Steven S. Wesolowski

Estimation of binding affinities for celecoxib analogues with COX-2 via Monte Carlo-extended linear response.

Monte Carlo (MC)-extended linear response (ELR) calculations have been used for prediction of binding affinities of celecoxib analogues with the COX-2 enzyme. Three physically motivated descriptors from the MC simulations were used in a regression equation to fit 45 experimental activities with r(2)=0.71 and q(2)=0.68. The ELR approach provides a promising screen for optimization of enzyme inhibitors.

Three- versus four-coordinate phosphorus in the gas phase and in solution: Treacherous relative energies for phosphine oxide and phosphinous acid

Previous ab initio studies have consistently predicted phosphine oxide (H3PO) to be less stable than its nearly isoenergetic cis- and trans-phosphinous acid isomers (H2POH). However, complete basis set extrapolations employing the coupled-cluster series show that phosphine oxide is actually ca. 1.0 kcal/mol more stable than its acid forms in the gas phase. Incorporation of tight d functions via Dunning’s core-valence (cc-pCVXZ) or newly constructed “plus d” [cc-pV(X+d)Z] basis sets is essential for rapid convergence of core polarization effects which are evident even at the SCF level. The precision to which the phosphorus hybridization is described in the three- and four-coordinate environments ultimately determines the predicted gas-phase relative energy orderings. Focal-point analyses demonstrate that this system represents a disturbing case where use of a conventional valence quadruple-ζ quality basis set (cc-pVQZ)—even at the CCSD(T) level—fails to provide the correct relative energy ordering for simp...

Coupled-cluster electronic spectra for the Ca+–acetylene π complex and comparisons to its alkaline earth analogs

Equation of motion coupled-cluster (EOM-CCSD) predictions of structures and electronic excitation energies for the recently detected Ca+–acetylene π-complex confirm three experimental state assignments, but suggest reinterpretation of the signals associated with the (2) 2B1 and (2) 2B2 states that correlate to the 2P←2S Ca+ atomic transition. The originally assigned 000 band for the (2) 2B1 state corresponds to the computed excitation energy to the (2) 2B2 state and simple reassignment is proposed. The true (2) 2B1 state was not assigned in the original spectrum. However, the computed oscillator strength is large and its optimized geometry is similar to that of the ground state. Furthermore, the experimental band tentatively attributed to the onset of the symmetric C–H stretching progression of the assigned state has a relative energy conspicuously close to the computed electronic energy for the unassigned (2) 2B1 state. Based on the computed energy separations of the optimized EOM-CCSD structures, reassi...

Coupled-cluster characterization of the ground and excited states of the CH2N and CH2P radicals

High-level coupled-cluster theory with large basis sets was used to determine the optimized geometries and harmonic vibrational frequencies for the ground and low-lying excited electronic states of the CH2N and CH2P radicals. Additionally, isotropic hyperfine splitting constants were determined for the C2v CH2X, trans-HCXH and cis-HCXH (where X=N and P) isomers as a gauge of the delocalization of the unpaired electron. The geometrical parameters of X 2B2 CH2X, the trans-HCXH and cis-HCXH conformers, and the first three excited states are reported at the coupled-cluster level with single, double, and perturbatively applied triple excitations [CCSD(T)] using Dunning’s correlation consistent polarized valence quadruple-ζ basis set (cc-pVQZ). The C2v structures on the ground state surface are predicted to lie 9.3 and 13.5 kcal/mol lower than the trans- and cis-isomers, respectively, for CH2N and 28.1 and 30.0 kcal/mol, respectively, for CH2P. There are many other important properties of CH2N and CH2P which a...

Scratching the surface of the water dication

The X 3Σg−, a 1Δg, and b 1Σg+ states of the water dication, H2O2+, have been investigated using several high-level ab initio methods and a range of basis sets. With Dunning’s augmented correlation consistent polarized valence quadruple-ζ (aug-cc-pVQZ) basis set at the complete active space self-consistent field second-order configuration interaction (CAS-SOCI) level, it is confirmed that the ground and first two excited states of H2O2+ are all of D∞h symmetry, in violation of Walsh’s rules for 6 valence electron AH2 systems. The singlet–triplet splitting (X 3Σg−—a 1Δg) is predicted to be 53.6 kcal/mol (2.32 eV, 18 700 cm−1), while the X 3Σg−—b 1Σg+ separation is predicted to be 91.1 kcal/mol (3.95 eV, 31 900 cm−1). The vertical double ionization potentials (IPs) from X 1A1 H2O to the X 3B1, 1 1A1, b 1B1, and 2 1A1 states of H2O2+ are predicted within the cc-pVQZ basis to be 40.1, 41.2, 42.6, and 46.1 eV, respectively, in good agreement with recent double-charge-transfer spectroscopic results. The ...

Isomerization pathway of the aluminum monocarbonyl/isocarbonyl pair, AlCO/AlOC: Evidence of a cyclic minimum

The isomerization pathway between AlOC and AlCO has been explored at the self-consistent field, configuration interaction, and coupled-cluster levels of theory. Five stationary points on the Al+CO potential energy surface were located and show that the path of Al migration from the isocarbonyl to the monocarbonyl involves a very small barrier to a perhaps unexpected cyclic minimum structure followed by a second barrier to the AlCO isomer. A quantitative analysis of the relative stabilities of the isomers as well as the ZPVE-corrected isomerization barriers are presented and compared to the boron carbonyl analogs. At the coupled-cluster level with single, double, and perturbatively applied connected triple substitutions [CCSD(T)] using a TZ2P+f basis set, the cyclic minimum is 9.4 kcal/mol higher in energy than AlCO but is 11.4 kcal/mol more stable than AlOC. The barriers from AlOC to the cyclic isomer and to the dissociation products 2P Al and X 1Σ+ CO are only 3.5 and 1.0 kcal/mol, respectively, and leav...

Definitive ab initio structure for the X̃ 2A′H2PO radical and resolution of the P–O stretching mode assignment

Previous ab initio studies of the X 2A′H2PO radical have reported dramatically differing P–O bond distances when using spin-restricted wave functions predicting two artifactual isomers of H2PO: a singly bonded oxygen-centered radical and a doubly bonded phosphorus-centered radical. We show that large basis sets coupled with high levels of dynamical electron correlation are required to correctly describe the P–O bond in H2PO as well as the unpaired electron density as evidenced by the Fermi contact terms and anisotropic components of the 31P, 1H, and 17O hyperfine splitting (hfs) constants. The optimized geometry, harmonic vibrational frequencies, and hfs constants of H2PO were determined at several coupled-cluster levels of theory using both spin-restricted (ROHF) and spin-unrestricted (UHF) Hartree–Fock reference wave functions. The geometrical parameters at the coupled-cluster level with single, double, and perturbatively applied triple substitutions [CCSD(T)] using Dunning’s correlation consistent pol...

Characterization of the and electronic states of CH2

Abstract The X 2 A 1 and a 4 A 2 electronic states of the methylene cation, CH 2 + , were investigated using the coupled cluster method with singles, doubles, and perturbatively applied triples [CCSD(T)] with Dunnings correlation consistent polarized valence basis set series (cc-pVXZ, where X=T, Q, and 5), core-valence basis sets (cc-pCVXZ, where X=T and Q), and augmented basis sets (aug-cc-pVXZ, where X=Q and 5). Explicit computation of the full set of triples (CCSDT) was also performed with the cc-pVTZ basis set. The most reliable equilibrium structures of r e =1.094 A and θ e =140.4° ( X 2 A 1 ) and r e =1.190 A and θ e =77.1° ( a 4 A 2 ) were obtained at the CCSD(T)/aug-cc-pV5Z level. The X 2 A 1 – a 4 A 2 classical energy separation is predicted to be 86.9 kcal/mol ( 30 400 cm −1 , 3.77 eV) at the CCSD(T)/cc-pCVQZ level of theory, and the zero-point vibrational energy corrected value is 84.5 kcal/mol ( 29 500 cm −1 , 3.66 eV).

Coupled-cluster studies of the hyperfine splitting constants of the thioformyl radical

Hyperfine splitting constants (hfs) of the X 2A′ electronic ground state of the thioformyl radical (HCS) have been determined at the coupled-cluster level with single, double, and perturbatively applied connected triple excitations [CCSD(T)] using 39 basis sets. Variation of the CCSD(T) hyperfine splittings with basis set was ascertained using a fixed geometry, optimized at the CCSD(T) level with Dunning’s correlation-consistent polarized valence quadruple-ζ basis set (cc-pVQZ). Pople basis sets, 6-311G++(2d,2p) and 6-311G++(3df,3pd), give 1H isotropic coupling constants (1H Aiso) in good agreement with the experimental vibrationally averaged value of 127.4 MHz, deviating by 5.5 and 9.3 MHz, respectively. Dunning’s valence correlation-consistent basis sets (cc-pVDZ, aug-cc-pVDZ, cc-pVTZ, aug-cc-pVTZ, cc-pVQZ, aug-cc-pVQZ) deviate 6.4 MHz (aug-cc-pVQZ) to 14.9 MHz (cc-pVDZ) from the experimental value. The correlation-consistent core valence analogues of these sets give very similar values with deviations...

The not-so-peculiar case of calcium oxide: a weakness in atomic natural orbital basis sets for calcium

Atomic natural orbital (ANO) basis sets for calcium may produce surprisingly poor atomic and molecular properties and energetics. The weaknesses in these basis sets may be traced primarily to deficiencies within the sets of d functions which are incapable of effectively correlating the 3s and 3p electrons. Examples are given which show that addition of tight d functions to the ANO basis is required to achieve qualitatively correct energetics and structures for conventionally bonded calcium compounds.