Indirect nuclear spin-spin coupling constants 1J( 17O,11B). First observation and calculation using density functional theory (DFT)

Article abstract of DOI:10.1515/znb-2006-0804

Coupling constants ¹J(¹⁷O,¹¹B) of borates, borane adducts and boranes with boron-oxygen bonds have been calculated on the basis of optimised molecular structures using the B3LYP/6-311+G(d,p) level of theory. This indicates that such coupling constants can be of either sign and that their magnitudes can be rather small. Since both ¹¹B and ¹⁷O are quadrupole nuclei, it is therefore difficult to measure representative data. In the cases of trimethoxyborane and tetraethyldiboroxanes, it proved possible to obtain experimental data ¹J( ¹⁷O,¹¹B) (22 and 18 Hz) by measurement of ¹⁷O NMR spectra at high temperature (120 °C and 160 °C) respectively. The magnitude of these coupling constants is in reasonable agreement with calculated data. In the case of the diboroxane, this points towards a bond angle B-O-B more close to 180° than to 140°.

Full text of DOI:10.1515/znb-2006-0804

Indirect Nuclear Spin-Spin Coupling Constants 1J(17O,11B). First
Observation and Calculation Using Density Functional Theory (DFT)
Bernd Wrackmeyer and Oleg L. Tok
Anorganische Chemie II, Universita¨t Bayreuth, D-95440 Bayreuth, Germany
Reprint requests to Prof. Dr. B. Wrackmeyer. E-mail: b.wrack@uni-bayreuth.de
Z. Naturforsch. 61b, 949 – 955 (2006); received March 3, 2006
Dedicated to Professor Dr. Max Herberhold on the occasion of his 70th birthday
Coupling constants 1J(17O,11B) of borates, borane adducts and boranes with boron-oxygen bonds
have been calculated on the basis of optimised molecular structures using the B3LYP/6-311+G(d,p)
level of theory. This indicates that such coupling constants can be of either sign and that their mag-
nitudes can be rather small. Since both 11B and 17O are quadrupole nuclei, it is therefore difficult to
measure representative data. In the cases of trimethoxyborane and tetraethyldiboroxanes, it proved
possible to obtain experimental data 1J(17O,11B) (22 and 18 Hz) by measurement of 17O NMR spec-
tra at high temperature (120 ◦C and 160 ◦C) respectively. The magnitude of these coupling constants
is in reasonable agreement with calculated data. In the case of the diboroxane, this points towards a
bond angle B-O-B more close to 180◦ than to 140◦.
Key words: Boranes, Borates, 17O NMR, Coupling Constants, DFT Calculations
Introduction
attempts, based on experiments and calculations, to
gain the missing information on 1J(17O,11B) for the
molecules of type 1 – 18 shown in Scheme 1. Many
representatives of these compounds have already been
prepared. Some have been reported as reactive inter-
mediates in the gas phase (e.g. 17) or may be present
in the solid state (18).
Boron-oxygen compounds are the natural resources
of the element boron, and they play an important role in
many applications of boranes. In addition to routinely
1
applied H and 13C NMR spectroscopy, many boron-
oxygen compounds have been studied extensively by
11B NMR [1, 2], allowing to distinguish readily be-
tween three- and four-coordinate boron atoms, and re-
vealing information on equilibria and also on the na-
ture of other substituents at the boron atom. Few stud-
ies have been carried out on boron-oxygen compounds
focusing on 17O NMR at natural abundance in solu-
tion [3 – 5], and it has been shown that the chemical
Results and Discussion
General remarks on 17O NMR and the search for
candidates showing resolved 17O-11B coupling in
17O NMR spectra
The nuclear magnetic properties of the 17O nu-
shifts δ17O cover a rather large range (ca. 250 ppm). cleus are not particularly inviting for NMR stud-
The δ17O values mirror certain properties of the B-O ies [8 – 10] considering its low natural abundance
bond, in particular the potential BO(pp)π interactions (0.037%) and its sizeable quadrupole moment (I = 5/2;
in trigonal boranes, complementary to the information Q = −2.6 10−2 [10−28 m2]). Enrichment of 17O in
from 11B NMR. In contrast to δ17O [3 – 5] and to the boron-oxygen compounds is relatively easy if labelled
large known data set of spin-spin couplings of 11B water H217O can be used for the synthesis, and ap-
with other nuclei J(11B,X) [1, 2], no examples of scalar plications of such labelled reagents have been demon-
17O-11B spin-spin coupling have been reported so far. strated [11 – 14]. However, the fast quadrupolar relax-
1
Sign and magnitude of coupling constants J(19F,11B) ation rate – T Q(17O) is generally in the order of a few
[6] and 1J(15N,11B) [7] are known and therefore, data ms or less – often prevents the observation of resolved
1
for J(17O,11B) would be highly welcome to estab- 17O-X spin-spin coupling in both 17O and X NMR
lish trends in spin-spin coupling. We report here on spectra except in favourable cases [15– 19]. If X is
c
0932–0776 / 06 / 0800–0949 $ 06.00 ꢀ 2006 Verlag der Zeitschrift fu¨r Naturforschung, Tu¨bingen · http://znaturforsch.com
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B. Wrackmeyer – O. L. Tok · Indirect Nuclear Spin-Spin Coupling Constants
Fig. 1. 54.2 MHz 17O NMR spectrum of trimethoxybo-
rane 1b in mesitylene (25%, V/V) at 120 ◦C. The sig-
nal shape is typical of a partially relaxed 1 : 1 : 1 : 1 quartet
[2π 1J(17O,11B) TQ(11B) ≈ 1], where the outer lines have
collapsed and the distance between the inner lines represents
the coupling constant 1J(17O,11B) = 22 4 Hz.
solved) 17O-11B spin-spin coupling contributes to the
line widths of the 17O NMR signals, an increase in
temperature could eventually lead to broadening of the
17O NMR signals as a result of the longer lifetimes
of both 17O and 11B spin states. Several of the com-
pounds shown in Scheme 1 (1b, 6b, 8b, 9, the B-ethyl
derivatives [4, 5] of 7, 10 and 12, and the diboroxanes
15b, 15c, 15d) were studied by 17O NMR at temper-
◦
atures between +25 to +160 C. In the cases of the
compounds 6b, 8b, 9, and the B-ethyl derivatives of
7, 10 and 12, a steady decrease in the line width of the
respective 17O NMR signal was observed with increas-
Scheme 1. Boron-oxygen compounds considered in this
study for calculations of NMR parameters.
ing temperature. This points towards a rather small
magnitude of 1J(17O,11B).
also a quadrupolar nucleus such as 11B (I = 3/2) in
an unsymmetrical environment, the situation becomes
worse, and even for 17O-enriched compounds resolved
splitting due to scalar coupling will be rare, and line
shape analysis as an alternative for deducing the cou-
pling information, becomes difficult. In any case, it is
advisable to measure the 17O NMR spectra for obser-
vation of 17O-X spin-spin coupling since enrichment
with 17O > 10% is rather expensive, and even with
25% enrichment of 17O it may be difficult to analyse
the splitting of the X NMR signal (at least six lines of
ideally equal intensity).
In the case of trimethoxyborane, B(OMe)3 (1b), dis-
◦
solved in mesitylene, the 17O NMR signal (25 C:
◦
h1/2 = 100 Hz) became slightly sharper up to 70 C
◦
(h1/2 = 90 Hz), and at 110 C, even better discernible
at 120 ◦C, the signal started to split in the top, typical of
partially relaxed scalar coupling of an X nucleus with
the 11B nucleus of I = 3/2, where the resolved split-
ting corresponds to the distance between the inner lines
of the 1 : 1 : 1 : 1 quartet with J(17O,11B) = 22 4 Hz
1
(Fig. 1). The relaxation time T Q(11B) of 1b in the same
◦
sample was found to increase from 0.011 s at 23 C to
0.046 s at 110 ◦C.
In our previous 17O NMR studies on boron-oxygen
The 17O NMR signal of 15b (δ 1◦7O 231.0) did not
compounds [3 – 5], we have noted sharpening of change markedly in going from 25 C (h1/2 = 75 Hz)
17O NMR signals when the 17O NMR spectra were to 70 ◦C (h1/2 = 70 Hz), and then the line width was in-
measured at higher temperature. This is in principle the creasing with increasing temperature, and a new strong
expected effect for the quadrupolar nucleus 17O, since signal appeared in the region for 6b (δ 17O 152.0). Ac-
the enhanced molecular mobility at elevated tempera- cording to 11B NMR spectra decomposition of 15b
ture leads to shorter correlation times τc, and by this (δ11B 52.0) into 6b and BMe3 (δ11B = 33.2 and 86.0)
the quadrupolar relaxation rate slows down [T Q(17O) sets in at elevated temperature. Therefore, the changes
becomes longer]. However, if partially relaxed (unre- in the 17O line widths did not provide unambiguous
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B. Wrackmeyer – O. L. Tok · Indirect Nuclear Spin-Spin Coupling Constants
951
15c, causes more efficient quadrupolar relaxation for
both 11B and 17O nuclei as shown by the parallel
changes of the line widths of 11B and 17O NMR sig-
nals (Fig. 3). The contribution arising from unresolved
17O-11B spin-spin coupling to the line widths◦of the
17O NMR signals is too small, even above 120 C.
Optimised geometries of boron-oxygen compounds
Fig. 2. 27.1 MHz
17O NMR spectrum
The molecules shown in Scheme 1 possess the ex-
pected gas phase structures. Major ambiguities arise in
of 17O enriched (5%)
tetraethyldiboroxane 15c
in mesitylene (∼ 3% the cases of 4 and 15. There are two isomers 4 and 4’
V/V) at 160 ◦C.
with a planar arrangement of all atoms (Scheme 2), of
which 4 with identical OH groups is slightly less sta-
ble. The optimised geometries of the diboroxanes 15a
[20] and 15b are close to D2d symmetry (almost lin-
ear B-O-B units: 179.6◦ in 15a and 178.7◦ in 15b).
In addition to 15a, the bent structure 15a’ with C2v
symmetry has been identified as a minimum on the
potential energy surface (PES) by MP4/6-311G(d,p)
calculations [20], and the energy difference between
15a and 15a’ was found to be extremely small [20]
(see also Scheme 2). In contrast to the present calcula-
tions, which predict a linear structure for 15b, the elec-
tron diffraction study has revealed a B-O-B angle of
144.4◦ for molecular C2 symmetry in 15b’ [21]. The
calculations predict that 15b is more stable than 15b’
(C2 symmetry) by 10 kcal/mol (Scheme 2). The calcu-
Fig. 3. Changes in the line widths of the 128.3 MHz 11B
lated structure of planar bent 15b” (with C2v molecu-
lar symmetry analogous to 15a’) is only a stationary
point on the PES). The calculation based on the exper-
imental geometry [31] of 15b’ leads to linear 15b as
the minimum on the PES. In the solid state, tetrameth-
yldiboroxane is a dimer [22]. Solid tetraphenyldibor-
oxane is a monomer and has a bent B-O-B unit (angle
B-O-B 152.7(2)◦) [23], whereas the only example of
a solid monomeric tetraalkyldiboroxane, derived from
1-boraadamantane,shows an almost linear B-O-B moi-
and 54.2 MHz 17O NMR signals of the diboroxane 15d as a
function of temperature. The almost parallel changes in the
functions indicate that contributions from 17O-11B coupling
to the line widths of the 17O NMR signals are small, even at
120 ◦C.
evidence for 17O-11B coupling. The tetraethyl deriva-
tive 15c proved to be thermally more stable than 15b,
in particular in diluted solutions (∼ 3% in benzene,
toluene, xylene or mesitylene). In order to observe the
17O NMR signal of such diluted solutions, 17O en-
riched (5%) 15c was used. The 17O NMR signal at
25 ◦C has a line width h1/2 = 90◦Hz, it became slightly
sharper (h1/2 = 75 Hz) at 70 C, then further heat-
◦
ing caused broadening, and at 160 C (in mesitylene;
TQ(11B) = 0.042 s) resolved splitting with the typical
pattern due to scalar one-bond spin-spin coupling of
1
17O with two 11B nuclei [| J(17O,11B)| = 18 2 Hz]
was observed (Fig. 2).
The measurements of 17O NMR spectra of 15d at
high temperature did not give the same result. The Scheme 2. Comparison of the energies of some alternative
structures of boron-oxygen compounds.
larger molecular size of 15d, when compared with
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B. Wrackmeyer – O. L. Tok · Indirect Nuclear Spin-Spin Coupling Constants
Table 1. Calculated and experimental 17O and 11B NMR parametersa,b of the boron oxygen compounds 1 – 18 (see Schemes 1
and 2 for the structures).
Compound
No.
1a
δ
17O
δ
17O
δ
11B
δ
11B
1J(17O,11B)
(calcd.) [Hz]
−13.9
FC
SD
PSO
(exp.)
+51.0
+11.0
(calcd.)
+26.3
+14.7
(exp.)
+19.6
+18.2
(calcd.)
+19.6
+18.1
(calcd.)
−17.8
−21.0
(calcd.)
−1.6
−0.2
(calcd.)
+4.1
+4.2
1b
−17.2
[(-)22] exp.
−21.0 (OH)
−5.3 (cis to H)
−11.8 (trans to H)
−6.9
2
–
+15.2
+46.6
+43.3
+23.0
+15.6
+89.1
+82.0
+72.3
+92.8
–
+23.2
−24.8
−9.4
−16.0
−8.5
−6.7
−17.8
−10.7
−8.2
−1.4
−3.3
−5.7
−2.7
−1.1
−0.2
−0.2
−0.2
−0.4
−0.3
−0.2
−0.1
−0.2
−0.3
−1.5
−0.2
−0.3
−0.2
+4.1
+4.5
+4.6
+2.1
+2.3
+5.4
+5.4
+5.3
+5.6
+6.1
+4.7
+6.0
+4.6
3a
3b
4
–
–
–
–
+1.1
+1.8
+10.7
+27.6
+26.9
−5.0
–
–
−12.5
4’
−5.6 (OH,BH, cis)
−3.3
5
–
–
+28.1
–
33.2
+38.6
+28.1
+27.4
+34.4
+38.7
+3.7
+1.5
6a
6b
7
+170.2
+160.1
+99.3
+152.0
−1.3
+88.0
+2.8
+222.4
+360.0 (C=O)
–
95.0
–
97.0
+161.8
–
+240.3
+383.6
+155.9
+120.2
+183.2
+106.4
+166.0
+298.8
+402.8 (C=O)
+145.9
−35.4
−3.1
8a
8b
8c
9
10
11
–
+49.8
+54.5
+49.0
+61.4
+57.2
+63.1
+1.3
−1.8
−5.3
−3.3
+5.5
+2.5
−6.1
−7.9
−12.3
−9.5
−5.9
−3.3
−0.2
−0.3
−0.1
−0.3
−0.4
−0.2
+7.7
+6.6
+7.2
+6.6
+7.0
+6.2
+53.5
–
+60.8
+57.0
–
12
13
+135
–
–
–
–
–
+48.0
+50.0
−12.3
+9.6
+0.7
+4.4
+9.7
−5.2
+4.5
−0.2
−5.3
−1.0
−0.6
−0.5
0.0
+6.2
+5.0
−1.3
−0.5
+6.3
+6.2
+4.4
+5.2
14a
14b
15a
15a’
15b
15b’
15c
15d
16a
16b
16cc
16d
16e
17a
17b
18
−30.2
–
+12.0
+15.2
−28.3
−8.0
−71.9
+2.5
+5.7
+13.8
−22.5
−1.8
+291.7
+259.9
+261.8
+238.3
–
–
–
+43.5
+57.5
+50.2
+60.2
–
+231
+52.0
−22.4
−13.0
[(-)19.5] (exp)
−27.3
−18.0
−0.4
+231
+52.0
−0.1
+235.0[m]
+52.0[m]
+225
–
+56.0
–
–
–
–
–
–
–
–
–
+460.7
+435.7
+332.8
+279.9
+237.2
+235.6
+211.3
−8.6
–
–
–
–
–
–
–
+67.2
+69.1
+68.2
+65.2
+57.0
+13.7
+21.2
+10.8
+28.4
+26.8
+12.6
+5.3
+16.7
+16.3
+5.4
+1.0
+0.9
0.0
+10.7
+9.7
+7.2
+6.3
+6.0
+2.6
+3.0
−2.3
−0.7
−0.1
+1.9
−4.0
0.0
−25.1
−24.0
−16.2
−22.4
−22.5
−18.1
−5.2
−4.6
−1.2
a
Calculated σ17O) data are converted to δ17O data by δ17O = σ(17O)[CO] − σ(17O) + 350.1, with σ(17O)[CO] = −72.3, δ17O[CO] =
350.1 and δ17O [H2O (liquid)] = 0; calculated σ(11B) data are converted to δ11B data by δ11B = σ(11B)[B2H6] − σ(11B) + 18 with
σ(11B)[B2H6] = 84.1, δ11B[B2H6] = 18.0 and δ11B[BF3-OEt2] = 0; experimental δ17O data are taken from ref. [3 – 5], and experimental
δ
11B data are taken from ref. [1, 2], if not stated otherwise. FC, SD and PSO mean Fermi contact, spin-dipole and paramagnetic spin-orbital
term. All contributions to the reduced coupling constant1K(17O,11B) have the opposite sign to 1J(17O,11B);
in some cases, experimental
b
data refer to the B-ethyl derivatives (see text); this work; measured at 160 ◦C in mesitylene; at 23 ◦C: δ17O +225.0; δ11B +52.6.
c
ety (angle B-O-B = 171.8(6)◦) [24]. All this indicates B-O-B in diboroxanes on the calculated coupling con-
a low barrier to the distortion from structures with stant 1J(17O,11B).
ideal D2d or C2v molecular symmetry in diboroxanes.
The cyclic compounds 16, of which the structure of
16b has been calculated previously [25], were included
in order to find out about the influence of the angle
In both borane adducts 14a and 14b, the sur-
roundings of the oxygen atom are pyramidal.
The oxides 17 and 18, isoelectronic with acylium
cations [26] and carbon dioxide, respectively, pos-
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B. Wrackmeyer – O. L. Tok · Indirect Nuclear Spin-Spin Coupling Constants
953
Table 2. Comparison of calculated 17O NMR parameters of
isoelectronic linear boron- and carbon-oxygen compounds.
sess the expected linear arrangement at the boron
atom.
Compound
No.
δ
17O
1J(17O,11B) FC
SD
PSO
Calculation of indirect nuclear spin-spin coupling
constants 1J(17O,11B)
(calcd.) 1J(17O,13C) (calcd.) (calcd.) (calcd.)
(calcd.) [Hz]
17a
+235.6 −25.1 (11B) −22.4
−5.2 +2.6
[H-C≡O]+
+316.2 −8.2 (13C)
+2.4 −12.2 +1.6
The calculated chemical shifts δ 11B and δ17O (Ta-
ble 1) are close to the experimental values when avail-
able [27 – 30]. It has been shown that calculation of
various indirect nuclear coupling constants nJ(X,Y) af-
fords fairly reliable data [29– 36]. The calculated cou-
17b
+211.3 −24.0 (11B) −22.5
−4.6 +3.0
−9.6 +3.7
−1.2 −2.3
−2.7 +5.0
[Me-C≡O]+ +303.2 +0.6 (13C)
+6.5
−18.1
18
O=C=O
−8.6 −16.2
+63.4 +17.1 (13C) +14.8
1
pling constants J(17O,11B) are given in Table 1 to-
(15a‘) to the D2d structure (15a). In the latter, both
lone pairs at the oxygen atom are involved in BO(pp)π
interactions and consequently, negative contributions
to the (reduced) FC term are less dominant. This can
also be seen by following the trend of the calculated
data for 16a – d, where the structure of 16d is com-
parable with that of 15a’. The calculated magnitude of
1J(17O,11B) = −13.0 for the experimentalC2 structure
15b’ is larger than for 15b’ with C2v structure (−7.6),
however, still less negative than for 15b with D2d sym-
metry (−22.4 Hz). Interestingly, the calculated chem-
ical shift δ11B of 15b (50.2) is much closer to the ex-
perimental value (+52.0) than the δ 11B value (60.2)
calculated using the experimentalC2 symmetry of 15b’
or the C2v structure (58.9). In the case of 15c, an exper-
gether with contributions from the Fermi contact term
(FC), the spin-dipole (SD) and the paramagnetic spin-
orbital term (PSO). Since γ11B) > 0 and γ(17O) < 0,
1
the total values J(17O,11B) as well as the FC-, SD-,
PSO contributions have the opposite sign relative to
1
the reduced coupling constants K(17O,11B) and their
respective reduced contributions [1K(17O,11B) = 4π2
1J(17O,11B) (γ(17O) γ(11B) h)−1].
The presence of lone pairs of electrons at the oxy-
gen atom should give rise to pronounced negative con-
tributions to the (reduced) Fermi-contact term [37].
This is obvious in the absence of BO(pp)π interac-
tions, e.g. for the borane adducts 14a, b; the FC term
becomes small in the case of the borate 13 and changes
its sign in the borates 3a, b. The situation for the
1
imental coupling constant J(17O,11B) = (−)18 Hz at
1
FC contribution to the coupling constants J(17O,13C)
160 ◦C (δ11B = 52.0) is available (Fig. 2). The experi-
mental δ11B values for 15c and 15d remain unchanged
with temperature, within the experimental error. The
experimental δ 17O value for 15c changes slightly from
+225 (23 ◦C) to +235 (+150 ◦C), into the direction of
the calculated shift of 15b (gas phase). It is well known
that there can be significant changes in δ 17O for the gas
and the liquid phase [38] and therefore, it is difficult to
draw a firm conclusion from the calculated δ 17O val-
ues of 15.
for all sp3-hybridized carbon atoms [36b] is similar
1
to that for J(17O,11B) in the borane adducts 14a, b.
The PSO contribution can be of either sign, and can-
not be neglected as is evident in the case of 13.
The SD contributions in the molecules containing
four-coordinate boron atoms are small, except again
for 13.
Any marked BO(pp)π interactions in the molecules
containing three-coordinate boron atoms will change
the nature of the lone pairs at the oxygen atom. The
inspection of Table 1 shows that the (reduced) FC con-
tributions are fairly large and positive in most cases,
except in slightly strained rings such as 5, 6, 7, 10 – 12,
16d, e, and in particular for small rings like 16a – c.
The contribution arising from the PSO term is quite
substantial for all trigonal boranes, whereas the SD
term is relatively small.
The oxides 17 have been proposed as reactive inter-
mediates [39]. The fairly large positive (reduced) FC
term indicates that BO(pp)π bonding suppresses most
of the usually negative influence of the lone pairs of
electrons at the oxygen atom. Table 2 shows a com-
parison of the calculated NMR data of 17 and 18
with the isoelectronic acylium cations and carbon diox-
ide. The 17O nuclei in the latter are more deshielded
The most intriguing question concerns the bond- and the (reduced) FC term is significantly less posi-
ing in the diboroxanes, where the bond angle B-O-B tive, typically of all comparable boron- and carbon-
is a matter of debate. The calculations show that the oxygen compounds [36b]. In contrast with the non-
value of 1J(17O,11B) increases from −1.8 to −22.5 Hz linear species, the SD term becomes important, and its
(1K(17O,11B) > 0 !) in going from the C2v structure magnitude is larger in the case of carbon because the
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B. Wrackmeyer – O. L. Tok · Indirect Nuclear Spin-Spin Coupling Constants
electrons are more attracted towards the carbon atom carefully dried solvents. The purity of the compounds was
checked by 1H and 13C NMR. Samples for measurements at
in the acylium cations.
high temperatures were sealed in NMR tubes (5 mm o. d.).
11B and 17O NMR measurements were carried out using
Conclusions
Bruker WP 200, ARX 250 and Varian Inova 400 spectrom-
eters, all equipped with multinuclear probe heads and vari-
able temperature units. Pulse widths were set close to 90◦
pulses and spectra were recorded without repetition delays.
Relaxation times TQ(X) (X =11B, 17O) given were deter-
mined from the line widths of the 11B or 17O NMR signals.
In several cases, the data were confirmed by applying the
inversion-recovery technique for determination of T1(X).
All calculations were performed using the program pack-
age Gaussian 03, revision B.02 [38]. Optimisation of the gas
phase geometries was done with DFT methods (B3LYP) [39]
and the 6-311+G(d,p) basis set [40]. The nature of calculated
optimised structures as minima or stationary points on the
PES was confirmed by the absence or presence of imaginary
frequencies. Table 1 contains the paramagnetic spin-orbital
(PSO) contribution; the diamagnetic spin-orbital (DSO) con-
tribution was < 1 Hz in all cases studied.
The magnitude of the coupling constants
1J(17O,11B) in borates, borane adducts and in
most boranes is small, and the coupling constants can
be of either sign. Experimental values 1J(17O,11B)
were obtained for two examples, trimethoxyborane
1b and tetraethyldiboroxane 15c, and the data were
found in reasonable agreement with calculated values.
Apparently, BO(pp)π bonding leads to less negative
contributions of the (reduced) Fermi contact term.
Trends for 17O-11B indirect nuclear spin-spin coupling
are now established. Expectedly, the data are in be-
tween those found for 15N-11B and 19F-11B spin-spin
coupling, and experimental and theoretical evidence
has become available for the first time.
Experimental Section
Acknowledgements
This work was supported by the Deutsche Forschungsge-
meinschaft. We are grateful to Prof. Dr. R. Ko¨ster for a gift
of 17O enriched compounds.
The compounds studied were commercial samples (1b)
or prepared as described previously [3 – 5, 11 – 13], and all
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