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)=-0.8, E (b/F, Cl)=-12.2, E (b/F,OH)=+13.6, E b/F,OH=+16.1, E b/F,SH=+2.8, Eb/F, NH=+32.8, and E b/F,PH =+19.7 kcal x mol(-1). For the reverse reactions E b/F,F= -0.8, Eb/Cl,F =+18.3, E b/CN,F=+12.2, E b/OH,F =-1.8, E b/SH,F =+13.2, E b/NH(2),=-1.5, and E b/PH(2) =+9.6 kcal x mol(-1). The change in energetics between the CCSD(T)/aug-cc-pVTZ reference prediction and the final extrapolated focal point value is generally 0.5-1.0 kcal mol(-1). The inclusion of a tight d function in the basis sets for second-row atoms, that is, utilizing the aug-cc-pV(X+d)Z series, appears to change the relative energies by only 0.2 kcal x mol(-1). Additionally, several decomposition schemes have been utilized to partition the ion-molecule complexation energies, namely the Morokuma-Kitaura (MK), reduced variational space (RVS), and symmetry adapted perturbation theory (SAPT) techniques. The reactant complexes fall into two groups, mostly electrostatic complexes (FCH(3).F(-) and ClCH(3).F(-)), and those with substantial covalent character (NCCH(3).F(-), CH(3)OH.F(-), CH(3)SH.F(-), CH(3)NH(2).F(-) and CH(3)PH(2).F(-)). All of the product complexes are of the form FCH(3).X(-) and are primarily electrostatic.

The molecular structures and electron affinities of the SiH n /SiH n - (n = 1−4) and Si2H n /Si2H n - (n = 0−6) molecules, as well as the silicon atom, have been investigated using density functional theory (DFT) and hybrid Hartree−Fock/density functional theory. Specifically, four different types of electron affinities are reported in this work:  the adiabatic electron affinity (EAad), zero-point corrected EAad (EAzero), the vertical electron affinity (EAvert), and vertical detachment energy (VDE). The basis set used in this work is of double-ζ plus polarization quality with additional s- and p-type diffuse functions, and is denoted as DZP++. Of the six different density functionals used in this work, the BHLYP functional predicted the best molecular structures, and the B3LYP functional predicted the best electron affinities. When compared to the available six experimental electron affinities, the B3LYP functional has an average absolute error of just 0.06 eV. We predict the unknown electron affinities for Si2H2 (dibridged), Si2H3, Si2H4 (disilene), and Si2H5 (disilyl radical) to be 0.45, 2.21, 1.34, and 1.85 eV, respectively. These predictions assume that the (unknown) molecular structure of each anion is analogous to the known structure of the corresponding neutral molecule. The most interesting aspect of the present research is that for Si2H2 - and Si2H4 -, the lowest energy structures are qualitatively different from those of the neutrals. For Si2H2 - the disilavinylidene structure H2SiSi-, is predicted to lie 24 kcal/mol below the dibridged or “butterfly” anion. For Si2H4 - the silylsilylene anion H3SiSiH-, is predicted to lie 3 kcal/mol below the disilene anion structure H2SiSiH2 -. The zero-point corrected adiabatic electron affinities of disilavinylidene, silylsilylidyne, and silylsilylene are 1.87, 1.01, and 1.71 eV, respectively. The saturated closed shell systems SiH4 and Si2H6 do not have conventional electron affinities.

The molecular structures, electron affinities, and dissociation energies of the BrF n /BrF n - (n = 1−7) molecules have been examined using hybrid Hartree−Fock/density functional theory (DFT). The three different types of electron affinities reported in this work are the adiabatic electron affinity (EAad), the vertical electron affinity (EAvert), and the vertical detachment energy (VDE). The first Br−F dissociation energies of the BrF n and BrF n - species have also been reported. The basis set used in this work is of double-ζ plus polarization quality with additional s- and p-type diffuse functions, and is denoted as DZP++. Four different density functionals were used in this work. Among these, the best for predicting molecular structures and energies was found to be BHLYP, while other methods generally overestimated bond lengths. The most reliable adiabatic electron affinities, obtained at the DZP++ BHLYP level of theory, are 3.41 (Br), 2.64 (BrF), 4.78 (BrF2), 3.77 (BrF3), 5.58 (BrF4), 4.24 (BrF5), and 5.59 eV (BrF6). The electron affinity of the Br atom predicted by this work is in good agreement with the experimental result, but not one of the molecular electron affinities (BrF n , n = 1−7) is known. The neutral BrFn bond distances range from 1.70 to 1.83 Å. However, the diatomic BrF- distance and the axial Br−F distances in BrF3 -and BrF5 - are much longer, 2.25−2.30 Å, suggesting that the bonding in these three anions is quite different from that of their neutral counterparts.

With several levels of multireference and restricted open-shell single-reference electronic structure theory, optimum structures, relative energetics, and spectroscopic properties of the low-lying (6)Delta, (6)Pi, (4)Delta, (4)Pi, and (4)Sigma(-) states of linear FeNC and FeCN have been investigated using five contracted Gaussian basis sets ranging from Fe[10s8p3d], C/N[4s2p1d] to Fe[6s8p6d3f2g1h], C/N[6s5p4d3f2g]. Based on multireference configuration interaction (MRCISD+Q) results with a correlation-consistent polarized valence quadruple-zeta (cc-pVQZ) basis set, appended with core correlation and relativistic corrections, we propose the relative energies: T(e)(FeNC), (6)Delta(0)<(6)Pi (2300 cm(-1))<(4)Delta (2700 cm(-1))<(4)Pi (4200 cm(-1))<(4)Sigma(-); and T(e)(FeCN), (6)Delta(0)<(6)Pi (1800 cm(-1))<(4)Delta (2500 cm(-1))<(4)Pi (2900 cm(-1))<(4)Sigma(-). The (4)Delta and (4)Pi states have massive multireference character, arising mostly from 11sigma-->12sigma promotions, whereas the sextet states are dominated by single electronic configurations. The single-reference CCSDT-3 (coupled cluster singles and doubles with iterative partial triples) method appears to significantly overshoot the stabilization of the quartet states provided by both static and dynamical correlation. The (4,6)Delta and (4,6)Pi states of both isomers are rather ionic, and all have dipole moments near 5 D. On the ground (6)Delta surface, FeNC is predicted to lie 0.6 kcal mol(-1) below FeCN, and the classical barrier for isocyanide/cyanide isomerization is about 6.5 kcal mol(-1). Our data support the recent spectroscopic characterization by Lei and Dagdigian [J. Chem. Phys. 114, 2137 (2000)] of linear (6)Delta FeNC as the first experimentally observed transition-metal monoisocyanide. Their assignments for the ground term symbol, isotopomeric rotational constants, and the Fe-N omega(3) stretching frequency are confirmed; however, we find rather different structural parameters for (6)Delta FeNC:r(e)(Fe-N)=1.940 A and r(N-C)=1.182 A at the cc-pVQZ MRCISD+Q level. Our results also reveal that the observed band of FeNC originating at 27 236 cm(-1) should have an analog in FeCN near 23 800 cm(-1) of almost equal intensity. Therefore, both thermodynamic stability and absorption intensity factors favor the eventual observation of FeCN via a (6)Pi<--(6)Delta transition in the near-UV.

The optimized geometries, adiabatic electron affinities, and IR-active vibrational frequencies have been predicted for the long linear carbon chains HC(2n)H. The B3LYP density functional combined with the DZP basis set was used in this theoretical study. The computed physical properties are discussed. The predicted electron affinities form a remarkably regular sequence: 1.78 (HC(12)H), 2.08 (HC(14)H), 2.32 (HC(16)H), 2.53 (HC(18)H), 2.69 (HC(20)H), 2.83 (HC(22)H), and 2.95 eV (HC(24)H). The predicted structures display an alternating triple and very short single bond pattern, with the degree of bond alternation significantly less for the radical anions.

The structures, energetics, and transition states of water clusters (trimer to pentamer, n = 3-5) are investigated as a function of electric field by using ab initio calculations. With an increasing strength of the field, the most stable cyclic structures of trimer, tetramer, and pentamer open up to align their dipole moments along the direction of the field. For the lower strength (below 0.3 V/angstroms) of the electric field, the dipole moment of each water monomer is along the same direction with the field, while it retains the cyclic structure. For the higher strength of the field, to have a higher dipole moment for the cluster along the field direction, each cyclic structure opens up to form a linear chain or "water wire." We have investigated the transition state structures between the cyclic and linear forms for the field strengths of 0.3-0.4 V/angstroms where both cyclic and linear forms are energetically comparable.

In contrast to the extensive theoretical investigation of the solvation phenomena, the dissolution phenomena have hardly been investigated theoretically. Upon the excitation of hydrated halides, which are important substances in atmospheric chemistry, an excess electron transfers from the anionic precursor (halide anion) to the solvent and is stabilized by the water cluster. This results in the dissociation of hydrated halides into halide radicals and electron-water clusters. Here we demonstrate the charge-transfer-to-solvent (CTTS)-driven femtosecond-scale dissolution dynamics for I-(H2O)n=2-5 clusters using excited state (ES) ab initio molecular dynamics (AIMD) simulations employing the complete-active-space self-consistent-field (CASSCF) method. This study shows that after the iodine radical is released from I-(H2O)n=2-5, a simple population decay is observed for small clusters (2

We review our theoretical work done on a variety of different chemical systems, which show different H-bonding characteristics. The systems include water clusters, its interactions with polar molecules and π -systems, organic nanotubes, enzymes, and ionophores/receptors. Special features of normal, short, short strong, and π -type H-bonding interactions in these systems are discussed in terms of structures, interaction energies, and spectra.