Andrzej Bil

C70 Oxides and Ozonides and the Mechanism of Ozonolysis on the Fullerene Surface. A Theoretical Study

A series of ab initio calculations have been carried out to determine why the a,b- and c,c-isomers are the most commonly observed mono-oxides of C(70) in ozonolysis reactions, when existing calculations in the literature report that these structures are not the most stable conformations. We show that the a,b- and c,c-isomers are the two most stable structures on the C(70)O(3) potential energy surface, which suggests that the reaction pathway toward oxide formation must proceed via the corresponding ozonide structure. From our calculations, we offer a mechanism for the thermally induced dissociation of C(70)O(3) that share the first two steps with the general mechanism for ozonolysis of alkenes proposed by Criegee. We suggest further steps that involve C(70)O(3) losing O(2) in its triplet or singlet state, thus leaving C(70)O in its triplet or singlet state, respectively. A pair of products in their singlet states seems to be more likely for the decomposition of a,b-C(70)O(3), which ultimately leads to the closed a,b-C(70)O epoxide structure. For c,c-C(70)O(3), the more thermodynamically favorable route is the triplet channel, resulting in the triplet open c,c-C(70)O oxidoannulene structure, which may subsequently decay to the singlet ground state c,c-C(70)O epoxide form. This finding offers an alternative interpretation of the experimental observations which reported an open d,d-C(70)O oxidoannulene structure as the metastable intermediate.

The hydroperoxy radical as a hydrogen bond acceptor. HOO-HCl complexes : Ab initio study

Protonacceptor properties of the HOO radical were investigated previously by means of ab initio as well as topological Atoms in Molecules (AIM) and Electron Localization Function (ELF) methods. It was pointed out that in the radical there are three nonequivalent positions most susceptible to protonation, and on this basis three structures of possible hydrogen bonded complexes were proposed. Results reported in the present article concern all possible 1:1 complexes formed by HCl and HOO molecules, and fully confirm suppositions given on the basis of the above‐mentioned investigations. There are three various complexes referring to the local minima, and the transition structure predicted by topological methods has been found as well. The cyclic structure appeared to be the most stable one, which confirms conclusions given in the experimental article. Apart from structure optimization, harmonic as well as anharmonic spectra of the complexes have been simulated. Anharmonicity of HCl stretching vibration was of special interest, as the frequency of this vibration characterizes the ClH · · · O hydrogen bond in these complexes. To obtain values of these frequencies the one‐dimensional Hamiltonian has been diagonalized numerically. The potential for this Hamiltonian has been taken from a set of single‐point scanning of the part of the Potential Energy Surface (PES) connected with this vibration. The potential calculated on the MP2 level leads to the result close to the experimental value, whereas the B3LYP method is inappropriate for the purpose of PES investigation of these complexes.

Modifying the fullerene surface using endohedral noble gas atoms: density functional theory based molecular dynamics study of C70O3.

We have performed a series of ab initio molecular orbital and molecular dynamics calculations to ascertain the influence of an endohedral noble gas atom on the reactivity of the surface of the model system C(70)O(3). Our simulations show that the minimum energy pathways for the ozone ring-opening reaction are influenced by the presence of the endohedral atom. The effect is isomer dependent, with the enthalpy of the reaction increasing for a,b-C(70)O(3) and decreasing for e,e-C(70)O(3) when doped with the heavy noble gas atoms Xe and Rn.

Dimethylalkoxygallanes: Monomeric versus Dimeric Gas-Phase Structures

The molecular structures of the vapors produced on heating dimethylalkoxygallanes of the type [Me(2)Ga(OR)](2) have been determined by gas electron diffraction and ab initio molecular orbital calculations. In the solid state [Me(2)Ga(OCH(2)CH(2)NMe(2))](2) (1) and [Me(2)Ga(OCH(2)CH(2)OMe)](2) (2) adopt dimeric structures, although only the monomeric forms [Me(2)Ga(OCH(2)CH(2)NMe(2))] (1a) and [Me(2)Ga(OCH(2)CH(2)OMe)] (2a) were observed in the gas phase. For comparison the structure of the vapor produced on heating [Me(2)Ga(O(t)Bu)](2) (3) was also studied by gas electron diffraction. In contrast to 1 and 2, compound 3 is dimeric in the gas phase, as well as in the solid state. The gas-phase structures of 1a and 2a exhibit five-membered rings formed by a dative bond between Ga and the donor atom (N or O) from the donor-functionalized alkoxide. In 3 there is no possibility of a monomeric structure being stabilized by the formation of such a dative bond since only a monofunctional alkoxide is present in the molecule.

Clustering of sulfamic acid: ESI MS and theoretical study

Sulfamic acid has wide application in industry and has been suggested to act as an effective nucleation agent for the formation of aerosols and cloud particles. From the point of view of the role that sulfamic acid may play in aerosol formation, the study of its homoaggregation is important. Gas phase clustering study was performed for sulfamic acid H3N·SO3, (ASA), from water and methanol-water solutions, by help of a TOF-Q spectrometer equipped with electrospray ionization source, in the negative-ion mode. The structure and stability of the (H3N·SO3)n and [(H3N·SO3)n-H](-) (n = 1-6) were studied using DFT/B3LYP/aug-cc-pVDZ method. The ESI MS study evidenced that both singly and doubly charged clusters are formed when the acids are electrosprayed from water solutions; they may be described as [(H3N·SO3)n-zH](z-) where z = 1 or 2. The largest identified clusters are built of 20 monomers. The theoretical studies showed that formation of higher order (ASA)n aggregates in the gas phase is energetically profitable. In contrast with the gas phase, aqueous solution does not favor the formation of (ASA)n aggregates. The study led to the conclusion that the ASA clusters are formed in the gas phase under the experimental conditions of the mass spectrometer. A hypothetical mechanism concerning the formation of the doubly negatively charged anionic aggregates is discussed. The obtained data suggest that small (NH3·SO3)n aggregates may also contribute to formation of aerosols in heavily polluted atmospheres with relatively large NH3 concentration.

Donor-Acceptor Complexes between Ammonia and Sulfur Trioxide: An FTIR and Computational Study.

The complexes of ammonia with sulfur trioxide have been studied using FTIR matrix isolation spectroscopy and DFT/B3LYP calculations with the aug-cc-pVTZ basis set. The NH3/SO3/Ar matrixes were prepared in two different ways. In one set of experiments the matrix was prepared by the simultaneous deposition of the NH3/Ar mixture and SO3 vapor from the thermal decomposition of K2S2O7. In the second set of experiments thermolysis products of sulfamic acid were trapped in an argon matrix. Both methods of matrix preparation led to the formation of the H3N·SO3 electron donor-acceptor complex that was characterized earlier. In the matrixes comprising thermolysis products of sulfamic acid, in addition to H3N·SO3, the H3N-SO3···NH3 complex (II(D)) was also identified. The identity of the complex was confirmed by comparison of the experimental and theoretical spectra of H3N-SO3···NH3 and D3N-SO3···ND3. The performed calculations show that in H3N-SO3···NH3 the two N atoms and the S atom are collinear; the two S-N bonds are nonequivalent, one is much shorter (2.230 Å) than the other one (2.852 Å). In the AIM topological analysis, the interaction energy decomposition and topological properties of the electron localizability indicator (ELI-D) allowed us to categorize the stronger N-S bond in the II(D) complex as a dative bond and to assume that the fragile N-S bond is a consequence of a weak electron-donor-acceptor interaction. The calculations indicate that the identified II(D) complex corresponds to a local minimum on the PES of the NH3/SO3 system of 2:1 stoichiometry. The (NH3)2SO3 complex, II(HB), corresponding to a global minimum is 7.95 kcal mol(-1) more stable than the II(D) complex. The reason that the II(D) complex is present in the matrix but not the II(HB) complex is discussed.

OH-Induced Oxidative Cleavage of Dimethyl Disulfide in the Presence of NO

We report the results of the theoretical study of (•)OH-induced oxidative cleavage of dimethyl disulfide (DMDS) and the experimental study of the CH3SSCH3 + (•)OH reaction in the presence of (•)NO. Infrared low temperature argon matrix studies combined with ab initio calculations allowed us to identify cis-CH3SONO, which evidences the formation of the CH3SO(•) and CH3SH molecules in the course of the CH3SSCH3 + (•)OH reaction. Ab initio/quantum chemical topology calculations revealed details of the oxidative cleavage of dimethyl disulfide, which is a complex multistep process involving an alteration of S-O and S-S covalent bonds as well as a hydrogen atom transfer. The ability of delocalization of the unpaired electron density by sulfur atoms and a formation of a hydrogen bond by CH3SO(•) and CH3SH are the factors which seem to explain antiradical properties of DMDS.

Photo-Induced Hydrogen Exchange Reaction between Methanol and Glyoxal: Formation of Hydroxyketene

We study the structure and photochemistry of the glyoxal-methanol system (G-MeOH) by means of FTIR matrix isolation spectroscopy and ab initio calculations. The FTIR spectra show that the non-hydrogen-bonded complex, G-MeOH-1, is present in an inert environment of solid argon. MP2/aug-cc-pVDZ calculations indicate that G-MeOH-1 is the most stable complex among the five optimized structures. The interaction energy partitioned according to the symmetry-adapted perturbation theory (SAPT) scheme demonstrates that the dispersion energy gives a larger contribution to the stabilization of a non-hydrogen-bonded G-MeOH-1 complex than compared to the hydrogen-bonded ones. The irradiation of G-MeOH-1 with the filtered output of a mercury lamp (lambda>370 nm) leads to its photo-conversion into the hydroxyketene-methanol complex HK-MeOH-1. The identity of HK-MeOH-1 is confirmed by both FTIR spectroscopy and MP2/aug-cc-pVDZ calculations. An experiment with deuterated methanol (CH(3)OD) evidences that hydroxyketene is formed in a photo-induced hydrogen exchange reaction between glyoxal and methanol. The pathway for the photo-conversion of G-MeOH-1 to HK-MeOH-1 is studied by a coupled-cluster method [CR-CC(2,3)]. The calculations confirm our experimental findings that the reaction proceeds via hydrogen atom exchange between the OH group of methanol and CH group of glyoxal.

The OH-Initiated Oxidation of CS2 in the Presence of NO: FTIR Matrix-Isolation and Theoretical Studies.

We studied the photochemistry of the carbon disulfide-nitrous acid system with the help of Fourier transform infrared (FTIR) matrix isolation spectroscopy and theoretical methods. The irradiation of the CS2···HONO complexes, isolated in solid argon, with the filtered output of the mercury lamp (λ > 345 nm) was found to produce OCS, SO2, and HNCS; HSCN was also tentatively identified. The (13)C, (15)N, and (2)H isotopic shifts as well as literature data were used for product identifications. The evolution of the measured FTIR spectra with irradiation time and the changes in the spectra after matrix annealing indicated that the identified molecules are the products of different reaction channels: OCS being a product of another reaction path than SO2 and HNCS or HSCN. The possible reaction channels between SC(OH)S/SCS(OH) radicals and NO were studied using DFT/B3LYP/aug-cc-pVTZ method. The SC(OH)S and/or SCS(OH) intermediates are formed when HONO attached to CS2 photodissociates into OH and NO. The calculations indicated that SC(OH)S radical can form with NO two stable adducts. The more stable SC(OH)S···NO structure is a reactant for a simple one-step process leading to OCS and HONS molecules. An alternative, less-stable complex formed between SC(OH)S and NO leads to formation of OCS and HSNO. The calculations predict only one stable complex between SCS(OH) radical and NO, which can dissociate along two channels leading to HNCS and SO2 or HSCN and SO2 as the end products. The identified photoproducts indicate that both SC(OH)S and SCS(OH) adducts are intermediates in the CS2 + OH + NO reaction leading to different reaction products.

Gas-phase structures of dithietane derivatives, including an electron diffraction study of 1,3-dithietane 1,1,3,3-tetraoxide

The gas electron diffraction structure of 1,3-dithietane 1,1,3,3-tetraoxide has been determined using the SARACEN method to restrain parameters that otherwise could not be refined. Quantum chemical calculations for this species showed that the potential-energy surface was extremely flat, and this was also observed from the diffraction experiments. The difference in goodness of fit for the diffraction experiment between a planar ring and one puckered by up to 9° was very small. Calculations were also performed for a variety of similar species with different numbers of oxygen atoms attached to the sulphur atoms. Topological analysis of the electron density, and electron localisation function studies of the relevant molecules, have given deeper insight into the nature of their bonding, and suggested how spatial localisation of electron pairs may influence the molecular structure.