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Crystals
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NQR of Polymorphic Crystals
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NQR of Polymorphic Crystals

A special issue of Crystals (ISSN 2073-4352). This special issue belongs to the section "Crystal Engineering".

Deadline for manuscript submissions: closed (30 April 2020) | Viewed by 22117

Special Issue Editor

Dr. Tomaž Apih

E-Mail Website
Guest Editor
Jozef Stefan Institute, Ljubljana, Slovenia
Interests: nuclear magnetic resonance; nuclear quadrupole resonance; fast field cycling relaxometry; active pharmaceutical ingredients; liquid crystals; hydrogen storage; quasicrystals; polymorphism

Special Issue Information

Dear Colleagues,

The phenomenon of polymorphism is well known to profoundly effect the solid-state properties of crystalline solids. For example, the polymorphic forms of solid active pharmaceutical ingredients (API) often manifest striking differences in solubility and dissolution rate, which directly affect the biological action of drugs. From an economic standpoint, the production and intellectual property protection aspects of polymorphism may be equally important.

Nuclear quadrupole resonance (NQR) spectroscopy is sometimes considered an 'exotic' variant of NMR and is not used as widely as NMR. However, it is becoming more and more clear that especially in the field of crystalline polymorphism, there are advantages of NQR spectroscopy that can help solve many questions more easily than NMR. For example, NQR does not require an external magnetic field, so powder samples exhibit the same high spectral resolution with narrow lines as single crystals. Different polymorphic structures often exhibit minuscule shifts or splitting of NMR lines; in NQR, the resonance frequencies are directly defined by the crystalline structure and the shifts are much more pronounced and easily distinguished. The usefulness of NQR is nowadays further enhanced by DFT calculations that allow for the calculation of the electric field gradient, thus establishing a link between the structural arrangement and the corresponding NQR spectra.

This Special Issue, entitled “NQR of Polymorphic Crystals”, aims to collect original research papers and review articles on NQR studies on polymorphic crystals, not just in the sense of different crystalline forms of the same molecules, but also other related solid state forms, such as co-crystals, solvates, salts and amorphous forms.

Dr. Tomaž Apih
Guest Editor

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Crystals is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2600 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • Nuclear Quadrupole Resonance
  • NQR
  • Electric Field Gradient
  • EFG
  • Polymorph
  • Cocrystal

Published Papers (5 papers)

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Research

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6 pages, 912 KiB  
Article
35Cl NQR Relaxation of the Molecular Ferroelectric 5,6-Dichloro-2-Methylbenzimidazole with Hydrogen-Bonded Molecular Chain
by Tetsuo Asaji
Crystals 2020, 10(2), 56; https://doi.org/10.3390/cryst10020056 - 21 Jan 2020
Viewed by 1934
Abstract
The 35Cl nuclear quadruple resonance (NQR) frequencies and spin-lattice relaxation times were measured in the temperature range of 4.2–420 K, of the above-room-temperature ferroelectric 5,6-dichloro-2-methylbenzimidazole, the ferroelectricity of which is thought to arise from the positional ordering of protons along chain of [...] Read more.
The 35Cl nuclear quadruple resonance (NQR) frequencies and spin-lattice relaxation times were measured in the temperature range of 4.2–420 K, of the above-room-temperature ferroelectric 5,6-dichloro-2-methylbenzimidazole, the ferroelectricity of which is thought to arise from the positional ordering of protons along chain of N-H...N hydrogen bonds. The first-order phase transition was clearly detected at around 400 K, as a discontinuity in the temperature dependence of NQR frequencies. The two NQR lines observed showed a much more significant difference in frequency in the high-temperature phase. This is consistent with the recently reported symmetry-lowering of crystal structure on warming. The relaxation mechanism in the low-temperature phase is discussed. Full article
(This article belongs to the Special Issue NQR of Polymorphic Crystals)
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14 pages, 464 KiB  
Article
The Predictive Power of Different Projector-Augmented Wave Potentials for Nuclear Quadrupole Resonance
by Jaafar N. Ansari, Karen L. Sauer and James K. Glasbrenner
Crystals 2019, 9(10), 507; https://doi.org/10.3390/cryst9100507 - 28 Sep 2019
Cited by 3 | Viewed by 4190
Abstract
The projector-augmented wave (PAW) method is used to calculate electric field gradients (EFG) for various PAW potentials. A variety of crystals containing reactive nonmetal, simple metal, and transition elements, are evaluated in order to determine the predictive ability of the PAW method for [...] Read more.
The projector-augmented wave (PAW) method is used to calculate electric field gradients (EFG) for various PAW potentials. A variety of crystals containing reactive nonmetal, simple metal, and transition elements, are evaluated in order to determine the predictive ability of the PAW method for the determination of nuclear quadrupole resonance frequencies in previously unstudied materials and their polymorphs. All results were compared to experimental results and, where possible, to previous density functional theory (DFT) calculations. The EFG at the 14N site of NaNO2 is calculated by DFT for the first time. The reactive nonmetal elements were not very sensitive to the variation in PAW potentials, and calculations were quite close to experimental values. For the other elements, the various PAW potentials led to a clear spread in EFG values, with no one universal potential emerging. Within the spread, there was agreement with other ab initio models. Full article
(This article belongs to the Special Issue NQR of Polymorphic Crystals)
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16 pages, 1939 KiB  
Article
Tris(2-Methoxyphenyl)Bismuthine Polymorphism Characterized by Nuclear Quadrupole Resonance Spectroscopy
by Hermann Scharfetter, Roland Fischer, Paul Krassnig, Martin Thonhofer, Felix Theyer and Christian Gösweiner
Crystals 2019, 9(9), 446; https://doi.org/10.3390/cryst9090446 - 28 Aug 2019
Viewed by 2867
Abstract
Based on the previous identification of metastable polymorphs in crystalline triphenylbismuth by nuclear quadrupole resonance spectroscopy (NQRS), the potential formation of similar phases was studied in Tris(2-Methoxyphenyl)Bismuthine. To this end, commercial samples with known NQRS properties were molten and re-crystallized at different speeds [...] Read more.
Based on the previous identification of metastable polymorphs in crystalline triphenylbismuth by nuclear quadrupole resonance spectroscopy (NQRS), the potential formation of similar phases was studied in Tris(2-Methoxyphenyl)Bismuthine. To this end, commercial samples with known NQRS properties were molten and re-crystallized at different speeds (shock freezing in different coolants versus slow cooling inside of a heater). In all recrystallization products we have identified a new crystal phase which has not been observed after synthesis from a solution. The new crystallographic structure has been confirmed by X-ray diffraction (XRD). The newly isolated polymorph crystallizes in the monoclinic space group P2(1)/c with only one molecule in the asymmetric unit and consequently only one 5/2-7/2 transition is observed at 88.75 MHz at 310 K. In contrast, the two transitions at 89.38 and 89.29 MHz for the well-known trigonal polymorph originate from two crystallographically distinct molecules of Tris(2-methoxy-Phenyl)Bismuthine in the asymmetric unit. Additional relaxometric NQRS shows distinctly different T2 relaxation times for the new polymorph when compared to the original samples. Additional phase transitions could not be observed during temperature sweeps between 153 K and 323 K. Full article
(This article belongs to the Special Issue NQR of Polymorphic Crystals)
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Review

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22 pages, 5459 KiB  
Review
Nuclear Quadrupole Resonance (NQR)—A Useful Spectroscopic Tool in Pharmacy for the Study of Polymorphism
by Zvonko Trontelj, Janez Pirnat, Vojko Jazbinšek, Janko Lužnik, Stane Srčič, Zoran Lavrič, Samo Beguš, Tomaž Apih, Veselko Žagar and Janez Seliger
Crystals 2020, 10(6), 450; https://doi.org/10.3390/cryst10060450 - 31 May 2020
Cited by 15 | Viewed by 9105
Abstract
Nuclear Quadrupole Resonance (NQR) spectroscopy has been known for 70 years. It is suitable for the study of measured (poly)crystalline chemical compounds containing quadrupole nuclei (nuclei with spin I ≥ 1) where the characteristic NQR frequencies represent the fingerprints of these compounds. In [...] Read more.
Nuclear Quadrupole Resonance (NQR) spectroscopy has been known for 70 years. It is suitable for the study of measured (poly)crystalline chemical compounds containing quadrupole nuclei (nuclei with spin I ≥ 1) where the characteristic NQR frequencies represent the fingerprints of these compounds. In several cases, 14N NQR can distinguish between the polymorphic crystalline phases of active pharmaceutical ingredients (APIs). In order to further stimulate 14N NQR studies, we review here several results of API polymorphism studies obtained in Ljubljana laboratories: (a) In sulfanilamide, a clear distinction between three known polymorphs (α, β, γ) was demonstrated. (b) In famotidine, the full spectra of all seven different nitrogen positions were measured; two polymorphs were distinguished. (c) In piroxicam, the 14N NQR data helped in confirming the new polymorphic form V. (d) The compaction pressure in the tablet production of paracetamol, which is connected with linewidth change, can be used to distinguish between producers of paracetamol. We established that paracetamol in the tablets of six different manufacturers can be identified by 14N NQR linewidth. (e) Finally, in order to get an extremely sensitive 14N NQR spectrometer, the optical detection of the 14N NQR signal is mentioned. Full article
(This article belongs to the Special Issue NQR of Polymorphic Crystals)
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12 pages, 3882 KiB  
Review
Nuclear Quadrupole Resonance Spectroscopy: Tautomerism and Structure of Functional Azoles
by Lyudmila I. Larina
Crystals 2019, 9(7), 366; https://doi.org/10.3390/cryst9070366 - 17 Jul 2019
Cited by 5 | Viewed by 3528
Abstract
The Nuclear Quadrupole Resonance spectroscopy data of functionalized azoles (imidazoles, triazoles and corresponding benzazoles) are reviewed and critically discussed. The possibility of studying the tautomerism of azoles by the NQR method is considered. Full article
(This article belongs to the Special Issue NQR of Polymorphic Crystals)
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