Free Energy Relationships for Reactions of Substituted Benzhydrylium Ions: From Enthalpy over Entropy to Diffusion Control

Second-order rate constants k(2) for the reactions of various donor- and acceptor-substituted benzhydrylium ions Ar(2)CH(+) with π-nucleophiles in CH(2)Cl(2) were determined by laser flash irradiation of benzhydryl triarylphosphonium salts Ar(2)CH-PAr(3)(+)X(-) in the presence of a large excess of the nucleophiles. This method allowed us to investigate fast reactions up to the diffusional limit including reactions of highly reactive benzhydrylium ions with m-fluoro and p-(trifluoromethyl) substituents. The rate constants determined in this work and relevant literature data were jointly subjected to a correlation analysis to derive the electrophilicity parameters E for acceptor-substituted benzhydrylium ions, as defined by the linear free energy relationship log?k(2)(20 °C) = s(N)(N + E). The new correlation analysis also leads to the N and s(N) parameters of 18 π-nucleophiles, which have only vaguely been characterized previously. The correlations of log?k(2) versus E are linear well beyond the range where the activation enthalpies ΔH(?) of the reactions are extrapolated to reach the value of ΔH(?) = 0, showing that the change from enthalpy control to entropy control does not cause a bend in the linear free energy relationship, a novel manifestation of the compensation effect. A flattening of the correlation lines only occurs for k(2) > 10(8) M(-1) s(-1) when the diffusion limit is approached.     

Tuning the Photophysical Properties of Symmetric Squarylium Dyes: Investigation on the Halogen Modulation Effects

A series of symmetric squarylium dyes (SQDPA-X) with different halogen (X=F, Cl, Br, I) substituents have been developed. The photophysical properties could be facilely tuned by the halogen modulation effects. The strategy of incorporating different halogen substitutions into AIE active luminogens enables a facile approach for exploring new intriguing organic fluorescent dyes.     

Solvolysis of 2‐chloro‐2(3,4‐disubstituted) phenylpropanes: Validity of Hammett–Brown σ+ constants in assessing additive effects of substituents

The objective of this study is to test the suitability of the extended Hammett–Brown equation, log (k_XX/k_HH) = ρ⁺Σσ⁺, in depicting satisfactorily additive effects of electronegative atom‐bearing substituents, which are known to possess diverse and multicomponent influences on the side chain reactions of polysubstituted benzenes. The equation has been used to correlate, for the first time, the additive effect of substituents in the specific rates of solvolysis of 2‐chloro‐2‐phenylpropanes ( 3b–3f ) having 3‐F,4‐Me, 3‐Br,4‐Me, 3‐I,4‐Me, 3‐Me,4‐Me, or 3‐MeO,4‐Me substituents. The rates were determined titrimetrically at 288, 298, and 308 K using 90% aqueous acetone as solvent. Measured additive effects of these substituents on the solvolysis rate and activation parameters of the parent cumyl chloride (2‐chloro‐2‐phenylpropane) are found to be well correlated using the equation given above. Plots of log (k_XX/k_HH) of 3b–3f together with mainly di‐, but also tri‐ and mono‐substituted cumyl chlorides from previous studies against Σσ⁺ give a linear correlation coefficient of 0.990 as a measure of the validity of the equation to depict such systems. The halogen substituents' extent of conformity with additivity reflected in their relative (k_obsd/k_calcd) rate ratios is found to correlate with the steric size of substituents. Plots of rate ratios against Taft's steric factor of each halogen give a linear correlation coefficient of 0.994 for the 3‐halo substituents. The 3,4‐dimethyl substituents' relative rate ratio of 1.03 shows excellent additivity, whereas the 3‐methoxy‐4‐methyl ratio of 1.43 shows the methoxy group to be far less deactivating than predicted. Similar trends were found for the free energy of activation (ΔG? – ΔG₀?) differences, which correlated linearly with a coefficient of 0.983 with Taft's steric factor of halogen atoms. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 514–523, 2012     

Theoretical study on the interactions of halogen‐bonds and pnicogen‐bonds in phosphine derivatives with Br2, BrCl,and BrF

The MP2 ab initio quantum chemistry methods were utilized to study the halogen‐bond and pnicogen‐bond system formed between PH₂X (X = Br, CH₃, OH, CN, NO₂, CF₃) and BrY (Y = Br, Cl, F). Calculated results show that all substituent can form halogen‐bond complexes while part substituent can form pnicogen‐bond complexes. Traditional, chlorine‐shared and ion‐pair halogen‐bonds complexes have been found with the different substituent X and Y. The halogen‐bonds are stronger than the related pnicogen‐bonds. For halogen‐bonds, strongly electronegative substituents which are connected to the Lewis acid can strengthen the bonds and significantly influenced the structures and properties of the compounds. In contrast, the substituents which connected to the Lewis bases can produce opposite effects. The interaction energies of halogen‐bonds are 2.56 to 32.06 kcal·mol^?1; The strongest halogen‐bond was found in the complex of PH₂OH???BrF. The interaction energies of pnicogen‐bonds are in the range 1.20 to 2.28 kcal·mol^?1; the strongest pnicogen‐bond was found in PH₂Br???Br₂ complex. The charge transfer of lp(P) ? σ*(Br? Y), lp(F) ? σ*(Br? P), and lp(Br) ? σ*(X? P) play important roles in the formation of the halogen‐bonds and pnicogen‐bonds, which lead to polarization of the monomers. The polarization caused by the halogen‐bond is more obvious than that by the pnicogen‐bond, resulting in that some halogen‐bonds having little covalent character. The symmetry adapted perturbation theory (SAPT) energy decomposition analysis showes that the halogen‐bond and pnicogen‐bond interactions are predominantly electrostatic and dispersion, respectively.     

Influence of halogen substituents on the catalytic oxidation of 2,4,6-halogenated phenols by Fe(III)-tetrakis(p-hydroxyphenyl) porphyrins and potassium monopersulfate

The influence of halogen substituents on the catalytic oxidation of 2,4,6-trihalogenated phenols (TrXPs) by iron(III)-porphyrin/KHSO? catalytic systems was investigated. Iron(III)-5,10,15,20-tetrakis(p-hydroxyphenyl)porphyrin (FeTHP) and its supported variants were employed, where the supported catalysts were synthesized by introducing FeTHP into hydroquinone-derived humic acids via formaldehyde poly-condensation. F (TrFP), Cl (TrCP), Br (TrBP) and I (TrIP) were examined as halogen substituents for TrXPs. Although the supported catalysts significantly enhanced the degradation and dehalogenation of TrFP and TrCP, the oxidation of TrBP and TrIP was not enhanced, compared to the FeTHP catalytic system. These results indicate that the degree of oxidation of TrXPs is strongly dependent on the types of halogen substituent. The order of dehalogenation levels for halogen substituents in TrXPs was F > Cl > Br > I, consistent with their order of electronegativity. The electronegativity of a halogen substituent affects the nucleophilicity of the carbon to which it is attached. The levels of oxidation products in the reaction mixtures were analyzed by GC/MS after extraction with n-hexane. The most abundant dimer product from TrFP via 2,6-difluoroquinone is consistent with a scenario where TrXP, with a more electronegative halogen substituent, is readily oxidized, while less electronegative halogen substituents are oxidized less readily by iron(III)-porphyrin/KHSO? catalytic systems.     

Evidence for Interfacial Halogen Bonding

A homologous series of donor–π–acceptor dyes was synthesized, differing only in the identity of the halogen substituents about the triphenylamine (TPA; donor) portion of each molecule. Each Dye‐X (X=F, Cl, Br, and I) was immobilized on a TiO₂ surface to investigate how the halogen substituents affect the reaction between the light‐induced charge‐separated state, TiO₂(e^?)/ Dye‐X⁺ , with iodide in solution. Transient absorption spectroscopy showed progressively faster reactivity towards nucleophilic iodide with more polarizable halogen substituents: Dye‐F < Dye‐Cl < Dye‐Br < Dye‐I . Given that all other structural and electronic properties for the series are held at parity, with the exception of an increasingly larger electropositive σ‐hole on the heavier halogens, the differences in dye regeneration kinetics for Dye‐Cl , Dye‐Br , and Dye‐I are ascribed to the extent of halogen bonding with the nucleophilic solution species.     

Structures and electronic properties of Al7x0,- and Al13 X1,2,12- clusters with X=F, Cl, and Br

The structures, binding energies, and electronic properties for Al7X, Al7X-, Al13X-, Al13X2-, and Al13X12- (X = F, Cl, Br) were studied at the B3LYP/6-311+G(2d,p) level. Among the systems studied, Al7 and Al13 clusters in Al7X and Al13X- reveal alkali-like and halogen-like superatom characters, respectively. Al7 can bind with one halogen atom to form a salt-like compound as Al7+delta-X-delta. Al13- can combine with one halogen atom to form a diatomic halogen anion Al13X-. However, when adding more halogens, the superatom structure would be destroyed, resulting in low-symmetry compounds with the center Al atom moving toward the cluster surface. The structures of Al13X1,2,12- (X = F, Cl, Br) are similar to those of X = I; however, their binding energies and electron structures are much different. In addition, the analyses of the calculated NBO charges show that Cl and Br have similar properties, but much different from F, when interacting with the Al clusters. The Al-Cl and Al-Br bonds have more covalent character in Al7X and Al13X2,12-, in contrast to the corresponding Al-F bond, which has prominent ionic character.     

Electronic structures and electron detachment energies of halogen substituted acetate anions, XCH2COO- (X=F,Cl,Br)

The electronic structures and the halogen inductive effects on the acetate anion were investigated in XCH2COO- (X=F,Cl,Br) by photoelectron spectroscopy (PES) and ab initio calculations. The PES spectra indicated that the electron binding energies increased in the order of FCl>Br. These systematic changes of detachment energy and IPs were explained by examining the charge redistributions upon detaching electrons.     

C40X2（X=H,F,Cl,Br）的理论研究

应用密度泛函理论在B3LYP/6-31G*水平上对C40X2(X=H,F,Cl,Br)进行研究.研究结果表明,C40X2(X=H,F,Cl,Br)在热力学上是稳定的,卤化衍生物的稳定性随卤素原子序数的增大而降低,最有利的衍生化方式是1-4加成,1-2与1-4加成的卤化和氢化衍生物在所研究的分子中较为稳定.这些研究有助于理解富勒烯衍生物的衍生化模式.     

Substituent Effects on Hydrogen Bonding in Watson–Crick Base Pairs. A Theoretical Study

We have theoretically analyzed Watson–Crick AT and GC base pairs in which purine C8 and/or pyrimidine C6 positions carry a substituent X = H, F, Cl or Br, using the generalized gradient approximation (GGA) of density functional theory at BP86/TZ2P. The purpose is to study the effects on structure and hydrogen bond strength if X = H is substituted by a halogen atom. Furthermore, we wish to explore the relative importance of electrostatic attraction versus orbital interaction in the above multiply hydrogen-bonded systems, using a quantitative bond energy decomposition scheme. We find that replacing X = H by a halogen atom has relatively small yet characteristic effects on hydrogen bond lengths, strengths and bonding mechanism. In general, it reduces the hydrogen-bond-accepting- and increases the hydrogen-bond-donating capabilities of a DNA base. The orbital interaction component in these hydrogen bonds is found for all substituents (X = H, F, Cl, and Br) to contribute about 41% of the attractive interactions and is thus of the same order of magnitude as the electrostatic component, which provides the remaining 59% of the attraction.     

Theoretical study and rate constant calculation for the reactions of SH (SD) with Cl2, Br2, and BrCl

The mechanisms of the SH (SD) radicals with Cl2 (R1), Br2 (R2), and BrCl (R3) are investigated theoretically, and the rate constants are calculated using a dual-level direct dynamics method. The optimized geometries and frequencies of the stationary points are calculated at the MP2/6-311G(d,p) and MPW1K/6-311G(d,p) levels. Higher-level energies are obtained at the approximate QCISD(T)/6-311++G(3df, 2pd) level using the MP2 geometries as well as by the multicoefficient correlation method based on QCISD (MC-QCISD) using the MPW1K geometries. Complexes with energies less than those of the reactants or products are located at the entrance or the exit channels of these reactions, which indicate that the reactions may proceed via an indirect mechanism. The enthalpies of formation for the species XSH/XSD (X = Cl and Br) are evaluated using hydrogenation working reactions method. By canonical variational transition-state theory (CVT), the rate constants of SH and SD radicals with Cl2, Br2, and BrCl are calculated over a wide temperature range of 200-2000 K at the a-QCISD(T)/6-311++G(3df, 2pd)//MP2/6-311G(d, p) level. Good agreement between the calculated and experimental rate constants is obtained in the measured temperature range. Our calculations show that for SH (SD) + BrCl reaction bromine abstraction (R3a or R3a') leading to the formation of BrSH (BrSD) + Cl in a barrierless process dominants the reaction with the branching ratios for channels 3a and 3a' of 99% at 298 K, which is quite different from the experimental result of k3a'/k3' = 54 +/- 10%. Negative activation energies are found at the higher level for the SH + Br2 and SH + BrCl (Br-abstraction) reactions; as a result, the rate constants show a slightly negative temperature dependence, which is consistent with the determination in the literature. The kinetic isotope effects for the three reactions are "inverse". The values of kH/kD are 0.88, 0.91, and 0.69 at room temperature, respectively, and they increase as the temperature increases.     

HCHO+X(X=F、Cl、Br)的反应机理

采用CCSD/6-311++G(d,p)//B3LYP/6-311++G(d,p)方法研究了HCHO与卤素原子X(X=F、Cl、Br)的反应机理. 计算结果表明, 卤素原子X(X=F、Cl、Br)主要通过直接提取HCHO中的H原子生成HCO+HX(X=F、Cl、Br). 另外还可以生成稳定的中间体, 中间体再通过卤原子夺氢和氢原子直接解离两个反应通道分别生成HCO+HX(X=F、Cl、Br)和H+XCHO(X=F、Cl、Br). 其中卤原子夺氢通道为主反应通道, HCO和HX(X=F、Cl、Br)为主要的反应产物; 且三个反应的活化能均较低, 说明此类反应很容易进行, 计算结果与实验结果符合很好. 电子密度拓扑分析显示, 在HCHO+X反应通道(b)中出现了T型结构过渡态, 结构过渡态(STS)位于能量过渡态(ETS)之后. 并且按F、Cl、Br的顺序, 结构过渡态出现得越来越晚.     

Comparative Computational Study of Hydrogen Abstraction Reactions of CY3H + XO− (X,Y = F,Cl, and Br)

The effects of halogen substituents on the reactivity are characterized by the hybrid B3LYP and BHandHLYP functionals of density functional theory using the aug‐cc‐pVDZ basis set. The species XO⁻ and CY₃H, where X, Y = F, Cl, and Br, have been chosen as model reactants in this work. Also, the mechanism of the hydrogen abstraction (HAT) reaction has been used to study the chemical reactivity of these anionic reactions. Our theoretical findings suggest that the relative reactivity of the CY₃H + XO⁻ reactions increases as Y goes from F to Br and decreases as X goes from F to Br. Moreover, among all reactions investigated in this study, the special role of the Y has very dominant effect on activation of the C–H bond in CY₃H when XO⁻ attacks the CY₃H. Again, through the transition state theory the rate constants at 298–1000 K are also evaluated for the HAT reactions, indicating the lower the temperature the faster is the chemical reaction.     

Suzuki-Miyaura cross-coupling reaction of monohalopyridines and l-aspartic acid derivative

Suzuki-Miyaura cross-coupling reaction of halogenated pyridines and a borated l-aspartic acid derivative was conducted. The reactivity of chloro-, bromo-, and iodo-pyridines with substituents at the C2, C3, and C4 positions was investigated. Electron density of halogenated pyridines was also estimated by density functional theory (DFT) calculations. The order of experimental yield of halogen substituents was Br?>?I?>>?Cl and C3?>?C2, C4 whereas the DFT results indicated the reactivity order as I?>>?Br, Cl and C4?>?C2, C3. Optimized experimental conditions (3-bromopyridine) afforded the coupling product quantitatively.     

Kinetische und Gleichgewichtsuntersuchungen zur Aquotisierung und Halogenid-Anation an Halogeno-Aquo-Komplexen von Osmium(IV)

Kinetic and Equilibrium Studies on Aquation and Halide-Anation of Osmium(IV)- Halogeno-Aquo Complexes Complexes of type [OsI₅X]^2? (X = Cl, Br, I) undergo hydrolysis in acidic, aqueous solutions already at room temperature according to [OsI₅X]^2? + H₂O ? [OsI₅(H₂O)]^? + X^?. Rate constants of aquation and anation reactions as well as equilibrium constants are determined by spectrophotometrical measurements. The kinetic and equilibrium experimental results are giving similar thermodynamic data. Kinetic stability increases from I over Br to Cl whereas the thermodynamic stability decreases in the same order. The monoaquo complex is less stable than the hexahalo complexes by 2 kcal/mol, and is present as an intermediate product in the halogen exchange reactions carried out in dilute solutions.     

The nature of halogen...halogen synthons: crystallographic and theoretical studies

A study of the halogen...halogen contacts in organic compounds using ab initio calculations and the results of previously reported crystallographic studies show that these interactions are controlled by electrostatics. These contacts can be represented by the geometric parameters of the C--X1...X2--C moieties (where theta1=C--X1...X2 and theta2=X1...X2--C; ri=X1...X2 distance). The distributions of the contacts within the sum of van der Waals radii (rvdW) versus thetai (theta1=theta2) show a maximum at theta approximately 150 degrees for X=Cl, Br, and I. This maximum is not seen in the distribution of F...F contacts. These results are in good agreement with our ab initio calculations. The theoretical results show that the position of the maximum depends on three factors: 1) The type of halogen atom, 2) the hybridization of the ipso carbon atom, and 3) the nature of the other atoms that are bonded to the ipso carbon atom apart from the halogen atom. Calculations show that the strength of these contacts decreases in the following order: I...I>Br...Br>Cl...Cl. Their relative strengths decrease as a function of the hybridization of the ipso carbon atom in the following order: sp2>sp>sp3. Attaching an electronegative atom to the carbon atom strengthens the halogen...halogen contacts. An electrostatic model is proposed based on two assumptions: 1) The presence of a positive electrostatic end cap on the halogen atom (except for fluorine) and 2) the electronic charge is anisotropically distributed around the halogen atom.     

Magnitude and origin of the attraction and directionality of the halogen bonds of the complexes of C6F5X and C6H5X (X = I, Br, Cl and F) with pyridine

The geometries and interaction energies of complexes of pyridine with C₆F₅X, C₆H₅X (X=I, Br, Cl, F and H) and R_FI (R_F=CF₃, C₂F₅ and C₃F₇) have been studied by ab initio molecular orbital calculations. The CCSD(T) interaction energies (E_int) for the C₆F₅X–pyridine (X=I, Br, Cl, F and H) complexes at the basis set limit were estimated to be ?5.59, ?4.06, ?2.78, ?0.19 and ?4.37 kcal mol^?1, respectively, whereas the E_int values for the C₆H₅X–pyridine (X=I, Br, Cl and H) complexes were estimated to be ?3.27, ?2.17, ?1.23 and ?1.78 kcal mol^?1, respectively. Electrostatic interactions are the cause of the halogen dependence of the interaction energies and the enhancement of the attraction by the fluorine atoms in C₆F₅X. The values of E_int estimated for the R_FI–pyridine (R_F=CF₃, C₂F₅ and C₃F₇) complexes (?5.14, ?5.38 and ?5.44 kcal mol^?1, respectively) are close to that for the C₆F₅I–pyridine complex. Electrostatic interactions are the major source of the attraction in the strong halogen bond although induction and dispersion interactions also contribute to the attraction. Short‐range (charge‐transfer) interactions do not contribute significantly to the attraction. The magnitude of the directionality of the halogen bond correlates with the magnitude of the attraction. Electrostatic interactions are mainly responsible for the directionality of the halogen bond. The directionality of halogen bonds involving iodine and bromine is high, whereas that of chlorine is low and that of fluorine is negligible. The directionality of the halogen bonds in the C₆F₅I– and C₂F₅I–pyridine complexes is higher than that in the hydrogen bonds in the water dimer and water–formaldehyde complex. The calculations suggest that the C? I and C? Br halogen bonds play an important role in controlling the structures of molecular assemblies, that the C? Cl bonds play a less important role and that C? F bonds have a negligible impact.     

Mechanisms for the Breakdown of Halomethanes through Reactions with Ground‐State Cyano Radicals

One route to break down halomethanes is through reactions with radical species. The capability of the artificial force‐induced reaction algorithm to efficiently explore a large number of radical reaction pathways has been illustrated for reactions between haloalkanes (CX₃Y; X=H, F; Y=Cl, Br) and ground‐state (²Σ⁺) cyano radicals (CN). For CH₃Cl+CN, 71 stationary points in eight different pathways have been located and, in agreement with experiment, the highest rate constant (10⁸ s^?1 M ^?1 at 298 K) is obtained for hydrogen abstraction. For CH₃Br, the rate constants for hydrogen and halogen abstraction are similar (10⁹ s^?1 M ^?1), whereas replacing hydrogen with fluorine eliminates the hydrogen‐abstraction route and decreases the rate constants for halogen abstraction by 2–3 orders of magnitude. The detailed mapping of stationary points allows accurate calculations of product distributions, and the encouraging rate constants should motivate future studies with other radicals.     

Rational Modification of the Hierarchy of Intermolecular Interactions in Molecular Crystal Structures by Using Tunable Halogen Bonds

A family of 16 isomolecular salts (3‐XpyH)₂[MX′₄] (3‐XpyH=3‐halopyridinium; M=Co, Zn; X=(F), Cl, Br, (I); X′=Cl, Br, I) each containing rigid organic cations and tetrahedral halometallate anions has been prepared and characterized by X‐ray single crystal and/or powder diffraction. Their crystal structures reflect the competition and cooperation between non‐covalent interactions: N? H???X′? M hydrogen bonds, C? X???X′? M halogen bonds and π–π stacking. The latter are essentially unchanged in strength across the series, but both halogen bonds and hydrogen bonds are modified in strength upon changing the halogens involved. Changing the organic halogen (X) from F to I strengthens the C? X???X′? M halogen bonds, whereas an analogous change of the inorganic halogen (X′) weakens both halogen bonds and N? H???X′? M hydrogen bonds. By so tuning the strength of the putative halogen bonds from repulsive to weak to moderately strong attractive interactions, the hierarchy of the interactions has been modified rationally leading to systematic changes in crystal packing. Three classes of crystal structure are obtained. In type A (C? F???X′? M) halogen bonds are absent. The structure is directed by N? H???X′? M hydrogen bonds and π‐stacking interactions. In type B structures, involving small organic halogens (X) and large inorganic halogens (X′), long (weak) C? X???X′? M interactions are observed with type I halogen–halogen interaction geometries (C? X???X′ ≈ X???X′? M ≈155°), but hydrogen bonds still dominate. Thus, minor but quite significant perturbations from the type A structure arise. In type C, involving larger organic halogens (X) and smaller inorganic halogens (X′), stronger halogen bonds are formed with a type II halogen–halogen interaction geometry (C? X???X′ ≈180°; X???X′? M ≈110°) that is electrostatically attractive. The halogen bonds play a major role alongside hydrogen bonds in directing the type C structures, which as a result are quite different from type A and B.     

Stereochemical dependence of 3JCH coupling constants in 2-substituted 4-t-butyl-cyclohexanone and their alcohol derivatives

Theoretical and experimental studies on (3)J(C2H6eq) NMR spin-spin coupling constants in both the 2-X-4-t-butyl-cyclohexanone (X = H, CH(3), F, Cl, and Br) and in their alcohol derivatives series are reported. Results thus found are rationalized in terms of the transmission of the Fermi contact contribution to such couplings. To this end, dependencies of (3)J(C2H6eq) couplings versus the C(2)-C(1)-C(6) angle are compared in both series for equatorial and axial X orientations. The main trend is described in terms of the rear lobes interaction. Besides, for X = halogen atom in equatorial orientation a rather strong interaction between oxygen and halogen lone pairs is observed, and its influence on (3)J(C2H6eq) couplings is discussed and rationalized in terms of different Fermi contact transmission pathways.