Angle-dependent terahertz time-domain spectroscopy of amino acid single crystals

The measurement of absorption spectra using angle-dependent terahertz (THz) time-domain spectroscopy for amino acid single crystals of l-cysteine and l-histidine is reported for the first time. Linearly polarized THz radiation enables us to observe angle-dependent far-infrared absorption spectra of amino acid single crystals and determine the direction of the oscillating dipole of the molecules in the 20-100 cm(-1) range. By comparing the THz spectra of a single crystal and powder, we found that there was a clear hydrogen-bond peak in the crystal spectrum as a result of the larger hydrogen-bond network. The low-temperature THz spectra of amino acid microcrystals showed more intermolecular vibrational modes than those measured at room temperature. An ab initio frequency calculation of a single amino acid molecule was used to predict the intramolecular vibrational modes. The validity of the calculation models was confirmed by comparing the results with experimentally obtained data in the Raman spectral region.     

Temperature-dependent far-infrared spectra of single crystals of high explosives using terahertz time-domain spectroscopy

Survey spectra of single-crystal HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine), RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine), and PETN (pentaerythritol tetranitrate) were acquired in the region from 10 to 80 cm(-1) using terahertz time-domain spectroscopy. The spectra were taken at temperatures ranging from 8.4 to 300 K. Generally, the spectra show multiple absorption peaks in the range 50-80 cm(-1), with PETN (110) showing strong absorption features at room temperature. RDX (210) is the most notable in the region 10-40 cm(-1), showing multiple spectral features, while HMX (010) shows a very broad absorption at 47.8 cm(-1) with a fwhm of 37.3 cm(-1). Future plans include polarization-dependent investigations for multiple crystallographic orientations over an increased spectral range and higher-level theoretical calculations.     

Terahertz spectroscopy and solid-state density functional theory simulations of the improvised explosive oxidizers potassium nitrate and ammonium nitrate

Terahertz spectroscopy provides a noninvasive and nondestructive method for detecting and identifying concealed explosives. In this work, the room-temperature and cryogenic terahertz spectra of two common improvised explosive oxidizers, namely, potassium nitrate (KN) and ammonium nitrate (AN), are presented, along with detailed solid-state density functional theory (DFT) analyses of the crystalline structures and spectral features. At both 294 and 78 K, KN exhibits two terahertz absorption features below 100 cm(-1) that have been assigned through DFT simulations to arise from hindered nitrate rotations in the KN-II crystalline polymorph. The terahertz spectrum of AN exhibits a pronounced temperature dependence. The 294 K spectrum is free of any absorptions, whereas the 78 K spectrum consists of several narrow and intense peaks. The origin of this large difference is the polymorphic transition that occurs during cooling of AN, where room-temperature AN-IV is converted to AN-V at 255 K. The 78 K terahertz spectrum of AN is assigned here to various ion rotations and translations in the AN-V polymorph lattice. The analysis of the room-temperature AN-IV terahertz spectrum proved to be more complicated. The solid-state DFT simulations predicted that the room-temperature crystal structure of AN is not very well described using the standard Pmmn space-group symmetry as previously believed. The AN-IV polymorph actually belongs to the Pmn2(1) space group, and the perceived Pmmn symmetry results from vibrational averaging through nitrate rotations. This newly observed Pmn2(1) crystal symmetry for room-temperature AN is the reason for the absence of absorption features in the 294 K terahertz spectrum of AN and provides new insight into the polymorphic transitions of this ionic solid.     

Theoretical analysis of the solid-state terahertz spectrum of the high explosive RDX

The solid-state terahertz (THz) spectrum (2–120 cm⁻¹) of α-form cyclotrimethylenetrinitramine (RDX) has been simulated using solid-state density functional calculations at a BP/DNP level of theory. BP/DNP features are in good agreement with both 298 K and a new 7 K polycrystalline RDX THz spectrum. The 7 K RDX spectrum is noteworthy for several mode shifts and spectral detail that greatly aids mode assignments. Previous RDX isolated-molecule calculations (with six calculated modes below 125 cm⁻¹) are incapable of accurately predicting the numerous features in this region, highlighting the importance of solid-state theoretical methods for solid-state terahertz feature assignments.     

The 1.28 μm transparency window of methane (7541-7919 cm⁻¹): empirical line lists and temperature dependence (80 K-300 K)

The high sensitivity absorption spectra of methane at room temperature and 80 K were recorded by CW-Cavity Ring Down Spectroscopy in the 1.28 μm transparency window (7541-7919 cm(-1)). The empirical line parameters of 7690 and 5794 transitions were retrieved at room temperature and at 80 K, respectively. The achieved sensitivity (α(min)≈ 10(-10) cm(-1)) allowed detecting transitions with intensities as small as 5 × 10(-30) cm per molecule. In order to facilitate identification of the CH(3)D transitions present in the CRDS spectrum of methane in "natural" isotopic abundance, the spectrum of a highly enriched CH(3)D sample was recorded by differential absorption spectroscopy at room temperature and at 80 K. The CH(3)D relative contribution in the considered transparency window is found to be significant only at 80 K (up to 15%) but more limited than in the 1.58 μm transparency window.The low energy values of the transitions observed at both room temperature and 80 K were derived from the variation of their line intensities. Empirical lower states and J values have been obtained for 2821 CH(4) transitions representing 94.1 and 98.5% of the absorbance in the region at room temperature and 80 K, respectively. The good quality of these derived energy values is demonstrated by the marked propensity of the corresponding CH(4) lower state J values to be close to integers. The constructed line lists extend to higher energies the WKC (Wang-Kassi-Campargue) line lists of methane in the near infrared (1.71-1.26 μm). They allow one accounting for the temperature dependence of methane absorption between 80 K and 300 K and are of importance for the analysis of the near infrared spectrum of several planetary bodies like Titan, Uranus and Neptune. The centers of the 3ν(2) + ν(3) and 6ν(4) bands responsible of the absorption in the studied region are discussed in relation with recent theoretical calculations.     

Theoretical analysis of the terahertz spectrum of the high explosive PETN.

The experimental solid-state terahertz (THz) spectrum (3 to 120 cm(-1)) of the high explosive pentaerythritol tetranitrate (PETN, C(5)H(6)N(4)O(12)) has been modeled using solid-state density functional theory (DFT) calculations. Solid-state DFT, employing the BP density functional, is in best qualitative agreement with the features in the previously reported THz spectrum. The crystal environment of PETN includes numerous intermolecular hydrogen-bonding interactions that contribute to large (up to 80 cm(-1)) calculated shifts in molecular normal-mode positions in the solid state. Comparison of the isolated-molecule and solid-state normal-mode calculations for a series of density functionals reveals the extent to which the inclusion of crystal-packing interactions and the relative motions between molecules are required for correctly reproducing the vibrational structure of solid-state THz spectra. The THz structure below 120 cm(-1) is a combination of both intermolecular (relative rotations and translations) and intramolecular (torsions, large amplitude motions) vibrational motions. Vibrational-mode analyses indicate that the first major feature (67.2 cm(-1)) in the PETN THz spectrum contains all of the optical rotational and translational cell modes and no internal (molecular) vibrational modes.     

Substituent Effects on the Low-Frequency Vibrational Modes of Benzoic Acid and Related Compounds

Well-resolved absorption spectra of benzoic acid and its derivatives with one hydrogen atom replaced by a substituent group CH3, OH, NH2 or NO2 were reported in the frequency region between 6 and 67 cm^-1 at room temperature with terahertz time-domain spectroscopy （THz-TDS）. These substances can be distinguished easily based on the terahertz absorption spectra. The measurements suggested that even minor changes in the molecular configuration and chemical composition lead to distinct differences in THz spectrum. Density functional theory （DFT） method was used to assist the analysis and assignment of the individual THz absorption spectra of benzoic acid and its methyl derivatives. Observed THz responses of samples can be assigned to the collective vibrations associated with intermolecular hydrogen bonds.     

Terahertz spectroscopy of the explosive taggant 2,3-dimethyl-2,3-dinitrobutane

The terahertz spectrum of the crystalline explosive taggant 2,3-dimethyl-2,3-dinitrobutane (C(6)H(12)N(2)O(4)) has been investigated as an alternative means of detecting solid-state explosives. The room-temperature spectrum exhibits two broad absorption features centered at 38.3 and 49.2 cm(-1). Once the sample is cooled to liquid-nitrogen temperatures, the resolution of three additional peaks occurs, with absorption maxima now appearing at 40.1, 47.5, 56.6, 63.9, and 73.6 cm(-1). Solid-state density functional theory simulations, both with and without London force dispersion corrections, have been used for the assignment of the experimental cryogenic THz spectrum to specific molecular motions in the crystalline solid. The B3LYP hybrid density functional paired with the 6-311G(2d,2p) basis set provides an excellent reproduction of the experimental data revealing that the THz spectrum arises from a mixture of intramolecular torsional vibrations localized primarily in the nitro groups and intermolecular lattice vibrations composed of rigid molecular rotations.     

VCSEL based detection of water vapor near 940nm

A vertical-cavity surface-emitting laser (VCSEL) was used to study the absorption spectrum of water vapor in the 940nm region. Measurements were performed in ambient air at room temperature and in a hydrogen-oxygen flame over the temperature range of 1500-1800K. Several rotational absorption lines within the 2v1 + v3 vibrational band were measured. The absorption spectra were well resolved, which demonstrates the feasibility of VCSEL-based spectroscopic measurements of water vapor at room and high-temperature in this spectral region. The results were in good agreement with the values obtained from the HITRAN-96 database.     

Terahertz absorption spectra of some saccharides and their metal complexes

In this work, THz absorption spectra of some saccharides and their metal complexes were measured. The main purpose of this work is to investigate the M-O vibrations, intermolecular and intramolecular hydrogen bonds and other vibrations in the FIR region using powerful spectroscopic techniques adopting the metal-sugar complexes prepared in our laboratory. The M-O vibrations in the FIR spectra of metal-sugar complexes indicate the formation of metal complexes. The THz spectrum of glucose below 100cm(-1) was measured at first to confirm the THz experimental method. Characteristic absorption bands in the spectra of various samples are observed. THz spectra of saccharides below 100cm(-1) often have several absorption bands, and different saccharides have various absorption peaks in the THz region, which may be used to distinguish different saccharides. The differences in the number of bands observed are related to different structures of the samples, and these absorption bands are related to the collective motion of molecules. But the THz spectra of their metal complexes are different from the ligands, and no band appears in the region below 50cm(-1) at the present experimental condition, which indicates that THz spectroscopy may also be helpful to identify the formation of metal-sugar complexes, and the changes after complexation in the THz spectra below 100cm(-1) may be related to different metal ions. The metal-sugar complexes with similar crystal structures resemble mid-IR spectra, but their THz spectra may have some differences.     

The near-infrared (1.30-1.70 microm) absorption spectrum of methane down to 77 K

The high resolution absorption spectrum of methane has been recorded at liquid nitrogen temperature by direct absorption spectroscopy between 1.30 and 1.70 microm (5850-7700 cm(-1)) using a newly developed cryogenic cell and a series of DFB diode lasers. The investigated spectral range includes part of the tetradecad and the full icosad regions for which only very partial theoretical analysis are available. The analysis of the low temperature spectrum will benefit from the reduction of the rotational congestion and from the narrowing by a factor of 2 of the Doppler linewidth allowing the resolution of a number of multiplets. Moreover, the energy value and rotational assignment of the angular momentum J of the lower state of a given transition can be obtained from the temperature variation of its line intensity. This procedure is illustrated in selected spectral regions by a continuous monitoring of the spectrum during the cell cool-down to 77 K, the temperature value being calculated at each instant from the measured Doppler linewidth. A short movie showing the considerable change of a spectrum during cool-down is attached as Supplementary Material. The method applied to a 30 cm(-1) section of the tetradecad spectrum around 6110 cm(-1) has allowed an unambiguous determination of the J values of part of the observed transitions.     

A spectroscopic and computational study on the effects of methyl and phenyl substituted phenanthroline ligands on the electronic structure of Re(I) tricarbonyl complexes containing 2,6-dimethylphenylisocyanide

[Re(CO)3(CNx)(L)]+, where CNx = 2,6-dimethylphenylisocyanide, forms complexes with L = 1,10-phenanthroline (1), 4-methyl-1,10-phenanthroline (2), 4,7-dimethyl-1,10-phenanthroline (3), 3,4,7,8-tetramethyl-1,10-phenanthroline (4), 2,9-dimethyl-1,10-phenanthroline (5) and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (6). The metal-ligand-to-ligand charge transfer transition (MLLCT) absorption bands follow the series: (27800 cm(-1)) > 1, 2, 4 and 5(27500 cm(-1)) > 6 (26600 cm(-1)). Density functional theory (DFT) geometry optimizations reveal elongated Re-N (L) distances of 2.28 and 2.27 A for 5 and 6, respectively, compared to 2.23 A for 1-4. The reversible reduction potentials (E(1/2(red))) of 1-4 are linearly dependent on the B3LYP calculated LUMO energies. Time-dependent (TD) DFT and conductor-like polarizable continuum model (CPCM) calculated singlet excited states deviate by 700 cm(-1) or less from the experimental absorption maxima and aid in the spectral assignments. The (3)MLLCT emitting state energies are within 900 cm(-1) of the experimental 77 K emission energies for 1-6. The 77 K emission energies, E(1/2(red)), and the room temperature emission quantum yields (phi(LUMO)(em)) decrease in the order 1 >2 >3 >4 whereas E(LUMO) and the room temperature emission energies follow the opposite trend. The emission lifetimes (tau(em)) decrease in the order 3 > 4 >2 >1 >5 with 3 having the highest emission lifetime values of 26.9 micros at room temperature and 384 micros at 77 K and complex 5 having the lowest emission lifetimes of 4.6 micros at room temperature and 61 micros and 77 K.     

Effects of torsion on O-H stretch overtone spectra and direct overtone photolysis of methyl hydroperoxide

We use laser photoacoustic spectroscopy to obtain overtone spectra at three through six quanta of O-H stretch excitation (3nu(OH)-6nu(OH)) for methyl hydroperoxide (MeOOH). Extending the spectral regions beyond our previous work reveals new features that can be attributed to transitions involving torsion about the O-O bond. Experimental spectral profiles (3nu(OH)-6nu(OH)) and cross sections (3nu(OH)-5nu(OH)) at room temperature show a good agreement with the simulated spectra that we obtain from ab initio calculations employing a vibration-torsion model at 298 K. A Birge-Sponer analysis yields experimental values for the O-H stretch frequency (omega=3773+/-15 cm(-1)) and anharmonicity (omegax=94+/-3 cm(-1)). We also detect OH radicals by laser-induced fluorescence and present photodissociation action spectra of MeOOH in the regions of 4nu(OH) and 5nu(OH). While the spectral profile at 5nu(OH) mimics the photoacoustic spectrum, the peak intensity for transitions to torsionally excited states is relatively more intense in the action spectrum at 4nu(OH), reflecting the fact that the 4nu(OH) excitation energy is below the literature dissociation energy (D0=42.6+/-1 kcal mol(-1)) so that features in the action spectrum come from thermally populated excited states. Finally, we use our calculations to assign contributions to individual peaks in the room-temperature spectra and relate our findings to a recent dynamics study in the literature.     

The quadruplex-duplex structural transition of polyriboguanylic acid

The vibrational infrared (IR) absorption and vibrational circular dichroism (VCD) spectral changes of polyriboguanylic acid (polyG) as a function of time, temperature and pH have been investigated to establish how changes in spectral features relate to the structural modifications of polyG. From the progression of IR and VCD spectral features with respect to time, it is observed that stabilization of the quadruplex structure at pH 6.4 (near-neutral environment) takes place within 5 days. This stabilization process is most clearly evidenced by a downshift of the carbonyl absorption band and the corresponding positive VCD couplet, from 1689 to approximately 1682 cm(-1) in time. Time-induced spectral modifications also indicated that, in an acidic environment (pH 3.1) and within a 5 day waiting period, polyG develops a duplex structure. An additional positive VCD couplet associated with an absorption band at 1589 cm(-1) is identified as a marker of the polyG duplex structure. From the progression of spectral features with respect to temperature at pH 6.4, it is found that heating induces structural changes that favor the formation of a duplex structure. This duplex structure, at pH 6.4, would not form at room temperature simply by the passage of time. When polyG is in an acidic environment (pH 3.1), heating accelerates the conversion to the duplex structure that could also be obtained with passage of time at that pH. On the basis of the comparison of experimental and quantum theoretical VCD spectra for polyG, the key spectral signature for the quadruplex form is considered to be a single positive VCD couplet, while the spectral signature for a duplex form is considered to contain an additional positive VCD couplet at a lower frequency.     

State-resolved unimolecular dissociation of cis-cis HOONO: Product state distributions and action spectrum in the 2nuOH band region

Nascent OH fragment product state distributions arising from unimolecular dissociation of room temperature HOONO, initiated by excitation in the region of the 2nu(OH) band, are probed using laser-induced fluorescence at sub-Doppler resolution. Phase-space simulations of the measured OH rotational distributions are consistent with the dissociation dynamics being statistical and confirm that all major features in the room temperature action spectrum belong to the cis-cis conformer. The phase-space simulations also allow us to estimate the HO-ONO bond dissociation energy of cis-cis HOONO to be D(0)=19.9+/-0.5 kcal/mol, which when combined with the known heat-of-formation data for the OH and NO(2) fragments gives DeltaH(f) (0)(cis-cis HOONO)=-2.5 kcal/mol. In addition to fragment energy release, spectral features in the cis-cis HOONO action spectrum are examined with respect to their shifts upon (15)N isotope substitution and through ab initio spectral simulation using a two-dimensional dipole surface that takes into account the influence of HOON torsional motion on the OH stretching overtone. The two-dimensional spectral simulations, using CCSD(T)/cc-pVTZ dipole surface, qualitatively reproduces features appearing in the action spectrum and suggest that the strong broad feature occurring approximately 570 cm(-1) to the blue of the cis-cis HOONO 2nu(OH) peak, likely involve excitation of HOON-torsion/OH-stretch combination bands originating from thermally populated excited torsional states. A closer examination of the predictions of the two-dimensional model with experiments also reveals its limitations and suggests that a more elaborate treatment, one which includes several additional modes, will likely be required in order to fully explain the room temperature action spectrum. Ab initio calculations of the HOON torsional potential at the CCSD(T)/cc-pVTZ level of theory are also presented and confirm that cis-perp configuration does not correspond to a bound localized minimum on the HOONO potential energy surface.     

Investigation of (1R,2S)‐(−)‐Ephedrine by Cryogenic Terahertz Spectroscopy and Solid‐State Density Functional Theory

The terahertz (THz) spectrum of the pharmaceutical (1R,2S)‐(?)‐ephedrine from 8.0 to 100.0 cm^?1 is investigated at liquid‐nitrogen (78.4 K) temperature. The spectrum exhibits several distinct features in this range that are characteristic of the crystal form of the compound. A complete structural analysis and vibrational assignment of the experimental spectrum is performed using solid‐state density functional theory (DFT) and cryogenic single‐crystal X‐ray diffraction. Theoretical modeling of the compound includes an array of density functionals and basis sets with the final assignment of the THz spectrum performed at a PW91/6‐311G(d,p) level of theory, which provides excellent solid‐state simulation agreement with experiment. The solid‐state analysis indicates that the seven experimental spectral features observed at low temperature consist of 13 IR‐active vibrational modes. Of these modes, nine are external crystal vibrations and provide approximately 57 % of the predicted spectral intensity. This study demonstrates that the THz spectra of complex pharmaceuticals may be well reproduced by solid‐state DFT calculations and that inclusion of the crystalline environment is necessary for realistic and accurate simulations.     

萘醌及其衍生物的太赫兹时域光谱研究

用太赫兹(THz)时域光谱技术研究了室温条件下的萘醌及其衍生物1,2-萘醌、1,2-萘醌-4-磺酸钠、甲萘醌、白花丹素、胡桃醌的光谱特征,得到了各自的吸收谱和折射率。结果表明,萘醌及其衍生物在此波段有不同的吸收特征,利用太赫兹时域光谱能够鉴别分子结构存在微小差别的化合物。在对样品的吸收谱进行比较的基础上,讨论了分子结构和分子间晶格振动与THz光谱特征吸收的关系。     

Far-infrared spectroscopy on free-standing protein films under defined temperature and hydration control

The functionality of proteins is governed by their dynamics. We have performed a systematic investigation on four different proteins in the far-infrared spectral region under control of the two external parameters that have the strongest influence on the dynamics, namely temperature and hydration. The absorption measurements covering the frequency range from 40 cm(-1) to 690 cm(-1) (1-20 THz) close the gap between the well-studied mid-infrared and the recent THz investigations. By preparing the proteins as free-standing films, we achieve unprecedented reproducibility. Besides a featureless slope in the THz range, we can identify absorption peaks characteristic for each protein and others common to several proteins. We fit the spectra to extract the peak positions and suggest assignments for them. The far-infrared absorption spectra of all proteins are basically independent on hydration. By a detailed analysis of the sorption isotherms this can be explained by the low absorption of biological water, which resembles more the behavior of ice than that of liquid water.     

L-和DL-福多司坦的太赫兹光谱分析

利用太赫兹时域光谱技术(THz-TDS)在室温下对L-福多司坦和DL-福多司坦进行测量,发现L-和DL-福多司坦在THz波段都有特征吸收峰,且两者的吸收谱有明显差异.运用密度泛函理论的B3LYP方法计算了L-和DL-福多司坦在太赫兹波段的吸收谱,并对L-和DL-福多司坦的特征吸收峰进行了指认,理论计算与实验结果基本吻合.此外,还对福多司坦胶囊成品药进行了测量,发现该胶囊的吸收谱与L-福多司坦非常吻合,证明胶囊药的主要成分为L-福多司坦.这项研究对手性物质的检测以及化合物有效成分的鉴别有一定的参考作用.     

High-resolution absorption cross sections of formaldehyde in the 30,285-32,890 cm(-1) (304-330 nm) spectral region

Absolute room temperature (294 ± 2 K) absorption cross sections for the ?(1)A(2)-X?(1)A(1) electronic transition of formaldehyde have been measured over the spectral range 30,285-32,890 cm(-1) (304-330 nm) using ultraviolet (UV) laser absorption spectroscopy. Accurate high-resolution absorption cross sections are essential for atmospheric monitoring and understanding the photochemistry of this important atmospheric compound. Absorption cross sections were obtained at an instrumental resolution better than 0.09 cm(-1), which is slightly broader than the Doppler width of a rotational line of formaldehyde at 300 K (～0.07 cm(-1)) and so we were able to resolve all but the most closely spaced lines. Comparisons with previous data as well as with computer simulations have been made. Pressure broadening was studied for the collision partners He, O(2), N(2), and H(2)O and the resulting broadening parameters have been measured and increase with the strength of intermolecular interaction between formaldehyde and the collision partner. The pressure broadening coefficient for H(2)O is an order of magnitude larger than the coefficients for O(2) and N(2) and will contribute significantly to spectral line broadening in the lower atmosphere. Spectral data are made available as Supporting Information.