3498
J. Phys. Chem. 1996, 100, 3498-3503
Acidities, Proton Affinities, and Other Thermochemical Properties of Hypohalous Acids HOX (X ) F-I): A High-Level Computational Study Mikhail N. Glukhovtsev,1a,b Addy Pross,*,1a,c and Leo Radom*,1d School of Chemistry, UniVersity of Sydney, Sydney, NSW 2006, Australia; Department of Chemistry, Ben-Gurion UniVersity of the NegeV, Beer SheVa, Israel; and Research School of Chemistry, Australian National UniVersity, Canberra, ACT 0200, Australia ReceiVed: August 31, 1995X
The acidities, proton affinities, ionization energies, dissociation energies, and heats of formation of the hypohalous and hydrohalic acids have been calculated at the G2 level of theory. Where reliable experimental data are available, our results are generally in good agreement but in other cases our predictions serve to fill important gaps. The calculated gas-phase acidities of the hypohalous acids (1507.9 (HOF), 1490.0 (HOCl), 1490.6 (HOBr), and 1487.0 (HOI) kJ mol-1 at 298 K) agree well with available experimental data and are close to one another (lying within a range of 20.9 kJ mol-1), showing that the nature of the halogen has relatively little impact on their acidity. In contrast, the ∆Hacid values for the hydrohalic acids HX increase by 232.9 kJ mol-1 in going from HF to HI. Hypohalous acids are more acidic than water. In addition, hypofluorous acid is a slightly stronger acid than HF. However, other hypohalous acids are weaker than the hydrohalic acids HX (X ) Cl-I). The calculated proton affinities at oxygen (HOX f H2OX+: 565.9 (F), 641.9 (Cl), 678.0 (Br), and 724.7 (I) kJ mol-1 at 298 K) and at the halogen (HOX f HOXH+: 488.7 (F), 581.5 (Cl), 601.0 (Br), and 642.3 (I) kJ mol-1 at 298 K) are larger than PA(HX) values (484.0 (F), 561.5 (Cl), 584.8 (Br), and 626.0 (I) kJ mol-1 at 298 K) for all the halogens. The HOXH+ structures are higher in energy than the O-protonated forms, H2OX+. The ionization energy (IE) values for HOX decrease from HOF (12.71 eV) to HOI (9.89 eV) in a manner parallel to that found for the IE values for HX (X ) F-I). The IE(HOX) values are all smaller than the corresponding IE(HX) values, but the IE difference decreases substantially in going from F to I. The G2 heats of formation for the hypohalous acids (-88.3 (HOF), -76.0 (HOCl), -58.3 (HOBr), and -48.9 (HOI) kJ mol-1 at 298 K) show good agreement with available experimental values.
1. Introduction A knowledge of the thermochemical properties of the hypohalous acids is important in assessing the impact that these species might have on the environment. In the stratosphere, hypochlorous and hypobromous acids, HOCl and HOBr, have been implicated in catalytic cycles that result in the seasonal depletion of ozone during the polar spring in Antarctica2a-f and during the winter in the Arctic.2g,h These compounds are thought to serve as temporary reservoirs for ClOx and BrOx in the stratosphere.3 Hypoiodous acid, HOI, has been suggested to be involved in tropospheric iodide chemistry, particularly in maritime regions.4 The potential role of HOI in the atmosphere as a volatile carrier of radioiodine species (considerable concentration of 131I is released in the ventilation air from uranium-fuelled nuclear reactors) is also noteworthy.5 The chemical properties of the hypohalous acids, which are powerful oxidizing agents, are of particular interest because they reflect an interplay between the σ-inductive role of the halogen atom in HOX and the repulsive interactions between the lone pairs of the adjacent oxygen and halogen atoms.6 The most prominent chemical feature of the hypohalous acids is their instability.6 For example, HOF, which can be prepared from the reaction of molecular fluorine with water, can decompose explosively,6b while the lifetime of gaseous HOI detected mass spectrometrically,7 is about 1-2 min in the reaction chamber.8 This hampers the accurate experimental determination of the properties of these species. As a consequence, some data, for example, estimates of the heats of formation of HOBr and HOI, X
Abstract published in AdVance ACS Abstracts, February 1, 1996.
0022-3654/96/20100-3498$12.00/0
remain controversial.9-12 In fact, there have been no direct experimental determinations of the heat of formation of HOI, although various estimates have been reported.11,12 Theory therefore has a potentially useful role to play in this area. In the present paper, we have considered for the first time the thermochemistry of the complete set of hypohalous acids, from HOF to HOI, focusing on their acidities and proton affinities as well as their ionization energies, dissociation energies, and heats of formation. We have carried out this broad study using G2 theory,13 recently extended by us to bromineand iodine-containing molecules using effective core potentials (ECP).14 This modification of G2 theory, denoted G2(ECP),14,15 is a higher computational level than that used in most of the previous calculations on the various individual members of this series. 2. Computational Methods Standard ab initio molecular orbital calculations16 were carried out using G2 theory13 with the GAUSSIAN 92 system of programs.17 G2 theory corresponds effectively to calculations at the QCISD(T)/6-311+G(3df,2p) level on MP2(full)/6-31G(d) optimized geometries with zero-point vibrational energy (ZPE) and higher level corrections.13 It was introduced by Pople and co-workers13 with the aim of producing molecular energies to within an accuracy of about 10 kJ mol-1. For bromine- and iodine-containing species, the G2(ECP) scheme,14 which includes the use of quasi-relativistic effective core potentials (ECP),18 was used.19 Geometries were optimized using analytic gradient techniques.20 The stationary points on the potential energy surfaces were characterized by calculations of vibrational © 1996 American Chemical Society
Thermochemical Properties of Hypohalous Acids TABLE 1: Calculated Total Energies (Hartrees) for HOX, H2OX+, HOXH+, OX-, HOX•+, HX, H2X+, X-, and X• Species (X ) F-I)a,b species
G2 (0 K)
G2 (298 K)
HOF H2OF+ HOFH+ OFHOF•+ HF H2F+ FF• HOCl H2OCl+ HOClH+ OClHOCl•+ HCl H2Cl+ ClCl• HOBr H2OBr+ HOBrH+ OBrHOBr•+ HBr H2Br+ BrBr• HOI H2OI+ HOIH+ OIHOI•+ HI H2I+ II•
-175.353 40a -175.566 61 -175.538 18 -174.781 03 -174.886 45 -100.350 01a -100.532 48 -99.760 69a -99.632 82a -535.408 58a -535.650 86 -535.628 05 -534.843 15 -535.001 33 -460.340 17a -460.552 13 -459.808 99a -459.676 64a -89.030 38 -89.286 47 -89.257 33 -88.464 68 -88.639 17 -13.945 10c -14.165 97c -13.429 88 -13.308 71 -87.068 02 -87.342 09 -87.310 75 -86.503 68 -86.704 53 -11.961 64 -12.197 77 -11.460 56 -11.351 84
-175.349 58 -175.562 75 -175.533 35 -174.777 63 -174.882 63 -100.346 71 -100.528 69 -99.758 33 -99.630 46 -535.404 70 -535.646 82 -535.623 81 -534.839 59 -534.997 50 -460.336 86 -460.548 36 -459.806 63 -459.674 28 -89.026 44 -89.282 30 -89.253 00 -88.461 06 -88.635 28 -13.941 80c -14.162 17c -13.427 52 -13.306 35 -87.064 05 -87.337 72 -87.306 34 -86.500 06 -86.700 59 -11.957 86 -12.193 94 -11.458 20 -11.349 48
a
Taken from ref 13. b G2(ECP) energies for Br- and I-containing systems. c These energies were incorrectly reported in ref 14a; corrected energies are presented in ref 14b.
frequencies, carried out analytically for fluorine- and chlorinecontaining species and numerically in ECP calculations of bromine- and iodine-containing species. Heats of formation were calculated from the atomization energies using experimental heats of formation of atoms.21 To obtain theoretical enthalpies of reaction at 298 K, enthalpy temperature corrections were derived using harmonic frequencies which were calculated at the HF/6-31G(d) level and scaled by 0.8929 according to the G2 scheme13 and using standard statistical thermodynamics formulas.16 We have used the results of G2 calculations for F- and Clcontaining molecules, and G2(ECP) calculations for Br- and I-containing molecules in our analysis. Throughout this paper, relative energies are presented as enthalpy changes (∆H) at 0 K or 298 K as indicated; bond lengths are in angstroms and bond angles in degrees. G2 total energies of hypohalous and hydrohalic acids, their protonated, deprotonated and ionized forms, as well as of the products of the HOX dissociations are listed in Table 1. 3. Results and Discussion Geometries. The geometries of the hypohalous acids, HOX, and of their O- and X-protonated forms are given in Tables 2-4. Comparison of the MP2/6-31G(d) geometries of HOX (X ) F-Br) with experimental structural data shows reasonable agreement, although the calculated H-O and O-X bonds are slightly too long (Table 2). The experimental indirect esti-
J. Phys. Chem., Vol. 100, No. 9, 1996 3499 TABLE 2: Calculated (MP2/6-31G(d)) and Experimental Geometries of HOX (X ) F-I) molecule HOF HOF HOCl HOCl HOBr HOBr HOI HOI
MP2/6-31G(d) exptlb MP2/6-31G(d) exptlc MP2/6-31G(d)-ECP exptld MP2/6-31G(d)-ECP exptl estimatee
R(H-O) (Å)
R(O-X) (Å)
∠HOX (deg)
0.979 0.966 0.978 0.964 0.980 0.961 0.979 0.96
1.444 1.442 1.717 1.689 1.869 1.834 2.043 1.959-1.995
97.1a 96.8 102.6 103.0 101.7 102.3 103.2 102.4
a This value of the ∠HOF bond angle differs slightly from the value of 97.8° reported in ref 22 calculated at the same theoretical level. b Reference 23a. c Reference 23b. d rs structure taken from ref 23c. e Estimated12 using the HO-X frequencies (X ) F-Br) and the HO-X and O-X bond lengths in HOX and OX• (X ) F-Br). Our calculated vibrational frequencies for HOI (after scaling by 0.8929) (ν1(OH stretch) ) 3657 cm-1, ν2(bend) ) 1092 cm-1 and ν3(OI stretch) ) 578 cm-1) are close to the available experimental values (ν1 (3620 cm-1) and ν2 (1068 cm-1)) for gaseous HOI.8 The frequencies reported for HOI in dilute nitrogen and argon matrices are 3597 (ν1), 1104 (ν2), and 575 (ν3) cm-1 (see ref 8 for a detailed discussion).
TABLE 3: Calculated (MP2/6-31G(d)) Geometries of H2OX+ (X ) F - I) species
R(H-O) (Å)
R(O-X) (Å)
∠XOH (deg)
∠HHXO (deg)
H2OF+ H2OCl+ a H2OBr+ H2OI+
1.008 0.997 0.994 0.989
1.426 1.760 1.930 2.138
102.0 109.4 109.6 112.8
114.2 120.2 119.8 123.5
a Calculations at the CCSD(T)/6-311G(2df,2p) level24 give R(HO) ) 0.980 Å, R(O-Cl) ) 1.752 Å and ∠ClOH ) 109.5°.
TABLE 4: Calculated (MP2/6-31G(d)) Geometries of HOXH+ (X ) F-I) species HOFH+ HOClH+ a HOBrH+ HOIH+
R(H-O) R(O-X) R(X-H) ∠HOX ∠OXH ∠HOXH (Å) (Å) (Å) (deg) (deg) (deg) 0.999 0.992 0.991 0.989
1.497 1.665 1.805 1.970
0.995 1.307 1.456 1.647
96.9 106.1 105.9 107.9
100.8 99.1 97.2 96.2
159.0 100.8 95.5 91.9
a Calculations at the CCSD(T)/6-311G(2df,2p) level24 give R(HO) ) 0.977 Å, R(O-Cl) ) 1.653 Å, R(Cl-H) ) 1.311 Å, ∠ClOH ) 105.1°, ∠OClH ) 99.7° and ∠HOClH ) 96.3°.
mates12 for HOI are close to the calculated geometry. As expected on the basis of the VSEPR model,25 the ∠HOX bond angles in hypohalous acids are smaller than the ∠HOH bond angle in the water molecule, the ∠HOF angle being the smallest. We are not aware of any experimental data on the structures of the protonated forms of hypohalous acids, although these species have been recently generated in the gas phase.26 Comparison of our results with the geometries of H2OCl+ and HOClH+ calculated at the CCSD(T)/6-311G(2df,2p) level24 suggests that our MP2/6-31G(d) H-O and O-Cl bond lengths are slightly overestimated (Tables 3 and 4). A previous analysis,27 using the Laplacian of the charge density in HOF, H2OF+, and HOFH+, has indicated that protonation at the more electronegative atom (F) results in considerable charge depletion in the O-F bond, which therefore becomes weaker and longer compared to the neutral HOF. Conversely, protonation at the less electronegative center (O) actually draws some electronic charge into the O-F bonding region, thereby slightly strengthening the O-F bond. Indeed, whereas the O-F bond length in HOFH+ is elongated with respect to that in HOF, the O-F bond length in H2OF+ is shorter than that in HOF (Tables 2-4). The opposite relationship between the O-X bond lengths is
3500 J. Phys. Chem., Vol. 100, No. 9, 1996
Glukhovtsev et al.
TABLE 5: Calculated (MP2/6-31G(d)) Geometries of HOX•+ Radical Cations (X ) F-I)
TABLE 8: Comparison of G2 Heats of Formation for OXwith Experimental Values (kJ mol-1)
R(H-O) (Å)
R(O-X) (Å)
∠HOX (deg)
aniona
G2 ∆Hf0
G2 ∆Hf298
exptl ∆Hf298b
species HOF•+ HOCl•+ HOBr•+ HOI•+
OF-
1.024 0.991 1.004 0.994
1.286 1.577 1.723 1.918
103.8 108.0 110.2 110.7
OClOBrOI-
-112.1 -118.0 -92.0 -91.8
-111.2 -116.9 -98.5 -92.7
-89 ( 13 -108 ( 18
