Rotational analysis of the v7,v11 (a′,a″) pair of bands of the infrared spectrum of the deuterium substituted propynal molecule C2H-CDO

The mid-infrared spectrum of the v₇,v₁₁ (a′,a″) pair of bands of the deuterium substituted propynal molecule C₂H-CDO was recorded at a resolution of about 0.08 cm⁻¹. An analysis of the pair of bands was completed using the method of simulation of the observed bands with synthetic spectra taking into account the effects of second order Coriolis interactions between the energy levels of the two bands. Best fit values for the changes in the rotational constants (A″ − A′), (B″ − B′) and (C″ − C′), the second order Coriolis constant ζ_7,11 and the δ_7,11 = v₁₁ − v₇ constant have been derived.     

Theoretical study of the stereodynamics of the reactions O(D) + H2, D2 and HD

Calculations of the dynamics of the reactions O(¹D) + H₂ → OH + H, O(¹D) + HD → OH + D, O(¹D) + HD → OD + H and O(¹D) + D₂ → OD + D have been performed using the quasi-classical trajectory (QCT) method with symplectic integration. The theoretical calculations were carried out on the ground state 1A′ potential energy surfaces (PES) by Dobbyn and Knowles. The distributions of the dihedral angle P(_r), the angle between k and j′, P(θ_r), and the product vibrational state are presented. The results show that the intermediate geometrical structures and lifetimes of the reactive collisions play a vital role in these reactions.     

Dynamic heat capacity and reaction enthalpy during the two-stage network growth in diepoxide–diamine compositions

The real and imaginary components of the dynamic heat capacity, C_p′ and C_p″, respectively, have been measured for a fixed frequency of 5 mHz during the polymerization of various compositions of a diepoxide–diamine, molecular liquid mixture to a network structure. The heat evolved during the polymerization was measured simultaneously. C_p′ decreased in two steps as the covalent bonds formed and the network structure grew. The steps became more separated when the amount of the already excess diepoxide was further increased. C_p″ showed a peak in its plot against the polymerization time, but only in the region where C_p′ showed a second step. This is attributed to the increase in the relaxation time leading to vitrification of the liquid. For the diepoxide-rich compositions, the enthalpy release also occurred in two steps and it was more for the second stage of the network's growth than for the first. Combined measurements of the exothermic effects and C_p′ and C_p″ thus delineated two stages of the network's growth by two chemical reactions. The nature of the second-stage network growth that ultimately vitrifies the stoichiometric liquid mixture is discussed. It is concluded that the second-stage growth is mass-controlled and occurs by an etherification reaction whose thermodynamic consequences have been elusive in past studies.     

Ansa-metallocenes with B---N and B---P linkages: the importance of N---HF---C hydrogen bonding in pentafluorophenyl boron compounds

The reaction of Cp′(Cp^B)ZrCl₂ [Cp^B=η⁵-C₅H₄B(C₆F₅)₂] with LiNHCMe₃ gave Cp′(Cp^B)(μ-NHCMe₃)ZrCl, with a constrained-geometry type Cp---B---N chelate ligand. The ¹⁹F-NMR spectrum of the zirconium complexes, as well as that of the titanium analogue, reveals C---FH---N hydrogen bonding to one of the ortho-F atoms of a C₆F₅ ring, strong enough to persist in solution at room temperature. The reaction of Cp′(Cp^B)TiCl₂ with LiPPh₂ affords the Cp---B---P chelate complex Cp′(Cp^B)(μ-PPh₂)TiCl, the first example of a crystallographically characterised Ti(IV) phosphido compound. A ¹⁹F-NMR study of a number of adducts of B(C₆F₅)₃ with prim- and sec-amines demonstrates the importance of intramolecular hydrogen bonding to C₆F₅ in this class of compounds, while there are no such interactions in B(C₆F₅)₃(PHR₂) (R=Cy, Ph). The crystal structures of Cp′(Cp^B)(μ-PPh₂)TiCl, B(C₆F₅)₃(NHMe₂) and B(C₆F₅)₃(PHCy₂) are reported.     

New strategy to the synthesis of (N → B) phenyl[N-alkyliminodiacetate-O,O′,N]boranes: The crystal structure of (N → B) phenyl[N-benzyliminodiacetate-O,O′,N]borane, (N → B)phenyl[N-(4-methyl)benzyliminodiacetate-O,O′,N]borane and (N → B) phenyl[N-phenacyliminodiacetate-O,O′,N]borane

A new, mild and friendly method for the synthesis of (N → B) phenyl[N-alkyliminodiacetate-O,O′,N]boranes 2–7 is reported. All compounds were identified by ¹H, ¹¹B, ¹³C NMR and their high resolution mass spectra (HRMS) are reported. The structure of the compounds 2, 4 and 5 were established by single crystal X-ray. Compounds 2 and 4 crystallized with two independent molecules 2A, 2B and 4A, 4B, respectively in the asymmetric unit. These molecular structures established the bicyclic structure showing a N → B bond length of 1.666 (2) Å for 2A, 1.675 (2) Å for 2B, 1.675 (3) Å for 4A, 1.663 (3) Å for 4B and 1.679 (2) Å for 5, as well as different torsion angles of the junction, 28.70 (2)° (C11–B1–N6–C17) for 2A, 21.50 (2)° (C11a–B1a–N6a–C17a) for 2B, 25.76 (0.26)° (C11–B1–N6–C17) for 4A, 21.96 (0.28)° (C11a–B1a–N6a–C17a) for 4B and −29.22 (0.20)° (C5–N1–B1–C13) for 5.     

A competition between carbo and hetero Diels–Alder reactions of E-2-phenyl-1-cyano-1-nitroethene to cyclopentadiene in the light of B3LYP/6-31G(d) computational study

The competitive reaction paths (Scheme 1) for the carbo and hetero Diels–Alder reaction of E-2-phenyl-1-cyano-1-nitroethene (1) to cyclopentadiene (2) were examined using the B3LYP exchange–correlation functional and the 6-31G(d) basis set. The calculated activation enthalpies indicate that preference of the paths increases in the order: A = C > B > D > F > E. In the gas phase, all reactions occur via pre-reaction complexes, which on the paths A, B, C and E resemble orientation complexes. On path A the initially formed 2-phenyl-3-cyano-4-aza-5-oxy-bicyclo-[3,4,0]-nona-3,7-diene N-oxide (5) is converted to endo-nitronorbornene 3 as a result of a [3.3]-sigmatropic shift. On path B, which yields exo-nitronorbornene 4, and pathways C–F, which yield 2-phenyl-3-cyano-4-aza-5-oxy-bicyclo-[3,4,0]-nonadienes N-oxides 6–8, the reaction proceeds according to a concerted mechanism. When a solvent is introduced into the reaction environment, activation barriers are slightly reduced and the degree of formation of new σ-bonds in transition structures is lowered. The solvent effect however is not sufficient to induce a change of the reaction mechanism or the reaction path preference. The global electrophilicity and electron chemical potential of the reagents 1 and 2 harmonise with the data of the B3LYP/6-31G(d) simulations.     

Photochromic Transformations of Spiro Compounds: 1. A New Approach to Determining the Photocoloration Quantum Yield

A new approach to determining a photocoloration quantum yield for photochromic compounds was considered. This approach is based on comparison between the calculated and experimental values of maximum absorbance A ^B _maxof a photocolored form upon monochromatic irradiation. Using spirooxazines as an example, the quantum yield of photocoloration was determined, and A ^B _maxwas examined as a function of a number of parameters that characterize the photocolored form (the quantum yields of photocoloring and photobleaching and the lifetime of the colored form). It was found that A ^B _maxnonlinearly increased with decreasing rate constant (<0.2 s^–1) of the thermal bleaching of spirooxazines.     

20‐Hydroxy­imino‐5α‐pregna‐9(11),16‐dien‐3β‐yl acetate and 17‐oxo‐5α‐androst‐9(11)‐en‐3β‐yl acetate

In the title compounds, C₂₃H₃₃NO₃ and C₂₁H₃₀O₃, respectively, the ester linkage in ring A is equatorial. In these steroids, the six‐membered rings A and B have chair conformations, but ring C can be better described as a half‐chair. The five‐membered ring D adopts a 14α‐envelop conformation. The A/B, B/C and C/D ring junctions are trans.     

Absorption and electroabsorption spectra of carotenoid cation radical and dication

Radical cations and dications of two carotenoids astaxanthin and canthaxanthin were prepared by oxidation with FeCl₃ in fluorinated alcohols at room temperature. Absorption and electroabsorption (Stark effect) spectra were recorded for astaxanthin cations in mixed frozen matrices at temperatures about 160 K. The D₀→D₂ transition in cation radical is at 835 nm. The electroabsorption spectrum for the D₀→D₂ transition exhibits a negative change of molecular polarizability, Δα=−1.2·10⁻³⁸ C·m²/V (−105 A³), which seems to originate from the change in bond order alternation in the ground state rather than from the electric field-induced interaction of D₁ and D₂ excited states. Absorption spectrum of astaxanthin dication is located at 715–717 nm, between those of D₀→D₂ in cation radical and S₀→S₂ in neutral carotenoid. Its shape reflects a short vibronic progression and strong inhomogeneous broadening. The polarizability change on electronic excitation, Δα=2.89·10⁻³⁸ C·m²/V (260 A³), is five times smaller than in neutral astaxanthin. This value reflects the larger energetic distance from the lowest excited state to the higher excited states than in the neutral molecule.     

A new type of piezoelectric detector in liquid: Part 2. Computation of the equivalent circuit parameters of a piezoelectric crystal with a separated electrode and of series piezoelectric sensors in a non-electrolyte solution

An improved equivalent circuit model is proposed for a piezoelectric crystal with a separated electrode. Equations are derived for the equivalent circuit parameters in a non-electrolyte solution and are verified experimentally. The resonance frequency f_s is given by f_s = f₀[1 + C₁/2(C′₀ + C_s)], where f₀, C₁ and C′₀ are the resonance frequency, motional capacitance and the shunt capacitance of the crystal respectively and C_s is the solution capacitance. The mechanical quality factor is the same as that of the crystal. The motional resistance, motional inductance, motional capacitance and shunt capacitance are respectively K², K², K⁻² and K⁻¹ times those of the crystal, where K = 1 + C′₀/C_s. The influence of the permittivity, density and viscosity of the solution and the configuration of the sensors on the equivalent parameters are investigated. The equivalent circuit parameters of a series piezoelectric crystal are also calculated and measured.     

Why not ICP as atom reservoir for AAS?

The sensitivity S_A of determination by inductively coupled plasma atomic absorption spectrometry (ICP—AAS) is discussed. All important factors influencing S_A are comprised in one single sensitivity formula, which allows an estimate to be made of the correct order of magnitude of S_A for both flame—AAS and ICP—AAS measurements. The most important analytical factors are the degree of dissociation and ionization (0≤f_C, and f_I≤1), the dilution factor f_D, which takes into account the dilution of the analysis element A by its transition from the solution to the ICP, and the absorption path length b. Like flame—AAS, an analytical approach using ICP—AAS has high selectivity and makes it possible to carry out determinations without chemical and ionization interferences. This important advantage of ICP—AAS in comparison to flame—AAS is based on the fact that the ideal condition f_C→1 and Δf→0 for the analysis and standard solutions can be much more easily realized in the ICP than in flames. Serious disadvantages of an ICP as an atomic reservoir for AAS are the reduced sensitivity and lower detection power compared to flame—AAS. The reduction of S_A is caused mainly by the reduction of b/f_D by a factor of about 0.1 and to a smaller degree by stronger broadening of the absorption line and the depopulation of the lower energy state of the atom A that absorbs the resonance radiation. The estimated S_A value for A = Ag, Al, Ca, Cd, Co, Cr, Cu, Pd and Pt agree with the corresponding experimental values to within a factor of about 3. No experimental values could be obtained for B and Si. An application field of ICP—AAS is the analysis of complex compounds that are difficult to dissociate into atoms using flames. In these determinations, a high sensitivity is generally not needed but a good selectivity is important. Some applications are shown.     

Neutron diffraction study of the β′ and γ phases of LiFeO2

The β, β′, γ and α phases of LiFeO₂, synthesized as powders, were annealed at different temperatures and characterized by X-ray measurements. The β′ and γ modifications were also studied by time-of-flight neutron diffraction (ISIS Facility, UK). The structure of the β′ phase was refined in the monoclinic C2/c space group (a=8.566(1), b=11.574(2), c=5.1970(5) Å, β=146.064(5)°) to wR_p=0.071–0.080 (data from four counter banks). Fe and Li atoms are ordered over two of the four independent sites, and partially disordered over the other two. The ordered Li has a distorted tetrahedral coordination. The γ structure was refined at RT (a=4.047(1), c=8.746(2) Å) and at 570 °C (a=4.082(3), c=8.822(6) Å) in the I4₁/amd symmetry, showing full order with Li in octahedral coordination at RT, and in a split-atom configuration at high temperature. On annealing, the β′ polymorph was found to transform to γ at 550 °C, thus suggesting that it is a metastable phase. Electrostatics is discussed as the driving force for the α→β′→γ ordering process of LiFeO₂.     

Theoretical study of stereodynamics for the O(D) + D2 → OD + D reaction

A quasi-classical trajectory method (QCT) running on the ¹A′ and ¹A″ potential energy surfaces (PESs) given by Dobbyn and Knowles [A.J. Dobbyn, P.J. Knowles, Mol. Phys. 91 (1997) 1107] has been employed to study the dynamical stereochemistry of the chemical reaction O(¹D) + D₂ → OD + D, especially the vector correlations between products and reagents. The results indicate that product rotational angular momentum j′ is not only aligned, but also oriented along the direction perpendicular to the scattering plane on both PESs, with different rotational polarization behaviors of product OD for the two PESs and for different collision energies. Calculations show that the alignment effect of products become weaker with an increase of the collision energy on the ¹A′ PES but is not sensitive to the collision energy on the ¹A″ PES. When the collision energy increases, the product OD mainly tends to the forward scattering on the ¹A′ PES and displays a switch from the backward scattering to the forward one on the ¹A″ PES. These differences are probably attributed to the different characteristics of the two PESs.     

3β‐Acetoxy‐5α,6β‐di­hydroxybisnorcholanic acid 2216 lactone

In the title compound, C₂₄H₃₆O₆, the ester linkage in ring A is equatorial. The six‐membered rings A, B and C have chair conformations. The five‐membered ring D adopts a 13β,14α‐half‐chair conformation and the E ring adopts an envelope conformation. The A/B, B/C and C/D ring junctions are trans, whereas the D/E junction is cis.     

Successive Synthesis of Well-Defined Star-Branched Polymers by “Convergent” Iterative Methodology Using Core-Functionalized 3-Arm Star-Branched Polymer and a Specially Designed 1,1-Diphenylethylene Derivative

The synthesis of well-defined regular and miktoarm star-branched polymers by a convergent iterative methodology using core-functionalized 3-arm star-branched polymer with 1,1-diphenylethylene (DPE) moiety and a specially designed DPE derivative is described. The methodology involves the following two reaction steps in the entire iterative synthetic sequence: 1) a coupling reaction of a star-branched polymer having an anion at the core with a DPE derivative with two benzyl bromide moieties, 1-{4-[5,5-bis(3-bromomethylphenyl)-7-methylnonyl]phenyl}-1-phenylethylene, and 2) an addition reaction of the resulting core-DPE-functionalized star-branched polymer with sec-BuLi to convert the DPE moiety to a DPE-derived anion. The iterative synthetic sequence including these two reaction steps, 1) and 2), was repeated to successively synthesize star-branched polymers with more arms. Iteration of this synthetic sequence doubled the number of the arms in the star-branched polymer. With this methodology, 6-arm, 12-arm, and 14-arm regular star-branched polystyrenes as well as 6-arm A₂B₂C₂, A₄B₂, and 12-arm A₄B₄C₄ and A₈B₄ miktoarm star-branched polymers with well-defined structures have been successfully synthesized.     

The electronic gas-phase spectrum of B3 radical revisited

The gas-phase electronic spectrum of cyclic-B₃ (D_3h) radical has been remeasured in a supersonic molecular beam using a mass-selective resonant 2-color 2-photon technique, leading to a revision of previously reported spectroscopic constants. The species was prepared by ablation of a boron nitride rod in the presence of helium. Ab intio calculations on the geometries and vertical electronic excitation energies, as well as mass identification, indicate that the detected band, centered at 21848.77(2) cm⁻¹, is the origin of the cyclic-¹¹B₃ system. A spectral fit yields the rotational constants as B″ = 1.2246(45) and C″ = 0.62131(72) cm⁻¹ in the ground state, and B′ = 1.1914(44) and C′ = 0.61173(69) cm⁻¹ in the excited 2 ²E′ state.     

Stereoselective total synthesis of (+)-(6R,2′S)-cryptocaryalactone and (−)-(6S,2′S)-epi cryptocaryalactone

The total synthesis of (+)-(6R,2′S)-cryptocaryalactone and (−)-(6S,2′S)-epi cryptocaryalactone is reported based on stereoselective reduction of δ-hydroxy β-keto ester to install 1,3-polyol system, cis Wittig olefination, and lactonization as the key steps. The synthesis of (−)-(6S,2′S)-epi cryptocaryalactone is also reported using syn-benzylidene acetal formation and a preferential Z-Wittig olefination reaction and lactonization as the key steps.     

Interaction Between Pd(RaaiR′)Cl<Subscript>2</Subscript> and Adenine: Reaction Dynamics and Mechanism [RaaiR′ = 1-alkyl-2-(arylazo)imidazole]

Nucleophilic substitution of Pd(RaaiR′)Cl₂ [RaaiR′=1-alkyl-2-(arylazo)imidazole, p-R—C₆H₄— N=N—C₃H₂NN-1-R′; where R= H(a)/Me(b)/Cl(c) and R′ = Et(1)/Bz(2)] with adenine (A) in MeCN–water (1:1) at 298 K, to form [Pd(A)₂]Cl₂, has been studied spectrophotometrically under pseudo-first-order conditions and the analyses support a nucleophilic association path. The reaction follows the rate law, rate = {a+k [A] ₀²[Pd(RaaiR′)Cl₂]: first-order in Pd(RaaiR′)Cl₂ and second-order in A. The rate increases as follows: Pd(RaaiEt)Cl₂(1) < Pd(RaaiBz)Cl₂(2) and Pd(MeaaiR′)Cl₂(b) < Pd(HaaiR′)Cl₂(a) < Pd(ClaaiR′)Cl₂(c). External addition of Cl⁻ (LiCl) suppresses the rate (rate 1/[Cl⁻]). The activation parameters, H⁰ and S⁰ of the reactions were calculated from the Eyring plot and support the proposed mechanism.     

The electronic states of 1,2,5-oxadiazole studied by VUV absorption spectroscopy and CI, CCSD(T) and DFT methods

The 1,2,5-oxadiazole VUV absorption spectrum in the range 5–11.5 eV, shows broad bands centred near 6.2, 7.1, 8.3, 8.8, 10.6 and 11.3 eV. Rydberg states associated with three ionisation energies (IE) were identified in the complex fine structure above 8.7 eV. Electronic vertical excitation energies for singlet and triplet valence, and Rydberg states were computed using ab initio multi-reference multi-root CI methods. There is generally a good correlation between the envelope of the theoretical intensities and the experimental spectrum. The nature of the more intense calculated Rydberg states, and positions of the main valence and Rydberg bands are discussed. The lowest triplet, singlet and Rydberg 3s excited states have equilibrium structures that are non-planar with C_S symmetry, in a chair-like orientation where the O and H atoms lie out of the NCCN plane. This finding is consistent with the doubling of the low energy UV spectral lines [B.J. Forrest, A.W. Richardson, Can. J. Chem., 50 (1972) 2088].The nearly degenerate IE of the UV-photoelectron spectrum (UV–PES, Palmer et al. 1977) makes analysis of the VUV spectrum difficult, leading to the necessity for reinvestigation. Vertical studies (IE_V) using CI, Tamm–Dancoff (TDA) and Green’s Function (GF) methods all gave similar results, with near degeneracy of the first 3IE_V confirming the earlier study. Studies of the adiabatic IE (IE_A) using CCSD(T) and B3LYP methods, showed the energy sequence ²A₂ < ²B₁ < ²B₂, but these states are all saddle points, in contrast to the 4th state (²A₁) which is a minimum. In contrast, MP2 study of the ²B₂ state showed a minimum, with only two saddle points.Complete minima were found after minor twisting of the structures. The lowest energy cationic state is ²A^″ (C_S), which closely resembles the ²B₂ state. The O–N–C–C skeleton is twisted by 8°. The corresponding ²A^′ state (C_S) is effectively identical to the ²B₁ state. Attempts to find minima for other symmetry states were unsuccessful.     

Palladium acetate complexes bearing chelating N-heterocyclic carbene (NHC) ligands: Synthesis and catalytic oxidative homocoupling of terminal alkynes

The imidazolium salts 1,1′-dibenzyl-3,3′-propylenediimidazolium dichloride and 1,1′-bis(1-naphthalenemethyl)-3,3′-propylenediimidazolium dichloride have been synthesized and transformed into the corresponding bis(NHC) ligands 1,1′-dibenzyl-3,3′-propylenediimidazol-2-ylidene (L₁) and 1,1′-bis(1-naphthalenemethyl)-3,3′-propylenediimidazol-2-ylidene (L₂) that have been employed to stabilize the Pd^II complexes PdCl₂(κ²-C,C-L₁) (2a) and PdCl₂(κ²-C,C-L₂) (2b). Both latter complexes together with their known homologous counterparts PdCl₂(κ²-C,C-L₃) (1a) (L₃ = 1,1′-dibenzyl-3,3′-ethylenediimidazol-2-ylidene) and PdCl₂(κ²-C,C-L₄) (1b) (L₄ = 1,1′-bis(1-naphthalenemethyl)-3,3′-ethylenediimidazol-2-ylidene) have been straightforwardly converted into the corresponding palladium acetate compounds Pd(κ¹-O-OAc)₂(κ²-C,C-L₃) (3a) (OAc = acetate), Pd(κ¹-O-OAc)₂(κ²-C,C-L₄) (3b), Pd(κ¹-O-OAc)₂(κ²-C,C-L₁) (4a), and Pd(κ¹-O-OAc)₂(κ²-C,C-L₂) (4b). In addition, the phosphanyl-NHC-modified palladium acetate complex Pd(κ¹-O-OAc)₂ (κ²-P,C-L₅) (6) (L₅ = 1-((2-diphenylphosphanyl)methylphenyl)-3-methyl-imidazol-2-ylidene) has been synthesized from corresponding palladium iodide complex PdI₂(κ²-P,C-L₅) (5). The reaction of the former complex with p-toluenesulfonic acid (p-TsOH) gave the corresponding bis-tosylate complex Pd(OTs)₂(κ²-P,C-L₅) (7). All new complexes have been characterized by multinuclear NMR spectroscopy and elemental analyses. In addition the solid-state structures of 1b·DMF, 2b·2DMF, 3a, 3b·DMF, 4a, 4b, and 6·CHCl₃·2H₂O have been determined by single crystal X-ray structure analyses. The palladium acetate complexes 3a/b, 4a/b, and 6 have been employed to catalyze the oxidative homocoupling reaction of terminal alkynes in acetonitrile chemoselectively yielding the corresponding 1,4-di-substituted 1,3-diyne in the presence of p-benzoquinone (BQ). The highest catalytic activity in the presence of BQ has been obtained with 6, while within the series of palladium-bis(NHC) complexes, 4b, featured with a n-propylene-bridge and the bulky N-1-naphthalenemethyl substituents, revealed as the most active compound. Hence, this latter precursor has been employed for analogous coupling reaction carried out in the presence of air pressure instead of BQ, yielding lower substrate conversion when compared to reaction performed in the presence of BQ. The important role of the ancillary ligand acetate in the course of the catalytic coupling reaction has been proved by variable-temperature NMR studies carried out with 6 and 7′ under catalytic reaction conditions.