Hydrogen bond donating ability of meta and para hydroxy phenoxyl radicals

H-bond complexes between 3- or 4-OH phenoxyl radicals and various H-bond accepting molecules were investigated by experimental and computational methods. The H-bond donating ability (α(2)(H)) of 2,6-di-tert-butyl-4-hydroxyphenoxyl radical (1) was determined as 0.79 ± 0.05 by measuring, using EPR spectroscopy, the variations of the hyperfine splitting constants of 1 as a function of the acceptor concentrations. A computational approach, based on DFT calculations, was employed to estimate the α(2)(H) values for OH-substituted phenoxyl radicals that were not persistent enough to be studied by EPR spectroscopy. The α(2)(H) value calculated for the 2,6-di-methyl analogue of 1 was 0.76, in good agreement with EPR experiments. The α(2)(H) values for 2-methoxy-4-hydroxy (3), 4-hydroxy (4), 4,6-di-methyl-3-hydroxy (5) and 3-hydroxy (6) phenoxyl radicals were computed as 0.77, 0.84, 0.66 and 0.71, respectively, indicating that α(2)(H) values were dependent on the presence of electron donating substituents and on the relative positions of the -OH and -O˙ groups. By correlating the α(2)(H) values for 4 and 6 with their water and gas-phase acidities, an unexpected role of water in promoting proton dissociation from these radicals was evidenced.     

Solution 1H NMR characterization of the distal H-bond network and the effective axial field in the resting-state, high-spin ferric, substrate-bound complex of heme oxygenase from N. meningitidis

The solution (1)H 1D and 2D NMR spectra of the high-spin ferric, resting-state, substrate-bound complex of heme oxygenase, HO, from the pathological bacterium N. meningitidis have been investigated to assess the prospects for definitive assignment of hyperfine shifted and relaxed residue protons and the interpretation of those shifts in terms of the anisotropy and orientation of the paramagnetic susceptibility tensor, chi. Appropriately tailored 1D/2D NMR data, together with analyses of paramagnetic relaxation and a preliminary estimate of the magnetic anisotropy, reveal a chi that is axially anisotropic and oriented along the Fe-His vector. Together with T(-)(2) dependence of the shifts, Deltachi(ax) yields a zero-field splitting constant, D = 9.1 cm(-)(1), which is expected to serve as a very sensitive probe of H-bond interactions between the iron-ligated water and a series of distal ordered water molecules implicated in the mechanism of HO action. The side chains, Gln49 and His53, involved in the stabilization of catalytically relevant water molecules, were found to exhibit orientations rotated by 180 degrees about the beta-gamma bonds in solution relative to those in the crystal. The implication of these reorientations on the details of the distal H-bond network is discussed. The H-bond donor strengths of Gln 49 and His53 were found to respond appropriately to H-bond donor (water) versus H-bond acceptor (cyanide) iron ligands. Very slow NH exchange for the N-terminal portion of the distal helix suggest that an intrinsically "unstable" distal helix may be valid only for the C-terminal portion.     

Pd-mediated activation of molecular oxygen in a nonpolar medium

The mechanism for direct insertion of O(2) in a toluene-solvated palladium-hydride bond (avoiding palladium zero) has been elucidated using quantum mechanics (B3LYP/LACVP** with the PBF polarizable continuum solvent model) for Pd(II)(-)-sparteine)(Cl)(H) and the model compound Pd(II)(bipyridine)(Cl)(H). We find that the process involves (1) the abstraction of the hydrogen atom by triplet oxygen, (2) the formation of a stable L(2)XPd(I)OOH triplet species, (3) a spin transition resulting in a stable L(2)XPd(II)OOH singlet species, and (4) the loss of H(2)O(2)and completion of the catalytic cycle upon the addition of HX. The limitations involved in the spin transition, the formation of the triplet Pd(I)-OOH species and the stability of that triplet species are all dependent on the presence of an H-bond acceptor cis to the hydride and the electronic characteristics of the other ligands which may or may not stabilize the Pd(I) species. Without this cis H-bond acceptor and/or electron-withdrawing ligands that can stabilize Pd(I), the reaction will not proceed via the palladium hydride insertion mechanism in a nonpolar environment.     

Reaction of O2 with the hydrogen atom in water up to 350 degrees C

The reaction of the H* atom with O2, giving the hydroperoxyl HO2* radical, has been investigated in pressurized water up to 350 degrees C using pulse radiolysis and deep-UV transient absorption spectroscopy. The reaction rate behavior is highly non-Arrhenius, with near diffusion-limited behavior at room temperature, increasing to a near constant limiting value of approximately 5 x 10(10) M(-1) s(-1) above 250 degrees C. The high-temperature rate constant is in near-perfect agreement with experimental extrapolations and ab initio calculations of the gas-phase high-pressure limiting rate. As part of the study, reaction of the OH* radical with H2 has been reevaluated at 350 degrees C, giving a rate constant of (6.0 +/- 0.5) x 10(8) M(-1) s(-1). The mechanism of the H* atom reaction with the HO2* radical is also investigated and discussed.     

Mechanism and Kinetics of the Reaction of Nitrosamines with OH Radical: A Theoretical Study

The reaction of nitrosodimethylamine, nitrosoazetidine, nitrosopyrrolidine, and nitrosopiperidine with the hydroxyl radical has been studied using electronic structure calculations in gas and aqueous phases. The rate constant was calculated using variational transition state theory. The reactions are initiated by H‐atom abstraction from the αC─H group of nitrosamines and leads to the formation of alkyl radical intermediate. In the subsequent reactions, the initially formed alkyl radical intermediate reacts with O₂ forming a peroxy radical. The reaction of peroxy radical with other atmospheric oxidants, such as HO₂ and NO radicals, is studied. The structures of the reactive species were optimized by using the density functional theory methods, such as M06‐2X, MPW1K, and BHandHLYP, and hybrid methods G3B3. The single‐point energy calculations were also performed at CCSD(T)/6‐311+G(d,p)// M062X/6‐311+G(d,p) level. The calculated thermodynamical parameters show that the reactions corresponding to the formation of intermediates and products are highly exothermic. We have calculated the rate constant for the initial H‐atom abstraction and subsequent favorable secondary reactions using canonical variational transition state theory over the temperature range of 150–400 K. The calculated rate constant for initial H‐atom abstraction reaction is ∼3 × 10⁻¹² cm³ molecule⁻¹ s⁻¹ and is in agreement with the previous experimental results. The calculated thermochemical data and rate constants show that the reaction profile and kinetics of the reactions are less dependent on the number of methyl groups present in the nitrosoamines. Furthermore, it has been found that the atmospheric lifetime of nitrosamines is around 5 days in the normal atmospheric OH concentration.     

Hydrogen bonding of formo- and thioformohydroxamic acid with methanethiol and methaneselenol as amino acid side chain groups

Hydrogen (H-) bonding ability of most stable keto and enol tautomers of formo- and thioformohydroxamic acids has been investigated by optimizing their 1:1 aggregates with MeSH and MeSeH as model molecules for sulfur and selenium containing amino acid side chains. Although enol is the most stable conformer of thioformohydroxamic acid, yet the most stable aggregate in both hydroxamic acids (HAs) being formed with keto conformer suggests that H-bonding can influence specific conformational dominance. In the aggregates, HAs preferably act as H-bond donor and S/Se of MeSH and MeSeH act as H-bond acceptor. The S···H and Se···H H-bonds although disfavored by electrostatics yet are favored by significant charge transfer. H-bonding preference and strength of interaction of HAs with MeSH and MeSeH is remarkably similar but markedly different from MeOH. AIM and NBO analysis have been employed to understand the role of electron delocalization, bond polarizations, charge transfer etc. as contributors to stabilization energy.     

Picosecond IR-UV pump-probe spectroscopic study of the dynamics of the vibrational relaxation of jet-cooled phenol. II. Intracluster vibrational energy redistribution of the OH stretching vibration of hydrogen-bonded clusters

A picosecond time-resolved IR-UV pump-probe spectroscopic study has been carried out for investigating the intracluster vibrational energy redistribution (IVR) and subsequent dissociation of hydrogen-bonded clusters of phenol (C6H5OH) and partially deuterated phenol (C6D5OH, phenol-d5) with various solvent molecules. The H-bonded OH stretching vibration was pumped by a picosecond IR pulse, and the transient S1-S0 UV spectra from the pumped level as well as the redistributed levels were observed with a picosecond UV laser. Two types of hydrogen-bonded clusters were investigated with respect to the effect of the H-bonding strength on the energy flow process: the first is of a strong "sigma-type H-bond" such as phenol-(dimethyl ether)(n=1) and phenol dimer, and the second is phenol-(ethylene)(n=1) having a weak "pi-type H-bond." It was found that the population of the IR-pumped OH level exhibits a single-exponential decay, whose rate increases with the H-bond strength. On the other hand, the transient UV spectrum due to the redistributed levels showed a different time evolutions at different monitoring UV frequency. From an analysis of the time profiles of the transient UV spectra, the following three-step scheme has been proposed for describing the energy flow starting from the IVR of the initially excited H-bonded OH stretching level to the dissociation of the H bond. (1) The intramolecular vibrational energy redistribution takes place within the phenolic site, preparing a hot phenol. (2) The energy flows from the hot phenol to the intermolecular vibrational modes of the cluster. (3) Finally, the hydrogen bond dissociates. Among the three steps, the rate constant of the first step was strongly dependent on the H-bond strength, while the rate constants of the other two steps were almost independent of the H-bond strength. For the dissociation of the hydrogen bond, the observed rate constants were compared with those calculated by the Rice, Ramsperger, Kassel, and Marcus model. The result suggests that dissociation of the hydrogen bond takes place much faster than complete energy randomization within the clusters.     

The role of hydrogen bonding on the h-atom-donating abilities of catechols and naphthalene diols and on a previously overlooked aspect of their infrared spectra

Catechols and 1,8-naphthalene diols contain one "free" hydroxyl and one intramolecularly H-bonded hydroxyl group. The "free" hydroxyls are strong hydrogen-bond donors (HBDs) with alpha2H values (Abraham et al. J. Chem. Soc., Perkin Trans. 2 1989, 699) ranging from 0.685 to 0.775, indicating that these compounds have similar HBD properties to those of strongly acidic phenols such as 4-chlorophenol (alpha2H = 0.670) and 3, 5-dichlorophenol (alpha2H = 0.774). Kinetic effects on H-atom abstractions from the diols in HB acceptor (HBA) solvents can be quantitatively accounted for over at least 50% of the available range of solvent HBA activities (as measured by their beta2H values; see Abraham et al. J. Chem. Soc. Perkin Trans. 2 1990, 521) on the basis of a single reactive OH group, the "free" OH. This free OH group is an outstanding H-atom donor in poor HBA solvents; e.g., in hexane rate constants for reaction with the DPPH* radical are 2.1 x 104 M-1 s-1 for 3,5-di-tert-butyl catechol and 2 x 106 M-1 s-1 for 4-methoxy-1,8-naphthalene diol, but only 7.4 x 103 M-1 s-1 for alpha-tocopherol (vitamin E). The diols are much more reactive than simple phenols because the O-H bond dissociation enthalpy of the "free" OH group is weakened by 5-9 kcal/mol by the intramolecular H-bond. The IR spectra of all the diols in CCl4 show two fairly sharp O-H stretching bands of roughly equal intensity separated by 42-138 cm-1. Addition of a low concentration of DMSO, a strong HBA, causes the band due to the intramolecularly H-bonded OH group to decrease in intensity to roughly half the extent that the "free" OH band loses intensity. The latter forms an intermolecular H-bond with the DMSO, the former does not. What has been overlooked in earlier work is that as the DMSO concentration is increased the band due to the intramolecularly H-bonded OH group first broadens and then evolves into a new, lower frequency (by 19-92 cm-1) band. The magnitude of the shift in the frequency of the intramolecular OH band caused by H-bonding of HBAs to the "free" OH group, Deltanu, increases linearly as the HBA activity of the additive increases, e.g., for 3,5-di-tert-butylcatechol, Deltanu/cm-1 = 33.8 beta2H (R 2 = 0.986). This may provide a new and simple method for determining beta2H values.     

Structural investigation of asymmetrical dimer radical cation system (H2O-H2S)+: proton-transferred or hemi-bonded?

Ab initio molecular orbital and hybrid density functional methods have been employed to characterize the structure and bonding of (H2O-H2S)+, an asymmetrical dimer radical cation system. A comparison has been made between the two-center three-electron (2c-3e) hemi-bonded system and the proton-transferred hydrogen-bonded systems of (H2O-H2S)+. Geometry optimization of these systems was carried out using unrestricted Hartree Fock (HF), density functional theory with different functionals, and second-order M?ller-Plesset perturbation (MP2) methods with 6-311++G(d,p) basis set. Hessian calculations have been done at the same level to check the nature of the equilibrium geometry. Energy data were further improved by calculating basis set superposition error for the structures optimized through MP2/6-311++G(d,p) calculations. The calculated results show that the dimer radical cation structure with H2O as proton acceptor is more stable than those structures in which H2O acts as a proton donor or the 2c-3e hemi-bonded (H2O thereforeSH2)+ system. This stability trend has been further confirmed by more accurate G3, G3B3, and CCSD(T) methods. On the basis of the present calculated results, the structure of H4OS+ can best be described as a hydrogen-bonded complex of H3O+ and SH with H2O as a proton acceptor. It is in contrast to the structure of neutral (H2O...H2S) dimer where H2O acts as a proton donor. The present work has been able to resolve the ambiguity in the nature of bonding between H2O and H2S in (H2O-H2S)+ asymmetrical dimer radical cation.     

NH4+ Charge Carrier Coordinated H-Bonded Organic Small Molecule for Fast and Superstable Rechargeable Zinc Batteries

Organic small molecules as high-capacity cathodes for Zn-organic batteries have inspired numerous interests, but are trapped by their easy-dissolution in electrolytes. Here we knit ultrastable lock-and-key hydrogen-bonding networks between 2, 7-dinitropyrene-4, 5, 9, 10-tetraone (DNPT) and NH₄⁺ charge carrier. DNPT with octuple-active carbonyl/nitro centers (H-bond acceptor) are redox-exclusively accessible for flexible tetrahedral NH₄⁺ ions (H-bond donator) but exclude larger and rigid Zn²⁺, due to a lower activation energy (0.14 vs. 0.31 eV). NH₄⁺ coordinated H-bonding chemistry conquers the stability barrier of DNPT in electrolyte, and gives fast diffusion kinetics of non-metallic charge carrier. A stable two-step 4e⁻ NH₄⁺ coordination with DNPT cathode harvests a high capacity (320 mAh g⁻¹), a high-rate capability (50 A g⁻¹) and an ultralong life (60,000 cycles). This finding points to a new paradigm for H-bond stabilized organic small molecules to design advanced zinc batteries.     

Detection of hydrogen bonding in solution: a 2H nuclear magnetic resonance method based on rotational motion of a donor/acceptor complex

The effect of hydrogen bonding on the rotational correlation time of an H-bond acceptor, pyridine N-oxide-d(5), in various solvents was investigated using the (2)H spin-lattice relaxation time (T(1)). The results demonstrate a linear relationship between viscosity and measured rotational correlation times, an example of Stokes-Einstein-Debye behavior. The results also clearly demonstrate reduced rotational rates for the probe in hydrogen bonding solvents in comparison to solvents incapable of forming hydrogen bonds with the probe. The utility of this observation was exploited to estimate the association constant (K(a)) through an NMR titration procedure. These results are presented as a new technique that can be applied to the characterization of hydrogen bonding in similar systems.     

低温条件下二氧化碳存在时羰基硫催化水解本征动力学

在自行设计和安装的一套微反-色谱联用装置上，以TGH为催化剂，进行了二氧化碳对羰基硫催化水解本征动力学影响的研究，得出了在低温50 ℃～70 ℃、高水汽摩尔比(H2O/COS=60～550)，分别对原料气中有无存在CO2的条件下,采用非线性Marquart法对实验数据进行回归，所建立的本征动力学方程式为：     

SENSIBILISIERTE PHOTOOXYDATION DURCH METHYLENBLAU THIOPYRONIN UND PYRONIN-III. MITTEILUNG: ÜBER DEN MECHANISMUS DER PHOTOSENSIBILISIERTEN OXYDATION DES GUANOSINS DURCH THIOPYRONIN

Abstract— The thiopyronin-sensitized photooxidation of guanosine was investigated, using flash photolysis techniques. The reaction kinetics of three short-lived intermediates (the triplet state, the reduced radical, and the oxidized radical) were followed by spectroscopy. The influence of guanosine concentration on the reaction rate shows that only the oxidized radical (reaction 7) of thiopyronin is affected by guanosine. This suggests that the primary step in the photodynamic destruction of DNA is the oxidation of guanosine by the oxidized radical of thiopyronin (reaction 9). This mechanism is discussed in terms of the redox potentials of the donor and the acceptor for three different cases: (1) where the oxidation potential of the donor is more negative than that of the triplet state, the acceptor being the triplet state ( F^T ); (2) where the oxidation potential of the donor is between the potentials of the triplet state and the oxidized radical, the acceptor being the oxidized radical (F_ox); and (3) where the oxidation potential of the donor is more positive than the reduction potential of both the triplet state and the oxidized radical; in this case no electron exchange takes place. The thiopyronin-guanosine system is an example of the second case.     

Kinetics of the HCCO + NO2 reaction

The kinetics of the HCCO + NO2 reaction were investigated using a laser photolysis/infrared diode laser absorption technique. Ethyl ethynyl ether (C2H5OCCH) was used as the HCCO radical precursor. Transient infrared detection of the HCCO radical was used to determine a total rate constant fit to the following expression: k1= (2.43 +/- 0.26) x 10(-11) exp[(171.1 +/- 36.9)/T] cm3 molecule(-1) s(-1) over the temperature range of 298-423 K. Transient infrared detection of CO, CO2, and HCNO products was used to determine the following branching ratios at 298 K: phi(HCO + NO + CO) = 0.60 +/- 0.05 and phi(HCNO + CO2) = 0.40 +/- 0.05.     

Ab initio study of the reaction of propionyl (C2H5CO) radical with oxygen (O2)

The reaction of propionyl radical with oxygen has been studied using the full coupled cluster theory with the complete basis set. This is the first time to gain a conclusive insight into the reaction mechanism and kinetics for this important reaction in detail. The reaction takes place via a chemical activation mechanism. The barrierless association of propionyl with oxygen produces the propionylperoxy radical, which decomposes to form the hydroxyl radical and the three-center alpha-lactone predominantly or the four-center beta-propiolactone. The oxidation of propionyl radical to carbon monoxide or carbon dioxide is not straightforward rather via the secondary decomposition of alpha-lactone and beta-propiolactone. Kinetically, the overall rate constant is almost pressure independent and it approaches the high-pressure limit around tens of torr of helium. At temperatures below 600 K, the rate constant shows negative temperature dependence. The experimental yields of the hydroxyl radical can be well reproduced, with the average energy transferred per collision -DeltaE=20-25 cm(-1) at 213 and 295 K (helium bath gas). At low pressures, together with the hydroxy radical, alpha-lactone is the major product, while beta-propiolactone only accounts for about one-fifth of alpha-lactone. At the high-pressure limit, the production of the propionylperoxy radical is dominant together with a fraction of the isomers. The infrared spectroscopy or the mass spectroscopy techniques are suggested to be employed in the future experimental study of the C2H5CO+O2 reaction.     

H原子和(CH3)2SiH2抽提反应的理论研究

采用UMP2/6-31G(d)理论水平优化了H原子和(CH3)2SiH2抽提反应势能面上的所有驻点,并在此水平基础上进行了内禀反应坐标(IRC)的计算,得到该反应的反应途径(MEP)。应用变分过渡态理论及最小能量途径半经典绝热基态隧道效应校正(MEPSAG)、小曲率半经典绝热基态隧道效应校正(SCSAG)等方法对上述反应进行了动力学研究,期望从理论上提供一套温度范围较宽、精度较高的动力学数据,为阐明反应机理和解释实验结果提供理论依据。     

HNCS与CH2(X2Π)反应微观动力学的理论研究

用量子化学密度泛函理论的UB3LYP/6-311+G**方法和高级电子相关的UQCISD(T)/6-311+G**方法研究了异硫氰酸(HNCS)与乙炔基自由基(C2H(X2Π))反应的微观机理. 采用双水平直接动力学方法IVTST-M, 获取反应的势能面信息, 应用正则变分过渡态理论并考虑小曲率隧道效应, 计算了在250～2500 K温度范围内反应的速率常数. 研究结果表明, HNCS与C2H(X2Π)反应为多通道、多步骤的复杂反应, 共存在三个可能的反应通道, 主反应通道为通过分子间H原子迁移, 生成主要产物NCS+C2H2. 反应速率常数随温度升高而增大, 表现为正温度效应. 速率常数计算中变分效果很小. 在低温区隧道效应对反应速率的贡献较大, 反应为放热反应.     

An EPR/ENDOR study of the asymmetric hydrogen bond between the quinone electron acceptor and the protein backbone in Photosystem I

Hydrogen bonding to the photoaccumulated secondary acceptor radical anion A₁^√− in photosystem (PS) I has been studied using pulsed Q-band ENDOR spectroscopy. With deuterated quinone in protonated PS I particles it is demonstrated that the observed radical anion has only one hydrogen-bond hyperfine coupling (hfc) tensor with tensor components above the 2 MHz range. Below 2 MHz the protein matrix protons dominate and a second weak H-bond could not be detected. The spectral resolution of pulsed Q-band ENDOR is critically required to separate the signals of the H-bond proton from those of the primary chlorophyll acceptor, A₀^√−, which cannot be avoided to be formed to some extent in the photoaccumulation procedure. The determined H-bond hfc tensor of A₁^√− is found to be close to axial symmetry with a small isotropic component, as expected from a predominantly dipolar electron–proton spin interaction in a hydrogen-bond. The principal tensor components are A=(+)7.7, MHz A=(−)4.9 MHz, A_iso=(−)0.7 MHz. The magnitude of the dipolar tensor corresponds to an unusually short H-bond which can be estimated from the point-dipole approximation (1.5±0.1 Å). Based on previous studies with A- and B-branch specific site-directed mutants of the A₁ site of PS I and the chosen photoaccumulation protocol, the observed A₁^√− radical anion can be assigned to the Q_K–A site of the A-branch. The observed H-bond hfc tensor is compared to those determined for related quinone radical anions observed in frozen protic solution as well as in the Q_A site of type II bacterial reaction centers.     

Kinetics of the reaction C2H5 + HO2 by time-resolved mass spectrometry

The overall rate constant for the radical-radical reaction C2H5 + HO2 --> products has been determined at room temperature by means of time-resolved mass spectrometry using a laser photolysis/flow reactor combination. Excimer laser photolysis of gas mixtures containing ethane, hydrogen peroxide, and oxalyl chloride was employed to generate controlled concentrations of C2H5 and HO2 radicals by the fast H abstraction reactions of the primary radicals Cl and OH with C2H6 and H2O2, respectively. By careful adjustments of the radical precursor concentrations, the title reaction could be measured under almost pseudo-first-order conditions with the concentration of HO2 in large excess over that of C2H5. From detailed numerical simulations of the measured concentration-time profiles of C2H5 and HO2, the overall rate constant for the reaction was found to be k1(293 K) = (3.1 +/- 1.0) x 10(13) cm3 mol(-1) s(-1). C2H5O could be confirmed as a direct reaction product.     

Kinetics of copolymerization—II. Kinetic investigation of the polymerization of methyl acrylate in solution

The kinetics of methyl acrylate polymerization initiated by azobisisobutyronitrile were investigated in dimethylformamide at 40–60. The polymerization was 1/2 order with respect to initiator; the rate of initiation was independent of the initial concentration of monomer. The rate and the overall rate constant of polymerization strongly depend on the medium: the overall rate constant decreases with increase of solvent concentration. The solvent dependence of overall rate constant can adequately be described by the hot radical theory.