Molecular Properties of Boron-Coinage Metal Dimers: BCu, BAg, BAu1

Molecular Properties of Boron-Coinage Metal Dirners: BCu, BAg, BAu Maria Barysz2, Miroslav Urban3 2Max-Planck-Inst itut fiir Ast rophysik, Garching, Germany2 3Department of Physical Chemistry, Comenius University, 84215 Bratislava, Slovak Republic Abstract Spectroscopic constants and dipole moment curves of the coinage metal diatomic molecules with boron , BCu, BAg, and BAu were investigated using high-level-correlated methods combined with quasi-relativistic DouglasKroll (No-pair) spin averaged approximation.

1. Introduction

2. Basis sets and computational methods 3. Results and discussions

4. Conclusions 5. Acknowledgments References 'This paper is dedicated to Professor Geerd H. F. Diercksen to mark his 60th birthday and to thank him cordially for all scientific support, collaboration and friendship.. 2Permanent address: Department of Chemistry, University of Silesia, ul. Szkolna 9, PL-40 006 Katowice, Poland

ADVANCES IN QUANTUM CHEMISTRY, VOLUME 28 Copyright8 1997 by Academic Press. Inc. All rights of reproduction in any form reserved 0065-3276/97 $25.00

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1.

Introduction

Dimers of metal atoms are the simplest building blocks useful in understanding of properties of bulk solids and alloys. The knowledge about the nature of the bonding between metals and other elements is directly related t o properties of many industrially important materials and catalysts, especially when transition metals are involved( 1). Experimental investigations of simple molecules involving metal atoms are principally oriented towards the question of their very existence (2) rather than concerned with the determination of their molecular properties. Hence, the importance of the theoretical work leading to the systematization, generalization, and prediction of properties of such molecules can hardly be overestimated. Of particular importance seem to be systematic investigations in the series of related atoms, as e.g. in the series of the coinage atoms which are studied in the present paper. Considerable attention has already been devoted to experimental (3, 4) and theoretical (5, 6) investigations of compounds of copper with different atoms, including aluminum. However, no systematic study of such dimers over the whole series of the coinage metals seems to be available. Such investigations are both interesting and challenging for theoreticians. One should note that already the properties of the coinage metal atoms, Cu, Ag, Au, show quite interesting patterns which reflect certain specific features of the electron correlation and relativistic effects within the series (7-11). The same holds for the series of the coinage metal hydrides (12, 13). Atomic properties like ionization potentials, electron affinities, atomic polarizabilities etc., are relevant for the explanation of the chemical behaviour of chemical compounds of these metals. Moreover, it is only the relativistic effects which can elucidate why the behaviour of the gold atom is different from that of the other members in the series. Of importance is the fact that highly sophisticated theoretical methods capable of simultaneously considering the electron correlation correlation and relativistic effects can be used to study diatomic molecules formed by the coinage metals and provide results in a good agreement with experiment. For example, the Coupled Cluster (CC) methods (14, 15) or Complete Active Space Self Consistent Field (CASSCF) method (16) followed by the second order perturbation correction (CASPTZ) (17) recover a substantial portion of the electron correlation effects. These methods combined with quasirelativistic schemes based on the No-pair (Np) Douglas Kroll (DK) transformation of the Dirac hamiltonian (18, 19) result in IP’s and EA’s of Cu, Ag, Au in reasonably good agreement with experiment(8, 11, 19). Also atomic polarizabilities resulting from CC, CASPTS and DK calculations show systematic patterns and are mutually consistent (10, 13, 20). All this gives a good basis for the credibility of molecular data predicted by high-level-correlated quasirelativistic theoretical calculations.

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259

The usefullness of theoretical ab initio methods in elucidating the bonding character and molecular properties of mixed dimers of the coinage metal atoms was convincingly demonstrated in the case of AICu. The theoretical results appear to compete with reliable experimental data (6). We did not find any theoretical calculations on dimers composed of the coinage metals and the boron atom. Since the valence electronic structure of the boron atom is similar to that of aluminum, one can expect the bonding situation in BMe compounds analogous to that found to be responsible for the bonding in AICu, i.e. the a(2p-ns) bond (3, 5, 6) and the ‘C+ ground state. Anther motivation for the present work is the very interesting chemistry of boron compounds and, from the theoretical point of view, the role of relativistic effects which are expected to significantly influence the bonding in the series BCu, BAg, BAu.

2.

Basis sets and computational methods

Basis sets and quasi-relativistic methods The non-relativistic PolMe (9) and quasirelativistic NpPolMe (10) basis sets were used in calculations reported in this paper. The size of the [uncontractd/contracted] sets for B, Cu, Ag, and Au is [10.6.4./5.3.2], [16.12.6.4/9.7.3.2], [19.15.9.4/11.9.5.2], and [21.17.11.9/13.11.7.4], respectively. The PolMe basis sets were systematically generated for use in nonrelativistic SCF and correlated calculations of electric properties (10, 21). They also proved to be successful in calculations of IP’s and EA’s (8, 22). Nonrelativistic PolMe basis sets can be used in quasirelativistic calculations in which the Mass-Velocity and Darwin (MVD) terms are considered (23). This follows from the fact that in the MVD approximation one uses the approximate relativistic hamiltonian as an external perturbation with the nonrelativistic wave function as a reference. At the SCF and CASSCF levels one can obtain the MVD quasi-relativistic correction as an expectation value of the MVD operator. In perturbative CASPT2 and CC methods one needs to use the MVD operator as an external perturbation either within the finite field approach or by the analytical derivative schems. The first approach leads to certain numerical accuracy problems. The NpPolMe basis sets were developed recently (10) for the investigation of relativistic effects using the DK transformed hamiltonim (13, 18-20). This is the spin-averaged no-pair approximation which reduces the 4-component relativistic one-electron hamiltonian to a 1-component form without introducing strongly singular operators. NpPolMe basis sets indirectly incorporate some relativistic effects on the wave function. Let us note that both PolMe and NpPolMe contracted sets share the same exponents of primitive Gaussians. Contraction coefficients are, however,

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different, even if the contraction scheme is essentially the same for both basis sets. Still, however, one can approximately estimate the relativistic contribution by comparing non-relativistic results (either SCF, CASSCF, CC, or CASPT2) calculated with PolMe basis sets and quasirelativistic DK data obtained with NpPolMe basis sets. Such a comparison is reasonable (10, 20) although, strictly speaking, only nonrelativistic and DK calculations with completely uncontracted basis sets would give the true DK relativistic contribution. The use of a fully uncontracted basis set is, obviously, not practical for molecular calculations. Another possibility of estimating the relativistic contribution would be to use the MVD approach and the PolMe basis sets for both nonrelativistic and relativistic calculations. This, however, is not applicable to atoms as heavy as gold (8, 13). Correlated methods Two different sets of theoretical methods were used at the correlated level of approximation for molecular wave functions. The CASSCF method (16) is well established as being capable of recovering most of the nondynamical correlation effects (near-degeneracy effects) and is suitable for the use over the full range of the potential energy surface. Usually, the dynamical correlation is not covered satisfactorily in CASSCF. A substantial portion of the dynamical correlation can be accounted for in the subsequent CASPT2 calculation. The CASPT2 method (17) is the second order approach to dynamical correlation with a single reference state given by a multiconfigurational (multideterminant) wave function. The selection of the active space in CASSCF and CASPT2 calculations is by no means trivial. In the present case we assumed that: (i) the selected active space should at least represent the valence bonding and antibonding n orbitals in BMe, (ii) the corresponding CAS scheme must lead to correct dissociation products (B: ' P , Me: 2S) and should be well balanced along the potential curve up to the dissociation limit, (iii) the CAS scheme should account for as much dynamical correlation as possible, and (iv) the same active space can be employed for all molecules in the series, i.e., BCu, BAg, and BAu. We did some preliminary calculations for BCu (which still could be treated with rather large active spaces) with different partitioning of the orbital space in CAS calculations. The notation is: (frozenlinactivelactive; n el) for orbital subspaces and n correlated electrons in the active space. The C2v symmetry was used in all computations. For most distances the wave function has definitively a two configuration form. The smallest active space considered is (0000~9331(2000;2 el) in the CASSCF calculation while in the subsequent CASPT2 calculation we used the (62201311112000; 2 el) space. The best would be to choose as the active space the valence orbitals of boron (2110) and the 3d,4s and the correlating 4d shell for Cu (5222).

Molecular Properties of Boron-Coinage Metal Dimers

26 1

This results in the active space (7332) with 14 correlated electrons, which is, unfortunately, too big to handle. It produces over lo6 determinants in the CAS wave function. For BCu the tractable active space was the one in which the 2p,(B) orbitals were eliminated from the active space, and the 2s(B) orbital was shifted to the inactive space, i.e. (00001722016222; 12 el). Thus, this partitioning of the orbital space in CASSCF calculations includes the optimization of the ls,2s of B and ls,2s,2pI3s,3porbitals of Cu in the inactive subspace, and u p * bond orbitals and 3d,4s,and 4d orbitals of Cu in the active subspace. The 2s(B) is then activated at the level of CASPT2 approximation which follows the CASSCF step. The partitioning in CASPT2 is thus (6220)100016222;12 el) with 1s of B, ls,2s,2p, 3s, 3p of Cu frozen, 2s(B) inactive, and u,u*, 3d, 4s, 4d (Cu) active. Unfortunately this partitioning can not be used for the whole series of BMe molecules. The pattern of correlating orbitals for BAg and BAu is different from that for BCu. The major contribution comes from the correlation effects on boron while that on the heavy metal is less important. Moreover, for Ag and Au the (n-l)d shell of the heavy atom has as the principal correlating shell the f orbitals (angular correlation). With all valence orbitals on boron, (n-l)d, ns, and the correlating f shell on the heavy atom one would need to use the active space (7442) for 14 electrons, which generates prohibitively large number of configurations in the CI expansion. The only possibility applicable t o all molecules BCu, BAg, and BAu is to use the active space comprising the valence orbitals of B and the ns orbital of the heavy atom, i.e. (3110) for 4 electrons. All subsequent CASSCF and CASPT2 results are calculated with this active space. The comparison of basic quantities representing BCu, i.e. its equilibrium bond distance, dissociation energy, and the harmonic vibrational frequency for these active spaces, calculated from a limited number of interatomic distances, is presented in Table 1. These results correspond to quasirelativistic DK CASSCF and DK CASPT2 levels of approximation. With the extension of the active space we observe a decrease of Re at the CASSCF level and increase of Re at the level of the CASPT2 method. The CASSCF values of D, and w e increase with the active space size and they decrease in the CASPT2 method with w e being insensitive to the increase of the active space from (3110) to (6222). This preliminary investigation of the selection of the active space gives us some confidence in the results obtained with relatively small active space. In CC calculations we used well established CCSD(T) method (24) in which the single and double excitations are considered iteratively, while triples are calculated in a perturbative way using the converged CCSD single and double excitation amplitudes. In CC calculations we correlated 2s and 2p electrons of boron and either (n-l)d electrons and ns electrons of the coinage metal or (n-l)p, (n-l)d, and ns electrons of the coinage metal.

Maria Barysz and Miroslav Urban

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Table 1 DK no-pair CASSCF and CASPT2 calculations for BCu with NpPol basis sets. A comparison of different active spaces.

CASSCF Frozen Inactive Active

(0000) (9331) (2000; 2eI)

(0000) (8331) (3110; 4eI)

(0000) (7220) (6222; 12eI)

Re@) D,(eV) we(cm-’)

2.176 0.96 339

2.191 1.02 343 CASPT2

2.017 1.33 406

Frozen Inactive Active

(6220) (3111) (2000; 2eI)

(6220) (2111) (3110; 4el)

(6220) (1000) (6222; 12el)

1.840 2.48 60 1

1.866 2.35 560

1.889 2.25 562

&(A)

De(eV) wdcm-

Computer programs were MOLCAS 3 program system (25) for SCF, CASSCF, and CASPT2 calculations and the program TITAN for closed shell calculations (26) . The new version of the COMENIUS program was used for open shell CCSD(T) calculations based on the spin adapted singly and doubly excited amplitudes (15,27-29). These codes were supplemented by the generator of the no-pair hamiltonian written by B. A. Hess in all DK calculations.

3. Results and discussion Spectroscopic constants of BCu, BAg, and BAu molecules are presented in Tables 2-4. The potential energy curves around the minima are shown in Figure 1 . For all molecules the correlation effects are very important. Some bonding is, however, obtained already at the CASSCF level without any dynamic electron correlation being taken into accounts. The analysis of molecular orbitals clearly shows that the bonding is mainly due to relatively strong single c bond orbital arising predominantly from 2p(B) and ns(Me) orbitals. One also finds quite significant contribution from 2s(B) and very little contribution from (n-l)p(Me) orbitals. This is fully compatible with the description of the bonding situation in AlCu (3, 5, 6).

Molecular Properties of Boron-Coinage Metal Dirners

263

Table 2

Spectroscopic constants of BCu. The (3110) active space with 4 correlated electrons w a s used in CAS calculations. In CC calculations 14 or 20 electrons were correlated.

N onrelat ivist ic CASSCF CASPT2

2.249 1.910

1.073 2.806

320 518

3.27 4.25

0.3555 0.4932

Relativistic CASSCF + M V D 2.188

1.019

343

3.45

0.3757

1.017 2.354 1.893 1.430 1.986 1.522

34 1 563 497 544 509 555

3.66 4.34 3.53 4.29 3.56 4.33

0.3753 0.5169 0.4689 0.4895 0.4748 0.4939

N o - pairDIC

CASSCF CASPT2 C C S D - 14 C C S D ( T ) - 14 CCSD - 20 C C S D ( T ) - 20

2.189 1.865 1.959 1.917 1.947 1.909

Table 3 Spectroscopic constants of BAg. The (31 10) active space with 4 correlated electrons was used in CAS calculations. In CC calculations 14 or 20 electrons were correlated.

N onrelativistic CASSCF CASPT2

2.547 2.258

0.655 1.248

261 341

3.00 2.32

0.2604 0.3313

Relativistic C A S S C F M V D 2.397

0.833

296

2.50

0.2940

0.850 1.684 1.403 0.774 1.507 0.910

302 425 404 416 423 440

3.22 3.41 2.58 3.01 2.75 3.26

0.2972 0.3833 0.3576 0.3674 0.3664 0.3775

+

N o - pairDIC CASSCF CASPT2 C C S D - 14 CCSD(T) - 14 C C S D - 20 CCSD(T) - 20

2.384 2.098 2.173 2.144 2.147 2.115

Maria Barysz and Miroslav Urban

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Table 4

Spectroscopic constants of BAu. The (3110) active space with 4 correlated electrons was used in CAS calculations. In CC calculations 14 or 20 electrons were correlated.

Re(A) D,(eV) Nonrelativistic CASSCF CASPT2

w , ( c ~ - ' ) weXe(Cm-')

B,(cm-')

2.546 2.256

0.708 1.261

266 362

2.95 3.06

0.2495 0.3177

Relativistic CASSCF + M V D 2.119

1.588

447

3.91

0.3601

2.078 3.519 3.107 2.529 3.248 2.709

521 676 633 649 646 663

3.93 4.76 3.96 4.04 3.94 4.01

0.3802 0.4338 0.4159 0.4187 0.4181 0.4210

N o - pairDK CASSCF CASPT2 CCSD - 14 CCSD(T) - 14 CCSD - 20 C C S D ( T )- 20

2.062 1.931 1.972 1.965 1.967 1.960

It has been found in the case of the coinage metal hydrides (12) that relativity induces some sd hybrides into the 6, orbital of AuH. In contrast to this finding we do not observe any sd hybridization in the BAu molecule. Both correlation and relativistic effect strengthen the bond. Results of CC calculations with 14 and 20 correlated electrons (i.e. not including or including (n-1)p electrons of the metal atom) are quite close to each other. However, the observed differences are by no means negligibe. Once again, the most sensitive is the dissociation energy where, e.g., for BAu the difference is about 0.1 eV at both CCSD and CCSD(T) levels of approximation. The importance of relativistic contributions for the shape of the potential energy curves of BCu, BAg, and BAu is clearly demonstrated by plots presented in Figure 1. The analysis of individual contributions is displayed in Table 5. In BAu the relativistic contribution is larger than the correlation contribution and for its dissociation energy the relativistic term, the correlation term and the mixed (correlation-relativistic) term are all of the same sign. For the equilibrium distance the mixed term partly compensates the shortenting of the The comparison of the two correlated methods, CASPT2 and CCSD(T) shows relatively good agreement for Re , w e , and Be and significant differences for D, and w e x e . bond resulting from both correlation and relativistic contributions.

Molecular Properties of Boron-Coinage Metal Dirners

4.n

m~ 48

Fig.1. CASPlZ potential energy curves of BCu, BAg, and BAIL

4.11

F i g 3 Dipole moment curves of BCIL

r. LU

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Maria Barysz and Miroslav Urban

266

Table 5 Correlation, relativistic and mixed contributions to Re and De of BCu, BAg and BAu (data from CASSCF, CASPT2 and no-pair DK calculations) relative to the corresponding nonrelativistic CASSCF values (100%). BCu CASSCF(100%)

Re

De

2.249

1.07

BAS Re 2.547

De 0.66

BAu Re 2.546

De 0.71

corr.con.

-0.339 +1.73 -0.289 +0.59 -0.290 +0.55 15 162 11 91 11 78

rei.con.

-0.060 2.7

(%I

(%I

mixed t e r m (%)

-0.06 -0.163 5 6

+0.20 -0.484 +1.37 30 19 194

+0.015 -0.396 +0.003 +0.240 +0.159 +0.888 .6 37 .I 37 6 125

One can expect that the wex, constant is better represented by the CASSCF and CASPT2 wave functions since these methods are applicable over the whole range of distances (our calculations have been carried out for distances through 20 a.u.). In contrast, the single reference C C methods are not expected to be well behaved for distances larger than about 1.5 of R e , i.e., larger than 5.0 - 5.5 a.u. Then, the multirefence character of the wave function deteriorates the quality of CC results. The convergence is worse and the TI diagnostic (30) increases. The dissociation energy is, however, calculated from energies at the equilibrium distance, where the wave function is very well represented by a single determinant reference and from energies of the separated atoms (using the ROHF reference with spin adapted amplitudes in CCSD, see Ref. (28)). Since three valence electrons at the boron atom and either 11 (n-l)d ns or 17 (n-l)p (n-l)d, ns electrons of the coinage atoms are correlated at the equal footing, the CC results are expected to be more reliable in the case of the dissociation energy. This is also supported by the decrease of the dissociation energy with the increasing active space and the number of correlated electrons for the test case of BCu with CASPT2 (see Table 1). Large contributions to the difference between CASPT2 and CCSD(T) dissociation energies seem to arise from triple excitations (see Tables 2-4). The dipole moment curves are presented in Figures 2-4 . The positive sign of the dipole moment means the polarity B(+)Me(-). The relativistically calculated CASPT2-DK curves differ considerably from their nonrelativistic counterparts. Moreover, the CASPT2-DK and CCSD(T)-DK

267

Molecular Properties of Boron-Coinage Metal Dimers

r,

-i

4.I

1

1. LIL

F i g 3 Dipole moment CLIJVCS ofBAg

L U

Fig.4. Dipole moment cwcs of BAP

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Maria Barysz and Miroslav Urban

curves differ considerably from each other. This reflects the larger sensitivity of the dipole moment (charge distribution) to the level of including the electron correlation effects. However, at least for BAg and BAu, the overall shape of the CASPT2-DK and CCSD(T)-DK curves is similar. For presumably most reliable CCSD(T) method and 20 correlated electrons the first derivatives of the dipole moment with respect to the bond lenghts in the vicinity of the equilibrium are: 0.15 a.u. for BCu , 0.20 a.u. for BAg, and 0.49 a.u. for BAu. More interesting than the absolute values are the trends in the series: the CASPTP-DK dipole moment curve oscillates around zero value for BCu and the CASPTP-DK curve for BAg is only slightly positive for distances longer than 3.9 a.u. In the relativistic approximation the equilibrium distance of BAg is 3.965 a.u. for CASPTZDK and slightly larger for CCSD(T)-DK. The CCSD(T)-DK dipole moment of BAg is negative up to the distance of 5 a.u. Quite different are the calculated dipole moments for the BAu molecule. The CASPTP-DK values are positive for distances above 3.4 a.u. (the equilibrium distance is 3.649 a.u.) and for CCSD(T) they are positive for distances larger than 4.0 a.u. This different behaviour of the dipole moment curves gives a key for the explanation of relativistic effects on the bond in the BAu molecule, which (i) has a shorter equilibrium distance than the BAg molecule, (ii) has the highest dissociation energy of all three molecules, and also (iii) has the largest harmonic vibrational frequency in the series. This indicates that in BAu the flow of electrons in the formation of the bond is different from that in BAg and BCu. In contrast to BCu and BAg, the transfer of electrons in BAu, at least for long enough interatomic separations, is in the direction from B to Au. This charge transfer is compatible with the relativistic enhancement of the electron affinity of the gold atom (8, 11, 19). In the CCSD(T) approximation with 17 correlated electrons EA of Au is 1.161 eV while the corresponding DK value amounts to 2.229 eV. For Cu and Ag this relativistic effect is much smaller (8). Quite instructive is the comparison of the CASSCF Mulliken charges for BAg and BAu obtained for instance, at the distance of 4.5 a.u. (2.38 A, an arbitrary bond distance in between the relativistic and nonrelativistic energy minima for both molecules): With the nonrelativistic CASSCF one finds a negative charge at Ag(-O.06el) whereas the CASSCF-DK method gives the formal charge of -0.13el. In BAu the charges are Au(-O.O8el) for nonrelativistic CASSCF and Au(-0.47el) for CASSCF-DK. The idea of the charge transfer was used in the interpretation of the mechanism of the interaction of Cu and Ag with the water molecule for different configurations of the complex (31) (see also (32, 33)). Another effect which should be considered is the relativistic shrinkage of the ns valence shell of the metal. This can contribute to the valence repulsion (making it weaker), although, on the other hand, this may also reduce the overlap between the boron 2p orbital and the ns orbital of the

269

Molecular Properties of Boron-Coinage Metal Dimers

metal. Once the atoms are close enough, the weaker valence repulsion prevails and brings about stronger attraction at shorter distances. The relativistic decrease of the polarizability (10) may influence the dispersion interaction but this effect is very small in comparison to other forces in the region of the equilibrium. Some of our results for BAu can be compared to experiment (2) but no experimental data seem to be available for BCu and BAg. To the best of our knowledge no other theoretical ab initio data are available as well. Experimental dissociation enthalpy for the process BAu(g) c--t B(g) A(g) varies from 337.8f16 kJ/mole [3.50f0.17 eV] to 364f11 [3.77f0.11 eV] and 356.5f20.9 [3.70f0.22 eV] (2, 34, 35, 36). The corresponding value with respect to the 298 K reference temperature is given as 368f11 kJ/mole [3.81f0.11 eV] (34). These values agree quite well with our CASPT2 values, but are much higher than CCSD(T) values although the later are considered to be more reliable. The bond lenghts and vibrational frequencies were only very roughly estimated using empirical formulae and vary from 1.91 A, 2.11 A up to 2.35 A for Re and 989 cm-', 650 cm-', and 554 cm-I for w e . To compare our theoretical data with experimental reaction enthalpies more precisely, we need t o add to our values some corrections. The zero point vibrational energy (ZPE) for BCu with CASPT2 and CCSD(T) (20 correlated electrons) are practically identical, 0.035 and 0.034 eV, respectively. With thermal vibrational correction, 0.005 eV, the change of translational and rotational energy and the P6V term for 298 K, all these corrections give the final enthalpy of the reaction BCu(g) +-+ B(g) Cu(g) identical to within 0.005 eV with D, listed in Table 2. The same holds for BAg and BAu with the ZPE (CASPT2) values of 0.026 eV and 0.042 eV, respectively. Almost the same values are obtained with the CCSD(T) method. Thus, the above mentioned comparison of our theoretical results with experiment for BAu does not change upon adding different corrections. Our data for BCu and BAg may predict their spectroscopic constants.

+

+

4.

Conclusions

The three molecules , BCu BAg, and BAu are stable species with a singlet 'C+ ground state. The strong binding in these molecules arises mainly due to a (T bond formed mainly by 2p(B) and ns(Me) orbitals with the significant contribution from the 2s(B) orbital. Both relativistic (mainly in the BAu molecule) and correlation effects must be considered very carefully if reasonably accurate results are to be obtained. Although a qualitative description of the studied molecules is already achieved at the level of the non-relativistic SCF approximation, any quantitative data can only follow from high-level-correlated calculations with relativistic effects being taken

Maria Barysz and Miroslav Urban

270

into account for heavier atoms. The relativistic effects change the order of dissociation energies. According to CASPT2-DK and CCSD(T)-DK (20 correlated electrons) methods most stable is BAu with the corresponding dissociation energies of 3.52 eV and 2.71 eV, followed by BCu (2.35 eV and 1.52 eV) . The least stable is BAg ( D , is 1.68 and 0.91 eV with CASPTSDK and CCSD(T)-DK, respectively). The nonrelativistic CASPTS values are 1.261 eV (BAu), 2.81 eV (BCu) and 1.25eV (BAg). Using the CASSCF and CASPTZ data one concludes that the pure correlation effects increase the dissociation energy of BCu by a factor of 2.6. When both correlation and relativistic effects are considered simultaneously, D, increases by a factor of 2.2. In BAg these factors are 1.9 and 2.6; with BAu the relativistic effect dominates, the respective factors being 1.8 and 5.0. The charge distribution in BCu and BAg is different from that in BAu. In contrast to BCu and BAg, the polarity of BAu indicates, at least for distance longer than the equilibrium separation, a shift of electrons from boron to the gold atom. This different behaviour is connected to the relativistic increase of the electron affinity of the gold atom. The relativistic shrinkage of the ns valence shell electrons also makes the bond stronger.

5.

Acknowledgements

A part of calculations reported in this paper has been carried out at the Department of Theoretical Chemistry of the University of Lund. One of us (MB) wishes to acknowledge the financial support from the Committee for Scientific Research (KBN) extended to her under the research grant KBN No. 2P30310407. MU thanks the Slovak grant agency for support of this work, under contract No. 1/1455/1994.

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Molecular Properties of Boron-Coinage Metal Dirners

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