Tautomerization of adenine facilitated by water: computational study of microsolvation

We present calculations for the mechanism and the barrier heights of tautomerization of adenine. We find various pathways for the 9(H) <--> 7(H) and 9(H) <--> 3(H) tautomerization. One mechanism for the 9(H) --> 7(H) tautomerization involves an sp(3)- or carbene-type intermediate, whereas the other proceeds via imine intermediates. Tautomerization from the 9(H) tautomer to 7(H) or 3(H) is predicted to occur with a very large activation barrier (60-70 kcal/mol), indicating that the processes may not occur readily in the gas phase. Interactions with the water molecule(s) are found to lower the barrier tremendously. We suggest that dramatic lowering of the 9(H) --> 3(H) and 9(H) --> 7(H) barriers by microsolvating water molecules may facilitate the formation and observation of the 7(H) and 3(H) tautomers in the solution phase.     

Insights into the absorption of carbon dioxide by zinc substrates: isolation and reactivity of di- and tetranuclear zinc complexes

The heptadentate Schiff base H(3)L can react with zinc acetate to form the discrete dinuclear complex Zn(2)L(OAc)(H(2)O), 1.H(2)O. The reaction of 1.H(2)O with NMe(4)OH.5H(2)O both in air and under an argon stream has been investigated. On one hand, this reaction in air yields the tetranuclear complex (Zn(2)L)(2)(CO(3))(H(2)O)(6), 2.5H(2)O, by spontaneous absorption of adventitious carbon dioxide. This process can be reverted in an acetic acid medium, whereas the treatment of 2.5H(2)O with methanoic acid yields crystals of [Zn(2)L(HCOO)].0.5MeCN.1.25MeOH.2H(2)O, 3.0.5MeCN.1.25MeOH.2H(2)O. On the other hand, the interaction under an argon atmosphere of 1.H(2)O with NMe(4)OH.5H(2)O in methanol allows the isolation of the dinuclear complex Zn(2)L(OMe)(H(2)O)(4), 4.4H(2)O. Recrystallisations of 1.H(2)O, 2.5H(2)O and 4.4H(2)O, in different solvents, yielded single crystals of 1.MeCN.2.5H(2)O, 2.4MeOH and 4.3MeOH.H(2)O, respectively. The crystal structure of 2.4MeOH can be understood as resulting from an unusual asymmetric tetranuclear self-assembly from two dinuclear units, and shows three different geometries around the four zinc atoms.     

Discovery of the potential of minimum mass for platinum electrodes

Electrochemical quartz-crystal nanobalance (EQCN) analysis of the behavior of Pt in aqueous H(2)SO(4) reveals that the interfacial mass reaches a minimum, the potential of minimum mass (E(pmm)), at 0.045 V. A similar behavior is observed for Pt in aqueous HClO(4) and NaOH. E(pmm) is a new parameter describing the electrochemical interface. The value of E(pmm) coincides with the completion of the saturation layer of electroadsorbed H (H(UPD)) and the commencement of H(2)(g) generation or H(2)(g) electro-oxidation. The value of E(pmm) and the structure of the Pt/electrolyte interface are discussed in terms of the interactions of the anions H(3)O(+), H(UPD), H(OPD), and H(2)O with Pt. The layer of H(UPD) embedded in the Pt surface lattice minimizes the surface dipole-water dipole and surface charge-water dipole interactions, thus reduces the wetting ability of Pt. Consequently, the discharge of H(3)O(+) in the electrolytic formation of H(2)(g) or the dissociative adsorption of H(2)(g) that precedes its electro-oxidation to H(3)O(+) proceed easily on Pt, because the species do not have to displace H(2)O molecules. Effective and inexpensive non-platinum electrocatalysts for the electrolytic H(2)(g) generation in water electrolyzers or H(2)(g) electro-oxidation in polymer electrolyte membrane fuel cells should mimic the interfacial behavior of Pt by minimizing the interaction of H(2)O molecules with the electrode.     

Synthesis and structural variations of substituted phenylamide complexes of the heavy alkaline earth metals calcium, strontium and barium

Starting material KN(H)C(6)H(3)-2,6-F(2) was prepared via a transamination reaction from KNH(2) and 2,6-F(2)C(6)H(3)NH(2) in THF and crystallized from 1,4-dioxane (diox) as the three-dimensional polymer [(diox)(1.5)K{N(H)-2,6-F(2)C(6)H(3)}.diox(0.5)](infinity) (1). The metathesis reaction of (THF)(4)CaI(2) with KN(Me)Ph in THF yields monomeric (THF)(4)Ca[N(Me)Ph](2) (2) with a nearly linear N-Ca-N moiety of 179.84(8) degrees . The metathesis reaction of (THF)(4)CaI(2) with KN(H)Mes yields trinuclear (THF)(6)Ca(3)[N(H)Mes](6) (3) with a linear Ca(3) fragment and bridging 2,4,6-trimethylphenylamido groups. The reaction of 1 with (THF)(4)CaI(2) gives dinuclear (THF)(5)Ca(2)[N(H)-2,6-F(2)C(6)H(3)](4).2THF (4) with three bridging and one terminally bound 2,6-difluorophenylamide. A similar reaction of (THF)(5)SrI(2) with KN(H)-2,6-F(2)C(6)H(3) yields dinuclear (THF)(6)Sr(2)[N(H)-2,6-F(2)C(6)H(3)](3)I.THF (5) in which the iodide anion binds terminally. This iodide ligand cannot be substituted as easily by excess KN(H)-2,6-F(2)C(6)H(3). The metathesis reaction of (THF)(5)BaI(2) with KN(H)-2,6-F(2)C(6)H(3) leads to the formation of [(THF)(2)Ba{N(H)-2,6-F(2)C(6)H(3)}(2)](infinity) (6) which crystallizes as a one-dimensional polymer with bridging 2,6-difluorophenylamide anions and additional Ba-F-bonds.     

Remarkable reactions of cyclooctatetraene diiron-bridging carbyne complexes with amino and amido compounds: nucleophilic addition to and breaking of the cyclooctatetraene ring

The reactions of the cationic, diiron-bridging carbyne complexes [Fe(2)(mu-CAr)(CO)(4)(eta(8)-C(8)H(8))]BF(4) (1, Ar=C(6)H(5); 2, Ar=p-CH(3)C(6)H(4); 3, Ar=p-CF(3)C(6)H(4)) with LiN(C(6)H(5))(2) in THF at low temperature gave novel N-nucleophilic-addition products, namely, the neutral, diiron-bridging carbyne complexes [Fe(2)(mu-CAr)(CO)(4)(eta(7)-C(8)H(8)N(C(6)H(5))(2))] (4, Ar=C(6)H(5); 5, Ar=p-CH(3)C(6)H(4); 6, Ar=p-CF(3)C(6)H(4))). Cationic bridging carbyne complexes 1-3 react with (C(2)H(5))(2)NH, (iC(3)H(7))(2)NH, and (C(6)H(11))(2)NH under the same conditions with ring cleavage of the COT ligand to produce the novel diiron-bridging carbene inner salts [Fe(2)[mu-C(Ar)C(8)H(8)NR(2)](CO)(4)] (7, Ar=C(6)H(5), R=C(2)H(5); 8, Ar=p-CH(3)C(6)H(4), R=C(2)H(5); 9, Ar=p-CF(3)C(6)H(4), R=C(2)H(5); 10, Ar=C(6)H(5), R=iC(3)H(7); 11, Ar=p-CH(3)C(6)H(4), R=iC(3)H(7); 12, Ar=p-CF(3)C(6)H(4), R=iC(3)H(7); 13, Ar=C(6)H(5), R=C(6)H(11); 14, Ar=p-CH(3)C(6)H(4), R=C(6)H(11), 15, Ar=p-CF(3)C(6)H(4), R=C(6)H(11)). Piperidine reacts similarly with cationic carbyne complex 3 to afford the corresponding bridging carbene inner salt [Fe(2)[mu-C(Ar)C(8)H(8)N(CH(2))(5)](CO)(4)] (16). Compound 9 was transformed into a new diiron-bridging carbene inner salt 17, the trans isomer of 9, by heating in benzene. Unexpectedly, the reaction of C(6)H(5)NH(2) with 2 gave a novel COT iron-carbene complex [Fe(2)[=C(C(6)H(4)CH(3)-p)NHC(6)H(5)](mu-CO)(CO)(3)(eta(8)-C(8)H(8))] (18). However, the analogous reactions of 2-naphthylamine with 2 and of p-CF(3)C(6)H(4)NH(2) with 3 produce novel chelated iron-carbene complexes [Fe(2)[=C(C(6)H(4)CH(3)-p)NC(10)H(7)](CO)(4)(eta(2):eta(3):eta(2)-C(8)H(9))] (19) and [Fe(2)[=C(C(6)H(4)CF(3)-p)NC(6)H(4)CF(3)-p](CO)(4)(eta(2):eta(3):eta(2)-C(8)H(9))] (20), respectively. Compound 18 can also be transformed into the analogous chelated iron-carbene complex [Fe(2)[=C(C(6)H(4)CH(3)-p)NC(6)H(5)](CO)(4)(eta(2):eta(3):eta(2)-C(8)H(9))] (21). The structures of complexes 6, 9, 15, 17, 18, and 21 have been established by X-ray diffraction studies.     

Deviation between the chemistry of Ce(IV) and Pu(IV) and routes to ordered and disordered heterobimetallic 4f/5f and 5f/5f phosphonates

The heterobimetallic actinide compound UO(2)Ce(H(2)O)[C(6)H(4)(PO(3)H)(2)](2)·H(2)O was prepared via the hydrothermal reaction of U(VI) and Ce(IV) in the presence of 1,2-phenylenediphosphonic acid. We demonstrate that this is a kinetic product that is not stable with respect to decomposition to the monometallic compounds. Similar reactions have been explored with U(VI) and Ce(III), resulting in the oxidation of Ce(III) to Ce(IV) and the formation of the Ce(IV) phosphonate, Ce[C(6)H(4)(PO(3)H)(PO(3)H(2))][C(6)H(4)(PO(3)H)(PO(3))]·2H(2)O, UO(2)Ce(H(2)O)[C(6)H(4)(PO(3)H)(2)](2)·H(2)O, and UO(2)[C(6)H(4)(PO(3)H)(2)](H(2)O)·H(2)O. In comparison, the reaction of U(VI) with Np(VI) only yields Np[C(6)H(4)(PO(3)H)(2)](2)·2H(2)O and aqueous U(VI), whereas the reaction of U(VI) with Pu(VI) yields the disordered U(VI)/Pu(VI) compound, (U(0.9)Pu(0.1))O(2)[C(6)H(4)(PO(3)H)(2)](H(2)O)·H(2)O, and the Pu(IV) phosphonate, Pu[C(6)H(4)(PO(3)H)(PO(3)H(2))][C(6)H(4)(PO(3)H)(PO(3))]·2H(2)O. The reactions of Ce(IV) with Np(VI) yield disordered heterobimetallic phosphonates with both M[C(6)H(4)(PO(3)H)(PO(3)H(2))][C(6)H(4)(PO(3)H)(PO(3))]·2H(2)O (M = Ce, Np) and M[C(6)H(4)(PO(3)H)(2)](2)·2H(2)O (M = Ce, Np) structures, as well as the Ce(IV) phosphonate Ce[C(6)H(4)(PO(3)H)(PO(3)H(2))][C(6)H(4)(PO(3)H)(PO(3))]·2H(2)O. Ce(IV) reacts with Pu(IV) to yield the Pu(VI) compound, PuO(2)[C(6)H(4)(PO(3)H)(2)](H(2)O)·3H(2)O, and a disordered heterobimetallic Pu(IV)/Ce(IV) compound with the M[C(6)H(4)(PO(3)H)(PO(3)H(2))][C(6)H(4)(PO(3)H)(PO(3))]·2H(2)O (M = Ce, Pu) structure. Mixtures of Np(VI) and Pu(VI) yield disordered heterobimetallic Np(IV)/Pu(IV) phosphonates with both the An[C(6)H(4)(PO(3)H)(PO(3)H(2))][C(6)H(4)(PO(3)H)(PO(3))]·2H(2)O (M = Np, Pu) and An[C(6)H(4)(PO(3)H)(2)](2)·2H(2)O (M = Np, Pu) formulas.     

Tuning of the internal energy and isomer distribution in small protonated water clusters H(+)(H2O)(4-8): an application of the inert gas messenger technique

Infrared spectroscopy of gas-phase hydrated clusters provides us much information on structures and dynamics of water networks. However, interpretation of spectra is often difficult because of high internal energy (vibrational temperature) of clusters and coexistence of many isomers. Here we report an approach to vary these factors by using the inert gas (so-called "messenger")-mediated cooling technique. Protonated water clusters with a messenger (M), H(+)(H(2)O)(4-8)·M (M = Ne, Ar, (H(2))(2)), are formed in a molecular beam and probed with infrared photodissociation spectroscopy in the OH stretch region. Observed spectra are compared with each other and with bare H(+)(H(2)O)(n). They show clear messenger dependence in their bandwidths and relative band intensities, reflecting different internal energy and isomer distribution, respectively. It is shown that the internal energy follows the order H(+)(H(2)O)(n) > H(+)(H(2)O)(n)·(H(2))(2) > H(+)(H(2)O)(n)·Ar > H(+)(H(2)O)(n)·Ne, while the isomer-selectivity, which changes the isomer distribution in the bare system, follows the order H(+)(H(2)O)(n)·Ar > H(+)(H(2)O)(n)·(H(2))(2) > H(+)(H(2)O)(n)·Ne ~ (H(+)(H(2)O)(n)). Although the origin of the isomer-selectivity is unclear, comparison among spectra measured with different messengers is very powerful in spectral analyses and makes it possible to easily assign spectral features of each isomer.     

Synthesis, characterization and optical properties of pi-conjugated systems incorporating closo-dodecaborate clusters: new potential candidates for two-photon absorption processes

Non-centrosymmetric pi-conjugated systems incorporating closo-dodecaborate clusters, [NC-C6H4-C(H=N(H)-B12H11]-(2), [NC-C6H4-C(H)=C(H)-C(6)H(4)-C(H)=N(H)-B12H11]-(3), and [NC-C6H4-C(H)=C(H)-C6H4-C(H)=C(H)-C6H4-C(H)=N(H)-B12H11]-(4) have been synthesized by reaction of the monoamino derivative of B12, [B12H11NH3]-(1), with various arylaldehydes, R-C6H4-CHO. These Schiff base-like compounds were fully characterized by multinuclear NMR spectroscopy and mass spectrometry. In order to evaluate these boron rich pi-systems as potential materials for two-photon absorption (TPA) processes, UV linear absorption curves were recorded for 3 and 4, and comparatively studied with those of the boron-free pi-systems NC-C6H4-C(H)=N-CH3(5) and NC-C6H4-C(H)=C(H)-C6H4-C(H)=N-CH3(6). The donor effect of the boron cluster was evidenced by a shift to the lower energy of the absorption band in the spectra of systems incorporating B12. The two photon absorption (TPA) spectrum of compound , obtained by the up-conversion method, shows a resonance at 720 nm with a cross-section sigma(TPA) of 35 x 10(-50) cm(4) s photon(-1) molecule(-1). This value suggests the potential of B12 clusters to be used as new donor groups for the synthesis of non-linear materials.     

Metallacycles of cadmium, mercury dicarboxylates

A series of linear coordination polymers, metallacycles of cadmium(II) and mercury(II) of flexible carboxylic acid ligands, RCH{3-CH(3)-,5-CH(3)-,6-(-OCH(2)CO(2)H)C(6)H(2)}(2), (when R = C(6)H(5), (H(2)L(1)); 2-NO(2)C(6)H(4)- (H(2)L(2)) and 3-NO(2)C(6)H(4)- (H(2)L(3))) are synthesized and characterized. [CdL(1) (py)(3)](n)·3nH(2)O (py = pyridine) is a linear coordination polymer, whereas [CdL(2)(py)(CH(3)OH)](2)·CH(3)OH is a dinuclear complex of cadmium with a Cd(2)O(2) type of core. The latter is obtained from reaction of cadmium(II) acetate with H(2)L(2) in methanol followed by reaction with pyridine. A similar reaction of cadmium(II) acetate with H(2)L(2) in dimethylformamide results in the formation of a cadmium metallacycle, namely [CdL(2) (py)(2)(H(2)O)](2)·H(2)O. The H(2)L(3) reacted with cadmium(II) acetate in the presence of pyridine to form a metallacycle [CdL(3)(py)(2)(H(2)O)](2)·3H(2)O. The ligand H(2)L(2) form mercury(II) metallacycle [HgL(2)(4-mepy)(2)](2) in the presence of 4-methylpyridine (4-mepy) and the ligand H(2)L(3) forms metallacycle [HgL(3)(3-mepy)(2)](2)·DMF in the presence of 3-methylpyridine (3-mepy). The potassium salts of H(2)L(1) and H(2)L(2) were found to be coordination polymers and these potassium coordination polymers were structurally characterized.     

新型螺环树状化合物的合成

利用环己二酮单腙芳构化反应, 得到9H, 9H, 11H, 11H, 12H, 12H-六氢化苯并[9,10]-(1H, 1H, 3H, 3H, 4H, 4H, 5H, 5H, 7H, 7H, 8H, 8H-十二氢)菲-2, 6, 10-三酮腙, 然后与季戊四醇反应, 得到9H, 9H, 11H, 11H, 12H, 12H-六氢化苯并[9,10]-(1H, 1H, 3H, 3H, 4H, 4H, 5H,5H, 7H, 7H, 8H, 8H-十二氢)菲-2,6,10-三酮缩三(2,2-二羟甲基-1,3-丙二醇), 再与9-[4-(2,6-二硫杂环己基)苯基]-3-[4-(二甲酯基甲基)苯基]-2,4,8,10-四氧杂螺[5.5]十一烷反应, 制成标题化合物. 中间体和标题化合物经过IR, 1H NMR, MS和元素分析表征.     

Dynamics of photodissociation of ethylene and its isotopomers at 157 nm: branching ratios and kinetic-energy distributions

We investigated the photodissociation of ethylene and its isotopomers at 157 nm in a molecular-beam apparatus using photofragment translational spectroscopy combined with synchrotron-based photoionization. The time-of-flight (TOF) spectra of all photofragments H, H(2), C(2)H(2), C(2)H(3), and their deuterium isotopic variants were recorded, from which kinetic-energy distributions P(E(t)) and branching ratios were obtained. Most C(2)H(3) spontaneously dissociates to C(2)H(2)+H and only C(2)H(3) with small internal energy survives. The C(2)H(2) fragment due to H(2) elimination is observed leading the C(2)H(2) fragment due to 2H elimination in TOF distribution because the former process has more kinetic-energy release. An analogous result is observed for C(2)D(4) photolysis. That elimination of molecular hydrogen is site-specific and is revealed from photolysis of three dideuterated ethylene isotopomers, in which an isotopic effect plays a significant role. Observations of C(2)D(2)+2H and C(2)H(2)+2D product channels in the photolysis of 1,1-CH(2)CD(2) provide evidence for migrations of H and D atoms. A comparison with previous experimental and theoretical results is made.     

Syntheses of functionalized ferratricarbadecaboranyl complexes

The reactions of nitriles (RCN) with arachno-4,6-C(2)B(7)H(12)(-) provide a general route to functionalized tricarbadecaboranyl anions, 6-R-nido-5,6,9-C(3)B(7)H(9)(-), R = C(6)H(5) (2(-)), NC(CH(2))(4) (4(-)), (p-BrC(6)H(4))(Me(3)SiO)CH (6(-)), C(14)H(11) (8(-)), and H(3)BNMe(2)(CH(2))(2) (10(-)). Further reaction of these anions with (eta(5)-C(5)H(5))Fe(CO)(2)I yields the functionalized ferratricarbadecaboranyl complexes 1-(eta(5)-C(5)H(5))-2-C(6)H(5)-closo-1,2,3,4-FeC(3)B(7)H(9) (3), 1-(eta(5)-C(5)H(5))-2-NC(CH(2))(4)-closo-1,2,3,4-FeC(3)B(7)H(9) (5), 1-(eta(5)-C(5)H(5))-2-[(p-BrC(6)H(4))(Me(3)SiO)CH]-closo-1,2,3,4-FeC(3)B(7)H(9) (7), 1-(eta(5)-C(5)H(5))-2-C(14)H(11)-closo-1,2,3,4-FeC(3)B(7)H(9) (9), and 1-(eta(5)-C(5)H(5))-2-H(3)BNMe(2)(CH(2))(2)-closo-1,2,3,4-FeC(3)B(7)H(9) (11). Reaction of 11 with DABCO (triethylenediamine) resulted in removal of the BH(3) group coordinated to the nitrogen of the side chain, giving 1-(eta(5)-C(5)H(5))-2-NMe(2)(CH(2))(2)-closo-1,2,3,4-FeC(3)B(7)H(9) (12). Crystallographic studies of complexes 3, 5, 7, 9, and 11 confirmed that these complexes are ferrocene analogues in which a formal Fe(2+) ion is sandwiched between the cyclopentadienyl and tricarbadecaboranyl monoanionic ligands. The metals are eta(6)-coordinated to the puckered six-membered face of the tricarbadecaboranyl cage, with the exopolyhedral substituents bonded to the low-coordinate carbon adjacent to the iron.     

Theoretical exploration of hydrogen loss from Al3H9

The Al(3)H(9) and Al(3)H(7) potential energy surfaces were explored using quantum chemistry calculations to investigate the H(2) loss mechanism from Al(3)H(9), which provide new insights into hydrogen production from bulk alane, [AlH(3)](x), a possible energy storage material. We present results of B3LYP/6-311++G(d,p) calculations for the various Al(3)H(9) and Al(3)H(7) optimized local minima and transition state structures along with some reaction pathways for their interconversion. We find the energy for Al(3)H(9) decomposition into Al(2)H(6) and AlH(3) is slightly lower than that for H(2) loss and Al(3)H(7) formation, but the calculations show that H(2) loss from Al(3)H(9) is a lower energy process than for losing hydrogen from either Al(2)H(6) or AlH(3). We found four transition state structures and reaction pathways for Al(3)H(9) → Al(3)H(7) + H(2), where the lowest energy activation barrier is around 25-73 kJ/mol greater than the experimental value for H(2) loss from bulk alane. Intrinsic reaction coordinate calculations show that the H(2) loss pathway involves considerable rearrangement of the H atom positions around a single Al center. Three of the pathways start with the formation of an AlH(3) moiety, which then enables a terminal H on the AlH(3) to get within 1.1 to 1.2 ? of a nearby bridging H atom. The bridging and terminal H atoms eventually combine to form H(2) and leave Al(3)H(9). One implication of these H(2) loss reaction pathways is that, since the H atoms in bulk alanes are all at bridging positions, if a similar H(2) loss mechanism were to apply to bulk alane, then H(2) loss would most likely occur on the bulk alane surface or at a defect site where there should be more terminal H atoms available for reaction with nearby bridging H atoms.     

Quantification of hydrogen sulphide in humid air by selected ion flow tube mass spectrometry

We report the results of a study of the reactions of H(3)O(+), NO(+) and O(2)(+.) ions with H(2)S. This study was undertaken to provide a thorough understanding of the ion chemistry required for accurate quantification of H(2)S in humid air by selected ion flow tube mass spectrometry (SIFT-MS). It shows that slow reactions occur between H(3)S(+), the primary product ions of the H(3)O(+)/H(2)S reaction, and the abundant H(2)O molecules present in humid air and breath. These reactions disturb somewhat the quantification of H(2)S by this analytical method, but the kinetic data obtained in this study facilitate precise quantification of H(2)S in humid air. This study also shows that NO(+) does not react with H(2)S, and that O(2)(+.) does react rapidly with H(2)S, but the product H(2)S(+.) ions react rapidly with H(2)O. Thus, NO(+) and O(2)(+.) cannot be used as precursor ion for analysis of H(2)S in moist air by SIFT-MS. A sample SIFT mass spectrum is shown from which H(2)S and several other volatile compounds have been quantified in a sample of cow rumen gas.     

Quantum reactive scattering of H + hydrocarbon reactions

A practical quantum-dynamical method is described for predicting accurate rate constants for general chemical reactions. The ab initio potential energy surfaces for these reactions can be built from a minimal number of grid points (average of 50 points) and expressed in terms of analytical functionals. All the degrees of freedom except the breaking and forming bonds are optimised using the MP2 method with a cc-pVTZ basis set. Single point energies are calculated on the optimised geometries at the CCSD(T) level of theory with the same basis set. The dynamics of these reactions occur on effective reduced dimensionality hyper-surfaces accounting for the zero-point energy of the optimised degrees of freedom. Bonds being broken and formed are treated with explicit hyperspherical time independent quantum dynamics. Application of the method to the H + CH(4)--> H(2)+ CH(3), H + C(2)H(6)--> H(2)+ C(2)H(5), H + C(3)H(8)--> H(2)+n-C(3)H(7)/H(2)+i-C(3)H(7) and H + CH(3)OH --> H(2)+ CH(3)O/H(2)+ CH(2)OH reactions illustrate the potential of the approach in predicting rate constants, kinetic isotope effects and branching ratios. All studied reactions exhibit large quantum tunneling in the rate constants at lower temperatures. These quantum calculations compare well with the experimental results.     

Physical organic chemistry of transition metal carbene complexes. 23. Kinetic and thermodynamic acidities of cationic benzothienyl- and selenylcarbene complexes of rhenium in aqueous acetonitrile.

The pK(a) values of a cationic selenyl- (5H(+)) and a benzothienylcarbene complex (6H(+)) and rate constants for the reversible deprotonation of these complexes by water, carboxylate ions, primary aliphatic amines, secondary alicyclic amines (5H(+) only), and OH(-) (5H(+) only) were determined in 50% MeCN-50% water (v/v) at 25 degrees C. In comparison with neutral Fischer-type carbene complexes such as 1H, the cationic complexes 5H(+) and 6H(+) are much more acidic, and the intrinsic barriers to proton transfer are substantially higher. This paper discusses a variety of factors that contribute to these differences, with the most important ones being that 5H(+) and 6H(+) are cationic, which makes the C(5)H(5)(NO)(PPh(3))Re moiety a stronger pi-acceptor than the (CO)(5)M moieties, coupled with the fact that the deprotonated forms of 5H(+) and 6H(+ )are aromatic molecules.     

DFT study on the geometric, electronic structure and Raman spectra of 5,15-diphenylporphine

The ground state geometric, electronic structure and Raman spectra of 5,15-diphenylporphine (H(2)DPP) have been studied using B3LYP/6-31G(d) method and compared with that of well-studied free base porphine (H(2)P) and meso-tetraphenylporphine (H(2)TPP). Calculation shows that 5,15-substitution causes remarkable in-plane distortion, whereas the resulting out-of-plane distortion is negligible. The calculated electronic structure of H(2)DPP is consistent with the absorption spectra compared with H(2)P and H(2)TPP. The calculated vibrational frequencies of H(2)DPP scaled with a single factor of 0.971 agree well with experimental data (the rms error is 8.0 cm(-1)). The assignment of experimental Raman bands of H(2)DPP was discussed on the basis of theoretical calculation and the comparison with that of H(2)P and H(2)TPP. The splitting of some vibrational modes involving the motion of C(m) atom, such as nu(1), nu(8), and nu(10), was observed and was attributed to the diversification of the environment around C(m) atoms. As the shift of absorption peaks, the shift of some structure-sensitive Raman bands of H(2)DPP form that of H(2)TPP and H(2)P was attributed to the in-plane nuclear reorganization (IPNR) induced by phenyl-substitution, though the contribution of nonplanarity mechanism could not be excluded completely.     

Syntheses and structures of quinuclidine-stabilized amido- and azidogallanes

Quinuclidine-stabilized amido- and azidogallanes, HGa[N(TMS)2]2(quin) (1), H2Ga[N(TMS)2](quin) (2), HGa-[N(H)(2,6-iPr2C6H3)]2(quin) (3), and H2GaN3(quin) (4), were synthesized from the quinuclidine adducts of mono- and dichlorogallane. Structural determinations revealed that all compounds were monomeric with four-coordinate gallium centers. Reactions of the five-coordinate compound, HGaCl2(quin)2, with 2 equiv of Li[N(TMS)2] or Li[N(H)(2,6-iPr2C6H3)] resulted in the isolation of compound 1 or 3. A ligand redistribution during the reaction of H2GaCl(quin) with Li[N(H)(2,6-iPr2C6H3)] produced compound 3 and H3Ga(quin) in a 1:1 molar ratio.     

Structural characterization of glycoporphyrins by electrospray tandem mass spectrometry

Porphyrin derivatives having a galactose or a bis(isopropylidene)galactose structural unit, linked by ester or ether bonds, were characterized by electrospray tandem mass spectrometry (ES-MS/MS). The electrospray mass spectra of these glycoporphyrins show the corresponding [M + H](+) ions. For the glycoporphyrins with pyridyl substituents and those having a tetrafluorophenyl spacer, the doubly charged ions [M + 2H](2+) were also observed in ES-MS with high relative abundance. The fragmentation of both [M + H](+) and [M + 2H](2+) ions exhibited common fragmentation pathways for porphyrins with the same sugar residue, independently of the porphyrin structural unit and type of linkage. ES-MS/MS of the [M + H](+) ions of the galactose-substituted porphyrins gave the fragment ions [M + H - C(2)H(4)O(2)](+), [M + H - C(3)H(6)O(3)](+), [M + H - C(4)H(8)O(4)](+) and [M + H - galactose residue](+). The fragmentation of the [M + 2H](2+) ions of the porphyrins with galactose shows the common doubly charged fragment ions [porphyrin + H](2+), [M + 2H - C(2)H(4)O(2)](2+), [M + 2H - C(4)H(8)O(4)](2+), [M + 2H - galactose residue](2+) and the singly charged fragment ions [M + H - C(3)H(6)O(3)](+) and [M + H - galactose residue](+). The fragmentation of the [M + H](+) ions of glycoporphyrins with a protected galactosyl residue leads mainly to the ions [M + H - CO(CH(3))(2)](+), [M + H - 2CO(CH(3))(2)](+), [M + H - 2CO(CH(3))(2) - CO](+), [M + H - C(10)H(16)O(4)](+) and [M + H - protected galactose](+). The doubly charged ions [M + 2H](2+) fragment to give the doubly charged ions [porphyrin + H](2+) and the singly charged ions [M + H - protected galactose residue](+) and [M + H - CO(CH(3))(2)](+). For the porphyrins where the sugar structural unit is linked by an ester bond, [M + 2H](2+), ES-MS/MS showed a major and typical fragmentation corresponding to combined loss of a sugar structural unit and further loss of water, leading to the ion [M + 2H - sugar residue - H(2)O](2+), independently of the structure of the sugar structural unit. These results show that ES-MS/MS can be a powerful tool for the characterization of the sugar structural unit of glycoporphyrins, without the need for chemical hydrolysis.     

A comparative study of the structures and reactivity of cyclometallated platinum compounds of N-benzylidenebenzylamines and cycloplatination of a primary amine

The reaction of cis-[PtCl(2)(dmso)2] with ligands 4-ClC(6)H(4)CHNCH(2)C(6)H(5) (1a) and 4-ClC(6)H(4)CHNCH(2)(4-ClC(6)H(4)) (1b) in the presence of sodium acetate and using either methanol or toluene as solvent produced the corresponding five-membered endo-metallacycles [PtCl{(4-ClC(6)H(3))CHNCH(2)C(6)H(5)}{SOMe(2)}] (2a) and [PtCl{(4-ClC(6)H(3))CHNCH(2)(4'-ClC(6)H(4))}{SOMe(2)}] (2b). An analogous reaction for ligands 2,6-Cl(2)C(6)H(3)CHNCH(2)C(6)H(5) (1c) and 2,6-Cl(2)C(6)H(3)CHNCH(2)(4-ClC(6)H(4)) (1d) produced five-membered exo-metallacycles [PtCl{(2,6-Cl(2)C(6)H(3))CHNCH(2)C(6)H(4)}{SOMe(2)}] (2c) and [PtCl{(2,6-Cl(2)C(6)H(3))CHNCH(2)(4'-ClC(6)H(3))}{SOMe(2)}] (2d) when the reaction was carried out in methanol and seven-membered endo-platinacycles [PtCl{(MeC(6)H(3))ClC(6)H(3)CHNCH(2)C(6)H(4)}{SOMe(2)}] (3c) and [PtCl{(MeC(6)H(3))ClC(6)H(3)CHNCH(2)(4'-ClC(6)H(3))}{SOMe(2)}] (3d) when toluene was used as a solvent. The reaction of 2,4,6-(CH(3))(3)C(6)H(2)CHNCH(2)(4-ClC(6)H(4)) (1e) produced in both solvents an exo-platinacycle [PtCl{(2,4,6-(CH(3))(3)C(6)H(2))CHNCH(2)(4'-ClC(6)H(3))}{SO(CH(3))(2)}] (2e). Cyclometallation of 4-chlorobenzylamine was also achieved to produce compound [PtCl{(4-ClC(6)H(3))CH(2)NH(2)}{SOMe(2)}] (2g). The reactions of endo- and exo-metallacycles with phosphines evidenced the higher lability of the Pt-N bond in exo-metallacycles while a comparative analysis of the crystal structures points out a certain degree of aromaticity in the endo-metallacycle.