Many molecules exhibit multiple rotational isomers (conformers) that interconvert thermally and are difficult to isolate. Consequently, a precise characterization of their role in chemical reactions has proven challenging. We have probed the reactivity of specific conformers by using an experimental technique based on their spatial separation in a molecular beam by electrostatic deflection. The separated conformers react with a target of Coulomb-crystallized ions in a trap. In the reaction of Ca(+) with 3-aminophenol, we find a twofold larger rate constant for the cis compared with the trans conformer (differentiated by the O-H bond orientation). This result is explained by conformer-specific differences in the long-range ion-molecule interaction potentials. Our approach demonstrates the possibility of controlling reactivity through selection of conformational states.

Water exists as two nuclear-spin isomers, para and ortho, determined by the overall spin of its two hydrogen nuclei. For isolated water molecules the conversion between these isomers is forbidden and they act as different molecular species. Yet, these species are not readily separated and no pure para sample has been produced. Accordingly, little is known about their specific physical and chemical properties, conversion mechanisms, or interactions. Here, we demonstrate the production of isolated samples of both spin isomers in pure beams of para and ortho water in their respective absolute ground state. These single-quantum-state samples are ideal targets for unraveling spin-conversion mechanisms, for precision spectroscopy and fundamentalsymmetry-breaking studies, and for spin-enhanced applications, e. g., laboratory astrophysics and -chemistry or hypersensitized NMR experiments.Significant efforts have been undertaken to separate and study the nuclear-spin isomers of water, motivated by their importance in a wide variety of scientific disciplines. This ranges from the astronomical importance of the ortho-para ratio [1][2][3][4][5] , to studies of nuclear-spin conversion [6,7] , selection rules and reactive collisions [8][9][10] or symmetry breaking [11] . Spin-enriched samples furthermore would allow for hypersensitized NMR experiments via polarization transfer reactions [12][13][14] . However, unlike other small polyatomic molecules exhibiting spin isomerism, such as fluoromethane or ethylene [15,16] , which were spin-isomerically enriched using the light induced drift technique [7] , this has not been achieved for water. Separation through selective adsorption on surfaces was reported [17] , but these results remain controversial and could not be reproduced [18][19][20][21] . Thus, studies of spin-conversion dynamics in water have been limited to water embedded in rare gas matrices, with relative spin populations modified by the sample temperature [22] . A recent study investigated nuclear-spin conversion in the gas-phase and found no spin conversion for water monomers [23] .Recently, the production of a single spin isomer of water in a magnetic-hexapole-focuser setup was demonstrated [24] . One of the magnetically active spin projections (m = +1) of ground-state ortho water was magnetically focused into the interaction volume, while all other spin-projection states were defocused or diverged unaffected by the field. The purity of the produced ortho beam was later evaluated as 93 %, with simulations suggesting an upper limit for the achievable purity of 97 % [24,25] .Here, we experimentally demonstrate the production of pure samples of both, para and ortho water, the latter further separated into its M = 0 and M = 1 angular momentum projections, in the gas-phase. The produced single-quantum-states are ideally suited for further experiments on nuclear-spin conversion under collision-free conditions, nuclear-spin-dependent reactivity [8] , trapping of single spin-isomer samples in electromagnetic traps [26] ...

Dedicated to Bretislav Friedrich on the occasion of his 60 th birthdayWe demonstrate strong adiabatic laser alignment and mixed-field orientation at kHz repetition rates. We observe degrees of alignment as large as cos 2 θ 2D = 0.94 at 1 kHz operation for iodobenzene. The experimental setup consist of a kHz laser system simultaneously producing pulses of 30 fs (1.3 mJ) and 450 ps (9 mJ). A cold 1 K state-selected molecular beam is produced at the same rate by appropriate operation of an Even-Lavie valve. Quantum state selection has been obtained using an electrostatic deflector. A camera and data acquisition system records and analyzes the images on a single-shot basis. The system is capable of producing, controlling (translation and rotation) and analyzing cold molecular beams at kHz repetition rates and is, therefore, ideally suited for the recording of ultrafast dynamics in so-called "molecular movies".