Ab initio study of the isomerism of (LiAB)(2) salt dimers with 24 valence electrons (AB(-) = NO-, PO-, NS-, PS-)

Ab initio MP2/6-31G*//HF/6-31G* + ZPE(HF/6-31G*) and MP4SDTQ/6-31G*//MP2/6- 31G* + ZPE(HF/6-31G*) calculations of potential energy surfaces (PESs) were performed in the vicinity of key structures of nonrigid lithium salt dimer s (LiNO)(2), (LiPO)(2), (LiNS)(2), and (LiPS)(2) with 24 valence electrons. A series of isomers was localized in which cis and trans ONNO2-, OPPO2-, S NNS2-, and SPPS2- dianions with double N-N and P-P bonds are incorporated i nto three-, four-, and five-membered cycles containing lithium bridges. For hyponitrite and hypophosphite dimers, cis isomer 5, C-2v with both cations bidentately coordinated to the chalcogen atoms was found to be the most fa vorable; other structures of these salt dimers are substantially less favor able. For the (LiNS)(2) dimer, another planar cis structure (4, C-2v) and t wo planar trans isomers (2, C-2h and 3, C-2h) are close in energy to struct ure 5. In structure 4, one cation is coordinated to two sulfur atoms and th e other cation, to the N-N bond. In structure 3; both cations are coordinat ed to the NS bonds. All structures possess gently sloping PESs and are stru cturally nonrigid to strong in-plane and out-of-plane angular distortions g iving rise to small energy changes. Nonplanar conformations are separated w ith low energy barriers and can invert to one another. The equilibrium geom etry parameters, relative and dissociation energies of isomers, and frequen cies and spectral intensities of their normal vibrations were determined. T he geometrical distortions and shifts of vibrational frequencies of cis and trans dianions under the action of cations in different coordination modes are analyzed, and the trends in molecular properties in different series o f (LiAB)(2) dimers are examined. The results of calculations are compared t o those of previous calculations of the monomer LiAB salts and Li(2)AB(+) i ons with N-val = 12 and of the (LiAB)(2) dimers with N-val = 20.