CI study of rovibrational dependence of nuclear quadrupole coupling constants of all isotopic variants of OH+ in the X3Σ− state

The ¹⁷O and ²H quadrupole coupling constants of rovibrational levels of ¹⁷O¹H⁺, ¹⁷O²H⁺, and ¹⁶O²H⁺ in their X³Σ⁻ state have been calculated from molecular wave functions that explicitly describe nuclear motion. The ¹⁷O quadrupole coupling is predicted to be strong and its vibrational dependence differs from that known for other nuclei A in the first-row hydrides AH or AH⁺. The deuterium coupling in ¹⁷O²H⁺ and ¹⁶O²H⁺ is found to be weak and its behavior is similar to that of other first-row hydrides. The change with rotational excitation is unimportant. The quadrupole hyperfine patterns of ¹⁷O²H⁺ in its ground state are dominated by the strong oxygen coupling. © 1996 John Wiley & Sons, Inc.     

Ab initio calculations of isotropic hyperfine coupling constants in β-ketoenolyl radicals

Ab initio unrestricted Hartree–Fock (UHF ), unrestricted second-order Møller–Plesset (UMP 2) perturbation, unrestricted coupled cluster (UCCD ), and unrestricted quadratic configuration interaction (UQCISD ) calculations have been performed on the organic radicals CH₃, CH₃CH₂, CH₂CHCH₂, CH₃CHCOO^?, HCOCHCOH, CH₃COCHCOH, CH₃COCHCOCH₃, and CH₃COC(CH₃)COCH₃, using double-zeta and split-valence-plus-polarization basis sets. These radicals are derived from common organic ligands and have been observed in recent experimental work on tris(β-ketoenolato)cobalt(III) complexes. Their geometry has been optimized at the UHF level using the two mentioned basis sets. From these calcuations, values for the isotropic hyperfine coupling constants at the hydrogen atoms are predicted and compared with the experimental results. The usefulness of semiempirical extrapolations based on limited basis sets and treatment of electron correlation effects is carefully analyzed in the examples considered. © 1994 John Wiley & Sons, Inc.     

Multireference CI calculation of nuclear quadrupole coupling constants of CN+ and CN-: rovibrational dependence

The 14N quadrupole coupling constants of rovibrational levels of the X1sigma+ and c1sigma+ states of CN+, and the ground electronic state of CN- are calculated from molecular wavefunctions which explicitly describe nuclear displacement. From the electronic states considered, the excited 1sigma+ state of CN is predicted to exhibit the strongest N coupling, at least in the ground vibrational state. Compared to the vibrational dependence of the 14N QCC's, which is found to be significant in all cases, the rotational dependence is predicted to be unimportant. Special attention is paid to the assessment of adequacy of the expectation value approach to the evaluation of the electric field gradient tensor within the applied multireference configuration interaction formalism. Spectroscopic constants are derived from corresponding potential energy curves to testify to the quality of the correlated wave functions used.     

Ab initio calculations on 5α-androstane

The electronic structure of 5α-androstane, the parent hydrocarbon of the hormonal steroids, has been computed by ab initio SCF methods in an STO -3G basis. The results are compared with existing MNDO computations and are used to discuss long-range electronic interactions between distant substituents that might be appended to rings A and D of 5α-androstane. It is thought that these interactions are mediated by the ribbonlike MO 'S of the parent molecule.     

Ab initio configuration interaction study of the low-lying 1Sigma+ electronic states of LiCl

Ab initio configuration interaction calculations have been performed for the X 1Sigma+ and B 1Sigma+ electronic states of LiCl. Potential energy curves, dipole moment functions, and dipole transition moments have been computed for internuclear distances between R = 2.5a0 and 50a0. Single- and double-excitation configuration interaction wave functions were constructed using molecular orbitals obtained from a two-state averaged multiconfiguration self-consistent-field calculation. This procedure yielded an accurate energy splitting between the covalent and ionic separated-atom limits. The calculated avoided crossing of the X and B state curves occurs at R = 16.2a0, in close agreement with previous calculations using a semiempirical covalent-ionic resonance model. X 1Sigma+ state spectroscopic constants are in excellent agreement with experimental values.     

Ab initio excited states calculations of Kr 3 + , probing semi-empirical modelling

The accuracy of the diatomics-in-molecules (DIM) model for the krypton ionic trimer is examined in a series of ab initio calculations. In the C_2v symmetry, the ground states of irreducible representations B₂ and A₁ were calculated using partially spin restricted open-shell coupled cluster method with perturbative triple connections (RHF-RCCSD-T), the relativistic effective core potential (RECP) and an extended basis set of atomic orbitals. Internally contracted multireference configuration interaction method (icMRCI) with the extended and restricted basis set was used to generate the potential energy surfaces (PESs) of the nine electronic states of Kr ₃ ⁺ corresponding to Kr(¹S) + Kr(¹S) + Kr⁺(²P) dissociation limit in a wide interval of nuclear geometries. The overall agreement of the accurate ab initio PESs and the diatomics-in-molecules PESs confirms the quality of the DIM Hamiltonian for the Kr ₃ ⁺ clusters and justifies its use in dynamical and spectroscopic studies of the Kr _n ⁺ clusters. Inclusion of the spin–orbit coupling into the ab initio PESs through a semi-empirical scheme is proposed.     

Ab initio calculations of spin-orbit coupling in the SnH+4 ion

Ab initio configuration interaction calculations (CI) have been performed in order to evaluate the spin-orbit coupling associated with the Jahn-Teller distortion of the SnH⁺₄ ion. Optimizing all geometric parameters at the CI (MP2) level leads to two stable forms, C_3v and C_2v. Spin-orbit coupling, which is 0.29 eV for the tetrahedral form of SnH⁺₄, decreases sharply with distortion by vibronic interaction and becomes very low in stable forms.     

Ab initio investigations of the bound rovibrational levels of NeH 2 + , NeHD+, and NeD 2 +

Summary Bound rovibrational levels have been calculated for NeH ₂ ⁺ , NeHD⁺, and NeD ₂ ⁺ using three recent fits to an accurateab initio PES. The NeH ₂ ⁺ molecule behaves essentially as a linear molecule, the predicted rotational constant is 2.57 cm^–1. The fundamental frequencies are 811, 1189, and 1748 cm^–1 for the Ne-H ₂ ⁺ stretch, the Ne-H ₂ ⁺ bend and H ₂ ⁺ stretching modes, respectively.Dedicated to the 60th birthday of Prof. W. Kutzelnigg, Bochum     

Ab initio calculations on low-lying electronic states of TeO2 and Franck-Condon simulation of the (1)1B2 <-- X1A1 TeO2 absorption spectrum including anharmonicity

Ab initio calculations have been carried out on low-lying singlet and triplet states of TeO2 at different levels of theory with basis sets of up to the augmented-polarized valence-quintuple-zeta quality. Equilibrium geometrical parameters, harmonic vibrational frequencies, and relative electronic energies of the X1A1, 1B1, 1B2, 1A2, 3A1, 3B1, 3B2, and 3A2 states of TeO2 have been calculated. Potential energy functions (PEFs) of the X1A1 and the (1)1B2 states were computed at the complete-active-space self-consistent-field multireference configuration interaction level, with a basis set of augmented-polarized valence-quadruple-zeta quality. Franck-Condon factors (FCFs) for the electronic transition between the X1A1 and (1)1B2 states of TeO2 were calculated with the above-mentioned ab initio PEFs. The (1)1B2 <-- X1A1 absorption spectrum of TeO2 was simulated employing the computed FCFs, which include Duschinsky rotation and anharmonicity, and compared with the recently published laser-induced fluorescence (LIF) spectrum of Hullah and Brown [J. Mol. Spectrosc. 200, 261 (2000)]. The ab initio results and spectral simulation reported here confirm the upper electronic state involved in the LIF spectrum to be the (1)1B2 state of TeO2 and also confirm the vibrational assignments of Hullah and Brown. However, our simulated spectrum suggests that the reported LIF spectrum from 345 to 406 nm represents only a portion of the full (1)1B2 <-- X1A1 absorption spectrum of TeO2, which extends from ca. 406 to 300 nm. Another dye other than the two used by Hullah and Brown is required to cover the 345-300 nm region of the LIF band. Ab initio calculations show strong configuration mixing of the (1)1B2 electronic surface with higher 1B2 states in a region of large TeO bond length (> or = 2.0 A) and OTeO bond angle (> or = 135.0 degrees).     

An ab initio calculation of the anisotropic hyperfine coupling constants in the low-lying vibronic levels of the X 2Pi electronic state of CCCH

The results of ab initio calculations of the vibronically averaged components of the anisotropic magnetic hyperfine tensor in the low-lying vibronic species of the X (2)Pi electronic state of CCCH and CCCD are reported. The electronically averaged hyperfine coupling constants for hydrogen and (13)C in (12)C (12)C (12)CH, (13)C (12)C (12)CH, (12)C (13)C (12)CH, (12)C (12)C (13)CH, and (12)C (12)C (12)CD are obtained as functions of two bending vibrational modes by the density functional theory method. The vibronic wave functions are calculated with help of a variational approach which takes into account the Renner-Teller effect and spin-orbit coupling. The results of the present study help to reliably interpret the experimental data previously published and predict the yet unobserved hyperfine structure in excited vibronic states of CCCH and CCCD.     

Global X2A′ potential energy surface of Li2H and quantum dynamics of H + Li2 (X1Σg+) → Li + LiH (X1Σ+) reaction

A global potential energy surface (PES) for the electronic ground state of Li₂H system is constructed over a large configuration space. About 30 000 ab initio energy points have been calculated by MRCI‐F12 method with aug‐cc‐pVTZ basis set. The neural network method is applied to fit the PES and the root mean square error of the current PES is only 1.296 meV. The reaction dynamics of the title reaction has been carried out by employing time‐dependent wave packet approach with second order split operator on the new PES. The reaction probability, integral cross section and thermal rate constant are obtained from the dynamics calculation. In most of the collision energy regions, the integral cross sections are in well agreement with the results reported by Gao et al. The rate constant calculated from the new PES increases in the temperature range of present investigation.     

Ab initio studies on several species of the formula X3YO and X3YOH

Ab initio molecular orbital calculations using both minimal and extended basis sets have been applied to two isoelectronic sets of molecules. One set corresponds to the 18 electron species H₃NO, H₃CO^– and H₃COH while the second set contains the 42 electron fluorinated molecules F₃NO, F₃CO^– and F₃COH. The geometries of these molecules have been optimized, using both the minimal STO-3G and the extended 4-31G basis sets. These comparative calculations reveal that the 4-31G basis produced structural parameters in much better agreement with experiment. The effect of includingd-orbitals in the basis set was also investigated. For the fluorinated oxides it has been found that the optimized 4-31G structures were only slightly altered by the addition ofd-orbitals. For H₃NO, on the other hand, the inclusion ofd-orbitals considerably shortens the N-O bond distance. Both H₃NO and CF₃OH, which are unknown experimentally, are theoretically predicted to be capable of existence. The electronic structures of these molecules have also been examined using electronic partitioning according to the Mulliken scheme.     

Ab initio calculations on SCl2 and low-lying cationic states of SCl2+: Franck-Condon simulation of the UV photoelectron spectrum of SCl2

Geometry optimization calculations were carried out on the (approximate)X (1)A(1) state of SCl(2) and the (approximate)X(2)B(1), (approximate)A(2)B(2), (approximate)B(2)A(1), (approximate)C(2)A(1), (approximate)D(2)A(2), and (approximate)E (2)B(2) states of SCl(2) (+) at the restricted-spin coupled-cluster single-double plus perturbative triple excitation [RCCSD(T)] level with basis sets of up to the augmented correlation-consistent polarized quintuple-zeta [aug-cc-pV(5+d)Z] quality. Effects of core electron correlation, basis set extension to the complete basis set limit, and relativistic contributions on computed minimum-energy geometrical parameters and/or relative electronic energies were also investigated. RCCSD(T) potential energy functions (PEFs) were calculated for the (approximate)X (1)A(1) state of SCl(2) and the low-lying states of SCl(2)(+) listed above employing the aug-cc-pV(5+d)Z basis set. Anharmonic vibrational wave functions of these neutral and cationic states of SCl(2), and Franck-Condon (FC) factors of the lowest four one-electron allowed neutral photoionizations were computed employing the RCCSD(T)aug-cc-pV(5+d)Z PEFs. Calculated FC factors with allowance for the Duschinsky rotation and anharmonicity were used to simulate the first four photoelectron (PE) bands of SCl(2). The agreement between simulated and observed He I PE spectra reported by Colton et al. [J. Electron Spectrosc. Relat. Phenom. 3, 345 (1974)] and Solouki et al. [Chem. Phys. Lett. 26, 20 (1974)] is excellent. However, our FC spectral simulations indicate that the first observed vibrational component in the first PE band of SCl(2) is a "hot" band arising from the SCl(2)(+)(approximate)X(2)B(1)(0,0,0)<--SCl(2)(approximate)X (1)A(1)(1,0,0) ionization. Consequently, the experimental adiabatic ionization energy of SCl(2) is revised to 9.55+/-0.01 eV, in excellent agreement with results obtained from state-of-the-art ab initio calculations in this work.     

Ab initio calculations of infrared transition probabilities in the electronic ground states of AlF and AlF+

Electric dipole moment functions and radiative transition probabilities have been calculated for the electronic ground states of AlF and AlF⁺ from highly correlated CEPA electronic wavefunctions. The dipole moments inv = 0 are calculated to be 1.56 D (experimental value is 1.53 ±0.1 D) for AlF and 5.49 D for AlF⁺. Intense transitions in the microwave and infrared spectral region are predicted for both species.     

Ab initio vibration—rotation coupling constants and the equilibrium geometries of NCCN and CNCN

The equilibrium geometries of NCCN and CNCN were calculated from experimental ground-state rotational constants and ab initio values for the vibration—rotation coupling constants. For NCCN, R_1e(NC) = 1.1578(5) Å and R_2e(CC) = 1.3839(5) Å were obtained, where estimated error bars are given in parentheses. The calculated equilibrium bond lengths of CNCN are R_1e(CN) = 1.1813(5) Å, R_2e(NC) = 1.3116(5) Å and R_3e(CN) = 1.1581(5) Å. Ground-state rotational and centrifugal distortion constants are predicted with high accuracy for various isotopomers of NCCN.     

Ab initio study of the electronic states of BS including spin–orbit coupling

The entire 30 Ω states generated from the 12 valence and two Rydberg Λ–S states of the BS radical have been studied at the MR-CISD + Q level of theory for the first time. The effects of spin–orbit coupling and the avoided crossing rule between electronic states of the same symmetry were analyzed. Spectroscopic constants of several excited states that have never been observed in experiment were obtained. The transition properties of several low-lying bound excited states to ground state transitions, including the transition dipole moments and the Franck–Condon factors, were also calculated, from which the corresponding single-channel radiative lifetimes were derived.     

Ab initio calculation of the Young's modulus of α‐polyamides

In earlier works the supermolecule model has been applied to the calculation of the Young's modulus of crystalline polyethylene and polyamide‐6. In the supermolecule model a crystalline polymer is represented as a single finite chain divided into a head, body, and tail part. The body contains a number of monomer units and is representative for a polymer chain. In this article, this model has been used to study the geometric properties and the elastic moduli of the α form of other polyamides: polyamide‐2 (or polyglycine), polyamide‐3, polyamide‐4, polyamide‐11, and polyamide‐6,6. All calculations have been performed with a linearly constrained body. The results have been compared to other theoretical and experimental results if available. © 2002 Wiley Periodicals, Inc.; DOI 10.1002/qua.2002;10121     

Conformation and reactivity of α-oxo-ketenes: Ab initio and semiempirical (AM1, PM3) calculations

Ab initio MP2/6-31G*//MP2/6-31G* and semiempirical AM1 and PM3 calculations on a series of differently substituted α-oxo-ketenes are used to investigate E/Z-isomerism and rotational barriers in these molecules. Sterically crowded derivatives are found to exist solely as s-E conformers. The unusual stability of these derivatives thus can be attributed to their inability to adopt the s-Z conformation required for the normal α-oxo-ketene reactions. With respect to structures and energies, the PM3 method (especially in the case of highly crowded molecules) is found to be less reliable than AM1. Ab initio HF/3-21G and PM3 vibrational frequencies appear to be of sufficient accuracy for a distinction between s-Z and s-E conformers. In this respect, the AM1 method appears less reliable. © 1994 by John Wiley & Sons, Inc.     

Ab initio calculations on SF2 and its low-lying cationic states: anharmonic Franck-Condon simulation of the UV photoelectron spectrum of SF2

Geometry optimization calculations were carried out on the (approximate)X(1)A(1) state of SF2 and the (approximate)X(2)B(1), (approximate)A(2)A(1), (approximate)B(2)B(2), (approximate)C(2)B(2), (approximate)D(2)A(1), and (approximate)E(2)A(2) states of SF2(+) employing the restricted-spin coupled-cluster single-double plus perturbative triple excitation [RCCSD(T)] method and basis sets of up to the augmented correlation-consistent polarized quintuple-zeta [aug-cc-pV(5+d)Z] quality. Effects of core electron (S 2s(2)2p(6) and F 1s(2) electrons) correlation and basis set extension to the complete basis set limit on the computed minimum-energy geometries and relative electronic energies (adiabatic and vertical ionization energies) were investigated. RCCSD(T) potential energy functions (PEFs) were calculated for the (approximate)X(1)A(1) state of SF2 and the low-lying states of SF2(+) listed above employing the aug-cc-pV(5+d)Z and aug-cc-pV5Z basis sets for S and F, respectively. Anharmonic vibrational wave functions of these neutral and cationic states of SF2, and Franck-Condon (FC) factors of the lowest four one-electron allowed neutral photoionizations were computed employing the RCCSD(T) PEFs. Calculated FC factors with allowance for Duschinsky rotation and anharmonicity were used to simulate the first four photoelectron bands of SF2. The agreement between the simulated and observed first bands in the He I photoelectron spectrum reported by de Leeuw et al. [Chem. Phys. 34, 287 (1978)] is excellent. Our calculations largely support assignments made by de Leeuw et al. on the higher ionization energy bands of SF2.     

Ab initio study of the J(CC) indirect nuclear spin-spin coupling constants in cyclobutane and related systems

Non-empirical calculations using the equations-of-motion approach, which incorporates the main portion of the electron correlation effects, are reported for the carbon-carbon nuclear spin-spin coupling constants in cyclobutane, bicyclobutane, tricyclobutane, cyclobutene, cyclobutyne, cyclobutadiene, bicyclobutene, methylenecyclopropane, and methylenecyclopropene. The results provide an overall picture of the influences exerted on sign and magnitude of the J(CC) by progressive condensation, unsaturation, and branching rearrangement of the cyclobutane frame.