C2h Symmetry Research Articles

Electronic structures of organometallic complexes of f elements LXXXIII: First comparison of experimental and calculated (on the basis of density functional theory) polarized Raman spectra of an oriented organometallic single crystal: Tris(pentamethylcyclopentadienyl)lanthanum

The polarized Raman spectra of an oriented La(η5-C5Me5)3 (1) single crystal (where the principal axes of the two molecules per unit cell are uniformly oriented) as well as the mid (ca. 90K) and far infrared spectra of pellets have been recorded. Applying the selection rules of C3h symmetry to the spectra obtained, the irreducible representations (irreps) of numerous lines/bands of intra-ligand character were derived. In the range <400cm−1, where 28 Raman-allowed lines and 20 FIR-allowed bands of both skeletal and intra-ligand character are expected, only few assignments based on symmetry considerations were possible. In order to increase the number of identifications, model calculations on the basis of density functional theory (DFT) were performed. In the intra-ligand range >400cm−1, the obtained results agree well with the experimental findings. Because of the strong mixing at lower wavenumbers, even the separation of calculated skeletal and intra-ligand modes and the identification of the former was only successful by comparing the calculated FIR and averaged Raman spectra of compound 1 with those of La(η5-C5Me4H)3 (2). Making use of both the calculated frequencies of normal modes and their polarizability tensors, the polarized Raman spectra of an oriented single crystal of 1 in the range <400cm−1 were calculated and compared to the experimental ones. Because of an overestimation of the mixing of normal vibrations of A′ symmetry, the experimental intensities of the lines of the symmetric stretch ν1(A′) were not reproduced by the calculation for compound 1 but by that for Sm(η5-C5Me5)3 (3). Skeletal and intra-ligand modes were separated and designated. Neglecting νC–H modes, the DFT calculation for 1 achieved an r.m.s. deviation of 17.9cm−1 for 72 assignments.

Facile synthesis and theoretical conformation analysis of a triazine-based double-decker rotor molecule with three anthracene blades.

The facile synthesis of a rotor-shaped compound with two stacked triazine units, which are symmetrically connected by three anthracene blades through oxygen linkers, is presented. This new double-decker, which is a potential monomer for two-dimensional polymerization, was synthesized by using readily available, cheap building blocks, exploiting the known selectivity difference for the nucleophilic substitution of cyanuric chloride. The crystal structure of a C3h symmetric rotor-shaped compound with 9,10-dihydroanthracene blades, which is a direct precursor to the targeted monomer, and the crystal structure of the new double-decker with the desired C3h symmetry, are also reported. The synthetic efforts were preceded by a computational analysis, which was triggered by the question of conformational stability of the potential monomer. Two stable conformers could be found, and the barrier for the transition path in the gas phase between these conformers was determined by quantum chemical calculations. Exploratory Born-Oppenheimer molecular-dynamics simulations revealed a strong influence of solvent-solute interactions on the stability of the conformers, which resulted in an energetic preference of the C3h symmetric conformation of the double-decker.

Electronic structures of organometallic complexes of f elements LXXXII: Comparison of assigned experimental and calculated (on the basis of density functional theory) frequencies of normal vibrations of the first homoleptic organometallic complex of the lanthanides: Tris(η5-tetramethylcyclopentadienyl)lanthanum

The far and mid infrared spectra of powdered La(η5-C5Me4H)3 as well as polarized Raman spectra of an oriented single crystal (where the principal axes of the two molecules per unit cell are uniformly oriented) have been recorded. Applying the selection rules of C3h symmetry to the experimental data, numerous signals were assigned which agree well with the predictions of a calculation based on density functional theory. Neglecting νC–H and γCp–H (out of plane) modes, an r.m.s. deviation of 21.9cm−1 for 71 assignments was achieved. Skeletal and intra-ligand vibrations were separated and an inspection of the vibronic sidebands of the purely electronic hypersensitive absorption transitions 4I9/2→4G5/2 of Nd(η5-C5Me4H)3 showed that mainly the latter are coupled.

Highly fluorinated hydrotris(indazolyl)borate calcium complexes: the structure and reactivity heavily depend on the ligand's electronic properties.

Two series of highly fluorinated 3-substituted hydrotris(indazolyl)borate (Fn-Tp(4Bo,3R); R = Ph, n = 12; R = CF3, n = 21) complexes of calcium were synthesized by salt metathesis reactions. CaI2 reacts with 2 and 1 equiv. of Tl(F12-Tp(4Bo,3Ph)) in THF to form the homo- and heteroleptic complexes [Ca(F12-Tp((4Bo,3Ph)*))2] (1) and [(F12-Tp(4Bo,3Ph))CaI(THF)] (2), respectively. 1 has C2h symmetry due to a 1,2-borotropic shift. The reaction of CaI2 with 2 equiv. of the more electron poor Tl(F21-Tp(4Bo,3CF3)) in THF, on the other hand, leads to the salt [Ca(THF)6][F21-Tp(4Bo,3CF3)]2 (3), with [F21-Tp(4Bo,3CF3)] acting as a counter-ion. This emphasizes the tuning ability of Fn-Tp(4Bo,3R) ligands and the consequences on their ability to bind hard electrophilic Ca(2+) centers.

Raman analysis of perrhenate and pertechnetate in alkali salts and borosilicate glasses

Sodium borosilicate glasses containing rhenium or technetium were fabricated and their vibrational spectra studied using confocal Raman microscopy. Glass spectra were interpreted relative to new high‐resolution spectra of pure crystalline NaReO4, KReO4, NaTcO4, and KTcO4 salts. Spectra of perrhenate and pertechnetate glasses exhibited sharp Raman bands, characteristic of crystalline salt species, superimposed on spectral features of the borosilicate matrix. At low concentrations of added KReO4 or KTcO4, the characteristic pertechnetate and perrhenate features are weak, whereas at high additions, sharp peaks from crystal field‐splitting and C4h symmetry dominate glass spectra, clearly indicating ReO4− or TcO4− is locally coordinated with K and/or Na. Peaks indicative of both K and Na salts are evident in many Raman spectra, with the Na form being favored at high concentrations of the source chemicals, where more K+ is available for ion exchange with Na+ from the base glass. The observed ion exchange likely occurred within depolymerized channels where nonbridging oxygens create segregation from the glass network in regions containing anions such as ReO4− and TcO4− as well as excess alkali cations. Although this anion exchange provides evidence of chemical mixing in the glass, it does not prove the added salts were homogeneously incorporated in the glass. The susceptibility to ion exchange from the base glass indicates that long‐term immobilization of Tc in borosilicate glass must account for excess charge compensating alkali cations in melt glass formulations. Published 2014. This article is a U. S. Government work and is in the public domain in the USA.

Zur Elektronenstruktur metallorganischer Komplexe der ƒ -Elemente, 81. Berechnung der Normalschwingungen von La(η5-C5H5)3(NCCH3)2 auf der Basis der Dichtefunktionaltheorie sowie vibronische Kopplungen und Auffindung weiterer rein elektronischer Absorptions- und Lumineszenzüberg¨ange bei [La(η5-C5H5)3(NCCH3)2:Nd3+] / Electronic Structures of Organometallic Complexes of f Elements, 81. Calculation of the Normal Modes of La(η5-C5H5)3(NCCH3)2 on the Basis of Density Functional Theory as well as Vibronic Couplings

The planned model calculations of normal modes of pseudo-trigonal-bipyramidal LaCp3(NCCH3)2 (Cp = η5-C5H5-) (1) adopting density functional theory (DFT), and using the molecular structure as suggested by X-ray investigations, did not converge. Alternatively, DFT calculations assuming molecular C3h symmetry were performed. Unfortunately, these calculations did not reproduce the experimentally derived frequencies of the skeletal modes very well, including a wrong energetic sequence of the four previously unambiguously assigned (close lying) Raman-active ν(La-Cp) skeletal modes. The same presumably also holds for DFT calculations made for LaCp3 · NCCH3 (2) and base-free LaCp3 (3), assuming molecular C3 and C3h symmetry, respectively. In order to check whether the calculated incorrect sequence is produced by the five-membered Cp rings, a DFT calculation also has been performed for the hypothetical model complex [La(η6-C6H6)3]3+ (4) of D3 symmetry. A closer examination of the vibronic sidebands of the hypersensitive absorption transition 4I9/2 → 4G5/2 of [LaCp3(NCCH3)2:Nd3+] (5) showed that first of all, totally symmetric intraligand and not skeletal vibrations are coupling as it was the case for [LaCp3(NCCH3)2:Ln3+] (Ln = Pr, Sm). Applying this result to the vibronic sidebands of some purely electronic crystal field (CF) transitions, which are hampered by strong binary combination vibrations, the energies of these CF levels could be determined. The CF state 1Γ8 of the ground multiplet 4I9/2 of [LaCp3(NCCH3)2:Nd3+], which previously could not be detected by absorption measurements, could be derived from the luminescence transition 4F3/2 → 4I9/2. Considering these additional assignments, the goodness of the fit increased from 32.3 to 29.9 cm-1 for 61 assignments

Mono‐ and Di‐Lanthanide Derivatives of 22‐Tungsto‐2‐antimonate(III), [Ln(H2O)4Sb2W21O72(OH)]10– and [Ln2(H2O)8Sb2W20O70)]8–

AbstractAbstract. The mono‐lanthanide‐containing [Ln(H2O)4Sb2W21O72(OH)]10– (Ln = Yb (1), Lu (2)), and the di‐lanthanide‐containing [Ln2(H2O)8Sb2W20O70)]8– (Ln = Yb (3), Lu (4), Y (5)) have been synthesized in aqueous‐acidic media and characterized in the solid state. Polyanions 1 and 2 were prepared by reaction of Ln3+ ions with [Sb2W22O74(OH)2]12– (Sb2W22), whereas 3–5 were prepared by using the trilacunary 9‐tungstoantimonate(III) [B‐α‐SbW9O33]9– (SbW9). Polyanions 1–5 crystallized as sodium salts in the triclinic system, space group P$\bar{1}$. All compounds were characterized in the solid state by FTIR, single‐crystal XRD, TGA, and elemental analysis. Polyanions 1–5 contain the lacunary [Sb2W20O70]14– unit (Sb2W20), which is coordinated to either a lanthanide and a tungsten ion, leading to a structure with Cs symmetry in polyanions 1 and 2, or to two lanthanide ions, leading to a structure with idealized C2h symmetry, in polyanions 3–5.

1,4-Cyclohexanedione. Composition, Molecular Structures, and Internal Dynamics of the Vapor: An Electron Diffraction Investigation Augmented by Molecular Orbital Calculations

Electron diffraction experiments on the vapor of 1,4-cyclohexanedione have been carried out at a nominal temperature of 435 K. The results are consistent with the presence of a mixture of a chair form of C2h symmetry and a twisted boat form of D2 symmetry. The former has the familiar dynamic properties of a semirigid molecule, but the D2 form undergoes a large-amplitude twisting motion (pseudorotation) that degrades the symmetry to C2. The analysis was designed to elucidate parameter values and internal dynamics of each conformer and the composition of the system. The large-amplitude motion of the twisted boat form was modeled by placement of 10 pseudoconformers at approximately 5° intervals along a pseudorotational coordinate that began at the D2 position and that reflected the angle between the C═O bond vectors. A Gaussian weighting of the pseudeoconformers centered on the (lowest-energy) D2 position was assumed. Differences in the interatomic distances and bond angles of these pseudoconformers were calculated via B3LYP/cc-pVTZ theory and introduced as constraints. The bond length averages over the twisted boat forms followed by values for the chair in square brackets are (rg/Å; [angle]α/deg) r(C-H) = 1.115(11) [1.124(11)], r(C═O) = 1.211(3) [1.233(6)], r(C1-C2) = 1.524(5) [1.526(5)], and r(C2-C3) = 1.533(11) [1.539(11)]. The corresponding ring angle values are [angle](C1C2C3) = 111.1(5) [111.0(4)] and [angle](C6C1C2) = 116.3(8) [115.7(8)]. In the twisted boat form, pseudorotation leads to a weighted average displacement of the angle between the C═O bond vectors, [angle]Δ(CO,CO), equal to 21.3° from the 180° value in the D2 form corresponding to an average angle between the CO bond vectors of 158.7(1)°. The amount of the chair form in the gas at 435 K is 24(10)%. The listed uncertainties are estimated at 2σ.

The potential energy surface of singlet cyclobutadiene and substituted analogs: a coupled-cluster study

Coupled-cluster investigations (CCSD/cc-pVDZ and CCSD/cc-pVQZ//CCSD/cc-pVDZ) of singlet cyclobutadiene and fifteen-substituted analogs were conducted. A local minimum with a square frame does not exist on their potential surfaces. The well-known rectangular D2h minimum, the square D4h transition state, and two additional stationary points were found on cyclobutadiene’s potential surface. This included a transition state with a rhombic carbon ring and C2h symmetry, separating two equivalent puckered C2v local minima. The predicted barriers were 19.7 and 19.8 kcal/mol at the CCSD/cc-pVDZ and CCSD/cc-pVQZ//CCSD/cc-pVDZ levels, respectively. The relative strain energies of rectangular D2h cyclobutadiene and all fifteen-substituted analogs were obtained from isodesmic reactions. Progressive substitution with methyl or BH2 groups continuously lowers ring strain while increasing substitution with fluorines or trifluoromethyl groups steadily increases ring strain. C4(BH2)4 is 16.6 and 13.3 kcal/mol less strained than cyclobutadiene while C4F4 is 17.7 and 21.5 kcal/mol more strained at the levels above. Cyclobutadiene is more strained than both cyclopropene and cyclobutene by 12.2 and 37.0 kcal/mol, respectively. Electron density contours indicate that fluorine substitution raised the electron density especially in the short C=C ring bonds above/below the ring plane (π-electrons) but not in the ring plane (σ-electrons). BH2-substitutions lower the ring π-electron density with little effect in the ring plane. Methyl substituents have little effect on electron densities. All rings retain a strong bond alternation tendency (rectangular) whether substituted with electron-donating or -attracting groups. One-bond coupling constants and the percent p-character in ring C-to-C and C-to-substituent bonds are described.

Synthesis, Properties, and Remarkable 2 D Self‐Assembly at the Liquid/Solid Interface of a Series of Triskele‐Shaped 5,11,17‐Triazatrinaphthylenes (TrisK)

A series of 5,11,17-triazatrinaphthylene (TrisK) derivatives, large disk-like π-conjugated molecules with C3h symmetry, has been synthesised by following an optimised synthetic pathway. The synthesis was performed by a four-step protocol based on the N-arylation of 1,3,5-tribromobenzene with appropriate anthranilate derivatives. This strategy permits the generation of either chlorinated (TrisK-Cl-OCn) or non-chlorinated (TrisK-H-OCn) alkoxy-substituted derivatives (OCn H2n+1 with n=3, 10, 12 and 16), thus providing additional versatility in the control of the structure-property relationships. The electronic properties of the various TrisK compounds have been characterised in solution by absorption and emission spectroscopies as well as cyclic voltammetry. The crystal structure of 2,8,14-propyloxy-5,11,17-triazatrinaphthylene TrisK-H-OC3 has been determined by X-ray diffraction analysis, which revealed the presence of stabilising weak intermolecular H bonds. Scanning tunnelling microscopy (STM) at the liquid/solid interface has revealed the remarkable 2D self-assembling properties of the TrisK compounds. In particular, it has shown that TrisK-H-OC12 forms three concomitant self-organised 2D phases with different row-packing arrangements. This 2D polymorphism arises from slow ordering due to the presence of three long dodecyloxy chains on the molecular backbone. Individual molecules can be imaged with spectacular intramolecular resolution, thus providing the possibility of correlating the STM features with the calculated charge density distribution.

Concealed Cyclotrimeric Polymorph of Lithium 2,2,6,6‐Tetramethylpiperidide Unconcealed: X‐Ray Crystallographic and NMR Spectroscopic Studies

Lithium 2,2,6,6-tetramethylpiperidide (LiTMP), one of the most important polar organometallic reagents both in its own right and as a key component of ate compositions, has long been known for its classic cyclotetrameric (LiTMP)4 solid-state structure. Made by a new approach through transmetalation of Zn(TMP)2 with tBuLi in n-hexane solution, a crystalline polymorph of LiTMP has been uncovered. X-ray crystallographic studies at 123(2) K revealed this polymorph crystallises in the hexagonal space group P63 /m and exhibited a discrete cyclotrimeric (C3h ) structure with a strictly planar (LiN)3 ring containing three symmetrically equivalent TMP chair-shaped ligands. The molecular structure of (LiTMP)4 was redetermined at 123(2) K, because its original crystallographic characterisation was done at ambient temperature. This improved redetermination confirmed a monoclinic C2/c space group with the planar (LiN)4 ring possessing pseudo (non-crystallographic) C4h symmetry. Investigation of both metalation and transmetalation routes to LiTMP under different conditions established that polymorph formation did not depend on the route employed but rather the temperature of crystallisation. Low-temperature (freezer at -35 °C) cooling of the reaction solution favoured (LiTMP)3 ; whereas high-temperature (bench) storage favoured (LiTMP)4 . Routine (1) H and (13) C NMR spectroscopic studies in a variety of solvents showed that (LiTMP)3 and (LiTMP)4 exist in equilibrium, whereas (1) H DOSY NMR studies gave diffusion coefficient results consistent with their relative sizes.

Pnicogen-Bonded Cyclic Trimers (PH2X)3with X = F, Cl, OH, NC, CN, CH3, H, and BH2

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out to determine the structures and binding energies of cyclic trimers (PH2X)3 with X = F, Cl, OH, NC, CN, CH3, H, and BH2. Except for [PH2(CH3)]3, these complexes have C3h symmetry and binding energies between -17 and -63 kJ mol(-1). Many-body interaction energy analyses indicate that the two-body terms are dominant, accounting for 97-103% of the total binding energy. Except for the trimer [PH2(OH)]3, the three-body terms are stabilizing. Charge transfer from the lone pair on one P atom to an antibonding σ* orbital of the P atom adjacent to the lone pair plays a very significant role in stabilization. The charge-transfer energies correlate linearly with the trimer binding energies. NBO, AIM, and ELF analyses have been used to characterize bonds, lone pairs, and the degree of covalency of the P···P pnicogen bonds. The NMR properties of chemical shielding and (31)P-(31)P coupling constants have also been evaluated. Although the (31)P chemical shieldings in the five most strongly bound trimers increase relative to the corresponding isolated monomers, there is no correlation between the chemical shieldings and the charges on the P atoms. EOM-CCSD (31)P-(31)P spin-spin coupling constants computed for four (PH2X)3 trimers fit nicely onto a plot of (1p)J(P-P) versus the P-P distance for (PH2X)2 dimers. A coupling constant versus distance plot for the four trimers has a second-order trendline which has been used to predict the values of (1p)J(P-P) for the remaining trimers.

Excitonic splitting and vibronic coupling in 1,2-diphenoxyethane: Conformation-specific effects in the weak coupling limit

Vibrationally and rotationally resolved electronic spectra of 1,2-diphenoxyethane (C6H5-O-CH2-CH2-O-C6H5, DPOE) are reported for the isolated molecule under jet-cooled conditions. The spectra demonstrate that the two excited surfaces are within a few cm(-1) of one another over significant regions of the torsional potential energy surfaces that modulate the position and orientation of the two aromatic rings with respect to one another. Two-color resonant two-photon ionization (2C-R2PI) and laser-induced fluorescence excitation spectra were recorded in the near-ultraviolet in the region of the close-lying S0-S1 and S0-S2 states (36,400-36,750 cm(-1)). In previous work, double resonance spectroscopy in the ultraviolet and alkyl CH stretch regions of the infrared was used to identify and assign transitions to two conformational isomers differing primarily in the central C-C dihedral angle, a tgt conformation with C2 symmetry and a ttt conformation with C2h symmetry [E. G. Buchanan, E. L. Sibert, and T. S. Zwier, J. Phys. Chem. A 117, 2800 (2013)]. Comparison of 2C-R2PI spectra recorded in the m∕z 214 (all (12)C) and m∕z 215 (one (13)C) mass channels demonstrate the close proximity of the S1 and S2 excited states for both conformations, with an upper bound of 4 cm(-1) between them. High resolution spectra of the origin band of the tgt conformer reveal it to consist of two transitions at 36,422.91 and 36,423.93 cm(-1), with transition dipole moments perpendicular to one another. These are assigned to the S0-S1 and S0-S2 origin transitions with excited states of A and B symmetry, respectively, and an excitonic splitting of only 1.02 cm(-1). The excited state rotational constants and transition dipole coupling model directions prove that the electronic excitation is delocalized over the two rings. The ttt conformer has only one dipole-allowed electronic transition (Ag→Bu) giving rise to a pure b-type band at 36,508.77 cm(-1). Here, the asymmetry induced by a single (13)C atom in one of the rings is sufficient to localize the electronic excitation in one or the other ring. Dispersed fluorescence (DFL) spectra are used to provide assignments for all vibronic structure in the first 200 cm(-1)of both conformers. In the tgt conformer, both "a" and "b" symmetry fundamentals are observed, consistent with extensive vibronic coupling between the two dipole-allowed, nearly degenerate excited states. In the ttt conformer, the lowest frequency vibronic transition located 46 cm(-1) above the Bu origin is assigned to a bu fundamental (labeled R[overline]) built off the dipole-forbidden Ag state origin. The DFL spectrum of the Ag(R[overline](1)) level contains strong transitions to v(")(R[overline]) = 0, 1, and 2, seemingly at odds with vibronic coupling models. Studies of the DFL spectrum of this band as a function of distance from the nozzle reveal that much of the intensity in v(") = 1 arises from collisions of DPOE while in the excited state Ag(vb' = 1) level with He, producing Bu(R[overline] = 1) levels with large collision cross section. The remaining intensity in the fundamental at large x∕D is ascribed to emission from the (13)C isotopomer, for which this emission is dipole-allowed.

Ground State Conformational Preferences and CH Stretch–Bend Coupling in a Model Alkoxy Chain: 1,2-Diphenoxyethane

1,2-Diphenoxyethane (C6H5-O-CH2-CH2-O-C6H5, DPOE) is a flexible bichromophore in which the two phenyl rings are separated from one another by an -O-CH2-CH2-O- chain with five flexible dihedral angles about which hindered rotation can occur. As such, it is a phenyl capped analog of dimethoxyethane (DMOE), which has served as a model compound for development of force fields for polyethylene glycol (PEG). The ground state conformational energy landscape of DPOE is explored using a combination of single-conformation spectroscopy of the jet-cooled molecule and calculations of the conformational minima and transition states. In the experimental UV spectrum, ultraviolet hole-burning establishes the presence of just two conformations with significant population in the supersonic jet expansion. Fluorescence dip infrared (FDIR) spectroscopy is used to record infrared spectra of the two conformers in the alkyl CH stretch, CH bend, and CO stretch regions. When compared with harmonic vibrational frequency calculations, the two isomers are determined to be of C2h and C2 symmetry, and labeled ttt and tgt to denote the three central dihedrals as trans or gauche. Infrared population transfer spectroscopy is used to determine fractional abundances for the two conformers (f(ttt) = 0.53 ± 0.01; f(tgt) = 0.47 ± 0.01). Relaxed potential energy curves along the three nonequivalent dihedral angles are used to map out the shape of the potential energy landscape that leads to these preferences. The Fermi resonance in the alkyl CH stretch spectrum is successfully modeled using a recently developed methodology [Buchanan et al., J. Chem. Phys. 2013, 138, 064308] employing a reduced dimension Hamiltonian. The scissor overtones couple to the CH2 symmetric stretch and only indirectly to the asymmetric stretch through symmetric stretch/asymmetric stretch coupling. The presence of the oxygen atoms in the chain shifts the CH scissor overtones to higher frequencies than in pure alkyl chains, qualitatively changing the spectral consequences of the Fermi resonance, with the scissor overtones now appearing as the highest frequency bands in the spectrum. The spectra are contrasted with those in 1,2-diphenylethane, a close analog with a very different appearance to its CH stretch spectrum, in which the scissor overtones appear as the lowest frequency bands.

A Nanospheric Polyhydrido Copper Cluster of Elongated Triangular Orthobicupola Array: Liberation of H2 from Solar Energy

An unprecedented air-stable, nanospheric polyhydrido copper cluster, [Cu20H11(S2P(O(i)Pr)2)9] (1H), which is the first example of an elongated triangular orthobicupola array of Cu atoms having C3h symmetry, was synthesized and characterized. Its composition was primarily determined by electrospray ionization mass spectrometry, and it was fully characterized by (1)H, (2)H, and (31)P NMR spectroscopy and single-crystal X-ray diffraction (XRD). The structure of complex 1H can be expressed in terms of a trigonal-bipyramidal [Cu2H5](3-) unit anchored within an elongated triangular orthobicupola containing 18 Cu atoms, which is further stabilized by 18 S atoms from nine dithiophosphate ligands and six capping hydrides. The positions of the 11 hydrides revealed by low temperature XRD were supported by a density functional theory investigation on the simplified model [Cu20H11(S2PH2)9] with C3h symmetry. 1H is capable of releasing H2 gas upon irradiation with sunlight, under mild thermal conditions (65 °C), or in the presence of acids at room temperature.

Pseudo Jahn–Teller Effect and Natural Bond Orbital Analysis of Structural Properties of Tetrahydridodimetallenes M2H4, (M = Si, Ge, and Sn)

The structural properties of ethene (1) and tetrahydridodimetallenes M(2)H(4) [M = Si (2), Ge (3), and Sn (4)] have been examined by means of CCSD(T)/Def2-TZVPP, MP4(SDTQ)/Def2-TZVPP, and B3LYP/Def2-TZVPP levels of theory and natural bond orbital analysis (NBO) interpretations. The results obtained showed the expected planar ground state structure for compound 1 (D(2h) symmetry) but trans-bent ground state structures for compounds 2-4 (C(2h) symmetry). The distortions of the high-symmetry configurations of compounds 2-4 are due to the pseudo Jahn-Teller effect (PJTE), which is the only source of instability of high-symmetry configurations in nondegenerate states. The distortions are due to the mixing of the ground A(g) and excited B(2g) states [i.e., HOMO(B(3u)) → LUMO + 3(B(1u)) for compound 1, HOMO(B(3u)) → LUMO + 2(B(1u)) for compound 2, and HOMO(B(3u)) → LUMO + 1(B(1u)) for compounds 3 and 4]. Importantly, the higher-lying B(1g), B(2u), and B(2g) states are not involved in the PJT interactions. The energy gaps between reference states (Δ) in the undistorted configurations decrease from compound 1 to compound 4, and the PJT stabilization energies increase. Therefore, the primary force constant of the ground state in the Q((b2g)) direction (K(0)) decreases from compound 1 to compound 4. This fact can be justified by the valence isoelectronic systems of these compounds (having similar vibronic coupling constants, F). For the purpose of more chemical transparency, the NBO results were analyzed, and their relation to the PJT interactions has been revealed. The NBO analysis showed that stabilization energy associated with π(M-H) (b(u)) → σ*(M═M) (b(u)) electron delocalization (i.e., the mixing of the distorted b(u) molecular orbitals along the b(2g) bending distortions) increases from compound 1 to compound 4. Also, by using the hybridized orbitals obtained, an n parameter is defined. The NBO results revealed that the n values in the mean hybrid orbitals (sp(n)) increase from compound 1 to 4. The correlations between the PJT stabilization energies, bond orders, n values, π(M-M) → σ*(M═M) electron delocalizations, and structural parameters of compounds 1-4 have been investigated.

Hyperfine interactions at nitrogen interstitial defects in diamond

Diamond has many extreme physical properties and it can be used in a wide range of applications. In particular it is a highly effective particle detection material, where radiation damage is an important consideration. The WAR9 and WAR10 are electron paramagnetic resonance centres seen in irradiated, nitrogen-containing diamond. These S = 1/2 defects have C2v and C1h symmetry, respectively, and the experimental spectra have been interpreted as arising from nitrogen split-interstitial centres. Based upon the experimental and theoretical understanding of interstitial nitrogen defect structures, the AIMPRO density functional code has been used to assess the assignments for the structures of WAR9 and WAR10. Although the calculated hyperfine interaction tensors are consistent with the measured values for WAR9, the thermal stability renders the assignment problematic. The model for the WAR10 centre yields principal directions of the hyperfine tensor at variance with observation. Alternative models for both centres are discussed in this paper, but no convincing structures have been found.

Dimeric aluminum methyl complex bridged by 2-oxy-2-methyl-1-(phenylamino)propane: Synthesis, structure, and use in ring opening polymerization of lactide

Abstract A dimeric aluminum complex bridged by alkoxide with pendant aniline group was synthesized and characterized. X-ray analysis revealed that the two four-coordinate aluminum centers possess a distorted tetrahedral geometry with C2h symmetry. The compound was used as an effective catalyst for the controlled ring opening polymerization of L-lactide, as shown by the linearity of the number average of the molecular weight of polylactides versus conversion, as well as narrow Mw/Mn values.

Tuning optical and electronic properties of star-shaped conjugated molecules with enlarged π-delocalization for organic solar cell application

Three structurally related conjugated molecules (BTT-BTD-0, BTT-BTD-1 and BTT-BTD-2) in star shape have been designed and synthesized as donor materials for small molecule based bulk heterojunction (BHJ) solar cells. The structural features of these molecules include a planarized benzo[1,2-b:3,4-b′:5,6-b″]trithiophene (BTT) with a C3h symmetry as the central core and three conjugated arms incorporating electron deficient benzo[2,1,3]thiadiazole (BTD) units, with arms being linked to the core via different number of thiophene connecting units (e.g., 0, 1, 2 corresponding to BTT-BTD-0, BTT-BTD-1 and BTT-BTD-2, respectively). Comparative analyses of optical and electronic properties indicate that the molecules bearing more thiophene units between the BTT core and the BTD arms possess higher-lying HOMO levels while their LUMO levels remain almost unchanged. The improvement of BHJ device performance, with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as the acceptor, is observed with increasing number of thiophene units between the BTT core and BTD arms, from BTT-BTD-0 to BTT-BTD-1 and BTT-BTD-2. The BTT-BTD-2:PC61BM based BHJ devices show the highest power conversion efficiency (PCE) of 0.74%, with an open-circuit voltage (Voc) of 0.69 V, a short-circuit current density (Jsc) of 2.93 mA cm−2, and a fill factor (FF) of 0.37 under 1 sun (100 mW cm−2) AM 1.5G simulated solar illumination. The PV performance of BTT-BTD-2 is further improved when [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) is used as the electron acceptor, yielding the best device performance with Jsc of 4.13 mA cm−2, Voc of 0.72 V, FF at 0.46 and PCE of 1.36%. The effect of the different number of thiophenes linking the BTT core and the conjugated BTD arms has been clearly demonstrated on regulating optical and electrochemical properties of the three molecules and their BHJ device performances.

Calix[4]arenes of Aluminum and Gallium with Benzimidazolyl Ligands: Steric Control of the Conformation via Substitution on the Ligand

Complexes [bzimAlR(2)](4) [bzim = benzimidazolate; R = Et (2), (i)Bu (3)], [mbzimAlR(2)](4) [mbzim = 2-methylbenzimidazolate; R = Et (6), (i)Bu (7)], [dmbzimAlR(2)](4) [dmbzim = 5,6-dimethylbenzimidazolate; R = Me (9), Et (10), (i)Bu (11)], and [tmbzimAlR(2)](4) [tmbzim = 2,5,6-trimethylbenzimidazolate; R = Me (12), Et (13), (i)Bu (14)] have been prepared via alkane elimination and coordinative self-assembly upon the reaction of benzimidazole ligands with aluminum alkyls in benzene, toluene, or xylene. Characterization of the complexes was achieved by spectroscopic methods, microanalysis, and X-ray crystallography of 2, 7, 10, 11, 13, and 14. The complexes reported herein and the aluminum and gallium analogues 1, 4, 5, and 8 reported in a previous paper (1) are predominantly tetranuclear aggregates related to calix[4]arenes in which the benzimidazolyl ligands bind two metal atoms in a η(1):η(1) fashion. X-ray crystallography demonstrates that modulation of the conformation adopted by these metallacalix[4]arenes is achieved by proper substitution on the C atom at the 2 position of the benzimidazolyl ligand. An H substituent for 1, 2, 4, 10, and 11 favors a chair conformation with a small cavity and approximate C(2h) symmetry, while a CH(3) substituent for 5, 7, 8, 13, and 14 introduces enough repulsion to switch the conformation to a 1,3-alternate or double cone with a concomitant larger cavity and approximate C(2v) symmetry.