Inter-ring Dihedral Angle Research Articles

Effect of torsional motion on the 13C, 1H and 19F NMR chemical shifts in 2,2′-difluorobiphenyl

Abstract Contribution of inter-ring torsional motion of 2,2′-difluorobiphenyl (DFBPh) to the NMR chemical shifts (CSs) and lineshapes is investigated using B3LYP/aug-cc-PVDZ computations. The anharmonic potential energy curve for this vibrational motion is calculated by a relaxed scan over the whole span of its coordinate, inter-ring dihedral angle D. The NMR CS curves for the most affected 1H, 13C and 19F nuclei are calculated as functions of D for these relaxed structures using IGAIM method. Lineshapes arising from the unaveraged NMR CSs are simulated for each vibrational state, and their corresponding state averages and the thermally averaged values at two temperatures are calculated. Lineshape broadening in slow regime is significant especially for the 19F nuclei. The averaged CSs differ slightly, but detectably, from their corresponding equilibrium values. It is thus concluded that for accurate evaluation of the NMR CSs and experimentally compatible NMR spectra, such vibrational analysis is necessary.

Applying strong external electric field to thiophene-based oligomers: A promising approach to upgrade semiconducting performance.

A key parameter dictating the rate of charge transfer (CT) is reorganization energy (λ), an energy associated with geometry changes during hole/electron transfer. We show that "ironing" the inter-ring dihedral angles of oligothiophenes via proper substitutions or insertions (e.g., -OR, -F or -C≡C-), decreases the λ and thus promotes CT according to Marcus equation. Our results demonstrate, to attain a smaller λ, extending oligomer length is only significant if the flattened backbone structure is realized. Of great interest is that external electric fields, which are ubiquitous in electronic devices yet commonly overlooked in the computation of λ, can have a significantly greater impact than conventional substitutions. It is important to emphasize, the responses of λ to external fields is system-dependent. Compared to fused-ring conjugated systems, single-bond connected thiophenes are more sensitive to external fields. Fx lowers the λ (552 meV) of quaterthiophene by almost 80% at the intensity of 1 V/Å, down to a value (125 meV) which is even lower than that of pentacene (154 meV) and rubrene (219 meV) at the same level of theory. © 2016 Wiley Periodicals, Inc.

Ultrafast photo-induced nuclear relaxation of a conformationally disordered conjugated polymer probed with transient absorption and femtosecond stimulated Raman spectroscopies.

A combination of transient absorption (TAS) and femtosecond stimulated Raman (FSRS) spectroscopies were used to interrogate the photo-induced nuclear relaxation dynamics of poly(3-cyclohexyl,4-methylthiophene) (PCMT). The large difference in inter-ring dihedral angles of ground and excited-state PCMT make it an ideal candidate for studying large-amplitude vibrational relaxation associated with exciton trapping. Spectral shifting in the S1 TA spectra on sub-ps timescales (110 ± 20 and 800 ± 100 fs) is similar to spectroscopic signatures of excited-state relaxation observed with related photoexcited conjugated polymers and which have been attributed to exciton localization and a combination of resonant energy transfer and torsional relaxation, respectively. Measurements made with both techniques reveal fast PCMT S1 decay and triplet formation (τS1 = 25-32 ps), which is similar to the excited-state dynamics of short oligothiophenes and highly twisted polyconjugated molecules. On ultrafast timescales FSRS of S1 PCMT offers a new perspective on the nuclear dynamics that underlie localization of excitons in photoexcited conjugated polymers: Spectral dynamics in the C=C stretching region (1400-1600 cm(-1)) include a red-shift of the in-phase C=C stretching frequency, as well as a change in the relative intensity of in-phase and out-of-phase stretch intensities on a timescale of ∼100 fs. Both changes indicate an ultrafast vibrational distortion that increases the conjugation length in the region of the localized excitation and are consistent with exciton self-localization or trapping. Wavelength-dependent excited-state FSRS measurements further demonstrate that the C=C stretching frequency provides a useful spectroscopic handle for interrogating the degree of delocalization in excited conjugated polymers given the selectivity achieved via resonance enhancement.

2,3-Diphenyl-2,3-di-hydro-4H-pyrido[3,2-e][1,3]thia-zin-4-one.

In the racemic title compound, C19H14N2OS, the two phenyl substituents on the 1,3-thia­zine ring are almost perpendicular to the pyridine ring which is fused to the thia­zine ring [inter-ring dihedral angles = 87.90 (8) and 85.54 (7)°]. The dihedral angle between the two phenyl rings is 75.11 (7)°. The six-membered thia­zine ring has an envelope conformation with the ortho-related C atom forming the flap. The crystals exhibit face-to-edge aromatic-ring interactions with the nearest C—H⋯C distance equal to 3.676 (3) Å.

2-(5-Methyl-1,3,4-oxadiazol-2-yl)phenyl acetate

In the title compound, C11H10N2O3, which is a potential bioactive compound, the benzene and oxa­diazole rings are approximately coplanar, with an inter-ring dihedral angle of 4.14 (2)°, while the ester plane is rotated out of the benzene plane [dihedral angle = 82.69 (9)°]. In the crystal, the mol­ecules form layers down the a axis with weak π–π inter­actions between the oxa­diazole and benzene rings [minimum ring centroid separation = 3.7706 (14) Å].

Torsional deformation effect on the N—H bond dissociation energy in diphenylamine

Abstract In this work, the influence of the inter-ring dihedral angles and their deformation on the energetics of diphenylamine molecule and its radical is investigated by the B3LYP/6-311++G** approach. Our approximated bond dissociation enthalpy of diphenylamine is 370.0 kJ mol-1 and it is in good agreement with the recently published experimental data. The potential functions of both the molecule and the radical with respect to the mutual aromatic ring orientations are presented. The potential function for the molecule is of a double-barrier type, whereas the radical possesses a single-barrier function. The calculated total electronic energies are used to approximate the change of the bond dissociation enthalpy with the twisting of the dihedral angle. The dependence of the bond dissociation enthalpy on the dihedral angle is represented by a single-barrier type function. The dependence of the nitrogen atom spin density on the studied dihedral angle is also discussed.

4-Methoxyanilinium 2-carboxy-4,5-dichlorobenzoate

In the title salt C7H10NO+·C8H3Cl2O4 − the benzene rings of the cation and anion are essentially parallel [inter-ring dihedral angle 4.8 (2)°]. In the anion the carb­oxy­lic acid and carboxyl­ate groups make dihedral angles of 19.0 (2) and 79.5 (2)°, respectively, with the benzene ring. Aminium N—H⋯O, carb­oxy­lic acid O—H⋯O and weak aromatic C—H⋯O hydrogen-bonding associations with carboxyl O-atom acceptors together with cation–anion π–π ring inter­actions [minimum ring centroid separation = 3.734 (3) Å] give rise to a sheet structure lying parallel to (001).

Ethyl N-[2-(4-phen-oxy-phen-oxy)eth-yl]carbamate.

The title compound, C17H19NO4, which is a non-toxic insect growth regulator with the common name fenoxycarb, contains two independent and conformationally different mol­ecules in the asymmetric unit. Although the inter-ring dihedral angles are similar [62.21 (15) and 63.00 (14)°], the side-chain orientations differ. In the crystal, the mol­ecules are linked through N—H⋯O hydrogen-bonding associations, giving chains which extend along [110], while intra- and inter­molecular aromatic C—H⋯π inter­actions give sheet structures parallel to [110].

(E)-1-[4-(Methylsulfanyl)phenyl]-2-(2,3,4-trimethoxyphenyl)ethene

In the title compound, C18H20O3S, the rings are almost coplanar [inter-ring dihedral angle = 6.6 (2)°]. In the crystal, weak C—H⋯O hydrogen bonds between the meth­oxy groups connect adjacent mol­ecules, giving chains which extend along [001].

Dimethyl 5,5′-methylenebis(2-hydroxybenzoate)

In the title compound, C17H16O6, the two methyl salicylate moieties are related by crystallographic twofold rotational symmetry with the two benzene rings close to being perpendicular [inter-ring dihedral angle = 86.6 (8)°]. Intra­molecular phenolic O—H⋯O hydrogen bonds with carboxyl O-atom acceptors are present, with these groups also involved in centrosymmetric cyclic inter­molecular O—H⋯O hydrogen-bonding associations [graph set R 2 2(4)], giving infinite chains extending across (101).

Rac-7,7′,9,9′-Tetraphenyl-9a,9a′-bi(7,8,9,9a-tetrahydro-6aH-pentaleno[1,2,3-ij]naphthalen-8-one)

The racemic title compound, C54H38O2, consists of two C-linked penta­leno[1,2,3-ij]naphthalenone moieties, the crowded aryl ring substitution on the cyclo­pentane rings forcing the two segments to assume a conformation which has pseudo-twofold rotational symmetry, with a dihedral angle between the naphthalene substituent groups of 55.30 (8)°. In each segment, the two phenyl rings have different conformational orientations, with inter-ring dihedral angles of 34.7 (2) and 49.63 (16)°. Each cyclo­pentane ring has the same relative configuration in its four chiral centres and together with the fused naphthalene ring assumes an overall chair-like conformation.

2,3-Dimethoxy-10-oxostrychnidinium 2-(2,4,6-trinitroanilino)benzoate monohydrate: a 1:1 proton-transfer salt of brucine witho-picraminobenzoic acid

In the structure of the title 1:1 proton-transfer compound of brucine with 2-(2,4,6-trinitroanilino)benzoic acid, C(23)H(27)N(2)O(4)(+)·C(13)H(7)N(4)O(8)(-)·H(2)O, the brucinium cations form classic undulating ribbon substructures through overlapping head-to-tail interactions, while the anions and the three related partial solvent water molecules (having occupancies of 0.73, 0.17 and 0.10) occupy the interstitial regions of the structure. The cations are linked to the anions directly through N-H...O(COO(-)) hydrogen bonds and indirectly by the three water molecules, which form similar conjoint cyclic bridging units [graph set R(2)(4)(8)] through O-H...O(C=O) and O-H...O(COO(-)) hydrogen bonds, giving a two-dimensional layered structure. Within the anion, intramolecular N-H...O(COO(-)) and N-H...O(nitro) hydrogen bonds result in the benzoate and picrate rings being rotated slightly out of coplanarity [inter-ring dihedral angle = 32.50 (14)°]. This work provides another example of the molecular selectivity of brucine in forming stable crystal structures, and also represents the first reported structure of any form of the guest compound 2-(2,4,6-trinitroanilino)benzoic acid.

1-(3-Chlorophenyl)-3-(4-nitrophenyl)urea

In the title compound, C13H10ClN3O3, prepared by the reaction of 1-chloro-3-isocyanato­benzene with 4-nitro­benzenamine, the two substituent benzene rings are roughly coplanar [inter-ring dihedral angle = 8.70 (7)°]. In the crystal, mol­ecules make cyclic inter­molecular associations through two urea–nitro N—H⋯O hydrogen bonds, forming a chain structure [give chain direction] in which there are also weak inter­molecular C—H⋯Cl inter­actions. The urea O atom has only intra­molecular aromatic ring C—H⋯O associations.

Exploiting Conformational Dynamics To Facilitate Formation and Trapping of Electron-Transfer Photoproducts in Metal Complexes

Three new photoinduced electron donor-acceptor (D-A) systems are reported which juxtapose a Ru(II) excited-state donor with a bipyridinium acceptor via a conformationally active asymmetric aryl-substituted bipyridine ligand participating in the bridge between D and A. Across the series of complexes 1-3, steric bulk is sequentially added to tune the inter-ring dihedral angle theta between the bipyridine and the aryl substituent. Driving forces for photoinduced electron transfer (DeltaG(ET)) and back electron transfer (DeltaG(BET)) are reported based on electrochemical measurements of 1-3 as well as Franck-Condon analysis of emission spectra collected for three new donor model complexes 1'-3'. These preserve the substitution patterns on the aryl substituent in their respective D-A complexes but remove the bipyridinium acceptor. Both DeltaG(ET) and DeltaG(BET) are invariant to within 0.02 eV across the series. Upon visible photoexcitation of each of the D-A systems with approximately 100 fs laser pulses at 500 +/- 10 nm, an electron-transfer (ET) photoproduct is observed to form with a time constant of tau(ET) = 29 ps (1), 37 ps (2), and 57 ps (3). That ET remains relatively rapid throughout this series, even as steric bulk significantly increases the inter-ring dihedral angle theta, is attributed to the effects of ligand-based torsional dynamics driven by intraligand electron delocalization in the D*-A excited state manifold prior to ET. The lifetimes of the charge-separated states (tau(BET)) are also reported with tau(BET) = 98 ps (1), 217 ps (2), and 789 ps (3), representing a more than 8-fold increase across the series. This is attributed to reverse conformational dynamics in D(+)-A(-) driven by steric repulsions, which serves to minimize electronic coupling to the ground state. Steric control of ligand geometry and the range over which theta changes during conformational dynamics provides a new strategy to facilitate the formation and storage of charge-separated excited states.

Modeling the Structural and Electronic Properties of an Optically Active Regioregular Polythiophene

A comprehensive theoretical study about the structural and electronic molecular properties of the optically active regioregular poly[(R)-3-(4-(4-ethyl-2-oxazolin-2-yl)phenyl)thiophene] and its model oligomers has been developed with use of quantum mechanical calculations. Results show two stable arrangements for the investigated oligomers. The first one consists of a plain structure with all the inter-ring dihedral angles arranged in a syn-gauche+ conformation rather than the anti-gauche+ typically found in substituted polythiophenes. This produces a reduction of the steric repulsions allowing the formation of a π-stacking alignment between phenyl groups of adjacent monomers. The second is a helical structure with six repeating units per turn that is constructed by alternating the syn-gauche+ and syn-gauche− conformations. The lowest π−π* transition energy predicted for an idealized polymer chain, which was derived from Density Functional Theory calculations, is in agreement with the reported experimental values obtained by UV−vis spectroscopy.

Poly(2-thiophen-3-yl-malonic acid), a Polythiophene with Two Carboxylic Acids Per Repeating Unit

A new substituted polythiophene derivative bearing malonic acid, poly(2-thiophen-3-yl-malonic acid), has been prepared and characterized using a strategy that combines both experimental and theoretical methodologies. The chemical structure of this material has been investigated using FTIR and (1)H NMR, and its molecular conformation has been determined using quantum mechanical calculations. Interestingly, the arrangement of the inter-ring dihedral angles was found to depend on the ionization degree of the material, that is, on the pH, which has been found completely soluble in aqueous base solution. Thus, the preferred anti-gauche conformation changes to syn-gauche when the negatively charged carboxylate groups transforms into neutral carboxylic acid. UV-vis experiments and quantum mechanical calculations on model systems with a head-to-tail regiochemistry showed that the lowest pi-pi* transition energy is 2.25 and 2.39 eV for the negatively charged and the neutral polymer, respectively. These values are slightly larger than those previously reported for other polythiophenes with bulky polar side groups. The polymer presents a good thermal stability with a decomposition temperature above 215 degrees C and an electrical conductivity of 10(-5) S/cm, which is characteristic of semiconductor materials. Scanning electron microscopy micrographs showed that, after doping, the surface of this material displays regular distribution pores with irregular sizes. This surface suggests that poly(2-thiophen-3-yl-malonic acid) is a candidate for potential applications such as selective membranes for electrodialysis, wastewater treatment, or ion-selective membranes for biomedical uses.

Parameterization of the torsional potential for calix[4]arene‐substituted poly(thiophene)s

Three different strategies have been followed to develop the torsional force-field parameters of the inter-ring dihedral angles for calix[4]arene-substituted poly(thiophene)s, a family of highly sensitive ion receptors. These procedures, which are based on the rotational profiles calculated using quantum mechanical methods, differ in the complexity of the model compounds and the processing applied to the quantum mechanical energies before the fitting. The performance of the three sets of developed parameters, which are essentially compatible with the General Amber Force Field, has been evaluated by computing the potential of mean forces for the inter-ring rotation of 2,2'-bithiophene, and its substituted analog bearing a calix[4]arene group in different environments. Finally, the ability of the new sets of torsional parameters to describe a calix[4]arene-substituted poly(thiophene) in tetrahydrofuran solution has been checked using Molecular Dynamics simulations. Specifically, the molecular shape, the polymer conformation, and the effects of the Na(+) ions trapped in the cavity of the receptor have been examined. Although the potential derived from unsubstituted 2,2'-bithiophene is able to reproduce the experimental free energies of the minima, the overall results indicate that the parameters derived from the analog bearing a calix[4]arene group provide the best description of the systems under study. This should be attributed to the strong constraints found in complex substituted poly(thiophene)s, which require parameterization strategies able to capture all the interactions and phenomena involved in their inter-ring rotations.

Characterization and properties of a polythiophene with a malonic acid dimethyl ester side group

A new polythiophene derivative bearing a malonic acid dimethyl ester substituent attached to the 3-position of the repeat unit has been prepared by chemical oxidative-coupling polymerization. The chemical structure of poly(2-thiophen-3-yl-malonic acid dimethyl ester) has been analyzed by FTIR and 1H NMR spectroscopy and, additionally, the distribution of the head-to-tail and head-to-head diads arising from polymerization was found to be a 75–25%. The glass transition temperature identified for this polymer was 17.6 °C lower than that recently determined for a closely related polythiophene derivative, in which the ester substituent arose from acrylic acid rather than from malonic acid. On the other hand, the electrical conductivity of the new material, which was soluble in polar solvents but not in water, was higher than that typically found for poly(3-alkylthiophene) derivatives. Ab initio quantum mechanical calculations on simple model compounds were used to predict the regiochemistry of the polymer chain, which was in excellent agreement with the experimental observation, and the conformational preferences of both the inter-ring dihedral angle and the bulky side group. Interestingly, calculations predict that the inter-ring dihedral angles adopt a syn-gauche conformation rather than the anti-gauche arrangement typically found in substituted polythiophenes. Thus, in this case the former conformation reduces the strong repulsive interactions induced by the bulky substituent. The lowest π–π ∗ transition energy derived from calculations on an idealized molecular model is in agreement with the experimental estimation determined using UV–vis spectroscopy. This electronic property is significantly higher for poly(2-thiophen-3-yl-malonic acid dimethyl ester) than for other substituted polythiophene derivatives, which is consequence of the geometrical distortions induced by bulky side group.

N-(Pyrimidin-2-yl)aniline

There are two molecules in the asymmetric unit of the title compound, C10H9N3, with inter-ring dihedral angles of 31.1 (1) and 35.3 (1)°. The bridging C—N—C bond angles are 128.2 (1) and 129.1 (1)°. In the crystal, the two independent mol­ecules are linked into a dimer by two N—H⋯N hydrogen bonds.

Structural and Electronic Properties of Crown Ether Functionalized Oligothiophenes

Quantum mechanical calculations at the B3LYP level combined with the 6-31G(d) and 6-31+G(d,p) basis sets have been performed on oligomers containing n repeating units (with n ranging from 2 to 8) of three crown ether functionalized polythiophenes, which differ in the size of the macrocycle. Results have been compared with those obtained for poly(3,4-ethylenedioxthiophene), which was calculated at the same theoretical levels. Results indicate that, independently of the size of the ether macrocycle, functionalized oligomers tend to adopt an antiplanar conformation with the inter-ring dihedral angles arranged in trans. Moreover, the rotational profile calculated by scanning the inter-ring angle is not significantly influenced by neither the size of the macrocycle nor its conformation. On the other hand, the electronic properties of the crown ether functionalized polythiophenes, which were obtained by extrapolating the results obtained for the oligomers, were very similar to those of poly(3,4-ethylenedioxythi...