A microwave and quantum chemical study of allyltrifluorosilane

The structural and conformational properties of allytrifluorsilane, H₂CCH-CH₂-SiF₃, have been explored by microwave (MW) spectroscopy and high-level ab initio and density functional theory quantum chemical calculations. The microwave spectrum was investigated in the 18-62 GHz spectral regions. The a-type R-branch transitions of one conformer were assigned for the ground as well as for 10 vibrationally excited states. The CC-C-Si chain of atoms in this rotamer takes an anti-clinal (‘skew’) conformation, with a dihedral angle calculated to be 111.6° from the syn-periplanar (0°) conformation. The question whether a CC-C-Si syn-periplanar conformer exists as a high-energy form in the gas phase remains open. In most of the quantum chemical calculations this conformation is predicted to be a transition state. However, in the most advanced calculations (B3LYP/aug-cc-pVTZ level of theory) the syn-periplanar conformer is predicted to be a stable rotamer that is calculated to be 6.5 kJ/mol higher in energy than the anti-clinal form. Since there is no indication in the MW spectrum for the presence of high-energy form(s), it is concluded that the anti-clinal conformer is at least 4 kJ/mol more stable than any other hypothetical rotamer.     

A microwave and quantum chemical study of cyclopropanethiol

The microwave spectra of cyclopropanethiol, C(3)H(5)SH, and one deuterated species C(3)H(5)SD, have been investigated in the 20 - 80 GHz frequency range. The spectra of the ground vibrational state and of three vibrationally excited states of the parent species of a conformer which has a synclinal ("gauche") arrangement for the H-C-S-H chain of atoms, was assigned. The H-C-S-H dihedral angle is 76(5)° from synperiplanar (0°). The b-type transitions of the ground and of the vibrationally excited states of the parent species were split into two components, which is assumed to arise from tunneling of the proton of the thiol group between two equivalent synclinal potential wells. No splitting was resolved in the spectrum of C(3)H(5)SD. The tunneling frequency of the ground vibrational state of C(3)H(5)SH is 1.664(22) MHz. The tunneling frequency of the first excited-state of the C-S torsion is 52.330(44) MHz, whereas this frequency is 26.43(13) and 3.286(61) MHz, respectively, for the first excited states of the two lowest bending vibrations. The dipole moment of the ground vibrational state of the parent species is μ(a) = 4.09(5), μ(b) = 2.83(11), μ(c) = 0.89(32), and μ(tot) = 5.06(16) × 10(-30) C m. The microwave study has been augmented by high-level density functional and ab initio quantum chemical calculations.     

A microwave and quantum chemical study of the conformational properties and intramolecular hydrogen bonding of 1-fluorocyclopropanecarboxylic acid

The structural and conformational properties of 1-fluorocyclopropanecarboxylic acid have been explored by microwave spectroscopy and a series of ab initio (MP2/6-311++G(d,p) level), density functional theory (B3LYP/aug-cc-pVTZ level), and G3 quantum chemical calculations. Four "stable" conformers, denoted conformers I-IV, were found in the quantum chemical calculations, three of which (conformers I -III) were predicted to be low-energy forms. Conformer I was in all the quantum chemical calculations predicted to have the lowest energy, conformer III to have the second lowest energy, and conformer II to have the third lowest energy. Conformers II and III were calculated to have relatively large dipole moments, while conformer I was predicted to have a small dipole moment. The microwave spectrum was investigated in the 18-62 GHz spectral range. The microwave spectra of conformers II and III were assigned. Conformer I was not assigned presumably because its dipole moment is comparatively small. Conformer II is stabilized by an intramolecular hydrogen bond formed between the fluorine atom and the hydrogen atom of the carboxylic acid group. Conformer III has a synperiplanar orientation for the F-C-C=O and H-O-C=O chains of atoms. Its dipole moment is: mua = 3.4(10), mub = 10.1(13), and muc = 0.0 (assumed) and mu(tot) = 10.6(14) x 10(-30) C m [3.2(4) D]. Several vibrationally excited states of the lowest torsional mode of each of II and III were also assigned. The hydrogen-bonded conformer II was found to be 2.7(2) kJ/mol less stable than III by relative intensity measurements. Absolute intensity measurements were used to show that the unassigned conformer I is the most abundant form present at a concentration of roughly 65% at room temperature. Conformer I was estimated to be ca. 5.0 kJ/mol more stable than the hydrogen-bonded rotamer (conformer II) and ca. 2.3 kJ/mol more stable than conformer III. The best agreement with the theoretical calculations is found in the MP2 calculations, which predict conformer I to be 5.1 kJ/mol more stable than III and 1.7 kJ/mol more stable than II.     

A quantum chemical study on electrophilic addition

Non-empirical SCF-MO calculations were carried out on two limiting structures of C₂H₄F⁺, corresponding to the cyclic and open valence tautomers, both of which are possible reaction intermediates of the electrophilic addition reaction of F₂ to CH₂ =CH₂. It was found that both species had thermodynamic stability, corresponding to two distinct minima on the energy surface. However, the 2-fluoroethyl carbonium ion showed a greater stability than the fluoronium ion by about 10 kcal/mole.     

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.     

A quantum chemical study of an isogermacrone-type molecule

A quantum chemical study of the electronic factors affecting the photochemical behaviour of 13-nor-E,E-isogermacrone, a sesquiterpene-type ketone, has been carried out. The results indicate that the parallel endo double bond intramolecular cyclization to cyclobutanes proceeding through n, π* excited states is strongly favoured, the interaction between the crossed endo double bonds also being possible.     

A quantum chemical NMR study of branched and unbranched fluoropolymers

The work demonstrates the results of quantum chemical calculations of ¹⁹F and ¹³C NMR spectra of model fluorocarbon C _n F_2n+2 molecules with various configurations and hydrocarbon chain molecules. The possibilities to determine the chain length, formation of branches, identification of fluorine substitution for hydrogen during the fluorination of hydrocarbon paraffins and polymers are discussed.     

Conformational composition of cyclopentadienylphosphine investigated by microwave spectroscopy and quantum chemical calculations

The properties of cyclopentadienylphosphine have been investigated by means of Stark-modulation microwave spectroscopy and quantum chemical calculations at the MP2/aug-cc-pVTZ, B3LYP/6-311++G(d,p), and G3 levels of theory. Spectra attributable to two rotamers denoted conformers I and II have been assigned. Conformer I has a symmetry plane (Cs symmetry) consisting of the bisectors of the cyclopentadiene ring and of the phosphino group with the lone electron pair of phosphorus pointing toward the carbon ring. In conformer II, the phosphino group is rotated approximately 120 degrees out of this plane. Relative intensity measurements have been made, and it was found that conformer II is more stable than I by 1.3(4) kJ/mol. The preferred conformer represents a borderline case of intramolecular hydrogen bond stabilization. The experimental and MP2/ aug-cc-pVTZ rotational constants differ by several percent, which indicates that the aug-cc-pVTZ basis set is not large enough to be able to predict an accurate structure for the two conformers that are close to the equilibrium geometries. 5-Substituted 1,3-cyclopentadienyl derivatives may undergo circumambulatory rearrangements. However, there is no manifestation of this effect in the microwave spectrum of cyclopentadienylphosphine.     

Electronic structure and reactivity of guanylthiourea: A quantum chemical study

Electronic structure analysis of guanylthiourea (GTU) and its isomers has been carried out using quantum chemical methods. Two major tautomeric classes (thione and thiol) have been identified on the potential energy (PE) surface. In both the cases conjugation of pi‐electrons and intramolecular H‐bonds have been found to play a stabilizing role. Various isomers of GTU on its PE surface have been analyzed in two different groups (thione and thiol). The interconversion from the most stable thione conformer ( GTU‐1 ) to the most stable thiol conformer ( GTU‐t1 ) was found to take place via bimolecular process which involves protonation at sulfur atom of GTU‐1 followed by subsequent C? N bond rotation and deprotonation. The detailed analysis of the protonation has been carried out in gas phase and aqueous phase (using CPMC model). Sulfur atom (S1) was found to be the preferred protonation site (over N4) in GTU‐1 in gas phase whereas N4 was found to be the preferred site of protonation in aqueous medium. The mechanism of S‐alkylation reaction in GTU has also been studied. The formation of alkylated analogs of thiol isomers (alkylated guanylthiourea) is believed to take place via bimolecular process which involves alkyl cation attack at S atom followed by C? N bond rotation and deprotonation. The reactive intermediate RS(NH₂)C? N? C(NH₂)₂⁺ belongs to the newly identified ^⊕N(←L)₂ class of species and provides the necessary dynamism for easy conversion of thione to thiol. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

A comparative quantum chemical study of ethylcarbonium ion and hydroxymethylcarbonium ion

Nonempirical molecular orbital calculations of the energies of CH₃CH (ethylcarbonium ion) and HOCH (hydroxymethylcarbonium ion) as a function of rotation about the C? C or C? O bonds and deviation from coplanarity at the carbonium ion center are reported. As expected, and in agreement with previous work, both carbonium centers are planar and there is no barrier to rotation in the planar ethylcarbonium ion. However, for the planar configuration at carbon, the conjugative interaction between oxygen and carbon produces a barrier to rotation about the C? O bond of HOCH of 19.6 Kcal/mole. When a pyramidal geometry is imposed upon the carbonium ion center of CH₃CH, a typical three-fold barrier results. As the deviation from coplanarity increases there is a regular increase in the barrier height (1.72 Kcal/mole at the tetrahedral geometry), but the energy minimum remains at the same position in each case (60°). For HOCH, imposition of a pyramidal geometry on the carbonium ion center causes a change in both rotational barriers. One decreases slightly (from 19.6 to 15.4 Kcal/mole) and the other increases to 30.5 Kcal/mole. There is an accompanying change in the position of the minimum of the rotational potential, from 90° towards the gauche structure.     

A quantum chemical study of the decomposition of Keggin-structured heteropolyacids

Heterpolyacids (HPAs) demonstrate catalytic activity for oxidative and acid-catalyzed hydrocarbon conversion processes. Deactivation and thermal instability, however, have prevented their widespread use. Herein, ab initio density functional theory is used to study the thermal decomposition of the Keggin molecular HPA structure through the desorption of constitutional water molecules. The overall reaction energy and activation barrier are computed for the overall reaction HnXM12O40-->Hn-2XM12O39+H2O. and subsequently used to predict the effect of HPA composition on thermal stability. For example, the desorption of a constitutional water molecule is found to be increasingly endothermic in the order silicomolybdic acid (H4SiMo12O40)相似文献   

A quantum chemical study of the mechanism of manganese catalase

The catalytic mechanism of manganese catalase has been studied using the Becke's three parameter hybrid method with the Lee, Yang and Parr correlation functional. On the basis of available experimental information on the geometric and electronic structure of the active manganese dimer complex, different possibilities were investigated. The mechanism finally suggested consists of eight steps. In the first steps, the first hydrogen peroxide becomes bound and its O–O bond is activated. This occurs in a spin-forbidden process found to be common in many biological processes where the O–O bond is cleaved, and two general rules are formulated for the requirements for a low activation energy in this type of reaction. As the O–O bond is cleaved a hydroxyl radical is initially formed in the overall rate-limiting step of the catalytic cycle. This radical is then immediately and irreversibly quenched in a strongly exothermic step. In the subsequent steps, the second hydrogen peroxide becomes bound and its two O–H bonds are broken, leading to the formation of an O₂ molecule, which is released. Parallels between the reversal of the present O–O cleavage mechanism in manganese catalase and the recently suggested O–O bond formation in photosystem II are drawn. Received: 12 July 2000 / Accepted: 12 July 2000 / Published online: 21 December 2000     

A quantum chemical study of three isomers of N20

Ab initio molecular orbital (MP2) and density functional theory (B3LYP) calculations using different basis sets have been employed to study the structures, energetics and vibrational frequencies of the large homonuclear polynitrogen compound, N₂₀. In the present study, three distinct forms were found to represent local minima on the potential energy surface. They are the fullerene-type cage form of I_h symmetry, a corannulene-like bowl form of C_5v symmetry, and a ring isomer with D₅ symmetry of which the cage form turns out to be the highest energy form. Both the bowl and ring forms are calculated to be more stable than the cage form by about 200 kcal/mol. The molecular properties calculated for these isomers may serve as valuable predictions for future experimental searches for new high energy density materials (HEDM).     

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.     

Structural studies on ethyl isovalerate by microwave spectroscopy and quantum chemical calculations

We observed the microwave spectrum of ethyl isovalerate by molecular beam Fourier transform microwave spectroscopy. The rotational and centrifugal distortion constants of the most abundant conformer were determined. Its structure was investigated by comparison of the experimental rotational constants with those obtained by ab initio methods. In a first step, the rotational constants of various conformers were calculated at the MP2/6-311++G** level of theory. Surprisingly, no agreement with the experimental results was found. Therefore, we concluded that in the case of ethyl isovalerate more advanced quantum chemical methods are required to obtain a reliable molecular geometry. Ab initio calculations carried out at MP3/6-311++G**, MP4/6-311++G**, and CCSD/6-311++G** levels and also density functional theory calculations using the B3LYP/6-311++G** method gave similar results for the rotational constants, but they were clearly distinct from those obtained at the MP2/6-311++G** level. With use of these more advanced methods, the rotational constants of the lowest energy conformer were in good agreement with those obtained from the microwave spectrum.     

A quantum chemical study of trimethoxyaluminum complexes with neutral molecules

The spatial and electronic structures of trimethoxyaluminum complexes with neutral molecules are calculated by the MP2/6-31(2d,p) method using the PC GAMESS-Firefly program package. The main characteristics of aluminum bonds in these molecules are determined by AIM and NBO methods. It is shown that these bonds can be characterized as the bonds between the atoms with closed shells.     

Sandwich compounds of second-row elements: A quantum chemical study

The structures and stabilities of a number of neutral and charged sandwich-type boron, carbon, and nitrogen compounds designed based on the cyclophane cage and obeying the “electron octet” rule were studied by the B3LYP/6−311+G** density functional method. The possibility of targeted modification of the electronic structures of such compounds by varying the basal or bridging atomic groups was investigated. Dedicated to Academician O. M. Nefedov on the occasion of his 75th birthday. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 1825–1835, November, 2006.     

A quantum chemical study of small beryllium hydrides and their radical anions

Molecular structures of neutral and anionic beryllium hydrides have been investigated using ab initio calculations at the MP 2 (geometries) and MP 4 (energies)/6 – 31⁺⁺G** levels. ZPE and (if possible) BSSE correction have been included. While BeH₂^? is not stable, the radical anion of the beryllium hydride dimer is significantly stabilized with respect to the neutral Be₂H₄ system. Moreover, this anion can appear in two isomeric forms with comparable stability. © 1995 John Wiley & Sons, Inc.     

A combined experimental and quantum chemistry study of selenium chemical shift tensors

A comprehensive investigation of selenium chemical shift tensors is presented. Experimentally determined chemical shift tensors were obtained from solid-state 77Se NMR spectra for several organic, organometallic, or inorganic selenium-containing compounds. The first reported indirect spin-spin coupling between selenium and chlorine is observed for Ph(2)SeCl(2) where 1J(77Se,35Cl)iso is 110 Hz. Selenium magnetic shielding tensors were calculated for all of the molecules investigated using zeroth-order regular approximation density functional theory, ZORA DFT. The computations provide the orientations of the chemical shift tensors, as well as a test of the theory for calculating the magnetic shielding interaction for heavier elements. The ZORA DFT calculations were performed with nonrelativistic, scalar relativistic, and scalar with spin-orbit relativistic levels of theory. Relativistic contributions to the magnetic shielding tensor were found to be significant for (NH4)2WSe4 and of less importance for organoselenium, organophosphine selenide, and inorganic selenium compounds containing lighter elements.