Rg Atoms Research Articles

Electronic spectroscopy of the Ẽ← X̃ transition of NO–Kr and shielding/penetration effects in Rydberg states of NO–Rg complexes

We report the results of a (2+1) resonance-enhanced multiphoton ionization (REMPI) study of the E2Sigma+(4ssigma) Rydberg state of NO-Kr. We present an assignment of the two-photon spectrum based on a simulation, and discuss it in the context of previously-reported spectra of NO-Ne and NO-Ar. In addition, we report on spectra in the region of the vNO=1 level of the E, F and H' 4s and 3d Rydberg states of NO-Rg (Rg=Ne-Kr). Since the NO vibrational frequency is affected by electron donation from the rare-gas (Rg) atom to the NO+ core, as well as by the penetration of the Rydberg electron, the fundamental NO-stretch frequency reflects the interactions in the complex. The results indicate that the 4s Rydberg state has a strong interaction between the NO+ core and the Kr atom, as was the case for NO-Ar and NO-Ne. For the 3d Rydberg states, although penetration is not as significant as for the 4s Rydberg states, it does play an important role, with subtle angular effects being notable.

Infrared spectroscopic studies of the rare gas atom perturbed S1(0) rovibron band of solid parahydrogen

We report high resolution infrared absorption studies of rare gas (Rg) atom doped solid parahydrogen in the hydrogen S 1(0) region around 4486 cm −1. At low Rg atom concentrations (∼0.1%), satellite transitions appear in the S 1(0) region due to rovibrational excitation of parahydrogen molecules with one nearest-neighbor Rg atom. The Ne satellite feature differs qualitatively from the Ar, Kr, and Xe satellite features for reasons described within. The frequency of the S 1(0) satellite features linearly shift to lower energy as the polarizability of the Rg atom increases while the absorption coefficients increase with the square of the Rg atom polarizability. Rotational calculations are performed for H 2 with a nearest-neighbor Rg atom assuming a rigid hexagonal close-packed lattice structure. The calculated fine structure of the S 1(0) satellite features agree qualitatively with lifting of the 2 J+1 degeneracy of the v = 1, J = 2 upper state caused by the anisotropy in the Rg–H 2 intermolecular potential. The discrepancy between the calculated and measured Rg atom S 1(0) satellite features may signal partial delocalization of the J = 2 roton onto neighboring parahydrogen molecules.

Theoretical Study of RgNO (Rg=He, Ne, Ar and Kr) Complexes

RgNO (Rg=He, Ne, Ar and Kr) complexes were studied using ab initio calculations. The neutral RgNO complex geometry and vibrational frequencies were calculated with the cc-pVDZ basis set at the CCSD(T) level of theory. The calculations show that the geometry of the RgNO complexes is a skewed T-shape with the Rg atom on the oxygen side of the NO molecule, and that the RgNO bond angle increases with mass. The dissociation energies (DE) and ionization energies (IE) of the neutral RgNO complexes, and the dissociation energies of RgNO+ ionic complexes were calculated using Gaussian-2 (G2) methods and a high accuracy energy model. The ionization energies of the neutral RgNO complexes range from 9.265 eV for HeNO to 9.132 eV for KrNO and the dissociation energies of RgNO+ range from 0.017 eV for HeNO+ to 0.156 eV for KrNO+, in line with the expectation based on the increasing polarizability of the Rg atom.

Insertion of rare gas atoms into BF3 and AlF3 molecules: An ab initio investigation

The structure, stability, charge redistribution, and harmonic vibrational frequencies of rare gas inserted group III-B fluorides with the general formula F-Rg-MF(2) (where M=B and Al; Rg=Ar, Kr, and Xe) have been investigated using ab initio quantum chemical methods. The Rg atom is inserted in one of the M-F bond of MF(3) molecules, and the geometries are optimized for ground as well as transition states using the MP2 method. It has been found that Rg inserted F-Rg-M portion is linear in both F-Rg-BF(2) and F-Rg-AlF(2) species. The binding energies corresponding to the lowest energy fragmentation products MF(3)+Rg (two-body dissociation) have been computed to be -670.4, -598.8, -530.7, -617.0, -562.1, and -494.0 kJmol for F-Ar-BF(2), F-Kr-BF(2), F-Xe-BF(2), F-Ar-AlF(2), F-Kr-AlF(2), and F-Xe-AlF(2) species, respectively. The dissociation energies corresponding to MF(2)+Rg+F fragments (three-body dissociation) are found to be positive with respect to F-Rg-MF(2) species, and the computed values are 56.3, 127.8, and 196.0 kJmol for F-Ar-BF(2), F-Kr-BF(2), and F-Xe-BF(2) species, respectively. The corresponding values for F-Ar-AlF(2), F-Kr-AlF(2), and F-Xe-AlF(2) species are also found to be positive. The decomposition of F-Rg-MF(2) species into the MF(3)+Rg (two-body dissociation) channel typically proceeds via a transition state involving F-Rg-M out-of-plane bending mode. The transition state barrier heights are 35.5, 62.7, 89.8, 22.0, 45.6, and 75.3 kJmol for F-Ar-BF(2), F-Kr-BF(2), F-Xe-BF(2), F-Ar-AlF(2), F-Kr-AlF(2), and F-Xe-AlF(2) species, respectively. The calculated geometrical parameters and the energy values suggest that these species are metastable and may be prepared and characterized using low temperature matrix isolation techniques, and are possibly the next new candidates for gas phase or matrix experiments.

Electronic spectroscopy of NO–(Rg)x complexes (Rg=Ne,Ar) via the 4s and 3d Rydberg states

We have employed (2 + 1) resonance enhanced multiphoton ionization spectroscopy to investigate the 3d and 4s Rydberg states of the NO molecule when bound to the surface of Rg(x) clusters (Rg = rare gas). We observe that the spectra of the NO-Ar(x) species converge in appearance as x increases, and this is discussed in terms of two Rg atoms interacting with the NO+ core, with other Rg atoms being "outside" the Rydberg orbital. We show that the interaction of each of the Rg atoms with the NO is essentially independent for the NO-Rg2 complexes: both by comparing our spectra for Rydberg states of NO-Rg and NO-Rg2, and from the results of ab initio calculations on NO+ - Rg and NO+ - Rg2. In addition, we discuss the disappearance of some electronic bands upon complexation in terms of Franck-Condon factors that are very sensitive to the angular coordinate. We relate our results to those of the bulk by comparing to the previously reported electronic spectroscopy of NO in both Rg matrices and He nanodroplets.

Interaction potentials for Br−–Rg (Rg=He–Rn): Spectroscopy and transport coefficients

High-level ab initio CCSD(T) calculations are performed in order to obtain accurate interaction potentials for the Br(-) anion interacting with each rare gas (Rg) atom. For the Rg atoms from He to Ar, two approaches are taken. The first one implements a relativistic core potential and an aug-cc-pVQZ basis set for bromine, an aug-cc-pV5Z basis set for Rg, and a set of bond functions placed at the midpoint of the Rg-Br distance. The second one uses the all-electron approximation with aug-cc-pV5Z bases further augmented by an extra diffuse function in each shell. Comparison reveals close similarity between both sets of results, so for Rg atoms from Kr to Rn only the second approach is exploited. Calculated potentials are assessed against the previous empirical, semiempirical, and ab initio potentials, and against available beam scattering data, zero electron kinetic energy spectroscopic data, and various sets of the measured ion mobilities and diffusion coefficients. This multiproperty analysis leads to the conclusion that the present potentials are consistently good for the whole series of Br(-)-Rg pairs over the whole range of internuclear distances covered.

Electronic spectroscopy of the 3d Rydberg states of NO–Rg (Rg=Ne,Ar,Kr,Xe) van der Waals complexes

We have employed (2+1) resonance-enhanced multiphoton ionization spectroscopy to record electronic absorption spectra of NO-Rg (Rg=Ne,Ar,Kr) van der Waals complexes. The nitric oxide molecule is the chromophore, and the excitation corresponds to an electron being promoted from the 2ppi* orbital to 3dsigma, 3dpi, and 3ddelta Rydberg states. We review the ordering of the 3dlambda states of NO and use this as a basis for discussing the 3d components in the NO-Rg complexes, in terms of the interactions between the Rydberg electron, the core, and the Rg atom. Predissociation of the H' 2Pi state occurs through the F2Delta state for NO-Ar and NO-Kr, and this will be considered. We shall also outline problems encountered when trying to record similar spectra for NO-Xe, related to the presence of atomic Xe resonances.

Rare Gas Effects on Hyperfine Coupling Constants of BO, AlO, and GaO

Using density functional theory methods and large basis sets, we calculated hyperfine coupling constants (HFCCs) for the (11)B, (17)O, (27)Al, and (69)Ga nuclei of the radicals BO, AlO, and GaO (XO), embedded in 2-14 rare gas (Rg) Ne and Ar atoms. Kr atoms were included for AlO. The distance of the Rg atoms from XO was varied from 4 to 12 bohr. Matrix effects cause A(iso)(X) to increase, accompanied by decreases in A(dip)(X) and A(dip)(O), while A(iso)(O) remains close to zero. Changes are largest for AlO, slightly smaller for GaO, and very small for BO, in line with the molecular polarizabilities. Observed changes of A(iso)(X) and A(dip)(X) for BO in Ne matrixes and for AlO in Ne, Ar, and Kr matrixes are reproduced in complexes with 12 Rg atoms at distances of 5-6 bohr or 14 Rg atoms at distances of 6-7 bohr. For GaO, experimental data are available only in Ne matrixes. Theoretical results obtained for HFCCs of (17)O could not be verified due to insufficient experimental information. Estimates of HFCCs in matrixes not yet experimentally studied and for GaO in the gas phase have been made. Due to the interaction with rare gas atoms, p-spin density on the X and O atoms of XO is converted into s-spin density on X, thereby causing an increase (in magnitude) of A(iso)(X), accompanied by decreases in A(dip) of X and O. The higher polarizability of XO along the bond axis is reflected in complexes that have axial Rg atoms showing larger changes in HFCCs than comparable complexes without axial Rg atoms.

Spectroscopy and dynamics of hydride radical van der Waals complexes

Spectroscopic and theoretical studies of hydride radical–rare gas atom complexes (Rg–HX) are reviewed. This family of van der Waals molecules is of interest as they can be used to explore the characteristics of the long-range forces associated with open-shell species. Orbitally degenerate states of HX radicals have an electronic anisotropy that results in van der Waals interactions that are qualitatively different from those exhibited by the corresponding closed-shell systems. Rg–HX complexes, where X is a first- or second-row p-block element, reveal systematic trends where the anisotropic components of the physical interactions are determined by the electronic orbital configuration. Radicals in Σ, Π and Δ states with singlet, doublet and triplet spin multiplicities have been examined. When Rg = He, Ne or Ar the interaction potential energy surfaces can be predicted using high-level ab initio methods. Theoretical studies have established the methods and basis sets that are capable of providing an accurate description of the long-range forces for open-shell molecules. Clusters consisting of an HX molecule with multiple rare gas atoms are model systems for studies of solvated radicals. Potential energy surfaces derived from the binary clusters are being used to construct approximate potentials for Rg n –HX clusters. The equilibrium structures and vibrational dynamics predicted for these systems show that solvated radicals exhibit unique properties. Contents PAGE 1. Introduction 376 2. Experimental methods 377 3. Theoretical considerations 378 4. Binary complexes 384 4.1. BH–Ar and AlH–Ar 384 4.2. CH–Rg complexes (Rg = He, Ne and Ar) 386 4.3. NH–Rg complexes (Rg = He, Ne and Ar) 397 4.4. OH–Rg and SH–Rg complexes 407 5. Complexes of HX radicals with multiple Rg atoms 413 6. Conclusions and future directions 417 Acknowledgements 418 References 418

Electron spin resonance g tensors for complexes of Ne and Ar with AlO: Theoretical studies related to the large matrix effect observed for AlO

For Ne(n)-AlO (n=2, 4, 6, 8, 10) and Ar(n)-AlO clusters (n=2, 4, 6, 8), the perpendicular (relative to AlO) component of the g tensor was calculated by second-order perturbation theory, using multireference configuration-interaction wave functions. The rare-gas (Rg) atoms were placed axially and/or off axially (one or two rings of four Rg atoms each), and the distance of the Rg atoms from the Al and O atoms, or from the AlO axis, was varied from 4 to 12 bohrs. Rg atoms placed axially mostly increase g(perpendicular), whereas off-axially placed ones lower it below the gas-phase value of AlO. The largest deviations from g(perpendicular) of isolated AlO occur at Ne-Al,O distances of 5-6 bohrs, and Ar-Al,O distances of 6-9 bohrs, with maximal lowerings of about 1600 ppm for Ne and about 2200 ppm (estimated) for Ar in the case of two axial and eight off-axial Rg atoms. Electron spin resonance studies by Knight and Weltner found large matrix effects for AlO, with downshifts of g(perpendicular) observed to be about 450 and 1150 ppm in Ne and Ar matrices, respectively.

Theory of Monte Carlo simulations of the magnetic circular dichroism spectra of alkali metal/rare gas systems

AbstractThe history of magnetic circular dichroism (MCD) spectroscopy in the study of alkali metal/rare gas (M/Rg) cryogenic systems is reviewed in the context of developing a better understanding of alkali metal/hydrogen systems of current interest to the U.S. Air Force as enhanced‐performance cryogenic rocket propellants. A new theory for simulating the MCD spectra of M/Rg systems is presented together with a careful discussion of the theory's implicit and explicit approximations and their implications. This theory uses a classical Monte Carlo (MC) simulation scheme to model the perturbing effects of the Rg environment on the 2S → 2P MCD‐active transition of the M atom. The theory sets up the MC–MCD simulation as a 6 × 6 matrix eigenvalue/eigenvector problem in the 2P manifold in which are included the effects of M–Rg interactions, metal atom spin‐orbit coupling in the 2P manifold, magnetic Zeeman perturbations of the 2S and 2P manifolds, Boltzmann temperature factors, and electric dipole transition moment integrals for left circularly polarized (LCP) and right circularly polarized (RCP) light. The theory may be applied to any type of trapping site of the host M in the guest Rg matrix; a single atom substitutional metal atom trapping site (one host Rg atom is replaced by one guest M atom) is modeled in this study for M = Na and Rg = Ar. Two temperature factors are used in these simulations; a lattice temperature to model the mobility of the Rg lattice and a magnetic temperature to model Boltzmann factors in the 2S ground manifold. The 6 × 6 eigenvalue/eigenvector problem is solved for a number of randomly generated and suitably averaged Rg configurations to yield the simulated MC‐MCD spectrum for the single substitutional Na/Ar system. The MC–MCD simulations of Na/Ar give the characteristic triplet MCD spectrum with the correct Boltzmann temperature dependence. The simulated MC–MCD spectrum correctly inverts when the direction of the applied magnetic field is reversed. Addition of the LCP and RCP absorbances gives rise to a characteristic 2S → 2P triplet absorption feature. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Theoretical study of chemical compounds formed by insertion of rare gas atoms into glycine molecule: a step towards bio-rare gas chemistry?

Stability, structural and vibrational properties of compounds where an Rg atom (Xe, Kr, or Ar) is inserted into different bonds of glycine molecule are investigated using accurate ab initio methods. The most stable configuration is found to correspond to insertion of Rg atoms into the O H bond of glycine. These NH 2CH 2COORgH compounds are metastable, but separated from the Rg + glycine dissociation products by significant potential barriers. Preliminary calculations show that NH 2CH 2COOXeH compound may be energetically stable with respect to the three-body dissociation (H + Rg + NH 2CH 2CO 2), while the corresponding Ar compound is energetically unstable in this respect. The compound with the inserted Kr is a borderline case, with the three-body dissociation products close in energy to the NH 2CH 2COOKrH minimum.

Rules of electron correlation energies of van der Waals' complexes RgX (Rg = Ar, Kr, X = F, Cl, Br)

AbstractFirst, the intrapair and interpair correlation energies of the Rg atom, X atom, and the optimized RgX (Rg = Ar, Kr, X = F, Cl, Br) complexes are calculated by the MELD program at the 6‐311++g(d), 6‐311++g(3df, 3pd), and cc‐pvqz basis sets (denoted by basis sets a, b, and c, respectively). It is found that the relationship Ecorr(RgX) ≈ Ecorr(Rg) + Ecorr(X) is correct for all the above systems but introducing an unsound absolute error for some RgX systems. Second, the same calculations are selectively carried out for ArF (the smallest system) and KrBr (the largest system) at their increasing interatomic distance. It was found that both the correlation energies of ArF and those of KrBr will decrease whenever the interatomic distance of them become larger. On the basis of our results, we provided an approach to quickly estimate the correlation energies of RgX complexes by which not only the absolute error becomes smaller but more computation work is saved than the direct calculation. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004

Spectroscopic and theoretical studies of CH+ 3-Rgn clusters (Rg=He, Ne, Ar): From weak intermolecular forces to chemical reaction mechanisms

This review summarizes the recent combined experimental and theoretical effort of high-resolution spectroscopy, mass spectrometry and quantum chemical calculations to characterize isolated CH+ 3-Rg n clusters (Rg=He, Ne, Ar; n ≤ 8). These complexes serve as a model system for the solvation of a fundamental reactive carbocation by non-polar ligands. The results provide unprecedented and detailed information about important properties of the interaction potential as a function of the interaction strength and the degree of microsolvation of the methyl cation. These include the geometries and binding energies of minima and transition states, the structure of solvation (sub)shells, the competition between various types of intermolecular bonding (p bonds versus H bonds), the change in the origin of the interaction as a function of the size of the Rg atom and the degree of solvation (induction versus charge transfer), the importance of monomer relaxation, the large angular-radial coupling and zero-point effects of the potential of these prototype disk-and-ball dimers, and the significance of non-cooperative three-body effects. The studies of CH+ 3-Rg and related dimers demonstrate that the inert Rg ligands can be used as a sensitive probe of the chemical reactivity of specific orbitals in molecular ions. In addition, the Rg-CH+ 3-Rgn-Rg trimers represent simple prototype intermediates in degenerate cationic nucleophilic substitution (SN2) reactions, and the results for Ar-CH+ 3-Ar provide the first spectroscopic evidence that such reactions proceed via a double minimum potential in the gas phase. The CH+ 3-Rg n results show that the fruitful combination of modern state-of-the-art spectroscopic and theoretical approaches provides a powerful route to the understanding of the physical and chemical properties of ion-solvent interactions at the molecular level.

A Comprehensive Computational Study on OCH+-Rg (Rg = He, Ne, Ar, Kr, Xe) Complexes

The equilibrium geometries, harmonic vibrational frenquencies, and the dissociation energies of the OCH+-Rg (Rg = He, Ne, Ar, Kr, and Xe) complexes were calculated at the DFT, MP2, MP4, CCSD, and CCSD(T) levels of theory. In the lighter OCH+-Rg (Rg = He, Ne, Ar) rare gas complexes, the DFT and MP4 methods tend to produce longer Rg-H+ distance than the CCSD(T) level value, and the CCSD-calculated Rg-H+ bond lengths are slightly shorter. DFT method is not reliable to study weak interaction in the OCH+-He and OCH+-Ne complexes. A qualitative result can be obtained for OCH+-Ar complex by using the DFT method; however, a higher-level method using a larger basis set is required for the quantitative predictions. For heavier atom (Kr, Xe)-containing complexes, only the CCSD method predicted longer Rg-H+ distance than that obtained at the CCSD(T) level. The DFT method can be applied to obtain the semiquantitative results. The relativistic effects are expected to have minor effect on the geometrical parameters, the H+-C stretching mode, and the dissociation energy. However, the dissociation energies are sensitive to the quality of the basis set. The nature of interaction between the OCH+ ion and Rg atoms was also analyzed in terms of the interaction energy components.

Dynamics of O(3Pj)+Rg collisions on ab initio and scattering potentials

Interaction potentials of the Π3 and Σ-3 electronic states of the Rg–O(3P) systems (Rg=He–Kr) are computed at the coupled cluster single, double (triple) level of ab initio theory using extended basis sets augmented by bond functions. The ab initio potentials agree well with the scattering potentials determined from experiments in molecular beams [Aquilanti et al., J. Chem. Phys. 89, 6157 (1988)]. Both sets of the interaction potentials are employed for accurate close-coupling calculations of cross sections and rate constants for intramultiplet transitions in collisions of O(3Pj) with Rg atoms and analytical approximations for temperature dependencies of rate constants over temperature interval 50–3500 K are proposed. The sensitivity of the dynamical results to the nature of Rg atoms and interaction potentials is analyzed and the dynamics of intramultiplet mixing in O(3Pj) is investigated in both high- and low-energy limits.

Rovibrational calculations for CH3+–Rg (Rg=He,Ne): The prototype disk-and-ball dimer

Rovibrational calculations in the intramolecular ground vibrational states of the CH3+–Rg dimers, Rg=He and Ne, are carried out on intermolecular ab initio potential energy surfaces (PESs) calculated at the MP2 level of theory using a basis set of aug-cc-pVTZ quality. The internal CH3+ coordinates in the dimer are kept frozen at the optimal monomer coordinates (D3h symmetry, rigid monomer approximation). The three-dimensional (3D) intermolecular PESs of both dimers feature pronounced global minima at p-bound equilibrium structures: the Rg atom is attached to one side of the 2pz orbital of the central C atom along the C3 symmetry axis (C3v symmetry). The intermolecular C–He and C–Ne bonds are characterized by separations of Re=1.93 and 2.21 Å and dissociation energies of De=672 and 935 cm−1, respectively. The PESs of these prototype disk-and-ball dimers reveal substantial angular–radial coupling in the region of the global minimum which leads to significant differences between the equilibrium and vibrationally averaged separations, Re and R0. The 3D rovibrational calculations on the rigid monomer PESs yield R0=2.54 and 2.43 Å and D0=193 and 474 cm−1 for CH3+–He and CH3+–Ne, respectively. In general, the spectroscopic constants derived for the ground vibrational states of both complexes are in good agreement with recent spectroscopic data obtained by infrared photodissociation spectroscopy.

M + / Rg bonding: The effects of M+ permanent quadrupole moments (M+= atomic metal ion; Rg=rare gas atom)

It has been shown, using a model-potential analysis, that the large permanent quadrupole moment of the excited Mg+(3p) ion can play a significant role in the strong physical M+/Rg bonding observed for Mg+(3pπ)⋅Rg[2Π] ionic states. The four permanent quadrupole terms included in the model potential (two proportional to 1/R6, two to 1/R8) contribute substantially to Mg+(3pπ)/Rg attraction near the bond distances Re. In fact, our analysis indicates that the leading charge/induced-dipole 1/R4 attractive term contributes only ∼25–30 % to the physical bonding in the Mg+(3pπ)⋅Ar excited state, in stark contrast to the conventional wisdom that this term is usually dominant in M+/Rg bonding. Empirically derived Ae−bR repulsive terms also show that electron/electron repulsion for a given Mg+(3pπ)⋅Rg excited state is less than for the analogous Mg+(3sσ)⋅Rg ground state, consistent with the fact that the Rg atoms approach the excited 3pπ orbital of Mg+ along its nodal axis. For the Mg+(3pσ)⋅Rg[2Σ+] excited states, however, three of the permanent quadrupole terms are repulsive (with twice the magnitude) and thus contribute significantly to the extremely weak bonds and very large bond distances for the 3pσ ionic states. In contrast, the much smaller quadrupole moments of open-shell d-orbital states of transition metal M+ ions appear to have very little effect on their physical bonding with the Ar atom, at least for the few states which have been well-characterized spectroscopically. For all the M+/Rg states discussed above, our model-potential analysis indicates that no substantial chemical or charge-transfer interactions are needed to rationalize the bond strengths, the bond lengths, and the vibrational frequencies (the “shapes” of the potential curves near their minima).

CCSD(T) calculation of the ground-state potential curves for the Zn-rare gas van der Waals molecules

The spectroscopic parameters ( R e, D e, ω e) for the ground state of the weakly bound Zn-rare gas (RG) van der Waals molecules have been derived from ab initio calculated potential curves. The potential curve calculations have been carried out using the coupled-cluster with single and double excitations and perturbative contribution of connected triple excitations (CCSD(T)) method for the 20 electrons of Zn and the valence electrons of RG atom, while the Zn 20+ and RG 8+ cores are replaced by ab initio quasirelativistic energy-consistent pseudopotentials. The theoretical potentials are discussed in the context of available experimental data. Very good agreement between theory and experiment has been obtained.

Heavy-element effect on the splitting of the [formula omitted] state of the LiRg (Rg=Ar, Kr and Xe) complexes

The spin–orbit splittings A SO ( R) of the A 2Π state of LiRg (Rg=Ar, Kr and Xe) complexes have been obtained from the configuration interaction calculations using relativistic effective core potentials and two-component spinors. The results confirm that the mixing between the p orbitals of the lithium atom and the valence np orbitals (and not the Rydberg orbitals) of the rare gas (Rg) atom produces a large A SO ( R) for the A 2Π states at short internuclear distances. The spectroscopic constants for the ground and A states of LiRg complexes are also reported.