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

The vibrational structure and binding motifs of vanadium cation-ethane clusters, V⁺(C₂H₆)_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 η² 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 N₂ 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 N₂ 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⁺.

Probing La and Ce Excited-State Reactivity with Resonant Two-Photon Ionization Spectroscopy

Dehydrogenation and C-C bond cleavage of 1-butyne by the excited states of La and Ce atoms are investigated in laser-ablation metal molecular beams. The excited states of the metal atoms are prepared by resonant excitation, detected by resonant two-photon ionization spectroscopy, and the reaction products are monitored by photoionization time-of-flight mass spectrometry. The reactivities of La* [5d²(³F)6p (⁴G_5/2°)] and Ce* [4f5d(³F°)6s6p(³P°) (⁵H₅)] excited states are observed to be higher than those of the initial states of the corresponding metal atoms. The higher reactivities of the excited states are attributed to their higher energies and favorable electron configurations to form two covalent bonds of the metal-insertion intermediates. Although both excited La and Ce atoms show increased reactivities, the enhancement for Ce is much more pronounced than that of La, which cannot be explained by electron configurations alone. The larger reactivity enhancement from the initial states to the excited state of the Ce atom than that of La is due to the longer lifetime of the Ce excited state.

Photo-control of bimolecular reactions: reactivity of the long-lived Rhodamine 6G triplet excited state with ˙NO

Photo-chemistry provides a non-intuitive but very powerful way to probe kinetically limited, sometimes thermodynamically non-favored reactions and, thus, access highly specific products. However, reactivity in the excited state is difficult to characterize directly, due to short lifetimes and challenges in controlling the reaction medium. Among photo-activatable reagents, rhodamine dyes find widespread uses due to a number of favorable properties including their high absorption coefficient. Their readily adaptable synthesis allows development of tailor-made dyes for specific applications. Remarkably, few studies have directly probed the chemical reactivity of their triplet excited state. Here we present a new conceptual approach to examine the specific chemistry of the triplet excited state. We have developed a pump (488 nm) - probe (600 nm) strategy to examine the gas-phase lifetime and reactivity of the triplet cation of Rhodamine 6G (³Rh6G⁺) in an ion trap mass spectrometer. The confounding effects of solvent, aggregation and formation of other reactive intermediates is thus avoided allowing fundamental reactivity to be explored. In the presence, in the ion trap, of helium seeded with 1% of nitric oxide (˙NO) (∼ 60 ion/˙NO collisions per second), the triplet lifetime is shortened from 1.9 s to 0.7 s. Simultaneously, the reaction products [Rh6G-H]˙⁺ and [Rh6G-H + NO]⁺ are observed. Reaction of ³Rh6G⁺ with ˙NO₂ yields [Rh6G-H]˙⁺, [Rh6G-H + NO₂]⁺ and [Rh6G-2H]⁺. None of these products are observed for the singlet, ¹Rh6G⁺. DFT calculations suggest a stepwise mechanism only allowed from ³Rh6G⁺, in which H atom abstraction by ˙NO_x (x = 1 or 2) yields [Rh6G-H]˙⁺ which, then, reacts with another ˙NO_x molecule. This illustrates the power of light to initiate specific chemical reactions, and the relevance of gas-phase ion-molecule reaction approaches to understand stepwise reaction mechanism from specific excited states.

High-Level ab Initio Predictions for the Ionization Energies, Bond Dissociation Energies, and Heats of Formation of Vanadium Methylidene, Vanadium Methyl Species, and Their Cations (VCH2/VCH2+, VCH3/VCH3+)

The ionization energies of VCH₂ and VCH₃, the various 0 K bond dissociation energies (D₀s) in their neutrals and cations, and their respective heats of formation at 0 and 298 K are computed by the single-reference, wave function-based CCSDTQ/CBS procedure. The core of the composite method is the approximation to the complete basis set (CBS) limit at the coupled cluster (CC) level which includes up to full quadruple excitations. The zero-point vibrational energy, core-valence correlation, spin-orbit coupling, and scalar relativistic effects have their contributions incorporated in an additive manner. For the species in the current study, this protocol requires geometry optimizations and harmonic frequency calculations practically no higher than the CCSD(T)/aug-cc-pwCVTZ and CCSD(T)/aug-cc-pVTZ levels, respectively. The present calculations successfully predict D₀(V⁺-CH₃) = 2.126 eV and D₀(V⁺-CH₂) = 3.298 eV in remarkable agreement with the data recently measured by a spin-orbit state selected V⁺ + CH₄ collision experiment (Phys. Chem. Chem. Phys. 2021, 23, 273-286). The good accord encourages the use of CCSDTQ/CBS protocol in thermochemical predictions of various feasible product channels identified in methane activation by transition metal species.

Structures of M+(CH4)n (M = Ti, V) Based on Vibrational Spectroscopy and Density Functional Theory

Photofragment spectroscopy is used to measure the vibrational spectra of M⁺(CH₄)(Ar) and M⁺(CH₄)_n (M = Ti, V; n = 1-4) in the C-H stretching region (2550-3100 cm^-1). Spectra were measured by monitoring the loss of Ar from M⁺(CH₄)(Ar) and loss of CH₄ from the larger clusters. The experimental spectra are then compared to simulations done at the B3LYP/6-311++G(3df,3pd) level of theory to identify the structures of the ions. The spectra all have a peak near 2800 cm^-1 due to the symmetric C-H stretch of the hydrogens adjacent to the metal. Some complexes also have a smaller peak due to the corresponding antisymmetric stretch. Most complexes also have a peak near 3000 cm^-1 due to the C-H stretch of hydrogens pointing away from the metal. The symmetric proximate C-H stretches of M⁺(CH₄)(Ar) to M⁺(CH₄)₄ are red-shifted from the symmetric stretch in bare CH₄ by 149, 152, 128, and 107 cm^-1 for the titanium complexes and 164, 175, 158, and 146 cm^-1, respectively, for the vanadium complexes. In M⁺(CH₄)(Ar) (M = Ti, V), the heavy atoms are collinear. Ti⁺(CH₄)(Ar) has η³ methane hydrogen coordination (∠M-C-H = 180°), while V⁺(CH₄)(Ar) has η² (∠M-C-H = 124°). The n = 2 complexes have C-M-C linear. Ti⁺(CH₄)₂ has C_2h symmetry with η³ CH₄ while V⁺(CH₄)₂ has methane coordination intermediate between η² and η³ (∠M-C-H = 156°). Both the M⁺(CH₄)₃ (M = Ti, V) complexes have C_2v symmetry with one methane farther away from the metal in an η² binding orientation and two methanes close to the metal with a nearly η² methane for vanadium and coordination between η² and η³ CH₄ for titanium (∠M-C-H = 150°). In Ti⁺(CH₄)₄ and V⁺(CH₄)₄ all of the methanes have η² coordination. The titanium complex has a distorted square planar geometry with two different Ti-C bond lengths and the vanadium complex is square planar.