DFT CALCULATIONS ON 1-HETERO-2,5-CYCLOHEXADIENE-1-OXIDE: MOLECULAR STRUCTURE AND STABILITY E. Vessally * Payame Noor University (PNU), Zanjan, Iran

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1 Bull. Chem. Soc. Ethiop. 21, 24(2), ISSN Printed in Ethiopia 21 Chemical Society of Ethiopia DFT CALCULATIONS ON 1HETERO2,5CYCLOHEADIENE1OIDE: MOLECULAR STRUCTURE AND STABILITY E. Vessally * Payame Noor University (PNU), Zanjan, Iran (Received September 28, 28; revised January 22, 21) ABSTRACT. Molecular structures of 1hetero2,5cyclohexadiene1oxide, O, are studied and compared with their corresponding deoxygenated compound, 1hetero2,5cyclohexadiene,, using DFT at B3LYP/6311G** level. The most stable boat conformers are found for O O, S O, Se O, P O, As O, S, Se, N, P, As, and the most stable planar conformers are also found for N O, C O, Si O, Ge O, O, C, Si, Ge. Isodesmic reactions to determine the stabilities of O and are considered. Nuclear independent chemical shifts,, are calculated for the investigation of the homoaromatic character of O and. The optimised geometries show the bonding in the molecules is explicable in terms of basic chemical concepts. The atomic charges calculated are also reasonable based on the concepts of electronegativity and conjugation. KEY WORDS: Molecular structure, Stability, 4HThiopyran, 4HThiopyran1oxide, 1Hetero2,5 cyclohexadiene1oxide, Isodesmic reaction, INTRODUCTION 1Hetero2,5cyclohexadienes,, specially 4Hthiopyrans are of interest to the chemists due to their corresponding photochromic and photoisomeric characters [16]. Therefore, we studied the structure of 1hetero2,5cyclohexadiene,, through DFT calculations [7]. Derivatives of 4Hthiopyran are simply converted to the corresponding 4Hthiopyran1oxides and/or 4Hthiopyran1,1dioxides [81]. The oxidation of organic sulfides to those corresponding sulfoxides and sulfones is a wellunderstood process for which a large variety of synthetic methods exist [1119]. The H 2 O 2 or trifluoroacetic acid is used for the selective oxidation of sulfides to sulfoxides [2]. Sulfones are produced through the oxidation of sulfides with peracids [2124]. In this work, the conformational structure and molecular stability of 1hetero 2,5cyclohexadiene1oxide, O, are studied and compared with their corresponding deoxygenated 1hetero2,5cyclohexadiene,. Isodesmic formal reactions and homoaromatic character indicate the molecular stability. The homoaromatic or aromatic character is not a directly measurable or computable quantity. Aromatic character is generally evaluated on the basis of magnetic, energetic and geometric criteria [2526]. Magnetic criterion is the important way for the determination of aromatic character. Magnetic criterion is measured through nuclear independent chemical shifts,. The concept of was introduced by Schleyer et al. in 1996 as a measure of aromaticity and antiaromaticity (or nonaromaticity) [27]. It is based on a probe with no basis functions (bq) which is placed at or above the geometrical center of a conjugated ring. Its calculated isotropic NMR chemical shift indicates the aromatic properties of the ring, either as an individual moiety in a polycyclic compound or as a molecule. Initially the probe was placed at the geometrical center of the molecules, but after realizing that in some systems the chemical shifts are influenced by the σ system (e.g., cyclopropane) it was placed.5 Å above the center (denoted as (.5)). The method has been used for the assignment of aromatic character in many systems, generally very successfully. *Corresponding author. e_vesali@yahoo.com

2 218 E. Vessally THEORETICAL METHODS Full geometric optimizations of 1hetero2,5cyclohexadiene1oxide and 1hetero2,5 cyclohexadiene were carried out by the density functional theory (DFT) (Scheme 1). Gaussian 98 offers a wide variety of DFT models [28]. B3LYP method is the best method of DFT model. The B3LYP method is formed through a combination of Becke s three parameters hybrid functional and the LYP semilocal correlation function. 6311G** basis set is used with the B3LYP method [293]. In order to find thermochemical parameters including thermal energy (E), thermal enthalpy (H) and Gibbs free energy (G), keyword Freq was used. = O O O, S S O, Se Se O, N N O, P P O, As As O, C C O, Si Si O, Ge Ge O Scheme 1. The molecular structures of 1hetero2,5cyclohexadiene1oxide, O, and 1hetero 2,5cyclohexadiene,. RESULTS AND DISCUSSION The molecular structures of 1hetero2,5cyclohexadiene1oxide ( O ), 4Hpyran1oxide (O O ), 4Hthiopyran1oxide (S O ), 4Hselenopyran1oxide (Se O ), 4Hpyridine1oxide (N O ), 4Hphosphinine1oxide (P O ), 4Harsinine1oxide (As O ), 2,5cyclohexadienone (C O ), 4Hsiline1 oxide (Si O ), 4Hgermine1oxide (Ge O ) are studied and compared with their deoxygenated analogues 1hetero2,5cyclohexadiene (), 4Hpyran (O), 4Hthiopyran (S), 4Hselenopyran (Se), 4Hpyridine (N), 4Hphosphinine (P), 4Harsinine (As), 1,4cyclohexadien (C), 4Hsiline (Si), and 4Hgermine (Ge) using B3LYP/6311G** level (Scheme 1). Table 1. Bond angle (degree), dihedral angles (degree) and the NBO charge atoms for thiopyran1oxide and their analogues at B3LYP/6311G** level H(O) 7 Compound A 2,1,6 D 2,1,6,5 D 2,3,4,5 D 2,1,6,7 1 C 2 C 3 C 4 O 7 (H 7) = O O =O = S O = S = Se O = Se = N O = N = P O = P = As O = As = C O = C = Si O = Si = Ge O = Ge Bull. Chem. Soc. Ethiop. 21, 24(2) 5 6

3 DFT calculations on 1hetero2,5cyclohexadiene1oxide 219 The geometrical parameters and NBO charge on atoms are calculated via B3LYP/6 311G** level. For O and, the bond angle A 2,1,6 is decreased from second row to fourth row elements of the table because of increasing the p character of heteroatom (Table 1). This is in good agreement with the simple textbook theory that orbital penetration causes a greater energy difference between the p and s atomic orbitals as one goes down the periodic table. Hence bonding to spn hybrid orbitals is favoured in the 1 st row and bonding to purer porbitals further down giving a bond angle nearer 9 degrees. The dihedral angles D 2,1,6,5 and D 2,3,4,5 indicate that the most stable boat conformer is obtained for O O, S O, Se O, P O, As O, S, Se, N, P, As and the most stable planar conformer is obtained for N O, C O, Si O, Ge O, O, C, Si, Ge. The NBO charge on atoms for 1hetero2,5cyclohexadiene1oxide, O, and 1hetero2,5 cyclohexadiene,, are presented (Table 1). For O, the charge on heteroatom, 1 is reasonably more than deoxygenated,, due to attaching to a more electronegative oxygen atom in O. Also, the charge on C 3 for O is more than that for due to electron donating resonance of C 2 =C 3 along with heteroatom,. For the charge on C 3 is increased from = O to = Se as well as from = N to = As due to diminishing the electron donating power of Se and As atoms. It should be noted that for = N, P, As, N O and P O compounds are radicals with an odd number of electrons (Scheme 1). 1) 2) 3) 4) 5) Scheme 2. Five isodesmic reactions for 1hetero2,5cyclohexadiene1oxide, O and 1hetero 2,5cyclohexadiene,. The compounds O and (except for C, C O, Si, Si O, Ge and Ge O ) have 6πelectrons in the ring. It was proposed that the overlap between the lone pair (AO) of heteroatom and the P z (AOs) of the doublebond carbon atoms (pπ overlap) improve the homoaromatic character for those compounds. In this work, the homoaromatic character of those compounds is estimated. Homoaromatic character is not a directly measurable or computable quantity. Homoaromaticity is generally evaluated on the basis of energetic, geometric, and magnetic criteria [2526]. For the energetic criteria, it usually requires the comparison with a nonaromatic model. There are some routes for the estimation of the homoaromatic stabilization energies (HASE) through various isodesmic reactions [31]. Isodesmic reactions consist of equal numbers Bull. Chem. Soc. Ethiop. 21, 24(2)

4 22 E. Vessally of formal single and double bonds in the products and reactants. Systems with negative HASE are homoaromatic, while those with positive HASE are nonaromatic. Five isodesmic reactions are presented (Scheme 2, Table 2 and 3). Equations 1 and 2 show that does not create a homoaromatic system and thus it is less stable than o (Scheme 2). Since sum of energy stabilization of the formal reactants is more than that of the products in equations 1 and 2. The equations 1 and 2 could not give reliable results because of being overlap of double bonds with heteroatom in the reactants of equations 1 and 2. Therefore, the equations 3 and 5 could give reliable results. However, the ringstrain is not considered in the Equation 5. Equations 3 and 5 indicate that the compounds with are more stable than o. For O and, stability is generally decreased from second row to fourth row elements of the table according to equation 3. The most stable compounds from isodesmic reaction 3 are found to be N (9.6 kcal/mol) and O (3.525 kcal/mol). This is in agreement with the idea that the 1 st row atoms have superior π bonding abilities to each other compared with heavier atoms. The high stability of with respect to O may be attributed to dipole moment. Compounds with higher dipole moment possess lower stability. Thus, O has high dipole moment and low stability. Table 2.Gibbs free energies, in kcal/mol, for typically isodesmic reactions of 1hetero2,5cyclohexadiene1oxide, O and 1 hetero2,5cyclohexadiene,. Compound =O O =O =S O =S =Se O =Se =N O =N =P O =P =As O =As =C O =C =Si O =Si =Ge O =Ge Bull. Chem. Soc. Ethiop. 21, 24(2)

5 DFT calculations on 1hetero2,5cyclohexadiene1oxide 221 Table 3. Gibbs free energy gaps, in kcal/mol, results of five isodesmic reactions for 1hetero2,5 cyclohexadiene1oxide, O and 1hetero2,5cyclohexadiene,. Compound HASE (eq. 1) HASE (eq. 2) HASE (eq. 3) HASE (eq. 4) HASE (eq. 5) = O O =O = S O = S = Se O = Se = N O = N = P O = P = As O = As = C O = C = Si O = Si = Ge O = Ge Table 4A. values, in ppm, of 1hetero2,5cyclohexadiene1oxide, O, and 1hetero2,5 cyclohexadiene,. Compound (.8) (.7) (.6) (.5) (.4) (.3) (.2) (.1) O O O S O S Se O Se N O N P O P As O As C O C Si O Si Ge O Ge Bull. Chem. Soc. Ethiop. 21, 24(2)

6 222 E. Vessally Table 4B. values, in ppm, of 1hetero2,5cyclohexadiene1oxide, O, and 1hetero2,5 cyclohexadiene,. Compound (.) (.1) (.2) (.3) (.4) (.5) (.6) (.7) (.8) O O O S O S Se O Se N O N P O P As O As C O C Si O Si Ge O Ge Another accurate method for determining the homoaromatic character is magnetic criteria. calculations are carried out on O and for determination of the aromatic character (Table 4, Figure 1). Negative and positive signs for indicate the aromatic and antiaromatic characters, respectively. (.5) calculations generally give reliable results. calculations achieve differential results for nonplanar molecules because of two differential electrons current in each surface. Therefore, we have two (.5,.5) data for nonplanar molecules on two surfaces (Table 4 Figure 1). The (.5) value is related to the boat region and (.5) is related to the opposite region. The (.5) calculation generally shows high aromatic character (Figure 1). For compounds, the (.5) value as well as aromatic character are increased from O to Se while (.5) for P is the highest with respect to those of As and N. For O compounds, the (.5) values are decreased from O O to Se O while (.5) for P O is the highest with respect to those of As O and N O. The calculation of (.5) indicates that O is the highest among O O, S O, Se O and N O. This is probably attributed to the carbocationic character of the heteroatom and conjugation of this positive charge with double bonds. The changes of nuclear independent chemical shift,, vs. distance of ghost atom are determined (Figure 1). The same groups of the periodic table produce similar dependence curves. Finally, in comparison of the isodesmic reactions and, we can conclude that the homoaromatic characters (from ) of O and do not affect in the stabilities (from isodesmic reactions) of O and. Bull. Chem. Soc. Ethiop. 21, 24(2)

7 2 DFT calculations on 1hetero2,5cyclohexadiene1oxide 223 O 1 S O O S O Se O N 4 6 Se N O P O 1 As O P 3 4 As Bull. Chem. Soc. Ethiop. 21, 24(2)

8 224 E. Vessally C O.5 Si O C Si Ge O Ge Figure 1. The changes vs. distance of the ghost atom for 1hetero2,5cyclohexadiene1 oxide, O, and 1hetero2,5cyclohexadiene,. CONCLUSION The molecular structure of 1hetero2,5cyclohexadiene1oxide, O, is studied and compared with that of the corresponding deoxygenated compound, 1hetero2,5cyclohexadiene,, at B3LYP/6311G** level. Isodesmic reactions to determine the stabilities of O and are considered. Isodesmic reactions indicate that the compound is more stable than o. For O and, the stability is generally decreased from second row to fourth row elements in the table. Nuclear independent chemical shifts,, are calculated for investigation of the homoaromatic characters of O and. For compound, the (.5) value as well as the aromatic character are increased from O to Se while (.5) for P is the highest with respect to those of As and N. Bull. Chem. Soc. Ethiop. 21, 24(2)

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