J . Phys. Chem. 1990, 94, 2146-2154
2746
of H,, that V atom is guarded against contact with H, by the surrounding Co atoms and H2 cannot be chemisorbed on the V atom. They cause the decrease in reactivity of Co13by a V atom substitution, which explains the observed stability of Co12V. When more than one V atom is substituted for a Co atom in the CoI2Vcluster, the reactivity is found to increase suddenly again. This comes from the fact that the second V atom is on the surface of the cluster. H2 can react with the surface V atom, to increase the reactivity for adsorption. Thus, this V atom plays the role of adsorption accelerator. When n is different from 13, but takes a value near 13, one V atom substitution generally causes a slight decrease of reactivity
and a second V atom induces the enhancement of adsorptivity. In consideration of the lack of rigid structure of clusters with n being other than 13, the observed results seem to support the particularly stable structure of Coi2V. A definite conclusion will be reached by the measurement of inner-shell electronic transition of the V atoms on Co,V, clusters. Acknowledgment. This work is supported by a Grant-in-Aid for Scientific Research on Priority Areas from the Ministry of Education, Science and Culture. S.N. expresses his gratitude for the donation of Fellowships of Japan Society for the Promotion of Science for Japanese Junior Scientists.
ARTICLES Nitrogen Molecule Actlvation by Excited States of Copper M. SPnchez-Zamora, 0. Novaro,*-tand M. E. Rdz lnstituto Mexican0 del Petrdeo, Investigacidn Bdsica de Procesos, A.P. 14-805, MPxico D.F. 07730, Mgxico (Received: September 14, 1988: In Final Form: September 14, 1989)
Ab initio molecular orbital studies that include variational (with a multiconfiguration reference state of 200 states) and perturbational (including over 3 million configurations) configuration interaction calculations were addressed to the interaction of nitrogen molecules with copper. The Cu ground state ,S and first two excited states ,P and ,D were studied as they interact in different geometrical approaches (including side-on and end-on geometries) with ground-state N2 molecules. The end-on approach is the most favorable, and even if none of the copper states are really effective in activating the triple bond of N,-which is only slightly relaxed by its interactions with copper-the 2P and ,D Cu states do attract N2, showing well-defined relative and absolute minima with respect to the energy of the isolated Cu and N2 moieties. These stable CU-NEN complexes are in good agreement with the experimental results obtained when a solid nitrogen matrix is substituted for rare-gas matrices in matrix isolation experiments for photoexcited reactions (Cu* + H2 + D2 CuH + CUD + H + D).
-
1. Introduction The interaction of nitrogen molecules with copper itself has not received too much attention, in spite of recent matrix isolation studies' that will be referred to the next section. The general subject of dinitrogen attachment to metal atoms or clusters, however, has a wealth of experimental and theoretical studies concerning structural and vibrational aspects.2-40 From these studies, it is concluded in the first place that there exist three forms in which N 2 attaches to the metal: end-on (Le., C,, symmetry), bent (i.e., C, symmetry), and side-on (i.e., C2, symmetry). Any geometrical arrangement for the molecule and the metal in these dinitrogen complexes is a combination of these three. End-on coordination is more common t h a n t h e side-on form.2-8,11-'4.18-26,30,3k40 The bent structure complexes have near linear (end-on) geometries6 Actually structures with angles no smaller than 1 7 5 O for mononuclear complexes and larger than 17 1 O for polynuclear have been On the other hand, the spectroscopic studies show that the u-donating property of dinitrogen is the weakest among several ligands, but its *-accepting capacity is of medium magnitude, falling between that of carbon monoxide and of the organic cyanides and isocyanides.6-8 These donator-acceptor properties play a critical role in the reduction of dinitrogen ligands. Due to its limited a-transfer capacity, the back-donation of the d-orbital orbitals of electronic density in the metal toward the empty 1rB 'On sabbatical leave from Instituto de F k a UNAM.
the dinitrogen becomes the main contributing role for the strengthening of the metal-nitrogen bond. ( I ) Ozin, G. A.; Mattar, S. M. Private communication. (2) Jiqing, X.; Lijuan, X.; Xisheng, L.; Zhigi, 2. Jilin University Sci. Sin. 1981, 24, 35-45. (3) Chi-Ching, H. (Jiqing, X.) A Quantum-ChemicalTheory of Tramition Metal-Dinitrogen Complexes. In Proceedings of the 3rd International Symposium on Nitrcgen Fixation; Newton, W. E., Orme-Johnson, W. H., Eds.; University Park Press: Baltimore, 1980; Vol. I, pp 317-341. (4) Veillard, H. Nouu. J . Chim. 1978, 2, 215-224. (5) Siegbahn, P. E. M.; Blomberg, M. R. A. Chem. Phys. 1984, 87, 189-20 1. (6) Pelikin, P.; Boca, R. Coord. Chem. Rev. 1984, 55, 55-1 12. (7) Chatt, J.; Dilworth, J. R.; Richards, R. L. Chem. Reo. 1978, 78, 589-625. (8) Kobayashi, H.; Yamaguchi, M.; Yoshida. S.; Yonezawa, T. J. Mol. Catal. 1983, 22, 205-218. (9) Ziegler, T.; Rauk, A. Inorg. Chem. 1979, 18, 1755-1759. (10) Isshiki, Y.; Hirashita, N.; Oguchi, T.; Yokoyama, G.; Yamazaki, H.; Kambara, T.; Gondaira, K. I. Surf. Sci. 1981, 102, 443-462. ( I I ) Murrell, J. N.; AI-Derzi, A.; Leigh, G. J.; Guest, M. F. J . Chem. SOC., Dalton Trans. 1980, 1425. (12) Yamabe, T.; Hori, K.; Minato, T.; Fukui, K. Inorg. Chem. 1980, 19, 2154-2159. (13) Yamabe, T.; Hori, K.; Fukui, K.Inorg. Chem. 1982,21,2046-2050. (14) Messmer, R. P. Surf. Sci. 1985, 158, 40-57. (15) Anderson, A. B. Chem. Phys. Lett. 1977, 49, 550-554. (16) Wedler, G.; Steidel, G.; Borgmann, D. Surf. Sci. 1980, 100, 507-518. (17) Anderson, A.; Hoffmann, R. J . Chem. Phys. 1974, 61,4545-4559. ( I 8) Rap*. A. K. Inorg. Chem. 1984, 23, 995-996. ( 1 9) RappE, A. K. Inorg. Chem. 1986, 25, 4686-4691,
QQ22-3654/9Q/2094-2146$02.5Q/Q0 1990 American Chemical Society
N2 Molecule Activation by Excited States of Copper
The Journal of Physical Chemistry, Vol. 94, No. 7, 1990 2141
For reasons of space, we cannot give more details of nitrogen activation studies. Therefore, we summed up the t h e ~ r e t i c a l ~ - , ~ and results as follows: 1. End-on coordination is typical of mono- and binuclear complexes while a mixing of both coordinations, end-on and side-on, is more common for tri- and tetranuclear metal clust e r ~ . ~ * As ~ ,concerns ~ ~ ~ . the ~ ~length * ~ ~of the N=N bond, a slight modification of the isolated dinitrogen normal length (1.097 A, 2.073 au) is observed.s-7~11~'4~1a~21-23~30~35"7 This modified NEN distance ranges from 1.03 to 1.298 A for the mononuclear end-on complexes. This form of coordination has been taken as a model for trapping of nitrogen molecules.s*26On the other hand, side-on coordination complexes generally present substantial relaxation (1.35 A) with respect to the natural N=N length.s-7v31 There is one case of mononuclear coordination in a side-on form for which a N=N distance of 0.83 A is found.6*' Such a short distance stems mostly from the great thermal movement of the atoms in the complex and the possible disorders in the crystalline structure. This form of approach is most probable for activation of dinitrogen and consequently for dissociative chemisorption.s 2. Another notable aspect worth mentioning is the dependence of the metal-nitrogen and NEN distances on the angle 0 of the bent structures. The metal atom-N distance is inversely proportional to this angle while the N=N distance is directly proportional to the angle variation. That is, as we shift away from the end-on structure, the angle grows making the N=N separation increase while the nitrogen gets closer to the metal atom. This is all consistent with the previous data.6,7 3. With respect to the vibrational properties, the dinitrogen complexes exhibit a strong band in their vibrational spectrum (IR and Raman) in the region around 1700-2200 cm-I that may be (20) Wheeler, R. A.; Whangbo, M. H.; Hughbanks, T.; Hoffmann, R.; Burdett, J. K.; Albright, T. A. J . Am. Chem. Soc. 1986, 108, 2222-2236. (21) Powell, C. B.; Hall, M. B. Inorg. Chem. 1984, 23, 4619-4627. (22) Sakaki, S.: Morokuma, K.: Ohkubo, K. J . Am. Chem. Soc. 1985,107, 2686-2693. (23) Dilworth, J. R.; Garcia-Rod&uez, A,; Leigh, G. J.; Murrell, J. N . J . Chem. Soc., Dalton Trans. 1984, 3, 455-461. (24) Muller, W.; Bagus, P. S. J . Vac. Sci. Techno/.A 1987.5, 1053-1056. (25) Tsai, K. R. Development of a Model Of Nitrogenase Active Center and Mechanism of Nitrogenase Catalysis. In Proceedings of the 3rd International Symposium on Nitrogen Fixation; Newton, W. E., Orme-Johnson, W. H., Eds.; University Park Press: Baltimore, 1980, Vol. I, pp 373-387. (26) Jiaxi, L. Composite "String Bag" Cluster Model for the Active Center of Nitrogenase. I n Proceedings of the 3rd International Symposium on Nitrogen Fixation; Newton, W. E., Orme-Johnson, W. H., Eds.; University Park Press: Baltimore, 1980: Vol. I, pp 343-371. (27) Enemark, J. H. Chemical Aspects of Nitrogen Fixation. In Proceedings of the 3rd International Symposium on Nitrogen Fixation; Newton, W. E., Orme-Johnson, W. H. Eds.; University Park Press: Baltimore, 1980: Vol. I, pp 297-315(1980). (28) Hodgson, K. 0. The Molybdenum Site in Nitrogenase-Structural Elucidation by X-Ray Absorption Spectroscopy. In Proceedings ojthe 3rd International Symposium on Nitrogen Fixation; Newton, W. E., OrmeJohnson, W. H., Eds.; University Park Press: Baltimore, 1980; Vol. I, pp 261-281, (29) Stiefel, E.I. Mechanism of Nitrogen Fixation. Recent Developments in Nifrogen Fixation; Newton, W. E., Postgate, J. R., Rodrigues-Barruecos, C.. Eds.; Academic Press: New York, 1977; pp 69-108. (30) Sato, M.; Enemark, J. H. Proc. Int. Conf: Coord. Chem. ZZnd, Mop50 Budapest 1982, 301-327. (31) Grunze, M.; Golze, M.; Hirschwald, W.; Freund, H. J.; Pulm, H.; Seip, U.;Tsai, M. C.; Ertl, G.; Kuppers, J. Phys. Reu. Lett. 1984, 53, 850-853. (32) Heskett, D.; Plummer, E. W.; Messmer, R. P., Surf. Sci. 1984, 139, 558-568. (33) Lee, J.; Madix, R. J.; Schaegel, J. E.; Auerbach, D. J. Surf: Sci. 1984, 143.626-638. (34) Grunze, M.; Golze, M.; Fuhler, J.; Neuman, M.; Schwarz, E. Int. Congr. Catal., [Proc.] 1985, 8th, 4, IV 133-IV 143 (1984). (35) Churchill, M. R.; Li, Y . J.; Blum, L.; Shrock, R. R., Oraanometalfics 1984, 3, 109-1 13. (36) Churchill. M. R.: Li. Y. J.; Theopold. K. H.: Shrock. R. R. Inora. Chem.' 1984. 23, 4472-4476. (37) Yamamoto, A,; Miura, Y.; Ito, T.; Chen, H. L.; Iri, K.; Ozawa, F.; Miki, K.; Sei, T.; Tanaka, N.; Kasai, N. Organometallics 1983, 2, 1429-1436. (38) Rao, C. N . R.; Rao, G. R.; Prabhakaran, K., Chem. Phys. Lett. 1987, 134, 47-50. (39) Heskett, D.; Plummer, E. W.; De Paola, R. A.; Eberhnardt, W.; Hoffman, F. M.; Moser, H. R. Surf. Sci. 1985, 164, 490-510. (40) De Paola, R. A.; Hoffman, F. M.; Heskett, D.; Plummer, E. W., Phys. Reu. B: Condens. Matter 1987, 35, 4236-4239.
attributed to the presence of a modified N=N bond (whose That this natural frequency Y ( N )is 233 1 cm-1).6,7,12,14,31,32,37,38 frequency shifts to smaller values is an indication of a weakening of the nitrogen molecular bond due to an increase in the population of the A* orbital generated by the metal d p orbital charge transfer toward the ligand.6-9*24,31q38 4. Both types of studies propose that the strengthening of the metal-dinitrogen bond and the weakening of the internal N2 bond has a direct relationship with the donation-back-donation properties both of the metal and of the ligand.6s7 That is, the more effective the a donation of the bond between metal and dinitrogen and the more effective the back-donation from the metal's d r orbitals to the A* states of the N2 molecule, the weaker the inner N 2 bond will be, and consequently the molecule will be activated to a higher degree.7-9,11-14,18,21-26.31.32,34,38-40 5. It should be noted that the bond weakening observed for chemisorbed dinitrogen is to be attributed mainly to the electron distribution in the conduction band of the metal rather than the occupation of the A* bonds in the N 2 molecule. In the case of matrix isolation the available electrons are completely utilized to fill the A* on N2during the back-donation process from the metal to the d i n i t r ~ g e n . ~ 6. Finally it might be said that under normal conditions N 2 only interacts with metal atoms that possess open d-subshells, which explains why there is no evidence of nitrogen complexes with ground-state copper.6 In other words, the formation of a Cu-N, complex is only possible through the establishment of an intermediate complex which contains the metal atom in an excited state as shall be demonstrated in the following. The ensuing process implies a nonradiative transition from an excited state to the ground state that allows the complex to accumulate enough energy to overcome a substantial activation barrier and form an activated ground state Cu-N, complex. This process, present in matrix isolation studies, is not common in traditional nitrogen fixation studies. Therefore, we understand why little information about copper-nitrogen complexes exists in the literature. 11. Experimental Precedents
Recently, a series of very interesting experiments involving the interaction of hydrogen molecules with photoexcited (320 nm) Cu atoms were published.4I This photokinetic process was carried out under low-temperature matrix isolation conditions using different solid matrices, and detailed ESR, FTIR, and UV-visible absorption spectroscopies, and kinetic studies allowed establishment of the formation of C u H H product^.^' Subsequently, a series424s of theoretical papers using the methods of the present work (which will be discussed in the following section) and which gave a satisfactory explanation4, of the main results of the photochemical and also the inverse recombination reaction45 were published. Some striking results concerning a strong dependence on the nature of the solid matrix utilized in the experiments, however, were not discussed in the original paper41 or in the theoretical studies. Ozin and co-workers1 found the interesting fact that the interactions between Cu and the different hydrogen isotopes
+
CU* + H2
+ D2
Kr
CUH
+ CUD+ H + D
-
with k H / k DE 1.5-2.0 and where Cu* denotes a photoexcited copper atom with 320-nm photons, Le., a 3d'04p1(2P) 3d104s1(2S)transition. In contrast, when a solid nitrogen matrix
(41) Ozin, G. A.; Mitchel, S. A,; Garcia-Prieto, J . Angew. Chem. Suppf. 1982, 785.
(42) Garcia-Prieto, J.; Ruiz, M. E.; Novaro, 0. J . Am. Chem. Soc. 1985, 107, 5635.. (43) Ruiz, M. E.: Garcia-Prieto, J.; Novaro, O., J . Chem. Phys. 1984.80, 1529. (44) Garcia-Prieto, J.; Ruk, M. E.; Poulain, E.; Ozin, G. A.; Novaro, 0. J . Chem. P j y s . 1984, 81, $920. (45) Ruiz, M. E.; Garcia-Prieto, J.: Poulain, E.; Ozin, G. A,; Pokier, R. A.; Matta, S. M.; Cizmadia, I . G.; Gracie, C.; Novaro, 0. J . Phys. Chem. 1986, 90, 279.
2748
Sdnchez-Zamora et al.
The Journal of Physical Chemistry, Vol. 94, No. 7 , 1990
was used CU*
+ H2 + D2
N2
CUH
+ H + D2
+
has a residual product ratio of C U D D scarcer by as much as IO' or more. In other words, the isotope effect with k H / k Don N 2 matrices is overwhelming. Under these circumstances Ozinl proposed, as a first attempt to explain these facts, the following hypothetical situation. If we suppose that the Cu*-H-H intermediate structure is formed in principle with Cu* in its ,P state (as proposed by us in ref 43), this implies a highly excited vibrational state. It thenceforth dissociates into the C u H + H products in rare-gas solids due to Y' from high-frean inefficient vibrational energy transfer Y quency normal modes of Cu*-H-H to low-frequency phonon modes. On the other hand, this vibrational energy transfer becomes more efficient when working with molecular solids with high-frequency internal modes (e& u C H in CH,, vN2 in N,). Further experiments' on Cu over N, matrices using EPR, optical, and 1R spectroscopies revealed that weak van der Waals complexes Cu-(N=N), were formed. Consequently, when a Cu 2P ,S excitation takes place in a N-N matrix, the u donation from the H2 us orbital to the empty Cu(4s) orbital faces a competing N2lone pair donation to this same Cu orbital. On the other hand the H donation from the semioccupied Cu(4p) to the u*" of H 2 diminishes because Cu(4p) is donating to the N, H* orbital. From the above arguments one may deduce, as Ozin states, that the activation barrier for the reaction Cu(,P) H2 over a Kr matrix can be substantially lower than for that Cu(,P) + H2 over a N 2 matrix, and this could then explain the large difference in the isotopic effects for H 2 over D, in N, matrices' (pending a quantitative analysis). In other words, the proton tunneling effects would be very dominant when an imposing barrier such as that present in a N 2 matrix exists, thus making the Cu + D, reaction very inefficient by comparison. A transition state similar to
-
-
+
H NGN-CU
/
\
H
complex is then implied.' The present theoretical results concerning the interaction of a copper atom (both in its ground and in the relevant excited states) and a nitrogen molecule have a direct bearing on this hypothetical situation. 111. Calculational Details The present theoretical method, which starts from a SCFLCAO scheme including extensive configuration interaction (CI), has been described in detail e l ~ e w h e r eand ~ ~ uses , ~ ~ the a b initio pseudopotential method of Durand and Barthelat& via the PSHONDO-CIPSI sequence of programs documented by D a ~ d e y . ~ ' We use here double-( quality basis sets for all the atoms as well as the pseudopotential parameters for copper and nitrogen that are reported in detail in ref 47. The copper basis set is that of ref 43, and it has more p-polarization functions than the set reported by Pili~sier.~* Thenceforth, a variational CI study using a large multiconfiguration space of over 200 states was used as the reference energy in a second-order Merller-Plesset perturbation scheme including over 3 million configurations. This is the well-known ClPSl alg~rithm.,~In effect all the calculations may be best understood by referring to the previous work of our group
(46) Durand. Ph.; Barthelat, J. C. Theor. Chim. Acfa 1975, 38, 283. (47) Daudey. J . P. Preprint of Universiti Paul Sabatier 1982 (available
upon request) (48) PClissier, M. J . Chem. Phys. 1981. 7.5, 775. (49) Huron, B.: Malrieu, J. P.: Rancurel. P. J . Chem. Phys. 1973. 58. 5735
TABLE I:' Results for the Asymptotes of Cu and N2 calcn total energy, A E , eV Cu ClPSl
exptl
state
au
(ground-excited)
data
2S
-50.203 124 -50.064 696 -50.128 104
-0.138 428 -0.075 020
-0. I 39 65 1 -0.074 971
D,(expt), eV
r,(expt), au
9.26 (9.9)
2.175 (2.075)
*P *D
calcn state
N,
total energy, au
'Eg+ -19.578618
ClPSI
"Copper results taken from ref 43. Experimental values. Moore, C. E. Atomic Energy Levels. Nut/. Bur. Stand. ( U S . ) Circ. 1949, 467. TABLE 11: Cu and N2 Orbital Classification According to the Irreducible Representation of the C,,, Ck?,and C, Point Groups for the CuN, Interactionso
c,,
~
copper nitrogen
s,
u
77
U
K
PT
4 2
Px.
dx,
py. d,,
u
c,
c2, a"
a'
6 6
al
d,
s,
dx2-p
dx2-y2
P2'
bg.
b, 4 2
="
Px.
a,
4,
"u
b, d,,
d,
Py3
"8
C", =g
"g
For linear molecules the z axis is the molecular axis; for the side-on geometry the z axis is perpendicular to the molecular axis.
+
+
on the Cu* H2 r e a ~ t i o n and ~ ~ -the ~ ~CuH H reaction.45 A set of calculations to establish the asymptotic reference levels for the Cu and N, moieties at infinite separations are reported in Table I . As concerns the analysis of the calculations for the interactions between the different atomic copper states and the nitrogen molecule, they shall be presented with the notation appropriate for the point groups associated to the geometries of approach selected for study. In all cases the interacting N, molecule is taken in its ground 'Bgf state, while the Cu atom may be in any of its *S,*D,and ,P states. For the side-on approach, the point group belongs to the C, symmetry. The end-on approach geometry implies a point group of C,, symmetry. Table I1 contains the main components of the molecular orbitals of the Cu N 2 system. These orbitals are ordered in each symmetry according to their irreducible representations within the most accepted reference system.
+
I V . Potential Energy Surfaces for the Cu* + N, Interaction (0)Side-OnApproach. The lowest energy curve found for the side-on approach of the Cu and N, moieties has the symmetry * A , . This state of the metal-dinitrogen system stems from the interaction of the ,S ground state of Cu with !Be+ of N,. The corresponding potential energy curve is repulsive at the CI level as shown in Figure 1, where the ,A, curve from the Cu(,P) and N, is also depicted. For sake of brevity, we omitted the other curve of the same symmetry that leads to the Cu(,D) N, final products which is also repulsive at the S C F level. Two other states of the Cu + N, interaction were thoroughly studied also, both of *B2 symmetry and which stem from the interactions of the ground-state N 2 molecule with the first two excited states of Cu, *D and *P. Their energy evolution is given in Figure 2 showing that both states are attractive. In fact both curves present energy minima, the highest one (stemming from Cu ,P) being attractive from the onset while the one corresponding to Cu ,D presents an energy barrier followed by a deeper minimum at shorter separations. Obviously this situation is a clear indication of an avoided crossing between both 2B2 potential energy curves, as may be inferred from Figure 2. The bonding for this side-on structure may be compared with the qualitative image of the Dewar-Chatt-Ducanson model which proposes u-charge donation from the lone pair of electrons in N, toward the metal atom with T back-donation from the metal d* orbitals to the H * orbital of N,. Our quantitative results bear out these predictions well. The classification of each orbital according
+
The Journal of Physical Chemistry, Vol. 94, No. 7, 1990 2749
N2 Molecule Activation by Excited States of Copper
TABLE Ill: Molecular Orbital Ordering and Composition of the CU-NI Distance for the Side-On Approach" MO
energy sequence
10.0
4.5
4 5
2a, - 30, 20, - 30, 1 nu - 30, - diz In" + 4, 20, In,
6
4,
1
2
3
IO II I2
+
+
+
dXZ dXY
d,z - dX2-y2+ s + In, d,, - 20, - 30, - 1 H , s - pz - 20,
dYZ
s - pz - 20, Py - dyz - In,
13
3.5 2a, - 3a, 20" + 30, 1 nu - 30, - s - dzz In, + dxz 30, 20, In, dX2-y2- dz2 - In,
dX2~y~ - d,2 - In, dXY dzz - dx2-y2t 1 a,
7
8 9
1 ng
14
PI - 1 Tu
HOMO-LUMO gap, eV
3.7306
sym class
1
la, la,
11
2a, Ib, Ib, 2al la, Ib, 2al OThe orbital classification is that of Table 11. I II Ill
distance, au, and composition 3.1
3al 3a, 3a,
Ib2 Ib2 Ib2
4a, 2b, 4a,
2b, 4a, 2b,
3 5891
3.4666
Ill
Ill
la2 5a, 5a,
2b2
la, la2
I
5a, 2b2 2b2
6a, 6a, 6aI
3b2 3b2 3b2
2a2 2a2 2a2
4b2 3b, 3bl
I
I
200 0
!I
I
- 1 160 0
E
2y 120 01p
v
h ?
$ w
I
CU(' P)+Nn
-
aooL
!
4
-
Cu('D)+N,
C U ( ' D)-N,
00-
\ 1
CU(' S)+N,
-
1
1
-
Cu(' S)+N,
-
00
20
40
Cu N,
60
80
100
distance ( a u )
Figure 2. Same as Figure 1 but for 2B2 curves of the C, interaction of N, with the 2D and 2P Cu states.
deduce that at these distances the total energy of the Cu-N2 system is simply the sum of the energy of the isolated Cu and N 2 moieties. Also, at these larger distances, the molecular orbitals, given in Table 111, do not show any admixture between the Cu atomic orbitals and the N, molecular orbitals, while for shorter separations such admixture becomes more and more evident. As the distance between the Cu and N, moieties is shortened, the interaction between them grows. This is reflected by the substantial mixing of their respective atomic and molecular orbitals, for the equilibrium distance shown in Table IV. In this table one can observe the existence of the 6a, molecular orbital that contributes to the u donation through the bonding combination of the Cu s and pz orbitals and 3ag and In, from N2. Also we have an orbital that contributes to A interaction and comes from the combination of the C u d,, orbital and the N 2 In, and 30,. The two ( 0 and n ) bonds just discussed are, however, too weak to permit that a stable CuN, complex is formed. This can be seen from a Mulliken population analysis, where a larger charge
2750
The Journal of Physical Chemistry, Vol. 94, No. 7, 1990
Sgnchez-Zamora et al.
TABLE IV: Atomic Orbital Main Contributions to the Molecular Orbitals for the Equilibrium Geometry of tbe SideOn Aooroach ICu(’D) orbital
+ N,1
LUMO
3bz 2a2 3bl
0.0521 0.0903
0.0912
(0.6)Py- (0.1)dy2- (o.I)s~, - (0.4 0.4)P,,~, (o.1)sN2+ (0.4 + O.4)pz,,, (O.I)d, + (0.5 + 0.6)Px,N,- (0.5 + 0.6)Px,~, (I.O)Px- (0.0 + O.I)Px,~, - (0.1 O . l ) p , ~ ,
transfer from the u orbital of dinitrogen toward the metal is present than the small back-donation from the metal d r orbital toward the empty r* orbital of the ligand. The derived charge transfers from the population analysis show that the molecule to metal transfer is 0.165e while the N 2 r* orbital receives only 0.070e from the metal. In the configuration interaction study the Cu-N2 states that derive from the nitrogen ground state and three lowest atomic copper states 2S,2D, and 2Pinteracting in a side-on structure were analyzed. The ground-state-ground-state interaction has a 2Ai symmetry and as shown in Figure 1 is repulsive. The excited states 2D and 2P also add two states of 2Ai symmetry which are repulsive. Also, the 2Bi states derived each from the N 2 approach to C U ( ~ D ) and C U ( ~ Pcan ) be shown to be nonbonding by simple orbital arguments and comparison with the CuH2 case.43 The 2B2states coming from Cu 2D and Cu 2P are reported in Figure 2 and both are capable of capturing N2. In short, we concentrated on four states, the CU(~S)-N,2Al state, the C U ( ~ P ) - N2Al ~ and 2B2states, and the C U ( ~ D ) - N 2B2 , state. The 2Al states are repulsive, and the lowest 2Al curve comes from a dominant configuration with a 0.97 coefficient that corresponds to the S C F configuration. Also, the other 2A, curve corresponding to the excited-state Cu(diop’)(2P)is repulsive and also has a dominant configuration with a large (0.96) coefficient. In both cases, the main contributions of the excited configurations represent mainly a relaxation of the closed d i osubshell, a situation previously found to be crucial for the stabilization of the Cu2 m o l e c ~ l and e ~ the ~ ~ Cu-H2 ~ ~ complex.43 The two attractive 2B2curves are quite interesting (see Figure 2). The lowest one corresponds to a CU(~D)-N,interaction, which is evident from the large (0.93) coefficient for the 2D excited-state configuration that dominates the CI scheme. Another important configuration in this CI scheme (coefficient 0.1) corresponds to the double excitation that tends to occupy the N2 r* orbital. This and other configurations, all tending to populate the r* orbital and interacting with the main configuration, are actually effective in diminishing the repulsive character of the Cu d subshell. Thus, this 2B2 curve becomes attractive after overcoming a 20 kcal/mol barrier. Its minimum, as shown in Figure 2, lies at a Cu-N2 separation of 3.7 au and with a depth of 5.4kcal/mol. The other 2B2 curve corresponds to the N,-CU(~P) interaction and also presents a minimum (see Figure 2 ) . This is confirmed by the large coefficient (0.96) that the (diopi)2Pconfiguration has in the CI scheme. The second most important coefficient in the CI represents an electron pair that jumps from the N, bonding a orbital to the N2 antibonding ?r* orbital. We, however, should warn that this large coefficient is probably being affected by the poor Hartree-Fock description of molecular nitrogen. In any case the Cu(’P) state is effective for capturing N 2 from the onset, and this is evident by the purely attractive curve for Cu(,P)-N, that reaches a minimum at 4.59 au and a depth of almost 20 kcal/mol. After (50) Bauschlicher, C. W.; Walch, S.P.; Siegbahn, P. E. M. J . Chem. Phys. 1982, 76. 61 5 .
v
I
I
\
Cu(‘ D)-N2
__ 20
-2
4 0
I~~
60
8 0
100
Cu N, distance ( a u )
Figure 3. %+potential curves for the C,, (end-on) interaction of unrelaxed N, to the 2S and zP states of Cu as a function of the nearest N to copper separation. Asymptotic energies are given on the right-hand side.
this distance the curve begins to rise mainly due to the avoided crossing between this curve and the lowest 2B2curve stemming from CU(~D)-N,. In spite of labilization of the N 2 ?r bond by admixture of its A* orbital, the effect in the strong triple bond of N 2 is quite small, and all attempts to relax the N==N distance even when N 2 is interacting with copper in the most favorable situations (Le., at the minima of curves 2B2 in Figure 2) were fruitless. N 2 is not really activated by copper even in its excited states due no doubt to the inefficient back-donation mentioned before. This explains why copper-dinitrogen complexes are scarce in the literature. The well-defined minima of Figure 2, however, do have an effect on the excited copper atom, diminishing its capacity of activating H2 when these atoms are deposited in a nitrogen, rather than a rare-gas, matrix. This shall be, however, better understood by studying the end-on Cu-N-N system. ( b ) End-On Approach. The results for this geometrical arrangement (Cu-NzN) are qualitatively similar to those of the side-on approach. The single-configuration calculations of the lowest energy state 2Z+were used as reference for the CI studies which concentrated on four states, two with 2Z+symmetry and another two with 211symmetry. The first state comes from the CU(~S)-N=N interaction, the remaining ,Z+ and one of the 211 come from CU(~P)-N=N, and the last 2rI is from CU(~D)-NSN.
The Journal of Physical Chemistry, Vol. 94, No. 7, 1990 2751
N z Molecule Activation by Excited States of Copper
TABLE V Molecular Orbital Ordering and Composition as a Function of the Cu-N, Distance for the End-On Approach distance, au, and composition MO energy sequence 10.0 4.1 3.4 3.35 3.25 1 20, 20, + 30, 20, + 30, 20, + 30, 20, + 30, 20, - 30" + s + pz + dzz 20, - 30, + s + d12 2 20, 20, + 30, 20, - 30, + s + dz2 20, - 30, - s - pz - dz2 20, - 30, - d,2 20, - 30, - dz2 3 30, 20, + 30, 4 IH, ITU 17, + dy, 17, + dyz I*u + d, 5 IH, IT, I*, + dxz 17, + dxz In, + 4,
6
dx2-yZ dxy dyz dxz dZ2
7 8
9 IO II 12 13
14 HOMO-LUMO gap, eV orbitalsequence
dxy
dXY
S
dxz-yz dyz - I*, dxz - I*, s + dz2 - 20, s pz + 20,
PX
P X +
PY
Py + IT, Px + d*z 3.9510
dx2-y2 dyz + P y - I*, dxz + P x - IT" s - pz - dz2 - 20, + 30, s - pz + d,2 - 20, + 30, PI - dxz + I*, Py - dyz + I*, PI + d,, - I*, 3.3442
Is,
5.3850 lo
20
+
+ 3a,
+ 30,
I*,
30
IT,
IH
IH
16
dXY dx2-y2 dxz - I*, dyz - I*,
s - pz - dzz - 20, s - pz d,2 - 20,
+
Px
Py - d y z + Px
+ 30,
+ 30,
- dxz + I*, 1*g
3.2925 40
2H
- 17, s - pz dz2 - 20, s - pz d,Z - 20,
dxz
+
+
+ 30,
+ 30,
Px - dxz + In, Py - dyz + IT, Px + dxz - I*, 3.2462
+ 4, - I*,
2~
16
dXY dx2-y2 dyz - I*,
50
3~
3~
4~
TABLE VI: Atomic Orbital Main Contributions to the Molecular Orbitals for the Equilibrium Geometry of the End-On Approach [Cu(*D) + N,]
2H 2a 40
energy, au -1.5951 -0.9 122 -0.7297 -0.706 2 -0.7062 -0.5265 -0.5265 -0.5168 -0.5167 -0.4929
atomic orbital configuration (0.5 + o . l ) s N , - (0.2)P,,NI + (0.5)SN2- (0.2)Pz,N2 (0.1)s + (0.2 + O.l)d,2 + (0.5 + 0 . 4 ) s -~ (0.4)Pz,~, ~ - (0.4 + 0 . 3 ) s ~ ~ (0.1 + 0.1)dz2- ( O . I ) S N , - (0.4 + O.I)P,,N, - (0.3 + 0.4)SN2+ (0.5 + O.I)P,,N, (0.1 + O.l)dyz+ (0.4 + 0.2)Px,N, + (0.4 + 0.2)px,3, (0.1 + O.l)d,, + (0.4 + 0.2)Px,~, + (0.4 + 0.2)Px,~, (0.7 + 0.5)dx2,2 (0.7 + 0.5)dxy (0.7 + 0.5)dy, - (O.l)Py,Nl- (0.1 + 0.1)Py,N2 (0.7 + 0.5)dxz- ( O . l ) P x , ~-, (0.1 + 0 . 1 ) P x , ~ , (0.1)s + (0.7 + 0.5)dz2 + (0.1 + 0.3)SNl - (O.I)Pz,NI+ (0.1 + 0.1)Pz,~2
50
-0.0687
HOMO (0.3 + 0.8)s - (0.2)Pz + (0.1 + 0.1)dz2- (0.1 + 0 . 2 ) s + ~ ~(0.1 + 0.2)pz,~, - ( 0 . 2 ) s -~ (~o . I ) p , , ~ ~
3* 377 477
0.0496 0.0496 0.1322
orbital la
20 30 IH IH
16 16
+ +
+
LUMO
+ +
+ +
(0.6)Px- (0.1 O.l)dxz (0.3 0.4)Px,N,- (0.4 0.5)Px,Nz - (0.4 + 0.5)Pyjq2 (0.6)Py- (0.1 + O.l)dy, + (0.3 + 0.4)Py,~, (0.9)Px (0.1 O.l)d,, - (0.4 0.6)Px,N, (0.3 0.5)Px,Nz
+
In Figure 3 the z8+curves are shown. They are repulsive but their total energy is substantially lower than that of the corresponding curves for the side-on approach (here we do not present the intermediate curve arising from the C U ( S ~ ~ ~ + ) ( Nz(l2,+) ~D) interaction due to its repulsive behavior). In Figure 4 we also obtain curves that have a lower energy than those of the side-on structures. The discussion of Figure 4 is given below. The molecular orbital analysis clearly shows the important hybridization of the s, pz, and dz2 orbitals of the metal and the u and ir orbitals of the molecule that contribute largely to the formation of the bond. In Tables V and V I one may see that this end-on combination of the orbitals has a bonding character that allows for the charge donation from the dinitrogen to the metal. Also worth mentioning is the fact that the atomic orbitals of each nitrogen contribute unevenly to the bond, the one closer to the metal interacting much more, a fact that is reflected by the coefficient of the corresponding N atomic orbital in the bond's molecular orbital. On the other hand, there does not exist practically any A charge back-donation toward N2,and no n,-bond contribution from the dinitrogen to the 2ir molecular orbital is evident. The population analysis shows that the molecule transfers a 0.237 charge toward the metal, which in turn back-donates a 0.101 charge to the nitrogen. Essentially all of the charge transfer toward the metal comes from the farthest N atom. I n effect the magnitude of this charge transfer is equal in magnitude to the charge lost by this atom; the other N atom keeps its total charge unmodified. In the extensive CI calculations the curves with z2+symmetry are completely repulsive (see Figure 3). The lowest curve has in its asymptote the isolated Cu(%) and N,('Z,+) entities and its main configuration is the (3d'04s')2S configuration (with coefficient 0.95). The other zZ+ curve dissociates to C U ( ~ Pand )
+
I ~
200.0 I
1
- 160.0 ,
Z I s 120.0
l-
L
h
2 -~ 2
L2 80.0
-
-
CU(* P)+N2
."
-
i
m
B
40 0
C U ( D)+N, ~
-
1 ~
Cu('S)+N,
-
0 0, I
2.0
1 10.0
c
6.0 8.0 C u . . .Nt distance ( a . u . )
4.0
Figure 4. Same as Figure 3 but for *II curves of the C,, reaction of the 2Dand 2P copper states with unrelaxed N2.
N2('Zg+),and its main configuration stems from the depopulation of the 4s to the 4p orbital, (Le., (3d104p1)2P, which has a coefficient of 0.96). Again the main interacting configurations in the re-
2152
The Journal of Physical Chemistry, Vol. 94, No. 7, 1990
Sinchez-Zamora et al.
TABLE VII: Main Contributions of the Atomic Orbitals to the Molecular Orbitals for the Equilibrium Geometry of the End-On Approach [Cu('D) + N2](with the Nitrogen-Nitrogen Distance Relaxed) orbital energy, au atomic orbital configuration lo -1.4416 (0.5 + O.I)SN, - (0.2)Pz,N, + ( 0 . 4 ) s ~ ~(0.2)pz,Nz 20 (0.1)s + (0.2 + 0.2)d22+ (0.4 + 0 . 3 ) s ~- ~(0.4 + O.l)Pz,~l - (0.4 + 0.3)SN2 -0.8415
2a 40
-0.63 17 -0.6161 -0.6088 -0.5447 -0.5447 -0.5264 -0.5209 -0.4947
(0.3 O.2)dz2- (0.1 O.l)sN, - (0.3 O.I)Pz.~I + (0.3 + 0 . 4 ) s +~ ~(0.5 + o.1)P2,~, (0.4 + 0.3)dx2+ (0.4 + 0.2)Px,Nl + (0.3 + o.l)Px,N2 (0.4 + 0.3)dyz+ (0.4 + 0.2)px,Nl+ (0.3 + 0 . 3 ) P x , ~ ~ (0.7 + 0.5)dxz,2 (0.7 + 0.5)dxy (0.6 + 0.4)dXz - (0.2 + O.I)p,N, - (0.3 + 0.2)Px,N2 (0.6 + 0.4)dyz - (0.3 + 0.2)Py,N1- (0.3 + 0.2)Py,N2 (0.2)s - (0.6 + O.4)dZz+ (0.1 + 0.2)sN, - (0.2)Pz,Nl + (0.2 + 0.1)pz,N2
3n
-0.0328
HOMO (0.1 + O.7)Px- (O.I)dxz+ (0.3 + 0.3)P,,~,- (0.4 + 0.5)Px,~,
5o 37
-0.0012 0.0732 0.1413
(0.3 + 0.7)s - (0.4)Pz + (0.1 + 0.1)dz2- (0.1)s~~ + (0.1 + 0.2)PZ,~, - (0.2)sN2+ (0.1)Pz.~2 (0.9)Py + (0.1 + o.l)Py,N, - (0.3 + 0 . 3 ) P y , ~ ~ (0.9)Px + (0.1 + O.l)d,, - (0.4 + 0.5)P,,~,+ (0.5 + O.~)P,,N,
30 la IT
li 16
2n
LUMO
4a
spective CI schemes for these states correspond to d-shell relaxation. The closed d subshell in effect repels the closed-shell N2 molecule, and naturally the polarization of this subshell greatly reduces the repulsion, albeit not enough to make the curves in Figure 3 attractive. The states with ,II symmetry, on the contrary, do represent attractions, as shown in Figure 4. Their CI schemes lead to the curves with well-defined potential energy wells, and the lower curve in fact has a double-well structure. This curve corresponds to the ( 3d94s2),D state of copper interacting with the nitrogen molecule ground state 'Eg+.It is very interesting that this state is attractive from the onset showing that the lone-pair electrons of the nearest N atom can interact with the open d subshell of ,D Cu. The outlying 4s2 subshell, however, prevents the N 2 molecule from coming too close to Cu, and at a comparatively long Cu-N separation of over 4.3 au the original attraction has already become a repulsion between the ground-state N 2 and 2DCu, but only to become attractive again after overcoming a potential energy barrier of 10.35 kcal/mol. This barrier obviously comes from the avoided crossing of this potential energy curve with the upper ,ll curve that stems from the interaction of the ,P Cu state and ground-state N,. This upper curve is strongly attractive at long distances and forms a deep well at about 4 au Cu-N separation. It is at this distance where the avoided crossing between the two curves of the same ,ll symmetry and the maximum for the barrier between the two minima of the lower ,II curve are. For the lowest state in Figure 4 we have that the most important element in the CI scheme (with 0.95 coefficient) leads to a 3d94s2 configuration of the Cu moiety. Other configurations also contribute significantly in this C1 scheme, the most relevant being the 1 7ru .-+ In8 excitation of the dinitrogen with a coefficient of 0.2. Similarly, all the other configurations that are characterized by their population of the N, r* starting from the a orbitals are also useful in diminishing the repulsive character of the Cu d subshell. Obviously, the interaction of the Cu(,D) state with N, in a end-on arrangement is extremely efficient as is evidenced by its double-well structure, with a long distance minimum that implies an attraction between the C ~ ( 3 d ~ 4 s ~ )and ( ~ Dground-state ) N2. Compare this situation with the approach of these same states in a ,B2 symmetry side-on approach (Figure 2) which is essentially repulsive prior to the avoided crossing with the upper curve. The inner minimum (3.25 au Cu-N separation) of the end-on 211curve in Figure 4 is also important, having a depth of almost 11 kcal/mol with respect to the activation barrier. It is therefore not surprising that Ozin' attributes to this Cu(,D) state the role of the one responsible for the interaction with the N z molecule. This would imply that, after the photoexcitation of the Cu atom to the 2Pstate, it would suffer a nonradiative transition to the ,D state that then would react with N,. The 2P state, however, is also capable of capturing Nz in a end-on geometry, as Figure 4 shows. This upper ,I'I curve is quite attractive with a potential energy well of 29.32 kcal/mol depth and Cu-N separation of 4.1 au. The branching
18
2 0
22
N
2 4
26
28
30
N distance ( a u )
Figure 5. Relaxation of the N-N distance for N, interacting with the 2S and 2Pstates of Cu in a 22+geometry. Asymptotic energies are given on the left-hand side of the figure.
ratio between the two exit channels for the nonadiabatic transition from the second ,II(,B2) state to the lower ZII(2Bz)state may lead to either capture into the bound well or to the dissociation to Cu(,D) + N2. The hopping probabilities were obtained by using a modified Landau-Zener-Stueckelberg formulation.s1 The ratio between the capture channel and the dissociation channel hopping probabilities is 0.9918. This means that both situations are almost equally probable. The largest coefficient in the CI scheme for the Cu(*P)-N2 interaction gives a 3dI04p' configuration for the Cu moiety with a coefficient of 0.96. The other contributions to the CI scheme are mainly d double excitations that reduce d-shell repulsions toward N2 and at shorter distances Cu s, dz2, and pz relaxation, Le., in the axis of approach of N,. We have then that both the ,P and ,D states of copper in a ,II end-on configuration are quite capable of capturing ground-state N,. The ground state of copper does not have an attractive ,z1+ curve with N, if we do not relax the N-N distance. If, however, (51) Landau, L. D. Phys. Z . Sowjetunion 1932,2,46. Zener, C . Proc. R. SOC.A 1932, 137,696. Stuckelberg, E. C. G. Helu. Phys. Acta 1932.5, 369. Preston, R. K.;Tully, J. C.J . Chem. Phys. 1971, 54, 4297, J . Chem. Phys. 1971, 55, 562. Bauschlicher, C. W.; @Neil, S.V.; Preston, R. K.;Schaefer 111, H. F.; Bender, C. F. J . Chem. Phys. 1973, 59, 1286.
N 2 Molecule Activation by Excited States of Copper
- 4 0 O1 8
20
22
N
2 4
26
The Journal of Physical Chemistry, Vol. 94, No. 7, 1990 2153
3C 0
26
N distance ( a u )
Figure 6. Same as Figure 5 , but for the 211 geometry of the N2 molecule attached to the 2D and 2P states of Cu. The N-N distance is relaxed after fixing the nearest N atom to Cu distance as that of the inner minimum of Figure 4, Le., a region for which the CU(~P) + N2 interactions already present a hard-core repulsive character. In the region of the outer minimum of Figure 4 the N-N bond cannot be relaxed; see text.
TABLE VIII: Summary of Properties for the Optimal Geometries of the Bound CUN, Speeies
CUNZ state
asymptotic species 2Z+ Cu(%) + N2(lZg+) 211 C U ( ~ D+) N2('Zg+) 211 C U ( ~ P+) N2(lZg+)
rcCu+,
4,
we,
W&,,
au au kcal/mol cm-I cm-I 2.22 3.25 15.19 1459.9" 100.3# 2.22 3.25 28.49 1307.9' 42.9" 2.07 4.16 29.32 197.3b 0.9b
aThese values correspond to the N-N stretching frequency. *These values correspond to the Cu-N stretching frequency. the N 2 molecule's internal distance is relaxed, the situation changes, as Figures 5 and 6 and Table VI1 show, and the interaction of relaxed N 2 with the ground state of Cu does reach a minimal energy structure. This structure is characterized by an electron charge donation of 0.325e from the copper d u orbitals toward N 2 r* orbital. There almost all of the charge is received by the nearest N atom (0.294e) while only a residual charge reaches the farthest removed N (0.031e). For the relaxed N2C U ( ~ Dminimal ) energy structure the orbital ordering according to Table VI1 is (1u)2(2u)2(3u)2(1r)4(16)4(27r)~(4u)~(37r)~
We shall discuss all of these results in more detail in the following section. V. Conclusions From the above results, we conclude that copper in an excited state can efficiently capture the dinitrogen molecule in an end-on geometry. The properties of the bound CuN2 species structure are summarized in Table VIII. This structure in itself allows us to explain the results of Ozin' because any Cu*-N=N interaction hinders the capacity of the excited copper atom to capture and activate H2, especially considering that the relevant Cu orbitals to capture and activate H2 are precisely those already involved in the Cu-NEN bonding situation. Therefore, the activation barrier for the Cu H2 reactions will be much larger over N 2 than over Kr matrices. On the other hand, if we analyze the Cu*-N=N interaction from the point of view of the nitrogen activation, it is obvious that
+
even if the excited state of copper is much more effective than its ground state, it is still true that even C U * ( ~ Pdoes ) not possess the capacity to sufficiently weaken the N 2 intramolecular bonds up to a point that its dissociation is at all possible. In effect, the attempts to relax the N-N distance from Cu in the linear arrangement depicted in Figure 4 were not very successful. This relaxation is described in Figures 5 and 6. Figure 5 shows that for the 2Z+ configurations the interaction energy actually goes down (remember that when the N 2 molecule approaches without relaxation, it is merely repelled at these configurations, as shown in Figure 3). This lowering of the energy is more notable for the ground-state 2Z+associated to the Cu 2S state which then is shown to be able to capture a relaxed N 2 molecule. The relaxation of the latter, however, is not too large (the N-N distance is only lengthened to 1.174 A, i.e., less than 1/10 A). For the upper 2Z+ curve the energy lowering barely allows the curve to graze the ) N2('2:) moieties. asymptotic value of the dissociated C U ( ~ Pand Figure 6 shows that the relaxation of N 2 also lowers the energy of the lowest 211 curve while for the upper 211 curve (N-N relaxation) is far less significant. Notice that in Figure 6 the N-N distance relaxations are done starting from the inner minimum of Figure 4, Le., the minimum of the C U ( ~ D ) - Ninteraction, ~ but which corresponds to the highly repulsive part of the CU(~P)-N, curve. If we on the other hand try to relax the N-N distance at the outer minimum, Le., that of the 2P curve, the effect is purely repulsive. But again, even for the lowest 211 curve the N-N distance is only relaxed by about 0.1 A, and in consequence we cannot expect N 2 activation to be very efficient even when the molecule is interacting with the C U ( ~ Dstate ) (Le., the one involved in the lower 211 curve). The possible mechanism of N 2 capture by copper after the photoexcitation of the latter to its second excited state (3d1°4p1)2P can now be analyzed following the information given by Figure 2 , 4 , and 5 . Evidently the capture of N 2 by the photoexcited state 2P of Cu is quite efficient because the approach of N 2 to Cu* is attractive both for the side-on and for the end-on arrangements of the Cu-N2 system, although the attraction is markedly more notable for the end-on one. In any case this attraction would lead to a potential energy well at a Cu-N separation of a little more than 4 au. Thenceforth a nonadiabatic transition to the lower curve of equal symmetry (211for the end-on case or 2B2 for the side-on, as the case may be), which presents a shorter distance minimum via an avoided crossing with the upper curve, can occur. Once the shorter minimum (3.25 a u for the lower 211 curve) is reached, a small measure of intermolecular N 2 relaxation can take place as discussed above. It is conceivable, however, that the photoexcited 2P state of Cu decays via an allowed nonradiative transition to the first excited Cu state (3d94s2)2Dbefore it can capture N2. From Figures 2 and 4 it is then evident that the Cu 2D itself is capable of capturing N2, specially in a end-on arrangement for which the C U ( ~ D ) N 2 interaction energy is attractive from the onset with a longdistance potential well. In any case, both the 2B2 (Figure 2) and 211 (Figure 4 ) lowest state curves have a shorter distance minimum that can easily be reached after surmounting their activation barriers, due to the substantial gain of kinetic energy after decay from the upper curves. A similar description of the reaction of C U ( ~ Dwith ) the hydrogen molecule has been presented before$2 except that there the C U ( ~ D ) H2 interactions were always repulsive at the beginning, while the curve in Figure 4 shows an initial attraction for the C U ( ~ D ) N 2 211 state. This may allow the explanation that competition of N 2 is quite efficient in disallowing the H, capture when Cu is deposited in N2 rather than noble-gas matrices. A third possibility is that the photoexcited Cu(IP) state decays back to the 2S ground state of copper. Considering that the C U ( ~ S ) state does present a minimum (see Figure 5 ) when it interacts with a relaxed N-N molecule, we would then have the possibility that this minimum for the end-on CU(~S)--N=N system can be reached after a nonradiative transition that brings us down from the photoexcited C U ( ~ Pstate. ) A transition from the C U ( ~ Dstate ) would of course liberate less energy, but in any case it is a for-
+
+ +
2754
J . Phys. Chem. 1990, 94, 2154-2163
bidden transition, so it should not enter our discussion here. In fact the inability to reach the 2D state directly by photoexcitation of 2S (due to its forbidden nature) was what kept Ozinl from testing directly the capacity of C U ( ~ Dto) capture H2, for in his mind this was the best candidate for both the Cu H, and Cu N2 reactions. We, however, have shown here that all three Cu states (2S, 2P, 2D) can actively capture H2 or N 2 after the original photoexcitation of the copper atom. This has been also discussed in detail in ref 42-45. For all of these three reaction pathways for N2 capture described above the final structures eventually reached are all stable (see Figure 4 and 5 ) . The binding energy for the complex formed with copper in its 2D state is 28.49 kcal/mol (the total energy is below the asymptotic value of the isolated entities), and for the complex formed with copper in its ground state 2Sthere exists a binding energy of 15.19 kcal/mol which is also below the asymptotic value. The above comment is relevant because in the results reported by Veillard4 and Siegbahd (they studied the interaction of dinitrogen with Co, Fe, and Ni) they find minimal energy structures that often are only relative minima. In the work of Veillard4 only S C F results are reported, so high accuracy cannot be expected. These results coincide well with our SCF values, and in consequence both their and our S C F results give a somewhat poor description and do not account correctly for the details of the interactions that take place when excited states are included. The
+
+
work of Siegbahn and Blomberg5 concerns Ni and Fe and includes configuration interaction. Their results differ substantially from those of Veillard;4 for example, the states of minimal energy for the end-on interaction for this author in the cases of FeN2 and NiN2 are 5rI and 3X+, respectively, while in ref 5 they are 32and IZ+.Of course in ref 5 these minima are absolute minima, as expected from the more sophisticated method used. On the other hand, these CI results for the side-on coordination lead to stable structures that are only relative minima; that is, the binding energy for the complexes is calculated to be above the asymptotic values when the molecule and the metal moieties are isolated. Summing up, we make the following remarks. The experiments of Ozin and Mattar' inducing a (3d'04p')2P (3d'04s')zS copper atom photoexcitation on low-temperature nitrogen matrices has led to the capture of N 2 molecules by Cu atoms in an end-on structure. Our present results give a theoretical explanation of these facts, because it is the C U ( ~ Pphotoexcited ) state that presents an attractive curve toward N 2 capture. However, the Cu(,P)-N2 system shows no relaxation of the inner bonds of the nitrogen molecule. The transition to the lower states of the CuN2 system, Cu(*D)-N2, and Cu(,S)-N, may lead to more relaxed bonds on the N 2 moiety, albeit this effect is too small to really allow us to talk about nitrogen activation.
-
Registry N o . Cu,7440-50-8; N2, 7727-37-9.
Ab Initio Study of the Force Field and Vibrational Assignment of N-Acetyl-#'-meth ylalaninamide Andris Balizs Department of Atomic Physics, Eotvos L. University, Puskin utca 5-7, H - 1088 Budapest, Hungary (Received: March 13, 1989; In Final Form: July 26, 1989)
The quadratic force field of N-acetyl-N'-methylalaninamide(AMAA) has been calculated at the ab initio Hartree-Fock level by using a 4-21 basis set. On the basis of this, a thorough discussion is presented on the previously observed vibrational spectrum in argon matrix. It is shown that the matrix contains this molecule in the C7q form, which is also known to be the most stable conformer in the isolated state. Evidence for the presence of the same conformation in the solid phase is demonstrated by the vibrational spectrum. Similarly, evidence is presented for the presence of the same conformer in the solid phase of N-acetyl-N'-methylglycinamide.
Introduction The far-reaching biological consequences of the manner of folding of polypeptides are well-known. The forces responsible for the folding are operative on several levels, and various approaches have been utilized in their investigation. The local forces which are major contributors to the establishment of stable folding patterns are amenable to study by quantum chemical methods, particularly making use of the dipeptide approximation which has become common in the past two decades. In this approximation it is assumed that a molecule made up of two peptide units with terminating methyl groups is large enough to exhibit the localized interactions which play the vital role in the determination of the pattern for large peptide chains. Of particular concern is the conformation around the C-Cu bond ($) and that around the N-C" bond together with an understanding of the interactions responsible for the relative stability of alternative minima (see Figure 1). The conformation of larger polypeptide units can be investigated by molecular mechanics, but only if appropriate force constants are known to describe the local forces. Thus a detailed investigation of forces at the dipeptide level can contribute to the necessary input information for study of much larger peptide chains. (c#J),
0022-3654/90/2094-27 54$02.50/0
Many communications, both experimental (IR,l NMR2) and theoretical (with empirical potential energy functions2 and also quantum mechanical3) have appeared on the conformation of peptides. Among the most relevant for the current work are the recent full a b initio geometry optimizations by Schafer's group and on N-acety1-N'on N-acetyl-N'-methylgly~inamide~ meth~lalaninamide.~ Experimental studies of N-acetyl-N'-methylalaninamide (hereinafter referred to as AMAA), particularly infrared work in the s ~ l i d , inert ~ . ~ solvents,8 water ( R a ~ n a n ) and , ~ in argon
(!) For a review see: Chung, M . T.; Marraud, M.; NCel, J. In Conformation of Biological Molecules and Polymers; Bergman, E. D..Pullman, B., Eds.; Israel Academy of Science Hum: Jerusalem, 1973; p 69. Also: Avignon, M.; Lascombe, J. Ibid. p 97. (2) Lewis, P. N.; Momany, F. A.; Scheraga, H . A. Isr. J . Cfiem. 1973, I / , 121.
(3) (a) Pullman, B.; Pullman, A. Adu. Protein Cfiem. 1974, 28,347. (b) Scheraga, H. A. In Peptides; Goodman, M., Meinehofer, J., Eds.; Wiley: New York, 1977; p 246. (4) Schafer, L.; Van Alsenoy, C.; Scarsdale, J . N . J . Cfiem. Pfiys. 1982. 76, 1439. (5) Scarsdale, J. N.; Van Alsenoy. C.; Klimkowski, V. J.; Schafer, L.; Momany. F. A . J . Am. Cfiem. Soc. 1983, 105. 3438.
0 1990 American Chemical Society
