Triple Bond Length Research Articles

Selective hydroboration of terminal alkynes catalyzed by heterometallic clusters with uranium–metal triple bonds

Selective hydroboration of terminal alkynes catalyzed by heterometallic clusters with uranium–metal triple bonds

A theoretical study on novel neutral noble gas compound F4XeOsF4.

A noble gas compound containing a triple bond between xenon and transition metal Os (i.e. F4XeOsF4, isomer A) was predicted using quantum-chemical calculations. At the MP2 level of theory, the predicted Xe-Os bond length (2.407 Å) is between the standard double (2.51 Å) and triple (2.31 Å) bond lengths. Natural bond orbital analysis indicates that the Xe-Os triple bond consists of one σ-bond and two π-bonds, a conclusion also supported by atoms in molecules (AIM) quantum theory, the electron density distribution (EDD) and electron localization function (ELF) analysis. The two-body (XeF4 and OsF4) dissociation energy barrier of F4XeOsF4 is 15.6 kcal mol-1. The other three isomers of F4XeOsF4 were also investigated; isomer B contains a Xe-Os single bond and isomers C and D contain Xe-Os double bonds. The configurations of isomers A, B, C and D can be transformed into each other.

Linearly Two-Coordinated Silicon: Transition Metal Complexes with the Functional Groups M≡Si—M and M═Si═M

A detailed experimental and theoretical analysis is presented of unprecedented molybdenum complexes featuring a linearly coordinated, multiply bonded silicon atom. Reaction of SiBr2(SIdipp) (SIdipp = C[N(C6H3-2,6- iPr2)CH2]2) with Na[Tp'Mo(CO)2(PMe3)] (Na-1) in the ratio 1:2 afforded the reddish-brown metallasilylidyne complex [Tp'(CO)2Mo≡Si-Mo(CO)2(PMe3)Tp'] (Tp' = κ3- N, N', N″-hydridotris(3,5-dimethylpyrazolyl)borate) (2), in which an almost linearly coordinated silicon atom (∠(Mo1-Si-Mo2) = 162.93(7)°) is bridging the 15VE metal fragment Tp'Mo(CO)2 with the 17VE metal fragment Tp'Mo(CO)2(PMe3) via a short Mo1-Si bond (2.287(2) Å) and a considerably longer Mo2-Si bond (2.438(2) Å), respectively. The reddish-orange silylidyne complex [Tp'(CO)2Mo≡Si-Tbb] (3) was also prepared from Na-1 and the 1,2-dibromodisilene ( E)-Tbb(Br)Si═Si(Br)Tbb (Tbb = C6H2-2,6-[CH(SiMe3)2]2-4- tBu) and contains as 2 a short Mo-Si bond (2.2614(9) Å) to an almost linearly coordinated Si atom (∠(Mo-Si-CTbb) = 160.8(1)°). Cyclic voltammetric studies of 2 in diglyme revealed an irreversible reduction of 2 at -1.907 V vs the [Fe(η5-C5Me5)2]+/0 redox couple. Two-electron reduction of 2 with potassium graphite yielded selectively the 1,3-dimetalla-2-silaallene dianion [Tp'(CO)2Mo═Si═Mo(CO)2Tp']2- (42-), which was isolated as the bright yellow dipotassium salt [K(diglyme)]2-4. Single crystal X-ray diffraction analysis revealed a centrosymmetric structure of 42-. The Mo-Si bond length of 42- (2.3494(2) Å) compares well with those of Mo-Si double bonds and lies in-between the Mo1-Si triple bond and Mo2-Si single bond length of 2. Compounds 2, 3 and [K(diglyme)2]-4 were characterized by elemental analyses, IR and multinuclear NMR spectroscopy. Comparative ELF (electron localization function), NBO (natural bond orbital) and NRT (natural resonance theory) analyses of 2, 3 and 42- shed light into the electronic structures of these compounds.

Lateral substituent effects on UV stability of high-birefringence liquid crystals with the diaryl-diacetylene core: DFT/TD-DFT study

ABSTRACTTo study the effect of the lateral substituents on the UV stability of high birefringence liquid crystals (LCs), computational chemistry was used to examine a series of high birefringence LCs based on a diphenyl-diacetylene (DPDA) central core, thiophene segments as elongated π-conjugated units and four electron-withdrawing groups (-F, -CF3, -OCF3, -CN) as lateral substituents. In the present study, geometry optimisations have been performed using the DFT/B3LYP/6-311G (d, p) method. Out of a series of functional and basis sets examined, the functional ωB97X-D and basis set 6-31G (d, p) are most successful in predicting charge transfer absorption. The theoretical study indicates that the enhancement of UV stability is related with the types, numbers and positions of the lateral substituents. The calculated results indicate that the electron-withdrawing groups can shorten triple bond length, decrease energy gap value and increase the absorption maxima of the high-Δn LCs, which is beneficial for good UV stability. With the introduction of increasing lateral electron-withdrawing substituent numbers, the DPDA derivatives would further improve UV stability. This work may provide an effective solution for the obstacle existed in the high-Δn LCs with DPDA structures and pave a way for their applications in LC photonics.

Improving UV stability of tolane-liquid crystals in photonic applications by the ortho fluorine substitution

A series of ortho fluoro-tolane liquid crystals was prepared via multi-step reactions based on 4-alkylcyclohexanecarboxylic acids. Their synthetic routes, thermotropic mesophases and UV stability properties are discussed by comparision with the non-fluorinated analogs. All of the measurements from DSC, UV-vis absorption and fluorescence emission spectra demonstrate that the presence of ortho fluorine can significantly improve the UV stability of tolane-liquid crystals. Meanwhile, the ultimate failure mechanism of tolane-LCs during UV exposure is deduced by a photochemical reaction. DFT calculations of molecular polarizabilities, triple bond lengths, electrostatic potential maps and molecular orbitals are used to correlate the experimental findings. The interaction mechanism for stabilization of the tolane structures by the ortho fluoro substitution is also explained. This work may provide an effective solution for the obstacle existed in tolane-liquid crystals and pave a way for their applications in liquid crystal photonics.

Silicon-based counterpart of alpha-graphyne

Abstract We present the first principles density functional calculations of electronic structure and energetics of silicon-based counterpart of α-graphyne, labeled as α-silicyne. Both LDA and GGA functionals are applied for exchange–correlation potentials. We show that graphyne-like silicon in 2D buckled structure (equilibrium buckling δ z ≅ 0.73 and Δ z ≅ 1.45 A ) has ≈ 2.33 eV and ≈ 1.96 eV lower energies than planar geometry for GGA and LDA functionals, respectively. The single and triple bond lengths of silicon are consistent with previously reported values. As a different case from graphyne, which is semimetallic, the electronic band structures of buckled α-silicyne do not show Dirac fermion indicating a metallic nature. The metallic character of the system is largely determined by p-electronic states of the triple bonded silicon atoms.

Theoretical studies on structures and electronic spectra of linear HC nN + ( n = 2–14)

With unrestricted B3LYP and CAM-B3LYP calculations, we have investigated the linear HC n N + ( n = 2–14) clusters focusing on the ground-state geometries, vibrational frequencies, rotational constants, dipole moments, energy differences (Δ E n ), and incremental binding energies (Δ E I). The results indicate that the odd-numbered HC n N + clusters have polyacetylene-like structures with significant single–triple bond length alternation, while the even-numbered analogues exhibit some sort of cumulenic character, with the former being more stable than the latter. The CASPT2/cc-pVTZ approach has been employed to estimate the vertical excitation energies for the dipole-allowed (2, 3) 2Π ← X 2Π and dipole-forbidden 1 2Φ ← X 2Π transitions in HC n N + ( n = 5–14) clusters. The predicted excitation energies of 2 2Π ← X 2Π transitions for HC n N + ( n = 5–13) clusters are 2.27, 2.33, 2.04, 2.02, 1.84, 1.76, 1.62, 1.58, and 1.49 eV, respectively, in good agreement with the available observed values (2.12, 2.17, 1.84, 1.89, 1.61, 1.66, 1.42, 1.49, and 1.27 eV). In addition, the higher electronic transitions of HC n N + ( n = 5–14) are also calculated. We expect that calculations for 3 2Π ← X 2Π transitions in odd- n clusters are able to contribute to further experimental and theoretical researches because of their relatively higher oscillator strengths. Finally, in the paper are discussed the possible dissociation channels and corresponding fragmentation energies of HC n N + clusters.

Theoretical Modeling of Molecular Spectra Parameters of Disubstituted Diacetylenes

Symmetrically disubstituted diacetylenes, X-C≡C-C≡C-X, were studied computationally by using the DFT B3LYP/aug-cc-pVDZ method. For more than 35 substituents the bond lengths, charge density and Laplacian in bond critical points, C≡C stretching vibrational frequencies, (13)C NMR chemical shifts and spin-spin CC coupling constants through diacetylene moiety were calculated and examined by using the substituent sEDA and pEDA descriptors. It is demonstrated that in disubstituted diacetylenes the triple bond length increases with the electron donating and decreases with the electron withdrawing properties of the substituents. The σ-electron repulsion is likely to be responsible for this phenomenon. The electron density of the C-X bond critical point decreases linearly with an increase of the σ-electron donating properties, whereas the Laplacian of electron density in the C≡C bond critical point increases as the sEDA descriptor i.e., σ-electron donating properties of the substituent, are increased. Thus, ρ(C≡C) is locally reduced with an increase of sEDA. The ν(as)(C≡C) and ν(s)(C≡C) mode frequencies decrease with the electron donating and increase with the electron withdrawing properties of the substituents. The calculated chemical shift of the C1-atom, to which the substituent is attached, does not correlate with the substituent descriptors, whereas the δ(C2) deshielding increases when the σ-electron donating properties and C≡C distance are increased. The calculated (1)J(CC) coupling constants decrease with an increase of the triple bond length, the sEDA parameter, and ρ(C-X), whereas they decrease with Laplacian in the C≡C BCP.

Unsymmetrically Substituted Disilyne Dsi2iPrSi—Si≡Si—SiNpDsi2 (Np = CH2tBu): Synthesis and Characterization

The unsymmetrically substituted disilyne, Dsi(2)(i)PrSi-Si≡Si-SiNpDsi(2) (Np = CH(2)(t)Bu) 2, was synthesized and characterized by X-ray crystallography to show a trans-bent structure with a silicon-silicon triple bond length of 2.0569(12) Å. The (29)Si chemical shifts of the triply bonded silicon atoms of 2 are quite different, being observed at 62.6 ppm for the Dsi(2)(i)PrSi side and 106.3 ppm for the Dsi(2)NpSi side, indicating different hybridizations on the triply bonded silicon atoms at each site.

Experimental and Theoretical Investigation of Simple Terminal Group 6 Arsenide As≡MF3 Molecules

Laser-ablated group 6 metal atoms react with NF3 and PF3 to form the simple lowest energy N[triple bond]MF3 and P[triple bond]MX3 products, and this investigation has been extended to AsF3. Mo and W atoms react with AsF3 upon excitation by laser ablation or UV irradiation to form stable trigonal As[triple bond]MF3 terminal arsenides. These molecules are identified by comparison of the closely related infrared spectra of the analogous phosphide species and with frequencies calculated by density functional theory and multiconfigurational second order perturbation theory (CASSCF/CASPT2). Computed CASSCF/CASPT2 triple bond lengths for the As[triple bond]MoF3 and As[triple bond]WF3 molecules are 2.240 A and 2.250 A, respectively. The natural bond orders calculated by CASSCF/CASPT2 decrease from 2.67 to 2.60 for P[triple bond]MoF3 to As[triple bond]MoF3 and from 2.74 to 2.70 for P[triple bond]WF3 to As[triple bond]WF3 as the arsenic valence orbitals are less effective than those of phosphorus in bonding to each metal atom and the larger metal orbital size becomes more compatible with the arsenic valence orbitals. The Cr atom reaction gives the arsinidene AsF=CrF2 product instead of the higher energy As[triple bond]CrF3 molecule as the Cr (VI) state is not supported by the softer pnictides.

Infrared Spectra, Structure, and Bonding of the GeH3—CrH, HGe≡MoH3, and HGe≡WH3 Molecules in Solid Neon and Argon

Laser ablated chromium, molybdenum, and tungsten atoms react with germane during condensation in excess noble gases. The chromium reaction stopped at the germyl metal hydride, molybdenum gave some hydride but mostly germylidyne, and tungsten reacted spontaneously to give only the germylidyne species. These molecules were identified by isotopic shifts, density-functional theory product energy and frequency calculations, and comparison to the analogous methane and silane reaction products. Effective bond orders for the HGe[triple bond]MoH3 and HGe[triple bond]WH3 molecules are 2.82 and 2.87 using the B3LYP density functional, and are slightly lower than their silicon and carbon analogues. Our calculated Ge[triple bond]M triple bond lengths for these simple trihydride complexes are 0.05 to 0.10 A shorter than those measured for larger group 6 organometallic complexes.

Vibrational dynamics of hydrogen-bonded HCN complexes with OH and NH acids: Computational DFT systematic study

Vibrational properties (band position, infrared [IR], and Raman intensities) of CN stretching mode were studied in 65 gas phase hydrogen-bonded 1:1 complexes of HCN with OH acids and NH acids using density functional theory (DFT) calculations at the B3LYP-6-311++G(d,p) level. Furthermore, general characteristics of the hydrogen bonds and vibrational changes in acids OH/NH stretching bands were also considered. Experimentally observed blue shift of the CN stretching band promoted by hydrogen bonding, which shortens the triple bond length, is very well reproduced and quantitatively depends on the hydrogen bond length. Both IR and Raman ν(CN) band intensities are enhanced, also in good agreement with the experimental results. IR intensity increase is a direct function of the hydrogen bond energy. However, the predicted Raman intensity raise is a more complex function, depending simultaneously on characteristics of both the hydrogen bond (CN bond length) and the H-donating acid (polarizability). With these two parameters, ν (CN) Raman intensities of the complexes are explained with a mean error of ±2.4%. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007

Carbon chains and the (5,5) single-walled nanotube: Structure and energetics versus length

Reliable thermochemistry is computed for infinite stretches of pure-carbon materials including acetylenic and cumulenic carbon chains, graphene sheet, and single-walled carbon nanotubes (SWCNTs) by connection to the properties of finite size molecules that grow into the infinitely long systems. Using ab initio G3 theory, the infinite cumulenic chain (:C[double bond]C[double bond]C[double bond]C:) is found to be 1.9+/-0.4 kcal/mol per carbon less stable in free energy at room temperature than the acetylenic chain (.C[triple bond]C-C[triple bond]C.) which is 24.0 kcal/mol less stable than graphite. The difference between carbon-carbon triple, double, and single bond lengths (1.257, 1.279, and 1.333 A, respectively) in infinite chains is evident but much less than with small hydrocarbon molecules. These results are used to evaluate the efficacy of similar calculations with the less rigorous PM3 semiempirical method on the (5,5) SWCNT, which is too large to be studied with high-level ab initio methods. The equilibrium electronic energy change for C(g)-->C[infinite (5,5) SWCNT] is -166.7 kcal/mol, while the corresponding free energy change at room temperature is -153.3 kcal/mol (6.7 kcal/mol less stable than graphite). A threefold alternation (6.866, 6.866, and 6.823 A) in the ring diameter of the equilibrium structure of infinitely long (5,5) SWCNT is apparent, although the stability of this structure over the constant diameter structure is small compared to the zero point energy of the nanotube. In general, different (n,m) SWCNTs have different infinite tube energetics, as well as very different energetic trends that vary significantly with length, diameter, and capping.

Effects of wave function modifications on calculated carbon–carbontriple bond lengths

Two-level factorial design (FD) and principal component (PC) models are used to determine the effects of wave function modifications on calculated carbon–carbon triple bond lengths for the HC CH, HC CF, HC CCH 3, CH 3C CCH 3 and HC CCCH molecules. Our results have shown that valence, diffuse and polarization main effects are significant at the three levels of calculation here employed: Hartree–Fock (HF), Møller–Plesset 2 (MP2) and density functional theory (DFT) with B3LYP exchange–correlation functional. The valence–polarization interaction effect is significant at only the HF and MP2 levels of calculation. When valence and polarization functions are introduced in the basis set, the calculated C C bond length values decrease, in contrast with the results obtained when diffuse functions are used. The latter increase the C C bond length by +0.0016 Å, at all levels of calculations. Changing the method from HF to B3LYP and from B3LYP to MP2 yields an increase (averaged values) of +0.0195 and +0.0169 Å, respectively, on the calculated C C bond length values. For each level of calculation it was possible to establish an algebraic model to explain how the calculated C C bond length values depend on the molecular orbital wave functions. Such algebraic models successfully reproduce calculated C C bond lengths for the HC CCN and HC CCl molecules, which were not included in our training set. Our FD and PC models lead to the B3LYP/6-31G(d,p) and B3LYP/6-311++G wave functions as the selected ones in order to better reproduce experimental C C bond lengths for all the seven molecules here studied (training and validation molecules). Their predicted values deviate by just 0.004 Å from the experimental ones.

Vibrational Spectroscopic Properties of Hydrogen Bonded Acetonitrile Studied by DFT

Vibrational properties (band position, Infrared and Raman intensities) of the acetonitrile C[triple bond]N stretching mode were studied in 27 gas-phase medium intensity (length range: = 1.71-2.05 angstroms; -deltaE range = 13-48 kJ/mol) hydrogen-bonded 1:1 complexes of CH3CN with organic and inorganic acids using density functional theory (DFT) calculations [B3LYP-6-31++G(2d,2p)]. Furthermore, general characteristics of the hydrogen bonds and vibrational changes in the OH stretching band of the acids were also considered. Experimentally observed blue-shifts of the C[triple bond]N stretching band promoted by the hydrogen bonding, which shortens the triple bond length, are very well reproduced and quantitatively depend on the hydrogen bond length. Both predicted enhancement of the infrared and Raman nu(C[triple bond]N) band intensities are in good agreement with the experimental results. Infrared band intensity increase is a direct function of the hydrogen bond energy. However, the predicted increase in the Raman band intensity increase is a more complex function, depending simultaneously on the characteristics of both the hydrogen bond (C[triple bond]N bond length) and the H-donating acid polarizability. Accounting for these two parameters, the calculated nu(C[triple bond]N) Raman intensities of the complexes are explained with a mean error of +/- 2.4%.

A Stable Compound Containing a Silicon-Silicon Triple Bond

The reaction of 2,2,3,3-tetrabromo-1,1,4,4-tetrakis[bis(trimethylsilyl)methyl]-1,4-diisopropyltetrasilane with four equivalents of potassium graphite (KC8) in tetrahydrofuran produces 1,1,4,4-tetrakis[bis(trimethylsilyl)methyl]-1,4-diisopropyl-2-tetrasilyne, a stable compound with a silicon-silicon triple bond, which can be isolated as emerald green crystals stable up to 100 degrees C in the absence of air. The SiSi triple-bond length (and its estimated standard deviation) is 2.0622(9) angstroms, which shows half the magnitude of the bond shortening of alkynes compared with that of alkenes. Unlike alkynes, the substituents at the SiSi group are not arranged in a linear fashion, but are trans-bent with a bond angle of 137.44(4) degrees.

Structure of [N,N´-bis(3,5-di-t-butyl-salicylidene)-2-benzyl-1,3-propanediaminato]nitridotechnetium(V) complex, TcN(bzdbsalpn), and nitrido-bridged linear polymeric chains in crystalline syn-isomer

Abstract A novel nitridotechnetium(V) complex with a tetradentate Schiff base ligand, TcN(bzdbsalpn), was synthesized by the reaction of TcNCl2(PPh3)2 and N , N´-bis(3,5-di- t-butyl-salicylidene)-2-benzyl-1,3-propanediamine (H2bzdbsalpn). Both the syn- and anti-isomers of TcN(bzdbsalpn) were formed in the solution, but recrystallization of a mixture of syn- and anti-TcN(bzdbsalpn) gave a single crystal only for the syn-isomer. X-ray crystallography showed that the syn-TcN(bzdbsalpn) was in a distorted octahedral structure. The planar tetradentate bzdbsalpn ligand coordinates to the technetium atom in an O-N-N-O equatorial plane and the six-membered Nimine-Nimine chelate ring that includes the propylene group adopts a chair conformation. The nitrido ligand (Tc≡N) is at an apical position and the Tc≡N triple bond length is 1.6 Å. Interestingly, the crystal of syn-TcN(bzdbsalpn) has a nitrido-bridged linear polymeric structure (···Tc≡N···Tc≡N···) running along the 21 screw axes parallel to the b axis. The intermolecular N···Tc distance in the syn-TcN(bzdbsalpn) crystal (∼2.8 Å) is fairly long compared with that reported for Cr(V) and Mn(V) nitrido complexes with analogous Schiff base ligands (∼2.5 Å).

Single (C–C) and triple (CC) bond-length dependence of the static electric polarizability and hyperpolarizability of H–CC–CC–H

We report an ab initio study of the static electric (hyper)polarizability of diacetylene and its dependence on the single (C–C) and triple (CC) bond length. At the CCSD(T) level of theory we find for the mean dipole polarizability and its derivatives α ̄ =49.10 e 2 a 0 2 E h −1,( ∂ α ̄ / ∂R C− C ) e=−4.41 and ( ∂ α ̄ / ∂R C C ) e=34.57 e 2 a 0 E h −1 . For the anisotropy Δα=54.45 e 2 a 0 2 E h −1 , (∂Δ α/∂ R C–C) e =−20.42 and ( ∂Δα/ ∂R C C ) e=64.56 e 2 a 0 E h −1 . The dependence of the mean hyperpolarizability on R C–C and R CC around the equilibrium is quite distinct. Varying the single bond by Δ R/ a 0 around the equilibrium entails changes of [ γ ̄ (R C– C )− γ ̄ (R e)]/ e 4 a 0 4 E h −3=−3643 ΔR−230 ΔR 2−184 ΔR 3+453 ΔR 4 The mean second hyperpolarizability increases strongly with R C≡C around the equilibrium [ γ ̄ (R C C )− γ ̄ (R e)]/ e 4 a 0 4 E h −3=22 259 ΔR+11 293 ΔR 2+2384 ΔR 3+6445 ΔR 4

Synthesis, Structures, and Donor−Acceptor Adducts of Tris(3,3-dimethyl-1-butynyl)borane

The synthesis and properties of tris(3,3-dimethyl-1-butynyl)borane (1), the first donor-free tris(alkynyl)borane, and of its adducts 2 [donor = pyridine (2a), triphenylphosphane (2b), tetrahydrofuran (2c)] are reported. X-ray structure analyses reveal that the B−C bond lengths in 1 are shorter than in 2; however, the C≡C triple bond lengths are similar to those of the donor-stabilized compounds 2a, 2b, and 2c. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Synthesis and characterization of a technetium nitrido dimer

In 1993, Baldas et al. [1] reported the synthesis of the technetium(VI) nitridodi (μ-oxo) dimer (AsPh 4) 2[{TcNCl 2} 2(μ-O) 2], which was synthesized from methanesulfonic acid and Cs 2[TcNCl 5] in water with heating. We report here, the facile synthesis and characterization of the tetrabutylammonium analog, (Bu 4N) 2[{TcNCl 2} 2(μ-O) 2] ( 1), which is synthesized from (Bu 4N)[TcNCl 4] and water in acetone at room temperature. This may have important ramifications for technetium based radiopharmaceutical development, since [ 99mTcNCl 4] − is frequently employed at the tracer level as a technetium nitrido synthon, and technetium nitrido chemistry continues to be a rapidly developing area in radiopharmaceutical research [1]. The infrared spectrum of 1 displays a pronounced absorption at 1062 cm −1, attributed to the TcN bond. The FAB(−) mass spectrum displays the parent ion of 642 m/ z which corresponds to {(Bu 4N)[{TcNCl 2} 2(μ-O) 2]} −. The X-ray crystal structure of 1 is similar to the previously reported tetraphenylarsonium analog [2]. The technetium nitrogen triple bond lengths are 1.601(7) and 1.597(7) Å. The X-ray structure solution for C 32H 72Cl 4N 2O 4Tc of molecular weight 886.72 g mol −1: Monoclinic space group P2 1/ n, a=15.2894(12), b=16.4321(12), c=17.9311(14) Å, β=105.5100(10)°, volume=4340.9(6) Å 3. Solution based on 6212 independent reflections to give a final R value of 0.0701 and GOF=1.124.