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The nucleophilic substitution reaction CN- + CH3I allows for two possible reactive approaches of the reactant ion onto the methyl halide, which lead to two different product isomers. Stationary point calculations predict a similar shape of the potential and a dominant collinear approach for both attacks. In addition, an H-bonded pre-reaction complex is identified as a possible intermediate structure. Submerged potential energy barriers hint at a statistical formation process of both CNCH3 and NCCH3 isomers at the experimental collision energies. Experimental angle- and energy differential cross sections show dominant direct rebound dynamics and high internal excitation of the neutral product. No distinct bimodal distributions can be extracted from the velocity images, which impedes the indication of a specific preference towards any of the product isomers. A forward scattering simulation based on the experimental parameters describes accurately the experimental outcome and shows how the possibility to discriminate between the two isomers is mainly hindered by the large product internal excitation. [ABSTRACT FROM AUTHOR]

The dissociative recombination of OPCl+ and OPCl2+ has been studied at the storage ring CRYRING. The rate constants as a function of electron temperature have been derived to be 7.63×10-7(Te/300)-0.89 and >1.2×10-6(Te/300)-1.22 cm3 s-1, respectively. The lower limit quoted for the latter rate constant reflects the experimental inability to detect all of the reaction products. The branching fractions from the reaction have been measured for OPCl+ at ≈0 eV interaction energy and are determined to be N(O+P+Cl)=(16±7)%, N(O+PCl)=(16±3)% and N(OP+Cl)=(68±5)%. These values have been obtained assuming that the rearrangement channel forming P+ClO is negligible, and ab initio calculations using GAUSSIAN03 are presented for the ion structures and energetics to support such an assumption. Finally, the limitations to using heavy ion storage rings such as CRYRING for studies into the dissociative recombination of large singly charged molecular ions are discussed. [ABSTRACT FROM AUTHOR]

We report an investigation into the dissociative recombination of the azide radical cation, N3+. The reaction rate constant has been measured to be 6.47×10-7 cm3 s-1 at room temperature. This value is smaller than those reported earlier for the ion-electron neutralization of N3+ at nitrogen atmospheric pressure. A strong propensity to dissociate through the N2+N channel has been observed. [ABSTRACT FROM AUTHOR]

Dimethyl disulfide (DMDS) and N-methylacetamide are two first choice model systems that represent the disulfide bridge bonding and the peptide bonding in proteins. These molecules are therefore suitable for investigation of the mechanisms involved when proteins fragment under electron capture dissociation (ECD). The dissociative recombination cross sections for both protonated DMDS and protonated N-methylacetamide were determined at electron energies ranging from 0.001 to 0.3 eV. Also, the branching ratios at 0 eV center-of-mass collision energy were determined. The present results give support for the indirect mechanism of ECD, where free hydrogen atoms produced in the initial fragmentation step induce further decomposition. We suggest that both indirect and direct dissociations play a role in ECD. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The reaction between atomic carbon and acetylene has been investigated using complementary crossed molecular beam techniques. Differential cross sections have been obtained for the reactions of both ground and excited carbon atoms, C(³P[sub J], ¹D[sub 2])+ C[sub 2]H[sub 2](X ¹Σ[sup +, sub g], in experiments conducted with continuous supersonic beams, mass spectrometric detection, and time-of-flight analysis at a relative translational energy of 29.3 kJmol[sup -1]. The reaction C(³P[sub J]) + C[sub 2]H[sub 2](X ¹Σ[sup +, sub g]) has been found to lead to C[sub 3]H+H and C[sub 3]+H[sub 2] products in comparable amounts. Both H and H[sub 2] elimination pathways are found to proceed through the formation of a C[sub 3]H[sub 2] long-lived intermediate complex whose lifetime may be comparable to its rotational period. The spin-forbidden H[sub 2] elimination channel is attributed to the occurrence of intersystem-crossing between the triplet and singlet manifolds of the C[sub 3]H[sub 2] potential-energy surfaces. The reaction C(¹D[sub 2])+C[sub 2]H[sub 2](X ¹Σ[sup +, sub g]) has been found to lead to formation of C[sub 3]H+H, with a C[sub 3]H center-of-mass angular distribution strongly forward peaked, indicating a short-lived intermediate complex. Integral cross sections have been obtained for the C(³P[sub J]j) + C[sub 2]H[sub 2] (X &sup1Σ[sup +, sub g]) → C[sub 3] H + H(²S[sub ½]) reaction in experiments conducted with pulsed, supersonic molecular beams in the range of relative translational energies E[sub T]=0.38 -25.5 kJ mol[sup -1], the H(²S[sub ½) product being detected by laser-induced fluorescence. The reaction has been found to be without a barrier, relative integral cross sections being proportional to (Er)[sup -0.80&plumsn;0.03] below E[sub T]=10 kJmol[sup -1]. These findings provide direct evidence that the C(³P[sub J]) +C[sub 2]H[sub 2] reaction can occur under... [ABSTRACT FROM AUTHOR]

Product branching ratios and thermal rate coefficients for the dissociative recombination of CO2+ have been measured in the cryogenic ion source ring ion storage ring. The rate constants were found to be 4.2×10-7(Te/300)-0.75 cm3 s-1. The 300-K result is in agreement with previous flowing afterglow values and is somewhat smaller than a recent determination made at the Aarhas storage ring in Denmark (ASTRID) storage ring. The electron temperature dependence is, however, in good agreement with the ASTRID result of T-0.8. The present results show that only CO plus O are formed, other product branching ratios are zero within experimental error. This is in contradiction to the ASTRID results which show that 9% of the reactivity goes to C+O2. The new results show that the C+O2 channel does not need to be included in the models of the ionospheres of Venus and Mars. [ABSTRACT FROM AUTHOR]

Product branching ratios and thermal rate coefficients for the dissociative recombination of C3D7+ and C4D9+ have been measured in the ion storage ring CRYRING. The results for C3D7+ are believed to be slightly more accurate than those obtained earlier for C3H7+. Only the C–C bond breaking channels could be measured for C4D9+ and were found to be in excellent agreement with earlier data. [ABSTRACT FROM AUTHOR]

Product branching ratios and thermal rate coefficients for the dissociative recombination of C3D7+ and C4D9+ have been measured in the ion storage ring CRYRING. The results for C3D7+ are believed to be slightly more accurate than those obtained earlier for C3H7+. Only the C–C bond breaking channels could be measured for C4D9+ and were found to be in excellent agreement with earlier data. [ABSTRACT FROM AUTHOR]

Product branching ratios and thermal rate coefficients for the dissociative recombination of CO2+ have been measured in the cryogenic ion source ring ion storage ring. The rate constants were found to be 4.2×10-7(Te/300)-0.75 cm3 s-1. The 300-K result is in agreement with previous flowing afterglow values and is somewhat smaller than a recent determination made at the Aarhas storage ring in Denmark (ASTRID) storage ring. The electron temperature dependence is, however, in good agreement with the ASTRID result of T-0.8. The present results show that only CO plus O are formed, other product branching ratios are zero within experimental error. This is in contradiction to the ASTRID results which show that 9% of the reactivity goes to C+O2. The new results show that the C+O2 channel does not need to be included in the models of the ionospheres of Venus and Mars. [ABSTRACT FROM AUTHOR]

The nucleophilic substitution reaction CN(-) + CH3I allows for two possible reactive approaches of the reactant ion onto the methyl halide, which lead to two different product isomers. Stationary point calculations predict a similar shape of the potential and a dominant collinear approach for both attacks. In addition, an H-bonded pre-reaction complex is identified as a possible intermediate structure. Submerged potential energy barriers hint at a statistical formation process of both CNCH3 and NCCH3 isomers at the experimental collision energies. Experimental angle- and energy differential cross sections show dominant direct rebound dynamics and high internal excitation of the neutral product. No distinct bimodal distributions can be extracted from the velocity images, which impedes the indication of a specific preference towards any of the product isomers. A forward scattering simulation based on the experimental parameters describes accurately the experimental outcome and shows how the possibility to discriminate between the two isomers is mainly hindered by the large product internal excitation.

The dissociative recombination of OPCl+ and OPCl2+ has been studied at the storage ring CRYRING. The rate constants as a function of electron temperature have been derived to be 7.63 x 10(-7)(Te/300)(-0.89) and >1.2 x 10(-6)(Te/300)(-1.22) cm3s(-1), respectively. The lower limit quoted for the latter rate constant reflects the experimental inability to detect all of the reaction products. The branching fractions from the reaction have been measured for OPCl+ at approximately 0 eV interaction energy and are determined to be N(O+P+Cl)=(16+/-7)%, N(O+PCl)=(16+/-3)% and N(OP+Cl)=(68+/-5)%. These values have been obtained assuming that the rearrangement channel forming P+ClO is negligible, and ab initio calculations using GAUSSIAN03 are presented for the ion structures and energetics to support such an assumption. Finally, the limitations to using heavy ion storage rings such as CRYRING for studies into the dissociative recombination of large singly charged molecular ions are discussed.

We report an investigation into the dissociative recombination of the azide radical cation, N(3) (+). The reaction rate constant has been measured to be 6.47 x 10(-7) cm(3) s(-1) at room temperature. This value is smaller than those reported earlier for the ion-electron neutralization of N(3) (+) at nitrogen atmospheric pressure. A strong propensity to dissociate through the N(2)+N channel has been observed.

Product branching ratios and thermal rate coefficients for the dissociative recombination of C3D(+)7 and C4D(+)9 have been measured in the ion storage ring CRYRING. The results for C3D(+)7 are believed to be slightly more accurate than those obtained earlier for C3H(+)7. Only the C-C bond breaking channels could be measured for C4D(+)9 and were found to be in excellent agreement with earlier data.