Sean T. Holmes, Cameron M. Boley, Angelika Dewicki, Zachary T. Gardner, Cameron S. Vojvodin, Robbie J. Iuliucci, Robert W. Schurko
This paper reports the principal values of the 13C chemical shift tensors for five nitrogen-dense compounds (i.e., cytosine, uracil, imidazole, guanidine hydrochloride, and aminoguanidine hydrochloride). Although these are all fundamentally important compounds, the majority do not have 13C chemical shift tensors reported in the literature. The chemical shift tensors are obtained from 1H→13C cross-polarization magic-angle spinning (CP/MAS) experiments that were conducted at a high field of 18.8 T to suppress the effects of 14N-13C residual dipolar coupling. Quantum chemical calculations using density functional theory are used to obtain the 13C magnetic shielding tensors for these compounds. The best agreement with experiment arises from calculations using the hybrid functional PBE0 or the double-hybrid functional PBE0-DH, along with the triple-zeta basis sets TZ2P or pc-3, respectively, and intermolecular effects modeled using large clusters of molecules with electrostatic embedding through the COSMO approach. These measurements are part of an ongoing effort to expand the catalog of accurate 13C chemical shift tensor measurements, with the aim of creating a database that may be useful for benchmarking the accuracy of quantum chemical calculations, developing nuclear magnetic resonance (NMR) crystallography protocols, or aiding in applications involving machine learning or data mining. This work was conducted at the National High Magnetic Field Laboratory as part of a 2-week school for introducing undergraduate students to practical laboratory experience that will prepare them for scientific careers or postgraduate studies.
本文报告了五种氮密集化合物(即胞嘧啶、尿嘧啶、咪唑、盐酸胍和盐酸氨基胍)的 13 C 化学位移张量的主要值。虽然这些都是基本的重要化合物,但大多数都没有 13 C 化学位移张量的文献报道。化学位移张量是从 1 H→13 C 交叉偏振魔角旋转(CP/MAS)实验中获得的,这些实验是在 18.8 T 的高磁场下进行的,以抑制 14 N-13 C 残余偶极耦合的影响。利用密度泛函理论进行的量子化学计算获得了这些化合物的 13 C 磁屏蔽张量。通过使用混合函数 PBE0 或双混合函数 PBE0-DH,以及三重zeta 基集 TZ2P 或 pc-3 分别进行计算,并通过 COSMO 方法使用具有静电嵌入的大分子簇模拟分子间效应,得出了与实验最吻合的结果。这些测量结果是正在进行的扩大 13 C 化学位移张量精确测量目录工作的一部分,目的是建立一个数据库,用于为量子化学计算的准确性设定基准、开发核磁共振(NMR)晶体学协议或协助涉及机器学习或数据挖掘的应用。这项工作是在国家高磁场实验室进行的,是为期两周的学校活动的一部分,目的是向本科生介绍实验室实践经验,为他们将来从事科学工作或攻读研究生做好准备。
{"title":"Carbon-13 chemical shift tensor measurements for nitrogen-dense compounds","authors":"Sean T. Holmes, Cameron M. Boley, Angelika Dewicki, Zachary T. Gardner, Cameron S. Vojvodin, Robbie J. Iuliucci, Robert W. Schurko","doi":"10.1002/mrc.5422","DOIUrl":"10.1002/mrc.5422","url":null,"abstract":"<p>This paper reports the principal values of the <sup>13</sup>C chemical shift tensors for five nitrogen-dense compounds (i.e., cytosine, uracil, imidazole, guanidine hydrochloride, and aminoguanidine hydrochloride). Although these are all fundamentally important compounds, the majority do not have <sup>13</sup>C chemical shift tensors reported in the literature. The chemical shift tensors are obtained from <sup>1</sup>H→<sup>13</sup>C cross-polarization magic-angle spinning (CP/MAS) experiments that were conducted at a high field of 18.8 T to suppress the effects of <sup>14</sup>N-<sup>13</sup>C residual dipolar coupling. Quantum chemical calculations using density functional theory are used to obtain the <sup>13</sup>C magnetic shielding tensors for these compounds. The best agreement with experiment arises from calculations using the hybrid functional PBE0 or the double-hybrid functional PBE0-DH, along with the triple-zeta basis sets TZ2P or pc-3, respectively, and intermolecular effects modeled using large clusters of molecules with electrostatic embedding through the COSMO approach. These measurements are part of an ongoing effort to expand the catalog of accurate <sup>13</sup>C chemical shift tensor measurements, with the aim of creating a database that may be useful for benchmarking the accuracy of quantum chemical calculations, developing nuclear magnetic resonance (NMR) crystallography protocols, or aiding in applications involving machine learning or data mining. This work was conducted at the National High Magnetic Field Laboratory as part of a 2-week school for introducing undergraduate students to practical laboratory experience that will prepare them for scientific careers or postgraduate studies.</p>","PeriodicalId":18142,"journal":{"name":"Magnetic Resonance in Chemistry","volume":"62 3","pages":"179-189"},"PeriodicalIF":2.0,"publicationDate":"2024-01-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139478773","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"化学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
James Daley, Joseph Siciliano, Vincent Ferraro, Elodie Sutter, Adam Lounsbery, Nicholas Whiting
The para spin isomer of hydrogen gas possesses high nuclear spin order that can enhance the NMR signals of a variety of molecular species. Hydrogen is routinely enriched in the para spin state by lowering the gas temperature while flowing through a catalyst. Although parahydrogen enrichments approaching 100% are achievable near the H2 liquefaction temperature of 20 K, many experimentalists operate at liquid nitrogen temperatures (77 K) due to the lower associated costs and overall simplicity of the parahydrogen generator. Parahydrogen that is generated at 77 K provides an enrichment value of ~51% of the para spin isomer; while useful, there are many applications that can benefit from low-cost access to higher parahydrogen enrichments. Here, we introduce a method of improving parahydrogen enrichment values using a liquid nitrogen-cooled generator that operates at temperatures less than 77 K. The boiling temperature of liquid nitrogen is lowered through internal evaporation into helium gas bubbles that are injected into the liquid. Changes to liquid nitrogen temperatures and parahydrogen enrichment values were monitored as a function of helium gas flow rate. The injected helium bubbles lowered the liquid nitrogen temperature to ~65.5 K, and parahydrogen enrichments of up to ~59% were achieved; this represents an ~16% improvement compared with the expected parahydrogen fraction at 77 K. This technique is simple to implement in standard liquid nitrogen-cooled parahydrogen generators and may be of interest to a wide range of scientists that require a cost-effective approach to improving parahydrogen enrichment values.
氢气的对位自旋异构体具有很高的核自旋阶次,可以增强各种分子物种的核磁共振信号。在氢气流经催化剂时,通过降低气体温度,可使氢气常规富集为对位自旋态。虽然在 20 K 的氢气液化温度附近可以实现接近 100% 的对位氢富集,但由于相关成本较低且对位氢发生器总体简单,许多实验人员在液氮温度(77 K)下进行操作。在 77 K 温度下生成的对氢提供了约 51% 的对位自旋异构体富集值;虽然有用,但许多应用可以从低成本获取更高的对氢富集值中获益。在此,我们介绍一种利用温度低于 77 K 的液氮冷却发生器提高对氢富集值的方法。液氮的沸腾温度通过内部蒸发进入注入液体的氦气泡而降低。液氮温度和对氢富集值的变化随氦气流速的变化而受到监测。注入的氦气泡将液氮温度降到了约 65.5 K,副氢富集度高达约 59%;与 77 K 时的预期副氢分数相比,提高了约 16%。这种技术在标准液氮冷却副氢发生器中实施起来非常简单,可能会引起需要以经济有效的方法提高副氢富集值的广大科学家的兴趣。
{"title":"Temperature lowering of liquid nitrogen via injection of helium gas bubbles improves the generation of parahydrogen-enriched gas","authors":"James Daley, Joseph Siciliano, Vincent Ferraro, Elodie Sutter, Adam Lounsbery, Nicholas Whiting","doi":"10.1002/mrc.5423","DOIUrl":"10.1002/mrc.5423","url":null,"abstract":"<p>The para spin isomer of hydrogen gas possesses high nuclear spin order that can enhance the NMR signals of a variety of molecular species. Hydrogen is routinely enriched in the para spin state by lowering the gas temperature while flowing through a catalyst. Although parahydrogen enrichments approaching 100% are achievable near the H<sub>2</sub> liquefaction temperature of 20 K, many experimentalists operate at liquid nitrogen temperatures (77 K) due to the lower associated costs and overall simplicity of the parahydrogen generator. Parahydrogen that is generated at 77 K provides an enrichment value of ~51% of the para spin isomer; while useful, there are many applications that can benefit from low-cost access to higher parahydrogen enrichments. Here, we introduce a method of improving parahydrogen enrichment values using a liquid nitrogen-cooled generator that operates at temperatures less than 77 K. The boiling temperature of liquid nitrogen is lowered through internal evaporation into helium gas bubbles that are injected into the liquid. Changes to liquid nitrogen temperatures and parahydrogen enrichment values were monitored as a function of helium gas flow rate. The injected helium bubbles lowered the liquid nitrogen temperature to ~65.5 K, and parahydrogen enrichments of up to ~59% were achieved; this represents an ~16% improvement compared with the expected parahydrogen fraction at 77 K. This technique is simple to implement in standard liquid nitrogen-cooled parahydrogen generators and may be of interest to a wide range of scientists that require a cost-effective approach to improving parahydrogen enrichment values.</p>","PeriodicalId":18142,"journal":{"name":"Magnetic Resonance in Chemistry","volume":"62 2","pages":"94-100"},"PeriodicalIF":2.0,"publicationDate":"2024-01-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139087444","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"化学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Tristan Maschmeyer, Breanna Conklin, Thomas C. Malig, David J. Russell, Kenji L. Kurita, Jason E. Hein, José G. Napolitano
Nuclear magnetic resonance (NMR) spectroscopy is a powerful analytical technique with the ability to acquire both quantitative and structurally insightful data for multiple components in a test sample. This makes NMR spectroscopy a desirable tool to understand, monitor, and optimize chemical transformations. While quantitative NMR (qNMR) approaches relying on internal standards are well-established, using an absolute external calibration scheme is beneficial for reaction monitoring as resonance overlap complications from an added reference material to the sample can be avoided. Particularly, this type of qNMR technique is of interest with benchtop NMR spectrometers as the likelihood of resonance overlap is only enhanced with the lower magnetic field strengths of the used permanent magnets. The included study describes a simple yet robust methodology to determine concentration conversion factors for NMR systems using single- and multi-analyte linear regression models. This approach is leveraged to investigate a pharmaceutically relevant amide coupling batch reaction. An on-line stopped-flow (i.e., interrupted-flow or paused-flow) benchtop NMR system was used to monitor both the 1,1′-carbonyldiimidazole (CDI) promoted acid activation and the amide coupling. The results highlight how quantitative measurements in benchtop NMR systems can provide valuable information and enable analysts to make decisions in real time.
{"title":"A reliable external calibration method for reaction monitoring with benchtop NMR","authors":"Tristan Maschmeyer, Breanna Conklin, Thomas C. Malig, David J. Russell, Kenji L. Kurita, Jason E. Hein, José G. Napolitano","doi":"10.1002/mrc.5421","DOIUrl":"10.1002/mrc.5421","url":null,"abstract":"<p>Nuclear magnetic resonance (NMR) spectroscopy is a powerful analytical technique with the ability to acquire both quantitative and structurally insightful data for multiple components in a test sample. This makes NMR spectroscopy a desirable tool to understand, monitor, and optimize chemical transformations. While quantitative NMR (qNMR) approaches relying on internal standards are well-established, using an absolute external calibration scheme is beneficial for reaction monitoring as resonance overlap complications from an added reference material to the sample can be avoided. Particularly, this type of qNMR technique is of interest with benchtop NMR spectrometers as the likelihood of resonance overlap is only enhanced with the lower magnetic field strengths of the used permanent magnets. The included study describes a simple yet robust methodology to determine concentration conversion factors for NMR systems using single- and multi-analyte linear regression models. This approach is leveraged to investigate a pharmaceutically relevant amide coupling batch reaction. An <i>on-line</i> stopped-flow (<i>i.e.</i>, interrupted-flow or paused-flow) benchtop NMR system was used to monitor both the 1,1′-carbonyldiimidazole (CDI) promoted acid activation and the amide coupling. The results highlight how quantitative measurements in benchtop NMR systems can provide valuable information and enable analysts to make decisions in real time.</p>","PeriodicalId":18142,"journal":{"name":"Magnetic Resonance in Chemistry","volume":"62 3","pages":"169-178"},"PeriodicalIF":2.0,"publicationDate":"2023-12-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1002/mrc.5421","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138799996","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"化学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Stefan Kuhn, Heinz Kolshorn, Christoph Steinbeck, Nils Schlörer
In October 2003, 20 years ago, the open-source and open-content database NMRshiftDB was announced. Since then, the database, renamed as nmrshiftdb2 later, has been continuously available and is one of the longer-running projects in the field of open data in chemistry. After 20 years, we evaluate the success of the project and present lessons learnt for similar projects.
{"title":"Twenty years of nmrshiftdb2: A case study of an open database for analytical chemistry","authors":"Stefan Kuhn, Heinz Kolshorn, Christoph Steinbeck, Nils Schlörer","doi":"10.1002/mrc.5418","DOIUrl":"10.1002/mrc.5418","url":null,"abstract":"<p>In October 2003, 20 years ago, the open-source and open-content database NMRshiftDB was announced. Since then, the database, renamed as nmrshiftdb2 later, has been continuously available and is one of the longer-running projects in the field of open data in chemistry. After 20 years, we evaluate the success of the project and present lessons learnt for similar projects.</p>","PeriodicalId":18142,"journal":{"name":"Magnetic Resonance in Chemistry","volume":"62 2","pages":"74-83"},"PeriodicalIF":2.0,"publicationDate":"2023-12-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1002/mrc.5418","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138800096","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"化学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Solid-state nuclear magnetic resonance (NMR) spectroscopy and quantum chemical density functional theory (DFT) calculations are widely used to characterize vanadium centers in biological and pharmaceutically relevant compounds. Several techniques have been recently developed to improve the accuracy of predicted NMR parameters obtained from DFT. Fragment-based and planewave-corrected methods employing hybrid density functionals are particularly effective tools for solid-state applications. A recent benchmark study involving molecular crystal compounds found that fragment-based NMR calculations using hybrid density functionals improve the accuracy of predicted 51V chemical shieldings by 20% relative to traditional planewave methods. This work extends the previous study, including a careful analysis of 51V chemical shift anisotropy, electric field gradient calculations, and a more extensive test set. The accuracy of planewave-corrected techniques and recently developed fragment-based methods using electrostatic embedding based on the polarized continuum model (PCM) are found to be highly competitive with previous methods. Planewave-corrected methods achieve a 34% improvement in the errors of predicted 51V chemical shieldings relative to planewave. Additionally, planewave-corrected and fragment-based calculations were performed using PCM embedding, improving the accuracy of predicted 51V chemical shielding (CS) tensor principal values by 30% and