Experimental and theoretical investigations of the vibration spectrum of λ-Hexanitrohexaazaisowurtzitane in the region of 0.2–2.5 terahertz are presented for the first time. The refraction index, absorption coefficient, and complex dielectric function of this sample are measured by terahertz time-domain spectroscopy. The simulated spectrum using density functional theory (DFT) is in agreement with the experimental data. The observed spectra features are assigned based on DFT calculation.
©2006 Optical Society of America
Terahertz (THz) or far infrared (far-IR) spectroscopy can reveal rich spectroscopic and structural information of NO2-containing compounds since many vibrational modes (e.g. out of plane vibrations, torsion of NO2 groups and phonon/intermolecular modes) of these compounds are located in THz range. The vibrational spectrum varies with the number and substitution position of the NO2 groups. Many THz spectroscopic studies on different explosives, such as 1,3,5-trinitro-s-triazine (RDX), 2,4-Dinitrobenzene (2,4-DNT), and 2,4,6-Trinitrotoluene (TNT) have been reported in recent years [1–9]. γ-Hexanitrohexaazaisowurtzitane. (γ-HNIW), a caged nitramine, was first synthesized by Nielsen. Its performance is better than HMX, a commonly used explosive. γ-HNIW has been studied in IR and far-IR ranging from 120 cm-1 to 3100 cm-1 via Fourier transform IR spectroscopy (FTIR) and laser Raman [10–12]. However, THz spectrum of γ-HNIW below 100 cm-1 (∼3 THz) still has not been explored previously.
As an attractive and unique spectroscopic technique in the far-IR range, THz time-domain spectroscopy (THz-TDS) has been utilized in a wide range of research fields in the past ten years, including chemical and biological detections and identifications [13, 14]. Different from conventional spectroscopic techniques, THz-TDS can provide both absorption coefficient and refraction index of a sample with high signal-to-noise ratio (SNR) and without using the Kramers-Kroning relation.
Theoretically, the caged structural compounds are of great interests due to high density, high energy, and high tension. DFT has proved to be a reliable theoretical method to predict accurate vibrational frequencies for medium size molecules . Due to the lack of THz spectrum of γ-HNIW down to 3 THz (∼100 cm-1), it is essential to investigate γ-HNIW in THz band both in experiment and theory.
In this letter, both the experiment and simulation of THz spectra for γ-HNIW in the range of 0.2–2.5 THz are presented. The refraction index, absorption coefficient, and complex dielectric function of this sample are measured by THz-TDS. Vibration frequencies were calculated based on DFT in the THz region. The characteristics of both the experimental and simulated spectra are analyzed and compared. DFT calculations allowed for assignments of the observed vibrational frequencies.
2. Experimental methods and materials
2.1 Experimental setup
The experimental setup for THz-TDS has been described in the literature . Where a repetition rate of 82 MHz, diode-pump mode-locked Ti: sapphire laser (MaiTai, Spectra Physics) provides the femtosecond pulses with duration of 100 fs and center wavelength of 810 nm. A p-type InAs wafer with <100> orientation is used as the THz emitter and a 2.8 mm-thick ZnTe with <110> orientation is employed as the sensor. The THz beam path is covered by a box and is purged with dry nitrogen to minimize the absorption of water vapor and to enhance the SNR. The humidity is kept less than 1% and temperature is kept at 293 K. The dynamic range is about 4000:1 and the spectral resolution is better than 40 GHz in the 0.2–2.5 THz region.
2.2 Sample preparation
The γ-HNIW sample (purity > 98%) was provided by the State Key Laboratory of Explosion Science and Technology at Beijing Institute of Technology, China. To reduce the scattering effect of the sample, it was ground into fine particles and then pressed into 1.6 mm-thick pellet.
3. Results and discussions
3.1 Experimental results
The refraction index, absorption coefficient, and complex dielectric function of γ-HNIW were obtained in the 0.2–2.5 THz region. The refraction index varies between 1.75 and 1.82 in the range of 0.2–2.5 THz. The average value is 1.79. Figure 1 shows four absorption peaks of γ-HNIW centered at 1.05, 1.52, 1.67 and 1.90 THz, respectively. The corresponding absorption coefficients are 14.2, 59.2, 38.4 and 36.2 cm-1. Figure 2 displays the real part (ε1) and imaginary part (ε2) of the dielectric function of γ-HNIW. The real part of the dielectric function indicates the resonance process related with a similar mechanism as optical absorption. Four main absorption peaks can be seen from the real part, consistent with the absorption spectrum in Fig. 1. The imaginary part describes the dielectric loss of this resonance process. The frequency dependant imaginary part of complex dielectric function is similar with the refraction index spectrum.
3.2 DFT calculation and analyses
In order to better understand the absorption spectrum, we did a theoretical calculation using Gaussian 03 package . This calculation was performed using the Becke-3–Lee–Yang–Parr (B3LYP) functional and the 6-311G (d, p) basis set [17, 18]. The initial geometry was adapted from x-ray crystallographic data, and subsequently optimized to find the minimum energy. It was confirmed by the vibrational analysis since no negative frequencies were found. The calculated vibration frequencies of one single γ-HNIW molecule agree well with the experimental FTIR data beyond 120 cm-1 [10, 11].
The predicted molecular structure of γ-HNIW is shown in Fig. 3. The skeleton of HNIW consists of two five-membered rings (1, 3-dinitro-1,3-dinitric-heterocyclic-pentane) and an six-membered ring (1,4-dinitro-1,4-dinitric-heterocyclichexane). Each five-membered ring is formed by two nitrogen and three carbon atoms. The optimized parameters agree well with those reported by Zhou et al  at the B3LYP/6-31G (d, p) level. The calculated geometrical parameters, total dipole moments, total energies, and literature values are tabulated in Table 1. It can be seen that the C25-C26, C27-C28 and C29-C30 bonds are 1.590, 1.595, and 1.585 Å, respectively, longer than the standard C-C bond (about 1.54 Å). All the N-N bonds ranging from 1.396 Å to 1.445 Å are larger than the standard N-N bond (about 1.36 Å). Consequently, N–N bonds are more fragile than the other bonds of γ-HNIW. The dihedral angles of five-membered rings which taken from τ(N13-C27-N14-C26), τ(N14-C27-N13-C25), τ(N16-C28-N18-C29), and τ(N18-C28-N16-C30) are 35.1, 35.7, 38.8, and 42.3, respectively. These dihedral angles are far from those of an ordinary pentagon ring (e.g. cyclopentane, <10°), which are caused by C27–C28 at the top of the five-membered rings.
3.3. Calculation and assignments of vibrational frequencies
The calculated frequencies as well as experimental results are plotted in Fig. 4. We use a Lorentzian shape with Full Width Half Maximum (FWHM) of 3 cm-1. The frequencies listed on Table 2 were not scaled since no scale factor is available for the B3LYP functional with the 6-311 G (d, p) basis set.
It can be seen from Fig. 4 that the calculated absorption peaks which located at 0.97, 1.70 and 1.88 THz fit well with the experimental absorption bands peaked at 1.05, 1.67 and 1.90 THz except some frequency shift. The discrepancy of vibrational frequencies between the calculation and experiment can be ascribed to the solid effect and the influence of the temperature. Based on the calculated results at the B3LYP/6-311G (d, p) level and with the aid of visualization of Gaussian View 3.09, the absorption bands at 1.05, 1.67 and 1.90 THz are tentatively assigned to intra-molecular modes of γ-HNIW. They are correlated to vibration of NO2 groups (see Table 2). The frequency differences between the experiment and the simulation are 0.08, 0.03 and 0.02 THz at 1.05, 1.67 and 1.90 THz respectively. The peak position of the band at 1.52 is absent in the calculated spectrum. It can possibly be attributed to a phonon or intermolecular mode other than an isolated-molecular vibration. The more precise calculation including inter-molecular force field will be done in future. The calculated absorption peaks located at 2.33 THz is absent in the experimental results. The reason might be we model the normal vibration modes of single molecule in gas phase at zero Kelvin while our experiment was performed at room temperature. The fine structure of the spectrum cannot be seen because room temperature spectrum possibly is a superposition of transition from the excited vibrational states. Shen et al. studied the temperature-dependent THz spectrum of biological molecules. More absorption peaks appeared at very low temperatures, and their intensities became stronger with the decrease of temperature . We will re-do the experiment in the future when the low-temperature device is available.
The experimental and simulated THz spectra of γ-HNIW in the region of 0.2–2.5 THz were presented. Frequency-dependent absorption coefficient, refraction index and complex dielectric function were obtained by THz-TDS. Theoretical simulation shows that the distinct features of the spectrum originated from low-frequency vibrational modes caused by intramolecular collective motion and phonon/intermolecular vibration. These THz fingerprints could be useful for explosive identification.
We gratefully thank Prof. Xun Wang, Prof. Fuhe Wang, Prof. Guozhong Zhao, Guanping Yu and Ning Li for their help in improving this manuscript. This project is supported by the Science Foundation of Education Commission of Beijing, China (Grant No. KM200310028115), Beijing Science Nova Program (No. 2004B35), the National Natural Science Foundation of China (Grant No. 10390160), and the Beijing Key Lab for Nano-photonics and Nano-structure.
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