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Articles 1 - 14 of 14
Full-Text Articles in Physical Sciences and Mathematics
Substituent Effects On The Gas-Phase Acidity Of Silane, Mark S. Gordon, David E. Volk, David R. Gano
Substituent Effects On The Gas-Phase Acidity Of Silane, Mark S. Gordon, David E. Volk, David R. Gano
Mark S. Gordon
In a previous paper, the gas-phase acidities of XHn compounds (X= C, N, 0, F, Si, P, S, Cl) were predicted with ab initio wave functions. At the MP4 level of theory with extended basis sets (6-311++G(3df,2pd)3 for second-period atoms and 6-3l++G- (3df,2pd) for third-period atoms], the calculated gas-phase acidities for these species were determined to be within 2 kcaljmol of experimental values. Similar results for the second period were obtained by DeFrees and McLean.
Predicted Heats Of Formation For Methylsilylene And Dimethylsilylene, Mark S. Gordon, Jarry A. Boatz
Predicted Heats Of Formation For Methylsilylene And Dimethylsilylene, Mark S. Gordon, Jarry A. Boatz
Mark S. Gordon
Ab initio calculations at the MP216-31G(d,p) level of computation, combined with a sequence of isodesmic reactions, have been used to predict the heats of formation of methylsilylene and dimethylsilylene to be 49.2 and 32-33 kcallmol, respectively. These results are discussed in the light of recent experimental determinations in the accompanying experimental papers.
Structures Of Two Organosilyl Azides, Steven S. Zigler, Kenneth J. Haller, Robert West, Mark S. Gordon
Structures Of Two Organosilyl Azides, Steven S. Zigler, Kenneth J. Haller, Robert West, Mark S. Gordon
Mark S. Gordon
X-ray crystal structures were determined for trimesitylazidosilane (3) and 1,1-dimesityl-2,2-diphenyl- 2-tert-butylazidodisilane (4). The NcN2 and N2-N3 bond lengths (pm) in both compounds are nearly equal, being 115.8 and 117.0 for 3 and 117.0 and 114.7 for 4, respectively. These silyl azides thus differ markedly from organic azides, in which N2-N3 is much shorter than NcN2• Molecular orbital calculations predict N2-N3 to be 6 pm shorter than NcN2 in H3SiN3 (5). The N-N-N angle is 173.7° for 3 and 174.9° for 4, in good agreement with the calculated value for 5.
Intrinsic Frequency Analysis Of The Generalized Normal-Mode Vibrations For The Reaction H 2 + Ch3 - H + Ch4, Jerry A. Boatz, Mark S. Gordon
Intrinsic Frequency Analysis Of The Generalized Normal-Mode Vibrations For The Reaction H 2 + Ch3 - H + Ch4, Jerry A. Boatz, Mark S. Gordon
Mark S. Gordon
Vibrational energy distributions and intrinsic frequencies are computed at selected points along the H 2 + CH 3 - H + CH4 intrinsic reaction coordinate, calculated at the UHF /ST0-3G level. The energy distributions and intrinsic frequencies are shown to provide a clear and intuitively pleasing picture of the evolution of generalized normal coordinates along the reaction path. Furthermore, these quantities aid in the identification of avoided crossings between distinct generalized normal modes and between generalized normal modes and the reaction coordinate.
Ab Initio Reaction Paths And Direct Dynamics Calculations, Kim K. Baldridge, Mark S. Gordon, Rozeanne Steckler, Donald G. Truhlar
Ab Initio Reaction Paths And Direct Dynamics Calculations, Kim K. Baldridge, Mark S. Gordon, Rozeanne Steckler, Donald G. Truhlar
Mark S. Gordon
A detailed study of methods for generating the minimum energy path of a chemical reaction using ab initio electronic structure calculations is presented; the convergence with respect to step size of the geometry and energy along this path is studied with several algorithms. The investigations are extended to the calculation of chemical reaction rate coefficients by interfacing the POLYRATE code for variational transition-state theory and semiclassical tunneling calculations with a locally modified GAUSSIAN82 electronic structure package that now contains reaction path following capabilities at both the Hartree-Fock and perturbation theory levels. This combined package is used to study the kinetics …
Molecular And Electronic Structure Of Siladiimide And Other Allenic X=Y=X Compounds, Mark S. Gordon, Michael W. Schmidt, S. Koseki
Molecular And Electronic Structure Of Siladiimide And Other Allenic X=Y=X Compounds, Mark S. Gordon, Michael W. Schmidt, S. Koseki
Mark S. Gordon
The preferred geometric structures of the allene analogues CH2=Si=CH2 (1), NH=Si=NH (2), PH=Si=PH (3), NH= C=NH (4), and NH=Si=CH2 (5) are examined at the RHF /6-31 G(d) level of theory, and that of 2 is further examined at the MP2/6-31G(d) computational level. The prediction that at the SCF level compounds 1, 3, 4, and 5 prefer orthogonal geometries like allene, while 2 prefers a planar structure, is examined with the aid of localized molecular orbitals, correlated energies, and, in the case of 2, a detailed analysis of the potential energy surface. The latter is very flat, and at the highest …
Theoretical Studies Of Three-Membered Ring Compounds Y2h4x (Y = C, Si; X = Ch2, Nh, O, Sih2, Ph, S), Jerry A. Boatz, Mark S. Gordon
Theoretical Studies Of Three-Membered Ring Compounds Y2h4x (Y = C, Si; X = Ch2, Nh, O, Sih2, Ph, S), Jerry A. Boatz, Mark S. Gordon
Mark S. Gordon
The heats of formation of the three-membered ring compounds Y 2H4X (Y = C, Si; X = CH2, NH, 0, SiH2, PH, S) and the thermodynamics of the insertion reactions X+ YH2=YH2 -- c-Y2H4X are predicted by using MP2/6-31G(d) energies at the 6-31G(d) geometries. Bent bond lengths are calculated by tracing the path of maximum electron density connecting two nuclei, with the 6-31G(2d) basis set at the 6-31G(d) structures. The short Si-Si internuclear distances in Si2H4X (X = CH2, NH, 0, PH, S) apparently are the result of severe bond bending rather than significant 1r character in the Si-Si bonds.
Theoretical Studies Of Bond Stretch Isomerism In Silabicyclobutanes, Jerry A. Boatz, Mark S. Gordon
Theoretical Studies Of Bond Stretch Isomerism In Silabicyclobutanes, Jerry A. Boatz, Mark S. Gordon
Mark S. Gordon
The structures and relative energies of the bond stretch isomers of tetrasilabicyclo[l.l.O]butane (1), 1,2,3-trisilabicyclo[ l:l.O]butane (2), and 1,3-disilabicyclo[l.l.O]butane (3) are predicted at the GVB/3-21G* level, with the u and u* bridge bond molecular orbitals correlated in the GVB wave function. The transition-state structure and intrinsic reaction coordinate connecting each pair of bond stretch isomers are calculated. Two transition states joining the bond stretch isomers of 3 are located. The more stable isomer in all three molecules is the one with the longer bridge bond.
Decomposition Of Normal-Coordinate Vibrational Frequencies, Jerry A. Boatz, Mark S. Gordon
Decomposition Of Normal-Coordinate Vibrational Frequencies, Jerry A. Boatz, Mark S. Gordon
Mark S. Gordon
A decomposition scheme for the partitioning of normal-coordinate vibrational frequencies into individual internal-coordinate "intrinsic~ frequencies is presented. This scheme assigns to each internal coordinate a frequency that is representative of a vibration described solely by motion of that particular coordinate. This facilitates the interpretation of harmonic vibrational frequencies and their associated normal-coordinate displacements, especially in those cases where the normal coordinates are best described by the coupling of several internal coordinates. The utility of the method is demonstrated via application to several simple hydrides and cycloalkanes, using ab initio SCF molecular structures and Cartesian force constant matrices.
Theoretical Studtes Of The Insertion Reactions Of Atomic Carbon And Silicon Into Methane And Silane, Shogo Sakai, John Deisz, Mark S. Gordon
Theoretical Studtes Of The Insertion Reactions Of Atomic Carbon And Silicon Into Methane And Silane, Shogo Sakai, John Deisz, Mark S. Gordon
Mark S. Gordon
The mechanisms for the insertions of atomic (ID and 3P) carbon and silicon into the C-Hand Si-H bonds of methane and silane are investigated by ab initio SCF methods, many body perturbation theory, and a localized molecular orbital (LMO) analysis. The LMO analysis shows that the insertion of 1D atoms into CH4 and SiH4 may be classified into two types: cationic hydrogen transfer and anionic hydrogen transfer. For the triplet atoms the LMO analysis suggests two different insertion reaction mechanisms: the near abstraction and the pull-push mechanisms.
Mechanistic Studies Of The Far-Uv Photochemical Ring-Opening And Cleavage Reactions Of 1, 1-Dimethyl-1-Silacyclobut-2-Ene, Mark G. Steinmetz, B. S. Udayakumar, Mark S. Gordon
Mechanistic Studies Of The Far-Uv Photochemical Ring-Opening And Cleavage Reactions Of 1, 1-Dimethyl-1-Silacyclobut-2-Ene, Mark G. Steinmetz, B. S. Udayakumar, Mark S. Gordon
Mark S. Gordon
Direct photolyses of 1,1-dimethyl-1-silacyclobut-2-ene (1) at 214 nm in tert-butyl alcohol gave 29% tert-butoxydimethyl(2-propenyl)silane (2), 4.3% (Z)-tert-butoxydimethyl(1-propenyl)silane [(Z)-3], 6.9% (E)-tert-butoxydimethyl(1-propenyl)silane [(E)-3], and 5.9% tert-butoxytrimethylsilane (4) at 49% conversion. Products 2-4 were primary, and the total quantum yield of formation was 0.11. Use of tert-butyl alcohol-0-d led to >98% monodeuteration: 2-dl had 93.5% of the label at cl and 6.5% at c3 of the 2-propenyl group, (E)- and (Z)-3-d1 [(E),(Z)-3-dd were labeled >95% at C3 of the 1-propenyl group, and 4-d1 was deuterated at a silyl methyl. Cleavage of a silacyclopropylmethylene intermediate to 1,1-dimethyl-1-silaethene accounts for the labeling of 4-d1 and …
Heats Of Formation Of Alkylsilanes, Mark S. Gordon, Jerry A. Boatz, Robin Walsh
Heats Of Formation Of Alkylsilanes, Mark S. Gordon, Jerry A. Boatz, Robin Walsh
Mark S. Gordon
Theoretical heats of formation at 298 K for several alkylsilanes, predicted at the MP2/6-31G(d) level of theory, are compared with recently obtained experimental and additivity values. Excellent agreement is obtained between the ab initio and additivity values and with the more reliable experimental values for acyclic alkylsilanes. The ab initio heats of formation for the silacycloalkanes permit the evaluation of strain energy increments for the additivity scheme. Comparison is made with limited experimental data.
Intrinsic Reaction Coordinate Calculations For Very Flat Potential Energy Surfaces: Application To Singlet Si2h2 (Disilenylidene) Isomerization, Shiro Koseki, Mark S. Gordon
Intrinsic Reaction Coordinate Calculations For Very Flat Potential Energy Surfaces: Application To Singlet Si2h2 (Disilenylidene) Isomerization, Shiro Koseki, Mark S. Gordon
Mark S. Gordon
Si2H2 is an important species that may appear in the chemical vapor deposition of silicon. This paper reports an intrinsic reaction coordinate (IRC), or a minimum energy path (MEP), of the isomerization from silasilene to bridged disilyne obtained by using the local cubic and quadratic approximations. These. new approximations generate a correct IRC for this isomerization, while some conventional methods fail to predict a reasonable IRC because of the very flat potential energy surface. This reaction path bifurcates to two identical IRCs to reach bridged disilyne. The activation energy for this isomerization is predicted to be less than 3 kcalfmol. …
Transition State Structure, Barrier Height, And Vibrational Frequencies For The Reaction Cl+Ch4→Ch3+Hcl, Thanh N. Truong, Donald G. Truhlar, Kim K. Baldridge, Mark S. Gordon, Rozeanne Steckler
Transition State Structure, Barrier Height, And Vibrational Frequencies For The Reaction Cl+Ch4→Ch3+Hcl, Thanh N. Truong, Donald G. Truhlar, Kim K. Baldridge, Mark S. Gordon, Rozeanne Steckler
Mark S. Gordon
We have carried out a b i n i t i o calculations using second‐ and fourth‐order Mo/ller–Plesset perturbation theory, scaled electron correlation, and several basis sets for the reaction Cl+CH4→CH3+HCl. We found that including electron correlation is essential for obtaining accurate barrier heights and vibrational frequencies. Furthermore, scaling the correlation energy further improves the barrier height predictions provided that the basis set being used is correlation balanced for both bonds involved in the reaction. Geometries and transition state frequencies calculated at the MP2 and MP‐SAC2 levels with the most extensive and best balanced basis set are in good agreement …