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Molecular beam experiments (lithium atom plus hydrogen fluoride) by both Becker and co-workers (C.H. Becker, P. Casavecchia, P.W. Tiedemann, J.J.Valentini, and Y.T. Lee, J. Chem. Phys. 73, 2833 (1980)) and Loesch and Stienkemeier (H.J. Loesch and F. Stienkemeier, J. Chem. Phys. 98, 9570 (1993)) deduced a van der Waals complex of type Li···HF. In this research, molecular electronic structure theory [aug-cc-pCVQZ CCSD(T)] has been used to predict a well depth of 0.86 kcal mol−1 relative to separated Li + HF. However, the barrier from this vdW well to the more strongly bound (∼6.2 kcal mol−1) HFLi complex lies 0.43 kcal mol−1 below separated Li + HF.

Although evanescent at best, the bare dimers of the elements Cr, Mo, and W have been identified as possible candidates for the sextuple metal–metal bond. The corresponding dibenzene sandwich compounds Cr2(C6H6)2, Mo2(C6H6)2, and W2(C6H6)2, satisfy the ‘18-Electron Rule’, and might achieve high-order metal–metal bonds and longer lifetimes at the same time. Twenty-two different DFT methods have been used to evaluate this possibility. Based on the present Wiberg bond index and molecular orbital analyses, however, only quadruple metal–metal bonds are predicted for the electronic ground states of Cr2(C6H6)2, Mo2(C6H6)2, and W2(C6H6)2, rather than the sextuple or even quintuple bonds, for both singlet and triplet electronic states. It is possible to force the hypothesised D 6h sextuple bond electron configuration, but the resulting energy is 39 kcal/mol above the D 2h quadruple bond structure for Cr2(C6H6)2. However, the constrained sextuple bond structure for Mo2(C6H6)2 lies 19 kcal/mol above the MoMo singlet....

Although all of the neutral alkaline earth dimers have been studied experimentally, only for the beryllium and strontium systems have the diatomic radical cations been subjected to modern spectroscopic techniques. In the present research, both coupled cluster and full configuration interaction methods were used to describe the M2 + systems. Basis sets as large as aug-cc-pCV5Z were chosen. For both Be2 + and Sr2 + agreement with the experiments is outstanding. Final predictions for the unknown dissociation energies are 10,651 (Mg2 +), 9605 (Ca2 +), and 8980 cm−1 (Ba2 +). The M2 + dissociation energies decrease monotonically going down the periodic except for the Sr2 + - Ba2 + pair. The unknown bond distances re are 3.015 (Mg2 +), 3.814 (Ca2 +), 4.194 (Sr2 +), and 4.587 A (Ba2 +).

The binuclear pentalene manganese carbonyl trans-(η5,η5-C8H6)Mn2(CO)6 has been synthesized by the reaction of the pentalene dianion with a manganese carbonyl halide. This species as well as its decarbonylation products (C8H6)Mn2(CO) n (n = 5, 4, 3) have been investigated by density functional theory. In all of these optimized structures both pentalene rings are bonded to manganese atoms as pentahapto ligands. The lowest energy (C8H6)Mn2(CO)6 structure is the experimentally observed singlet trans structure but a closely related cis structure lies ∼7 kcal/mol higher. The lowest energy (C8H6)Mn2(CO)5 structure is a singlet structure having a four-electron donor bridging CO group and a Mn–Mn single bond of length ∼2.9 A. The lowest energy (C8H6)Mn2(CO)4 structure is a singlet structure having all terminal CO groups and a short Mn ≡ Mn triple bond of length ∼2.3 A. For (C8H6)Mn2(CO)3 low energy structures are found with either triplet or quintet spin states. Low energy structures with pentalene rings only part...

Transition metal complexes with bridging PF3 groups are unknown analogous to metal carbonyls with bridging carbonyl groups. The possibility of bridging PF3 groups in binuclear Co2(PF3) n complexes (n = 9, 8, 7, 6, 5) and the tetranuclear Co4(PF3)12 has now been investigated by density functional theory (DFT). The lowest energy Co2(PF3)8, Co2(PF3)7, and Co4(PF3)12 structures are predicted to be unbridged structures with cobalt-cobalt bond distances of ∼2.8 A, ∼2.5 A, and ∼2.6 A, respectively. The lowest energy Co2(PF3)6 structure is an unsymmetrical unbridged structure with a Co–Co distance of ∼2.5 A. For the more highly unsaturated Co2(PF3)5 both triplet and singlet structures are found, consisting of two Co(PF3)2 units linked by two bridges, namely a PF2 group and a F atom. Such Co2(PF3)5 structures have square planar cobalt coordination and a Co…Co distance too long for direct bonding. In addition, a singlet Co2(PF3)5 structure is found, with two semi-bridging PF3 groups and a very short Co ··· Co dista...

The mixed sandwich compounds (η5-C5H5)M(η7-C7H7) (M = Ti, V, Cr) have been known for approximately 50 years. The vanadium derivative (‘trovacene’) has recently been shown to be a useful precursor for the preparation of a variety of paramagnetic sandwich compounds. In this connection the structures of the complete series of the first row transition metal mixed sandwich compounds C5H5MC7H7 (M = Ti, V, Cr, Mn, Fe, Co, Ni) have been investigated by density functional theory. The experimentally known parallel ring sandwich structures (η5-C5H5)M(η7-C7H7) are found for the Ti, V, and Cr derivatives. Analysis of the frontier molecular orbitals of these complexes indicates that, for the metal d-orbitals, the z 2 orbital is essentially non-bonding, the x 2–y 2 and xy orbitals are δ bonding to the seven-membered ring, and the xz and yz orbitals are π bonding to the five-membered ring. For the later transition metals, (η5-C5H5)M(η5-C7H7) structures with pentahapto seven-membered rings are preferred for iron and manga...

The optimized geometries, total energies, and vibrational frequencies of the polyacenes and perfluoropolyacenes in their closed-shell singlet and lowest triplet states have been studied using the B3LYP method in conjunction with double-ζ plus polarization (DZP) basis sets. The equilibrium structures of all the polyacenes and perfluoropolyacenes are planar, with D2h symmetry, and the singlet and triplet geometries display interesting patterns for each family of molecules. The largest singlet–triplet structural changes appear in the outermost C–C distances. The small perfluoropolyacene systems have ground state singlet states, and the singlet–triplet energy gaps decrease as the number of linearly fused benzene rings increases. With the number of rings greater than seven, the triplet state of the perfluorinated species falls below the closed-shell singlet. For the parent polyacenes, the same shift occurs between n = 8 and n = 9. Correspondingly, the LUMO–HOMO gap for the singlet state decreases with increasi...

The structures, energetics, and electron affinities of 18 H-addition cytosine radical isomers (C + H)· are predicted using carefully calibrated density functional methods. Radical rO1 in which H is attached at the N3 site of amino-oxo cytosine, has the lowest energy. The lowest energy anion is aO2, in which, qualitatively, H− is attached to the C6 site of amino-oxo cytosine. The theoretical adiabatic electron affinities (AEAs) for the 18 radicals range from −0.20 to 2.59 eV. Radical rO4, where the additional H atom is at the C4 site has the largest AEA. In contrast, when H is attached to the N3 site of the trans amino-hydroxy form, the resulting radical rA1 has a negative AEA, −0.20 eV. The radical rB1, in which H is appended on the N1 atom of the cis amino-hydroxy form, also has a negative AEA value, −0.16 eV. Generally, the AEA values for the cytosine H-addition radicals are rather smaller than those of the H-abstraction cytosine radicals studied earlier, which range from 2.22 to 3.00 eV.

The first structural characterization of the highly unsaturated octacarbonyldivanadium, V2(CO)8, is reported using density functional theory (DFT) with the B3LYP and BP86 functionals. Various combinations of vanadium–vanadium multiple bonding, four-electron bridging CO groups, and metal electron configurations less than the 18-electron rare gas configuration are found to accommodate the high unsaturation implied by the V2(CO)8 stoichiometry. Thus, the global minimum for V2(CO)8 is a C2h structure (1) with two four-electron bridging CO groups and a vanadium–vanadium distance of 2.896 (B3LYP) or 2.858 A (BP86), suggestive of a V–V single bond and thus 16-electron configurations for the vanadium atoms. The next lowest-lying structure (2) for V2(CO)8 at 10.6 (B3LYP) or 12.5 kcal mol−1 (BP86) above the global minimum (1) also has C2h symmetry with two bridging CO groups, but a much shorter vanadium–vanadium distance of 2.440 (B3LYP) or 2.460 A (BP86), suggestive of a V≡V triple bond. Of similar energy to 2 is ...

Four selected density functional theory (DFT) and hybrid Hartree-Fock/DFT methods have been used to carry out an investigation of the molecular structures, thermochemistry and the neutral-anion energy separations of the bromine oxygen fluorides BrOF n /BrO (n = 1–5). Double-ζ plus polarization quality basis sets augmented with the diffuse functions, labelled as DZP++, were adopted. Thirty-four stationary points have been located on their potential energy hypersurfaces. Several previously unknown and novel structures are discovered in this research. For example, a beautiful, nearly octahedral C4v structure is predicted to be a local minimum for BrOF5. Most global minima are found to be Br-centred, O-terminal structures, except for OBrF5, which has a higher energy than Br(OF)F4 by ∼6 kcal mol−1. The adiabatic electron affinities (EAad) with ZPVE correction are predicted by BP86 to be 3.46 eV (BrOF), 3.90 eV (BrOF2), 4.02 eV (BrOF3), 6.23 eV (BrOF4) and 5.04 eV (BrOF5). Except for BrOF4, these EAad values ar...

The molecular structures and energetics of the potent greenhouse gas SF5CF3/SF5CF3− species have been examined using nine hybrid and pure density functional theory (DFT) methods, with the basis sets of double-ζ plus polarization quality plus additional diffuse s- and p-type functions, denoted as DZP++. The geometries are fully optimized with each DFT method independently. The three different types of the neutral-anion energy separations reported in this work are the adiabatic electron affinity (EAad), the vertical electron affinity (EAvert) and the vertical detachment energy (VDE). The dissociation energies of the SF5CF3 and species as well as the harmonic vibrational frequencies are reported. The neutral SF5CF3 global minimum has Cs symmetry in its electronic ground state. The S–C bond distance for SF5CF3 is predicted to be 1.898 A (BHLYP) and the torsional barrier around the S–C bond is 17 cm−1 (B3LYP). For the anion, there are three minima. Structure a is geometrically similar to the neutral, while str...

Six different hybrid and pure density functional theory (DFT) methods have been employed to study the structures, electron affinities, and dissociation energies of the (n = 1–6) species. The basis set used is of double-ζ plus polarization quality with additional s- and p-type diffuse functions, denoted DZP++. The geometries are fully optimized with each DFT method independently. Located were 20 structures of the neutral Ge n clusters and 12 structures of the anionic clusters. The ground states of Ge n and clusters in this work are in good agreement with available experiments. Three types of electron affinities reported are the adiabatic electron affinity (EAad), the vertical electron affinity (EAvert), and the vertical detachment energy (VDE). The first Ge–Ge dissociation energies D e(Ge n −1–Ge) for Ge n and both D e( –Ge) and D e(Ge n −1–Ge−) for the species have also been reported. Previously observed trends in the prediction of bond lengths by DFT methods are also demonstrated for the germanium cluste...

The molecular structures, vibrational frequencies, dissociation energies and electron affinities of the Br 2 /Br - 2 , Br 2 O/Br 2 O - , Br 2 O 2 /Br 2 O - 2 , Br 2 O 3 /Br 2 O - 3 , and Br 2 O 4 /Br 2 O - 4 systems have been investigated using density functional theory (DFT) and hybrid Hartree-Fock/density functional theory. The methods used have been carefully calibrated against a comprehensive tabulation of experimental electron affinities, (Rienstra-Kiracofe, J. C., Tschumper, G. S., Shaefer, H. F., Nandi, S. and Ellison, G. B., 2002, Chem. Rev., 102, 231). Four different types of neutral/anion energetic difference are reported in this work: the adiabatic electron affinity (EA a d ), the zero-point corrected EA a d (EA z e r o ), the vertical electron affinity (EA v e r t ), and the vertical detachment energy (VDE). The basis set used in this work is of double-zeta plus polarization quality with additional s- and p-type diffuse functions, and is denoted as DZP + + . The BHLYP method does well in reproducing the very limited experimental energetics, while the B3LYP method does well for the few known vibrational frequencies. The final predicted electron affinities with the BHLYP method are 3.41 eV (Br, experiment 3.36eV), 3.02eV (Br 2 , experiment 2.6′0.2eV), 3.27 eV (Br 2 O), 2.93 eV (Br 2 O 2 ), 3.07eV (Br 2 O 3 ), and 2.54 eV (Br 2 O 4 ). The global minimum structures for several of the larger dibromine oxides and their anions are unusual. For neutral Br 2 O 2 the peroxide structure (BrOOBr) lies lowest, but for the anion a loosely bound C s symmetry BrO-BrO - structure lies lowest for the hybrid functionals, while a C 2 symmetry peroxide BrOOBr - structure lies lowest for the pure functionals. Furthermore, the C 2 structures are found to exhibit an inverse symmetry breaking problem, and should be interpreted with caution. For neutral Br 2 O 3 , a chain structure Br-O 3 -Br lies lowest, while the complex Br...O 3 ...Br - lies lowest for the negative ion. For neutral Br 2 O 4 , a chain structure Br-O 4 -Br lies lowest, while the complex BrO - ...BrO 3 lies lowest for the negative ion.

Four independent density functional theory (DFT) methods have been employed to study the structures and electron affinities of the methyl and F-, CI- and Br-substituted methyl radicals and their anions. The methods used have been carefully calibrated against a comprehensive tabulation of experimental electron affinities (Chemical Reviews, 2002, 102, 231). The first dissociation energies together with the vibrational frequencies of these species are also reported. The basis sets used in this work are of double-ζ plus polarization quality with additional s- and p-type diffuse functions, labelled as DZP++. Previously observed trends in the prediction of bond lengths by the DFT methods are also demonstrated for the F-, Cl- and Br-substituted methyl radicals and their anions. Generally, the Hartree-Fock/DFT hybrid methods predict shorter and more reliable bond lengths than the pure DFT methods. Neutral-anion energy differences reported in this work are the adiabatic electron affinity (EAad), the vertical elect...

A systematic investigation of Si2Fn/Si2F− n systems is carried out with five density functional (DFT) methods in conjunction with DZP++ basis sets. For each system, various structures, including minima, transition states, and energetically low lying saddle points, are optimized. The geometries and the relative energies are discussed and compared. Three kinds of electron affinity and dissociation energy pertaining to the global minimum for each compound are reported. The theoretical predictions are in good agreement with the limited experimental results. The zero-point vibrational energy (ZPVE) corrected adiabatic electron affinities (EAad) are predicted as 1.97 (Si2F), 1.92 (Si2F2), 2.39 (Si2F3), 2.02 (Si2F4), 2.68 (Si2F5), and 0.73 (Si2F6)eV by the BHLYP method, which is considered to be the most reliable method in the present study for predicting the EAs. These theoretical predictions are quite different from those for the analogous silicon hydrides and fluorocarbons. For example, both Si2F2 and its ani...

Bromine oxides are of significant interest due to their importance in atmospheric chemistry. Density functional theory (DFT) methods have been used in conjunction with a DZP++ (double-ζ plus polarization with diffuse functions) basis set to study the molecular geometries and total energies of BrOn and BrO- n (n = 1-4). The adiabatic electron affinity (EAad), the vertical electron affinity (EAvert) of the bromine oxide and the vertical detachment energy (VDE) of each anion are reported. Harmonic vibrational frequencies and zero point energies are also reported. Five different DFT methods were employed for comparison. Among these, the BHLYP method predicts the geometries and the vibrational frequencies in best agreement with available experimental data, while the the other methods do better in predicting the limited number of energetic quantities determined observationally. The predicted adiabatic electron affinities are 2.38 eV (BrO, experiment 2.35 eV), 2.36 eV (BrO2), 3.35 eV (BrOO), 4.32 eV (BrO3), 2.91...

Since symmetry breaking occurs in the ONNO+ cation with the self-consistent field (SCF) method and inverse symmetry breaking occurs when density functional theory (DFT) methods are used, Brueckner double excitation coupled cluster methods (BD) and BD plus perturbative triple-excitation contributions [BD(T)]have been used to study the geometries and vibrational frequencies for the trans and cis structures of the ONNO+ cation. Double-ζ plus polarization (DZP) basis sets and triple-ζ plus double polarization with f functions (TZ2Pf) basis sets were utilized. The ground state of the trans-ONNO cation is of 2Ag symmetry, which has a slightly (0.36 kcalmol-1) lower energy than the cis conformer (2A1). The controversial vibrational frequency corresponding to the asymmetric N–O stretching mode for both trans and cis structures is predicted as about 1600cm-1. This value is discussed in the context of the many (sometimes variant) experimental assignments.

Formulae in terms of integrals and orbital energies are presented for the evaluation of the second-, third- and fourth-order energies in open-shell spinrestricted Moller-Plesset (RMP) perturbation theory. The performance of RMP theory is evaluated by calculating equilibrium structures and harmonic vibrational frequencies of a large number of open-shell systems and by calculating energies of reactions which have been shown to be poorly described by spinunrestricted Moller-Plesset (UMP) theory. RMP theory removes the spin contamination present in the unrestricted Hartree-Fock (UHF) reference of UMP theory. In cases where the spin contamination gives rise to large errors in UMP calculations, such as for the harmonic frequencies of NO, CN, CO+ and FO2 and the energies of reactions involving CN and FO2, RMP theory is shown to offer a substantial improvement. In cases where the Moller-Plesset series exhibits large oscillations, such as for O2, O+ 2 and F+ 2, RMP theory is shown to offer no improvement over UMP ...

Ab initio quantum mechanical methods have been applied to the S6 and O6 molecules at their respective D3d hexagonal chair equilibrium geometries. Double zeta plus polarization (DZ + P) and triple zeta plus double polarization (TZ + 2P) basis sets have been used in conjunction with the self-consistent field (SCF) method and second-order perturbation theory. Equilibrium geometries, harmonic vibrational frequencies, infrared intensities, and Raman intensities have been predicted for the two cyclic molecules. Two previous vibrational difficulties between theory and experiment for S6 have been resolved. The O6 molecule appears to be similar to the well-characterized S6 in several respects. However, its dissociation energy and vibrational frequencies reveal a much flatter potential energy surface for O6 in the region of the equilibrium geometry. While still predicted to correspond to a genuine potential minimum, O6 nevertheless lies about 100 kcal mol-1 above three separated O2 molecules. From a methodological ...

Four distinct methods for the evaluation of anharmonic vibrational frequencies have been used to study the infrared spectrum of the, as yet, undetected molecular ion HO2 +. Three methods are variational with the other being based upon second-order vibrational perturbation theory. Results are given for two different potential energy surfaces: (a) an SCF surface utilizing up to fourth derivatives of the energy and (b) a CISD surface formed by replacing the SCF second derivatives by the appropriate CISD second derivatives. All calculations have been performed with DZ, DZP and TZ2P basis sets. Equilibrium geometries have also been determined with TZ2P + f and QZ3P basis sets in order to examine the convergence of properties with expansion of the basis set. The best estimates of the vibration frequencies for HO2 + are v 1 = 3140 ± 100, v 2 = 1140 ± 40, v 3 = 1535 ± 40 cm-1.

The complete harmonic vibrational force field of pyrrole has been calculated by the ab initio gradient method at the Hartree-Fock level using the 4–21 basis set. The force field was then scaled with a set of six factors transferred from benzene, and the vibrational spectrum of pyrrole was calculated. This a priori prediction, made with no reference to observations on pyrrole, agreed with the known experimental fundamental frequencies with a mean deviation of 12 cm-1 for in-plane modes and 20 cm-1 for out-of-plane modes except for the NH wagging and NH stretch. A new set of ten scale factors was next obtained by direct fitting of the computed force field to the observed pyrrole spectrum, producing the best force field obtainable by combined use of the theoretical and experimental information. This force field reproduced the entire pyrrole spectrum with mean deviations of 4·2 cm-1 (in-plane) and 5·9 cm-1 (out-of-plane). The spectra of three deuterated forms of pyrrole were also computed. Infrared absorption...