Primary Dissociation Channels Research Articles

A CASSCF/MRCI trajectory surface hopping simulation of the photochemical dynamics and the gas phase ultrafast electron diffraction patterns of cyclobutanone.

We present the simulation of the photochemical dynamics of cyclobutanone induced by the excitation of the 3s Rydberg state. For this purpose, we apply the complete active space self-consistent field method together with the spin-orbit multireference configuration interaction singles treatment, combined with the trajectory surface hopping for the inclusion of nonadiabatic effects. The simulations were performed in the spin-adiabatic representation, including nine electronic states derived from three singlet and two triplet spin-diabatic states. Our simulations reproduce the two previously observed primary dissociation channels: the C2 pathway yielding C2H4 + CH2CO and the C3 pathway producing c-C3H6 + CO. In addition, two secondary products, CH2 + CO from the C2 pathway and C3H6 from the C3 pathway, both of them previously reported, are also observed in our simulation. We determine the ratio of the C3:C2 products to be about 2.8. Our findings show that most of the trajectories reach their electronic ground state within 200fs, with dissociation events finished after 300fs. We also identify the minimum energy conical intersections that are responsible for the relaxation and provide an analysis of the photochemical reaction mechanism based on multidimensional scaling. Furthermore, we demonstrate a minimal impact of triplet states on the photodissociation mechanism within the observed timescale. In order to provide a direct link to experiments, we simulate the gas phase ultrafast electron diffraction patterns and connect their features to the underlying structural dynamics.

Beyond vanilla: The dissociation mechanism of vanillin in four charge states

We compare the unimolecular decay mechanism of vanillin in four charge states. The unimolecular thermal decomposition of the neutral and dissociative ionization reactions in the cation have already been addressed in synchrotron-based photoionization studies. The picture is completed here by studying the collision-induced dissociation mechanism of protonated vanillin cations and deprotonated vanillin anions in the 5–30 eV collision energy range. The mass spectrometric observations are rationalized by ab initio calculations, which reveal the mechanism and the energetics of dissociation pathways. In the positive ion mode, the aldehyde oxygen is protonated, after which CO is lost. In contrast to previous results, CO loss is found to be the sole primary fragmentation pathway of protonated vanillin. Two hydrogen migration steps to the benzene ring open sequential CH3OH and CH3 loss channels to yield C6H5O+ and C6H6O2+, respectively. The former fragment ion can continue to lose CO, associated with ring contraction and producing the cyclopentadiene cation, C5H5+. In the negative ion mode, the proton is removed from the hydroxyl group. The primary dissociation channel of deprotonated vanillin corresponds to (C)OCH3 bond breaking in the methoxy group to form C7H4O3− at m/z 136. This species goes on to produce C6H4O2− and C6H4O− anions via CO and CO2 loss, respectively, in sequential dissociation processes at higher collision energies. We compare the protonated and deprotonated vanillin fragmentation pathways with those observed in the decomposition of neutral vanillin and with the dissociation pathways of the vanillin cation. The initial decomposition step for neutral, cationic, and deprotonated vanillin is methyl loss by direct CO bond breaking, often followed by loss of CO. In contrast, protonated vanillin first loses CO after isomerization.

Ethylcarbene versus Direct Propene Formation in the Near-UV Photodissociation of Ethylketene

The competing pathways in the photodissociation of gaseous ethylketene at excitation wavelengths of 320.0, 340.0, and 355.1 nm were studied using photofragment translational energy spectroscopy. The primary dissociation channel was C═C bond fission producing ethylcarbene (CH3CH2CH; also known as propylidene) and CO. Product translational energy distributions are consistent with theoretical predictions that ground state ethylcarbene lies ∼34 kJ/mol higher in energy than its isomer dimethylcarbene (CH3CCH3). A second dissociation channel involved direct formation of propene prior to or concurrent with CO elimination. The measured product branching ratios indicate that the effective potential energy barrier for the direct propene channel lies below the energetic threshold for ethylcarbene formation. A minor C-C bond fission channel was also observed, leading to CH3 + CH2CHCO products. Comparisons are made to the results of our recent studies of methylketene and dimethylketene photodissociation.

Dissociation channels of c-C4F8 to C2F4 in reactive plasma

Progress in computational methods and personal computing has made possible more accurate estimations for primary dissociation channels and energies. The main dissociation route is revealed to be via the 7E excited state with an energy of 12.23 eV, which is composed of transitions from the highest occupied molecular orbital with b1 symmetry to some degenerate unoccupied e molecular orbitals. The main contributing e orbitals consisted of antibonding combination of C2F4 π-bonding orbitals. This degenerate 7E state is lowered by non-adiabatic transitions through the conical interactions on the dissociating route to 2C2F4, so the energy is finally relaxed at the dissociative second lowest 1E excited state leading to 2C2F4 production. In the electron attachment process, the calculated results show that the F− ion is produced from the excited states of the D4h c-C4F8 − ion through conical interactions at the energies of 4.3 eV, 5.6 eV, and 5.0 eV, along the C–F dissociation route.

Ultraviolet and vacuum ultraviolet photo-processing of protonated benzonitrile (C6H5CNH+)

Context. The recent detection in pre-stellar sources of cyano-substituted and pure hydrocarbon cycles has emphasized the importance of aromatic chemistry in the earliest stages of star formation. Ultraviolet (UV) and vacuum-UV (VUV) radiation is ubiquitous in space and thus the photo-processing of small cyclic ions may open a window onto rich chemical networks and lead to the formation of larger aromatics in space. Aims. The aim is to investigate the fate of protonated benzonitrile species after UV and VUV photoexcitation and the subsequent potential impact on stellar and interstellar chemistry. Methods. Protonated benzonitrile was isolated in a linear ion trap prior to irradiation with UV and VUV radiation (4.5–13.6 eV) from the DESIRS beamline at synchrotron SOLEIL. The study was extended down to 3.5 eV using a cryogenic Paul ion trap coupled to an OPO laser at the PIIM laboratory. Photodissociation action spectra were obtained by monitoring the photofragment yields as a function of photon energy. Results. The UV/VUV photodissociation action spectra of protonated benzonitrile show structured bands from 3.8 to 9 eV. The primary dissociation channel of protonated benzonitrile corresponds to HCN/HNC loss and formation of the phenylium cation (C6H5+); whereas at high energies, a minor channel is observed that correlates with HC3N loss and formation of C4H5+. Conclusions. The UV and VUV photodestruction of protonated benzonitrile leads to the formation of a highly reactive cationic species, C6H5+, predicted to be an important precursor of larger aromatic molecules in space, such as polycyclic aromatic hydrocarbons. The inclusion of C6H5+ – a precursor of benzene and, by extension, of benzonitrile – as the result of formation via the photodissociation of protonated benzonitrile in current astrochemical models could improve the predicted abundance of benzonitrile, which is currently underestimated.

Electronic properties and primary dissociation channels of fluoromethane compounds

We investigated fluoromethane compounds (CF4, CF3, CHF3, CH2F2, and CH3F) to examine their electronic properties and primary dissociation channels by using computational chemistry. For the electron attachment process, it was very important to represent the observed negative mass spectrum as a function of electron energy that calculation by the MP2 method for the negative ion and subsequent calculation by the EOMCCSD method were used to more accurately estimate the ground and the first excited states of the negative ion. In the evaluation of dissociation channels through the excitation process, the structure change of the excited fragment (from pyramidal to planar) was taken into account. Evaluation of Jahn–Teller distortion for the highly symmetrical CHF3 and CH3F was also very important to estimate the fragmentation process through the excitation. The calculated results after these treatments gave satisfactory representation of the experimental values. Moreover some predictions of experimentally unknown values are proposed.

Thermochemistry and Initial Decomposition Pathways of Triazole Energetic Materials.

A thorough investigation of the initial decomposition pathways of triazoles and their nitro-substituted derivatives has been conducted using the MP2 method for optimization and DLPNO-CCSD(T) method for energy. Different initial thermolysis mechanisms are proposed for 1,2,4-triazole and 1,2,3-triazole, the two kinds of triazoles. The higher energy barrier of the primary decomposition path of 1,2,4-triazole (H-transfer path, ∼52 kcal/mol) compared with that of 1,2,3-triazole (ring-open path, ∼45 kcal/mol) shows that 1,2,4-triazole is more stable, consistent with experimental observations. For nitro-substituted triazoles, more dissociation channels associated with the nitro group have been obtained and found to be competitive with the primary decomposition paths of the triazole skeleton in some cases. Besides, the effect of the nitro group on the decomposition pattern of the triazole skeleton has been explored, and it has been found that the electron-withdrawing nitro group has an opposite effect on the primary dissociation channels of 1,2,4-triazole derivatives and 1,2,3-triazole derivatives.

Experimental and computational studies of monovalent metal cation-peptide interactions in [M·GlyGlyHis]+ (M = Li, Na, K, Rb, Cs, and Ag) complexes.

[M·GlyGlyHis]+ (M = Li, Na, K, Rb, Cs, and Ag) complexes were generated using the electrospray ionization method and were subjected to collision-induced dissociation. Metal ion loss is the primary dissociation channel for [Cs·GlyGlyHis]+ whereas other complexes yield metal-bound peptide sequence ions and dehydrated ions as the main products. [Li·GlyGlyHis]+ and [Ag·GlyGlyHis]+ also generate product ions that are not observed for other complexes. Density functional theory calculations suggest that metal ion-peptide ligand interaction occurs through covalent interactions in [Li·GlyGlyHis]+ and [Ag·GlyGlyHis]+, and through electrostatic attraction in [Na·GlyGlyHis]+, [K·GlyGlyHis]+, [Rb·GlyGlyHis]+, and [Cs·GlyGlyHis]+. The calculations also suggest that fragmentation behavior of these complexes is affected by charge transfer to the ligand and ion-ligand interaction energy, and to a lesser extent by the ion size.

A XMS-CASPT2 non-adiabatic dynamics study on pyrrole

The photoinduced dynamics of pyrrole is revisited employing the independent trajectory surface hopping methodology based on extended multi-state second order perturbation theory (XMS-CASPT2). Excitation at 200 nm into a high density of states region gives rise to ultrafast deactivation via internal conversion. The primary dissociation channel was found to be NH hydrogen abstraction, resulting in pyrrolyl radical formation. The associated time constant for hydrogen dissociation was determined to be 64 ± 13 fs, in good agreement with the experimental value of 52 ± 12 fs (Roberts et al., 2013). A total kinetic energy spectrum was also computed, that is in qualitative agreement with experiment. Radiationless decay via the ring-puckering crossing seam was identified as a minor deactivation channel.

Trajectory surface hopping study of propane photodissociation dynamics at 157 nm.

The photodissociation dynamics of propane molecules has been studied using the quasiclassical trajectory surface hopping (TSH) method in conjunction with Tully's fewest switches algorithm. The trajectories are propagated on potential energy surfaces computed on-the-fly using the multiconfiguration and multireference ab initio method starting in the lowest excited singlet state (HOMO → 3s Rydberg state) of propane at 157 nm with the emphasis on the site specificity of atomic hydrogen elimination, molecular hydrogen elimination, and their product branching ratios. Our dynamics simulation revealed that there are three primary dissociation channels: the atomic hydrogen elimination, the molecular hydrogen elimination, and the C-C bond scission. The trajectories indicate that the H2 elimination from the internal carbon atom (2,2-H2 elimination) and terminal carbon atom (1,1-H2 elimination) is the major process and follows a three centred synchronous concerted mechanism. 1,2-H2 and 1,3-H2 eliminations on the other hand are minor processes and exclusively follow the roaming mediated nonadiabatic dynamics. The probability of elimination of the hydrogen atom from two terminal groups (terminal hydrogen elimination) is greater than that from the internal CH2 group (internal hydrogen elimination). Almost 83% of atomic hydrogen elimination occurs through the asynchronous concerted mechanism from the terminal carbon atom via triple dissociation leading to CH3 + C2H4 + H products. This finding is in good agreement with a recent experimental observation. The present TSH study indicates that approximately one-third of the trajectories those resulted in a triple dissociation channel, CH3 + C2H4 + H completed in the ground singlet state following a nonadiabatic path (hopping from the first excited singlet S1 to the ground state S0) via the C-C and C-H dissociation coordinate conical intersection S1/S0. The products CH3(1 2A2″) + C2H4(1Ag) + H, obtained are ground state methyl radicals and ground state ethylene. The trajectories those ended in a triple dissociation channel CH3 + C2H4 + H adiabatically in the S1 state lead to CH3(1 2A2″) + C2H4 (1 3B1) + H, where singlet methyl radicals and triplet ethylene are formed in their corresponding lowest electronic state via a spin conserving route. Two channels, CH4 + CH3CH and C2H6 + CH2, are found to have minor contributions. In the case of methane elimination, the trajectories that follow an adiabatic path lead to CH3CH(1 1A″) + CH4,(1 1A1), where ethylidene is in the excited state and methane is in the ground state. Methane elimination via nonadiabatic path leads to CH3CH(11A') + CH4(1 1A1), where both ethylidene and methane are in the ground electronic state. Ethane eliminations follow the adiabatic path leading to C2H6(1 1A1g) + CH2(1 1B1) where ethane is in the ground state and methylene is in the first excited state.

The collision-free photochemistry of methyl azide at 157 nm: Mechanism and energy release.

Synchrotron radiation VUV-photoionization based photofragment translational spectroscopy was used to identify the primary and secondary photodissociation reactions of methyl azide (CH3N3) at 157 nm under collision-free conditions. Two primary dissociation channels are identified, leading to CH3 + N3 (the radical channel) and CH3N + N2 (the molecular elimination channel). The last channel is the major dissociation pathway, but unlike work at longer photolysis wavelengths, here, the radical channel exclusively produces the higher energy isomer cyclic-N3. Product time-of-flight data for both channels were obtained and compared with earlier work on methyl azide photochemistry at 193 nm based on electron impact ionization, allowing us to estimate a product branching ratio ΦCH3-N3 ΦCH3N-N2 =2.3%±0.6%97.7%±0.6%.

Cl2 Elimination in 248 nm Photolysis of (COCl)2 Probed with Cavity Ring-Down Absorption Spectroscopy.

Cavity ring-down absorption spectroscopy (CRDS) is employed to investigate one-photon dissociation of (COCl)2 at 248 nm obtaining a primary Cl2 elimination channel. A ratio of vibrational population is estimated to be 1:(0.12 ± 0.03):(0.011 ± 0.003) for the v = 0, 1, and 2 levels. The quantum yield of Cl2 molecular channel is obtained to be 0.8 ± 0.4 initiated from the X̃1Ag ground state surface (COCl)2 via internal conversion. The obtained total quantum yield is attributed to both primary ((COCl)2 + hν → 2CO + Cl2) and secondary reactions (dominated by Cl + COCl → Cl2 + CO). The former is estimated to share a yield of >0.14, while the latter contributes up to 0.66. The photodissociation pathway to the molecular products is calculated to proceed via a four-center transition state (TS) from which Cl2 is eliminated synchronously. Installation of the mirrors with reflectivity of 99.995% in the CRDS apparatus prolongs the ring-down time to 70 μs, thus allowing for the contribution from 17% up to 66% of the total Cl2 yield from secondary reaction depending on the reaction temperature. Despite uncertainty in determining the product yield, the primary Cl2 dissociation channel eliminated from (COCl)2 is observed for the first time.

Ultrafast dynamics of the lowest-lying neutral states in carbon dioxide

We present a study of the ultrafast dissociation dynamics of the lowest-lying electronic excited states in ${\mathrm{CO}}_{2}$ by using ultraviolet (UV) and extreme-ultraviolet (XUV) pulses from high-order harmonic generation. We observe two primary dissociation channels: a direct dissociation channel along the $^{1}{\mathrm{\ensuremath{\Pi}}}_{g}$ electronically excited manifold, and a second channel which results from the mixing of electronic states. The direct dissociation channel is found to have a lifetime which is shorter than our experimental resolution, whereas the second channel has a significantly longer lifetime of nearly 200 fs. In this long-lived channel we observe a beating of the vibrational populations with a period of $\ensuremath{\sim}133$ fs.

Computational study on SiH4 dissociation channels and H abstraction reactions

The primary dissociation channels of SiH4 were investigated using computational chemistry. The results showed properties very similar to those of CH4. The main dissociation product was SiH2 and the second dissociation product was SiH3. SiH was produced through SiH3 to SiH + H2 dissociation by electronic excitation. H abstraction reactions by H and SiH3 were also calculated for SiH4, Si2H6, Si3H8, and Si9H14(100) cluster models. The energy barriers of H abstraction reactions were lower than those of SiH3 abstraction reactions. This result is considerably important for deposition in SiH4/H2 process plasma.

Tracking the photodissociation dynamics of liquid nitromethane at 266 nm by femtosecond time-resolved broadband transient grating spectroscopy

Femtosecond time-resolved transient grating (TG) technique was employed to get insight into the photodissociation mechanism of liquid nitromethane (NM). Broadband white-light continuum was introduced as the probe to observe the evolution of electronic excited states of NM molecules and the formation of photodissociation products simultaneously. The reaction channel of liquid NM under 266nm excitation was obtained that NM molecules in excited state S2 relax through two channels: about 73% relax to low lying S1 state through S2/S1 internal conversion with a time constant of 0.24ps and then go back to the ground state through S1/S0 internal conversion; the other 27% will dissociate with a time constant of 2.56ps. NO2 was found to be one of the products from the experimental TG spectra, which confirmed that C–N bond rupture was the primary dissociation channel of liquid NM.

Collision-induced dissociation mass spectrometry of phosphorus cluster anions [formula omitted] (3 ≤ m ≤ 20)

Phosphorus cluster anions were generated and investigated using collision-induced dissociation mass spectrometry. Results show that the primary dissociation channels of P2m+1− (3≤m≤11) are generally characterized by the loss of a P2 unit, which distinguishes them from their counterpart cations. However, P11− and P13− are two exceptions. The former is characterized by the loss of a P4 unit, and the latter is characterized by two parallel dissociation channels, which lead to the loss of P2 and P6 units. The dissociation behaviors of larger cluster anions of P8k+1− (k=3–5) are more complex. The primary dissociation pathway of P25− is the loss of a P8 unit, and those of P33− and P41− are the loss of P16 and P2 units respectively. Theoretical calculations were performed for anions of P2m+1− (3≤m≤9). Results show that their dissociation channels characterized by the loss of the P2 unit are more thermodynamically favored, which is generally consistent with the experimental results.

Medium-sized phosphorus cluster cations P+ 2m+1 (6 ≤ m ≤ 32) studied by collision-induced dissociation mass spectrometry.

Medium-sized phosphorus cluster cations were generated by laser ablation of red phosphorus and investigated by the method of collision-induced dissociation mass spectrometry. Experimental results show that the primary dissociation channels of phosphorus cluster cations of P(+) 2m+1 (6 ≤ m ≤ 11) are all characterized by the loss of P4 unit. For larger cluster cations, their dissociation pathways were more complex. For those magic cations of P(+) 8k+1 observed previously, their dissociation pathways progressively change from the loss of P4 unit (for k = 3) to the loss of P8 unit (for k = 4, 5). A new dissociation pathway characterized by the loss of P10 unit was also indentified for larger cations of P(+) 8k+1 (6 ≤ k ≤ 8). Theoretical calculation also shows that, for cations of P(+) 2m+1 (4 ≤ m ≤ 10), the dissociation channel characterized by the loss of P4 unit is more energetically favorable than other dissociation channels, which is in good agreement with the experimental results.

Dimetal hydride generated from dimetalated glycine: Formation mechanism and electronic structure

Collision-induced dissociation (CID) of [MaMb(Gly-H)]+ precursors (where Ma=Ag, Cu; Mb=Ag, Cu) results in formation of novel dimetal hydrides, [MaHMb]+ and elimination of MaH in the gas phase. Deuterium labeling studies reveal that the hydride only originates from the CH2 group. Energy-resolved CID of [Ag2(Gly-H)]+ shows that the formation of [Ag2H]+ and the elimination of AgH are two primary dissociation channels, and the latter is always more prevalent than the former. H2 loss, only a minor channel, was also observed. This was confirmed by the correlation of the relative abundance of dimetal hydride with the difference of silver ion affinity between the neutral (Gly-2H) and AgH. Dimetal hydrides are of particular interest due to the three-center two-electron (3c–2e) bond. DFT calculations predict that [Cu2H]+ and [CuAgH]+ have bent structures, while [Ag2H]+ has linear structures. However, bending deformation energies are only 2.6kcal/mol for [Cu2H]+. Population analysis and atom in molecule (AIM) calculations show that in [Cu2H]+ the metal–metal bond is present, while for [Ag2H]+ there is no metal–metal bond. Energies from high-level theory calculations provide direct evidence to explain why the dimetal radical cation cannot be observed in the CID of the dimetal hydride.

Thermodynamics and Mechanism of Protonated Cysteine Decomposition: A Guided Ion Beam and Computational Study

A quantitative molecular description of the decomposition of protonated cysteine, H(+)Cys, is provided by studying the kinetic energy dependence of threshold collision-induced dissociation (CID) with Xe using a guided ion beam tandem mass spectrometer (GIBMS). Primary dissociation channels are deamidation (yielding both NH3 loss and NH4(+) formation) and (H2O + CO) loss reactions, followed by an additional six subsequent decompositions. Analysis of the kinetic energy-dependent CID cross sections provides the 0 K barriers for six different reactions after accounting for unimolecular decay rates, internal energy of reactant ions, multiple ion-molecule collisions, and competition among the decay channels. To identify the mechanisms associated with these reactions, quantum chemical calculations performed at the B3LYP/6-311 + G(d,p) level were used to locate the transition states (TSs) and intermediates for these processes. Single point energies of the reactants, products, and key optimized TSs and intermediates are calculated at B3LYP, B3P86, and MP2(full) levels using a 6-311 + G(2d,2p) basis set. The computational characterization of the elementary steps of these reactions, including the structures of the final products, is validated by quantitative agreement with the experimental energetics. In agreement with previous work, deamidation is facilitated by anchimeric assistance of the thio group, which also leads to an interesting rearrangement of the intact amino acid identified computationally.

The Effect of a Covalent and a Noncovalent Small-Molecule Inhibitor on the Structure of Abg β-Glucosidase in the Gas-Phase

The effects of binding two small-molecule inhibitors to Agrobacterium sp. strain ATCC 21400 (Abg) β-glucosidase on the conformations and stability of gas-phase ions of Abg have been investigated. Biotin-iminosugar conjugate (BIC) binds noncovalently to Abg while 2,4-dinitro-2-deoxy-2-fluoro-β-D-glucopyranoside (2FG-DNP) binds covalently with loss of DNP. In solution, Abg is a dimer. Mass spectra show predominantly dimer ions, provided care is taken to avoid dissociation of dimers in solution and dimer ions in the ion sampling interface. When excess inhibitor, either covalent or noncovalent, is added to solutions of Abg, mass spectra show peaks almost entirely from 2:2 inhibitor-enzyme dimer complexes. Tandem mass spectrometry experiments show similar dissociation channels for the apo-enzyme and 2FG-enzyme dimers. The +21 dimer produces +10 and +11 monomers. The internal energy required to dissociate the +21 2FG-enzyme to its monomers (767 ± 30 eV) is about 36 eV higher than that for the apo-enzyme dimer (731 ± 6 eV), reflecting the stabilization of the free enzyme dimer by the 2FG inhibitor. The primary dissociation channels for the noncovalent BIC-enzyme dimer are loss of neutral and charged BIC. The internal energy required to induce loss of BIC is 482 ± 8 eV, considerably less than that required to dissociate the dimers. For a given charge state, ions of the covalent and noncovalent complexes have about 15 % and 25 % lower cross sections, respectively, compared with the apo-enzyme. Thus, binding the inhibitors causes the gas-phase protein to adopt more compact conformations. Noncovalent binding surprisingly produces the greatest change in protein ion conformation, despite the weaker inhibitor binding. ᅟ