High-symmetry Geometries Research Articles

Nature of the Metal–Ligand Bonding in Bis(Cyclopentadienyl) and Bis(Cyclobutadienyl) Lutetium(III) Metallocenes

Metal-ligand bonding in bis(cyclopentadienyl) and bis(cyclobutadienyl) lutetium(iii) complexes was studied in a variety of geometries using density functional theory. Based on analysis of the orbitals, the Morokuma–Ziegler–Rauk extended transition state method and natural orbitals of the chemical valence, the bonding interaction is dominantly ionic with a very important covalent contribution. The relative contribution of covalency is so large that it should not be neglected even in qualitative treatments of the electronic structure. The covalency arises from electron donation from the ligand HOMOs and HOMO–1 orbitals to the vacant 5d, 6s and 6p orbitals at the Lu3+ ion. While 5d and the 6s orbitals play the most important part, the 6p orbitals also make a comparable contribution. A similar bonding picture emerges both in realistic low-symmetry metallocenes and idealized high-symmetry geometries. The results put to question the common view of the metal–ligand bonding in lanthanide complexes as completely or almost completely of ionic character, and have implications in the design of crystal-fields in lanthanide complexes when targeting specific magnetic properties.

Analytic Gradients for Selected Configuration Interaction

We develop analytic gradients for selected configuration interaction wave functions. Despite all pairs of molecular orbitals now potentially having to be considered for the coupled perturbed Hartree-Fock equations, we show that degenerate orbital pairs belonging to different irreducible representations in the largest abelian subgroup do not need to be included and instabilities due to degeneracies are avoided. We introduce seminumerical gradients and use them to validate the analytic approach even when near degeneracies are present due to high-symmetry geometries being slightly distorted to break symmetry. The method is applied to carbon monoxide, ammonia, square planar H4, hexagonal planar H6, and methane for a range of bond lengths where we demonstrate that analytic gradients for selected configuration interaction can approach the quality of full configuration interaction yet only use a very small fraction of its determinants.

Energetic 1,2,4-Triazines: 3,5-Diamino-6-nitro-1,2,4-triazine and Its Oxide

3,5-Diamino-6-nitro-1,2,4-triazine (DANT) was identified as a potentially insensitive energetic material with high symmetry and planar geometry yielding improved π–π stacking. The straightforward generation of this energetic 1,2,4-triazine was accomplished by azide to amine reduction of 5-amino-3-azido-6-nitro-1,2,4-triazine (AANT) with triphenylphosphine via a modified Staudinger reaction. Oxidation of DANT using potassium peroxymonosulfate triple salt produces the triazine N-oxide 3,5-diamino-6-nitro-1,2,4-triazine-2-oxide (DANTX). DANT and DANTX were characterized using mass spectroscopy, multinuclear NMR spectroscopy, infrared spectroscopy, and elemental analysis. The molecular structure and density of DANT (1.778 g cm–3) were determined by single-crystal X-ray diffraction indicating a graphite-like packed structure. Crystallization of DANTX yielded low-density (1.747 g cm–3) and high-density (1.852 g cm–3) polymorphs. Both DANT and DANTX were characterized as energetic materials in terms of sensitivity toward impact, friction, and thermal stimuli, and calculated detonation parameters are reported.

Growth behavior and electronic and optical properties of IrGen (n = 1–20) clusters

First principles calculations are performed to investigate the structural and electronic properties of small IrGen (n = 1–20) clusters. Cage-like configurations where the iridium atom is encapsulated inside a germanium cage are predicted to be favored for n ≥ 12. Doping Ir atom enhances the stability of the corresponding germanium frame. Our results highlight the great stability of IrGe13 which presents a high-symmetry cage-like geometry and a peculiar electronic structure in which the valence electrons of Ir and Ge atoms are delocalized and exhibit a shell structure. Absorption spectra, vertical ionization potentials, and electron affinities are also calculated and discussed.Graphical abstract

Design of a 1 ampere high-precision thin-film resistive current transducer with negligible frequency dependence from DC to 100 KHz

Currently, non-linear loads are found virtually anywhere with the promise of high electrical efficiency. Examples of this type of non-linear loads are compact fluorescent lamps and light-emitting diode lamps which can now be found in any home. However, they produce highly distorted currents that pollute the power grid and cause stability problems, and making the measurement of the distorted electrical current a non-trivial issue. For the reliable measurement of distorted waveforms within a wide bandwidth, magnetic current transducers present disadvantages over resistive current transducers, such as those caused by the magnetic material which attenuates the high-frequency components while producing heating on the magnetic material. This research presents the design principles to develop a thin-film wideband current transducer. Principles such as the selection of high-purity materials, high-symmetry coaxial design, size, geometry, and aspect ratios were used to obtain a linear relationship between its input and output, i.e.: a flat frequency response from DC to 100 kHz, and the ability to operate continuously with a custom passive thermal system for heat dissipation and reliable measurement. An exhaustive effort has been made on the refinement of the design aimed at understanding the effects that govern the frequency behavior of the transducer and the ways to compensate them. The manufacturing feasibility of the proposed design is well confirmed by the results obtained from the simulation process.

Non-planarity in four-membered homo-cyclic compounds A4 (A = O, S, Se, Te, Po) and restoring their planarity: a study of the pseudo-Jahn–Teller effect

The bent D2d structures of the four-membered homo-cyclic compounds A4 (A = O, S, Se, Te, Po) were examined computationally to understand the pseudo-Jahn–Teller effect (PJTE). To do this, ab initio geometry optimizations and corresponding frequency calculations (at the MP2/cc-pVQZ-(PP) level of theory) show that all A4 compounds under-consideration are unstable in their planar (D4h) configuration. The ground state and six low-lying non-degenerate and degenerate electronic excited states were computed at the CASSCF (6,7)/cc-pVQZ-(PP) along the bending normal coordinate connecting the D4h and D2d geometries; these represent the adiabatic potential energy surfaces (APESs). Based on the APESs, the coupling between the ground state (1A1g) and the 1B2u excited state is demonstrated to be the reason for the planar structure bends from the high-symmetry D4h geometry into the lower-symmetry D2d stable equilibrium configuration. The solution to the PJTE (1A1g + 1B2u) ⊗ b2u problem is useful to answer the question of “how instability rises in A4 planar configuration?”. Although all A4 compounds in the series are non-planar with D4h symmetry, but geometrical optimizations and frequency calculations show that coordination of two noble gas cations (NG+ = He+, Ne+ and Ar+) above and below the σh plane of the A4 (A = O, S, Se) ring could restore ring planarity in (A4 NG)2+ complexes. The PJTE is also quenched in the A42+ (A = O, S, Se, Te, Po) cation and dication series, and planarity of the rings is also restored, i.e., the high-symmetry D4h structure becomes the equilibrium configuration.

An organic chromophore -modified samarium-containing polyoxometalate: excitation-dependent color tunable behavior from the organic chromophores to the lanthanide ion.

Here, an organic-inorganic hybrid lanthanide-based polyoxometalate (Ln-POM) [N(CH3)4]3K2Sm(C7H5O2)(H2O)2(α-PW11O39)·11H2O (1) was successfully synthesized and characterized well by several physicochemical techniques. In the polyanion of 1, the benzoic acid ligand directly binds a Sm3+ ion for the formation of an organic-inorganic hybrid polyanion. This organic-inorganic hybrid structure can effectively sensitize the emissions of the Sm3+ ion, which was confirmed by the photoluminescence and time-resolved emission spectroscopy of 1. The photoluminescence study demonstrates that the Sm3+ ion possesses a relatively high-symmetry coordination geometry, which is consistent with the structural analysis of Sm3+ in 1. Furthermore, the polyanion of 1 can form a 3D 4,8-topology framework through the linkage of K1 and K2 ions. In addition, the photoluminescence properties of 1 have been explored, revealing that 1 shows reversible color-tunable photoluminescence based on the excitation from 260 nm to 350 nm, and emitting colors from blue to pink.

Divining the shape of nascent polymer crystal nuclei.

We demonstrate that nascent polymer crystals (i.e., nuclei) are anisotropic entities with neither spherical nor cylindrical geometry, in contrast to previous assumptions. In fact, cylindrical, spherical, and other high symmetry geometries are thermodynamically unfavorable. Moreover, postcritical transitions are necessary to achieve the lamellae that ultimately arise during the crystallization of semicrystalline polymers. We also highlight how inaccurate treatments of polymer nucleation can lead to substantial errors (e.g., orders of magnitude discrepancies in predicted nucleation rates). These insights are based on quantitative analysis of over four million crystal clusters from the crystallization of prototypical entangled polyethylene melts. New comprehensive bottom-up models are needed to capture polymer nucleation.

The Role of Pseudo Jahn–Teller Effect in Geometry and Electronic Parameters of Tetrahalodigermene Ge2X4 (X= Cl, Br, I)

The symmetry breaking of molecules depends on its context and origin. It is well shown the pseudo Jahn – Teller effect (PJTE) describes the origin of symmetry breaking of planar configurations. Based on the PJTE theory, the configurations, electronic parameters and conformational changes in molecular systems are predicted. Optimization of the D2h and C2h configurations of tetrahalodigermene Ge2X4 (X = Cl, Br, I) by ab initio calculations showed that the C2h configurations are more stable than planar ones. The different energy between high-symmetry D2h geometries and low-symmetry C2h geometries decrease from Ge2Cl4 to Ge2I4. Instability of the high-symmetry D2h configurations is arising from the PJT coupling of the ground Ag state and the exited B2g state in the Qb2g direction. The PJTE problem $$({{A}_{g}} + {{B}_{{2g}}}) \otimes {{b}_{{2g}}}$$ creates stable geometries of Ge2X4 (X = Cl, Br, I) analogs with C2h configurations. It is expected that when the energy gap between the desired states with D2h configurations decreases, the PJT stabilization energy (EPJT) increases. Despite this expectation, with increasing the energy gap from Ge2Cl4 to Ge2I4, EPJT decreases. According to the results of the canonical molecular orbital (CMO) analysis, the contributions of the b3u and b1u molecular orbitals in the vibronic coupling constant (F) decrease from Ge2Cl4 to Ge2I4. Consequently, assuming the primary force constant is positive, the negative curvatures of ground state electronic configuration curves in adiabatic potential energy surfaces (APESs) decreases from Ge2Cl4 to Ge2I4. The trend observed for the variations of the chemical hardness (η) of Ge2X4 analogs obey the maximum hardness principle.

Multimode Jahn-Teller effect in bulk systems: A case of the NV0 center in diamond

The multimode Jahn-Teller (JT) effect in a bulk system of a neutral nitrogen-vacancy ($\mathrm{N}{\mathrm{V}}^{0}$) center in diamond is investigated via first-principles density-functional-theory calculations and the intrinsic distortion path (IDP) method. The adiabatic potential energy surface of the electronic ground state of the $\mathrm{N}{\mathrm{V}}^{0}$ center is calculated based on the local spin-density approximation. Our calculations confirm the presence of the dynamic Jahn-Teller effect in the ground $^{2}E$ state of the $\mathrm{N}{\mathrm{V}}^{0}$ center. Within the harmonic approximation, the IDP method provides the reactive path of JT distortion from unstable high-symmetry geometry to stable low-symmetry energy minimum geometry, and it describes the active normal modes participating in the distortion. We find that there is more than one vibrational mode contributing to the distortion, and their contributions change along the IDP. Several vibrational modes with large contributions to JT distortion, especially those modes close to 44 meV, are clearly observed as the phonon sideband in photoluminescence spectra in a series of experiments, indicating that the dynamic Jahn-Teller effect plays an important role in the optical transition of the $\mathrm{N}{\mathrm{V}}^{0}$ center.

Potential energy and dipole moment surfaces of the triplet states of the O2(X3Σg-) - O2(X3Σg-,a1Δg,b1Σg+) complex.

We compute four-dimensional diabatic potential energy surfaces and transition dipole moment surfaces of O2-O2, relevant for the theoretical description of collision-induced absorption in the forbidden X3Σg- → a1Δg and X3Σg- → b1Σg+ bands at 7883 cm-1 and 13 122 cm-1, respectively. We compute potentials at the multi-reference configuration interaction (MRCI) level and dipole surfaces at the MRCI and complete active space self-consistent field (CASSCF) levels of theory. Potentials and dipole surfaces are transformed to a diabatic basis using a recent multiple-property-based diabatization algorithm. We discuss the angular expansion of these surfaces, derive the symmetry constraints on the expansion coefficients, and present working equations for determining the expansion coefficients by numerical integration over the angles. We also present an interpolation scheme with exponential extrapolation to both short and large separations, which is used for representing the O2-O2 distance dependence of the angular expansion coefficients. For the triplet ground state of the complex, the potential energy surface is in reasonable agreement with previous calculations, whereas global excited state potentials are reported here for the first time. The transition dipole moment surfaces are strongly dependent on the level of theory at which they are calculated, as is also shown here by benchmark calculations at high symmetry geometries. Therefore, ab initio calculations of the collision-induced absorption spectra cannot become quantitatively predictive unless more accurate transition dipole surfaces can be computed. This is left as an open question for method development in electronic structure theory. The calculated potential energy and transition dipole moment surfaces are employed in quantum dynamical calculations of collision-induced absorption spectra reported in Paper II [T. Karman et al., J. Chem. Phys. 147, 084307 (2017)].

Imposing high-symmetry and tuneable geometry on lanthanide centres with chelating Pt and Pd metalloligands.

Exploitation of HSAB preferences allows for high-yield, one-pot syntheses of lanthanide complexes chelated by two Pd or Pt metalloligands, [MII(SAc)4]2- (SAc- = thioacetate, M = Pd, Pt). The resulting complexes with 8 oxygen donors surrounding the lanthanides can be isolated in crystallographically tetragonal environments as either [NEt4]+ (space group: P4/mcc) or [PPh4]+ (space group: P4/n) salts. In the case of M = Pt, the complete series of lanthanide complexes has been structurally characterized as the [NEt4]+ salts (except for Ln = Pm), while the [PPh4]+ salts have been structurally characterized for Ln = Gd-Er, Y. For M = Pd, selected lanthanide complexes have been structurally characterized as both salts. The only significant structural difference between salts of the two counter ions is the resulting twist angle connecting tetragonal prismatic and tetragonal anti-prismatic configurations, with the [PPh4]+ salts approaching ideal D4d symmetry very closely (φ = 44.52-44.61°) while the [NEt4]+ salts exhibit intermediate twist angles in the interval φ = 17.28-27.41°, the twist increasing as the complete 4f series is traversed. Static magnetic properties for the latter half of the lanthanide series are found to agree well in the high temperature limit with the expected Curie behavior. Perpendicular and parallel mode EPR spectroscopy on randomly oriented powder samples and single crystals of the Gd complexes with respectively Pd- and Pt-based metalloligands demonstrate the nature of the platinum metal to strongly affect the spectra. Consistent parametrization of all of the EPR spectra reveals the main difference to stem from a large difference in the magnitude of the leading axial term, B02, this being almost four times larger for the Pt-based complexes as compared to the Pd analogues, indicating a direct Pt(5d z2 )-Ln interaction and an arguable coordination number of 10 rather than 8. The parametrization of the EPR spectra also confirms that off-diagonal operators are associated with non-zero parameters for the [NEt4]+ salts, while only contributing minimally for the [PPh4]+ salts in which lanthanide coordination approximates D4d point group symmetry closely.

Fluctuating-surface-current formulation of radiative heat transfer: Theory and applications

We describe a novel fluctuating-surface current formulation of radiative heat transfer between bodies of arbitrary shape that exploits efficient and sophisticated techniques from the surface-integral-equation formulation of classical electromagnetic scattering. Unlike previous approaches to non-equilibrium fluctuations that involve scattering matrices---relating "incoming" and "outgoing" waves from each body---our approach is formulated in terms of "unknown" surface currents, laying at the surfaces of the bodies, that need not satisfy any wave equation. We show that our formulation can be applied as a spectral method to obtain fast-converging semi-analytical formulas in high-symmetry geometries using specialized spectral bases that conform to the surfaces of the bodies (e.g. Fourier series for planar bodies or spherical harmonics for spherical bodies), and can also be employed as a numerical method by exploiting the generality of surface meshes/grids to obtain results in more complicated geometries (e.g. interleaved bodies as well as bodies with sharp corners). In particular, our formalism allows direct application of the boundary-element method, a robust and powerful numerical implementation of the surface-integral formulation of classical electromagnetism, which we use to obtain results in new geometries, including the heat transfer between finite slabs, cylinders, and cones.

Infrared Plasmonic Transmission Resonances of Gold Film with Hexagonally Ordered Hole Arrays on ZnSe Substrate

The high fractional open area of metal thin film coatings, with two dimensional, hexagonally ordered, close packed arrays of holes, makes them of interest for the incorporation of plasmonic effects into a variety of optical devices. Gold films with hexagonal patterns of circular holes have been created on ZnSe infrared windows. The films have 2.50 μm diameter holes and a hexagonal lattice parameter of 3.06 μm which places the primary transmission resonances of the ZnSe/gold interface at ~1,400 cm−1 (7.14 μm) and that of the air/gold interface at ~3,800 cm−1 (2.63 μm). This geometry produces useful transmission across the whole traditional mid-infrared range. The dispersion of these resonances has been measured by changing the angle of incident light. The data is modeled with explicit momentum matching equations in two different, high symmetry geometries, allowing the effective index of refraction to vary with wavelength. The response of these resonances to the addition of an acetaldehyde coating is described.

A structural paradigm for 3-periodic semiregular (46.69)-hxg net with high-symmetry hexagonal geometry, constructed from the linear [Cd2NaO6(H2O)6] SBUs and a flexible 6,6’-dithiodinicotinate linker

A 3-D metal-organic framework constructed from the linear trimetallic [Cd(2)NaO(6)(H(2)O)(6)] secondary building units (SBUs) and a long flexible 6,6'-dithiodinicotinate tecton is presented, which has a 6-connected (4(6).6(9))-hxg network with high-symmetry hexagonal geometries.

Interdependence of structure and chemical order in high symmetry (PdAu)N nanoclusters

In this work we study the stabilities of high-symmetry AuN, PdN and (AuPd)N clusters, for N < 1500, using mathematical constructs, a semi-empirical potential with two different parameter sets, and a quasi-Newtonian minimisation technique. For PdN clusters, both parameter sets tested result in preferences for icosahedral (Ih) structures for N < 1000 over other high-symmetry 12-vertex geometries; for AuN clusters we find a tendency towards face-centred cubic (FCC) structures at values of N lower than seen for PdN: parameter set I of Cleri and Rosato [Cleri and Rosato, Phys. Rev. B, 1993, 48, 22] gave a transition at N ≈ 650 to the I-Dh, whilst for parameter set II of Baletto et al. [Baletto et al., J. Chem. Phys., 2002, 116, 3856] this value was lower still. For (AuPd)N clusters we found that the preferred arrangement is (PdcoreAushell)N, with thin (monolayer) surface coverings of Au being most energetically favourable compared to the homogeneous clusters; however for parameter set II multiple layers of Au lead to energetic instability. (AucorePdshell)N clusters are not energetically favourable with thin coatings of Pd, however as the shell coating thickens so the stability improves. Ih structures are unfavourable compared to the Ino-decahedron and cuboctahedron for (AucorePdshell)N, whereas the FCC-type structures are strongly preferred for (PdcoreAushell)N. Overall, the strong tendency towards core–shell segregation is emphasised for parameter set I more than II, agreeing with previous work on smaller (AuPd)N clusters.

A SIMPLE ALGORITHM TO ENFORCE DIRICHLET BOUNDARY CONDITIONS IN COMPLEX GEOMETRIES

We present a new algorithm to implement Dirichlet boundary conditions for diffusive processes in arbitrarily complex geometries. In this approach, the boundary conditions around the diffusing object is replaced by the fictitious phase transition of a pure substance where the energy cost of the phase transition largely overwhelms the amount of energy stored in the system. The computing cost of this treatment of the boundary condition is independent of the topology of the boundary. Moreover, the implementation of this new approach is straightforward and follows naturally from enthalpy-based numerical methods. This algorithm is compatible with a wide variety of discretization methods, finite differences, finite volume, lattice Boltzmann methods and finite elements, to cite a few. We show, here, using both lattice Boltzmann and finite-volume methods that our model is in excellent agreement with analytical solutions for high symmetry geometries. We also illustrate the advantages of the algorithm to handle more complex geometries.

Pseudo Jahn–Teller origin of cis–trans and other conformational changes. The role of double bonds

Based on the pseudo Jahn-Teller effect (PJTE) theory, an approach is developed to rationalize and predict the conformations and conformational changes in molecular systems with a common pattern, a double bond. It is shown that starting with the high-symmetry geometry of the environment (in many cases D(2d)), the double bond descends from an e(2) electronic configuration (e is a twofold degenerate MO) which produces a variety of PJT distortions, the main of which is the rotational (b(1)) transformation D(2d) → D(2h) accompanied by the formation of the double bond. Further PJT interactions with higher energy E-states may trigger additional distortions which in D(2h) symmetry are classified as in-plane (e(i)) cis and trans, and out-of-plane (e(o)) chair and boat. The realization of these conformations depends on the positions of the excited E-states and the PJTE parameter values. The two emerging PJTE problems, ((3)A(2) + (3)E(1) + (3)E(2)) ⊗ (e(i) + e(o)) and ((1)A(1) + (1)B(1) + (1)B(2) + (1)E(1) + (1)E(2)) ⊗ (b(1) + e(i) + e(o)), are formulated in the matrix form and provide a general picture of the ground and excited adiabatic potential energy surfaces. Following this scheme in combination with ab initio calculations, the possible conformations and conformational transitions are analyzed for several specific systems including (in increasing complexity) N(2)H(2), C(2)H(4), N(2)(NH(2))(2) and N(2)(C(6)H(5))(2) (azobenzene). The family of molecular systems with a double bond is vast, but the importance of the PJT approach developed here is also in its general validity as it can be applied to any other systems.

Spectral-product methods for electronic structure calculations

Progress is reported in development, implementation, and application of a spectral method for ab initio studies of the electronic structure of matter. In this approach, antisymmetry restrictions are enforced subsequent to construction of the many-electron Hamiltonian matrix in a complete orthonormal spectral-product basis. Transformation to a permutation-symmetry representation obtained from the eigenstates of the aggregate electron antisymmetrizer is seen to enforce the requirements of the Pauli principle ex post facto, and to eliminate the unphysical (non-Pauli) states spanned by the product representation. Results identical with conventional use of prior antisymmetrization of configurational state functions are obtained in applications to many-electron atoms. The development provides certain advantages over conventional methods for polyatomic molecules, and, in particular, facilitates incorporation of fragment information in the form of Hermitian matrix representatives of atomic and diatomic operators which include the non-local effects of overall electron antisymmetry. An exact atomic-pair expression is obtained in this way for polyatomic Hamiltonian matrices which avoids the ambiguities of previously described semi-empirical fragment-based methods for electronic structure calculations. Illustrative applications to the well-known low-lying doublet states of the H3 molecule in a minimal-basis-set demonstrate that the eigensurfaces of the antisymmetrizer can anticipate the structures of the more familiar energy surfaces, including the seams of intersection common in high-symmetry molecular geometries. The calculated H3 energy surfaces are found to be in good agreement with corresponding valence-bond results which include all three-center terms, and are in general accord with accurate values obtained employing conventional high-level computational-chemistry procedures. By avoiding the repeated evaluations of the many-centered one- and two-electron integrals required in construction of polyatomic Hamiltonian matrices in the antisymmetric basis states commonly employed in conventional calculations, and by performing the required atomic and atomic-pair calculations once and for all, the spectral-product approach may provide an alternative potentially efficient ab initio formalism suitable for computational studies of adiabatic potential energy surfaces more generally.

Electronic quenching of OH AΣ+2 radicals in single collision events with molecular hydrogen: Quantum state distribution of the OH XΠ2 products

We report a combined experimental and theoretical investigation of the nonreactive quenching channel resulting from electronic quenching of OH A 2Sigma+ by molecular hydrogen. The experiments utilize a pump-probe scheme to determine the OH X 2Pi population distribution following collisional quenching in a pulsed supersonic expansion. The pump laser excites OH A 2Sigma+ (nu'=0, N'=0), which has a significantly reduced fluorescence lifetime due to quenching by H2. The probe laser monitors the OH X 2Pi (nu", N") population via laser-induced fluorescence on various A-X transitions under single collision conditions. The experiments reveal a high degree of rotational excitation (N") of the quenched OH X 2Pi products observed in nu"=1 and 2 as well as a pronounced propensity for quenching into the Pi(A') Lambda-doublet level. These experiments have been supplemented by extensive multireference, configuration-interaction calculations aimed at exploring the topology of the relevant potential energy surfaces. Electronic quenching of OH A 2Sigma+ by H2 proceeds through conical intersections between two potentials of A' reflection symmetry (in planar geometry) that correlate with the electronically excited A 2Sigma+ and ground X 2Pi states of OH. The conical intersections occur in high-symmetry geometries, in which the O side of OH points toward H2. Corroborating and extending earlier work of Hoffman and Yarkony [J. Chem. Phys. 113, 10091 (2000)], these calculations reveal a steep gradient away from the OH-H2 conical intersection as a function of both the OH orientation and interfragment distance. The former will give rise to a high degree of OH rotational excitation, as observed for the quenched OH X 2Pi products.