Carbon-13 chemical shifts of inverted carbon atoms

The inverted carbons in 2,4-methano-2,4-dehydroadamantane are shielded by 14.3 ppm relative to the corresponding carbons in 2,4-methanoadamantane. This was explained by the balance of the inverted carbon hybridization deshielding, the shielding effect of the cyclopropane rings formed, and the change in the influence of the neighbouring atoms.     

Spectroscopy and fragmentation of undercoordinated bromoiridates

We report gas-phase electronic photodissociation spectra of the undercoordinated bromoiridate complexes IrBr(4)(-) and IrBr(5)(-) at photon energies from 1 to 5.6 eV. Both ions have open-shell ground states with low-symmetry structures. The fragmentation is characterized by thresholds for the loss of one Br atom for IrBr(4)(-) and one or two Br atoms for IrBr(5)(-). The experimental spectra consist of ligand-to-metal charge transfer transitions and reveal a large density of electronic states that can be recovered by time-dependent density functional theory.     

1S core-level shifts of Al and Ar atoms in aluminum clusters

Theoretical 1s core-electron binding energies are presented for Al and Ar atoms in free space and in AlAl₁₂, AlAl₁₂Al₆, and ArAl₁₂ clusters. The binding energies have been calculated by the self-consistent field Xα scattered-wave (SCF Xα SW ) method using various exchange parameters and different atomic-sphere overlaps. The atom/cluster binding-energy shifts have been obtained both from the Slater's transition-state energies and from the total-energy differences; these values are in better agreement with each other if calculated with proper overlapping than if with nonoverlapping spheres. A comparison with available experimental and theoretical data is given as well.     

Core excitations of naphthalene: vibrational structure versus chemical shifts

High-resolution x-ray photoelectron emission (XPS) and near-edge x-ray absorption fine structure (NEXAFS) spectra of naphthalene are analyzed in terms of the initial state chemical shifts and the vibrational fine structure of the excitations. Carbon atoms located at peripheral sites experience only a small chemical shift and exhibit rather similar charge-vibrational coupling, while the atoms in the bridging positions differ substantially. In the XPS spectra, C-H stretching modes provide important contributions to the overall shape of the spectrum. In contrast, the NEXAFS spectrum contains only vibrational progressions from particular C-C stretching modes. The accuracy of ab initio calculations of absolute electronic transition energies is discussed in the context of minute chemical shifts, the vibrational fine structure, and the state multiplicity.     

Unexpected Pt(II) migration between the calixarene oxygen atoms

Reaction between the 1,3-bis(trimethylsilyl) ether of calix[4]arene and platinum(II) difluoro complexes unexpectedly results in the formation of the 1,2-bridging platinum(II) calixarene complex, which, upon treatment with 2 equiv of acyl chloride, reinstates the 1,3-disubstitution pattern in the calixarene moiety.     

Core electrons and hydrogen atoms in chemical graph theory

General and complete graphs have recently been used to free chemical graph theory, and especially molecular connectivity theory, from spurious concepts, which belonged to quantum chemistry with no direct counterpart in graph theory. Both types of graph concepts allow the encoding of multiple bonds, non-bonding electrons, and core electrons. Furthermore, they allow the encoding of the bonded hydrogen atoms, which are normally suppressed in chemical graphs. This suppression could sometimes have nasty consequences, like the impossibility to differentiate between compounds, whose hydrogen-suppressed chemical graphs are completely equivalent, like for the CH₂F₂ and BHF₂ compounds. At the computational level the new graph concepts do not introduce any dramatic changes relatively to previous QSPR/QSAR studies. These concepts can nevertheless help in encoding the many electronic features of a molecule, achieving, as a bonus, an improved quality of the modeled properties, as it is here exemplified with a set of properties of different classes of compounds.     

Xanes of the Pd and Pt atoms in square and octahedral chloride complexes

The fine structure near the ionization threshold in metal X-ray L-absorption spectra of the clusters PdCl²⁻₄, PtCl²⁻₄, PdCl²⁻₆ and PtCl²⁻₆ simulating solid complex compounds of the type K₂PdCl₄ and so on has been calculated using the SCF Xα SW method. Pt L₃ absorption spectra for the complexes K₂PtCl₄ and K₂PtCl₆ have been obtained using synchrotron radiation. The pre-edge white line and the near-edge fine structure (XANES) in Pd and Pt spectra of these square and octahedral chloride complexes have been shown to arise, respectively, from electron excitation to a vacant bound molecular orbital (e_g for octahedral and b_1g for square complexes) and from photoelectron resonance elastic scattering by the metal atom and its first coordination sphere. The calculations indicated no intense pre-edge transitions for the metal L₁ spectra of these complexes and the XANES was shown to be due to several intense narrow shape resonances of the t_1u and a_2u, e_u for octahedral and square complexes, respectively.     

Core level study of alanine and threonine

Core level X-ray photoemission spectra (XPS) and near edge X-ray absorption fine structure (NEXAFS) spectra of alanine and threonine in the gas phase have been measured at the carbon, nitrogen, and oxygen K edges and interpreted in the light of theoretical calculations. For the computations, a set of approximations is made which allows sufficiently accurate calculations of several conformers to be performed in reasonable computing time. The accuracy has been checked by comparing results obtained for proline to our previous, higher level calculations. The photoemission spectra at the carbon and oxygen edges are assigned and compared. The nitrogen 1s photoemission peaks show anomalous broadening which we relate to the populations and types of conformers. The carbon K-edge NEXAFS spectra of alanine and threonine are compared with our previous data on glycine and resonances assigned accordingly. The nitrogen K-edge NEXAFS spectra of alanine and threonine do not show measurable effects due to the population of conformers, in contrast to the photoemission results. At the oxygen K edge, the spectra of these amino acids are similar with two prominent peaks assigned to transitions of O 1s electrons from the oxo and hydroxyl groups to vacant pi* and sigma* orbitals and additional intensity for threonine due to the second OH group. Conformer effects are observable in photoemission but appear to be more difficult to resolve in photoabsorption. We explain this by energetic shifts of opposite sign for the core hole states and unoccupied orbitals, which causes partial cancelation in NEXAFS but not in photoemission.     

Core level studies of calcite and dolomite

Conventional X‐ray photoelectron spectroscopy (XPS) and synchrotron‐based X‐ray photoelectron spectroscopy (HRXPS) have been used to study Iceland spar calcite (CaCO₃) and dolomite (CaMg(CO₃)₂). The obtained full widths at half maximum (FWHMs) are mostly narrower than in the previous results, which together with the symmetry of the fitted peaks indicate effective neutralisation of surface charging. Some previously unidentified features observed in the Ca 2p, C 1s and O 1s spectra of calcite have been suggested to be bulk plasmons. Also, surface core level shifts in Ca 2p (in calcite) and Mg 2p (in dolomite) spectra have been obtained and found to be consistent between XPS and HRXPS measurements. A peak attributed to carbide (CaC₂) has been suggested to indicate beam‐assisted interaction with hydrocarbons found on the surface. Copyright © 2014 John Wiley & Sons, Ltd.     

Correlations of chemical shifts of saturated carbon atoms that have a directly bonded substituent

The dependence of the chemical shifts, δ, of saturated carbon atoms upon the nature of the directly bonded substituent is attributed to three factors (1) electronegativity E, (2) field effect, W, and (3) heavy atom ettect, Q. Ten series of compounds (five aliphatic and five cyclic) have been correlated with the equation δ = aE + W + Q+ c where a and c are constants.     

Pseudoequivalent nitrogen atoms in aqueous imidazole distinguished by chemical shifts in photoelectron spectroscopy

The photoelectron spectra of aqueous imidazole are presented, and the N 1s and C 1s binding energies are assigned aided by density functional theory calculations. The chemical equivalency of the two nitrogens of the cationic form is directly identified by the occurrence of a single N 1s photoelectron peak, which results from the delocalization of the positive charge over the molecule as a consequence of the Cv symmetry of the system. In contrast to NMR measurements, the photoemission process is faster than the rapid proton exchange in the aqueous environment, making the pseudoequivalent nitrogens of the neutral state clearly distinguishable with a N 1s binding energy shift of 1.7 eV.     

Reversible hydrogenation of surface N atoms to form NH on Pt(111)

The formation and dissociation chemistry of the NH species on Pt(111) was characterized with reflection absorption infrared spectroscopy and temperature programmed desorption. Irradiation of a chemisorbed bilayer of ammonia with a 100 eV electron beam at 85 K leads to a mixture of NH, N, and H on the surface. Annealing to temperatures in the range of 200-300 K leads to reaction of N and H to form additional NH. The NH species has an intense and narrow NH stretch peak at 3320 cm(-1), while no peak due to the PtNH bend is observed above 800 cm(-1). The NH species is stable up to a temperature of approximately 400 K. The surface N atoms produced from NH dissociation are readily hydrogenated back to NH by exposure of the surface to H2. However, NH cannot be further hydrogenated to generate adsorbed NH2 or to NH3 under the conditions used here. Exposure of the NH/Pt(111) surface to D2 at 380 K produces the ND species. Comparison with the results of density functional theory calculations based on small Pt clusters indicates that NH occupies three-fold hollow sites with the molecular axis perpendicular to the surface.     

Analyzing Pt chemical shifts calculated from relativistic density functional theory using localized orbitals: the role of Pt 5d lone pairs

Pt chemical shifts were calculated from two-component relativistic density functional theory (DFT). The shielding tensors were analyzed by using a recently developed method to decompose the spin-orbit DFT results into contributions from spin-free localized orbitals (here: natural localized molecular orbitals (NLMOs) and natural bond orbitals (NBOs)). Seven chemical shifts in six Pt complexes with Pt oxidation states II, III, and IV; and halide, amino, and amidate ligands were analyzed, with particular focus on the role of nonbonding Pt 5d orbitals. A simple d-orbital 'rotation' model has been used to rationalize some of the observed trends such as the main difference between Pt(II) and Pt(IV) chemical shifts. The localized orbital analysis data showed that most of this difference as well as trends among different Pt complexes with similar coordination can be rationalized by comparing properties of the nonbonding Pt 5d orbitals. We have also analyzed the spin-orbit effects on the chemical shifts of [PtCl4](2-) compared to [PtBr4](2-).     

Core distortions in metal atoms and the use of effective core potentials

The recent experimental determination of the geometry of Ti(CH₃)₂Cl₂ shows it to be inconsistent with the VSEPR model, a result not uncommon for molecules containing transition metal atoms. The valence shell charge concentrations (CCs) that appear as maxima in L(r)=−²ρ(r), provide a physical basis for the VSEPR model of molecular geometry for main group molecules. The same model accounts for the geometry of transition metal molecules with the proviso that the CCs are formed within the outer shell of the core of the metal atom, as defined by the shell structure of L(r). This observation appears to be in conflict with calculations for Ti(CH₃)₂Cl₂ showing that its geometry can be predicted using an effective core potential for the metal atom, a procedure that would appear to preclude the presence of core distortions. The apparent contradiction is resolved by distinguishing between the definition of the core using L(r) and one based on the orbital model.     

Core and valence electrons in atoms and ions: configuration interaction calculations

Summary The partitioning of ground-state atoms or ions into inner spherical cores with radius _b and outer valence regions extending from _b to infinity is explored with the help of the expression for the valence-region energy (where T ^v and V ^v are, respectively, the kinetic and potential energies of the valence electrons N ^v found beyond the boundary surface defined by _b) using also the appropriate expression for E ^ion, the energy of the ion left behind after removal of the valence electrons. E ^v and E ^ion are meaningful only for discrete numbers, N ^c, of electrons assigned to the core, namely, when the exchange integrals, K ^cv, between N ^c and N ^v total (or at least closely approach) 0, i.e., for N ^c = 2 e or N ^c = 2 and 10 e for the first- or second-row elements, respectively.Part of the projected Ph.D. dissertation of N. D.     

Core ionization energies of atoms and ions calculated using the generalized Sturmian method

The physical event of the umbrella inversion of ammonia has been studied in detail by application of the formalisms of frontier orbital theory, the density functional theory, the localized molecular orbital method, and the energy partitioning analysis. An intuitive structure for the transition state and dynamics of the physical process of structural reorganization prior to inversion have been suggested. The computation starts with the CNDO/2 equilibrium geometry, and thereafter the cycle proceeds through all the conformations of ammonia obtained by varying the ∠HNH angle in steps of 2° from its equilibrium value up to the transition state. The geometry of each conformation is optimized with respect to the length of the N–H bond. The glimpses of the charge density reorganization during the movement of the molecule from equilibrium conformation toward the transition state is computed in terms of dipole moment and the quantum mechanical hybridizations of bond pair and lone pair of N atom through the localized molecular orbitals (LMOs) of all the conformations. Results demonstrate that as the geometry of the molecule begins to evolve through the reorganization of structure, the N–H bond length and the dipole moment begin to decrease, and the trend continues up to the transition state. The dipole moment of the molecule at the suggested transition state is zero. The computed nature of quantum mechanical hybridization of bond pair and lone pair of the N atom as a function of reaction coordinates of the ∠HNH angles reveals that the percentage of s character of the lone pair hybrid decreases and that of the bond pair hybrid forming the σ(N–H) bond increases during the process of geometry reorganization from the equilibrium shape to the transition state. The rationale of the zero dipole moment of the transition state for inversion is not straightforward from its point‐group symmetry, but is self‐evident from its electronic structure drawn in terms of LMOs. The electronic structure of the transition state, which may be drawn in terms of the LMOs, seems to closely reproduce its suggested intuitive structure. The pattern of variation of dipole moment and nature of the changes of the percentage of the s character in the lone pair hybrid creating dipole with the evolution of geometry during the physical process of structural reorganization for the inversion are found to be nicely correlated according to the suggestion of Coulson. The profiles of the increasing strength of the N–H bond and the increasing percentage of s character of the bond pair hybrid of N atom forming this bond as a function of reaction coordinates are also found to be correlated in accordance with the suggestion of Coulson. The profile of global hardness as a function of reaction coordinate seems to demonstrate that the dynamics of the evolution of the molecular structure from equilibrium shape to the transition state following the course of suggested mode of structural reorganization conforms to the principle of maximum hardness (PMH). The profiles of parameters like the energies of highest occupied and lowest unoccupied molecular orbital (HOMO and LUMO), the gap in energy between HOMO and LUMO, the global hardness, the global softness, and chemical potential as a function of reaction coodinates of a continuous structural evolution from equilibrium shape to the transition state mimic the potential energy diagram of the total energy. Both the frontier orbital parameters and the density functional quantities are found to be equally effective and reliable to monitor the process of necessary structural reorganization prior to the inversion of mofecules. An energy partitioning analysis demonstrates that the origin of barrier has no unique single source rather as many as four mutually exclusive but interacting one‐ and two‐center energy terms within the molecule entail the origin and the height of the barrier. From a close analysis of the results, it seems highly probable that the necessary structural reorganization prior to umbrella inversion of ammonia most realistically occurs following the course of normal modes of vibration of the molecule. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 80: 1–26, 2000     

Correlation of dihedral angles with 13C chemical shifts of quaternary carbon atoms of some phenothiazine derivatives

A direct relationship between the ¹³C nmr chemical shifts of the quaternary carbon atoms of the central ring of phenothiazine and the dihedral angle of the respective derivatives has been observed. This correlation allows for the useful and quick estimation of the dihedral angles of novel phenothiazines from readily obtainable ¹³C nmr solution measurements. In addition, the change in the dihedral angle appears to be directly related to the S-C_4a-C_10a bond angle; however, the C_4a-S-C_5a bond angle is not affected by changes in the dihedral angle. This indicates that the “flattening” of the phenothiazine tricyclic ring system is compensated for by the vertical displacement of the sulfur atom, by changes in hybridization of N₁₀, and by other angular distortions of the middle ring.     

Origin of the carbon 1s-core level shifts in polyimide model compounds

Carbon 1s (C1s) core-level positions have been calculated for two model compounds which are similar in structure to the PMDA part of the PMDA-ODA monomer of polyimide. It is found that either one or two N-phenyl imide groups fused to a benzene ring will lower the C1s energy levels of the benzene ring. The difference in previous interpretation of the origin of the C1s (ESCA) spectra between the two model compounds examined is shown to be consistent with the difference in calculated magnitude of this shift between the two compounds.