IMPACT-HMBC for Bruker (Julien Furrer)

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Basic facts about IMPACT

Synopsis: IMPACT-HMBC

Family: 2D HMBC

Code authors:  Julien Furrer

Copyright: University of Neuchatel, Julien Furrer

Introduction

NOTE:The pulse sequence is not yet available for download, but the author has been contacted and we expect to publish it in the future.

The IMPACT-HMBC (improved and accelerated constant-time heteronuclear multiple-bond correlation) is a pulse sequence developed by Julien Furrer[1]. It is a variation of 2D HMBC technique where it is possible to obtain a high quality 2D spectrum with minimal artifacts, enhanced sensivity and less time consuming.

The pulse sequence combines tree-fold low pass J filter for a better supression of ¹J_CH with constant time (CT) to improve signal shape and ASAP method for quick recover delay. Another advantage of this HMBC is in fact the transformation of an broadband experiment in a semiselective experiment changing only the shaped pulse (spnam7).

Delays

d1 - recovery delay (1-5 x T1) (usually 0.2 s due to ASAP sequence)

d6 - Calculed autmatically based in cnst13 - evolution of long range coupling.

d9 - mixing time for ASAP method (recommended 40 ms for DIPSI-2 mixing time)

d16 - gradient recovery delay (depends on the probe) may be on the order of 100-200 μs.

d20 - delay for constant time period

Acquisition in the indirect dimension and number of scans

The incremented delay used to record evolution in the indirect dimension is 2xd0. d0 is automatically calculated within the experiment using in0.

in0 equals 1/(2*SW) one half of the inverse of the spectral window width in X dimension. (1/2) is due to nd0=2 (below).

td1 - number of experiments to record in the X dimension

nd0 = 2 - there are two delays d0 within the X dimension evolution delay.

ns - number of scans must be a multiple of 2.

ds - number od dummy scans must be 16

Carrier frequencies

O1 (or O1P for the value in ppm) - in the middle of ¹H spectrum.

O2 (or O2P for the value in ppm) - in the middle of ¹³C spectrum.

Hard RF pulses

¹H channel

p1 - hard ¹H at power level pl1 - Length of 180^o pulse (p2) is calculated automatically.

p0 - hard ¹H for Enerst angle. By default, can be used an 90^o pulse angle. For improve sensivity, this value can be optimized utilized the 1D version of the sequence. This values can varies from 90^o to 120-130 pulse angle depending on the substance being analyzed.

p6 - ¹H pulse for ASAP method (recommended - 50 μs)

X-nucleus channel

p3 - hard X-nucleus 90^o at power level pl2.

Shaped 180 pulses

This sequence has one shaped 180 pulses at the X-nucleus channel - p24

p24 - is a refocusing pulse at power level sp7.

spnam7 must be set to the name of the waveform file (Crp60comp.4 for a broadband excitation, G3 for band selective experiment or gaus1.1000 for selective 1D experiment).

p24 of 2 ms is recommended for spnam7 set to Crp60comp.4.

If waveform Crp60comp.4, G3 or gaus1.1000 are not available on the system they need to be prepared using the stdisp tool or an appropriate replacement used instead.

Gradient pulses

Duration of all gradients is set by:

• p16 (recommended default - 1 ms)

Shapes of all gradients are recommended to be sine bell implemented in 100 steps, i.e.:

• gpnam1 SINE.100
• gpnam2 SINE.100
• gpnam3 SINE.100
• gpnam4 SINE.100
• gpnam5 SINE.100
• gpnam6 SINE.100

Strengths of gradients are set by parameters:

• gp1:gp2:gp3:gp4:gp5:gp6 - 80:37:-19:-11:-7:-17

Setup Constants

• cnst6 - minimal coupling constant for ¹J_CH (recommended - 120 Hz)

• cnst7 - maximum coupling constant for ¹J_CH (recommended - 170 Hz)

• cnst13 - coupling constant for long range J_CH (recommended - 8 Hz)

Processing

Set PH_mod (F1) to pk or no

Process using xfb and xf2m

References

1. Furrer, J. A robust, sensitive, and versatile HMBC experiment for rapid structure elucidation by NMR: IMPACT-HMBC. Chemical Communications 46:3396--3398, 2010. BibTeX [impact-hmbc]