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Ab initio CI study of the stability and electronic spectrum of the HOCl molecule
Volume 52, number 3
CHEMKAL
PHYSiCS LETTERS
AB INITKO CI STUDY OF THE STAdILITY AND ELECTRONiC SPECTRUM OF THE HOC1 MOLECULE
Received 25 August 1977
J,%e ~~arac~cris~es of the UoCI m&c& and its various dissociation praducts are investigated with the help of the ab initio MRD CX method; the HOC1 dipole moment as well as the structural data for the isomeric HCIO species are also ealc&ted. The results obtained for the vertical spectrum of HOC1 suggest that a feature observed experimentally at 3.87 eV daes not correspand to a bona fide HOC1 transition; the calculations indicate further thal all low-lying excited states of this molecule are unstable relative to dissociation into OH(*n) + Cl<*P). The enthalpy of reaction for the process 0(3P) + HCI (IX*) -+ 0H(2Ki) + Clt2P) is obtained to be *3.6 k&/mole (experimental +I.0 kcal/moIe) and the heat of formation of HOCL is estimated to be - 19.2 & 3.9 kcaifmofe.
1. Introduction The fist of substances possibly affecting the stratospheric ozone budget has recently been extended to include chlorine and its compounds. As a result of this development there is special interest in the HOC1 molecule since there is good reason to believe that this spe-
cies may be formed in the stratosphere [ 1] as an intermediate in the reaction of the Cl0 with HO, or H,Oz. The destruction ofHOC1 under stratospheric conditions is assumed ta occur via reaction with oxygen or by the photode~omposition process HOCf + Itv + OH + Cf. Although the absorption spectrum of HOCl was measured as early as f 936 [Z] * no comparable exper&n&al study is found in the recent literature, and thus no detailed information concenhing the excited states of HOC1 is available today because of the relatively low resolution attained in the early work [Z] _ Hence only rough estimated for the absorption cross sections and the photodecomposition rates of this system can be dc3uced from the experimental information presently at hand. i: Present address: ~e~m~ochsch~e
Woppert~, Lehrstuhl Theorctische Chernie, D-5600 WuppertaI-Eiberfeld, Germany.
443,
Since theoretical ab initio c~cuiations have been found to be very successful in recent years for the determination of bonding properties in simple molecules in their ground as weU as electronically excited states, it would appear useful to carry out such an SCF and CI study to consider the various points raised above. The absorption spectrum and the relative energetics of the HOC1 system compared to its dissociation into the various products OH -I-Cl, H + CIO and H f Ci + 0 is thereby given special attention in the current work while details of the ~~rr~sp~~dingenergy surfaces (in a variety of states) and analogous :esults for the isomerit compound HOCl will be reported at a later time.
2_ Procedure of calcutations Four different A0 basis sets are ,employed in the present calculations, with each of them using four s and two p groups of Cartesian gaussian functions on oxygen (in the contraction given by Dunning [3] ) and six s and four p groups located on the chlorine atom (taken from the work of Veillard; contraption no. 8 [4] ). Two gaussian s groups with four and one components respectively f5] are centered at the hydrogen site with scaling factor q 2 = 2 along with onecomponent
Volume 52, number 3
15 December 1977
CHEMICAL PHYSICS LETTERS
pX, pr and pz functions with exponent OL= 0.735824, while an s bond function (0 = 1.0) is located in the OH bond in all the basis sets to account for polarization in this region. The basis sets employed are distinguishable primarily on the basis of the way in which they account for polarization effects. Basis A includes s (c~= 0.70) and px, p,, , pz (CY= 0.60) bond functions at the center of the Cl0 axis for this purpose, as well as a six-member Cartesian d function with an exponent (OS 188) chosen to be approximately the mean of the corresponding optimized values for atom-centered Cl and 0 species [3]. From previous work on 0, and N, [6] it is known that such a combination of s, p and d bond functions is very adequate for the description of a molecule at small internuclear separations but that it tends to overestimate the corresponding bond energy somewhat [6] _ In addition this basis contains Rydberg s and pX, p,, , pZ functions (all with (Y= 0.01 S), also located at the center of the Cl0 bond, making a total of 48 contracted Cartesian gaussians. Basis B is wholly similar to A except that it does not contain the six d components in the Cl0 bond, hence consisting of only 42 functions. The C basis does not use the bond d functions either but instead locates one d set (cu = 0.53733) on oxygen and another (o = 0.502 i 1) on the chlorine atom. This set is especially constructed for proper description of the relative energetics between the molecule at equilibrium and the various HOC1 dissociation products; since the Rydberg functions are not necessary for this purpose, they are omitted in basis C (thus 50 contracted species in all). Finally a fourth basis, D, is employed to give further consideration to dissociation effects; it comprises the entirety of basis C but also includes the six-member bond d species found in basis A. The CT procedure used is of the multi-reference double excitation (MRD CI) type, with configuration selection and energy extrapolation [7] _ 3. HOC1 at equilibrium The bond length OH = 1.83 ao, OCI = 3.194 a0 and the internuclear angle LHOCl = 102_4O determined from microwave measurements [8] are employed in the present work for the ground state equilibrium HOC1 geometry; these values differ only very slightly from those determined earlier from similar experiments
Table 1 Orbital energies of the HOC1 molecule in its ground state at the experimental equilibrium geometry (all values arc given in hartree units, basis C) la’ 2a’ 3a’ 4a’
-104.9014 -20.6444 -10.6018 -8.07842
5a’ 6a’ 7a’ 8a’ 9a’ 1 Oa’
-8.07551 -1.40916
la”
-8.07530
2a”
-0.60905
3a”
-0.43989
($
-1.05612 -0.7 1469 -0.59607 -0.46437 (r@
[9]. The ground state equilibrium configuration is ‘A’ (!Oa’)2(3a”)2, which in linear geometry would correspond to 1 E ‘( 1‘IT)4 _According to qualitative MO theory this occup%ion scheme leads to the smallest bond angle known among HAB systems [IO,1 I] , similar to the situation in HOF [12,13] _The molecule is
thus expected to show a definite opening of the internuclear angle in all low-lying electronically excited states. The orbital energies (basis C) and the total energies obtained in the SCF and CI treatment employing the various basis sets discussed in section 2 are given in tables 1 and 2. As should be expected the total energy varies only slightly from one basis set to the other at the SCF level, while the choice of the various d functions is seen to be of much greater importance for the description of electronic correlation. The present energy values are also considerably lower than that given in a previous study (-534.7097 hartree SCF) on the HOC1 molecule [ 14]_ The two components (3a” and 10a’) of the ?rg-type MO are relatively close in energy; application of Koopmans’ theorem to both yields ionization potentials of 11.96 eV and 12.63 eV respectively, while the scaled values (multiplication by a factor of 0.92) of 1 1 .OI eV (2A”) and I 1.62 eV (2A’) should be more realistic predictions according to previous experience [ 151. The dipole moment of HOC1 in its equilibrium nuclear arrangement is calculated to be 1.633 debye (basis C) in the CI treatment with components ,+ = 0.223 and r_‘u= -1.618 debye?_ Hence the dipole moment vector ?
The molecule
is thereby oriented such that the oxygen atom is at the origin of the coordinate system dnd the Cl0 bond is collinear with the x axis while the hydrogen coordinates are positive iny and negative in x at LHOCI = 102.4”. 443
Volume 52. number 3
15 December 1977
CHEMICAL PHYSICS LETTERS
Tabic 2
Total enegics (in hartrcc) obtained for the HOC1ground state equilibrium in various treatments using three different basis sets Trcatmcnt
Basis A
Basis B
Basis Cb)
SCF
-534.87876 -535.1129 -535.127
-534.87494 -534.0719 -535.083
-534.88349 -535.2002 -535.221 5
CI . estimated”) CI basis limit
a) C&i&ted as (1 - XC~)AE, where AE is the difference between the calculated CI energy and the value obtained from the reference configurations only; the sum runs over all reference species. b) If the optimum HOC1geometry obtained from the CI calculation is employed, an energy lowering of 2.7 kcal is observed in the SCF trcatmcnt compared to the present value while the energy in the CI calculation is lower by 0.6 kcal (or 0.7 kcal in the CI basis limit).
forms an angle of 7.8” with they axis, i.e. making an angle of 97.8O with the O-Cl axis, with the oxygen corresponding to the negative center of charge. Since the dipole moment of an isolated OH molecule is 1.54 debye or 1.65 debye [16] , it is clear that this bond
ably higher energy (8.92 eV) whereby transitions to this state should be quite weak. Comparison with the measured data [2] suggests very strongly a 10a’ + 11 a’ (o*) assignment for the peak around ?20 mp (5.63 ev), while no comparable
component
transition
makes
by far the most
dominant
contribu-
tion to the HOC1 dipole (its p,, value would be -1.65 cos(102.4 -90) = -1.61 debye in the present geometrical arrangement)_ By contrast the separation of charges along the Cl0 bond is much less in HOC1
than it is in the isolated Cl0 molecule (with a dipole moment of 1.2 debye [ 17,18])_ This result thus indicates that the oxygen atom withdraws electronic charge primarily from the hydrogenic region rather than from the chlorine atom, so that the charge distribution in the Cl0 bond is considerably more evenly balanced in the HOC1 molecule than in the parent diatomic species.
4. Electronically
excited states
The excited states arising from excitation out of the two components 3a’ and 10a’ of the ng-type MO into the lowest-lying valence orbital and some additional Rydberg species are treated; details of the requisite CI calculations (using basis A) are listed in table
3. The lowest transitions result from population of the strongly Cl-0 antibonding o,,-type orbital 1 la’. The allowed singlet transitions thereto occur at an energy of 4.54 eV and 5.75 eV (vertical) respectively, with a calculatedfvalue of0.006 for *A’ and 0.0005 for 1A", i.e. the latter is approximately one order of magnitude weaker than the first transition $_ The doubleexcitation state 3aff2 + 1 la’2 is calculated at consider444
is found
theoretically
which
would
account
for the second experimental broad feature (reported to be almost equally as strong as the first) with a maximum around 3.87 eV. It is clear that some underlying intensity from the 1A" + LA' transition is found in this energy region (neglecting the 3A” f 1A' species for spin-conservation reasons), but both energy and intensity arguments rule out an assignment in terms of 1A"
for the second peak. Especially since similar results have been obtained in a related theoretical treatment [19], a more detailed experimental study seems to be called for to determine whether the HOC1 system is really responsible for this low-energy feature noted in the previous experimental study. Only a few representative Rydberg states are calculated in the present work. The first series converges to the 2A” IP which is calculated to occur at 10.93 eV. The calculated term value for 1A’ (3a” + 4p,) is 18790 cm-‘, a value which is in good agreement with the usual empirical rule [20] that the first member of a p Rydberg series lies in the order of 20000-21000 $ In order to calculate the oscillator strengths the ground
state SCF MO’s have been employed for the representation of both ’ A’ and ‘A” upper states to allow for the use
of a mutually orthogonal basis in all cases; a calculation was necessary for ‘A’ for this purpose, while five such species were required to represent the ‘A” state within an accuracy of less than 0.1 eV compared to the values given in table 3. with three reference confiiurations
CHEMICAL PHYSICS LETTERS
Volume 52, number 3 Table 3 Calculated vertical transition
energies
AE for various
15
States of HOCI, along with some technical
December 1977
details of the corresponding
CI
treatment State
% ‘A’ 3A” 1 A” 1 3A’
‘A’ 1
Number of reference configurations
Transition
...(10a’)2(3a” 3a” 10a’ -
I2
1 la’(o*) lla’(a*)
3a” - 4a” (4&J
‘A’ ‘A’ 3A” ‘A” t 3A’ 2A” ZA’
3a” ’ -
1 la’2(o*)
1Oa’ +
4a” (4~~)
7-a”, 3a” 3a” + m 10a’ + -
a) The notation
1 la” (o*)
m/n indicates
that m configurations
(symmetry
SCF
MO basis
Secular
AE(cV)b)
equationsa)
X’A
2353/17181
0.0
3A”(3a” + 1 la’)
2829/63498 2785138160
3.50 4.54
2804153264 2789139933 2781/19446 2566/l 7775 1 77/l 13668
4.73 5.75 8.60 8.92 9.45
3A’(10a’
+ 1 la’)
3A’(3a” tA’(3a”2
4 4a”) - lla’2)
3A”(10a’
--L~J”)
3A’(2a”, 2A” ion 2A’ ion
3.1” -
adapted
functions)
1 la’2)
are selected
2679168644 2542/33891
9.53 11.24
2760/l 14634 264016478 1
10.93 11.95
whde extrapolation
is with respect
to a
total of n configurations. b) AE is taken to be the difference between the electronic energies of excited and ground states respectively.
cm-l below the corresponding IP $1. Since singlettriplet splittings of corresponding Rydberg states are very small, the 3A’ (3a” + 4p,) state should be found in the immediate neighborhood of the 1A', as should be the Rydberg states involving the 38” and the other spatial 4p components_ The lowest Rydberg state (3a” + 4s) is expected to lie in the order of 28000 cm-t below the corresponding ionization limit, i.e. 1 S-2 eV above the low valence-shell
excitations_
The situa-
tion is very similar for the other Rydberg series converging to 2A’; the term-value (195 18 cm-t) for the calculated 10a’ + 4pz Rydberg member is wholly comparable to that obtained for the state originating from the 3a” MO. Similar relations as are found to exist between the various 3a” Rydberg states are also expected to hold for the other 4p and the 4s Rydberg species obtained by depopulating the lOa’. The calculated IF% furthermore support the empirical rule of scaling the Koopmans’ theorem results by 0.92, as discussed in section 3. Finally, the geometrical behavior of the various ex$$ This conclusion seems especially justified once it is realized that calculations at the present level generally underestimate the IP In the order 0.2-0.3
cV.
cited states with respect to separation into OH + Cl has been investigated. The Rydberg states show their minima at shorter HO-Cl distances than the ground state, in full agreement with the simple MO model [IO] , since depopulating the Cl-O antibonding ng-,-type orbital favors contraction of the corresponding bond. All calculated states populating the strongly antibonding u* show completely repulsive curves with respect to OH + Cl decomposition and correlate with the products in their corresponding ground state: OH(’ fl) and C1(2P). Thus it can be concluded that transitions into the states lying below the first Rydberg species (7.07.5 eV) lead immediately to photodissociation of the HOC1 molecule.
5. Stability and heat of formation of HOC1 The total energies of the various possible dissociation products of HOC1 have also been calculated and these results are listed in table 4. In addition the energy of the HCIO isomer in its geometrically optimized form (calculated structural parameters are Cl0 = 3.10 ao, HCl = 2-48 “o and LHClO = 104AO”) is also given. Experience with similar calculations for the N2 mole445
CHEMICAL PHYSICS LE’ITERS
Volume 52, number 3
15 December 1977
Table 4 Calculated
total energies (in hartree units) for various products of HOC1 obtained with basis C (or its equivalent)
Cl(2P) ClO(*n) o(3P) HCl(‘z+)
CI Iirnitb)
CI
SCF OH(*n)
-75.40690
-75.5482
-75.5538
-459.45568 -534.260 -74.80063 -460.07869
-459.5780 -534.5441 -74.8963 -460.2336 -534.5642 -0.5000
-459.5835 -534.5666 -74.8995 -460.2420 -534.5843 -0.5000
~210 (*np)
-
H(*S)
-0.50000
3) This rcsuh is obtained with basis D, i.e. with an additional bond d function compared to basis C. No SCF value is obtained for Cl0 in this case; the MO’s of ClO- are used in the CL b) Calculated as (1 - C CO”, Ah*, where PE is the difference cnce configurations
cule [6] indicates
only;
between
the sum runs over all reference
that for a generally consistent
scription of the combined
the calculated
CT energy
and the value obtained
from the refer-
species.
ever, since the Cl0 bond is relatively unaffected by this type of dissociation; hence results of this nature have been obtained with and without atom-centered d functions (i.e. using both basis A and C). The stabilities of HOC1 and its dissociation products are then compared in tables Sa and 5b for various basis sets. In line with the comments given above it is seen that the relative energetics of the systems in which the
de-
molecule and its dissociarion
fragments it is desirable to include both bond- and atom-centered polarization functions in the A0 bask.
Consequently the results of table 4 have been obtained using basis C (and in the case of Cl0 also basis D), in which d functions are located on the Cl and 0 atoms to complement the Cl0 bond-type functions (see section 2). These considerations are expected to be much less critical for hydrogen abstraction processes, how-
Cl0 bond remains intact are nearly unaffected by the manner in which polarization effects at the heavy atoms
Table 5 Relative stabilities (in kcJ/mole) taincd from various treatments.
(a)
of various products compared to HOC1 in its calculated ground state equilibrium (a) Corresponds to De values from basis A, results in (b) are from basis C)
SCF
CI
CI limit
HOC1 (’ A’) H(*S) + ClO(*fI)
0.0 79.6
0.0 95.5
0.0 97.9
HClO(‘A’)
68.7
66.3
63.7
(b)
CI
CI limit
best CI a)
best CIb)
IlOr’l(‘A’) H(%)+ClO(*II)
0.0 80.4
0.0 98.6
0.0 97.9
0.0 97.9
0.0 91.1
HCIO(‘A’) OHCn) +Cl(*P) 0(3P) + HCl(‘C+)
68.5 16.0 5.5
65.2 47.1 44.7
63.7 53.6 50.9
63.7 64.7 62.0
~60 62.0 58.3
H(*S)
82.8
142.4
150.4
161.5
153.5
+ C1(2P)
+ O(3P)
ob-
DO
De SCF
geometry
exptl. 0.0 9&2c),
98.0d)
? 60.3c) 59.3 d) 165.8e)
a) The value for the Cl0 bond dissociation De obtained from the more adequate basis set (see table 4) is taken in each instance, n.uncly 56.4 kcal/mole for CI and 63.6 kcal/mole for the CI limit instead of the CI limit value of 52.5 kcal obtained in basis B (see discussion in text). b) Zero point energies are taken into account, whereby the values for HCI, Cl0 and OH are taken from refs- [21,22] values Q (OH) = 36 17 cm-‘, ~2 (ClO) = 739 cm-’ and ~3 (LCIOH) = 1242 cm-’ [9,23] are employed for HOCI. c) Ref. [ 1] , sometimes only estimates. d) Ref. [ 19]_ e, Ref. 1211 on the basis of 64.6 kcal for the dissociation of Cl0 into Cl + 0.
while the
Volume 52, number 3
15 December 1977
CHEMICAL PHYSICS LElTERS
estimated full CI limit the results using basis A and C
mentally. The most striking result of table 5b is the essentially random nature of the SCF De values com-
respectively for the energy differences among the systems HOCl, HCIO and H(*S) + C10(2 ll) agree with one another to within 0.1 kcal/mole in each case (De values in tables Sa and Sb). The situation with Cl0 bondbreaking processes is more complicated, however, as illustrated by the De results for Cl0 itself. In this case
pared with their experimental counterparts. The only case for which the SCF method appears to be at all adequate is for simple hydrogen abstraction of HOCl, and even in this case the error relative to experiment is in the order of 20 kcal/mole. The description of the Cl0 bond-breaking process is very much more distort-
the extra d-type function centered in the Cl0 bond has a relatively large effect on the dissociation energy, with basis C yielding a CI limit De value of only 52.5 kcal/
ed at the SCF level, with discrepancies in excess of 50 kcal/mole being noted. By contrast the best CI Do values obtained in this work are in generally good agreement with experiment, showing errors of only 7,2 and 1 kcal/mole respectively for dissociation into H(2S) + CIO(211), OH(?II) + C@P) and OCP) + HCl(l X’)_ The error in the case of complete atomization is 12.3 kcal/mole or 7% of the total D, value. The isomeriza-
are represented in the present basis sets. In fact at the
mole compared
to that of basis D (with d functions
at
each atom and in the Cl0 bond; see section 2) of 63.6
kcal/mole. The latter result is in much better agreement with the experimental value of 64.6 k&/mole [2 I] and hence it appears that addition of d AO’s at the atomic centers without simultaneous inclusion of bond-center d species lead to a significant bias in favor of the separated atoms. These results are consistent with the N2 study [6] mentioned earlier, in which it has been found that bond functions through d type are required in conjunction with atom-centered d ,40’s in order to achieve a generally accurate balance between the molecule and its lowest three dissociation limits?‘. Since this basis set effect appears to be quite general it therefore is desirable to correct the De results obtained with
basis C for all systems in which the Cl0 bond is broken, while taking the corresponding uncorrected values at face value for species in which this bond remains in-
tact; based on the results for Cl0 the value of the former correction is thus taken to be 11 .l kcal/mole, i.e. the difference between the Cl0 De values obtained inbasis C and D respectively. These findings are‘ then summarized in table Sb under the heading “best Cl” results. Finally it should be noted that the calculated minimum in the CI energy of HOC1 used to construct table 5 corresponds to the geometry: Cl0 = 3.22 ao, OH = 1.88 a0 and LHOCl = 104. lo, as derived from a polynomial fit to the various data points obtained_ The resulting energy is only 0.6 kcal/mole lower than the previous calculated CI value given in table 2 corresponding to the equilibrium geometry observed experi? It is also
worth noting that the Cl0 De value obtained with basis B, with no d AO’s at all but with s and p bond func-
tions is 62.0 kcal/mole, in good agreement with both the experimental result and that obtained with basis D, but well above the corresponding basis C value.
tion energy of HOC1 and HClO is not known experimentally but on the basis of the present Cl calculations it is estimated to be approximately 60 kcal/mole. From these results it is also possible to calculate the heat of reaction for the process: 0(3P) + HCl(‘Z’) + OH(2n) + Cl(“P), namely i-3.7 kcal/mole for Mt and +3.6 kcal/mole for M&. These findings agree well with the corresponding
measured values of +0.9 kcal/mole [24] and +0.96 kcal/mole [25] . They also allow for a number of pathways for calculation of the heat of formation of HOCl, as indicated in table 6. The average of the four AHzg8 values found therein is -19.2 kcal/mole with a standard deviation of 3.9 kcal/moleii. which results in turn also compare favor?? Whereby
the higher absolute
weighted
somewhat
abstrdction Cl0
process
bond-breaking
values should
more because is described
actually be the calculated hydrogen
less satisfactorily
than the
(5ee table 5).
Table 6 Calculated
heat of formation
(in kcal/mole)
the assumption ofvarious formation schemes CI DO vdues of table 5b”) Reaction
partners
H + Cl0 OH+CI O+HCl H+Cl+O
for HOC1 under
using the best
A-G
AJ+La
-15.3
-15.9 -24.1 -21.9 -14.8
-23.4 -21.3 -14.3
a) An ided gas correction for the temperature dependence of AHO for HOC1 has been applied [26] to obtain the results of the last column;
experimental values for Af&a are taken from standard tables [ 27 ] for each of the dlatomic and . atomic spccics involved therein.
447
Yolumc
52, number
3
CHEMICAL
PHYSICS LETTERS
abiy with the value of -22 + 3 kcal/mole adopted recently by Warnek [l] upon consideration of a variety of experimental observations. In summary by using a flexible A0 basis with both bond- and atom-centered polarization functions the CT calculations carried out in this study are capable of describing the relative energetics of HOC1 and its various dissociation products to within error limits of 0. I-03 eV in al1 instances except that of complete atomization, in which case the error increases to 0.5 eV. After this paper had been submitted it was pointed out to us that the dipole moment of HOC1 has been determined from microwave spectra by D.G. Lister and D.J. Miller (Trans. Faraday Sot. 67 (1971) 601). The experimental quantity of 1.3 -t 0.3 debye is in good agreement with our calculated value of 1.633 debye and the angle of 73 f 3” between the dipole moment vector-and the Cl0 bond reported in the experiment compares quite well with the calculated angle of 82.2” (i.e. the complement of 97.8O given in the text).
Acknowledgement The authors wish to thank Professor K.H. Becker, Cesamthochschule Wuppertal, for pointing out the possible importance of the HOC1 molecule in the stratosphere_ One of us (G-H.) wishes to express his gratitude to the Studienstiftung des Deutschen Volkes for financial support. The services and computer time made available by the University of Bonn Computer Center have been essential for this study and are gratefully acknowledged.
References [I] C. Warneck, preprint. [2] WC. Fcrgusson, L. SIotin and D.W.G. Style, Trans. Faraday Sot. 32 (1936) 956. [3] T-H. Dunning, J. Chcm. Phys. 53 (1970)
448
2823.
15 December 1977
A. Veillard, Theoret. Chim. Acta 12 (1968) 405. S.D. Peyerimhoff, R.J. Buenker and L.C. Allen, J. Chem. Phys. 45 (1966) 734, appendix I. W. Butscher, S. Shih, R-J. Buenker and SD. Peyerimhoff, submitted for publication. R.J. Buenker, S-D. Peyerimhoff and W. Butscher, sub-
mitted for publication; R.J. Buenker and S-D. Peyerimhoff, Theoret. Chim. Acta 39 (1975) 217. A.M. Mirri, F. Scappini and G. Cazzoli, J. Mol. Spectry. 38 (1971) 218. 191 R-A. Ashby, J. Mol. Spectry. 23 (1967) 439. [lOI A.D. Walsh, J. Chem- Sac. (1953) 2260. t111 R.J. Buenker and S.D. Peyerimhoff, Chem. Rev. 74 (1974) 127. 1121 R.J. Buenker and S.D. Peyerimhoff, J. Chem. Phys. 45 (1966) 3682. [13] H. Kim and J.R. Sabin, Chem. Phys. Letters 20 (1973) 215. [ 141 G.L. Bendazzoli, D.G. Lister and P. Palmiero, I. Chem. Sot. Faraday II 69 (1973) 791. [ 151 M.B. Robin, Higher excited states of potyatomic molecules (Academic Press, New York, 1974). [ 161 A.L. McClellan, Tables of experimental dipole moments (Freeman, San Francisco, 1963). [ 171 A. Carrington, P-N. Dyner and D.H. Levy, J. Chem. Phys. 47 (1967) 1756. [ 181 T. Amano, S. Saito, E. Hirota and Y. Merino, J. Mol. Spectry. 30 (1969) 275. [ 191 R-L. Jaffe and S-R. Langhoff, paper presented at the 32nd Symposium on Molecular Spectroscopy, Columbus, 19771201 C. Sandorfy, Lecture given at the VIIth International Conference on Photochemistry, Jerusalem, August 1973. [21] J.A. Coxon, W-E. Jones and E.G. Skolnick, Can. J. Phys. 54 (1976) 1043. (221 G. Herzberg, Spectra of diatomic molecules (Van Nostrand, Princeton, 1950). [23] K. Hedberg and R.M. Badger, J. Chem. Phys. 19 (1951) 508. [24] R.D.H. Brown and I.W.M. Smith, Intern. J_ Chem. Kinetics 8 (1975) 301. [2.5] J. Wolfrum. Ber. Bunsenges. Physik. Chem. 81 (1977) 114. [26] S-W. Benson, Thermomechanical kinetics (Wiley, New York, 1976). [27] V.I. Vedeneyev, L.V. Gurvich, V-N. Kondrat’yen, V.A. Medvedev and Ye-L. Frankevich, Bond energies, ionization potentials and electron affinities (Arnold, London, 1962).
CHEMKAL
PHYSiCS LETTERS
AB INITKO CI STUDY OF THE STAdILITY AND ELECTRONiC SPECTRUM OF THE HOC1 MOLECULE
Received 25 August 1977
J,%e ~~arac~cris~es of the UoCI m&c& and its various dissociation praducts are investigated with the help of the ab initio MRD CX method; the HOC1 dipole moment as well as the structural data for the isomeric HCIO species are also ealc&ted. The results obtained for the vertical spectrum of HOC1 suggest that a feature observed experimentally at 3.87 eV daes not correspand to a bona fide HOC1 transition; the calculations indicate further thal all low-lying excited states of this molecule are unstable relative to dissociation into OH(*n) + Cl<*P). The enthalpy of reaction for the process 0(3P) + HCI (IX*) -+ 0H(2Ki) + Clt2P) is obtained to be *3.6 k&/mole (experimental +I.0 kcal/moIe) and the heat of formation of HOCL is estimated to be - 19.2 & 3.9 kcaifmofe.
1. Introduction The fist of substances possibly affecting the stratospheric ozone budget has recently been extended to include chlorine and its compounds. As a result of this development there is special interest in the HOC1 molecule since there is good reason to believe that this spe-
cies may be formed in the stratosphere [ 1] as an intermediate in the reaction of the Cl0 with HO, or H,Oz. The destruction ofHOC1 under stratospheric conditions is assumed ta occur via reaction with oxygen or by the photode~omposition process HOCf + Itv + OH + Cf. Although the absorption spectrum of HOCl was measured as early as f 936 [Z] * no comparable exper&n&al study is found in the recent literature, and thus no detailed information concenhing the excited states of HOC1 is available today because of the relatively low resolution attained in the early work [Z] _ Hence only rough estimated for the absorption cross sections and the photodecomposition rates of this system can be dc3uced from the experimental information presently at hand. i: Present address: ~e~m~ochsch~e
Woppert~, Lehrstuhl Theorctische Chernie, D-5600 WuppertaI-Eiberfeld, Germany.
443,
Since theoretical ab initio c~cuiations have been found to be very successful in recent years for the determination of bonding properties in simple molecules in their ground as weU as electronically excited states, it would appear useful to carry out such an SCF and CI study to consider the various points raised above. The absorption spectrum and the relative energetics of the HOC1 system compared to its dissociation into the various products OH -I-Cl, H + CIO and H f Ci + 0 is thereby given special attention in the current work while details of the ~~rr~sp~~dingenergy surfaces (in a variety of states) and analogous :esults for the isomerit compound HOCl will be reported at a later time.
2_ Procedure of calcutations Four different A0 basis sets are ,employed in the present calculations, with each of them using four s and two p groups of Cartesian gaussian functions on oxygen (in the contraction given by Dunning [3] ) and six s and four p groups located on the chlorine atom (taken from the work of Veillard; contraption no. 8 [4] ). Two gaussian s groups with four and one components respectively f5] are centered at the hydrogen site with scaling factor q 2 = 2 along with onecomponent
Volume 52, number 3
15 December 1977
CHEMICAL PHYSICS LETTERS
pX, pr and pz functions with exponent OL= 0.735824, while an s bond function (0 = 1.0) is located in the OH bond in all the basis sets to account for polarization in this region. The basis sets employed are distinguishable primarily on the basis of the way in which they account for polarization effects. Basis A includes s (c~= 0.70) and px, p,, , pz (CY= 0.60) bond functions at the center of the Cl0 axis for this purpose, as well as a six-member Cartesian d function with an exponent (OS 188) chosen to be approximately the mean of the corresponding optimized values for atom-centered Cl and 0 species [3]. From previous work on 0, and N, [6] it is known that such a combination of s, p and d bond functions is very adequate for the description of a molecule at small internuclear separations but that it tends to overestimate the corresponding bond energy somewhat [6] _ In addition this basis contains Rydberg s and pX, p,, , pZ functions (all with (Y= 0.01 S), also located at the center of the Cl0 bond, making a total of 48 contracted Cartesian gaussians. Basis B is wholly similar to A except that it does not contain the six d components in the Cl0 bond, hence consisting of only 42 functions. The C basis does not use the bond d functions either but instead locates one d set (cu = 0.53733) on oxygen and another (o = 0.502 i 1) on the chlorine atom. This set is especially constructed for proper description of the relative energetics between the molecule at equilibrium and the various HOC1 dissociation products; since the Rydberg functions are not necessary for this purpose, they are omitted in basis C (thus 50 contracted species in all). Finally a fourth basis, D, is employed to give further consideration to dissociation effects; it comprises the entirety of basis C but also includes the six-member bond d species found in basis A. The CT procedure used is of the multi-reference double excitation (MRD CI) type, with configuration selection and energy extrapolation [7] _ 3. HOC1 at equilibrium The bond length OH = 1.83 ao, OCI = 3.194 a0 and the internuclear angle LHOCl = 102_4O determined from microwave measurements [8] are employed in the present work for the ground state equilibrium HOC1 geometry; these values differ only very slightly from those determined earlier from similar experiments
Table 1 Orbital energies of the HOC1 molecule in its ground state at the experimental equilibrium geometry (all values arc given in hartree units, basis C) la’ 2a’ 3a’ 4a’
-104.9014 -20.6444 -10.6018 -8.07842
5a’ 6a’ 7a’ 8a’ 9a’ 1 Oa’
-8.07551 -1.40916
la”
-8.07530
2a”
-0.60905
3a”
-0.43989
($
-1.05612 -0.7 1469 -0.59607 -0.46437 (r@
[9]. The ground state equilibrium configuration is ‘A’ (!Oa’)2(3a”)2, which in linear geometry would correspond to 1 E ‘( 1‘IT)4 _According to qualitative MO theory this occup%ion scheme leads to the smallest bond angle known among HAB systems [IO,1 I] , similar to the situation in HOF [12,13] _The molecule is
thus expected to show a definite opening of the internuclear angle in all low-lying electronically excited states. The orbital energies (basis C) and the total energies obtained in the SCF and CI treatment employing the various basis sets discussed in section 2 are given in tables 1 and 2. As should be expected the total energy varies only slightly from one basis set to the other at the SCF level, while the choice of the various d functions is seen to be of much greater importance for the description of electronic correlation. The present energy values are also considerably lower than that given in a previous study (-534.7097 hartree SCF) on the HOC1 molecule [ 14]_ The two components (3a” and 10a’) of the ?rg-type MO are relatively close in energy; application of Koopmans’ theorem to both yields ionization potentials of 11.96 eV and 12.63 eV respectively, while the scaled values (multiplication by a factor of 0.92) of 1 1 .OI eV (2A”) and I 1.62 eV (2A’) should be more realistic predictions according to previous experience [ 151. The dipole moment of HOC1 in its equilibrium nuclear arrangement is calculated to be 1.633 debye (basis C) in the CI treatment with components ,+ = 0.223 and r_‘u= -1.618 debye?_ Hence the dipole moment vector ?
The molecule
is thereby oriented such that the oxygen atom is at the origin of the coordinate system dnd the Cl0 bond is collinear with the x axis while the hydrogen coordinates are positive iny and negative in x at LHOCI = 102.4”. 443
Volume 52. number 3
15 December 1977
CHEMICAL PHYSICS LETTERS
Tabic 2
Total enegics (in hartrcc) obtained for the HOC1ground state equilibrium in various treatments using three different basis sets Trcatmcnt
Basis A
Basis B
Basis Cb)
SCF
-534.87876 -535.1129 -535.127
-534.87494 -534.0719 -535.083
-534.88349 -535.2002 -535.221 5
CI . estimated”) CI basis limit
a) C&i&ted as (1 - XC~)AE, where AE is the difference between the calculated CI energy and the value obtained from the reference configurations only; the sum runs over all reference species. b) If the optimum HOC1geometry obtained from the CI calculation is employed, an energy lowering of 2.7 kcal is observed in the SCF trcatmcnt compared to the present value while the energy in the CI calculation is lower by 0.6 kcal (or 0.7 kcal in the CI basis limit).
forms an angle of 7.8” with they axis, i.e. making an angle of 97.8O with the O-Cl axis, with the oxygen corresponding to the negative center of charge. Since the dipole moment of an isolated OH molecule is 1.54 debye or 1.65 debye [16] , it is clear that this bond
ably higher energy (8.92 eV) whereby transitions to this state should be quite weak. Comparison with the measured data [2] suggests very strongly a 10a’ + 11 a’ (o*) assignment for the peak around ?20 mp (5.63 ev), while no comparable
component
transition
makes
by far the most
dominant
contribu-
tion to the HOC1 dipole (its p,, value would be -1.65 cos(102.4 -90) = -1.61 debye in the present geometrical arrangement)_ By contrast the separation of charges along the Cl0 bond is much less in HOC1
than it is in the isolated Cl0 molecule (with a dipole moment of 1.2 debye [ 17,18])_ This result thus indicates that the oxygen atom withdraws electronic charge primarily from the hydrogenic region rather than from the chlorine atom, so that the charge distribution in the Cl0 bond is considerably more evenly balanced in the HOC1 molecule than in the parent diatomic species.
4. Electronically
excited states
The excited states arising from excitation out of the two components 3a’ and 10a’ of the ng-type MO into the lowest-lying valence orbital and some additional Rydberg species are treated; details of the requisite CI calculations (using basis A) are listed in table
3. The lowest transitions result from population of the strongly Cl-0 antibonding o,,-type orbital 1 la’. The allowed singlet transitions thereto occur at an energy of 4.54 eV and 5.75 eV (vertical) respectively, with a calculatedfvalue of0.006 for *A’ and 0.0005 for 1A", i.e. the latter is approximately one order of magnitude weaker than the first transition $_ The doubleexcitation state 3aff2 + 1 la’2 is calculated at consider444
is found
theoretically
which
would
account
for the second experimental broad feature (reported to be almost equally as strong as the first) with a maximum around 3.87 eV. It is clear that some underlying intensity from the 1A" + LA' transition is found in this energy region (neglecting the 3A” f 1A' species for spin-conservation reasons), but both energy and intensity arguments rule out an assignment in terms of 1A"
for the second peak. Especially since similar results have been obtained in a related theoretical treatment [19], a more detailed experimental study seems to be called for to determine whether the HOC1 system is really responsible for this low-energy feature noted in the previous experimental study. Only a few representative Rydberg states are calculated in the present work. The first series converges to the 2A” IP which is calculated to occur at 10.93 eV. The calculated term value for 1A’ (3a” + 4p,) is 18790 cm-‘, a value which is in good agreement with the usual empirical rule [20] that the first member of a p Rydberg series lies in the order of 20000-21000 $ In order to calculate the oscillator strengths the ground
state SCF MO’s have been employed for the representation of both ’ A’ and ‘A” upper states to allow for the use
of a mutually orthogonal basis in all cases; a calculation was necessary for ‘A’ for this purpose, while five such species were required to represent the ‘A” state within an accuracy of less than 0.1 eV compared to the values given in table 3. with three reference confiiurations
CHEMICAL PHYSICS LETTERS
Volume 52, number 3 Table 3 Calculated vertical transition
energies
AE for various
15
States of HOCI, along with some technical
December 1977
details of the corresponding
CI
treatment State
% ‘A’ 3A” 1 A” 1 3A’
‘A’ 1
Number of reference configurations
Transition
...(10a’)2(3a” 3a” 10a’ -
I2
1 la’(o*) lla’(a*)
3a” - 4a” (4&J
‘A’ ‘A’ 3A” ‘A” t 3A’ 2A” ZA’
3a” ’ -
1 la’2(o*)
1Oa’ +
4a” (4~~)
7-a”, 3a” 3a” + m 10a’ + -
a) The notation
1 la” (o*)
m/n indicates
that m configurations
(symmetry
SCF
MO basis
Secular
AE(cV)b)
equationsa)
X’A
2353/17181
0.0
3A”(3a” + 1 la’)
2829/63498 2785138160
3.50 4.54
2804153264 2789139933 2781/19446 2566/l 7775 1 77/l 13668
4.73 5.75 8.60 8.92 9.45
3A’(10a’
+ 1 la’)
3A’(3a” tA’(3a”2
4 4a”) - lla’2)
3A”(10a’
--L~J”)
3A’(2a”, 2A” ion 2A’ ion
3.1” -
adapted
functions)
1 la’2)
are selected
2679168644 2542/33891
9.53 11.24
2760/l 14634 264016478 1
10.93 11.95
whde extrapolation
is with respect
to a
total of n configurations. b) AE is taken to be the difference between the electronic energies of excited and ground states respectively.
cm-l below the corresponding IP $1. Since singlettriplet splittings of corresponding Rydberg states are very small, the 3A’ (3a” + 4p,) state should be found in the immediate neighborhood of the 1A', as should be the Rydberg states involving the 38” and the other spatial 4p components_ The lowest Rydberg state (3a” + 4s) is expected to lie in the order of 28000 cm-t below the corresponding ionization limit, i.e. 1 S-2 eV above the low valence-shell
excitations_
The situa-
tion is very similar for the other Rydberg series converging to 2A’; the term-value (195 18 cm-t) for the calculated 10a’ + 4pz Rydberg member is wholly comparable to that obtained for the state originating from the 3a” MO. Similar relations as are found to exist between the various 3a” Rydberg states are also expected to hold for the other 4p and the 4s Rydberg species obtained by depopulating the lOa’. The calculated IF% furthermore support the empirical rule of scaling the Koopmans’ theorem results by 0.92, as discussed in section 3. Finally, the geometrical behavior of the various ex$$ This conclusion seems especially justified once it is realized that calculations at the present level generally underestimate the IP In the order 0.2-0.3
cV.
cited states with respect to separation into OH + Cl has been investigated. The Rydberg states show their minima at shorter HO-Cl distances than the ground state, in full agreement with the simple MO model [IO] , since depopulating the Cl-O antibonding ng-,-type orbital favors contraction of the corresponding bond. All calculated states populating the strongly antibonding u* show completely repulsive curves with respect to OH + Cl decomposition and correlate with the products in their corresponding ground state: OH(’ fl) and C1(2P). Thus it can be concluded that transitions into the states lying below the first Rydberg species (7.07.5 eV) lead immediately to photodissociation of the HOC1 molecule.
5. Stability and heat of formation of HOC1 The total energies of the various possible dissociation products of HOC1 have also been calculated and these results are listed in table 4. In addition the energy of the HCIO isomer in its geometrically optimized form (calculated structural parameters are Cl0 = 3.10 ao, HCl = 2-48 “o and LHClO = 104AO”) is also given. Experience with similar calculations for the N2 mole445
CHEMICAL PHYSICS LE’ITERS
Volume 52, number 3
15 December 1977
Table 4 Calculated
total energies (in hartree units) for various products of HOC1 obtained with basis C (or its equivalent)
Cl(2P) ClO(*n) o(3P) HCl(‘z+)
CI Iirnitb)
CI
SCF OH(*n)
-75.40690
-75.5482
-75.5538
-459.45568 -534.260 -74.80063 -460.07869
-459.5780 -534.5441 -74.8963 -460.2336 -534.5642 -0.5000
-459.5835 -534.5666 -74.8995 -460.2420 -534.5843 -0.5000
~210 (*np)
-
H(*S)
-0.50000
3) This rcsuh is obtained with basis D, i.e. with an additional bond d function compared to basis C. No SCF value is obtained for Cl0 in this case; the MO’s of ClO- are used in the CL b) Calculated as (1 - C CO”, Ah*, where PE is the difference cnce configurations
cule [6] indicates
only;
between
the sum runs over all reference
that for a generally consistent
scription of the combined
the calculated
CT energy
and the value obtained
from the refer-
species.
ever, since the Cl0 bond is relatively unaffected by this type of dissociation; hence results of this nature have been obtained with and without atom-centered d functions (i.e. using both basis A and C). The stabilities of HOC1 and its dissociation products are then compared in tables Sa and 5b for various basis sets. In line with the comments given above it is seen that the relative energetics of the systems in which the
de-
molecule and its dissociarion
fragments it is desirable to include both bond- and atom-centered polarization functions in the A0 bask.
Consequently the results of table 4 have been obtained using basis C (and in the case of Cl0 also basis D), in which d functions are located on the Cl and 0 atoms to complement the Cl0 bond-type functions (see section 2). These considerations are expected to be much less critical for hydrogen abstraction processes, how-
Cl0 bond remains intact are nearly unaffected by the manner in which polarization effects at the heavy atoms
Table 5 Relative stabilities (in kcJ/mole) taincd from various treatments.
(a)
of various products compared to HOC1 in its calculated ground state equilibrium (a) Corresponds to De values from basis A, results in (b) are from basis C)
SCF
CI
CI limit
HOC1 (’ A’) H(*S) + ClO(*fI)
0.0 79.6
0.0 95.5
0.0 97.9
HClO(‘A’)
68.7
66.3
63.7
(b)
CI
CI limit
best CI a)
best CIb)
IlOr’l(‘A’) H(%)+ClO(*II)
0.0 80.4
0.0 98.6
0.0 97.9
0.0 97.9
0.0 91.1
HCIO(‘A’) OHCn) +Cl(*P) 0(3P) + HCl(‘C+)
68.5 16.0 5.5
65.2 47.1 44.7
63.7 53.6 50.9
63.7 64.7 62.0
~60 62.0 58.3
H(*S)
82.8
142.4
150.4
161.5
153.5
+ C1(2P)
+ O(3P)
ob-
DO
De SCF
geometry
exptl. 0.0 9&2c),
98.0d)
? 60.3c) 59.3 d) 165.8e)
a) The value for the Cl0 bond dissociation De obtained from the more adequate basis set (see table 4) is taken in each instance, n.uncly 56.4 kcal/mole for CI and 63.6 kcal/mole for the CI limit instead of the CI limit value of 52.5 kcal obtained in basis B (see discussion in text). b) Zero point energies are taken into account, whereby the values for HCI, Cl0 and OH are taken from refs- [21,22] values Q (OH) = 36 17 cm-‘, ~2 (ClO) = 739 cm-’ and ~3 (LCIOH) = 1242 cm-’ [9,23] are employed for HOCI. c) Ref. [ 1] , sometimes only estimates. d) Ref. [ 19]_ e, Ref. 1211 on the basis of 64.6 kcal for the dissociation of Cl0 into Cl + 0.
while the
Volume 52, number 3
15 December 1977
CHEMICAL PHYSICS LElTERS
estimated full CI limit the results using basis A and C
mentally. The most striking result of table 5b is the essentially random nature of the SCF De values com-
respectively for the energy differences among the systems HOCl, HCIO and H(*S) + C10(2 ll) agree with one another to within 0.1 kcal/mole in each case (De values in tables Sa and Sb). The situation with Cl0 bondbreaking processes is more complicated, however, as illustrated by the De results for Cl0 itself. In this case
pared with their experimental counterparts. The only case for which the SCF method appears to be at all adequate is for simple hydrogen abstraction of HOCl, and even in this case the error relative to experiment is in the order of 20 kcal/mole. The description of the Cl0 bond-breaking process is very much more distort-
the extra d-type function centered in the Cl0 bond has a relatively large effect on the dissociation energy, with basis C yielding a CI limit De value of only 52.5 kcal/
ed at the SCF level, with discrepancies in excess of 50 kcal/mole being noted. By contrast the best CI Do values obtained in this work are in generally good agreement with experiment, showing errors of only 7,2 and 1 kcal/mole respectively for dissociation into H(2S) + CIO(211), OH(?II) + C@P) and OCP) + HCl(l X’)_ The error in the case of complete atomization is 12.3 kcal/mole or 7% of the total D, value. The isomeriza-
are represented in the present basis sets. In fact at the
mole compared
to that of basis D (with d functions
at
each atom and in the Cl0 bond; see section 2) of 63.6
kcal/mole. The latter result is in much better agreement with the experimental value of 64.6 k&/mole [2 I] and hence it appears that addition of d AO’s at the atomic centers without simultaneous inclusion of bond-center d species lead to a significant bias in favor of the separated atoms. These results are consistent with the N2 study [6] mentioned earlier, in which it has been found that bond functions through d type are required in conjunction with atom-centered d ,40’s in order to achieve a generally accurate balance between the molecule and its lowest three dissociation limits?‘. Since this basis set effect appears to be quite general it therefore is desirable to correct the De results obtained with
basis C for all systems in which the Cl0 bond is broken, while taking the corresponding uncorrected values at face value for species in which this bond remains in-
tact; based on the results for Cl0 the value of the former correction is thus taken to be 11 .l kcal/mole, i.e. the difference between the Cl0 De values obtained inbasis C and D respectively. These findings are‘ then summarized in table Sb under the heading “best Cl” results. Finally it should be noted that the calculated minimum in the CI energy of HOC1 used to construct table 5 corresponds to the geometry: Cl0 = 3.22 ao, OH = 1.88 a0 and LHOCl = 104. lo, as derived from a polynomial fit to the various data points obtained_ The resulting energy is only 0.6 kcal/mole lower than the previous calculated CI value given in table 2 corresponding to the equilibrium geometry observed experi? It is also
worth noting that the Cl0 De value obtained with basis B, with no d AO’s at all but with s and p bond func-
tions is 62.0 kcal/mole, in good agreement with both the experimental result and that obtained with basis D, but well above the corresponding basis C value.
tion energy of HOC1 and HClO is not known experimentally but on the basis of the present Cl calculations it is estimated to be approximately 60 kcal/mole. From these results it is also possible to calculate the heat of reaction for the process: 0(3P) + HCl(‘Z’) + OH(2n) + Cl(“P), namely i-3.7 kcal/mole for Mt and +3.6 kcal/mole for M&. These findings agree well with the corresponding
measured values of +0.9 kcal/mole [24] and +0.96 kcal/mole [25] . They also allow for a number of pathways for calculation of the heat of formation of HOCl, as indicated in table 6. The average of the four AHzg8 values found therein is -19.2 kcal/mole with a standard deviation of 3.9 kcal/moleii. which results in turn also compare favor?? Whereby
the higher absolute
weighted
somewhat
abstrdction Cl0
process
bond-breaking
values should
more because is described
actually be the calculated hydrogen
less satisfactorily
than the
(5ee table 5).
Table 6 Calculated
heat of formation
(in kcal/mole)
the assumption ofvarious formation schemes CI DO vdues of table 5b”) Reaction
partners
H + Cl0 OH+CI O+HCl H+Cl+O
for HOC1 under
using the best
A-G
AJ+La
-15.3
-15.9 -24.1 -21.9 -14.8
-23.4 -21.3 -14.3
a) An ided gas correction for the temperature dependence of AHO for HOC1 has been applied [26] to obtain the results of the last column;
experimental values for Af&a are taken from standard tables [ 27 ] for each of the dlatomic and . atomic spccics involved therein.
447
Yolumc
52, number
3
CHEMICAL
PHYSICS LETTERS
abiy with the value of -22 + 3 kcal/mole adopted recently by Warnek [l] upon consideration of a variety of experimental observations. In summary by using a flexible A0 basis with both bond- and atom-centered polarization functions the CT calculations carried out in this study are capable of describing the relative energetics of HOC1 and its various dissociation products to within error limits of 0. I-03 eV in al1 instances except that of complete atomization, in which case the error increases to 0.5 eV. After this paper had been submitted it was pointed out to us that the dipole moment of HOC1 has been determined from microwave spectra by D.G. Lister and D.J. Miller (Trans. Faraday Sot. 67 (1971) 601). The experimental quantity of 1.3 -t 0.3 debye is in good agreement with our calculated value of 1.633 debye and the angle of 73 f 3” between the dipole moment vector-and the Cl0 bond reported in the experiment compares quite well with the calculated angle of 82.2” (i.e. the complement of 97.8O given in the text).
Acknowledgement The authors wish to thank Professor K.H. Becker, Cesamthochschule Wuppertal, for pointing out the possible importance of the HOC1 molecule in the stratosphere_ One of us (G-H.) wishes to express his gratitude to the Studienstiftung des Deutschen Volkes for financial support. The services and computer time made available by the University of Bonn Computer Center have been essential for this study and are gratefully acknowledged.
References [I] C. Warneck, preprint. [2] WC. Fcrgusson, L. SIotin and D.W.G. Style, Trans. Faraday Sot. 32 (1936) 956. [3] T-H. Dunning, J. Chcm. Phys. 53 (1970)
448
2823.
15 December 1977
A. Veillard, Theoret. Chim. Acta 12 (1968) 405. S.D. Peyerimhoff, R.J. Buenker and L.C. Allen, J. Chem. Phys. 45 (1966) 734, appendix I. W. Butscher, S. Shih, R-J. Buenker and SD. Peyerimhoff, submitted for publication. R.J. Buenker, S-D. Peyerimhoff and W. Butscher, sub-
mitted for publication; R.J. Buenker and S-D. Peyerimhoff, Theoret. Chim. Acta 39 (1975) 217. A.M. Mirri, F. Scappini and G. Cazzoli, J. Mol. Spectry. 38 (1971) 218. 191 R-A. Ashby, J. Mol. Spectry. 23 (1967) 439. [lOI A.D. Walsh, J. Chem- Sac. (1953) 2260. t111 R.J. Buenker and S.D. Peyerimhoff, Chem. Rev. 74 (1974) 127. 1121 R.J. Buenker and S.D. Peyerimhoff, J. Chem. Phys. 45 (1966) 3682. [13] H. Kim and J.R. Sabin, Chem. Phys. Letters 20 (1973) 215. [ 141 G.L. Bendazzoli, D.G. Lister and P. Palmiero, I. Chem. Sot. Faraday II 69 (1973) 791. [ 151 M.B. Robin, Higher excited states of potyatomic molecules (Academic Press, New York, 1974). [ 161 A.L. McClellan, Tables of experimental dipole moments (Freeman, San Francisco, 1963). [ 171 A. Carrington, P-N. Dyner and D.H. Levy, J. Chem. Phys. 47 (1967) 1756. [ 181 T. Amano, S. Saito, E. Hirota and Y. Merino, J. Mol. Spectry. 30 (1969) 275. [ 191 R-L. Jaffe and S-R. Langhoff, paper presented at the 32nd Symposium on Molecular Spectroscopy, Columbus, 19771201 C. Sandorfy, Lecture given at the VIIth International Conference on Photochemistry, Jerusalem, August 1973. [21] J.A. Coxon, W-E. Jones and E.G. Skolnick, Can. J. Phys. 54 (1976) 1043. (221 G. Herzberg, Spectra of diatomic molecules (Van Nostrand, Princeton, 1950). [23] K. Hedberg and R.M. Badger, J. Chem. Phys. 19 (1951) 508. [24] R.D.H. Brown and I.W.M. Smith, Intern. J_ Chem. Kinetics 8 (1975) 301. [2.5] J. Wolfrum. Ber. Bunsenges. Physik. Chem. 81 (1977) 114. [26] S-W. Benson, Thermomechanical kinetics (Wiley, New York, 1976). [27] V.I. Vedeneyev, L.V. Gurvich, V-N. Kondrat’yen, V.A. Medvedev and Ye-L. Frankevich, Bond energies, ionization potentials and electron affinities (Arnold, London, 1962).