Search

In the current work, for the first time, the conjugation structure, electronic properties, and UV–visible spectral characteristics of a series of 1-amino-9,10-anthraquinone oligomers, oligo(AAQs), are studied using the density functional theory (DFT) method. The required computations are performed on the 2AAQ, 4AAQ, 6AAQ, and 8AAQ oligomers in their ground state and radical ion forms as the potential candidates for the conducting polymers. The geometric characteristics, frontier molecular band gap energy, ionization potential, electron affinity, and UV–visible spectral characteristics of the understudied oligo(AAQs) are investigated by the B3LYP/6-31G(d,p) level of theory and the time-dependent DFT method. The results obtained show that the oligo(AAQ) chains are twisted from the planarity structure due to the steric strain of the AAQ repeating units along the oligomer chain. This deviation from planarity decreases as the oligomer chain increases. Our calculations show that the n- and p-doped oligo(AAQ)s are more favorable than their ground states and that the doped phases have more satisfactory properties, reflecting the electron and hole transport characteristics for conductivity. The results obtained allow one to understand the qualitative structure-property correlation and a new vision into the conduction and the emission process of poly(AAQ) for a better use in the opto-electronic applications.

The determined structure of benzyl methyl (3,4-dichlorobenzoyl)carbonohydrazonodithioate, C16H14Cl2N2OS2, has monoclinic (P21/c) symmetry. It is of interest with respect to antibacterial properties. A set of 24 dithioesters was examined in respect of their structural and antibacterial properties. Our studies show that the substitution of the aromatic ring with the hydroxyl group in the ortho position influences strongly the biological activity of the studied compounds against the tested strains.

New sets of unsymmetric calamitic molecules with uracil unit at one end and biphenyl core at another end were synthesized. Liquid crystal property of these molecules was investigated by POM and DSC techniques. All compounds exhibit LC property depending on the spacer and terminal alkoxy chain of the molecules. First set shows smectic phase in lower members and nematic phase appeared in higher members. The second set favour nematic liquid crystalline phase with respect to spacer alkyl chain length. Molecules are escaped from the planarity as a result disturbing the layer stacking leads to nematic phase in higher analogues. Theoretical studies have been performed for all the compounds and are found to be in agreement with the results of the current studies.

In this paper, we present the synthesis of three chalcone analogues and their spectroscopic, structural and optical characterization. The influence of the different substituents on the crystalline structure and on their optical properties was evaluated. The effects of derivatization on the chalcones, both at the molecular and supramolecular levels, were evaluated. Also, the molar absorptivity, the first hyperpolarizability β and the two-photon absorption cross-section σ2PA were obtained. The extended structures in each compound are stabilized by hydrogen-bonded dimers and by a pseudo-ring formed from an intramolecular H-bond. It was observed that more voluminous substituents, such as chlorine, contribute to the deviation from planarity. Although many effects were observed in molecular structures of these analogues when in solid form, experimental results of linear and nonlinear optical (NLO) properties showed that the optical properties were not as much influenced in solution. In addition, and to support some experimental results, the theoretical first hyperpolarizability was calculated and showed good agreement with experimental results, which are approximately 10 × 10 − 30 c m 5 / e s u .

The pseudo Jahn-Teller effect (PJTE) is employed to explore the origin of the puckering structure of cyclohexasulfur (S 6 ), cyclohexaselenium (Se 6 ) and cyclohexatellurium (Te 6 ) and their nondegenerate and degenerate vibronic excited states and their planar structure instabilities have investigated. The ab initio geometry optimization and frequency calculations show that all these cyclohexahomoatomic molecules chose D 6h symmetry in the planar configuration, and according the S 6 and Se 6 experimental structure, the chair form of the molecules is stable structure. The vibronic coupling between the ground state 1 A 1g and excited state 1 B 2g is the cause of chair puckering in all these series compounds and the numerical solutions of the PJTE ( 1 A 1g + 1 B 2g )⊗ b 2g problems describe their instability. The adiabatic potential energy surfaces (APES) cross sections of low-lying electronic states along the b 2g puckering normal coordinates have calculated by the state-average complete active space self-consistent field (SA-CASSCF) method. The calculation results show that, the chair puckering instability in the S 6 from unstable planar configuration with D 6h symmetry to a stable D 3d distorted geometry, is stronger than others, whereas it is weaker in Te 6 . Additionally, coordination two canions (X = H + , He 2+ ) to the S 6 chair structure restore the planarity of S 6 puckered ring in the S 6 X 2 systems, although the D 6h symmetry in S 6 planar ring configuration changes to the C s symmetry in the systems.

The gas-phase molecular structure of the magnesium octa(m-trifluoromethylphenyl)porphyrazine (MgC 72 H 32 N 8 F 24 ) has been studied by a synchronous gas-phase electron diffraction and mass spectrometric experiment at T = 667(10) K in combination with DFT calculations using B3LYP hybrid method and triple-ζ valence basis sets. The molecule has an equilibrium structure of D 4 symmetry. The following values of selected internuclear distances have been determined: r h1 , A: r(Mg–N) = 1.979(5), r(N–C) = 1.363(3), r(N mezo –C) = 1.334(4), r(C pyr –C Ph ) = 1.469(3), r(C Ph –CF 3 ) = 1.510(5), r(C–F) = 1.349(3), r(C α –C β ) = 1.466(3), r(C β –C β ) = 1.380(7). A slight (less than 1 to 2 degrees) twisting deformation of the macrocycle from planarity, caused by the presence of the eight bulky PhCF 3 substituents, planes of which are turned by 132.6(9) degrees relative to the adjacent pyrrole rings, has been found. The deviation of phenyl ring planes from 90 degrees orientation is caused by stabilizing donor–acceptor interactions between π-natural orbitals of pyrrole and phenyl moieties. Substitution effects and coordination bonding in magnesium porphyrazine complexes, MgPz, MgPzPh 8 and MgPz(CF 3 Ph) 8 , are discussed. Sensitivity of GED data to long range interatomic distances of large molecules has been shown.

Experimental and theoretical studies on the structure and vibrational properties of the possible isomers of dinitropyrazoles have been investigated. The infrared and Raman spectra of 1,3-dinitropyrazole, 1,4-dinitropyrazole, 3,4-dinitropyrazole and 3,5-dinitropyrazole were recorded in the solid state. To interpret the experimental infrared and Raman spectra, the vibrational frequency calculations were carried out at the HF/6-311++G(df,pd), B3PW91/6-311++G(df,pd), B3LYP/6-311++G(df,pd) and B3LYP/aug-cc-pVDZ levels of theory. The combined use of experiments and computations allowed a firm assignment of the majority of vibrational bands of dinitropyrazoles. The calculated stretching frequencies were found to be in good agreement with the experimental values. However, the structural parameters and fundamental vibrational frequencies calculated at the B3LYP/aug-cc-pVDZ level are superior compared with those values that are obtained from the HF/311++G(df,pd), B3PW91/311++G(df,pd) and B3LYP/311++G(df,pd) levels. The discrepancies in the structural and vibrational properties are restricted to the relative positions of NO2 groups that present the greatest deviation from the planarity.

In this work, we have analyzed the vibrational, optical and emission properties of soluble copolymer denoted poly(2,5-di-butoxyethoxy-1,4-phenylene-alt-2,5-thienylene) (PBuPT) by combining experimental and theoretical studies. Extrapolated Ionization Potentials (IPs), Electron Affinities (EAs) and energy gaps were determined employing Density Functional Theory (DFT) method. Theoretical analyses have been performed in order to understand deeply the interesting role played by the intra-molecular S⋯O interactions in determining the planarity of the copolymer. Further, an intra-molecular charge transfer for the copolymer has been proposed through the alternating donor–acceptor conjugated systems. In addition, vibrational, optical and emission spectra of three repeat units model compound have been simulated and compared to the experiments. Based on these results, we have described attentively the structure–property relationships of those samples which could be exploited as an active layer in organic optoelectronic devices.

Two isomeric compounds of methoxy substituted benzo[de]benzo[4,5]imidazo[2,1-a] -isoquinolin-7-one were obtained from the one-step ring closure reaction of 2-(2-acetamino-5-methoxyphenyl)benzo[de]isoquinoline-1,3-dione under acidic condition. Their structures were characterized by using HRMS, NMR spectra analyses together with single crystal X-ray diffraction. Compound 2 crystallizes in the monoclinic P2 (1)/n space group with the crystal cell parameters a = 8.3311(11) A, b = 7.6646(10) A, c = 21.201(2) A, α = 90.00°, β = 95.7380(10)°, γ = 90.00°, V = 1347.0(3) A3 and Z = 4. Compound 3 crystallizes in the monoclinic space group P2 (1)/c with the crystal cell parameters a = 3.8270(3) A, b = 15.8361(14) A, c = 28.559(3) A, α = 90.00°, β = 94.1090(10)°, γ = 90.00°, V = 1726.4(3) A3 and Z = 4. X-ray results indicated that it exist non-classically intermolecular interactions in the molecules of the two compounds. The substitution position of methoxy group has no obvious influence on the whole molecular planarity, but results in great different formation modes of their intermolecular interactions and structural stabilization.

Mixtures of trans -hexatriene-1,1-d 2 , - cis -1-d 1 , and - trans -1-d 1 have been synthesized. Anharmonic frequencies and harmonic intensities were predicted with the B3LYP/cc-pVTZ model for the out-of-plane ( a ″) modes of the three isotopologues. Assignments are proposed for most of the a ″ vibrational modes above 500 cm −1 . Ground state (GS) rotational constants have been determined for the 1,1-d 2 and cis -1-d 1 species from the analysis of rotational structure of C-type bands in the high-resolution (0.0015 cm −1 ) infrared spectra in a mixture of the three isotopologues. The GS constants for the 1,1-d 2 species are A 0 = 0.8018850(6), B 0 = 0.0418540(6), and C 0 = 0.0397997(4) cm −1 . The GS constants for the cis -1-d 1 species are A 0 = 0.809388(1), B 0 = 0.043532(2), and C 0 = 0.041320(1) cm −1 . Small inertial defects confirm planarity for both species. These ground state rotational constants are intended for use in determining a semiexperimental equilibrium structure and evaluating the influence of chain length on π-electron delocalization in polyenes.

The infrared spectrum (3200–400 cm −1 ) of the gas and the Raman spectrum (3200–50 cm −1 ) of liquid and solid 1,1-difluoro-1-silacyclopentane and 1,1-dichloro-1-silacyclopentane have been recorded. In all of these physical states only the twisted (C 2 ) conformer was detected. The conformational energies have been calculated with the Moller–Plesset perturbation method to the second order; MP2(Full) as well as the density functional theory by the B3LYP method utilizing a variety of basis sets up to 6-311 + G(2df,2pd). All the calculations predict only the twisted form as the stable conformer of 1,1-difluoro-1-silacyclopentane with the average barrier to planarity of 2548 cm −1 (30.49 kJ/mol) from the MP2 calculations and a significantly lower value of 1422 cm −1 (17.01 kJ/mol) from the density functional calculations. Neither calculation was significantly affected by the inclusion of diffuse functions. The C s conformer has a lower energy of 1703 cm −1 (MP2) and 1334 cm −1 (B3LYP) than the planar form. Thus the path between the two identical C 2 conformers is by pseudorotational motion rather than through the planar form. Similar results obtain for 1,1-dichloro-1-silacyclopentane. The optimized geometry calculated with the 6-311 + G(2df,2pd) basis is given together with a complete vibrational assignment for the twisted (C 2 ) conformer. These assignments are supported by normal coordinate calculations with scaled force constants from MP2(Full)/6-31G(d) calculations. The results of these spectroscopic and theoretical studies are discussed and compared to the corresponding results for silacyclopentane.

N,N ′-Diphenyl- N,N ′-di( m -tolyl)benzidine (TPD) is a well-known hole-transport material for organic light-emitting diodes (OLEDs). Here, we studied the planarity of triphenylamine moieties of TPD in the amorphous state, by the combined use of solid-state 15 N NMR experiments and density functional theory (DFT) calculations. It was shown that the planarity is of crucial importance for hole-transport performance of TPD, because of the strong effects for the shape of HOMO and for the intermolecular repulsion energy. It was also found that the molecular structure around the nitrogen in its amorphous state is not pyramidal, but is instead planar, which is favorable for hole-transport. Natural bond orbital calculation indicates that the nitrogen atom is sp 2 hybridized. This is unlike most amine compounds, which have pyramidal structures with sp 3 hybridized nitrogens.

The ring puckering in the four-membered 3-chloro-1,3-thiaphosphetane 3-oxide, its 3-sulfide derivative and 1,3-dithietane 1,1-dioxide hetero rings were investigated by DFT and ab initio MP2 calculations using a 6-311+G ∗∗ basis set. From the calculations at both levels of theory the asymmetric ring puckering potential energy curves in the two phosphetanes were consistent with an almost single minimum that corresponds to a pseudo - axial configuration (P Cl bond is in axial position) with a puckering angle of about 15–20°. In the case of the dithietane dioxide the symmetric ring puckering potential was consistent at the B3LYP level with a flat minimum that corresponds to a planar ring but at the MP2 level with a double minimum with a very low barrier of about 185 cal/mol to ring planarity. The potential functions that describe the ring puckering in the three molecules were derived at the B3LYP/6-311+G ∗∗ and the MP2/6-311+G ∗∗ levels of calculations. The vibrational wavenumbers were calculated and the potential energy distributions PED among the symmetry coordinates of the normal modes were computed for the stable conformers of each of the molecules. Complete vibrational assignments were provided on the basis of the calculated PED values. The experimental infrared and Raman spectra of the two phosphetane molecules were simulated and compared to the calculated ones.

The infrared spectra (3500–50 cm −1 ) of the gas and solid and the Raman spectrum (3200–30 cm −1 ) of liquid germacyclopentane, c -C 4 H 8 GeH 2 , have been recorded. Additionally, the infrared spectra (3200–400 cm −1 ) of liquid xenon solutions have been recorded at −65 and −95 °C. In all of the physical states only the twisted C 2 conformer was detected. The conformational energetics have been calculated with the Moller–Plesset perturbation method to the second order, (MP2(full)) as well as with density functional theory by the B3LYP method utilizing a variety of basis sets up to 6-311G(2d,2p). All of these calculations predict only the twisted conformer as the stable form with an average barrier to planarity of 2756 cm −1 (32.97 kJ/mol) from the MP2 calculations and a significantly lower value of 2128 cm −1 (25.45 kJ/mol) from the DFT calculations with neither calculations being significantly effect by inclusion of diffuse functions. The C s conformer (envelope) has a lower energy of 700 cm −1 (MP2) and 400 cm −1 (B3LYP) than the planar form. Thus, the path between the two identical C 2 conformers is by a pseudorotational motion rather than through the planar form. From the isolated GeH frequency from the GeHD isotopomer the GeH distance was calculated to be 1.532(2) A. By utilizing the previously reported microwave rotational constants for four isotopomers ( 70 Ge, 72 Ge, 73 Ge, 74 Ge) combined with the structural parameters predicted from the MP2(full)/6-311+G(d,p) calculations, adjusted r 0 structural parameters were obtained. The determined heavy atom distances are: r 0 (GeC) = 1.972(5); r 0 (C 2 C 4 ) and (C 3 C 5 ) = 1.541(5); and r 0 (C 4 C 5 ) = 1.533(5) A and the angles in degrees: ∠CGeC = 93.6(5)°; ∠GeCC = 102.6(5)°; ∠CCC = 109.4(5)° with the two dihedral angles ∠GeCCC = −38.4(3)° and ∠CCCC = 53.7(3)°. A complete vibrational assignment is given for the twisted (C 2 ) conformer for the normal species and the GeD 2 isotopomer which are supported by normal coordinate calculations with scaled force constants from MP2(full)/6-31G(d) calculations. The results of these spectroscopic and theoretical studies are discussed and compared to the corresponding results for some similar molecules.

With the aid of artificial neural network (ANNs), MM2 force field parameters were derived for the molecular mechanics modelling of the metalloporphyrins of five- and six-coordinate Mn(II), Mn(III), Mn(IV), Mn(V), Co(I), Co(II), Co(III), Ni(II) and Cu(II). The previously-derived parameters for modelling the porphyrin ring itself reproduced, on average, bond lengths to within 0.003 A, angles to within 0.3° and torsions to within 2.4° of the mean crystallographic values. As these compounds have a wide variety of axial ligands, and there are relatively few structures available which contain the same metal in the same oxidation state and with the same axial ligands, only preliminary parameters are reported for the modelling of the axial ligands. These were typically derived by setting the strain free bond length and bond angles involving the metal and the axial donor atom to the crystallographically observed value, and varying the bond stretching and angle bending parameter, or the strain free bond length and bond angles iteratively, until the bond length and angles were reproduced to within 0.01 A and 2.5°, respectively. Since, we have previously shown the relative insensitivity of the equatorial metal–ligand parameters to the axial metal–ligand parameters and vice-versa, the necessarily preliminary values of the parameters for the modelling of the axial ligands did not preclude the development of parameters for the modelling of the metal–Nporph bond. To develop these, the strain free bond length lo, and the stretching force constant ks for the bond length was varied in a grid-like pattern and the absolute mean difference between the metal–Nporph bond length observed experimentally and determined by MM was defined as the error function, the minimum of which was found using ANNs. When the structures were modelled with the values of ks and lo that correspond to the minimum of the error function, the mean metal–Nporph bonds differed from crystallographically observed values by at most 0.008 A, which is comfortably within the experimental standard deviation of this metric. The deviations of the porphyrin ring from planarity were often, but not always, reproduced in the modelling. We show that where significant differences were found, these could be due to packing effects in the solid state.

The molecular and crystal structures of 3,8-diphenyl-3a,4,5,5a,8a,8b-hexahydro-benzo[1,2-d: 3,4-d′]diisoxazole have been XRD determined. The compound crystallizes in the monoclinic system (space group P 2 1 / c ) with cell dimensions a =15.278(1), b =9.839(1), c =10.912(2) A, β =92.15(1)°. The structure was solved from 2193 reflections with I ≥2 σ ( I ). The final R was 0.039 for 289 variables. DFT calculations at the B3LYP/6-31G* level afforded a twist-boat conformer almost identical with the X-ray structure as well as a less stable half-chair conformer which lies 9.7 kJ/mole higher in enthalpy. The near planarity of the two fused isoxazoline rings destabilizes the staggered cyclohexane chair and forces the cyclohexane ring to adopt uncommon conformations.

In order to develop the design rules for producing specific conformations of peptides with α, β-dehydro-residues a peptide Cbz-ΔVal-ΔPhe-Ala-OCH 3 was synthesized in solution phase. The crystal structure has been determined by X-ray diffraction method. The structure was refined to an R -value of 0.050. The peptide adopts a type I β-turn conformation with backbone torsion angles of two corner residues, ϕ 1 =−53.9(6)°, ψ 1 =−33.0(6)°, ϕ 2 =−73.7(5)° and ψ 2 =−12.2(6)°. The conformation is stabilized by an intramolecular 4→1 hydrogen bond involving NH of Ala residue as a donor and carbonyl oxygen atom of Cbz group as an acceptor. The torsion angles, χ 1 1 , 1 = 172.8 ( 6 ) and χ 1 1 , 2 = − 6.9 ( 9 ) of ΔVal residue indicate that its side chain is planar while the torsion angles, χ 2 1 = − 9.0 ( 9 ) , χ 2 2 , 1 = − 43.4 ( 10 ) and χ 2 2 , 2 = 130.1 ( 9 ) show that the side chain of ΔPhe deviates considerably from the planarity. This is the first sequence in which ΔVal and ΔPhe are introduced at adjacent positions and the structure reveals clearly that the side chain of ΔPhe is a relatively less rigid than that of ΔVal. The molecules are packed in columns parallel to c -axis.

Artificial neural networks (ANNs) are used as an aid in developing force field parameters for modelling, using molecular mechanics methods and the MM2 force field, the porphyrins of the early transition metals, Sc(III), Ti(II), Ti(III), Ti(IV), V(II), V(III), V(IV), Cr(III), Cr(IV) and Cr(V), in which the metal is either five- or six-coordinate. The porphyrin ring itself was modelled with previously derived parameters and attention was focussed on deriving parameters to model the coordination sphere of the metal. By modelling five-coordinate Zn(II) porphyrins with an axial pyridine ligand, for which there are many structures, we demonstrate that the length of the metal–N porph bond is relatively insensitive to the length of the metal–N axial bond, and vice versa. There are relatively few crystal structures of the early 3d metalloporphyrins available and very few which contain the same axial ligand. Hence, preliminary parameters for modelling a wide variety of axial ligands are reported; these were typically derived by initially setting the strain free bond length and bond angles involving the metal and the axial donor atom to the crystallographically observed value, and varying the bond stretching and angle bending parameter, or the strain free bond length and bond angles iteratively, until the bond length and angles were reproduced to within 0.01 A and 2.5°, respectively. Since the modelling of the [Zn(porphyrin)(pyridine)] complexes demonstrated the relative insensitivity of the equatorial metal–ligand parameters to the axial metal–ligand parameters and vice-versa, the paucity of structures and hence the necessarily preliminary values of the parameters for the modelling of the axial ligands did not preclude the development of parameters for the equatorial ligands. The strain free bond length l o , and the stretching force constant k s for the metal–N porph bond length was varied in a grid-like pattern. The mean difference between the metal–N porph bond length observed experimentally and determined by MM was defined as the error function. The minimum value of the error function was found using ANNs. Modelling the structures with the values of k s and l o that correspond to the minimum of the error function gave mean metal–N porph bonds that differed from crystallographically observed values by at most 0.008 A, within the experimental standard deviation of this parameter. The deviations from planarity found in many of the modelled structures were usually well reproduced in the modelling. Where significant differences were noted, these could sometimes (but not always) be shown to be due to packing forces in the crystal lattice. As expected, the orientation of axial ligands and substituents on the periphery of the porphyrin ring was often significantly different in the modelled and the solid-state structure because of the conformational freedom of these groups.

The X-ray and spectroscopic results clearly indicate that the (+)-2-thiono-17-oxosparteine ( 1 ) and (+)-2,17-ditihionosparteine ( 2 ) are conformationally rigid. In order to analyze deviations of lactam/thiolactam groups from planarity induced by ring constraints, the Dunitz–Winkler approach has been used. The lactam and thiolactam groups are close to planarity, only the lactam group in one of the two independent molecules of 1 is markedly non-planar. The bond angles in the thiolactam and lactam groups are highly diverse. Rings A and C adopt a distorted sofa conformation in both compounds. The distortions in the molecules of 2 as compared with those in the related monothiolactams correspond to the unusual chemical shifts of H5(eq), H5(ax) and H11, as well as to the extremely low J 5ax-6 and extremely large J 5eq-6 coupling constants. Also chemical shifts show a similar regularity being extremely high and low for C2 and C17, respectively.

The calculated geometry of the 2,2′,7,7′-tetraphenyl-9,9′-spirobifluorene, 2,2′,7,7′-tetrakis(biphenyl-4-yl)-9,9′-spirobifluorene and 2,2′,7,7′-tetrakis(9,9′-spirobifluoren-2-yl)- 9,9′-spirobifluorene in the electronic ground and lowest singlet excited states was optimised using the semiempirical Austin Model 1 (AM1) method. The electron absorption and luminescence spectra obtained by ZINDO/S method and the electronic polarisabilities and second-order hyperpolarisabilities obtained by the finite-field method in AM1 approach show that the departure from planarity leads to a reduction of the effective conjugation length. This causes the decrease of the above quantities whereas the mutual orientation of torsion angles is much less important. The good agreement between the theoretical (in planar arrangement) and experimental optical data indicate that the spectral properties of the studied molecules originate in the molecular structure and are well interpreted using the semiempirical methods of quantum chemistry.

2-Thioureidobenzoxazole ( 1 ) was synthesized using 5-amino-3H-1,2,4-dithiazole-3-thione and 2-aminophenol and characterized by X-ray crystallography. The molecule of 1 is essentially planar due to π conjugation and an intramolecular N–H⋯N hydrogen bond also contributes towards its planarity. The crystal lattice is built up of stacks of dimers of 1 generated by N–H⋯S hydrogen bonds. The supramolecular assembly is dictated by weaker N–H⋯S interactions to the exclusion of more stronger N–H⋯X (X=O, N) hydrogen bonds which are normally expected.

Parallel experimental and computational studies of phenyl cis - and trans -n-alkenyl amides (tuberines) have been carried out to probe the structural and dynamic properties with particular attention to interaction between the conjugated enamides chain and the aromatic ring. The cis isomers are non-planar with 40–50° due to steric interactions between the enamide chain and aromatic ring,whereas the trans form is essentially planar. Rotation barriers for the carbonyl group about the C–N bond are predicted to be 18–22 kcal/mol at HF, B3LYP, and MP2 levels using the cc-pVDZ basis set. Despite its significant non-planarity,the cis conformation of tuberine is not found to be significantly less stable than the trans conformation.

Raman spectra of pure synthetic indigo and of Maya blue of a Mexican clay sculpture are compared. The Raman spectrum of Maya blue shows extra bands assigned to vibrational modes of Bu symmetry as well as an increasing intensity of some other bands. The partial removal of the mutual exclusion rule for the centrosymmetric indigo molecule is supposed to indicate a loss of its planarity due to strong adsorption at the palygorscite matrix.

The infrared and Raman spectra of vapor-phase and liquid-phase tetralin (TET) and 1,4-benzodioxan (14BZD) have been recorded and assigned. Calculations for the structures were carried out using the MP2/cc-pvtz (triple zeta) basis set, and the twisting angles were calculated to be 31.4° for TET and 30.1° for 14BZD. The barriers to planarity were calculated to be 4809 and 4093 cm −1 , respectively. Density functional theory calculations for both planar ( C 2 v ) and twisted ( C 2 ) structures were carried out to predict the vibrational frequencies. After scaling, the agreement with the experimental values was excellent for C 2 symmetry. Almost all frequencies below 1350 cm −1 were calculated to be within 10 cm −1 of the experimental values. Frequencies calculated for the C 2 v structures preclude the possibility of many vibrational interactions and hence agree more poorly.

2-Hydroxyacetophenone nicotinic acid hydrazone (H2ApNH) was synthesized as a part of our work, in search for non-linear optical crystal based on hydrazones, and studied spectroscopically. Complete NMR assignments for the hydrazone was made using COSY homonuclear and HMQC heteronuclear correlation techniques. Solid state reflectance was also studied in order to understand the electronic structure of the synthesized compound. The crystal and molecular structures of H2ApNH were determined. The compound crystallizes into an orthorhombic lattice with a non-centrosymmetric space group Pca21 with two crystallographically unique molecules of in an asymmetric unit. The geometry reveals quasi co planarity in the whole molecular skeleton with localization of the double bonds in the CN–N–CO with an E-configuration.

N-2-(4-picolyl)-N′-4-chlorophenylthiourea, 4PicTu4ClPh, triclinic, P-1, a=7.5235(3), b=9.1585(5), c=10.5158(7) A , α=76.015(3), β=70.015(4), γ=82.010(4)°, V=1309.8(2) A 3 and Z=2; N-2-(5-picolyl)-N′-4-chlorophenylthiourea, 5PicTu4ClPh, monoclinic, P21/c, a=15.139(2), b=4.8386(3), c=17.338(2) A , β=90.661(4)°, V=1270.0(2) A 3 and Z=4 and N-2-(6-picolyl)-N′-4-chlorophenylthiourea, 6PicTu4ClPh, monoclinic, C2/c, a=33.520(6), b=4.0750(3), c=18.658(4) A , β=97.500(6)°, V=2526.8(7) A 3 and Z=8. The most striking difference between the structures of the three thioureas is the difference in planarity, and among the four N-2-(picolyl)-N′-4-chlorophenylthioureas, their values for ΔHfus.

The far-infrared, Raman and ultraviolet spectra of phthalan and 1,3-benzodioxole were recorded and analyzed. In addition, the fluorescence excitation spectra and the dispersed fluorescence spectra of the jet-cooled molecules were also investigated. The far-infrared spectrum of phthalan shows single, double, and triple quantum jump transitions between the various ring-puckering energy levels. Many hot bands involving the ring-puckering and ring-flapping vibrations were also observed. The ring-puckering energy level spacings possess an irregular pattern and calculations show this to arise from the kinetic energy interactions between the puckering and flapping vibrations. A two-dimensional potential energy surface, which nicely fits all the observed data, was determined. This has a barrier to planarity of 35 cm−1. Dispersed fluorescence spectra of jet-cooled phthalan molecules helped to confirm the far-infrared assignments, and the fluorescence excitation spectra were recorded to determine the vibrational energy levels from the puckering in the S1(π,π*) electronic excited state. Ultraviolet absorption spectra were used to better understand the excited state energy levels. The far-infrared spectra of 1,3-benzodioxole were also reanalyzed. Because of the anomeric effect, 1,3-benzodioxole is puckered with a barrier to planarity of 125 cm−1. Analysis of the ultraviolet absorption spectra and the fluorescence excitation spectra of the jet-cooled molecules is also in progress and shows that the barrier increases in the S1(π,π*) state.

X-ray diffraction data are reported for two 1-thia-2,3,4-triazolo-5-methylides: I C 22 H 15 N 3 O 2 S, Pbca, a = 19.567(5) A , b = 8.197(2) A , c = 23.299(5) A , V = 3737.0(16) A 3 , Z = 8; II C 13 H 13 N 3 O 3 S, C 2 c , a = 11.605(3) A , b = 22.230(2) A , c = 11.579(2) A , β = 107.94(2)°, V = 2841.9(9) A 3 , Z = 8. The results reveal charge separation between the thiatriazole ring and the exocyclic group, accompanied by partial delocalization of the positive charge over the thiatriazole ring. The geometrical features of the heterocyclic rings in the two compounds are very similar to each other and to those found in analogous compounds. In both structures the geometry of the exocyclic group suggests delocalization of the negative charge directed mainly towards the C  O bond being cis with respect to the ring sulphur atom, resulting in a partial negative charge δ − on the oxygen atom.

The microwave spectrum of acetophenone was observed in the frequency region 9–18 GHz. The rotational and centrifugal distortion constants of acetophenone in the ground and first three excited states of torsion around the CCOCH3 bond have been determined. The determined constants for the ground state are A = 3688.040(11) MHz, B = 1215.048(1) MHz, C = 919.919(1) MHz, ΔJK = −0.250(44) kHz, ΔK = 1.95(68) kHz, and δK = 0.440(22) kHz. The residual inertial defect calculated from the inertial defects of the ground and torsional excited states indicates that the structure of acetophenone is skeletal planar.

The molecular and crystal structure of 2-(2′-hydroxyphenyl)imidazole (2) and 1-methyl-2-(2′-hydroxyphenyl)imidazole (5) have been determined by X-ray analysis. Compound (2) presents a strong intramolecular hydrogen bond (IMHB) responsible for the planarity of the molecule. In both compounds the molecules form chains through NH…O (compound 2) and OH…N hydrogen bonds (compound 5) but giving rise to the same packing mode. Ab initio calculations (6–31G∗∗) have been carried out on both compounds in order to study the effect of the IMHB on the structure.

The structures of six molecules (dibenzodioxine, phenoxathiin, thianthrene, 1-azathianthrene, 1,4-diazathianthrene and 1,4,6,9-tetraazathianthrene) previously studied by X-ray and electron diffraction have been calculated using MM3 and ab initio (3-21G∗ basis set) methods. These computational methods reproduce correctly the experimental bond lengths, bond angles and conformations. Calculations helped to classify all the molecules into two families, where the thianthrene is a common member of both. The calculated barriers to planarity are found to correlate with both the calculated and the observed puckering angles of the molecules.

The molecular structures of styrene and (Z)-β-bromostyrene have been studied in the gas phase at nozzle temperatures of 303 and 338 K respectively. For both molecules the electron diffraction data were consistent with the results from ab initio calculations which described the vinyl torsional motion, near the planar configurations, in terms of a double minimum potential function with barriers of 243 cal mol−1 (styrene) and 430 cal mol−1 (bromostyrene) at the planar form, and with the minimum energy forms 27° (styrene) and 39° (bromostyrene) away. The perpendicular barriers were calculated to 2.73 kcal mol−1 (styrene) and 1.10 kcal mol−1 (bromostyrene). The important distances (ra) and angles (∠α) obtained from least squares refinements of the electron diffraction data are as follows: styrene, r(CH)Av = 1.102(7) A, r( CC ) = 1.355(16) A , r( CC ) Ph = 1.399(2) A , r( CC ) = 1.475(23) A , ∠CCC = 126.9(24)°; and bromostyrene, r(CH)Av = 1.082(13) A, r(CC) = 1.331(20) A, r( CC ) Ph = 1.400(2) A , r(CC) = 1.465(20) A, r( CBr ) = 1.893(8) A , ∠CCC = 132.8(23)°, ∠BrCC = 125.7(15)°, ∠C2C1C7 = 123.9(33).

The electron density distribution calculated for 2,2,6,6-tetramethyl-1-piperidinyloxy ( 1 ), 4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy ( 2 ) and 3-carbamoyl-2,2,5,5-tetramethyl-3-pyrrolin-1-yloxy ( 3 ) free radicals on the basis of low-temperature X-ray single crystal diffraction experiments revealed some differences in region of the >N–O groups. Generally, the experimental bonding density along the axis of the N–O bond is relatively low and asymmetric in cases of the pyramidal geometry of the aminoxyl group ( 1 and 2 ) and almost symmetrical for the planar geometry ( 3 ). The shape and distribution of electron densities in the region of the oxygen lone-pairs suggest that this atom is sp 3 hybridized but the spatial orientation of these maxima depends on hydrogen bond interactions (>N–O⋯H) in studied crystals. Also the relation between the direction of the relatively strong hydrogen bonds (O⋯H

The dependence of N–H stretching-mode frequencies in representative di- and trialkyl ureas on the conformational state of the ureido group has been studied by ab initio MO calculations using HF/3-21G and HF/6-31G** basis sets. The molecules studied were 1,3-dimethylurea, 1-methyl-3,3-dimethylurea and 1-methyl-3,3-di- iso -propylurea. The principal conclusions from the ab initio results are: 1. the trans – trans conformer (N–H bonds trans to the CO bond) has N–H stretching bands with about 20–30 cm −1 higher frequency than the respective cis – cis structure, in accord with earlier literature assignments based on experimental data; 2. the N–H stretching frequency interval in tri-substituted ureas is 15–20 cm −1 higher than the N–H band position in the 1,3-disubstituted molecule studied, the effect being determined mostly by the higher N–H stretching force constant; 3. in the absence of the steric hindrance the stable rotameric forms of the ureido grouping are almost planar at HF/3-21G level of calculations, while HF/6-31G** calculations predict a slightly pyramidal structure at the nitrogen atoms in the trans – trans conformer; 4. in 1-methyl-3,3-di- iso -propylurea the steric influence of the two bulky iso -propyl groups cause a deviation from planarity of the N–H bond. The non-planar conformation is accompanied by a shift of the N–H stretching mode frequency towards higher values; and 5. the variations of the theoretically estimated N–H stretching-mode frequencies appear to be principally determined by changes in the N–H stretching force constants in the different molecules. © 1997 Elsevier Science B.V.

The thermal-average parameters of BBr 3 at 21(1) °C were obtained from a conventional analysis of gas electron diffraction (GED) data ( r g (BBr) = 190.0(4) pm). The equilibrium structure and the force constants were refined from a joint analysis of the GED intensities and vibrational frequencies using different approximations. The simplest approximation (quadratic potential function in rectilinear coordinates) is suitable for the refinements of the equilibrium bond length ( r h e (BBr) = 189.6(4) pm) and the force constants of BBr 3 . The molecule is planar within the error limits. Quantum-chemical density-functional calculations supported planarity of the molecule.

The microwave spectrum of propionamide has been investigated in the 21.4–39 GHz spectral range. One conformer has been assigned. This rotamer has a C s equilibrium conformation and the methyl group is syn to the carbonyl group. The dipole moments are μ a = 2.121(20), μ b = 11.66(10) and μ tot. = 11.85(10) × 10 −30 C m. The nuclear quadrupole coupling constants of the 14 N nucleus are χ aa = 2.2(8) and χ bb = 2.3(5)MHz. The barrier to internal rotation of the methyl group is 9.1(5) kJ mol −1 . Six vibrationally excited states of the torsion around the C2–C6 bond were assigned and this fundamental frequency was found to be 45(7)cm −1 . These vibrationally excited states were used to approximate the potential function for torsion near its bottom as V = 7.0 (〈 z 4 〉 + 2.0〈 z 2 〉) cm −1 . This function implies that there is no (or a very small) potential hump at the heavy-atom planar conformation. The first vibrationally excited state of the methyl-group torsion was also assigned. Ab initio computations have been made at the 6–31G ∗ , 6&3ndash;11G ∗∗ , 6–311+ G ∗ , MP 2 6–311 ++ G ∗∗ and MP 3 6–311 ++ G ∗∗ levels of theory. The ab initio calculations imply that there is no second stable form of propionamide. These computations also predict a non-planar heavy-atom conformation at all levels of theory. The Hartree-Fock calculations predict a very small barrier to planarity which is not in complete disagreement with the microwave data. However, the computations involving electron correlation are in obvious disagreement with the experimental findings because a rather high barrier to planarity of 1.6 kJ mol −1 is predicted.

The molecular structure of diphenylchloroborane, (C 6 H 5 ) 2 BCl, has been determined by gas-phase electron diffraction (GED) at 415 K and by ab initio MO calculations at the HF/3-21G∗ level, both being consistent with C 2 molecular symmetry. Corresponding ab initio fundamental frequencies are reported and an approximate HF/3-21G∗//HF/3-21G∗ scaled quantum mechanical (SQM) force field was established, transferring scale factors from a concurrent study of dichlorophenylborane, C 6 H 5 BCl 2 . Vibrational amplitude quantities needed in the GED structural analysis were computed from the SQM force field. Planarity of the phenyl groups and the ClBC 2 framework was assumed in addition to C 2 molecular symmetry, and the geometrical parameters determined in the GED analysis, with effects from correlation in the data and uncertainty in the s -scale (0.1%) included in the standard deviations are: r a (CH)(para) = 110.8(6) pm, r a (CC)(ortho) = 141.0(2) pm, r a (BC) = 155.5(7) pm, r a (BCl) = 178.1(8) pm, ∡ α C  B  Cl = 117.2(5)° , and ф α ( B  C ) = 29.5(1.8)° . Other geometry-defining parameters such as tilt of the phenyl groups and their distortion from the benzene structure, for example ∡ α C  C  C(ipso ) = 117.9° , were taken from the computational structure. For comparison purposes the HF/3-21G∗ computational structures are also reported for the three other molecules in the four-molecule series of (C 6 H 5 ) 3− n BCl n , n = 0, 1, 2, 3. For the three phenylboranes ( n = 2, 1, 0) there is an elongation of the BC bonds as the number of phenyl groups increases (154.4, 155.8 and 157.2 pm, with ф-values of 0, 27.7 and 33.7°), whereas for the chloroboranes ( n = 1, 2, 3) the BCl bonds become shorter (179.8, 176.9 and 174.7 pm) with an increasing number of chlorines attached to boron.

Infrared and Raman spectra of the vapor, liquid, and solid phases of 1,3-dioxole have been recorded and analyzed. Much of the spectra can be interpreted assuming C 2v symmetry. However, several combination bands with the ring-puckering vibration along with the observation of an otherwise inactive mode confirm the non-planarity of this molecule. The observed frequencies are compared with predicted values from molecular mechanics (MM3) and ab initio (STO3-21G∗) calculations. These calculated values provide useful estimates but about half of them differ from the observed values by more than 50 cm −1 . Several predicted values disagree by more than 200 cm −1 .

Ab initio and semi-empirical calculations have been carried out on six cyclic cross-conjugated compounds: fulvene, cyclopentadienone, 2,5-cyclohexadienone, 4,4-dimethyl-2,5-cyclohexadienone, 3-methylene-1,4-cyclohexadiene and 3,3-dimethyl-6-methylene-1,4-cyclohexadiene. The results are compared with experimental data as far as it is available.

The unstable 3-methylene-1,4-cyclohexadiene, an isomer of toluene, has been prepared from benzoic acid. The hydrocarbon can be handled quite easily in the solid state, and in the gas phase at low pressures. Rotational transitions have been measured from 12 to 40 GHz. Sixty-three lines were assigned with J-quantum numbers up to 60. From the least-squares fit with a standard deviation of 23 kHz the three rotational constants and all five quartic centrifugal distortion constants from Watson's A-reduction could be determined as A = 5177.8216(37) MHz, B = 2613.1518(10) MHz, C = 1755.8422(9) MHz, Δj = 0.1384(65) kHz, ΔJK = 0.135(16) kHz, ΔK = 1.074(50) kHz, δJ = 0.0440(11) kHz and δK = 0.333(24) kHz (representation IR used). The dipole moment was found to be 0.8645(31) D from 32 frequency shifts in total of the M components of the transition 835 ← 734. The 6-monodeuterated species has also been prepared and its microwave spectrum measured. Using Kraitchman's equations, the coordinates of the ring methylene hydrogens could be determined. The pseudoinertial defect was found to be −0.1435(56) A2 which proves that the molecule possesses a planar ring. The results are compared with the data for 4,4-dimethyl-2,5-cyclohexadien-1-one.

The molecular structure of methylene cyclobutane has been studied by combined analysis of electron diffraction and spectroscopic data in the vapour phase. The molecule was found to undergo large-amplitude ring-puckering motion with the barrier at the planar form. The minima are determined to be 21.6(23)° out of planarity. The results for some of the more important bond lengths( r g ) and valence angles (∠ α ) are: r g (CH)=1.101(3) A, r g (CC)=1.331(2) A, r g (CC)= 1.524(4) A, r g (CC) = 1.557(3) A, ∠HCH av = 106(2)° and [∠C 4 C 1 C 2 ] 0 = 92.0(3)°.

A vibrational analysis of the electron diffraction intensity data of iron trifluoride provides firm evidence for the planarity of this molecule. The optimized deformation frequencies of iron trifluoride, ν 2 110(17) and ν 4 158(17) cm −1 , appear to be much lower than the available estimates. The optimized symmetric streching frequency for aluminium trifluoride is 683(54) cm −1 from the joint analysis of electron diffraction intensities and vibrational frequencies. Force constants are also obtained for both molecules.

Results of X-ray structure and FTIR investigations for 2-(2,4,6-trimethylphenyl)-adamantan-2-ol are presented. C 19 H 26 O, M r = 270.41, monoclinic, C 2/ c , a = 35.458(5), b = 8.2142(9), c = 10.0701(6) A, β = 97.928(9)°, V = 2905.0(6) A 3 , Z = 8, D x = 1.237 g cm −3 , Mo K α, λ = 0.71073 A, μ = 0.7 cm −1 , F (000) = 1184, T = 150 K, R = 0.042 for 2015 observed reflections. The methyl groups ortho to the ring junction cause severe geometric distortions. The phenyl ring deviates from hexagonal symmetry and planarity. The ortho methyl groups display large angular deformations. There are several very short intramolecular H---H contacts ranging from 2.00(3) to 2.29(3) A. The hydroxyl group is not involved in any OH⋯O type hydrogen bond; however the FTIR results (the position and dichroic ratio of the v (OH) band) indicate that a weak intermolecular OH⋯π type interaction is present in the crystal.

The microwave spectrum of ( 35 Cl, 11 B)-1-chloroborepin was obtained and the rotational constants in the ground state derived. The principal moments of inertia obtained from the rotational constants were used to calculate the inertia defect Δ =−0.19 uA 2 . From this evidence, we conclude that 1-chloroborepin is a planar molecule.

The molecular structures of 1-silacyclopent-3-ene and 1,1-difluoro- and 1,1-dichloro-substituted species have been studied by electron diffraction. All three molecules have structures of C 2v symmetry, but there are apparent deviations from planarity of the rings caused by ring-puckering vibrations, the amplitudes of which are greatest for the fluoride, and smallest for the chloride. For 1-silacyclopent-3-ene important parameters ( r a ) are as follows: r (SiC) = 189.9(3) pm; r (CC) = 153.3(4) pm; r (CC) = 135.9(6) pm; ∠ (CCC) = 118.4(2)°. For 1,1-difluoro-1-silacyclopent-3-ene, r (SiF) = 158.3(3) pm; r (SiC) = 184.7(3) pm; r (CC) = 137.8(7) pm; ∠ (CCC) = 117.9(3)°. For 1,1-dichloro-1-silacyclopent-3-ene, r (SiCl) = 206.6(4) pm; r (SiC) = 187.6(6) pm; r (CC) = 152.6(7) pm; r (CC) = 133.0(10) pm; ∠ (CCC) = 120.3(10)°; ∠ (ClSiCl) = 103.9(5)°.

A normal coordinate analysis for the AlCl 3 · NH 3 complex is performed. Calculations for a hypothetical complex with a planar AlCl 3 ligand are included. A deviation from planarity is found in the realistic complex. Force fields for free NH 3 and AlCl 3 molecules are employed in the construction of an initial approximate force field. The theory of kinematic coupling is used to study the frequency shifts from free to complexed ligands. A final force field is developed so as to reproduce exactly a set of observed frequencies from the literature. Calculations on mean amplitudes and perpendicular amplitude correction coefficients are reported.

The microwave spectrum of p -fluorostyrene has been studied in the frequency region 18.0–26.5 GHz. Rotational transitions in the ground and several vibrationally excited states have been identified. The molecule has been shown to be planar in the ground state. From intensity measurements, the frequency of the first torsional transition has been estimated to be 30 ± 15 cm −1 . The potential barrier to the internal rotation is briefly discussed.

The molecular structure of gaseous dichloromaleic anhydride has been investigated by electron diffraction at a nozzle-tip temperature of 164–170°C. The molecule is planar to within experimental error, but small deviations from planarity corresponding to torsion up to about 10° around the carbon-carbon single bonds cannot be ruled out. Values of the more important r α distances and angles with estimated 2σ uncertainties are r (CO) = 1.188(2) A, r (CC) = 1.332(5) A, r (C-O) = 1.389(3) A, r (C—C) = 1.495(3) A, r (C—Cl) = 1.685(2) A, ∠CC-Cl = 129.4(2)°, ∠C-CO = 128.5(4)° and ∠CC—C = 107.9(2)°. The shortening of the carbonyl bond relative to that in maleic anhydride itself is discussed in terms of a possible general effect of vicinal substitution.

The molecular structure and conformation of 1,2,6-cyclononatriene were studied by the gas electron diffraction method. Two different models, differing primarily in the degree of distortion from planarity of the bonds at the isolated CC double bond, give satisfactory agreement with the experimental data.