Early work on the nuclear magnetic resonance (NMR) spectra of boron compounds (
11 B-12.83 MHz) was done and showed a range of chemical shifts and
some 1-bond
11 B1 H and
11 B2 H coupling constants, using BF 3 ⋅ Et 2 O as a reference [37]. Of interest in this study are the
11 B chemical shifts of NaBO 2 (aq)
(−1.3 ± 0.5 ppm, due to [B(OH) 4 ]
−
), NaB 5 O 8 (aq) (−1.3 and −14.4 ± 1.0 ppm),
K 2 B 4 O 7 (−7.5 ± 1.0 ppm), Na 2 B 4 O 7 (−8.0 ± 0.5 ppm), (NH 4 ) 2 B 4 O 7 (−10.3 ±
0.5 ppm), KB 5 O 8 (aq) (−13.0 ± 0.5 ppm), and B(OH) 3 (aq) (−18.8 ± 1.0 ppm).
These were interpreted as being due to a dynamic equilibrium between B(OH) 4
−
and B(OH) 3 . A later study by Momii and Nachtrieb (sat. B(OH) 3 (aq) reference,
11 B-14 MHz) reexamined these results and gave 0.090–0.900 M NaBO 2 (aq) at
17.4 ± 0.5 ppm and 0.090–0.900 M KBO 2 at 15.5 ± 0.5 ppm [38]. These were
interpreted as due to [B(OH) 4 ]
− . For sodium pentaborate solutions, the peak at
15.0 ppm was assigned to [B 5 O 6 (OH) 4 ]
−
, and the peak at 1.1 ppm assigned to a
rapid equilibrium between B(OH) 3 , B(OH) 4
− and B 3 O 3 (OH) 4
− . For the tetraborates, a single peak is observed whose chemical shift increases with concentration
from 8 to 11 ppm, and this was assigned to a rapid equilibrium between B(OH) 3 ,
B(OH) 4
− and at least two other ions. How and coworkers showed that the chemical
shift of a 50 g/L solution at 33 °C varied from −2 to −20 ppm between pH 12–2
respectively (
11 B, 12.83 MHz, BF 3 ⋅ Me 2 O ref.) [39]. Smith and Wiersema
(
11 B-80 MHz) noted that one NMR peak in all borate solutions was linearly related
to the sodium to boron ratio and could this be interpreted as the peak of rapidly
exchanging B(OH) 3 and [B(OH) 4 ]
− [40]. For tetraborate solutions, three peaks
could be observed, with the 5.0 ppm peak assigned to [B 3 O 3 (OH) 4 ]
−
. Pentaborate
solutions also showed three peaks, with the 5 ppm peak assigned to [B 3 O 3 (OH) 4 ]
−
,
and the peak at 18 ppm assigned to [B 5 O 6 (OH) 4 ]
− . Covington and Newman
examined the
11 B spectra (28.87 MHz, rel. to infinite dilution [B(OH) 4 ]
−
) of
sodium and potassium borate in water and in ∼0.1 mol/L added [OH
−
] in an effort
to determine the pK b of borate [41]. Henderson et al., in their study of the complexation of borate with diols, showed the
11 B NMR (12.83 MHz) of borax, boric
acid, and sodium metaborate from pH 2–12, along with the line width at half height
[42]. Janda and Heller examined the
11 B spectra (60 MHz) of sodium, potassium,
and ammonium polyborates as a function of concentration and pH (0.5–13.8) and
either one or two lines were observed [43]. Epperlein et al. examined the
10 B
spectra (1.807T, 8.267 MHz) of some boron species and found B(OH) 3 at 0 ppm
(reference), B 5 O 6 (OH) 4
− at 17 ppm, B 4 O 5 (OH) 4
2− between 70–85 ppm, and B
(OH) 4
− at around 140 ppm [44]. Salentine confirmed earlier results (
11 B, 127 and
160 MHz, external reference BF 3 ⋅ Et 2 O) on the pentaborate (18, 13, 1 ppm) and
tetraborate (12, 8, 1 ppm) [45]. It was proposed that the resonance at 13 ppm, due
to triborate ion, and at 1 ppm, due to pentaborate ion, are due to the tetrahedral
boron atoms, and the trigonal boron atoms are not observed because of quadrupolar
relaxation.
We have reviewed the crystallography of boric acid and monomeric borates, and
the boron NMR of boric-acid/borate containing solutions. Our remaining goals are
to compare the ab initio energy, structure, and vibrational spectra to experiment
(where known) of orthoboric acid, metaboric acid, and tetrahydroxoborate, and to
148
C. C. Pye
11 B-12.83 MHz) was done and showed a range of chemical shifts and
some 1-bond
11 B1 H and
11 B2 H coupling constants, using BF 3 ⋅ Et 2 O as a reference [37]. Of interest in this study are the
11 B chemical shifts of NaBO 2 (aq)
(−1.3 ± 0.5 ppm, due to [B(OH) 4 ]
−
), NaB 5 O 8 (aq) (−1.3 and −14.4 ± 1.0 ppm),
K 2 B 4 O 7 (−7.5 ± 1.0 ppm), Na 2 B 4 O 7 (−8.0 ± 0.5 ppm), (NH 4 ) 2 B 4 O 7 (−10.3 ±
0.5 ppm), KB 5 O 8 (aq) (−13.0 ± 0.5 ppm), and B(OH) 3 (aq) (−18.8 ± 1.0 ppm).
These were interpreted as being due to a dynamic equilibrium between B(OH) 4
−
and B(OH) 3 . A later study by Momii and Nachtrieb (sat. B(OH) 3 (aq) reference,
11 B-14 MHz) reexamined these results and gave 0.090–0.900 M NaBO 2 (aq) at
17.4 ± 0.5 ppm and 0.090–0.900 M KBO 2 at 15.5 ± 0.5 ppm [38]. These were
interpreted as due to [B(OH) 4 ]
− . For sodium pentaborate solutions, the peak at
15.0 ppm was assigned to [B 5 O 6 (OH) 4 ]
−
, and the peak at 1.1 ppm assigned to a
rapid equilibrium between B(OH) 3 , B(OH) 4
− and B 3 O 3 (OH) 4
− . For the tetraborates, a single peak is observed whose chemical shift increases with concentration
from 8 to 11 ppm, and this was assigned to a rapid equilibrium between B(OH) 3 ,
B(OH) 4
− and at least two other ions. How and coworkers showed that the chemical
shift of a 50 g/L solution at 33 °C varied from −2 to −20 ppm between pH 12–2
respectively (
11 B, 12.83 MHz, BF 3 ⋅ Me 2 O ref.) [39]. Smith and Wiersema
(
11 B-80 MHz) noted that one NMR peak in all borate solutions was linearly related
to the sodium to boron ratio and could this be interpreted as the peak of rapidly
exchanging B(OH) 3 and [B(OH) 4 ]
− [40]. For tetraborate solutions, three peaks
could be observed, with the 5.0 ppm peak assigned to [B 3 O 3 (OH) 4 ]
−
. Pentaborate
solutions also showed three peaks, with the 5 ppm peak assigned to [B 3 O 3 (OH) 4 ]
−
,
and the peak at 18 ppm assigned to [B 5 O 6 (OH) 4 ]
− . Covington and Newman
examined the
11 B spectra (28.87 MHz, rel. to infinite dilution [B(OH) 4 ]
−
) of
sodium and potassium borate in water and in ∼0.1 mol/L added [OH
−
] in an effort
to determine the pK b of borate [41]. Henderson et al., in their study of the complexation of borate with diols, showed the
11 B NMR (12.83 MHz) of borax, boric
acid, and sodium metaborate from pH 2–12, along with the line width at half height
[42]. Janda and Heller examined the
11 B spectra (60 MHz) of sodium, potassium,
and ammonium polyborates as a function of concentration and pH (0.5–13.8) and
either one or two lines were observed [43]. Epperlein et al. examined the
10 B
spectra (1.807T, 8.267 MHz) of some boron species and found B(OH) 3 at 0 ppm
(reference), B 5 O 6 (OH) 4
− at 17 ppm, B 4 O 5 (OH) 4
2− between 70–85 ppm, and B
(OH) 4
− at around 140 ppm [44]. Salentine confirmed earlier results (
11 B, 127 and
160 MHz, external reference BF 3 ⋅ Et 2 O) on the pentaborate (18, 13, 1 ppm) and
tetraborate (12, 8, 1 ppm) [45]. It was proposed that the resonance at 13 ppm, due
to triborate ion, and at 1 ppm, due to pentaborate ion, are due to the tetrahedral
boron atoms, and the trigonal boron atoms are not observed because of quadrupolar
relaxation.
We have reviewed the crystallography of boric acid and monomeric borates, and
the boron NMR of boric-acid/borate containing solutions. Our remaining goals are
to compare the ab initio energy, structure, and vibrational spectra to experiment
(where known) of orthoboric acid, metaboric acid, and tetrahydroxoborate, and to
148
C. C. Pye