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Jeanluc Bredas - One of the best experts on this subject based on the ideXlab platform.

  • cyanine like dyes with large Bond Length alternation
    Chemistry: A European Journal, 2013
    Co-Authors: Karl J Thorley, Joseph W Perry, Joel M Hales, Hyeongeu Kim, Shino Ohira, Jeanluc Bredas, Harry L Anderson
    Abstract:

    Herein, the synthesis and properties of alkyne-bridged carbocations, which are analogous in structure to cyanine dyes, are reported. An alkene-bridged dye, linked at the third position of the indole, was also synthesized as a reference compound. These new carbocations are stable under ambient conditions, allowing characterization by UV/Vis and NMR ((1)H and (13)C) spectroscopies. These techniques revealed a large degree of delocalization of the positive charge, similar to a previously reported porphyrin carbocation. The linear and nonlinear optical properties are compared with cyanine dyes and triarylmethyl cations, to investigate the effects of the Bond-Length alternation and the overall molecular geometry. The value of Re(γ), the real part of the third-order microscopic polarizability, of -1.3×10(-33)  esu for the alkyne-linked cation is comparable to that of a cyanine dye of similar Length. Nondegenerate two-photon absorption spectra showed that the alkene-bridged dye exhibited characteristics of cyanines, whereas the alkyne-bridged dye is reminiscent of octupolar chromophores, such as the triarylmethyl carbocation brilliant green. Such attributes were confirmed and rationalized by quantum chemical calculations.

  • on the relationship between Bond Length alternation and many electron self interaction error
    Journal of Chemical Physics, 2012
    Co-Authors: Thomas Korzdorfer, Robert M Parrish, John S Sears, David C Sherrill, Jeanluc Bredas
    Abstract:

    Predicting accurate Bond-Length alternations (BLAs) in long conjugated molecular chains has been a major challenge for electronic-structure theory for many decades. While Hartree-Fock (HF) overestimates BLA significantly, second-order perturbation theory and commonly used density functional theory (DFT) approaches typically underestimate it. Here, we discuss how this failure is related to the many-electron self-interaction error (MSIE), which is inherent to both HF and DFT approaches. We use tuned long-range corrected hybrids to minimize the MSIE for a series of polyenes. The key result is that the minimization of the MSIE alone does not yield accurate BLAs. On the other hand, if the range-separation parameter is tuned to yield accurate BLAs, we obtain a significant MSIE that grows with chain Length. Our findings demonstrate that reducing the MSIE is one but not the only important aspect necessary to obtain accurate BLAs from density functional theory.

  • a new class of cyanine like dyes with large Bond Length alternation
    Journal of the American Chemical Society, 2009
    Co-Authors: Shino Ohira, Joseph W Perry, Karl J Thorley, Joel M Hales, Harry L Anderson, Jeanluc Bredas
    Abstract:

    Cyanines, which represent a class of charged chromophores with an odd number of pi-conjugated carbons, display unique electronic and optical properties attributed to the strong electronic delocalization and the absence of any significant carbon-carbon Bond-Length alternation (BLA) along their backbones. The flatness of the corresponding electronic potential makes cyanine dyes the compounds to which simple free-electron theory can be applied in the most relevant way. Recently, cations of porphyrin dimers linked by a pi-conjugated bridge with an odd number of carbons and presenting alternating single and triple Bonds were shown to possess linear and nonlinear optical properties analogous to those of cyanines. Here, by using a joint theoretical and experimental approach, we demonstrate the correspondence between cyanines and the new class of alkyne carbocations, in spite of their marked difference in BLA.

Sean A C Mcdowell - One of the best experts on this subject based on the ideXlab platform.

  • correlation of the Bond Length change and vibrational frequency shift in model hydrogen Bonded complexes of pyrrole
    Chemical Physics Letters, 2017
    Co-Authors: Sean A C Mcdowell
    Abstract:

    Abstract An MP2 computational study of model hydrogen-Bonded pyrrole⋯YZ (YZ = NH3, NCH, BF, CO, N2, OC, FB) complexes was undertaken in order to examine the variation of the N H Bond Length change and its associated vibrational frequency shift. The chemical hardness of Y, as well as the YZ dipole moment, were found to be important parameters in modifying the Bond Length change/frequency shift. The basis set effect on the computed properties was also assessed. A perturbative model, which accurately reproduced the ab initio N H Bond Length changes and frequency shifts, was useful in rationalizing the observed trends.

  • on the correlation between Bond Length change and vibrational frequency shift in hydrogen Bonded complexes a computational study of y hcl dimers y n2 co bf
    Journal of the American Chemical Society, 2005
    Co-Authors: Sean A C Mcdowell, David A Buckingham
    Abstract:

    The H−Cl Bond-Length change and the harmonic vibrational frequency shift of the H−Cl stretch on formation of the linear isoelectronic Y···H−Cl complexes (Y = N2, CO, BF) have been determined by ab initio computations at different levels of theory. These shifts are in agreement with predictions from a model based on perturbation theory and involving the first and second derivatives of the interaction energy with respect to displacement of the H−Cl Bond Length from its equilibrium value in the isolated monomer. At the highest level of theory, blue shifts were obtained for BF···HCl and CO···HCl, while red shifts were obtained for FB···HCl, OC···HCl, and N2···HCl. These vibrational characteristics are rationalized by considering the balance between the interaction energy derivatives obtained from the perturbative model. The widely believed correlation between the Bond-Length change and the sign of the frequency shift obtained on complexation is discussed and found to be unreliable.

Tapas Ganguli - One of the best experts on this subject based on the ideXlab platform.

  • Bond Length variation in zn substituted nio studied from extended x ray absorption fine structure
    Solid State Communications, 2017
    Co-Authors: S D Singh, A K Poswal, C Kamal, Parasmani Rajput, Aparna Chakrabarti, S N Jha, Tapas Ganguli
    Abstract:

    Abstract Bond Length behavior for Zn substituted NiO is determined through extended x-ray absorption fine structure (EXAFS) measurements performed at ambient conditions. We report Bond Length value of 2.11±0.01 A for Zn-O of rock salt (RS) symmetry, when Zn is doped in RS NiO. Bond Length for Zn substituted NiO RS ternary solid solutions shows relaxed behavior for Zn–O Bond, while it shows un-relaxed behavior for Ni–O Bond. These observations are further supported by first-principles calculations. It is also inferred that Zn sublattice remains nearly unchanged with increase in lattice parameter. On the other hand, Ni sublattice dilates for Zn compositions up to 20% to accommodate increase in the lattice parameter. However, for Zn compositions more than 20%, it does not further dilate. It has been attributed to the large disorder that is incorporated in the system at and beyond 20% of Zn incorporation in the cubic RS lattice of ternary solid solutions. For these large percentages of Zn incorporation, the Ni and the Zn atoms re-arrange themselves microscopically about the same nominal Bond Length rather than systematically increase in magnitude to minimize the energy of the system. This results in an increase in the Debye-Waller factor with increase in the Zn concentration rather than a systematic increase in the Bond Lengths.

Ernesto Carmona - One of the best experts on this subject based on the ideXlab platform.

Ad Bax - One of the best experts on this subject based on the ideXlab platform.

  • nmr determination of amide n h equilibrium Bond Length from concerted dipolar coupling measurements
    Journal of the American Chemical Society, 2008
    Co-Authors: Lishan Yao, Beat Vogeli, Jinfa Ying, Ad Bax
    Abstract:

    The N-H Bond Length in backbone peptide groups of the protein GB3 has been studied by liquid crystal NMR, using five structurally conserved mutants of this protein. In the absence of additional information, the impact of dynamic fluctuations of the N-H vector orientation on the 15N-1H dipolar interaction cannot be separated from a change in N-H Bond Length. However, a change in N-H Bond Length directly impacts the orientation of C′-H vectors in the peptide group, and simultaneous analysis of 13C′-HN and 15N-HN residual dipolar couplings, measured under five different alignment orientations, permits modelfree determination of the average equilibrium N-H Bond Length in GB3, yielding rNHeq = 1.008 ± 0.006 A. Anharmonicity of the Bond stretching results in a slightly longer time-averaged Bond Length = 1.015 ± 0.006 A, and an effective Bond Length reff = -1/3 = 1.023 ± 0.006 A pertinent for NMR relaxation analysis, not including the impact of zero-point or other angular fluctuations in N-H orientation. Using a reference frame defined by the backbone Cα-C′ vectors of the protein, angular fluctuations for N-H vectors in elements of secondary structure are approximately 1.5 fold larger for out-of-plane fluctuations than motions within the peptide plane and not much larger than anticipated on the basis of quantum mechanical analysis of their zero-point librations.

  • nmr determination of amide n h equilibrium Bond Length from concerted dipolar coupling measurements
    Journal of the American Chemical Society, 2008
    Co-Authors: Lishan Yao, Beat Vogeli, Jinfa Ying, Ad Bax
    Abstract:

    The N−H Bond Length in backbone peptide groups of the protein GB3 has been studied by liquid-crystal NMR, using five structurally conserved mutants of this protein. In the absence of additional information, the impact of dynamic fluctuations of the N−H vector orientation on the 15N−1H dipolar interaction cannot be separated from a change in N−H Bond Length. However, a change in N−H Bond Length directly impacts the orientation of C′−H vectors in the peptide group, and simultaneous analysis of 13C′−HN and 15N−HN residual dipolar couplings, measured under five different alignment orientations, permits modelfree determination of the average equilibrium N−H Bond Length in GB3, yielding rNHeq = 1.008 ± 0.006 A. Anharmonicity of the Bond stretching results in a slightly longer time-averaged Bond Length = 1.015 ± 0.006 A, and an effective Bond Length reff = −1/3 = 1.023 ± 0.006 A pertinent for NMR relaxation analysis, not including the impact of zero-point or other angular fluctuations in N−H orienta...

  • correlation between 3hjnc and hydrogen Bond Length in proteins
    Journal of the American Chemical Society, 1999
    Co-Authors: Gabriel Cornilescu, Benjamin E Ramirez, Kirsten M Frank, Marius G Clore, And Angela M Gronenborn, Ad Bax
    Abstract:

    Establishing a quantitative relationship between backbone−backbone hydrogen-Bond (H-Bond) Length observed in protein crystal structures and recently observed 3hJNC‘ couplings across such Bonds is limited by the coordinate precision of the X-ray structure. For an immunoglobulin binding domain of streptococcal protein G, very high-resolution X-ray structures are available. It is demonstrated that over the small range of N−O H-Bond Lengths (2.8−3.3 A) for which 3hJNC‘ couplings are observable, the 32 measured 3hJNC‘ values can be fit to:  3hJNC‘ = −59000 exp(−4RNO) ± 0.09 Hz, or RNO = 2.75 − 0.25 ln(−3hJNC‘) ± 0.06 A. Backbone amide to side-chain carboxyl hydrogen Bonds were also investigated, and the measured 3hJNC‘ values tend to be smaller than expected from their crystallographically determined H-Bond Lengths. The sign of 3hJNC‘, determined from a zero-quantum/double-quantum experiment, is found to be the same as that of the 1JNH coupling, i.e., negative.