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Spectroscopic Analyses for Surface Characterization of Electrospun Fibers
Published in K.M. Praveen, Rony Thomas Murickan, Jobin Joy, Hanna J. Maria, Jozef T. Haponiuk, Sabu Thomas, Electrospun Nanofibers from Bioresources for High-Performance Applications, 2023
At a particular magnetic field strength, the energy required to flip the proton matches with the energy of radiation. At this point, absorption occurs, and a signal is observed. The proton is said to be ‘shielded’ if the resonance occurs at higher field strength and ‘deshielded’ if the resonance is at lower field strength. Shift in the position of NMR absorptions due to the shielding and deshielding of protons is termed as chemical shift (Kumar, P. et al; 2014).
Identification of Microplastics and Nanoplastics and Associated Analytical Challenges
Published in Hyunjung Kim, Microplastics, 2023
An NMR spectrum provides a lot of information about the molecules present in the sample. First, chemical shift values can identify chemical groups within a molecule. For example, acetic acid (H3C–COOH) has four protons, and the three protons of the methyl group (CH3) are magnetically equivalent and therefore have the same chemical shift. So one signal comes from the CH3 group and the other from the proton in the carboxylic acid group (COOH). Secondly, in 1H-NMR spectra, the signal area is proportional to the number of atomic nuclei producing that signal (this does not apply to 13C-NMR spectra). In this example, if the areas of both signals were to be calculated, the most intense signal would be three times larger than the other. It is by the fact that one signal represents the three protons from the CH3 group (signal at δ = 2.0 ppm) and the other one the proton from the COOH group (signal at δ = 11.5 ppm) (Jacobsen, 2016; Bible, 2013).
Nuclear Magnetic Resonance Spectroscopy
Published in Thomas J. Bruno, Paris D.N. Svoronos, CRC Handbook of Basic Tables for Chemical Analysis, 2020
Thomas J. Bruno, Paris D.N. Svoronos
The following table contains 15N chemical shifts of various organic nitrogen compounds. Chemical shifts are expressed relative to different standards (NH3, NH4Cl, CH3NO2, NH4NO3, HNO3, etc.) and are interconvertible. Chemical shifts are sensitive to hydrogen bonding and are solvent dependent as seen in the case of pyridine (see note b below). Consequently, the reference as well as the solvent should always accompany chemical shift data. No data are given on peptides and other biochemical compounds. All shifts are relative to ammonia unless otherwise specified. A section of “miscellaneous” data gives the chemical shift of special compounds relative to unusual standards [1–15].
Four new cycloartane-type triterpenoids from the leaves of Combretum mellifluum Eichler: assessment of their antioxidant and antileishmanial activities
Published in Journal of Toxicology and Environmental Health, Part A, 2022
Jaelson Santos Silva, Éverton Leandro França Ferreira, Amanda Maciel Lima, Ruth Raquel Soares de Farias, Bruno Quirino Araújo, José Carlos Quilles Junior, Rodolfo Ritchelle Lima Santos, Fernando Aécio de Amorim Carvalho, Mahendra Rai, Gerardo Magela Vieira Júnior, Mariana Helena Chaves
Solvents with analytical standards (AS) were used. Folin–Ciocalteu reagent and pyridine were purchased from Merck, radical 2,2-diphenyl-1-picrylhidrazila (DPPH) and rutin, aluminum chloride (Fluka Analytical), glacial acetic acid (Vetec), sodium carbonate (Synth) were purchased from Sigma. The absorption measurements were conducted with UV-Vis Lambda 25 PerkinElmer spectrophotometer. The chromatographic plates were prepared using silica gel (Macherey-Nagel) and sprayed with a solution of cerium sulfate. The atmospheric pressure chromatographic columns were prepared with silica gel 60 (0.060–0.200 mm) from Acros Organics, Sephadex LH-20 (Aldrich) and Strata C18 cartridges (10 g, Phenomenex). One-dimensional nuclear magnetic resonance (1D-NMR) and two-dimensional nuclear magnetic resonance (2D-NMR) spectra were recorded on a Varian Inova (400 and 600 MHz) spectrometer. Chemical shifts (relative to TMS) are expressed in parts per million (ppm). Mass spectra were measured using a Bruker Daltonics (microTOF-QII) HRMS spectrometer. Acquisition of mass spectra was performed in MS/MS, with ESI (+) detection mode, and samples diluted in methanol/toluene (4:1). The silylated steroids derivatives were analyzed using GC-MS (GC-2010 Plus and GC-MS-QP2010 SE, Shimadzu) equipped with an SLBTM-5 ms (Supelco, 30 m × 0.25 mm × 0.25 µm).
Estimation of carbon-13 chemical shift anisotropy from aligned phase parameters in liquid crystals
Published in Phase Transitions, 2022
R. V. Sudheer Kumar, M. Srinivasa Reddy, Ravi Arukula, Y. Vijayakumar
Chemical shift is perhaps the most important NMR parameter as it enables site-specific information about the structure and dynamics of a molecule to be obtained. In solids, the anisotropic chemical shift (CSA) provides additional information such as molecular orientation and hydrogen bonding. In partially ordered systems, it can be used to obtain information on the dynamics and degree of ordering. Over the years, a variety of experiments has been designed to determine the CSA parameters [1–5]. One of the available methods to determine CSA parameters is by observing the intensities of the spinning sidebands in slow spinning MAS spectra. This method, known as the Herzfeld and Berger method [6], has been well utilized, but has the disadvantage that spinning side bands from different resonances overlap and several spectra have to be recorded at different spinning speeds to overcome the overlapping problem. However, this approach is useful for systems with long T1 relaxation delay, for which the applications of the more sophisticated 2D experiments will be time consuming. The 2D CSA recoupling techniques known by the acronyms SUPER [7] and ROCSA [8] have been proposed in recent years. One can determine the CSA parameters from the CSA powder patterns. The experimental CSA powder patterns obtained from the 2D spectrum are then fitted with simulated patterns to get the best fit values of CSA parameters. Also, quantum chemical calculations such as the density functional theory method have made significant success in providing the information of CSA in many liquid crystalline systems [9,10].
Synthesis of thiazolylidenethiazoloquinazolinone hybrids from monocarbonyl curcumin analogues. Characterization, bio-evaluation and DFT study
Published in Journal of Sulfur Chemistry, 2022
Soufiane Benreka, Fatima-Zohra Zradni, Fatiha Madi, Gilbert Kirsch, Souad Kasmi-Mir
Melting points were determined with a Büchi530 digital melting point apparatus and were uncorrected. The spectra of 1H NMR and 13C NMR were recorded in solution in CDCl3 or DMSO-d6 on a Bruker spectrometer (1H at 400 MHz, 13C at 100 MHz). The chemical shifts are expressed in parts per million (ppm) by using tetramethylsilane (TMS) as internal reference. The multiplicities of the signals are indicated by the following abbreviations: s, singlet; d, doublet; t, triplet; q, quadruplet; and m, multiplet, and coupling constants are expressed in Hertz. Elemental analyses were done in a LECO CHNS 932 equipment. Thin-layer chromatography (TLC) was performed using silica gel plates (POLYGRAM SIL G/UV254, 0.20 mm), which were visualized under UV light. All synthesized compounds gave satisfactory elemental analysis