Bruno Lunelli

The vibrational spectrum of 1,2-difluorobenzene and 1,2-difluorobenzene-d4

Abstract The infrared spectra of pure and isotopic solution single crystal, liquid, and vapor samples have been registered for the previously unknown 1,2-difluorobenzene- d 4 . On the basis of these and the analogous measurements for 1,2-difluorobenzene we discuss an assignment for all 30 fundamentals of both these isotopically related molecules. The use of several techniques for spectral measurements has allowed us to generally avoid the exploitation of any one method to its borderline significance. A normal coordinate analysis of the planar fundamentals has led to a force field for the title compound based on several mutually supporting sets of frequencies, and therefore of correspondingly increased reliability.

Geometry and molecular parameters of 3,4-bis(dimethylamino)-3-cyclobutene-1,2-dione and its isomer bis(dimethylamino)squaraine. Combined study by IR spectroscopy, XRD and ab initio MO calculations

Absorption FTIR spectra have been recorded for solutions and solid phases of 3,4-bis(dimethylamino)-3-cyclobutene-1,2-dione and its isomer bis(dimethylamino)squaraine. The crystal structures of the two compounds have been established by XRD and compared with those of other bis(dialkylamino) analogues already studied by the same technique. Ab initio calculations for the squaraine led to a 6–31G** geometry substantially similar to the experimental one, indicating that crystal packing only marginally perturbs the equilibrium configuration of the isolated molecule. In contrast, the geometry of the 3-cyclobutene-1,2-dione derivative shows a degree of planarity considerably greater in the crystal than in the free molecule at the MP2/6–31G* minimum-energy nuclear configuration. The squaraine is predicted to be more stable, at all basis set levels here adopted, than its isomer. Differences and similarities between experimental IR frequencies and their ab initio estimates are presented.

Effect of Methoxy Substituents on the Structural and Electronic Properties of Fluorinated Cyclobutenes: A Study of Hexafluorocyclobutene and Its Vinyl Methoxy Derivatives by XRD and Periodic DFT Calculations

The effect of the methoxy substituent on the structure, crystal packing, and electrostatic properties of hexafluorocyclobutene (C(4)F(6)) was investigated in the solid-state with DFT-B3LYP calculations. Full geometry optimizations were done for the parent compound and its two vinyl methoxy derivatives C(4)F(5)OCH(3) and C(4)F(4)(OCH(3))(2), starting from the structures obtained by single-crystal X-ray diffraction at low temperature. A full topological analysis, followed by the calculation of several electrostatic properties, was performed on the periodic electron density using the quantum theory of atoms in molecules. Eventually, the cohesive energies of the three crystals were estimated. In the cyclobutene plane, the methoxy substitution yields a significant electronic rearrangement involving the pi-electrons. The solid-state (periodic) results agree with those obtained by gas-phase calculations on C(4)F(6) and its derivatives at a comparable level of theory. It was found that the substitution of one or two vinylic fluorine atoms with the OCH(3) group considerably influences the molecular dipole moment, which undergoes an enhancement in both the solid and the gas phase as large as 200% and 235% for C(4)F(5)OCH(3) and C(4)F(4)(OCH(3))(2), respectively, with respect to that calculated for C(4)F(6). The charge rearrangement due to the substituents provides a significant electrostatic contribution to the lattice energy, and in turn it can be related to the change in the observed crystal packing on going from C(4)F(6) (space group P2(1)/c) to both of its derivatives (space group P1). It is also shown that the dispersion energy significantly contributes to the lattice stability in all three compounds. Since the DFT calculations, in the limit of large separations, entirely miss the dispersion term, this was estimated by applying a recently proposed dampening function to the semiempirical atom-atom C(6) R(-6) potentials in the mainframe of Spackmans energy decomposition scheme for Mulliken multipoles.

A study on the electrochemical behaviour of the 1,2-carbo dicyanocyclobuten-3,4-dione structure

Abstract The electrochemical behaviour of the 1,2-carbodicyano cyclo buten-3,4-dione structure was investigated both in acetonitrile (AN) and in aqueous medium. This structure can accommodate a different number of electrons giving rise to a dianion CDCB 2− , a radical anion CDCB − · and a neutral species CDCB, all with the same composition and geometry. These species in AN give two subsequent one electron redox systems CDCB 2− /CDCB − · and CDCB − ·/CDCB, whose E 1/2 are respectively +0.68 and +1.47V vs. Ag/AgCl. While the dianion and the radical anion are stable in AN, the neutral species undergoes an irreversible reaction with k =5×10 −4 s −1 at −30°C. Also, in water at pH 2− /CDCB − · is reversible, owing to the low reactivity of CDCB − · in these conditions; the further oxidation of CDCB − · is a three-electron process, since CDCB undergoes a nucleophilic attack by water with formation of intermediates oxidizable at the potential of formation. At pH>10 the high reactivity of the radical anion towards hydroxyls causes the direct oxidation of CDCB 2− in a single, irreversible, four-electron wave. Kinetic measurements of the radical decay in aqueous medium at different pH values show that its disappearance follows a first-order rate law with respect to the radical concentration and with a k app =4{1×10 −5 [H 2 O]+2×10 +4 [OH − ]}s −1 .

Gas phase vibrational fundamentals and vib-rotational parameters of AsCl3

Abstract Contour simulation of the gas phase infrared fundamental bands of AsCl 3 led to estimated values for the first-order Coriolis constants, the vibrational frequencies, and the relative values of the vibrational transition moments. Improved values of the six independent force constants of a general quadratic force field were determined on the basis of the observed frequencies, mean amplitudes of vibration, and Coriolis and centrifugal distortion constants. The potential energy distribution over the normal modes is reported.

Equilibrium geometry, force field and stability of the 1,2-dihalogenocyclobutene-3,4-diones

Abstract The equilibrium geometry, heat of formation, force fields and vibrational frequencies of the four symmetric 1,2-dihalogenocyclobuten-3,4-diones have been computed using the MNDO method. The results are discussed and compared with experimental distances and frequencies and the empirical force fields proposed earlier.

General purpose absorption cell for high‐sensitivity spectrometry of liquid samples

A priori requirements and compatible, optimized design considerations for a high‐performance low‐cost absorption cell for the spectrometry of liquid samples are discussed. A new cell model is presented.

The vibrational spectra of quinones—III2: Infrared spectra of single crystals of 1,4-benzoquinone and 1,4-benzoquinone-d4

Abstract The polarized infrared spectra of 1,4-benzoquinone, of 1,4-benzoquinone- d 4 single crystals and of their dilute isotopic solutions have been measured for the (201) crystal plane. The crystal spectra are analysed in terms of the previously reported assignment of the infrared active fundamentals which is largely confirmed. The dichroism is carefully examined as regards some aspects of its measurement, its empirical trends and its predictability, and the limits of the more commonly used interpretative schemes are discussed.

Physicochemical characterization of the 1,2-bis(cyanimido)cyclobutene-3,4-dione dianion and its twin radical anions

Controlled oxidation of the 1,2-bis(cyanoimido)cyclobutene-3,4-dione (CMCB) dianion in aprotic solvents gives electron paramagnetic resonance (EPR) spectra characteristic of the presence of two radical species in a fixed mole ratio, identified as two planar conformers of the monoanion of CMCB differing in the orientation of one of the two N—CN groups. Consideration of the nature and symmetry of the molecular orbitals, and of the IR and 13C NMR spectra, suggests that similar isomers should also be present in the solution of the dianion.Cyclic voltammograms of the dianion in N,N-dimethylformamide exhibit two oxidation peaks, the first being due to a one-electron diffusion-controlled reversible process, and the second to an irreversible process. Computer simulation of the cyclic voltammetric curves for the first process indicates that the observed behaviour is consistent with the four-member square scheme suggested by EPR, IR and NMR experiments. The irreversibility of the second peak is due to the presence of fast chemical reactions involving the product of one-electron reversible oxidation of the radical anion.

Excess molar enthalpies and excess molar volumes of binary mixtures containing p-xylene + cyclic ethers

Abstract Excess molar enthalpies H E , and excess molar volumes V E , of mixturesof p -xyleae with the cyclic ethers oxolane, oxane, 1,3-dioxolane, and 1,4-dioxane have been determined by means of a flow microcalorimeter and a digital density meter at 298. 15″ K and afinospheric pressure. The experimental values are correlated by polynomiah equations. V E is positive over the entire range of mole fractions for p -xylene-1,4-dioxane, but is negative for the other mixtures. H E is positive for the mixtures involving the diethers-1,3-dioxolane and 1,4-dioxane, and negative for the monoethers. The heat effects are qualitatively discussed in terms of molecular interactions.