Photoelectron spectroscopy and ab initio study of mixed cluster anions of [(CO2)(1-3)(Pyridine)(1-6)](-): Formation of a covalently bonded anion coreof (C5H5N-CO2)(-)

Anions of mixed cluster between carbon dioxide (CO2) and pyridine (C5H5N, d enoted by Py), each possessing a negative adiabatic electron affinity, were studied. The minimum size cluster anion was the 1:1 complex of [(CO2)(1)(P y)(1)](-), which displayed highest mass spectral intensity in the series. I t was shown by photoelectron spectroscopy that [(CO2)(1)(Py)(1)](-) is not a typically expected ion-molecule complex of the type (CO2)(1)(-)(Py)(1) or (CO2)(1)(Py)(1)(-). Ab initio calculation in the UHF/6-311++G** level reve aled that [(CO2)(1)(Py)(1)](-) is indeed a totally new anion of the type (C 5H5N-CO2)(-) with a planar C-2v structure, bound by an intermolecular bond of a strongly covalent character. The calculation predicted 1.24 eV for the vertical detachment energy of this anion, which is in reasonably good agre ement with the measured value of 1.46 eV by photoelectron spectroscopy. A M ulliken analysis showed that the excess negative charge is fully delocalize d throughout the entire molecular frame. Further calculation on intramolecu lar rotational barrier predicted a considerable barrier height of similar t o 0.68 eV, which could be due to an additional, weak pi-bond character of t he newly formed C-N sigma-bond. Such extra pi-bond network was suggested to provide a path for the charge delocalization and thus lead to the stabilit y of the anion core. (C) 2000 American Institute of Physics. [S0021-9606(00 )30226-4].