Showing papers by "Paul von Ragué Schleyer published in 1993"

Generalized anomeric effects and hyperconjugation in CH2(OH)2, CH2(SH)2, CH2(SeH)2, and CH2(TeH)2

Abstract: The origin of generalized anomeric effects (energy differences between gauche and anti structures) is methanediol and its sulfur, selenium, and tellurium derivatives has been examined in detail by ab initio calculations through the MP2 level and by NBO analyses of the Hartree-Fock wave functions. Gauche preferences of SH, SeH, and TeH substituents decrease but do not vanish. The X lone pairs as well as the CH, CX, and XH bonding electrons are slighly delocalized (X=O, S, Se, and Te). If the wave functions are expanded in terms of localized and orthogonal molecular orbitals, these delocalizations are represented by hyperconjugative orbital interactions. p X →σ * CX and σ CH →σ * XH hyperconjugation strongly favors gauche structures

CH+5: The never‐ending story or the final word?

Abstract: The closely related Cs(1), Cs(2), and C2v(3) structures of CH5+ have been reinvestigated with high level ab initio theory through the coupled cluster with single and double substitutions (CCSD), and CCSD with perturbatively included connected triple excitations [CCSD(T)] levels, employing a triple‐ζ plus double polarization functions basis set, with f‐functions on carbon as well as d‐functions on the hydrogens [TZ2P(f,d)]. Vibrational frequencies have been computed up to TZ2P+f CCSD; the inclusion of f‐functions on carbon is critical for the configuration interaction with single and double excitations (CISD) and coupled cluster methods using the triple‐ζ basis sets. The changes in geometries between the CISD and CCSD levels are very small, e.g., the C–H bond lengths vary by at most 0.005 A. Thus, the optimizations are essentially converged within theoretical limits. The differences in energies of 1, 2, and 3 decrease and essentially vanish at the most sophisticated levels when the zero point vibrational e...

Ab initio study of structures and stabilities of substituted lead compounds. Why is inorganic lead chemistry dominated by PbII but organolead chemistry by PbIV

Abstract: The influence of electronegative substituents on the structures and relative stabilities of lead(IV) versus lead(II) species has been assessed by ab initio pseudopentential computations for a series of halogenated lead hydrides and methyllead compounds R n PbX 4-n (R=H, CH 3 ; X=F, Cl; n=0-4) and R n Pb 2-n (n=0-2). The calculated energies of various model reactions reveal the drastic degree of destabilization of tetravalent lead compounds by electronegative substituents X (X=F, Cl).

Ab initio study of the electronic structures of lithium containing diatomic molecules and ions

Abstract: Ab inifio calculations are used to provide bond lengths, harmonic frequencies, and dissociation energies of low-lying electronic states for LiX, LiX+, and LiX- (with X = Li through F and Na through Cl). Most of these species represent hitherto experimentally unknown molecules or ions, which provides the focus of the work presented here. All of these species are stable to dissociation and the anions are stable to loss of an electron. Differences among the electronic structures of the valence isoelectronic LiX; and HX, LiX+, and HX+; and LiX- and HXspecies are analyzed. Optimized geometries, dissociation energies, ionization potentials, and electron affinities were calculated for the following ground states of the respective species: rZ+ for Li2( ‘1:) LiNa, LiBe+, LiBe-, LiMg+, LiMg-, LiF, LiAl, LiS-, and LiCl* *8+ for Liz (“2: ), Li, (*2:) LiBe, LiB+, LiF-, LiNa+, LiNa-, LiMg, LiAl+, and LiCl’; *IIr for LIB-, LiAl-; ‘Hi for LiO, LiF+, LiS, and LiCl+; 3H, for LiB, LiC+, and LiSi+; 32- for LiN, LiO+, LiSi-, Lip, and LiS , i-e 42- for LiC, LiN+, LiN-, LiSi, Lip+, and Lip-; and 52for LiC-.

The tert-butyl cation (C4H9+) potential energy surface

Abstract: The CH 49 * potential energy surface (PES) is investigated using level ab initio molecular orbital theory. All structures were fully optimized at Hartree-Fock (HF) and correlated levels (HF/6-31G * , MP2(full)/6-31G * , and MP2-(full)/6-31G ** ) followed by single point energy calculations at MP4sdtq/6-31G ** //MP2(full)/6-31G ** . The C s 1, C 3h 2, and C 3v 3 forms of the tert-butyl cation were investigated

Decisive electron correlation effects on computed boron-11 and carbon-13 NMR chemical shifts. Application of the GIAO-MP2 method to boranes and carbaboranes

Abstract: The performance of 11 B NMR chemical shift computations at a correlated (GIAO-MP2/TZP') level of ab initio theory is assessed for the first time. For molecules where SCF based methods (e.g. IGLO or GIAO-SCF) perform well, correlation effects on δ are negligible and IGLO, GIAO-SCF, and GIAO-MP2 results are equally satisfactory. But for problem cases e.g. 1,5-C 2 B 3 H 5 (1) and B 4 R 4 (3c, R=t-Bu), the large (10-35 ppm) errors obtained with uncorrelated wave functions are corrected at GIAO-MP2: excellent accord is achieved with the experimental values

Rotational barriers of 1,1'-binaphthyls: a computational study

Abstract: The activation energies for rotation about the σ-bonds of 1,1'-binaphthyl (1) and 2,2'-dibromo-1,1'-binaphthyl (2) have been computed with MNDO, AM1, and PM3 and of 2,2'-dilithio-1,1'-binaphthyl (3)-2EDA (ethylenediamine) (3a) with MNDO. All methods find that 1 showed racemize preferably through the anti path, in agreement with previous force field calculations. The PM3 rotational barrier for 1 (23.1 kcal/mol) matches the experimental value (22.5 kcal/mol) best; the'ground-state bond lengths correspond well with the X-ray data. We developed a procedure which evaluates the distortion and the steric repulsion effects in the transition structures roughly

Pentaaza- and pentaphosphacyclopentadienide anions and their lithium and sodium derivatives: structures and stabilities

Abstract: Both the pentazole, N 5 - (4), and pentaphosphole, P 5 - (5), anions favor planar D 5h geometries, in contrast to hexazine (N 6 ) and to hexaphosphabenzene (P 6 ), which have D 2 twist-boat structures. While the pentazole anion 4 is thermodynamically unstable relative to N 3 - +N 2 , a barrier of 19.4 kcal/mol at MP4SDTQ/6-31+G * //MP2(full)/6-31G * +ΔZPE(HF/6-31G * ) inhibits the dissociation

Green function calculation of ionization energies of hypermetallic molecules

Abstract: Vertical ionization and electron capture energies (electron affinities) are obtained for the hypermetallic molecules Al 2 O, Al 3 O, Al 4 O, Al 4 C, CAl 2 Si 2 , NAl 3 Si and BAlSi 3 and some related compounds from ab initio calculations with large basis sets. Many-body effects are taken into account by the Green function technique. The results agree with the photoelectron data on Al 2 O. Koopmans' approximation gives good agreement with the Green function results for the first few ionization energies. However, the one-particle ionization model breaks down for the lower valence levels. Electron correlation and relaxation contributes strongly to the electron capture energies.

The N4 molecule has an open-chain triplet C2h structure

Abstract: The lowest-energy N 4 is computed ab initio to be the planar C 2h ( 3 B u ) open-chain structure 13. The open-chain N 4 singlet-state structures dissociate on geometry optimization. The tetraazatetrahedrane T d structure 1 and the tetrazete D 2h structure 2 are minima at MP2/6-31G * . However, both are higher in energy than 13 (24.1 and 21.2 kcal/mol [UQCISD(T) (full)/6-311+G * //MP2/6-31G * + ZPE(MP2/6-31G * ), respectively

Ab initio models for multiple-hydrogen exchange: comparison of cyclic four- and six-center systems

Abstract: High-level ab initio calculations {QCISD(T)/6-311 +G**//MP2(fu)/6-31 +G**, with corrections for higher polarization [evaluated at MP2/6-311 +G(3df,2p)] and ΔZPE//MP2(fu)/6-31 +G**, i.e., comparable to Gaussian-2 theory} indicate concerted mechanisms for double- and triple-hydrogen exchange reactions in HF and HCl dimers and trimers, in mixed dimers and trimers containing one NH3, and in mixed dimers of HF, HCl, and NH3 with formic acid. All these reactions proceed via cyclic four- or six-center transition structures, the latter being generally more favorable. Calculated activation barriers (ΔHd at 0 K, kcal/mol) are 42.3 for (HF)2, 20.3 for (HF)3, 41.2 for (HCl)2, 25.6 for (HCl)3, 36.0 for NH3-HF, 10.6 for NH3(HF)2, 19.9 for NH3-HCl, 2.3 for NH3(HCl)2, 9.7 for HCO2H-HF, 7.0 for HCO2H-HCl, and 11.3, for HCO2H-NH3. The barriers are lower for the more ionic systems and when more ion pair character is present. © John Wiley & Sons, Inc.

Theoretical investigation of four-center two-electron bonding involving boron derivatives

Abstract: Factors contributing to the stability of 4c2e bonds have been investigated by means of ab initio calculations on boranes and carboranes. Electron density plots and «deformation energies», the energy required to deform the deprotonated species into a geometry optimal for interacting with the added proton, have been used to help evaluate the propensity of 4c2e vs 3c2e bonding. Two molecules exhibiting 4c2e bonding, CB 5 H 7 (1) and B 6 H 7 - (2), have been optimized at MP2/6-31G * with the «extra» hydrogen capping a triangular face or bridging two boron atoms. Face-capping is predicted to be more stable than BHB bridging by 16.8 kcal/mol (CB 5 H 7 ) and by 12.1 kcal/mol (B 6 H 7 - )

Molecular structure of 1-aza-closo-dodecaborane(12). Experimental and theoretical refinement

Abstract: The gas-phase electron diffraction data (GED) for 1-aza-closo-dodecaborane, 1-NB 11 H 12 , can be fit by four models with C 5υ symmetry almost equally well. The MP2/6-31G * single point calculations, as well as IGLO (individual gauge for localized orbitals) computed chemical shifts were able to decide among these possibilities. The final experimental geometry was selected on the basis of the agreement between the IGLO 11 B chemical shifts calculated for the various GED models and the experimental values. This optimal choice was supported by the energetic criteria

Experimental and theoretical investigations on the structure and reactivity of .alpha.-lithiomethoxyallene and its Grignard analog

Abstract: 13 C NMR, 6 Li, 1 H-HOESY-NMR, and IR spectroscopy of α-lithiomethoxyallene (7) and 1-lithio-1-ethoxy-3-tert-butylallene (8) as well as ab initio model calculations on monomeric and dimeric α-lithiohydroxyallene prove 7 and 8 to be dimeric in THF (7 forms a tetramer in diethyl ether) with a nonclassically 1,3-bridged structure. Alternative oxygen-coordinated structwes could be excluded definitely. Ab initio calculations indicate that the methoxyallene α-Grignard derivative either has a classic allenic structure or an O-coordinated allenic form (or is a mixture of both) in solution. An alternative propargylic-type isomer is unfavorable energetically

Even more reliable NMR chemical shift computations by the GIAO-MP2 method

Abstract: Correlated ab initio(GIAO-MP2) chemical shift computations agree excellently with the newly remeasured 11B and 13C NMR values for the problem carboranes, 1 and 2, which had given poor IGLO results.

Reinvestigation of the SNi reaction. The ionization of chlorosulfites

Abstract: The decomposition of allyl chlorosulfites (ROSOCI) has been investigated both computationally and experimentally. Semiempirical (AM1 and PM3) as well as ab initio (HF/3-21G( * ), HF/6-31G * , and MP2(full)/6-31G * //MP2(full)/6-31G * ) methods were employed, and the results were confirmed experimentally by NMR spectroscopy. The computations indicated that certain alkyl sulfinyl cations (ROSO + ) are stable and might be involved in the decomposition of chlorosulfites. Detection of these ions by 1 H and 13 C NMR spectroscopy in polar solvents such as acetone-d 6 and acetonitrile-d 3 as well as kinetic studies allowed important conclusions to be drawn about the mechanistic details of the S N i reaction

Structure and nonrigidity of undecahydrodecaborate(1-). An ab initio/IGLO/NMR study

Abstract: Ab initio calculations on B 10 H 11 - , i.e. protonated B 10 H 10 2- , at the MP2/6-31G * //HF/3-21G+ZPE(3-21G) level suggest that the «additional», polycoordinate proton (designated H * ) lies above a triangular face in the «polar region» near B1. The marked lengthening (to 2.00 A) of one of the basal B-B bonds due to coordination with the additional proton in B 10 H 11 - noteworthy

Polyatomic Molecules without Electron‐Pair Bonds: High‐Spin Trigonal, Tetrahedral, and Octahedral Lithium Clusters

Abstract: Bound polyatomic molecules without any spin-paired valence electrons are exemplified by lithium clusters in high-spin states. Three unpaired valence electrons are able to bind the (4A1) quartet states of Li3 and of Li4+ in trigonal (D3h) geometries. The tetrahedral Li4 and Li5+ (5A1) quintet state structures illustrate bound molecules with four unpaired valence electrons. Five, six, and seven spin-uncoupled valence electrons bind the square pyramidal Li5 (6B1) structure, the Li6 (7A2u) distorted octahedral structure, and the Li6−(8A1g) octahedral (Oh) octuplet, respectively. However, these high-multiplicity structures are only local minima; forms with lower spin states are more favorable energetically. Other high symmetry, high multiplicity clusters, e.g., triangular Be3+ (6A1′), also have positive atomization energies. The results demonstrate that chemical bonding in multiatom species does not require spin-coupling.

The peculiar coordination of barium: ab initio study of the molecular and electronic structures of the Group II dihydride dimers M2H4 (M = magnesium, calcium, strontium, barium)

Abstract: Ab initio pseudopotential calculations at the SCF and MP2 levels of theory on the dimeric alkaline-earth dihydrides M 2 H 4 (M=Mg, Ca, Sr, Ba) reveal interesting trends down group 2. In particular, barium prefers coordination environments which contradict simple ionic or VSEPR model expectations. The lowest-energy structure for Ba 2 H 4 , and for Sr 2 H 4 , is a triply hydride-bridged C 3v arrangement 4 with only one terminal hydrogen instead of the doubly bridged planar D 2h structure 1

Polysila analogs of aromatic hydrocarbon ions: Structures and energies of Si3H3+, Si4H 42+, and Si5H5−

Abstract: Silicon analogs of aromatic monocyclic ions, (SiH) 3 + (4), (SiH) 4 2+ (5), and (SiH) 5 - (6) have been studied ab initio at Mp2(full)/6-31G * . The D 3h structure of Si 3 H 3 + is the global minimum, whereas other two ions are nonplanar. The D 2d structure of (SiH) 4 2+ is less folded than the carbon analog and possesses a higher stabilization energy. Stabilization energies for the monocharged ions are diminished with respect to the corresponding carbons