The vibrational spectra, molecular structure and conformation of organic azides Part III. 2,3-Diazido-1,3-butadiene

A sample of 2,3-diazido-1,3-butadiene has been synthesized from 1,4-dibromo-2-butyne and tetramethylguanidinium azide. The highly explosive sample has been studied by gaseous electron diffraction and by IR spectroscopy. Only incomplete Raman spectra have been recorded due to sample decomposition in the laser beam. The title compound is found to be planar with the CNN angle equal to 114.5°, oriented syn to the adjacent C=C double bond; the NNN angle is ca. 167°, oriented anti to the C---N bond. The following bond distances (r_a) are obtained: N---N(N), 114.1; N---N(C), 124.2; C---N, 143.2; C=C 134.8; and C---C, 148.5 pm. The vibrational spectra are tentatively assigned in terms of C_2h molecular symmetry, supported by force constant calculations.     

The vibrational spectra, molecular structure and conformation of organic azides : Part VI. Azidoacetonitrile

Azidoacetonitrile (azidoethanenitrile) has been synthesized and the structure determined by electron diffraction from the vapour. IR spectra have been recorded of the vapour, liquid, solutions, solid at ca. 90 K and of the molecule trapped in argon and nitrogen matrices at 15 K. Raman spectra of the cooled liquid and of the amorphous and crystalline solids have also been obtained.

The electron diffraction results show the conformation to be gauche around the C---N bond with a dihedral angle of 52°(5) from syn and the NNN angle 173°(3) oriented anti to the C---N bond.

The following bond distances (r_a) between the heavy atoms were obtained: r_CN = 115.4(5) pm, r_C---C = 146.5(15) pm, r_C---N = 147.5(6) pm, r_N---N = 124.5(5) pm and r_NN = 113.5(4) pm.

Very large changes in both frequency and intensity have been observed in the vibrational spectra upon crystallization which may be due to intermolecular associations formed by the azido group but which can also be explained by a change of the molecular conformation from only gauche in the vapour and liquid phases to anti in the crystal phase.     

X-ray diffraction study of the smectic mesophase of some azobenzene-containing polyacrylates

An X-ray study is performed on powder specimens and on stretched oriented fibres of two liquid crystalline polyacrylates containing the azobenzene mesogenic unit with either a 4'-n-pentyloxy (sample 1-4) or 4'-n-hexyloxy (sample 1-5) substituent. The X-ray diffraction patterns of both samples showed the presence of a bilayer smectic C mesophase with the mesogneic groups tilted by an angle β ∼ 45° (1-4) or β ∼ 38° (1-5) with respect to the layer normal. The electron density profile p(z) along the direction normal to the smectic layers was calculated by Fourier inversion and possible structural models of the smectic mesophase are discussed. A partial interdigitation of the terminal alkyloxy substituents appears to occur.     

The conformation and vibrational spectra of 2-ethynyl-1,3-butadiene (3-methylene-4-pentene-yne)

A sample of 2-ethynyl-1,3-butadiene was synthesized by a thermal rearrangement of 1,2-hexadiene-5-yne at ca. 770 K. Infrared spectra were recorded of the vapour, the liquid and of the amorphous and crystalline solids at 90 K in the region 4000-50 cm⁻¹. Raman spectra were obtained of the cooled liquid, including semiquantitative polarization measurements, and of the crystalline solid at 90 K. The spectral data indicate that 2-ethynyl-1,3-butadiene exists as the s-trans conformer in the various states of aggregation but the possibility of small amounts of a second conformer cannot be excluded.     

The synthesis and liquid crystal transition temperatures of some weakly polar nematic methyl (E)-[trans-4-substituted-cyclohexyl]-allyl ethers

We have introduced an oxygen atom and a carbon-carbon double bond with a trans-configuration (E) into the terminal alkyl chain of a wide variety of liquid crystalline cyclohexane derivatives to produce a variety of new methyl (E)-allyl ethers. The melting points and tendency to form smectic mesophases are often low, while nearly all of the compounds prepared exhibit a nematic phase. Thus, even two-ring derivatives can exhibit nematic phases over a wide temperature range (≤80°C), sometimes starting below room temperature (T_m≈10°C). Comparisons with the corresponding derivatives incorporating either just an oxygen atom or just a carbon-carbon double bond in the same position indicate that synergetic effects lead to broader nematic phases than would otherwise have been expected. Thus many of the new methyl (E)-allyl ethers exhibit nematic phases over a wider temperature range than the corresponding materials with an unsubstituted alkyl chain attached to the cyclohexyl ring. The new compounds are easily prepared from known starting materials. Many intermediates are themselves liquid crystalline. This allows investigation of the relationship between liquid crystal transition temperatures and the nature of the terminally substituted alkyl chain (for example, incorporating C=C, OH, CO₂C₂H₅ and OCH₃ groups).     

The vibrational spectra, molecular structure and conformation of organic azides : Part IV. An ab initio study of hydrazoic acid, azidomethane, azidoethane, azidoethene and azidomethanal

Fully optimized geometries have been calculated for the title compounds at the Hartree—Fock SCF level and compared with existing experimental data. A basis set of double zeta quality has been employed. For hydrazoic acid, a calculation with a larger basis set, expected to give results near the Hartree—Fock limit, has also been performed. All of the calculations show the azide group to be slightly bent with a trans configuration around the central NN bond. Azidoethane is predicted to exist in two conformations, gauche (71°) and anti, with a negligible energy difference of 0.26 kJ mol⁻¹ between them. Azidoethene and azidomethanal both prefer the syn orientation of the azide group with respect to the C---C or C---O bonds, the computed energy difference between the anti and syn conformations being 3.31 and 30.3 kJ mol⁻¹ respectively.

The barrier to rotation around the C---N bond has been calculated to be 3.75 kJ mol⁻¹ in azidomethane while in azidoethane it was 3.30 and 9.40 kJ mol⁻¹ in the eclipsed anti-clinal (120°) and syn positions, respectively.

Complete harmonic force fields and dipole moment derivatives have been calculated for hydrazoic acid, azidomethane and for the two stable conformations of azidoethane. For azidoethane and azidomethanal only the azide part of the harmonic force field has been calculated. The theoretical harmonic force fields have been modified through scaling by a least squares refinement to the observed wavenumbers of hydrazoic acid, azidomethane and azidoethane (anti and gauche). Infrared vapour phase intensities have been calculated and theoretical spectra are presented for azidomethane and azidoethane.     

Thermal switching of the optical anisotropy of a macroscopically aligned film of a discotic liquid crystal

We have studied the temperature dependence of anisotropy in the optical absorption and charge transport properties of an aligned film of hexakis-dodecyl-hexa-peri-hexabenzocoronene (HBC-C12) formed by zone-casting on a quartz substrate. At room temperature the film displays a large anisotropy in (photo)conductivity, as determined using the flash photolysis time-resolved microwave conductivity technique, with charge transport in the casting direction favoured by a factor of at least 10. The anisotropy in the optical absorption is however negligible. At the temperature corresponding to the transition from the crystalline solid to the liquid crystalline mesophase (c. 110°C), the optical anisotropy increases abruptly, with absorption of light polarized in the direction perpendicular to the alignment direction favoured by a factor of c. 3. On cooling, the dichroism reverts to its initial very low value with a hysteresis of c. 30°C. The results are explained in terms of a reversible change in the orientation of the molecules with respect to the axis of the aligned columnar stacks from tilted (at c. 45°) in the crystalline phase to close to orthogonal in the liquid crystalline phase.     

Electron diffraction investigation of the molecular structures of isopropyl isocyanate and isopropyl isothiocyanate

The molecular structures of isopropyl isocyanate and isopropyl isothiocyanate have been determined by electron diffraction in the gas phase. For both molecules the data are consistent with the presence of a single conformer at room temperature, with the NCX group eclipsing one of the C---C bonds of the isopropyl group (a skew conformation). The structures found are consistent with the limited information available from microwave studies. The skeletal parameters found are (NCO, NCS; distances in pm, angles in degrees): r(C---N) 146.0(8), 145.9(13); r(C---C) 153.4(5), 15.8(7); r(N=C) 121.4(6), 120.1(6); r(C=O) 118.4(4); r(C=S) 159.8(5); NCC 110.0(5), 110.4(10); CCC 114.7(9), 115.7(13); C---N=C 132.6(10), 135.9(17); NCX 159(3), 166(3); C---N torsion 125(3), 115(6) (where a torsion angle of 120° corresponds to the skew form). The results are compared with those for some related molecules.     

Ionic interaction induced novel ordered columnar mesophases in asymmetric discotic triphenylene salts

Asymmetric triphenylene imidazolium salts with different spacer lengths were successfully synthesized through quarternization of ω-bromo-substituted triphenylenes with 1-methyl imidazole. The asymmetry in ω-bromo-substituted triphenylenes tended to destroy liquid crystallinity in the sample. However, highly ordered columnar mesophases with a lamellar microphase segregation were induced by ionic interactions among the imidazolium salts, and the lamellar morphology was visualized by transmission electron microscopy. On the basis of an X-ray diffraction study on shear oriented samples, a novel rectangular columnar phase with a plane group of pm was observed for a triphenylene imidazolium salt with a spacer length of C₁₁, while an oblique columnar phase was determined for a triphenylene imidazolium salt with a C₈ spacer. Due to the asymmetric molecular shape and ionic interactions in the triphenylene imidazolium salts, the columnar liquid crystalline phase was extended to below room temperature (c. -20°C) for samples with spacer lengths of C₈ and C₁₁.     

Infrared and Raman spectra, conformational stability, ab initio calculations, and vibrational assignments for methylaminothiophosphoryl difluoride

Infrared spectra (4000–50 cm⁻¹) of the vapor, amorphous and crystalline solids and Raman spectra (3600–10 cm⁻¹) of the liquid with qualitative depolarization data as well as the amorphous and crystalline solids of methylaminothiophosphoryl difluoride, CH₃N(H)P(=S)F₂, and three deuterated species, CD₃N(H)P(=S)F₂, CH₃N(D)P(=S)F₂, and CD₃N(D)P(=S)F₂, have been recorded. The spectra indicate that in the vapor, liquid and amorphous solid a small amount of a second conformer is present, whereas only one conformer remains in the low temperature crystalline phase. The near-infrared spectra of the vapor confirms the existence of two conformers in the gas phase. Asymmetric top contour simulation of the vapor shows that the trans conformer is the predominant vapor phase conformer. From a temperature study of the Raman spectrum of the liquid the enthalpy difference between the trans and near-cis conformers was determined to be 368±15 cm⁻¹ (4.41±0.2 kJ/mol), with the trans conformer being thermodynamically preferred. Ab Initio calculations with structure optimization using the 6-31G(d) and 6-311+G(d,p) basis sets at the restricted Hartree–Fock (RHF) and/or with full electron correlation by the perturbation method to second order (MP2) support the occurrence of near-trans (5° from trans) and near-cis (20° from cis) conformers. From the RHF/6-31G(d) calculation the near-trans conformer is predicted to be the more stable form by 451 cm⁻¹ (5.35 kJ/mol) and from the MP2/6-311+G(d,p) calculation by 387 cm⁻¹ (4.63 kJ/mol). All of the normal modes of the near-trans rotamer have been assigned based on infrared band contours, depolarization values and group frequencies and the assignment is supported by the normal coordinate calculation utilizing harmonic force constants from the MP2/6-31G(d) ab initio calculations.     

Characterization and decomposition mechanism of an unusual nickel, thiocyanate and 4-methylpyridine polymeric complex

The synthesis, characterization, and thermal decomposition of the [Ni(SCN)₂(H⁺SCN)₂(4-mepy)₂] compound with an octahedral structure in polymeric chain were reported, in which SCN groups form bridges among Ni(II) ions. The compound decomposes in water resulting in a pH<4 solution. The FT-IR spectrum presented doublet bands at 2117; 2128 cm⁻¹, 788; 773 cm⁻¹ assigned to ν(C---N) and ν(C---S) stretching modes, respectively, and δ(SCN) deformation modes at 468; 476 cm⁻¹. The Raman spectrum of the compound presented the ν(C---N) stretching as a strong doublet at 2122; 2128 cm⁻¹, ν(C---S) at 783; 770 cm⁻¹, and δ(SCN) at 468; 477 cm⁻¹. No significant changes were observed in the 4-mepy ligand bands compared with the vibrational frequencies of the pure compound or the compound in aqueous solution 0.2 mol l⁻¹. The crystal UV–vis reflectance spectrum presented two bands centered in 626 and 424 nm tentatively assigned to the d→d type transitions, ³A_2g→³T_1g and ³A_2g→³T_1g, for a symmetry close to O_h. The TG curve showed a mass loss between 120 and 200 °C assigned to the loss of the two 4-mepy molecules; from 200 to 265 °C, the loss of the two H⁺SCN groups; and from 265 to 450 °C, the loss of the two SCN groups that formed the bridges among the nickel atoms. Based on these mass loss data, a mechanism of thermal decomposition for the compound was proposed.     

An electron diffraction determination of the molecular structure of BIS(trifluoromethyl) diselenide

The molecular structure of F₃CSeSeCF₃ has been determined in the vapour phase by the sector microphotometer method of electron diffraction. The data are consistent with a C₂ model having the dimensions: C-F 1.326±0.005, C-Se 2.018±0.020, Se-Se 2.292±0.010 Å, angle F-C-Se 109.1±0.8°, angle C-Se-Se 98.0±0.5°, C-Se-Se-C dihedral angle 84.5±3.0°, and an angle of twist of 11.8±1.0° of both F₃C- groups from the conformation in which they are staggered with respect to the Se-Se bond.     

Molecular structure and conformational stability of allylisocyanate—post-Hartree–Fock and density functional theory studies

The molecular structure and conformational stability of allylisocyanate (CH₂CHCH₂NCO) molecule was studied using the ab initio and DFT methods. The geometries of possible conformers, C-gauche (δ=120°, θ=0°) (δ=C=C–C–N and θ=C–C–N=C) and C-cis N-trans (δ=0° and θ=180°) were optimized employing HF/6-31G^*, MP2/6-31G^* levels of theory of ab initio and BLYP, B3LYP, BPW91 and B3PW91 methods of DFT implementing the atomic basis set 6-311+G(d,p). The structural and physical parameters of the above conformers were discussed with the experimental and theoretical values of the related molecules, methylisocyanate and 3-fluoropropene. It has been found that the N=C=O bond angle is not linear as the experimental result for both the conformers and the theoretical bond angle is 173°. The rotational potential energy surfaces have been performed at the HF/6-31G^*, and MP2/6-31G^* levels of theory. The Fourier decomposition potentials were analysed at the HF/6-31G^*, and MP2/6-31G^* levels of theory. The HF/6-31G^* level of theory predicted that the C-gauche conformer is more stable than the C-cis N-trans conformer by 0.41 kJ/mol, but the MP2 and DFT methods predicted the C-cis N-trans conformer is found to be more stable than the C-gauche conformer. The calculated chemical hardness value at the HF/6-31G^* level of theory predicted the C-cis N-trans form is more stable than C-gauche form, whereas the chemical hardness value at the MP2/6-31G^* level of theory favours the slight preference towards the C-gauge conformer.     

Far infrared spectra, conformational equilibria, vibrational assignments, ab initio calculations and structural parameters for 2-bromoethanol

The far infrared spectrum from 370 to 50 cm⁻¹ of gaseous 2-bromoethanol, BrCH₂CH₂OH, was recorded at a resolution of 0.10 cm⁻¹. The fundamental O–H torsion of the more stable gauche (Gg′) conformer, where the capital G refers to internal rotation around the C–C bond and the lower case g to the internal rotation around the C–O bond, was observed as a series of Q-branch transitions beginning at 340 cm⁻¹. The corresponding O–H torsional modes were observed for two of the other high energy conformers, Tg (285 cm⁻¹) and Tt (234 cm⁻¹). The heavy atom asymmetric torsion (rotation around C–C bond) for the Gg′ conformer has been observed at 140 cm⁻¹. Variable temperature (−63 to −100°C) studies of the infrared spectra (4000–400 cm⁻¹) of the sample dissolved in liquid xenon have been recorded. From these data the enthalpy differences have been determined to be 411±40 cm⁻¹ (4.92±0.48 kJ/mol) for the Gg′/Tt and 315±40 cm⁻¹ (3.76±0.48 kJ/mol) for the Gg′/Tg, with the Gg′ conformer the most stable form. Additionally, the infrared spectrum of the gas, and Raman spectrum of the liquid phase are reported. The structural parameters, conformational stabilities, barriers to internal rotation and fundamental frequencies have been obtained from ab initio calculations utilizing different basis sets at the restricted Hartree–Fock or with full electron correlation by the perturbation method to second order. The theoretical results are compared to the experimental results when appropriate. Combining the ab initio calculations with the microwave rotational constants, r₀ adjusted parameters have been obtained for the three 2-haloethanols (F, Cl and Br) for the Gg′ conformers.     

An NMR study of the conformational flexibility of phenyl acetate dissolved in a nematic liquid crystalline solvent

The ¹H, ²H and ¹³C NMR spectra of phenyl acetate, phenyl acetate-[¹³CO] and phenyl acetate-[C²H₃] dissolved in a nematic liquid crystalline solvent have been analysed to yield dipolar coupling, D_ij. These have been interpreted using the additive potential model to provide information on the molecular conformation, resulting in three possible shapes for V(φ), the potential energy for rotation about the ring-oxygen bond. A comparison with the results of molecular orbital calculations leads to the conclusion that a potential with a minimum at 54.4° ± 0.1° is the most probable.     

Surface properties of ω-perfluorooctyl-alkyl polyacrylates: odd–even effect

A series of ω-perfluorooctyl-alkyl polyacrylates has been prepared and analysed. The odd–even effect, already observed in the case of liquid crystalline polymers has been exhibited for perfluorinated ones. Values of the contact angles to advanced (θ_a), with withdrawal (θ_r), as well as the contact angle hysteresis (Δθ) of various ω-perfluorooctyl-alkyl polyacrylates in water at 20 °C are described. Contrary to the advancing contact angle which is almost independent of spacer length, the receding one varies strongly with it.     

The molecular structure and conformational composition of epichlorohydrin as determined by gas phase electron diffraction

The molecular structure of gaseous epichlorohydrin has been investigated using electron diffraction data obtained at 67°C. The conformational composition at this temperature is such that the molecules exist predominantly in a gauche-2 conformer (where the C---Cl bond is 160° away from the C---O) bond). Refinements showed that 33% (σ = 4) of the molecule exist in the gauche-1 form. The important distances (r_g) and angle () with the associated uncertainties are r(C---H) = 1.095(5) Å, r(C---O) = 1.442(3) Å, r(C---C) = 1.475(8) Å, r(C---C_M) = 1.523(7) Å, r(C---Cl) = 1.788(2) Å, CCO = 114° (1), CCC_M = 119°(1), ClCC = 108.9° (7), and Tau(ClCCO) = −150°(10) (gauche-2) and Tau(ClCCO) = 78° (10) (gauche-1).     

Microwave spectrum, structure, dipole moment and internal rotation of allylsilane

Microwave spectra of allylsilane and its ¹³C and deuterium substituted species have been measured and assigned for the skew isomer. The r_s structure was determined with the aid of several assumptions. Some of the parameters determined are; r(C=C) = 1.328 ± 0.007 Å, r(C---C) = 1.492 ± 0.008 Å, (CCC) = 126.7 ± 0.8°, (CCSi) = 111.6 ± 0.5° and τ(CCCSi) = 106.8 ± 1.1°. Dipole moments and their components were also determined for the CH₂ = CHCH₂SiH₃ and CH₂=CHCH₂SiD₃ species. Hyperconjugation between the C=C π bond and the C---Si σ bond is discussed.     

Synthesis and characterization of main chain liquid crystalline ionomers containing sulphonate groups

A series of main chain liquid crystalline ionomers containing sulphonate groups pendent to the polymer backbone were synthesized by an interfacial condensation reaction of 4,4'-bis(1,10-sebacyloxo)benzoic acid, brilliant yellow (BY), and 4,4'-biphenyldiol. 4,4'-Bis(1,10-sebacyloxo)benzoic acid exhibited nematic schlieren texture during heating and cooling. The ionomers are thermotropic liquid crystalline polymers and thermally stable to about 270°C. They exhibit broad mesophase regions over a range of 220°C and the same nematic mesomogen with a colourful thread texture as B₀-LCP, which implies that the introduction of an ionic group did not change the texture of the B₀-LCP. However, the thermotropic liquid crystalline properties were somewhat weakened when the concentration of BY was more than 5%. The inherent viscosity in N,N-dimethylformamide solution suggested that intermolecular associations of sulphonate groups occurred at low concentration, and intermolecular associations predominated at higher concentration.     

Evidence for the importance of intermolecular coupling in the OD band''s vibrational structure in deeply supercooled liquid D2O

We present the Raman spectrum of pure liquid D₂O at −27.0°C, 17°C lower than any previously reported Raman spectrum of liquid D₂O. The liquid's OD stretch band at −27.0°C displays a prominent feature which is clearly analogous to the ν₁ in-phase mode in D₂O ice Ih providing strong evidence that the structure of cold liquid D₂O's OD band is principally due to strong intermolecular coupling. Deeply supercooled liquid D₂O displays this ν₁ in-phase feature more clearly than deeply supercooled liquid H₂O due to the smaller disparity in the D/O mass ratio compared to that of H/O which enhances this coupling.