Homopolar and Heteropolar Bond Dissociation Energies and Heats of Formation of Radicals and Ions in the Gas Phase II. The relationship between structure and bond dissociation in organic molecules

The preceeding paper presents a critical tabular compilation of 2-center homo- and heteropolar bond dissociation energies. This paper deals with some empirically derived general aspects of these data, particularly regarding relationships between structure and reactivity, i.e. substituent effects on bond dissociation energies. ‘Extra’ stabilization energies generated in electronically delocalized radicals or ions derived from these experimental data are also presented.     

Bond Dissociation Energies and Electronic Structures in a Series of Peroxy Radicals: A Theoretical Study

Quantum chemical calculations are performed to estimate the bond dissociation energies (BDEs) for 18 peroxy radicals. Since DFT methods are researched to have low basis sets sensitivity, these radicals are studied by utilizing the hybrid density functional theory (DFT) (B3LYP, B3P86, B3PW91 and PBE1PBE) in conjunction with the 6‐311G** basis set and the complete basis set (CBS‐Q) method. On the basis of comparisons of the computational results and the experimental values, we evaluate the effectiveness of above methods. It is demonstrated that CBS‐Q method is the best method for computing the reliable BDEs of C—OO bond, with the average absolute errors of 2.1 kcal/mol. So CBS‐Q method is suitable to predict accurate BDEs of C‐OO bond for peroxy compounds. The computational energy gaps between the HOMO and LUMO of studied compounds are almost identical from the point of view of stability and substantial HOMO‐LUMO gaps for all molecules suggest their electronic stability. In addition, substituent effect on the C—OO BDE of peroxy radicals is analyzed. It is noted that the effects of substitution on the C—OO BDE of peroxy radicals are significant. Our results will shed lights on future theoretical and experimental work.     

Surprising Homolytic Gas Phase Co−C Bond Dissociation Energies of Organometallic Aryl-Cobinamides Reveal Notable Non-Bonded Intramolecular Interactions

Aryl-cobalamins are a new class of organometallic structural mimics of vitamin B₁₂ designed as potential ‘antivitamins B₁₂’. Here, the first cationic aryl-cobinamides are described, which were synthesized using the newly developed diaryl-iodonium method. The aryl-cobinamides were obtained as pairs of organometallic coordination isomers, the stereo-structure of which was unambiguously assigned based on homo- and heteronuclear NMR spectra. The availability of isomers with axial attachment of the aryl group, either at the ‘beta’ or at the ‘alpha’ face of the cobalt-center allowed for an unprecedented comparison of the organometallic reactivity of such pairs. The homolytic gas-phase bond dissociation energies (BDEs) of the coordination-isomeric phenyl- and 4-ethylphenyl-cobinamides were determined by ESI-MS threshold CID experiments, furnishing (Co−C )-BDEs of 38.4 and 40.6 kcal mol⁻¹, respectively, for the two β-isomers, and the larger BDEs of 46.6 and 43.8 kcal mol⁻¹ for the corresponding α-isomers. Surprisingly, the observed (Co−C )-BDEs of the Co_β-aryl-cobinamides were smaller than the (Co−C )-BDE of Co_β-methyl-cobinamide. DFT studies and the magnitudes of the experimental (Co−C )-BDEs revealed relevant contributions of non-bonded interactions in aryl-cobinamides, notably steric strain between the aryl and the cobalt-corrin moieties and non-bonded interactions with and among the peripheral sidechains.     

密度泛函法计算C1-C14正构烃生成焓及C-C键裂解能

采用密度泛函理论(DFT)方法研究了费托石脑油裂解反应中涉及到C₁-C₁₄正构烃和自由基中间体的生成焓及其C-C键解离能(BDE)。 结果表明,在所有评价的密度泛函理论方法（B97-1、BB1K、B1B95、MPWB1K和MPW1B95）中,MPW1B95/6-311G(d,p)方法计算最精确。 以此方法为基准,进一步对高碳烃及其裂解产物的标准生成焓和C-C键解离能进行了预测。 与可得到的实验数据相比,MPW1B95/6-311G(d,p)方法预测的烃和自由基的平均生成焓分别为0.8和2.7 kJ/mol,C-C键解离能的平均绝对误差只有3.1 kJ/mol,表明此方法不仅可准确计算正构烃标准生成焓和C-C键解离能,而且还能正确预测C-C键解离能变化趋势。     

碱金属离子对甘氨酸五肽气相解离过程的影响

为了获得更多的多肽结构信息,采用结构简单的甘氨酸五肽(简写为GGGGG或G5)作为模型,研究了碱金属离子(Li+、Na+、K+、Rb+)对甘氨酸五肽GGGGG气相解离过程的影响.将一定化学计量比的甘氨酸五肽分别和四种碱金属盐溶液混合后,静置10h,使反应达到平衡.电喷雾质谱结果表明,四种碱金属离子均可以在溶液中与甘氨酸五肽形成非共价复合物,其中主要组分为碱金属离子与G5配合比为1:1和2:1的非共价复合物.质谱碰撞诱导解离(CID)时的碰撞能量为25eV.气相碰撞诱导解离实验结果表明,在配合比为1:1的复合物中,其碎片化程度按照Li+、Na+、K+、Rb+的次序依次减小,Rb+的复合物碎裂过程中生成了不常见的c、z离子;在配合比为2:1的复合物中,其碎片化程度按照Li+、Na+、K+、Rb+的次序依次增大.与1:1的非共价复合物相比,Na+、K+、Rb+的2:1复合物的气相解离显得更加容易.除Li+外,两个碱金属离子对G5的活化能力明显较单个碱金属离子强,它们可以诱导多肽在更多位点断裂,生成更多类型的碎片离子.     

Heterolytic and Homolytic N-NO Bond Dissociation Energies of N-Nitroso-benzenesulfonylmethylamines in Acetonitrile

Great interests have been accumulated in recent years in the chemistry and biochemistry of nitric oxide (NO) since the remarkable discoveries of its key roles in a wide range of human physiological processes. To elucidate the mechanistic details of NO migration from its donor to its acceptor, it is necessary to determine the Y-NO bind energy that registers the thermodynamic driving force for NO release and capture. In this paper the heterolytic and homolytic N-NO bond dissociation energies [ i. e., △Hhet(N-NO) and △Hhomo(N-NO)] for ten N-nitroso-p-substituted-benzensulfonyl methylamines in acetonitrile are offered, which were obtained from titration calorimetry and thermodynamic cycles, respectively (Scheme 1).     

乙腈介质中S-NO键异裂能和均裂能的测定

近年来, 大量研究表明, 一氧化氮在许多生命过程(如免疫、神经传导和血管扩张等)中发挥着十分重要的作用[1～4]. 然而, 文献[5]的研究表明, NO在生命体内很少以游离形式存在, 绝大部分都是与生命体内活线性分子结合着, 其中NO与有机硫以S-NO键方式结合形成的NO蕴合物(Nitric oxide-generating agent)被认为是NO在生命体内贮存、 转移和释放最主要的分子源[5]. 因此, 系统研究各种类型硫键NO蕴合物中S-NO键的断裂能, 可以诠释和预测NO在生命体内的转移方向和转移机制. 由于这一原因, S-NO键断裂能的研究是当今许多键能化学家正致力于解决的最热门课题之一. 10多年来, 我们从事的化学键键能研究, 已为这一领域的深入研究奠定了基础. 前文[6～8]根据热力学循环利用滴定量热法成功地测得了多个系列的N-NO键的异裂能和均裂能. 最近, 我们在此工作的基础上, 通过改变热力学循环方式又成功地利用滴定量热法测得了12个S-亚硝基化合物中的S-NO键的异裂能和均裂能, 其中9个为芳香体系, 3个为脂肪体系, 分别模拟生命体系内芳香体系和脂肪体系中的S-NO键. 本文首次报道其实验结果, 并进行一些讨论.     

DFT-PCM Study of the Bond Dissociation Energies of N-nitrosoindole Compounds in Acetonitrile

Quantum chemical calculations are used to estimate the equilibrium N–NO bond dissociation energies (BDEs) in acetonitrile (MeCN) for seven N-nitrosoindole compounds. These compounds are studied by employing the hybrid density functional theory (B3LYP, B3P86 and B3PW91) methods together with 6-31G^∗∗ basis sets. The obtained results are compared with the available experimental results. It is demonstrated that the B3PW91 method is the best of these methods to compute the bond dissociation energies of N-nitrosoindole compounds. The solvent effects on the BDEs of the N–NO bond are analyzed and it is shown that the N–NO BDEs in a vacuum, computed by the B3LYP method, are the closest to the computed values in MeCN and the average solvent effect is 4.0 kJ⋅mol⁻¹. The substituent effects on the N–NO BDEs were further analyzed and it is found that the N–NO BDE increases with increments of the Hammett constants of substituent groups on the benzene ring for N-nitrosoindole compounds, except the compound with 5-NO₂−C₈H₅N–NO. Finally, N-nitrosoindole compounds and the other NO-donors (N-nitroso compounds) are compared.     

ChemInform Abstract: Gas Phase Water and Hydroxyl Binding Energies for Monopositive First-Row Transition-Metal Ions.

M+-OH2 and M+-OH binding energies of first-row transition metals are determined by means of collision-induced dissociation in a triple-quadrupole mass spectrometer.     

Chemical Empiricism 2.0 at Age of Big Data: Large-scale Prediction of Reaction Pathways Based on Bond Dissociation Energies

A programmable algorithm using bond dissociation energies has been proposed for the prediction of reaction pathways. It has been successfully applied to a gas-phase reaction of F₂+CH₃Cl, with the accurate revelation of the most favorable product CF₄ and its corresponding reaction pathway. This acts as an inspiring example of chemical empiricism 2.0, and may open the door for the large-scale prediction of reaction pathways at the age of big data.     

N-亚硝基吲哚系列化合物及其自由基负离子在乙腈介质中N-NO键断裂能的测定

利用滴定量热技术并结合适当的热力学循环测定了乙腈溶液中7个取代的N-亚硝基吲哚化合物中N—NO键的异裂能和均裂能, 能量范围分别为206.1~246.2 kJ/mol和119.1~124.6 kJ/mol. 表明N-亚硝基吲哚均裂释放NO自由基(NO·)比异裂释放NO正离子(NO+)要容易得多, 通过热力学循环得到的相应自由基负离子中N—NO键的异裂能和均裂能的能量范围分别为25.5~34.4和5.0~40.5 kJ/mol, 表明所研究化合物的自由基负离子在室温下很不稳定.     

Catalytic Effect of Cesium Cation Adduct Formation on the Decarboxylation of Carboxylate Ions in the Gas Phase

The effect of Cs⁺ ligation on the decarboxylation of malonic acids (unsubstituted and methyl‐, dimethyl‐, ethyl‐, and phenyl‐substituted) in their carboxylate form was studied in the gas phase using tandem mass spectrometry. The study is based on the comparison of the decarboxylation of the bare monoanion (hydrogen malonates) and of the cesium adduct of the cesium salt (Cs⁺[cesium hydrogen malonates]) under collisional activation. Energy‐resolved dissociation curves of the negative and positive ions exhibit major differences. Decarboxylation of the cationic adducts of substituted malonic acid salts occurs at significantly lower collisional activation than for the corresponding bare hydrogen malonate anions. The conclusions from these experiments are supported by DFT calculations. The calculated activation parameters (enthalpy and Gibbs energy) confirm that the cesium cation coordination assists the decarboxylation of the carboxylate form.     

Bond Energy Scheme for Estimating Heats of Formation of Monomers and Polymers. VI. Sulfur Compounds

The bond energy scheme is extended to sulfur compounds and heats of formation and atomization energy terms derived from thermochemical data reviewed to 1977, for bonds of sulfur with carbon, hydrogen, halogens, and oxygen atoms. A precision of ± 1 kcal/mole was attainable for the covalent bonds of divalent sulfur in the lowest oxidation state S(± II). The higher valency states: S(IV) and S(VI) involve polar contributions depending upon the electrouegativity of the combining atom as well as (d^π -p^π) orbital promotion energies which are specific to the compound and transferable to other molecules only with a limited precision, no better than about ± 3 kcal/mole. The atomization energy terms (E_a ^25°C) of various bonds of sulfur a are found consistent with the experimental bond dissociation energies and bear a relationship with bond lengths and force constants as observed in the previous work. Heats of polymer-forming reactions and heats of formation of sulfur-containing monomers and polymers are estimated from the newly derived bond energy terms.     

Bond Energy Scheme for Estimating Heats of Formation of Monomers and Polymers. VII. Nitrogen Compounds

The bond energy scheme is extended to nitrogen compounds by correlating experimental thermochemical data reviewed to 1980. Heats of formation and atomization energy terms are provided for bonds of nitrogen with other elements: H, C, O, N, S, and halogens. An overall precision of 3 kcal/mol was attainable at the best, which is rather low for chemical reaction kinetics purpose. This is attributable mainly to the intrinsically unpredictable bond energies of the nitrogen atom due to the “lone-pair” electrons participation in the valence bonding, rendering nitrogen bonds specific and less transferable. The nearest-neighbor interactions on nitrogen atom are also severe but predictable if sufficient energy terms are generated. The concept of ring strain in five-membered rings (about 5 × 2 kcal/mol in the background of thermochemical data) has been reviewed and amended by providing a special set of energy terms for the (C, O, N, S) -ring skeleton which is considered strain-free if the hydrogen atom is the only substituent. Heats of formation of some common molecular structures are predicted.

Heats of formation of nitrogen-containing polymers and heats of polymer-forming and polymer-modification reactions are estimated and compared with available calorimetric data.     

Gas-Phase and Solution-Phase Homolytic Bond Dissociation Energies of H-N(+) Bonds in the Conjugate Acids of Nitrogen Bases

The oxidation potentials of 19 nitrogen bases (abbreviated as B: six primary amines, five secondary amines, two tertiary amines, three anilines, pyridine, quinuclidine, and 1,4-diazabicyclo[2,2,2]octane), i.e., E(ox)(B) values in dimethyl sulfoxide (DMSO) and/or acetonitrile (AN), have been measured. Combination of these E(ox)(B) values with the acidity values of the corresponding acids (pK(HB)(+)) in DMSO and/or AN using the equation: BDE(HB)(+) = 1.37pK(HB)(+) + 23.1 E(ox)(B) + C (C equals 59.5 kcal/mol in AN and 73.3 kcal/mol in DMSO) gave estimates of solution phase homolytic bond dissociation energies of H-B(+) bonds. Gas-phase BDE values of H-B(+) bonds were estimated from updated proton affinities (PA) and adiabatic ionization potentials (aIP) using the equation, BDE(HB(+))(g) = PA + aIP - 314 kcal/mol. The BDE(HB)(+) values estimated in AN were found to be 5-11 kcal/mol higher than the corresponding gas phase BDE(HB(+))(g) values. These bond-strengthening effects in solution are interpreted as being due to the greater solvation energy of the HB(+) cation than that of the B(+*) radical cation.     

Iron‐Promoted CC Bond Formation in the Gas Phase

An unusual iron transfer and carbon–carbon coupling take place in gas‐phase ionized mixtures containing ferrocene and dichloromethane. Ferrous chloride and the protonated benzenium ion are eventually formed by a thermal and efficient reaction, through stable intermediates that undergo a remarkable reorganization. The mechanism of the concerted iron extrusion, carbon–chlorine bond activation and carbon–carbon bond formation is elucidated by electronic structure calculations that show the crucial role of iron.     

Formation of y + 10 and y + 11 Ions in the Collision-Induced Dissociation of Peptide Ions

Tandem mass spectra of peptide ions, acquired in shotgun proteomic studies of selected proteins, tissues, and organisms, commonly include prominent peaks that cannot be assigned to the known fragmentation product ions (y, b, a, neutral losses). In many cases these persist even when creating consensus spectra for inclusion in spectral libraries, where it is important to determine whether these peaks represent new fragmentation paths or arise from impurities. Using spectra from libraries and synthesized peptides, we investigate a class of fragment ions corresponding to y_n-1 + 10 and y_n-1 + 11, where n is the number of amino acid residues in the peptide. These 10 and 11 Da differences in mass of the y ion were ascribed before to the masses of [+ CO – H₂O] and [+ CO – NH₃], respectively. The mechanism is suggested to involve dissociation of the N-terminal residue at the CH-CO bond following loss of H₂O or NH₃. MS³ spectra of these ions show that the location of the additional 10 or 11 Da is at the N-terminal residue. The y_n-1 + 10 ion is most often found in peptides with N-terminal proline, asparagine, and histidine, and also with serine and threonine in the adjacent position. The y_n-1 + 11 ion is observed predominantly with histidine and asparagine at the N-terminus, but also occurs with asparagine in positions two through four. The intensities of the y_n-1 + 10 ions decrease with increasing peptide length. These data for y_n-1 + 10 and y_n-1 + 11 ion formation may be used to improve peptide identification from tandem mass spectra.