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

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 2Σ state in odd clusters may play an important role in these reactions. This opposes the previous interpretation that the 2Πg/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 2Σ state Cn+. In contrast, a carbene-like insertion forming CnD2+ occurs through a sideways approach for the 2Πg/u state Cn+. We have concluded that the higher rate constants for the odd clusters come from the existence of symmetry broken 2Σ states which are absent in even linear clusters.