J . Phys. Chem. 1994, 98, 6980-6987
6980
G2 ab Initio Calculations on the F+
+ OH2 Singlet and Triplet Potential Energy Surfaces
A. Luna, M. Manuel, 0. M6,' and M. YBiiez Departamento de Quimica, C-9, Universidad Autbnoma de Madrid, Cantoblanco, 28049- Madrid, Spain Received: February 9, 1994; In Final Form: May 4, 1994"
The Gaussian-2 (G2) theoretical procedure, based on ab initio molecular orbital theory, is used to study the potential energy surfaces corresponding to [H2,0,F]+ singlet and triplet state cations which can be generated in the gas-phase reactions between F+(3P)and F+('D) with water. Important differences between singlets and triplets, regarding both their bonding and their stabilities, have been found. The most outstanding result is that although the first 'D excited state of F+ is about 2.6 eV above the 3Pground state, the most stable product of the reaction between F+ and water corresponds to a singlet state cation, l(S). This is so because, in general, triplet state cations are ion-dipole complexes, while the singlets are covalently bound species. Single charge transfer is the most likely process in F+ water reactions. Very likely, this resonant charge transfer occurs into an excited state of OHz+. The formations of OH+, FH+, and O+ are also exothermic processes. The estimated heat of formation of the most stable H20F+ species is 21 1.4 kcal/mol.
+
Introduction The reactions of halogen positive ions in the gas phase are of great interest because (a) they are involved in the formation of many species in interstellar clouds,' (b) in some cases the exoergic charge-transfer reactions between halogen cations and neutrals produce electronically excited molecular ions which eventually decay in a luminescentprocess,2-6(c) they areclassical prototypes to investigate electrophilic substitution rea~tions,~-I~ (d) some halonium derivatives are also paradigmatic examples of nonclassical bridged structures,12-20 and (e) some of them have lifetimes sufficiently long to be involved in a manifold of chemical reactions which are believed to have important implications in stratospheric chemistry.21v22 The first member of the series, F+, either in its ground 3P or in its first ID excited state, has exceptionallylarge recombination energies (17.42 and 14.83 eV, respectively), only surpassed by those of helium and neon atoms. Since these recombination energies are significantly greater than the ionization potentials of most stable molecules, charge transfer is energeticallypossible with practically all molecular gases, and therefore F+ is very reactive with most molecular specie^.^^-^^ In particular, recent experimentalresults23for the reaction of F+ with water at thermal energies show that the major ion product is OHz+ (66%). Nevertheless, the existence of other products such as HF+, 0+, and OH+ clearly indicates that at least a fraction of the F+ collisions with water are intimate since chemical bonds are broken and made. These experimental results prompted us to investigate the correspondingpotential energy surface, since although some high ab initio calculations have been reported on the potential energy surfaces of other halonium derivatives,12-20,22,26to the best of our knowledge no similar studies have been reported so far for the [H20F]+ potential energy surface (PES). Since in many of the experimentalsetups the ion beams contain not only the ground state cation but also one or two of the metastable states (lD and 1S),27we have included in this paper a similar study of the singlet PES.
Computational Details G2 ab initio molecular orbital calculations were carried out by using the Gaussian-9OZB and the Gaussian-92program pa~kages.2~ G2 theory30 is a composite procedure based in Hartree-Fock (HF) and Moller-Plesset perturbation theory at second and fourth Abstract published in Aduance ACS Abstracts, June 15, 1994.
orders (MP2 and MP4) and quadratic configuration interaction includingsingle, double,and triple excitations(QCISD(T)) levels of theory. In the G2 procedure the initial equilibrium structures are optimized at the UHF/6-31G* level, in order to obtain the corresponding zero-point vibrational energies (ZPE) and to characterize the stationary points of the potential energy surface as either minima or transition states. These geometries are then refined by a reoptimization at the MP2/6-31G* level. Assuming additivity of different basis set enhancements at the MP4 level and additivity of basis set and correlation effects between MP4 and QCISD(T), relative energies are obtained at effectively the QCISD(T) level with the 6-31 1+G(2df,p) basis set. Further calculations at the MP2/6-31 l+G(3df,2p) level permit one to correct some shortcomings which arise from the assumption of additivity of individual basis set improvements described above. The final energies, which are effectively of QCISD(T)/6-3 11+G(3df,2p) quality, are obtained after adding a high level correction which accounts for the isogyric effect and the ZPEvalueobtained in the SCF/6-3 lG* calculation scaled by the empirical factor 0.893. This theoretical scheme has been proved to yield ionization energies, atomization energies, proton affinities, and heats of formation in agreement with the experimentalvalues within fO.1 eV. Nevertheless, since as shown by Curtiss et al.31the removal of the additivity assumptions in G2 theory reduces the overall absolute deviation between theory and experiment from 1.1 to 1.2 kcal/mol, we have also obtained the total energies of the global minima of both PES (singlet and triplet) by performing QCISD(T) calculations using the 6-31l+G(3df,2p) basis set. For these high level calculations the geometries were previously refined at the QCISD(T)/6-31 lG(d,p) level. In this respect it should be indicated that the structural changes found with respect to the MP2/6-31G* optimized geometries are not relevant, for either the singlet or the triplet. In the latter case this is likely a consequence of the negligible spin contamination of the unrestricted wave function.' In our study we have also included other species such as FOH+ and FH2+ which are possible dissociation products of the F+ + OH2 reaction. The G2 energies of the remaining products (FOH, FH, FH+, OH, OH+, and OH2+) were taken from ref 30. The natureof the bonding interactionsof the different molecular ions of both PESs was investigated by means of a topological analysis of the electronic charge density, p, and its Laplacian, V2p. As has been shown by Bader and c o - ~ o r k e r s , ~ ~V2p -3~ identifies regions of space where the electronic charge is locally
0022-365419412098-6980%04.50/0 0 1994 American Chemical Society
F+
+ OH2 Potential Energy Surfaces
The Journal of Physical Chemistry, Vol. 98, No. 28, 1994 6981
TABLE 1: C2 Total Energies (hartrees) of [HkO,F‘I+Singlet and Triplet State Cations and of Some of the Conceivable Products Of the F+ OH2 Reactions sytem G2 energy system G2 energy
+
1(S)
-175.566 -175.560 Z(S) -175.538 3(S) -175.475 TS1l’(S)b -175.553 TSlZ(S) -175.506 system G2 energy -175.559 919 1(T) -175.553 387’ 2(T) -175.544 894 3(T) -175.510947 4(T) -175.508 677 TSll’(T)b -175.516 714
592 102‘ 176 469 945 316
.!Pc
TS22’(S)b TS23(S) TS33’(S)b FC(’D)
FH2+
system
2.00 TS12(T) TS23(T) 2.00 TS33’(T)b 2.00 TS34(T) 2.00 TS44’(T)b 2.00
-175.526 -175.465 -175.448 -98.895 -100.532 G2 energy -175.545 739 -175.504 015 -175.497 776 -175.507 316 -175.497 635
006
403 727 014 473 s2c
2.00 2.00 2.00 2.00 2.00
Values obtained at the QCISD(T)/6-31 l+G(3df,Zp) level by using QCISD(T)/6-31 lG(d,p) obtimized geometries. These values include the ZPE and the HLC corrections. b Transitionstates between equivalent minima of the PES. C Expectation value. depleted (V2p > 0) or built up (V2p < 0). The former situation is typically associated with interactions between closed-shell systems (ionic bonds, hydrogen bonds, and van der Waals molecules), while the latter characterizes covalent bonds, where the electronic charge concentrates between the nuclei. We have also located the relevant bond critical points (bcp), i.e., points where the electronic charge density is minimum along the bond and maximum in the other two directions, because the values of p and V 2 p at these points offer quantitative information on the strength and nature of the bonding. This topological analysis was carried out on the wave function correct to first order, to take explicitly into account electronic correlation effects. For this purpose we have employed the AIMPAC series of programs.35
Results and Discussion The G2 total energies of the species under consideration have been summarized in Table 1. For the particular case of triplet state cations we have also included the 9 expectation values, to illustrate that in no case is the spin contamination of the unrestricted wave function significant. For the sake of clarity, the different minima of both the singlet and the triplet PESs are identified by a number followed by (S)or (T), respectively. A similar convention was adopted to identify the different transition states. In this case the two digits indicate the two minima which are connectedby a particular TS. Some of these transition states, namely all the symmetrictransition structures between equivalent minima as TS1 l’(T), etc., were obtained in the normal searching for stationary points of the correspondingPESs. The remaining ones were found by using the eigenvalue following algorithm implemented in the Gaussian-90 series of programs. All of them were characterized by a harmonic frequency calculation. An inspection of the reaction coordinate vector associated with the sole imaginary frequency allowed us to establish which minima were associate with each transition structure. In this respect, it should be mentioned that TSll’(T) was predicted to bea minimum at the SCF level rather than a TS. In this case the frequencies, as well as the ZPE, were evaluated at the MP2 level and scaled by the empirical factor 0.93. Structures and Bonding of the Stable [Hz,O,FI+ Species. The optimized structures are shown schematically in Figure 1. The global minimum of the triplet PES evidences that some F+ OH2 collisions must imply an intimate (chemical) reaction, in agreement with the experimental evidence.2s Species 1(T) could be viewed, in principle, as either a HzF+- -O(3P) or a HF- -OH+(3z)complex. However, the Laplacian of its charge density (see Figure 2) clearly shows that this species is a hydrogen bond complex between FH and OH+. In fact, Vzp is negative between
+
the fluorine atom and one of the hydrogen atoms, as well as between the oxygen and the other hydrogen atom, but clearly positive between the latter and the fluorine. Furthermore, the corresponding spin densities also indicate that both unpaired electrons are located at the O H moiety, which subtends also the positive charge. Thisis alsoconsistentwith the fact that, according to our G2 total energies for FH2+and those reported previously30 for OH+(3E), FH, and O(3P), the proton affinity of the oxygen atom (116.1 kcal/mol) is predicted to be higher than that of hydrogen fluoride (1 14.5 kcal/mol) . The 2(T) minimumpresentssimilarcharacteristics, in the sense that it has to be viewed as a tightly bound ion-dipole complex between FH and OH+, as it is clearly illustrated by both the Laplacian of its electronic charge density (see Figure 3) and the fact that both unpaired electrons and the positive charge are located on the OH moiety. Further, from the data given in Table 1 and the G2 energies of FH and OH+ taken from ref 30, an interaction energy of 17.5 kcal/mol between the two subunits is estimated. For 1(T) this interaction energy is of 26.9 kcal/mol, due to the formation of the strong ionichydrogen bond mentioned above, which is also reflected in a sizable charge density at the F-H bond critical point (see Figure 2). It is worth noticing that other fluoronium ions, such as FzN-FH+ and FN-FH+ which are minima of the correspondingpotential energy surfaces, have been found22926 to be also ion-dipole complexes. The other two minima 3(T) and 4(T) correspond to the interaction between a neutral fluorine atom and a OH2+(2BI) doublet state cation. As in previous cases, the Laplacian of the electroniccharge densityis clearly consistent with this description. Figure 4 shows that the equilibrium structure 4(T) corresponds to a practically spherical fluorine atom in a weak interaction with the OH2+ cation. In agreement with this, the correspondingspin densities show that one of the unpaired electrons is located on the fluorine atom, while the other is on the oxygen atom of the OH2+ moiety. Also, the positive charge is associated almost exclusively with the OH2+ subunit. Furthermore, from the G2 energiesgoof the fluorine atom and OHz+,interaction energies of 6.4 and 4.9 kcal/mol, respectively, for the two building subunits are found, which are consistent with a polarization interaction ion-neutral. This clearly indicates, in agreementwith the experimental findings, that one important channel in the interaction between F+ in its 3Pground state and water is a single charge transfer process. The existence of a hydrogen bond in species 3(T) is responsible for the greater stability of this form with respect to 4(T). The insertion of F+ into the OH bonds of the water molecule is a more complicated process, because it results in a fragmentation of the system into two moieties, FH and OH+, which interact electrostatically. In other words, the insertion of F+ into the 0-H bond of water is accompanied by a drastic reorganization of the electronic charge density of the system, as revealed by Figure 3, with the result that the positive charge is accumulated almost exclusively on the OH block, the FH one being practically neutral within the complex. This charge reorganization can be easily understood if one considersthat 2(T) can be also visualized as the result of the fluorine protonation of HOF neutral molecule in its lowest triplet state. HOFCA’) presentsan equilibrium structure with a very long F-O bond (2.4 A), since this triplet species can be viewed as a weak interaction between F(2P) and OH(2II). Protonation at the fluorine atom implies a significant charge transfer to the incoming bare proton, which results in a considerable enhancement of fluorine electronegativity, which favors a single charge transfer from the OH moiety to the FH+ one. The ion-dipole interaction between the resulting OH+ and FH subunits is responsiblefor the shortening of the F-O distance, which in complex 2(T) is only of 2.0 A. The singlet minima are only apparently similar to those found for the triplet PES. The global minimum 1(S), apparently similar to 4(T) species, corresponds to the adduct at the oxygen atom of
6982
Luna et al.
The Journal of Physical Chemistry, Vol. 98, No. 28, 1994
Tq+
124.6
7I
1 '
I
It155
1
1 '
F i
1.68 2
1
12.269
12.047
i
12. 55
&--,o,
0
1a5
4 ( T ) 3A' 1
4
I F
I
\
2.273
\
'2360 1b4 2 1
t
la00 \,
6 - :&
\ I
0
H'
T S l l l ( T ) 3B1
I
IF \
TS 1 2 ( T ) 3A"
f
1 1
',2.292
1068
I+
12.97 2
\
I
110.1
T S 2 3 ( T )'A
I+
I
G2 I' H'
4I
1
1
H
1
1 . 0 1 ~ 0 1 0
TS 1
1.4 26
-
44'(T)3B1
L
I'
0.993
2(S 1 'A
l(S)'A1
T S ll'(S)'Ai
T S 12(S)'A
TS 2
?(SI 'A'
l+ 2164\
TS 23(S)'A
12.164
T S 3 3'(S)'A'
Figure 1. Optimized geometries for the stationary points of the singlet and triplet [H2,0,F]+ PESs. Bond lengths are in angstroms and bond angles in degrees. The structures shown were optimized at the MP2/6-31G0 level, with the exception of 1(T)and 1(S) species which were optimized at the QCISD(T)/6-31 lG(d,p) level.
water. However, in the former the 0-F distance is 0.8 A shorter than in the latter as it correspondsto a typical covalent linkage.
Hence, 1(S) cannot beviewed as an interaction betweena fluorine atom and a OH2+moiety. The map of the Laplacian of its charge
F+
+ OH2 Potential Energy Surfaces
The Journal of Physical Chemistry, Vol. 98, No. 28, 1994 6983
P
P
"?P
F-H,
I1.285
-2 Ill4
F-0
0.029
VIP 0.136
F-H,
0124
-111l211
0-H
0.295
-1.707
O-H,
0.172
-11sii
Figure 2. Contour maps of the Laplacian of the charge density of species 1(T). Positive values of V2p are denoted by solid lines and negative values by dashed lines. Contour values in au are 10.05, 10.25, 10.50, 10.75, and 10.95. The charge density, p in e.a~-~, and its Laplacian, Vzp in
Figure 4. Contour maps of the Laplacian of the charge density of species 4(T). Same conventions as in Figure 2.
e a r 5 ,at the corresponding bond critical points are given.
\ i--O
F-o
o.nm
0.265
F-H
0.m
-2.047
O H
0.287
-1.648
Figure 3. Contour maps of the Laplacian of the charge density of species 2(T). Same conventions as in Figure 2.
density (see Figure 5) corroborates this picture. The fluorine atom is no longer a sphericalchargedistribution,and the Laplacian within the 0-F region is characteristicof covalent bonds between highly electronegativeatoms.34 Furthermore, the charge density at the 0-F bond critical point in 1(S) is almost 10 times greater than in 4(T). Something similar can be said regarding the 2(S) minimum with respect to the triplet analog (2(T)). As it is clearly illustrated in Figure 6,the 2(S) species coresponds to a covalent interaction between the FH and OH blocks. The values of p at the bond critical points are consistent with this description. Both the 0-H and F-H bonds present smaller charge densities than in species 2(T), while the charge density at the 0-F linkage is 5 times greater. These significant differences in the bonding of singlet and triplet species will have a dramatic effect on the energetics of these molecular cations (see below).
Figure 5. Contour maps of the Laplacian of the charge density of species 1(S). Same conventions as in Figure 2.
It can be seen that for singlets the insertion of F+ into the 0-H bonds of water (2(S)) is energetically less favorable than the formation of the oxygen adduct (l(S)),in contrast to what is found for the triplets. This can be understood if one considers that both systems can be also viewed as the result of the gasphase protonation of HOF in its ground 'A' state. As it has been shown in similar protonation at the most electronegative atom (F) implies a significant charge depletion of the F-O bond which accordingly becomes weaker and longer. Quite on the contrary, protonation at the less electronegativecenter (0)leads to a reinforcement of the F-O bond, which becomes stronger and shorter. Actually the F-O bond lengths of 1(S) and 2(S) are predicted to be 0.02 A shorter and 0.052 A longer, respectively, than that of neutral HOF.39 This relative reinforcement of the F-O bond of 1(S) is one of the reasons for its greater stability. Harmonic Vibrational Frequencies. We present in Table 2 the harmonic vibrational frequenciesof both singlet and triplet global minima. It is worth noticing that these values are coherent with the bonding characteristics discussed in the previous section.
Luna et al.
6984 The Journal of Physical Chemistry, Vol. 98, No. 28, 1994
B
F-0
0216
0407
F-H 0-H
0.250
.I 715
0309
-I 742
experimentally accessible, perhaps with the only exception of 3(S), due to the relatively small barrier between this minimum and 2(S). One of the most significant results is that although F+(ID) lies 59.7 kcal/mol above F+(3P), the most stable [H2,0,F]+ cation corresponds to a singlet species (1(S)) which lies 4.2 kcal/mol below the triplet local minimum (l(T)). In this respect it should be noticed that the relative stabilities of both global minima do not change significantly when the additivity assumptions of the G2 theory are removed (see Table 1). The greater stability of 1(S) species means that F+-OH2 interactions arevery stabilizing for singlets, in agreement with the bonding characteristics discussed in the previous section. Let us take the oxygen adducts, 1(S) and 4(T), as suitable examples. As mentioned above, the triplet species 4(T) corresponds to an electrostatic (polarization) interaction between OH2+ and neutral fluorine, while species 1(S) is a covalently bound molecular ion. This quite different behavior can be rationalized as follows. As we have mentioned above, the recombination energies of either F+(3P) or F+(lD) are much higher than the ionization energy of water. Thus, the first step in either the F+(3P)+ OH2 or the F+(lD) + OHZprocesses is a charge transfer which yields F OHz+. Since this charge transfer preserves the spin, in the case of the triplets one of the unpaired electrons must be on the fluorine atom and the other one on the OH2+ moiety, in agreement with our results. Hence, a covalent interaction between both subunits would necessarily require the promotion of one of the electrons, either of fluorine or of OH2+,to a vacant orbital in order to have an additional unpaired electron to be shared in the formation of the covalent bond. In other words, if one takes into account that the electronic configuration of OH2+is la122a121bz23a121b11, a covalent interaction with a F(2P) atom, preserving a triplet multiplicity, would necessarily involve the first unoccupied 4al MO of the OH2+ moiety, which lies very high in energy due to its antibonding character, or alternatively the 3s unoccupied A 0 of the fluorine atom. Since the energeticcost of such interactions is much higher than the energy gained in the formation of the new covalent bond, both subunits remain in their ground state, and only an electrostatic interaction is possible. Of course this situation does not apply to the singlets, since the unpaired electrons on fluorine and on OHz+ have different spins, and they may form a bonding electron pair without violating the Pauli exclusion principle. Similar arguments are applicable to the other species of the PES, explaining why systematically the [H2,0,F]+triplet state cations are ion-dipole or polarization complexes, while the corresponding singlet statecations are covalently bound species. We have found a similar behavior for Cl+-OH2 adducts. The CI-0 bond length for the triplet is 2.2 vs 1.7 A for the singlet. However, in this case there is not a complete charge transfer from OH2 to C1+ because chlorine is less electronegative than fluorine, and the positive charge is shared by both subunits. It must be also observed, however, that this is not the case for other triplet state cations as, for instance, those formed in the reactions involving P+(3P).37 This is so because after a charge transfer process the P atom would still have one empty p orbital, while that is not the case for F. Hence, the energetic cost necessary to open the p2 shell is considerably lower for P than for F. Thecharge-transfer processes (1) and (2) are highly exothermic, both for triplets and singlets. The reaction exothermicity is about 4.7 eV for triplets and about 7.4 eV for singlets.
+
Figure 6. Contour maps of the Laplacian of the charge density of species 2(S). Same conventions as in Figure 2.
TABLE 2 Harmonic Vibrational Frequencies’ (cm-l) Calculated for the Singlet and Triplet Global Minima (l(T) and 1 (S)) of the [H*O,F]+ PESs 1(S) 1(T) V assignment V assignment 1008 OH2 waging 375 F-H stretching 1070 FO stretching 463 FHO bending (in-plane) 1229 OH2 twisting 686 FHO bending (out-of-plane) 1577 OH2 bending 929 HFH bending 3277 OH sym stretching 1598 OH stretching 3348 OH asym stretching 3609 FH stretching a Values are scaled by the empirical factor 0.893. The set of harmonic vibrational frequencies of 1(T) species confirms that this molecular ion can be viewed as a tightly bound ion-dipole complex between FH and OH+, whose stability is reinforced by an ionic hydrogen bond between both building units. The existence of the F-H hydrogen bond is also reflected in the F-H stretching frequency of 375 cm-1. Significantly, the formation of this ionic hydrogen bond implies an electron density depletion of the bonds of both building blocks, FH and OH+, although this effect is much greater in the latter. Actually, the FH stretching frequency is red-shifted by 283 cm-I with respect to the FH isolated molecule, while the OH stretching frequency of 1(T) is 1389 cm-1 smaller than that of isolated OH+ cation. This is consistent also with the topology of the charge densities, which indicates (see Figure 2) that the value of p(r) at the 0-H bond critical point of 1(T) is 0.17 e.au-3, while for the OH+ triplet state cation is much larger (0.28 e . a ~ - ~ ) . For the singlet global minimum 1(S) all harmonic vibrational frequencies are greater than 1000 cm-1, and in particular the F-0 stretching frequency appears at 1070 cm-1, confirming the covalent nature of this linkage. Thermochemistry. Figures 7 and 8 present schematic representations of the triplet and singlet PESs, respectively. The former presents four different minima while the latter has only three. This is so because the structure analog to 3(T) is not stable as a sinlet, and it collapses to 2(S). It must be also observed that among the triplets only the global minimum is expected to be experimentally observable because the activation barriers which connect the other three minima, namely, 2(T), 3(T), and 4(T), among them and with the global minimum are rather small. Quite on thecontrary, the large barriers separating the different minima of the singlet PES indicate that most of them should be
F+(3P)+ OH,
F+(’D)
+ OH,
-
F(’P)
+ OH;(’BI) AH: = -109.7 kcal/mol (1)
F(’P)
+ OH:(’B1) AH: = -171.8 kcal/mol (2)
The fact that processes 1 and 2 are strongly exothermic implies
F+
+ OH2 Potential Energy Surfaces
The Journal of Physical Chemistry, Vol. 98, No. 28, 1994 6985
-
F
OH + HF'
86.6
- 1
30.7
I
30.7
H
\
F
I
F H
1
w
0.0
0.0
F'+ H,O
210.0-
213.1
-
/\
cm
...
100.0-
..
Y4
\i
1;:H (I'
TWS)
00.0-
',o/
/H F
F....
O H + HF'
.. ...
102.8
H\F
TW(S3
73.9
0
I
i 0
83.5
i
YS)
W)
57.2
57.2
H:;::::l
W
074
Tl:)1H--I
30.0 -
1A i
1-H
0+322(S)li~O 25.4
2 0 17.8
W) 17.8
#bo /Tsll~(s)\oa# 0.0-
1(SI
7.9
1'(S)
that, very likely, the OH,+ molecule will be produced in a vibrationally excited state. Furthermore,process 2 may eventually lead to the dissociation of the OH2+ molecule, since the G2 estimated dissociation energies of OH2+ in either OH+ + H or OH + H+ are 126.1 and 140.6 kcal/mol, respectively. On the other hand, the vertical excitation energy from the ground ( ~ B I ) state of OH2+ state into its first excited (2A1) state (estimated by means of a CIS/6-31G* calculation) is quite small (2.2 eV) comoared with theexothermicitvof reactions 1 and 2. Therefore. the formation of the OHz+speci-es in its first excited state cannot be discarded, in principle. The reaction channels yielding OH+,FH+, and O+(3)-(8) are
also exothermic.
p(3p)
OH2
p ( ' D ) + OH,
F+cP)
+ OH,
+
FH
-
+ OH+(32-) AH: = -1 19.9 kcal/mol (3)
FH + OH+(32-)
FH+(211)
AH: = -1 8 1.9 kcal/mol (4)
+ OH(,II) AH: = -48.2 kcal/mol ( 5 )
6986 The Journal of Physical Chemistry, Vol. 98, No. 28, 199'4
+ OH,
F+('D)
F+(3P)
+ OH,
F+('D)
-
+ OH,
FH+('II)
+ OH('II) AH: = -1 10.3 kcal/mol (6)
FH + H + ' 0
-
AH: = -5.7 kcal/mol (7) FH + H + ' 0
AH: = -67.7 kcal/mol (8) This is in agreement with the fact that all of them have been found25 as products of the reaction of F+ with water at thermal energies. The formation of HOF (reaction 9) and FH2+ (reaction 10) are also exothermic processes. F+(3P)
F'(3P)
+ OH, + OH,
-
HOF + H+
AH: = -17.2 kcal/mol (9)
FH2++ 0
AH: = -1 18.3 kcal/mol
(10) However, these processes might involve quite high activation barriers. Actually, none of the minima present a structure which would favor the formation of either HOF or FH2+. HOF could be produced only from 2(T) by breaking a strong F-H bond. Similarly, the formation of FH2+ could be possible from l(T), but this would involve the breaking of a reasonably strong 0-H linkage. Proton Affinities and Heats of Formation. Our results show that HOF in its ground state is an oxygen base, with a proton affinity of 133.8 kcal/mol. The protonation at the fluorine atom is also exothermic by 115.9 kcal/mol. Unfortunately, we are not aware of experimental values to compare with. We have also estimated the heat of formation of the most stable [H2,0,F]+ cation, Le., species 1(S). For this purpose we have considered the following alternative processes:
-
+ H+ FOH: FH + OH+ FOH: F + OH: FOH: FH: + 0 FOH:
HOF
AH: = -133.8 kcal/mol
(1 1)
AH: = -3 1.1 kcal/mol
(12)
-
AH: = -41.3 kcal/mol (1 3) AH: = -32.7 kcal/mol (14)
The experimentalheats of formationof H+, FH, OH+,F, OH2+, FH2+,and 0 were taken from ref 38. That of HOF was taken from ref 39. The estimated heat of formation of FOH,+ varies form210.3 to212.8 kcal/mol,according to thereactionemployed, the average value being 21 1.4 kcal/mol. From our calculations we can also estimate the proton affinity of FH as well as the heat of formation of FH2+ by using the protonation reaction
FH
+ H+
-
FH;
(15) Thesecalculatedvalues,114.5and 185.6 kcal/mol, respectively, are in good agreement with the experimental ones38(1 17 and 184 kcal/mol, respectively). Similarly, the theoretical estimated proton affinity of O(3P), 116.1 kcal/mol, is also very close to the value estimated (1 15.8 kcal/mol) from the experimental heats of formation38 of OCP), H+, and OH+(%-).
Conclusions Our survey of the [H2,0,F]+ singlet and triplet PESs shows important differences between singlets and triplets, regarding both their bonding and their stabilities. In this respect the most
Luna et ai. outstanding result is that although the first lD excited state of F+ is about 2.5 eV above the 3P ground state, the most stable product of the reaction between F+ and water corresponds to a singlet state cation 1(S). This result may bear some relevance to the experimental observation of spin-forbiddentransitions in collisions between F+ and different neutrals.27 In this respect, it should be mentioned that this result is not found at the H a r t r e e Fock level of theory which predicts, on the contrary, a greater stability of the triplet global minimum l(T). This confirms that the inclusion of electron correlation effects is unavoidable when a proper description of open-shell cations is req~ired.~' Thelarge stability ofsingletstate [Hz,O,F]+cationswith respect to the triplet ones is a consequence of the drastic differences between their bonding characteristics. The latter are, in general, ion-dipole complexes, while the former are covalently bound species. In agreement with the experimental evidence our results show that single charge transfer is the most likely process in F+ + water reactions. Since these processes are highly exothermic, in particular when F+('D) is involved, the production of OHz+ in its ground state would necessitatethe large reaction exothermicity to appear as translational energy of this ion and the F atom which is quite improbable. Hence, very likely the OH2+ molecule is vibrationally excited and, for the particular case of F+(lD) + HzO reactions, may even dissociate. On the other hand, the possibility of a resonant charge transfer into the first excited state of OH2+cannot be discarded.
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