3.5 Hydrated Borate, B(OH) 4
−
⋅ nH 2 O
The monoborate anion may be hydrated. The monohydrate has C 1 symmetry; the
dihydrate, either C 1 or C 2 ; the trihydrate, C 1 ; and the tetrahydrate, S 4 (see Fig. 8).
Upon hydration, the average B-O distance slightly decreases (0.002 Å) but the
deviation from the mean can be as much as 0.025 Å (Fig. 9).
The vibrational frequencies (MP2/6-311+G*) of hydrated monoborate are
plotted in Fig. 10. The OH stretching frequencies (not shown) decrease by about
50 cm
−1 upon going from the anhydrate to the tetrahydrate. The restricted translations of the water molecules appear below 250 cm
−1 , whereas the restricted
rotations appear at approximately 300–440 cm
−1 (rock), 580–730 cm
−1 (twist), and
C 1
C 2
C 1
C 1
S 4
Fig. 8 Structure of hydrated borate, B(OH) 4
− ⋅ nH 2 O
Fig. 9 Variation of B-O distance of B(OH) 4
− as a function of hydration and structure
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−
⋅ nH 2 O
The monoborate anion may be hydrated. The monohydrate has C 1 symmetry; the
dihydrate, either C 1 or C 2 ; the trihydrate, C 1 ; and the tetrahydrate, S 4 (see Fig. 8).
Upon hydration, the average B-O distance slightly decreases (0.002 Å) but the
deviation from the mean can be as much as 0.025 Å (Fig. 9).
The vibrational frequencies (MP2/6-311+G*) of hydrated monoborate are
plotted in Fig. 10. The OH stretching frequencies (not shown) decrease by about
50 cm
−1 upon going from the anhydrate to the tetrahydrate. The restricted translations of the water molecules appear below 250 cm
−1 , whereas the restricted
rotations appear at approximately 300–440 cm
−1 (rock), 580–730 cm
−1 (twist), and
C 1
C 2
C 1
C 1
S 4
Fig. 8 Structure of hydrated borate, B(OH) 4
− ⋅ nH 2 O
Fig. 9 Variation of B-O distance of B(OH) 4
− as a function of hydration and structure
An Ab Initio Study of Boric Acid, Borate …
163