Ab initio scf and ci study of the electronic structure of BH3CO

Results of ab initio SCF and CI calculations employing a Gaussian basis set of double-zeta quality are reported for BH₃CO. The heat of formation for the gas-phase reaction, BH₃ + CO → BH₃CO, is calculated as ?10.98 kcal mol^?1 within the SCF approximation, and as ?14.56 kcal mol^?1 if the CI treatment is included. This is in good agreement with the estimated experimental value of ?16.6 kcal mol^?1. The energy of rearrangement of the BH₃ fragment from D_3h to C_3v symmetry in BH₃CO is calculated as 15.97 kcal mol^?1. Molecular properties have been studied in terms of the calculated electron populations, the dipole moment and the electric Field gradient of ¹¹B in BH₃CO.     

Finite-field method calculations. IV. Higher-order moments,dipole moment gradients,polarisability gradients and field-induced shifts in molecular properties: Application to N2, CO,CN−, HCN and HNC

In this paper, theoretical methods developed in III are applied in calculating polarisabilities, polarisability gradients and field-induced shifts, by the finite-field method. Values of dipole moment gradients and higher-order moments, calculated from the unperturbed wavefunctions, are also reported. Results for N₂, CO, CN^?, HCN and HNC have been obtained at the SCF level; some CI results for the N₂ polarisability components and moments and for the dipole moment gradients of HCN are also given. The calculated polarisability gradients and dipole moment gradients have been used to estimate the Raman scattering intensities and depolarisation ratios and the IR absorption intensities. Model calculations of field-induced shifts in bond length, vibrational levels, spectroscopic constants, force constants and dipole moment gradient are reported for N₂ and CO.The discrepancy between the SCF and experimental bond dipole moment gradients for HCN, previously noted in the literature, has been re-examined and resolved by our CI results.     

Sign ambiguity in dipole moment derivatives of Br2CO

Signs of the dipole moment derivatives, ?p_x/?S₄ and ?p_x/?S₅ where p_x is the dipole moment vector along an axis perpendicular to the CO bond in the plane of the molecule and S_j are the symmetry coordinates for the B₁ vibrations of Br₂CO have been re-evaluated from the reported ?p/?Q_i values (p being the dipole moment of the molecule and Q_i the normal coordinates) and the L matrix elements. The new set of dipole moment derivatives fits well with similar parameters for Cl₂CO and F₂CO.     

Bonding in nickel cluster carbonyls

Semiempirical (INDO) and ab initio valence only calculations of the electronic structure of Ni _n and Ni _n (CO) _m clusters support the idea that the bonding with the CO ligands destabilizes the 4s-like MOs of the Ni _n fragment thus eliminating their contribution to the metal-metal bonding in the carbonylated form. This process is accompanied by a complete quenching of the magnetic moment of the bare cluster.     

Dipole moment derivatives from MINDO/3 semi-empirical MO calculations

Dipole moment derivatives for CO, NO, CO₂, H₂O, HCN, BF₃, CH₄, C₂H₄, C₂H₆, CH₃F, F₂CO and H₂CO molecules have been evaluated using MINDO/3 MO calculations. The values are compared with those obtained by other semi-empirical MO methods.     

Potential energy curve of 1Σ+ Li+/He

The potential energy curve of the system Li⁺/He has been determined with moderately large basis sets for 0.5 ? r ? 10.0 a₀ both at the SCF level and including correlation. The present SCF results predict a deeper well (?0.00248 au) at a smaller r(3.66 a₀) compared with earlier calculations. Correlation deepens the well further (?0.00274 au), but pulls it inward slightly (3.63 a₀). In the repulsive part the calculated curve lies above the experimental one, especially at shorter distances. A similar behavior has been noted in the systems Li⁺/H₂, Li⁺/CO and Li⁺/N₂, suggesting that the experimental determinations may underestimate the interaction in this region by 10–20%.     

Syntheses et isomerisation de complexes de la serie des derives halocarbonyle due fer: [FeX(CO)5−nLn]+, FeX2(CO)4−nLn et [FeX3(CO)3]- (L = PMe3; n = 1, 2, 3; X = Cl,Br, I)

The synthesis of a number of neutral, cationic and anionic halocarbonyl complexes of iron having the formula [FeX_m(CO)_6?m?nL_n]^2?m is described. The determination of their molecular configuration by infrared, NMR and dipole moment measurements allowed the study of several of their photochemical isomerization reactions, the mechanism of these reactions is explained in some cases by the intermediary formation of cationic complexes.     

Vibrational matrix elements of the quadrupole moment functions of H2, N2 and CO

The quadrupole moment functions (molecular quadrupole moment versus internuclear distance) have been determined by quantum mechanical calculations for H₂ (by Kolos and Wolniewicz), N₂ (by Wahl and Nesbet), and CO (by Nesbet). These functions are used with numerical vibrational wave functions to compute matrix elements which are useful for calculations of scattering cross sections, energy transfer rates and excitation probabilities, and infrared intensities of forbidden bands.     

Synthesis, characterisation of two hexa-iron clusters with {Fe2S2(CO)x} (x = 5 or 6) fragments and investigation into their inter-conversion

Reaction of a trithiol ligand, 2-(mercaptomethyl)-2-methylpropane-1,3-dithiol (H₃L), with tri-iron dodecacarbonyl in toluene produces two hexa-iron clusters (1 and 2). The two clusters are characterised crystallographically and spectroscopically. NMR spectroscopy reveals that the cluster 2 exists in two conformations in equilibrium 2_anti ⇔ 2_syn and the equilibrium constant K_eq = 0.55 under CO atmosphere. In the cluster 2, the central {Fe₂S₂(CO)₆} sub-unit is flanked by two identical {Fe₂S₂(CO)₆} satellite sub-units through thiolate linkages whereas one of the thiolate linkages can further form Fe-S bond with the proximal Fe atom in one of the two satellite sub-units to produce the cluster 1 by expelling one CO. This conversion can be entirely reversed by continuously purging CO through the solution of the cluster 1. As suggested by DFT calculations, the conversion features a key step, the rotation of the Fe_prox(CO)₃ to expose a vacant site for exogenic ligand binding (the S atom from the central sub-unit in this case) with concomitant switch for one of the three CO ligands in the unit of Fe_prox(CO)₃ from terminal to bridging orientation. The conversion from the clusters 1-2 involving one CO uptake is much faster than its reverse process since the latter is an endergonic process characterised by large reaction barriers, as revealed by the DFT calculations.     

Theoretical study on interaction of different coordination modes of BH4 ligand with transition metal in [TM(BH4)(CO)4] (TM = Cr, Mo)

Density functional calculations were performed on bonding and structural features of [(ηⁿ-BH₄)TM(CO)₄]⁻ (n = 1, 2, 3; TM = Cr, Mo) complexes. Calculations show that the ground state is bidentate which is in good agreement with experimental results. It has been found that the bridge and terminal hydrogen atoms will interchange by two pathways: (i) twist of BH₄ about one of the bridge B-H and (ii) twist of BH₄ about one of the terminal B-H. The molecular orbital calculations and natural bond orbital methodologies for different isomers of these complexes have been evaluated. The final results indicate that case (i) is more preferable relative to another case.     

Characterization of [Rh(PhCOCHCOCH2CH2CH3)(CO)2] by X-ray crystallography, a computational and a statistical study

[Rh(PhCOCHCOCH₂CH₂CH₃)(CO)₂] is characterised by crystallographic and density functional theory computational methods. The experimental structure is compared to the calculated structure, as well as to the structurally similar compound [Rh(PhCOCHCOCH₂CH₃)(CO)₂] using root-mean-square calculations and a half-normal probability plot analyses.     

Experimental and ab initio evidence for preferred syn-conformation of thionyl amines, , X = H,CH3, and C6H5

The conformational syn—anti problem with respect to the

bond in thionylamines,

, has only partially been solved for X = H and CH₃ [1, 2]. By providing new experimental material for C₆H₅NSO and p-FC₆H₄NSO (vibrational spectra, low-resolution microwave spectra, dipole moment determinations) and by performing ab initio calculations for HNSO and CH₃NSO it is shown that these two molecules are exclusively syn conformers The experimental data here presented strongly suggest the same conclusion for C₆H₅NSO. The result is consistent with a previous investigation by van Woerden and Bijl-Vlieger [3] comprising 13 mono- and polysubstituted sulfinylanilines.     

Étude lcao-mo-cndo/2 des liaisons po et no dans des composés X3PO ET X3NO

The authors have studied the electronic structure of X₃PO and X₃NO compounds (with X = F, Cl, CH₃), using the semi-empirical CNDO/2 method. All the calculations have been made with and without 3d functions on the phosphorus atom. The comparison between the calculated and experimental values, especially in the case of bond length, dipole moment, and orbital level order, shows the influence of the 3d orbitals in the PO bond, which contains a sigma donation P → O and a pπ(O)-dπ(P) back bonding. The NO bond has sigma character in trimethylamine oxide, but is partially a double bond in trifluoramine oxide.     

On the splitting of the CO 1π level in the chemisorption system CO/Ni(111): substrate or lateral interaction?

LCGTO Xα model cluster calculations have been carried out to rationalize the shape of the CO 1π band of the chemisorption system CO/Ni(111) observed in angular resolved photoemission. The splitting induced by substrate interaction at twofold bridging sites (0.4 eV) is much smaller than the value deduced from experiment (1.2 eV). From calculations on (CO) _n clusters lateral adsorbate interaction is estimated to cause a sizable broadening of the 1π band (1.2eV, in satisfactory agreement with experiment), but essentially no splitting (?0.1 eV). Therefore, rehybridisation due to local interaction does not seem to suffice as an explanation for the observed shape of the CO 1π band.     

On the conformational structure of cyclohexadecane,the corresponding ketone,the 1,9-dione and the corresponding ketals

The title compounds have been prepared, and their structures have been studied using molecular mechanics calculations, together with dipole moment measurements, and thermodynamic and spectroscopic data reported in the literature. It is concluded that the parent hydrocarbon exists to a very large extent in a “square” diamondoid conformation (D_2d symmetry). Other conformations (rectangular, nearly D₂, and puckered, (D_2d) are considerably higher in energy. The ketone consists of a mixture of both square and rectangular conformations with the CO group in several different positions. The monoethylene ketal exists primarily in a conformation which is square, with the ketal group on a corner. The diketone, diketal, and monoketal of the dione are considered in the light of the above, and conclusions are drawn regarding their conformations. The dipole moments of the difunctional compounds are in agreement with the conformational conclusions reached by the calculations.     

Study of the electric dipole moments of the (C2H5)n PX3−n and P{EIVB (CH3)3}3 compounds (X = Cl,Br, I; EIVB = C,Si, Sn; n = 0, 1, 2, 3)

The dipole moments of the (C₂H₅)_nPX_3?n (X = Cl, Br, I; n = 0, 1, 2, 3) and P{E^IVB(CH₃)₃}₃ compounds (E^IVB = C, Si, Sn) have been determined by dielectric measurements in benzene at 25°C by Higasi's method. The results are interpreted in terms of an additive vector model from bond moment calculations and a maximum phosphorus lone pair contribution of 1.75 D as calculated for P(C₂H₅)₃. The high dipole moment of the mixed PA₂B-type compounds in comparison with PA₃ moments is mainly ascribed to a loss of C_3v symmetry, characterized by a quite asymmetric orientation of the phosphorus lone pair with respect to the phosphorus bonding orbitals.     

A CASPT2 study of the dipole moment surfaces of hydrogen sulphide molecule

The ability of the CASPT2 method to yield accurate H₂S dipole moment surfaces that could be further used for conclusive predictions about rovibrational calculations, has been evaluated using ANO-L basis set. The optimised geometry, permanent dipole moment, linearity barrier as well as general features of the μ_x and μ_z dipole moment components in the vicinity of the equilibrium configuration agree favourably with available empirical determination and with recent accurate ab initio calculations. Dipole moment functions behaviour is also well reproduced for those geometrical configurations which are far from equilibrium in dissociation path. Computational technical aspects are discussed.     

On the apex angle in C2v-‘V’ structure of B2O3

Results of valence electron SCF-MO calculations in the INDO approximation predict a value of 102° for the apex angle of C_2v-‘V’ structure of B₂O₃, assuming bond lengths obtained from electron diffraction work. Calculated results for the dipole moment are reported and electron population analysis is used to discuss bonding.     

Structural and magnetic characterization of Mn3IrGe and Mn3Ir(Si1−xGex): experiments and theory

The structural and magnetic properties of a new ternary Ir-Mn-Ge phase, Mn₃IrGe, as well as the solid solution Mn₃Ir(Si_1−xGe_x), 0?x?1, have been investigated by means of X-ray and neutron powder diffraction, magnetization measurements and first principles calculations. The crystal structure is cubic, of the AlAu₄-type (an ordered form of the β-Mn structure), Z=4, space group P2₁3, and the unit-cell dimension varies linearly with the silicon content. For all compositions, antiferromagnetic ordering is found below a critical temperature of about 225 K. The magnetic structure is noncollinear, as a result of frustrated magnetic interactions on a triangular network of Mn atoms, on which the moments rotate 120° around the triangle axes. The magnitude of the magnetic moment at 10 K is 3.39(4) μ_B for Mn₃IrGe. The theoretical calculations reproduce with very good accuracy the magnitudes as well as the directions of the experimentally observed magnetic moments.     

Comparison of ab initio,CNDO/2 and experimental dipole moment derivatives for C2 hydrocarbons and formaldehyde

Dipole moment derivatives determined by ab initio and CNDO/2 calculations are compared with the corresponding data obtained from infrared intensities. For ethane, ethylene and formaldehyde, the recent results of quadratic force field calculations have been used to calculate experimental derivatives; for the latter two molecules, the individual intensities of certain overlapping bands were determined from the results of rovibrational analysis. The experimental dipole moment derivative with respect to the rocking symmetry coordinate, S₁₀, of ethylene has been found to be 0.03 D, as opposed to the value of ca. 0.4 D reported previously. CNDO results agree both in sign and magnitude with ab initio dipole moment derivatives.