Molecular structure and barriers to internal rotation of α-naphthalenesulfonyl chloride: a study by gas-phase electron diffraction and quantum chemical calculations

α-Naphthalenesulfonyl chloride, α-NaphSC, was studied by gas-phase electron diffraction (GED) and quantum chemical calculations (HF/6-311 + G**, HF/aug-cc-pVDZ, B3LYP/cc-pVDZ, B3LYP/cc-pVTZ, B3LYP/aug-cc-pVDZ, B3LYP/aug-cc-pVTZ, MP2/cc-pVDZ, and MP2/cc-pVTZ). The calculations predict the existence of two conformers with C ₁ (I) and C _s (II) symmetries. The most stable conformer I has an enantiomer. The experimental data of α-NaphSC obtained at 370(5) K could be best fitted by a C ₁ symmetry model indicating that only this form exists in the gas-phase. In this model the C_α–S–Cl plane deviates from the perpendicular orientation relative to the plane of the naphthalene skeleton. Under the applied experimental conditions, the mole fraction of a second less stable conformer II of α-NaphSC predicted by calculations is no more than 1 %. The following geometrical parameters of conformer I were obtained from the experiment (Å and °; uncertainties are in parentheses): r _h1(C–H) = 1.082(6), r _h1(C–C)_cp = 1.407(3), r _h1(C–S) = 1.764(5), r _h1(S–O)_av = 1.425(3), r _h1(S–Cl) = 2.051(5), ∠C–C_α–C = 122.5(1), ∠C_α–S–Cl = 101.5(10); C9–C1–S–Cl = 71.4(21). The calculated barriers to internal rotation of the sulfonyl chloride group exceed considerably the thermal energy values corresponding to the temperatures of the GED experiments. Natural bond orbitals analysis of the electron density distribution was carried out to explain the peculiarities of the molecular structure of the studied compound and the deviation from the structures of β-NaphSHal molecules and their benzene analogs.     

Dependency of the delocalized charge density and of the structural parameters on the pseudorotational parameter phi in 1,1-dicyanocyclopentane

Encouraged by the results we recently obtained from the exploration of the dependency of the structural parameters of 1,1-dichlorocyclopentane (J. Chem. Phys. A 2004, 108, 4658) on the pseudorotational parameter phi, we decided to reinvestigate the structure and the potential function governing the conformational equilibrium of 1,1-dicyanocyclopentane (DCCP) in the light of these novel results. The improved potential function we developed describes more adequately the dependency of the geometrical parameters on the pseudorotational phase angle phi. In the present work, we also incorporated additional terms into the equations we developed earlier (J. Chem. Phys. A 2004, 108, 4658; J. Mol. Struct. 2002, 612, 181) for describing the dependency of the distribution of the delocalized net charges throughout the ring on phi to account for the observed systematic deviations between the computed atomic distances and those provided by these equations. Although the overall fit of the electron diffraction was not significantly different from that which we presented previously, however, applying these extended equations has led to a better fit by refining a smaller number of parameters.     

The molecular structure of fluoromalononitrile as determined by gas-phase electron diffraction and ab initio calculations

The molecular structure of fluoromalononitrile was studied by means of gas-phase electron diffraction and quantum mechanical methods using HF/6-31G(d), MP2/6-311++G(2df,2pd) and DFT/B3LYP/6-31G(d), B3PW91/6-31G(d), B3LYP/6-311++G(2df,2pd) and B3PW91/6-311++G(2df,2pd). The r(g) and angle(alpha) structural parameters we obtained from the present analysis are: CC=1.487(5) A, CN=1.157(3) A, CF=1.386(5) A, CH=1.096 A (ass.), angleCCC=106.7(1.0) degrees , angleCCF=108.0(0.7) degrees , angleCCN=177.6(2.0) degrees . Uncertainties in parenthesis are 3sigma.     

The gas-phase molecular structure of 1,1-diethynylsilacyclobutane as determined by means of electron diffraction and ab initio calculations

As a continuation of our systematic investigation of the effect of substituents on the ring geometry and dynamics in silacyclobutanes and in order to explore the role of the silicon atom as a mediator for electronic interactions between the attached fragments, we studied the molecular structure of 1,1-diethynylsilacyclobutane (DESCB) by means of gas-phase electron diffraction and ab initio calculations. The structural refinement of the electron diffraction data yielded the following bond lengths (r_a) and bond angles (uncertainties are 3σ): r(Si–C)=1.874(2) Å, r(Si–C)=1.817(1) Å, (C–Si–C)=79.2(6)°, (C–Si–C)=106.5(6)°. The geminal Si–CC moieties were found to be bent outwards by 3.1(15)° and the puckering angle was determined to be 30.0(15)°. The evidently short Si–C bond length, which was also reproduced by the ab initio calculations, could be rationalized as being the consequence of the electronic interaction between the outer π charges of the triple bond and the 3pπ orbitals at the silicon atom. It is also likely that the conjugation of the geminal ethynyl groups leads to an enhancement of this bond contraction. Electrostatic interactions and the subsequent reduction of the covalent radius of the silicon atom may also contribute to this bond shortening. It has been found that the endocyclic Si–C bond length fits nicely within a scheme describing a monotonous decrease of the Si–C bond length with the increase of the electronegativity of the substituent in various geminally substituted silacyclobutanes.A series of related silacyclobutanes and acyclic diethynylsilanes have been studied by applying various ab initio methods and their optimized structures were compared to the structure of DESCB. Among these compounds are 1,1-dicyanosilacyclobutane (DCYSCB), which is isoelectronic to DESCB, 1,1-diethynylcyclobutane (DECB) which is isovalent to DESCB, monoethynylsilacyclobutane (MESCB) and monocyanosilacyclobutane (MCYSCB). Searching for reasonable support for the explanation of the structural results of DESCB we performed detailed natural population analysis as well as Mulliken population analysis (MPA) on DESCB and other related molecules. In contrast to the Mulliken charges, the natural atomic charges provided helpful information concerning the bonding properties in DESCB and the corresponding compounds. By varying the size of some basis sets, we could demonstrate the validity of the repeatedly discussed dependency of the Mulliken MPA on the basis set.For the performance of the quantum mechanical calculations we employed the following methods and basis sets: HF/6-31G(d,p), DFT/B3PW91/6-31G(d), DFT/B3PW91/6-311++G(d,p), MP2/6-31G(d,p) and MP2/6-311++G(d,p).     

Theoretical analysis of bonding properties in 1,1-dicyanocyclopentane and 1,1-dichlorocyclopentane by applying the AIM and NBO approaches

Using Bader’s quantum-topological theory of atoms in molecules (AIM) and Weinhold’s Natural Bond Orbital (NBO) analysis we could rationalize the impact of the geminal substitution by C≡N and Cl on the geometry and electronic structure of the cyclopentane ring in 1,1-dicyanocyclopentane (DCCP) and 1,1-dichlorocyclopentane (DClCP). Among the crucial results we obtained are: 1. The topological quantities, particularly the bond ellipticity of 0.035 for the C–CN bond, indicate that this bond possesses higher bond order than a single bond. This conclusion is clearly supported by the NBO results. 2. The AIM theory as well as the NBO analysis confirm uniformly the non-linearity of the C–C≡N moiety. 3. Regardless the quantum mechanical method that has been employed for a variety of nitriles the sign of the Laplacian of the charge density, ∇²ρ(r), of the C≡N group changes its sign from negative to positive upon moving from the triple zeta to the double zeta basis set. A possible explanation for this striking behavior has been provided. By invoking the AIM and NBO approaches the different endocyclic C–C bond lengths in DCCP and DClCP as a consequence of the geminal substitution could be explained. Also the variations of these bond lengths upon moving from the more stable C _s to the energetically less favorable C ₂ conformer of both compounds could be rationalized. For the purpose of comparison and verification of some findings of this work, we also carried out AIM and NBO calculations on various related cyclic and non-cyclic compounds.     

The surface structure of flame-annealed Au(100) in aqueous solution: An STM study

The initial structure of flame-annealed Au(100) surfaces has been studied in air and in 0.1 M H₂SO₄ by scanning tunnelling microscopy (STM). It is shown that before, during and after contact with the electrolyte, at potentials sufficiently negative to prevent specific adsorption of anions, the flame-annealed Au(100) surface is reconstructed into exactly the same “hex” form as a surface which has been prepared by annealing in ultrahigh vacuum (UHV). However, the quality of the reconstructed surface depends sensitively on the sample preparation and on the experimental conditions of the flame-annealing procedure. The influence of the cooling procedure after flame annealing on the initial surface structure of the Au(100) electrode is demonstrated and briefly discussed in the light of results published previously.     

Gas-Phase Molecular Structure of 2-Halo-1,3-dihetero-2-phospholanes

The molecular structure of 2-chloro-1,3-dithia-2-phospholane was determined using gas-phase electron diffraction and ab initio calculations. The heteroring in the molecule has an asymmetric structure like a symmetric P-envelope twisted about the C-C bond with an axial P-Cl bond. Geometric parameters of the molecule and mean vibration amplitudes were determined. The molecular structure of 2-chloro-1,3-oxathia-2-phospholane was predicted. The molecule in the gas phase has two conformers [twisted C(O)- and C(S)-envelope] with an axial P-Cl bond.