Molecular Orbital Contributions Research Articles

KL.01.01 Prosocial primates: cooperation and empathy

The nature and origin of ion-π and ion-σ interactions has been systematically investigated using dispersion-corrected density functional theory and the recently developed noncovalent interaction (NCI) method. A detailed analysis of these interactions is performed with the aim to identify the requirements that have to be fulfilled by the molecular system for strong ion-ligand interactions. Interestingly, our results indicate that aliphatic systems, such as cyclohexane, can interact as strong as aromatic ones with both cations and anions, despite of having a negligible quadrupole moment. In fact, cyclohexane binds anions stronger than benzene itself but slightly weaker that hexafluorobenzene. The NCI method reveals that the interaction between the ions and three C–H bonds of the saturated fragment are responsible for the surprisingly strong ion-σ interaction. A weakening of the ion-σ interactions is observed in the order: Li+ > F− > Na+ > Cl− > Br− ≈ K+. In addition, a complete Ziegler–Rauk type energy decomposition analysis has been carried out in order to reveal the origins of the thermodynamic driving force for complex formations. The electron density deformation upon complex formation has been scrutinized with a complementary NOCV analysis allowing the identification of molecular orbital interaction contributions to the stabilization. Based on these analysis, it is shown that the formally anion-π interaction is rather an anion-σ∗ interaction.

Schiff base fluorescence probes for Cu2+ based on imidazole and benzimidazole

Two novel Schiff base fluorescence probes for Cu2+ were designed and synthesized by reactions between p-fluorobenzaldehyde, p-phenylenediamine and heterocyclic compound (including imidazole and benzimidazole). The optimized geometries, frontier orbitals, molecular orbital contributions and excitation energies for the probes were predicted at B3LYP/6-31G* by theoretical calculation. And the probes were characterized by FTIR, 1H NMR, UV–vis and fluorescence spectroscopy. Test results showed the probes had a high selectivity for Cu2+ compared to other metal ions measured (Ag+, Co2+, Mg2+, Ni2+, Pb2+, Zn2+, Cd2+ and Fe3+). From test results, a high selectivity for Cu2+ was discovered in this type of probes, especially, the probe based on benzimidazole exhibited more significant enhancement in fluorescence intensity compared with imidazole-based due to its relative large conjugate rigid plane structure. The detection mechanism of the synthesized probes with high selectivity for Cu2+ was also discussed.

Theoretical study of acene-bridged dyes for dye-sensitized solar cells.

The electronic structures and absorption spectra for a series of acene-based organic dyes and the adsorption energy and optical properties for these dyes adsorbed on (TiO2)38 have been investigated using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. The effects of acene units and different substitution positions of electron donors on the optoelectronic properties of the acene-modified dyes are demonstrated. The photophysical properties of tetracene- and pentacene-based dyes are found to be tuned by changing the size of acene and the substitution position of the donor. The donor sites have a significant influence on the absorption wavelength mainly because of different molecular orbital (MO) contributions of the highest occupied molecular orbital (HOMO) on the bridging acene units, and the increasing MO contribution would lead to the red shift in the absorption spectra. Meanwhile, the donor is located close to the center of the π-conjugated bridge, and the absorption spectra are extended. The adsorption energy and optical properties of tetracene- and pentacene-based dyes adsorbed on (TiO2)38 suggest that acene-bridged dyes could be adsorbed on the TiO2 surface and inject electrons into semiconductors effectively. Then the results obtained from the hexacene-based dyes confirm the conclusions proposed from the tetracene- and pentence-based dyes. This study will provide a useful reference to the future design and optimization of acene dyes for dye-sensitized solar cell applications.

Electronic structure and chemical bond nature in Cs2PuO2Cl4

X-ray photoelectron spectral analysis of dicaesiumtetrachlorodioxoplutonate (Cs2PuO2Cl4) single crystal was done in the binding energy range 0-~35 eV on the basis of binding energies and structure of the core electronic shells (~35 eV-1250 eV), as well as the relativistic discrete variation calculation results for the PuO2Cl4 (D4h). This cluster reflects Pu close environment in Cs2PuO2Cl4 containing the plutonyl group PuO2. The many-body effects due to the presence of cesium and chlorine were shown to contribute to the outer valence (0-~15 eV binding energy) spectral structure much less than to the inner valence (~15 eV- ~35 eV binding energy) one. The filled Pu 5f electronic states were theoretically calculated and experimentally con- firmed to present in the valence band of Cs2PuO2Cl4. It corroborates the suggestion on the direct participation of the Pu 5f electrons in the chemical bond. The Pu 6p atomic orbitals were shown to participate in formation of both the inner and the outer valence molecular orbitals (bands), while the filled Pu 6p and O 2s, Cl 3s electronic shells were found to take the largest part in formation of the inner valence molecular orbitals. The composition of molecular orbitals and the sequence order in the binding energy range 0-~35 eV in Cs2PuO2Cl4 were established. The quantitative scheme of molecular orbitals for Cs2PuO2Cl4 in the binding energy range 0-~15 eV was built on the basis of the experimental and theoretical data. It is fundamental for both understanding the chemical bond nature in Cs2PuO2Cl4 and the interpretation of other X-ray spectra of Cs2PuO2Cl4. The contributions to the chemical binding for the PuO2Cl4 cluster were evaluated to be: the contribution of the outer valence molecular orbitals -66 %, the contribution of the inner valence molecular orbitals -34 %.

From Charge State to Isosurfaces to Spectra: Unraveling the Mystery of Lys-Trp Dipeptide Fluorescence

The quest for factors controlling the partition of excitation energy between fluorescence and nonradiative processes in tryptophan is of perennial interest to spectroscopists because its fluorescence has distinct intensity, color and environmental sensitivity. Solving the relationship between specifics of molecular environment and characteristics of tryptophan fluorescence would provide the key for emission spectra interpretation of a wide variety of proteins. Our quantum mechanical calculations and molecular modeling reveal substantial differences in ground state isosurface charge distribution on the indole ring for Lys-Trp dipeptides when backbone and residue charge is varied. These isosurfaces represent the superposition of all ground state molecular orbital contributions to charge distribution and thereby provide a visual representation of the contributions of all relevant orbitals. Comparison of Lys-Trp species isosurfaces with experimentally derived quantum yields and fluorescence lifetimes reveals a correlation: high π-electron density on the indole ring is associated with high quantum yield and long average lifetime, both of which are found in the zwitterionic and anionic states. Conversely, low quantum yield and short average lifetime is associated with low, uneven π-electron density on the indole ring, which is found in the cationic and highly anionic state where the indole amine is deprotonated. This interpretation explains fluorescence results for tryptophan-containing proteins. Where x-ray crystal structures have shown proteinaceous tryptophans to be hydrogen bonded at the indole amine, very low quantum yields are observed, and isosurfaces resembling those with a deprotonated indole amine are anticipated. Thus isosurfaces reveal losses in π-electron density over the indole ring or a loss of aromaticity for low fluorescing tryptophans. Isosurfaces are the key to solving the relationship between molecular environment and tryptophan fluorescence.

Communication: Localized molecular orbital analysis of the effect of electron correlation on the anomalous isotope effect in the NMR spin-spin coupling constant in methane

We discuss the effect of electron correlation on the unexpected differential sensitivity (UDS) in the (1)J(C-H) coupling constant of CH4 using a decomposition into contributions from localized molecular orbitals and compare with the (1)J(N-H) coupling constant in NH3. In particular, we discuss the well known fact that uncorrelated coupled Hartree-Fock (CHF) calculations are not able to reproduce the UDS in methane. For this purpose we have implemented for the first time a localized molecular orbital analysis for the second order polarization propagator approximation with coupled cluster singles and doubles amplitudes--SOPPA(CCSD) in the DALTON program. Comparing the changes in the localized orbital contributions at the correlated SOPPA and SOPPA(CCSD) levels and at the uncorrelated CHF level, we find that the latter overestimates the effect of stretching the bond between the coupled atoms on the contribution to the coupling from the localized bonding orbital between these atoms. This disturbs the subtle balance between the molecular orbital contributions, which lead to the UDS in methane.

Scrutinizing ion-π and ion-σ interactions using the noncovalent index and energy decomposition analysis

The nature and origin of ion-π and ion-σ interactions has been systematically investigated using dispersion-corrected density functional theory and the recently developed noncovalent interaction (NCI) method. A detailed analysis of these interactions is performed with the aim to identify the requirements that have to be fulfilled by the molecular system for strong ion-ligand interactions. Interestingly, our results indicate that aliphatic systems, such as cyclohexane, can interact as strong as aromatic ones with both cations and anions, despite of having a negligible quadrupole moment. In fact, cyclohexane binds anions stronger than benzene itself but slightly weaker that hexafluorobenzene. The NCI method reveals that the interaction between the ions and three C–H bonds of the saturated fragment are responsible for the surprisingly strong ion-σ interaction. A weakening of the ion-σ interactions is observed in the order: Li+>F−>Na+>Cl−>Br−≈K+. In addition, a complete Ziegler–Rauk type energy decomposition analysis has been carried out in order to reveal the origins of the thermodynamic driving force for complex formations. The electron density deformation upon complex formation has been scrutinized with a complementary NOCV analysis allowing the identification of molecular orbital interaction contributions to the stabilization. Based on these analysis, it is shown that the formally anion-π interaction is rather an anion-σ∗ interaction.

NMR and DFT analysis of trisaccharide from heparin repeating sequence.

NMR and density functional theory (DFT) have afforded detailed information on the molecular geometry and spin-spin coupling constants of a trisaccharide from the heparin repeating-sequence. The fully optimized molecular structures of two trisaccharide conformations (differing from each other in the form of the central iduronic acid residue) were obtained using the B3LYP/6-311+G(d,p) level of theory in the presence of solvent, the latter included as either explicit water molecules or via a continuum solvent model. NMR spin-spin coupling constants were also computed using various basis sets and functionals and then compared with measured experimental values. Optimized structures for both conformers showed differences in geometry at the glycosidic linkages and in the formation of intramolecular hydrogen bonds. Three-bond proton-proton coupling constants ((3)JH-C-C-H), based on fully optimized geometry computed using the B3LYP/6-311+G(d,p)/UFF level of theory and hydrated with 57 water molecules, showed that the best agreement with experiment was obtained with the 6-311+G(d,p) basis set and a weighted average of 55:45 ((1)C4:(2)S0) of the IdoA2S forms. Other basis sets, DGDZVP and TZVP, also gave acceptable data for most coupling constants, with DGDZVP outperforming the TZVP. Detailed analysis of Fermi-contact contributions to (3)JH-C-C-H showed that important contributions arise from oxygen at both glycosidic linkages, as well as from oxygen atoms on the neighboring monosaccharide units. Their contribution to the Fermi term cannot be neglected and must be taken into account for a correct description of coupling constants. The analysis also showed that the magnitude of paramagnetic (PSO) and diamagnetic (DSO) spin-orbit contributions is comparable to the magnitude of the Fermi-contact contribution in some coupling constants in the IdoA2S residue. Calculations of the localized molecular orbital contributions to the DSO terms from separate conformational residues showed that the contribution from adjacent residues is not negligible and can be important for the spin-spin coupling constants between protons located close to the geometrical center of the molecule. These contributions should be taken into account when interpreting DSO terms in spin-spin coupling constants especially in large molecules.

An investigation of chlorine ligands in transition-metal complexes via ³⁵Cl solid-state NMR and density functional theory calculations.

Chlorine ligands in a variety of diamagnetic transition-metal (TM) complexes in common structural motifs were studied using (35)Cl solid-state NMR (SSNMR), and insight into the origin of the observed (35)Cl NMR parameters was gained through first-principles density functional theory (DFT) calculations. The WURST-CPMG pulse sequence and the variable-offset cumulative spectrum (VOCS) methods were used to acquire static (35)Cl SSNMR powder patterns at both standard (9.4 T) and ultrahigh (21.1 T) magnetic field strengths, with the latter affording higher signal-to-noise ratios (S/N) and reduced experimental times (i.e., <1 h). Analytical simulations were performed to extract the (35)Cl electric field gradient (EFG) tensor and chemical shift (CS) tensor parameters. It was found that the chlorine ligands in various bonding environments (i.e., bridging, terminal-axial, and terminal-equatorial) have drastically different (35)Cl EFG tensor parameters, suggesting that (35)Cl SSNMR is ideal for characterizing chlorine ligands in TM complexes. A detailed localized molecular orbital (LMO) analysis was completed for NbCl5. It was found that the contributions of individual molecular orbitals must be considered to fully explain the observed EFG parameters, thereby negating simple arguments based on comparison of bond lengths and angles. Finally, we discuss the application of (35)Cl SSNMR for the structural characterization of WCl6 that has been grafted onto a silica support material. The resulting tungsten-chloride surface species is shown to be structurally distinct from the parent compound.

The R–7 Dispersion Interaction in the General Effective Fragment Potential Method

The R(-7) term (E7) in the dispersion expansion is developed in the framework of the general effective fragment potential (EFP2) method, formulated with the dynamic anisotropic Cartesian polarizability tensors over the imaginary frequency range. The E7 formulation is presented in terms of both the total molecular polarizability and the localized molecular orbital (LMO) contributions. An origin transformation from the center of mass to the LMO centroids is incorporated for the computation of the LMO dipole-quadrupole polarizability. The two forms considered for the damping function for the R(-7) dispersion interaction, the overlap-based and Tang-Toennies damping functions, are extensions of the existing damping functions for the R(-6) term in the dispersion expansion. The R(-7) dispersion interaction is highly orientation dependent: it can be either attractive or repulsive, and its magnitude can change substantially as the relative orientation of two interacting molecules changes. Although the R(-7) dispersion energy rotationally averages to zero, it may be significant for systems in which rotational averaging does not occur, such as rotationally rigid molecular systems as in molecular solids or constrained surface reactions.

Quantum mechanical study of the structure and spectroscopic (FTIR, FT-Raman), first-order hyperpolarizability and NBO analysis of 1,2-benzoxazol-3-ylmenthane sulfonamide

Fourier transform infrared (FTIR) and FT-Raman spectra of 1,2-benzoxazol-3-ylmenthane sulfonamide in the solid phase were recorded and analyzed. The molecular geometry, vibrational frequencies, infrared intensities, Raman activities and atomic charges were calculated using HF and density functional theory calculation (B3LYP) with standard 6-31G(d, p) basis set. Complete vibrational assignment and analysis of the fundamental modes of the compound were carried out using the observed FTIR and FT-Raman spectra. The thermodynamic functions of the title compound were also performed. Stability of the molecule arising from hyper-conjugative interactions, charge delocalization has been analyzed using natural bond orbital (NBO) analysis. The dipole moment (μ), polarizability (α) and the hyperpolarizability (β) values of the molecule has been computed. Potential Energy Distribution (PED) were computed for the assignment of unambiguous vibrational fundamental modes. UV–vis spectrum of the compound was also recorded. The theoretical electronic absorption spectra have been calculated by TD-DFT/B3LYP using 6-31G(d,p) basis set. The HOMO and LUMO energy gap reveals that the chemical activity of the molecule. The molecular orbital contributions were studied by density of energy states (DOSs). Thermodynamic properties (heat capacity, entropy and enthalpy) of the title compound at different temperatures were calculated. Finally, simulated FTIR and FT-Raman spectra of 1,2-benzoxazol-3-ylmenthane sulfonamide showed good agreement with the observed spectra.

Interpretation of Electron Delocalization in Benzene, Cyclobutadiene, and Borazine Based on Visualization of Individual Molecular Orbital Contributions to the Induced Magnetic Field

The magnetic response of the valence molecular orbitals (MOs) of benzene, cyclobutadiene, and borazine to an external magnetic field has been visualized by calculating the chemical shielding in two-dimensional grids of points on the molecular plane and on a plane perpendicular to it, using gauge-including atomic orbitals (GIAOs). The visualizations of canonical MO contributions to the induced magnetic field (CMO-IMF) provide a clear view of the spatial extension, the shape, and the magnitude of shielding and deshielding areas within the vicinity of the molecule, originating from the induced currents of each valence orbital. The results are used to investigate the delocalization of each valence MO and to evaluate its contribution to the aromatic character of systems under study. The differentiation of the total magnetic response among the three molecules originates exclusively from π-HOMO orbitals because the magnetic response of the subsets of the remaining MOs is found to be almost identical. Borazine is classified as nonaromatic as the four electrons that occupy the π-HOMO are found to be strongly localized on nitrogen centers. CMO-IMF can clarify the interpretation of various NICS indexes and can be applied for the investigation of various types of electron delocalization.

Ionization of water molecules by ion beams. On the relevance of dynamic screening and the influence of the description of the initial state

Single ionization from water molecules by impact of protons, alpha particles and C6 + ions is studied. The post- and prior-versions of the continuum distorted wave-eikonal initial state (CDW-EIS) model within an independent electron approximation are employed to compute double differential cross sections. To avoid the complexity of using numerical molecular continuum states in the cross-section calculations, effective Coulombic continuum wavefunctions are employed. However, this may lead to the appearance of post–prior discrepancies and this fact is examined in detail. Moreover, the influence of the dynamic screening on this behaviour is studied. In addition, the contribution of different molecular orbitals to the angular spectrum is analysed for several ejection electron energies. Finally, the sensitivity of CDW-EIS calculations to the representation of the initial bound molecular orbitals is investigated.

DFT computations and spectroscopic analysis of p-bromoacetanilide

This work presents the characterization of p-bromoacetanilide (PBA) by quantum chemical calculations and spectral techniques. The spectroscopic properties were investigated by FT-IR, FT-Raman and UV–Vis techniques. The structural and spectroscopic data of the molecule were obtained from B3LYP/6-311++ G(d,p) and MPW1PW91/6-311++G(d,p) basis set calculations. The theoretical wavenumbers were scaled and compared with experimental FT-IR and FT-Raman spectra. The complete assignments were performed on the basis of the normal co-ordinate analysis (NCA), experimental results and potential energy distribution (PED) of the vibrational modes, calculated with scaled quantum mechanics (SQM) method, interpreted in terms of fundamental modes. The stability of molecule has been analyzed by NBO/NLMO analysis. The molecular orbital contributions were studied by using the density of states. The electronic properties like UV–Vis spectral analysis and HOMO–LUMO energies were reported. The calculated HOMO and LUMO energies shows that charge transfer interactions taking place within the molecule. Mulliken population analysis on atomic charges, Statistical thermodynamic properties at various temperatures of the PBA is also calculated.

Sensitivity of X-ray Core Spectroscopy to Changes in Metal Ligation: A Systematic Study of Low-Coordinate, High-Spin Ferrous Complexes

In order to assess the sensitivity and complementarity of X-ray absorption and emission spectroscopies for determining changes in the metal ligation sphere, a systematic experimental and theoretical study of iron model complexes has been carried out. A series of high-spin ferrous complexes, in which the ligation sphere has been varied from a three-coordinate complex, [L(tBu)Fe(SPh)] (1) (where L(tBu) = bulky β-diketiminate ligand; SPh = phenyl thiolate) to four-coordinate complexes [L(tBu)Fe(SPh)(X)] (where X = CN(t)Bu (2); 1-methylimidazole (3); or N,N-dimethylformamide (DMF) (4)), has been investigated using a combination of Fe K-edge X-ray absorption (XAS) and Kβ X-ray emission (XES) spectroscopies. The Fe K XAS pre-edge and edge of all four complexes are consistent with a high-spin ferrous assignment, with the largest differences in the pre-edge intensities attributed to changes in covalency of the fourth coordination site. The X-ray emission spectra show pronounced changes in the valence to core region (V2C) as the identity of the coordinated ligand is varied. The experimental results have been correlated to density functional theory (DFT) calculations, to understand key molecular orbital contributions to the observed absorption and emission features. The calculations also have been extended to a series of hypothetical high-spin iron complexes to understand the sensitivity of XAS and XES techniques to different ligand protonation states ([L(tBu)Fe(II)(SPh)(NHn)](3-n) (n = 3, 2, 1, 0)), metal oxidation states [L(tBu)Fe(SPh)(N)](n-) (n = 3, 2, 1), and changes in the ligand identity [L(tBu)Fe(IV)(SPh)(X)](n-) (X = C(4-), N(3-), O(2-); n = 2, 1, 0). This study demonstrates that XAS pre-edge data have greater sensitivity to changes in oxidation state, while valence to core (V2C) XES data provide a more sensitive probe of ligand identity and protonation state. The combination of multiple X-ray spectroscopic methods with DFT results thus has the potential to provide for detailed characterization of complex inorganic systems in both chemical and biological catalysis.

Measuring dipolar and J coupling between quadrupolar nuclei using double-rotation NMR

Among the interactions which govern NMR spectra, spin-spin coupling interactions provide the most direct form of structural information which is of interest to chemists. Dipolar coupling may be used to measure internuclear distances directly and J coupling may be used to identify bonding interactions and provide insights into the nature of the bonds. It is well known that the presence of a quadrupolar interaction reintroduces the dipolar interaction in spinning samples; however, similarly to the J coupling, this information is often lost if the observed nucleus is quadrupolar due to quadrupolar spectral broadening. Here we show for multiple spin pairs that double-rotation (DOR) NMR fully removes the effects of the quadrupolar interaction on the NMR spectrum leaving only the effects of dipolar and J couplings. We also demonstrate that the J coupling multiplets do not disappear for quadrupolar A2 spin pairs as they do for spin-1/2 nuclei. With DOR NMR, it is then straightforward to measure homonuclear J coupling constants between magnetically equivalent quadrupolar nuclei. A deeper understanding of the origins of the magnitudes and dominant mechanisms of J coupling for quadrupolar spin pairs in a series of related compounds is obtained by decomposing computed J coupling constants into their major molecular orbital contributions.

Validating and Analyzing EPR Hyperfine Coupling Constants with Density Functional Theory.

This paper focuses on the calculation of the (isotropic) hyperfine coupling tensor, AisoK, which consists of the Fermi contact term (AFCK) and a spin orbit correction, the pseudocontact term (APCK). Using the aug-cc-pVTZ-J basis set, we test a range of correlation exchange functionals against experimental values for a series of first row transition metal complexes. This has been done both with (AisoK = AFCK + APCK) and without (AisoK = AFCK) spin orbit coupling included. Overall, hybrid functionals perform best, although some exceptions are found. Furthermore, we analyze molecular orbital contributions to the Fermi contact term. We find a great difference in the relative magnitude of contributions from frontier orbitals and inner or outer-core orbitals. Complexes, where the frontier orbital contribution exceeds the core-orbital contributions, are always small, ionic complexes (“class 1”). For these complexes, the computational requirements with respect to the one-electron basis set are not severe, and reg...

Novel Multinuclear Cyclic and V‐shaped Cuprous Clusters with Short and Unsupported Cu(I)‐Cu(I) Separations

AbstractA series of novel cyclic and V‐shaped cuprous clusters, (HNEt3)2 [Cu6(bmsapy)4] (1), [Cu8(banlny)4] (2) and [Cu6(ansny)4Cl] (CF3SO3) (3), were synthesized. Compounds (1) and (2) possess a similar cyclic metal structural arrangement, and the metallic arrangement of compound (2) is as in (1) with an insertion of a pair of ligand‐unbridged Cu(I) atoms. Compounds (1) and (2) both have unusual ligand‐unbridged Cu(I)‐Cu(I) separations, but compound (3) does not. The ligand‐unbridged Cu(I)‐Cu(I) distances in (2) are shorter than those in (1). In absorption spectra, compound (2) displayed red shifts from (1) due to the increase in nuclearity and the number of the aromatic rings in ligands. TDDFT calculations suggest that in HOMOs, the major molecular orbital contribution is from the six Cu(I) atoms implying that the photoluminescent property of (1) is dependent on the cyclic hexacopper arrangement rather than that of (HNEt3) [Cu3(btsapy)2] (4).

Orbital tomography for highly symmetric adsorbate systems

Orbital tomography is a new and very powerful tool to analyze the angular distribution of a photoemission spectroscopy experiment. It was successfully used for organic adsorbate systems to identify (and consequently deconvolute) the contributions of specific molecular orbitals to the photoemission data. The technique was so far limited to surfaces with low symmetry like fcc(110) oriented surfaces, owing to the small number of rotational domains that occur on such surfaces. In this letter we overcome this limitation and present an orbital tomography study of a 3,4,9,10-perylene-tetra-carboxylic-dianhydride (PTCDA) monolayer film adsorbed on Ag(111). Although this system exhibits twelve differently oriented molecules, the angular resolved photoemission data still allow a meaningful analysis of the different local density of states and reveal different electronic structures for symmetrically inequivalent molecules. We also discuss the precision of the orbital tomography technique in terms of counting statistics and linear regression fitting algorithm. Our results demonstrate that orbital tomography is not limited to low-symmetry surfaces, a finding which makes a broad field of complex adsorbate systems accessible to this powerful technique.

The rotational barrier in ethane: a molecular orbital study.

The energy change on each Occupied Molecular Orbital as a function of rotation about the C-C bond in ethane was studied using the B3LYP, mPWB95 functional and MP2 methods with different basis sets. Also, the effect of the ZPE on rotational barrier was analyzed. We have found that σ and π energies contribution stabilize a staggered conformation. The σs molecular orbital stabilizes the staggered conformation while the stabilizes the eclipsed conformation and destabilize the staggered conformation. The πz and molecular orbitals stabilize both the eclipsed and staggered conformations, which are destabilized by the πv and molecular orbitals. The results show that the method of calculation has the effect of changing the behavior of the energy change in each Occupied Molecular Orbital energy as a function of the angle of rotation about the C–C bond in ethane. Finally, we found that if the molecular orbital energy contribution is deleted from the rotational energy, an inversion in conformational preference occurs.