Empirical Solvent Polarity Parameter Research Articles

Photophysical characteristics of three novel benzanthrone derivatives: Experimental and theoretical estimation of dipole moments

The effect of solvents on absorption and fluorescence spectra and dipole moments of novel benzanthrone derivatives such as 3- N-( N′, N′-Dimethylformamidino) benzanthrone (1), 3- N-( N′, N′-Diethylacetamidino) benzanthrone (2) and 3-morpholinobenzanthrone (3) have been studied in various solvents. The fluorescence lifetime of the dyes (1–3) in chloroform were also recorded. Bathochromic shift observed in the absorption and fluorescence spectra of these molecules with increasing solvent polarity indicates that the transitions involved are π→ π ⁎. Using the theory of solvatochromism, the difference in the excited-state ( μ e ) and the ground-state ( μ e ) dipole moments was estimated from Lippert–Mataga, Bakhshiev, Kawski–Chamma–Viallet, and McRae equations by using the variation of Stokes shift with the solvent’s relative permittivity and refractive index. AM1 and PM6 semiempirical molecular calculations using MOPAC and ab-initio calculations at B3LYP/6-31 G ⁎ level of theory using Gaussian 03 software were carried out to estimate the ground-state dipole moments and some other physicochemical properties. Further, the change in dipole moment value (Δ μ) was also calculated by using the variation of Stokes shift with the molecular-microscopic empirical solvent polarity parameter ( E T N ). The excited-state dipole moments observed are larger than their ground-state counterparts, indicating a substantial redistribution of the π-electron densities in a more polar excited state for all the systems investigated.

Estimation of the ground and the first excited singlet-state dipole moments of 1,4-disubstituted anthraquinone dyes by the solvatochromic method

The both, ground-state ( μ g ) and the excited-state ( μ e ) dipole moments of three 1,4-disubstituted anthraquinones, namely 1,4-diaminoanthracene-9,10-dione (1,4-DAAQ), 1-amino-4-hydroxyanthracene-9,10-dione (1,4-AHAQ), and 1,4-dihydroxyanthracene-9,10-dione (1,4-DHAQ) were estimated in binary solvent mixtures (methylcyclohexane–ethyl acetate and ethyl acetate–acetonitrile). The dipole moments ( μ g and μ e ) were estimated from Lippert–Mataga, Bakhshiev, Kawski–Chamma–Viallet, McRae, and Suppan equations by using the variation of Stokes shift with the solvent's relative permittivity and refractive index. The ground-state dipole moments were also calculated theoretically by Gaussian 03 software using B3LYP/6-31 G* level of theory. Further, the change in dipole moment values Δ μ were also calculated by using the variation of Stokes shift with the molecular-microscopic empirical solvent polarity parameter ( E T N ) . The excited-state dipole moments observed are larger than their ground-state counterparts, indicating a substantial redistribution of the π-electron densities in a more polar excited state for all the molecules investigated.

Solvent Effects on the Solvatochromism of 7-Aminocoumarin Derivatives in Neat and Binary Solvent Mixtures: Correlation of the Electronic Transition Energies with the Solvent Polarity Parameters

The change in the electronic absorption and emission energy of 7-aminocoumarin derivatives in binary solvent mixtures has been studied. The electronic transition energy along with the Stokes' shift is correlated with the orientation polarizability of the solvent as well as the empirical solvent polarity parameters E T (30). It is observed that the emission peak shift traces the change of E T (30) value very closely in binary solvent mixtures. The emission transition more strongly depends on the solvent polarity than the absorption, which indicates the dipole moment gets larger on excitation. From the dependence of the Stokes' shift of 7-aminocoumarins with the solvent polarity parameters and the ground state dipole moment obtained by the semi-empirical calculations, the excited state dipole moment was estimated. The fluorescence lifetime change of 7-aminocoumarins in binary solvent mixtures was measured and the results are explained in terms of molecular conformation and solvent polarity. The study indicates the empirical solvent polarity E T (30) is a good measure of microscopic solvent polarity and it probes in general the non-specific solvent interactions.

Estimation of dipole moments of some biologically active coumarins by solvatochromic shift method based on solvent polarity parameter, [formula omitted

The electronic absorption and fluorescence spectra of three newly synthesized coumarin derivatives viz., 4-(5-methyl-3-furan-2-yl-benzofuran-2-yl)-7-methyl-chromen-2-one (MFBMC), 4-(5-chloro-3-furan-2-yl-benzofuran-2-yl)-6-methyl-chromen-2-one (ClFBMC) and 4-(5-methyl-3-phenyl-benzofuran-2-yl)-6-chloro-chromen-2-one (MPBClC) have been recorded at room temperature (296 K) in solvents of different polarities. The effects of the solvents upon the spectral properties are discussed. Solvatochromic correlations were used to estimate the ground-state ( μ g) and excited-state ( μ e) dipole moments. The excited-state dipole moments for all the three molecules are found to be larger than their corresponding ground-state dipole moments. Further, the changes in dipole moment (Δ μ) were calculated both from solvatochromic shift method and on the basis of microscopic empirical solvent polarity parameter ( E T N ) , and the values are compared.

Pyridinium-N-phenolate betaine dyes as empirical indicators of solvent polarity: Some new findings

Abstract Solutions of the zwitterionic betaine dye 2,6-diphenyl-4-(2,4,6-triphenylpyridinium-1-yl)phenolate (hereinafter called standard betaine dye) and its derivatives are solvatochromic, thermochromic, piezochromic, and halochromic. That is, the position of its longest-wavelength intramolecular charge-transfer (CT) absorption band depends on solvent polarity, solution temperature, external pressure, and the type and concentration of salts (ionophores) added to the betaine dye solution. The outstanding large negative solvatochromism of this standard betaine dye has been used to establish UV/vis spectroscopically a comprehensive set of empirical parameters of solvent polarity, called E T(30) resp. E T N values, now known for many molecular and ionic solvents as well as for a great variety of solvent mixtures. This report describes relevant physicochemical properties of this standard betaine dye as well as the definition and some more recent practical applications of these solvent polarity parameters, derived from the standard betaine dye and its derivatives. In particular, the perichromism of the standard betaine dye can be used to study the polarity of microheterogeneous solutions (e.g., micelles and other organized media), surfaces (e.g., silica, alumina, cellulose), glasses (e.g., sol-gel systems), and solids (e.g., polymers), and for the construction of chemical sensors. As extension to solvatochromism, the more general term perichromism describes UV/vis band shifts of chromophore-containing solutes which are caused not only by changes in the surrounding solvent sphere, but also by their embedding in other surroundings such as micelles, vesicles, glasses, polymers, solids, interfaces, and surfaces. Some representative examples for such extended applications of the perichromic standard betaine dye are given.

Lysozyme gelation in mixtures of tetramethylurea with protic solvents: Use of solvatochromic indicators to probe medium microstructure and solute–solvent interactions

This work investigated the relationship between the structure of binary mixtures of tetramethylurea and protic solvents and their capacity to induce lysozyme gelation. In order to get an insight into the mechanism of gel formation, the solvatochromic behavior of zwitterionic probes, employed as simple models for the protein, was investigated. We studied two probes of similar p K a’s, but different hydrophobic character, namely 2,6-diphenyl-4-(2,4,6-triphenylpyridinium-1-yl) phenolate, RB, and 4-[2-(1-methylpyridinium-4-yl) ethenyl] phenolate, MC. The protic solvents used included water, 1-propanol and 2- n-butoxyethanol in the temperature range from 10 to 60 °C, and methanol, from 10 to 40 °C. In all cases, the dependence of the empirical solvent polarity parameter, E T, on mixture composition was non-ideal with negative deviation for TMU-water and positive deviation for TMU-organic solvent. For all binary mixtures, the deviation from linearity decreased as a function of increasing the temperature. In TMU/alcohol, the effect became more pronounced with increasing alcohol hydrophobicity.

Interactions of organic molecules at grain boundaries in ice: A solvatochromic analysis

Empirical solvent polarity parameters were used to evaluate the nature and magnitude of the intermolecular interactions of eight dipolar organic solvatochromic indicators in aqueous solutions frozen at 253 or 77 K, using the concept that is generally employed to study the polarity of liquid solvents or solid surfaces. E T(30), E T(33), and E T N as well as α, acceptor number (AN) (hydrogen-bond donation ability), β (hydrogen-bond acceptor ability), and π * (polarity/polarizability) parameters were obtained by measuring the differences in the shifts of the absorption spectra of the probes. It was found that hydrogen-bond and electron-pair donating interactions were significant contributors to the polarity of a probe environment in ice and, at the same time, they were found to be substantially larger than those measured in liquid aqueous solutions and relatively insensitive to the sample temperature. While the former interaction type is attributed rather to the presence of water in a close vicinity of the probe molecules, the latter is evidently connected with the inter-probe interactions within the self-assembled molecular aggregations in conjunction with the water–probe interactions. The solvatochromic analysis revealed very weak dipole–dipole interactions ( π *) but the results are inconclusive. The data are consistent with a model according to which, upon freezing the aqueous solutions, the organic solute molecules are ejected to the grain boundaries to form highly concentrated liquid or frozen mixtures of organic and water molecules, having a high degree of complexity and exhibiting specific intermolecular interactions. Evaluation of the intermolecular polar interactions at the grain boundaries in ice should be of a great value in advancing our understanding of physical and chemical processes occurring in natural ice and snow.

Kinetics of the pH‐independent hydrolyses of 4‐nitrophenyl chloroformate and 4‐nitrophenyl heptafluorobutyrate in water‐acetonitrile mixtures: consequences of solvent composition and ester hydrophobicity

AbstractThe pH‐independent hydrolyses of 4‐nitrophenyl chloroformate, NPCF and 4‐nitrophenyl heptafluorobutyrate, NPFB in aqueous acetonitrile were studied spectrophotometrically from 15 to 45 °C. The binary solvent composition covers water concentrations from 2.349 to 53.207 and from 2.745 to 53.333 mol L−1 for NPCF and NPFB, respectively. For both esters, the dependence of log (kobs), the observed rate constant, on log [water] is sigmoidal. The approximate kinetic orders with respect to water were found to be 2 and 3 for NPCF and NPFB, respectively. ΔG≠ gradually decreases as a function of increasing [water], due to a complex, quasi‐mirror image compensation of ΔH≠ and ΔS≠; both parameters increase. The structures of the transition states were probed by a proton inventory study, carried out in the presence of L2O mole fractions (L = H or D) of 0.190, 0.540, 0.890 and 0.180, 0.529, 0.890, for NPCF and NPFB, respectively. Plots of observed rate constants versus the atom fraction of deuterium in the solvent curve downward. Cyclic transition state models were fitted to the kinetic data; these models contain the ester and two water molecules (NPCF) or three water molecules (NPFB). Thus, the sigmoidal dependences of log (kobs) on log [water] are not due to a change in the number of water molecules in the transition states as a function of increasing [water]. The binary solvent mixture is micro‐heterogeneous; there exists two “micro‐domains,” one consists predominantly of coordinated water molecules, the other consists mostly of acetonitrile hydrogen‐bonded to water molecules. NPCF is 232 times more soluble in water than NPFB. That is, the former ester is dissolved in the outer, more polar periphery of these micro‐domains whereas the more hydrophobic NPFB is dissolved in their inner, less polar interiors. This conclusion is corroborated by comparing the dependence on log [water] of log [kobs], and of ET, the empirical solvent polarity parameter, as measured by solvatochromic probes of increasing hydrophobicity. Copyright © 2006 John Wiley & Sons, Ltd.

Probing Surface Basicity of Solid Acids with an Aminobenzodifurandione Dye as the Solvatochromic Probe

Solvatochromism and sorptiochromism of the dye 3-(4-amino-3-methylphenyl)-7-phenyl-benzo1,2b:4,5b'difuran-2,6-dione (1) are studied with an extended set of solvents and various solid acids including silicas, aluminas, and alumosilicates. 1 shows a positive solvatochromism with increasing basicity and dipolarity/polarizability of the solvent; its solvent-induced bathochromic UV-vis absorption band shift ranges from formic acid (upsilon(max) = 21 630 cm(-1)) to hexamethylphosphoric acid triamide (upsilon(max) = 14 200 cm(-1)). Multiple square analyses of upsilon(max) of the solvent-dependent solvatochromic UV-vis absorption band of 1 with several empirical solvent polarity parameters prove that a composite of basicity, acidity, and dipolarity/polarizability of the environment must be taken into account. For the analysis of the solvent-dependent UV-vis shift of 1, the Kamlet-Taft and Catalan solvent parameters have been evaluated. It could be shown that the Catalan solvent parameter set is more suitable to reflect multiple solvation processes involving both strong basic and strong acidic solvents. Quantum chemical calculations indicate that an interaction of the silanol oxygen atom with the protons of the amino group is clearly favored over various acidic attacks of silanol groups upon 1. Accordingly, surface basicity of silica, alumina, and alumosilicates can be determined using the linear solvation energy relationship derived from the solvent-dependent UV-vis band of 1. An unambiguous interpretation of the UV-vis spectroscopic data of 1 adsorbed on surfaces containing Lewis-acid sites is sometimes difficult. UV-vis monitoring of 1-loaded solid acids during surface titration with 2,6-di-tert-butyl pyridine allows the discrimination of whether Brønsted- or Lewis-acid sites interfere with 1. Additionally, adsorbed water has an important influence on the actual surface basicity of solid acids. 1 is recommended as a sensitive probe for checking both basicity and acidity when directly compared with solvatochromism of the established hydrogen-bond-donating indicator (cis-dicyano)bis(1,10-phenanthroline)iron(II) (2).

Solvatochromism of 3-[2-(aryl)benzoxazol-5-yl]alanine derivatives

The photophysical properties of N-Boc-3-[2-(9-anthryl)benzoxazol-5-yl]- l-alanine methyl ester (BoxAnt) and N-Boc-3-[2-[4-(9′-(10′-butyl)anthryl)phenyl]benzoxazol-5-yl]- l-alanine methyl ester (BoxPhAnt) were studied in a series of solvents. Their absorption spectra are less sensitive to the solvent polarity than the corresponding fluorescence spectra which show a pronounced solvatochromic effect leading to large Stokes shifts. Using an efficient solvatochromic method, based on the empirical solvent polarity parameter E T N , a large change of the dipole moment on excitation for BoxPhAnt has been found. From an analysis of the solvatochromic behaviour of the absorption and fluorescence spectra in terms of bulk solvent polarity functions, f( ɛ r, n) and g( n), a larger excited-state dipole moment (about 8 D, ψ = 56) was obtained for BoxPhAnt than for BoxAnt (about 3 D, ψ = 0). Both applied methods gave similar values of the excited-state dipole moments for both compounds studied.

Solvatochromism of 3-[2-(4-diphenylaminophenyl)benzoxazol-5-yl]alanine methyl ester: A new fluorescence probe

The photophysical properties of 3-[2-(4-diphenylaminophenyl)benzoxazol-5-yl]alanine methyl ester ( 1b) and its Boc derivative ( 1a) were studied in a series of solvents. Its UV–Vis absorption spectra are less sensitive to the solvent polarity than the corresponding fluorescence spectra which show pronounced solvatochromic effect leading to large Stokes shifts. Using an efficient solvatochromic method, based on the molecular-microscopic empirical solvent polarity parameter E T N , a large change of the dipole moment on excitation has been found. From an analysis of the solvatochromic behaviour of the UV–Vis absorption and fluorescence spectra in terms of bulk solvent polarity functions, f( ɛ r, n) and g( n), a large excited-state dipole moment ( μ e = 11 D), almost perpendicular to the smaller ground-state dipole moment, was observed. This demonstrates the formation of an intramolecular charge-transfer excited state. Large changes of the fluorescence quantum yields as well as the fluorescence lifetimes with an increase of a solvent polarity cause that the new non-proteinogenic amino acid, 3-[2-(4-diphenylaminophenyl)benzoxazol-5-yl]-alanine methyl ester, is a new useful fluorescence probe for biophysical studies of peptides and proteins.

Electronic spectroscopy, photophysical properties, and emission quenching studies of an oxidatively robust perfluorinated platinum porphyrin.

The highly electron-deficient, beta-octafluorinated meso-tetrakis(pentafluorophenyl)-porphyrin (H(2)F(28)TPP) was metalated with platinum to afford the oxidatively robust luminophore [PtF(28)TPP], and its X-ray structure shows that the porphyrin core exists in a slightly saddle-shaped conformation. The absorption spectrum of [PtF(28)TPP] in CH(2)Cl(2) displays a near-UV Soret band (B) at 383 nm (epsilon = 2.85 x 10(5) dm(3) mol(-1) cm(-1)) and two visible Q(1,0) and Q(0,0) bands at 501 (epsilon = 1.45 x 10(4) dm(3) mol(-1) cm(-1)) and 533 (epsilon = 1.36 x 10(4) dm(3) mol(-1) cm(-1)) nm, respectively. These absorption bands of [PtF(28)TPP] are blue-shifted from those in [PtF(20)TPP] (390, 504, and 538 nm, respectively) and [PtTPP] (401, 509, and 539 nm, respectively). Excitation of [PtF(28)TPP] (complex concentration = 1.5 x 10(-6) mol dm(-3)) in dichloromethane at the Soret or Q(1,0) or Q(0,0) band gave a phosphorescence with peak maximum at 650 nm (lifetime = 5.8 micros) and a weak shoulder at 712 nm. Both the emission lifetime and quantum yield vary with solvent polarity, and plots of tau versus E(K) and Phi versus E(K) (where E(K) is the empirical solvent polarity parameter based on the hypsochromic shift of the longest wavelength absorption of the [Mo(CO)(4)[(C(5)H(4)N)HC[double bond]NCH(2)C(6)H(5)]] complex with increasing solvent polarity; see: Kamlet, M. J.; Abboud, J. L. M.; Taft, R. W. Prog. Phys. Org. Chem. 1981, 13, pp 485-630) show linear correlation, indicating that the emission is sensitive to the local environment/medium. Electrochemical studies on [PtF(28)TPP] by cyclic voltammetry showed no porphyrin-centered oxidation at potential < or = 1.5 V versus Ag/AgNO(3), demonstrating that [PtF(28)TPP] is more resistant toward oxidation than [PtF(20)TPP] (E(1/2) = 1.33 V) and [PtTPP] (E(1/2) = 0.97 V). The porphyrin-centered reduction of [PtF(28)TPP] occurs at -0.75 and -1.18 V, which is anodically shifted from those at -1.06 and -1.55 V in [PtF(20)TPP], and -1.51 V in [PtTPP], respectively. The excited-state reduction potential of [PtF(28)TPP] is estimated to be 1.49 V versus Ag/AgNO(3). Over 97% of the emission intensity of [PtF(28)TPP] was retained after irradiation with a high power mercury arc lamp (500 W) for 14 h, compared to 90% and 12% for [PtF(20)TPP] and [PtTPP], respectively; hence, [PtF(28)TPP] exhibits superior photostability. Quenching of the emission of [PtF(28)TPP] by oxygen, alcohol, catechol, and butylamine reveals that [PtF(28)TPP] is an oxidatively robust material with medium-sensitive photoluminescence properties.

A fluorescence study of (4-{(1E,3E)-4-[4-(dimethylamino)phenyl]buta-1,3-dienyl}phenyl)methanol and its bioconjugates with bovine and human serum albumins

Electronic absorption and fluorescence properties of (4-{(1E,3E)-4-[4-(dimethylamino)phenyl]buta-1,3-dienyl}phenyl)methanol in different media, including organic solvents of varying polarity at 298 K, an ethanol–methanol matrix at 77 K and protein environments, have been examined. The diene shows solvatochromic fluorescence, which is correlated with empirical solvent polarity parameters. The red-shifted fluorescence emission of the diene is attributed to a polar, conformationally relaxed intramolecular charge transfer excited state. Further, the diene has been covalently attached to bovine serum albumin and human serum albumin and the fluorescence probe properties of the protein-diene conjugates have been examined in phosphate buffer.

Thermo‐solvatochromism of betaine dyes in aqueous alcohols: explicit consideration of the water–alcohol complex

AbstractThermo‐solvatochromism of 2,6‐dichloro‐4‐(2,4,6‐triphenylpyridinium‐1‐yl)‐phenolate, 1‐methylquinolinium‐8‐olate and 4‐[2‐(1‐methylpyridinium‐4‐yl)ethenyl]‐phenolate, in the temperature ranges 10–45°C (methanol) and 10–60°C (1‐ and 2‐propanol) was investigated in binary water–alcohol mixtures. Thermo‐solvatochromic data were treated according to a modified model that explicitly considers the presence of 1:1 water–alcohol species in bulk solution, and its exchange reactions with water and alcohol in the solvation micro‐sphere of the probe employed. Concentrations of these complex species were calculated from density data. Plots of the empirical solvent polarity parameter, ET, versus effective mole fraction of water in the binary mixtures indicate that the probes are preferentially solvated by the alcohol, except for one case. A temperature increase causes gradual desolvation of the probe, due to a decrease in the H‐bonding abilities of all components of the binary solvent mixture. Copyright © 2003 John Wiley &amp; Sons, Ltd.

Solvatochromic azamethine dyes for probing the polarity of gold-cluster-functionalized silica particles.

Azamethine dyes of the merocyanine type [4-(N,N-di-n-butylamino)-2-methylphenyl][2,4-di-keto-3-[N'-(n-hexyl)]-5-cyano-6-methyl-3-pyridinio]-1-azamethine (1) and [4-(N,N-diethylamino)-2-(N'-tert-butylcarboxy)-amidophenyl]-[2,4-diketo-3-[N"-(n-hexyl)]-5-cyano-6-methyl-3-pyridinio]-1-azamethine (2) have been used as surface-polarity indicators for gold-cluster-functionalized silica particles. Their UV/Vis absorption maxima range from about lambda=600 to 700 nm as a function of solvent polarity and are clearly separated from the surface plasmon UV/Vis absorption band of gold (lambda approximately 520-540 nm). Solvatochromism of both dyes has been investigated in 26 solvents of different polarity. The positive solvatochromic band shifts of 1 and 2 can be well expressed in terms of the empirical Kamlet-Taft solvent polarity parameters alpha and pi*. They are mainly sensitive to the dipolarity/polarizability (pi* term; 70-75 %) and HBD (hydrogen-bond donating) acidity (alpha term) of the solvent. Both dyes adsorb readily on functionalized silica samples from solutions in 1,2-dichloroethane or cyclohexane. The surface polarities of gold-cluster-functionalized silica particles, with and without co-adsorbed L-cysteine and poly(ethylenimine), have been investigated by using these solvatochromic dyes. The specific interaction of dye 2 with cysteine has been examined independently by quantum-chemical calculations by using the AM1 and PM3 methods.

Quantitative correlation of solvent polarity with the alpha-/3(10)-helix equilibrium: a heptapeptide behaves as a solvent-driven molecular spring.

The long and the short of it: Just by changing the polarity of the solvent the highly folded heptapeptide Ac-[Aib-L-(αMe)Val-Aib]2-L-His-NH2 converts from an α into a 310 helix (see picture). The equilibrium constant between the two conformations correlates with the empirical solvent polarity parameter E. Molecular dynamics calculations show that the peptide elongates (310 helix) or shortens (α helix) on switching from one conformation to the other.

UV/Vis Spectroscopic Properties of N -(2?-Hydroxy-4?- N , N -dimethyl-aminobenzylidene)-4-nitroaniline in Various Solvents and Solid Environments

N-(2′-Hydroxy-4′-N,N-dimethylaminobenzylidene)-4-nitroaniline [HDBN] has been used as a model for investigating intra- and intermolecular D–A (donor–acceptor) interactions in various environments by means of UV/Vis spectroscopy. UV/Vis spectra of HDBN have been measured in various solvents, ethanolic solutions of different pH, adsorbed on silica, and in the solid state. A bathochromic shift of νmax is observed with increasing the dipolarity/polarizability and HBD (hydrogen bond donor) capacity of the solvent, which is described by means of a multiple LSE (linear solvation energy) relationship in terms of the empirical Kamlet-Taft solvent polarity parameters. The adsorption of HDBN on Aerosil® 300-silica particles in non-HBA (hydrogen bond acceptor) solvents is explained in the same sense. Mobile protons and sol–gel entrapping cause a hypsochromic shift due to protonation of the lone electron pair of the 4′-N,N-dimethylamino group. Hydroxide ions attack the 2′-hydroxy group which causes a bathochromic shift. A strong intramolecular hydrogen bond between the 2′-hydroxyl hydrogen and the imine nitrogen atom is present in the solid-state structure causing an unprecedented bathochromic shift.

1H NMR Investigation of Solvent Effects in Aromatic Stacking Interactions

One of the marquis challenges in modern Organic Chemistry concerns the design and synthesis of abiotic compounds that emulate the exquisite complex structures and/or functions of biological macromolecules. Oligomers possessing the propensity to adopt well-defined compact conformations, or foldamers, have been attained utilizing hydrogen bonding, torsional restriction, and solvophobic interactions.(1) In this laboratory, aromatic electron donor--acceptor interactions have been exploited in the design of aedamers--foldamers that adopt a novel, pleated secondary structure in aqueous solution. Herein is reported detailed (1)H NMR binding studies of aedamer monomers that were carried out in solvents and solvent mixtures covering a broad polarity range. Curve-fitting analysis of the binding data using a model that incorporated the formation of higher order and self-associated complexes yielded a linear free energy relationship between the free energy of complexation and the empirical solvent polarity parameter, E(T)(30). From these studies, the association of electron-rich and electron-deficient aedamer monomers was seen to be driven primarily by hydrophobic interactions in polar solvents. However, the magnitude of these interactions is modulated to a significant extent by the geometry of the donor--acceptor complex, which, in turn, is dictated by the electrostatic complementarity between the electron-deficient and electron-rich aromatic faces of the monomers.

Experimental Evidence for Intramolecular Attractive Nonbonded C–F...H–C Interactions in 2′,3′-Dideoxy-4′-(fluoromethyl)nucleosides – Through-Space JCF and JHF NMR Coupling Constants, Correlation with Empirical Parameters of Solvent Polarity and Single-Crystal X-ray Structures

Experimental Evidence for Intramolecular Attractive Nonbonded C–F...H–C Interactions in 2′,3′-Dideoxy-4′-(fluoromethyl)nucleosides – Through-Space JCF and JHF NMR Coupling Constants, Correlation with Empirical Parameters of Solvent Polarity and Single-Crystal X-ray Structures

Experimental Evidence for Intramolecular Attractive Nonbonded C–F…H–C Interactions in 2′,3′-Dideoxy-4′-(fluoromethyl)nucleosides – Through-SpaceJCF andJHF NMR Coupling Constants, Correlation with Empirical Parameters of Solvent Polarity and Single-Crystal X-ray Structures

A collection of 5′-O-benzyl-2′,3′-dideoxy-4′-(fluoromethyl)nucleosides carrying both purinic and pyrimidinic nucleobases (uracil, 5-Br-uracil, 5-O2N-uracil, 6-Cl-purine and inosine) were synthesized in both the α and the β form. Through-space-transmitted 6JCF NMR coupling constants between F and C-6 (pyrimidinic base) or C-8 (purinic base) were observed for all of the α anomers of the compounds examined, whilst the corresponding 7JHF coupling constants were resolved only for the 5-substituted uracil derivatives. The absolute values of all the through-space couplings were found to decrease monotonically with increasing solvent polarity (CDCl3, MeOD, [D6]acetone, [D6]DMSO). This trend suggests that the through-space interaction is mediated by an intramolecular (sp3)C–F…H–C(sp2) hydrogen bond. The possibility of any relevant solvent-induced conformational change influencing the F/base mutual spatial relationship in the molecules investigated was ruled out by heteronuclear steady-state 1H{19F}-NOE experiments. A linear correlation was observed between 6JCF and 7JHF coupling constants and the Kamlet–Taft's hydrogen bond basicity parameter β. The crystal structures of the α and β anomers of the 5-nitrouracil nucleoside show evidence that the H-6 of the nucleobase forms hydrogen-bond-like interactions involving the O-benzyl oxygen atom in the β anomer, and that in the case of the α anomer this is replaced by the F atom of the fluoromethyl group.