ISSN 0012-5016, Doklady Physical Chemistry, 2007, Vol. 414, Part 2, pp. 162–165. © Pleiades Publishing, Ltd., 2007. Published in Russian in Doklady Akademii Nauk, 2007, Vol. 414, No. 6, pp. 784–787. 162 1 Formation of negative molecular ions in the gas phase is often studied by negative ion mass spectrome- try with resonant electron capture (REC) by molecules [1] in combination with other methods [2–9]. This necessitates deep insight into the regularities of the REC processes. However, there are problems in the field. One of them is an anomalously long (on the order of microseconds) lifetime (τ) (relative to the electron autodetachment) of molecular ions formed by poly- atomic molecules at electron energies (E el ) of 1–2 eV. Generally, only ions formed in the ground state ( GS M – ) at E el close to zero have τ values on the order of micro- seconds. For the ions formed in the gas phase at E el > 0 eV, τ values on the order of 10 –8 –10 –9 μs are charac- teristic since such ions, being involved in single colli- sions, cannot dissipate the energy excess obtained with the additional electron. Parabenzoquinone (PBQ) is a classical example of this anomaly where negative molecular ions with τ = 30 μs are observed at E el = 1.35 eV [10–12]. Various hypotheses have been put for- ward in order to explain this phenomenon [10–12], but no one of these hypotheses has received wide accep- tance. For this reason, one more hypothesis has been proposed in the present work. In our opinion, the effect of abnormally long τ’s is associated with the transfor- mation of the doublet ions ( D M – ) forming at E el > 0 eV in the REC process into quartet ions ( Q M – ) with three unpaired electrons with parallel spins and, correspond- ingly, with a multiplicity of four (M = 4). Such a Q M – ion is similar to the doublet ion formed by the intershell resonance mechanism, which also has three unpaired electrons, but with anti-parallel spins [8, 13]. The trans- formation of the “initial” D M – ion to the Q M – ion is a result of intersystem crossing, i.e., the radiationless transition of the system from the potential energy sur- 1 The article was translated by the authors. face (PES) of the former to the PES of the latter, with an electron spin flip ( ) at the intersection point of the PESs: D M – → Q M – . The main feature of the Q M – ion is that it is unable to rapidly decay to the ground state molecule ( GS M 0 ) even if the molecule lies lower on the energy scale than the ion because this process also involves spin flip: Q M – → GS M 0 + e. As a result, the electron autodetachment is delayed in time. This mech- anism is analogous to that of phosphorescence; there- fore, the autodetachment delay should have the same microsecond time scale. The proposed concept has been confirmed in the present study by B3LYP/6-311++G** and B3LYP/6- 311+G* calculations, which allow us to estimate the relative positions of the PESs of the molecule and dif- ferent M – ions in their ground and excited states for PBQ and pyromellitic acid imide (PYRAI), where the analogous effect of abnormally long-lived M – ions with τ = 20 μs was revealed experimentally at E el = 1.65 eV, and for phthalic acid imide (PHAI), which has a struc- ture close to that of PYRAI, but lacks the second peak of long-lived M – ions at E el > 0. Figure 1 and 2 show two-dimensional representa- tions of the PESs of the most important (from the stand- point of the concept suggested) objects for PBQ and PYRAI, respectively. In order to construct the PESs, in particular, for PBQ, calculations with geometry optimi- zation to minimize the total energy were first performed for the GS M 0 molecule; the first triplet; the GS M – ion; the Q M – ion, for which two equilibrium conformations were found—the planar ( ) and bent ( ) con- formations (the same was obtained for PYRAI and PHAI); the transition state between and ; and the molecular ion with the charge q = –2 and M = 5 (Fig. 1, verticals I–VII). Then, in each of the geometries thus found, single point calculations were carried out of all objects other than the optimized one. In addition, the electronic spectra were calculated in all cases, which allowed us to estimate the energies of the resonances responsible for the formation of molecular ions in dif- ferent electronic states. It is worth noting that the calcu- lations reproduce well the experimental resonance ↔ ↔ ↔ M Q – pl M Q – bnt M Q – pl M Q – bnt Doublet–Quartet Intersystem Crossing in Negative Molecular Ions with an Abnormally Long Lifetime 1 O. G. Khvostenko a , G. M. Tuimedov a , and Corresponding Member of the RAS U. M. Dzhemilev b Received October 23, 2006 DOI: 10.1134/S0012501607060115 a Institute of Physics of Molecules and Crystals, Ufa Scientific Center, Russian Academy of Sciences, pr. Oktyabrya 151, Ufa, 450075 Bashkortostan, Russia b Institute of Petroleum Chemistry and Catalysis, Ufa Scientific Center, Russian Academy of Sciences, pr. Oktyabrya 141, Ufa, 450075 Bashkortostan, Russia PHYSICAL CHEMISTRY