The anomeric effect: Ab-initio studies on molecules of the type X?CH2?O?CH3

The anomeric effect of the functional groups X = C?N, C?CH, COOH, COO^?, O? CH₃, NH₂, and NH⁺₃ has been studied with ab initio techniques. Geometry effects upon rotation around the central C? O bond in X? CH₂? O? CH₃ have been compared in the various compounds. The energy differences between the conformers with a gauche and trans (X? C? O? C) arrangement were calculated at the 6-31G* level in the fully optimized 4-21G geometries. Energy differences calculated at the 4-21G level appeared not to be reliable, especially for the groups X that contain non-sp³ hybridized atoms. The 6-31G* energy differences indicate a normal anomeric effect for X = COO^?, O? CH₃, and NH₂(g⁺) (ca. 13 kJ/mol) and a small anomeric effect for X = COOH, C?N, and C?CH (ca. 6 kJ/mol). For X = NH₂(t) and NH⁺₃ a reverse anomeric effect occurs. These observations are in line with experimental results and evidence is given for a competition among various stereoelectronic interactions that occur at the same anomeric center. Geometry variations can be understood in terms of simple rules associated with anomeric orbital interactions. Trends followed when the group X is varied cannot be related in a straightforward way to the energy differences between the trans and the gauche forms in these compounds. Only the variation in the gauche torsion angle X? C? O? C follows roughly the same trend.     

Force field parameters for sulfates and sulfamates based on ab initio calculations: Extensions of AMBER and CHARMm fields

Ab initio self-consistent field (SCF) Hartree-Fock calculations of sulfates R? O? SO₃(?1) (R = Me, Et, i-Pr) and sulfamates R? NHSO₃(?1) (R = H, Me, Et, i-Pr) were performed at the 4-31G(*S*N) //3-21G(*S*N) basis set levels, where asterisks indicate d functions on sulfur and nitrogen atoms. These standard levels were determined by comparing calculation results with several basis sets up to MP2/6-31G*//6-31G*. Several conformations per compound were studied to obtain molecular geometries, rotational barriers, and potential derived point charges. In methyl sulfate, the rotational barrier around the C? O bond is 1.6 kcal/mol at the MP2 level and 1.4 kcal/mol at the standard level. Its ground state has one of three HCOS torsion angles trans and one of three COSO torsion angles trans. Rotation over 60° around the single O? S bond in the sulfate group costs 2.5 kcal/mol at the MP2 and 2.1 kcal/mol at the standard level. For ethyl sulfate, the calculated rotational barrier in going from the ground state, which has its CCOS torsion angle trans, to the syn-periplanar conformation (CCOS torsion angle cis) is 4.8 kcal/mol. However, a much lower barrier of 0.7 kcal/mol leads to a secondary gauchelike conformation about 0.4 kcal/mol above the ground state, with the CCOS torsion angle at 87.6°. Again, one of the COSO torsion angles is trans in the ground state, and the rotational barrier for a 60° rotation of the sulfate group amounts to 1.8 kcal/mol. For methyl sulfamate, the rotational barriers are 2.5 kcal/mol around the C? N bond and 3.3 kcal/mol around the N? S bond. This is noteworthy because sulfamate itself has a calculated rotational barrier around the N? S bond of only 1.7 kcal/mol. These and other data were used to parameterize the well-known empirical force fields AMBER and CHARMm. When the new fields were tested by means of vibrational frequency calculations at the 6-31G*//6-31G* level for methyl sulfate, sulfamate, and methyl sulfamate ground states, the frequencies compared favorably with the AMBER and CHARMm calculated frequencies. The transferability of the force parameters to β-D -glucose-6-sulfate and isopropyl sulfate appears to be better than to isopropyl sulfamate. © 1995 by John Wiley & Sons, Inc.     

Energetics of proton transfer between carbon atoms (H3CH ? CH3)−

Ab initio calculations were carried out to study the potential energy surface of (H₃C? H? CH₃)^?. The 6–31G* basis set is supplemented by a set of diffuse p functions on both C and H (with a range of exponents for the latter). The binding energy of CH₄ and CH₃^? to form the (H₃CH? CH₃)^? complex is about 2 kcal/mol, much smaller than for comparable ionic H-bonded systems involving O or N atoms. Nearly half of this interaction energy is due to correlation effects, computed at second and third orders of Møller-Plesset perturbation theory. Correlation is also responsible for substantial reductions in the energy barrier to proton transfer within the complex. This barrier is computed to be 13?15 kcal/mol at the MP3 level, depending upon the exponent used for the H p functions.     

Theoretical study of substituent effects on the acidity of the methyl group: Structure of anions CH2X−

Geometries have been optimized using molecular-orbital calculations (a) with a 4-31G Gaussian basis set for carbanions CH₂X^? where X = H, CH₃, NH₂, OH, F, C?CH, CH?CH₂, CHO, COCH₃, CN, and NO₂; and (b) with an STO -3G basis set for methyl acetate and acetyl deprotonated methyl acetate. All the carbanions containing unsaturated substituents are planar, with a considerable shortening of the C? X bond. Carbanions containing saturated substituents are pyramidal with the out-of-plane angle α increasing with the electronegativity of the substituent. Double-zeta basis set calculations give proton affinities over the range 449 (for CH₃CH₂^?) to 355 kcal/mol (for CH₂NO₂^?), with all unsaturated anions having smaller affinities than saturated anions. The correlation of proton affinities with 1s binding energies, and with charges on both the carbon of the anion and on the acidic proton of the neutral molecule are examined.     

Ab initio calculations on the barrier height for the hydrogen addition to ethylene and formaldehyde. The importance of spin projection

Heats of reaction and barrier heights have been computed for H + CH₂CH₂ → C₂H₅, H + CH₂O → CH₃O, and H + CH₂O → CH₂OH using unrestricted Hartree-Fock and Møller–Plesset perturbation theory up to fourth order (with and without spin annihilation), using single-reference configuration interaction, and using multiconfiguration self-consistent field methods with 3-21G, 6-31G(d), 6-31G(d,p), and 6-311G(d,p) basis sets. The barrier height in all three reactions appears to be relatively insensitive to the basis sets, but the heats of reaction are affected by p-type polarization functions on hydrogen. Computation of the harmonic vibrational frequencies and infrared intensities with two sets of polarization functions on heavy atoms [6-31G(2d)] improves the agreement with experiment. The experimental barrier height for H + C₂H₄ (2.04 ± 0.08 kcal/mol) is overestimated by 7?9 kcal/mol at the MP2, MP3, and MP4 levels. MCSCF and CISD calculations lower the barrier height by approximately 4 kcal/mol relative to the MP4 calculations but are still almost 4 kcal/mol too high compared to experiment. Annihilation of the largest spin contaminant lowers the MP4SDTQ computed barrier height by 8?9 kcal/mol. For the hydrogen addition to formaldehyde, the same trends are observed. The overestimation of the barrier height with Møller-Plesset perdicted barrier heights for H + C₂H₄ → C₂H₅, H + CH₂O → CH₃O, and H + CH₂O → CH₂OH at the MP4SDTQ /6-31G(d) after spin annihilation are respectively 1.8, 4.6, and 10.5 kcal/mol.     

Structures and stabilities of [C3H3O]+ ions in the gas phase: A molecular orbital study

The relative energies of 11 [C₃H₃O]⁺ ions are calculated by different molecular orbital methods (MINDO/3, MNDO, ab initio with 3-21G and 4-31G* basis set and configuration interaction). The four most stable structures are: a ([CH₂?CH? CO]⁺), b c ([CH?C? CHOH]⁺) and d ([CH₂?C?COH]⁺); their relative energies at the CI/4-31G*//3-21G level are 0, 117, 171 and 218 kJ mol^?1, respectively. The isomerizations c→[CH?CH? CHO]⁺→[CH₂?C? CHO]⁺→ a and dissociations into [C₂H₃]⁺+CO and [HCO]⁺+C₂H₂ are explored. The calculated potential energy profile reveals that the energy-determining step is the 1,3-H migration c→[CH?CH? CHO]⁺. This explains the value of unity of the branching ratio and the spread of kinetic energy released for the two dissociation channels.     

Degenerate lithium-hydrogen exchange reactions: Ab initio models for metallation mechanisms involving H2, CH4, NH3, H2O,and HF

Hydrogen exchange reactions between lithium and sodium compounds, MX (M=Li: X=H, CH₃, NH₂, OH, F; M=Na: X=CH₃), and the corresponding hydrides, HX, have been modelled by means of ab initio calculations including electron correlation and zero point energy (ZPE) corrections. Small or no activation barriers (from the initial complexes) are encountered in systems involving lone pairs (10.8, 2.4, 0.0 kcal/mol for X=NH₂, OH, F, respectively). Since the association energies of the initial complexes are much larger (21.0, 20.4, 23.5 kcal/mol, respectively; MP2/6–31+G*/6–31+G* + ZPE), such exchange reactions should occur spontaneously in the gas phase. The methyl systems (X=CH₃) have the largest barriers: 26.7 (M=Li) and 31.7 (M=Na) kcal/mol (MP2/6–31+G*/6–31G* + ZPE), and the initial complexes are only weakly bound. The significance of these systems as models for hydrogen exchange reactions in complexes of electropositive transition metals is discussed. However, the gegenion-free exchange of hydrogen between CH₃ and CH₄ has a much lower, 11.8 kcal/mol barrier (MP2/6–31+G*/6–31+G* + ZPE). All the transition structures are highly ionic (charges on the metals > +0.8). The effect of aggregation has been considered by examining the hydrogen exchange between (LiX)₂ and HX(X=H, CH₃, NH₂, OH). Although these dimer reactions formally involve six, instead of four electrons, no “aromatic” preference is observed.     

Ab initio studies of structural features not easily amenable to experiment. 25. Conformational analysis of methyl propanoate and comparison with the methyl ester of glycine

The molecular geometries of three conformations of methyl propanoate (MEP) (C? C? C?O torsions of 0°, 120°, and 180°) and the potential-energy surfaces of MEP (C? C? C?O torsions) and of the methyl ester of glycine (MEG) (N? C? C?O torsions) have been determined by ab initio gradient calculations at the 4-21G level. MEP has conformational energy minima at 0° and 120° of the C? C? C?O torsion, while the 60–90° range and 180° are energy maxima. For MEG there are two minima (at 0° and 180°) and one barrier to N? C? C?O rotation in the 60–90° range. The N? C? C?O barrier height is about twice as high (4 kcal/mol) as the C? C? C?O barrier. The 180° N? C? C?O minimum is characteristically wide and flat allowing for considerable flexibility of the N? C? C?O torsion in the 150–210° range. This flexibility could be of potential importance for polypeptide systems, since the N? C? C?O angles of helical forms are usually found in this region. The molecular structures of the methyl ester group CH₃OC(?O)CHRR′ in several systems are compared and found to be rather constant when R ? H and R′ ? H, CH₃, CH₃CH₂; or when R ? NH₂ and R′ ? H, CH₃, or CH(CH₃)₂.     

An SCF and CI Study of the 1,3 Shift in the HX?CHY⇌XCH?YH Isoelectronic Series: X,YCH2, NH,and O

Ab initio molecular orbital calculations at SCF level with the 3-21G, 6-31G, and 6-31G** basis sets and CI level with the 6-31G basis set have been carried out for an isoelectronic series HX? CH?Y and X?CH? YH, where X, Y can be CH₂, NH, and O. Optimized structures (3-21G and 6-31G**) for both tautomers and the 1,3 hydrogen shift transition states are reported. The relative stabilities of the isomers and the barriers of the 1,3 shift are discussed in terms of proton affinities and bond orders. It is shown that both the relative stabilities of the tautomers and the relative barrier heights can be explained qualitatively using simple proton affinity arguments and that the barrier heights are quantitatively related to bond orders.     

The remarkably invariant interaction energies of lithium first-row compounds with water and with ammonia

Solvation energies of lithium first-row compounds LiX (X ? H, Li, BeH, BH₂, CH₃, NH₂, OH, F) and of the lithium cation with the model solvents, water and ammonia, have been calculated ab inito (MP2/6-31 + G^*//6-31G^* with zero-point vibrational energy corrections at 3-21G//3-21G). The solvation energies are found to be remarkably constant: ?18.0 ± 1.2 and ?21.5 ± 1.3 kcal/mol for the hydrates and ammonia solvates, respectively. This independence on the nature of X is due largely to the ionic character of the LiX compounds (dipole moments 4.7–6.6 debye). The unexpectedly high solvation energies of the lithium molecule (?14.3 and ?17.8 kcal/mol, respectively) are due to the polarizability of Li₂. At the same level, the lithium cation has interaction energies with H₂O and NH₃ of ?34.1 and ?39.7 kcal/mol, respectively. For the hydrates of LiOH and LiF cyclic structures with hydrogen bonds and somewhat increased solvation energies also are described.     

Hetero‐π‐systems from 2 + 2 cycloreversion,part 2.1Ab initio thermochemical study of heterocyclobutanes 2 + 2 cycloreversion to form heteroethenes H2C=X (X=NH,O, SiH2, PH,S)

Ab initio and DFT thermochemical study of diradical mechanism of 2 + 2 cycloreversion of parent heterocyclobutanes and 1,3‐diheterocyclobutanes, cyclo‐(CH₂CH₂CH₂X), and cyclo‐(CH₂XCH₂X), where X = NH, O, SiH₂, PH, S, was undertaken by calculating closed‐shell singlet molecules at three levels of theory: MP4/6‐311G(d)//MP2/6‐31G(d)+ZPE, MP4/6‐311G(d,p)//MP2/6‐31G (d,p)+ZPE, and B3LYP/6‐311+G(d,p)+ZPE. The enthalpies of 2 + 2 cycloreversion decrease on going from group 14 to group 16 elements, being substantially higher for the second row elements. Normally endothermic 2 + 2 cycloreversion is predicted to be exothermic for 1,3‐diazetidine and 1,3‐dioxtane. Strain energies of the four‐membered rings were calculated via the appropriate homodesmic reactions. The enthalpies of ring opening via the every possible one‐bond homolysis that results in the formation of the corresponding 1,4‐diradical were found by subtracting the strain energies from the central bond dissociation energies of the heterobutanes CH₃CH₂—CH₂XH, CH₃CH₂—XCH₃, and HXCH₂—XCH₃. The latter energies were determined via the enthalpies of the appropriate dehydrocondensation reactions, using C—H and X—H bond energies in CH₃XH calculated at G2 level of theory. Except 1,3‐disiletane, in which ring‐opening enthalpy attains 69.7 kcal/mol, the enthalpies of the most economical ring openings do not exceed 60.7 kcal/mol. The 1,4‐diradical decomposition enthalpies found as differences between 2 + 2 cycloreversion and ring‐opening enthalpies were negative, the least exothermicity was calculated for ⋅ CH₂SiH₂CH₂CH₂. The only exception was 1,3‐disiletane, which being diradical, CH₂SiH₂CH₂SiH₂, decomposed endothermically. Since decomposition of the diradical containing two silicon atoms required extra energy, raising the enthalpy of the overall reaction to 78.9 kcal/mol, 1,3‐disiletane was predicted to be highly resisting to 2 + 2 cycloreversion. © 2007 Wiley Periodicals, Inc. Heteroatom Chem 18:704–720, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20377     

Long‐range π‐type hydrogen bond in dimers CH2?HF,CH2O?H2O,and CH2O?NH3

Using four basis bets, (6‐311G(d,p), 6‐31+G(d,p), 6‐31++G(2d,2p), and 6‐311++G(3df,3pd), the optimized structures with all real frequencies were obtained at the MP2 level for the dimers CH₂O? HF, CH₂O? H₂O, CH₂O? NH₃, and CH₂O? CH₄. The structures of CH₂O? HF, CH₂O? H₂O, and CH₂O? NH₃ are cycle‐shaped, which result from the larger bend of σ‐type hydrogen bonds. The bend of σ‐type H‐bond O…H? Y (Y?F, O, N) is illustrated and interpreted by an attractive interaction of a chemically intuitive π‐type hydrogen bond. The π‐type hydrogen bond is the interaction between one of the H atoms of CH₂O and lone pair(s) on the F atom in HF, the O atom in H₂O, or the N atom in NH₃. In contrast with the above three dimers, for CH₂O? CH₄, because there is not a π‐type hydrogen bond to bend its linear hydrogen bond, the structure of CH₂O? CH₄ is noncyclic shaped. The interaction energy of hydrogen bonds and the π‐type H‐bond are calculated and discussed at the CCSD (T)/6‐311++G(3df,3pd) level. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Structure and formation of gaseous [C4H6O]+˙ ions: 1—The enolic ions [CH2C(OH)?CHCH2]+˙ and [CH2CH?CHCH(OH)]+˙ and their relationship with their keto counterparts

The [C₄H₆O]^+˙ ion of structure [CH₂?CHCH?CHOH]^+˙ (a) is generated by loss of C₄H₈ from ionized 6,6-dimethyl-2-cyclohexen-1-ol. The heat of formation ΔH_f of [CH₂?CHCH?CHOH]^+˙ was estimated to be 736 kJ mol^?1. The isomeric ion [CH₂?C(OH)CH?CH₂]^+˙ (b) was shown to have ΔH_f, ? 761 kJ mol^?1, 54 kJ mol^?1 less than that of its keto analogue [CH₃COCH?CH₂]^+˙. Ion [CH₂?C(OH)CH?CH₂]^+˙ may be generated by loss of C₂H₄ from ionized hex-1-en-3-one or by loss of C₄H₈ from ionized 4,4-dimethyl-2-cyclohexen-1-ol. The [C₄H₆O]^+˙ ion generated by loss of C₂H₄ from ionized 2-cyclohexen-1-ol was shown to consist of a mixture of the above enol ions by comparing the metastable ion and collisional activation mass spectra of [CH₂?CHCH?CHOH]^+˙ and [CH₂?C(OH)CH?CH₂]^+˙ ions with that of the above daughter ion. It is further concluded that prior to their major fragmentations by loss of CH₃˙ and CO, [CH₂?CHCH?CHOH]⁺˙ and [CH₂?C(OH)CH?CH₂]^+˙ do not rearrange to their keto counterparts. The metastable ion and collisional activation characteristics of the isomeric allenic [C₄H₆O]^+˙ ion [CH₂?C?CHCH₂OH]^+˙ are also reported.     

On the Effect of Tether Composition on cis/trans Selectivity in Intramolecular Diels–Alder Reactions

Intramolecular Diels–Alder (IMDA) transition structures (TSs) and energies have been computed at the B3LYP/6‐31+G(d) and CBS‐QB3 levels of theory for a series of 1,3,8‐nonatrienes, H₂C?CH? CH?CH? CH₂? X? Z? CH?CH₂ [? X? Z? =? CH₂? CH₂? ( 1 ); ? O? C(?O)? ( 2 ); ? CH₂? C(?O)? ( 3 ); ? O? CH₂? ( 4 ); ? NH? C(?O)? ( 5 ); ? S? C(?O)? ( 6 ); ? O? C(?S)? ( 7 ); ? NH? C(?S)? ( 8 ); ? S? C(?S)? ( 9 )]. For each system studied ( 1 – 9 ), cis‐ and trans‐TS isomers, corresponding, respectively, to endo‐ and exo‐positioning of the ? C? X? Z? tether with respect to the diene, have been located and their relative energies (E_rel^TS) employed to predict the cis/trans IMDA product ratio. Although the E_rel^TS values are modest (typically <3 kJ mol^?1), they follow a clear and systematic trend. Specifically, as the electronegativity of the tether group X is reduced (X?O→NH or S), the IMDA cis stereoselectivity diminishes. The predicted stereochemical reaction preferences are explained in terms of two opposing effects operating in the cis‐TS, namely (1) unfavorable torsional (eclipsing) strain about the C4? C5 bond, that is caused by the ? C? X? C(?Y)? group’s strong tendency to maintain local planarity; and (2) attractive electrostatic and secondary orbital interactions between the endo‐(thio)carbonyl group, C?Y, and the diene. The former interaction predominates when X is weakly electronegative (X?N, S), while the latter is dominant when X is more strongly electronegative (X?O), or a methylene group (X?CH₂) which increases tether flexibility. These predictions hold up to experimental scrutiny, with synthetic IMDA reactions of 1 , 2 , 3 , and 4 (published work) and 5 , 6 , and 8 (this work) delivering ratios close to those calculated. The reactions of thiolacrylate 5 and thioamide 8 represent the first examples of IMDA reactions with tethers of these types. Our results point to strategies for designing tethers, which lead to improved cis/trans‐selectivities in IMDAs that are normally only weakly selective. Experimental verification of the validity of this claim comes in the form of fumaramide 14 , which undergoes a more trans‐selective IMDA reaction than the corresponding ester tethered precursor 13 .     

Theory-enforced Re-investigation of the Origin of the Large Metal–Carbon Bond Strength in PdCH2I+ and its reactions with unsaturated hydrocarbons

State-of-the-art ab initio studies demonstrate that the reaction Pd⁺ + CH₃I → PdCH₂I⁺ + H^. is endothermic by ca. 20 kcal/mol, which translates into a bond dissociation energy (BDE) of ca. 83 kcal/mol for the Pd⁺? CH₂I bond. This figure is in agreement with an experimental bracket of 68 kcal/mol < BDE(Pd⁺? CH₂I) < 92 kcal/mol. Based on these findings, the previously studied Pd⁺/CH₃I system was re-investigated, and double-resonance experiments demonstrate that the formation of PdCH₂I⁺ occurs stepwise via PdCH as a reactive intermediate. Further, ion/molecule reactions of PdCH₂I⁺ with unsaturated hydrocarbons are studied, which reveal the formation of carbon–carbon bonds in the gas phase.     

Quantum-Chemical Study of Alkyl Carbenium Ions in 100% Sulfuric Acid

A quantum-chemical study of neutral and protonated monoalkyl sulfates RHSO₄and [RH₂SO₄]⁺(where R = CH₃, C₂H₅, iso-C₃H₇, and tert-C₄H₉) is carried out. Calculations are performed using the Hartree–Fock method in the 6-31G** and 6-31++G** basis sets taking into account electron correlation according to the Müller–Plesset perturbation theory MP2/6-31+G*//6-31+G*. Protonated tert-butyl sulfate was also calculated by the DFT B3LYP/6-31++G** method. It was found that monoalkyl sulfates are covalent compounds, and the complete abstraction of alkyl carbenium ions from them has substantial energy cost: 196.4, 161.7, 150.8 and 136.0 kcal/mol, respectively. Protonated methyl and ethyl sulfates are also covalent compounds according to the calculation. They have lower but still high energies of heterolytic dissociation (65.0 and 33.5 kcal/mol, respectively). The energy of R⁺abstraction from protonated isopropyl sulfate is much lower: 23.6 kcal/mol. The main covalent state and the ion–molecular pair, which is a carbenium ion [C(CH₃)₂H]⁺solvated by the H₂SO₄molecule, have about the same energy. The ground state of protonated tert-butyl sulfate corresponds to the ion–molecular complex [C(CH₃)⁺ ₃H₂SO₄] with still lower energy of carbenium ion [C(CH₃)₃]⁺abstraction, which is equal to 10.0 kcal/mol. Calculation according to the DFT B3LYP/6-31++G** method shows the absence of a minimum for the protonated tert-butyl sulfate with a covalent structure on the potential energy surface.     

A theoretical study of C4H5N isomers: Comparison of cyano and isocyano as substituents

Ab initio molecular orbital calculations using a 3-21G basis set have been used to optimize geometries for pyrrole, CH₃(X)CCH₂, CH₃(H)CCHX (both cis and trans), c-C₃H₅X, and CH₂CHCH₂X, where X is CN and NC. In all the alkenyl derivatives methyl groups are found to adopt the conformation in which the methyl hydrogen eclipses the double bond. 6-31G*∥3-21G level calculations show the alkenyl cyanides to be of similar energy to pyrrole, but the isocyanides are ～20 kcal mol^?1 higher in energy. For both substituents the cyclopropyl derivatives are higher in energy by ～10 kcal mol^?1. At the 6-31G* level ring strain is 27.7 kcal mol^?1 for the cyanide and 30.6 kcal mol^?1 for the isocyanide. Data on the relative energies of RCN and RNC are compared when R is (i) a saturated hydrocarbon, (ii) an unsaturated hydrocarbon, (iii) an α-carbenium ion, (iv) an allyl cation, and (v) an α-carbanion.     

A Theoretical Study of Complex Formation between Formaldehyde and Lithium

Ab initio SCF and CI calculations on the cationic and neutral complexes of formaldehyde and lithium are reported. For the cationic complex CH₂O/Li⁺, the stabilization energy of 41.7 kcal/mol obtained from the SCF calculation increases to 51.6 kcal/mol if a configuration interaction is introduced. For the neutral complex CH₂O^?/Li⁺, the C_2v-conformer of the ²A₁-state with the equilibrium bond distances of d(C? O) = 1.23 Å and d (O? Li) = 1.90 Å is calculated to be more stable than the ₂B₁-state with d (C? O) = 1.34 Å, and d (O? Li) = 1.65 Å. Charge transfer and polarization effects upon complex formation are discussed.     

Substituent effect on electron affinity,gas‐phase basicity,and structure of monosubstituted propynyl radicals and their anions: A theoretical study

The substituent effect of electron‐withdrawing groups on electron affinity and gas‐phase basicity has been investigated for substituted propynl radicals and their corresponding anions. It is shown that when a hydrogen of the α‐CH₃ group in the propynyl system is substituted by an electron‐withdrawing substituent, electron affinity increases, whereas gas‐phase basicity decreases. These results can be explained in terms of the natural atomic charge of the terminal acetylene carbon of the systems. The calculated electron affinities are 3.28 eV (?C?C? CH₂F), 3.59 eV (?C?C? CH₂Cl) and 3.73 eV (?C?C? CH₂Br), and the gas‐phase basicities of their anions are 359.5 kcal/mol (^?:C?C? CH₂F), 354.8 kcal/mol (:C?C? CH₂Cl) and 351.3 kcal/mol (^?:C?C? CH₂Br). It is concluded that the larger the magnitude of electron‐withdrawing, the greater is the electron affinity of radical and the smaller is the gas‐phase basicity of its anion. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2009     

An Ab Initio Molecular Orbital Theory and Density Functional Theory (DFT) Study of Conformers and Rotamers of 4-Substituted (Methyl, Hydroxymethyl, Methanoyl, Ethanoyl, Cyano, Fluoro, Chloro, Bromo, Acetoxy) Tetrahydro-2H-thiopyran-1,1-dioxides

The structures and energies of axial and equatorial conformers and rotamers of 4-substituted tetrahydro-2H-thiopyran-1,1-dioxides (tetrahydrothiopyran-1,1-dioxides, thiacyclohexane-1,1-dioxides, thiane-1,1-dioxides, and 1,1-dioxothianes; CH₃, CH₂OH, CHO, COCH₃, CN, F, Cl, Br, and OCOCH₃) were calculated using the hybrid density functionals B3LYP, B3P86, and B3PW91, as well as MP2 and the 6-31G(d), 6-31G(2d), 6-31G(3d), 6-31G(d,p), and 6-31+G(d) basis sets. MP2/6-31+G(d)/ /HF/6-31+G(d) [–G° = 1.73 kcal/mol], B3P86/6-31G(d) [–G° = 1.75 kcal/mol], and B3PW91/6-31G(d) [–G° = 1.85 kcal/mol] gave conformational free energy (G°) values at 180 K for 4-methyltetrahydro-2H-thiopyran-1,1-dioxide which were similar to the reported experimental values for methylcyclohexane (–G° = 1.80 kcal/mol), 4-methyltetrahydro-2H-thiopyran (–G° = 1.80 kcal/mol), and other 4-methyl-substituted heterocycles. All levels of theory showed that the conformational preferences of the 4-methanoyl (4-formyl), 4-ethanoyl (4-acetyl), and 4-cyano substituents were small. The HF calculations gave conformational free energy (G°) values for 4-chlorotetrahydro-2H-thiopyran-1,1dioxide which were closer to the experimental value than the MP2 and density functional methods. The best agreement with available experimental data for 4-bromotetrahydro-2H-thiopyran-1,1-dioxide was obtained from the HF/6-31G(2d), HF/6-31G(3d), and B3LYP/6-31G(2d) calculations, and, for 4-acetoxytetrahydro-2H-thiopyran-1,1-dioxide, from the HF/6–31G(3d) calculations. The conformational free energies (G°) and relative energies (E) of the conformers and rotamers have been compared with the correspondingly substituted cyclohexanes and tetrahydro-2H-thiopyrans and are discussed in terms of dipole–dipole (electrostatic) interactions and repulsive nonbonded interactions (steric) in the most stable axial and equatorial conformers. The axial S=O bond lengths are shorter than the equatorial S=O bond lengths and the C2–C3 bond lengths in the substituents with carbon-bonded to the ring are shorter than the C3–C4 and C4–C-5 bond lengths. In contrast, the C2–C3 bond lengths in the 4-halogen and 4-acetoxy substituents are longer than the C3–C4 and C4–C-5 bond lengths.