Preferable solvatation of decane and benzene in 1-octanol-N,N-dimethylformamide mixed solvent

Heat effects of the dissolution of decane and benzene in a model system of 1-octanol (OctOH)-N,N-dimethylformamide are measured at 298 and 318 K using a variable temperature calorimeter with an isotermic shell. The state of hydrocarbon molecules in the mixed solvent is studied using an extended coordination model and is compared to earlier data for ethyl acetate (EtOAc), DMF, OctOH, and tetramethyl hematoporphyrin (TMHP). It is shown that the polar carboxylic groups of porphyrin are preferably solvated by amide molecules due to stronger interaction with DMF, while nonpolar aliphatic groups are solvated by alcohol molecules. We conclude that a solvate shell of aromatic benzene is strongly enriched with DMF over the range of compositions, suggesting that the weakening of the preferable solvatation of porphyrin relative to EtOAc is due primarily to the influence of nonpolar substituents.     

Photophysical properties of some phenetidines in non-polar and hydrogen bonding solvents at room temperature and 77 K

The present study was undertaken to investigate the photophysical processes in o-, m- and p-phenetidines, when dissolved in nonpolar and hydrogen bonding solvents, in their ground state and excited electronic state S₁, both at 300 and 77 K. In the ground as well as in the S₁ state it is proposed that the o-phenetidine molecule possesses a structure in which NH₂ and OC₂H₅ groups are away from each other, both in nonpolar cyclohexane (CH) and H-bond acceptor solvent triethylamine (TEA). The formation of a transient or nonemissive charge transfer (CT) complex resulting from strong excited state hydrogen bonding interaction with TEA is found to be responsible for the observed fluorescence quenching of the proton donor phenetidines at 300 K. From the room as well as low (77 K) temperature electronic absorption and steady state fluorescence studies, it was deduced that nonplanarity in the structure of the molecules increases as one moves from aniline to the phenetidines. It is suggested that in the solvent stiffening temperature 77 K, triplet states of all the phenetidines (o-, m- and p-) acquire some nπ* character due to conformational changes, whereas ππ* character is retained in their S₁ state. This facilitates a larger intersystem crossing (ISC) rate in phenetidines relative to the situation in aniline where both S₁ and T₁ possess the same nπ* nature at 77 K due to its more planar structure. However, ISC efficiency in phenetidines at 77 K is found to be impeded, especially in the case of o- and m-isomers, in the presence of TEA as inferred from the lowering of φ_p/φ_⨍ values and the increment of τ_p. In p-phenetidine, rapid equilibrium between a triplet state hydrogen bonded species and free molecules during the triplet excited state lifetime is suggested.     

A new equation of state for polar and nonpolar pure fluids

Chung, T.H., Khan, M.M., Lee, L.L. and Starling, K.E., 1984. A new equation of state for polar and nonpolar pure fluids. Fluid Phase Equilibria, 17: 351–372A new equation of state based on the concept of perturbation theory and the hard-convex-body equation of state has been developed successfully for nonpolar compounds. The equation can predict the thermodynamic properties (density, enthalpy departure and vapor pressure) of a wide range of pure fluids from small, spherical (argon-like) molecules to large, structurally complex molecules. For nonpolar compounds, the equation employs three parameters: the shape, size and energy parameters. For normal paraffins, the size parameter (hard-core volume) is related to the measurable van der Waals volume given by Bondi. For most other compounds, it is related to the critical volume. The shape-parameter values reflect the structure and degree of acentricity of the compound of interest. The equation has been extended to polar and associating compounds by using the mean-potential model. For polar compounds, a fourth parameter is required. The equation has been tested extensively for polar (dipolar and quadrupolar) and hydrogen-bonding compounds. The applicability of this equation for such a wide variety of substances provides an important first step in the development of a composition-dependent equation of state for mixtures.     

Comparison of two hard-sphere reference systems for perturbation theories for mixtures

The Carnahan—Starling equation of state for hard spheres can be extended to mixtures using either a one-fluid theory, or the generalization of scaled-particle (or Percus—Yevick theory) proposed by Boublik and by Mansoori and coworkers. The two reference systems are combined with a perturbation term of the van der Waals form; they are then used to correlate the phase behavior of binary mixtures of nonpolar molecules differing significantly in molecular size. In each case, one adjustable binary parameter (a₁₂) is used to correlate vapor—liquid equilibria over the entire composition range. Predicted Henry's constants and liquid densities for the saturated mixture are compared with experiment. The Boublik—Mansoori hard-sphere-mixture equation is superior to the Carnahan—Starling One-Fluid theory, expecially in the dilute region.     

Thermodynamics of associated solutions: Henry's constants for nonpolar solutes in water

Hu, Y., Azevedo, D., Lüdecke, D. and Prausnitz, J., 1984. Thermodynamics of associated solutions: Henry's constants for nonpolar solutes in water. Fluid Phase Equilibria,17: 303–321.A systematic derivation is presented for the Helmholtz energy of a van der Waals fluid mixture whose nonideality is ascribed to both chemical and physical interactions; this derivation, applicable to all fluid densities, leads to an equation of state which contains chemical equilibrium constants in addition to the customary physical van der Waals constants a and b. Attention is given to the need for simplifying assumptions and to the variety of symplifying assumptions that can lead to useful results. A particular equation of state is used to correlate Henry's constants for nonpolar solutes in water over a wide temperature range. The correlation, however, is only partly successful, because a one-fluid van der Waals theory of mixtures is not satisfactory for mixtures containing molecules that differ appreciably in size, especially in the dilute region.     

Thermochemical properties of some vinyl chloride-induced DNA lesions: detailed view from NBO & AIM analysis

Etheno-damaged DNA adducts such as 3,N ⁴-ethenocytosine, N ²,3-ethenoguanine, and 1,N ²-ethenoguanine are associated with carcinogenesis and cell death. These inevitable damages are counteracted by glycosylase enzymes, which cleave damaged nucleobases from DNA. Escherichia coli alkyl purine DNA glycosylase is the enzyme responsible for excising damaged etheno adducts from DNA in humans. In an effort to understand the intrinsic properties of these molecules, we examined gas-phase acidity values and proton affinities (PA) of multiple sites of these molecules as well as equilibrium tautomerization and base pairing properties by quantum mechanical calculations. We also used calculations to compare the acidic and basic properties of these etheno adduct with those of the normal bases—cytosine and guanine nucleobases. We hypothesize that alkyl DNA glycosylase may cleave certain damaged nucleobases as anions and that the active site may take advantage of a nonpolar environment to favor deprotonated cytosine or guanine as a leaving group versus damaged nucleobases.     

Monte Carlo study of the pair correlation function for a liquid with non-central forces

The pair correlation function g(R), giving the probability that the centers of two nonspherical molecules are a distance R apart irrespective of their orientations, has been studied by the Monte Carlo method. The pair potential model studied is of the type u_o + u_a, where u_o is the isotropic Lennard-Jones (12,6) potential and u_a is either a dipole-dipole, quadrupole-quadrupole, or anisotropic overlap interaction. Dipolar and quadrupolar forces are found to have a small effect on g(R), whereas anisotropic overlap forces have a large effect.     

Solvation of decane and benzene in mixtures of 1-octanol and <Emphasis Type="Italic">N</Emphasis>,<Emphasis Type="Italic">N</Emphasis>-dimethylformamide

The heats of dissolution of decane and benzene in a model system of octanol-1 (OctOH) and N,N-dimethylformamide (DMF) at 308 K are measured using a variable temperature calorimeter equipped with an isothermal shell. Standard enthalpies are determined and standard heat capacities of dissolution in the temperature range of 298–318 K are calculated using data obtained in [1, 2]. The state of hydrocarbon molecules in a binary mixture is studied in terms of the enhanced coordination model (ECM). Benzene is shown to be preferentially solvated by DMF over the range of physiological temperatures. The solvation shell of decane is found to be strongly enriched with 1-octanol. It is obvious that although both hydrocarbons are nonpolar, the presence of the aromatic π-system in benzene leads to drastic differences in their solvation in a lipid–protein medium.     

Cavitation free energy for organic molecules having various sizes and shapes

Cavitation free energy DeltaG(cav), corresponding to the formation of an excluded volume cavity in water, is calculated for a large set of organic molecules employing the thermodynamic integration procedure, which is realized as the original two-step algorithm for growing the interaction potential between the hard cavity wall and the water molecules. A large variety of solute systems is considered. Their characteristic radii change in the range 3-7 A; spherical cavities with radii 3-6 A are also studied. The interaction between water molecules is described by the four-site nonpolarizable TIP4P model. The diversity of the trial molecular set is provided by using a specially formulated nonspherical criterion classifying the cavity shapes according to their deviation from a sphere. Molecular objects were partly taken from the data base NCI Diversity with the aid of this criterion. The so-computed free energies are approximated by the linear volume dependence DeltaG(cav)V = XiV, where V is the cavity volume. This relation works fairly well until the cavity size becomes very large (the effective radius larger than 7 A). The volume dependence valid for solutes of arbitrary shapes can be included in a calculation of the nonpolar free energy component as required in the implicit water model.     

4-Dimethylaminochalcone as fluorescent probe: effect of the medium polarity on relaxation processes in the excited state

Relaxation processes in a 4-dimethylaminochalcone molecule after excitation with a light pulse of duration 70 fs were studied. During 0.4–1 ps after excitation, an absorbance of an excited state S₁ with a maximum at 460 nm is formed in both polar and nonpolar media. Subsequent relaxation processes depend on the polarity of the medium. In nonpolar hexane, the 4-dimethylaminochalcone molecule transits to the triplet state having an absorption maximum at 570 nm (lifetime longer than 600 ps) for 20 ps. In polar aprotic acetonitrile, the absorbance at 460 nm decreases slowly (during hundreds of picoseconds), indicating that the molecules return to the ground state. The induced emission from the level S₁ in a region of 520–550 nm and fluorescence from the same level with a maximum at 537 nm are also observed in acetonitrile. Thus, a reason for a sharp decrease in the fluorescence yield on going from polar to nonpolar media was found. The mechanism of fluorescence quenching of 4-dimethylaminochalcone in nonpolar media is confirmed by the data on phosphorescence. The phosphorescence of 4-dimethylaminochalcone is observed at–196 °C in nonpolar solvents, indicating a triplet excited state, while no phosphorescence is revealed in polar solvents.Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1607–1610, August, 2004.     

Photodissociation and geminate dynamics in solution phase: Picosecond transient absorption studies of tetraphenylhydrazines and diphenyl disulfides

Photodissociation of aromatic molecules and the following geminate dynamics of the photochemically produced fragment radical pair in liquid phase are discussed in relation to the results of ultrafast transient absorption measurements. Photodissociation of tetraphenylhydrazine occurs from the unequilibrated excited singlet state competing with the relaxation to the fluorescence state. In addition to the rapid fragmentation, the N?N bond rupture is clearly demonstrated to take place in the fluorescence state of tetraphenyl-and tetratolyl-hydrazine in tens of picoseconds time scale. In the case of di-p-aminophenyl disulfide in nonpolar solvents, geminate recombination of the p-aminophenylthiyl radical and the formation of the radical dimer were observed in tens of picoseconds. The observed slow geminate recombination can be explained by the repulsion between the large dipole of the fragment radical preventing the reformation of the S?S bond and by the restricted conformation required for the dimer formation at the encounter of radicals.     

Modification of the predictive capability of the PRSV-2 equation of state for critical volumes of binary mixtures

The predictive capability of the Peng–Robinson–Stryjek–Vera (PRSV-2) equation of state for critical properties of binary mixtures showing continuous critical lines has been investigated. The procedure adopted by Heidemann and Khalil and discussed by Abu-Eishah et al., in a previous paper, has been followed. The effect of using the pure-component parameters of the PRSV-2 equation of state (κ₁, κ₂ and κ₃), new values of κ₁, revised values of κ₁, or giving zero values for these parameters have been investigated. The effect of using zero values or optimized values for the binary interaction parameter on the PRSV-2 predictive capability of critical properties have also been investigated. The standard and the average of the absolute relative deviations in critical properties are included. The predicted critical temperature and pressure for the 20 nonpolar and 18 polar systems studied here agree well with experimental data, and are always better than those predicted by the group-contribution method. A correction has been introduced here to the critical volume predicted by PRSV-2 equation of state that makes the average deviations between the predicted and experimental values very close to or even better than those predicted by the group-contribution method.     

Specifics of photoinduced excited-state intramolecular proton transfer in o-tosylaminobenzoic and o-acetylaminobenzoic acids

In the ground state, o-tosylaminobenzoic and o-acetylaminobenzoic acids exist in the form of two rotamers with intramolecular hydrogen bonds N-H...O=C (cis) and N-H...O(OH)-C (trans). In nonpolar solvents, the formation of dimers with hydrogen bonding between carboxyl groups takes place. Efficient barrierless excited state intramolecular proton transfer (ESIPT) occurs along the N-H...O=C hydrogen bond upon excitation of o-tosylaminobenzoic acid. The efficiency of ESIPT in o-acetylaminobenzoic acid is lower because of the low acidity of the substituted amino group.     

Hydrodynamic characteristics of the capsular polysaccharides ofKlebsiella

Investigations of the acidic polysaccharides isolated from the capsules ofK. scleromatis andK. ozaenae under the conditions of high-speed sedimentation and an approximation to equilibrium (Archibald's method) has made it possible to regard them as polydisperse homogeneous substances with different molecular masses and a nonspherical form of the molecules entering their composition.     

Cyclodextrin complexation as a probe of molecular photophys1cs

Three examples are described in which complexation of organic guests by a cyclodextrin (CD) alters the photophysical paths for excited state decay of the host-guest complex. The fluorescence properties ol the complexes between β- and γ-CD and the bichromophoric molecule 1.1'-bis-(α-naphthylmethyl) dithiane are presented to illustrate how complexation alters the dynamics of conformational interconversion. The complexation of β-naphthol by β-CD is shown by time-correlated emission spectroscopy to prevent the adiabatic deprotonation of the excited alcohol normally observed in aqueous solution. Lastly, solid complexes of CD's and aromatic donor-acceptor molecules such as p-nitroaniline (PNA) are shown to be active for generation of second harmonic radiation (SHG) when irradiated by 1.06 μ light from a Nd-YAG laser. Since PNA itself crystallizes in a centrosymmetric space group incapable of producing SHG, the complexation method effectively induces SHG activity in this polarizable, but nonpolar material.     

Excited state dynamics of Michler's ketone: a laser flash photolysis study

Steady state absorption and fluorescence as well as the time resolved absorption studies in the pico and subpicosecond time domain have been performed to characterize the excited singlet and triplet states of Michler's ketone (MK). The nature of the lowest excited singlet (S₁) and triplet (T₁) states depends on the polarity of the solvent - in nonpolar solvents they have either pure nπ * character or mixed character of nπ * and ππ * states but in more polar solvents the states have CT character. Concentration dependence of the shapes of the fluorescence as well the excited singlet and triplet absorption spectra provide the evidence for the association of the MK molecules in the ground state.     

Three Modes of Proton Transfer in One Chromophore: Photoinduced Tautomerization in 2‐(1H‐Pyrazol‐5‐yl)Pyridines,Their Dimers and Alcohol Complexes

Studies of 2‐(1H‐pyrazol‐5‐yl)pyridine (PPP) and its derivatives 2‐(4‐methyl‐1H‐pyrazol‐5‐yl)pyridine (MPP) and 2‐(3‐bromo‐1H‐pyrazol‐5‐yl)pyridine (BPP) by stationary and time‐resolved UV/Vis spectroscopic methods, and quantum chemical computations show that this class of compounds provides a rare example of molecules that exhibit three types of photoreactions: 1) excited‐state intramolecular proton transfer (ESIPT) in the syn form of MPP, 2) excited‐state intermolecular double‐proton transfer (ESDPT) in the dimers of PPP in nonpolar media, as well as 3) solvent‐assisted double‐proton transfer in hydrogen‐bonded 1:1 complexes of PPP and MPP with alcoholic partners. The excited‐state processes are manifested by the appearance of a dual luminescence and a bimodal irreversible kinetic coupling of the two fluorescence bands. Ground‐state syn–anti equilibria are detected and discussed. The fraction of the higher‐energy anti form varies for different derivatives and is strongly dependent on the solvent polarity and hydrogen‐bond donor or acceptor abilities.     

Stark quantum beat spectroscopy of polyatomic molecules

We derive analytical expressions for Stark quantum beat signals of polyatomic molecules and discuss them with regard to molecular and geometrical parameters. The general treatment is specified for near prolate asymmetric rotor molecules and a method for determining rotational constants and all components of the dipole moment of electronically excited polyatomic molecules is presented. The method is tested and illustrated for the vibrationlessS ₁ state of deuterated propynal (HC ≡ CCDO,C _s symmetry) and its lowest frequency non-totally symmetric state 12¹. The results of the vibrationless state are compared with structural data reported in the literature. For the 12¹ state we obtainA=1.5004(43) cm^?1,B=0.16131(34) cm^?1,C=0.14623(34) cm^?1, and the components of the electric dipole moment in the molecular plane μ _a =?0.88(2)D, μ _b =1.03(2)D. Furthermore, it is shown that the modulation depth of Stark quantum beat signals can be utilized to quantify the contribution of the individual components of the transition moment to the total emission.     

Influence of solvent on the thermal stability and organization of self-assembling fibrillar networks in methyl-4,6-O-(p-nitrobenzylidene)-α-d-glucopyranoside gels

Sugar based low-molecular-mass organogelator (LMOG) methyl-4,6-O-(p-nitrobenzylidene)-α-d-glucopyranoside, is a unique gelator because its small and weakly-interacting molecules can form large supramolecular structures in nonpolar, but also in polar, solvents and cause their gelation. The self-assembling properties of the gelator were studied in selected nonpolar and polar solvents. It was shown that the driving forces for both types of solvent are the intermolecular hydrogen bond interaction. The effect of the nature of the solvent on the thermal stability of the gels and on the three-dimensional network organization was determined. Different solvent parameters, such as dielectric constant, one-component solubility parameter, the polarity parameter and the Kamlet-Taft parameters were considered to quantify solvent effects on the gelation. Some correlation between these parameters and the gel stability, microstructure and the enthalpy of the phase transition were established. The effort to correlate the Kamlet-Taft parameters to the thermal stability and gelation ability is also possible but applies only to the studied gelator.