Measure Of Solvent Polarity Research Articles

Investigation of solute-solvent interactions of 4′-alkyl-4-cyanobiphenyl liquid crystals using Raman spectroscopy

Liquid crystal materials possess hybrid liquid and solid-like properties with high response to stimuli. The 4′-alkyl-4-cyanobiphenyls (nCB) are an important class of liquid crystals that are widely used for various applications. In this study, six alkylcyanobiphenyl liquid crystal samples (5CB to 10CB) were examined using Raman spectroscopy in a total of twelve solvents of various polarities. The distinctive bands contributed by the LC sample are from C≡N stretch, C–C aromatic ring breathing, C–C biphenyl linking stretch, and C–H in-plane deformation. These modes are found to be responsive to different solvent environments by shifting positions. For instance, the cyano stretching mode of 5CB is blue-shifted from 2229 cm−1 for the pure sample to 2233 cm−1 when measured in hexane due to the repulsive interaction between the mode and the solvent bath. On the other hand, this mode undergoes a red-shift to 2220 cm−1 in methanol due to hydrogen bond interaction between the mode and solvent molecules. Overall, the shift in the position of the vibrational modes of nCB molecules was correlated with solvent properties such as acceptor number, donor number, and Kamlet-Taft dipolarity constants, which are a measure of solvent polarity. In samples with longer alkyl chains (8CB, 9CB, and 10CB), the wavenumber of the stretching modes was generally shifted to around the same position in all the solvents. Such observations are related to the folding back effect and enhanced dimerization constant with an increase in the chain length.

Solvation and Solvatochromism: An Overview#

Different examples of solvatochromic shifts are shown and the theory that explain the non-specific general solvent effects are discussed here. The use of solvatochromic shifts for the determination of excited state dipole moment has been presented in detail in connection with a brief overview of different solvent polarity functions. The lack of proper theoretical expressions to explain the specific solvent effect stimulates to introduce the concept of “empirical measures of solvent polarity”, based on well known, convenient, solvent sensitive model processes. Various solvent sensitive polarity probes are discussed in this context. So far as quantitative estimation of various parameters responsible for the observed solvatochromic shifts are concerned, introduction of multiparameter approaches, its application in recent years has been carefully analyzed. The review also focuses some of the recent developments on the theoretical calculations of various solute-solvent interaction parameters based on quantum mechanical approaches. This issue solvation interaction by the use of mixed solvents has been discussed in detail, starting from the early development of the theories related to preferential solvation to the recent scenario, with a critical survey on the application of the concept of preferential solvation in the solubilization of drugs, polymers considering the emerging ‘green approach’ in recent years.

Guanidinium solvents with exceptional hydrogen bond donating abilities.

Guanidinium chloride-based solvents have been prepared using deep eutectic solvent principles. Strong hydrogen-bond (H-bond) donating abilities are established based on a range of measures of solvent polarity, including a novel 31P NMR chemical shift method. The physical properties and origin of the strong H-bonding ability of these solvents have been explored.

Kinetic solvent effects in the reduction of alkyl halides by {Sm[N(SiMe3)2]2(THF)2}

The rate of reduction of two representative alkyl halides, 1-bromododecane and 1-chlorododecane by {Sm[N(SiMe3)2]2(THF)2} was examined in five different solvents. Reductions were several orders of magnitude faster in nonpolar, non-coordinating solvents than in polar coordinating solvents. Good correlations between the rate of reduction and coordination index of each solvent was obtained whereas excellent correlations were obtained between rates and dielectric (as a measure of solvent polarity). The basis of this effect is proposed to be a consequence of solvent coordination to the inner sphere of Sm(II) leading to a deceleration of ET in coordinating solvents and possible transition state stabilization by nonpolar solvents.

Twisting-Based Spectroscopic Measure of Solvent Polarity: ThePTScale

The P(T) scale, which is correlated well with the E(T)(30), Y, and Z scales, is the first twisting-based spectroscopic measure of solvent polarity. It is based on two combined mechanisms: (1) the solvent-dependent intramolecular charge-transfer absorption that displays a regular hypsochromic band shift in polar solvents and (2) overcoming the intramolecular hydrogen-bond of o-1 by its differentiated solvation in polar solvents, causing a C-C bond to twist that leads to a regular hypsochromic shift of its lowest energy electronic absorption band.

Influence of Solvent on Carbene Intersystem Crossing Rates

The influence of coordinating solvents on singlet-to-triplet carbene intersystem crossing (ISC) rates has been studied with diphenylcarbene (DPC) and para-biphenyltrifluoromethylcarbene (BpCCF 3) by using ultrafast time-resolved spectroscopy. DPC has a triplet ground state in all of the solvents considered, and the concentration of singlet carbene at equilibrium is too small to be measured. It is found that the lifetime of (1)DPC is extended in acetonitrile, benzene, tetrahydrofuran, dichloromethane, and halobenzene solvents relative to cyclohexane. The solvent effect does not well correlate with bulk measures of solvent polarity. The singlet-triplet energy separation of BpCCF 3 is close to zero. The data demonstrates that BpCCF 3 has a triplet ground state in benzene, fluorobenzene, and hexafluorobenzene. Halogenated solvents are found to dramatically retard the rate of ISC in (1)BpCCF 3. We postulate that the empty p orbital of a singlet carbene coordinates with a nonbonding pair of electrons of a halogen atom of the solvent to form a pseudoylide solvent complex, stabilize the singlet carbene, and decrease the singlet-triplet (S-T) energy gap. The "golden rule" of radiationless transitions posits that the smaller the energy gap between the two states, the faster their rate of interconversion. To explain the apparent violation of the golden rule of radiationless transitions for the carbene ISC processes monitored in this study, we propose that the significantly different specific solvation of the singlet and triplet carbenes imposes a Franck-Condon-like factor on the ISC process. Those solvents that most solvate the singlet carbene will also have the greatest structural difference between singlet carbene-solvent complex and their triplet spin isomer-solvent complex, the smallest S-T gap, and the slowest ISC rate. Alternatively, one can propose that a highly solvated singlet carbene must desolvate prior to ISC, and that this requirement decelerates the radiationless transition.

Solvents and Solvent Effects in Organic Chemistry, Third Edition (Christian Reichardt)

The third edition of Reichardt’s book contains seven well-organized chapters on a wealth of topics including in-depth discussions of solvation, equilibria, kinetics, spectroscopy, and empirical measures of solvent polarity. Every chapter is richly larded with tables, graphs, diagrams, and equations. In addition, each topic is extensively referenced for further reading.

Quantitative Measures of Solvent Polarity

AbstractFor Abstract see ChemInform Abstract in Full Text.

Quantitative measures of solvent polarity.

Quantitative measures of solvent polarity.

Role of hydroxyl concentrations in solvatochromic measures of solvent polarity of alcohols and alcohol-water mixtures-evidence that preferential solvation effects may be overestimated.

For single solvents, primary alcohols and water, there is a good linear correlation (r = 0.994) between the solvent polarity index ET(30) and the molar concentration of OH groups (or 1000/Vm, where Vm is the solvent molar volume). The corresponding correlations for alcohol-water mixtures are plots vs. the sum of molar concentrations of alcohol and water, alternatively expressed as plots of ET(30)vs. volume fraction. Our quantitative treatment is an extension of recent theoretical and experimental results. In contrast, previous studies of alcohol-water mixtures have relied on plots of ET(30)vs. mole fraction, and have overestimated the effect of preferential solvation of solvatochromic dyes by the more hydrophobic alcohols.

Effect of the Media on the Quantum Yield of Singlet Oxygen (O2(1Δg)) Production by 9H‐Fluoren‐9‐one: Solvents and Solvent Mixtures

AbstractWe have investigated the effect of a series of 18 solvents and mixtures of solvents on the production of singlet molecular oxygen (O2(1Δg), denoted as 1O2) by 9H‐fluoren‐9‐one (FLU). The normalized empirical parameter E derived from ET(30) has been chosen as a measure of solvent polarity using Reichardt's betaine dyes. Quantum yields of 1O2 production (ΦΔ) decrease with increasing solvent polarity and protic character as a consequence of the decrease of the quantum yield of intersystem crossing (ΦISC). Values of ΦΔ of unity have been found in alkanes. In nonprotic solvents of increasing polarity, ΦISC and, therefore, ΦΔ decrease due to solvent‐induced changes in the energy levels of singlet and triplet excited states of FLU. This compound is a poor 1O2 sensitizer in protic solvents, because hydrogen bonding considerably increases the rate of internal conversion from the singlet excited state, thus diminishing ΦΔ to values much lower than those in nonprotic solvents of similar polarity. In mixtures of cyclohexane and alcohols, preferential solvation of FLU by the protic solvent leads to a fast decrease of ΦΔ upon addition of increasing amounts of the latter.

Solvent effect on electronic absorption spectra of some 2-aminobenzimidazoles

The effect of protic and aprotic solvents on electronic apsorption spectra of 1-(3-X-benzil)-2-aminobenzimidazoles (X=CH3;OCH3; Cl) was examined. UV-apsorption spectra (200-400 nm) were recorded in five protic and four aprotic solvents. Batochromic shift of absorption maxima, ?max, occurs from the solvent with the highest proton-donor ability to the proton acceptor solvent (from water to DMSO). Positions of absortion maxima in various solvents are in correlation with the dielectric constant of the solvent. In order to explain the obtained results, the ultraviolet absorption frequences of the electronic transitions of the compounds were correlated using a total solvatochromic equation of the form: ?max = ?0 + s?* + a? + b?, where??* is the measure of solvent polarity. Prepresents the scale of solvent hydrogen bond acceptor basicities and ? represents the scale of solvent hydrogen bond donor acidities. Correlation of spectroscopic data was carried out by means of multiple linear regression analysis.

Calibration of the triplet–triplet absorption of xanthone as a microenvironment sensor

The wavelength of maximum absorption of triplet xanthone ( λ max) is strongly dependent on the nature of the solvent, shifting to the red as the solvent polarity decreases. Thus triplet xanthone can be employed as a probe for polarity of microenvironments such as micelles, cyclodextrins and zeolites. Using nanosecond laser flash photolysis we have measured λ max in a variety of solvents using a Gaussian fitting routine to obtain an objective value for the triplet maximum. These data were compared with two different measures of solvent polarity: the solvent dielectric constant and the pyrene fluorescence band ratio ( I 1/ I 3). There is a modest correlation ( R = 0.89) with dielectric constant. The value of λ max shows two different correlations with I 1/ I 3, one with hydrogen bonding solvents and one with non-hydrogen bonding solvents. The triplet lifetime and absorption band width (FWHM) also exhibit some solvent dependence. We propose the triplet xanthone scale as a convenient tool for probing supramolecular systems using laser flash photolysis; this tool complements nicely the fluorescence of pyrene, which requires a different experimental technique and probes a shorter time scale.

Medium Effects on the Rates of Stereomutation of a Pair of Diastereomeric Cyclopropanones. Ground State Stabilization in Nucleophilic Solvents Induces Deviation from Solvent Polarity Controlled Behavior

The synthesis of the two stereoisomers of spiro(bicyclo[2.2.1]heptane-2,1‘-cyclopropan)-2‘-one, 3a and 4a, from diazomethane and the ketene 2-carbonylylbicyclo[2.2.1]heptane in ether at 195 K yields a ∼1.6 to 1 ratio. At 245 K, the ratio changes in a first-order manner, with an observed rate constant of 1.7 × 10-4 s-1, to an equilibrium ratio of 0.8 to 1. The temperature dependence of the interconversion of 3a and 4a (GC method) and that of their dideuterio derivatives 3b and 4b (NMR method) have been determined and yield activation parameters Ea = 16.3 ± 1.4 kcal/mol and log A = 10.4 ± 1.4 (A in s-1) (GC method) and Ea = 15.3 ± 1.4 and log A = 9.6 ± 1.4 (NMR method). The free energies of activation at 239 K have been determined in four solvents: dichloromethane (16.1 kcal/mol), acetone (17.7), hexane (17.9), and ether (19.1). The solvent dependence does not correlate well with commonly used measures of solvent polarity, and the reaction is unexpectedly slow in acetone and ether. This deceleration is exp...

Electronic Effects in Transition Metal Porphyrins. 8. The Effect of Porphyrin Substituents, Axial Ligands, "Steric Crowding", Solvent, and Temperature on the 57Fe Chemical Shifts of a Series of Model Heme Complexes

The 57Fe chemical shifts of a series of 94.5% 57Fe enriched model heme complexes (those of tetraphenylporphyrin, TPP, and two of its p-phenyl substituted derivatives, tetramesitylporphyrin, TMP, and octaethylpoxphyrin, OEP), for which trimethylphosphine serves as at least one of the axial ligands, have been measured by recording the 3iP NMR spectrum using double resonance at the appropriate 57Fe frequency. The discovery of an approximate correlation between 31P and 57Fe chemical shifts makes it easy to predict the 57Fe chemical shift of new complexes, thus simplifying the search for the proper decoupling frequency. The 57Fe chemical shifts of substituted phenyl TPP complexes increase as the electron-donating nature of the substituents increases, and most OEP complexes have chemical shifts that are 200-250 ppm larger than their TPP counterparts; both of these trends are attributed to increased JC donor characteristics that increase the magnitude of the paramagnetic screening constant, dpara. Mixed axial ligand complexes where trimethylphosphine is one of the ligands have larger 57Fe chemical shifts when the other ligand (L) is a strong u donortweak JC acceptor, but within a group of closely related ligands such as 4-substituted pyridines, the weakest u donorhtrongest n acceptor ligand produces the largest 57Fe chemical shift (4-CNPy > Py > 4-NMezPy), in line with the predictions of the Ramsey formula for dpxa. Solvent effects on the 57Fe chemical shift of [TMPFe(PMe&] are small and, in distinction to 6-coordinate low-spin cobalt(II1) porphyrinates, do not follow any common measures of solvent polarity or donor strength (dielectric constant, Gutmann’s donor number, Reichardt’s ET). 31P and 57Fe chemical shifts both show linear temperature dependence, but with opposite slopes. The structures of two (PMe3)z complexes of iron(I1) porphyrinates, [@OCH3)4TPPFe(PMe3)2]-2CsDs and [OEPFe(PMe3)2], have also been determined. In both cases the porphyrinate core is essentially planar and the axial trimethylphosphine ligands have their methyl groups in staggered conformation.

Pyridinium N‐phenoxide betaines and their application to the characterization of solvent polarities, XXI Determination of new and corrections of old ET(30) values as empirical measures of solvent polarity for 40 organic solvents

AbstractBy means of the negative solvatochromic betaine dyes 1a and 1b empirical solvent polarity parameters ET(30) for 40 organic solvents of current interest were newly determined or corrected (Table 1).

Solvent polarity of aqueous polymer solutions as measured by the solvatochromic technique

Ultraviolet–visible absorption spectra of the anionic pyridinium-N-phenoxide betaine dye 1 and of the triphenyl-methane dye thymol blue 2 in aqueous solutions of dextran, poly(ethylene glycol)[(PEG)-6000] and Ficoll-400 have been studied as functions of the polymer concentration. The molar transition energy, ET, for the long-wavelength solvatochromic absorption band of both dyes was used as a measure of solvent polarity of the aqueous medium in the solutions studied. ET was found to decrease linearly with increasing polymer concentration up to a limiting concentration specific to a given polymer. Several arguments in favour of the absence of direct polymer–dye interactions in the solutions examined are presented. The influence of ethylene glycol and PEG samples of different molecular weight up to 20 000 on the polarity of the medium was examined. In the presence of the solutes studied, the polarity of the aqueous medium is found to decrease with increasing solute size and solute concentration. It is suggested that the polarity decrease is due to the structuring action of the solutes on water. The results obtained are viewed as an indication that the changes in the water structure lead to alterations in the hydration capability of water.

ChemInform Abstract: The Charge‐Transfer Band of N‐Alkylpyridinium Iodides in Mixed Aqueous Solvents

The charge-transfer band of N-alkylpyridinium iodides has been studied in mixed aqueous solvents. For water–alcohol (up to C3) mixtures a linear relation between the transition energy corresponding to the band maximum (EmaxT) and the mole fraction of water holds good. [The transition energy denoted by EmaxT is not related to the ET(30) values of Dimroth et al.(K. Dimroth, C. Reichardt, T. Siepmann and F. Bohlmann, Annalen, 1963, 661, 1) involving a pyridinium betaine as the measure of solvent polarity.] For dipolar aprotic solvents deviation from linearity appears at the organic end. The results have been explained by a suitable model of solvation. The linear correlation for water–alcohol system enables the Z value of water to be re-examined; a value of 91.4 kcal mol–1 for water has been advocated.

Estimations of the heats of vaporization of simple hydrocarbon derivatives at 298 K

A simple method of estimating the enthalpies of vaporization of monofunctional hydrocarbon derivatives within 5% of literature values is described. The technique utilizes the analytical expression previously developed to estimate vaporization enthalpies of hydrocarbons, AHv = 1.12fiC + 0.31~~~ + 0.71, where nQ and fi, refer to the number of quaternary and nonquaternary carbon atoms, respectively, modified by introducing an additional parameter, b, that is characteristic of the functional group in question. Values of b for 21 different functional groups are derived from 433 critically reviewed vaporization enthalpies. They range from a low value of 1.19 kcal/mol for ethers to a high value of 12.74 kcal/mol for sulfones. Results obtained by other estimation techniques are compared. Advantages of this method include the ease with which estimations can be made, the general applicability of the method to compounds of diverse structure, and the overall accuracy of the estimations. In addition, the magnitude of b, a characteristic of the attractive interactions of a functional group, can be correlated to ET values that are often used as a measure of solvent polarity.

The charge-transfer band of N-alkylpyridinium iodides in mixed aqueous solvents

The charge-transfer band of N-alkylpyridinium iodides has been studied in mixed aqueous solvents. For water–alcohol (up to C3) mixtures a linear relation between the transition energy corresponding to the band maximum (EmaxT) and the mole fraction of water holds good. [The transition energy denoted by EmaxT is not related to the ET(30) values of Dimroth et al.(K. Dimroth, C. Reichardt, T. Siepmann and F. Bohlmann, Annalen, 1963, 661, 1) involving a pyridinium betaine as the measure of solvent polarity.] For dipolar aprotic solvents deviation from linearity appears at the organic end. The results have been explained by a suitable model of solvation. The linear correlation for water–alcohol system enables the Z value of water to be re-examined; a value of 91.4 kcal mol–1 for water has been advocated.