Potential Energy Curves and Electronic Structure of Copper Nitrides CuN and CuN+Versus CuO and CuO+

Journal of Molecular Spectroscopy 194, 8 –16 (1999) Article ID jmsp.1998.7596, available online at http://www.idealibrary.com on

Potential Energy Curves and Electronic Structure of Copper Nitrides CuN and CuN1 Versus CuO and CuO1 A. Daoudi,* A. Touimi Benjelloun,* J. P. Flament,† and G. Berthier‡ *Laboratoire de Chimie The´orique, Faculte´ des Sciences Dhar Mehraz, BP. 1796, Fe`s, Morocco; †Laboratoire de Dynamique Mole´culaire et Photonique, Universite´ USTL de Lille 1, 59 655 Villeneuve d’Ascq Cedex, France; and ‡Laboratoire de Radioastronomie Millime`trique, Ecole Normale Supe´rieure, 24 Rue Lhomond, 75 231 Paris Cedex 05, France Received September 23, 1997; in revised form April 4, 1998

Ab initio configuration interaction calculations for the diatomic CuO, CuN, CuO1, and CuN1 have been carried out, and potential energy curves are reported for several low-lying states of these systems. The electronic structure and bonding, not yet known for the neutral copper nitride and its cation, are examined and compared to those of the copper oxide systems. We find that the ground states of these systems are X2P (CuO), X3S2 (CuN), X3S2 (CuO1), and X4S2 (CuN1); their first low-lying excited states Y2S1, 1D, 1D, and 2P are located at 0.69, 1.45, 2.32, and 2.09 eV, respectively. For CuO we found two unobserved quartet states, namely 4S2 (Te 5 1.09 eV) and 4P (Te 5 1.86 eV). We report equilibrium structural parameters for various electronic states of the studied systems and compare them with experimental values, where it is possible. © 1999 Academic Press

Key Words: copper nitride; copper oxide; ab initio; CIPSI; potential curves; spectroscopic constants; electronic structure.

puted potential energy curves of the 5D, 5P, and 3S2 low-lying states; their calculations suggest that these states are quasidegenerate and weakly bound by about 0.6 eV. Several electronic states of ScN have been theoretically investigated by Kunze and Harrison (10a); they predicted the ground state to be a strongly bound triplet X1S1 state with a bond energy (De) of 2.73 eV. For ScN1 species, the calculated ground state has a X2S1 symmetry and a dissociation energy of 2.75 eV (11, 12), identical to that found for ScN (10a). A similar calculated value of 2.10 eV is also obtained for the CrN1 ion in the 3S2 state at an optimized bond length of 3.02 a0 (12). Siegbahn and Blomberg (13) have studied NiN(2P), FeN(2D), and FeN1(1S1); their optimized bond distances are 3.43, 3.15, and 2.82 a0, and their calculated dissociation energies are 1.36, 0.90, and 0.99 eV, respectively. In the complete absence of electronic and structural information on CuN and CuN1, we report the ab initio results of these nitride systems and of the oxides CuO and CuO1 for comparison. Theoretical potential energy curves and corresponding spectroscopic constants are given for each system in their low-lying electronic states.

I. INTRODUCTION

Coordinately unsaturated transition metal compounds ML or ML1, involving the interaction of metal M or M1 with simple ligand L, are of particular interest with regard to their role as potential intermediates in the homogeneous and/or heterogeneous catalysis (1). The knowledge of the electronic and geometrical structures, as well as the binding energy of such systems, is one of the most important pieces of information necessary to experiment. In the absence of experimental data, it is possible to resort to theoretical calculations. The purpose of this paper and others in this series (2– 6) is to use ab initio methods in order to provide insight into the bonding of the diatomic nitrides CuN and CuN1 and oxides CuO and CuO1. Considerable effort has been devoted theoretically and experimentally to the study of the metal oxides MO and MO1; much less attention has been focused on the metal nitrides MN and MN1. Experimentally, some neutral nitrides MN involving the left transition-metal in periodical classification, such as ScN (7a), TiN (7b), and VN (7c), have been observed and characterized, whereas, for CuN, to our knowledge, no experimental studies have been reported. The copper nitride CuN1 and copper oxide CuO1 ions have been recently generated in gas phase experiments by Su¨lzle and co-workers (8) using neutralization–reionization mass spectrometry. These experiments provide the only information now available for these diatomic molecules; their electronic structures and molecular properties are unknown. Theoretically, there are several studies devoted to predicting the electronic structure of MN and MN1 species (9 –14). For the neutral ScN molecule, Jeung and Koutecky (9) have com-

II. METHODS

The valence molecular SCF orbitals are obtained with the PSHONDO program (15), which includes the pseudopotentials of Durand and Barthelat (16) implemented in the HONDO package (17). The pseudopotential parameters utilized for Cu (18), N (19), and O (19) were the same as in Ref. (6). The symmetry and spin-adapted open-shell calculations are done with RHF (restricted Hartree Fock) using the exponential op8

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FIG. 1. CIPSI variation of the Fr force constant of NO with the d-exponent values of the oxygen and the nitrogen.

timization technique (20) incorporated in a recent PSHONDO version of Flament (15). The configuration interaction (CI) used in the present study is the CIPSI method (configuration interaction by perturbation of a multiconfiguration wavefunction selected iteratively) with configuration selection according to Huron and Malrieu (21). This method is based on a variational subspace S(0) and complementary perturbational subspace S(1). The variational zeroth-order wavefunction is constructed in an iterative way. The determinants having a first-order coefficient larger than the threshold h are included in the subspace S(0) for the next step of the calculation. The determinants forming this subspace are generators from which the wavefunction is perturbed to the first-order. The energy is treated through Rayleigh–Schro¨dinger expansion up to the second-order. The Epstein–Nesbet partition is chosen in this work to select the main determinants and to calculate the electronic energy of studied systems. Starting from the 37 occupied and virtual molecular orbitals, the threshold value of 0.04 lead to wavefunctions developed upon 30 –200 determinants for the variational subspace and 450 000 –2 700 000 for the perturbational subspace. The norm of the first-order correction to the wavefunction is typically ;0.08 in the equilibrium region. The population analysis calculated from the CIPSI wavefunction is obtained by means of the ONEL program (22). The dissociation energies (De) have been computed with respect to the asymptotic system obtained when the atoms are separated by a large distance of 10 a0. The excitation energies (Te) and ionization potentials (IP) computed in this work are adiabatic, that is, minimum to minimum using the optimum geometries found in each species. The equilibrium bond lengths (Re) and force constants (FR) have been evaluated by computing three points on the potential energy curve near the calculated equilibrium distance and fitting these points to a parabola. For the calculations of the stretching frequencies (ne), we have selected

the masses of the most abundant isotopes: 63Cu (M 5 63.5463), 14N (M 5 14.0067), and 16O (M 5 15.9994). For Cu we used a Gaussian basis analogous to that reported previously in Ref. (6); the (3s3p6d) primitive Gaussian basis is contracted in triple-zeta form for the d orbitals, double-zeta for the p orbitals, and uncontracted for the s orbitals. For N and O, the (4s4p) Gaussian basis (19) is contracted in double-zeta form for s and p and enlarged by d polarization functions. The best d-exponents (zd 5 0.80 for N and zd 5 1.175 for O) were calculated at the CIPSI level from a new proposed process (32, 33). It is well known that the optimum exponents of d functions depend on a particular molecule or on the ‘‘chemical environment’’ of the atoms as well as on the employed basis set, particularly in pseudopotential methods. Our process has recently been proposed for the first elements (C, N, O, . . .) in order to reproduce well the experimental spectroscopic constants, essentially the force constant of the diatomic molecules (CO, CN, NO, . . .) (32, 33). In case of the NO radical, this process consists of the following steps: (i) In a first step, the optimizations of the zd(N) and zd(O) in the NO molecule (with r bond length fixed at the experimental value of 2.175 a0) lead to the following values: zd(N) 5 0.80 and zd(O) 5 1.24. Starting with these values and with three points on the potential curve E(r), the CIPSI calculation of the force constant Fr gives 20.81 mdyn/Å which in this step is larger than the experimental value (15.95 mdyn/Å). (ii) In a second step and in order to improve the Fr value, the zd(N) is fixed at 0.80 and the zd(O) is varied with the chosen values of 1.10, 1.15, 1.20, and 1.25. The force constant Fr is calculated at each value of the exponent, and the results are represented in Fig. 1a. We see from this figure that the variation of Fr with respect to the zd(O) exponent is found to be linear in nature:

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III. RESULTS AND DISCUSSION

1. CuN and CuN1

FIG. 2. CuN.

CIPSI potential energy curves for the ground and excited states of

F r 5 20.51 2 3.88 z d ~O! . This linear behavior allows us to obtain the appropriate exponent value by simple interpolation of the experimental force constant, i.e., to Fr 5 15.95 mdyn/Å corresponds zd(O) 5 1.175. (iii) In a third step, we start with the before extrapolated value of the oxygen exponent (zd(O) 5 1.175) and calculate the Fr at different values of zd(N) 5 0.70, 0.75, 0.85, and 0.90. As shown in Fig. 1b, the Fr variation with respect to zd(N) is also linear in nature:

To classify and investigate electronic configurations, we started with the model of atomic occupation of molecular orbitals. In this model, the molecular orbitals are assumed to have a certain atomic character. For CuN the valence configuration describing the interaction of Cu(2S) and N(4S) is 4 2s2(N)2p1s(N)2pp2 (N)3d2s(Cu)3dp (Cu)3d4d(Cu)4s1(Cu). The formation of the chemical bond is possible between 2ps(N) and 4s(Cu) by singlet coupled pairs of electrons or by charge transfers; our CIPSI calculations favor this latter possibility. The resulting molecular state is possibly a triplet 3S2 or a quintet 5S. For performing the further CI calculations, we have chosen the 5S2 state, arising from the lowest asymptote Cu(2S) 1 N(4S), as SCF reference having the molecular configuration (1s)2(2s)2(1p)4(1d)4(2p)2(3s)1(4s)1, where the dominant characters of these orbitals, indicated in parentheses, are 1s(2sN), 2s(3dCu), 1p(3dCu), 1d(3dCu), 3s(2pN), 2p(2pN), and 4s(4sCu). For CuN1, the 4S2 state was chosen as SCF reference; it is obtained by removing one electron from the 4s orbital in CuN(5S2) and arises from the fundamental asymptote Cu1(1S) 1 N(4S). The result of CI calculations of the potential energy curves of the lowest states of CuN are shown in Fig. 2. The minimum energies (Ee), dissociation energies (De), adiabatic excitation energies (Te), bond lengths (Re), and vibrational frequencies (ve) are given in Table 1. Included in Table 2 are the valence orbital populations calculated at the equilibrium geometry from the natural orbitals of the multiconfigurational reference function of CIPSI. From Fig. 2, we see that the ground state of the CuN molecule is a triplet X3S2 arising from the Cu(2S) 1 N(4S) TABLE 1 Structural Data for CuN and CuN1 in the Lowest Electronic States

F r 5 18.13 2 2.72 z d ~N! . The extrapolated experimental value of Fr leads to a zd(N) 5 0.80. We remark that the zd(N) 5 0.80 value obtained by this process is identical to that recommended by Dunning and Hay (34a). In contrast, the value zd(O) 5 1.175 is intermediate between 1.30 of Ross and Siegbahn (34b) and 0.85 of Dunning and Hay (34a). Finally, with the obtained value zd(N) 5 0.80 and zd(O) 5 1.175, the CIPSI calculations give the excellent values of the spectroscopic constants of NO: the force constant is 15.949 mdyn/Å (versus 15.950 mdyn/Å), the bond length is 1.162 Å (versus 1.151 Å), and bond energy is 6.20 eV (versus 6.50 eV). Copyright © 1999 by Academic Press

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TABLE 2 Mulliken Populations* Obtained from Natural Orbitals of CuN and CuN1

For CuN1(X4S2), we obtain a bond length increased to 4.05 a0 and a dissociation energy decreased to 0.34 eV with respect to the CuN(X3S2). The longer Cu–N bond length and weaker bond energy in CuN1 is a consequence of the electrostatic character of the chemical bond, i.e., resulting from the interaction between closed shell 3d10 of charged Cu1 with open shell 2p3 of neutral N. The strength of the chemical bond in CuN is interpreted by a dominated covalent character resulting of the charge transfers from 4s of Cu to 2ps of N. This description is supported by the Mulliken population analysis given in Table 2. The charge distribution calculated for CuN(X3S2) shows that the Cu–N interaction has a pure s type. The occupancies of the 3dp(Cu) and 2pp(N) orbitals are almost unchanged with respect to the free atomic states. As a consequence, the formation of a chemical bond is interpreted in terms of a strong s interaction and a very weak p interaction. The weights (square of the coefficients summed over the configuration state functions) of the most important configurations participating in the CI wavefunction of the X3S2 and 1D states are (see Table 7 for other states) as follows:

X 3 S 2 5 68% ~3 s ! 1 ~2 p ! 2 ~4 s ! 1 1 25% ~3 s ! 2 ~2 p ! 2 1 2% ~2 p ! 2 ~4 s ! 2

* From multireference zeroth-order CIPSI wavefunctions.

fundamental asymptote. The 1D first excited state of CuN arising from the Cu(2S) 1 N(2D) asymptote lies above the ground state at 1.45 eV with a De 5 2.47 eV, Re 5 3.29 a0, and ve 5 769 cm21 (see Table 1). The optimized bond length at 3.43 a0 and the calculated dissociation energy at 1.20 eV for CuN(X3S2) are very close to the values of Re 5 3.43 a0 and De 5 1.36 eV calculated for NiN(2P) by Siegbahn and Blomberg (13). For ScN, the calculated dissociation energy at 2.73eV (10a) shows that this nitride is more strongly bonded than CuN. Figure 3 shows the CI potential energy curves of six lowest electronic states of the CuN1 ion. The ground state of CuN1 is found to be a quartet X4S2 arising from the Cu1(1S) 1 N(4S) lowest asymptote. The doublet states 2S1, 2S2, 2P, and 2D are found to be higher in energy and arising from the excited asymptotes Cu1(1S) 1 N(2D), Cu1(3D) 1 N(4S), and Cu1(1S) 1 N(2P). We note that the high-spin character of the ground state (X4S2) is preferred in the CuN1 ion (cf. Fig. 3), whereas the CuN molecule (cf. Fig. 1) exhibits an intermediate nature (X3S2).

FIG. 3. CuN1.

CIPSI potential energy curves for the ground and excited states of

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TABLE 3 Electronic Configuration and Structures Composing the CI Wavefunction for CuNa

1

D 5 61% ~3s2 ~2p2x 2 2p2y ! 1 29% 3s1 ~2p2x 2 2p2y !4s1 1 2% 3s1 ~2p2x 2 2p2y !3p1 .

The bonding of CuN in the X3S2 ground state involves a strong covalent component (68%) arising from Cu(2S) 1 N(4S), whereas in the 1D first excited state this is predominantly ionic (61%) arising from Cu1(1S) 1 N2(1D). As shown in Table 3, the X3S2 ground state involves a mixture of two structures: z 2px 4s z Cu–N z 2p s z 2py

z 2px Cu2N z 2py

· · · .~3s !1 ~2px !1 ~2py !1 ~4s !1

· · · .~3s !2 ~2px !1 ~2py !1 .

The dominant first structure is best described as arising out of Cu(3d104s1) and N(2pp1 x2pp1 y2p1s), and the bonding is consistent with a delocalized s Cu 3 N. The second structure involves a s bond formed by singlet coupled pairs of electron between 4s(Cu) and 2ps(N). The natural orbital analysis shows that the global structure in X3S2 is 2px z N Cu 3 z 2py and the Cu–N interaction is of s Cu 3 N type.

The total population of the 3d(Cu) and 4sp(Cu) orbitals in the ground state are 9.92 and 0.81, respectively, showing that about 0.27 electrons are transferred from Cu to N. The ionicity of Cu10.27 N20.27 is very similar to that of Ni10.30 N20.30 (13). In the first excited state the ionicity is Cu10.65 N20.65. For CuN1(X4S2), the total population of the 3d(Cu) orbitals is almost 10 and that of 4sp(Cu) is 0.09. The charge of 0.09 electron is transferred from N to Cu1, in particular to the 4sp vacant shell of Cu1. The electrostatic nature of the interaction between Cu1 and N is dominating in this ion. From Table 1, the CI results show that the Cu–N stretching frequencies (ve) are 614 cm21 for CuN (X3S2) and 144 cm21 for CuN1(X4S2), whereas in CuNO1(X2P) (6) the corresponding frequency is found at 484 cm21 using the same method of calculation. We have also calculated the CI adiabatic ionization potential (IP) for CuN at 7.86 eV; the experimental IP for this molecule has not yet been measured. However, we note that this value is very close to the IP computed here at 7.33 eV and measured at 7.72 eV (23) for the Cu atom; this is a consequence of the extraction of the electron from the 4s orbital of CuN having the predominantly 4s(Cu) character. For the N and O atoms, the IP calculated at 14.17 and 12.88 eV, respectively, are in reasonable agreement with the corresponding experimental values of 14.54 and 13.61 eV (23). Since this parameter is about 5% smaller than experimental values, we expect the same percentage error for CuN and thus estimate the experimental value for this molecule to be IP ; 8.25 eV. 2. CuO and CuO1 To investigate the nature of the ground and first excited states of CuO and CuO1, we have performed CIPSI calculations of potential energy curves for several states. The electronic spectrum of CuO is well known (23, 24), whereas for CuO1, to our knowledge, no experimental and theoretical studies have been reported. Experimentally and theoretically, the transition energy between the ground state and the first excited state of CuO is not definitively established. Huber and Herzberg (23) have reported A2S1 lying at 2.04 eV above the X2P ground state as being the first excited state of CuO. Lefebvre et al. (24b) have observed a new first excited state at 0.98 eV, which they denote as Y2S1; these authors also report the results of the X2P–Y2S1 separation energy (Te) at 0.76 eV obtained from SCF 1 CI calculations. Basch and Osman (25), using a semiempirical effective core potential (ECP), have performed MCSCF 1 CI calculations for the lowest 2S1, 2P, and 2D states of CuO. They find that the term energy Te of the 2S1 state is 1.88 eV with respect to the X2P ground state. The SDCI (single and double configuration interaction) and CPF (coupled pair functional) calculations performed by Bauschlicher and co-workers (26, 28) found that the term

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energy Te of 2S1 is 1.05 and 1.08, eV, respectively. The SDCI wavefunction calculations (28) also show that the bonding in X2P and A2S1 is predominantly ionic and arises from Cu1(3d10) and O2(2p5); whereas the CPF wavefunctions (28) indicate that this bonding is a mixture of both ionic Cu1(3d10) 1 O2 (2p5) and covalent Cu(3d10s1) 1 O(2p4) components. The Mulliken population analysis shows a reduction of the net charge on O from 0.7 at the SDCI level to 0.5 at the CPF level (28). Experimentally, a number of doublet states X2P, Y2S1, b2D, 2 g P, and C2P are observed (23, 24a), but no quartet state has been identified or analyzed. The primary purpose of the present work is to calculate a number of doublet and quartet states in order to identify their position relative to the X2P ground state of CuO. To achieve a comparably accurate description at the CI level of the large number of states, the choice of the SCF molecular orbitals (MO) is very important (30). It was found that the SCF MO basis optimized for the ground state, particularly for predominantly ionic systems such as CuO and CuH (31), led to a slowly converging CI expansion requiring a large number of configurations. The use of this MO basis also produces a poor description of the excited states. On the other hand, the MO basis optimized for the excited state corresponding formally to a good dissociation into neutral asymptotic products was found to be a very good starting point for the description of all the states of interest. Therefore, we chose a 4S2 state, which dissociates properly into the ground state neutral atoms Cu(2S) 1 O(3P), as a SCF reference for the CI description of the doublet and quartet low-lying states of CuO; the corresponding configuration with the dominant characters of the orbitals in parentheses is written 1s2(2sO)2s2(3dCu)1p4(3dCu)1d4(3dCu)3s2(2pO)2p2(2pO)4s1(4sCu). In Fig. 4, we report the calculated CIPSI potential energy curves of the various electronic states of CuO. The deduced spectroscopic constants are collected in Table 4 and compared as far as possible with the experimental findings. From these results, we note that for most of the states the potential energy curves are complicated by various avoided crossings which take place between covalent states and/or ionic states of CuO. The methodology adopted in this work for describing the avoided crossing states consists of the inclusion in the zerothorder CIPSI wavefunctions of all important configurations selected from the equilibrium point, and the crossing region, as well as from the asymptotic region. The equilibrium bond lengths (Re) and dissociation energies (De), as well as the transition energies (Te), agree quite well with experimental values of observed states X2P, Y2S1, b2D, g2P, and C2P (see Table 4). The bond length Re of the X2P ground state is found to be 3.24 a0 very close to the experimental value of 3.26 a0 (23, 24a). The Y2S1 state is found also with the bond length of 3.26 a0; experimentally, Re for Y2S1 is not known. The Y2S1 4 X2P transition energy is computed at 0.69 eV by our CIPSI calculations. This calculated transition energy is rather close to the values of 0.98 and 0.97 eV

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FIG. 4. CIPSI potential energy curves for the lowest states of CuO.

observed, respectively, by Lefebvre et al. (24b) and by Appelblad (24a), but very far from the value of 2.04 eV reported in tables of Huber and Herzberg (23). We mention that the difference between our transition energy and the corresponding experimental value may be due to the omission of f-function from the Cu basis set and also to the lack of core–valence correlation in the present calculations. As indicated in Fig. 4, we see that the X2P, 2S2, 4P, and 4 2 S states dissociate into the same asymptote Cu(2S) 1 O(3P), whereas the Y2S1, b2D, and g2P states dissociate into the Cu(2D) 1 O(3P) asymptote. The dissociation energy (De) of X2P was computed to be 2.83 eV, in excellent agreement with the experimental value (Table 4). For Y2S1 the dissociation energy is 4.52 eV with respect to the lowest state asymptote Cu(2D) 1 O(3P). Experimentally, De for Y2S1 is not known. Therefore, we may use the observed excitation energies of Cu(2S–2D) and of CuO (X2P–Y2S1), as well as the experimental dissociation energy of CuO(X2P), for estimating an experimental De of Y2S1. We obtain a value of 3.37 eV, below the CIPSI value of 4.52 eV. From the same considerations, we can also suggest experimental values. Our CIPSI results exhibit the new quartet states 4S2 and 4P (Fig. 4) calculated at 1.09 and 1.86 eV above the X2P ground state, respectively, but not observed in the experimental electronic spectrum. An interesting feature of the 4S2 and 4P potential curves is the potential maximum that occurs near 4.0 and 3.7 a0, respectively, on the internuclear distance R; their

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X 2P : ~1 s ! 2 ~2 s ! 2 ~1 p ! 4 ~1 d ! 4 ~3 s ! 2~2 p ! 3

TABLE 4 Structural Data for CuO in the Lowest Electronic States

Y 2 S 1 : ~1 s ! 2 ~2 s ! 2 ~1 p ! 4 ~1 d ! 4 ~3 s ! 1 ~2 p ! 4 4

S 2 : ~1 s ! 2 ~2 s ! 2 ~1 p ! 4 ~1 d ! 4 ~3 s ! 2 ~2 p ! 2 ~4 s ! 1 4

P : ~1 s ! 2 ~2 s ! 2 ~1 p ! 4 ~1 d ! 4 ~3 s ! 1 ~2 p ! 3 ~4 s ! 1 .

Examination of the CIPSI wavefunctions at equilibrium distances shows that these states are composed primarily of the following components: X 2 P 5 74% ~3 s ! 2 ~2 p ! 3 1 14% ~3 s ! 2 ~2 p ! 3 ~4 s ! 1 1 6% ~3 s ! 2 ~2 p ! 2 ~3 p ! 1 Y 2 S 1 5 79% ~3 s ! 1 ~2 p ! 4 1 13% ~3 s ! 1 ~2 p ! 3 ~3 p ! 1 1 3% ~3 s ! 2 ~2 p ! 4 ~4 s ! 1 S 2 5 76% ~3 s ! 2 ~2 p ! 2 ~4 s ! 1

4

1 17% ~3 s ! 2 ~1 p x ! 1 ~1 p y ! 1 @~1 p y ! 1 ~2 p x ! 1 2 ~1 p x ! 1 ~2 p y ! 1 #~2 p x ! 1 ~2 p y ! 1 ~4 s ! 1 a

Relative to separated atoms. Ref. (24a); c Ref. (24b). d Proposed experimental values (cf. text). e Calculated values relative to maximum of potential curve (cf. Figure 3).

P 5 83% ~3 s ! 1~2 p ! 3 ~4 s ! 1

4

b

minima are found at 3.24 and 3.02 a0, respectively. Defining the potential barrier as the energy difference between minimum and maximum, we obtain a value of 0.09 eV (710 cm21) for the 4 2 S state and 0.52 eV (4180 cm21) for the 4P state. These energy barriers, located in the infrared region, are necessary for dissociating these states into the ground state asymptote Cu(2S) 1 O(3P). Thus, the location of the maximum at a distance R near the equilibrium bond length Re of the X2P and with small energy above the Cu(2S) 1 O(3P) dissociation limit suggests that 4S2 is probably a predissociative state. The Cu–O stretching frequencies (ve) for the X2P state is obtained at 565 cm21; the corresponding experimental values is 640 cm21 (23). For the Y2S1, 4S2, and 4P states the stretching frequencies are calculated at 491, 582, and 787 cm21, respectively; the experimental ve for these states have not yet been measured. The Mulliken population analysis obtained from natural orbitals is given in Table 4 for CuO in several low-lying electronic states. The unpaired electron in the X2P and in Y2S1 states is primarily O2pp and O2ps, respectively. In the 4S2 state, found as being the second excited state of CuO in this work, the unpaired three electrons are primarily O2ppx, O2ppy, and Cu4s in character. Their predominant configurations are found to be

1 13% ~2 s ! 1 ~3 s ! 2 ~2 p ! 3 ~4 s ! 1 . The bonding of CuO in the X2P and Y2S1 states predicted by our calculations involves a strong ionic component, whereas in the 4S2 and 4P excited states this is predominantly covalent. The population analysis shows that the X2P ground state is somewhat polar in the Cu10.59O20.59 sense with a large s charge transfer of 0.59 e from Cu to O; this polarity is more pronounced in the Y2S1 first excited state with a charge transfer of 0.74 e (Table 5). Our results are in good agreement with the analysis of the CPF wavefunction of Langhoff and

TABLE 5 Mulliken Populations* Obtained from Natural Orbitals of CuO

* From multireference zeroth-order CIPSI wavefunctions.

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TABLE 7 Mulliken Populations* Obtained from Natural Orbitals of CuO1

* From multireference zeroth-order CIPSI wavefunctions.

FIG. 5. CuO1.

CIPSI potential energy curves for the ground and excited states of

Bauschlicher (28). The bonding in the 4S2 excited state is viewed as arising from two processes: s Cu 4 O donation of 0.13 e from 2sps(O) to 4sps(Cu), and p Cu 3 O backdonation of 0.18 e from 3dp(Cu) to 2pp(O). The potential energy curves of five lowest electronic states of the CuO1 ion are represented in Fig. 5, and the corresponding structural data are collected in Table 6. The triplet 3S2 and 3 P states arising from the Cu1(1S) 1 O(3P) lowest asymptote are found to be more stable than the singlet 1D, 1P, and 1S1 states arising from the Cu1(1S) 1 O(1D) lowest excited asymptote. As seen from Fig. 4 and Table 6, the ground state of CuO1 is found to be 3S2 with an equilibrium bond length Re

TABLE 6 Structural Data for CuO1 in the Lowest Electronic States

a b

Relative to CuO1 at RCuO1 5 10.0 a.u. Calculated value relative to maximum of potential curve (cf. Figure 4).

5 4.03 a0 and a dissociation energy De 5 0.56 eV. These values are very close to the corresponding parameters computed for the CuON1(2P) ion (6) at Re 5 4.03 a0 and De 5 0.59 eV. This similarity is also reproduced in the CuN1 ion with Re 5 4.05 a0 and De 5 0.34 eV (Table 1); CuN1 appear more weakly bound than CuO1 but have similar a bond length. We note that the high-spin nature of the ground state (X3S2) is preferred in the CuO1 ion, whereas the low-spin nature (X2P) is preferred in the CuO molecule (cf. Fig. 5). The Mulliken population analysis of CuO1 (Table 7) shows that the two unpaired electrons in the X3S2 ground state are predominantly O2ppx and O2ppy characters. The electronic distribution of X3S2 on Cu is 3d10.00 4s0.06 4p0.05 and on O is 2s1.98 2p3.91, showing that about 0.11 e are transferred from O to Cu1. IV. CONCLUSION

We have found, from CIPSI calculations, the ground states for CuO, CuN, CuO1, and CuN1 to be X2P, X3S2, X3S2, and X4S2, respectively, with dissociation energies equal to 2.83, 1.20, 0.56, and 0.34 eV. Their respective first low-lying excited states Y2S1, 1D, 1D, and 2P are bound by 4.52, 2.47, 0.54, and 0.91 eV, respectively. They are located above the corresponding ground states by 0.69, 1.45, 2.32, and 2.09 eV. Though the CuN and CuO1 isoelectronic systems possess the same ground states (X3S2) and the same first excited states (1D), they have rather different structural and bonding characteristics. The bonding in CuO is somewhat analogous to the bonding in CuN, but the high electronegativity of oxygen leads to a more ionic system than CuN. The s bond between Cu and O or N involves the 4sps on Cu and the 2sps on O or N. The extent of electron transfer in the s systems is consistent with the electronegativity of the ligand, being larger for O and smaller

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16

DAOUDI ET AL.

for N. These calculations suggest also that we have an excess of 0.58 electrons on O and 0.27 electrons on N. Moreover, the computed dissociation energies (De) for the ground state of CuN is consistently smaller than that for CuO. Since the CuO and CuN molecules have formal single bonds, this trend in the De’s reflects the importance of the ionic component in the bond. The stability of both CuO1 and CuN1 ions is smaller than the corresponding CuO and CuN neutral molecules. The large internuclear distance and small dissociation energies appears to be a common feature of these ions. From these calculations we remark that the intermediate spin states in the copper nitride are preferred to the high or low spin states. In contrast, the low spin states in the copper oxide are preferred. In conclusion, it appears that the number of open shells in the ground state X3S2 of CuN (four decoupled electrons) and in X4S2 of CuN1 (three decoupled electrons) suggests that these nitrides are highly reactive in the gas phase. Accordingly, compounds resulting from the accretion of a third atom or ligand, for instance the CuNO or CuNO1 molecules (6), are more stable systems.

12. 13. 14. 15.

16. 17. 18. 19.

20. 21. 22.

23. 24.

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