untitled European Journal of Chemistry 5 (2) (2014) 343‐350 European Journal of Chemistry ISSN 2153‐2249 (Print) / ISSN 2153‐2257 (Online)  2014 Eurjchem Publishing ‐ Printed in the USA http://dx.doi.org/10.5155/eurjchem.5.2.343‐350.959 European Journal of Chemistry Journal homepage: www.eurjchem.com Ab initio calculations of 13C NMR chemical shielding in some N4O2, N4S2 and N6 Schiff base ligands containing piperazine moiety Majid Rezaeivala a,* and Sam Daftari b a Department of Chemical Engineering, Hamedan University of Technology, Hamedan 65157, Iran b Department of Environmental Sciences, University of Omran and Toseeh, Hamedan, 65157, Iran *Corresponding author at: Department of Chemical Engineering, Hamedan University of Technology, Hamedan 65155, Iran. Tel.: +98.811.8411501. Fax: +98.811.8411407. E‐mail address: mrezaeivala@hut.ac.ir (M. Rezaeivala). ARTICLE INFORMATION ABSTRACT DOI: 10.5155/eurjchem.5.2.343‐350.959 Received: 27 October 2013 Received in revised form: 01 February 2014 Accepted: 04 February 2014 Online: 30 June 2014 KEYWORDS The calculation of 13C isotropic shielding constants by means of GIAO and CSGT methods of eight Schiff base ligands containing piperazine moiety at the Hartree‐Fock and B3LYP levels of theory are presented. Good linear correlations between the calculated chemical shielding at gas‐phase and experimental shift values in CDCl3 solution were obtained. Density functional theory (DFT) calculations at the B3LYP/6‐31G(2d,p) level of theory is used to optimize the geometry of ligands. Calculated nuclear magnetic resonance (NMR) chemical shifts 13C are reported for the some N4O2, N4S2 and N6 Schiff base ligands containing piperazine moiety. In order to establish a convenient and consistent protocol to be employed for confirming the experimental 13C NMR spectra of Schiff base ligands, different combinations of models and basis sets were considered. The most reliable results were obtained at B3LYP/6‐311G++ (d,p) level and CSGT method which can be used to predict 13C NMR chemical shifts with a very high accuracy for latter compounds. These results show the agreement between theoretical and experimental 13C NMR chemical shielding of mentioned ligands. DFT GIAO CSGT Ab initio Schiff base Piperazine 1. Introduction Nuclear magnetic resonance spectroscopy is an important experimental tool to probe the local geometric and electronic structure of molecules. For quantum chemical calculations on large molecules, density functional theory has become a popular and powerful tool [1‐5]. The chemical shift in nuclear magnetic resonance is closely linked to the environment of the nuclei. Thus, the measurement and theoretical calculation of the chemical shift tensors can provide useful information of the electronic structures of molecules [6,7]. In the next few years, it is likely that theoretical predictions of chemical shifts will become routinely used in the structural study of molecules. On the other hand, experimental improvements enable the determination of the principal values of 13C chemical tensors in complex molecules to be made [8]. Even though we see a steady advancement in NMR techniques, this information may not lead to an unambiguous structure: among many possible causes there are difficulties in resolving crowded spectral regions, in determining small long‐ range couplings, or in assigning them. As a result, the confirmation of the proposed structure through X‐ray analysis or total synthesis is required, but these avenues are not always available [9‐11]. On the other hand, it is clear that the good correlation between the experimental and theoretical properties of the molecules will be achieved when the theoretically considered structure is the same or very close to the “actual” structure. In this work we have considered the latter structure for NMR study on upcoming ligands, and we were interested to know whether we can obtain a good correlation between the experimental and theoretical 13C isotropic shielding constants. Schiff base compounds have attracted considerable attention due to their impressive and useful chemical and physical properties [12‐15]. The instant and enduring popularity of Schiff base compounds undoubtedly stem from the ease with which they can be synthesized and their wide range complexing ability [16‐19]. Besides, a wide variety of Schiff base ligands and their metal complexes have been extensively investigated because of their potential applicability as catalysts [20‐23] and their magnetic properties [24]. On the other hand, the comparison between experimental and theoretical NMR data may be helpful in making correct assignments and understanding the relationship between chemical shielding and molecular structure. Therefore, nowadays the Gauge Independent Atomic Orbitals/Density Functional Theory (GIAO/DFT) [23,25] and Continuous Set of Gauge Transformations/Density Functional Theory (CSGT/ DFT) [26,27] approaches are widely used to calculate chemical shifts for a variety of compounds [28‐31]. 344 Rezaeivala and Daftari / European Journal of Chemistry 5 (2) (2014) 343‐350 L1 L2 L3 Figure 1. B3LYP/6‐31G(2d,p) geometry optimized structure of L1, L2 and L3. A comparison between the experimental and theoretical investigations on carbon‐13 isotropic shielding constants of some tripodal tetraamine ligands has been reported [32]. The present work is focused on predicting NMR shielding tensors at the DFT/B3LYP level of theory with two different models: gauge‐including atomic orbital (GIAO) and continuous sets of gauge transformations and two different basis sets, 6‐ 31G (2d,p) and 6‐311++G(d,p). According to our knowledge, there are only few computational NMR studies on Schiff base compounds [33‐35]. In this work, the 13C NMR isotropic chemical shielding of eight Schiff base ligands in gas phase (Scheme 1) studied systema‐ tically by the GIAO [23,25] and CSGT [26,27] methods at the level of density functional theory (DFT) with two different basis sets, 6‐31G(2d,p) and 6‐311++G(d,p). 2. Experimental The geometries of all Schiff base ligands investigated here, in gas‐phase were fully optimized at the DFT (B3LYP) [36] level of theory using the Gaussian 03 package [37]. The standard 6‐ 31G(2d,p) basis set was used for geometry optimization (Figure 1‐3) and vibrational frequency analyses calculated at the same level of theory, indicate that optimized structures are at the stationary points corresponding to local minima without any imaginary frequencies. The NMR calculations were performed at the Hartree‐Fock (HF) level of theory using the standard 6‐31G(2d,p) and 6‐311++G(d, p) basis sets. L1, n=2, R=H, R’=H L2, n=2, R=H, R’=Br L3, n=2, R=t‐Bu, R’=t‐Bu L5, n=3, R=H, R’=H L6, n=3, R=t‐Bu, R’=t‐Bu L7, n=3, R=Ph, R’=H L4, n=2 L8, n=3 Scheme 1 Rezaeivala and Daftari / European Journal of Chemistry 5 (2) (2014) 343‐350 345 L4 L5 L6 Figure 2. B3LYP/6‐31G(2d,p) geometry optimized structure of L4, L5 and L6. The calculations of NMR shielding tensors were done using two common procedures, namely GIAO and CSGT. The obtained shielding tensors were referenced against tetramethylsilane (TMS) to yield relative chemical shifts {δ (cal) = δ (TMS, calc) ‐ δ iso (calc)} [38]. Calculations were performed on a Pentium‐PC computer with a 3200 MHz processor. A starting molecular‐ mechanics structure for the ab initio calculations was obtained using the HyperChem 5.02 program [39]. The experimental 13C NMR data of Schiff base ligands derived from a previous paper [40]. 3. Result and discussion Piperazine is a water soluble cyclic diazine with rigid preorganized cyclohexane conformation [41]. As an amine, piperazine readily undergoes nucleophilic substitution reactions with proper halides. The so‐called reinforced compounds have been introduced, namely, systems in which two amine groups are linked by a further aliphatic chain [42]. It has been experimentally/theoretically confirmed that the chair conformation is usually more stable than boat confor‐ mation for saturated six‐membered rings [43]. In particular, when the ligand is complexed, the piperazine moiety should exist as a boat conformer, but when the ligand demetallates, the piperazine fragment assumes the thermodynamically more favorable chair conformation [44]. The standard 6‐31G(2d,p) basis set was used for geometry optimization and vibrational frequency analyses, calculated at the same level of theory, indicate that optimized structures are at the stationary points corresponding to local minima without any imaginary frequencies (Figure 1‐3). The theoretical calculations of 13C NMR chemical shifts for eight Schiff base ligands containing piperazine moiety were performed with GIAO and CSGT models, through HF and B3LYP level of theory applying two different basis sets. The computed 13C NMR chemical shifts at 6‐31G(2d,p) and 6‐311++G(d,p) levels of theory in comparison with experi‐ mental data for all Schiff base ligands are given in Tables 1‐8. It is clear that the results of the B3LYP method are in a better agreement with experimental values in L3, L6 and L8 and HF method have good agreement with experimental data in L1, L2 and L5. In L3 and L8 with GIAO, B3LYP is better and with CSGT, HF is better. 346 Rezaeivala and Daftari / European Journal of Chemistry 5 (2) (2014) 343‐350 L7 L8 Figure 3. B3LYP/6‐31G(2d,p) geometry optimized structure of L7 and L8. Table 1. Comparison between experimental and calculated chemical shifts (ppm) for L1. Atoms GIAO CSGT Experimental HF B3LYP HF B3LYP C1 111.0 a 110.6 113.4 110.7 117.0 116.3 b 117.4 112.3 113.3 C2 134.6 126.8 135.6 126.0 118.7 142.2 136.7 138.9 133.6 C3 154.7 151.5 158.6 153.6 161.1 163.8 163.5 158.9 158.1 C5 118.2 117.0 119.9 116.5 113.3 124.7 125.6 122.0 122.8 C6 120.3 120.0 122.6 120.4 118.6 127.1 129.2 121.7 123.1 C9 134.0 125.1 135.1 124.6 132.3 141.7 134.9 136.5 130.2 C10 156.1 150.0 158.6 151.1 165.8 163.2 159.5 159.0 155.4 C16 56.9 63.3 59.2 65.9 58.5 59.8 68.3 57.6 66.2 C17 50.4 55.4 52.2 57.7 56.8 52.9 59.9 51.1 58.2 C23 49.1 55.1 50.8 57.0 53.1 51.7 59.7 50.4 58.4 C24 43.6 48.8 45.5 50.9 53.1 45.7 52.7 44.4 51.3 C25 42.7 47.1 44.5 49.3 53.1 44.5 50.3 43.5 49.5 C26 44.2 48.0 46.0 50.1 53.1 46.1 51.3 44.9 50.2 C36 53.3 59.6 55.39 61.8 56.8 55.5 63.8 53.48 61.8 C37 47.0 51.9 55.4 54.7 58.5 48. 8 55.5 47.0 53.5 C43 159.4 155.6 162.8 157.9 165.8 167.1 166.2 162.7 162.0 C44 118.7 118.9 121.9 120.3 118.6 125.6 127.8 120.3 121.7 C46 131.9 124.9 133.2 124.5 132.3 139.8 134.9 135.5 130.9 C47 152.4 148.9 156.3 150.7 161.1 160.9 160.3 156.2 154.8 C48 118.4 116.0 120.2 116.1 131.3 125.2 124.9 122.5 122.5 C49 114.7 111.5 116.5 111.3 117.0 121.8 120.4 117.4 116.0 C52 133.7 126.4 134.6 125.6 118.7 a Computed data at 6‐31G(2d,p) for all atoms. b Computed data at 6‐311++G(d,p) for all atoms. Table 2. Comparison between experimental and calculated chemical shifts (ppm) of L2. Atoms GIAO CSGT Experimental HF B3LYP HF B3LYP C1 112.1 a 111.5 114.6 112.1 109.9 117.3 b 118.3 113.2 C2 137.9 129.8 139.4 129.9 134.9 145.7 139.7 140.4 C3 154.4 150.7 158.6 153.2 160.4 163.2 162.3 158.3 C5 123.7 128.4 125.9 128.3 120.1 130.4 137.6 129.2 C6 121.8 121.7 124.3 122.4 119.9 128.3 130.6 123.1 C9 136.6 127.7 138.3 128.2 134.9 144.3 137.3 137.1 C10 155.3 149.6 157.8 150.6 164.4 162.4 159.2 158.1 C16 56.9 63.2 59.1 65.8 58.4 59.7 68.1 57.3 C17 50.4 55.3 52.3 57.9 56.8 52.8 59.8 51.1 C23 48.9 54.9 50.5 57.0 53.3 51.3 59.6 49. 8 C24 43.3 49.6 44.8 51.3 53.3 45.1 53.2 43.6 C25 40.8 45.2 42.9 47.4 53.3 42.3 47.9 41.3 C26 39.8 43. 9 41. 7 45.5 53.3 41.0 46.0 40.1 C36 53.6 60.0 55.7 62.4 56.8 55.9 64.5 54.5 C37 51.9 58.0 53.7 60.0 58.4 54.0 61.9 51.9 C43 160.1 155.6 163.3 157.9 164.4 168.7 166.9 164.4 C44 119.1 119.7 122.1 121.4 119.9 125.7 128.2 120.1 C46 135.3 128.1 136.9 128.4 134.9 143.0 137.6 137.0 C47 152. 8 148.4 156.9 150.7 160.4 161.2 159.6 156.4 C48 125.3 129.1 127.6 129.5 120.1 132.4 138.8 130.2 C49 115.5 112.6 117.4 112.7 109.9 122.2 121.2 117.9 C52 137. 9 130.3 139.1 130.2 134.9 145.7 140.3 140.6 a Computed data at 6‐31G(2d,p) for all atoms. b Computed data at 6‐311++G(d,p) for all atoms. Rezaeivala and Daftari / European Journal of Chemistry 5 (2) (2014) 343‐350 347 Table 3. Comparison between experimental and calculated chemical shifts (ppm) of L3. Atoms GIAO CSGT Experimental HF B3LYP HF B3LYP C1 125.2 a 125.0 128. 8 126.8 125.9 132.8 b 134.6 128. 8 126.8 C2 134. 7 126.8 137.7 128.9 127.0 142.8 137. 7 137.7 128.9 C3 151.4 149.2 157.2 153. 8 158.2 159.4 160.2 157.2 153. 8 C4 31.5 39.1 33.9 43.5 35.1 33.6 43.0 33.9 43.5 C5 136.0 135.0 138.2 135.7 140.1 145.3 147.2 138.2 135.7 C6 121.6 121.9 124.5 122. 118.0 128.1 130.7 124.5 122.4 C8 32.4 34.4 32.2 34.1 31.6 34.1 36.4 32.2 34.1 C9 27.7 29.2 28.4 29.6 31.6 28.5 30.2 28.4 29.6 C10 32.5 34.9 32.3 34.2 31.6 34.2 37.0 32.3 34.2 C12 128.5 120.4 131.3 122.2 125.9 135.2 129.3 131.3 122.2 C13 157. 7 151.6 160.1 152.4 166.8 165.0 161.1 160.1 152.4 C14 29.8 36.5 32.2 41.2 34.2 31. 9 40.5 32.2 41.2 C26 30.8 32.5 30.7 32.2 29.1 32.4 34.4 30. 7 32.2 C27 27.2 29.1 27.8 29.3 29.1 28.4 30.3 27.8 29.3 C28 30.9 32. 6 30.7 32.3 29.1 32. 5 34.4 30.7 32.3 C31 56.9 63.6 59.2 65.9 59.0 59.8 68.4 59.2 65.9 C41 50.5 55.6 52.3 57.8 57.1 53.1 60.2 52.3 57.8 C47 50.1 56.4 51.9 58.2 53.5 53.1 61.4 51.9 58.2 C48 43.2 48.2 45.0 50.3 53.5 45.4 52.2 45.0 50.3 C49 47.3 51. 8 49.2 54.1 53.5 49.6 55.8 49.2 54.1 C50 42.3 46.2 43.7 48.1 53.5 44.4 49.8 43.7 48.1 C60 53.4 59.3 55.3 61.6 57.1 56.2 64.5 55.3 61.6 C61 52.2 57.7 54.2 60.2 59.0 55.0 62.4 54.2 60.2 C67 157.3 154.0 160.6 156.0 166.8 165.4 165.0 160.6 156.0 C68 124. 6 123.1 127.5 124.0 118.0 132.0 132.5 127.5 124.0 C70 130.3 123.1 133.2 125.1 126.9 137.5 132.7 133.2 125.1 C71 148.6 146.1 154.4 150.0 158.2 156.6 157.3 154.4 151.0 C72 136.8 135.5 139.0 135.7 136.8 146.4 148.3 139.0 135.7 C73 125.6 124.4 128.7 126.3 125.9 133.1 134.1 128.7 126.3 C76 129.8 123.5 133.0 125.4 126.9 137.0 133.1 133.0 125.4 C77 29.9 37.7 32.3 41.3 34.2 32.1 41.3 32.3 41.3 C78 31.7 39.7 34.1 43.6 35.1 33.8 43.4 34.1 43.6 C80 31.7 32.6 30.2 31.9 29.1 32.0 34.4 30.2 31.9 C81 27.2 29.1 27.9 29.3 29.1 28.3 30.4 27.9 29.3 C82 31.0 32.7 30.8 32.6 29.1 32.8 34.7 30.8 32.6 C83 32.1 34.7 31.8 33.9 31.6 33.9 36.5 31.8 33.9 C84 27.7 29.4 28.4 29.6 31.6 28.5 30.5 28.4 29.6 C85 32.4 34.8 32.1 34.2 31.6 34.1 36.6 32.1 34.2 a Computed data at 6‐31G(2d,p) for all atoms. b Computed data at 6‐311++G(d,p) for all atoms. Table 4. Comparison between experimental and calculated chemical shifts (ppm) of L4. Atoms GIAO CSGT Experimental HF B3LYP HF B3LYP C1 150.5 a 145.0 152.9 145.6 149.3 158.2 b 155.1 152.5 149.6 C2 121.2 120.0 122.4 118.8 120.7 127.6 128.5 124.6 125.5 C5 138.5 130.3 140.0 130.2 136.9 146.1 140.3 142.0 136.6 C6 157.8 151. 7 159.7 152.1 162.4 165.8 162.4 159.3 155.9 C8 120.0 118.2 121.7 117.9 125.1 126.4 126.5 121.3 121.7 C9 161.2 157.5 164.0 159.0 153.9 169.1 168.6 164.6 164.1 C14 57.0 63.4 59.3 66.0 57.7 59.9 68.2 57.6 66.1 C15 49.9 55.1 51.8 57.4 57.7 52.5 59.4 50.7 57.8 C21 50.5 57.1 52.3 59.0 52.8 53.3 61.8 51.7 60.5 C22 43.2 48.5 44.9 50.5 52.8 45.2 52.1 43.6 50. 8 C23 41.5 45.6 43.7 48.4 52.8 43.2 49.0 42.2 48.2 C24 40.2 44.1 42.2 46.3 52.8 41.5 46.8 40.6 45.9 C34 54.1 59.5 56.4 62.4 57.7 56.9 64.8 55.4 63.2 C35 52.7 59.7 54.9 62.3 58.0 55.2 64.4 52.9 62.0 C41 153.6 151.5 156.9 153.8 153.9 160.8 162.2 157.2 158.7 C42 155.1 148.3 157.0 148.2 162.4 163.0 158.6 156.3 151.2 C44 125.1 124.7 126.6 124.2 125.1 131.3 133.0 127.4 129.2 C46 139.6 131.7 140.8 131.3 136.9 147.4 142.0 143.0 137.9 C47 151.4 145.3 153.7 145.8 149.3 158.9 155.1 153.0 149.6 C49 120.8 118.7 121.9 117.5 120.7 127.2 127.3 123.8 123.9 a Computed data at 6‐31G(2d,p) for all atoms. b Computed data at 6‐311++G(d,p) for all atoms. Table 5. Comparison between experimental and calculated chemical shifts (ppm) of L5. Atoms GIAO CSGT Experimental HF B3LYP HF B3LYP C1 111.2 a 110.8 113.6 110.8 116.9 142.1 b 142.1 138.8 133.6 C2 134.6 127.1 135.5 126.0 131.0 131.0 142.1 142.1 138.8 C3 154.8 151.6 158.7 153. 7 161.3 163.9 163.9 159.0 158.1 C5 118.2 116.7 119.9 116.4 118.3 124.7 124.7 122.0 122.7 C6 120.3 119.6 122.7 120.3 118.8 127.2 127.2 121.8 123.0 C9 133.5 124.4 134.5 124.2 132.0 141.1 141.1 135.9 129.6 C10 155.5 149.3 158.0 150.3 164.9 162.6 162.6 158.4 154.6 C16 52.6 59.5 54.7 61.8 57.3 55.3 55.3 53.6 62.4 C17 30.0 32.8 29.7 33.1 27.8 31.9 31.9 31.0 35.6 C20 42.7 47.2 44.8 49.9 55.7 44.0 44.0 43.0 49.3 C26 50.1 56.8 51.8 58.5 53.1 52.6 52.6 51.1 59.8 C27 42.9 48.4 44.7 50.5 53.1 44.8 44.8 43.6 51.1 C28 42.4 47.1 44.4 49.4 53.1 44.3 44.3 43.2 49.5 C29 39.4 43.3 41.3 45.5 53.1 40.9 40.9 39. 9 45.2 C39 51.0 57.2 53.1 59.9 55.7 53.6 53.6 52.7 61.2 a Computed data at 6‐31G(2d,p) for all atoms. b Computed data at 6‐311++G(d,p) for all atoms. 348 Rezaeivala and Daftari / European Journal of Chemistry 5 (2) (2014) 343‐350 Table 5. Continued. Atoms GIAO CSGT Experimental HF B3LYP HF B3LYP C40 31.9 36.6 31.4 37.1 27.8 33.9 40.2 33.1 40.2 C43 47.7 53.4 50.0 56.3 57.3 49.7 49.7 48.0 55.3 C49 158.8 154.7 161.9 156.8 164.9 166.5 166.5 161.9 161.1 C50 117.6 117.6 120.8 119.3 118.8 124.3 124.3 118.9 120.4 C52 131.7 124.8 133.0 124.5 132.0 138.8 138.8 134.89 130.4 C53 153.7 149. 9 157.5 151.8 161.3 162.3 162.3 157.4 155.9 C54 118.1 116.0 119.8 115.7 118.3 124.8 124.8 122.0 122.2 C55 114.9 112.0 116.7 111.6 116.9 122.1 117.6 116.4 122.1 C58 134.6 127.3 135.4 126.2 131.0 142.2 138. 7 133.9 142.2 a Computed data at 6‐31G(2d,p) for all atoms. b Computed data at 6‐311++G(d,p) for all atoms. Table 6. Comparison between experimental and calculated chemical shifts (ppm) of L6. Atoms GIAO CSGT Experimental HF B3LYP HF B3LYP C1 125.6 a 124.5 129.1 127.1 126.8 144.0 b 134.1 129.0 130.5 C2 134.8 126.5 129.1 129.1 136.7 147.9 137.5 139.3 134.4 C3 151.5 149.6 157.3 153.9 158.2 169.7 160.5 155.5 156.2 C4 31.5 39.9 33.9 43.5 35.0 59.2 43.4 34.9 45.7 C5 135.9 135.3 138.1 135.6 136.7 156.5 147.5 141.4 143.0 C6 121.4 121.7 124.3 122.2 117.8 139.4 130.6 124.2 125.9 C8 32.6 35.0 32.3 34.2 31.5 49.2 36.9 32.5 35.7 C9 27.7 29.2 28.3 29.6 31.5 41.8 30.0 26.0 28.3 C10 32.4 34.8 32.1 34.0 31.5 49.0 36.7 32.3 35.5 C12 127.9 120.4 130.7 121.7 126.8 137.7 129.0 129.5 124.2 C13 157.3 151.2 159.8 151.8 166.0 169.9 160.6 160.5 156.4 C14 29.8 37.1 32.2 41.2 34.1 56.8 40.9 33.2 43.3 C26 30.8 32.8 30.7 32.3 29.4 47.1 34.6 30.8 33.5 C27 27.2 28.4 27.8 29.2 29.4 42.4 29.9 26.3 28.9 C28 30.8 32.7 30.6 32.2 29.4 47.0 34.4 30.8 33.5 C31 52.4 59.2 54.6 61.6 57.6 75.8 63.7 53.5 62.2 C41 30.0 32.7 29.7 33.2 28.1 49.2 35.6 31.0 35.7 C44 42.9 47.8 44.9 50.3 56.1 63.3 50.8 43.2 49.8 C50 49.5 55.6 51.3 57.5 53.3 72.3 60.0 50.8 58.8 C51 42.7 48.0 44.5 50.1 53.3 64.2 51.9 43.6 50.7 C52 42.5 46.9 44.4 49.2 53.3 63.0 50.2 43.4 49.5 C53 44.4 48.7 46.2 50.6 53.3 64.2 51.8 45.1 50.7 C63 48.7 55.3 50.7 57.3 56.1 71.3 58.8 49.3 57.8 C64 26.8 29.3 27.1 30.3 28.1 45.4 31.9 27.4 32.0 C67 45.5 51.6 47.8 53.9 57.6 66.7 54.8 45.9 53.2 C73 157.2 154.3 160.6 156.4 166.0 174.4 165.0 160.7 160.9 C74 123.6 121.8 127.1 123.3 117.8 140.1 131.7 127.1 126.6 a Computed data at 6‐31G(2d,p) for all atoms. b Computed data at 6‐311++G(d,p) for all atoms. Table 6. Continued. Atoms GIAO CSGT Experimental HF B3LYP HF B3LYP C76 130.7 124.2 133.4 125.4 117.8 143.6 133.9 134.3 130.0 C77 150.2 147.1 155.4 151.6 139.8 168.5 158.1 154.8 154.9 C78 137.0 135.6 138.9 135.8 158.2 157.2 148.1 142.6 143.6 C79 126.9 125.4 129.9 127.5 136.7 144.8 135.7 130.0 131.25 C82 126.6 120.7 129.9 122.7 126.8 138.6 129.2 128.5 125.1 C83 30.0 36.5 32.4 41.4 126.8 57.0 40.5 33.4 43.5 C84 29.6 37.5 31.5 40.9 34.1 56.7 41.1 32.6 43.1 C86 31.0 32.9 30.8 32.5 35.0 47.5 35.2 31.0 34.0 C87 27.2 28.9 27.8 29.3 29.4 42.3 30.3 26.2 28.8 C88 31.2 33.2 30.8 32.5 29.4 47.6 35.2 31.1 34.1 C89 27.3 29.1 27.2 28.8 29.4 43.0 30.4 26.8 29.5 C90 27.1 29.2 27.0 28.5 31.5 42.5 30.3 26.5 29.0 C91 30.1 32.6 30.6 32.3 31.5 45.6 33.9 29.0 32.1 a Computed data at 6‐31G(2d,p) for all atoms. b Computed data at 6‐311++G(d,p) for all atoms. Table 7. Comparison between experimental and calculated chemical shifts (ppm) of L7. Atoms GIAO CSGT Experimental HF B3LYP HF B3LYP C1 112.9 a 111.9 116.1 113.3 117.7 118.1 b 119.0 115.0 115.8 C2 137.1 129.3 138.8 129.2 137.3 144.3 138.7 140.9 135.5 C3 156.0 152.2 160.1 155.1 137.3 164.0 163.7 159.7 159.0 C5 126.4 124.6 128.8 125.1 126.2 134.4 134.9 129.3 129.6 C6 111.3 111.8 115.2 113.8 106.5 117.3 119.9 112.6 114.3 C9 136.7 130.0 139.2 131.1 134.0 144.2 139.6 138.6 134.3 C10 159.5 153.3 161.9 153.7 158.2 166.6 162.8 162.4 158.3 C11 128.4 124.0 130.6 124.5 127.9 135.3 133.2 132.2 130.3 C14 123.1 120.1 124.9 120.0 125.2 129.7 129.1 127.0 126.2 C15 128.0 125.2 129.8 125.4 122.6 135.1 134.42 130.8 130.4 C18 54.0 61.3 56.2 63.5 54.8 56.8 65.6 55.1 64.1 C19 129.87 124.9 131.3 124.6 129.3 136.7 134.2 133.4 131.0 C22 30.2 32.8 29.8 33.2 27.4 32.0 35.6 31.1 35.6 C26 43.2 47.7 45.2 50.4 50.7 44.5 50.6 43.5 49.8 C32 50.3 57.2 52.1 58.9 53.0 52.9 61.7 51.4 60.2 C33 42.9 48.5 44.7 50.6 53.0 44.8 52.2 43.6 51.1 C34 42.4 47.2 44.4 49.4 53.0 44.2 50.5 43.1 49.4 C35 39.7 43.6 41.6 45.8 53.0 41.3 46.6 40.2 45.5 C45 51.2 57.3 53.3 60.0 50.7 53.8 62.1 52.8 61.3 C46 32.0 36.4 31.5 37.21 27.4 34.0 40.3 33.3 40.5 C49 48.1 53.9 50.3 56.8 54.8 50.1 57.6 48.6 56.1 C55 159.3 154.4 162.6 157.1 158.2 167.3 165.6 162.5 161.0 C56 108.4 110.4 113.0 114.0 106.5 114.4 118.2 109.3 113.0 a Computed data at 6‐31G(2d,p) for all atoms. b Computed data at 6‐311++G(d,p) for all atoms. Rezaeivala and Daftari / European Journal of Chemistry 5 (2) (2014) 343‐350 349 Table 7. Continued. Atoms GIAO CSGT Experimental HF B3LYP HF B3LYP C58 134.2 128.6 136.7 129.7 134.0 141.8 138.4 136.2 132.9 C59 152.4 148.2 156.9 151.1 137.3 160.3 159.0 156.2 154.5 C60 125.9 123.8 128.4 124.5 126.2 133.9 134.3 128.7 128.7 C61 116.1 113.5 118.8 114.1 117.7 122.7 122.2 119.3 118.5 C63 121.8 119.3 123.9 119.7 122.6 128.3 127.9 125.0 124.6 C64 135.1 128.7 137.0 128.7 137.3 142.3 138.1 139.2 135.0 C65 129.5 123.4 130.8 123.0 129.3 136.7 132.9 133.3 129.8 C66 130.1 125.4 132.1 125.3 127.9 137.2 134.5 133.7 131.0 C70 122.5 119.9 124.3 119.6 125.2 129.2 128.9 124.3 119.6 a Computed data at 6‐31G(2d,p) for all atoms. b Computed data at 6‐311++G(d,p) for all atoms. Table 8. Comparison between experimental and calculated chemical shifts (ppm) of L8. Atoms GIAO CSGT Experimental HF B3LYP HF B3LYP C1 150.8 a 145.3 153.1 145.7 148.4 158.5 b 155.4 152.7 149.8 C2 121.4 120.0 122.5 119.0 123.7 127.7 128.7 124.7 125.7 C5 138.7 130.3\ 140.0 130.2 135.5 146.2 140.4 142.1 136.6 C6 157.6 151.5 159.5 151.9 153.7 165.6 162.2 159.0 155.6 C8 119.6 117.9 121.4 117.6 120.2 125.9 126.1 120.9 121.3 C9 160.9 156.9 163.6 158.4 161.1 168.7 167.8 164.2 163.4 C14 52.3 58.8 54.4 61.5 58.4 54.9 63.3 53.2 62.0 C15 29.4 32.2 29.1 32.6 27.0 31.3 35.0 30.3 35.0 C18 42.8 47.2 44.8 50.0 55.2 44.1 50.1 43.0 49.3 C24 50.2 56.8 51.9 58.6 52.3 52.8 61.3 51.3 59.9 C25 42.7 48.0 44.5 50.2 52.3 44.6 51.7 43.3 50.7 C26 42.2 46.9 44.2 49.2 52.3 44.0 50.2 42.9 49.2 C27 39.6 43.6 41.6 45.8 52.3 41.3 46.6 40.2 45.5 C37 51.1 57.6 53.3 60.3 55.2 53.7 62.3 53.0 61.6 C38 32.3 36.5 32.1 37.6 27.0 34.2 40.4 33.3 40.3 C41 48.5 54.9 50.9 57.8 58.4 50.9 59.2 48.9 57.1 C47 152.9 150.6 156.2 152.7 161.1 160.2 161.3 156.4 157.6 C48 155.2 148.3 157.0 148.2 153.7 163.1 158.6 156.3 151.2 C50 124.8 124.5 126.4 124.0 120.2 131.0 132.9 127.2 129.1 C52 139.5 131.6 140.7 131.2 135.5 147.3 141.9 142.9 137.8 C53 151.4 145.4 153.6 145.7 148.4 158.9 155.2 153.0 149.5 C55 120.6 118.3 121.7 117.3 123.7 127.1 127.0 123.6 123.5 a Computed data at 6‐31G(2d,p) for all atoms. b Computed data at 6‐311++G(d,p) for all atoms. The results show that when the molecular weight of ligands is higher, B3LYP is more appropriate than HF as it is obvious in comparison of L3 and L6, L4 and L8. B3LYP/6‐311G(2d, p) is the recommended method by Cheeseman et al. for calculating isotropic NMR chemical shifts [40]. The best basis set for 13C NMR chemical shifts calculation of these ligands is B3LYP/6‐311G++(d,p) because all of my ligands are neutral. To clarify the relation between theoretical and experimental values of NMR shielding tensors, the experimental data are plotted versus computed values. The r2 values as shown in Table 9. There are good linear relationships between experimental and theoretical chemical shifts. Differences between the calculated and measured values may be a result of solvent interactions. The convergence of GIAO and CSGT methods with respect to the basis set is demonstrated in Tables 2‐9 for absolute shielding and refers to TMS calculated at the Hartree‐Fock and DFT levels of theory. The shielding constants are found to converge to the same value as the basis set used here. The overall quality of linear correlation between the experimental carbon shift and calculated shielding is very good. Regarding the method for achievement of 13C chemical shifts, for the present case at the B3LYP level, the CSGT algorithm is slightly superior to GIAO. One of the most valuable properties of the chemical shielding is their sensitivity to the molecular geometry and environment. The experiments provided only the values of chemical shifts, which were later assigned into the molecular framework by comparison with the calculated values. The high accuracy achievable by modern chemical shielding calculations allows its use in revising questionable assignments. An important finding is that the deviations from experimental data appear to be related to the atomic weight of the ligands, the higher the atomic weight the larger deviation. Table 9. Correlation between theoretical (ppm, refer to TMS) and experimental chemical shifts (ppm) for ligands. Methods L1 L2 L3 L4 L5 L6 L7 L8 GIAO HF 0.989 a 0.988 b 0.985 0.984 0.991 0.990 0.992 0.992 0.992 0.985 0.989 0.988 0.982 0.982 0.991 0.989 GIAO B3LYP 0.975 0.973 0.977 0.976 0.993 0.991 0.987 0.987 0.987 0.987 0.991 0.990 0.977 0.976 0.991 0.990 CSGT HF 0.992 0.988 0.987 0.984 0.992 0.991 0.992 0.990 0.990 0.985 0.991 0.988 0.981 0.982 0.992 0.991 CSGT B3LYP 0.991 ‐ 0.979 0.979 0.992 0.990 0.986 0.984 0.984 0.987 0.992 0.989 0.972 0.976 0.991 0.989 a 6‐31G(2d,p). b 6‐311G++(d,p). 4. Conclusion In order to suggest a convenient and consistent protocol to be employed to confirm the experimental 13C spectra of Schiff base ligands, different combinations of models and basis sets were considered. The most reliable results were obtained at B3LYP/6‐311++G(d,p) level and CSGT model and can be used to calculate 13C NMR chemical shifts with high accuracy for latter ligands. These results show that the agreement between theoretical and experimental 13C NMR chemical shielding of Schiff base ligands containing piperazine moiety could be used to evaluate the intrinsic relationship between structure and exclusive properties. References [1]. Gauss, J.; Bunsen‐Ges, B. Phys. Chem. 1995, 99, 1001‐1008. [2]. Ziegler, T. Chem. Rev. 1991, 91, 651‐667. [3]. Hohenberg, P.; Kohn, W. Phys. Rev. B. 1964, 136, 864‐871. [4]. Vignale, G.; Rasolt, M.; Geldard, D. J. W. Adv. Quant. Chem. 1990, 21, 235‐253. [5]. Lee, A. M.; Handy, N. C.; Colwell, S. M. J. Chem. Phys. 1995, 103, 10095‐ 10109. [6]. Ando, I.; Webb, G. A. Theory of NMR Parameters, Academic Press, London, 1983. [7]. Facelli, J. C.; Hu, J. Z.; Orendt, A. M.; Arif, A. M.; Pugmire, R. J.; Grant, D. M. J. Phys. Chem. 1994, 98, 12186‐12190. [8]. Zheng, G.; Hu, J.; Zhang, X.; Shen, L.; Yea, C.; Webb, G. A. J. Mol. Struct. (Theochem) 1998, 428, 283‐286. [9]. Bagno, A.; Rastrelli, F.; Saielli, G. Chem. Eur. J. 2006, 12, 5514‐5525. [10]. Price, W. S.; Hwamg, J. L. P. J. Chin. Chem. Soc. 1992, 39, 497‐507. 350 Rezaeivala and Daftari / European Journal of Chemistry 5 (2) (2014) 343‐350 [11]. Jiang, B. C.; Miao, X. J.; Hwang, L. P.; Ye, C. H. J. Chin. Chem. Soc. 1995, 42, 887‐892. [12]. Borisova, N. E.; Reshetova, M. D.; Ustynyuk, Y. A. Chem. Rev. 2007, 107, 46‐79. [13]. Vigato, P. A.; Tamburini, S.; Bertolo, L. Coord. Chem. Rev. 2007, 251, 1311‐1620. [14]. Beckmann, U.; Brooker, S. Coord. Chem. Rev. 2003, 245, 17‐29. [15]. Radecka‐Paryzek, W.; Patroniak, V.; Lisowski, J. Coord. Chem. Rev. 2005, 249, 2156‐2175. [16]. Bouwman, E.; Reedijk, J. Coord. Chem. Rev. 2005, 249, 1555‐1581. [17]. Vigato, P. A.; Tamburini, S. Coord. Chem. Rev. 2008, 252, 1871‐2154. [18]. Okawa, H.; Furutachi, H.; Fenton, D. E. Coord. Chem. Rev. 1998, 174, 51‐75. [19]. Cozzi, P. G. Chem. Soc. Rev. 2004, 33, 410‐421. [20]. Grigoropoulou, G.; Clark, J. H.; Elings, J. A. Green Chem. 2003, 5, 1‐7. [21]. Sheldon, R. A.; Kochi, J. K. In Metal‐catalyzed Oxidations of Organic Compounds, Academic Press, New York, 1981. [22]. Hudlicky, M. In Oxidation in Organic Chemistry, ACS Monographs 186. Washington, DC, 1990. [23]. Ditchfield, R. Mol. Phys. 1974, 27, 789‐807. [24]. Keith, T.; Bader, R. Chem. Phys. Lett. 1993, 179, 479‐482. [25]. Wolinski, K.; Hinton, J. F.; Pulay, P. J. Am. Chem. Soc. 1990, 112, 8251‐ 8260. [26]. Keith, T. A.; Bader, R. F. W. Chem. Phys. Lett. 1992, 194, 1‐8. [27]. Keith, T. A, Bader, R. F. W. Chem. Phys. Lett. 1993, 210, 223‐231. [28]. Prakash, R. V.; Rasul, G.; Surya, P. G. K.; Olah, G. A. J. Org. Chem. 2003, 68, 3507‐3510. [29]. Katritzky, A. R.; Akhmedov, N. G.; Güven, A.; Doskocz, J.; Akhmedova, R. G.; Majumder, S.; Dennis Hall, C. J. Mol. Struct. 2006, 787, 131‐147. [30]. Meng, Z.; Carper, W. R. J. Mol. Struct. (Theochem.) 2002, 588, 45‐53. [31]. Gallant, A. J.; Hui, J. K. H.; Zahariev, F. E.; Wang, Y. A.; MacLachlan, M. J. J. Org. Chem. 2005, 70, 7936‐7946. [32]. Salehzadeh, S.; Bayat, M. J. Chinese Chem. Soc. 2007, 54, 1145‐1150. [33]. Khanmohammadi, H.; Erfantalab, M. Spectrochim. Acta A 2010, 75, 127‐133. [34]. Sheikhshoaie, I.; Saheb, V. Spectrochim. Acta A 2010, 77, 1069‐1076. [35]. Infante‐Castillo, R. J. Mol. Struct. (Theochem) 2010, 940, 124‐128. [36]. Becke, A. D. J. Chem. Phys. 1993, 98, 5648‐5652. [37]. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.; Stratman, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Menucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ciolowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al‐Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzales, C.; Challacombe, M.; Gill, P. M. W.; Jonhson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head‐Gordon, M.; Repogle, E. S.; Pople, J. A., Gaussian 03, Gaussian Inc., 2003, Pittsburgh, PA, USA. [38]. Cheeseman, J. R. Trucks, G. W.; Keith, T. A.; Frisch, M. J. J. Chem. Phys. 1996, 104, 5497‐5509. [39]. Hyper Chem. Released on May 2nd; Hypercube, INC: Gainesville 1997. [40]. Keypour, H.; Rezaeivala, M.; Fall, Y.; Dehghani‐Firouzabadi, A. A. Arkivoc 2009, 10, 292‐301. [41]. Larkins, H. L.; Hamilton, A. D. Tetrahedron Lett. 1986, 27, 2721‐2724. [42]. Hubin, T. J.; McComick, J. M.; Collinson, S. R.; Buchalova, M.; Perkins, C. M.; Alcock, N. W.; Kahol, P. K.; Raghunathan, A.; Busch, D. H. J. Am. Chem. Soc. 2000, 122, 2512‐2522. [43]. Keypour, H.; Rezaeivala, M.; Valencia, L.; Salehzadeh, S.; Perez‐ Lourido, P.; Khavasi, H. R. Polyhedron 2009, 28, 3533‐3541. [44]. Boiocchi, M.; Bonizzoni, M.; Fabbrizzi, L.; Foti, F.; Licchelli, M.; Taglietti, A.; Zema, M. J. Chem. Soc. Dalton Trans. 2004, 2616‐2620.