Electronic spectra of the MgC4H and MgC6H radicals

Electronic transitions of the linear MgC 4H and MgC 6H radicals have been observed in the gas phase using laser-induced fluorescence spectroscopy. The species were prepared in a supersonic expansion by ablation of a magnesium rod in the presence of acetylene, diacetylene, or methane gas. The transitions were recorded in the 445-446 nm region and assigned to the A (2)Pi- X (2)Sigma (+) systems ( T 0 = 22 431.978(7) and 22 090.08(7) cm (-1)) based on previously reported mass-selective resonance-enhanced ionization spectra and the rotational structure. A spectral fit in MgC 4H yields the rotational constants B' = 0.04619(19) cm (-1) for the X (2)Sigma (+) state and B' = 0.04748(20) cm (-1) for A (2)Pi. Astrophysical implications are briefly addressed.     

Electronic spectrum of the AlC(2) radical

An electronic transition of the AlC2 radical (C2v structure) has been observed using laser-induced fluorescence spectroscopy. The molecule was prepared in a supersonic expansion by ablation of an aluminum rod in the presence of acetylene gas. A spectrum was recorded in the 451-453 nm region and assigned to the C 2B2-X 2A1 system (T0 = 22,102.7 cm(-1)) based on a rotational analysis and agreement with calculated molecular parameters and excitation energies. Ab initio results obtained using couple cluster methods are in accord with previous theoretical work which concludes that ground-state AlC2 possesses a T-shaped C2v 2A1 geometry, with the linear 2Sigma+ AlCC isomer 0.70 eV higher in energy. A fit of the experimental spectrum yields rotational constants in the ground and electronically excited states that are in reasonable agreement with the calculated values: A' = 1.7093(107), B' = 0.4052(50), C' = 0.3228(49) cm(-1) for the X 2A1 state, and A' = 1.5621(137), B' = 0.4028(46), C' = 0.3201(54) cm(-1) for C 2B2. Variation in individual fluorescence lifetimes suggests that the emitting C 2B2 state undergoes rovibronic mixing with lower lying electronic states.     

Theoretical study of the equilibrium structure, vibrational spectrum, and thermochemistry of the peroxynitrate CF2BrCFBrOONO2

The results of a theoretical study of the molecular structure and conformational mobilities of the peroxynitrate CF(2)BrCFBrOONO(2) and its radical decomposition product CF(2)BrCFBrOO are reported in this paper. The most stable structures were calculated from ab initio G3(MP2)B3 and G4(MP2) methods and from density functional theory at the B3LYP/6-311+G(d) and B3LYP/6-311+G(3df) levels of theory. The equilibrium conformation of CF(2)BrCFBrOONO(2) indicates that the bromine atoms lie in position anti to each other and possess a COON dihedral angle of 114°. A quantum statistical analysis shows that about 40% of the internal rotors can freely rotate at room temperature. Our best values for the standard enthalpies of formation of CF(2)BrCFBrOONO(2) and CF(2)BrCFBrOO at 298 K obtained from isodesmic reactions at the G3(MP2)//B3LYP/6-311+G(3df) level of theory are -144.7 and -127.0 kcal mol(-1). From these values and the enthalpy of formation of the NO(2) radical, a CF(2)BrCFBrOO-NO(2) bond dissociation enthalpy of 26.0 ± 2 kcal mol(-1) was estimated.     

SCF-CI studies of the equilibrium structure and the proton transfer barrier H3O 2 −

Large-scale configuration interaction (CI) calculations have been performed in order to study the effect of the correlation energy on the equilibrium geometrical structure, the stability, and on the energy barrier of the proton transfer reaction in the hydrogen bonded system HO^– · HOH. An extended Gaussian basis set including polarization functions on each nuclear centre has been employed to approximate the molecular Orbitals. All possible single and double replacements resulting from a single determinant Hartree-Fock reference state have been taken into account in the CI wavefunction. Compared to the SCF results the equilibrium oxygen/oxygen distance has been obtained from the CI calculations to be smaller by about 0.08 Å and the correlation energy has been found to stabilize the composed system by 3.6 kcal/mole. An almost symmetric equilibrium structure with the hydrogen bonding H-atom midway between the two oxygen centres has been obtained in the CI treatment, whereas SCF calculations yield an asymmetric geometrical configuration with a small energy barrier of 1.4 kcal/mole for the proton transfer process.     

On the mechanism of proton transfer in the 2-hydroxypyridine α 2-pyridone tautomeric equilibrium

The saddle point for the proton transfer involved in the equilibrium 2-hydroxypyridine α 2-pyridone has been calculated in a 3-21G basis to be 206 kJ mol^?1 above the lactim form, for a unimolecular mechanism. This barrier is estimated to be reduced to 46 kJ mol^?1 in the self-associated dimer.     

Rotational spectrum and equilibrium structure of silanethione, H2Si=S

Unsubstituted silanethione, H(2)Si=S, has been characterized experimentally for the first time by means of rotational spectroscopy; the equilibrium structure of this fundamental molecule has been evaluated through a combination of experimental data from a total of ten isotopologues and results of high-level coupled-cluster calculations.     

V2AlC, V4AlC3-x (x approximately 0.31), and V12Al3C8: synthesis, crystal growth, structure, and superstructure

Single crystals of V2AlC and the new carbides V4AlC3-x and V12Al3C8 were synthesized from metallic melts. V2AlC was formed with an excess of Al, while V4AlC3-x (x approximately 0.31) and V12Al3C8 require the addition of cobalt to the melt. All compounds were characterized by XRD, EDX, and WDX measurements. Crystal structures were refined on the basis of single-crystal data. The crystal structures can be explained with a building-block system consisting of two types of partial structures. The intermetallic part with a composition VAl is a two-layer cutting of the hexagonal closest packing. The carbide partial structure is a fragment of the binary carbide VC1-x containing one or three layers. V2AlC is a H-phase (211-phase) with space group P63/mmc, Z=2, and lattice parameters of a=2.9107(6) A, and c=13.101(4) A. V4AlC3-x (x approximately 0.31) represents a 413-phase with space group P63/mmc, Z=2, a=2.9302(4) A, and c=22.745(5) A. The C-deficit is limited to the carbon site of the central layer. V12Al3C8 is obtained at lower temperatures. In the superstructure (P63/mcm, Z=2, a=5.0882(7) A, and c=22.983(5) A) the vacancies on the carbon sites are ordered. The ordering is combined to a small shift of the V atoms. This ordered structure can serve as a structure model for the binary carbides TMC1-x as well. V4AlC3-x (x approximately 0.31) and V12Al3C8 are the first examples of the so-called MAX-phases (MX)nMM' (n=1, 2, 3), where a deficit of X and its ordered distribution in a superstructure is proven, (MX1-x)nMM'.     

Ionization and protonation of MgC3: A theoretical study

A theoretical study of the MgC₃⁺ and MgC₃H⁺ species has been carried out. Predictions for their geometries and vibrational frequencies have been made at both second‐order Møller–Plesset (MP2) and B3LYP levels, whereas electronic energies have been computed at G2 and coupled cluster single and double excitation model augmented with a noniterative triple excitation conection (CCSD(T)) levels. The predicted global minimum for MgC₃⁺ is a rhombic structure (²A₁ electronic state), whereas a T‐shaped structure and an open‐chain isomer lie about 10 and 12 kcal/mol, respectively, higher in energy. In the case of MgC₃H⁺ the predicted global minimum is also a four‐membered ring obtained upon protonation of the most stable neutral isomer. Low ionization potentials and high proton affinities are generally obtained, especially for the most stable MgC₃⁺ isomer. The estimated values at the CCSD(T) level for the predicted global minimum are 7.20 eV [ionization potential (IP)] and 256.5 kcal/mol [proton affinities (PA)]. Therefore, if present in the interstellar medium, MgC₃ should be easily ionized and would react quite easily to give the protonated species. © 2002 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Ta3AlC2 and Ta4AlC3--single-crystal investigations of two new ternary carbides of tantalum synthesized by the molten metal technique

Single crystals of the new ternary carbides Ta4AlC3 and Ta3AlC2 were synthesized from molten aluminum and characterized XRD, EDX, and WDX measurements. Crystal structures were refined for the first time on the basis of single-crystal data. Both compounds crystallize in a hexagonal structure with space group P63/mmc and Z = 2. The lattice constants are a = 3.1131(3) A and c = 24.122(3) A for Ta4AlC3 and a = 3.0930(6) A and c = 19.159(4) A for Ta3AlC2. The crystal structures can be explained with a building block system consisting of two types of partial structures. The intermetallic part with a composition TaAl is a two layer cutting of a hexagonal closest packing. The carbide partial structure is a fragment of the binary carbide TaC (NaCl type). It consists of three (Ta4AlC3) or two layers (Ta3AlC2) of CTa6-octahedra linked via common corners and edges. Both compounds are members of the series (TaC)nTaAl. The crystal quality of Ta3AlC2 is improved by using a Al/Sn melt for crystal growth leading to small quantities of Sn in the crystal: Ta3Al1-xSnxC2, x approximately 0.04. On the basis of reliable data a detailed discussion of structural parameters is possible. According to the building principle structure models can be developed for the whole series (MX)nMM' including coordinates for all atoms.     

The equilibrium structure of ferrocene.

The molecular structures of ferrocene in the eclipsed (equilibrium) and staggered (saddle‐point) conformations have been determined by full geometry optimizations at the levels of second‐order Møller–Plesset (MP2) theory, coupled‐cluster singles‐and‐doubles (CCSD) theory, and CCSD theory with a perturbative triples correction [CCSD(T)] in a TZV2P+f basis set. Existing experimental results are reviewed. The agreement between the CCSD(T) results and experiment is in all cases excellent; the calculated structure parameters and the barrier to internal rotation of the ligand rings differ from the most accurate experimental values by less than two estimated standard deviations. The CCSD(T) calculations for single‐configuration‐dominated transition metal complexes such as ferrocene thus appear to have an accuracy comparable to that observed for molecules containing only first‐ and second‐row atoms, and to be of a quality similar to that obtained experimentally. A comparison with previous DFT results indicates that the B3LYP model gives overall the best DFT results, with a deviation of around 2 pm for the metal–carbon distance and smaller errors for the cyclopentadienyl rings.     

On the phase equilibrium Stockmayer fluids

The fluid phase equilibrium of the Stockmayer fluid is investigated using a thermodynamic perturbation theory approach. The reference and the perturbation potential are the Lennard–Jones potential and the dipolar–dipolar interactions, respectively. They are assumed to be represented by the modified Benedict–Webb–Rubin equation of state [J.K. Johnson, J.A. Zollweg, K.E. Gubbins, Mol. Phys. 78 (1993) 591–618] and the Padé approximant [G. Stell, J.C. Rasaiah, H. Narang, Mol. Phys. 27 (1974) 1393–1414], respectively. The asymmetry found in an analogous study [M.E. van Leeuwen, B. Smit, E.M. Hendriks, Mol. Phys. 78 (1993) 271–283] based on the BWR equation of state [J.J. Nicolas, K.E. Gubbins, W.B. Streett, D.J. Tildesley, Mol. Phys. 37 (1979) 1429–1454] is now not observed on the vapour–liquid equilibrium coexistence curves of Stockmayer fluids with dipolar strength of μ^*2 = 1, 2, 3, and 4. Results agree with computer simulations for dipolar strength of μ^*2 = 1; however as strength dipole increases, liquid densities are over-estimated.     

On equilibrium structures of the water molecule

Equilibrium structures are fundamental entities in molecular sciences. They can be inferred from experimental data by complicated inverse procedures which often rely on several assumptions, including the Born-Oppenheimer approximation. Theory provides a direct route to equilibrium geometries. A recent high-quality ab initio semiglobal adiabatic potential-energy surface (PES) of the electronic ground state of water, reported by Polyansky et al. [ ibid. 299, 539 (2003)] and called CVRQD here, is analyzed in this respect. The equilibrium geometries resulting from this direct route are deemed to be of higher accuracy than those that can be determined by analyzing experimental data. Detailed investigation of the effect of the breakdown of the Born-Oppenheimer approximation suggests that the concept of an isotope-independent equilibrium structure holds to about 3 x 10(-5) A and 0.02 degrees for water. The mass-independent [Born-Oppenheimer (BO)] equilibrium bond length and bond angle on the ground electronic state PES of water is r(e) (BO)=0.957 82 A and theta e (BO)=104.48(5) degrees , respectively. The related mass-dependent (adiabatic) equilibrium bond length and bond angle of H2 (16)O is r(e) (ad)=0.957 85 A and theta e (ad)=104.50(0) degrees , respectively, while those of D2 (16)O are r(e) (ad)=0.957 83 A and theta e (ad)=104.49(0) degrees . Pure ab initio prediction of J=1 and 2 rotational levels on the vibrational ground state by the CVRQD PESs is accurate to better than 0.002 cm(-1) for all isotopologs of water considered. Elaborate adjustment of the CVRQD PESs to reproduce all observed rovibrational transitions to better than 0.05 cm(-1) (or the lower ones to better than 0.0035 cm(-1)) does not result in noticeable changes in the adiabatic equilibrium structure parameters. The expectation values of the ground vibrational state rotational constants of the water isotopologs, computed in the Eckart frame using the CVRQD PESs and atomic masses, deviate from the experimentally measured ones only marginally, especially for A0 and B0. The small residual deviations in the effective rotational constants are due to centrifugal distortion, electronic, and non-Born-Oppenheimer effects. The spectroscopic (nonadiabatic) equilibrium structural parameters of H2 16O, obtained from experimentally determined A'0 and B'0 rotational constants corrected empirically to obtain equilibrium rotational constants, are r(e) (sp)=0.957 77 A and theta e (sp)=104.48 degrees .     

Spin-state equilibrium and 1-D heisenberg chain structure in 2-methylimidazolatocobalt(II)

Summary Magnetic susceptibility measurements have been carried out on 2-methylimidazolatocobalt(II) in the polycrystalline state from room to liquid helium temperature (4 K). The data support the spin-state equilibrium between the thermally accessible states⁴ T _2g and² E _g from room temperature to 40 K. However, below 40 K, antiferromagnetic exchange interactions are operative and the system has the 1-D Heisenberg infinitely chained structure. Some of the magnetic parameters have also been calculated.     

On the crystal structure of Bi2Te2O7

Bi₂Te₂O₇ is orthothobic, space group Pbcn. The lattice parameters derived from a Guinier powder pattern (Si standard) are: a= 22.794 ± 0.005, b= 5.526 ± 0.001, c= 22.065 ± 0.005Å, z= 16. The structure was solved by analysis of single crystal X-ray data and refined on the basis of neutron powder diffraction data. It is an anion-deficient 4: 1: 4 superstructure of fluorite in which the electron lone-pairs of tellurium are stereochemically active. The structural relationships with fluorite and with Bi₂TeO₅ (and other anion-deficient fluorite-related superstructures) have been shown and discussed.     

Surface-hopping dynamics and decoherence with quantum equilibrium structure

In open quantum systems, decoherence occurs through interaction of a quantum subsystem with its environment. The computation of expectation values requires a knowledge of the quantum dynamics of operators and sampling from initial states of the density matrix describing the subsystem and bath. We consider situations where the quantum evolution can be approximated by quantum-classical Liouville dynamics and examine the circumstances under which the evolution can be reduced to surface-hopping dynamics, where the evolution consists of trajectory segments exclusively evolving on single adiabatic surfaces, with probabilistic hops between these surfaces. The justification for the reduction depends on the validity of a Markovian approximation on a bath averaged memory kernel that accounts for quantum coherence in the system. We show that such a reduction is often possible when initial sampling is from either the quantum or classical bath initial distributions. If the average is taken only over the quantum dispersion that broadens the classical distribution, then such a reduction is not always possible.