Nuclear Magnetic Resonance Studies of Multi-site Chemical Exchange. III. Hindered Rotation in Dimethylacetamide, Dimethyl Trifluoro-acetamide, and Dimethyl Benzamide

1971 ◽  
Vol 49 (22) ◽  
pp. 3683-3691 ◽  
Author(s):  
L. W. Reeves ◽  
R. C. Shaddick ◽  
K. N. Shaw
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Chemical Exchange ◽  
Chemical Shifts ◽  
Activation Parameters ◽  
Type Systems ◽  
Hindered Rotation ◽  
Fitting Procedure ◽  
Nuclear Magnetic

The experimental application of a multi-site exchange theory developed recently has been made to three restricted rotation barriers in amido-type systems. Inclusion of 8 spin-sites in the analyses of the line shapes for spectra of N,N-dimethyl trifluoroacetamide and dimethyl acetamide leads to improved values for the activation parameters. Variation of chemical shifts with temperature is included in the iterative fitting procedure. The determination of the relative signs of long range couplings and the inclusion and measurement of unresolved couplings has been accomplished. The barrier for hindered rotation in N,N-dimethyl benzamide has been re-investigated including the effect of relatively large chemical shift changes with temperature. The internal consistency of the measurements from all aspects indicates that systematic errors common in the determination of energy barriers by steady-state nuclear magnetic resonance (n.m.r.) have been largely eliminated and hence the activation parameters obtained form a reliable addition to a compilation of experimental data pertaining to the bonding in a related series of simple C-substituted amides.

Nuclear Magnetic Resonance Studies of Multi-site Chemical Exchange. II. Hindered Rotation in N,N-Dimethyl Carbamyl Fluoride

1971 ◽  
Vol 49 (22) ◽  
pp. 3671-3682 ◽  
Author(s):  
L. W. Reeves ◽  
K. N. Shaw
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Chemical Exchange ◽  
Chemical Shifts ◽  
Activation Parameters ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Matrix Formulation ◽  
Hindered Rotation ◽  
Nuclear Magnetic

The matrix formulation of the Bloch equations including chemical exchange reported earlier is extended to include indirect spin–spin coupling in first-order spectra. The ABX (JAB = 0) spin system is treated in detail and particular attention is paid to the determining effect on the nuclear magnetic resonance (n.m.r.) lineshapes of the relative signs of the coupling constants JAX and JBX. The hindered rotation for N,N-dimethyl carbamyl fluoride in CCl4 as solvent has been studied using a complete 1H n.m.r. lineshape analysis and the activation parameters obtained are: ΔG≠ = 18.1 ± 0.6 kcal mol−1, ΔH≠ = 17.7 ± 0.6 kcal mol−1, and ΔS≠ = −1.4 ± 2.1 cal deg−1 mol−1 at 25 °C.The complete lineshape fits give very precise values of the relative shifts (16.5 mol% in CCl4) of the methyl groups and of the coupling constants JAX (0.30 ± 0.05 Hz) and JBX (0.80 ± 0.05 Hz) at all temperatures. A 40% change in JBX (1.10 ± 0.05 Hz) is observed in neat DMCF from a lineshape fit at −15 °C. The origin of changes in chemical shifts with temperature and JBX with solvent is discussed.

Nuclear Magnetic Resonance Studies of some Trimethyl Tin Carbamates

1972 ◽  
Vol 50 (9) ◽  
pp. 1386-1389 ◽  
Author(s):  
A. E. Lemire ◽  
J. C. Thompson
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Low Temperature ◽  
Chloroform Solution ◽  
Activation Parameters ◽  
Hindered Rotation ◽  
Magnetic Resonance Studies ◽  
Hexane Solution ◽  
Nuclear Magnetic

The low temperature n.m.r. spectra of some trimethylstannyl esters of N,N-dimethylcarbamic acid, N,N-dimethyldithiocarbamic acid, and N,N-dimethylmonothiocarbamic acid have been examined. Activation parameters for hindered rotation about the C—N bond in the trimethylstannyl ester of N,N-dimethylmonothiocarbamic acid have been determined to be: in hexane solution, Ea = 17.0 + 0.5 kcal/mol, [Formula: see text][Formula: see text][Formula: see text] in chloroform solution, Ea = 19.9 ± 0.5 kcal/mol, [Formula: see text][Formula: see text][Formula: see text]not available

scholarly journals Zero-Field Nuclear Magnetic Resonance of Chemically Exchanging Systems

2019 ◽  
Author(s):  
Danila Barskiy ◽  
Michael C. D. Tayler ◽  
Irene Marco-Rius ◽  
John Kurhanewicz ◽  
Daniel B. Vigneron ◽  
...  
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
High Field ◽  
Nmr Spectra ◽  
Chemical Exchange ◽  
Chemical Shifts ◽  
Proton Exchange ◽  
Zero Field ◽  
Resonance Frequencies ◽  
Nuclear Magnetic

Zero- and ultralow-field (ZULF) nuclear magnetic resonance (NMR) is an emerging tool for precision chemical analysis. Unlike conventional (high-field) NMR, which relies on chemical shifts for molecular identification, zero-field analog reports <i>J</i>-spectra that depend on the nuclear spin-spin coupling topology of molecules under investigation. While chemical shifts are usually a small fraction of the resonance frequencies, <i>J</i>-spectra for various spin systems are completely different from each other. In this work, we use zero-field NMR to study dynamic chemical processes and investigate the influence of chemical exchange on ZULF NMR spectra. We develop a computation approach that allows quantitative calculation of ZULF NMR spectra in the presence of chemical exchange and apply it to study aqueous solutions of [<sup>15</sup>N]ammonium as a model system. In this system, proton exchange rates span more than three orders of magnitude depending on acidity (pH), as monitored by high-field and ZULF NMR. We show that chemical exchange substantially affects the <i>J</i>-coupled NMR spectra and, in some cases, can lead to degradation and complete disappearance of the spectral features. To demonstrate potential applications of ZULF NMR for chemistry and biomedicine, we show a ZULF NMR spectrum of [2-<sup>13</sup>C]pyruvic acid hyperpolarized via dissolution dynamic nuclear polarization (dDNP). The metabolism of pyruvate provides valuable biochemical information and its monitoring by zero-field NMR could give spectral resolution that is hard to achieve at high magnetic fields. We foresee applications of affordable and scalable ZULF NMR coupled with hyperpolarization modalities to study chemical exchange phenomena in vivo and in situations where high-field NMR detection is not possible to implement.<br>

Carbon-13 Nuclear Magnetic Resonance Chemical Shift Calculation Protocol Applied to Rigid Triterpenes Molecules

2020 ◽  
Vol 12 (8) ◽  
pp. 995-1001
Author(s):  
Rênica Alves de Morais Rocha ◽  
Thaís Forest Giacomello ◽  
Antonio Maia de Jesus Chaves Neto ◽  
Gunar Vingre Da Silva Mota ◽  
Fabio Luiz Paranhos Costa
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Natural Products ◽  
Magnetic Resonance ◽  
Chemical Shifts ◽  
Scaling Factor ◽  
Resonance Spectroscopy ◽  
Theoretical Point ◽  
Structural Determination ◽  
Nuclear Magnetic

Nuclear magnetic resonance spectroscopy is one of the most powerful experimental techniques for obtaining three-dimensional structures of complex molecules, mainly for the analysis of the relative and absolute configurations of organic compounds. For this reason, this has become one of the most promising tools in the field of chemistry. From the theoretical point of view, advanced computational protocols have been developed for calculating nuclear magnetic resonance, mainly hydrogen-1 and carbon-13, parameters of isolated molecules, in which the environmental effects are neglected. These effects are predominantly related to the inherently large size of such systems, making conventional ab initio theories either very computationally demanding or even prohibitive. Despite the current advances in spectroscopic techniques, instances of revision of structures erroneously established for natural products are still common in the literature. Therefore, it is still necessary the development of quantum-chemical protocols that may assist in the correct structural determination of these compounds. This work aimed to test a universal scaling factor, based on a linear regression, for the calculation of carbon-13 nuclear magnetic resonance chemical shifts for rigid molecules, which has low computational cost and great accuracy to aid in the structural determination of natural products. The carbon-13 chemical shifts were calculated using the mPW1PW91/3-21G level of theory. Scaled chemical shifts were obtained according to the relation: 1.14x(calculated chemical shifts)–4.71. To test the application of the created scaling factor to problems related to stereochemistry, we investigated its ability to differentiate pentacyclic triterpenes regioisomers. Our results show that the mPW1PW91/3-21G//PM7 level of theory applied to the calculations, together with the use of the scaling factor, is an efficient and low-cost tool as an alternative to computational requirement approaches, usually applied to the calculation of carbon-13 nuclear magnetic resonance chemical shifts.

The semiempirical derivation of 13C nuclear magnetic resonance chemical shifts. Hydrocarbons, alcohols, amines, ketones, and olefins

1977 ◽  
Vol 55 (15) ◽  
pp. 2813-2828 ◽  
Author(s):  
Helmut Beierbeck ◽  
John K. Saunders ◽  
John W. Apsimon
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Chemical Shifts ◽  
Saturated Hydrocarbons ◽  
13C Nuclear Magnetic Resonance ◽  
Nuclear Magnetic

Substituent parameters were derived for the semiempirical determination of 13C chemical shifts in saturated hydrocarbons, alcohols, amines, ketones, and olefins. The olefin parameters are valid for six-membered rings and the remaining parameters for six-membered rings in chair conformations. The use of these parameters for the calculation of carbon resonances is illustrated with a number of examples.

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