FT‐IR kinetic and product study of the OH radical and Cl‐atom–initiated oxidation of dibasic esters

Using a relative kinetic technique, rate coefficients have been measured, at 296 ± 2 K and 740 Torr total pressure of synthetic air, for the gas‐phase reaction of OH radicals with the dibasic esters dimethyl succinate [CH₃OC(O)CH₂CH₂C(O)OCH₃], dimethyl glutarate [CH₃OC(O)CH₂CH₂CH₂C(O)OCH₃], and dimethyl adipate [CH₃OC(O)CH₂CH₂CH₂CH₂C(O)OCH₃]. The rate coefficients obtained were (in units of cm³ molecule^?1 s^?1): dimethyl succinate (1.89 ± 0.26) × 10^?12; dimethyl glutarate (2.13 ± 0.28) × 10^?12; and dimethyl adipate (3.64 ± 0.66) × 10^?12. Rate coefficients have been also measured for the reaction of chlorine atoms with the three dibasic esters; the rate coefficients obtained were (in units of cm³ molecule^?1 s^?1): dimethyl succinate (6.79 ± 0.93) × 10^?12; dimethyl glutarate (1.90 ± 0.33) × 10^?11; and dimethyl adipate (6.08 ± 0.86) × 10^?11. Dibasic esters are industrial solvents, and their increased use will lead to their possible release into the atmosphere, where they may contribute to the formation of photochemical air pollution in urban and regional areas. Consequently, the products formed from the oxidation of dimethyl succinate have been investigated in a 405‐L Pyrex glass reactor using Cl‐atom–initiated oxidation as a surrogate for the OH radical. The products observed using in situ Fourier transform infrared (FT‐IR) absorption spectroscopy and their fractional molar yields were: succinic formic anhydride (0.341 ± 0.068), monomethyl succinate (0.447 ± 0.111), carbon monoxide (0.307 ± 0.061), dimethyl oxaloacetate (0.176 ± 0.044), and methoxy formylperoxynitrate (0.032–0.084). These products account for 82.4 ± 16.4% C of the total reaction products. Although there are large uncertainties in the quantification of monomethyl succinate and dimethyl oxaloacetate, the product study allows the elucidation of an oxidation mechanism for dimethyl succinate. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 431–439, 2001     

Perylene π‐Bridges Equally Delocalize Anions and Cations: Proportioned Quinoidal and Aromatic Content

A complete experimental and theoretical study has been carried out for aromatic and quinoidal perylene‐based bridges substituted with bis(diarylamine) and bis(arylimine) groups respectively. The through‐bridge inter‐redox site electronic couplings (V_AB) have been calculated for their respective mixed‐valence radical cation and radical anion species. The unusual similitudes of the resulting V_AB values for the given structures reveal the intervention of molecular shapes with balanced semi‐quinoidal/semi‐aromatic structures in the charge delocalization. An identical molecular object equally responding to the injection of either positive or negative charges is rare in the field of organic π‐conjugated molecules. However, once probed herein for perylene‐based systems, it can be extrapolated to other π‐conjugated bridges. As a result, this work opens the door to the rational design of true ambipolar bulk and molecular conductors.     

Graphitic molecules with partial "zig/zag" periphery

A new synthetic protocol that affords novel hexa-peri-hexabenzocoronene (HBC) derivatives with partial "zig/zag" periphery and dramatically different electronic properties has been developed, which represents an important step toward understanding structure-property relationships of graphitic materials. The scope of this methodology was broadened by application to a much larger polycyclic aromatic hydrocarbon (PAH) with a more extended "zig/zag" character.     

Interanionic (-)O-H...O(-) interactions: a solid-state and computational study of the ring and chain motifs

The (-)O-H...O(-) interaction formed by the anions HCO3-, HCO4, HC4O4 and HC5O5- (HA-), obtained upon monodeprotonation of the corresponding carbonic, oxalic, squaric and croconic acids (H2A), has been investigated theoretically and experimentally. The ring (RING) and chain (CHAIN) hydrogen bond motifs established between these anions have been analysed in terms of geometry and energy and their occurrence in crystalline salts investigated by searching the Cambridge Structural Database (CSD) and the Inorganic Chemistry Structural Database (ICSD). It has been shown that hydrogen carbonates form RINGs, with the notable exception of NaHCO3, while only CHAINs are known for hydrogen oxalates. Hydrogen squarates and hydrogen croconates can form both RINGs and CHAINs. The structures of Rb- and Cs- hydrogen croconates, which present the two alternative motifs, have been discussed together with that of the hydrated salt NaHC5O5.H2O. The relationship between RING and CHAIN has been examined in the light of ab initio calculations. A rigorous quantum chemical study of the nature of the interanionic (-)O-H...O(-) interaction in both vacuum and condensed phase has shown that the interaction energy is dominated by the electrostatic component which becomes attractive at short O...O distances (less than 2.5 A) if the net ionic charge on the anion is delocalised away from the -OH group. It has been demonstrated that the RING motif is slightly metastable with respect to dissociation in the gas phase, but becomes stable in the crystal owing to the influence of the Madelung field. However, the CHAIN motif is unstable both in the gas phase and in the crystal. It is argued that interanionic (-)O-H...O(-) interactions ought to be regarded as stabilising bonding interactions rather than proper intermolecular hydrogen bonds because the RING and CHAIN aggregates are not energetically stable on an absolute scale of bonding energy (i.e., in the absence of counterions). The presence of very short non-hydrogen-bridged O...O contacts resulting from charge compression of polyatomic anions bridged by alkali cations is also discussed.     

Polymorphs from supramolecular gels: four crystal forms of the same silver(I) supergelator crystallized directly from its gels

A new silver(I) complex with 1-phenyl-3-(quinolin-5-yl)urea has been prepared, acting as a supramolecular supergelator in the presence of polar solvents. In open vials temperature dependent, reversible gel-to-xerogel transition is observed, while in sealed vials direct crystallization of four polymorphs of the same gelling compound is achieved, depending on the gelling solvent.