INTRODUCTION

The Diels Alder reaction is an electrocyclic reaction which involve [4+2] cycloaddition of 4π electrons of the conjugated diene and 2π electrons of the dienophile. The reaction involves the formation of new sigma(ϭ) bonds which are energetically more stable than π bonds. The Diels Alder cycloaddition reaction does not require any metal catalyst, it is one of the most useful reactions in synthetic organic chemistry and material design. Diels Alder is used for synthesis of polymers and dendrimers for drug delivery, preparation of functionalized surfaces, bio-conjugation techniques, in nanotechnology and for the preparation of hydrogels for drug delivery and tissue engineering.

The Density Functional Theory (DFT) preforms quantum chemical calculations to investigate electronic structure^2-4. Computational and DFT approach is widely applied in organic and inorganic chemistry, material sciences like metallurgy and for electronic materials⁵. The discovery, success and applicability of DFT lies in mid-1960’s by Walter Kohn, Pierre Hohenberg and Lu Jeu Sham. In recent decades DFT became and essential tool for material sciences, chemistry and many other fields^6-8. Investigation of electronic activities of a molecule is a challenging task today. Each molecule has some hidden properties which are based on their electronic interactions. To determine such interactions and the behaviour of the molecule, Density Functional Theory (DFT) is the most commonly used method for analysis of HOMO LUMO and muliken charges.

Gara and co-workers⁹ studied the investigation of solvent substituents and catalyst effect on the intermolecular Dias and reaction. Abdulai et.al¹⁰ studied DFT on mechanism on oxidative dehydrogenative diels alder reaction of alkyl benzenes. Harmandez¹¹ explained the reactivity of chiral anthracene template with symmetrical an unsymmetrical dienophile. Where as the influence of noncovalent interaction in the exo and regioselective interaction of aza diels alder reaction, experimental and DFT calculations were summarized by Gallardo Fuentes¹² and co-workers.

Intermolecular Dias elder approaches to the decline core of veronginolide was analyse by Maiga Wandiam¹³. DFT study of mechanism and stereochemistry of the Rh catalysed reactions between electronically neutral dienes and dienophiles explained by Liao et.al.¹⁴ Understanding the high reactivity of triazolidinone in Diels Alder reaction is summarized by Fernandes¹⁵ Ab initio study of Diels Alder reaction of cyclopentadiene with acrolein in ionic liquid by KS-DFT theory Cippe et.al.¹⁶ DFT study of Diels Alder reaction on single wall carbon nanotube is studied by Nunzi et.al.¹⁷ Domingo and Co-workers reported unexpected regioselectivity of hetero Diels Alder reaction of a symmetric tetrazine with electron rich ethylene by the DFT study. Fernandes and coworkers reported DFT study between cycloaddition of alkenyls metal carbene complex and neutral 1,3 dienes.¹⁸ DFT study of dials under reaction of o-quinone methides with various substituted ethenes selectivity and reaction mechanism was reported by Wang et.al.¹⁹

In our pursuit for optimised structure of methyl 2-methoxycyclohex-3-ene-1-carboxylate we have carried out DFT study for the structure optimization, transition state, vibrational, electronic and structural analysis. PES scan is helpful for analysis of conformational isomer by changing the dihedral angle, bond angle, bond length of a particular set of atoms in a molecule.

MATERIALS AND METHODS:

Materials:

In order to find the most stable conformer of Diels-Alder adduct methyl 2-methoxycyclohex-3-ene-1-carboxylate the DFT study was carried out by Gaussian (9W) software using B3LYP method with 6-311G (d,p) basis set in gas phase. The optimized parameters performed as vibrational frequency calculation in DFT to characterize all the stationary points as minima. The B3LYP method was used to calculate molecular electrostatic potential of reactive sites of the molecule. Also, the HOMO LUMO energy gap was determined. PES scan was performed by changing the dihedral angle between (C19,C5,C6,O26) two substituents with 10 degree increment. From DFT calculations bond parameters like bond angle, bond length can be calculated.

Fig 1: Diels-Alder reaction between 1-methoxy 1,3butadiene (1) and methyl acrylate (2)

Fig 2: Diels-Alder reaction products

OPTIMIZED STRUCTURE:

The optimized molecular structure with numbering of atoms of methyl 2-methoxycyclohex-3-ene-1-carboxylate are shown in fig.3. The parameters like bond length, bond angle, dihedral angle are calculated by B3LYP/6-311G (d,p) method and represented in table no.1

Fig 3: Optimized Structures of stereoisomers of methyl 2-methoxycyclohex-3-ene-1-carboxylate, A (e,e), B (a,e) and C (aa).

Table No 1. Optimised bond parameters of (e,e) methyl 2-methoxycyclohex-3-ene-1-carboxylate (A)

Sr. No.	Atom Symbol	NA	NB	NC	Bond length	Bond angle	Dihedral angle
1	C
2	C	1			1.3406986
3	C	2	1		1.5113109	123.3951679
4	C	3	2	1	1.5408772	112.2253986	13.1937063
5	C	4	3	2	1.5528283	110.8067813	-43.2338181
6	C	1	2	3	1.5081372	123.7265165	1.1712385
7	H	3	2	1	1.0989448	109.9792267	135.4382654
8	H	2	1	6	1.0884724	119.7619722	-178.1061519
9	H	1	2	3	1.0875169	120.2799669	-179.9788109
10	H	4	3	2	1.0984886	109.7573447	76.8225669
11	H	4	3	2	1.0954940	110.5094607	-164.9806840
12	H	5	4	3	1.0952971	108.6520921	-57.7179612
13	H	6	1	2	1.1007635	110.8071458	-106.2027182
14	H	3	2	1	1.1019742	109.2802338	-108.9716924
15	C	6	1	2	2.4564881	107.1611549	167.8779563
16	H	15	6	1	1.0912927	138.8986838	-105.0202589
17	H	15	6	1	1.0990292	95.0913112	21.7436594
18	H	15	6	1	1.0982711	92.3260327	131.1927353
19	C	5	4	3	1.5117177	111.1795167	-176.4128895
20	C	19	5	4	2.4234540	144.0892109	84.0249698
21	H	20	19	5	1.0881581	135.2697781	3.2025553
22	H	20	19	5	1.0920060	93.3103383	128.0988891
23	H	20	19	5	1.0923122	93.7567163	-122.3128598
24	O	19	5	4	1.2368964	126.1581993	-91.9713572
25	O	19	5	4	1.3753668	111.2950677	85.6087194
26	O	15	6	1	1.4501573	32.9699036	-102.2331152

Molecular electronic potential (MEP):

Molecular Electrostatic Potential (MEP) is related to electronic density. It is useful to determine the electrophilic and nucleophilic reactions. (20) The charge distribution of molecule arranged in 3D orientation is shown in fig.4. The electrostatic potential plot is based electron density with molecule at different sites. (21) The key role of MEP is to determine molecular size, shapes, and positive, negative and neutral electrostatic potential regions also charge densities and reactive sites. Different values of electrostatic potential are given by different colours. The lone pair of oxygen are covered by red and yellow region sites. Red/Yellow shows most electronegative electrostatic potential and electrophilic reactivity whereas green shows positive electrostatic potential and nucleophilic reactivity.

Fig 4: Molecular Electrostatic Potential (MEP) surfaces of (e,e) methyl 2-methoxycyclohex-3-ene-1-carboxylate

Vibrational spectra:

The result of DFT calculated experimental FTIR spectra of the methyl 2-methoxycyclohex-3-ene-1-carboxylate are represented in fig.5. We compare the experimental and DFT calculated IR vibrational spectra of the compound (3). The results are in good agreement with each other. The appearance of peak at 1055 cm^-1, 1196cm^-1 are indicated the C-O stretching characteristic peak. The peak appears at 1744 cm^-1 indicate that C=O stretching of the ester group and 3000cm^-1, 3100cm^-1 appeared peak indicate characteristic peak for C-H stretching shown in fig.3.

Fig 5: Theoretical FTIR spectra of (e,e) methyl 2-methoxycyclohex-3-ene-1-carboxylate

Potential energy surface (PES) scan:

Potential Energy Surface (PES) Scan describes the energy of a system especially a collection of atoms in terms of parameters normally the position of atoms. The PES curve is investigated to determine the stable structures of methyl 2-methoxycyclohex-3-ene-1-carboxylate as shown in fig.6. From PES scan we analyzed that the molecule has minimum total energy at70 degree (e,e) position. As we go on changing the dihedral angle by 10 degrees, we can see the change in total energy. At 160 degree the energy is maximum at (a,a) position . The relative energies of this configuration are given in table no.2

Fig 6: Potential Energy Surface (PES) Scan of (e,e) methyl 2-methoxycyclohex-3-ene-1-carboxylate

Table No 2. Optimised bond parameters of (e,e) methyl 2-methoxycyclohex-3-ene-1-carboxylate

Sr.No	Dihedral angle	Total energy
1.	70.0000288685	-576.8636478050
2.	80.0000288685	-576.8630808500
3.	90.0000288685	-576.8614172960
4.	100.0000288685	-576.8592637770
5.	110.0000288685	-576.8573607270
6.	120.0000288685	-576.8562054940
7.	130.0000288685	-576.8556967390
8.	140.0000288685	-576.8554487390
9.	150.0000288685	-576.8550528320
10.	160.0000288685	-576.8544602460
11.	170.0000288685	-576.8608709090
12.	180.0000288685	-576.8576788330

Frontier Molecular Orbital (FMO):

The Frontier Molecular Orbital offers a reasonable qualitative information of the excitation properties and the ability of electron transfer (22). The electronic absorption basically means the transition from the ground state to the excited state and mainly described by electron transition from the Highest Occupied Molecular Orbital (HOMO) to the Lowest Unoccupied Molecular Orbital (LUMO). The HOMO and LUMO energies refer to the ability of donating and accepting electron respectively. The large energy gap between HOMO & LUMO indicates that the molecule is more stable but less reactive as the electron transfer from the ground state to excited state will occur slowly due to large energy gap whereas the small energy gap indicates that the electron transfer from the ground state to excited state will take place fast and as a result the molecule will be more reactive but less stable. The HOMO-LUMO energy gap for methyl 2-methoxycyclohex-3-ene-1-carboxylate molecule for equatorial-equatorial(e,e) conformer is E=6.516ev and for axial-axial (a,a) conformer is E=6.492ev.( fig 7)

Fig 7: Frontier Molecular Orbital of (e,e) methyl 2-methoxycyclohex-3-ene-1-carboxylate

CONCLUSION:

The analysis indicated that the dihedral angle 70 degree between two substituent is optimal therefore ee conformer is most stable having least total energy (-576.864 hartree) and higher HOMO LUMO gap (6.516ev). The FTIR analysis shows prominent stretching for C-H, C-O and C=O vibrations. In MEP mapping the red and yellow regions designates the negative electrostatic potential and electrophilic reactivity, whereas blue or green regions for nucleophilic reactivity. By using DFT study we concluded that the experimental and the theoretical values are related with each other. therefore, DFT study is an important tool for analysis of structural parameter of any reaction or product.

CONFLICT OF INTEREST:

The authors have no conflicts of interest regarding this investigation.

ACKNOWLEDGMENTS:

The authors express sincere gratitude towards Management, Principal, Anekant Education Society’s Tuljaram Chaturchand College Baramati. Head, and P.G. Coordinator Department of Chemistry and ARC T.C. College for facility and financial support through student minor research project. The authors are also grateful to coordinator, CFC for DFT and FTIR analysis.

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Received on 22.12.2022 Modified on 18.03.2023

Asian J. Research Chem. 2023; 16(3):216-220.

DOI: 10.52711/0974-4150.2023.00034