A theoretical study of the CuOH molecule

Volume 18.5, number 5,6

CHEMICAL PHYSICS LETTERS

25 October 1991

A theoretical study of the CuOH molecule Yuji Mochizuki ‘, Toshikazu Takada Department Exploratory Research, NEC Tsukuba Research Laboratory, Miyukigaoka 34, Tsukuba 305, Japan

qf

and Akinori Murakami Research Cenrer, Mitsubishi Kasei Corporation. Kamoshida-cho 1000, Mldoriku, Yokohama 227, Japan Received 22 May 199 1; in final form I7 June 199 1

.4b initio configuration-interaction calculations were carried out on the copper hydroxide (CuOH) molecule in its ground and excited states. The characteristics of these states are discussed in terms of the excitation energies and the one-electron properties. Electron correlation is found to be an important factor in the energetics of this molecule.

I. Introduction From a theoretical point of view, the importance of the electron correlation effect is well known, especially for transition metal (TM) compounds [ 11. The highly localized nature of the d orbitals is responsible for the significant level of correlation in TM compounds. The correlation energy of systems containing TM atom(s) depends strongly upon the population of d-electron shells. Smaller d-shell populations lead to smaller correlation energies. Thus, the theoretical energetics of TM compounds require the proper consideration of electron correlation in cases that the d-electron population changes. The extensive calculation on CuF, by Bauschlicher and Roos [ 21 has given a good example of this. Systems containing Cu atom(s) have been studied extensively by spectroscopy. Many of these systems show visible absorption or emission [ 31. It is well known that the 3d electrons of Cu are mainly responsible for visible transitions. LMCT (ligand-tometal charge transfer) transitions have also been observed in the ultraviolet region for such complexes. If the system contains direct Cu-0 bonds, the 0 2p electrons might contribute to the excitation due to I To whom correspondence should be addressed. 0009-2614/91/$

the near degeneracy of 3d and 2p levels. This situation has been demonstrated by an ab initio CI (configuration-interaction) study on the copper dioxide (CuOZ) molecule [4]. The green emission from flames containing copper salts is known to be due to the CuOH molecule [ 51. Trkula and Harris reported the high-resolution laser spectra of the green-colored perpendicular transition X ‘A’-‘A” (I) of this molecule under the C, molecular symmetry, where the transition energy is 2.28 eV [6]. They noted that the transition might be caused by a 3d-+4s excitation within the Cu atom. Very recently, Jarman et al. carried out a similar spectroscopic study of the same system [ 71. They observed additional transitions at 1.97 and 2.36 eV [ 7,8]. The molecular structures of CuOH were predicted for the X ‘A’ and ‘A” (I) states by rotational analyses [6,7]. The binding energy of the Cu+ OH-+CuOH process was determined experimentally to be 2.69 If 0.13 eV by Belyaer [ 9 1. Only two ab initio studies of the CuOH molecule have been published to date. Illas et al. optimized the ground-state geometry using a RHF (restricted Hartree-Fock) calculation, and evaluated the binding energy by a configuration interaction (CI) procedure at RHF optimized geometry [ lo]. Klimenko et al. carried out a similar RHF calculation [ 111. However, it has

03.50 0 1991 Elsevier Science Publishers B.V. All rights reserved.

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been suggested that electron correlation affects the molecular properties for such a TM compound [ 11. The characteristics of the excited states have been discussed by the authors of refs. [ 6-81, but have not yet been analyzed theoretically. Furthermore, the dipole moment and the vibrational frequency of the system are unknown. It is, therefore, desirable to investigate the CuOH molecule for both ground and excited states by the use of quantum-chemical calculations, including electron correlation. This paper reports ab initio CI calculations on the CuOH molecule. The geometry and the vibrational frequency are determined by full second-order polynomials of the three internal coordinates for the X ‘A’ and ‘A” (I) states. The binding energy and the excitation energy are evaluated to demonstrate the important effect of electron correlation on these quantities. The one-electron properties: dipole moment, transition moment, and electron population, are calculated to illuminate the nature of the states. Mixing of the Cu 3d and 0 2p orbitals in the excited states is discussed.

2. Rlethod of calculation 2.1. Basis set A contracted Gaussian basis set is adopted in the calculations. The basis functions [ 6s5p3dlf] and [ 4s3pld] are used for the Cu atom and the 0 atom, respectively, as in the previous study by Mochizuki et al. [4] of the CuO, molecule. It is noted that in these sets, the Cu 3s3p and 0 1s core functions are split into two to describe the relaxation properly. An extra diffuse s-type function is added for each atom. The exponents are 0.0 13 for Cu and 0.090 for 0. For the H atom, the 4s primitive basis set of van Duijneveldt [ 121 is contracted to a 3s set in the standard manner, and a set of p-type polarization functions (exponent 1.O) is added. The total number of basis functions used is 69. 2.2. Geometry optimizarion and vibrational ana1y.Q The molecular structure of CuOH is assumed to belong to the point group C, with the three internal 536

25 October 1991

center

Fig. 1. Molecular structure of CuOH. Center of moment on Cu atom.

coordinates, R ( Cu-0)) R (O-H), and 0( Cu-O-H) as determined by experiments [ 6-8 1. The structure is shown in fig. 1. The CuOH molecule lies on the X-Y plane, where the Cu-0 bond is directed along the X axis. Geometry optimizations are carried out for the ‘A’ ground state and the perpendicular excited state ‘A” (I), which would be the origin of the green transition. The ten-termed full second-order polynomials of the internal coordinates are used to determine the optimal geometries. The ten coeflicients of the polynomials are obtained by the ieastsquares fitting from the CI energies of the 27 grid points. The detailed CI scheme is discussed in section 2.3. The grid, points result from all possible combinations of R(Cu-0): 1.68, 1.78 and 1.92 A; R(O-H): 0.90, 0.95 and 1.00 A, and .9(Cu-O-H): 94 >, 114” and 134’. The vibrational frequencies are evaluated by diagonalizing the socalled GF matrix [ 13 1, where the atomic weights in the G matrix are as follows: 63.544 amu for Cu, 15.999 amu for 0, and 1.008 amu for H.

2.3. CI calculation for geometry optimization An SDCI (singly and doubly excited CI) calculation with two reference configurations is used to obtain the total energies for geometry optimizations. Hereafter, the above calculation is denoted as SDCI (2 ), The initial molecular orbitals ( MOs)are prepared by RHF calculations for the X ‘A’ and ‘A” (I) states, since RHF electronic configurations are dominant with the weights greater than 90% in both these SDCI(2) wavefunctions for all geome-

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CHEMICAL PHYSICS LETTERS

tries scanned. The RHF configuration of X ‘A’ is the closed-shell type, X ‘A’...(9a’-14a’)‘2(3a”-5a”)6, and that of the singly excited ‘A” (I) open shells, ‘A”(I)...Sa”+lSa’

,

(1) state

has two

(2)

where the frozen-core parts of the Cu ls-3p and 0 Is orbitals are not shown. The 18 valence electrons of the Cu 3d4s, 0 2s2p and H 1s shells are correlated in the CI wavefunction. The highest a’ orbital (49a’ ), which represents the out-of-phase mixing of the two 0 Is basis functions, is discarded. For convenience, the 3d-type orbitals are classified symbolically by their symmetries along the Cu-0 (or X) axis, as follows: 3do(a’), 3dlc(a’, a”): and 3d6(a’, a”). The bonding in the X ‘A’ state of CuOH would include both ionic CU+(~~“)-OH-(~(OH)~~~~C~) and covalent Cu( 3d”‘4s’)-OH(o(OH)*2pn3) components in the molecular plane (a’ symmetry). The RHF closed-shell configuration of this state is dominated by ionic bonding. However, it is known that RHF wavefunctions tend to emphasize the ionic character. The low-lying configurations of the Cu atom, 3d94s’, 3d’O4p’, ... also contribute to the covalency through correlation. For an ionic CuF molecule which is characterized by Cu+ (3d”)-F-, the Cu+ ( 3d94s’)-F--type configuration with a 3do hole makes a considerable contribution to the CI wavefunction [ 141. Thus, the Cu+ ( 3d94s’)-OH--type configuration is chosen as the second reference function. Since the RHF canonical orbitals of CuOH do not have the appropriate character to describe this configuration, the canonical MO set is modified using the corresponding orbital technique [ 151. The RHF orbitals of a polarized Cu+ ion (3d94s’) are separately prepared, setting a point charge of -0.4 on the 3do axis at a distance of 5 au from the Cu atom. A 3do-type MO is projected out from the occupied valence MO space and a 4s-type MO is generated from a linear combination of the unoccupied MOs by maximizing the overlap with the corresponding MO of a polarized Cu+ ion for each unitary transformation. The RHF configuration of the ‘A” (I) excited state is roughly characterized by the 3dlc( a” ) +4s( a’) excitation within the copper ion. From an RHF single-

25 October 1991

reference SDCl test calculation, the 0-2px( a” ) -4s excited configuration, which is of LMCT type, is found to be an important component in the resultant CI wavefunction with the weight of about 2%. Thus, this configuration is chosen as the second reference. The singly and doubly excited CSFs (configuration state functions) are generated from the two reference CSFs. In the CSF generation, the restriction of the first-order interacting space [ 161 is adopted. The expansion lengths of the SDCI(2) wavefunctions are 98605 for X ‘A’ and 117639 for ‘A”(I), but these are too large to store the whole matrix elements on the available peripheral storage. The second-order perturbation theory (B, [ 171) is, therefore, applied to select the important doubly excited CSFs. An energy threshold of - 5 x 10m6 au is used upon selecting individual CSFs. It is well known that the use of K orbitals [ 181 accelerates the convergence of the CI expansion in case the perturbation selection is applied. Thus, the K orbitals are used as the correlating MOs. The dimensions to be solved variationally are reduced to about 5500-7000 for both states. The effect of multiple excitations is estimated from the variational solution using the Davidson formula [ 19 1. Finally, the energy lowering due to rejected CSFs is evaluated by a second-order perturbational technique [ 4,201. 2.4. Cl calculation for the transition moment An MRSDCI (multi-reference SDCI) calculation is performed to obtain the transition moments at the optimized geometry of X ‘A’. The orbitals are prepared by an SACASSCF (state-average complete-active-space self-consistent-field) method. The ground state and seven excited states are treated simultaneously in the SACASSCF/MRSDCI calculation. It is noted that the lowest symmetry (C, ) is used due to program limitations. The active space of SACASSCF consists of 1Oa’15a’ and 3a”-5a” orbitals, where the 15a’ orbital is of the Cu 4s type and is unoccupied in the ground state. The CASCI dimension is 45 due to the problem of distributing 16 electrons over more than nine orbitals. The weights in the state average are 7 for the ground state and 1 for each of the seven excited states. When convergence is obtained, the inactive and secondary orbitals are transformed within their 537

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CHEMICALPHYSICSLETTERS

Volume 185,number 5,6

respective spaces. The inactive MO space is rotated to diagonalize the so-called average Fock operator [ 2 11. The secondary MOs are modified to K orbitals. All CSFs whose weights are greater than 0.5% in the SACASSCF wavefunctions are used as reference CSFs for MRSDCI. This gives a total of 17 reference CSFs. The internal MO set is composed of two groups of orbitals. The electrons of the first group, which contains all active MOs and the 0 2s-like 9a’ inactive MO obtained by SACASSCF, are allowed to be doubly excited. The second group of MOs (6a’-Sa’, 2a”) are Cu 3s3p core orbitals. The 3s3p electron distribution is expected to relax according to the change in the 3d electron population due to excitation. Thus, the single excitations from the second internal set are allowed to take the relaxation ofthe Cu 3s3p core into account. The external MO set is also split into two classes. To reduce the length of the MRSDCI expansion, the first set of external MOs, into which two electrons are excited, is limited to the first ten K orbitals. Only a single electron excitation is allowed for the remaining external MOs. Using the above definitions for orbitals, we generate excited CSFs which span the first-order interacting space. The MRSDCI dimension of 534929 is too large to solve explicitly. This parent dimension is thus reduced to 29243 by the & selection for doubly excited CSFs, where the selection threshold is - 1.Ox 1Ok’ au. The reduced Hamiltonian matrix is diagonalized to obtain eight variational solutions. For each solution, the Davidson correction for multiple excitations and the energy lowering due IO rejected

CSFs are added in a similar way as in the SDCI (2 ) calculation. The electronic transition moments (,uT) from the ground state are evaluated using the above MRSDCI wavefunctions. The components of pT are given by PTx.v./r = ( Pglground s&WI (X y>Z) I ~chxcm3 State > 9 (3 ) and the oscillator strength cf) is defined by (4)

f=WPTIZ> where AE is the excitation energy [22].

3. Result and discussion 3.1. The X ‘A’ and ‘A”(I) states Before discussing the results for CuOH, we consider the electron correlation in the Cu atom. The RHF and SDCI energies relative to the ‘S( 3d”4s’ ) ground state are summarized in table 1. The RHF procedure gives poor results in casesthat the 3d population changes. In contrast, the SDCI results compare favorably to the experimental data [ 23 1. Table 1 shows that it is essential to consider electron correlation for making calculations of energetics. Langhoff and Bauschlicher have reported that the contribution of d-d correlation with a weight greater than 90% dominates the total correlation energy in the ‘S state of Cu [ 241. The d-d correlation energy (&) may be given by (5)

Table 1 Energydifferences relative to ?Sstate of Cu atom a’

Relativeenergy (eV) w

CU

*S(3dt04s’) 2D(3d94sz) CU+ ‘S(3d’O) ‘D (3d94s’) ‘D(3d94s’)

Correlation energy (eV)

RHF

SDCI

exp. c’

gross

difference a)

0.0 0.37 6.42 7.75 8.14

0.0 1.45

0.0 I .49 1.12 10.44 10.97

-6.55 - 5.46 -5.14 -4.12 -4.16

0.0 1.09 0.81(0.0) 2.43( 1.62) 2.29( 1.58)

7.23 10.18 10.52

a1Energiesare calculated by RHF and SDCI methods under point group DZh.The Davidson correction is added to SDCI energy. b, Total energies of % are - 1638.4905au by RHF and - 1638.731I au by SDCI. cJ From ref. [23]. d, Valuesmean the reduced amount of correlation energy, and those in parentheses represent differences based on Cut (3).

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where N is the d population and epair is the correlation energy per one d-d electron pair. It should be noted that Ed-d is second-order with respect to N. Thus, the change in 3d population leads to a large difference in correlation energies as shown in table 1. The configuration change 3dr0+3d94s’ for the Cu+ ion gives a correlation-energy reduction of 1.6 eV. In summary, the difference in the correlation energies between states is important in discussing a quantitative energetic even for the Cu atom. Table 2 summarizes the optimized geometries and the energy lowerings obtained by SDCI (2 ) calculations for the ‘A’ ground state and the perpendicular excited state ‘A” (I). The geometries determined experimentally by Jarman et al. [ 71 are also shown in this table. The calculated geometries are comparable to the experimental values, The calculated Cu-0 distance is slightly longer than that obtained by the experiment. One possible reason for this is that relativistic effects are not included in the present calculations. Pyykkid concluded that the bond contraction due to relativity is 0.02 A for Cu compounds [ 251. The present study of CuOH can be seen as a prelude for more accurate treatments including relativistic effects. Since the water molecule is known as a covalent system, it is interesting to note that the calculated

25 October 1991

Cu-O-H bond angle of X ‘A’ ( 114” ) is close to that of Hz0 ( lOSo). This correspondence suggests that covalency makes a substantial contribution to the Cu-OH bonding in CuOH. In table 2, one finds quite a poor result for the excitation energy at the RHF level of theory, where the value is only 0.47 eV in comparison with the experimental value of 2.28 eV [6,7]. The SDCI( 2) calculation, however, shows remarkable agreement with the experiment, demonstrating the significance of electron correlation for CuOH. It is impressive that the transition energy of X ‘A’-’ A” (I) is dominated by the difference in correlation energies with a weight of 78%. The origin of the green emission has now been verified as the ‘A” (I) state. The properties at optimized geometries are summarized in table 3 for each state. The calculated binding energy for the ground state is 2.57 eV. With the correction for ionicity (see footnote a to table 3 ), the value of 2.63 eV is estimated. These results agree with the 2.69 &0.13 eV determined experimentally by Belyaer [ 9 1. It is noted that the RHF binding energy at the X ‘A’ optimized geometry is only 1.39 eV. Electron correlation is again found to be important. The large dipole moment of the X ‘A’ state of 5.4 debye is in line with the ionic character of Cu+-OH-,

Table 2 Optimized geometries a) and energy lowerings bJ for X ‘A’ and ‘A” (I) states X ‘A’ talc. R (Cu-0) (A) R(O-H) (A) B(Cu-O-H) (deg)

RHF total energy SDCI(2) total energy d’ correlation energy e’

‘A”(1) exp. ”

I.811 0.960 114.1

1.769 0.952 110.2

exp. ‘)

1.803 0.963 120.2

1.784 0.951 117.7

X ‘A’

‘A”(I)

diff. (eV)

- 1713.9513 - 1714.3831 -0.4318

-1713.9339 - 17 14.3042 -0.3703

0.47 2.15 1.67 2.28 ”

exp. energy diff. a) ‘) ” d,

talc.

Geometries determined by SDCl(2) calculations (see text). Energy in hartree except differences between the two states (eV). From ref. 171. Weight of reference space for X ‘A’ is 92.03% in variational wavefunction whose length is 6542, and that for ‘A’

termed wavefunction. ei Weights ofcorrelation

energy due to rejected CSFs estimated perturbationally

(I) is 92.46% in 6568-

are 4.4% for X ‘A’ and 5.9% for ‘A” (I ).

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Volume 185, number 5.6 Table 3 SDCI(2) results for X ‘A’ and ‘A”( I) states a)

‘A”(1)

X ‘A’ binding energy (eV) ‘) excitation energy (eV) dipole moment (debye) norm Xd’ Yd’ Mulliken population e) Cu s

2.57(2.63)

p(a’) p(a”) d(a’) d(a”) f net OS p(a’ p(a” d net Hs P net

) 1

2.15 1.713 - 1.020 1.376 7.08 8.37 4.05 5.99 3.12 0.02 Z&63(+0.37)

5.405 -5.265 1.221 6.35 8.12 4.05 5.91 3.99 0.02 28.44(+0.56) 3.82 3.08 1.92 0.03 8.85(-0.85)

Exp. ” 2.69kO.13 2.28

3.77 3.08 1.81 0.03 8.69(-0.69) 0.63 0.05 0.68(+0.32)

0.66 0.05 0.71(+0.29)

a’ Properties are calculated at each optimized geometry. h, Binding energy from ref. [ 91 and excitation energy from ref. [ 71.

” Value in parentheses presents corrected value for ionicity ofCu+( 3d”)-OH-

forthe following reason: Calculated iomzation potential of Cu is lower than experimental value [23] by 0.49 eV as shown in table 1. Electron affinity of OH is calculated to be 1.28 eV, this being smaller than the 1.83 eV known experimentally [26]. Present SDCI(2) scheme may, thus, underestimate binding energy by 0.06 (=0.55-0.49) eV. d, Both Xand Y components mean negative polarity of 0 atom. ” Values in parentheses denote net charge of each atom.

where the X component is dominant. The population analysis shows that the 0.56 electron transfers from Cu to OH. The 4s population is estimated to be 0.35, and that of the 3d shell is 9.90. The nature of the Cu atom is thus similar to 3d” for the ground state of CuOH. Mulliken population analysis shows that the rough character of the ‘A”(I) excited state is given by Cu+ ( 3d94s’)-OH- resulting from Cu 3d+4s excitation. This is in agreement with the speculations of refs. [ 6-81. Thus, the change in 3d population (roughly 10-19) is the main reason for the large difference in correlation energies of 1.67 eV as shown in table 2. The contribution of Cu 4p and 0 2p to the transition is found to be considerable. The 4p(a’) orbitals with a population of 0.37 describe the polarization of a 4s electron away from the OH region. This polarized 4s electron would be responsible for the notably reduced dipole moment of the system. 540

The contribution of 0 2p is reflected in the mixing of the second reference CSF of the 2px(a”)+4s LMCT type to the first 3dlr(a”)+4s, where the weights are 2.4% for the former and 90.1% for the latter in the SDCI (2) wavefunction. Thus, the hole orbital due to transition has a somewhat hybridized character of 3d-2p. The predicted vibrational frequencies are summarized in table 4. The coefficients of the second-order polynomials are given in table 5. The O-H stretching mode has a far-higher frequency around 4000 cm-’ than the other two frequencies for both X ‘A’ and ‘A” (I) states. In the ‘.4”(I) state, the modes of Cu-0 stretching and Cu-O-H bending are found to be remarkably coupled. The bending motion is expected to affect electron repulsions more strongly in the ‘A” (I) excited state than the case in X ‘A’ due to the polarized 4s electron in the molecular plane. This may lead to synchronized Cu-0

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Table 4 Vibrational analyses for X ‘A’ and ‘A” (I) states a) (in cm-‘)

frequency No. 1 ‘A’ 4050 ‘A” 3784 frequency No. 2 ‘A’ 666 ‘PI” 652 frequency No. 3 ‘A’ 620 ‘A” 647

R(Cu-0)

R(O-H)

B(Cu-O-H)

0.028 0.030

-0.999 -0.999

-0.030 -0.018

0.062 0.699

0.03 I 0.008

-0.998 0.715

0.998 0.715

0.026 0.034

0.063 - 0.698

Table 5 Coefficients of SDCI(2) second-order polynomials a) ‘A” (I)

IX x Y

0.097417 0.293893 0.027444 -0.012500 0.033364 0.017338 -0.678625 - 1.089875

0.111722 0.256923 0.025496 0.01 1655 0.018217 0.022665 -0.830074 - 1.012872

I constant

-0.229148 1.993728

-0.217330 2.258823

x2 Y2 ZZ -KY .V:

stretching with the Cu-O-H bending for ‘A”(I), where the Cu-0 bond is “softer” than the O-H bond (see the coeflkients in table 5). 3.2. The transition moments for several excited states

‘) Analyses are based on SDCl(2) second-order surfaces whose coefficients are given in table 5.

X ‘A’

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a) Variables are x: R(Cu-O), y: R(O-H), and Z: B(Cu-O-H). Length in au and angle in radian. Total energy shifted by t 17 14 au.

In table 6, the vertical excitation energies and the transition moments calculated by the SACASSCF/ MRSDCI procedure are summarized. The five excited states of 3d+4s type are found in the energy region of 2-3 eV. The energy order is as follows according to the hole positions: 3drt(a” ), 3do(a’ ), 3dn(a’), 3dS(a”), and 3dS(a’). It is noted that the contributions of 0 2p orbitals exist for these states as discussed in section 3.1. Firstly, it should be noted that the MRSDCI calculation gives the vertical excitation energy of 2.30 eV for the 3dk( a”)-’ or ‘A” (I) state. This value correlates well with 2.19 eV, which is calculated by SDCI( 2) at a geometry of X ‘A’. It is expected, therefore, that the present MRCI computation has an acceptable reliability. The transition moment (pT) for this perpendicular system X ‘A’ + ‘.4” (I) shows a fairly large norm of 0.345 au. The in-plane 3da(a’)-’ state, ‘A’ (III), has a similar value of 0.288 au, where the calculated excitation energy is 2.70 eV. This calculation agrees with the assignment by Jarman et al. [ 71, who assumed that the ‘A’ (III) state is responsible for the emission at 2.36 eV due to a parallel transition to the ground state. They ob-

Table 6 One-electron properties ofseven excited states by MRSDCI calculation ai No.

I 2 3 4 5 6 7

‘A” (I) ‘A’(H) ‘A’ (III) ‘A” (II) ‘A’(IV) ‘A” (III) ‘A’ (V) ‘)

Hole b,

AE c)

!G

G

/G

I

3dx(a”) 3do(a’) 3dn(a’) 3dS(a”) 3dS(a’ ) Zpn(a” ) 2&a’ )

2.30 2.31 2.70 2.77 2.95 4.30 5.66

0.0 0.678 -0.122 0.0 -0.020 0.0 1.253

0.0 -0.018 0.261 0.0 0.001 0.0 -0.022

0.345 0.0 0.0 -0.002 0.0 -0.1 I5 0.0

6.7x 2.7x 5.5x 2.2x 2.8x 1.4x 2.2x

4(Cu) d’ lo-’ lo-’ 10-l lo-’ lO-5 lo-’ 10-l

to.27 t 0.45 t 0.40 t 0.40 t 0.40 -0.10 - 0.02

a) Excitation energy (AI?) in eV and transition moment (fir) in au. ‘I Particle orbital due to excitation is usually of polarized 4s type. Characterization of states thus given by hole orbital. ‘) Reference energy of X ‘A’ is - 17 14.2852 au. d, Net charge of Cu. e) For ‘A’ (V) state, weight of reference is only 81%, contrasting with all other states which are about 90%. Quantities of this state are, thus, less reliable.

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served another parallel transition at the red region of 1.97 eV and found that the direction of the transition moment for this process is very close to the Cu0 (x) axis [8]. For the ‘A’(II) state of 3da(a’)-’ type, the X component completely dominates the calculatedpT norm of0.678 au. This is in accord with the result reported by Jarman et al. The transition, ‘A’ (11)+X ‘A’, is thus attributable to the origin of the “lowest” emission, although the calculated energy of 2.37 eV is slightly high relative to the experimentally observed value ( 1.97 eV). Table 6 shows that the intensities of transitions related to the 3d6-’ excited states are expected to be much weaker than those for 3do-’ and 3dz-’ types, being consistent with the selection rule of transition. The ‘A” (III) state is calculated to be a 2prr( a” ) -4s LMCT type, whose excitation energy is 4.30 eV in the ultraviolet region with an f value of 0.0014. It is noted that in this state, the counter-mixing of 3drc(a” ) + 4s is considerable. The net charge on the Cu atom changes from positive to slightly negative due to LMCT excitation. The character of another LMCT state ‘.4’ ( V ) is given by 2pn:(a’ ) + 4s. The weight of the reference space for this state is only 8 1Win the resultant MRSDCI wavefunction. The estimated quantities of ‘A’ (V) are thus less reliable.

I

mation System Development Ltd. for his technical assistance.

References [ I ] A. Veillard, ed., Quantum chemistry: the challenge of transition metals and coordination chemistry (Reidel, Dordrecht, 1986). [2] C.W. Bauschlicher Jr. and B.O. Roos. J. Chem. Phys. 91 (1989) 4785. [3] F.A. Cotton and G. Wilkinson,

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[ 14) A. Ramirez-Solis and J.P. Daudey, Chem. Phys. 134 ( 1989) 4. Concluding remarks Ab initio CI computations have been performed on the CuOH molecule. The transition energy for the ‘A” (1)+X ‘A’ process is calculated to be 2.15 eV, which agrees to a remarkable degree with the experimental value of 2.28 eV. The transition moments and the vibrational frequencies were also evaluated. The present study demonstrates the importance of electron correlation for this molecule.

111. [ I5 ] A.T. Amos and G.G. Hall, Proc. Roy. Sot. A 263 483.

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[ 161 A.D. McLean and B. Liu, J. Chem. Phys. 58 (1973) 1066. [ 171 Z. Gershgorn and I. Shavitt, Intern. J. Quantum Chem. 2 (1968) 751.

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Acknowledgement .411calculations were carried out using the program systems, JAMOL4 [ 27 ] ,4ASON2 [ 28 ] (modified by author (YM)), and MICA3 [29] on the NEC-SX2 system at the SupercomputerCenter of the NEC Tsukuba Research Laboratory. The authors are grateful to Mr. T. Sakuma of NEC Scientific Infor542

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(25 1P. Pyykkb;, Chem. Rev. 88 ( 1988) 563. [26] K.P. Huber and G. Herzberg, Molecular spectra and molecular structure, Vol. 4. Constants of dlatomic molecules (Van Nostrand Reinhold, New York, 1979).

Volume 185, number 5,6

CHEMICAL PHYSICS LETTERS

[ 271 H. Kashlwagi, T. Takada, E. Miyoshi, S. Obara and F. Sasaki, RHF program JAMOU (1989). [ 28 ] S.Yamamoto, U. Nagashima, T. Aoyama and H. Kashiwagi, CASSCF program JASON2 ( 1987).

25 October 199 I

[29] A. Murakami, H. Iwaki, H. Terashima, T. Shoda, T. Kawaguchi and T. Noro, MRSDCI program MICA 3 (1985).

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