Ab initio molecular orbital studies of polyethylene deformation

A molecular LCAO Hartree-Fock procedure was used to calculate total energies of axially stretched normal paraffins containing up to nine carbon atoms. The results are used to model the mechanical properties of polyethylene.     

Exploring molecular complexity: conical intersections and NH3 photodissociation

The role of conical intersections in the photodissociation of the A 1A2" state of NH3 is investigated using extended atomic basis sets and a configuration state function expansion of approximately 8.5 million terms. A previously unknown portion of the 1 1A-2 1A seam of conical intersections with only C(s) symmetry is located. This portion of the seam is readily accessible from the equilibrium geometry of the A 1A2" state. These conical intersections are expected to play a role in the competition between adiabatic and nonadiabatic pathways for NH3(A 1A2") photodissociation.     

Ab initio studies of isomerization and dissociation reactions of methyl peroxynitrate

Using the CCSD(T)/cc-pVDZ//B3LYP/6-311G(2d,2p) method, we calculated the detailed potential energy surfaces (PESs) for the unimolecular isomerization and decomposition of methyl peroxynitrate (CH₃O₂NO₂). The results show that there are the two most stable isomers, IS1a and IS1b, which are a pair of mirror image isomers. From IS1a and IS1b, different isomerization and unimolecular decomposition reaction channels have been studied and discussed. Among them, the predominant thermal decomposition pathways are those leading to CH₃O₂ + NO₂ and cis-CH₃ONO + O₂. The former is the lowest-energy path through the direct O–N bond rupture in IS1a or IS1b. The PES along the O–N bond in IS1a has been scanned, where the energy of IS1a reaches maximum value of 23.5 kcal/mol when the O–N bond is stretched to about 2.8 Å. This energy is 2.7 kcal/mol larger than the O–N bond dissociation energy (BDE) and 2.8 kcal/mol larger than the experimental active energy. In addition, because the energy barriers of IS1a isomerization to IS2a are 23.8 kcal/mol, close to the 20.8 kcal/mol O–N BDE in IS1a or IS1b, the isomerization reaction may compete with the direct bond rupture dissociation reaction.     

Ab initio nonadiabatic dynamics study of ultrafast radiationless decay over conical intersections illustrated on the Na3F cluster

We present a theoretical approach for the ultrafast nonadiabatic dynamics based on the ab initio molecular dynamics carried out "on the fly" in the framework of the configuration interaction method combined with Tully's surface hopping algorithm for nonadiabatic transitions. This approach combined with our Wigner distribution approach allows us to perform accurate simulations of femtosecond pump-probe spectra in the systems where radiationless transitions among electronic states take place. In this paper we illustrate this by theoretical simulation of ultrafast processes and nonradiative relaxation in the Na(3)F cluster, involving three excited states and the ground electronic state. Furthermore, we show that our accurate simulation of the photoionization pump-probe spectrum is in full agreement with the experimental signal. Based on the nonadiabatic dynamics at high level of accuracy and taking into account all degrees of freedom, the nonradiative lifetime for the 1 (1)B(1) excited state of Na(3)F has been determined to be approximately 900 fs.     

Ab initio studies on the thermolysis of azetidine

The reaction mechanism of the thermolysis of azetidine to form ethylene and methylen-imine has been studied by ab initio SCF MO method at STO--3G and 3-21G levels. Two possible step-wise pathways are explored. One is the breaking of C--C bond as the first step, while the other is thebreaking of C--N bond. All the stationary points on the potential energy surface (PES) are fully optimiz-ed. MP2 / 3-21G single point calculations on all stationary points and MCSCF / STO-3G computationsfor some stationary points are also carried out. The calculations indicate that azetidine decomposesvia biradicaloid intermediates and the cleavage of C--N bond is preferable to that of C--C bond.     

Ab initio study on the mechanism of reaction HNCO+NH_2

Ab initio UMP2 and UQCISD(T) calculations, with 6-311G** basis sets, were performed for the titled reactions. The results show that the reactions have two product channels: NH2+ HNCO→NH3+NCO (1) and NH2+HNCO-N2H3+CO (2), where reaction (1) is a hydrogen abstraction reaction via an H-bonded complex (HBC), lowering the energy by 32.48 kJ/mol relative to reactants. The calculated QCISD(T)//MP2(full) energy barrier is 29.04 kJ/mol, which is in excellent accordance with the experimental value of 29.09 kJ/mol. In the range of reaction temperature 2300-2700 K, transition theory rate constant for reaction (1) is 1.68 × 1011- 3.29 × 1011 mL · mol-1· s-1, which is close to the experimental one of 5.0 ×1011 mL× mol-1· s-1 or less. However, reaction (2) is a stepwise reaction proceeding via two orientation modes, cis and trans, and the energy barriers for the rate-control step at our best calculations are 92.79 kJ/mol (for cis-mode) and 147.43 kJ/mol (for trans-mode), respectively, which is much higher than     

Ab initio studies of arginine and deaminoarginine

L-Arginine and Deaminoarginine were studied via quantum chemical ab initio calculations using the STO -3G basis set for arginine and the STO -3G and 3–21G basis sets for deaminoarginine. It was found that the most stable conformations are the ones featuring the carboxyl group slightly twisted versus the rest of the molecule, which adopts an extended conformation. It was also found that the cyclic zwitterion is less stable than the neutral cyclic conformation and that there is a barrier to the proton transfer from the carboxyl's oxygen to one of guanidine moiety's nitrogens.     

Assigning the NH stretches of the guanine tautomers using adiabatic separation: CCSD(T) benchmark calculations

Using an adiabatic separation of the NH stretching vibration from the remaining vibrational molecular motions, the NH fundamental frequencies and absolute intensities of several keto/enol and 7/9NH tautomers of guanine are evaluated ab initio within the framework of a one-dimensional "semirigid" stretching Hamiltonian. The frequencies (calculated by means of the standard MP2, CCSD(T) and DFT procedures) are in a close one-to-one harmony with their experimental counterparts, thus evidencing the adequacy of the used separation for reliable assigning of the NH stretches in the vibrational spectra of very large molecular systems.     

Ab initio study on deactivation pathways of excited 9H-guanine

The complete active space with second-order perturbation theory/complete active space self-consistent-field method was used to explore the nonradiative decay mechanism for excited 9H-guanine. On the 1pipi* (1L(a)) surface we determined a conical intersection (CI), labeled (S0pipi*)(CI), between the 1pipi* (1L(a)) excited state and the ground state, and a minimum, labeled (pipi*)min. For the 1pipi* (1L(a)) state, its probable deactivation path is to undergo a spontaneous relaxation to (pipi*)min first and then decay to the ground state through (S0pipi*)(CI), during which a small activation energy is required. On the 1n(N)pi* surface a CI between the 1n(N)pi* and 1pipi* (1L(a)) states was located, which suggests that the 1n(N)pi* excited state could transform to the 1pipi* (1L(a)) excited state first and then follow the deactivation path of the 1pipi* (1L(a)) state. This CI was also possibly involved in the nonradiative decay path of the second lowest 1pipi* (1L(b)) state. On the 1n(O)pi* surface a minimum was determined. The deactivation of the 1n(O)pi* state to the ground state was estimated to be energetically unfavorable. On the 1pisigma* surface, the dissociation of the N-H bond of the six-membered ring is difficult to occur due to a significant barrier.     

Ab initio molecular dynamics of protonated dialanine and comparison to infrared multiphoton dissociation experiments

Finite temperature Car-Parrinello molecular dynamics simulations are performed for the protonated dialanine peptide in vacuo, in relation to infrared multiphoton dissociation experiments. The simulations emphasize the flexibility of the different torsional angles at room temperature and the dynamical exchange between different conformers which were previously identified as stable at 0 K. A proton transfer occurring spontaneously at the N-terminal side is also observed and characterized. The theoretical infrared absorption spectrum is computed from the dipole time correlation function, and, in contrast to traditional static electronic structure calculations, it accounts directly for anharmonic and finite temperature effects. The comparison to the experimental infrared multiphoton dissociation spectrum turns out very good in terms of both band positions and band shapes. It does help the identification of a predominant conformer and the attribution of the different bands. The synergy shown between the experimental and theoretical approaches opens the door to the study of the vibrational properties of complex and floppy biomolecules in the gas phase at finite temperature.     

二元低价铟溴化合物结构不稳定的从头算研究

应用较大基组的从头算法,以八配位立方构型多面体为例,研究了二元低价铟溴化合物不稳定性的微观结构原因,分析了构型的势能曲线状况和分子轨道性质．结果表明, 高对称的八配位立方构型多面体势能曲线异常平缓且分布各向异性,中心力场很弱,导致构型极易在外界因素的影响下发生变化．二元低价铟溴化合物具有无机固态化合物中少见的二阶Ｊａｈｎ－ Ｔｅｌｌｅｒ 效应     

宝塔烷电子结构与张力的ab initio研究

利用MNDO程序优化出宝塔烷、宝塔烷双正离子及宝塔二烯的构型参数。从头算的电荷分布及分子轨道均表明: 宝塔烷与其价键异构体宝塔二烯性质相近。双正离子的中间四个碳原子则形成了一种与环丁烷双正离子类似的四中心双电子大π键。张力研究表明, 宝塔烷的键张力要比不能稳定存在的小螺桨烷小得多, 说明了宝塔烷骨架的刚性及稳定性。     

Ab initio study on the adsorption and dissociation process of methanol on a silver surface

The geometries of methanol adsorbed on an oxygen-free silver surface, a promoted silver surface and an oxygen preadsorbed silver surface were optimized at the MP2 level and the energies were calculated at the MP4 level. Our calculations showed that weak physisorption of methanol occurs on the clean silver surface, but stable molecular chemisorption occurs in the other two cases. The adsorption and dissociation process of methanol was postulated to occur via two pathways, i.e. the Eley-Rideal mode and the Langmuir-Hinshelwood mode. The calculations also showed that the presence of atomic oxygen at a silver surface is essential for the cleavage of the OH bond in the methanol. The dissociation of methanol in the Langmuir-Hinshelwood mode has a small energy barrier but has no energy barrier in the Eley-Rideal mode.     

Ab initio studies of methylenecarbene and isoelectronic species

Optimum equilibrium geometries, energetics, harmonic vibrational frequencies, and infared intensities within the double harmonic approximation are computed for methylenecarbene, CCH₂, and isoelectronic species involving silicon and germanium at both the SCF level of theory and the level of second-order perturbation theory using a 6-311G(2df, 2p) basis set or its equivalent. Optimum equilibrium geometries and energetics are also computed at both levels of theory using a smaller 6-311G(d, p) basis set or its equivalent. This investigation of these species is the first to include all of the systems with germanium. In addition, this present work is the first study to includef-type polarization functions in a systematic investigation of the molecular structure and properties of all the molecules in the series. Acetylenic structures are also computed for energy comparisons. Of all the linear isomers, only acetylene is found to be a minimum on the potential energy surface. However, all of the C_2v structures are found to be local minima. Both the C_2v and linear structures will serve as a basis for future work involving mapping the entire hyperenergy surfaces of all of the molecular systems in the series.     

Desolvation effects on the dissociation energy of diatomic molecules:Ab initio study of the dissociation of Li-F in polar media

Summary The potential curve of the ground state dissociation of Li-F in water has been studied by a combination of a standardab initio Hartree-Fock procedure and a perturbative reaction field approach. The electrostatic solute-solvent interaction is accounted for by the generalized Born formalism introduced through a perturbation approach. The calculations were carried out at a 6–311+G* basis set level. Diffuse functions ofs symmetry were included to model a desolvation potential. A double well potential curve was obtained for the dissociation of this molecule in the presence of a highly polarizable medium. The first minimum, corresponding to an ion pair, electrostatically bound, is found at aR(Li-F)<6.0 Å distance. As the two ions come together, a desolvation barrier of about 30 kcal/mol is to be overcome before the formation of the neutral Li-F at 1.56 Å. The barrier to ionization towards the ion pair is calculated to be about 14 kcal/mol. The dissociation of the ion pair towards the free ions is discussed in terms of the electrostatic solvation entropy changes.Contribution No 6 from Centro de Mecánica Cuantica Aplicada (CMCA)     

Ab initio and DFT studies on hydrolyses of phosphorus halides

Hydrolyses of phosphorus halides, (RO)(2)POX where R = H or Me and X = F or Cl, in the gas phase and in the reaction field have been investigated theoretically with ab initio and the density functional theory (DFT). The free energy of activation in the reaction field was also estimated using the Onsager method with a correction of entropy change and basis set superposition error (BSSE). The reaction of (MeO)(2)POF proceeds through a path with bifunctional catalysis regardless of the medium, but the reaction of (MeO)(2)POCl proceeds through bifunctional and general base catalysis in the gas phase and in water, respectively. The estimated free energy barrier of 23 kcal/mol for the hydrolysis of (MeO)(2)POF is in good agreement with the experimental values of 24 kcal/mol, and relative barrier of 3 kcal/mol to the (MeO)(2)POCl is also in good agreement with the experimental values of 5 kcal/mol of diisopropyl phosphorus halides ((Pr(i)O)(2)POX, X = F and Cl).     

Ab initio studies on the polarizability of lithium clusters: Some unusual results

The polarizability of lithium clusters was studied using ab initio as well as Density Functional Theory (DFT)‐based methods. The electron correlation was found not to play any major role in the evaluation of polarizability of the lithium clusters, contrary to the case of sodium and other types of clusters. The theoretical methods used in the present study are not able to reproduce the unusual features in the size evolution of the polarizability observed experimentally. An interesting feature is that the polarizability of lithium clusters calculated by various methods at 0K temperature is found to be higher than the experimental results measured at higher temperature, which is quite unusual. The significance of these issues is highlighted and the need of highly accurate DFT exchange‐correlation functionals and ab initio methods in the study of the electronic properties of lithium clusters is emphasized. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Infrared experimental and ab initio quantum mechanical studies of 2-mercaptopyrimidine tautomers

Results are presented from ab initio SCF(3-21G*) calculations for the geometries and vibrational spectra (wavenumbers and absolute intensifies) of the thiol and thione tautomers of 2-mercaptopyrimidine. The results of calculations are compared with available experimental data, particularly with the reported vibrational spectra of the molecule isolated in inert gas matrices (Ar, N₂) and in crystalline state. The calculations of the normal modes predicted the experimental spectrum close enough to allow reliable assignment of most of the bands. The thiol⇌⇌thione tautomerism of the molecule is discussed. Matrix isolated monomers were observed in the thiol form only. That agrees with the results of ab initio calculations of internal energies of the tautomers [SCF(6-31Gu*) + MBPT(2)(6-31G*) + vib(0)(3-21G*); at the SCF(3-21G*) geometries] which predict the energy of thiol form to be ≈33 kJ mol⁻¹ lower than that of thione form. In the crystalline state the hydrogen-bonded associations in the thione form dominate while in disordered amorphous layers, in matrices with a high guest-to-host ratio and in annealed matrices the associations both in thiol and thione form were observed.