Sublimation Enthalpies of Some Methyl Derivatives of Uracil from Vapor Pressure Measurements

Abstract

The standard sublimation enthalpies for uracil, 1-methyluracil, 3-methyluracil, and 1,3-dimethyluracil, ∆ sub H°(298 K) ) (128 ( 2), (124 ( 5), (121 ( 4), and (118 ( 4) kJ mol -1 , respectively, were determined from their vapor pressures measured by the torsion method. The results of vapor pressure measurements were fit to the following linear equations: uracil, log(p/kPa) ) (12.29 ( 0.15) -(6634 ( 100) K/T (from 384 K to 494 K); 1-methyluracil, log(p/kPa) ) (13.75 ( 0.15) -(6357 ( 150) K/T (from 343 K to 428 K); 3-methyluracil, log(p/kPa) ) (13.59 ( 0.10) -(6210 ( 100) K/T (from 344 K to 419 K); 1,3-dimethyluracil, log(p/kPa) ) (15.10 ( 0.10) -(6049 ( 100) K/T (from 311 K to 364 K). The different sublimation behavior of some compounds can be due to the presence of hydrogen bonds in their solid state.

Sublimation Enthalpies of 5-Haloderivatives of 1,3-Dimethyluracil

“…We have used the following relationships to determine the enthalpies of sublimation at melting point: The values in Table 5 are discordant and difficult to compare. For both substances, the higher sublimation enthalpies are those determined by us in previous works 17,18 by the torsion-effusion method and are different from those reported in this work. These discrepancies are evident also in Figures 4 and 5, which compare the relative percent differences Δp/p calc = (p − p calc )/p calc ·100 between the experimental vapor pressures (p) and the vapor pressures calculated with the corresponding equation reported in Table 3 (p calc ) for present and previous works.…”

“…These discrepancies are evident also in Figures 4 and 5, which compare the relative percent differences Δp/p calc = (p − p calc )/p calc ·100 between the experimental vapor pressures (p) and the vapor pressures calculated with the corresponding equation reported in Table 3 (p calc ) for present and previous works. 17,18,27 The error bars reported in Figures 4 and 5 are combined standard uncertainties of Δp/p calc , obtained combining uncertainties of the experimental vapor pressures reported by the authors and the uncertainties on p calc derived from standard uncertainties on the 27 Knudsen 328.16 95.9 ± 0.6 b 96.8 ± 0.6 Brunetti et al 18 torsion-effusion 355 119 ± 4 b 120.7 c this work Knudsen-effusion 98.1 ± 1.4…”

“…24 To correct the evaporation enthalpy of DMU was used also the Δ c l H(398 K) = 14.6 kJ·mol −1 reported by Sakiyama and Imamura. 33 Table 3 for DMU: •, this work; Δ, Brunetti et al 17 The error bars represent the combined standard uncertainties calculated from the uncertainties of the experimental vapor pressures reported by the authors and the uncertainties on p calc derived from standard uncertainties on the fitted ln(p calc /Pa). Table 3 for 5-FDMU: •, this work; Δ, Amaral et al; 27 ○, Brunetti et al 18 The error bars represent the combined standard uncertainties calculated from the uncertainties of the experimental vapor pressures reported by the authors and the uncertainties on p calc derived from standard uncertainties on the fitted ln(p calc /Pa).…”

Role of Weak Hydrogen Bonds and Halogen Bonds in 5-Halo-1,3-dimethyluracils and Their Cocrystals—A Combined Experimental and Computational Study

“…6 Despite the work already performed, there remains a need to investigate small biomolecular systems where XBs and non-conventional HBs, such as C-H … O, are simultaneously present, as the latter interactions are ubiquitous in many small molecule crystal structures 24,25 and advantageously used for enhancing the efficiency of weak base pairing in DNA. [26][27][28][29][30][31][32][33] Due to our continuing interest in the potential of pyrimidine nucleobases for crystal engineering strategies underpinned by multiple HBs [34][35][36][37][38][39][40][41][42][43][44][45] and our involvement in studies of systems exhibiting halogen bonding via alternative donors (halogen atom not polarized by fluorine) and acceptors (such as anions), [46][47][48][49][50][51][52][53][54][55][56][57][58][59] we were interested in search for systems that could be used to investigate the role of concurrent XBs and non-conventional HBs in the control of sequence, structure and flexibility of DNA halogenated within the natural tract. For this purpose, 5-halogenated derivatives (DMXU; X=F, Cl, Br, I) of N,N-1,3-dimethyluracil (DMHU) and their mixed cocrystals are ideal candidates because of several advantageous properties.…”

Low energy positron interactions with uracil—Total scattering, positronium formation, and differential elastic scattering cross sections

“…The uncertainty in the former is known to be small (<1%), while for the latter, the uncertainty may be considerable for the present experiment. The gas number density is proportional to the pressure in the gas cell (P) which is related to the temperature of the cell (T) and, according to Brunetti et al, 14 it is given by the formula log (P(kPa)) = 12.29 (±0.15) − 6634(±100)/T(K). (1) Applying this formula in the temperature region between 120 • C and 130 • C indicates that the uncertainties in the equation parameters lead to a possible uncertainty in the number density of more than 50%.…”