- Email: [email protected]
A model study on the adsorption of hydrogen, oxygen and carbon monoxide on organic molecules and polymers
Volume 134. number 2
CHEMICAL PHYSICS LETTERS
20 February 1987
A MODEL STUDY ON THE ADSORPTION OF HYDROGEN, OXYGEN AND CARBON MONOXIDE ON ORGANIC MOLECULES AND POLYMERS SC. LIU I, P. OTTO, J. LADIK and M. GIES 2 Chairfor Theoretical Chemistry, Friedrich-Alexander-University Erlangen-Niirnberg, Egerlandstrasse 3, D-8520 Erlangen, Federal Republic of Germany Received 8 October 1986; in final form 3 December 1986
The ab initio Hartree-Fock method and a London-type approximation to calculate the dispersion energy have been employed to study the chemisorption of HZ, O2 and CO on benzene and model macromolecules built up from phenylene units. Different orientations of the adsorbed molecule relative to the aromatic hydrocarbon have been considered. The intermolecular distance has been optimized with respect to the energy for each composite system. The dependence of the interaction energy on the intermolecular distance and on the number of phenyl units has been investigated.
1. Introduction The problem of the interaction between molecules in the gas phase and a solid surface has received considerable attention both theoretically and experimentally in recent years [ l-41. Important processes such as chemical reactions on a catalyst (e.g. molecular hydrogen +graphite [ 51) and oxidation [ 61 depend on surface properties. Chemisorption of oxygen on organic polymers plays an important role in transforming the originally insulating form into a highly conductive oxidized material (e.g. polypyrrole [ 71). A complete theoretical treatment requires as a first step a description of the interaction between the adsorbate and the perfect crystal surface. Different quantum-chemical methods have been applied to study adsorption of small molecules on polymers [B] . The problem for a periodic polymer with an impurity (represented by the elementary cell together with the adsorbed molecule) which modified a number of neighbouring cells was solved using an SCF treatment and assuming a localized perturbation [ 8,9]. An alternative approach involves the Green function formalism [ lo- 121. In this work results are reported ’ Permanent address: Faculty of Chemistry, Liaoning University, Shenyang, People’s Republic of China. 2 Present address: Siemens UBMed, Henkestrasse 127, D-8520 Erlangen, Federal Republic of Germany.
on the chemisorption of hydrogen, oxygen and carbon monoxide on benzene and models for the corresponding periodic macromolecule by performing ab initio Hartree-Fock calculations. Of special interest was the question as to whether the properties of a molecular complex, e.g. binding energy, charge transfer and equilibrium geometry, can be transferred when the same molecule is adsorbed on a polymer. A further intention was to obtain information on the cooperative effect in the adsorption process, which may be energetically favored (autocooperative), impeded (anticooperative) or not influenced at all. The aromatic hydrocarbon benzene has been chosen to examine complex formation with Hz, O2 and CO. The periodic polymer built up from the pphenylene unit ( -C6H4-) would be poly (pphenylene ) . As a model for this macromolecule p-diphenylphenylene has been used. A series of molecular calculations for the system phenyl-( phenylene),-phenyl with n = 1,2,3 has shown that the electronic charge distribution in the phenylene units has effectively converged for n = 1. Furthermore the contribution to the total energy per C6H4 subunit did not change very much as n increased. The adsorption of one and two CO molecules has been studied for diphenyl. Standard bond lengths and bond angles [ 131 were used for all molecules. It was further assumed that
0 009-2614/87/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
133
Volume 134, number 2
CHEMICAL PHYSICS LETTERS
of the molecules A and B, respectively. According to the London approximation C, ( i, j) can be written as
Y
R2&Rl
G(i,j) Rl=R2=
H
RI =C6H5,
R2=H
R1=R2=CgH5 X=Y=H X=Y= 0 x=c X=0,
X,Y
Y=C
Fig. 1. The structure of the investigated adsorption complexes.
the internal structure of the components in the complex remains the same as in the isolated molecules. For each chosen configuration of the admolecule relative to the hydrocarbon (shown in fig. 1) the binding energy has been computed for different intermolecular distances to obtain the equilibrium position for stable adducts. The ab initio Hartree-Fock method (HF) has been employed for the molecular clusters involving H2 and CO. In the case of oxygen unrestricted Hartree-Fock (UHF) calculations have been carried out. Clementi’s 7s/3p minimal basis set [ 141 was used throughout. For two of the benzene complexes with Hz and CO, respectively, the basis set superposition error (BSSE) has been computed using the counterpoise technique. The correction amounts to about 20% of the Hartree-Fock binding energy. However, because the absolute value of the interaction energy is of the order of 0.5 kcal/mol and less, the BSSE correction only amounts to a few hundredth of a kcal, which does not cause a qualitative change in the results. Because the intermolecular dispersion interaction is assumed to be a sum of atom-atom contributions the dispersion coefficient can be expressed by adding atomic dispersion terms such as 13
G(&j)
,
where i and j are the atoms in particular valence states 134
=~~,cWJ(Z,
+Zi) ,
where a! and Zare the atomic static polarizability and the ionization potential, respectively, taken from ref. [ 151. The values for (Y have been obtained from experimental polarizabilities of molecules. The calculated average molecular polarizabilities and dispersion coefficients for a variety of molecules are in excellent agreement with experimental results [ 15 1.
) Y=O
,2&R,
MA, B) =Xx
20 February 1987
i
J
Since our calculations have shown that the Rp6 dispersion energy is 1 kcal mol- ’ or less, the shorterrange dispersion terms are certainly still smaller and have therefore been neglected.
2. Hydrogen adducts We have chosen two configurations of the adsorbate H2 relative to the aromatic hydrocarbon, which are shown in fig. 1. In the first structure bond between the two H atoms coincides with the C, symmetry axis of the benzene ring and the distance is measured from the hydrocarbon plane to the nearest H atom. In the second the hydrogen atoms are in a plane parallel to that of benzene at different interplane distances parallel to the line which connects two opposite carbon atoms with the midpoint of the bond located on the C, axis. The results for the H2 adducts in both configurations are listed in table 1 for some of the calculated intermolecular distances. Column three contains the Hartree-Fock interaction energy, which is obtained as the difference between the total energy of the complex and the energy of its constituents. In column four the dispersion energy is added to AEHF and AEUHF, respectively. In the case of benzene the results predict a stable complex only for the perpendicular orientation of the H2 molecule. The energy minima occur at a distance of about 2.5 au and the stabilization energy is of the order of - 0.6 kcallmol. The dispersion energy contribution, however, also stabilizes the parallel complex and its energy minimum is only about 10% higher than for the perpen-
Volume 134, number 2
CHEMICAL PHYSICS LETTERS
20 February 1987
Table 1 The interaction energies (in kcal/mol) of H2 and O2 complexes with benzene, diphenyl and pdiphenylphenylene intermolecular distance (in au)
as a function of the
System
GM6+ X2 perpendicular
‘S-L+&
1.80 2.50 3.00 3.50 4.00 5.00
4.49
-0.61” -0.30 -0.12
-1.29 -2.20 -1.01 -0.43
1.80 2.50 3.00 3.50 4.00 5.00
17.49 0.59 0.02 0.01
7.83 -1.85 -1.03 -0.41
H&-C6HS + X2 perpendicular
1.80 2.50 3.00 3.50
4.38 -0.62 - 0.29 -0.11
-1.55 -2.29 - 1.06 -0.49
H,Cs-C6HS + X2 parallel
1.80 2.50 3.00 3.50
17.38 0.58 -0.01 0.06
7.53 - 1.99 -1.13 -0.49
HSC6-C6H&6Hs + X2 perpendicular
1.80 2.50 3.00 3.50 4.00 5.00
4.26 -0.62 -0.04 -0.01
-1.81 -2.39 -1.25 -0.56
H&-C6H4-C6HS + Xx parallel
1.80 2.50 3.00 4.00 5.00 5.50
17.31 0.57 -0.02 0.00
7.26 -2.12 -1.23 -0.62
parallel
4.03 0.57 0.16 0.08 0.04 8.96 0.83 -0.02 -0.06 -0.03
3.76 0.42 0.09 0.06 0.03 8.82 0.76 -0.04 -0.07 -0.03
2.17 -0.26 -0.24 -0.13 -0.03 5.75 -0.53 -0.66 -0.38 -0.13
1.76 -0.51 -0.48 -0.26 -0.09 5.21 -0.82 -0.96 -0.48 -0.27
a) Value corrected for basis set superposition error: -0.57 kcabmol.
dicular case. Substituting one hydrogen atom (or two in the para position) in benzene by a phenyl group the calculations again yield a stable adduct for the complex in which H2 approaches the substrate along the C, axis at the same intermolecular distance as in benzene. The binding energy slightly increases with the number of rings. A similar trend is found when the Hz molecule is parallel to the ring plane.
3. Oxygen adducts For the position of the O2 molecule relative to the ring plane of benzene and diphenylphenylene the same two orientations have been chosen as in the case of Hz. On the basis of the UHF results (see table 1) a very low stabilization is predicted for the planar arrangement. When the dispersion energy is included both complex configurations are stable, with the parallel position of the O2 molecule relative to the plane 135
Volume
134, number
2
CHEMICAL
PHYSICS
of the ring again favoured. The energy minima are obtained at a distance of about 3.5 au. The extension of the x-electron system by increasing the number of phenyl rings affects the interaction with O2 in a similar manner as in the adsorption of Hz. The potential curves of both configurations are shifted to lower energies, the position of the minima are unchanged and the relative order of stability is maintained. The amount of charge transferred to the oxygen atoms is very small for all geometries of the complex (0.1 e for d= 3.50 au) and decreases rapidly with increasing distance. The adsorption of atomic oxygen on a graphite surface under pressure and high temperature, leading to different oxidation products, is experimentally interesting. Theoretical work, however, has been restricted to an investigation of the primary steps of the adsorption process. Extensive computations using either empirical potential functions [ 161 or the CNDO method [4] have been performed to simulate the chemisorption and diffusion of molecular and atomic oxygen on different graphite surfaces.
20 February
LETTERS
Table 2 The interaction energies (in kcal/mol) of CO complexes with benzene, diphenyl and p-diphenylphenylene as a function of the intermolecular distance (in au)
GH6
s
HSC6-C6H4-C6HS
g
C6H6 CO
136
AEHF
AJpw
3.00 3.50 4.00
2.13 1.19 0.64
0.39 0.36 0.22
3.00 3.50 4.00
2.02 1.10 0.59
0.15 0.12 0.09
3.00 3.50 4.00
1.92 1.02 0.53
0.10 0.00 -0.03
3.00 3.50 4.00
1.94 0.75 0.44
0.92 0.25 0.17
3.00 3.50 4.00
1.86 0.69 0.39
0.74 0.12 0.10
3.00 3.50 4.00
1.76 0.58 0.21
0.62 0.03 0.01
2.50 3.00 3.50 4.00 4.50
11.15 0.37 -0.12 -0.31 =I -0.24
7.69 -0.73 -0.64 -0.52 -0.37
3.00 3.50 4.00 4.50
0.29 -0.17 -0.31 -0.28
-0.92 -0.16 -0.65 -0.44
3.00 3.50 4.00 4.50
0.20 -0.19 -0.39 -0.29
-0.97 -0.79 -0.61 -0.45
2.50 3.00 3.50
11.28 4.21 2.48
2.87 0.45 0.63
3.00 3.50 4.00
4.09 2.02 1.12
1.09 0.52 0.23
d
System
4. Carbon monoxide adducts As for the H2 and O2 adsorbates equivalent relative orientations of the CO molecule with respect to the hexagonal plane have been studied. One configuration of CO is characterized by the orientation of the CO bond parallel to the line connecting two opposite carbon atoms of benzene in a plane parallel to the plane of the ring. As the diatomic adsorbate consists of two different atoms there are two possibilities of placing the CO molecule perpendicular to the plane on the C, axis, with either the C or the 0 atom pointing to the plane, respectively. In table 2 the calculated interaction energies are summarized as a function of the intermolecular distance. For the complex with the parallel arrangement (denoted by C6H6 CO) the shape of the potential curve is very similar to that of the O2 adduct. The energy minimum occurs at about 3.0 au and the binding energy is -0.7 kcal/mol. The potential curves of the two vertical configurations (denoted C6H6 g and CsH6 $ ), however, do not have minima and the complexes are unstable for all intermolecular distances. In CsH6 g the CO molecule acts as a weak
1987
H&-C6HS
CO
H&-C6H‘,-HSC6
a) Value corrected kcal/mol.
CO
for basis
set superposition
error:
-0.25
electron acceptor, whereas it acts as an electron donor in the C6H6 $j complex. In the case of CdHs CO carbon monoxide plays the role of an electron acceptor over the whole range of intermolecular distances.
Volume 134, number 2
CHEMICAL PHYSICS LETTERS
We now compare the results for the molecular adducts formed with diphenyl and pdiphenylene. From table 2 we find that the binding energies become more attractive with increasing number of phenyl units for CO in the parallel position. The difference is of the same order of magnitude as in the comparable O2 systems. The equilibrium distances are unchanged. The unstable complexes between benzene and CO in a vertical position also remain unstable for the more extended substrate, although the repulsive energy is reduced. We also investigated possible adsorption complexes of two CO molecules with diphenyl in two different configurations. All structures have in common that the two CO molecules are placed along the two C, axes of the hexagonal rings. In the first case the carbon atoms are the nearest atoms to the plane. In the second configuration the two CO molecules are inverted. These computations have been carried out in order to obtain some information about the cooperative effect in the adsorption process. Comparing the calculated energies (see table 2) for the complex of two CO molecules with diphenyl with the energies of the complexes formed between a single CO and a diphenyl molecule, we can conclude that in both cases an anticooperative effect occurs. Namely, in these systems the repulsive interaction energy of two CO molecules with diphenyl is larger than twice the energy of a single CO adsorbed to diphenyl. We have checked that the repulsion between the CO molecules (0.3 kcal/mol) is not responsible for this effect. Once a CO molecule has been adsorbed on the macromolecule, the binding of a second one to the neighboring aromatic ring is not favored. The cause of the anticooperative effect is due to the small but non-negligible shift of the electronic charge from the carbon atoms to the hydrogen atoms in the phenyl unit in the presence of a CO molecule. Consequently the interaction energy of CO with the distant phenyl ring is more repulsive in the case of a second adsorbed CO molecule (anticooperative). In a parallel study the abovementioned systems have been investigated using the semi-empirical CNDO/Z method. Without giving detailed numbers for the interaction energies and the structural properties we can state that the results are contrary to the ab initio calculations. According to the CNDO calculations all complexes are strongly stabilized. The
20 February 1987
interaction energies are chemically unreasonably large and the intermolecular equilibrium distances are too small. A complete geometry optimization of the benzene-CO complex leads to a structure with non-polar benzene, which again is far from reality. Repeating the calculations with the MIND0/2 method, however, gives results which are comparable to the Hartree-Fock computations. It is clear that model calculations at the level of the CND0/2 approximation for the chemisorption of carbon monoxide on the basal plane of graphite [ 17,181 (which can be roughly compared with the benzene models) have to be looked at with some caution. Experimental data are not available for a direct comparison with the theoretical results. However, it is known that aromatic systems may form complexes with non-polar molecules which are weakly stabilized through dispersion interactions. Similar results have been obtained from investigations on hydrogen molecule adducts. Although calculations were performed with a minimal basis set comparisons with other interaction energy calculations [ 191 suggest that our results give the right trends, even though the numbers themselves are not reliable due to basis set limitations and the fact that one has to deal with small differences of large numbers.
Acknowledgement We thank the Fonds der Chemischen Industrie for financial support. One of us (XL) gratefully acknowledges the Hanns-Seidel-Stiftung for obtaining his Scholarship.
References [ 1] T.A. Belcher and G.J. Ehrlich, J. Chem. Phys. 42 (1965) 2686. [2] G.J. Ehrlich and F. Hudde, J. Chem. Phys. 44 (1966) 1039. [3] A.J. Bennett and L.M. Falicov, Phys. Rev. 151 (1966) 512; A.J. Bennett, Surface Sci. 24 (1971) 191; Phys. Rev. 83 (1971) 1397. [4] M.R. Hayns, Theoret. Chim. Acta 39 (1975) 61. [ 51M. Balooch and D.R. Olander, J. Chem. Phys. 63 (1975) 4772; B. McCarroll and D.W. McKee, Nature 225 (1970) 722;
137
Volume 134, number 2
CHEMICAL PHYSICS LETTERS
L. Mattera, F. Rosatelli, C. Salvo and F. Tommasini, Natural Sci. 93 (1980) 5 15. [ 61 D.R. Olander, R.H. Jones, J.A. Schwarz and W.J. Siekhaus, J. Chem. Phys. 57 (1972) 428; D.R. Olander, W.J. Siekhaus, R.H. Jones and J.A. Schwarz, J. Chem. Phys. 57 (1972) 408. [7] P. Pfluger, M. Krounbi and G.B. Street, J. Chem. Phys. 78 (1983) 3212. [8] J. Ladikand M. Seel, Phys. Rev. B13 (1976) 5338. [9] J. Ladik, Phys. Rev. B17 (1978) 1663. [lo] G. de1 Reand J. Ladik, Chem. Phys. 49 (1980) 321. [ 111 M. Seel, Intern. J. Quantum Chem. 19 ( 198 1) 1083. [ 121 M. Seel, G. de1 Re and J. Iadik, J. Comput. Chem. 3 (1982) 451.
138
20 February 1987
[ 13 ] L.E. Sutton, in: Tables of interatomic distances and contigurations of molecules and ions (Chem. Sot., London, 1958). [ 141 E. Clementi, F. Cavallone and R. Scordamaglia, J. Am. Chem. Sot. 99 (1977) 5531. [ 151 Y.K. Yang and M.S. Jhong, Theoret. Chim. Acta 61 (1981) 41. [ 161 S. Beran, J. Dubsky and Z. Slanina, Surface Sci. 79 (1979) 39; J. Dubsky and S. Beran, Surface Sci. 92 (1979) 53. [ 171 C. Pisani and F. Ricca, Surface Sci. 92 ( 1980) 48 1. [ 18) Th. Weller and W. Miiller, Chem. Phys. Letters 98 (1983) 451. [ 191 W. Flimer, P. Otto and J. Ladik, Chem. Phys. 86 (1984) 49.
CHEMICAL PHYSICS LETTERS
20 February 1987
A MODEL STUDY ON THE ADSORPTION OF HYDROGEN, OXYGEN AND CARBON MONOXIDE ON ORGANIC MOLECULES AND POLYMERS SC. LIU I, P. OTTO, J. LADIK and M. GIES 2 Chairfor Theoretical Chemistry, Friedrich-Alexander-University Erlangen-Niirnberg, Egerlandstrasse 3, D-8520 Erlangen, Federal Republic of Germany Received 8 October 1986; in final form 3 December 1986
The ab initio Hartree-Fock method and a London-type approximation to calculate the dispersion energy have been employed to study the chemisorption of HZ, O2 and CO on benzene and model macromolecules built up from phenylene units. Different orientations of the adsorbed molecule relative to the aromatic hydrocarbon have been considered. The intermolecular distance has been optimized with respect to the energy for each composite system. The dependence of the interaction energy on the intermolecular distance and on the number of phenyl units has been investigated.
1. Introduction The problem of the interaction between molecules in the gas phase and a solid surface has received considerable attention both theoretically and experimentally in recent years [ l-41. Important processes such as chemical reactions on a catalyst (e.g. molecular hydrogen +graphite [ 51) and oxidation [ 61 depend on surface properties. Chemisorption of oxygen on organic polymers plays an important role in transforming the originally insulating form into a highly conductive oxidized material (e.g. polypyrrole [ 71). A complete theoretical treatment requires as a first step a description of the interaction between the adsorbate and the perfect crystal surface. Different quantum-chemical methods have been applied to study adsorption of small molecules on polymers [B] . The problem for a periodic polymer with an impurity (represented by the elementary cell together with the adsorbed molecule) which modified a number of neighbouring cells was solved using an SCF treatment and assuming a localized perturbation [ 8,9]. An alternative approach involves the Green function formalism [ lo- 121. In this work results are reported ’ Permanent address: Faculty of Chemistry, Liaoning University, Shenyang, People’s Republic of China. 2 Present address: Siemens UBMed, Henkestrasse 127, D-8520 Erlangen, Federal Republic of Germany.
on the chemisorption of hydrogen, oxygen and carbon monoxide on benzene and models for the corresponding periodic macromolecule by performing ab initio Hartree-Fock calculations. Of special interest was the question as to whether the properties of a molecular complex, e.g. binding energy, charge transfer and equilibrium geometry, can be transferred when the same molecule is adsorbed on a polymer. A further intention was to obtain information on the cooperative effect in the adsorption process, which may be energetically favored (autocooperative), impeded (anticooperative) or not influenced at all. The aromatic hydrocarbon benzene has been chosen to examine complex formation with Hz, O2 and CO. The periodic polymer built up from the pphenylene unit ( -C6H4-) would be poly (pphenylene ) . As a model for this macromolecule p-diphenylphenylene has been used. A series of molecular calculations for the system phenyl-( phenylene),-phenyl with n = 1,2,3 has shown that the electronic charge distribution in the phenylene units has effectively converged for n = 1. Furthermore the contribution to the total energy per C6H4 subunit did not change very much as n increased. The adsorption of one and two CO molecules has been studied for diphenyl. Standard bond lengths and bond angles [ 131 were used for all molecules. It was further assumed that
0 009-2614/87/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
133
Volume 134, number 2
CHEMICAL PHYSICS LETTERS
of the molecules A and B, respectively. According to the London approximation C, ( i, j) can be written as
Y
R2&Rl
G(i,j) Rl=R2=
H
RI =C6H5,
R2=H
R1=R2=CgH5 X=Y=H X=Y= 0 x=c X=0,
X,Y
Y=C
Fig. 1. The structure of the investigated adsorption complexes.
the internal structure of the components in the complex remains the same as in the isolated molecules. For each chosen configuration of the admolecule relative to the hydrocarbon (shown in fig. 1) the binding energy has been computed for different intermolecular distances to obtain the equilibrium position for stable adducts. The ab initio Hartree-Fock method (HF) has been employed for the molecular clusters involving H2 and CO. In the case of oxygen unrestricted Hartree-Fock (UHF) calculations have been carried out. Clementi’s 7s/3p minimal basis set [ 141 was used throughout. For two of the benzene complexes with Hz and CO, respectively, the basis set superposition error (BSSE) has been computed using the counterpoise technique. The correction amounts to about 20% of the Hartree-Fock binding energy. However, because the absolute value of the interaction energy is of the order of 0.5 kcal/mol and less, the BSSE correction only amounts to a few hundredth of a kcal, which does not cause a qualitative change in the results. Because the intermolecular dispersion interaction is assumed to be a sum of atom-atom contributions the dispersion coefficient can be expressed by adding atomic dispersion terms such as 13
G(&j)
,
where i and j are the atoms in particular valence states 134
=~~,cWJ(Z,
+Zi) ,
where a! and Zare the atomic static polarizability and the ionization potential, respectively, taken from ref. [ 151. The values for (Y have been obtained from experimental polarizabilities of molecules. The calculated average molecular polarizabilities and dispersion coefficients for a variety of molecules are in excellent agreement with experimental results [ 15 1.
) Y=O
,2&R,
MA, B) =Xx
20 February 1987
i
J
Since our calculations have shown that the Rp6 dispersion energy is 1 kcal mol- ’ or less, the shorterrange dispersion terms are certainly still smaller and have therefore been neglected.
2. Hydrogen adducts We have chosen two configurations of the adsorbate H2 relative to the aromatic hydrocarbon, which are shown in fig. 1. In the first structure bond between the two H atoms coincides with the C, symmetry axis of the benzene ring and the distance is measured from the hydrocarbon plane to the nearest H atom. In the second the hydrogen atoms are in a plane parallel to that of benzene at different interplane distances parallel to the line which connects two opposite carbon atoms with the midpoint of the bond located on the C, axis. The results for the H2 adducts in both configurations are listed in table 1 for some of the calculated intermolecular distances. Column three contains the Hartree-Fock interaction energy, which is obtained as the difference between the total energy of the complex and the energy of its constituents. In column four the dispersion energy is added to AEHF and AEUHF, respectively. In the case of benzene the results predict a stable complex only for the perpendicular orientation of the H2 molecule. The energy minima occur at a distance of about 2.5 au and the stabilization energy is of the order of - 0.6 kcallmol. The dispersion energy contribution, however, also stabilizes the parallel complex and its energy minimum is only about 10% higher than for the perpen-
Volume 134, number 2
CHEMICAL PHYSICS LETTERS
20 February 1987
Table 1 The interaction energies (in kcal/mol) of H2 and O2 complexes with benzene, diphenyl and pdiphenylphenylene intermolecular distance (in au)
as a function of the
System
GM6+ X2 perpendicular
‘S-L+&
1.80 2.50 3.00 3.50 4.00 5.00
4.49
-0.61” -0.30 -0.12
-1.29 -2.20 -1.01 -0.43
1.80 2.50 3.00 3.50 4.00 5.00
17.49 0.59 0.02 0.01
7.83 -1.85 -1.03 -0.41
H&-C6HS + X2 perpendicular
1.80 2.50 3.00 3.50
4.38 -0.62 - 0.29 -0.11
-1.55 -2.29 - 1.06 -0.49
H,Cs-C6HS + X2 parallel
1.80 2.50 3.00 3.50
17.38 0.58 -0.01 0.06
7.53 - 1.99 -1.13 -0.49
HSC6-C6H&6Hs + X2 perpendicular
1.80 2.50 3.00 3.50 4.00 5.00
4.26 -0.62 -0.04 -0.01
-1.81 -2.39 -1.25 -0.56
H&-C6H4-C6HS + Xx parallel
1.80 2.50 3.00 4.00 5.00 5.50
17.31 0.57 -0.02 0.00
7.26 -2.12 -1.23 -0.62
parallel
4.03 0.57 0.16 0.08 0.04 8.96 0.83 -0.02 -0.06 -0.03
3.76 0.42 0.09 0.06 0.03 8.82 0.76 -0.04 -0.07 -0.03
2.17 -0.26 -0.24 -0.13 -0.03 5.75 -0.53 -0.66 -0.38 -0.13
1.76 -0.51 -0.48 -0.26 -0.09 5.21 -0.82 -0.96 -0.48 -0.27
a) Value corrected for basis set superposition error: -0.57 kcabmol.
dicular case. Substituting one hydrogen atom (or two in the para position) in benzene by a phenyl group the calculations again yield a stable adduct for the complex in which H2 approaches the substrate along the C, axis at the same intermolecular distance as in benzene. The binding energy slightly increases with the number of rings. A similar trend is found when the Hz molecule is parallel to the ring plane.
3. Oxygen adducts For the position of the O2 molecule relative to the ring plane of benzene and diphenylphenylene the same two orientations have been chosen as in the case of Hz. On the basis of the UHF results (see table 1) a very low stabilization is predicted for the planar arrangement. When the dispersion energy is included both complex configurations are stable, with the parallel position of the O2 molecule relative to the plane 135
Volume
134, number
2
CHEMICAL
PHYSICS
of the ring again favoured. The energy minima are obtained at a distance of about 3.5 au. The extension of the x-electron system by increasing the number of phenyl rings affects the interaction with O2 in a similar manner as in the adsorption of Hz. The potential curves of both configurations are shifted to lower energies, the position of the minima are unchanged and the relative order of stability is maintained. The amount of charge transferred to the oxygen atoms is very small for all geometries of the complex (0.1 e for d= 3.50 au) and decreases rapidly with increasing distance. The adsorption of atomic oxygen on a graphite surface under pressure and high temperature, leading to different oxidation products, is experimentally interesting. Theoretical work, however, has been restricted to an investigation of the primary steps of the adsorption process. Extensive computations using either empirical potential functions [ 161 or the CNDO method [4] have been performed to simulate the chemisorption and diffusion of molecular and atomic oxygen on different graphite surfaces.
20 February
LETTERS
Table 2 The interaction energies (in kcal/mol) of CO complexes with benzene, diphenyl and p-diphenylphenylene as a function of the intermolecular distance (in au)
GH6
s
HSC6-C6H4-C6HS
g
C6H6 CO
136
AEHF
AJpw
3.00 3.50 4.00
2.13 1.19 0.64
0.39 0.36 0.22
3.00 3.50 4.00
2.02 1.10 0.59
0.15 0.12 0.09
3.00 3.50 4.00
1.92 1.02 0.53
0.10 0.00 -0.03
3.00 3.50 4.00
1.94 0.75 0.44
0.92 0.25 0.17
3.00 3.50 4.00
1.86 0.69 0.39
0.74 0.12 0.10
3.00 3.50 4.00
1.76 0.58 0.21
0.62 0.03 0.01
2.50 3.00 3.50 4.00 4.50
11.15 0.37 -0.12 -0.31 =I -0.24
7.69 -0.73 -0.64 -0.52 -0.37
3.00 3.50 4.00 4.50
0.29 -0.17 -0.31 -0.28
-0.92 -0.16 -0.65 -0.44
3.00 3.50 4.00 4.50
0.20 -0.19 -0.39 -0.29
-0.97 -0.79 -0.61 -0.45
2.50 3.00 3.50
11.28 4.21 2.48
2.87 0.45 0.63
3.00 3.50 4.00
4.09 2.02 1.12
1.09 0.52 0.23
d
System
4. Carbon monoxide adducts As for the H2 and O2 adsorbates equivalent relative orientations of the CO molecule with respect to the hexagonal plane have been studied. One configuration of CO is characterized by the orientation of the CO bond parallel to the line connecting two opposite carbon atoms of benzene in a plane parallel to the plane of the ring. As the diatomic adsorbate consists of two different atoms there are two possibilities of placing the CO molecule perpendicular to the plane on the C, axis, with either the C or the 0 atom pointing to the plane, respectively. In table 2 the calculated interaction energies are summarized as a function of the intermolecular distance. For the complex with the parallel arrangement (denoted by C6H6 CO) the shape of the potential curve is very similar to that of the O2 adduct. The energy minimum occurs at about 3.0 au and the binding energy is -0.7 kcal/mol. The potential curves of the two vertical configurations (denoted C6H6 g and CsH6 $ ), however, do not have minima and the complexes are unstable for all intermolecular distances. In CsH6 g the CO molecule acts as a weak
1987
H&-C6HS
CO
H&-C6H‘,-HSC6
a) Value corrected kcal/mol.
CO
for basis
set superposition
error:
-0.25
electron acceptor, whereas it acts as an electron donor in the C6H6 $j complex. In the case of CdHs CO carbon monoxide plays the role of an electron acceptor over the whole range of intermolecular distances.
Volume 134, number 2
CHEMICAL PHYSICS LETTERS
We now compare the results for the molecular adducts formed with diphenyl and pdiphenylene. From table 2 we find that the binding energies become more attractive with increasing number of phenyl units for CO in the parallel position. The difference is of the same order of magnitude as in the comparable O2 systems. The equilibrium distances are unchanged. The unstable complexes between benzene and CO in a vertical position also remain unstable for the more extended substrate, although the repulsive energy is reduced. We also investigated possible adsorption complexes of two CO molecules with diphenyl in two different configurations. All structures have in common that the two CO molecules are placed along the two C, axes of the hexagonal rings. In the first case the carbon atoms are the nearest atoms to the plane. In the second configuration the two CO molecules are inverted. These computations have been carried out in order to obtain some information about the cooperative effect in the adsorption process. Comparing the calculated energies (see table 2) for the complex of two CO molecules with diphenyl with the energies of the complexes formed between a single CO and a diphenyl molecule, we can conclude that in both cases an anticooperative effect occurs. Namely, in these systems the repulsive interaction energy of two CO molecules with diphenyl is larger than twice the energy of a single CO adsorbed to diphenyl. We have checked that the repulsion between the CO molecules (0.3 kcal/mol) is not responsible for this effect. Once a CO molecule has been adsorbed on the macromolecule, the binding of a second one to the neighboring aromatic ring is not favored. The cause of the anticooperative effect is due to the small but non-negligible shift of the electronic charge from the carbon atoms to the hydrogen atoms in the phenyl unit in the presence of a CO molecule. Consequently the interaction energy of CO with the distant phenyl ring is more repulsive in the case of a second adsorbed CO molecule (anticooperative). In a parallel study the abovementioned systems have been investigated using the semi-empirical CNDO/Z method. Without giving detailed numbers for the interaction energies and the structural properties we can state that the results are contrary to the ab initio calculations. According to the CNDO calculations all complexes are strongly stabilized. The
20 February 1987
interaction energies are chemically unreasonably large and the intermolecular equilibrium distances are too small. A complete geometry optimization of the benzene-CO complex leads to a structure with non-polar benzene, which again is far from reality. Repeating the calculations with the MIND0/2 method, however, gives results which are comparable to the Hartree-Fock computations. It is clear that model calculations at the level of the CND0/2 approximation for the chemisorption of carbon monoxide on the basal plane of graphite [ 17,181 (which can be roughly compared with the benzene models) have to be looked at with some caution. Experimental data are not available for a direct comparison with the theoretical results. However, it is known that aromatic systems may form complexes with non-polar molecules which are weakly stabilized through dispersion interactions. Similar results have been obtained from investigations on hydrogen molecule adducts. Although calculations were performed with a minimal basis set comparisons with other interaction energy calculations [ 191 suggest that our results give the right trends, even though the numbers themselves are not reliable due to basis set limitations and the fact that one has to deal with small differences of large numbers.
Acknowledgement We thank the Fonds der Chemischen Industrie for financial support. One of us (XL) gratefully acknowledges the Hanns-Seidel-Stiftung for obtaining his Scholarship.
References [ 1] T.A. Belcher and G.J. Ehrlich, J. Chem. Phys. 42 (1965) 2686. [2] G.J. Ehrlich and F. Hudde, J. Chem. Phys. 44 (1966) 1039. [3] A.J. Bennett and L.M. Falicov, Phys. Rev. 151 (1966) 512; A.J. Bennett, Surface Sci. 24 (1971) 191; Phys. Rev. 83 (1971) 1397. [4] M.R. Hayns, Theoret. Chim. Acta 39 (1975) 61. [ 51M. Balooch and D.R. Olander, J. Chem. Phys. 63 (1975) 4772; B. McCarroll and D.W. McKee, Nature 225 (1970) 722;
137
Volume 134, number 2
CHEMICAL PHYSICS LETTERS
L. Mattera, F. Rosatelli, C. Salvo and F. Tommasini, Natural Sci. 93 (1980) 5 15. [ 61 D.R. Olander, R.H. Jones, J.A. Schwarz and W.J. Siekhaus, J. Chem. Phys. 57 (1972) 428; D.R. Olander, W.J. Siekhaus, R.H. Jones and J.A. Schwarz, J. Chem. Phys. 57 (1972) 408. [7] P. Pfluger, M. Krounbi and G.B. Street, J. Chem. Phys. 78 (1983) 3212. [8] J. Ladikand M. Seel, Phys. Rev. B13 (1976) 5338. [9] J. Ladik, Phys. Rev. B17 (1978) 1663. [lo] G. de1 Reand J. Ladik, Chem. Phys. 49 (1980) 321. [ 111 M. Seel, Intern. J. Quantum Chem. 19 ( 198 1) 1083. [ 121 M. Seel, G. de1 Re and J. Iadik, J. Comput. Chem. 3 (1982) 451.
138
20 February 1987
[ 13 ] L.E. Sutton, in: Tables of interatomic distances and contigurations of molecules and ions (Chem. Sot., London, 1958). [ 141 E. Clementi, F. Cavallone and R. Scordamaglia, J. Am. Chem. Sot. 99 (1977) 5531. [ 151 Y.K. Yang and M.S. Jhong, Theoret. Chim. Acta 61 (1981) 41. [ 161 S. Beran, J. Dubsky and Z. Slanina, Surface Sci. 79 (1979) 39; J. Dubsky and S. Beran, Surface Sci. 92 (1979) 53. [ 171 C. Pisani and F. Ricca, Surface Sci. 92 ( 1980) 48 1. [ 18) Th. Weller and W. Miiller, Chem. Phys. Letters 98 (1983) 451. [ 191 W. Flimer, P. Otto and J. Ladik, Chem. Phys. 86 (1984) 49.