Single-bond Character Research Articles

Planar hexacoordinate phosphorous and arsenic

Here the first planar hexacoordinate phosphorous and arsenic (phP and phAs) atoms are reported. Neutral 18 valence electron EBe3Li3H6 clusters (E = P, As) adopt planar D3h symmetry in their lowest energy singlet state. Chemical bonding reveals partial double bond character in P(As)-Be bonds and single bond character in P(As)-Li bonds. Born-Oppenheimer molecular dynamics simulation as well as their reluctance towards decomposition reveals high kinetic stability of these clusters.

On the ineffectiveness of grubbs-type iron olefin metathesis catalysts: Role of spin-state isomerism and cyclopropanation

Unlike Grubbs-type Ru(II) catalysts, the corresponding Fe(II) catalysts are ineffective for olefin metathesis. A DFT study on the mechanistic pathways of Fe(II) analogues of the Grubbs first and second generation catalysts has been done on the basis of 16-electron dissociative (16eD) and 18-electron associative (18eA) pathways to unravel the reasons behind the ineffectiveness of the catalysts to perform CC bond metathesis. Both the singlet and triplet spin states of the catalysts and the intermediates have been considered for the evaluation of the reaction profile. The [2 + 2] cycloaddition step to form the singlet ironacyclobutane, FeCB is very facile for Fe(II) systems and the behavior is very similar to the Grubbs catalysts. Though FeCB systems showed activated agostic-type CC bonds, metathesis is prevented due to the spin crossover transition during the cycloaddition to significantly more stable 3FeCB. Formation of 3FeCB is exergonic with respect to the olefin coordinated Fe(II) complex. Also the CC bonds of 3FeCB show normal inactivated single bond character suggesting high energy barrier for the cycloreversion to produce a new olefin. Moreover, cyclopropane formation from 3FeCB is very facile, exergonic and leads to the degradation of the catalyst.

Keto-enol tautomerism from the electron delocalization perspective.

The equilibrium between keto and enol forms in acetylacetone and its derivatives is studied using electron delocalization indices and delocalization tensor density. We demonstrate how electron delocalization governs the equilibrium between keto and enol forms. The less stable enols have more distinct double and single bond character in the CCC fragment, while electron delocalization in this fragment is more pronounced in more stable enols. Looking for the origin of such behavior, we considered the one-electron potentials entering the Euler equation for the electron density. We found that electron delocalization is mainly governed by the static exchange potential, which depends on the three-dimensional atomic structure. It, however, does not distinguish differences in electron delocalization in more and less stable enols, the effect arising from the kinetic exchange contribution, which reflects spin-dependent effects in the electron motion. The local depletion of kinetic exchange in the conjugated fragment yields the enhanced electron delocalization along the CCC bonds in more stable enols. Thus, a combination of considered descriptors allowed us to reveal the influence of electron delocalization on the equilibrium between keto and enol forms and showed the significant features of this phenomenon.

Copper(II) Prevents the Saccarine-Dialkylcyanamide Coupling by Forming Mononuclear (Saccharinate)(Dialkylcyanamide)copper(II) Complexes

The reaction in the system CuII/sacNa(H)/NCNR2 (sacNa(H) = sodium saccharinate (saccharin); R = Me, Et) results in the formation of the complexes [Cu(sac)2(NCNR2)(H2O)2] (R = Me 1, Et 2) instead of the expected products derived from the saccharin–cyanamide coupling. Complexes 1, 2, and hydrate 1·2H2O were characterized by IR, AAS (Cu%), TGA, and also by single-crystal X-ray diffraction for 1 and 1·2H2O. An integrated computational study of model structure 1 in the gas phase demonstrates that the Cu–Ncyanamide and Cu–Nsac coordination bonds exhibited a single bond character, polarized toward the N atom and almost purely electrostatic, with the calculated vertical total energies for the Cu–Ncyanamide and Cu–Nsac of 43.6 and 156.4 kcal/mol, respectively. These data confirmed that the copper(II) completely blocks the nucleophilic centers of ligands via coordination, thus preventing the saccharin–cyanamide coupling.

Synthesis of Unsupported Primary Phosphido Complexes of Aluminum(III)

AbstractPrimary phosphido complexes of aluminum(III) are rare, particularly those that are not supported by interactions with Lewis bases or stabilizing cations. Here we report two new examples of unsupported primary phosphido complexes of Al(III). The ancillary ligand on Al is a pincer ligand, diiminopyridine (denoted as I2P). Solid‐state structures show distorted tetrahedral geometry about Al and single bond character in the Al−P bond. Near infra‐red spectra display low energy absorption bands near 1050 nm that are consistent with pincer ligand – Al charge transfer transitions and metalloaromatic character in the “(I2P2−)Al” fragment of the molecules. The phosphido ligands lie out of the I2P ligand plane by up to 15° and this is consistent with our previous reports where π‐donor halide ligands occupy the fourth coordination site on Al.

Theoretical research on mercury-laden halogenated activated carbon adsorbent bonding nature

Halogenated activated carbon adsorbents exhibit superior mercury capture performance in the flue gas. Not only the fabrication of effective adsorbents, but the used adsorbents stability is also crucial. It is essentially determined by product bonding nature. Here, Density functional theory was employed to investigate spent halogenated activated carbon adsorbents bonding nature at the atomic level. Seven possible mercury-containing product molecular geometries and effects of halogen species (X = Cl/Br/I) were considered. Specifically, bond length value, and bond order of Hg-C, Hg-X and C-X bonds of interest were calculated to investigate product stability in the gaseous phase. It was found that mercury was attached on the carbonaceous surface with C-Hg Mayer bond order around 0.5 in product structures G and H, suggesting a weak interaction. In other possible product molecular geometries, C-Hg or X-Hg exhibited a single bond character. Moreover, effects of halogen atoms on X-Hg and C-X bond strength were discussed. Electron localization function (ELF) and Localized orbital locator (LOL) were utilized to determine electron high localization regions. Results indicated C-Hg and C-X on the carbonaceous surface are covalent bonds and formed X-Hg bonds are ionic bonds. Furthermore, atom composition in the localized molecular orbital was calculated via the Hirshfeld method. In summary, this study investigated the bonding nature of used halogenated activated carbon adsorbents. The results are beneficial for fabricating more environmental-friendly mercury removal adsorbents in the future.

Structural Changes Induced by Quinones: High-Resolution Microwave Study of 1,4-Naphthoquinone.

1,4‐Naphthoquinone (1,4‐NQ) is an important product of naphthalene oxidation, and it appears as a motif in many biologically active compounds. We have investigated the structure of 1,4‐NQ using chirped‐pulse Fourier transform microwave spectroscopy and quantum chemistry calculations. The rotational spectra of the parent species, and its 13C and 18O isotopologues were observed in natural abundance, and their spectroscopic parameters were obtained. This allowed the determination of the substitution r s, mass‐weighted r m and semi‐experimental r e SE structures of 1,4‐NQ. The obtained structural parameters show that the quinone moiety mainly changes the structure of the benzene ring where it is inserted, modifying the C−C bonds to having predominantly single or double bond character. Furthermore, the molecular electrostatic surface potential reveals that the quinone ring becomes electron deficient while the benzene ring remains a nucleophile. The most electrophilic areas are the hydrogens attached to the double bond in the quinone ring. Knowledge of the nucleophilic and electrophilic areas in 1,4‐NQ will help understanding its behaviour interacting with other molecules and guide modifications to tune its properties.

Donor-Acceptor Conjugated Macrocycles with Polyradical Character and Global Aromaticity.

SummaryPolyradical character and global aromaticity are fundamental concepts that govern the rational design of cyclic conjugated macromolecules for optoelectronic applications. Here, we report donor-acceptor (D−A) conjugated macromolecules with and without π-spacer derivatives to tune the antiferromagnetic couplings between the unpaired electrons. The macromolecules without π-spacer have a closed-shell electronic configuration and show global nonaromatic character in the singlet and lowest triplet states. However, the derivatives with π-spacer develop a nearly pure open-shell diradical and a very high polyradical character, not reported for D−A type macromolecules. Furthermore, the π-spacer derivatives display global nonaromaticity in the singlet ground state, but global aromaticity in the lowest triplet state, according to Baird's rule. The absorption spectra of the open-shell macromolecules calculated with time-dependent density functional theory indicate intensive light absorption in the near-infrared region and broadening to 2,500 nm, making these materials suitable for numerous optoelectronic applications.

Facile Synthesis of Aryl-Substituted Cycloarenes via Bismuth(III) Triflate-Catalyzed Cyclization of Vinyl Ethers

Cycloarenes are an essential class of polycyclic aromatic hydrocarbons with unique electronic structure, but their synthesis is very challenging. Herein, we report a facile synthetic strategy prima...

First principles high pressure studies on group-14 element pernitrides

The search of ultra hard materials is inevitable in high pressure device applications. Nitrides of group-14 elements have been foreseen as potential candidates in replacing existing hard materials. In a recent experiment, pyrite structures of SiN2, GeN2 and SnN2 have been synthesized at high pressure and are shown to have high bulk modulus. Though their existence and bulk modulus are known, limited studies have been devoted to SiN2, GeN2 and SnN2. In the present work, we have performed the first principles calculations to investigate structural, electronic, mechanical and vibrational properties at ambient as well as high pressure condition. The physical properties of SnN2 and high pressure lattice dynamical properties of SiN2 and GeN2 are explored for the first time. SiN2 has higher bulk modulus among all MN2 (where M = Si, Sn and Ge), and increases further with increase in pressure. The increase in elastic moduli of MN2 have been related to the shortening of M-N bond length at high pressure. Electronic properties of these pyrites suggest that the bandgap increases with pressure. We further characterize these MN2 at high pressure using theoretically calculated Raman spectra. Frequency of N–N stretching Ag and Tg modes confirms the single bond character of nitrogen dimer in all studied MN2 compounds.

Oxido‐Hydroxido‐ and Oxido‐Aminato‐Osmium(V) Complexes with a Cyclohexanediamine‐Based Tetradentate Ligand as Active Oxidants for Dihydroxylation and Aminohydroxylation of Alkenes

Six‐coordinate osmium(III) complexes coordinated by a cyclohexanediamine‐based tetradentate ligand [trans‐N,N′‐dimethyl‐N,N′‐bis(2‐pyridylmethyl)‐1,2‐cyclohexanediamine: BPMCN] are synthesized. The remaining two coordination sites are occupied by chloride (Cl–), triflate (OTf–), or hydroxide/water (OH–/H2O) to complete the octahedral osmium(III) centers; [OsIIIX2(BPMCN)]+ (X = Cl– or OTf–) and [OsIII(OH)(H2O)(BPMCN)]2+. The OsIII(OH)(H2O) complex catalyzes cis‐dihydroxylation and cis‐aminohydroxylation of styrene with hydrogen peroxide (H2O2) and chloramine‐T (TsNClNa) to yield the corresponding diol and aminoalcohol, respectively. The oxido‐hydroxido‐osmium(V) complex {[OsV(O)(OH)(BPMCN)]2+} is isolated as the active oxidant of the dihydroxylation. The oxidation of the OsIII(OH)(H2O) complex with chloramine‐T yields oxido‐aminato‐osmium(V) complex, [OsV(O)(NHTs)(BPMCN)]2+. Comparison of the crystal structure with those of the oxido‐hydroxido‐osmium(V) complex and related imido‐ and amino‐osmium complexes indicates that the –NHTs group works as a monoanionic aminate ligand and the osmium(V)–NHTs bond has a single bond character [Os–N; 2.058(4) Å]. The oxido‐aminato‐osmium(V) complex is revealed as the active oxidant for the aminohydroxylation by the direct reaction of the complex with styrene. The characterization of the complex allows us to propose the catalytic mechanism of the aminohydroxylation of alkenes.

Enthalpy-Driven Transition of Liquid Crystalline Textures of Poly(2-cyano-p-phenylene terephthalamide) in N,N-Dimethyl Acetamide/Lithium Chloride

The solution of poly(2-cyano-p-phenylene terephthalamide) (CY-PPTA) in N,N-dimethyl acetamide/lithium chloride exhibited a concentration-dependent transition of liquid crystalline textures from nematic-rich to cholesteric-rich with increasing concentration above the critical concentration. The 2D correlation analysis of the FTIR spectra revealed that the dipolar interactions between cyano groups of the polymer increased as concentration increased. The polarized cyano group withdrew the delocalized π-electrons in the C-N bond of the amide group of the polymer, increasing its single bond character. The molecular simulation based on this spectroscopic evidence proposed a change of molecular shapes of CY-PPTA from linear rigid rod to semi-rigid helix. The semi-rigid helical conformation where the single bond character was prevalent over the delocalized one in the C-N bond would develop a cholesteric liquid crystalline texture.

Strong pH-Dependent Near-Infrared Fluorescence in a Microbial Rhodopsin Reconstituted with a Red-Shifting Retinal Analogue.

Near-infrared (NIR)-driven rhodopsins are of great interest in optogenetics and other optobiotechnological developments such as artificial photosynthesis and deep-tissue voltage imaging. Here we report that the proton pump proteorhodopsin (PR) containing a NIR-active retinal analogue (PR:MMAR) exhibits intense NIR fluorescence at a quantum yield of 3.3%. This is 130 times higher than native PR (LenzM. O.; Biophys J.2006, 91, 255−26216603495) and 3–8 times higher than the QuasAr and PROPS voltage sensors (KraljJ.; Science2011, 333, 345−34821764748; HochbaumD. R.; Nat. Methods2014, 11, 825–83324952910). The NIR fluorescence strongly depends on the pH in the range of 6–8.5, suggesting potential application of MMAR-binding proteins as ultrasensitive NIR-driven pH and/or voltage sensors. Femtosecond transient absorption spectroscopy showed that upon near-IR excitation, PR:MMAR features an unusually long fluorescence lifetime of 310 ps and the absence of isomerized photoproducts, consistent with the high fluorescence quantum yield. Stimulated Raman analysis indicates that the NIR-absorbing species develops upon protonation of a conserved aspartate, which promotes charge delocalization and bond length leveling due to an additional methylamino group in MMAR, in essence providing a secondary protonated Schiff base. This results in much smaller bond length alteration along the conjugated backbone, thereby conferring significant single-bond character to the C13=C14 bond and structural deformation of the chromophore, which interferes with photoinduced isomerization and extends the lifetime for fluorescence. Hence, our studies allow for a molecular understanding of the relation between absorption/emission wavelength, isomerization, and fluorescence in PR:MMAR. As acidification enhances the resonance state, this explains the strong pH dependence of the NIR emission.

Ab initio ro-vibronic spectroscopy of the Π2 PCS radical and Σ+1PCS- anion.

Near-equilibrium potential energy surfaces have been calculated for both the PCS radical and its anion using a composite coupled cluster approach based on explicitly correlated F12 methods in order to provide accurate structures and spectroscopic properties. These transient species are still unknown and the present study provides theoretical predictions of the radical and its anion for the first time. Since these species are strongly suggested to play an important role as intermediates in the interstellar medium, the rotational and vibrational spectroscopic parameters are presented to help aid in the identification and assignment of these spectra. The rotational constants produced will aid in ground-based observation. Both the PCS radical and the PCS- anion are linear. In the PCS- anion, which has a predicted adiabatic electron binding energy (adiabatic electron affinity of PCS) of 65.6 kcal/mol, the P-C bond is stronger than the corresponding neutral radical showing almost triple bond character, while the C-S bond is weaker, showing almost single bond character in the anion. The PCS anion shows a smaller rotational constant than that of the neutral. The ω3 stretching vibrational frequencies of PCS- are red-shifted from the radical, while the ω1 and ω2 vibrations are blue-shifted with ω1 demonstrating the largest blue shift. The ro-vibronic spectrum of the PCS radical has been accurately calculated in variational nuclear motion calculations including both Renner-Teller (RT) and spin-orbit (SO) coupling effects using the composite potential energy near-equilibrium potential energy and coupled cluster dipole moment surfaces. The spectrum is predicted to be very complicated even at low energies due to the presence of a strong Fermi resonance between the bending mode and symmetric stretch, but also due to similar values of the bending frequency, RT, and SO splittings.

The nature of M–B and B–N bonding in iminoboryl complexes of rhodium and iridium cis,mer-[(L)3(Br)2M(B[tbnd]NSiMe3)] (L = PMe3, CO): Dispersion corrected DFT study

Dispersion corrected quantum-chemical calculations of iminoboryl complexes cis,mer-[(L)3(Br)2M(BNSiMe3)] (M = Rh, Ir; L = PMe3, CO) were studied with and without dispersion interactions using density functionals BP86, BLYP, PBE, PW91 and TPSS. The calculations of the 11B NMR chemical shifts were carried out at DFT-D3(BJ)/BP86/TZ2P/ZORA with scalar and spin orbit relativistic level of theory in solvent C6D6. The calculated geometrical parameters of the rhodium iminoboryl complex cis,mer-[(Me3P)3(Br)2Rh(BNSiMe3)] (Braunschweig et al., J. Am. Chem. Soc. 130 (2008) 7974–7983) are in good agreement with the available experimental values. The calculated M−B and B–N bond distances, Mayer bond orders and Pauling bond orders suggest that the M–B bond has more than single bond character and B–N bond has less than triple bond character. The calculated bond dissociation energies (BDEs) of the M−B bonds follow the order BLYP < BP86 < TPSS < PBE < PW91. The absolute value of orbital interactions is significantly larger than the electrostatic interactions. The 11B NMR shielding is larger for carbonyl complexes cis,mer-[(CO)3Br2M(BNSiMe3)] than the phosphine complexes cis,mer-[(PMe3)3Br2M(BNSiMe3)]. On going from rhodium to iridium in the complexes cis,mer-[(L)3(Br)2M(BNSiMe3)] (L = PMe3, CO), the 11B NMR chemical shift decreases. The calculation of 11B chemical shifts by means of the ZORA relativistic scalar yields reliable results.

Crystal structure of N-[3-(di-methyl-aza-nium-yl)prop-yl]-N',N',N'',N''-tetra-methyl-N-(N,N,N',N'-tetra-methyl-form-am-id-in-ium-yl)-guanidinium dibromide hydroxide monohydrate.

The asymmetric unit of the title hydrated salt, C15H37N6 3+·2Br−·OH−·H2O, contains one cation, three partial-occupancy bromide ions, one hydroxide ion and one water mol­ecule. Refinement of the site-occupancy factors of the three disordered bromide ions converges with occupancies 0.701 (2), 0.831 (2) and 0.456 (2) summing to approximately two bromide ions per formula unit. The structure was refined as a two-component inversion twin with volume fractions 0.109 (8):0.891 (8) for the two domains. The central C3N unit of the bis­amidinium ion is linked to the aliphatic propyl chain by a C—N single bond. The other two bonds in this unit have double-bond character as have the four C—N bonds to the outer NMe2 groups. In contrast, the three C—N bonds to the central N atom of the (di­methyl­aza­nium­yl)propyl group have single-bond character. Delocalization of the two positive charges occurs in the N/C/N and C/N/C planes, while the third positive charge is localized on the di­methyl­ammonium group. The crystal structure is stabilized by O—H⋯O, N—H⋯Br, O—H⋯Br and C—H⋯Br hydrogen bonds, forming a three-dimensional network.

Crystal structure of N''-benzyl-N''-[3-(benzyl-dimethyl-aza-nium-yl)prop-yl]-N,N,N',N'-tetra-methyl-guanidinium bis-(tetra-phenyl-borate).

In the crystal structure of the title salt, C24H38N4 2+·2C24H20B−, the C—N bond lengths in the central CN3 unit of the guanidinium ion are 1.3364 (13), 1.3407 (13) and 1.3539 (13) Å, indicating partial double-bond character. The central C atom is bonded to the three N atoms in a nearly ideal trigonal–planar geometry and the positive charge is delocalized in the CN3 plane. The bonds between the N atoms and the terminal methyl groups of the guanidinium moiety and the four C—N bonds to the central N atom of the (benzyl­dimethyl­aza­nium­yl)propyl group have single-bond character. In the crystal, C—H⋯π inter­actions between the guanidin­ium H atoms and the phenyl C atoms of the tetra­phenyl­borate ions are present, leading to the formation of a two-dimensional supra­molecular pattern parallel to the ac plane.

Crystal structure of (1-eth-oxy-ethyl-idene)di-methyl-aza-nium tetra-phenyl-borate.

In the cation of the title salt, C6H14NO+·C24H20B−, the C—N bond lengths are 1.297 (2), 1.464 (2) and 1.468 (2) Å, indicating double- and single-bond character, respectively. The C—O bond length of 1.309 (2) Å shows double-bond character, pointing towards charge delocalization within the NCO plane of the iminium ion. In the crystal, C—H⋯π inter­actions between the iminium H atoms and the phenyl C atoms of the anion are present. The phenyl rings form aromatic pockets, in which the iminium ions are embedded.

Structural and energetic characterization of anhydrous and hemihydrated 2-mercaptoimidazole: Calorimetric, X-ray diffraction, and computational studies

This paper reports an experimental and theoretical study on the structural and energetic characterization of the 2-mercaptoimidazole (2-MI) in the solid and in the gaseous phases. The single crystal X-ray diffraction determinations on the anhydrous and hemihydrate 2-MI forms were carried out at T=(296±2)K and T=(150±2)K, respectively, and suggest that in both forms the 2-MI molecule is closer to the thione conformation, albeit some single bond character is possible.The energy of combustion of the title compound was measured by rotating-bomb combustion calorimetry, being used to derive the corresponding enthalpy of formation in the crystalline-phase. The enthalpy of sublimation of 2-MI, at T=298.15K, was obtained from high temperature Calvet microcalorimetry measurements. These two parameters yielded the gas-phase enthalpy of formation, allowing the inherent energetic analysis of the molecule. This result was discussed together with the corresponding predictions for 2-MI and its tautomer, 1,3-dihydro-2H-imidazole-2-thione, by the G3 method.The dehydration reaction of 2-MI·0.5H2O(cr) was also investigated and the corresponding enthalpy of dehydration was determined by Calvet microcalorimetry.

Crystal structure of (eth-oxy-ethyl-idene)di-methyl-aza-nium ethyl sulfate.

In the title salt, C6H14NO+·C2H5SO4 −, the C—N bond lengths in the cation are 1.2981 (14), 1.4658 (14) and 1.4707 (15) Å, indicating double- and single-bond character, respectively. The C—O bond length of 1.3157 (13) Å shows double-bond character, indicating charge delocalization within the NCO plane of the iminium ion. In the crystal, C—H⋯O hydrogen bonds between H atoms of the cations and O atoms of neighbouring ethyl sulfate anions are present, generating a three-dimensional network.