i-HMBC : Identifying two bond correlations

The HMBC experiment provides ¹H-¹³C correlations over multiple bonds and is essential for assigning small molecules. The main drawback to the HMBC is that it does not identify the length of the correlation and in some cases may be ambiguous. A recent publication describes a method, called i-HMBC, for distinguishing two bond HMBC correlations from longer ones.

NUS for HMBC and LR-HSQMBC experiments

Non Uniform Sampling (NUS) of multidimensional NMR data can greatly reduce the time taken to record a spectrum by recording only a subset of the normal data. A variety of algorithms are available to reconstruct the omitted data based on the data that was recorded. The most commonly used algorithm is Iterative Soft Thresholding (IST). Most implementations of the IST algorithm rely on the peaks in the detected dimension being phased correctly and positive. For most modern experiments this is not a problem, but in the HMBC experiment it is not possible to phase the peaks. For this reason I have not recommended using NUS with HMBCs. The LR-HSQMBC experiment, however, can be phased and I recommend using NUS with it. In this post, spectra recorded with different levels of NUS were recorded to determine how NUS affects HMBC and LR-HSQMBC experiments.

Testing the NOAH BSC sequence

One of the many recent innovations to make acquiring NMR data faster is the NOAH technique. NOAH concatenates several pulse sequences behind a single relaxation delay to reduce the total acquisition time. Various combinations of experiments are available, but perhaps the most useful is the NOAH_BSC variant which provides a gCOSY, ¹³C HSQC and ¹³C HMBC. In this post the NOAH_BSC spectra are compared with those collected using the standard Facility parameters.

Constant time

Collecting multi-dimensional spectra requires recording chemical shift information via the use of an incremented delay. Since the length of the delay increases over the course of the experiment the signal is recorded at different times for each increment. This allows coupling in the indirect dimension to evolve, which broadens the peaks and decreases resolution. The "constant time" method was developed to eliminate this problem.

Magnitude processing

Most modern NMR experiments are run in a phase sensitive mode, which means that after fourier transformation the peaks require phase correction to become all positive. The HMBC experiment, however, generates signals that cannot all be phased simultaneously. The commonly used solution is magnitude processing.

15N HMBC sensitivity

The HMBC experiment is normally used to detect ¹H-¹³C correlations, but it can also be used for ¹⁵N. An ¹⁵N HMBC experiment may provide structural information that cannot be obtained from a ¹³C HMBC, and it can also be used to determine the number of nitrogen atoms present in a compound. Recently I was asked, "How does the sensitivity of an ¹⁵N HMBC compare with that of a ¹³C HMBC?", so I decided to try and find out.

Single bond correlations in HMBC spectra

HMBC spectra are designed to show correlations between hydrogens and heteronuclei separated by two or more bonds, however, single bond correlations are often present in HMBC spectra as well. Like the multiple bond correlations in HSQC spectra discussed in the previous post, single bond correlations in HMBC spectra can be confusing if you are not expecting them, but once aware that they may be present they are fairly easy to identify.

ASAP - Acceleration by Sharing Adjacent Polarization

NMR is a non destructive technique and so reducing acquisition time can be used either to increase sample throughput, or to lower the detection limit. To lower the detection limit a given amount of time is used to acquire a greater number of scans than would normally be acquired. A relatively recent method for reducing acquisition time is ASAP (Acceleration by Sharing Adjacent Polarization)¹. The ASAP technique speeds acquisition by greatly reducing the length of the relaxation delay.

Sensitivity in HMBC and LR-HSQMBC experiments

Long range correlations are critical for structure elucidation. They are typically identified using HMBC experiments¹, but a relatively recent alternative is the LR-HSQMBC experiment². Recent attempts to confirm the structure of several challenging molecules in the Facility prompted an examination of the sensitivity of these two experiments to determine which is most likely to provide the crucial long range correlations.