A quantum chemical study of the protonation of phenylphosphine and its halogenated derivatives

The protonation and methylation of phenylphosphine (C(6)H(5)PH(2)) and its mono-halogenated derivatives have been studied using ab initio quantum chemical calculations. Density functional theory (B3LYP) calculations using the 6-311++G(d,p) basis set consistently confirm that protonation of phenylphosphines takes place at the phosphorus atom; the C(4)-protonated phenylphosphine lying about 66 kJ mol(-1) above the P-protonated isomer. Similarly, methylation of phosphines consistently occurs at phosphorus. The proton and methyl cation affinities are estimated as follows: PA(phenylphosphine) = 863 +/- 10 kJ mol(-1) and MCA(phenylphosphine) = 515 -/+ 12 kJ mol(-1). Mono-halogen substitution appears to reduce the proton affinites by up to 20 kJ mol(-1). In this context, following P-protonation of either ameta- or a para-X-C(6)H(4)-PH(2), an elimination of the halogen X-atom under collisional activation (CA) conditions is expected to lead to a distonic radical cation, a low-energy isomer being 50 kJ mol(-1) above ionized phenylphosphine.     

Tautomerism of peroxyacetic acid derivatives: a quantum chemical study

The electronic and molecular structures and the relative stabilities of organic peracids X=C(R)-COOH and their cyclic tautomers, dioxiranes

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, with R = Me, CF₃; X = O, NH, were studied using the ab initio Hartree-Fock method and the density functional theory (B3LYP version) as well as at the MP2-MP4 Møller-Plesset levels of perturbation theory. Geometry optimization was performed by the UHF and B3LYP methods with the 6-31G** basis set and at the MP2/cc-pvtz level of theory. The acyclic form of the peracid is more stable than the cyclic dioxirane form irrespective of the nature of the substituent. The energy difference between these tautomers increases as the CF₃ and NH groups are replaced by Me and O, respectively. Parameters of the activation barrier to tautomeric conversion increase in parallel with enhancement of the electron-accepting properties of both substituents. The transition state of tautomeric interconversion, which is topologically similar to the acyclic structure of the carbonyl oxide derivative R(HX)C=O⁺-O^?, was found and characterized taking peroxyacetic acid as an example. The characteristic features of the transition state are an intramolecular “multicenter” H-bond and the charge distribution that is inconsistent with the canonical structure mentioned above. An appropriate reaction coordinate for the transformation of the quasi-tetrahedral dioxirane structure into a planar peroxyacetic acid structure is provided by the dihedral angle. Deprotonated anionic systems are characterized by much smaller differences between the relative stabilities of the open and closed forms of isomers and by much lower activation barriers to isomeric conversions.

     

二乙炔及其衍生物的聚合和光学性能的量子化学研究

主要报道利用半经验分子轨道方法PM3研究的五个二乙炔衍生物的结构, 并预计了其分子晶体的光学和聚合性能。这些化合物的几何构型参数以及由此形成的晶体的堆积参量均用能量梯度方法优化。同时, 从实验上研究了其中三个化合物的晶体在热和光环境下的聚合状况。理论预计和实验结果基本吻合。计算表明, 可聚合晶体应具有适当的堆积距d(～0.55nm)和倾斜角θ(～50°)。     

Tautomeric and conformational properties of acetoacetamide: electron diffraction and quantum chemical study

The tautomeric properties of acetoacetamide, CH3C(O)CH2C(O)NH2, have been investigated by gas electron diffraction (GED) and quantum chemical calculations (B3LYP and MP2 approximations with 6-31G(d,p) and 6-311++G(3df,pd) basis sets). GED results in a mixture of 63(7)% enol tautomer and 37(7)% diketo form at 74(5) degrees C. Only one enol form with the O-H bond adjacent to the methyl group (CH3C(OH)=CHC(O)NH2) and only one diketo conformer (with dihedral angles tau(O=C(CH3)-C-C) = 31.7(7.5) degrees and tau(O=C(NH2)-C(H2)-C(O)) = 130.9(4.5) degrees ) are present. The calculated tautomeric composition varies in a wide range depending on the quantum chemical method and basis set. Only the B3LYP method with small basis sets reproduces the experimental composition correctly.     

Tautomers of cytosine and their excited electronic states: a matrix isolation spectroscopic and quantum chemical study

We have measured the IR and UV spectra of cytosine in a low-temperature argon matrix. An attempt was made to determine the tautomeric ratios existing in the matrix, making use of the matrix-isolation IR spectrum and computed IR intensities of the tautomers in a least squares fitting procedure. The mole fractions are about 0.22 for oxo(-amino) form, 0.26 and 0.44 for the two rotamers, respectively, of the hydroxy(-amino) form and 0.08 for the (oxo-)imino tautomer. These ratios were then used to simulate the matrix-isolation UV spectrum as a composite of the individual spectra, the latter calculated ab initio at high levels of electron correlation theory. The agreement between simulated and experimental UV spectra seems satisfactory. This indicates that, in contrast to the solid state and solution spectra described up to now by the oxo(-amino) form alone, the reproduction of the matrix-isolation UV spectrum needs at least the hydroxy(-amino) and oxo(-amino) forms, and probably also the (oxo-)imino form.     

The sodium ion affinities of cytosine and its methylated derivatives

The sodium ion affinities of cytosine (Cyt), 5-methylcytosine (5MeCyt) and 1-methylcytosine (1MeCyt) have been determined by experimental and quantum chemical methods. Na(+)-bound heterodimers were produced carrying one cytosine or methylated cytosine ligand (designated as C) and one peptide or amino acid reference base (designated as Pep); the Pep molecules included the peptides GlyLeu, GlyPhe, SerGly, and PheGly, and the amino acid His. The dissociation kinetics of these C--Na(+)--Pep ions were determined by collisionally activated dissociation (CAD) and converted to relative and absolute Na(+) affinities via kinetic method approaches. Relative Na(+) affinities increase in the order (kJ/mol): GlyLeu (0) < Cyt (3) < GlyPhe (4) < SerGly (6) < 5MeCyt (8) < PheGly (11) < 1MeCyt (13) < His (17). Anchoring the relative values of the nucleobases to the absolute affinities of the reference bases leads to absolute Na(+) affinities of 214 +/- 8, 219 +/- 8, and 224 +/- 8 kJ/mol for Cyt, 5MeCyt, and 1MeCyt, respectively. Ab initio calculations were used to confirm these results. The computed affinities of Cyt (213 kJ/mol) and 1MeCyt (217 kJ/mol) are in very good agreement with the experiments. These values unambiguously correspond to Na(+) complexes with the keto form of cytosine and its methyl derivatives. Ab initio calculations on tautomerization mechanisms in the gas versus condensed phase are used to discuss why the sodiated keto isomers were formed in the present electrospray ionization (ESI) experiments, but the enol isomers in previous fast atom bombardment (FAB) experiments.     

Molecular structure of magnesium dibromide: an electron diffraction and quantum chemical study

The molecular structure of magnesium dibromide was investigated by high-level computational techniques and gas-phase electron diffraction. The vapor consisted of about 88% monomeric and 12% dimeric species at the electron diffraction experimental conditions at 1065 K. The geometrical parameters and vibrational characteristics of monomeric, dimeric, and trimeric magnesium dibromide species were determined by computations. Very high level computations with extended basis sets and relativistic pseudopotentials on bromine were needed to reach an agreement between computed and estimated experimental equilibrium geometries for the monomer. For both the dimer and the trimer, different geometrical arrangements were tested. Their ground-state structures have halogen bridges with four-membered ring geometries and D2h and D2d symmetry, respectively. Thermodynamic parameters have also been calculated.     

A possible contribution to the population of triplet states in electron impact measurements of total cross sections for triplet state excitation

It is proposed that low energy secondary electrons produced in medium energy electron impact experiments may play an important role in the excitation of triplet states even at low sample gas densities. A model calculation is carried out which shows that the population of the 4³S, S³P and 4³D triplet states in He from secondary electron excitation can be comparable to the population of these states by direct excitation at an incident electron energy of 1000 eV and sample gas pressures as low as 10^?3 torr. The model calculations show that the secondary electron mechanism becomes more important as the incident energy increases and that the produced populations are of a similar magnitude for the 3³P and 4³D states which in turn are about a factor of 4 larger than the population found for the 4³S state. The results indicate that the effect of secondary electron excitation in careful experimental measurements of electron impact total triplet state cross sections may have to be considered when incident electron excitation energies in the range of 1 keV or higher are used.     

The ground and excited state electron affinities of cytosine and trans-azobenzene

The electron capture detector, reduction potential, electron transfer and photon methods of determining electron affinities are compared. The adiabatic electron affinities are (in eV): t-azobenzene(O₂), 1.578(5); t-azobenzene, 1.378(5); cytosine, 1.043(5) from anion photoelectron spectra. The largest or ground state value for trans-azobenzene and an excited state electron affinity for cytosine, 0.70 eV are also determined by reduction potentials. Other excited state energies are (in eV): t-azobenzene, 0.328(5), 0.589(5), 0.690(5), 0.768(5), 0.954(5), 1.038(5), 1.150(5), 1.275(5) and cytosine, 0.089(5), 0.098(5), 0.198(5), 0.235(5). The cytosine values are consistent with electron transport and radiation damage and repair in DNA.     

A quantum chemical study of alkoxy derivatives of three-coordinate aluminum

DFT and HF methods using the PC GAMESS-Firefly program are employed to calculate the spatial and electronic structures of molecules: alkoxy derivatives of aluminum. By NBO and AIM methods the main characteristics of Al-O, C-O, and Al-X bonds in these molecules are determined. It is shown that Al and O atoms interact with each other as atoms with closed shells. The C-O bonds are close to covalent, whereas the Al-X bonds are the intermediate type bonds.     

Characteristic ortho-methoxycarboxylate fragmentation of yerrinquinone and its derivatives on electron impact

Mass Spectrometry of a natural product, yerrinquinone, isolated from fungal-infested Diospyros montana, and isomeric 1-tetralonecarboxylates has revealed fragmentation pathways characteristic of a methoxy group flanked by a carboalkoxy and a carbonyl group.     

Ab initio study of a biradical radiationless decay channel of the lowest excited electronic state of cytosine and its derivatives

A theoretical model for the ultrafast S1-->S0 internal conversion of cytosine is presented, in which a state switch from the initially prepared 1pipi* state to the out-of-plane deformed excited state of biradical character controls the rate of the S1(1pipi*) decay. This mechanism successfully accounts for the dramatically longer S1 lifetimes of 5-fluorocytosine and N-acetylcytosine relative to cytosine. The replacement of the C5 hydrogen atom by a methyl group is predicted to lead to a substantial, but not dramatic, increase in the S1 lifetime, also consistent with experiment. It is this ability to correctly predict the substituent effects that distinguishes the present model from the previously proposed mechanisms.     

A quantum chemical study of tricyclooctane and its congeners

A PBE0/6-311G(3d ₅ f ₇,p) quantum chemical method is used to determine the structural parameters of the molecules of sin- and anti-tricyclo[4.2.0.0^2.5]octane, [2.2.2]propellane, tricyclo[3.3.0.0^2.6]octane, prismanes (CH)_2n (n = 1–7), and dicubane C₁₂H₈. Bond lengths in anti-tricyclo[4.2.0.0^2.5]octane amount to 1.572 ?, The tetratomic ring in tricyclo[3.3.0.0^2.6]octane is a flattened tetrahedron with internuclear distances of 1.551? and 2.037 ?. The symmetry of C₈H₈ sin-tricyclo[4.2.0.0^2.5]octa-3,4,7,8-tetrayl moieties in prismanes and metal organic compounds (C₈H₈RhCl₂RhC₈H₈, C₈H₈RhCl₂RhC₇H₈, and C₈H₈PdCl₂) is higher than the symmetry of a free sin-tricyclo[4.2.0.0^2.5]octane molecule.     

Acetone in its lowest excited triplet state: an isotope dependent ODMR study

The lowest excited triplet state T₁ of acetone-h₆ and acetone-d₆ was investigated with a pulsed dye laser equipped Optical Detection of Magnetic Resonance spectrometer. The results obtained for this prototype aliphatic carbonyl compound are discussed with reference to a recent ab initio calculation on formaldehyde. The peculiarities of the acetone T₁ are largely determined by its distinct non-planar geometry: Our data indicate that the carbonyl group is rotated out of the plane defined by the three C-atoms, and the distortion angle is found to be appoximately 38°C, similar to the formaldehyde value. This is a central result of our investigation: All of the observed zero-field splitting parameters, ODMR linewidths, kinetic data, and the deuterium effects can be satisfactorily rationalized with this out-of-plane distortion.     

Polynuclear sandwich derivatives of [10]annulene: A quantum chemical study

Density-functional theory (DFT) calculations at B3LYP/6-311+G(df,p) and M06/6-311+G(df,p) levels of theory predict the stability of new polynuclear sandwich complexes [Cu₅(C₁₀H₁₀)₂]⁺ and Ni₅(C₁₀H₁₀)₂ based on flat [10]annulene cycles.     

高分子三重态敏化剂的研究IV: 高分子苯乙酮及其衍生物的光谱, 磷光量子产率及光敏化光稳定问题的研究

本工作合成和研究了高分子苯乙酮及其衍生物的光谱、磷光量子产率、磷光寿命以及光敏化、光稳定等问题。结量表明高分子光敏化剂和光稳定剂保持着它们原来相应小分子的光化学和光物理行为。高分子敏化剂由于T-T湮灭, 分子链中的光敏基团的存在一最佳值, 此时磷光量子产率最高, 敏化效率最好。但高分子光稳定剂则光稳定基团含量愈高光稳定能力愈强。     

Structural variations and electronic substituent effects in phenylcubane derivatives: a quantum chemical study

Electronic substituent effects in 4-substituted 1-phenylcubane derivatives, Ph–C₈H₆–X, have been investigated from the structural changes caused by the substituent X. The molecular structures of 34 derivatives with charged or dipolar substituents have been determined from quantum chemical calculations at the HF/6-31G* and B3LYP/6-311++G** levels of theory. Geometrical variations caused by substitution appear both in the cubane framework and in the benzene ring, but the two kinds of changes show no correlation. The rather small changes in the benzene ring geometry are caused by long-range polar effects (field effects), while the larger changes in the cubane cage are controlled primarily by electronegativity effects. A structural parameter measuring the long-range polar effect of the substituent, S _F^CUB, has been derived from the geometry of the phenyl group acting as a probe. This parameter correlates well with the calculated gas-phase acidities of 4-substituted cubane-1-carboxylic acids, HOOC–C₈H₆–X, and with other indicators of long-range polar effects obtained from bicyclo[2.2.2]octane derivatives. The correlations can further be improved by introducing a resonance parameter as an additional explanatory variable. This indicates that the electron delocalization resulting from hyperconjugative interactions between substituent and cage modifies the long-range polar effect of the substituent. Strong hyperconjugative interactions between some charged substituents and the cubane cage result in remarkable variations in the cage geometry, superimposed onto those ascribed to electronegativity effects.     

Morpholine-phthalocyanine (donor-acceptor) construct: photoinduced intramolecular electron transfer and triplet formation from its charge separation state

Silicon phthalocyanine (SiPc) with two axially attached morpholine (MP) units was prepared, and its photophysics was studied by laser flash photolysis, steady state and time-resolved fluorescence methods. Both the fluorescence efficiency and lifetime of SiPc moiety were remarkably quenched, because of the efficient intramolecular photoinduced electron transfer (PET) from morpholine donors to SiPc moiety. The generated charge separation state (CSS), SiPc(?-)-MP(?+), which was observed by transient absorption spectra, showed a lifetime of 4.8 ns. The triplet quantum yield of SiPc unit in the supra-molecule is unexpectedly high, and the predominant spectral signal in microsecond-scale is triplet-triplet (T(1)-T(n)) absorption. This high triplet yield is due to the charge recombination of CSS that generates T(1) in 32% efficiency: SiPc(?-)-MP(?+) → (3)SiPc-MP. The T(1) formation process occurred efficiently because the CSS SiPc(?-)-MP(?+) has a higher energy (1.65 eV) than that of the triplet state (3)SiPc-MP (1.0 eV). Emission from the CSS was also observed: SiPc(?-)-MP(?+) → SiPc-MP + hν'.     

Structural study of 8-azole derivatives of protoberberine alkaloids: experimental and quantum chemical approach

New derivatives of protoberberine alkaloids were prepared by nucleophilic addition of some azoles (differing in bulkiness) to the iminium functionality of the quaternary protoberberine alkaloids. Compounds were structurally characterized mainly by ¹H and ¹³C NMR spectroscopy, and the structure of 8-carbazol-1-yl-7,8-dihydroberberine was determined using single-crystal X-ray diffraction. Additionally, conformational behaviors of five derivatives varying in bulkiness of the azole moiety have been investigated by low temperature NMR spectroscopy and quantum chemical calculations. Ring current effects of pyrrole and carbazole moieties on selected ¹H NMR resonances have been characterized, visualized, and discussed.