电子激发态SO2(A^1A2, B^1B1)分子的碰撞猝灭动力学研究

用四倍频YAG激光(266nm)把SO2分子从电子基态X^1A1激励到电子激发态A^1A2和B^1B1的高振动耦合区, 通过检测自发辐射SO2(B^1B1→X^1A1)的时间分辨信号, 测定了室温(290K)下SO2(A^1A2, B^1B1)被He, 氯代甲烷分子和某些烷烃分子猝灭的速率常数。此外, 还从碰撞配合物模型出发, 对SO2(A^1A2, B^1B1)的猝灭机制进行了讨论。     

Reaction mechanism and kinetics of the NCN+NO reaction: comparison of theory and experiment

The rate constants for the NCN + NO reaction have been measured by laser photolysis/laser-induced fluorescence technique in the temperature range of 254-353 K in the presence of He (40-600 Torr) and N2 (30-528 Torr) buffer gases. The NCN radical was produced from the photodissociation of NCN3 at 193 nm and monitored with a dye laser at 329.01 nm. The reaction was found to be strongly positive-pressure dependent with negative-temperature dependence, as was reported previously. The experimental data could be reasonably accounted for by dual-channel Rice-Ramsperger-Kassel-Marcus calculations based on the predicted potential-energy surface using the modified Gaussian-2 method. The reaction is predicted to occur via weak intermediates, cis- and trans-NCNNO, in the 2A" state which crosses with the 2A' state containing more stable cis- and trans-NCNNO isomers. The high barriers for the fragmentation of these isomers and their trapping in the 2A' state by collisional stabilization give rise to the observed positive-pressure dependence and negative-temperature effect. The predicted energy barrier for the fragmentation of the cis-NCNNO (2A') to CN + N2O also allows us to quantitatively account for the rate constant previously measured for the reverse process CN + N2O --> NCN + NO.     

CH(A^2△和B^2∑^-)和醇类分子碰撞过程动力学研究

本文报道用四倍频YAG激光(266nm)光解CHBr3产生电子激发态CH(A,B)自由基和测量自发辐射CH(A,B→X)的时间分辨信号的方法测定了室温(290K)下CH(A,B)被醇类分子(乙醇、异丙醇、正丁醇、异戊醇和叔戊醇)猝灭的速率常数, 实验测定的CH(A)和CH(B)猝灭速率常数k~q^A和k~q^B(单位为10^-^1^0cm^3.molecule^-^1.s^-^1)值如下(误差为线性拟合的标准偏差):此外, 还从碰撞配合物模型出发, 就醇分子中OH基对猝灭速率常数的影响作了讨论。     

Quasi-classical trajectory study of the S + OH → SO + H reaction: from reaction probability to thermal rate constant

First quasi-classical trajectory calculations have been carried out for the S((3)P) + OH(X?(2)Π) → SO(X?(3)Σ(-)) + H((2)S) reaction on an ab initio global potential energy surface for the ground electronic state, X?(2)A', of HSO. Cross sections, computed for collision energies up to 1 eV, show no energy threshold and decrease with the increasing collision energy. Rate constants have been calculated in the 5-500 K temperature range. The thermal rate constant is in good agreement with approximate quantum results, while a disagreement is found at 298 K with the experimental data. Product energy distributions have also been reported at four collision energies from 0.001 to 0.5 eV. The shapes of the rovibrational and angular distributions suggest the formation of an intermediate complex that is more and more long-lived as the collision energy increases.     

NH2(A2A1,090,423)的电子猝灭和转动态-态传能

在束 气和束 束实验条件下,详细研究了NH2(A1,090,423)自由基分别与Ar,N2,O2和NH3碰撞引起的电子态猝灭和转动态 态传能,获得了总的猝灭截面σQ (分别为≤0.17、0.26、0.30和0.48 nm2),以及相对转动态 态传能截面.利用碰撞络合物模型计算的电子猝灭截面与实验测得的截面具有基本相同的趋势,表明长程吸引势在猝灭过程中起着重要的作用.同时还发现,转动态 态传能中相对截面随着碰撞对的折合质量的减小而下降.由于NH3具有较大的偶极矩以及O2的开壳层电子结构使得猝灭截面增大,而转动态 态传能截面减小.     

烷烃类分子对NCO(A2∑+)自由基的猝灭

采用激光光解-激光诱导荧光(LP-LIF)的方法,用266 nm激光光解CHBr3分子产生CH自由基,再与N2O继续反应作为NCO自由基的产生源,用438.6 nm激光将电子基态X2∏i(0010)的NCO激励到激发态A2∑+(00°0)上,通过检测激发态NCO时间分辨荧光信号,测得室温(298 K)下NCO(A2∑+)被烷烃类分子猝灭的实验结果,获得了A2∑+(00°0)态猝灭速率常数.实验发现,随着烷烃分子中C-H键数增加,其猝灭截面也近线性增加,但随着分子体积增大,这种增加趋缓.     

Cross sections and rate constants for the C(3P)+OH(X 2Pi)-->CO(X 1Sigma+)+H(2S) reaction using a quasiclassical trajectory method

First quasiclassical trajectory calculations have been carried out for the C(3P)+OH(X 2Pi)-->CO(X 1Sigma+)+H(2S) reaction using a recent ab initio potential energy surface for the ground electronic state, X 2A', of HCO/COH. Total and state-specific integral cross sections have been determined for a wide range of collision energies (0.001-1 eV). Then, thermal and state-specific rate constants have been calculated in the 1-500 K temperature range. The thermal rate constant varies from 1.78x10(-10) cm3 s-1 at 1 K down to 5.96x10(-11) cm3 s-1 at 500 K with a maximum value of 3.39x10(-10) cm3 s-1 obtained at 7 K. Cross sections and rate constants are found to be almost independent of the rovibrational state of OH.     

F与CH~3F反应的可见化学发光

利用分子束装置研究了F与CH~3F反应可见光范围(450-900nm)的化学发光.观察到HCF(A~1A"-X2A')的七个振动带和HF^+电子基态振动广频跃迁的四个振动带和它们的强度随反应物流量的变化.求得HF分子的V'=4,5,6能级相对振动布居和V'=3的转动温度.分析表明两种光谱都是第二步反应(F+CH~2F)引起的,这步反应造成了HF高振动能级的统计性粒子分布和转动能级的玻尔兹曼分布.     

Radiative association of He+ with H2 at temperatures below 100 K

The paper presents a theoretical study of the low-energy dynamics of radiative association processes in the He+ + H2 collision system. Formation of the triatomic HeH2(+) ion in its bound rotation-vibration states on the potential-energy surfaces of the ground and of the first excited electronic states is investigated. Close-coupling calculations are performed to determine detailed state-to-state characteristics (bound <-- free transition rates, radiative and dissociative widths of resonances) as well as temperature-average characteristics (rate constants, photon emission spectra) of the two-state (X <-- A) reaction He+(2S) + H2(X1sigma(g)+) --> HeH2(+)(X2A') + h nu and of the single-state (A <-- A) reaction He+(2S) + H2(X1sigma(g)+) --> HeH2(+)(A2A') + h nu. The potential-energy surfaces of the X- and A-electronic states of HeH2(+) and the dipole moment surfaces determined ab initio in an earlier work [Kraemer, Spirko, and Bludsky, Chem. Phys. 276, 225 (2002)] are used in the calculations. The rate constants k(T) as functions of temperature are calculated for the temperature interval 1 < or = T < or = 100 K. The maximum k(T) values are predicted as 3.3 x 10(-15) s(-1) cm3 for the X <-- A reaction and 2.3 x 10(-20) s(-1) cm3 for the A <-- A reaction at temperatures around 2 K. Rotationally predissociating states of the He+-H2 complex, correlating with the upsilon = 0, j = 2 state of free H2, are found to play a crucial role in the dynamics of the association reactions at low temperatures; their contribution to the k(T) function of the X <-- A reaction at T < 30 K is estimated as larger than 80%. The calculated partial rate constants and emission spectra show that in the X <-- A reaction the HeH2(+)(X) ion is formed in its highly excited vibrational states. This is in contrast with the vibrational state population of the ion when formed via the (X <-- X) reaction He(1S) + H2(+)(X2sigma(g)+) --> HeH2(+)(X2A') + h nu.     

Crossed beam study of the atom-radical reaction of ground state carbon atoms (C(3P)) with the vinyl radical (C2H3(X2A'))

The atom-radical reaction of ground state carbon atoms (C((3)P)) with the vinyl radical (C(2)H(3)(X(2)A')) was conducted under single collision conditions at a collision energy of 32.3 ± 2.9 kJ mol(-1). The reaction dynamics were found to involve a complex forming reaction mechanism, which is initiated by the barrier-less addition of atomic carbon to the carbon-carbon-double bond of the vinyl radical forming a cyclic C(3)H(3) radical intermediate. The latter has a lifetime of at least 1.5 times its rotational period and decomposes via a tight exit transition state located about 45 kJ mol(-1) above the separated products through atomic hydrogen loss to the cyclopropenylidene isomer (c-C(3)H(2)) as detected toward cold molecular clouds and in star forming regions.     

CH(A^2Δ)自由基被醇类分子猝灭动力学的温度效应研究

用266nm激光光解CHBr3产生CH自由基, 利用精密控温仪控制反应温度, 测定CH(A→X)荧光的时间分辨信号, 测量了290K至653K范围内CH(A)被乙醇、丙醇、丁醇猝灭的速率常数, 利用修正的碰撞配合物模型, 定性解释了在多极吸引势与活化势垒的双重影响下, 猝灭截面与温度间的关系。     

烷烃和胺类分子对电子激发态CH(A~2Δ和B~2Σ~-)自由基猝灭

用266nm激光光解CHBr_3分子产生CH(A,B)态自由基,通过测量CH(A,B→X)自发辐射的时间分辨信号测定室温下(CH_3)_2NH、(C_2H_5)_2NH、(C_2H_5)_3N、n-C_5H_(12)、n-C_6H_(14)和n-C_7H_(16)对CH(A,B,v'=0)的猝灭速率常数.发现猝灭速率常数与猝灭剂烷烃分子中的C-H键数近似成线性关系,但对大的烷烃分子,这种增加逐渐趋缓.用碰撞络合物模型计算胺类分子及烷烃分子与CH形成碰撞络合物时的生成截面,结果表明,在电子激发态CH自由基的猝灭过程中,碰撞对子间的多极相互吸引势和色散力作用势可能起重要作用.     

Reaction dynamics of Y + O2--> YO(X,A',A)+ O(3P(J)) studied by the crossed-beam technique

The dynamics of the reaction, Y + O2--> YO + O was studied by using the crossed-beam technique at several collision energies from 10.3 to 52.0 kJ mol(-1). The Y atomic beam was generated by laser vaporization and crossed with the O2 beam at a right angle. Among the energetically accessible electronic states of YO, the formation of the A2Pi and A'2Delta states was observed by their chemiluminescence at all collision energies. By analyzing the chemiluminescence spectra of YO(A2Pi(1/2,3/2)-X2Sigma+), vibrational state distributions and relative populations of spin-orbit states were determined for YO(A2Pi(1/2,3/2)). At low collision energies, the vibrational distributions agree quite well with those expected from the statistical energy partitioning, while a little deviation from the statistical expectation was observed at the highest energy, 52.0 kJ mol(-1). The populations of two spin-orbit states are in good agreement with the statistical expectations at all collision energies. The vacuum ultraviolet laser-induced fluorescence (VUV-LIF) technique was employed to determine the distributions of spin-orbit states of the product O(3P(J)) at two collision energies, 20.7 and 52.0 kJ mol(-1). The line shapes of the O atom transitions were analyzed to determine relative branching ratio of the ground state to the excited states of YO, i.e. YO(X2Sigma+)+ O(3P(J))vs. YO(A2Pi and A'2Delta)+ O(3P(J)). The results showed that the electronically excited YO was formed with comparable amount with the ground state which is statistically more favorable, and suggested the occurrence of two mechanisms taking place in the title reaction.     

Quantum chemical and statistical rate investigation of the CF2(a3B1)+NO(X2Pi) reaction: a fast chemical quenching process

The reaction of CF2(a3B1) with NO(X2Pi) was theoretically investigated using the B3LYP, MP2, CCSD(T), G2M, CASSCF, and CASPT2 quantum chemical methods with various basis sets including 6-31G(d), 6-311G(d), 6-311+G(3df), cc-pVDZ, and cc-pVTZ. In agreement with the experimental kinetic data, the CF2(a3B1)+NO(X2Pi) reaction is found to proceed via a fast, barrier-free combination. This process, occurring on the doublet potential energy surface, leads to the electronically excited adduct F2C-NO(22A'), which readily undergoes a surface hopping to the 12A' electronic surface, with a Landau-Zener transition probability estimated to be close to 90% per C-N vibration. The metastable adduct F2C-NO(12A') can then either spontaneously decompose into CF2(X1A1)+NO(X2Pi) in a direct chemical quenching mechanism or relax to its ground-state equilibrium structure F2CNO(X2A'). The product distribution resulting from the latter, chemically activated intermediate was evaluated by solution of the master equation (ME), under different reaction conditions, using the exact stochastic simulation method; microcanonical rate constants were computed using Rice-Ramsperger-Kassel-Marcus (RRKM) theory, based on the potential energy surfaces (PESs) constructed using both G2M and CASPT2 methods. The RRKM/ME analysis reveals that the hot F2CNO(X2A') rapidly fragments almost exclusively to the same products as above, CF2(X1A1)+NO(X2Pi), which amounts to an indirect chemical quenching mechanism. The reaction on the quartet PES is unlikely to be significant except at very high temperatures. The high crossing probability (up to 90%) between the two "avoided" doublet PESs points out the inherent difficulty in treating chemically activated reactions with fast-moving nuclei within the Born-Oppenheimer approximation.     

Unraveling the formation of HCPH(X2A') molecules in extraterrestrial environments: crossed molecular beam study of the reaction of carbon atoms, C(3Pj), with phosphine, PH3(X1A1)

The reaction between ground state carbon atoms, C(3P(j)), and phosphine, PH3(X(1)A1), was investigated at two collision energies of 21.1 and 42.5 kJ mol(-1) using the crossed molecular beam technique. The chemical dynamics extracted from the time-of-flight spectra and laboratory angular distributions combined with ab initio calculations propose that the reaction proceeds on the triplet surface via an addition of atomic carbon to the phosphorus atom. This leads to a triplet CPH3 complex. A successive hydrogen shift forms an HCPH2 intermediate. The latter was found to decompose through atomic hydrogen emission leading to the cis/trans-HCPH(X(2)A') reaction products. The identification of cis/trans-HCPH(X(2)A') molecules under single collision conditions presents a potential pathway to form the very first carbon-phosphorus bond in extraterrestrial environments like molecular clouds and circumstellar envelopes, and even in the postplume chemistry of the collision of comet Shoemaker-Levy 9 with Jupiter.     

Collision energy dependence of the O(1D) + HCl --> OH + Cl(2P) reaction studied by crossed beam scattering and quasiclassical trajectory calculations on ab initio potential energy surfaces

The dynamics of the O(1D) + HCl --> OH + Cl(2P) reaction are investigated by a crossed molecular beam ion-imaging method and quasiclassical trajectory calculations on the three ab initio potential energy surfaces, the ground 1(1)A' and two excited (1(1)A' and 2(1)A') states. The scattering experiment was carried out at collision energies of 4.2, 4.5, and 6.4 kcal/mol. The observed doubly differential cross sections (DCSs) for the Cl(2P) product exhibit almost no collision energy dependence over this inspected energy range. The nearly forward-backward symmetric DCS indicates that the reaction proceeds predominantly on the ground-state potential energy surface at these energies. Variation of the forward-backward asymmetry with collision energy is interpreted using an osculating complex model. Although the potential energy surfaces obtained by CASSCF-MRCI ab initio calculations exhibit relatively low potential barriers of 1.6 and 6.5 kcal/mol for 1(1)A' and 2(1)A', respectively, the dynamics calculations indicate that contributions of these excited states are small at the collision energies lower than 15.0 kcal/mol. Theoretical DCSs calculated for the ground-state reaction pathway agree well with the observed ones. These experimental and theoretical results suggest that the titled reaction at collision energies less than 6.5 kcal/mol is predominantly via the ground electronic state.     

Photoreduction of Ru(bpy)(3)2+ by amines in aqueous solution. Kinetics characterization of a long-lived nonemitting excited state

The photochemistry of Ru(bpy)(3)+2 in the presence of amines was investigated in water by laser flash photolysis. N,N'-Dimethylaniline and p-phenylenediamine quench the luminescent metal to ligand charge transfer (MLCT) excited state of the complex by an electron transfer reaction that produces the semireduced form Ru(bpy)3+ in relatively high yields. On the other hand, triethylamine (TEA) and aniline do not quench the MLCT. Nevertheless, when laser flash irradiation at 532 nm is carried out in the presence of these amines, the formation of Ru(bpy)3+ is clearly detected by its transient absorption at 510 nm. These results are interpreted by an electron transfer reaction with the participation of a nonemitting excited state of the complex, formed independently of the MLCT from the Franck-Condon or the relaxed singlet excited state. The rate constants for the quenching of this state by TEA and aniline and the quantum yields for Ru(bpy)(3)+ were determined. The new state is formed in a very fast process and has a lifetime of ca 4 micros in water.     

Determination of the internal state distribution of the SD product from the S(1D)+D2 reaction

The S(1D)+D2-->SD+D reaction has been studied through a photolysis-probe experiment in a cell. S(1D) reagent was prepared by 193 nm photolysis of CS2, and the SD(X 2Pi) product was detected by laser fluorescence excitation. The nascent rotational/fine-structure state distribution of the SD(X 2Pi) product was determined. This reaction, previously studied theoretically and in a crossed molecular beam experiment, is known to proceed through formation and decay of a long-lived collision complex involving the deep well in the H2S ground electronic state. The determined SD rotational state distribution in the v=0 vibrational level was found to be approximately statistical, with a small preference for formation of the F1 (Omega=3/2) fine-structure manifold over F2 (Omega=1/2). The branching into the Lambda doublet levels was also investigated, and essentially equal populations of levels of A' and A" symmetry were found. The present results are compared with previous investigations of this reaction and the analogous O(1D)+D2 reaction.     

Photoinduced intramolecular electron-transfer reactions of reconstituted met- and zinc-myoglobins appending acridine and methylacridinium ion as DNA-binders

Three types of reconstituted met- and zinc-myoglobin (metMb and ZnMb) dyads, ZnMbAc(4)Me+, ZnMbAc(6)Me+, and metMbAc(6) have been prepared by incorporating chemically modified metalloporphyrin cofactor appending an acridine (Ac) or a methylacridinium ion ([AcMe]+) into apo-Mb. In the bimolecular system between ZnMb and [AcMe]+, the photoexcited triplet state of ZnMb, 3(ZnMb)*, was successfully quenched by [AcMe]+ to form the radical pair of ZnMb cation (ZnMb*+) and reduced methylacridine ([AcMe]*), followed by a thermal back ET reaction. The rate constants for the intermolecular quenching ET (kq) and the back ET reaction (kb) at 25 degrees C were successfully obtained as kq = (8.8 +/- 0.4) x 10(7) M(-1) s(-1) and kb = (1.2 +/- 0.1) x 10(8) M(-1) s(-1), respectively. On the other hand, in case of the intramolecular photoinduced ET reactions of ZnMbAc(4)Me+ and ZnMbAc(6)Me+ dyads, the first-order quenching rate constants (kET) of 3(ZnMb)* by [AcMe]+ moiety were determined to be kET = 2.6 x 10(3) and 2.5 x 10(3) s(-1), respectively. When such ET occurs along the alkyl spacer via through-bond mechanism at the surface of Mb, the obtained kET is reasonable to provide decay constant of beta (1.0-1.3 A(-1)). Upon photoirradiation of [AcMe]+ moiety, kinetic studies also presented the intramolecular quenching reactions from the excited singlet state, 1([AcMe]+)*, whose likely process is the photoinduced energy-transfer reaction. For metMbAc(6) dyad, steady-state fluorescence was almost quenched, while the signal around 440 nm gradually appeared in the presence of various concentrations of DNA. Our study implies that synthetic manipulation at the Mb surface, by using an artificial DNA-binder coupled with photoinduced reaction, may provide valuable information to construct new Mb-DNA complex and sensitive fluorescent for DNA.     

Br2 elimination in 248-nm photolysis of CF2Br2 probed by using cavity ring-down absorption spectroscopy

By using cavity ring-down absorption spectroscopy technique, we have observed the channel of Br2 molecular elimination following photodissociation of CF2Br2 at 248 nm. A tunable laser beam, which is crossed perpendicular to the photolyzing laser beam in a ring-down cell, is used to probe the Br2 fragment in the B 3Piou+-X1Sigmag+ transition. The vibrational population is obtained in a nascent state, despite ring-down time as long as 500-1000 ns. The population ratio of Br2(v=1)/Br2(v=0) is determined to be 0.4+/-0.2, slightly larger than the value of 0.22 evaluated by Boltzmann distribution at room temperature. The quantum yield of the Br2 elimination reaction is also measured to be 0.04+/-0.01. This work provides direct evidence to support molecular elimination occurring in the CF2Br2 photodissociation and proposes a plausible pathway with the aid of ab initio potential-energy calculations. CF2Br2 is excited probably to the 1B1 and 3B2 states at 248 nm. As the C-Br bond is elongated upon excitation, the coupling of the 1A'(1B1) state to the high vibrational levels of the ground state X 1A'(1A1) may be enhanced to facilitate the process of internal conversion. After transition, the highly vibrationally excited CF2Br2 feasibly surpasses a transition barrier prior to decomposition. According to the ab initio calculations, the transition state structure tends to correlate with the intermediate state CF2Br+Br(CF2Br...Br) and the products CF2+Br2. A sequential photodissociation pathway is thus favored. That is, a single C-Br bond breaks, and then the free-Br atom moves to form a Br-Br bond, followed by the Br2 elimination. The formed Br-Br bond distance in the transition state tends to approach equilibrium such that the Br2 fragment may be populated in cold vibrational distribution. Observation of a small vibrational population ratio of Br2(v=1)Br2(v=0) agrees with the proposed mechanism.