Stabilization Enthalpy Research Articles

The influence of substituents in governing the strength of the P-X bonds of substituted halophosphines R1R2P-X (X = F and Cl).

In this study, the gas-phase homolytic P-F and P-Cl bond dissociation energies (BDEs) of a set of thirty fluorophosphine (R1R2P-F) and thirty chlorophosphine-type (R1R2P-Cl) molecules have been obtained using the high-level W2 thermochemical protocol. For the R1R2P-F species, the P-F BDEs (at 298K) differ by up to 117.0kJmol-1, with (H3Si)2P-F having the lowest BDE (439.5kJmol-1) and F2P-F having the largest BDE (556.5kJmol-1). In the case of the chlorophosphine-type molecules, the difference in BDEs is considerably smaller (i.e., 72.6kJmol-1), with (NC)2P-Cl having the lowest P-Cl BDE (299.8kJmol-1) and (HO)2P-Cl having the largest (372.4kJmol-1). We have further analyzed the effect of substituents in governing the P-F and P-Cl BDEs by considering the effect of substituents in the parent halogenated precursors (using molecule stabilization enthalpies) and the effect of substituents in the product radicals (using radical stabilization enthalpies). Finally, we have also assessed the performance of a wide range of DFT methods for their ability to compute the gas-phase P-F and P-Cl BDEs contained in this dataset. We find that, overall, the double hybrid functional DSD-PBEB95 offers the best performance for both bond types, with mean absolute deviations of just 2.1 (P-F BDEs) and 2.2 (P-Cl BDEs) kJmol-1.

A Systematic Exploration of B-F Bond Dissociation Enthalpies of Fluoroborane-Type Molecules at the CCSD(T)/CBS Level.

Fluoroborane-type molecules (R1R2B-F) are of interest in synthetic chemistry, but to date, apart from a handful of small species (such as H2BF, HBF2, and BF3), little is known concerning the effect of substituents in governing the strength of the B-F bonds of such species toward homolytic dissociation in the gas phase. In this study, we have calculated the bond dissociation enthalpies (BDEs) of thirty unique B-F bonds at the CCSD(T)/CBS level using the high-level W1w thermochemical protocol. The B-F bonds in all species considered are very strong, ranging from 545.9 kJ mol-1 in (H2B)2B-F to 729.2 kJ mol-1 HBF2. Nevertheless, these BDEs still vary over a wide range of 183.3 kJ mol-1. The structural properties that affect the BDEs are examined in detail, and the homolytic BDEs are rationalized based on molecule stabilization enthalpies and radical stabilization enthalpies. Since polar B-F bonds may represent a challenging test case for density functional theory (DFT) methods, we proceed to examine the performance of a wide range of DFT methods across the rungs of Jacob's Ladder for their ability to compute B-F BDEs. We find that only a handful of DFT methods can reproduce the CCSD(T)/CBS BDEs with mean absolute deviations (MADs) below the threshold of chemical accuracy (i.e., with average deviations below 4.2 kJ mol-1). The only functionals capable of achieving this feat were (MADs given in parentheses): ωB97M-V (4.0), BMK (3.5), DSD-BLYP (3.8), and DSD-PBEB95 (1.8 kJ mol-1).

Resolving the enthalpy of protein stabilization by macromolecular crowding.

Proteins in the cellular milieu reside in environments crowded by macromolecules and other solutes. Although crowding can significantly impact the protein folded state stability, most experiments are conducted in dilute buffered solutions. To resolve the effect of crowding on protein stability, we use 19 F nuclear magnetic resonance spectroscopy to follow the reversible, two-state unfolding thermodynamics of the N-terminal Src homology 3 domain of the Drosophila signal transduction protein drk in the presence of polyethylene glycols (PEGs) of various molecular weights and concentrations. Contrary to most current theories of crowding that emphasize steric protein-crowder interactions as the main driving force for entropically favored stabilization, our experiments show that PEG stabilization is accompanied by significant heat release, and entropy disfavors folding. Using our newly developed model, we find that stabilization by ethylene glycol and small PEGs is driven by favorable binding to the folded state. In contrast, for larger PEGs, chemical or soft PEG-protein interactions do not play a significant role. Instead, folding is favored by excluded volume PEG-protein interactions and an exothermic nonideal mixing contribution from release of confined PEG and water upon folding. Our results indicate that crowding acts through molecular interactions subtler than previously assumed and that interactions between solution components with both the folded and unfolded states must be carefully considered.

Propene oligomerization on Beta zeolites: Development of a microkinetic model and experimental validation

The microkinetic modelling methodology that we developed previously to describe Brønsted acid-catalyzed propene oligomerization on medium-pore MFI zeolites has been extended successfully to large-pore Beta zeolites. The extension of the model was supported by the identification of the key descriptors that account for the different topologies and acid strengths of the zeolite frameworks (physisorption enthalpies, stabilization enthalpies, frequency factors). The model is validated with experimental conversion and selectivity data measured in a plug-flow reactor on a commercial Beta zeolite over a range of operating conditions. Analysis of net reaction rates allowed identifying the preferred pathways that increase oligomerization selectivity toward C9 species with increasing propene pressure. The model was additionally used to investigate how the stabilization enthalpies of chemisorbed intermediates, an important catalyst descriptor, influenced the selectivity and surface coverage at iso-conversion. This analysis provides mechanistic insights into the propene oligomerization reaction network and its dependence on zeolite topology, and demonstrates how microkinetic models can describe catalyst behaviour and aid in catalyst and process optimization.

A quantum chemical study of the gas-phase homolytic S–Br bond dissociation enthalpies of sulfenyl bromides

Abstract This study reports the gas-phase homolytic S–Br bond dissociation enthalpies (at 298 K) for a set of 44 sulfenyl bromide derivatives (RSBr) obtained using the high-level G4 thermochemical protocol. For the species in this set, we find that the BDEs range from 178.0 kJ mol–1 (F3CSSBr) to 239.5 kJ mol–1 (HC(=NH)SBr). We have additionally reported molecule and radical stabilization enthalpies for all species, allowing for insights to be gained as to the effect of substituents in both the sulfenyl bromide reactants and the sulfur-centered radical products. In addition, we have also reported the BDEs of six sufinyl bromides (RS(=O)Br) and six sulfonyl bromides (RSO2Br). For the sulfinyl bromides, the BDEs range from 156.6 kJ mol–1 (F3CS(=O)Br) to 173.8 kJ mol–1 (tBuS(=O)Br), whilst for the sulfonyl bromides, the BDEs range from 211.0 kJ mol–1 (F3CSO2Br) to 226.2 kJ mol–1 (FSO2Br).

Bonding and Diels–Alder reactions of substituted 2-borabicyclo(1.1.0)but-1(3)-enes: a theoretical study

Substituted 2-borabicyclo(1.1.0)but-1(3)-enes (2BB’s) are a kind of novel molecules with inverted carbon–carbon bonds whose orders range between one and two. Considering the significant interest in the Diels–Alder (D–A) reactions of substituted cyclopropenes, here we studied the bonding and D–A reactions of 24 substituted 2BB molecules at the B3LYP-D3/6-311G** theoretical level. Computations showed that the inverted C=C bond lengths fall in the range of 1.46–1.50 A and 2BB is of significant characteristic of a singlet biradical with two σ single electrons and π aromatic conjugation over the three-membered rings. Yet ab initio valence bond computations showed that the inverted bonds are charge-shifted, and the ionic resonance structures arising from charge-shifting are of carbene characteristics as well. Thus, substituted 2BB’s can also be considered as B-heterocyclic carbenes. Since the LUMOs of the 2BB derivatives are similar to those of ethene and cyclopropene, we studied their D–A reactions with butadiene. All cycloaddition reactions kinetically and thermodynamically favor the formation of the derivatives of 7-bora-oct-3-ene[4.1.1]propellane undergoing endo transition states. Notably, there is a good correlation between the reaction barriers and the hyperconjugative aromatic and antiaromatic stabilization enthalpies for all substituted 2BB’s. In other words, the differences in their reactivity result from the difference in their aromatic stability.

Insights into the Stability of Siloxy Carbene Intermediates and Their Corresponding Oxocarbenium Ions.

Siloxy carbenes, formed thermally or photochemically from acyl silanes via a 1,2-Brook rearrangement, are intriguing reactive intermediates that partake in a range of chemical reactions. To gain further insight into the properties of this class of carbenes, the thermodynamic stabilities of a series of known siloxy carbenes were explored on the basis of hydrogenation enthalpies. Calculations were conducted at the B3LYP-D3(BJ) level (using dispersion-corrected DFT) on siloxy carbenes (X-C-OSiR3, singlet and triplet state), oxocarbenium ions (X-CH-OSiR3+), and their hydrogen addition products (X-CH2-OSiR3). Overall, strong correlation between singlet-triplet gaps and hydrogenation enthalpies was observed. Carbene stabilization enthalpy (CSE) values were also determined to provide additional insight into the structural features that influence the stability of siloxy carbenes.

Gas phase basicity of biguanides – Comparison of the equilibrium and the kinetic methods

The gas-phase basicities (GB) of 7 biguanides were measured by the equilibrium and the entropy corrected kinetic methods. The experimental data were compared with the GB values calculated at B3LYP/6-311+G(2df,p)//B3LYP/6-31G(d) level of theory. Calculated GB values for biguanides correlate well with the experimental data and lie on the same regression line as guanidine derivatives. All biguanides belong to the class of superbases having GB above 1000 kJ mol−1 and being comparable or more basic than the most basic guanidines known. The highest GB of 1103 kJ mol−1 was determined for the bis-dimethylaminomethylene-2,3-bis-(3-dimethylaminopropyl)guanidine (biguanide 4) in which two intramolecular hydrogen bonds (IMHB) can be formed in the protonated form. For selected biguanides and guanidines, contribution of IMHB to the GB was calculated for two conformations with different hydrogen bonding motifs (6- and 8- membered pseudocyclic substructure). In conjugated acids of biguanides having two 3-dimethylaminopropyl or 3-methoxypropyl substituents, conformational preference is determined by maximizing number of possible IMHBs. Hydrogen bond stabilization enthalpies were also calculated using aforementioned B3LYP as well as M06-2X/6-311++G(3df,2p)//B3LYP/6-31G(d) model and compared to the values obtained for guanidine derivatives. Slightly weaker hydrogen bonds in biguanides with respect to guanidines were ascribed to larger steric congestion while the impact of more extensive delocalization area was found negligible.

Trapping CO2 by Adduct Formation with Nitrogen Heterocyclic Carbenes (NHCs): A Theoretical Study.

Carbon dioxide can form compounds with nitrogen heterocyclic carbenes (NHCs) based on azoles through noncovalent interactions or by covalent bonding. A narrow dependence on the carbene structure has been observed for the preference for one or the other type of bonding, as revealed by a series of physicochemical descriptors. In our survey, a set of NHCs based on the azole family (three classical, three abnormal, and one remote) was shown to bind CO2 at the accurate G4MP2 computational level. In most cases, exothermic reaction profiles towards the covalently bound form were found, which reached stabilization enthalpies of up to -77 kJ mol-1 for the remote carbene case. Both noncovalent and covalent minima and the corresponding transition state that connects them have been identified as stationary points along the reaction coordinate.

Oxidative Degradation of Thiaproline Derivatives in Aqueous Solutions Induced by •OH Radicals

AbstractThe participation of neighboring amino and carboxyl groups in thiazolidyne‐4‐carboxylic acid (thiaproline) and its two derivatives, thiazolidyne‐2‐carboxylic acid and thiazolidyne‐2,4‐dicarboxylic acid, is demonstrated during .OH‐radical‐induced oxidation in aqueous solutions. The reaction of .OH radicals with these compounds does not lead to the expected one‐electron oxidized sulfur‐centered radical cations and subsequently to the intermolecularly (S∴S)‐bonded dimeric radical cations (even at very high concentration of thiaprolines (>50 mM)), but rather to the elimination of CO2 with the parallel formation of αN radicals. The latter radicals formed in thiaproline and thiazolidyne‐2,4‐dicarboxylic acid subsequently undergo β‐fragmentation of the CS bond, leading to the thiyl radicals (RS.). The αN and RS. radicals were probed and quantified by one‐electron reduction of p‐nitroacetophenone (PNAP) and by one‐electron oxidation of the monoanion of ascorbic acid (AH−), respectively. The calculated stabilization enthalpies of the αN radicals show that the loss of CO2 is thermodynamically a favorable process. The Gibbs free energies of β‐fragmentation processes in thiaproline and thiazolidyne‐2,4‐dicarboxylic acid are equal to −3.7 and −10.2 kJ mol−1, respectively, showing that at room temperature both are exergonic. A general .OH‐radical‐induced oxidation mechanism of thiaproline derivatives in aqueous solutions is proposed.

The Complexation of the Anticancer Drug ThioTEPA with Methylated DNA Base Guanine: Combined Ab Initio and QTAIM Investigation.

Non-covalent complexes of methylated nitrogenous DNA base guanine (m(9) Gua) with 1 to 6 molecules of anticancer drug ThioTEPA (1,1',1''-phosphorothioyltriaziridine) have been investigated by molecular modeling techniques (molecular docking and DFT geometry optimization), ab initio wavefunction calculations and the quantum theory of atoms in molecules (QTAIM). The accuracy of complex structures predicted by standard molecular docking techniques have been assessed by comparing them with ab initio calculations, and the most important differences have been discussed. Obtained stabilization enthalpies (kcal/mol) for the m(9) Gua⋅⋅⋅(ThioTEPA)n complexes with n=1…6 have been found to be -15.6, -26.5, -38.4, -49.6, -60.5 and -69.3 respectively. The non-covalent interactions revealed by the QTAIM method have been shown to be a dominating factor responsible for the complex stability, with hydrogen bonds of NH⋅⋅⋅N type being the most important interactions in small (n=1 to 4) and CH⋅⋅⋅N bonds - in large (n=5, 6) complexes. The obtained results may help to understand ThioTEPA-DNA interactions and clarify the mechanism of the drug action.

Thermochemistry of 2,2,5,7,8-pentamethylchroman-6-ol (PMC) and 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (trolox)

2,2,5,7,8-Pentamethylchroman-6-ol (PMC, CAS number 950-99-2) and 6-hydroxy-2,5,7,8-tetramethyl chroman-2-carboxylic acid (trolox, CAS number 53188-07-1) are perhaps the two most widely used model compounds for α-tocoferol, the main active antioxidant form of vitamin E. Because the antioxidant activity of α-tocoferol, and of phenol derivatives (ArOH) in general (such as PMC and trolox), is intimately related to the breaking of the ArOH bond, considerable efforts have been made to investigate how the energetics of that bond is linked to the structure and energetics of both the ArO radical and the ArOH parent compound, which are reflected by the corresponding enthalpies of formation. In this work the standard molar enthalpies of formation of crystalline PMC (triclinic, space group P1¯ phase) and trolox (monoclinic, space group P21/c phase), at T=298.15K, were obtained as ΔfHmo(PMC,cr)=−(513.7±3.4)kJ·mol−1 and ΔfHmo(trolox,crI)=−(892.5±4.1)kJ·mol−1 by combustion calorimetry. The results of drop-sublimation Calvet microcalorimetry experiments and vapor pressure vs. temperature measurements by the Knudsen effusion method, combined with Cp,mo(cr) and Cp,mo(g) data obtained by differential scanning calorimetry and calculated at the B3LYP/6-31+G(d,p) level of theory, respectively, led to ΔsubHmo(PMC)=(107.4±0.8)kJ·mol−1 and ΔsubHmo(trolox)=(136.9±2.5)kJ·mol−1 at T=298.15K. From the obtained ΔsubHmo(cr) and ΔsubHmo values it was possible to derive ΔsubHmo(PMC, g)=−(406.3±3.5)kJ·mol−1 and ΔsubHmo=−(755.6±4.8)kJ·mol−1 at T=298.15K. These results led to the conclusion that, in terms of the homodesmotic stabilization enthalpy criteria the replacement of a CH3 group in PMC by a COOH group in trolox approximately conforms to group additivity with no significant contribution from the possible presence of a weak intramolecular hydrogen bond in trolox.

Calculated stabilities and structures for carbocations and singlet carbenes bearing electron‐withdrawing groups

Carbocations and carbenes, as electron‐deficient species, require electron donation from the remainder of the molecule to the carbon center by whatever means available. Classical interactions include resonance, polar and polarizability effects, but neighboring group participation of several sorts can also serve as stabilizing factors. Simple carbocations with directly attached electron‐withdrawing groups (EWGs), that is, EWG–CH2+ ions, comprise one group by which these interactions may be probed. This article provides computational evidence at the MP2/6‐311 + G** level for variable but significant stabilizing interactions between the carbocation center and common EWGs, many via bridging (partial or symmetrical), neighboring group participation, homoconjugation or π interactions. Bridging from atoms possessing nonbonding electron pairs is a common motif. Removal of bridging by application of geometric constraints nullifies bridging stabilization, but does not eliminate the possibility of other stabilizing interactions, for example polar, polarizability and π donation from the EWG. The potential for π donation from strong EWGs has not been widely appreciated in the past. A carbocation stabilization enthalpy, CSE+, is defined as the enthalpy of the isodesmic reaction CH3+ + R–H → CH4 + R+. Comparisons are made with singlet carbenes bearing EWGs for which many structural features are similar. Copyright © 2013 John Wiley & Sons, Ltd.

Thermodynamic parameters of stabilization of the Yersinia pestis Caf113-149 subunit in compact form

Several experimental methods (circular dichroism, viscosity, intrinsic fluorescence, and fluorescence labeling) were used to study the conformational folding/unfolding transitions in a compact monomeric form of the Caf113-149 subunit under the action of guanidine hydrochloride in the temperature range 5–45°C. It has been shown that transitions always occur between two major states (unfolded and compact). This has made it possible to determine all the main thermodynamic functions that characterize the compact state of the Caf113-149 subunit: stability temperature Tm, free energy of stabilization ΔGst, enthalpy ΔHtr, and heat capacity jump ΔC in collapse of the structure. These data have been confirmed by an independent experiment on melting of fluorescently labeled protein.

Effects of Ethynyl Substitution on Cyclobutadiene

The effects of ethynyl substitution on cyclobutadiene are explored via density functional theory and coupled-cluster calculations. The computed singlet-triplet gaps indicate a monotonic dependence on the degree of ethynyl substitution, which differentially stabilizes the triplet relative to the singlet ground state and reduces the gap. A series of isodesmic, homodesmotic, and hyperhomodesmotic equations are employed to quantify the stabilization upon ethynyl substitution. Analyses that rely on a simple isodesmic equation and/or B3LYP/6-31G(d) values are found to be problematic. Analyses that rely on homodesmotic or hyperhomodesmotic equations, in conjunction with CCSD/cc-pVDZ values, are more robust. Using a hyperhomodesmotic equation to assess the stabilization enthalpies of tetra-substituted singlet cyclobutadienes, our analysis predicts tetramethylcyclobutadiene (ΔH(0)(rxn) = -17.3 kcal/mol) to be more stable than tetraethynylcyclobutadiene (ΔH(0)(rxn) = -11.7 kcal/mol), which, in turn, is substantially more stable than tetracyanocyclobutadiene (ΔH(0)(rxn) = +12.7 kcal/mol).

Stabilities of Carbenes: Independent Measures for Singlets and Triplets

Thermodynamic stabilities of 92 carbenes, singlets and triplets, have been evaluated on the basis of hydrogenation enthalpies calculated at the G3MP2 level. The carbenes include alkyl-, aryl-, and heteroatom-substituted structures as well as cyclic 1,3-diheteroatom carbenes. Over a wide energy range, a good correlation is seen between the singlet-triplet gaps and the hydrogenation enthalpies of the singlets, but there are some clear outliers, which represent cases where the triplet has unusual stability or instability. By use of hydrogenation enthalpies, separate carbene stabilization enthalpy scales (CSEs) have been developed for singlets and triplets, and these highlight structural features that affect the stability of each. The treatment also allows estimates of aromaticity in cyclic carbenes. In this way, imidazol-2-ylidene is estimated to have an aromatic stabilization energy of about 20 kcal/mol.

The Thermodynamic Conjugation Stabilization of 1,3-Butadiyne Is Zero

Many textbooks point out that the thermodynamic stabilization enthalpy of 1 mol of 1,3-butadiene relative to 2 mol of 1-butene or to 1 mol of 1,4-pentadiene is slightly less than 4 kcal mol−1, owing to conjugation between the double bonds in the 1,3 configuration. It is reasonable to suppose that the analogous thermochemical stabilization enthalpy of 1,3-butadiyne would be somewhat greater than 4 kcal mol−1, perhaps as high as 7 or 8 kcal mol−1. Thus it was with some surprise that we discovered, as subsequently verified by other groups, that the thermodynamic stabilization enthalpy of 1,3-butadiyne is zero.

ChemInform Abstract: Intramolecular Charge Transfer and Thermal Exciplex Dissociation with p-Aminobenzonitriles in Toluene.

The rate constants of the reversible intramolecular excited-state charge-transfer (CT) reaction of 4-(dimethylamino)benzonitrile (DMABN;I) in toluene were determined as a function of temperature, from double-exponential fluorescence decays measured by time-correlated single-photon counting. The same decay times were found for the locally excited (LE) and the CT state, as shown by global analysis. For the CT state, the stabilization enthalpy {Delta}H ({minus}6 kJ mol{sup {minus}1}) and the change in entropy {Delta}S ({minus}24 J K{sup {minus}1} M{sup {minus}1}) were determined. A correlation of the charge-transfer time of DMABN with the longitudinal dielectric relaxation time was not observed. The presence of dimers or solute/solvent complexes in the ground state was excluded by {sup 1}H NMR experiments. A reversible excited-state reaction was shown to occur in toluene for the planar molecules 1-methyl-5-cyanoindoline (NMCI; II) and 1-methyl-6-cyano-1,2,3,4-tetrahydroquinoline (NMCQ;III) as well as for 3,5-dimethyl-4-(dimethylamino)benzonitrile (DMADBN;IV), based on the wavelength dependence of the double-exponential fluorescence decays. Such a dependence was not found for N-methyl-4-aminobenzonitrile (MABN; IH), 4-aminobenzonitrile (ABN; IHH), and 3,5-dimethyl-4-aminobenzonitrile (ADBN; IVHH), which is attributed to an increase in energy of the CT state relative to the LE state in these secondary and primary aromatic amines.

Thermodynamic basis of the thermostability of CYP175A1 from Thermus thermophilus

Detailed circular dichroism studies have been carried out to monitor thermal as well as denaturant induced unfolding of CYP175A1 from Thermus thermophilus and its mesophilic homologue, CYP101 from Pseudomonas putida. The unfolding midpoint temperatures for tertiary and secondary structures of the substrate-free CYP175A1 were found to be 83.7 degrees C and 87 degrees C respectively, while the corresponding midpoint temperatures for substrate-free CYP101 were at 47.7 degrees C and 51 degrees C respectively. The apparent C(m) value for GdnHCl induced unfolding of secondary structure of CYP175A1 was found to be 2.6M. The thermodynamic stability curves determined from GdnHCl induced unfolding of the enzymes at different temperatures, showed that CYP175A1 had higher free energy compared to CYP101 at temperature >10 degrees C. The results clearly established that the high thermostability of CYP175A1 arises predominantly due to higher enthalpy of the thermostable enzyme compared to CYP101. The larger number of salt bridges was proposed to be responsible for higher enthalpy of stabilization of CYP175A1.

An ab initio study of the structures and stabilities of the complexes of the bases N2O, CO2, and CO with the acids FH, H+, and Li+

The geometries of the complexes of the bases N2O, CO2, and CO with the acids FH, H+, and Li+ have been optimized at second-order Moller–Plesset perturbation theory with the 6–31+G(d, p) basis set. Interaction energies were computed at QCISD(T)/6–31+G(2d,2p). The complexes FH … ONN and FH … NNO have bent and linear geometries, respectively, in agreement with experimental data. The complex FH … OCO also has a bent equilibrium structure, although at room temperature a vibrationally averaged linear structure is observed. FH … CO and FH … OC have linear structures. N-and O-protonated N2O and protonated CO2 have bent structures, whereas C- and O-protonated CO are linear. The Li+ complexes of all of these bases are also linear. Association of H+, Li+, and FH with N2O occurs preferentially at O, although interaction energies for association at N are similar. In contrast, association of these acids with CO is significantly stronger at C than at O. The most stable hydrogen bonded complexes FH … ONN, FH … OCO, and FH … CO have similar stabilization enthalpies at 298 K of −2.2–2.5 kcal/mol. The order of proton affinities of these bases is CO > N2O > CO2, while the order of lithium ion affinities is reversed.