Convergent Concordant Mode Approach for Molecular Vibrations: CMA-2.

The concordant mode approach (CMA) is a promising new scheme for dramatically increasing the system size and level of theory achievable in quantum chemical computations of molecular vibrational frequencies. Here, we achieve advances in the CMA hierarchy by computations targeting CCSD(T)/cc-pVTZ (coupled cluster singles and doubles with perturbative triples using a correlation-consistent polarized-valence triple-ζ basis set) benchmarks within the G2 molecular test set, executing a statistical analysis for 1501 frequencies from 111 compounds and then separately solving the refractory case of pyridine. First, MP2/cc-pVTZ (second-order Møller-Plesset perturbation theory with the same basis set) proves to be an excellent and preferred choice for generating the underlying (Level B) normal modes of the CMA scheme.

Reactivity in Friedel-Crafts aromatic benzylation: the role of the electrophilic reactant.

Density functional theory is employed in understanding the reactivity in the TiCl catalyzed Friedel-Crafts benzylation of benzene with substituted benzyl chlorides in nitromethane solvent. A series of ten substituted (in the aromatic ring) benzyl chlorides are characterized by theoretical reactivity indices. The theoretical parameters are juxtaposed to experimental relative rates of benzylation.

Highly Strained Pn(CH) (Pn = N, P, As, Sb, Bi) Tetrahedranes: Theoretical Characterization.

Recent experimental research by Cummins and co-workers has established the existence of a tetrahedrane molecule with one CH moiety replaced by phosphorus. We present here the first theoretical studies of the entire Pn(CH) (Pn = N, P, As, Sb, Bi) class of molecules. Geometries are obtained at the highly reliable CCSD(T)/aug-cc-pwCVTZ(-PP) level of theory.

The Recently Synthesized Dimagnesiabutadiene and the Analogous Dimetalla-Beryllium, -Calcium, -Strontium, and -Barium Compounds.

The 2014 synthesis of the remarkable dimagnesium compound Mg [C (CH ) (Si(CH ) ) ](C H ) (C H O) may point the way to a new chapter in alkaline earth organometallic chemistry. Accordingly, we have studied the known Mg compound and the analogous Be, Ca, Sr, and Ba structures. Although most of our theoretical predictions come from density functional methods, the latter have been benchmarked using coupled cluster theory including single, double, and perturbative triplet excitations, CCSD(T) using cc-pVTZ basis sets.

Characterizing a nonclassical carbene with coupled cluster methods: cyclobutylidene.

Carbenes represent a special class of reactive compounds that possess a lone pair of electrons on a carbon atom. Among the myriad examples of carbenes in the literature, cyclobutylidene stands out as a unique nonclassical compound that includes transannular interaction between opposing C1 and C3 carbon atoms within a four-membered ring. On its lowest potential energy surface (X[combining tilde](1)A'), cyclobutylidene quickly rearranges, following three reaction paths: (i) 1,2-H migration; (ii) 1,2-C migration; and, (iii) 1,3-H migration.

The methylsulfinyl radical CH3SO examined.

Methylsulfinyl radical, a key intermediate in marine atmospheric chemistry, plays a central role in the oxidation of dimethyl sulfide. CH3SO has been extensively studied here with ab initio quantum mechanical methods, with methods as complete as CCSDT(Q) in conjunction with basis sets as large as cc-pV(5+d)Z. In this research, we report high-level computations for the ground and first excited electronic states of the methylsulfinyl radical.

Exploring the effect of axial ligand substitution (X = Br, NCS, CN) on the photodecomposition and electrochemical activity of [MnX(N-C)(CO)3] complexes.

The synthesis, electrochemical activity, and relative photodecomposition rate is reported for four new Mn(i) N-heterocyclic carbene complexes: [MnX(N-ethyl-N'-2-pyridylimidazol-2-ylidine)(CO)3] (X = Br, NCS, CN) and [MnCN(N-ethyl-N'-2-pyridylbenzimidazol-2-ylidine)(CO)3]. All compounds display an electrocatalytic current enhancement under CO2 at the potential of the first reduction, which ranges from -1.53 V to -1.

Butterfly and rhombus structures for binuclear cobalt carbonyl sulfur and phosphinidene complexes of the type Co2(CO)6E2 (E = S, PX).

Theoretical studies on Co(2)(CO)(6)(PX)(2) derivatives (X = H, Cl, OH, OMe, NH(2), NMe(2)) predict the lowest energy structures to be butterfly structures containing five two-electron two-center bonds in the central Co(2)P(2) unit. Among these butterfly structures the energy increases as the unique bond forming the "body" of the butterfly changes from Co-Co to Co-P and then P-P. Higher energy rhombus structures are also found for Co(2)(CO)(6)(PX)(2) with only Co-P bonds in the Co(2)P(2) framework without any Co-Co or P-P bonds.

Binuclear manganese and rhenium carbonyls M2(CO)n (n = 10, 9, 8, 7): comparison of first row and third row transition metal carbonyl structures.

The equilibrium geometries, thermochemistry, and vibrational frequencies of the homoleptic binuclear rhenium carbonyls Re2(CO)n (n = 10, 9, 8, 7) were determined using the MPW1PW91 and BP86 methods from density functional theory (DFT) with the effective core potential basis sets LANL2DZ and SDD. In all cases triplet structures for Re2(CO)n were found to be unfavorable energetically relative to singlet structures, in contrast to corresponding Mn2(CO)n derivatives, apparently owing to the larger ligand field splitting of rhenium. For M2(CO)10 (M = Mn, Re) the unbridged structures (OC)5M-M(CO)5 are preferred energetically over structures with bridging CO groups.

Understanding electron attachment to the DNA double helix: the thymidine monophosphate-adenine pair in the gas phase and aqueous solution.

Electron attachment to the 2'-deoxythymidine-5'-monophosphate-adenine pairs (5'-dTMPH-A and 5'-dTMP(-)-A) has been investigated at a carefully calibrated level of theory (B3LYP/DZP++) to investigate the electron-accepting properties of thymine (T) in the DNA double helix under physiological conditions. All molecular structures have been fully optimized in vacuo and in solution. The adiabatic electron affinity of 5'-dTMPH-A in the gas phase has been predicted to be 0.

The 2'-deoxyadenosine-5'-phosphate anion, the analogous radical, and the different hydrogen-abstracted radical anions: molecular structures and effects on DNA damage.

The 2'-deoxyadenosine-5'-phosphate (5'-dAMP) anion and its related radicals have been studied by reliably calibrated theoretical approaches. This study reveals important physical characteristics of 5'-dAMP radical related processes. One-electron oxidation of the 5'-dAMP anion is found on both the phosphoryl group and the adenine base with electron detachment energies close to that of phosphate.

Assessing alkyl-, silyl-, and halo-substituent effects on the electron affinities of silyl radicals.

Neutral anion energy differences for a large class of alpha-substituted silyl radicals have been computed to determine the effect of alkyl, silyl, and halo substituents on their electron affinities. In particular, we report theoretical predictions of the adiabatic electron affinities (AEAs), vertical electron affinities (VEAs), and vertical detachment energies (VDEs) for a series of methyl-, silyl-, and halo-substituted silyl radical compounds. This work utilizes the carefully calibrated DZP++ basis set, in conjunction with the pure BLYP and OLYP functionals, as well as with the hybrid B3LYP, BHLYP, PBE1PBE, MPW1K, and O3LYP functionals.

Effects of fluorine on the structures and energetics of the propynyl and propargyl radicals and their anions.

[reaction: see text] The adiabatic electron affinity (EA(ad)) of the CH(3)-C[triple bond]C(*) radical [experiment = 2.718 +/- 0.008 eV] and the gas-phase basicity of the CH(3)-C[triple bond]C:(-) anion [experiment = 373.

The dichotomy of dimetallocenes: coaxial versus perpendicular dimetal units in sandwich compounds.

Theoretical studies of the dimetallocene (eta5-C5H5)2Zn2 lead to optimized D5h or D5d structures in which the Zn-Zn bond is coaxial with the C5 axes of the two Cp rings, with a Zn-Zn distance of 2.33 A, corresponding to a Zn-Zn single bond. (eta5-C5H5)2Ni2 (singlet state) and (eta5-C5H5)2Cu2 (triplet) have similar structures with a NiNi triple bond (2.

DNA nucleosides and their radical anions: molecular structures and electron affinities.

The deoxyribonucleosides have been studied to determine the properties of combinations of 2-deoxyribose with each of the isolated DNA bases for both neutral and anionic species. We have used a carefully calibrated theoretical method [Chem. Rev.

Definitive ab initio studies of model SN2 reactions CH(3)X+F- (X=F, Cl, CN, OH, SH, NH(2), PH(2)).

The energetics of the stationary points of the gas-phase reactions CH(3)X+F(-)-->CH(3)F+X(-) (X=F, Cl, CN, OH, SH, NH(2) and PH(2)) have been definitively computed using focal point analyses. These analyses entailed extrapolation to the one-particle limit for the Hartree-Fock and MP2 energies using basis sets of up to aug-cc-pV5Z quality, inclusion of higher-order electron correlation [CCSD and CCSD(T)] with basis sets of aug-cc-pVTZ quality, and addition of auxiliary terms for core correlation and scalar relativistic effects. The final net activation barriers for the forward reactions are: E (b/F,F)=-0.

Odd carbon long linear chains HC2n+1H (n = 4-11): properties of the neutrals and radical anions.

The optimized geometries, adiabatic electron affinities, vertical electron affinities, vertical electron detachment energies (for the anions), and IR-active vibrational frequencies have been predicted for the long linear carbon chains HC(2)(n)()(+1)H (n = 4-11). The B3LYP density functional combined with DZP and TZ2P basis sets was used in this theoretical study. These methods have been extensively calibrated versus experiment for the prediction of electron affinities (Chem.

Resurrection of neutral tris-homoaromaticity.

Neutral in-plane tris-homoaromaticity is evaluated in tris(bismethano)benzene (15) and modifications of this parent structure in which the pi-orbitals might interact in the plane established by the unsaturated carbon atoms (in-plane conjugation). On the basis of magnetic susceptibility exaltation, nucleus-independent shift (NICS), and aromatic stabilization energy (ASE, evaluated via homodesmotic and isodesmic equations using B3LYP/6-311+G + ZPVE energies, as well as by MM3 and MM4 force field computations), we identified triene 17, a triply bridged analogue of 15, as the system where homoaromaticity is most effective. The NICS(total) in the center of 17 is -30.

Aza[10]annulene: Next Higher Aromatic Analogue of Pyridine.

The aza[10]annulene isomers (CH)(9)N derived from "twist", "heart", "naphthalene-like", and "azulene-like" [10]annulenes by replacement of a CH group by nitrogen were investigated at the CCSD(T)/DZd//B3LYP/6-311+G level of theory. Except for the twist structure, all aza[10]annulenes studied were found to be aromatic. As the transannular repulsion is reduced in the aza[10]annulenes, the aromatic isomers become competitive energetically with the twist form.

Examination of the Stabilities of Group 14 (C, Si, Ge, Sn, Pb) Congeners of Dihydroxycarbene and Dioxirane. Comparison to Formic Acid and Hydroperoxycarbene Congeners.

The relative energetics of four XH(2)O(2) (X = C, Si, Ge, Sn, Pb) isomers, dihydroxycarbene, formic acid, dioxirane, and hydroperoxycarbene, were determined using the BLYP and B3LYP density functionals with DZP and TZ2P basis sets, as well as CCSD and CCSD(T) single-point energies at the BLYP/TZ2P optimized geometries. Relative to dihydroxycarbene, formic acid was 41.8 kcal/mol lower in energy while dioxirane and hydroperoxycarbene were 51.

Structure, Spectra, and Reaction Energies of the Aluminum-Nitrogen (HAl-NH)(2) and (H(2)Al-NH(2))(2) Rings and the (HAl-NH)(4) Cluster.

Rings of four-coordinate aluminum and nitrogen are easily synthesized and well studied, as are clusters of four-coordinate Al and N. Only recently, however, have rings that are derivatives of the model compounds (HAl-NH)(n)() (n = 2, 3) with three-coordinate Al and N been synthesized. Ab initio investigations of the structure, bonding, vibrational spectra, and reaction energies for the three-coordinate ring (HAl-NH)(2), the four-coordinate ring (H(2)Al-NH(2))(2), and the four-coordinate cluster (HAl-NH)(4) are presented.

Carbene Rearrangements Unsurpassed: Details of the C(7)H(6) Potential Energy Surface Revealed.

The rearrangement of phenylcarbene (1) to 1,2,4,6-cycloheptatetraene (3) has been studied theoretically, using SCF, CASSCF, CASPT2N, DFT (B3LYP), CISD, CCSD, and CCSD(T) methods in conjunction with the 6-31G, 6-311+G, 6-311G(2d,p), cc-pVDZ, and DZd basis sets. Stationary points were characterized by vibrational frequency analyses at CASSCF(8,8)/6-31G and B3LYP/6-31G. Phenylcarbene (1) has a triplet ground state ((3)A") with a singlet-triplet separation (DeltaE(ST)) of 3-5 kcal mol(-)(1).

The S(6) Point Group Conformers of the Hexamethylchalcogens: Me(6)S, Me(6)Se, Me(6)Te.

The synthesis of Me(6)Te in 1990 stimulated the exploration of hexamethylchalcogen potential energy surfaces. This earlier ab initio work focused only on the D(3) conformers, but it has been noted that the pseudooctahedral X(CH(3))(6) compounds show either D(3) or S(6) symmetry. Here are reported the results of an ab initio molecular orbital study of the hexamethylchalcogens confined to S(6) symmetry.