Methyl Fluorocarbonyl Disulfide (FC(O)SSMe): A Theoretical Study on the Structural and Conformational Properties of Its Neutral Ground State and Lowest‐Lying Cationic State

In this work, we present a theoretical study on the structural and conformational properties of FC(O)SSMe in its neutral and cationic ground states. The structure of the neutral molecule, as deduced from Hartree? Fock (HF), Density Functional Theory (DFT), and Møller? Plesset (MP2) methods, agrees with the experimentally determined value for the CSSC dihedral angle (C? S bonds gauche with respect to each other) and with the syn preference of the SSCO dihedral angle (C?O bond syn with respect to the S? S bond). The calculated values for these two dihedral angles are 81.9 and 4.2 degrees, respectively. From the energy difference of the anti vs. syn conformer computed at the CCSD(T)/6‐311++G** level of theory, a 3% contribution of a less‐stable conformer at room temperature is proposed. The potential barrier of rotation about the S? S bond is 5.7 kcal/mol (B3PW91/6‐311++G** approximation). The FC(O)SSMe molecule adopts a planar structure after ionization, the anti conformer (CSSC dihedral angle 180°) being the most‐stable form. For the first ionization of the title compound, the adiabatic ionization potential (IP^ad) derived from the three mentioned theoretical methods (using the 6‐311++G** basis sets) is 8.48, 9.06, and 8.99 eV, whereas the vertical ionization potential (IP^ver) is 8.96, 9.79, and 9.62 eV, respectively (experimental value: 9.0 eV). The results are compared with previous experimental studies carried out for the neutral and charged species interpreted on the basis of the Natural Bond Orbital (NBO) analysis. From these calculations, the importance of the anomeric and mesomeric effects becomes evident. The preferred conformation can be quantitatively explained by evaluation of donor/acceptor interaction energies.