G2 Level Of Theory Research Articles

Electronic Requirements for Oxygen Atom Transfer from Alkyl Hydroperoxides. Model Studies on Multisubstrate Flavin-Containing Monooxygenases

Density functional calculations at the B3LYP/6-31+G(d,p) level are used to study the mechanism of 4a-hydroperoxyflavin oxidation of a series of heteroatom nucleophiles. The oxidation of xenobiotics catalyzed by flavin-containing monooxygenases (FMOs) is modeled by the oxidation of N-, S-, P-, and Se-containing nucleophiles. A mechanism for distinguishing oxygen atom versus hydroxyl transfer from RO−OH to (CH3)3N, (CH3)2S, (CH3)3P, and (CH3)2Se is presented. The nature of these oxidative processes is related to the magnitude of the single imaginary frequency for the transition state for oxygen atom transfer. Classical activation barriers for oxygen atom transfer from a tricyclic isoalloxazine C-4a-hydroperoxide 3 (FlHOOH) to dimethyl sulfide, trimethylamine, trimethylphosphine, and dimethylselenide suggest that the reactivity of this biologically important oxidizing agent is intermediate between that of tert-butyl hydroperoxide and a peracid. The gas-phase reactivity of FlHOOH toward these four nucleophiles is estimated to be 107−1012 greater than that of t-BuO−OH but 102−106 less than that of peroxyformic acid. The intrinsic gas-phase barriers for the oxidation of (CH3)3N and (CH3)2S with 3 (FlHOOH) differ by only 1 kcal/mol. The experimentally observed 106 rate difference for amine oxidation with protein-bound FlHOOH versus synthetic flavinhydroperoxide (FlEtOOH) in solution is attributed simply to solvent effects; no special activation of the amine by the local enzymatic environment is required for this xenobiotic oxidation. A comparison of estimated O−O bond dissociation energies for CH3O−OH and CH3O−OCH3, at the AM1, Hartree−Fock, B3LYP, CBS-Q, and G3 levels of theory is presented. The performance of AM1, Hartree−Fock, versus CASSCF and QCISD(T) computational methods for the O−O bond elongation in CH3O−OH and CH3O−OCH3 is discussed. Our results provide an estimate of 40 kcal/mol for the O−O bond energy in 4a-flavinhydroperoxide (FlHOOH).

Computational Study of the Protonation of AlXH2 and AlX2H (X = F, Cl, and Br). Structures of AlXH3+ and AlX2H2+ and Their Dihydrogen Complexes AlXH5+ and AlX2H4+

Structures of protonated AlXH2 and AlX2H (AlXH3+ and AlX2H2+) and their dihydrogen complexes AlXH5+ and AlX2H4+ (X = F, Cl, and Br) were investigated using the ab initio method at the G2 level of theory. All the dihydrogen complexes involving hypercoordinated aluminum atom with a three-center two-electron (3c−2e) bond. The G2 calculated protonation energies of AlXH2 and AlX2H to form AlXH3+ and AlX2H2+, respectively, were found to be highly exothermic. The possible dissociation of the cations AlXH5+ and AlX2H4+ into AlXH3+ and AlX2H2+ and molecular H2, respectively, are calculated to be endothermic.

Alkane‐ and areneoxodiazonium ions: experimental results leading to an ab initio theoretical investigation

AbstractAlkaneoxodiazonium ions (R—NNO)+ and (R—O—N2)+, which are closely analogous to alkanediazonium ions (R—N2)+, were detected as reactive intermediates in acid‐catalysed reactions of nitrosohydroxylamines and deamination‐type reactions. The isomers were compared by high‐level MO investigations of the reactions of nitrous oxide with cationic electrophiles H+, CH3+, Me3C+ and PhCH2+. Strongly bonded species are formed in reactions with the powerfully electrophilic H+ and CH3+, and the resulting ions differ considerably in stabilities; less stable species are formed with Me3C+ and PhCH2+ and the complexes so formed have similar stabilities. Two isomers of Ph(N2O)+, analogous to phenyldiazonium ion, PhN2+, are calculated to be stable ions with the end‐N‐bonded isomer being preferred over the O‐bonded ion at the MP4(SDQ)/6–311G(d,p)//MP2(fc)/6–31G(d,p) and G3 levels of theory. Both are non‐planar with the phenyl ring at 90° to the non‐linear N2O residue. Syntheses of salts of Ar(N2O)+ are now planned. Copyright © 2003 John Wiley & Sons, Ltd.

The dynamics of formation of HCl products from the reaction of Cl atoms with methanol, ethanol, and dimethyl ether

The dynamics of ground state Cl(2P3/2) atom reactions with methanol, methanol-d1, ethanol, and dimethyl ether have been studied both experimentally and theoretically. The reactions were photoinitiated by 355 nm photolysis of Cl2 to produce monoenergetic Cl(2P3/2) atoms that react with ground electronic state organic molecules under single collision conditions. The rotational quantum state population distributions of the nascent HCl(ν′) products were probed by 2+1 resonance-enhanced multiphoton ionization in a time-of-flight mass spectrometer. Nascent HCl(ν′=0) products from reaction of Cl atoms with methanol, methanol-d1 (CH3OD), ethanol, and dimethyl ether, at mean collision energies in the range of 5.6–6.7 kcal/mol, exhibit distributions of population over rotational levels that all peak at J′=3–5. The average rotational energies of the HCl(ν′=0) products for the respective reactions are 〈Erot〉=330±29, 300±24, 340±24, and 256±17 cm−1 (1σ uncertainties). Ab initio calculations were performed in order to examine the mechanisms of Cl atom abstraction of hydrogen from the alcohols and ether. Optimized geometrical structures and vibrational frequencies of molecular complexes and transition states on the reaction pathways were obtained at the MP2/6-311G(d,p) level and their energies were further refined at the G2 level of theory. Comparisons are drawn between the mechanisms and energetic pathways of the various reactions. The degree of rotational excitation of the HCl, which is significantly greater than for Cl atom abstraction of an H atom from alkanes, is attributed to a dipole–dipole interaction between the HCl and RCHOR′ (R, R′=H or CH3) moieties in the products’ region of the potential energy surface.

Electron-spin conservation and methyl-substitution effects on bonds in closed- and open-shell systems — A G3 ab initio study of small boron-containing molecules and radicals

High level ab initio molecular orbital theory calculations have been used to study the geometries and thermochemistry of molecules and free radicals substituted by BH2, BHCH3, and B(CH3)2. The heats of formation and RR'B—X bond strengths (RR' = H, H; H, CH3; CH3, CH3 and X = CH3, NH2, OH, F, SiH3, PH2, SH, and Cl) together with those for the open-shell systems RR'B—Y· (RR' = H, H; H, CH3; CH3, CH3 and Y = CH2, NH, O, SiH2, PH, and S) have been calculated at the G3 level of theory. The trends observed for the homolytic bond strengths in the closed-shell systems are those expected from electronegativity arguments, i.e., as the difference in electronegativity between the two atoms in the B—X bond increases, the bond strength increases. Methyl substitution on B in the closed- and open-shell species increases the ionic contribution to the bond thereby decreasing the bond strength. The lowest possible homolytic dissociation energy for the free radicals RR'BY· is lower than those of their closed-shell counterparts, yet the B—Y· bonds are shorter. This is due to the demands of spin conservation in the dissociation of the radicals favouring the formation of higher energy products.Key words: ab initio calculations, bond dissociation energy, organoboron compounds, thermochemistry.

Calculation of aqueous dissociation constants of 1,2,4-triazole and tetrazole: A comparison of solvation models

The aqueous acidity constants of 1,2,4-triazole and tetrazole have been calculated using quantum chemical methods with four solvation models. Gas-phase proton transfer reaction energies and enthalpies of formation were calculated at the G3 level of theory. For tetrazole, the G3 energies all differ from the experimental values by more than 10 kJ mol−1, while for 1,2,4-triazole the agreement is significantly better. Solvation energies were calculated using the semi-empirical quantum mechanical method SM5.42R/A, the iterative Langevin dipole method iLD, the ab initio quantum mechanical polarisable continuum method IEF-PCM and the Poisson–Boltzmann method PB. The PB method consistently provided better agreement with experiment than the other methods when applied to the G3 energies, although the difference in pKa was as large as 2.6 for tetrazole. The acidity constants obtained using PB and iLD solvation energies applied to the experimental gas-phase proton dissociation energies are in good agreement with experiment.

Computational studies of the reaction of the hydroxyl radical with hydrofluorocarbons (HFCs) and hydrofluoroethers (HFEs)

The reaction rate constants of a hydroxyl radical with some hydrofluorocarbons and hydrofluoroethers have been calculated using “transition state theory”. A new computational method has been developed for transferring along the reaction coordinate the very high accuracy corrections, obtained at G2 level of theory, from small models (CH 4 and CF 3H) to larger molecules (halogenated ethanes and methyl ethers). The substituent effects on the CH bond strengths and on the reaction rate constants have been examined, as well as the geometrical and electronic features of the transition states.

The ultraviolet photochemistry of phenylacetylene and the enthalpy of formation of 1,3,5-hexatriyne.

The ultraviolet photochemistry of phenylacetylene was studied in a molecular beam at 193 nm. The only primary photofragments observed were HCCH (acetylene) and C(6)H(4). Some of the C(6)H(4) molecules were found to decompose to 1,3,5-hexatriyne and molecular hydrogen. An enthalpy of formation of DeltaH(f) < or = 160 +/- 4 kcal mol(-1) was determined for 1,3,5-hexatriyne from the energetic threshold for this process. This experimentally determined value agrees well with our ab initio calculations performed at the G2 level of theory. Angular distribution measurements for the HCCH + C(6)H(4) channel yielded an isotropic distribution and were attributed to a long-lived intermediate and ground-state dissociation. An exhaustive search yielded no evidence for the phenyl + ethynyl or the atomic hydrogen elimination channels even though these were observed in the pyrolytic studies of phenylacetylene [Hofmann, J.; Zimmermann, G.; Guthier, K.; Hebgen, P.; Homann, K. H. Liebigs Ann. 1995, 631, 1995. Guthier, K.; Hebgen, P.; Hofmann, K. H.; Zimmermann, G. Liebigs Ann. 1995, 637, 1995].

Generation and characterization of ionic and neutral (CH 3OBH) +/· and (CH 3BOH) +/· in the gas phase by tandem mass spectrometry

The isomeric dicoordinated borinium ions CH 3O–B–H + and CH 3–B–OH + are generated upon electron ionization of trimethylborate and methyl boronic acid, respectively. The connectivity of the ions is confirmed by collision-induced dissociation experiments on magnetic deflection type tandem mass spectrometers. Neutralization–reionization experiments on these structurally characterized ions indicate that the neutral radicals CH 3O–B–H · and CH 3–B–OH · are viable species in the gas phase. Calculations at the G2 level of theory were performed to obtain thermochemical data on the title isomers and their main dissociation products. The calculations also provide a rationale for the moderate yield of the neutrals generated in the experiments: the vertical electron transfer processes for both systems are associated with particularly unfavourable Franck-Condon factors.

G2 Molecular Orbital Study of [H3AlXH]- (X = NH, PH, AsH, O, S, and Se) and H3AlYH (Y = OH, SH, SeH, F, Cl, and Br) Donor−Acceptor Complexes

[H3AlXH]- (X = NH, PH, AsH, O, S, and Se) and H3AlYH (Y = OH, SH, SeH, F, Cl, and Br) have been investigated as donor−acceptor complex type at the G2 level of theory. Both staggered and eclipsed conformations have been examined. The G2 energetic results show that the anionic complexes are more stable than the neutral ones. They also show that this stability decreases when descending in the corresponding periodic table column, from nitrogen (or oxygen and fluorine) to arsenic (or selenium and bromine) atoms. The interaction diagrams prove that the evolution of complexation energy depends on the coordination mode. In fact, this is a simple “HOMO−LUMO” interaction for [H3AlXH]- anionic complexes while for the H3AlYH neutral ones it is a result of two interaction types: interaction between a‘ symmetry fragment molecular orbital (stabilizing) and interaction between a‘ ‘ symmetry fragment molecular orbitals (destabilizing). The NBO analysis shows that there is no correlation between the G2 complexation energy...

Origin of the Inversion of the Acidity Order for Haloacetic Acids on Going from the Gas Phase to Solution

The change in the relative acidities of the haloacetic acids on going from the gas phase to aqueous solution has been studied via ab initio calculations for the gas phase, the SCIPCM reaction field model for an aprotic polar solvent, and Monte Carlo statistical mechanics for aqueous solution. The relative gas phase acidities of acetic acid, the haloacetic acids and trifluoroacetic acid are well reproduced at the B3P86/6-311+G** and G2 levels of theory. The reaction field model reduced the relative acidities of fluoro- and bromoacetic acids to a value close to that found in aqueous solution, but it only reproduced half of the observed net effect of going from the gas phase to aqueous solution. Since the remainder of the solvent effect was probably due to hydrogen bonding, Monte Carlo calculations were carried out and they were in satisfactory accord with the aqueous solution pKa's.

Protonated Cyanogen Fluoride. Structure, Stability, and Reactivity of (FCN)H+ Ions

Gaseous (FCN)H+ ions were obtained by protonation of cyanuric fluoride by strong Bronsted acids such as H3+ (D3+) and CnH5+ (n = 1, 2), and their structure and reactivity were examined by FT-ICR mass spectrometry and computational methods at the G2 level of theory. The results show that FCNH+ is considerably more stable than the HFCN+ protomer, the gap amounting to 360 kJ mol-1 at the G2 level. The gas-phase basicity (GB) of FCN was estimated by “bracketing” experiments by measuring the efficiency of H+ transfer from FCNH+ to reference bases. Utilizing a general correlation between the rate and the ΔG° change of the proton-transfer we obtain GB(FCN) = 668.4 ± 10 kJ mol-1, to be compared with the G2 computed GB of 651.8 ± 8 kJ mol-1, both values being referred to N protonation. The reactions of FCNH+ with isolated (C2H4) and conjugated (C6H6) π systems were also investigated. The ion behaves as a FC+ = NH electrophile promoting aromatic cyanation, e.g., yielding C6H5CNH+ from benzene. Consideration is give...

Gas-Phase Acidities of Nine Sulfur Oxoacids of Composition [H2,S,On] (n = 1–4)

The gas-phase acidities of the most stable conformations of nine sulfur oxo acids were determined by ab initio MO calculations at the CBS-Q, G2(MP2), and G2 levels of theory. The most accurate G2 results are as follows (ΔG°298 of the deprotonation reaction in kJ mol–1): H–S–O–H 1468, H–S–O–H 1449, (HO)2S 1426, H–S(=O)OH 1406, H2S/O 1394, H–S(=O)OH 1361, (HO)2S/O 1324, H2S(=O)2 1321, H2O→S 1294, H–S(=O)2OH 1287, H–S(=O)2OH 1279, (HO)2S(=O)2 1268. These values are in excellent agreement with the few experimental data that can be considered reliable. For molecules of analogous structure, the acidity increases with increasing oxidation number of the sulfur atom. While sulfenic acid, HSOH, is as weak an acid as HOCl, sulfurous acid, (HO)2SO, is as strong a proton donor as nitric acid, HNO3, and sulfonic acid, HS(O)2OH, is even stronger than hydrogen iodide, HI, surpassed only by sulfuric acid, H2SO4. These results are of relevance to phenomena such as acid rain and aerosol formation in the earth′s atmosphere.

Generation and characterization of ionic and neutral dihydroxy boron B(OH) 2+/0 in the gas phase

The dicoordinated borinium ion, dihydroxyborinium, B(OH) 2 + is generated from methyl boronic acid CH 3B(OH) 2 by dissociative electron ionization and its connectivity confirmed by collisional activation. Neutralization–reionization (NR) experiments on this ion indicate that the neutral B(OH) 2 radical is a viable species in the gas phase. Both vertical neutralization of B(OH) 2 + and reionization of B(OH) 2 in the NR experiment are, however, associated with particularly unfavorable Franck-Condon factors. The differences in adiabatic and vertical electron transfer behavior can be traced back to a particular π stabilization of the cationic species compared to the sp 2-type neutral radical. Thermochemical data on several neutral and cationic boron compounds are presented based on calculations performed at the G2 level of theory.

Protonation as a means of manufacturing light trications: a case study of B 2H 33+

Multiple protonation of a stable species is investigated as an effective way of forming light, highly charged systems. As a test of these ideas, we report the structures, frequencies and thermochemistry of the metastable species B 2H 3 3+ at the G2 level of theory with additional calculations at the QCISD(T)/6-311G(d,p) level. The lowest energy isomer is a linear, singly bridged species while the second lowest isomer is triply bridged. In examining the thermochemistry related to the fragmentation of B 2H 3 3+, we report results for most of the mono-, di- and triprotonated forms of B, B 2 and B 2 +. Suprisingly, we note a positive proton affinity for B 2H +.

Can O−H Acid be More Acidic Than Its S−H Analog? A G2 Study of Fluoromethanols and Fluoromethanethiols

Acidities and homolytic O−H bond dissociation energies of CHnF3-nOH and CHnF3-nSH (n = 0−3) were calculated at the G2 level of theory. The variation of the geometry of the neutral species and products of heterolytic and homolytic proton abstraction processes, as well as the energetics (gas-phase acidities and homolytic bond dissociation energies of the title compounds), upon successive fluorine substitution has been studied. The results show that the progressive introduction of fluorine atoms into methanol and methanethiol reduces significantly the acidity gap between the representatives of these two series. Therefore, the calculations indicate that the acidities of CF3OH and CF3SH are predicted to be rather close: the former compound is expected to be 0.1 kcal/mol more acidic than its SH counterpart. It was concluded that negative (anionic) hyperconjugation and electrostatic effects are mainly responsible for such behavior.

Unimolecular Reactions of Proton-Bound Cluster Ions: Competition between Dissociation and Isomerization in the Methanol−Acetonitrile Dimer

The metastable proton-bound dimer of acetonitrile and methanol, (CH3CN)(CH3OH)H+, exhibits two unimolecular reactions on the microsecond time scale, a simple bond cleavage reaction to form CH3CNH+ and CH3OH, and the loss of water to form CH3CNCH3+. The latter process is preceded by the isomerization of the proton-bound dimer to a second isomer, (CH3CNCH3)(H2O)+. Collision-induced dissociation mass spectrometry was used to identify the two sets of reaction products, and the results of isotopic labeling experiments suggest that there is a rate-limiting isomerization step going from the proton-bound dimer to the second complex. The competition between the two channels was modeled with ab initio calculations and RRKM rate theory to obtain relative energies for the reaction surface. The transition structure for the rate-determining isomerization could not be located with ab initio calculations, and so its relative energy was estimated using RRKM theory. The 0 K binding energy of the (CH3CN)(CH3OH)H+ complex was calculated to be 121 kJ mol-1 at the G2 level of theory (relative to the dissociation products CH3CNH+ and CH3OH).

A G2 study of H 3B XH n ( X=N, O, F, P, S, and Cl) donor–acceptor complexes

Complexation energies of H 3B XH n complexes ( X=N, O, F, P, S, and Cl; n=1, 2, 3) have been computed at the G2 level of theory. G2 results show that the H 3B XH 3 ( X=N, P) complexes are more stable than H 3B XH 2 ( X=O, S) and H 3B XH ( X=F, Cl) ones. This stability was related completely either to the nature of donor compounds, and to the pyramidalization of the monoborane. Two linear correlations were established. The first one is between experimental proton affinity of the XH n donor compounds, and complexation energies of the H 3B XH n complexes. The second correlation is between the ∠HBH bond angles and the complexation energies of the H 3B XH n complexes calculated at the G2 level of theory.

PCCP

The optimized geometry of PCCP(X1Σg+) was calculated at a number of levels of theory, yielding a best D∞h geometry at the QCISD/6-311G(2d) level with RP-C=1.56Åand RC-C=1.37Å; harmonic vibrational frequencies were also calculated. The four lowest cationic states were studied, allowing the lowest four ionization energies to be obtained. At the G2 level of theory, the first adiabatic ionization energy was calculated to be 9.5 eV. The ordering of the four lowest photoelectron bands is found to be different to NCCN. The adiabatic electron affinity of PCCP is calculated at the G2 level to be 1.1 eV. Finally, the approximate positions of the lowest singlet excited electronic states of the neutral are computed.

Theoretical investigations of reactions between AlH(1Σ) and XνH (X=Cl, OH)

Reaction paths of AlH(1Σ) with X–H (X=Cl, OH) are characterized by the HF/6-31G(d), MP2(fc)/6-31G(d) and MP2(full)/6-31G(d) levels, respectively; energies of reactions and barriers are calculated by the G2 level of theory and MP4SDQ/6-31G(d)//MP2(fc)/6-31G(d). The calculations show that there are two parallel reaction channels: one is an addition reaction to give H2AlX via the three-membered ring transition state (TS); the other is a dehydrogenation reaction to give (AlX+H2) via the four-membered ring TS. General statistical thermodynamics (GST) and Eyring transition state theory (TST) with Wigner correction are also used to compute the thermodynamic functions, equilibrium constants, A factors, and rate constants of these reaction channels at 100–1100 K. The results show that the unique product (AlX+H2) is to be obtained.