Mikhail N. Glukhovtsev

The performance of B3-LYP density functional theory in describing SN2 reactions at saturated carbon

Abstract The performance of the B3-LYP variant of density functional theory when used in conjunction with the 6-31G(d) and 6-311 + G(3df, 2p) basis sets in describing the prototypical gas-phase SN2 reactions of Cl− + CH3Cl and CH3Br has been examined in detail. Reasonable values of the complexation energies (ΔHcomp) for the ion-molecule complexes formed in these reactions are obtained. However, the overall (ΔHovr#) and central (ΔHcent#) barriers for these reactions calculated using the B3-LYP functional are significantly underestimated when compared with G2(+) or experimental results. This implies that the B3-LYP energies for the Cl(H3C)Cl− (D3h) and Cl(H3C)Br− (C3v) transition structures are relatively too low. The B3-LYP errors appear to be systematic, with similar errors being found for corresponding quantities for the two reactions examined.

Is the stereomutation of methane possible

Large basis set ab initio calculations at correlated levels, including MP2, single reference, as well as multireference configuration interaction, carried out on the methane potential energy surface, have located and characterized a transition structure for stereomutation (one imaginary frequency). This structure is best described as a pyramidal complex between singlet methylene and a side‐on hydrogen molecule with Cs symmetry. At the single reference CI level, it lies 105 kcal/mol above the methane Td‐ground state but is stable relative to dissociation into CH2(1A1) and H2 by 13 kcal/mol at 0 K (with harmonic zero point energy (ZPE) corrections for all structures). Dissociation of the transition state into triplet methylene and hydrogen also is endothermic (by 4 kcal/mol), but single bond rupture to give CH  3. and H. is 3 kcal/mol exothermic. Thus, it does not appear likely that methane can undergo stereomutation classically beneath the dissociation limit. Confirming earlier conclusions, side‐on insertion of 1A1 CH2 into H2 in a perpendicular geometry occurs without activation energy. Planar (D4h) methane (130.5 kcal/mol) has four imaginary frequencies. Two of these are degenerate and lead to equivalent planar C2v structures with one three‐center, two‐electron bond and two two‐electron bonds and two imaginary frequencies. The remaining imaginary frequencies of the D4h form lead to tetrahedral (Td) and pyramidal (C4v) methane. The latter has three negative eigenvalues in the force‐constant matrix; one of these leads to the Td global minimum and the other to the Cs (parallel) stereomutation transition structure. Multireference CI calculations with a large atomic natural orbitals basis set produce similar results, with the electronic energy of the Cs stereomutation transition state 0.7 ± 0.5 kcal/mol higher than that of CH  3. + H. dissociation products, and a ZPE‐corrected energy which is 5 ± 1 kcal/mol higher. Also considered are photochemical pathways for stereomutation and the possible effects of nuclear spin, inversion tunneling, and the parity‐violating weak nuclear interaction on the possibility of an experimental detection of stereomutation in methane.

Ab initio study on the thermochemistry of vinyl radical and cation

Abstract The G2 calculations have been used to resolve current discrepancies in the experimental data on the thermochemistry of vinyl radical ( 1 ) and cation ( 3 ). The calculated enthalpies of formation for 1 and 3 (Δ H f 298 ( 1 ) =301.4 and Δ H f 298 ( 3 ) =1116.8 kJ/mol) agree well with some recent experimental estimates. While the G2 value of the adiabatic ionization energy for 1 (8.61 eV) differs from the recommended value of 8.25 eV, it is in very good agreement with the earlier experimental data of J. Berkowitz, C.A. Mayhew and R. Ruscic (J. Chem. Phys. 88 (1988) 7396): 8.59±0.03 eV.

Isodesmic and homodesmotic stabilization energies of [n]annulenes and their relevance to aromaticity and antiaromaticity: is absolute antiaromaticity possible?

Abstract Isodesmic stabilization energies (ISEs) for conjugated molecules, which are determined by a balance of the stabilizing π-electron non-cyclic delocalization and either stabilizing aromatic or destabilizing antiaromatic π-electron cyclic delocalization, do not yield a reasonable estimation of aromaticity or antiaromaticity. Homodesmotic stabilization energies (HSEs) should be used for this purpose. The ISEs for [n]annulenes are larger by the ( n 2 ) ISE(s-trans-1,3-butadiene) value than the corresponding HSEs. The experimental value of the ISE(s-trans-1,3-butadiene) is 59.4 ± 4.1 kJ mol−1 at 298 K. While cyclooctatetraene has only the relative antiaromatic character, cyclobutadiene features both relative and absolute antiaromaticity. Since the HSE and ISE of cyclobutadiene, corrected for its strain energy, are still negative (−174.0 and −55.2 kJ mol−1, respectively, at the MP4SDTQ/6-31G (d,p)//MP2/6-31G(d,p) + ZPE(HF/6-31G(d)) level), π-antiaromaticity rather than σ-strain is at the origin of the remarkable instability of cyclobutadiene. At the MP4SDTQ/6-31G(d,p)//MP2/6-31G(d,p) + ZPE(HF/6-31G(d)) level, the antiaromatic destabilization of cyclobutadiene corrected for the ring-strain and normalized per π-electron is −43.5 kJ mol−1, a value that is larger (in absolute value) than the antiaromatic destabilization of planar D4h cyclooctatetraene (−14.9 kJ mol−1 per π-electron) and the aromatic stabilization of benzene (17.7 kJ mol−1 per π-electron). In contrast to cyclobutadiene, planar D4h cyclooctatetraene has a positive ISE (118.8 kJ mol−1) indicating that the stabilizing π-electron non-cyclic delocalization overbalances the destabilizing antiaromatic π-electron cyclic delocalization. Nevertheless, planar cyclooctatetraene is assigned to the class of antiaromatic molecules since its HSE is −118.8 kJ mol−1 at the MP4SDTQ/6-31G(d,p)//MP2/6-31G(d,p) + ZPE(HF/6-31G(d)) level.

High-level computational study on the thermochemistry of saturated and unsaturated three- and four-membered nitrogen and phosphorus rings

The heats of formation and strain energies for saturated and unsaturated three- and four-membered nitrogen and phosphorus rings have been calculated using G2 theory. G2 heats of formation (ΔHf298) of triaziridine [(NH)3], triazirine (N3H), tetrazetidine [(NH)4], and tetrazetine (N4H2) are 405.0, 453.7, 522.5, and 514.1 kJ mol−1, respectively. Tetrazetidine is unstable (121.5 kJ mol−1 at 298 K) with respect to its dissociation into two trans-diazene (N2H2) molecules. The dissociation of tetrazetine into molecular nitrogen and trans-diazene is highly exothermic (ΔH298 = −308.3 kJ mol−1 calculated using G2 theory). G2 heats of formation (ΔHf298) of cyclotriphosphane [(PH)3], cyclotriphosphene (P3H), cyclotetraphosphane [(PH)4], and cyclotetraphosphene (P4H2) are 80.7, 167.2, 102.7, and 170.7 kJ mol−1, respectively. Cyclotetraphosphane and cyclotetraphosphene are stabilized by 145.8 and 101.2 kJ mol−1 relative to their dissociations into two diphosphene molecules or into diphosphene (HP(DOUBLE BOND)PH) and diphosphorus (P2), respectively. The strain energies of triaziridine [(NH)3], triazirine (N3H), tetrazetidine [(NH)4], and tetrazetine (N4H2) were calculated to be 115.0, 198.3, 135.8, and 162.0 kJ mol−1, respectively (at 298 K). While the strain energies of the nitrogen three-membered rings in triaziridine and triazirine are smaller than the strain energies of cyclopropane (117.4 kJ mol−1) and cyclopropene (232.2 kJ mol−1), the strain energies of the nitrogen four-membered rings in tetrazetidine and tetrazetine are larger than those of cyclobutane (110.2 kJ mol−1) and cyclobutene (132.0 kJ mol−1). In contrast to higher strain in cyclopropane as compared with cyclobutane, triaziridine is less strained than tetrazetidine. The strain energies of cyclotriphosphane [(PH)3, 21.8 kJ mol−1], cyclotriphosphene (P3H, 34.6 kJ mol−1), cyclotetraphosphane [(PH)4, 24.1 kJ mol−1], and cyclotetraphosphene (P4H2, 18.5 kJ mol−1), calculated at the G2 level are considerably smaller than those of their carbon and nitrogen analog. Cyclotetraphosphene containing the P(DOUBLE BOND)P double bond is less strained than cyclotetraphosphane, in sharp contrast to the ratio between the strain energies for the analogous unsaturated and saturated carbon and nitrogen rings.

METHYLENE-IODONIUM YLIDE : AN ISOMER OF DIIODOMETHANE

Abstract Iso-diiodomethane, H 2 CII is a minimum at the B3LYP/6–31G(d) and MP2/6–31G(d) levels and is separated from conventional isomer, diiodomethane, by a barrier of 134.6 kJ mol −1 calculated using G2 theory. The G2 calculated heats of formation for diiodomethane and iso-diiodomethane are 136.1 and 317.8 kJ mol −1 , respectively. The enthapy change of the H 2 CII → H 2 CI + I( 3 P) dissociation is only 19.4 kJ mol −1 . The charge distribution for H 2 CII does not agree with the description of this species as a hypervalent structure with a positive charge on the carbon and a negative charge on the central iodine.

Is the most stable gas-phase isomer of the benzenium cation a face-protonated π-complex?

The recent suggestion, based on gas-phase experimental data, that the most stable isomer of protonated benzene has a face-protonated π-complex structure is not supported by our detailed computations which indicate that the π-complex is a second-order saddle point on the potential energy surface, lying 199 kJ mo–1 higher in energy than the well-established C2vσ-protonated structure.

Computational study on nature of transition structure for oxygen transfer from dioxirane and carbonyloxide

The relative reactivity of a series of nucleophiles that includes ethylene, sulfides, sulfoxides, amines, and phosphines toward dioxirane, dimethyldioxirane, carbonyloxide and dimethylcarbonyloxide has been examined at the MP4/6‐31G*//MP2/6‐31G*, QCISD(T)/6‐31G*//MP2/6‐31G*, and B3‐LYP/6‐31G* levels of theory. The barriers for the oxidations with dimethyldioxirane are higher (up to 2.5 kcal/mol for the oxidation of H2S) than those for the oxidations with the parent dioxirane. The oxidation barriers for dioxirane are larger than those for the oxidations with peroxyformic acid, except the barriers for the oxidation of sulfoxides. The reactivity of dimethylsulfide toward dimethyldioxirane was found to be comparable to that of dimethylsulfoxide both in the gas phase and in solution (chloroform). The classical gas phase barrier for the oxidation of trimethylamine to trimethylamine oxide was higher (6.3 kcal/mol at the MP4//MP2/6‐31G* level) than that for oxygen atom transfer to trimethylphosphine. When the transition states were examined by self‐consistent reaction field (SCRF) methods, the predicted barriers for the oxidation of amines and phosphines were found to be in good agreement with experiment. The general trend in reactivity for oxidation by dioxirane was R2S≈R2SO, R3P>R3N in the gas phase, and R2S≈R2SO, R3N≈R3P (R=Me) in solution. The oxidation barriers calculated using the B3‐LYP functional were lower than those computed at the MP4 and QCISD(T) levels. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 1353–1369, 1998

The structure of the methanol radical cation: an artificially short CO bond with MP2 theory

Abstract A detailed reinvestigation of the structure of the methanol radical cation has been undertaken using a variety of correlated theoretical procedures. These include the B3-LYP density functional method, second-order Moller-Plesset theory (MP2), quadratic configuration interaction (QCISD, QCISD(T)), Brueckner doubles (BD) and coupled-cluster theory (CCSD(T)). At the highest level of theory employed in this study (CCSD(T)/6–311G(df,p)), the preferred structure of the methanol radical cation has an eclipsed conformation and a CO bond length of 1.370 A. The structure is found to be strongly influenced by hyperconjugation. MP2, in combination with large basis sets, overestimates the effects of hyperconjugation, predicting CH 3 OH ·+ to have a CO bond length of less than 1.3 A. The use of a moderately large basis set that includes f-functions on the heavy atoms, and a high-level electron correlation procedure, are important in accurately determining the structure of the methanol radical cation.

Ab initio study on the thermochemistry of diphosphine (P2H4) and diphosphine radical cation (P2H+4)

Abstract The G2 calculated heat of formation of diphosphine, Δ H f298 (H 2 PPH 2 ) is 35.3 ± 8.3 kJ mol −1 . This is closer to the earlier experimental estimates of 20.9 ± 4.2 kJ mol −1 and 41.4 kJ mol −1 than to the ΔH f298 value of 69 kJ mol −1 given in the widely used compilation of Lias et al. The diphosphine radical cation, H 2 PPH + 2 ; has a non-planar C 2h ground state structure and its heat of formation of H 2 PPH + 2 ( Δ H f28 ) is 893.2 ± 2.0 kJ mol −1 . The adiabatic ionization energy of diphosphine, H 2 P-PH 2 , was calculated using G2 theory to be 8.87 eV.