Étude des isomères (CO)Fe(PF3)4

Assignments of a certain number of vibrational bands of (CO)Fe(PF₃)₄ are made with reference to the C_3v and C_2v isomers. A study of the Raman and spectra at low temperature revealed that in the solid state only one isomer exists. Furthermore, a comparative study of liquid (CO)Fe(PF₃)₄ and HCo(PF₃)₄ of molecular symmetry C_3v was made. These two approaches made it possible to determine the difference in enthalpy between the two isomers of the iron complex (+0-52±0.05 kcal.mole⁻¹) in the liquid state, as well as to verify the assignments proposed previously. The greater stability of the C_2v isomer is discussed.     

Vibrational spectra of hexafluorodisiloxane and internal rotation around the SiO bond. An ab initio study

The structure of hexafluorodisiloxane was optimized at different levels of theory up to MP2/6-31G^*, and the vibrational spectrum was calculated and compared with the experimental one. It was shown that at the Hartree-Fock level the simple SCF/3-21 (O^*) basis set gives satisfactory results. The geometry was also optimized at two C_2v structures which appear to be saddle points on the SiF₃ rotor potential curve. The significant change of the SiOSi angle equilibrium value in the course of the SiF₃ group rotation was explained by assuming repulsion between lone pair electrons of fluorines and oxygen. This effect is also revealed in the predicted change in the v_asSiOSi frequency on going from the C₂ to the C_2v structure. This provides the explanation of some unusual features in the vibrational spectra of hexahalodisiloxanes, namely large gas-solid shifts of v_as SiOSi and splitting of this mode in the matrices.     

Molecular vibrations of complexes with trigonal ligands. Part V. Symmetry coordinates for tetrahedral complexes

Ligands of C_3V symmetry are assumed to be a part of a T_d complex. Suitable combinations of C_3v (a₂ + e) symmetry coordinates, which fit into the T_d model, are studied. The results constitute a useful supplement to the C_3V (a₁ + e) type coordinates studied previously. Torsional coordinates in the Ni(PF₃)₄ type model are treated in particular.     

Cis- and trans-3-hexene: infrared spectrum in liquid argon solution, ab initio calculations of equilibrium geometry, normal coordinate analysis, and vibrational assignments

The infrared spectra of cis-3-hexene and trans-3-hexene dissolved in liquid argon have been obtained at temperatures from 93 to 120 K. The absorptions were observed with a low-temperature cell and a Fourier transform infrared spectrophotometer. Ab initio molecular orbital calculations were performed to obtain the equilibrium geometry, vibrational frequencies, force fields, and infrared intensities. The calculations were done at the Hartree-Fock level using 6-31G basis set. The Cartesian force fields from ab initio calculations have been converted to the force field in symmetry coordinates. The scale factors of ab initio calculated force fields were determined. Normal coordinate calculations were performed using a scaled quantum mechanical (SQM) force field. Vibrational normal modes calculated for the lowest energy rotamers of cis- and trans-3-hexene have been assigned to infrared absorption bands observed in liquid argon solution. The assignments were based on calculated frequencies and potential energy distributions. The equilibrium geometries of the two lowest energy rotamers (symmetry C₂ and C_s) of cis-3-hexene and of the three lowest energy rotamers (symmetry C_i, C₂, and C₁) of trans-3-hexene were calculated. Variable temperature studies of the infrared spectrum of cis- and trans-3-hexenes dissolved in liquid argon were done to obtain the ΔH of conversion between the rotamers C₂ and C_s of cis-3-hexene and between the rotamers C_i, C₂, and C₁ of trans-3-hexene.     

Ab initio calculations of the vibrational force field and IR intensities of the ammonia dimers

Ab initio SCF calculations using the 4-31G basis set have been carried out to determine the equilibrium geometry, force constants and dipole moment derivatives of the linear (C_s and cylic (C_2h) ammonia dimers. The results are compared with monomer calculations and experimental data.     

A computational study of adducts between atomic chlorine and carbon dioxide, carbonyl sulfide and carbon disulfide

The geometries and vibrational frequencies of the adducts ClCO₂, ClCOS and ClCS₂ were derived at the Hartree-Fock (HF) 3-21G (^*) level. The C_a, structure of ClCO₂ corresponds to one C-O bond and one C=O bond. Similarly, C_a, ClCS₂ has one C-S and one C=S bond, and ClCOS has one C-S and one C-O bond. Single-point spin-projected fourth-order Møller-Plesset (MP4) 3-21G (^*) calculations at these geometries were used in bond-separation reactions to derive ΔH^o₀ for adduct formation, which is calculated to be about 39 kJ mol⁻¹ exothermic for ClCOS and ClCS₂, but about 39 kJ mol⁻¹ endothermic for ClCO₂. The C_2v structures for ClCO₂ and ClCS₂ were also characterized. The geometry of ClCS₂ has not been determined experimentally; comparison with an available measured entropy for ClCS₂ suggests that the C_2v structure is the one formed by addition of Cl to CS₂, although the energy relative to the C_a form is not reliably calculated because of instability in the HF wavefunction.     

Microwave spectrum and conformation of isopropyl fluoroformate

The microwave spectrum of isopropyl fluoroformate is characterized by intense a-type R-branch transitions from one conformational species. The rotational constants of the ground state, A₀ = 4967.0(8) MHz, B₀ = 1704.69(2) MHz, C₀ = 1468.86(1) MHz and κ = −0.8651(2) are consistent with a τ₁ (O=COC) = 0°, τ₂(COCH) ˜35° structure. This structure can be viewed as a combination of the two conformational species found in ethyl fluoroformate. Two vibrational satellites having rotational constants A₀ = 4963(5) MHz, B₀ = 1694.11(7) MHz. C₀ = 1471.43(4) MHz and A₀=4998(6) MHz, B₀ = 1705.21(7) MHz, C₀ = 1471.10(4) MHz have been assigned.     

Protonation constants of some substituted 2-hydroxy-1-naphthalideneanilines in dioxan-water mixtures

The protonation constants of ten Schiff's bases synthesized by the condensation of 2-hydroxy-1-naphthaldehyde with aniline and o-, m-, and p-substituted methylanilines, ethylanilines and methoxyanilines in various dioxan-water mixtures have been determined potentiometrically using an electrode system calibrated in concentration units of the hydrogen ion. The influence of substituents in the molecular structure on the protonation constants is discussed. The log K₂ values relating to the protonation of imine nitrogen atoms are in accordance with the Hammett relationship for all solvent compositions. A reverse relationship is observed between log K₂ for all Schiff's bases and the mole fraction of dioxan. However, the log K₁ values corresponding to the protonation of naphtholate O-ions have been found to increase with the increase in concentration of dioxan. Both electrostatic and non-electrostatic contributions influencing these constants are briefly discussed.     

Ab initio MO calculation of force constants and dipole derivatives for the formamide dimer. An estimation of hydrogen-bond force constants

Ab initio SCF MO calculations (with the 4-31G basis set) have been carried out to determine the equilibrium geometry, vibrational frequencies, dipole-moment derivatives, and force constants for intermolecular modes of the formamide dimer and its d₄ and d₆ derivatives. The results are correlated with monomer calculations and experimental data for crystalline formamide.     

Vibrational spectra of σ-allylmanganese pentacarbonyl

The vibrational spectra of σ-(C₃H₅)Mn(CO)₅ are reported. Assignment of bands is made and carbonyl force constants are calculated. The results indicate that the Mn(CO)₅ moiety has C_4ν symmetry. The calculated angle between the axial and equatorial carbonyl groups is approximately 95°. The bonding in this compound is very similar to that in (CH₃)Mn(CO)₅.

In the far-infrared region, seven bands are expected in C_4ν symmetry (3A₁ + 4E), and all are observed.     

Vibrational spectra of zinc halide complexes with trimethylphosphine oxide

The Raman and IR spectra are reported for the complexes ZnCl₂.2TMPO, ZnBr₂.2TMPO and ZnI₂.2TMPO. The observed frequencies for polycrystalline samples are assigned on the basis of a C_3v structure for the TMPO ligand and a C_2y structure for the ZnX₂O₂ skeletal fundamentals. To discuss the coordination effects of TMPO, the ligand vibrations are compared with those of free TMPO and the skeletal vibrations with those of acetonitrile complexes. Using the ratios of G matrix elements and of observed frequencies for symmetric and asymmetric Zn-X stretching modes, the ZnX₂O₂ skeletal structures are found to be tetrahedral.     

Valency-bond studies of some conjugated hydrocarbons—II Pentalene, and some preliminary results on heptalene

Valency-bond studies of the still unknown molecule pentalene C₈H₆ in which the molecular symmetry may be either D_2h or C_2h show that a more stable molecule (lower ground state energy) is found for the lower symmetry, and that marked alternation of bond lengths occurs. The corresponding Penney-Dirac bond orders have been determined. Preliminary results for heptalene forecast a similar conclusion.     

Ab initio study of H2SO4 rotamers

This work uses ab initio calculations to obtain harmonic frequencies and anharmonic constants for the O–H symmetric and asymmetric stretches of H₂SO₄ in its C₂, C_s, C_1a, and C_1b configurations. In addition, a high-resolution potential energy surface is calculated as a function of both O=S–O–H dihedral angles in order to accurately obtain minimum and saddle point energies. The resulting peak positions and Boltzmann populations are compared to experimental frequencies and intensities and provide evidence for the assignment of rotamers in H₂SO₄ as suggested in recent work.     

Molecular vibrations of complexes with trigonal ligands : VI: Tetrammines with a square-planar-skeleton- the infrared spectrum of [Cu(NH3)4]

A vibrational analysis is performed for [Cu(NH₃)₄]²⁺, which has a square-planar frame work structure. Two versions of symmetry coordinates are produced assuming the C_4V and C_4h structures, respectively. The IR spectrum of [Cu(¹⁵NH₃)₄]²⁺ was recorded, and the observed frequencies used as supplementary data in a normal coordinate analysis along with data available for other isotopic compounds. An approximate force field which reproduces satisfactorily the observed frequencies for all the isotopic compounds is developed. The force constants may be classified as pertaining to the (a) ligand vibrations, (b) ligand-framework couplings and (c) framework vibrations. These calculations for the whole complex are compared with (i) results for free NH₃, (ii) the analysis of the XY₄ (D_4h) point-mass model and (iii) calculations for the pyramidal-axial CuNH₃ (C_3V) fragment. It is concluded that the effects of kinematic coupling are small. The validity of the point-mass model approximation is confirmed.     

Thermal degradation behavior of cellulose fibers partially esterified with some long chain organic acids

The thermal degradation behavior of cellulose fibers and some fibrous cellulose esters with partial degree of substitution has been studied by thermogravimetry analysis (TG) and differential scanning calorimetry (DSC). Cellulose esters were prepared by heterogeneous esterification in Py/TsCl with unsaturated or saturated long chain organic acids [undecylenic (C₁₁), undecanoic (C₁₁), oleic (C₁₈) and stearic (C₁₈)]. The thermal degradation of cellulose fibers follows a one-step process. The thermal stability of cellulose esters is inferior to that of unmodified cellulose fibers and the thermograms show a two-step degradation process, probably controlled by crosslinking reactions, which occur during thermal decomposition. Exothermic peaks in the DSC thermograms are also an indication of such reactions. Kinetic parameters such as the activation energy E, order of decomposition n, and frequency factor Z were obtained following the Friedman method. The cellulose sample followed first order of decomposition, however for cellulose esters higher orders were observed.     

Preparation, thermal stability and luminescence properties of selected rare earth oxycarbonates

A comparative TG and DTG study of the preparation and thermal stability of selected rare earth oxycarbonates, (REO)₂CO₃ (RE = La and Gd), shows that the ease of formation as well as the stability of these compounds decreases strongly with increasing atomic number of the host cation. According to X-ray powder diffraction analyses, the RE oxycarbonates obtained as decomposition products of acetate and carbonate hydrates belong to the tetragonal IA-type. UV- and dye laser-excited luminescence studies of Eu³⁺-doped (LaO)₂CO₃ and (GdO)₂CO₃ reveal the presence of two different sites for the host cation. One of the sites resembles closely that prevailing in the tetragonal RE oxysalts, i.e. oxyhalides, oxysulphates, oxymolybdates and oxynitrates. A crystal field analysis carried out on the ⁷F₁ and ⁷F₂ level schemes according to a C_2v site symmetry confirms this hypothesis.     

C94 IPR isomeric set: large-scale computations of relative stabilities based on the Gibbs function

At present C₉₄ is one of the highest sets of isomeric fullerenes that has been characterized by ¹³C NMR spectra. This contribution reports quantum-chemical computations on the C₉₄ system. The complete set of 134 isolated-pentagon-rule isomers of C₉₄ is described by four semiempirical quantum-chemical methods (MNDO, AM1, PM3, and SAM1). The C₉₄ energetics is also checked with Hartree–Fock SCF calculations in the standard 4-31G basis set (HF/4-31G). The calculations point out a C₂ structure as the system ground state. As energetics itself cannot produce reliable relative stabilities at high temperatures, entropy terms are also computed and the relative-stability problem is entirely treated in terms of the Gibbs function. The lowest-energy structure remains the most populated isomer at higher temperatures. However, several other structures show significant populations at higher temperatures. The six most populated species at the AM1 computational level read: C₂, C₂, C₁, C₁, C_s, and C₂. This selected six-membered isomeric set indeed contains the four symmetries observed in the available experiment (C₂, C_s, C₂, and C₂). This incidence represents a good agreement with the experiment and can be viewed as another evidence that the supposed inter-isomeric thermodynamic equilibrium does exist in experiments.     

Long-range dispersion coefficients for like centrosymmetric linear molecules and an application to H2-H2

The real spherical tensor theory of long-range intermolecular coefficients developed in previous papers is applied to derive explicit formulae for the first three dispersion coefficients for like centrosymmetric linear molecules. The expansion of angle-dependent coefficients in associated Legendre polynomials allows one to identify the isotropic and anisotropic components of the dispersion interaction in terms of London dispersion constants, the treatment of higher coefficients being simplified by the coupling of the elementary (l, l′)-polarizations to resultant angular momenta L_A and L_B onto each molecule. The contributions from all coupling schemes are given explicitly for C₆, C₈, C₁₀, and numerical results are presented for H₂-H₂ using two-term reduced spectra values from the Kaiserlautern group.     

Relationship between the pitch and elastic constants of the transient planar state in cholesteric liquid crystals

We investigate experimentally the relationship between the pitch and the elastic constants of a transient planar state in cholesteric liquid crystals. The pitch of the transient planar state is measured by the optical reflection technique. The twist and bend elastic constants of cholesteric materials are determined from the measured values of the threshold voltages for the transitions between a focal-conic state and a homeotropic state. By comparing these values, we experimentally confirm the relationship for the pitch of a transient planar state; P_transient=(K₃₃/K₂₂)P₀, which has been suggested theoretically.     

Vibrational spectrum of [(CD3)3S]I and normal coordinate calculations for [(CH3)3S] and [(CD3)3S] cations

IR (4000-30 cm⁻¹) and Raman (4000-0 cm⁻) spectra of [(CD₃)₃S]I have been observed, together with those of [(CH₃)₃S]I. By assuming a C_3v molecular symmetry for the cations [(CH₃)₃S]⁺ and [(CD₃)₃S]⁺, all the active fundamentals of [(CD₃)₃s]⁺ have been assigned and normal coordinate calculations have been carried out by a symmetry force field for [(CH₃)₃S]⁺ and [(CD₃)₃S]⁺. The strength of the S---C and C---H bonds in the compound has been compared with that in dimethyl sulfide by using their valence stretching force constants.