An interpretation of the nuclear quadrupole coupling constant for the HCl molecule on the basis of an “ab initio” calculation

The results of a LCAO MO SCF calculation of the quadrupole coupling constant of chlorine in the HCl molecule (carried out with a fairly extended set of basic orbitals) are analyzed. Sensible deformations of the internal shells and of the p -bonding, p -lone pair and p type orbitals of the valence shell of chlorine are evidenced.

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Zusammenfassung Die Ergebnisse einer verhÄltnismÄ\ig ausgedehnten LCAO-MO-SCF-Rechnung zur Bestimmung der Quadrupolkopplungskonstanten für Chlor in HCl werden untersucht. Die Verzerrungen, die der Rumpf und die verschiedenen ZustÄnde der Valenzschale erleiden, werden dabei gesondert wiedergegeben.

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Résumé Les résultats d'un calcul LCAO-MO-SCF (avec un jeu d'orbitales assez étendu) de la constante de couplage quadripolaire du Cl dans la molécule de HCl sont analysés. On trouve que et les couches internes et les orbitales p liante, p libre et p de la couche de valence du chlore sont sensiblement déformées.

     

Potential energy surfaces for the benzene–rare gas systems

A harmonic expansion representation of the intermolecular interaction has been exploited to obtain the potential energy surface (PES) for the C₆H₆–He, –Ne, –Ar, –Kr and –Xe systems in an analytical form. Basic data employed are binding energy, equilibrium distance and long-range attraction predicted by a semi-empirical method for selected configurations of the complexes. For those favorable cases where additional information are available the proposed PESs exhibit features in good agreement with those derived from spectroscopy and scattering experiments and/or ab initio calculations. The availability of realistic PESs expressed in an analytical form opens new perspectives of calculations in molecular dynamics and spectroscopic simulations where the benzene molecule and rare gas atoms are involved.     

Ab initio calculations for the potential curves and spin–orbit coupling of Mg2

 The ground state and several low-lying excited states of the Mg₂ dimer have been studied by means of a combination of the complete-active-space multiconfiguration self-consistent-field (CASSCF)/CAS multireference second-order perturbation theory (CASPT2) method and coupled-cluster with single and double excitations and perturbative contribution of connected triple excitations [CCSD(T)] scheme. Reasonably good agreement with experiment has been obtained for the CCSD(T) ground-state potential curve but the dissociation energy of the only experimentally known A¹Σ _u ⁺ excited state of Mg₂ is somewhat overestimated at the CASSCF/CASPT2 level. The spectroscopic constants D _e, R _e and ω_e deduced from the calculated potential curves for other states are also reported. In addition, some spin–orbit matrix elements between the excited singlet and triplet states of Mg₂ have been evaluated as a function of internuclear separation. Received: 10 May 2001 / Accepted: 15 August 2001 / Published online: 30 October 2001     

Is large-scale ab initio Hartree-Fock calculation chemically accurate? Toward improved calculation of biological molecule properties

Numerical errors in total energy values in large-scale Hartree–Fock calculations are discussed. To obtain total energy values within chemical accuracy, 0.01 kcal/mol, stricter numerical accuracy is required as basis size increases. In molecules with 10,000 basis sizes, such as proteins, numerical accuracy for total energy values must be retained to at least 11 digits (i.e., to the order of 1.0D-10) to keep accumulation of numerical errors less than the chemical accuracy (0.01 kcal/mol). With this criterion, we examined the sensitivity analysis of numerical accuracy in Hartree–Fock calculation by uniformly replacing the last bit of the mantissa part of a double-precision real number by zero in the Fock matrix construction step, the total energy calculation step, and the Fock matrix diagonalization step. Using a partial summation technique in the Fock matrix generation step, the numerical error for total energy value of molecules with basis size greater than 10,000 was within chemical accuracy (0.01 kcal/mol), whereas with the conventional method the numerical error with several thousand basis sets was larger than chemical accuracy. Computation of one Fock matrix element with parallel machines can include the partial summation technique automatically, so that parallel calculation yields not only high-performance computing but also more precise numerical solutions than the conventional sequential algorithm. We also found that the numerical error of the Householder-QR diagonalization routine is equal to or less than chemical accuracy, even with a matrix size of 10,000. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 443–454, 1999     

An ab initio Study of Intermolecular Potential for Ne-HBr Complex

ＩｎｔｒｏｄｕｃｔｉｏｎＴｈｅｒａｒｅ ｇａｓｈｙｄｒｏｇｅｎｈａｌｉｄｅ (Ｍ—ＨＸ)ｃｏｍｐｌｅｘｅｓｈａｖｅｌｏｎｇｂｅｅｎｓｔｕｄｉｅｄｆｏｒｉｎｓｉｇｈｔｓｉｎｔｏｔｈｅｎａｔｕｒｅｏｆｉｎ ｔｅｒｍｏｌｅｃｕｌａｒｆｏｒｃｅｓａｎｄｄｅｔａｉｌｓｏｆｉｎｔｅｒ ａｎｄｉｎｔｒａｍｏｌｅｃｕｌａｒｄｙｎａｍｉｃｓ .1,2 ＳｙｓｔｅｍａｔｉｃｉｎｖｅｓｔｉｇａｔｉｏｎｓｏｆｔｈｅｃｏｍｐｌｅｘｅｓｏｆＸｅ ,Ｋｒ,Ａｒ,ａｎｄｌａｔｅｌｙＮｅｗｉｔｈＨＦ ,ＨＣｌ,ＨＢｒａｎｄｔｈｅｉｒｄｅｕｔｅｒａ…     

The ab initio calculation of the band origin and vibrational frequencies of the Ã-X̃ system of HNCl using the non-rigid bender hamiltonian

A three-dimensional fit of ab initio MRD CI potential data has been made for the lowest two electronic states of the HNC1 molecule (X̃ ²A″ and à ²A'), and the corresponding vibrational frequencies and rotational energies have been computed using the non-rigid bender Hamiltonian. For the ground state the vibrational frequencies obtained are ν₁ = 2942 cm⁻¹, ν₂ = 1232 cm⁻¹, and ν₃ = 549 cm⁻¹, while the corresponding values for the first excited state are 3524,947 and 836 cm⁻¹ respectively. We calculate T_c(à ²A') 16200 cm⁻¹, T_o(òA') = 16400 cm⁻¹, and the Franck-Condon maximum, Ã(0,3,1)-X̃(0,0.0), is calculate at 19200 cm⁻¹(5200 Å).     

Geometrical structure of benzene and naphthalene: ultrahigh-resolution laser spectroscopy and ab initio calculation

Geometrical structures of the isolated benzene and naphthalene molecules have been accurately determined by using ultrahigh-resolution laser spectroscopy and ab initio calculation in a complementary manner. The benzene molecule has been identified to be planar and hexagonal (D(6h)) and the structure has been determined with accuracies of 2 × 10(-14) m (0.2 mA?; 1 A? = 1 × 10(-10) m) for the C-C bond length and 1.0 × 10(-13) m (1.0 mA?) for the C-H bond length. The naphthalene molecule has been identified to be symmetric with respect to three coordinate axes (D(2h)) and the structure has been determined with comparable accuracies. We discuss the effect of vibrational averaging that is a consequence of zero-point motions on the uncertainty in determining the bond lengths.     

Fluorine spin–spin coupling constants of pentafluorobenzene revisited at the ab initio correlated levels

All possible spin–spin coupling constants, ¹⁹F–¹⁹F, ¹⁹F–¹³C, and ¹⁹F–¹H, of pentafluorobenzene were calculated at five different levels of theory, HF, DFT, SOPPA (CCSD), CCSD, and the SOPPA (CCSD)-based composite scheme with taking into account solvent, vibrational, relativistic, and correlation corrections. Most corrections were next to negligible for the long-range couplings but quite essential for the one-bond carbon–fluorine coupling constants. Hartree–Fock calculations were found to be entirely unreliable, while DFT results were comparable in accuracy with the data obtained using the wave function-based methods.     

An ab initio study of the E 3Πg state of the iodine molecule

The E (3)Π(g) state of the iodine molecule is studied by ab initio multireference methods coupled with effective core potentials and large basis sets. Two potential minima are found, a global featuring an ion-pair character, and a local presenting a purely Rydberg nature. Four avoided crossings along the dissociation coordinate attribute an interesting topology to its potential energy curve, and their effect on the vibrational levels of I(2) is discussed.     

Characterization of [M–H] cations,radicals and anions of glycine in the gas phase: a combined experimental and ab initio study

The gas phase structures of the [M–H] cations and anions of glycine have been studied by using a combination of ab initio calculations (at the MP2(FC)/6–31+G1 level of theory) and tandem mass spectrometry (MS/MS). It was found that the ab initio stability order for the anions is [H₂NCH₂CO₂]⁻ > [H₂NCHCO₂H]⁻ > [HNCH₂CO₂H]⁻. In contrast, the cations exhibit different behaviour, whereas [H₂NCHCO₂H]⁺ is predicted to be a stable structure, [H₂NCH₂CO₂]⁺ spontaneously fragments to the ion–molecule complex [H₂NCH₂⁺ ⋯ (OCO)] and the singlet [HNCH₂CO₂H]⁺ isomer is predicted to undergo a skeletal rearrangement to form [CH₂NHCO₂H]⁺. MS/MS spectra of [M–H]⁺ cations of various glycine isotopomers were obtained via: (i) collisional activation of electron impact generated cations and (ii) charge reversal of anions formed via HO⁻ negative ion chemical ionization. The resulting spectra were significantly different, suggesting different structures were involved. Neutralization–reionization experiments were performed on [M–H]⁻ anions in order to gain insights into the structures of the intermediate radicals.     

The use of the cohen–turnbull model for calculation of the self-diffusion coefficients of liquid dichloroalkanes

The paper presents the self-diffusion coefficients calculated for liquid dichloroalkanes C₆H₁₂Cl₂, C₈H₁₆Cl₂, C₁₀H₂₂Cl₂ and C₁₂H₂₄Cl₂, with the use of the Cohen and Turnbull model. Determination of self-diffusion coefficients permits a separate analysis of intra- and intermolecular motions and provides information on geometrical and dynamical properties of molecules. The self-diffusion coefficients of selected dichloroalkanes have been determined by X-ray diffraction and compared with the corresponding NMR results. The suitability of the Cohen–Turnbull model of the translating motion for prediction of self-diffusion coefficients for molecules whose shape significantly differs from the spherical symmetry is analysed. Angular distributions of X-ray scattered intensity were measured, and differential radial distribution functions of electron density (DRDFs) were calculated. The mean coordination numbers were obtained from the area delimited by the minima of the DRDFs, and their dependence on the length of the methylene chain is also presented subsequently. On the basis of the DRDFs the average free volume of the molecules and total free volume of the liquids were calculated. The activation volume of the diffusion was found to make about 0.6 of the van der Waals volume of the molecule. As expected the diffusion coefficients decrease with increasing molecular weight. The equation relating the self-diffusion coefficient with the volume of the coordination spheres in the liquid has been derived.     

Assignment of the molecular absolute configuration through the ab initio Hartree-Fock calculation of the optical rotation: can the circular dichroism data help in reducing basis set requirements?

In this paper the calculation of the optical rotation (OR) of some rigid organic molecules, using the Hartree-Fock method with small (6-31G, DZP) basis sets, has been studied thoroughly to carefully evaluate the scope and limitations of this method, previously introduced by other authors. Calculations on test molecules (compounds 1-13) together with a careful analysis of their CD spectra allow a simple criterion for the reliability of this approach to be formulated: for unsaturated and/or aromatic (i.e., absorbing in the near-UV region) molecules, if the [alpha](D) is quantitatively determined by the lowest energy Cotton effect (at wavelengths >220 nm), then the HF/6-31G result is reliable. The usefulness of this method for the experimental organic chemist has been further demonstrated because the OR (sign and order of magnitude) of compounds 14-19 (i.e., large molecules having considerable interest in organic chemistry), which fulfill the above criterion and for which an extended basis set treatment is not feasible owing to their size, is correctly predicted.     

Ab initio MRD CI calculation of the zero-field splitting of the 2Π ground state of the CBr molecule

The zero-field spin-orbit splitting of the ²Π ground state of CBr is computed by means of an ab initio MRD CI treatment employing the Breit-Pauli formalism. The choice of the one-electron basis is found to be important, but the results are seen to be relatively insensitive to the number of core electrons employed in the CI computations.     

N–H stretching frequencies and the conformation of substituted ureas: an ab initio MO study

The dependence of N–H stretching-mode frequencies in representative di- and trialkyl ureas on the conformational state of the ureido group has been studied by ab initio MO calculations using HF/3-21G and HF/6-31G** basis sets. The molecules studied were 1,3-dimethylurea, 1-methyl-3,3-dimethylurea and 1-methyl-3,3-di-iso-propylurea. The principal conclusions from the ab initio results are:

• 1.the trans–trans conformer (N–H bonds trans to the CO bond) has N–H stretching bands with about 20–30 cm⁻¹ higher frequency than the respective cis–cis structure, in accord with earlier literature assignments based on experimental data;
• 2.the N–H stretching frequency interval in tri-substituted ureas is 15–20 cm⁻¹ higher than the N–H band position in the 1,3-disubstituted molecule studied, the effect being determined mostly by the higher N–H stretching force constant;
• 3.in the absence of the steric hindrance the stable rotameric forms of the ureido grouping are almost planar at HF/3-21G level of calculations, while HF/6-31G** calculations predict a slightly pyramidal structure at the nitrogen atoms in the trans–trans conformer;
• 4.in 1-methyl-3,3-di-iso-propylurea the steric influence of the two bulky iso-propyl groups cause a deviation from planarity of the N–H bond. The non-planar conformation is accompanied by a shift of the N–H stretching mode frequency towards higher values; and
• 5.the variations of the theoretically estimated N–H stretching-mode frequencies appear to be principally determined by changes in the N–H stretching force constants in the different molecules.

© 1997 Elsevier Science B.V.     

The application of the perrin–agishev model for calculation of the translational diffusion coefficients of liquid dichloroalkanes

The article presents the translational diffusion coefficients calculated for dichloroalkanes series C _n H_{2 n} Cl₂ (where n =?6, 8, 10, 12) in the liquid state, with the use of the Perrin and Agishev model. It has been shown that the molecules of dichloroalkanes assume a mutually parallel arrangement in three possible coordinations. The model of arrangement, orientations and packing of the molecules has been presented. The activation parameters of the compounds studied have been discussed. The physical and structural properties of the liquids studied (macroscopic density, electron density and molecular weight) are correctly described within the van der Waals model predicting their orientations and packing. The formulae linking the diffusion, viscosity and temperature for the liquids have been presented. The assumption that each molecule can be approximated by an ellipsoid of the semiaxes lengths a, b and c has been justified. The translations become slower with increasing volume and weight of the molecule. The diffusion coefficients decrease with increasing molecular weight.