Quantum mechanical studies on the singlet and triplet potential energy surface of the reactions CF3O2 + I,CF3O + OI and CF3 + OIO

The singlet and triplet potential energy surfaces for the reactions CF₃O₂ + I (1), CF₃O + OI (2) and CF₃ + OIO (3) are investigated using ab initio quantum mechanical methods. Four important isomeric energy minima were found, three on the singlet surface, CF₃OOI, CF₃OIO and CF₃IO₂ and one on the triplet surface ³CF₃OIO. CF₂O + FOI are shown to be the most probable products for all reactions, CF₃O +I and CF₃O + O(³P) are possible for reactions (2) and (3) while the reaction pathway leading to CF₃O +OI is also possible for reaction (3).     

A density functional theory study of CF3CH2I adsorption and reaction on Ag(111)

The adsorption and reaction behaviors of CF₃CH₂I on Ag(111) were systematically studied by density functional theory (DFT) calculations. Physical adsorption of CF₃CH₂I on Ag(111) occurs due to the weak interactions between surface Ag atoms and iodine atom of CF₃CH₂I; while strong chemisorption occurs for CF₃CH₂ fragment on Ag(111). Electronic analysis indicates that the singly occupied molecular orbital (SOMO) of CF₃CH₂ strongly interacts with the surface Ag atoms. It is very interesting to find that the most stable structures of CF₃CH₂ on Ag(111) locate at the top site, instead of the hollow sites. This might be attributed to the facts that CF₃CH₂ adsorbed at the top site will maximize the sp³-type hybridization, and the possible weak interaction between the fluorine lone pair electrons of p orbitals for CF₃CH₂ and surface Ag(111) occurs, which is supported by the charge density difference (CDD) analysis with a low isosurface value. We propose that the charge density difference (CDD) analysis with a high or low isosurface value can be widely applied to analyze the strong or weak electronic interactions upon adsorption. Transition state calculations suggested that the energy barrier of CF bond rupture for CF₃CH₂I on Ag(111) (1.44 eV) is much higher than that of CI bond breakage for CF₃CH₂I (0.43 eV); and the activation energy of the CF bond dissociation for CF₃CH₂(a) is 0.67 eV.     

Reactive scattering of a supersonic deuterium atom beam : D+Br2, CF3I,C2F5I,C3F7I

Reactive scattering of D atoms with Br₂, CF₃I, C₂F₅I and C₃F₇I molecules has been studied at an initial translational energy E = 5·3 kJ mol^-1 using a supersonic beam of D atoms seeded in He. Deuterium halide product is scattered in the backward direction with respect to the incident D atoms, with substantial energy disposed into product translation in all cases. The fraction of the total available energy which is disposed into product translation, is greater for the fluoralkyl iodides than for Br₂ and decreases slightly along the series CF₃I, C₂F₅I, C₃F₇I. These results demonstrate that the reaction of D atoms with fluoroalkyl iodides occurs only in collisions at small impact parameters with strong repulsion between the reaction products. The potential energy surfaces for the reactions of the fluoroalkyl iodides are similar to that for the D + Br₂ reaction but involve the release of a still greater proportion of the reaction exoergicity in the exit valley of the surface. Only a small fraction, if any, of the product repulsion is transferred to internal excitation of the product fluoroalkyl radical.     

Electron spectroscopy with excited atoms and photons.: X. Halogen-substituted methanes and methyl amines

Electron binding energy spectra have been measured for CH₃Cl, CH₃Br, CH₃I, CFCl₃, CF₂Cl₂, CF₃Cl, CF₄, CH₃NH₂, (CH₃)₂ NH and (CH₃)₃N. Measurements have been made using 584 Å (21.22 eV) photons as well as with 2³S(19.82 eV) and 2¹S(20.62 eV) metastable helium atoms. Relative spectral intensities are compared for photoionization and Penning ionization.     

Quantitative infrared photochemistry with CO2 laser of different temporal shape: Dissociation of CF3I with nanosecond pulses

Infrared laser pulses of different temporal pulse shape with a length of 2–50 ns and a nominal power of 100–1000 MW are generated by a TEA CO₂ laser oscillator combination using saturable intracavity absorbers and a fast CdTe electro-optical switch. The multiphoton dissociation of CF₃I + nhv → CF₃ + I at 1074.6 cm⁻¹ and p(CF₃I) = 10 Pa was studied using these pulses. The product yields and the absolute rate constants were determined for different temporal pulse profiles, showing clear intensity effects at low intensities and a transition to a linear intensity regime at higher intensities.     

Features of kinetic processes in the active medium of a pulsed chemical oxygen-iodine laser

Conclusion It can be concluded from our experiments and calculations that the product CF₃O₂ of the interaction between the CF₃ radical and the O₂ molecule quenches the oxygen O₂(¹) more strongly. At low chlorine admixture density in the singlet-oxygen stream this output energy of the oxygen-iodine laser with CF₃I as the atomic iodine donor is lower compared with CH₃I. The rate constant of quenching singlet oxygen by CF₃O₂ molecules is (3–5)·10^–11 cm³·sec^–1. It would be possible to decrease the influence of CF₃O₂ by adding to the initial O₂ ^*–O₂–CF₃I–Ar active mixture some other substance causing the CF₃ radicals to enter in a chemical reaction with a shorter characteristic time than that for CF₃O₂ formation. Of course, neither the initial substance nor the reaction products should quench O₂ ^* noticeably. This role can be possibly assumed by the NO molecule.The influence of the chlorine additive on the output energy of a laser with CH₃I and CF₃I differs greatly. The choice of the chlorine donor must therefore be determined by the amount of this additive. CH₃I is preferable if the chlorine is fully utilized in the singlet-oxygen laser, and CF₃I in the opposite case.Quantum Radiophysics Division, Lebdev Physics Institute. Translation of Preprint No. 21 of the Lebedev Physics Institute, Moscow, 1991.     

Reactions induced in (CF3I) n clusters by femtosecond UV laser pulses

The excitation and ionization of CF₃I molecules and their clusters by femtosecond UV laser pulses is studied. It is concluded that the types of excitation of free CF₃I molecules and their clusters by femtosecond UV laser pulses are different. The composition and kinetic energy of ion products observed upon the ionization of (CF₃I) _n clusters by femtosecond pulses are found to differ considerably from those obtained upon ionization by nanosecond pulses. It is shown that the molecular I ₂ ⁺ ion is produced in reactions induced in (CF₃I) _n clusters by UV radiation. Using the pump-probe method, we found the two channels of producing I ₂ ⁺ ions with characteristic times ??₁ ?? 1 ps and ??₂ ?? 7 ps. A model of the reactions under study proposed in the paper is consistent with our experimental results.     

Multiphoton dissociation of CF<Subscript>3</Subscript>I clusters by IR laser radiation

The results of investigation of dissociation of (CF₃I) _n clusters upon resonant excitation of vibrations of CF₃I molecules by pulsed CO₂ laser radiation are reported. The kinetics of dissociation of such clusters is studied and the dissociation yield is measured. It is shown that its value is completely controlled by fluence Φ_IR of transmitted IR radiation energy and its time dependence is exponential. The results of measurements show that the dissociation lifetime of clusters is shorter than 10^?8 s. The velocity and translational temperature of free CF₃I molecules formed during dissociation of clusters are measured, as well as the dependence of these parameters on Φ_IR. It is concluded that, under these conditions of IR excitations, the dissociation of (CF₃I) _n clusters can be treated as consecutive evaporation of molecules.     

Improved isotope separation in laser dissociation of trifluoromethyl halides: Scavenging of CF3 with HI

The efficiency of¹³C isotope separation by multiphoton dissociation using a CO₂ laser has been shown to increase by ∼33% upon addition of small amounts of HI to the starting mixture. The reason for this increase is that the CF₃ radicals are scavenged by HI more rapidly than they recombine with free iodine atoms to reform the starting material. For Torr and for 0.5J/cm² roughly 65% of the CF₃ and I radicals reform the starting material in the absence of HI. The results of this study indicate that at −80°C the rate constants for CF₃+CF₃→C₂F₆ and CF₃+I→CF₃I are about equal and are roughly 8 times higher than that for CF₃+HI→CF₃H+I.     

Isotopically selective infrared multiphoton dissociation of 2-chloro-1,1,1-trifluoroethane: 13C selectivity and mechanism

13 C-selective infrared multiphoton dissociation of CF₃CH₂Cl has been studied by analyzing the distribution of ¹³C concentrations of the main products CF₂=CHCl, CF₂=CH₂, CF₂=CHF, C₂F₆, and the trace products CF₃CH₂CF₃ and CF₃CH=CHF₃. The mechanism mainly concerns the dissociation of energized CF₃CH₂Cl, the collisional stabilization of excited CF₃CH and CF₃CH₂ and the recombination of the nascent radicals. No significant radical–molecule reactions degrade the intrinsic ¹³C dissociation selectivity. High ¹³C production yield and ¹³C concentration can be attained at a laser fluence of 1.6 J/cm². Such low fluence can be used to improve focus condition and enhance photon utilization efficiency for practicable ¹³C separation. Received: 10 March 1998/Revised version: 17 September 1998     

Multiphoton ionization of iodine atoms and CF<Subscript> 3</Subscript>I molecules by XeCl laser radiation

We report about effective ionization of iodine atoms and CF₃I molecules under the action of intense XeCl laser radiation (308 nm). The only ion fragment resulting from the irradiation of the CF₃I molecules is the I⁺ ion. We have studied the influence of the intensity, spectral composition, and polarization of the laser radiation used on the intensity of the ion signal and the shape of its time-of-flight peak. Based on the analysis of the results obtained, we have suggested the mechanism of this effect. The conclusion drawn is that the ionization of the iodine atoms by the ordinary XeCl laser with a nonselective cavity results from a three- (2 + 1)-photon REMPI process. This process is in turn due to the presence of accidental two-photon resonances between various spectral components of the laser radiation and the corresponding intermediate excited states of the iodine atom. The probability of ionization of the atoms from their ground state I(²P_3/2) by the radiation of the ordinary XeCl laser is more than two orders of magnitude higher than the probability of their ionization from the metastable state I^*(²P_1/2). The ionization of the CF₃I molecules by the XeCl laser radiation occurs as a result of a four-photon process involving the preliminary one-photon dissociation of these molecules and the subsequent (2 + 1)-photon REMPI of the resultant neutral iodine atoms.     

A study of the valence shell photoionisation dynamics of CH3CN and CF3CN

Valence shell angle resolved photoelectron spectra of CH₃CN and CF₃CN have been recorded in the photon energy range 14–120 eV, thereby allowing asymmetry parameters and branching ratios to be derived. The carbon and nitrogen K-shell photoabsorption spectra of these two molecules exhibit features ascribed to shape resonantly enhanced transitions. Energy dependent variations observed in the asymmetry parameters and branching ratios provide evidence of shape resonances influencing the valence shell photoionisation dynamics. In addition to the main lines associated with single-hole states, complex satellite structure appears in the inner valence region of the photoelectron spectrum due to many-electron effects. The experimental spectra have been interpreted using previously reported ionisation energies and spectral intensities obtained from Green's function calculations.     

Photolysis of compounds with P−I and As−I bonds

Detailed experimental investigations are reported of the parameters of lasing on the transition²P_1/2→²P_3/2, λ = 1.3 μm of atomic iodine upon photodissociation of the molecules (CF₃)₂AsI and (CF₃)₂PI. The light source was an IFP-5000 flash lamp. The experimental data are used to determine the sequence of the basic reactions that accompany the photolysis of the molecules (CF₃)₂AsI and (CF₃)₂PI and to determine the energy parameters of the generated radiation.     

Optoacoustic cross-section measurements for ir pulses: CO2 laser absorption by CF3I

The pulsed optoacoustic method is studied to measure absolute infrared multiphoton absorption cross sections. The influence of the thermalisation by the walls of the cell is shown to be very important at low pressure. This influence is analysed both experimentally and theoretically by solving the coupled diffusion and relaxation equations for vibrational and translational energies. The sensitivity of the method is limited by a spurious pressure signal present even with non-absorbing gases. This parasitic signal is attributed to an absorption located on the inner surface of the windows of the cell. For instance, using KCl windows, the observed spurious signal corresponds to about 10⁻⁴ of the total laser energy, transmitted from the windows to the gas. This proportion is independent on the energy fluence. Taking care of these limitations, we have measured the absorption cross section of CF₃I at the different wavelengths of the (001–020) transitions of the CO₂ laser. The typical energy fluences in these experiments were varied from 10⁻³ to 1J/cm². To be in collision free conditions, the CF₃I pressure was made equal to 0.2 Torr.     

Energy relaxation through the π*-state in C,F and O K-shell excited CF3COCH3

Total photoabsorption spectra of CF₃COCH₃ were measured in the C, F and O K-shell regions and the peak assignments were tentatively given. The K-shell electrons of C, F and O atoms were selectively excited into the π^* orbital. The kinetic energy (KE) distribution of CF₃⁺ formed through the π^* states gave the maxima at KE = 0 and 0.43 eV. The yield of CF₃⁺ with KE = 0 eV increased from 10 to 50% by changing the excitation sites from F 1s to O 1s. This finding was reasonably understood by considering that intramolecular energy flows from the initially excited K-shell electron to vibrational modes of CF₃ group. The KE distribution of CH₃⁺ showed a mirror image of that for CF₃⁺.     

Infrared absorption of iodomethylperoxy (syn-ICH2OO) radical generated upon photolysis of CH2I2 and O2 in solid para-H2

Irradiation of a para-hydrogen (p-H₂) matrix containing diiodomethane (CH₂I₂) and O₂ at 3.2 K with light at 280 ± 20 nm, followed by annealing of the matrix at 4.0 K, yielded infrared (IR) absorption lines at 2982.4, 1408.9, 1231.8, 1226.5/1225.6, 1085.6, 917.7, 841.6/841.1, 550.5, and 490.2 cm^?1 that are assigned to the syn-iodomethylperoxy (syn-ICH₂OO) radical. Further irradiation of the matrix at 365 nm diminished these features. Experiments with CH₂I₂ and ¹⁸O₂ yielded lines of syn-ICH₂¹⁸O¹⁸O at 1407.3, 1228.1/1227.7, 1217.7/1217.0, 1031.5, 899.9/899.4, 836.7/836.0, and 473.6 cm^?1. The assignments are based on the photolytic behaviour and comparisons of observed vibrational wavenumbers, IR intensities, and ¹⁸O-isotopic shifts with those predicted with the B3LYP/aug-cc-pVTZ-pp method. The observation is consistent with a mechanism that, upon photolysis of CH₂I₂ at 280 nm, CH₂I was formed and subsequently reacted with O₂ to yield syn-ICH₂OO. Compared with the gaseous reaction CH₂I + O₂ → CH₂OO + I at low pressure, observation of ICH₂OO in a p-H₂ matrix instead of CH₂OO in the gaseous phase indicates that the excess energy of internally excited ICH₂OO, produced upon reaction of CH₂I + O₂, was rapidly quenched in the matrix so that ICH₂OO became readily stabilised without further decomposition to form CH₂OO + I.     

From quantum chemistry to dissociation kinetics: what we need to know †

The relationship between rate constants for dissociation and the reverse association reactions and their potential energy surfaces is illustrated. The reaction systems e^? + SF₆ ? SF₆ ^? →SF₅ ^? + F, H + CH₃ ?CH₄, 2 CF₂ ? C₂F₄, H + O₂ →HO₂, HO + O ?HO₂ ? H + O₂, and C + HO →CHO are chosen as representative examples. The necessity to know precise thermochemical data is emphasised. The interplay between attractive and anisotropic components of the potentials influences the rate constants. Spin–orbit and electronic–rotational coupling in reactions between electronic open-shell radicals so far generally has been neglected, but is shown to have a marked influence on low temperature rate constants.     

克里奇中间体CH2OO与CF3CF=CF2反应的动力学研究

本文使用OH激光诱导荧光方法研究了结构最简单的克里奇中间体CH₂OO和CF₃CF=CF₂的反应动力学. 在压强为10 Torr条件下,测量了温度在283,298,308和318 K的反应速率常数,分别为(1.45±0.14)×10^-13,(1.18±0.11)×10^-13,(1.11±0.08)×10^-13和(1.04±0.08)×10^-13 cm³·molecule^-1·s^-1. 根据阿伦尼乌斯方程,获得该反应的活化能为(-1.66±0.21) kcal/mol. 在6.3∽70 torr压力范围内,未观察到该反应的速率常数存在压力相关.     

Theoretical study on the reactions of CH3NHNH2 with ground state O(3P) atom and excited state O(1D) atom

The reaction mechanisms of methylhydrazine (CH₃NHNH₂) with O(³P) and O(¹D) atoms have been explored theoretically at the MPW1K/6-311+G(d,p), MP2/6-311+G(d,p), MCG3-MPWPW91 (single-point), and CCSD(T)/cc-pVTZ (single-point) levels. The triplet potential energy surface for the reaction of CH₃NHNH₂ with O(³P) includes seven stable isomers and eight transition states. When the O(³P) atom approaches CH₃NHNH₂, the heavy atoms, namely N and C atoms, are the favourable combining points. O(³P) atom attacking the middle-N atom in CH₃NHNH₂ results in the formation of an energy-rich isomer (CH₃NHONH₂) followed by migration of O(³P) atom from middle-N atom to middle-H atom leading to the product P6 (CH₃NNH₂+OH), which is one of the most favourable routes. The estimated major product CH₃NNH₂ is consistent with the experimental measurements. Reaction of O(¹D) + CH₃NHNH₂ presents different features as compared with O(³P) + CH₃NHNH₂. O(¹D) atom will first insert into C–H2, N1–H4, and N2–H5 bonds barrierlessly to form the three adducts, respectively. There are two most favourable paths for O(¹D) + CH₃NHNH₂. One is that the C–N bond cleavage accompanied by a concerted H shift from O atom to N atom (mid-N) leads to the product P_I (CH₂O + NH₂NH₂), and the other is that the N–N bond rupture along with a concerted H shift from O to N (end-N) forms P_IV (CH₃NH₂ + HNO). The similarities and discrepancies between two reactions are discussed.     

Collisional enhancement of the I(52P12) emission at 1.3μm by parent molecules CF3I,C2F5I,i-C3F7I and n-C3F7I

The magnetic dipole transition I(5²P_{$\text{1}\text{2}$}-5²P_{$\text{3}\text{2}$}) at 1.3152 μm is shown to be enhanced by collisions of the metastable iodine atoms with the parent RI molecules CF₃I, C₂F₅I, i-C₃F₇I and n-C₃F₇I. The enhancing mechanism is exciplex emission of the RI·I(5²P_{$\text{1}\text{2}$}) molecule at 1.3 μm, with different rates for each iodide. The influence of this effect on the measurement of the quantum yields to I(5²P_{$\text{1}\text{2}$}) and of the respective reaction rates by infrared fluorescence is discussed.