V. Suni et al. / Journal of Molecular Structure 749 (2005) 177–182
181
1
protons is carried out with the help of H–1H correlation
data available from the COSY spectrum.
well-defined spin states of the nitrogen nucleus, to which the
proton responds by giving a triplet in the 1H NMR spectrum.
The singlets at appreciable downfield in the previous
reports were the result of rapid exchange and resultant
decoupling of the –NH protons, which is found to be absent
for the N3 and N4 protons in the present compound.
However, the N5H proton shows exchangeability at a low
rate and turns up as a broad peak at 7.33 ppm. At low –NH
exchange rates, 14N nucleus imparts moderate spin
relaxation resulting in an intermediate lifetime for the spin
states of nitrogen. Response of the –NH proton to these
three spin states which are changing at a moderate rate is
usually observed as a broad peak [26]. As the cyclohexyl
moiety is observed in the chair conformation, the axial and
equatorial hydrogens show slightly varied chemical shift
values with the equatorial protons observed at more
downfield (2.10–1.73 ppm) compared to their axial counter-
parts (1.42–1.22 ppm). However, the C13–H is deshielded
by the adjacent electronegative nitrogen resulting in a
multiplet at 4.21 ppm, which couples with the proton on N5.
Assignment of the broad peak to N5 proton is also
confirmed by the 1H–1H correlation experiments, since
coupling of the N5–H proton with the C13–H proton of the
cyclohexyl ring is clearly evident from the COSY.
Two sets of double doublets observed at 8.79 and
8.60 ppm for protons a to pyridyl nitrogens attribute
1
interesting features to the compound under study. The H
NMR spectrum of the present compound follows strictly
first order with well-separated chemical shift positions for
protons with different environments. For example, the
signal for for C1–H proton appears as a double doublet
around 8.60 ppm due to the following coupling pattern
observed between adjacent protons. At first, the proton at C1
couples with C2 proton, which splits the C1–H signal into a
doublet. But C1–H also couples with C3-H so that each line
of the C1–H is further split into two, giving a double doublet
[20]. Although doublets and double doublets are reported
previously [24,25], two well-defined sets of double doublets
corresponding to two pyridyl protons in identical chemical
environment are observed for the first time in a compound
prepared form di-2-pyridyl ketone.
More distinctive features are associated with the
resonance peaks of the protons on N3 and N4 atoms,
which appear as a triplet at 7.84 ppm corresponding to the
two protons of identical chemical environments. Contrary to
this, in all the previous observations of structurally related
compounds, a sharp singlet at appreciable downfield values
(d 13–15 ppm) is observed for the N3–H proton [22]. Hence,
in order to confirm the assignment of these triplets to –NH
protons, the 1H–13C correlation spectra were recorded.
4. Supplementary data
Absence of coupling of the triplets at 7.84 ppm with the 13
C
Crystallographic data for structural analysis has been
deposited with the Cambridge Crystallographic Data
Center, CCDC for compound HL. Copies of this infor-
mation maybe obtained free of charge from The Director,
CCDC 271996, 12 Union Road, Cambridge, CB2, IEZ, UK
peaks in the HMQC experiments (Fig. 5) authenticated its
assignment to –NH protons. The triplet is in accordance
with the 2IC1 splitting (IZ1 for 14N) of the resonance peak
of the proton attached to the 14N nucleus. The appreciable
electrical quadrupole moment at N3 and N4 nitrogens is
able to induce an efficient spin relaxation to observe three
Acknowledgements
The authors are thankful to Sophisticated Instruments
Facility, IISc Bangalore, India for NMR measurements.
DST IRPHA is kindly acknowledged for X-ray data
collection. V. Suni is thankful to the State Council for
Science, Technology and Environment, Kerala, India for
financial support in the form of a fellowship.
References
[1] T.H. Lowry, K.S. Richardson, Mechanism and Theory in Organic
Chemistry, third ed., Harper Collins, New York, 1987.
[2] A. Sreekanth, H.-K. Fun, M.R.P. Kurup, Inorg. Chem. Commun. 7
(2004) 1250.
[3] V. Philip, V. Suni, M.R.P. Kurup, Acta Crystallogr. C 60 (2004) o856.
[4] M. Joseph, V. Suni, M.R.P. Kurup, M. Nethaji, A. Kishore, S.G. Bhat,
Polyhedron 23 (2004) 3069.
Fig. 5. 1H–13C HMQC spectrum of HL.