Theoretical study on coordination of CO2 to third row metal atoms (CaMn, Cu, Zn)

20January1995

CHEMICAL PHYSICS LETTERS

ELSEVIER

Chemical PhysicsLetters 232 ( 1995) 319-327

Theoretical study on coordination of CO2 to third row metal atoms (Ca-Mn, Cu, Zn) Gwang-Hi Jeung Laboratoire de Chimie Quantique (CNRS UPR-139), Institut Le Bel, UniversitOLouis Pasteur, 4 rue Blaise Pascal, 6 7000 &rasbourg, France

Received 24 October 1994

Abstract

Restricted Hartree-Fock, configurationinteraction and perturbation calculationswere performed to establish the bond strengths, the stable geometries, and the electron distributions of the MCO2moleculeswhere M = Ca-Mn, Cu, Zn. All the MCO2 molecules except the CuCO2and ZnCO2 are shown to be stable with respect to dissociation. The ground states have maximum spin electron configurations. Other high-spin states, which only differ from the ground states by electron permutations within the nonbonding d subshell, are very close to the ground state. The spin populations show a transfer of one metal valence electron to the CO2 group, which is essentially localized at the carbon atom.

1. Introduction

The use of carbon dioxide as an alternative source of organic substances is one of the chemists' dreams. The activation of the carbon dioxide molecule may be done by photochemical, electrochemical, or catalytic processes. The fixation of CO2 on the metal center is supposed to be a key step for the ultimate reduction of COs (for reviews, see Refs. [ 1-4] ). It is thus necessary to understand the metal-COs complexes. A small number of metal-COs complexes involving one, two or three metal centers have been isolated and their structures were characterized by Xray diffraction. There is plenty of IR and NMR spectroscopic evidence for the existence of various M COs intermediates. Unfortunately, the short lifetime of these species and the presence of solvent molecules in the samples made detailed analysis difficult. The one-to-one MCO 2 complexes without any ligands can be formed and have been analyzed by IR and ESR techniques for the alkali metals [ 5-12 ], and

transition metals [ 13-15 ]. However, the matrix environment in which these species were formed ( C 0 2 or COs/rare gas), and the presence of water molecules made it impossible to deduce reliable structural and energetical data from these measurements. Some quantum chemical calculations for the MCO2 molecules with alkali metal atoms [ 16-18], and transition-metal atoms [ 13,19-21 ] have also been reported. The aim of this work was to provide the first systematic study of the MCO2 molecules to understand the nature of the bonding, the most stable geometry, and the common features and differences among these species, using quantum chemical calculations. The Ca, Sc, Ti, V, Cr, Mn, Cu and Zn metal atoms were chosen in this paper. The properties of the MgCO2 [22 ], TiCO2 [21 ] and CFCO2 [ 20 ] molecules were reported before in detail.

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320

G.-H. Jeung / Chemical Physics Letters 232 (1995) 319-32 7

2. Methods

The all-electron restricted Hartree-Fock (RHF) calculations were done using the ASTERIX program package [23,24]. The dynamic and non-dynamic electron correlation energies were calculated using two different programs. One is a program originally written by Brooks and Schaefer [ 25 ], the other is a direct configuration interaction (CI) program with contractions included in the MOLCAS program package [26]. The canonical MOs resulting from the RHF calculation were employed for CI calculations. All valence occupied and virtual orbitals were used in the CI calculations, except the la~ ( l a ' ) and lb~ (2a') MOs (corresponding to the log and 1 ou orbitals of CO2, i.e. essentially the 2s AOs of the two oxygen atoms) which were kept frozen in the RHF canonical MOs. The correction energies were also evaluated according to the simple formula by Langhoff and Davidson [27], to improve the size consistency. For the metal atoms, 14s I 1p Gaussian-type orbitals (GTOs) contracted to 8s6p atomic basis functions (ABFs) by Wachters [28] and the 6d GTOs--.3d ABFs by Rapp6 et al. [29 ] were used, except for the Ca atom, where 5d GTOs optimized for the 4s~3d ~ (3D) were contracted to 3d ABFs. This basis set is quite flexible to describe the 3dn4s 2, 3dn4s ~, 3d~+~4s ~, and 3d ~+l atomic states. For carbon and oxygen atoms, 1 ls7p GTOs contracted to 5s3p ABFs explained in Ref. [ 22 ] were used. The first CIs were done by including the 7a'-I la' and 3a"-4a" MOs in Cs or the 4al - 7a~, 2a2, 4bl and 2b2 MOs in C2I to the active space. These MOs are essentially the metal valence AOs, 3d and 4s, or the 4at MOs of (CO2)-. This kind of CI considers the quasi-degeneracy of the MOs by using multiconfigurational description of the wavefunctions. The energy differences between these CIs and the RHF calculations represent the so-called nondynamic correlation effect. In our second CIs, the wavefunction is developed, starting from the first CI, in an ensemble of configuration state functions (CSFs) obtained by zero, single and double substitutions. This multi-reference single-and-double (MRSD) CI includes a further correlation effect, which is often called the dynamic correlation effect. In all cases, the single CSF resulting from the RHF calculation remains largely predominant over other singly and doubly substi-

tuted CSFs in the MRSDCI calculation. The weights of the RHF-CSFs are typically in the range of 90%95%. The MRSDCI only slightly lowered the potential energy with respect to the single-reference SDCI, and the relative energy differences between different electronic states did not change significantly. A Moller-Plesset perturbation method was also used to get the correlated energies. The wavefunction up to the second order was renormalized and the energy was calculated up to the third order (MP3). The result proves that the RHF calculation gives a qualitatively correct description for the high-spin states calculated in this work. For the CO2 complexes whose structures have been measured by X-ray diffraction, the C-O distance varies from 117 to 128 pm and the O - C - O angle varies from 133 ° to 136 °. The angle a ( O - C - O ) and the bond distances R (C-O) were varied for the CrCO2 molecule. The lowest potential energies were obtained near 135 ° and 127 pm in all forms of coordination, as if the CO2 fragment in the complex sticked to keep the (CO2)- geometry [30,31]. Subsequently, the internal geometry of the CO2 moiety was kept constant with R ( C - O ) = 127 pm and or(O-CO ) = 135 °. Four planar forms of CO2 coordination to the metal atoms, as have been previously explained [ 22 ], were considered in this work (see Fig. 1 ). These are: the rl~ form (A), the !12o form (B), the ~ form (C), and the ~3oco form (D). The circles representing the mean radii of the AOs in Fig. 1 are useful to understand the electronic distribution in the MCO2 molecule. The relatively diffuse character of the 4s

A

B

C

D

Fig. 1. Four modes of coordination of the MCO2molecules:Tl~ (A), "q~o (B), "q~)(C), and ~l~co (D). The atomic radii representing the (r) valuesfor the 2p AOs of carbon and oxygenatoms, and the 3d (inner circle) and 4s (outer circle) AOs of the Ti atom (M) are to scale.

G.-H. Jeung / Chemical Physics Letters 232 (1995) 319-327

AO of the metal atoms in comparison with the 3d AO of the metal atoms and the 2p AOs of carbon and oxygen atoms plays an essential role for metal/carbon dioxide interaction. All possible high-spin (with 2S+ 1 = V+ 1 where V is the number of metal valence electrons), and lowspin (with 2S+ 1 = V - 1, and 2S+ 1 = V - 3 when possible) molecular electronic configurations were calculated to find the lowest states for each form. The RHF calculation gave higher potential energies for the low-spin states with 2S+ 1 = V - 1 in comparison with the high-spin states. The low-spin states with 2S+ 1 = V - 3 lie still much higher. It is due to a loss of the d-d exchange energies in the latter states with respect to the former states. No further CI calculation was done for the states with 2S+ 1 = V - 3. The 'high-spin' refers to 2S= V and the 'low-spin' refers to 2S= V - 2 hereafter. Only the valence electrons have been considered in this work to establish the ordering of the MOs. The population analysis is done using the natural molecular orbitals (NMOs) resulting from the MRSDCI calculation. The M-CO2 vibrational frequencies were calculated regarding these molecules as two-body systems. The resulting numbers may be regarded as an approximation for the lowest-frequency normal mode of the real systems. The computational method employed in this work takes into account the main energetical effects, and it is believed to give reasonably precise description of the M-CO2 bonds.

3. Result and discussion

The symmetries, the equilibrium M-CO2 distances, the dissociation energies into M + CO2, and the M - C O 2 vibrational wavenumbers for the lowest electronic states of the MCO2 molecules in the four forms are summarized in Table 1. For all the molecules studied in this work, the "q~ form corresponds to electronic states that are much higher in energy than those formed by other forms which dissociate into M q-CO2. The symmetries of the excited electronic states and the corresponding excitation energies are summarized in Table 2. The spin populations and the net charges on each atom in MCO2 are reported in

321

Table 3. No bonding orbital between M and CO2 exists in the q~ and rl~co forms. There exists one bonding orbital between M and CO2 in the q2o and rl~ forms, which is singly occupied in the high-spin (2S+ 1 = V+ 1 ) states and doubly occupied in the lower-spin (2S+ 1 = V - 1 ) states. The spin densities of the MnCO2 molecule near the equilibrium distance for each geometrical form are drawn in Fig. 2. The carbon dioxide molecule cannot conserve its linear geometry when it forms a bond with the metal atoms. Only a bent carbon dioxide molecule can ligate to metal atoms. This is shown in Fig. 3, using M n C O / a s an example. The potential energy curve labeled as Mn+CO2 is a 6Z+ state, representing a collinear approach of the CO2 molecule to the Mn atom keeping the R (C-O) = 116 pm. There is a repulsion between COz and Mn in this state. In reality, this curve should have a shallow van der Waals well at long Mn-CO2 distances when the electron correlation between the electrons belonging to the metal atom and those of the carbon dioxide molecule is better taken into account. The metal atom retains its atomic ground state 4s23d 5 at all distances and no metal electron is donated to the CO2 moiety. The energy curve labeled as Mn*+CO/, is a BE+ state, resulting from the same collinear approach. This state, which is in higher potential energy than the 6•+ state all along, is also repulsive between the metal atom and the carbon dioxide molecule. In this state, the metal atom is essentially in the 4s~4p13d 5 state and the spin population of the CO2 moiety is nearly zero. This neutral character can be explained by the energy difference between hypothetical linear (CO2)- and linear CO2. The former is 150 kJ mol- ~ higher than the latter in MRSDCI. The curve labeled as (Mn+) + ( C Q ) - is a 8A' state, where the C-O distance is maintained at 127 pm and the O - C - O angle to 135 °. The spin population of the metal atom in the unit of ½h is 6 and that of CO2 moiety is 1. Fig. 3 clearly shows the importance of the CO2 bending in bond formation. The O - C - O angle is quite rigid because the straightening of the CO2 moiety in the metal complex requires a substantial input of energy, as can be seen in Fig. 3. This energy varies from one species to the next. The CrCO2 (riCo) is a relatively less rigid example where the straightening energy is 75 kJ mol- '. Varying all coplanar geometrical parameters (the C-O distances, the O - C - O angle and the M -

~

BE(M-CO2)

TI~

~

q~o

RHF CI

RHF CI/+Q MP3 RHF CI/+Q MP3 RHF CI/+Q MP3 RHF CI/+Q MP3 RHF

CI CI CI CI RHF

390

-12

43

119

139 103/80

253(3A1) b

210(aA,) b 265(3At) b 241 (3A') b

Method a CaCO2

425 420

144 87/62 100 96 57/37 43 38(4A") 35/21 31 -16(2A ') 43/51 62 -36

201 (4A', 4A") 254(4B2) 235 (4A-) 201(2A ') 246(4B2)

ScCO2

465 425

112 59/55 70 80 58/36 35 22(SA ") 37/21 24 -72(3A ") 2/15 -4 -42

241 (SA") 252(5B2) 229 (SA") 208(3A ") 235(5B2)

TiCO2

580 630

54 33 6/3 -8 12(eA") 20/24 15 -84(4A ') -15/16 -8 -66

98

195 (6A") 247(6B2) 224(6A') 200(4A ') 233(6B2)

VCO2

540

440

-110

- 18c7(6A ')

-172(SA ')

-96

-67(aA ')

-3

34 -40/-36

195 b(aA') 251 b(SAi) 228 b(SA') 208(6A ') 235 b(SAl)

MnCO2

-84(7A ')

-28

-1

192 b(7A') 257 b(7Al) 230 b(TA') 198(5A ') 232 b(TAt)

CrCO2

a CI ( multireference single and double CI) / + Q ( with Davidson's correction energy added to the CI energy ). b RHF values.

p(M--CO2)

~

R(M-C)

~co

q~ q~ ~o

R(M-O) R(M-C) R(M-C, O)

Geometry

340 325

-106

-49 -46 -33/-27 -39 -90 -50/-40 -45

-45

215(2Ai)

188(2A ' ) 244(2Ai) 207(2A ')

CuCO2

137

420

--217

-- 183

--

--93

217(3A1) b

186(3A') b 241(3A1) b 221(3A') b

ZnCO2

Table 1 Symmetries, minimum-energy interatomic distances (R, in pm), binding energies with respect to the lowest M + CO2 (BE, in kJ mol -~ ) and M-CO2 vibration wave numbers ( p in cm- t ) for the lowest electronics states of MCO2

",4

t~

G.-H. Jeung / Chemical Physics Letters 232 (1995) 319-327

323

Table 2 Symmetries and relative potential energies (in kJ mol -~ ) of the excited electronic states with respect to the lowest states ( 1 ) in each geometrical form of MCO2 Geometry RHF

rl~)

MRSDCI

ScCO2

TiCO2

0(1 4A'--~l 4A")

VCO2

11~0

- 1 (1 4B2---,4At) 1 ( ! 4B2-*4A2) 49 ( 14B2--,4BI )

9(15A"-* 1 5A') 17(1 5A"--,2 5A') 7(1 5B2--, 1 5A2) 12( 1 5B2"* 1 5BI ) 10( l 5B2-, 1 5Al )

112-O

6(1 4A"-, 14A ' ) 54(1 4A"-, 1 2A' ) 39( 1 2A'-, 1 2A")

19(1 5A"--, 1 5A' ) 20(15A"--,2 ~A' ) 93( 1 5A"--,3A")

11~)

1(1 4A'--, 1 4A")

1130

2(14B2-, 14AI) 7(1 4B2---,1 4A2) 52(1 4B2--, 1 4BI)

10(1 11(1 13(i 14(1 14(1

112o

8(1 2A'--, 1 4A") 4(1 4A"--, 14A ' ) 43(1 2A'-, l 2A" )

13(1 5A"~ 15A') 20( 15A"-,2 5A' ) 34( 15A"--. 1 3A" )

5A"--, 1 5A') 5A"--,2 5A') 5B2-* 15A2) 5B2-* 1 5BI) 5B2--, 1 5Al)

CrCO2

71(1 6A"---~1 6A')

45(1 7A'-, 1 7A")

- 7 ( 1 6B2--, 1 6Ai) - 0 ( 1 6B2---*1 6A2) -- l ( 1 6B2--,2 6A2) - 0 ( 1 6B2---,26B2) 10(1 6A'-, 1 6A" ) 90(1 6A'~I 4A' ) 10( 1 4A'---~1 4A")

31(1 7AI--* 1 7BI) 71 ( 1 7AI-* 1 7B2) 73 ( 1 7A1~ 17A2) 78(1 7A'-, 1 7A" ) 88(1 7A'---,15A ' ) 100( 1 5A'--, 1 5A")

76(1 6A"--, 1 6A') 6(16B2--, 16AI) 7(1 6B2--*1 6A2) 7(1 6B2--,2 6A2) 9( 16B2--,2 6B2) 17(1 6A'--, l 6A") 35(1 6A'-,l 4A' ) 2(1 4A'--, 14A" )

I l l ( 1 5A'-,1 5A" )

Table 3 Electron spin populations (Qs) and net atomic charges (AQ) of the lowest states near the minimum-energy geometries of MCO2 (in au 1.602× l0 -tg) I

11~

Q,(M)

Q,(C)

CaCO 2

SeCO2

TiCO2

VCO2

CrCO2

MnCO2

CuCO2

ZnCO2

d s p p s

0.00 0.70 0.20 0.45 0.40 1.00 0.35 -0.90 -0.45

1.00 0.70 0.25 0.45 0.40 0.90 0.15 -0.70 -0.35

2.00 0.75 0.20 0.45 0.40 0.80 0.20 -0.70 -0.30

3.00 0.85 0.15 0.45 0.40 0.75 0.20 -0.65 -0.30

4.05 0.80 0.10 0.40 0.45 0.70 0.35 -0.65 -0.40

5.00 0.80 0.20 0.50 0.40 0.65 0.35 -0.65 -0.35

0.05 0.05 0.00 0.45 0.40 0.75 0.10 -0.50 -0.35

0.00 0.90 0.15 0.45 0.35 0.50 0.25 -0.40 -0.40

d s p p s

0.00 0.80 0.25 0.55 0.30 0.50 0.45 --0.50

1.00 0.80 0.25 0.50 0.25 0.45 0.25 --0.35

2.05 0.80 0.25 0.45 0.25 0.40 0.20 --0.30

3.10 0.50 0.10 0.45 0.25 0.45 0.15 --0.30

4.95 0.10 0.05 0.50 0.25 0.60 0.20 --0.40

5.00 0.80 0.25 0.55 0.30 0.50 0.20 --0.35

0.05 0.10 0.05 0.50 0.25 0.50 0.25 --0.35

0.00 0.90 0.20 0.55 0.25 0.55 0.20 --0.40

AQ(M) AQ(C) AQ(O) b AQ(O') c rl~co

Q,(M)

Qs(C) AQ(M) AQ(C) AQ(O) b

• Rounded to the nearest multiples of 0.05. b O is the oxygen atom(s) which is (are) in direct contact with the metal atom. c (y is the uncoordinated terminal oxygen atom.

324

G.-H. Jeung / Chemical Physics Let ters 232 (1995) 319- 327

6.0O-

(a)

---

0.64 0,32 0.16 0.08 004

4.0O ---

6oo~-(b)

--

,ooI

Oo :

0.64

2.00

I'

0.~

o

,I

"2,~ .0 C

Mn

-4,00

.0

-6.00

i

i

[

. . . .

I

"7.50

,

,

-5.00

I

,

I

-2 50

I

,

0.00

,

[

m

2.50

L

~

-4.00 .

m

I

5.00

m

~

m

m

~

~

m

~

m

7.50

6.00 5.00-~

~

-6.00

-7.60

-5.06

-2.50

0.00

2.50

5.00

7.50

600-

(c)

4.00 -

---

0,64 0.32

- -

0,16

(d) -4.00-

0.06 0.04

----

3.00--

0,64 0,32 0.16 0.08 0.04

----

2.00-

2,00 -1.000.00 -

0.00

-1.00 -

-6.00-

-2.00-

p

-3.00n

O C

°

C

Ma

.o

-4.00-

-4,00~

-6.00 ~ -6.00

r

i

I

-7.50

.

.

.

.

I

-6.00

I

h

i

[

I

i

I

-2.50

I

i

0.00

i , 5.00

J

2.50

i 760

-6.00

I

-7.60

-5,00

-260

0.00

2.50

I

S O0

I

1

1

I

7.60

Fig. 2. Electron spin density contour maps of the MnCO2 near the minimum-energy geometries corresponding to the four modes of coordination. Mn - CO 2

300-

Mn++

C02

200-

100-

Mn*÷ CO 2

o

Mn I

1 50

200

+

i

I

i

I

250

300

350

400

CO 2

R

Fig. 3. Superimposition of two sections of the potential energy surfaces for the ground state of the MnCO2 molecule in the tl~) mode of coordination. The relative potential energies (E) are in kJ tool -1, and the Mn-O and Mn-C distances (R) are in pm. The curves labeled Mn+ + CO£, Mn*+ CO2 and Mn + CO2 have respectively SA',6Z+ and sZ+ symmetries.

CO2 distance) indicates the existence of an energy barrier. This means that all the MCO2 molecules are energetically bound with respect to dissociation into the ground state of M + CO2. Many electronic states can be made according to the distribution o f nonbonding d electrons into the ten d-spin-orbitals. This can be illustrated taking an example o f the q~ form o f ScCO2. The 4A' state with a predominant CSF of ((7a') ] (8a') l (9a,) l } is degenerate with the 4A" state with a predominant CSF of { ( 7a' ) ~( 8a' ) ~(3a") 1} at all levels of calculation within the precision of this work. The 4A' and 4A" states differ by the single nonbonding 3d electron o f the metal: the dy2_z2 AO is occupied in the 4A' state, while the dyz AO is occupied in the 4A" state (see Fig. 1 for the coordinate axes). Two other electronic states made by putting the single d electron to the dxy or dx= AOs lie higher than the former two states. Finally, the single occupation o f the dx2 ( a to the O - C ) leads to the highest valence state. The energetically close charac-

G.-H. Jeung / CheraicalPhysics Letters 232 (1995) 319-327

325 Mn - C O 2

ter of these five states proves the nearly identical interaction of the five types of 3d electrons with the remaining electrons. This is also an indirect proof that the (CO2) - part and the M + part of the electron distribution do not overlap significantly. The cases of TiCO2 and VCO2 generate many more electronic states, because two (TiCO2) or three (VCO2) electrons should be distributed among the ten d-spinorbitals. Fig. 3 allows us to imagine the reaction paths for the bonding between a metal atom and the carbon dioxide molecule. When the carbon dioxide and the metal atom are in their electronic ground states, their collision may benefit from the spin-orbit interaction to encounter the potential energy hypersurface coming from the (M + ) + (CO2) -. Another possibility is to prepare (e.g. photochemically) the metal atom, whose ground state is sEd ", to the excited state with sld n+! during the collision. The spin population of the CO2 moiety in the rl~, rico, and I13c0 forms remain close to ½h for a wide range of the M - C O 2 distances. (In the example of MnCO2 in Fig. 4, the maximum electron transfer seems to occur at around 230 pm.) Except the case ofCrCO2 ( ~ , rico ) where a d electron (which is c for the incoming CO2) is taken, one s electron is taken from the metal atom. This transferred electron is essentially localized on the carbon atom, suggesting a formation of carbanion. This electron is stabilized upon coordination (see Fig. 4, upper drawing) and has nearly an invariant spatial distribution all along the reaction coordinate (Fig. 4, lower drawing). In this respect, the bonding between the metal atom and the carbon dioxide molecule is purely ionic, M + (CO2)-. The electron transfer from the metal atom to the CO2 moiety accompanies a rearrangement of the closed-shell electron distribution, resulting in a reinforcement of the electron density to the oxygen atoms. This may be interpreted as a strengthening of the basic character of the oxygen atoms on coordination to the metal atom. Other factors intervene in the M - C O 2 bonding. (i) One factor is the stabilization of the closed shell electrons belonging to the CO2 moiety (see Refs. [20,21 ] ). The CO2 electrons are polarized due to the effective charge of the metal atom. This polarization appears in the population analysis as Q s ( C O E ) AQ(M) where AQ(M) is the net charge of the metal

0.000-

-0.050-

J

-0.100

0.150-

-0.200-

-0,250-0.300-

.... -1

3d

-0,350 i 150

200

250

300

350

400

1.00

I..= ~

-

-

-

~

slMn)

0.80-

0.60-

*

° ---~-----__~

p(C)

0.40

s(C)

°2°i 0.00

~

150

--

200

~ - - - r ~

'

250

-I

300

' ~

250

400

R Fig. 4. Valence orbital energy levels (upper part, in hartree ), and spin populations (lower part, in au) of MnCO2 in rl~ as functions of the Mn-O distance (in pm ).

atom and Qs (CO2) is the spin population of the CO2 moiety, Q s ( C O z ) > A Q ( M ) > 0 . In the example of MnCO2 (q~) this quantity is 0.39 e - at a Mn-O distance of 3.2 bohr, 0.36 at 3.6 (near the equilibrium distance), 0.28 at 4, 0.15 at 5, and 0.03 at 7. (ii) The second factor is the repulsion between the metal d electrons and the closed-shell electrons of CO2. This is clearly shown in Fig. 4 (upper drawing), where destabilization of the d energy levels occurs when the M - C O 2 distance is decreased. This factor may explain the decreasing bond strength C a C O 2 > . . . > ZnCO2. The nearly degenerate character of the different d AOs up to very short distances is also remarkable. (iii) The third factor is the destabilization of the remaining metal s electron due to the incoming CO2 electrons, save in the CuCO2 and the CrCO2 (q~co) molecules. This is also shown in Fig. 4 (upper drawing). To minimize the repulsion, a polari-

326

G.-H. Jeung / Chemical Physics Letters 232 (1995) 319-327

zation (long distance) or hybridization (short distance) occurs as shown in the same figure (lower drawing). This electron is pushed away from the CO2 moiety, as can be seen in Fig. 2. These factors which contribute to the potential energy are of course not strictly independent. There exist few experimental or theoretical studies for MCO2 to compare with the present work. Mascetti and Tranquille [ 15 ] did a Fourier transform infrared study of cocondensation of the Ti, V, Cr, Fe, Co, Ni and Cu metal vapors with CO2 in matrices at low temperature. They identified different species of metal complexes of O, CO, CO2 and CO3. Most metal complexes were thermally unstable and decomposed above 60 K. Unfortunately, the extremely intricate nature of the metal complexes in the matrices did not allow them to precisely determine geometries and bond energies. However, they observed IR band frequency of 455 cm-1 for the Ti/CO2/Ar matrices annealed to 40 K. They attributed this to v ( T i - O ) for the rlI coordination of TiCO2. This value is close to our calculated vibrational frequency, 465 cm -1 ( R H F ) or 425 cm - I (SDCI). Caballol et al. [ 19 ] computed the structure of the CuCO2 by using a truncated CI based upon the U H F (spin-unrestricted) MOs and using a second-order perturbation-extrapolation technique. Their U H F calculation gave no chemical bonding between the copper atom and the carbon dioxide. They also found nearly one electron transferred from the copper atom to the carbon dioxide moiety. This part of their result is in agreement with the present study. In contrast, their truncated CI + second-order perturbation + extrapolation technique showed that the 112o and ~ forms are stable for dissociation into the ground state of Cu + CO2 with dissociation energies of 21 and 19 kJ m o l - 1, respectively, while the 113o and ~ forms have negative dissociation energies of - 35 and - 193 kJ m o l - l , respectively. This means that the bond energy between Cu and CO2 comes from the electron correlation effect instead of the ionic contribution. However, our MRSDCI or perturbation calculation up to the third order and the use of Pad6 approximation shows no electronic state having negative potential energy with respect to the ground state of Cu + CO2. Our calculation also gives the q~o less stable than the rl3co. Huber et al. [ 13,14 ] studied UV-VIS spectra for

cocondensation of silver and gold atoms with CO2/ Ar. They observed broad absorption bands. The corresponding IR data showed a weakly perturbed CO2 molecule in the metal CO2 complexes. They observed also small but significant frequency shifts and Raman activity of v2 and v3 IR-type modes. Even though they could not resolve the molecular geometry and assign their spectra, they concluded that the nature of the AgCO2 and AuCO2 interaction must be very weak. The MCO2 molecules with the metal atoms whose ground states are s2d" (Sc, Ti, V, Mn) are more strongly bonded than those with the metal atoms whose ground states are sld m (Cr, Cu). This may be explained by the lower s i d ~__,s Id ~- 1 ionization energies [ 32 ] of the former group, 5.12 eV (Sc), 6.00 eV (Ti), 6.82 eV (V), 5.28 eV (Mn), in comparison with the sldS~sld 4 energy of Cr (8.28 eV) or sldl°--,d 1° energy of Cu (7.72 eV).

Acknowledgement

The author would like to thank Dr. JoElle Mascetti and Professor W.C. Stwalley for many useful discussions, and Dr. Amanda J. Ross for critically reading the manuscript. The calculations presented in this paper have been done with a grant from the CNRS.

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