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ABSTRACT: Atomization energies at 0 K and heats of formation at 0 and 298 K are predicted for the MH(x)Cl(y) compounds (M = Si, P, As, and Sb) and for a number of trivalent, tetravalent, and pentavalent fluorides (SbF(3), BiF(3), GeF(4), SnF(4), PbF(4), AsF(5), SbF(5)) from coupled cluster theory (CCSD(T)) calculations using correlation consistent basis sets and extrapolation to the complete basis set limit. Small-core, relativistic effective core potentials were used for the heavier elements (Ge, As, Sn, Sb, Pb, and Bi), including correlation of the outer core electrons. Additional scalar relativistic (for the lighter elements) and atomic spin-orbit corrections are included in order to achieve near chemical accuracy of ±1.5 kcal/mol. Vibrational zero point energies were computed from scaled harmonic frequencies at the second order Møller-Plesset perturbation theory (MP2) level where possible. Agreement between theory and the available experimental data is excellent. We present a revised heat of formation of the antimony atom in the gas phase. The calculated values will be of use in predicting the behavior of chemical vapor deposition systems.
The Journal of Physical Chemistry A 03/2012; 116(14):3717-27. · 2.95 Impact Factor
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ABSTRACT: Structures, vibrational frequencies, atomization energies at 0 K, and heats of formation at 0 and 298 K are predicted for the compounds As(2), AsH, AsH(2), AsH(3), AsF, AsF(2), and AsF(3) from frozen core coupled cluster theory calculations performed with large correlation consistent basis sets, up through augmented sextuple zeta quality. The coupled cluster calculations involved up through quadruple excitations. For As(2) and the hydrides, it was also possible to examine the impact of full configuration interaction on some of the properties. In addition, adjustments were incorporated to account for extrapolation to the frozen core complete basis set limit, core/valence correlation, scalar relativistic effects, the diagonal Born-Oppenheimer correction, and atomic spin orbit corrections. Based on our best theoretical D(0)(As(2)) and the experimental heat of formation of As(2), we propose a revised 0 K arsenic atomic heat of formation of 68.86 ± 0.8 kcal/mol. While generally good agreement was found between theory and experiment, the heat of formation of AsF(3) was an exception. Our best estimate is more than 7 kcal/mol more negative than the single available experimental value, which argues for a re-examination of that measurement.
The Journal of Physical Chemistry A 11/2011; 115(51):14667-76. · 2.95 Impact Factor
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ABSTRACT: Atomization energies at 0 K and heats of formation at 0 and 298 K are predicted for the neutral and ionic N(x)F(y) and O(x)F(y) systems using coupled cluster theory with single and double excitations and including a perturbative triples correction (CCSD(T)) method with correlation consistent basis sets extrapolated to the complete basis set (CBS) limit. To achieve near chemical accuracy (±1 kcal/mol), three corrections to the electronic energy were added to the frozen core CCSD(T)/CBS binding energies: corrections for core-valence, scalar relativistic, and first order atomic spin-orbit effects. Vibrational zero point energies were computed at the CCSD(T) level of theory where possible. The calculated heats of formation are in good agreement with the available experimental values, except for FOOF because of the neglect of higher order correlation corrections. The F(+) affinity in the N(x)F(y) series increases from N(2) to N(2)F(4) by 63 kcal/mol, while that in the O(2)F(y) series decreases by 18 kcal/mol from O(2) to O(2)F(2). Neither N(2) nor N(2)F(4) is predicted to bind F(-), and N(2)F(2) is a very weak Lewis acid with an F(-) affinity of about 10 kcal/mol for either the cis or trans isomer. The low F(-) affinities of the nitrogen fluorides explain why, in spite of the fact that many stable nitrogen fluoride cations are known, no nitrogen fluoride anions have been isolated so far. For example, the F(-) affinity of NF is predicted to be only 12.5 kcal/mol which explains the numerous experimental failures to prepare NF(2)(-) salts from the well-known strong acid HNF(2). The F(-) affinity of O(2) is predicted to have a small positive value and increases for O(2)F(2) by 23 kcal/mol, indicating that the O(2)F(3)(-) anion might be marginally stable at subambient temperatures. The calculated adiabatic ionization potentials and electron affinities are in good agreement with experiment considering that many of the experimental values are for vertical processes.
Inorganic Chemistry 01/2011; 50(5):1914-25. · 4.60 Impact Factor
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ABSTRACT: Aromatic and single-olefin six-membered BN heterocycles were synthesized, and the heats of hydrogenation were measured calorimetrically. A comparison of the hydrogenation enthalpies of these compounds revealed that 1,2-azaborines have a resonance stabilization energy of 16.6 ± 1.3 kcal/mol, in good agreement with calculated values.
Journal of the American Chemical Society 12/2010; 132(51):18048-50. · 9.91 Impact Factor
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ABSTRACT: N(2)F(+) salts are important precursors in the synthesis of N(5)(+) compounds, and better methods are reported for their larger scale production. A new, marginally stable N(2)F(+) salt, N(2)F(+)Sn(2)F(9)(-), was prepared and characterized. An ordered crystal structure was obtained for N(2)F(+)Sb(2)F(11)(-), resulting in the first observation of individual N[triple bond]N and N-F bond distances for N(2)F(+) in the solid phase. The observed N[triple bond]N and N-F bond distances of 1.089(9) and 1.257(8) A, respectively, are among the shortest experimentally observed N-N and N-F bonds. High-level electronic structure calculations at the CCSD(T) level with correlation-consistent basis sets extrapolated to the complete basis limit show that cis-N(2)F(2) is more stable than trans-N(2)F(2) by 1.4 kcal/mol at 298 K. The calculations also demonstrate that the lowest uncatalyzed pathway for the trans-cis isomerization of N(2)F(2) has a barrier of 60 kcal/mol and involves rotation about the N=N double bond. This barrier is substantially higher than the energy required for the dissociation of N(2)F(2) to N(2) and 2 F. Therefore, some of the N(2)F(2) dissociates before undergoing an uncatalyzed isomerization, with some of the dissociation products probably catalyzing the isomerization. Furthermore, it is shown that the trans-cis isomerization of N(2)F(2) is catalyzed by strong Lewis acids, involves a planar transition state of symmetry C(s), and yields a 9:1 equilibrium mixture of cis-N(2)F(2) and trans-N(2)F(2). Explanations are given for the increased reactivity of cis-N(2)F(2) with Lewis acids and the exclusive formation of cis-N(2)F(2) in the reaction of N(2)F(+) with F(-). The geometry and vibrational frequencies of the F(2)N=N isomer have also been calculated and imply strong contributions from ionic N(2)F(+) F(-) resonance structures, similar to those in F(3)NO and FNO.
Inorganic Chemistry 05/2010; 49(15):6823-33. · 4.60 Impact Factor
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ABSTRACT: We describe a new hydrogen storage platform based on well-defined BN heterocyle materials. Specifically, we demonstrate that regeneration of the spent fuel back to the charged fuel can be accomplished using molecular H(2) and H(-)/H(+) sources. Crystallographic characterization of intermediates along the regeneration pathway confirms our structural assignments and reveals unique bonding changes associated with increasing hydrogen content on boron and nitrogen. Synthetic access to the fully charged BN cyclohexane fuels will now enable investigations of these materials in hydrogen desorption studies.
Journal of the American Chemical Society 03/2010; 132(10):3289-91. · 9.91 Impact Factor
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ABSTRACT: High level ab initio electronic structure calculations at the coupled cluster level with a correction for triples extrapolated to the complete basis set limit have been made for the thermodynamics of the BrBrO2, IIO2, ClBrO2, ClIO2, and BrIO2 isomers, as well as various molecules involved in the bond dissociation processes. Of the BrBrO2 isomers, BrOOBr is predicted to be the most stable by 8.5 and 9.3 kcal/mol compared to BrBrO2 and BrOBrO at 298 K, respectively. The weakest bond in BrOOBr is the O−Br bond with a bond dissociation energy (BDE) of 15.9 kcal/mol, and in BrBrO2, it is the Br−Br bond of 19.1 kcal/mol. The smallest BDE in BrOBrO is for the central O−Br bond with a BDE of 12.6 kcal/mol. Of the IIO2 isomers, IIO2 is predicted to be the most stable by 3.3, 9.4, and 28.9 kcal/mol compared to IOIO, IOOI, and OIIO at 298 K, respectively. The weakest bond in IIO2 is the I−I bond with a BDE of 22.2 kcal/mol. The smallest BDEs in IOIO and IOOI are the terminal O−I bonds with values of 19.0 and 5.2 kcal/mol, respectively.
02/2010;
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ABSTRACT: High level ab initio electronic structure calculations at the coupled cluster level with a correction for triples extrapolated to the complete basis set limit have been made for the thermodynamics of the BrBrO(2), IIO(2), ClBrO(2), ClIO(2), and BrIO(2) isomers, as well as various molecules involved in the bond dissociation processes. Of the BrBrO(2) isomers, BrOOBr is predicted to be the most stable by 8.5 and 9.3 kcal/mol compared to BrBrO(2) and BrOBrO at 298 K, respectively. The weakest bond in BrOOBr is the O-Br bond with a bond dissociation energy (BDE) of 15.9 kcal/mol, and in BrBrO(2), it is the Br-Br bond of 19.1 kcal/mol. The smallest BDE in BrOBrO is for the central O-Br bond with a BDE of 12.6 kcal/mol. Of the IIO(2) isomers, IIO(2) is predicted to be the most stable by 3.3, 9.4, and 28.9 kcal/mol compared to IOIO, IOOI, and OIIO at 298 K, respectively. The weakest bond in IIO(2) is the I-I bond with a BDE of 22.2 kcal/mol. The smallest BDEs in IOIO and IOOI are the terminal O-I bonds with values of 19.0 and 5.2 kcal/mol, respectively.
The Journal of Physical Chemistry A 02/2010; 114(12):4254-65. · 2.95 Impact Factor
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ABSTRACT: Atomization energies at 0 K and heats of formation at 0 and 298 K are predicted for XeF3+, XeF3−, XeF5+, XeF7+, XeF7−, and XeF8 from coupled cluster theory (CCSD(T)) calculations with effective core potential correlation-consistent basis sets for Xe and including correlation of the nearest core electrons. Additional corrections are included to achieve near chemical accuracy of ±1 kcal/mol. Vibrational zero point energies were computed at the MP2 level of theory. Unlike the other neutral xenon fluorides, XeF8 is predicted to be thermodynamically unstable with respect to loss of F2 with the reaction calculated to be exothermic by 22.3 kcal/mol at 0 K. XeF7+ is also predicted to be thermodynamically unstable with respect to the loss of F2 by 24.1 kcal/mol at 0 K. For XeF3+, XeF5+, XeF3−, XeF5−, and XeF7−, the reactions for loss of F2 are endothermic by 14.8, 37.8, 38.2, 59.6, and 31.9 kcal/mol at 0 K, respectively. The F+ affinities of Xe, XeF2, XeF4, and XeF6 are predicted to be 165.1, 155.3, 172.7, and 132.5 kcal/mol, and the corresponding F− affinities are 6.3, 19.9, 59.1, and 75.0 kcal/mol at 0 K, respectively.
12/2009;
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ABSTRACT: Thermochemical parameters of a set of small-sized neutral (B(n)) and anionic (B(n)(-)) boron clusters, with n = 5-13, were determined using coupled-cluster theory CCSD(T) calculations with the aug-cc-pVnZ (n = D, T, and Q) basis sets extrapolated to the complete basis set limit (CBS) plus addition corrections and/or G3B3 calculations. Enthalpies of formation, adiabatic electron affinities (EA), vertical (VDE), and adiabatic (ADE) detachment energies were evaluated. Our calculated EAs are in good agreement with recent experiments (values in eV): B(5) (CBS, 2.29; G3B3, 2.48; exptl., 2.33 +/- 0.02), B(6) (CBS, 2.59; G3B3, 3.23; exptl., 3.01 +/- 0.04), B(7) (CBS, 2.62; G3B3, 2.67; exptl., 2.55 +/- 0.05), B(8) (CBS, 3.02; G3B3, 3.11; exptl., 3.02 +/- 0.02), B(9) (G3B3, 3.03; exptl., 3.39 +/- 0.06), B(10) (G3B3, 2.85; exptl., 2.88 +/- 0.09), B(11) (G3B4, 3.48;, exptl., 3.43 +/- 0.01), B(12) (G3B3, 2.33; exptl., 2.21 +/- 0.04), and B(13) (G3B3, 3.62; exptl., 3.78 +/- 0.02). The difference between the calculated adiabatic electron affinity and the adiabatic detachment energy for B(6) is due to the fact that the geometry of the anion is not that of the ground-state neutral. The calculated adiabatic detachment energies to the (3)A(u), C(2h) and (1)A(g), D(2h) excited states of B(6), which have geometries similar to the (1)A(g), D(2h) state of B(6)(-), are 2.93 and 3.06 eV, in excellent agreement with experiment. The VDEs were also well reproduced by the calculations. Partitioning of the electron localization functions into pi and sigma components allows probing of the partial and local delocalization in global nonaromatic systems. The larger clusters appear to exhibit multiple aromaticity. The binding energies per atom vary in a parallel manner for both neutral and anionic series and approach the experimental value for the heat of atomization of B. The resonance energies and the normalized resonance energies are convenient indices to quantify the stabilization of a cluster of elements.
The Journal of Physical Chemistry A 12/2009; 114(2):994-1007. · 2.95 Impact Factor
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ABSTRACT: Atomization energies at 0 K and heats of formation at 0 and 298 K are predicted for XeF(3)(+), XeF(3)(-), XeF(5)(+), XeF(7)(+), XeF(7)(-), and XeF(8) from coupled cluster theory (CCSD(T)) calculations with effective core potential correlation-consistent basis sets for Xe and including correlation of the nearest core electrons. Additional corrections are included to achieve near chemical accuracy of +/-1 kcal/mol. Vibrational zero point energies were computed at the MP2 level of theory. Unlike the other neutral xenon fluorides, XeF(8) is predicted to be thermodynamically unstable with respect to loss of F(2) with the reaction calculated to be exothermic by 22.3 kcal/mol at 0 K. XeF(7)(+) is also predicted to be thermodynamically unstable with respect to the loss of F(2) by 24.1 kcal/mol at 0 K. For XeF(3)(+), XeF(5)(+), XeF(3)(-), XeF(5)(-), and XeF(7)(-), the reactions for loss of F(2) are endothermic by 14.8, 37.8, 38.2, 59.6, and 31.9 kcal/mol at 0 K, respectively. The F(+) affinities of Xe, XeF(2), XeF(4), and XeF(6) are predicted to be 165.1, 155.3, 172.7, and 132.5 kcal/mol, and the corresponding F(-) affinities are 6.3, 19.9, 59.1, and 75.0 kcal/mol at 0 K, respectively.
Inorganic Chemistry 12/2009; 49(1):261-70. · 4.60 Impact Factor
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ABSTRACT: Atomization energies at 0 K and heats of formation at 0 and 298 K are predicted from high level ab initio electronic structure calculations using the coupled cluster CCSD(T) method with augmented correlation-consistent basis sets extrapolated to the complete basis set (CBS) limit for the H1,2OmSn (m, n = 0−3) compounds, as well as various radicals involved in different bond breaking processes. To achieve near chemical accuracy (±1.0 kcal/mol), additional corrections were added to the CBS binding energies based on the frozen core CCSD(T) energies including corrections for core−valence, scalar relativistic, and first-order atomic spin−orbit effects. Geometries were optimized up through the CCSD(T)/aV(T+d)Z level. Vibrational zero point energies were computed at the MP2/aV(T+d)Z level. The calculated heats of formation are in excellent agreement with the available experimental data and allow the prediction of adiabatic bond dissociation energies (BDEs) to within ±1.0 kcal/mol. The decomposition mechanisms were largely determined by a preference to maintain a strong S═O bond in the dissociated products as opposed to O═O and S═S bonds, exactly matching the ordering of the BDEs in the diatomics. For the H2X2 and H2X3 systems, as well as the HX3 radicals, the energetically favorable decomposition pathway leads to the formation of XH radicals and breaking the X—X bond as opposed to breaking the X—H bond. For the HX2 radicals, however, the more thermodynamically favorable pathway leads to a breaking of the H—X bond and forming X2 molecules.
10/2009;
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ABSTRACT: Atomization energies at 0 K and enthalpies of formation at 0 and 298 K are predicted for the BH4−nXn− and the BH3−nXnF− compounds for (X = F, Cl, Br, I, NH2, OH, and SH) from coupled cluster theory (CCSD(T)) calculations with correlation-consistent basis sets and with an effective core potential on I. To achieve near chemical accuracy (±1.0 kcal/mol), additional corrections were added to the complete basis set binding energies. The hydride, fluoride, and X− affinities of the BH3−nXn compounds were predicted. Although the hydride and fluoride affinities differ somewhat in their magnitudes, they show very similar trends and are both suitable for judging the Lewis acidities of compounds. The only significant differences in their acidity strength orders are found for the boranes substituted with the strongly electron withdrawing and back-donating fluorine and hydroxyl ligands. The highest H− and F− affinities are found for BI3 and the lowest ones for B(NH2)3. Within the boron trihalide series, the Lewis acidity increases monotonically with increasing atomic weight of the halogen, that is, BI3 is a considerably stronger Lewis acid than BF3. For the X− affinities in the BX3, HBX2, and H2BX series, the fluorides show the highest values, whereas the amino and mercapto compounds show the lowest ones. Hydride and fluoride affinities of the BH3−nXn compounds exhibit linear correlations with the proton affinity of X− for most X ligands. Reasons for the correlation are discussed. A detailed analysis of the individual contributions to the Lewis acidities of these substituted boranes shows that the dominant effect in the magnitude of the acidity is the strength of the BX3−−F bond. The main contributor to the relative differences in the Lewis acidities of BX3 for X, a halogen, is the electron affinity of BX3 with a secondary contribution from the distortion energy from planar to pyramidal BX3. The B−F bond dissociation energy of X3B−F− and the distortion energy from pyramidal to tetrahedral BX3− are of less importance in determining the relative acidities. Because the electron affinity of BX3 is strongly influenced by the charge density in the empty pz lowest unoccupied molecular orbital of boron, the amount of π-back-donation from the halogen to boron is crucial and explains why the Lewis acidity of BF3 is significantly lower than those of BX3 with X = Cl, Br, and I.
09/2009;
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ABSTRACT: The heats of formation for BN, HBNH, H2BNH2, and H3BNH3 and their radicals, anions, cations, and protonated species were predicted using the CCSD(T) method with correlation consistent basis sets extrapolated to the complete basis set and core−valence, scalar relativistic, spin−orbit, and zero-point energy corrections. Chemical accuracy (±1 kcal/mol) is obtained for these heats of formation, allowing reliable results for electron and hydride affinities, ionization energies, basicities (neutral proton affinities), acidities (anion proton affinities), and bond dissociation energies of the BNHn (n = 0−6) molecules. The closed shell BNHn molecules, except for H3BNH3 (double diffuse functions on B and N required), do not bind an electron and behave as nitrogen acids and bases. Protonation of H3BNH3 leads to nearly spontaneous H2 release. The closed shell BNHn species exhibit ionization energies similar or smaller than those of hydrocarbons, and for the H2BNH radical, its ionization energy (5.9 eV) is close to that of the alkali elements. Except for H3BNH3, the NH bonds are systematically stronger than the BH bonds, and the BDE(NH) values tend to increase with increasing multiple character of the BN bonds. Trends and comparisons between properties obtained through reactions on the N and B atoms are given. Comparisons with properties of hydrocarbons show similarities and differences between the two isoelectronic series.
09/2009;
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ABSTRACT: Atomization energies at 0 K and enthalpies of formation at 0 and 298 K are predicted for the BH(4-n)X(n)(-) and the BH(3-n)X(n)F(-) compounds for (X = F, Cl, Br, I, NH(2), OH, and SH) from coupled cluster theory (CCSD(T)) calculations with correlation-consistent basis sets and with an effective core potential on I. To achieve near chemical accuracy (+/-1.0 kcal/mol), additional corrections were added to the complete basis set binding energies. The hydride, fluoride, and X(-) affinities of the BH(3-n)X(n) compounds were predicted. Although the hydride and fluoride affinities differ somewhat in their magnitudes, they show very similar trends and are both suitable for judging the Lewis acidities of compounds. The only significant differences in their acidity strength orders are found for the boranes substituted with the strongly electron withdrawing and back-donating fluorine and hydroxyl ligands. The highest H(-) and F(-) affinities are found for BI(3) and the lowest ones for B(NH(2))(3). Within the boron trihalide series, the Lewis acidity increases monotonically with increasing atomic weight of the halogen, that is, BI(3) is a considerably stronger Lewis acid than BF(3). For the X(-) affinities in the BX(3), HBX(2), and H(2)BX series, the fluorides show the highest values, whereas the amino and mercapto compounds show the lowest ones. Hydride and fluoride affinities of the BH(3-n)X(n) compounds exhibit linear correlations with the proton affinity of X(-) for most X ligands. Reasons for the correlation are discussed. A detailed analysis of the individual contributions to the Lewis acidities of these substituted boranes shows that the dominant effect in the magnitude of the acidity is the strength of the BX(3)(-)-F bond. The main contributor to the relative differences in the Lewis acidities of BX(3) for X, a halogen, is the electron affinity of BX(3) with a secondary contribution from the distortion energy from planar to pyramidal BX(3). The B-F bond dissociation energy of X(3)B-F(-) and the distortion energy from pyramidal to tetrahedral BX(3)(-) are of less importance in determining the relative acidities. Because the electron affinity of BX(3) is strongly influenced by the charge density in the empty p(z) lowest unoccupied molecular orbital of boron, the amount of pi-back-donation from the halogen to boron is crucial and explains why the Lewis acidity of BF(3) is significantly lower than those of BX(3) with X = Cl, Br, and I.
Inorganic Chemistry 09/2009; 48(18):8811-21. · 4.60 Impact Factor
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ABSTRACT: Atomization energies at 0 K and heats of formation at 0 and 298 K are predicted from high level ab initio electronic structure calculations using the coupled cluster CCSD(T) method with augmented correlation-consistent basis sets extrapolated to the complete basis set (CBS) limit for the H(1,2)O(m)S(n) (m, n = 0-3) compounds, as well as various radicals involved in different bond breaking processes. To achieve near chemical accuracy (+/-1.0 kcal/mol), additional corrections were added to the CBS binding energies based on the frozen core CCSD(T) energies including corrections for core-valence, scalar relativistic, and first-order atomic spin-orbit effects. Geometries were optimized up through the CCSD(T)/aV(T+d)Z level. Vibrational zero point energies were computed at the MP2/aV(T+d)Z level. The calculated heats of formation are in excellent agreement with the available experimental data and allow the prediction of adiabatic bond dissociation energies (BDEs) to within +/-1.0 kcal/mol. The decomposition mechanisms were largely determined by a preference to maintain a strong S=O bond in the dissociated products as opposed to O=O and S=S bonds, exactly matching the ordering of the BDEs in the diatomics. For the H(2)X(2) and H(2)X(3) systems, as well as the HX(3) radicals, the energetically favorable decomposition pathway leads to the formation of XH radicals and breaking the X-X bond as opposed to breaking the X-H bond. For the HX(2) radicals, however, the more thermodynamically favorable pathway leads to a breaking of the H-X bond and forming X(2) molecules.
The Journal of Physical Chemistry A 09/2009; 113(42):11343-53. · 2.95 Impact Factor
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ABSTRACT: Atomization energies at 0 K and heats of formation at 0 and 298 K are predicted for (CH3)H2N-BH3, (CH3)HN=BH2, (BH3)HN=CH2, (CH3)H2B-NH3, (CH3)HB=NH2, and (NH3)HB=CH2, as well as various molecules involved in the different bond-breaking processes, from coupled cluster theory (CCSD(T)) calculations. In order to achieve near-chemical accuracy (+/-1 kcal/mol), three corrections were added to the complete basis set binding energies based on frozen core CCSD(T) energies, corrections for core-valence, scalar relativistic, and first-order atomic spin-orbit effects. Scaled vibrational zero-point energies were computed with the MP2 method. The heats of formation were predicted for the respective dimethyl- and trimethyl-substituted ammonia boranes, their dehydrogenated derivatives, and the various molecules involved in the different bond breaking processes, based on isodesmic reaction schemes calculated at the G3(MP2) level. Thermodynamics for dehydrogenation pathways in the monomethyl-substituted molecules were predicted. Dehydrogenation across the B-N bond is more favorable as opposed to dehydrogenation across the B-C and N-C bonds. Methylation at N reduces the exothermocity of the dehydrogenation reaction and makes the reaction more thermoneutral, while methylation at B moves it away from thermoneutral. Various mixtures of CH3NH2BH3 and NH3BH3 were made, and their melting points were measured. The lowest melting mixture contained approximately 35% NH3BH3 by weight and melted at 35-37 degrees C.
The Journal of Physical Chemistry A 06/2009; 113(21):6121-32. · 2.95 Impact Factor
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ABSTRACT: Atomization energies at 0 K and heats of formation at 0 and 298 K are predicted for SiH(3)X, SiH(2)XCH(3), and SiH(3)CH(2)X with X = F, Cl, Br, and I from coupled cluster theory (CCSD(T)) calculations with effective core potential correlation-consistent basis sets for Br and I. To achieve near chemical accuracy (+/-1 kcal/mol), three corrections were added to the complete basis set binding energies based on frozen core coupled cluster theory energies: a correction for core-valence effects, a correction for scalar relativistic effects, and a correction for first order atomic spin-orbit effects. Vibrational zero point energies were computed at the CCSD(T) level of theory and the C-H and Si-H stretches scaled to experiment. The C-H, Si-H, Si-C, C-X, and Si-X (X = F, Cl, Br, and I) bond dissociation energies (BDEs) in the halosilanes, halomethysilanes, and methylhalosilanes were predicted. Except for methyliodosilane, methyl substitution leads to an increase in Si-X BDE when compared to the Si-X BDE in the halosilanes. Except for methyliodosilane, halide substitution leads to an increase in the Si-C BDE in comparison to the Si-C BDE in methylsilane of 86.9 kcal/mol at 0 K. Unlike the methylhalosilanes, the halomethylsilanes all show a decrease in the Si-C BDE when compared to the Si-C BDE in methylsilane. The trends correlate with the electronegativity of the substituent.
The Journal of Physical Chemistry A 05/2009; 113(15):3656-61. · 2.95 Impact Factor
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ABSTRACT: Thermochemical properties of a set of small boron (B(n)) and boron oxide (B(n)O(m)) clusters, with n = 1-4 and m = 0-3, their anions, and the B(4)(2-) dianion, were calculated by using coupled-cluster theory CCSD(T) calculations with the aug-cc-pVnZ (n = D, T, Q, 5) basis sets extrapolated to the complete basis set limit with additional corrections. Enthalpies of formation, bond dissociation energies, singlet-triplet or doublet-quartet separation gaps, adiabatic electron affinities (EA), and both vertical electron attachment and detachment energies were evaluated. The predicted heats of formation show agreement close to the error bars of the literature results for boron oxides with the largest error for OBO. Our calculated adiabatic EAs are in good agreement with recent experiments: B (calc, 0.26 eV; exptl, 0.28 eV), B(2) (1.95, 1.80), B(3) (2.88, 2.820 +/- 0.020), B(4) (1.68, 1.60 +/- 0.10), BO (2.50, 2.51), BO(2) (4.48, 4.51), BOB (0.07), B(2)O(2) (0.37), B(3)O (2.05), B(3)O(2) (2.94, 2.94), B(4)O (2.58), and B(4)O(2) (3.14, 3.160 +/- 0.015). The BO bond is strong, so this moiety is maintained in most of the clusters. Thermochemical parameters of clusters are not linearly additive with respect to the number of B atoms. The EA tends to be larger in the dioxides. The growth mechanism of small boron oxides should be determined by a number of factors: (i) formation of BO bonds, (ii) when possible, formation of a cyclic B(3) or B(4), and (iii) combination of a boron cycle and a BO bond. When these factors compete, the strength of the BO bonds tends to compensate the destabilization arising from a loss of binding in the cyclic boron clusters, in such a way that a linear boron oxide prevails. When the B(2) moiety is present in these linear clusters, the oxide derivatives prefer a high spin state.
The Journal of Physical Chemistry A 04/2009; 113(17):4895-909. · 2.95 Impact Factor
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ABSTRACT: Atomization energies at 0 K and heats of formation at 0 and 298 K are predicted for diammoniosilane, H(4)Si(NH(3))(2), and its dehydrogenated derivates at the CCSD(T) and G3(MP2) levels. To achieve near chemical accuracy (+/-1 kcal/mol), three corrections were added to the complete basis set binding energies based on frozen core coupled cluster theory energies: a correction for core-valence effects, a correction for scalar relativistic effects, and a correction for first-order atomic spin-orbit effects. Vibrational zero-point energies were computed at the CCSD(T) or MP2 levels. The edge inversion barrier of silane is predicted to be 88.9 kcal/mol at 298 K at the CCSD(T) level and a substantial amount, -63.6 kcal/mol, is recovered upon complexation with 2 NH(3) molecules, so that the diammoniosilane complex is only 25.6 kcal/mol at 298 K above the separated reactants SiH(4) + 2NH(3). The complex is a metastable species characterized by all real frequencies at the MP2/aV(T+d)Z level. We predict the heat of reaction for the sequential dehydrogenation of diammoniosilane to yield H(3)Si(NH(2))(NH(3)) and H(2)Si(NH(2))(2) to be exothermic by 33.6 and 12.2 kcal/mol at 298 K at the CCSD(T) level, respectively. The cumulative dehydrogenation reaction yielding two molecules of hydrogen at 298 K is -45.8 kcal/mol at the CCSD(T) level. The sequential release of H(2) from H(2)Si(NH(2))(2) consequently yielding HN=SiH(NH(2)) and HN=Si=NH are predicted to be largely endothermic reactions at 45.3 and 55.7 kcal/mol at the CCSD(T) level at 298 K. If the endothermic reaction for the third step and the exothermic reactions for the release of the first two H(2) were coupled effectively, loss of three H(2) molecules from H(4)Si(NH(3))(2) would be almost thermoneutral at 0 K.
The Journal of Physical Chemistry A 02/2009; 113(4):750-5. · 2.95 Impact Factor
