Tapas Kar

Three-center bond index

Abstract The bond index for a multi-center bond has been defined from a generalized SCF density matrix. Using this definition, we have calculated the three-center bond indices ( I ABC ) for a number of molecules. It has been observed that I ABC is positive and appreciably high (⩾ 0.1) only for those three-center bonds which can be obtained from localized MO calculations. In other cases, I ABC is either negative or very close to zero.

Comparison between hydrogen and dihydrogen bonds among H3BNH3, H2BNH2, and NH3

Several possible binary complexes among ammonia-borane, aminoborane, and ammonia, via hydrogen and/or dihydrogen bonds, have been investigated to understand the effect of different hybridization. Moller–Plesset second-order perturbation theory with aug-cc-pVDZ basis set was used. The interaction energy is corrected for basis set superposition error, and the Morokuma–Kitaura method was employed to decompose the total interaction energy. Like H3BNH3, the sp2 hybridized H2BNH2 also participates in H- and dihydrogen bond formation. However, such bonds are weaker than their sp3 analogs. The contractions of BN bonds are associated with blueshift in vibrational frequency and stretches of BH and NH bonds with redshift. The polarization, charge transfer, correlation, and higher-order energy components are larger in dihydrogen bonded complexes, compared to classical H-bonded ammonia dimers.

Effect of Adjoining Aromatic Ring upon Excited State Proton Transfer. o-Hydroxybenzaldehyde

Abstract The ground and first few excited states of o-hydroxybenzaldehyde (oHBA) are computed at the CIS and MP2/CIS levels with a 6-31+G** basis set, with emphasis on its intramolecular H-bond. These results are compared with those for malonaldehyde, which differs from oHBA in that it lacks an adjoining benzene ring. In most respects, the addition of the latter aromatic system exerts surprisingly little influence upon the properties of malonaldehyde. With the exception of the 1ππ* state, electronic excitation weakens the H-bond and simultaneously raises the barrier to proton transfer in either system. Unlike the symmetric transfer potential in malonaldehyde, the enol and keto tautomers of oHBA are chemically distinct. π→π* excitation reverses the preference for the enol tautomer in the ground state. This reversal is connected with the changing degree of aromaticity in the benzene ring of oHBA. The asymmetric transfer potential in oHBA leads to forward and reverse barriers of different magnitude. When this factor is accounted for by an averaging procedure, the transfer barriers in oHBA are remarkably similar to those of the corresponding states of malonaldehyde.

Spectroscopic and Structural Signature of the CH-O Hydrogen Bond

It has been observed that the vibrational stretching frequency of a C-H covalent bond commonly shifts to the blue and suffers intensity loss, when the CH engages in a hydrogen bond. However, the shift does not always occur in this direction, and there are cases when a CH blue shift may be present even in the absence of a CH...O interaction. Ab initio quantum calculations are used to analyze the structure, and vibrational and NMR spectra of small model systems containing both conventional and CH...O H-bonds, and thereby identify patterns that unambiguously signal the presence of a CH...O interaction.

Ab initio theoretical study of three-centre bonding on the basis of bond index

Abstract Various methods of derivation of a multi-centre bond index and its physical significance are briefly discussed. The usefulness of the bond index model for the study of three-centre (3c) bonding is illustrated by taking a number of examples chosen from earlier calculations. The origin of the sign of 3c bond indices ( I ABC ) predicted for three-centre two-electron (3c-2e) and three-centre four-electron (3c-4e) bonds is investigated using simple models with different bonding topologies. Some examples are given highlighting the role of I ABC in elucidating the nature of bonding in controversial molecules. The bond index and the effective pair population models are compared and shown to be equivalent for an RHF wave function and even-centre bond indices. It is concluded that despite some inherent limitations such as the basis set sensitivity the bond index model provides a quite satisfactory description of 3c bonding.

Effects of Peripheral Substituents on the Electronic Structure and Properties of Unligated and Ligated Metal Phthalocyanines, Metal = Fe, Co, Zn

The effects of peripheral, multiple -F as well as -C2F5 substituents, on the electronic structure and properties of unligated and ligated metal phthalocyanines, PcM, PcM(acetone)2 (M = Fe, Co, Zn), PcZn(Cl), and PcZn(Cl(-)), have been investigated using a DFT method. The calculations provide a clear explanation for the changes in the ground state, molecular orbital (MO) energy levels, ionization potentials (IP), electron affinities (EA), charge distribution on the metal (QM), axial binding energies, and in electronic spectra. While the strongly electron-withdrawing -C2F5 groups on the Pc ring change the ground state of PcFe, they do not influence the ground state of PcCo. The IP is increased by ∼1.3 eV from H16PcM to F16PcM and by another ∼1.1 eV from F16PcM to F48PcM. A similar increase in the EA is also found on going from H16PcM to F48PcM. Substitution by the -C2F5 groups also considerably increases the binding strength between PcM and the electron-donating axial ligand(s). Numerous changes in chemical and physical properties observed for the F64PcM compounds can be accounted for by the calculated results.

B3N3 Borazine Substitution in Hexa-peri-Hexabenzocoronene: Computational Analysis and Scholl Reaction of Hexaphenylborazine

The doping of graphene molecules by borazine (B(3)N(3)) units may modify the electronic properties favorably. Therefore, the influence of the substitution of the central benzene ring of hexa-peri-hexabenzocoronene (HBC, C(42)H(18)) by an isoelectronic B(3)N(3) ring resulting in C(36)B(3)N(3)H(18) (B3N3HBC) is investigated by computational methods. For comparison, the isoelectronic and isosteric all-B/N molecule B(21)N(21)H(18) (termed BN) and its carbon derivative C(6)B(18)N(18)H(18) (C6BN), obtained by substitution of a central B(3)N(3) by a C(6) ring, are also studied. The substitution of C(6) in the HBC molecule by a B(3)N(3) unit results in a significant change of the computed IR vibrational spectrum between 1400 and 1600 cm(-1) due to the polarity of the borazine core. The properties of the BN molecule resemble those of hexagonal boron nitride, and substitution of the central B(3)N(3) ring by C(6) changes the computed IR vibrational spectrum only slightly. The allowed transitions to excited states associated with large oscillator strengths shift to higher energy upon going from HBC to B3N3HBC, but to lower energy upon going from BN to C6BN. The possibility of synthesis of B3N3HBC from hexaphenylborazine (HPB) using the Scholl reaction (CuCl(2)/AlCl(3) in CS(2)) is investigated. Rather than the desired B3N3HBC an insoluble and X-ray amorphous polymer P is obtained. Its analysis by IR and (11)B magic angle spinning NMR spectroscopy reveals the presence of borazine units. The changes in the (11)B quadrupolar coupling constant C(Q), asymmetry parameter η, and isotropic chemical shift δ(iso)((11)B) with respect to HPB are in agreement with a structural model that includes B3N3HBC-derived monomeric units in polymer P. This indicates that both intra- and intermolecular cyclodehydrogenation reactions take place during the Scholl reaction of HPB.

Analysis of the Reactivities of Protein C-H Bonds to H Atom Abstraction by OH Radical

Ab initio and density functional theory calculations are used to monitor the process wherein a OH· radical is allowed to approach the various CH groups of a Leu dipeptide, with its CH(2)CH(CH(3))(2) side chain. After forming an encounter complex, the OH· abstracts the pertinent H atom, and the resulting HOH is then dissociated from the complex. The energy barriers for H· abstraction from the β, γ, and δ CH groups are all less than 8 kcal/mol, but a significantly higher barrier is computed for the C(α)H removal. This higher barrier is the result of the strong H-bonds formed in the encounter complex between the OH· and the NH and C═O groups of the peptide units that surround the C(α) atom. This low-energy complex represents a kinetic trap which raises the energy needed to surmount the ensuing H· transfer barrier.

Theoretical study of the nonlinear polarizabilities in H2N and NO2 substituted chromophores containing two hetero aromatic rings

Abstract The static first- and second-hyperpolarizabilities of a number of amino- and nitro-substituted chromophores containing two hetero aromatic rings have been calculated by ab initio time dependent Hartree–Fock (TDHF) method. The computed nonlinear polarizabilities correlate well with frontier orbital energies and hardness parameter ( η ). Furan and thiophene rings at the donor site and the thiazole and pyridine at the acceptor site are predicted to enhance the first hyperpolarizability ( β ) – but not 〈 γ 〉 values. The nature of the dependence of β and 〈 γ 〉 on the twist angle between the bridging rings have been explored.

Borazine and Benzene Homo- and Heterodimers

The homodimers of benzene and borazine as well as a heterodimer consisting of one benzene (bz) and one borazine (bor) molecule are investigated using MP2, SCS-MP2, and CCSD(T) theories in conjunction with basis sets of up to quadruple-zeta quality. Dimer geometries were completely optimized using the resolution of the identity approximation of MP2 with a QZVPP basis set and characterized by computation of harmonic vibrational frequencies using triple-zeta basis sets. While significant higher order correlation effects beyond MP2 are important for the benzene dimer, these are very small for the borazine dimer and intermediate for the heterodimer. The spin-component scaling (SCS) correction of MP2 produces binding energies for the borazine dimer that are too low but yields very good agreement with CCSD(T) for the heterodimer. The decrease in the intermolecular distance in the sandwich (S) configurations from bz(2) via bz-bor to bor(2) is accompanied by an increased binding energy and a change from second-order stationary points to a minimum for bor(2). The T isomer is less stable than the S configuration for bor(2), but it is preferred over the S and a parallel-displaced (PD) arrangement in the heterodimer. The following order of stability is obtained for the minima at the extrapolated CCSD(T) level: T(bz-bor) > S(bor(2)) > PD(bz-bor) > PD(bor(2)) > T(bor(2)) > PD(bz(2)). The most stable isomer at all levels of theory, T(bz-bor), features a NH...pi interaction.