Photofragmentation translational spectroscopy of methyl azide (CH3N3) photolysis at 193 nm: molecular and radical channel product branching ratio

We describe molecular-beam photofragment translational spectroscopy (PTS) experiments using electron impact (EI) ionization product detection to investigate the 193 nm photodissociation of methyl azide (CH(3)N(3)) under collision-free conditions. These experiments are used to derive the branching ratio between channels 1 and 2 [(1) radical channel: CH(3)N(3) + hν (λ = 193 nm) → CH(3) + N(3); (2) molecular channel: CH(3)N(3) + hν (λ = 193 nm) → CH(3)N + N(2)], which have been reported in a previous VUV-photoionization based PTS study. (1) Using electron impact ionization cross sections and ion fragmentation ratios for the various detected products, we derive the branching ratio (X(CH(3)-N(3)))/(X(CH(3)N-N(2))) = (0.017 ± 0.004)/(0.983 ± 0.004). Based on analysis of the kinetic energy release in the radical channel, we find that the cyclic form of N(3) is the dominant product in the radical channel. Only a small fraction of the radical channel produces ground state linear N(3).     

One-photon mass-analyzed threshold ionization spectroscopy of CH2BrI: extensive bending progression, reduced steric effect, and spin-orbit effect in the cation

One-photon mass-analyzed threshold ionization (MATI) spectrum of CH2BrI was obtained using coherent vacuum-ultraviolet radiation generated by four-wave difference-frequency mixing in Kr. Unlike CH2ClI investigated previously, a very extensive bending (Br-C-I) progression was observed. Vibrational frequencies of CH2BrI+ were measured from the spectra and the vibrational assignments were made by utilizing frequencies calculated by the density-functional-theory (DFT) method using relativistic effective core potentials with and without the spin-orbit terms. A noticeable spin-orbit effect on the vibrational frequencies was observed from the DFT calculations, even though its influence was not so dramatic as in CH2ClI+. A simple explanation based on the bonding characteristics of the molecular orbitals involved in the ionization is presented to account for the above differences between the MATI spectra of CH2BrI and CH2ClI. The 0-0 band of the CH2BrI spectrum could be identified through the use of combined data from calculations and experiments. The adiabatic ionization energy determined from the position of this band was 9.5944+/-0.0006 eV, which was significantly smaller than the vertical ionization energy reported previously.     

Photoionization of 1-alkenylperoxy and alkylperoxy radicals and a general rule for the stability of their cations

The photoionization of 1-alkenylperoxy radicals, which are peroxy radicals where the OO moiety is bonded to an sp2-hybridized carbon, is studied by experimental and computational methods and compared to the similar alkylperoxy systems. Quantum chemical calculations are presented for the ionization energy and cation stability of several alkenylperoxy radicals. Experimental measurements of 1-cyclopentenylperoxy (1-c-C5H7OO) and propargylperoxy (CH2=C=CHOO) photoionization are presented as examples. These radicals are produced by reaction of an excess of O2 with pulsed-photolytically produced alkenyl radicals. The kinetic behavior of the products confirms the formation of the alkenylperoxy radicals. Electronic structure calculations are employed to give structural parameters and energetics that are used in a Franck-Condon (FC) spectral simulation of the photoionization efficiency (PIE) curves. The calculations also serve to identify the isomeric species probed by the experiment. Adiabatic ionization energies (AIEs) of 1-c-C5H7OO (8.70 +/- 0.05 eV) and CH2=C=CHOO (9.32 +/- 0.05 eV) are derived from fits to the experimental PIE curves. From the fitted FC simulation superimposed on the experimental PIE curves, the splitting between the ground state singlet and excited triplet cation electronic states is also derived for 1-c-C5H7OO (0.76 +/- 0.05 eV) and CH2=C=CHOO (0.80 +/- 0.15 eV). The combination of the AIE(CH2=C=CHOO) and the propargyl heat of formation provides Delta f H(0)(o) (CH2=C=CHOO+) of (1162 +/- 8) kJ mol-1. From Delta f H(0)(o) (CH2=C=CHOO+) and Delta f H (0)(o) (C3H3+) it is also possible to extract the bond energy D(0)(o)(C3H3+-OO) of 19 kJ mol-1 (0.20 eV). Finally, from consideration of the relevant molecular orbitals, the ionization behavior of alkyl- and alkenylperoxy radicals can be generalized with a simple rule: Alkylperoxy radicals dissociatively ionize, with the exception of methylperoxy, whereas alkenylperoxy radicals have stable singlet ground electronic state cations.     

分子拓扑学方法估算多环芳香烃类化合物的电离能

It was found that the ionization potentials (Ip) is related with the polarizability effect index (PEI) for the fragments CH, CH2, and CH3 of polycyclic aromatic hydrocarbon. Therefore a kind of adjacent matrix of molecular graph was constructed, in which the characteristics of the diagonal elements were expressed with the PEI of the fragments C, CH, CH2, and CH3 in molecular graph. The research result shows that there is a good correlation between the eigenvalue of the matrix and the ionization potential for the title compounds: Ipi=4.756+2.870OMOi, R=0.9853, s=0.1765, n=446. This new calculation method has only one parameter for calculating ionization potentials of polycyclic aromatic hydrocarbon. The obtained result shows that the topologic molecular method is convenient and reliable.     

Trajectory surface hopping study of the O(3P) + ethylene reaction dynamics

Spin-orbit coupling (SOC) induced intersystem crossing (ISC) has long been believed to play a crucial role in determining the product distributions in the O(3P) + C2H4 reaction. In this paper, we present the first nonadiabatic dynamics study of the title reaction at two center-of-mass collision energies: 0.56 eV, which is barely above the H-atom abstraction barrier on the triplet surface, and 3.0 eV, which is in the hyperthermal regime. The calculations were performed using a quasiclassical trajectory surface hopping (TSH) method with the potential energy surface generated on the fly at the unrestricted B3LYP/6-31G(d,p) level of theory. To simplify our calculations, nonadiabatic transitions were only considered when the singlet surface intersects the triplet surface. At the crossing points, Landau-Zener transition probabilities were computed assuming a fixed spin-orbit coupling parameter, which was taken to be 70 cm-1 in most calculations. Comparison with a recent crossed molecular beam experiment at 0.56 eV collision energy shows qualitative agreement as to the primary product branching ratios, with the CH3 + CHO and H + CH2CHO channels accounting for over 70% of total product formation. However, our direct dynamics TSH calculations overestimate ISC so that the total triplet/singlet ratio is 25:75, compared to the observed 43:57. Smaller values of SOC reduce ISC, resulting in better agreement with the experimental product relative yields; we demonstrate that these smaller SOC values are close to being consistent with estimates based on CASSCF calculations. As the collision energy increases, ISC becomes much less important and at 3.0 eV, the triplet to singlet branching ratio is 71:29. As a result, the triplet products CH2 + CH2O, H + CH2CHO and OH + C2H3 dominate over the singlet products CH3 + CHO, H2 + CH2CO, etc.     

Dynamics of the O(3P) + C2H4 reaction: identification of five primary product channels (vinoxy, acetyl, methyl, methylene, and ketene) and branching ratios by the crossed molecular beam technique with soft electron ionization

The crossed molecular beam scattering technique with soft electron ionization (EI) is used to disentangle the complex dynamics of the polyatomic O(3P) + C2H4 reaction, which is of great relevance in combustion and atmospheric chemistry. Exploiting the newly developed capability of attaining universal product detection by using soft EI, at a collision energy of 54.0 kJ mol(-1), five different primary products have been identified, which correspond to the five exoergic competing channels leading to CH2CHO(vinoxy) + H, CH3CO(acetyl) + H, CH3(methyl) + HCO(formyl), CH2(methylene) + HCHO(formaldehyde), and CH2CO(ketene) + H2. From laboratory product angular and velocity distributions, center-of-mass product angular and translational energy distributions and the relative branching ratios for each channel have been obtained, affording an unprecedented characterization of this important reaction.     

Geometries and molecular properties of phosphaethyne cations

Franck-Condon analyses were carried out on vibrational intensity distributions of the first two photoelectron bands of phosphaethyne (HCP). The CH and CP bond lengths of HCP⁺ were found to be respectively 1.073 Å and 1.600 A in the X²π state and 1.077 Å and 1.572 Å in the A²Σ⁺ state. The calculated structural parameters were compared with those from experimental work and molecular orbital calculations. In addition, force constants for the first two lowest energy ionic states were determined from observed vibrational frequencies. Bonding properties of the ions are discussed in the light of the molecular parameters obtained.     

Threshold-photoelectron spectroscopic study of methyl-substituted hydrazine compounds

The valence shell electronic structures of methylhydrazine (CH(3)NHNH(2)), 1,1-dimethylhydrazine ((CH(3))(2)NNH(2)) and tetramethylhydrazine ((CH(3))(4)N(2)) have been studied by recording threshold and conventional (kinetic energy resolved) photoelectron spectra. Ab initio calculations have been performed on ammonia and the three methyl substituted hydrazines, with the structures being optimized at the B3-LYP/6-31+G(d) level of theory. The ionization energies of the valence molecular orbitals were calculated using the Green's function method, allowing the photoelectron bands to be assigned to specific molecular orbitals. The ground-state adiabatic and vertical ionization energies, as determined from the threshold photoelectron spectra, were IE(a) = 8.02 +/- 0.16 eV and IE(v) = 9.36 +/- 0.02 eV for methylhydrazine, IE(a) = 7.78 +/- 0.16 eV and IE(v) = 8.86 +/- 0.01 eV for 1,1-dimethylhydrazine and IE(a) = 7.26 +/- 0.16 eV and IE(v) = 8.38 +/- 0.01 eV for tetramethylhydrazine. Due to the large geometry change that occurs upon ionization, these IE(a) values are all higher than the true thresholds. New features have been observed in the inner valence region and these have been compared with similar structure in the spectrum of hydrazine. The effect of resonant autoionization on the threshold photoelectron yield is discussed. New heats of formation (Delta(f)H) are proposed for the three hydrazines on the basis of G3 calculations: 107, 94, and 95 kJ/mol for methylhydrazine, 1,1-dimethyhydrazine and tetramethylhydrazine, respectively. The previously reported Delta(f)H for tetramethylhydrazine is shown to be erroneous.     

Competing channels in the thermal decomposition of azidoacetone studied by pyrolysis in combination with molecular beam mass spectrometric techniques

The thermal decomposition of azidoacetone (CH3COCH2N3) was studied using a combined experimental and computational approach. Flash pyrolysis at a range of temperatures (296-1250 K) was used to induce thermal decomposition, and the resulting products were expanded into a molecular beam and subsequently analyzed using electron bombardment ionization coupled to a quadrupole mass spectrometer. The advantages of this technique are that the parent molecules spend a very short time in the pyrolysis zone (20-30 mus) and that the subsequent expansion permits the stabilization of thermal products that are not observable using conventional pyrolysis methods. A detailed analysis of the mass spectra as a function of pyrolysis temperature revealed the participation of five thermal decomposition channels. Ab initio calculations on the stable structures and transition states of the azidoacetone system in combination with an analysis of the dissociative ionization pattern of each channel allowed the identity and mechanism of each channel to be elucidated. At low temperatures (296-800 K) the azide decomposes principally by the loss of N2 to yield the imine (CH3COCHNH), which can further decompose to CH3CO and CHNH. At low and intermediate temperatures a process involving the loss of N2 to yield CH3CHO and HCN is also open. Finally, at high temperatures (800-1250 K) a channel in which the azide decomposes to a stable cyclic amine (CO(CH2)2NH) (after loss of N2) is active. The last channel involves subsequent thermal decomposition of this cyclic amine to ketene (H2CCO) and methanimine (H2CNH).     

Vacuum ultraviolet-infrared photo-induced Rydberg ionization spectroscopy: C-H stretching frequencies for trans-2-butene and trichloroethene cations

We have demonstrated the two-color vacuum ultraviolet (VUV)-infrared (IR) photoinduced Rydberg ionization (PIRI) experiment. Trichloroethene (ClCH=CCl2) and trans-2-butene (trans-CH3CH=CHCH3) were prepared in Rydberg states in the range of effective principal quantum number n* approximately 7-93 by VUV excitation prior to IR-induced autoionization. The observed VUV-IR-PIRI spectra are found to be independent of n*, suggesting that the electron Rydberg orbital is conserved, i.e., the Rydberg electron is behaving as a spectator during the excitation process. The observed IR active C-H stretching vibrational frequencies nu12+ = 3072+/-5 cm(-1) for ClCH=CCl2+ and nu23+ =2908+/-3 cm(-1), nu25+ =2990+/-10 cm(-1) and nu30+ =3022+/-10 cm(-1) for trans-CH3CH=CHCH3+ are compared with predictions based on ab initio quantum-chemical procedures and density functional calculations.     

Density functional study of molecular properties of hydrazoic acid and methyl azide

Density functional calculations for hydrazoic acid HN₃ and methyl azide CH₃N₃ and for the respective singly ionized structures HN⁺₃ and CH₃N⁺₃ are reported. An analysis of the electrostatic solvent effects, based on the self-consistent reaction field approach, on the molecular properties and conformational equilibrium of CH₃N₃ is also reported. The results are sensitive to the basis set quality and show some dependence on the different representations for the exchange-correlation functions. For HN₃ very good agreement with experiment is observed for several properties, such as the geometry, dipole moment, vibrational frequencies and for the adiabatic first ionization energy. For CH₃N₃ the energy difference between eclipsed (ec) and staggered (st) conformers (δ_ec-st) is 2.5 kJ mol⁻¹, in good agreement with the experimental value (2.9 kJ mol⁻¹). However, for CH₃N⁺₃, δ_ec-st is −3.2 kJ mol⁻, reflecting a significant modification of the methyl group rotational potential after ionization. Solvent effects on the molecular properties of CH₃N₃ are important when it is solvated in a polar medium. The most significant modifications concern the dipole moment and the frequencies related to the CH₃ symmetric stretch and torsion vibrational modes.     

Electronic spectroscopy of the deuterated isotopomers of the NO.methane molecular complex

The molecular complexes formed between a nitric oxide molecule and the various deuterated isotopomers of the methane molecule have been studied in a supersonic jet expansion. The electronic spectrum arising from the transition corresponding to a 3s<--pi* excitation (approximately A (2)Sigma(+)<-- approximately X (2)Pi) located on the NO chromophore has been recorded employing resonance-enhanced multiphoton ionization spectroscopy, with each of CH(4), CH(3)D, CH(2)D(2), CHD(3), and CD(4) as the complexing partner. Rich spectra are obtained, whose appearance changes in a systematic way as the amount of deuteration increases. Unexpectedly, it was possible to record spectra not only in the parent mass channel, but also in various fragment channels; this also led to the identification of some O atom resonances; and their origin is discussed. Discussion is presented of the structure in the spectra, and its possible sources including hindered internal rotation of the methane and NO moieties, overall rotation of the complex, and tunneling. In addition, some guidance has been gleaned from ab initio calculations, and these are discussed in the light of the experimental results.     

Intramolecular SN2 reaction caused by photoionization of benzene chloride-NH3 complex: direct ab initio molecular dynamics study

Ionization processes of chlorobenzene-ammonia 1:1 complex (PhCl-NH3) have been investigated by means of full dimensional direct ab initio molecular dynamics (MD) method, static ab initio calculations, and density functional theory (DFT) calculations. The static ab initio and DFT calculations of neutral PhCl-NH3 complex showed that one of the hydrogen atoms of NH3 orients toward a carbon atom in the para-position of PhCl. The dynamics calculation for ionization of PhCl-NH3 indicated that two reaction channels are competitive with each other as product channels: one is an intramolecular SN2 reaction expressed by a reaction scheme [PhCl-NH3]+-->SN2 intermediate complex-->PhNH3++Cl, and the other is ortho-NH3 addition complex (ortho complex) in which NH3 attacks the ortho-carbon of PhCl+ and the trajectory leads to a bound complex expressed by (PhCl-NH3)+. The mechanism of the ionization of PhCl-NH3 is discussed on the basis of the theoretical results.     

A vacuum ultraviolet photoionization mass spectrometric study of acetone

The photoionization and dissociative photoionization of acetone have been studied at the photon energy range of 8-20 eV. Photoionization efficiency spectra for ions CH3COCH3+, CH3+, C2H3+, C3H3+, C3H5+, CH(2-)CO+, CH3CO+, C3H4O+, and CH3COCH2+ have been measured. In addition, the energetics of the dissociative photoionization has been examined by ab initio Gaussian-3 (G3) calculations. The computational results are useful in establishing the dissociation channels near the ionization thresholds. With the help of G3 results, the dissociation channels for the formation of the fragment ions CH3CO+, CH2CO+, CH3+, C3H3+, and CH3COCH2+ have been established. The G3 results are in fair to excellent agreement with the experimental data.     

Development of a cooled He*(23S) beam source for measurements of state-resolved collision energy dependence of Penning ionization cross sections: Evidence for a stereospecific attractive well around methyl group in CH3CN

A low-temperature discharge nozzle source with a liquid-N(2) circulator for He*(2(3)S) metastable atoms has been developed in order to obtain the state-resolved collision energy dependence of Penning ionization cross sections in a low collision energy range from 20 to 80 meV. By controlling the discharge condition, we have made it possible to measure the collision energy dependence of partial ionization cross sections (CEDPICS) for a well-studied system of CH(3)CN+He*(2(3)S) in a wide energy range from 20 to 350 meV. The anisotropic interaction potential energy surface for the present system was obtained starting from an ab initio model potential via an optimization procedure based on classical trajectory calculations for the observed CEDPICS. A dominant attractive well depth was found to be 423 meV (ca. 10 kcal/mol) at a distance of 3.20 A from the center of mass of CH(3)CN in the N-atom side along the CCN axis. In addition, a weak attractive well (ca. 0.9 kcal/mol) surrounding the methyl group (-CH(3)) has been found and ascribed to the interaction between an unoccupied molecular orbital of CH(3)CN and 2s atomic orbital of He*(2(3)S).     

On the accuracy of molecular properties by coupled-cluster methods for some difficult examples: Oxygen atom,iron atom,and cyano radical

Coupled-cluster (CC ) methods at the level of CCSD , CCSD +T (CCSD ), CCSD (T ), CCSDT -1, and CCSDT -3 are applied to calculations of the dipole moment and polarizability of the CN molecule, ionization potentials and electron affinities of the oxygen and iron atoms and CN molecule, and the energy splitting of the ⁵D and ⁵F states of the iron atom. Both UHF and ROHF references are applied. Extended basis sets are used in some comparison of CC data to experiment. All calculated atomic and molecular properties are known as challenging problems, suitable for a careful analysis of the performance of sophisticated versions of the CC approach. Attention is paid to energy terms distinguishing CCSD (T ) from CCSD +T (CCSD ). We exploit results from various iterative and noniterative high-level CC methods in the assessment of error bars in calculations of atomic and molecular properties. © 1994 John Wiley & Sons, Inc.     

Electron ionization of methane: the dissociation of the methane monocation and dication

Time-of-flight mass spectrometry and two-dimensional coincidence techniques have been used to determine, for the first time, the relative precursor-specific partial ionization cross sections following electron-methane collisions. Precursor-specific partial ionization cross sections quantify the contribution of single, double, and higher levels of ionization to the partial ionization cross section for forming a specific ion (e.g. CH(+)) following electron ionization of methane. Cross sections are presented for the formation of H(+), H(2)(+), C(+), CH(+), CH(2)(+), and CH(3)(+), relative to CH(4)(+), at ionizing electron energies from 30 to 200 eV. We can also reduce our dataset to derive the relative partial ionization cross sections for the electron ionization of methane, for comparison with earlier measurements. These relative partial ionization cross sections are in good agreement with recent determinations. However, we find that there is significant disagreement between our partial ionization cross sections and those derived from earlier studies. Inspection of the values of our precursor-specific partial ionization cross sections shows that this disagreement is due to the inefficient collection of energetic fragment ions in the earlier work. Our coincidence experiments also show that the lower energy electronic states of CH(4)(2+) populated by electron double ionization of CH(4) at 55 eV are the same (ground (3)T(1), first excited (1)E(1)) as those populated by 40.8 eV photoionization. The (3)T(1) state dissociating to form CH(3)(+) + H(+) and CH(2)(+) + H(2)(+) and the (1)E(1) to form CH(2)(+) + H(+) and CH(+) + H(+). At this electron energy, we also observe population of the first excited triplet state of CH(4)(2+) ((3)T(2)) which dissociates to both CH(2)(+) + H(+) + H and CH(+) + H(+) + H(2).     

He I photoelectron spectroscopy and theoretical investigation on diaceto disulfide, CH3C(O)OSSOC(O)CH3

A novel species, diaceto disulfide (CH3C(O)OSSOC(O)CH3), has been generated through the heterogeneous reaction between sulfur monochloride (S2Cl2) and silver acetate (AgOC(O)CH3). Photoelectron spectroscopy (PES) and theoretical calculations are performed to investigate its electronic and geometric structures. This molecule exhibits gauche conformation with both C=O groups syn to the S-O bond. The dihedral angle around the S-S bond is calculated to be -93.1 degrees at the B3LYP/6-311++G(3df,3pd) level. After structural optimizations of the most stable conformer, a theoretical study involving the calculation of the ionization energies using orbital valence Green's functional (OVGF) was performed. The ionization energies of different bands in the photoelectron spectrum are in good agreement with the calculated values from the OVGF method. The first vertical ionization energy of CH3C(O)OSSOC(O)CH3 is determined to be 9.83 eV by photoelectron spectroscopy, which corresponds to the ionization of an electron mainly localized on the sulfur 3p lone pair molecular orbital.     

脂肪胺分子离子及质子化脂肪胺分子的碎裂反应

应用碰撞诱导解离(CID)技术研究了电子轰击方法产生的脂肪胺分子离子和化学电离方法产生的质子化脂肪胺分子的碎裂反应。质子化脂肪胺碰撞活化后的主要碎裂通道包括丢失C~XH~2~X、C~XH~2~X~+~1、C~XH~2~X~+~2单元及NH~3和生成[C~yH~2~y~+~1]^+及CH~3CH=NH~2^+离子。脂肪胺分子离子碰撞活化后的主要碎裂通道是丢失C~XH~2~X、C~XH~2~X~+~1及NH~3和生成[C~mH~2~m~-~1]^+、CH~2NH~2^+及CH~3CHNH~2^+离子。随着碰撞能的增加，远电荷碎裂反应和电荷诱导碎裂反应之间竞争引起产物离子的分布发生变化，如[C~mH~2~m~-~1]^+和[C~yH~2~y~+~1]^+离子。自由基机理可以解释质子化脂肪胺分子的远电荷反应。分子内氢抽取可以解释脂肪胺分子离子的碎裂反应。