Raman Spectroscopic Study and DFT Calculations on the Conformation of 5-azonia-spiro[4.4]nonane Cation in Crystal and Dimethyl Carbonate Solution

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5-Azonia-spiro[4.4]nonane tetrafluoroborate [(CH2)4N(CH2)4]+BF4− is dissolved in dimethyl carbonate (DMC) to yield two liquid phases, DMC and salt-rich DMC solutions. The molecular structure of the [(CH2)4N(CH2)4]+ ion in the crystalline state and in the salt-rich DMC solution was studied by means of Raman spectroscopy at 298 K and theoretical DFT calculations. The [(CH2)4N(CH2)4]+ ion involves two pyrrolidinium (CH2)4N rings bridged through the N atom, and each pyrrolidinium ring involves various types of conformation. Theoretical DFT calculations for the [(CH2)4N(CH2)4]+ ion show that the distorted envelope-envelope conformers of the type E1-E1 and E1-E3 give relatively small energies. Observed Raman spectra of [(CH2)4N(CH2)4]+BF4− crystals and the salt-rich DMC solution were satisfactorily explained in terms of the favorable presence of the sole envelope-envelope conformer of the type E1-E1.