Ab initio SCF and CI calculations for ground and low

Ab initio SCF and CI calculations for ground and low-lying valence and Rydberg excited states of HOCl and HClO in linear and bent nuclear conformations PABLO J . BRUNA, GERHARD HIRSCH, A N D SIGRID D. PEYERIMHOFF Lehrsrrrhl j i i r T i ~ ~ o r . e ~ i s cChemie he rler Utlicersiriir Bot111, WegelersrrtrJe 12, 0-5300 Botltl. Wesr Gertntrtly

AND

ROBERT J. BUENKER Can. J. Chem. Downloaded from www.nrcresearchpress.com by 88.72.127.217 on 11/04/15 For personal use only.

L e h r s ~ r r hjzir l Theor~erischeChetnie der. Gestrtnrhochschrtle W~rpperrtrl,Gnl@srrr$e 20, 0-5600 Wrrpperrtrl, Wesr Gertntrtly

Received December 28, 1978 PABLOJ. BRUNA,GERHARD HIRSCH,SIGRIDD. PEYERIMHOFF, and ROBERTJ. BUENKER. Can. J. Chem. 57. 1839 (1979). Potential curves are reported for the ground and various valence and Rydberg excited states of HOCl and HClO using ab inirio SCF and CI methods. The states of HOCl are found to be generally favored over their counterparts in HCIO, but the calculations indicate that small barriers nevertheless exist for angular interconversion between at least some pairs of such states; they also show that dissociation processes involving CIO bond breaking are universally favored for both HOCl and HC10. Additional calculations for the ions have also been carried out, from which the heat of formation of HOCI+ is calculated to be 237 kcal/mol and it is pointed out that a previous estimate for the corresponding HOCl quantity has subsequently been substantiated by experiment; the relative energies of the first four states of HOCl+ are found to be in good agreement with experiment as well. PABLOJ. BRUNA,GERHARD HIRSCH,SIGRIDD. PEYERIMHOFF et ROBERTJ. BUENKER. Can. J. Chem. 57, 1839 (1979). On rapporte les courbes de potentiel pour les Ctats fondamentaux, divers Ctats de valence et pour des Ctats excites Rydberg du HOCl et du HClO CvaluCes par des mtthodes SCF et CI. On a trouvC que les Ctats de HOCl sont generalement favorises par rapport a leurs contreparties de HClO mais les calculs indiquent toutefois qu'il existe de faibles barritres a l'interconversion angulaire entre au moins quelques-uns de ces etats; ils montrent aussi que le processus de dissociation impliquant le bris de la liaison C10 est universellement favorise pour HOCl et HC10. On a effectue des calculs additionnels sur ces moltcules et l'on peut en deduire que la chaleur de formation de HOCl+ est tgale a 237 kcal/mol et l'on signale que I'Cvaluation anterieure de la quantite correspondante pour HOCl a depuis ttC confirmee experimentalement; on a trouve que les energies relatives des quatre premiers Ctats du HOCl+ sont aussi en bon accord avec les valeurs experimentales. [Traduit par le journal]

1. Introduction between HClO in I additional interest of a more theoretical nature has arisen, especially since isoa previous (1) the relative stability of the me'ization energies for HAB systems composed HOCl molecule with respect to the HClO nuclear arrangement and the various fragments H + C10, exclusively of first-row atoms have quite generally has been been found to be much smaller. It thus becomes OH + ~ 1 0, + H C ~ ,and H + ~1 + investigated by means of ab inirio configuration important to understand what differences in elecinteraction treatments. l-he first four low-lying tronic structure cause the HOCl arrangement to be states of H O C ~were also studied in I, and it SO preferred in nature and furthermore to establish has been found, as confirmed in a similar study by whether a similar situation exists for the excited Jaffe and Langhoff (2), that population of all these States these systems. For part of the present work only the SCF level species promotes dissociation of the molecule into of treatment is employed since potential energy OH and C1. Interest in the HOCl molecule has increased con- Curves obtained for ground and pertinent low-lying siderably in the past few years because of its possible excited states in this approximation are often satisrole as an intermediate in the chlorine chemistry of factory for gaining information On the general the troposphere and stratosphere (3, 4). Further- structural behavior of the system under more, in view of the large stability difference found nuclear displacements. These calculations are supplemented by CI treatments at various key points along the potential energy surfaces when necessary in 'Hereafter referred to as I. 0008-4042/79/141839-13$01.OO/O @ 1979 National Research Council of Canada/Conseil national de recherches du Canada

1840

CAN. J . CHEM. V O L . 57, 1979

Can. J. Chem. Downloaded from www.nrcresearchpress.com by 88.72.127.217 on 11/04/15 For personal use only.

order to obtain information of quantitative significance. At the same time the original study (I) is extended in order to predict the location of additional excited states of valence and Rydberg character and their possible role in photochemical reactions of this system.

2. Technical Details The HOCl system is placed in the xy plane for the present investigations. The A 0 basis set chosen is of double-zeta plus polarization quality and for the majority of calculations consists of 48 contracted cartesian gaussian functions (basis A of I). The (4, 2) oxygen contraction is thereby taken from Dunning (5) and a (6,4) chlorine set is adopted from the work of Veillard (6), while a (2,l) set with scaling factor q 2 = 2.0 is centered at the hydrogen site (1). Polarization is taken into account by bond functions of s, p, and d type centered in the C10 bond and an s species located between hydrogen and the respective heavier atom. Long-range functions of s and p,, p,, pZ type (a = 0.015) are also located at the center of the C10 bond in order to represent the first members of corresponding Rydberg series. In addition for a few cases which are critical for destruction of the C1-0 bond the d bond functions are replaced by a set of d functions centered at each of the heavy atoms 0 and C1. The CI procedure employed is of the multireference double-excitation (MRD-CI) type with configuration selection and energy extrapolation (7); it is wholly analogous to the treatment in I and maintains a core of 12 electrons (corresponding to the K and L shells in chlorine and the K shell in oxygen) uncorrelated. 3. Comparison of the HOCl Structures A . Orbital Characteristics The experimental (8) structural data for HOCl are: OH = 1.83ao, C10 = 3.194ao, and L H O C ~= 102.4" while those determined by calculations in I for HOCl are: OH = 1.876a0, C10 = 3.220a0, and LHOCl = 104.0°, and for HClO: HCI = 2.48a0, C10 = 3.10ao, and LHCIO = 104.4". The orbital energies obtained for both isomers in the same basis are contained in Table 1 and these data show that the charge distribution around the respective oxygen and chlorine atoms in the two species are markedly different from one another; the distinctly higher inner-shell orbital energy for chlorine (la') in HOCl compared to HClO reflects the larger negative charge around the chlorine atom when it is in the terminal position than what is found when it is located in the middle; similarly the lower 2a' energy in HOCl

TABLE 1. Orbital energies (in hartree) for HOCl and HClO in their equilibrium nuclear arrangements (basis A of I) E

1a' 2a' 3a' 4a' 5a' 60' 7a' 8a1(n) 9a1(o) 1Oaf(n*) In" 2a"(n) 3a"(x *)

HOCl

- 104.9041 - 20.6489 - 10.6045 - 8.08093 - 8.07827 -1.41331 - 1 ,05870 -0.71491 -0.59538 -0.46619 - 8.07805 -0.61110 -0.44231

HClO - 104.9785

-20.5593 - 10.6797 -8.15586 -8.15428 - 1.32464 - 1.10042 - 0.70503 -0.56702 - 0.45642 -8.15371 -0.59456 - 0.42776

indicates that oxygen is less negative in HOCl than in HClO. Both observations taken together show that the more negatively charged atom is preferred at the terminal position in each case, a finding which is quite consistent with what has been observed generally in other triatomic systems, including molecules of A , type (9). Similarly large differences in the valence orbital energies are obvious from the results of Table 1. A comparison of the various charge density contour diagrams shows that the 6a' is to a large extent the o,(2s + 3s) type MO in both systems, while the 7a' is the antibonding o,(2s - 3s) counterpart and the 9a' can be looked upon as the o,(po) bonding orbital (Fig. 1). The 8a' and 10a' species correspond to n and n* respectively with a typical hydrogen admixture. It is interesting that in the bent equilibrium geometry the o-type orbital 9a' lies between the n and n* species (for both in-plane and out-ofplane components), in contrast to the situation for the linear HOCl geometry, for which the energy order is G(E= -0.695 hartree), TC(E = -0.577), and n *(E = -0.430). The lowest unoccupied MO (1 la') is of o* type, as can also be seen from Fig. 1. It is strongly Cl-0 antibonding and shows a definite preference for the linear nuclear arrangement, as will become clear from consideration of the geometrical characteristics of electronic states which occupy this species. Finally the out-of-plane MO's (Fig. 2) exhibit clear differences in charge distribution; in HOCl oxygen character is dominant in the 2a" (n-type MO) and that of chlorine is more important in the antibonding 3a" MO while the situation is reversed in HCIO. A localized MO representation (10) indicates quite clearly that the C10 bond is stronger in the more stable HOCl species (Fig. 3), since the charge distribution is much more evenly distributed between


BRUN

FIG.1. Canonical orbital charge density contours for the three highest occupied a' MO's (8a' to 10a' from bottom to top) of (a) HOCl and (b) HClO in their respective ground state equilibrium geometries, and for the lowest unoccupied (o*)M O in the same systems, (c) and (d).


CAN. J. CHEM. VOL. 57, 1979

FIG.2. Canonical orbital charge density contours for t h e n and n * MO's of HOCl (left) and HClO (right) respectively.

FIG.3. Localized orbital charge density contours for the OH and C10 bonds in HOCl (left) and for the HCI and CIO bonds of HClO (right).

the two nuclei in the corresponding orbital than in its HClO counterpart. Since the respective dissociation products (OH + C1 and HCl + 0 ) are known (11, 12) to be nearly isoenergetic, the difference in C10 bond strengths is essentially equal to the stability difference of the two systems, which has been calculated to be 60 kcal/mol in I. The localized OH and ClH bonding orbitals (also in Fig. 3) show quite normal behavior; their relation to the canonical MO's is given in Table 2 and shows that there are always four MO's which contribute to the localized bonds. The o,-type species 6af and 9a' (Fig. 1) contribute in the main to the C10 bond, while the OH species gains its dominant character from the 6a' and

8a' canonical MO's and the 7a' and 8a' MO's predominate in forming the HCl bond. B. Angular Potential Energy Curves A series of SCF calculations is carried out at various internuclear angles for all low-lying electronic states of HClO and HOCl which are accessible via the Roothaan procedure, and the results are summarized in Fig. 4; the various bond lengths are held constant thereby at their optimal values in the respective equilibrium conformations. The correlation between the various states is obvious for the ground and those states resulting from n:" + o * (lla') excitation; the relation between

B R U N A ET A L .

TABLE 2. Expansion coefficients for the localized orbitals representing the H-X bonds in the basis of the canonical MO's

HOCl


HClO

Bond

6a'

0-H CI-0 C1-0 CI-H

and C1-0

7a'

8a'

9a'

10a'

0.56 0.55

0.36 -0.30

0.71 -0.33

-

-0.21

-0.69

-0.52 0.36

-0.11 0.52

+0.30 0.76

-0.79

-

-

-0.10

FIG. 4. S C F potential curves for the angular interconversion of HOCl and HClO in ground and various excited states (OH = 1.830~and C10 = 3.194ao for HOCl and HCI = 2.48a0 and C10 = 3.10ao for HCIO). The enumeration of the potential curves is made according to the notation of Table 3. The 10a' + 1la"A' state has not been treated in the S C F calculations but an estimate of the nature of its potential curve (as indicated wlth a dashed line) is given, based on subsequent C I calculations.

the various species in different symmetry and the abbreviations used in the figure are indicated in Table 3, along with details of the corresponding CI calculations. The SCF potential energy curves show that all the states under discussion are higher in energy in the less stable HClO form than in HOCl, and CI calculations (Fig. 5) carried out for the four representative points, linear HOCl, equilibrium HOCl, equilibrium HC10, and linear HClO, demonstrate that the relative spacing and form of the SCF curves is not unrealistic in this case (HOCl and HClO vertical transition energies are given in Table 4). A barrier for interconversion from HClO to HOCl is indicated for each state, however, but optimization of the various structural parameters is expected to reduce these quantities relative to Fig. 4

and probably eradicate them entirely in some instances. On the other hand, in states such as the 3 , 1 ~ '(10a' + lla') pair (curves 4 and 5) which strongly prefer the linear nuclear arrangement in HClO it is quite unlikely that the corresponding barriers disappear upon optimization of the internuclear distances at each bond angle. Nonetheless since the HClO ground state is considerably more stable in the bent geometry it is possible that no barrier to angular interconversion is actually present in this important case. In this connection it is interesting that a substantial barrier for HNC + HCN interconversion (of the order of 35 kcal(l3)) and also for HOC' + HCO' (14) is found in calculations, while only a very small barrier results for the related HSiN + HNSi process (of the order of 4 kcal(l5))

1844

CAN. J . CHEM. VOL. 57, 1979

TABLE3. Characterization of HOCl states treated and technical details of the CI calculations. The corresponding HClO states are treated in a similar manner


State

Label

XIA' 3AI, 'A" A' 'A' 3Aft 'A" 'A" A' 'A' 'A' 'A" 'A" 'A' 3A" lAtf 'A' 'A' 3A" All 'A' A'

Treatmentb (nMlmR)

Secular equation generated/solved

211 211 211 211 311 612 612 311 511 911 511

1718112353 6349812829 3816012785 5326412804 3993312789 11912714739 7216814795 7969012733 11412112581 8456312540 10878112773

+ 4p(a1)

512

130204/5061

4p(a")

111 511 511

1944612781 11366812877 6864412679

912

21061913537

Excitationa

1

:t it t 1 8

1; 11 12 13 14

3a"

+

lla'

10a'

+

lla'

2a"

+

lla'

3a"

+

4s

9a'

+

lla'

10a'

+

4s

3a" 3a"

+

17 18

;;1

512 10966215232 512 6628815227 1777512566 211 Estimated from a less extensive calculation 111 3389112542 412 12067013950 412 9018114053 412 12067013950 412 9018114053

21 22

Corresponding C,, symmetry

1 1

nn*

'o * Z

1

OExcitation is always taken relative to the ground state electronic configuration 8a'22a"29a'210n'23a"211a'o0 bSelection of configurations is made for n~ roots while rr main (or reference) species are employed for the configuration generation.

TABLE 4. Calculated vertical excitation energies in eV for low-lying states of HOCl and HClO A E, State

Labela

HOCl

XIA'

1

0.0

1 :j:: 1 :{: 1 :I:: 1 :;: 1 :j: 1:;: A' 'A' A'

42 3 5 76 -

8 109 19 20 21 22 23

3.50 4.54 4.73 5.75 7.49 8.19 Not ~ a l c . ~ 7.16 7.92 9.97 9.42 9.93 9.92' 11.30 11.24

AE, HClO 0.0 1.87 3.20 2.52 3.92 8.64 9.20 5.73 (7.30) -

6.57 8.46 7.78 -

State A' 'A' 'A" 'A"

{

:!

Label -

11 12 13

HOCl

HClO

Not ~ a l c . ~ 8.34 7.25 8.36 8.55 -

A' 'A'

8.60 9.45 9.53 9.33 9.40

7.00

14 15 16 17 18

ZA" ZA' ZA" ZA'

30 31 32 33

10.93 11.95 14.55 15.48

10.05 10.77 15.30 15.42

-

7.62 -

-

10.32

OThe numbering refers to the states given in Table 3. bSince the singlet-triplet splitting is very small in Rydberg states, generally only one multiplet is calculated explicitly. =This value is misprinted in I (8.92 instead of 9.92).


CAN. J . CHEM. VOL. 57, 1979

. ...

4 ' 1 3 L P

Ion

/a 111 '11 '11 I//

7

'A'

-

---*

10.';

oo

7

,,Ion . . .

O//

0

r

r


-3a"L

HOCL

HOCI,,

m

HCLoaq.

//

4'

4 4/

HCIO

FIG. 6 . Comparison of calculated CI energy levels for the lowest-lying Rydberg states and ions of bent and linear HOCl and HClO (fixed bond lengths of the same magnitudes as in Fig. 4). Estimated appearance of the corresponding angular potential curves is indicated in the figure with dashed lines; note that fewer states have actually been treated for the less stable HClO conformer.

species (Fig. lc), whereby the n and n') MO's exhibit very similar stability in both linear conformers. This situation. on the other hand. is not unusual since similar observations have been made in related molecule pairs such as HNO-HON (17, 18) or HCO -HOC+ (14), for example, in which an exchange of singlet and triplet ground states or the reverse ordering of (n,n*) and (o,n:'.) states in the respective pairs of isomeric forms has also been noted. The end result of the various changes in relative orbital stability in the two systems is that the vertical transition energies of stable HOCl are generally much higher than for HC10, as can be seen in Table 4 as well as in Fig. 5. A similar but even more extreme example of this phenomenon has been found in the study of the HNSi-HSiN pair (15, 19), for which the lowest excited state becomes nearly isoenergetic with the closed shell ground state in the less stable (HSiN) form. Since as mentioned above there is at most a small barrier for angular interconversion of the HClO ground state to the much more stable HOCl species it is doubtful whether this clear distinction between these two systems can be observed experimentally. In this connection it should be noted, however, that there is still an open question concerning the HOCl spectrum, as observed by Fergusson et al. (20, 2). Although the first strong experimental band at 5.64 eV is quite plausibly +

assigned as the 2'A'-'A' transition of HOCl (calculated at 5.75 eV; Table 4), a second weaker system with an absorption maximum at 3.87 eV does not fit in well at all with any other dipole-allowed species for this system. The analogous (2'A'-'A') transition in HClO does lie in the proper range (3.92 eV; Table 4), as does the 3A"-'~' HOCl species (3.50 eV), but it is certainly unclear whether either of these transitions is in any way related to the above experimental findings. The analysis of the more highly excited states involving the n MO or double substitutions from n and n * is more difficult since a number of states lying in the same energy region are mixed (in the CI or already at the SCF level of treatment) because of the lower C, symmetry of the bent molecule. There is also the additional complication that although o (8a') and n (9a',2a1') are energetically well separated in the linear nuclear framework, the in-plane 8a' and 9a' components can mix so that for the equilibrium bent structure (see Fig. 1, a and 6) it is the 9a' MO which possesses o character in both isomers while the 8at is predominantly an in-plane n orbital; hence identification of the dominant configuration in a given state must be coupled with the knowledge of the specific character of the MO's in order to allow for a proper microscopic description of the state. A relatively clear pattern is present for the lowest members of the Rydberg states (Fig. 6). All first


BRUNA ET AL.

singlet members for the most stable equilibrium form of HOCl are treated by CI calculations (3a" -+ 4s, 4p,, 4py, and 4p, and 10a' -+ 4s, 4p,, 4py, 4pz), while calculation of the corresponding triplet species (with the exception of the 10a' -+ 4pz) is not undertaken because of the well-known small triplet-singlet separation for such Rydberg states. The typical features of Rydberg states are obvious: the 4s state is found well below the 4p species, of which all spatial components lie very close in energy; furthermore the energy difference between the series originating from the 3a" and the 10a' MO and their respective ionic limits is very similar (approximately 30 000 cm-' for the s series). The pattern of Rydberg levels for the other arrangements of nuclei is equivalent (even though not all states have been treated explicitly in CI calculations), especially once it is realized that 3a" and 10a' become degenerate in the linear molecular geometry. The Rydberg states lying in the energy range between 7 and 13 eV might possibly mix with valence states (21) of the proper symmetry and energy (Fig. 5), although this feature has not been explicitly treated in the present work. On the other hand, such mixing is partially accounted for in the SCF data of Fig. 4, and as a result the corresponding potential curves exhibit a rather unconventional form with several minima in some areas. The valence-shell states involving higher than 7 C e (3:q ransitions are also calculated for HOCl and HClO in the CI treatment and the results are collected in Fig. 5 and Table 4. Correlation with corresponding states in the four nuclear conformations is shown, while crossing of states is only indicated; details concerning the various states are contained in Table 3. It is generally seen that the higher valence-shell states prefer a linear structure, while the Rydberg states (Fig. 6) are generally more stable in the bent nuclear arrangement (as are the corresponding ions), leading to the interactions of various states mentioned above. -+

4. The Molecules HOCl and HClO and their Fragments In order to investigate the possible destruction of HOCl into its various components or formation therefrom, potential energy curves connecting the different partners in a number of electronic states are calculated and will be discussed in what follows. A . HOCl and its Components HO + CI ~h~ calculated SCF curves for separation of H O C ~ into OH and a C1 atom are contained in Fig. while the CI values for the pertinent end points are given in Fig. 8. It is obvious that the first valence states

E~~~ -534.3A(hartree)

-534.4-

6 3

0 - 534.5-

- 534.63~

-534.7-

-534.8-

-534.9

1 ,

2.7

FIG, and

3.2

3.8

4.4 R(Ct-OHl5.O a.u.

?, SCFexcited potential curves for Cl-0 states of H ~ ~ , 'A

CI(?P, I +OH?XI

~

.

in ground ,

,,---

AEc,(eVl

.'

'XX""

,o.o-

I,=

H('Sgl + Cl0lA7lL1

R

In"

' -I

,,K\,

=1Rf

.,

,,I=

-

1~3. //

I,I~-~

x,

CI(~P~I

,/ /

1

--_ 'A'

I /1

/--.

,,I\ $-,/,,, -2,

/ I-, I /,/-A~'

-

,,-, -Y-Y 8 /I H

I

Z\\

-

OHI'TCI+ CI (?P, I

I

In"

(7s91

\\ \

.c101x'lL1

I

\.,--

'A'

I I

\

I I I/

\\

-

]A-

I

\\

\\

0.0

/,%'--\----

,,181\\

,I

1I5/ /

- -T

/I--.----

/ /

\

,$,+,"-' &-

2.0

\

/

//

4.0

/I

f'w-7

'

(121

OH

6,0-

Id -IP;.

'A%

CL*OH

HOCL

H*CIO

HClO

FIG. 8. Comparison of calculated CI energy levels of HOCl and HClO in ground and various excited states with those of the dissociation products H C10 and OH C1 respectively. (The degeneracies of the various fragment energy levels are given in parentheses in each case.)

+

+


CAN. J . CHEM. VOL. 57, 1979

FIG. 9. SCF potential curves for hydrogen removal of (a) HOCl and (b) HClO in ground and various excited states (calculated equilibrium values for the C10 bond length and corresponding bond angle are used in each case).

3,1A' and 3 , 1 A" arising from n * + CJ*excitations promote immediate decomposition into 0H(2rI) and CI(2P,) as a consequence of populating the strongly C10 antibonding o ' h r b i t a l , as has been pointed out earlier in I, as well as by Jaffe and Langhoff (2). The Rydberg states (Figs. 7 and 8) are not repulsive and show potential curves similar to that of the positive ion, i.e. they prefer a somewhat smaller C1-0 bond length than does the ground state, which is also stable with respect to C1 + OH separation. Various high-energy states (Figs. 7 and 8) populating the antibonding CJ* twice are strongly repulsive and furnish the remaining curves leading to the ground state products (Fig. 8). The conclusion from these results is that any excitation of HOCl below the first Rydberg species (7 eV) will immediately lead to fragmentation into OH and C1 (calculated fragment total energies are given in Table 5 of I for comparison with the HOCl vertical excitation energies in Table 4).

remove the hydrogen atom from HOCl in its ground state (Table 5 of I).4 All excited states in Fig. 9 a and b with minima in the same general energy range as the ground state fragments H ( 2 ~ , )+ C10 (2rI) exhibit a potential barrier for larger hydrogen distances. Although the calculated barriers in Fig. 9 a and b will most probably be reduced considerably when relaxation of the C10 bond is taken into account in the process, it is still likely that a small barrier remains in each case in analogy to what is found for similar states in HCO, HN,, or HNO (for example see ref. 24). All potential energy curves involving HOCl or HClO Rydberg states correlate with electronically excited fragments and the SCF calculations of Fig. 9 again suggest that substantial energy is required before dissociation into H C10 can take place via these states. Finally consideration of Fig. 8, which shows the realistic relative positions of the various energy levels, makes it clear that loss of hydrogen in any of the HOCl states treated is extremely unlikely, especially since in the excited states dissociation into 0H(2rI) + C1(2P,,) can take place so easily instead. Similarly formation of the molecule in its ground state from H ( 2 ~ , )+ ~ 1 0 ( , n ) is probably not

+

B. Hydrogen Removal Processes The calculated SCF potential energy curves for hydrogen removal HOCl -t H + C10 and HClO -t H + C10 are contained in Fig. 9, aand b ; the corresponding CI data are also presented in Fig. 8. Both ground state curves show the typical behavior of a 4The measurement of a heat of formation for HOCl of - 18 bound state with one minimum3, and approximately kcal/mol by Molina and Molina (22) and of - 19 kcal/mol by 95 kcal/mol must be added to the system in order to Timmons (private communication, value of R. Timmons 3The long-range SCF part must be corrected in the standard way to obtain a realistic description of the bond-breaking process involving the open-shell fragments.

transferred to authors by D. Phillips (NASA)) as compared to the predicted value for this quantity in I of - 19.2 ) 3.9 kcal, gives a t least some indication of the overall reliability of the present nb irlitio results.


BRUNA ET AL.

favored since the excess energy of this reaction would readily promote dissociation into OH and C1. Furthermore although the OH + C1 destruction channel is not directly open to the HClO isomer, consideration of Fig. 5 suggests that a small supply of energy in the bending vibrational mode might favor interconversion from the HClO excited states into HOCl (not even considering the HCl + 0 pathway), which again would be followed by fragmentation into OH + C1 rather than dissociation from the excited HClO states into H + C10. C. HClO and the Components 0 + HC1 The SCF potential energy curves connecting the fragments 0 and HCl with the system HClO are given for various states in Fig. 10 (whereby the internuclear angle of 102.4" is held constant) while the complementary CI data are contained in Fig. 11. In this case the adiabatic correlation of states is especially simple since all low-lying triplet HClO states correlate with 0(3Pg) HCl('C+), while the corresponding singlet species must be connected with O('D,) + HCl('C+). It is seen that except for that of the ground state the low-energy curves are all

+

10.0

8.0

6.0 -

--'nn* - -

1

-.'\

'a'

___.--,\ Id'

4.0 -

.

'x,'p:

\

---.---.

',,

. -, Id>

--'"+\

O I ' D ~HIC L ( ' T ' J +

(51

/1

\\

1

\..___ ', ; 1 I

.., ; ,\ \

In'.

7 A , ,

-\b- ( 9 1

\

2.0-

+

HCI('T+J

I

\

I I

\ I

\ I

\

1

\ \ \

O.O-

O('P~J

I

!

1

'7' A'

I

HClO

€5,~ (hartree)

HOCl

O+HCl

FIG. 11. Comparison of calculated CI energy levels of HOCl and HClO with those of various 0 HC1 dissociation products. (The degeneracies of the various fragment energy levels are given in parentheses in each case.)

+

'CI

-0

repulsive, favoring dissociation into 0 + HC1 in a manner quite similar to the HOCl dissociation (Fig. 7) into OH + C1. The reaction O('D,) + HCl has been studied experimentally (25) and it has been found to yield the products OH + C1 rather than C10 + H. According to the present study this reaction could proceed with very little activation energy (if any) in a favorable geometrical position via the lowest excited 'A" state (which is essentially isoenergetic with O('Dg) HCl ('C') if the oxygen inserts to give the HOCl arrangement; see Fig. ll), which has been shown to be strongly repulsive with respect to the OH + C1 fragments. Reaction via the ground state HClOHCI ( 1' I HOCl surface could lead to an HOCl entity provided the high excess energy of this reaction can be set free appropriately. Again, as discussed earlier (Fig. 8), dissociation into OH + C1 is preferred over that in H + C10 in any of the low-energy HOCl states, in accordance with experimental observation (2, 25). The rate of the corresponding triplet reaction 0(3Pg) HCI('C+) -t 0H(211) CI(2P,) has been FIG. 10. SCF potential curves for C1-0 stretch in ground and various excited states of the HClO system (calculated investigated by various experimental techniques equilibrium values for the H-C1 bond length and HClO bond (11, 26-29) and an activation barrier of approxangle are assumed throughout). imately 0.25 eV (11, 25) has been estimated. From

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CAN. J . CHEM. VOL. 57, 1979

the overall experimental evidence there is an indication (25, 30), although not conclusive (ll), that this reaction proceeds via a non-adiabatic transition from the triplet to the lowest-lying singlet surface, as discussed by Nikitin and co-workers (30, 31). Furthermore the absence of vibrational to translational and rotational energy transfer is taken as an indication (28) that the representative pathway cuts the corner of the potential energy surface and does not penetrate into the deep HOCl equilibrium well. According to the present potential energy curves a pathway involving the 3A" state is only conceivable with a low activation energy of a few kcal/mol if the hydrogen migration occurs for large 0-C1 distances and requires very little energy for such structures; since the corresponding potential energy surfaces (Figs. 4 and 5) are only calculated for structures close to equilibrium, the present study cannot support or discard this possibility. Because of the strongly repulsive nature of the 3A" curve an intersection with the corresponding singlet to yield a stable HOCl molecule via a spin-orbit curve crossing must also occur at quite large 0-C1 distances, unless (as in the previous case) the repulsive nature changes considerably with the angle of approach of oxygen to HCl, a situation which is unlikely to occur. A curve crossing is found in CI calculations for L HClO = 140°, HCl-0 = 3.45ao, for example, but this point is already 1.0 eV above the initial products, so that a study of this phenomenon must involve larger HC1-0 separations.

5. Vertical Ionization of HOCl and HClO The ground state for both the HOClf and HCIOf isomers is a 2A" species, arising from the loss of an electron from the corresponding 3a" MO in the respective neutral system. Because of the nodal characteristics of the latter orbital it can be predicted from Walsh's rules that little change in equilibrium geometry accompanies ionization in this instance. Hence it is expected that the values for the vertical minimum IP for both HOCl and HClO will be only slightly higher than for the corresponding adiabatic quantities. The lowest ionization potential of HOCl has recently been measured experimentally to be 11.22 eV (32), in good agreement with the present calculated value of 10.90 eV (Table 4), as well as with another theoretical result given in the literature (33). Furthermore because of the above geometrical considerations for such ions it is possible to identify the energy difference between lowest and higher vertical IP's with the vertical transition energies of the ion itself. In the case of HOCl+ the results obtained in this manner (1.08, 3.62, and 4.55 eV) correspond to

transitions into the 3a" MO from the 10al, 2a", and 9a' orbitals respectively and are found to be in good agreement with experimental values for these quantities (32). If the lowest adiabatic IP is taken to be 11.1 eV (32) the heat of formation of HOCl' can thus be estimated to be in the order of 237 kcal/mol, based on the AHfOvalue for HOCl of - 19 kcal/mol discussed earlier (1, 22). The HOClf heat of formation can then be combined with known AH,' values for possible dissociation fragments of this system to obtain predictions of its bond strengths. For example, heats of reaction of the order of 100 kcal/mol each are thereby indicated for decomposition into both Clf(3Pg) 0H(2H) and C1('PU) 0H'(3C-). Comparison of these results with the corresponding value for the neutral reaction, HOCl + C1(2Pu) + 0H(2rI), of 57 kcal/mol leads to the conclusion that the C10 bond is some 43 kcal/mol stronger in HOClf than in neutral HOC1; such a finding is clearly consistent with the C10 antibonding characteristics of the 3a" (IT:" species from which ionization takes place. On the other hand, similar calculations indicate that the OH bond strength is very nearly the same (95 kcal/mol) before and after the 3a" ionization, as again seems quite reasonable based on the lack of H-atom character in this molecular orbital. Analogous calculations can be made for the HClO' system, for which a AH,' value of 260 5 kcal/mol results. Taking the difference between these two AHfOvalues leads to an estimate for the isomerization energies of HOCl' and HClO' of 22 f 5 kcal/mol, markedly less than for the corresponding neutral systems. By comparison the isomerization energy for the HSO-SOH pair of the same number of electrons as the ions has been calculated at a similar level of CI treatment (23) to be 12 kcal/mol, again with the system containing the second-row atom in the terminal position indicated to be the more stable. Finally it is found that the C1-0 bond of HC10' should be 75 kcal/mol stronger than in HC10, while its H-Cl counterpart should be about 35 kcal/mol stronger than in the neutral system.

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6. Conclusion The present study has investigated various points of the potential energy surface of the HOCl-HCIO system in ground and a number of excited states in connection with interconversion of the two isomeric forms and their fragmentation into (or formation from) OH, C1, H, C10, HCl, 0 ( 3 ~ , ) and , O('D,). It is found that the ground state and the four lowest singlet and triplet states occupying the o" MO can be followed very easily from products to various fragments, while higher excited species, including the


B R U N A ET A L .

Rydberg states, will generally change their energy ordering among one another upon structural deformations so that no definite reaction channels will be preferred in the higher-energy range of fragments and products. The calculations indicate further that isomer conversion into the more stable HOCl form is likely in the ground state without substantial activation barrier, but that in at least some of its excited states HClO prefers a linear nuclear arrangement and should not be easily converted into HOCl in these cases. In general the antibonding n Z kand o" MO's in these systems are found to be relatively more stable in HClO than in HOCI, while the opposite is true for the corresponding bonding orbitals. As a result the vertical transition energies in the less stable HClO conformer are often markedly smaller than for the corresponding excitations in HOC1. All four first excited states 3 * 1 ~ ' rand 3.'A' (and also higher species) exhibit strongly repulsive potential energy curves and hence decidedly favor dissociation into OH('n) C1('PU) as soon as one of these excited states is populated. Since furthermore this fragment energy is considerably lower than that of H('S,) C10(X2H), decomposition into OH C1 is preferred over hydrogen removal; in addition it is found that barriers towards hydrogen removal are present even in the excited states, as has been observed in related systems, and this eventuality makes such processes even less likely. Dissociation of HOCl in its first two triplet excited states also seems possible via repulsive energy curves into O(jP,) HC1('Cf), i.e. fragments which are practically isoenergetic with OH('n) + C1('PU); on the other hand, crossing with the ground state singlet curve might introduce (via spin-orbit coupling) some complications. Decomposition of HOCl in its first two singlet states into O('D,) + HC1(ICf) via repulsive potential energy surfaces is also conceivable, although the energetically more favorable channel OH + C1 is probably preferred in this case.

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Acknowledgements One of us (G.H.) wishes to thank the Studienstiftung des Deutschen Volkes for a stipend to work on the present problem. The services and computer time made available by the Computer Center (RHRZ) of the University of Bonn are hereby gratefully acknowledged. I. G. HIRSCH.P. J. BRUNA.S. D. PEYERIMHOFF, and R. J. BUENKER. Chem. Phys. Lett. 52,442 (1977). J. Chem. Phys. 68, 1638 2. R. L . JAFFEand S. R. LANGHOFF. (1978).

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3. F. C. FEHSENFELD, P. J. CRUTZEN, A. L. SCHMELTEKOPF, C. J . HOWARD. D. L. ALBRITTON. and E. E. FERGUSON. J. Geophys. Res. 81.4454 (1976). 2 . Natur-forsch. 32a, 1254 (1977). 4. P. WARNECK. J .. Chern. Phys. 53.2823(1970). 5. T . H. D U N N I N G 6. A. VEILLARD. Theor. Chirn. Acta, 12,405 (1968). S., D. P E Y E R I M H O F and F , W. B u - r s c t r ~ n . 7. R. J . B U E N K E R Mol. Phys. 35.771 (1978). I . G . C A Z Z O LJI.. Mol. Spec8. A. M. MIRRI,F. S C A P P I Nand trosc. 38,218 (1971). . J .FBFU~E~N~K E JR. .Chern. Phys. 47, 9. S. D. P E Y E R ~ X ~ HRO 1953 (1967). 10. S . F. BOYS.Theory of atoms, molecules and solids. Edited by P. 0. Lowdin. Academic Press, New York. NY. 1966. I I. R. D. H. BROWNand I. W. M. S M I T HInt. . J . Chem. Kinet. 7,301 (1975). 12. J . WOLFRUM. Ber. Bunsenges. Phys. Chem. 81, 114(1977). 13. P. K . PEARSON, H. F. SCHAEFER 111, and U . WAHLCREN. J. Chern. Phys. 62. 350(1975). 14. P. J. BKUNA.S. D. PEYERIMHOFF, and R. J . B U E N K E R . Chem. Phys. 10.323 (1975). , S. D. P E Y E R I M H O FJ.F . 15. R. PREUB.R. J . B U E N K E Rand Mol. Struct. 49, 171 (1978). , D. PEYERIMHOFF, and R. J . BUENKER. P. J . B R U N AS. Chern. Phys. 27.33 (1978). and C . M. M A R I A NResults . of this labo~xtot.y. P. J . BRUNA G. A. GALLUP. Inorg. Chem. 14.563 (1975). R. PREUR,R. J . B U E N K E and R . S . D. P E Y E R I M H O Chem. FF. Phys. Lett. 62,21 (1979). W. C. FERGUSON. L. SLOTIN,and D. W. G . STYLE.Trans. Fwaday Soc. 32.956 (1936). R. J. B U E N K E R ~ ID.~ PEYERIMHOFF. ~S. Chem. Phys. Lett. 36. 415 (1975); S. D. PEYERIMHOFF. G ~ z z Chim. . Ital. 108, 41 l(1978). L. T. M O L ~ Nand A M. J . MOLINA. J. Phys. Chern. 82,2410 (1978). A. B. S A N N I G R A HS.I . D. PEYERIMHOFF. and R. J. B U E N K E RChem. . Phys. 20. 381 (1977); R. J . B U E N K E R , P. J . BRUNA.and S . D. PEYERIMHOFF. Isr. J. Chem. To be published. S. D. PEYERIMHOFF. Ber. Bunsenges. Phys. Chern. 78. 119 (1974): K. V A S U D E V ASN. , D. PEYERIMHOFF, and R. J . BUENKER J. .Mol. Struct. 29,285 (1975); P. J . B R U N AR. , J. BUENKER and , S. D. PEYERIMHOFF. J . Mol. Struct. 32,217 (1976); A. A. WU, S . D. P E Y E R I M H O FR. F J. , ~B~U~E~N K E R . Chem. Phys. Lett. 35,316 (1977). M. C. ADDISON. R. J. DONOVAN, and H. M. GILLESPIE. Chem. Phys. Lett. 44.602 (1976). W. HACK,G. MEX,and H. G. WAGNER. Ber. Bunsenges. 81.677 (1977). S M I T HR. , T . WATSON.and D. D. A. R. RAVISKANKARA.G. DAVIS.J . Phys. Chem. 81,2220(1977). B. A. B L A C K W E LJ.LC. , P O L A N Yand I , J. J. SLOAN. Chem. Phys. 24.25 (1977). J . E. BUTLER,J . W. HUDGENS, M. C. L I N , and G. K. SMITH.Chem. Phys. Lett. 58.216(1978). E. E. N I K I T ~and N A. V. N. KONRAT'EV. Translation from Dokl. Akad. Nauk SSSR, 212, 149(1973). E. E. N I K I T I Nand S. YA. U M A N S K IDiscuss. I. Faraday SOC.53, 7 (1972). D. COLBOURNE. D. C. FROST. C. A. MCDOWELL, and J . Chern. Phys. 68,3574 (1978). N. P. C. WESTWOOD. D. P. CHONG.F. G. HERRING. and Y. TAKAHATA. J. Electron Spectrosc. Relat. Phenom. 13.39 (1978).