Significant change of alignment effect by dimer formation in the dissociative energy transfer reaction of Ar(3P2)+(N2O)n and (H2O)n

An alignment effect in the dissociative energy transfer reaction of Ar((3)P(2))+(X(2)O)(n)(X=N,H) was directly measured using an oriented Ar((3)P(2),M(J)=2) beam. The chemiluminescence intensity of N(2)(B,(3)Pi(g)) for (N(2)O)(n) and OH(A,(2)Sigma(+)) for (H(2)O)(n) was measured as a function of the magnetic orientation field direction in the collision frame. The relative reaction cross section for each magnetic substate in the collision frame, sigma(M(J) (') ), was determined. In both the reaction systems, it is observed that the dimer formation significantly enhances the alignment effect and decreases the reactivity, especially for sigma|1| and sigma|2|. A significant contribution of rank 4 moment is recognized in the dimer reaction.     

Steric effect in the energy transfer reaction of N2 + Rg (3P2) (Rg = Kr, Ar)

Steric effect for the formation of N 2 (B, (3)Pi u ) in the energy transfer reaction of Kr ( (3)P 2) + N 2 has been measured using an oriented Kr ( (3)P 2, M J = 2) beam at a collision energy of 0.07 eV. The N 2 (B, (3)Pi u ) emission intensity was measured as a function of the magnetic orientation field direction in the collision frame. A significant atomic alignment effect on the energy transfer probability was observed. This result was compared with that for the formation of N 2 (C, (3)Pi g ) in the Ar ( (3)P 2) + N 2 reaction. Despite the large difference on the energy transfer cross-section, the atomic alignment dependence for Kr ( (3)P 2) + N 2 is found to be analogous to that for Ar ( (3)P 2) + N 2. It is revealed that the configuration of inner 4p (3p) orbital in the collision frame gives an important role for the stereoselectivity on electron transfer process via the curve-crossing mechanism.     

Deuterium isotope effect on the atomic orbital alignment dependence in the reaction of the oriented Ar (3P2) with CH3CN (CD3CN)

Atomic orbital alignment effect was observed for the CN (B2Sigma+) formation in the reaction of oriented Ar (3P2) with CH3CN (CD3CN). The relative cross-sections for each magnetic MJ' substrate in collision frame sigmaH|MJ'| for CH3CN and sigmaD|MJ'| for CD3CN, were determined to be sigmaH0:sigmaH|1|:sigmaH|2|:sigmaD0:sigmaD|1|:sigmaD|2| = 1.00:0.81:0.84:2.01:1.92:1.87. A significant atomic orbital alignment effect was observed. In addition, a notable deuterium isotope effect was observed on both the cross-section and the atomic orbital alignment effect.     

Deuterium isotope effect on the atomic alignment dependence in the reaction of oriented Ar (3P2) with (CH3CN)2 and (CD3CN)2 dimers

The effect of atomic alignment on CN (B2Sigma+) formation has been studied in the reaction of oriented Ar (3P2) with (CX3CN)2 (X = H, D). The reaction cross-section for each magnetic M'(J) substate in the collision frame sigma|M'(J)|(H(D),d) relative to the cross-section sigma0(H,m) in the CH(3)CN reaction was determined to be sigma0(H,d)/sigma|1|(H,d)/sigma|2|(H,d)/sigma0(D,d)/sigma|1|(D,d):/sigma|2|(D,d)= 0.87/1.00/0.98/1.58/1.93/1.78. A notable deuterium isotope effect was observed. In contrast with the monomer reactions, a significant decrease of sigma0(H(D),d) relative to the other cross-sections of sigma|M'(J)|(H(D),d) was observed.     

Effect of molecular rotation on the atomic alignment dependence in the oriented Ar (3P2) + CF3H reaction

Atomic alignment effect for the CF3* formation in the oriented Ar (3P2, MJ = 2) + CF3H reaction has been investigated at different two CF3H beam conditions: effusive and supersonic beams. The chemiluminescence intensity of CF3* was measured as a function of the magnetic orientation field direction in the collision frame. A significant contribution of rank 4 moment was recognized. The cross-section for each magnetic M'(J) substate in the collision frame, sigma|M'(J)|, was determined to be sigma(|M'(J)|=0):sigma(|M'(J)|=1):sigma(|M'(J)|=2) = 1.00:0.84 +/- 0.02:0.88 +/- 0.02 for the effusive CF3H beam condition. The atomic alignment effect was found to significantly depend on the CF3H beam condition. For the supersonic beam condition, sigma(|M'(J)|=0&1) was changed to be smaller than sigma(|M'(J)|=2).     

Atomic alignment effect on the branching to ArCl* and CCl2* formation in the reaction of oriented Ar(3P2,MJ=2) + CCl4

Atomic alignment effects for the formation of ArCl*(C) and CCl2*(A) in the reaction of Ar((3)P 2) + CCl 4 have been measured by using an oriented Ar( (3)P2, M J=2) beam at a collision energy of 0.08 eV. The emission intensity for ArCl*(C) and CCl2*(A) has been measured as a function of the magnetic orientation field direction in the collision frame. A significant atomic alignment effect is observed for the atom transfer process [ArCl*(C) formation]. Formation of ArCl*(C) is modestly enhanced when the electron angular momentum of the Ar((3)P 2) reactant is aligned along the relative velocity vector, while the excitation transfer process [CCl2*(A) formation] shows little alignment effect.     

Atomic alignment effect in the dissociative energy transfer reaction of metal carbonyls (Fe(CO)5, Ni(CO)4) with oriented Ar (3P2, M(J) = 2)

The atomic alignment effect has been studied for the dissociative energy transfer reaction of metal carbonyls (Fe(CO)(5), Ni(CO)(4)) with the oriented Ar ((3)P(2), M(J) = 2). The emission intensity from the excited metal products (Fe*, Ni*) has been measured as a function of the atomic alignment in the collision frame. The selectivity of the atomic orbital alignment of Ar ((3)P(2), M(J) = 2) (rank 2 moment, a(2)) is found to be opposite for the two reaction systems; the Fe(CO)(5) reaction is favorable at the Π configuration (positive a(2)), while the Ni(CO)(4) reaction is favorable at the Σ configuration (negative a(2)). Moreover, a significant spin alignment effect (rank 4 moment, a(4)) is recognized only in the Ni(CO)(4) reaction. The atomic alignment effect turns out to be essentially different between the two reaction systems; the Fe(CO)(5) reaction is controlled by the configuration of the half-filled 3p atomic orbital of Ar ((3)P(2)) in the collision frame (L dependence), whereas the Ni(CO)(4) reaction is controlled by the configuration of the total angular moment J (including spin) of Ar ((3)P(2)) in the collision frame (J dependence). As the origin of J dependence observed only in the Ni(CO)(4) reaction, the correlation (and/or the interference) between two electron exchange processes via the electron rearrangements is proposed.     

Alignment effect of N2(A3Σu+) in the energy transfer reaction of aligned N2(A3Σu+) + NO(X2Π) → NO(A2Σ+) + N2(X1Σg+)

Steric effect in the energy transfer reaction of N(2)(A(3)Σ(u)(+)) + NO(X(2)Π) → NO(A(2)Σ(+)) + N(2)(X(1)Σ(g)(+)) has been studied under crossed beam conditions at a collision energy of ~0.07 eV by using an aligned N(2)(A(3)Σ(u)(+)) beam prepared by a magnetic hexapole. The emission intensity of NO(A(2)Σ(+)) has been measured as a function of the magnetic orientation field direction (i.e., alignment of N(2)(A(3)Σ(u)(+))) in the collision frame. A significant alignment effect on the energy transfer probability is observed. The shape of the steric opacity function turns out to be most reactive at the oblique configuration of N(2)(A(3)Σ(u)(+)) with an orientation angle of γ(v(R)) ~ 45° with respect to the relative velocity vector (v(R)), which has a good correlation with the spatial distribution of the 2pπ(g)* molecular orbital of N(2)(A(3)Σ(u)(+)). We propose the electron exchange mechanism in which the energy transfer probability is dominantly controlled by the orbital overlap between N(2)(2pπ(g)*) and NO(6σ).     

Correlation between atomic orientation and product angular momentum alignment in the reaction of oriented Ar (3P2) with CH3OH

Atomic orientation effect for the CH(3)O(*) formation has been studied for the dissociative energy transfer reaction of oriented Ar ((3)P(2)) with CH(3)OH. The degree of polarization of CH(3)O(*) chemiluminescence was determined as a function of each magnetic M(J) (') substate in the collision frame. A drastic change of the product angular momentum alignment due to atomic orientation was recognized.     

Substituent-dependent negative hyperconjugation in 2-aryl-1,3-N,N-heterocycles. fine-tuned anomeric effect?

The epimerization reactions of conformationally inflexible 2-aryl-1,3-N,N-heterocycles were used as model systems to study the role of the nitrogen lone pair-C2 associated antibonding orbital hyperconjugative interactions in the experimentally observed substituent-dependent generalized anomeric effect. The measured reaction free enthalpies were found to correlate well with the sum of the hyperconjugative stabilization energies of all the vicinal donor-acceptor orbital overlaps around C2, obtained from ab initio NBO analysis, and both quantities correlated linearly with the Hammett-Brown substituent constant. The individual stereoelectronic interactions (n(N)-sigma(C2)(-)(N), n(N)-sigma(C2)(-)(Ar), n(N)-sigma(C2)(-)(H)) were also observed to exhibit a substituent dependence, despite their distance from the 2-aryl substituent and their nonperiplanar arrangement. The higher the electron-withdrawing effect of the 2-aryl substituent, the larger was the stabilization for n(N)-sigma(C2)(-)(Ar), while the overlaps n(N)-sigma(C2)(-)(N) and n(N)-sigma(C2)(-)(H) changed in the opposite sense. The different polarization of the acceptor sigma orbitals, caused by the 2-aryl substituent, accounted for the observed propagation of the substituent effect. These results promote a detailed explanation of the useful tautomeric behavior of the 2-aryl-1,3-X,N-heterocycles, and reveal the nature of the connection between the anomeric effect and the Hammett-type linear free energy relationship.     

Multidimensional molecular steric opacity function for XeCl*(B, C) formation in the oriented Xe* (³P₂, MJ = 2) + oriented CCl₃F reaction

The steric effect for the XeCl*(B, C) formations in the oriented Xe* (3P?, MJ = 2) + oriented CCl?F reaction has been observed as a function of the mutual configuration between the molecular orientation and the atomic orbital alignment in the collision frame. Molecular steric opacity functions have been determined as a function of the atomic orbital alignment (M(L)') in the collision frame. The XeCl*(B, C) channels show similar molecular steric opacity functions at M(L)' = 0 but not at |M(L)'| = 1. The large molecular alignment dependence (i.e., the reactivity of the Cl? end and the F end is comparable, but a very poor reactivity for the sideway) is recognized for the XeCl*(B, C) channels except for the XeCl*(C) channel at |M(L)'| = 1, which shows an almost isotropic molecular orientation dependence. The M(L)' selectivity is different between the XeCl*(B, C) channels. At the molecular axis direction, the XeCl*(B) channel has little M(L)' selectivity whereas the XeCl*(C) channel is significantly favorable at M(L)' = 0. On the other hand, |M(L)'| = 1 is favorable at the sideway for the XeCl*(B, C) channels.     

Probing anisotropic interaction potentials of unsaturated hydrocarbons with He*(2 3S) metastable atom: attractive-site preference of sigma-direction in C2H2 and pi-direction in C2H4

State-resolved collision energy dependence of Penning ionization cross sections of acetylene (C2H2) and ethylene (C2H4) with He*(2 3S) metastable atoms was observed in a wide collision energy range from 20 to 350 meV. A recently developed discharge nozzle source with a liquid N2 circulator was employed for the measurements in the low-energy range from 20 to 80 meV. Based on classical trajectory calculations for the energy dependence of the partial ionization cross sections, anisotropic potential energy surfaces for the present systems were obtained by optimizing ab initio model potentials for the chemically related systems Li+C2H2 and C2H4. In the case of C2H2, the global minimum was found to be located around the H atom along the molecular axis with a well depth of 48 meV (ca. 1.1 kcal/mol). On the other hand, a dominant attractive well with a depth of 62 meV (ca. 1.4 kcal/mol) was found in the piCC electron region of C2H4. These findings were discussed in connection with orbital interactions between molecular orbitals of the target molecules and atomic orbitals of the metastable atom. It is concluded that sigma-type unoccupied molecular orbitals of C2H2 and a piCC-type highest occupied molecular orbital of C2H4 play a significant role for the attractive-site preference of sigma direction in C2H2 and pi direction in C2H4, respectively.     

Ab initio potential energy surfaces of the ion-molecule reaction: C2H2 + O+

High level ab initio calculations using complete active space self-consistent field and multi reference single and double excitation configuration interaction methods with cc-pVDZ (correlation consistent polarized valence double zeta) and cc-pVTZ (triple zeta) basis sets have been performed to elucidate the reaction mechanism of the ion-molecule reaction, C2H2(1Sigmag+) + O+(4S), for which collision experiment has been performed by Chiu et al. [J. Chem. Phys. 109, 5300 (1998)]. The minor low-energy process leading to the weak spin-forbidden product C2H2+ (2Piu) + O(1D) has been studied previously and will not be discussed here. The major pathways to form charge-transfer (CT) products, C2H2+ (2Piu) + O(3P) (CT1) and C2H2+ (4A2) + O(3P) (CT2), and the covalently bound intermediates are investigated. The approach of the oxygen atom cation to acetylene goes over an energy barrier TS1 of 29 kcal/mol (relative to the reactant) and adiabatically leads the CT2 product or a weakly bound intermediate Int1 between CT2 products. This transition state TS1 is caused by the avoided crossing between the reactant and CT2 electronic states. As the C-O distance becomes shorter beyond the above intermediate, the C1 reaction pathway is energetically more favorable than the Cs pathway and goes over the second transition state TS2 of a relative energy of 39 kcal/mol. Although this TS connects diabatically to the covalent intermediate Int2, there are many states that interact adiabatically with this diabatic state and these lead to the other charge-transfer product CT1 via either of several nonadiabatic transitions. These findings are consistent with the experiment, in which charge transfer and chemical reaction products are detected above 35 and 39 kcal/mol collision energies, respectively.     

Comparison of keV N2(+*)/He and N2(+*)/Ar collisions by emission spectroscopy and theory

Collision-induced emission spectra from 190-1020 nm were obtained for N 2 (+*)/Ar collisions at laboratory frame collision energies from 2 to 8 keV. The relative emission intensities of N 2 (+*), N (+), and N (*) are independent of the ion translational energy within the studied energy range, supporting the curve-crossing mechanism for collisional excitation. The role of the target gas in keV N 2 (+*)/He and N 2 (+*)/Ar collisions was investigated by both emission spectroscopy and theoretical calculations. Adiabatic potential energy surfaces calculated at the CISD/6-311+G(2df) level of theory for N 2 (+*)/He and N 2 (+*)/Ar collisions indicate that excitation to the A (2)Pi u and B (2)Sigma u (+) states involves curve-crossing first to the C (2)Sigma u (+) state, and the difference in N 2 (+*) emission intensities from the two systems can be accounted for by the slope difference at the crossing points based on the Landau-Zener curve-crossing model.     

Kinetics of C2(a3Piu) radical reactions with alkanes by LIF

The reactions of C2(a3Piu) radicals with a series of alkanes have been studied at room temperature and 6.5 torr total pressure using the pulsed laser photolysis/laser-induced fluorescence technique. C2(a3Piu) radicals were generated by photolysis of C2Cl4 with the focused output from the fourth harmonic of a Nd: YAG laser at 266 nm. The relative concentration of C2(a3Piu) radicals was monitored on the (0,0) band of the C2(d3Pig <-- a3Piu) transition at 516.5 nm by laser-induced fluorescence. From the analysis of the relative concentration-time behavior of C2(a3Piu) under pseudofirst-order conditions, the rate constants for the reactions of C2(a3Piu) with alkanes (C1-C8) were determined. The rate constant increases linearly with the increasing of the number of CH2 groups in the alkanes. The experimental results indicate that the reaction of C2(a3Piu) with small alkanes (C1-C8) follows the typical hydrogen abstraction process. Based on the correlation of the experimental results with the bond dissociation energy of the alkanes, the reactions of C2(a3Piu) with small alkanes likely proceed via the mechanism of hydrogen abstraction.     

Crossed beam studies of elementary reactions of N and C atoms and CN radicals of importance in combustion

The dynamics of some elementary reactions of N(2D), C(3P,1D) and CN(X2 sigma +) of importance in combustion have been investigated by using the crossed molecular beam scattering method with mass spectrometric detection. The novel capability of producing intense, continuous beams of the radical reagents by a radio-frequency discharge beam source was exploited. From angular and velocity distribution measurements obtained in the laboratory frame, primary reaction products have been identified and their angular and translational energy distributions in the center-of-mass system, as well as branching ratios, have been derived. The dominant N/H exchange channel has been examined in the reaction N(2D) + CH4, which is found to lead to H + CH2NH (methylenimine) and H + CH3N (methylnitrene); no H2 elimination is observed. In the reaction N(2D) + H2O the N/H exchange channel has been found to occur via two competing pathways leading to HNO + H and HON + H, while formation of NO + H2 is negligible. Formation of H + H2CCCH (propargyl) is the dominant pathway, at low collision energy (Ec), of the C(3P) + C2H4 reaction, while at high Ec formation of the less stable C3H3 isomers (cyclopropenyl and/or propyn-1-yl) also occurs; the H2 elimination channel is negligible. The H elimination channel has also been found to be the dominant pathway in the C(3P,1D) + CH3CCH reaction leading to C4H3 isomers and, again, no H2 elimination has been observed to occur. In contrast, both H and H2 elimination, leading in comparable ratio to C3H + H and C3(X1 sigma g+) + H2(X1 sigma g+), respectively, have been observed in the reaction C(3P) + C2H2(X1 sigma g+). The occurrence of the spin-forbidden molecular pathway in this reaction, never detected before, has been rationalized by invoking the occurrence of intersystem crossing between triplet and singlet manifolds of the C3H2 potential energy surfaces. The reaction CN(X2 sigma +) + C2H2 has been found to lead to internally excited HCCCN (cyanoacetylene) + H. For all the reactions the dynamics have been discussed in the light of recent theoretical calculations on the relevant potential energy surfaces. Previous, lower resolution studies on C and CN reactions carried out using pulsed beams are noted. Finally, throughout the paper the relevance of these results to combustion chemistry is considered.     

Velocity map imaging of ion-molecule reactive scattering: the Ar(+)+ N(2) charge transfer reaction

We present a velocity map imaging spectrometer for the study of crossed-beam reactive collisions between ions and neutrals at (sub-)electronvolt collision energies. The charge transfer reaction of Ar(+) with N(2) is studied at 0.6, 0.8 and 2.5 eV relative collision energies. Energy and angular distributions are measured for the reaction product N. The differential cross section, as analyzed with a Monte Carlo reconstruction algorithm, shows significant large angle scattering for lower collision energies in qualitative agreement with previous experiments. Significant vibrational excitation of N(+)(2) is also observed. This theoretically still unexplained feature indicates the presence of a low energy scattering resonance.     

Unusual concerted Lewis acid-Lewis base mechanism for hydrogen activation by a phosphine-borane compound

Density functional theory calculations have been carried out to investigate the possible reaction mechanisms for the reversible dihydrogen activation catalyzed by a phosphine-borane compound, (C6H2Me3)2P(C6F4)B(C6F5)2 (Welch, G. C.; Juan, R. R. S; Masuda, J. D.; Stephan, D. W. Science. 2006, 314, 1124-1126). The present calculations show that an unusual concerted Lewis acid-Lewis base mechanism is more favorable than the proton transfer or hydride transfer mechanisms suggested previously. In the concerted Lewis acid-Lewis base mechanism, the H-H heterolytic cleavage is achieved through the simultaneous electron transfer from the lone-pair orbital of the Lewis base P center to the sigma* orbital of H2 and from the sigma orbital of H2 to the empty orbital of the Lewis acid B center. The solvent is found to dramatically change the potential energy surface. The proposed mechanism can account well for the bimolecular H-D exchange process observed in deuteration experiments and the experimental fact that the H2 activation is reversible at mild conditions.     

Collision-induced harpooning observed in the excimer formation in the oriented NF3 + oriented Kr*(3P2, MJ = 2) reaction

We have studied how the KrF* formation in the NF3 t Kr*(3P2) reaction depends on the mutual configuration between the orientation of the NF3 molecule and the alignment of the Kr*(3P2, M(J) = 2) atom in the collision frame. The molecular steric opacity function has been determined as a function of the atomic orbital alignment (M'(L)) in the collision frame. The molecular steric opacity function turns out to depend remarkably on M'(L) ; the |M'(L)| = 1 alignment is favorable at the molecular axis direction, whereas the M'(L) = 0 alignment is favorable at the sideways direction with a very poor reactivity at the molecular axis direction. The influence of deformation of the NF3 geometry on the electron affinity has been evaluated by ab initio calculation, and the M'(L) dependent intermolecular potential has been estimated from the interaction potential for the bromine-rare gas system. We propose the "collision-induced harpooning mechanism" as a novel process for the harpooning in which collisional deformation of the NF3 geometry with C(s) symmetry plays an important role as an initiating factor on electron transfer for the formation of NF3(-) due to increasing the electron affinity of NF3 and due to localizing the negative charge on the closest F-atom of NF3(-) anion. All experimental observations can support the collision-induced harpooning mechanism.     

Stereoelectronic and inductive effects on 1H and 13C NMR chemical shifts of some cis-1,3-disubstituted cyclohexanes

This work presents the substituent effects on the 1H and 13C NMR chemical shifts in the cis-isomer of 3-Y-cyclohexanols (Y = Cl, Br, I, CH3, N(CH3)2 and OCH3) and 3-Y-1-methoxycyclohexanes (Y = F, Cl, Br, I, CH3, N(CH3)2 and OCH3). It was observed that the H-3 chemical shift, due to the substituent alpha-effect, increases with the increase of substituent electronegativity when Y is from the second row of the periodic table of elements, (CH3 *sigma(C3--H3a) interaction energy. This interaction energy, for the halogenated compounds, decreases with an increase in size of the halogen, and this is a possible reason for the largest measured chemical shift for H-3 of the iodo-derivatives. The beta-effect of the analyzed compounds showed that the chemical shift of hydrogens at C-2 and C-4 increases with the decrease of n(Y) --> *sigma(C2-C3) and n(Y) --> *sigma(C3-C4) interaction energies, respectively, showing a behavior similar to H-3. The alpha-effect on 13C chemical shifts correlates well with substituent electronegativity, while the beta-effect is inversely related to electronegativity in halogenated compounds. NBO analysis indicated that the substituent inductive effect is the predominant effect on 13C NMR chemical shift changes for the alpha-carbon. It was also observed that C-2 and C-4 chemical shifts for compounds with N(CH3)2, OCH3 and F are more shielded in comparison to the compounds having a halogen, most probably because of the larger interaction of the lone pair of more electronegative atoms (n(N) > n(O) > n(F)) with *sigma(C2-C3), *sigma(C3-C4) and *sigma(C3-H3a) in comparison with the same type of interaction with the lone pair of the other halogens.