Enthalpies of fusion and solid-to-solid transition,entropies of fusion for urea and twelve alkylureas

Enthalpies and temperatures of fusion have been measured by differential scanning calorimetry for urea and a number of its mono- and di-alkyl derivatives. Enthalpies obtained are: urea, 14.79 kj mol⁻¹; monomethylurea, 15.75 kJ mol⁻¹ ; monoethylurea, 13.94 kJ mol⁻¹ ; monopropylurea, 14.63 kJ mol⁻¹ ; monoisopropylurea, 17.40 kJ mol⁻¹ ; monobutylurea, 14.55 kJ mol⁻¹ ; monotertbutylurea, 33.13 kJ mol⁻¹ ; dimethyl-1,1 urea, 29.61 kJ mol⁻¹ ; dimethyl-1,3 urea, 13.62 kJ mol⁻¹; diethyl-1,1 urea, 16/78 kJ mol⁻¹ ; diethyl-1,3 urea, 12.46 kJ mol⁻¹ ; dibutyl-1,3 urea, 14.87 kJ mol⁻¹; trimethyl-1,1,3 urea, 14.30 kJ mol⁻¹. Entropies of fusion have been derived from the experimental results.By temperature scanning starting from r.t. some solid-to-solid transitions for four alkylureas have also been detected, all hitherto unreported. Temperatures and enthalpies of transition are: for monoisopropylurea, 375.5 K and 2.31 kJ mol ⁻¹ ; for monobutylurea (two transitions), 313.1 K and 7.02 kJ mol⁻¹ , 344.9 K and 0.88 kJ mol⁻¹ ; for diethyl-1,3 urea, 339.4 K and 1.87 kJ mol⁻¹ ; for dibutyl-1,3 urea, 311.5 K and 11.10 kJ mol⁻¹.     

Enthalpies of sublimation and fusion for N-acetyl substituted glycine,L-alanine,and D-leucine amides

Enthalpies and entropies of sublimation for N-acetylglycine amide (NAGA), N-acetyl-L-alanine amide (L-NAAA), and N-acetyl-D-leucine amide (D-NALA) were determined from the dependence of their vapour pressures on temperature, as measured by the torsion-effusion method. Enthalpies and temperatures of fusion were measured by differential scanning calorimetry (DSC) and entropies of fusion were derived. No solid-to-solid transitions were detected from r.t. to fusion. Enthalpies of sublimation and fusion were combined to evaluate enthalpies of vaporization of the melts. The experimental results decreased inversely to the molecular mass. An interpretation of this trend in terms of the crystalline structure of these compounds is proposed.     

Vacuum sublimation kinetics of urea nitrate

Urea nitrate completely sublimes in a continuously pumped vacuum at a rate dependent upon the extent of the surface area. The fraction sublimed (α) vs. time (t) curve is sigmoidal in shape with an inflection point fluctuating between 5.3 and 8.1 % weight loss in the temperature range 56 to 97°. The experimental data fit the grain burning model $$\begin{gathered} 1 - (1 - \beta )^{{1 \mathord{\left/ {\vphantom {1 3}} \right. \kern-\nulldelimiterspace} 3}} = k_s (t - t_0 ) \hfill \\ \beta = \alpha - \beta _0 ,\beta = 0 at inf{\text{lection}} {\text{point}} \hfill \\ \end{gathered} $$ where k_s is a sublimation rate constant, and the constantsβ ₀ andt ₀ are the respective values ofα andt at the inflection point. This equation yielded an activation enthalpy of sublimation of 79±5.4kJ/mole.     

Enthalpies of sublimation of ferrocene and nickelocene measured by calorimetry and the method of Langmuir

A quick and accurate methodology that is based on Langmuir’s equation and that is developed by utilising a DSC7 device is proposed for the measurement of the enthalpies of sublimation of substances characterised by vapour pressures of approximately 1.0 Pa at room temperature. The procedure was applied to ferrocene and nickelocene; the accuracy and uncertainty associated with the experimental results show that the reliability of the developed indirect method is comparable to the direct calorimetric measurements also performed in this work. Furthermore, the melting data and crystal-phase heat capacities for both metallocenes were calorimetrically measured, whereas the gas-phase heat capacity for each metallic bis(cyclopentadienyl) was theoretically estimated by DFT calculations.     

Enthalpies and entropies of fusion of some substituted dipeptides

Enthalpies and temperatures of fusion or transition for four substituted dipeptides (Nacetylamides of glycyl-L-alanine (NAGAA),L-alanyl-L-alanine (NAA₂A),L-prolyl-glycine (NAPGA) andL-leucyl-L-proline monohydrate (NALPA·H₂O)) were determined by differential scanning calorimetry and the entropies of fusion derived. The results obtained have been compared with those of the corresponding substituted aminoacids and some of their racemic crystalline mixtures. The enthalpies and entropies of fusion of some substituted aminoacids have been redetermined. The results are discussed in comparison with crystal structural data, which has been reported in the literature or determined recently by some of the authors. Rationalization of the fusion parameters was attempted mainly on the basis of the number of intramolecular hydrogen bonds and the packing densities in the crystals.This work was carried out with the financial support from the Italian C.N.R. (Rome) and from the Ministry of University and Scientific and Technological Research.     

Enthalpies of solution of urea in water-methanol mixtures at 298.15 K

The standard enthalpies of solution of urea in water-methanol mixtures were determined over the entire range of solvent compositions at 298.15 K. A comparison of the results obtained with the published data on mixtures of water with ethanol and n-propanol revealed a differentiating effect of the alcohol concentration on the enthalpy of solution of urea. Only for water-methanol solutions, an increase in the alkanol content in the mixture (0.6 mole fractions < x ₂ < 1.0 mole fractions) caused an increase in the degree of solvation of urea.     

Enthalpies of dilution of urea solutions in six polar solvents at several temperatures

Using the isothermal displacement calorimeter, enthalpies of dilution are obtained for solutions of urea in water, methanol, ethanol, formamide, N,N-dimethyl-formamide, and dimethyl sulfoxide. The results are discussed in terms of solute-solvent and solute-solute interactions.Now Diane Beaumont.     

Thermodynamics of bolaform electrolytes. V. Enthalpies and heat capacities in aqueous urea solutions

The partial molal heats of solution at infinite dilution of 1,4-bis(triethylammonium)butane dibromide and 1,10-bis(triethylammonium)decane dibromide in aqueous urea (up to 8m urea) have been determined calorimetrically in the temperature range 18–33°C. These data have been used to derive the partial molal heat capacities at infinite dilution, the enthalpies of transfer, and heat capacities of transfer at infinite dilution from water to urea-water solutions. The results show that the enthalpies of transfer are negative and decrease with increasing urea concentrations. The heat capacities of transfer are negative at low urea concentrations and increase in magnitude at higher urea concentrations. In the case of the smaller cation the partial molal heat capacity in 8m aqueous urea solution is greater than in pure water. The results are discussed in terms of structural changes in the solvents on dissolution.     

The fluorescence of diphenyl- and retinol-polyenes

The unusual fluorescence properties of the polyenes are listed. The fluorescence lifetimes and/or quantum yields of the first four all-trans diphenylpolyenes have been determined in seven solvents between ?50 and 50°C. A model is proposed to account for the unusual fluorescence behaviour in terms of a low-lying ¹Ag state, coupled to ¹B_u, and which shifts to lower energies on intramolecular rotation.     

Enthalpies of interaction of urea in mixtures of water and N,N-dimethylformamide at 25°C

Enthalpies of dilution of urea dissolved in mixtures of water and N,N-dimethylformamide with mole fractions of water 0.33, 0.50 and 0.67, respectively, have been measured calorimetrically at 25°C. From these, enthalpic interaction coefficients were obtained. The present results are compared with those of urea dissolved in pure water and in pure N,N-dimethylformamide.     

A consistent estimation of sublimation pressures using a cubic equation of state and fusion properties

In many cases of industrial fluid–solid separation process design, a thermodynamic key parameter may be the sublimation pressure of pure components. A new trend in chemical applications is the use of supercritical solvents either in purifying operations on mixtures of complex pharmaceutical molecules or in stripping on polluted stuff. Measurements of very low sublimation pressures of heavy components are very difficult to perform although their values are of most importance in the process evaluation. Unfortunately, the prediction tools available in the literature for the estimation of sublimation pressures are poor. This paper deals with a consistent approach of sublimation pressure estimation, applicable to any pure material using on one hand easy measurements of normal fusion temperature and fusion enthalpy, and on the other hand vapor pressure data. The influence of all the uncertainties is discussed and the method is proposed as a new reference with emphasis on extrapolating reliably to very heavy compounds. By computing vapor liquid equilibrium using a cubic equation of state (EOS), the estimation of sublimation pressures is discussed in a new perspective.     

A one-step fusion of 1,3-thiazine and pyrimidine cycles

The chlorotrimethylsilane-promoted Biginelli type reactions of aldehydes, thiourea, and cyanoketones led to a diverse set of tetrahydropyrimidine-2(1H)-thiones. Under similar conditions, thioureas, benzaldehyde, and cyanoacetamide reacted to give first representatives of hexahydro-5H-pyrimido[5,4-e][1,3]thiazin-5-ones in high preparative yield.     

Photophysical properties of diphenyl-2,3-dihydroxychlorin and diphenylchlorin

The photophysical properties of 5,15-diphenyl-2,3-dihydroxychlorin (DPCOH) and 5,15-diphenyl-chlorin (DPC) in organic solution were studied. Absorption, fluorescence, triplet state and photobleaching experiments are reported. The ground states of both compounds show strong absorbance in red region (lambda = 638 nm, epsilon = 35,000 M(-1) cm(-1) and lambda = 645 nm, epsilon = 42,000 M(-1) cm(-1), respectively) and the singlet excited states show low fluorescence quantum yields of 0.0802 and 0.150 in benzene and the lifetimes are 7.38 and 10.18 ns, respectively. Absorption spectra of the triplet states were also measured and they have nearly the same triplet state lifetimes of 53 micros (DPCOH) and 50 micros (DPC). The triplet quantum yields are 0.82 and 0.75, respectively. The data of photobleaching quantum yields show that the presence of oxygen does not significantly affect the photobleaching. All the results demonstrate that both diphenylchlorines are good candidates for second-generation photosensitizer in photodynamic therapy.     

An examination of vaporization,fusion and sublimation enthalpies of tolazoline using correlation gas chromatography and differential scanning calorimetry

Journal of Thermal Analysis and Calorimetry - Tolazoline is a vasodilator that is used to treat spasms in peripheral blood vessels and to treat convulsive symptoms. The vaporization, fusion and...     

o-Hydroxyphenylureas. Intermediates in the urea fusion synthesis of 2-benzoxazolinones

o-Hydroxyphenylureas have been identified as intermediates in the urea fusion synthesis of 2-benzoxazolinones. The pyrolysis of these intermediates represents a new alternate synthesis of the benzoxazolinone ring system.     

Spin–spin couplings in doubly 15N labelled urea and 1,3-dimethylurea

The ¹H n.m.r. spectra of doubly ¹⁵N labelled urea and 1,3-dimethylurea have been analysed with the aid of computer simulation. ¹H? ¹H, ¹H? ¹⁵N, ¹⁵N? ¹⁵N and ¹³C? ¹⁵N (from ¹³C Ft spectra) couplings are reported.     

Synthesis of methyl N-phenyl carbamate from dimethyl carbonate and 1,3-diphenyl urea under mild conditions

Synthesis of methyl N-phenyl carbamate from dimethyl carbonate and 1,3-diphenyl urea was investigated under atmospheric pressure. The results showed that homogenous catalyst sodium methoxide had the excellent activity to efficiently catalyze the synthesis of methyl N-phenyl carbamate under atmospheric pressure.