Ab Initio Molecular Orbital Study of the First Four Si/C Alternately Substituted Annulenes.

The ground and low-lying excited states of four alternating Si/C annulenes, H nSi n/2C n/2 with n = 4, 6, 8, and 10, have been investigated by ab initio molecular orbital methods and compared to those of their all-carbon and all-silicon analogues. In the ground state, all of the Si/C-mixed annulenes, except for the largest 10-membered annulene (H10Si5C5), assume equal-bond-length structures by adopting a closed-shell electronic structure in the possible highest symmetry. For the largest H10Si5C5, the trend of the bond delocalization still remains but the circular structure is considerably distorted and nonplanar due to severe angle strain. In the low-lying singlet (S1) and triplet (T1) states, the geometry of the compounds tends to be nonplanar as the excitations produce silyl radical character. Relative energies of the T1 and S1 states of the 6-membered ring, compared to those of the respective ground states (S0), are higher than those of the 4- and 8-membered rings, suggesting a special stability for H6Si3C3. The planar rhombus shape of the formally antiaromatic H4Si2C2 suggests that a synthetic effort is merited. Bonding analyses are given to support the conclusions reached on the basis of geometric structures and excited-state energetics.

Spin-Orbit Coupling Constants in Atoms and Ions of Transition Elements: Comparison of Effective Core Potentials, Model Core Potentials, and All-Electron Methods.

The spin-orbit coupling constants (SOCC) in atoms and ions of the first- through third-row transition elements were calculated for the low-lying atomic states whose main electron configuration is [ nd] q ( q = 1-4 and 6-9, n = the principal quantum number), using four different approaches: (1) a nonrelativistic Hamiltonian used to construct multiconfiguration self-consistent field (MCSCF) wave functions utilizing effective core potentials and their associated basis sets within the framework of second-order configuration interaction (SOCI) to calculate spin-orbit couplings (SOC) using one-electron Breit-Pauli Hamiltonian (BPH), (2) a nonrelativistic Hamiltonian used to construct MCSCF wave functions utilizing model core potentials and their associated basis sets within the framework of SOCI to calculate SOC using the full BPH, (3) nonrelativistic and spin-independent relativistic Hamiltonians used to construct MCSCF wave functions utilizing all-electron (AE) basis sets within the framework of SOCI to calculate SOC using the full BPH, and (4) a relativistic Hamiltonian given by the exact two-component (X2C) transformation for construction of Kramers-restricted relativistic configuration interaction wave functions. In this investigation, these four approaches are referred to as ECP, MCP, AE, and X2C methods, respectively. The ECP, MCP, and AE methods are so-called two-step approaches (TSA), while the X2C method is a one-step approach (OSA). In the AE method, three different calculations-relativistic elimination of small components (RESC), third-order Douglas-Kroll-Hess (DKH3), and infinite-order two-component (IOTC) relativistic correction-were performed for the estimation of the scalar relativistic components in addition to those of the nonscalar relativistic (NSR) contributions. The calculated SOCC are compared to the available experimental data via the Landé interval rule. Although there are several exceptions, including states whose main configuration is [ nd]5, the average differences between the ECP and AE (IOTC) SOCC and between the ECP and the X2C SOCC are mostly less than 20%. The differences between the ECP and the experimental SOCC are even smaller. No serious discrepancy was found between the TSA and OSA predictions of SOCC for the first- and second-row transition elements in comparison to experiment. For atoms and ions of the third-row transition elements, the SOCC calculated through the Landé interval rule are not reliable. The low-energy spin-mixed (SM) states originating from a [5d] q configuration ( q = 2-4) have a larger energy lowering due to the SOC effects, in comparison with those for atoms and ions of the first- and second-row transition elements. For the spin-mixed (SM) states originating from a [5d] q configuration ( q = 6-8), the energy lowering of all 4F7/2, 5D1, and 5D3 states due to the SOC effects is smaller than those of the other SM states. This difficulty, which also arises for the MCP, AE, and X2C (OSA) approaches, suggests that the LS-coupling scheme is inappropriate.

Numerical Estimation of the Pseudo-Jahn-Teller Effect Using Nonadiabatic Coupling Integrals in Monocyclic and Bicyclic Conjugated Molecules.

The pseudo-Jahn-Teller (pJT) effect in monocyclic and bicyclic conjugated molecules was investigated by using the state-averaged multiconfiguration self-consistent field (MCSCF) method, together with the 6-31G(d,p) basis sets. Following the perturbation theory, the force constant along a normal mode Q is given by the sum of the classical force constant and the vibronic contribution (VC) resulting from the interaction of the ground state with excited states. The latter is given as the sum of individual contributions arising from vibronic interactions between the ground state and excited states. In the present work, each VC was calculated on the basis of nonadiabatic coupling (NAC) integrals. Furthermore, the classical force constant was estimated by taking advantage of the VC and the force constant obtained by vibrational analyses. For pentalene and heptalene, the present method seems to overestimate the VC in absolute value because of the small energy gap between the ground state and the lowest excited state. However, we are confident that the VC and the classical force constant for the other molecules are reasonable in magnitude in comparison with available literature information. Thus, it is proved that the present method is applicable and useful for numerical estimation of pJT effect.

Accurate potential energy curve for B2. Ab initio elucidation of the experimentally elusive ground state rotation-vibration spectrum.

The electron-deficient diatomic boron molecule has long puzzled scientists. As yet, the complete set of bound vibrational energy levels is far from being known, experimentally as well as theoretically. In the present ab initio study, all rotational-vibrational levels of the X (3)Σ(g)(-) ground state are determined up to the dissociation limit with near-spectroscopic accuracy (<10 cm(-1)). Two complete sets of bound vibrational levels for the (11)B(2) and (11)B-(10)B isotopomers, containing 38 and 37 levels, respectively, are reported. The results are based on a highly accurate potential energy curve, which also includes relativistic effects. The calculated set of all vibrational levels of the (11)B(2) isotopomer is compared with the few results derived from experiment [Bredohl, H.; Dubois, I.; and Nzohabonayo, P. J. Mol. Spectrosc. 1982, 93, 281; Bredohl, H.; Dubois, I.; and Melen, F. J. Mol. Spectrosc. 1987, 121, 128]. Theory agrees with experiment within 4.5 cm(-1) on average for the four vibrational level spacings that are so far known empirically. In addition, the present theoretical analysis suggests, however, that the transitions from higher electronic states to the ground state vibrational levels v = 12-15 deserve to be reanalyzed. Whereas previous experimental investigators considered them to originate from the v' = 0 vibrational level of the upper state (2)(3)Σ(u)(-), the present results make it likely that these transitions originate from a different upper state, namely the v' = 16 or the v' = 17 vibrational level of the (1)(3)Σ(u)(-) state. The ground state dissociation energy D(0) is predicted to be 23164 cm(-1).

Tetrahydrides of third-row transition elements: spin-orbit coupling effects on the stability of rhenium tetrahydride.

The potential energy surfaces of low-lying states in rhenium tetrahydride (ReH(4)) were explored by using the multiconfiguration self-consistent field (MCSCF) method together with the SBKJC effective core potentials and the associated basis sets augmented by a set of f functions on rhenium atom and by a set of p functions on hydrogen atoms, followed by spin-orbit coupling (SOC) calculations to incorporate nonscalar relativistic effects. The most stable structure of ReH(4) was found to have a D(2d) symmetry and its ground state is (4)A(2). It is found that this is lower in energy than the dissociation limit, ReH(2)+H(2), after dynamic correlation effects are taken into account by using second-order multireference Møller-Plesset perturbation (MRMP2) calculations. This reasonably agrees with previous results reported by Andrews et al. [J. Phys. Chem. 107, 4081 (2003)]. The present investigation further revealed that the dissociation reaction of ReH(4) cannot occur without electronic transition from the lowest quartet state to the lowest sextet state. This spin-forbidden transition can easily occur because of large SOC effects among low-lying states in such heavy metal-containing compounds. The minimum-energy crossing (MEX) point between the lowest quartet and sextet states is proved to be energetically and geometrically close to the transition state for the dissociation reaction on the potential energy surface of the lowest spin-mixed state. The MEX point (C(2) symmetry) was estimated to be 9184 cm(-1) (26.3 kcal/mol) higher than the (4)A(2) state in D(2d) symmetry at the MRMP2 level of theory. After inclusion of SOC effects, an energy maximum on the lowest spin-mixed state appears near the MEX point and is recognized as the transition state for the dissociation reaction to ReH(2)+H(2). The energy barrier for the dissociation, evaluated to be MEX in the adiabatic picture, was calculated to be 5643 cm(-1) (16.1 kcal/mol) on the lowest spin-mixed state when SOC effects were estimated at the MCSCF level of theory.

Heats of formation of organic compounds by a simple calculation.

Heats (enthalpies) of formation of organic compounds are obtained by adding a constant for a functional group to the heat of formation of its hydrocarbon precursor obtained by counting hydrogens, as previously described. A single molecule is used to obtain the contribution of each functional group. Unlike all other empirical schemes, no global fitting is used. The calculation is done with pencil and paper or a hand calculator. The accuracy of the results is demonstrated by their agreement with more than 360 species of experimentally known heats of formation with a mean average deviation of 0.75 kcal mol(-1), very nearly equal to the mean experimental uncertainty. Alcohols, hydroperoxides, ethers, amines, ketones, aldehydes, carboxylic acids, esters, chlorides, phenols, anilines, aryl ethers, nitriles, amides, and alkyl radicals are used as examples of the method. This success allows the calculation of experimentally unknown heats of formation. The method is limited to compounds that are not subject to strain or effects of conjugation. In such cases, the experimental heat of formation of the hydrocarbon precursor may be used. The method can be extended to functional groups not treated here.

Shortcomings of basing radical stabilization energies on bond dissociation energies of alkyl groups to hydrogen.

Stabilization energies (SE(H)) of carbon radicals (R(*)) are traditionally defined as the difference between the bond dissociation energy (BDE) of CH(3)-H, as a reference point, and of R-H. The term "stabilization energy" implies that it is an intrinsic property of the radical and a quantitative measure of stability. Applicable only to carbon-centered radicals, SE(H) stabilization energies are not transferable and cannot be used to estimate carbon-carbon BDE[R-R'], symmetrical BDE[R-R], or any other BDE[R-X]. SE(H) values by themselves are neither an intrinsic property nor a quantitative measure of stability. There is available an alternative that is not limited only to carbon-carbon and carbon-hydrogen bonds, does not depend on any one particular molecule or BDE as a reference point, and is accurate with several hundred different types of bonds.

Accurate ab initio potential energy curve of O2. II. Core-valence correlations, relativistic contributions, and vibration-rotation spectrum.

In the first paper of this series, a very accurate ab initio potential energy curve of the (3)Sigma(g)(-) ground state of O(2) has been determined in the approximation that all valence shell electron correlations were calculated at the complete basis set limit. In the present study, the corrections arising from core electron correlations and relativity effects, viz., spin-orbit coupling and scalar relativity, are determined and added to the potential energy curve. From the 24 points calculated on this curve, an analytical expression in terms of even-tempered Gaussian functions is determined and, from it, the vibrational and rotational energy levels are calculated by means of the discrete variable representation. We find 42 vibrational levels. Experimental data (from the Schumann-Runge band system) only yield the lowest 36 levels due to significant reduction in the transition intensities of higher levels. For the 35 term values G(v), the mean absolute deviation between theoretical and experimental data is 12.8 cm(-1). The dissociation energy with respect to the lowest vibrational energy is calculated within 25 cm(-1) of the experimental value of 41,268.2+/-3 cm(-1). The theoretical crossing between the (3)Sigma(g)(-) state and the (1)Sigma(g)(+) state is found to occur at 2.22 A and the spin-orbit coupling in this region is analyzed.

Halogenated catechols from cycloaddition reactions of η-(2-ethoxyvinylketene)iron(0) complexes with 1-haloalkynes.

1-chloroalkynes and 1-bromohexyne undergo cycloaddition reactions with ethoxyvinylketeneiron(0) complexes to form chloro and bromocatechols. With most substituents, the halogen is incorporated ortho to the phenolic hydroxyl group regioselectively. With chloroethyne, chlorohexyne, and methyl chloropropiolate, the reverse regioselection is observed. Ab initio calculations reveal that the products are, in most cases, nearly isoenergetic, which indicates that the intermediate ketene-alkyne adduct geometry must be important in determining the product distribution.

Effects of molecule stabilization energies on radical reactions: G3 and G3(MP2) model chemistries applied to benzylic systems.

We have carried out G3 and G3(MP2) calculations of the molecule stabilization energies (MSEs) brought about by 11 common substituent groups in the meta and para positions of benzyl fluoride. We find that MSE is a function of the tendency of the substituent to donate or to withdraw electrons, such that a classic Hammett plot can be drawn. We propose that, in general, the direction of the benzylic Z-X dipole of YC6H4ZX is the major factor controlling the sign of the slope of Hammett plots of benzylic atom abstractions by radicals. When the Z-X dipole is pointed away from the substituted ring, electron withdrawing substituents destabilize the molecule, contributing to a decrease of the Z-X bond dissociation energy, and electron donating substituents stabilize it. The reverse is true when the dipole is reversed. This proposal is supported by 13C NMR results and by a survey of relevant benzylic and quasi-benzylic hydrogen or halogen atom abstractions studied experimentally. Calculations at the G3 level of theory demonstrate an increase in the bond dissociation energy of p-YC6H4CH2-H with increasing electron withdrawing ability of Y, contrary to results of previous lower level calculations. MSE values of substituted benzyl fluorides (p-YC6H4CH2F) correlate well with allylic MSE (trans-YCH=CHCH2F) and quantify the relative efficacy of transmission of electronic effects by vinylogy.

Enthalpies of formation of hydrocarbons by hydrogen atom counting. Theoretical implications.

Standard enthalpies of formation at 298 K of unstrained alkanes, alkenes, alkynes, and alkylbenzenes can be expressed as a simple sum in which each term consists of the number of hydrogen atoms n of one of eight different types (n1-n8) multiplied by an associated coefficient (c1-c8) derived from the known enthalpy of formation of a typical molecule. Alkylbenzenes require one additive constant for each benzene ring, accounting for a possible ninth term in the sum. Terms are not needed to account for repulsive or attractive 1,3 interactions, hyperconjugation, or for protobranching, rendering them irrelevant. Conjugated eneynes and diynes show thermodynamic stabilizations much smaller than that observed for 1,3-butadiene, bringing into question the usual explanation for the thermodynamic stabilization of conjugated multiple bonds (p orbital overlap, pi electron delocalization, etc.).

Remote substituent effects on allylic and benzylic bond dissociation energies. Effects on stabilization of parent molecules and radicals.

The effect of remote substituents on bond dissociation energies (BDE) is examined by investigating allylic C-F and C-H BDE, as influenced by Y substituents in trans-YCH=CHCH2-F and trans-YCH=CHCH2-H. Theoretical calculations at the full G3 level model chemistry are reported. The interplay of stabilization energies of the parent molecules (MSE) and of the radicals formed by homolytic bond cleavage (RSE) and their effect on BDE are established. MSE values of allyl fluorides yield an excellent linear free energy relationship with the electron-donating or -withdrawing ability of Y and decrease by 4.2 kcal mol-1 from Y = (CH3)2N to O2N. RSE values do not follow a consistent pattern and are of the order of 1-2 kcal mol-1. A decrease of 4.1 kcal mol-1 is found in BDE[C-F] from Y = CH3O to NC. BDE[YCH=CHCH2-H] generally increases with decreasing electron-donating ability of Y for electron-donating groups and does not follow a consistent pattern with electron-withdrawing groups, the largest change being an increase of 3.6 kcal mol-1 from Y = (CH3)2N to CF3. The G3 results are an indicator of benzylic BDE in p-YC6H4CH2-F and p-YC6H4CH2-H, via the principle of vinylogy, demonstrated by correlating MSE of the allylic compounds with physical properties of their benzylic analogues.

Effect of spacer attachment sites and pH-sensitive headgroup expansion on cationic lipid-mediated gene delivery of three novel myristoyl derivatives.

The transfection activity and physicochemical properties of the dimyristoyl derivatives from three novel series of double-chained tertiary cationic lipids were compared. Two of the derivatives were constructed as isomers with different linkages of the same bis-(2-dimethylaminoethane) polar headgroup and hydrophobic chains to the diaminopropanol backbone, while the third was designed with a hydrophilic region containing only a single ionizable amine group. Such systematic molecular changes offer a great opportunity to delineate factors critical for transfection activity, which in this work include the intramolecular distance between the hydrophobic chains and pH-expandability of the polar headgroup. The physical studies comprised a variety of techniques, including pKa determination, Langmuir monolayer studies, fluorescence anisotropy, gel electrophoresis mobility shift assay, ethidium bromide displacement assay, particle size distribution, and zeta potential. These studies are crucial in the development of lipid-based gene delivery systems with improved efficacy. Physicochemical characterization revealed that a symmetric bivalent pH-expandable polar headgroup in combination with greater intramolecular space between the hydrophobic chains provide for high transfection activity through efficient binding and compaction of pDNA, increased acyl chain fluidity, and high molecular elasticity.

Generation of oxynitrenes and confirmation of their triplet ground states.

New sulfoximine- and phenanthrene-based photochemical precursors to oxynitrenes have been developed. These precursors have been used to examine the chemistry and spectroscopy of oxynitrenes. The first EPR spectra of oxynitrenes are reported and are consistent with their triplet ground states. Additional support for the triplet ground state of oxynitrenes is provided by trapping and reactivity studies, nanosecond time-resolved IR investigations, and computational studies.

Stabilization energies of extensively conjugated propargylic radicals.

G3(MP2) and other model chemistry calculations indicate that stabilization energies of extensively conjugated allylic radicals H2(C=C)nCH2*, n = 1-4, increase monotonically as the number of repeating C=C units increase. In contrast, stabilization energies of the analogous propargylic radicals, H(C[triple bond]C)nCH2*, decrease beyond n = 2. Breaking up the number of contiguous conjugated C[triple bond]C units in conjugation with the odd electron enhances rather than diminishes stability. These results complement previous findings of significant differences in the stabilization of conjugated ground-state polyenes vs polyynes.

G3(MP2) enthalpies of hydrogenation, isomerization, and formation of extended linear polyacetylenes.

Motivated by our recent finding that, in contrast to their olefinic counterparts, linear alternant polyacetylenes (polyynes) show no appreciable thermodynamic evidence of conjugation stabilization, we have extended our G3(MP2) calculations of standard enthalpies of hydrogenation, delta(hyd), formation, delta(f), and isomerization, delta(isom), as far as isomeric dodecadiynes. We show that thermochemical stabilization of conjugated polyalkynes is about 1 kcal mol(-1) over most of this range, and that the progression from one polyalkyne to the next is regular and additive. The longest chain polyalkynes, however, begin to revert to classical conjugation stabilization energies. For example, 5,7-dodecadiyne has a thermochemical stabilization enthalpy of 3.1 kcal mol(-1), approaching that of 1,3-butadiene. We also point out some of the difficulties encountered when one departs from Kistiakowsky's operational definition of conjugation stabilization. A cautionary example is drawn from the recent literature in which arguments of hyperconjugation and "virtual states" are used to arrive at, among other things, a value of 8.5 kcal mol(-1) of conjugative stabilization in 1,3-butadiene.

On the lack of conjugation stabilization in polyynes (polyacetylenes).

By analogy to conjugated polyenes, conjugative stabilization of polyynes with the -CC-CC- group might be expected to be substantial. On the contrary, consistent with our recent report of a surprising lack of conjugative stabilization in butadiyne, we find by G3(MP2) calculations and by comparisons with available experimental data from these and other laboratories that the ground-state stabilization of conjugated polyynes is in fact quite small, amounting to <1 kcal mol(-)(1). By similar calculations, the 2,4-pentadiyn-1-yl radical shows no enhanced stabilization relative to 2-propyn-1-yl radical, despite the potential stabilization of the odd electron by two conjugated triple bonds and unlike the behavior of 2,4-pentadien-1-yl radical. The thermochemistry of straight-chain alkynes and polyynes is very self-consistent. Enthalpies of hydrogenation, leading to enthalpies of formation, are predictable with a high degree of accuracy (absolute mean deviation = +/-0.39 kcal mol(-)(1) vs theoretical values and +/-0.52 vs experimental) from three molecular structure enthalpies and one conjugation stabilization parameter.

Comparison of spectroscopic potentials and an a priori analytical function. The potential energy curve of the ground state of the sodium dimer, X1Sigmag(+) Na2.

The results of a "universal" potential energy function, one that incorporates electronegativity and Slater's effective nuclear charge into a Morse-type function, are compared to spectroscopically derived potential energy curves of the X1Sigmag(+) state of Na2. The function is a priori in that it does not require prior knowledge of the actual potential and has no adjustable parameters. Criteria used to evaluate the performance of the function are comparisons of predicted versus spectroscopic energies at Rydberg-Klein-Rees (RKR) procedure turning points, predicted distances at measured energies versus RKR distances, and eigenvalues derived from the a priori potential versus spectroscopically deduced energy levels. The a priori function describes the Na2 potential with deviations approaching the magnitude of those found among some spectroscopic potentials from different sources. By examining the behavior of the "spectroscopic" parameter of the Morse function, irregularities are found in five of the seven spectroscopic potentials examined. A new procedure is demonstrated for correcting irregularities on the inner branch of spectroscopic potentials at high extents of dissociation and for extending reliably the potential in this region beyond the domain of the measurements.

Symmetry and geometry considerations of atom transfer: deoxygenation of (silox)3WNO and R3PO (R = Me, Ph, (t)Bu) by (silox)3M (M = V, NbL (L = PMe3, 4-picoline), Ta; silox = (t)Bu3SiO).

Deoxygenations of (silox)(3)WNO (12) and R(3)PO (R = Me, Ph, (t)Bu) by M(silox)(3) (1-M; M = V, NbL (L = PMe(3), 4-picoline), Ta; silox = (t)Bu(3)SiO) reflect the consequences of electronic effects enforced by a limiting steric environment. 1-Ta rapidly deoxygenated R(3)PO (23 degrees C; R = Me (DeltaG degrees (rxn)(calcd) = -47 kcal/mol), Ph) but not (t)Bu(3)PO (85 degrees, >2 days), and cyclometalation competed with deoxygenation of 12 to (silox)(3)WN (11) and (silox)(3)TaO (3-Ta; DeltaG degrees (rxn)(calcd) = -100 kcal/mol). 1-V deoxygenated 12 slowly and formed stable adducts (silox)(3)V-OPR(3) (3-OPR(3)) with OPR(3). 1-Nb(4-picoline) (S = 0) and 1-NbPMe(3) (S = 1) deoxygenated R(3)PO (23 degrees C; R = Me (DeltaG degrees (rxn)(calcd from 1-Nb) = -47 kcal/mol), Ph) rapidly and 12 slowly (DeltaG degrees (rxn)(calcd) = -100 kcal/mol), and failed to deoxygenate (t)Bu(3)PO. Access to a triplet state is critical for substrate (EO) binding, and the S --> T barrier of approximately 17 kcal/mol (calcd) hinders deoxygenations by 1-Ta, while 1-V (S = 1) and 1-Nb (S --> T barrier approximately 2 kcal/mol) are competent. Once binding occurs, significant mixing with an (1)A(1) excited state derived from population of a sigma-orbital is needed to ensure a low-energy intersystem crossing of the (3)A(2) (reactant) and (1)A(1) (product) states. Correlation of a reactant sigma-orbital with a product sigma-orbital is required, and the greater the degree of bending in the (silox)(3)M-O-E angle, the more mixing energetically lowers the intersystem crossing point. The inability of substrates EO = 12 and (t)Bu(3)PO to attain a bent 90 degree angle M-O-E due to sterics explains their slow or negligible deoxygenations. Syntheses of relevant compounds and ramifications of the results are discussed. X-ray structural details are provided for 3-OPMe(3) (90 degree angle V-O-P = 157.61(9) degrees), 3-OP(t)Bu(3) ( 90 degree angle V-O-P = 180 degrees ), 1-NbPMe(3), and (silox)(3)ClWO (9).

The conjugation stabilization of 1,3-butadiyne is zero.

[reaction: see text] In contrast to 1,3-butadiene, the textbook example of "conjugation stabilization", G3(MP2) calculations yielding the enthalpy of hydrogenation Delta(hyd)H(298) of 1,3-butadiyne indicate that it is not stabilized by the conjugated configuration of its triple bonds. Differences between ethylenic and acetylenic pi bonds are examined in the light of CAS-MCSCF calculations on 1,3-butadiene and 1,3-butadiyne.