A HeI photoelectron spectroscopy and theoretical study of 2,6-dichloropyrazine, 2,3-dichloropyrazine, 4,6-dichloropyrimidine and 3,6-dichloropyridazine

The electronic structures of C₄H₂Cl₂N₂ isomers have been studied by HeI photoelectron spectroscopy (PES) combined with the outer valence Green’s function (OVGF) calculations at 6-311++G(d,p) basis sets. The vertical first ionization potentials for 2,6-dichloropyrazine, 2,3-dichloropyrazine, 4,6-dichloropyrimidine and 3,6-dichloropyridazine are determined to be 9.93, 9.89, 10.45 and 10.07 eV, respectively, and the PE spectra were assigned based on molecular orbital analysis and by comparison with related compounds. The effects of chlorine substituent and different positions of nitrogen atoms in these C₄H₂Cl₂N₂ isomers have been carefully analyzed by comparing the C₄H₂Cl₂N₂ isomers with their matrixes (pyrazine, pyridazine, pyrimidine). The inductive effect of chlorine substituent leads to the stabilization of the nitrogen lone pair orbitals. The resonance effect predominates over the inductive effect, and the consequence is the destabilization of the π orbitals. Owing to the different positions, the effects of nitrogen atoms are different and lead to the different sequence of π orbitals.     

Gaseous nitryl azide N4O2: A joint theoretical and experimental study

Gaseous nitryl azide N₄O₂ is generated by the heterogeneous reaction of gaseous ClNO₂ with freshly prepared AgN₃ at −50 °C. The geometric and electronic structure of the molecule in the gas phase has been characterized by in situ photoelectron spectroscopy (PES) and quantum chemical calculations. The experimental first vertical ionization energy of N₄O₂ is 11.39 eV, corresponding to the ionization of an electron on the highest occupied molecular orbital (HOMO) {4a″(π_{nb(N4–N5–N6)})}⁻¹. An apparent vibrational spacing of 1600 ± 60 cm⁻¹ (ν_asO1N2O3) on the second band at 12.52 eV (π_{nb(O1–N2–O3)}) further confirms the preference of energetically stable chain structure in the gas phase. To complement the experimental results, the potential-energy surface of this structurally novel transient molecule is discussed. Both calculations and spectroscopic results suggest that the molecule adopts a trans-planar chain structure, and a five-membered ring decomposition pathway is more favorable.     

FC(O)SNCO的电子结构和光电离解离过程

由于拥有―C(O)S―和―NCO基团, FC(O)SNCO的分子和电子结构是非常有趣的. 利用FC(O)SCl和AgNCO制备了FC(O)SNCO,并利用HeI光电子能谱(PES)、光电离质谱(PIMS)以及理论计算研究了其分子和电子结构. 通过将实验、理论计算以及自然键轨道(NBO)分析结合起来, 获得了FC(O)SNCO的最稳定分子构型. 利用外壳层格林函数(OVGF)方法以及与相似化合物的比较, 对其光电子能谱进行了指认. 理论计算表明,对于中性分子最稳定的构型为syn-syn非平面构型, 而电离后的离子最稳定构型为syn-syn平面构型. 实验结果表明, 第一电离能来自于S的孤对电子轨道, 为10.33 eV. 第二至第六电离能分别为12.03、13.23、13.77、14.78、15.99 eV, 并对这些电离能进行了指认. 在光电离质谱中产生了六个质谱峰, 分别为SN⁺、FC(O)⁺、SNCO⁺、FC(O)SN⁺、C(O)SNCO⁺、FC(O)SNCO^+·, 其中FC(O)SNCO^+·的峰是最强峰. 结合HeI光电子能谱和理论计算, 对PIMS进行了分析,并研究了可能的电离和解离过程并对其进行了讨论.     

几种硫醚化合物的紫外光电子能谱及量子化学研究

报导了硫酸系列化合物DMS（二甲基硫酸），DpCPS（二对氯苯硫酸），DpTS（二对甲基苯硫醚）气相HcI紫外光电子能谱（UPs），其中DpCPS，DpTS的UPS谱为首次获得．对各体系利用MNDO方法进行了分子构型优化，对优化得到的优势构型实施RHF／6－31G量子化学计算，并利用计算结果对各个分子体系的UPS谱进行了指认，计算结果分析显示：a）S原子的孤对电厂在DpCPS和DpTS中起到阻碍形成遍及整个分子体系π轨道的阻断作用，故此不存在遍及整个分子体系的π轨道；b）通过对三体系第一电高能的对比分析表明，第一电高能所激发出的电子主要是受S原子的束缚；c）还得到另一个有意义的结论──各分子体系的第一电离能大小与HOMO中3Pz轨道所占成份成有很好的线性关系．表明体系的第一电离出的电子主要是受S原子的3Pz轨道束缚     

1,2,5-噻二唑衍生物电子结构的紫外光电子能谱研究

首次报导了１,２,５－噻二唑衍生物３－氯－１,２,５－噻二唑（Ａ）和３,４＝二氯－１,２,５－噻二唑（Ｂ）化合物的紫外光电子能谱（ＵＰＳ）,谱带的指认建筑在对谱带形状、相对强度、实验电离能（ＩＰＳ）的分析以对研究分子Ｇａｕｓｓｉａｎ ９４ＳＴ０－６Ｇ从头计算电离能（－ε１）。化合物Ｂ ＵＰＳ谱带的ＩＰＳ比相应的化合物Ａ的ＩＰＳ均你芝归结为Ｂ分子中两个取代Ｃ１原子上电子的拥挤效应。计算的Ｂ的 有量（     

B(OCH3)3电子结构的Hel紫外光电子能谱研究

人们知道，Hel紫外光电子能借（PES）提供研究分子轨道能量、能级次序、成键类型以及由光电子峰强度所反映的电离轨道特性等信息是其他手段没有的，因而PES技术已广泛地用于众多化合物分子电子结构的研究中．有机础化合物由于它们高的反应活性作为合成试剂而信受人们重视［‘－     

Electronic structure of H2CS3 and H2CS4: an experimental and theoretical study

The HeI photoelectron spectra of H2CS3 and H2CS4 in the gas phase have been obtained for the first time. A complete theoretical study involving the calculation of the ionization energies using orbital valence Green's functional (OVGF) and population analysis was performed. Calculations of cation-radical forms were carried out in order to interpret the main characters of the six highest occupied molecular orbitals (HOMOs). The first vertical ionization potentials are 8.74 and 8.56eV for H2CS3 and H2CS4, and attributed to {9b2(nS(C=S))}-1 and {8a"(3ppi*(S-S), nS)}-1, respectively. Meanwhile, the energy sequence of three types of sulfur 3p lone-pair have been discussed: 3ppi(S-S)*相似文献   

An electronic structure investigation of the BNO-BON-NBO system

The potential energy hypersurface of the ground triplet states of the BNO-BON-NBO system has been investigated using traditional ab initio electronic structure theory. The molecules studied have the molecular formula BON and include three linear and three angular species, and two transition states for the isomerization of an angular N-B-O to an angular B-O-N and a linear B-NO, respectively. All stationary points on the BNO-BON-NBO isomerization potential energy surface have been characterized employing UMP2, UMP4, and Gaussian-2 (G2) theory with the 6-311G(d), 6-311G(2d), and TZ2P basis sets. The isomerization for an angular N-BO to the linear B-NO has a lower energy barrier than that of the former to an angular B-ON. Energetics are presented with G2 energies. Two sets of resonance structures for both bent B-NO (boron nitrosyl) and B-ON (boron isonitrosyl) were proposed and the bonding in the two species was analyzed. For the purpose of comparison, the density functional theory based hybrid methods B3LYP/6-311G(d) and B3LYP/TZ2P have also been applied to both geometry optimization and single-point calculations. It is found that the B3LYP prediction of the nature of the linear B-O is contradictory to that made by all MPn(n = 2 and 4) calculations. The cause for this contradiction is discussed.     

Reinvestigation of the GaMn structure and theoretical studies of its electronic and magnetic properties

The crystal structure of the binary gallide compound GaMn is reinvestigated using X-ray diffraction. The structure is quite different from that proposed previously. Although GaMn is reported to crystallize with the Al₈Cr₅ structure type, space group R3m, we found that the centrosymmetric space group , with a=12.605(2) Å and c=8.0424(11) Å, was more accurate. Moreover, the atomic positions and the atomic displacement parameters, which are missing in the previous study, are now refined. Thereafter, band structure calculations have been performed using the TB-LMTO-ASA method to understand the electronic and magnetic properties of this compound. Analyses from the band structure, the density of states and the magnetic moments obtained using spin-polarized calculations show the stability of two different magnetic models relative to the nonmagnetic one.     

Ionization Energies and Dyson Orbials of Allyl Alcohol and Allyl Mercaptan Conformers

The conformers of allyl alcohol and allyl mercaptan were studied with B3LYP/aug-cc-pVTZ method. Their relative energies were calculated at MP3, MP4(SDQ), and CCSD(T) levels. The most stable conformers for these two molecules are Gauche-gauche' (Gg'). The theo-retical photoelectron spectra simulated with the calculated ionization energies demonstrate that there are at least four conformers in allyl alcohol and four conformers in allyl mercaptan in the gas-phase experiments. The Dyson orbitals of the highest occupied molecular orbital (HOMO) and the next HOMO (HOMO-1) of allyl mercaptan Gg' conformer show strongly mixing n_S and π_C=C characteristics, which may be due to the resonance and inductive effects between π_C=C and n_S in HOMO-1 and HOMO.     

Theoretical electronic structure of the molecule ScI

Theoretical investigation of the 18 lowest electronic states of the molecule ScI in the representation ^2S+1Λ^(±) has been performed via CASSCF and MRCI (single and double excitation with Davidson correction) calculations. To the best of our knowledge these calculated electronic states are the first ones from ab initio methods. Thirteen electronic states between 4,500 cm^?1 and 21,000 cm^?1 have been studied for the first time and have not yet been observed experimentally. The harmonic frequency ω_e, the internuclear distance R_e, the electronic transition energy with respect to the ground state T_e, and the rotational constant B_e have been calculated for the considered electronic states. By using the canonical functions approach the eigenvalues E_υ and the rotational constants B_υ have also been calculated for the six lowest‐lying electronic states. The comparison of these results with the theoretical and the experimental data available in the literature shows a good agreement. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Exocyclic push–pull conjugated compounds. Part 3. An experimental NMR and theoretical MO ab initio study of the structure, the electronic properties and barriers to rotation about the exocyclic partial double bond in 2-exo-methylene- and 2-cyanoimino-quinazolines and -benzodiazepines

The structure of a number of 2-exo-methylene substituted quinazolines and benzodiazepines, respectively, 1, 3a,b, 4 (X=–CN,–COOEt) and their 2-cyanoimino substituted analogues 2, 3c,d (X=–CN,–SO₂C₆H₄–Me(p) was completely assigned by the whole arsenal of 1D and 2D NMR spectroscopic methods. The E/Z isomerism at the exo-cyclic double bond was determined by both NMR spectroscopy and confirmed by ab initio quantum chemical calculations; the Z isomer is the preferred one, its amount proved dependent on steric hindrance. Due to the push–pull effect in this part of the molecules the restricted rotation about the partial C²,C¹¹ and C²,N¹¹ double bonds, could also be studied and the barrier to rotation measured by dynamic NMR spectroscopy. The free energies of activation of this dynamic process proved very similar along the compounds studied but being dependent on the polarity of the solvent. Quantum chemical calculations at the ab initio level were employed to prove the stereochemistry at the exo-cyclic partial double bonds of 1–4, to calculate the barriers to rotation but also to discuss in detail both the ground and the transition state of the latter dynamic process in order to better understand electronic, inter- and intramolecular effects on the barrier to rotation which could be determined experimentally. In the cyanoimino substituted compounds 2, 3c,d, the MO ab initio calculations evidence the isomer interconversion to be better described by the internal rotation process than by the lateral shift mechanism.     

Spectral-product methods for electronic structure calculations

Progress is reported in development, implementation, and application of a spectral method for ab initio studies of the electronic structure of matter. In this approach, antisymmetry restrictions are enforced subsequent to construction of the many-electron Hamiltonian matrix in a complete orthonormal spectral-product basis. Transformation to a permutation-symmetry representation obtained from the eigenstates of the aggregate electron antisymmetrizer is seen to enforce the requirements of the Pauli principle ex post facto, and to eliminate the unphysical (non-Pauli) states spanned by the product representation. Results identical with conventional use of prior antisymmetrization of configurational state functions are obtained in applications to many-electron atoms. The development provides certain advantages over conventional methods for polyatomic molecules, and, in particular, facilitates incorporation of fragment information in the form of Hermitian matrix representatives of atomic and diatomic operators which include the non-local effects of overall electron antisymmetry. An exact atomic-pair expression is obtained in this way for polyatomic Hamiltonian matrices which avoids the ambiguities of previously described semi-empirical fragment-based methods for electronic structure calculations. Illustrative applications to the well-known low-lying doublet states of the H₃ molecule in a minimal-basis-set demonstrate that the eigensurfaces of the antisymmetrizer can anticipate the structures of the more familiar energy surfaces, including the seams of intersection common in high-symmetry molecular geometries. The calculated H₃ energy surfaces are found to be in good agreement with corresponding valence-bond results which include all three-center terms, and are in general accord with accurate values obtained employing conventional high-level computational-chemistry procedures. By avoiding the repeated evaluations of the many-centered one- and two-electron integrals required in construction of polyatomic Hamiltonian matrices in the antisymmetric basis states commonly employed in conventional calculations, and by performing the required atomic and atomic-pair calculations once and for all, the spectral-product approach may provide an alternative potentially efficient ab initio formalism suitable for computational studies of adiabatic potential energy surfaces more generally. Contribution to the Mark S. Gordon 65th Birthday Festschrift Issue.     

IR, Raman and theoretical ab initio RHF study of aminoglutethimide — an anticancer drug

A comparative analysis of the IR and Raman spectra of aminoglutethimide (AG) dissolved in CCl₄, CHCl₃ and CH₃CN was performed. Most of the absorption bands were assigned to characteristic group vibrations with the use of the IR and Raman spectra of deuterated AG, glutethimide, N-methyl glutethimide and glutarimide. The AG samples very weakly interacting with the environment were studied with the use of the Ar matrix isolation IR spectra. For comparison, the IR and Raman spectra of the crystalline samples formed by hydrogen-bonded AG molecules were recorded. The spectra were analyzed also in terms of normal modes and the harmonic approximation with the use of the ab initio restricted Hartree-Fock theory. It was found that increasing the solute concentration in CCl₄ and CHCl₃ leads to formation of the autoassociates. In CH₃CN the solute–solvent AG–CH₃CN dimers occur. Possible structures of the associates were theoretically studied on the model systems: the centrosymmetric glutarimide dimer and the linear AG–CH₃CN dimer. By a comparison of the theoretical and experimental spectra we were able to identify several peaks originating from the solute–solvent interactions.     

Experimental and theoretical study of the atmospherically important O2-H2O complex

The theoretically predicted water-oxygen van der Waals adduct has been experimentally confirmed by vibrational characterization using matrix isolation spectroscopic studies at 10 K. Vibrational bands for asymmetric and symmetric OH-stretching for this adduct have been found at 3728 cm(-1) and 3639 cm(-1), respectively. Theoretical calculations performed with Gaussian 98 software at the MP2/6-311++G(2d,2p) level of theory support the alternative structure of the hydrated complex proposed by this study.     

Ionization and protonation of NaC3: a theoretical study

A theoretical study of the NaC₃⁺ and NaC₃H⁺ systems has been carried out. Predictions have been made for some of the molecular properties, which could help in their possible experimental detection. The predicted global minimum for NaC₃⁺ is the linear isomer 1s (¹Σ). The lowest-lying triplet state is a three-membered ring 3t (³B₂), lying about 27.1 kcal/mol higher in energy than the predicted global minimum at the G2(P) level. In the case of NaC₃H⁺, there are two isomers that lie close in energy: a linear species, 1d (²Π), and a three-membered ring, 4d (²A′). The most reliable levels of theory employed predict that 1d (²Π) is the global minimum, whereas 4d (²A′) is predicted to lie 5.3 kcal/mol higher in energy at the G2(P) level. In any case it seems that both structures could be accessible to experimental detection. Low ionization potential and high proton affinities are obtained for the most stable NaC₃ isomers. Therefore, if present in the interstellar medium, NaC₃ should be easily ionized and would react quite easily to give the protonated species.     

取代基对氮自由基稳定性影响的ab initio研究

用UHF／4-31G基组，全构型优化，研究了九个氮自由基NHR（R＝CH3，CF3，CCl3，CN，NH2，CHO，OH，COOH，F）的构型和稳定化能ΔE，它们的ΔE分别为25．24，－38．53，－20．59，21．46，19．96，58．82，73．69，31．75，63．85kJ•mol-1，表明，除CF3，CCl3以外，其余七个取代基对氮自由基起稳定化作用．     

Matrix-isolation FT-IR study and theoretical calculations of the vibrational, tautomeric and H-bonding properties of hypoxanthine

The neutral compound hypoxanthine is investigated using the technique of matrix-isolation FT-IR spectroscopy combined with density functional theory (DFT) and ab initio methods. Two theoretical methods (RHF and DFT/B3-LYP) are compared for vibrational frequency prediction, and four methods (RHF//RHF, MP2//RHF, DFT//DFT and MP2//DFT) for prediction of the relative energies of the tautomers and the interaction energies of the complexes. All the possible tautomeric forms have been considered theoretically, and the results indicate that two oxo forms (O17 and O19) and one hydroxy form (H9-r1) are the three most stable forms. The experimental FT-IR spectra are consistent with this prediction, and nearly all the characteristic spectral features of these forms have been identified in the spectrum. A theoretical study of the H-bonded complexes of these three tautomers with water is also performed. Several structures have been found for each form and the results demonstrate that the closed complexes with two H-bonds are the most stable systems due to the H-bond cooperative effect.