3.2 Metaboric Acid, B 3 O 3 (OH) 3
The structures of α-metaboric acid are analogous in symmetry to orthoboric acid
(Fig. 2). The barriers are quite similar to that of orthoboric acid (Table 7), but the
C s #2 form is much more stable (only 3–4 kJ/mol above the minimum energy C 3h
structure). If metaboric acid is ever observed in the gas phase, it should exist as a
mixture of an appreciable amount of both conformers (assuming it does not
decompose to form the monomer HBO 2 ). It might even be possible to observe the
microwave spectrum of the C s conformation of the molecule. It is the C s form that is
observed in the crystal structure (Table 1), so one strategy for gas-phase observation might be laser ablation.
3.3 Hydrated Orthoboric Acid, H 3 BO 3 ⋅ nH 2 O
Four C s forms of hydrated boric acid with a water molecule directly bound to the
boron atom were attempted. Alternatively, this may be viewed as protonated
monoborate anion. In all cases, the water molecule dissociated. In the first two
cases, a solvated boric acid was obtained, neither of which was an energy minimum
(based on C s #1 and C s #3 naked boric acid, see Fig. 3). In the final two cases,
Table 6 Theoretical vibrational frequencies (cm
−1
) of the C s form of boric acid, B(OH) 3
BO 3
def
BOH
tors
BOH
tors
BO 3
oop
def
BO
str
HOB
def
HOB
def
BO
str
OH
str
OH
str
A′
A″
A″
A″
A′
A′
A′
A′
A′
A′
HF/6-31G*
471
488
306
508
549
729
930 1006
1116
1131
1517
1576
4141
4156
4168
HF/6-31+G*
468
483
324
514
558
733
928
979
1093
1099
1495
1554
4136
4154
4166
HF/6-311+G*
471
486
313
506
560
735
923
992
1107
1111
1488
1543
4195
4211
4221
B3LYP/6-31G*
441
458
260
476
527
661
877
947
1042
1058
1430
1494
3808
3814
3830
B3LYP/6-31+G*
436
450
295
482
539
665
873
912
1012
1022
1400
1463
3806
3816
3833
B3LYP/6-311+G* 438
452
274
466
536
668
871
926
1019
1031
1392
1454
3838
3847
3862
MP2/6-31G*
439
457
274
491
542
670
879
963
1057
1074
1446
1510
3825
3842
3853
MP2/6-31+G*
435
451
289
487
546
672
872
928
1024
1036
1410
1473
3797
3819
3830
MP2/6-311+G*
443
459
262
467
540
673
874
942
1037
1047
1411
1472
3875
3894
3904
154
C. C. Pye
The structures of α-metaboric acid are analogous in symmetry to orthoboric acid
(Fig. 2). The barriers are quite similar to that of orthoboric acid (Table 7), but the
C s #2 form is much more stable (only 3–4 kJ/mol above the minimum energy C 3h
structure). If metaboric acid is ever observed in the gas phase, it should exist as a
mixture of an appreciable amount of both conformers (assuming it does not
decompose to form the monomer HBO 2 ). It might even be possible to observe the
microwave spectrum of the C s conformation of the molecule. It is the C s form that is
observed in the crystal structure (Table 1), so one strategy for gas-phase observation might be laser ablation.
3.3 Hydrated Orthoboric Acid, H 3 BO 3 ⋅ nH 2 O
Four C s forms of hydrated boric acid with a water molecule directly bound to the
boron atom were attempted. Alternatively, this may be viewed as protonated
monoborate anion. In all cases, the water molecule dissociated. In the first two
cases, a solvated boric acid was obtained, neither of which was an energy minimum
(based on C s #1 and C s #3 naked boric acid, see Fig. 3). In the final two cases,
Table 6 Theoretical vibrational frequencies (cm
−1
) of the C s form of boric acid, B(OH) 3
BO 3
def
BOH
tors
BOH
tors
BO 3
oop
def
BO
str
HOB
def
HOB
def
BO
str
OH
str
OH
str
A′
A″
A″
A″
A′
A′
A′
A′
A′
A′
HF/6-31G*
471
488
306
508
549
729
930 1006
1116
1131
1517
1576
4141
4156
4168
HF/6-31+G*
468
483
324
514
558
733
928
979
1093
1099
1495
1554
4136
4154
4166
HF/6-311+G*
471
486
313
506
560
735
923
992
1107
1111
1488
1543
4195
4211
4221
B3LYP/6-31G*
441
458
260
476
527
661
877
947
1042
1058
1430
1494
3808
3814
3830
B3LYP/6-31+G*
436
450
295
482
539
665
873
912
1012
1022
1400
1463
3806
3816
3833
B3LYP/6-311+G* 438
452
274
466
536
668
871
926
1019
1031
1392
1454
3838
3847
3862
MP2/6-31G*
439
457
274
491
542
670
879
963
1057
1074
1446
1510
3825
3842
3853
MP2/6-31+G*
435
451
289
487
546
672
872
928
1024
1036
1410
1473
3797
3819
3830
MP2/6-311+G*
443
459
262
467
540
673
874
942
1037
1047
1411
1472
3875
3894
3904
154
C. C. Pye