J Infrared Milli Terahz Waves (2014) 35:871–880 DOI 10.1007/s10762-014-0092-x
Terahertz Spectra of L-Ascorbic Acid and Thiamine Hydrochloride Studied by Terahertz Spectroscopy and Density Functional Theory Ling Jiang & Miao Li & Chun Li & Haijun Sun & Li Xu & Biaobin Jin & Yunfei Liu
Received: 5 March 2014 / Accepted: 3 July 2014 / Published online: 16 July 2014 # Springer Science+Business Media New York 2014
Abstract We have investigated the terahertz spectra of L-ascorbic acid and thiamine hydrochloride measured by terahertz time-domain spectroscopy (THz-TDS) and Fourier transform infrared spectroscopy (FTIR). The measured absorption spectra were demonstrated to be in good agreement with the results simulated by Density Functional Theory (DFT) using hybrid functional B3LYP with basis set of 6-31G (d), except with slight frequency shift and few peaks missing. We presented the comparison of measured spectra by the FTIR spectroscopy employing low temperature silicon bolometer as detector and the TDS system. The measured spectra of the L-ascorbic acid showed shoulder bands at 0.25, 1.1, 1.5, 1.82, 2.03, 2.30, 2.44, 2.67, 2.97, 3.12, and 3.40 THz, respectively. The spectra of the thiamine hydrochloride show shoulder bands at 0.48, 1.11, 1.57, 1.75, 1.92, 2.08, 2.31, 2.53, 2.69, 2.85, 3.12, 3.22, and 3.31 THz. Most absorption peaks of the two samples agree with the results simulated by Density Function Theory (DFT) method of Gaussian 09 software. In our work, more spectral peaks based on experimental and theoretical results were found in comparison to that of other groups, since we employed higher sensitive FTIR measurement system and considered the effect of number of molecule unit in simulation. The study suggests that the effect of intermolecular vibration is stronger than intramolecular interaction on the absorption bands in THz region. Keyword FTIR . TDS . L-ascorbic acid . thiamine hydrochloride . DFT method
L. Jiang : M. Li : C. Li : Y. Liu (*) College of Information Science and Technology, Nanjing Forestry University, Nanjing, Jiangsu 210037, China e-mail:
[email protected] H. Sun : L. Xu Advanced Analysis and Testing Center, Nanjing Forestry University, Nanjing, Jiangsu 210037, China B. Jin School of Electronic Science and Engineering, Nanjing University, Nanjing, Jiangsu 210000, China
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1 Introduction To date, the terahertz (THz) spectroscopy has been widely applied in various fields of research to understand the THz frequency dynamics of biological molecular systems related to intermolecular vibrations and large-amplitude intramolecular modes [1]. Extensive spectroscopic studies in this region have been carried out, and THz spectroscopy has demonstrated that this method is complementary to other spectroscopic techniques. THz time-domain spectroscopy (THz-TDS) and Fourier transform infrared spectroscopy (FTIR) are two useful methods for evaluating the optical properties of materials and biological molecules in infrared range. THz-TDS focus on the spectral analysis in the low-frequency range between 0.1 and 3 THz, which obtains the simultaneous information of both the amplitude and phase of the THz electric field, making it an ideal tool in analyzing the subpicosecond and picosecond dynamics of various molecular species. THz-TDS have been applied in many fields, such as medical diagnosis, pharmaceutical and biomolecular analysis, and security enhancement [2–10], especially in probing low frequency modes for various chemical and biological systems such as vitamins [11–13]. Beside low-frequency THz range, the far-infrared at higher frequencies, middle-infrared and near-infrared region are very important to study the molecular vibrations and determine the presence of certain characteristic groups in large biological molecules [14, 15], which can be measured by FTIR spectroscopy. In a FTIR spectrometer, all frequencies can be scanned simultaneously because of all elements of the source reaching the detector at once, which is called multiplexing or Fellgett’s advantage [16]. And with less optics and no slits compared to dispersive instruments, so a much higher throughput can be achieved and hence increasing the signal-to-noise ratio (SNR), which is called Jacquinot advantage [17]. L-ascorbic acid (Vitamin C) and thiamine hydrochloride (Vitamin B1) are two kinds of important vitamins for human health. Using THz-TDS spectroscopy, the absorption spectra of the L-ascorbic acid and thiamine hydrochloride were studied. Yu Bin et al. showed that large amounts of spectral and structural information exist for such samples, and it were found that five absorption peaks appeared at 1.08, 1.49, 1.82, 2.02, 2.34 THz for the L-ascorbic acid, and two frequency peaks appeared at 1.11 and 1.50 THz for the thiamine hydrochloride [11]. However, the theoretical calculation results were not in agreement with the measurement results. The same study was done in the Vitamin C by using THz-TDS, but showing different absorption peaks [12]. In paper [13], Yan Zhigang investigated the thiamine hydrochloride with THz-TDS, and more absorption peaks were found. The experimental and theoretical studies have been done in above papers, however, the experimental results are not consistent between these papers, and the reason of disagreement with theoretical calculation is not given in these papers. In our work, we employed both of the THz-TDS and the FTIR spectroscopy to study the absorption spectra of the L-ascorbic acid and thiamine hydrochloride samples. The FTIR spectroscopy used low temperature silicon bolometer as detector which worked at 4-kelvin in order to further increase system SNR. The low temperature detector has much higher sensitivity compared with room temperature ones in the far-infrared range. Experimental results demonstrated that the SNR and measurement accuracy of the FTIR spectrometer was enhanced largely below 4 THz. Furthermore, spectral calculations based on density functional theory (DFT) using Gaussian 09 [18] were done for the L-ascorbic acid and thiamine hydrochloride samples. The density functional theory (DFT) calculation has proven to be a reliable theoretical method to predict accurate vibrational frequencies for typical small-size and medium-size molecules and interpret the measured infrared spectra in physical origin of these signatures [19]. In the simulation,
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we calculated the absorption spectra for various number of duplicate molecule unit in order to understand intermolecular effect on terahertz spectra. The calculated absorption spectra are well consistent with the experimental results except with slight frequency shift and few peaks missing.
2 Sample Preparation and Measurement Method The L-ascorbic acid and thiamine hydrochloride measured in our experiment were purchased from Sigma Aldrich Co. and used without further purification. Samples were prepared by weighing 50-100 mg of each solid and homogenizing the material in a mortar and pestle. This procedure ensured particle sizes sufficiently smaller than THz wavelengths to reduce baseline offsets at higher frequencies arising from non-resonant light scattering. The samples were pressed as a pellet in a 13 mm diameter vacuum die at the lowest possible pressures to minimize decomposition from transient heating. The pellets have thickness between 0.5 and 1 mm which is suitable for the measurement of the THz-TDS and FTIR spectroscopy. The samples were not diluted with, for example, polyethylene powder. This is because we aimed to investigate the weak absorption band in the terahertz region. In the conventional measurement of the THz absorption spectrum, the purpose is to observe prominent bands. Therefore, the samples are diluted with polyethylene powder to reduce the intensity of the absorption peak to an appropriate level. The absorption spectra in the 0.1-4 THz range were obtained with the THz-TDS system working in transmission mode, provided by Advantest Co. [20]. Figure 1 shows the schematic of THz-TDS measurement system (type: TAS7500SP). The THz-TDS spectroscopy employs two ultra short pulse fiber lasers self developed by Advantest, which are ensured to synchronized control. These pulses are centered at 1550 nm with maximum output power of 50 mW, and provides extremely short pulse width less than 50 f. and low jitter below 50 fs. The THzTDS system achieves the sampling rate with 8 ms per scan and ultra-wide frequency band extending to 4 THz. The whole system purged by dry-air has sufficient sensitivity to generate high quality spectra in the frequency range of 0.1-4 THz. The spectral resolution of 0.25 cm-1 were obtained and averaged 2048 times for one point.
Fig. 1 Schematic of THz-TDS measurement system
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The infrared absorption spectra covering far-infrared to near-infrared were measured by Bruker v80 Fourier transform infrared spectrometer (FTIR) [21]. In the far-infrared band, we employed low temperature silicon bolometer as detector working at 4-kelvin instead of DTGS room temperature detector in order to enhance the SNR of system. The vacuumized FTIR spectroscopy provides 2 cm-1 spectral resolution for the far-infrared band and 4 cm-1 for the middle-infrared band. The principle of the FTIR has been described in other papers [15, 22]. Reproducibility of spectral features was demonstrated well by measuring several pellets with various amounts of the L-ascorbic acid and thiamine hydrochloride for both of the THz-TDS and FTIR.
3 Measurement Results Absorption measurement using the THz-TDS was conducted under a continuous flow of dry air, and the FTIR measurement was done in vacuum condition. The effect of water absorption was eliminated for both of measurement. We measured the time domain spectra for the Lascorbic acid with different thickness by using the THz-TDS, as shown in Fig. 2. It can be seen that the time delay of the signal increase with the thickness of the sample, which represented the transmission time of terahertz wave in the sample. The frequency domain absorption spectra were obtained after Fourier transform, as shown in Fig. 3. The calculated refractive indices of the samples were included as well. The thickness of the measured sample is 0.763 mm for the L-ascorbic acid and 0.558 mm for the thiamine hydrochloride, respectively. Several absorption bands were observed in the frequency range of 0.1-4 THz. For Lascorbic acid, there are two weak bands at 1.07 and 1.47 THz, and several strong bands at 1.78, 2.01, 2.31, 2.38, 2.73, 2.84, 2.92, and 3.02 THz, respectively. Thiamine hydrochloride shows two weak bands at 2.10 and 2.18 THz, and two strong bands at 1.11 and 1.71 THz. In comparison to the results in paper [11], two new absorption peaks at 2.10 and 2.18 THz were found for the thiamine hydrochloride. Moreover, some fluctuations were observed in the spectra, particularly at higher frequencies for thiamine hydrochloride. It is probably due to low SNR of the THz-TDS system at high frequencies. Hence, we turned to adopt the FTIR which use low temperature silicon bolometer with higher sensitivity as detector. The
Fig. 2 THz time domain spectra of L-ascorbic acid for different thickness measured by the THz-TDS
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(a) L-ascorbic acid
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(b) thiamine hydrochloride
Fig. 3 THz spectra and calculated refractive indices of L-ascorbic acid (a) and thiamine hydrochloride (b) measured by the THz-TDS
measurement of far-infrared spectra employed Mylar film with three thickness of 125 μm, 50 μm, 25 μm as beamsplitter. The 125-μm thick Mylar works for 0.15-0.6 THz, the 50-μm Mylar 0.3-1.8 THz, the 25-μm Mylar 0.6-3.6 THz. We combined the measured spectra for the three bands into one long band varying from 0.15-3.6 THz. Figure 4 displays the measurement results of the FTIR, the TDS results were included as well for comparison. The absorption frequencies measured by the FTIR are almost consistent with these measured by the TDS in the low-frequency range. However, at high frequency above 2.5 THz, the absorption spectra measured by the FTIR displayed clearer fingerprint peaks. The measured absorption frequencies were shown in Table 1. For the thiamine hydrochloride, we measured 12 absorption frequencies, which are much more than reported by papers [11, 12].
4 Theoretical Results In order to demonstrate the measurement results and analyze the intermolecular and intramolecular vibration modes, we calculate the absorption spectra with density functional theory (DFT) method embedded in Gaussian 09 program set. It has been demonstrated that DFT predicts molecular structures and harmonic vibrational frequencies of higher accuracy than obtained by predictions of MP2 calculations [23]. The hybrid functional B3LYP with the 6-
(a) L-ascorbic acid
(b) thiamine hydrochloride
Fig. 4 THz spectra of L-ascorbic acid (a) and thiamine hydrochloride (b) measured by the FTIR and THz-TDS
Thiamine hydrochloride
1.47
0.48
0.68
0.50
0.44
FTIR
One molecule
Two molecules
Four molecules
0.20
Three molecules THz-TDS
1.02
0.91
1.11
1.38
1.41
1.57
1.38 1.71
1.41
0.36
Two molecules 0.93 1.11
1.31
1.50
One molecule 1.15
1.07 1.10
FTIR
0.25
THz-TDS
L-ascorbic acid
Absorption frequency (THz)
Method
Sample
1.64
1.65
1.63
1.75
1.83
1.82
1.78
Table 1 Comparison of the absorption frequencies of measurement and simulation
2.01
1.92
1.92
2.01
2.01
2.12
2.03
2.31
2.21
2.08
1.97
2.08
2.25 2.10
2.30
2.30
2.38
2.38
2.35
2.36
2.31
2.58 2.18
2.58
2.44
2.73
2.50
2.53
2.71
2.74
2.67
2.69
2.84
2.92
2.82
2.80
2.80
2.85
2.97
3.02
3.12
3.12
3.02
2.98
3.11
3.12
3.22
3.55
3.40 3.9
3.33
3.31
3.31
3.89
4
3.66 3.66
3.67
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31G (d) basis set has been often used in the THz spectroscopic calculation of biological molecules [24]. The structure parameters were included as Gaussian calculation setup in the Gaussian 09 software. The original one molecule structure was originated from the X-Ray data, and we optimized the molecule structure of L-ascorbic acid and thiamine hydrochloride with functional method B3LYP and 6-31G (d) basis set. The ground state of these structures has been confirmed by using conformational analysis. Based on the optimized structures and same functional method and basis set, we calculated the absorption spectra of L-ascorbic acid and thiamine hydrochloride. The simulated number of absorption frequency is four for Lascorbic acid and five for thiamine hydrochloride, both of them are much less than measured value. For the one molecule calculation, only intramolecular vibration and rotation were considered. However, in practical experiment, an amount of molecules were mixed, the interaction between molecules should affect the absorption spectra. In order to study the effect of intermolecular interaction on the absorption properties, we also calculated the absorption spectra of duplicate molecule unit such as two molecules, three molecules and four molecules. The absorption frequencies are summarized in Table 1. It should be noted that no imaginary frequency appears for all calculations. The simulated absorption peak position of the three molecules of L-ascorbic acid sample and the two molecules of thiamine hydrochloride sample are in good agreement with the measured results by FTIR except with slight frequency shift and few frequencies missing. The discrepancy between simulation and experiment results originates from that the molecular structure used in the simulation is gas phase molecular model, which is different from crystal structure. The unit cell of the L-ascorbic acid contains four molecules, and each molecule consists of an almost planar five-membered ring plus a side chain [25]. We simulated the absorption spectra of three molecules and four molecules of the L-ascorbic acid, and found similar frequency peaks, but with blue shift below 100 GHz while increasing one molecule. The blue shift may result from the reaction of intermolecular hydrogen bonds. The unit cell of the thiamine hydrochloride is monoclinic, and the packing of molecules in the crystal structure is complicated [26]. It’s hard to simulate the THz spectra of such large crystal structure. We simulated the two molecules and four molecules with relatively small molecular weight. The number of absorption frequency substantially increases with molecule size, and becomes constant up to four molecule structure. Figure 5 show the molecule structures employing in the simulation, which were drawn in the Gaussian view (Gaussian drawing interface). Table 2 and 3 show the initial and optimized bond lengths, angles, and dihedral of three molecules structure of L-ascorbic acid and two molecules structure of thiamine hydrochloride. The simulated absorption spectra are shown in Fig. 6. The low-frequency absorption below 0.5 THz arises from the intermolecular vibration modes.
(a) L-ascorbic acid
(b) thiamine hydrochloride
Fig. 5 Three molecules structure of L-ascorbic acid (a) and two molecules structure of thiamine hydrochloride (b) drawn by Gaussian view
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Table 2 Initial and optimized bond length, angle, and dihedral of three molecules structure of L-ascorbic acid Bond
Initial structure
Optimized structure
Length
Degrees
Dihedral
Length
Degrees
Dihedral
O21-O6-O5-O4
2.407109
130.7138
-109.454
2.68109
129.9246
-123.519
C27-O21-O6-O5
1.295917
-100.554
1.22383
108.9266
H33-O22-O6-O5
1.104155
O43-O42-O41-O3
3.205578
O46-O6-O5-O4
2.529149
99.87433 6.807323 102.7791 99.03015
C48-O42-O41-O3
1.411691
52.87822
H53-O42-O41-O3 H60-O46-O6-O5
1.027601 1.059268
48.72978 6.059021
-49.6579
0.99253
6.93848
-89.4432 -79.4802
30.92803
3.08015
104.0554
6.17109
33.70731
2.87543
114.6888
43.59036
36.51169 -146.085 168.201
1.35728
53.54031
0.97476 0.97861
53.35232 5.0736
7.23447 -175.039 -120.509
However, the combination of intramolecular and intermolecular vibration and rotation lead to the absorptions observed above 0.5 THz. As an example of the L-ascorbic acid, the absorption at 0.2 THz and 0.93 THz exhibits the torsion and vibration between three molecules, and the absorption at 2.58 THz indicates the intramolecular Benzene rotation. For the thiamine hydrochloride, the peak at 1.41 THz originates from the rotation of methyl and C—H…O, and the peak at 2.35 THz is assigned to the rotation of weak H—Cl hydrogen bond. The absorption peaks at other region, display the overlap of weak intramolecular and strong intermolecular interactions [27]. Since we measured the samples at room temperature, the dependence of the absorption frequency on temperature was not considered in the paper. Indeed S. C. Shen et al found the absorption frequency shift downward with the increase of temperature in the temperature of 5300K for the sample of α-L-alanine, and some new absorption bands appear at low temperatures [28]. Due to the frequency band limitation of low temperature detector, the absorption peaks were not observed above 3.6 THz by the FTIR. To observe the spectrum feature of the two samples at higher-frequency region, we use thinner 6 μm Mylar film as the beamsplitter and room temperature DTGS detector instead of low temperature detector. Figure 7 shows the measured and simulated wide band absorption spectra of the thiamine hydrochloride in the frequency range from 1 to 15 THz. It can be seen that three absorption bands were measured, Table 3 Initial and optimized bond length, angle, and dihedral of two molecules structure of thiamine hydrochloride Bond
Initial structure Length
Degrees
Optimized structure Dihedral
Length
Degrees
C9-C8-C7-N6
1.380293
122.9526
-44.5516
1.366824
125.8393
Cl19-N13-C12-N11 H23-C1-C2-C3
2.997397 1.106283
127.814 108.9356
177.4795 80.60492
2.938572 1.097079
123.9416 111.1371
Dihedral 49.47976 164.4786 105.4493
H28-C7-N6-C5
1.11863
107.5948
164.0653
1.098259
108.5996
122.001
H38-N13-C12-N11
1.705324
120.3482
178.4729
1.091156
119.9938
171.0998
C47-C46-C45-N44
1.391382
119.1391
151.4174
1.374896
119.8521
109.578
Cl57-N49-C48-C47
3.624724
131.326
170.2999
2.917748
103.1691
177.4559
H76-N49-C48-C47
2.547571
144.2656
172.6962
1.095057
115.0937
178.4466
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(a) L-ascorbic acid
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(b) thiamine hydrochloride
Fig. 6 Comparison of absorption spectra measured by FTIR and simulated by Gaussian software for L-ascorbic acid (a) and thiamine hydrochloride (b) samples
located at 8.38, 9.94, and 10.95 THz. However, the number of the simulated absorption peak is much more than the measured peak, which is probably caused the sensitivity limitation of the room temperature detector in the FTIR.
5 Summary To investigate the broad-band terahertz spectrum in the frequency range of 0.1-4 THz for Lascorbic acid and thiamine hydrochloride, we employed both of terahertz spectroscopy measurement methods of THz-TDS and FTIR. The measurement results were demonstrated to be in good agreement with theoretical simulation by Density Functional Theory (DFT). The low frequency absorption peak below 0.5 THz was observed while using high sensitive FTIR with low temperature silicon bolometer detector. The low-frequency absorption mainly results from the intermolecular vibration and rotation. The absorption bands above 0.5 THz arise from the combination of intramolecular and intermolecular vibration and rotation. The simulated
Fig. 7 Absorption spectra measured by the FTIR and simulated by Gaussian software for thiamine hydrochloride at higher-frequency region
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results exhibit that the number of absorption peak increases with the number of duplicate molecule. The absorption spectra of multiple molecules are consistent with the measured results by the FTIR, such as three molecules of L-ascorbic acid and two molecules of thiamine hydrochloride, except with slight frequency shift and few frequency missing. The number of absorption peak nearly keeps constant while number of molecule increase to four, which is the molecule number in a unit cell. Acknowledgements This work is supported by the National Natural Science Foundation of China under Contracts 31170668 and 31200541, by the Natural Science Foundation of Jiangsu province under contract BK2012417, by the returned personnel foundation of ministry of education, and by the fund of high level and returned personnel of Nanjing Forestry University.
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