HMBC‐1,1‐ADEQUATE via generalized indirect covariance: a high sensitivity alternative to n,1‐ADEQUATE

1,1‐ADEQUATE and the related long‐range 1,n‐ and n,1‐ADEQUATE variants were developed to provide an unequivocal means of establishing ²J_CH and the equivalent of ⁿJ_CH correlations where n = 3,4. Whereas the 1,1‐ and 1,n‐ADEQUATE experiments have two simultaneous evolution periods that refocus the chemical shift and afford net single quantum evolution for the carbon spins, the n,1‐variant has a single evolution period that leaves the carbon spin to be observed at the double quantum frequency. The n,1‐ADEQUATE experiment begins with an HMBC‐type ⁿJ_CH magnetization transfer, which leads to inherently lower sensitivity than the 1,1‐ and 1,n‐ADEQUATE experiments that begin with a ¹J_CH transfer. These attributes, in tandem, serve to render the n,1‐ADEQUATE experiment less generally applicable and more difficult to interpret than the 1,n‐ADEQUATE experiment, which can in principle afford the same structural information. Unsymmetrical and generalized indirect covariance processing methods can complement and enhance the structural information encoded in combinations of experiments e.g. HSQC‐1,1‐ or ?1,n‐ADEQUATE. Another benefit is that covariance processing methods offer the possibility of mathematically combining a higher sensitivity 2D NMR spectrum with for example 1,1‐ or 1,n‐ADEQUATE to improve access to the information content of lower sensitivity congeners. The covariance spectrum also provides a significant enhancement in the F₁ digital resolution. The combination of HMBC and 1,1‐ADEQUATE spectra is shown here using strychnine as a model compound to derive structural information inherent to an n,1‐ADEQUATE spectrum with higher sensitivity and in a more convenient to interpret single quantum presentation. Copyright © 2012 John Wiley & Sons, Ltd.     

Additional pitfalls of using 1,1‐ADEQUATE for structure elucidation

1,1‐ADEQUATE is a powerful and robust NMR experiment to establish carbon–carbon connectivities using modest sample quantities when cryogenic probe technology is available. Yet potential pitfalls of applying this method are not widely appreciated, such as weak or missing ¹J_CC correlations in strongly coupled ¹³C‐¹³C AB spin systems and unusually large multi‐bond (ⁿJ_CC) correlations associated with particular functional groups. These large ⁿJ_CC correlations observed in 1,1‐ADEQUATE spectra could be mistaken for ¹J_CC correlations. Copyright © 2016 John Wiley & Sons, Ltd.     

Observation of potentially troublesome 2JCC correlations in 1,1‐ADEQUATE spectra

Despite the tremendous usage of HMBC to establish long‐range ¹H–¹³C and ¹H–¹⁵N heteronuclear correlations, an inherent drawback of the experiment is the indeterminate nature of the ⁿJ_XH correlations afforded by the experiment. A priori there is no reliable way of determining whether a given ⁿJ_CH correlation is, for example, via two‐, three‐, or sometimes even four‐bonds. This limitation of the HMBC experiment spurred the development of the ADEQUATE family of NMR experiments that rely on, in the case of 1,1‐ADEQUATE, an out‐and‐back transfer of magnetization via the ¹J_CC homonuclear coupling constant, which is significantly larger than ⁿJ_CC (where n = 2–4) couplings in most cases. Hence, the 1,1‐ADEQUATE experiment has generally been assumed to unequivocally provide the equivalent of ²J_CH correlations. The recent development of the 1,1‐ and 1,n‐HD‐ADEQUATE experiments that can provide homodecoupling for certain ¹J_CC and ⁿJ_CC correlations has increased the sensitivity of the ADEQUATE experiments significantly and can allow acquisition of these data in a fraction of the time required for the original iterations of this pulse sequence. With these gains in sensitivity, however, there occasionally come unanticipated consequences. We have observed that the collapse of proton multiplets, in addition to providing better s/n for the desired ¹J_CC correlations can facilitate the observation of typically weaker ²J_CC correlations across intervening carbonyl resonances in 1,1‐HD‐ADEQUATE spectra. Several examples are shown, with the results supported by the measurement of the ²J_CC coupling constants in question using J‐modulated‐HD‐ADEQUATE and DFT calculations. Copyright © 2016 John Wiley & Sons, Ltd.     

HMBC‐1,n‐ADEQUATE spectra calculated from HMBC and 1,n‐ADEQUATE spectra

Unsymmetrical and generalized indirect covariance processing methods provide a means of mathematically combining pairs of 2D NMR spectra that share a common frequency domain to facilitate the extraction of correlation information. Previous reports have focused on the combination of HSQC spectra with 1,1‐, 1,n‐, and inverted ¹J_CC 1,n‐ADEQUATE spectra to afford carbon–carbon correlation spectra that allow the extraction of direct (¹J_CC), long‐range (ⁿJ_CC, where n ≥ 2), and ¹J_CC‐edited long‐range correlation data, respectively. Covariance processing of HMBC and 1,1‐ADEQUATE spectra has also recently been reported, allowing convenient, high‐sensitivity access to ⁿJ_CC correlation data equivalent to the much lower sensitivity n,1‐ADEQUATE experiment. Furthermore, HMBC‐1,1‐ADEQUATE correlations are observed in the F₁ frequency domain at the intrinsic chemical shift of the ¹³C resonance in question rather than at the double‐quantum frequency of the pair of correlated carbons, as visualized by the n,1, and m,n‐ADEQUATE experiments, greatly simplifying data interpretation. In an extension of previous work, the covariance processing of HMBC and 1,n‐ADEQUATE spectra is now reported. The resulting HMBC‐1,n‐ADEQUATE spectrum affords long‐range carbon–carbon correlation data equivalent to the very low sensitivity m,n‐ADEQUATE experiment. In addition to the significantly higher sensitivity of the covariance calculated spectrum, correlations in the HMBC‐1,n‐ADEQUATE spectrum are again detected at the intrinsic ¹³C chemical shifts of the correlated carbons rather than at the double‐quantum frequency of the pair of correlated carbons. HMBC‐1,n‐ADEQUATE spectra can provide correlations ranging from diagonal (⁰J_CC or diagonal correlations) to ⁴J_CC under normal circumstances to as much as ⁶J_CC in rare instances. The experiment affords the potential means of establishing the structures of severely proton‐deficient molecules. Copyright © 2013 John Wiley & Sons, Ltd.     

A broadband ADEQUATE pulse sequence using chirp pulses

Recently, we have introduced the ADEQUATE pulse sequence as a sensitive method to observe ¹³C,¹³C correlations in natural products. This kind of experiment suffers from offset‐dependent effects of the 180°(¹³C) pulses. Here we describe an application of smoothed chirp pulses in the ADEQUATE pulse sequence which allows ¹³C,¹³C correlations to be run without any offset dependences. This experiment is called chirp ADEQUATE and was applied to δ‐valerolactone and cholesteryl acetate. This modification will allow a general application of the ADEQUATE pulse sequence. Copyright © 2002 John Wiley & Sons, Ltd.     

Inversion of 1JCC correlations in 1,n‐ADEQUATE spectra

ADEQUATE experiments provide an alternative to the more commonly employed GHMBC experiment for the establishment of long‐range heteronuclear connectivities. The 1,1‐ADEQUATE experiment allows the unequivocal identification of both protonated and non‐protonated carbon resonances adjacent to a protonated carbon. The 1,n‐ADEQUATE experiment establishes correlations via an initial ¹J_CH heteronuclear transfer followed by an ⁿJ_CC out‐and‐back transfer, most typically, via three carbon–carbon bonds. Hence, the 1,n‐ADEQUATE experiment allows the equivalent of ⁴J_CH heteronuclear correlations to be probed when they are not observed in a GHMBC spectrum. Aside from the lower sensitivity of the 1,n‐ADEQUATE experiment relative to GHMBC experiments, the interpretation of the former is also complicated by the ‘leakage’ of ¹J_CC correlations into the spectrum that must be identified. A method for the inversion of ¹J_CC correlations to facilitate the interpretation of 1,n‐ADEQUATE spectra is presented that allows a single experiment to be performed to access ¹J_CC and ⁿJ_CC correlation information. Copyright © 2012 John Wiley & Sons, Ltd.     

SQ—SQ experiment for determination of relative signs of small nJ(29Si,13C) couplings in a wide variety of silicon compounds

A modification of double quantum–zero quantum (DQ—ZQ) experiment termed single‐quantum–single‐quantum (SQ—SQ) experiment is proposed for the determination of relative signs and magnitudes of coupling constants. The modification replaces the multiple‐quantum evolution period by two synchronously incremented single‐quantum periods. Similarly to DQ—ZQ experiment, the sequence requires only two coupling constants that share one nucleus, the one to be measured and a reference one. This allows application to a larger variety of molecular fragments than traditional 2D sequences producing E.COSY or TROSY pattern. The SQ—SQ experiment eliminates the effects of some other couplings during t₁, thereby simplifying the 2D pattern and increasing the signal intensity in comparison with DQ—ZQ experiment. The presented sequence is particularly designed for the determination of silicon–carbon coupling constants across several bonds at natural abundance using silicon–hydrogen couplings as the sign reference. The signs of silicon–carbon couplings across two and three bonds in dimethyl(phenoxy)silane which cannot be detected by traditional methods and which have not yet been determined are established by the SQ—SQ method here: ²J(Si,C) = +2.2 Hz and ³J(Si,C) = ?1.7 Hz. Copyright © 2009 John Wiley & Sons, Ltd.     

HSQC-ADEQUATE: an investigation of data requirements

Utilizing ¹³C‐¹³C connectivity networks for the assembly of carbon skeletons from HSQC‐ADEQUATE spectra was recently reported. HSQC‐ADEQUATE data retain the resonance multiplicity information of the multiplicity‐edited GHSQC spectrum and afford a significant improvement in the signal‐to‐noise (s/n) ratio relative to the 1,1‐ADEQUATE data used in the calculation of the HSQC‐ADEQUATE spectrum by unsymmetrical indirect covariance (UIC) processing methods. The initial investigation into the computation of HSQC‐ADEQUATE correlation plots utilized overnight acquisition of the 1,1‐ADEQUATE data used for the calculation. In this communication, we report the results of an investigation of the reduction in acquisition time for the 1,1‐ADEQUATE data to take advantage of the s/n gain during the UIC processing to afford the final HSQC‐ADEQUATE correlation plot. Data acquisition times for the 1,1‐ADEQUATE spectrum can be reduced to as little as a few hours, while retaining excellent s/n ratios and all responses contained in spectra computed from overnight data acquisitions. Concatenation of multiplicity‐edited GHSQC and 1,1‐ADEQUATE data also allows the interrogation of submilligram samples with 1,1‐ADEQUATE data when using spectrometers equipped with 1.7‐mm Micro CryoProbes ?. Copyright © 2011 John Wiley & Sons, Ltd.     

1JCC‐edited HSQC‐1,n‐ADEQUATE: a new paradigm for simultaneous direct and long‐range carbon–carbon correlation

Establishing the carbon skeleton of a molecule greatly facilitates the process of structure elucidation, leaving only heteroatoms to be inserted, heterocyclic rings to be closed, and stereochemical features to be defined. INADEQUATE, and more recently PANACEA, have been the only means of coming close to the goal of totally defining the carbon skeleton of a molecule. Unfortunately, the extremely low sensitivity and prodigious sample requirements of these experiments and the multiple receiver requirement for the latter experiment have severely restricted the usage of these experiments. Proton‐detected ADEQUATE experiments, in contrast, have considerably higher sensitivity and more modest sample requirements. By combining experiments such as 1,1‐ADEQUATE and 1,n‐ADEQUATE with higher sensitivity experiments such as GHSQC through covariance processing, sample requirements can be further reduced with a commensurate improvement in the s/n ratio and F₁ resolution of the covariance processed spectrum. We now wish to report the covariance processing of an inverted ¹J_CC 1,n‐ADEQUATE experiment with a non‐edited GHSQC spectrum to afford a spectrum that can trace the carbon skeleton of a molecule with the exception of correlations between quaternary carbons. Copyright © 2012 John Wiley & Sons, Ltd.     

1H Indirect detected 13C Low-Abundance Single-transition correlation Spectroscopy (HICLASS)-13C homonuclear correlation at natural abundance

A novel proton-detected (13)C homonuclear correlation experiment is reported at natural abundance, viz., (1)H Indirect detected (13)C Low-Abundance Single-transition correlation Spectroscopy (HICLASS). HICLASS is based on the evolution of (13)C single-quantum single transitions, followed by their mixing, and (1)H detection subsequent to heteronuclear transfer. Reduced relaxation losses during the evolution time and partial selectivity in the (1)H multiplet structure result in enhanced sensitivity of HICLASS. The superior performance of HICLASS is demonstrated for (1)H-detected (13)C correlation work.     

NMR studies on 1,2‐dihydro‐2‐(4‐aminophenyl)‐4‐[4‐(4‐aminophenoxy)‐4‐phenyl]‐(2H)phthalazin‐1‐one

An unsymmetrical heterocyclic diamine, 1,2‐dihydro‐2‐(4‐aminophenyl)‐4‐[4‐(4‐aminophenoxy)‐4‐phenyl]‐(2H)phthalazin‐1‐one, was synthesized. Its ¹H and ¹³C NMR spectra were completely assigned by utilizing the two‐dimensional heteronuclear ¹³C–¹H multiple‐bond coherence (HMBC) spectroscopy, and heteronuclear ¹³C–¹H one‐bond correlation spectroscopy, homonuclear shift correlation spectroscopy (H,H‐COSY) and rotating frame Overhauser enhancement spectroscopy (ROESY). The structure of the compound was shown to be the phthalazinone rather than the phthalazine ether from cross peaks and chemical shifts of the protons. Copyright © 2002 John Wiley & Sons, Ltd.     

Observation of untoward 3Jcc correlations in 1,1‐ADEQUATE spectra of pyrimidine analogs: Avoiding potential interpretation pitfalls

Recently, it has been reported that large ⁿJ_CC correlations can sometimes be observed in 1,1‐ADEQUATE spectra with significant intensity, which opens the possibility of structural misassignment. In this work, we have focused on pyrimidine‐based compounds, which exhibit multiple bond correlations in the 1,1‐ADEQUATE experiment as a consequence of ³J_CC coupling constants greater than 10 Hz. Results are supported by both the experimental measurement of ³J_CC coupling constants in question using J‐modulated‐ADEQUATE and density functional theory calculations.     

3D Heteronuclear long‐range 1H–13C scalar correlation at natural abundance: application to oligosaccharide analysis

A 3D ¹H–¹³C–¹H refocused INEPT transfer experiment is proposed in which the initial coherence transfer of ¹H longitudinal to ¹³C transverse magnetization is tuned to the long‐range ¹H, ¹³C couplings while the reverse INEPT component transfers the magnetization to the directly bonded ¹H. Integration of a constant time ¹H evolution period into the long‐range coherence transfer interval provides absorption mode signals for each dimension. A ¹³C purge component at the beginning of the sequence selects for ¹²C‐bound ¹H magnetization that is then transferred to a ¹³C‐bound hydrogen, thus strongly suppressing the diagonal signals. This experiment is expected to be of particular value for situations in which resonance overlap in the ¹³C dimension renders 2D long‐range heteronuclear correlation data ambiguous. In combination with a diagonal‐suppressed 3D ¹H–¹³C–¹H TOCSY‐HSQC experiment, complete assignment of the ring resonances of the Lewis‐b hexasaccharide was obtained on a 4.2 mM sample using a conventional 500 MHz probe (0.1% ethylbenzene signal‐to‐noise ratio of 600), suggesting its applicability to sub‐millimolar samples using cryoprobe technology. Copyright © 2002 John Wiley & Sons, Ltd.     

(119/117/115)Sn low-abundance single-transition correlation spectroscopy (LASSY): sensitivity-enhanced homonuclear correlation experiments for Sn NMR

We report the implementation of our novel rare-spin homonuclear correlation experiment, namely, Low-Abundance Single-transition correlation SpectroscopY (LASSY), for (119/117/115)Sn NMR at natural abundance. Our pulse sequence results in diagonal suppressed COSY-style display and outperforms the optimal homonuclear correlation experiment for rare spins, which involves double quantum evolution (INADEQUATE CR). The new experiment maximizes efficiency both in respect of pulse transformations as well as relaxation effects, and gives rise to a simplified two-dimensional (2D) spectrum with considerably reduced crowding, exhibiting only one transition in each cross peak, instead of four. Performance optimization of LASSY is carried out in light of the relatively 'large' line widths typical of Sn NMR in solution state. The superior performance of the sequence is demonstrated on dimeric tetraorganodistannoxane samples.     

Combination of DQ and ZQ Coherences for Sensitive Through‐Bond NMR Correlation Experiments in Biosolids under Ultra‐Fast MAS

A double‐zero quantum (DZQ)‐refocused INADEQUATE experiment is introduced for J‐based NMR correlations under ultra‐fast (60 kHz) magic angle spinning (MAS). The experiment records two spectra in the same dataset, a double quantum–single quantum (DQ‐SQ) and zero quantum–single quantum (ZQ‐SQ) spectrum, whereby the corresponding signals appear at different chemical shifts in ω₁. Furthermore, the spin‐state selective excitation (S³E) J‐decoupling block is incorporated in place of the second refocusing echo of the INADEQUATE scheme, providing an additional gain in sensitivity and resolution. The two sub‐spectra acquired in this way can be treated separately by a shearing transformation, producing two diagonal‐free single quantum (SQ‐SQ)‐type spectra, which are subsequently recombined to give an additional sensitivity enhancement, thus offering an improvement greater than a factor of two as compared to the original refocused INADEQUATE experiment. The combined DZQ scheme retains transverse magnetization on the initially polarized (I) spin, which typically exhibits a longer transverse dephasing time (T₂′) than its through‐bond neighbour (S). By doing so, less magnetization is lost during the refocusing periods in the sequence to give even further gains in sensitivity for the J correlations. The experiment is demonstrated for the correlation between the carbonyl (CO) and alpha (CA) carbons in a microcrystalline sample of fully protonated, [¹⁵N,¹³C]‐labelled Cu^II, Zn^II superoxide dismutase, and its efficiency is discussed with respect to other J‐based schemes.     

Deuterium MAS NMR Studies of Dynamics on Multiple Timescales: Histidine and Oxalic Acid

Deuterium (²H) magic‐angle spinning (MAS) nuclear magnetic resonance is applied to monitor the dynamics of the exchanging labile deuterons of polycrystalline L ‐histidine hydrochloride monohydrate‐d₇ and α‐oxalic acid dihydrate‐d₆. Direct experimental evidence of fast dynamics is obtained from T_1Z and T_1Q measurements. Further motional information is extracted from two‐dimensional single‐quantum (SQ) and double‐quantum (DQ) MAS spectra. Differences between the SQ and DQ linewidths clearly indicate the presence of motions on intermediate timescales for the carboxylic moiety and the D₂O in α‐oxalic acid dihydrate, and for the amine group and the D₂O in L ‐histidine hydrochloride monohydrate. Comparison of the relaxation rate constants of Zeeman and quadrupolar order with the relaxation rate constants of the DQ coherences suggests the co‐existence of fast and slow motional processes.     

Structural determination of ε‐lactams by 1H and 13C NMR

The thermodynamic products (ε‐lactams) of the degradation of ten different spirocyclic oxaziridines were analyzed by ¹H and ¹³C NMR spectroscopy. The preferred conformations were determined by examining the homonuclear spin–spin coupling constant and the chemical shift effects of the N‐substituent and the alkyl group of the aliphatic ring on ¹H and ¹³C NMR spectra. Copyright © 2009 John Wiley & Sons, Ltd.     

Synthesis of rhodium(III) complexes with tris/tetrakis‐benzimidazoles and benzothiazoles—quick identification of cyclometallation by nuclear magnetic resonance spectroscopy

Reactions of rhodium(III) halides with multidentate N,S‐heterocycles, (LH₃) 1,3,5‐tris(benzimidazolyl)benzene (L¹H₃; 1 ), 1,3,5‐tris(N‐methylbenzimidazolyl) benzene (L²H₃; 2 ) and 1,3,5‐tris(benzothiazolyl)benzene (L³H₃; 3 ), in the molar ratio 1:1 in methanol–chloroform produced mononuclear cyclometallated products of the composition [RhX₂(LH₂)(H₂O)] (X = Cl, Br, I; LH₂ = L¹H₂, L²H₂, L³H₂). When the metal to ligand ( 1–3 or 1,2,4,5‐tetrakis(benzothiazolyl)benzene [L⁴H₂; 4 ]) molar ratio was 2:1, the reactions yielded binuclear complexes of the compositions [Rh₂Cl₅(LH₂)(H₂O)₃] (LH₂ = L¹H₂, L²H₂, L³H₂) and [Rh₂X₄(L⁴)(H₂O)₂] (X = Cl, Br, I). Elemental analysis, IR and ¹H nuclear magnetic resonance (NMR) chemical shifts supported the binuclear nature of the complexes. Cyclometallation was detected by conventional ¹³C NMR spectra that showed a doublet around ～190 ppm. Cyclometallation was also detected by gradient‐enhanced heteronuclear multiple bond correlation (g‐HMBC) experiment that showed cross‐peaks between the cyclometallated carbon and the central benzene ring protons of 1–3 . Cyclometallation was substantiated by two‐dimensional ¹H? ¹H correlated experiments (gradiant‐correlation spectroscopy and rotating frame Overhauser effect spectroscopy) and ¹H? ¹³C single bond correlated two‐dimensional NMR experiments (gradient‐enhanced heteronuclear single quantum coherence). The ¹H? ¹⁵N g‐HMBC experiment suggested the coordination of the heterocycles to the metal ion via tertiary nitrogen. Copyright © 2009 John Wiley & Sons, Ltd.     

1H and 13C NMR signal assignments of carboxyl‐linked glucosides of bile acids

Complete ¹H and ¹³C resonance assignments were carried out for a new type of carboxyl‐linked glucosides of chenodeoxycholic (3α,7α‐dihydroxy‐5β‐cholan‐24‐oic) and hyodeoxycholic (3α,6α‐dihydroxy‐5β‐cholan‐24‐oic) acids by using several homonuclear (¹H–¹H) and heteronuclear (¹H–¹³C) 2D NMR techniques. Differences in the ¹H and ¹³C resonances between the α‐ and β‐anomers of the ester glucosides of bile acids were clarified for the first time. A comparison of the ¹H and ¹³C signal shifts induced by β‐D ‐glucosidation at the 24‐carboxyl and 3α‐hydroxyl groups in the parent 5β‐cholanoic acid was also made. Copyright © 2003 John Wiley & Sons, Ltd.     

Coherence transfer delay optimisation in PSYCOSY experiments

PSYCOSY is an f1 broadband homonuclear decoupled version of the COSY nuclear magnetic resonance pulse sequence. Here, we investigate by a combination of experimental measurements, spatially distributed spin dynamics simulations, and analytical predictions the coherence evolution delay necessary in PSYCOSY experiments to ensure intensity discrimination in favour of the correlations typically arising from short range (ⁿJ, n ≤ 3) ¹H–¹H couplings and show that, in general, a coherence evolution delay of around 35 ms is optimum.