Even–odd product variation of the C_n⁺ + D₂ (n = 4–9) reaction: complexity of the linear carbon cation electronic states

Abstract

We have studied reactions between linear Cn(+) (n = 4-9) and D2, using ion mobility mass spectrometry techniques and quantum chemical calculations in order to understand the complex reactivity of the linear cluster cations. Only linear CnD(+) products were observed for the odd (n = 5, 7, 9) linear clusters, while CnD2(+) was the main product for the even clusters. For the reaction rate constants determined for these two channels, we obtained the following two features: (1) the rate constant decreases with the size n, and (2) even-sized clusters have lower rate constants than neighboring odd-sized clusters. In the theoretical calculations using the CCSD(T) and B3LYP methods with the cc-pVTZ basis, we found that a low lying (2)Σ state in odd clusters may play an important role in these reactions. This opposes the previous interpretation that the (2)Πg/u state is the dominant electronic state for linear Cn(+) (n = 4-9) clusters. We showed that a barrierless radical abstraction forming CnD(+) occurs through a direct head on approach for the (2)Σ state Cn(+). In contrast, a carbene-like insertion forming CnD2(+) occurs through a sideways approach for the (2)Πg/u state Cn(+). We have concluded that the higher rate constants for the odd clusters come from the existence of symmetry broken (2)Σ states which are absent in even linear clusters.

Electronic spectra of positively charged carbon clusters—C2n+ (n = 6–14)

“…Third, linear clusters, if they do form through isomerization of cyclic clusters following their passage through the ion mobility spectrometer drift region -either when they were stored in the hexapole, passing through the octopole ion guide, or contained in the cryogenic QIT -would be expected to react rapidly with trace H 2 O or H 2 to form linear C n H + or HC n H + molecules. [5][6][7][8] We tested this by recording spectra of the C + n clusters with both H 2 /N 2 (100:1) and He/N 2 (100:1) buffer gas mixtures in the QIT and found no substantial differences in ToF mass spectra or in the electronic spectra. Last, preliminary experiments with mobility selection of the C + n clusters clearly show that the spectral features apparent in FIG.…”

Odd–Even Reactivity Variation Due to Dynamical Effects around the Roaming Saddle Points of the Reaction Between Cn– Chain (n = 2–8) and H2

“…Our result for the chain structures of the odd n is in agreement with the previous systematic study for C n H – , and in particular for n = 3, electronic energy differences between the lowest energy states of C 3 H – isomers are consistent with the previous studies. , For the cyclic geometries, we could not locate a transition state from the C n – + H 2 reactant. Thus, we consider the reaction path for forming C n H 2 – or C n H – by a single collision process where the H 2 approaches the edge carbon atom having the radical electron, similar to the case of C n + …”

“…This is opposite to the case of C n + linear chain. 6 These rate coefficients are obtained under the establishment of an equilibrium between the reactants and the TS. In some astrochemical environment where the concentration of surrounding gases is very low, this condition may not be satisfied.…”

“…Recently, carbon cluster cation C n + linear chain was reported to exhibit a size-dependent reactivity that odd number C n + has higher reactivity with D 2 compared to even ones, while the reaction rate decreases with increasing the cluster size. 6 In addition, a unique dissociation reaction path was reported for double charged carbon hydride system C 2 H 2 2+ , 7 and the mechanism was described by complex transition states and intermediate molecular states. On the other hand, the chemical property of carbon cluster anion C n − has been also investigated due to its interest in astrochemistry and astrophysics.…”

“…Small neutral as well as charged carbon chain clusters show complex reactivity associated with their geometrical property. Recently, carbon cluster cation C n + linear chain was reported to exhibit a size-dependent reactivity that odd number C n + has higher reactivity with D 2 compared to even ones, while the reaction rate decreases with increasing the cluster size . In addition, a unique dissociation reaction path was reported for double charged carbon hydride system C 2 H 2 2+ , and the mechanism was described by complex transition states and intermediate molecular states.…”