Report No xxxx - Instytut Fizyki JÄ drowej PAN

ESTIMATION OF h2 J PH AND 2 J PH SCALAR COUPLINGS
BY DFT/FPT METHOD TO RATIONALISE TWO DISTINCT
NMR OBSERVATIONS
Ryszard B. Nazarski
Department of Organic Chemistry, Institute of Chemistry, University of Łódź,
90-950 Łódź 1, P.O. Box 376, Poland. E-mail: rynaz@chemul.uni.lodz.pl
Four years ago, our report appeared [1] about first probable observation of two-bond scalar
31 P− 1 H coupling constants J across the N−H···O − −P + intramolecular hydrogen bond (H
bond) in the Z forms of four O,O-dialkyl 1-oxoalkanephosphonate hydrazones 1 (R = Me, t-
Bu, CH 2 Ph, Ph; J PH 2.9 ± 0.5 Hz, from 1 H and/or 31 P proton-coupled NMR spectra), investigated
previously by NMR [2]. Our work was subsequently followed by two experimental
findings of similar H-bond-mediated interactions involving a P nucleus in protein-DNA complexes
[3] or D. vulgaris flavodoxin (flavoprotein) [4], supported later by deMon-NMR code
based DFT computations [5] for the models of aforementioned biological systems. In addition,
other related papers were published (see e.g., refs 68-72 in recent review on J-coupling
calculations [6]). In the case of compounds (Z)-1, two possibilities exist for explaining the observed
heteronuclear J PH coupling, namely, the P↔H interaction across the N−H···O − −P +
hydrogen bond ( h2 J) in the formed 6-membered ring, or traditional coupling along a longrange
pathway via the molecular backbone ( 4 J). The best simple method to choose between
these two pathways was a theoretical study. Results of presented DFT/FPT computations suggest
that the foregoing interactions in 1 are in fact due to an across-H-bond J coupling. As the
h2 J PH coupling seems to be a sensitive probe of the H-bond angle and distance [5], an interesting
question of its sign will be discussed in detail in the poster version of this communication.
The second problem related to this issue concerns our recent NMR study on phosphonium
Ph
N
N
R
H
P
O
O-iPr
O-iPr
Ph
Ph
Pα
Ph
C
1 2
Hβ'
ylide 2 [7], i.e. a proper assignment of the 1 H-spectra observable 2 J PH coupling (≅ 5.2 Hz) and
tentative explanation of an “absence” of the second coupling of this type in terms of fast pyramidalisation
of the Cβ-anionic site. Thus, it was possible to arrange a large variety of ϕ-
constrained conformers of the small-sized model 3 to simulate reversible inversion of the geometry
at its β-carbon, and then to estimate 2 J PH for each arrangement. The simplest ylide 3
was thoroughly examined before, including the J-sign determination ( 2 J PH +6.5 Hz, in C 6 D 6 )
[8,9]. In this way, we can test the reliability of such computations for 2 and 3 and, at the same
time, answer the question of which orientation of the β-proton in such systems gives a minimal
magnitude of vicinal J PH coupling. Gas-phase DFT/FPT data for 2 J PH in ylide 3 plotted as
a function of the X–P α –C β –H torsion angle (ϕ), where X is dummy atom lying on an axis perpendicular
to the C S symmetry plane, are shown in Fig. 1 (squares); the related DFT-energy
well is also pictured (circles). These results drive us to the conclusion that for 3 only qualitative
agreement of calculations is observed with 2 J PH data in solution (13.8 vs 6.5 Hz) and that
for ϕ near 28 o this more than twice overestimated NMR parameter most likely is of a negligible
value in P-ylides. Obviously, various motional processes (e.g. pyramidalisation or rotation
around the P α –C β bond) can modify these values of ϕ and J, for the reason that all low-energy
forms of such carbanionic systems exist only part of the time (Boltzmann averaging). Above
Pγ
O
OPh
OPh
Me
Me
Me
Pα
X
Cβ
trans-bent conformation [9]
3
H
H'
62