Self,nitrogen and oxygen broadening of the 115-GHz line of carbon monoxide

Self nitrogen, oxygen and air-broadened half-widths of the 115-GHz line of CO have been measured at various temperatures between 293 K (room temperature) and 220 K. The temperature dependence of the broadening parameter CCO-XW is described by a power law CCO-XW (T) = CCO-XW(293 K)(T/293)-n co-x. The values of CCOW (293 K) and nCO-X are presented for each broadening gas X, X - CO, N2 and O2. The usual relation CCO-airW (T) = 0.78CCO−N2W(T) + 0.21CCO−O2 W(T) is found to be valid in the temperature and pressure ranges of the present experiments.     

Vibrational study of layered ZnPS3 compounds intercalated with [Co(η5 -C5H5)2+] and [Cr(η6 -C6H6)2+] CATIONS

Infrared and Raman spectra (10–3100 cm^?1) of the layered ZnPSs compound and of intercalates with [Co(η⁵ ? C₅H₅)₂⁺] and [Cr(η⁶ ? C₆H₆)₂⁺] cations in the polycrystalline state have been recorded 300–10 K temperature range. A complete assignment of the spectra is proposed in terms of PSs group motions, Zn²⁺ ion translations and [Co(η⁵ ? C₅H₅)₂⁺] or [Cr(η⁶ ? C₆H₆)₂⁺] internal vibrations. New low frequency for the [Co(η⁵-C₅H₅)₂⁺] intercalate at low temperature are assigned to librational and torsional modes of C₅H₅ rings. Moreover, the preresonance Raman spectra of this intercalate show a selective enhancement for the metal-ligand vibrations when the charge transfer band of the cobalticenium is approached. One concludes that guest molecules are intercalated under their cationic form, are weakly interacting with the host lattice and seem to be dynamically disordered at room temperature.     

Stoichiometric synthesis of fullerene compounds with lithium and sodium and analysis of their IR and EPR spectra

A modified method is proposed for preparing fullerene compounds with alkali metals in a solution. The compounds synthesized have the general formula Me _n C₆₀(THF)_x, where Me = Li or Na; n=1–4, 6, 8, or 12; and THF = tetrahydrofuran. The use of preliminarily synthesized additives MeC₁₀H₈ makes it possible to prepare fullerene compounds with an exact stoichiometric ratio between C ₆₀ ^n? and Me ⁺. The IR and EPR spectra of the compounds prepared are analyzed and compared with the spectra of their analogs available in the literature. The intramolecular modes T _u (1)-T _u (4) for the C ₆₀ ^n? anion are assigned. The splitting of the T _u (1) mode into a doublet at room temperature for Me _n C₆₀(THF)_x (n=1, 2, 4) compounds indicates that the fullerene anion has a distorted structure. An increase in the intensity of the T _u (2) mode, a noticeable shift of the T _u (4) mode toward the long-wavelength range, and an anomalous increase in the intensity of the latter mode for the Li₃C₆₀(THF)_x complex suggest that, in the fullerene anion, the coupling of vibrational modes occurs through the charge-phonon mechanism. The measured EPR spectra of lithium-and sodium-containing fullerene compounds are characteristic of C ₆₀ ^? anions. The g factors for these compounds are almost identical and do not depend on temperature. The g factor for the C ₆₀ ^n? anion depends on the nature of the metal and differs from the g factor for the C ₆₀ ^? anion.     

Non-radiative energy transfer from Sm3+→Nd3+ in sodium borate glass

A study of energy transfer from optically excited Sm³⁺ to Nd³⁺ in borate glass has been performed. Contrary to the observations made by Cabezas and DeShazer, the Sm³⁺ → Nd³⁺ energy transfer has been observed as non-radiative. Energy transfer probabilities (P_da) and transfer efficiencies (η_T) have been calculated from our measurements of donor fluorescence intensity and decay times. The mechanism governing the transfer is electrostatic dipole-dipole in nature, contrary to the conclusions made by Nakazawa and Shionoya. At low acceptor (Nd³⁺) concentrations a linear dependence of P_da on acceptor concentration (C) has been observed which suggests the migration of excitation energy among donors. At high acceptor (Nd³⁺) concentration a plot of P_da vs (C_o + C)², where C_o is donor ion concentration, presents a linear dependence which is consistent with the Fong-Drestler theory of dipole-dipole energy transfer mechanism and interaction of one donor (Sm³⁺) with two acceptors (Nd³⁺).     

Pressure-Tuning Raman Spectra of the Arene Chromium(0) Chalcocarbonyl Complexes, (η6-C6H5CO2CH3)Cr(CO)2(CX) (X = O,S)

The pressure-tuning Raman spectra of the two methylbenzoate complexes, (η⁶-C₆H₅CO₂CH₃)Cr(CO)₂(CX) (X = O, S), have been examined up to ～35 kbar. Structural changes occurred for both complexes in the 10–15 kbar pressure range, most probably as the result of second-order phase transitions. From the observed pressure dependences, replacement of a CO group in the piano-stool (η⁶-C₆H₅CO₂CH₃)Cr(CO)₃ molecule by a CS group has a marked influence on the Cr-arene ring vibrational modes, and the arene ring clearly plays a role in determining the nature of the Cr-CO and Cr-CS bonding interactions.     

A comparison between shake-up and UV charge-transfer transitions

The He(II) spectra of the unsubstituted metallocenes {M(η-C₅H₅)₂}, M  V, Cr, Mn, Fe, Co, Ni and Ru, and of {Mn(η-C₅H₄Me)₂} are reported; both He(I) and He(II) spectra of some decamethylmetallocenes {M(η-C₅Me₅)₂}, where M  Mg, V, Cr, Mn, Fe, Co and Ni, are also given. Intensity changes between the He(I) and He(II) spectra are shown to provide a reliable guide to band assignment. A ligand field treatment of the decamethylmanganocene cation, including limited configuration interaction, gives values for Δ₂, B and C; these values are also in good agreement with the photoelectron spectra of {M(η-C₅Me₅)₂} where M  V, Cr and Fe. Overlap between the ligand and metal “d” band structures prevents complete assignment in the cases of Co and Ni.     

The Magnetic structure of UIr

Low-temperature neutron-diffraction and magnetization measurements carried out on poly- and monocrystalline UIr are explained by a simple collinear ferromagnetic structure with magnetic moments of 0.6(3)μ_B/U atom oriented along [010] of the monoclinic cell described in space group P2₁. The bulk magnetic behavior of UIr (T_C = 46 K) is not substantially altered by a partial substitution of Ir by Rh, Pt (increase of T_C) or Os (decrease of T_C) and it is virtually reproduced in the isoelectronic compound UOs_0.5Pt_0.5.     

Study of molecular motions in liquids by electron spin-lattice relaxation measurements,I: Semiquinone ions in hydrogen bonding solvents

The electron spin-lattice relaxation times (T ₁) of a variety of semiquinone ions in hydrogen bonding solvents have been measured by the pulsed saturation recovery technique as a function of temperature (T) and viscosity (η) of the solvent. Also linewidths (ΔH) have been measured in suitable cases in such solvents at low radical concentrations (～10^?4 M). It is observed that (i) the temperature and viscosity dependence ofT ₁ can be fitted to an equation of the form 1/T ₁=A(T/η)+Bexp(-ΔE/RT) whereA andB are constants and ΔE is an activation energy of the order of 1 kcal mole^?1 for these systems; (ii)T ₁ is essentially independent of the radical concentration within the range 10^?3 to 5×10^?2 M; (iii) the concentration independent part of the linewidth (ΔH) increases linearly with (η/T) at sufficiently low temperatures, and (iv) the (η/T) dependent part ofT ₁ is sensitive to the size of the semiquinone as well as that of the solvent molecule, whereas the linewidth which is proportional to (η/T) at high viscosity, low temperature region is not sensitive to the size of the semiquinone and that of the solvent. Based on these observations, it is postulated that in hydrogen bonding solvents, three types of motion contribute significantly to electron spin relaxation:

1. A restricted small step diffusional motion, not involving large changes in the orientation of the molecule, leading to the dominant viscosity dependent contributions toT ₁ and ΔH, due to spin rotation interaction;
2. a large amplitude reorientation of the semiquinone, coupled to translational diffusion, resulting in viscosity dependent contributions toT ₁ and ΔH, throughg-modulation;
3. a hindred rotation of the semiquinone within the solvent cage, contributing toT ₁ due to spin rotation interaction.

The fact thatT ₁ is not sensitive to the concentration of the radicals, is ascribed to the formation of the solvent cage that prevents the close approach of radicals, thereby rendering radical-radical interactions to be weak mechanisms for relaxation, even at relatively high radical concentrations.     

Muon spin relaxation as a probe of molecular dynamics of organometallic compounds

LF‐Muon Spin Relaxation data are reported for the organometallic compounds Pb(C₆H₅)₄, (C₆H₆)Cr(CO)₃ and (C₅H₅)₂Ru. In each case the change in relaxation rate with temperature shows a peak analogous to the T_1 minimum in NMR. The activation parameters were calculated, and the mechanism of muon spin relaxation in the case of (C₆H₆)Cr(CO)₃ is shown to be the reorientation motion of the benzene ring. This revised version was published online in August 2006 with corrections to the Cover Date.     

Preparation and superconductivity of (B1−xCx)(Sr1−yBay)2Ca2Cu3Oz

Preparation of (B_1−xC_x)(Sr_1−yBa_y)₂Ca₂Cu₃O_z has been studied using high-pressure synthesis technique to improve sample quality. Samples were prepared from various starting compositions of 0.2≤x≤0.8 and 0≤y≤1. Nearly single-phased samples were obtained for (B_0.6C_0.4)(Sr_1−yBa_y)₂Ca₂Cu₃O_z (0≤y≤0.75) and (B_1−xC_x)-(Sr_0.25Ba_0.75)₂Ca₂Cu₃O_z (0.3≤x≤0.6). We have found that the partial substitution of Ba is effective to improve the sample quality as well as to enhance the T_c. The C substitution was also demonstrated to affect the sample preparation and the physical properties. Based on the substitution study, a maximum T_c of 120 K was observed for the sample with a starting composition of (B_0.65C_0.35)(Sr_0.3Ba_0.7)₂Ca₂Cu₃O_9+δ. Critical current density (J_c) and irreversibility field (H_irr) were estimated from magnetization measurements. The J_c at 77 K in a field of 1 T was about 1.1×10⁴ A/cm³ and the H_irr at 77 K was about 2.5 T. The H_irr was well-described by H_irr=a(1−T/T_c)ⁿ with a=39.1 and n=2.38.     

Thermal activation and reaction of allyl alcohol on Ni(100)

The thermal chemistry of allyl alcohol (CH₂CHCH₂OH) on a Ni(100) single-crystal surface was studied by the temperature programmed desorption (TPD) and the X-ray photoelectron spectroscopy (XPS). The allyl alcohol adsorbs molecularly on the metal surface at 100 K. Intact molecular desorption from the surface occurs at temperatures around 180 K, but some molecules exhibit chemical reactivity on the surface: activation of the OH, CC, and CO bonds produces η¹(O)-allyloxy CH₂CHCH₂O_(a), η²(C, C) allyl alcohol (C_(a)H₂C_(a)HCH₂OH), and η³(C, C, O)-alkoxide (C_(a)H₂C_(a)CH₂ O_(a)) intermediates. Further thermal activation of allyl alcohol on the surface yields propylene (CH₂CHCH₃), 1-propanol (CH₃CH₂CH₂OH), propanal (CH₃CH₂CHO), and combustion and dehydrogenation products (H₂O, H₂, and CO). Propylene desorbs from the surface at temperatures of around 270 K. Hydrogenation to the η³(C, C, O)-alkoxide intermediate leads to the production of propanal which desorbs from the surface around 320 K, while hydrogenation of the η²(C, C) allyl alcohol intermediate produces 1-propanol, which desorbs at around 310 K. The co-adsorption of hydrogen atoms on the surface enhances the formation of the saturated alcohol, while co-adsorption of oxygen enhances the formation of both the saturated alcohol and the saturated aldehydes.     

Pyrolysis of diethyl carbonate: Shock-tube and flow-reactor measurements and modeling

Shock-tube and flow-reactor experiments were used to study the thermal decomposition of diethyl carbonate (C₂H₅OC(O)OC₂H₅; DEC). The formation of CO₂, C₂H₄, and C₂H₅OH was measured with gas chromatography/mass spectrometry (GC/MS) and high-repetition-rate time-of-flight mass spectrometry (HRR-TOF-MS) behind reflected shock waves. The same products were also detected by GC/MS in flow reactor experiments. All experiments combined span a temperature range of 663–1203 K at pressures between 1.0 and 2.0 bar. Time-resolved species concentration profiles from HRR-TOF-MS and product compositions from GC/MS measurements were simulated applying a detailed reaction mechanism for DEC combustion. A master-equation analysis was conducted based on computed energies from G4 calculations. Quantum chemical calculations confirm that DEC primarily decomposes by six-center elimination, C₂H₅OC(O)OC₂H₅ → C₂H₄ + C₂H₅OC(O)OH (1a), followed by rapid decomposition of the alkoxy acid, C₂H₅OC(O)OH → C₂H₅OH + CO₂ (1b). Measured DEC decomposition rate constants k(T) at p ≈ 1.5 bar can be represented by the Arrhenius equation k(T) = 10^13.64±0.12 exp(?204.24±1.95 kJ/mol/RT) s ^{? 1}. Theoretical predictions for k_1a were in good agreement with experimentally derived values. The theoretical analysis also included dipropyl carbonate (C₃H₇OC(O)OC₃H₇; DPC) decomposition and the reactivities of DEC and DPC are compared and discussed in the context of reactivity of dialkyl carbonates under pyrolytic conditions.     

The chemisorption of Co on Rh(111)

The adsorption of CO on Rh(111) has been studied by thermal desorption mass spectrometry and low-energy electron diffraction (LEED). At temperatures below 180 K, CO adsorbs via a mobile precursor mechanism with sticking coefficient near unity. The activation energy for first-order CO desorption is 31.6 kcal/mole (ν_d = 10^13.6s^?1) in the limit of zero coverage.As CO coverage increases, a (√3 ×√3)R30^u overlayer is produced and then destroyed with subsequent formation of an overlayer yielding a (2 × 2) LEED pattern in the full coverage limit. These LEED observations allow the absolute assignment of the full CO coverage as 0.75 CO molecules per surface Rh atom. The limiting LEED behavior suggests that at full CO coverage two CO binding states are present together.     

Effect of γ irradiation on the structural and thermal properties of [N(C2H5)4]2ZnBr4 in the vicinity of a first-order phase transition

The unit cell parameters a and c of nonirradiated [N(C₂H₅)₄]₂ZnBr₄ crystals in the temperature region 90–300 K and of samples irradiated with γ rays to doses of 10⁶ and 5 × 10⁶ R in the 270-to 300-K interval were measured using x-ray diffraction. The data obtained were used to derive the thermal expansion coefficients α_a and α_c. It is shown that the parameter a increases and the parameter c decreases with increasing temperature. In the vicinity of the phase transition (PT) at T = 285 K, the temperature dependences of a(T) and c(T) reveal anomalies in the form of jumps and the α_a(T) and α_c(T) curves have a maximum and a minimum, respectively. The heat capacity of nonirradiated and irradiated [N(C₂H₅)₄]₂ZnBr₄ samples was measured by adiabatic calorimetry. A maximum was found in the C _p(T) curve at T = 285 K. Both x-ray diffraction and heat capacity measurements showed that the PT temperature decreased after γ irradiation.     

A calorimetric study of the dimerized phase of C60 fullerene

The temperature dependence of the heat capacity at a constant pressure C _p ⁰ = f(T) for the dimerized phase of the C₆₀ fullerene in the temperature range 300–575 K and the thermodynamic characteristics for depolymerization of this phase under normal pressure are investigated using precision differential scanning calorimetry. It is established that thermal depolymerization is a kinetically hindered process. The final products of thermal depolymerization are identified as a partially crystalline monomer face-centered cubic phase of C₆₀ with a degree of crystallinity α = 67 mol %. The results obtained in this study and our previous experimental data on the low-temperature heat capacity are used in the calculations of standard thermodynamic functions for the (C₆₀)₂ crystalline dimer, namely, the heat capacity C _p ⁰ (T), the enthalpy H ⁰(T) ? H ⁰(0), the entropy S ⁰(T), and the Gibbs function G ⁰(T) ? H ⁰(0) in the temperature range from T → 0 to 394 K.     

Note on the two inequivalent spin 1/2 baryon field operators

There are two inequivalent spin 1/2 local baryon field operators that can be constructed from 3 quarks. A priori theJ ^P =1/2⁺ baryons can couple to any linear combination of these operators. We show however that the coupling of the 1/2⁺ baryons to these operators is determined by the value of theSU(3) ratio ofF toD type pseudoscalar-baryon couplings. The experimental value of this ratio implies, for example, that the proton couples strongly to (u ^T C _γ d)u and weakly to (u ^T C d)γ _s u. This is of interest in QCD sum rule applications.     

(T1u+T1g)⊗(hg+τ1u)vibronic interaction and superconductivity in C 60 n− fullerides

The closeness of low-lying T_1u and T_1g levels of C ₆₀ ^? could enable their mixing under an odd parity vibration of (T_{1 u} + T_{1 g} ? (h_g + τ_{1 u})type. In addition, the two levels are susceptible to Jahn-Teller interaction due to five-fold degenerate h_g vibrations. This complex problem of (T_1u+T_1g)?(h_g+τ_1u) vibronic interaction is transformed to a form similar to T_2g ? (ε_g + τ_2g) vibronic problem of octahedral symmetry. The problem is analysed in an infinite coupling model and compared with the experimental spectroscopic results for the C ₆₀ ^? radical. The resulting parameters are used to calculate the pair-binding energy and superconducting transition temperature in C ₆₀ ^n? fullerides. Vibronic mixing with the T_1g level is found to be responsible for maximising the pair-binding energy at the doping level n=3. It is also found to be an important source of T_c enhancement.     

Hartree–Fock and density functional theory study of remote substituent effects on heterolytic Fe–N bond energies of p‐G‐C6H4NHFe(CO)2(η5‐C5H5) and p‐G‐C6H4N(COMe)Fe(CO)2(η5‐C5H5)

The nature and strength of metal–ligand bonds in organotransition‐metal complexes are crucial to the understanding of organometallic reactions and catalysis. Quantum chemical calculations at different levels of theory have been used to investigate heterolytic Fe–N bond energies of para‐substituted anilinyldicarbonyl(η⁵‐cyclopentadienyl)iron [p‐G‐C₆H₄NH(η⁵‐C₅H₅)Fe(CO)₂, abbreviated as p‐G‐C₆H₄NHFp (1), where G = NO₂, CN, COMe, CO₂Me, CF₃, Br, Cl, F, H, Me, MeO, and NMe₂] and para‐substituted α‐acetylanilinyldicarbonyl(η⁵‐cyclopentadienyl)iron [p‐G‐C₆H₄N(COMe)(η⁵‐C₅H₅)Fe(CO)₂, abbreviated as p‐G‐C₆H₄N(COMe)Fp (2)] complexes. The results show that BP86 and TPSSTPSS can provide the best price/performance ratio and more accurate predictions in the study of ΔH_het(Fe–N)'s. The linear correlations [r = 0.98 (g, 1a), 0.93 (g, 2b)] between the substituent effects of heterolytic Fe–N bond energies [ΔΔH_het(Fe–N)'s] of series 1 and 2 and the differences of acidic dissociation constants (ΔpK_a) of N–H bonds of p‐G‐C₆H₄NH₂ and p‐G‐C₆H₄NH(COMe) imply that the governing structural factors for these bond scissions are similar. And the linear correlations [r = ?0.99 (g, 1c), ?0.92 (g, 2d)] between ΔΔH_het(Fe–N)'s and the substituent σ_p^? constants show that these correlations are in accordance with Hammett linear free energy relationships. The polar effects of these substituents and the basis set effects influence the accuracy of ΔH_het(Fe–N)'s. ΔΔH_het(Fe–N)'s(1, 2) follow the captodative principle. ME_{α‐COMe, para‐G}s include the influences of the whole molecules. The correlation of ME_{α‐COMe, para‐G}s with σ_p^? is excellent. ME_{α‐COMe, para‐G}s rather than ΔΔH_het(Fe–N)'s in series 2 are more suitable indexes for the overall substituent effects on ΔH_het(Fe–N)'s(2). Insight from this work may help the design of more effective catalytic processes. Copyright © 2012 John Wiley & Sons, Ltd.     

On the mechanism of decomposition of the benzyl radical

The C₇H₇ potential energy surface was studied from first principles to determine the benzyl radical decomposition mechanism. The investigated high temperature reaction pathway involves 15 accessible energy wells connected by 25 transition states. The analysis of the potential energy surface, performed determining kinetic constants of each elementary reaction using conventional transition state theory, evidenced that the reaction mechanism has as rate determining step the isomerization of the 1,3-cyclopentadiene, 5-vinyl radical to the 2-cyclopentene,5-ethenylidene radical and that the fastest reaction channel is dissociation to fulvenallene and hydrogen. This is in agreement with the literature evidences reporting that benzyl decomposes to hydrogen and a C₇H₆ species. The benzyl high-pressure decomposition rate constant estimated assuming equilibrium between the rate determining step transition state and benzyl is k₁(T) = 1.44 × 10¹³T^0.453exp(−38400/T) s⁻¹, in good agreement with the literature data. As fulvenallene reactivity is mostly unknown, we investigated its reaction with hydrogen, which has been proposed in the literature as a possible decomposition route. The reaction proceeds fast both backward to form again benzyl and, if hydrogen adds to allene, forward toward the decomposition into the cyclopentadienyl radical and acetylene with high-pressure kinetic constants k₂(T) = 8.82 × 10⁸T^1.20exp(1016/T) and k₃(T) = 1.06 × 10⁸T^1.35exp(1716/T) cm³/mol/s, respectively. The computed rate constants were then inserted in a detailed kinetic mechanism and used to simulate shock tube literature experiments.