Nonlinear absorption of new mesoionic compounds

The nonlinear absorption of new mesoionic compounds (MIC) was investigated using nanosecond laser pulses with wavelengths at 570, 605 and 618 nm. Nonlinear absorption cross-sections, σ₂ ≈ 10⁻⁴⁵ cm⁴ s/photon, larger than any recorded in the literature, were obtained due to the introduction of p-CF₃-C₆H₄ electron-acceptor and p-CH₃-C₆H₄ or p-CH(CH₃)₂-C₆H₄ electron-donor groups in the MIC ring. The large values of σ₂, the ease of synthesis of these compounds and their marked stability, make them promising candidates for photonic applications.     

Binuclear transition metal complexes on gold: Molecular orientation by angular dependent NEXAFS spectroscopy

The influence of the transition metals on the molecular orientation and molecule-substrate interaction has been investigated by angular dependent NEXAFS spectroscopy for the recently synthesised dialkynyl bridged complexes M₂-DEBP (Cl(PBu₃)₂M-CC-C₆H₄-C₆H₄-CCM(PBu₃)₂Cl, M = Pt, Pd; DEBP = diethynyl-biphenyl, i.e. CC-C₆H₄-C₆H₄-CC-). Thin films of both samples have been deposited on Au/Si(1 1 1), and the angular dependent analysis of the main π^∗ feature deriving by the superimposition of the resonances due to benzene and acetylene carbon orbitals showed a polarisation effect for Pd₂-DEBP only. A tendency to a preferential molecular orientation at nearly 50° to the surface was calculated. Furthermore, for Pd₂-DEBP, the two π^∗ resonances already assigned to benzene and acetylene carbon atoms showed different angular effects; a likely explanation for this behaviour bear in mind the interaction between sp and sp² carbons of the organic DEBP moieties with Pd centres of neighbouring macromolecules, giving rise to interchain interactions then leading to an enhancement of the already assessed self-assembling properties.     

A shock tube investigation of H atom production from the thermal dissociation of ortho-benzyne radicals

The thermal dissociation of ortho-benzyne (o-C₆H₄) has been studied behind reflected shock waves under very isolated conditions. In the shock tube experiments 1,2-diiodobenzene was employed as a thermal source for the o-C₆H₄ radical. For different series of experiments the temperature ranged from 1600 to 2400 K at pressures between 1.4 and 2 bar. Very low initial concentrations of the radical precursor, 0.5-4 ppm diluted in argon, were used. In situ atomic resonance absorption measurement of iodine atoms formed during the thermal dissociation of the radical precursor molecule provides for a precise determination of the initial 1,2-diiodobenzene concentration. ARAS (atomic resonance absorption spectroscopy) was also used to record the absorption profiles of hydrogen atoms obtained during the pyrolysis of ortho-benzyne. From the measured H atom absorption profiles and by taking into account recent results from literature, it is shown that the thermal dissociation of ortho-benzyne occurs via two pathways: besides the molecular route:

(R1a)     

Orientation polarization and dielectric dispersion characteristics of nonpolar liquid solutions

Taking account of dipole-multipole interactions of molecules, the Debye dispersion _p — _p is computed in nonpolar liquid solutions. The influence of the multipole interactions between molecules of different components of the solution on the Debye dispersion is demonstrated. It is shown that the theory is in good agreement with experiment for the solutions CCl₄-C₆H₆, CCl₄-CS₂, C₆H₆-CS₂, and C₄H₈O₂-C₆H₆.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 13–17, November, 1986.     

Site-blocking effects of preadsorbed H on Pt(1 1 1) probed by 1,3-butadiene adsorption and reaction

The influence of hydrogen coadsorption on hydrocarbon chemistry on transition metal surfaces is a key aspect to an improved understanding of catalytic selective hydrogenation. We have investigated the effects of H preadsorption on adsorption and reaction of 1,3-butadiene (H₂CCHCHCH₂, C₄H₆) on Pt(1 1 1) surfaces by using temperature-programmed desorption (TPD) and Auger electron spectroscopy (AES). Preadsorbed hydrogen adatoms decrease the amount of 1,3-butadiene chemisorbed on the surface and chemisorption is completely blocked by the hydrogen monolayer (saturation) coverage (θ_H = 0.92 ML). No hydrogenation products of reactions between coadsorbed H adatoms and 1,3-butadiene were observed to desorb in TPD experiments over the range of θ_H investigated (θ_H = 0.6-0.9 ML). This is in strong contrast to the copious evolution of ethane (CH₃CH₃, C₂H₆) from coadsorbed hydrogen and ethylene (CH₂CH₂, C₂H₄) on Pt(1 1 1). Hydrogen adatoms effectively (in a 1:1 stoichiometry) remove sites from interaction with chemisorbed 1,3-butadiene, but do not affect adjacent sites. The adsorption energy of coadsorbed 1,3-butadiene is not affected by the presence of hydrogen on Pt(1 1 1). The chemisorbed 1,3-butadiene on hydrogen preadsorbed Pt(1 1 1) completely dehydrogenates to H₂ and surface carbon upon heating without any molecular desorption detected, which is identical to that observed on clean Pt(1 1 1). In addition to revealing aspects of site blocking that should have broad implications for hydrogen coadsorption with hydrocarbon molecules on transition metal surfaces in general, these results also provide additional basic information on the surface science of selective catalytic hydrogenation of butadiene in butadiene-butene mixtures.     

A computational study on the adsorption and ring cleavage of para-chlorophenol on anatase TiO2 surface

In this work, a computational technique based on semiempirical SCF MO method MSINDO, has been used for investigation of the adsorption and photocleavage of para-chlorophenol (p-CP) molecule on the anatase TiO₂ (0 0 1) and (1 0 0) surfaces. The surfaces have been modeled with two saturated clusters Ti₂₁O₅₈H₃₂ and Ti₃₆O₉₀H₃₆. The optimization of the perpendicular conformation of p-CP molecule relative to the anatase TiO₂ (1 0 0) surface, has resulted in a linkage of the molecule to the surface titanium atom via phenolic oxygen atom. We studied the aromatic ring cleavage by singlet oxygen (¹O₂) and superoxide radical anion () and accordingly, relevant mechanisms are suggested. The results reveal that the ring opening path of p-CP molecule on TiO₂ (1 0 0) surface, following the single electron transfer/ mechanism, is energetically more favourable than the ¹O₂/dioxetane mechanism.     

Structural impact on the methano bridge in norbornadiene, norbornene and norbornane

An electronic structural study of the ground electronic states for the chemically similar bicyclic norbornadiene (NBD, C₇H₈, X¹A₁), norbornene (NBN, C₇H₁₀, X¹A′) and norbornane (NBA, C₇H₁₂, X¹A₁) molecules is provided quantum mechanically. Initially, the unique orbital imaging capability of electron momentum spectroscopy is used to validate which of the quantum mechanical models available to us for these calculations best represents these species. Thereafter, individual molecular point group symmetry is incorporated in the calculations with energy minimization in the search for equilibrium geometries of the species using MP2/TZVP and B3LYP/TZVP models. The optimized geometries compare favourably with available crystallographic results and also build confidence in cases where the crystallographic results are ambiguous. The present study aims to reveal the particular subtle structural deviation of the species, which results in significant molecular property differences among these organic compounds. This work intends to probe bonding information of the species and the impact, on the seven member carbon skeleton, as the CC double bonds of NBD are progressively saturated by hydrogen atoms to give NBN and NBA. Significant changes observed through the present work include: (i) the seven member carbon skeleton tends to relax the strain whenever possible and (ii) the ethano ring experiences greater structural changes than the methano bridge. The methano bridge (C₍₁₎-C₍₇₎-C₍₄₎) of the less symmetric NBN molecule (C_s) tilts to the single C-C bond side of the ethano ring of the molecule (rather than the CC side), producing a dihedral angle of 8.7° between plane H-C₍₁₎-C₍₄₎ (the yz-plane) and plane C₍₁₎-C₍₇₎-C₍₄₎. Our work suggests that it is this unique dihedral angle in NBN which causes the molecules exo-reactivity and is also responsible for the extra activity of its CC bond.     

Crystal structure and structural transition caused by charge-transfer phase transition for iron mixed-valence complex (n-C3H7)4N[FeFe(dto)3] (dto=C2O2S2)

(n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] shows a new type of first order phase transition called charge-transfer phase transition around 120 K, where the charge transfer between Fe^II and Fe^III occurs reversibly. Recently, we have succeeded in obtaining single crystals of the title complex and determined the crystal structure at room temperature. Crystal data: space group P6₃, Z=2. Moreover, we have investigated the structural transition caused by the charge-transfer phase transition by means of powder X-ray diffraction measurement. When the temperature is decreased, the a-axis, which corresponds to the hexagonal ring size in two-dimensional honeycomb network structure of [Fe^IIFe^III(dto)₃]_∞, contracts by 0.1 Å at the charge-transfer transition temperature (T_CT), while the c-axis, perpendicular to the honeycomb network layer, elongates by 0.1 Å at T_CT. Consequently, when the temperature is decreased, the unit cell volume decreases without noticeable anomaly around T_CT, which is responsible for the quite small vibrational contribution to the entropy change, compared with usual spin crossover transition. Thus, the charge-transfer phase transition around 120 K for (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] is regarded as spin entropy driven phase transition.     

Electrochemical stability of self-assembled monolayers of biphenyl based thiols studied by cyclic voltammetry and second harmonic generation

The reductive desorption of self-assembled monolayers (SAMs) of ω-(4′-methyl-biphenyl-4-yl)-alkanethiols (CH₃-C₆H₄-C₆H₄-(CH₂)_n-SH, BPn) on Au(1 1 1) on mica was studied in 0.5 M KOH solution as a function of the length of the aliphatic spacer chain (n = 1-6 and 12) and for two different preparations temperatures (295 K and 343 K). Second harmonic generation (SHG) was applied in situ parallel to cyclic voltammetry (CV). Odd-even differences in the structure of the BPn monolayers are clearly reflected in the electrochemical stability, as well as by the charge and shape of the desorption peak. For n = 1-5 a single desorption peak is detected whereas multiple peaks occur for BP6 similar to hexadecane thiol which was also studied for comparison. An increased preparation temperature affects the shape and width of the desorption peak but not the position. BP1 exhibits a temperature dependence different from the other homologues. The relationship between coverage monitored by SHG and desorption charge determined from the CVs is found to be linear and surprisingly independent from the details of the SAMs. The combined SHG and CV experiments suggest that capacitive and faradaic current are always closely coupled even for BP6 and hexadecane thiol which exhibit multiple desorption peaks.     

Ultrasound irradiation promotes the synthesis of new 1,2,4-triazolo[1,5-a]pyrimidine

Ultrasonic irradiation was used in the synthesis of a series of novel 1,2,4-triazolo[1,5-a]pyrimidines. The products were synthetized from the cyclocondensation reaction of 1,1,1-trifluoro-4-metoxy-3-alken-2-one [CF₃C(O)CHC(R)OMe, where R = Ph, 4-F-C₆H₄, 4-Br-C₆H₄, 4-I-C₆H₄, 4-CH₃-C₆H₄, 4-CH₃O-C₆H₄, Thien-2-yl, Biphen-4-yl] or β-enaminones [RC(O)CHCHNMe₂, where R = Ph, 4-F-C₆H_4, 4-Br-C₆H_4, 4-I-C₆H_4, 4-CH₃-C₆H₄, 4-CH₃O-C₆H₄, 4-NO₂-C₆H₄, Thien-2-yl, Biphen-4-yl, Naphth-2-yl, Pyrrol-2-yl, CCl₃] with 5-amino-1,2,4-triazole in acetic acid at 99 °C with 5–17 min of ultrasound irradiation. This methodology has shown several advantages, such as shorter reaction times, mild conditions, high regioselectivity, and excellent yields, when compared with conventional thermal heating (oil bath).     

UV excimer laser photochemistry of hybrid organometallic compounds of gallium

The gas phase ultraviolet (UV) excimer laser induced photolysis of the gallium-alkyls Ga(t-C₄H₉) _n ^– (CH₃)_3–n (n=0, 1, 2, 3) was studied, using photolysis wavelengths of 308, 248, and 193 nm. The photofragments Ga, GaH, and GaCH₃ were detected by laser ionization time-of-flight mass spectroscopy, while the hydrocarbon products CH₄, C₂H₆, HC(CH₃)₃ and H₂C=C(CH₃)₂ were identified using Fourier transform infrared (FTIR) spectroscopy. The formation of the GaH photofragment, and a high olefin-to-alkane product ratio, for Ga(t-C₄H₉)₂(CH₃) and Ga(t-C₄H₉)₃ are interpreted to indicate a -hydrogen elimination process. However, -hydrogen elimination only occurs after fission of the weakest Ga-C bond, thus no -hydride elimination is observed for Ga(t-C₄H₉)(CH₃)₂. Detection of C₂H₆ for Ga(CH₃)₃ and Ga(t-C₄H₉)(CH₃)₂, but not for Ga(t-C₄H₉)₂(CH₃), shows that under our experimental conditions the formation of ethane is as a result of the reductive elimination of the methyl groups, and is not due to the recombination of two free methyl radicals.     

The elastic behavior of dense C3N4 under high pressure: First-principles calculations

We have carried a detailed theoretical study on the geometry, density of states, elastic properties, sound velocities and Debye temperature of α-, β-, c- and p-C₃N₄ compounds under a maximum of pressure up to 100 GPa by using first principles calculations. The optimized lattice constants under zero pressure and zero temperature agreed well with the previous experimental and theoretical results. The band gaps of the four types of dense C₃N₄ were widened gradually with the increase of pressure. The calculated Poisson’s ratio γ and B/G values suggest α-, c- and p-C₃N₄ are brittle materials under 0–100 GPa, whereas β-C₃N₄ will become a ductile material as external pressure reaches 57 GPa. We found that the Debye temperature of the four dense C₃N₄ gradually reduces in the order of c-C₃N₄>p-C₃N₄>α-C₃N₄>β-C₃N₄ at 0 GPa and 0 K. However, the Debye temperature of c-C₃N₄ was lower than p-C₃N₄ when external pressure exceeds 6.3 GPa. It may hint that the results could be served as a valuable prediction for further experiments.     

Cyclopentadiene combustion in a plug flow reactor near 1150 K

Cyclopentadienyl (CPDyl) was generated for study by oxidizing and pyrolizing 1,3-cyclopentadiene (CPD) in Princeton’s adiabatic, atmospheric pressure flow reactor. This study used nitrogen carrier gas, initial CPD concentrations from 1000 to 3000 ppm by volume (ppmv), equivalence ratios from fuel lean (? = 0.6) to pyrolytic conditions (? = 100) and initial temperatures from 1100 to 1200 K. The reaction progress was followed from 5 to 150 ms using a water cooled sample probe and GC-FID analysis of C₁-C₁₄ species. The oxidation results show that CPD and CPDyl react via 19 pathways to yield 22 hydrocarbon intermediates. Analysis of the oxidative CPDyl ring opening pathways reveals the importance of the 2,4-cyclopentadienoxy (c-C₅H₅O) β-scission reaction: c-C₅H₅O ↔ CHCH-CHCH-CHO. The fastest theoretical mechanism has a calculated unimolecular high-pressure rate constant of 2.00 × 10¹³e^−7215/T s⁻¹ which is seven orders of magnitude larger at 1150 K than the previous literature estimate. Cyclopentadienone (CPDone) has been assumed to be an important intermediate in C₅ ring oxidation even though it has not been unambiguously identified in the combustion environment. A detection limit of 20 ppmv for CPDone in the present apparatus failed to note any CPDone. A set of mechanistic pathways for the C₅ ring oxidation includes steps to avoid unrealistic CPDone production is presented. The complex mechanism illustrates the need for detailed models to understand the combustion of aromatics and soot precursors. The article stresses the importance of CPDyl in the formation of aromatic rings during combustion, which subsequently leads to polycyclic aromatic hydrocarbons (PAH) and soot precursors.     

Crystal structure and ferromagnetism of (n-C3H7)4N[CoFe(dto)3] (dto=C2O2S2)

Recently, we have discovered a new type of first order phase transition around 120 K for (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] (dto=C₂O₂S₂), where the charge transfer transition between Fe^II and Fe^III occurs reversibly. In order to elucidate the origin of this peculiar first order phase transition. Detailed information about the crystal structure is indispensable. We have synthesized the single crystal of (n-C₃H₇)₄N[Co^IIFe^III(dto)₃] whose crystal structure is isomorphous to that of (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃], and determined its detailed crystal structure. Crystal data: space group P6₃, a=b=10.044(2) Å, c=15.960(6) Å, α=β=90°, γ=120°, Z=2 (C₁₈H₂₈NS₆O₆FeCo). In this complex, we found a ferromagnetic transition at T_c=3.5 K. Moreover, on the basis of the crystal data of (n-C₃H₇)₄N[Co^IIFe^III(dto)₃], we determined the crystal structure of (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] by simulation of powder X-ray diffraction results.     

Dielectric relaxation in single crystal NH4H2PO4 in the high-temperature regime

The dispersion curves of the dielectric response in single crystal NH₄H₂PO₄ were obtained in the radio frequency range and below the high-temperature transition at T_p−160 °C. The results reveal dielectric relaxation at low frequency, which is about 10⁵ Hz at 70 °C, and it shifts to higher frequencies (∼3×10⁶ Hz) as the temperature increases. The relaxation frequency was determined from the peak obtained in the imaginary part of the permittivity as well as from the derivative of the real part of the permittivity. The activation energy E_a=0.55 eV, obtained from the relaxation frequency is very close to that derived from the dc conductivity. We suggest that this dielectric relaxation could be due to the proton jump and phosphate reorientation that cause distortion and change the local lattice polarizability inducing dipoles like      

Density functional study of hypophosphite adsorption on Ni (1 1 1) and Cu (1 1 1) surfaces

Surface structures and electronic properties of hypophosphite, H₂PO₂⁻, molecularly adsorbed on Ni(1 1 1) and Cu(1 1 1) surfaces are investigated in this work by density functional theory at B3LYP/6-31++g(d, p) level. We employ a four-metal-atom cluster as the simplified model for the surface and have fully optimized the geometry and orientation of H₂PO₂⁻ on the metal cluster. Six stable orientations have been discovered on both Ni (1 1 1) and Cu (1 1 1) surfaces. The most stable orientation of H₂PO₂⁻ was found to have its two oxygen atoms interact the surface with two PO bonds pointing downward. Results of the Mulliken population analysis showed that the back donation from 3d orbitals of the transition metal substrate to the unfilled 3d orbital of the phosphorus atom in H₂PO₂⁻ and 4s orbital's acceptance of electron donation from one lone pair of the oxygen atom in H₂PO₂⁻ play very important roles in the H₂PO₂⁻ adsorption on the transition metals. The averaged electron configuration of Ni in Ni₄ cluster is 4s^0.634p^0.023d^9.35 and that of Cu in Cu₄ cluster is 4s^1.004p^0.033d^9.97. Because of this subtle difference of electron configuration, the adsorption energy is larger on the Ni surface than on the Cu surface. The amount of charge transfers due to above two donations is larger from H₂PO₂⁻ to the Ni surface than to the Cu surface, leading to a more positively charged P atom in Ni_nH₂PO₂⁻ than in Cu_nH₂PO₂⁻. These results indicate that the phosphorus atom in Ni_nH₂PO₂⁻ complex is easier to be attacked by a nucleophile such as OH⁻ and subsequent oxidation of H₂PO₂⁻ can take place more favorably on Ni substrate than on Cu substrate.     

Benzene thermal synthesis and characterization of crystalline carbon nitride

A new solvothermal route has been successfully used to prepare crystalline carbon nitride powder from 1,3,5-trichlorotriazine (C₃N₃Cl₃) and lithium nitride (Li₃N) in benzene at 360 °C and 6–7 MPa. The as-prepared sample was brown and was analyzed by X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR). The results show that the powder mainly consists of -C₃N₄, -C₃N₄ and some unidentified carbon-nitrogen crystalline phases. The experimental lattice constants of -C₃N₄ (a=6.48 Å,c=4.72 Å) and -C₃N₄ (a=6.43 Å,c=2.47 Å) match the latest ab-initio calculations (a=6.47 Å and c=4.72 Å for -C₃N₄, a=6.40 Å and c=2.40 Å for -C₃N₄) quite well. The relative nitrogen-to-carbon composition ratio is 0.76. Only C–N and C=N bonds were demonstrated by XPS and FTIR. The feasibility of this synthetic method is discussed and this approach may provide a possible and very effective way to realize the growth of pure crystalline carbon nitride materials, which is quite different from the conventional solid-state reactions (SSR). PACS 81.10.-h; 81.10.Dn; 81.05.Zx; 61.66.Fn; 42.70.Nq     

Crystal structure and thermal behavior of two new organic monosulfates NH3(CH2)5NH3SO4 1.5H2O and NH3(CH2)9NH3SO4 H2O

The chemical preparation, the calorimetric studies and the crystal structure are given for two new organic sulfates NH₃(CH₂)₅NH₃SO₄ 1.5H₂O (DAP-S) and NH₃(CH₂)₉NH₃SO₄·H₂O (DAN-S). DAP-S is monoclinic P2₁/n with unit cell dimensions: a=11.9330(2) Å; b=10.9290(2) Å; c=17.5260(2) Å; β=101.873(1)°; V=2236.77(6) Å³; and Z=8. Its atomic arrangement is described as inorganic layers of units and water molecules separated by organic chains. DAN-S is monoclinic P2₁/c with unit cell parameters: a=5.768(2) Å; b=25.890(10) Å; c=11.177(5) Å; β=115.70(4)°; V=1504.0(11) Å³ and Z=4. Its structure exhibits infinite chains, parallel to the [100] direction where the organic cations are interconnected. In both structures a network of strong and weak hydrogen bonds connects the different components in the building of the crystal.     

Microwave and high resolution infrared spectra of vinyl chloride, ab initio anharmonic force field and equilibrium structure

The quadratic, cubic, and semi-diagonal quartic force field of vinyl chloride has been calculated at the MP2 level of theory employing a basis set of triple-ζ quality. The spectroscopic constants derived from this force field are compared with the experimental values. To make this comparison more complete, the rotational constants of the lowest excited state, v₉ = 1 at 395 cm⁻¹ have been determined by microwave spectroscopy and the ν₁₂ band (around 618 cm⁻¹) has been investigated by high-resolution infrared Fourier transform spectroscopy. The equilibrium structure has been derived from experimental ground state rotational constants and ab initio rovibrational interaction parameters. This semi-experimental structure is in excellent agreement with the ab initio structure calculated at the CCSD(T) level of theory using a basis set of quintuple-ζ quality and a core correlation correction. The experimental mass-dependent r_m structures are also determined and their accuracy is discussed. The recommended equilibrium geometry is: r (CC) = 1.3262(10), r (CCl) = 1.7263(10), r (CH_g) = 1.0784(10), r (CH_c) = 1.0795(10), r (CH_t) = 1.0797(10), ∠(CCCl) = 122.77(10)°, ∠(CCH_g) = 123.86(10)°, ∠(CCH_c) = 121.80(10)°, ∠(CCH_t) = 119.29(10)°.     

Stranski-Krastanov like oxide growth on Ag(1 1 1) at atmospheric oxygen pressures

The oxidation behavior of Ag(1 1 1) was studied by means of in situ surface X-ray diffraction at atmospheric oxygen pressure. Exposure to 1 bar oxygen at 773 K reveals a competing growth of three different oxygen-induced structures on Ag(1 1 1), namely the well-known p(4 × 4) reconstruction, a surface oxide in a p(7 × 7) coincidence structure and the bulk oxide Ag₂O in orientation. The latter two exhibit the same honeycomb on hexagon arrangement of the Ag sublattice with respect to the Ag(1 1 1) surface. An inverted stacking of Ag planes in the bulk oxide islands is observed as compared to the Ag(1 1 1) substrate, which sheds new light on the Ag₂O formation process. Finally, we present a structural model of the p(7 × 7) reconstruction, based on a three-layer O-Ag-O slab of Ag₂O(1 1 1).