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Zvonimir B Maksic - One of the best experts on this subject based on the ideXlab platform.
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high basicity of phosphorus Proton Affinity of tris tetramethylguanidinyl phosphine and tris hexamethyltriaminophosphazenyl phosphine by dft calculations
Chemical Communications, 2006Co-Authors: Borislav Kovacevic, Zvonimir B MaksicAbstract:It is shown by approximate but reliable DFT calculations that the title compounds represent very strong superbases in gas phase and MeCN. In particular, tris-(hexamethyltriaminophosphazenyl)phosphine has a Proton Affinity, PA, of 295.5 kcal mol−1 and records a pKa(MeCN) of 50 ± 1 units.
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the intramolecular hydrogen bond and intrinsic Proton Affinity of neutral organic molecules n n n tris 3 aminopropyl guanidine and some related systems
Journal of Physical Organic Chemistry, 2002Co-Authors: Borislav Kovacevic, Zoran Glasovac, Zvonimir B MaksicAbstract:Multiple aminopropyl substitutions of the initial N,N′,N″-trimethylguanidine lead to high intrinsic absolute Proton affinities (APA) and basicities culminating in APA = 268.4 kcal mol−1 (1 kcal = 4.184 kJ) in N,N′,N″-tris(3-aminopropyl)guanidine (9). The reason behind a high Proton Affinity is identified as a strong cationic resonance in the central guanidine moiety and the strength of the intramolecular hydrogen bonding (IMHB), which is enhanced upon Protonation. A cooperative IMHB effect in 9 and 9H+ realized by three N(sp3)H···N(sp3) H-bridges is estimated to be as high as 14.7 and 33.0 kcal mol−1, respectively. It follows that the IMHB effect contributes 18.3 kcal mol−1 to the absolute Proton Affinity of 9, which is a respectable amount. The basicity of 9 in acetonitrile is estimated to be very high, as evidenced by the corresponding pKa = 28.8. A derivative of 9, N,N′,N″-tris(3-dimethylaminopropyl)guani-dine (10), assumes an even higher superbasic Proton Affinity of 275 kcal mol−1 due to an additional relaxation effect caused by the methyl groups. The corresponding pKa(MeCN) = 29.4. Copyright © 2002 John Wiley & Sons, Ltd.
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predicted high Proton Affinity of poly 2 5 dihydropyrrolimines the aromatic domino effect
Journal of Physical Organic Chemistry, 2002Co-Authors: Zvonimir B Maksic, Zoran Glasovac, Ines DespotovicAbstract:The basicity of a family of 2,5-dihydropyrrolimines is examined by the Hartree-Fock model. It is found that these systems exhibit high Proton Affinity, which increases with the number of the five-membered rings present. The origin of the pronounced basicity is identified as the tandem or domino aromatic effect occurring in the conjugate acids. The aromatization is triggered by Protonation and spread over whole systems via mobile π-electrons. Proton affinities as high as 300 kcal/mol or more can be achieved in the gas phase, if the aromatization is combined with the favourable substituent effects and with the intramolecular hydrogen bond corona effect. Copyright © 2002 John Wiley & Sons, Ltd.
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the Proton Affinity of the superbase 1 8 bis tetramethylguanidino naphthalene tmgn and some related compounds a theoretical study
Chemistry: A European Journal, 2002Co-Authors: Borislav Kovacevic, Zvonimir B MaksicAbstract:The spatial and electronic structure of the very strong neutral organic bases TMGN, TMGF and some related compounds are explored by ab initio computational methods. Their Affinity towards the Proton is scrutinized both in the gas phase and in acetonitrile. The Protonation at the most basic center (imine nitrogen) yields asymmetric and relatively strong intramolecular hydrogen bonds (IHB). It is found that the angular strain effect and steric repulsion practically vanish in TMGN implying that its high absolute Proton Affinity (APA) has its origin in the inherent basicity of the guanidine fragment and a relatively strong IHB in [TMGN]H++. The nonbonded repulsions in TMGF are higher than in TMGN, which in conjunction with a slightly stronger IHB in the corresponding conjugate acid makes it more basic: APA(TMGF) > APA(TMGN). An interesting new phenomenon is observed in both TMGN and TMGF: the Proton triggers the resonance stabilization not only in the directly bonded guanidine moiety, but also in the other guanidine fragment placed more distantly from Proton, albeit in a less pronounced manner. The latter feature is termed as partial Protonation. It supports the hydrogen bonding and contributes to the IHB stabilization. Convincing evidence is presented that the solvent effect in acetonitrile is determined by two antagonistic factors: (1) the intrinsic (gas phase) Proton Affinity and (2) the size effect given by the ratio between the positive charge in molecular cation (conjugate acid) and the magnitude of the molecular surface. The resulting pK_a values are given by an interplay of these factors.
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absolute Proton Affinity of some polyguanides
Journal of Organic Chemistry, 2000Co-Authors: Zvonimir B Maksic, Borislav KovacevicAbstract:The problem of the absolute Proton Affinity (APA) of some polyguanides is addressed by the MP2(fc)/6-311+G//HF/6-31G theoretical model. It is shown that the linear chain polyguanides exhibit increased basicity as a function of the number of guanide subunits. However, the saturation effect yields an asymptotic APA value of 254 kcal/mol. Branched polyguanides on the other hand have higher APAs than their linear counterparts. The largest Proton Affinity is found in a doubly bifurcated heptaguanide, being as high as 285 kcal/mol, thus potentially representing one of the strongest organic bases. Finally, it is found that all polyguanides Protonate at imino nitrogen atoms, since they are apparently susceptible the most to the Proton attack. The origin of their very high intrinsic basicity is traced down to a dramatic increase in the resonance interaction of the corresponding conjugate bases. For instance, the increase in the resonance energy in the Protonated guanidine is estimated to be in a range of 24-27 kcal/mol, which is higher than the aromatic stabilization in benzene. The Proton Affinity of some polycyclic guanides including Schwesinger Proton sponge and porphine is briefly discussed.
Borislav Kovacevic - One of the best experts on this subject based on the ideXlab platform.
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high basicity of phosphorus Proton Affinity of tris tetramethylguanidinyl phosphine and tris hexamethyltriaminophosphazenyl phosphine by dft calculations
Chemical Communications, 2006Co-Authors: Borislav Kovacevic, Zvonimir B MaksicAbstract:It is shown by approximate but reliable DFT calculations that the title compounds represent very strong superbases in gas phase and MeCN. In particular, tris-(hexamethyltriaminophosphazenyl)phosphine has a Proton Affinity, PA, of 295.5 kcal mol−1 and records a pKa(MeCN) of 50 ± 1 units.
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the intramolecular hydrogen bond and intrinsic Proton Affinity of neutral organic molecules n n n tris 3 aminopropyl guanidine and some related systems
Journal of Physical Organic Chemistry, 2002Co-Authors: Borislav Kovacevic, Zoran Glasovac, Zvonimir B MaksicAbstract:Multiple aminopropyl substitutions of the initial N,N′,N″-trimethylguanidine lead to high intrinsic absolute Proton affinities (APA) and basicities culminating in APA = 268.4 kcal mol−1 (1 kcal = 4.184 kJ) in N,N′,N″-tris(3-aminopropyl)guanidine (9). The reason behind a high Proton Affinity is identified as a strong cationic resonance in the central guanidine moiety and the strength of the intramolecular hydrogen bonding (IMHB), which is enhanced upon Protonation. A cooperative IMHB effect in 9 and 9H+ realized by three N(sp3)H···N(sp3) H-bridges is estimated to be as high as 14.7 and 33.0 kcal mol−1, respectively. It follows that the IMHB effect contributes 18.3 kcal mol−1 to the absolute Proton Affinity of 9, which is a respectable amount. The basicity of 9 in acetonitrile is estimated to be very high, as evidenced by the corresponding pKa = 28.8. A derivative of 9, N,N′,N″-tris(3-dimethylaminopropyl)guani-dine (10), assumes an even higher superbasic Proton Affinity of 275 kcal mol−1 due to an additional relaxation effect caused by the methyl groups. The corresponding pKa(MeCN) = 29.4. Copyright © 2002 John Wiley & Sons, Ltd.
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the Proton Affinity of the superbase 1 8 bis tetramethylguanidino naphthalene tmgn and some related compounds a theoretical study
Chemistry: A European Journal, 2002Co-Authors: Borislav Kovacevic, Zvonimir B MaksicAbstract:The spatial and electronic structure of the very strong neutral organic bases TMGN, TMGF and some related compounds are explored by ab initio computational methods. Their Affinity towards the Proton is scrutinized both in the gas phase and in acetonitrile. The Protonation at the most basic center (imine nitrogen) yields asymmetric and relatively strong intramolecular hydrogen bonds (IHB). It is found that the angular strain effect and steric repulsion practically vanish in TMGN implying that its high absolute Proton Affinity (APA) has its origin in the inherent basicity of the guanidine fragment and a relatively strong IHB in [TMGN]H++. The nonbonded repulsions in TMGF are higher than in TMGN, which in conjunction with a slightly stronger IHB in the corresponding conjugate acid makes it more basic: APA(TMGF) > APA(TMGN). An interesting new phenomenon is observed in both TMGN and TMGF: the Proton triggers the resonance stabilization not only in the directly bonded guanidine moiety, but also in the other guanidine fragment placed more distantly from Proton, albeit in a less pronounced manner. The latter feature is termed as partial Protonation. It supports the hydrogen bonding and contributes to the IHB stabilization. Convincing evidence is presented that the solvent effect in acetonitrile is determined by two antagonistic factors: (1) the intrinsic (gas phase) Proton Affinity and (2) the size effect given by the ratio between the positive charge in molecular cation (conjugate acid) and the magnitude of the molecular surface. The resulting pK_a values are given by an interplay of these factors.
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absolute Proton Affinity of some polyguanides
Journal of Organic Chemistry, 2000Co-Authors: Zvonimir B Maksic, Borislav KovacevicAbstract:The problem of the absolute Proton Affinity (APA) of some polyguanides is addressed by the MP2(fc)/6-311+G//HF/6-31G theoretical model. It is shown that the linear chain polyguanides exhibit increased basicity as a function of the number of guanide subunits. However, the saturation effect yields an asymptotic APA value of 254 kcal/mol. Branched polyguanides on the other hand have higher APAs than their linear counterparts. The largest Proton Affinity is found in a doubly bifurcated heptaguanide, being as high as 285 kcal/mol, thus potentially representing one of the strongest organic bases. Finally, it is found that all polyguanides Protonate at imino nitrogen atoms, since they are apparently susceptible the most to the Proton attack. The origin of their very high intrinsic basicity is traced down to a dramatic increase in the resonance interaction of the corresponding conjugate bases. For instance, the increase in the resonance energy in the Protonated guanidine is estimated to be in a range of 24-27 kcal/mol, which is higher than the aromatic stabilization in benzene. The Proton Affinity of some polycyclic guanides including Schwesinger Proton sponge and porphine is briefly discussed.
Joseph S Francisco - One of the best experts on this subject based on the ideXlab platform.
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Proton Affinity of methyl peroxynitrate
Journal of Physical Chemistry A, 2008Co-Authors: Rose M Ravelo, Joseph S FranciscoAbstract:The equilibrium structures, harmonic vibrational frequencies of methyl peroxynitrate, and structures of Protonated methyl peroxynitrate have been investigated using ab initio methods. The methods include the single- and double-excitation quadratic configuration (QCISD) methods and the QCISD(T) method, which incorporates a perturbational estimate of the effects of corrected triple excitation. The lowest-energy gas-phase form of Protonated methyl peroxynitrate is a complex between CH3OOH and NO2+. The CH3OOH·NO2+ complex is bound by 22 ± 2 kcal/mol. The estimated Proton Affinity of methyl peroxynitrate is 178.8 ± 3 kcal/mol. A general trend for the Proton Affinity of ROO−NO2 (peroxynitrates) compounds is discussed.
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Proton Affinity of Peroxyacetic Acid
The Journal of Physical Chemistry A, 2004Co-Authors: Charles E. Miller, Joseph S FranciscoAbstract:The Proton Affinity of peroxyacetic acid has been studied using ab initio methods. The most stable peroxyacetic acid structure forms a nearly planar five-membered ring in which the carbonyl oxygen ...
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A quadratic configuration interaction study of the Proton Affinity of acetic acid
Chemical Physics Letters, 2002Co-Authors: Charles E. Miller, Joseph S FranciscoAbstract:Abstract Optimized geometries and harmonic vibrational frequencies have been obtained for neutral and Protonated acetic acid using quadratic configuration interaction methods. The Proton Affinity of acetic acid calculated at the QCISD(T)/6-311++G(3df,3pd)//QCISD/6-311+G(2d,2p) level of theory is 187.9±0.7 kcal mol −1 , in excellent agreement with the experimental value of 187.3 kcal mol −1 . Thermochemical calculations using the CBS-Q, CBS-QB3, CBS-APNO, G2 and G2(MP2) model chemistries also display excellent agreement with the experimental Proton Affinity, suggesting that these model chemistries will provide accurate Proton affinities for carboxylic acids that are too large to treat with high level ab initio methods.
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Protonated nitrous acid h2ono molecular structure vibrational frequencies and Proton Affinity
Journal of Chemical Physics, 2001Co-Authors: Joseph S FranciscoAbstract:The equilibrium structures, harmonic vibrational frequencies of HONO, and Protonated HONO have been investigated using ab initio methods. The ab initio methods include second-order Moller-Plesset (MP2) perturbation theory, single-and double-quadratic configuration interaction (QCISD) theory, and the QCIDS(T) method, which incorporates a perturbational estimate of the effects of connected triple excitations. The QCISD(T) equilibrium geometry and vibrational frequencies for nitrous acid are in good agreement with experiment. The lowest energy form of Protonated HONO is a complex between water and NO+. The next lowest isomer is 35.1 kcal mol−1 higher in energy. The ab initio Proton Affinity (PA) of HONO is predicted to be 191.5 kcal mol−1 at the QCISD(T)/6-311++G(3df,3pd) level of theory.
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Proton Affinity of Peroxyacetyl Nitrate (PAN)
The Journal of Physical Chemistry A, 2001Co-Authors: Charles E. Miller, Joseph S FranciscoAbstract:Ab initio calculations for the Proton Affinity of peroxyacetyl nitrate (PAN) reveal the complicated Protonation chemistry associated with the unusual multifunctional structure of this molecule. Optimized molecular structures, relative energies, decomposition energies, and Proton affinities have been determined for four chemically distinguishable PANH+ protomers. Calculations performed at the CBS-Q level of theory predict that the most stable structure is the peracetic acid-nitronium ion complex, [CH3C(O)OOH···NO2+], and yield (PAN) = −773.4 ± 10 kJ mol-1. This result disagrees with the experimental determination of PA298 K(PAN) = −798 ± 12 kJ mol-1 that was attributed to Protonation of the carbonyl oxygen but agrees with the experimental and theoretical results for the Proton affinities of the nitrates HONO2, CH3ONO2, C2H5ONO2, and HOONO2, where the most stable protomers are also of the form [ROH···NO2+].
Jeehiun K Lee - One of the best experts on this subject based on the ideXlab platform.
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the acidity and Proton Affinity of the damaged base 1 n6 ethenoadenine in the gas phase versus in solution intrinsic reactivity and biological implications
Journal of Organic Chemistry, 2008Co-Authors: Min Liu, Jeehiun K LeeAbstract:1,N6-Ethenoadenine (ϵA) is a highly mutagenic lesion that is excised from human DNA by the enzyme alkyladenine DNA glycosylase (AAG). In an effort to understand the intrinsic properties of 1,N6-ethenoadenine, we examined its gas phase acidity and Proton Affinity using quantum mechanical calculations and mass spectrometric experimental methods. We measure two acidities for ϵA: a more acidic site (ΔHacid = 332 kcal mol−1; ΔGacid = 325 kcal mol−1) and a less acidic site (ΔHacid = 367 kcal mol−1; ΔGacid = 360 kcal mol−1). We also find that the Proton Affinity of the most basic site of 1,N6-ethenoadenine is 232−233 kcal mol−1 (GB = 224 kcal mol−1). These measurements, when compared to calculations, establish that, under our experimental conditions, we have only the canonical tautomer of 1,N6-ethenoadenine present. We also compare the gas phase acidic properties of ϵA with that of the normal bases adenine and guanine and find that ϵA is the most acidic. This supports the theory that AAG and other related enzyme...
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acidity and Proton Affinity of hypoxanthine in the gas phase versus in solution intrinsic reactivity and biological implications
Journal of Organic Chemistry, 2007Co-Authors: Xuejun Sun, Jeehiun K LeeAbstract:Hypoxanthine is a mutagenic purine base that most commonly arises from the oxidative deamination of adenine. Damaged bases such as hypoxanthine are associated with carcinogenesis and cell death. This inevitable damage is counteracted by glycosylase enzymes, which cleave damaged bases from DNA. Alkyladenine DNA glycosylase (AAG) is the enzyme responsible for excising hypoxanthine from DNA in humans. In an effort to understand the intrinsic properties of hypoxanthine, we examined the gas-phase acidity and Proton Affinity using quantum mechanical calculations and gas-phase mass spectrometric experimental methods. In this work, we establish that the most acidic site of hypoxanthine has a gas-phase acidity of 332 ± 2 kcal mol-1, which is more acidic than hydrochloric acid. We also bracket a less acidic site of hypoxanthine at 368 ± 3 kcal mol-1. We measure the Proton Affinity of the most basic site of hypoxanthine to be 222 ± 3 kcal mol-1. DFT calculations of these values are consistent with the experimental d...
Zvonimir B. Maksić - One of the best experts on this subject based on the ideXlab platform.
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Absolute Proton Affinity of some polyguanides
The Journal of organic chemistry, 2000Co-Authors: Zvonimir B. Maksić, Borislav KovačevićAbstract:The problem of the absolute Proton Affinity (APA) of some polyguanides is addressed by the MP2(fc)/6-311+G**//HF/6-31G* theoretical model. It is shown that the linear chain polyguanides exhibit increased basicity as a function of the number of guanide subunits. However, the saturation effect yields an asymptotic APA value of 254 kcal/mol. Branched polyguanides on the other hand have higher APAs than their linear counterparts. The largest Proton Affinity is found in a doubly bifurcated heptaguanide, being as high as 285 kcal/mol, thus potentially representing one of the strongest organic bases. Finally, it is found that all polyguanides Protonate at imino nitrogen atoms, since they are apparently susceptible the most to the Proton attack. The origin of their very high intrinsic basicity is traced down to a dramatic increase in the resonance interaction of the corresponding conjugate bases. For instance, the increase in the resonance energy in the Protonated guanidine is estimated to be in a range of 24−27 ...
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Intrinsic Proton Affinity of substituted aromatics
Theoretical and Computational Chemistry, 1998Co-Authors: Zvonimir B. Maksić, Mirjana Eckert-maksićAbstract:It is shown that the MP2(fc) /6-31 G * // HF /6-31 G * + ZPE (HF )/6-31 G * ) model reproduces very well the experimental Proton affinities in a large number of substituted benzenes and naphthalenes. Extensive applications of this model revealed that the Proton Affinity of polysubstituted aromatics followed a simple additivity rule, which have been rationalized by the ISA (independent substitutent approximation) model. Performance of this model is surprisingly good. Applications of Proton affinities, obtained by the transparent and intuitively appealing ISA model, in interpreting directional ability of substituents in the electrophilic substitution reactions of aromatics are briefly discussed.
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Additivity of the Proton Affinity of polysubstituted benzenes: the ipso position
Chemical Physics Letters, 1996Co-Authors: Zvonimir B. Maksić, Mirjana Eckert-maksić, Martin KlessingerAbstract:It is shown, by the MP2(fc)/6-31G∗ ∗/ /HF/6-31G∗ +ZPE(HF/6-31G∗) theoretical model and concomitant use of homodesmic reactions, that the ipso Proton affinities in polyfluorinated benzenes follow a simple additivity rule. Performance of the latter is good, as evidenced by a low average absolute deviation Δabs = 0.8 kcal/mol from the accurate ab initio results. Additional evidence supporting the additivity concept is provided by good accordance with the experimental Proton Affinity (PA) for perfluorobenzene. The present approach enables estimates of the ipso PAs of multiply substituted aromatics. It is particularly useful in those systems which involve or atomic groupings with lone pairs of electrons proximate to the aromatic π moiety. The additivity rule of thumb offers a simple rationalization of the ipso Proton affinities. The origin of the PA additivity is briefly discussed.
