Σ-type Interactions Research Articles

Characterization of hydrated Na+(phenol) and K+(phenol) complexes using infrared spectroscopy

Hydrated alkali metal ion-phenol complexes were studied to model these species in aqueous solution for M=Na and K. IR predissociation spectroscopy in the O-H stretch region was used to analyze the structures of M+(Phenol)(H2O)n cluster ions, for n = 1-4. The onset of hydrogen bonding was observed to occur at n=4. Ab initio calculations were used to qualitatively explore the types of hydrogen-bonded structures of the M+(Phenol)(H2O)4 isomers. By combining the ab initio calculations and IR spectra, several different structures were identified for each metal ion. In contrast to benzene, detailed in a previous study of Na+(Benzene)n(H2O)m [J. Chem. Phys. 110, 8429 (1999)], phenol is able to bind directly to Na+ even in the presence of four waters. This is likely the result of the sigma-type interaction between the phenol oxygen and the ion. With K+, the dominant isomers are those in which the phenol O-H group is involved in a hydrogen bond with the water molecules, while with Na+, the dominant isomers are those in which the phenol O-H group is free and the water molecules are hydrogen-bonded to each other. Spectra and ab initio calculations for the M+(Phenol)Ar cluster ions for M=Na and K are reported to characterize the free phenol O-H stretch in the M+(Phenol) complex. While pi-type configurations were observed for binary M+(Phenol) complexes, sigma-type configurations appear to dominate the hydrated cluster ions.

Reverse pulse polarography for EC processes. Electrochemistry of 1,1′-trimethylene-bis-(4-carbomethoxypyridinium) ion

The 1,1′-trimethylene-bis-(4-carbomethoxypyridinium) ion ( 3 ++ ) is reduced in tetrabutylammonium perchlorate (TBAP)+CH 3 CN at the dropping mercury electrode to the biradical 3 in an EC-type reaction, consisting of a reversible charge transfer 3 ++ +2e − = 3 op , followed by intramolecular cyclization ( 3 op being the open form of the biradical) to cyclomers. Depending on the time scale, different cyclomers are formed: in the sub-millisecond time range, the coupled chemical reaction results in the reversible formation of fast forming species 3 ff , cyclomers with π-type interactions ( cis -cyclomer B). In the time range of seconds, the slow forming species 3 sf with σ-type interactions ( trans -cyclomer) are dominant. The forward and backward rate constants of the reaction 3 ff = 3 sf are determined by the reverse pulse polarographic method: k f =3.0 s −1 , k b =1.0 s −1 . In the presence of metallic ions, 1:1 biradical–metal complexes are formed reversibly. The stoichiometry and formation constants are determined by the reverse pulse polarographic method: K 3 Li/op =1.2×10 4 l mol −1 ; K 3 Mg/op =1.1×10 5 l mol −1 .

Spectroscopic studies of mass selected clusters of Sr+ solvated by H2O and D2O

In this paper we present the results of spectroscopic studies on mass selected Sr+(H2O)n, n=1–4 and Sr+(D2O)n, n=1–6 clusters. Mass spectra of nascent clusters formed in our laser vaporization source show that hydrated metal ion species are predominant for n⩽4. Clusters larger than this size are more abundant in the hydrogen loss form SrOH+(H2O)n−1. The cluster size at which the product switching occurs is slightly larger (n=5) in the deuterated species. Photodissociation of all clusters results in both ligand loss and H/D atom loss occurring via an intracluster reaction. The monomer and dimer cluster species exhibit distinct absorption bands attributable to electronic excitation of the 5s valence electron of Sr+. Metal–ligand stretching frequencies are extracted from Franck–Condon progressions in the excited state. Spectroscopic parameters agree well with ab initio calculations [Bauschlicher et al., J. Chem. Phys. 96, 4453 (1992), and Sodupe et al., Chem. Phys. Lett. 212, 624 (1993)]. Analysis of the product branching ratios allows us to conclude that, when below the threshold for excited state dissociation, rapid internal conversion followed by an intracluster reaction preferentially occurs on the ground state surface, while evaporation occurs primarily in the excited state. We find evidence in the spectral profiles of clusters with n⩾3 for a consistent “substructure” characterized by a pπ state lacking σ-type interactions with the ligand molecules.

From chemisorption to mechanism on surfaces: An exploration of the pyrolysis of triisobutylaluminum in the chemical vapor deposition of aluminum thin films

The chemisorption of H, CH3, C2H5, and C2H4 is examined on Al(111) and Al(100) surfaces by the extended Hückel method, using a tight-binding formalism. A local chemical viewpoint is sought through fragment analyses, decompositions of the density of states and overlap populations. Various adsorption sites have been studied. On both surfaces, the first three species prefer to bind in the on-top site by a σ-type interaction with the coordinated Al atom. Ethylene, on the other hand, favors the twofold bridging site by a di-σ bonded interaction with its highest occupied molecular orbital and lowest unoccupied molecular orbital. In addition, the mobility of all these species on surfaces is investigated. Finally, the mechanism for the thermal decompositions of the triisobutylaluminum is studied. A β-hydride elimination rate-determining step is established, in which the activation barrier of the reaction in Al(111) is lower than Al(100). This result is in agreement with the experimental findings.

Thermochemical parameters for organic radicals and radical ions. Part 4. The relationship between bond dissociation enthalpy and radical stability in benzenoid systems

It is shown that isodesmic reactions used to assess interaction energies between substituents (X and Y) in benzenoid compounds have not been corrected for X/H and Y/H interactions. Excellent linear Mulliken charge – interaction energy relationships are established for meta- and para-substituted toluenes. These studies illustrate the relative importance of σ- and π-effects on a quantitative basis. The ρ-values obtained show an equal importance of -π-effects in meta- and para-substituted toluenes. The signs of the ρ(π)-values confirm the presence of resonance type -π-interactions. The role of ring hydrogens in σ-type interactions cannot be assessed from this study. At present, BDH values in benzenoid systems cannot be corrected such that they can be used to define radical stability. 13C chemical shifts are found not to correlate with isodesmic interaction energies. The application of 13C chemical shifts is limited by the inability to independently relate these shifts to σ- and -π-effects.

Nature of S ⋯· O interaction in short X –S ⋯· O contacts: charge density experimental studies and theoretical interpretation

2-(2-Chlorobenzoylimino)-1,3-thiazolidine (I) and 3-benzoylimino-4-methyl-1,2,4-oxathiazane (II) present short intramolecular S ⋯· O contacts of 2.68 and 2.24 A, respectively. X-Ray and neutron diffraction at 122 K performed on (II), for comparison with (I), already studied, led to experimental charge density deformation maps which exhibit a strong peak around the sulphur atom. Using simple molecular orbital theory, it is possible to interpret the shortening of S ⋯· O distances in these compounds by σ-type interaction between the oxygen p and the sulphur p and d orbitals. The significant variation of the equilibrium S ⋯· O distance with the nature of the atom bonded to S is explained in terms of the strength of the coupling between X–S antibonding orbital and oxygen ione-pair orbitals.

Electronic Structure of Transition Metal Complexes. III. A Comparative Study of Bondings in Metal Porphyrins

Abstract In order to elucidate π-type interaction between the metal and the ligand, the electronic structure of various metal porphyrins, [M(Por)], where M=Mn, Fe, Co, Ni, and Cu, has been investigated by the CNDO/2 method. The present calculation shows that the nature of σ-type interaction in M–N bond of porphyrin complex is similar to that of the corresponding ammine complex. The calculated ionization potentials reproduce the experimental data. The electron population of macrocycle π-electron systems in metal porphyrins is about 18 which accords with Hückel’s 4n+2 rule.

Magnetic and electrical properties of Ba 2MReO 6 ordered perovskites

Crystals of Ba 2MReO 6 ordered perovskites have been prepared and studied for M = Mn, Fe, Co, Ni, Mg and Y. Ferrimagnetism is found when M is Mn, Fe or Ni, and antiferromagnetism is found when M is Co. Ba 2FeReO 6 is a metallic conductor, and Mössbauer measurements show that the iron is high-spin and essentially trivalent. It is suggested, however, that a Fe +2Re +6 combination is nearly degenerate with this Fe +3Re +5 combination. All the other Ba 2MReO 6 ordered perovskites are semiconductors with low activation energies. Ordinary superexchange rules would predict ferromagnetism for these Ba 2MReO 6 compounds when M is Mn, Fe, Co or Ni. However, these rules assume σ-type interactions to be stronger than π-type interactions. The ferrimagnetism is rationalized by showing that in these compounds the π-type interactions dominate. Polycrystalline samples of Ba 2GdReO 6 and Ba 2ErReO 6 showed no magnetic order down to 4·2°K.