The calculation of London coefficients by the variation-perturbation method

The London coefficients for the dispersion interaction between LiLi, BeBe and LiBe are calculated by the variation-perturbation method using the whole atomic hamiltonian as H₀ and the Hartree-Fock approximation for the upper turbed wavefunction ?₀. A single excited valence configuration with optimized orbital exponents gives accurate results for Li, whereas comparable accuracy for Be is obtained when part of the valence correlation energy in the ground as well as in the excited state is accounted for through a limited configuration interaction.     

Exchange reactions of hydrogen halides with hydrogenic atoms

Calculations on the D + HBr → DBr + H and D + HI → DI + H reactions are reported. A three-dimensional, quantum-dynamical approximation is used which involves applying the energy sudden approximation to the entrance channel hamiltonian and the centrifugal sudden approximation to the exit channel hamiltonian. Results of integral and differential cross sections, rate coefficients and rotational distributions are presented. Diatomics-in-molecules potential-energy surfaces have been used in the computations. The HBrH potential has been optimesed so that the calculated room-temperature rate coefficient agrees with experiment. This potential has a barrier height of 0.237 eV. Rate coefficient computations for the four reactions H′ + H″ Cl → - H′Cl + H″ (H′, H″ = H or D) are also reported. These results, for a LEPS surface, agree well with those obtained in quasiclassical trajectory and variational transition state theory calculations.     

Estimates for systematic empirical corrections of consistent 4–21G ab initio geometries and their correlations to total energy group increments

The structural parameters of the completely relaxed 4–21G ab initio geometries of more than 30 basic organic compounds are compared to experimental results. Some ranges for systematic empirical corrections, which relate 4–21G bond distances to experimental parameters, are associated with total energy increments. In general, for the currently feasible comparisons, the following corrections can be given which relate calculated distances to experimental r_g parameters and calculated angles to r_s-structures For CC single bond distances, deviations between calculated and observed parameters (r_g) are in the ranges of ?0.006(2) to ?0.010(2) Å for normal or unstrained hydrocarbons; ?0.011(3) to ?0.016(3) Å for cyclobutane type compounds; and +0.001(5) to +0.004(4) Å for CH₃ conjugated with CO. For CO single bonds the ranges are ?0.006(9) to +0.002(3) Å for CO conjugated with CO; and ?0.019(3) to ?0.027(9) Å for aliphatic and ether compounds. A very large and exceptional discrepancy exists for the highly strained ethylene oxide, r_s — r_e = ?0.049(5) Å and in CH₃OCH₃ and C₂H₅OCH₃ the r_s — r_e differences are ?0.029(5), ?0.040(10) and ?0.025(10) Å. Some of these discrepancies may also be due to deficiencies of the microwave substitution method caused by atomic coordinates close to inertial planes. For CN bonds, two types of NCH₃ corrections are from +0.005(6) to ?0.006(6) and from ?0.009(2) to ?0.014(6) Å; and the range for NCO is +0.012(3) to +0.028(4) Å. For isolated CC double bonds the range is + 0.025(2) to +0.028(2) Å. For conjugated CC double bonds the correction is less positive (+0.014(1) Å for benzene). For CO double bonds the corrections are ?0.004(3) to +0.003(3) Å. For bond angles of type HCH, CCH, CCC, CCO, CCO, OCO, NCO and CCC the corrections are of the order of magnitude about 1–2° (or better). Angles centered at heteroatoms are less accurate than that, when hydrogen atoms are involved. Differences in HOC and NHC angles were found in a range of ?2.3(5)° to ?6.2(4)°.     

Second-order properties of bonds

By defining a localized hamiltonian for a saturated molecule its second order molecular properties may be regarded as a sum of bond contributions. Static and dynamic polarizabilities are calculated for the CH bond in methane and the latter one used to calculate the van der Waals interaction between two CH₄ molecules.     

Enhancement of reactivity through near-resonant radiative excitation of an overtone: model study of an isomerization by hydrogen tunneling

The enolization of 2-methyl acetophenone derivatives through hydrogen tunneling is simply modeled to study the possibility of increasing the reaction yield by exciting the v = 7 CH overtone. Vibrational relaxation is assumed to occur via Fermi resonances transforming one CH quantum into two quanta of the H bending. The role of the quasi-continuum of CC modes is taken into account via an effective hamiltonian. It is shown that the only way to increase significantly the reaction yield is using light detuned a few hundred cm⁻¹ from the overtone level, as suggested by Tannor et al.     

Dynamical effects of mode specific excitation in unimolecular decomposition: A trajectory study of C2H6

Classical trajectories on a well coupled model potential energy surface have been used to study the effects of mode localized excitation on unimolecular decomposition in C₂H₆. The results display both apparent and intrinsic non-statistical effects that can be ascribed to restrictions in intramolecular energy transfer both among the CH vibrations and between these CC motion.     

Sensitivity of the H + CH3 → CH4 recombination rate constant to the shape of the CH stretching potential

Using a potential-energy surface obtained in part from ab initio calculations, the H + CH₃ → CH₄ bimolecular rate constant at T = 300 K is determined from a Monte Carlo classical trajectory study. Representing the CH stretching potential with a standard Morse function instead ofthe ab initio curve increases the calculated rate constant by an order of magnitude. The experimental recombination rate constant is intermediate of the rate constants calculated with the Morse and ab initio stretching potentials.Two properties of the H + CH₃ α CH₄ potential-energy surface which significantly affect the recombination rate constant are the shape of the CH stretching potential and the attenuation of the H₃CH bending frequencies. Ab initio calculations with a hierarchy of basis sets and treatment of electron correlation indicate the latter is properly described [13]. The exact shape of the CH stretching potential is not delineated by the ab initio calculations, since the ab initio calculations are not converged for bond lengths of 2.0–3.0 Å [12]. However, the form of this stretching potential deduced from the highest-level ab initio calculations, and fit analytically by eq. (2), is significantly different from a Morse function. The experimental recombination rate constant is intermediate of the rate constants calculated with the Morse and ab initio CH stretching potentials. This indicates that the actual CH potential energy curve lies between the Morse and ab initio curves. This is consistent with the finding that potential energy curves for diatomics are not well described by a Morse function [12].     

Halometallates of 1-methyl-4,4-dimercapto-piperidinium. Evidence of strong hydrogen bonds with participating thiol groups

Chlorometallates of transition and B subgroup elements are readily prepared and precipitated by reaction of the corresponding metallic salts with 1-methyl-4,4-dimercaptopiperidinium chloride. These chlorometallates investigated were [ZnCl₄]^2?, [CdCl₃]^?, [CoCl₄]^2?, [CuCl₅]^3? and [FeCl₄]^2?. Strong SH … Cl interactions, but not NH … Cl, have been evidenced by IR spectroscopy in the zinc, cadmium and cobalt complexes. The SH and NH absorptions are observed at ? 2480 cm^?1 and 3060 cm^?1, respectively. Partial deuteration of the [ZnCl₄]^2? complex with d₁-methanol, shifted these IR signals to 1800 and 2260 cm^?1, clearly evidencing a X-hydrogen type of bond. The SH … Cl interaction is smaller in the [FeCl₄]^2? complex, and practically nonexistent in the [CuCl₅^3? complex.     

The molecular structure of chlorothiomethoxymethyltriphenylphosphinepalladium(II) at −160 and 20°C

The molecular structure of [PdCl(CH₂SCH₃)(PPh₃)] has been determined from three-dimensional X-ray diffraction data collected at both ?160 and 20°C. The crystal belongs to the monoclinic system, space group P2₁/c, with four formula units in a cell of dimensions: a 11.398(2), b 9.788(1), c 17.267(2) Å and β 95.38(1)° at ?160°C; a 11.454(3), b 9.880(2), c 17.459(2) Å and β 95.84(1)° at 20°C. The structure was solved by the conventional heavy atom method, and refined by the least-squares procedure to R = 0.049 (?160°C) and 0.068 (20°C) for observed reflections. No essential difference is observed between molecular structures at ?160 and 20°C. The geometry around the palladium atom is square-planar. The CH₂SCH₃ group, bonded to the palladium atom through PdC and PdS bonds, forms a metallocyclic three-membered ring [PdC(1) 2.042(6), PdS 2.371(1), SC(1) 1.756(6) and SC(2) 1.807(7) Å, PdC(1)S 76.9(2), PdC(1)H 113(3) and 122(4)°, SC(1)H 115(3) and 112(4) and HC(1)H 113(5)° at ?160°C].     

Dirac—Fock one-centre calculations. VI. The tetrahedral and octahedral model systems CeH4, ThH4, CrH6, MoH6, WH6, UH6 and (106)H6

Relativistic and non-relativistic Hartree—Fock calculations are reported for the tetrahedral model systems CeH₄ and ThH₄ and the octahedral model systems CrH₆, MoH₆, WH₆, UH₆ and (106)H₆. The effects of relativity on bond strengths and lengths are obtained from fits to a Morse potential. The calculated CrH, MoH and WH bond lengths are comparable to those measured in the organometallic systems. Their relativistic contractions are 0.36, 0.8 and 2.8%, respectively. For the MH_n systems (M = Ti, Zr, Hf, Th, Cr, Mo, W), the calculated MH bond lengths differ in the average from the experimental MX by 16, 39, ?3, 42, 55 and 75 pm for X = Hb(BH₄), C(σ), F, Cl, Br and I, respectively, suggesting a “hydridic” hydrogen covalent radius of 58 pm. A comparison of the bond lengths for CeH₄ and HfH₄ yields the value of 19 pm for the lanthanoid contraction. A corresponding non-relativistic calculation gives 16 pm. Thus the lanthanoid contraction is predominantly a non-relativistic shell-structure effect. A similar comparison of ThH₄ and (104)H₄ or of UH₆ and (106)H₆ predicts an actinoid contraction of 30 pm for compounds of the present type. The fact that WH bonds are stronger than MoH bonds is probably due to relativistic effects. Strong s-p-d hybridization is found for CeH₄ and ThH₄ with only weak f AO contributions. For UH₆ the f AO participation is four times larger and has a double-humped radial distribution suggesting “true” 5f participation in bonding. Adding the 5f:s shortens the bond length by 8 and 22 pm for ThH₄ and UH₆, respectively, also indicating s-p-d-f hybridization for uranium. The 7p radius for Th is larger than the 6d radius and the norm N(7p), of the t₂ MO increases with increasing bond length R. Therefore this t₂ MO causes a potential V(R), that is more attractive than in the case without 7p functions, even at R of the order of 500 pm. Possible connection with hydrogenation catalysis by elements like Th and Ti is discussed.     

An ab initio study of the structure and torsional modes of the H2SO4 molecule

Ab initio calculations at the STO—3G and 4—31G levels have been carried out for the H₂SO₄ molecule as a function of the pair of twist angles of the HO bonds about the respective OS bonds. Values for the remaining bond angles and lengths were taken from the recent microwave structural determination by Kuczkowski et al. The results indicate a minimum energy for a structure with a (sc, sc) conformation and C₂ symmetry, where sc denotes synclinal, or gauche. This structure corresponds to that observed. At a higher energy of 11.5 kJ mol^?1 (4—31G) there is a structure with a (+sc, ?sc) conformation and C_s symmetry. The torsional modes corresponding to the a and b irreducible representations of the C₂ point group are estimated to have frequencies of 280 and 265 cm^?1, respectively.     

The thermodynamic effect of fluorine as a substituent: Allylic CF3 and CF2H

Cope rearrangements of 7,7,7-trifluoro-1,5-heptadiene and 7,7-difluoro-1,5-heptadiene were examined to gain quantitative understanding of the thermodynamic effect of allylic fluorine substitution. Group value contributions to ΔH_f's were thus able to be determined for allylic fluorine-substituted carbon groups: C(F)₃(C_d) = ?166.0, C(F)₂(H)(C_d)= ?107.6 and C(F)(H)₂(C_d) = ?52.2 kcal/mole.     

Energy surface calculations for the reaction HF + H+ ⇌ HFH+

Various GTO basis sets were investigated for their effectiveness in determining the SCF energy and geometry of the HFH⁺ molecule. A double zeta set augmented with a p_z function on each H atom was used to calculate the potential energy surface for the collinear protonation of HF. Limited configuration interaction calculations gave an energy of ?100.27365 E_a for an HF separation of 1.819 a₀ and a bond angle of 118.1°, and an energy of protonation of 119.5 kcal/mol.     

Pulsed fourier transform NMR of substituted aryltrimethyltin derivatives : III. Proton data at 100 MHz and the derived Hammett σ-constant of the trimethyltin group

Proton NMR data at 100 MHz are reported for thirteen para- and meta-substituted phenyltrimethyltin compounds, XC₆H₄Sn(CH₃)₃, where X = para-N(CH₃)₂, para-OCH₃, para-OC₂H₅, para-CH₃, meta-CH₃, -H, para-F, meta-OCH₃, para-Cl, para-Br, meta-F, meta-Cl and para-Sn(CH₃)₃. Correlation coefficients with Hammett σ-constants of greater than 0.95 are obtained with the methyltin proton chemical shifts and coupling constants to carbon [¹J(¹³C¹H)] and tin [²J(SnC¹H)]. Solvent effects and other extraneous factors invalidate comparisons of ? values in terms of the relative attenuation of the transmission of substituent effects through homologous carbon, silicon, germanium and tin systems, but coupling constant data reflect a diminution of ca. one tenthfold per bond in the order ?[C(1)Sn] > ? [SnC] > ? [CH]. Satisfactory correlations (r > 0.95) are obtained in this series of closely-related compounds among the conventionally recorded two-bond, ²J(SnC¹H) and the constituent, one-bond ¹J (Sn¹³C) and J(¹³C¹H) coupling constants, but the correlation coefficient for the comparison between the two one-bond couplings, ¹J(Sn¹³C) and ¹J(¹³C¹H) is lower (r = 0.872). Changes in the couplings at the methyltin carbon bond tin-119 atoms are interpreted in terms of isovalent hybridization; a model based upon effective nuclear charges is tested with respect to both NMR coupling constants and ¹¹⁹Sn Mössbauer Isomer shifts at tin and is invalidated. Proton and carbon-13 NMR, chemical shift and coupling constant data are used to derive a Hammett σ-constant for the para-trimethyltin group of ?0.14, and the significance of this value is discussed.     

Calculation of a power spectrum and definition of wave functions of an ion Cr3+ in antiferromagnetic crystal Cr2O3 in the model of a crystalline field

The antiferromagnetic crystal Cr₂O₃ contains magnetoactive ions Cr3⁺ and has C3v local symmetry. In this crystal exist cross‐effects, for example linear magnetoelectric effect and others. It is necessary to know the energy levels and wave functions of an ion Cr3⁺ exact simulation of this effect. In the present paper the problem of calculation of the crystalline field potential is solved in the single‐ion model. For Cr₂O₃ the splitting of energy levels are determined and wave functions for ground and exited states are calculated. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

Vibration-to-rotation and vibration-to-vibration energy transfer between diatomic molecules

A classical model for VR and VV energy transfer between two dissimilar diatomic molecules is proposed. For the exponential repulsive potential, in the limit as the rotation of the molecules goes to zero, the present model reduces to the previous results for VT and VV energy transfer. For the hydrogen halides, whose relaxation is generally accepted as being governed by VR energy transfer, the Pr's predicted by the present theory agree reasonably well with the experimental data. In particular, the positive and negative temperature dependence of the VV Pr's for N₂HI and N₂DI are predicted.     

Gaussian representations of charge overlap effects in intermolecular forces

The ground state H? H⁺ and H? H interactions are used as model interactions for investigating the feasibility of using Gaussian basis sets for representing charge overlap effects in intermolecular forces. The non-expanded charge-induced dipole energy and the non-expanded dipole-dipole dispersion energy, respectively, for these interactions are calculated using two types of Gaussian basis functions to represent the first order wave function, Ψ⁽¹⁾. Very good results for these interaction energies, which include charge overlap effects, are obtained for all interatomic separations by using small Gaussian basis sets to represent the interaction, that is Ψ⁽¹⁾, and/or the isolated atoms (the zeroth order wave function).     

Applications of an adiabatic rotational model to the Ar…O2 van der Waals molecule

In this paper a separation of vibrational and rotational motions is applied to describe the metastable levels of the ArO₂ van der Waals molecule. The potential energy surface is written as a sum of atom—atom pairwise Morse functions, the parameters being obtained by fitting the potential of Pirani and Vecchiocattivi at the equilibrium configuration. The results agree fairly well with experimental data about infrared absorption, assuming a J = 2 → 1 transition, where J is the quantum number associated to the total angular momentum of the complex. Also, close coupling three-dimensional calculations about the ArO₂ collision show a good agreement with some energies previously calculated. The widths for rotational predissociation have been estimated by this procedure to be of the order of 1 cm^?1.     

Structure of Ti1−yVyHx alloys studied by X-ray diffraction and by 1H and 51V NMR

The structure of the TiVH system is studied by X-ray diffraction and ¹H and ⁵¹V NMR measurements. It is shown that the solid solution of TiV is separated into a few phases by hydrogenation. They are α-TiVH, β-TiVH, γ-TiVH, and γ-TiH phases, which are assumed to have their origins either in TiH_x or in VH_x. The concentration of each phase can be estimated by NMR, which is dependent on the composition of the system. The phase separation caused by hydrogenation is due to the large stability of the γ-TiH phase.