Molecular structure, conformation, and potential to internal rotation of 2,6- and 3,5-difluoronitrobenzene studied by gas-phase electron diffraction and quantum chemical calculations

3,5-Difluoronitrobenzene (3,5-DFNB) and 2,6-difluoronitrobenzene (2,6-DFNB) have been studied by gas-phase electron diffraction (GED), MP2 ab initio, and by B3LYP density functional calculations. Refinements of r h1 and r e static and r h1 dynamic GED models were carried out for both molecules. Equilibrium r e structures were determined using anharmonic vibrational corrections to the internuclear distances ( r e - r a) calculated from B3LYP/cc-pVTZ cubic force fields. 3,5-DFNB possesses a planar structure of C 2 v symmetry with the following r e values for bond lengths and bond angles: r(C-C) av = 1.378(4) A, r(C-N) = 1.489(6) A, r(N-O) = 1.217(2) A, r(C-F) = 1.347(5) A, angleC6-C1-C2 = 122.6(6) degrees , angleC1-C2-C3 = 117.3(3) degrees , angleC2-C3-C4 = 123.0(3) degrees , angleC3-C4-C5 = 116.9(6) degrees , angleC-C-N = 118.7(3) degrees , angleC-N-O = 117.3(4) degrees , angleO-N-O = 125.5(7) degrees , angleC-C-F = 118.6(7) degrees . The uncertainties in parentheses are three times the standard deviations. As in the case of nitrobenzene, the barrier to internal rotation of the nitro group in 3,5-DFNB, V 90 = 10 +/- 4 kJ/mol, is substantially lower than that predicted by quantum chemical calculations. The presence of substituents in the ortho positions force the nitro group to rotate about the C-N bond, out of the plane of the benzene ring. For 2,6-DFNB, a nonplanar structure of C 2 symmetry with a torsional angle of phi(C-N) = 53.8(14) degrees and the following r e values for structural parameters was determined by the GED analysis: r(C-C) av = 1.383(5) A, r(C-N) = 1.469(7) A, r(N-O) = 1.212(2) A, r(C-F) = 1.344(4) A, angleC6-C1-C2 = 118.7(5) degrees , angleC1-C2-C3 = 121.2(2) degrees , angleC2-C3-C4 = 119.0(2) degrees , angleC3-C4-C5 = 121.1(4) degrees , angleC-C-N = 120.6(2) degrees , angleC-N-O = 115.7(4) degrees , angleO-N-O = 128.6(7) degrees , angleC-C-F = 118.7(5) degrees . The refinement of a dynamic model led to barriers V 0 = 16.5 +/- 1.5 kJ/mol and V 90 = 2.2 +/- 0.5 kJ/mol, which are in good agreement with values predicted by B3LYP/6-311++G(d,p) and MP2/ cc-pVTZ calculations. The values of C-F bond lengths are similar in both molecules. This is in contrast to the drastic shortening of the C-F bond in the ortho position in 2-fluoronitrobenzene compared to the C-F bond length in the meta and para position in 3- and 4-fluoronitrobenzene observed in an earlier GED study.     

Conformational stability of cyclobutanol from temperature dependent infrared spectra of xenon solutions, r0 structural parameters, ab initio calculations and vibrational assignment

Variable temperature (-55 to -100 degrees C) studies of the infrared spectra (4000-400 cm(-1)) of cyclobutanol, c-C4H7OH dissolved in liquid xenon have been carried out. The infrared spectrum (4000-100 cm(-1)) of the gas has also been recorded. From these data two of the four possible stable conformers have been confidently identified and their order of stabilities has been experimentally determined where the first indicator is for the position of attachment of the hydroxyl group on the bent cyclobutyl ring (Eq=equatorial or Ax=axial) and the second one (t=trans, g=gauche) is the relative position of the hydroxyl rotor, i.e. rotation around the ring C-O bond. The enthalpy difference between the most stable Eq-t conformer and the second most stable rotamer, Eq-g, has been determined to be 200+/-50 cm(-1) (2.39+/-0.60 kJ/mol). This experimentally determined order is consistent with the order of stability predicted by ab initio calculations Eq-t>Eq-g>Ax-g>Ax-t. Evidence was obtained for the third conformer Ax-g which is predicted by ab initio calculations to be less stable by more than 650cm(-1) than the Eq-t form. The percentage of each conformer at ambient temperature is estimated to be Eq-t (50%), Eq-g (47%) and Ax-g (3%). The conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios and vibrational frequencies have been obtained for all of the conformers from MP2(full)/6-31G(d) ab initio calculations. The optimized geometries and conformational stabilities have been obtained from ab initio calculations utilizing several different basis sets up to MP2(full)/aug-cc-pVTZ and from density functional theory calculations by the B3LYP method. By utilizing previously reported microwave rotational constants for the Eq-t conformer combined with ab initio MP2(full)/6-311+G(d,p) predicted structural values, adjusted r0 parameters have been obtained. The determined heavy atom structural parameters for the Eq-t conformer are: the distances C1-C4=1.547(5) angstroms, C4-C6=1.552(5)angstroms, C-O=1.416(5) angstroms and angles angleC6C4C1=86.6(5) degrees , angleC4C1C5=88.9(5) degrees and angleC6C5C1C4=22.8(5) degrees . The results are discussed and compared to the corresponding properties of some similar molecules.     

Molecular structures and compositions of trans-1,2-dichlorocyclohexane and trans-1,2-difluorocyclohexane in the gas phase: an electron-diffraction investigation

The structures and compositions of gaseous trans-1,2-dichloro- (DCCH) and trans-1,2-difluorocyclohexane (DFCH), each of which may exist with the halogen atoms in a diaxial (aa) or diequatorial (ee) conformation, have been investigated by electron diffraction. The analysis was aided by rotational constants from microwave spectroscopy for the ee form of DFCH and by ab initio and density functional theory molecular orbital calculations for all species. The skeletons of the molecules have similar parameter values, but for the Cl-C-C-Cl and F-C-C-F fragments there are significant differences between the corresponding C-C-X bond angles and the X-C-C-X torsion angles in the two systems. There are also significant differences between the values of these parameters in the aa and ee forms of the same system. The composition of DCCH at 100 degrees C was measured to be 60(4)% aa, and that of DFCH at 70 degrees C was 42(7)% aa; the uncertainties are estimated 2sigma. From the preferred B3LYP/aug-cc-pVTZ calculations, the predicted theoretical composition is 51.2% aa for DCCH and 40.8% aa for DFCH. (Calculations at the levels B3LYP/6-31G(d) and MP2/6-31G(d) give similar results for DCCH, but both predict more aa than ee for DFCH.) Values (r(g)/A and angle(alpha)/degree) for some of the more important parameters of the aa/ee forms of DCCH are = 1.525(4)/1.525(6), C-Cl = 1.806(2)/1.787(2), angleC2-C1-Cl = 107.3(3)/111.5(3), angleC1-C2-C3 = 113.9(5)/111.6(5), angleC2-C3-C4 = 111.3(12)/109.9(12), and Cl-C2-C3-Cl = 165.3(9)/-59.4(9); and for DFCH C-C = 1.525(6)/1.520(9), C-F = 1.398(2)/1.390(2), angleC2-C1-F = 106.5(6)/109.2(6), angleC1-C2-C3 = 111.4(9)/110.9(9), angleC2-C3-C4 = 113.1(10)/113.1(10), and F-C2-C3-F = 171.1(37)/-67.2(37). The structures and compositions are discussed.     

丁烯和丁烷自由基阳离子的分子结构和超精细结构的 理论研究

用密度函数理论B3LYP方法和6-31G(d,p),6-311G(d,p)及6-311+G(d,p)基组，分别对1-C4H^+~8,2-C4H^+~8和C4H^+~10进行了构型优化和频率分析计算，预言1-C4H^+~8具有非平面构型，与以往报道的从头算和密度函数理论计算结果不同。在各自由基阳离子的B3LYP构型上，进行了B3LYP、MP2及MRSDCI方法的超精细偶合常数计算，得到了比以往更好的结果，特别是MP2/B3LYP计算值是至今与实验值符合得最好的理论计算结果。     

Vibrational spectrum, conformational stability, structural parameters and ab initio calculations of dimethylaminodifluorophosphine

From analysis of the infrared and Raman spectra along with support from the ab initio predictions it is concluded that there is only one stable conformer of dimethylaminodifluorophosphine, (CH(3))(2)NPF(2), in the gaseous and liquid phases which has a planar PNC(2) moiety with C(s) symmetry. The adjusted r(0) structural parameters have been obtained by combining the MP2(full)/6-311+G(d) predicted values with the previous reported rotational constants for four isotopomers obtained from previously reported microwave studies. The difference in the two NC distances is 0.002A whereas, these two parameters were previously assumed to have the same values from the microwave and electron diffraction studies but a reported difference of 0.025A from the structural parameters of the crystal. The adjusted r(0) heavy atom distances and angles are: r(PF)=1.593(3); r(NP)=1.654(3); r(NC(i))=1.455(3); r(NC(o))=1.453(3) A; angleFPF=93.5(5); angleNPF=100.8(5); angleCNC=116.0(5); angleC(i)NP=124.1(5); angleC(o)NP=120.0 degrees . The planar bonding around the nitrogen atom is consistent with the previously reported structural information from the microwave study but differs from the slightly pyramidal bonding obtained in the electron diffraction investigation. To support the vibrational assignment MP2(full) ab initio calculations with the 6-31G(d) basis set were carried out to predict the fundamental vibrational frequencies, infrared intensities, Raman activities, depolarization values, infrared band contours, and centrifugal distortion constants. Vibrational assignments are given for (CH(3))(2)NPF(2) and (CD(3))(2)NPF(2) and comparisons are made with the predicted intensities, frequencies and centrifugal distortion constants. Frequencies of some of the lattice modes are reported from both the infrared and Raman spectra with suggested assignments based on the factor group symmetry of the crystal of D(2h)(16) (Pnma) with four molecules per primitive cell. These results are compared to the corresponding quantities of some similar molecules.     

Tautomeric properties and gas-phase structure of 3-chloro-2,4-pentanedione

The tautomeric properties of alpha-chlorinated acetylacetone, 3-chloro-2,4-pentanedione CH3C(O)-CHCl-C(O)CH3, have been investigated by gas electron diffraction (GED) and quantum chemical calculations (B3LYP and MP2 approximations with different basis sets up to cc-pVTZ). Analysis of the GED intensities resulted in the presence of 100(2)% enol tautomer at 269(8) K. The following skeletal geometric parameters (rh1 values) of the molecule, which possesses Cs symmetry, were derived: r(C=C) = 1.378(3) A, r(C-C) = 1.450(3) A, r(C=O) = 1.243(3) A, r(C-O) = 1.319(3) A, r(O-H) = 1.001(4) A, r(C-Cl) = 1.752(4) A, angleC-C=C = 121.3(1.0) degrees , angleC=C-O = 119.9(1.2) degrees , angleC-C=O = 119.1(1.2) degrees . Due to very small contributions of the keto tautomer in alpha-chlorinated acetylacetone and its parent species, the effect of alpha-chlorination on tautomeric properties cannot be derived from experimental data. Quantum chemical calculations (B3LYP/6-31G**, B3LYP/cc-pVTZ, and MP2/cc-pVTZ) predict that alpha-chlorination of acetylacetone has no pronounced effect on the tautomeric properties. On the other hand, similar calculations for 1-chloro-1,3-butanedion, ClC(O)-CH2-C(O)CH3, demonstrate that chlorination in one beta position destabilizes the enol tautomer. In both chlorinated species the enol form is strongly preferred.     

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.     

A new scheme for determining the intramolecular seven-membered ring N-H...O=C hydrogen-bonding energies of glycine and alanine peptides

In this paper a new scheme was proposed to calculate the intramolecular hydrogen-bonding energies in peptides and was applied to calculate the intramolecular seven-membered ring N-H...O=C hydrogen-bonding energies of the glycine and alanine peptides. The density-functional theory B3LYP6-31G(d) and B3LYP6-311G(d,p) methods and the second-order Moller-Plesset perturbation theory MP26-31G(d) method were used to calculate the optimal geometries and frequencies of glycine and alanine peptides and related structures. MP26-311++G(d,p), MP26-311++G(3df,2p), and MP2/aug-cc-pVTZ methods were then used to evaluate the single-point energies. It was found that the B3LYP6-31G(d), MP26-31G(d), and B3LYP6-311G(d,p) methods yield almost similar structural parameters for the conformers of the glycine and alanine dipeptides. MP2/aug-cc-pVTZ predicts that the intramolecular seven-membered ring N-H...O=C hydrogen-bonding strength has a value of 5.54 kcal/mol in glycine dipeptide and 5.73 and 5.19 kcal/mol in alanine dipeptides, while the steric repulsive interactions of the seven-membered ring conformers are 4.13 kcal/mol in glycine dipeptide and 6.62 and 3.71 kcal/mol in alanine dipeptides. It was also found that MP26-311++G(3df,2p) gives as accurate intramolecular N-H...O=C hydrogen-bonding energies and steric repulsive interactions as the much more costly MP2/aug-cc-pVTZ does.     

Synthesis and characterization of the first examples of perfluoroalkyl-substituted trialkyloxonium salts, [(CH3)2OCF3]+[Sb2F11]- and [(CH3)2OCF(CF3)2]+[Sb2F11]-

In the superacidic HF/SbF(5) system, methyl trifluoromethyl ether forms at -78 degrees C the new tertiary oxonium salt [(CH(3))(2)OCF(3)](+)[Sb(2)F(11)](-), which was characterized by Raman and multinuclear NMR spectroscopy and its crystal structure. The same oxonium salt was also obtained by methylation of CH(3)OCF(3) with CH(3)F and SbF(5) in HF solution at -30 to -10 degrees C. Replacement of one methyl group in the trimethyloxonium cation by the bulkier and more electronegative trifluoromethyl group increases the remaining O-CH(3) bond lengths by 0.037(1) A and the sum of the C-O-C bond angles by about 4.5 degrees. Methylation of CH(3)OCF(CF(3))(2) with CH(3)F in HF/SbF(5) solution at -30 degrees C produces [(CH(3))(2)OCF(CF(3))(2)](+)[Sb(2)F(11)](-). The observed structure and vibrational and NMR spectra were confirmed by theoretical studies at the B3LYP/6-311++G(2d,2p) and the MP2/6-311++G(2d,p) levels.     

New insights into an old reaction. High-resolution x-ray powder diffraction of Wiberg's aminoalane intermediate

In accordance with the procedure described by E. Wiberg, Me(3)Al-NH(3) was heated as a bulk material in inert atmosphere to give a colorless liquid which slowly loses methane. Close to the end of this elimination reaction, the melt crystallized to give a microcrystalline powder of (Me(2)AlNH(2))(x)(). The structure of this intermediate has been solved by the method of high-resolution X-ray powder diffraction. The compound crystallizes in the monoclinic space group C2/c with the cell parameters of a = 15.0047(6) A, b = 8.7500(2) A, c = 24.4702(8) A, and beta = 107.290(2) degrees, with eight trimers (Me(2)AlNH(2))(3) per unit cell. These trimers crystallize in a boat conformation in contrast to the known trimers of the same composition where a twist-boat conformation had been found by single crystal determination. Different conformers of (Me(2)AlNH(2))(3) have been investigated by theoretical methods (HF/6-31G(d), B3LYP/6-31G(d), B3LYP/6-311G(d,p), MP2(fc)/6-31G(d), and MP2(fc)/6-311G(d,p)). The twist-boat and the chair conformer correspond to minima at the potential energy surface, whereas the boat conformer corresponds to a first-order transition state (relative energies of 0.45-2.56 kJ/mol (boat) and 6.66-11.91 kJ/mol (chair)). Relaxed scans of the potential energy surface at the HF/6-31G(d) and B3LYP/6-31G(d) levels have shown that the boat conformer (C(s)() symmetry) connects two enantiomers of the twist-boat form (C(2) symmetry).     

tert-Butylphosphonic acid: from the bulk to the gas phase

The structure of tert-butylphosphonic acid in the solid, in solution, and in the gas phase was studied by single-crystal X-ray diffraction, (1)H and (31)P NMR spectroscopic studies in solution, solid-state (31)P NMR spectroscopy, and electrospray ionization mass spectrometry. In addition, density functional theory (DFT) calculations at the B3LYP/6-31G*, B3LYP/6-31+G*, and B3LYP/6-311+G* level of theory for a large number of H-bonded aggregates of the type (tBuPO(3)H(2))(n) (C(n), P(n); n=1-7) support the experimental work. Crystallization of tBuPO(3)H(2) from polar solvents such as CH(3)CN or THF gives the H-bonded one-dimensional polymer 2, whereas crystallization from the less polar solvent CDCl(3) favors the formation of the H-bonded cluster (tBuPO(3)H(2))(6).CDCl(3) (1). In CDCl(3) the hexamer (tBuPO(3)H(2))(6) (C(6)) is replaced by smaller aggregates down to the monomer with decreasing concentration. DFT calculations and natural bond orbital (NBO) analyses for the clusters C(1)-C(7) and the linear arrays P(1)-P(7) reveal the hexamer C(6) to be the energetically favored structure resulting from cooperative strengthening of the hydrogen bonds in the H-bonded framework. However, the average hydrogen bond strengths calculated for C(6) and P(2) do not differ significantly (42-43 kJ mol(-1)). The average distances r(O.O), r(Obond;H), r(Pdbond;O), and r(Pbond;OH) in C(1)-C(7) and P(1)-P(7) are closely related to the hydrogen bond strength. Electrospray ionization mass spectrometry shows the presence of different anionic species of the type [(tBuPO(3)H(2))(n)-H](-) (A(1)-A(7), n=1-7) depending on the instrumental conditions. DFT calculations at the B3LYP/6-31G* level of theory were carried out for A(1)-A(6). We suggest the dimer [(tBuPO(3)H(2))(2)-H](-) (A(2)) and the trimer [(tBuPO(3)H(2))(3)-H](-) (A(3)) are the energetically favored anionic structures. A hydrogen bond energy of approximately 83 kJ mol(-1) was calculated for A(2). Electrospray ionization mass spectrometry is not suitable to study the assembling process of neutral H-bonded tert-butylphosphonic acid since the removal of a proton from the neutral aggregates has a large influence on the hydrogen bond strength and the cluster structure.     

Tautomeric and conformational properties of acetoacetamide: electron diffraction and quantum chemical study

The tautomeric properties of acetoacetamide, CH3C(O)CH2C(O)NH2, have been investigated by gas electron diffraction (GED) and quantum chemical calculations (B3LYP and MP2 approximations with 6-31G(d,p) and 6-311++G(3df,pd) basis sets). GED results in a mixture of 63(7)% enol tautomer and 37(7)% diketo form at 74(5) degrees C. Only one enol form with the O-H bond adjacent to the methyl group (CH3C(OH)=CHC(O)NH2) and only one diketo conformer (with dihedral angles tau(O=C(CH3)-C-C) = 31.7(7.5) degrees and tau(O=C(NH2)-C(H2)-C(O)) = 130.9(4.5) degrees ) are present. The calculated tautomeric composition varies in a wide range depending on the quantum chemical method and basis set. Only the B3LYP method with small basis sets reproduces the experimental composition correctly.     

Conformational stability, r0 structural parameters, and vibrational assignments of mono-substituted cyclobutanes: fluorocyclobutane

Variable temperature (-55 to -100°C) studies of the infrared spectra (3500-400 cm(-1)) of fluorocyclobutane, c-C(4)H(7)F, dissolved in liquid xenon have been carried out as well as the infrared spectra of the gas. By utilizing eight pairs of conformers at 10 different temperatures, the enthalpy difference between the more stable equatorial conformer and the axial form has been determined to be 496±40 cm(-1) (5.93±0.48 kJ/mol). The percentage of the axial conformer present at ambient temperature is estimated to be 8±1%. The ab initio MP2(full) average predicted energy difference from a variety of basis sets is 732±47 cm(-1) (9.04±0.44 kJ/mol) and the average value of 602±20 cm(-1) from density functional theory predictions by the B3LYP method are significantly larger than the experimentally determined enthalpy value. By utilizing previously reported microwave rotational constants for the equatorial and axial conformers combined with ab initio MP2(full)/6-311+G(d,p) predicted structural values, adjusted r(0) parameters have been obtained. The determined heavy atom structural parameters for the equatorial [axial] conformer are: distances (?) C-F=1.383(3) [1.407(3)], C(α)-C(β)=1.543(3) [1.546(3)], C(β)-C(γ)=1.554(3) [1.554(3)] and angles (°) ∠C(α)C(β)C(γ)=85.0(5) [89.2(5)], ∠C(β)C(α)C(β)=89.3(5) [89.2(5)], ∠F-(C(β)C(α)C(β))=117.4(5) [109.2(5)] and a puckering angle of 37.4(5) [20.7(5)]. The conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios and vibrational frequencies have been obtained for both conformers from MP2(full)/6-31G(d) ab initio calculations and compared to experimental values where available. The results are discussed and compared to the corresponding properties of some other monosubstituted cyclobutanes with halogen and pseudo-halogen substituents.     

Molecular structure and conformational composition of 2-chloro-1-phenylethanone, ClH2C-C(=O)Ph, obtained using gas-phase electron-diffraction data and results from theoretical calculations

The structure and conformation of 2-chloro-1-phenylethanone, ClH(2)C-C(=O)Ph (phenacyl chloride), have been determined by gas-phase electron diffraction (GED), augmented by results from ab initio molecular orbital calculations, employing the second-order M?ller-Plesset (MP2) level of theory and the 6-311+G(d) basis set. The molecules may exist as a mixture of different conformers with the C-Cl bond either syn (torsion angle phi = 0 degrees ) or gauche to the carbonyl bond. At 179 degrees C, the majority of the molecules (90 +/- 11%) have the gauche conformation (phi = 112(3) degrees). Torsion is also possible about the C-Ph single bond. Both experimental and theoretical data indicated, however, that the phenyl ring is coplanar or nearly coplanar with the carbonyl group. The results for the principal distances (r(g)) and angles (angle(alpha)) for the gauche conformer from a combined GED/ab initio study (with estimated 2sigma uncertainties) are the following: r(C-C)(phenyl) = 1.394(2) (average value) A, r(C(phenyl)-C(carbonyl)) = 1.484(5) A, r(C(carbonyl)-C(alkyl)) = 1.513(5) A, r(C-Cl) = 1.790(5) A, r(C=O) = 1.218(6) A, r(C-H)(phenyl) = 1.087(9) (average value) A, r(C-H)(alkyl) = 1.090(9) A (average value), angle C(phenyl)-C=O = 119.5(9) degrees, angle C(phenyl)-C(carbonyl)-C(alkyl) = 119.2(10) degrees, angle C-C-Cl = 109.8(12) degrees, angle C(2)-C(1)-C(carbonyl) = 122.8(15) degrees, angle C-C(alkyl)-H = 111.2 degrees (ab initio value).     

Tautomeric and conformational properties of malonamide, NH2C(O)-CH2-C(O)NH2: electron diffraction and quantum chemical study

The geometric structure of malonamide, NH2C(O)-CH2-C(O)NH2, has been investigated by gas electron diffraction (GED) and quantum chemical calculations (B3LYP and MP2 approximations with 6-311++G(3df,pd) basis sets). Both GED and quantum chemistry result in the existence of a single diketo conformer in the gas phase. According to GED refinement this conformer possesses (sc,ac) conformation with one C=O bond in synclinal orientation (dihedral angle tau(O=C-C-C)=49.0(3.0) degrees) and the other C=O bond in anticlinal orientation (dihedral angle tau(O=C-C-C)=139.5(3.3) degrees). The experimental geometric parameters are reproduced very closely by the B3LYP method.     

NMR spectra, GIAO and charge density calculations of five-membered aromatic heterocycles

The B3LYP/6-31+G(d) molecular geometry optimized structures of 17 five-membered heterocycles were employed together with the gauge including atomic orbitals (GIAO) density functional theory (DFT) method at the B3LYP/6-31+G(d,p), B3LYP/6-311++G(d,p) and B3LYP/6-311+G(2d,p) levels of theory for the calculation of proton and carbon chemicals shifts and coupling constants. The method of geometry optimization for pyrrole (1), N-methylpyrrole (2) and thiophene (7) using the larger 6-311++G(d,p) basis sets at the B3LYP/6-31+G(d,p), B3LYP/6-311++G(d,p), B3LYP/6-31+G(2d,p) and B3LYP/cc-pVTZ levels of theory gave little difference between calculated and experimental values of coupling constants. In general, the (1)H and 13C chemical shifts for all compounds are in good agreement with theoretical calculations using the smaller 6-31 basis set. The values of nJHH(n=3, 4, 5) and rmnJ(CH)(n=1, 2, 3, 4) were predicted well using the larger 6-31+G(d,p) and 6-311++G(d,p) basis sets and at the B3LYP/6-31+G(d,p), B3LYP/6-311++G(d,p), B3LYP/6-31+G(2d,2p) levels of theory. The computed atomic charges [Mülliken; Natural Bond Orbital Analysis (NBO); Merz-Kollman (MK); CHELP and CHELPG] for the B3LYP/6-311++G(d,p) geometry optimized structures of 1-17 were used to explore correlations with the experimental proton and carbon chemical shifts.     

Conformational stability, structural parameters and vibrational assignment from variable temperature infrared spectra of krypton solutions and ab initio calculations of ethylisothiocyanate

Variable temperature (-105 to -150 degrees C) studies of the infrared spectra (3500-400 cm(-1)) of ethylisothiocyanate, CH(3)CH(2)NCS, dissolved in liquid krypton have been recorded. Additionally the infrared spectra of the gas and solid have been re-investigated. These spectroscopic data indicate a single conformer in all physical states with a large number of molecules in the gas phase at ambient temperature in excited states of the CN torsional mode which has a very low barrier to conformational interchange. To aid in the analyses of the vibrational and rotational spectra, ab initio calculations have been carried out by the perturbation method to the second order (MP2) with valence and core electron correlation using a variety of basis sets up to 6-311+G(2df,2pd). With the smaller basis sets up to 6-311+G(d,p) and cc-PVDZ, the cis conformer is indicated as a transition state with all larger basis sets the cis conformer is the only stable form. The predicted energy difference from these calculations between the cis form and the higher energy trans conformer is about 125 cm(-1) which represents essentially the barrier to internal rotation of the NCS group (rotation around NC axis). Density functional theory calculation by the B3LYP method with the same basis sets predicts this barrier to be about 25 cm(-1). By utilizing the previously reported microwave rotational constants with the structural parameters predicted by the ab initio MP2(full)/6-311+G(d,p) calculations, adjusted r(0) structural parameters have been obtained for the cis form. The determined heavy atom parameters are: r(NC)=1.196(5), r(CS)=1.579(5), r(CN)=1.439(5), r(CC)=1.519(5)A for the distances and angles of angleCCN=112.1(5), angleCNC=146.2(5), angleNCS=174.0(5) degrees . The centrifugal distortion constants, dipole moments, conformational stability, vibrational frequencies, infrared intensities and Raman activities have been predicted from ab initio calculations and compared to experimental quantities when available. These results are compared to the corresponding quantities of some similar molecules.     

Marked counteranion effects on single-site olefin polymerization processes. Correlations of ion pair structure and dynamics with polymerization activity, chain transfer, and syndioselectivity

Counteranion effects on the rate and stereochemistry of syndiotactic propylene enchainment by the archetypal C(s)-symmetric precatalyst [Me(2)C(Cp)(Flu)]ZrMe(2) (1; Cp = C(5)H(4); Flu = C(13)H(8), fluorenyl) are probed using the cocatalysts MAO (2), B(C(6)F(5))(3) (3)(,) B(2-C(6)F(5)C(6)F(4))(3) (4)(,) Ph(3)C(+)B(C(6)F(5))(4)(-) (5), and Ph(3)C(+)FAl(2-C(6)F(5)C(6)F(4))(3)(-) (6), offering greatly different structural and ion pairing characteristics. Reaction of 1 with 3 affords [Me(2)C(Cp)(Flu)]ZrMe(+) MeB(C(6)F(5))(3)(-) (7). In the case of 4, this reaction leads to formation the micro-methyl dinuclear diastereomers [([Me(2)C(Cp)(Flu)]ZrMe)(2)(micro-Me)](+) MeB(2-C(6)F(5)C(6)F(4))(3)(-) (8). A similar reaction with 6 results in diastereomeric [Me(2)C(Cp)(Flu)]ZrMe(+) FAl(2-C(6)F(5)C(6)F(4))(3)(-) (10) ion pairs. The molecular structures of 7 and 10 have been determined by single-crystal X-ray diffraction. Reorganization pathways available to these species have been examined using EXSY and dynamic NMR, revealing that the cation-MeB(C(6)F(5))(3)(-) interaction is considerably weaker/more mobile than in the FAl(2-C(6)F(5)C(6)F(4))(3)(-)-derived analogue. Polymerizations mediated by 1 in toluene over the temperature range of -10 degrees to +60 degrees C and at 1.0-5.0 atm propylene pressure (at 60 degrees C) reveal that activity, product syndiotacticity, m and mm stereodefect generation, and chain transfer processes are highly sensitive to the nature of the ion pairing. Thus, the complexes activated with 4 and 5, having the weakest ion pairing, yield the highest estimated propagation rates, while with 6, having the strongest pairing, yields the lowest. The strongly coordinating, immobile FAl(2-C(6)F(5)C(6)F(4))(3)(-) anion produces the highest/least temperature-dependent product syndiotacticity, lowest/least temperature-dependent m stereodefect abundance, and highest product molecular weight. These polypropylene microstructural parameters, and also M(w), are least sensitive to increased propylene pressure for FAl(2-C(6)F(5)C(6)F(4))(3)(-), but highest with MeB(C(6)F(5))(3)(-). In general, mm stereodefect production is only modestly anion-sensitive; [propylene] dependence studies reveal enantiofacial propylene misinsertion to be the prevailing mm-generating process in all systems at 60 degrees C, being most dominant with 6, where mm stereodefect abundance is lowest. For 1,3-dichlorobenzene as the polymerization solvent, product syndiotacticity, as well as m and mm stereodefects, become indistinguishable for all cocatalysts. These observations are consistent with a scenario in which ion pairing modulates the rates of stereodefect generating processes relative to monomer enchainment, hence net enchainment syndioselectivity, and also dictates the rate of termination relative to propagation and the preferred termination pathway. In comparison to 3-6, propylene polymerization mediated by MAO (2) + 1 in toluene reveals an estimated ordering in site epimerization rates as 5 > 4 > 2 > 3 > 6, while product syndiotacticities rank as 6 > 2 > 5 approximately 4 > 3.     

Ab initio and density functional calculations on the pericyclic vs pseudopericyclic mode of conjugated nitrile ylide 1, 5-electrocyclizations

Ab initio (MP2/6-311+G and MP4(SDTQ)/6-311+G//MP2/6-311+G) and density functional (B3LYP/6-311+G) calculations on the ring closure reactions of conjugated nitrile ylides 1a-e, 3, and 6 to the corresponding oxazoles 2a, 5, 7, and 8; thiazoles 2b and 4; imidazole 2c; and pyrroles 2d and 2e, respectively, are reported. Vinyl nitrile ylides 1d and 1e cyclize with a substantially higher barrier than nitrile ylides containing a heteroatom. Geometric features as well as electronic structures as obtained by NBO analysis are indicative of a pericyclic, monorotatory 1, 5-electrocyclization of 1d and 1e. For nitrile ylides where X = heteroatom, a pseudopericyclic heteroelectrocyclization pathway, characterized by in-plane attack of the heteroatom's lone pair at the nitrile ylide group, is found. For 3 and 6, where two different cyclization products are possible, the calculated barriers and reaction energies are in line with the experimentally observed direction of reaction. Vinyl nitrile ylides 1d and 1e are characterized by an allene, acyl substituted derivatives 1a, 1b, 3, and 6 by a propargyl type structure. The nitrogen derivative 1c represents an intermediate case.     

Study of the thymine molecule: equilibrium structure from joint analysis of gas-phase electron diffraction and microwave data and assignment of vibrational spectra using results of ab initio calculations

Thymine is one of the nucleobases which forms the nucleic acid (NA) base pair with adenine in DNA. The study of molecular structure and dynamics of nucleobases can help to understand and explain some processes in biological systems and therefore it is of interest. Because the scattered intensities on the C, N, and O atoms as well as some bond lengths in thymine are close to each other the structural problem cannot been solved by the gas phase electron diffraction (GED) method alone. Therefore the rotational constants from microvawe (MW) studies and differences in the groups of N-C, C=O, N-H, and C-H bond lengths from MP2 (full)/cc-pVQZ calculations were used as supplementary data. The analysis of GED data was based on the C(s) molecular symmetry according to results of the structure optimizations at the MP2 (full) level using 6-311G (d,p), cc-pVTZ, and cc-pVQZ basis sets confirmed by vibrational frequency calculations with 6-311G (d,p) and cc-pVTZ basis sets. Mean-square amplitudes as well as harmonic and anharmonic vibrational corrections to the internuclear distances (r(e)-r(a)) and to the rotational constants (B(e)(k)-B(0)(k), where k = A, B, C) were calculated from the quadratic (MP2 (full)/cc-pVTZ) and cubic (MP2 (full)/6-311G (d,p)) force constants (the latter were used only for anharmonic corrections). The harmonic force field was scaled using published IR and Raman spectra of the parent and N1,N3-dideuterated species, which were for the first time completely assigned in the present work. The main equilibrium structural parameters of the thymine molecule determined from GED data supplemented by MW rotational constants and results of MP2 calculations are the following (bond lengths in Angstroms and bond angles in degrees with 3sigma in parentheses): r(e) (C5=C6) = 1.344 (16), r(e) (C5-C9) = 1.487 (8), r(e) (N1-C6) = 1.372 (3), r(e) (N1-C2) = 1.377 (3), r(e) (C2-N3) = 1.378 (3), r(e) (N3-C4) = 1.395 (3), r(e) (C2=O7) = 1.210 (1), r(e) (C4=O8) = 1.215 (1), angle e (N1-C6=C5) = 123.1 (5), angle e (C2-N1-C6) = 123.7 (5), angle e (N1-C2-N3) = 112.8 (5), angle e (C2-N3-C4) = 128.0 (5), angle e (N3-C4-C5) = 114.8 (5), angle e (C6=C5-C9) = 124.4 (9). The experimental structural parameters are in good agreement with those from MP2 (full) calculations with use of cc-pVTZ and cc-pVQZ basis sets.