Vibrational Spectroscopy and Structural Analysis of V⁺(C₂H₆)_n Clusters (n = 1–4)

Abstract

The vibrational structure and binding motifs of vanadium cation−ethane clusters, V + (C 2 H 6 ) n , for n = 1−4 are probed using infrared photodissociation spectroscopy in the C−H stretching region (2550−3100 cm −1 ). Comparison of spectra to scaled harmonic frequency spectra obtained using density functional theory suggests that ethane exhibits two primary binding motifs when interacting with the vanadium cation: an end-on η 2 configuration and a side-on configuration. Determining the denticity of the side on isomer is complicated by the rotational motion of ethane, implying that structural analysis based solely on Born−Oppenheimer potential energy surface minimizations is insufficient and that a more sophisticated vibrationally adiabatic approach is necessary to interpret spectra. The lower-energy side-on configuration predominates in smaller clusters, but the end-on configuration becomes important for larger clusters as it helps to maintain a roughly square-planar geometry about the central vanadium. Proximate C−H bonds exhibit elongation and large red-shifts when compared to bare ethane, particularly in the case of the side-on isomer, demonstrating initial effects of C−H bond activation, which are underestimated by scaled harmonic frequency calculations. Tagging several of the clusters with argon and nitrogen results in nontrivial effects. The high binding energy of N 2 can lead to the displacement of ethane from a side-on configuration into an end-on configuration. The presence of either one or two Ar or N 2 can impact the overall symmetry of the cluster, which can alter the potential energy surface for ethane rotation in the side-on isomer and may affect the accessibility of low-lying electronic excited states of V + .