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Marcos N. Eberlin - One of the best experts on this subject based on the ideXlab platform.

  • Sulfur trifluoride Cation (SF3+) affinities of pyridines determined by the kinetic method : Stereoelectronic effects in the gas phase
    Journal of the American Society for Mass Spectrometry, 1997
    Co-Authors: Philip S. H. Wong, Sheng Sheng Yang, Shuguang Ma, R. Graham Cooks, Fabio C. Gozzo, Marcos N. Eberlin
    Abstract:

    Ion/molecule reactions performed by pentaquadrupole mass spectrometry are used to generate cluster ions in which neutral pyridines are bound to the Polyatomic Cation SF3 +. The dimeric ions Py1SF3 +Py2, where Py1 and Py2 represent substituted pyridines, are shown to have loosely bound structures by collision-induced dissociation (MS3) experiments and by semiempirical AMI and ab initio RHF/6-31G(d, p) molecular orbital calculations. In the case of dimers comprised of meta- and/or para-substituted pyridines (unhindered pyridines), there is an excellent linear correlation between the logarithm of the fragment ion abundance ratio ln[Py1(SF3 +)]/[Py(SF3 +)] and the proton affinities (PA) of the constituent pyridines. Semiempirical calculations are used to estimate the SF3 + affinities of pyridines which are found to be in the range of 25–31 kcal/mol. The SF3 + affinities show an excellent linear correlation with the proton affinities of the pyridines, and the relationship SF3 + affinity (kcal/mol) = 0. 73PA — 135. 8 between the two affinities is derived. The effective temperature of the dimeric ions is determined to be 595 ± 69 K, which is in good agreement with values of around 600 K obtained experimentally in studies on many other systems activated under similar conditions. Ortho-substituted pyridines show lower than expected affinities due to stereoelectronic effects that decrease the Cation affinities. Gas-phase Stereoelectronic parameters (Sk) are measured from the deviation from the PA correlation and are ordered as 2-MePy (−1.09) < 2,6-diMePy (−1.11) < 2-EtPy (−1.91) < 2,3-diMePy (−2. 15) < 2,5-diMePy (− 2. 25) < 2,4-diMePy (− 2. 40). Overall, the steric effects are larger than those in the corresponding Cl+-bound dimers but smaller than those in the bulky [OCNCO+] system. Calculations show evidence for agostic bonding that offsets the steric effects in some cases. The eclipsed conformation of 2-methylpyridine/SF3 + adduct is found to be more stable than the staggered form by 0. 8 kcal/mol, due to auxiliary agostic bonding between the hydrogen of the ortho methyl substituent and the sulfur atom. Calculations on atomic charge distribution reveal that the positive charge is mainly on the sulfur atom (+1. 99) and the charge on the bonding hydrogen S-H-C (+ 0. 07) is considerably lower than that on the other two methyl hydrogens ( + 0. 14), which appears to be a good indiCation of agostic binding. The most stable form of the 2-ethylpyridine/SF3 + adduct is found when the N-C1-Cα-Cβ dihedral angle is approximately 60 °, where the ethyl hydrogen is directed toward the SF3 group via an interesting six-membered ring alignment. The experiments show a remarkably small steric effect in 2,6-dimethylpyridine, probably due to strong agostic bonding enhanced by the buttressing effect that shortens the S-H distance. In addition, the face-to-face interactions of the F atoms and the H atoms further stabilize this form.

  • Sulfur trifluoride Cation (SF_3 ^+) affinities of pyridines determined by the kinetic method: Stereoelectronic effects in the gas phase
    Journal of the American Society for Mass Spectrometry, 1997
    Co-Authors: Philip S. H. Wong, Sheng Sheng Yang, Shuguang Ma, R. Graham Cooks, Fabio C. Gozzo, Marcos N. Eberlin
    Abstract:

    Ion/molecule reactions performed by pentaquadrupole mass spectrometry are used to generate cluster ions in which neutral pyridines are bound to the Polyatomic Cation SF_3 ^+. The dimeric ions Py_1SF_3 ^+Py_2, where Py_1 and Py_2 represent substituted pyridines, are shown to have loosely bound structures by collision-induced dissociation (MS^3) experiments and by semiempirical AMI and ab initio RHF/6-31G( d, p ) molecular orbital calculations. In the case of dimers comprised of meta- and/or para-substituted pyridines (unhindered pyridines), there is an excellent linear correlation between the logarithm of the fragment ion abundance ratio ln[Py_1(SF_3 ^+)]/[Py(SF_3 ^+)] and the proton affinities (PA) of the constituent pyridines. Semiempirical calculations are used to estimate the SF_3 ^+ affinities of pyridines which are found to be in the range of 25–31 kcal/mol. The SF_3 ^+ affinities show an excellent linear correlation with the proton affinities of the pyridines, and the relationship SF_3 ^+ affinity (kcal/mol) = 0. 73PA — 135. 8 between the two affinities is derived. The effective temperature of the dimeric ions is determined to be 595 ± 69 K, which is in good agreement with values of around 600 K obtained experimentally in studies on many other systems activated under similar conditions. Ortho-substituted pyridines show lower than expected affinities due to stereoelectronic effects that decrease the Cation affinities. Gas-phase Stereoelectronic parameters (S^k) are measured from the deviation from the PA correlation and are ordered as 2-MePy (−1.09) < 2,6-diMePy (−1.11) < 2-EtPy (−1.91) < 2,3-diMePy (−2. 15) < 2,5-diMePy (− 2. 25) < 2,4-diMePy (− 2. 40). Overall, the steric effects are larger than those in the corresponding Cl^+-bound dimers but smaller than those in the bulky [OCNCO^+] system. Calculations show evidence for agostic bonding that offsets the steric effects in some cases. The eclipsed conformation of 2-methylpyridine/SF_3 ^+ adduct is found to be more stable than the staggered form by 0. 8 kcal/mol, due to auxiliary agostic bonding between the hydrogen of the ortho methyl substituent and the sulfur atom. Calculations on atomic charge distribution reveal that the positive charge is mainly on the sulfur atom (+1. 99) and the charge on the bonding hydrogen S-H-C (+ 0. 07) is considerably lower than that on the other two methyl hydrogens ( + 0. 14), which appears to be a good indiCation of agostic binding. The most stable form of the 2-ethylpyridine/SF_3 ^+ adduct is found when the N-C_1-C_α-C_β dihedral angle is approximately 60 °, where the ethyl hydrogen is directed toward the SF_3 group via an interesting six-membered ring alignment. The experiments show a remarkably small steric effect in 2,6-dimethylpyridine, probably due to strong agostic bonding enhanced by the buttressing effect that shortens the S-H distance. In addition, the face-to-face interactions of the F atoms and the H atoms further stabilize this form.

  • Relative carbonyl isocyanate Cation [OCNCO]+ affinities of pyridines determined by the kinetic method using multiple‐stage (MS3) mass spectrometry
    Journal of Mass Spectrometry, 1995
    Co-Authors: Sheng Sheng Yang, Guodong Chen, Shuguang Ma, R. Graham Cooks, Fabio C. Gozzo, Marcos N. Eberlin
    Abstract:

    Ion-molecule reactions in a pentaquadrupole mass spectrometer are used to generate cluster ions in which neutral pyridine molecules are bound to a linear Polyatomic Cation, [OCNCO]+. The dimeric adduct, viz. Py1[OCNCO]+Py2 where Py1 and Py2 represent substituted pyridines, formed upon reaction of mass-selected [OCNCO]+ with a mixture of pyridines, has a loosely bound structure, as suggested by triple stage mass spectrometric (MS3) experiments. Dimeric adducts comprised of meta- and/or para-substituted pyridines (unhindered pyridines) display an excellent linear correlation between the fragment ion abundance ratio ln [Py1[OCNCO]+/Py2[OCNCO]+] and the proton affinity difference of the pyridines. On the assumption that the effective temperature of the [OCNCO]+ -bound dimers is similar to that of the corresponding Cl+-bound dimers, [OCNCO]+ affinities of the substituted pyridines relative to pyridine are estimated to be 3-MePy 2.3, 4-MePy 3.2, 3-EtPy 3.5, 4-EtPy 4.1, 3,5-diMePy 4.9 and 3,4-diMePy 5.6 kcal mol−1 (1 kcal = 4.184 kJ). A linear relationship between the relative [OCNCO]+ Cation affinity and the relative proton affinity (PA) is derived as relative [OCNCO]+ affinity (kcal mol−1) = 0.96 ΔPA, using the assumed effective temperature of 555 K. Dimers consisting of ortho-substituted pyridines display substantial steric effects between the ortho-substituted alkyl group and the central [OCNCO]+ Cation. A set of gas-phase steric parameters (Sk) is determined and steric effects are ordered 2-MePy (-1.39)

Philip S. H. Wong - One of the best experts on this subject based on the ideXlab platform.

  • Sulfur trifluoride Cation (SF3+) affinities of pyridines determined by the kinetic method : Stereoelectronic effects in the gas phase
    Journal of the American Society for Mass Spectrometry, 1997
    Co-Authors: Philip S. H. Wong, Sheng Sheng Yang, Shuguang Ma, R. Graham Cooks, Fabio C. Gozzo, Marcos N. Eberlin
    Abstract:

    Ion/molecule reactions performed by pentaquadrupole mass spectrometry are used to generate cluster ions in which neutral pyridines are bound to the Polyatomic Cation SF3 +. The dimeric ions Py1SF3 +Py2, where Py1 and Py2 represent substituted pyridines, are shown to have loosely bound structures by collision-induced dissociation (MS3) experiments and by semiempirical AMI and ab initio RHF/6-31G(d, p) molecular orbital calculations. In the case of dimers comprised of meta- and/or para-substituted pyridines (unhindered pyridines), there is an excellent linear correlation between the logarithm of the fragment ion abundance ratio ln[Py1(SF3 +)]/[Py(SF3 +)] and the proton affinities (PA) of the constituent pyridines. Semiempirical calculations are used to estimate the SF3 + affinities of pyridines which are found to be in the range of 25–31 kcal/mol. The SF3 + affinities show an excellent linear correlation with the proton affinities of the pyridines, and the relationship SF3 + affinity (kcal/mol) = 0. 73PA — 135. 8 between the two affinities is derived. The effective temperature of the dimeric ions is determined to be 595 ± 69 K, which is in good agreement with values of around 600 K obtained experimentally in studies on many other systems activated under similar conditions. Ortho-substituted pyridines show lower than expected affinities due to stereoelectronic effects that decrease the Cation affinities. Gas-phase Stereoelectronic parameters (Sk) are measured from the deviation from the PA correlation and are ordered as 2-MePy (−1.09) < 2,6-diMePy (−1.11) < 2-EtPy (−1.91) < 2,3-diMePy (−2. 15) < 2,5-diMePy (− 2. 25) < 2,4-diMePy (− 2. 40). Overall, the steric effects are larger than those in the corresponding Cl+-bound dimers but smaller than those in the bulky [OCNCO+] system. Calculations show evidence for agostic bonding that offsets the steric effects in some cases. The eclipsed conformation of 2-methylpyridine/SF3 + adduct is found to be more stable than the staggered form by 0. 8 kcal/mol, due to auxiliary agostic bonding between the hydrogen of the ortho methyl substituent and the sulfur atom. Calculations on atomic charge distribution reveal that the positive charge is mainly on the sulfur atom (+1. 99) and the charge on the bonding hydrogen S-H-C (+ 0. 07) is considerably lower than that on the other two methyl hydrogens ( + 0. 14), which appears to be a good indiCation of agostic binding. The most stable form of the 2-ethylpyridine/SF3 + adduct is found when the N-C1-Cα-Cβ dihedral angle is approximately 60 °, where the ethyl hydrogen is directed toward the SF3 group via an interesting six-membered ring alignment. The experiments show a remarkably small steric effect in 2,6-dimethylpyridine, probably due to strong agostic bonding enhanced by the buttressing effect that shortens the S-H distance. In addition, the face-to-face interactions of the F atoms and the H atoms further stabilize this form.

  • Sulfur trifluoride Cation (SF_3 ^+) affinities of pyridines determined by the kinetic method: Stereoelectronic effects in the gas phase
    Journal of the American Society for Mass Spectrometry, 1997
    Co-Authors: Philip S. H. Wong, Sheng Sheng Yang, Shuguang Ma, R. Graham Cooks, Fabio C. Gozzo, Marcos N. Eberlin
    Abstract:

    Ion/molecule reactions performed by pentaquadrupole mass spectrometry are used to generate cluster ions in which neutral pyridines are bound to the Polyatomic Cation SF_3 ^+. The dimeric ions Py_1SF_3 ^+Py_2, where Py_1 and Py_2 represent substituted pyridines, are shown to have loosely bound structures by collision-induced dissociation (MS^3) experiments and by semiempirical AMI and ab initio RHF/6-31G( d, p ) molecular orbital calculations. In the case of dimers comprised of meta- and/or para-substituted pyridines (unhindered pyridines), there is an excellent linear correlation between the logarithm of the fragment ion abundance ratio ln[Py_1(SF_3 ^+)]/[Py(SF_3 ^+)] and the proton affinities (PA) of the constituent pyridines. Semiempirical calculations are used to estimate the SF_3 ^+ affinities of pyridines which are found to be in the range of 25–31 kcal/mol. The SF_3 ^+ affinities show an excellent linear correlation with the proton affinities of the pyridines, and the relationship SF_3 ^+ affinity (kcal/mol) = 0. 73PA — 135. 8 between the two affinities is derived. The effective temperature of the dimeric ions is determined to be 595 ± 69 K, which is in good agreement with values of around 600 K obtained experimentally in studies on many other systems activated under similar conditions. Ortho-substituted pyridines show lower than expected affinities due to stereoelectronic effects that decrease the Cation affinities. Gas-phase Stereoelectronic parameters (S^k) are measured from the deviation from the PA correlation and are ordered as 2-MePy (−1.09) < 2,6-diMePy (−1.11) < 2-EtPy (−1.91) < 2,3-diMePy (−2. 15) < 2,5-diMePy (− 2. 25) < 2,4-diMePy (− 2. 40). Overall, the steric effects are larger than those in the corresponding Cl^+-bound dimers but smaller than those in the bulky [OCNCO^+] system. Calculations show evidence for agostic bonding that offsets the steric effects in some cases. The eclipsed conformation of 2-methylpyridine/SF_3 ^+ adduct is found to be more stable than the staggered form by 0. 8 kcal/mol, due to auxiliary agostic bonding between the hydrogen of the ortho methyl substituent and the sulfur atom. Calculations on atomic charge distribution reveal that the positive charge is mainly on the sulfur atom (+1. 99) and the charge on the bonding hydrogen S-H-C (+ 0. 07) is considerably lower than that on the other two methyl hydrogens ( + 0. 14), which appears to be a good indiCation of agostic binding. The most stable form of the 2-ethylpyridine/SF_3 ^+ adduct is found when the N-C_1-C_α-C_β dihedral angle is approximately 60 °, where the ethyl hydrogen is directed toward the SF_3 group via an interesting six-membered ring alignment. The experiments show a remarkably small steric effect in 2,6-dimethylpyridine, probably due to strong agostic bonding enhanced by the buttressing effect that shortens the S-H distance. In addition, the face-to-face interactions of the F atoms and the H atoms further stabilize this form.

Sheng Sheng Yang - One of the best experts on this subject based on the ideXlab platform.

  • Sulfur trifluoride Cation (SF3+) affinities of pyridines determined by the kinetic method : Stereoelectronic effects in the gas phase
    Journal of the American Society for Mass Spectrometry, 1997
    Co-Authors: Philip S. H. Wong, Sheng Sheng Yang, Shuguang Ma, R. Graham Cooks, Fabio C. Gozzo, Marcos N. Eberlin
    Abstract:

    Ion/molecule reactions performed by pentaquadrupole mass spectrometry are used to generate cluster ions in which neutral pyridines are bound to the Polyatomic Cation SF3 +. The dimeric ions Py1SF3 +Py2, where Py1 and Py2 represent substituted pyridines, are shown to have loosely bound structures by collision-induced dissociation (MS3) experiments and by semiempirical AMI and ab initio RHF/6-31G(d, p) molecular orbital calculations. In the case of dimers comprised of meta- and/or para-substituted pyridines (unhindered pyridines), there is an excellent linear correlation between the logarithm of the fragment ion abundance ratio ln[Py1(SF3 +)]/[Py(SF3 +)] and the proton affinities (PA) of the constituent pyridines. Semiempirical calculations are used to estimate the SF3 + affinities of pyridines which are found to be in the range of 25–31 kcal/mol. The SF3 + affinities show an excellent linear correlation with the proton affinities of the pyridines, and the relationship SF3 + affinity (kcal/mol) = 0. 73PA — 135. 8 between the two affinities is derived. The effective temperature of the dimeric ions is determined to be 595 ± 69 K, which is in good agreement with values of around 600 K obtained experimentally in studies on many other systems activated under similar conditions. Ortho-substituted pyridines show lower than expected affinities due to stereoelectronic effects that decrease the Cation affinities. Gas-phase Stereoelectronic parameters (Sk) are measured from the deviation from the PA correlation and are ordered as 2-MePy (−1.09) < 2,6-diMePy (−1.11) < 2-EtPy (−1.91) < 2,3-diMePy (−2. 15) < 2,5-diMePy (− 2. 25) < 2,4-diMePy (− 2. 40). Overall, the steric effects are larger than those in the corresponding Cl+-bound dimers but smaller than those in the bulky [OCNCO+] system. Calculations show evidence for agostic bonding that offsets the steric effects in some cases. The eclipsed conformation of 2-methylpyridine/SF3 + adduct is found to be more stable than the staggered form by 0. 8 kcal/mol, due to auxiliary agostic bonding between the hydrogen of the ortho methyl substituent and the sulfur atom. Calculations on atomic charge distribution reveal that the positive charge is mainly on the sulfur atom (+1. 99) and the charge on the bonding hydrogen S-H-C (+ 0. 07) is considerably lower than that on the other two methyl hydrogens ( + 0. 14), which appears to be a good indiCation of agostic binding. The most stable form of the 2-ethylpyridine/SF3 + adduct is found when the N-C1-Cα-Cβ dihedral angle is approximately 60 °, where the ethyl hydrogen is directed toward the SF3 group via an interesting six-membered ring alignment. The experiments show a remarkably small steric effect in 2,6-dimethylpyridine, probably due to strong agostic bonding enhanced by the buttressing effect that shortens the S-H distance. In addition, the face-to-face interactions of the F atoms and the H atoms further stabilize this form.

  • Sulfur trifluoride Cation (SF_3 ^+) affinities of pyridines determined by the kinetic method: Stereoelectronic effects in the gas phase
    Journal of the American Society for Mass Spectrometry, 1997
    Co-Authors: Philip S. H. Wong, Sheng Sheng Yang, Shuguang Ma, R. Graham Cooks, Fabio C. Gozzo, Marcos N. Eberlin
    Abstract:

    Ion/molecule reactions performed by pentaquadrupole mass spectrometry are used to generate cluster ions in which neutral pyridines are bound to the Polyatomic Cation SF_3 ^+. The dimeric ions Py_1SF_3 ^+Py_2, where Py_1 and Py_2 represent substituted pyridines, are shown to have loosely bound structures by collision-induced dissociation (MS^3) experiments and by semiempirical AMI and ab initio RHF/6-31G( d, p ) molecular orbital calculations. In the case of dimers comprised of meta- and/or para-substituted pyridines (unhindered pyridines), there is an excellent linear correlation between the logarithm of the fragment ion abundance ratio ln[Py_1(SF_3 ^+)]/[Py(SF_3 ^+)] and the proton affinities (PA) of the constituent pyridines. Semiempirical calculations are used to estimate the SF_3 ^+ affinities of pyridines which are found to be in the range of 25–31 kcal/mol. The SF_3 ^+ affinities show an excellent linear correlation with the proton affinities of the pyridines, and the relationship SF_3 ^+ affinity (kcal/mol) = 0. 73PA — 135. 8 between the two affinities is derived. The effective temperature of the dimeric ions is determined to be 595 ± 69 K, which is in good agreement with values of around 600 K obtained experimentally in studies on many other systems activated under similar conditions. Ortho-substituted pyridines show lower than expected affinities due to stereoelectronic effects that decrease the Cation affinities. Gas-phase Stereoelectronic parameters (S^k) are measured from the deviation from the PA correlation and are ordered as 2-MePy (−1.09) < 2,6-diMePy (−1.11) < 2-EtPy (−1.91) < 2,3-diMePy (−2. 15) < 2,5-diMePy (− 2. 25) < 2,4-diMePy (− 2. 40). Overall, the steric effects are larger than those in the corresponding Cl^+-bound dimers but smaller than those in the bulky [OCNCO^+] system. Calculations show evidence for agostic bonding that offsets the steric effects in some cases. The eclipsed conformation of 2-methylpyridine/SF_3 ^+ adduct is found to be more stable than the staggered form by 0. 8 kcal/mol, due to auxiliary agostic bonding between the hydrogen of the ortho methyl substituent and the sulfur atom. Calculations on atomic charge distribution reveal that the positive charge is mainly on the sulfur atom (+1. 99) and the charge on the bonding hydrogen S-H-C (+ 0. 07) is considerably lower than that on the other two methyl hydrogens ( + 0. 14), which appears to be a good indiCation of agostic binding. The most stable form of the 2-ethylpyridine/SF_3 ^+ adduct is found when the N-C_1-C_α-C_β dihedral angle is approximately 60 °, where the ethyl hydrogen is directed toward the SF_3 group via an interesting six-membered ring alignment. The experiments show a remarkably small steric effect in 2,6-dimethylpyridine, probably due to strong agostic bonding enhanced by the buttressing effect that shortens the S-H distance. In addition, the face-to-face interactions of the F atoms and the H atoms further stabilize this form.

  • Relative carbonyl isocyanate Cation [OCNCO]+ affinities of pyridines determined by the kinetic method using multiple‐stage (MS3) mass spectrometry
    Journal of Mass Spectrometry, 1995
    Co-Authors: Sheng Sheng Yang, Guodong Chen, Shuguang Ma, R. Graham Cooks, Fabio C. Gozzo, Marcos N. Eberlin
    Abstract:

    Ion-molecule reactions in a pentaquadrupole mass spectrometer are used to generate cluster ions in which neutral pyridine molecules are bound to a linear Polyatomic Cation, [OCNCO]+. The dimeric adduct, viz. Py1[OCNCO]+Py2 where Py1 and Py2 represent substituted pyridines, formed upon reaction of mass-selected [OCNCO]+ with a mixture of pyridines, has a loosely bound structure, as suggested by triple stage mass spectrometric (MS3) experiments. Dimeric adducts comprised of meta- and/or para-substituted pyridines (unhindered pyridines) display an excellent linear correlation between the fragment ion abundance ratio ln [Py1[OCNCO]+/Py2[OCNCO]+] and the proton affinity difference of the pyridines. On the assumption that the effective temperature of the [OCNCO]+ -bound dimers is similar to that of the corresponding Cl+-bound dimers, [OCNCO]+ affinities of the substituted pyridines relative to pyridine are estimated to be 3-MePy 2.3, 4-MePy 3.2, 3-EtPy 3.5, 4-EtPy 4.1, 3,5-diMePy 4.9 and 3,4-diMePy 5.6 kcal mol−1 (1 kcal = 4.184 kJ). A linear relationship between the relative [OCNCO]+ Cation affinity and the relative proton affinity (PA) is derived as relative [OCNCO]+ affinity (kcal mol−1) = 0.96 ΔPA, using the assumed effective temperature of 555 K. Dimers consisting of ortho-substituted pyridines display substantial steric effects between the ortho-substituted alkyl group and the central [OCNCO]+ Cation. A set of gas-phase steric parameters (Sk) is determined and steric effects are ordered 2-MePy (-1.39)

R. Graham Cooks - One of the best experts on this subject based on the ideXlab platform.

  • Sulfur trifluoride Cation (SF3+) affinities of pyridines determined by the kinetic method : Stereoelectronic effects in the gas phase
    Journal of the American Society for Mass Spectrometry, 1997
    Co-Authors: Philip S. H. Wong, Sheng Sheng Yang, Shuguang Ma, R. Graham Cooks, Fabio C. Gozzo, Marcos N. Eberlin
    Abstract:

    Ion/molecule reactions performed by pentaquadrupole mass spectrometry are used to generate cluster ions in which neutral pyridines are bound to the Polyatomic Cation SF3 +. The dimeric ions Py1SF3 +Py2, where Py1 and Py2 represent substituted pyridines, are shown to have loosely bound structures by collision-induced dissociation (MS3) experiments and by semiempirical AMI and ab initio RHF/6-31G(d, p) molecular orbital calculations. In the case of dimers comprised of meta- and/or para-substituted pyridines (unhindered pyridines), there is an excellent linear correlation between the logarithm of the fragment ion abundance ratio ln[Py1(SF3 +)]/[Py(SF3 +)] and the proton affinities (PA) of the constituent pyridines. Semiempirical calculations are used to estimate the SF3 + affinities of pyridines which are found to be in the range of 25–31 kcal/mol. The SF3 + affinities show an excellent linear correlation with the proton affinities of the pyridines, and the relationship SF3 + affinity (kcal/mol) = 0. 73PA — 135. 8 between the two affinities is derived. The effective temperature of the dimeric ions is determined to be 595 ± 69 K, which is in good agreement with values of around 600 K obtained experimentally in studies on many other systems activated under similar conditions. Ortho-substituted pyridines show lower than expected affinities due to stereoelectronic effects that decrease the Cation affinities. Gas-phase Stereoelectronic parameters (Sk) are measured from the deviation from the PA correlation and are ordered as 2-MePy (−1.09) < 2,6-diMePy (−1.11) < 2-EtPy (−1.91) < 2,3-diMePy (−2. 15) < 2,5-diMePy (− 2. 25) < 2,4-diMePy (− 2. 40). Overall, the steric effects are larger than those in the corresponding Cl+-bound dimers but smaller than those in the bulky [OCNCO+] system. Calculations show evidence for agostic bonding that offsets the steric effects in some cases. The eclipsed conformation of 2-methylpyridine/SF3 + adduct is found to be more stable than the staggered form by 0. 8 kcal/mol, due to auxiliary agostic bonding between the hydrogen of the ortho methyl substituent and the sulfur atom. Calculations on atomic charge distribution reveal that the positive charge is mainly on the sulfur atom (+1. 99) and the charge on the bonding hydrogen S-H-C (+ 0. 07) is considerably lower than that on the other two methyl hydrogens ( + 0. 14), which appears to be a good indiCation of agostic binding. The most stable form of the 2-ethylpyridine/SF3 + adduct is found when the N-C1-Cα-Cβ dihedral angle is approximately 60 °, where the ethyl hydrogen is directed toward the SF3 group via an interesting six-membered ring alignment. The experiments show a remarkably small steric effect in 2,6-dimethylpyridine, probably due to strong agostic bonding enhanced by the buttressing effect that shortens the S-H distance. In addition, the face-to-face interactions of the F atoms and the H atoms further stabilize this form.

  • Sulfur trifluoride Cation (SF_3 ^+) affinities of pyridines determined by the kinetic method: Stereoelectronic effects in the gas phase
    Journal of the American Society for Mass Spectrometry, 1997
    Co-Authors: Philip S. H. Wong, Sheng Sheng Yang, Shuguang Ma, R. Graham Cooks, Fabio C. Gozzo, Marcos N. Eberlin
    Abstract:

    Ion/molecule reactions performed by pentaquadrupole mass spectrometry are used to generate cluster ions in which neutral pyridines are bound to the Polyatomic Cation SF_3 ^+. The dimeric ions Py_1SF_3 ^+Py_2, where Py_1 and Py_2 represent substituted pyridines, are shown to have loosely bound structures by collision-induced dissociation (MS^3) experiments and by semiempirical AMI and ab initio RHF/6-31G( d, p ) molecular orbital calculations. In the case of dimers comprised of meta- and/or para-substituted pyridines (unhindered pyridines), there is an excellent linear correlation between the logarithm of the fragment ion abundance ratio ln[Py_1(SF_3 ^+)]/[Py(SF_3 ^+)] and the proton affinities (PA) of the constituent pyridines. Semiempirical calculations are used to estimate the SF_3 ^+ affinities of pyridines which are found to be in the range of 25–31 kcal/mol. The SF_3 ^+ affinities show an excellent linear correlation with the proton affinities of the pyridines, and the relationship SF_3 ^+ affinity (kcal/mol) = 0. 73PA — 135. 8 between the two affinities is derived. The effective temperature of the dimeric ions is determined to be 595 ± 69 K, which is in good agreement with values of around 600 K obtained experimentally in studies on many other systems activated under similar conditions. Ortho-substituted pyridines show lower than expected affinities due to stereoelectronic effects that decrease the Cation affinities. Gas-phase Stereoelectronic parameters (S^k) are measured from the deviation from the PA correlation and are ordered as 2-MePy (−1.09) < 2,6-diMePy (−1.11) < 2-EtPy (−1.91) < 2,3-diMePy (−2. 15) < 2,5-diMePy (− 2. 25) < 2,4-diMePy (− 2. 40). Overall, the steric effects are larger than those in the corresponding Cl^+-bound dimers but smaller than those in the bulky [OCNCO^+] system. Calculations show evidence for agostic bonding that offsets the steric effects in some cases. The eclipsed conformation of 2-methylpyridine/SF_3 ^+ adduct is found to be more stable than the staggered form by 0. 8 kcal/mol, due to auxiliary agostic bonding between the hydrogen of the ortho methyl substituent and the sulfur atom. Calculations on atomic charge distribution reveal that the positive charge is mainly on the sulfur atom (+1. 99) and the charge on the bonding hydrogen S-H-C (+ 0. 07) is considerably lower than that on the other two methyl hydrogens ( + 0. 14), which appears to be a good indiCation of agostic binding. The most stable form of the 2-ethylpyridine/SF_3 ^+ adduct is found when the N-C_1-C_α-C_β dihedral angle is approximately 60 °, where the ethyl hydrogen is directed toward the SF_3 group via an interesting six-membered ring alignment. The experiments show a remarkably small steric effect in 2,6-dimethylpyridine, probably due to strong agostic bonding enhanced by the buttressing effect that shortens the S-H distance. In addition, the face-to-face interactions of the F atoms and the H atoms further stabilize this form.

  • Relative carbonyl isocyanate Cation [OCNCO]+ affinities of pyridines determined by the kinetic method using multiple‐stage (MS3) mass spectrometry
    Journal of Mass Spectrometry, 1995
    Co-Authors: Sheng Sheng Yang, Guodong Chen, Shuguang Ma, R. Graham Cooks, Fabio C. Gozzo, Marcos N. Eberlin
    Abstract:

    Ion-molecule reactions in a pentaquadrupole mass spectrometer are used to generate cluster ions in which neutral pyridine molecules are bound to a linear Polyatomic Cation, [OCNCO]+. The dimeric adduct, viz. Py1[OCNCO]+Py2 where Py1 and Py2 represent substituted pyridines, formed upon reaction of mass-selected [OCNCO]+ with a mixture of pyridines, has a loosely bound structure, as suggested by triple stage mass spectrometric (MS3) experiments. Dimeric adducts comprised of meta- and/or para-substituted pyridines (unhindered pyridines) display an excellent linear correlation between the fragment ion abundance ratio ln [Py1[OCNCO]+/Py2[OCNCO]+] and the proton affinity difference of the pyridines. On the assumption that the effective temperature of the [OCNCO]+ -bound dimers is similar to that of the corresponding Cl+-bound dimers, [OCNCO]+ affinities of the substituted pyridines relative to pyridine are estimated to be 3-MePy 2.3, 4-MePy 3.2, 3-EtPy 3.5, 4-EtPy 4.1, 3,5-diMePy 4.9 and 3,4-diMePy 5.6 kcal mol−1 (1 kcal = 4.184 kJ). A linear relationship between the relative [OCNCO]+ Cation affinity and the relative proton affinity (PA) is derived as relative [OCNCO]+ affinity (kcal mol−1) = 0.96 ΔPA, using the assumed effective temperature of 555 K. Dimers consisting of ortho-substituted pyridines display substantial steric effects between the ortho-substituted alkyl group and the central [OCNCO]+ Cation. A set of gas-phase steric parameters (Sk) is determined and steric effects are ordered 2-MePy (-1.39)

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  • Sulfur trifluoride Cation (SF3+) affinities of pyridines determined by the kinetic method : Stereoelectronic effects in the gas phase
    Journal of the American Society for Mass Spectrometry, 1997
    Co-Authors: Philip S. H. Wong, Sheng Sheng Yang, Shuguang Ma, R. Graham Cooks, Fabio C. Gozzo, Marcos N. Eberlin
    Abstract:

    Ion/molecule reactions performed by pentaquadrupole mass spectrometry are used to generate cluster ions in which neutral pyridines are bound to the Polyatomic Cation SF3 +. The dimeric ions Py1SF3 +Py2, where Py1 and Py2 represent substituted pyridines, are shown to have loosely bound structures by collision-induced dissociation (MS3) experiments and by semiempirical AMI and ab initio RHF/6-31G(d, p) molecular orbital calculations. In the case of dimers comprised of meta- and/or para-substituted pyridines (unhindered pyridines), there is an excellent linear correlation between the logarithm of the fragment ion abundance ratio ln[Py1(SF3 +)]/[Py(SF3 +)] and the proton affinities (PA) of the constituent pyridines. Semiempirical calculations are used to estimate the SF3 + affinities of pyridines which are found to be in the range of 25–31 kcal/mol. The SF3 + affinities show an excellent linear correlation with the proton affinities of the pyridines, and the relationship SF3 + affinity (kcal/mol) = 0. 73PA — 135. 8 between the two affinities is derived. The effective temperature of the dimeric ions is determined to be 595 ± 69 K, which is in good agreement with values of around 600 K obtained experimentally in studies on many other systems activated under similar conditions. Ortho-substituted pyridines show lower than expected affinities due to stereoelectronic effects that decrease the Cation affinities. Gas-phase Stereoelectronic parameters (Sk) are measured from the deviation from the PA correlation and are ordered as 2-MePy (−1.09) < 2,6-diMePy (−1.11) < 2-EtPy (−1.91) < 2,3-diMePy (−2. 15) < 2,5-diMePy (− 2. 25) < 2,4-diMePy (− 2. 40). Overall, the steric effects are larger than those in the corresponding Cl+-bound dimers but smaller than those in the bulky [OCNCO+] system. Calculations show evidence for agostic bonding that offsets the steric effects in some cases. The eclipsed conformation of 2-methylpyridine/SF3 + adduct is found to be more stable than the staggered form by 0. 8 kcal/mol, due to auxiliary agostic bonding between the hydrogen of the ortho methyl substituent and the sulfur atom. Calculations on atomic charge distribution reveal that the positive charge is mainly on the sulfur atom (+1. 99) and the charge on the bonding hydrogen S-H-C (+ 0. 07) is considerably lower than that on the other two methyl hydrogens ( + 0. 14), which appears to be a good indiCation of agostic binding. The most stable form of the 2-ethylpyridine/SF3 + adduct is found when the N-C1-Cα-Cβ dihedral angle is approximately 60 °, where the ethyl hydrogen is directed toward the SF3 group via an interesting six-membered ring alignment. The experiments show a remarkably small steric effect in 2,6-dimethylpyridine, probably due to strong agostic bonding enhanced by the buttressing effect that shortens the S-H distance. In addition, the face-to-face interactions of the F atoms and the H atoms further stabilize this form.

  • Sulfur trifluoride Cation (SF_3 ^+) affinities of pyridines determined by the kinetic method: Stereoelectronic effects in the gas phase
    Journal of the American Society for Mass Spectrometry, 1997
    Co-Authors: Philip S. H. Wong, Sheng Sheng Yang, Shuguang Ma, R. Graham Cooks, Fabio C. Gozzo, Marcos N. Eberlin
    Abstract:

    Ion/molecule reactions performed by pentaquadrupole mass spectrometry are used to generate cluster ions in which neutral pyridines are bound to the Polyatomic Cation SF_3 ^+. The dimeric ions Py_1SF_3 ^+Py_2, where Py_1 and Py_2 represent substituted pyridines, are shown to have loosely bound structures by collision-induced dissociation (MS^3) experiments and by semiempirical AMI and ab initio RHF/6-31G( d, p ) molecular orbital calculations. In the case of dimers comprised of meta- and/or para-substituted pyridines (unhindered pyridines), there is an excellent linear correlation between the logarithm of the fragment ion abundance ratio ln[Py_1(SF_3 ^+)]/[Py(SF_3 ^+)] and the proton affinities (PA) of the constituent pyridines. Semiempirical calculations are used to estimate the SF_3 ^+ affinities of pyridines which are found to be in the range of 25–31 kcal/mol. The SF_3 ^+ affinities show an excellent linear correlation with the proton affinities of the pyridines, and the relationship SF_3 ^+ affinity (kcal/mol) = 0. 73PA — 135. 8 between the two affinities is derived. The effective temperature of the dimeric ions is determined to be 595 ± 69 K, which is in good agreement with values of around 600 K obtained experimentally in studies on many other systems activated under similar conditions. Ortho-substituted pyridines show lower than expected affinities due to stereoelectronic effects that decrease the Cation affinities. Gas-phase Stereoelectronic parameters (S^k) are measured from the deviation from the PA correlation and are ordered as 2-MePy (−1.09) < 2,6-diMePy (−1.11) < 2-EtPy (−1.91) < 2,3-diMePy (−2. 15) < 2,5-diMePy (− 2. 25) < 2,4-diMePy (− 2. 40). Overall, the steric effects are larger than those in the corresponding Cl^+-bound dimers but smaller than those in the bulky [OCNCO^+] system. Calculations show evidence for agostic bonding that offsets the steric effects in some cases. The eclipsed conformation of 2-methylpyridine/SF_3 ^+ adduct is found to be more stable than the staggered form by 0. 8 kcal/mol, due to auxiliary agostic bonding between the hydrogen of the ortho methyl substituent and the sulfur atom. Calculations on atomic charge distribution reveal that the positive charge is mainly on the sulfur atom (+1. 99) and the charge on the bonding hydrogen S-H-C (+ 0. 07) is considerably lower than that on the other two methyl hydrogens ( + 0. 14), which appears to be a good indiCation of agostic binding. The most stable form of the 2-ethylpyridine/SF_3 ^+ adduct is found when the N-C_1-C_α-C_β dihedral angle is approximately 60 °, where the ethyl hydrogen is directed toward the SF_3 group via an interesting six-membered ring alignment. The experiments show a remarkably small steric effect in 2,6-dimethylpyridine, probably due to strong agostic bonding enhanced by the buttressing effect that shortens the S-H distance. In addition, the face-to-face interactions of the F atoms and the H atoms further stabilize this form.

  • Relative carbonyl isocyanate Cation [OCNCO]+ affinities of pyridines determined by the kinetic method using multiple‐stage (MS3) mass spectrometry
    Journal of Mass Spectrometry, 1995
    Co-Authors: Sheng Sheng Yang, Guodong Chen, Shuguang Ma, R. Graham Cooks, Fabio C. Gozzo, Marcos N. Eberlin
    Abstract:

    Ion-molecule reactions in a pentaquadrupole mass spectrometer are used to generate cluster ions in which neutral pyridine molecules are bound to a linear Polyatomic Cation, [OCNCO]+. The dimeric adduct, viz. Py1[OCNCO]+Py2 where Py1 and Py2 represent substituted pyridines, formed upon reaction of mass-selected [OCNCO]+ with a mixture of pyridines, has a loosely bound structure, as suggested by triple stage mass spectrometric (MS3) experiments. Dimeric adducts comprised of meta- and/or para-substituted pyridines (unhindered pyridines) display an excellent linear correlation between the fragment ion abundance ratio ln [Py1[OCNCO]+/Py2[OCNCO]+] and the proton affinity difference of the pyridines. On the assumption that the effective temperature of the [OCNCO]+ -bound dimers is similar to that of the corresponding Cl+-bound dimers, [OCNCO]+ affinities of the substituted pyridines relative to pyridine are estimated to be 3-MePy 2.3, 4-MePy 3.2, 3-EtPy 3.5, 4-EtPy 4.1, 3,5-diMePy 4.9 and 3,4-diMePy 5.6 kcal mol−1 (1 kcal = 4.184 kJ). A linear relationship between the relative [OCNCO]+ Cation affinity and the relative proton affinity (PA) is derived as relative [OCNCO]+ affinity (kcal mol−1) = 0.96 ΔPA, using the assumed effective temperature of 555 K. Dimers consisting of ortho-substituted pyridines display substantial steric effects between the ortho-substituted alkyl group and the central [OCNCO]+ Cation. A set of gas-phase steric parameters (Sk) is determined and steric effects are ordered 2-MePy (-1.39)