Infrared spectrum of Hg(OH)2 in solid neon and argon

Article abstract of DOI:10.1021/ic048673w

Mercury(II) hydroxide molecules have been prepared upon mercury arc lamp irradiation of Hg, H₂, and O₂ mixtures in solid neon and argon. The strongest three infrared absorptions are identified through isotopic substitution (D₂, HD, ¹⁸O₂,¹⁶O ¹⁸O) and comparison to frequencies from DFT calculations. The isolated Hg(OH)₂ molecule is stable and has a linear O-Hg-O linkage in a C₂ structure with an 86° dihedral angle. However, in aqueous solution Hg²⁺ and 2OH^- may form an Hg(OH)₂ intermediate, which eliminates water and precipitates solid HgO: The solid Hg(OH)₂ compound is not known.

Full text of DOI:10.1021/ic048673w

Inorg. Chem. 2005, 44, 108−113
Infrared Spectrum of Hg(OH)2 in Solid Neon and Argon
Xuefeng Wang and Lester Andrews*
Department of Chemistry, UniVersity of Virginia, McCormick Road, P.O. Box 400319,
CharlottesVille, Virginia 22904-4319
Received September 22, 2004
Mercury(II) hydroxide molecules have been prepared upon mercury arc lamp irradiation of Hg, H2, and O2 mixtures
in solid neon and argon. The strongest three infrared absorptions are identified through isotopic substitution (D2,
HD, 18O2, 16O18O) and comparison to frequencies from DFT calculations. The isolated Hg(OH)2 molecule is stable
+
and has a linear O−Hg−O linkage in a C2 structure with an 86°
dihedral angle. However, in aqueous solution Hg2
and 2OH- may form an Hg(OH)2 intermediate, which eliminates water and precipitates solid HgO: The solid
Hg(OH)2 compound is not known.
Introduction
The most closely related compound HgH2 is reported to
be an unstable solid, which decomposes at -125 °C.13 The
HgH2 molecule has been formed by 247 nm laser irradiation
of Hg, H2 samples in excess argon, hydrogen, and nitro-
gen14,15 and by mercury arc lamp irradiation of Hg atoms in
solid hydrogen.16 Recent work has shown that irradiation of
mercury/water mixtures in excess argon gives the HHgOH
molecule.17 Here we employ a mercury arc street lamp to
form Hg(OH)2 molecules from mixtures of Hg, H2, and O2
in excess neon and argon and report the first experimental
evidence for the Hg(OH)2 molecule. In addition, we compare
the infrared spectrum with vibrational frequencies calculated
by density functional theory for the stable isolated Hg(OH)2
molecule.
Mercury is an exotic element. Divalent mercury forms a
number of binary compounds, and many of these are toxic.1-3
The common HgCl2 compound, calomel, is a volatile
covalent molecular solid stabilized by “metallophilic” at-
traction, which gives a linear molecule in the vapor.4-7 The
addition of OH- to Hg2+ in aqueous solution precipitates
the yellow solid HgO, which consists of zigzag -O-Hg-
O-Hg- chains with linear O-Hg-O units, and it is possible
that Hg(OH)2 is a transient intermediate in this precipitation
reaction.1,8 The Hg(OH)2 molecule has been computed,9 but
there is no evidence for the gaseous molecule nor the solid
compound1,2,8 although Hg(OH)2 is thought to coprecipitate
with Fe(OH)3.10 The presence of mercury in industrial wastes
is a significant environmental problem, and precipitation
reactions provide a means of recovery. Furthermore, gas
phase elemental mercury is rapidly oxidized to Hg(II)
compounds in the arctic regions,11 and Hg(OH)2 may play
an intermediate role in this process.12
Experimental and Computational Methods
The technique for reactions of metal atoms with hydrogen during
condensation in excess hydrogen and neon has been described in
detail previously.18,19 Mercury atoms were evaporated from the
liquid at 50 °C in a stainless steel, valved system. Mercury atoms
were thus codeposited with hydrogen (2 to 8%) and oxygen (0.2-
2.0%) in excess neon, argon, or pure deuterium onto a 5 K CsI
cryogenic window at 2-4 mmol/h (Ne, Ar for 1 h and pure D2 for
* Corresponding author. E-mail: lsa@virginia.edu.
(1) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. AdVanced
Inorganic Chemistry, 6th ed.; Wiley: New York, 1999.
(2) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, 2nd ed.;
Butterworth-Heinemann: Oxford, U.K., 1997.
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ed.; Merck, Englewood Clifts, NJ, 2001; pp 1050-1053.
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(11) Schroeder, W. H.; Anlauf, K. G.; Barrie, L. A.; Lu, J. Y.; Steffen, A.;
Schneeberger, D. R.; Berg, T. Nature 1998, 394, 331.
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(13) Wiberg, E.; Henle, W. Z. Naturforsch., B: Chem. Sci. 1951, 6b, 461.
(14) Legay-Sommaire, N.; Legay, F. Chem. Phys. Lett. 1993, 207, 123.
(15) Legay-Sommaire, N.; Legay, F. J. Phys. Chem. 1995, 99, 16945.
(16) Wang, X.; Andrews, L. Inorg. Chem. 2004, 43, 7146.
(17) Macrae, V. A.; Greene, T. M.; Downs, A. J. Phys. Chem. Chem. Phys.
2004, 6, 4586.
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(6) Balabanov, N. B.; Peterson, K. A. J. Chem. Phys. 2003, 119, 12271
and references therein.
(7) Bu¨chler, A.; Stauffer, J. L.; Klemperer, W. J. Chem. Phys. 1964, 40,
3471.
(8) Holleman, A. F.; Wiberg, E. Lehrbuch der Anorganischen Chemie;
Wiberg, N., Ed.; Walter de Gruyter: Berlin, 1995; Vol. 101, p 1389.
(9) Cundari, T. R.; Yoshikawa, A. J. Comput. Chem. 1998, 19, 902.
(10) Inove, I.; Munemori, M. EnViron. Sci. Technol. 1979, 13, 443.
(18) Andrews, L.; Citra, A. Chem. ReV. 2002, 102, 885 and references
therein.
(19) Andrews, L. Chem. Soc. ReV. 2004, 33, 123 and references therein.
108 Inorganic Chemistry, Vol. 44, No. 1, 2005
10.1021/ic048673w CCC: $30.25
© 2005 American Chemical Society
Published on Web 11/18/2004

IR Spectrum of Hg(OH)2 in Solid Ne and Ar
Figure 1. Infrared spectra of neon matrix samples codeposited at 5 K
with mercury vapor: (a) Ne and Hg with 2% H2 and 0.2% O2, (b) after λ
> 220 nm mercury arc irradiation for 25 min, (c) after annealing to 12 K,
and (d) Ne and Hg with 4% H2 after irradiation. W denotes water, and WC
identifies water complex absorptions.
Figure 2. Infrared spectra of neon and argon matrix samples codeposited
with mercury vapor, hydrogen, and isotopic oxygen molecules, and irradiated
at λ > 220 nm: (a) Ne, 2% H2, 0.2% 16O2, (b) Ne, 3% H2, 0.4% 16,18O2,
(c) Ne, 2% H2, 0.3% 18O2, (d) Ar, 8% H2, 0.8% 16O2, (e) Ar, 10% H2, 1%
16,18O2, and (f) 8% H2, 0.8% 18O2. W denotes water.
30 min). The D2, HD (Cambridge Isotopic Laboratories), 18O2, and
16,18O2 (Yeda) reagents were used in different experiments. FTIR
spectra were recorded at 0.5 cm-1 resolution with 0.1 cm-1 accuracy
on a Nicolet 750 using an HgCdTe detector. Matrix samples were
annealed at different temperatures, and selected samples were
subjected to broadband photolysis by a medium-pressure mercury
arc street lamp (Philips, 175W) with globe removed.
Density functional theoretical calculations of mercury hydroxide
are given for comparison. The Gaussian 98 program20 was employed
to calculate the structures and frequencies of expected product
molecules using the B3LYP functional. The 6-311++G(3df,3pd)
basis set for O and H atoms and SDD pseudopotential for Hg were
used. All the geometrical parameters were fully optimized, and the
harmonic vibrational frequencies were obtained analytically at the
optimized structures.
3737.5 cm-1 water band does not grow on irradiation.
Annealing to 12 K slightly increased the 3702 and 1612 cm-1
bands but decreased the other bands by 20-30%. Without
oxygen the spectrum shows only the HgH2 reaction product
(Figure 1d). A similar experiment with Ne, Hg, O2, and no
H2 gave no observable product absorptions.
Similar investigations with 18O2 substitution shifted the
new absorptions to 3695, 3631.7, 925.2 and 614.1 cm-1. The
weak 2118.2 cm-1 band was not shifted. A scrambled 16,18O2
sample gave doublet absorptions at 3702, 3695 cm-1 and
3642, 3672 cm-1, a partially resolved triplet at 929, 927.1,
925 cm-1, and a triplet at 644.0, 632.3, 614.0 cm-1. Spectra
showing the effect of oxygen isotopic substitution are
compared in Figure 2, and the product absorptions are listed
in Table 1.
Results
Experiments with D2 and 16O2 gave strong HgD2 absorp-
tions at 1378 and 562 cm-1 and new product absorptions at
2733, 2685.0, 1519.2, 711.5, and 619.9 cm-1. The new
product bands again shifted with 18O2 as listed in Table 1,
and 16,18O2 also gave doublet absorptions in the upper region
and partially resolved triplet absorptions in the lower region,
which are illustrated in Figure 3.
An investigation with 16O2 and HD was particularly
productive: Strong absorptions were observed for HgHD at
1968, 1407, and 683 cm-1. New absorptions were observed
at 3697, 3643.1, 2687.5, 934.3, 701.8, and 632.4 cm-1. The
latter, lower frequency bands are also shown in Figure 3.
Although no absorption was observed at 2118 cm-1, a weak
deuterium counterpart was observed at 1520.6 cm-1.
Argon. Analogous experiments were done with higher
reagent gas concentrations and Hg vapor in excess argon
codeposited at 8 K. The samples were irradiated by the full
mercury arc for 15 min, and HgH2 absorptions14,21 at 1895
and 773 cm-1 and new product absorptions at 3629.4, 927.1,
and 637.3 cm-1 were produced along with water and water
dimer bands. Argon matrix bands were weaker and sharper
Infrared spectra and density functional calculations will
be presented for the major products in the photochemical
reaction of mercury with hydrogen and oxygen.
Neon. Figure 1 illustrates infrared spectra of the products
formed by the reaction of mercury with hydrogen and oxygen
in excess neon. The first sample with 2% H2, 0.2% O2, and
Hg vapor (0.1-0.2%) codeposited at 5 K has no infrared
absorption except for the usual trace impurities (H2O, CO2,
CO, CH4). Full mercury arc irradiation (λ > 220 nm) for 25
min produces strong HgH2 absorptions16 at 1918.1 and 781.7
cm-1 and new absorptions at 3702, 3642.3, 2118.2, 1612
(not shown), 928.8, and 644.2 cm-1. Note that the weak
(20) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels,
A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone,
V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.;
Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R.
L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara,
A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.;
Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle,
E. S.; Pople, J. A. Gaussian 98, revision A.11.4; Gaussian, Inc.:
Pittsburgh, PA, 1998.
(21) Greene, T. M.; Brown, E.; Andrews, L.; Downs, A. J.; Chertihin, G.
V.; Runeberg, N.; Pyykko¨, P. J. Phys. Chem. 1995, 99, 7925.
Inorganic Chemistry, Vol. 44, No. 1, 2005 109

Wang and Andrews
Table 1. Infrared Absorptions (cm-1) Produced by Mercury Arc Irradiation of Solid Neon or Argon Containing Hydrogen, Oxygen, and Mercury
H2, 16O2
H2, 18O2
H2, 16,18O2
HD, 16O2
D2, 16O2
D2, 18O2
D2, 16,18O2
identity
Neon Matrix
3702
3642.3
2118.2
1918
1612
928.8
782
3695
3631.7
2118.2
1918
1605
925.2
782
3702, 3695
3642, 3632
2118.2
3697 br
3643.1, 2687.5
1520.6
2733
2684.0
1519.2
1368
1180
711.5
562
2717
2668.7
1519.2
1368
1171
688.4
562
2732, 2717
2685, 2668
1519.2
HOH‚‚‚X
Hg(OH)2
HHgOH
HgH2
HOH‚‚‚X
Hg(OH)2
HgH2
1918
1968, 1407
1368
1612, 1605
929, 927.1, 925
782
1180, 1171
712, 704, 689
562
934.3
683
644.2
551
614.1
524
644.0, 632.3, 614.0
701.8, 632.4
619.9
598.2
512
620, 612, 599
Hg(OH)2
[Hg(OH)2]2
Argon Matrix
3629.4
1896
927.1
773
3618.6
1896
923.7
773
3629.5, 3618.4
1986
927, 924.9, 924
773
3629.7, 2677.9
1948, 1392
932.1
2677.7
1363
704.5
555
Hg(OH)2
HgH2
Hg(OH)2
HgH2
675
637.3
607.8
637.3, 626.2, 607.8
698.9, 626.9
Hg(OH)2
Table 2. Infrared Absorptions (cm-1) Produced by Mercury Arc
Irradiation of Solid Deuterium Containing O2 and Hg
were observed at 2690.6, 2679.6, 1523.9, 709.4, 652, 617.9,
and 569.1 cm-1. Continued irradiation substantially increased
the HgD2, D2O, 2690.6, 1523.9, 652, and 569.1 cm-1
absorptions and slightly increased the 2679.6, 709.4, and
617.9 cm-1 bands as shown in Figure 4. A similar irradiation
of solid D2 containing O2 without Hg failed to produce D2O.
16O2
identity
D2O
18O2
2782
2762
2773.9
2690.6
2679.6
2661.5
2611
2531.1
1523.9
1368.9
1178.3
1124.2
1024.6
709.4
652
D2O
2754.0
2674.3
2663.4
2650.8
2599
2515.3
1523.8
1368.9
1169.9
DHgOD
DOHgOD
D2O
(D2O)2
DO2
DHgOD
HgD2
D2O
DO2
DO2
1002.9
691.2
644
596.5
544.7
556.0
554.7
DOHgOD
DHgOD
DOHgOD
DHgOD
HgD2
617.9
569.1
556.0
554.7
HgD2
than neon matrix counterparts. The samples were annealed
to 12 K and photolyzed for another 20 min: The product
absorptions doubled their intensity. The samples were
annealed to 14 K and photolyzed again, and the product
absorptions increased another 50%. The new product absorp-
tions in solid argon are illustrated in Figure 2. Isotopic
precursor molecules were reacted in a like manner; infrared
spectra are compared in Figure 2, and band positions are
listed in Table 1.
Figure 3. Infrared spectra of neon matrix samples codeposited at 5 K
with mercury vapor and irradiated at λ > 220 nm: (a) Ne and Hg with 2%
H2 and 0.2% O2, (b) Ne and Hg with 3% HD and 0.3% O2, (c) Ne and Hg
with 4% D2 and 0.4% 16O2, (d) Ne and Hg with 3% D2 and 0.4% 16,18O2,
and (e) Ne and Hg with 2% and 0.2% 18O2. WC indicates water complex.
Deuterium. Mercury and O2 were subjected to irradiation
in two solid deuterium samples, and the spectra revealed a
different distribution of analogous product absorptions.
Strong HgD2 bands at 1398 and 555 cm-1 and D2O
absorptions at 2782, 2773.9, 2661.5, and 1178.3 cm-1 and
(D2O)2 at 2611 cm-1 were observed along with DO2 at
2531.1, 1124.2, and 1024.6 cm-1.22-24 Additional weak bands
(22) The solid D2 values for DO2 frequencies are nearer to those for the
neon matrix: Thompson, W. E.; Jacox, M. E. J. Chem. Phys. 1989,
91, 3826. Values for the argon matrix: Smith, D. W.; Andrews, L. J.
Chem. Phys. 1974, 60, 81.
(23) Wang, X.; Andrews, L. J. Am. Chem. Soc. 2003, 125, 6581.
(24) For comparison of D2O in p-H2 see: Fajardo, M. E.; Tam, S.; DeRose,
M. E. J. Mol. Struct. 2004, 696, 111. For D2O and (D2O)2 in argon
see: Ayers, G. P.; Pullen, A. D. E. Spectrochim. Acta 1976, 32A,
1629 and references therein.
Figure 4. Infrared spectra of deuterium matrix sample codeposited at 5
K with mercury vapor: (a) D2 and 0.1% O2, (b) after λ > 220 nm irradiation
for 22 min, and (c) after λ > 220 nm irradiation for 20 additional min. X
denotes common unknown photolysis product.
110 Inorganic Chemistry, Vol. 44, No. 1, 2005

IR Spectrum of Hg(OH)2 in Solid Ne and Ar
Table 3. Vibrational Frequencies (cm-1) and Intensities (km/mol) of Hg(OH)2 and HHgOH Calculated at the B3LYP/6-311++G(3df,3pd)/SDD Level
a
Hg(OH)2
mode
HOHgOH
DOHgOD
HOHgOD
H18OHg18OH
H16OHg18OH
D18OHg18OD
D16OHg18OD
O-H str
3828.0(13)
3826.8(101)
957.2(10)
2787.0(8)
2785.8(65)
706.2(99)
705.0(6)
3827.4(57)
2786.4(36)
951.6(44)
705.7(56)
598.4(87)
3815.3(12)
3814.2(98)
953.6(9)
942.0(75)
578.2(104)
3827.4(53)
3814.8(59)
955.7(11)
943.7(76)
595.7(104)
2769.5(8)
2768.4(61)
698.0(6)
695.5(78)
566.0(74)
2786.4(34)
2768.8(36)
705.4(49)
697.0(46)
580.0(68)
O-H str
Hg-O-H bend
Hg-O-H bend
Hg-O str
946.1(81)
606.9(112)b
588.7(67)
HHgOHc
mode
HHgOH
DHgOD
HHgOD
DHgOH
HHg18OH
DHg18OD
O-H str
3846.3(42)
2126.0(123)
871.8(40)
584.7(5)
579.2(11)
556.2(74)
2800.1(29)
1507.7(64)
647.5(45)
543.9(53)
422.0(4)
2800.1(29)
2125.8(122)
688.9(30)
583.2(7)
557.8(7)
525.1(2)
3846.2(42)
1508.1(66)
860.2(40)
556.2(70)
424.8(9)
3833.6(41)
2126.0(122)
868.1(38)
583.8(4)
578.8(12)
527.5(67)
2782.6(27)
1507.7(64)
639.3(36)
519.2(54)
420.9(4)
Hg-H str
Hg-O-H bend
H-Hg-O bend
H-Hg-O bend
Hg-O str
414.2(7)
424.2(2)
413.8(7)
a Structure given in Figure 5 for 1A ground state. b Four lower frequencies: 541.2(0), 167.0(5), 152.1(3), and 123.2(78) cm-1 (km/mol). c Planar 1A′
ground state. H-Hg, 1.601 Å; Hg-O, 2.021 Å; O-H, 0.962 Å; HHgO, 176.7°; HgOH, 111.3°.
and 2677.7 cm-1: These bands exhibit large H/D ratios
(1.357 and 1.355) that are characteristic of O-H stretching
modes. Crucial information is obtained with the HD reagent,
which gives new bands at 3643.1 and 2687.5 cm-1 in neon
Figure 5. Structure of Hg(OH)2 calculated at the B3LYP/6-311++G-
(3df,3pd)/SDD level. Bond lengths in angstroms and angles in degrees.
and at 3629.7 and 2677.9 cm-1 in argon in the upper region,
and at 701.8 and 632.4 cm-1 in neon and at 698.9 and 626.9
cm-1 in argon in the lower region. Thus, with HD we observe
two slightly different O-H and O-D stretching modes and
Hg-O-H and Hg-O-D bending modes, and the Hg(OH)2
molecule is thereby identified. There is precedent for
dihydroxide molecules: the Be(OH)2 molecule is a major
product of the laser-ablated Be atom and H2O reaction,26 and
the Ca(OH)2 molecule is formed by photolysis of Ca and
H2O in excess argon.27
Assignment of the 3642.3, 928.8, and 644.2 cm-1 absorp-
tions to the Hg(OH)2 molecule is confirmed by B3LYP
harmonic frequency calculations, which predict the strongest
three infrared absorptions at 3826.8, 946.1, and 606.9 cm-1.
These predictions, 5.0% high, 1.9% high, 5.8% low, respec-
tively, deviate more than previous results for first row
transition metals,28 but the Hg-O stretching mode is difficult
to model. The 18O shifts, however, are predicted accurately
for these modes (calculated/observed: 12.6/10.6 cm-1, 3.6/
3.6 cm-1, 28.7/28.1 cm-1), respectively.
DHgOD. The solid deuterium experiment was performed
for the ultimate high concentration, and similar bands were
observed at 2679.6, 709.4, and 617.9 cm-1, which are
between the above neon and argon matrix observations for
Hg(OD)2. Three additional strong, sharp bands at 2690.6,
1523.9, and 569.1 cm-1 and a weaker, broad band at 652
cm-1 can be assigned to the related DHgOD molecule. First,
the 1523.9, 652, and 569.1 cm-1 bands are slightly higher
than 1519.2, 646.7, and 565.6 cm-1 argon matrix counterparts
for the DHgOD product of the photochemical Hg + D2O
reaction where the 2700 cm-1 region was drowned in excess
D2O.17 Our B3LYP calculation predicts the O-D stretch 14.3
cm-1 higher than that for Hg(OD)2, and our new associated
A like investigation of Hg and 18O2 in solid D2 gave the
same HgD2 bands, D218O, and D18O2: Additional weak bands
are listed in Table 2. Continued irradiation behaved as
described above.
Calculations. DFT calculations were performed on the
two anticipated product species HHgOH and Hg(OH)2, and
the results are given in Table 3. Our structures are similar
to those computed previously.9,17 The 1A Hg(OH)2 molecule
has C2 symmetry, a linear O-Hg-O linkage, and a 85.8°
dihedral angle as illustrated in Figure 5. The high energy
3
-
OHgO molecule is calculated to be stable in a Σg state.25
Discussion
The new absorptions will be assigned on the basis of
observed isotopic shifts and splittings and comparison with
isotopic frequencies computed by density functional theory.
Hg(OH)2. The major product requires three elements, Hg,
H2, and O2, and the transient molecule Hg(OH)2 is identified
from the mixed isotopic spectra shown in Figure 2. The
644.0, 632.3, 614.0 cm-1 triplet in neon and the correspond-
ing 637.3, 626.2, 607.8 cm-1 triplet in argon using scrambled
isotopic oxygen (16O2 + 16O18O + 18O2) clearly demonstrate
that two equivalent oxygen atoms are involved in this
vibrational mode. The large 16O2 to 18O2 shift (16/18 ratio
1.0489 (neon) and 1.0485 (argon)) identifies this predomi-
nately antisymmetric HO-Hg-OH motion. The weaker
928.8 cm-1 absorption also gives a triplet band contour, and
the small 16O2 to 18O2 shift and large H2 to D2 shift
characterize an Hg-O-H bending mode. The associated
band at 3642.3 cm-1 shifts to 2684.0 cm-1 with D2 in neon,
and the corresponding argon matrix counterparts are 3629.4
(26) Thompson, C. A.; Andrews, L. J. Phys. Chem. 1996, 100, 12214.
(27) Kauffman, J. W.; Hauge, R. H.; Margrave, J. L. High Temp. Sci. 1984,
18, 97.
(25) The OHgO (3Σg-) molecule has computed 1.894 Å Hg-O length and
frequencies (intensities) 698 cm-1 (15 km/mol), 625 (0), 184 (12 ×
2).
(28) Bytheway, I.; Wong, M. W. Chem. Phys. Lett. 1998, 282, 219.
Inorganic Chemistry, Vol. 44, No. 1, 2005 111

Wang and Andrews
band at 2690.6 cm-1 is 11.0 cm-1 higher. The 16.3 cm-1
observed 18O shift is in good agreement with the 17.5 cm-1
calculated value, which for a single O-D bond is necessarily
less than the 19.7 cm-1 18O shift observed for the sharp
2772.2 cm-1 antisymmetric D2O stretching mode. The DHg-
OD stretching mode observed here at 569.1 cm-1 shifts to
544.7 cm-1 with 18O substitution, in excellent agreement with
the calculated 24.7 cm-1 shift. Unfortunately, interaction with
the D2 matrix broadened the 652 cm-1 DHgOD bending
mode.
Our B3LYP calculation predicts harmonic frequencies in
good agreement with observed values for DHgOD as does
a previous BP calculation.17 The O-D stretch is predicted
4.0% high, which is in the range for this functional, but the
Hg-D stretch and Hg-O stretch are predicted 1.0% and 4.4%
too low. Our density functional calculations for mercury
compounds are only approximate, but they support the recent
argon matrix assignments.17
The strongest band, the Hg-H stretching mode, was
observed for HHgOH at 2118.2 cm-1 in solid neon, just 1.7
cm-1 higher than the recent argon matrix observation,17 and
the DHgOD counterpart was observed at 1519.2 cm-1. We
note that with Hg, HD, and O2 in neon only the DHgOH
isotopomer is formed. We provide additional information on
the DHgOD transient species, namely the O-D stretching
mode and 18O shift for the Hg-O stretching mode, and its
photochemical synthesis by a different route, which sub-
stantiates its recent identification.17
The HHgOH molecule may be compared with HXeOH
formed in the 193 nm induced reaction of H2O in pure
xenon.29 The most interesting mode observed here for
DHgOD in solid deuterium, the O-D stretching mode, was
not observed for DXeOD, but the calculated O-D stretching
frequency and bond length for these two species are nearly
the same.
Other Absorptions. Two broad absorptions are produced
at 3702 and 1612 cm-1 on irradiation in neon experiments.
Their position near water absorptions invites our consider-
ation of water, and their shift down from isolated water at
3737.5 cm-1 and up from H2O at 1596.3 cm-1 shows that
hydrogen bonding is involved.30 Hence, the 3702 and 1612
cm-1 bands are due to a water complex HOH‚‚‚X where
H2O and X are photolysis products. The isotopic shifts are
in agreement with this assignment. As expected, the photo-
chemical yield of water is highest in the solid D2 experiments.
In addition, DO2 radicals22,23 are observed in the higher yield
pure D2 investigations, which shows that D atoms are formed
in the photochemical reactions with Hg atoms.
Figure 6. Relative energies computed at the B3LYP/6-311++G(3df,3pd)/
SDD level for important Hg, O2, H2 species.
Reaction Mechanisms. The mercury dihydroxide mol-
ecule is prepared here by the reaction of excited Hg with O2
and H2, reaction 1, in the matrix cage. The excited Hg(3P)
state (+112.6 kcal/mol)31 is formed directly by mercury arc
resonance radiation (254 nm) to activate this reaction. The
stable 1A Hg(OH)2 molecule is calculated to be 54 kcal/mol
lower in energy than Hg(2S) + O2 + H2 at the B3LYP level.
In addition, we have evidence for H2O and HHgOH
formation, which are likely products of reaction 2 with more
dihydrogen. Molecular oxygen is not photodissociated at 254
nm,32 and full arc irradiation of solid H2 containing O2 gives
no photolysis products, but when Hg is added, HgH2 is
formed and O2 can react with both H2 and HgH2 to give
H2O and HHgOH. Reaction 2 can, of course, give Hg and
2H2O as well. Water and water dimer (Figures 2d and 4c)
are also produced on full arc irradiation.
On the other hand, HXeOH is formed by a different
mechanism in the 193 nm induced reaction of Xe and H2O,
which involves H and OH radicals.29 We do not detect OH
radicals33 in this work, and OH is not a likely reagent in our
254 nm excited photochemical reactions.
Hg(3P) + O2 + H2 f (OHgO)* + H2 f HOHgOH (1)
Hg(3P) + O2 + 2H2 f HHgOH + H2O
(2)
Finally, a weak 551 cm-1 band shifts to 524 cm-1 with
18O and exhibits a 1.052 16O/18O ratio, and the latter band
has a 1.023 H/D ratio, which are comparable to the values
observed for Hg(OH)2. This suggests that the 551 cm-1 band
is due to a Hg-O vibration, coupled with H, and the Hg-
(OH)2 dimer is a likely carrier for this band.
An interesting question is the mechanism of reaction 1.
The reaction of Hg (3P) and H2 proceeds directly to HgH2,
as shown in previous work14-16 and in Figure 1d. It is
counterintuitive to suggest that reaction 1 proceeds through
(31) Moore, C. E. Atomic Energy LeVels; Circular 467; National Bureau
of Standards: Washington, DC, 1952.
(29) Petterson, M.; Khriachtchev, L.; Lundell, J.; Rasanen, M. J. Am. Chem.
Soc. 1999, 121, 11904.
(30) Pimentel, G. C.; McClellan, A. L. The Hydrogen Bond; W. H.
Freeman: San Francisco, 1960.
(32) Huber, K. P.; Herzberg, G. Constants of Diatomic Molecules; Van
Nostrand Reinhold: New York, 1979.
(33) Cheng, M. B.-M.; Lee, Y.-P.; Ogilvie, J. F. Chem. Phys. Lett. 1988,
109, 151.
112 Inorganic Chemistry, Vol. 44, No. 1, 2005

IR Spectrum of Hg(OH)2 in Solid Ne and Ar
HgH2 although HgO is very weakly bound34 and evidence
for HgO2 is tenuous.35 Following the preparation of stable
and the total energy of HgO and H2O molecules is ap-
proximately 57 kcal/mol above that for the Hg(OH)2
molecule. Hence, the thermodynamic driving force for the
precipitation of HgO from solution must arise mainly from
the stability of the solid covalent chain zigzag HgO
compound.37
3Σg states for the OZnO and OCdO molecules,36 and the
-
recent formation of stable Zn(OH)2 and Cd(OH)2 molecules
-
in this laboratory, we have calculated a physically stable 3Σg
state for OHgO, which is 72.5 kcal/mol higher in energy
-
than Hg(1S) + O2, but this does not preclude OHgO (3Σg )
Conclusions
as a reactive intermediate in reaction 1. The relative
computed energies of important Hg, O2, and H2 species are
given in Figure 6: Note that OHgO is 40 kcal/mol more
stable than Hg(3P) + O2. We suggest that OHgO produced
in the initial reaction then inserts into dihydrogen to form
HOHgOH. Recall that no mercury oxides are observed when
Hg and O2 are irradiated in excess neon. Hence, H2
chemically “traps” the OHgO subunit as the stable HOHgOH
molecule. The strongest Hg(OH)2 absorption at 637.3 cm-1
was also observed but not assigned in recent photochemical
studies of Hg and H2O in solid argon.17 Therefore, the Hg-
(OH)2 molecule is stable enough to result from the reaction
of excited Hg and two water molecules.
We have prepared the stable Hg(OH)2 molecule by
mercury arc irradiation of Hg, H2, and O2 mixtures in solid
neon and argon. The strongest three infrared absorptions are
identified through isotopic substitution (D2, HD, 18O2, 16O18O)
and comparison to frequencies from DFT calculations. The
isolated Hg(OH)2 molecule is stable and has a linear
O-Hg-O linkage in a C2 structure with a 85.8° dihedral
angle. A like irradiation of pure D2 doped with Hg and O2
formed Hg(OD)2 and DHgOD. The photochemical reaction
is believed to proceed through a linear OHgO intermediate
to the stable HOHgOH molecule. In aqueous solution, Hg2+
and 2OH- may form the Hg(OH)2 intermediate molecule,
which eliminates H2O and precipitates solid HgO; hence,
solid Hg(OH)2 is not known.1,2,8
Even though Hg(OH)2 is a very stable molecule, solid Hg-
(OH)2 does not precipitate from aqueous solution.1 The HgO
molecule is computed to be bound by about 4 kcal/mol,34
Acknowledgment. We gratefully acknowledge support
from NSF Grant CHE 03-52487 and a preprint of ref 17
from T. M. Greene.
(34) Shepler, B. C.; Peterson, K. A. J. Phys. Chem. A 2003, 107, 1783.
(35) Butler, R.; Katz, S.; Snelson, A. J. Phys. Chem. 1979, 83, 2579.
(36) Chertihin, G. V.; Andrews, L. J. Chem. Phys. 1997, 106, 3457.
(37) Aurivillius, K. Acta Chem. Scand. 1964, 18, 1305.
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