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.     

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.     

Reliable predictions of the thermochemistry of boron-nitrogen hydrogen storage compounds: BxNxHy, x = 2, 3

Thermochemical data calculated using ab initio molecular orbital theory are reported for 16 BxNxHy compounds with x = 2, 3 and y > or = 2x. Accurate gas-phase heats of formation were obtained using coupled cluster with single and double excitations and perturbative triples (CCSD(T)) valence electron calculations extrapolated to the complete basis set (CBS) limit with additional corrections including core/valence, scalar relativistic, and spin-orbit corrections to predict the atomization energies and scaled harmonic frequencies to correct for zero point and thermal energies and estimate entropies. Computationally cheaper calculations were also performed using the G3MP2 and G3B3 variants of the Gaussian 03 method, as well as density functional theory (DFT) using the B3LYP functional. The G3MP2 heats of formation are too positive by up to approximately 6 kcal/mol as compared with CCSD(T)/CBS values. The more expensive G3B3 method predicts heats of formation that are too negative as compared with the CCSD(T)/CBS values by up to 3-4 kcal/mol. DFT using the B3LYP functional and 6-311+G** basis set predict isodesmic reaction energies to within a few kcal/mol compared with the CCSD(T)/CBS method so isodesmic reactions involving BN compounds and the analogous hydrocarbons can be used to estimate heats of formation. Heats of formation of c-B3N3H12 and c-B3N3H6 are -95.5 and -115.5 kcal/mol at 298 K, respectively, using our best calculated CCSD(T)/CBS approach. The experimental value for c-B3N3H6 appears to be approximately 7 kcal/mol too negative. Enthalpies, entropies, and free energies are calculated for many dehydrocoupling and dehydrogenation reactions that convert BNH6 to alicyclic and cyclic oligomers and H2(g). Generally, the reactions are highly exothermic and exergonic as well because of the release of 1 or more equivalents of H2(g). For c-B3N3H12 and c-B3N3H6, available experimental data for sublimation and vaporization lead to estimates of their condensed phase 298 K heats of formation: DeltaHf degrees [c-B3N3H12(s)] = -124 kcal/mol and DeltaHf degrees [c-B3N3H6(l)] = -123 kcal/mol. The reaction thermochemistries for the dehydrocoupling of BNH6(s) to c-B3N3H12(s) and the dehydrogenation of c-B3N3H12(s) to c-B3N3H6(l) are much less exothermic compared with the gas-phase reactions due to intermolecular forces which decrease in the order BNH6 > cyclo-B3N3H12 > cyclo-B3N3H6. The condensed phase reaction free energies are less negative compared with the gas-phase reactions but are still too favorable for BNH6 to be regenerated from either c-B3N3H12 or c-B3N3H6 by just an overpressure of H2.     

Enthalpies of formation of gas-phase N3, N3-, N5+, and N5- from Ab initio molecular orbital theory, stability predictions for N5(+)N3(-) and N5(+)N5(-), and experimental evidence for the instability of N5(+)N3(-)

Ab initio molecular orbital theory has been used to calculate accurate enthalpies of formation and adiabatic electron affinities or ionization potentials for N3, N3-, N5+, and N5- from total atomization energies. The calculated heats of formation of the gas-phase molecules/ions at 0 K are DeltaHf(N3(2Pi)) = 109.2, DeltaHf(N3-(1sigma+)) = 47.4, DeltaHf(N5-(1A1')) = 62.3, and DeltaHf(N5+(1A1)) = 353.3 kcal/mol with an estimated error bar of +/-1 kcal/mol. For comparison purposes, the error in the calculated bond energy for N2 is 0.72 kcal/mol. Born-Haber cycle calculations, using estimated lattice energies and the adiabatic ionization potentials of the anions and electron affinities of the cations, enable reliable stability predictions for the hypothetical N5(+)N3(-) and N5(+)N5(-) salts. The calculations show that neither salt can be stabilized and that both should decompose spontaneously into N3 radicals and N2. This conclusion was experimentally confirmed for the N5(+)N3(-) salt by low-temperature metathetical reactions between N5SbF6 and alkali metal azides in different solvents, resulting in violent reactions with spontaneous nitrogen evolution. It is emphasized that one needs to use adiabatic ionization potentials and electron affinities instead of vertical potentials and affinities for salt stability predictions when the formed radicals are not vibrationally stable. This is the case for the N5 radicals where the energy difference between vertical and adiabatic potentials amounts to about 100 kcal/mol per N5.     

Thermochemistry of new molecular species: SBr and HSBr

Total energies, optimized geometries, and vibrational frequencies of SBr and HSBr have been evaluated at the coupled cluster level of theory with the correlation consistent basis sets. Extrapolated to the complete basis set limit and with corrections for core-valence, scalar relativistic, and spin-orbit effects, atomization energies were computed and then combined with the experimental heats of formation of the atomic species to generate very accurate heats of formation for the species SBr and HSBr. For SBr, we predict 37.45 and 36.07 kcal/mol for DeltaHf(0 K) and DeltaHf (298.15 K), respectively, in very good agreement with the inferred experimental values of 37.98 and 36.15 kcal/mol. For HSBr, the estimate turns out to be 10.38 and 8.29 kcal/mol for DeltaHf (0 K) and DeltaHf (298.15 K), respectively. Using the more recent HBrO experimental heat of formation at 298.15 K of Lock et al., [J. Phys. Chem. 100, 7972 (1996)] the inferred experimental value for HSBr is predicted to be 8.15 kcal/mol, compared with 8.65 kcal/mol derived from the data of Ruscic and Berkowitz [J. Chem. Phys. 101, 7795 (1994)]. Considering the better agreement of the result with that predicted using the experimental value of DeltaHf(298.15 K) of Lock et al., the author also supports the suggestion made by Denis [J. Phys. Chem. A. 110, 5887 (2006)] that the result of Lock et al. should be preferred over the one of Ruscic and Berkowitz. For DeltaHf(0 K), the author found 10.38 and 10.56 kcal/mol, respectively, for the theoretical and inferred experimental estimates.     

Heats of formation of triplet ethylene, ethylidene, and acetylene

Heats of formation of the lowest triplet state of ethylene and the ground triplet state of ethylidene have been predicted by high level electronic structure calculations. Total atomization energies obtained from coupled-cluster CCSD(T) energies extrapolated to the complete basis set limit using correlation consistent basis sets (CBS), plus additional corrections predict the following heats of formation in kcal/mol: DeltaH0r(C2H4,3A1) = 80.1 at 0 K and 78.5 at 298 K, and DeltaH0t(CH3CH,3A' ') = 86.8 at 0 K and 85.1 at 298 K, with an error of less than +/-1.0 kcal/mol. The vertical and adiabatic singlet-triplet separation energies of ethylene were calculated as DeltaES-T,vert = 104.1 and DeltaES-T,adia = 65.8 kcal/mol. These results are in excellent agreement with recent quantum Monte Carlo (DMC) values of 103.5 +/- 0.3 and 66.4 +/- 0.3 kcal/mol. Both sets of computational values differ from the experimental estimate of 58 +/- 3 kcal/mol for the adiabatic splitting. The computed singlet-triplet gap at 0 K for acetylene is DeltaES-T,adia(C2H2) = 90.5 kcal/mol, which is in notable disagreement with the experimental value of 82.6 kcal/mol. The heat of formation of the triplet is DeltaH0tC2H2,3B2) = 145.3 kcal/mol. There is a systematic underestimation of the singlet-triplet gaps in recent photodecomposition experiments by approximately 7 to 8 kcal/mol. For vinylidene, we predict DeltaH0t(H2CC,1A1) = 98.8 kcal/mol at 298 K (exptl. 100.3 +/- 4.0), DeltaH0t(H2CC,3B2) = 146.2 at 298 K, and an energy gap DeltaES-T-adia(H2CC) = 47.7 kcal/mol.     

Thermodynamic properties of the C5, C6, and C8 n-alkanes from ab initio electronic structure theory

The heats of formation for the n-alkanes C(n)H(n+2) for n = 5, 6, and 8 have been calculated using ab initio molecular orbital theory. Coupled-cluster calculations with perturbative triples (CCSD(T)) were employed for the total valence electronic energies. Correlation-consistent basis sets were used, up through the augmented quadruple zeta, to extrapolate to the complete basis set limit. Geometries were optimized at the B3LYP/TZVP and MP2/aug-cc-pVTZ levels. The MP2 geometries were used in the CCSD(T) calculations. Frequencies were determined at the density functional level (B3LYP/TZVP), and scaled zero point energies were calculated from the B3LYP frequencies. Core/valence, scalar relativistic, and spin-orbit corrections were included in an additive fashion to predict the atomization energies. The core/valence corrections are not small, (approximately 1.1 kcal/mol per carbon unit) and cannot be neglected for chemical accuracy. The calculated deltaH(298)f values are -35.0, -40.2, and -50.2 kcal/mol for C5H12, C6H14, and C8H18, respectively, in excellent agreement with the respective experimental values of -35.11 +/- 0.19, -39.89 +/- 0.19, and -49.90 +/- 0.31 kcal/mol. Isodesmic reaction energies are presented for some simple reactions involving C8H18 and are shown not to be strongly method dependent.     

The enthalpies of formation of o-, m-, and p-benzoquinone: gas-phase ion energetics, combustion calorimetry, and quantum chemical computations combined

Radical anions of o-, m-, and p-benzoquinone were produced in a Fourier transform mass spectrometer by low energy electron attachment or collision-induced dissociation and were differentiated. Classical derivatization experiments also were carried out to authenticate the ortho and meta anions. Gas-phase techniques were used to measure the proton affinities of all three radical anions and the electron affinities of o- and m-benzoquinone. By combining these results in thermodynamic cycles, we derived heats of hydrogenation of o-, m-, and p-benzoquinone (Delta(hyd)H degrees (1o, 1m, and 1p) = 42.8 +/- 4.1, 74.8 +/- 4.1, and 38.5 +/- 3.0 kcal mol(-)(1), respectively) and their heats of formation (Delta(f)H degrees (1o, 1m, and 1p) = -23.1 +/- 4.1, 6.8 +/- 4.1, and -27.7 +/- 3.0 kcal mol(-)(1), respectively). Good accord with the literature value for the para derivative was obtained. Combustion calorimetry and heats of sublimation also were measured for benzil and 3,5-di-tert-butyl-o-benzoquinone. The former heat of formation agreed with previous determinations, while the latter result (Delta(f)H degrees (g) = -73.09 +/- 0.87 kcal mol(-)(1)) was transformed to Delta(f)H degrees (1o) = -18.9 +/- 2.2 kcal mol(-)(1) by removing the effect of the tert-butyl groups via isodesmic reactions. This led to a final value of Delta(f)H degrees (1o) = -21.0 +/- 3.1 kcal mol(-)(1). Additivity was found to work well for m-benzoquinone, but BDE1 and BDE2 for 1,2- and 1,4-dihydroxybenzene differed by a remarkably small 14.1 +/- 4.2 and 23.5 +/- 3.7 kcal mol(-)(1), respectively, indicating that o- and p-benzoquinone should be excellent radical traps.     

Heats of formation of beryllium, boron, aluminum, and silicon re-examined by means of W4 theory

Benchmark total atomization energies (TAE0 values) were obtained, by means of our recent W4 theory [Karton, A.; Rabinowitz, E.; Martin, J. M. L.; Ruscic, B. J. Chem. Phys. 2006, 125, 144108], for the molecules Be2, BeF2, BeCl2, BH, BF, BH3, BHF2, B2H6, BF3, AlF, AlF3, AlCl3, SiH4, Si2H6, and SiF4. We were then able to deduce "semi-experimental" heats of formation for the elements beryllium, boron, aluminum, and silicon by combining the calculated TAE0 values with experimental heats of formation obtained from reactions that do not involve the species Be(g), B(g), Al(g), and Si(g). The elemental heats of formation are fundamental thermochemical quantities that are required whenever a molecular heat of formation has to be derived from a calculated binding energy. Our recommended DeltaH degrees f,0 [A(g)] values are Be 76.4+/-0.6 kcal/mol, B 135.1+/-0.2 kcal/mol, Al 80.2+/-0.4 kcal/mol, and Si 107.2+/-0.2 kcal/mol. (The corresponding values at 298.15 K are 77.4, 136.3, 80.8, and 108.2 kcal/mol, respectively.) The Be value is identical to the CODATA recommendation (but with half of the uncertainty), while the B, Al, and Si values represent substantial revisions from established earlier reference data. The revised B and Si values are in agreement with earlier semi-ab initio derivations but carry much smaller uncertainties.     

Thermodynamic and ab initio analysis of the controversial enthalpy of formation of formaldehyde.

There are two values, -26.0 and -27.7 kcal mol(-1), that are routinely reported in literature evaluations for the standard enthalpy of formation, Delta(f) H(o)(298), of formaldehyde (CH(2)=O), where error limits are less than the difference in values. In this study, we summarize the reported literature for formaldehyde enthalpy values based on evaluated measurements and on computational studies. Using experimental reaction enthalpies for a series of reactions involving formaldehyde, in conjunction with known enthalpies of formation, its enthalpy is determined to be -26.05+/-0.42 kcal mol(-1), which we believe is the most accurate enthalpy currently available. For the same reaction series, the reaction enthalpies are evaluated using six computational methods: CBS-Q, CBS-Q//B3, CBS-APNO, G2, G3, and G3B3 yield Delta(f) H(o)(298)=-25.90+/-1.17 kcal mol(-1), which is in good agreement to our experimentally derived result. Furthermore, the computational chemistry methods G3, G3MP2B3, CCSD/6-311+G(2df,p)//B3LYP/6-31G(d), CCSD(T)/6-311+G(2df,p)//B3LYP/6-31G(d), and CBS-APNO in conjunction with isodesmic and homodesmic reactions are used to determine Delta(f) H(o)(298). Results from a series of five work reactions at the higher levels of calculation are -26.30+/-0.39 kcal mol(-1) with G3, -26.45+/-0.38 kcal mol(-1) with G3MP2B3, -26.09+/-0.37 kcal mol(-1) with CBS-APNO, -26.19+/-0.48 kcal mol(-1) with CCSD, and -26.16+/-0.58 kcal mol(-1) with CCSD(T). Results from heat of atomization calculations using seven accurate ab initio methods yields an enthalpy value of -26.82+/-0.99 kcal mol(-1). The results using isodesmic reactions are found to give enthalpies more accurate than both other computational approaches and are of similar accuracy to atomization enthalpy calculations derived from computationally intensive W1 and CBS-APNO methods. Overall, our most accurate calculations provide an enthalpy of formation in the range of -26.2 to -26.7 kcal mol(-1), which is within computational error of the suggested experimental value. The relative merits of each of the three computational methods are discussed and depend upon the accuracy of experimental enthalpies of formation required in the calculations and the importance of systematic computational errors in the work reaction. Our results also calculate Delta(f) H(o)(298) for the formyl anion (HCO(-)) as 1.28+/-0.43 kcal mol(-1).     

Competitive C-H and O-D bond fission channels in the UV photodissociation of the deuterated hydroxymethyl radical CH2OD

Photodissociation studies of the CH2OD radical in the region 28,000-41,000 cm(-1) (357-244 nm), which includes excitation to the 3s, 3p(x), and 3p(z) states, are reported. H and D photofragments are monitored by using resonance-enhanced multiphoton ionization (REMPI) from the onset of H formation at approximately 30,500 cm(-1) to the origin band region of the 3pz(2A")<--1 2A" transition at 41,050 cm(-1). Kinetic energy distributions P(ET) and recoil anisotropy parameters as a function of kinetic energy, beta(eff)(ET), are determined by the core sampling technique for the channels producing H and D fragments. Two dissociation channels are identified: (I) D+CH2O and (II) H+CHOD. The contribution of channel II increases monotonically as the excitation energy is increased. Based on the calculations of Hoffmann and Yarkony [J. Chem. Phys. 116, 8300 (2002)], it is concluded that conical intersections between 3s and the ground state determine the final branching ratio even when initial excitation accesses the 3px) and 3pz states. The different beta(eff) values obtained for channels I and II (-0.7 and approximately 0.0, respectively) are attributed to the different extents of out-of-plane nuclear motions in the specific couplings between 3s and the ground state (of A' and A' symmetry, respectively) that lead to each channel. The upper limit to the dissociation energy of the C-H bond, determined from P(ET), is D0(C-H)=3.4+/-0.1 eV (79+/-2 kcal/mol). Combining this value with the known heats of formation of H and CH2OD, the heat of formation of CHOD is estimated at DeltaHf(0)(CHOD)=24+/-2 kcal/mol.     

Extension of the PDDG/PM3 and PDDG/MNDO semiempirical molecular orbital methods to the halogens

The new semiempirical methods, PDDG/PM3 and PDDG/MNDO, have been parameterized for halogens. For comparison, the original MNDO and PM3 were also reoptimized for the halogens using the same training set; these modified methods are referred to as MNDO' and PM3'. For 442 halogen-containing molecules, the smallest mean absolute error (MAE) in heats of formation is obtained with PDDG/PM3 (5.6 kcal/mol), followed by PM3' (6.1 kcal/mol), PDDG/MNDO (6.6 kcal/mol), PM3 (8.1 kcal/mol), MNDO' (8.5 kcal/mol), AM1 (11.1 kcal/mol), and MNDO (14.0 kcal/mol). For normal-valent halogen-containing molecules, the PDDG methods also provide improved heats of formation over MNDO/d. Hypervalent compounds were not included in the training set and improvements over the standard NDDO methods with sp basis sets were not obtained. For small haloalkanes, the PDDG methods yield more accurate heats of formation than are obtained from density functional theory (DFT) with the B3LYP and B3PW91 functionals using large basis sets. PDDG/PM3 and PM3' also give improved binding energies over the standard NDDO methods for complexes involving halide anions, and they are competitive with B3LYP/6-311++G(d,p) results including thermal corrections. Among the semiempirical methods studied, PDDG/PM3 also generates the best agreement with high-level ab initio G2 and CCSD(T) intrinsic activation energies for S(N)2 reactions involving methyl halides and halide anions. Finally, the MAEs in ionization potentials, dipole moments, and molecular geometries show that the parameter sets for the PDDG and reoptimized NDDO methods reduce the MAEs in heats of formation without compromising the other important QM observables.     

The correlation consistent composite approach (ccCA): an alternative to the Gaussian-n methods

An alternative to the Gaussian-n (G1, G2, and G3) composite methods of computing molecular energies is proposed and is named the "correlation consistent composite approach" (ccCA, ccCA-CBS-1, ccCA-CBS-2). This approach uses the correlation consistent polarized valence (cc-pVXZ) basis sets. The G2-1 test set of 48 enthalpies of formation (DeltaHf), 38 adiabatic ionization potentials (IPs), 25 adiabatic electron affinities (EAs), and 8 adiabatic proton affinities (PAs) are computed using this approach, as well as the DeltaHf values of 30 more systems. Equilibrium molecular geometries and vibrational frequencies are obtained using B3LYP density functional theory. When applying the ccCA-CBS method with the cc-pVXZ series of basis sets augmented with diffuse functions, mean absolute deviations within the G2-1 test set compared to experiment are 1.33 kcal mol(-1) for DeltaHf,0.81 kcal mol(-1) for IPs, 1.02 kcal mol(-1) for EAs, and 1.51 kcal mol(-1) for PAs, without including the "high-level correction" (HLC) contained in the original Gn methods. Whereas the HLC originated in the Gaussian-1 method as an isogyric correction, it evolved into a fitted parameter that minimized the error of the composite methods, eliminating its physical meaning. Recomputing the G1 and G3 enthalpies of formation without the HLC reveals a systematic trend where most DeltaHf values are significantly higher than experimental values. By extrapolating electronic energies to the complete basis set (CBS) limit and adding G3-like corrections for the core-valence and infinite-order electron correlation effects, ccCA-CBS-2 often underestimates the experimental DeltaHf, especially for larger systems. This is desired as inclusion of relativistic and atomic spin-orbit effects subsequently improves theoretical DeltaHf values to give a 0.81 kcal mol(-1) mean absolute deviation with ccCA-CBS-2. The ccCA-CBS method is a viable "black box" method that can be used on systems with at least 10-15 heavy atoms.     

Thermodynamic properties of molecular borane amines and the [BH4-][NH4+] salt for chemical hydrogen storage systems from ab initio electronic structure theory

The heats of formation for the borane amines BH3NH3, BH2NH2, and HBNH, tetrahedral BH4-, and the BN molecule have been calculated by using ab initio molecular orbital theory. Coupled cluster calculations with single and double excitations and perturbative triples (CCSD(T)) were employed for the total valence electronic energies. Correlation consistent basis sets were used, up through the augmented quadruple-zeta, to extrapolate to the complete basis set limit. Core/valence, scalar relativistic, and spin-orbit corrections were included in an additive fashion to predict the atomization energies. Geometries were calculated at the CCSD(T) level up through at least aug-cc-pVTZ and frequencies were calculated at the CCSD(T)/aug-cc-pVDZ level. The heats of formation (in kcal/mol) at 0 K in the gas phase are Delta Hf(BH3NH3) = -9.1, Delta Hf(BH2NH2) = -15.9, Delta Hf(BHNH) = 13.6, Delta Hf(BN) = 146.4, and Delta Hf(BH4-) = -11.6. The reported experimental value for Delta Hf(BN) is clearly in error. The heat of formation of the salt [BH4-][NH4+](s) has been estimated by using an empirical expression for the lattice energy and the calculated heats of formation of the two component ions. The calculations show that both BH3NH3(g) and [BH4-][NH4+](s) can serve as good hydrogen storage systems which release H2 in a slightly exothermic process. The hydride affinity of BH3 is calculated to be 72.2 kcal/mol, in excellent agreement with the experimental value at 298 K of 74.2 +/- 2.8 kcal/mol.     

Heats of formation of [2.2]paracyclophane-1-ene and [2.2]paracyclophane-1,9-diene - an experimental study

The enthalpies of formation [Delta(g)] of tricyclo[8.2.2.2(4,7)]hexadeca-1(13),2,4(16),5,7(15),10(14),11-heptaene (2, 1,2-dehydro[2.2]paracyclophane or [2.2]paracyclophane-1-ene) and tricyclo[8.2.2.2(4,7)]hexadeca-1(13),2,4(16),5,7(15),8,10(14),11-octaene (3, 1,2,9,10-dehydro[2.2]paracyclophane or [2.2]paracyclophane-1,9-diene) have been determined by measuring their heats of combustion in a microcalorimeter and their heats of sublimation by the transpiration method. Values of the strain energies (SE) [SE(2) = 34.7 kcal mol(-)(1), SE(3) = 42.0 kcal mol(-)(1)] have been derived from the gas-phase heats of formation and are compared with those from MM3 and PM3 calculations and with the corresponding value SE(1) = 30.1 kcal mol(-)(1) for the parent tricyclo[8.2.2.2(4,7)]hexadeca-1(13),4(16),5,7(15),10(14),11-hexaene (1, [2.2]paracyclophane). The higher strain energies of 2 and 3 (by 4.6 and 11.9 kcal mol(-)(1)) are in accord with the well-known increased reactivities of their aromatic rings as a consequence of their increased bending. As revealed by an X-ray crystal structure analysis, the bending in the monoene 2 corresponds to that of 1 and 3 at one of two bridging corners.     

Quantum Monte Carlo study of the reaction: Cl+CH3OH-->CH2OH+HCl

A theoretical study is reported of the Cl+CH3OH-->CH2OH+HCl reaction based on the diffusion Monte Carlo (DMC) variant of the quantum Monte Carlo method. Using a DMC trial function constructed as a product of Hartree-Fock and correlation functions, we have computed the barrier height, heat of reaction, atomization energies, and heats of formation of reagents and products. The DMC heat of reaction, atomization energies, and heats of formation are found to agree with experiment to within the error bounds of computation and experiment. M?ller-Plesset second order perturbation theory (MP2) and density functional theory, the latter in the B3LYP generalized gradient approximation, are found to overestimate the experimental heat of reaction. Intrinsic reaction coordinate calculations at the MP2 level of theory demonstrate that the reaction is predominantly direct, i.e., proceeds without formation of intermediates, which is consistent with a recent molecular beam experiment. The reaction barrier as determined from MP2 calculations is found to be 2.24 kcal/mol and by DMC it is computed to be 2.39(49) kcal/mol.     

Energetic azolium azolate salts

Energetic salts comprising substituted imidazolium and 1,2,4-triazolium cations and 4,5-dinitro-imidazolate and 5-nitrotetrazolate anions were synthesized and characterized. On the basis of experimentally obtained heats of combustion, the calculated heats of formation range from deltaHf(o) degrees = 80 (3) to 1071 kJ/mol (13). Imidazolate salts are more dense but have lower heats of formation than their tetrazolate analogues. Salts 4, 5, 8, and 11-14 fall into the ionic liquid class (mp < 100 degrees C). The structure of 1,2,4-triazolium 5-nitrotetrazolate (10) was confirmed by X-ray analysis.     

Polycyano-anion-based energetic salts

Energetic salts based on polycyano anions and cations with a high nitrogen content tend to have extensive hydrogen bonding and exhibit heats of formation up to DeltaH(f)=1579.1 kJ mol(-1). Based on theoretical calculations, some of the new salts may be considered to be low-energy monopropellants. One of the salts can be used as a precursor to a carbon nitride.     

Comparison of SCC-DFTB and NDDO-based semiempirical molecular orbital methods for organic molecules

Extensive testing of the SCC-DFTB method has been performed, permitting direct comparison to data available for NDDO-based semiempirical methods. For 34 diverse isomerizations of neutral molecules containing the elements C, H, N, and O, the mean absolute errors (MAE) for the enthalpy changes are 2.7, 3.2, 5.0, 5.1, and 7.2 kcal/mol from PDDG/PM3, B3LYP/6-31G(d), PM3, SCC-DFTB, and AM1, respectively. A more comprehensive test was then performed by computing heats of formation for 622 neutral, closed-shell H, C, N, and O-containing molecules; the MAE of 5.8 kcal/mol for SCC-DFTB is intermediate between AM1 (6.8 kcal/mol) and PM3 (4.4 kcal/mol) and significantly higher than for PDDG/PM3 (3.2 kcal/mol). Similarly, SCC-DFTB is found to be less accurate for heats of formation of ions and radicals; however, it is more accurate for conformational energetics and intermolecular interaction energies, though none of the methods perform well for hydrogen bonds with strengths under ca. 7 kcal/mol. SCC-DFTB and the NDDO methods all reproduce MP2/cc-pVTZ molecular geometries with average errors for bond lengths, bond angles, and dihedral angles of only ca. 0.01 A, 1.5 degrees , and 3 degrees . Testing was also carried out for sulfur containing molecules; SCC-DFTB currently yields much less accurate heats of formation in this case than the NDDO-based methods due to the over-stabilization of molecules containing an SO bond.     

BAC-MP4 predictions of thermochemistry for gas-phase indium compounds in the In-H-C-O-Cl system

The BAC-MP4 methodology has been used to calculate the heats of formation and associated thermodynamic parameters for a family of 51 different one-, two-, and three-coordinate indium compounds. The ligands explored were H, CH3, C2H5, OH, OCH3, and Cl. The BAC-MP4 methodology was calibrated by both experimental data and high-level coupled-cluster calculations. The results agree well with the small amount of published experimental data on InHn and InCln species. A linear variation in compound heats of formation with ligand substitution is identified. For example, the heat of formation gradient, Delta(DeltaHf degrees)/Deltan, in the series InHn-3Cln (n=0-3), is -89.6 kcal/mol, with R=1.000. Additionally, trends in bond strengths, bond angles, and bond dissociation energies (BDEs) depending on ligand identity are identified. The BDEs decrease monotonically with increasing atomic number in group III and show similar variations among ligand systems for different metals. These observations and results can be combined with the data from previous studies to guide the selection of CVD precursors and experimental parameters for thin-film deposition of indium-containing materials for semiconductor and optical thin-film applications.