Zero-point average structures of acetyl chloride and acetyl bromide

The zero-point average structures of acetyl chloride and acetyl bromide have been determined by the combined use of their moments of inertia and average distances, obtained by means of microwave spectroscopy and electron diffraction. The r_z parameters determined are as follows: r_z(CO) = 1.185 ± 0.003 Å, r_z(C-Cl) = 1.796 ± 0.002 Å, r_z(C-C) = 1.505 ± 0.003 Å, r_z(C-H) = 1.092 ± 0.005 Å, φ_z(OCCl) = 121.2 ± 0.6°, φ_z(CCCl) = 111.6 ± 0.6°, φ_z(HCH) = 108.8 ± 0.8° and tilt(CH3) = 1.3 ± 1.0°, for chloride; r_z(CO) = 1.181 ± 0.003 Å, r_z(C-Br) = 1.974 ± 0.003 Å, r_z(C-C) = 1.516 ± 0.003 Å, φ_z(OCBr) = 122.3 ± 1.5°, φ_z(CCBr) = 111.0 ± 1.5°, φ_z(HCH) = 109.9 ± 1.1°, tilt(CH₃) = 1.9 ± 1.0°, for bromide. The barriers V₃ to internal rotation have been revised to 1260 and 1256 cal mol⁻¹ for the chloride and bromide, respectively.     

The structures of acetylacetone,trifluoroacetyl-acetone and trifluoroacetone

The molecular geometries of three structurally related compounds have been determined by electron diffraction in the gas phase. Acetylacetone, which exists primarily as the enol tautomer, was found to have a planar symmetric ring with the following r_g values: C₁-C₂ = 1.405± 0.005 Å, C₂-C₄ = 1.510± 0.005 Å, C-O = 1.287± 0.006 Å, C-H = 1.090± 0.010 Å, ∠C₂C₁C₃ = 118.3 ± 1.8°, ∠C₁C₃O₁ = 123.2± 1.7°, ∠C₁C₃C₅ = 122.0± 1.2°, and ∠CCH = 110.2 ± 2.1°. A model in which the enol proton is in the ring plane located symmetrically between the oxygen atoms is in best agreement with the diffraction data. The structure of trifluoroacetylacetone is similar to that of acetylacetone. The r_g values for this compound are C₁-C₃ = 1.4164 ± 0.006 Å, C₃-C₅ = 1.511 ± 0.021 Å, C₂-C₄ = 1.536 ± 0.018 Å, C-O = 1.270 ± 0.008 Å, C-H = 1.088 ± 0.039 Å, C-F = 1.340 ± 0.005 Å, ∠C₂C₁C₃ = 117.2 ± 1.8°, ∠C₁C₂O₂ = 123.6 ± 1.7°, ∠C₁C₃C₅ = 118.1 ± 2.3°, ∠C₁C₂C₄ = 123.0 ± 1.4°, ∠CCH = 109.0± 4.8°, and ∠CCF = 110.6± 0.8°. The r_g values for the trifluoroacetone are: C₁-C₂ = 1.481 ± 0.019 Å, C₁-C₃ = 1.562 ± 0.011 Å, CO = 1.207 ± 0.006 Å, C-H = 1.089 ± 0.024 Å, C-F = 1.339 ± 0.003 Å, ∠C₂C₁O = 122.0 ± 1.1°, ∠C₃C₁O = 116.8 ± 0.7 °, ∠CCH = 105.0 ± 2.2 °, and ∠CCF = 110.7 ± 0.3°. The significance of the error estimates is discussed briefly.     

Structural determination of carvone, a component of spearmint, by means of gas electron diffraction augmented by theoretical calculations

The molecular structure and conformation of carvone, a compound with a minty odor, were investigated by means of gas electron diffraction supported by theoretical calculations. Electron diffraction patterns were recorded by heating the nozzle up to 128 °C to obtain enough scattering intensity. The infrared spectrum was also measured by using an absorption cell with a path length of 10 m. The obtained molecular scattering intensities were analyzed with the aid of theoretical calculations and infrared spectroscopy. It was revealed that the experimental data are well reproduced by assuming that carvone consists of a mixture of three conformers that have the isopropenyl group in the equatorial position and mutually differ in the torsional angle around the single bond connecting the ring and the isopropenyl group. It was also found that the puckering amplitude of the ring of carvone is close to those of menthol and isomenthol, a minty compound and its nonminty isomer. The determined structural parameters (r_g and ∠_α) of the most abundant conformer of carvone are as follows: 〈r(C-C)〉=1.520(3) Å; 〈r(CC)〉=1.360(5) Å; r(CO)=1.225(5) Å; 〈r(C-H)〉=1.104(4)Å; 〈∠CC-C〉=121.1(5)°; 〈∠C-C-C〉=110.4(5)°; ∠C-CO-C=117.1(14)°; 〈∠C-C-H〉=111.1(13)°. Angle brackets denote average values and parenthesized values are the estimated limits of error (3σ) referring to the last significant digit.     

Spectra and structure of organophosphorus compounds: XLIX. Microwave,infrared, and Raman spectra,electric dipole moment,molecular structure,and ab initio calculations of dimethylphosphonothioic fluoride

The microwave spectra of (CH₃)₂PSF, (CH₃)(CD₃)PSF, (CD₃)₂PSF, and (CH₃)₂P³⁴SF have been investigated from 20.0 to 40.0 GHz. Botha-type R branch andc-type Q branch transitions have been measured in the ground states of each isotopic species. From a least-square adjustment to fit 12 rotational constants, the following structural parameters were obtained:r(P–F)=1.582 ± 0.003 Å;r(P=S)=1.902 ± 0.001 Å;r(P-C)=1.800 ± 0.001 Å;r(C-H)=1.088 ± 0.002 Å; HCP=109.28 ± 0.12°; SPF=114.50 ± 0.13°; and SPC=116.33 ± 0.06°. From Stark effect measurements, the dipole moment components have been determined to be ¦ _a ¦ =3.556 ± 0.005; ¦ _c ¦=2.026 ± 0.009; and ¦ _t ¦=4.093 ± 0.009 (D). The Raman spectra (3200 to 100 cm^–1) of each isotopic species have been measured for the solid, and liquid and qualitative depolarization values obtained. Additionally, the mid-infrared spectra (3200 to 500 cm^–1) of the solids have been recorded. Proposed assignments of the normal modes have been made on the basis of Raman depolarization values and group frequencies which are supported by normal coordinate analysis utilizing an ab initio force field. Optimized structural parameters have been obtained with both the 3-21G^* and 6-31G^* basis sets. These results are compared to the corresponding quantities for several similar molecules.For part XLVIII, seeJ. Raman Spectrosc.1922,23, 107.     

Molecular structure of ethynylbenzene from electron diffraction and ab initio molecular orbital calculations

The molecular structure and benzene ring distortions of ethynylbenzene have been investigated by gas-phase electron diffraction and ab initio MO calculations at the HF/6-31G^* and 6-3G^** levels. Least-squares refinement of a model withC _2v, symmetry, with constraints from the MO calculations, yielded the following important bond distances and angles:r _g(C _i -C _o )=1.407±0.003 Å,r _g(C _o -C _m )=1.397±0.003 Å,r _g(C _m -C _p )=1.400±0.003 Å,r _g(Cr _i -CCH)=1.436 ±0.004 Å,r _g(C=C)=1.205±0.005 Å, C _o -C _i -C _o =119.8±0.4°. The deformation of the benzene ring of ethynylbenzene given by the MO calculations, including o-C_i-C_o=119.4°, is insensitive to the basis set used and agrees with that obtained by low-temperature X-ray crystallography for the phenylethynyl fragment, C₆H₅-CC-, in two different crystal environments. The partial substitution structure of ethynylbenzene from microwave spectroscopy is shown to be inaccurate in the ipso region of the benzene ring.     

Equilibrium Structure and Internal Rotation in B2F4 from Electron Diffraction and Spectroscopic Data and Quantum Chemical Calculations

Electron diffraction (ED) data for B₂F₄ recorded by Hedberg et al. over the temperature range –80 to +150°C have been used to obtain equilibrium geometry of this molecule in the framework of a large-amplitude motion model. The torsional coordinate has been adiabatically separated from the rest of vibrations. Two types of constraints applied to obtain ab initio torsional potential energy function (PEF) and the parameters of the geometry relaxation are discussed. The relations between anharmonic interaction force constants and the parameters of the geometry relaxation are briefly considered. Ab initio force constant matrices for rigid vibrational coordinates as well as large-amplitude torsional PEF have been scaled in the procedure of simultaneous fitting to the ED data and experimental vibrational frequencies. The resulting equilibrium geometry and potential function provided good fit to both ED and spectroscopic data. As expected, the results for the equilibrium geometry obtained from separate ED patterns recorded at different temperatures did not show noticeable temperature trend. The determined equilibrium structural parameters for B₂F₄ are: r _e (B–B) = 1.719(4) Å, r _e (B–F) = 1.309(2) Å, BBF = 121.1(1)°. Uncertainties given in parentheses include three times standard deviation and a systematic error. The rotational barrier height was evaluated as 160(50) cm^–1.     

Microwave spectrum and structure of CF3OOCl

The microwave spectrum of chloroperoxytrifluoromethane has been recorded from 12.5 to 40.0 GHz. Only a-type transitions were observed. The R-branch assignments have been made for both the CF₃OO³⁵C1 and CF₃OO³⁷Cl species for the ground vibrational state. The rotational constants are: A=4808± 12, B=1318.55±0.02, C=1278.28±0.02 MHz for the ³⁵CI species, and A=4748±300,B=1285.28±0.96, C=1246.80±0.96 MHz for the ³⁷Cl species. From a diagnostic least-squares adjustment to fit the six rotational constants the following structural parameters were obtained: r(C-0)=1.377±0.03 Å, r(O-O)=1.445± 0.049 Å, r(Cl-O)=1.69±0.04 Å, ∠COO=108.1±4.2°, ∠ClOOC=99.5±2.0°, and ∠tilt = 6.0±0.9° with reasonable assumptions for the three other structural parameters. The relatively large uncertainty in these structural parameters results from the large uncertainty in the A rotational constants. These parameters are compared to the corresponding ones in some other peroxides. The quadrupole coupling constants have been obtained and are discussed.     

Preparation and properties of iron vanadium telluride,FeV2Te4

Iron vanadium telluride, FeV₂Te₄, has been prepared and found to have the monoclinic defect NiAs type structure with unit cell parameters a₀ = 6.641 ± 0.005Å, b₀ = 3.727 ± 0.005Å, c₀ = 12.36 ± 0.01Å, β = 91.75 ± 0.05°. Electrical resistance measurements show FeV₂Te₄ to be metallic, and magnetic susceptibility vs. temperature data indicate magnetic ordering at approximately 5°K.     

Gas-phase electron diffraction study of cyclic dimer of dimethylphosphinic acid (Me2P(O)OH)2 using quantum chemical data and a priori force field

The molecular structure of the hydrogen bonded cyclic dimer of dimethylphosphinic acid (Me₂P(O)OH)₂ was determined by gas-phase electron diffraction (GED) at 433 K. The presence of monomer cannot be determined at this temperature within the error limits for the GED method. Structural analysis was performed with consideration of non-linear kinematic effects at the first-order level of perturbation theory (h1). The vibrational characteristics of internuclear distances were calculated from a priori scaled quantum chemical (RHF/6-311G**) force field. The analysis aided by a constraint based on the RHF/6-311G** calculations yielded the following r_h1-parameters of the C₂-symmetry dimer configuration: PO 1.497(3); P-O 1.573(4); P-C 1.806(1) and 1.811(1) Å; (C-H)_av. 1.109(3) Å; ∠O-PO 120(1)°. Unlike PO and P-O bonds, whose lengths in the gas phase and in the solid state differ insignificantly, the -O?O distance in the gas phase (r_h1 2.81(4) Å) is considerably longer than in the solid state (r_α 2.48(2) Å). The latter is in accordance with the conclusion based on the IR spectra that transition from gas to a solid sample leads to strengthening of the H-bonds. Due to its small contribution to the diffraction pattern, the donor O-H bond length (r_h1 0.99(1) Å) was forcedly bound up with the parameters of C-H bonds. With this assumption, the other geometrical parameters characterizing the H-bond fragment have the following r_h1 values: O?H 1.84(4) Å, ∠-OHO164(6)°, and ∠P-O-H 117(4)°. Conformational flexibility of the non-planar eight-atom ring of the dimer is experimentally verified by absence of any apparent peaks of the f(r) curve at the r-region of more than 4.2 Å.     

Microwave spectra of isotopic glycolaldehydes,substitution structure,intramolecular hydrogen bond and dipole moment

The microwave spectra of ¹³CH₂OH-CHO, CH₂OH-¹³CHO, and CH₂OH-CH¹⁸O are reported and have been used in combination with previously published data on other monosubstituted glycolaldehydes to determine the substitution structure of the molecule as r(CO) = 1.209 Å, r(C-O) = 1.437 Å, r(C-C) = 1.499 Å, r(O-H) = 1.051 Å, r(C-H_ald) = 1.102 Å, r(C-H_alc) = 1.093 Å, r(O β H) = 2.007 Å, r(O β O) = 2.697 Å, ∠(C-CO) = 122°44', ∠(C-C-H_ald) = 115°16', ∠(C-C-O) = 111°28', ∠(C-O-H) = 101°34', ∠(C-C-H_alc) = 109°13', ∠(H-C-H) = 107°34', ∠(O-H β O) = 120°33', ∠(H β OC) = 83°41', and ∠(O-H, C0) = 24°14'. The intramolecular hydrogen bond and the other structural parameters are discussed and compared to related molecules. The dipole moment is redetermined to be μ_a = 0.262 ±0.002 D, μ_b = 2.33 ± 0.01 D, and μ_tot = 2.34 ± 0.01 D. Relative intensity measurements yielded 195 ± 30 cm^?1 for the C-C torsional fundamental and 260±40 cm^?1 for the lowest in-plane skeletal bending mode. Computations performed by the CNDO/2 method correctly predict the observed cis hydrogen-bonded conformer to be the energetically favoured one and in addition yield some indication of the existence of at least two other non-hydrogen-bonded forms of higher energy.     

Molecular structure and microwave spectra of isotopic chloro-, bromo-, and iodocyanoacetylene

The microwave spectra of the halogeno-cyanoacetylenes, X-CC-CN (X = ¹²⁷I, ⁸¹Br, ⁷⁹Br, ³⁷Cl, ³⁵Cl), have been investigated. The molecules were found to be linear. The vibration-rotation constants of the three bending vibrations and the lower stretching vibration were determined. Lines belonging to the monosubstituted ¹³C and ¹⁵N species in their natural abundances were measured and the rotational constants obtained. The bond distances based on the substitution coordinates were: for I-CC-CN r(I-C) = 1.984₆ Å, r(CC) = 1.207_o Å, r(C-C) = 1.370₂ Å, r(CN) = l.l60₄ Å; for Br-CC-CN, r(Br-C) = 1.785₈ Å, r(CC) = 1.204₁ Å, r(C-C) = 1.369₉ Å, r(CN) = 1.159₃ Å; and for C1-CC-CN, r(Cl-C) = 1.624₅ Å, r(CC) = 1.209_o Å, r(C-C) = 1.369_o Å, r(CN) = 1.160₂ Å.     

Four new hydroxymonophosphates with closely related intersecting tunnels structures: The series AM(PO3(OH))2 with A=Rb, Cs; M=Fe, Al, Ga, In

The family of hydroxymonophosphates of generic formula AM^III(PO₃(OH))₂ has been revisited using hydrothermal techniques. Four new phases have been synthesized: CsIn(PO₃(OH))₂, RbFe(PO₃(OH))₂, RbGa(PO₃(OH))₂ and RbAl(PO₃(OH))₂. Single crystal diffraction studies show that they exhibit two different structural types from previously observed other phases with A=H₃O, NH₄, Rb and M=Al, V, Fe. The “Cs-In” and “Rb-Fe” phosphates crystallize in the triclinic space group , with the cell parameters a=7.4146(3) Å, b=9.0915(3) Å, c=9.7849(3) Å, α=65.525(3)°, β=70.201(3)°, γ=69.556(3)° and V=547.77(4) Å³ (Z=3) for CsIn(PO₃(OH))₂ and a=7.2025(4) Å, b=8.8329(8) Å, c=9.4540(8) Å, α=65.149(8)°, β=70.045(6)°, γ=69.591(6)° and V=497.44(8) Å³ (Z=3) for α-RbFe(PO₃(OH))₂. The “Rb-Al” and “Rb-Ga” phosphates crystallize in the Rc space group, with a=8.0581(18) Å and c=51.081(12) Å (V=2872.5(11) Å³ and Z=18) for RbAl(PO₃(OH))₂ and a=8.1188(15) Å and c=51.943(4) Å (V=2965(8) Å and Z=18) for RbGa(PO₃(OH))₂. These two structural types are closely related. Both are built up from M^IIIO6 octahedra sharing their apices with PO₃(OH) tetrahedra to form [M₃(PO₃OH)₆] units, but the latter exhibits a different configuration of their tetrahedra. The three-dimensional host-lattices result from the connection of the [M₃(PO₃OH)₆] units and they present numerous intersecting tunnels containing the monovalent cations.     

Microwave spectrum and molecular structure of arsenic tribromide1

The microwave spectrum of arsenic tribromide has been recorded in the frequency range 26.5–40.0 GHz. From the rotational constants for the symmetric top species ⁷⁹AsBr₃ and ⁸¹AsBr₃ the following structural parameters have been deduced: r_z(AsBr) = 2.324 ± 0.003 Å, θ_z(BrAsBr) = 99.8 ± 0.2°. These results are in excellent agreement with the parameters obtained by Samdal et al., in a concurrent electron diffraction study.     

Synthesis and characterization of two-dimensional fluorinated metal phosphates incorporating 2,2′-bipyridine ligands: M2F2(2,2′-bpy)(HPO4)2(H2O) (M=Fe, Ga)

Two fluorinated metal phosphates, M₂F₂(2,2′-bpy)(HPO₄)₂(H₂O) (M=Fe, Ga), have been synthesized under hydrothermal conditions and characterized by single-crystal X-ray diffraction, thermogravimetric analysis, and magnetic susceptibility. The two compounds are isostructural and crystallize in the triclinic space group , a=7.6595(8)Å, b=10.101(1)Å, c=11.260(1)Å, α=107.555(2)°, β=105.174(2)°, γ=98.975(2)°, V=775.1(2)ų and Z=2 for the Fe compound, and a=7.5816(6)Å, b=9.9943(7)Å, c=11.1742(8)Å, α=107.333(1)°, β=105.014(1)°, γ=99.261(1)° and V=754.2(2)ų for the Ga compound. They are the first fluorinated metal phosphates which incorporate 2,2′-bipyridine ligands. The structure consists of edge-sharing octahedral dimers with the composition Fe₂F₄(H₂O)₂O₄ and discrete FeN₂O₄ octahedra, which are linked into two-dimensional sheets through corner-sharing phosphate tetrahedra. The 2,2′-bpy ligands bind in a bidentate fashion to the metal atoms and project into interlamellar region. The layers are extended into a three-dimensional supramolecular array via π-π stacking interactions of the 2,2′-bpy ligands. Magnetic susceptibility of the iron compound confirms the presence of Fe^III.     

The Molecular Structure and the Puckering Potential Function of Octamethylcyclotetrasilane,Si4Me8, Determined by Gas Electron Diffraction and Relaxation Constraints from Ab Initio Calculations

Gas electron diffraction data are applied to determine the geometrical parameters of the octamethylcyclotetrasilane molecule using a dynamic model in which the ring puckering is treated as a large amplitude motion. The structural parameters and parameters of the potential function were refined, taking into account the relaxation of the molecular geometry estimated from ab initio calculations at the Hartree–Fock level of theory using a 6-311G** basis set. The potential function has been described as V() = V ₀[(/ _e )² – 1]² with V ₀ = 1.0 ± 0.5 kcal/mol and _e = 28.3 ± 1.9°, where is the puckering angle of the ring. The geometric parameters at the minimum of V() (r _a in Å, in degrees and errors given as three times the standard deviations including a scale error) are as follows: r(Si—C)_av = 1.894(3), r(Si—Si) = 2.363(3), r(C—H) = 1.104(3), CSiC = 109.5(6), SiSiSi = 88.2(2), SiCH = 111.7(6), C = 4.1, where the tilt C was estimated from ab initio constraints. The structural parameters are compared with those obtained for related compounds.     

Hydrothermal synthesis, crystal structure and properties of 2-D and 3-D lanthanide sulfates

Two new lanthanum sulfates DySO₄(OH) 1 and Eu₂(SO₄)₃(H₂O)₈2 have been hydrothermally synthesized. The colorless crystals were characterized by IR, TGA, ICP and XRD. The structure was determined by single-crystal X-ray diffraction. 1 crystallizes with monoclinic symmetry, space group P2(1)/n [a=7.995(4) Å, b=10.945(5) Å, c=8.164(4) Å, α=90°, β=93.619(6)°, γ=90°, V=713.0(5) Å³, Z=8]. It displays a three-dimensional framework, based on the novel Dy-O chains connected by the sulfate groups through helical chains. 2 crystallizes with monoclinic symmetry, space group C2/c, [a=13.5605(17) Å, b=6.7676(8) Å, c=18.318(2) Å, α=90°, β=102.265(2)°, γ=90°, V=1642.7 (4) Å³, Z=4]. Its layered framework is attained by the europium atoms connected by the sulfate groups arranged in a helical manner.     

Vibrational spectra,conformational stability,barriers to internal rotation,r0 structural parameters and ab initio calculations of fluoromethyl methyl ether

The far-infrared spectrum of gaseous fluoromethyl methyl ether, FCH₂OCH₃, along with three of the deuterium isotopes, has been recorded at a resolution of 0.10 cm^–1 in the 350 to 50 cm^–1 region. The fundamental asymmetric torsional and methyl torsional modes are extensively mixed and have been observed at 182 and 132 cm^–1, respectively, for the stablegauche conformer with the lower frequency band having several excited states falling to lower frequency. An estimate is given for the potential function governing the asymmetric rotation. On the basis of a one-dimensional model the barrier to internal rotation of the methyl moiety is determined to be 527±9 cm^–1 (1.51±0.03 kcal/mol). A complete assignment of the vibrational fundamentals for all four isotopic species observed from the infrared (3500 to 50 cm^–1) spectra of the gas and solid and from the Raman (3200 to 10 cm^–1) spectra of the gas, liquid, and solid is proposed. No evidence could be found in any of the spectra for the high-energytrans conformer. All of these data are compared to the corresponding quantities obtained from ab initio Hartree-Fock gradient calculations employing the 3-21G and 6-31G* basis sets along with the 6-31G* basis set with electron correlation at the MP2 level. Additionally, completer ₀ geometries have been determined from the previously reported microwave data and carbon-hydrogen distances determined from infrared studies. The heavy-atom structural parameters (distances in Å, angles in degrees) arer(C₁-F) = 1.395 ± 0.005;r(C₁-O) = 1.368 ± 0.007;r(C₂-O) = 1.426 ±0.003; FC₁O = 111.33 ± 0.25; C₁OC₂ = 113.50 ± 0.18 and dih FC₁OC₂ = 69.12 ± 0.26. All of these results are discussed and compared with the corresponding quantities obtained for some similar molecules.     

Microwave spectrum,structure, quadrupole coupling constants and dipole moment of carbon monoxide-borane

The J = 0 → 1 and J = 1 → 2 transitions of thirteen isotopic species of carbon monoxide-borane (BH₃CO) have been measured. The heavy atom r₂ structural parameters have been calculated in several ways so as to minimize the effects of the small carbon coordinate. The structural parameters found are: d(B-C) = 1.534 ± 0.01 Å, d(C-O)= 1.135 ± 0.01 Å. d(B-H)= 1.221 ± 0.001 Å, ∠HBC = 103.79 ± 0.06°, and ∠HBH = 114.50 ± 0.15°. In addition, a complete r₀ structure has been calculated by least-squares fitting the moments of inertia of all the isotopic species. A dipole moment of 1.698 ± 0.02 D was determined.     

Molecular structure of nitrobenzene in the planar and orthogonal conformations

The molecular structure and ring distortions of nitrobenzene have been determined by gas-phase electron diffraction and ab initio molecular orbital (MO) calculations as well as from the structures of six derivatives studied by X-ray crystallography. The experimental value of the ring angle at the ipso position is = 123.4 ± 0.3° in the free molecule; this is about 1.5° less than the hitherto reported values. Regression analysis of the ring angles in the six derivatives studied by X-ray crystallography yields = 122.7(1)° for nitrobenzene in a crystalline environment. The small difference in the two values of a is interpreted as an effect of intermolecular interactions in the crystal. The value produced by the MO calculations, = 122.3° at the 6–31G^* (5D) level, is smaller than either of the experimental results. As regards the ring angles at the meta and para positions, the three techniques of structure determination consistently indicate that these are larger than 120° by a few tenths of a degree. Other important geometrical parameters from the electron diffraction study are r _g (C-C) = 1.399 ± 0.003 Å,r _g (C-N) = 1.486 ± 0.004 Å,r _g (N-O) = 1.223 ± 0.003 Å, and A sO-N-O = 125.3 ± 0.2°. X-ray diffraction experiments on 3,5-dimethyl-4-nitrobenzoic acid and 3,5-dimethylbenzoic acid and ab initio MO calculations provide solid evidence that the geometry of nitrobenzene is little affected when the nitrogroup is twisted by 90° out of the planar equilibrium conformation. This indicates that the extent of -electron transfer from the benzene ring to the nitro group is small. The barrier to rotation is estimated to be 17 ± 4 kJ mol^–1 from the electron diffraction data.