An ab initio study of the E 3Πg state of the iodine molecule

The E (3)Π(g) state of the iodine molecule is studied by ab initio multireference methods coupled with effective core potentials and large basis sets. Two potential minima are found, a global featuring an ion-pair character, and a local presenting a purely Rydberg nature. Four avoided crossings along the dissociation coordinate attribute an interesting topology to its potential energy curve, and their effect on the vibrational levels of I(2) is discussed.     

Theoretical studies on the potential energy surfaces and vibrational energy levels of HXeF and HXeCl

The potential energy surfaces for the electronic ground state of the HXeCl and HXeF molecules areconstructed by using the internally contracted multi-reference configuration interaction with theDavidson correction(icMRCI Q)method and large basis sets.The stabilities and dissociation barriersare identified from the potential energy surfaces.The three-body dissociation channel is found to bethe dominate dissociation channel for HXeCl,while two dissociation channels are possible and com-petitive for HXeF.Based on the obtained potentials,vibrational energy levels of HXeCl and HXeF arecalculated using the Lanczos algorithm.Our theoretical results are in good agreement with the avail-able observed values.Particularly,the calculated fundamental frequency of the H—Xe stretching vi-bration including the Xe matrix effect of HXeCl is found to be 1666.6 cm-1,which is only 17.6 cm-1higher than the recently observed value of 1649 cm-1.     

Ab initio study of the intermolecular potential energy surface in the ion-induced-dipole hydrogen-bonded O2(-)(X2Πg)-H2(X1Σg(+)) complex

This work presents the first investigation on the intermolecular potential energy surface of the ground electronic state of the O2(-)(2Πg)-H2(1Σg(+)) complex. High level correlated ab initio calculations were carried out using the Hartree-Fock spin-unrestricted coupled cluster singles and doubles including perturbative triples correction [RHF-UCCSD(T)]/aug-cc-pVXZ levels of calculations, where XZ = DZ, TZ, QZ, and 5Z. Results of full geometry optimization and the intermolecular potential energy surface (IPES) calculations show four equivalent minimum energy structures of L-shaped geometry with Cs symmetry at equilibrium along the 2A″ surface of the complex. For these equilibrium minimum energy structures, the most accurate value for the dissociation energy (De) was calculated as 1407.7 cm(-1), which was obtained by extrapolating the counterpoise (CP) corrected De values to the complete basis set (CBS) limit. This global minimum energy structure is stabilized by an ion-induced-dipole hydrogen bond. Detailed investigations of the IPES show that the collinear structure is unstable, while the C2v geometries present saddle points along the 2A″ surface. The barrier height between the two equivalent structures that differs in whether the hydrogen-bonded hydrogen atom is above or below the axis that connects centers of masses of the H2 and O2(-) moieties within the complex was calculated as 70 cm(-1). This suggests that the complex exhibits large amplitude motion. The barrier height to rotation of the H2 moiety by 180° within the complex is 1020 cm(-1). Anharmonic oscillator calculations predicted a strong H-H stretch fundamental transition at 3807 cm(-1). Results of the current work are expected to stimulate further theoretical and experimental investigations on the nature of intermolecular interactions in complexes that contain the superoxide radical and various closed-shell molecules that model atmospheric and biological molecules. These studies are fundamental to understanding the role of the O2(-) anion in chemistry in the atmosphere and in biological systems.     

Ab initio potential energy surface and excited vibrational states for the electronic ground state of Li_2H

A 285-pomt multi-reference configuration-interaction involving single and double excitations ( MRS DCI) potential energy surface for the electronic ground state of L12H is determined by using 6-311G (2df,2pd)basis set.A Simons-Parr-Finlan polynomial expansion is used to fit the discrete surface with a x2 of 4.64×106 The equn librium geometry occurs at Rc=0.172 nm and,LiHL1=94.10°.The dissociation energy for reaction I2H(2A)→L12(1∑g)+H(2S) is 243.910 kJ/mol,and that for reaction L12H(2A')→HL1(1∑) + L1(2S) is 106.445 kl/mol The inversion barrier height is 50.388 kj/mol.The vibrational energy levels are calculated using the discrete variable representation (DVR) method.     

An ab initio study of the rotation—vibration energy levels of GeH2 in the ā3B1 state

Thirty-five points on the potential energy surface of the ā³B₁ first excited state for the GeH₂ radical have been calculated using the (ab initio) MRD CI technique. Thirteen parameters in an analytic expression for the potential have been adjusted (by least-squares optimization) so that the surface fits these points. The rotation-vibration energy levels of GeH₂. GeD₂ and GeHD have been calculated using the non-rigid bender Hamiltonian, and we determine for GeH₂ that ν₁ = 1991 cm^-1. ν₂ = 763 cm^-1, and ν₃ = 2012 cm^-1. The equilibrium structure is found to be r_e = 1.545 Å and α_z = 119.8°, and the singlet-triplet splitting is calculated to be 22.8 kcal/mole (7975 cm^-1).     

The 1(5)Π(g) state of C2

We report ab initio spectroscopic constants for the recently identified 1(5)Π(g) state of C(2) [P. Bornhauser, Y. Sych, G. Knopp, T. Gerber, and P. P. Radi, J. Chem. Phys. 134, 044302 (2011)]. The calculations are performed at the multi-reference configuration interaction level of theory with Davidson's correction using aug-cc-pV6Z basis sets and include core-valence correlation and relativistic corrections obtained with quadruple-zeta bases. Such treatment accurately reproduces the experimentally observed constants of the a(3)Π(u) and other states. Thus, we expect our calculated ω(e) value for the 1(5)Π(g) state to be within a few cm(-1), and rotational constants to be within 0.1% of experiment. Agreement with available spectroscopic data is excellent, with the calculations strongly suggesting that the 1(5)Π(g) vibrational level observed by Bornhauser et al. is v = 0.     

Comparative semiempirical and ab initio study of the harmonic vibrational frequencies of aniline—I. The ground state

The harmonic vibrational frequencies of the ground state S₀ of aniline obtained from various ab initio methods [6-31G, 6-31G(*) and 6-31G* basis sets] and semiempirical methods (MINDO/3, MNDO, AM1 and PM3) have been compared to the experimental vibrational spectra. Detailed theoretical analyses of the atomic Cartesian displacements of all normal modes are presented. The semiempirical PM3 method reproduces the experimental frequencies of aniline with comparable accuracy to the ab initio methods. Ale PM3 method will be useful in predicting the vibrational spectra of larger aromatic amines.     

Role of the 3(ππ*) state in photolysis of lumisantonin: insight from ab initio studies

The CASSCF and CASPT2 methodologies have been used to explore the potential energy surfaces of lumisantonin in the ground and low-lying triplet states along the photoisomerization pathways. Calculations indicate that the (1)(nπ*) state is the accessible low-lying singlet state with a notable oscillator strength under an excitation wavelength of 320 nm and that it can effectively decay to the (3)(ππ*) state through intersystem crossing in the region of minimum surface crossings with a notable spin-orbital coupling constant. The (3)(ππ*) state, derived from the promotion of an electron from the π-type orbital mixed with the σ orbital localized on the C-C bond in the three-membered alkyl ring to the π* orbital of conjugation carbon atoms, plays a critical role in C-C bond cleavage. Based on the different C-C bond rupture patterns, the reaction pathways can be divided into paths A and B. Photolysis along path A arising from C1-C5 bond rupture is favorable because of the dynamic and thermodynamic preferences on the triplet excited-state PES. Path B is derived from the cleavage of the C5-C6 bond, leading first to a relatively stable species, compared to intermediate A-INT formed on the ground state PES. Accordingly, path B is relatively facile for the pyrolytic reaction. The present results provide a basis to interpret the experimental observations.     

Do traditional, chlorine-shared, and ion-pair halogen bonds exist? An ab initio investigation of FCl:CNX complexes

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out to determine the structures, binding energies, and bonding of complexes FCl:CNX, with X = CN, NC, NO(2), F, CF(3), Cl, Br, H, CCF, CCH, CH(3), SiH(3), Li, and Na. Equation-of-motion coupled cluster calculations have also been carried out to determine the coupling constants (1)J(F-Cl), (1X)J(Cl-C), and (2X)J(F-C) across these halogen bonds. As the strength of the base is systematically increased, the nature of the halogen bond changes from traditional, to chlorine-shared, to ion-pair. The type of halogen bond present in a complex can be readily determined from its structure, binding energy, AIM bonding analyses, and spin-spin coupling constants. Coupling constants across halogen bonds are compared with corresponding coupling constants across traditional, proton-shared, and ion-pair hydrogen bonds.     

Formation of ultracold Rb2 molecules in the v' = 0 level of the a(3)Σ+(u) state via blue-detuned photoassociation to the 1(3)Π(g) state

We report on the observation of blue-detuned photoassociation in Rb(2), in which vibrational levels are energetically above the corresponding excited atomic asymptote. (85)Rb atoms in a MOT were photoassociated at short internuclear distance to levels of the 1(3)Π(g) state at a rate of approximately 5 × 10(4) molecules s(-1). We have observed most of the predicted vibrational levels for all four spin-orbit components; 0(+)(g), 0(-)(g), 1(g), and 2(g), including levels of the 0(+)(g) outer well. These molecules decay to the metastable a(3)Σ(+)(u) state, some preferentially to the v' = 0 level, as we have observed for photoassociation to the v' = 8 level of the 1(g) component.     

Double-well potential surface and associated vibrational frequencies in the ā3A2 nπ* state of formaldehyde

A double-well potential surface has been computed for the out-of-plane bending and the CO stretching vibrations of the lowest ā³A₂ excited electronic state of H₂CO. The vibrational levels of the coupled oscillators have been calculated using a variational treatment. The barrier height of inversion as well as the symmetric (+)-antisymmetric (−) splittings and the vibrational energies are reported.     

Global ab initio potential energy surfaces for the O2(3Σg-)+O2(3Σg-) interaction

Completely ab initio global potential energy surfaces (PESs) for the singlet and triplet spin multiplicities of rigid O(2)((3)Σ(g)(-))+O(2)((3)Σ(g)(-)) are reported for the first time. They have been obtained by combining an accurate restricted coupled cluster theory with singles, doubles, and perturbative triple excitations [RCCSD(T)] quintet potential [Bartolomei et al., J. Chem. Phys. 128, 214304 (2008)] with complete active space second order perturbation theory (CASPT2) or, alternatively, multireference configuration interaction (MRCI) calculations of the singlet-quintet and triplet-quintet splittings. Spherical harmonic expansions, containing a large number of terms due to the high anisotropy of the interaction, have been built from the ab initio data. The radial coefficients of these expansions are matched at long range distances with analytical functions based on recent ab initio calculations of the electric properties of the monomers [M. Bartolomei, E. Carmona-Novillo, M. I. Hernández, J. Campos-Martínez, and R. Hernández-Lamoneda, J. Comput. Chem. (2010) (in press)]. The singlet and triplet PESs obtained from either RCCSD(T)-CASPT2 or RCCSD(T)-MRCI calculations are quite similar, although quantitative differences appear in specific terms of the expansion. CASPT2 calculations are the ones giving rise to larger splittings and more attractive interactions, particularly in the region of the absolute minima (in the rectangular D(2h) geometry). The new singlet, triplet, and quintet PESs are tested against second virial coefficient B(T) data and, their spherically averaged components, against integral cross sections measured with rotationally hot effusive beams. Both types of multiconfigurational approaches provide quite similar results, which, in turn, are in good agreement with the measurements. It is found that discrepancies with the experiments could be removed if the PESs were slightly more attractive. In this regard, the most attractive RCCSD(T)-CASPT2 PESs perform slightly better than the RCCSD(T)-MRCI counterpart.     

High-resolution observation and analysis of the I(2) (+) A(2)Π(3∕2,u)-X (2)Π(3∕2,g) system

Rotationally resolved absorption spectra of I(2) (+) were recorded in 12 065-13 062 cm(-1) region by employing optical heterodyne velocity modulation absorption spectroscopy. In total, 4054 lines were assigned to 24 bands in the A(2)Π(3∕2,u)-X(2)Π(3∕2,g) system spanning the vibrational levels υ(') = 1-4 and υ(n) (') = 11-19. The assigned lines were globally fitted and an error of 0.003 cm(-1) was obtained. Rotational constants, B(υ), were used to derive equilibrium parameters B(e) (') = 0.03977725(77) cm(-1), a(e) (') = 1.1819(24)×10(-4) cm(-1), r(e) (') = 2.584386(25) A? of the X(2)Π(3∕2,g) state, and B(e) (') = 0.0305787(37) cm(-1), a(e) (') = 1.2353(23)×10(-4) cm(-1), r(e) (') = 2.94758(18) A? of the A(2)Π(3∕2,u) state. Vibrational energies were used to derive ω(e) (') = 239.0397(55) cm(-1), ω(e)x(e) (') = 0.64951(87) cm(-1) of the X(2)Π(3∕2,g) state and ω(e) (') = 138.103(11) cm(-1), ω(e)x(e) (') = 0.45027(34) cm(-1) of the A(2)Π(3∕2,u) state. The A(2)Π(3∕2,u) (υ(n) = 13) state was found to be rotationally perturbed by the a(4)Σ(1/2,u) (-) (υ(n) = 17) state through second-order spin-orbit coupling.     

Predissociation and hyperfine structure of the B3Πou+ state of Br2

Using molecular-beam laser spectroscopy, the hyperfine structure of the B-X system of ⁷⁹Br₂ was measured to obtain reliable data on hyperfine and rotational predissociation for the levels B(0_u⁺) ν = 11, 13 and 16. We report the first measurement of the hyperfine predissociation with no rotational predissociation on P(1) transitions. The unexpected strongly J-dependent variation of the predissociation parameters leads to the conclusion that the familiar theory describing all known data for the B state of iodine may be inadequate for bromine.     

Vibrational analysis of N2(B 3Πg and C 3Πu) and CO(X) excited in N2 discharge and post discharge

A spectroscopic characterization of a N₂ radiofrequency discharge and N₂CO post discharge has been performed. The relative vibrational distribution of the excited B ³Π_g and C ³Π_u states of nitrogen and their correlation with the ground state have been analyzed. The analysis confirms the importance of the metastable molecules. N₂(A ³Σ⁺_u), in affecting the vibrational distribution of nitrogen in its ground state in the discharge and post discharge. The vibrational analysis of the CO ground state, excited in the post discharge by vibrationally excited N₂ molecules, confirms the high degree of vibrational non-equilibrium in the ground state of nitrogen, in the presence of a low first-level vibrational temperature.     

A crossed beam and ab initio investigation of the reaction of boron monoxide ((11)BO; X(2)Σ+) with acetylene (C2H2; X(1)Σ(g)+)

The reaction dynamics of boron monoxide (BO; X(2)Σ(+)) with acetylene (C(2)H(2); X(1)Σ(g)(+)) were investigated under single collision conditions at a collision energy of 13 kJ mol(-1) employing the crossed molecular beam technique; electronic structure RRKM calculations were conducted to complement the experimental data. The reaction was found to have no entrance barrier and proceeded via indirect scattering dynamics initiated by an addition of the boron monoxide radical with its boron atom to the carbon-carbon triple bond forming the O(11)BHCCH intermediate. The latter decomposed via hydrogen atom emission to form the linear O(11)BCCH product through a tight exit transition state. The experimentally observed sideways scattering suggests that the hydrogen atom leaves perpendicularly to the rotational plane of the decomposing complex and almost parallel to the total angular momentum vector. RRKM calculations indicate that a minor micro channel could involve a hydrogen migration in the initial collision to form an O(11)BCCH(2) intermediate, which in turn can also emit atomic hydrogen. The overall reaction to form O(11)BCCH plus atomic hydrogen from the separated reactants was determined to be exoergic by 62 ± 8 kJ mol(-1). The reaction dynamics were also compared with the isoelectronic reaction of the cyano radical (CN; X(2)Σ(+)) with acetylene (C(2)H(2); X(1)Σ(g)(+)) studied earlier.     

Global potential energy surface, vibrational spectrum, and reaction dynamics of the first excited (Ã (2)A(')) state of HO(2)

The authors report extensive high-level ab initio studies of the first excited (A??(2)A(')) state of HO(2). A global potential energy surface (PES) was developed by spline-fitting 17?000 ab initio points at the internal contracted multireference configuration interaction (icMRCI) level with the AVQZ basis set. To ascertain the spectroscopic accuracy of the PES, the near-equilibrium region of the molecule was also investigated using three interpolating moving least-squares-based PESs employing dynamically weighted icMRCI methods in the complete basis set limit. Vibrational energy levels on all four surfaces agree well with each other and a new assignment of some vibrational features is proposed. In addition, the dynamics of both the forward and reverse directions of the H+O(2)(a??(1)Δ(g))?OH+O reaction (J=0) were studied using an exact wave packet method. The reactions are found to be dominated by sharp resonances.     

The transition probabilities from the ground state to the excited J = 0 1Σu(+) levels of H2: measurements and ab initio quantum defect study

The energies, widths and absolute intensities of the P(1) v' = 0, J' = 1 absorption transitions of H(2) have been measured in the spectral range of 81-75 nm using monochromatized synchrotron radiation. This work completes and extends previous observations, in particular those of Herzberg and Jungen [J. Mol. Spectrosc. 41, 425 (1972)]. Ab initio multichannel quantum defect theory (MQDT) is used to corroborate the spectral analysis of the experimental data. Line intensities and decay widths are also calculated using MQDT.     

Ab initio/empirical potential energy functions and stretching vibrational frequencies of the X̃ 2Π and à 2Π states of the chloroacetylene cation

Anharmonic potential energy functions for the stretching vibrational motions of the two lowest doublet states of chloro-acetylene cation have been constructed using ab initio RHF SCF calculations and some experimental information. Stretching vibrational frequencies are calculated variationally for four different isotopomers. The ν₁ vibrations of H¹²C₂³⁵Cl⁺ are predicted to occur at 3176 (X̃ ²Π) and 3231 (à ²Π) and the ν₂ vibration of the à state at 2081 cm⁻¹.