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Density functional theory: 1H- and 13C-NMR spectra of some ... - fulir

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culations; this is obtained by the calculation of NMR chemical shifts for some structure and ... The samples were measured in DMSO-d6 solutions at 298 and.

J. Serb. Chem. Soc. 79 (11) 1405–1411 (2014) JSCS–4674

UDC 547.587.51+532.14:544.176:544.18 Original scientific paper

Density functional theory: 1H- and 13C-NMR spectra of some coumarin derivatives SELMA ŠPIRTOVIĆ-HALILOVIĆ1*, MIRSADA SALIHOVIĆ2, SNEŽANA TRIFUNOVIĆ3, SUNČICA ROCA4, ELMA VELJOVIĆ1, AMAR OSMANOVIĆ1, MARIJANA VINKOVIĆ4 and DAVORKA ZAVRŠNIK1 1Department

of Pharmaceutical Chemistry, Faculty of Pharmacy, University of Sarajevo, Zmaja od Bosne 8, 71000 Sarajevo, Bosnia and Herzegovina, 2Department of Organic Chemistry, Faculty of Pharmacy, University of Sarajevo, Zmaja od Bosne 8, 71000 Sarajevo, Bosnia and Herzegovina, 3Faculty of Chemistry, University of Belgrade, Studentski trg 12–16, 11000 Belgrade, Serbia and 4NMR Centre, Ruđer Bošković Institute, Bijenička cesta 54, 10000 Zagreb, Croatia (Received 24 February, revised 13 March, accepted 13 March 2014) Abstract: For some synthesized coumarin derivatives, 1H- and 13C-NMR isotropic chemical shifts and some other molecular properties were calculated using the density functional theory. The calculations yielded reliable results that were in good correlation with experimental data. This is a good basis for collaboration between experimentalists and quantum chemists. Keywords: NMR spectra; Spartan 10 software; quantum chemistry; chemical shifts. INTRODUCTION

NMR spectroscopy is one of the key analytical methods in modern organic and synthetic chemistry. It is widely used for structure elucidation and in investigations of the reactivity and reaction mechanisms involving organic molecules. In addition, the irreplaceable role of NMR spectroscopy is commonly recognized in biochemistry, biology and pharmacy for structure determination and in studies of biomolecular dynamics.1 After preliminary NMR measurements have been realized, it is then decided whether a direct structure determination using other methods should be performed.2 Experimentalists expect additional information from quantum chemical calculations; this is obtained by the calculation of NMR chemical shifts for some structure and comparing them to existing NMR data. If the two sets of NMR data * Corresponding author. E-mail: [email protected] doi: 10.2298/JSC140221023S

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correlate well, all the molecular properties that were obtained by the calculations can be assigned to the new compound and accordingly, the experimentalist obtains a detailed description. In this way, the calculation of NMR chemical shifts provides an ideal basis for collaboration between experimentalists and quantum chemists.2 In recent years, density functional theory (DFT) calculations have been used extensively for the calculation of a wide variety of molecular properties, such as equilibrium structure, charge distribution, NMR spectra, and to provide reliable results that are in good correlation with experimental data.3,4 A series of 3-cinnamoyl-4-hydroxycoumarin derivatives were prepared, as reported in a previous paper.5 In the present research, 13C-NMR spectra and 1H-NMR spectra of these derivatives were recorded. The Beck three-parameter exchange functional6 with the Lee, Yang and Parr7 correlation functional (B3LYP), developed by Truhlar et al.8 were used to perform theoretical calculations on the structure, the 1H- and 13C-NMR spectra and some additional properties of the investigated compounds. Correlation coefficients were used to compare the experimentally observed and theoretically computed shifts for each synthesized compound. MATERIALS AND METHODS Investigated compounds The experimental and theoretical properties of three 3-substituted derivatives of 4-hydroxycoumarin were studied. The structures of the investigated compounds are presented in Fig. 1.

Fig. 1. Structure of the synthesized compounds. NMR measurements The one- and two-dimensional homo- and heteronuclear 1H- and 13C-NMR spectra were recorded on a Bruker AV-600 spectrometer, operating at 600.133 MHz for the 1H nucleus and 150.917 MHz for the 13C nucleus. The samples were measured in DMSO-d6 solutions at 298 and 373 K in 5 mm NMR tubes. The chemical shifts, ppm, are referred to TMS as an internal standard. The following measurement techniques were used: standard 1H-, attached proton test (APT), correlation spectroscopy (COSY), and heteronuclear multiple quantum correlation (HMQC) and heteronuclear multiple bond correlation (HMBC) spectroscopy. The 2D NMR spectra were measured in the pulsed field gradient mode (z-gradient). Theoretical calculations All the calculations were realized using Spartan 10 software.

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NMR SPECTRA OF COUMARIN DERIVATIVES

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The geometries were optimized at the B3LYP level of theory along with standard 1, 2 and 3 G (d) basis set as shown in Fig. 2.

Fig. 2. The optimized geometry of the compounds. The harmonic vibration frequencies were calculated by this method and the results were compared with the experimental spectra. This method was used for calculating 1H- and 13C- NMR chemical shifts at the B3LYP/6-31*G (d) level for the three synthesized coumarin derivatives. RESULTS AND DISCUSSION

Spectral data of the prepared compounds are given in the Supplementary material to this paper. Bond lengths and bond angles The optimized structural parameters, bond lengths and bond angles for the thermodynamically preferred geometry of compounds 1–3 determined at the B3LYP/6-31*G level are presented in Table I in accordance with the atom numbering scheme of the molecules shown in Fig. 2. From the structural data, it was observed that the various C–C bond distances calculated between the carbon atoms and the C–H and C–O bond length are found to be nearly the same for all three structures. The influence of the substituent on the molecular parameters, particularly on the C–C bond distance of ring carbon atoms, seems to be negligibly small. The calculated bond angles are very similar to each other in these molecules. An investigation of bond angles O7–C8–O14, O7–C8–C9, C9–C12–C15 and C9–C12–C13 showed that these atoms were sp2 hybridized and that they were completely planar. This data clearly indicates that there is a pronounced tautomer structure between the OH and the C=O group, which is shown in Fig. 3. Comparison of the experimentally measured and theoretically computed shifts Chemical shifts calculated using the B3LYP level with the 6-31*G (d) basis sets can be utilized to eliminate the uncertainties in the fundamental assignments of

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Fig. 3. Tautomerization of the coumarin derivatives.

the spectra. The 1H- and 13C- theoretical and experimental chemical shifts, isotropic shielding tensors and assignments of 1–3 are presented in Tables II and III, respectively. TABLE I. Selected structural parameters calculated for compounds 1–3 by the DFT B3LYP method with 6-31*G basis sets

Bond O7–C4 C4–C5 C3–C4 C5–C6 O7–C8 C8–O14 C8–C9 C9–C12 C10–O11 C12–C13 C13–C16 C16–C17 C17–C18 C17–C22 C22–C21 C20–C19

Bond length, nm Compound 1 2 0.138 0.136 0.141 0.141 0.139 0.140 0.141 0.141 0.140 0.141 0.122 0.121 0.144 0.145 0.152 0.152 0.136 0.135 0.146 0.147 0.135 0.135 0.146 0.146 0.141 0.140 0.141 0.141 0.139 0.138 0.140 0.140

3 0.136 0.140 0.140 0.141 0.140 0.121 0.146 0.148 0.131 0.147 0.135 0.146 0.141 0.141 0.139 0.140

Bond O7–C4–C5 C4–O7–C8 C4–C3–H26 C5–C6–H27 O7–C8–O14 O7–C8–C9 C9–C10–C5 C9–C12–C15 C10–O11–H28 C9–C12–C13 C13–C16–H29 C17–C16–H30 C17–C18–H31 C17–C22–H34 C20–C19–H32 C21–C20–C19

Bond angles, ° Compound 1 2 121.80 121.86 122.13 122.64 118.88 118.56 120.96 120.57 116.87 116.88 117.00 116.70 120.84 120.89 119.48 119.79 110.19 110.40 119.27 119.79 117.63 117.63 114.94 114.75 119.43 119.03 120.18 119.88 120.33 121.11 121.36 119.56

3 121.40 123.48 119.02 118.64 114.96 117.18 120.92 118.00 104.72 120.20 118.18 114.90 120.10 119.16 120.26 121.01

TABLE II. The experimental and calculated 1H-isotropic chemical shifts, ppm, with respect to DMSO-d6 for compounds 1–3 1

Assignment H24 H25 H26 H27 H28 H29 H30 H31,34 for compound 2 H31 H32,33 for compound 2 H32,35 H33,34 H36,37,38

Calcd. 7.36 7.99 7.07 8.32 – 8.41 7.93 7.67 7.56 – –

2 Exp. 7.40 7.79 7.36 8.02 – 8.22 7.92 7.65 7.65 – –

Calcd. – 7.07 7.40 7.10 7.80 – 8.30 7.68 7.70 6.80 3.7

3 Exp. – 7.38 7.76 7.33 8.00 – 8.14 7.98 7.68 7.03 3.84

Calcd. 7.12 7.50 7.00 8.00 – 8.05 7.98 7.46 7.11 – –

Exp. 7.40 7.78 7.35 8.01 – 8.21 7.93 7.71 7.49 – –

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NMR SPECTRA OF COUMARIN DERIVATIVES

TABLE III. The experimental and calculated 13C-isotropic chemical shifts, ppm, with respect to DMSO-d6 for compounds 1–3 Assignment C1 C2 C3 C4 C5 C6 C8 C9 C10 C12 C13 C16 C17 C18, C22 C19, C21 C20 C24

1 Calcd. 129.20 134.21 119.53 158.60 115.70 130.60 167.60 107.60 177.20 199.30 127.40 146.00 139.10 130.20 132.00 126.50 –

2 Exp. 124.00 135.90 116.20 153.80 115.40 124.80 158.90 100.50 179.60 190.80 123.20 144.00 132.00 130.00 131.60 124.20 –

Calcd. 121.7 131.3 118.0 156.4 115.7 118.4 160.9 112.1 179.8 192.2 125.3 146.4 127.6 130.5 114.0 160.7 54.3

3 Exp. 123.9 135.7 116.1 153.7 115.7 124.7 159.0 100.0 180.1 190.4 119.3 146.2 126.7 130.5 114.4 162.0 55.0

Calcd. 122.68 134.43 116.34 157.17 116.80 127.20 161.80 104.90 178.00 199.00 126.40 146.56 133.30 130.40 129.00 137.60 –

Exp. 124.00 135.00 116.20 153.80 115.30 124.80 158.90 100.50 179.60 190.80 123.00 143.90 132.80 129.80 128.60 135.50 –

The aromatic ring carbons gave resonances in the region from 100–150 ppm in the 13C-NMR spectra and from 6.0–8.5 ppm in the 1H-NMR spectra of 1–3. At the higher temperature, the hydrogen of the hydroxy group on the coumarin moiety (OH at C4 for compounds 1–3) was not visible in the 1H-NMR spectra (Table II), because of fast hydrogen/deuterium exchange. It is also interesting to note that this hydrogen was also not visible on the theoretical spectra obtained from the software. The software does not estimate shifts for hydrogen atoms attached to heteroatoms because it also takes into consideration the effects of the solvent. Due to the influence of the electronegative atom, the values of the chemical shift of C2, C17, C18 and C22 in compounds differed significantly in the shift positions in range 126.7–135.9 ppm and the corresponding values of chemical shifts related to C2, C17, C18 and C22 were 134.21, 139.10 and 130.20 ppm for compound 1, 131.30, 127.60 and 130.50 ppm for compound 2 and 134.43, 133.30 and 130.40 ppm for compound 3, respectively. The higher chemical shift of C20 is due to the attachment of the electron withdrawing substituent (Br, OCH3 or Cl) at C20. The downfield signals at 7.67 and 7.56 ppm for compound 1, 7.68 and 7.70 ppm for compound 2 and 7.46 and 7.11 ppm for compound 3 are assigned to protons H31,34 and H32,33 for 1 and 3, and H31 and H32,35 for 2, respectively. This strongly indicates the electron withdrawing nature of Br, methoxy and Cl in their neighborhood. The electron withdrawing methoxy group inc-

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reased the deshielding of the olefinic proton H31 and aromatic protons H32–H35 more than Br and Cl. A downfield shift was observed for C9 compared with C8, C10 and C12 for all three compounds, as given in Table III. This was due to the tautomerization between C8–C9–C10–C12. The calculated and experimental chemical shift values given in Tables II and III show good correspondence. The correlation factors of the linear regression used to compare the experimentally measured and theoretically computed NMR shifts for each compound (1–3) are presented in Table IV. TABLE IV. The correlation factors of compared chemical shifts for compounds 1–3 R2 for 1H-NMR 0.939 0.984 0.942

Compound 1 2 3

R2 for 13C-NMR 0.979 0.983 0.990

CONCLUSIONS

Selected structural parameters of the optimized geometries of the coumarin compounds were obtained by DFT calculations. The vibration frequencies of the fundamental modes of the compounds was precisely assigned and analyzed, and the theoretical results were compared with the experimental vibrations. The 1Hand 13C-NMR chemical shifts were calculated and the assignments were compared with the experimental values. The conducted research provided complete vibration assignments, structural information and NMR chemical shifts of these compounds. SUPPLEMENTARY MATERIAL Spectral data of the prepared compounds are available electronically from http:// //www.shd.org.rs/JSCS/, or from the corresponding author on request. ИЗВОД

ТЕОРИЈА ФУНКЦИОНАЛА ГУСТИНЕ: 1H- И 13C-NMR СПЕКТРИ ДЕРИВАТА КУМАРИНА 1

1

2

3

SELMA ŠPIRTOVIĆ-HALILOVIĆ , MIRSADA SALIHOVIĆ , SNEŽANA TRIFUNOVIĆ , SUNČICA ROCA , ELMA 1 3 1 VELJOVIĆ1, AMAR OSMANOVIĆ , MARIJANA VINKOVIĆ и DAVORKA ZAVRŠNIK 1Department of Pharmaceutical Chemistry, Faculty of Pharmacy, University of Sarajevo, Zmaja od Bosne 8, 71000 Sarajevo, Bosnia and Herzegovina, 2Department of Organic Chemistry, Faculty of Pharmacy, University of Sarajevo, Zmaja od Bosne 8, 71000 Sarajevo, Bosnia and Herzegovina, 3Хемијски факултет, Универзитет у Београду, Студентски трг 12–16, 11000 Београд и 4NMR Centre, Ruđer Bošković

Institute, Bijenička cesta 54, 10000 Zagreb, Croatia

За неке синтетизоване деривате кумарина, 1H- и 13C-NMR изотропна хемијска померања и нека друга молекулска својства израчуната су помоћу теорије функционала густине. Нађено је да су резултати поуздани и у доброј сагласности са експерименталним подацима. Ово представља добру основу за сарадњу између експериментатора и квантних хемичара. (Примљено 24. фебруара, ревидирано 13. марта, прихваћено 13. марта 2014)

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NMR SPECTRA OF COUMARIN DERIVATIVES

1. 2. 3. 4. 5. 6. 7. 8.

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