A theoretical study of the structure and bonding of UOX4 (X = F, Cl, Br, I) molecules: the importance of inverse trans influence.

During nitroxide-mediated polymerization (NMP) in the presence of a nitroxide R2(R1)NO*, the reversible formation of N-alkoxyamines [P-ON(R1)R2] reduces significantly the concentration of polymer radicals (P*) and their involvement in termination reactions. The control of the livingness and polydispersity of the resulting polymer depends strongly on the magnitude of the bond dissociation energy (BDE) of the C-ON(R1)R2 bond. In this study, theoretical BDEs of a large series of model N-alkoxyamines are calculated with the PM3 method. In order to provide a predictive tool, correlations between the calculated BDEs and the cleavage temperature (T(c)), and the dissociation rate constant (k(d)), of the N-alkoxyamines are established. The homolytic cleavage of the N-OC bond is also investigated at the B3P86/6-311++G(d,p)//B3LYP/6-31G(d), level. Furthermore, a natural bond orbital analysis is carried out for some N-alkoxyamines with a O-C-ON(R1)R2 fragment, and the strengthening of their C-ON(R1)R2 bond is interpreted in terms of stabilizing anomeric interactions.     

Theoretical Study of the N-NO2 Bond Dissociation Energies for Energetic Materials with Density Functional Theory

The N-NO2 bond dissociation energies （BDEs） for 7 energetic materials were computed by means of accurate density functional theory （B3LYP, B3PW91 and B3P86） with 6-31G＊＊ and 6-311G＊＊ basis sets. By comparing the computed energies and experimental results, we find that the B3P86/6-311G＊＊ method can give good results of BDE, which has the mean absolute deviation of 1.30kcal/mol. In addition, substituent effects were also taken into account. It is noted that the Hammett constants of substituent groups are related to the BDEs of the N-NO2 bond and the bond dissociation energies of the energetic materials studied decrease when increasing the number of NO2 group.     

Bond dissociation energies and equilibrium structures of Cu+(MeOH)x, x = 1-6, in the gas phase: competition between solvation of the metal ion and hydrogen-bonding interactions

The solvation of Cu+ by methanol (MeOH) was studied via examination of the kinetic energy dependence of the collision-induced dissociation of Cu+(MeOH)x complexes, where x = 1-6, with Xe in a guided ion beam tandem mass spectrometer. In all cases, the primary and lowest-energy dissociation channel observed is the endothermic loss of a single MeOH molecule. The primary cross section thresholds are interpreted to yield 0 and 298 K bond dissociation energies (BDEs) after accounting for the effects of multiple ion-neutral collisions, kinetic and internal energy distributions of the reactants, and lifetimes for dissociation. Density functional theory calculations at the B3LYP/6-31G* level are performed to obtain model structures, vibrational frequencies, and rotational constants for the Cu+(MeOH)x complexes and their dissociation products. The relative stabilities of various conformations and theoretical BDEs are determined from single-point energy calculations at the B3LYP/6-311+G(2d,2p) level of theory using B3LYP/6-31G*-optimized geometries. The relative stabilities of the various conformations of the Cu+(MeOH)x complexes and the trends in the sequential BDEs are explained in terms of stabilization gained from sd hybridization, hydrogen-bonding interactions, electron donor-acceptor natural bond orbital stabilizing interactions, and destabilization arising from ligand-ligand repulsion.     

Noncovalent interactions of Zn+ with N-donor ligands (pyridine, 4,4'-dipyridyl, 2,2'-dipyridyl, and 1,10-phenanthroline): collision-induced dissociation and theoretical studies

The binding interactions in complexes of Zn(+) with nitrogen donor ligands, (N-L) = pyridine (x = 1-4), 4,4'-dipyridyl (x = 1-3), 2,2'-dipyridyl (x = 1-2), and 1,10-phenanthroline (x = 1-2), are examined in detail. The bond dissociation energies (BDEs) for loss of an intact ligand from the Zn(+)(N-L)(x) complexes are reported. Experimental BDEs are obtained from thermochemical analyses of the threshold regions of the collision-induced dissociation cross sections of Zn(+)(N-L)(x) complexes. Density functional theory calculations at the B3LYP/6-31G* level of theory are performed to determine stable structures of these species and to provide molecular parameters needed for the thermochemical analysis of experimental data. Relative stabilities of the various conformations of these N-donor ligands and their complexes to Zn(+) as well as theoretical BDEs are determined from single point energy calculations at the B3LYP/6-311+G(2d,2p) and M06/6-311+G(2d,2p) levels of theory using the B3LYP/6-31G* optimized geometries. The experimental BDEs for the Zn(+)(N-L)(x) complexes are in reasonably good agreement with values derived from density functional theory calculations. BDEs derived from M06 calculations provide better agreement with the measured values than those based on B3LYP calculations. Trends in the sequential BDEs are explained in terms of sp polarization of Zn(+) and repulsive ligand-ligand interactions. Comparisons are made to the analogous Cu(+)(N-L)(x) and Ni(+)(N-L)(x) complexes previously studied.     

CH and Chalogen bond dissociation energies for fluorinated and chlorinated methane evaluated with hybrid B3LYP density functional theory methods and their comparison with experimental data and the CBS-Q ab initio computational approach

The CH and Chalogen bond dissociation energies (BDEs) were computed with the hybrid B3LYP/6-311 + G(2d,2p) theory model for chlorinated and fluorinated methane. All computed values were substantially lower (5–10 kcal mol⁻¹) than the experimental values. To obtain better agreement, a correlation factor was introduced. When this factor was applied, excellent agreement between the B3LYP/6-311 + G(2d,2p) computed energies and the experimental BDEs was observed. On the other hand, the CBS-Q ab initio computational approach generated BDEs which are in good agreement with experimental values without a correction factor.     

A simple model for metal cation-phosphate interactions in nucleic acids in the gas phase: Alkali metal cations and trimethyl phosphate

Threshold collision-induced dissociation techniques are employed to determine the bond dissociation energies (BDEs) of complexes of alkali metal cations to trimethyl phosphate, TMP. Endothermic loss of the intact TMP ligand is the only dissociation pathway observed for all complexes. Theoretical calculations at the B3LYP/6-31G* level of theory are used to determine the structures, vibrational frequencies, and rotational constants of neutral TMP and the M+(TMP) complexes. Theoretical BDEs are determined from single point energy calculations at the B3LYP/6-311+G(2d,2p) level using the B3LYP/6-31G* optimized geometries. The agreement between theory and experiment is reasonably good for all complexes except Li+(TMP). The absolute M+-(TMP) BDEs are found to decrease monotonically as the size of the alkali metal cation increases. No activated dissociation was observed for alkali metal cation binding to TMP. The binding of alkali metal cations to TMP is compared with that to acetone and methanol.     

Absolute thermodynamic measurements of alkali metal cation interactions with a simple dipeptide and tripeptide

Absolute bond dissociation energies (BDEs) of glycylglycine (GG) and glycylglycylglycine (GGG) to sodium and potassium cations and sequential bond energies of glycine (G) in Na+G2 were determined experimentally by threshold collision-induced dissociation (TCID) in a guided ion beam tandem mass spectrometer. Experimental results showed that the binding energies follow the order of Na+ > K+ and M+GGG > M+GG > M+G. Theoretical calculations at the B3LYP/6-311+G(d) level show that all complexes had charge-solvated structures (nonzwitterionic) with either [CO,CO] bidentate or [N,CO,CO] tridentate coordination for M+GG complexes, [CO,CO,CO] tridentate or [N,CO,CO,CO] tetradentate coordination for M+GGG complexes, and [N,CO,N,CO] tetradentate coordination for Na+G2. Ab initio calculations at three different levels of theory (B3LYP, B3P86, and MP2(full) using the 6-311+G(2d,2p) basis set with geometries and zero-point energies calculated at the B3LYP/6-311+G(d) level) show good agreement with the experimental bond energies. This study demonstrates for the first time that TCID measurements of absolute BDEs can be successfully extended to biological molecules as complex as a tripeptide.     

An evaluation of various computational methods for the treatment of organoselenium compounds

A reliable computational method for the prediction of organoselenium geometries and bond dissociation energies (BDEs) has been determined on the basis of the performance of density functional theory (DFT: B3LYP and B3PW91) and ab initio molecular orbital procedures (Hartree-Fock (HF)) in conjunction with various Pople basis sets including (but not limited to) the 6-31G(d), 6-31G(d,p), 6-311G(d), 6-311G(d,p), 6-311G(2df,p), and 6-311G(3df,3pd) sets. Predicted geometries and BDEs are compared with available experimental data and quadratic configuration interaction including single and double substitutions (QCISD) results. The B3PW91/6-311G(2df,p) level of theory is recommended for the prediction of the geometries and energetics of organoselenium compounds.     

Periodic trends in bond dissociation energies. A theoretical study

Bond dissociation energies (BDEs) of all possible A-X single bonds involving the first- and second-row atoms, from Li to Cl, where the free valences are saturated by hydrogens, have been estimated through the use of the G3-theory and at the B3LYP/6-311+G(3df,2pd)//B3LYP/6-31G(2df,p) DFT level of theory. BDEs exhibit a periodical behavior. The A-X (A = Li, Be, B, Na, Mg, Al, and Si) BDEs show a steady increase along the first and the second row of the periodic table as a function of the atomic number Z(X). For A-X bonds involving electronegative atoms (A = C, N, O, F, P, S, and Cl) the bond energies achieve a maximum around Z(X) = 5. The same behavior is observed when BDEs are plotted against the electronegativity chi(X) of the atom X. Thus, for A-X bonds (A = Li, Be, B, Na, Mg, Al, Si), the BDEs for a fixed A increases, grosso modo, as the electronegativity differences between X and A increase, with some exceptions, which reflect the differences in the relaxation energies of the radicals produced upon the bond cleavage. A similar trend, albeit less pronounced, is found for single A-X bonds, where A = C, N, O, F, P, S, and Cl. However, there is an additional feature embodied in the enhancement of the strength of the A-boron bonds due to the ability of boron to act as a strong electron acceptor. The trends in bond lengths and charge densities at the bond critical points are in line with the aforementioned behavior.     

Density Functional Calculations of C-NO2 Bond Dissociation Energies for Nitroalkanes Molecules

Bond dissociation energies for the removal of nitrogen dioxide group in some nitroalkane energetic materials have been calculated by using the three hybrid density functional theory （DFT） methods B3LYP, B3PW91 and B3P86 with 6-31g^＊＊ and 6-311g^＊＊ basis sets. The computed BDEs have been compared with the available experimental results. It is found that the B3P86 method with 6-31g^＊＊ and 6-311g^＊＊ basis sets can obtain satisfactory bond dissociation energies （BDEs）, which are in extraordinary agreement with the experimental data. Considering the smaller mean absolute deviation and maximum difference, the reliable B3P86/6-311g^＊＊ method was recommended to compute the BDEs for the removal of nitrogen dioxide group in the nitroalkane energetic materials. Using the method, the BDEs of 8 other nitroalkane energetic materials have been calculated and the maximum difference from experimental value is 1.76 kcal.mo1^-1 （for the BDE of tC4Hg-NOz）, which further proves the reliability of B3P86/6-311g^＊＊ method. In addition, it is noted that the BDEs of C-NO2 bond change slightly for main chain nitroalkane compounds with the maximum difference of only 3.43 kcal mo1^-1.     

DFT study of the C-Cl bond dissociation enthalpies and electronic structure of substituted chlorobenzene compounds

Quantum chemical calculations were used to estimate the bond dissociation energies (BDEs) for 13 substituted chlorobenzene compounds. These compounds were studied by employing the hybrid density functional theory methods (B3LYP, B3PW1, B3P86) with 6-31G** and 6-311G** basis sets. It was demonstrated that B3P86/6-311G** method is the best method for computing the reliable BDEs for substituted chlorobenzene compounds which contain the C-Cl bond. It was found that the C-Cl BDE depends strongly on a computational method and basis set used. Substitution effect on the C-Cl BDE of substituted chlorobenzene compounds is further discussed. It is shown that the effects of substitution on the C-Cl BDE of substituted chlorobenzene compounds are very insignificant. Frontier orbital energy gap of studied compounds was also investigated. From the data on frontier orbital energies gap, we estimated the relative thermal stability of substituted chlorobenzene compounds.     

The C-H and alpha(C-X) bond dissociation enthalpies of toluene, C6H5-CH2X (X = F, Cl), and their substituted derivatives: a DFT study

The homolytic C-H bond dissociation enthalpies (BDEs) of toluene and its para- and meta-substituted derivatives have been estimated by using the (RO)B3LYP/6-311++G(2df,2p)//(U)B3LYP/6-311G(d,p) procedure. The performance of two other hybrid functionals of DFT, namely, B3PWP91 and O3LYP, has also been evaluated using the same basis sets and molecules. Our computed results are compared with the available experimental values and are found to be in good agreement. The (RO)B3LYP and (RO)O3LYP procedures are found to produce reliable BDEs for the C-H bonds in toluene and the C-X (X = F, Cl) bond in alpha-substituted toluene (C6H5-CH2X) and their substituted derivatives. The substituent effect on the BDE values has been analyzed in terms of the ground-state effect and the radical effect. The effect of polarization of the C-H bond on the substituent effect is also analyzed. The BDE(C-H) and BDE(C-X) values for alpha-substituted (X = F and Cl) toluenes with a set of para substituents are presented for the first time.     

Density Functional Calculations of C–NO_2 Bond Dissociation Energies for Nitroalkanes Molecules

Bond dissociation energies for the removal of nitrogen dioxide group in some nit- roalkane energetic materials have been calculated by using the three hybrid density functional theory (DFT) methods B3LYP, B3PW91 and B3P86 with 6-31g** and 6-311g** basis sets. The computed BDEs have been compared with the available experimental results. It is found that the B3P86 method with 6-31g** and 6-311g** basis sets can obtain satisfactory bond dissociation energies (BDEs), which are in extraordinary agreement with the experimental data. Considering the smaller mean absolute deviation and maximum difference, the reliable B3P86/6-311g** method was recommended to compute the BDEs for the removal of nitrogen dioxide group in the nitroalkane energetic materials. Using the method, the BDEs of 8 other nitroalkane energetic materials have been calculated and the maximum difference from experimental value is 1.76 kcal·mol-1 (for the BDE of tC4H9–NO2), which further proves the reliability of B3P86/6-311g** method. In addition, it is noted that the BDEs of C–NO2 bond change slightly for main chain nitroalkane compounds with the maximum difference of only 3.43 kcal mol-1.     

环境污染物中碳氯键离解能的理论研究

利用六种密度泛函理论方法（B3LYP, B3P86, MPW1K, TPSS1KCIS, X3LYP, BMK）对碳氯键离解能进行理论计算,结果发现几种新发展的密度泛函(DFT)方法用于碳氯键离解能的计算比传统的B3LYP有较大的改善,其中对能量估算相对准确的B3P86方法对碳氯键离解能的计算精度最高,对17个分子中碳氯键离解能计算的平均绝对偏差为6.58 kJ/mol。最后运用B3P86方法对一系列环境危害较大,但可通过光化学降解和生物降解的氯代有机物的碳氯键离解能值进行预测,并讨论了影响碳氯键离解能的结构性质关系。     

N—O键解离焓的密度泛函理论研究

在高精度计算方法G3和G3B3的基础上,比较了密度泛函理论(DFT)十几种方法对N—O键解离焓(BDE)相对于实验值的计算精度,发现用B3P86方法计算15种化合物N—O键的BDE,均方根误差最小,仅为6.36kJ·mol-1,计算值与实验值的线性相关系数为0.991.在此基础上,用该方法分别计算了非芳香化合物及芳香化合物的N—O键BDE.通过自然键轨道分析,发现部分N—O键的BDE与N—O键的键长、原子电荷密度及键级之间存在定量关系.此外,在B3P86方法的基础上预测了几种典型的杂环芳香化合物N—O键BDE值.     

Sodium Cation Affinities of Commonly Used MALDI Matrices Determined by Guided Ion Beam Tandem Mass Spectrometry

The sodium cation affinities of six commonly used MALDI matrices are determined here using guided ion beam tandem mass spectrometry techniques. The collision-induced dissociation behavior of six sodium cationized MALDI matrices, Na⁺(MALDI), with Xe is studied as a function of kinetic energy. The MALDI matrices examined here include: nicotinic acid, quinoline, 3-aminoquinoline, 4-nitroaniline, picolinic acid, and 3-hydroxypicolinic acid. In all cases, the primary dissociation pathway corresponds to endothermic loss of the intact MALDI matrix. The cross section thresholds are interpreted to yield zero and 298 K Na⁺−MALDI bond dissociation energies (BDEs), or sodium cation affinities, after accounting for the effects of multiple ion-neutral collisions, the kinetic and internal energy distributions of the reactants, and dissociation lifetimes. Density functional theory calculations at the B3LYP/6-311+G(2d,2p)//B3LYP/6-31G* and MP2(full)/6-311+G(2d,2p)//B3LYP/6-31G* levels of theory are used to characterized the structures and energetics for these systems. The calculated BDEs exhibit very good agreement with the measured values for most systems. The experimental and theoretical Na⁺−MALDI BDEs determined here are compared with those previously measured by cation transfer equilibrium methods.     

山奈酚的电喷雾质谱裂解途径

采用电喷雾质谱技术对山奈酚的质谱过程进行了表征,并用量子化学方法对山奈酚的质谱裂解途径进行了理论研究。 依据密度泛函理论,在B3LYP/6-31G(d)水平,对山奈酚的各质谱碎片离子进行了几何结构优化,确定了各碎片的稳定结构;然后,在ROB3LYP/ 6-311++G(2d,2p)水平计算了形成各碎片离子所需的键断裂能,进而推导出了山奈酚的质谱碎裂途径。 计算结果显示,山奈酚主要通过C环碎裂而发生裂解,出现碎片m/z 284.7、256.7、228.7、210.7、184.8、168.7和150.7,其中m/z 210.7的碎片离子键断裂能最小,m/z 150.7的碎片离子键断裂能最大,说明前者很容易由母离子碎片形成,后者较难由母离子碎片形成。     

Sodium cation affinities of MALDI matrices determined by guided ion beam tandem mass spectrometry: application to benzoic acid derivatives

Threshold collision-induced dissociation of Na(+)(xBA) complexes with Xe is studied using guided ion beam mass spectrometry. The xBA ligands studied include benzoic acid and all of the mono- and dihydroxy-substituted benzoic acids: 2-, 3-, and 4-hydroxybenzoic acid and 2,3-, 2,4-, 2,5-, 2,6-, 3,4-, and 3,5-dihydroxybenzoic acid. In all cases, the primary product corresponds to endothermic loss of the intact xBA ligand. The cross section thresholds are interpreted to yield 0 and 298 K bond dissociation energies (BDEs) for Na(+)-xBA after accounting for the effects of multiple ion-neutral collisions, internal and kinetic energy distributions of the reactants, and dissociation lifetimes. Density functional theory calculations at the B3LYP/6-31G* level of theory are used to determine the structures of these complexes and provide the molecular constants necessary for the thermodynamic analysis of the experimental data. Theoretical BDEs are determined at the B3LYP/6-311+G(2d,2p) and MP2(full)/6-311+G(2d,2p) levels using the B3LYP/6-31G* optimized geometries. The trends in the measured BDEs suggest two very different binding modes for the Na(+)(xBA) complexes, while theory finds four. In general, the most stable binding conformation involves the formation of a six-membered chelation ring via interaction with the carbonyl and 2-hydroxyl oxygen atoms. The ground state geometries of the Na(+)(xBA) complexes in which the ligand does not possess a 2-hydroxyl group generally involve binding of Na(+) to either the carbonyl oxygen atom or to both oxygen atoms of the carboxylic acid group. These binding modes tend to be competitive because the enhancement in binding associated with the chelation interactions in the latter is mediated by steric repulsion between the hydroxyl and ortho hydrogen atoms. When possible, hydrogen bonding interactions with the ring hydroxyl group(s) enhance the stability of these complexes. The agreement between the theoretical and experimental BDEs is quite good for B3LYP and somewhat less satisfactory for MP2(full).     

Sulfur and selenium ylide bond enthalpies

The bond dissociation enthalpies (BDEs) of sulfur and selenium ylides have been estimated by applying MP2/6-311++G(3df,2p)//MP2/6-31G(d,p), G3, and other computational methods. Computed sulfoxide bond enthalpies were compared to experimental results to ensure the reliability of the computational methods before extending to related compounds. The examined ylides include the following: sulfoxides, sulfilimines, S,C-sulfonium ylides, and selenoxides. Selenoxides have BDEs about 10 kcal/mol smaller than the corresponding sulfoxides. N-H sulfilimines and CH2-S,C-sulfonium ylides have low BDEs, unless the sulfilimine or S,C-sulfonium ylide is stabilized by an electronegative substituent on N or C, respectively. Incorporation of the S or Se into a thiophene or selenophene-type ring lowers the BDE for the ylide.     

PCM Study of Bond Dissociation Energies of the S–NO Bond: A DFT Study

Quantum chemical calculations are used to estimate the equilibrium S–NO bond dissociation energies (BDEs) for seven S-nitro-N-acetyl-d,l-penicillamine dipeptides (SNAP-two peptides) in acetonitrile solution. These compounds were studied by employing the hybrid density functional theory (B3LYP, B3P86 and B3PW91) methods together with the 6-31G** basis set. The obtained results are compared with available experimental results. It is demonstrated that B3PW91 method is the best method to compute the bond dissociation energies of SNAP-two peptides. The substituent and solvent effects of the S–NO BDEs are further analyzed. The results show that S–NO BDE increases with the increment of isoelectric points of substituted groups. In addition, the S–NO BDE decreases due to the inclusion of solvent effects. Furthermore, SNAP-two peptides and the other NO-donors are compared.