Theoretical Study of the Interaction between the Guanidinium Cation and Chloride and Sulfate Anions.

The interactions between the guanidinium cation and the chloride and sulfate anions have been explored theoretically. As a basis for this study, the structure of the guanidinium cation has been reexamined and confirmed to possess a nonplanar propeller structure. Thus, computations at the MP2/6-31+G* level yielded several guanidinium-chloride complexes in 1:1 and 2:1 stoichiometry, with the most stable being those in which the guanidinium cation(s) interact in pincerlike form with the spherical chloride anion. In the case of guanidinium-sulfate complexes, a large number of stable 1:1 dimers and 2:1 trimers were found at the same level of computation. The nature of all the interactions established has been analyzed by means of the AIMs and NBO approaches, and several correlations have been found between electron density at the bond critical points, hydrogen bond distances, and orbital interaction energies. The guanidinium moiety has been widely exploited as an anion receptor in supramolecular chemistry and bears relation to the arginine amino acid, which interacts with anionic substrates within proteins. Thus, the current study provides insight into the nature of the interactions involving the guanidinium cation within both abiotic and natural systems.