Charge-charge Flux-overlap Research Articles

The interaction strengths and spectroscopy parameters of the C2H2∙∙∙HX and HCN∙∙∙HX complexes (X = F, Cl, CN, and CCH) and related ternary systems valued by fluxes of charge densities: QTAIM, CCFO, and NBO calculations.

This theoretical work exhibits a new systematic study of structural parameters, electronic properties, infrared vibration modes, and molecular topography of hydrogen complexes, namely linear-type HCN⋯HX and T-type C2H2⋯HX (X = F, Cl, CN, and CCH). Ideally, the knowledge of the ternary systems of C2H2⋯HCN⋯HF and HCN⋯HCN⋯HF whose subparts integrate the linear and T-shaped complexes were used to give support in this current research. By means of computational calculations carried out in both levels B3LYP and MP2, the variations of the HX bond lengths are clearly overestimated in the HCN⋯HX linear complexes. In agreement with the analyses of the electrostatic potentials, the higher intermolecular energies of these complexes agree with the larger red-shifts in the stretch frequencies in HX. Also, the QTAIM descriptors and NBO calculations were used to inspect the interaction strength as well as to confirm the π cloud as a proton accepting center. By taking into account the absorption intensity ratio as a standard parameter to predict the interaction strength and intermolecular characterization, the formalism of the charge-charge flux-overlap modified (CCFO) was applied.

Comparative study of Green's function matrix elements and charge transfers obtained from different partitioning schemes of molecular charge in hydrogen-bonded complexes

RHF and MP2 ab initio molecular orbital calculations using the 4-31G**, 6-311G** and cc-pVTZ basis sets have revealed that the Green's function matrix element (G D,A) values show a good correlation with the amount of intermolecular transferred charges obtained from different charge partitioning schemes for the CNH…CNH, NCH…CNH, CNH…NCH and NCH…NCH hydrogen bonded complexes. This is evident specially when the hydrogen bond distance is progressively increased from the equilibrium position until 4.5 Å. However, G D,A values show a better linear correlation with deltaQ values using corrected Mülliken charges, which are obtained from the charge-charge flux-overlap (CCFO) model for infrared intensities. In this case, both G D,A and deltaQcorr form two practically superposed exponential curves. On the other hand, G D,A values show a smaller agreement with deltaQ values obtained from atomic charges derived from natural bonding orbitals. This is clearly verified when considering the first order exponential decay rate of G D,A versus deltaQ obtained from different charge partitioning schemes.

Hydrogen bonds between hydrogen fluoride and aromatic azines: An ab initio study

AbstractMP2/6‐31++G(d,p) ab initio molecular orbital calculations have been employed to characterize hydrogen‐bonded complexes between hydrogen fluoride as proton donor and aromatic azines (pyridine, pyrimidine, pyridazine, pyrazine, 1,3,5‐triazine, and 1,2,4‐triazine). Our calculations have shown that the H‐bond strength in these complexes depends on both the number of nitrogen atoms as well as the position of these atoms in aromatic ring. The binding energies of azines‐HF with or without BSSE and ZPE corrections decrease with the increasing number of nitrogen atoms in the ring. For example, its ΔEBSSE,ZPE value is 40.9 kJ · mol−1 in pyridine‐HF, whereas its corresponding values in pyrazine‐HF and 1,3,5‐triazine‐HF are 34.0 kJ · mol−1 and 26.6 kJ · mol−1, respectively. With respect to position of the nitrogen atom in the aromatic ring, the more pronounced effect is verified when it is at meta‐position relative to the pyridine ring. There is a high linear correlation between the H‐bond strength in these complexes and the intermolecular charge transfer using corrected Mulliken charges obtained from the charge‐charge flux‐overlap (CCFO) modified model for infrared intensities. Finally, our MP2/6‐31++G(d,p) calculations have revealed that stronger hydrogen bonds are associated with smaller H‐bond length values, larger intermolecular charge transfers and greater HF downward stretching frequency displacements. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006

A new partition of the atomic polar tensor: the benzene molecule

Gradient corrected exchange-correlation functional (BLYP) gives excellent predictions of the dipole moment derivatives and IR intensities of benzene. A new partition, generalized charge-charge flux overlap (GCCFO) analysis of the atomic polar tensor (APT) has been proposed based on Cioslowski's generalized APT and compared with CCFO, The GCCFO shows near independence on the choice of the basis sets. There is no qualitative difference between the APT for the in plane and the out of plane modes, as would be required by the rehybridization moment model. The purely quantum mechanical overlap term is too large to be neglected.

An ab initio study of hydrogen complexes of the X-H … π type between acetylene and HF or HCl

Abstract Ab initio calculations are performed at the HF/6-311G, HF/6-311G∗∗, HF/6-311++G∗∗, MP2/6-311G∗∗ and MP2/6-311++G∗∗ theoretical levels to obtain geometries, H-bond energies and vibrational properties for the symmetrical T-shaped structures of hydrogen-bonded complexes involving acetylene (as the proton acceptor) and the halides HF and HCl (as the proton donors). The calculated H-bond lengths are a little overestimated at the Hartree-Fock level, whereas MP2 results are in very close agreement with the experimental values. For acetylene-HF the H-bond energy is predicted to be 11 kJ mol−1 at the HF level (including BSSE) and 19 kJ mol−1 at the MP2 level, indicating that the complex is more strongly bound than a simple van der Waals complex. The acetylene-HCl complex is less strongly bound than acetylene-HF, with a ΔE value of 5.8 kJ mol−1 at the HF/6-311++G∗∗ level (including BSSE) and of 13.2 kJ mol−1 at the MP2/6-311++G∗∗ level. The strengths of the H-bonds in both complexes can be interpreted in terms of intermolecular charge transfers by means of atomic charges derived from infrared intensity parameters. The intermolecular vibrational modes exhibit the usual effects of H-bond formation, which are an increased intensity in the X-H stretching of the proton-donor mode together with little change in the electron-donor spectrum. These normal modes are adequately interpreted in terms of the charge-charge flux-overlap modified model for infrared intensities. The new vibrational modes arising from complexation show several interesting features, especially the bending modes of the proton-donor molecule and the intermolecular stretching mode.

Rotational isomerism in acrylic acid. A combined matrix-isolated IR, Raman and ab initio molecular orbital study

The results of a combined vibrational and structural study of the acrylic acid monomer undertaken by matrix-isolated low-temperature IR spectroscopy and ab initio SCF-HF and MP2 MO calculations are presented. In addition, both Raman and IR spectra of liquid acrylic acid and the Raman spectrum of the crystal are also reported and interpreted. It is shown that in both argon and krypton matrices acrylic acid monomer exists as a mixture of two conformers of similar energies, differing by the relative orientation of the CC—CO axis. Upon irradiation at λ= 243 nm by a xenon lamp, the s-cis form (CC—CO dihedral angle equal to 0 °), corresponding to the conformational ground state, converts to the s-trans form (CC—CO dihedral angle equal to 180 °). In the liquid phase, dimeric structures strongly predominate, but the existence in this phase of the two conformational states referred to above can also be inferred from the corresponding vibrational spectra. In turn, in the crystal only the thermodynamically most stable form (s-cis) exists. Results of ab initio SCF-HF and MP2 molecular orbital (MO) calculations, in particular optimised geometries, relative stabilities, dipole moments and harmonic force fields, for the relevant conformational states of acrylic acid are also presented and the conformational dependence of some relevant structural parameters is used to characterise the most important intramolecular interactions present in the studied conformers. Finally, the calculated vibrational spectra and both the results of a normal-mode analysis based on the theoretical harmonic force fields and of IR intensity studies based on the charge–charge flux–overlap (CCFO) model were used to help interpret the experimental vibrational data, enabling a detailed assignment of the acrylic acid spectra obtained in the different conditions considered.

Infrared intensity parameters for furan and thiophene

Determination of atomic charges and charge fluxes, derived from the charge-charge flux-overlap modified (CCFOM) model by using 4-31G ab initio calculations for furan and thiophene.

Ab initio studies of the linear complex HC 3N⋯HC 3N

Ab initio calculations using an STO-3G basis set have been performed to obtain the optimized geometry of the complex HC 3N⋯HC 3 N. A 6–31G basis set is then used at the STO-3G optimized geometry to estimate hydrogen bond energies, dipole moments, charge transfer effects, vibrational ground state rotational spectroscopic constants, and to analyze the perturbation of the N⋯H interaction on the IR intensity parameters of these atoms from their atomic polar tensors using the charge-charge flux-overlap (CCFO) model. These results support the conclusion from previous work, using the monomer experimental geometry and optimized r N⋯H bond distance, that the deformation modes are practically unaltered owing to the formation of the H bond and that the elements associated with the CN b and H b-C stretching modes increase substantially, mainly due to the charge flux term P zz (CF) in which charge transfer and polarization effects have important contributions. It is interesting that in this largest cyanopolyacetylene H-bonded complex of the series previously investigated, smaller basis sets and fixed monomer geometries yield similar trends in the physical interpretations as compared to the present more elaborate calculations using larger basis sets and a fully optimized geometry. This may be important regarding computational feasibility of larger complexes of the series under investigation, considering the large amount of computer time required to obtain optimized geometries of these complexes even at the STO-3G level.

Ab initio studies on hydrogen-bonded complexes involving hydrogen cyanide and cyanoacetylene

Ab initio calculations using the 4-31G and STO-3G basis sets have been performed to estimate the hydrogen-bond energies and distances of the HCN...HCN, HC3N...HCN, HCN...HC3N, and HC3N...Cchemically bondHC3N complexes. These calculations indicate that both are determined essentially by the nature of the proton donor molecule (PD). An accentuated increase of polarity has been found on each complex due to the H-bond formation. The perturbation of the N...H interaction on the infrared intensity parameters of these atoms has been analyzed from their atomic polar tensors (APTs) using the charge-charge flux-overlap (CCFO) model. This analysis indicates that the P/sub xx//sup (Nb)/ and P/sub xx//sup (Hb)/ elements associated with the deformation modes are practically unaltered due to the formation of the H bond. On the other hand, the P/sub zz//sup Nb)/ and P/sub zz//sup (Hb)/ elements associated with the C identical with N/sub b/ and H/sub b/-C stretching modes are increased by a factor of approx. 2. This increase is principally due to the charge flux term. Their results also show that approximately half the increase in the charge flux term P/sub zz//sup (Hb)/(CF) comes from the charge transfer, whereas the other half comes from the polarization contribution. However, the major contribution tomore » the P/sub zz//sup (Nb)/(CF) term comes from the polarization contribution.« less

The dipole moment derivatives of nitrosyl cyanide— theoretical calculations and analysis

The results of an ab initio (9 s5 p/3 s2 p) molecular orbital calculation of the dipole moment derivatives and gas phase IR intensities of ONCN are reported for all the five in-plane IR active modes. The results are compared with the corresponding values previously obtained from an MNDO calculation. The directions of the dipole derivatives calculated by the two methods agree very well, whereas the intensities differ significantly. The MNDO results, which are in better agreement with the experimental spectrum, are analyzed for the charge—charge flux—overlap (CCFO) electronic contributions to the dipole derivatives. The resulting CCFO partition is used to study the electronic substituent effect on the CN stretching.

Prediction of the vibrational spectra of interacting water molecules

A system of computer programs has been developed to predict and interpret the infrared spectra of molecules, ions, radicals and of chemically interacting species. The frequency parameters (or force constants) and the intensity parameters (or atomic polar tensors (APTs)) are calculated using the Gaussian-76 program for ab initio quantum mechanical calculations with a 4–31G basis set. The resultant wavenumbers and intensities have been used to predict the fundamental infrared spectra of water molecules interacting with different molecular species. The corresponding wavenumber shifts and intensity changes from the spectrum of the non-interacting water molecule are dependent upon both the geometrical orientation and the electron-donating properties of the interacting species. The intensity parameters can be further analyzed by using the charge-charge flux-overlap (CCFO) interpretation of the APTs. This interpretation is helpful in pinpointing the exact source of the perturbation that produces the intensity change. The significance is discussed of each of these CCFO terms in predicting the infrared intensities of molecules undergoing intermolecular interaction.

The charge-charge flux-overlap model for the interpretation of atomic polar tensors and infrared intensities

The interpretation of infrared intensities using the concepts of atomic polar tensors divided according to contributions from charge-charge flux-overlap (CCFO) terms appears in general to be quite useful. Here we shall illustrate this analysis for the F and H atoms in chemically related molecules to show how their properties vary for CF, CH, OF and OH bonds as R varies in RCF, RCH and ROH molecules.