Accepted Manuscript Vibrational spectroscopy and density functional theory study of ninhydrin Ran Li, Sui Huimin, Peipie Liu, Lei Chen, Jianbo Cheng, Bing Zhao PII: DOI: Reference:
S1386-1425(14)01556-X http://dx.doi.org/10.1016/j.saa.2014.10.059 SAA 12874
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: Revised Date: Accepted Date:
1 September 2014 9 October 2014 12 October 2014
Please cite this article as: R. Li, S. Huimin, P. Liu, L. Chen, J. Cheng, B. Zhao, Vibrational spectroscopy and density functional theory study of ninhydrin, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa.2014.10.059
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Vibrational spectroscopy and density functional theory study of ninhydrin Ran Li,a Sui Huimin,a Peipie Liu,a Lei Chen,a Jianbo Cheng,b and Bing Zhaoa,* a
State Key Laboratory of Supramolecular Structure and Materials, Jilin University,
Changchun 130012, China b
The Laboratory of Theoretical and Computational Chemistry, School of Chemistry
and Chemical Engineering, Yantai University, Yantai 264005, China Corresponding author: Bing Zhao Telephone: +86-431-8516-8473; E-mail: [email protected]
Abstract： ： In this paper, ninhydrin was designed as a model molecule for theoretical and experimental studies of the molecule structure. Density functional theory (DFT) calculations have been performed to predict the IR and Raman spectra for the molecule. In addition, Fourier transform infrared (FTIR) and Raman spectra of the compound have been obtained experimentally. Based on the modeling results obtained at the B3LYP/6-311++G** level, all FTIR and Raman bands of the compound obtained experimentally were assigned. Our calculated vibrational frequencies are in good agreement with the experimental vales. The molecular electrostatic potential surface calculation was performed and the result suggested that the ninhydrin had two potential hydrogen bond donors and four potential hydrogen bond acceptors. HOMO-LUMO gap was also obtained theoretically at B3LYP/6-311++G** level.
Keywords: ninhydrin, FTIR, Raman, DFT, HOMO-LUMO gap, hydrogen bond.
1. Introduction Ninhydrin is usually considered as a tricarbonyl compound since it stable in equilibrium of indane-1,2,3-trione [1]. Ninhydrin was first synthesized by Ruhemann in 1910 [2], and then, well known as a common coloration reagent for amino acid. In 1954, Oden and von Hofsten recognized that it was a useful reagent for developing fingerprints [3]. Fingerprints are still one of the most useful forms of physical evidence in identification. Latent prints are normally invisible without development. The techniques used for fingerprint identification vary according to the surface where the fingerprints are applied. The particular interest to ninhydrin-based development is the concentration of amino acids, which has been reported to be between 0.3 to 2.59 mg L21 of sweat. This value corresponds to an average amino acid content of about 250 ng per print. Small concentrations of amino acids in sweat are sufficient for development on paper [4]. Latent prints have highly potential applications, however the low amino acid content limit the detection probability. Surface-enhanced Raman scattering (SERS) has been widely used as a powerful tool for ultrasensitive chemical analysis, which allows this technique sensitive enough to detect single-molecules [5-6]. Applications of SERS range from nanostructure characterization to chemical-biochemical analysis [7-9]. Ninhydrin is an important analytical tool in various fields, however the concentration of the ninhydrin are usually rather low, which limit its application. The limit of the low concentration could be broke though by taking the advantage of SERS analysis. For the selected enhancement in SERS-based study of molecules, the orientation of the studied molecule on the substrate can be identified and the vibrational information is the basis of SERS analysis. Therefore, the molecular structure information, the vibration mode assignment, and the
frontier molecular orbital information of a probe molecular are important for analyzing the complex structure of molecule and the mechanism of the enhancement. Although molecular conformation and the frontier molecular orbital data are very significant for many applications, they were extremely hard to attain by employing experiment method. Fortunately, such information can be calculated theoretically by using the density functional theory (DFT) which describes the electronic states of atoms, molecules, and materials in terms of the three-dimensional electronic density of the system. DFT is generally accepted as a reliable means for predict the spectrum information and molecular conformation. Our previous works [10-11] also showed the advantage of DFT method in application of obtaining the vibrational information. M. Arivazhagan [19] et al. and D. Sajan [20] et al. proposed the ninhydrin monomer and dimmer, respectively. However the SERS information is lack. The Herein, we report the conformational, IR, Raman, and SERS study of ninhydrin based on DFT calculations at the B3LYP/6-311++G** level. FT-IR and confocal Raman spectra of the compound have also been obtained experimentally and accurately assigned by using the results of the theoretical calculations. For further application, the energy difference between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), the HOMO/LUMO gap, has also been calculated, which is valuable for explaining the enhancement mechanism of SERS. In addition, the hydrogen bond donors and receptors are predicted based on the theoretical calculations.
2. Experimental 2.1 Materials Ninhydrin (99%) was obtained from Sigma. All other chemicals were of analytical grade
and were purchased from Beijing Chemical Reagent Factory and were used without further purification.
2.2 Instruments Raman spectra of ninhydrin were recorded on a LabRam Aramis Raman Microscope system (Horiba-Jobin Yvon) equipped with a multichannel air cooled charge-coupled device (CCD) detector. Spectra were excited using the 633 nm line of a HeNe narrow bandwidth laser (Melles Griot). The Raman spectra were collected at room temperature with the laser power at the sample position typically at 400 µW with an average spot size diameter of 1 µm. The typical acquisition time used in this work was 30s. The FT-IR spectra of ninhydrin were recorded as KBr disks at room temperature by a Bruker IFS-66V FT-IR spectrometer, equipped with a DTGS detector at a resolution of 4 cm−1 .
3. Theoretical Method All the geometries we got in this work were optimized by DFT method of B3LYP which is the hybrid of Becke’s three-parameter exchange functional [12-13] with the Lee–Yang–Parr correlation functional [14]. The triple split valence basis set of 6-311+G** for the H, C, and O atoms and lanl2dz for Ag atom was adapted. The frequency calculation was performed at the same level. All calculations were carried out with the aid of the Gaussian 09 program [15]. The molecular electrostatic potential (MEP) were obtained by the WFA [16] software package. Potential energy distribution (PED) calculation was carried out by the VEDA 4 (Vibrational Energy Distribution Analysis) [17]. The method for calculating scaling factors was same as that proposed by Scott and Radom [18].
4. Results and Discussion
The title compound, ninhydrin, is an indene derivative containing two ketoes and two hydroxyls groups. The optimized geometry of ninhydrin was characterized as potential energy minima at the same level by verifying that all vibrational frequencies are real.
4.1 Molecular geometry The optimized geometry of ninhydrin is shown in Fig.1, and the corresponding structural parameter of bond lengths, bond angles, and dihedral angles are shown in Table 1. The atom numerical labels in the following discussion refer to Fig. 1. All atoms lay in the same plane except the O16, H17, O18, and H19, thus the twofold symmetry axis survived whereas the mirror planes no longer exist. Therefore, the theoretically obtained title molecule is a C2 symmetric molecule. The C2 symmetry equates the two rotation-free hydroxyls, thus the DFT calculated potential energy curve which is shown in Fig. 2 is obtained by twisting one of the hydroxyls. The horizontal and axes refer to torsion angles and the potential energy difference, respectively. The torsional potential energy curve shows two rotation barriers with different barrier heights of 23.5 and 15.8 kJ/mol; the saddle implied another structure in particular condition.
4.2 Vibrational assignments The ninhydrin molecule consists of 19 atoms that, undergoes 51 normal modes of vibrations. 21of which were assigned in this manuscript. The observed and calculated vibration modes are listed in Table 2. The frequencies 1000-1800 cm-1 were multiplied by the scaling factors of 0.9734. The IR spectra in Fig. 3 shows a number of bands appearing at 741, 795, 887, 943, 1001, 1012, 1063, 1078, 1153, 1186, 1258, 1292, 1338, 1359, 1391, 1593, 1720, and 1749 cm-1. The Raman bands in Fig. 4 are observed at 698, 741, 888, 943, 979, 1012, 1064, 1153, 1185, 1257, 1291, 1587,
1719, and 1747 cm-1. As it can be seen from Figs. 2 and 3, most of the fundamentals vibrations observed in Raman and FTIR spectra agreed well with the theoretically predicted frequencies. Most fundamentals were observed both in IR and Raman spectra, while 7 normal modes were observed only in IR spectra and 3 normal modes were observed only in Raman spectra. The fundamental ν43, ν38, and ν31 arise from 9b, 19b, and 8b, respectively, and were observed only in IR spectra at ca. 1593, 1359, and 1078 cm-1 , respectively. The fundamental ν39 was assigned to the hydroxyl bending vibration whereas the fundamental ν37 was assigned to 15 coupled with the hydroxyl bending vibration. The fundamental ν39 and ν37 were observed only in IR spectra at ca. 1391 and 1338 cm-1, respectively. The fundamental ν28 and ν21 were also observed only in IR spectra at ca. 1001 and 795 cm-1, respectively, and the former was assigned to 5 whereas the later was assigned to 5 coupled with C=O out-of-plane bending vibration. The fundamental ν42 and ν26 were assigned to 9a and 17b respectively, and observed only in Raman spectra at ca. 1587 and 979 cm-1, respectively. The fundamental ν18 was assigned to 9a coupled with breathing vibration of the five-membered ring and observed only in Raman spectra at ca. 698 cm-1. The other 11 fundamentals were observed in both IR and Raman spectra. The fundamental ν45 (1749 cm-1 in IR spectra and 1747 cm-1 in Raman spectra) and ν44 were both assigned to C=O stretching vibration, the former one was symmetric and the later one was asymmetric. The fundamental ν36 (1292 cm-1 in IR spectra and 1291 cm-1 in Raman spectra) and ν24 (887 cm-1 in IR spectra and 888 cm-1 in Raman spectra) were attributed to 3 and 10b respectively. The fundamental ν35 (1258 cm-1 in IR spectra and 1257 cm-1 in Raman spectra) arises from 18b coupled with the O-H bending vibration whereas the ν32 (1153 cm-1 both in IR spectra and Raman spectra) arises from 18b coupled with the O-H rocking vibration. The fundamental ν30 (1063 cm-1 in IR spectra and 1064 cm-1 in Raman
spectra) arises from the O-H wagging vibration. The fundamental ν33 (1186 cm-1 in IR spectra and 1185 cm-1 in Raman spectra) arises from motion related to benzene ring of 14 whereas the fundamental ν29 (1012 cm-1 both in IR spectra and Raman spectra) arises from 14 coupled with 9b. The fundamental v25 (943 cm-1 both in IR spectra and Raman spectra) arises from motion related to benzene ring of 1 coupled with C-O stretching vibration and C=O in-plane bending vibration. The fundamental v20 (741 cm-1 both in IR spectra and Raman spectra) arises from motion related to benzene ring of 11 coupled with C=O out-of-plane bending vibration.
4.3 SERS assignments The SERS structure of ninhydrin connected with Ag nanoparticles are illustrated in Fig.5. The SERS spectra are quite different from the bulk spectra, especially the band intensities, relative intensities, and band positions. Even some new bands in the SERS spectra arise while some bands in the bulk spectra disappear. For instance, in the SERS spectra, the 9a and 9b vibrational modes are red shifted whereas the 14 vibrational mode is blue shifted; a new band of O-C-O bending arises whereas the 17b vibrational mode in the bulk spectra disappeared in the SERS spectra. These differences are caused by the interaction between ninhydrin and the nanoparticles. The structure change of ninhydrin, which caused by chemically connected with the nanoparticles, is an ignorable parameter to these differences too. The observed and calculated vibration modes are listed in Table 3. The frequencies 1000-1800 cm-1 were multiplied by the scaling factors of 0.9618. The SERS bands in Fig. 6 were observed at 507, 541, 697, 890, 950, 1016, 1145, 1162, 1187, 1230, 1275, 1294, 1383, 1497, 1562, 1586, and 1651 cm-1.
The fundamental ν46 arises from the O-H wagging vibration was observed at 1651 cm-1. The fundamental ν44 and ν43 observed at 1586 and 1562 cm-1, respectively, were assigned to 9b and 9a, respectively. The fundamental ν42 (1497 cm-1) and ν41 (1383 cm-1) were assigned to 18b and 18a, respectively. The fundamental ν40 (1294 cm-1), ν38 (1187 cm-1) and ν34 (1145 cm-1) were contributed to 15, 3, and 14, respectively. The fundamental v39 (1275 cm-1) was assigned to O-H bending whereas the fundamental v36 (1162 cm-1) was assigned to O-C-O bending. The fundamental ν37 observed at 1187 cm-1 was assigned to 19b; the fundamental ν31 observed at 950 cm-1 was assigned to 19b coupled with C-O asymmetric stretching; the fundamental ν32 observed at 1016 cm-1 was assigned to 19b coupled with C-O asymmetric stretching and C-C-C bending of the five membered ring. The fundamental ν27 (890 cm-1 ) and ν20 (697 cm-1) were both assigned to C-C symmetric stretching vibration of the five membered ring, the former was coupled with 1 whereas the later was coupled with 6a. The fundamental ν17 (541 cm-1) was assigned to C-C-C bending of the five membered ring coupled with C-C-C bending of the six membered ring whereas the fundamental ν16 (507 cm-1) assigned to C-C-C bending of the five membered ring coupled with 18b.
4.4 HOMO–LUMO gap HOMO–LUMO gap results in a significant degree of electronic excitation and charge transfer. In most cases, even in the absence of inversion symmetry, the strongest band in the Raman spectrum is weak in the IR spectrum and vice versa. Changes in the HOMO–LUMO gap by connecting with some noble metal, semiconductor or some other means result in the change of the charge transfer degree, intensity and position of the peak. The frontier molecular orbitals are
shown in Fig. 7 and the HOMO–LUMO gap estimated to be 4.43 eV at B3LYP/6-311++G** level.
4.5 Hydrogen bond donor and accepter Hydrogen bond is an important intermolecular interaction in supramolecular systems. Charge transfer and the corresponding change in electric structure have direct effect on the spectra. The natural population analysis results are shown in Fig. 8, the atom number labeling consist with Fig. 1. The mapping of molecular electrostatic potential onto the iso-electron density surface simultaneously displays electrostatic potential distribution, molecular shape, size, and dipole moments of the molecule which provides a visual method to understand the relative polarity of the molecule. The total electron density and molecular electrostatic potential (MEP) surfaces of the molecules under investigation are constructed by using B3LYP/6-311++G** method. The total electron density mapped with electrostatic potential surface of ninhydrin is shown in Fig. 9. The color scheme in Fig. 9 of MEP is red > 0.0352 a.u. > yellow > 0.0 a.u. > green > -0.0197 a.u. > blue. The MEP map indicates that ninhydrin has two potential hydrogen bond donors (the H atoms in the hydroxyl functional groups) and four hydrogen bond accepters (two O atoms in the ketone carbonyl functional groups and two O atoms in the hydroxyl functional groups).
5. Conclusion In this work, the structural parameters of ninhydrin have been obtained at the B3LYP/6-311++G** level of theory. The title molecule was found to lie in the same plane except two hydroxyls with C2 symmetric. The torsional potential energy curve shows two rotation barriers. Modeling performed for ninhydrin and SERS structure of ninhydrin at the B3LYP/6-311++G** level allowed the assignments of the IR, Raman, and SERS bands of the
compound, and it was accurate in predicting harmonic vibrational frequencies, then the normal modes along with the vibrational spectra were well resolved. The SERS spectra are quite different from the bulk spectra, especially the band intensities, relative intensities, and band positions. Even some new bands in the SERS spectra arise while some bands in the bulk spectra disappear. The HOMO-LUMO gap is predicted to be 4.43 eV at the B3LYP/6-311++G** level. Furthermore, based on the MEP map, we found that ninhydrin has two potential hydrogen donors and four potential hydrogen acceptors.
Acknowledgement This work was supported by the National Natural Science Foundation (Grant Nos. 21272991, 21221063) of P. R. China; Specialized Research Fund for the Doctoral Program of Higher Education(20110061110017); the 111 project (B06009), the Development Program of the Science and Technology of Jilin Province(20110338), the Natural Science Foundation of China (21103062) and the Specialized Research Fund for the Doctoral Program of Higher Education of China (20100061120087).
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Table 1 The optimized structural parameters of ninhydrin calculated at the B3LYP/6-311+G** level. BL C2-C1 C3-C2 C4-C3 C5-C4 C6-C1 H7-C1 H8-C4 H9-C5 H10-C6 C11-C3 C12-C2 O13-C11 O14-C12 C15-C12 O16-C15 H17-O16 O18-C15 H19-O18
Value/Å 1.3940 1.4018 1.3940 1.3915 1.3915 1.0834 1.0834 1.0841 1.0841 1.4823 1.4823 1.2074 1.2074 1.5521 1.3991 0.9681 1.3991 0.9681
BA C3-C2-C1 C4-C3-C2 C5-C4-C3 C6-C1-C2 H7-C1-C6 H8-C4-C3 H9-C5-C4 H10-C6-C1 S11-C3-C2 C12-C2-C1 O13-C11-C3 O14-C12-C2 C15-C12-C2 O16-C15-C12 H17-O16-C15 O18-C15-C12 H19-O18-C15
Value/ ̊ 121.0 121.0 118.0 118.0 121.6 120.4 119.8 119.8 110.6 128.4 128.0 128.0 107.4 111.0 107.5 108.7 107.5
Dihedral
C4-C3-C2-C1 C5-C4-C3-C2 C6-C1-C2-C3 H7-C1-C6-C5 H8-C4-C3-C2 H9-C5-C4-C3 H10-C6-C1-C2 S11-C3-C2-C1 C12-C2-C1-C6 O13-C11-C3-C2 O14-C12-C2-C1 C15-C12-C2-C1 O16-C15-C12-C2 H17-O16-C15-C12 O18-C15-C12-C2 H19-O18-C15-C12
Value/ ̊
0.1 -0.1 -0.1 -179.9 180.0 -180.0 -179.9 179.3 -179.1 -174.9 4.2 -179.7 -117.0 -54.7 117.8 -168.3
Note: BL is bond length, BA is bond angle. The data in parentheses are from phenol and thiophenol calculated at the same level.
Table 2 Experimental and theoretical vibrational frequencies (cm-1) of ninhydrin at the B3LYP/6-311+G** level. **
Experimental No.
FT-IR
Raman
Vibrational
*
Theo.
PED
Assignments
1820 ν45 1749 1747 s.vC=O s.vC=O (88) 1790 ν44 1720 1719 as.vC=O As.vC=O 88) 1630 9b (94) ν43 1593 9b ν42 1587 1622 9a 9a (96) 1388 ν39 1391 βOH βOH (70) 1379 ν38 1359 19b 19b (70)； 1356 ν37 1338 βOH&15 βOH (52)；15 (29) 1308 3 (90) ν36 1292 1291 3 1255 ν35 1258 1257 18b&βOH 18b (52); βOH (34) 1187 14 (87) ν33 1186 1185 14 1149 ν32 1153 1153 γOH&18b γOH (43); 18b (40) 1110 8b (89) ν31 1078 8b 1096 ν30 1063 1064 ωOH ωOH (88) ν29 1012 1012 1037 14&9b 14 (64); 9b (30) 1020 5 (99) ν28 1001 5 979 993 17b (97) ν26 17b 943 950 ν25 943 1& vC-O& i.p.βC=O 1 (40); vC-O (34); i.p. βC=O (18) 888 912 10b (96) ν24 887 10b 815 ν21 795 5&o.p.βC=O 5 (57); o.p.βC=O (34) 741 746 11 (60); o.p.βC=O (34) ν20 741 11&o.p.βC=O 698 707 9a (75); Ring5 breathing (18) ν18 9a&Ring5 breathing *The theoretical frequencies are scaled. **The Wilson notation is employed. ***s., symmetry; as., asymmetry; v, stretching vibration; β, bending; γ, rocking; ω, wagging; i.p., in-plane; o.p., out-of-plane.
Table 3 Experimental and theoretical vibrational frequencies (cm-1) of ninhydrin SERS at the B3LYP/6-311++G** level. No.
Experimental
Theoretical
Assignment
1651 1651 ν46 vC=O 1586 1585 9b ν44 1562 1562 9a ν43 1497 1493 18b ν42 1383 1490 18a ν41 1294 1302 15 ν40 1275 1276 ν39 βO-H 1230 1231 3 ν38 1187 1188 19b ν37 1162 1164 ν36 βO-C-O 1145 1143 14 ν34 1016 1007 ν32 19b & as.vC-O & βC-C-CR5 950 950 ν31 19b & as.vC-O 890 890 ν27 1 & s.vC-CR5 697 683 ν20 6a & s.vC-CR5 541 540 ν17 βC-C-CR6 & βC-C-CR5 507 513 ν16 3 & 18b & βC-C-CR5 *The theoretical frequencies are scaled. **The Wilson notation is employed. ***s., symmetry; as., asymmetry; v, stretching vibration; β, bending.
Figure captions Fig. 1. Sketch map for ninhydrin structure calculated at the B3LYP/6-311++G** level. Fig. 2. Torsional potential energy curve for ninhydrin calculated at the B3LYP/6-311++G** level. Fig. 3. The comparison of the experimental FT-IR spectrum (black) of ninhydrin with the theoretical IR spectrum (red, B3LYP/6-311++G**). The theoretical spectrum has been shifted to improve the visual comparison. Fig. 4. The comparison of the experimental Raman spectrum (black) of ninhydrin with the theoretical Raman spectrum (red, B3LYP/6-311++G**). The theoretical spectrum has been shifted to improve the visual comparison. Fig. 5. Illustrated SERS structure of ninhydrin calculated at the B3LYP/6-311++G** level. Fig. 6. The comparison of the experimental SERS spectrum (black) of ninhydrin with the theoretical SERS spectrum (red, B3LYP/6-311++G**). The theoretical spectrum has been shifted to improve the visual comparison. Fig. 7. Plots of the frontier orbitals of ninhydrin calculated at the B3LYP/6-311++G** level. Fig. 8. The natural charge population of ninhydrin calculated at the B3LYP/6-311++G** level. Fig. 9. The molecular electrostatic potential (MEP) of ninhydrin calculated at the B3LYP/6-311++G** level from different observational orientation. A: top, B: bottom, C: left, D: right, E: front, F: back. The color scheme is red > 0.0352 a.u. > yellow > 0.0 a.u. > green > -0.0197 a.u. > blue.
Fig. 1
Fig. 2
Fig. 3
Fig.4
Fig. 5
Fig. 6
Fig.7
Fig. 8
Fig.9
Graphical abstract
Ninhydrin was designed as a model molecule for theoretical and experimental studies of the molecule structure. Fourier transform infrared (FTIR) and Raman spectra of the compound have been obtained experimentally. Most of the fundamentals vibrations agree well with the predicted frequencies.
Most of the fundamentals vibrations agree well with the predicted frequencies. Hydrogen bond donors and acceptors are predicted. The vibrational spectra of ninhydrin SERS are investigated experimentally and theoretically.
S1386-1425(14)01556-X http://dx.doi.org/10.1016/j.saa.2014.10.059 SAA 12874
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: Revised Date: Accepted Date:
1 September 2014 9 October 2014 12 October 2014
Please cite this article as: R. Li, S. Huimin, P. Liu, L. Chen, J. Cheng, B. Zhao, Vibrational spectroscopy and density functional theory study of ninhydrin, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa.2014.10.059
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Vibrational spectroscopy and density functional theory study of ninhydrin Ran Li,a Sui Huimin,a Peipie Liu,a Lei Chen,a Jianbo Cheng,b and Bing Zhaoa,* a
State Key Laboratory of Supramolecular Structure and Materials, Jilin University,
Changchun 130012, China b
The Laboratory of Theoretical and Computational Chemistry, School of Chemistry
and Chemical Engineering, Yantai University, Yantai 264005, China Corresponding author: Bing Zhao Telephone: +86-431-8516-8473; E-mail: [email protected]
Abstract： ： In this paper, ninhydrin was designed as a model molecule for theoretical and experimental studies of the molecule structure. Density functional theory (DFT) calculations have been performed to predict the IR and Raman spectra for the molecule. In addition, Fourier transform infrared (FTIR) and Raman spectra of the compound have been obtained experimentally. Based on the modeling results obtained at the B3LYP/6-311++G** level, all FTIR and Raman bands of the compound obtained experimentally were assigned. Our calculated vibrational frequencies are in good agreement with the experimental vales. The molecular electrostatic potential surface calculation was performed and the result suggested that the ninhydrin had two potential hydrogen bond donors and four potential hydrogen bond acceptors. HOMO-LUMO gap was also obtained theoretically at B3LYP/6-311++G** level.
Keywords: ninhydrin, FTIR, Raman, DFT, HOMO-LUMO gap, hydrogen bond.
1. Introduction Ninhydrin is usually considered as a tricarbonyl compound since it stable in equilibrium of indane-1,2,3-trione [1]. Ninhydrin was first synthesized by Ruhemann in 1910 [2], and then, well known as a common coloration reagent for amino acid. In 1954, Oden and von Hofsten recognized that it was a useful reagent for developing fingerprints [3]. Fingerprints are still one of the most useful forms of physical evidence in identification. Latent prints are normally invisible without development. The techniques used for fingerprint identification vary according to the surface where the fingerprints are applied. The particular interest to ninhydrin-based development is the concentration of amino acids, which has been reported to be between 0.3 to 2.59 mg L21 of sweat. This value corresponds to an average amino acid content of about 250 ng per print. Small concentrations of amino acids in sweat are sufficient for development on paper [4]. Latent prints have highly potential applications, however the low amino acid content limit the detection probability. Surface-enhanced Raman scattering (SERS) has been widely used as a powerful tool for ultrasensitive chemical analysis, which allows this technique sensitive enough to detect single-molecules [5-6]. Applications of SERS range from nanostructure characterization to chemical-biochemical analysis [7-9]. Ninhydrin is an important analytical tool in various fields, however the concentration of the ninhydrin are usually rather low, which limit its application. The limit of the low concentration could be broke though by taking the advantage of SERS analysis. For the selected enhancement in SERS-based study of molecules, the orientation of the studied molecule on the substrate can be identified and the vibrational information is the basis of SERS analysis. Therefore, the molecular structure information, the vibration mode assignment, and the
frontier molecular orbital information of a probe molecular are important for analyzing the complex structure of molecule and the mechanism of the enhancement. Although molecular conformation and the frontier molecular orbital data are very significant for many applications, they were extremely hard to attain by employing experiment method. Fortunately, such information can be calculated theoretically by using the density functional theory (DFT) which describes the electronic states of atoms, molecules, and materials in terms of the three-dimensional electronic density of the system. DFT is generally accepted as a reliable means for predict the spectrum information and molecular conformation. Our previous works [10-11] also showed the advantage of DFT method in application of obtaining the vibrational information. M. Arivazhagan [19] et al. and D. Sajan [20] et al. proposed the ninhydrin monomer and dimmer, respectively. However the SERS information is lack. The Herein, we report the conformational, IR, Raman, and SERS study of ninhydrin based on DFT calculations at the B3LYP/6-311++G** level. FT-IR and confocal Raman spectra of the compound have also been obtained experimentally and accurately assigned by using the results of the theoretical calculations. For further application, the energy difference between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), the HOMO/LUMO gap, has also been calculated, which is valuable for explaining the enhancement mechanism of SERS. In addition, the hydrogen bond donors and receptors are predicted based on the theoretical calculations.
2. Experimental 2.1 Materials Ninhydrin (99%) was obtained from Sigma. All other chemicals were of analytical grade
and were purchased from Beijing Chemical Reagent Factory and were used without further purification.
2.2 Instruments Raman spectra of ninhydrin were recorded on a LabRam Aramis Raman Microscope system (Horiba-Jobin Yvon) equipped with a multichannel air cooled charge-coupled device (CCD) detector. Spectra were excited using the 633 nm line of a HeNe narrow bandwidth laser (Melles Griot). The Raman spectra were collected at room temperature with the laser power at the sample position typically at 400 µW with an average spot size diameter of 1 µm. The typical acquisition time used in this work was 30s. The FT-IR spectra of ninhydrin were recorded as KBr disks at room temperature by a Bruker IFS-66V FT-IR spectrometer, equipped with a DTGS detector at a resolution of 4 cm−1 .
3. Theoretical Method All the geometries we got in this work were optimized by DFT method of B3LYP which is the hybrid of Becke’s three-parameter exchange functional [12-13] with the Lee–Yang–Parr correlation functional [14]. The triple split valence basis set of 6-311+G** for the H, C, and O atoms and lanl2dz for Ag atom was adapted. The frequency calculation was performed at the same level. All calculations were carried out with the aid of the Gaussian 09 program [15]. The molecular electrostatic potential (MEP) were obtained by the WFA [16] software package. Potential energy distribution (PED) calculation was carried out by the VEDA 4 (Vibrational Energy Distribution Analysis) [17]. The method for calculating scaling factors was same as that proposed by Scott and Radom [18].
4. Results and Discussion
The title compound, ninhydrin, is an indene derivative containing two ketoes and two hydroxyls groups. The optimized geometry of ninhydrin was characterized as potential energy minima at the same level by verifying that all vibrational frequencies are real.
4.1 Molecular geometry The optimized geometry of ninhydrin is shown in Fig.1, and the corresponding structural parameter of bond lengths, bond angles, and dihedral angles are shown in Table 1. The atom numerical labels in the following discussion refer to Fig. 1. All atoms lay in the same plane except the O16, H17, O18, and H19, thus the twofold symmetry axis survived whereas the mirror planes no longer exist. Therefore, the theoretically obtained title molecule is a C2 symmetric molecule. The C2 symmetry equates the two rotation-free hydroxyls, thus the DFT calculated potential energy curve which is shown in Fig. 2 is obtained by twisting one of the hydroxyls. The horizontal and axes refer to torsion angles and the potential energy difference, respectively. The torsional potential energy curve shows two rotation barriers with different barrier heights of 23.5 and 15.8 kJ/mol; the saddle implied another structure in particular condition.
4.2 Vibrational assignments The ninhydrin molecule consists of 19 atoms that, undergoes 51 normal modes of vibrations. 21of which were assigned in this manuscript. The observed and calculated vibration modes are listed in Table 2. The frequencies 1000-1800 cm-1 were multiplied by the scaling factors of 0.9734. The IR spectra in Fig. 3 shows a number of bands appearing at 741, 795, 887, 943, 1001, 1012, 1063, 1078, 1153, 1186, 1258, 1292, 1338, 1359, 1391, 1593, 1720, and 1749 cm-1. The Raman bands in Fig. 4 are observed at 698, 741, 888, 943, 979, 1012, 1064, 1153, 1185, 1257, 1291, 1587,
1719, and 1747 cm-1. As it can be seen from Figs. 2 and 3, most of the fundamentals vibrations observed in Raman and FTIR spectra agreed well with the theoretically predicted frequencies. Most fundamentals were observed both in IR and Raman spectra, while 7 normal modes were observed only in IR spectra and 3 normal modes were observed only in Raman spectra. The fundamental ν43, ν38, and ν31 arise from 9b, 19b, and 8b, respectively, and were observed only in IR spectra at ca. 1593, 1359, and 1078 cm-1 , respectively. The fundamental ν39 was assigned to the hydroxyl bending vibration whereas the fundamental ν37 was assigned to 15 coupled with the hydroxyl bending vibration. The fundamental ν39 and ν37 were observed only in IR spectra at ca. 1391 and 1338 cm-1, respectively. The fundamental ν28 and ν21 were also observed only in IR spectra at ca. 1001 and 795 cm-1, respectively, and the former was assigned to 5 whereas the later was assigned to 5 coupled with C=O out-of-plane bending vibration. The fundamental ν42 and ν26 were assigned to 9a and 17b respectively, and observed only in Raman spectra at ca. 1587 and 979 cm-1, respectively. The fundamental ν18 was assigned to 9a coupled with breathing vibration of the five-membered ring and observed only in Raman spectra at ca. 698 cm-1. The other 11 fundamentals were observed in both IR and Raman spectra. The fundamental ν45 (1749 cm-1 in IR spectra and 1747 cm-1 in Raman spectra) and ν44 were both assigned to C=O stretching vibration, the former one was symmetric and the later one was asymmetric. The fundamental ν36 (1292 cm-1 in IR spectra and 1291 cm-1 in Raman spectra) and ν24 (887 cm-1 in IR spectra and 888 cm-1 in Raman spectra) were attributed to 3 and 10b respectively. The fundamental ν35 (1258 cm-1 in IR spectra and 1257 cm-1 in Raman spectra) arises from 18b coupled with the O-H bending vibration whereas the ν32 (1153 cm-1 both in IR spectra and Raman spectra) arises from 18b coupled with the O-H rocking vibration. The fundamental ν30 (1063 cm-1 in IR spectra and 1064 cm-1 in Raman
spectra) arises from the O-H wagging vibration. The fundamental ν33 (1186 cm-1 in IR spectra and 1185 cm-1 in Raman spectra) arises from motion related to benzene ring of 14 whereas the fundamental ν29 (1012 cm-1 both in IR spectra and Raman spectra) arises from 14 coupled with 9b. The fundamental v25 (943 cm-1 both in IR spectra and Raman spectra) arises from motion related to benzene ring of 1 coupled with C-O stretching vibration and C=O in-plane bending vibration. The fundamental v20 (741 cm-1 both in IR spectra and Raman spectra) arises from motion related to benzene ring of 11 coupled with C=O out-of-plane bending vibration.
4.3 SERS assignments The SERS structure of ninhydrin connected with Ag nanoparticles are illustrated in Fig.5. The SERS spectra are quite different from the bulk spectra, especially the band intensities, relative intensities, and band positions. Even some new bands in the SERS spectra arise while some bands in the bulk spectra disappear. For instance, in the SERS spectra, the 9a and 9b vibrational modes are red shifted whereas the 14 vibrational mode is blue shifted; a new band of O-C-O bending arises whereas the 17b vibrational mode in the bulk spectra disappeared in the SERS spectra. These differences are caused by the interaction between ninhydrin and the nanoparticles. The structure change of ninhydrin, which caused by chemically connected with the nanoparticles, is an ignorable parameter to these differences too. The observed and calculated vibration modes are listed in Table 3. The frequencies 1000-1800 cm-1 were multiplied by the scaling factors of 0.9618. The SERS bands in Fig. 6 were observed at 507, 541, 697, 890, 950, 1016, 1145, 1162, 1187, 1230, 1275, 1294, 1383, 1497, 1562, 1586, and 1651 cm-1.
The fundamental ν46 arises from the O-H wagging vibration was observed at 1651 cm-1. The fundamental ν44 and ν43 observed at 1586 and 1562 cm-1, respectively, were assigned to 9b and 9a, respectively. The fundamental ν42 (1497 cm-1) and ν41 (1383 cm-1) were assigned to 18b and 18a, respectively. The fundamental ν40 (1294 cm-1), ν38 (1187 cm-1) and ν34 (1145 cm-1) were contributed to 15, 3, and 14, respectively. The fundamental v39 (1275 cm-1) was assigned to O-H bending whereas the fundamental v36 (1162 cm-1) was assigned to O-C-O bending. The fundamental ν37 observed at 1187 cm-1 was assigned to 19b; the fundamental ν31 observed at 950 cm-1 was assigned to 19b coupled with C-O asymmetric stretching; the fundamental ν32 observed at 1016 cm-1 was assigned to 19b coupled with C-O asymmetric stretching and C-C-C bending of the five membered ring. The fundamental ν27 (890 cm-1 ) and ν20 (697 cm-1) were both assigned to C-C symmetric stretching vibration of the five membered ring, the former was coupled with 1 whereas the later was coupled with 6a. The fundamental ν17 (541 cm-1) was assigned to C-C-C bending of the five membered ring coupled with C-C-C bending of the six membered ring whereas the fundamental ν16 (507 cm-1) assigned to C-C-C bending of the five membered ring coupled with 18b.
4.4 HOMO–LUMO gap HOMO–LUMO gap results in a significant degree of electronic excitation and charge transfer. In most cases, even in the absence of inversion symmetry, the strongest band in the Raman spectrum is weak in the IR spectrum and vice versa. Changes in the HOMO–LUMO gap by connecting with some noble metal, semiconductor or some other means result in the change of the charge transfer degree, intensity and position of the peak. The frontier molecular orbitals are
shown in Fig. 7 and the HOMO–LUMO gap estimated to be 4.43 eV at B3LYP/6-311++G** level.
4.5 Hydrogen bond donor and accepter Hydrogen bond is an important intermolecular interaction in supramolecular systems. Charge transfer and the corresponding change in electric structure have direct effect on the spectra. The natural population analysis results are shown in Fig. 8, the atom number labeling consist with Fig. 1. The mapping of molecular electrostatic potential onto the iso-electron density surface simultaneously displays electrostatic potential distribution, molecular shape, size, and dipole moments of the molecule which provides a visual method to understand the relative polarity of the molecule. The total electron density and molecular electrostatic potential (MEP) surfaces of the molecules under investigation are constructed by using B3LYP/6-311++G** method. The total electron density mapped with electrostatic potential surface of ninhydrin is shown in Fig. 9. The color scheme in Fig. 9 of MEP is red > 0.0352 a.u. > yellow > 0.0 a.u. > green > -0.0197 a.u. > blue. The MEP map indicates that ninhydrin has two potential hydrogen bond donors (the H atoms in the hydroxyl functional groups) and four hydrogen bond accepters (two O atoms in the ketone carbonyl functional groups and two O atoms in the hydroxyl functional groups).
5. Conclusion In this work, the structural parameters of ninhydrin have been obtained at the B3LYP/6-311++G** level of theory. The title molecule was found to lie in the same plane except two hydroxyls with C2 symmetric. The torsional potential energy curve shows two rotation barriers. Modeling performed for ninhydrin and SERS structure of ninhydrin at the B3LYP/6-311++G** level allowed the assignments of the IR, Raman, and SERS bands of the
compound, and it was accurate in predicting harmonic vibrational frequencies, then the normal modes along with the vibrational spectra were well resolved. The SERS spectra are quite different from the bulk spectra, especially the band intensities, relative intensities, and band positions. Even some new bands in the SERS spectra arise while some bands in the bulk spectra disappear. The HOMO-LUMO gap is predicted to be 4.43 eV at the B3LYP/6-311++G** level. Furthermore, based on the MEP map, we found that ninhydrin has two potential hydrogen donors and four potential hydrogen acceptors.
Acknowledgement This work was supported by the National Natural Science Foundation (Grant Nos. 21272991, 21221063) of P. R. China; Specialized Research Fund for the Doctoral Program of Higher Education(20110061110017); the 111 project (B06009), the Development Program of the Science and Technology of Jilin Province(20110338), the Natural Science Foundation of China (21103062) and the Specialized Research Fund for the Doctoral Program of Higher Education of China (20100061120087).
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Table 1 The optimized structural parameters of ninhydrin calculated at the B3LYP/6-311+G** level. BL C2-C1 C3-C2 C4-C3 C5-C4 C6-C1 H7-C1 H8-C4 H9-C5 H10-C6 C11-C3 C12-C2 O13-C11 O14-C12 C15-C12 O16-C15 H17-O16 O18-C15 H19-O18
Value/Å 1.3940 1.4018 1.3940 1.3915 1.3915 1.0834 1.0834 1.0841 1.0841 1.4823 1.4823 1.2074 1.2074 1.5521 1.3991 0.9681 1.3991 0.9681
BA C3-C2-C1 C4-C3-C2 C5-C4-C3 C6-C1-C2 H7-C1-C6 H8-C4-C3 H9-C5-C4 H10-C6-C1 S11-C3-C2 C12-C2-C1 O13-C11-C3 O14-C12-C2 C15-C12-C2 O16-C15-C12 H17-O16-C15 O18-C15-C12 H19-O18-C15
Value/ ̊ 121.0 121.0 118.0 118.0 121.6 120.4 119.8 119.8 110.6 128.4 128.0 128.0 107.4 111.0 107.5 108.7 107.5
Dihedral
C4-C3-C2-C1 C5-C4-C3-C2 C6-C1-C2-C3 H7-C1-C6-C5 H8-C4-C3-C2 H9-C5-C4-C3 H10-C6-C1-C2 S11-C3-C2-C1 C12-C2-C1-C6 O13-C11-C3-C2 O14-C12-C2-C1 C15-C12-C2-C1 O16-C15-C12-C2 H17-O16-C15-C12 O18-C15-C12-C2 H19-O18-C15-C12
Value/ ̊
0.1 -0.1 -0.1 -179.9 180.0 -180.0 -179.9 179.3 -179.1 -174.9 4.2 -179.7 -117.0 -54.7 117.8 -168.3
Note: BL is bond length, BA is bond angle. The data in parentheses are from phenol and thiophenol calculated at the same level.
Table 2 Experimental and theoretical vibrational frequencies (cm-1) of ninhydrin at the B3LYP/6-311+G** level. **
Experimental No.
FT-IR
Raman
Vibrational
*
Theo.
PED
Assignments
1820 ν45 1749 1747 s.vC=O s.vC=O (88) 1790 ν44 1720 1719 as.vC=O As.vC=O 88) 1630 9b (94) ν43 1593 9b ν42 1587 1622 9a 9a (96) 1388 ν39 1391 βOH βOH (70) 1379 ν38 1359 19b 19b (70)； 1356 ν37 1338 βOH&15 βOH (52)；15 (29) 1308 3 (90) ν36 1292 1291 3 1255 ν35 1258 1257 18b&βOH 18b (52); βOH (34) 1187 14 (87) ν33 1186 1185 14 1149 ν32 1153 1153 γOH&18b γOH (43); 18b (40) 1110 8b (89) ν31 1078 8b 1096 ν30 1063 1064 ωOH ωOH (88) ν29 1012 1012 1037 14&9b 14 (64); 9b (30) 1020 5 (99) ν28 1001 5 979 993 17b (97) ν26 17b 943 950 ν25 943 1& vC-O& i.p.βC=O 1 (40); vC-O (34); i.p. βC=O (18) 888 912 10b (96) ν24 887 10b 815 ν21 795 5&o.p.βC=O 5 (57); o.p.βC=O (34) 741 746 11 (60); o.p.βC=O (34) ν20 741 11&o.p.βC=O 698 707 9a (75); Ring5 breathing (18) ν18 9a&Ring5 breathing *The theoretical frequencies are scaled. **The Wilson notation is employed. ***s., symmetry; as., asymmetry; v, stretching vibration; β, bending; γ, rocking; ω, wagging; i.p., in-plane; o.p., out-of-plane.
Table 3 Experimental and theoretical vibrational frequencies (cm-1) of ninhydrin SERS at the B3LYP/6-311++G** level. No.
Experimental
Theoretical
Assignment
1651 1651 ν46 vC=O 1586 1585 9b ν44 1562 1562 9a ν43 1497 1493 18b ν42 1383 1490 18a ν41 1294 1302 15 ν40 1275 1276 ν39 βO-H 1230 1231 3 ν38 1187 1188 19b ν37 1162 1164 ν36 βO-C-O 1145 1143 14 ν34 1016 1007 ν32 19b & as.vC-O & βC-C-CR5 950 950 ν31 19b & as.vC-O 890 890 ν27 1 & s.vC-CR5 697 683 ν20 6a & s.vC-CR5 541 540 ν17 βC-C-CR6 & βC-C-CR5 507 513 ν16 3 & 18b & βC-C-CR5 *The theoretical frequencies are scaled. **The Wilson notation is employed. ***s., symmetry; as., asymmetry; v, stretching vibration; β, bending.
Figure captions Fig. 1. Sketch map for ninhydrin structure calculated at the B3LYP/6-311++G** level. Fig. 2. Torsional potential energy curve for ninhydrin calculated at the B3LYP/6-311++G** level. Fig. 3. The comparison of the experimental FT-IR spectrum (black) of ninhydrin with the theoretical IR spectrum (red, B3LYP/6-311++G**). The theoretical spectrum has been shifted to improve the visual comparison. Fig. 4. The comparison of the experimental Raman spectrum (black) of ninhydrin with the theoretical Raman spectrum (red, B3LYP/6-311++G**). The theoretical spectrum has been shifted to improve the visual comparison. Fig. 5. Illustrated SERS structure of ninhydrin calculated at the B3LYP/6-311++G** level. Fig. 6. The comparison of the experimental SERS spectrum (black) of ninhydrin with the theoretical SERS spectrum (red, B3LYP/6-311++G**). The theoretical spectrum has been shifted to improve the visual comparison. Fig. 7. Plots of the frontier orbitals of ninhydrin calculated at the B3LYP/6-311++G** level. Fig. 8. The natural charge population of ninhydrin calculated at the B3LYP/6-311++G** level. Fig. 9. The molecular electrostatic potential (MEP) of ninhydrin calculated at the B3LYP/6-311++G** level from different observational orientation. A: top, B: bottom, C: left, D: right, E: front, F: back. The color scheme is red > 0.0352 a.u. > yellow > 0.0 a.u. > green > -0.0197 a.u. > blue.
Fig. 1
Fig. 2
Fig. 3
Fig.4
Fig. 5
Fig. 6
Fig.7
Fig. 8
Fig.9
Graphical abstract
Ninhydrin was designed as a model molecule for theoretical and experimental studies of the molecule structure. Fourier transform infrared (FTIR) and Raman spectra of the compound have been obtained experimentally. Most of the fundamentals vibrations agree well with the predicted frequencies.
Most of the fundamentals vibrations agree well with the predicted frequencies. Hydrogen bond donors and acceptors are predicted. The vibrational spectra of ninhydrin SERS are investigated experimentally and theoretically.
