to no basic amino acids in either negative or positive ion mode.     

Comparative theoretical study of the dissociation process of the isoelectronic molecules BH3CO,CH2CO,HNCO, CO2 and BH3N2, CH2N2, HN3, N2O

Anab initio study of the electronic structure of several 22-electrons molecules is presented. The equilibrium geometries of their ground state are calculated at the SCF level using the 6–31G basis set and are found to be in good agreement with the experimental geometries. The dissociation process of these molecules leading to the isoelectronic products CO or N₂ on the one hand and BH₃, CH₂, NH and O on the other hand is studied. The least-energy dissociation paths of the ground states determined at the SCF level are compared on the basis of electron density interactions. The dissociation energies corresponding to the two lowest dissociation channels are calculated. In these calculations, the correlation energy is taken into account using a non-variational method developed previously. The calculated values of dissociation energies are in good agreement with the existing experimental values. The results permit to predict values for HNCO, BH₃CO and CH₂N₂ and to confirm the instability of BH₃N₂.Aspirant du Fonds National Belge de la Recherche Scientifique.     

A theoretical study of the effects of transition metal dopants on the adsorption and dissociation of hydrogen on nickel clusters

The structure, stability, adsorption, and dissociation of H₂ on nickel clusters doped with late transition metals were investigated using density functional theory with the BP86 functional. Molecular hydrogen physisorption occurred at a vertex atom with a low coordination number. Charge transfer between clusters and the H₂ molecule stabilized the physisorption. The chemisorption of H₂ occurred at the bridge sites, without any structural or spin change of the clusters. Among the pentamer clusters, Cd, Zn, and Au had the lowest chemisorption energies, while Ir and Pt had higher chemisorption energies for hydrogen. The computed reaction energies and activation barriers for the dissociation mechanism showed that dopants such as Rh, Pd, Pt, and Au have endothermic reaction energies and low activation barriers. This facilitates the reversible adsorption/dissociation of the H₂ molecule on these metal‐doped clusters. The dopant atoms play a major role in modulating the physisorption, chemisorption, and dissociation mechanism of H₂ on nickel clusters. © 2013 Wiley Periodicals, Inc.     

Dissociative Adsorption of H2S on Li(110) Surface Using Density Functional Theory Calculations and Car-Parrinello Molecular Dynamics Simulations

The information concerning dissociative adsorption of H₂S on Li surface is inadequate and the mechanistic insight for its complete dissociation is yet to be explored. The present investigation aims to scrutinize the dissociative adsorption of H₂S on Li(110) surface using density functional theory calculations. The climbing image nudged elastic band calculation was employed to unveil the relative energy profiles for S−H dissociation. To elucidate the components of interaction energy responsible for stabilizing the adsorbed moieties on the surface, periodic energy decomposition analysis was performed. A Car-Parrinello molecular dynamics (CPMD) simulation was performed to understand the dynamic behaviour of H₂S on Li(110). Results vividly demonstrates: (i) partially dissociated product with perpendicular S−H is comparatively stable than the parallel SH, (ii) completely dissociated moieties H/H/S are the most stable among all, (iii) dissociation of first S−H is barrierless and the second S−H dissociation is a low energy barrier reaction, (iv) complete dissociation of H₂S occurs in a stepwise manner, (v) orbital and electrostatic contributions of the interaction energy plays a vital role in stabilizing the dissociated moieties, and (vi) stepwise dissociation of H₂S was further reinforced by CPMD.     

Measuring internal energy deposition in collisional activation using hydrated ion nanocalorimetry to obtain peptide dissociation energies and entropies

The internal energy deposited in both on- and off-resonance collisional activation in Fourier transform ion cyclotron resonance mass spectrometry is measured with ion nanocalorimetry and is used to obtain information about the dissociation energy and entropy of a protonated peptide. Activation of Na⁺(H₂O)₃₀ results in sequential loss of water molecules, and the internal energy of the activated ion can be obtained from the abundances of the product ions. Information about internal energy deposition in on-resonance collisional activation of protonated peptides is inferred from dissociation data obtained under identical conditions for hydrated ions that have similar m/z and degrees-of-freedom. From experimental internal energy deposition curves and Rice-Ramsperger-Kassel-Marcus (RRKM) theory, dissociation data as a function of collision energy for protonated leucine enkephalin, which has a comparable m/z and degrees-of-freedom as Na⁺(H₂O)₃₀, are modeled. The threshold dissociation energies and entropies are correlated for data acquired at a single time point, resulting in a relatively wide range of threshold dissociation energies (1.1 to 1.7 eV) that can fit these data. However, this range of values could be significantly reduced by fitting data acquired at different dissociation times. By measuring the internal energy of an activated ion, the number of fitting parameters necessary to obtain information about the dissociation parameters by modeling these data is reduced and could result in improved accuracy for such methods.     

Nb(110)表面氧原子覆盖度对氧分子解离的影响

用基于密度泛函理论的第一性原理方法研究了Nb(110)表面氧原子覆盖度分别为0.25、0.50、0.75 和1.00单层时对氧分子解离的影响. 结果表明, 在氧原子覆盖度不大于0.50单层时, 由于氧分子和表面铌原子的较强相互作用, 使它们能够自发解离. 然而在氧原子覆盖度为0.75单层时, 氧分子只能够在未占据的洞位附近解离, 同时发生严重的晶格畸变. 在形成一个氧原子单层后(1.00 单层), 氧分子只能弱吸附在Nb(100)表面上,此时氧原子向内扩散成为氧分子继续解离的速率决定步骤. 这些结果从理论上解释了在形成一个氧原子单层后, Nb(110)表面氧分子吸收速率迅速下降的原因.     

An extended-valence MC SCF procedure: determination of the dissociation energies of C2, N2, O2, and F2

A series of MC SCF calculations have been carried out on C₂, N₂, O₂, and F₂ with the goal of obtaining compact wavefunctions which recover a significant fraction of the electron correlation effects important for bond dissociation. The active orbital space is varied in size, with the largest spaces including the molecular orbitals derived from 2s, 2p, 3s, 3p and 4p atomic orbitals. Several basis sets ranging in size from 5s3p to 5s4p2d1f are investigated to determine the flexibility in the basis set needed with various choices of the active orbital space. The best extended-valence MC SCF (EVMC) dissociation energies are 0.2–0.5 eV less than the experimental values, indicating that further enlargement of the active orbital space is necessary to achieve 0.1 eV accuracy in the computed dissociation energies. The EVMC calculations reveal that, for the calculation of the dissociation energies, inclusion of non-valence orbitals is much more important for O₂ and F₂ than for C₂ and N₂. The EVMC results are compared with the predictions of full fourth-order perturbation theory, coupled cluster theory, and with the best available CI calculations.     

Structure, stability and dissociation of silanitriles RSiN (R = H2B, H2N, H2P)

Structure, stability, and dissociation of H₂BSiN, H₂NSiN, H₂PSiN and their isomers H₂BNSi, H₂NNSi, H₂PNSi have been studied in detail using ab initio MP2 and CCSD(T) methods. After dissociation of H₂BNSi, H₂NNSi, H₂PNSi and their isomers, the fragmented atoms have been considered to be either in their ground state or in their valence excited state in various dissociation channels. Only allowed dissociations of these molecules are considered. Various dissociation channels of H₂BNSi, H₂NNSi, H₂PNSi and their isomers have been explored and interesting trends are observed for the dissociation of stable isomers H₂BNSi, H₂NNSi, H₂PNSi and less stable isomers H₂BSiN, H₂NSiN, H₂PSiN. The effect of substituents on their structural properties has been discussed. The potential energy surfaces for the RSiN ? RNSi isomerization reactions have been analyzed. The structural properties of these molecules agree well with the theoretical values wherever available.