Spectroscopic (FT-Raman, FT-IR, UV-Vis, and NMR) and Theoretical Analysis of 1-Methylindole: Structural Characterization, Non-Covalent Interactions, and Electronic Properties

The structural and spectroscopic characterization of 1-Methylindole (1MI) by favors of FT-Raman (4000–200 cm−1), FT- IR (4000–400 cm−1), UV-Vis, and 1H and 13C NMR techniques is presented. The experimental observations were enlightened by density functional theory (DFT) calculations at the B3LYP/6-311G(d,p) level of theory. Geometrical structure of the molecule was obtained, and bond order analysis was performed based on the optimized structure. Non-Covalent Interactions (NCIs) were analyzed by using the Reduced Density Gradient (RDG) with energy density descriptors to visualize the limiting regions of these interactions. Furthermore, molecular charge distribution and isosurface mappings with local extrema were obtained and the critical regions on the molecular surface were visualized. The vibrational spectra were calculated, and the normal modes were assigned based on total energy distribution (TED) calculations. The electronic properties of 1MI were explored experimentally through UV-Vis spectroscopy and analyzed in detail via Atoms in Molecules (AIMs) methodology. Total and partial density of state (TDOS and PDOS) and overlap population density of state (OPDOS) diagrams were calculated and fractional contributions of nonpolar methyl group and aromatic indole to frontier molecular orbitals were obtained through this methodology. Theoretical NMR chemical shifts were assigned based on DFT calculations that use the gauge-invariant atomic orbital (GIAO) method. Inclusion of solvents effect in NMR calculations produces twice less dispersive data and better fitting results to experimental observations. Non-linear optical properties: polarizability, anisotropy of polarizability, and first hyperpolarizability of the molecule were also computed to explore the potential of 1MI as nonlinear spectroscopy agent.