ISSN:
0894-3230
Keywords:
Organic Chemistry
;
Physical Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
,
Physics
Notes:
The molecular and electronic structures of the cyclic 1,2-dithiolium ylides (2) and their acyclic dithiocarbonyl valence isomers (4) were studied by ab initio methods. All molecular structures were fully optimized at the R(U)HF/6-31G* and R(U)MP2/6-31G* levels of theory. The results of the restricted and unrestricted Hartree-Fock (HF) calculations are less reliable since the RHF wavefunctions of the cyclic compounds proved to be singlet/triplet unstable. A marked ground-state energy depression occurs on passing from restricted to unrestricted HF methods. The title compounds are of diradicaloid nature and belong to the non-classical class of structures. The heterocycles are planar and display C2v symmetry, while the cisoid open-chain dithiocarbonyl isomers are slightly distorted (C2 symmetry). According to the calculated bond lengths and charge distributions, the title compounds are in fact more or less zwitterionic. The thiolate is the most polar compound of this series. According to RMP2 calculations, the thiolate (2, X = S) and olate (2, X = O) should exist in the single ground rather than in the triplet state. However, in these cases the isomeric acyclic compounds are predicted to be almost isoenergetic with the cyclic compounds. Since the cyclic ylides have higher dipole moments than the acyclic dithiocarbonyls, the ylides will gain stability in polar solvents. However, the continuum quantum chemical model used in estimating the solvent effect does not favour the ring compounds sufficiently to exclude valence isomerization between the ring-closed and ring-open compounds.
Additional Material:
1 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/poc.610080804
