污泥热解中HCN与CaO的反应机理:密度泛函理论研究

采用密度泛函理论对污泥热解中CaO与HCN在低温段的反应进行了研究。在B3LYP/6-311++(3df,2p)水平上计算得到了反应路径上各驻点的几何构型与频率,并在此构型上使用CCSD(T)/cc-pVQZ进行单点能计算。结果表明,两个HCN分子吸附于CaO后,质子发生转移时出现反应路径中最大能垒(310.33 kJ/mol)。使用经典过渡态理论拟合了反应中各步骤的阿累尼乌斯公式,计算了三种典型温度下各步骤的反应速率,发现质子转移为该反应的决速步骤,且温度越高CaO对HCN的作用效果越好。     

HCN(HNC)与NH3, H2O和HF分子间相互作用的理论研究

在MP2/aug-cc-pVTZ水平上, 对HCN(HNC)与NH3, H2O和HF分子间可能存在的氢键型复合物进行了全自由度能量梯度优化, 通过在相同水平上的频率验证分析发现了稳定的分子间相互作用形式是HCN(HNC)作为质子供体或作为质子受体形成的复合物. 基组重叠误差对总相互作用能的影响均小于3.34 kJ/mol. 通过自然键轨道(NBO)分析, 研究了单体和复合物中的原子电荷和电荷转移对分子间相互作用的影响. 对称性匹配微扰理论(SAPT, Symmetry Adapted Perturbation Theory)能量分解结果表明, 在分子间相互作用中, 静电作用与诱导作用占主导地位, 而诱导作用与复合物的电荷转移之间具有良好的正相关性.     

A theoretical prediction about harnessing ESPT process for HBO derivatives

The different excited-state behaviors involved in excited-state proton transfer (ESPT) process of a series of 2-(2-hydroxyphenyl)benzoxazole (HBO) derivatives have been theoretically investigated. The primary bond lengths and bond angles were analyzed. Coupling with the infrared (IR) vibrational spectra, we confirmed that the intramolecular hydrogen bond O–H···N should be strengthened in the S₁ state, which might provide the possibility for ESPT reaction, whereas introducing the fused rings may weaken the hydrogen bond in excited state. By investigating the vertical excitation process, the charge redistribution was explored. It is found that the electron-accepting –NO₂ and –COOH would facilitate the ESPT reaction. With adding fused rings to HBO, less charge transfer exists in the transition process, which can reasonably explain the weakening hydrogen bond phenomenon in excited states. Via constructing the potential energy curves of both S₀ and S₁ states, we further confirm that electron-accepting substitutions could promote the ESPT process for HBO systems. And fused rings do inhibit ESPT reaction to a great extent. We believe this work not only elaborates the different excited-state proton transfer behaviors for a series of HBO derivatives but also presents a new harnessing ESPT process through substitutional effects.     

Quantification of hydrogen cyanide in humid air by selected ion flow tube mass spectrometry

Following our recent observation that Pseudomonas bacteria in vitro emit hydrogen cyanide, we have found it necessary to investigate the ion chemistry of this compound and to extend the kinetics database for selected ion flow tube mass spectrometry (SIFT-MS) to allow the accurate quantification of HCN in moist air samples, including exhaled breath. Because of the proximity of the proton affinities of HCN and H2O molecules, the presence of water vapour can significantly distort HCN analysis in the presence of water vapour and a more sophisticated analytical procedure has to be developed. Thus, the reactions of H3O+(H2O)0,1,2,3 ions with HCN molecules have been studied in the presence of varying concentrations of water vapour, reactions on which SIFT-MS analysis of HCN relies. The results of these experiments have allowed an analytical procedure to be developed which has extended the kinetics database of SIFT-MS, such that HCN can now be quantified in humid air and in exhaled breath.     

Rotational energy barrier of 2-(2',6'-dihydroxyphenyl)benzoxazole: a case study by NMR

2-(2'-Hydroxyphenyl)benzoxazole (HBO) derivatives represent an important class of luminescent materials, as they can undergo excited state intramolecular proton transfer (ESIPT). The material's ESIPT properties are dependent on the ratio of two different rotamers, whose interconversion is poorly understood. By using HBO derivative 4, the rotational energy barrier of 2- (2',6'-hydroxyphenyl)benzoxazole is determined to be 10.5 kcal/mol by variable-temperature NMR. Although a HBO derivative typically exhibits two rotamers with O···H-O (e.g., 1a) and N···H-O bonding (e.g., 1b), correlation of NMR with fluorescence data reveals that the rotamer with N···H-O bonding is predominant in the solution.     

Theoretical study of photoacidity of HCN: the effect of complexation with water

The character of the hydrogen bonding and the excited state proton transfer (ESPT) in the model system HCN...H(2)O is investigated. The PES of the two lowest excited states of the H(2)O...HCN complex was calculated using the CASPT2 method. The nonadiabatic coupling of the two states of the (pi-->pi*) and (pi-->sigma*) character is responsible for the excited state proton/hydrogen transfer. Compared to the ground state, the barrier for this process is significantly smaller. An increased number of water molecules in the complex with cyclic hydrogen-bonded network causes a large blue shift of the state of the (pi-->sigma*) character. The question of the dissociation of the complex in its excited state is also addressed.     

Collision-induced dissociation of HS-(HCN): unsymmetrical hydrogen bonding in a proton-bound dimer anion

The energy-resolved competitive collision-induced dissociation of the proton-bound complex [HS.H.CN](-) is studied in a guided ion beam tandem mass spectrometer. H(2)S and HCN have nearly identical gas-phase acidities, and therefore, the HS(-) + HCN and the CN(-) + H(2)S product channels exhibit nearly the same threshold energies, as expected. However, the HS(-) + HCN channel has a cross section up to a factor of 50 larger than CN(-) + H(2)S at higher energies. The cross sections are modeled using RRKM theory and phase space theory. The complex dissociates to HS(-)+ HCN via a loose transition state, and it dissociates to CN(-) + H(2)S via a tight transition state. Theoretical calculations show that the proton-transfer potential energy surface has a single minimum and that the hydrogen bonding in the complex is strongly unsymmetrical, with an ion-molecule complex of the form HS(-)..HCN rather than CN(-)..H(2)S or an intermediate structure. The requirement for proton transfer before dissociation and curvature along the reaction path impedes the CN(-) + H(2)S product channel.     

Unearthing the Mechanism of Prebiotic Nitrile Bond Reduction in Hydrogen Cyanide through a Curious Association of Two Molecular Radical Anions

HCN is clearly associated with the prebiotic chemical evolution of life. It has been known for decades that the radiolysis of HCN solutions produces sugars, amino acids and nucleobases. Remarkably, recent experimental studies have shown that the photolytic reduction of aqueous HCN by a photoredox reagent [Cu(CN)₃]^2? specifically yields sugars, which are the essential building blocks of RNA. Although a mechanistic understanding of such reductions with solvated electrons is poor, the general consensus is that they involve neutral free radicals. We show herein through the use of electronic structure studies and molecular simulations that the reduction of the nitrile bond of HCN is initiated through the formation of a molecular dipole‐bound anion from the photoredox reagent. Our theoretical studies show how HCN binds to the photoexcited reagent and then extracts an electron from the reagent and is ultimately detached as a dipole‐bound anion. The dipole‐bound anionic form of [HCN]^? can easily convert into a solvated valence‐bound form of [HCN]^?. After the formation of solvated [HCN]^?, an extraordinary chemical event ensues through a counter‐intuitive coupling of two valence‐bound anions to form a solvated molecular dianionic intermediate, [HCN]₂^2?. Finally, a proton‐coupled electron transfer occurs within the dianionic entity to complete the reduction. This mechanistic scenario is applicable to the reduction of other prebiotic nitrile species and avoids neutral radical‐based pathways, thereby preventing the proliferation of reactive species and preserving chemical selectivity. Furthermore, we show how such similar nitrile reduction pathways operate to yield the sugar precursors.     

Ab initio GB study of prebiotic synthesis of purine precursors from aqueous hydrogen cyanide: dimerization reaction of HCN in aqueous solution

Ab initio MO GB theory which includes the continuum model of solvent effect using generalized Born formula has been applied to the dimerization reaction of HCN in aqueous solution which is the starting step in prebiotic synthesis of purine precursors from aqueous hydrogen cyanide. Three steps considered were: (i) the reaction of HCN and H₂O to produce the CN⁻ anion, (ii) the reaction of CN⁻ with HCN to give the NC–CH=N⁻ anion, and (iii) the addition of a proton to the anion to give iminoacetonitrile. The formation of CN⁻ ion from HCN in aqueous solution requires 15.1 kcal/mol (the experimental value estimated from the dissociation constant of HCN in water is 14.8 kcal/mol). The reaction of CN⁻ with HCN requires the activation energy of 32.2 kcal/mol (MP2/6-31++G^**//HF/6-31++G^**) to give the dimer. This barrier height is reduced to 26.1 kcal/mol when HCN is associated with H₃O⁺. In the presence of NH₃ in aqueous solution, CN⁻ is produced easily by the reaction of HCN and NH₃ with a low activation energy of 4.3 kcal/mol. It was shown that the formation of CN⁻ becomes easier in ammoniacal solution, and the dimerization occurs efficiently in aqueous solutions which contain NH₃.     

The long-lived molecular ion of propionitrile: An ion cyclotron resonance study

Evidence has been reported that primary loss of H and of HCN from the molecular ions of propionitrile, isobutyronitrile and butyronitrile in the mass spectrometer is preferentially preceded by hydrogen migration from C-2 to C-1. Ion cyclotron double resonance spectra of proton (or deuteron-) transfer products derived from propionitrile-2-d2 and -3-d3 and a series of bases provide evidence that such migration occurs also in long-lived propionitrile molecular ions.     

Basis set and correlation effects on computed proton affinities of some oxygen and nitrogen bases

Basis set expansion and correlation effects on the computed proton affinities of the oxygen and nitrogen bases CH₃OH, H₂CO, CO, CH₃NH₂, CH₂NH, and HCN have been evaluated. Basis set enhancements lead to systematic changes in computed proton affinities. These effects appear to be additive, and are greater for correlated proton affinities than for Hartree-Fock energies. Inclusion of correlation decreases proton affinities, with fourth-order Møller-Plesset energies bracketed by second and third order energies.     

The quantum mechanical calculation of rotational spectra. A comparison of methods for C2H2, HCN, HNC, HCO+, N2H+, CO and N2. Predictions for HCNH+, COH+, HBO, HBNH, and HBF+

Rotational frequencies determined with ab initio molecular orbital theory can play an important role in guiding spectroscopic searches for new molecules and in corroborating the assignment of unidentified lines, from the laboratory and from space. In a systematic study of 22 levels of molecular orbital theory, CISD/6-311G** gave rotational frequencies to an accuracy of +/- 0.4 GHz when an empirical correction is applied to the results for C2H2,HCN, HNC, HCO+, N2H+, CO, and N2. Larger errors can be expected when there are large vibrational effects on the rotational constants, as exemplified by COH+. Predicted J = 0--> 1 rotational frequencies using these methods are 73.9 +/- 0.4 GHz for HCNH+, 78.6 +/- 0.4 GHz for HBO, 65.8 +/- 0.4 GHz for HBNH, and 72.1 +/- 0.4 GHz for HBF+.     

Absorption and Emission Sensitivity of 2‐(2′‐Hydroxyphenyl)benzoxazole to Solvents and Impurities

2‐(2′‐Hydroxyphenyl)benzoxazole (HBO) is known for undergoing intramolecular proton transfer in the excited state to result in the emission of its tautomer. A minor long‐wavelength absorption band in the range 370–420 nm has been reported in highly polar solvents such as dimethylsulfoxide (DMSO). However, the nature of this species has not been entirely clarified. In this work, we provide evidence that this long‐wavelength absorption band might have been caused by base or metal salt impurities that are introduced into the spectral sample during solvent transport using glass Pasteur pipettes. The contamination by base or metal salt could be avoided by using borosilicate glass syringes or nonglass pipettes in sample handling. Quantum chemical calculations conclude that solvent‐mediated deprotonation is too energetically costly to occur without the aid of a base of an adequate strength. In the presence of such a base, the deprotonation of HBO and its effect on emission are investigated in dichloromethane and DMSO, the latter of which facilitates deprotonation much more readily than the former. Finally, the absorption and emission spectra of HBO in 13 solvents are reported, from which it is concluded that ESIPT is hindered in polar solvents that are also strong hydrogen bond acceptors.     

Proton transfer of NH3-HCl catalyzed by only one molecule

The proton transfer in NH(3)-HCl by only one molecule of catalyst was studied by using the MP2 method with the large 6-311++G(2d,2p) basis set. The 18 structures are obtained for the smallest units, NH(3)-HCl-A trimers, for which the proton transfer maybe occurred. The final results show that the proton transfers have occurred in the 15 cyclic shape structures for A = H(2)SO(4), H(2)SO(3), HCOOH (a), HF, H(2)O(2), HNO(3), HNO(2) (a), CH(3)OH, HCl, HNC, H(2)O, HNO(2) (b), NH(3), HCOOH (b), and HCHO, and not occurred in another 3 trimer structures for A = HCN, H(2)S, and PH(3). These results show that the proton transfer occurs from HCl to NH(3) when catalyst molecule A (acidic, neutral, or basic) not only as a proton donor strongly donates the proton to the Cl atom but as an acceptor strongly accepts the proton from the NH(3) molecule in the cyclic H-bond structure. In this work, a proton circumfluence model is proposed to explain the mechanism of the proton transfer. We find that, for the trimer, when the sum of two hydrogen bond lengths (R = R(1) + R(2)) is shorter than 5.0 A, molecule A has the ability to catalyze the proton transfer. In addition, we also find that the interaction energy E(int) between NH(3)-HCl and A is nearly related to the extent (R(H1)(-)(Cl)) of proton transfer, that is, the interaction energy E(int) increases with the proton transfer.     

Temperature-dependent ratiometric fluorescence from an organic aggregates system

The aggregates of 2-(2'-hydroxyphenyl)benzoxazole (HBO), a typical molecule exhibiting excited-state intramolecular proton transfer (ESIPT), were prepared and the photophysical properties of the aqueous dispersion of aggregates were investigated. It is found that the aggregates and the solvated enols coexist in the aqueous dispersion system. Furthermore, the aggregates undergo ESIPT to give rise to keto for green emission, while the solvated enols give rise to blue emission. The temperature effects on the aqueous dispersion of the HBO aggregates system were also explored. It shows a fluorescent ratiometric change in a range of temperature from 15 to 60 degrees C. A mechanism of a temperature-dependent equilibrium between the aggregates and the solvated enols is proposed for the fluorescence change. The reversibility and robustness as well as the stability of the aqueous dispersion of aggregates show very good performances, which may be useful in the applications of molecular fluorescent temperature sensors or molecular thermometers.     

Proton affinity correlations between hydrogen and dihydrogen bond acceptors

Several series of hydrogen- and dihydrogen-bonded complexes with HCN, C2H2, HF, H2O, CH3CONH2, and CH3COOH as donors and H2O, MeOH, EtOH, MeOMe, NH3, NH2Me, NHMe2, NMe3, NEtMe2, and BH3-NMe3 as acceptors were investigated using the MP2/6-311++G(d,p) level of theory. The total lowering of the X-H stretching frequencies in the hydrogen-bonded complexes were linearly correlated with the proton affinities of the accepting bases. From comparison of hydrogen- and dihydrogen-bonded complexes, a scaling factor to estimate the exact proton affinity of a dihydrogen bond acceptor was developed. Further, the scaling factor involving linear donors (1.204) is marginally higher than that involving nonlinear donor molecules (1.162). Finally, it was found that, given identical conditions, a hydrogen bond will be about 16-20% stronger than a corresponding dihydrogen bond.     

Electronic properties of hydrogen-bonded complexes of benzene(HCN)(1-4): comparison with benzene(H2O)(1-4)

The electronic properties, specifically, the dipole and quadrupole moments and the ionization energies of benzene (Bz) and hydrogen cyanide (HCN), and the respective binding energies, of complexes of Bz(HCN)(1-4), have been studied through MP2 and OVGF calculations. The results are compared with the properties of benzene-water complexes, Bz(H(2)O)(1-4), with the purpose of analyzing the electronic properties of microsolvated benzene, with respect to the strength of the CH/π and OH/π hydrogen-bond (H-bond) interactions. The linear HCN chains have the singular ability to interact with the aromatic ring, preserving the symmetry of the latter. A blue shift of the first vertical ionization energies (IEs) of benzene is observed for the linear Bz(HCN)(1-4) clusters, which increases with the length of the chain. NBO analysis indicates that the increase of the IE with the number of HCN molecules is related to a strengthening of the CH/π H-bond, driven by cooperative effects, increasing the acidity of the hydrogen cyanide H atom involved in the π H-bond. The longer HCN chains (n ≥ 3), however, can bend to form CH/N H-bonds with the Bz H atoms. These cyclic structures are found to be slightly more stable than their linear counterparts. For the nonlinear Bz(HCN)(3-4) and Bz(H(2)O)(2-4) complexes, an increase of the binding energy with the number of solvent molecules and a decrease of the IE of benzene, relative to the values for the Bz(HCN) and Bz(H(2)O) complexes, respectively, are observed. Although a strengthening of the CH/π and OH/π H-bonds, with increasing n, also takes place for the Bz(H(2)O)(2-4) and Bz(HCN)(3-4) nonlinear complexes, Bz proton donor, CH/O, and CH/N interactions are at the origin of this decrease. Thus CH/π and OH/π H-bonds lead to higher IEs of Bz, whereas the weaker CH/N and CH/O H-bond interactions have the opposite effect. The present results emphasize the importance of both aromatic XH/π (X = C, O) and CH/X (X = N, O) interactions for understanding the structure and electronic properties of Bz(HCN)(n) and Bz(H(2)O)(n) complexes.     

Gas-phase behaviour of negative ions produced from thiazidic diuretics under electrospray conditions

A systematic mass spectrometric study of 10 thiazidic diuretics and related compounds was undertaken by mass spectrometry (MS) with electrospray ionization in the negative ion mode. Collisional dissociation 'in-source' (CID-MS) and in a low-pressure collision cell (CID-MS/MS) were compared in both excitation regions. Spectra obtained by CID-MS and by CID-MS/MS were matched. Using the two methods, loss of HCl and consecutive dissociations from 2HCl losses were exhibited from compounds such as methyclothiazide and trichlormethiazide but not from other thiazidic diuretics that contain chlorine substituents in the aromatic moiety. However, deprotonated dichlorphenamide gave rise to loss of HCl by CID-MS and CID-MS/MS. For other diuretics such as hydroflumethiazide and hydrochlorothiazide, the loss of HCN and [HCN + SO(2)] was relevant. Reaction mechanisms were checked by means of deuterium-hydrogen exchange, which showed that deprotonation took place regioselectively on the heterocyclic moiety. The cleavage pathways require molecular isomerization forming ion-dipole complexes prior to decompositions, allowing long-distance proton transfer for neutral elimination. Identifications of the most specific fragmentations presented in this paper were applied to the screening and unambiguous identification of diuretics for horse doping control.     

HCN消除反应机理的理论研究

采用密度泛函理论方法从HCN氧化和水解两个方面研究了HCN消除反应机理,并考虑了HCN的直接消除反应(途径Ⅰ和途径Ⅱ)和CuO上的HCN消除反应(途径Ⅲ和途径Ⅳ)。途径Ⅰ为HCN与2个O2分子生成CO2、NO和H原子;途径Ⅱ为HCN与1个O2分子和1个H2O分子生成 CO2和NH3;途径Ⅲ为CuO上HNCO水解为CO2和NH3;途径Ⅳ为CuO上HCN水解为CO和NH3。研究发现,途径III速控步骤的活化自由能垒为157.32 kJ/mol,比途径Ⅱ中HNCO水解降低12.34 kJ/mol;比途径Ⅳ降低了63.8 kJ/mol。可见,HNCO是HCN净化过程中的重要中间体,CuO的加入降低了反应能垒,促进了HCN消除。     

Gas Phase Reactions of 1,3,5-Triazine: Proton Transfer,Hydride Transfer,and Anionic σ-Adduct Formation

The gas phase reactivity of 1,3,5-triazine with several oxyanions and carbanions, as well as amide, was evaluated using a flowing afterglow-selected ion flow tube mass spectrometer. Isotopic labeling, H/D exchange, and collision induced dissociation experiments were conducted to facilitate the interpretation of structures and fragmentation processes. A multi-step (→ HCN + HC₂N₂⁻ → CN⁻ + 2 HCN) and/or single-step (→ CN⁻ + 2 HCN) ring-opening collision-induced fragmentation process appears to exist for 1,3,5-triazinide. In addition to proton and hydride transfer reactions, the data indicate a competitive nucleophilic aromatic addition pathway (S_NAr) over a wide range of relative gas phase acidities to form strong anionic σ-adducts (Meisenheimer complexes). The significant hydride acceptor properties and stability of the anionic σ-adducts are rationalized by extremely electrophilic carbon centers and symmetric charge delocalization at the electron-withdrawing nitrogen positions. The types of anion-arene binding motifs and their influence on reaction pathways are discussed.