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Conformation of NH3 and C2H4 molecules approaching a metal surface
Volume 85A, number 8,9
PHYSICS LETFERS
19 October 1981
CONFORMATION OF NH3 AND C2H4 MOLECULES APPROACHING A METAL SURFACE F. FLORES, I. GABBAY and N.H. MARCH Theoretical Chemistry Department, University of Oxford, Oxford OXI 3TG, England
Received 13 August 1981
Using a semiempirical approach to the conformation of free space molecules, the HNH angle in NH3 adsorbed on Ru(OO1) is estimated to increase by about 1°.For C2H4, both HCC and HCH angles are predicted to decrease.
Although the conformation of molecules approaching a metal surface has received a certain amount of attention experimentally [1,2], little has been done theoretically. However, in a recent paper [3] we have discussed this problem for the water molecule weakly chemisorbed on a ruthenium metal surface. The purpose of the present work is to consider the conformation of NTI3 and C2H4 near a metal surface. Specifically, for ammonia we shall discuss the shape of the molecule by considering the interaction between th~lone pair and its image, while for ethylene the equivalent approach is to consider the interaction between the ir.orbitals and the surface. The orientation of an NH3 molecule approaching a metal surface is illustrated in fig. 1. The attractive interaction between the lone pair orbital and its image changes the hybridization in the molecule and hence HNH angle. In order to discuss this effect, we follow the work of Pauling [4] on free space molecules. The 3 four ammonia sp hybrids shown also m fig. 1 can be .
written
~(1 11)
=
=
(x’2 +x)”2
.
(~
1/2 +~—i-
.
Pj~))~
x \1/2 / 1 (-j—~---,~ ~Is>~—i-7~2
z N
29
—cos 9’ satisfying 3x’2 = 1
—
H
H
/
(1,1,1)
___________________
X
“I
.
Fig. 1. Orientation of NH3
molecule approaching metal surface. Attractive interaction between lone pair orbital and its image is also depicted3 (inset). hybrids.Main figure shows notation used
(1)
for four ammonia sp
(2)
According to Pauling, the energy associated with the N—H bond in free space can be written, per bond,
andsimilarlyfor ~~and ~~withx=—cos28,x’ =
H
2x. In this notation
~
E
b
~~—~-—
r
/3\1/212 [1 +~—) —
j
~ 1—~---p’(E~ — E5).
the lone pair is associated with the 111 hybrid direction and Ip,) denotes a p wave function along direction i.
0031-9163/81/0000—0000/$ 02.75 © 1981 North-Holland
(3)
The first term in Pauling’s bond strength while the second allows for hybridization by virtue of the energy 433
Volume 85A, number 8,9
PHYSICS LETT’ERS
19 October 1981
required to promote an electron from an s to a p orbital. The factor p’ corrects for the interaction of the hybrids [3] In our own calculation of the energy, we propose to use eq. (3) with p’(E~— E5) taken to be the value we obtained for water [3] scaled by the ratio of the adjusting s—p separation N to namely ~ll/l6. By b in eq.in(3) tothat give in an0, energy minimum equal to the experimental value, we obtain
the adsorption of ethylene on planar metal surfaces, assuming that prior to interaction the molecule is orientated with its molecular plane paraliel to the sur-
b = 116 eV
uration as ethylene deformed it is standard. molecule We turn interacting immediately with the to the sur-
.
,
20
=
95 6°
(4)
The result for 20 in eq. (4) implies that the difference between the observed H—N—H angle of 107.3°,and 90°,is partly due to hybridization (5.60) and partly to the H—H repulsions (11 .7°if we attribute the remaining angle increase solely to this mechanism). Turning to NH 3 in interaction with the metal surface, we can use hybrid orbitals in eqs. (1) and (2) to express the dipole associated with the lone pair [3], and hence the interaction between the lone pair and its image, the latter taking the form 1.62 x(l —2~c) 3(1 -t-x)2 (R~N)
(5)
where R~Nis the distance between the N atom and its image and the energy is in atomic units. While eq. (5) represents an interaction characteristic of physisorption, we can generalize it to chemisorption in the formx(l — 2x)d/(1 +x)2, to obtain the energy per bond for the chemisorbed molecule as Ehem
=
b
~
face. The major difference between ethylene and ammonia or water is that the latter molecules possess a net electric dipole moment to which the lone pair hybrids make a significant contribution. As discussed earlier, the interaction between the lone pairs and their images is largely responsible for the deformation of these molecules on metal surfaces. Ethylene, though having no net dipole, can interact with the surface via its polarizable ir-electron system. Referring to fig. 2, which shows a possible rehybridized distorted ethylene configuration in the inset, we take into account the following contributions to the total energy of the molecule: (a) from the C—H bonds; this is given by r
/3 \1/2~2
Ii + (,~_)
4A ~
x = —cos 20; (7) L x (b) from the CC bond; this is given in terms of the and /~hybrids ~infig. 2 as X
[1 +(~~~)h/2]2
~p’(E _E 2 5)+x(1 —2x)d, (6) 1+x P (1+x) Where the constant d evidently measures the interaction energy between the lone pair and the surface. Eq. (6) gives the HNH angle 20 as a function of the chemisorption interaction. Thus, if the chemisorption energy is known, it is possible, by adjusting the parameter d to fit this, to estimate the change in angle due to the interaction. For NH 3 on Ru(001) [5] we find ~.(20) = 1.2°.This is less than the increase we obtained for water [3] on the same surface; the difference is due to different dipole—dipole interactions which in turn reflect the different geometries of the two molecules. We have also studied by similar chemical methods 434
face. Adopting the conventional picture of bonding in ethylene, we describe carbon—carbon double bond 2 hybridsthe along the C—C axis, with p~ orin terms of sp bitals perpendicular to the molecular plane. We shall not go into detail on the free space config.
z
~ -.~
- — —
~
/
,~
/
-
H
C
w X
\
~
-
Y Fig. 2. Notation used in text for hybrids. The hybrids ~ and v lie in the x—z plane. Inset shows geometrical configuration of C2H4 near planar metal surface.
Volume 85A, number 8,9
PHYSICS LE~FERS
19 October 1981
Table 1 Chemisorption energies E and heat of chemisorption H for deformed C 2H4 molecule for different molecule—surface interactions.
Table 2 Angles inferred from photoemission [2]. Surface
HCH
HCC
‘y
HCH
0 10 20 30 40 50
115.9 115.9 115.7 115.4 114.7 113.7
Cu(111) Ni(111) Pd(111) Pt(111)
120 117:4 106.8—109.5 106.8
120—117.4 120 106.8—119.5 106.8—109.5
HCC 122 121.5 120 117.6 114.4 110.7 112sin ~)2
E (eV)
H (eV)
0 0.55 1.34 3.57 5.15
Ag Cu Ni Fe W, Cr Ta
15.56
(8)
B(cos i~+ 3 and B[cos p — 3112sin v sin(7 +
0.37 0.8 2.51 2.94 4.37 5.97
+~
2
C[3~”2sins’ cos(y + ci)} 2
(9)
respectively. The hybrids i,1i,~,are taken to be parallel to the surface. In deriving eq. (9) for the bonding interaction between the two p hybrids we take into account the fact that they are not parallel to each other by projecting the p,~,wave function into components along the C—C bond direction and perpendicular to it. Using these equations, and calculating the constants A, B and C from the free space molecule as A 1 .08, B = 0.92, C 0.91, all in eV, we obtain Edeformed molecule
= 432
~—
[1
l+xL
0.92 (cos r~+ 31/2 sin ~)2 2sin p sin(y + 0.92 [cos v — 3 ~ + 0.91 [3~2sin z cos(y + ~ 2
+ (~~1/2]2
In summary, ammonia on Ru(00l) is expected to exhibit a slight increase in the HNH angle from its free space value, due to the interaction of the lone pair electrons with their image in the metal surface. For ethylene, the main effects of chemisorption are (i) the
ct)] 2
tilting upwards of the C—H bonds so that the H atoms are not in the plane containing the C atoms and (ii) a decrease HCC and the HCH angles, the HCC angle being of thethe most affected.
\X/
+
+
b(cos v
+ 31”2sin
v)2,
+
ficiently large distortions, the bonding character would be so markedly altered that the present model would no the longer be appropriate, so thattothe of large. validity of present work is restricted Hrange not too According to table 1, the effect of the surface is (a) to tilt the C—H bonds, so that the H atoms lie above the plane containing the C atoms and (b) to decrease the HCC and HCH angles, the former to a greater extent. It is of interest to make contact with the work of Demuth [2], obtained by inference from photoemission spectra, as listed in table 2. Larger deformations of C 2H4 occur on Pd(l 11) and Pt(l 11), suggesting single bond character in the C—C linkage. Comparison of tables 1 and 2 shows that the model proposed here describes the main trends of the observed data.
(10)
the final term proportional to b measuring the molecule—surface interaction. Evidently the energy in eq. (10) is a function of two angles, say ‘y and ~, and of the parameter b. Table 1 summarizes the different angles for the deformed molecule, obtained by minimizing the energy expression (10) with respect to ‘y and 0 for various values of b which we need not list. In this table,Eis calculated from eq. (10) while His taken from experiment for the metals shown. As can be seen, the metals have been ordered in increasing H, which one expects to involve increasing distortion ‘y. For suf-
One of us (F.F.) wishes to acknowledge the award of a Senior Visiting Fellowship by the Science Research Council (SRC) while another of us (I.G.) was in receipt also of an SRC Postgraduate Studentship. References [1] T.E. Madey and J.T. Yates, Chem. Phys. Lett. 51(1977) 77. [2] J.E. Demuth, IBM 1. Res. Dev. 22 (1978) 265. [3] F. Flores, I. Gabbay and N.H. March, Surf. Sci. 107 (1981) 127. [4] L. Pauling, The nature of the chemical bond (Cornell U.P., 1948). [5] L.R. Danielson, M.J. Dresser, E.E. Donaldson and J.T. Dickinson, Surf. Sci. 71(1978) 599.
435
PHYSICS LETFERS
19 October 1981
CONFORMATION OF NH3 AND C2H4 MOLECULES APPROACHING A METAL SURFACE F. FLORES, I. GABBAY and N.H. MARCH Theoretical Chemistry Department, University of Oxford, Oxford OXI 3TG, England
Received 13 August 1981
Using a semiempirical approach to the conformation of free space molecules, the HNH angle in NH3 adsorbed on Ru(OO1) is estimated to increase by about 1°.For C2H4, both HCC and HCH angles are predicted to decrease.
Although the conformation of molecules approaching a metal surface has received a certain amount of attention experimentally [1,2], little has been done theoretically. However, in a recent paper [3] we have discussed this problem for the water molecule weakly chemisorbed on a ruthenium metal surface. The purpose of the present work is to consider the conformation of NTI3 and C2H4 near a metal surface. Specifically, for ammonia we shall discuss the shape of the molecule by considering the interaction between th~lone pair and its image, while for ethylene the equivalent approach is to consider the interaction between the ir.orbitals and the surface. The orientation of an NH3 molecule approaching a metal surface is illustrated in fig. 1. The attractive interaction between the lone pair orbital and its image changes the hybridization in the molecule and hence HNH angle. In order to discuss this effect, we follow the work of Pauling [4] on free space molecules. The 3 four ammonia sp hybrids shown also m fig. 1 can be .
written
~(1 11)
=
=
(x’2 +x)”2
.
(~
1/2 +~—i-
.
Pj~))~
x \1/2 / 1 (-j—~---,~ ~Is>~—i-7~2
z N
29
—cos 9’ satisfying 3x’2 = 1
—
H
H
/
(1,1,1)
___________________
X
“I
.
Fig. 1. Orientation of NH3
molecule approaching metal surface. Attractive interaction between lone pair orbital and its image is also depicted3 (inset). hybrids.Main figure shows notation used
(1)
for four ammonia sp
(2)
According to Pauling, the energy associated with the N—H bond in free space can be written, per bond,
andsimilarlyfor ~~and ~~withx=—cos28,x’ =
H
2x. In this notation
~
E
b
~~—~-—
r
/3\1/212 [1 +~—) —
j
~ 1—~---p’(E~ — E5).
the lone pair is associated with the 111 hybrid direction and Ip,) denotes a p wave function along direction i.
0031-9163/81/0000—0000/$ 02.75 © 1981 North-Holland
(3)
The first term in Pauling’s bond strength while the second allows for hybridization by virtue of the energy 433
Volume 85A, number 8,9
PHYSICS LETT’ERS
19 October 1981
required to promote an electron from an s to a p orbital. The factor p’ corrects for the interaction of the hybrids [3] In our own calculation of the energy, we propose to use eq. (3) with p’(E~— E5) taken to be the value we obtained for water [3] scaled by the ratio of the adjusting s—p separation N to namely ~ll/l6. By b in eq.in(3) tothat give in an0, energy minimum equal to the experimental value, we obtain
the adsorption of ethylene on planar metal surfaces, assuming that prior to interaction the molecule is orientated with its molecular plane paraliel to the sur-
b = 116 eV
uration as ethylene deformed it is standard. molecule We turn interacting immediately with the to the sur-
.
,
20
=
95 6°
(4)
The result for 20 in eq. (4) implies that the difference between the observed H—N—H angle of 107.3°,and 90°,is partly due to hybridization (5.60) and partly to the H—H repulsions (11 .7°if we attribute the remaining angle increase solely to this mechanism). Turning to NH 3 in interaction with the metal surface, we can use hybrid orbitals in eqs. (1) and (2) to express the dipole associated with the lone pair [3], and hence the interaction between the lone pair and its image, the latter taking the form 1.62 x(l —2~c) 3(1 -t-x)2 (R~N)
(5)
where R~Nis the distance between the N atom and its image and the energy is in atomic units. While eq. (5) represents an interaction characteristic of physisorption, we can generalize it to chemisorption in the formx(l — 2x)d/(1 +x)2, to obtain the energy per bond for the chemisorbed molecule as Ehem
=
b
~
face. The major difference between ethylene and ammonia or water is that the latter molecules possess a net electric dipole moment to which the lone pair hybrids make a significant contribution. As discussed earlier, the interaction between the lone pairs and their images is largely responsible for the deformation of these molecules on metal surfaces. Ethylene, though having no net dipole, can interact with the surface via its polarizable ir-electron system. Referring to fig. 2, which shows a possible rehybridized distorted ethylene configuration in the inset, we take into account the following contributions to the total energy of the molecule: (a) from the C—H bonds; this is given by r
/3 \1/2~2
Ii + (,~_)
4A ~
x = —cos 20; (7) L x (b) from the CC bond; this is given in terms of the and /~hybrids ~infig. 2 as X
[1 +(~~~)h/2]2
~p’(E _E 2 5)+x(1 —2x)d, (6) 1+x P (1+x) Where the constant d evidently measures the interaction energy between the lone pair and the surface. Eq. (6) gives the HNH angle 20 as a function of the chemisorption interaction. Thus, if the chemisorption energy is known, it is possible, by adjusting the parameter d to fit this, to estimate the change in angle due to the interaction. For NH 3 on Ru(001) [5] we find ~.(20) = 1.2°.This is less than the increase we obtained for water [3] on the same surface; the difference is due to different dipole—dipole interactions which in turn reflect the different geometries of the two molecules. We have also studied by similar chemical methods 434
face. Adopting the conventional picture of bonding in ethylene, we describe carbon—carbon double bond 2 hybridsthe along the C—C axis, with p~ orin terms of sp bitals perpendicular to the molecular plane. We shall not go into detail on the free space config.
z
~ -.~
- — —
~
/
,~
/
-
H
C
w X
\
~
-
Y Fig. 2. Notation used in text for hybrids. The hybrids ~ and v lie in the x—z plane. Inset shows geometrical configuration of C2H4 near planar metal surface.
Volume 85A, number 8,9
PHYSICS LE~FERS
19 October 1981
Table 1 Chemisorption energies E and heat of chemisorption H for deformed C 2H4 molecule for different molecule—surface interactions.
Table 2 Angles inferred from photoemission [2]. Surface
HCH
HCC
‘y
HCH
0 10 20 30 40 50
115.9 115.9 115.7 115.4 114.7 113.7
Cu(111) Ni(111) Pd(111) Pt(111)
120 117:4 106.8—109.5 106.8
120—117.4 120 106.8—119.5 106.8—109.5
HCC 122 121.5 120 117.6 114.4 110.7 112sin ~)2
E (eV)
H (eV)
0 0.55 1.34 3.57 5.15
Ag Cu Ni Fe W, Cr Ta
15.56
(8)
B(cos i~+ 3 and B[cos p — 3112sin v sin(7 +
0.37 0.8 2.51 2.94 4.37 5.97
+~
2
C[3~”2sins’ cos(y + ci)} 2
(9)
respectively. The hybrids i,1i,~,are taken to be parallel to the surface. In deriving eq. (9) for the bonding interaction between the two p hybrids we take into account the fact that they are not parallel to each other by projecting the p,~,wave function into components along the C—C bond direction and perpendicular to it. Using these equations, and calculating the constants A, B and C from the free space molecule as A 1 .08, B = 0.92, C 0.91, all in eV, we obtain Edeformed molecule
= 432
~—
[1
l+xL
0.92 (cos r~+ 31/2 sin ~)2 2sin p sin(y + 0.92 [cos v — 3 ~ + 0.91 [3~2sin z cos(y + ~ 2
+ (~~1/2]2
In summary, ammonia on Ru(00l) is expected to exhibit a slight increase in the HNH angle from its free space value, due to the interaction of the lone pair electrons with their image in the metal surface. For ethylene, the main effects of chemisorption are (i) the
ct)] 2
tilting upwards of the C—H bonds so that the H atoms are not in the plane containing the C atoms and (ii) a decrease HCC and the HCH angles, the HCC angle being of thethe most affected.
\X/
+
+
b(cos v
+ 31”2sin
v)2,
+
ficiently large distortions, the bonding character would be so markedly altered that the present model would no the longer be appropriate, so thattothe of large. validity of present work is restricted Hrange not too According to table 1, the effect of the surface is (a) to tilt the C—H bonds, so that the H atoms lie above the plane containing the C atoms and (b) to decrease the HCC and HCH angles, the former to a greater extent. It is of interest to make contact with the work of Demuth [2], obtained by inference from photoemission spectra, as listed in table 2. Larger deformations of C 2H4 occur on Pd(l 11) and Pt(l 11), suggesting single bond character in the C—C linkage. Comparison of tables 1 and 2 shows that the model proposed here describes the main trends of the observed data.
(10)
the final term proportional to b measuring the molecule—surface interaction. Evidently the energy in eq. (10) is a function of two angles, say ‘y and ~, and of the parameter b. Table 1 summarizes the different angles for the deformed molecule, obtained by minimizing the energy expression (10) with respect to ‘y and 0 for various values of b which we need not list. In this table,Eis calculated from eq. (10) while His taken from experiment for the metals shown. As can be seen, the metals have been ordered in increasing H, which one expects to involve increasing distortion ‘y. For suf-
One of us (F.F.) wishes to acknowledge the award of a Senior Visiting Fellowship by the Science Research Council (SRC) while another of us (I.G.) was in receipt also of an SRC Postgraduate Studentship. References [1] T.E. Madey and J.T. Yates, Chem. Phys. Lett. 51(1977) 77. [2] J.E. Demuth, IBM 1. Res. Dev. 22 (1978) 265. [3] F. Flores, I. Gabbay and N.H. March, Surf. Sci. 107 (1981) 127. [4] L. Pauling, The nature of the chemical bond (Cornell U.P., 1948). [5] L.R. Danielson, M.J. Dresser, E.E. Donaldson and J.T. Dickinson, Surf. Sci. 71(1978) 599.
435