Localized molecular orbital studies in momentum space. I. compton profiles of 1s core electrons and hydrocarbons

The calculations of momentum space properties for the polyatomic molecules CH₄, C₂H₄ and C₂H₆ using localized molecular orbitals of double zeta quality basis sets are presented. The LMO analysis shows that localized and canonical core electrons have different momentum space properties, and that in agreement with the experimental data of Eisenberger and Marra one can distinguish the momentum properties of the CC single and double bonds. The effect of environment on a bond is seen by comparing the CH bond in these three molecules.The concept of electron pair size is introduced as a quantitative guide for interpreting momentun space properties.     

Evolution of the H2 + electron wavepacket under magnetic and electric fields of ultrashort intense laser pulse

Magnetic interaction was included in the simulation of the evolution of the electron wave-packet of the hydrogen molecular ion H₂ ⁺ in femtosecond intense pulsed laser fields applied along the molecular axis. This evolution was followed by solving 2-D time-dependent Schrödinger equation at some fixed inter-nuclear separations. Magnetic interaction effects at non-relativistic intensities induced a phase shift in the time evolution of the electron wave-packet, and an excess z-component angular momentum as compared with the results obtained in the absence of magnetic interaction. Furthermore, the H₂ ⁺ electron WP displacement showed a drift and wiggling in the propagation direction which was different from that observed under pure electric field of the laser pulse. The local fluxes at different points of the 2-D space borders and the time-dependent induced angular momentum are calculated and analyzed.     

Ab Initio Calculations of the Rotational Barriers in H2Te2 and (CH3)2Te2

The heights of the rotational barriers of the ditelluride bridge in H₂Te₂ and (CH₃)₂Te₂ have been calculated at the Hartree–Fock level with the 3-21G basis set. The minima in the rotational potential energy functions occur at torsional angles of 87.58° and 89.32°, respectively. The barriers were determined by complete geometry optimization at each point along the potential surface. In addition, preliminary results on the stability of H₂Te₂ are reported. The molecule is found to be stable with respect to dissociation into H₂Te + Te or H₂ + Te₂. © 1997 John Wiley & Sons, Inc. Heteroatom Chem 8 : 199–202, 1997.     

OH stretch IR spectra of (H2O)3 and benzene-(H2O)3

The OH stretch IR spectrum of (H₂O)₃ is significantly altered upon complexation of the cluster with a benzene molecule. Theoretical calculations show that the differences in the OH stretch spectra of (H₂O)₃ and benzene-(H₂O)₃ are primarily due to mode localization induced by the benzene molecule. Received: 27 January 1997 / Accepted: 28 January 1997     

A novel approach to molecular similarity

Summary We review briefly the general problem of assessing the similarity between one molecule and another. We propose a novel approach to the quantitative estimation of the similarity of two electron distributions. The procedure is based on momentum space concepts, and avoids many of the difficulties associated with the usual position space definitions. Results are presented for the model systems CH₃CH₂CH₃, CH₃OCH₃, CH₃SCH₃, H₂O and H₂S.     

Internal conrotation and disrotation in H2BCH2BH2 and diborylmethane 1,3 H exchange

The paths of correlated internal disrotation (barrier less than 0.4 kcal/mol) and conrotation (barrier around 1.9 kcal/mol) of the two BH₂ groups in H₂BCH₂BH₂ have been computed employing ab initio [MP2(full)/6–31G**] and density functional theory (Becke3LYP/6–311+G**) methods. Two B(SINGLE BOND)C(DOTTED BOND)B(p) hyperconjugative interactions stabilize the C_s symmetric H₂BCH₂BH₂ isomer ( 1 ). The B(SINGLE BOND)C(DOTTED BOND)B(p) hyperconjugative stabilization, evaluated by homodesmotic reactions and using the orbital deletion procedure (which “deactivates” the “vacant” born p orbital), is less than 6 kcal/mol in diborylmethane. The B(SINGLE BOND)C(DOTTED BOND)B(p) stabilization is shown to be remarkably large in C₄B₆H₁₀ (Td). At MP2(fu)/6–31G**, disproportionation into 1 and methane is only 5.6 kcal/mol exothermic. The 1,3 H exchange in diborylmethane is an asynchronous process and proceeds via a doubly bridged cyclic intermediate with 9.3 kcal/mol barrier. Structures with “planar tetracoordinate” carbon are stabilized considerably by BH₂ substituents, but they are still high in energy. © 1997 John Wiley & Sons, Inc. J Comput Chem 18 : 1792–1803, 1997     

Ab initio studies of internal rotation barriers and vibrational frequencies of (C2H2)2, (CO2)2, and C2H2-CO2

We have performed large-scaleab initio calculations using second order Møller-Plesset perturbation theory (MP2) on the three van der Waals dimers formed from acetylene and carbon dioxide. Intermolecular geometrical parameters are reliably computed at this level of theory. Calculations of vibrational frequencies of the van der Waals modes, currently unobtainable by experimental means, give important information about the intermolecular potential and predict significant large-amplitude motion. Zero point energy contributions are shown to be vital in assessing the relative stability of conformations which are close in energy. Our studies suggest that the barrier to interconversion tunnelling in (CO₂)₂ is significantly smaller than previously inferred and is approximately the same as in (C₂H₂)₂. The reason for the rigidity of (CO₂)₂ is the difference in monomer centre-of-mass separation between ground state and transition state. We also show that, in addition to the previously observedC _2v form, the collinear form of C₂H₂-CO₂ is a local minimum on its potential energy surface.     

Looking at orbitals in the laboratory: The experimental investigation of molecular wavefunctions and binding energies by electron momentum spectroscopy

The experimental technique of electron momentum spectroscopy (EMS ) (i.e., binary (e, 2e) spectroscopy) is discussed together with typical examples of its applications over the past decade in the area of experimental quantum chemistry. Results interpreted within the framework of the plane wave impulse and the target Hartree—Fock approximations provide direct measurements of, spherically averaged, orbital electron momentum distributions. Results for a variety of atoms and small molecules are compared with calculations using a range of Fourier transformed SCF position space wavefunctions of varying sophistication. Measured momentum distributions (MD ) provide a “direct” view of orbitals. In addition to offering a sensitive experimental diagnostic for semiempirical molecular wavefunctions, the MD's provide a chemically significant, additional experimental constraint to the usual variational optimization of wavefunctions. The measured MD's clearly reflect well known characteristics of various chemical and physical properties. It appears that EMS and momentum space chemistry offer the promise of supplementary perspectives and new vistas in quantum chemistry, as suggested by Coulson more than 40 years ago. Binding energy spectra in the inner valence region reveal, in many cases, a major breakdown of the simple MO model for ionization in accord with the predictions of many-body calculations. Results are considered for atomic targets, including H and the noble gases. The measured momentum distribution for H₂ is also compared with results from Compton scattering. Results for H₂ and H are combined to provide a direct experimental assessment of the bond density in H₂, which is compared with calculations. The behavior of the outer valence MD ''s for small row two and row three hydride molecules such as H₂O and H₂S, NH₃, HF, and HCl are consistent with well known differences in chemical and physical behavior such as ligand-donor activity and hydrogen bonding. MD measurements for the outermost valence orbitals of HF, H₂O and NH₃ show significant differences from those calculated using even very high-quality wavefunctions. Measurements of MD's for outer σ_g orbitals of small polyatomic molecules such as CO₂, COS, CS₂, and CF₄ show clear evidence of mixed s and p character. It is apparent that EMS is a sensitive probe of details of electronic structure and electron motion in atoms and molecules.     

Localized molecular orbital studies in momentum space. II. Correspondence between coordinate and momentum space properties of various electron pairs

The momentum space properties of the ten-electron systems Ne, HF, H₂O, NH₃ and CH₄ as well as those of CH₃CH₃, CH₃NH₂, CH₃OH and FCH₂OH were investigated using localized molecular orbitals (LMO) obtained from ab initio self-consistent-field (SCF) wavefunctions constructed from double zeta quality gaussain basis sets.Compton profiles of various LMO electron pairs (CC, CN, CO, CF; CH, NH, OH, FH bond pairs and C, N, O, F lone pairs) are tabulated. In order to understand the correspondence between the momentum and the coordinate space properties of those electron pairs, the concept of the size and the shape of an LMO electron pair charge distribution has been utilized. The use of the intermediate expectation values of pⁿ is introduced for the purpose of interpreting the momentum space properties.The dependence of molecular property partitioning on different localization schemes and on different basis sets is also studied by using the H₂O profile as an example.     

Preparation,Crystal Structure and Vibrational Spectra of Ca2P2O6·2H2O and [Ca(H2O)3(H2P2O6)]·0.5(C12H24O6)·H2O

The calcium salts Ca₂P₂O₆ · 2H₂O ( 1 ) and [Ca(H₂O)₃(H₂P₂O₆)] · 0.5(C₁₂H₂₄O₆) · H₂O ( 2 ) were prepared and structurally characterized by single‐crystal X‐ray diffraction. Compound 1 crystallizes in the orthorhombic space group Pbca and compound 2 in the monoclinic space group P2₁/n. The crystal structure of compound 1 consists of chains of edge‐sharing [CaO₇] polyhedra linked by hypodiphosphate(IV) anions to form a three‐dimensional network. The crystal structure of compound 2 consists of alternated layers of crown ether and water molecules and respective ionic units. Within the layers of ionic units the Ca²⁺ cations are octahedrally coordinated by three monodentate dihydrogenhypodiphosphate(IV) anions and three water molecules. The IR/Raman spectra of the title compounds were recorded and interpreted, especially with respect to the [P₂O₆]^4– and [H₂P₂O₆]^2– groups. The phase purity of 2 was verified by powder diffraction measurements.     

Synthesis and Crystal Structures of Two New Layered Manganese(II) Diphosphonates: Mn2[{(O3PCH2)2NHR}(H2O)F]·H2O

Two manganese phosphonates Mn₂[{(O₃PCH₂)₂NHR}(H₂O)F]·H₂O (R = ?C₆H₁₁ (1) , ?CH₂C₅H₄N (2) ) have been synthesized by hydrothermal reaction and characterized by single–crystal X–ray diffraction as well as with infrared spectroscopy, elemental analysis and thermogravimetric analysis. The two isomorphous compounds feature layered structures in ab plane, in which the tetramers comprised of two MnO₄F₂ and two MnO₅F polyhedra are cross–linked via phosphonate oxygen atoms to form Mn^II phosphonate layers. The organic groups of the ligands are orientated toward the interlayer space.     

Synthesis and Characterization of the New Lacunar Zincophosphate [C6H10N2][ZnP2O8H2]·0.6H2O

The new zincophosphate of chemical formula [C₆H₁₀N₂][ZnP₂O₈H₂] · 0.6H₂O was hydrothermally synthesized with p‐phenylenediamine as structure‐directing agent. The title compound crystallizes in the trigonal symmetry (proposed space group P3m1), where inorganic zincophosphate chains form layers due to the half occupancy of the unique crystallographic zinc site. The layers are separated from each other by p‐phenylenediammonium dications with hydrogen bonding scheme involving the ammonium protons that reveals a pillar‐like 3D structure aspect. The compound was characterized by powder X‐ray diffraction, multinuclear solid‐state NMR, scanning electron microscopy, chemical analysis, and thermogravimetric analysis.     

A computer study of the formation of H3+ and its vibrational deactivation using a statistical model

The formation of H₃⁺ and its vibrational deactivation have been modeled using a statistical model for energy partitioning. The theory is a restricted phase space calculation which neglects conservation of angular momentum. The model is fitted to experimental data on the dissociative reactions of H₃⁺ with CH₄, C₂H₄, and NH₃. Interpretation of these experiments using this model allows us to infer that several collisions of H₃⁺* with H₂ are required to relax the internal energy of the ion. It is also concluded that only a fraction of the reactants with enough energy to populate endothermic channels actually do so.     

An investigation of the valence orbitals of water by high momentum resolution electron momentum spectroscopy

The momentum distributions of the valence orbitals for water well as the binding energy spectra in the region 10–45 eV have been reinvestigated with a high momentum resolution (≈0.1 a₀^?1 fwhm) binary (e.2e) spectrometer. The binding energy spectra show considerable satellite structure in the region > 25 eV which is consistent with theoretical predictions of final state configuration interaction (many-body effects) involving the (2a₁)^?1 hole state. An investigation of the momentum distribution in the satellite region confirms this assignment. This is in accord with a variety of recent theoretical studies and also consistent with earlier experiments. Differences suggested in earlier comparisons between theory and low momentum resolution experiments for the momentum distributions of the 1b₁ and 3a₁ orbitals have been verified. Several possible theoretical studies are suggested to investigate further this discrepancy between experiment and theory. Bonding effects and thenature of the molecular orbitals of H₂O in momentum space are also discussed.     

Ground state correlations in H2 measured by the (e,2e) technique

The relative intensities for exciting the 1sσ_g, 2pσ_u, 2pπ_u, and 2sσ_g states of H₂ are measured in a 1200 eV noncoplanar symmetric (e,2e) experiment on H₂. Momentum distributions are obtained at separation energies corresponding to the various transitions. The ratio of transition probability to the excited states relative to the ground state is strongly dependent on the ion recoil momentum q, having a minimum value of approximately 2% at small q. The excited state cross sections are sensitive to electron correlation effects and the data are compared with calculated cross sections using a configuration interaction wavefunction for H₂.     

Synthese und Struktur neuer Natriumhydrogensulfate Na(H3O)(HSO4)2; Na2(HSO4)2(H2SO4) und Na(HSO4)(H2SO4)2

Synthesis and Structure of New Sodium Hydrogen Sulfates Na(H₃O)(HSO₄)₂, Na₂(HSO₄)₂(H₂SO₄), and Na(HSO₄)(H₂SO₄)₂ Three acidic sodium sulfates have been synthesized from the system sodium sulfate/sulfuric acid and have been crystallographically characterized. Na(H₃O)(HSO₄)₂ ( A ) crystallizes in the space group P2₁/c with the unit cell parameters a = 6.974(2), b = 13.086(2), c = 8.080(3) Å, α = 105.90(4)°, V = 709.1 Å3, Z = 4. Na₂(HSO₄)₂(H₂SO₄) ( B ) is orthorhombic (space group Pna2₁) with the unit cell parameters a = 9.970(2), b = 6.951(1), c = 13.949(3) Å, V = 966.7 ų and Z = 4. Na(HSO₄)(H₂SO₄)₂ ( C ) crystallizes in the triclinic space group P1 with the unit cell parameters a = 5.084(1), b = 8.746(1), c = 11.765(3) Å, α = 68.86(2)°, β = 88.44(2)°, γ = 88.97(2)°, V = 487.8 Å3 and Z = 2. All three compounds contain SO₄ tetrahedra as HSO₄^? anions and additionally in B and C in form of H₂SO₄ molecules. The ratio H:SO₄ determines the connectivity degree in the hydrogen bond system. In A , there are zigzag chains and dimers additionally connected via oxonium ions. Complex chains consisting of cyclic trimers (two HSO₄^? and one H₂SO₄) are present in B . In structure C , several parallel chains are connected to columns due to the greater content of H₂SO₄. Sodium cations show a distorted octahedral coordination by oxygen in all three structures, the NaO₆ octahedra being “isolated” (connected via SO₄ tetrahedra only) in A . Pairs of octahedra with common edge form Na₂O₁₀ dimeric units in C . Such double octahedra are connected via common corners forming zigzag chains in B .     

Electron transfer oxidation kinetics of Na2[Mo2O4EDTA] by S2O82− ion in dilute acid medium

Na₂(Mo₂^vO₄EDTA).4H₂O crystals have been prepared in pure form. Kinetics for the oxidation of the compound by S₂O₈²⁻ have been studied spectrophotometrically in dilute sulphuric acid medium. The effects of hydrogen ion concentration, metal ion concentration, S₂O₈²⁻ ion concentration, and temperature on the process have been studied. Rate equations have been derived to explain the experimental observations. On the basis of the observations, suitable mechanisms have been suggested. The kinetic parameters E^*, ΔH^≠, and ΔS^≠ of the process have also been determined. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 269–274, 1997     

On the sudden approximation of cross: computational tests and factorization of cross sections and related scattering phenomena

A sudden approximation recently derived by Cross using a semiclassical treatment of the orbital motion is recast into a form which permits factorization of differential and integral degeneracy averaged cross sections, opacities as a function of final angular momentum quantum number, the scattering amplitude, and the phenomenological cross section which describes spectral line broadening. Calculations are done using an average of initial and final orbital angular momentum quantum numbers for the partial wave parameter for ArN₂, ArTIF, H⁺H₂ and Li⁺H₂. The results indicate that the method is a good approximation for integral cross sections and opacities when the energy sudden approximation is valid and when the coupling of the orbital motion is important.     

A Novel μ2‐(H2O)‐Bridged Double‐Chain Coordination Polymer [Cd(pc)(phen)(H2O)]n with Rhombic Grids (H2pc = pamoic acid,phen = 1,10‐phenanthroline)

A novel coordination polymer [Cd(pc)(phen)(H₂O)]_n (H₂pc = pamoic acid, phen = 1,10‐phenanthroline) has been synthesized under hydrothermal conditions. Single crystal X‐ray diffraction analysis reveals that the compound crystallizes in triclinic space group P1. All the Cd^II atoms in the compound are hexacoordinate and are linked by pamoicate ligands to form a one‐dimensional zigzag chain. Furthermore, two adjacent zigzag chains are connected by the μ₂‐(H₂O) molecules to form a double‐chain with rhombic grids. There exist intermolecular C–H ··· π contacts, π–π stacking and hydrogen‐bonding interactions. Compound 1 displays strong fluorescent emission in the solid state at room temperature.     

Experimental and theoretical binding energy spectra and momentum distributions for the valence orbitals of H2O

The binding energy spectra (10–46 eV) and momentum distributions of the valence orbitals of H₂O have been measured using a new high-sensitivity binary (e,2e) electron spectrometer employing position-sensitive detectors. The binding energy spectrum shows a previously unreported feature at = 27 eV which is shown to be associated with the (2a₁)^?1 ionization process. The region between 25 and 46 eV is compared with previous (e,2e) and X-ray photoelectron measurements as well as with several existing and new many-body calculations indicating a splitting of the 2a₁ ionization pole strength. In addition the separate momentum distributions of the three outer valence orbitals of H₂O have been obtained from deconvoluted binding energy spectra run at a series of azimuthal angles. The results, which show considerably improved signal-to-noise ratio over earlier measurements using single-channel instrumentation are compared with spherically averaged momentum distributions calculated with a variety of wavefunctions.