Ab Initio Study on the Potential Energy Surfaces of B4 Cluster

The singlet and triplet potential energy surfaces (PES) for the isomerization and dissociation reactions of B₄ isomers have been investigated using ab initio methods. Ten B4 isomers have been identified and of these 10 species, 4 have not been reported previously. The singlet rhombic structure ¹1 is found to be the most stable on the B₄ surface, in agreement with the results of previous reports. Several isomerization and dissociation pathways have been found. On the singlet PES, the linear ¹3b can rearrange to rhombus ¹1 directly, while ¹3c rearranges to ¹1 through two‐step reactions involving a cyclic intermediate. On the triplet PES, the capped triangle structure ³2 undergoes ring opening to the linear isomer ³3b with a barrier of 34.8 kcal/mol and 44.9 kcal/mol, and the latter undergoes ring closure to the square structure ³1 with a barrier of 30.4 kcal/mol and 33.0 kcal/mol at the MP4/6–311+G(3df)//MP2/6–311G(d) and CCSD/aug‐cc‐pVTZ//MP2/6–311G(d) levels of theory, respectively. The direct decomposition of singlet B₄ yielding to B₃+B is shown to have a large endothermicity of 87.3 kcal/mol (CCSD), and that producing 2B₂ to have activation energy of 133.4 kcal/mol (CCSD).     

Theoretical study on the reaction mechanisms of CH2SH + NO reaction

The mechanisms for the CH₂SH + NO reaction were investigated on both of the singlet and triplet PES at the BMC-CCSD//B3LYP/6-311+G(d,p) level. The results indicate that the singlet PES is much lower than the triplet PES energetically; therefore, the reaction occurs on the singlet PES dominantly. The most favorable channel on the singlet PES takes place by a barrierless addition of N atom to CH₂SH radical to form HSCH₂NO. Subsequently, the rearrangement of the initial adduct HSCH₂NO (IM1) to form another intermediate IM3 via a four-center transition state, followed by the C–O bond fission in IM3 leading to the major product CH₂S + HNO. Due to high barriers, other product including HC(N)SH + HO, HON + CH₂S, and HNO + CHSH could be negligible. The direct abstraction channel was also determined to yield CH₂S + HON. With high barrier (33.3 kcal/mol), it is not competitive with the addition channel, in which all stationary points are lower than reactant energetically. While on the triplet PES, with the lowest barrier height (18.8 kcal/mol), the direct N-abstracted channel to form CH₂S + HNO is dominant. However, it is not competitive with the channels on the singlet PES. Our results are in good accordance with experimental conclusions that the reaction proceeds via addition mechanism.     

Quantum mechanical study of the potential energy surface of the ClO + NO2 reaction

In the study of the reaction pathways of the ClO + NO2 reaction including reliable structures of the reactants, products, intermediates, and transition states as well as energies the MP2/6-311G(d), B3LYP/6-311G(d), and G2(MP2) methods have been employed. Chlorine nitrate, ClONO2, is formed by N-O association without an entrance barrier and is stabilized by 29.8 kcal mol(-1). It can undergo either a direct 1,3 migration of Cl or OCl rotation to yield an indistinguishable isomer. The corresponding barriers are 45.8 and 7.1 kcal mol(-1), respectively. ClONO2 can further decompose into NO3 + Cl with an endothermicity of 46.4 kcal mol(-1). The overall endothermicity of the NO2 + ClO --> NO3 + Cl reaction is calculated to be 16.6 kcal mol(-1). The formation of cis-perp and trans-perp conformer of chlorine preoxynitrite, ClOONO(cp) and ClOONO(tp), are exothermic by 5.4 and 3.8 kcal mol(-1), respectively. Calculations on the possible reaction pathways for the isomerization of ClOONO to ClONO2 showed that the activation barriers are too high to account for appreciable nitrate formation from peroxynitrite isomerization. All quoted relative energies are related to G2(MP2) calculations.     

Kinetic and thermodynamic properties of the aminoxyl (NH2O*) radical

The product of one-electron oxidation of (or H-atom abstraction from) hydroxylamine is the H2NO* radical. H2NO* is a weak acid and deprotonates to form HNO-*; the pKa(H2NO*) value is 12.6+/-0.3. Irrespective of the protonation state, the second-order recombination of the aminoxyl radical yields N2 as the sole nitrogen-containing product. The following rate constants were determined: kr(2H2NO*)=1.4x10(8) M-1 s-1, kr(H2NO*+HNO-*)=2.5x10(9) M-1 s-1, and kr(2HNO-*)=4.5x10(8) M-1 s-1. The HNO-* radical reacts with O2 in an electron-transfer reaction to yield nitroxyl (HNO) and superoxide (O2-*), with a rate constant of ke(HNO-*+O2-->HNO+O2-*)=2.2x10(8) M-1 s-1. Both O2 and O2-* seem to react with deprotonated hydroxylamine (H2NO-) to set up an autoxidative chain reaction. However, closer analysis indicates that these reactions might not occur directly but are probably mediated by transition-metal ions, even in the presence of chelators, such as ethylenediamine tetraacetic acid (EDTA) or diethylenetriamine pentaacetic acid (DTPA). The following standard aqueous reduction potentials were derived: E degrees (H2NO*,2H+/H3NOH+)=1.25+/-0.01 V; E degrees (H2NO*,H+/H2NOH)=0.90+/-0.01 V; and E degrees (H2NO*/H2NO-)=0.09+/-0.01 V. In addition, we estimate the following: E degrees (H2NOH+*/H2NOH)=1.3+/-0.1 V, E degrees (HNO, H+/H2NO*)=0.52+/-0.05 V, and E degrees (HNO/HNO-*)=-0.22+/-0.05 V. From the data, we also estimate the gaseous O-H and N-H bond dissociation enthalpy (BDE) values in H2NOH, with BDE(H2NO-H)=75-77 kcal/mol and BDE(H-NHOH)=81-82 kcal/mol. These values are in good agreement with quantum chemical computations.     

Ab initio kinetics of the reaction of HCO with NO: abstraction versus association/elimination mechanism

The kinetics and mechanism for the reaction of HCO with NO occurring by both singlet and triplet electronic state potential-energy surfaces (PESs) have been studied at the modified Gaussian-2 level of theory based on the geometric parameters optimized by the Becke-3 Lee-Yang-Parr/6-311G(d,p) method. There are two major reaction channels on both singlet and triplet PESs studied: one is direct H abstraction producing CO+HNO and the other is association forming a stable HC(O)NO (nitrosoformaldehyde) molecule. The dominant reaction is predicted to be the direct H abstraction occurring primarily by the lowest-energy path via a loose hydrogen-bonding singlet molecular complex, ON...HCO, with a 2.9-kcal/mol binding energy and a small decomposition barrier (1.9 kcal/mol). The commonly assumed HC(O)NO intermediate, predicted to lie below the reactants by 27.7 kcal/mol, has a high HNO-elimination barrier (34.5 kcal/mol). Bimolecular rate constants for the formation of the singlet products and their branching ratios have been calculated in the temperature range of 200-3000 K. The rate constant for the disproportionation process producing HNO+CO, found to be affected strongly by multiple reflections above the well of the complex at low temperature, is predicted to be k(HNO)=3.08 x 10(-12) T(0.10) exp(242T) for 200-500 K, and 1.72 x 10(-16) T(1.47) exp(888T) for 500-3000 K in units of cm(3) molecule(-1) s(-1). The high- and low-pressure rate constants for the association process forming HC(O)NO can be represented by k(infinity)=4.42 x 10(-11) T(0.25) exp(-28T) cm(3) molecule(-1) s(-1) (200-3000 K) and k(0)=7.30x10(-16) T(-5.75) exp(-719T) (200-1000 K) and 1.82 x 10(2) T(-11.92) exp(1846T) (1000-3000 K) cm(6) molecule(-2) s(-1) for N(2)-buffer gas. The absolute values of total rate constant, predicted to be weakly dependent negatively on temperature but positively on pressure, are in close agreement with most experimental data within their reported errors.     

1-Germavinylidene (Ge═CH2), germyne (HGeCH), and 2-germavinylidene (H2Ge═C) molecules and isomerization reactions among them: anharmonic rovibrational analyses

Theoretical investigations of three equilibrium structures and two associated isomerization reactions of the GeCH(2) - HGeCH - H(2)GeC system have been systematically carried out. This research employed ab initio self-consistent-field (SCF), coupled cluster (CC) with single and double excitations (CCSD), and CCSD with perturbative triple excitations [CCSD(T)] wave functions and a wide variety of correlation-consistent polarized valence cc-pVXZ and cc-pVXZ-DK (where X = D, T, Q) basis sets. For each structure, the total energy, geometry, dipole moment, harmonic vibrational frequencies, and infrared intensities are predicted. Complete active space SCF (CASSCF) wave functions are used to analyze the effects of correlation on physical properties and energetics. For each of the equilibrium structures, vibrational second-order perturbation theory (VPT2) has been utilized to obtain the zero-point vibration corrected rotational constants, centrifugal distortion constants, and fundamental vibrational frequencies. The predicted rotational constants and anharmonic vibrational frequencies for 1-germavinylidene are in good agreement with available experimental observations. Extensive focal point analyses, including CCSDT and CCSDT(Q) energies and basis sets up to quintuple zeta, are used to obtain complete basis set (CBS) limit energies. At all levels of theory employed in this study, the global minimum of the GeCH(2) potential energy surface (PES) is confirmed to be 1-germavinylidene (GeCH(2), 1). The second isomer, germyne (HGeCH, 2) is predicted to lie 40.4(41.1) ± 0.3 kcal mol(-1) above the global minimum, while the third isomer, 2-germavinylidene (H(2)GeC, 3) is located 92.3(92.7) ± 0.3 kcal mol(-1) above the global minimum; the values in parentheses indicate core-valence and zero-point vibration energy (ZPVE) corrected energy differences. The barriers for the forward (1→2) and reverse (2→1) isomerization reactions between isomers 1 and 2 are 48.3(47.7) ± 0.3 kcal mol(-1) and 7.9(6.6) ± 0.3 kcal mol(-1), respectively. On the other hand, the barriers of the forward (2→3) and reverse (3→2) isomerization reactions between isomers 2 and 3 are predicted to be 55.2(53.2) ± 0.3 kcal mol(-1) and 3.3(1.6) ± 0.3 kcal mol(-1), respectively.     

Reaction mechanism of the CCN radical with nitric oxide

To investigate the possibility of the carbyne radical CCN in removal of nitric oxide, a detailed computational study is performed at the Gaussian-3//B3LYP/6-31G(d) level on the CCN + NO reaction by constructing the singlet and triplet electronic state [C(2)N(2)O] potential energy surfaces (PESs). The barrierless formation of the chain-like isomers NCCNO (singlet at -106.5, triplet cis at -48.2 and triplet trans at -47.6 kcal/mol) is the most favorable entrance attack on both singlet and triplet PESs. Subsequently, the singlet NCCNO takes an O-transfer to form the branched intermediate singlet NCC(O)N (-85.6), which can lead to the fragments CN + NCO (-51.2) via the intermediate singlet NCOCN (-120.3). The simpler evolution of the triplet NCCNO is the direct N-O rupture to form the weakly bound complex triplet NCCN...O (-56.2) before the final fragmentation to NCCN + (3)O (-53.5). However, the lower lying products (3)NCN + CO (-105.6) and (3)CNN + CO (-74.6) are kinetically much less competitive. All the involved transition states for generation of CN + NCO and NCCN + (3)O lie much lower than the reactants. Thus, the novel reaction CCN + NO can proceed effectively even at low temperatures and is expected to play a role in both combustion and interstellar processes. Significant differences are found on the singlet PES between the CCN + NO and CH + NO reaction mechanisms.     

Radical reaction C3H+NO: a mechanistic study

Although a number of hydrocarbon radicals including the heavier C(3)-radicals C(3)H(3) and C(3)H(5) have been experimentally shown to deplete NO effectively, no theoretical or experimental attempts have been made on the reactivity of the simplest C(3)-radical towards NO. In this article, we report our detailed mechanistic study on the C(3)H+NO reaction at the Gussian-3//B3LYP/6-31G(d) level by constructing the singlet and triplet electronic state [H,C(3),N,O] potential energy surfaces (PESs). The l-C(3)H+NO reaction is shown to barrierlessly form the entrance isomer HCCCNO followed by the direct O-elimination leading to HCCCN+(3)O on triplet PES, or by successive O-transfer, N-insertion, and CN bond-rupture to generate the product (1)HCCN+CO on singlet PES. The possible singlet-triplet intersystem crossings are also discussed. Thus, the novel reaction l-C(3)H+NO can proceed effectively even at low temperatures and is expected to play an important role in both combustion and interstellar processes. For the c-C(3)H+NO reaction, the initially formed H-cCCC-NO can most favorably isomerize to HCCCNO, and further evolution follows that of the l-C(3)H+NO reaction. Quantitatively, the c-C(3)H+NO reaction can take place barrierlessly on singlet PES, yet it faces a small barrier 2.7 kcal/mol on triplet PES. The results will enrich our understanding of the chemistry of the simplest C(3)-radical in both combustion and interstellar processes, which to date have received little attention despite their importance and available abundant studies on its structural and spectroscopic properties.     

Quantum chemical study of the mechanism of reaction between NH (X 3sigma-) and H2, H2O, and CO2 under combustion conditions

Reactions of ground-state NH (3sigma-) radicals with H2, H2O, and CO2 have been investigated quantum chemically, whereby the stationary points of the appropriate reaction potential energy surfaces, that is, reactants, products, intermediates, and transition states, have been identified at the G3//B3LYP level of theory. Reaction between NH and H2 takes place via a simple abstraction transition state, and the rate coefficient for this reaction as derived from the quantum chemical calculations, k(NH + H2) = (1.1 x 10(14)) exp(-20.9 kcal mol(-1)/RT) cm3 mol(-1) s(-1) between 1000 and 2000 K, is found to be in good agreement with experiment. For reaction between triplet NH and H2O, no stable intermediates were located on the triplet reaction surface although several stable species were found on the singlet surface. No intersystem crossing seam between triplet NH + H2O and singlet HNO + H2 (the products of lowest energy) was found; hence there is no evidence to support the existence of a low-energy pathway to these products. A rate coefficient of k(NH + H2O) = (6.1 x 10(13)) exp(-32.8 kcal mol(-1)/RT) cm3 mol(-1) s(-1) between 1000 and 2000 K for the reaction NH (3sigma-) + H2O --> NH2 (2B) + OH (2pi) was derived from the quantum chemical results. The reverse rate coefficient, calculated via the equilibrium constant, is in agreement with values used in modeling the thermal de-NO(x) process. For the reaction between triplet NH and CO2, several stable intermediates on both triplet and singlet reaction surfaces were located. Although a pathway from triplet NH + CO2 to singlet HNO + CO involving intersystem crossing in an HN-CO2 adduct was discovered, no pathway of sufficiently low activation energy was discovered to compare with that found in an earlier experiment [Rohrig, M.; Wagner, H. G. Proc. Combust. Inst. 1994, 25, 993.].     

Theoretical study on the reaction mechanism of the methyl radical with nitrogen oxides

The radical-molecule reaction mechanism of CH3 with NOx (x = 1, 2) has been explored theoretically at the B3LYP/6-311Gd,p and MC-QCISD (single-point) levels of theory. For the singlet potential energy surface (PES) of the CH3 + NO2 reaction, it is found that the carbon to middle nitrogen attack between CH3 and NO2 can form energy-rich adduct a (H3CNO2) with no barrier followed by isomerization to b1 (CH3ONO-trans), which can easily convert to b2 (CH3ONO-cis). Subsequently, starting from b (b1, b2), the most feasible pathway is the direct N-O bond cleavage of b (b1, b2) leading to P1 (CH3O + NO) or the 1,3-H-shift and N-O bond rupture of b1 to form P2 (CH2O + HNO), both of which may have comparable contribution to the reaction CH3 + NO2. Much less competitively, b2 can take a concerted H-shift and N-O bond cleavage to form product P3 (CH2O + HON). Because the intermediates and transition states involved in the above three channels are all lower than the reactants in energy, the CH3 + NO2 reaction is expected to be rapid, as is consistent with the experimental measurement in quality. For the singlet PES of the CH3 + NO reaction, the major product is found to be P1 (HCN + H2O), whereas the minor products are P2 (HNCO + H2) and P3 (HNC +H2O). The CH3 + NO reaction is predicted to be only of significance at high temperatures because the transition states involved in the most feasible pathways lie almost above the reactants. Compared with the singlet pathways, the triplet pathways may have less contributions to both reactions. The present study may be helpful for further experimental investigation of the title reactions.     

Coupled-cluster study of the electronic structure and energetics of tetrasulfur, S4

Ab initio electronic structure calculations are reported for S4. Geometric and energetic parameters are calculated using the singles and doubles coupled-cluster method, including a perturbutional correction for connected triple excitation, CCSD(T), together with systematic sequences of correlation consistent basis sets extrapolated to the complete basis set limit. The geometry for the ground state singlet C2v structure of S4 is in good agreement with the microwave structure determined for S4. There is a low-lying D2h transition state at 1.6 kcal/mol which interchanges the long S-S bond. S4 has a low-lying triplet state (3B 1u) in D2h symmetry which is 10.8 kcal/mol above the C2v singlet ground state. The S-S bond dissociation energy for S4 into two S2(3Sigma*g) molecules is predicted to be 22.8 kcal mol(-1). The S-S bond energy to form S3+S(3P) is predicted to be 64 kcal/mol.     

Singlet and triplet potential surfaces for the O2+C2H4 reaction

Electronic structure calculations at the CASSCF and UB3LYP levels of theory with the aug-cc-pVDZ basis set were used to characterize structures, vibrational frequencies, and energies for stationary points on the ground state triplet and singlet O(2)+C(2)H(4) potential energy surfaces (PESs). Spin-orbit couplings between the PESs were calculated using state averaged CASSCF wave functions. More accurate energies were obtained for the CASSCF structures with the MRMP2/aug-cc-pVDZ method. An important and necessary aspect of the calculations was the need to use different CASSCF active spaces for the different reaction paths on the investigated PESs. The CASSCF calculations focused on O(2)+C(2)H(4) addition to form the C(2)H(4)O(2) biradical on the triplet and singlet surfaces, and isomerization reaction paths ensuing from this biradical. The triplet and singlet C(2)H(4)O(2) biradicals are very similar in structure, primarily differing in their C-C-O-O dihedral angles. The MRMP2 values for the O(2)+C(2)H(4)→C(2)H(4)O(2) barrier to form the biradical are 33.8 and 6.1 kcal/mol, respectively, for the triplet and singlet surfaces. On the singlet surface, C(2)H(4)O(2) isomerizes to dioxetane and ethane-peroxide with MRMP2 barriers of 7.8 and 21.3 kcal/mol. A more exhaustive search of reaction paths was made for the singlet surface using the UB3LYP/aug-cc-pVDZ theory. The triplet and singlet surfaces cross between the structures for the O(2)+C(2)H(4) addition transition states and the biradical intermediates. Trapping in the triplet biradical intermediate, following (3)O(2)+C(2)H(4) addition, is expected to enhance triplet→singlet intersystem crossing.     

New insight into the gas-phase bimolecular self-reaction of the HOO radical

The singlet and triplet potential energy surfaces (PESs) for the gas-phase bimolecular self-reaction of HOO*, a key reaction in atmospheric environments, have been investigated by means of quantum-mechanical electronic structure methods (CASSCF and CASPT2). All the reaction pathways on both PESs consist of a first step involving the barrierless formation of a prereactive doubly hydrogen-bonded complex, which is a diradical species lying about 8 kcal/mol below the energy of the reactants at 0 K. The lowest energy reaction pathway on both PESs is the degenerate double hydrogen exchange between the HOO* moieties of the prereactive complex via a double proton transfer mechanism involving an energy barrier of only 1.1 kcal/mol for the singlet and 3.3 kcal/mol for the triplet at 0 K. The single H-atom transfer between the two HOO* moieties of the prereactive complex (yielding HOOH + O2) through a pathway keeping a planar arrangement of the six atoms involves a conical intersection between either two singlet or two triplet states of A' and A" symmetries. Thus, the lowest energy reaction pathway occurs via a nonplanar cisoid transition structure with an energy barrier of 5.8 kcal/mol for the triplet and 17.5 kcal/mol for the singlet at 0 K. The simple addition between the terminal oxygen atoms of the two HOO* moieties of the prereactive complex, leading to the straight chain H2O4 intermediate on the singlet PES, involves an energy barrier of 7.3 kcal/mol at 0 K. Because the decomposition of such an intermediate into HOOH + O2 entails an energy barrier of 45.2 kcal/mol at 0 K, it is concluded that the single H-atom transfer on the triplet PES is the dominant pathway leading to HOOH + O2. Finally, the strong negative temperature dependence of the rate constant observed for this reaction is attributed to the reversible formation of the prereactive complex in the entrance channel rather than to a short-lived tetraoxide intermediate.     

Ditantalum dinitrogen complex: reaction of H2 molecule with "end-on-bridged" [Ta(IV)]2(μ-η(1):η(1)-N2) and Bis(μ-nitrido) [Ta(V)]2(μ-N)2 complexes

To elucidate (i) the physicochemical properties of the {(η(5)-C(5)Me(5))[Ta(IV)](i-Pr)C(Me)N(i-Pr)}(2)(μ-η(1):η(1)-N(2)), I, [Ta(IV)](2)(μ-η(1):η(1)-N(2)), and {(η(5)-C(5)Me(5))[Ta(V)](i-Pr)C(Me)N(i-Pr)}(2)(μ-N)(2), II, [Ta(V)](2)(μ-N)(2), complexes; (ii) the mechanism of the I → II isomerization; and (iii) the reaction mechanism of these complexes with an H(2) molecule, we launched density functional (B3LYP) studies of model systems 1, 2, and 3 where the C(5)Me(5) and (i-Pr)C(Me)N(i-Pr) ligands of I (or II) were replaced by C(5)H(5) and HC(NCH(3))(2), respectively. These calculations show that the lower-lying electronic states of 1, [Ta(IV)](2)(μ-η(1):η(1)-N(2)), are nearly degenerate open-shell singlet and triplet states with two unpaired electrons located on the Ta centers. This finding is in reasonable agreement with experiments [J. Am Chem. Soc. 2007, 129, 9284-9285] showing easy accessibility of paramagnetic and diamagnetic states of I. The ground electronic state of the bis(μ-nitrido) complex 2, [Ta(V)](2)(μ-N)(2), is a closed-shell singlet state in agreement with the experimentally reported diamagnetic feature of II. The 1-to-2 rearrangement is a multistep and highly exothermic process. It occurs with a maximum of 28.7 kcal/mol free energy barrier required for the (μ-η(1):η(1)-N(2)) → (μ-η(2):η(2)-N(2)) transformation step. Reaction of 1 with H(2) leading to the 1,4-addition product 3 proceeds with a maximum of 24.2 kcal/mol free energy barrier associated by the (μ-η(1):η(1)-N(2)) → (μ-η(2):η(1)-N(2)) isomerization step. The overall reaction 1 + H(2) → 3 is exothermic by 20.0 kcal/mol. Thus, the addition of H(2) to 1 is kinetically and thermodynamically feasible and proceeds via the rate-determining (μ-η(1):η(1)-N(2)) → (μ-η(2):η(1)-N(2)) isomerization step. The bis(μ-nitrido) complex 2, [Ta(V)](2)(μ-N)(2), does not react with H(2) because of the large energy barrier (49.5 kcal/mol) and high endothermicity of the reaction. This conclusion is also in excellent agreement with the experimental observation [J. Am Chem. Soc. 2007, 129, 9284-9285].     

Computational study of the energetics of 3Fe(CO)4, 1Fe(CO)4 and 1Fe(CO)4(L), L = Xe, CH4, H2 and CO

Large basis CCSD(T) calculations are used to calculate the energetics of 3Fe(CO)4, 1Fe(CO)4 and 1Fe(CO)4(L), L = Xe, CH4, H2 and CO. . The relative energy of the excited singlet state of Fe(CO)4 with respect to the ground triplet state is not known experimentally, and various lower levels of theory predict very different results. Upon extrapolating to the infinite basis set limit, and including corrections for core-core and core-valence correlation, scalar relativity, and multi-reference character of the wavefunction, the best CCSD(T) estimate for the spin-state splitting in iron tetracarbonyl is 2 kcal mol(-1). Calculation of the dissociation energy of 1Fe(CO)4(L) into singlet fragments, taken together with known experimental behaviour of triplet Fe(CO)4, provides independent evidence for the fact that the spin-state splitting is smaller than 3 kcal mol(-1). These calculations highlight some of the challenges involved in benchmark calculations on transition metal containing systems.     

Theoretical reduction potentials for nitrogen oxides from CBS-QB3 energetics and (C)PCM solvation calculations

The complete basis set method, CBS-QB3, is used in combination with two continuum solvation models for aqueous solvation to compute reduction potentials previously determined experimentally for 36 nitrogen oxides and related species of the general formula H(V)C(W)N(X)O(Y)Cl(Z). The PCM model led to the correlation E(o)exp (vs NHE) = 0.84E(o)calc + 0.03 V with an average error of 0.12 V (2.8 kcal/mol) and a maximum error of 0.32 V (7.4 kcal/mol). The CPCM/UAKS model gave E(o)exp (vs NHE) = 0.83E(o)calc + 0.11 V with the same average error. This general method was used to predict reduction potentials (+/-0.3 V) for nitrogen oxides for which reduction potentials are not known with certainty: NO2/NO2- (0.6 V), NO3/NO3- (1.9 V), N2O3-/N2O3(2-) (0.5 V), HN2O3/HN2O3- (0.9 V), HONNO,H+/HONNOH (1.6 V), 2NO,H+/HONNO (0.0 V), 2NO/ONNO- (-0.1 V), ONNO-/ONNO(2-) (-0.4 V), HNO,H+/H2NO (0.6 V), H2NO,H+/H2NOH (0.9 V), HNO,2H+/H2NOH (0.8 V), and HNO/HNO- (-0.7 V).     

Theoretical interpretation of the kinetics and mechanisms of the HNO + HNO and HNO + 2NO reactions with a unified model

Previously measured decay rates of HNO in the presence of NO have been kinetically modeled on the basis of thermochemical data calculated with the BAC-MP4 technique. The results of this modeling, aided by TST-RRKM calculations for the association of HNO and the isomerization, decomposition, and stabilization of the many dimers of HNO, reveal that the decay of HNO under NO-lean conditions occurs primarily by association forming cis- and trans-(HNO)₂ at temperatures below 420 K. N₂O, which is a relatively minor product, is believed to be formed by H₂O elimination from cis-HON ? NOH, a product of succesive isomerization reactions: trans-(HNO)₂^? → HN(OH)NO^? → HN(O)NOH^? → cis-HON NOH^?. The calculated rate constants, which fit experimental data quantitatively, can be represented by k = 10^16.2 × T^?2.40e^?590/T cm³/mol sec for the HNO recombination reaction and k = 10^?2.44T^3.98e^?600/T cm³/mol sec for N₂O formation in the temperature range 80–420 K, at a total pressure of 710 torr H₂ or He. Under NO-rich conditions, HNO reacts predominantly by the exothermic termolecular reaction, HNO + 2NO → HN(NO)ONO → HN NO + NO₂, with a rate contant of (6 ± 1) × 10⁹ cm⁶/mol² sec at room temperature, based on both HNO decay and NO₂ production. All existing thermal kinetic data on HNO + HNO and HNO + 2NO processes can be satisfactorily rationalized with a unified model based on the thermochemical data obtained by BAC-MP4 calculations.     

Ab initio MO study of potential energy surface of NH2 with CN reaction

An extensive quantum chemical study of the potential energy surfaces (PES) for the association reaction of NH₂ with CN and the subsequent isomerization and dissociation reactions has been carried out using density functional theory (DFT)/B3LYP/6‐311++G(3df,2p) level of theory on both singlet and triplet states. The reaction mechanism on the triplet surface is more complicated than that on the singlet surface. A total of 19 isomers and 46 transition states have been identified and characterized on the triplet PES. Among them, IM2 (IM2a), IM3 (IM3a, IM3b), and IM10 are the lowest‐lying isomers with thermodynamic stability. Twenty available dissociation channels, depending on the different initial isomers, have been identified. On the singlet surface, only 12 isomers and 16 transition states have been found, and among them IM1(S) and IM2(S) are the lowest‐lying isomers. The higher isomerization and dissociation barriers on the singlet surface indicate that the addition and the subsequent reactions of NH₂+CN are most likely to occur on the triplet PES because of the lower barriers. A prediction can be made for the possible mechanism explaining the production of H+HNCN. Besides HNCN, other major products are NH+HCN and NH+HNC, which are produced by direct dissociation reactions from triplet IM2 and IM3, respectively. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Theoretical study on the reaction of VS+(3∑-, 1Γ) with CO

Two possible reaction mechanisms of VS (3Ε-, 1Γ) with CO in the gas phase have been studied by using B3LYP/TZVP and CCSD(T)/6-311 G (3df, 3pd) methods: the O/S exchange reaction (VS CO → VO CS) and the S-transfer reaction (VS CO → V COS). The two reactions proceed via two-step and one-step mechanism, respectively. The barriers of the triplet and singlet PESs are 30.6 and 50.9 kcal/mol, respectively, for O/S exchange reaction and 7.3 and 50.2 kcal/mol, respectively, for the S-transfer reaction. The results indicate that the triplet ground state reaction is more favorable, and the S-transfer reaction is more favorable than the O/S exchange reaction, which is in good agreement with the experimental observation.     

Nitrosation, thiols, and hemoglobin: energetics and kinetics

Nitrosothiols are powerful vasodilators. Although the mechanism of their formation near neutral pH is an area of intense research, neither the energetics nor the kinetics of this reaction or of subsequent reactions have been addressed. The following considerations may help to guide experiments. (1) The standard Gibbs energy for the homolysis reaction RSNO → RS(?) + NO(?)(aq) is +110 ± 5 kJ mol(-1). (2) The electrode potential of the RSNO, H(+)/RSH, NO(?)(aq) couple is -0.20 ± 0.06 V at pH 7. (3) Thiol nitrosation by NO(2)(-) is favorable by 37 ± 5 kJ mol(-1) at pH 7. (4) N(2)O(3) is not involved in in vivo nitrosation mechanisms for thermodynamic--its formation from NO(2)(-) costs 59 kJ mol(-1)--or kinetic--the reaction being second-order in NO(2)(-)--reasons. (5) Hemoglobin (Hb) cannot catalyze formation of N(2)O(3), be it via the intermediacy of the reaction of Hb[FeNO(2)](2+) with NO(?) (+81 kJ mol(-1)) or reaction of Hb[FeNO](3+) with NO(2)(-) (+88 kJ mol(-1)). (6) Energetically and kinetically viable are nitrosations that involve HNO(2) or NO(?) in the presence of an electron acceptor with an electrode potential higher than -0.20 V. These considerations are derived from existing thermochemical and kinetics data.