Ab initio characterization of C5

In this paper, the structure and spectroscopic parameters of the C5 cluster are determined using multiconfigurational quantum chemical methods as implemented in the MOLCAS software. A number of spectroscopic properties (band center positions, l-doubling parameters, and rotational constants) have been characterized. From the new results, the assignments of previous astrophysical observations [J. Goicoechea et al., Astrophys. J. 609, 225 (2004)] are discussed. A detailed exploration of the global potential energy surface confirms that C5 has a X1Sigmag+ linear isomer of prominent stability and, at least, three minimum energy structures showing singlet electronic ground states. Two of them are cyclic and one has a nonplanar geometry. Vertical and adiabatic electronic transitions and vibrational spectroscopic parameters are determined for the most stable linear isomer using multiconfigurational second order perturbation theory (CASPT2) using an active space containing 12 valence orbitals with 12 active electrons and extended ANO-type basis sets. The infrared spectrum has been analyzed from an anharmonic force field derived form the local surface, determined from the energies of a grid of 1350 geometries. The force field includes four coupling terms. The CASPT2 band center position of the nu7(piu) anharmonic fundamental has been calculated to be at 102 cm(-1), which validates the assignment to C5 of the pattern of bands centered at 102 cm(-1) observed with the ISO telescope.     

Ab initio study of hydrogen migration across n-alkyl radicals

A thorough ab initio investigation is conducted on all possible hydrogen migration pathways for the 1-ethyl, 1-propyl, 1-butyl, 1-pentyl, 1-hexyl, 1-heptyl, and 1-octyl radicals in order to determine underlying trends in reaction enthalpies, activation energies, Arrhenius A-factors, tunneling, and rate coefficients. The G4, G2, and CBS-Q composite methods are used to determine the enthalpy of reaction and activation energy barrier for each reaction. Each method shows excellent agreement with eight experimental enthalpy of reaction values, with root mean squared values of 0.8, 0.9, and 0.6 kcal mol(-1) for CBS-Q, G2, and G4, respectively. Differences in barrier heights, A-factors, tunneling, and rate coefficients are observed for axial and equatorial arrangements as well as between secondary hydrogen migration sites, depending on the location of the secondary site relative to the terminal carbon. The validity of using cycloalkane model systems to estimate rate parameters is also assessed. The failure of two key assumptions inherent to the cycloalkane models, resulting in a breakdown in the accuracy of these methods for larger transition states, is discussed. This study has significant ramifications for future theoretical, experimental, and modeling studies involving the decomposition of n-alkanes.     

Ab initio study of azolides: Energetic and spectroscopic properties

We report the ab initio study of twenty‐four azolides derived from pyrrole, imidazole, pyrazole, both triazoles, tetrazole, pentazole, indole and carbazole bearing at the nitrogen atom the groups COMe, CHO, COCF₃ and CO₂Me. Theoretical values (isomerism, barriers, dipole moments, C=O stretching) are compared with experimental ones, when available, and also internally compared. A special effort has been devoted to the calculation of the absolute shieldings for the different nuclei present in azolides. At the level of calculation used (RHF/6‐311G**) the results are satisfactory. To complete the nmr data from the literature, some ¹H, ¹³C, ¹⁵N, ¹⁷O and 19F chemical shifts have been determined.     

Ab initio study of hydrogen migration in 1-alkylperoxy radicals

Alkylperoxy and hydroperoxyalkyl radicals are key reactive intermediates in hydrocarbon oxidation mechanisms. An understanding of the interconversion of these two species via a hydrogen migration reaction is of fundamental importance to the prediction of chain branching reactions and end product composition. An extensive ab initio investigation of the hydrogen migration reaction in 1-ethyl, 1-propyl, 1-butyl, 1-pentyl, and 1-hexylperoxy radicals is conducted to assess the validity of using cycloalkanes to model the ring strain of their transition states as well as the effect of both location of the migrating hydrogen and directionality of the remaining alkyl chain in the transition state of the reaction involving a secondary hydrogen. The G2 and CBS-Q composite methods are used to determine the activation energy and enthalpy of reaction relative to the alkylperoxy radical. Both methods show good agreement with five experimentally determined reaction enthalpies, having root mean squared deviations of 0.7 and 1.3 kcal mol(-1) for the CBS-Q and G2 methods, respectively. The effect of hydrogen abstraction site and transition state geometry, particularly axial and equatorial geometries of the remaining alkyl chain, on the activation energy, Arrhenius A-factor, tunneling, and rate coefficient are discussed. Differences between terminal adjacent and nonterminal adjacent secondary sites result in small but consistent differences in barrier height. Failure of key assumptions within the cycloalkane based estimation method leads to the break down in the accuracy for both small and large transition states. For large transition states, the breakdown of these assumptions also results in the failure of the current cycloalkane method as a conceptual model. Of great interest is the observed alteration in the preferred H-migration from the 1,5 to the 1,6 H-migration within the temperature region where these reactions are particularly important to the combustion mechanism.     

Ab initio rotation-vibration energy levels of triatomics to spectroscopic accuracy

The factors that need to be taken into account to achieve spectroscopic accuracy for triatomic molecules are considered focusing on H3+ and water as examples. The magnitude of the adiabatic and non-adiabatic corrections to the Born-Oppenheimer approximation is illustrated for both molecules, and methods of including them ab initio are discussed. Electronic relativistic effects are not important for H3+, but are for water for which the magnitude of the various effects is discussed. For H3+ inclusion of rotational non-adiabatic effects means that levels can be generated to an accuracy approaching 0.01 cm(-1); for water the error is still dominated by the error in the correlation energy in the electronic structure calculation. Prospects for improving this aspect of the calculation are discussed.     

Ab initio study on the decomposition of first excited state HOOO radicals

The HOOO radical plays a crucial role in atmospheric processes involving the OH radical and O(2) molecule. We present an ab initio molecular orbital theory study on the decomposition reaction of the first excited state HOOO((2)A') with respect to OH and O(2). The geometries and harmonic vibrational frequencies of all stationary points are calculated at the CASSCF and MRCI levels of theory in conjunction with the 6-31+G(d,p) basis set. The potential energy profile of the decomposition reaction is studied at the CASSCF/6-31+G(d,p) level of theory, in which the complete valence orbitals and electrons are included in the active space. The energies of the potential energy profile are further refined at the CASPT2 and MRCI levels of the theory. Additionally, we have determined the interesting reaction process: the HOOO((2)A') radical with C(s) symmetry does not dissociate to OH((2)Pi) and O(2)((3)Sigma(-)(g)) directly as this is forbidden by orbital symmetry, but dissociates to OH((2)Pi) and O(2)((3)Sigma(-)(g)) via the change in symmetry from C(s) to C(infinity v) symmetry with a low barrier.     

Ab initio spectroscopic study for the NaRb molecule in ground and excited states

A wide adiabatic study is performed for NaRb molecule, involving 15¹Σ⁺ electronic states including the ionic state Na^?Rb⁺, as well as 14³Σ⁺, 1–9^1,3Π, and 1–5^1,3Δ states. This investigation is performed using an ab initio approach which involves the effective core potential, the core polarization potential with l‐dependent cut‐off functions. The NaRb system has been treated as a two‐electron system and the full valence configuration interaction is easily achieved. The spectroscopic constants R_e, D_e, T_e, ω_e, ω_ex_e, B_e, and D₀ for all these states are derived. We have also computed the vibrational levels as well their spacing for different values of J. In addition, permanent and transition dipole moments are determined and analyzed. The Dunham coefficients have been used to perform experimental spacing to compare directly with our results. The present calculations on NaRb extend previous theoretical works to numerous electronic excited states in the various symmetries. © 2014 Wiley Periodicals, Inc.     

Ab initio characterization of the Mg-HF van der Waals complex

The equilibrium structure and the three-dimensional potential energy surface of the Mg-HF van der Waals complex in its ground electronic state have been determined from accurate ab initio calculations using the coupled-cluster method, CCSD(T), in conjunction with the basis sets of triple- through quintuple-zeta quality. The core-electron correlation, high-order valence-electron correlation, and scalar relativistic effects were investigated. The Mg-HF complex was confirmed to be linear at equilibrium, with a vibrationless dissociation energy (into Mg and HF) D(e) of 280 cm(-1). The vibration-rotation energy levels of two isotopologues, (24)Mg-HF and (24)Mg-DF, were predicted using the variational method. The predicted spectroscopic constants can be useful in a further analysis of high-resolution vibration-rotation spectra of the Mg-HF complex.     

Ab initio UHF and SWX α study of the GeF3 and SiF3 radicals

Results of ab initio UHF and SWX _α calculations concerning the geometrical and electronic properties of the GeF₃ and SiF₃ radicals are presented. Comparison of the theoretical estimates of inversion barriers, ionization potentials and electron affinities obtained for three XF₃ radicals (X=Ge, Si, C) is made.     

Ab initio molecular orbital studies of flavin radicals and the lowest triplet state of isoalloxazine

Ab initio calculations have been performed on the anion, neutral and cation radicals of isoalloxazine (isoalloxazine^?, 5H-isoalloxazine and 1,5-dihydroisoalloxazinium respectively) and on the lowest excited triplet state of isoalloxazine within the RHF and UHF SCF method. The calculated spin distribution is in reasonable agreement with experiments, provided the molecules are assumed to be planar. However, total energy considerations suggest that the radicals are non-planar in the gaseous phase.     

Ab initio and DFT investigation of the structures and properties of chloromethyl and chlorofluoromethyl peroxyl radicals

Ab initio and Density Functional Theory (DFT) calculations were performed to determine the equilibrium geometries, charge distributions, spin density distributions, dipole moments, electron affinities (EAs), and C–O bond dissociation energies (BDEs) of CH₂ClO₂^? CHCl₂O₂^?, CCl₃O₂^?, CF₂ClO₂^?, CFCl₂O₂^?, and CHFClO₂^? peroxyl radicals. The C–H BDEs of the parent methanes were calculated using the same levels of theories. Both MP2(full) and B3LYP methods, using the 6‐31G(d,p) basis set, were found to be capable of accurately predicting the geometries of peroxyl radicals. The B3LYP/6‐31G(d,p) method was found to be comparable to high ab initio levels in predicting C–O BDEs of studied peroxyl radicals and C–H BDEs of the parent alkanes. The progressive chlorine substitution of hydrogen atoms in methyl peroxyl radicals results in an increase (decrease) of the spin density on the terminal (inner) oxygen, a decrease in dipole moments, and an increase in electron affinities. The substitution of fluorine by chlorine in the series CF₃O₂^? – CCl₃O₂^? was found to lengthen (destabilize) the C–O bonds. Both C–O BDEs and EAs of peroxyl radicals (RO₂^?) were found to correlate well with Taft σ* substituent constants for the R groups. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Ab initio study of spectroscopic constants and anharmonic force field of 74GeCl2

The equilibrium structure, spectroscopy constants, and anharmonic force field of germanium dichloride have been calculated at MP2, B3LYP, and CCSD(T) levels of theory employing two basis sets, cc-pVDZ and cc-pVTZ, respectively. The computed geometries, rotational constants, and vibration-rotation interaction constants, and quartic centrifugal distortion constants are compared with the available experimental data. The harmonic frequencies, anharmonic constants, and cubic and quartic force constants are predicted. The calculated results show that the MP2 results are in excellent agreement with experiment and represent a substantial improvement over the results obtained from B3LYP. The CCSD(T) method is also an advisable choice to study anharmonic force field of molecules.     

Ab initio MO analysis of interaction paths between radicals in ferromagnetic organic systems

Interaction path analyses for pi-conjugated organic systems were performed at the ab initio molecular orbital level to examine the relationship between inter-radical interactions and the high-spin stability of the system. It was found that the high-spin stability results from through-bond interactions between radicals, not from through-space interactions, in relation to the stabilization of a low-spin state due to the effects of electron correlation. L(ij)(min) value for estimating the mixing of nonbonding molecular orbitals well predicted the relationship between the through-bond interactions and the high-spin stability. Furthermore, molecular orbital calculations revealed that the all-trans type interaction path between radicals produces long-range exchange interactions, and the additivity of high-spin stability is observed by keeping short-range through-bond interaction paths.     

Ab initio study of n-tetrasilane cation and anion radicals as models of doped linear polysilanes

The geometric and electronic structures of n-tetrasilane cation and anion radicals as models of doped linear polysilanes are studied theoretically using an ab initio molecular orbital method at the UMP2/6–31 + G(d, p) level of calculations. It is found that the trans-conformations in these molecules are the most stable structures in each ground state and that the energy differences between the cis- and trans-conformations are 3 kcal/mol for the cation radical and 11 kcal/mol for the anion radical. There exists no stable gauche-conformation in these molecules in contrast to neutral n-tetrasilane. It seems that the weakening of the central silicon-silicon bond on doping is connected to the concentration of the charge distributions on central silicon atoms with the change from trans- to cis-conformations in both n-tetrasilane cation and anion radicals. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 62 : 393–401, 1997     

Ab initio studies of

Ab initio calculations of the [1,5]-H shift in (3Z)-penta-1,3-diene and other substituted pentadienes and heteroanalogues using the hybrid density functional Becke3LYP with the 6-31G basis set are presented. Electron-donating substituents, such as methoxy in (3Z)-3-methoxypenta-1,3-diene 1, or heteroatoms such as a nitrogen atom in (Z)-ethylidenevinylamine 2, (1Z)-buta-1,3-dienylamine 3, (2Z)-but-2-enylideneamine 4, (Z)-allylidenemethylamine 5, and methylene-(Z)-propenylamine 6 are introduced. The electron-withdrawing fluoride is substituted for the hydrogen atoms in (3Z)-3-fluoropenta-1,3-diene 7, (3Z)-2,4-difluoropenta-1,3-diene 8, (3Z)-1,1',2,3,4,5,5'-heptafluoropenta- 1,3-diene 10, (1E,3E)-1,3,5-trifluoropenta-1,3-diene 11, and (1Z,3E)-1,3,5- trifluoropenta-1,3-diene 13. A detailed analysis of the geometries, energies, and electronic characteristics of the sigmatropic transposition compared to those of the unsubstituted case provides insights into substituent effects of this prototype of pericyclic reaction. The inductive and mesomeric effects of heteroatoms or heterosubstituents are of a great importance and in a continuous balance in the energetics of the transformation. Sterics can also play an important role due to the geometrical constraints of the reaction. As a general trend, decreasing the electron density of the phi system destabilizes the aromatic transition structure and increases the activation energy, and vice versa.     

Ab initio study of spectroscopic properties of CuAr, CuAr(+) and CuAr(-)

The spectroscopic properties of CuAr, CuAr(+) and CuAr(-) have been studied in detail using ab initio MP2, CCSD and CCSD(T) methods. The effect of basis set on spectroscopic properties of these molecular systems has also been investigated. Among these molecules, CuAr(+) is found to be more strongly bound than CuAr and CuAr(-). The spectroscopic properties of CuAr and CuAr(-) are calculated in Lennard-Jones potential and the spectroscopic properties of CuAr(+) are calculated in Morse potential. Most of the spectroscopic constants of CuAr, CuAr(+) and all the spectroscopic constants of CuAr(-) are first reported. Our calculated bond length, harmonic frequency and dissociation energy of CuAr and CuAr(+) agree very well with the existing theoretical results.     

Ab initio Ehrenfest dynamics

We present an ab initio direct Ehrenfest dynamics scheme using a three time-step integrator. The three different time steps are implemented with nuclear velocity Verlet, nuclear-position-coupled midpoint Fock integrator, and time-dependent Hartree-Fock with a modified midpoint and unitary transformation algorithm. The computational cost of the ab initio direct Ehrenfest dynamics presented here is found to be only a factor of 2-4 larger than that of Born-Oppenheimer (BO) dynamics. As an example, we compute the vibration of the NaCl molecule and the intramolecular torsional motion of H2C=NH2+ by Ehrenfest dynamics compared with BO dynamics. For the vibration of NaCl with an initial kinetic energy of 1.16 eV, Ehrenfest dynamics converges to BO dynamics with the same vibrational frequency. The intramolecular rotation of H2C=NH2+ produces significant electronic excitation in the Ehrenfest trajectory. The amount of nonadiabaticity, suggested by the amplitude of the coherent progression of the excited and ground electronic states, is observed to be directly related to the strength of the electron-nuclear coupling. Such nonadiabaticity is seen to have a significant effect on the dynamics compared with the adiabatic approximation.     

Ab initio and DFT study of the electronic structures and spectroscopic properties of pyrene ligands and their cyclometalated complexes

Two ligands 1‐diphenylphosphinopyrene (1‐PyP) ( L ₁ ), 1,6‐bis(diphenylphosphino)‐pyrene (1,6‐PyP) ( L ₂ ) and their cyclometalated complexes [Pt(dppm)(1‐PyP‐H)]⁺ ( 1 ), [Pt₂(dppm)₂(1,6‐PyP‐H₂)]²⁺ (dppm = bis(diphenylphosphino)methane ( 2 ), and [Pd(dppe)(1‐PyP‐H)⁺ (dppe = bis(diphenylphosphino)ethane) ( 3 ) are investigated theoretically to explore their electronic structures and spectroscopic properties. The ground‐ and excited‐state structures are optimized by the density functional theory (DFT) and single‐excitation configuration interaction method, respectively. At the time‐dependent DFT (TDDFT) and B3LYP level, the absorption and emission spectra in solution are obtained. As revealed from the calculations, the lowest‐energy absorptions of 1 and 3 are attributed to the mixing ligand‐to‐metal charge transfer (CT)/intraligand (IL)/ligand‐to‐ligand CT transitions, while that of 2 is attributed to the IL transition. The lowest‐energy phosphorescent emissions of the cyclometalated complexes are attributed to coming from the ³ILCT transitions. With the increase of the spin‐orbit coupling effect, the phosphorescence intensities and the emissions wavelength are correspondingly increased. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Ab initio study of KN

The potential energy curves for the lowest (3)Sigma(-), (3)Pi, and (5)Sigma(-) states of the KN molecule have been calculated by the multireference singles and doubles configuration interaction method, including Davidson's corrections for quadruple excitations [MRCI(+Q)]. It is shown that the former two are bound, while the last one is repulsive. The electronic ground state of KN is predicted as (3)Sigma(-) state, although the term energy of the (3)Pi state is very small, 177.3 cm(-1). The binding energy for the (3)Sigma(-) state is evaluated as 0.838 eV, the rotational constant B(0) as 0.250 63 cm(-1), and harmonic frequency as 324.4 cm(-1). The spin-orbit coupling effects between the (3)Sigma(-) and (3)Pi states of KN are evaluated and discussed. The same MRCI(+Q) computational procedures are applied to the isovalent LiN, KC, KO, and KCl to confirm the accuracy of present calculations. Theoretical spectroscopic constants presented here will inspire experimental studies of KN.