Femtosecond Laser Mass Spectrometry and High Harmonic Spectroscopy of Xylene Isomers

Simultaneous Recognition and Separation of Organic Isomers Via Cooperative Control of Pore-Inside and Pore-Outside Interactions

Despite the desirability of organic isomer recognition and separation, current strategies are expensive and complicated. Here, a simple strategy for simultaneously recognizing and separating organic isomers using pillararene-based charge-transfer cocrystals through the cooperative control of pore-inside and pore-outside intermolecular interactions is presented. This strategy is illustrated using 1-bromobutane (1-BBU), which is often produced as an isomeric mixture with 2-bromobutane (2-BBU). According to its structure, perethylated pillar[5]arene (EtP5) and 3,5-dinitrobenzonitrile (DNB) are strategically chosen as a donor and an acceptor. As a result, their cocrystal exhibited stronger pore-inside interactions and much weaker pore-outside interactions with 1-BBU than with 2-BBU. Consequently, nearly 100% 1-BBU selectivity is achieved in two-component mixtures, even in those containing trace 1-BBU (1%), whereas free EtP5 only achieved 89.80% selectivity. The preference for linear bromoalkanes is retained in 1-bromopentane/3-bromopentane and 1-bromohexane/2-bromohexane mixtures, demonstrating the generality of this strategy. Selective adsorption of linear bromoalkanes induced a naked-eye-detectable color change from red to white. Moreover, the cocrystal are used over multiple cycles without losing selectivity.

Two-Body Metastable Dissociation of n-Pentane and n-Hexane Triplet Dications in Intense Femtosecond-Laser Fields

Mass spectra of n-pentane and n-hexane ionized through femtosecond-laser pulses were measured using a time-of-flight mass spectrometer. Fragment ions ejected with large kinetic energies were identified as side peaks in which a two-body dissociation pathway, C₅H₁₂⁺⁺ → C₂H₅⁺ + C₃H₇⁺, was identified for n-pentane, and two for n-hexane, C₆H₁₄⁺⁺ → C₂H₅⁺ + C₄H₉⁺ and C₃H₇⁺ + C₃H₇⁺, based on momentum matching of the fragments. The two-body dissociation pathways were observed when the polarization direction of the linearly polarized laser light was perpendicular to the molecular axis. However, when the polarization direction was parallel to the molecular axis or the laser light was circularly polarized, these signals were weak or difficult to identify. These results suggest that the two-body dissociation pathways are caused by nonsequential double ionization (NSDI), which begins with ionization from the π-type second highest occupied molecular orbital (HOMO-1) via the laser electric field perpendicular to the molecular axis rather than bonding the σ-type HOMO. Quantum chemical calculations show that the dication has a triplet metastable state with the same formula as the neutral state (i.e., ³[CH₃-(CH₂)_n-CH₃]⁺⁺). Therefore, the relevant two-body dissociation channels open through transition states with the (HOMO)¹(HOMO-1)¹ electron configuration and the estimated kinetic energy release values correlate with those observed.