On the role of localized molecular orbitals in the formation of bond dipoles

The hypothesis of the classical chemistry about bond dipoles resulting from shifts of separate pairs of electrons is proved using the non-canonical method of molecular orbitals (MOs). To this end, a relation is sought between the total charge distribution inside an individual chemical bond of a polyatomic molecule and the square of the respective single localized MO (LMO). General expressions for these MOs are obtained directly on the basis of the Brillouin theorem without invoking additional localization criteria. The two characteristics under comparison are presented in an explicit algebraic form in terms of meaningful components. Reshaping of square of the ‘own’ LMO of the given bond is shown to play the decisive role in the formation of secondary dipoles of initially homopolar bonds (e.g. of C–C and C–H bonds in substituted alkanes), as well as of bonds of relatively low initial polarity. Thus, representability of these dipoles by shifts of the ‘own’ pairs of electrons of respective bonds is supported. For bonds of a high initial polarity, the secondary dipoles are shown to originate mainly from contributions of LMOs of other bonds extending over the antibonding basis orbital of the given bond. Moreover, the actual secondary bond dipole takes an opposite direction vs. that predicted by the shift of the respective ‘own’ pair of electrons in this case. The latter result serves to account for the known low nucleofugality of highly electronegative heteroatoms in the S_N2 reactions.     

On the possibility of complex formation of Group IIIB organic compounds with molecular oxygen

The likelihood of the formation of R₃E·O₂ complexes (E is an element of Group IIIB) in R₃E/O₂ systems has been shown experimentally and theoretically. The most probable energetic parameters, structure, problems of stability are discussed.     

On the formation mechanism of hexafluoropropylene oxide in low-temperature liquid-phase oxidation of hexafluoropropylene by molecular oxygen

The effect of initiation rate (dose-rate of -radiation) on the formation rate of hexafluoropropylene oxide has been studied. Experimental data confirm that the epoxide forms in the reaction R–O–C₃F ₆ ^· R^·+C₃F₆O, as contrasted to the generally accepted fragmentation of -peroxyalkyl radicals R–O–O–C₃F ₆ ^· RO^·+C₃F₆O.

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( -) . , , R–O–C₃F₆R+C₃F₆O R–O–O–C₃F₆RO+C₃F₆O.

     

On the enhanced reactivity and selectivity of triazole formation in molecular flasks. A theoretical rationale

The azide-alkyne cycloaddition assisted by a self-assembled molecular flask developed by Rebek and coworkers (Org. Lett., 2002, 4, 327) has been simulated by means of the ONIOM methodology, thereby evidencing the reliability of this theoretical approach to model such large encapsulated systems. Experimental evidences accounting for this transformation within the supramolecular assembly such as the significant rate enhancement, complete regioselectivity, and product inhibition as the reaction proceeds have been qualitatively disentangled through estimation of the energy barriers and the structural characteristics of the corresponding host-guest complexes.     

On the relationship between fluorescence and molecular geometry solvent-assisted formation of luminescent intramolecular exciplexes from anthronyl-substituted anthracenes

The fluorescence efficiency of 1′,2′-dihydro-spiro[anthracene-9(10H),3′-[3H]benz[de]anthracen]-10-one in 10 solvents of varying dielectric constant has been investigated. The observed exciplex luminescence is attributed to a solvent-assisted intramolecular charge-transfer interaction between the ground-state anthrone and excited-state anthracene moietis. The geometry of the exciplex is suggested to resemble that of a σ-π complex.     

On the molecular structure of cycloheptadiene

Molecular mechanics calculations on cycloheptadiene indicate that the molecule has a structure which undergoes a wide pseudorotational motion between two C₁ forms, and a C₁ form, and this structure is in equilibrium with the C₂ form. It is shown that this equilibrium mixture is consistent with all of the available experimental data.     

Monomer adsorption on terraces and nanotubes

We construct a nonsparse transfer matrix (T-matrix) for a lattice gas model of monomers adsorbed on planar and nanotube surfaces of arbitrary geometry. The model can accommodate any number of higher-order pairwise adsorbate-adsorbate interactions. The technique is sufficiently general for application to nonequivalent adsorption sites and coadsorption of two or more monomer species. The T-matrices for monomer adsorption on a finite width terrace and for monomer adsorption on a nanotube, both of the same lattice geometry, share a basic G-matrix. First, the G-matrix is diagrammatically and recursively constructed. Then, its elements are modified to provide the T-matrix elements for either the terrace or the nanotube. The T-matrices for several particular lattice geometries previously studied as special cases are easily recovered with the generalized technique presented here. This generalization also provides a vectorized algorithm for efficient use on multi-parallel processors and supercomputers.     

On the molecular structure of HOOO

The molecular structure of trans, planar hydridotrioxygen (HOOO) has been examined by means of isotopic spectroscopy using Fourier transform microwave as well as microwave-millimeter-wave double resonance techniques, and high-level coupled cluster quantum-chemical calculations. Although this weakly bound molecule is readily observed in an electrical discharge of H(2)O and O(2) heavily diluted in an inert buffer gas, we find that HOOO can be produced with somewhat higher abundance using H(2) and O(2) as precursor gases. Using equal mixtures of normal and (18)O(2), it has been possible to detect three new isotopic species, H(18)OOO, HO(18)O(18)O, and H(18)O(18)O(18)O. Detection of these species and not others provides compelling evidence that the dominant route to HOOO formation in our discharge is via the reaction OH + O(2) → HOOO. By combining derived rotational constants with those for normal HOOO and DOOO, it has been possible to determine a fully experimental (r(0)) structure for this radical, in which all of the structural parameters (the three bond lengths and two angles) have been varied. This best-fit structure possesses a longer central O-O bond (1.684 A?), in agreement with earlier work, a markedly shorter O-H bond distance (0.913 A?), and a more acute [angle]HOO angle (92.4°) when compared to equilibrium (r(e)) structures obtained from quantum-chemical calculations. To better understand the origin of these discrepancies, vibrational corrections have been obtained from coupled-cluster calculations. An empirical equilibrium (r(e) (emp)) structure, derived from the experimental rotational constants and theoretical vibrational corrections, gives only somewhat better agreement with the calculated equilibrium structure and large residual inertial defects, suggesting that still higher order vibrational corrections (i.e., γ terms) are needed to properly describe large-amplitude motion in HOOO. Owing to the high abundance of this oxygen-chain radical in our discharge expansion, a very wide spectral survey for other oxygen-bearing species has been undertaken between 6 and 25 GHz. Only about 50% of the observed lines have been assigned to known hydrogen-oxygen molecules or complexes, suggesting that a rich, unexplored oxygen chemistry awaits detection and characterization. Somewhat surprisingly, we find no evidence in our expansion for rotational transitions of cis HOOO or from low-lying vibrationally excited states of trans HOOO under conditions which optimize its ground state lines.     

The formation and molecular structure of acetylcyclopentadienylsodium-tetrahydrofuranate

Acetylcyclopentadienylsodium has been isolated in crystalline form as a THF adduct from a reaction between cyclopentadienylsodium and methyl acetate in THF solution. The product has been characterized by means of a single-crystal X-ray diffraction study. {[C₅H₄CMeO]Na·THF}_n crystallizes in the monoclinic space group P2₁/c with unit cell parameters a 6.698(3), b 16.095(4), c 10.661(3) Å, β 92.93(3)° and D_c 1.17 g cm^?3 for Z = 4. Least-squares refinement led to a final R value of 0.080 based on 661 independent observed reflections. The coordination sphere around each sodium atom consists of the oxygen atoms from two C₅H₄CMeO ligands, the oxygen atom of the THF molecule, and an ion contact pair between the sodium and the five ring carbon atoms of the C₅H₄CMeO ligand.     

Defect-engineered surfaces to investigate the formation of self-assembled molecular networks

Herein we report the impact of covalent modification (grafting), inducing lateral nanoconfinement conditions, on the self-assembly of a quinonoid zwitterion derivative into self-assembled molecular networks at the liquid/solid interface. At low concentrations where the compound does not show self-assembly behaviour on bare highly oriented pyrolytic graphite (HOPG), close-packed self-assembled structures are visualized by scanning tunneling microscopy on covalently modified HOPG. The size of the self-assembled domains decreases with increasing the density of grafted molecules, i.e. the molecules covalently bound to the surface. The dynamics of domains are captured with molecular resolution, revealing not only time-dependent growth and shrinkage processes but also the orientation conversion of assembled domains. Grafted pins play a key role in initiating the formation of on-surface molecular self-assembly and their stabilization, providing an elegant route to study various aspects of nucleation and growth processes of self-assembled molecular networks.

We showcase the use of covalently modified HOPG for the investigation of domain size controlled 2D self-assembly, nucleation and growth kinetics, molecular adsorption/desorption thermodynamics, and tip-induced selective recrystallization.     

On the formation of the so-called arsanthrene

The structure of Kalb's “arsanthrene oxide” and “arsanthrene” have been reinvestigated. The “arsanth-rene oxide” is monomeric and is therefore properly termed 5,10-epoxy-5,10-dihydroarsanthrene(6). A spectroscopic study of “arsanthrene” revealed that this compound has the dimeric structure 12 corresponding to the photodimer of anthracene. Mechanisms of formation of dimeric “arsanthrene” and the possible dissociation of dimeric “arsanthrene” into its monomer by reaction with a dienophile at elevated temperature are discussed. An attempt to synthesize arsanthrene (7) by dehalogenation of 5,10-dichloro-5,10-dihydroarsanthrene (5) was unsuccessful. The mass spectral fragmentation patterns of some 5,10-dihydroarsanthrenes are recorded.     

On the enthalpy of formation of thiophene

Thiophene is an important contaminant of petroleum-derived fuels, and it also plays an important role in molecular electronics. We have calculated the enthalpy of formation of thiophene employing the CCSD(T) methodology and the cc-pV(X + d)Z X = T,Q,5 basis sets. At the CCSD(T)/CBS limit and including corrections for scalar relativistic effects, anharmonic effects, spin–orbit and core-valence correlation effects, the estimated enthalpy of formation is 25.15 _-1 ^+0.5  kcal/mol. Our estimation is 2.3 kcal/mol lower than the experimental value. The discrepancies between experiment and theory are expected to be increased if higher-order correlation effects are taken into account. Thus, a new determination of the experimental value is highly recommended. Finally, we discuss the problems faced to make this estimation, in particular the determination of accurate Zero-point energy corrections and the evaluation of core-valence correlation effects.     

Enthalpy of formation of crystalline surfactant molecular complexes

The standard enthalpy of formation of novel chemical species — crystalline cationic surfactant molecular complexes — was studied to elucidate the bonding nature, serially scanning over the different surfactant chain-length homologs and various additive species. The enthalpy was not large, but was obviously dependent on the surfactant chain length and the chemical nature of the additive species. The typical complexes comprising long alkyl chain surfactants were formed endothermally, while in short alkyl chain homologs the process was exothermic. By examining the thermal aspect, it was suggested that the typical complexes of long alkyl-chain surfactants were derived not from attractive energetic force factors, but rather from entropic factors associated with the occurrence of severe disorder caused by heavy thermal agitation in the complex crystalline state.     

On the computation of molecular electronic affinities

Using a multireferent MBPT method (CIPSI) the electronic affinity (EA) of F, CN and HCC is computed. Results show how UMP2 gives unbalanced truncation of the MP series, while ROMP2 has the correct (balanced) behaviour. The good agreement with the experimental EA found for some compounds is accidental and associated to an error compensation. The good agreement with the experimental data found for the ROMP2 and CIPSI EAs is analysed.This paper was presented at the International Conference on The Impact of Supercomputers on Chemistry, held at the University of London, London, UK, 13–16 April 1987     

On the molecular origins of volumetric data

We use a statistical thermodynamic approach and a simple thermodynamic model of hydration to examine the molecular origins of the volumetric properties of solutes. In this model, solute-solvent interactions are treated as a binding reaction. The free energy of hydration of the noninteracting solute species coincides with the free energy of cavity formation, while the free energy of solute-solvent interactions is given by the binding polynomial. By differentiating the relationship for the free energy of hydration with respect to temperature and pressure, one obtains the complete set of equations describing the thermodynamic profile of hydration, including enthalpy, entropy, volume, compressibility, expansibility, and so forth. The model enables one to rigorously define in thermodynamic terms the hydration number and the related concept of hydration shell, which are both widely used as operational definitions in experimental studies. Hydration number, nh, is the effective number of water molecules solvating the solute and represents the derivative of the free energy of hydration with respect to the logarithm of water activity. One traditional way of studying hydration relies on the use of volumetric measurements. However, microscopic interpretation of macroscopic volumetric data is complicated and currently relies on empirical models that are not backed by theory. We use our derived model to link the microscopic determinants of the volumetric properties of a solute and its statistical thermodynamic parameters. In this treatment, the partial molar volume, V degrees, of a solute depends on the cavity volume, hydration number, and the properties of waters of hydration. In contrast, the partial molar isothermal compressibility, K degrees T, and expansibility, E degrees, observables, in addition to the intrinsic compressibility and expansibility of the cavity enclosing the solute, hydration number, and the properties of waters of hydration, contain previously unappreciated relaxation terms that originate from pressure- and temperature-induced perturbation of the equilibrium between the solvated solute species. If significant, the relaxation terms may bring about a new level of nonadditivity to compressibility and expansibility group contributions that goes beyond the overlap of the hydration shells of adjacent groups. We apply our theoretical results to numerical analyses of the volume and compressibility responses to changes in the distribution of solvated species of polar compounds.