The acidity of polyhalogenated silanes and silyl radicals

1992 ◽  
Vol 70 (8) ◽  
pp. 2234-2240 ◽  
Author(s):  
C. F. Rodriquez ◽  
A. C. Hopkinson
Keyword(s):  
Gas Phase ◽  
Molecular Orbital ◽  
Electron Affinity ◽  
Substituent Effects ◽  
Linear Interpolation ◽  
Molecular Orbital Calculations ◽  
Electron Affinities ◽  
Silyl Radicals ◽  
Experimental Values ◽  
Cl Atom

The results of abinitio molecular orbital calculations at the MP4SDTQ/6-31++G(d,p)//HF/6-31++G(d,p) level have been used to calculate acidities of fluoro- and chloro-substituted silanes and silyl radicals. The radicals are more acidic than the silanes and substituent effects are also slightly larger in the radicals. For the gas phase deprotonation of fluorosilanes at 298 K, ΔHr (kcal/mol) values are SiH4, 378.5; SiH3F, 374.5; SiH2F, 366.7, and SiHF3, 351.0, i.e., interaction between fluorine atoms leads to increased enhancement of acidity. For chlorosilanes substituent effects are larger but strictly additive (13 kcal/mol for each Cl atom) with ΔHr values SiH3Cl, 365.4; SiH2Cl2 352.5, and SiHCl3 339.4. The electron affinities of silyl radicals calculated using isogyric reactions at the MP4SDTQ/6-31++G(d,p) level are too low by ~0.3 eV, but at the MP4SDTQ/6-311++G(2df,p) level the calculated electron affinity of SiH3 is 1.39 eV, compared with an experimental value of 1.44 ± 0.03 eV. This higher level of theory gives calculated electron affinities of 1.53 eV for SiH2F and 1.92 eV for SiH2Cl. Heats of formation obtained by using isogyric reactions to calculate atomization energies at the MP4SDTQ/6-311++G(2df,p) level are within 3 kcal/mol of experimental values except for SiH2F (where the "experimental" value was obtained from linear interpolation between SiH3 and SiF3). [Formula: see text] (kcal/mol) calculated for the anions are SiH3−, 14.4; SiH2F−, −78.0; and SiH2Cl−, −37.6.

Substituent effects at silicon in cations SiX+, HSiX+•, and H2SiX+, and in radicals H2SiX+•

1989 ◽  
Vol 67 (6) ◽  
pp. 991-997 ◽  
Author(s):  
A. C. Hopkinson ◽  
M. H. Lien
Keyword(s):  
Linear Relationship ◽  
Molecular Orbital ◽  
Single Point ◽  
Substituent Effects ◽  
Molecular Orbital Calculations ◽  
Amino Group ◽  
Ionisation Potential ◽  
Silyl Radicals

Abinitio molecular orbital calculations at the 6-31G* level have been used to optimise structures for ions SiX+, HSiX+•, and H2SiX+, and for neutrals HSiX (singlets), H2SiX•, and H3SiX, where X is H, CH3, NH2, OH, F, CN, and NC. Single point calculations at the MP4(SDTQ)/6-31G* level were used to calculate substituent stabilisation energies.The amino group is the strongest π-donor and also is the most stabilising group in the cations, the silylenes, and the silyl radicals. Stabilisation is greatest in ions SiX+. Ions HSiX+• and H2SiX+ are stabilised by similar but smaller amounts, although CN and NC are destabilising in these ions. Substituent stabilisation energies in radicals H2SiX• are almost zero. There is a linear relationship between the stabilisation energies of ions H2SiX+ and the ionisation potentials of radicals H2SiX•, but a similar plot correlating stabilisation energies for ions HSiX+۟• with the ionisation potential of HSiX (singlet) shows considerable scatter. Keywords: silications, silyl radicals, stabilisation energies.

Ab initio Predictions of the Gas-Phase Acidities of Diazirine and Diazomethane

1995 ◽  
Vol 48 (2) ◽  
pp. 401 ◽  
Author(s):  
AJ Russell ◽  
AP Scott ◽  
L Radom
Keyword(s):  
Ab Initio ◽  
Gas Phase ◽  
Molecular Orbital ◽  
Calculated Result ◽  
Molecular Orbital Calculations ◽  
Experimental Values

Absolute gas-phase acidities of diazirine, diazomethane, dimethylamine, ethylamine and methylamine have been obtained with ab initio molecular orbital calculations at the G2 and G2(MP2) levels of theory. Values at the G2 and G2(MP2) levels are all within 1 kJ mol-1 of one another. The calculated gas-phase acidities for diazomethane, dimethylamine, ethylamine and methylamine (and other reference molecules considered) are all within 3 kJ mol-1 of experiment. For diazirine the calculated result, although consistent with experiment, would suggest an acidity towards the lower end of the range of possible experimental values.

Gas-Phase Tautomerism in the Triazoles and Tetrazoles: A Study by Photoelectron Spectroscopy and ab Initio Molecular Orbital Calculations

1981 ◽  
Vol 36 (11) ◽  
pp. 1246-1252 ◽  
Author(s):  
Michael H. Palmer ◽  
Isobel Simpson ◽  
J. Ross Wheeler
Keyword(s):  
Ab Initio ◽  
Gas Phase ◽  
Molecular Orbital ◽  
Photoelectron Spectroscopy ◽  
Relative Stability ◽  
Photoelectron Spectra ◽  
Equilibrium Geometry ◽  
Molecular Orbital Calculations ◽  
Methyl Derivatives

The photoelectron spectra of the tautomeric 1,2,3,- and 1,2,4-triazole and 1,2,3,4-tetrazole systems have been compared with the corresponding N-methyl derivatives. The dominant tautomers in the gas phase have been identified as 2 H-1,2,3-triazole, 1 H-1,2,4-triazole and 2H-tetrazole.Full optimisation of the equilibrium geometry by ab initio molecular orbital methods leads to the same conclusions, for relative stability of the tautomers in each of the triazoles, but the calculations wrongly predict the tetrazole tautomerism.

Molecular orbital structures of sulfones

1982 ◽  
Vol 60 (6) ◽  
pp. 730-734 ◽  
Author(s):  
Russell J. Boyd ◽  
Jeffrey P. Szabo
Keyword(s):  
Crystal Structure ◽  
Gas Phase ◽  
Molecular Orbital ◽  
Geometry Optimization ◽  
Membered Ring ◽  
Molecular Orbital Calculations ◽  
Bond Lengths ◽  
Comparison With Experiment ◽  
The Difference

Abinitio molecular orbital calculations are reported for several cyclic and acyclic sulfones. The geometries of XSO2Y, where X, Y = H, F, or CH3 are optimized at the STO-3G* level. Similar calculations are reported for the smallest cyclic sulfone, thiirane-1,1 -dioxide, as well as the corresponding sulfoxide, thiirane-1-oxide, and the parent sulfide, thiirane. Where comparison with experiment is possible, the agreement is satisfactory. In order to consider the possibility of substantial differences between axial and equatorial S—O bonds in the gas phase, as observed in the crystal structure of 5H,8H-dibenzo[d,f][1,2]-dithiocin-1,1-dioxide, STO-3G* calculations are reported for a six-membered ring, thiane-1,1-dioxide, and a model eight-membered ring. Limited geometry optimization of the axial and equatorial S—O bonds in the chair conformations of the six- and eight-membered rings leads to bond lengths of 1.46 Å with the difference being less than 0.01 Å.

Absolute gas-phase acidities of CH3NH2, C2H5NH2, (CH3)2 NH, and (CH3)3N

1976 ◽  
Vol 54 (10) ◽  
pp. 1624-1642 ◽  
Author(s):  
Gervase I. Mackay ◽  
Ronald S. Hemsworth ◽  
Diethard K. Bohme
Keyword(s):  
Equilibrium Constant ◽  
Gas Phase ◽  
Equilibrium Constants ◽  
Heats Of Formation ◽  
Molecular Orbital Calculations ◽  
Electron Affinities ◽  
Flowing Afterglow ◽  
Experimental Assessment ◽  
Gas Phase Acidity ◽  
Conjugate Bases

The flowing afterglow technique has been employed in measurements of the rate and equilibrium constants at 296 ± 2 K for reactions of the type[Formula: see text]and[Formula: see text]where R1 and R2 may be H, CH3, or C2H5. The equilibrium constant measurements provided absolute values for the intrinsic (gas-phase) acidities of the Brønsted acids CH3NH2, C2H5NH2, (CH3)2NH, and (CH3)3N, the heats of formation of their conjugate bases, and the electron affinities of the corresponding radicals R1R2N. Proton removal energies, ΔG0298/(kcal mol−1), were determined to be 395.7 ± 0.7 for [Formula: see text] 391.7 ± 0.7 for [Formula: see text] 389.2 ± 0.6 for [Formula: see text] and > 396 for [Formula: see text] Heats of formation, ΔH0f.,298, were determined to be 30.5 ± 1.5 for CH3NH−, 21.2 ± 1.5 for C2H5NH−, and 24.7 ± 1.4 for (CH3)2N−. Electron affinities (in kcal mol−1) were determined to be 13.1 ± 3.5 for CH3NH, 17 ± 4 for C2H5NH, and 14.3 ± 3.4 for (CH3)2N. These results quantify earlier conclusions regarding the intrinsic effects of substituents on the gas-phase acidity of amines and provide an experimental assessment of recent molecular orbital calculations of proton removal energies for alkylamines.

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