Towards a Molecular Understanding of Cation–Anion Interactions—Probing the Electronic Structure of Imidazolium Ionic Liquids by NMR Spectroscopy,X‐ray Photoelectron Spectroscopy and Theoretical Calculations

Ten [C₈C₁Im]⁺ (1‐methyl‐3‐octylimidazolium)‐based ionic liquids with anions Cl^?, Br^?, I^?, [NO₃]^?, [BF₄]^?, [TfO]^?, [PF₆]^?, [Tf₂N]^?, [Pf₂N]^?, and [FAP]^? (TfO=trifluoromethylsulfonate, Tf₂N=bis(trifluoromethylsulfonyl)imide, Pf₂N=bis(pentafluoroethylsulfonyl)imide, FAP=tris(pentafluoroethyl)trifluorophosphate) and two [C₈C₁C₁Im]⁺ (1,2‐dimethyl‐3‐octylimidazolium)‐based ionic liquids with anions Br^? and [Tf₂N]^? were investigated by using X‐ray photoelectron spectroscopy (XPS), NMR spectroscopy and theoretical calculations. While ¹H NMR spectroscopy is found to probe very specifically the strongest hydrogen‐bond interaction between the hydrogen attached to the C² position and the anion, a comparative XPS study provides first direct experimental evidence for cation–anion charge‐transfer phenomena in ionic liquids as a function of the ionic liquid’s anion. These charge‐transfer effects are found to be surprisingly similar for [C₈C₁Im]⁺ and [C₈C₁C₁Im]⁺ salts of the same anion, which in combination with theoretical calculations leads to the conclusion that hydrogen bonding and charge transfer occur independently from each other, but are both more pronounced for small and more strongly coordinating anions, and are greatly reduced in the case of large and weakly coordinating anions.     

Enantioseparation of rabeprazole and omeprazole by nonaqueous capillary electrophoresis with an ephedrine-based ionic liquid as the chiral selector

An ephedrine‐based chiral ionic liquid, (+)‐N,N‐dimethylephedrinium‐bis(trifluoromethanesulfon)imidate ([DMP]⁺[Tf₂N]^‐), served as both chiral selector and background electrolyte in nonaqueous capillary electrophoresis. The enantioseparation of rabeprazole and omeprazole was achieved in acetonitrile–methanol (60:40 v/v) containing 60 mm [DMP]⁺[Tf₂N]^‐. The influences of separation conditions, including the concentration of [DMP]⁺[Tf₂N]^‐, the electrophoretic media and the buffer, on enantioseparation were evaluated. The mechanism of enantioseparation was investigated and discussed. Ion‐pair interaction and hydrogen bonding may be responsible for the main separation mechanism. Copyright © 2010 John Wiley & Sons, Ltd.     

Abrasive Stripping Voltammetric Studies of Lignin and Lignin Model Compounds

Lignin is potentially a major renewable, nonfossil source of aromatic and cyclohexyl compounds. In this study, we have investigated the abrasive stripping voltammetry of lignin and four lignin model compounds in the room temperature ionic liquids (RTILs) [C₄mim][NTf₂], [N_6,2,2,2][NTf₂] and [C₄mim][OTf] (where [C₄mim]⁺=1‐butyl‐3‐methylimidazolium, [N_6,2,2,2]⁺=n‐hexyltriethylammonium, [NTf₂]^?=bis(trifluoromethanesulfonyl)imide and [OTf]^? =trifluoromethanesulfonate) on a gold macrodisk and in 0.1 M H₂SO₄ and 0.1 M NaOH on a boron‐doped diamond (BDD) macroelectrode, with the hope of using the voltammetry to fingerprint the functional groups within the lignin molecule. The use of RTILs on metal electrodes, or either acidic or basic media in combination with BDD electrodes allows solvent systems with wide electrochemical potential windows, useful for studying voltammetry which may be difficult to observe in systems where early breakdown of the solvent occurs.     

Physical and CO2-Absorption Properties of Imidazolium Ionic Liquids with Tetracyanoborate and Bis(trifluoromethanesulfonyl)amide Anions

Ionic liquids with tetracyanoborate ([TCB]^?) and bis(trifluoromethanesulfonyl)amide ([Tf₂N]^?) anions generally have low viscosities and high CO₂ capacities, and thus they are attractive solvents for CO₂-related applications. Herein, we have investigated physical and CO₂-absorption properties of 1-ethyl-3-methylimidazolium tetracyanoborate ionic liquid ([emim][TCB]) to discuss the anion effects of [TCB]^? in comparison with the previous results of [emim][Tf₂N]. The density, viscosity, electrical conductivity, and isobaric molar heat capacity were measured as a function of temperature at atmospheric pressure. [emim][TCB] has both lower density and isobaric molar heat capacity than [emim][Tf₂N]. [emim][TCB] shows superior transport properties (lower viscosity and higher electrical conductivity) compared to [emim][Tf₂N], whereas the Walden plots of molar conductivity against fluidity (reciprocal of viscosity) have smaller values in [emim][TCB] than in [emim][Tf₂N] at certain fluidities. The high-pressure CO₂ solubilities were also determined in [emim][TCB]. The mole fraction scaled solubility of CO₂ in [emim][TCB] is slightly larger than that in [emim][Tf₂N] at certain pressures and temperatures. The former ionic liquid shows much higher molarity scaled solubility of CO₂ than the latter because of the smaller molar volume. It is suggested that both anions have similar strength of intermolecular interaction with CO₂ and comparable changes in the solvent structure between neat and CO₂ solution, in view of the thermodynamic parameters of dissolution.     

Physical Vapor Deposition of [EMIM][Tf2N]: A New Approach to the Modification of Surface Properties with Ultrathin Ionic Liquid Films

Ultrathin films of the ionic liquid (IL) 1‐ethyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide, [EMIM][Tf₂N], are prepared on a glass substrate by means of an in situ thermal‐evaporation/condensation process under ultrahigh‐vacuum conditions. By using X‐ray photoelectron spectroscopy (XPS), it is demonstrated that the first layer of the IL film grows two dimensionally, followed by the three‐dimensional growth of successive layers. The first molecular layer consists of a bilayer, with the [EMIM]⁺ cations in contact to the surface and the [Tf₂N]^? anions at the vacuum side. The ultrathin IL films are found to be stable under ambient conditions.     

Characterization of N‐Boc/Fmoc/Z‐N′‐formyl‐gem‐diaminoalkyl derivatives using electrospray ionization multi‐stage mass spectrometry

N‐Boc/Fmoc/Z‐N′‐formyl‐gem‐diaminoalkyl derivatives, intermediates particularly useful in the synthesis of partially modified retro‐inverso peptides, have been characterized by both positive and negative ion electrospray ionization (ESI) ion‐trap multi‐stage mass spectrometry (MSⁿ). The MS² collision induced dissociation (CID) spectra of the sodium adduct of the formamides derived from the corresponding N‐Fmoc/Z‐amino acids, dipeptide and tripeptide acids show the [M + Na‐NH₂CHO]⁺ ion, arising from the loss of formamide, as the base peak. Differently, the MS² CID spectra of [M + Na]⁺ ion of all the N‐Boc derivatives yield the abundant [M + Na‐C₄H₈]⁺ and [M + Na‐Boc + H]⁺ ions because of the loss of isobutylene and CO₂ from the Boc protecting function. Useful information on the type of amino acids and their sequence in the N‐protected dipeptidyl and tripeptidyl‐N′‐formamides is provided by MS² and subsequent MSⁿ experiments on the respective precursor ions. The negative ion ESI mass spectra of these oligomers show, in addition to [M‐H]^?, [M + HCOO]^? and [M + Cl]^? ions, the presence of in‐source CID fragment ions deriving from the involvement of the N‐protecting group. Furthermore, MSⁿ spectra of [M + Cl]^? ion of N‐protected dipeptide and tripeptide derivatives show characteristic fragmentations that are useful for determining the nature of the C‐terminal gem‐diamino residue. The present paper represents an initial attempt to study the ESI‐MS behavior of these important intermediates and lays the groundwork for structural‐based studies on more complex partially modified retro‐inverso peptides. Copyright © 2013 John Wiley & Sons, Ltd.     

Reduction of Digallane [(dpp‐bian)Ga?Ga(dpp‐bian)] with Group 1 and 2 Metals

The reduction of digallane [(dpp‐bian)Ga? Ga(dpp‐bian)] ( 1 ) (dpp‐bian=1,2‐bis[(2,6‐diisopropylphenyl)imino]acenaphthene) with lithium and sodium in diethyl ether, or with potassium in THF affords compounds featuring the direct alkali metal–gallium bonds, [(dpp‐bian)Ga? Li(Et₂O)₃] ( 2 ), [(dpp‐bian)Ga? Na(Et₂O)₃] ( 3 ), and [(dpp‐bian)Ga? K(thf)₅] ( 7 ), respectively. Crystallization of 3 from DME produces compound [(dpp‐bian)Ga? Na(dme)₂] ( 4 ). Dissolution of 3 in THF and subsequent crystallization from diethyl ether gives [(dpp‐bian)Ga? Na(thf)₃(Et₂O)] ( 5 ). Ionic [(dpp‐bian)Ga]^?[Na([18]crown‐6)(thf)₂]⁺ ( 6 a ) and [(dpp‐bian)Ga]^?[Na(Ph₃PO)₃(thf)]⁺ ( 6 b ) were obtained from THF after treatment of 3 with [18]crown‐6 and Ph₃PO, respectively. The reduction of 1 with Group 2 metals in THF affords [(dpp‐bian)Ga]₂M(thf)_n (M=Mg ( 8 ), n=3; M=Ca ( 9 ), Sr ( 10 ), n=4; M=Ba ( 11 ), n=5). The molecular structures of 4 – 7 and 11 have been determined by X‐ray crystallography. The Ga? Na bond lengths in 3 – 5 vary notably depending on the coordination environment of the sodium atom.     

The First Homoleptic Bis(trifluoromethanesulfonyl)amide Complex of Yttrium: [bmim][Y(Tf2N)4]

1‐Butyl‐3‐methylimidazolium tetrakis‐(bis(trifluoromethanesulfonyl)amide)yttrium(III), [bmim][Y(Tf₂N)₄], was obtained from the ionic liquid 1‐butyl‐3‐methylimidazoliumbis(trifluoromethanesulfonyl)amide, [bmim][Tf₂N] and yttrium(III)bis(trifluoromethanesulfonyl)amide, Y(Tf₂N)₃. The crystal structure [monoclinic, C2/c (no. 15), a = 2096.(1), b = 1451.5(1), c = 1608.55(9) pm, β = 109.669(6)°, V = 4608.1(5)·10⁶ pm³, Z = 4, R₁ for 3874 symmetry independent reflections with I₀>2σ(I₀): 0.0438] contains Y^III coordinated by four Tf₂N‐ligands which all adopt a transoid‐conformation with respect to their –CF₃ groups. The oxygen coordination polyhedron around Y^III can be best described as a trigonal dodecahedron.One 1‐butyl‐3‐methylimidazolium cation compensates for the charge of the complex [Y(Tf₂N)₄]^? anion.     

Electrospray ionization multiple-stage linear ion-trap mass spectrometry for structural elucidation of triacylglycerols: Assignment of fatty acyl groups on the glycerol backbone and location of double bonds

Linear ion-trap multiple-stage mass spectrometric approach (MS ⁿ ) towards nearly complete structural elucidation of triacylglycerol (TAG) including (1) assignment the fatty acid substituents on the glycerol backbone and (2) location of the double bond(s) on the unsaturated fatty acyl groups is reported. The characterization is established by the findings that MS² on the [M+Li]⁺ ions of TAG yields more abundant ions reflecting losses of the outer fatty acid substituents either as free acids (i.e., [M+Li-R₁CO₂H]⁺ and [M+Li-R₃CO₂H]⁺ ions) or as lithium salts (i.e., [M+Li-R₁CO₂Li]⁺ and [M+Li-R₃CO₂Li]⁺ ions) than the ions reflecting the similar losses of the inner fatty acid substituent (i.e., [M+Li-R₂CO₂Li]⁺ and [M+Li-R₂CO₂Li]⁺ ions). Further dissociation (MS³) of [M+Li-R _n CO₂H]⁺ (n=1, 2, or 3) gives rise to the ion series locating the double bonds along the fatty acid chain. These ions arise from charge-remote fragmentations involving β-cleavage with γ-H shift, analogous to those seen for the unsaturated long-chain fatty acids characterized as initiated ions. Significant differences in abundances in the ion pairs reflecting the additional losses of the fatty acid moieties, respectively, were also seen in the MS³ spectra of the [M+Li-R _n CO₂H]⁺ and [M+Li-R _n CO₂Li]⁺ ions, leading to confirmation of the fatty acid substituents on the glycerol backbone. MS ⁿ on the [M+Na]⁺ and [M+NH₄]⁺ adduct ions also affords location of fatty acid substituents on the glycerol backbone, but not the position of the double bond(s) along the fatty acid chain. Unique ions from internal losses of the glycerol residues were seen in the MS³ spectra of [M+Alk-R _n CO₂H]⁺ (n=1, 2, 3) and of [M+Alk-R _n CO₂Alk]⁺ (Alk=Li, Na, NH₄; n=1, 3). They are signature ions for glycerides and the pathways leading to their formation may involve rearrangements.     

Mechanism of Dissolution of a Lithium Salt in an Electrolytic Solvent in a Lithium Ion Secondary Battery: A Direct Ab Initio Molecular Dynamics (AIMD) Study

The mechanism of dissolution of the Li⁺ ion in an electrolytic solvent is investigated by the direct ab initio molecular dynamics (AIMD) method. Lithium fluoroborate (Li⁺BF₄^?) and ethylene carbonate (EC) are examined as the origin of the Li⁺ ion and the solvent molecule, respectively. This salt is widely utilized as the electrolyte in the lithium ion secondary battery. The binding of EC to the Li⁺ moiety of the Li⁺BF₄^? salt is exothermic, and the binding energies at the CAM–B3LYP/6‐311++G(d,p) level for n=1, 2, 3, and 4, where n is the number of EC molecules binding to the Li⁺ ion, (EC)_n(Li⁺BF₄^?), are calculated to be 91.5, 89.8, 87.2, and 84.0 kcal mol^?1 (per EC molecule), respectively. The intermolecular distances between Li⁺ and the F atom of BF₄^? are elongated: 1.773 Å (n=0), 1.820 Å (n=1), 1.974 Å (n=2), 1.942 Å (n=3), and 4.156 Å (n=4). The atomic bond populations between Li⁺ and the F atom for n=0, 1, 2, 3, and 4 are 0.202, 0.186, 0.150, 0.038, and 0.0, respectively. These results indicate that the interaction of Li⁺ with BF₄^? becomes weaker as the number of EC molecules is increased. The direct AIMD calculation for n=4 shows that EC reacts spontaneously with (EC)₃(Li⁺BF₄^?) and the Li⁺ ion is stripped from the salt. The following substitution reaction takes place: EC+(EC)₃(Li⁺BF₄^?)→(EC)₄Li⁺?(BF₄^?). The reaction mechanism is discussed on the basis of the theoretical results.     

Theoretical study on nonlinear optical properties of the Li+[calix[4]pyrrole]Li−dimer,trimer and its polymer with diffuse excess electrons

The static (hyper)polarizabilities of the dimer and trimer with diffuse excess electrons, [Li⁺[calix[4]pyrrole]Li^?]_n, are firstly investigated by the DFT(B3LYP) method in detail. For the dimer and trimer, a Li atom inside each calix[4]pyrrole unit is ionized to form a diffuse excess electron. The results show that the dimer and trimer containing two and three excess electrons, respectively, have very large first hyperpolarizablities as 2.3 × 10⁴ and 4.0 × 10⁴ au, which are 30 and 40 times larger than that of the corresponding [calix[4]pyrrole]_n (n = 2, 3) without Li atom. Also, β values of dimer and trimer are twice and four times as large as that of monomer containing one excess electron. Obviously, not only excess electron but also the number of excess electron plays an important role in increasing the first hyperpolarizability. Moreover, the (hyper)polarizabilities of the [Li⁺[calix[4]pyrrole]Li^?]_n polymer are investigated at ab initio level by using the elongation finite‐field (elongation FF) method. All the oligomers of the [Li⁺[calix[4]pyrrole]Li^?]_n with many excess electrons exhibit very large first hyperpolarizability and large second hyperpolarizability. The present investigation shows that by introducing several and more excess electrons into the nonlinear optical (NLO) materials will be an important strategy for improving their NLO properties, which will be helpful for design of NLO materials. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

Electron impact mass spectra of some salts of carboxylic acids with alkali (group Ia) metals

Thirteen of the salts of the alkali metals (Li, Na, K, Rb, Cs) with acetic, 2,2-dimethylpropionic, trifluoroacetic and heptafluorobutyric acid have been found to be sufficiently volatile to give mass spectra under normal electron impact conditions. The metal containing ions observed include (M=metal): [M]⁺, [MO]⁺, [MCO₂]⁺, [M₂]^+·, [M₂O]^+·, [M₂CO₂]^+· and the cluster ions [M_n (carboxylate)_n-1]⁺ for n = 2–8.     

Ether-Functionalized Pyrrolidinium-Based Room Temperature Ionic Liquids: Physicochemical Properties,Molecular Dynamics,and the Lithium Ion Coordination Environment

The physicochemical properties of room temperature ionic liquids (RTILs) consisting of bis(trifluoromethanesulfonyl)amide (TFSA⁻) combined with 1-hexyl-1-methylpyrrolidinium (Pyr_1,6⁺), 1-(butoxymethyl)-1-methylpyrrolidinium (Pyr_1,1O4⁺), 1-(4-methoxybutyl)-1-methyl pyrrolidinium (Pyr_1,4O1⁺), and 1-((2-methoxyethoxy)methyl)-1-methylpyrrolidinium (Pyr_1,1O2O1⁺) were investigated using both experimental and computational approaches. Pyr_1,1O2O1TFSA, which contains two ether oxygen atoms, showed the lowest viscosity, and the relationship between its physicochemical properties and the position and number of the ether oxygen atoms was discussed by a careful comparison with Pyr_1,1O4TFSA and Pyr_1,4O1TFSA. Ab initio calculations revealed the conformational flexibility of the side chain containing the ether oxygen atoms. In addition, molecular dynamics (MD) calculations suggested that the ion distributions have a significant impact on the transport properties. Furthermore, the coordination environments of the Li ions in the RTILs were evaluated using Raman spectroscopy, which was supported by MD calculations using 1000 ion pairs. The presented results will be valuable for the design of functionalized RTILs for various applications.     

Vapochromic Ionic Liquids from Metal–Chelate Complexes Exhibiting Reversible Changes in Color,Thermal, and Magnetic Properties

Vapor‐ and gas‐responsive ionic liquids (ILs) comprised of cationic metal‐chelate complexes and bis(trifluoromethanesulfonyl)imide (Tf₂N) have been prepared, namely, [Cu(acac)(BuMe₃en)][Tf₂N] ( 1 a ), [Cu(Bu‐acac)(BuMe₃en)][Tf₂N] ( 1 b ), [Cu(C₁₂‐acac)(Me₄en)][Tf₂N] ( 1 c ), [Cu(acac)(Me₄en)][Tf₂N] ( 1 d ), and [Ni(acac)(BuMe₃en)][Tf₂N] ( 2 a ) (acac=acetylacetonate, Bu‐acac=3‐butyl‐2,4‐pentanedionate, C₁₂‐acac=3‐dodecyl‐2,4‐pentanedionate, BuMe₃en=N‐butyl‐N,N′,N′‐tetramethylethylenediamine, and Me₄en=N,N,N′,N′‐trimethylethylenediamine). These ILs exhibited reversible changes in color, thermal properties, and magnetic properties in response to organic vapors and gases. The Cu^II‐containing ILs are purple and turn blue‐purple to green when exposed to organic vapors, such as acetonitrile, methanol, and DMSO, or ammonia gas. The color change is based on the coordination of the vapor molecules to the cation, and the resultant colors depend on the coordination strength (donor number, DN) of the vapor molecules. The vapor absorption caused changes in the melting points and viscosities, leading to alteration in the phase behaviors. The IL with a long alkyl chain ( 1 d ) transitioned from a purple solid to a brown liquid at its melting point. The Ni^II‐containing IL ( 2 a ) is a dark red diamagnetic liquid, which turned into a green paramagnetic liquid by absorbing vapors with high DN. Based on the equilibrium shift from four‐ to six‐coordinated species, the liquid exhibited thermochromism and temperature‐dependent magnetic susceptibility after absorbing methanol.     

Rapid characterization of triterpene saponins from Conyza blinii by liquid chromatography coupled with mass spectrometry

Conyza blinii Le'vl is a medicinal herb used for the treatment of inflammation in Chinese folk medicine. Its major bioactive constituents are triterpene saponins, most of which contain 6–8 sugar residues. In this report, electrospray ionization tandem mass spectrometry fragmentation behaviors of bisdesmosidic triterpene saponins (conyzasaponin A, B, and C) were studied in both positive and negative ion modes with an ion‐trap mass spectrometer. In full scan mass spectrometry, these saponins gave predominant [M–H]^? and [M+Na]⁺ ions, which determined the molecular weights. In tandem mass spectrometry (MSⁿ, n = 2–4), the [M–H]^? and [M+Na]⁺ ions yielded fragments [Y_0α–H]^? and [B_α+Na]⁺, which were diagnostic for the structures of the triterpene skeleton and sugar chains. The structural elucidation was approved by accurate mass data using IT‐TOF‐MS. An interpretation guideline based on MSⁿ (n = 2–4) diagnostic ions was proposed in order to elucidate the chemical structures of unknown triterpene saponins in C. blinii extract. The saponins in C. blinii were separated by liquid chromatography with a methanol/acetonitrile/water solvent system, and then analyzed by ion‐trap and IT‐TOF mass spectrometers. Based on the interpretation guideline, a total of 35 triterpenoid saponins were tentatively identified. Among them, 15 saponins had been previously reported, and the other 20 saponins were reported from Conyza species for the first time. This study indicates that LC/MS is a powerful technology for the rapid characterization of complicated saponins in herbal extracts. Copyright © 2010 John Wiley & Sons, Ltd.     

The Temperature Effect on the Transport Properties of 1-Alkyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide Ionic Liquids

The temperature dependences of specific and equivalent conductivities, viscosity, density, and crystallization temperature are determined for three 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([C _n MeIm] [Tf₂N], n = 2, 3, 4) ionic liquids saturated with water vapor at room temperature. It is established that in the area of positive temperatures, the relationship between viscosity and conductivity obeys the fractional Walden rule with exponents of 0.97, 0.92, and 0.92 for ionic liquids with ethyl-, propyl-, butylradicals, respectively. The temperature dependences of conductivity and viscosity are approximated using the Vogel–Fulcher–Tammann equation (R² > 0.999), and ideal glass transition temperatures T₀ are calculated for the investigated liquids. The obtained values of T₀ depend largely on the chosen range of temperatures. It is shown that [C₂MeIm][Tf₂N] occupies a separate position with regard to [C₃MeIm][Tf₂N] and [C₄MeIm][Tf₂N].     

Estimation of peptide N–Cα bond cleavage efficiency during MALDI‐ISD using a cyclic peptide

Matrix‐assisted laser desorption/ionization in‐source decay (MALDI‐ISD) induces N–C_α bond cleavage via hydrogen transfer from the matrix to the peptide backbone, which produces a c′/z? fragment pair. Subsequently, the z? generates z′ and [z + matrix] fragments via further radical reactions because of the low stability of the z?. In the present study, we investigated MALDI‐ISD of a cyclic peptide. The N–C_α bond cleavage in the cyclic peptide by MALDI‐ISD produced the hydrogen‐abundant peptide radical [M + 2H]⁺? with a radical site on the α‐carbon atom, which then reacted with the matrix to give [M + 3H]⁺ and [M + H + matrix]⁺. For 1,5‐diaminonaphthalene (1,5‐DAN) adducts with z fragments, post‐source decay of [M + H + 1,5‐DAN]⁺ generated from the cyclic peptide showed predominant loss of an amino acid with 1,5‐DAN. Additionally, MALDI‐ISD with Fourier transform‐ion cyclotron resonance mass spectrometry allowed for the detection of both [M + 3H]⁺ and [M + H]⁺ with two ¹³C atoms. These results strongly suggested that [M + 3H]⁺ and [M + H + 1,5‐DAN]⁺ were formed by N–C_α bond cleavage with further radical reactions. As a consequence, the cleavage efficiency of the N–C_α bond during MALDI‐ISD could be estimated by the ratio of the intensity of [M + H]⁺ and [M + 3H]⁺ in the Fourier transform‐ion cyclotron resonance spectrum. Because the reduction efficiency of a matrix for the cyclic peptide cyclo(Arg‐Gly‐Asp‐D‐Phe‐Val) was correlated to its tendency to cleave the N–C_α bond in linear peptides, the present method could allow the evaluation of the efficiency of N–C_α bond cleavage for MALDI matrix development. Copyright © 2016 John Wiley & Sons, Ltd.     

Theoretical studies of structures and stabilities of endohedral fullerenes X0/n+ @C32 (X = H,Li, Na,K, Be,Mg, Ca,B, Al,C, Si,N, P,n = 1–3)

Based on the D₃ C₃₂ fullerene, the equilibrium geometries, electronic structures, and binding energies of the endohedral fullerenes X^0/n+@C₃₂ (X = H, Li, Na, K, Be, Mg, Ca, B, Al, C, Si, N, P, n = 1–3) have been calculated using the DFT/B3LYP/6‐31G(d) method. The results show that the C₃₂ cages are slightly enlarged due to encapsulation, and the sizes of non‐neutral molecules are smaller than the corresponding neutral ones. Cages containing Li, Na, and Ca and most of the cations, except Na⁺ and K⁺, are energetically favorable. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

A comparison of positive and negative ion collision‐induced dissociation for model heptapeptides with one basic residue

The effects of the identity and position of basic residues on peptide dissociation were explored in the positive and negative modes. Low‐energy collision‐induced dissociation (CID) was performed on singly protonated and deprotonated heptapeptides of the type: XAAAAAA, AAAXAAA, AAAAAXA and AAAAAAX, where X is arginine (R), lysine (K) or histidine (H) residues and A is alanine. For [M + H]⁺, the CID spectra are dominated by cleavages adjacent to the basic residues and the majority of the product ions contain the basic residues. The order of a basic residue's influence on fragmentation of [M + H]⁺ is arginine > histidine ≈ lysine, which is also the order of decreasing gas‐phase basicity for these amino acids. These results are consistent with the side chains of basic residues being positive ion charge sites and with the more basic arginine residues having a higher retention (i.e. sequestering) of the positive charge. In contrast, for [M ? H]^? the identity and position of basic residues has almost no effect on backbone fragmentation. This is consistent with basic residues not being negative mode charge sites. For these peptides, more complete series of backbone fragments, which are important in the sequencing of unknowns, can be found in the negative mode. Spectra at both polarities contain C‐terminal y‐ions, but y_n″⁺ has two more hydrogens than the corresponding y_n^?. Another major difference is the production of the N‐terminal backbone series b_n⁺ in the positive mode and c_n^? in the negative mode. Thus, comparison of positive and negative ion spectra with an emphasis on searching for pairs of ions that differ by 2 Da (y_n″⁺ vs y_n^?) and by 15 Da (b_n⁺ vs c_n^?) may be a useful method for determining whether a product ion is generated from the C‐terminal or the N‐terminal end of a peptide. In addition, a characteristic elimination of NH?C?NH from arginine residues is observed for deprotonated peptides. Copyright © 2010 John Wiley & Sons, Ltd.     

Mass Distribution and Diffusion of [1‐Butyl‐3‐methylimidazolium][Y] Ionic Liquids Adsorbed on the Graphite Surface at 300–800 K

The structure and diffusion behavior of 1‐butyl‐3‐methylimidazolium ([bmim]⁺) ionic liquids with [Cl]^?, [PF₆]^?, and [Tf₂N]^? counterions near a hydrophobic graphite surface are investigated by molecular dynamics simulation over the temperature range of 300–800 K. Near the graphite surface the structure of the ionic liquid differs from that in the bulk and it forms a well‐ordered region extending over 30 Å from the surface. The bottom layer of the ionic liquid is stable over the investigated temperature range due to the inherent slow dynamics of the ionic liquid and the strong Coulombic interactions between cation and anion. In the bottom layer, diffusion is strongly anisotropic and predominantly occurs along the graphite surface. Diffusion perpendicular to the interface (interfacial mass transfer rate k_t) is very slow due to strong ion–substrate interaction. The diffusion behaviors of the three ionic liquids in the two directions all follow an Arrhenius relation, and the activation barrier increases with decreasing anion size. Such an Arrhenius relation is applied to surface‐adsorbed ionic liquids for the first time. The ion size and the surface electrical charge density of the anions are the major factors determining the diffusion behavior of the ionic liquid adjacent to the graphite surface.