Assignment of the First Photoelectron Band of CH3CHBr(X2A) Using Ab-initio and Density Functional Theory (DFT) Computational Calculations

HeI photoelectron spectra have been recorded for the reaction of atomic fluorine with ethyl bromide at different reaction times. A structured band associated with a short-lived primary reaction product has been recorded at a mixing distance of 12 mm above the photon beam. The adiabatic and vertical ionization energies of this band was measured as 7.78 ± 0.01 and 8.05 ± 0.01 eV, respectively . The average vibrational separation of 700 ± 30 cm⁻¹ was observed in this band. Vertical ionization energies were computed in this work for CH₃CHBr(X²A) and CH₂CH₂Br(X²A) via ΔSCF, ΔMP2 (full) and Δ(B3LYP) levels of theory using different basis sets. Mulliken population analysis and force constant calculations have also been carried out for CH₃CHBr(X²A) and CH₂CH₂Br(X²A) and their singlet cationic states. Comparison between the experimental vertical ionization energies and the corresponding values computed for CH₃CHBr (X²A) and CH₂CH₂Br(X²A) at different levels of theory led to the assignment of the observed first photoelectron band to the ionization of CH₃CHBr(X²A). The observed vibrational structure was assigned to the excitation of C–Br stretching mode in CH₃CHBr⁺ (X¹A).     

A theoretical study of the photoelectron spectra of dichloroketene with accurate computation of ionization energies via complete basis set limit extrapolation

We calculated the equilibrium geometries and harmonic vibrational frequencies of the ground state and five cationic states of dichloroketene using (TD-)B3LYP, PBE0, and M06/M06-2X approaches. The photoelectron spectra of dichloroketene were simulated by computing Franck-Condon factors. The ionization energies were computed using the CCSD(T) approach with extrapolation to the complete basis set (CBS) limit. We propose two new CBS energy formulas (E = E_CBS + Aexp(-x) + B/(x−1) ⁿ, n = 2 or 3) and compare the performance of different CBS approaches. A new ionic state of dichloroketene belonging to the C_s point group is reported. This state is identified as the first excited state of Cl₂CCO⁺ having a double-well potential-energy curve along the CCO bending mode with a barrier height of 1.335 eV. The simulated photoelectron spectra are in agreement with the experiment. The vertical ionization energies calculated via spectral simulation are more accurate compared with those obtained at the ground-state structure. Among the CBS formulas used, the proposed ansatz with n = 2 performs best, with a mean absolute error of 0.021 and 0.012 eV for the adiabatic and vertical ionization energies, respectively.     

VUV photoionization of acetamide studied by electron/ion coincidence spectroscopy in the 8-24 eV photon energy range

A VUV photoionization study of acetamide was carried out over the 8-24 eV photon energy range using synchrotron radiation and photoelectron/photoion coincidence (PEPICO) spectroscopy. Threshold photoelectron photoion coincidence (TPEPICO) measurements were also made. Photoion yield curves and branching ratios were measured for the parent ion and six fragment ions. The adiabatic ionization energy of acetamide was determined as I.E. (1²A′) = (9.71 ± 0.02) eV, in agreement with an earlier reported photoionization mass spectrometry (PIMS) value. The adiabatic energy of the first excited state of the ion, 1²A″, was determined to be ≈10.1 eV. Assignments of the fragment ions and the pathways of their formation by dissociative photoionization were made. The neutral species lost in the principal dissociative photoionization processes are CH₃, NH₂, NH₃, CO, HCCO and NH₂CO. Heats of formation are derived for all ions detected and are compared with literature values. Some astrophysical implications of these results are discussed.     

Photoelectron spectroscopic study of simple hydrogen-bonded dimers. II. The methanol dimer

HeI photoelectron spectra of a supersonic jet of methanol vapor have been obtained by using the temperature-controlled supersonic nozzle beam photoelectron spectrometer recently constructed in our laboratory. A HeI spectrum attributable to the methanol dimer (CH₃OH)₂ has been deduced by spectrum stripping. The first ten vertical ionization energies and the first adiabatic ionization energy of (CH₃OH)₂ have been determined from the stripped spectrum. Ab initio SCF MO calculations of ionization energies have also been carried out for (CH₃OH)₂ on the basis of Koopmans' theorem. The lower bound of the dissociation energy of (CH₃OH)₂⁺ has been estimated to be 1.2 ± 0.2 eV from the adiabatic ionization energies of the monomer and dimer. The equilibrium structure of (CH₃OH)₂ is also discussed.     

Spectroscopy of the Simplest Criegee Intermediate CH2OO: Simulation of the First Bands in Its Electronic and Photoelectron Spectra

CH₂OO, the simplest Criegee intermediate, and ozone are isoelectronic. They both play very important roles in atmospheric chemistry. Whilst extensive experimental studies have been made on ozone, there were no direct gas‐phase studies on CH₂OO until very recently when its photoionization spectrum was recorded and kinetics studies were made of some reactions of CH₂OO with a number of molecules of atmospheric importance, using photoionization mass spectrometry to monitor CH₂OO. In order to encourage more direct studies on CH₂OO and other Criegee intermediates, the electronic and photoelectron spectra of CH₂OO have been simulated using high level electronic structure calculations and Franck–Condon factor calculations, and the results are presented here. Adiabatic and vertical excitation energies of CH₂OO were calculated with TDDFT, EOM‐CCSD, and CASSCF methods. Also, DFT, QCISD and CASSCF calculations were performed on neutral and low‐lying ionic states, with single energy calculations being carried out at higher levels to obtain more reliable ionization energies. The results show that the most intense band in the electronic spectrum of CH₂OO corresponds to the ${{\rm{\tilde B}}}$ ¹A′ ← ${{\rm{\tilde X}}}$ ¹A′ absorption. It is a broad band in the region 250–450 nm showing extensive structure in vibrational modes involving O–O stretching and C‐O‐O bending. Evidence is presented to show that the electronic absorption spectrum of CH₂OO has probably been recorded in earlier work, albeit at low resolution. We suggest that CH₂OO was prepared in this earlier work from the reaction of CH₂I with O₂ and that the assignment of the observed spectrum solely to CH₂IOO is incorrect. The low ionization energy region of the photoelectron spectrum of CH₂OO consists of two overlapping vibrationally structured bands corresponding to one‐electron ionizations from the highest two occupied molecular orbitals of the neutral molecule. In each case, the adiabatic component is the most intense and the adiabatic ionization energies of these bands are expected to be very close, at 9.971 and 9.974 eV at the highest level of theory used.     

线性BC2nB (n＝1～12)的结构特征和电子光谱的理论研究

应用密度泛函理论, 在B3LYP/6-31G^*水平上优化得到了线性簇合物BC_2nB (n＝1～12, D_(h)的平衡几何构型, 并计算了它们的谐振动频率. 在优化平衡几何构型下, 通过TD-B3LYP/cc-pvDZ和TD-B3LYP/cc-pvTZ计算, 分别得到了n＝1～12和n＝1～7的电子跃迁的垂直激发能和对应的振子强度. 在B3LYP/6-311＋G^*水平上计算得到了簇合物BC_2nB (n＝1～12, D_(h)的电离能. 基于计算结果, 导出了BC_2nB体系电子跃迁能以及第一电离能与体系大小n的解析表达式.     

Multiconfigurational study on the synchronous mechanisms of the ClO self-reaction leading to Cl or Cl<Subscript>2</Subscript>

For studying the adiabatic and nonadiabatic mechanisms of the ClO (X ²Π) + ClO (X ²Π) → ClOOCl → 2Cl (² P _u) + O₂ (X ³Σ _g ⁻) reaction (1) and the ClO (X ²Π) + ClO (X ²Π) → ClOOCl → Cl₂ (X ¹Σ _g ⁺) + O₂ (X ³Σ _g ⁻) reaction (2), we calculated, by partial geometry optimizations under the C₂ constraint, the O–O and O–Cl dissociation potential energy curves (PECs) from the five low-lying states of ClOOCl at the CASPT2 level. The CASSCF minimum-energy crossing point (MECP) between the potential energy surfaces of the 1 ¹A ground state [correlating with the product of reaction (1)] and the 1 ³B state [correlating with the product of reaction (2)] states was also determined. Based on the CAS calculation results (PECs, energies, and spin–orbit coupling at the MECP), we predict that reaction (1) occurs along pathway 1: ClO (X ²Π) + ClO (X ²Π) → ClOOCl (1 ¹A) → 2Cl (² P _u) + O₂ (X ³Σ _g ⁻) and that reaction (2) occurs along pathway 2: ClO (X ²Π) + ClO (X ²Π) → ClOOCl (1 ¹A) → 1 ¹A/1 ³B MECP (142.4 cm⁻¹) → ClOOCl (1 ³B) → Cl₂ (X ¹Σ _g ⁺) + O₂ (X ³Σ _g ⁻). The needed energies (relative to the reactant) for pathways 1 and 2 are predicted to be 5.3 and 11.1 kcal/mol, respectively, which indicates that reaction (1) is more favorable than reaction (2). The present work supports the traditional photochemical model for ozone degradation: ClOOCl (1 ¹A), formed by two ClO (X ²Π), can directly produce O₂ plus two Cl atoms.     

A study of the phenyl radical by vacuum ultraviolet photoelectron spectroscopy

Part of a photoelectron band assigned to the phenyl radical, produced from the F + C₆H₆ reaction, has been recorded. The adiabatic ionization energy was measured as 8.32±0.04 eV. Two vibrational components separated by 2790±100 cm⁻¹ were observed in the C₆H₅ band corresponding to excitation of a CH stretching mode in the ion. For C₆D₅ this separation decreases to 2370±110 cm⁻¹. Ab initio CI calculations on the low-lying C₆H⁺₅ states, ³B₁ and ¹A₁, show that state of C₆H⁺₅ is ¹A₁ and the first adiabatic ionization energy is estimated as 8.0±0.1 eV. Evidence is presented to show that the observed photoelectron band should be assigned to the ionization of C₆H⁺₅ (a ³B₁) ← C₆H₅(X²A₁). In the F + C₆D₆ reaction, a photoelectron band assigned to C₆D₆F is observed at 7.80±0.02 eV which is seen only very weakly in the F + C₆H₆ reaction.     

Theoretical computation of low‐lying electronic states of HCNS: A CASPT2 study

Complete active space self‐consistent field (CASSCF) and complete active space second‐order perturbation theory (CASPT2) calculations in conjunction with the aug‐cc‐pVTZ basis set have been used to investigate the low‐lying electronic states of thiofulminic acid (HCNS), HCNS⁺, and HCNS^?. The result of geometry optimization using CASPT2/aug‐cc‐pVTZ shows that theoretically determined geometric parameters and harmonic vibrational frequencies for the HCNS ground state X¹Σ⁺(X¹A′) are in agreement with previous studies. The ionization energies, the electron affinity energies, the adiabatic excitation energies, and vertical excitation energies have been calculated and the corresponding cation and anion states are identified. By calculating adiabatic electron affinity, the states of HCNS^? have been identified to contain both π orbital states (X²A′ and 1²A″) and dipole‐bond states (1⁴A′ and 1⁴A″). © 2012 Wiley Periodicals, Inc.     

Improving convergence of experimental and computed vertical ionization energies using the ionization potential equation-of-motion coupled-cluster with singles and doubles method

Comparison of statistically evaluated experimental vertical ionization energies (IEs) for 53 medium-sized molecules (6-34 atoms) with ionization potential equation-of-motion coupled-cluster with singles and doubles (IP-EOMCCSD) computations shows that discrepancies between computed and experimental results can be accounted for with a combination of experimental and theoretical contributions. Discrepancies can be minimized by extrapolating computations to the complete basis set limit and correcting for vibrational zero-point energy (ZPE) while comparing with experimental IEs calculated as the intensity-weighted mean band position to account for band asymmetries. This procedure reduced the average discrepancy for ethylene, (E)-2-butene, 2,5-dihydrofuran, and pyrrole from 0.25 to 0.05 eV. Agreement between reported vertical IEs and computations without either making adjustments as described in this paper or using complete simulation of the ionization spectrum should be considered fortuitous. The comparisons made in this work show that estimates of vertical and adiabatic IE made using IP-EOMCCSD extrapolated to the complete basis set limit and corrected for vibrational ZPE can be used with reasonable confidence when experimental values are not available.     

CH2＝CHCl与O(3P)反应的理论研究

用量子化学密度泛函理论和QCISD (Quadratic configuration interaction calculation)方法, 对O(³P)与CH₂CHCl的反应进行了理论研究. 在UB3LYP/6-311＋＋G(d,p), UB3LYP/6-31＋＋G(3df, 3pd)计算水平上, 优化了反应物、产物、中间体和过渡态的几何构型, 并在UQCISD(T)/6-311＋＋G(2df,2pd)水平上计算了单点能量. 为了确证过渡态的真实性, 在UB3LYP/6-311＋＋G(3df,3pd)水平上进行了内禀坐标(IRC)计算和频率分析, 并确定了反应机理. 研究结果表明, 反应主要产物为CH₂CHO和Cl.     

Difluorocarbene Studied with Threshold Photoelectron Spectroscopy (TPES): Measurement of the First Adiabatic Ionization Energy (AIE) of CF2

The first photoelectron band of difluorocarbene CF₂, has been studied by threshold photoelectron (TPE) spectroscopy. CF₂ was prepared by microwave discharge of a flowing mixture of hexafluoropropene, C₃F₆, and argon. A vibrationally resolved band was observed in which at least twenty‐two components were observed. In the first PE band of CF₂, the adiabatic ionization energy differs significantly from the vertical ionization energy because, for the ionization CF₂⁺ (X?²A₁)+e^? ← CF₂ (X?¹A₁), there is an increase in the FCF bond angle (by ≈20°) and a decrease in the C? F bond length (by ≈0.7 Å). The adiabatic component was not observed in the experimental TPE spectrum. However, on comparing this spectrum with an ab initio/Franck–Condon simulation of this band, using results from high‐level ab initio calculations, the structure associated with the vibrational components could be assigned. This led to alignment of the experimental TPE spectrum and the computed Franck–Condon envelope, and a determination of the first adiabatic ionization energy of CF₂ as (11.362±0.005) eV. From the assignment of the vibrational structure, values were obtained for the harmonic and fundamental frequencies of the symmetric stretching mode (ν₁′) and symmetric bending mode (ν₂′) in CF₂⁺ (X?²A₁).     

Heat capacities and absolute entropies of UTi<Subscript>2</Subscript>O<Subscript>6</Subscript> and CeTi<Subscript>2</Subscript>O<Subscript>6</Subscript>

Summary As part of a larger study of the physical properties of potential ceramic hosts for nuclear wastes, we report the molar heat capacity of brannerite (UTi₂O₆) and its cerium analog (CeTi₂O₆) from 10 to 400 K using an adiabatic calorimeter. At 298.15 K the standard molar heat capacities are (179.46±0.18) J K^-1 mol^-1 for UTi₂O₆ and (172.78±0.17) J K^-1 mol^-1 for CeTi₂O₆. Entropies were calculated from smooth fits of the experimental data and were found to be (175.56±0.35) J K^-1 mol^-1 and (171.63±0.34) J K^-1 mol^-1 for UTi₂O₆ and CeTi₂O₆, respectively. Using these entropies and enthalpy of formation data reported in the literature, Gibb’s free energies of formation from the elements and constituent oxides were calculated. Standard free energies of formation from the elements are (-2814.7±5.6) kJ mol^-1 for UTi₂O₆ and (-2786.3±5.6) kJ mol^-1 for CeTi₂O₆. The free energy of formation from the oxides at T=298.15 K are (-5.31±0.01) kJ mol^-1 and (15.88±0.03) kJ mol^-1 for UTi₂O₆ and CeTi₂O₆, respectively.     

Die Photoelektron-Spektren der Monohalogenacetylene

The photoelectron spectra of the four monohaloacetylenes X? C?C? H with X ? F, Cl, Br, I have been recorded. The first four bands of these spectra are assigned (in order of increasing ionization potentials) to the following states: band 1: ²Π_3/2(1), ²Π_1/2(1); band 2: ²Π_3/2(2), ²Π_1/2(2); band 3: ²Σ⁺(3); band 4: ²Σ⁺(4). A correlation diagram based on a simple ZDO-MO model shows that the observed band positions and the size of the splits due to spin-orbit coupling can be satisfactorily explained in terms of such a model. It is found that the orbital energies A_X of the postulated halogen n p λ(X)-basis orbitals are a linear function of the ionization potentials I(X) of the free atoms X. The validity of the ZDO-MO-model is confirmed by the excellent qualitative agreement between the observed and predicted spacings of the vibrational fine structure of the π-bands.     

间甲基苯甲醚分子旋转异构体的质量分辨阈值电离光谱

间甲基苯甲醚分子有顺式和反式两个转动异构体. 利用单光共振双光子电离技术和质量分辨阈值电离技术,研究了间甲基苯甲醚分子顺反异构体的基态到第一电子激发态（S₁←S₀）的跃迁和阈值电离. 得到顺式、反式间甲基苯甲醚分子S₁态的激发能（E₁）分别为（36049±2）和（36117±2）cm^-1,绝热电离能（I_p）分别为（64859±5）和（65110±5）cm^-1. 结合从头算法和密度泛函理论的量子化学计算,解释了顺式、反式间甲基苯甲醚分子E₁和I_p存在差异的原因,并且对S₁态和离子基态D0态出现的谱峰进行了标识. 间甲基苯甲醚分子顺反异构体在S₁态和D₀态的活性振动主要是甲基转动、面内环的运动和与取代基相关的弯曲振动. 间甲基苯甲醚分子的S₁态振动光谱、D₀态离子光谱以及理论计算均表明这两个转动异构体在D₀态的几何构型与S₁态的中性几何构型相比有较大改变,取代基与取代基、取代基与苯环间的相互作用强度高低次序为：S₀<S₁<D₀.     

A CASSCF/CASPT2 study on the low‐lying electronic states of the CH3SS and its cation

Complete active space self‐consistent field (CASSCF) and multiconfiguration second‐order perturbation theory (CASPT2) calculations with contracted ANO‐RCC basis set were performed for low‐lying electronic states of CH₃SS and its cation in C_s symmetry. For the ground state X²A″ of CH₃SS, the calculated S‐S stretching mode is in good agreement with experimental reports. The electron transitions of CH₃SS⁺, X¹A′ → 1¹A″, X¹A′ → 2¹A′, and X¹A′ → 2¹A″, are predicted at 1.055, 3.247, and 3.841 eV. Moreover, the calculated adiabatic and vertical ionization potential and adiabatic affinity are in reasonable agreement with the experimental data. The CASSCF/CASPT2 potential energy curves (PECs) were calculated for S₂‐loss dissociation from the X²A″, 1²A′, and 2²A″ states. The electronic states of the CH₃ radical and S₂ molecule as the dissociation products were carefully determined by checking energies and geometries of the asymptote products. The S₂‐loss PEC for CH₃SS indicate that S₂‐loss dissociation occurs from the X²A″ state leading to CH₃ (1²A″) + S₂ (X³Σ), the 1²A′ and 2²A″ leading to CH₃ (1²A″) + S₂ (¹Δ_g). © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012.     

DFT(B3LYP) calculations of silatranes and their radical cations. First ionization potentials

DFT(B3LYP) and 2 quantum chemical calculations have been performed for 1-substituted silatranes XSi(OCH₂CH₂)N (X = H, CH₃, CH₂Cl, F), their radical cations, and first ionization potentials (IP₁) of these silatranes. The calculated values of IP₁ agree well with the experiment and make it possible to assign the first band to IP₁ in the photoelectron spectra. Analysis of spin density distribution and electronic charges in the radical cations suggests that ionization occurs mainly due to the lone electron pair of nitrogen, participating in intramolecular coordination. The N → Si interaction is broken, and the N...Si distance increases to 335–340 pm.     

A study of the atmospherically relevant reaction between molecular chlorine and dimethylsulfide (DMS): Establishing the reaction intermediate and measurement of absolute photoionization cross-sections

In a study using UV photoelectron spectroscopy (PES) of the atmospherically relevant reaction

CH₃SCH₃ + Cl₂ → CH₃SCH₂Cl + HCl

Vertical ionization energies of halogen anions in solution

Based on the constrained equilibrium state theory, the nonequilibrium solvation energy is derived in the framework of the continuum model. The formula for spectral shift and vertical ionization energy are deduced for a single sphere cavity with the point charge assumption. The new model is adopted to investigate the vertical ionization for halogen atomic and molecular anions X^? (X = Cl, Br, I, Cl₂, Br₂, I₂) in aqueous solution. According to the calculation using the CCSD-t/aug-cc-pVQZ method in vacuum, our final estimated vertical ionization energies in solution are very close to the experimental observations, while the traditional nonequilibrium solvation theory overestimates these vertical ionization energies.     

Vacuum ultraviolet photoelectron spectroscopy of transient species. XVII. The SiH3(X 2A1) radical

The first band in the vacuum ultraviolet photoelectron spectrum of the silyl radical, corresponding to the process SiH₃ (X ¹A′₁) ← SiH₃(X ²A₁), has been observed with HeI radiation. Extensive vibrational fine structure associated with the SiH₃⁺ deformation vibration was observed in this band and analysis of the structure gave a value of ω = 820 = 40 cm^-1 for the out-of-plane deformation mode in the ion. The vertical and adiabatic ionization energies were measured as 8.74 = 0.01 eV and 8.14 ± 0.01 eV respectively and use of the latter value together with the established heat of formation of the silyl radical allows an improved heat of formation of SiH₃^-. ΔH₂₉₈⁰ (SiH₃^-) to be derived as 980 ± 7 kJ mol^?1.