(HEED) Pulse Sequences

Coupling constants V (I5N 13CR) and V (15N 'H R) in 2-substituted pyridines [R = Me (1), CH = CH2 (2), C = C H (3), C (0)H (4), C (0)M e (5)] have been measured by using 7/ahn-echo extended (H EED) pulse sequences for one(ID ) and two-dimensional (2D ) l3C/'H N M R (HEEDINEPT, HEED-HETCOR). The magnitude of | 2/ ( ,5N 13Cr)| is hardly affected by the hybridiza­ tion o f 13Cr . 15N N M R spectra, measured under conditions o f ultra high resolution (UHR) con­ firm the values 2/ ( 15N 13CR). 2D 13C /‘H HEEDHETCOR experiments show that the sign of 3/ ( 15N ‘H r) is negative in 1, whereas the coupling constants V (15N 'H R) in 4 and 4/ ( 15N 'H R) in 3 have a positive sign. is no other source of signal broadening, the trans­ verse X-magnetization connected with X 15N coupling is little affected during the time of the Hahn-echo, whereas the X 14N magnetization has decayed to a considerable extend. Depending on the length of the Hahn-echo delay and on the mag­ nitude of scalar X 14N coupling, the parent line can be completely or at least partially suppressed. This allows to take advantage of the full dynamic range of the spectrometer and to distinguish the 15N satellites in order to measure coupling con­ stants 7(I5NX) as well as isotope induced shifts J I5/I4N(X), all at natural abundance of 15N and X (assuming higher NMR sensitivity of X than 15N). Furthermore, two-dimensional (2D) X /’H HEED heteronuclear shift correlations (HEED-HET­ COR) allow to measure long range coupling con­ stants V (I5N'H) and to determine relative signs of coupling constants / ( 15NX) and ./(^N’H). In this work, we have used HEED pulse se­ quences for the first time to measure two-bond coupling constants 2/ ( 15N 13CR) in the 2-substituted pyridines C5H4N-2-R (1 to 5). These were selected to compare the influence of different groups R and the influence of the lone pair of electrons at nitro­ gen [2,3] on the magnitude of the geminal coupling constants |2/ ( 15N 13CR) | .