Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 120 (2014) 106–118
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Study on molecular structure, spectroscopic investigation (IR, Raman and NMR), vibrational assignments and HOMO–LUMO analysis of L-sodium folinate using DFT: A combined experimental and quantum chemical approach Linwei Li, Tiancheng Cai, Zhiqiang Wang, Zhixu Zhou, Yiding Geng, Tiemin Sun ⇑ Key Laboratory of Structure-Based Drug Design and Discovery, Shenyang Pharmaceutical University, Ministry of Education, Shenyang 110016, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Conformers of L-sodium folinate have
In the present work, the experimental and theoretical studies on the structure, vibrations, NMR of L-sodium folinate are presented. The molecular geometry, harmonic vibrational frequencies and NMR spectra of the title compound have been calculated by using the density functional method B3LYP at 6-311++G (d, p) level. Potential energy distribution of the normal modes of the vibrations was done using VEDA program. Finally all of the calculation results were applied to simulate IR, Raman, 1H NMR and 13C NMR spectrum of the L-sodium folinate which show excellent agreement with observed spectrum. All results indicated that B3LYP method was able to provide a satisfactory way for the structural identification of organic compounds.
been analyzed. IR, Raman, NMR spectra of L-sodium folinate have been recorded and analyzed. The computed vibrational wavenumbers and chemical shifts were seen to be in good agreement with the experimental data. Potential energy distribution of the normal modes of the vibrations has been done. Mulliken atomic charges and HOMO– LUMO were performed.
a r t i c l e
i n f o
Article history: Received 5 June 2013 Received in revised form 19 September 2013 Accepted 4 October 2013 Available online 12 October 2013 Keywords: L-Sodium folinate IR Raman NMR Vibrational assignment DFT
a b s t r a c t In the present work, an exhaustive conformational search of N-[4-[[(2-Amino-5-formyl-(6S)-3,4,5,6,7,8hexahydro-4-oxo-6-pteridinyl)methyl]amino]benzoyl]-L-glutamic acid disodium salt (L-SF) has been preformed. The optimized structure of the molecule, vibrational frequencies and NMR spectra studies have been calculated by density functional theory (DFT) using B3LYP method with the 6-311++G (d, p) basis set. IR and FT-Raman spectra for L-SF have been recorded in the region of 400–4000 cm1 and 100–3500 cm1, respectively. 13C and 1H NMR spectra were recorded and 13C and 1H nuclear magnetic resonance chemical shifts of the molecule were calculated based on the gauge-independent atomic orbital (GIAO) method. Finally all of the calculation results were applied to simulate IR, Raman, 1 H NMR and 13C NMR spectrum of the title compound which showed excellent agreement with observed spectrum. Furthermore, reliable vibrational assignments which have been made on the basis of potential energy distribution (PED) and characteristic vibratinonal absorption bands of the title
⇑ Corresponding author. Tel./fax: +86 24 23986398. E-mail address: [email protected] (T. Sun). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.10.011
107
L. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 120 (2014) 106–118
compound in IR and Raman have been figured out. HOMO–LUMO energy and Mulliken atomic charges have been evaluated, either. Ó 2013 Elsevier B.V. All rights reserved.
Introduction The title compound N-[4-[[(2-Amino-5-formyl-(6S)-3,4,5,6,7,8hexahydro-4-oxo-6-pteridinyl)methyl]amino]benzoyl]-L-glutamic acid disodium salt (L-SF) is the reduction hydroformylation derivative of folic acid and the activated form of folic acid in body [1]. It is used as an anti-anemia drug (treats folic acid deficiency megaloblastic anemia) and antitumor adjuvant (cures colon and rectum cancer) in recent years [2–4]. L-SF as a new salt of sub folic, without calcium, it can infusion with 5-fluorouracil, dose not produce precipitation [1]. In addition, L-SF is the effective pharmaceutical conformation of sodium folinate. It will achieve equal efficacy in lower dose, can also avoid potential side-effects at the same time than sodium folinate racemic product [5]. Considering these aspects, a better understanding of its structure and proprieties are very important. To the best of our knowledge, there are deals of researches have been applied in L-SF pharmacological mechanism and synthesis, however, no vibrational data of L-sodium folinate molecule in the literature. Nowadays IR, FT-Raman spectroscopy combined with quantum chemical computations has been used as an effective tool in the vibrational analysis of drug molecules [6], since vibrational spectra and the computed results can help unambiguous identification of vibrational modes as well as the bonding and structural features of complex organic molecular systems [7,8]. The results presented in this study provide clear evidence of the benefits of IR and Raman spectroscopy in L-sodium folinate drug analysis, which can be used for efficient quality control, forensic analysis and analytical chemistry. The main objective of our work is to find theoretical methods that would offer a higher certainty of finding molecular structure parameters and vibrational wavenumbers to elucidate the structural information of molecules. In the present study, we investigated a more complete and reliable assignment of the vibrational spectra by DFT/B3LYP methods with 6-311++G (d, p) as basis set. At first, we performed geometry optimization calculations for the L-SF using the above method and obtain the most stable conformation of L-SF. The optimized structural parameters of stable conformer were used for the vibrational frequencies calculations. In addition, NMR, Mulliken population and HOMO–LUMO analysis have been used to support the information of structural properties. Experimental details Synthesis of L-sodium folinate N-[4-[[(2-Amino-5-formyl-(6S)-3,4,5,6,7,8-hexahydro-4-oxo-6pteridinyl)methyl]amino]benzoyl]-L-glutamic acid disodium salt (L-SF) was synthesized according to the literature procedure [5,9,10]. IR, Raman and NMR spectra The IR spectrum of the title compound was recorded in the region 4000–400 cm1 using KBr pellet technique with 1.0 cm1 resolution on a BRUKER IFS-55V IR spectrometer. The FT-Raman spectra of L-SF was recorded using 1064 nm line of Nd:YAG laser as excitation wavelength in the region 3500–100 cm1 on a BRUKER RFS 100/S FT-Raman spectrometer. The detector was liquid nitrogen cooled Ge detector. Two hundred scans were accumulated
at 4 cm1 resolution using a laser power of 50 mW. 1H NMR spectra at 600 MHz was recorded at 293.4 K on BRUKER AVANCE600 MHz NMR spectrometer, and D2O was used as a solvent. The 13 C NMR spectrum was recorded at 294.2 K in the same instrument taking TSP as an internal standard. Computational details The first task for the computational work was to establish the optimized geometry of L-SF molecule. The stable possible conformations of the title molecule were scanned with the Spartan 08 program [11]. Then geometry optimizations of all of the possible conformers was performed at B3LYP level of theory using 6311++G (d, p) basis set. The Gibbs free energies were also calculated at the same level and frequency calculations based on the previously optimized geometries in order to ensure the minimum energy of the structure. Relative population of each conformer was valued on the basis of Boltzmann weighting factor at 298 K [12]. The Boltzmann weighting factor Pi (P i ¼ PexpðGi =RTÞ 100%), j
expðGj =RTÞ
where Gi, R and T stand for the Gribbs free energy, gas constant and absolute temperature (298 K) [13]. After the most stable conformer of the title compound determined, optimized structural parameters of this conformer were used for the vibrational frequencies calculations. All of the possible conformers were optimized at B3LYP level with 6-311++G (d, p) basis set and optimized structural parameters of these conformers were used GIAO method for simulate the 1H and 13C NMR spectra. All the quantum chemical calculation (molecular structure, vibrational frequencies and the energies of the optimized structures of L-SF) were performed by the same level of DFT using the Gaussian 09 software package [14]. The GaussView 5.0.8 [15] program was used for visualization the structure and also for the verification of the normal modes assignment. The theoretical force constants in Cartesian representation have been computed at the full optimized geometry. PED calculations, which show the relative contributions of the redundant internal coordinates to each normal vibrational mode of the molecule and it is possible to describe the character of each mode accurately, were calculated by VEDA4 (Vibrational Energy Distribution Analysis) [16] program. Theoretical wavenumbers in the ranges from 4000 to 1700 cm1 and lower than 1700 cm1 were scaled with 0.958 and 0.983 respectively [17]. By combining the results of the GaussView 5.0.8 [15] program with PED, there is a high degree of confidence between calculated and experimental data. The Raman activities (Si) calculated with the Gaussian 09 [14] program, then converted them to relative Raman intensities (Ii) using the following relation from the basic theory of Raman scattering [18,19]:
Ii ¼
h
f ðm0 mi Þ4 Si
i
mi 1 expð hckTv i Þ
ð1Þ
where m0 is the laser exciting wavenumber in cm1 (in this work, we have used the excitation wavenumber m0 = 9398.5 cm1, which corresponds to the wavelength of 1064 nm of a Nd:YAG laser), mi is the vibrational wavenumber of the ith normal mode (cm1), Si is the Raman scatting activity of the normal mode mi, f (is a constant equal to 1012) is a suitably chosen common normalization factor for all peak intensities, h, k, c and T are Planck and Boltzmann constants, speed of light and temperature in Kelvin, respectively.
108
L. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 120 (2014) 106–118
The isotropic chemical shifts are usually used as an aid in identification of organic compounds and accurate predictions of molecular geometries. The B3LYP method allows calculating the shielding constants [20]. The 13C and 1H NMR isotropic shielding were calculated with the GIAO method [21,22] using the optimized parameters obtained from B3LYP/6-311++G (d, p) level. Relative chemical shifts were then estimated by using the corresponding TSP shielding calculated in advance at the same theoretical level as the reference. The predicated 13C and 1H NMR chemical shifts were derived from the equation:
mann weighting factors of conformers are presented in Table 1. From both calculated energies and Boltzmann weighting factors of five conformers, given in Table 1, the conformer (a) is the most stable conformation and is the most conformation in solid. For these reasons, the molecular geometry, Mulliken atomic charge, hydrogen bond and vibrational analysis will be based on conformer (a), further in this paper.
disoX ¼ risoTSPX risoX
where disoXi is the isotropic chemical shift of ith conformer, Pi is the Boltzmann weighting factor of ith conformer, dtotal isoX is the isotropic chemical shift.
To clarity the vibrational frequencies, it is essential to understand the geometry of any compound as structure determines properties. The optimized structure of the molecule has been calculated by density functional theory (DFT) using B3LYP method at the 6-311++G (d, p) basis set, given in Fig. 2. The whole molecule presents a folded form which looks like a Ring. The ringA and ringC forms p–p conjugation. The ringA forms a big p bond, so that the entire ring is in a plane. Other amide bonds in the molecule also become conjugated systems, and the two carboxylates form conjugated conformations, either. These small conjugated systems in the L-SF molecule reduce the energy of L-SF molecule greatly. Furthermore, H39 forms a hydrogen bond with O53. Na55 is attracted by carboxylate and O11 at the same time and Na57 atom is attracted by carboxylate and O27, either. These effects lead to the molecule in the smallest energy post. In addition, the optimized geometry whether to a true minimum can be proved by the lack of imaginary wavenumbers.
Results and discussion
Mulliken population analysis
Conformational stability
The electronic charge on an atom determines the bonding capability and molecular conformation. The atomic charge values were obtained by the Mulliken population analysis [23]. The Mulliken population analysis of a single L-sodium folinate molecule is calculated using B3LYP level of theory with 6-311++G (d, p) basis set. The Mulliken atomic charges on the component atoms of the title molecule are presented in Table 2. The calculated Mulliken atomic charge of O11 and O27 is 0.3201 and 0.4193 e, respectively. Na55 and Na57 display positive charge, the calculated Mulliken atomic charge of them are 0.5493 and 0.6447 e, respectively. Hence, O11 atom attracts Na55 atom strongly; while O27 atom attracts Na57, either. To validate the reliability of our results, the bond length analysis of a single L-SF molecule is calculated at DFT/B3LYP/6-311++G (d, p) basis set, given in Table 2. The calculated length of O11-Na55 and O27-Na57 are 2.5814 and 2.2486 Å
ð2Þ
where disoX is the isotropic chemical shift, riso is the isotropic shielding. The calculated average value for 13C isotropic magnetic shielding of TSP is 184.02 ppm and 31.90 ppm for 1H isotropic magnetic shielding of TSP at 6-311++G (d, p) basis set. 1 H and 13C NMR spectra were recorded in liquid state (D2O) in that the molecular conformation can change every time. For this reason, theoretical isotropic chemical shift depends on the Boltzmann distribution of each conformation:
dtotal isoX ¼
n X P i disoX i
ð3Þ
i¼1
An accurate conformational analysis was carried out to found stable conformers of the title compound. Rotating each free rotation bonds, conformational space of the title compound was searched by molecular mechanic calculations with Spartan 08 program [11]. At first, we obtained 22 possible conformers of L-SF molecule. Then we selected the relative Gibbs free energy of these possible conformations less than 4 kcal/mol, and then full geometry optimizations of these obtained structures were performed by DFT/B3LYP/6-311++G (d, p) method. Boltzmann weighting factors of these possible conformations were calculated depended on the relative Gibbs free energy of optimized structures. These values indicated that the title compound has five conformers as shown in Fig. 1. Gibbs free energies, relative Gibbs free energies and Boltz-
Molecular geometry
Fig. 1. Stable conformers of L-sodium folinate.
109
L. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 120 (2014) 106–118 Table 1 Gibbs free energies (G), relative Gibbs free energies (DG)a and Boltzmann weighting factor (P%)b of L-SF conformers by using the DFT/B3LYP/6-311++G (d, p) method.
a b
Conf.
G (kcal/mol)
DG (kcal/mol)
Pi%
(a) (b) (c) (d) (e)
1,261,017.713 1,261,015.642 1,261,015.641 1,261,015.412 1,261,014.061
0 2.071 2.072 2.301 3.652
0.922 0.028 0.028 0.019 0.002
Which related to the most stable conformer. Boltzmann weighting factor (Pi%) based on DG.
respectively showing the possibility of attractions of OANa. The calculated Mulliken atomic charge of H39 and O53 are 0.4757 and 0.3865 e, respectively. On the other aspect, the theoretical bond length of N12H39. . .O53 is 1.8313 Å also illustrates the hydrogen bond between H39 atom and O53 atom. Hydrogen bonding The optimized structural of L-sodium folinate (Fig. 2) shows the presence of intramolecular hydrogen bond. In our present study, the geometrical parameters of hydrogen bonds in the title molecule has been carried out by B3LYP/6-311++G (d, p) method, as shown in Table 2. The theoretical distance between H39 and O53 is 1.8313 Å; is well within the range of
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Study on molecular structure, spectroscopic investigation (IR, Raman and NMR), vibrational assignments and HOMO–LUMO analysis of L-sodium folinate using DFT: A combined experimental and quantum chemical approach Linwei Li, Tiancheng Cai, Zhiqiang Wang, Zhixu Zhou, Yiding Geng, Tiemin Sun ⇑ Key Laboratory of Structure-Based Drug Design and Discovery, Shenyang Pharmaceutical University, Ministry of Education, Shenyang 110016, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Conformers of L-sodium folinate have
In the present work, the experimental and theoretical studies on the structure, vibrations, NMR of L-sodium folinate are presented. The molecular geometry, harmonic vibrational frequencies and NMR spectra of the title compound have been calculated by using the density functional method B3LYP at 6-311++G (d, p) level. Potential energy distribution of the normal modes of the vibrations was done using VEDA program. Finally all of the calculation results were applied to simulate IR, Raman, 1H NMR and 13C NMR spectrum of the L-sodium folinate which show excellent agreement with observed spectrum. All results indicated that B3LYP method was able to provide a satisfactory way for the structural identification of organic compounds.
been analyzed. IR, Raman, NMR spectra of L-sodium folinate have been recorded and analyzed. The computed vibrational wavenumbers and chemical shifts were seen to be in good agreement with the experimental data. Potential energy distribution of the normal modes of the vibrations has been done. Mulliken atomic charges and HOMO– LUMO were performed.
a r t i c l e
i n f o
Article history: Received 5 June 2013 Received in revised form 19 September 2013 Accepted 4 October 2013 Available online 12 October 2013 Keywords: L-Sodium folinate IR Raman NMR Vibrational assignment DFT
a b s t r a c t In the present work, an exhaustive conformational search of N-[4-[[(2-Amino-5-formyl-(6S)-3,4,5,6,7,8hexahydro-4-oxo-6-pteridinyl)methyl]amino]benzoyl]-L-glutamic acid disodium salt (L-SF) has been preformed. The optimized structure of the molecule, vibrational frequencies and NMR spectra studies have been calculated by density functional theory (DFT) using B3LYP method with the 6-311++G (d, p) basis set. IR and FT-Raman spectra for L-SF have been recorded in the region of 400–4000 cm1 and 100–3500 cm1, respectively. 13C and 1H NMR spectra were recorded and 13C and 1H nuclear magnetic resonance chemical shifts of the molecule were calculated based on the gauge-independent atomic orbital (GIAO) method. Finally all of the calculation results were applied to simulate IR, Raman, 1 H NMR and 13C NMR spectrum of the title compound which showed excellent agreement with observed spectrum. Furthermore, reliable vibrational assignments which have been made on the basis of potential energy distribution (PED) and characteristic vibratinonal absorption bands of the title
⇑ Corresponding author. Tel./fax: +86 24 23986398. E-mail address: [email protected] (T. Sun). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.10.011
107
L. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 120 (2014) 106–118
compound in IR and Raman have been figured out. HOMO–LUMO energy and Mulliken atomic charges have been evaluated, either. Ó 2013 Elsevier B.V. All rights reserved.
Introduction The title compound N-[4-[[(2-Amino-5-formyl-(6S)-3,4,5,6,7,8hexahydro-4-oxo-6-pteridinyl)methyl]amino]benzoyl]-L-glutamic acid disodium salt (L-SF) is the reduction hydroformylation derivative of folic acid and the activated form of folic acid in body [1]. It is used as an anti-anemia drug (treats folic acid deficiency megaloblastic anemia) and antitumor adjuvant (cures colon and rectum cancer) in recent years [2–4]. L-SF as a new salt of sub folic, without calcium, it can infusion with 5-fluorouracil, dose not produce precipitation [1]. In addition, L-SF is the effective pharmaceutical conformation of sodium folinate. It will achieve equal efficacy in lower dose, can also avoid potential side-effects at the same time than sodium folinate racemic product [5]. Considering these aspects, a better understanding of its structure and proprieties are very important. To the best of our knowledge, there are deals of researches have been applied in L-SF pharmacological mechanism and synthesis, however, no vibrational data of L-sodium folinate molecule in the literature. Nowadays IR, FT-Raman spectroscopy combined with quantum chemical computations has been used as an effective tool in the vibrational analysis of drug molecules [6], since vibrational spectra and the computed results can help unambiguous identification of vibrational modes as well as the bonding and structural features of complex organic molecular systems [7,8]. The results presented in this study provide clear evidence of the benefits of IR and Raman spectroscopy in L-sodium folinate drug analysis, which can be used for efficient quality control, forensic analysis and analytical chemistry. The main objective of our work is to find theoretical methods that would offer a higher certainty of finding molecular structure parameters and vibrational wavenumbers to elucidate the structural information of molecules. In the present study, we investigated a more complete and reliable assignment of the vibrational spectra by DFT/B3LYP methods with 6-311++G (d, p) as basis set. At first, we performed geometry optimization calculations for the L-SF using the above method and obtain the most stable conformation of L-SF. The optimized structural parameters of stable conformer were used for the vibrational frequencies calculations. In addition, NMR, Mulliken population and HOMO–LUMO analysis have been used to support the information of structural properties. Experimental details Synthesis of L-sodium folinate N-[4-[[(2-Amino-5-formyl-(6S)-3,4,5,6,7,8-hexahydro-4-oxo-6pteridinyl)methyl]amino]benzoyl]-L-glutamic acid disodium salt (L-SF) was synthesized according to the literature procedure [5,9,10]. IR, Raman and NMR spectra The IR spectrum of the title compound was recorded in the region 4000–400 cm1 using KBr pellet technique with 1.0 cm1 resolution on a BRUKER IFS-55V IR spectrometer. The FT-Raman spectra of L-SF was recorded using 1064 nm line of Nd:YAG laser as excitation wavelength in the region 3500–100 cm1 on a BRUKER RFS 100/S FT-Raman spectrometer. The detector was liquid nitrogen cooled Ge detector. Two hundred scans were accumulated
at 4 cm1 resolution using a laser power of 50 mW. 1H NMR spectra at 600 MHz was recorded at 293.4 K on BRUKER AVANCE600 MHz NMR spectrometer, and D2O was used as a solvent. The 13 C NMR spectrum was recorded at 294.2 K in the same instrument taking TSP as an internal standard. Computational details The first task for the computational work was to establish the optimized geometry of L-SF molecule. The stable possible conformations of the title molecule were scanned with the Spartan 08 program [11]. Then geometry optimizations of all of the possible conformers was performed at B3LYP level of theory using 6311++G (d, p) basis set. The Gibbs free energies were also calculated at the same level and frequency calculations based on the previously optimized geometries in order to ensure the minimum energy of the structure. Relative population of each conformer was valued on the basis of Boltzmann weighting factor at 298 K [12]. The Boltzmann weighting factor Pi (P i ¼ PexpðGi =RTÞ 100%), j
expðGj =RTÞ
where Gi, R and T stand for the Gribbs free energy, gas constant and absolute temperature (298 K) [13]. After the most stable conformer of the title compound determined, optimized structural parameters of this conformer were used for the vibrational frequencies calculations. All of the possible conformers were optimized at B3LYP level with 6-311++G (d, p) basis set and optimized structural parameters of these conformers were used GIAO method for simulate the 1H and 13C NMR spectra. All the quantum chemical calculation (molecular structure, vibrational frequencies and the energies of the optimized structures of L-SF) were performed by the same level of DFT using the Gaussian 09 software package [14]. The GaussView 5.0.8 [15] program was used for visualization the structure and also for the verification of the normal modes assignment. The theoretical force constants in Cartesian representation have been computed at the full optimized geometry. PED calculations, which show the relative contributions of the redundant internal coordinates to each normal vibrational mode of the molecule and it is possible to describe the character of each mode accurately, were calculated by VEDA4 (Vibrational Energy Distribution Analysis) [16] program. Theoretical wavenumbers in the ranges from 4000 to 1700 cm1 and lower than 1700 cm1 were scaled with 0.958 and 0.983 respectively [17]. By combining the results of the GaussView 5.0.8 [15] program with PED, there is a high degree of confidence between calculated and experimental data. The Raman activities (Si) calculated with the Gaussian 09 [14] program, then converted them to relative Raman intensities (Ii) using the following relation from the basic theory of Raman scattering [18,19]:
Ii ¼
h
f ðm0 mi Þ4 Si
i
mi 1 expð hckTv i Þ
ð1Þ
where m0 is the laser exciting wavenumber in cm1 (in this work, we have used the excitation wavenumber m0 = 9398.5 cm1, which corresponds to the wavelength of 1064 nm of a Nd:YAG laser), mi is the vibrational wavenumber of the ith normal mode (cm1), Si is the Raman scatting activity of the normal mode mi, f (is a constant equal to 1012) is a suitably chosen common normalization factor for all peak intensities, h, k, c and T are Planck and Boltzmann constants, speed of light and temperature in Kelvin, respectively.
108
L. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 120 (2014) 106–118
The isotropic chemical shifts are usually used as an aid in identification of organic compounds and accurate predictions of molecular geometries. The B3LYP method allows calculating the shielding constants [20]. The 13C and 1H NMR isotropic shielding were calculated with the GIAO method [21,22] using the optimized parameters obtained from B3LYP/6-311++G (d, p) level. Relative chemical shifts were then estimated by using the corresponding TSP shielding calculated in advance at the same theoretical level as the reference. The predicated 13C and 1H NMR chemical shifts were derived from the equation:
mann weighting factors of conformers are presented in Table 1. From both calculated energies and Boltzmann weighting factors of five conformers, given in Table 1, the conformer (a) is the most stable conformation and is the most conformation in solid. For these reasons, the molecular geometry, Mulliken atomic charge, hydrogen bond and vibrational analysis will be based on conformer (a), further in this paper.
disoX ¼ risoTSPX risoX
where disoXi is the isotropic chemical shift of ith conformer, Pi is the Boltzmann weighting factor of ith conformer, dtotal isoX is the isotropic chemical shift.
To clarity the vibrational frequencies, it is essential to understand the geometry of any compound as structure determines properties. The optimized structure of the molecule has been calculated by density functional theory (DFT) using B3LYP method at the 6-311++G (d, p) basis set, given in Fig. 2. The whole molecule presents a folded form which looks like a Ring. The ringA and ringC forms p–p conjugation. The ringA forms a big p bond, so that the entire ring is in a plane. Other amide bonds in the molecule also become conjugated systems, and the two carboxylates form conjugated conformations, either. These small conjugated systems in the L-SF molecule reduce the energy of L-SF molecule greatly. Furthermore, H39 forms a hydrogen bond with O53. Na55 is attracted by carboxylate and O11 at the same time and Na57 atom is attracted by carboxylate and O27, either. These effects lead to the molecule in the smallest energy post. In addition, the optimized geometry whether to a true minimum can be proved by the lack of imaginary wavenumbers.
Results and discussion
Mulliken population analysis
Conformational stability
The electronic charge on an atom determines the bonding capability and molecular conformation. The atomic charge values were obtained by the Mulliken population analysis [23]. The Mulliken population analysis of a single L-sodium folinate molecule is calculated using B3LYP level of theory with 6-311++G (d, p) basis set. The Mulliken atomic charges on the component atoms of the title molecule are presented in Table 2. The calculated Mulliken atomic charge of O11 and O27 is 0.3201 and 0.4193 e, respectively. Na55 and Na57 display positive charge, the calculated Mulliken atomic charge of them are 0.5493 and 0.6447 e, respectively. Hence, O11 atom attracts Na55 atom strongly; while O27 atom attracts Na57, either. To validate the reliability of our results, the bond length analysis of a single L-SF molecule is calculated at DFT/B3LYP/6-311++G (d, p) basis set, given in Table 2. The calculated length of O11-Na55 and O27-Na57 are 2.5814 and 2.2486 Å
ð2Þ
where disoX is the isotropic chemical shift, riso is the isotropic shielding. The calculated average value for 13C isotropic magnetic shielding of TSP is 184.02 ppm and 31.90 ppm for 1H isotropic magnetic shielding of TSP at 6-311++G (d, p) basis set. 1 H and 13C NMR spectra were recorded in liquid state (D2O) in that the molecular conformation can change every time. For this reason, theoretical isotropic chemical shift depends on the Boltzmann distribution of each conformation:
dtotal isoX ¼
n X P i disoX i
ð3Þ
i¼1
An accurate conformational analysis was carried out to found stable conformers of the title compound. Rotating each free rotation bonds, conformational space of the title compound was searched by molecular mechanic calculations with Spartan 08 program [11]. At first, we obtained 22 possible conformers of L-SF molecule. Then we selected the relative Gibbs free energy of these possible conformations less than 4 kcal/mol, and then full geometry optimizations of these obtained structures were performed by DFT/B3LYP/6-311++G (d, p) method. Boltzmann weighting factors of these possible conformations were calculated depended on the relative Gibbs free energy of optimized structures. These values indicated that the title compound has five conformers as shown in Fig. 1. Gibbs free energies, relative Gibbs free energies and Boltz-
Molecular geometry
Fig. 1. Stable conformers of L-sodium folinate.
109
L. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 120 (2014) 106–118 Table 1 Gibbs free energies (G), relative Gibbs free energies (DG)a and Boltzmann weighting factor (P%)b of L-SF conformers by using the DFT/B3LYP/6-311++G (d, p) method.
a b
Conf.
G (kcal/mol)
DG (kcal/mol)
Pi%
(a) (b) (c) (d) (e)
1,261,017.713 1,261,015.642 1,261,015.641 1,261,015.412 1,261,014.061
0 2.071 2.072 2.301 3.652
0.922 0.028 0.028 0.019 0.002
Which related to the most stable conformer. Boltzmann weighting factor (Pi%) based on DG.
respectively showing the possibility of attractions of OANa. The calculated Mulliken atomic charge of H39 and O53 are 0.4757 and 0.3865 e, respectively. On the other aspect, the theoretical bond length of N12H39. . .O53 is 1.8313 Å also illustrates the hydrogen bond between H39 atom and O53 atom. Hydrogen bonding The optimized structural of L-sodium folinate (Fig. 2) shows the presence of intramolecular hydrogen bond. In our present study, the geometrical parameters of hydrogen bonds in the title molecule has been carried out by B3LYP/6-311++G (d, p) method, as shown in Table 2. The theoretical distance between H39 and O53 is 1.8313 Å; is well within the range of
