Extensive exploration of the potential energy surfaces of protonated Nα-acetylhistidine hydrated ... more Extensive exploration of the potential energy surfaces of protonated Nα-acetylhistidine hydrated by 0 to 3 molecules of water was performed. The methodology combined hierarchical and genealogical (Darwin family tree) approaches using polarizable AMOEBA force field and M06 functional. It is demonstrated that this mixed approach allows recovering a number of conformers larger than when using any one of the two methods alone. Hydration enthalpies of protonated Nα-acetylhistidine and of model compounds have been computed using higher theoretical methods, up to the G4MP2 procedure. Excellent agreement with experiment is observed for successive hydration of methylamonium and imidazolium cations using MP2/6-311++G(2d,2p)//M06/6-311++G(d,p) and G4MP2 methods, thereby validating the theory levels used for hydrated protonated Nα-acetylhistidine. It is found that first hydration enthalpy of protonated Nα-acetylhistidine is ca. 10 kJ.mol-1 lower than that of imidazolium, a result explained by t...
Protonated 4-bromophenol and 4-bromoanisole produced by methane chemical ionization are found to ... more Protonated 4-bromophenol and 4-bromoanisole produced by methane chemical ionization are found to easily be dehalogenated upon high (8 keV) or low (20-30 eV) energy collisional activation giving essentially phenol and anisole radical cations, respectively. Under similar conditions, protonated unsubstituted anisole is also readily demethylated generating the phenol ion but not cyclohexadienone ions. Other nonconventional isomers of ionized phenol are only detected by MS/MS/MS experiments performed on [M-CO] •+ ions from salicylaldehyde. (U)B3LYP/6-311++G(d,p) and CASPT2/6-31G(d,p) calculations indicate the higher stability of the phenol radical cation with respect to the other six-membered-ring isomers. The least energy demanding fragmentation, namely, the decarbonylation, is shown to involve the intermediacy of six-membered ketones, open-chain ketenes, and five-membered cyclopentadiene isomeric ions. The rate determining step corresponds to the enol-keto interconversion with an energy barrier of about 276 kJ/mol relative to the phenol ion, which is markedly smaller than that required for hydrogen atom loss, deprotonation, or CO loss from an open-chain form. This suggests a crucial role played by the solvent in the readiness of the deprotonation of phenol ions in nonpolar media. The adiabatic ionization energy of phenol is evaluated as IE a (C 6 H 5 O) ) 8.35 ( 0.2 eV (exptl: 8.49 eV), and the proton affinity of the phenoxy radical is evaluated as PA(C 6 H 5 O) ) 863 ( 10 kJ/mol (exptl: 860 kJ/mol), PA(phenol) ) 826 ( 10 kJ/mol (exptl: 818 kJ/mol), and PA(anisole) ) 848 ( 10 kJ/mol.
A microcanonical analysis of the thermokinetic method is performed using statistical rate calcula... more A microcanonical analysis of the thermokinetic method is performed using statistical rate calculations based on orbiting transition state theory in order to model a proton transfer process: MH(+) + B(i) --> M + B(i)H(+). The reaction efficiency is calculated as a function of the difference in zero point energy of reactants and products. Several models of reactions were investigated in order to simulate situations where the base of interest M exhibits loss of entropy upon protonation of up to approximately 40 J mol(-1) K(-1). It is shown that the standard thermokinetic method would predict correct 298 K gas phase basicities, GB(298)(M), even for polydentate molecules M, if experiments are conducted at this temperature. Proton affinity, PA(298)(M), and protonation entropy may be obtained by the thermokinetic method only in special circumstances such as, for example, experiments conducted at various temperatures.
An evaluation of the results obtained by the extended kinetic method for a series of representati... more An evaluation of the results obtained by the extended kinetic method for a series of representative bases is presented here. Analysis of the original experimental data is conducted using the orthogonal distance regression (ODR) statistical treatment. A comparison with the proton affinities and protonation entropies obtained from variable temperature equilibrium constant measurements demonstrate deviations, which may be ascribed to random and systematic errors. Considerable random errors are associated with the extended kinetic method if the number of reference bases and the range of effective temperatures are too low. It is also confirmed that large systematic errors on proton affinities and protonation entropies are obtained when large protonation entropy is associated with the considered system. It is, however, encouraging to note that the gas phase basicities obtained by the extended kinetic method are generally comparable to that obtained by other methods within a few kJ/mol.
ABSTRACT An experimental and theoretical study of the protonation of representative dicarbonyl co... more ABSTRACT An experimental and theoretical study of the protonation of representative dicarbonyl compounds M = 2,3-butanedione (biacetyl), 1, 2,4-pentanedione (acetylacetone), 2, 2,5-hexanedione (acetonylacetone), 3, and methyl-acetoacetate, 4 has been carried out. The experimental proton affinities and protonation entropies have been obtained by the extended kinetic method using the orthogonal distance regression (ODR) treatment. Theoretical proton affinities are calculated at the G2MP2 level of theory while protonation entropies were estimated after a detailed treatment of the internal rotations. The data show that protonation of 1 and 2 is associated with negligible protonation entropies while significant negative values are obtained for molecules 3 and 4. Protonation of 2,3-butanedione, 1a, is associated with a tautomerisation inside the proton transfer complex thus leading to protonated 2-hydroxy-butenone, 1bH+. Protonation thermochemistry of 2,4-pentanedione 2 may be simply rationalized by the protonation of its most stable tautomer, the 4-hydroxy-3-pentene-2-one, 2b, to give its most stable protonated form 2bH+ stabilized by a strong intramolecular hydrogen bond. Protonation of 2,5-hexanedione 3a most probably produces a cyclic structure stabilized by a covalent bonding, 3aHc+. The structure of neutral methyl acetoacetate 4 sampled during protonation in mass spectrometry experiments appears to be its diketonic form 4a; its protonation leading to an internally hydrogen bonded stabilized structure 4aH+.
ABSTRACT Using a combination of tandem mass spectrometry methodologies, collisional activation of... more ABSTRACT Using a combination of tandem mass spectrometry methodologies, collisional activation of fast (8 keV) or slow (20-30 eV) ion beams, neutralization-reionization and ion-molecule reactions, it is shown that thiosulfoxides X2S=S (X = H, CH3, C2H5) are stable in the gas phase as radical cations as well as neutral molecules, The absence of isomerization into the more conventional disulfane structure, XSSX, is firmly demonstrated by ion-molecule reactions of the isomeric ions with methyl isocyanide and by collisional activation of "survivor" ions generated in neutralization-reionization experiments. All the mass spectral data have been recorded with a single hybrid mass spectrometer of sectors-quadrupole-sectors configuration. Molecular orbital calculations demonstrate that ionized thiosulfoxides and disulfanes are local minima on the MP2/6-31G* potential energy surface. The disulfane structures are found to be the most stable species by 91, 65 and 59 kJ/mol for X = H, CH3 and C2H5, respectively. Heat of formation values of 1000, 810 and 760 kJ/mol are estimated for X2SS.+ ions (X = H, CH3 and C2H5, respectively). (C) 2000 Elsevier Science B.V.
The generation of carbene radical cations (R C R·+) and related ions in the gas phase using var... more The generation of carbene radical cations (R C R·+) and related ions in the gas phase using various mass spectrometric techniques is critically reviewed. Their reactions as well as their thermochemical properties obtained either experimentally or from ab initio quantum ...
ABSTRACT Ion-molecule reactions between ionized carbonyl compounds or ionized enols with t-butyl ... more ABSTRACT Ion-molecule reactions between ionized carbonyl compounds or ionized enols with t-butyl nitrite allow a clear-cut distinction to be made between both these isomeric structures. The most relevant difference between the observed reactions is the formal substitution of a hydrogen atom by nitric oxide in the enol ions. Collisional activation experiments on the product ions are interpreted in terms of alpha-nitroso carbonyl structures. However, a quantum chemical investigation at the B3LYP/6-311++G(d,p) level of theory shows that ionized alpha-nitroso carbonyl and vinyl nitrite structures may isomerize provided they contain ca 60 kj mol(-1) of internal energy. It is, therefore, possible that both structures are generated in the substitution reaction.
Some nonheme hydroperoxoiron(III) species have been tris(2-pyridylmethyl)ethane-1,2-diamine] give... more Some nonheme hydroperoxoiron(III) species have been tris(2-pyridylmethyl)ethane-1,2-diamine] gives a blue highspin species which we characterized as the η 2 -peroxoiron(III) recently characterized by several groups. The reported examples were obtained by adding H 2 O 2 in excess to an Fe II species [L 5 Fe III O 2 ] + . It seems that with such an auxiliary ligand, the hydroperoxo group is acidic in contrast with its complex with a neutral polypyridine ligand. We show here that on deprotonation, the purple low-spin hydroper-basicity when it is coordinated to a heme group. oxoiron(III) complex [L 5 Fe III OOH] 2+ NЈ,
Quantum chemistry calculations using composite G3B3, G3MP2B3 and CBS-QB3 methods were performed f... more Quantum chemistry calculations using composite G3B3, G3MP2B3 and CBS-QB3 methods were performed for benzaldehyde, 1, tropone, 2, ortho-quinone methide, 3, para-quinone methide, 4, their protonated forms 1H+–4H+ and the isomeric meta-hydroxybenzyl cation 5H+. The G3B3 298K heats of formation values obtained in this work are: −39, 61, 52, 39, 661, 679, 699, 680 and 733kJmol−1 for 1–4, 1H+–5H+, respectively. At
... GUY BOUCHOUX et MARCEL FBTIZON Laboratoire de StBrBochimie, Universite de Paris-Sud, Centre d... more ... GUY BOUCHOUX et MARCEL FBTIZON Laboratoire de StBrBochimie, Universite de Paris-Sud, Centre d'Orsay, 91405 Orsay, France ... d'activation dans les approximations de Rabinovitch et Vestal Les approximations de Vestal et de Rabinovitch conduisent 21 basse Cnergie B ...
International Journal of Mass Spectrometry and Ion Processes, 1993
Ab initio molecular orbital calculations were carried out to determine the activation energy of t... more Ab initio molecular orbital calculations were carried out to determine the activation energy of the methane loss from the dimethyloxonium cation. At the approximate MP4SDTQ/6-311 + +G(2df,2p) + ZPE level based on MP2/6-31G(d,p)-geometries, the energy barrier is calculated to be 267 ± 15 kJ mol-1.
Page 1. Organic Mass Spectrometry. 1976. Vol. 11. pp. 712 to 721. @ Heyden & Son Limited.... more Page 1. Organic Mass Spectrometry. 1976. Vol. 11. pp. 712 to 721. @ Heyden & Son Limited. Printed in Northern Ireland REARRANGEMENTS 1,3 EN SERIE HETEROCYCLIQUE. VI-FRAGMENTATION, EN SPECTROMETRIE ...
Journal of Mass Spectrometry - J MASS SPECTROMETRY, 2007
Proton affinity and protonation entropy of methionine (Met) were determined by the extended kinet... more 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 1 p S°(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 1 p S°(Met) = −35 ± 15 J mol −1 K −1 and GB(Met) = 900 ± 2 kJ mol −1 .
tive for systematic gaining of unknown so far values of the enthalpies of formation of free radic... more tive for systematic gaining of unknown so far values of the enthalpies of formation of free radicals. The following new or corrected values of the free radicals' enthalpies of formation were obtained (kcal mol 21 ): 2bicyclo[2.2.2]octyl z (18), PhCH 2 CH 2 z (54.5), BrCH 2 CH 2 z (33.5), ICH 2 CH 2 z (46.5), CH 2 yCHOCH 2 z (17), z CH 2 O-COCH 3 (254), z CH 2 CH 2 COOEt (263), z CH 2 OCOPh (224.5), z CH 2 OPh (26), H 2 NNHCH 2 z (65), NCCH 2 CH 2 z (60.5), O 2 NCH 2 CH 2 z (24.5), F 2 NCH 2 CH 2 z (19), 3-cyanocyclobutyl z (83) HSCH 2 CH 2 z (36), EtS(O)CH 2 CH 2 z (21.5), EtSO 2 CH 2 CH 2 z (254), z CH 2 PH 2 (41.2), z CH 2 At (68), H 3 ECH 2 z (39.5, 57.5, 77.5, 97) (E Si, Ge, Sn, Pb, respectively). The DH f 0 values for z CH 2 SH and Me 3 CCH 2 z free radicals 42 and 5 kcal mol 21 , respectively, were derived by using all three procedures as compared with earlier found values of 36.3 and 8.7 kcal mol 21 , respectively. From the known enthalpic shift DDH f 0 7.2 kcal mol 21 for RCHyCH 2 ! RPh replacement and the regularity in thermochemistry of RCH 2 z /RCHyCH 2 species, the new or corrected values of the molecules' enthalpies of formation were found (kcal mol 21 ): BrCHyCH 2
Ab initio molecular orbital calculations at the G2(MP2,SVP) level have been employed to explore a... more Ab initio molecular orbital calculations at the G2(MP2,SVP) level have been employed to explore a large part of the H 3 ; C; N; O Å potential energy surface. Ionized aminohydroxycarbene, NH 2 ±C Å ±OH, 1, is found to correspond to the global minimum of the surface. The other stable species are also unconventional structures: ion±neutral complexes OC Á Á Á NH Å 3 , 2, and CO Á Á Á NH Å 3 , 2 H , and the distonic ion, H 3 N C Å O, 3. The more classical structures HCONH 2 Å , 4, and HCOHNH Å , 5 are higher in energy. The heat of formation of the ®ve radical cations have been determined using their atomization energies. The various isomerization reactions connecting 1±5 as well as their dissociation by H or CO losses have been theoretically investigated and compared with the available experimental data. Ó
Extensive exploration of the potential energy surfaces of protonated Nα-acetylhistidine hydrated ... more Extensive exploration of the potential energy surfaces of protonated Nα-acetylhistidine hydrated by 0 to 3 molecules of water was performed. The methodology combined hierarchical and genealogical (Darwin family tree) approaches using polarizable AMOEBA force field and M06 functional. It is demonstrated that this mixed approach allows recovering a number of conformers larger than when using any one of the two methods alone. Hydration enthalpies of protonated Nα-acetylhistidine and of model compounds have been computed using higher theoretical methods, up to the G4MP2 procedure. Excellent agreement with experiment is observed for successive hydration of methylamonium and imidazolium cations using MP2/6-311++G(2d,2p)//M06/6-311++G(d,p) and G4MP2 methods, thereby validating the theory levels used for hydrated protonated Nα-acetylhistidine. It is found that first hydration enthalpy of protonated Nα-acetylhistidine is ca. 10 kJ.mol-1 lower than that of imidazolium, a result explained by t...
Protonated 4-bromophenol and 4-bromoanisole produced by methane chemical ionization are found to ... more Protonated 4-bromophenol and 4-bromoanisole produced by methane chemical ionization are found to easily be dehalogenated upon high (8 keV) or low (20-30 eV) energy collisional activation giving essentially phenol and anisole radical cations, respectively. Under similar conditions, protonated unsubstituted anisole is also readily demethylated generating the phenol ion but not cyclohexadienone ions. Other nonconventional isomers of ionized phenol are only detected by MS/MS/MS experiments performed on [M-CO] •+ ions from salicylaldehyde. (U)B3LYP/6-311++G(d,p) and CASPT2/6-31G(d,p) calculations indicate the higher stability of the phenol radical cation with respect to the other six-membered-ring isomers. The least energy demanding fragmentation, namely, the decarbonylation, is shown to involve the intermediacy of six-membered ketones, open-chain ketenes, and five-membered cyclopentadiene isomeric ions. The rate determining step corresponds to the enol-keto interconversion with an energy barrier of about 276 kJ/mol relative to the phenol ion, which is markedly smaller than that required for hydrogen atom loss, deprotonation, or CO loss from an open-chain form. This suggests a crucial role played by the solvent in the readiness of the deprotonation of phenol ions in nonpolar media. The adiabatic ionization energy of phenol is evaluated as IE a (C 6 H 5 O) ) 8.35 ( 0.2 eV (exptl: 8.49 eV), and the proton affinity of the phenoxy radical is evaluated as PA(C 6 H 5 O) ) 863 ( 10 kJ/mol (exptl: 860 kJ/mol), PA(phenol) ) 826 ( 10 kJ/mol (exptl: 818 kJ/mol), and PA(anisole) ) 848 ( 10 kJ/mol.
A microcanonical analysis of the thermokinetic method is performed using statistical rate calcula... more A microcanonical analysis of the thermokinetic method is performed using statistical rate calculations based on orbiting transition state theory in order to model a proton transfer process: MH(+) + B(i) --> M + B(i)H(+). The reaction efficiency is calculated as a function of the difference in zero point energy of reactants and products. Several models of reactions were investigated in order to simulate situations where the base of interest M exhibits loss of entropy upon protonation of up to approximately 40 J mol(-1) K(-1). It is shown that the standard thermokinetic method would predict correct 298 K gas phase basicities, GB(298)(M), even for polydentate molecules M, if experiments are conducted at this temperature. Proton affinity, PA(298)(M), and protonation entropy may be obtained by the thermokinetic method only in special circumstances such as, for example, experiments conducted at various temperatures.
An evaluation of the results obtained by the extended kinetic method for a series of representati... more An evaluation of the results obtained by the extended kinetic method for a series of representative bases is presented here. Analysis of the original experimental data is conducted using the orthogonal distance regression (ODR) statistical treatment. A comparison with the proton affinities and protonation entropies obtained from variable temperature equilibrium constant measurements demonstrate deviations, which may be ascribed to random and systematic errors. Considerable random errors are associated with the extended kinetic method if the number of reference bases and the range of effective temperatures are too low. It is also confirmed that large systematic errors on proton affinities and protonation entropies are obtained when large protonation entropy is associated with the considered system. It is, however, encouraging to note that the gas phase basicities obtained by the extended kinetic method are generally comparable to that obtained by other methods within a few kJ/mol.
ABSTRACT An experimental and theoretical study of the protonation of representative dicarbonyl co... more ABSTRACT An experimental and theoretical study of the protonation of representative dicarbonyl compounds M = 2,3-butanedione (biacetyl), 1, 2,4-pentanedione (acetylacetone), 2, 2,5-hexanedione (acetonylacetone), 3, and methyl-acetoacetate, 4 has been carried out. The experimental proton affinities and protonation entropies have been obtained by the extended kinetic method using the orthogonal distance regression (ODR) treatment. Theoretical proton affinities are calculated at the G2MP2 level of theory while protonation entropies were estimated after a detailed treatment of the internal rotations. The data show that protonation of 1 and 2 is associated with negligible protonation entropies while significant negative values are obtained for molecules 3 and 4. Protonation of 2,3-butanedione, 1a, is associated with a tautomerisation inside the proton transfer complex thus leading to protonated 2-hydroxy-butenone, 1bH+. Protonation thermochemistry of 2,4-pentanedione 2 may be simply rationalized by the protonation of its most stable tautomer, the 4-hydroxy-3-pentene-2-one, 2b, to give its most stable protonated form 2bH+ stabilized by a strong intramolecular hydrogen bond. Protonation of 2,5-hexanedione 3a most probably produces a cyclic structure stabilized by a covalent bonding, 3aHc+. The structure of neutral methyl acetoacetate 4 sampled during protonation in mass spectrometry experiments appears to be its diketonic form 4a; its protonation leading to an internally hydrogen bonded stabilized structure 4aH+.
ABSTRACT Using a combination of tandem mass spectrometry methodologies, collisional activation of... more ABSTRACT Using a combination of tandem mass spectrometry methodologies, collisional activation of fast (8 keV) or slow (20-30 eV) ion beams, neutralization-reionization and ion-molecule reactions, it is shown that thiosulfoxides X2S=S (X = H, CH3, C2H5) are stable in the gas phase as radical cations as well as neutral molecules, The absence of isomerization into the more conventional disulfane structure, XSSX, is firmly demonstrated by ion-molecule reactions of the isomeric ions with methyl isocyanide and by collisional activation of "survivor" ions generated in neutralization-reionization experiments. All the mass spectral data have been recorded with a single hybrid mass spectrometer of sectors-quadrupole-sectors configuration. Molecular orbital calculations demonstrate that ionized thiosulfoxides and disulfanes are local minima on the MP2/6-31G* potential energy surface. The disulfane structures are found to be the most stable species by 91, 65 and 59 kJ/mol for X = H, CH3 and C2H5, respectively. Heat of formation values of 1000, 810 and 760 kJ/mol are estimated for X2SS.+ ions (X = H, CH3 and C2H5, respectively). (C) 2000 Elsevier Science B.V.
The generation of carbene radical cations (R C R·+) and related ions in the gas phase using var... more The generation of carbene radical cations (R C R·+) and related ions in the gas phase using various mass spectrometric techniques is critically reviewed. Their reactions as well as their thermochemical properties obtained either experimentally or from ab initio quantum ...
ABSTRACT Ion-molecule reactions between ionized carbonyl compounds or ionized enols with t-butyl ... more ABSTRACT Ion-molecule reactions between ionized carbonyl compounds or ionized enols with t-butyl nitrite allow a clear-cut distinction to be made between both these isomeric structures. The most relevant difference between the observed reactions is the formal substitution of a hydrogen atom by nitric oxide in the enol ions. Collisional activation experiments on the product ions are interpreted in terms of alpha-nitroso carbonyl structures. However, a quantum chemical investigation at the B3LYP/6-311++G(d,p) level of theory shows that ionized alpha-nitroso carbonyl and vinyl nitrite structures may isomerize provided they contain ca 60 kj mol(-1) of internal energy. It is, therefore, possible that both structures are generated in the substitution reaction.
Some nonheme hydroperoxoiron(III) species have been tris(2-pyridylmethyl)ethane-1,2-diamine] give... more Some nonheme hydroperoxoiron(III) species have been tris(2-pyridylmethyl)ethane-1,2-diamine] gives a blue highspin species which we characterized as the η 2 -peroxoiron(III) recently characterized by several groups. The reported examples were obtained by adding H 2 O 2 in excess to an Fe II species [L 5 Fe III O 2 ] + . It seems that with such an auxiliary ligand, the hydroperoxo group is acidic in contrast with its complex with a neutral polypyridine ligand. We show here that on deprotonation, the purple low-spin hydroper-basicity when it is coordinated to a heme group. oxoiron(III) complex [L 5 Fe III OOH] 2+ NЈ,
Quantum chemistry calculations using composite G3B3, G3MP2B3 and CBS-QB3 methods were performed f... more Quantum chemistry calculations using composite G3B3, G3MP2B3 and CBS-QB3 methods were performed for benzaldehyde, 1, tropone, 2, ortho-quinone methide, 3, para-quinone methide, 4, their protonated forms 1H+–4H+ and the isomeric meta-hydroxybenzyl cation 5H+. The G3B3 298K heats of formation values obtained in this work are: −39, 61, 52, 39, 661, 679, 699, 680 and 733kJmol−1 for 1–4, 1H+–5H+, respectively. At
... GUY BOUCHOUX et MARCEL FBTIZON Laboratoire de StBrBochimie, Universite de Paris-Sud, Centre d... more ... GUY BOUCHOUX et MARCEL FBTIZON Laboratoire de StBrBochimie, Universite de Paris-Sud, Centre d'Orsay, 91405 Orsay, France ... d'activation dans les approximations de Rabinovitch et Vestal Les approximations de Vestal et de Rabinovitch conduisent 21 basse Cnergie B ...
International Journal of Mass Spectrometry and Ion Processes, 1993
Ab initio molecular orbital calculations were carried out to determine the activation energy of t... more Ab initio molecular orbital calculations were carried out to determine the activation energy of the methane loss from the dimethyloxonium cation. At the approximate MP4SDTQ/6-311 + +G(2df,2p) + ZPE level based on MP2/6-31G(d,p)-geometries, the energy barrier is calculated to be 267 ± 15 kJ mol-1.
Page 1. Organic Mass Spectrometry. 1976. Vol. 11. pp. 712 to 721. @ Heyden & Son Limited.... more Page 1. Organic Mass Spectrometry. 1976. Vol. 11. pp. 712 to 721. @ Heyden & Son Limited. Printed in Northern Ireland REARRANGEMENTS 1,3 EN SERIE HETEROCYCLIQUE. VI-FRAGMENTATION, EN SPECTROMETRIE ...
Journal of Mass Spectrometry - J MASS SPECTROMETRY, 2007
Proton affinity and protonation entropy of methionine (Met) were determined by the extended kinet... more 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 1 p S°(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 1 p S°(Met) = −35 ± 15 J mol −1 K −1 and GB(Met) = 900 ± 2 kJ mol −1 .
tive for systematic gaining of unknown so far values of the enthalpies of formation of free radic... more tive for systematic gaining of unknown so far values of the enthalpies of formation of free radicals. The following new or corrected values of the free radicals' enthalpies of formation were obtained (kcal mol 21 ): 2bicyclo[2.2.2]octyl z (18), PhCH 2 CH 2 z (54.5), BrCH 2 CH 2 z (33.5), ICH 2 CH 2 z (46.5), CH 2 yCHOCH 2 z (17), z CH 2 O-COCH 3 (254), z CH 2 CH 2 COOEt (263), z CH 2 OCOPh (224.5), z CH 2 OPh (26), H 2 NNHCH 2 z (65), NCCH 2 CH 2 z (60.5), O 2 NCH 2 CH 2 z (24.5), F 2 NCH 2 CH 2 z (19), 3-cyanocyclobutyl z (83) HSCH 2 CH 2 z (36), EtS(O)CH 2 CH 2 z (21.5), EtSO 2 CH 2 CH 2 z (254), z CH 2 PH 2 (41.2), z CH 2 At (68), H 3 ECH 2 z (39.5, 57.5, 77.5, 97) (E Si, Ge, Sn, Pb, respectively). The DH f 0 values for z CH 2 SH and Me 3 CCH 2 z free radicals 42 and 5 kcal mol 21 , respectively, were derived by using all three procedures as compared with earlier found values of 36.3 and 8.7 kcal mol 21 , respectively. From the known enthalpic shift DDH f 0 7.2 kcal mol 21 for RCHyCH 2 ! RPh replacement and the regularity in thermochemistry of RCH 2 z /RCHyCH 2 species, the new or corrected values of the molecules' enthalpies of formation were found (kcal mol 21 ): BrCHyCH 2
Ab initio molecular orbital calculations at the G2(MP2,SVP) level have been employed to explore a... more Ab initio molecular orbital calculations at the G2(MP2,SVP) level have been employed to explore a large part of the H 3 ; C; N; O Å potential energy surface. Ionized aminohydroxycarbene, NH 2 ±C Å ±OH, 1, is found to correspond to the global minimum of the surface. The other stable species are also unconventional structures: ion±neutral complexes OC Á Á Á NH Å 3 , 2, and CO Á Á Á NH Å 3 , 2 H , and the distonic ion, H 3 N C Å O, 3. The more classical structures HCONH 2 Å , 4, and HCOHNH Å , 5 are higher in energy. The heat of formation of the ®ve radical cations have been determined using their atomization energies. The various isomerization reactions connecting 1±5 as well as their dissociation by H or CO losses have been theoretically investigated and compared with the available experimental data. Ó
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Papers by Guy Bouchoux