Search

Introductory chemistry textbooks often present valence bond (VB) theory as useful, but incorrect and inferior to molecular orbital (MO) theory, citing the electronic structure of O[subscript 2] and electron delocalization as evidence. Even texts that initially present the two theories on equal footing use language that biases students toward the MO approach. However, these "failures" of VB are really just misconceptions and/or misapplications of the theory. At their theoretical limits, both VB and MO are equivalent; they simply approach that limit from different sides. Certain concepts may be easier to grasp with one theory or the other so that having a commanding knowledge of both is extremely beneficial. However, presenting one theory as superior to the other suppresses the ability to look at a problem from both sides and is therefore detrimental to students and the whole of chemistry. It is time for VB and MO to be taught on equal footing like the complementary theories they are.

The role of d-orbital hybridization in the chemistry curriculum and a qualitative description of bonding in SF[subscript 6] are described. The sp[cubed]d[squared] hybridization model found to be helpful in understanding the chemical phenomenon of chemical bonding in SF[subscript 6] and are not applicable to all situations.

(8/8) CASSCF and (8/8) CASPT2 calculations are performed in order to investigate the potential surface for the ring expansion of the (super1) A(sub2) state of phenlphosphinidene to 1phospha-1,2,4,6-cycloheptatetraene. Unlike the comparable ring expansion of the (super1) A(sub2) state of phenylnitrene to 1-aza-1,2,4,6-cycloheptatetraene, ring expansion is computed to be quite endothermic.

Valence bond theory has been used to investigate the transition metal hydride cations TMH+. It was established that modern valence bond methods are valuable for studies involving transition metals.

A spectroscopic characterization of nitrosyl azide (N4O) shows that the 6-pi-electron ring isomer is the most stable structure on the potential energy hypersurface. Isolation is difficult as the barrier to dissociation may be in the range 1-2 kcal/mol-1. A trans-chain structure is the lowest energy chain isomer. A study uses different theoretical methods to characterize geometrical parameters and vibrational frequencies of both the chain and ring isomers. The results are in agreement with the experimental results.

Theoretical studies and predictions at various levels for a series of substituted oxirenes with the formula X2C2O, where X = CH2, CH3, NH2, OH and F, reveal that only the dimethyloxirene molecule can be a true minimum on the surface of double-plus polarization single and double coupled clusters with perturbative triples corrections. All other substituted oxirenes might be transition states or lower levels of theory. Initial results on the benzoxirene study are provided.

Charge‐shift bonds (CSBs) constitute a new class of bonds different than covalent/polar‐covalent and ionic bonds. Bonding in CSBs does not arise from either the covalent or the ionic structures of the bond, but rather from the resonance interaction between the structures. This Essay describes the reasons why the CSB family was overlooked by valence‐bond pioneers and then demonstrates that the unique status of CSBs is not theory‐dependent. Thus, valence bond (VB), molecular orbital (MO), and energy decomposition analysis (EDA), as well as a variety of electron density theories all show the distinction of CSBs vis‐à‐vis covalent and ionic bonds. Furthermore, the covalent–ionic resonance energy can be quantified from experiment, and hence has the same essential status as resonance energies of organic molecules, e.g. benzene. The Essay ends by arguing that CSBs are a distinct family of bonding, with a potential to bring about a Renaissance in the mental map of the chemical bond, and to contribute to productive chemical diversity. [ABSTRACT FROM AUTHOR]

The valence bond theory (VB) is applied to the coordinate covalent N-B bonds in the Lewis acid/base adducts R3N-BR3 where R represent an electron-donating methyl group or an electron-withdrawing (F). VB weights, in conjunction with bond decomposition results, show that the donor-acceptor bonds are charge shift bonds where mixing of configurations, rather than any one configuration alone, is responsible for the bonding event.

Inspired by the recent experimental study of the radical anions HCCN- and HCNC- and by earlier examinations of HCCN, the equilibrium geometry of the HCNC molecule has been investigated using both self-consistent field (SCF) and configuration interaction methods including single and double excitations (CISD). The largest basis set used was a triple-ζ plus double polarization with diffuse functions and higher angular momentum functions appended to each atom [TZ2P(f,d)+diff]. Using this basis, the H–C–N equilibrium angle is predicted to be 128.5° at the CISD level of theory. Additionally, the zero point vibrational energy (ZPVE) corrected energy separation of the bent and linear conformations was predicted to be 10.1 kcal mol-1 at the CISD level of theory with the largest basis set employed. The barrier to linearity is 7.7 kcal mol-1 at the CCSD level of theory and 6.9 kcal mol-1 at the CCSD(T) level of theory, employing the CISD optimized geometries with a basis that was comprised of triple-ζ plus double polarization with higher angular momentum functions appended to each atom [TZ2P(f,d)]. These results were compared to those obtained in previous ab initio investigations of HCCN, which has been dubbed a quasilinear molecule by the most recent experimental investigators. HCNC is predicted to lie 22.2 kcal mol-1 above HCCN at the CISD level of theory, with a the TZ2P(f,d) basis. The differences between the two isomers are discussed and HCNC is predicted to be a definitively bent molecule, rather than quasilinear. [ABSTRACT FROM AUTHOR]

Using sophisticated ab initio methods, the equilibrium structure of the ClF2 radical has been predicted to be bent, confirming previous experimental and theoretical work. The single, double and perturbative triple excitation coupled cluster level of theory along with a triple-ζ plus double polarization basis set gives the geometrical parameters re=1.756 Å and θe=151.8°. At the same level of theory, harmonic vibrational frequencies were predicted to be 536, 243, and 568 cm-1 for the symmetric stretch, bend and asymmetric stretch, respectively. In addition to this high level study of the harmonic vibrational frequencies, theoretical analyses of anharmonic terms have been included in order to help settle the controversy surrounding experimental assignments of the fundamental vibrational frequencies. This research supports Mamantov’s experimental assignment of the bending and symmetric stretching fundamental, over the experimental objections of Prochaska. The vibrational frequency splitting due to 37ClF2 was also studied indicating that only the splitting of the bending and asymmetric stretching modes should be readily observable experimentally. Dipole moments and infrared (IR) intensities were also predicted. [ABSTRACT FROM AUTHOR]

There is concern within theoretical chemistry that density functional methods are fundamentally in error for negative ions. We have tested this hypothesis by evaluating electron affinities for F and F2 with a variety of density functionals and extremely large, diffuse basis sets. In addition, we have observed the behavior of a known unbound system Ne-. We have found no convincing evidence to support the claims of negative ion instability with density functional methods. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Structural weights and bond dissociation energies have been determined for H-F, H-X, and F-X molecules (-X = -OH, -NH2, and -CH3) at the valence bond self-consistent field (VBSCF) and breathing orbital valence bond (BOVB) levels of theory with the aug-cc-pVDZ and 6-31++G(d,p) basis sets. At the BOVB level, the aug-cc-pVDZ basis set yields a counterintuitive ordering of ionic structural weights when the initial heavy atoms-type basis functions are included. For H-F, H-OH, and F-X, the ordering follows chemical intuition when these basis functions are not included. These counterintuitive weights are shown to be a result of the diffuse polarisation function on one VB fragment being spatially located, in part, on the other VB fragment. Except in the case of F-CH3, this problem is corrected with the 6-31++G(d,p) basis set. The initial heavy atoms-type functions are shown to make an important contribution to the VB orbitals and bond dissociation energies and, therefore, should not be excluded. It is recommended to not use diffuse basis sets in valence bond calculations unless absolutely necessary. If diffuse basis sets are needed, the 6-31++G(d,p) basis set should be used with caution and the structural weights checked against VBSCF values which have been shown to follow the expected ordering in all cases. [ABSTRACT FROM AUTHOR]

Predictions of molecular structure and energies have been made for each of the members in the ClFn/ClFn- (n = 1-7) series of molecules using density functional theory (DFT). The three different types of prediction for electron affinities determined in this work are: the adiabatic electron affinity (EAad), the vertical electron affinity (EAvert), and the vertical detachment energy (VDE) of the anion. Also reported are the first Cl-F dissociation energies for both the neutral and the anion (DCIFa). Self-consistent Kohn-Sham orbitals were obtained using four different functional forms and a double-zeta plus polarization (DZP) basis set. When diffuse s- and p-type functions were added to the basis set, the anions were stabilized significantly with respect to the neutrals, and the electron affinity values were increased significantly. Overall, the best agreement with experimental structures was obtained with the DZP++ BHLYP method based upon Becke's half-and-half exchange functional and the Lee-Yang-Parr correlation functional, while the other three functionals tended to overshoot bond lengths and bond angles. Although a number of experimental energetic results for the ClFn/ClFn series exist, many are ruled out of bounds by this study.The most reliable adiabatic electron affinities predicted are 2.74 eV (ClF), 4.93 eV (ClF2), 4.32 eV (ClF8), 5.73 eV (ClF4), 5.17 eV (ClF5), 5.51 eV (ClF6) and 8.65 eV (ClF7). It is hoped that the abundance of theoretical predictions reported in this work will provide a stimulus for future experimental studies in the important area of chlorine fluoride chemistry. [ABSTRACT FROM AUTHOR]

In an effort to elucidate the nature of general reactions between carbynes and hydrocarbons, the reactants, products and transition state for the reaction of methylidyne (CH) with methane (CH4) have been characterized using ab-initio quantum-mechanical methods. Both the self-consistent field (SCF) method and the method of configuration interaction of including single and double excitations (CISD) were employed initially. The largest basis used was a triple-zeta plus double-polarization plus f-functions basis (TZ2P + f). All levels of theory predict that this reaction proceeds through a transition state of Cs symmetry. With the TZ2P + f basis, the SCF method predicts an activation energy of 25.8 kcal mol-1 while CISD predicts 9.5 kcal mol-1. Coupled cluster methods including all single and double excitations (CCSD) applied to the optimized TZ2P + f CISD geometries yield an activation energy of 5.8 kcal mol-1. When connected triple excitations are added perturbatively to the CCSD energy (CCSD(T)), the activation energy is further reduced to 4.3 kcal mol-1. The estimated exact activation energy is 3.5 +/- 0.5 kcal mol-1. [ABSTRACT FROM AUTHOR]

Charge-shift bonds (CSBs) constitute a new class of bonds different than covalent/polar-covalent and ionic bonds. Bonding in CSBs does not arise from either the covalent or the ionic structures of the bond, but rather from the resonance interaction between the structures. This Essay describes the reasons why the CSB family was overlooked by valence-bond pioneers and then demonstrates that the unique status of CSBs is not theory-dependent. Thus, valence bond (VB), molecular orbital (MO), and energy decomposition analysis (EDA), as well as a variety of electron density theories all show the distinction of CSBs vis-à-vis covalent and ionic bonds. Furthermore, the covalent-ionic resonance energy can be quantified from experiment, and hence has the same essential status as resonance energies of organic molecules, e.g., benzene. The Essay ends by arguing that CSBs are a distinct family of bonding, with a potential to bring about a Renaissance in the mental map of the chemical bond, and to contribute to productive chemical diversity., (© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)