苏氨酸对甲苯磺酸盐及其酯化物的微波合成、表征及量化计算

以苏氨酸(Thr)和对甲苯磺酸(p-TsOH)为原料, 用一步法在微波反应仪中合成了离子液体苏氨酸对甲苯磺酸盐(Thr-TsOH), 并采用对甲苯磺酸法一步制得其甲酯和乙酯化合物ThrC1-TsOH和ThrC2-TsOH. 对合成的离子液体的理化性质进行了表征, 它们均具有较低的熔点(低于100 ℃), 但热稳定性相对较差. 结合量化计算理论方法初步探索了氨基酸酯化对熔点的影响. 采用DFT的B3LYP/6-311++G**方法对苏氨酸对甲苯磺酸及其甲酯、乙酯化合物3种分子进行构型优化及振动频率分析, 通过对分子中阴、阳离子间结合能的研究发现, 酯化不仅能减弱分子间氢键相互作用, 还能明显减弱分子内离子间相互作用, 从而降低体系的熔点.     

缬氨酸阳离子型离子液体的实验与理论研究

以缬氨酸（Val）和氟硼酸、对甲苯磺酸、硝酸、盐酸、硫酸和磷酸为原料,通过混合适当摩尔比的缬氨酸和相应的强酸于水中,采用一步法在微波反应仪中合成缬氨酸阳离子型离子液体,得到了缬氨酸氟硼酸盐（ValBF4）、缬氨酸盐酸盐（ValCl）、缬氨酸硝酸盐（ValNO3）、缬氨酸对甲苯磺酸盐（ValTsO）、缬氨酸硫酸盐（Val2SO4）、缬氨酸磷酸盐（Val3PO4）共6种产物,并对其理化性质进行了表征,其中ValBF4、Val3PO4具有较低的熔点（低于100 ℃) ,但热稳定性较传统的离子液体差。此外研究了缬氨酸阳离子型离子液体在汽油脱硫中的应用,以正庚烷作溶剂、噻吩作含硫杂质组成模拟汽油体系,将缬氨酸硝酸盐作为脱硫剂加入模拟油中,在进行超声波振荡之后利用色谱仪检测不同振荡时间的脱硫效果。同时采用量子化学方法对合成的6种化合物的单个分子进行了理论研究,在B3LYP/6-11++G**水平下获得了最稳能量构型,并在此基础上进行了红外光谱、离子间相互作用、自然键轨道分析等,从理论上对上述实验结果进行验证及补充。     

咪唑氟硼酸类离子液体熔点与分子内相互作用能的关系

运用密度泛函理论B3LYP方法及6-311++G(d,p)基组对11种咪唑氟硼酸离子液体进行了研究.选择相应化合物的离子体系{[XIM][BF4]n}(n-1)-(n=2,3)作为研究对象,即研究体系由一个烷基咪唑阳离子XIM+和2-3个BF4-阴离子构成,对其进行结构优化.在优化得到的最低能量构型的基础上计算了分子内阳离子与阴离子间的相互作用能,同时考虑了基组重叠误差的修正.结果表明所研究离子体系的离子间相互作用能与离子液体的实验熔点之间存在明确的线性关系,并且所得到的线性方程与氨基酸阳离子型离子液体中存在的线性关系相近.我们的工作为今后借助量子化学方法设计功能化离子液体提供了一定的理论基础.     

Protic ionic liquids: preparation, characterization, and proton free energy level representation

We give a perspective on the relations between inorganic and organic cation ionic liquids (ILs), including members with melting points that overlap around the borderline 100 degrees C. We then present data on the synthesis and properties (melting, boiling, glass temperatures, etc.) of a large number of an intermediate group of liquids that cover the ground between equimolar molecular mixtures and ILs, depending on the energetics of transfer of a proton from one member of the pair to the other. These proton-transfer ILs have interesting properties, including the ability to serve as electrolytes in solvent-free fuel cell systems. We provide a basis for assessing their relation to aprotic ILs by means of a Gurney-type proton-transfer free energy level diagram, with approximate values of the energy levels based on free energy of formation and pK(a) data. The energy level scheme allows us to verify the relation between solvent-free acidic and basic electrolytes, and the familiar aqueous variety, and to identify neutral protic electrolytes that are unavailable in the case of aqueous systems.     

Melting properties of molten salts and ionic liquids. Chemical homology,correlation, and prediction

The application of the concept of chemical homology to describe melting properties of molten salts and ionic liquids (ILs) is analyzed. This concept was used several years ago to correlate and predict properties of solids and more recently to correlate melting temperatures of ILs. To analyze the characteristics of the extended method, this is first applied to melting properties of organic substances for which abundant data are available. The method is extended to analyze its applicability for properties of molten salts and ILs such as glass transition temperature, heat of melting, and entropy of melting. The foundation of the chemical homology concept is revised, and the difficulties for extending the method to correlate and predict melting properties of ILs are presented. Despite the difficulties, the homology concept can still be used with some conditions and limitations that are analyzed in this article. Several correlations are proposed.     

Macrocyclic Ionic Liquids with Amino Acid Residues: Synthesis and Influence of Thiacalix[4]arene Conformation on Thermal Stability

Novel thiacalix[4]arene based ammonium ionic liquids (ILs) containing amino acid residues (glycine and L-phenylalanine) in cone, partial cone, and 1,3-alternate conformations were synthesized by alkylation of macrocyclic tertiary amines with N-bromoacetyl-amino acids ethyl ester followed by replacing bromide anions with bis(trifluoromethylsulfonyl)imide ions. The melting temperature of the obtained ILs was found in the range of 50–75 °C. The effect of macrocyclic core conformation on the synthesized ILs’ melting points was shown, i.e., the ILs in partial cone conformation have the lowest melting points. Thermal stability of the obtained macrocyclic ILs was determined via thermogravimetry and differential scanning calorimetry. The onset of decomposition of the synthesized compounds was established at 305–327 °C. The compounds with L-phenylalanine residues are less thermally stable by 3–19 °C than the same glycine-containing derivatives.     

离子液体增塑生物基聚丁内酰胺的热分解机理

采用疏水性1-丁基-3-甲基咪唑六氟磷酸盐([BMIM]PF₆)和亲水性1-丁基-3-甲基咪唑四氟硼酸盐 ([BMIM]BF₄)两种咪唑类离子液体(IL)增塑聚丁内酰胺(PBL), 探讨了IL对PBL结晶性能及热性能的影响. 研究发现, 两种IL都会削弱PBL分子间氢键, 并抑制PBL晶体在(200)晶面的生长, 降低PBL结晶度. 当IL添加质量分数为5%时, 增塑膜熔点下降7~8 ℃. 与纯PBL膜相比, [BMIM]BF₄增塑PBL膜热稳定性下降, 而[BMIM]PF₆增塑PBL膜的热稳定性提高. [BMIM]PF₆增塑PBL膜热分解过程的热动力学分析结果表明, 其热分解反应活化能为46.68 kJ/mol, 反应级数为1, 热分解最概然机理函数模型符合Mampel单行法则(一级), 即PBL受到热刺激后, 在聚合物和分解产物界面无规律成核, 反应核心具备反应活性, 随后反应逐步扩大, 直至结束.     

Structural,Thermal, and Storage Stability of Rapana Thomasiana Hemocyanin in the Presence of Cholinium-Amino Acid-Based Ionic Liquids

Novel biocompatible compounds that stabilize proteins in solution are in demand for biomedical and/or biotechnological applications. Here, we evaluated the effect of six ionic liquids, containing mono- or dicholinium [Chol]_1or2 cation and anions of charged amino acids such as lysine [Lys], arginine [Arg], aspartic acid [Asp], or glutamic acid [Glu], on the structure, thermal, and storage stability of the Rapana thomasiana hemocyanin (RtH). RtH is a protein with huge biomedicinal potential due to its therapeutic, drug carrier, and adjuvant properties. Overall, the ionic liquids (ILs) induce changes in the secondary structure of RtH. However, the structure near the Cu-active site seems unaltered and the oxygen-binding capacity of the protein is preserved. The ILs showed weak antibacterial activity when tested against three Gram-negative and three Gram-positive bacterial strains. On the contrary, [Chol][Arg] and [Chol][Lys] exhibited high anti-biofilm activity against E. coli 25213 and S. aureus 29213 strains. In addition, the two ILs were able to protect RtH from chemical and microbiological degradation. Maintained or enhanced thermal stability of RtH was observed in the presence of all ILs tested, except for RtH-[Chol]₂[Glu].     

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 (Tf(2) N) have been prepared, namely, [Cu(acac)(BuMe(3) en)][Tf(2) N] (1?a), [Cu(Bu-acac)(BuMe(3) en)][Tf(2) N] (1?b), [Cu(C(12) -acac)(Me(4) en)][Tf(2) N] (1?c), [Cu(acac)(Me(4) en)][Tf(2) N] (1?d), and [Ni(acac)(BuMe(3) en)][Tf(2) N] (2?a) (acac=acetylacetonate, Bu-acac=3-butyl-2,4-pentanedionate, C(12) -acac=3-dodecyl-2,4-pentanedionate, BuMe(3) en=N-butyl-N,N',N'-tetramethylethylenediamine, and Me(4) en=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.     

Crystal Structures,Thermal Analysis and Electrochemical Behaviors of Functionalized Pyridinium Ionic Liquids Comprising One 1-Ethyl Acetate Group

Pyridinium ionic liquids(ILs, 1-ethyl acetate pyridinium hexfluorophosphate[EAPy][PF₆] and 1-ethyl acetate-3-methyl pyridinium hexfluorophosphate[EAMPy][PF₆]), were synthesized by a two-step process involving introduction of one ethyl acetate group and anion metathesis. Colorless single crystals of the two ILs were initially obtained using the solvent-evaporation method in mixed solvents. Single-crystal X-ray diffraction was used to determine the crystal structures.[EAPy][PF₆] crystallizes in the monoclinic space group C2/c with a=2.2748(16) nm, b=0.6204(4) nm, c=1.8552(12) nm and Z=8, whereas[EAMPy][PF₆] crystallizes in the orthorhombic space group P2₁2₁2₁ with a=0.7126(17) nm, b=1.2792(3) nm, c=1.5327(3) nm and Z=4. The structure of[EAPy][PF₆] contains double zigzag chains formed by alternately pairing large organic cations with the octahedral anions of[P1F₆]^- or[P2F₆]^-. The[P1F₆]^- and[P2F₆]^- anions occupy respectively two distinct crystallographic sites in crystal packing models. The structure of[EAMPy][PF₆] includes ladder-type chains constructed through pairing pyridinium cations with inorganic anions of[PF₆]^-. The[PF₆]^- anion in[EAMPy][PF₆] shows a distorted octahedron structure and is sandwiched by ethyl acetate groups in crystallographic stacking. This study reveals the influence of chemical mo-dification involving the methyl group(CH₃) onto crystallographic structure of pyridinium ILs. Thermal analysis indicates that the difficult crystallization of the two ILs is related to the low void filling of ion pairs in crystal structure, leading to relatively low melting point and evident supercooling during the cooling process. Additionally, the experimental results indicate that the two ILs have electrochemical activity. The ethyl acetate group also allows downward shifting of electrochemical windows to less negative positions and the ionic conductivities of the two ILs follow an Arrhenius-type behavior.     

Melting point depression of ionic liquids confined in nanospaces

A new physical method was proposed to control the liquid properties of room temperature ionic liquids (RT-ILs) in combination with nanoporous materials; the melting point of ILs confined in nanopores remarkably decreases in proportion to the inverse of the pore size.     

Straightforward Synthesis of the Brønsted Acid hfipOSO3H and its Application for the Synthesis of Protic Ionic Liquids

The easily accessible hexafluoroisopropoxysulfuric acid ( 1 , hfipOSO₃H ; hfip=C(H)(CF₃)₂) was synthesized by the reaction of hexafluoroisopropanol and chlorosulfonic acid on the kilogram scale and isolated in 98 % yield. The calculated gas‐phase acidity (GA) value of 1 is 58 kJ mol^?1 lower in ΔG° than that of sulfuric acid (GA value determined by a CCSD(T)‐MP2 compound method). Considering the gas‐phase dissociation constant as a measure for the intrinsic molecular acid strength, a hfipOSO₃H molecule is more than ten orders of magnitude more acidic than a H₂SO₄ molecule. The acid is a liquid at room temperature, distillable at reduced pressure, stable for more than one year in a closed vessel, reactive towards common solvents, and decomposes above 180 °C. It is a versatile compound for further applications, such as the synthesis of ammonium‐ and imidazolium‐based air‐ and moisture‐stable protic ionic liquids (pILs). Among the six synthesized ionic compounds, five are pILs with melting points below 100 °C and three of them are liquids at nearly room temperature. The conductivities and viscosities of two representative ILs were investigated in terms of Walden plots, and the pILs were found to be little associated ILs, comparable to conventional aprotic ILs.     

Why are ionic liquids liquid? A simple explanation based on lattice and solvation energies

We have developed a simple and quantitative explanation for the relatively low melting temperatures of ionic liquids (ILs). The basic concept was to assess the Gibbs free energy of fusion (Delta(fus)G) for the process IL(s) --> IL(l), which relates to the melting point of the IL. This was done using a suitable Born-Fajans-Haber cycle that was closed by the lattice (i.e., IL(s) --> IL(g)) Gibbs energy and the solvation (i.e., IL(g) --> IL(l)) Gibbs energies of the constituent ions in the molten salt. As part of this project we synthesized and determined accurate melting points (by DSC) and dielectric constants (by dielectric spectroscopy) for 14 ionic liquids based on four common anions and nine common cations. Lattice free energies (Delta(latt)G) were estimated using a combination of Volume Based Thermodynamics (VBT) and quantum chemical calculations. Free energies of solvation (Delta(solv)G) of each ion in the bulk molten salt were calculated using the COSMO solvation model and the experimental dielectric constants. Under standard ambient conditions (298.15 K and 10(5) Pa) Delta(fus)G degrees was found to be negative for all the ILs studied, as expected for liquid samples. Thus, these ILs are liquid under standard ambient conditions because the liquid state is thermodynamically favorable, due to the large size and conformational flexibility of the ions involved, which leads to small lattice enthalpies and large entropy changes that favor melting. This model can be used to predict the melting temperatures and dielectric constants of ILs with good accuracy. A comparison of the predicted vs experimental melting points for nine of the ILs (excluding those where no melting transition was observed and two outliers that were not well described by the model) gave a standard error of the estimate (s(est)) of 8 degrees C. A similar comparison for dielectric constant predictions gave s(est) as 2.5 units. Thus, from very little experimental and computational data it is possible to predict fundamental properties such as melting points and dielectric constants of ionic liquids.     

Lanthanoid-based ionic liquids incorporating the dicyanonitrosomethanide anion

A series of low-melting-point salts with hexakisdicyanonitrosomethanidolanthanoidate anions has been synthesised and characterised: (C(2) mim)(3) [Ln(dcnm)(6)] (1?Ln; 1?Ln=1?La, 1?Ce, 1?Pr, 1?Nd), (C(2) C(1) mim)(3) [Pr(dcnm)(6)] (2?Pr), (C(4) C(1) pyr)(3) [Ce(dcnm)(6)] (3?Ce), (N(1114))(3) [Ln(dcnm)(6)] (4?Ln; 4?Ln=4?La, 4?Ce, 4?Pr, 4?Nd, 4?Sm, 4?Gd), and (N(1112OH) )(3) [Ce(dcnm)(6)] (5?Ce) (C(2) mim=1-ethyl-3-methylimidazolium, C(2) C(1) mim=1-ethyl-2,3-dimethylimidazolium, C(4) C(1) py=N-butyl-4-methylpyridinium, N(1114) =butyltrimethylammonium, N(1112OH) =2-(hydroxyethyl)trimethylammonium=choline). X-ray crystallography was used to determine the structures of complexes 1?La, 2?Pr, and 5?Ce, all of which contain [Ln(dcnm)(6)](3-) ions. Complexes 1?Ln and 2?Pr were all ionic liquids (ILs), with complex 3?Ce melting at 38.1?°C, the lowest melting point of any known complex containing the [Ln(dcnm)(6)](3-) trianion. The ammonium-based cations proved to be less suitable for forming ILs, with complexes 4?Sm and 4?Gd being the only salts with the N(1114) cation to have melting points below 100?°C. The choline-containing complex 5?Ce did not melt up to 160?°C, with the increase in melting point possibly being due to extensive hydrogen bonding, which could be inferred from the crystal structure of the complex.     

1-烷基-3-甲基咪唑氯化物焓变的热重分析

The structures of ionic liquids (ILs) based on 1-alkyl-3-methylimidazolium chloride [C_nmim]Cl (n = 2, 4, 6), (1-ethyl-3-methylimidazolium chloride [C₂mim]Cl, 1-butyl-3-methylimidazolium chloride [C₄mim]Cl, and 1-hexyl-3-methylimidazolium chloride [C₆mim]Cl) were elucidated by ¹H NMR and ¹³C NMR experiments. The vaporization characteristics of these ILs were studied by thermogravimetric analysis. Dynamic and isothermal thermogravimetric experiments were conducted in this study. The purpose of the dynamic experiments was to determine the initial decomposition temperature of the experimental sample and the temperature range for the isothermal thermogravimetric experiments. The purpose of the isothermal experiments was to record the mass dependence of the sample on time in the experimental temperature range. The Langmuir equation and Clausius-Clapeyron equation were used to fit the experimental data and obtain the vaporization enthalpies of these ILs at the average temperature within the experimental temperature range. However, in order to expand the applicability of the estimated values and to compare them with the literature data, the vaporization enthalpy ΔH_vap(T_av) measured at the average temperature was converted into vaporization enthalpy ΔH_vap(298) at ambient temperature. The difference between the heat capacities of the ILs in the gaseous and liquid states at constant pressure, Δ_l^gC_pm^ө proposed by Verevkin, was used in this conversion process. The experimental data for substance density and surface tension at other temperatures were obtained by referring to the literature. In addition, the data for density and surface tension at T = 298.15 K were obtained by applying the extrapolation method to the literature values for other temperatures. The vaporization enthalpy of the 1-octyl-3-methylimidazolium chloride IL [C₈mim]Cl was estimated by using the new vaporization model we had proposed in our previous work and compared with the reference value. The estimated value for [C₈mim]Cl was on the same order of magnitude as the reference value. We compared the vaporization enthalpies in the present study with those for the carboxylic acid imidazolium and amino acid imidazolium ILs ([C_nmim]Pro (n = 2-6) and [C_nmim]Thr (n = 2-6), respectively in our previous work. The results revealed that a change in the anion type affects the vaporization enthalpy of the ILs in the order amino acid imidazolium > carboxylic acid imidazolium > halogen imidazolium, when the cation is the same. Considering the structural differences between the three kinds of ILs, the abovementioned order may be related to the intermolecular hydrogen bonds. There were no intermolecular hydrogen bonds in the [C_nmim]Cl (n = 2, 4, 6) ILs studied here. Therefore, the vaporization enthalpy of [C_nmim]Cl (n = 2, 4, 6) was the lowest among the three kinds of ILs considered.     

Origin of low melting point of ionic liquids: dominant role of entropy

Ionic liquids (ILs) are salts with an extremely low melting point. Substantial efforts have been made to address their low melting point from the enthalpic standpoint (i.e. interionic interactions). However, this question is still open. In this study, we report our findings that entropic (large fusion entropy), rather than enthalpic, contributions are primarily responsible for lowering the melting point in many cases, based on a large thermodynamic dataset. We have established a computational protocol using molecular dynamics simulations to decompose fusion entropy into kinetic (translational, rotational, and intramolecular vibrational) and structural (conformational and configurational) terms and successfully applied this approach for two representatives of ILs and NaCl. It is revealed that large structural contribution, particularly configurational entropy in the liquid state, plays a deterministic role in the large fusion entropy and consequently the low melting point of the ILs.

Large structural entropy makes salts liquid at room temperature.     

Determination of phase transition points of ionic liquids by combination of thermal analysis and conductivity measurements at very low heating and cooling rates

The determination of phase transition points of nine different ionic liquids (ILs) was performed by thermal analysis with simultaneous recording of conductivity. Conductivity of electrolyte solutions and ILs drastically changes during phase transitions and thus is an additional and very sensitive indicator for measuring phase transition points. Evaluation of temperature–time functions and conductivity–time functions with our computer-coupled automated equipment enabled the determination of melting temperatures with high accuracy and reliability. This claim is based on large samples, low temperature change rates and by regularly repeated measurements, i.e. at least seven measurements per IL. The melting temperatures of 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate, 1-butyl-1-methylpyrrolidinium tris(penta-fluoroethyl)trifluorophosphate, and 1-methyl-3-propylimidazolium iodide were, to our knowledge, determined for the first time. The melting temperatures of the other 1-butyl-1-methylpyrrolidinium-, 1-ethyl-3-methylimidazolium-, 1-hexyl-3-methylimidazolium-, and trimethylsulfonium-based ILs showed either a very good accordance with values published in literature or were distinctly higher.     

Reactive half-metallocenium ionic liquids that undergo solventless ligand exchange

The piano-stool half-metallocenium cations [Fe(C(5)R(5))(CO)(2)L](+) (C(5)R(5) = C(5)H(5), C(5)Me(5), C(5)Me(4)Et; L = 1-pentene, nBuCN, MeCN, Me(2)S, NH(3), NMe(3), pyridine) provide ionic liquids (ILs) with the bis(trifluoromethanesulfonyl)imide (Tf(2)N) anion without introducing long alkyl chains. Their melting points are affected by molecular symmetry, and their thermal stabilities reflect the strength of the metal-ligand bonding. These are reactive liquids that show solventless ligand exchange reactions by gas absorption. The direction of the ligand-exchange reaction is correlated with the stabilities. Based on the variation of the melting points, these ILs undergo transformations between the liquid and solid phases associated with the reaction.     

Synthesis and Physical Properties of Tunable Aryl Alkyl Ionic Liquids (TAAILs)

Tunable aryl alkyl ionic liquids (TAAILs) based on the imidazolium cation were first reported in 2009. Since then, a series of TAAILs with different properties due to the electron-donating or -withdrawing effect of the substituents at the aryl ring has been developed. Herein, a wide variety of those ionic liquids (ILs) is presented in terms of their cation structure. The authors synthesized ILs containing the bromide or bis(trifluoromethane)sulfonimide anion and 1-aryl-3-alkyl imidazolium cations with various substituents in the ortho and/ or para position of the phenyl ring and alkyl chains of different lengths varying from butyl to dodecyl. The differences of their physical properties (melting point, thermal decomposition, viscosity, electro-chemical window) of these ILs are reported according to their structure.     

Structure and properties of high stability geminal dicationic ionic liquids

Thirty-nine geminal dicationic ILs were synthesized and characterized in terms of their surface tensions, densities, melting points, refractive indices, viscosities, and miscibilities with a polar and nonpolar solvent. Two imidazolium or pyrrolidinium cations were joined via different length hydrocarbon linkage chains (from 3 to 12 carbons long). The various geminal dications were paired with up to four different anions. The effect of the dication type, linkage chain, alkyl substituents, and anion type on the physicochemical properties of these compounds was examined. Among the more interesting findings for this class of compounds was that their liquid and thermal stability ranges generally exceeded those of the more conventional, better known ILs. Indeed, this range was from -4 to >400 degrees C for one of the pyrrolidinium-based geminal dicationic liquids. X-ray crystallography of the smaller solid ionic compounds indicated that there may be a correlation between the configurational degrees of freedom of the ILs and their melting points/glass transition temperatures. In one case, the crystal structure showed that a dicationic moiety had three distinct conformations in an asymmetric unit cell. The solvation properties of the geminal dicationic ILs tend to be similar to those of their monocationic analogues.