Gradient of the ZAPT2 energy

 An explicit expression for the analytical first derivative of the Z-averaged perturbation theory taken to second order energy, due to Lee and Jayatilaka, is presented for application to high-spin systems described by a restricted open-shell Hartree–Fock wavefunction. The use of frozen core orbitals is incorporated into the derivation. Received: 23 April 2001 / Accepted: 31 August 2001 / Published online: 9 January 2002     

Structure and stability of gold-substituted diborane, boranes, and borohydride ions

Pseudopotential ab initio calculations were performed for species of the type BH _n (AuPH₃) _m ^k , where n+m=3 or 4, and the charge k is −2,…,+1. Some derivatives of these and diaurated diboranes were also studied. The structural data agree well with the available experimental evidence. Factors affecting the stability of these systems, including the role of aurophilic attraction, are discussed. The singly charged anions and the diaurated diboranes are predicted to be the most stable members of these series. Received: 22 January 1999 / Accepted: 2 June 1999 / Published online: 4 October 1999     

Molecular dynamics, density functional and second-order Møller–Plesset theory study of the structure and conformation of acetylcholine in vacuo and in solution

 Molecular mechanics minimizations based on the CVFF force field and molecular dynamics simulation for a time of 2.5 ns were performed to examine the conformational behaviour and the molecular motion of acetylcholine in vacuo and in aqueous solution. Five low-lying conformations, namely the TT, TG, GG, G*G and GT, were obtained from molecular mechanics computations with the GT structure as the absolute minimum. Molecular dynamics trajectories in vacuo and in water show that only four (GT, GG, G*G and TG) and three (TG, TT and GT) conformations are present in the simulation time, respectively. Density functional B3LYP and second-order M?ller–Plesset (MP2) methods were then used to study all the five lowest-lying conformers of acetylcholine neurotransmitter in vacuo and in water by the polarizable continuum model approach. The B3LYP and MP2 computations show that in the gas phase all minima lie in a narrow range of energy with the G*G conformer as the most stable one. The relative minima GG, GT, TG and TT are located at 1.1 (3.3), 1.8 (4.2), 2.1 (4.5) and 4.3 (7.3) kcal/mol above the absolute one at the B3LYP (MP2) level. The preferred conformation in water is the TG. Solvation reduces the relative energy differences between the five minima in both computations. Received: 4 April 2001 / Accepted: 5 July 2001 / Published online: 30 October 2001     

Influence of diffuse and polarization functions on the second-order Møller–Plesset optimized dihedral angle of biphenyl

 Using 6-31G and 6-311G basis sets to which diffuse and polarization functions were added in a stepwise fashion (a total of 16 basis sets), Hartree–Fock (HF), MP2 and B3LYP geometry optimizations were performed on biphenyl. With the MP2 method, diffuse functions raise the dihedral angle φ, for example, from 46.3° for 6-31G to 54.1° for 6-311++G, while polarization functions lower it, for example, from 54.1° for 6-311++G to 42.1° for 6-311++G(2d,2p). For a single set of polarization functions, φ(MP2) lies close to or above φ(HF) (44–47°), but for a double set it is below φ(HF) and is close to B3LYP values (38–42°) which show little basis set dependence. The most reliable value for φ, 42.1° [MP2/6-311++G(2d,2p)], is expected to increase slightly by adding more diffuse functions. The corresponding best calculated energy barrier at 0° (coplanar conformation) is 2.83 kcal/mol, much higher than the experimental estimate (1.4 ± 0.5 kcal/mol). The barrier at 90° is 1.82 kcal/mol, in line with the experimental estimate (1.6 ± 0.5 kcal/mol) and with previous theoretical results. Received: 9 September 2002 / Accepted: 15 November 2002 / Published online: 1 April 2003 Correspondence to: Friedrich Grein e-mail: fritz@unb.ca Acknowledgement. The author would like to thank NSERC (Canada) for financial support.     

<Emphasis Type="Italic">Ab initio</Emphasis> study of small coinage metal telluride clusters Au<Subscript>n</Subscript>Te<Subscript>m</Subscript> (<Emphasis Type="Italic">n,m</Emphasis> = 1, 2)

The geometries of the most stable isomers of gold telluride systems AuTe, Au₂Te, and AuTe₂ are determined using the MP2 method. The aspect of gold—telluride interaction, the electron correlation, and relativistic effects on geometry and stability are investigated at the MP2 and CCSD(T) theoretical levels. The results show that the electron correlation and relativistic effects are responsible not only for gold—gold attraction but also for additional gold—telluride interaction. The gold—telluride interaction is strong enough to modify the known pattern of bare gold clusters. Both effects are essential for determining the geometry and relative stability of this type of systems.     

Local-orbital-based correlated ab initio band structure calculations in insulating solids: LiF

 A local-orbital-based ab initio approach to calculate correlation effects on quasi-particle energies in insulating solids is presented. The use of localized Wannier-type Hartree–Fock orbitals allows correlation effects to be efficiently assessed. First a Green's function approach based on exact diagonalization is introduced and this is combined with an incremental scheme, while subsequently different levels of perturbative approximations are derived from the general procedure. With these methods the band structure of LiF is calculated and good agreement with experiment is found. By comparing the different approximations proposed, including the exact diagonalization procedure, their relative quality is established. Received: 25 June 2001 / Accepted: 31 August 2001 / Published online: 19 December 2001     

Synthesis and characterization of diorganotin(IV) complexes of tridentate Schiff bases: crystal structure of [N-(3-hydroxypyridine-2-yl)-5-chlorosalicylideneiminato]dimethyltin(IV)

Some five-coordinated dimethyltin(IV) complexes of the type Me₂SnL (where L is the anion of a bifunctional tridentate Schiff base) were synthesized. These Schiff bases are N-(3-hydroxypyridine-2-yl)-3-methoxysalicylideneimine, HOC₆H₃OCH₃CH=NC₅H₃NOH (1), N-(3-hydroxypyridine-2-yl)-3-ethoxysalicylideneimine, HOC₆H₃OC₂H₅CH=NC₅H₃NOH (2), N-(3-hydroxypyridine-2-yl)-5-chlorosalicylideneimine, HOC₆H₃ClCH=NC₅H₃NOH (3), and N-(3-hydroxypyridine-2-yl)-3-methoxy-5-bromosalicylideneimine, HOC₆H₂OCH₃BrCH=NC₅H₃NOH (4). Dimethyltin(IV) complex of 3 (3a) was characterized by single crystal X-ray diffraction method and a coordination geometry that is nearly halfway between trigonal–bipyramidal and square pyramidal arrangement was found. Dimethyltin complexes of (1), (2), and (4) were also prepared and characterized by the comparison of their elemental analysis and ¹H-NMR-, IR-, UV- and mass spectral data with those of (3a). For example, in the ¹H-NMR spectra, the ²J(¹¹⁹Sn-¹H) in the Sn-CH₃ moiety have values between 80 Hz and 90 Hz, typical of five-coordinated tin species. By using these values in Lockart’s Equations, H₃C–Sn–CH₃ angles in the complexes were estimated to lie between 130° and 145°. X-ray diffraction value for 3a, confirms this estimate within 3.4% relative deviation (129.7° exp. Vs. 134.9° estimate).     

Interaction energies for the water dimer by supermolecular methods and symmetry-adapted perturbation theory: the role of bond functions and convergence of basis subsets

 Using a systematic series of basis sets in supermolecular and symmetry-adapted intermolecular perturbation theory calculations it is examined how interaction energies of various water dimer structures change upon addition and shifting of bond functions. Their addition to augmented double- and triple-zeta basis sets brings the sum of the electron correlation contributions to the second-order interaction energy nearly to convergence, while accurate first-order electrostatic and exchange contributions require better than augmented quadruple-zeta quality. A scheme which combines the different perturbation energy contributions as computed in different basis subsets performs uniformly well for the various dimer structures. It yields a symmetry-adapted perturbation theory value of −21.08 kJ/mol for the energy of interaction of two vibrationally averaged water molecules compared to −21.29 kJ/mol when the full augmented triple-zeta basis set is used throughout. Received: 4 November 1999 / Accepted: 8 February 2000 / Published online: 12 May 2000     

A density functional theory study of the conformational properties of 1,2-ethanediamine: protonation and solvent effects

 The structures and the conformational energies of nonprotonated, monoprotonated and diprotonated 1,2-ethanediamine have been investigated through density functional theory. The relative performance of local and gradient-corrected functionals is discussed. The existence of hydrogen-bond formation has been determined with electron localisation function calculations. Proton affinities for nonprotonated and monoprotonated 1,2-ethanediamine have been calculated and are in agreement with experimental data. The influence of solvation has been accounted for through the self-consistent isodensity polarisable continuum model. The results for the nonprotonated conformers show that solvation stabilises those conformers which have the lone pair in an antiperiplanar conformation. Solvation of the monoprotonated conformer stabilises significantly the “anti” conformation, which is unstable in the gas phase. For the di-protonated species, solvation stabilises slightly the gauche conformer, which is unstable in the gas phase. Received: 28 September 1999 / Accepted: 2 May 2000 / Published online: 27 September 2000     

Monte Carlo simulation of 2-ethoxyethanol in continuum configurational biased procedure: conformational analysis and association in aqueous and non-aqueous media

 Monte Carlo simulations have been carried out for 2-ethoxyethanol (C₂E₁) in isothermal-isobaric ensemble (NPT) at different temperatures and 1 atm pressure with a continuum configurational biased procedure in water and chloroform media. Hydrogen bond bridges were formed between adjacent oxygen atoms in C₂E₁ (CH₃CH₂OCH₂CH₂OH) through water molecules. We also found that the stable conformers of C₂E₁ in water and CHCl₃ are different and the effect of temperature on solute-solvent interaction energies is considerable. The self-association of C₂E₁ in aqueous and nonaqueous media has been studied by statistical perturbation theory, and the relative free energy has been obtained at different reaction coordinates by double-wide sampling method. Two minima were found in water solvent in the potential of mean force (PMF), corresponding to the contact and solvent-separated pairs, but only one minimum was found in CHCl₃ solvent corresponding to a contact pair complex. Received: 18 January 2001 / Accepted: 22 October 2001 / Published online: 21 January 2002     

Comparison of CEPA and CP-MET methods

The known CEPA variants CEPA (v) withv = 0,1,2,3 and two new ones withv = 4, 5 are compared both formally and for various numerical examples with CP-MET. The main conclusions are: 1. In those situations where both CP-MET and the CEPA variants are justified (i.e. for “good” closed shell states) the correlation energies obtained with the 7 different schemes differ very little (by something like ±2%), with CEPA (1) closest to CP-MET (difference usually a fraction of 1%) and CEPA (4) nearly as close; this is rather insensitive to whether one uses canonical or localized orbitals. Even CEPA (3) is not too far from CP-MET, which confirms an earlier suggestion of Kelly. 2. In those cases where one of the 7 schemes fails (e.g. due to near degeneracy as in covalent molecules at large internuclear distances) the other 6 usually fail as well, though CEPA (0) is then somewhat poorer than the other schemes. Then no longer CEPA (1) but rather CEPA (3) is closest to CP-MET and then all schemes converge much better in a localized representation. 3. CEPA (2) usually leads to best agreement with experiment since it simulates to some extent triple substitutions. In none of the studied examples does CP-MET show a significant superiority as compared to the other schemes. Possible improvements to extend the domain of applicability of these methods are discussed.     

(π)C—H···S, (allyl)C—H···C(π) and π···π intermolecular interactions: synthesis, characterization, and crystal structure of 2,2′-bipyridine bis(diallyldithiocarbamato)zinc(II)

A mixed-ligand Zn(II) complex formulated as [Zn(aldtc)₂(bipy)] (aldtc=diallyldithiocarbamate; bipy=2,2′-bipyridine) was synthesized and characterized by IR, ¹H and ¹³C NMR spectral measurements and X-ray crystallography. The crystal structure of this complex indicates that Zn has a distorted octahedral geometry. The Zn—N distances are invariant (2.168(2) Å), while those of the Zn—S are slightly different (2.5408(9) and 2.5440(9) Å). The N—Zn—N, S—Zn—S and N—Zn—S bond angles are in the range 75.35(13)–99.75(7)°, 70.48(3)–161.02(5)° and 95.26(7)–160.32(7)°, respectively. The crystal packing of the complex shows different motifs of supramolecularity resulting from both hydrophilic ((π)C—H···S) and hydrophobic ((allyl)C—H···C(π)) intermolecular interactions. These interactions result in a chain arrangement of molecules along crystallographic c axis and the chains are further connected via π···π stacking along with ((π)C—H···S along b axis leading to an overall crystal packing that can be regarded as layers of complexes along bc plane, which are held together through nonconventional hydrogen bonding and π···π stacking.