Perpendicular Conformation Research Articles

Solvent effects on the torsional dynamics of a twisted intramolecular charge transfer (TICT) molecule: bianthryl in acetonitrile

Molecular dynamics simulations have been used to study the torsional potential surface and dynamics of 9,9′-bianthryl (BA) in the polar aprotic solvent acetonitrile. In the gas phase, the ground state (S 0) torsional potential has a minimum at the perpendicular conformation with a twist angle of 90°, while for the first excited singlet electronic state (S 1), the 90° twist angle corresponds to a local maximum with symmetric minima located at ≈ 70° and ≈ 110°, respectively. The solvent coupling to the torsional coordinate in both the ground and excited states results in increased twisting with respect to the gas phase potential. Two models for the charge distribution in the electronically excited state were studied corresponding to a locally excited (LE) and a charge transfer (CT) state, respectively. The partial atomic charges representative of the S 0, LE, and CT states of BA were obtained from fits to molecular electrostatic potentials created by electronic structure calculations employing the semi-empirical INDO/1S Hamiltonian. The transfer of an electron between the two anthracene rings does not have a large effect on the torsional potential of mean force in acetonitrile. This surprising result is partly attributed to delocalization of the electronic charge occurring over a large molecular region without substantial atomic charge buildup or depletion occurring on any specific atomic site fully exposed to the solvent. The torsional dynamics of bianthryl following photoexcitation was modeled by running nonequilibrium trajectories on the excited state surfaces with initial configurations chosen from the ground state distribution. The relaxation time τ t for bianthryl on the torsional surface in the excited state is estimated to be between 1.5 and 2 ps in acetonitrile. We have also calculated the time correlation function C( t) for the fluctuations in the bianthryl-acetonitrile electrostatic potential. The relaxation time is estimated to be τ s≈ 200 fs. The possible consequences of the relation τ s<τ t are discussed.

Some molecular orbital computations of the internal rotational barrier heights in benzaldehyde and its 4-fluoro, 4-cyano and 4-hydroxy derivatives

Some molecular orbital calculations on the title molecules imply that correlation-gradient computations will be necessary if the internal barriers are to agree with those deduced from torsional frequencies. For example, the energy difference between the planar and perpendicular conformers of benzaldehyde is 34.7 kJ mol −1 for an MP2/6-311G ∗//6-31G ∗(5D) computation. A calculation of the difference of the zero point energies of the two conformers, using somewhat less flexible basis sets, suggests that the enthalpy difference of the two conformers will not be substantially less than 32.5 kJ mol −1. The barrier height deduced from torsional frequencies is 19.3 kJ mol −1. Computations of the torsional frequencies, using STO-3G and 6-31G bases, suggests that the ensuing two-fold barriers are substantially lower than the computed energy and enthalpy differences of the two conformers for all the compounds. Comparisons are made with the measured barriers in condensed media. The computed dipole moments of the planar and perpendicular conformers imply that the solvent effects on the internal barriers differ in sign among the compounds. For 4-hydroxybenzaldehyde this sign depends on the orientation of the hydroxyl group relative to the CO bond. The need for certain new measurements and computations is emphasized.

Triplet pathways in the cis⇆trans photoisomerization of 4-acetyl-and 4-benzoyl-stilbene

The photochemical properties of 4-acetylstilbene (AS) and 4-benzoylstilbene (BS) have been studied in various solvents using steady-state irradiation and ns laser flash photolysis. The quantum yields of trans ⇄ cis photoisomerization (Φt → c, Φc → t) are substantial (0.3) over a wide temperature range, while the quantum yield of fluorescence is small even at –196 °C (e.g.Φf⩽ 0.04). Similar transient absorption spectra, showing maxima at ca. 380 and ca. 415 nm in fluid and frozen media, respectively, were recorded for trans-AS and also for cis-AS. The transient is assigned to the triplet state in either case; its lifetime between 25 and –196 °C varies by a factor of ca. 105. These two virtually identical T–T absorption spectra are suggested to originate from the trans(3t*) and the cis(3c*) triplet state, respectively, since at –196 °C intersystem crossing occurs without significant configurational changes. Apart from small spectral differences BS behaves similarly. Quenching measurements by ferrocene of Φt → c, in comparison with those of the triplet decay, support that the triplet state is intermediate in the trans → cis photoisomerization pathway. The spectral and kinetic features of the triplet state are interpreted on the basis of the intermediacy of the twisting step from 3c* to the perpendicular conformation (3p*) in the cis → trans photoisomerization pathway, the 3t*⇄3p* equilibrium and decay to the ground states via the 3p*→1p intersystem-crossing step.

The solvent dependence of the small internal barrier to rotation in 3,5-dichlorobenzyl fluoride

The 1H and 19F nuclear magnetic resonance spectra of 3,5-dichlorobenzyl fluoride, dissolved in solvents of varying polarity, are analyzed to yield precise values of the long-range coupling constants between ring protons and the nuclei in the fluoromethyl group. The conformational properties of these coupling constants, dependent on the angle by which the C—H and C—F bonds of the side chain twist out of the aromatic plane, are used to determine the solvent dependence of the predominantly twofold barrier to rotation about the Csp2—Csp3 bond. In all solvents, the conformer in which the C—F bond lies in the aromatic plane represents the minimum in the internal rotational potential. The magnitude of the twofold potential changes from 3.2(2) kJ/mol in the nonpolar medium, C6D12, to 0.6(2) kJ/mol in the polar solvent, CD3CN. Intermediate magnitudes occur for solvents of intermediate dielectric constant. Comparisons are made with the parent molecule, benzyl fluoride, for which the perpendicular conformer is most stable in the same solvents. Such comparisons indicate that for the free molecules, the in-plane conformer of the dichloro derivative is also more stable than the same conformer of benzyl fluoride. This indication is in qualitative agreement with molecular orbital predictions.

Long-range 1H,19F and 13C,19F coupling constants and molecular orbital computations as indicators of internal motion in C6H5OCF3 and its 4-fluoro derivative

Internal rotational potentials for rotation about the Csp2—O bond in C6H5OCF3 and its 4-fluoro derivative are computed at the STO-3G and 3-12G levels of molecular orbital theory. As for anisole, the perpendicular conformer is calculated as least stable for C6H5OCF3 but the height of the internal barrier is considerably lower than in anisole. The 4-fluoro substituent reduces the twofold component significantly, indicating reduced p …π conjugation, but does not much change the values of the higher terms in the theoretical potentials. The internal potentials are rather flat with minima between about 30° and 45°, these angles measuring the twist away from planarity. The 1H, 13C, and 19F nuclear magnetic resonance spectra in polar and nonpolar solution yield long-range coupling constants, nJ(1H,19F) and nJ(13C,19F). These parameters are qualitatively consistent with relatively low barriers to rotation about the Csp2—O bonds in these molecules and with only a small dependence on solvent polarity. Key words: C6H5OCF3 and 4-F-C6H4OCF3: 1H NMR, 19F NMR, conformations, MO computations.

2-Phenyladamantane as a model for axial phenylcyclohexane. 1H NMR and molecular orbital studies of motion about the Csp2—Csp3 bond

The 1H NMR spectra of the aromatic groups of 2-phenylcyclohexane and 2-phenyladamantane, in CS2/C6D12 solution at 300 K, are analyzed to yield the long-range coupling constants between the α and ring protons. The coupling over six bonds is related to the internal rotational potential about the Csp2—Csp3 bond in these molecules. It is confirmed that the equatorial isomer of phenylcyclohexane has the parallel conformer, that in which the aromatic plane lies in the symmetry plane bisecting the cyclohexane moiety, as the most stable. The apparent twofold barrier to rotation about the exocyclic carbon–carbon bond follows as 7.1 kJ/mol from the six-bond coupling constant. For 2-phenyladamantane, the six-bond coupling constant strongly implies that the perpendicular conformer, perhaps slightly skewed, is that of lowest energy and that the apparent twofold barrier to rotation about the Csp2—Csp3 bond is about 7.5 kJ/mol. Insofar as 2-phenyladamantane mimics axial phenylcyclohexane, these results confirm recent conclusions about the conformation of the latter and provide evidence for its internal mobility. Geometry-optimized AMI and STO-3G MO computations are reported for the internal motion in both isomers of phenylcyclohexane. The former agree best with experiment for the equatorial isomer, but both imply a significant fourfold, of opposite sign to the twofold, component of the internal rotational potential. For the axial isomer, the two sets of computations find a skewed perpendicular conformer as most stable, in rough agreement with force-field results. However, the barrier to rotation about the Csp2—Csp3 bond is computed as small and AMI has the parallel conformer as more stable than the perpendicular. Key words: 2-phenyladamantane, 1H NMR and internal rotation; phenylcyclohexane, 1H NMR and internal rotation; MO computations, 2-phenyladamantane and phenylcyclohexane.

Theoretical and experimental approaches to the barrier to rotation about the Csp2—Csp3 bond in benzyl silane. Hyperconjugative stabilization of the perpendicular conformation

The 1H nuclear magnetic resonance spectra of benzyl silane and benzyl trichlorosilane, obtained in CS2 and benzene-d6 solutions, are analyzed. The long-range coupling constants between the methylene and para ring protons are used to derive apparent twofold barriers about the Csp2—Csp3 bonds of 7.4 ± 1.6 and 8.1 ± 1.1 kJ/mol for the silane and the trichlorosilane, respectively. These are higher than that for ethylbenzene and are attributed mainly to the stabilization of the perpendicular conformer, that with the C—Si bond in a plane perpendicular to the phenyl plane, by σ–π conjugation (hyperconjugation) of the C—Si bond and the π electron system. Molecular orbital computations confirm the predominantly twofold nature of the internal barrier in benzyl silane and also for benzyl germane and stannane. The calculated barriers for the silane derivatives are rather higher than the experimental values. The computed barriers have magnitudes that appear to change with X in much the same order as do the hyperconjugative interactions deduced in other ways for CH2X(CH3)3 groups (X = Sn, Ge, Si, C). The angles CCX in benzyl-X (X = CH3, SiH3, SiCl3, GeH3, SnH3) are all computed to decrease smoothly as sin2ψ, where ψ is the angle by which the C—X bond twists out of the phenyl plane. Key words: conformations, benzyl silane and trichlorosilane; NMR, benzyl silane and trichlorosilane; MO calculations, benzyl silane and trichlorosilane.

Theoretical Studies on Mechanism of Mptp Action: Et Interference by Mpp+(1-Methyl-4-Phenylpyridinium) With Mitochondrial Respiration vs. Oxidative Stress

This report demonstrates that ease of electron uptake by 1-methyl-4-phenylpyridinium (MPP+), apparently the active agent derived from MPTP, is influenced by conformation of the phenyl ring. From quantum mechanical calculations on MPP+, electron affinity is most negative for the nearly coplanar arrangement, indicating that the molecule is most readily reduced in this geometry. Ionization potential is largest in the perpendicular conformation, thus making for most facile oxidation in that form. Site binding would be expected to alter conformation in comparison with the situation in solution, and, hence, to influence reduction potential. We suggest that electron transfer by MPP+ may play a role in inhibition of mitochondrial respiration and in oxidative stress.

Comment on: The torsional potential function of dimethylaminobenzonitrile and related compounds in their S1 states

An S1 ring-N(CH3)2 torsional potential function with V2≂3700 and V4≂−1450 cm−1 is deduced from the published spectrum [Grassian et al., J. Chem. Phys. 90, 3994 (1989)] of 4-N,N-dimethylaminobenzonitrile. The equilibrium conformation is twisted by 25°, and there are barriers of 185 and 3900 cm−1 at the planar and perpendicular conformations. Similar results are obtained for the (CD3)2 derivative, and for N,N-dimethylaniline, 3-N,N-dimethylaminobenzonitrile, and 4-(trifluoromethyl)-N,N-dimethylaniline.

29Si and 13C NMR spectra of 4‐substituted 2‐methoxytrimethylsiloxybenzenes. Factors determining the chemical shifts in models of lignin constituents

Abstract29Si and 13C NMR chemical shifts are reported for a series of twenty 4‐substituted 2‐methoxytrimethyl‐siloxybenzenes; the set of substituents incorporates a basic set of ten substituents differing in their relative polar and resonance effects and ten other substituents which model substituents encountered in lignins. The factors affecting the chemical shifts are discussed using a comparison with the NMR data for a similar series of compounds, correlations with substituent parameters and electronic charges calculated by a quantum chemical method (MNDO calculations on force‐field optimized molecular geometries) for the basic set. It is concluded that the methoxy group in the ortho position to the trimethylsiloxy group takes, for most of its time, the conformation in which the methyl group lies in the plane of the benzene ring and is turned away from the trimethylsiloxy group which assumes a perpendicular conformation. Proximity deshielding effects observed on both the methoxy and trimethylsiloxy groups are influenced by the substituents in position 4, which is in agreement with the notion that these effects are due to an interaction of the vicinal oxygen atoms or their unshared electrons. The dependence of the 29Si chemical shifts on σpis not substantially affected by the presence of the ortho methoxy group.

Stereoelectronic Aspects of the Anomeric Effect in Fluoromethylamine

AbstractHigh‐level ab initio calculations have been made for fluoromethylamine to study structural and energetic effects of the relative orientation of the N lone pair to the CF bond. The anti‐conformer (N lone pair anti‐planar to the CF bond) corresponds to the global energy minimum. It has the longest CF distance, the shortest CN distance, and is 7.5 kcal·mol−1 more stable than the related perpendicular conformation (lone pair perpendicular to the CF bond). The syn‐conformation also shows hallmarks of the anomeric effect: long CF bond, short CN bond, and energetic stability when allowance is made for the two pairs of eclipsed hydrogens. The transition state for N inversion is close to the syn‐structure; rotation about the CN bond is strongly coupled with this inversion process. Small bond distance changes of ca. 0.02 Å between parallel and perpendicular conformations are associated with dissociation energy differences of ca. 30 kcal·mol−1. Various criteria for assessing the strength of the anomeric effect are discussed.

Molecular modeling studies of novel heteroarotinoids

AbstractThe molecular structure and conformational properties of structurally related oxo and thio heteroarotinoids have been calculated by employing AM1 molecular orbital and both MM2P and Chem‐X “optimize” molecular mechanics methods, and the results have been compared with crystal structure data. For the cis and trans oxo heteroarotinoids, MM2P gives values of the bridge torsion angles ϕ1 and ϕ2 in closest agreement with the crystal structure, and all three computational methods yield values of ϕ1 and ϕ2 within about 10° of that found in the crystal structures. All three computational methods locate a minimum‐energy conformation for the trans isomer corresponding to the two bridged aryl rings being mutually perpendicular, in agreement with the crystal structure and similar to that found for the structurally analogous trans‐stilbene. The calculated heteroring geometries also reproduce the twist‐sofa conformation observed for the crystal structure. Calculated conformational energies versus ϕ1 and ϕ2 indicate broad energy wells about the minimum‐energy conformation with barriers to rotation at the planar and perpendicular conformations, and with higher barriers found for the more sterically congested cis isomer. The corresponding cis and trans thio heteroarotinoids exhibit conformational properties similar to their oxo analogues. Both AM1 and MM2P fare poorly in reproducing the crystal structure values of the sulfur‐containing bond lengths and bond angles. The C‐S bonds found in these thio heteroarotinoids may possess more double‐bond character than accounted for in the calculations. Also, the results suggest that the MM2P sulfur‐related force‐field parameters adopted for these calculations may require further refinement.

Experimental and theoretical estimates of the internal rotational barrier of benzyl fluoride in the vapor phase

The twofold barrier to rotation about the [Formula: see text] bond in benzyl fluoride is deduced from the long-range 1H,1H; 1H,19F; and 13C,19F nuclear spin–spin coupling constants in solution. The barrier changes from 3.2(2) kJ/mol in the polar solvent, acetonitrile-d3, to 0.7(2) kJ/mol in the nonpolar environment provided by cyclohexane-d12. In all solutions the conformer of greatest stability has the C—F bond in a plane perpendicular to that of the phenyl group. Extrapolation of the barrier to the vapor phase, using a simple reaction field model, indicates that the most stable conformer for the free (unclustered) molecule is now that with the C—F bond in the phenyl plane and that the barrier to internal rotation is 1.1(7) kJ/mol. Molecular orbital calculations with the basis sets STO-3G, 4-21G, 4-31G, 6-31G, and 6-31G* all predict the latter conformer as that of lowest energy. However, they disagree significantly among themselves as to the height of the internal barrier. The complete geometries are given for both conformers, as computed with the 6-31G basis, and the side-chain geometries are tabulated for the planar and perpendicular conformers, as given by all the bases. Keywords: benzyl fluoride, internal rotational potential; 13C,19F spin–spin coupling constants in benzyl fluoride; benzyl fluoride, molecular orbital computations.

An improved potential for Xe+HCl from rainbow scattering

We have measured an angular distribution of HCl scattered from Xe at a collision energy of E tr= 86.5 meV which exhibits a clearly resolved damped rainbow structure. To analyse the measurement we have performed classical (QCT) and quantum (IOSA) scattering calculations using Hutson and Howard's M5-potential function. We find that the set of potential parameters deduced from their spectroscopic data is not compatible with our scattering data and that two parameters need to be revised to recover all essential features of the measured angular distribution. These parameters are responsible for the depth of the potential minimum and its anisotropy. The revised values provide additional support for the existence of a double minimum potential and lead to deeper wells at both the collinear Xe-HCl (−35.8 meV ) and Xe-ClH (−22.2 meV) conformations. The analysis of our data based on an isotropic potential provides a potential which is very similar to the one for the perpendicular XeHCl conformation.

Observation and geometry assignment of the conformation of benzyl alcohol in the gas phase

Supersonic molecular jet laser spectroscopy is used to establish the perpendicular conformation of benzyl alcohol and a number of sterically unencumbered derivatives in the gas phase.

The possibility of molecular switching: Theoretical study of [(NH 3) 5Ru-4,4′-bipy-Ru(NH 3) 5] 5+

The possibility of molecular switching is investigated theoretically on a model system in which two redox active sites are connected by 4,4′-bipyridine. The electronic coupling between these sites is determined by two methods: the “dimer splitting” method and the effective Hamiltonian method. The pathways for electronic coupling are analyzed and computed for all dihedral angles between the pyridine cycles. It is found that the π-π coupling dominates the electronic interaction for most angle values. However, in strictly perpendicular conformation, the electronic interaction does not vanish, due to the existence of crossed σ-π interactions.

Structure and conformation of polynitrodiphenylmethanes and related carbanions

1 H and 13C n.m.r. data are reported for a series of ortho and para nitro-substituted diphenylmethanes and related diphenylmethyl anions. In the case of the neutral molecules, a predominance of the skew conformation is suggested for mono- and di-o nitro-substituted derivatives, e.g. 2,4,4′-trinitro- and 2,2′4,4′-tetranitro-diphenylmeth anes, while a perpendicular conformation is preferred by the tri-ortho nitro-substituted derivative, e.g. 2,2′,4,4′,6-pentanitrodiphenylmethane. 13C shifts and 1JcαHα coupling constants indicate that, in all carbanions studied, the exocyclic α-carbon is sp2 hybridized and carries only a small fraction of the negative charge. For carbanions with different substituents on the aromatic rings, e.g. 2,4,4′-trinitro- and 2,2′,4,4′,6-pentanitrodiphenylmethyl carbanions, the negative charge appears to be preferentially delocalized in the more substituted aromatic ring.From arguments based upon the observed n.m.r. and u.v. spectroscopic data, the two rings in diphenylmethyl anions with up to two ortho nitro substituents are essentially coplanar, whereas steric interactions prevent such coplanarity in those with three or four such substituents.

The crystal and molecular structure of hordenine sulfate, (C10H16NO)2+SO42−· 2 H2O

The crystal structure of hordenine sulfate, a sympathomimetic amine, has been determined by X-ray diffraction methods and refined to R = 0.075 (Rw = 0.079) for 1683 observed reflections. The space group is P21/c with a = 11.807(3) Å, b = 8.245(4) Å, c = 24.545(9) Å, β = 101.60(2)°, V = 2340.6 Å3, Z = 4, Mr = 464.58, Dc = 1.318 g/cm3, Dm = 1.343 g/cm3, λ = 1.5418 Å, μ = 15.91 cm−1, F(000) = 1000. The two independent hordenine molecules in the asymmetric unit exhibit the usual perpendicular trans conformation found in most of the sympathomimetic amines. The nitrogen atoms in both the molecules are protonated. The crystal structure is stabilized by a three-dimensional network of hydrogen bonds.

Thiocyanate Ion Promoted E ⇌ Z Isomerization of a‐Bromo‐β,β‐Diactivated Electrophilic Olefins: An Apparent High Nucleofugality of NCS−

Abstract(E)‐β‐Chloro‐α‐phenylcinnamaldehyde and (E)‐ and (Z)‐methyl α‐tert‐butoxycarbonyl‐p‐nitro‐β‐bromocinnamates in the presence of NCS− ion in acetonitrile undergo an (E) ⇌ (Z) isomerization which is faster than the vinylic substitution. (E)‐ and (Z)‐methyl α‐cyano‐β‐mesyloxy‐ (and β‐chloro‐) p‐nitrocinnamates undergo substitution under similar conditions without preceding isomerization. MO calculations gave the energy differences between the eclipsed and the perpendicular conformers of carbanions −CH2CH2X and (NC)2ČH2X (X = Cl, NCS). These rotational barriers denote the hyperconjugation stabilization energy resulting from interaction of the C‐X bond and the 2p(C−) orbital. Both HCA values are high, those for X = Cl being 15–20% higher than for X = NCS. The limited literature data on the nucleofugality of NCS− suggests that it can have a nucleofugality which is not much lower than that of Cl− or Br−. Consequently, NCS−‐catalyzed isomerizations of electrophilic vinyl chlorides and bromides, which involve a 180° internal rotation followed by a faster expulsion of NCS− than of Cl− from the initially formed intermediate carbanion Y'Y−C‐C(R)(Cl)SCN, are not unreasonable, especially if solvation, which was not included in the calculations, plays an important role in the nucleofuge expulsion process.

Conformational Flexibility of the Methoxyphenyl Group Studied by Statistical Analysis of Crystal Structure Data

AbstractIn a large sample of observed methoxyphenyl groups, the twist angle τ about the MeO‐CPh bond measuring internal rotation of the MeO group shows a continuous distribution with maxima at (0°) (coplanar conformation) and (∼90°) (perpendicular conformation). The preferred conformation of methoxyphenyl depends on the nature of the ortho‐‐substituents: In general, it is coplanar in the case of one or two ortho‐hydrogens, and perpendicular in the case of two substituents. The internal rotation of the MeO group is accompained by systematic variations in bond angles and bond distances: 1 if MeO is twisted out of plane, the bond angle CH3OCPh decreases from 117.7°, until it reaches a minimum of 114.9° at τ = ±90°. The OCC angle which is syn to CH3 for τ = 0° decreases from 124.6° to a minimum of 115.4° at τ = ±180°. These angles changes keep the nonbonded distance CH3 …︁ ortho substituent maximal during internal rotation of MeO and tend to minimize the corresponding strain energy. (2) In the perpendicular conformation, the O‐atom is ∼ 0. 06 Å displaced from the Ph plane, O and CH3 and being on opposite sides of this plane. In addition, small but systematic increases of bond lengths MeOCPh and CH3O are observed. These variations indicate a decrease in conjugation with increasing twist angle. Their interdependence during twisting and the magnitudes of the changes are close values obtained by ab initio calculations.