Energetics of C-F, C-Cl, C-Br, and C-I bonds in 2-haloethanols. enthalpies of formation of XCH(2)CH(2)OH (X = F, Cl, Br, I) compounds and of the 2-hydroxyethyl radical

The energetics of the C-F, C-Cl, C-Br, and C-I bonds in 2-haloethanols was investigated by using a combination of experimental and theoretical methods. The standard molar enthalpies of formation of 2-chloro-, 2-bromo-, and 2-iodoethanol, at 298.15 K, were determined as Delta(f)H(degree)m(CH2CH2OH, l) = -315.5 +/- 0.7 kJ.mol-1, Delta(f)H(degree)mBrCH2CH2OH, l) = -275.8 +/- 0.6 kJ.mol-1, Delta(f)H(degree)m(ICH2CH2OH, l) = -207.3 +/- 0.7 kJ.mol-1, by rotating-bomb combustion calorimetry. The corresponding standard molar enthalpies of vaporization, Delta(vap)H(degree)m(ClCH2CH2OH) = 48.32 +/- 0.37 kJ.mol-1, Delta(vap)H(degree)m(BrCH2CH2OH) = 54.08 +/- 0.40 kJ.mol-1, and Delta(vap)H(degree)m(ICH2CH2OH) = 57.03 +/- 0.20 kJ.mol-1 were also obtained by Calvet-drop microcalorimetry. The condensed phase and vaporization enthalpy data lead to Delta(f)H(degree)m(ClCH2CH2OH, g) = -267.2 +/- 0.8 kJ.mol-1, Delta(f)H(degree)m(BrCH2CH2OH, g) = -221.7 +/- 0.7 kJ.mol-1, and Delta(f)H(degree)m(ICH2CH2OH, g) = -150.3 +/- 0.7 kJ.mol-1. These values, together with the enthalpy of selected isodesmic and isogyric gas-phase reactions predicted by density functional theory (B3LYP/cc-pVTZ) and CBS-QB3 calculations were used to derive the enthalpies of formation of gaseous 2-fluoroethanol, Delta(f)H(degree)m(FCH2CH2OH, g) = -423.6 +/- 5.0 kJ.mol-1, and of the 2-hydroxyethyl radical, Delta(f)H(degree)m(CH2CH2OH, g) = -28.7 +/- 8.0 kJ.mol-1. The obtained thermochemical data led to the following carbon-halogen bond dissociation enthalpies: DHo(X-CH2CH2OH) = 474.4 +/- 9.4 kJ.mol-1 (X = F), 359.9 +/- 8.0 kJ.mol-1 (X = Cl), 305.0 +/- 8.0 kJ.mol-1 (X = Br), 228.7 +/- 8.1 kJ.mol-1 (X = I). These values were compared with the corresponding C-X bond dissociation enthalpies in XCH2COOH, XCH3, XC2H5, XCH=CH2, and XC6H5. In view of this comparison the computational methods mentioned above were also used to obtain Delta(f)H(degree)m-594.0 +/- 5.0 kJ.mol-1 from which DHo(F-CH2COOH) = 435.4 +/- 5.4 kJ.mol-1. The order DHo(C-F) > DHo(C-Cl) > DHo(C-Br) > DHo(C-I) is observed for the haloalcohols and all other RX compounds. It is finally concluded that the major qualitative trends exhibited by the C-X bond dissociation enthalpies for the series of compounds studied in this work can be predicted by Pauling's electrostatic-covalent model.     

Hyperconjugation effect in substituted methyl boranes: an orbital deletion procedure analysis

The hyperconjugation effect in the substituted methyl boranes, XCH(2)BH(2) (X = H, CH(3), NH(2), PH(2), OH, SH, F, Cl, Br), has been quantitatively evaluated by using the orbital deletion procedure (ODP), where the p(pi) orbital on boron is deactivated. Except for the case of X = NH(2), which forms a three-membered ring, the magnitude of the hyperconjugative stabilization in all other substituted methylborane ranges from 6.8 to 3.4 kcal/mol. Significant structural changes are observed, particularly the shortening of the central B-C bond distance and the reducing of the corresponding XCB and HCB bond angles. In general, the strength of the hyperconjugative interaction between the occupied sigma(C-X) bond and the vacant p(pi) orbital on boron is correlated to the electronegativity of X, and the competition between the donation ability of the sigma(C-X) and the sigma(C-H) bonds determines the preference of the staggered or eclipsed structure as the energy minimum state. When the donation abilities of the C-X and C-H bonds are comparable, other factors such as electron correlation and steric effect may play elaborate roles in the geometrical propensity of the most stable structures.     

A concerted three-body formation X+Y+C2H4 in the photodissociation of CH2XCH2Y (X,Y=Br,Cl) at 193 nm

The photodissociation of CH2XCH2Y (X,Y=Br,Cl) through absorption of 193 nm photons was investigated using product translational spectroscopy. No stable CH2BrCH2 or CH2ClCH2 was detected. The recorded time-of-flight spectra indicate that these internally excited radicals dissociated into Y+C2H4 in a concerted reaction with the first C-X bond rupture. Product anisotropy implies that the overall reaction time for three-body formation is in a fraction of rotational period. According to an asynchronous concerted reaction model, the measured spectra were simulated with product translational energy distributions coupled by asymmetric angular distributions. For the mixed halide, CH2BrCH2Cl, triple products Br+Cl+C2H4 can be originated from the cleavage of either the C-Br bond or the C-Cl bond. The results are discussed and where appropriate, comparisons with previous investigations of the related molecules are included.     

Electron deficient carbon-titanium triple bonds: formation of triplet XC/TiX3 methylidyne complexes

Laser-ablated titanium atoms react with CX4 (X = F and Cl) to produce triplet state XC/TiX3 complexes trapped in an argon matrix. Products are identified by their infrared spectra and comparison to theoretically predicted vibrations. Density functional theory calculations converge to C(3v) symmetry structures for these lowest-energy products. The two unpaired electrons in the carbon 2p orbitals are shared with empty titanium d orbitals leading to degenerate singly occupied pi molecular orbitals and an electron-deficient triple bond between the carbon and titanium centers, on the basis of DFT bonding analysis and spin density calculations. The carbon-titanium distances are near typical C=Ti double bond lengths, and the C-X bonds in the resulting products are shorter than in the CX4 precursors. It appears that X lone-pair conjugation contributes to the C-X bond strength and somewhat to the C-Ti bond, and Cl does better in this regard than F.     

多硝基甲烷Mannich碱稳定性的理论研究

用MNDO SCF-MO方法全优化了系列多硝基甲烷Mannich碱的几何构型, 计算了它们的电子结构。在胺、醛组分相同时, 标题物>N-CH2-Y的特征键CH2-Y的键级随酸组分亲核性的增强而增大。该CH2-Y键级较小是造成Mannich碱在溶液中不稳定的重要原因。C-NO2键的键级在分子中较小, 可能是热或光解等受激分解的引发键, 从电子结构特征上阐明了以硝仿为酸组分的Mannich碱稳定性较差的原因。     

Infrared and DFT investigations of the XC[triple bond]ReX3 and HC[triple bond]ReX3 complexes: Jahn-Teller distortion and the methylidyne C-X(H) stretching absorptions

The XC[triple bond]ReX3 complexes (X = F, Cl) are produced by CX(4) reaction with laser-ablated Re atoms, following oxidative C-X insertion and alpha-halogen migration in favor of the carbon-metal triple bond and are identified through the observation of characteristic absorptions in the argon matrix infrared spectra and comparison with vibrational frequencies calculated by density functional theory. The methylidyne C-F and C-Cl stretching absorptions are observed near 1584 and 1328 cm-1, and the C-H stretching modes for HC[triple bond]ReX3 at 3104 and 3097 cm(-1), respectively, which are substantially higher than the precursor stretching modes and in agreement with the general trend that higher s-orbital character in carbon hybridization leads to a higher stretching frequency. The Jahn-Teller effect in the doublet-state XC[triple bond]ReX3 and HC[triple bond]ReX3 complexes gives rise to distorted structures with Cs symmetry and two equivalent longer Re-X bonds and one slightly shorter Re-X bond.     

C6H5—H…X分子间氢键的理论计算

利用密度泛函理论B3LYP方法, 在6-311+G(3df,2p)水平上对C6H5—H…X型分子间氢键进行了几何构型优化、氢键相互作用能、电子密度分布等计算. 其中C6H6为质子供体, HCOH、H2O、NH3、CH2NH和HCN为质子受体. 从电荷布居分析、自然键轨道等角度详细地讨论了C6H5—H…X 体系中, 共轭π键、O和N的不同键型结构对氢键形成的影响以及孤电子对与C—H 反键轨道之间的相互作用(n→σ*)等.     

Steric nature of the bite angle. A closer and a broader look

The bite angle (ligand-metal-ligand angle) is known to greatly influence the activity of catalytically active transition-metal complexes towards bond activation. Here, we have computationally explored how and why the bite angle has such effects in a wide range of prototypical C-X bonds and palladium complexes, using relativistic density functional theory at ZORA-BLYP/TZ2P. Our model reactions cover the substrates H(3)C-X (with X = H, CH(3), Cl) and, among others, the model catalysts, Pd[PH(2)(CH(2))(n)PH(2)] (with n = 2-6) and Pd[PR(2)(CH(2))(n)PR(2)] (n = 2-4 and R = Me, Ph, t-Bu, Cl), Pd(PH(3))X(-) (X = Cl, Br, I), as well as palladium complexes of chelating and non-chelating N-heterocyclic carbenes. The purpose is to elaborate on an earlier finding that bite-angle effects have a predominantly (although not exclusively) steric nature: a smaller bite angle makes more room for coordinating a substrate by bending away the ligands. Indeed, the present results further consolidate this steric picture by revealing its occurrence in this broader range of model reactions and by identifying and quantifying the exact working mechanism through activation strain analyses.     

Theoretical study on the bromomethane-water 1:2 complexes

Bromomethane-water 1:2 complexes have been theoretically studied to reveal the role of hydrogen bond and halogen bond in the formation of different aggregations. Four stable structures exist on the potential energy surface of the CH3Br(H2O)2 complex. The bromine atom acts mainly as proton acceptor in the four studied structures. It is also capable of participating in the formation of the halogen bond. The properties and characteristics of the hydrogen bond and the halogen bond are investigated employing several different quantum chemical analysis methods. Cooperative effects for the pure hydrogen bonds or the mixed hydrogen bonds with halogen bonds and the possibility of describing cooperative effects in terms of the topological analysis of the electronic density or the charge-transfer stabilization energy are discussed in detail. An atoms-in-molecules study of the hydrogen bond or the halogen bond in the bromomethane-water 1:2 complexes suggests that the electronic density topology of the hydrogen bond or the halogen bond is insensitive to the cooperative effect. The charge-transfer stabilization energy is proportional to the cooperative effect, which indicates the donor-acceptor electron density transfer to be mainly responsible for the trimer nonadditive effect.     

Factors influencing the reductive cleavage of C-x multiple bonds in their reactions with meal-meal multiple bonds (X = C, N, O, S)

Though metal-metal multiple bonds of the transition elements are redox active, their reactivity towards C-X multiple bonds (X = C, N, O, S) vary greatly depending principally on: 1. The coordination geometry of the metal. 2. The oxidation state of the metal and the electronic configuration of the M-M bond. 3. The nature of the attendant ligands. Specific examples of C-X multiple bond activation at dimolybdenum and ditungsten centers are presented that illustrate the importance of these factors. Evidence is presented to support the view that reductive cleavage of a C-X multiple bond can be considered to be equivalent to an intramolecular redox reaction within a [M2CX] "cluster complex," for which the frontier orbital energies of the C-X and M-M multiple bonds are of paramount importance. Some applications of these C-X reductive cleavage reactions toward organic synthesis are described.     

Computational investigation of the CH3XC=S...S (X = H, HO, HS, PH2, CH3) bonding type

The CH₃XC=S...S (X = H, HO, HS, PH₂, CH₃) bonding types are investigated using the second order Møller-Plesset perturbation approximation with the cc-pVDZ basis set. Electrostatic density potential maps of CH₃XC=S (X = H, HO, HS, PH₂, CH₃) are generated at the MP2/cc-pVDZ level of theory. The interaction energy and topological property are theoretically encompassed for the five complexes. Electrostatic density potential maps of five monomers are generated for the determination of attractive interaction sites. There are different misshaped electron clouds. The red-shifting character is obtained for the CH₃XC=S...S (X = H, HO, HS, PH₂) interaction. For all complexes the S...S bonds are typical closedshell interactions, and the topological properties of the S...S bond fall short of three criteria for the existence of the hydrogen bond. Theoretical values are in very good agreement with the experimental results.     

Ketalization of phosphonium ions by 1,4-dioxane: selective detection of the chemical warfare agent simulant DMMP in mixtures using ion/molecule reactions

Phosphonium ions CH(3)P(O)OCH(3)(+) (93 Th) and CH(3)OP(O)OCH(3)(+) (109 Th) react with 1,4-dioxane to form unique cyclic ketalization products, 1,3,2-dioxaphospholanium ions. By contrast, a variety of other types of ions having multiple bonds, including the acylium ions CH(3)CO(+) (43 Th), CH(3)OCO(+) (59 Th), (CH(3))(2)NCO(+) (72 Th), and PhCO(+) (105 Th), the iminium ion H(2)C[double bond]NHC(2)H(5)(+) (58 Th) and the carbosulfonium ion H(2)C[double bond]SC(2)H(5)(+) (75 Th) do not react with 1,4-dioxane under the same conditions. The characteristic ketalization reaction can also be observed when CH(3)P(OH)(OCH(3))(2)(+), viz. protonated dimethyl methylphosphonate (DMMP), collides with 1,4-dioxane, as a result of fragmentation to yield the reactive phosphonium ion CH(3)P(O)OCH(3)(+) (93 Th). This novel ion/molecule reaction is highly selective to phosphonium ions and can be applied to identify DMMP selectively in the presence of ketone, ester, and amide compounds using a neutral gain MS/MS scan. This method of DMMP analysis can be applied to aqueous solutions using electrospray ionization; it shows a detection limit in the low ppb range and a linear response over the range 10 to 500 ppb.     

含杂三元环化合物键强变化规律

用键强参数f~i, ~A~-B, f~A~-~B可定量地表达含杂三元环化合物CH2-CH2-X(X=Be, BH, CH2, NH, O及S)中成键情况及键强变化规律: 1. 无论在三元环中, 还是在C-X-C中, X=Be→O时, f~c~-~x随X中重原子的核电荷数Z递增而增大。其原因: X中重原子实对于价电子的有效核电荷数随其核电荷数Z的增大而增大。C-X的"键电荷"也随X的Z值递增。2. 当X=Be→O, 在形成CH2-CH2-X时, 较强键被削弱, 较弱键则增强, 致环内各-键键强和键长都有平均化的趋势。其原因为: 三元环分子中形成σ共轭的三中心键, C-X与C-C键共享"键电荷"。这种"σ-共轭效应"与π-共轭效应有相似处。3.由于硫的价电子云平均半径较大, 可向c-C提供更多共享电荷, 故在含S三元环化合物CH2-CH2-S中C-S键受到更大程度的削弱, C-C则更被增强。     

Supramolecular chemistry of halogens: complementary features of inorganic (M-X) and organic (C-X') halogens applied to M-X...X'-C halogen bond formation

Electronic differences between inorganic (M-X) and organic (C-X) halogens in conjunction with the anisotropic charge distribution associated with terminal halogens have been exploited in supramolecular synthesis based upon intermolecular M-X...X'-C halogen bonds. The synthesis and crystal structures of a family of compounds trans-[MCl(2)(NC(5)H(4)X-3)(2)] (M = Pd(II), Pt(II); X = F, Cl, Br, I; NC(5)H(4)X-3 = 3-halopyridine) are reported. With the exception of the fluoropyridine compounds, network structures propagated by M-Cl...X-C halogen bonds are adopted and involve all M-Cl and all C-X groups. M-Cl...X-C interactions show Cl...X separations shorter than van der Waals values, shorter distances being observed for heavier halogens (X). Geometries with near linear Cl...X-C angles (155-172 degrees ) and markedly bent M-Cl...X angles (92-137 degrees ) are consistently observed. DFT calculations on the model dimers {trans-[MCl(2)(NH(3))(NC(5)H(4)X-3)]}(2) show association through M-Cl...X-C (X not equal F) interactions with geometries similar to experimental values. DFT calculations of the electrostatic potential distributions for the compounds trans-[PdCl(2)(NC(5)H(4)X-3)(2)] (X = F, Cl, Br, I) demonstrate the effectiveness of the strategy to activate C-X groups toward halogen bond formation by enhancing their electrophilicity, and explain the absence of M-Cl...F-C interactions. The M-Cl...X-C halogen bonds described here can be viewed unambiguously as nucleophile-electrophile interactions that involve an attractive electrostatic contribution. This contrasts with some types of halogen-halogen interactions previously described and suggests that M-Cl...X-C halogen bonds could provide a valuable new synthon for supramolecular chemists.     

A comparison of C-F and C-H bond activation by zerovalent ni and pt: a density functional study

Density functional theory indicates that oxidative addition of the C-F and C-H bonds in C6F6 and C6H6 at zerovalent nickel and platinum fragments, M(H2PCH2CH2PH2), proceeds via initial exothermic formation of an eta2-coordinated arene complex. Two distinct transition states have been located on the potential energy surface between the eta2-coordinated arene and the oxidative addition product. The first, at relatively low energy, features an eta3-coordinated arene and connects two identical eta2-arene minima, while the second leads to cleavage of the C-X bond. The absence of intermediate C-F or C-H sigma complexes observed in other systems is traced to the ability of the 14-electron metal fragment to accommodate the eta3-coordination mode in the first transition state. Oxidative addition of the C-F bond is exothermic at both nickel and platinum, but the barrier is significantly higher for the heavier element as a result of strong 5dpi-ppi repulsions in the transition state. Similar repulsive interactions lead to a relatively long Pt-F bond with a lower stretching frequency in the oxidative addition product. Activation of the C-H bond is, in contrast, exothermic only for the platinum complex. We conclude that the nickel system is better suited to selective C-F bond activation than its platinum analogue for two reasons: the strong thermodynamic preference for C-F over C-H bond activation and the relatively low kinetic barrier.     

Observation of adducts in the reaction of Cl atoms with XCH2I (X = H, CH3, Cl, Br, I) using cavity ring-down spectroscopy

The reactions of Cl atoms with XCH2I (X = H, CH3, Cl, Br, I) have been studied using cavity ring-down spectroscopy in 25-125 Torr total pressure of N2 diluent at 250 K. Formation of the XCH2I-Cl adduct is the dominant channel in all reactions. The visible absorption spectrum of the XCH2I-Cl adduct was recorded at 405-632 nm. Absorption cross-sections at 435 nm are as follows (in units of 10(-18) cm2 molecule(-1)): 12 for CH3I, 21 for CH3CH2I, 3.7 for CH2ICl, 7.1 for CH2IBr, and 3.7 for CH2I2. Rate constants for the reaction of Cl with CH3I were determined from rise profiles of the CH3I-Cl adduct. k(Cl + CH3I) increases from (0.4 +/- 0.1) x 10(-11) at 25 Torr to (2.0 +/- 0.3) x 10(-11) cm3 molecule(-1) s(-1) at 125 Torr of N2 diluent. There is no discernible reaction of the CH3I-Cl adduct with 5-10 Torr of O2. Evidence for the formation of an adduct following the reaction of Cl atoms with CF3I and CH3Br was sought but not found. Absorption attributable to the formation of the XCH2I-Cl adduct following the reaction of Cl atoms with XCH2I (X = H, CH3, Br, I) was measured as a function of temperature over the range 250-320 K.     

A challenge to chemical intuition: donor-acceptor interactions in H3B-L and H2B+-L (L=CO; EC5H5, E=N-Bi)

The equilibrium geometries and bond energies of the complexes H(3)B-L and H(2)B(+)-L (L=CO; EC(5)H(5): E=N, P, As, Sb, Bi) have been calculated at the BP86/TZ2P level of theory. The nature of the donor-acceptor bonds was investigated by energy decomposition analysis (EDA). The bond strengths of H(3)B-L have the order CO>N>P>As>Sb>Bi. The calculated values are between D(e)=37.1 kcal mol(-1) for H(3)B-CO and D(e)=6.9 kcal mol(-1) for H(3)B-BiC(5)H(5). The bond dissociation energies of the cations H(2)B(+)-CO and H(2)B(+)-EC(5)H(5) are larger than for H(3)B--L, particularly for complexes of the heterobenzene ligands. The calculated values are between D(e)=51.9 kcal mol(-1) for H(2)B(+)-CO and D(e)=122.1 kcal mol(-1) for H(2)B(+)-NC(5)H(5). The trend of the BDE of H(2)B(+)-CO and H(2)B(+)-EC(5)H(5) is N>P>As>Sb>Bi>CO. A surprising result is found for H(2)B(+)-CO, which has a significantly stronger and yet substantially longer bond than H(3)B-CO. The reason for the longer but stronger bond in H(2)B(+)-CO compared with that in H(3)B-CO comes mainly from the change in electrostatic attraction and pi bonding at shorter distances, which increases more in the neutral system than in the cation, and to a lesser extent from the deformation energy of the fragments. The H(2)B(+)<--NC(5)H(5) pi( perpendicular) donation plays an important role for the stronger interactions at shorter distances compared with those in H(3)B-NC(5)H(5). The attractive interaction in H(2)B(+)--CO further increases at bond lengths that are shorter than the equilibrium value, but this is compensated by the energy which is necessary to deform BH(2) (+) from its linear equilibrium geometry to the bent form in the complex. The EDA shows that the contributions of the orbital interactions to the donor-acceptor bonds are always larger than the classical electrostatic contributions, but the latter term plays an important role for the trend in bond strength. The largest contributions to the orbital interactions come from the sigma orbitals. The EDA calculations suggest that heterobenzene ligands may become moderately strong pi donors in complexes with strong Lewis acids, while CO is only a weak pi donor. The much stronger interaction energies in H(2)B(+)-EC(5)H(5) compared with those in H(3)B-EC(5)H(5) are caused by the significantly larger contribution of the pi(perpendicular) orbitals in H(2)B(+)-EC(5)H(5) and by the increase of the binding interactions of the sigma+pi( parallel) orbitals.     

Mechanisms of reductive eliminations in square planar Pd(II) complexes: nature of eliminated bonds and role of trans influence

The trans influence of various phosphine ligands (L) in direct as well as dissociative reductive elimination pathways yielding CH(3)CH(3) from Pd(CH(3))(2)L(2) and CH(3)Cl from Pd(CH(3))(Cl)L(2) has been quantified in terms of isodesmic reaction energy, E(trans), using the MPWB1K level of density functional theory. In the absence of a large steric effect, E(trans) correlated linearly with the activation barrier (E(act)) of both direct and dissociation pathways. The minimum of molecular electrostatic potential (V(min)) at the lone pair region of phosphine ligands has been used to assess their electron donating power. E(trans) increased linearly with an increase in the negative V(min) values. Further, the nature of bonds that are eliminated during reductive elimination have been analyzed in terms of AIM parameters, viz. electron density (ρ(r)), Laplacian of the electron density (?(2)ρ(r)), total electron energy density (H(r)), and ratio of potential and kinetic electron energy densities (k(r)). Interestingly, E(act) correlated inversely with the strength of the eliminated metal-ligand bonds measured in terms of the bond length or the ρ(r). Analysis of H(r) showed that elimination of the C-C/C-Cl bond becomes more facile when the covalent character of the Pd-C/Pd-Cl bond increases. Thus, AIM details clearly showed that the strength of the eliminated bond is not the deciding factor for the reductive elimination but the nature of the bond, covalent or ionic. Further, a unified picture showing the relationship between the nature of the eliminated chemical bond and the tendency of reductive elimination is obtained from the k(r) values: the E(act) of both direct and dissociative mechanisms for the elimination of CH(3)CH(3) and CH(3)Cl decreased linearly when the sum of k(r) at the cleaved bonds showed a more negative character. It means that the potential electron energy density dominates over the kinetic electron energy density when the bonds (Pd-C/Pd-Cl) become more covalent and the eliminated fragments attain more radical character leading to the easy formation of C-C/C-Cl bond.     

气相中开壳型(CH3)2S(O)…HOO红移氢键复合物的结构与性质

在DFT-B3LYP/6-311++G**水平上分别求得(CH3)2S…HOO和(CH3)2O…HOO开壳型氢键复合物势能面上的稳定构型. 频率分析表明, 与单体HOO自由基相比, 复合物中H10-O11键伸缩振动频率发生显著的红移, 红移值分别为424.21和374.22 cm-1. 在MP2/6-311++G**水平计算得到, 含基组重叠误差(BSSE)校正和零点振动能(ZPVE)校正的相互作用能分别为-24.68和-31.01 kJ·mol-1. 自然键轨道(NBO)理论分析表明, 在(CH3)2S…HOO复合物中, 引起H10-O11键变长的因素包括两种电荷转移: (1) LP(S1)1→σ*(H10-O11); (2) LP(S1)2→σ*(H10-O11), 其中LP(S1)2→σ*(H10-O11)占主要作用, 总的结果是使σ*(H10-O11)的自然布居数增加了37.27 me; 在(CH3)2O…HOO中也有相似的电荷转移的超共轭作用. AIM理论分析表明, S1…H10间和O1…H10间都存在键鞍点, ▽2ρ(r)分别为0.06196和0.03745, 说明这种相互作用介于共价键和离子键之间, 偏于静电作用.     

Endgroup-assisted siloxane bond cleavage in the gas phase

Unimolecular dissociation of H(2)N(CH(2))(3)SiOSi(CH(2))(3)NH(3)(+) generates SiC(5)H(16)NO(+) and SiC(5)H(14)N(+). The formation of SiC(5)H(16)NO(+) involves dissociation of a Si[bond]O bond and formation of an O[bond]H bond through rearrangement. The fragmentation mechanism was investigated utilizing ab initio calculations and Fourier transform ion cyclotron resonance (FTICR) mass spectrometry in combination with hydrogen/deuterium (H/D) exchange reactions. Sustained off-resonance irradiation collision-induced dissociation (SORI-CID) studies of the fully deuterated ion D(2)N(CH(2))(3)SiOSi(CH(2))(3)ND(3)(+) provided convincing evidence for a backbiting mechanism which involves hydrogen transfer from the terminal amine group to the oxygen to form a silanol-containing species. Theoretical calculations indicated decomposition of H(2)N(CH(2))(3)SiOSi(CH(2))(3)NH(3)(+) through a backbiting mechanism is the lowest energy decomposition channel, compared with other alternative routes. Two mechanisms were proposed for the fragmentation process which leads to the siloxane bond cleavage and the SORI-CID results of partially deuterated precursor ions suggest both mechanisms should be operative. Rearrangement to yield a silanol-containing product ion requires end groups possessing a labile hydrogen atom. Decomposition of disiloxane ions with end groups lacking labile hydrogen atoms yielded product ions from direct bond cleavages.