Aqueous pyridine plays an important role in a variety of catalytic processes aimed at harnessing solar energy. In this work, the pyridine-water interaction is studied by microwave spectroscopy and density functional theory calculations. Water forms a hydrogen bond to the nitrogen with the oxygen tilted slightly toward either of the ortho-hydrogens of the pyridine, and a tunneling motion involving in-plane rocking of the water interconverts the resulting equivalent structures. A pair of tunneling states with severely perturbed rotational spectra is identified and their energy separation, ΔE, is inferred from the perturbations and confirmed by direct measurement. Curiously, values of ΔE are 10404.45 and 13566.94 MHz for the HO and DO complexes, respectively, revealing an inverted isotope effect upon deuteration. Small splittings in some transitions suggest an additional internal motion making this complex an interesting challenge for theoretical treatments of large amplitude motion. The results underscore the significant effect of the ortho-hydrogens on the intermolecular interaction of pyridine.

Understanding and controlling ultrafast charge carrier dynamics is of fundamental importance in diverse fields of (quantum) science and technology. Here, we create a three-dimensional hot electron gas through two-photon photoemission from a copper surface in vacuum. We employ an ultrafast electron microscope to record movies of the subsequent electron dynamics on the picosecond-nanosecond time scale. After a prompt Coulomb explosion, the subsequent dynamics is characterized by a rapid oblate-to-prolate shape transformation of the electron gas, and periodic and long-lived electron cyclotron oscillations inside the magnetic field of the objective lens. In this regime, the collective behavior of the oscillating electrons causes a transient, mean-field lensing effect and pronounced distortions in the images. We derive an analytical expression for the time-dependent focal length of the electron-gas lens, and perform numerical electron dynamics and probe image simulations to determine the role of Coulomb self-fields and image charges. This work inspires the visualization of cyclotron dynamics inside two-dimensional electron-gas materials and enables the elucidation of electron/plasma dynamics and properties that could benefit the development of high-brightness electron and X-ray sources.

The rotational spectrum of the weakly bound complex 3,5-difluoropyridine•••CO 2 has been observed using pulsed-nozzle Fourier transform microwave spectroscopy. Spectroscopic constants are reported for the parent and 13 CO 2 isotopologues. The data indicate a planar structure in which the nitrogen approaches the carbon of the CO 2 with either a C 2v or effectively C 2v geometry in the ground vibrational state. The N•••C van der Waals bond distance is 2.8245(16) Å and the oxygen•••ortho-hydrogen distance is 3.091(2) Å. The N•••C van der Waals bond length is 0.027(8) Å longer than that previously determined for pyridineCO 2 , but is still considerably shorter than the 2.998 Å distance in HCN•••CO 2. M06-2X/6-311++G(3df,3pd) calculations place the binding energy of the complex at 4.3 kcal/mol (4.1 kcal/mol with counterpoise correction). The calculations further indicate that a secondary interaction between the ortho-hydrogens of the ring and the CO 2 oxygens account for ~50% of the total binding energy.

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