Vibrational analysis of 1,4-diiodobutane

Infrared and Raman spectra were obtained for 1,4-diiodobutane, and normal-coordinate calculations were made using a transferred 48-parameter modified v force field. This compound sometimes crystallizes in the GG' conformation with C₂ symmetry and sometimes in the TG conformation. No evidence was obtained for the presence of the TT (C_2h symmetry) or GG (C_i) conformers, but one or two additional conformers are present that must have a nonplanar chain of carbon atoms.     

Phase diagram and crystal phases of trans-1,3 butadiene probed by FTIR and Raman spectroscopy

In the present work the first spectroscopic data concerning solid butadiene are presented. FTIR and Raman spectra of solid trans-1,3 butadiene in the ordered phase I were recorded at atmospheric pressure down to 12 K. The FTIR technique was used to characterize the phase diagram of butadiene in the 0–7 GPa and 150–300 K pressure–temperature range. Besides phase I another solid phase (phase II), orientationally disordered, was found to be stable at pressures above 0.5 GPa between the liquid and the ordered phase I. A monoclinic C_2h⁵ crystal structure with two molecules per cell sitting on C_i sites is proposed for phase I.     

Polymorphs of the antiviral drug ganciclovir

Ganciclovir (GCV; systematic name: 2‐amino‐9‐{[(1,3‐dihydroxypropan‐2‐yl)oxy]methyl}‐6,9‐dihydro‐1H‐purin‐6‐one), C₉H₁₃N₅O₄, an antiviral drug for treating cytomegalovirus infections, has two known polymorphs (Forms I and II), but only the structure of the metastable Form II has been reported [Kawamura & Hirayama (2009). X‐ray Struct. Anal. Online , 25 , 51–52]. We describe a successful preparation of GCV Form I and its crystal structure. GCV is an achiral molecule in the sense that its individual conformers, which are generally chiral objects, undergo fast interconversion in the liquid state and cannot be isolated. In the crystalline state, GCV exists as two inversion‐related conformers in Form I and as a single chiral conformer in Form II. This situation is similar to that observed for glycine, also an achiral molecule, whose α‐polymorph contains two inversion‐related conformers, while the γ‐polymorph contains a single conformer that is chiral. The hydrogen bonds are exclusively intermolecular in Form I, but both inter‐ and intramolecular in Form II, which accounts for the different molecular conformations in the two polymorphs.     

The Fourier Transform Raman and infrared spectra of naphthazarin

The FT Raman spectra of the three naphthazarin polymorphs have been recorded for the first time and compared with the FTIR spectra of the polymorphs and of naphthazarin with deuterated hydroxyl groups. It is possible to distinguish between the polymorphs, to deduce that the room temperature static symmetry point group of naphthazarin is C_2v, to assign the ν(CH), σ(OH), γ(OH) vibrations and to confirm the absence of the ν(OH) mode.     

Vibrational analysis of 1,4-dibromopentane

Infrared and Raman spectra have been obtained for 1,4-dibromopentane and interpreted with the aid of normal coordinate calculations. It is concluded that all six conformers resulting from rotation about the C₁-C₂ and C₃-C₄ bonds probably exist in the neat liquid and amorphous solid, and that the compound crystallizes in the P_HS_HH conformer that has the two bromine atoms on opposite sides of the plane of carbon atoms. The compound therefore behaves like 1,3-dibromopropane and 1,4-dibromobutane.     

Formation of H and D atoms in pyrolysis of 1,3-butadiene and 1,3 butadiene-1,1,4,4,-d4 behind shock waves

Highly dilute mixtures of 1,3-butadiene and 1,3-butadiene-1,1,4,4-d₄ were pyrolyzed behind reflected and incident shock waves, respectively. Concentrations of H and D atoms were measured by resonance absorption spectroscopy. In the early stages of the reaction, nearly equal amounts of H and D were formed from CD₂CHCHCD₂, indicating that loss of H from C₂ followed by loss of D from C₁ is a more important reaction than breaking of the central C? C bond. Overall, rate constants for atom-forming reactions are much slower than rate constants for disappearance of butadiene in earlier experiments, suggesting that most of the butadiene disappears by processes that do not involve H or D atoms or by radicals that produce them rapidly.     

Vibrational spectra of 1-chloro-2-methylpropane and 1-chloro-2,2-dimethylpropane

Infrared and Raman spectra are obtained for 1-chloro-2-methylpropane and 1-chloro-2,2-dimethylpropane. The former compound exists as a mixture of P_C and P_H' conformers in the liquid and unannealed solid states, but only the P_C conformer is present in the crystalline solid. Vibrational assignments are made for both conformers of 1-chloro-2-methylpropane and for 1-chloro-2,2-dimethylpropane with the aid of normal coordinate calculations.     

Polarized Absorption,Spontaneous and Stimulated Blue Light Emission of J‐type Tetraphenylbutadiene Monocrystals

Blue amplified spontaneous emission at room temperature is demonstrated from the exposed face of the strongly emitting organic semiconductor 1,1,4,4‐tetraphenyl‐1,3‐butadiene in single crystal form. The symmetry of the crystal and calculation of lattice sums indicate the J‐type organization of the molecular transition moments. The minimum in the lowest exciton dispersion branch, from which emission takes place, is found at the edge of the Brillouin zone leading to a dominant vibronic emission since the zero‐phonon line is forbidden. The observed gain narrowed line is attributed to the vibronic replica which becomes amplified with increased pumping. The reported emission is along the normal to the exposed crystal face, important for the development of vertical cavity geometry lasers based on organic single crystals. The threshold excitation fluence of 400 μJ cm^?2 is comparable to other organic crystalline systems, even if the amplification path is much reduced as a consequence of the vertical geometry. Considering these relevant aspects, the optical characterization of this material is provided. The polarized absorption spectra are reported and the properties of the lowest‐energy excitonic state investigated. Calculation of the electronic transitions for the isolated molecule, lattice sums for the transition at lowest energy, and the symmetry of the crystal allow attributing the largest face of the samples and the observed optical bands in the spectra. Polarized time‐resolved spectra are also reported allowing to identify the intrinsic excitonic emission.     

Vibrational analysis of 1,2-dichloroalkanes

IR and Raman spectra have been obtained for 1,2-dichlorobutane and 1,2-dichloro- pentane. The butane crystallizes in the P_XS_XH conformation, but the pentane could not be made to crystallize. Normal coordinate calculations were made for the P_XS_XH, P_CS_HH, and P_HS_HH conformers of these two compounds and for 1,2-dichloropropane. The ob- served spectra were interpreted with the aid of these calculations, and it was concluded that all three conformers exist for each of these compounds. The force field that was obtained should be applicable to other 1,2-dichloroalkanes.     

Vibrational analysis of alkyl bromides: Part III. Branched-chain bromides: 1-bromo-3-methylbutane and 1-bromo-4-methylpentane

Liquid-state IR and Raman spectra and solid-state IR spectra were obtained for 1-bromo-3-methylbutane and 1-bromo-4-methylpentane. The butane exists as a mixture of P_C and P_H conformers in the liquid and amorphous solid, but only the P_H conformer is present in the crystalline solid. The pentane exists as a mixture of P_C,P_H, and P'_H conformers in the liquid and amorphous solid. The solid could not be made to crystallize. The observed bands are assigned to the appropriate conformers with the aid of normal coordinate calculations.     

First‐principles conformational analysis of the C36H36 spheriphane,a prototype hydrocarbon host cage

Comprehensive B3LYP/6‐311G** electronic structure calculations establish that, unlike closely related species such as cyclophanes, the C₃₆H₃₆ spheriphane (heptacyclo[13.13.2^1,15.2^8,22.1^3,27.1^6,10.1^13,17.1^20,24] hexatriaconta‐1,3(33),6,8,10(34),13,15,17(35),20,22,24(36),27‐dodecaene) possesses only seven energetically distinct conformers, out of which five exist in enantiomeric pairs and two are achiral. These local energy minima are interrelated through an intricate net of 20 reaction paths involving single inversions at the ? CH₂? CH₂? bridges. In particular, the T and C₃ conformers, which are predicted to coexist in comparable concentrations at ambient temperatures, are linked through three consecutive single‐bridge inversions that proceed through the overall barrier of only 5.3 (kcal/mol). This barrier is compatible with coalescence temperatures well below those employed in the recently measured ¹H and ¹³C NMR spectra, explaining the observed lack of line splittings. Both the T→C₃ pathway and the racemizations of the low‐energy conformers involve C₁ and C₂ intermediates that are expected to be present in detectable amounts. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1279–1286, 2001     

Copolymerization of propylene with 1,3‐butadiene using isospecific zirconocene catalysts

Poly(propylene‐ran‐1,3‐butadiene) was synthesized using isospecific zirconocene catalysts and converted to telechelic isotactic polypropylene by metathesis degradation with ethylene. The copolymers obtained with isospecific C₂‐symmetric zirconocene catalysts activated with modified methylaluminoxane (MMAO) had 1,4‐inserted butadiene units ( 1,4‐BD ) and 1,2‐inserted units ( 1,2‐BD ) in the isotactic polypropylene chain. The selectivity of butadiene towards 1,4‐BD incorporation was high up to 95% using rac‐dimethylsilylbis(1‐indenyl)zirconium dichloride (Cat‐A)/MMAO. The molar ratio of propylene to butadiene in the feed regulated the number‐average molecular weight (M_n) and the butadiene contents of the polymer produced. Metathesis degradations of the copolymer with ethylene were conducted with a WCI₆/SnMe₄/propyl acetate catalyst system. The ¹H NMR spectra before and after the degradation indicated that the polymers degraded by ethylene had vinyl groups at both chain ends in high selectivity. The analysis of the chain scission products clarified the chain end structures of the poly(propylene‐ran‐1,3‐butadiene). © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5731–5740, 2007     

Gas‐Phase Synthesis of the Benzyl Radical (C6H5CH2)

Dicarbon (C₂), the simplest bare carbon molecule, is ubiquitous in the interstellar medium and in combustion flames. A gas‐phase synthesis is presented of the benzyl radical (C₆H₅CH₂) by the crossed molecular beam reaction of dicarbon, C₂(X¹Σ_g⁺, a³Π_u), with 2‐methyl‐1,3‐butadiene (isoprene; C₅H₈; X¹A′) accessing the triplet and singlet C₇H₈ potential energy surfaces (PESs) under single collision conditions. The experimental data combined with ab initio and statistical calculations reveal the underlying reaction mechanism and chemical dynamics. On the singlet and triplet surfaces, the reactions involve indirect scattering dynamics and are initiated by the barrierless addition of dicarbon to the carbon–carbon double bond of the 2‐methyl‐1,3‐butadiene molecule. These initial addition complexes rearrange via multiple isomerization steps, leading eventually to the formation of C₇H₇ radical species through atomic hydrogen elimination. The benzyl radical (C₆H₅CH₂), the thermodynamically most stable C₇H₇ isomer, is determined as the major product.     

Differently coloured polymorphs of 4‐[(2‐nitrophenyl)azo]phenol

The crystal structures of the brown–yellow and orange polymorphs of the title compound, 4‐[(2‐nitro­phenyl)­diazenyl]­phenol, C₁₂H₉N₃O₃, have been determined and their visible reflection spectra recorded. Both structures adopt a stacking arrangement with interstack hydrogen bonds. Ab initio and semi‐empirical (AM1 and INDO‐CISD) calculations were performed in order to rationalize the difference in colour. It can be attributed neither to the subtle distinctions in molecular geometry nor to the effect of intermolecular electrostatic interactions. The most probable origin of this difference is the mixing of intramolecular n π* and intermolecular charge‐transfer excitations.     

Structural analysis of sterically hindered 1,4‐diols from the naturally occurring lignan hydroxymatairesinol a quantum chemical study

The structures of TADDOL‐like α‐conidendrin‐based chiral 1,4‐diols (LIGNOLs) have been studied at molecular mechanics, Hartree‐Fock (HF)/6‐31G* and DFT/B3LYP/TZVP level of theory. The molecules included were 1,1‐diphenyl, two diastereomers of 1,1,4‐triphenyl, 1,1,4,4‐tetraphenyl, and 1,1,4,4‐tetramethyl 1,4‐diol. Several conformers of each molecule were studied thorougly also including the entropy contributions. For the triphenyl 1,4‐diols, which can form π ? π interactions between phenyl rings, the DFT optimized structures differed significantly from the HF optimized ones. A property for the most stable structures, in addition to the ability to form π ? π interactions, seemed to be the possibility to have the aliphatic six‐membered ring in a boat conformation. For all of the studied LIGNOLs some conformers were found, where the two OH groups pointed almost to the same direction. By this an intramolecular hydrogen bond can be formed between them. The bridging hydrogen atom falls at the same place as a chelate‐bonded metal ion would be situated, as in the case of the analogous molecules, TADDOLs, but only a few of these molecules would be able to work well as ligands for asymmetric catalysis. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Synthesis and oxidative degradation of poly(ethene‐ran‐1,3‐butadiene)

Copolymerization of ethene and 1,3‐butadiene was conducted over SiO₂‐supported CpTiCl₃ catalyst using Ph₃CB(C₆F₅)₄ or B(C₆F₅)₃ combined with triisobutylaluminium (ⁱBu₃Al) or trioctylaluminium (Oct₃Al). When the copolymerization was carried out at 0°C, the Ph₃CB(C₆F₅)₄/ⁱBu₃Al and B(C₆F₅)₃/Oct₃Al systems selectively produced copolymers which contained about 0.5–2.5 mol‐% of trans‐1,4‐inserted butadiene units. The number‐average molecular weight (M_n) of the copolymers was around 80 000 with polydispersities in the range from 6 to 8. Oxidative degradation of the vinylene units with potassium permanganate decreased the M_n values to several thousands with polydispersities of ca. 2. This indicates that the butadiene units are randomly distributed in the copolymers. NMR analysis clarified that the decomposed product is a polyethene with carboxyl groups at both chain ends.     

Charge Fluctuations and Ethylene‐Group‐Ordering Transition in β′′‐(BEDT‐TTF)4[(H3O)Fe(C2O4)3]⋅Y Molecular Charge‐Transfer Salts

The polarized infrared reflectance and Raman spectra of the three quasi‐two‐dimensional β′′‐(BEDT‐TTF)₄[(H₃O)Fe(C₂O₄)₃]?Y bifunctional charge‐transfer salts, where BEDT‐TTF=bis(ethylenedithio)tetrathiafulvalene and Y=C₆H₅Br, (C₆H₅CN)_0.17(C₆H₅Br)_0.83, (C₆H₅CN)_0.4(C₆H₅F)_0.6, have been measured as a function of the temperature. Signatures of charge inhomogenity have been found in both Raman and infrared spectra of the β′′‐(BEDT‐TTF)₄[(H₃O)Fe(C₂O₄)₃]?Y superconductors. A 100 K transition to a mixed insulating/metallic state is clearly seen for the first time in the temperature dependence of the electronic spectra of superconducting β′′‐(BEDT‐TTF)₄[(H₃O)Fe(C₂O₄)₃]?C₆H₅Br. We suggest that this phase transition is due to subtle changes in the ethylene groups ordering, which are related to a structural phase transition in the anionic layer. The infrared and Raman spectra of quasi‐two‐dimensional metal α‐′pseudo‐κ′‐(BEDT‐TTF)₄[(H₃O)Fe(C₂O₄)₃]C₆H₄Br₂ are also investigated.     

The vibrational spectra of 1,1,4,4-tetrafluoro- and 1,1,4,4-tetrachlorobutadiene

Raman data are reported for gas, liquid and solid 1,1,4,4-tetrafluorobutadiene, F₂CCHCHCF₂, and IR data for gas and solid. The molecule has a planar trans conformation of C_2h symmetry. With the aid of Raman depolarization ratios and IR band contours, twenty of the twenty-four spectroscopically-active fundamentals can be assigned with assurance. Frequencies are suggested for the remaining four modes.Raman and IR data are reported for the liquid and solid 1,1,4,4-tetrachlorobutadiene. Mid-IR gas phase data are also reported. Again the data can be satisfactorily explained under C_2h symmetry. Fourteen fundamentals can be assigned with confidence. Suggestions have been made for the frequencies of nine other fundamentals.     

Concomitant polymorphic forms of 3‐cyclopropyl‐5‐(2‐hydrazinylpyridin‐3‐yl)‐1,2,4‐oxadiazole

The dipharmacophore compound 3‐cyclopropyl‐5‐(2‐hydrazinylpyridin‐3‐yl)‐1,2,4‐oxadiazole, C₁₀H₁₁N₅O, was studied on the assumption of its potential biological activity. Two concomitant polymorphs were obtained on crystallization from isopropanol solution and these were thoroughly studied. Identical conformations of the molecules are found in both structures despite the low difference in energy between the four possible conformers. The two polymorphs differ crucially with respect to their crystal structures. A centrosymmetric dimer formed due to both stacking interactions of the `head‐to‐tail' type and N—H…N(π) hydrogen bonds is the building unit in the triclinic structure. The dimeric building units form an isotropic packing. In the orthorhombic polymorphic structure, the molecules form stacking interactions of the `head‐to‐head' type, which results in their organization in a column as the primary basic structural motif. The formation of N—H…N(lone pair) hydrogen bonds between two neighbouring columns allows the formation of a double column as the main structural motif. The correct packing motifs in the two polymorphs could not be identified without calculations of the pairwise interaction energies. The triclinic structure has a higher density and a lower (by 0.60 kcal mol^?1) lattice energy according to periodic calculations compared to the orthorhombic structure. This allows us to presume that the triclinic form of 3‐cyclopropyl‐5‐(2‐hydrazinylpyridin‐3‐yl)‐1,2,4‐oxadiazole is the more stable.     

Conformational Polymorphism in N‐(4′‐methoxyphenyl)‐ 3‐bromothiobenzamide

Three conformational polymorphs of N‐(4′‐methoxyphenyl)‐3‐bromothiobenzamide, yellow α, orange β, and yellow γ, have been identified by single‐crystal X‐ray diffraction. The properties and structure of the polymorphs were examined with FT Raman, FTIR (ATR), and UV/Vis spectroscopy, as well as differential scanning calorimetry. Computational data on rotational barriers in the isolated gas‐phase molecule indicate that the molecular conformation found in the α form is energetically preferred, but only by around 2 kJ mol^?1 over the γ conformation. The planar molecular structure found in the β form is destabilized by 10–14 kJ mol^?1, depending on the calculation method. However, experimental evidence suggests that the β polymorph is the most stable crystalline phase at room temperature. This is attributed to the relative planarity of this structure, which allows more and stronger intermolecular interactions, that is, more energetically effective packing. Calculated electronic‐absorption maxima were in agreement with experimental spectra.