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Comparative Theoretical Studies on Natural Atomic Orbitals

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Comparative Theoretical Studies on Natural Atomic Orbitals Indian Journal of Pure & Applied Physics Vol. 51, March 2013, pp. 178-184 Comparative theoretical studies on natural atomic orbitals, natural bond orbitals and simulated UV-visible spectra of N-(methyl)phthalimide and N-(2 bromoethyl)phthalimide V Balachandran1,*, T Karthick2, S Perumal3 & A Nataraj4 1P G & Research Department of Physics, Arignar Anna Government Arts College, Musiri, Tiruchirapalli 621 211, Tamil Nadu, India 2Department of Physics, Vivekanandha College for Women, Tiruchengode 637 205, Tamil Nadu, India 3P G & Research Department of Physics, S T Hindu College, Nagarcoil 629 002, Tamil Nadu, India 4P G & Research Department of Physics, Thanthai Hans Roever College, Perambalur 621 212, Tamil Nadu, India *E-mail: [email protected] Received 10 October 2012; revised 3 December 2012; accepted 11 January 2013 The charge delocalization patterns of phthalimide derivatives such as N-(methyl)phthalimide and N-(2-bromoethyl)- phthalimide have been performed with the help of natural bond orbital (NBO) analyses and simulated UV-visible spectra. The second order perturbation energies of the most interacting NBOs and the population of electrons in core, valance and Rydberg sub-shells have been predicted by density functional theory (DFT) computations in GAUSSIAN 03W software package. In the present study, the natural atomic orbital occupancies showed the presence of charge delocalization within the molecule. The natural hybrid atomic orbital studies enhance us to know about the type of orbitals and its percentage of s-type and p-type character. In addition, the excited electronic transitions along with their absorption wavelengths, oscillator strengths and excitation energies for the gaseous phase of the title molecules have been computed at TD-DFT/B3LYP/6- 311++G(d,p) method. The energetic behaviour of the compound in different solvent medium (water, ethanol, and methanol) has been examined by applying polarizable continuum model (PCM). The complete molecular orbital simulations and theoretical UV-visible spectra have been carried out in this study which yield better understanding of charge delocalization pattern and stability of the title molecules to a greater extent. Keywords: Charge transfer, Density functional theory, Natural bond orbital, Natural atomic orbital, Simulated UV-visible spectra 1 Introduction N-(methyl)phthalimide and N-(2-bromoethyl)phtha- In recent years, the charge delocalization patterns limide to estimate their delocalization patterns of and the chemical reactivity of polyatomic molecules charge, electron densities of atoms and excited have been focused widely. The information about the electron transition. charge delocalization and the chemical reactivity is Phthalimide and its N-substituted phthalimide being helpful to the drug designers to design new type derivatives are found in wide applications in of drugs. The natural bond orbital (NBO) is an pharmacy, medicinal chemistry and industry. In effective tool for the elucidation of residual resonance pharmaceutical, they are being used as an delocalization effects of a molecule and it also intermediate material. In medical field, the illustrates the deciphering of the molecular wave phthalimide analogues are being used in the synthesis function in terms of Lewis structures, charge, bond of anti-microbial activity, androgens and other agents order, bond type, hybridization, resonance, donor- for treating tumor necrosis factor8. Apart from their acceptor interactions, etc1. Therefore, the NBO well recognized applications in pharmacy, the analysis of certain pharmaceutical compounds has phthalimide analogues take an important role as anti- been performed by various spectroscopists2-7. For the convulsant, anti-inflammatory, analgesic, hypoli- complete assignments of charge delocalization pidimic and immunomodulatory activities9 in patterns of a molecule, the allowed excited electron medicinal chemistry. Certain phthalimide derivatives transitions and maximum absorption wavelenghts are are used as herbicides for reducing bacterial to be measured. Thus, in the present study, we have contamination. In industry, they are being widely used performed NBO and the simulated UV-Visible in the production of pesticides, dyes, plastics, high spectral analysis on the electronic structures of performance ion exchange resins, surfactants. In BALACHANDRAN et al.: NATURAL ATOMIC ORBITALS OF PHTHALIMIDE 179 particular, N-(methyl) phthalimide (abbreviated as Moreover, the information about excited electron NMP) and N-(2-bromoethyl)phthalimide (abbreviated transitions along with their absorption wavelengths, as NBEP) are being most extensively used as an excitation energies and oscillator strengths was intermediate material for the synthesis of a drug10,11. predicted at TD-DFT/B3LYP method with In our previous work, we have studied their 6-311++G(d,p) basis set. The solvent effects of the vibrational characteristics, non-linear optical molecules have also been examined by using properties and thermodynamic parameters10,11. To polarizable continuum model (PCM) with establish our previous work, the natural population TD-DFT/B3LYP/6-311++G(d,p) method. The analysis, natural bond orbital analysis and simulated simulated UV-Visible spectra of both gaseous and in UV-Visible spectra of NMP and NBEP have been different solvent phases have also been provided. performed in the present work. 3 Results and Discussion 2 Computational Details 3.1 Natural atomic orbitals Initially, the electronic structures of these The occupancies and energies of bonding molecules were optimized by solving self-consistent molecular orbitals of both the title molecules are field equation and the optimized structures predicted at B3LYP/6-311++G(d,p) level of theory corresponding to a single point minima were obtained and presented in Table 1. The variations in by density functional B3LYP method with standard occupancies and energies of NBEP in comparison high level 6-311++G(d,p) basis set. In order to obtain with those of NMP directly give the evidence for the the complete information regarding population of delocalization of charge upon substitution and this electrons in sub-shells of atomic orbitals and leads to the variation of bond lengths between the delocalization of charge and electron densities of molecules as shown in Figs 1 and 2. atoms in the title molecules, the molecular orbital calculations have been carried out using NBO 3.1 3.2 Natural population analysis program12 as employed in Gaussian 03W software The natural population analysis14 performed on the package13 at the DFT/B3LYP/6-311++G(d,p) level. electronic structures of NMP and NBEP clearly Table 1 — Comparison of occupancies and energies of bonding molecular orbitals between NMP and NBEP Atomic orbitalsa NMP Atomic orbitalsb NBEP Occupancies (e) Energies (a.u) Occupancies (e) Energies (a.u) BD (C1−N2) 1.9855 −0.8108 BD (C1−N2) 1.9839 −0.8344 BD (C1−C8) 1.9708 −0.6735 BD (C1−C8) 1.9764 −0.7162 BD (C1−O10) 1.9945 −1.1062 BD (C1−O10) 1.9922 −0.8005 BD (N2−C3) 1.9855 −0.8108 BD (N2−C3) 1.9836 −0.8387 BD (N2−C11) 1.9900 −0.7694 BD (N2−C11) 1.9880 −0.7865 BD (C3−C9) 1.9708 −0.6735 BD (C3−C9) 1.9773 −0.7179 BD (C3−O15) 1.9945 −1.1061 BD (C3−O18) 1.9923 −0.8191 BD (C4−C5) 1.9787 −0.7211 BD (C4−C5) 1.9764 −0.7215 BD (C4−C9) 1.9765 −0.7364 BD (C4−C9) 1.9751 −0.7481 BD (C4−H16) 1.9796 −0.5429 BD (C4−H19) 1.9786 −0.5508 BD (C5−C6) 1.9808 −0.7256 BD (C5−C6) 1.9813 −0.7376 BD (C5−H17) 1.9805 −0.5417 BD (C5−H20) 1.9805 −0.5547 BD (C6−C7) 1.9787 −0.7211 BD (C6−C7) 1.9787 −0.7238 BD (C6−H18) 1.9805 −0.5417 BD (C6−H21) 1.9804 −0.5539 BD (C7−C8) 1.9765 −0.7364 BD (C7−C8) 1.9767 −0.7496 BD (C7−H19) 1.9796 −0.5429 BD (C7−H22) 1.9786 −0.5519 BD (C8−C9) 1.9643 −0.7147 BD (C8−C9) 1.9559 −0.6815 BD (C11−H12) 1.9873 −0.5307 BD (C11−H12) 1.9712 −0.5540 BD (C11−H13) 1.9886 −0.5318 BD (C11−H13) 1.9800 −0.5500 BD (C11−H14) 1.9886 −0.5318 BD (C11−C14) 1.9875 −0.6559 BD (C14−H15) 1.9868 −0.5503 BD (C14−H16) 1.9846 −0.5554 BD (C14−Br17) 1.9870 −0.6327 a For numbering of atoms of NMP refer Fig. 1 b For numbering of atoms of NBEP refer Fig. 2 180 INDIAN J PURE & APPL PHYS, VOL 51, MARCH 2013 Fig. 1 — Optimized molecular structure of NMP Fig. 2 — Optimized molecular structure of NBEP describes the distribution of electrons in various sub- electron. Whereas, the most electropositive atoms shells of their atomic orbitals. The accumulation of such as; C1 and C3 have a tendency to accept an charges on the individual atom and the accumulation electron. Further, the natural population analysis of electrons in the core, valence and Rydberg showed that 84 electrons in the NMP molecule are sub-shells are also presented in Table 2. In the case of distributed on the sub-shells as follows: NMP, the most electronegative charge of −0.5589 e is accumulated on O10 and O15 atom and −0.5146 e is Core : 23.9907 (99.9614% of 24) accumulated on the nitrogen atom N2. According to Valence : 59.7553 (99.5922% of 60) an electrostatic point of view of the molecule, these electronegative atoms have a tendency to donate an Rydberg : 0.2540 (0.3023% of 84) BALACHANDRAN et al.: NATURAL ATOMIC ORBITALS OF PHTHALIMIDE 181 Table 2 — Accumulation of natural charges, population of electrons in core, valance, Rydberg orbitals of NMP and NBEP NMP NBEP Atoma Charge (e) Natural population (e) Total (e) Atomb Charge (e) Natural population (e) Total (e) core valence Rydberg Core valance Rydberg C1 0.6925 1.9993 3.2655 0.0428 5.3076 C1 0.6608 1.9989 3.3080 0.0322 5.3392 N2 −0.5146 1.9992 5.5002 0.0152
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