OCBBCO: a neutral molecule with some boron-boron triple bond character

Molecules that contain boron-boron multiple bonds are extremely rare due to the electron-deficient nature of boron. Here we report experimental and theoretical evidence of a neutral OCBBCO molecule with some boron-boron triple bond character. The molecule was produced and unambiguously characterized by matrix isolation infrared spectroscopy. Quantum chemical calculations indicate that the molecule has a linear singlet ground state with a very short boron-boron bond length.     

Isolation and characterization of phenyl viologen as a radical cation and neutral molecule

The chemical synthesis, isolation, and characterization of phenyl viologen (PV) as a dication, radical cation, and neutral species are described. Single-crystal X-ray diffraction of PV(2+)2Cl(-.)2H2O and PV(.+)PF(6)(-).pyridine reveals the expected differences in bond lengths and also a structural change from two coplanar central rings in PV(.+) to a twist of 36 degrees between the two central rings in PV(2+). The phenyl viologen radical cation exhibits behavior characteristic of many radical cations, including weak pi-dimerization in the solid state and reversible pi-dimerization in solution. Electrical conductivity measurements of neutral phenyl viologen, the first such measurements of a neutral viologen, reveal that it is a significantly better conductor than the radical cation. Differences in geometric relaxation during charge transfer offer a possible explanation for the higher conductivity of the neutral viologen.     

Spectroscopic study of the conformation of 1,3,2-dioxaphosphorinanes with a P=S bond

5,5- Dimethyl-2-thio-2-phenoxy- and 5,5-dimethyl-2-thio-2-ethoxy-1,3,2-dioxaphosphorinanes exist in the liquid state and in solution in a three-component equilibrium featuring two chair forms with equatorial orientation of the P=S bond differing in the orientation of the OPh or OEt groups and a chair form with an axial P=S bond. The P=S bond in 4-methyl-2-thio-2-phenoxy-1,3,2-dioxaphosphorinane occupies the equatorial position.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 479–482, February, 1990.     

New group 2 chemistry: a multiple barium-nitrogen bond in the CsNBa molecule

The existence of a series of triatomic molecules with the general formula MNM', where M is an alkaline metal (K, Rb, Cs), and M' is an alkaline earth metal (Ca, Sr, Ba), has been predicted by quantum chemical methods. Among these, the CsNBa molecule shows a feature not found before, the presence of a multiple bond between barium and nitrogen. As a consequence of this novel bonding situation, the molecule is linear. The same holds for all Ba triatomics, MNBa, independent of the nature of the alkali M atom, and for all Sr compounds, MNSr. The presence of a multiple bond makes CsNBa, and other related Ba and Sr molecules, particularly stable and appealing experimentally. The systems with the alkaline earth metal M' = Ca, on the other hand, turned out to be bent. Calculations have also been performed on the negative ions BaN(-) and CaN(-), which form a well-defined entity in the MNM' systems (M' = Ba, Ca). The results show that the two ions have a different electronic structure in the ground state, which is one reason for the different properties of the MNM' systems and explains why the molecules containing the BaN(-) moiety are linear, while those containing CaN(-) are bent.     

Spectroscopic studies of compounds containing a P —H bond

IR and Raman spectra of liquid di-tert-butylphosphorous acid and its two low-temperature crystalline modifications are studied. It is found that at least three stable conformations of acid molecules can exist in the liquid state.For report 3, seeIzv. Akad. Nauk SSSR, Ser. Khim., 1990, 564 [Bull. Acad. Sci. USSR, Div. Chem. Sci., 1990,39, 489 (Engl. Transl.)].Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 285–288, February, 1995.     

Hydrogen bonding in 2-hydroxybenzoyl and 2-hydroxythiobenzoyl compounds: Spectroscopic characterization and spectroscopic bond strength sequences

Summary A survey of IR and NMR spectroscopic data is given for 30 2-hydroxybenzoyl and 2-hydroxy-thiobenzoyl compounds, which are representative for various kinds of (thio)carbonyl groups. The data shall provide a basis for a comparison between corresponding intramolecular O-H··O and O-H··S hydrogen bonds and for the evaluation of systematic dependencies. Correlations between v_OH frequencies, which are considered as the main characteristic of hydrogen bond strengths, and several other spectroscopic quantities are shown. For O-H··O type associations the bond strength sequence is acid chloridesOH-Frequenzen, die als wesentliches Kriterium zur Charakterisierung der Bindungsstärken herangezogen werden, und verschiedenen anderen spektroskopischen Daten werden gezeigt. Bei den Wasserstoffbrückenbindungen des Typs O-H··O nehmen die Bindungsstärken in der Reihenfolge SäurechlorideCO-Frequenzen. Die auffällige Sonderstellung der tertiären Amide ist auf sterische Effekte zurückzuführen. Die v_OH-Frequenzen der O-H··S Wasserstoffbrückenbindungen sind denen der entsprechenden O-H··O Bindungen sehr ähnlich, ausgenommen Thiosäuren und Thioester, für die wesentlich niedrigere Frequenzen gefunden wurden.     

Valence bond functions for the hydrogen molecule

Valence bond wavefunctions were constructed for H₂. Use of Slater exponents resulted in very slow convergence to the ground state energy. Convergence was improved by optimizing exponents which were found to increase as the principal quantum number n. However, this gave problems of linear dependence since optimum orbitals were strikingly similar for all n. The best function without angular correlation contained 27 terms constructed from 1s, 3s, 2p ₀, 3d ₀, 4f ₀, and 5g ₀ orbitals and gave an energy of –1.1594a.u. The best function with angular correlation gave E=-1.1656 a.u.

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Zusammenfassung Für das H₂-Molekül werden Wellenfunktionen nach der Valenzstrukturmethode konstruiert. Die Benutzung von Exponenten nach Slater führt zu einer sehr langsamen Konvergenz zur Grundzustandsenergie. Die Konvergenz wurde durch Optimierung der Exponenten verbessert, wobei diese mit der Hauptquantenzahl ansteigen. Dabei ergab sich jedoch das Problem linearer Abhängigkeit der Funktionen, da die optimalen Orbitale sehr ähnlich für alle n waren. Die beste Funktion ohne Winkelkorrelation enthielt 27 Terme, die aus den Orbitalen 1s, 3s, 2p ₀, 3d ₀, 4f ₀ und 5g ₀ konstruiert waren, und ergab eine Energie von –1,1594A.E. Die beste Funktion mit Winkelkorrelation ergab E=–1,1656 A.E.

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Résumé Des fonctions d'onde de liaison de valence sont construites pour H₂. L'emploi d'exposants de Slater entraîne une très faible convergence vers l'énergie de l'état fondamental. La convergence a été améliorée par optimisation des exposants qui croissent comme le nombre quantique principal n. Cependant, ceci crée des problèmes de dépendance linéaire car les orbitales optimales sont étonnement similaires pour tous les n. La meilleure fonction sans corrélation angulaire contient 27 termes construits à partir d'orbitales 1s, 3s, 2p ₀,3d ₀,4f ₀ et 5g ₀ et donne une énergie de –1,1594 u.a. La meilleure fonction avec corrélation angulaire donne E= –1,1656 u.a.

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Presented in part at the Symposium on Molecular Structure and Spectroscopy, The Ohio State University, Sept., 1968.

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N.I.H. Predoctoral Fellow 1964–1967.     

An intrinsic radical stability scale from the perspective of bond dissociation enthalpies: a companion to radical electrophilicities

Bond dissociation enthalpies (BDEs) of a large series of molecules of the type A-B, where a series of radicals A ranging from strongly electrophilic to strongly nucleophilic are coupled with a series of 8 radicals (CH2OH, CH3, NF2, H, OCH3, OH, SH, and F) also ranging from electrophilic to nucleophilic, are computed and analyzed using chemical concepts emerging from density functional theory, more specifically the electrophilicities of the individual radical fragments A and B. It is shown that, when introducing the concept of relative radical electrophilicity, an (approximately) intrinsic radical stability scale can be developed, which is in good agreement with previously proposed stability scales. For 47 radicals, the intrinsic stability was estimated from computed BDEs of their combinations with the strongly nucleophilic hydroxymethyl radical, the neutral hydrogen atom, and the strongly electrophilic fluorine atom. Finally, the introduction of an extra term containing enhanced Pauling electronegativities in the model improves the agreement between the computed BDEs and the ones estimated from the model, resulting in a mean absolute deviation of 16.4 kJ mol(-1). This final model was also tested against 82 experimental values. In this case, a mean absolute deviation of 15.3 kJ mol(-1) was found. The obtained sequences for the radical stabilities are rationalized using computed spin densities for the radical systems.     

Free radical polymerization with catalytic chain transfer: Using NMR to probe the strength of the cobalt–carbon bond in small molecule model reactions

This work examines cobalt–carbon bond formation between the cobalt (II) macrocycle, (tetrakis(p‐methoxyphenyl)porphyrinato)cobalt (II), (TAP)Co, and a variety of radicals derived from vinyl compounds to facilitate a better understanding of the various factors affecting the cobalt–carbon bond strength in catalytic chain transfer polymerization. The reaction of (TAP)Co with the following vinylic molecules was studied: methacrylonitrile, cyclohexene, methyl methacrylate, styrene, methyl acrylate, vinyl acetate, vinyl benzoate, methyl crotonate, cis‐2‐pentenenitrile, and ethyl α‐hydroxymethacrylate. Different concentrations of each vinylic compound were added to (TAP)Co and 2,2′‐azobis(isobutyronitrile) in CDCl₃ at 60 °C. The ratio of Co(III) to Co(II) and the nature of the radical bound to the cobalt macrocycle were determined via nuclear magnetic resonance measurements. Several factors are shown to affect the reaction of the radical and the cobalt (II) species (and hence the strength of the cobalt–carbon bond in the resulting compound). These factors are as follows: the number of pathways by which a radical may be derived from the vinyl compound; the variety of radicals that can be produced from the vinylic molecule; the stability of the radical(s) generated; and the relative propagation rate of the vinyl compound. A discussion on the relevance of this study to the behavior of different monomers in catalytic chain transfer reactions is included. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6171–6189, 2006     

A valence bond study of the oxygen molecule

Ab initio valence bond calculations are performed for the three lowest states of the oxygen molecule (³Σ⁻_g, ¹Δ_g, and ¹Σ⁺_g). One objective of the present study was to make a contribution to previous valence bond discussions about the oxygen “double” bond. Further, we study the origin of a small barrier in the potential energy surface of the ground state. Two compact models are employed to maintain the clear picture that can be offered by the valence bond method. The first model has only the Rumer structures that are essential for bonding and a proper dissociation. The second model, in addition, has structures which represent excited atoms. These prove to be important for the dissociation energies. For both models, the orbitals are fully optimized. The spectroscopic data obtained are significantly better than are the (few) valence bond results on O₂ that have been published and have the quality of multiconfiguration self-consistent field calculations in which the same valence space is used. The “hump” in the potential energy surface of the ground state is shown to arise from a spin recoupling. The free atoms correspond to a spin coupling that is incapable of describing the formation of bonds. Only at short distances, an alternative spin coupling provides bonding and the repulsive curve is converted into an attractive one. Our results on this subject support a valence bond explanation previously given by McWeeny [R. McWeeny, Int. J. Quantum Chem. Symp. 24 , 733 (1990)]. © 1996 John Wiley & Sons, Inc.     

Spectroscopic and structural signature of the CH-O hydrogen bond

It has been observed that the vibrational stretching frequency of a C-H covalent bond commonly shifts to the blue and suffers intensity loss, when the CH engages in a hydrogen bond. However, the shift does not always occur in this direction, and there are cases when a CH blue shift may be present even in the absence of a CH...O interaction. Ab initio quantum calculations are used to analyze the structure, and vibrational and NMR spectra of small model systems containing both conventional and CH...O H-bonds, and thereby identify patterns that unambiguously signal the presence of a CH...O interaction.     

Spectroscopic characterization of mechanisms of oxidation of Phe by SO4 - radical: A pulse radiolysis study

By using time-resolved kinetic spectrophotometry and pulse radiolysis technique, the oxidation of Phe by SO₄ ^- radical has been investigated both in aqueous and water/acetonitrile mixed solutions. The results reveal that attack of the oxidizing SO₄ ^- radical on Phe leads directly to the formation of Phe cation radical 3 with a strong absorption peak at 310 nm, then it proceeds in three competitive reactions via either hydroxylation, deprotonation or decarboxylation, which were found to be strongly dependent upon the ionization state of the substitutes —COOH and —NH₂ and the nature of the solvents. Decarboxylation takes place only when the carboxyl group is deprotonated. At high pH deprotonation of Phe cation radical 3 is much easier to occur than that in neutral or acid solutions. Moreover, with addition of acetonitrile, deprotonation is more predominant than hydroxylation, whereas in aqueous solutions hydroxylation is much easier to occur.     

Spectroscopic observation of the resonance-stabilized 1-phenylpropargyl radical

The gas-phase laser-induced fluorescence (LIF) spectrum of a 1-phenylpropargyl radical has been identified in the region 20,800-22,000 cm(-1) in a free jet. The radical was produced from discharges of hydrocarbons including benzene. Disregarding C2, C3, and CH, this radical appears as the most strongly fluorescing product in a visible wavelength two-dimensional fluorescence excitation-emission spectrum of a jet-cooled benzene discharge. The structure of the carrier was elucidated by measurement of a matching resonant two-color two-photon ionization spectrum at m/z = 115 and density functional theory. The assignment was proven conclusively by observation of the same excitation spectrum from a low-current discharge of 3-phenyl-1-propyne. The apparent great abundance of the 1-phenylpropargyl radical in discharges of benzene and, more importantly, 1-hexyne may further underpin the proposed importance of the propargyl radical in the formation of complex hydrocarbons in combustion and circumstellar environments.     

beta-scission of the N-O bond in alkyl hydroxamate radicals: a fast radical trap

[reaction--see text ] The rate of the beta-scission of the N-O bond in the alkyl hydroxamate radical is faster than 2 x 10(8) s(-)(1). This reaction may be useful as a radical trap.