On the Heterogeneous Nature of Cisplatin-1-Methyluracil Complexes: Coexistence of Different Aggregation Modes and Partial Loss of NH3 Ligands as Likely Explanation

The conversion of the 1 : 1-complex of Cisplatin with 1-methyluracil (1MeUH), cis-[Pt(NH₃)₂(1MeU-N3)Cl] ( 1 a ) to the aqua species cis-[Pt(NH₃)₂(1MeU-N3)(OH₂)]⁺ ( 1 b ), achieved by reaction of 1 a with AgNO₃ in water, affords a mixture of compounds, the composition of which strongly depends on sample history. The complexity stems from variations in condensation patterns and partial loss of NH₃ ligands. In dilute aqueous solution, 1 a , and dinuclear compounds cis-[(NH₃)₂(1MeU-N3)Pt(μ-OH)Pt(1MeU-N3)(NH₃)₂]⁺( 3 ) as well as head-tail cis-[Pt₂(NH₃)₄(μ-1MeU-N3,O4)₂]²⁺ ( 4 ) represent the major components. In addition, there are numerous other species present in minor quantities, which differ in metal nuclearity, stoichiometry, stereoisomerism, and Pt oxidation state, as revealed by a combination of ¹H NMR and ESI-MS spectroscopy. Their composition appears not to be the consequence of a unique and repeating coordination pattern of the 1MeU ligand in oligomers but rather the coexistence of distinctly different condensation patterns, which include μ-OH, μ-1MeU, and μ-NH₂ bridging and combinations thereof. Consequently, the products obtained should, in total, be defined as a heterogeneous mixture rather than a mixture of oligomers of different sizes. In addition, a N₂ complex, [Pt(NH₃)(1MeU)(N₂)]⁺ appears to be formed in gas phase during the ESI-MS experiment. In the presence of Na⁺ ions, multimers n of 1 a with n=2, 3, 4 are formed that represent analogues of non-metalated uracil quartets found in tetrastranded RNA.     

Retardation of monomolecular reactions in the solid phase

The rate constants of initial monomolecular stages of thermal decomposition in the solid phase were measured for 22 organic compounds. The ratio of rate constants of decomposition in the melt and solid state, characterizing the reaction retardation in the crystal lattice, was determined. The retardation effect was compared to the physical properties of the crystal. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1261–1264, July, 1999.     

Photoemission study of glycine adsorption on Cu/Au(1 1 1) interfaces

The adsorption of glycine on Au(1 1 1) pre-deposited with different amounts of Cu was investigated with both conventional X-ray photoelectron spectroscopy (XPS) and synchrotron-based photoemission. In the Cu submonolayer range, glycine physically adsorbs on the Cu/Au(1 1 1) surfaces in its zwitterionic form and completely desorbs at 350 K. The C 1s, O 1s and N 1s core level binding energies monotonically increase with Cu coverage. This indicates that, in the Cu submonolayer range, the admetal is alloyed with Au rather than forming overlayers on the Au(1 1 1) substrate, consistent with our recent experimental and theoretical results [X. Zhao, P. Liu, J. Hrbek, J.A. Rodriguez, M. Pérez, Surf. Sci. 592 (2005) 25]. Upon increasing the amount of deposited Cu over 1 ML, part of the glycine overlayer transforms from the zwitterionic form to the anionic form (NH₂CH₂COO⁻) and adsorbs chemically on the Cu/Au(1 1 1) surface with the N 1s binding energy shifted by −2.3 eV. When the amount of deposited Cu is at 3.0 or 6.0 ML, the intensity of the N 1s chemisorption peak increases with aging time at 300 K. It indicates that glycine adsorption induces Cu segregation from the subsurface region onto the top layer of the substrate. Judging from the initial N 1s peak intensities, it is concluded that 64% and 36% of the top layer are still occupied by Au atoms before glycine adsorption even when the amounts of deposited Cu are 3.0 and 6.0 ML, respectively. On the Au(1 1 1) surface pre-dosed with 6.0 ML of Cu, part of the chemisorbed glycine will desorb and part will decompose upon heating to 450-500 K. In addition, about 20% of the glycine exists in the neutral form when the glycine overlayer was dosed on Cu/Au(1 1 1) held at 100 K.     

Multicomponent synthesis of dihydropyrimidines and thiazines

A broad range of differently substituted dihydropyrimidines and thiazines can be efficiently prepared by using a four-component reaction between phosphonates, nitriles, aldehydes, and iso(thio)cyanates. The scope and limitations of this multicomponent reaction are fully described. Variation of all four components has been investigated. The nitrile and aldehyde inputs can be varied extensively, but variation of the phosphonate input remains limited. An interesting rearrangement leading to phosphoramidates has been observed. Furthermore, the multicomponent reaction seems to be restricted to the use of isocyanates with strongly electron-withdrawing substituents, but an interesting additional exchange reaction under microwave conditions leads to dihydropyrimidines with less electron-withdrawing substituents at N3. In addition, a diastereoselective formation of dihydropyrimidines has been observed when using a chiral aldehyde as the input. Finally, by changing the isocyanate component to an isothiocyanate, thiazines are efficiently formed instead of the corresponding thio-dihydropyrimidines.     

The first application of an imidazole o-quinodimethane in Diels-Alder reactions leading to the synthesis of benzimidazoles

An efficient procedure for the generation of the imidazole-4,5-quinodimethane intermediate 4 from 2-bromo-4,5-bis(bromomethyl)imidazole derivative 3 in boiling toluene in the presence of 18-crown-6 is described. o-Quinodimethane 4 was captured for the first time by several symmetrically and unsymmetrically substituted dienophiles to afford the corresponding Diels-Alder benzimidazole adducts.     

Density functional theory study of hydrogen sulfide dissociation on bi-metallic Ni-Mo catalysts

This work presents results on the dissociation of H₂S over Ni-Mo catalysts suggesting that the presence of surface Mo-atom(s) has a significant impact on both the energetics of the process and the reaction mechanism. The presence of one Mo atom provides an additional energetic advantage of 10.2 kcal/mol overall. While increasing the energetic advantage of the process, the presence of Mo atom also increases the activation barriers by at most 3 kcal/mol. The large exothermic nature of this process combined with the comparatively small activation barriers suggests that the H₂S dissociation process is a facile process on all of the surfaces studied here. Additionally, analysis was provided to explain the difference in catalytic behavior between a bi-metallic alloy and a bi-metallic sulfide. It was determined that the bi-metallic alloy binds sulfur strongly (>100 kcal/mol) which can be compared with the results of Sun and co-workers [M. Sun, A.E. Nelson, J. Adjaye, Catal. Today 105 (2005) 36] who predict that S adsorption on the metal sulfide phase is not energetically favorable. It is suggested that the sulfide surface does not bind S in an energetically favorable manner because the sulfide surface structure does not possess a binding site that can emulate the hollow site on a metal surface.     

Dynamic thermal analysis of solid-state reactions

There are many reactions of interest in which one or more of the reactants belong to some solid phases. Modern thermoanalytical instruments can conveniently provide reaction kinetic data of high precision and accuracy, from which the underlying activation energyE may be derived in principle. Unfortunately, no best method yet exists for the derivation when the data have been collected with a programmed linear increase in sample temperature, unlike the case of isothermal measurements, which however suffer from experimental limitations [1]. Here we propose a method for extractingE from non-isothermal data, that promises general validity.     

Pre-equilibrium effects in (n, 2n) reactions at 14.2 MeV+

With a view to study the pre-equilibrium effects in neutron-induced reactions, the activation cross-sections for (n, 2n) reactions at 14.2±0.2 MeV in the heavy mass region have been measured using the versatile mixed powder technique and high resolution Ge(Li) detection. The experimental cross-sections are found to be consistently smaller than the predictions based on the statistical theory and this is attributed to the effect of pre-equilibrium decay in these reactions. The cross-sections due to pre-equilibrium decay were estimated using exciton, hybrid and unified models. When this cross-section was included in comparing the experimental cross-sections with theory, we obtained better agreement, within the limitations of the present-day preequilibrium theories. Preliminary results of this work were reported at the Nuclear Physics and Solid State Physics Symposium, Pune (1978).     

Quantum dynamics of gas-phase SN2 reactions.

Understanding the state-resolved dynamics of elementary chemical reactions involving polyatomic molecules, such as the well-known reaction mechanism of nucleophilic bimolecular substitution (SN2), is one of the principal goals in chemistry. In this Review, the progress in the quantum mechanical treatment of SN2 reactions in the gas phase is reviewed. The potential energy profile of this class of reactions is characterized by two relatively deep wells, which correspond to pre- and post-reaction chargedipole complexes. As a consequence, the complex-forming reaction is dominated by Feshbach resonances. Calculations in the energetic continuum constitute a major challenge because the high density of resonance states imposes considerable requirements on the convergence and the energetic resolution of the scattering data. However, the effort is rewarding because new insights into the details of multimode quantum dynamics of elementary chemical reactions can be obtained.