Nomenclature, pulse sequence

There is no standard pulse sequence nomenclature so that the system adopted in this book is a combination of the acronyms used in literature and the relevant adjectives to highlight the differences between similar experiments. A pulse sequence name must convey the following information  

The directly detected nucleus is given as the last nucleus so the expressions IH COSY indicates a detected experiment. 

The complete expression or the abbreviations ps phase cycled or gs gradient selected. 

Well-known sequence elements like DEPT, TOCSY or BIRD are added without any further comment, 

The quadrature detection mode is given as a suffix to the main sequence name me magnitude calculated, TPPI time proportional phase increment, E/A Echo / Antiecho The term selective is reserved exclusively for sequences using selective pulses. If selectivity is achieved using other methods this is defined using a different term. 

Virtually all NMR experiments can described in terms of a pulse sequence, which, as the name suggests, is a notation which describes the series of radiofrequency (rf) or field-gradient pulses used to manipulate nuclear spins and so tailor the experiment to provide the desired information. Over the years, a largely (although not completely) standard pictorial format has evolved for representing these sequences, not unlike the way a musical score is used to encode a symphony. As these crop up repeatedly throughout the text, the format and conventions used in this book deserve explanation. Only the definitions of the various 

90° high-power (hard) pulse 180° high-power (hard) pulse 

Low-power pulse or train of pulses Shaped low-power (soft) pulse Frequency swept (adiabatic) puise 

Through-bond interactions scalar (/) spin coupling via bonding electrons. 

Through-space interactions the NOE mediated through dipole-dipole coupling and spin 

H JcUll H- HCOSY Proton J-coupling typically over 2 or 3 bonds. 5 

H IJJ 1 X H-X HMQC H-X HSQC One-bond heteronuclear couplings with proton observation. 6 

J x x X-X COS Y X-X INAESQUATE COSY only used when X-spin natural abundance 20%. Sensitivity problems when X has low natural abundance. 5 

NOE H H ( 1 H- H NOE difference 1/2D NOE3 Y 1/2D RCEcY Through-space correlations. NOE difference only applicable to mid-sized molecules with masses ca. 1-2 kDa. 8 

NOE Ox H-X NOE difference 2D HOESY Sensitivity limited by X-spin observation. Care required to make NOEs specific in presence of proton decoupling. 8 

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Figure U. Pulse sequence nomenclature, (a) A complete pulse sequence and (b) the reduced representation used throughout the remainder of the book.

There are two approaches to pulse sequence classification depending on the user s occupation. For the chemist who has to solve a structural question or characterize a new compound it is the spectra obtained from the pulse sequence that is of primary importance. The NMR spectroscopist is usually more concerned with the pulse sequence structure and choice of experimental parameters and whether a particular pulse sequence can be improved or modified to solve a specific problem. These two different approaches lead to confusion in pulse sequence nomenclature such that names are often a combination of the purpose of the experiment and the sequence layout. For example the commonly used acronyms HMQC, HSQC and HMBC imply a consistent abbreviation system yet HMQC and HSQC describe the coherence state during the evolution time whilst HMBC denotes an experiment to correlate nuclei using multiple bond heteronuclear scalar coupling. 

A short remark about the relevance of HMQC and HMBC experiments and nomenclatures should be done here. Initially, Bax et al. [9] introduced the HMQC technique for specific editing of H- C pairs correlated by direct V( C, H) couplings. The HMBC technique was proposed subsequently by Bax and Summers [11] to edit specifically multiple bond correlations through "/( C, H) couplings (n = 2 and 3), which explains the HMBC acronym. From the point of view of the pulse sequence the introduction of the low-pass filter, which consists of a supplementary 90° pulse and an extended phase cycle, is... 

Hundreds of pulse sequences [5.1 - 5.4] have been developed and every month improvements and new combinations of sequence units are published. For a number of different reasons there is no systematic nomenclature for the naming of pulse sequences. A name based on the exact combination of sequence units used in a pulse sequence would be very unwieldy whilst spectrometer manufacturers have built up their own pulse sequences library using their own nomenclature system. In addition the number and type of characters in the name depends upon the spectrometer operating system. 

Reference to the decoupler channel(s) is often used when referring to these additional channels but this should not be taken too literally as they may only be used for the application of only a few pulses rather than a true decoupling sequence. This nomenclature stems from the early developments of NMR spectrometers when the additional channel was only capable of providing noise decoupling , usually of protons. 

The simple one-pulse experiment is shown below, the delay dl is called the relaxation delay and it ensures that in a multi-scan experiment the spin system has returned to equilibrium before the next pulse (see Check it 5.2.1.5). The delay dlO is not normally part of the ordinary sequence scheme and represents the pre-acquisition delay (Bruker nomenclature del, de2). This delay is automatically inserted to enable time for switching between transmit and receive mode to minimize pulse breakthrough. For further aspects the reader is referred to section S.2.3.4 Check its 3.2.3.4 and 3.2.3.6). 

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