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Using an Electrostatic Accelerator to Determine the Stereochemical Structures of Molecular Ions

Traditional experimental techniques (e.g. studies on photon absorption or emission) for determining the stereochemical structures of neutral molecules are extremely difficult to apply to molecular ions because of problems in obtaining a sufficient spatial density of the ions to be studied. Recent high-resolution measurements on the energy and angle distributions of the fragments produced when fast (MeV) molecular-ion beams from an electrostatic accelerator dissociate ("Coulomb explode") in thin foils and in gases, offer promising possibilities for deducing the stereochemical structures of the molecular ions constituting the incident beams. Bond lengths have been determined in this way for several diatomic projectiles (H2+, HeH+, CH+, NH+, OH+, N2+, O2+, etc.) with an accuracy of ~0.01 Å. H3+ has been demonstrated (for the first time) to be equilateral triangular and the interproton distance measured. Measurements on single fragments from CO2+, N2O+, C3H3+, and CHn+ have revealed the gross structures of the projectiles. An apparatus has recently been constructed at Argonne to permit precise measurements on fragments in coincidence. The apparatus has been tested on a known structure (OH2+) The O-H bond length was found to be 1.0 ± 0.04 Å and the H-O-H bond angle was measured as 110 ± 2°. These values are in excellent agreement with those found in optical experiments (0.999 Å and 110.5°). This "Coulomb explosion" technique can be expected to be refined in accuracy and to be extended to a wide range of molecular ions whose structures are inaccessible by other means.