Organocatalysis: quantum chemical modeling of nucleophilic addition to α,β‐unsaturated carbonyl compounds

One of the successful transformations within the field of organocatalysis, the organocatalytic asymmetric addition of nitromethane to α,β‐unsaturated aldehydes and ketones, has been studied by quantum chemical modeling. The level of accuracy of the hybrid density functional theory method B3LYP/6‐31G(d) was compared to a high level ab initio benchmark for this reaction. It is concluded that B3LYP/6‐31G(d) performs very well for this reaction type, giving good estimates of critical energies. The reaction between acrolein and nitromethane was studied in detail. The reaction mechanism revealed an intermediate oxazolidin structure, which is currently unknown. Alkyl substitution in various positions on the amine catalyst or α,β‐unsaturated carbonyl compound influences the reactivity in a predictive fashion. The iminium ion, prop‐2‐en‐iminium, is less activated towards nucleophilic attack compared to protonated acrolein. Copyright © 2007 John Wiley & Sons, Ltd.     

The π N → e +e−N reaction close to the vector-meson production threshold

The π^? p→e ⁺ e ^? n and π⁺ n→e ⁺ e ^? p reaction cross sections are calculated below and in the vicinity of the vector-meson (?⁰,ω) production threshold. These processes are largely responsible for the emission of e ⁺e^? pairs in pion-nucleus reactions and contribute to the dilepton spectra observed in relativistic heavy ion collisions. They are dominated by the decay of low-lying baryon resonances into vector-meson-nucleon channels. The vector mesons materialize subsequently into e ⁺ e^? pairs. Using πN→?⁰ N and πN→ωN, amplitudes calculated in the center of mass energy interval 1.4 < √s<1.8 GeV, we compute the π^? p→e ⁺ e ^? n and π⁺ n→e ⁺ e ^? p reaction cross sections in these kinematics. Below the vector-meson production threshold, the π⁰?ω interference in the e ⁺ e^? channel appears largely destructive for the π^? p→e ⁺ e ^? n cross section and constructive for the π⁺ n→e ⁺ e ^? p cross section. The pion beam and the HADES detector at GSI offer a unique possibility to measure these effects. Such data would provide strong constraints on the coupling of vector-meson-nucleon channels to low-lying baryon resonances.     

Determination of calcium ion in oil-based drilling fluids with an ion-selective electrode

Calcium ion is determined in the internal phase of oil-based drilling fluids with a calcium ion-selective electrode after treatment with a 1:1 (v/v) xylene/isopropanol mixture and 4 M potassium chloride. Results obtained by this potentiometric method and by the Magcobar titration on laboratory-prepared muds and on a North Sea field mud (Statfjord) were in satisfactory agreement.     

Formation of adenine-N3/guanine-N7 cross-link in the reaction of trans-oriented platinum substrates with dinucleotides

The reactions of the anticancer complex trans-[PtCl2{(E)-HN=C(OMe)Me}2] (trans-EE) with a series of ribo and deoxyribodinucleotides have been studied by HPLC and 2D [1H, 15N] HMQC NMR spectroscopy and compared with those of the inactive trans isomer of cisplatin, trans-[PtCl2(NH3)2] (trans-DDP). Reactions of trans-EE with r(ApG) and d(ApG) take place through solvolysis of the starting substrate and subsequent formation of trans G-N7/monochloro and G-N7/monoaqua adducts. Slowly, the monofunctional adducts evolve to a bifunctional adduct forming an unprecedented and unexpected A-N3/G-N7 platinum cross-link spanning two trans positions. For stereochemical reasons, trans platinum complexes cannot form N7/N7 cross-links between adjacent purines in di- or polynucleotides. For the reverse sequence r(GpA), no chelate structure was formed even after a two-week reaction. The reaction of trans-DDP with r(ApG) produces many more products than the analogous reaction with trans-EE. One of these products was identified as the A-N3/G-N7 trans-chelate.     

Volume and compressibility changes of complex formation between 18-Crown-6 and NaCl,KCl, and CsCl in water

The apparent molal volumes and compressibilities of NaCl, KCl, and CsCl in mixtures of 18-Crown-6 and water have been calculated from density and speed-of-sound measurements at 25°C. The partial molal volumes and compressibilities of the salts when all cations have formed complexes with 18-Crown-6 molecules have been evaluated. The sign and magnitude of the volume and compressibility changes of complex formation strongly suggest that the charge of the cation becomes very effectively screened by the crown ether.     

The unimolecular chemistry of protonated glycine and the proton affinity of glycine: a computational model

The potential energy hypersurface of protonated glycine, GH⁺, has been investigated. The calculated G2(MP2) value for the proton affinity (PA) of glycine, PA _calc=895kJ mol⁻¹, is in good agreement with the experimental value which has been estimated to lie in the range 864kJ mol⁻¹ < PA _exp <891kJ mol⁻¹. Ab initio quantum chemical calculations of relevant parts of the potential energy surface of GH⁺ give a reaction model which is consistent with the observed mass spectrometric fragmentation pattern. The lowest energy unimolecular reactions of GH⁺ are two distinct processes: (1) loss of CO, which has a substantial barrier for the reverse reaction, and (2) loss of CO plus H₂O, which has no barrier for the reverse reaction. Received: 15 November 1996 / Accepted: 6 May 1997     

On the gas-phase reactivity of complexed OH+ with halogenated alkanes

OH(+) is an extraordinarily strong oxidant. Complexed forms (L--OH(+)), such as H(2)OOH(+), H(3)NOH(+), or iron-porphyrin-OH(+) are the anticipated oxidants in many chemical reactions. While these molecules are typically not stable in solution, their isolation can be achieved in the gas phase. We report a systematic survey of the influence on L on the reactivity of L--OH(+) towards alkanes and halogenated alkanes, showing the tremendous influence of L on the reactivity of L--OH(+). With the help of with quantum chemical calculations, detailed mechanistic insights on these very general reactions are gained. The gas-phase pseudo-first-order reaction rates of H(2)OOH(+), H(3)NOH(+), and protonated 4-picoline-N-oxide towards isobutane and different halogenated alkanes C(n)H(2n+1)Cl (n=1-4), HCF(3), CF(4), and CF(2)Cl(2) have been determined by means of Fourier transform ion cyclotron resonance measurements. Reaction rates for H(2)OOH(+) are generally fast (7.2x10(-10)-3.0x10(-9) cm(3) mol(-1) s(-1)) and only in the cases HCF(3) and CF(4) no reactivity is observed. In contrast to this H(3)NOH(+) only reacts with tC(4)H(9)Cl (k(obs)=9.2x10(-10)), while 4-CH(3)-C(5)H(4)N-OH(+) is completely unreactive. While H(2)OOH(+) oxidizes alkanes by an initial hydride abstraction upon formation of a carbocation, it reacts with halogenated alkanes at the chlorine atom. Two mechanistic scenarios, namely oxidation at the halogen atom or proton transfer are found. Accurate proton affinities for HOOH, NH(2)OH, a series of alkanes C(n)H(2n+2) (n=1-4), and halogenated alkanes C(n)H(2n+1)Cl (n=1-4), HCF(3), CF(4), and CF(2)Cl(2), were calculated by using the G3 method and are in excellent agreement with experimental values, where available. The G3 enthalpies of reaction are also consistent with the observed products. The tendency for oxidation of alkanes by hydride abstraction is expressed in terms of G3 hydride affinities of the corresponding cationic products C(n)H(2n+1) (+) (n=1-4) and C(n)H(2n)Cl(+) (n=1-4). The hypersurface for the reaction of H(2)OOH(+) with CH(3)Cl and C(2)H(5)Cl was calculated at the B3 LYP, MP2, and G3(m*) level, underlining the three mechanistic scenarios in which the reaction is either induced by oxidation at the hydrogen or the halogen atom, or by proton transfer.     

C-H activation of alkanes on Rh+ n (n=1-30) clusters: size effects on dehydrogenation

The rate coefficients for the dehydrogenation of ethane, propane, and isobutane with cationic rhodium atoms Rh+ and clusters Rh+ n of up to 30 atoms were measured under single-collision conditions in a Fourier-transform ion cyclotron resonance mass spectrometer. The reaction rates are cluster size dependent and parallel for all the three alkanes. While the reactions proceed close to the theoretical collision rates for a large number of clusters, characteristic minima are observed for Rh+ (5/6/9/19/28). The degree of dehydrogenation varies with the cluster size with maxima for 10< or =n< or =15 for the three alkanes and for n=3 and 2-4 in the cases of ethane and propane, respectively. However, complete dehydrogenation is only observed for the reaction of Rh+ 11 with propane. Dehydrogenation is remarkably selective and no other neutral products than H2 are observed. The results are interpreted in terms of likely cluster geometries.     

Spatial dependence of meson correlation functions at high temperature

The spatial dependence of meson correlation functions at high temperature is studied in perturbative QCD, keeping only the lowest order term. We obtain analytic results for the static correlation function in this approximation. Problems connected with the regularization of the divergent expressions are discussed in detail. The meson screening mass is determined from the form of the correlation function at large distances. We obtain , which agrees with the results of Eletskii and Ioffe, for massless quarks (M=0). Finally, the correlation function in the temporal direction is briefly discussed. For massless quarks, we find the corresponding screening massm _scr=2T.Research supported in part by the Polish State Committee for Scientific Research under Grant No. 2.0204.91.01