Phase Proton Affinities Research Articles

Gas phase proton affinity and basicity of methylated uracil-derivatives

The gas phase proton affinity (PA) and basicity (GB) of methylated uracil derivatives have been evaluated experimentally by the Extended Kinetic Method (EKM) using ESI-TQ mass spectrometry. The experimental PA values, in kJ·mol-1, for 1,5-dimethyluracil (890.0 ± 8.4), 5,6-dimethyluracil (896.4 ± 8.4) and 5,6-dihydro-6-methyluracil (856.0 ± 8.4) were in the same order as those determined theoretically by calculations performed at the B3LYP/6–311++G(3df,2p) level of theory. From the results, it can be concluded that most stable protonated tautomers correspond to the enol structures. Furthermore, it should be noted that the methylation of uracil and 5,6-dihydrouracil has little effect on the proton affinity and basicity of their corresponding methylated derivatives.

Toward tailoring of robust organobases based on extended π-systems: A density functional theory study of the carbonyl basicity

The gas phase proton affinity (PA) and basicity (GB) of a series of extended π-systems, possessing carbonyl as the most basic sites, were calculated using the DFT-B3LYP theoretical method. The backbone of the polycyclic π-electron networks contains either pyrone, pyrolidine or thiopyrane substructures, which can be in α or λ isomer forms. The PAs of designed molecules were reported in the range of 868–1089 kJ mol−1, indicating that some of the molecules have the basicity higher than 1,8-bis(dimethylamino) naphthalene. Such high basicity is a consequence of stabilization in protonated forms, due to the formation of the six-membered aromatic rings upon protonation which stabilize the positive charge. The aromaticity indices of the rings before and after protonation were calculated. The results show that aromaticity indices in the protonated form of the designed molecules is significantly higher than the neutral one.

Investigation of Bridgehead Effects on Reduction Potential in Alkyl and Aryl Azadithiolate‐Bridged (µ‐SCH2XCH2S) [Fe(CO)3]2 Synthetic Analogues of [FeFe]‐H2ase Active Site

Factors affecting the FeIFeI/FeIFe0 reduction potentials (E0) in a series of propane and azadithiolate bridged (µ‐SRS)[Fe(CO)3]2 synthetic model complexes of diiron hydrogenases are investigated. The E0 is found to vary from 60 to 390 mV depending on the nature of the substituent (Alkyl or Aryl) on the nitrogen atom at the bridgehead of the azadithiolate. The E0 is found to shift to more negative values as the average C‐O stretching vibrations ν(CO)avg of the CO ligands decreases, i.e., with increasing backbonding from the iron centers. This trend is accompanied by a linear dependence of the E0 on the σ Hammett parameter of the para substituents in a series of Arylamine bridged complexes and on the gas phase proton affinities (PA) of a series of Alkylamine bridged complexes. The unexpected dependence of E0 on the substituents on the bridgehead nitrogen is addressed by computational chemistry. Density functional theory (DFT) calculations accurately reproduce the geometric structures and the correlations between E0, ν(CO)avg, σ and PA observed in the experimental data. It is concluded that while for Alkyl‐amine bridged complexes an interaction between the nitrogen lone pair with C–S σ* affects ca. 60 mV shift in the E0, the inductive and mesomeric effects of the substituents in the Aryl ring are responsible for ca. 300 mV shift in the E0 of the Aryl‐amine bridged complexes.

The pKa values of amine based solvents for CO2 capture and its temperature dependence—An analysis by density functional theory

A computational chemistry study has been employed to calculate the pKa values of alkylamines (primary, secondary and tertiary) used in the removal of carbon dioxide. A DFT-B3LYP level of theory with 6-311++G (d, p) basis set is used for the computational analysis. The pKa values of amines are of great importance as it has a major role in carbon dioxide capturing mechanism. The temperature dependence of pKa values has also been evaluated and it has been observed that the pKa values of alkylamines shows an inverse relation with the temperature. The Secondary and tertiary amines show maximum and minimum pKa values respectively. The gas phase proton affinity (PA) and gas the phase basicity (GB), which is difficult to measure experimentally, have been computed easily and accurately by this method. It is observed that the PA and GB values increase from primary to tertiary alkylamines. The linear relationship between the pKa values and the free energy of protonation has been computationally verified; the results are in good agreement with the experimental results and the method is simpler and less expensive. The work can be extended to study the carbamate stability and carbon dioxide capture mechanism and there by screening the candidates for the Carbon dioxide Capture and Storage.

Fast and accurate prediction of proton affinities: revisiting the extended Koopmans' theorem for protons.

In this work we propose schemes based on the extended Koopmans' theorem for quantum nuclei (eKT), in the framework of the any particle molecular orbital approach (APMO/KT), for the quantitative prediction of gas phase proton affinities (PAs). The performance of these schemes has been tested on a set of 300 organic molecules containing diverse functional groups. The APMO/KT scheme scaled by functional group (APMO/KT-SC-FG) displays an overall mean absolute error of 1.1 kcal mol-1 with respect to experimental data. Its performance in PA calculations is similar to that of post-Hartree-Fock composite methods or that of the APMO second order proton propagator (APMO/PP2) approach. The APMO/KT-SC-FG scheme is also employed to predict PAs of polyfunctional molecules such as the Nerve Agent VX and the 20 common α-amino acids, finding excellent agreement with available theoretical and/or experimental data. The accuracy of the predictions demonstrates that the APMO/KT-SC-FG scheme is a low-cost alternative to adiabatic methods for the calculation of accurate PAs. One of the most appealing features of the APMO/KT-SC-FG scheme, is that PAs can be derived from one single-point APMO Hartree-Fock calculation.

Prediction of proton affinities of organic molecules using the any-particle molecular-orbital second-order proton propagator approach.

We assess the performance of the recently developed any-particle molecular-orbital second-order proton propagator (APMO/PP2) scheme [M. Díaz-Tinoco, J. Romero, J. V. Ortiz, A. Reyes and R. Flores-Moreno, J. Chem. Phys., 2013, 138, 194108] on the calculation of gas phase proton affinities (PAs) of a set of 150 organic molecules comprising several functional groups: amines, alcohols, aldehydes, amides, ketones, esters, ethers, carboxylic acids and carboxylate anions. APMO/PP2 PAs display an overall mean absolute error of 0.68 kcal mol-1 with respect to experimental data. These results suggest that the APMO/PP2 method is an alternative approach for the quantitative prediction of gas phase proton affinities. One novel feature of the method is that a PA can be obtained from a single calculation of the optimized protonated molecule.

Effect of Solvent Dielectric Constant and Acidity on the OH Vibration Frequency in Hydrogen-Bonded Complexes of Fluorinated Ethanols.

Infrared spectroscopy measurements were used to characterize the OH stretching vibrations in a series of similarly structured fluoroethanols, RCH2OH (R = CH3, CH2F, CHF2, CF3), a series which exhibits a systematic increase in the molecule acidity with increasing number of F atoms. This study, which expands our earlier efforts, was carried out in non-hydrogen-bonding solvents comprising molecules with and without a permanent dipole moment, with the former solvents being classified as polar solvents and the latter designated as nonpolar. The hydrogen bond interaction in donor-acceptor complexes formed in solution between the fluorinated ethanol H-donors and the H-acceptor base DMSO was investigated in relation to the solvent dielectric and to the differences ΔPA of the gas phase proton affinities (PAs) of the conjugate base of the fluorinated alcohols and DMSO. We have observed that νOH decreases as the acidity of the alcohol increases (ΔPA decreases) and that νOH varies inversely with ε, exhibiting different slopes for nonpolar and polar solvents. These 1/ε slopes tend to vary linearly with ΔPA, increasing with increasing acidity. These experimental findings, including the ΔPA trends, are described with our recently published two-state Valence Bond-based theory for acid-base H-bonded complexes. Lastly, the correlation of the alcohol's conjugate base PAs with Taft σ* values of the fluorinated ethyl groups CH(n)F(3-n)CH2- provides a connection of the inductive effects for these groups with the acidity parameter ΔPA associated with the H-bonded complexes.

Conformational preferences and basicities of monofluorinated cyclopropyl amines in comparison to cyclopropylamine and 2-fluoroethylamine

Fluorine substituents in organic molecules do dramatically influence the electronic structure of neighbouring functional groups and the conformation of molecules. Hence the presence of fluorine in a compound changes its chemical reactivity and biological activity. On the basis of MP2 and SCS-MP2 calculations, we discuss the conformational preferences and basicity of the diastereoisomeric 2-fluorocyclopropylamines (cis-2 and trans-2) in comparison to those of cyclopropylamine (1) and 2-fluoroethylamine (3). 1 and 2 are viewed as model compounds for the antidepressant drug tranylcypromine (trans-2-phenylcyclopropylamine, 1a) and its fluorinated derivatives 2. The potential energy profile for the rotation of the amino group in cis-2 differs from that of trans-2 and 1 which have very similar rotational curves. For 2 the global minimum conformer is trans-2a and the lowest energy cis-conformer 2c is less stable by 2.57 kcal mol(-1). The calculated enthalpy differences between the conformers gauche-1b and s-trans-1a (2.0 kcal mol(-1)) as well as between gauche-3b and gauche-4a (0.2 kcal mol(-1)) agree well with the available experimental data of 2.0 kcal mol(-1) and 0.1 +/- 0.3 kcal mol(-1), respectively. The calculated gas phase proton affinities (PA) of 1 (217.6 kcal mol(-1)), cis-2c (215.6 kcal mol(-1)), and trans-2a (209.3 kcal mol(-1)) follow the trends of the pKa values measured in solution for the diastereomeric 2-phenylcyclopropylamines 1a and 1b and their fluorinated derivatives cis-2 and trans-2. It is shown that the conformational preferences and basicity of the investigated molecules are due to stereoelectronic effects from hyperconjugative interactions which lead to different local charge distributions and different hybridization of the nitrogen lone-pair. The basicity of gauche-3a (PA = 215.3 kcal mol(-1)) and anti-3b (PA = 210.1 kcal mol(-1)) is controlled by the charge of the nitrogen atom, while that of cis-2c and trans-2a is overlap controlled as a result of different hybridization of the nitrogen lone-pair [sp4.34 (cis-2c), sp4.07 (trans-2a)].

The proton affinity of a series of substituted acetophenones in their low-lying excited triplet state: A DFT study

Gas phase proton affinities (PA) and transition energies of a series of para- substituted acetophenones and their O-protonated counterparts(aromatic conjugated carbonyl systems) have been theoretically studied by DFT [B3LYP 6–311G (d,p)] calculations with complete geometry optimizations of the relevant excited state.The gas phase O-protonation turns out to be exothermic case and the local stereochemical disposition of the proton is found to be almost the same in each case. The presence of para substituent is seen to cause very little change of the proton affinities relative to unsubstituted acetophenones. Computed proton affinities are sought to be correlated with a number of computed system parameters such as the net charge on the carbonyl oxygen of the unprotonated and protonated species, charge on the proton of the protonated species and the computed hardness of the unprotonated species in the relevant excited state. The proton induced shifts are in general red shifts for the low–lying excited triplet state. The overall basicity is explained by distant atom contribution in addition to the contribution from the carbonyl group.

Cooperative Intramolecular Hydrogen Bonding Effect and Basicity –An Ab Initio and DFT Study of the Superbasic Properties ofN‐[(Dimethylamino)alkyl]‐2,3‐diaminocycloprop‐2‐ene‐1‐imines

AbstractThe MP2(fc)/6‐311+G**//HF/6‐31G* and B3LYP/6‐311+G**//HF/6‐31G* calculations show that N‐[3‐(dimethylamino)propyl] and N‐[4‐(dimethylamino)butyl] derivatives of 2,3‐diamino‐cycloprop‐2‐ene‐1‐imine (CPI) possess high gas phase proton affinities (PA) and pronounced basicities in acetonitrile (MeCN). Tris[N‐dimethylaminopropyl(‐butyl)]‐CPI derivatives achieve superbasic PA and pKa values. The reason behind these remarkable features is the stability of the resulting conjugate acids. They are stabilized by aromatization of the three‐membered ring accompanied by the cationic resonance between the cyclopropyl ring and amino nitrogen atoms of the side chains, and, last but not least, by the cooperative multiple intramolecular hydrogen bonding effect. The latter is particularly enhanced in tris‐substituted CPIs. It was found that the N‐[4‐(dimethylamino)butyl] side chains contribute more to the stability of the conjugate acids than those of the corresponding compound with a propyl chain, because they undergo a somewhat larger relaxation upon protonation. The studied compounds might be helpful in building up a dense ladder of strong bases and superbases, which could contribute to the development of the chemistry of superacids and superbases. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)