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Henry F Schaefer

Publications and source records attributed to Henry F Schaefer.

At least 19 recordsLinked to original sources
PubMed2007

The existence of secondary orbital interactions.

B3LYP/6-311+G** (and MP2/6-311+G**) computations, performed for a series of Diels-Alder (DA) reactions, confirm that the endo transition states (TS) and the related Cope-TSs are favored energetically over the respective exo-TSs. Likewise, the computed magnetic properties (nucleus-independent chemical shifts and magnetic susceptibililties) of the endo- (as well as the Cope) TS's reveal their greater electron delocalization and greater aromaticity than the exo-TS's. However, Woodward and Hoffmann's original example is an exception: their endo-TS model, involving the DA reaction of a syn- with an anti-butadiene (BD), actually is disfavored energetically over the corresponding exo-TS; magnetic criteria also do not indicate the existence of SOI delocalization in either case. Instead, a strong energetic preference for endo-TSs due to SOI is found when both BDs are in the syn conformations. This is in accord with Alder and Stein's rule of "maximum accumulation of double bonds:" both the dienophile and the diene should have syn conformations. Plots along the IRC's show that the magnetic properties typically are most strongly exalted close to the energetic TS. Because of SOI, all the points along the endo reaction coordinates are more diatropic than along the corresponding exo pathways. We find weak SOI effects to be operative in the endo-TSs involved in the cycloadditions of cyclic alkenes, cyclopropene, aziridine, cyclobutene, and cyclopentene, with cyclopentadiene. While the endo-TSs are only slightly lower in energy than the respective exo-TSs, the magnetic properties of the endo-TS's are significantly exalted over those for the exo-TS's and the Natural Bond Orbitals indicate small stabilizing interactions between the methylene cycloalkene hydrogen orbitals (and lone pairs in case of aziridine) with pi-character and the diene pi MOs.

Journal Article↗
PubMed2006

Manganese carbonyl nitrosyls: comparison with isoelectronic iron carbonyl derivatives.

The manganese carbonyl nitrosyls Mn(NO)(CO)4, Mn2(NO)2(CO)n (n = 7, 6, 5, 4), and Mn3(NO)3(CO)9 have been studied by density functional theory (DFT) using the B3LYP and BP86 methods for comparison of their predicted structures with those of isoelectronic iron carbonyl derivatives. DFT predicts a trigonal bipyramidal structure for Mn(NO)(CO)4 with an equatorial NO group very close to the experimental structure. The predicted lowest energy structure for Mn2(NO)2(CO)7 has two bridging NO groups in contrast to the known structure of the isoelectronic Fe2(CO)9, which has three bridging CO groups. The structures for the unsaturated binuclear Mn2(NO)2(CO)n (n = 6, 5, 4) derivatives are similar to those of the corresponding binuclear iron carbonyls Fe2(CO)n+2 derivatives but always with a preference of bridging NO groups over bridging CO groups. The trinuclear Mn3(NO)3(CO)9 is predicted to have a structure analogous to the known structure for Fe3(CO)12 but with two bridging NO groups rather than two bridging CO groups across one of the metal-metal edges of the M3 triangle. The dark red solid photolysis product of Mn(NO)(CO)4 characterized by its nu(CO) and nu(NO) frequencies approximately 45 years ago is suggested by these DFT studies not to be the originally assumed Mn2(NO)2(CO)7 analogous to Fe2(CO)9. Instead, this photolysis product appears to be Mn2(NO)2(CO)5 with a Mn(triple bond)Mn formal triple bond analogous to (eta5-C5H5)2V2(CO)5 obtained from the photolysis of (eta5-C5H5)V(CO)4.

Journal Article↗
PubMed2006

Successive attachment of electrons to protonated Guanine: (G+H)* radicals and (G+H)- anions.

The structures, energetics, and vibrational frequencies of nine hydrogenated 9H-keto-guanine radicals (G+H)(*) and closed-shell anions (G+H)(-) are predicted using the carefully calibrated (Chem. Rev. 2002, 102, 231) B3LYP density functional method in conjunction with a DZP++ basis set. These radical and anionic species come from consecutive electron attachment to the corresponding protonated (G+H)(+) cations in low pH environments. The (G+H)(+) cations are studied using the same level of theory. The proton affinity (PA) of guanine computed in this research (228.1 kcal/mol) is within 0.7 kcal/mol of the latest experiment value. The radicals range over 41 kcal/mol in relative energy, with radical r1, in which H is attached at the C8 site of guanine, having the lowest energy. The lowest energy anion is a2, derived by hydride ion attachment at the C2 site of guanine. No stable N2-site hydride should exist in the gas phase. Structure a9 was predicted to be dissociative in this research. The theoretical adiabatic electron affinities (AEA), vertical electron affinities, and vertical detachment energies were computed, with AEAs ranging from 0.07 to 3.12 eV for the nine radicals.

Anions↗
PubMed2006

Elementary constituents of microdevices: the Ge2H fragment.

Highly correlated ab initio electronic structure theory has been used to systematically investigate the linear (X 2Pi) GeGeH and H-bridged (X 2B1 and A 2A1) GeHGe structures and the isomerization transition state (A 2A') connecting X 2Pi with A 2A1. The equilibrium structures and physical properties have been predicted employing self-consistent field, configuration interaction with single and double excitations, coupled cluster with single and double excitations (CCSD), and CCSD with perturbative triple excitations [CCSD(T)]. Four correlation-consistent polarized valence-[cc-pVXZ and aug-cc-pVXZ (X=T and Q)] type basis sets were used in the study. For the H-bridged GeHGe isomer, the X 2B1 ground state is predicted to lie only 0.74 [0.95 with zero-point vibrational energy (ZPVE) corrections] kcal mol-1 below the A 2A1 excited state at the CCSD(T) level of theory with the augmented correlation-consistent polarized valence quadruple zeta (aug-cc-pVQZ) basis set. The X 2B1 state of the H-bridged GeHGe isomer lies 8.6 kcal mol-1 below the X 2Pi ground state of the linear GeGeH isomer. The forward isomerization barrier from the linear ground state to the A 2A' transition state is predicted to be 3.7 kcal mol-1. The reverse isomerization barrier between the A 2A1 GeHGe structure and the X 2Pi GeGeH structure is predicted to be 11.6 (10.8 with the ZPVE corrections) kcal mol-1 at the aug-cc-pVQZ CCSD(T) level of theory.

Journal Article↗
PubMed2006

Hydrogen bridging in the compounds X2H (X=Al,Si,P,S).

X2H hydrides (X=Al, Si, P, and S) have been investigated using coupled cluster theory with single, double, and triple excitations, the latter incorporated as a perturbative correction [CCSD(T)]. These were performed utilizing a series of correlation-consistent basis sets augmented with diffuse functions (aug-cc-pVXZ, X=D, T, and Q). Al2H and Si2H are determined to have H-bridged C2v structures in their ground states: the Al2H ground state is of 2B1 symmetry with an Al-H-Al angle of 87.6 degrees, and the Si2H ground state is of 2A1 symmetry with a Si-H-Si angle of 79.8 degrees. However, P2H and S2H have nonbridged, bent Cs structures: the P2H ground state is of 2A' symmetry with a P-P-H angle of 97.0 degrees, and the S2H ground state is of 2A' symmetry with a S-S-H angle of 93.2 degrees. Ground state geometries, vibrational frequencies, and electron affinities have been computed at all levels of theory. Our CCSD(T)/aug-cc-pVQZ adiabatic electron affinity of 2.34 eV for the Si2H radical is in excellent agreement with the photoelectron spectroscopy experiments of Xu et al. [J. Chem. Phys. 108, 7645 (1998)], where the electron affinity was determined to be 2.31+/-0.01 eV.

Journal Article↗
PubMed2006

High-order excitations in state-universal and state-specific multireference coupled cluster theories: model systems.

For the first time high-order excitations (n>2) have been studied in three multireference couple cluster (MRCC) theories built on the wave operator formalism: (1) the state-universal (SU) method of Jeziorski and Monkhorst (JM) (2) the state-specific Brillouin-Wigner (BW) coupled cluster method, and (3) the state-specific MRCC approach of Mukherjee (Mk). For the H4, P4, BeH(2), and H8 models, multireference coupled cluster wave functions, with complete excitations ranging from doubles to hextuples, have been computed with a new arbitrary-order string-based code. Comparison is then made to corresponding single-reference coupled cluster and full configuration interaction (FCI) results. For the ground states the BW and Mk methods are found, in general, to provide more accurate results than the SU approach at all levels of truncation of the cluster operator. The inclusion of connected triple excitations reduces the nonparallelism error in singles and doubles MRCC energies by a factor of 2-10. In the BeH(2) and H8 models, the inclusion of all quadruple excitations yields absolute energies within 1 kcal mol(-1) of the FCI limit. While the MRCC methods are very effective in multireference regions of the potential energy surfaces, they are outperformed by single-reference CC when one electronic configuration dominates.

Journal Article↗
PubMed2006

Effects of microsolvation on uracil and its radical anion: uracil(H2O)n (n = 1-5).

Microsolvation effects on the stabilities of uracil and its anion have been investigated by explicitly considering the structures of complexes of uracil with up to five water molecules at the B3LYPDZP++ level of theory. For all five systems, the global minimum of the neutral cluster has a different equilibrium geometry from that of the radical anion. Both the vertical detachment energy (VDE) and adiabatic electron affinity (AEA) of uracil are predicted to increase gradually with the number of hydrating molecules, qualitatively consistent with experimental results from a photodetachment-photoelectron spectroscopy study [J. Schiedt et al., Chem. Phys. 239, 511 (1998)]. The trend in the AEAs implies that while the conventional valence radical anion of uracil is only marginally bound in the gas phase, it will form a stable anion in aqueous solution. The gas-phase AEA of uracil (0.24 eV) was higher than that of thymine by 0.04 eV and this gap was not significantly affected by microsolvation. The largest AEA is that predicted for uracil(H2O)5, namely, 0.96 eV. The VDEs range from 0.76 to 1.78 eV.

Anions↗
PubMed2006

Structure of the boronic acid dimer and the relative stabilities of its conformers.

Despite the widespread use of boronic acids in materials science and as pharmaceutical agents, many aspects of their structure and reactivity are not well understood. In this research the boronic acid dimer, [HB(OH)(2)](2), was studied by second-order Møller-Plesset (MP2) perturbation theory and coupled cluster methodology with single and double excitations (CCSD). Pople split-valence 6-31+G*, 6-311G**, and 6-311++G** and Dunning-Woon correlation-consistent cc-pVDZ, aug-cc-pVDZ, cc-pVTZ, and aug-cc-pVTZ basis sets were employed for the calculations. A doubly hydrogen-bonded conformer (1) of the dimer was consistently found to be lowest in energy; the structure of 1 was planar (C(2h)) at most computational levels employed but was significantly nonplanar (C(2)) at the MP2/6-311++G** and CCSD/6-311++G** levels, the result of an intrinsic problem with Pople-type sp-diffuse basis functions on heavy atoms. The dimerization energy, enthalpy, and free energy for the formation of (1) from the exo-endo conformer of the monomer were -10.8, -9.2, and +1.2 kcal/mol, respectively, at the MP2/aug-cc-pVTZ level. Several other hydrogen-bonded conformers of the dimer were local minima on the potential energy surface (PES) and ranged from 2 to 5 kcal/mol higher in energy than 1. Nine doubly OH-bridged conformers, in which the boron atoms were tetracoordinated, were also local minima on the PES, but they were all greater than 13 kcal/mol higher in energy than 1; doubly H-bridged structures proved to be transition states. MP2 and CCSD results were compared to those from the BLYP, B3LYP, OLYP, O3LYP, PBE1PBE, and TPSS functionals with the 6-311++G** and aug-cc-pVTZ basis sets; the PBE1PBE functional performed best relative to the MP2 and CCSD results. Self-consistent reaction field (SCRF) calculations predict that boronic acid dimerization is less favorable in solution than in vacuo.

Boronic Acids↗
PubMed2006

Characterization of singlet ground and low-lying electronic excited states of phosphaethyne and isophosphaethyne.

The singlet ground ((approximate)X(1)Sigma1+) and excited (1Sigma-,1Delta) states of HCP and HPC have been systematically investigated using ab initio molecular electronic structure theory. For the ground state, geometries of the two linear stationary points have been optimized and physical properties have been predicted utilizing restricted self-consistent field theory, coupled cluster theory with single and double excitations (CCSD), CCSD with perturbative triple corrections [CCSD(T)], and CCSD with partial iterative triple excitations (CCSDT-3 and CC3). Physical properties computed for the global minimum ((approximate)X(1)Sigma+HCP) include harmonic vibrational frequencies with the cc-pV5Z CCSD(T) method of omega1=3344 cm(-1), omega2=689 cm(-1), and omega3=1298 cm(-1). Linear HPC, a stationary point of Hessian index 2, is predicted to lie 75.2 kcal mol(-1) above the global minimum HCP. The dissociation energy D0[HCP((approximate)X(1)Sigma+)-->H(2S)+CP(X2Sigma+)] of HCP is predicted to be 119.0 kcal mol(-1), which is very close to the experimental lower limit of 119.1 kcal mol(-1). Eight singlet excited states were examined and their physical properties were determined employing three equation-of-motion coupled cluster methods (EOM-CCSD, EOM-CCSDT-3, and EOM-CC3). Four stationary points were located on the lowest-lying excited state potential energy surface, 1Sigma- -->1A", with excitation energies Te of 101.4 kcal mol(-1) (1A"HCP), 104.6 kcal mol(-1)(1Sigma-HCP), 122.3 kcal mol(-1)(1A" HPC), and 171.6 kcal mol(-1)(1Sigma-HPC) at the cc-pVQZ EOM-CCSDT-3 level of theory. The physical properties of the 1A" state with a predicted bond angle of 129.5 degrees compare well with the experimentally reported first singlet state ((approximate)A1A"). The excitation energy predicted for this excitation is T0=99.4 kcal mol(-1) (34 800 cm(-1),4.31 eV), in essentially perfect agreement with the experimental value of T0=99.3 kcal mol(-1)(34 746 cm(-1),4.308 eV). For the second lowest-lying excited singlet surface, 1Delta-->1A', four stationary points were found with Te values of 111.2 kcal mol(-1) (2(1)A' HCP), 112.4 kcal mol(-1) (1Delta HPC), 125.6 kcal mol(-1)(2(1)A' HCP), and 177.8 kcal mol(-1)(1Delta HPC). The predicted CP bond length and frequencies of the 2(1)A' state with a bond angle of 89.8 degrees (1.707 A, 666 and 979 cm(-1)) compare reasonably well with those for the experimentally reported (approximate)C(1)A' state (1.69 A, 615 and 969 cm(-1)). However, the excitation energy and bond angle do not agree well: theoretical values of 108.7 kcal mol(-1) and 89.8 degrees versus experimental values of 115.1 kcal mol(-1) and 113 degrees. of 115.1 kcal mol(-1) and 113 degrees.

Journal Article↗
PubMed2006

Remarkable aspects of unsaturation in trinuclear metal carbonyl clusters: the triiron species Fe3(CO)n (n = 12, 11, 10, 9).

The trinuclear iron carbonyls Fe(3)(CO)(n) (n = 12, 11, 10, 9) have been studied by density functional theory using the B3LYP and BP86 functionals. The experimentally known C(2)(v) isomer of Fe(3)(CO)(12), namely Fe(3)(CO)(10)(mu-CO)(2), is found to be the global minimum below the unbridged D(3)(h) isomer analogous to the known structures for Ru(3)(CO)(12) and Os(3)(CO)(12). The lowest-energy isomer found for Fe(3)(CO)(11) is Fe(3)(CO)(9)(mu(3)-CO)(2) with iron-iron distances in the Fe(3) triangle, suggesting the one double bond (2.460 A by B3LYP and 2.450 A by BP86) and two single bonds (2.623 A by B3LYP and 2.604 A by BP86) required to give each Fe atom the favored 18-electron configuration. Two different higher-energy dibridged structures Fe(3)(CO)(9)(mu(2)-CO)(2) are also found for Fe(3)(CO)(11). The lowest-energy isomer found for Fe(3)(CO)(10) is Fe(3)(CO)(9)(mu(3)-CO) with equivalent iron-iron distances in the Fe(3) ring (2.47 A by B3LYP or BP86). The lowest-energy isomer found for Fe(3)(CO)(9) is Fe(3)(CO)(6)(mu-CO)(3) with distances in the Fe(3) triangle possibly suggesting one single bond (2.618 A by B3LYP and 2.601 A by BP86), one weak double bond (2.491 A by B3LYP and 2.473 A by BP86), and one weak triple bond (2.368 A by B3LYP and 2.343 A by BP86). A higher-lying isomer of Fe(3)(CO)(9), i.e., Fe(3)(CO)(8)(mu-CO), at approximately 21 kcal/mol above the global minimum, has iron-iron distances strongly suggesting two single bonds (2.6 to 2.7 A) and one quadruple bond (2.068 A by B3LYP and 2.103 A by BP86). Wiberg Bond Indices are also helpful in evaluating the iron-iron bond orders.

Journal Article↗
PubMed2006

An efficient computational approach for the evaluation of substituent constants.

Density functional theory computations at the B3LYP/6-311+G(2d,2p) and BPW91/6-311G(d,p) levels were carried out for a series of 15 monosubstituted benzene derivatives to study dependencies between electronic structure parameters and experimental reactivity constants. An efficient and accurate computational approach for the evaluation of sigma(0) substituent constants for substituted benzene systems is outlined. It is based on the excellent linear correlation between the experimental reactivity constants and the theoretical electrostatic potential values (EPN) at the carbon atoms in the para and meta positions. The results underline the usefulness of the EPN as a local reactivity descriptor. Theoretical computations to assess the influence of water solvent using the SCIPCM method showed that the solvent enhances the overall effect of polar substituents by about 30%. The results obtained indicate also that the relative values of the sigma(0) constants are predominantly determined by intramolecular influences.

Journal Article↗
PubMed2006

Molecular oxygen dianions trapped in lithium cluster cages.

In an important recent report, Coles and Hitchcock (Chem. Commun. 2005, 3165) have synthesized a unique compound of the form Li9O2(hpp)7, with an O2 unit encapsulated in the Li cluster. The group hpp is the bicyclic guanidinate 1,3,4,6,7,8-hexahydropyrimido[1,2-a]pyrimidinate. Coles and Hitchcock raise the interesting possibility that the trapped species is the O2 dianion. We have theoretically investigated (with full geometry optimization) the structural, vibrational, and electronic properties of this fascinating system with 165 atoms. Every indication supports the qualitative conclusion that this system is best represented as an O2 dianion embedded in a Li cluster. Several smaller model systems, including replacement of the hpp ligands by Cl atoms, are also examined and provide some interesting comparisons.

Journal Article↗
PubMed2006

Popular theoretical methods predict benzene and arenes to be nonplanar.

Planar (D6h) benzene has one (1181i cm-1, b2g) and three (1844i cm-1, b2g; 462i cm-1, e2u) imaginary vibrational frequencies at the MP2/6-311++G(d,p) and MP2/6-311++G levels of theory, respectively! The spurious frequencies correspond to D3d chair (b2g) and C2v boat (e2u) out-of-plane distortions. Numerous, similar examples where planar benzene is not a minimum are documented at the MP2, MP3, and CISD levels with popular Pople-type basis sets, while the RHF, B3LYP, and BLYP methods exhibit no such problems. We show that, in the sp and spd atomic-orbital limits of MP2 theory, benzene is nonplanar. The observed failure of electron correlation methods with unbalanced basis sets to predict planar minima is not unique to benzene but is also found for other pi-delocalized molecules, including pyridine, naphthalene, anthracene, the cyclopentadienyl and indenyl anions, and the tropylium cation. Detailed mathematical analysis reveals that an insidious, geometry-dependent, two-electron basis set incompleteness error (BSIE) is responsible for the problem and that balanced, correlation-consistent constructions of basis sets are generally required to ensure reliable predictions for arenes with correlated wave functions.

Journal Article↗
PubMed2006

Hydrogen atom and hydride anion addition to adenine: structures and energetics.

The radicals and anions derived from the 9H tautomer of adenine by adding a hydrogen atom to one of the four double bonds of the adenine framework have been studied. Computations were carried out using a carefully calibrated density functional (B3LYP) method and basis set (DZP++). Optimized geometries, energies, and vibrational frequencies are predicted for eight radicals and anions. The radicals are found to lie in a range of 22 kcal mol(-1), with the radical derived by addition to the C(8) carbon atom being the lowest lying energetically. The anions are predicted to be bound species in the gas phase with an energetic range of 43 kcal mol(-1). Anions produced by addition of a hydride ion to adenine carbon atoms are found to be the most favorable. Six of the anions are predicted to be stable species with respect to electron detachment. The adiabatic electron affinities, vertical electron affinities, and vertical detachment energies are computed for the first time. Electron affinities for these radicals range from 0.0 to 2.0 eV. Radicals produced by addition to a nitrogen atom have near-zero adiabatic electron affinities, while radicals produced by addition at carbon atoms have considerably higher electron affinities.

Adenine↗
PubMed2006

Binuclear cyclopentadienylmetal nitrosyls of iron, cobalt, and nickel: comparison with related carbonyl derivatives.

The binuclear cyclopentadienylmetal nitrosyls and carbonyls Cp2M2(AO)n (A = N, M = Fe, Co, Ni; A = C, M = Ni; n = 2, 1) are studied by density functional theory using the B3LYP and BP86 functionals. In general, structures with bridging AO ligands are energetically preferred over those with terminal AO ligands. Thus, the global minima for Cp2M2(AO)2 are all found to have closely related axial dimetallocene structures with two symmetrically bridging AO ligands but variable planarity of the central M(mu-AO)2M units. Similarly, the single AO ligands in the global minima for Cp2M2(AO) are found to bridge symmetrically the pair of metal atoms. However, structures with terminal AO groups and a single bridging Cp ligand are also found at accessible energies for CpM2(NO)(mu-Cp) (M = Fe and Co) and CpNi2(CO)(mu-Cp). The metal-metal bond distances in Cp2M2(AO)n derivatives correlate reasonably well with the requirements of the 18-electron rule. In this connection, the unusual dimer Cp2Ni2(mu-NO)2 has a Ni-Ni bond distance suggestive of a single bond and geometry suggesting one one-electron donor bridging NO group and one three-electron donor bridging NO group. However, dissociation of Cp2Ni2(mu-NO)2 into the well-known stable monomer CpNiNO is highly favored energetically.

Journal Article↗
PubMed2006

Advances in methods and algorithms in a modern quantum chemistry program package.

Advances in theory and algorithms for electronic structure calculations must be incorporated into program packages to enable them to become routinely used by the broader chemical community. This work reviews advances made over the past five years or so that constitute the major improvements contained in a new release of the Q-Chem quantum chemistry package, together with illustrative timings and applications. Specific developments discussed include fast methods for density functional theory calculations, linear scaling evaluation of energies, NMR chemical shifts and electric properties, fast auxiliary basis function methods for correlated energies and gradients, equation-of-motion coupled cluster methods for ground and excited states, geminal wavefunctions, embedding methods and techniques for exploring potential energy surfaces.

Algorithms↗
PubMed2006

Molecular mechanics (MM4) study of fluorinated hydrocarbons.

A molecular mechanics study of small saturated hydrocarbons (up to C-6) substituted by up to six fluorines has been carried out with the MM4 force field. A parameter set has been developed for use in the calculation of bond lengths, bond angles, torsion angles, conformational energies, barriers to rotation, dipole moments, moments of inertia, and vibrational frequencies for these compounds. The results are mostly in fair to good agreement with experiment and ab initio calculations. The high electronegativity of fluorine leads to serious geometric consequences in these compounds, but these consequences can be dealt with adequately by suitable cross-terms in the force constant matrix, and by recognizing that some of the reference bond lengths and angles (l(0), theta(0)) and the corresponding stretching and bending constant parameters (k(s), k(theta)) that are usually thought of as constants must in fact be treated as functions of the electronegativity of the substituents. Additionally, the heavy mass of the fluorine (relative to the mass of hydrogen in alkanes) leads to large values for other cross-terms that were found to be unimportant in hydrocarbons. Conformational equilibria for polyfluorinated compounds are affected by the delta-two effect well-known in carbohydrates. A few larger fluorinated and polyfluorinated alkanes, including perfluoropropane, perfluorobutane, and Teflon, have also been studied.

Journal Article↗