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El consumo y dependencia de sustancias, es un serio problema social con alta morbilidad materno-fetal. El aumento de la oferta y el contexto social favorecedor, permitió que la difusión del consumo sustancias tóxicas ilícitas, conlleve una incidencia creciente en gestantes consumidoras de sustancias y aumento de recién nacidos afectados por las prácticas tóxicas de sus madres. La evaluación del riesgo cuando la gestación es expuesta a las drogas es difícil, los resultados pueden estar sesgados por el consumo concomitante de otros tóxicos o por factores sicológicos y socio-sanitarios desfavorables. Aunque tampoco se definió un patrón específico de anomalías congénitas, se considera que el abuso de drogas, en general, comporta mayor riego de desenlace anómalo del embarazo, por un incremento del riesgo de malformaciones congénitas, debido a la probable teratogenicidad de algunas sustancias o de la morbilidad perinatal afectando el crecimiento fetal o el normal desarrollo del embarazo. También existen posibles repercusiones a largo plazo en la capacidad de aprendizaje y comportamiento de los niños expuestos intraútero, aunque no demostró efectivamente. Por lo tanto, todo embarazo en el que se detecto un hábito tóxico se debe considerar de mayor riesgo, tomando las medidas oportunas para lograr que las pacientes se alejen de éstas prácticas, apoyadas por un equipo multidisciplinario, idealmente antes del inicio del embarazo, lo que implica la adopción de medidas profilácticas de información y concienciación de las mujeres en edad fértil y de apoyo durante el embarazo y la lactancia para el abandono de la dependencia

Molecules of the general form Tp*MoO(OR)(2) [where Tp* = hydrotris(3,5-dimethyl-1-pyrazolyl)borate and (OR)(2) = (OMe)(2), (OEt)(2), and (O(n)Pr)(2) for alkoxide ligands and (OR)(2) = O(CH(2))(3)O, O(CH(2))(4)O, and O[CH(CH(3))CH(2)CH(CH(3))]O for diolato ligands] were studied using gas-phase photoelectron spectroscopy, cyclic voltammetry, and density functional theory (DFT) calculations to examine the effect of increasing ligand size and structure on the oxomolybdenum core. Oxidation potentials and first ionization energies are shown to be sensitive to the character of the diolato and alkoxide ligands. A linear correlation between the solution-phase oxidation potentials and the gas-phase ionization energies resulted in an unexpected slope of greater than unity. DFT calculations indicated that this unique example of a system in which oxidation potentials are more sensitive to substitution than vertical ionization energies is due to the large differences in the cation reorganization energies, which range from 0.2 eV or less for the molecules with diolato ligands to around 0.5 eV for the molecules with alkoxide ligands.

The charge-transport parameters of the perfluoropentacene and perfluorotetracene crystals are studied with a joint experimental and theoretical approach that combines gas-phase ultraviolet photoelectron spectroscopy and density functional theory. To gain a better understanding of the role of perfluorination, the results for perfluoropentacene and perfluorotetracene are compared to those for their parent oligoacenes, that is, pentacene and tetracene. Perfluorination is calculated to increase the ionization potentials and electron affinities by approximately 1 eV, which is expected to reduce significantly the injection barrier for electrons in organic electronics devices. Perfluorination also leads to significant changes in the crystalline packing, which greatly affects the electronic properties of the crystals and their charge-transport characteristics. The calculations predict large conduction and valence bandwidths and low hole and electron effective masses in the perfluoroacene crystals, with the largest mobilities expected along the pi-stacks. Perfluorination impacts as well both local and nonlocal vibrational couplings, whose strengths increase by a factor of about 2 with respect to the parent compounds.

Understanding the electronic properties and electron transfer characteristics of functionalized oligoacenes is of great importance for the fabrication of organic electronic devices. Charge transfer parameters of bis-triisopropylsilylethynyl-substituted anthracene, tetracene, and pentacene have been investigated based on the analysis of their ionization energies and radical cation reorganization energies. High-resolution gas-phase photoelectron spectroscopy measurements and first-principles quantum-mechanical calculations at the density functional theory level have been performed. The results indicate that the ionization energies in the gas phase and the inner-sphere reorganization energies are sensitive to the number of fused rings and the substitution by the triisopropylsilylethynyl (TIPS) group. This TIPS functional group shifts the first ionization band of these molecules to lower energy in the gas phase due to mixing between the frontier occupied orbitals of the TIPS group with the highest occupied ac...

Synthetic analogs, μ-(RS)2Fe2(CO)6, of the active site of [Fe–Fe] hydrogenases do not have the semi-bridged CO and “rotated” structure found in the enzyme. However, recent studies have shown that cations of dithiolatodiiron complexes adopt this rotated structure. This paper reports the use of photoelectron spectroscopy in combination with density functional theory calculations to show that two previously reported complexes: μ-(1,2-benzenedithiolato)Fe2(CO)6 and μ-(1,3-propanedithiolato)Fe2(CO)6 and two new complexes: μ-(2,3-pyridinodithiolato)Fe2(CO)6 and μ-(norbornane-2-exo,3-exo-dithiolato)Fe2(CO)6 favor the “rotated” structure in their corresponding cations. Furthermore, these methods provide a measure of the reorganization energy between the “rotated” and “unrotated” structures in the gas phase. The results provide insight on the entatic state of the dithiolatodiiron site in the enzyme, in which the protein controls the structure of the active site. This structure influences the redox energy and reorganization energy enabling fast electron transfer.

By using gas-phase ultraviolet photoelectron spectroscopy, vis/NIR spectroscopy, and electronic-structure calculations, we have investigated the electron-vibration and electronic interactions in a series of bisdimethylamino mixed-valence systems: N,N,N′,N′-tetramethyl-p-phenylenediamine, N,N,N′,N′-tetramethylbenzidine, and N,N,N′,N′-tetramethyltolane-4,4′-diamine. Experiment and theory concur to indicate that the electron-vibration coupling in these systems is dominated by interactions with symmetric modes. The results reveal that the strength of both electronic and electron-vibration couplings decreases as the molecular bridge lengthens. The parameters derived for the present compounds have been compared to those of diarylamino-based structural analogs. This comparison underlines that the replacement of the methyl terminal groups with p-anisyl groups has a significant effect on the electronic and electron-vibrational interactions.

The Cp(2)VCl(2) molecule is a prototype for bent metallocene complexes with a single electron in the metal d shell, but experimental measure of the binding energy of the d electron by photoelectron spectroscopy eluded early attempts due to apparent decomposition in the spectrometer to Cp(2)VCl. With improved instrumentation, the amount of decomposition is reduced and subtraction of ionization intensity due to Cp(2)VCl from the Cp(2)VCl(2)/Cp(2)VCl mixed spectrum yields the Cp(2)VCl(2) spectrum exclusively. The measured ionization energies provide well-defined benchmarks for electronic structure calculations. Density functional calculations support the spectral interpretations and agree well with the ionization energy of the d(1) electron and the energies of the higher positive ion states of Cp(2)VCl(2). The calculations also account well for the trends to the other Group V bent metallocene dichlorides Cp(2)NbCl(2) and Cp(2)TaCl(2). The first ionization energy of Cp(2)VCl(2) is considerably greater than the first ionization energies of the second- and third-row transition metal analogues.

Lone pair ionization energies for 1,5-dichalcogenocanes containing endocyclic 3,7-R2Si groups and the parent 1,5-dichalcogenocanes, estimated from the TCNE charge-transfer wavelength maxima, are in good agreement with ionization energies directly obtained from photoelectron spectroscopy. These data indicate the occurrence of substantial intra-annular interaction between the chalcogen atoms and silyl groups, consistent with the well-known β -effect of silicon and the novel β-disilyl effect, when R (in R 2 Si) is Me 3 Si.

The metal–sulfur bonding present in the transition metal–thiolate complexes CpFe(CO)2SCH3, CpFe(CO)2StBu, CpRe(NO)(PiPr3)SCH3, and CpRe(NO)(PPh3)SCH3 (Cp = η5-C5H5) is investigated via gas-phase valence photoelectron spectroscopy. For all four complexes a strong dπ–pπ interaction exists between a filled predominantly metal d orbital of the [CpML2]+ fragment and the purely sulfur 3pπ lone pair of the thiolate. This interaction results in the highest occupied molecular orbital having substantial M–S π∗ antibonding character. In the case of CpFe(CO)2SCH3, the first (lowest energy) ionization is from the Fe–S π∗ orbital, the next two ionizations are from predominantly metal d orbitals, and the fourth ionization is from the Fe–S π orbital. The pure sulfur pπ lone pair of the thiolate fragment is less stable than the filled metal d orbitals of the [CpFe(CO)2]+ fragment, resulting in a Fe–S π∗ combination that is higher in sulfur character than the Fe–S π combination. Interestingly, substitution of a tert-butyl group for the methyl group on the thiolate causes little shift in the first ionization, in contrast to the shift observed for related thiols. This is a consequence of the delocalization and electronic buffering provided by the Fe–S dπ–pπ interaction. For CpRe(NO)(PiPr3)SCH3 and CpRe(NO)(PPh3)SCH3, the strong acceptor ability of the nitrosyl ligand rotates the metal orbitals for optimum backbonding to the nitrosyl, and the thiolate rotates along with these orbitals to a different preferred orientation from that of the Fe complexes. The initial ionization is again the M–S π∗ combination with mostly sulfur character, but now has considerable mixing among several of the valence orbitals. Because of the high sulfur character in the HOMO, ligand substitution on the metal also has a small effect on the ionization energy in comparison to the shifts observed for similar substitutions in other molecules. These experiments show that, contrary to the traditional interpretation of oxidation of metal complexes, removal of an electron from these metal–thiolate complexes is not well represented by an increase in the formal oxidation state of the metal, nor by simple oxidation of the sulfur, but instead is a variable mix of metal and sulfur content in the highest occupied orbital.

The photoelectron spectrum and a density functional computational analysis of the first p-block paddlewheel complex, Bi2(tfa)4, where tfa = (O2CCF3)−, are reported. The photoelectron spectrum of Bi2(tfa)4 contains an ionization band between the region of metal-based ionizations and the region of overlapping ligand ionizations that is not seen in the photoelectron spectra of d-block paddlewheel complexes. This additional ionization arises from an a1g symmetry combination of the tfa ligand orbitals that is directed for σ bonding with the metals, and the unusual energy of this ionization follows from the different interaction of this orbital with the valence s and p orbitals of Bi compared to the valence d orbitals of transition metals. There is significant mixing between the Bi–Bi σ bond and this a1g M–L σ orbital. This observation led to a re-examination of the ionization differences between Mo2(tfa)4 and W2(tfa)4, where the metal–metal σ and π ionizations are overlapping for the Mo2 molecule but a separate and sharp σ ionization is observed for the W2 molecule. The coalescing of the σ and π bond ionizations of Mo2(tfa)4 is due to greater ligand orbital character in the Mo–Mo σ bond (∼7%) versus the W–W σ bond (∼1%).

The ability of neighboring C-Si, C-Sn, and Si-Si groups in conformationally constrained cyclic molecules to reduce the lowest ionization energies of sulfur, selenium, and tellurium compounds has been determined by charge-transfer spectroscopy of complexes with tetracyanoethylene. For selected compounds, ionization energies were determined by gas-phase photoelectron spectroscopy. The lowest ionization energies measured by photoelectron spectroscopy, with one exception, correlate with the charge-transfer spectroscopic data. In addition, theoretical analysis has provided insight into the photoelectron spectra and the geometry-dependent interaction between C-Si or C-Sn bonds and chalcogen lone pairs. Substantial lowering of ionization energies is found which is anticipated to have important consequences in the chemistry of these and related species.

More than 40 new 4- to 12-membered ring heterocycles containing various combinations of Group 14 elements (Si and Sn) and Group 16 elements (S, Se, and Te) have been synthesized and fully characterized. Synthesis of these small-ring as well as medium-ring (mesocyclic) heterocycles from α, ω-dihalides was facilitated by the presence of gem-dialkylsilyl and gem-dialkylstannyl groups in the precursors. Solid-state conformations of the new ring systems have been determined by X-ray crystallography. Oxidation of mixed S(Se, Te)/Si eight-membered ring mesocycles as well as 1,5-dithia-, 1,5-diselena-, and 1,5-ditelluracyclooctane with NOPF6 gave dications, which can be characterized by NMR. On treatment with nucleophiles, mesocyclic dications or the corresponding radical cations underwent ring contraction to give five- or six-membered ring heterocycles. The ionization energies of the above conformationally constrained β-disilanyl sulfides and selenides were determined by photoelectron spectroscopy. These ionization energies reflect substantial (0.53--0.75 eV) orbital destabilizations. The basis for these destabilizations was investigated by theoretical calculations, which reveal geometry-dependent interaction between sulfur or selenium lone pair orbitals and σ-orbitals, especially Si–Si σ-orbitals. These results suggest facile redox chemistry for these compounds and significantly extend the concept of σ-stabilization of electron-deficient centers. © 2007 Wiley Periodicals, Inc. 18:509–515, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20333

The ionization energies of conformationally constrained, newly synthesized beta-disilanyl sulfides and selenides were determined by photoelectron spectroscopy. These ionization energies reflect substantial (0.53-0.75 eV) orbital destabilizations. The basis for these destabilizations was investigated by theoretical calculations, which reveal geometry-dependent interaction between sulfur or selenium lone pair orbitals and sigma-orbitals, especially Si-Si sigma-orbitals. These results presage facile redox chemistry for these compounds and significantly extend the concept of sigma-stabilization of electron-deficient centers.

Electrochemical oxidation of the extended tetrathiafulvalene 9,10-bis(1,3-dithiole-2-ylidene)-9,10-dihydroanthracene (2) was studied in N,N-dimethylformamide. A single, two-electron oxidation peak occurs, and on the return sweep of a cyclic voltammogram, a two-electron reduction peak is seen. The oxidation of 2 to its cation radical and dication occurs with potential inversion (i.e., removal of the second electron occurs more easily than removal of the first). The extent of potential inversion was estimated by cyclic voltammetry to be 0.28 V by analysis of the process in terms of concerted structural change and electron transfer. Failure to detect the cation radical by EPR of an equimolar mixture of neutral 2 and the dication is consistent with this value. The inner reorganization energy of the cation radical was determined by gas-phase photoelectron spectroscopy (PES) to be 0.31-0.35 eV. Calculations, consistent with earlier experimental data, show rather large changes in structure associated with the oxidation processes. These large structural changes contrast with the relatively small inner reorganization energy found by PES. This observation prompted an analysis of voltammetry in terms of two-step processes, with structural change either preceding or following electron transfer. Agreement of simulations based on this mechanism with experimental voltammograms was equally as good as with the concerted mechanism. Notably, the two-step mechanism produced more realistic values of the transfer coefficient and electron-transfer rate constant for the first step of oxidation.

The electronic structures of molecules of the form Cp*Ru(η5-Pdl) (where Cp* = η5-pentamethylcyclopentadienyl and Pdl = 2,4-dimethylpentadienyl and various heteropentadienyl ligands, including the azapentadienyl ligand 1-tert-butyl-3,5-dimethyl-1-azapentadienyl and the oxopentadienyl ligands 2,4-dimethyl-1-oxopentadienyl and 2,4-di-tert-butyl-1-oxopentadienyl) have been investigated using photoelectron spectroscopy and computational methods. The photoelectron spectra of these half-open metallocenes allow direct insight into the bonding capabilities of pentadienyl ligands and the influences of heteroatom substitution on the electronic structure. The number of separate valence ionization bands that are observed corresponds directly to the number of occupied valence metal d orbitals and highest occupied π orbitals of the Cp* and Pdl ligands plus (in the case of the heteropentadienyl ligands) an ionization that derives from the heteroatom (oxygen or nitrogen) lone pair orbital that is in the plane of the ligan...

Gas-phase photoelectron spectroscopy and density functional theory have been utilized to investigate the interactions between the p orbitals of dithiolate ligands and d orbitals of titanium in bent...

Molecules of the form Cp(6,6-dmch)ZrX(2) (Cp = eta(5)-cyclopentadienyl, X = Cl, Br, I; 6,6-dmch = eta(5)-6,6-dimethylcyclohexadienyl) have been synthesized, and the molecular and electronic structures have been investigated. These molecules allow direct comparison of the bonding and properties of pentadienyl and cyclopentadienyl ligands in the same high-oxidation-state metal complexes. Unlike the well-known Cp(2)ZrX(2) analogues, these Cp(6,6-dmch)ZrX(2) molecules are intensely colored, indicating significantly different relative energies of the frontier orbitals. Also unusual, the average Zr-C distances to the 6,6-dmch pentadienyl ligand are about 0.1 A longer than the average Zr-C distances to the cyclopentadienyl ligand for these Zr(IV) complexes, opposite of what is observed for the Zr(II) complex Cp(2,6,6-tmch)Zr(PMe(3))(2) (tmch = eta(5)-2,6,6-trimethylcyclohexadienyl), reflecting a dramatic reversal in the favorability of the bonding depending on the metal oxidation state. The experimental and computational results indicate that the color of the Cp(6,6-dmch)ZrX(2) complexes is due to a 6,6-dmch ligand-to-metal charge-transfer band. Compared to the Cp(2)ZrX(2) analogues, the Cp(6,6-dmch)ZrX(2) molecules have a considerably less stable HOMO that is pentadienyl-based and an essentially unchanged metal-based LUMO. Also, the lowest unoccupied orbital of pentadienyl is stabilized relative to cyclopentadienyl and becomes a better potential delta electron acceptor, thus contributing to the differences in structure and reactivity of the low-valent and high-valent metal complexes.

The molecular parameters that govern charge transport in anthradithiophene (ADT) are studied by a joint experimental/theoretical approach involving high-resolution gas-phase photoelectron spectroscopy and quantum-mechanical methods. The hole reorganization energy of ADT has been determined by an analysis of the vibrational structure of the lowest ionization band in the gas-phase photoelectron spectrum as well as by density-functional theory calculations. In addition, various dimers and clusters of ADT molecules have been considered in order to understand the effect of molecular packing on the hole and electron intermolecular transfer integrals. The results indicate that the intrinsic electronic structure, the relevant intramolecular vibrational modes, and the intermolecular interactions in ADT are very similar to those in pentacene.

The electronic structures of the title complexes—VO(acen) and VS(acen)—and the free H2(acen) ligand are probed using gas-phase UV-photoelectron spectroscopy [acen = N,N � -ethylenebis(acetylacetonylideiminato)]. The effect of the different axial donors on the metal center is examined, as is the effect that the oxo and thio ligands have on the acen orbitals. We find that the oxo and thio donors primarily affect the metal center and that the ligand periphery remains mostly unchanged. © 2003 Elsevier B.V. All rights reserved.

The electronic structures of the potassium salts of the homoscorpionates hydrotris(3,5-dimethylpyrazol-1-yl)borate (Tp*, 1), hydrotris(4-chloro-3,5-dimethylpyrazol-1-yl)borate (Tp*Cl, 2) and hydrotris(3,5-bis(trifluoromethyl)pyrazol-1-yl)borate (Tp(CF3)2, 3) are compared using gas-phase photoelectron spectroscopy and density functional theory (DFT). DFT calculations also are reported for the generic scorpionate potassium (hydrotris(pyrazol-1-yl)borate) (KTp). This is the first such experimental probe of the electronic structure of halogen containing scorpionate ligands and subtle differences in the ionizations from the frontier orbitals in the photoelectron spectra of 1 and 3 are observed that give insight into the influence of substituents upon metal–scorpionate bonding. Distinct assignments of the ionizations from the nitrogen σ-donor orbitals (σN) and σBH molecular orbitals are possible experimentally by the use of variable (He I and He II) excitation energies. The experimentally observed first ionization energy of 3 is stabilized by ∼2.0 eV relative to 1 due to the strong electron withdrawing effect of the trifluoromethyl groups. The photoelectron spectroscopic studies of NaTp(CF3)2 further confirm the assignments of ionizations from σN orbitals for 3 associated with the a and e sets in C3 symmetry. The X-ray crystal structure of 2 as the (μ-aqua)3(potassium hydrotris(4-chloro-3,5-dimethylpyrazol-1-yl)borate)2 dimer is also reported.

Gold surfaces modified with C3−C18-alkanethiols (CH3(CH2)X-1SH; HXSH; x = 3, 8, 12, 16, 18) and C16-alkanethiols, fluorinated at the outer 1, 2, 4, and 10 methylene positions (CF3(CF2)Y-1(CH2)XSH; FyHxSH; y = 1, x = 15; y = 2, x = 14; y = 4, x = 12; y = 10, x = 6) were characterized by He(I) UV-photoelectron spectroscopy (UPS). (Detailed X-ray photoelectron spectroscopic characterization of the partially fluorinated thin films is given in the Supporting Information). Long incubation times of the gold surface with the alkanethiol solutions lead to compact monolayer films for all of the alkanethiols, as indicated by the exponential decrease in emission intensity versus alkyl chain length for both the gold Fermi edge (UPS data), and by a parallel decrease in Au(4f) photoemission intensity using X-ray photoelectron spectroscopy. Changes in the effective work function of these surfaces due to the presence of significant interfacial dipoles are observed (i) as alkyl chain length is increased, and (ii) as the fr...

Gas-phase photoelectron spectroscopy is used to investigate metal–metal interactions and the mixed-valence positive ion states of biferrocene and bis(μ-fulvalenediyl)diiron. The spectra of phenylferrocene and 1,1′-diphenylferrocene are used to show that, in comparison to ferrocene, the extension of the ligand π system and the reduced ligand symmetry do not have an appreciable effect on the band profile of the metal-based ionizations. In contrast, the initial ionization bands of both bimetallic molecules, which derive from the metal-based 2 E 2g ionizations of ferrocene, are spread over a wide energy range, indicating delocalization across the two metal halves of the molecule and formal oxidation states of +2 1 / 2 for each metal atom in these cation states. The broadening and splitting of this ionization band for bis(μ-fulvalenediyl)diiron is twice that observed for biferrocene, consistent with a through-bond ligand-mediated mechanism of interaction. Ionizations of the bimetallic molecules that derive from the metal-based 2 A 1g ionizations of ferrocene occur in a single narrow band, indicating that both through-space and through-ligand interactions are not appreciable for the d z 2 -based orbitals. The difference between the metal–metal interactions in these positive ion states follows from the different overlap and energy match of the metal orbitals with fulvalendiyl orbitals of the appropriate symmetry. Most important to the metal–metal interaction in the ground ion state are empty fulvalendiyl orbitals with two nodes perpendicular to the C 5 planes and gerade and ungerade symmetries with respect to the inversion centers of the molecules. In the gas phase, both species are found to be strongly interacting, delocalized mixed-valence compounds in their ground ion states.

The electronic structures of CpRe(NO)(L)R and Cp*Re(NO)(L)R (Cp = η5-C5H5, Cp* = η5-C5(CH3)5; L = CO, P(C6H5)3; R = H, CH3) are studied using gas-phase photoelectron spectroscopy and density functional theory. Separate valence ionizations from the three occupied metal-based orbitals of the d6 Re center, the Re−R σ bond orbitals, and the predominantly Cp e1‘ ‘ pπ orbitals are clearly observed. Comparison of the shapes and energies of the Cp and σ(Re−R) ionizations indicates an additional direct interaction between these orbitals that is sensitive to energy matching. This interaction results in a more delocalized σ-bonding framework for the methyl complexes than for the analogous hydrides and halides. The energy shifts and cross-sections of the metal-based ionizations provide quantitative measures of the different abilities of the nitrosyl, carbonyl, and phosphine ligands to delocalize and stabilize the metal electron density through π back-bonding. In these molecules the stabilization of a metal-based ioni...

The electronic structure of derivatives of VO(acac)(2) is probed using gas-phase UV-photoelectron spectroscopy [acac = 2,4-pentanedione]. The effect on the metal center of electron-donating and -withdrawing groups on the acac ligands is examined. Ionizations from metal-based, acac-based, and V=O-based orbitals are clearly observed. We find that changes at the ligand periphery lead to equivalent changes in the energies of the metal-based and ligand-based ionizations. Additionally, we find that a linear correlation exists between the ionization energies of the metalated complexes and the pK(a) values of the free ligands, indicating that the pK(a) is a good indication of the overall electron-donating abilities of acac ligands.

The transferability of the valence ionization intensities of chemical functional groups is investigated with three common laboratory ionization sources (Ne Iα, 16.7 eV; He Iα, 21.2 eV; and He IIα, 40.8 eV) for a series ofmolecules. Each molecule contains two different functional groups that are well separated electronically and spatially by an alkane chain linker. The optimum length of the alkane chain between the functional groups is investigated. In this initial study, the functional groups are RCl, RBr, RSH, and RFc, where R is an alkane chain and Fc is ferrocene with an alkane chain bonded to one cyclopentadienyl ring. An additional feature of this study is that each functional group contains a molecular orbital with nearly pure atomic character, allowing comparison to theoretical atomic ionization cross-section ratios. In general, the observed photoelectron intensity changes are more gradual than those predicted by either theoretical atomic photoionization cross sections or by a complete Gelius analysis based on molecular orbital compositions obtained from electronic structure calculations. Experimentally, there is a high degree of transferability of the relative ionization intensities of the chemical functional groups between the molecules of this set. These results indicate that an empirical library of the relative ionization cross sections of chemical functional groups as a function of photon energy will be a useful aid to the study of more complex molecules and will provide an experimental foundation for further theoretical studies of molecular photoelectron cross sections.

We report the synthesis of novel phenylenevinylene derivatives that allow for the introduction of meta versus para connections on the phenylene rings, as well as the incorporation of nitrogen atoms within the conjugated backbone and the attachment of electroactive substituents. We assess the impact of the various derivatization schemes on the electronic and optical properties by means of gas-phase ultraviolet photoelectron spectroscopy (UPS) and optical absorption and emission measurements; the evolution of the experimental data is further supported by the results of quantum-chemical calculations. We demonstrate that the electronic and optical properties of phenylenevinylene chains, and by extension those of many conjugated materials, can be tuned over a large energy range by a tailored design of the molecular structures.

He I photoelectron spectra of phenanthrene (1), 1,10-phenanthroline (2), phenazine (3), dibenzo[a,c]anthracene (4), dibenzo[a,c]phenazine (5), and dipyrido[3,2-a;2‘3‘-c]phenazine (6) have been obtained. Assignment of the π ionization states was aided by electronic structure calculations: the first ionization state of 1, ^2B_1(π_1), is observed at 7.888 ± 0.002 eV, ^2B_2(π_1) of 2 is at 8.342 ± 0.002 eV, and ^2B_(1g)(π_1) of 3 is at 8.314 ± 0.002 eV. Spectra of 4−6 are reported for the first time: ^2A_2(π_1) of 4 is at 7.376 ± 0.002 eV, and both 5 (7.983 ± 0.002 eV) and 6 (8.289 ± 0.002 eV) exhibit quasi-degenerate first and second ionization states. Quantum-mechanical reorganization energies, λ^(QM), were extracted from analyses of vibrational structure: values are 149 ± 5 (1), 167 ± 5 (2), 68 ± 2 (3), and 92 ± 4 (4) meV. Low-frequency modes were treated semiclassically: values of λ^(SC) are estimated to be 21 ± 1 (1), 13 ± 1 (2), 22 ± 1 (3), 66 ± 1 (4), 27 ± 9 (5), and 16 ± 1 (6) meV. Reorganization energies (λ = λ^(QM) + λ^(SC)) of isolated molecules are 170 ± 5 (1), 180 ± 5 (2), 90 ± 2 (3), and 158 ± 4 (4) meV. Density functional calculations (B3LYP/6-311G++(d,p)) give λ values that are on average 63 meV lower than experimentally derived energies.

The first examples of vibrational structure in metal-ligand sigma-bond ionizations are observed in the gas-phase photoelectron spectra of CpRe(NO)(CO)H and CpRe(NO)(CO)H [Cp = eta(5)-C(5)H(5), Cp = eta(5)-C(5)(CH(3))(5)]. The vibrational progressions are due to the Re-H stretch in the ion states formed by removal of an electron from the predominantly Re-H sigma-bonding orbitals. A vibrational progression is also observed in the corresponding ionization of the deuterium analogue, CpRe(NO)(CO)D, but with lower vibrational energy spacing as expected from the reduced mass effect. The vibrational progressions in these valence ionizations are directly informative about the nature of the metal-hydride bonding and electronic structure in these molecules. Franck-Condon analysis shows that for these molecules the Re-H or Re-D bond lengthens by 0.25(1) A when an electron is removed from the Re-H or Re-D sigma-bond orbital. This bond lengthening is comparable to that of H(2) upon ionization. Removal of an electron from the Re-H or Re-D bonds leads to a quantum-mechanical inner sphere reorganization energy (lambda(QM)) of 0.34(1) eV. These observations suggest that even in these low symmetry molecules the orbital corresponding to the Re-H sigma bond and the Re-H vibrational mode is very localized. Theoretical calculations of the electronic structure and normal vibrational modes of CpRe(NO)(CO)H support a localized two-electron valence bond description of the Re-H interaction.

Ultraviolet photoelectron spectroscopy was performed on phthalocyanine samples in the gas phase and as thin vapor-deposited solid films. The results are compared to those of molecular orbital calculations and electrochemical redox experiments in solutions and as solid films. Electron-withdrawing groups or substituents are introduced to the phthalocyanine system to increase the ionization potential and electron affinity. An almost parallel shift of photoelectron spectra is obtained. Unsubstituted phthalocyaninatozinc (PcZn), complexes of tetrapyridotetraazaporphyrins (TPyTAPZn, TPyTAPVO) and hexadecafluorophthalocyanines (F16PcZn, F16PcVO, F16PcAlF) have been studied. The alignment of energy levels in organic heterojunctions prepared from the substituted phthalocyanines and the unsubstituted zinc complex is discussed in detail. The offset between the highest occupied electronic levels, the establishment of a surface dipole, and the redistribution of charge in the heterointerface region beyond molecular con...

We investigate by means of gas-phase ultraviolet photoelectron spectroscopy complemented by quantum-chemical calculations how the frontier levels of the 4,4‘-bis(N-m-7tolyl-N-phenylamino)biphenyl (TPD) molecule are affected upon substitution of the terminal aryl rings with methoxy groups or fluorine atoms. These results provide strategies to modulate energy barriers at metal/organic or organic/organic interfaces involving TPD and its derivatives; it is shown that the change in the energy of the HOMO level of TPD upon derivatization is strongly affected by inductive effects taking place in the σ skeleton.

A number of organic pigments were identified by semiempirical molecular orbital calculations, using the PM3 method, as having ionization potential (IP) values of 7.0–9.5 eV. Based on photostability, solubility and commercial availability twelve (quinacridone, isoviolanthrone, indanthrone, indigo, 3,4,9,10 perylenetetracarboxylic dianhydride, bis( p -chlorophenyl)1,4-diketopyrrolo (3,4-C) pyrrole, pyranthrone, indanthrene yellow GCN, 16,17-dimethoxyviolanthrone, indanthrene gold orange, 4,4′-diamino-9,9′,10,10′-tetrone [1,1′ bianthracene], and N,N ′ ditridecyl-3,4,9,10-perlenetetracarboxylic diimide) were chosen for further study. The accuracy of the MO calculations was confirmed by experimental measurement of the ionization potentials for eight of the pigments, using gas phase ultraviolet photoelectron spectroscopy. For compounds having at least three fused rings and containing oxygen, nitrogen, or both, the theoretical and experimental IPs have a linear relationship defined by the equation IP exp =0.694IP Calc +1.9049. Lewis acid pigment solubilization (LAPS) was shown to be a viable approach to preparing electrodes for cyclic voltammetry of pigment solid films. The results of the cyclic voltammetry experiments were utilized to formulate the equation E ox (V vs. NHE)=0.5488 IP Calc −3.042, which relates the experimental oxidation potential to the theoretical IP.

Gas-phase photoelectron spectroscopy and theoretical calculations are used to study the electronic structure of 1,2-dichalcogenins. Photoelectron spectra are reported for 1,2-dithiin, 3,6-dimethyl-...

The compounds (L-N3)MoO(qdt) and (L-N3)MoO(tdt) [(L-N3) = hydrotris(3,5-dimethyl-1-pyrazolyl)borate; tdt = toluene-3,4-dithiolate; qdt = quinoxaline-2,3-dithiolate] have been studied by cyclic voltammetry and photoelectron, magnetic circular dichroism, and electronic absorption spectroscopies, and the experimental data have been interpreted in the context of ab initio molecular orbital calculations on a variety of dithiolate dianion ligands. The PES data reveal very substantial differences between (L-N3)MoO(qdt) and (L-N3)MoO(tdt) in that the first ionization (originating from the Mo dxy orbital) for (L-N3)MoO(qdt) is about 0.8 eV to deeper binding energy than that of (L-N3)MoO(tdt). This stabilizing effect is also reflected in the solution reduction potentials, where (L-N3)MoO(qdt) is approximately 220 mV easier to reduce than (L-N3)MoO(tdt). A direct correlation between the relative donating ability of a given dithiolate ligand and the reduction potential of the (L-N3)MoO(dithiolate) complex has been observed, and a linear relationship exists between the calculated Mulliken charge on the S atoms of the dithiolate dianion and the Mo reduction potential. The study confirms previously communicated work (Helton, M. E.; Kirk, M. L. Inorg. Chem. 1999, 38, 4384-4385) that suggests that anisotropic covalency contributions involving only the out-of-plane S orbitals of the coordinated dithiolate control the Mo reduction potential by modulating the effective nuclear charge of the metal, and this has direct relevance to understanding the mechanism of ferricyanide inhibition in sulfite oxidase. Furthermore, these results indicate that partially oxidized pyranopterins may play a role in facilitating electron and/or atom transfer in certain pyranopterin tungsten enzymes which catalyze formal oxygen atom transfer reactions at considerably lower potentials.

We have previously shown that the arene complex (η6-C6H6)Mo(TRIPOD), where TRIPOD = 1,1,1-tris((diphenylphosphino)methyl)ethane, is protonated by exo addition of H+ to the arene ring to give the transient cyclohexadienyl complex [(η5-C6H7)Mo(TRIPOD)]+, which eventually yields the thermodynamic molybdenum hydride [(η6-C6H6)Mo(TRIPOD)(H)]+. The present study is a combined experimental and theoretical investigation that reveals the fundamental basis for this mechanism. Photoelectron spectroscopy (PES) is used to probe the electronic structure of (η6-C6H6)Mo(TRIPOD) and the production of the [(η6-C6H6)Mo(TRIPOD)]+ cation in the gas phase. The initial ionizations of (η6-C6H6)Mo(TRIPOD) are from energetically closely spaced orbitals of predominantly metal d character (2A1 and 2E cation states using C3v symmetry) that are shifted over 2 eV to lower energy with respect to the comparable ionizations of (η6-C6H6)Mo(CO)3. The oxidized species [(η6-C6H6)Mo(TRIPOD)]+ is also prepared in solution by electrochemical mea...

Gas-phase photoelectron spectroscopy is used to investigate the bonding between early transition metals and carbonyl and cyclopentadienyl ligands for the molecules (η5-C5H4R)M(CO)4 (R = H, M = V, Nb, Ta; R = SiMe3, M = Nb, Ta; R = COCH3, M = Nb). The lowest ionization energy region contains two overlapping ionizations that arise from the two orbitals that are occupied according to the formal d4 metal configuration. However, the character of these ionizations is dominated by the carbonyls rather than by the metals, as evidenced by the extensive C−O stretching vibrational progressions observed with these ionizations, by the trends in the ionization cross sections between the molecules and with different ionization sources, and by the relative lack of shifts of these ionizations with metal substitution from vanadium to niobium to tantalum or with trimethylsilyl and acetyl substitutions on the cyclopentadienyl. The second group of ionizations for these molecules corresponds to orbitals with predominantly cycl...

The reactions of mer–trans-ReOCl3(PPh3)2 with PMe3 and PEt3 provide a route to the isomers fac–cis-ReOCl3(PR3)2 and mer–trans-ReOCl3(PR3)2 (R=Me or Et) which can be converted to ReH7(PR3)2 (R=Me or Et) in ca. 60% yield by reaction with LiAlH4 in THF and subsequent hydrolysis of the reaction mixtures. The properties of these heptahydrides have been examined, including studies of the remarkable stability of aqueous solutions of ReH7(PMe3)2 and the measurement of the gas-phase He(I) photoelectron spectrum of this complex. While the fac–cis and mer–trans isomers of ReOCl3(PR3)2 (R=Me or Et) do not convert to ReO(OEt)Cl2(PR3)2 upon reaction with ethanol, these ethoxides can be prepared by the reaction of mer–trans-ReO(OEt)Cl2(PPh3)2 with PMe3 and PEt3. The reactions of the isomers of ReOCl3(PMe3)2 with ethylamine, followed by the treatment of the reaction products with acetone, afford the salt [Me3PCMe2CH2C(O)CH3]ReO4. The compounds fac–cis-ReOCl3(PEt3)2, trans-ReO(OEt)Cl2(PMe3)2 and [Me3PCMe2CH2C(O)CH3]ReO4 have been structurally characterized by X-ray crystallography.

A comparison between models for the substrate−catalyst adduct in hydrodenitrogenation (HDN) catalysis is made with respect to oxygen vs sulfur ancillary ligands. Reacting [η2(N,C)-NC5tBu3H2]Ta(OAr)...

The first ionization energy in the gas-phase photoelectron spectra (PES) of Tp*Mo(E)(tdt) complexes (where E = O, S, NO; Tp* = hydrotris(3,5-dimethyl-1-pyrazolyl)borate; tdt = 3,4-toluenedithiolate) is essentially independent of the nature of E, even though the formal oxidation state of the Mo center ranges from +2 to +5. The PES data for the tdt complexes contrast with the results for analogous complexes with alkoxide ligands, which show large variations in first ionization energy (Westcott, B. L.; Enemark, J. H. Inorg. Chem. 1997, 36, 5404−5405). For the tdt complexes the relative intensities of the two lowest energy ionizations do not substantially change as the excitation source is varied among Ne I, He I, and He II radiation, even though the atomic photoionization cross sections for Mo 4d and S 3p orbitals change dramatically over this energy region. These results all point to substantial covalency in the Mo−S bonds. It is proposed that the S atoms of the tdt ligand act as an “electronic buffer” to t...

The compounds of W2(NMe2)2(OCMe2CF3)4, 1, W2(NMe2)2(OCMe(CF3)2)4, 2, and W2(NMe2)2 (OC(CF3)3)4, 3, have been obtained from reactions between W2(NMe2)6 and the respective fluoroalcohol in hydrocarbon solvents. The compounds 2 and 3 appear inert to further reaction with their fluoroalcohol at room temperature while 1 reacts further to give W2(OCMe2CF3)6. In solution NMR data indicate that all three compounds exist in a mixture of anti- and gauche-rotamers with restricted rotations about both WN and WW bonds on the NMR time-scale. The solid-state molecular structure of 2 reveals a centrosymmetric NO2WWO2N core with W  W = 2.3107(6) A , W z.sbnd; N = 1.914(4) A , W  O = 1.959(3) aand 1.907(3) A and angles WWO/N within the range 100 to 110°. The gas phase photoelectron spectra for 1, 2 and 3 were obtained and are compared with that for W2(NMe2)6. The introduction of the fluoroalkoxide ligands has a pronounced effect in stabilizing the metal and nitrogen lone-pair based ionizations, and this effect is amplified by the successive replacement of each methyl group by a trifluoromethyl group. Also for the compounds 1, 2, and 3 the lowest ionization bands clearly reveal the removal of the degeneracy of the WW π MOs as expected for anti and gauche complexes.

The electronic structure perturbations caused by cyclopentadienyl substitutions in the series of complexes (η5-C5H5)2Ru, (η5-C5Me5)(η5-C5H5)Ru, (η5-C5Me5)2Ru, (η5-C5Me5)(η5-C5Cl5)Ru, and (η5-C5Me5)(η5-C5F5)Ru are measured by gas-phase photoelectron spectroscopy. The shifts of the valence metal- and cyclopentadienyl-based ionizations give an indication of the overall electronic effects of methyl and halogen substitutions on the cyclopentadienyl rings. The halogen substituent interaction is an admixture of inductive σ-electron-withdrawing and filled−filled π-electron-overlap effects, which act in opposite directions. The π-overlap interaction is relatively weak in the case of chlorine substitution for hydrogen, and the combined σ and π interactions give rise to an overall withdrawal of electron density from the metal center and increase in the metal d-based ionization energies. Fluorine substituents on cyclopentadienyl make the ring only slightly more electron withdrawing than η5-C5Cl5, despite the much gre...

Conformational analysis of 1,4,7-trithiacyclononane, [9]aneS3, in the gas phase was done using three techniques: ab initio molecular orbital calculations at the HF and MP2 levels as well as microwave and photoelectron spectroscopies. The photoelectron spectroscopic data show evidence for at least two conformations with different ionization energies. Using the calculated photoelectron spectra, the observed sulfur 3p ionization peaks can be assigned to C1 and C2 conformations. Forty of the observed microwave transitions can be assigned to a C1 symmetry, asymmetric top, rigid rotor spectrum with rotational constants A = 1155.651(3) MHz, B = 998.442(3) MHz, and C = 629.426(2) MHz. Additional microwave lines are believed to be due to a nonrigid C2 symmetry conformation. No significant populations of conformers of higher symmetry are found.

The gas-phase photoelectron spectroscopy of compounds of the general form CpM(CO) 2 L (Cp = η 5 -C 5 H 5 ; M = Cr, Mn, or Fe; L = ligand of interest) provides a direct experimental measure of the relative bonding capabilities of ligands to metals. The metal-based ionization energies of the formal d 6 metal centers of these compounds are sensitive to the electron donation/acceptance of the ligands, and the splitting patterns of these ionizations are sensitive to the relative π-donation/acceptance properties of the ligands. This is the only experimental technique that can separate the ligand π-bonding effects from the σ-bonding and charge potential effects. A general formula is described by which the π-bonding capabilities of ligands can be compared.

We report a high-resolution gas-phase UPS spectrum of zinc phthalocyanine (ZnPc) together with a detailed analysis of the vibronic structure of the first ionization band, showing that ZnPc presents the lowest value of the intramolecular reorganization energy experimentally reported for a molecular organic semiconductor.

Gas phase photoelectron spectroscopy (PES) is used to investigate the bonding and electronic structure in (fv) [M(CO) 2 ] 2 (fv = fulvalene, η 5 : η 5 -C 10 H 8 2− ; M = Co, Rh). The results for these bimetallic complexes are also compared to those for the analogous monometallic complexes CpM(CO) 2 (Cp = η 5 −C 5 H 5 − ; M = Co, Rh) which have been reported previously. The low valence ionization patterns observed for CpCo(CO) 2 and (fv)[Co(CO) 2 ] 2 are very similar, indicating that there is little electronic interaction between the two metals of the dicobalt complex. The spectrum of (fv)[Rh(CO) 2 ] 2 also is very similar to the spectrum of CpRh(CO) 2 , except that the first metal ionizations in the bimetallic rhodium compound show a significant splitting (0.45 eV). This splitting is due to electronic interaction between the two metal centers which occurs via communication through the fulvalene π system. The differences in electronic structure are compared to the differences in electrochemical behavior of the Co and Rh fulvalene complexes.

The electron distribution and orbital interactions of C 60 with metals coordinated at different sites on the outside of the fullerene are evaluated. These sites include the position of a metal atom directly above a carbon atom (η 1 site), the metal atom centered above two carbons of a pentagon or above two carbons between two pentagons (both η 2 sites), the metal atom centered above a pentagon (η 5 site), and the metal atom centered above a hexagon (η 6 site). The frontier orbitals of C 60 are illustrated first with three-dimensional orbital contour plots. A palladium atom is then used to probe the bonding at the different sites on the C 60 surface. The results with Pd 0 are compared to our earlier study with the harder Ag + ion in order to examine the effects of metal electron richness and size. In addition, these results are compared with the bonding to more traditional ligands that represent the hapticity of these sites, such as methyl (η 1 ), ethylene (η 2 ), cyclopentadienyl (η 5 ), and benzene (η 6 ). The strength of the metal-C 60 interaction and the amount of charge delocalized from the metal to C 60 is sensitive to the site of coordination, the electron richness of the metal, and distortions in the geometry of C 60 . As discussed in our previous work, the frontier orbitals of C 60 are well-suited for synergistic bonding of a metal atom to a carbon-carbon pair in an alkene-like fashion, in which the HOMO of C 60 donates carbon-carbon π bonding electron density to the metal, and the LUMO of C 60 accepts electron density from the metal into a carbon-carbon π* antibonding orbital. Although the HOMO and LUMO of C 60 describe the basic interaction, many frontier orbitals are involved. The site above the CC bond between two pentagons is favored over the site above the CC bond within a pentagon, and the interaction above the other sites is indicated to be net repulsive by these calculations. The differentiation between these sites increases with the electron richness of the metal center. The bonding of the metal to C 60 is generally weaker than to the small ligands, except for very electron rich metal centers where the bonding to the η 2 site between pentagons apparently becomes stronger than the bonding to ethylene.

The gas-phase free energies of ionization, [Delta]G[sub i][degrees] for Cp*[sub 2]Mn, Cp*[sub 2]Fe, Cp*[sub 2]Ni, Cp*[sub 2]Os, Cp[sub 2]Cr, and Cp[sub 2]Co (Cp = [eta][sup 5]-cyclopentadienyl, Cp[sup *] = [eta][sup 5]-pentamethylcyclopentadienyl) have been determined by using the electron-transfer equilibrium (ETE) technique and Fourier transform ion cyclotron resonance mass spectrometry. The high-resolution valence photoelectron spectra of bis(benzene)chromium(0), Bz[sub 2]Cr, Cp*[sub 2]Os, and Cp*[sub 2]Ru have also been measured. Most of the [Delta]G[sub i][degrees] values are referenced to the estimated [Delta]G[sub i][degrees] value of Bz[sub 2]Cr, for which the narrow first ionization band at 5.473 [+-] 0.005 eV is assigned as the adiabatic ionization potential. The [Delta]S[sub i][degrees] for ionization of Bz[sub 2]Cr is assumed to be equal to the electronic entropy change, [Delta]S[sub elec][degrees] (=1.4 cal mol[sup [minus]1] K[sup [minus]1]), and the difference between the integrated heat capacities for Bz[sub 2]Cr and Bz[sub 2]Cr[sup +] is also assumed to be negligible near room temperature [Delta]H[sub i,0][degrees] [approx] [Delta]H[sub i,350][degrees], leading to [Delta]G[sub i][degrees] (Bz[sub 2]-Cr) = 125.6 [+-] 1.0 kcal mol[sup [minus]1]. Through the use of thermochemical cycles, estimates are given for the average heterolytic and homolytic M-Cp bond disruption enthalpies of Cp[sub 2]Cr[sup +/0] and Cp[sub 2]Co[sup +/0]. 46 refs.,more » 7 figs., 4 tabs.« less

The electron distribution and orbital interactions of C 60 with metals coordinated at different sites on the outside of the fullerene are evaluated with the Fenske-Hall molecular orbital method. The characters and nodal properties of the frontier orbitals of C 60 are first evaluated in terms of basis transformations to the C 2 units that join the pentagons and to the C 5 units of the pentagons in the C 60 molecule. The highest occupied molecular orbital (HOMO, h u symmetry) of C 60 is largely π bonding between the carbon atom pairs that join adjacent pentagons. The lowest unoccupied molecular orbital (LUMO, t 1u symmetry) is predominantly π antibonding between these carbon atom pairs. These orbital characters and energies are well situated for synergistic bonding of a metal atom to the carbon-carbon pair between the pentagons, in which the HOMO of C 60 donates σ electron density to the metal, and the LUMO of C 60 accepts π electron density from the metal. The electron donation and acceptance between the individual molecular orbitals of the C 60 molecule and the orbitals of a metal at different possible bonding sites of C 60 are probed with a Ag + ion. It is found that the bonding is favored at the site between the pentagons and that many different orbitals of C 60 are involved in the interaction. The net bonding of Ag + to C 60 is weaker than to ethylene. Calculations are also carried out on the organometallic complexes C 60 Pt(PH 3 ) 2 and (C 2 H 4 )Pt(PH 3 ) 2 . The net bonding of ethylene and C 60 to platinum is found to be very similar in these cases. A significant difference in this case is that the net negative charge on C 60 is more delocalized in the carbon cluster in contrast to the localized charge on ethylene.