Establishing Spectral Windows

A fundamental consideration of any long-range heteronuclear shift correla- 

One of the fundamental approaches to acquiring heteronuclear 2D NMR data for unknown molecules in many laboratories is to acquire data using survey spectrum conditions. For survey conditions, pre-established spectral windows are routinely used in both frequency domains. In the case of HSQC and HMBC experi- 

Further, the Fi spectral windows generally used to acquire survey spectra 

For the chemist attempting to elucidate a chemical structure, the N chemical shift is a sensitive probe of the nitrogen environment. For the purpose of structure verification, a common approach is to review the literature for related species and use their chemical shifts and couplings as models to allow estimates of these properties for the new species. While there are a number of texts, reviews, and publications available that have brought together the spectral properties of tens to hundreds of molecules, these paper-based collections are cumbersome to use when it comes to searching for a particular chemical shift or a chemical structure or 

The database is updated on an annual basis with new data extracted from the literature. This database is also the foundation of data supporting the prediction algorithms that are required to predict NMR spectral properties for chemical structures not contained within the database. 

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As an example of a more dispersed system, H 2Q- and 3Q-HoMQC spectra of the cyclic antibiotic primycin [30] recorded at 800 MHz [31] are shown in fig. 6. This molecule has a sequence of alternating CH(OR) and CH2 groups, establishing correlations both within, and between the two extreme regions of the spectrum. The spectral windows could be still chosen to be equal with no serious overlap introduced on the final spectrum. 

The determination of the charge, and thereby the valency, of a boron atom in an organic compound is usually straightforward if its "B NMR chemical shift within the 300+ ppm spectral window is compared to that of a close standard with a firmly established solution stmcture. But, there is a need for caution The structure of many boron-containing compounds depends on the nature of the solvent, and so multisolvent (i.e., aprotic vs. protic) analyses are often essential for a definitive characterization. Sadly, aqueous solution "B NMR spectral analyses are seldom reported—even, surprisingly, for compounds clearly prepared for their potential biological value. 

The last equation tells us what value of the dwell time we have to use to establish a particular spectral width. In practice, the user enters a value for SW and the computer calculates DW and sets up the ADC to digitize at that rate. It is important to understand that with the simultaneous (Varian-type) acquisition mode, there is a wait of 2 x DW between acquisition of successive pairs of data points. The average time to acquire a data point (DW) is the total time to acquire a data set divided by the number of data points acquired whether they are acquired simultaneously or alternately. The spectral window is fixed once the sampling rate and the reference frequency have been set up. The spectral window must not be confused with the display window, which is simply an expansion of the acquired spectrum displayed on the computer screen or printed on a paper spectrum (Fig. 3.15, bottom). The display window can be changed at will but the spectral window is fixed once the acquisition is started. 

We have also attempted to highlight the value of N chemical shift calculations for establishing Fj spectral windows when performing long-range heteronuc-... 

When it is impossible to select analyte lines that are completely free from spectral overlap by concomitant spectral lines, array detectors may be used to reduce the adverse effect of such spectral interferences. To accomplish this task, the spectrum of the known concomitant is first measured and stored. One or more concomitant lines that are known to be located in an interference free zone within the measured spectral window are then used as internal reference lines. The relative intensities of these lines in the sample spectrum are then used to establish a correction factor by which the stored concomitant spectrum is multiplied and subtracted from the sample spectrum. This correction procedure can be performed for each known concomitant present in the sample. A prior knowledge of the concentration of each concomitant is not required. 

Previous work by these authors suggests that using the NNMR program to calculate expected N chemical shifts of a potential molecular structure with which an investigator has no experience and then adding a buffer of 20 ppm to the calculated range of potential chemical shifts is a reasonable way to establish a starting Fj spectral window in most cases [30]. An example of the value of this approach is seen with the simple alkaloid harmaline (5). 

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