Molecular structures of tris(dipivaloylmethanato) complexes of the lanthanide metals, Ln(dpm)3, studied by gas electron diffraction and density functional theory calculations: a comparison of the Ln-O Bond distances and enthalpies in Ln(dpm)3 complexes and the cubic sesquioxides, Ln2O3

The molecular structures of tris(dipivaloylmethanato)neodymium(III), Nd(dpm)3, and tris(dipivaloylmethanato)ytterbium(III), Yb(dpm)3, have been determined by gas electron diffraction (GED) and structure optimizations through density functional theory (DFT) calculations. Both molecules were found to have D3 molecular symmetry. The most important structure parameters (r(a) structure) are as follows (GED/DFT): Nd-O = 2.322(5)/2.383 A, Yb-O = 2.208(5)/2.243 A, O-Nb-O = 72.1(3)/71.3 degrees , and O-Yb-O = 75.3(2)/75.8 degrees . The twist angles of the LnO6 coordination polyhedron, defined as zero for prismatic and 30 degrees for antiprismatic coordination, were theta = 19.1(3)/14.2 degrees for Nd and 20.4(2)/19.2 degrees for Yb. Structure optimizations of La(dpm)3, Gd(dpm)3 Er(dpm)3, and Lu(dpm)3 by DFT also yielded equilibrium structures of D3 symmetry with bond distances of La-O = 2.438 A, Gd-O = 2.322 A, Er-O = 2.267 A, and Lu-O = 2.232 A. The Ln-O bond distances in 12 Ln(dpm)3 complexes studied by GED decrease in a nearly linear manner with the increasing atomic number (Z) of the metal atom, as do the Ln-O bond distances in the cubic modifications of 14 sesquioxides, Ln2O3. The bond distances in the dpm complexes are, however, about 2% shorter. The mean Ln-O bond rupture enthalpies of the cubic sesquioxides calculated from thermodynamic data in the literature vary in an irregular manner with the atomic number; the La-O, Gd-O, Tb-O, and Lu-O bonds are nearly equally strong, and the remaining bonds are significantly weaker. The Ln-O bond rupture enthalpies previously reported for 11 Ln(dpm)3 complexes are on the average 13 kJ mol(-1) or about 5% smaller than in the sesquioxides, but they vary in a similar manner along the series: it is suggested that the pattern reflects variations in the absolute enthalpies of the gaseous Ln atoms.     

Vibrational spectra and rotational isomerism of bis(N-2-chloroethyl)nitramine

The IR and Raman spectra of bis(N-2-chloroethyl)nitramine (BCENA) in the liquid and crystalline states and in CCl₄ and CH₃CN solutions are studied. The spectra are compared, and it is concluded that BCENA exists as a mixture of conformers of different polarities in the liquid state and as one less polar conformer in the crystalline state. To determine the conformations corresponding to the total electron energy minima and interpret the vibrational spectrum of BCENA, we performed an ab initio quantum chemical calculation of the BCENA molecule in the Hartree-Fock approximation using the 3–21G* and 6–31G* basis sets. Out of twelve possible conformations five are stable; the most stable conformer is C₂(GG). The frequencies and forms of normal vibrations of stable conformers are calculated using scaled quantum chemical force fields. The calculated and experimental frequencies are compared, and the relations between the frequencies of skeletal stretching and bending vibrations are analyzed. It is concluded that the BCENA crystal is formed by the C₂ (GG) conformer. The vibrational spectrum is interpreted, and the frequencies are assigned to vibrations of conformers differing in form. Translated fromZhumal Struktumoi Khimii, Vol. 38, No. 2, pp. 303–317, March–April, 1997.     

Molecular Structure of Diphenylamine by Gas-Phase Electron Diffraction and Quantum Chemistry

Geometric parameters of the diphenylamine molecule were determined by gas-phase electron diffraction and quantum-chemical calculations. By gas-phase electron diffraction, the molecule has an asymmetric structure with torsion angles about N-C bonds of ?45.6(23)° and 173.4(46)°, which agrees with RHF/6-31G** calculations. Density functional theory (DFT) calculations at the B3LYP/6-31G** level of theory lead to a C ₂ molecular conformation in the ground electronic state. The principal experimental geometric parameters are as follows: bond lengths: C-N 1.417(1), C-C_av 1.403(1) Å; and bond angles: CNC 123.9(5)°, and NCC 121.5° (assumed) and 116.4°.     

The structure of 1-thia-closo-decaborane(9), 1-SB(9)H(9), as determined by microwave spectroscopy and quantum chemical calculations

The microwave spectrum of 1-thia-closo-decaborane(9), 1-SB(9)H(9), has been investigated in the 12-61 GHz spectral region. The molecule has C(4v) symmetry. The spectra of five isotopomers have been assigned, and a precise substitution structure of the non-hydrogen atoms has been determined. It was found that the axial sulfur atom causes a substantial expansion of the B(4) belt adjacent to sulfur and hence leads to a significant distortion from a regular bicapped square antiprismatic structure. The experimental work has been supplemented by high-level ab initio (MP2/6-311G**) and density functional theory calculations (B3LYP/6-311G** and B3LYP/cc-pVTZ). The agreement between the substitution structure and the two DFT calculations is very good in each case. The agreement is considerably poorer for the MP2/6-311G** calculations, particularly for the sulfur-boron bond length.     

An electron diffraction determination of the molecular structures of phosphoryl bromide and thiophosphoryl bromide

Gas-phase electron diffraction structures have been determined for phosphoryl bromide (OPBr₃ thiophosphoryl bromide (SPBr₃Normal coordinate analyses were carried out for the two molecules using a valence force field, and the resulting amplitude terms used for transformations between r_a and r_ga. An unconstrained refinement of the OPBr₃ intensities gives the parameters r_g(PO) = 1.455(7) Å and r_g(PBr) = 2.175(3) Å. The weighted average, geometrically-consistent valence angles derived from the four internuclear distances, r_α, are θ_α(OPBr) = 114.4(2)° and θ_α(BrPBr) = 104.1(2)°. For SPBr₃ a constrained fit to a self-consistent rα structure gives the parameters r_g(PS) = 1.895(4) Å, r_g(PBr) = 2.193(3) Å, θ_α(SPBr) = 116.2(2)°, and θ_α(BrPBr) = 101.9(2)°. Electron diffraction and spectroscopic vibrational amplitudes are reported for both molecules. The electron diffraction structures are compared with those predicted by simple models previously developed to describe main group V trihalides and trihalogen oxides and sulfides. Treatment of valence angles in four-coordinate molecules is found to be the least satisfactory feature of these models.     

Antibodies with broad specificity to azaspiracids by use of synthetic haptens

The development of general, sensitive, portable, and quantitative assays for the azaspiracid (AZA) class of marine toxins is urgently needed. Use of a synthetic hapten containing rings F-I of AZA to generate antibodies that cross-react with the AZAs via their common C28-C40 domain and use of these antibodies in ELISA and immunoaffinity columns are reported. This approach has many advantages over using intact azaspiracids (AZAs) derived from environmental samples or total synthesis as haptens for antibody development. A derivative of the levorotatory C28-C40 azaspiracid domain (1) was synthesized efficiently using a one-pot Staudinger reduction/intramolecular aza-Wittig reaction-imine capture sequence to form the H-I ring spiroaminal and a double intramolecluar hetero-Michael addition to assemble the F-G ring ketal. Conjugation of the hapten 1 to cBSA and immunization in sheep generated antibodies that recognized and bound to ovalbumin-conjugated 1 in the absence of AZA1. This binding was inhibited by 1 in a concentration-dependent manner. A mixture of AZA1, AZA2, AZA3, and AZA6 caused a degree of inhibition of antibody binding consistent with its total AZA content, rather than just its content of AZA1. This result suggests that the antibodies also have a similar affinity for AZA2, AZA3, and AZA6 as they do for AZA1 and that such antibodies are suitable for analysis of AZAs in shellfish samples.     

Electron diffraction study of the molecular structure ofo-chloroanisole using a dynamic nonparametrized model

The geometrical parameters of the o-chloroanisole molecule were determined by gas phase electron diffraction in terms of the dynamic model using vibrational spectroscopy data and quantum chemical calculations. A new approach based on Tikhonov's regularization method is used to explicitly define the internal rotation potential of the methoxy group. It was found that the nonparametric internal rotation potential has two minima, one of which corresponds to the planar (?=0°) and another to orthogonal (?=90°) orientation of the O?CH₃ bond relative to the plane of the benzene ring. The difference between the energies of the orthogonal and planar conformers is 0.9–1.0 kcal/mole, and the height of rotation barriers at ??65° is 1.4–1.6 kcal/mole, which confirms the results of quantum chemical calculations, indicating that the orthogonal conformer is present in substantial amounts (～30%). The following basic geometrical parameters were found (r_a in Å, ∠_α in deg, the error equals 3σ): r(C?C)_ave=1.398(4); r(O?C_Ph)=1.358(36); r(O?C_Me)=1.426(21);r(C?Cl)=1.733(4);r(C?H)_Ph=1.086(6);r(C?H)_Me=1.095(6); ∠CC_OC_Cl=118.7(2.2); ∠C_OCC=119.9(2.5); ∠C_OC_ClC=121.5(1.1); ∠COC=117.6(2.6); ∠C_OCCl=119.1(2.1); ∠CCO=124.7(1.2). The results are compared with the data for related compounds. Stereochemical features of o-anisoles that are responsible for the orthogonal conformer are discussed.     

The molecular structures of gaseous tetramethylurea and tetramethylthiourea as determined by the electron-diffraction method

Tetramethylurea and tetramethylthiourea have been studied by electron-diffraction in the gas phase. A pyramidal configuration about the N-atoms is found for both molecules with pyramid heights of 27.2 and 11.3 pm for tetramethylurea and tetramethylthiourea, respectively. The most important structural parameters are:

(see Fig. 1 for numbering of the atoms and the definition of φ₁ and φ₂). Values in parentheses are one standard deviation where correlation among data and uncertainty in the electron wavelength have been included.     

Microwave spectrum and conformational composition of 1-vinylimidazole

The microwave spectrum of 1-vinylimidazole has been investigated in the 21-80 GHz spectral region. The spectra of two conformers have been assigned. One of these forms is planar, while the other is nonplanar with the imidazole ring and the vinyl group forming an angle of 15(4)° from coplanarity. The planar form is found to be 5.7(7) kJ/mol more stable than the nonplanar rotamer by relative intensity measurements. The spectra of 10 vibrationally excited states of the planar form and one excited-state spectrum of the nonplanar form were assigned. The vibrational frequencies of several of these states were determined by relative intensity measurements. The microwave work has been augmented by quantum chemical calculations at the CCSD/cc-pVTZ, MP2/cc-pVTZ, and B3LYP/cc-pVTZ levels of theory. The B3LYP calculations predict erroneously that both forms of 1-vinylimidazole are planar, whereas the MP2 and CCSD calculations correctly predict the existence of a planar and a nonplanar conformer of this compound.     

Microwave spectrum, conformational composition, and intramolecular hydrogen bonding of (2-chloroethyl)amine (ClCH2CH2NH2)

The microwave spectrum of (2-chloroethyl)amine, ClCH(2)CH(2)NH(2), has been investigated in the 22-120 GHz region. Five rotameric forms are possible for this compound. In two of these conformers, denoted I and II, the Cl-C-C-N chain of atoms is antiperiplanar, with different orientations of the amino group. The link of the said atoms is synclinal in the three remaining forms, III-V, which differ with respect to the orientation of the amino group. The microwave spectra of four of these conformers, I-IV, have been assigned. In two of these rotamers, III and IV, the amino group is oriented in such a manner that rare and weak five-membered N-H···Cl intramolecular hydrogen bonds are formed. The geometries of conformers I and II preclude a stabilization by this interaction. The energy differences between the conformers were obtained from relative intensity measurements of spectral lines. The hydrogen-bonded conformer IV represents the global energy minimum. This rotamer is 0.3(7) kJ/mol more stable than the other hydrogen-bonded conformer III, 4.1(11) kJ/mol more stable than II, and 5.5(15) kJ/mol more stable than I. The spectroscopic work has been augmented by quantum chemical calculations at the CCSD/cc-pVTZ and MP2/6-311++G(3df,3pd) levels of theory. The CCSD rotational constants and energy differences are in good agreement with their experimental counterparts.