DFT Studies on the Molecular Structure, Regioisomerism, Ground and Excited state Charge Transfer Properties of Spiro‐heterocycles

Abstract

The electronic structure, stereochemical aspects and charge transfer properties of the spiro compound (5S,8S) spiro‐{6‐phenyl‐1‐aza‐3‐oxa‐7‐thia‐bicyclo[3,3,0]‐octene‐8,3’(2’H)‐indol}‐2’‐one (spiro bicyclo adduct) synthesized using the multi‐component reaction via 1,3‐dipolar cycloaddition of isatin and thioproline with the dipolarophile phenylacetylene have been studied. The two lowest energy regio‐isomers of the spiro heterocycle formed from the stereospecific dipolar reaction have been analysed along with the electron‐withdrawing and releasing substituents using DFT method at the CAM‐B3LYP/6‐311++G(d,p) level of theory. The atomic charges and the stabilization energy due to the intramolecular charge delocalization process calculated using the natural orbitals at the different molecular sites reveal the different donor‐acceptor interactions occurring in the system. The intramolecular electronic transitions (specifically n to π* and π to π*) is found to occur from the localized donor end component to the acceptor end components in the multi‐component system. The hole produced by the ionization of the entities is found to migrate from the thioproline component to the phenyl acetylene part in a time scale of around 4 fs. The electron‐withdrawing and donating substituents on the spirobicyclo adduct have been proven to tune the excited and ground state charge transfer properties uniquely. The results can shed light on how multi‐component reactions can be directed to produce spiro adducts with versatile applications.