Four-dimensional quantum study on exothermic complex-forming reactions: Cl- + CH3Br-->ClCH3+Br-

The exothermic gas-phase bimolecular nucleophilic substitution (S(N)2) reaction Cl(-)+CH(3)Br (upsilon1',upsilon2',upsilon3')-->ClCH(3) (upsilon1,upsilon2,upsilon3)+Br- and the corresponding endothermic reverse reaction have been studied by time-independent quantum scattering calculations in hyperspherical coordinates on a coupled-cluster potential-energy surface. The dimensionality-reduced model takes four degrees of freedom into account [Cl-C and C-Br stretching modes (quantum numbers upsilon3' and upsilon3); totally symmetric modes of the methyl group, i.e., C-H stretching (upsilon1' and upsilon1) and umbrella bending vibrations (upsilon2' and upsilon2)]. Diagonalization of the Hamiltonian was performed employing the Lanczos algorithm with a variation of partial reorthogonalization. A narrow grid in the total energy was employed so that long-living resonance states could be resolved and extracted. While excitation of the reactant umbrella bending mode already leads to a considerable enhancement of the reaction probability, its combination with vibrational excitation of the broken C-Br bond, (0, 1, 1), results in a strong synergic effect that can be rationalized by the similarity with the classical transitional normal mode. Exciting the C-H stretch has a non-negligible effect on the reaction probability, while for larger translational energies this mode follows the expected spectatorlike behavior. Combination of C-Br stretch and symmetric C-H, (1,0,1), stretch does not show a cooperative effect. Contrary to the spectator mode concept, energy originally stored in the C-H stretching mode is by no means conserved, but almost completely released in other modes of the reaction products. Products are most likely formed in states with a high degree of excitation in the new C-Cl bond, while the internal modes of the methyl group are less important. Reactants with combined umbrella/C-Br stretch excitation, (0, 1, 1), may yield products with two quanta in the umbrella mode.     

Reaction dynamics and vibrational spectroscopy of CH3D molecules with both C-H and C-D stretches excited

State-resolved reactions of CH3D molecules containing both C-H and C-D stretching excitation with Cl atoms provide new vibrational spectroscopy and probe the consumption and disposal of vibrational energy in the reactions. The vibrational action spectra have three different components, the combination of the C-H symmetric stretch and the C-D stretch (nu1 + nu2), the combination of the C-D stretch and the C-H antisymmetric stretch (nu2 + nu4), and the combination of the C-D stretch and the first overtone of the CH3 bend (nu2 + 2nu5). The simulation for the previously unanalyzed (nu2 + nu4) state yields a band center of nu0 = 5215.3 cm(-1), rotational constants of A = 5.223 cm(-1) and B = 3.803 cm(-1), and a Coriolis coupling constant of zeta = 0.084. The reaction dynamics largely follow a spectator picture in which the surviving bond retains its initial vibrational excitation. In at least 80% of the reactive encounters of vibrationally excited CH3D with Cl, cleavage of the C-H bond produces CH2D radicals with an excited C-D stretch, and cleavage of the C-D bond produces CH3 radicals with an excited C-H stretch. Deviations from the spectator picture seem to reflect mixing in the initially prepared eigenstates and, possibly, collisional coupling during the reaction.     

Vibrationally controlled chemistry: mode- and bond-selected reaction of CH3D with Cl

Selective vibrational excitation controls the competition between C-H and C-D bond cleavage in the reaction of CH(3)D with Cl, which forms either HCl + CH(2)D or DCl + CH(3). The reaction of CH(3)D molecules with the first overtone of the C-D stretch (2nu(2)) excited selectively breaks the C-D bond, producing CH(3) exclusively. In contrast, excitation of either the symmetric C-H stretch (nu(1)), the antisymmetric C-H stretch (nu(4)), or a combination of antisymmetric stretch and CH(3) umbrella bend (nu(4) + nu(3)) causes the reaction to cleave only a C-H bond to produce solely CH(2)D. Initial preparation of C-H stretching vibrations with different couplings to the reaction coordinate changes the rate of the H-atom abstraction reaction. Excitation of the symmetric C-H stretch (nu(1)) of CH(3)D accelerates the H-atom abstraction reaction 7 times more than excitation of the antisymmetric C-H stretch (nu(4)) even though the two lie within 80 cm(-1) of the same energy. Ab initio calculations and a simple theoretical model help identify the dynamics behind the observed mode selectivity.     

Bond and mode selectivity in the reaction of atomic chlorine with vibrationally excited CH2D2

The title reaction is investigated by co-expanding a mixture of Cl2 and CH2D2 into a vacuum chamber and initiating the reaction by photolyzing Cl2 with linearly polarized 355 nm light. Excitation of the first C-H overtone of CH2D2 leads to a preference for hydrogen abstraction over deuterium abstraction by at least a factor of 20, whereas excitation of the first C-D overtone of CH2D2 reverses this preference by at least a factor of 10. Reactions with CH2D2 prepared in a local mode containing two quanta in one C-H oscillator /2000>- or in a local mode containing one quantum each in two C-H oscillators /1100> lead to products with significantly different rotational, vibrational, and angular distributions, although the vibrational energy for each mode is nearly identical. The Cl+CH2D2/2000>- reaction yields methyl radical products primarily in their ground state, whereas the Cl+CH2D2/1100> reaction yields methyl radical products that are C-H stretch excited. The HCl(v=1) rotational distribution from the Cl+CH2D2/2000>- reaction is significantly hotter than the HCl(v=1) rotational distribution from the Cl+CH2D2/1100> reaction, and the HCl(v=1) differential cross-section (DCS) of the Cl+CH2D2/2000>- reaction is more broadly side scattered than the HCl(v=1) DCS of the Cl+CH2D2/1100> reaction. The results can be explained by a simple spectator model and by noting that the /2000>- mode leads to a wider cone of acceptance for the reaction than the /1100> mode. These measurements represent the first example of mode selectivity observed in a differential cross section, and they demonstrate that vibrational excitation can be used to direct the reaction pathway of the Cl+CH2D2 reaction.     

State-to-state reaction dynamics of CH3I photodissociation at 304 nm

The detailed reaction dynamics of CH(3)I photodissociation at 304 nm were studied by using high-resolution long time-delayed core-sampling photofragment translation spectroscopy. The vibrational state distributions of the photofragment, i.e., CH(3), are directly resolved due to the high kinetic resolution of this experiment for the first time. CH(3) radicals produced from I((3)Q(0+)), I((1)Q(1) <--( 3)Q(0+)), and I((3)Q(1)) channels are populated in different vibrational state distributions. The I((3)Q(0+)) and I((3)Q(1)) channels show only progressions in the nu2'(a2") umbrella bending mode, and the I((1)Q(1) <-- (3)Q(0+)) channel shows both progression in the nu2' umbrella bending mode and a small amount of excitation in the nu1'(a1') C-H stretching mode. The photodissociation processes from the vibrational hot band of CH(3)I (upsilon3 = 1, upsilon3 = 2) were also detected, primarily because of the absorption probability from the vibrational excited states, i.e., hot bands are relatively enhanced. Photofragments from the hot bands of CH(3)I show a cold vibrational distribution compared to that from the vibrational ground state of CH(3)I. The I* quantum yield and the curve crossing possibility were also studied for the ground vibrational state of CH(3)I. The potential energy at the curve crossing point was calculated to be 32 790 cm(-1) by using the one-dimensional Landau-Zener model.     

Comparing the dynamical effects of symmetric and antisymmetric stretch excitation of methane in the Cl+CH4 reaction

The effects of two nearly isoenergetic C-H stretching motions on the gas-phase reaction of atomic chlorine with methane are examined. First, a 1:4:9 mixture of Cl(2), CH(4), and He is coexpanded into a vacuum chamber. Then, either the antisymmetric stretch (nu(3)=3019 cm(-1)) of CH(4) is prepared by direct infrared absorption or the infrared-inactive symmetric stretch (nu(1)=2917 cm(-1)) of CH(4) is prepared by stimulated Raman pumping. Photolysis of Cl(2) at 355 nm generates fast Cl atoms that initiate the reaction with a collision energy of 1290+/-175 cm(-1) (0.16+/-0.02 eV). Finally, the nascent HCl or CH(3) products are detected state-specifically via resonance enhanced multiphoton ionization and separated by mass in a time-of-flight spectrometer. We find that the rovibrational distributions and state-selected differential cross sections of the HCl and CH(3) products from the two vibrationally excited reactions are nearly indistinguishable. Although Yoon et al. [J. Chem. Phys. 119, 9568 (2003)] report that the reactivities of these two different types of vibrational excitation are quite different, the present results indicate that the reactions of symmetric-stretch excited or antisymmetric-stretch excited methane with atomic chlorine follow closely related product pathways. Approximately 37% of the reaction products are formed in HCl(v=1,J) states with little rotational excitation. At low J states these products are sharply forward scattered, but become almost equally forward and backward scattered at higher J states. The remaining reaction products are formed in HCl(v=0,J) and have more rotational excitation. The HCl(v=0,J) products are predominantly back and side scattered. Measurements of the CH(3) products indicate production of a non-negligible amount of umbrella bend excited methyl radicals primarily in coincidence with the HCl(v=0,J) products. The data are consistent with a model in which the impact parameter governs the scattering dynamics.     

Quasi-classical trajectory study of the role of vibrational and translational energy in the Cl(2P) + NH3 reaction

A detailed state-to-state dynamics study was performed to analyze the effects of vibrational excitation and translational energy on the dynamics of the Cl((2)P) + NH(3)(v) gas-phase reaction, effects which are connected to such issues as mode selectivity and Polanyi's rules. This reaction evolves along two deep wells in the entry and exit channels. At low and high collision energies quasi-classical trajectory calculations were performed on an analytical potential energy surface previously developed by our group, together with a simplified model surface in which the reactant well is removed to analyze the influence of this well. While at high energy the independent vibrational excitation of all NH(3)(v) modes increases the reactivity by a factor ≈1.1-2.9 with respect to the vibrational ground-state, at low energy the opposite behaviour is found (factor ≈ 0.4-0.9). However, when the simplified model surface is used at low energy the independent vibrational excitation of all NH(3)(v) modes increases the reactivity, showing that the behaviour at low energies is a direct consequence of the existence of the reactant well. Moreover, we find that this reaction exhibits negligible mode selectivity, first because the independent excitation of the N-H symmetric and asymmetric stretch modes, which lie within 200 cm(-1) of each other, leads to reactions with similar reaction probabilities, and second because the vibrational excitation of the reactive N-H stretch mode is only partially retained in the products. For this "late transition-state" reaction, we also find that vibrational energy is more effective in driving the reaction than an equivalent amount of energy in translation, consistent with an extension of Polanyi's rules. Finally, we find that the non-reactive events, Cl((2)P)+NH(3)(v) → Cl((2)P) + NH(3)(v'), lead to a great number of populated vibrational states in the NH(3)(v') product, even starting from the NH(3)(v = 0) vibrational ground state at low energies, which is unphysical in a quantum world. This result is interpreted on the basis of non-conservation of the ZPE per mode.     

The photodissociation reaction dynamics of CF(3)I at 304 nm ((3)Q(0+), (1)Q(1)<--(3)Q(0+), and (3)Q(1))

The photodissociation of CF(3)I at 304 nm has been studied using long time-delayed core-sampling photofragment translational spectroscopy. Due to its capability of detecting the kinetic energy distribution of iodine fragments with high resolution, it is able to directly assign the vibrational state distribution of CF(3) fragments. The vibrational state distributions of CF(3) fragments in the I(*)((2)P(12)) channel, i.e., (3)Q(0+) state, have a propensity of the nu(2) (') umbrella mode with a maximum distribution at the vibrational ground state. For the I((2)P(32)) channel, i.e., (1)Q(1)<--(3)Q(0+), the excitation of the nu(2) (') umbrella mode accounts for the majority of the vibrational excitation of the CF(3) fragments. The 1 nu(1) (') (symmetric CF stretch) +nnu(2) (') combination modes, which are associated with the major progression of the nu(2) (') umbrella mode, are observed for the photodissociation of CF(3)I at the I channel, i.e., (3)Q(1) state. The bond dissociation energy of the CI bond of CF(3)I is determined to be D(0)(CF(3)-I)相似文献   

Infrared vacuum-ultraviolet laser pulsed field ionization-photoelectron study of CH3Br+ (X(2)E(3/2))

By preparing methyl bromide (CH3Br) in selected rotational levels of the CH3Br(X(1)A1; v1 = 1) state with infrared (IR) laser excitation prior to vacuum-ultraviolet (VUV) laser pulsed field ionization-photoelectron (PFI-PE) measurements, we have observed rotationally resolved photoionization transitions to the CH3Br(+)(X(2)E(3/2); v1(+) = 1) state, where v1 and v1(+) are the symmetric C-H stretching vibrational mode for the neutral and cation, respectively. The VUV-PFI-PE origin band for CH3Br(+)(X(2)E(3/2)) has also been measured. The simulation of these IR-VUV-PFI-PE and VUV-PFI-PE spectra have allowed the determination of the v1(+) vibrational frequency (2901.8 +/- 0.5 cm(-1)) and the ionization energies of the origin band (85 028.3 +/- 0.5 cm(-1)) and the v1(+) = 1 <-- v1 = 1 band (84 957.9 +/- 0.5 cm(-1)).     

Secondary kinetic isotope effect in nucleophilic substitution: a quantum-mechanical approach

Four-dimensional time-independent quantum scattering calculations have been carried out on the perdeuterated exothermic and complex-forming gas-phase S(N)2 reaction Cl- + CD3Br --> ClCD3 + Br- and the reverse process Br- + CD3Cl --> BrCD3 + Cl-, employing a fine energetic resolution to resolve all scattering resonances. The two totally symmetric modes of the methyl group, C-D symmetric stretch and umbrella bend, are explicitly taken into account. Converged state-selected reaction probabilities and product distributions have been calculated up to 2960 cm(-1) above the vibrational ground state of CD3Br, i.e., up to initial vibrational excitation of the second overtone of the umbrella bending vibration. The inverse secondary kinetic isotope effect found experimentally is nicely confirmed by the calculated state-selected reaction probabilities. One contribution to this originates from excitation of the high-frequency symmetric C-D stretching vibration, which increases the reaction probability as a function of translational energy more than the corresponding vibration in the undeuterated system. Although transition state theory (TST) suffices to explain this effect qualitatively, the dynamics of S(N)2 reactions is well-known to show strong nonstatistical features. A striking example is given by the umbrella mode: Contrary to estimates obtained from TST, we find a significant enhancement of the reactivity in the perdeuterated system that is attributed to the increased density of states and the higher number of avoided crossings of the hyperspherical adiabats compared to the undeuterated system. Furthermore, compared to the system Cl- + CH3Cl'/CD3Cl', the influence of tunneling is negligible in this net-barrierless reaction. In the reverse endothermic reaction, the kinetic isotope effect of the umbrella mode is normal.     

Vibrationally quantum-state-specific dynamics of the reactions of CN radicals with organic molecules in solution

The dynamics of reactions of CN radicals with cyclohexane, d(12)-cyclohexane, and tetramethylsilane have been studied in solutions of chloroform, dichloromethane, and the deuterated variants of these solvents using ultraviolet photolysis of ICN to initiate a reaction. The H(D)-atom abstraction reactions produce HCN (DCN) that is probed in absorption with sub-picosecond time resolution using ～500 cm(-1) bandwidth infrared (IR) pulses in the spectral regions corresponding to C-H (or C-D) and C≡N stretching mode fundamental and hot bands. Equivalent IR spectra were obtained for the reactions of CN radicals with the pure solvents. In all cases, the reaction products are formed at early times with a strong propensity for vibrational excitation of the C-H (or C-D) stretching (v(3)) and H-C-N (D-C-N) bending (v(2)) modes, and for DCN products there is also evidence of vibrational excitation of the v(1) mode, which involves stretching of the C≡N bond. The vibrationally excited products relax to the ground vibrational level of HCN (DCN) with time constants of ～130-270 ps (depending on molecule and solvent), and the majority of the HCN (DCN) in this ground level is formed by vibrational relaxation, instead of directly from the chemical reaction. The time-dependence of reactive production of HCN (DCN) and vibrational relaxation is analysed using a vibrationally quantum-state specific kinetic model. The experimental outcomes are indicative of dynamics of exothermic reactions over an energy surface with an early transition state. Although the presence of the chlorinated solvent may reduce the extent of vibrational excitation of the nascent products, the early-time chemical reaction dynamics in these liquid solvents are deduced to be very similar to those for isolated collisions in the gas phase. The transient IR spectra show additional spectroscopic absorption features centered at 2037 cm(-1) and 2065 cm(-1) (in CHCl(3)) that are assigned, respectively, to CN-solvent complexes and recombination of I atoms with CN radicals to form INC molecules. These products build up rapidly, with respective time constants of 8-26 and 11-22 ps. A further, slower rise in the INC absorption signal (with time constant >500 ps) is attributed to diffusive recombination after escape from the initial solvent cage and accounts for more than 2/3 of the observed INC.     

Infrared and DFT investigations of the XC[triple bond]ReX3 and HC[triple bond]ReX3 complexes: Jahn-Teller distortion and the methylidyne C-X(H) stretching absorptions

The XC[triple bond]ReX3 complexes (X = F, Cl) are produced by CX(4) reaction with laser-ablated Re atoms, following oxidative C-X insertion and alpha-halogen migration in favor of the carbon-metal triple bond and are identified through the observation of characteristic absorptions in the argon matrix infrared spectra and comparison with vibrational frequencies calculated by density functional theory. The methylidyne C-F and C-Cl stretching absorptions are observed near 1584 and 1328 cm-1, and the C-H stretching modes for HC[triple bond]ReX3 at 3104 and 3097 cm(-1), respectively, which are substantially higher than the precursor stretching modes and in agreement with the general trend that higher s-orbital character in carbon hybridization leads to a higher stretching frequency. The Jahn-Teller effect in the doublet-state XC[triple bond]ReX3 and HC[triple bond]ReX3 complexes gives rise to distorted structures with Cs symmetry and two equivalent longer Re-X bonds and one slightly shorter Re-X bond.     

Reaction dynamics of CN radicals with tetrahydrofuran in liquid solutions

Transient, broadband infra-red absorption spectroscopy with picosecond time resolution has been used to study the dynamics of reactions of CN radicals with tetrahydrofuran (THF) and d(8)-THF in liquid solutions ranging from neat THF to 0.5 M THF in chlorinated solvents (CDCl(3) and CD(2)Cl(2)). HCN and DCN products were monitored via their v(1) (C≡N stretching) and v(3) (C-H(D) stretching) vibrational absorption bands. Transient spectral features indicate formation of vibrationally excited HCN and DCN, and the onsets of absorption via the fundamental bands of HCN and DCN show short (5-15 ps) delays consistent with vibrational relaxation within the nascent reaction products. This interpretation is confirmed by non-equilibrium molecular dynamics simulations employing a newly derived analytic potential energy surface for the reaction in explicit THF solvent. The rate coefficient for reactive formation of HCN (as determined from measurements on both the 1(1)(0) and 3(1)(0) fundamental bands) decreases with increasing dilution of the THF in CDCl(3) or CD(2)Cl(2), showing pseudo-first order kinetic behaviour for THF concentrations in the range 0.5-4.5 M, and a bimolecular rate coefficient of (1.57 ± 0.12) × 10(10) M(-1) s(-1) is derived. Simultaneous analysis of time-dependent HCN 1(1)(0) and 3(1)(0) band intensities following reaction of CN with THF (3.0 M) in CD(2)Cl(2) suggests that C-H stretching mode excitation is favoured, and this deduction is supported by the computer simulations. The results extend our recent demonstration of nascent vibrational excitation of the products of bimolecular reactions in liquid solution to a different, and more strongly interacting class of organic solvents. They serve to reinforce the finding that dynamics (and thus the topology of the reactive potential energy surface) play an important role in determining the nascent product state distributions in condensed phase reactions.     

Photolysis of oxalyl chloride (ClCO)2 at 193 nm: emission of CO(v<or=6, J<or=60) detected with time-resolved Fourier-transform spectroscopy

Upon photolysis of oxalyl chloride at 193 nm, time-resolved and rotationally resolved emission of CO(v相似文献   

Four-mode quantum calculations of resonance states in complex-forming bimolecular reactions: C- + CH3Br

The vibrational resonance states of the complexes formed in the nucleophilic bimolecular substitution (S(N)2) reaction Cl(-)+CH(3)Br-->ClCH(3)+Br(-) were calculated by means of the filter diagonalization method employing a coupled-cluster potential-energy surface and a Hamiltonian that incorporates an optical potential and is formulated in Radau coordinates for the carbon-halogen stretching modes. The four-dimensional model also includes the totally symmetric vibrations of the methyl group (C-H stretch and umbrella bend). The vast majority of bound states and many resonance states up to the first overtone of the symmetric stretching vibration in the exit channel complex have been calculated, analyzed, and assigned four quantum numbers. The resonances are classified into entrance channel, exit channel, and delocalized states. The resonance widths fluctuate over six orders of magnitude. In addition to a majority of Feshbach-type resonances there are also exceedingly long-lived shape resonances, which are associated with the entrance channel and can only decay by tunneling. The state-selective decay of the resonances was studied in detail. The linewidths of the resonances, and thus the coupling to the energetic continuum, increase with excitation in any mode. Due to the strong mixing of the many progressions in the intermolecular stretching modes of the intermediate complexes, this increase as a function of the corresponding quantum numbers is not monotonic, but exhibits pronounced fluctuations.     

Planar study of H2O+Cl<-->HO+HCl reactions

The first four dimensional (4D) quantum scattering calculations on the tetra-atomic H2O+Cl<-->HO+HCl reactions are reported. With respect to a full (6D) treatment, only the planar constraint and a fixed length for the HO spectator bond are imposed. This work explicitly accounts for the bending and local HO stretching vibrations in H2O, for the vibration of HCl and for the in-plane rotation of the H2O, HO and HCl molecules. The calculations are performed with the potential energy surface of Clary et al. and use a Born-Oppenheimer type separation between the motions of the light and the heavy nuclei. State-to-state cross sections are reported for a collision energy range 0-1.8 eV measured with respect to H2O+Cl. For the H2O+Cl reaction, present results agree with previous (3D) non planar calculations and confirm that excitation of the H2O stretching promotes more reactivity than excitation of the bending. New results are related to the rotation of the H2O molecule: the cross sections are maximal for planar rotational states corresponding to 10相似文献   

Imaging the nature of the mode-specific chemistry in the reaction of Cl atom with antisymmetric stretch-excited CH4

Following up our preliminary communication [Kawamata et al., Phys. Chem. Chem. Phys. 10, 4378 (2008)], the effects of the antisymmetric-stretching excitation of methane on the Cl((2)P(3/2))+CH(4) reaction are examined here over a wide range of initial collision energy in a crossed molecular beam imaging experiment. The antisymmetric stretch of CH(4) is prepared in a single rovibrational state of (v(3)=1, j=2) by direct infrared absorption, and the major product states of CH(3)(v=0) are probed by a time-sliced velocity-map imaging method. We find that at fixed collision energies, the stretching excitation promotes reaction rate. Compared to the ground-state reaction, this vibrational enhancement factor is, however, no more effective than the translational enhancement. The correlated HCl(v'=1) vibrational branching fraction shows a striking dependence on collision energies, varying from 0.7 at E(c)=2 kcal mol(-1) to about 0.2 at 13 kcal mol(-1). This behavior resembles the previously studied Cl+CH(2)D(2)(v(6)=1), but is in sharp contrast to the Cl+CHD(3)(v(1)=1) and CH(2)D(2)(v(1)=1) reactions. Dependences of experimental results on the probed rotational states of CH(3)(v=0) are also elucidated. We qualitatively interpret those experimental observations based on a conceptual framework proposed recently.     

Two-color resonant four-wave mixing spectroscopy of highly predissociated levels in the A 2A1 state of CH3S

We report results of two-color resonant four-wave mixing experiments on highly predissociated levels of the methylthio (or thiomethoxy) radical CH3S in its first excited electronic state A 2A1. Following photolysis of jet-cooled dimethyl disulfide at 248 nm, the spectra were measured with a hole-burning scheme in which the probe laser excited specific rotational transitions in band 3(3). The spectral simplification afforded by the two-color method allows accurate determination of line positions and homogeneous linewidths, which are reported for the C-S stretching states 3v(v=3-7) and combination states 1(1)3v(v=0-2), 2(1)3v(v=3-6), and 1(1)2(1)3v(v=0,1) involving the symmetric CH3 stretching (nu1) mode and the CH3 umbrella (nu2) mode. The spectra show pronounced mode specificity, as the homogeneous linewidth of levels with similar energies varies up to two orders of magnitude; nu3 is clearly a promoting mode for dissociation. Derived vibrational wave numbers omega1', omega2', and omega3' of the A state agree satisfactorily with ab initio predictions.