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Stability and the CO stretching vibrational frequency of molecular AgCo
Volume 2 15, number 6
CHEMICAL PHYSICS LETTERS
17 December 1993
Stability and the CO stretching vibrational frequency of molecular AgCO Chnstel M. Manan InstttuteofPhysrcaland TheoretrcalChemrstry,Unrversrtyof Bonn, Wegelerstrasse12, D-531I5 Bonn, Germany Received 24 September 1993, m final form 14 October 1993
Quantum chemical calculations mcludmg relatlvlstlc effects and electron correlation have been performed on sliver monocarbony1 AgCO IS found to be a van der Waals complex bound by 40-50 cm-’ The fundamental transltlon energy of the CO stretchmg vlbratlon m the complex 1sidentical to the value of free CO The results of the present study rule out that the red-shlfis of z 200 cm-’ observed m rare-gas matrices m which both Ag and CO are present are due to AgCO monomers
1. Introduction Carbon monoxrde 1s known to chemlsorb on surfaces of several transitron metals In most of these cases the frequency of the CO stretchmg vrbratron m the chemtsorbed state is lowered with respect to its gas-phase value Depending on the adsorption sate, rt takes values typically between 2000 and 2 100 cm- ’ m the on-top positron [ 1 ] The effect ISexplained by a model similar to the bonding m carbonyls [ 2 ],I e a weak electron transfer from the 50 orbital to the surface and a stronger back-donatron from the transttton metal d band to the CO 2x* orbrtal Occupation of the 2rP orbital leads to a bond weakenmg and a CO stretching vrbratronal frequency lower than m free CO Silver, on the other hand, has trghtly bound d electrons, which manifests itself m the fact that the first excited atomic state 1s an s+p excrtatlon and not d+s as m Cu and Au Accordingly, the mteraction of silver with CO is weak For CO on Ag ( 111) surfaces only a phystsorbed state 1sknown [ 3 ] whtle there 1s an mdrcatton of weak chemlsorptton on Ag( 110) below T= 165 K [4] The fundamental band of the CO stretching vtbratron for low coverages of CO adsorbed on silver films [ 5 ] and on large and medmm silver clusters [ 6 ] IS close to Its gasphase value of 2143 274 cm-’ [ 71 Wrth increasing CO coverage the band is shifted by = 20-40 cm-’ to lower wavenumbers Because of the high coverage 582
dependence these frequency shifts are believed to orlgmate from lateral mteractrons of the CO molecules [ 51 Several new features m the infrared spectrum are observed d CO and Ag are cocondensed m an argon matnx Among them, under condrtrons where silver atoms and small Ag,, clusters (n < 10) are present in the matnx, an absorptron band appears at 2159 cm-’ with a small shoulder growmg out at 2 168 cm-’ when the metal concentration is mcreased [ 6 ] Isotopic substltutrons show that this band 1s due to a compound containing a single CO umt The question arises whether the observed transrtions are due to AgCO molecules formed by thermal aggregation of silver and carbon monoxrde. Compared wrth the energy of the fundamental vrbratronal transitron of a CO monomer m argon (2138 cm-‘) [6,8,9] the band exhrblts a blue-shift of ~20-30 cm-’ Older matnx infrared studies by McIntosh and Ozm [ lo] postulate large red-shifts (of the order of 200 cm-’ ) of the CO fundamental m Ag-CO However, these authors also state that rt 1s practically rmpossrble to obtain pure silver monocarbonyl because under the condmons m which Ag-CO 1s formed stlver dlcarbony1 and tncarbonyl are also produced. Extrapolating the CO stretching fun&mental frequencies observed for Ag-CO in Ar, Kr, and Xe matrices,, McIntosh and Ozm [lo] are led to estimate a gas-
0009-2614/93/$ 06 00 0 1993 Elsevler Science Publishers B V All nghts reserved
Volume 2 15, number 6
CHEMICAL PHYSICS LETTERS
phase value of 1966 cm- ’ mlcatmg a strong Ag-CO interaction So far, no AgCO molecules have been observed m the gas phase nor does Agl react wtth CO at room temperature m a fast-flow reactor as the isovalent Cuz and Au* do [ 111 Because of the repulsion between the 50 lone-pair electrons of CO and the partially filled 5s shell of silver and the tightly bound silver d electrons, AgCO may be expected to be only weakly bound (or even unbound) m its electromc ground state Its positive ion, on the other hand, is remarkably stable (80 3 kJ/mol) [ 121 The bmdmg in AgCO+ may be attributed to a polarization of CO by the positive silver ion, similar to the binding between N2 and CO+ [ 131, combmed with a much smaller Pauh repulsion between Ag+ and the 50 orbital of CO compared with the neutral species. To our knowledge, no quantum chemical investigation on isolated AgCO has been published Theoretical studies on the isovalent CuCO are contradictory Merchan et al [ 141 predict CuCO to be a weakly bound van der Waals complex, whereas Berthier et al [ 151 find a bmdmg energy of as much as 19 4 kcal/mol. Both studies employ pseudo-potenttals combined with small valence basis sets. Small valence basis sets, especially a lack of polarization functions, inherently bear the risk of large basis set superposition errors (BSSEs) In order to avoid these problems Merchan et al. work m a basrs of localized molecular orbitals whrle Berthier et al. do not take any precautions so that a large contribution to the calculated bmdmg energy is probably due to BSSEs The followmg quantum chemical study is concerned with the question of whether the AgCO molecule is bound m its electronic ground state and mvestigates the frequency shift of its CO stretching vibrational mode with respect to free CO.
2. Methods The choice of basis sets and computational methods has been guided by the following criteria A proper descriptton of the triple bond m CO requires a multi-configurational SCF (MCSCF) optimization of molecular orbitals and a subsequent multireference treatment of the electron correlation For van der Waals-type bondmg it is important that the
17 December 1993
BSSE ISsmall and that the energy scales properly with the particle number, 1 e the electron correlation treatment should be size extensive. Last but not least, relativistic effects have to be taken mto account, because they heavily mfluence the dr”s1+d9s2 excitation energies m coinage metals The atomic orbital basis is a [4s3p2dlf] atomic natural orbital (ANO) contraction of a ( 13s8p6d4f) primitive Gaussian basis on C and 0 [ 161 The Ag ( 18s14p9d4f) primitive basis started from a ( 17~1lp8d) Huzmaga basis [ 171, augmented with a diffuse s function (exp 0 015), three pnmrtrve p functions with exponents 0 16,O 056, and 0.02, whrch describe the 5p shell and the polarization of the slver 5s shell, and a semr-diffuse d function (exp 0 1) which is especially important m the d” contiguration Four f polarization functions were used wrth exponents taken from ref [ 181. The contraction coefficients of the Ag [7s6p5d3f] AN0 basis were determmed in one-component relativistic coupledpair functional calculations on the d”s’ state of atomic silver The Hamiltonian employed m the molecular calculations is a spin-free relativtsttc no-parr Hamiltoman with external field projection [ 19-221 Since the electronic ground state is a *E+ state and energetically well separated, spin-orbit coupling is expected to be small and is therefore neglected. Only linear arrangements of nuclei have been mvestigated The MCSCF is of the CAS (complete active space) type [ 23 1, where five electrons are freely distnbuted m five acttve orbitals (Ag 5s, CO 1xX,, CO 2x&) Attempts to add the CO 50 and a correspondmg correlatmg orbital to the active space were not successful smce the CO 50 retained an occupation number close to 2 0 Electron correlation is included wnhm the averaged coupled-pair functional (ACPF) approach [ 241 #’ where we have chosen a g factor of 1 for all mtemal configurattons and 2/n elsewhere The number of correlated electrons n is 2 1, 1 e the 5s and 4d electrons of silver and the 3-50, lx electrons of CO Four reference configurations give rise to 3390377 CSFs treated explicitly m the diagonalization The size of the BSSE is esttmated by employing the counterpoise correction procedure of Boys and Bernard1 [ 25 ] *’ We use the implementation of Blomberg and Slegbahn
583
Volume 2 15, number 6
17 December 1993
CHEMICAL PHYSICS LETTERS
All calculations were performed employmg the MOLECULE-SWEDEN suite of programs #2 which have been interfaced #3with the no-pair one-electron mtegral code [ 261
Table 1 Mull&en populations of the most important natural orbltals from ACPF calculations on AgCO for vanous Ag-C mtemuclear separations The CO bond length was kept fixed at 2 132 & &s-c (a01
3. Results and discussion For companng the frequency of the CO stretching vibration m AgCO and free CO, the latter has been treated completely analogously to the compound molecule. At a fixed Ag-C separation of 100 ao, the CO bond distance was varied A third degree polynomial fit to the ACPF energies at four data points ((2 0,2.132,2 25,2 4) ao) gave an equihbnum bond length of 2 148 ao, shghtly longer than expenment (2 132044 ao) [ 271 and the value of 2 138 a0 obtamed m a previous treatment [ 28 ] with the same primitive A0 basis, but a more flexible [ 6s5p4d2f] AN0 contraction The harmonic frequency computed from the second derivative at the mmimal energy point is 2165 925 cm-‘, only %4 cm-’ smaller than the expertmental gas-phase harmomc frequency of 2169.75589 cm-’ [27] The accuracy of the current treatment is thus sufficient to be able to detect a frequency shift of the order of 20-30 cm-‘. AgCO is not bound at the level of the present CASSCF treatment The Ag-CO bond is a typical van der Waals bond where the bmdmg is achieved merely by electron correlation effects. Interestmgly, AgOC is also bound, a test calculation at an Ag-OC bond dtstance of 8 a0 gives almost the same binding energy as for the Ag-CO isomer A Mulhken population analysis of the natural orbitals generated from the ACPF expansion vectors shows that there is almost no charge transfer from CO to Ag or vice versa The populattons of the CO 50, 1x, and 2x* orbitals listed for three different Ag-C distances m table 1 are almost unchanged with respect to the ACPF natural orbital occupattons m isolated CO For Ag the 5% 5p, and 4d populatrons are shown m table 1 As for CO, the orbital occupations of silver change little as
Go 80 40
co
Ag 5s
5p
4d
50
lr
2~~
0 98 0 99 0 99 0 98
0 04 0 04 004 021
9 90 9 90 9 90 9 74
1 94 1 94 1 94 1 94
3 87 3 a7 3 a7 3 87
0 0 0 0
14 14 14 14
the CO approaches At bond distances of 4-5 uo, where the mteraction of Ag and CO is repulsive, the 4d,, and 4d, orbitals polarize away from CO by mtxmg m 5p contributions The ACPF bmdmg energy before BSSE correction amounts to 61.23 cm-’ with an Ag-C equll~bnum bond distance of 8 27 a0 With the Boys-Bernard1 correction applied, the ACPF bmdmg energy ts reduced to 34 24 cm-’ wth an Ag-C eqmhbrmm bond distance of 8 67 a0 Both potential energy curves are displayed m fig 1 The large change m the calculated equilibnum bond distance is not a serious concern because the potential energy curve 1s flat and the actual bond distance is sensitive to small changes in the energy The Boys-Bernard1 correction tends to overestimate the BSSE slightly and we are thus led to estimate the Ag-CO bmdmg energy to be of the order of 40-50 cm-i It should be noted that m the present calculations there is no BSSE at all at the CASSCF level Keeping the Ag-C separation fixed to 9 a0 - not far from the equihbnum bond distance - the CO bond length has been varied m the same way as m the free CO molecule The harmonic frequency of the CO stretching vibration m the Ag-CO complex is found to be 2165 922. It differs from the value obtamed at R,++= 100 a0 only m the third decimal place, 1 e there is no frequency shift at all within the error bars of this calculation
X2MOLECULE-SWEDEN is an electronic structure program system wntten by J Almlof, C W Bauschhcher III, M R A Blomberg, D P Chong, A Helberg, S R Langhoff, P-A Malmqvlst, A P Rendell, B 0 Roos, P E M Slegbahn and P R Taylor g3 The interfaces were wntten by C M Manan
584
4. Conclusions Summarizmg, I should like to mention that there is no mdication for a strong Ag-CO mteraction lead-
Volume 2 15, number 6
CHEMICAL PHYSICS LETTERS
17 December 1993
shift at a double-substltutlonal site due to the shorter separation from the nearest-nelghbour Ar atoms and the concomitant higher repulsion by the Ar cage Interestingly, CO with an adlacent water molecule m an argon matm expenences a blue-shlfi of about 6 cm- ’ with respect to gaseous CO [ 8,9 1. There 1s no mdlcatlon for the presence of water m the expenments of Jackschath [ 61; It could not be ruled out, however, that the frequency shift might ongmate from AgCO mteractmg wth oxygen Another possib&y 1s that the observed species 1s not AgCO but A&CO, with n being small Further studies are necessary to decide upon this questlon
Acknowledgement I should like to thank Dr. Chnstlan Jackschath from the Frele Umversltat Berlin for raising this problem and for makmg available hu expenmental results pnor to pubhcatlon All calculations were performed on the Convex C2 of the “Sonderforschungsberelch 334” at the University of Bonn Fig 1 Potential energy curve for the X ‘Z+ ground state of AgCO (- - -) ACPF enewes, (-) ACPF energies including counterpoise correction
mg to a significant lowermg of the CO fundamental stretchmg frequency m molecular AgCO The large red-shifts observed by McIntosh and Ozm [ lo] m rare-gas matrices m which both Ag and CO were present are certainly not due to AgCO monomers Nevertheless, the present study cannot defimtely rule out that the two structures observed by Jackschath [ 6 ] m the infrared spectrum at 2 159 and 2 168 cm- ’ might ongmate from AgCO trapped m an argon matrlx Matnx effects can probably not be neglected m this case The distance between a molecule at a smgle-substitutional site m an Ar crystal and the nearest.nelghbour Ar atoms 1s = 7 a0 [ 29],1 e somewhat shorter than the calculated bond distance of the free Ag-CO complex The mfluence of the Ar lattice on a CO monomer 1s to shift the fundamental vlbratlonal transltlon by x 5 cm-’ towards lower wavenumbers compared to the gas-phase value [6,8,9] On the other hand, AgCO might experience a blue-
References [l] R Ryberg, Advan Chem Phys 76 (1989) 1 [2] W Muller and P S Bagus, J Vacuum Scl Technol A 5 (1987) 1053 [3] W Hansen, M Bertolo and K Jacobi, Surface Scl 253 (1991) 1 [4] M Canepa, L Mattera, M Polese and S Terrem, Chem Phys Letters 177 (1991) 123 (51 C Pettenkofer and A Otto, Surface Scl 151 (1985) 37 [ 61 C Jackschath, Ph D Thesis, Frele Umversltat Berlin (1993) [ 71 S N Suchard, Spectroscopic data, Vol 1 (IFI/Plenum Data, New York, 1975) [ 81 R N Perutz and J J Turner, J Chem Sot Faraday Trans 1169 (1973)452 [ 91 H Dubost and L Abouaf-Margum, Chem Phys Letters 17 (1972) 269 [lo] D McIntosh and G A Ozm, J Am Chem Sot 98 (1976) 3167 [ 111 L Llan, P A Hackett and D M Rayner, J Chem Phys 99 (1993) 2583 [ 121 L A Barnes, M Rosl and C W Bauschbcher III, J Chem Phys 93 ( 1990) 609 [ 131 J Baker and A D Buckmgham, J Chem Sot Faraday Trans 83 (1987) 1609 [ 141 M MerchBn, I Nebot-Gil, R GonzBlez-Luque and E Ortl, J Chem Phys 87 (1987) 1690
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Volume 2 15, number 6
CHEMICAL PHYSICS LETTERS
[ 151 G Berthler, A Daoudl and M Suard, J Mol Struct THEOCHEM 179 ( 1988) 407 [ 161 J AlmlBf and P R Taylor, J Chem Phys 86 ( 1987) 4070 [ 171 S Huzmaga, J Chem Phys 66 (1977) 4245 [ 18 ] S R Langhoff, L G M Pettersson, C W Bauschhcher III and H Partndge, J Chem Phys 86 (1987) 268 [ 191 M Douglas and NM Kroll, Ann Phys (NY) 82 (1974) 89 1201J Sucher, Phys Rev A 22 (1980) 348 [21] G Hardekopf and J Sucher, Phys Rev A 30 ( 1984) 703 1221 B A Hess, Phys Rev A 33 (1986) 3742
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[23] B 0 Roos, P R Taylor and P E M Slegbahn, Chem Phys 48 (1980) 157 [24] R J Gdamtz and R Ahlnchs, Chem Phys Letters 143 (1988) 413 (251 S F Boys and F Bemar&, Mol Phys 19 (1970) 553 [ 261 B A Hess and P Chandra, Phyaca Scnpta 36 ( 1987) 412 [27]A LeFloch,Mol Phys 72 (1991) 133 [ 28 ] M Emzerhof, C M Manan and S D Peyenmhoff, Chem Phys Letters 204 (1993) 59 [29] R A Aziz, in Inert gases, potent& dynamics and energy transfer m doped crystals, ed M L Klein (Spnnger, Berlm, 1984)
CHEMICAL PHYSICS LETTERS
17 December 1993
Stability and the CO stretching vibrational frequency of molecular AgCO Chnstel M. Manan InstttuteofPhysrcaland TheoretrcalChemrstry,Unrversrtyof Bonn, Wegelerstrasse12, D-531I5 Bonn, Germany Received 24 September 1993, m final form 14 October 1993
Quantum chemical calculations mcludmg relatlvlstlc effects and electron correlation have been performed on sliver monocarbony1 AgCO IS found to be a van der Waals complex bound by 40-50 cm-’ The fundamental transltlon energy of the CO stretchmg vlbratlon m the complex 1sidentical to the value of free CO The results of the present study rule out that the red-shlfis of z 200 cm-’ observed m rare-gas matrices m which both Ag and CO are present are due to AgCO monomers
1. Introduction Carbon monoxrde 1s known to chemlsorb on surfaces of several transitron metals In most of these cases the frequency of the CO stretchmg vrbratron m the chemtsorbed state is lowered with respect to its gas-phase value Depending on the adsorption sate, rt takes values typically between 2000 and 2 100 cm- ’ m the on-top positron [ 1 ] The effect ISexplained by a model similar to the bonding m carbonyls [ 2 ],I e a weak electron transfer from the 50 orbital to the surface and a stronger back-donatron from the transttton metal d band to the CO 2x* orbrtal Occupation of the 2rP orbital leads to a bond weakenmg and a CO stretching vrbratronal frequency lower than m free CO Silver, on the other hand, has trghtly bound d electrons, which manifests itself m the fact that the first excited atomic state 1s an s+p excrtatlon and not d+s as m Cu and Au Accordingly, the mteraction of silver with CO is weak For CO on Ag ( 111) surfaces only a phystsorbed state 1sknown [ 3 ] whtle there 1s an mdrcatton of weak chemlsorptton on Ag( 110) below T= 165 K [4] The fundamental band of the CO stretching vtbratron for low coverages of CO adsorbed on silver films [ 5 ] and on large and medmm silver clusters [ 6 ] IS close to Its gasphase value of 2143 274 cm-’ [ 71 Wrth increasing CO coverage the band is shifted by = 20-40 cm-’ to lower wavenumbers Because of the high coverage 582
dependence these frequency shifts are believed to orlgmate from lateral mteractrons of the CO molecules [ 51 Several new features m the infrared spectrum are observed d CO and Ag are cocondensed m an argon matnx Among them, under condrtrons where silver atoms and small Ag,, clusters (n < 10) are present in the matnx, an absorptron band appears at 2159 cm-’ with a small shoulder growmg out at 2 168 cm-’ when the metal concentration is mcreased [ 6 ] Isotopic substltutrons show that this band 1s due to a compound containing a single CO umt The question arises whether the observed transrtions are due to AgCO molecules formed by thermal aggregation of silver and carbon monoxrde. Compared wrth the energy of the fundamental vrbratronal transitron of a CO monomer m argon (2138 cm-‘) [6,8,9] the band exhrblts a blue-shift of ~20-30 cm-’ Older matnx infrared studies by McIntosh and Ozm [ lo] postulate large red-shifts (of the order of 200 cm-’ ) of the CO fundamental m Ag-CO However, these authors also state that rt 1s practically rmpossrble to obtain pure silver monocarbonyl because under the condmons m which Ag-CO 1s formed stlver dlcarbony1 and tncarbonyl are also produced. Extrapolating the CO stretching fun&mental frequencies observed for Ag-CO in Ar, Kr, and Xe matrices,, McIntosh and Ozm [lo] are led to estimate a gas-
0009-2614/93/$ 06 00 0 1993 Elsevler Science Publishers B V All nghts reserved
Volume 2 15, number 6
CHEMICAL PHYSICS LETTERS
phase value of 1966 cm- ’ mlcatmg a strong Ag-CO interaction So far, no AgCO molecules have been observed m the gas phase nor does Agl react wtth CO at room temperature m a fast-flow reactor as the isovalent Cuz and Au* do [ 111 Because of the repulsion between the 50 lone-pair electrons of CO and the partially filled 5s shell of silver and the tightly bound silver d electrons, AgCO may be expected to be only weakly bound (or even unbound) m its electromc ground state Its positive ion, on the other hand, is remarkably stable (80 3 kJ/mol) [ 121 The bmdmg in AgCO+ may be attributed to a polarization of CO by the positive silver ion, similar to the binding between N2 and CO+ [ 131, combmed with a much smaller Pauh repulsion between Ag+ and the 50 orbital of CO compared with the neutral species. To our knowledge, no quantum chemical investigation on isolated AgCO has been published Theoretical studies on the isovalent CuCO are contradictory Merchan et al [ 141 predict CuCO to be a weakly bound van der Waals complex, whereas Berthier et al [ 151 find a bmdmg energy of as much as 19 4 kcal/mol. Both studies employ pseudo-potenttals combined with small valence basis sets. Small valence basis sets, especially a lack of polarization functions, inherently bear the risk of large basis set superposition errors (BSSEs) In order to avoid these problems Merchan et al. work m a basrs of localized molecular orbitals whrle Berthier et al. do not take any precautions so that a large contribution to the calculated bmdmg energy is probably due to BSSEs The followmg quantum chemical study is concerned with the question of whether the AgCO molecule is bound m its electronic ground state and mvestigates the frequency shift of its CO stretching vibrational mode with respect to free CO.
2. Methods The choice of basis sets and computational methods has been guided by the following criteria A proper descriptton of the triple bond m CO requires a multi-configurational SCF (MCSCF) optimization of molecular orbitals and a subsequent multireference treatment of the electron correlation For van der Waals-type bondmg it is important that the
17 December 1993
BSSE ISsmall and that the energy scales properly with the particle number, 1 e the electron correlation treatment should be size extensive. Last but not least, relativistic effects have to be taken mto account, because they heavily mfluence the dr”s1+d9s2 excitation energies m coinage metals The atomic orbital basis is a [4s3p2dlf] atomic natural orbital (ANO) contraction of a ( 13s8p6d4f) primitive Gaussian basis on C and 0 [ 161 The Ag ( 18s14p9d4f) primitive basis started from a ( 17~1lp8d) Huzmaga basis [ 171, augmented with a diffuse s function (exp 0 015), three pnmrtrve p functions with exponents 0 16,O 056, and 0.02, whrch describe the 5p shell and the polarization of the slver 5s shell, and a semr-diffuse d function (exp 0 1) which is especially important m the d” contiguration Four f polarization functions were used wrth exponents taken from ref [ 181. The contraction coefficients of the Ag [7s6p5d3f] AN0 basis were determmed in one-component relativistic coupledpair functional calculations on the d”s’ state of atomic silver The Hamiltonian employed m the molecular calculations is a spin-free relativtsttc no-parr Hamiltoman with external field projection [ 19-221 Since the electronic ground state is a *E+ state and energetically well separated, spin-orbit coupling is expected to be small and is therefore neglected. Only linear arrangements of nuclei have been mvestigated The MCSCF is of the CAS (complete active space) type [ 23 1, where five electrons are freely distnbuted m five acttve orbitals (Ag 5s, CO 1xX,, CO 2x&) Attempts to add the CO 50 and a correspondmg correlatmg orbital to the active space were not successful smce the CO 50 retained an occupation number close to 2 0 Electron correlation is included wnhm the averaged coupled-pair functional (ACPF) approach [ 241 #’ where we have chosen a g factor of 1 for all mtemal configurattons and 2/n elsewhere The number of correlated electrons n is 2 1, 1 e the 5s and 4d electrons of silver and the 3-50, lx electrons of CO Four reference configurations give rise to 3390377 CSFs treated explicitly m the diagonalization The size of the BSSE is esttmated by employing the counterpoise correction procedure of Boys and Bernard1 [ 25 ] *’ We use the implementation of Blomberg and Slegbahn
583
Volume 2 15, number 6
17 December 1993
CHEMICAL PHYSICS LETTERS
All calculations were performed employmg the MOLECULE-SWEDEN suite of programs #2 which have been interfaced #3with the no-pair one-electron mtegral code [ 261
Table 1 Mull&en populations of the most important natural orbltals from ACPF calculations on AgCO for vanous Ag-C mtemuclear separations The CO bond length was kept fixed at 2 132 & &s-c (a01
3. Results and discussion For companng the frequency of the CO stretching vibration m AgCO and free CO, the latter has been treated completely analogously to the compound molecule. At a fixed Ag-C separation of 100 ao, the CO bond distance was varied A third degree polynomial fit to the ACPF energies at four data points ((2 0,2.132,2 25,2 4) ao) gave an equihbnum bond length of 2 148 ao, shghtly longer than expenment (2 132044 ao) [ 271 and the value of 2 138 a0 obtamed m a previous treatment [ 28 ] with the same primitive A0 basis, but a more flexible [ 6s5p4d2f] AN0 contraction The harmonic frequency computed from the second derivative at the mmimal energy point is 2165 925 cm-‘, only %4 cm-’ smaller than the expertmental gas-phase harmomc frequency of 2169.75589 cm-’ [27] The accuracy of the current treatment is thus sufficient to be able to detect a frequency shift of the order of 20-30 cm-‘. AgCO is not bound at the level of the present CASSCF treatment The Ag-CO bond is a typical van der Waals bond where the bmdmg is achieved merely by electron correlation effects. Interestmgly, AgOC is also bound, a test calculation at an Ag-OC bond dtstance of 8 a0 gives almost the same binding energy as for the Ag-CO isomer A Mulhken population analysis of the natural orbitals generated from the ACPF expansion vectors shows that there is almost no charge transfer from CO to Ag or vice versa The populattons of the CO 50, 1x, and 2x* orbitals listed for three different Ag-C distances m table 1 are almost unchanged with respect to the ACPF natural orbital occupattons m isolated CO For Ag the 5% 5p, and 4d populatrons are shown m table 1 As for CO, the orbital occupations of silver change little as
Go 80 40
co
Ag 5s
5p
4d
50
lr
2~~
0 98 0 99 0 99 0 98
0 04 0 04 004 021
9 90 9 90 9 90 9 74
1 94 1 94 1 94 1 94
3 87 3 a7 3 a7 3 87
0 0 0 0
14 14 14 14
the CO approaches At bond distances of 4-5 uo, where the mteraction of Ag and CO is repulsive, the 4d,, and 4d, orbitals polarize away from CO by mtxmg m 5p contributions The ACPF bmdmg energy before BSSE correction amounts to 61.23 cm-’ with an Ag-C equll~bnum bond distance of 8 27 a0 With the Boys-Bernard1 correction applied, the ACPF bmdmg energy ts reduced to 34 24 cm-’ wth an Ag-C eqmhbrmm bond distance of 8 67 a0 Both potential energy curves are displayed m fig 1 The large change m the calculated equilibnum bond distance is not a serious concern because the potential energy curve 1s flat and the actual bond distance is sensitive to small changes in the energy The Boys-Bernard1 correction tends to overestimate the BSSE slightly and we are thus led to estimate the Ag-CO bmdmg energy to be of the order of 40-50 cm-i It should be noted that m the present calculations there is no BSSE at all at the CASSCF level Keeping the Ag-C separation fixed to 9 a0 - not far from the equihbnum bond distance - the CO bond length has been varied m the same way as m the free CO molecule The harmonic frequency of the CO stretching vibration m the Ag-CO complex is found to be 2165 922. It differs from the value obtamed at R,++= 100 a0 only m the third decimal place, 1 e there is no frequency shift at all within the error bars of this calculation
X2MOLECULE-SWEDEN is an electronic structure program system wntten by J Almlof, C W Bauschhcher III, M R A Blomberg, D P Chong, A Helberg, S R Langhoff, P-A Malmqvlst, A P Rendell, B 0 Roos, P E M Slegbahn and P R Taylor g3 The interfaces were wntten by C M Manan
584
4. Conclusions Summarizmg, I should like to mention that there is no mdication for a strong Ag-CO mteraction lead-
Volume 2 15, number 6
CHEMICAL PHYSICS LETTERS
17 December 1993
shift at a double-substltutlonal site due to the shorter separation from the nearest-nelghbour Ar atoms and the concomitant higher repulsion by the Ar cage Interestingly, CO with an adlacent water molecule m an argon matm expenences a blue-shlfi of about 6 cm- ’ with respect to gaseous CO [ 8,9 1. There 1s no mdlcatlon for the presence of water m the expenments of Jackschath [ 61; It could not be ruled out, however, that the frequency shift might ongmate from AgCO mteractmg wth oxygen Another possib&y 1s that the observed species 1s not AgCO but A&CO, with n being small Further studies are necessary to decide upon this questlon
Acknowledgement I should like to thank Dr. Chnstlan Jackschath from the Frele Umversltat Berlin for raising this problem and for makmg available hu expenmental results pnor to pubhcatlon All calculations were performed on the Convex C2 of the “Sonderforschungsberelch 334” at the University of Bonn Fig 1 Potential energy curve for the X ‘Z+ ground state of AgCO (- - -) ACPF enewes, (-) ACPF energies including counterpoise correction
mg to a significant lowermg of the CO fundamental stretchmg frequency m molecular AgCO The large red-shifts observed by McIntosh and Ozm [ lo] m rare-gas matrices m which both Ag and CO were present are certainly not due to AgCO monomers Nevertheless, the present study cannot defimtely rule out that the two structures observed by Jackschath [ 6 ] m the infrared spectrum at 2 159 and 2 168 cm- ’ might ongmate from AgCO trapped m an argon matrlx Matnx effects can probably not be neglected m this case The distance between a molecule at a smgle-substitutional site m an Ar crystal and the nearest.nelghbour Ar atoms 1s = 7 a0 [ 29],1 e somewhat shorter than the calculated bond distance of the free Ag-CO complex The mfluence of the Ar lattice on a CO monomer 1s to shift the fundamental vlbratlonal transltlon by x 5 cm-’ towards lower wavenumbers compared to the gas-phase value [6,8,9] On the other hand, AgCO might experience a blue-
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