Chemical Physics 311 2005 177 186 www elsevier com locate chemphys Multiple bonds to gold a theoretical investigation of XAuC X F Cl Br I molecules Cristina Puzzarini 1 Kirk A Peterson Department of Chemistry Washington State University Pullman WA 99164 4630 USA Received 28 May 2004 accepted 4 October 2004 Available online 5 November 2004 Abstract Structures and spectroscopic properties are reported for the linear XAuC X F Cl Br I series of molecules and their related diatomic species at a high level of accuracy The singles and doubles coupled cluster method including a perturbational correction for connected triple excitations CCSD T with systematic sequences of new correlation consistent basis sets have been employed Scalar relativistic e ects have been accurately included by making use of relativistic e ective core potentials Extrapolation to the complete basis set limit has been used with accurate treatments of core valence correlation and spin orbit e ects in order to accurately predict spectroscopic properties as well as dissociation and atomization energies at 0 K of AuC AuC AuF AuCl AuBr AuI and the XAuC molecules The Au C bond length in the FAuC molecule is predicted to be nearly identical to that calculated for AuC which makes it the shortest known for a neutral gold molecule The Au C and Au F 0 K bond dissociation energies in FAuC are predicted to be 92 5 and 93 1 kcal mol respectively also making them some of the strongest known bonds to gold 2004 Elsevier B V All rights reserved Keywords Gold carbides Gold halides Ab initio theory Pseudopotentials Spectroscopic properties Thermodynamic properties 1 Introduction In recent years the chemistry of gold has received considerable attention in inorganic as well as theoretical chemistry see for example 1 4 and other references therein Even though the chemistry of gold is dominated by single chemical bonds it is well known that gold can form unusually strong chemical bonds due to relativistic e ects 5 Nevertheless no genuine multiple bonds to gold are experimentally known apart from certain types of carbenes e g Au CL and 2 L2 C Au CL 6 8 The strongest partial triple bond 2 Corresponding author Tel 1 509 335 7867 fax 1 509 335 8867 E mail addresses cristina puzzarini unibo it C Puzzarini kipeters wsu edu K A Peterson 1 Permanent address Dipartimento di Chimica G Ciamician Universita di Bologna Via Selmi 2 I 40126 Bologna Italy 0301 0104 see front matter 2004 Elsevier B V All rights reserved doi 10 1016 j chemphys 2004 10 004 was predicted in ab initio calculations to exist in AuC 2 3 In 1998 this species was the subject of two theoretical studies both carried out by Pyykko and co workers 2 3 they evaluated the equilibrium bond length and dissociation energy using both all electron quasirelativistic Douglas Kroll DK and relativistic e ective core potential RECP methods These computations revealed a short Au C bond distance of about 1 77 A with an equilibrium dissociation energy to ground state atoms of 3 8 eV In 3 an attempt to predict further triple bonds to gold in particular for neutral species was also reported and multiple bond character was observed for Cl2AuCH and Cl2AuN More recently Pyykko et al 4 carried out additional calculations on AuC at the B3LYP and MP2 levels of theory as calibration calculations for the subsequent investigation on Au C Au 2 species The present work was stimulated by the lack of data on neutral molecules containing multiple bonds to gold 178 C Puzzarini K A Peterson Chemical Physics 311 2005 177 186 as well as the recent development of correlation consistent basis sets for Au The series of XAuC molecules where X is a halogen atom F Cl Br and I were chosen since they were expected to perhaps have a multiple bond between Au and C by analogy with AuC assuming that the halogen atom would withdraw electron density from Au In order to fully characterize these species the molecular and spectroscopic properties have been evaluated at the coupled cluster level with the accurate inclusion of relativistic core valence and spin orbit effects In order to determine the thermodynamic properties of these compounds namely dissociation and atomization energies the related diatomic and atomic fragments have also been characterized at the same level of accuracy ergy gradients the hessian matrix was calculated numerically 20 21 by nite di erences For the diatomics near equilibrium potential energy functions PEFs were calculated by accurately tting a total of 7 points to polynomial expansions in displacement coordinates Spectroscopic parameters were then determined from the tted PEFs by the usual second order perturbation theory expressions 22 In the R UCCSD T calculations on the atomic fragments fully symmetry equivalenced orbitals were used If not otherwise mentioned the frozen core approximation has been used throughout core de nitions C F 1s Cl 1s2s2p Br 3s3p3d I 4s4p4d Au 5s5p Complete basis set limits of the total energies have been obtained by using the mixed exponential Gaussian extrapolation formula 23 24 2 Methodology E n ECBS Be n 1 Ce n 1 The standard aug cc pVnZ basis sets 9 10 have been employed for the C F and Cl atoms The newly developed aug cc pVnZ PP sets 11 have been used for the Br and I atoms these are a series of correlation consistent basis sets in conjunction with small core relativistic pseudopotentials that leave 25 electrons to be handled explicitly for both bromine 3s23p63d104s24p5 and iodine 4s24p64d105s25p5 The Au basis sets corresponded to preliminary aug cc pVnZ PP sets 12 that use a new small core energy consistent relativistic pseudopotential 13 that leaves 19 electrons 5s25p65d106s1 to be explicitly correlated in the ab initio calculations The Hartree Fock spd parts of the Au basis sets are analogous in size to those presented previously 11 14 for Tl Rn but the shells of correlating functions corresponded to 2d1f 3d2f1g 4d3f2g1h and 5d4f3g2h1i for the cc pVDZ PP through cc pV5Z PP basis sets Additional di use augmenting functions were also added in each angular symmetry to form the aug cc pVnZ PP basis sets These will be fully described in another publication but are currently available upon request from the authors In the remainder of the text all of the basis sets mentioned above will be denoted as aVnZ Geometry optimizations and energy evaluations have been performed with the coupled cluster singles and doubles method with a perturbative correction for connected triple