n-C_3H_7I和i-C_3H_7I在266nm的光解:烷基自由基分支化对C—I解离动力学的影响(英文)

结合共振增强多光子电离(REMPI)方案,利用离子影像技术研究了n-C3H7I和i-C3H7I分子的光解动力学.分析和比较了它们光解过程中所涉及的能量分配和解离态间的非绝热跃迁信息.它们的I(2P3/2)产物通道的内能所占百分比要大于I*(2P1/2)产物通道的.随着烷烃自由基变得更加的分支化,一方面,原子碎片(I和I*)的能量分布明显变宽,暗示了α-碳原子上的烷基具有更复杂的振转模式;另一方面,在266nm光子的泵浦下,尽管两分子3Q0邝X跃迁的谐振强度表现出很小的差别,但是,产生I*碎片的几率明显降低,从n-C3H7I的0.72降到i-C3H7I的0.46.这可以归因于在光解i-C3H7I过程中弯曲振动模式对产生I和I*的贡献要比n-C3H7I光解过程中弯曲振动模式对I和I*的贡献更明显,使得3Q0与1Q1态之间的非绝热跃迁得到增强.此外,n-C3H7I和i-C3H7I的3Q0邝X跃迁并不完全是平行跃迁,对应的跃迁偶极矩与键轴间的夹角分别约为15°和18°.     

Vibrationally resolved photofragment translational spectroscopy of CH3I from 277 to 304 nm with increasing effect of the hot band

The photodissociation dynamics of CH(3)I from 277 to 304 nm is studied with our mini-TOF photofragment translational spectrometer. A single laser beam is used for both photodissociation of CH(3)I and REMPI detection of iodine. Many resolved peaks in each photofragment translational spectrum reveal the vibrational states of the CH(3) fragment. There are some extra peaks showing the existence of the hot-band states of CH(3)I. After careful simulation with consideration of the hot-band effect, the distribution of vibrational states of the CH(3) fragment is determined. The fraction σ of photofragments produced from the hot-band CH(3)I varies from 0.07 at 277.38 nm to 0.40 at 304.02 nm in the I* channel and from 0.05 at 277.87 nm to 0.16 at 304.67 nm in the I channel . E(int)/E(avl) of photofragments from ground-state CH(3)I remains at about 0.03 in the I* channel for all four wavelengths, but E(int)/E(avl) decreases from 0.09 at 277.87 nm to 0.06 at 304.67 nm in the I channel . From the ground-state CH(3)I, the quantum yield Φ(I*) is determined to be 0.59 at 277 nm and 0.05 at 304 nm. The curve-crossing probability P(cc) from the hot-band CH(3)I is lower than that from the ground-state CH(3)I. The potential energy at the curve-crossing point is determined to be 32,740 cm(-1).     

Photoionization of hot radicals: C2H5, n-C3H7, and i-C3H7

The combination of ion-imaging and vacuum-ultraviolet (vuv) single-photon ionization is used to study the internal energy dependence of the relative photoionization yields of the C(2)H(5),n-C(3)H(7), and i-C(3)H(7) radicals following the 266 nm photodissociation of the corresponding alkyl iodides. The comparison of the ion images obtained by vuv photoionization of the radical with those obtained by two-photon-resonant, three-photon ionization of the complementary I (2)P(32) and I*(2)P(12) atoms allows the extraction of the internal energy dependence of the cross sections. Factors influencing the appearance of the ion images in the different detection channels are discussed, including the secondary fragmentation of the neutral radicals, Franck-Condon factors for the photoionization process, and the unimolecular fragmentation of the parent photoions.     

High-resolution photofragment translational spectroscopy for the UV photodissociation of C<Subscript>2</Subscript>H<Subscript>5</Subscript>I

The photodissociation of ethyl iodide at 279.71, 281.73, 304.02 and 304.67 nm has been studied on our new mini-photofragment translational spectrometer with a total flight path of only 5 cm. Some vibrational peaks are firstly resolved in the TOF spectra of I*(2P1/2) and I(2P3/2) channels. These vibrational peaks are assigned to the excitation states (v2 = 0, 1, 2,…) of the umbrella mode (v2, 540 cm-1) of the photofragment C2H5, and the distribution of the vibrational states is obtained. The dissociation energy has been determined to be D0(C-I)=2.314 ±0.03 eV. The energy partitioning of the available energy (Eavl=ET Eint=ET EV,R) calculated from our experimental data (-E)int/Eavl= 22.1% at 281.73 nm, 22.4% at 304.02 nm for the I* channel, and (-E)int/Eavl = 25.2% at 279.71 nm, 25.9% at 304.67 nm for the I channel, seem to be more reliable.     

Measurements and modeling of HO2 formation in the reactions of n-C3H7 and i-C3H7 radicals with O2

The formation of HO(2) in the reactions of C(2)H(5), n-C(3)H(7), and i-C(3)H(7) radicals with O(2) is investigated using the technique of laser photolysis/long-path frequency-modulation spectroscopy. The alkyl radicals are formed by 266 nm photolysis of alkyl iodides. The formation of HO(2) from the subsequent reaction of the alkyl radicals with O(2) is followed by infrared frequency-modulation spectroscopy. The concentration of I atoms is simultaneously monitored by direct absorption of a second laser probe on the spin-orbit transition. The measured profiles are compared to a kinetic model taken from time-resolved master-equation results based on previously published ab initio characterizations of the relevant stationary points on the potential-energy surface. The ab initio energies are adjusted to produce agreement with the present experimental data and with available literature studies. The isomer specificity of the present results enables refinement of the model for i-C(3)H(7) + O(2) and improved agreement with experimental measurements of HO(2) production in propane oxidation.     

Kinetics of the reactions of Cl*(2P(1/2)) and Cl(2P(3/2)) atoms with CH3OH, C2H5OH, n-C3H7OH, and i-C3H7OH at 295 K

The title reactions were studied using laser flash photolysis/laser-induced-fluorescence (FP-LIF) techniques. The two spin-orbit states, Cl*(2P(1/2)) and Cl(2P(3/2)), were detected using LIF at 135.2 and 134.7 nm, respectively. Measured reaction rate constants were as follows (units of cm3 molecule(-1) s(-1)): k(Cl(2P(3/2))+CH3OH) = (5.35 +/- 0.24) x 10(-11), k(Cl(2P(3/2))+C2H5OH) = (9.50 +/- 0.85) x 10(-11), k(Cl(2P(3/2))+n-C3H7OH) = (1.71 +/- 0.11) x 10(-10), and k(Cl(2P(3/2))+i-C3H7OH) = (9.11 +/- 0.60) x 10(-11). Measured rate constants for total removal of Cl*(2P(1/2)) in collisions with CH3OH, C2H5OH, n-C3H7OH, and i-C3H7OH were (1.95 +/- 0.13) x 10(-10), (2.48 +/- 0.18) x 10(-10), (3.13 +/- 0.18) x 10(-10), and (2.84 +/- 0.16) x 10(-10), respectively; quoted errors are two-standard deviations. Although spin-orbit excited Cl*(2P(1/2)) atoms have 2.52 kcal/mol more energy than Cl(2P(3/2)), the rates of chemical reaction of Cl*(2P(1/2)) with CH3OH, C2H5OH, n-C3H7OH, and i-C3H7OH are only 60-90% of the corresponding Cl(2P(3/2)) atom reactions. Under ambient conditions spin-orbit excited Cl* atoms are responsible for 0.5%, 0.5%, 0.4%, and 0.7% of the observed reactivity of thermalized Cl atoms toward CH3OH, C2H5OH, n-C3H7OH, and i-C3H7OH, respectively.     

Chemically activated reactions on the C7H5 energy surface: propargyl + diacetylene, i-C5H3 + acetylene, and n-C5H3 + acetylene

This study uses computational chemistry and statistical reaction rate theory to investigate the chemically activated reaction of diacetylene (butadiyne, C(4)H(2)) with the propargyl radical (C˙H(2)CCH) and the reaction of acetylene (C(2)H(2)) with the i-C(5)H(3) (CH(2)CCCC˙H) and n-C(5)H(3) (CHCC˙HCCH) radicals. A detailed G3SX-level C(7)H(5) energy surface demonstrates that the C(3)H(3) + C(4)H(2) and C(5)H(3) + C(2)H(2) addition reactions proceed with moderate barriers, on the order of 10 to 15 kcal mol(-1), and form activated open-chain C(7)H(5) species that can isomerize to the fulvenallenyl radical with the highest barrier still significantly below the entrance channel energy. Higher-energy pathways are available leading to other C(7)H(5) isomers and to a number of C(7)H(4) species + H. Rate constants in the large multiple-well (15) multiple-channel (30) chemically activated system are obtained from a stochastic solution of the one-dimensional master equation, with RRKM theory for microcanonical rate constants. The dominant products of the C(4)H(2) + C(3)H(3) reaction at combustion-relevant temperatures and pressures are i-C(5)H(3) + C(2)H(2) and CH(2)CCHCCCCH + H, along with several quenched C(7)H(5) intermediate species below 1500 K. The major products in the n-C(5)H(3) + C(2)H(2) reaction are i-C(5)H(3) + C(2)H(2) and a number of C(7)H(4) species + H, with C(7)H(5) radical stabilization at lower temperatures. The i-C(5)H(3) + C(2)H(2) reaction predominantly leads to C(7)H(4) + H and to stabilized C(7)H(5) products. The title reactions may play an important role in polycyclic aromatic hydrocarbon (PAH) formation in combustion systems. The C(7)H(5) potential energy surface developed here also provides insight into several other important reacting gas-phase systems relevant to combustion and astrochemistry, including C(2)H + the C(3)H(4) isomers propyne and allene, benzyne + CH, benzene + C((3)P), and C(7)H(5) radical decomposition, for which some preliminary analysis is presented.     

离子速度影像法研究n-C7H15Br分子光解反应动力学

利用离子速度成像方法, 研究n-C7H15Br分子在231～239 nm范围内几个波长处的光解离动力学. 通过同一束激光经(2+1)共振多光子电离(REMPI)过程探测光解碎片Br(2P3/2)和Br*(2P1/2), 得到了不同激光波长处的离子速度分布图像, 从而获得C7H15Br光解产物的能量分配和角度分布. 结合各向异性参数和量子产率, 计算了n-C7H15Br分子在234 nm波长下不同解离通道的比例. 实验表明光解产物的能量分配可以用冲击模型中的软碰撞模型来解释. 实验还发现, 各向异性参数β(Br*)的值对光波长变化很敏感, 这是由电子激发态的绝热和非绝热过程决定的.     

Measurements and modeling of DO2 formation in the reactions of C2D5 and C3D7 radicals with O2

Time-resolved production of HO2 and DO2 from the reactions of nondeuterated and deuterated ethyl and propyl radicals with O2 are measured as a function of temperature and pressure in the "transition region" between 623 and 748 K using the technique of laser photolysis/long path frequency modulation spectroscopy. Experimental measurements, using both pulsed-photolytic Cl-atom-initiated oxidation of ethane and propane and direct photolysis of ethyl, n-propyl, and isopropyl iodides, are compared to kinetic models based on the results of time-dependent master equation calculations with ab initio characterization of stationary points. The formation of DO2 and HO2 from the subsequent reaction of the alkyl radicals with O2 is followed by infrared frequency modulation spectroscopy. The concentration of I atoms is simultaneously monitored by direct absorption of a second laser probe on the spin-orbit transition. The kinetic models accurately describe the time scale and amplitude of the DO2 and HO2 formation resulting from C2D5 + O2, n-C3D7 + O2, i-C3D7 + O2, and i-C3H7 + O2. Overall, a very good level of agreement is found between theory and experiments over a wide range of temperatures, pressures, and O2 concentrations. Good agreement is also found between previous literature studies and the theory presented in this work except in the case of the high-temperature rate coefficients for the reaction of i-C3H7 + O2 to form propene. A reinvestigation of the high-temperature kinetics of the i-C3H7 + O2 reaction appears warranted. The results from the present work suggest that the theory for formation of HO2 from the reactions of ethyl and both isomeric forms of propyl radicals with O2 are very well established at this time. It is hoped that these reactions can now form the groundwork for the study and interpretation of larger and more complex R + O2 systems.     

Photoion photoelectron coincidence spectroscopy of primary amines RCH2NH2 (R = H, CH3, C2H5, C3H7, i-C3H7): alkylamine and alkyl radical heats of formation by isodesmic reaction networks

Alkylamines (RCH(2)NH(2), R = H, CH(3), C(2)H(5), C(3)H(7), i-C(3)H(7)) have been investigated by dissociative photoionization by threshold photoelectron photoion coincidence spectroscopy (TPEPICO). The 0 K dissociation limits (9.754 +/- 0.008, 9.721 +/- 0.008, 9.702 +/- 0.012, and 9.668 +/- 0.012 eV for R = CH(3), C(2)H(5), C(3)H(7), i-C(3)H(7), respectively) have been determined by preparing energy-selected ions and collecting the fractional abundances of parent and daughter ions. All alkylamine cations produce the methylenimmonium ion, CH(2)NH(2)+, and the corresponding alkyl free radical. Two isodesmic reaction networks have also been constructed. The first one consists of the alkylamine parent molecules, and the other of the alkyl radical photofragments. Reaction heats within the isodesmic networks have been calculated at the CBS-APNO and W1U levels of theory. The two networks are connected by the TPEPICO dissociation energies. The heats of formation of the amines and the alkyl free radicals are then obtained by a modified least-squares fit to minimize the discrepancy between the TPEPICO and the ab initio values. The analysis of the fit reveals that the previous experimental heats of formation are largely accurate, but certain revisions are suggested. Thus, Delta(f)Ho(298K)[CH(3)NH(2)(g)] = -21.8 +/- 1.5 kJ mol-1, Delta(f)Ho(298K)[C(2)H(5)NH(2)(g)] = -50.1 +/- 1.5 kJ mol(-1), Delta(f)Ho(298K)[C(3)H(7)NH(2)(g)] = -70.8 +/- 1.5 kJ mol(-1), Delta(f)Ho(298K)[C(3)H(7)*] = 101.3 +/- 1 kJ mol(-1), and Delta(f)Ho(298K)[i-C(3)H(7)*] = 88.5 +/- 1 kJ mol(-1). The TPEPICO and the ab initio results for butylamine do not agree within 1 kJ mol-1; therefore, no new heat of formation is proposed for butylamine. It is nevertheless indicated that the previous experimental heats of formation of methylamine, propylamine, butylamine, and isobutylamine may have been systematically underestimated. On the other hand, the error in the ethyl radical heat of formation is found to be overestimated and can be decreased to +/- 1 kJ mol(-1); thus, Delta(f)Ho(298K)[C(2)H(5).] = 120.7 +/- 1 kJ mol(-1). On the basis of the data analysis, the heat of formation of the methylenimmonium ion is confirmed to be Delta(f)Ho(298K)[CH(2)NH(2)+] = 750.3 +/- 1 kJ mol(-1).     

辅酶B_(12)模型化合物的研究(V)RCo(Salen)L配合物Co—C键断裂的动力学

运用光度法研究了RCo(Salen)L配合物在甲醇中热分解反应动力学;测定了Co—C键断裂的速率常数及活化能,得到表观速率常数顺序为i-C3H7>i-C4H9>n-C4H9>n-C3H7>C2H5,活化能顺序为i-C3H7相似文献   

Two- and three-body photodissociation of gas phase I(3) (-)

The photodissociation dynamics of I3- from 390 to 290 nm (3.18 to 4.28 eV) have been investigated using fast beam photofragment translational spectroscopy in which the products are detected and analyzed with coincidence imaging. At photon energies < or = 3.87 eV, two-body dissociation that generates I- + I2(A 3Pi1) and vibrationally excited I2- (X 2Sigmau+) + I(2P(3/2)) is observed, while at energies > or = 3.87 eV, I*(2P(1/2)) + I2- (X 2Sigmau+) is the primary two-body dissociation channel. In addition, three-body dissociation yielding I- +2I(2P(3/2)) photofragments is seen throughout the energy range probed; this is the dominant channel at all but the lowest photon energy. Analysis of the three-body dissociation events indicates that this channel results primarily from a synchronous concerted decay mechanism.     

State-to-state correlated study of CD3I photodissociation at 266 and 304 nm

High-resolution photofragment translational spectroscopy is used in this work to measure the translational and internal energy distributions in the CD3 and iodine fragments produced from the photodissociation of CD3I at 266 and 304 nm. Channel selected detection, via resonantly enhanced multiphoton ionization, combined with one-dimensional core sampling provides detailed information about vibrational state distributions of the CD3 fragments. The vibrational state distributions of CD3 fragments in the I*(2P12) channel have a propensity of nu2 ' umbrella bending mode with a maximum at nu2 ' = 1 for 266 nm photodissociation. For I*(2P12) channel at 304 nm photodissociation, vibrational state distributions of CD3 fragment have a maximum in the vibrational ground state. For the I(2P32) channel (1Q1 <-- 3Q(0+)), nu2 ' umbrella bending vibrational distribution is measured as the predominant vibrational mode but has a much broader distribution when compared to that of the I* channel. The vibrational state distributions of the CD3 fragment produced from the perpendicular transition, i.e., 3Q1, which was determined at 304 nm photodissociation, has a maximum at nu2 ' = 1. The curve crossing possibility between the 1Q1 and 3Q(0+) adiabatic potentials is determined as 0.19 for 266 and 0.85 for 304 nm. The trend in reaction dynamics in 266 and 304 nm photodissociation of CD3I is compared with theoretical calculations. A bond dissociation energy D0(C-I) = 56.60+/-0.5 kcal/mol was derived by applying laws of energy conservation.     

Structures, electron affinities, and harmonic vibrational frequencies of the simplest alkyl peroxyl radicals and their anions

The molecular structures and electron affinities of the R-OO/R-OO(-) (R = CH3, C2H5, n-C3H7, n-C4H9, n-C5H11, i-C3H7, t-C4H9) species have been determined using seven different density functional or hybrid Hartree-Fock density functional methods. The basis set used in this work is of double-zeta plus polarization quality with additional diffuse s-type and p-type functions, denoted DZP++. The geometries are fully optimized with each density functional theory method. Harmonic vibrational frequencies were found to be within 3.1% of available experimental values for most functionals. Two different types of the neutral-anion energy separations reported in this work are the adiabatic electron affinity and the vertical detachment energy. The most reliable adiabatic electron affinities obtained at the DZP++ BP86 level of theory are 1.150 (CH3OO), 1.124 (C2H5OO), 1.146 (n-C3H7OO), 1.173 (n-C4H9OO), 1.184 (n-C5H11OO), 1.145 (i-C3H7OO), and 1.114 eV (t-C4H9OO). Compared with the experimental values, the average absolute error of the BPW91 method is 0.05 eV.     

The addition of hydrogen atoms to diacetylene and the heats of formation of i-C4H3 and n-C4H3

In this article, we discuss in detail the addition of hydrogen atoms to diacetylene and the reverse dissociation reactions, H + C(4)H(2)<==>i-C(4)H(3) (R1) and H + C(4)H(2)<==>n-C(4)H(3) (R2). The theory utilizes high-level electronic structure methodology to characterize the potential energy surface, Rice-Ramsperger-Kassel-Marcus (RRKM) theory to calculate microcanonical/J-resolved rate coefficients, and a two-dimensional master-equation approach to extract phenomenological (thermal) rate coefficients. Comparison is made with experimental results where they are available. The rate coefficients k1(T, p) and k2(T, p) are cast in forms that can be used in chemical kinetic modeling. In addition, we predict values of the heats of formation of i-C(4)H(3) and n-C(4)H(3) and discuss their importance in flame chemistry. Our basis-set extrapolated, quadratic-configuration-interaction with single and double excitations (and triple excitations added perturbatively), QCISD(T), predictions of these heats of formation at 298 K are 130.8 kcal/mol for n-C(4)H(3) and 119.3 kcal/mol for the i-isomer; multireference CI calculations with a nine-electron, nine-orbital, complete-active-space (CAS) reference wavefunction give just slightly larger values for these parameters. Our results are in good agreement with the recent focal-point analysis of Wheeler et al. (J. Chem. Phys. 2004, 121, 8800-8813), but they differ substantially for DeltaH0(f 298)(n-C(4)H(3)) with the earlier diffusion Monte Carlo predictions of Krokidis et al. (Int. J. Chem. Kinet.2001, 33, 808-820).     

Product branching ratios in photodissociation of phenyl radical: a theoretical ab initio/Rice-Ramsperger-Kassel-Marcus study

Ab initio CCSD(T)/CBS//B3LYP/6-311G** calculations of the potential energy surface for possible dissociation channels of the phenyl radical are combined with microcanonical Rice-Ramsperger-Kassel-Marcus calculations of reaction rate constants in order to predict statistical product branching ratios in photodissociation of c-C(6)H(5) at various wavelengths. The results indicate that at 248 nm the photodissociation process is dominated by the production of ortho-benzyne via direct elimination of a hydrogen atom from the phenyl radical. At 193 nm, the statistical branching ratios are computed to be 63.4%, 21.1%, and 14.4% for the o-C(6)H(4) + H, l-C(6)H(4) ((Z)-hexa-3-ene-1,5-diyne) + H, and n-C(4)H(3) + C(2)H(2) products, respectively, in a contradiction with recent experimental measurements, which showed C(4)H(3) + C(2)H(2) as the major product. Although two lower energy pathways to the i-C(4)H(3) + C(2)H(2) products are identified, they appeared to be kinetically unfavorable and the computed statistical branching ratio of i-C(4)H(3) + C(2)H(2) does not exceed 1%. To explain the disagreement with experiment, we optimized conical intersections between the ground and the first excited electronic states of C(6)H(5) and, based on their structures and energies, suggested the following photodissociation mechanism at 193 nm: c-C(6)H(5) 1 → absorption of a photon → electronically excited 1 → internal conversion to the lowest excited state → conversion to the ground electronic state via conical intersections at CI-2 or CI-3 → non-statistical decay of the vibrationally excited radical favoring the formation of the n-C(4)H(3) + C(2)H(2) products. This scenario can be attained if the intramolecular vibrational redistribution in the CI-2 or CI-3 structures in the ground electronic state is slower than their dissociation to n-C(4)H(3) + C(2)H(2) driven by the dynamical preference.     

两种烷基碘化物分子理论研究及其发射谱测量

利用密度泛函理论(DFT)的B3LYP方法, 对烷基碘化物分子C2H2F3I和n-C3H4F3I的C—I解离势能曲线进行了理论计算, 并采用B3LYP方法和MPn(n=2, 3, 4)方法精确计算了C—I键解离能. 解离能计算进行了零点振动能(ZPVE)校正, 并运用完全均衡校正法对基函数重叠误差(BSSE)进行校正. 利用微波放电激励方法, 对C2H2F3I和n-C3H4F3I的发射谱进行观测. 实验结果表明, 通过微波放电激励这两种分子, 均可产生1315 nm发射谱, 说明利用微波放电可使C2H2F3I和n-C3H4F3I分子的C—I键解离, 从而产生碘原子.     

n-C3H7I和i-C3H7I的激光裂解

Studies on the photofragmentation of n-C_3H_7I and i-C_3H_7I have been carried out by a photofragment spectrometer with rotatable pulsed molecular beam crossed with KrF excimer laser beam. TOF spectra of the iodine atom fragments (Fig.1, 2) which show the separation of the primary photodissociation channels n-C_3H_7I→n-C_3H_7+I~*(~2P_(1/2)) n-C_3H_7+I(~2p_(3/2)) i-C_3H_7I→i-C_3H_7+I~*(~2P_(1/2)) i-C_3H_7+I(~2P_(3/2)) are obtatned at 12 different angles. The distribution of total translational energy E_(CM) of recoiling photofragments are then determined. The ratios I~*/I of the photodissociation channels of n-C_3H_7I and i-C_3H_7I are measured to be 1.61 and 0.96 respectively (Table 1). The ratios I~*/I obtained by photofragment translational energy measurement in this Lab~[3]. are good in agreement with most of results abtained by IR emission~[5] and LIF~[4] measurement except the data of i-C_3H_7I, which is much different from I~*/I=0.35 reported by Bershon~[4] using LIF method.The extent of internal alkyl fragment excitation E_(int)~R is also determined (Table 2) by energy balance. The fraction of the available energy (E_(av1)=E_(CM)+E_(int)~R) which goes into internal exciatation of the alkyl fragment increases from 12.5% for I~* channe of CH_3I to 64% for both channels of i-C_3H_7I. The results are consistent with direct impulsive dynamic model of unimolecular decomposition based on “soft” alkyl radicals.The facts that the ratio I~*/I decreases with increasing carbon atoms and that the difference of the internal excitation of alkyl radicals between the I~* (~2P_(1/2)) and I (~2P_(3/2)) channelsincreases with increasing carbon atoms should be related and important for better understanding the origin of the I(~2P_(3/2)) channel caused by the potential energy surface crossing~[3]. The results of the internal excitation of the alkyl radicals in the photodissociation are also valuable for prediction of secondary products during the UV photolysis of those alkyl iodides in the gas cell.     

Effects of alkyl substitution on the energetics of enolate anions and radicals.

The photoelectron spectra of the structural isomers of the three- and four-carbon enolate anions, n-C3H5O(-), i-C3H5O(-), n-C4H7O(-), s-C4H7O(-), and i-C4H7O(-) have been measured at 355 nm. Both the X(2A' ') ground and A(2A') first excited states of the corresponding radicals were accessed from the X(1A') ground state of the enolate anions. The separation energies of the ground and first excited states (T0) were determined: T0[(E)-n-C3H5O] = 1.19 +/- 0.02 eV, T0[(Z)-n-C3H5O] = 0.99 +/- 0.02 eV, T0[i-C3H5O] = 1.01 +/- 0.02 eV, T0[n-C4H7O] = 1.19 +/- 0.02 eV, T0[(2,3)-s-C4H7O] = 1.25 +/- 0.02 eV, T0[(1,2)-s-C4H7O] = 0.98 +/- 0.02 eV, and T0[i-C4H7O] = 1.36 +/- 0.02 eV. The effects of alkyl substitution on the vibronic structure and energetics previously observed in the vinoxy radical are discussed. The X(1A')-X(2A' ') relative stability is strongly influenced by substitution whereas the X(1A')-A(2A') relative stability remains nearly constant for all of the observed structural isomers. Alkyl substitution at the carbonyl carbon affects vibronic structure more profoundly than the energetics, while the converse is observed upon alkyl substitution at the alpha carbon.     

Photodissociation of isobutene at 193 nm

The collisionless photodissociation dynamics of isobutene (i-C(4)H(8)) at 193 nm via photofragment translational spectroscopy are reported. Two major photodissociation channels were identified: H + C(4)H(7) and CH(3) + CH(3)CCH(2). Translational energy distributions indicate that both channels result from statistical decay on the ground state surface. Although the CH(3) loss channel lies 13 kcal mol(-1) higher in energy, the CH(3):H branching ratio was found to be 1.7 (5), in reasonable agreement with RRKM calculations.