Gas-phase protonation thermochemistry of arginine

The gas-phase basicity (GB), proton affinity (PA), and protonation entropy (DeltapS degrees (M)=S degrees (MH+)-S degrees (M)) of arginine (Arg) have been experimentally determined by the extended kinetic method using an electrospray ionization quadrupole time-of-flight (ESI-Q-TOF) mass spectrometer. This method provides GB(Arg)=1004.3+/-2.2 (4.9) kJ.mol(-1) (indicated errors are standard deviations, and in parentheses, 95% confidence limits are given). Consideration of previous experimental data using a fast atom bombardment ionization tandem sector mass spectrometer slightly modifies these estimates since GB(Arg)=1005.9+/-3.1 (6.6) kJ.mol(-1). Lower limits of the proton affinity, PA(Arg)=1046+/-4 (7) kJ.mol(-1), and of the "protonation entropy", DeltapS degrees (Arg)=S degrees (ArgH+)-S degrees (Arg)=-27+/-7 (15) J.mol(-1).K(-1), are also provided by the experiments. Theoretical calculations conducted at the B3LYP/6-311+G(3df,2p)//B3LYP/6-31+G(d,p) level, including 298 K enthalpy correction, predict a proton affinity value of ca. 1053 kJ.mol-1 after consideration of isodesmic proton-transfer reactions with guanidine as the reference base. Computations including explicit treatment of hindered rotations and mixing of conformers confirm that a noticeable entropy loss does occur upon protonation, which leads to a theoretical DeltapS degrees (Arg) term of ca. -45 J.mol(-1).K(-1). The following evaluated thermochemical parameter values are proposed: GB(Arg)=1005+/-3 kJ.mol(-1); PA(Arg)=1051+/-5 kJ.mol(-1), and DeltapS degrees (Arg)=-45+/-12 J.mol(-1).K(-1).     

Gas-phase protonation thermochemistry of adenosine

The goal of this work was to obtain a detailed insight on the gas-phase protonation energetic of adenosine using both mass spectrometric experiments and quantum chemical calculations. The experimental approach used the extended kinetic method with nanoelectrospray ionization and collision-induced dissociation tandem mass spectrometry. This method provides experimental values for proton affinity, PA(adenosine) = 979 +/- 1 kJ.mol (-1), and for the "protonation entropy", Delta p S degrees (adenosine) = S degrees (adenosineH (+)) - S degrees (adenosine) = -5 +/- 5 J.mol (-1).K (-1). The corresponding gas-phase basicity is consequently equal to: GB(adenosine) = 945 +/- 2 kJ.mol (-1) at 298K. Theoretical calculations conducted at the B3LYP/6-311+G(3df,2p)//B3LYP/6-31+G(d,p) level, including 298 K enthalpy correction, predict a proton affinity value of 974 kJ.mol (-1) after consideration of isodesmic proton transfer reactions with pyridine as the reference base. Moreover, computations clearly showed that N3 is the most favorable protonation site for adenosine, due to a strong internal hydrogen bond involving the hydroxyl group at the 2' position of the ribose sugar moiety, unlike observations for adenine and 2'-deoxyadenosine, where protonation occurs on N1. The existence of negligible protonation entropy is confirmed by calculations (theoretical Delta p S degrees (adenosine) approximately -2/-3 J.mol (-1).K (-1)) including conformational analysis and entropy of hindered rotations. Thus, the calculated protonation thermochemical properties are in good agreement with our experimental measurements. It may be noted that the new PA value is approximately 10 kJ.mol (-1) lower than the one reported in the National Institute of Standards and Technology (NIST) database, thus pointing to a correction of the tabulated protonation thermochemistry of adenosine.     

Gas-phase basicity of methionine

Proton affinity and protonation entropy of methionine (Met) were determined by the extended kinetic method from ESI-Q-TOF tandem mass spectrometry experiments. The values, PA(Met) = 937.5 +/- 2.9 kJ mol(-1) and Delta(p)S degrees (Met) = - 22 +/- 5 J mol(-1) K(-1), lead to gas-phase basicity GB(Met) = 898.2 +/- 3.2 kJ.mol(-1). Quantum chemical calculations using density functional theory confirm that the proton affinity of Met is indeed in the 940 kJ mol(-1) range and that a significant entropy loss, of at least - 25 J mol(-1) K(-1), occurs upon protonation. This last point is evidenced here for the first time and suggests revision of the tabulated protonation thermochemistry of Met. A comparison with previous experimental data allows us to propose the following evaluated thermochemical values: PA(Met) = 943 +/- 4 kJ mol(-1) and Delta(p)S degrees (Met) = - 35 +/- 15 J mol(-1) K(-1) and GB(Met) = 900 +/- 2 kJ mol(-1).     

Structure, reactivity and thermochemical properties of protonated lactic acid

The protonation energetics of lactic acid (LA) were experimentally determined by the kinetic method including the entropy effect. The values (proton affinity, PA(LA) = 817.4 +/- 4.3 kJ mol(-1); protonation entropy, DeltaS degrees (p)(LA) = -2 +/- 5 J K(-1) mol(-1); gas-phase basicity, GB(LA) = 784.5 +/- 4.5 kJ mol(-1)) agree satisfactorily with computed G2(MP2) expectations (PA(LA) = 811.8 kJ mol(-1); DeltaS degrees (p)(LA) = -7.1 J K(-1) mol(-1); GB(LA) = 777.4 kJ mol(-1)). The fragmentation behaviour of protonated lactic acid (LAH(+)) is dominated by carbon monoxide loss followed by elimination of a water molecule. Direct dehydration of LAH(+) is only a high-energy process hardly competitive with the CO loss. A complete mechanistic scheme, based on MP2/6-31G* calculations, is proposed; it involves isomerization of the various protonated forms of LA and the passage through the ion-neutral complex between the 2-hydroxypropyl acylium cation and a water molecule.     

Proton affinity of β-oxalylaminoalanine (BOAA): Incorporation of direct entropy correction into the single-reference kinetic method

A new version of the single-reference-extended kinetic method is presented in which direct entropy correction is incorporated. Results of calibration experiments with the monodentate base pyridine and the bidentate base ethylenediamine are presented for which the method provides proton affinities in excellent agreement with published values and reasonable predictions for the protonation entropies. The method is then used to determine the proton affinity and protonation entropy of the non-protein amino acid beta-oxalylaminoalanine (BOAA). The PA of BOAA is found to be 933.1 +/- 7.8 kJ/mol and a prediction for the protonation entropy of -39 J mol(-1) K(-1) is also obtained, indicating a significant degree of intramolecular hydrogen bonding in the protonated form. These results are supported by hybrid density functional theory calculations at the B3LYP/6-311++G**//B3LYP/6-31+G* level. They indicate that the preferred site of protonation is the alpha-nitrogen atom (PA = 935.0 kJ/mol) and that protonated BOAA has a strong hydrogen bond between the hydrogen on the alpha-amino group and one of the carbonyl oxygen atoms on the side chain.     

Effects of internal hydrogen bonds between amide groups: protonation of alicyclic diamides

The proton affinity (PA) of cyclopentane carboxamide 1, cyclohexane carboxamide 2 and their secondary and tertiary amide derivatives S1, S2, T1 and T2, was determined by the thermokinetic method and the kinetic method [PA(1) = 888 +/- 5 kJ mol(1); PA(2) = 892 +/- 5 kJ mol(1); PA(S1) = 920 +/- 6 kJ mol(1); PA(S2) = 920 +/- 6 kJ mol(1); PA(T1) = 938 +/- 6 kJ mol(1); PA(T2) = 938 +/- 6 kJ mol(1)]. Special entropy effects are not observed. Additionally, the effects of protonation have been studied using an advanced kinetic method for all isomers 37 of cyclopentane dicarboxamides and cyclohexane dicarboxamides (with the exception of cis-cyclopentane-1,2-dicarboxamide) and their bis-tertiary derivatives T3T7 by estimating the PA and the apparent entropy of protonation Delta(DeltaS(app)). Finally, the study was extended to bicyclo[2.2.1]hepta-2,5-diene-2,3-dicarboxamide 8 and its bis-tertiary derivative T8, to all stereoisomers of bicyclo[2.2.1]heptane-2,3-dicarboxamide 9, their secondary and tertiary amide derivatives S9 and T9, and to endoendobicyclo[2.2.1]heptane-2,5-dicarboxamide 10 and the corresponding secondary and tertiary derivatives S10 and T10. Compared with 1 and 2, all alicyclic diamides exhibit a significant increase of the PA (DeltaPA) and special entropy effects on protonation. For alicyclic diamides, which can not accommodate a conformation appropriate for building a proton bridge, the values of DeltaPA and Delta(DeltaS(app)) are small to moderate. This is explained by ion / dipole interactions between the protonated and neutral amide group which stabilize the protonated species but hinder the free rotation of the amide groups. If any of the conformations of the alicyclic diamide allows formation of a proton bridge, DeltaPA and Delta(DeltaS(app)) increase considerably. A spectacular case is cis-cyclohexane-1,4-dicarboxamide 7c which is the most basic monocyclic diamide, although generation of the proton bridge requires the unfavorable boat conformation with both amide substituents at a flagpole position. A pre-orientation of the two amide groups in such a 1,4-position in 10 results in a particularly large PA of < 1000 kJ mol(1). The observation of comparable values for Delta(DeltaS(app)) for linear and monocyclic diamides indicates that a major part of the entropy effects originates from freezing the free rotation of the amide groups by formation of the proton bridge. This is corroborated by observing corresponding effects during the protonation of dicarboxamides containing the rigid bicyclo[2.2.1]heptane carbon skeleton, where the only internal movements of the molecules corresponds to rotation of the amide substituents.     

Structural and energetic aspects of the protonation of phenol,catechol, resorcinol,and hydroquinone

The various protonated forms of phenol (1), catechol (2), resorcinol (3), and hydroquinone (4) were explored by ab initio quantum chemical calculations at the MP2/6-31G(d) and B3LYP/6-31G(d) levels. Proton affinities (PA) of 1-4 were calculated by the combined G2(MP2,SVP) method, and their gas-phase basicities were estimated after calculation of the change in entropy on protonation. These theoretical data were compared with the corresponding experimental values determined in a high-pressure mass spectrometer. This comparison confirmed that phenols are essentially carbon bases and that protonation generally occurs in a position para to the hydroxyl group. Resorcinol is the most effective base (PA = 856 kJ mol-1) due to the participation of both oxygen atoms in the stabilization of the protonated form. Since protonation is accompanied by a freezing of the two internal rotations, a significant decrease in entropy is observed. The basicity of catechol (PA = 823 kJ mol-1) is due to the existence of an intramolecular hydrogen bond, which is strengthened upon protonation. The lower basicity of hydroquinone (PA = 808 kJ mol-1) is a consequence of the fact that protonation necessarily occurs in a position ortho to the hydroxyl group. When the previously published data are reconsidered and a corrected protonation entropy is used, a proton affinity value of 820 kJ mol-1 is obtained for phenol.     

Protonation thermochemistry of alpha-aminoacids bearing a basic residue

The proton affinity, PA, and protonation entropy, Delta(p)S degree, of glycine (Gly), 1, aspartic acid (Asp), 2, asparagine (Asn), 3, histidine (His), 4, lysine (Lys), 5, glutamic acid (Glu), 6, and glutamine (Gln), 7, have been reinvestigated by the extended kinetic method, using the "isothermal point" method and the orthogonal distance regression, ODR, technique. The proton affinity values of a-aminoacids bearing a basic residue (PA = 926.8; 965.2; 996.0; 993.9; 981.8 and 988.1 kJ.mol(-1) for 2-7, respectively) show significant deviation from the tabulated values. As expected from the effect of a strong intramolecular hydrogen bond in the protonated forms of these peculiar aminoacids, negative protonation entropies are detected (Delta(p)S degree = 36; 43; 37; 29; 95 and 55 J mol(-1) K(-1) for for 27 respectively).     

Protonated p-Benzoquinone

The structure and energetics of protonated p-benzoquinone (pBQ) have been investigated using high-pressure mass spectrometry and ab initio calculations. The experimental proton affinity of pBQ is 801.4 +/- 8.9 kJ/mol (191.5 +/- 2.1 kcal/mol) (1sigma) from bracketing measurements and hydration thermochemistry. This value is supported by theory and by analogies with related compounds. In its protonation chemistry, pBQ behaves as an aliphatic ketone, both structurally and energetically. The dissociation of the hydrate (pBQH(+)).(H(2)O) is characterized by DeltaH degrees (D) = 90.0 +/- 2.3 kJ/mol and DeltaS degrees (D) = 123.4 +/- 4.9 J/mol.K (95% confidence).     

The gas-phase basicity and proton affinity of 1,3,5-cycloheptatriene--energetics,structure and interconversion of dihydrotropylium ions

The hitherto unknown gas-phase basicity and proton affinity of 1,3,5-cycloheptatriene (CHT) have been determined by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Several independent techniques were used in order to exclude ambiguities due to proton-induced isomerisation of the conjugate cyclic C(7)H(9)(+) ions, [CHT + H](+). The gas-phase basicity obtained by the thermokinetic method, GB(CHT) = 799 +/- 4 kJ mol(-1), was found to be identical, within the limits of experimental error, with the values measured by the equilibrium method starting with protonated reference bases, and with the values resulting from the measurements of the individual forward and reverse rate constants, when corrections were made for the isomerised fraction of the C(7)H(9)(+) population. The experimentally determined gas-phase basicity leads to the proton affinity of cycloheptatriene, PA(CHT) = 833 +/- 4 kJ mol(-1), and the heat of formation of the cyclo-C(7)H(9)(+) ion, deltaH(f)(0)([CHT + H](+)) = 884 +/- 4 kJ mol(-1). Ab initio calculations are in agreement with these experimental values if the 1,2-dihydrotropylium tautomer, [CHT + H((1))](+), generated by protonation of CHT at C-1, is assumed to be the conjugate acid, resulting in PA(CHT) = 825 +/- 2 kJ mol(-1) and deltaH(f)(0)(300)([CHT + H((1))](+)) = 892 +/- 2 kJ mol(-1). However, the calculations indicate that protonation of cycloheptatriene at C-2 gives rise to transannular C-C bond formation, generating protonated norcaradiene [NCD + H](+), a valence tautomer being 19 kJ mol(-1) more stable than [CHT + H((1))](+). The 1,4-dihydrotropylium ion, [CHT + H((3))](+), generated by protonation of CHT at C-3, is 17 kJ mol(-1) less stable than [CHT + H((1))](+). The bicyclic isomer [NCD + H](+) is separated by relatively high barriers, 70 and 66 kJ mol(-1) from the monocyclic isomers, [CHT + H((1))](+) and [CHT + H((3))](+), respectively. Therefore, the initially formed 1,2-dihydrotropylium ion [CHT + H((1))](+) does not rearrange to the bicyclic isomer [NCD + H](+) under mild protonation conditions.     

A quantum chemical study of the protonation of phenylphosphine and its halogenated derivatives

The protonation and methylation of phenylphosphine (C(6)H(5)PH(2)) and its mono-halogenated derivatives have been studied using ab initio quantum chemical calculations. Density functional theory (B3LYP) calculations using the 6-311++G(d,p) basis set consistently confirm that protonation of phenylphosphines takes place at the phosphorus atom; the C(4)-protonated phenylphosphine lying about 66 kJ mol(-1) above the P-protonated isomer. Similarly, methylation of phosphines consistently occurs at phosphorus. The proton and methyl cation affinities are estimated as follows: PA(phenylphosphine) = 863 +/- 10 kJ mol(-1) and MCA(phenylphosphine) = 515 -/+ 12 kJ mol(-1). Mono-halogen substitution appears to reduce the proton affinites by up to 20 kJ mol(-1). In this context, following P-protonation of either ameta- or a para-X-C(6)H(4)-PH(2), an elimination of the halogen X-atom under collisional activation (CA) conditions is expected to lead to a distonic radical cation, a low-energy isomer being 50 kJ mol(-1) above ionized phenylphosphine.     

Calorimetric and computational study of thiacyclohexane 1-oxide and thiacyclohexane 1,1-dioxide (thiane sulfoxide and thiane sulfone). Enthalpies of formation and the energy of the S=O bond

A rotating-bomb combustion calorimeter specifically designed for the study of sulfur-containing compounds [J. Chem. Thermodyn. 1999, 31, 635] has been used for the determination of the enthalpy of formation of thiane sulfone, 4, Delta(f)H(o) m(g) = -394.8 +/- 1.5 kJ x mol(-1). This value stands in stark contrast with the enthalpy of formation reported for thiane itself, Delta(f)H(o) m(g) = -63.5 +/- 1.0 kJ x mol(-1), and gives evidence of the increased electronegativity of the sulfur atom in the sulfonyl group, which leads to significantly stronger C-SO2 bonds. Given the known enthalpy of formation of atomic oxygen in the gas phase, Delta(f)H(o) m(O,g) = +249.18 kJ x mol(-1), and the reported bond dissociation energy for the S=O bond in alkyl sulfones, BDE(S=O) = +470.0 kJ x mol(-1), it was possible to estimate the enthalpy of formation of thiane sulfoxide, 5, a hygroscopic compound not easy to use in experimental calorimetric measurements, Delta(f)H(o) m(5) = -174.0 kJ x mol(-1). The experimental enthalpy of formation of both 4 and 5 were closely reproduced by theoretical calculations at the G2(MP2)+ level, Delta(f)H(o) m(4) = -395.0 kJ x mol(-1) and Delta(f)H(o) m(5) = -178.0 kJ x mol(-1). Finally, calculated G2(MP2)+ values for the bond dissociation energy of the S=O bond in cyclic sulfoxide 5 and sulfone 4 are +363.7 and +466.2 kJ x mol(-1), respectively.     

Obtaining thermochemical data by the extended kinetic method

A microcanonical analysis of the extended kinetic method is performed using statistical rate calculations based on orbiting transition state theory. The model systems simulate polydentate bases M which exhibit losses of entropy upon protonation of up to 35 kJ mol(-1) K(-1). It is shown that the correlations using the natural logarithm of the ratio of rate constants vs the proton affinity of the reference bases, at several effective temperatures, lead to correct proton affinity and protonation entropy of the base M of interest. A systematic underestimate of the latter quantity (by 5-15%), mainly due to the use of a linear rather than a polynomial curve fitting procedure, is noted, however. When considering experimental data, more severe underestimates are observed for the protonation entropies of polydentate bases (by 50-90%). The origins of these considerable discrepancies are beyond the limits of the present modeling and remain to be determined.     

Synthesis, crystal structure, and H/D exchange of the inside protonated form of the cage imine 4,8,12-triaza-1-azoniatricyclo[6.6.3.2(4,12)]nonadec-1(15)-ene. A model for proton transfer through an aliphatic membrane

The reaction of the inside protonated form of the tricyclic amine 1,4,8,12-tetraazatricyclo[6.6.3.2(4,12)]nonadecane (1) with iron(III) affords the inside monoprotonated form of the corresponding imine 4,8,12-triaza-1-azoniatricyclo[6.6.3.2(4,12)]nonadec-1(15)-ene (2), which was isolated as the tetrabromozincate salt (2a) in a yield of 78%. The crystal structure of 2a has been solved by X-ray diffraction at T = 120 K. In the imine cation the acidic hydrogen atom and the lone pairs of the nitrogen atoms are oriented toward the inside of the cavity. The acidic hydrogen atom is bound to a nitrogen atom belonging to the triazacyclononane entity. The imine double bond is situated between the N-atom of the triazacyclononane entity and the C-atom belonging to one of the three trimethylene bridges. The imine 2 is stable in acidic solution and the inside coordinated proton is very robust in acidic solution. In basic solution the imine reacts fast to give a quantitative formation of the inside protonated form of the hemiaminal 1,4,8,12-tetraazatricyclo[6.6.3.2(4,12)]nonadecan-5-ol (3). The equilibrium constant K(im) = [3][H(+)]/[2] was determined at three different temperatures from potentiometric measurements, which gave K(im) = 1.57(1) x 10(-5) M at 25 degrees C, Delta S degrees = -83(1) J mol(-1) K(-)(1),and Delta H degrees = 2.6(3) kJ mol(-1) at I = 1.0 M (NaCl). The inside coordinated proton in 3 is labile in basic solution and the rate for NH/ND exchange was determined by (1)H NMR at three different temperatures. The reaction followed the expression k(obs) = k(ex)[OD(-)] with k(ex) = 0.0978(30) dm(3) mol(-1) s(-1) at 25 degrees C, Delta S(++) = 87(4) J mol(-1) K(-1), and Delta H(++) = 104.9(11) kJ mol(-1) at I = 1.0 M (NaCl). The exchange rate is more than 5 x 10(6) times faster than that of the parent saturated cage 1. This extreme enhancement of reactivity is explained by an intramolecular proton transfer reaction mediated by hydroxy and oxy groups flipping in and out of the cavity, which mechanistically has resemblance to the transport of ions in a biological system.     

Isoelectronic homologues and isomers: tropolone, 5-azatropolone, 1-H-azepine-4,5-dione, saddle points, and ions

Computational studies of 12 64-electron homologues and isomers of tropolone in the S(0) electronic ground state are reported. Three minimum-energy structures, tropolone (Tp), 5-azatropolone (5Azt), and 5-H-5-azatropolonium (5AztH(+)), have an internal H-bond and planar C(s)) geometry, and three, tropolonate (TpO(-)), 5-azatropolonate (5AzO(-)), and 1-H-azepine-4,5-dione (45Di), lack the H-bond and have twisted C(2) geometry. All 6 substances have an equal double-minimum potential energy surface and a saddle point with planar C(2)(v) geometry. The energy for the gas-phase isomerization reaction 45Di --> 5Azt is near +4 kJ mol(-1) at the MP4(SDQ)/6-311++G(df,pd)//MP2/6-311++G(df,pd) (energy//geometry) theoretical level and around -20 kJ mol(-1) at lower theoretical levels. The dipole moments computed for 45Di and 5Azt are 9.6 and 2.1 D, respectively, and this large difference contributes to MO-computed free energies of solvation that strongly favor--as experimentally observed--45Di over 5Azt in chloroform solvent. The MO-computed energy for the gas-phase protonation reaction 45Di + H(+) --> 5AztH(+) is -956.4 kJ mol(-1), leading to 926.8 kJ mol(-1) as the estimated proton affinity for 45Di at 298 K and 1 atm. The intramolecular dynamical properties predicted for 5Azt and 5AztH(+) parallel those observed for tropolone. They are therefore expected to exhibit spectral tunneling doublets. Once they are synthesized, they should contribute importantly to the understanding of multidimensional intramolecular H transfer and dynamical coupling processes.     

Gas‐phase basicity of 2‐furaldehyde

2‐Furaldehyde (2‐FA), also known as furfural or 2‐furancarboxaldehyde, is an heterocyclic aldehyde that can be obtained from the thermal dehydration of pentose monosaccharides. This molecule can be considered as an important sustainable intermediate for the preparation of a great variety of chemicals, pharmaceuticals and furan‐based polymers. Despite the great importance of this molecule, its gas‐phase basicity (GB) has never been measured. In this work, the GB of 2‐FA was determined by the extended Cooks's kinetic method from electrospray ionization triple quadrupole tandem mass spectrometric experiments along with theoretical calculations. As expected, computational results identify the aldehydic oxygen atom of 2‐FA as the preferred protonation site. The geometries of O‐O‐cis and O‐O‐trans 2‐FA and of their six different protomers were calculated at the B3LYP/aug‐TZV(d,p) level of theory; proton affinity (PA) values were also calculated at the G3(MP2, CCSD(T)) level of theory. The experimental PA was estimated to be 847.9 ± 3.8 kJ mol^?1, the protonation entropy 115.1 ± 5.03 J mol^?1 K^?1 and the GB 813.6 ± 4.08 kJ mol^?1 at 298 K. From the PA value, a ΔH°_f of 533.0 ± 12.4 kJ mol^?1 for protonated 2‐FA was derived. Copyright © 2012 John Wiley & Sons, Ltd.     

Solvation properties of N-substituted cis and trans amides are not identical: significant enthalpy and entropy changes are revealed by the use of variable temperature 1H NMR in aqueous and chloroform solutions and ab initio calculations

The cis/trans conformational equilibrium of N-methyl formamide (NMF) and the sterically hindered tert-butylformamide (TBF) was investigated by the use of variable temperature gradient 1H NMR in aqueous solution and in the low dielectric constant and solvation ability solvent CDCl3 and various levels of first principles calculations. The trans isomer of NMF in aqueous solution is enthalpically favored relative to the cis (deltaH(o) = -5.79 +/- 0.18 kJ mol(-1)) with entropy differences at 298 K (298 x deltaS(o) = -0.23 +/- 0.17 kJ mol(-1)) playing a minor role. The experimental value of the enthalpy difference strongly decreases (deltaH(o) = -1.72 +/- 0.06 kJ mol(-1)), and the contribution of entropy at 298 K (298 x deltaS(o) = -1.87 +/- 0.06 kJ mol(-1)) increases in the case of the sterically hindered tert-butylformamide. The trans isomer of NMF in CDCl3 solution is enthalpically favored relative to the cis (deltaH(o) = -3.71 +/- 0.17 kJ mol(-1)) with entropy differences at 298 K (298 x deltaS(o) = 1.02 +/- 0.19 kJ mol(-1)) playing a minor role. In the sterically hindered tert-butylformamide, the trans isomer is enthalpically disfavored (deltaH(o) = 1.60 +/- 0.09 kJ mol(-1)) but is entropically favored (298 x deltaS(o) = 1.71 +/- 0.10 kJ mol(-1)). The results are compared with literature data of model peptides. It is concluded that, in amide bonds at 298 K and in the absence of strongly stabilizing sequence-specific inter-residue interactions involving side chains, the free energy difference of the cis/trans isomers and both the enthalpy and entropy contributions are strongly dependent on the N-alkyl substitution and the solvent. The significant decreasing enthalpic benefit of the trans isomer in CDCl3 compared to that in H2O, in the case of NMF and TBF, is partially offset by an adverse entropy contribution. This is in agreement with the general phenomenon of enthalpy versus entropy compensation. B3LY/6-311++G** and MP2/6-311++G** quantum chemical calculations confirm the stability orders of isomers and the deltaG decrease in going from water to CHCl3 as solvent. However, the absolute calculated values, especially for TBF, deviate significantly from the experimental values. Consideration of the solvent effects via the PCM approach on NMF x H2O and TBF x H2O supermolecules improves the agreement with the experimental results for TBF isomers, but not for NMF.     

Thermodynamic relationship between structural isomers of the thermochromic compound bis(N-Isopropyl-5,6-benzosalicylideneiminato)nickel(II)

A pair of structural isomers was isolated at room temperature for the thermochromic nickel complex bis( N-isopropyl-5,6-benzosalicylideneiminato)nickel(II); one is a diamagnetic green form with square-planar coordination geometry (G phase), and the other is a paramagnetic brown form with a tetrahedral geometry (B phase). However, a question as to which form is thermodynamically stable was left open. To solve this problem, thermal and magnetic properties of this complex were investigated by adiabatic heat capacity calorimetry in the 6-508 K temperature range and magnetic measurements in the 2-400 K region. In addition to the two forms previously reported, two metastable crystal forms (G' and B' phases) were found. The stable phase sequence was G phase, B phase, and then liquid upon heating. The supercooled B phase gave rise to a small phase transition with nonmagnetic origin at around 50 K. By rapidly cooling the liquid, a glassy liquid state was realized below approximately 290 K. The order of thermodynamic stability at 298.15 K was revealed to be the G, B, G', and then the B' phase. The entropy, enthalpy, and Gibbs energy differences between the B and the G phases at 298.15 K were S degrees (B) - S degrees (G) = 32.8 J K (-1) mol (-1), H degrees (B) - H degrees (G) = 16.0 kJ mol (-1), and G degrees (B) - G degrees (G) = 6.25 kJ mol (-1), respectively.     

Density functional computations of proton affinity and gas-phase basicity of proline

The proton affinity and gas-phase basicity of proline were evaluated by using density functional theory coupling the B3-LYP hybrid functional with the extended 6--311++G** basis set. Cis and trans conformations of the carboxyl moiety for both exo and endo ring structures were considered for the neutral proline. The results show that the most stable structure of proline has the endo ring conformation with the carboxyl group in the cis position. The structure at the global minimum is stabilized by an intramolecular hydrogen bond. The nitrogen of the ring in the exo form is the preferred protonation site. The calculated proton affinity (924.3 kJ mol(-1)) and gas-phase basicity (894.4 kJ mol(-1)) are in very good agreement with the experimental counterparts.     

Acid-catalyzed nucleophilic aromatic substitution: experimental and theoretical exploration of a multistep mechanism

The mechanism for the acid-mediated substitution of a phenolic hydroxyl group with a sulfur nucleophile has been investigated by a combination of experimental and theoretical methods. We conclude that the mechanism is distinctively different in nonpolar solvents (i.e., toluene) compared with polar solvents. The cationic mechanism, proposed for the reaction in polar solvents, is not feasible and the reaction instead proceeds through a multistep mechanism in which the acid (pTsOH) mediates the proton shuffling. From DFT calculations, we found a rate-determining transition state with protonation of the hydroxyl group to generate free water and a tight ion pair between a cationic protonated naphthalene species and a tosylate anion. Kinetic experiments support this mechanism and show that, at moderate concentrations, the reaction is first order with respect to 2-naphthol, n-propanethiol, and p-toluenesulfonic acid (pTsOH). Experimentally determined activation parameters are similar to the calculated values (Delta H exp not equal =105+/-9, Delta H calcd not equal =118 kJ mol(-1); Delta G exp not equal =112+/-18, Delta G calcd not equal =142 kJ mol(-1)).