Atomic And Molecular Spectroscopy By Rajkumar 163

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Synthesis, thermodynamic characterization, and energetic properties of three novel high-nitrogen bishomocubane-based compounds DADMBHC, DTetzBHC and DPTrizDMBHC are reported here. These compounds have higher heats of formation (HoFs) and higher energy densities as compared to traditional hydrocarbon fuels. Densities, gas phase HoF and their optimized molecular structure geometries were calculated with various levels of theory. In general, the calculated HoFs of these compounds turn out to be extremely high. Ballistic properties such as vacuum specific impulse and density vacuum specific impulse were calculated using the NASA Chemical Equilibrium and Applications utility. Propulsive properties were compared with liquid bipropellants (RP1) and solid propellants (AP) and explosive properties were compared with RDX. The density specific impulse demonstrated an improvement of 35 s for DADMBHC and DTetzBHC over standard liquid hydrocarbon HTPB, thus showing promise as possible monomers to replace HTPB as a fuel-binder. The density specific impulses of these compounds were also found to be significantly higher than that of RP1, e.g. that of DADMBHC was found to be higher by 84 s, making them potentially good candidates as propellants for use under volume-limited conditions. The detonation properties showed that these compounds have low potential as explosives. TGA, coupled with IR spectroscopy, revealed that DADMBHC and DPTrizDMBHC evaporate readily while DTetzBHC decomposes partially.

Solutions of zirconium salts are often used as chemical modifiers in electrothermal atomization atomic absorption spectrometry and electrothermal atomization molecular absorption spectrometry using novel high-resolution continuum-source atomic absorption spectrometer (ET-AAS and ET-AMA).

Thus, the number of objects demanding the determination of zirconium content is great, but at present there are no works that present generalized information on modern methods for zirconium determination. Some sufficiently detailed reviews published in the 1960s and 1970s (Elinson & Petrov, 1965; Mukherji, 1970) do not, naturally, include the descriptions of such methods as atomic absorption, inductively coupled plasma atomic emission spectroscopy, mass spectrometry (MS), etc., as they began to be developed in the decades that followed. Since then, only one review has been published about the spectrophotometric techniques for zirconium determination, developed between 1991 and 2004 (Dalawat, Chauhan, and Goswami 2005).

A relatively new method of atomic spectroscopy, laser-induced breakdown spectroscopy (LIBS) in its double-pulse variant (for increasing the sensitivity of determination), was tested in Xin et al. (2016) for zirconium determination in a new type of magnesium alloys. The distance from the laser to the sample was 1.89 m. The detection limit of zirconium of this method was quite high, 0.09 wt.%.

In the Surface Plasmon Resonance (SPR) sensor, surface plasmon polaritons are excited at the interface of a thin metal film such as gold, silver, and dielectric. One of the factors that affect the surface plasmon oscillation of gold or silver nanoparticles is the effect of the refractive index of the environment in which the nanoparticles are located. A change in the refractive index of nanoparticles alters the emission constant of surface plasmons and causes some changes in the coupling between light and surface plasmon, which are visible as optical characteristics at the output (Kneipp et al. 2008; Sunmook 2007). This property is used to fabricate many sensors in the field of medicine and industry. The first high-sensitivity SPR sensor was developed in 1999 by Homola et al. (Homola et al. 1999) without the application of molecular labeling. The SPR biosensors have since been extensively used in the analysis of biomolecular interactions and the detection of chemical and biological analytes (Nooke et al. 2010; Homola et al. 2002; Koubová et al. 2001). With the advancement of the industry, the interest in the fabrication of sensors shifted to some sensors capable of sensing specific responses (due to targeted analyte molecules) from unspecified responses because of temperature fluctuations, analyte composition, and molecular absorption (Homola et al. 2005; Patskovsky et al. 2010). In later years, the development of multi-channel sensors, such as prism coupled with SPR structure, was investigated for simultaneous measurement of different analytes (Hoa et al. 2007). Surface-enhanced Raman spectroscopy has grown increasingly over the past four decades and is rapidly expanding as applications for diagnostics in the fields of chemistry, materials science, biochemistry, and life sciences. Advances in the construction of SERS-based biosensors are major breakthroughs in the detection of biological analytes and chemicals (Zong et al. 2018).

Photons are often absorbed or scattered when they collide with a reflected molecule. In Raman spectroscopy, monochromatic radiation photons (single wavelength light) scatter in different directions after colliding with the sample. In this type of spectroscopy, the scattered photons of the sample are of great importance. Most of the photons that interact with the molecule are scattered elastically. This type of scattering is called Rayleigh scattering, in which the scattered photons of the sample have the same energy or wavelength as the photons that collide with the sample. In 1928, Chandrasekhara Venkata Raman, an Indian physicist, discovered a phenomenon called Raman. In this phenomenon, the energy or wavelength of the beam scattered by the molecules is different from the wavelength of the initial beam that collides the sample; this type of scattering of light beams is called inelastic scattering. Approximately one in ten million photons scatter inelastically after colliding with the matter. Moreover, the difference in energy or wavelength of scattered inelastic light depends on the molecular structure of the compound. In fact, Raman spectroscopy is on the basis of analyzing these differences and with the aim of determining the molecular structure of different compounds. The change in wavelength or energy of the initial radiation provides important information regarding the molecular movements within the system. In Raman scattering, a photon collides with the matter, and after scattering, its wavelength shifts to shorter or longer wavelengths. In this type of beam scattering, the transmission to higher wavelengths is predominant, which is called the Stokes Raman shift. Moreover, transmission to lower wavelengths is also called anti-Stokes shift (Downes and Elfick 2010; Nemecek et al. 2013). The intensity ratio of the anti-Stokes shift to Stokes has been reported to increase with increasing the temperature. The incident photon collides with the electron cloud of the functional group bonds, exciting the electrons to a virtual mode. Afterward, the electron returns from the virtual mode to an excited vibrational or rotational mode. This phenomenon causes the photon to lose a part of its energy and appear as a Stokes Raman shift. The lost energy is directly related to the chemical identity of the functional group, the molecular structure attached to it, the type of molecule's atoms, and its surrounding medium. Therefore, Raman spectra are specific to each molecule and can be used as a "fingerprint" in the chemical identification of molecular compounds in a liquid or air or on a surface (Nemecek et al. 2013; Loudon 1964).

The SERS method is now used to obtain information from molecules adsorbed on the surface of gold, silver, and noble metals. Currently, the SERS enhancement is known for the reasons of two main phenomena: (1) The electromagnetic effect (field effect) in which the molecule is exposed to an external field; this larger alternative field is created by electromagnetic amplification near the metal surface, (2) chemical effect (molecular effect) in which molecular polarization is affected by the interaction between the surfaces of the molecule and the metal (Dieringer et al. 2006). By applying SERS, it is even possible to investigate single molecules that amplify the Raman signal up to 1010 through binding nanoparticles to single molecules (Qian and Nie 2008; Stranahan and Willets 2010; Ochoa et al. 2017; Ru et al. 2006). Consequently, the detection of biological substances in small quantities and their early detection are of great importance. Currently, several methods are used to quantify biomaterials, such as electrochemistry (Li et al. 2014), gas chromatography, High-Performance Liquid Chromatography (HPLC) (Heidbreder et al. 2001), gravimetry (Fourati et al. 2014; Maouche et al. 2015), and optical spectroscopy (Qian and Nie 2008; Stranahan and Willets 2010; Ochoa et al. 2017; Ru et al. 2006). These methods are destructive, challenging, polluting, in-laboratory, time-consuming, and expensive, as well as require sample preparation, trained specialists, and well-equipped laboratories. Therefore, it is essential to develop a non destructive method, simple to use, fast, cost-effective, unpolluted, portable, and applicable outside the laboratory environment that needs less sample preparation. On the other hand, the detection of significantly low amounts of biological analytes and chemicals is of great importance. Materials and biological analytes can be detected using infrared spectroscopy and Raman spectroscopy, both of which are fingerprint spectroscopies and used for studying the molecular vibrations of matter (Ivanov et al. 2002; Alizadeh 2009). In infrared spectroscopy, due to the active molecular vibrations of water, biological species are difficult to detect, and the sensitivity of their detectors is low. In Raman spectroscopy, due to the inherent weakness of the signal from Raman scattering, the study of molecules with low concentrations is practically impossible (Duan et al. 2016). One of the methods that can enhance the Raman signal is the use of metal nanostructures that can create a strong electric field near the nanostructures due to the intensification of surface plasmons or effectively improve the scattering signal by increasing the light scattering from these nanostructures, which will be followed by molecular vibrations with better and higher signals. The SERS is a sensitive and selective method that results in enhanced Raman scattering of molecules that are adsorbed on metal structures (Wang and Fang 2006b). Moreover, by irradiating light (laser) to a rough metal surface, enhanced electric fields are created around the metal by intensifying the surface plasmons of metal nanostructures by the electromagnetic field of laser (Ren et al. 2007; Matricardi et al. 2018), as if the electric field resulted from laser light radiation is amplified. Therefore, the molecule placed in this amplified electric field becomes more polarized, resulting in an enhanced Raman signal (Lin et al. 2016) In this method, when the target biological analytes and chemicals are placed near the metal surface or physically adsorb the metal nanoparticles, the intensity of the Raman signal increases due to the interaction of biological analytes and chemicals and metal surface plasmons; hence, SERS can be applied for rapid and accurate detection of microbiological analytes. Enhancement of the electromagnetic field in the plasmonic resonance mode of the nanoparticles increases the excitation and emission of enhanced Raman in the SERS mode. In metal nanoparticles, such as gold nanoparticles, silver nanoparticles, and nanoparticles of noble metals that have appropriate morphology and dimensions, the electromagnetic enhancement can be increased by a high factor called the hot spot, which is directly related to the increased sensitivity and amplification of SERS. Furthermore, nanostructured arrays can usually be adjusted so that many hot spots to be placed on them (Lin et al. 2016; Radziuk and Moehwald 2015) to enhance the sensitivity of SERS-based nanosensors and plasmonic resonance. 2b1af7f3a8