Accurate theoretical near-equilibrium potential energy and dipole moment surfaces of HgClO and HgBrO

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1 JOURNAL OF CHEMICAL PHYSICS VOLUME 120, NUMBER 14 8 APRIL 2004 Accurate theoretical near-equilibrium potential energy and dipole moment surfaces of HgClO and HgBrO Nikolai B. Balabanov a) and Kirk A. Peterson b) Department of Chemistry, Washington State University, Pullman, Washington Received 3 December 2003; accepted 12 January 2004 The complexes HgBrO and HgClO have been previously determined by ab initio methods to be strongly bound and were suggested to be important intermediates during mercury depletions events observed in the polar troposphere. In the present work accurate near-equilibrium potential energy surfaces PESs of these species are reported. The PESs are determined using accurate coupled cluster methods and a series of correlation consistent basis sets with subsequent extrapolation to the complete basis set limit. Additive corrections for both core valence correlation energy and relativistic effects are also included. The anharmonic ro-vibrational spectra of HgBrO and HgClO have been calculated in variational calculations. Strong infrared band strengths are predicted for all fundamentals in these species. The spin orbit splitting dominates over the vibronic coupling effect in both HgClO and HgBrO. The Renner Teller vibronic energy levels corresponding to the bending mode of these molecules are calculated via perturbation theory American Institute of Physics. DOI: / I. INTRODUCTION The springtime mercury depletion events MDE observed in both the Arctic and Antarctic tropospheres have been of growing interest over the past several years. 1 3 During the MDEs mercury is believed to be converted from its elemental vapor form to reactive gaseous mercury RGM species with short atmospheric lifetimes, which then become deposited on snow and ice surfaces. 2,3 The mechanism for MDE formation suggested in previous experimental studies 1 3 is based on the observation that MDEs resemble analogous depletions of ozone concentrations 4 at the onset of polar spring. The first step of this process consists of the generation of chlorine and bromine atoms through photochemical processes. The halogen radicals Br and Cl may than either directly oxidize elementary mercury to form RGM species HgBr 2, HgCl 2, etc., 2,3 or first react with tropospheric ozone to produce halogenated radicals BrO and ClO that in turn oxidize the gaseous elemental mercury. 2,3 The latter chemical path was supported by the presence of strong correlations in the concentrations of Hg and BrO during MDEs. 2 In our previous work, 5 the heats of reaction of Hg with halogen species, including BrO and ClO, were accurately determined in ab initio calculations. The abstraction reactions Hg XO HgX O and Hg XO HgO X X Br, Cl were found to be endothermic by kcal/mol and therefore are unlikely to play significant roles in the oxidation of atmospheric mercury. In addition, the insertion reactions resulting in the formation of the stable linear Cl Hg O and Br Hg O complexes were calculated to be exothermic by 20 kcal/mol, however, they are expected to have high barriers. 6 However, both the HgBrO and HgClO complexes are strongly bound by kcal/mol compared to both a Electronic mail: nick@mail.wsu.edu b Electronic mail: kipeters@wsu.edu the HgO X and HgX O X Cl, Br asymptotes. Therefore, it was proposed that HgBrO and HgClO should be considered important RGM species produced during MDEs through barrierless 6 reactions such as HgX O HgXO X Br, Cl. The HgO X channel is not expected to be important based on the small stability of HgO. See Ref. 7. The information about the structure and spectra of both HgBrO and HgClO that could help in detection of these species and in modeling the mercury oxidation processes in the troposphere, however, is largely missing. To our knowledge there are no experimental data on the structure and spectra of either HgClO or HgBrO and the ab initio calculations of Ref. 5 are believed to be the first focusing on these species. The present work is concerned with the calculation of highly accurate ab initio near-equilibrium potential energy surfaces PESs and electric dipole moment surfaces for Hg- BrO and HgClO in order to reliably predict the spectroscopic parameters of these species. The PESs are determined by the CCSD T method with a series of correlation consistent basis sets up to 5Z quality. The extrapolations to the complete basis set limits were then determined and additive corrections for core valence correlation effects, spin orbit coupling, and scalar relativistic effects were also taken into account. The band origins in the vibrational spectra of HgBrO and HgClO together with their infrared intensities were predicted by the variational method. The analysis of the bending modes of these molecules were also carried out with an account of both the Renner Teller effect and spin orbit coupling. A methodology similar to that described in this work was recently employed in calculations of the nearequilibrium PESs of HgBr 2, HgCl 2, and HgBrCl. 8 The equilibrium geometries and spectroscopic constants obtained from those PESs were in excellent agreement with the available experimental data on those molecules /2004/120(14)/6585/8/$ American Institute of Physics

2 6586 J. Chem. Phys., Vol. 120, No. 14, 8 April 2004 N. B. Balabanov and K. A. Peterson II. METHODOLOGY All electronic structure calculations in this work were carried out with the MOLPRO suite of programs. 9 At each molecular geometry chosen to sample the potential surface see below the total energy, E, was determined as E E CBS E CV E SO E SR, where E CBS is the complete basis set limit obtained within the frozen core approximation with a series of correlation consistent basis sets, E CV is the correction for the core valence electron correlation energy, E SO is the contribution of spin orbit coupling, and E SR is a correction for scalar relativistic effects due to Cl and O. The total energies in the frozen core approximation were found by the spin unrestricted ROHF/UCCSD T method, 10 i.e., spin unrestricted coupled cluster singles and doubles with perturbative triples based on restricted open shell Hartree Fock orbitals. In these calculations only the valence 2s2p O, 3s3p Cl, 4s4p Br, and 5d6s Hg electrons were correlated. The oxygen basis sets corresponded to the standard aug-cc-pvnz sets. 11 The chlorine basis corresponded to the aug-cc-pv(n d)z basis sets, i.e., the aug-cc-pvnz chlorine basis sets from Ref. 12 with modified sets of tight exponents for the d-shell. 13 The Br 1s 2 2s 2 2p 6 electrons were described with a small core relativistic effective core potential RECP and the valence Br electrons were treated with the new aug-cc-pvnz-pp basis sets. 14 In addition, the RECP of Häussermann et al. 15 and the recently developed aug-cc-pvnz-pp valence basis sets 16 were employed on mercury. Overall, basis sets with n 2 DZ, 3 TZ, 4 QZ, and 5 5Z were used. In the following these basis sets will be denoted simply as AVnZ. In all cases only the pure spherical d-, f-, g-, and h-function sets were employed in the calculations. The complete basis set extrapolations of the CCSD T total energies were obtained by using both the mixed exponential/gaussian function 17 E n E CBS Be n 1 Ce n 1 2, where n 2 4 DTQ or n 3 5 TQ5 and a twoparameter extrapolation formula 18 that depends on the reciprocal of n l max, E n E CBS B/n 3, where n 3, 4 TQ or n 4, 5 Q5. The corrections for core valence electron correlation effects were calculated with the correlation consistent weighted core valence basis sets, 19 aug-cc-pwcvtz, on both O and Cl with analogous aug-cc-pwcvtz-pp basis sets on Br and Hg. In comparison to the standard aug-cc-pvtz and aug-cc-pvtz-pp basis sets, the aug-cc-pwcvtz and aug-cc-pwcvtz-pp sets contained extra tight functions in each shell optimized explicitly for core valence correlation. The net core valence correction E CV is obtained as E core val E val, where E core val is the CCSD T /aug-ccpwcvtz total energy calculated with all electrons correlated and E val is the CCSD T /aug-cc-pwcvtz total energy obtained in the frozen core approximation In order to calculate the correction due to spin orbit coupling, E SO, the spin orbit eigenstates were obtained by diagonalizing a spin orbit matrix H el H SO in a basis of pure-spin electronic eigenstates, i.e., within the stateinteracting approach. The H el H SO matrix was constructed from the 2 x and 2 y components of the 2 electronic state for the linear geometries and the 2 A and 2 A states for the bent structures. The energies of the electronic states and spin orbit matrix elements were obtained using the internally contracted multireference configuration interaction MRCI method. 20 The molecular orbitals used in the MRCI calculations were obtained from state-averaged CASSCF calculations. The Hg 5d, O 2s, Cl 3s, and Br 4s orbitals were not correlated in either the CASSCF or MRCI calculations and the basis sets corresponded to AVTZ. Relativistic pseudopotentials 21 were also applied on O (1s 2 in core and Cl (1s 2 2s 2 2p 6 in core in order to employ the effective spin orbit potentials of these RECPs. In these cases the s and p shells of the AVTZ basis sets on O and Cl were recontracted in the presence of the RECPs. In all cases the SO operators used for setting up H SO were derived from the pseudopotentials. Finally, E SO was found as the difference of the lowest spin orbit eigenvalue and the MRCI total energy of the lowest pure-spin electronic state. The scalar relativistic correction to the total energy, E SR, which accounted for the small effects due to the O and Cl atoms, was obtained as a sum of the matrix elements of the one-electron Darwin and mass-velocity MV terms in the Breit Pauli Hamiltonian 22 calculated with the single reference configuration interaction singles and doubles CISD method and the aug-cc-pvtz and aug-cc-pvtz-pp basis sets. While there is a possibility of double counting the scalar relativistic effects due to the nonzero contributions of the atoms with RECPs to these expectation values, it has recently been demonstrated by Feller et al. 23 in calculations on Br 2 that this effect is negligible for energy differences. We have also investigated this point in regards to the bond length of Br 2 and found that the calculated r e values with and without the inclusion of the MV Darwin correction in the presence of the RECP differed by just Å. Overall 74 points 50 symmetry unique points around the approximate equilibrium geometries were computed for each molecule in the range 0.4a 0 R 0.4a 0 for shifts R R R e in the HgX X Cl, Br and HgO distances and for the X Hg O valence angles. Polynomial functions of the form V Q 1,Q 2,Q 3 ijk C ijk Q 1 i Q 2 j Q 3 k 4 were fitted to the obtained grids. In Eq. 4, the Simons Parr Finlan coordinate 24 (R R e )/R was used for Q 1 and Q 2, which corresponded to the Hg X and Hg O distances, respectively, and Q 3 (X Hg O) 180. The surface fitting and calculation of the force constants and spectroscopic parameters via second order perturbation theory were carried out with the SURFIT program. 25 At each geometry chosen for the determination of the potential surfaces, the components of the electric dipole moment were also calculated using the CCSD T method and

3 J. Chem. Phys., Vol. 120, No. 14, 8 April 2004 Energy of HgClO and HgBrO 6587 TABLE I. CCSD T equilibrium geometries and harmonic frequencies for HgClO and HgBrO. Approximation level r e (HgX Å) r e (HgO Å) 1 (cm 1 ) 2 (cm 1 ) 3 (cm 1 ) HgClO AVDZ AVTZ AVQZ AV5Z CBS1 DTQ CBS1 TQ CBS2 TQ CBS2 Q CBS2 TQ CV CBS2 TQ CV SO CBS2 TQ CV SO SR HgBrO AVDZ AVTZ AVQZ AV5Z CBS1 DTQ CBS1 TQ CBS2 TQ CBS2 Q CBS2 TQ CV CBS2 TQ CV SO CBS2 TQ CV SO SR the AVTZ basis sets. The values were obtained within a finite field approach with field strengths of a.u. The computed dipole moments were transformed into an Eckart frame 26 where the B axis was defined to be orthogonal to the molecular C v axis A axis of the equilibrium geometry, the origin being assigned to the center of mass. A quartic polynomial function analogous to Eq. 4, but with simple displacement coordinates for the stretches, was then fitted to the grids of Eckart frame dipole moment components with resulting coefficients D (x,y) ijk. The analytical potential energy and dipole moment surfaces were then used in three-dimensional variational calculations of the vibrational energy levels and IR band intensities using the program packages of Tennyson and co-workers DVR1D, ROTLEVD, and DIPOLE. 27,28 The variational calculations were carried out in a Jacobi (r,r, ) coordinate system, where r is the Hg X distance, R is the distance from the O atom to the center of mass on the Hg X axis and the coordinate is the angle between r and R. The basis set consisted of 12 Morse-type functions for the radial coordinates r and R. The angle coordinate was treated with 120 discrete variable representation DVR points in the range on associated Legendre polynomials. The twodimensional 2D effective radial Hamiltonian was solved at each DVR point and the lowest 2000 eigenvalues were then selected to construct the full 3D Hamiltonian matrix. The ro-vibrational calculations were performed for rotational quantum number J 0 and 1, where the solutions for J 1 were found in a two-step procedure. 27 The obtained rovibrational wave functions were then used to calculate the electric dipole moment matrix elements, and the infrared intensities were found in terms of the band strength S, 29 S 4 2 N A 3 c , 5 S km/mol cm , where is the frequency of the band origin and are the components of the electric dipole moments in atomic units. III. RESULTS AND DISCUSSION Both HgBrO and HgClO have linear X Hg O equilibrium geometries, and in the absence of spin orbit coupling they have 2 ground electronic states. The main contribution to the singly occupied MOs are from the 2p x /2p y oxygen atomic orbitals assuming the molecules to lie along the z axis. With bending of the molecules and concomitant lowering of the molecular symmetry from C v to C s, the energetically degenerate 2 state splits into 2 A and 2 A states due to the Renner Teller effect, 30 the 2 A state having the lowest total energy for bent molecular geometries. After spin orbit coupling is taken into account, the linear 2 state splits into 2 3/2 and 2 1/2 spin orbit states. First we consider the structural and spectroscopic parameters of only the lowest 2 electronic or the lowest 2 3/2 spin orbit state, i.e., we assume the low-lying energy levels of these Renner Teller molecules can be approximately found within the adiabatic approximation. A. The adiabatic approximation The equilibrium internuclear distances and harmonic vibrational frequencies of HgBrO and HgClO calculated at the CCSD T level of theory with a series of correlation consis-

4 6588 J. Chem. Phys., Vol. 120, No. 14, 8 April 2004 N. B. Balabanov and K. A. Peterson tent basis sets are displayed in Table I. Note that due to a different fitting procedure for the PESs in the present work, these results differ slightly from the subset of results published in our previous work. 5 Like in the analogous HgXY species 8 X, Y Br, Cl the systematic expansion of the correlation consistent basis sets from double to quintuple zeta quality leads to a shortening of both the mercury-halogen and mercury-oxygen distances with an increase in the stretching vibrational frequencies. The convergence of the structural parameters is fast: the changes in distances and vibrational frequencies decrease at least by a factor of 2 with each basis set expansion step. At the last step from the AVQZ to AV5Z basis, the internuclear distances shorten by less than Å and the stretching frequencies increase by just 3 cm 1. The values of the bending harmonic frequencies do not change either significantly or systematically with the extension of the basis sets. The use of extrapolation formulas for the total energies consistently results in a further decrease in the bond lengths and an increase in the stretching frequencies. Relative to the distances obtained at the CCSD T /AV5Z level the mixed exponential Gaussian formula 2 with use of the AVDZ, AVTZ, and AVQZ energies CBS1 DTQ predicts only slightly Å shorter distances. The n 3 extrapolation formula 3 with use of the AVQZ and AV5Z total energies CBS2 Q5 yields distances Å shorter than the CCSD T /AV5Z results. Two other extrapolation schemes listed in Table I, CBS1 TQ5 and CBS2 TQ, lead to only moderate shortening of the distances by Å compared to the CCSD T /AV5Z values. We believe that the latter results are more consistent with the distances calculated in the AVnZ basis set series. Hence, all of following calculations are based on the more conservative CBS2 TQ total energies. The inclusion of core valence electron correlation effects results in additional strong shrinkages of Å of both Hg X and Hg O bond distances and corresponding increases of 3 6 cm 1 of the stretching vibrational frequencies with respect to the CBS2 TQ results. Due to the open shell electronic structure of HgClO and HgBrO, the spin orbit coupling effects in these molecules are fairly strong. In both species the SO splitting of the 2 state near equilibrium amounts to 500 cm 1. Therefore one can expect the spectroscopic properties of HgClO and Hg- BrO to be affected by the inclusion of spin orbit coupling. Indeed, the addition of the SO correction to the total energy results in a shortening by as much as Å of the Hg O distances. The Hg X distances, however, only change by less than Å. The CBS2 TQ CV SO stretching frequencies in HgBrO and HgClO also do not change significantly compared to the CBS2 TQ CV values. The SO effect on the bending harmonic frequencies in HgBrO and HgClO is to increase them by 2 3 cm 1. In regards to scalar relativistic effects, the large effects due to the Hg and Br atoms were already taken into account by RECPs on these atoms. The residual scalar relativistic effects due to Cl and O, which were explicitly recovered via the mass velocity and Darwin terms SR, give negligibly small corrections to the bond distances and harmonic vibrational frequencies. As in our previous work on the structures and spectra of TABLE II. The expansion coefficients in a.u. of the final ab initio potential energy functions for HgClO and HgBrO. a i j k HgClO b C ijk HgBrO c a CCSD T /CBS2 TQ CV SO SR level of theory. See the text. b Expanded around R(Hg Cl) a 0, R(Hg O) a 0. c Expanded around R(Hg Br) a 0, R(Hg O) a 0. HgXY species X, Y Cl, Br, 8 the effect of the pseudopotential approximation on the calculated equilibrium structures has been investigated by carrying out all-electron CCSD T calculations using the Douglas Kroll Hess Hamiltonian DK 31 with the aug-cc-pvqz-dk AVQZ-DK basis sets described in Ref. 8. The results of these optimizations; r e (HgCl) Å, r e (HgO) Å for HgClO and r e (HgBr) Å, r e (HgO) Å for HgBrO; are nearly identical to the AVQZ SR values that can be extracted from Table I, namely differences of just and Å for r e (HgCl) and r e (HgBr), respectively, and negligible differences for the HgO distances. Hence, the PP approximation is not a significant source of error in the present calculations. The expansion coefficients of the final near equilibrium potential energy surfaces of HgBrO and HgClO obtained by fitting to the CBS2 TQ CV SO SR total energies are given in Table II. The quartic force fields of HgBrO and HgClO in internal coordinates are presented in Table III. Inspection of these force fields indicate the presence of two relatively strong bonds, Hg X and Hg O. Both HgBrO and HgClO are dominated by their harmonic terms and exhibit a lack of significant coupling between the coordinates. This dominance of the harmonic force constants in HgBrO and HgClO makes it possible to accurately calculate the lowlying vibrational energy levels by second order perturbation theory. The spectroscopic parameters of HgBrO and HgClO determined in this manner are listed in Table IV, while selected anharmonic vibrational levels found in the variational calculations are shown in Table V. As seen from Tables IV and V, the values of the fundamental frequencies obtained

5 J. Chem. Phys., Vol. 120, No. 14, 8 April 2004 Energy of HgClO and HgBrO 6589 TABLE III. Quartic internal coordinate force fields for HgXO X Br, Cl in aj Å n rad m ). a Force constants HgClO HgBrO f rr f RR f f rr f rrr f RRR f rrr f rrr f r f R f rrrr f RRRR f f rrrr f rrrr f rrrr f rr f RR f rr a Using the potential function of Table II. The definition of force constants correspond to the expansion V V e (1/2!) ij f ij q i q j (1/3!) ijk f ijk q i q j q k (1/4!) ijkl f ijkl q i q j q k q l, where q 1 r, q 2 R, q 3 with r R(Hg X), X Br, Cl, and R R(Hg O). 1 aj J. with the variational and perturbation methods are essentially identical. The near-equilibrium electric dipole moment surfaces of HgClO and HgBrO calculated at the CCSD T /AVTZ level are given in Table VI. At its equilibrium geometry HgClO has a small dipole moment of D, while the HgBrO molecule has a larger dipole moment of D. In both TABLE IV. Spectroscopic constants of HgClO and HgBrO derived from the CCSD T /CBS2 TQ CV SO SR potential energy functions. a Constant HgClO HgBrO r e (HgX Å) r e (HgO Å) B e (MHz) B 0 (MHz) (MHz) (MHz) (MHz) D e (KHz) q e (MHz) (cm 1 ) (cm 1 ) (cm 1 ) (cm 1 ) (cm 1 ) (cm 1 ) X 11 (cm 1 ) X 22 (cm 1 ) X 33 (cm 1 ) X 13 (cm 1 ) X 12 (cm 1 ) X 23 (cm 1 ) X ll (cm 1 ) G(000) (cm 1 ) a 1 HgX stretch, 2 O Hg X bend, 3 HgO stretch. TABLE V. Selected vibrational band origins cm 1 and infrared band intensities S km/mol. Band HgClO HgBrO v 1 v 2 v 3 S S e e e e e e e e e e e e 1 species the positive end of the dipole along the linear O Hg X axis is directed towards the halogen atom. The first and second derivatives of the Eckart frame dipole moment components with respect to normal coordinates are collected in Table VII. The derivatives of the dipole moment compo- TABLE VI. The expansion coefficients in a.u. of the Eckart frame electric dipole moment functions x and y for HgBrO and HgClO. a i j k HgClO HgBrO x D ijk y D ijk a The molecules are oriented with the x axis along the molecular C v axis.

6 6590 J. Chem. Phys., Vol. 120, No. 14, 8 April 2004 N. B. Balabanov and K. A. Peterson TABLE VII. First and second derivatives of the Eckart frame dipole moments in debye with respect to dimensionless normal coordinates of HgClO and HgBrO. nents for both species are well described by the linear terms, which implies only weak overtone intensities. Furthermore, the coupling between the dipole moment components is negligible in HgBrO and very small in HgClO. Therefore the combination bands in the spectra of these species will also likely have small IR intensities. The intensities of selected bands in the infrared spectra of these molecules are given in Table V. The infrared spectra of both molecules are dominated by the fundamental transitions. The largest IR intensities are 30 km/mol and are observed for the bands corresponding to the Hg O stretches. In HgClO the bands due to bending and the Hg Cl stretching fundamental are predicted to have intensities of approximately the same magnitude and are about half as intense as the Hg O stretch. In HgBrO the Hg Br stretching band is predicted to be much weaker than the bending one. The intensities of the overtone and combination bands are calculated to be very small less than 1 km/mol. In addition, the calculations predict that the 2 2 and 2 3 bands are the strongest ones among the overtones and combinations in the HgClO spectra. The 2 3 overtone has the largest IR intensity among the overtone and combination bands in HgBrO. B. Renner Teller analysis HgClO HgBrO A component B component Generally the description of the energy levels of a linear molecule with an open shell degenerate electronic state should be carried out beyond the adiabatic approximation by taking into account both the Renner Teller effect and spinorbit coupling. The vibronic levels of such a molecule can be found either by second order perturbation theory or by a variational method within the effective Hamiltonian approach. 30 In the present work we have used the perturbation scheme derived by Peric and Peyerimhoff 32 for a simplified model Hamiltonian, where the bending vibrational mode is assumed to be completely separated from the stretches and the anharmonicity of the vibrations is neglected. This approximation is expected to be suitable for both HgBrO and HgClO since the coupling between the bending and stretching coordinates, as well as the anharmonicity of the vibrations, is very small in both of these molecules. TABLE VIII. The vibronic and spin orbit constants of HgClO and HgBrO. Constant HgClO HgBrO k, aj/rad k, aj/rad ,cm A SO,cm The HgClO and HgBrO vibronic and spin-orbit coupling parameters for the model Hamiltonian are found in Table VIII. The 2, k, k, and parameters shown in Table VIII are calculated from the CCSD T /AVTZ PESs. The Renner parameter is defined as k k / k k 6 where k and k are the harmonic bending force constants for the 2 A and 2 A electronic states, respectively. Parameter 2 denoted further as is the harmonic bending frequency for the PES obtained by averaging the total energies of the 2 A and 2 A electronic states. The spin orbit constant A SO denoted further as A, i.e., the equilibrium spin orbit splitting between the 2 1/2 and 2 3/2 states, was previously found in accurate single point calculations at the MRCI/ AVTZ level of theory. 5 In both HgClO and HgBrO the spin orbit coupling significantly dominates the Renner Teller effect and the vibronic coupling itself is small (,A). In the case of a 2 electronic state of a triatomic molecule where the spin orbit coupling is much stronger than the Renner Teller interaction, the vibronic levels can be found by the formulas: 32 E v 1 A K K 1 2 2, 7 8 A E 1,2 v 1 A 1 v K 1 v K A v K 2 1 K 2 4 A v 1 2 A, 8 where v is the bending vibrational quantum number, K is the non-negative vibronic angular momentum quantum number (K v 1, v 1 2,...,1 or 0, i.e., K l, and, l, and are the projections of the electronic, vibrational, and spin angular momenta along the linear axes of the molecules. Equation 7 describes the unique vibronic levels for the case K v 1, while formula 8 describes the pair of vibronic levels that correspond to K v 1. Equations 7 and 8 were obtained in the work of Ref. 32 by using a perturbation scheme where the zeroth-order Hamiltonian included the effective spin orbit operator. Several bending vibronic levels in HgClO and HgBrO are listed in Table IX. The vibronic splittings of the v 1 vibrational levels in HgBrO and HgClO are calculated to be just 0.14 and 0.19 cm 1, respectively. The fundamental transitions 2 3/2 2 5/2 are cm 1 for HgClO and cm 1 for HgBrO, which should be compared to the values and cm 1, respectively, of the harmonic bending frequencies found at the CCSD T /AVTZ level.

7 J. Chem. Phys., Vol. 120, No. 14, 8 April 2004 Energy of HgClO and HgBrO 6591 TABLE IX. Selected bending mode vibronic energy levels in cm 1 for HgClO and HgBrO and their correlation to the harmonic vibrational levels. provide motivation for new spectroscopy experiments involving these species since they have not been observed previously, and particularly since they could be important reactive mercury compounds in the polar troposphere. ACKNOWLEDGMENT This work was supported by the National Science Foundation Grant No. CHE As seen from Table I the values of the bending frequencies in HgBrO and HgClO are changed by 5 cm 1 after including both core valence electron correlation effects and the geometrical dependence of the spin orbit splitting. Therefore these effects should be taken into account in more accurate calculations of the vibronic levels in HgBrO and HgClO. These more accurate determinations of the vibronic levels and intensities of transitions will presumably require a sophisticated variational technique for the treatment of the Renner Teller problem, as has been previously carried out for triatomic molecules by several groups cf. Ref. 33. IV. SUMMARY Accurate near-equilibrium potential energy surfaces of HgBrO and HgClO have been constructed using the CCSD T method and sequences of correlation consistent basis sets with subsequent extrapolation to the complete basis set limits. Additional additive corrections for core valence electron correlation, spin orbit coupling, and scalar relativistic effects were also included. Using these PESs the spectroscopic parameters of the complexes are accurately determined via second order perturbation theory, as well as in variational calculations. Employing three-dimensional CCSD T dipole moment surfaces and anharmonic vibrational wave functions, all of the fundamental bands in the HgClO and HgBrO vibrational spectra are predicted to have strong IR strengths. In addition, it is found that the spin orbit coupling dominates over the Renner Teller effect in both species. The predictions from this work will hopefully 1 W. H. Schroeder, K. G. Anlauf, L. A. Barrie, J. Y. Lu, A. Steffen, D. R. Schneeberger, and T. Berg, Nature London 394, ; T. Berg, J. Bartnicki, J. Munthe, H. Lattila, J. Hrehoruk, and A. Mazur, Atmos. Environ. 35, ; R. Ebinghaus, H. H. Kock, C. Temme, J. W. Einax, A. G. Lowe, A. Richter, J. P. Burrows, and W. H. Schroeder, Environ. Sci. Technol. 36, S. E. Lindberg, S. Brooks, C.-J. Lin, K. J. Scott, M. S. Landis, R. K. Stevens, M. Goodsite, and A. Richter, Environ. Sci. Technol. 36, C. Temme, J. W. Einax, R. Ebinghaus, and W. H. Schroeder, Environ. Sci. Technol. 37, H. Boudries and J. W. Bottenheim, Geophys. Res. Lett. 27, N. B. Balabanov and K. A. Peterson, J. Phys. Chem. A 107, N. B. Balabanov, B. C. Shepler, and K. A. Peterson unpublished. 7 B. C. Shepler and K. A. Peterson, J. Phys. Chem. A 107, N. B. Balabanov and K. A. Peterson, J. Chem. Phys. 119, MOLPRO is a package of ab initio programs written by H.-J. Werner and P. J. Knowles with contributions from J. Almlöf, R. D. Amos, A. Bernhardsson et al. 10 G. E. Scuseria, Chem. Phys. Lett. 176, ; J. D. Watts, J. Gauss, and R. J. Bartlett, J. Chem. Phys. 98, ; P. J. Knowles, C. Hampel, and H.-J. Werner, ibid. 99, T. H. Dunning, Jr., J. Chem. Phys. 90, ; R. A. Kendall, T. H. Dunning, Jr., and R. J. Harrison, ibid. 96, D. E. Woon and T. H. Dunning, Jr., J. Chem. Phys. 98, T. H. Dunning, Jr., K. A. Peterson, and A. K. Wilson, J. Chem. Phys. 114, K. A. Peterson, D. Figgen, E. Goll, H. Stoll, and M. Dolg, J. Chem. Phys. 119, U. Häussermann, M. Dolg, H. Stoll, H. Preuss, P. Schwerdtfeger, and R. M. Pitzer, Mol. Phys. 78, K. A. 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