Conformations of dimethylhydrogen phosphonate (DMHP): a matrix isolation infrared and ab initio study

The infrared spectra of dimethylhydrogen phosphonate (DMHP) isolated in nitrogen, argon and krypton matrices using an effusive source at 298 and 373 K have been recorded. Experiments were also performed using a supersonic jet source to look for conformational cooling in the expansion process. As a result of these experiments, infrared spectral characteristics of the ground and higher energy conformers of the DMHP have been identified for the first time. The structures of DMHP were optimized at the hybrid B3LYP and Hartree fock (HF) levels of theory using the 6-31++G** basis sets. Computationally, four minima were obtained corresponding to DMHP conformers with G (+/-)G (-/+), G (-)G (-), TG (+) and TG (-) structures in the order of increasing energy. Frequency calculations were done to confirm that the structures were indeed minima on the potential energy surface (PES). The computed frequencies corroborated well with the experimental matrix isolation infrared frequencies leading to definite assignments of the infrared features of DMHP, for the G (+/-)G (-/+) and TG (+) conformers. At B3LYP/6-31++G** level, the energy difference between the G (+/-)G (-/+) and G (-)G (-) conformer was 1.53 kcal/mol, and that between G (+/-)G (-/+) and TG (+), G (+/-)G (-/+) and TG (-) were 1.65 and 1.95 kcal/mol. Transition-state calculations were also carried out at B3LYP/6-31++G** level connecting the G (+/-)G (-/+) to G (-)G (-), TG (+) and TG (-) conformers. Computations indicated that the conformer interconversion between G (-)G (-) --> G (+/-)G (-/+) is barrierless, whereas the barriers for TG (+) --> G (+/-)G (-/+) and TG (-) --> G (+/-)G (-/+) are 1.47 and 0.88 kcal/mol, respectively.     

Matrix isolation infrared and ab initio study of the conformations of 2,2-dimethoxypropane

Conformations of 2,2-dimethoxypropane (DMP) were studied using matrix isolation infrared spectroscopy. An effusive source maintained at different temperatures (298, 388 and 430 K) was used to deposit DMP in a nitrogen matrix. As a result of these experiments, spectrally resolved infrared features of the ground and first higher energy conformer of DMP have been recorded, for the first time. The experimental studies were supported by ab initio computations performed at B3LYP/6-31++G** level. Computationally, four minima were identified corresponding to conformers with G+/-G-/+, TG+/-, G+/-G+/- and TT structures. The computed frequencies at the B3LYP level were found to compare well with the experimental matrix isolation frequencies, leading to a definitive assignment of the infrared features of DMP, for the G+/-G-/+ and TG+/- conformers. At the B3LYP/6-31++G** level, the energy difference between the G+/-G-/+ and TG+/- conformers was computed to be 3.25 kcal x mol(-1). The barrier for conformation interconversion, TG+/--->G+/-G-/+, was calculated to be 1.29 kcal x mol(-1). The magnitude of this barrier is consistent with the experimental observation that the spectral features due to the TG+/- decreased considerably in intensity when the matrix was annealed.     

Conformations of dimethoxymethane: matrix isolation infrared and ab initio studies

Conformations of dimethoxymethane (DMM) were studied using matrix isolation infrared spectroscopy. DMM was trapped in an argon matrix using an effusive source at 298, 388 and 430 K. Experiments were also done using a supersonic jet source to look for conformational cooling in the expansion process. As a result of these experiments, spectrally resolved infrared features of the ground and first higher energy conformer of DMM have been recorded, for the first time. The experimental studies were supported by ab initio computations performed at HF and B3LYP levels, using a 6-31++G** basis set. Computationally, four minima were identified corresponding to conformers with GG, TG, G+G- and TT structures. The computed frequencies at the B3LYP level were found to compare well with the experimental matrix isolation frequencies, leading to a definitive assignment of the infrared features of DMM, for the GG and TG conformers. At the B3LYP/6-31++G** level, the energy difference between the GG and TG conformers was computed to be 2.30 kcal mol(-1). The barrier for conformation interconversion, TG-->GG level was calculated to be 0.95 kcal mol(-1). This value is consistent with the experimental observation that the spectral features due to the TG conformer disappeared in the matrix on annealing.     

A kinetic study of Mg+ and Mg-containing ions reacting with O3, O2, N2, CO2, N2O and H2O: implications for magnesium ion chemistry in the upper atmosphere

Reactions between Mg(+) and O(3), O(2), N(2), CO(2) and N(2)O were studied using the pulsed laser photo-dissociation at 193 nm of Mg(C(5)H(7)O(2))(2) vapour, followed by time-resolved laser-induced fluorescence of Mg(+) at 279.6 nm (Mg(+)(3(2)P(3/2)-3(2)S(1/2))). The rate coefficient for the reaction Mg(+) + O(3) is at the Langevin capture rate coefficient and independent of temperature, k(190-340 K) = (1.17 ± 0.19) × 10(-9) cm(3) molecule(-1) s(-1) (1σ error). The reaction MgO(+) + O(3) is also fast, k(295 K) = (8.5 ± 1.5) × 10(-10) cm(3) molecule(-1) s(-1), and produces Mg(+) + 2O(2) with a branching ratio of (0.35 ± 0.21), the major channel forming MgO(2)(+) + O(2). Rate data for Mg(+) recombination reactions yielded the following low-pressure limiting rate coefficients: k(Mg(+) + N(2)) = 2.7 × 10(-31) (T/300 K)(-1.88); k(Mg(+) + O(2)) = 4.1 × 10(-31) (T/300 K)(-1.65); k(Mg(+) + CO(2)) = 7.3 × 10(-30) (T/300 K)(-1.59); k(Mg(+) + N(2)O) = 1.9 × 10(-30) (T/300 K)(-2.51) cm(6) molecule(-2) s(-1), with 1σ errors of ±15%. Reactions involving molecular Mg-containing ions were then studied at 295 K by the pulsed laser ablation of a magnesite target in a fast flow tube, with mass spectrometric detection. Rate coefficients for the following ligand-switching reactions were measured: k(Mg(+)·CO(2) + H(2)O → Mg(+)·H(2)O + CO(2)) = (5.1 ± 0.9) × 10(-11); k(MgO(2)(+) + H(2)O → Mg(+)·H(2)O + O(2)) = (1.9 ± 0.6) × 10(-11); k(Mg(+)·N(2) + O(2)→ Mg(+)·O(2) + N(2)) = (3.5 ± 1.5) × 10(-12) cm(3) molecule(-1) s(-1). Low-pressure limiting rate coefficients were obtained for the following recombination reactions in He: k(MgO(2)(+) + O(2)) = 9.0 × 10(-30) (T/300 K)(-3.80); k(Mg(+)·CO(2) + CO(2)) = 2.3 × 10(-29) (T/300 K)(-5.08); k(Mg(+)·H(2)O + H(2)O) = 3.0 × 10(-28) (T/300 K)(-3.96); k(MgO(2)(+) + N(2)) = 4.7 × 10(-30) (T/300 K)(-3.75); k(MgO(2)(+) + CO(2)) = 6.6 × 10(-29) (T/300 K)(-4.18); k(Mg(+)·H(2)O + O(2)) = 1.2 × 10(-27) (T/300 K)(-4.13) cm(6) molecule(-2) s(-1). The implications of these results for magnesium ion chemistry in the atmosphere are discussed.     

Molecular structure, infrared spectrum, and photochemistry of squaric acid dimethyl ester in solid argon

Squaric acid dimethyl ester (C(6)O(4)H(6); 3,4-dimethoxycyclobut-3-ene-1,2-dione; DCD) was studied by matrix isolation infrared spectroscopy and by density functional theory (B3LYP) and ab initio (MP2) calculations with the 6-31++G(d,p) and 6-311++G(d,p) basis sets. Three conformers of the compound were theoretically predicted. The two most stable conformers were identified in low-temperature argon matrixes and the energy gap between them was determined. The trans-trans conformer (C(2)(v)) was found to be more stable than the cis-trans form (C(s)) by 4.2 kJ mol(-1), in consonance with the theoretical predictions (MP2 calcd = 3.9 kJ mol(-1)). In situ broadband UV irradiation (lambda > 337 nm) of the matrix-isolated compound was found to induce the ring-opening reaction leading to production of the bisketene, 2,3-dimethoxybuta-1,3-diene-1,4-dione as well as the trans-trans --> cis-trans conformational isomerization. The latter phototransformation allowed separation of the infrared spectra of the two conformers initially trapped into a low-temperature matrix. Upon higher energy irradiation (lambda > 235 nm), the main observed photoproducts were CO and deltic acid dimethyl ester (C(5)O(3)H(6); 2,3-dimethoxycycloprop-2-en-1-one), the latter being obtained in two different conformations (trans-trans and cis-trans). According to the experimental data, deltic acid dimethyl ester is produced by decarbonylation of the initially formed bisketene and not by direct CO extrusion from DCD.     

Thermal decomposition mechanism of 1-ethyl-3-methylimidazolium bromide ionic liquid

In order to better understand the volatilization process for ionic liquids, the vapor evolved from heating the ionic liquid 1-ethyl-3-methylimidazolium bromide (EMIM(+)Br(-)) was analyzed via tunable vacuum ultraviolet photoionization time-of-flight mass spectrometry (VUV-PI-TOFMS) and thermogravimetric analysis mass spectrometry (TGA-MS). For this ionic liquid, the experimental results indicate that vaporization takes place via the evolution of alkyl bromides and alkylimidazoles, presumably through alkyl abstraction via an S(N)2 type mechanism, and that vaporization of intact ion pairs or the formation of carbenes is negligible. Activation enthalpies for the formation of the methyl and ethyl bromides were evaluated experimentally, ΔH(?)(CH(3)Br) = 116.1 ± 6.6 kJ/mol and ΔH(?)(CH(3)CH(2)Br) = 122.9 ± 7.2 kJ/mol, and the results are found to be in agreement with calculated values for the S(N)2 reactions. Comparisons of product photoionization efficiency (PIE) curves with literature data are in good agreement, and ab initio thermodynamics calculations are presented as further evidence for the proposed thermal decomposition mechanism. Estimates for the enthalpy of vaporization of EMIM(+)Br(-) and, by comparison, 1-butyl-3-methylimidazolium bromide (BMIM(+)Br(-)) from molecular dynamics calculations and their gas phase enthalpies of formation obtained by G4 calculations yield estimates for the ionic liquids' enthalpies of formation in the liquid phase: ΔH(vap)(298 K) (EMIM(+)Br(-)) = 168 ± 20 kJ/mol, ΔH(f,?gas)(298 K) (EMIM(+)Br(-)) = 38.4 ± 10 kJ/mol, ΔH(f,?liq)(298 K) (EMIM(+)Br(-)) = -130 ± 22 kJ/mol, ΔH(f,?gas)(298 K) (BMIM(+)Br(-)) = -5.6 ± 10 kJ/mol, and ΔH(f,?liq)(298 K) (BMIM(+)Br(-)) = -180 ± 20 kJ/mol.     

Liquid structure of room-temperature ionic liquid, 1-Ethyl-3-methylimidazolium bis-(trifluoromethanesulfonyl) imide

The liquid structure of 1-ethyl-3-methylimidazolium bis-(trifluoromethanesulfonyl) imide (EMI(+)TFSI(-)) has been studied by means of large-angle X-ray scattering (LAXS), (1)H, (13)C, and (19)F NMR, and molecular dynamics (MD) simulations. LAXS measurements show that the ionic liquid is highly structured with intermolecular interactions at around 6, 9, and 15 A. The intermolecular interactions at around 6, 9, and 15 A are ascribed, on the basis of the MD simulation, to the nearest neighbor EMI(+)...TFSI(-) interaction, the EMI(+)...EMI(+) and TFSI(-)...TFSI(-) interactions, and the second neighbor EMI+...TFSI(-) interaction, respectively. The ionic liquid involves two conformers, C(1) (cis) and C(2) (trans), for TFSI(-), and two conformers, planar cis and nonplanar staggered, for EMI(+), and thus the system involves four types of the EMI(+)...TFSI(-) interactions in the liquid state by taking into account the conformers. However, the EMI(+)...TFSI(-) interaction is not largely different for all combinations of the conformers. The same applies alsoto the EMI(+)...EMI(+) and TFSI(-)...TFSI(-) interactions. It is suggested from the 13C NMR that the imidazolium C(2) proton of EMI(+) strongly interacts with the O atom of the -SO(2)(CF(3)) group of TFSI(-). The interaction is not ascribed to hydrogen-bonding, according to the MD simulation. It is shown that the liquid structure is significantly different from the layered crystal structure that involves only the nonplanar staggered EMI(+) and C(1) TFSI(-) conformers.     

Oxidation of a guanine derivative coordinated to a Pt(IV) complex initiated by intermolecular nucleophilic attacks

In this study we report that fac-[Pt(IV)(dach)(9-EtG)Cl(3)](+) (dach = d,l-1,2-diaminocyclohexane, 9-EtG = 9-ethylguanine) in high pH (pH 12) or phosphate solution (pH 7.4) produces 8-oxo-9-EtG and Pt(II) species. The reaction in H(2)(18)O revealed that the oxygen atom in hydroxide or phosphate ends up at the C8 position of 8-oxo-G. The kinetics of the redox reaction was first order with respect to both Pt(IV)-G and free nucleophiles (OH(-) and phosphate). The oxidation of G initiated by hydroxide was approximately 30～50 times faster than by phosphate in 100 mM NaCl solutions. The large entropy of activation of OH(-1) (ΔS(?) = 26.6 ± 4.3 J mol(-1) K(-1)) due to the smaller size of OH(-) is interpreted to be responsible for the faster kinetics compared to phosphate (ΔS(?) = -195.5 ± 11.1 J mol(-1) K(-1)). The enthalpy of activation for phosphate reaction is more favorable relative to the OH(-) reaction (ΔH(?) = 35.4 ± 3.5 kJ mol(-1) for phosphate vs. 96.6 ± 11.4 kJ mol(-1) for OH(-1)). The kinetic isotope effect of H8 was determined to be 7.2 ± 0.2. The rate law, kinetic isotope effect, and isotopic labeling are consistent with a mechanism involving proton ionization at the C8 position as the rate determining step followed by two-electron transfer from G to Pt(IV).     

Structures, internal rotor potentials, and thermochemical properties for a series of nitrocarbonyls, nitroolefins, corresponding nitrites, and their carbon centered radicals

Structures, enthalpy (Δ(f)H°(298)), entropy (S°(T)), and heat capacity (C(p)(T)) are determined for a series of nitrocarbonyls, nitroolefins, corresponding nitrites, and their carbon centered radicals using the density functional B3LYP and composite CBS-QB3 calculations. Enthalpies of formation (Δ(f)H°(298)) are determined at the B3LYP/6-31G(d,p), B3LYP/6-31+G(2d,2p), and composite CBS-QB3 levels using several work reactions for each species. Entropy (S) and heat capacity (C(p)(T)) values from vibration, translational, and external rotational contributions are calculated using the rigid-rotor-harmonic-oscillator approximation based on the vibration frequencies and structures obtained from the density functional studies. Contribution to Δ(f)H(T), S, and C(p)(T) from the analysis on the internal rotors is included. Recommended values for enthalpies of formation of the most stable conformers of nitroacetone cc(═o)cno2, acetonitrite cc(═o)ono, nitroacetate cc(═o)no2, and acetyl nitrite cc(═o)ono are -51.6 kcal mol(-1), -51.3 kcal mol(-1), -45.4 kcal mol(-1), and -58.2 kcal mol(-1), respectively. The calculated Δ(f)H°(298) for nitroethylene c═cno2 is 7.6 kcal mol(-1) and for vinyl nitrite c═cono is 7.2 kcal mol(-1). We also found an unusual phenomena: an intramolecular transfer reaction (isomerization) with a low barrier (3.6 kcal mol(-1)) in the acetyl nitrite. The NO of the nitrite (R-ONO) in CH(3)C(═O')ONO moves to the C═O' oxygen in a motion of a stretching frequency and then a shift to the carbonyl oxygen (marked as O' for illustration purposes).     

Photoinduced charge separation in three-layer supramolecular nanohybrids: fullerene-porphyrin-SWCNT

Photoinduced charge separation processes of three-layer supramolecular hybrids, fullerene-porphyrin-SWCNT, which are constructed from semiconducting (7,6)- and (6,5)-enriched SWCNTs and self-assembled via π-π interacting long alkyl chain substituted porphyrins (tetrakis(4-dodecyloxyphenyl)porphyrins; abbreviated as MP(alkyl)(4)) (M = Zn and H(2)), to which imidazole functionalized fullerene[60] (C(60)Im) is coordinated, have been investigated in organic solvents. The intermolecular alkyl-π and π-π interactions between the MP(alkyl)(4) and SWCNTs, in addition, coordination between C(60)Im and Zn ion in the porphyrin cavity are visualized using DFT calculations at the B3LYP/3-21G(*) level, predicting donor-acceptor interactions between them in the ground and excited states. The donor-acceptor nanohybrids thus formed are characterized by TEM imaging, steady-state absorption and fluorescence spectra. The time-resolved fluorescence studies of MP(alkyl)(4) in two-layered nanohybrids (MP(alkyl)(4)/SWCNT) revealed efficient quenching of the singlet excited states of MP(alkyl)(4) ((1)MP*(alkyl)(4)) with the rate constants of charge separation (k(CS)) in the range of (1-9) × 10(9) s(-1). A nanosecond transient absorption technique confirmed the electron transfer products, MP˙(+)(alkyl)(4)/SWCNT˙(-) and/or MP˙(-)(alkyl)(4)/SWCNT˙(+) for the two-layer nanohybrids. Upon further coordination of C(60)Im to ZnP, acceleration of charge separation via(1)ZnP* in C(60)Im→ZnP(alkyl)(4)/SWCNT is observed to form C(60)˙(-)Im→ZnP˙(+)(alkyl)(4)/SWCNT and C(60)˙(-)Im→ZnP(alkyl)(4)/SWCNT˙(+) charge separated states as supported by the transient absorption spectra. These characteristic absorptions decay with rate constants due to charge recombination (k(CR)) in the range of (6-10) × 10(6) s(-1), corresponding to the lifetimes of the radical ion-pairs of 100-170 ns. The electron transfer in the nanohybrids has further been utilized for light-to-electricity conversion by the construction of proof-of-concept photoelectrochemical solar cells.     

The role of Zn-OR and Zn-OH nucleophiles and the influence of para-substituents in the reactions of binuclear phosphatase mimetics

Analogues of the ligand 2,2'-(2-hydroxy-5-methyl-1,3-phenylene)bis(methylene)bis((pyridin-2-ylmethyl)azanediyl)diethanol (CH(3)H(3)L1) are described. Complexation of these analogues, 2,6-bis(((2-methoxyethyl)(pyridin-2-ylmethyl)amino)methyl)-4-methylphenol (CH(3)HL2), 4-bromo-2,6-bis(((2-methoxyethyl)(pyridin-2-ylmethyl)amino)methyl)phenol (BrHL2), 2,6-bis(((2-methoxyethyl)(pyridin-2-ylmethyl)amino)methyl)-4-nitrophenol (NO(2)HL2) and 4-methyl-2,6-bis(((2-phenoxyethyl)(pyridin-2-ylmethyl)amino)methyl)phenol (CH(3)HL3) with zinc(II) acetate afforded [Zn(2)(CH(3)L2)(CH(3)COO)(2)](PF(6)), [Zn(2)(NO(2)L2)(CH(3)COO)(2)](PF(6)), [Zn(2)(BrL2)(CH(3)COO)(2)](PF(6)) and [Zn(2)(CH(3)L3)(CH(3)COO)(2)](PF(6)), in addition to [Zn(4)(CH(3)L2)(2)(NO(2)C(6)H(5)OPO(3))(2)(H(2)O)(2)](PF(6))(2) and [Zn(4)(BrL2)(2)(PO(3)F)(2)(H(2)O)(2)](PF(6))(2). The complexes were characterized using (1)H and (13)C NMR spectroscopy, mass spectrometry, microanalysis, and X-ray crystallography. The complexes contain either a coordinated methyl- (L2 ligands) or phenyl- (L3 ligand) ether, replacing the potentially nucleophilic coordinated alcohol in the previously reported complex [Zn(2)(CH(3)HL1)(CH(3)COO)(H(2)O)](PF(6)). Functional studies of the zinc complexes with the substrate bis(2,4-dinitrophenyl) phosphate (BDNPP) showed them to be competent catalysts with, for example, [Zn(2)(CH(3)L2)](+), k(cat) = 5.70 ± 0.04 × 10(-3) s(-1) (K(m) = 20.8 ± 5.0 mM) and [Zn(2)(CH(3)L3)](+), k(cat) = 3.60 ± 0.04 × 10(-3) s(-1) (K(m) = 18.9 ± 3.5 mM). Catalytically relevant pK(a)s of 6.7 and 7.7 were observed for the zinc(II) complexes of CH(3)L2(-) and CH(3)L3(-), respectively. Electron donating para-substituents enhance the rate of hydrolysis of BDNPP such that k(cat)p-CH(3) > p-Br > p-NO(2). Use of a solvent mixture containing H(2)O(18)/H(2)O(16) in the reaction with BDNPP showed that for [Zn(2)(CH(3)L2)(CH(3)COO)(2)](PF(6)) and [Zn(2)(NO(2)L2)(CH(3)COO)(2)](PF(6)), as well as [Zn(2)(CH(3)HL1)(CH(3)COO)(H(2)O)](PF(6)), the (18)O label was incorporated in the product of the hydrolysis suggesting that the nucleophile involved in the hydrolysis reaction was a Zn-OH moiety. The results are discussed with respect to the potential nucleophilic species (coordinated deprotonated alcohol versus coordinated hydroxide).     

Guided-ion beam and theoretical study of the potential energy surface for activation of methane by W+

A guided-ion beam tandem mass spectrometer is used to study the reactions, W(+) + CH(4) (CD(4)) and [W,C,2H](+) + H(2) (D(2)), to probe the [W,C,4H](+) potential energy surface. The reaction W(+) + CH(4) produces [W,C,2H](+) in the only low-energy process. The analogous reaction in the CD(4) system exhibits a cross section with strong differences at the lowest energies caused by zero-point energy differences, demonstrating that this reaction is slightly exothermic for CH(4) and slightly endothermic for CD(4). The [W,C,2H](+) product ion reacts further at thermal energies with CH(4) to produce W(CH(2))(x)(+) (x = 2-4). At higher energies, the W(+) + CH(4) reaction forms WH(+) as the dominant ionic product with smaller amounts of WCH(3)(+), WCH(+), and WC(+) also formed. The energy dependent cross sections for endothermic formation of the various products are analyzed and allow the determination of D(0)(W(+)-CH(3)) approximately 2.31 +/- 0.10 eV, D(0)(W(+)-CH(2)) = 4.74 +/- 0.03 eV, D(0)(W(+)-CH) = 6.01 +/- 0.28 eV, and D(0)(W(+)-C) = 4.96 +/- 0.22 eV. We also examine the reverse reaction, [W,C,2H](+) + H(2) (D(2)) --> W(+) + CH(4) (CH(2)D(2)). Combining the cross sections for the forward and reverse processes yields an equilibrium constant from which D(0)(W(+)-CH(2)) = 4.72 +/- 0.04 eV is derived. Theoretical calculations performed at the B3LYP/HW+/6-311++G(3df,3p) level yield thermochemistry in reasonable agreement with experiment. These calculations help identify the structures and electronic states of the species involved and characterize the potential energy surface for the [W,C,4H](+) system.     

7Li NMR studies on complexation reactions of lithium ion with cryptand C211 in ionic liquids: comparison with corresponding reactions in nonaqueous solvents

(7)Li NMR spectra of DEME-TFSA [DEME=N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium; TFSA=bis(trifluoromethanesulfonyl)amide], EMI-TFSA (EMI=1-ethyl-3-methylimidazolium), MPP-TFSA (MPP = N-methyl-N-propylpyridinium), DEME-PFSA [PFSA=bis(pentafluoroethanesulfonyl)amide], and DEME-HFSA [HFSA=bis(heptafluoropropanesulfonyl)amide] ionic liquid (IL) solutions containing LiX (X=TFSA, PFSA, or HFSA) and C211 (4,7,13,18-tetraoxa-1,10-diazabicyclo[8.5.5]eicosane) were measured at various temperatures. As a result, it was found that the uncomplexed Li(I) species existing as [Li(X)(2)](-) in the present ILs exchange with the complexed Li(I) ([Li·C211](+)) and that the exchange reactions proceed through the bimolecular mechanism, [Li·C211](+) + [*Li(X)(2)](-)=[*Li·C211](+) + [Li(X)(2)](-). Kinetic parameters [k(s)/(kg m(-1) s(-1)) at 25 °C, ΔH(++)/(kJ mol(-1)), ΔS(++)/(J K(-1) mol(-1))] are as follows: 5.57×10(-2), 69.8 ± 0.4, and -34.9 ± 1.0 for the DEME-TFSA system; 5.77×10(-2), 70.6 ± 0.2, and -31.9 ± 0.6 for the EMI-TFSA system, 6.13×10(-2), 69.0 ± 0.3, and -36.7 ± 0.7 for the MPP-TFSA system; 1.35 × 10(-1), 65.2 ± 0.5, and -43.1 ± 1.4 for the DEME-PFSA system; 1.14×10(-1), 64.4 ± 0.3, and -47.1 ± 0.6 for the DEME-HFSA system. To compare these kinetic data with those in conventional nonaqueous solvents, the exchange reactions of Li(I) between [Li·C211](+) and solvated Li(I) in N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) were also examined. These Li(I) exchange reactions were found to be independent of the concentrations of the solvated Li(I) and hence proposed to proceed through the dissociative mechanism. Kinetic parameters [k(s)/s(-1) at 25 °C, ΔH(++)/(kJ mol(-1)), ΔS(++)/(J K(-1) mol(-1))] are as follows: 1.10 × 10(-2), 68.9 ± 0.2, and -51.3 ± 0.4 for the DMF system; 1.13×10(-2), 76.3 ± 0.3, and -26.3 ± 0.8 for the DMSO system. The differences in reactivities between ILs and nonaqueous solvents were proposed to be attributed to those in the chemical forms of the uncomplexed Li(I) species, i.e., the negatively charged species ([Li(X)(2)](-)) in ILs, and the positively charged ones ([Li(solvent)(n)](+)) in nonaqueous solvents.     

Effect of the N-terminal basic residue on facile Cα-C bond cleavages of aromatic-containing peptide radical cations

Fragmentation of radical cationic peptides [R(G)(n-2)X(G)(7-n)]˙(+) and [R(G)(m-2)XG]˙(+) (X = Phe or Tyr; m = 2-5; n = 2-7) leads selectively to a(n)(+) product ions through in situ C(α)-C peptide backbone cleavage at the aromatic amino acid residues. In contrast, substituting the arginine residue with a less-basic lysine residue, forming [K(G)(n-2)X(G)(7-n)]˙(+) (X = Phe or Tyr; n = 2-7) analogs, generates abundant b-y product ions; no site-selective C(α)-C peptide bond cleavage was observed. Studying the prototypical radical cationic tripeptides [RFG]˙(+) and [KFG]˙(+) using low-energy collision-induced dissociation and density functional theory, we have examined the influence of the basicity of the N-terminal amino acid residue on the competition between the isomerization and dissociation channels, particularly the selective C(α)-C bond cleavage viaβ-hydrogen atom migration. The dissociation barriers for the formation of a(2)(+) ions from [RFG]˙(+) and [KFG]˙(+)via their β-radical isomers are comparable (33.1 and 35.0 kcal mol(-1), respectively); the dissociation barrier for the charge-induced formation of the [b(2)- H]˙(+) radical cation from [RFG]˙(+)via its α-radical isomer (39.8 kcal mol(-1)) was considerably higher than that from [KFG]˙(+) (27.2 kcal mol(-1)). Thus, the basic arginine residue sequesters the mobile proton to promote the charge-remote selective C(α)-C bond cleavage by energetically hindering the competing charge-induced pathways.     

Photoelectron spectroscopy of higher bromine and iodine oxide anions: electron affinities and electronic structures of BrO(2,3) and IO(2-4) radicals

This report details a photoelectron spectroscopy (PES) and theoretical investigation of electron affinities (EAs) and electronic structures of several atmospherically relevant higher bromine and iodine oxide molecules in the gas phase. PES spectra of BrO(2)(-) and IO(2)(-) were recorded at 12 K and four photon energies--355 nm/3.496 eV, 266 nm/4.661 eV, 193 nm/6.424 eV, and 157 nm/7.867 eV--while BrO(3)(-), IO(3)(-), and IO(4)(-) were only studied at 193 and 157 nm due to their expected high electron binding energies. Spectral features corresponding to transitions from the anionic ground state to the ground and excited states of the neutral are unraveled and resolved for each species. The EAs of these bromine and iodine oxides are experimentally determined for the first time (except for IO(2)) to be 2.515 ± 0.010 (BrO(2)), 2.575 ± 0.010 (IO(2)), 4.60 ± 0.05 (BrO(3)), 4.70 ± 0.05 (IO(3)), and 6.05 ± 0.05 eV (IO(4)). Three low-lying excited states along with their respective excitation energies are obtained for BrO(2) [1.69 (A (2)B(2)), 1.79 (B (2)A(1)), 1.99 eV (C (2)A(2))], BrO(3) [0.7 (A (2)A(2)), 1.6 (B (2)E), 3.1 eV (C (2)E)], and IO(3) [0.60 (A (2)A(2)), 1.20 (B (2)E), ～3.0 eV (C (2)E)], whereas six excited states of IO(2) are determined along with their respective excitation energies of 1.63 (A (2)B(2)), 1.73 (B (2)A(1)), 1.83 (C (2)A(2)), 4.23 (D (2)A(1)), 4.63 (E (2)B(2)), and 5.23 eV (F (2)B(1)). Periodate (IO(4)(-)) possesses a very high electron binding energy. Only one excited state feature with 0.95 eV excitation energy is shown in the 157 nm spectrum. Accompanying theoretical calculations reveal structural changes from the anions to the neutrals, and the calculated EAs are in good agreement with experimentally determined values. Franck-Condon factors simulations nicely reproduce the observed vibrational progressions for BrO(2) and IO(2). The low-lying excited state information is compared with theoretical calculations and discussed with their atmospheric implications.     

Isoelectronic homologues and isomers: tropolone, 5-azatropolone, 1-H-azepine-4,5-dione, saddle points, and ions

Computational studies of 12 64-electron homologues and isomers of tropolone in the S(0) electronic ground state are reported. Three minimum-energy structures, tropolone (Tp), 5-azatropolone (5Azt), and 5-H-5-azatropolonium (5AztH(+)), have an internal H-bond and planar C(s)) geometry, and three, tropolonate (TpO(-)), 5-azatropolonate (5AzO(-)), and 1-H-azepine-4,5-dione (45Di), lack the H-bond and have twisted C(2) geometry. All 6 substances have an equal double-minimum potential energy surface and a saddle point with planar C(2)(v) geometry. The energy for the gas-phase isomerization reaction 45Di --> 5Azt is near +4 kJ mol(-1) at the MP4(SDQ)/6-311++G(df,pd)//MP2/6-311++G(df,pd) (energy//geometry) theoretical level and around -20 kJ mol(-1) at lower theoretical levels. The dipole moments computed for 45Di and 5Azt are 9.6 and 2.1 D, respectively, and this large difference contributes to MO-computed free energies of solvation that strongly favor--as experimentally observed--45Di over 5Azt in chloroform solvent. The MO-computed energy for the gas-phase protonation reaction 45Di + H(+) --> 5AztH(+) is -956.4 kJ mol(-1), leading to 926.8 kJ mol(-1) as the estimated proton affinity for 45Di at 298 K and 1 atm. The intramolecular dynamical properties predicted for 5Azt and 5AztH(+) parallel those observed for tropolone. They are therefore expected to exhibit spectral tunneling doublets. Once they are synthesized, they should contribute importantly to the understanding of multidimensional intramolecular H transfer and dynamical coupling processes.     

Understanding the acid-base properties of adenosine: the intrinsic basicities of N1, N3 and N7

Adenosine (Ado) can accept three protons, at N1, N3, and N7, to give H(3) (Ado)(3+) , and thus has three macro acidity constants. Unfortunately, these constants do not reflect the real basicity of the N sites due to internal repulsions, for example, between (N1)H(+) and (N7)H(+). However, these macroconstants are still needed for the evaluations and the first two are taken from our own earlier work, that is, pK(H)(H(3))((Ado)) = -4.02 and pK(H)(H(2))((Ado)) = -1.53; the third one was re-measured as pK(H)(H)((Ado)) = 3.64 ± 0.02 (25 °C; I=0.5 M, NaNO(3)), because it is the main basis for evaluating the intrinsic basicities of N7 and N3. Previously, contradicting results had been published for the micro acidity constant of the (N7)H(+) site; this constant has now been determined in an unequivocal manner, and that of the (N3)H(+) site was obtained for the first time. The micro acidity constants, which describe the release of a proton from an (N)H(+) site under conditions for which the other nitrogen atoms are free and do not carry a proton, decrease in the order pk(N7-N1)(N7(Ado)N1·H)) = 3.63 ± 0.02 > pk(N7-N1)(H·N7(Ado)N1) = 2.15 ± 0.15 > pk(N3-N1,N7)(H·N3(Ado)N1,N7) =1.5 ± 0.3, reflecting the decreasing basicity of the various nitrogen atoms, that is, N1>N7>N3. Application of the above-mentioned microconstants allows one to calculate the percentages (formation degrees) of the tautomers formed for monoprotonated adenosine, H(Ado)(+) , in aqueous solution; the results are 96.1, 3.2, and 0.7% for N7(Ado)N1·H(+), (+)H·N7(Ado)N1, and (+)H·N3(Ado)N1,N7, respectively. These results are in excellent agreement with theoretical DFT calculations. Evidently, H(Ado)(+) exists to the largest part as N7(Ado)N1·H(+) having the proton located at N1; the two other tautomers are minority species, but they still form. These results are not only meaningful for adenosine itself, but are also of relevance for nucleic acids and adenine nucleotides, as they help to understand their metal ion-binding properties; these aspects are briefly discussed.     

Kinetics and mechanisms of the homogeneous, unimolecular gas-phase elimination of trimethyl orthoacetate and trimethyl orthobutyrate

The gas-phase elimination kinetics of the title compounds have been examined over the temperature range of 310-369 degrees C and pressure range of 50-130 Torr. The reactions, in seasoned vessels, are homogeneous, unimolecular, and follow a first-order rate law. The products are methanol and the corresponding methyl ketene acetal. The rate coefficients are expressed by the Arrhenius equation: for trimethyl orthoacetate, log k1 (s(-1)) = [(13.58 +/- 0.10) - (194.7 +/- 1.2) (kJ mol(-1))](2.303RT)(-1)r = 0.9998; and for trimethyl orthobutyrate, log k1(s(-1)) = [(13.97 +/- 0.37) - (195.3 +/- 1.6) (kJ mol(-1))](2.303RT)(-1)r = 0.9997. These reactions are believed to proceed through a polar concerted four-membered cyclic transition state type of mechanism.     

Zinc-thiolate complexes of the bis(pyrazolyl)(thioimidazolyl)hydroborate tripods for the modeling of thiolate alkylating enzymes

The new tripod ligands bis(pyrazolyl)(3-tert-butyl-2-thioimidazol-1-yl)hydroborate (L(1)) and bis(pyrazolyl)(3-isopropyl-2-thioimidazol-1-yl)hydroborate (L(2)), together with zinc nitrate or zinc chloride and the corresponding thiolates, have yielded a total of 17 zinc-thiolate complexes. These comprise aliphatic as well as aromatic thiolates and a cysteine derivative. Structure determinations have confirmed the tetrahedral ZnN(2)S(2) coordination in the complexes. Upon reaction with methyl iodide, the species L(1).Zn-SR are slowly converted to L(1).Zn-I and the free thioethers CH(3)SR. A kinetic analysis has shown these alkylations to be about 1 order of magnitude slower than those of the tris(pyrazolyl)borate complexes Tp(Ph,Me)Zn-SR. Alkylations with trimethyl phosphate were found to proceed very slowly even in DMSO at 80 degrees C.