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1.
ZHANG Xinzhuang ZHEN Zhen & LIU Xinhou Technical Institute of Physics Chemistry of Chinese Academy of Sciences Beijing China 《中国科学B辑(英文版)》2005,48(4)
HNCO is a convenient photolytic source of NCO and NH radicals for laboratory kinetics studies of elementary reaction[1] and plays an important role in the combustion and atmosphere chemistry. It can re- move deleterious compounds rapidly from exhausted ga… 相似文献
2.
Ab initio UMP2 and UQCISD(T) calculations, with 6-311G** basis sets, were performed for the titled reactions. The results show that
the reactions have two product channels: NH2+ HNCO?NH3+NCO (1) and NH2+HNCO?N2H3+CO (2), where reaction (1) is a hydrogen abstraction reaction via an H-bonded complex (HBC), lowering the energy by 32.48
kJ/mol relative to reactants. The calculated QCISD(T)//MP2(full) energy barrier is 29.04 kJ/mol, which is in excellent accordance
with the experimental value of 29.09 kJ/mol. In the range of reaction temperature 2300–2700 K, transition theory rate constant
for reaction (1) is 1.68×1011–3.29×1011 mL·mol-1·s-1, which is close to the experimental one of 5.0×1011mL·mol-1·s-1or less. However, reaction (2) is a stepwise reaction proceeding via two orientation modes,cis andtrans, and the energy barriers for the rate-control step at our best calculations are 92.79 kJ/mol (forcis-mode) and 147.43 kJ/mol (fortrans-mode), respectively, which is much higher than reaction (1). So reaction (1) is the main channel for the titled reaction. 相似文献
3.
We report ab initio UMP2 calculations of the reaction of CN with HNCO using 6-311G(d,p) basis sets. The obtained results show that the reaction
has two product channels: HNCO+CN→HCN+NCO (1) and HNCO+CN→HNCN+CO (2). Channel (1) is a hydrogen abstraction reaction, which
is a concerted process. The calculated potential energy barrier is 20.80 kJ/mol at UMP2(full)/6-311G(d,p) level. In the range
of reaction temperature (1000-2100 K), the conventional transition theory rate constant for channel (1) ranges from 0.32×10−11 to 6.9×10−11cm3· mol−1· s−1, which is close to the experimental value. Channel (2) is a stepwise reaction involving an intermediate during the process
of reaction. The UMP2(full)/6-311G(d,p) potential energy barrier is 83.42 kJ/mol for the rate-controlling step, which is much
higher than that of channel (1). 相似文献
4.
Direct measurements of the rate constants of the reactions NCN + NO and NCN + NO2 behind shock waves
The high-temperature rate constants of the reactions NCN + NO and NCN + NO(2) have been directly measured behind shock waves under pseudo-first-order conditions. NCN has been generated by the pyrolysis of cyanogen azide (NCN(3)) and quantitatively detected by sensitive difference amplification laser absorption spectroscopy at a wavelength of 329.1302 nm. The NCN(3) decomposition initially yields electronically excited (1)NCN radicals, which are subsequently transformed to the triplet ground state by collision-induced intersystem crossing (CIISC). CIISC efficiencies were found to increase in the order of Ar < NO(2) < NO as the collision gases. The rate constants of the NCN + NO/NO(2) reactions can be expressed as k(NCN+NO)/(cm(3) mol(-1)s(-1)) = 1.9 × 10(12) exp[-26.3 (kJ/mol)/RT] (±7%,ΔE(a) = ± 1.6 kJ/mol, 764 K < T < 1944 K) and k(NCN+NO(2))/(cm(3) mol(-1)s(-1)) = 4.7 × 10(12) exp[-38.0(kJ/mol)/RT] (±19%,ΔE(a) = ± 3.8 kJ/mol, 704 K < T < 1659 K). In striking contrast to reported low-temperature measurements, which are dominated by recombination processes, both reaction rates show a positive temperature dependence and are independent of the total density (1.7 × 10(-6) mol/cm(3) < ρ < 7.6 × 10(-6) mol/cm(3)). For both reactions, the minima of the total rate constants occur at temperatures below 700 K, showing that, at combustion-relevant temperatures, the overall reactions are dominated by direct or indirect abstraction pathways according to NCN + NO → CN + N(2)O and NCN + NO(2) → NCNO + NO. 相似文献
5.
HNCO与CX(X=F,Cl,Br)自由基反应机理的密度泛函理论研究 总被引:7,自引:0,他引:7
用量子化学密度泛函理论的B3LYP方法,在6-31+G~*水平上按BERNY能量梯度解 析全参数优化了HNCO与CX(X=F,Cl,Br)反应势能面上各驻点的几何构型,通过 振动频率分析确认了中间体和过渡态,内禀反应坐标(IRC)对反应物、中间体、 过渡态和产物的相关性予以证实,对各驻点进行了零点能校正(ZPE)在此基础上 计算了反应能垒。研究结果表明,与HNCO和其它小分子自由基反应不同,HNCO与 CX自由基反应首先发生分子间H原子迁移,随后N与CX的C(1)原子相互靠近成键并 生成较稳定的中间体,再发生N-C(2)键的断裂,完成N向C(1)上的迁移并进一 步解离为产物。反应按反应物→TS1→IM→TS2→产物通道进行。反应为放热反应。 相似文献
6.
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。 相似文献
7.
HNCO+OH——NH~2+CO~2反应理论研究 总被引:1,自引:0,他引:1
用从头算UHF/6-31G基组研究了异氰酸和羟基生成氨基和二氧化碳即HNCO+OH--NH~2+CO~2的反应机理.优化得到了反应途径上的过渡态和中间体,并通过振动分析对过渡态和中间体进行了确认.在UMP4/6-31G水平上计算了它们的能量,同时对零点能进行了较正.计算结果表明:此反应是多步反应,先后通过3个过渡态(TS1,TS2,TS3),2个内旋转位垒(TSI,TSII),4个中间体(IM1,IM2,IM3,IM4),其中,IM3--TS2这一步为整个反应的决速步骤,速控步的活化能为202.388kJ/mol.与异氰酸和羟基作用的另一反应通道(即HNCO+OH--H~2O+NCO)的活化能(69.038kJ/mol)比较,可看出所研究反应通道为次要反应通道,这与实验结果是一致的。 相似文献
8.
The syntheses, single crystal X-ray structures, and magnetic properties of the homometallic μ?-oxo trinuclear clusters [Fe?(μ?-O)(μ-O?CCH?)?(4-Phpy)?](ClO?) (1) and [Fe?(μ?-O)(μ-O?CAd)?(4-Mepy)?](NO?) (2) are reported (Ad = adamantane). The persistence of the trinuclear structure within 1 and 2 in CD?Cl? and C?D?Cl? solutions in the temperature range 190-390 K is demonstrated by 1H NMR. An equilibrium between the mixed pyridine clusters [Fe?(μ?-O)(μ-O?CAd)?(4-Mepy)(3-x)(4-Phpy)(x)](NO?) (x = 0, 1, 2, 3) with a close to statistical distribution of these species is observed in CD?Cl? solutions. Variable-temperature NMR line-broadening made it possible to quantify the coordinated/free 4-Rpy exchanges at the iron centers of 1 and 2: k(ex)2?? = 6.5 ± 1.3 × 10?1 s?1, ΔH(?) = 89.47 ± 2 kJ mol?1, and ΔS(?) = +51.8 ± 6 J K?1 mol?1 for 1 and k(ex)2?? = 3.4 ± 0.5 × 10?1 s?1, ΔH(?) = 91.13 ± 2 kJ mol?1, and ΔS(?) = +51.9 ± 5 J K?1 mol?1 for 2. A limiting D mechanism is assigned for these ligand exchange reactions on the basis of first-order rate laws and positive and large entropies of activation. The exchange rates are 4 orders of magnitude slower than those observed for the ligand exchange on the reduced heterovalent cluster [Fe(III)?Fe(II)(μ?-O)(μ-O?CCH?)?(4-Phpy)?] (3). In 3, the intramolecular Fe(III)/Fe(II) electron exchange is too fast to be observed. At low temperatures, the 1/3 intermolecular second-order electron self-exchange reaction is faster than the 4-Phpy ligand exchange reactions on these two clusters, suggesting an outer-sphere mechanism: k?2?? = 72.4 ± 1.0 × 103 M?1 s?1, ΔH(?) = 18.18 ± 0.3 kJ mol?1, and ΔS(?) = -90.88 ± 1.0 J K?1 mol?1. The [Fe?(μ?-O)(μ-O?CCH?)?(4-Phpy)?](+/0) electron self-exchange reaction is compared with the more than 3 orders of magnitude faster [Ru?(μ?-O)(μ-O?CCH?)?(py)?](+/0) self-exchange reaction (ΔΔG(exptl)(?298) = 18.2 kJ mol?1). The theoretical estimated self-exchange rate constants for both processes compare reasonably well with the experimental values. The equilibrium constant for the formation of the precursor to the electron-transfer and the free energy of activation contribution for the solvent reorganization to reach the electron transfer step are taken to be the same for both redox couples. The larger ΔG(exptl)(?298) for the 1/3 iron self-exchange is attributed to the larger (11.1 kJ mol?1) inner-sphere reorganization energy of the 1 and 3 iron clusters in addition to a supplementary energy (6.1 kJ mol?1) which arises as a result of the fact that each encounter is not electron-transfer spin-allowed for the iron redox couple. 相似文献
9.
Koppenol WH 《Inorganic chemistry》2012,51(10):5637-5641
Nitrosothiols are powerful vasodilators. Although the mechanism of their formation near neutral pH is an area of intense research, neither the energetics nor the kinetics of this reaction or of subsequent reactions have been addressed. The following considerations may help to guide experiments. (1) The standard Gibbs energy for the homolysis reaction RSNO → RS(?) + NO(?)(aq) is +110 ± 5 kJ mol(-1). (2) The electrode potential of the RSNO, H(+)/RSH, NO(?)(aq) couple is -0.20 ± 0.06 V at pH 7. (3) Thiol nitrosation by NO(2)(-) is favorable by 37 ± 5 kJ mol(-1) at pH 7. (4) N(2)O(3) is not involved in in vivo nitrosation mechanisms for thermodynamic--its formation from NO(2)(-) costs 59 kJ mol(-1)--or kinetic--the reaction being second-order in NO(2)(-)--reasons. (5) Hemoglobin (Hb) cannot catalyze formation of N(2)O(3), be it via the intermediacy of the reaction of Hb[FeNO(2)](2+) with NO(?) (+81 kJ mol(-1)) or reaction of Hb[FeNO](3+) with NO(2)(-) (+88 kJ mol(-1)). (6) Energetically and kinetically viable are nitrosations that involve HNO(2) or NO(?) in the presence of an electron acceptor with an electrode potential higher than -0.20 V. These considerations are derived from existing thermochemical and kinetics data. 相似文献
10.
Traeger JC 《The journal of physical chemistry. A》2007,111(21):4643-4649
Photoionization mass spectrometry has been used to measure appearance energies for immonium cation formation from 25 alkyl amine precursors. A number of the unimolecular fragmentation processes are shown to involve excess energy at threshold so that, of the 11 different cations investigated, it is only possible to derive reliable 298 K heats of formation for CH2=NH2+ (749.0 +/- 0.9 kJ mol(-1)), CH(3)CH=NH2+ (666.1 +/- 1.1 kJ mol(-1)), C(2)H(5)CH=NH2+ (636.8 +/- 2.5 kJ mol(-1)), CH2=NH(CH3)+ (706.1 +/- 1.0 kJ mol(-1)), CH2=NH(C(2)H(5))+ (668.4 +/- 1.3 kJ mol(-1)), and CH2=N(CH3)2+ (668.0 +/- 2.5 kJ mol(-1)). When these are compared to those calculated by the G3, G3B3, G2, G2(MP2), CBS-APNO, and W1U composite ab initio methods, it is found that the smallest mean absolute deviation of 1.2 +/- 0.8 kJ mol(-1) is obtained from the G2 calculations. 相似文献
11.
Decomposition of nitrous acid in aqueous solution has been studied by stopped flow spectrophotometry to resolve discrepancies in literature values for the rate constants of the decomposition reactions. Under the conditions employed, the rate-limiting reaction step comprises the hydrolysis of NO(2). A simplified rate law based on the known elementary reaction mechanism provides an excellent fit to the experimental data. The rate constant, 1.34 × 10(-6) M(-1) s(-1), is thought to be of higher accuracy than those in the literature as it does not depend on the rate of parallel reaction pathways or on the rate of interphase mass transfer of gaseous reaction products. The activation energy for the simplified rate law was established to be 107 kJ mol(-1). Quantum chemistry calculations indicate that the majority of the large activation energy results from the endothermic nature of the equilibrium 2HNO(2) ? NO + NO(2) + H(2)O. The rate constant for the reaction between nitrate ions and nitrous acid, which inhibits HNO(2) decomposition, was also determined. 相似文献
12.
13.
To explore the details of the reaction mechanisms of Zr atoms with acetonitrile molecules, the triplet and singlet spin-state potential energy surfaces have been investigated. Density functional theory (DFT) with the relativistic zero-order regular approximation at the PW91/TZ2P level has been applied. The complicated minimum energy reaction path involves four transition states (TS), stationary states 1-5 and one spin inversion (indicated by ?): (3)Zr + NCCH(3) → (3)Zr-η(1)-NCCH(3) ((3)1) → (3)TS(1/2) → (3)Zr-η(2)-(NC)CH(3) ((3)2) → (3)TS(2/3) → (3)ZrH-η(3)-(NCCH(2)) ((3)3) → (3)TS(3/4) → CNZrCH(3) ((3)4) ? (1)TS(4/5) → CN(ZrH)CH(2) ((1)5). The minimum energy crossing point was determined with the help of the DFT fractional-occupation-number approach. The spin inversion leading from the triplet to the singlet state facilitates the activation of a C-H bond, lowering the rearrangement-barrier by 78 kJ/mol. The overall reaction is calculated to be exothermic by about 296 kJ/mol. All intermediate and product species were frequency and NBO analyzed. The species can be rationalized with the help of Lewis type formulas. 相似文献
14.
The geometry of N(2)S was obtained at the CCSD(T)/aug-cc-pV(T + d)Z level of theory and energies with coupled-cluster single double triple (CCSD(T)) and basis sets up to aug-cc-pV(6 + d)Z. After correction for anharmonic zero-point energy, core-valence correlation, correlation up to CCSDT(Q) and relativistic effects, D(0) for the N-S bond is estimated as 71.9 kJ mol(-1), and the corresponding thermochemistry for N(2)S is Δ(f)H(0)(°)=205.4 kJ mol(-1) and Δ(f)H(298)(°)=202.6 kJ mol(-1) with an uncertainty of ±2.5 kJ mol(-1). Using CCSD(T)/aug-cc-pV(T + d) theory the minimum energy crossing point between singlet and triplet potential energy curves is found at r(N-N) ≈ 1.105 ? and r(N-S) ≈ 2.232 ?, with an energy 72 kJ mol(-1) above N(2) + S((3)P). Application of Troe's unimolecular formalism yields the low-pressure-limiting rate constant for dissociation of N(2)S k(0) = 7.6 × 10(-10) exp(-126 kJ mol(-1)/RT) cm(3) molecule(-1) s(-1) over 700-2000 K. The estimated uncertainty is a factor of 4 arising from unknown parameters for energy transfer between N(2)S and Ar or N(2) bath gas. The thermochemistry and kinetics were included in a mechanism for CO/H(2)/H(2)S oxidation and the conclusion is that little NO is produced via subsequent chemistry of NNS. 相似文献
15.
We examine the adsorption of CH(4) on the MgO(001) surface by a hybrid approach. It combines MP2 calculations with extrapolation to the complete basis set limit for the adsorption site and the CH(4)-CH(4) pair interactions in the adsorbate layer, with DFT+dispersion calculations under periodic boundary conditions for the whole system. To the total binding energy of 10.7 kJ mol(-1), the DFT+D(ispersion) correction contributes 0.7 kJ mol(-1) only, showing that the Mg(9)O(9) two-layer surface model is an excellent choice and that the interaction between the CH(4) molecules in the adsorbate layer is dominated by pair interactions. Contributions due to relaxation of the atom positions of 0.6 kJ mol(-1) (evaluated at DFT+dispersion) and of higher order correlation effects of 2.0 kJ mol(-1) (evaluated by CCSD(T)) yield a final estimate of 13.3 kJ mol(-1). To this total adsorption energy, the lateral interactions between the CH(4) molecules in the adsorbate layer contribute substantially, 4.1 kJ mol(-1)."Observed" desorption energies of 15.3 and 16.0 kJ mol(-1) have been derived from the observed Arrhenius desorption barriers (12.6 and 13.1 kJ mol(-1)) using thermal enthalpy contributions and a substantial zero-point energy (4.2 kJ mol(-1)) calculated from DFT+D vibrational frequencies. The comparison shows that our final hybrid MP2?:?PBE+D+ΔCCSD(T) estimate has reached chemical accuracy. It misses 2-3 kJ mol(-1) of binding only, which is most likely due to missing higher order correlation effects.PBE+D(ispersion) itself yields an adsorption energy that agrees within 1 kJ mol(-1) with our final hybrid MP2?:?PBE+D+ΔCCSD(T) estimate. 相似文献
16.
17.
Theoretical calculations (B3LYP/6-311+G(3df,2p)//B3LYP/6-31G) of the 1,3 migration of NR(2) transforming alpha-oxoketenimines 1 to alpha-imidoylketenes 3 and vice versa indicate that this process is a pseudo-pericyclic reaction with a low activation energy (NH(2) 97 kJ mol(-1), N(CH3)(2) 62 kJ mol(-1)). The oxoketenimines were found to be more stable (by 18-35 kJ mol(-1)) which is in line with experimental observations. The hindered amine rotation in the amide and amidine moieties adjacent to the cumulenes are important in the migration of the NR(2) group, as one of the rotation transition states is close to the 1,3 migration pathway. This gives an interesting potential energy surface with a valley-ridge inflection (VRI) between the orthogonal hindered amine rotation and 1,3 migration transition states. The imidoylketene may also undergo ring closure to an azetinone 5; however, this is metastable, and under the conditions that allow the 1,3-migration, the oxoketenimine 1 will be favored. The imine NH E/Z-interconversion of the ketenimine group takes place by inversion and has a low activation barrier ( approximately 40 kJ mol(-1)). In all the amidines examined the E/Z-interconversion of the imine function was predicted to be by rotation with a high barrier (>80 kJ mol(-1)), in contrast to all other reported imine E/Z-interconversions which are by inversion. 相似文献
18.
T. G. Denisova 《Kinetics and Catalysis》2008,49(3):331-343
A large body of experimental data on the reactions of ozone with C-H bonds of polar molecules in the liquid and gas phases is analyzed in the framework of the intersecting parabolas model. The reactions are considered as the abstraction reaction O3 + RH → HOOO. + R.. The contribution from the polar effect to the activation energy of such reactions is calculated. This contribution is ?6.8 kJ/mol for the reactions of ozone with aliphatic alcohols, and is ?8.1, ?11.7, ?6.8, and ?2.2 kJ/mol for the reactions of ozone with ketones, ethers, 1,3-dioxolanes, and 1,3-dioxanes, respectively. The contribution is insignificant in the reactions of ozone with aldehydes. The interatomic distances in the transition state of these reactions r #(C…H) and r #(O…H) and the angle between the C…H and O…H bonds are calculated. For the reactions in polar solvents, the contribution from solvation to the activation energy is calculated. In most of the systems considered, this contribution is insignificant (from ?1 to ?3 kJ/mol). The reactions involving ozone are compared to the reactions of peroxy radicals with the same classes of compounds. 相似文献
19.
Demeter A Druzhinin SI Kovalenko SA Senyushkina TA Zachariasse KA 《The journal of physical chemistry. A》2011,115(9):1521-1537
6-N,N-Dimethyl-9-methyladenine (DMPURM) and 6-N,N-dimethyladenine (DMPURH) show dual fluorescence from a locally excited (LE) and an intramolecular charge transfer (ICT) state in solvents of different polarity over extended temperature ranges. The fluorescence quantum yields are very small, in particular those of LE. For DMPURM in acetonitrile (MeCN) at 25 °C, for example, Φ'(ICT) = 3.2 × 10(-3) and Φ(LE) = 1.6 × 10(-4). The large value of Φ'(ICT)/Φ(LE) indicates that the forward LE → ICT reaction is much faster than the back reaction. The data obtained for the intersystem crossing yield Φ(ISC) show that internal conversion (IC) is the dominant deactivation channel from LE directly to the ground state S(0). For DMPURM in MeCN with Φ(ISC) = 0.22, Φ(IC) = 1 - Φ(ISC) - Φ'(ICT) - Φ(LE) = 0.78, whereas in cyclohexane an even larger Φ(IC) of 0.97 is found. The dipole moment gradually increases upon excitation, from 2.5 D (S(0)), via 6 D (LE) to 9 D (ICT) for DMPURM and from 2.3 D (S(0)), via 7 D (LE) to 8 D (ICT) for DMPURH. From the temperature dependence of Φ'(ICT)/Φ(LE), a reaction enthalpy -ΔH of 11 kJ/mol is obtained for DMPURM in n-hexane (ε(25) = 1.88), increasing to 17 kJ/mol in the more polar solvent di-n-butyl ether (ε(25) = 3.05). With DMPURM in diethyl ether, an activation energy of 8.3 kJ/mol is determined for the LE → ICT reaction (k(a)). The femtosecond excited state absorption spectra at 22 °C undergo an ultrafast decay: 1.0 ps in CHX and 0.63 ps in MeCN for DMPURM, still shorter (0.46 ps) for DMPURH in MeCN. With DMPURM in n-hexane, the LE fluorescence decay time τ(2) increases upon cooling from 2.6 ps at -45 °C to 6.9 ps at -95 °C. The decay involves ICT and IC as the two main pathways: 1/τ(2) ? k(a) + k(IC). As a model compound (no ICT) is not available, its lifetime τ(0)(LE) ~ 1/k(IC) is not known, which prevents a separate determination of k(a). The excited state reactions of DMPURM and DMPURH are treated with a two-state model: S(0) → LE ? ICT. With 6-N-methyl-9-methyladenine (MPURM) and 9-methyladenine (PURM), the fluorescence quantum yield is very low (<5 × 10(-5)) and dominated by impurities, due to enhanced IC from LE to S(0). 相似文献