Heats of formation of diphosphene, phosphinophosphinidene, diphosphine, and their methyl derivatives, and mechanism of the borane-assisted hydrogen release

The heats of formation of diphosphene (cis- and trans-P2H2), phopshinophosphinidene (singlet and triplet H2PP) and diphosphine (P2H4), as well as those of the P2H and P2H3 radicals resulting from PH bond cleavages, have been calculated by using high-level ab initio electronic structure theory. Energies were calculated using coupled-cluster theory with a perturbative treatment for triple excitations (CCSD(T)) and employing augmented correlation consistent basis sets with additional tight d-functions on P (aug-cc-pV(n+d)Z) up to quadruple- or quintuple-zeta, to perform a complete basis set extrapolation for the energy. Geometries and vibrational frequencies were determined with the CCSD(T) method. Core-valence and scalar relativistic corrections were included, as well as scaled zero-point energies. We find the following heats of formation (kcal/mol) at 298 [0] K: DeltaH(degree)(f)(P2H) = 53.4 [54.4]; DeltaH(degree)(f)(cis-P2H2) = 32.0 [33.9]; DeltaH(degree)(f)(trans-P2H2) = 28.7 [30.6]; DeltaH(degree)(f)(H2PP) = 53.7 [55.6]; DeltaH(degree)(f)(3H2PP) = 56.5 [58.3]; DeltaH(degree)(f)(P2H3) = 32.3 [34.8]; DeltaH(degree)(f)(P2H4) = 5.7 [9.1] (expt, 5.0 +/- 1.0 at 298 K); and DeltaH(degree)(f)(CH3PH2) = -5.0 [-1.4]. We estimate these values to have an accuracy of +/-1.0 kcal/mol. In contrast to earlier results, we found a singlet ground state for phosphinophosphinidene (H2PP) with a singlet-triplet energy gap of 2.8 kcal/mol. We calculated the heats of formation of the methylated derivatives CH3PPH, CH3HPPH2, CH3PPCH3, CH3HPP, (CH3)2PP, (CH3)2PPH2, and CH3HPPHCH3 by using isodesmic reactions at the MP2/CBS level. The calculated results for the hydrogenation reactions RPPR + H2 --> RHPPHR and R2PP + H2 --> R2PPH2 show that substitution of an organic substituent for H improves the energetics, suggesting that secondary diphosphines and diphosphenes are potential candidates for use in a chemical hydrogen storage system. A comparison with the nitrogen analogues is given. The mechanism for H2-generation from diphosphine without and with BH3 as a catalyst was examined. Including tunneling corrections, the rate constant for the catalyzed reaction is 4.5 x 1015 times faster than the uncatalyzed result starting from separated catalyst and PH2PH2.     

Experimental and quantum-chemical studies on photoionization and dissociative photoionization of CH2Br2

The dissociative photoionization of CH2Br2 in a region approximately 10-24 eV was investigated with photoionization mass spectroscopy using a synchrotron radiation source. An adiabatic ionization energy of 10.25 eV determined for CH2Br2 agrees satisfactorily with predictions of 10.26 and 10.25 eV with G2 and G3 methods, respectively. Observed major fragment ions CH2Br+, CHBr+, and CBr+ show appearance energies at 11.22, 12.59, and 15.42 eV, respectively; minor fragment ions CHBr2+, Br+, and CH2+ appear at 12.64, 15.31, and 16.80 eV, respectively. Energies for formation of observed fragment ions and their neutral counterparts upon ionization of CH2Br2 are computed with G2 and G3 methods. Dissociative photoionization channels associated with six observed fragment ions are proposed based on comparison of determined appearance energies and predicted energies. An upper limit of DeltaH0f,298(CHBr+) < or = 300.7 +/- 1.5 kcal mol(-1) is derived experimentally; the adiabatic ionization energy of CHBr is thus derived to be < or = 9.17 +/- 0.23 eV. Literature values for DeltaH0f,298(CBr+) = 362.5 kcal mol(-1) and ionization energy of 10.43 eV for CBr are revised to be less than 332 kcal mol(-1) and 9.11 eV, respectively. Also based on a new experimental ionization energy, DeltaH0f,298(CH2Br2+) is revised to be 236.4 +/- 1.5 kcal mol(-1).     

Accurate heats of formation of the "Arduengo-type" carbene and various adducts including H2 from ab initio molecular orbital theory

The heats of formation of saturated and unsaturated diaminocarbenes (imadazol(in)-2-ylidenes) have been calculated by using high levels of ab initio electronic structure theory. The calculations were done at the coupled cluster level through noniterative triple excitations with augmented correlation consistent basis sets up through quadruple. In addition, four other corrections were applied to the frozen core atomization energies: (1) a zero point vibrational correction; (2) a core/valence correlation correction; (3) a scalar relativistic correction; (4) a first-order atomic spin-orbit correction. The value of DeltaHf( 298) for the unsaturated carbene 1 is calculated to be 56.4 kcal/mol. The value of DeltaHf( 298) for the unsaturated triplet carbene (3)1 is calculated to be 142.8 kcal/mol, giving a singlet-triplet splitting of 86.4 kcal/mol. Addition of a proton to 1 forms 3 with DeltaHf( 298)(3) = 171.6 kcal/mol with a proton affinity for 1 of 250.5 kcal/mol at 298 K. Addition of a hydrogen atom to 1 forms 4 with DeltaHf( 298)(4) = 72.7 kcal/mol and a C-H bond energy of 35.8 kcal/mol at 298 K. Addition of H- to 1 gives 5 with DeltaHf( 298)(5) = 81.2 kcal/mol and 5 is not stable with respect to loss of an electron to form 4. Addition of H2 to the carbene center forms 6 with DeltaHf( 298)(6) = 41.5 kcal/mol and a heat of hydrogenation at 298 K of -14.9 kcal/mol. The value of DeltaHf( 298) for the saturated carbene 7 (obtained by adding H2 to the C=C bond of 1) is 47.4 kcal/mol. Hydrogenation of 7 to form the fully saturated imidazolidine, 8, gives DeltaHf( 298)(8) = 14.8 kcal/mol and a heat of hydrogenation at 298 K of -32.6 kcal/mol. The estimated error bars for the calculated heats of formation are +/-1.0 kcal/mol.     

Active Thermochemical Tables: accurate enthalpy of formation of hydroperoxyl radical, HO2

Through the use of the Active Thermochemical Tables approach, the best currently available enthalpy of formation of HO2 has been obtained as delta(f)H(o)298 (HO2) = 2.94 +/- 0.06 kcal mol(-1) (3.64 +/- 0.06 kcal mol(-1) at 0 K). The related enthalpy of formation of the positive ion, HO2+, within the stationary electron convention is delta(f)H(o)298 (HO2+) = 264.71 +/- 0.14 kcal mol(-1) (265.41 +/- 0.14 kcal mol(-1) at 0 K), while that for the negative ion, HO2- (within the same convention), is delta(f)H(o)298 (HO2-) = -21.86 +/- 0.11 kcal mol(-1) (-21.22 +/- 0.11 kcal mol(-1) at 0 K). The related proton affinity of molecular oxygen is PA298(O2) = 100.98 +/- 0.14 kcal mol(-1) (99.81 +/- 0.14 kcal mol(-1) at 0 K), while the gas-phase acidity of H2O2 is delta(acid)G(o)298 (H2O2) = 369.08 +/- 0.11 kcal mol(-1), with the corresponding enthalpy of deprotonation of H2O2 of delta(acid)H(o)298 (H2O2) = 376.27 +/- 0.11 kcal mol(-1) (375.02 +/- 0.11 kcal mol(-1) at 0 K). In addition, a further improved enthalpy of formation of OH is briefly outlined, delta(f)H(o)298 (OH) = 8.93 +/- 0.03 kcal mol(-1) (8.87 +/- 0.03 kcal mol(-1) at 0 K), together with new and more accurate enthalpies of formation of NO, delta(f)H(o)298 (NO) = 21.76 +/- 0.02 kcal mol(-1) (21.64 +/- 0.02 kcal mol(-1) at 0 K) and NO2, delta(f)H(o)298 (NO2) = 8.12 +/- 0.02 kcal mol(-1) (8.79 +/- 0.02 kcal mol(-1) at 0 K), as well as H(2)O(2) in the gas phase, delta(f)H(o)298 (H2O2) = -32.45 +/- 0.04 kcal mol(-1) (-31.01 +/- 0.04 kcal mol(-1) at 0 K). The new thermochemistry of HO2, together with other arguments given in the present work, suggests that the previous equilibrium constant for NO + HO2 --> OH + NO2 was underestimated by a factor of approximately 2, implicating that the OH + NO2 rate was overestimated by the same factor. This point is experimentally explored in the companion paper of Srinivasan et al. (next paper in this issue).     

Theoretical prediction of the heats of formation of C2H5O* radicals derived from ethanol and of the kinetics of beta-C-C scission in the ethoxy radical

Thermochemical parameters of three C(2)H(5)O* radicals derived from ethanol were reevaluated using coupled-cluster theory CCSD(T) calculations, with the aug-cc-pVnZ (n = D, T, Q) basis sets, that allow the CC energies to be extrapolated at the CBS limit. Theoretical results obtained for methanol and two CH(3)O* radicals were found to agree within +/-0.5 kcal/mol with the experiment values. A set of consistent values was determined for ethanol and its radicals: (a) heats of formation (298 K) DeltaHf(C(2)H(5)OH) = -56.4 +/- 0.8 kcal/mol (exptl: -56.21 +/- 0.12 kcal/mol), DeltaHf(CH(3)C*HOH) = -13.1 +/- 0.8 kcal/mol, DeltaHf(C*H(2)CH(2)OH) = -6.2 +/- 0.8 kcal/mol, and DeltaHf(CH(3)CH(2)O*) = -2.7 +/- 0.8 kcal/mol; (b) bond dissociation energies (BDEs) of ethanol (0 K) BDE(CH(3)CHOH-H) = 93.9 +/- 0.8 kcal/mol, BDE(CH(2)CH(2)OH-H) = 100.6 +/- 0.8 kcal/mol, and BDE(CH(3)CH(2)O-H) = 104.5 +/- 0.8 kcal/mol. The present results support the experimental ionization energies and electron affinities of the radicals, and appearance energy of (CH(3)CHOH+) cation. Beta-C-C bond scission in the ethoxy radical, CH(3)CH2O*, leading to the formation of C*H3 and CH(2)=O, is characterized by a C-C bond energy of 9.6 kcal/mol at 0 K, a zero-point-corrected energy barrier of E0++ = 17.2 kcal/mol, an activation energy of Ea = 18.0 kcal/mol and a high-pressure thermal rate coefficient of k(infinity)(298 K) = 3.9 s(-1), including a tunneling correction. The latter value is in excellent agreement with the value of 5.2 s(-1) from the most recent experimental kinetic data. Using RRKM theory, we obtain a general rate expression of k(T,p) = 1.26 x 10(9)p(0.793) exp(-15.5/RT) s(-1) in the temperature range (T) from 198 to 1998 K and pressure range (p) from 0.1 to 8360.1 Torr with N2 as the collision partners, where k(298 K, 760 Torr) = 2.7 s(-1), without tunneling and k = 3.2 s(-1) with the tunneling correction. Evidence is provided that heavy atom tunneling can play a role in the rate constant for beta-C-C bond scission in alkoxy radicals.     

Methyl cation affinities of rare gases and nitrogen and the heat of formation of diazomethane

The methyl cation affinities of the rare gases have been calculated at 0 and 298 K by using coupled cluster theory including noniterative, quasiperturbative triple excitations with the new correlation-consistent basis sets for Xe up through aug-cc-pV5Z in some cases. To achieve near chemical accuracy (+/-1 kcal/mol) in the thermodynamic properties, we add to the estimated complete basis set valence binding energies, based on frozen core coupled cluster theory energies, two corrections: (1) a core/valence correction and (2) a scalar relativistic correction. Vibrational zero-point energies were computed at the coupled cluster level of theory at the CCSD(T)/aug-cc-pVDZ level. The calculated rare gas methyl cation affinities (MCA in kcal/mol) at 298 K are the following: MCA(He) = 1.7, MCA(Ne) = 2.5, MCA(Ar) = 16.9, MCA(Kr) = 25.5, and MCA(Xe) = 36.6. Because of the importance of the MCA(N(2)) in the experimental measurements of the MCA scale, we calculated a number of quantities associated with CH(3)N(2)(+) and CH(2)N(2). The calculated values for diazomethane at 298 K are: DeltaH(f)(CH(2)N(2)) = 65.3 kcal/mol, PA(CH(2)N(2)) = 211.9 kcal/mol, and MCA(N(2)) = 43.2 kcal/mol.     

Accurate computational determination of the binding energy of the SO3 x H2O complex

Reliable thermochemical data for the reaction SO3 + H2O<-->SO3 x H2O (1a) are of crucial importance for an adequate modeling of the homogeneous H2SO4 formation in the atmosphere. We report on high-level quantum chemical calculations to predict the binding energy of the SO3 x H2O complex. The electronic binding energy is accurately computed to De = 40.9+/-1.0 kJ/mol = 9.8+/-0.2 kcal/mol. By using harmonic frequencies from density functional theory calculations (B3LYP/cc-pVTZ and TPSS/def2-TZVP), zero-point and thermal energies were calculated. From these data, we estimate D0 = -Delta H(1a)0(0 K) = 7.7+/-0.5 kcal/mol and Delta H(1a)0(298 K) = -8.3+/-1.0 kcal/mol.     

The heats of formation of diazene, hydrazine, N2H3+, N2H5+, N2H, and N2H3 and the Methyl Derivatives CH3NNH, CH3NNCH3, and CH3HNNHCH3

The heats of formation of N(2)H, diazene (cis- and trans-N(2)H(2)), N(2)H(3), and hydrazine (N(2)H(4)), as well as their protonated species (diazenium, N(2)H(3)(+), and hydrazinium, N(2)H(5)(+)), have been calculated by using high level electronic structure theory. Energies were calculated by using coupled cluster theory with a perturbative treatment of the triple excitations (CCSD(T)) and employing augmented correlation consistent basis sets (aug-cc-pVnZ) up to quintuple-zeta, to perform a complete basis set extrapolation for the energy. Geometries were optimized at the CCSD(T) level with the aug-cc-pVDZ and aug-cc-pVTZ basis sets. Core-valence and scalar relativistic corrections were included, as well as scaled zero point energies. We find the following heats of formation (kcal/mol) at 0 (298) K: DeltaH(f)(N(2)H) = 60.8 (60.1); DeltaH(f)(cis-N(2)H(2)) = 54.9 (53.2); DeltaH(f)(trans-N(2)H(2)) = 49.9 (48.1) versus >/=48.8 +/- 0.5 (exptl, 0 K); DeltaH(f)(N(2)H(4)) = 26.6 (23.1) versus 22.8 +/- 0.2 (exptl, 298 K); DeltaH(f)(N(2)H(3)) = 56.2 (53.6); DeltaH(f)(N(2)H(3)(+)) = 231.6 (228.9); and DeltaH(f)(N(2)H(5)(+)) = 187.1 (182.7). In addition, we calculated the heats of formation of CH(3)NH(2), CH(3)NNH, and CH(3)HNNHCH(3) by using isodesmic reactions and at the G3(MP2) level. The calculated results for the hydrogenation reaction RNNR + H(2) --> RHNNHR show that substitution of an organic substituent for H improved the energetics, suggesting that these types of compounds may be possible to use in a chemical hydrogen storage system.     

Thermochemistry of the hypobromous and hypochlorous acids, HOBr and HOCl

The enthalpies of formation of HOBr and HOCl have been estimated by employing coupled cluster theory in conjunction with the correlation consistent basis sets and corrections for core-valence, relativistic, and anharmonic effects. We have employed three different reactions to estimate the DeltaH(o)(f,298)(HOBr), namely, the atomization reaction and two homodesmic reactions. Our best estimation is DeltaH(o)(f,298) (HOBr) = -15.3 +/- 0.6 kcal/mol and is very likely to lie toward the more negative values. The present value is 1.4 kcal/mol lower than the widely used experimental determination of Ruscic and Berkowitz (J. Chem. Phys. 1994, 101, 7795), DeltaH(o)(f,298)(HOBr) > -13.93 +/- 0.42 kcal/mol. However, it is closer to the more recent measurement of Lock et al. (J. Phys. Chem. 1996, 100, 7972), DeltaH(o)(f,298)(HOBr) = -14.8 +/- 1 kcal/mol. In the case of HOCl we have determined DeltaH(o)(f,298)(HOCl) = -18.1 +/- 0.3 kcal/mol, just in the middle of the two experimental values proposed, -17.8 +/- 0.5 kcal/mol (JANAF), obtained from equilibrium constant measurements, and -18.36 +/- 0.03 kcal/mol (Joens, J. A. J. Phys. Chem. A 2001, 105, 11041), determined from the measurements of the Cl-OH bond energy. If our conclusions are correct, several enthalpies of formation that have been determined by experimental chemists, Orlando and Burholder (J. Phys. Chem. 1995, 99, 1143), and theoretical chemists, Lee (J. Phys. Chem. 1995, 99, 15074), need to be revised, since a larger value was used for DeltaH(o)(f,298)(HOBr). Employing the results obtained by Orlando and Burkholder for Br(2)O we propose DeltaH(o)(f,298)(Br(2)O) = 24.9 +/- 0.6 kcal/mol, and employing Lee's enthalpies of reaction we propose the following DeltaH(o)(f,298): for BrBrO, HBrO, ClOBr, ClBrO, BrClO, BrCN, BrNC, BrNO, BrON, FOBr, and FBrO, 39.5 +/- 1, 41.0 +/- 1, 22.7 +/- 1.5, 34.2 +/- 1.5, 40.9 +/- 1.5, 43.7 +/- 1.5, 80.1 +/- 1.5, 22.3 +/- 1, 46.2 +/- 1, 17.3 +/- 1.5, and 6.3 +/- 1.5 kcal/mol, respectively. We expect that this work will stimulate new experimental measurements of the thermodynamic properties of HOBr and HOCl.     

Negative-ion photoelectron spectroscopy, gas-phase acidity, and thermochemistry of the peroxyl radicals CH(3)OO and CH(3)CH(2)OO

Methyl, methyl-d(3), and ethyl hydroperoxide anions (CH(3)OO(-), CD(3)OO(-), and CH(3)CH(2)OO(-)) have been prepared by deprotonation of their respective hydroperoxides in a stream of helium buffer gas. Photodetachment with 364 nm (3.408 eV) radiation was used to measure the adiabatic electron affinities: EA[CH(3)OO, X(2)A' '] = 1.161 +/- 0.005 eV, EA[CD(3)OO, X(2)A' '] = 1.154 +/- 0.004 eV, and EA[CH(3)CH(2)OO, X(2)A' '] = 1.186 +/- 0.004 eV. The photoelectron spectra yield values for the term energies: Delta E(X(2)A' '-A (2)A')[CH(3)OO] = 0.914 +/- 0.005 eV, Delta E(X(2)A' '-A (2)A')[CD(3)OO] = 0.913 +/- 0.004 eV, and Delta E(X(2)A' '-A (2)A')[CH(3)CH(2)OO] = 0.938 +/- 0.004 eV. A localized RO-O stretching mode was observed near 1100 cm(-1) for the ground state of all three radicals, and low-frequency R-O-O bending modes are also reported. Proton-transfer kinetics of the hydroperoxides have been measured in a tandem flowing afterglow-selected ion flow tube (FA-SIFT) to determine the gas-phase acidity of the parent hydroperoxides: Delta(acid)G(298)(CH(3)OOH) = 367.6 +/- 0.7 kcal mol(-1), Delta(acid)G(298)(CD(3)OOH) = 367.9 +/- 0.9 kcal mol(-1), and Delta(acid)G(298)(CH(3)CH(2)OOH) = 363.9 +/- 2.0 kcal mol(-1). From these acidities we have derived the enthalpies of deprotonation: Delta(acid)H(298)(CH(3)OOH) = 374.6 +/- 1.0 kcal mol(-1), Delta(acid)H(298)(CD(3)OOH) = 374.9 +/- 1.1 kcal mol(-1), and Delta(acid)H(298)(CH(3)CH(2)OOH) = 371.0 +/- 2.2 kcal mol(-1). Use of the negative-ion acidity/EA cycle provides the ROO-H bond enthalpies: DH(298)(CH(3)OO-H) = 87.8 +/- 1.0 kcal mol(-1), DH(298)(CD(3)OO-H) = 87.9 +/- 1.1 kcal mol(-1), and DH(298)(CH(3)CH(2)OO-H) = 84.8 +/- 2.2 kcal mol(-1). We review the thermochemistry of the peroxyl radicals, CH(3)OO and CH(3)CH(2)OO. Using experimental bond enthalpies, DH(298)(ROO-H), and CBS/APNO ab initio electronic structure calculations for the energies of the corresponding hydroperoxides, we derive the heats of formation of the peroxyl radicals. The "electron affinity/acidity/CBS" cycle yields Delta(f)H(298)[CH(3)OO] = 4.8 +/- 1.2 kcal mol(-1) and Delta(f)H(298)[CH(3)CH(2)OO] = -6.8 +/- 2.3 kcal mol(-1).     

Heats of formation and singlet-triplet separations of hydroxymethylene and 1-hydroxyethylidene

Thermochemical parameters of hydroxymethylene (HC:OH) and 1-hydroxyethylidene (CH3C:OH) were evaluated by using coupled-cluster, CCSD(T), theory, in conjunction with the augmented correlation consistent, aug-cc-pVnZ, basis sets, with n = D, T, Q, and 5, extrapolated to the complete basis set limit. The predicted value at 298 K for Delta Hf(CH2O) is -26.0 +/- 1 kcal/mol, as compared to an experimental value of -25.98 +/- 0.01 kcal/mol, and for Delta Hf(CH:OH) it is 26.1 +/- 1 kcal/mol. The hydroxymethylene-formaldehyde energy gap is 52.1 +/- 0.5 kcal/mol, the singlet-triplet separation of hydroxymethylene is Delta E(ST)(HC:OH) = 25.3 +/- 0.5 kcal/mol, the proton affinity is PA(HC:OH) = 222.5 +/- 0.5 kcal/mol, and the ionization energy is IEa(HC:OH) = 8.91 +/- 0.04 eV. The predicted value at 298 K for Delta Hf(CH3CHO) is -39.1 +/- 1 kcal/mol as compared to an experimental value of -40.80 +/- 0.35 kcal/mol, and for Delta Hf(CH3C:OH) it is 11.2 +/- 1 kcal/mol. The hydroxyethylidene-acetaldehyde energy gap is 50.6 +/- 0.5 kcal/mol, the singlet-triplet separation of 1-hydroxyethylidene is Delta E(ST)(CH3C:OH) = 30.5 +/- 0.5 kcal/mol, the proton affinity is PA(CH3C:OH) = 234.7 +/- 0.5 kcal/mol, and the ionization energy is IEa(CH3C:OH) = 8.18 +/- 0.04 eV. The calculated energy differences between the carbene and aldehyde isomers, and, thus, the heats of formation of the carbenes, differ from the experimental values by 2.5 kcal/mol.     

Heats of formation of boron hydride anions and dianions and their ammonium salts [BnHmy-][NH4+]y with y=1-2

Thermochemical parameters of the closo boron hydride BnHn2- dianions, with n=5-12, the B3H8- and B11H14- anions, and the B5H9 and B10H14 neutral species were predicted by high-level ab initio electronic structure calculations. Total atomization energies obtained from coupled-cluster CCSD(T)/complete basis set (CBS) extrapolated energies, plus additional corrections were used to predict the heats of formation of the simplest BnHmy- species in the gas phase in kcal/mol at 298 K: DeltaHf(B3H8-)=-23.1+/-1.0; DeltaHf(B5H52-)=119.4+/-1.5; DeltaHf(B6H62-)=64.1+/-1.5; and DeltaHf(B5H9)=24.1+/-1.5. The heats of formation of the larger species were evaluated by the G3 method from hydrogenation reactions (values at 298 K, in kcal/mol with estimated error bars of+/-3 kcal/mol): DeltaHf(B7H72-)=51.8; DeltaHf(B8H82-)=46.1; DeltaHf(B9H92-)=24.4; DeltaHf(B10H102-)=-12.5; DeltaHf(B11H112-)=-11.8; DeltaHf(B12H122-)=-86.3; DeltaHf(B11H14-)=-57.3; and DeltaHf(B10H14)=18.7. A linear correlation between atomization energies of the dianions and energies of the BH units was found. The heats of formation of the ammonium salts of the anions and dianions were predicted using lattice energies (UL) calculated from an empirical expression based on ionic volumes. The UL values (0 K) of the BnHn2- dianions range from 319 to 372 kcal/mol. The values of UL for the B3H8- and B11H14- anions are 113 and 135 kcal/mol, respectively. The calculated lattice energies and gas-phase heats of formation of the constituent ions were used to predict the heats of formation of the ammonium crystal salts [BnHmy-][NH4+]y. These results were used to evaluate the thermodynamics of the H2 release reactions from the ammonium hydro-borate salts.     

Peculiar structure of the HOOO(-) anion

The HOOO(-) anion (1) can adopt a triplet state (T-1) or a singlet state (S-1), where the former is 9.8 kcal/mol (DeltaH(298) = 10.3 kcal/mol) more stable than the latter. S-1 possesses a strong O-OOH bond with some double bond character and a weakly covalent OO-OH bond (1.80 A) according to CCSD(T)/6-311++G(3df,3pd) calculations (the longest O-O bond ever found for a peroxide). In aqueous solution, S-1 adopts a geometry closely related to that of HOOOH (OO(O), 1.388 A; (O)OO(H), 1.509 A; tau(OOOH), 78.3 degrees ), justifying that S-1 is considered the anion of HOOOH. Dissociation into HO anion and O(2)((1)Delta(g)) requires 15.4 (DeltaH(298) = 14.3; DeltaG(298) = 8.9) kcal/mol. Structure T-1 corresponds to a van der Waals complex between HO anion and O(2)((3)Sigma(g)(-)) having a binding energy of 2.7 (DeltaH(298) = 2.1) kcal/mol. Modes of generating S-1 in aqueous solution are discussed, and it is shown that S-1 represents an important intermediate in ozonation reactions.     

Thinking out of the black box: accurate barrier heights of 1,3-dipolar cycloadditions of ozone with acetylene and ethylene

Accurate barriers for the 1,3-dipolar cycloadditions of ozone with acetylene and ethylene have been determined via the systematic extrapolation of ab initio energies within the focal point approach of Allen and co-workers. Electron correlation has been accounted for primarily via coupled cluster theory, including single, double, and triple excitations, as well as a perturbative treatment of connected quadruple excitations [CCSD, CCSD(T), CCSDT, and CCSDT(Q)]. For the concerted [4 + 2] cycloadditions, the final recommended barriers are DeltaH(0K) = 9.4 +/- 0.2 and 5.3 +/- 0.2 kcal mol(-1) for ozone adding to acetylene and ethylene, respectively. These agree with recent results of Cremer et al. and Anglada et al., respectively. The reaction energy for O3 + C2H2 exhibits a protracted convergence with respect to inclusion of electron correlation, with the CCSDT/cc-pVDZ and CCSDT(Q)/cc-pVDZ values differing by 2.3 kcal mol-1. Recommended enthalpies of formation (298 K) for cycloadducts 1,2,3-trioxole and 1,2,3-trioxolane are +32.8 and -1.6 kcal mol(-1), respectively. Popular composite ab initio approaches [CBS-QB3, CBS-APNO, G3, G3B3, G3(MP2)B3, G4, G4(MP3), and G4(MP2)] predict a range of barrier heights for these systems. The CBS-QB3 computed barrier for ozone and acetylene, DeltaH(0K) = 4.4 kcal mol(-1), deviates by 5 kcal mol(-1) from the focal point value. CBS-QB3 similarly underestimates the barrier for the reaction of ozone and ethylene, yielding a prediction of only 0.7 kcal mol(-1). The errors in the CBS-QB3 results are significantly larger than mean errors observed in application to the G2 test set. The problem is traced to the nontransferability of MP2 basis set effects in the case of these reaction barriers. The recently published G4 and G4(MP2) approaches perform substantially better for O3 + C2H2, predicting enthalpy barriers of 9.0 and 8.4 kcal mol(-1), respectively. For the prediction of these reaction barriers, the additive corrections applied in the majority of the composite approaches considered lead to worse agreement with the reference focal point values than would be obtained relying only on single point energies evaluated at the highest level of theory utilized within each composite method.     

The lowest-lying electronic singlet and triplet potential energy surfaces for the HNO-NOH system: energetics, unimolecular rate constants, tunneling and kinetic isotope effects for the isomerization and dissociation reactions

The lowest-lying electronic singlet and triplet potential energy surfaces (PES) for the HNO-NOH system have been investigated employing high level ab initio quantum chemical methods. The reaction energies and barriers have been predicted for two isomerization and four dissociation reactions. Total energies are extrapolated to the complete basis set limit applying focal point analyses. Anharmonic zero-point vibrational energies, diagonal Born-Oppenheimer corrections, relativistic effects, and core correlation corrections are also taken into account. On the singlet PES, the (1)HNO → (1)NOH endothermicity including all corrections is predicted to be 42.23 ± 0.2 kcal mol(-1). For the barrierless decomposition of (1)HNO to H + NO, the dissociation energy is estimated to be 47.48 ± 0.2 kcal mol(-1). For (1)NOH → H + NO, the reaction endothermicity and barrier are 5.25 ± 0.2 and 7.88 ± 0.2 kcal mol(-1). On the triplet PES the reaction energy and barrier including all corrections are predicted to be 7.73 ± 0.2 and 39.31 ± 0.2 kcal mol(-1) for the isomerization reaction (3)HNO → (3)NOH. For the triplet dissociation reaction (to H + NO) the corresponding results are 29.03 ± 0.2 and 32.41 ± 0.2 kcal mol(-1). Analogous results are 21.30 ± 0.2 and 33.67 ± 0.2 kcal mol(-1) for the dissociation reaction of (3)NOH (to H + NO). Unimolecular rate constants for the isomerization and dissociation reactions were obtained utilizing kinetic modeling methods. The tunneling and kinetic isotope effects are also investigated for these reactions. The adiabatic singlet-triplet energy splittings are predicted to be 18.45 ± 0.2 and 16.05 ± 0.2 kcal mol(-1) for HNO and NOH, respectively. Kinetic analyses based on solution of simultaneous first-order ordinary-differential rate equations demonstrate that the singlet NOH molecule will be difficult to prepare at room temperature, while the triplet NOH molecule is viable with respect to isomerization and dissociation reactions up to 400 K. Hence, our theoretical findings clearly explain why (1)NOH has not yet been observed experimentally.     

Thermochemistry of the gaseous vanadium chlorides VCl, VCl2, VCl3, and VCl4

Gaseous equilibria in the V-Ag-Cl system were studied at elevated temperatures by effusion-beam mass spectrometry, where the pertinent species were generated by reaction of Cl 2(g) with V + Ag granules in the effusion cell source. Reaction enthalpies were derived from the equilibrium data, and the standard enthalpies of formation at 298 K of gaseous VCl, VCl2, and VCl3 were found to be +49.7, -34.8, and -85.6 kcal mol(-1), respectively. The corresponding bond dissociation energies at 298 K are D(V-Cl) = 102.9 kcal, D(ClV-Cl) = 113.5 kcal, D(Cl2V-Cl) = 79.8 kcal, and D(Cl3V-Cl) = 69.5 kcal. From these data, the dissociation energy D degrees 0(VCl) = 101.9 kcal mol(-1) or 4.42 eV is obtained. An alternate value, Delta(f)H(o)298(VCl 3,g) = -87.0 kcal mol (-1) was derived from third-law analysis of literature sublimation data for VCl3(s). In addition, literature thermochemical data on VCl4(g) were re-evaluated, leading to Delta(f)H(o)298 = -126.1 kcal mol (-1). The results are compared with various estimates in the literature.     

Cheletropic decomposition of cyclic nitrosoamines revisited: the nature of the transition states and a critical role of the ring strain

The cheletropic decompositions of 1-nitrosoaziridine (1), 1-nitroso-Delta(3)-pyrroline (2), 7-nitroso-7-azabicyclo[2.2. 1]hepta-2,5-diene (3), and 6-nitroso-6-azabicyclo[2.1.1]hexa-4-ene (4) have been studied theoretically using high level ab initio computations. Activation parameters of the decomposition of nitrosoaziridine 1 were obtained experimentally in heptane (DeltaH()(298) = 18.6 kcal mol(-)(1), DeltaS()(298) = -7.6 cal mol(-)(1) K(-)(1)) and methanol (20.3 kcal mol(-)(1), 0.3 cal mol(-)(1) K(-)(1)). Among employed theoretical methods (B3LYP, MP2, CCD, CCSD(T)//CCD), the B3LYP method in conjunction with 6-31+G, 6-311+G, and 6-311++G(3df,2pd) basis sets gives the best agreement with experimental data. It was found that typical N-nitrosoheterocycles 2-4 which have high N-N bond rotation barriers (>16 kcal mol(-)(1)) extrude nitrous oxide via a highly asynchronous transition state with a planar ring nitrogen atom. Nitrosoaziridine 1, with a low rotation barrier (<9 kcal mol(-)(1)) represents a special case. This compound can eliminate N(2)O via a low energy linear synperiplanar transition state (DeltaH()(298) = 20.6 kcal mol(-)(1), DeltaS()(298) = 2.5 cal mol(-)(1) K(-)(1)). Two higher energy transition states are also available. The B3LYP activation barriers of the cheletropic fragmentation of nitrosoheterocycles 2-4 decrease in the series: 2 (58 kcal mol(-)(1)) > 3 (18 kcal mol(-)(1)) > 4 (12) kcal mol(-)(1). The relative strain energies increase in the same order: 2 (0 kcal mol(-)(1)) < 3 (39 kcal mol(-)(1)) < 4 (52 kcal mol(-)(1)). Comparison of the relative energies of 2-4 and their transition states on a common scale where the energy of nitrosopyrroline 2 is assumed as reference indicates that the thermal stability of the cyclic nitrosoamines toward cheletropic decomposition is almost entirely determined by the ring strain.     

Direct determination of the ionization energies of PtC, PtO, and PtO2 with VUV radiation

Photoionization efficiency curves were measured for gas-phase PtC, PtO, and PtO2 using tunable vacuum ultraviolet (VUV) radiation at the Advanced Light Source. The molecules were prepared by laser ablation of a platinum tube, followed by reaction with CH4 or N2O and supersonic expansion. These measurements provide the first directly measured ionization energy for PtC, IE(PtC) = 9.45 +/- 0.05 eV. The direct measurement also gives greatly improved ionization energies for the platinum oxides, IE(PtO) = 10.0 +/- 0.1 eV and IE(PtO2) = 11.35 +/- 0.05 eV. The ionization energy connects the dissociation energies of the neutral and cation, leading to greatly improved 0 K bond dissociation energies for the neutrals: D0(Pt-C) = 5.95 +/- 0.07 eV, D0(Pt-O) = 4.30 +/- 0.12 eV, and D0(OPt-O) = 4.41 +/- 0.13 eV, as well as enthalpies of formation for the gas-phase molecules DeltaH(0)(f,0)(PtC(g)) = 701 +/- 7 kJ/mol, DeltaH(0)(f,0)(PtO(g)) = 396 +/- 12 kJ/mol, and DeltaH(0)(f,0)(PtO2(g)) = 218 +/- 11 kJ/mol. Much of the error in previous Knudsen cell measurements of platinum oxide bond dissociation energies is due to the use of thermodynamic second law extrapolations. Third law values calculated using statistical mechanical thermodynamic functions are in much better agreement with values obtained from ionization energies and ion energetics. These experiments demonstrate that laser ablation production with direct VUV ionization measurements is a versatile tool to measure ionization energies and bond dissociation energies for catalytically interesting species such as metal oxides and carbides.     

Mechanism of 1,3-migration in allylperoxyl radicals: computational evidence for the formation of a loosely bound radical-dioxygen complex

The three pathways postulated for 1,3-migration of the peroxyl group in the allylperoxyl radical (1a), a key reaction involved in the spontaneous autoxidation of unsaturated lipids of biological importance, have been investigated by means of quantum mechanical electronic structure calculations. According to the barrier heights calculated from RCCSD(T)/6-311+G(3df,2p) energies with optimized molecular geometries and harmonic vibrational frequencies determined at the UMP2/6-311+G(3df,2p) level, the allylperoxyl rearrangement proceeds by fragmentation of 1a through a transition structure (TS1) with a calculated DeltaH++(298 K) of 21.7 kcal/mol to give an allyl radical-triplet dioxygen loosely bound complex (CX). In a subsequent step, the triplet dioxygen moiety of CX recombines at either end of the allyl radical moiety to convert the complex to the rearranged peroxyl radical (1a') or to revert to the starting peroxyl radical 1a. CX shows an electron charge transfer of 0.026 e in the direction allyl --> O(2). The dominant attractive interactions holding in association the allyl radical-triplet dioxygen pair in CX are due chiefly to dispersion forces. The DeltaH(298 K) for dissociation of CX in its isolated partners, allyl radical and triplet dioxygen, is predicted to be at least 1 kcal/mol. The formation of CX prevents the diffusion of its partners and maintains the stereocontrol along the fragmentation-recombination processes. The concerted 1,3-migration in allylperoxyl radical is predicted to take place through a five-membered ring peroxide transition structure (TS2) showing two long C-O bonds. The DeltaH++(298 K) calculated for this pathway is less favorable than the fragmentation-recombination pathway by 1.9 kcal/mol. The cyclization of 1a to give a dioxolanyl radical intermediate (2a) is found to proceed through a five-membered ring transition structure (TS3) with a calculated DeltaH++(298 K) of 33.9 kcal/mol. Thus, the sequence of ring closure 1a --> 2a and ring opening 2a --> 1a' is unlikely to play any significant role in allylperoxyl rearrangement 1a --> 1a'. In the three pathways investigated, the energy of the transition structure is predicted to be somewhat lower in either heptane or aqueous solution than in the gas phase. Although the energy lowering calculated for TS1 is smaller than the calculated for TS2 and TS3, it is very unlikely that the solvent effects may reverse the predicted preference of the fragmentation-recombination pathway over the concerted and stepwise ring closure-ring opening mechanisms.     

Carboranes and baskets from reaction of B4H10 with allene

The reaction of the boron hydride B4H10 with allene was studied at the CCSD(T)/6-311+G(d)//MP2/6-31G(d) level. The mechanism is surprisingly complex with 44 transition states and several branching points located. The four carboranes and one basket that have been observed experimentally are all connected by pathways that have very similar free energies of activation. In addition, two new structures, a basket (2,4-(CH2CH2CH2)B4H8, 5a) and a "classical" structure (1,4-(Me2C)bisdiborane, 7), which might be obtained from the B4H10 + C3H4 reaction under the right conditions (hot/cold, quenched, etc.) have been identified. The first branch point in the reaction is the competition between H2 elimination from B4H10 (DeltaG(298 K) = 32.2 kcal/mol) and the hydroboration of allene by B4H10 (DeltaG(298 K) = 31.3 kcal/mol). The next branch point in the hydroboration mechanism controls the formation of 2,4-(MeCHCH2)B4H8 (1) (DeltaG(298 K) = 31.5 kcal/mol) and arachno-1,2/arachno-1,3-Me2-1-CB4H7 (8 and 8a) (DeltaG(298 K) = 34.3 kcal/mol). Another branch point in the H2-elimination mechanism controls the formation of 1-Me-2,5-micro-CH2-1-CB4H7 (29) (DeltaG(298 K) = 0.1 kcal/mol) and 2,5-micro-CHMe-1-CB4H7 (25/26) (DeltaG(298 K) = 7.3 kcal/mol). Formation of 2-Me-2,3-C2B4H7, a carborane observed in the reaction of methylacetylene with B4H10, is calculated to be blocked by a high barrier for H2 elimination. All free energies are relative to B4H10 + allene. An interesting reaction step discovered is the "reverse hydroboration step" in which a hydrogen atom is transferred from carbon back to boron, which allows a CH hydrogen to shuttle between the terminal and central carbon of allene.