Direct evidence of inter-molecular vibrations by THz spectroscopy

Transcription

1 Chemical Physics Letters (5) Direct evidence of inter-molecular vibrations by THz spectroscopy Naoto Nagai a, *, Ryoichi Kumazawa b, Ryoichi Fukasawa c a Industrial Research Institute of Niigata Prefecture, Kaetsu Technical Center, -- Abumi Nishi, Niigata 95-95, Japan b Toray Research Center Inc., --7 Sonoyama, Otsu, Shiga , Japan c Tochigi Nikon Co., 77 Midori, Ohtawara, Tochigi -865, Japan Received 9 July 5; in final form 7 August 5 Available online August 5 Abstract The time-dependent THz spectra of some chemical materials after being dipped in solvents or after application of spots of solvent were observed. Some specific peaks disappeared, increased in intensity, or newly appeared. These peaks showed different features after spots of different solvents were applied. The evidence suggests that these peaks can be attributed to the vibration of inter-molecules, and shows that the solvents significantly affect inter-molecular vibrations. This method will help in assigning the THz band, and will be useful in researching molecular interactions. Ó 5 Elsevier B.V. All rights reserved.. Introduction Recently, material characterization using THz spectroscopy has been applied to biochemicals [,], pharmaceuticals [], polymers [] and semiconductors [5,6] and has given us important information. Moreover, THz imaging has progressed and is expected to be applicable for the identification of narcotics and explosives [7]. The most important and characteristic point of THz spectroscopy is said to be its ability to observe inter-molecular vibrations in contrast to infrared spectroscopy (IR), which observes intra-molecular vibrations. Many researchers have observed inter-molecular interaction in studying the temperature dependence of the peak positions and spectral shapes of THz spectra or when making comparisons to the results of the calculation using the molecular orbital (MO) and/or molecular dynamics (MD) methods [8]. Generally, information on molecular interaction is obtained as a peak shift of intra-molecular vibrations appearing in the infrared region. Low-frequency intra-molecular vibrations such * Corresponding author. Fax: address: photon_nagai@yahoo.co.jp (N. Nagai). as breathing and rotation are also thought to appear in the THz frequency region as well as inter-molecular vibrations. Therefore, it is difficult to distinguish intramolecular vibrations and inter-molecular vibrations by simple THz spectroscopic techniques without careful researching of the temperature dependence of the bands or the measurements by changing the molecular environment (solvents effect). Changes of the intensity of molecular interaction are observed as a peak shift in intra-molecular vibrations. On the other hand, the intensity changes of molecular interaction are thought to be observed as generation and disappearance and/or intensity changes of the peaks at THz frequency region in direct inter-molecular vibrations. However, no report on the intensity changes of the peaks in the THz frequency region in view of changes of the degree of molecular interactions from the changes of the molecular environment (solvents) has appeared. There have been detailed studies on liquids in the THz frequency region. It is well known that the plural Debye relaxation time appears in this frequency region in water [9]. A broad background peak was observed in this frequency region in some polar liquids [], and the absorption coefficients of non-polar liquids are 9-6/$ - see front matter Ó 5 Elsevier B.V. All rights reserved. doi:.6/j.cplett.5.8.

2 96 N. Nagai et al. / Chemical Physics Letters (5) 95 5 extremely low. In solutions, observations of changes of the background peak depending on the kinds and the concentrations of the solutes dissolved in the solvent are possible. However, observation of changes of the solutes in the initial stage of dissolving in THz spectroscopy has been thought to be difficult. The dissolution is thought to start at the interaction between the molecules of solute and solvent, therefore, if we can observe the initial stages of the molecular interaction between the solute and the solvent, we will obtain very useful information. The interaction is expected to help us understand the effects of medicines on the human body from the viewpoint of molecular interaction levels in future. The authors tried to observe the inter-molecular vibrations by detecting the changes of THz spectra introducing a small amount of solvent to the solute, in contrast to seeing the change of THz spectra of the solution by dissolution of the solute into the solvent. In particular, changes of the spectra using different solvents were observed, and the peaks associated with the molecular interaction of solutes were successively detected. In this Letter, some evidence of inter-molecular interaction and the tentative interpretation will be shown.. Experimental The samples used for this study are some chemical materials (L-(+)-arginine monohydrochloride, L-glutamic acid, Na salicylate, coronene and chrysene). All samples were purchased as powders, and made into disks of mm diameter by the press method. About lm thick disks are prepared from mg L-(+)-arginine monohydrochloride, and the disks of L-glutamic acid samples were also made by the same method. The 5 mg Na salicylate was mixed with mg polyethylene powder; which is transparent in the frequency region, in order to avoid the saturation of the peak absorption. It was then mixed in a mortar with a pestle, and was made into lm thick disks. This preparation was done due to the hardness of making the thin selfstanding disk of pure Na salicylate without absorption saturation. In addition, mm thick disks were also made from coronene and chrysene powders without diluents like polyethylene. Each sample was fixed on a sample holder plate and the THz spectra were measured by transmission mode. In order to observe the interaction between the solute and the solvent, the THz transmission measurements were performed after applying spots of about 5 ml of solvent onto the sample as shown in Fig.. Sequential measurements were performed after various time intervals without additional solvent spotting. The preparation method mixed with polyethylene powder mentioned above is useful for the large absorption samples, because the solvents are well penetrated to the whole disk and seem to fully interact with the samples. On the other hand, the measurements of coronene and chrysene were performed after dipping in benzene, and sequential measurements were also performed after various time intervals. Between the THz measurements, the dipping in benzene was continued. All measurements were performed at C room temperature and 5% humidity condition. The measurements were performed by a THz TDS (time-domain spectroscopy) system RT- produced by Tochigi Nikon Rayfact. A pumping fiber-laser with fsec width of pulse and 78 nm wavelength irradiated the LT (low-temperature grown)-gaas epi-layer THz light Solvent Sample Sample Holder Fig.. The experimental setup of the solvent spotting THz measurements.

3 N. Nagai et al. / Chemical Physics Letters (5) as a photoconductive dipole antenna by biased metal on the hemispherical Si lens, and a THz pulse emitted from the epi-layer. The same structural device was also used as a detector. When gated by the laser pulse a current proportional to the instantaneous field strength of the THz pulse can be measured in the external circuit. A lock-in amplifier referenced to the frequency of a mechanical chopper placed in the THz beam path to detect this current was used. Optical setup is almost same shown in some literature [,8]. The optical system was set in the evacuated compartment, and the optical path length of the sample compartment partitioning to the optical compartment by Teflon window is about cm in order to minimize the affect of the water vapor absorption. The transmission measurements were done in the region. THz (6.7 7 cm ). The resolution was THz.. Results and discussion.. L-(+)-Arginine monohydrochloride Fig. a shows the time-dependent THz spectra of L-(+)-arginine monohydrochloride after spotting with water. At least five peaks (.79,.88,.99,.,.65 THz) can be observed. As time advances after spotting with water, only the. THz peak decreases in intensity and disappears min later. The disk thickness is not drastically changed after spotting with water. The interference fringes observed below.7 THz indicate a slight diminishing in the thickness of the sample disk or the change in refractive index. However, the decrease in intensity of the. THz peak is drastic as compared to other peaks in spite of the slight decrease in thickness. Drastic changes of the other peaks are not observed. Since HCl is thought to be well dissolved in the added water, the molecular interaction between arginin and HCl is decreased by adding water. This evidence suggests that the. THz peak originates from the vibration between arginine HCl molecules. Fig. b shows the time-dependent THz spectra of L-(+)-arginine monohydrochloride after spotting with ethanol. The. THz peak is not changed in intensity in this case but the relative intensity of the.99 THz peak is thought to be changed. This suggests that the ethanol molecules interact with the arginine molecules and affect vibration of the inter-molecular of arginine molecules each other rather than the inter-molecular vibration of arginine HCl... L-Glutamic acid Fig. shows the time-dependent THz spectra of L-glutamic acid after spotting with acetone. The intensity of the peak at. THz increases after spotting with acetone and gradually decreases. The spectrum of 9 min after spotting shows the maximum intensity. A broad new peak at.6 THz is observed after spotting with the acetone, and this peak also decreases in intensity over 9 min after spotting, and has almost disappeared 6 min later. This evidence suggests that the acetone molecules interact with glutamic acid molecules and show the newly appeared vibration between acetone and glutamic acid (.6 THz), and the conformational changes induce an increase in molecular interaction 6 a 6 b min. min. 6 min. 7 min. 8 min. 7 min. min. min. 5 min. 7 min. Fig.. The time-dependent THz spectra of L-(+)-arginine monohydrochloride after spotting with: (a) water and (b) ethanol.

4 98 N. Nagai et al. / Chemical Physics Letters (5) between glutamic acids (. THz). The disappearance of the.6 THz peak will be explained by the evaporation of the acetone... Na salicylate 9 min. 9min. min. 6min...6 Fig. a shows the time-dependent THz spectra of Na salicylate after spotting with ethanol. Two peaks (..8 Fig.. The time-dependent THz spectra of L-glutamic acid after spotting with acetone... and THz) are observed. The intensity of the THz peak seems to decrease in intensity as time advances, and shows the minimum intensity min later. The peak gradually increases and recovers in intensity. The functional groups of COO Na + and OH are thought to interact with the inter- or intra- molecules of each other. Since the interactions COO Na + HO were intercepted by the OH functional group of ethanol, the intensity of the peak is thought to decrease. The molecular interaction of Na salicylate may recover as the ethanol is evaporated. However, the data gets noisy at around THz, therefore, the more appropriate sample preparation and the additional experiments may be needed. Fig. b shows the time-dependent spectra of Na salicylate after spotting with water. A broad background peak appeared after spotting with water and was due to the absorption of water. The spectrum of 7 min later shows the largest background, and as time advances the intensity of the background gradually decreases. The. THz peak also disappears after 5 min. The peak at THz cannot easily be observed due to the large noise level caused by the water absorption, however, the peak is also thought to disappear. The background of water decreases after 88 min. However, since the water is strongly fixed to the Na salicylate molecules, the recovery in intensity of the. THz peak is slight. This evidence shows that the intermolecular interaction of Na salicylate may be cancelled due to the strong interaction between COONa and H O, and/or OH from Na salicylate and H O induced by the water spotting. a b 8 min. 9 min. min. min. 7 min. 5 min. min. 88 min. Fig.. The time-dependent THz spectra of Na salicylate after spotting with: (a) ethanol and (b) water.

5 N. Nagai et al. / Chemical Physics Letters (5) Coronene and chrysene Fig. 5a shows the time-dependent spectra of coronene after continuous dipping in benzene. The times displayed in the figure are the measured times after drying in the room atmosphere in a few seconds subsequent dipping the sample into the benzene. Between the THz measurements, the samples were kept in the benzene solvent. Coronene is generally dissolvable in benzene when it is dipped in the liquid for a long time. The disk samples evaluated in these measurements did not apparently change in thickness. As shown in the figure, coronene shows two peaks (. and 5 THz) in this THz frequency region. As we continued with the measurements, the peak intensity at 5 THz was almost unchanged, but the intensity of. THz increased gradually. This evidence is confirmed by comparing the relative intensity of the 5 THz peak and the. THz peak in Fig. 5a. The relative intensity of the. THz band to the 5 THz band as a function of the dipping time is shown in the inset of Fig. 5a. The coronene molecule has a large flat surface constituted of seven benzene rings, therefore, it is thought that it has a large Van der Waals force by stacking. The benzene molecule is thought to intercalate into the stacked coronene layers, and this phenomenon affects the inter-molecular interactions of coronene layers. The peaks at 5 THz may be the in-plane mode or other vibrational modes. Fig. 5b shows the time-dependent THz spectra of chrysene after dipping in benzene. Chrysene has a higher soluble rate than coronene, therefore, the data shown for chrysene are restricted to a shorter time range than those for coronene. The disk thickness of chrysene is almost unchanged up to min. Chrysene shows a peak at.75 THz in this frequency region, and the intensity of this peak is almost unchanged. Chrysene is constituted of four benzene rings, therefore the degree of stacking is thought to be lower than coronene. The unclear intensity change of chrysene is thought to originate from the lower degree of layer by layer stacking of chrysene molecules. Therefore, the.75 THz peak is also thought to be an in-plane mode or another vibrational mode.. Summary The time-dependent THz spectra of some chemical materials after spotting with solvents or dipping in solvents were studied. Some specific peaks showed characteristic features after spotting. This evidence suggests that the peaks can be attributed to the vibration of inter-molecules, and shows that the solvents significantly affect the inter-molecular vibrations. The interpretation of the results is phenomenological and rather complicated, and the detailed calculations and an assignment of the studied low-frequency vibrational modes will be needed. The phenomena shown in this study will serve to the future detailed study of these peaks. 7 6 a Relative intensity b 5 5min. 5 Time (min.) min. 77min. min. 78min. min. 5min. 7min. 5min. min. before dipping 5min. 5min. 7min. before dipping Fig. 5. The time-dependent THz spectra of: (a) coronene and (b) chrysene after dipping in benzene. The relative intensity of the. THz band to the 5 THz band as a function of the dipping time is also shown in (a).

6 5 N. Nagai et al. / Chemical Physics Letters (5) 95 5 References [] M. Walther, B. Fischer, P. Uhd Jepsen, Chem. Phys. 88 () 6. [] M. Walther, B. Fischer, M. Schall, H. Helm, P. Uhd Jepsen, Chem. Phys. Lett. () 89. [] P.F. Taday, I.V. Bradley, D.D. Arnone, M. Pepper, J. Pharm. Sci. 9 () 8. [] N. Nagai, T. Imai, R. Fukasawa, K. Kato, K. Yamauchi, Appl. Phys. Lett. 85 (). [5] M. van Exter, D. Grischkowsky, Appl. Phys. Lett. 6 (99) 69. [6] P. Uhd Jepsen, W. Schairer, I.H. Libon, U. Lemmer, N.E. Hecker, M. Birkholz, K. Lips, M. Schall, Appl. Phys. Lett. 79 () 9. [7] K. Kawase, Opt. Photon. News (Oct) (). [8] B. Fischer, M. Walther, P. Uhd Jepsen, Phys. Med. Biol. 7 () 87. [9] C. Rønne, P.O. Åstrand, S.R. Keiding, Phys. Rev. Lett. 8 (999) 888. [] B.N. Flanders, R.A. Cheville, D. Grischkowsky, N.F. Scherer, J. Phys. Chem. (996) 8.