Spectroscopic, quantum chemical, and topological calculations of the phenylephrine molecule using density functional theory

Abstract

In this work, Density Functional Theory (DFT) on Gaussian 09 W software was utilized to investigate the phenylephrine (PE) molecule (C9H13NO2). Firstly, the optimized structure of the PE molecule was obtained using B3LYP/6-311 + G (d, p) and CAM-B3LYP/6-311 + G (d, p) basis sets. The electron charge density is shown in Mulliken atomic charge as a bar chart and also as a color-filled map in Molecular Electrostatic Potential (MEP). Using these properties, the possibility of different charge transfers occurring within the molecule was evaluated. The calculated values of the energy gap from HOMO-LUMO mapping, illustrated in Frontier Molecular Orbitals (FMO) and Density of State (DOS), were found to be similar for both the neutral and anion states in the gaseous and water solvent phases. Both the global and local reactivity were studied to understand the reactivity of the PE molecule. Using the thermodynamic parameters, the thermochemical property of the title molecule was understood. Non-covalent interaction was studied to understand the Van der Waals interactions, hydrogen bonds, and steric repulsion in the title molecule. Natural Bond Orbital (NBO) Analysis was performed to understand the strongest stabilization interaction. In the vibrational analysis, Total Electron Density (TED) assignments were done in the intense region where the frequency of the title molecule was shifted distinctly. For vibrational spectroscopy, FT-IR and Raman spectra in the neutral and anion states were plotted and compared. Using the TD-DFT technique, the UV-Vis spectra along with Tauc's plot were studied. Finally, topological analysis, electron localized function (ELF), and localized orbital locator (LOL) were performed in the PE molecule.

Keywords: DOS; ELF and LOL; HOMO-LUMO; MEP; NBO; TD-DFT; TED; UV-Vis.

Fig. 1

Optimized structure of PE molecule using basis set 6-311 + G(d, p) with B3LYP model in Gaussian09 software. The structure represents all bond lengths with its labled atoms and symbols. The molecule has one benzene ring with a planar structure; the hydroxyl and amino groups exhibit torsional angles.

Fig. 2

MEP maps for title molecule (a) in the neutral state and (b) in the anion state were obtained by using model B3LYP with basis set 6-311 + G (d, p) in Gaussian09 software. The different color gradient represents the electrostatic potential; the red color region indicates negative potential, and the blue color represents positive potential.

Fig. 3

The Mulliken charge distribution of the PE molecules in the neutral and anion states is performed with the model B3LYP in the basis set 6311 + G (d, p) in the Gaussian09 package.

Fig. 4

The calculated HOMO-LUMO and the energy gap of the PE molecule in (a) the gaseous phase and (b) the water solvent phase using the CAM-B3LYP model with a scaling parameter of 0.961 in the basis set 6-311 + G (d, p). In the neutral state with the water solvent phase, the energy gap between HOMO and LUMO is greater, indicating more stability and less reactivity.

Fig. 5

HOMO-LUMO and energy gap of the PE molecule in anion state (alpha and beta) in (a) the gaseous phase and (b) solvent water phase are obtained using the CAM-B3LYP model in the basis set 6-311 + G (d, p). The energy gap between HOMO and LUMO in the anion state in the water solvent phase has more than an anion phase.

Fig. 6

The PE molecule’s density of states (DOS) shows the range of electronic states that can be occupied in the neutral state in (a) the gaseous phase and (b) the water solvent phase, from − 15 eV to 15 eV. The DOS was calculated using the CAM-B3LYP model in the basis set 6-311 + G (d, p) using Gaussian09. The gap between the valance (occupied) and conduction (virtual) states in the neutral gas phase is 8.02 eV and in the water solvent phase 8.30 eV.

Fig. 7

The density of states (DOS) of the title molecule reveals electronic states that can be occupied in the neutral state in (a) the gaseous phase and (b) the water solvent phase, within the range of -15 eV to 15 eV, using the CAM-B3LYP model in the basis set 6-311 + G (d, p) with Gaussian09.

Fig. 8

Reduced density gradient analysis RDG and NCI (a) 2D and (b) 3D determines weak and strong interactions in title molecules in neutral state.

Fig. 9

FT-IR spectrum of the PE molecule for transmittance with wavenumber ranges from (0–4000 cm^-1) (a) in the neutral state (b) in the anion state.

Fig. 10

Raman spectra of the PE molecule with wavenumber ranges from (0–4000 cm^-1) for (a) the neutral state and (b) the anion state.

Fig. 11

The UV-Vis absorption spectra of the PE molecule in its neutral state within an aqueous solvent phase, along with the optical energy gap derived from Tauc’s figure, are presented.

Fig. 12

The color-filled map of the PE molecule for (a) ELF shows the electrons localized and delocalized with a color gradient and (b) LOL shows the orbital localized and delocalized with a color gradient.