Heteronuclear long‐range scalar coupling constants (nJCH) are a valuable tool for solving problems in organic chemistry and are especially suited for stereochemical and configurational analyses of small molecules and natural products. This tutorial will focus on the step‐by‐step implementation of several 2D 1H frequency selective HSQMBC experiments for the easy and accurate measurement of either the magnitude or both the magnitude and the sign of long‐range nJCH couplings. The performance of these experiments will be showcased with several scenarios in a range of different experimental conditions.
Bruker pulse program code for selHSQMBC experiments available here.
Bruker pulse program code for selHSQMBC-TOCSY experiment available here.
Nolis, Pau & Teodor Parella. 2019. Practical aspects of the simultaneous collection of COSY and TOCSY spectra. Magnetic Resonance in Chemistry. DOI: 10.1002/mrc.4835
The practical aspects of some NMR experiments designed for the simultaneous acquisition of 2D COSY and 2D TOCSY spectra are presented and discussed. Several techniques involving afterglow‐based, CTP‐based and NOAH‐based strategies for the collection of different FIDs within the same scan are evaluated and compared. These methods offer a faster recording of these spectra in small‐molecule NMR when sensitivity is not a limiting factor, with a reduction in spectrometer time about 45%‐60% when compared to the conventional sequential acquisition of the parent experiments. It is also shown how the optimized design of an extended three‐FID approach yields one COSY and two TOCSY spectra simultaneously by combining CTP and NOAH principles in the same experiment, affording substantial sensitivity enhancements per time unit.
Nolis, Pau, Kumar Motiram-Corral, Míriam Pérez-Trujillo & Teodor Parella. 2019. Simultaneous acquisition of two 2D HSQC spectra with different 13C spectral widths. Journal of Magnetic Resonance. DOI: 10.1016/j.jmr.2019.01.004
ABSTRACT: A time-efficient NMR strategy that involves the interleaved acquisition of two 2D HSQC spectra having different spectral widths in the indirect 13C dimension is presented. We show how the two equivalent coherence transfer pathways involved in sensitivity-enhanced HSQC experiments are managed selectively and detected separately in different FID periods within the same scan. The feasibility of this new SADA-HSQC (Spectral Aliasing in Dually Acquired HSQC) technique is demonstrated by recording simultaneously two complementary datasets, conventional and highly-resolved spectral-aliased 2D HSQC spectra, in a single NMR experiment. Combining the information from both datasets, accurate chemical shift determination and excellent signal dispersion is achieved in a unique measurement using only few t1 increments.
Bergemann, N., C. Pistidda, C. Milanese, M. Aramini, S. Huotari, P. Nolis, A. Santoru, M. R. Chierotti, A.-L. Chaudhary, M. D. Baro, T. Klassen & M. Dornheim. 2018. A hydride composite featuring mutual destabilisation and reversible boron exchange: Ca(BH4)2–Mg2NiH4. Journal of Materials Chemistry A 6(37). 17929–17946. DOI: 10.1039/c8ta04748k
The system Ca(BH4)2–Mg2NiH4 is used as a model to prove the unique possibility to fully reverse the borohydride decomposition process even in cases where the decomposition reaction leads to undesired stable boron containing species (boron sinks). The formation of MgNi2.5B2 directly from Ca(BH4)2 or from CaB12H12 and amorphous boron allows an unexpectedly easy transfer of the boron atoms to reversibly form Ca(BH4)2 during rehydrogenation. In addition, to the best of our knowledge, the mutual destabilisation of the starting reactants is observed for the first time in Ca(BH4)2 based Reactive Hydride Composite (RHC) systems. A detailed account of dehydrogenation and rehydrogenation reaction mechanisms as the function of applied experimental conditions is given.
Karimi, Fahim, María V.C. Riglos, Antonio Santoru, Armin Hoell, Vikram S. Raghuwanshi, Chiara Milanese, Nils Bergemann, Claudio Pistidda, Pau Nolis , Maria D. Baro, Gökhan Gizer, Thi-Thu Le, P. Klaus Pranzas, Martin Dornheim, Thomas Klassen, Andreas Schrey & Julián Puszkiel. 2018. In Situ Formation of TiB2 Nanoparticles for Enhanced Dehydrogenation / Hydrogenation Reaction Kinetics of LiBH4–MgH2 as a Reversible Solid-State Hydrogen Storage Composite System. The Journal of Physical Chemistry C 122(22). 11671–11681. DOI: 10.1021/acs.jpcc.8b02258
To enhance the dehydrogenation/rehydrogenation kinetic behavior of the LiBH4–MgH2 composite system, TiF4 is used as an additive. The effect of this additive on the hydride composite system has been studied by means of laboratory and advanced synchrotron techniques. Investigations on the synthesis and mechanism upon hydrogen interaction show that the addition of TiF4 to the LiBH4–MgH2 composite system during the milling procedure leads to the in situ formation of well-distributed nanosized TiB2 particles. These TiB2 nanoparticles act as nucleation agents for the formation of MgB2 upon dehydrogenation process of the hydride composite system. The effect of TiB2 nanoparticles is maintained upon cycling.
Magnetic Resonance in Chemistry 2017 (DOI: 10.1002/mrc.4695)
A 1H-1H TOCSY experiment incorporating 13C multiplicity information is proposed. In addition, broadband 1H homodecoupling in the indirect dimension can be implemented using a perfect BIRD module that affords exclusive 1H chemical shift evolution with full decoupling of all heteronuclear and homonuclear (including 2JHH) coupling constants. As a complement to the normal TOCSY and the recent PSYCHE-TOCSY experiments, this novel multiplicity-edited TOCSY experiment distinguishes between CH/CH3 (phased up) and CH2 (phased down) cross-peaks which facilitates resonance analysis and assignment.
Stereoselectivity of Proline / Cyclobutane Amino Acid-Containing Peptide Organocatalysts for Asymmetric Aldol Additions: a Rationale
Ona Illa, Oriol Porcar-Tost, Carme Robledillo, Carlos Elvira, Pau Nolis, Oliver Reiser, Vicenç Branchadell, and Rosa M. Ortuño
J. Org. Chem., Just Accepted Manuscript
Publication Date (Web): November 29, 2017
Several α,β,α- or α,γ,α-tripeptides, consisting of a central cyclobutane β- or γ-amino acid being flanked by two (D)- or (L)-proline residues, have been synthesized and tested as organocatalysts in asymmetric aldol additions. High yields and enantioselectivities have been achieved with α,γ,α-tripeptides, being superior to the peptides containing a cyclobutane β-amino acid residue. This can probably be due to their high rigidity, which hinders the peptide catalysts to adopt the proper active conformation. This reasoning correlates with the major conformation of the peptides in the ground state, as suggested by 1H NMR and computational calculations. The configuration of the aldol products is controlled by the proline chirality, and consequently, the R/S configuration of aldol products can be tuned by the use of either commercially available (D)- or (L)-proline enantiomers. The enantioselectivity in the aldol reactions is reversed if the reactions are carried out in the presence of water or other protic solvents such as methanol. Spectroscopic and theoretical investigations revealed that this effect is not the consequence of conformational changes in the catalyst but rather caused by the participation of a water molecule in the rate determining transition state, in such a way that the preferential nucleophilic attack is oriented to the opposite enantiotopic aldehyde face.
Four new series of diastereomeric β,γ-di- and β,γ-tetrapeptides derived from conveniently protected (1R,2S)- and (1S,2S)-2-aminocyclobutane-1-carboxylic acid and cis- and trans-γ-amino-l-proline joined in alternation have been synthesized. High resolution NMR experiments show that peptides containing trans-cyclobutane amino acid residues adopt a more folded structure in solution than those containing a cis-cyclobutane residue, which adopt a strand-like structure. The cis/trans relative configuration of the cyclobutane residue is the origin of the folding pattern of each peptide due to either intra- or inter-residue hydrogen-bonded ring formation, whereas the cis/trans isomerism of the γ-amino-l-proline residue does not have a significantly relevant role on the folding ability of these peptides.