Molecular structure and vibrational and chemical shift assignments of 3,5-bis-(4-methylbenzoyl)-2,6-bis(4-methylphenyl)-4H-pyran-4-one: A combined experimental and theoretical analysis

Abstract

The molecular geometry, vibrational frequencies, gauge-including atomic orbital (GIAO) ¹H and ¹³C chemical shift values of 3,5-bis-(4-methylbenzoyl)-2,6-bis(4-methylphenyl)-4H-pyran-4-one in the ground state have been calculated by using the Hartree–Fock (HF) and density functional method (DFT/B3LYP) with 6-31G(d) basis set. And this structure has been confirmed by IR, ¹³C and ¹H spectroscopy. The results of the optimized molecular structure are presented and compared with the experimental X-ray diffraction. The computed vibrational frequencies are used to determine the types of molecular motions associated with each of the experimental bands observed. Also, calculated ¹³C and ¹H chemical shift values are compared with the experimental ones. The data of the title compound display significant molecular structure. Moreover, its IR and NMR spectroscopic analysis provide the basis for future design of efficient materials having the pyran core.

Introduction

The derivatives of the 4H-pyran-4-one heterocyclic system which are known as 4- or γ-pyrones are usually quite stable crystalline compounds. However, it is used for different derivatives in the literature [1]. Besides, 4H-pyran-4-ones and their various derivatives have been reported to exhibit diverse biological activities [2], [3], [4], [5], [6]. Furthermore, the synthesis and molecular structure of the title compound were reported to have as terasubstituted-4H-pyran-4-one with 2,6-bis(p-tolyl) and 3,5-bis(p-toluyl) substituents [7].

The aim of the present work is to describe and characterize the molecular structure, experimental and theoretical vibrational properties and chemical shifts on 3,5-bis-(4-methylbenzoyl)-2,6-bis(4-methylphenyl)-4H-pyran-4-one crystalline-structure. A number of papers have recently been appeared in the literature concerning the calculation of NMR chemical shift (c.s.) by quantum-chemistry methods [8], [9], [10], [11], [12], [13], [14]. These papers indicate that geometry optimization is a crucial factor in an accurate determination of computed NMR chemical shift. Moreover, it is known that the DFT (B3LYP) method adequately takes into account electron correlation contributions, which are especially important in systems containing extensive electron conjugation and/or electron lone pairs. However, considering that, as molecular size increases, computing-time also increases. It was proposed that the single-point calculation of magnetic shielding by DFT methods was combined with a fast and reliable geometry-optimization procedure at the molecular mechanics level [12].

The gauge-including atomic orbital (GIAO) [15], [16] method is one of the most common approaches for calculating nuclear magnetic shielding tensors. It has been shown to provide results that are often more accurate than those calculated with other approaches, at the same basis set size [17]. In most cases, in order to take into account correlation effects, post-Hartree–Fock calculations of organic molecules have been performed using: (i) Møller–Plesset perturbation methods, which are higly time consuming and hence applicable only to small molecular systems, and (ii) density functional theory (DFT) methods, which usually provide significant results at a relatively low computational cost [18]. In this regard, DFT methods have been preferred in the study of large organic molecules [19], metal complexes [20] and organometallic compounds [21] and for GIAO ¹³C c.s. calculations [17] in which cases the electron correlation contributions were not negligible.

In the previous work, the crystal structure of the title compound was studied as experimental [7]. To best of our knowledge, no theoretical results exist for the title compound. In this study, the IR, ¹H and ¹³C (in the CDCl3 solvent) of the title compound are analyzed. Besides, the geometrical parameters, fundamental frequencies and NMR (¹H and ¹³C) chemical shifts of the title compound have been calculated in the ground state by using the HF and DFT (B3LYP) method with 6-31G(d) basis set. A comparison of the experimental and theoretical spectra can be very useful in making correct assignments and understanding the basic chemical shift-molecular structure relationship. And so, these calculations are valuable for providing an insight into molecular analysis.