Effect of substitutions on the geometry and intramolecular hydrogen bond strength in meta-benziporphodimethenes: A new porphyrin analogue

Molecular Tailoring Approach for the Direct Estimation of Individual Noncovalent Interaction Energies in Molecular Systems

The noncovalent interactions (NCIs) are omnipresent in chemistry, physics, and biology. The study of such interactions offers insights into various physicochemical phenomena. Some indirect approaches proposed in the literature for exploring the NCIs are briefly reviewed in Section 1 of this Perspective. These include: (i) Shift in the stretching frequency of an X-Y bond involved in X-Y···Z interaction. (ii) Topological analysis of molecular electron density. (iii) Empirical equations derived employing experimental and theoretical quantities. However, a direct method for estimating individual intramolecular/intermolecular interaction energies has been conspicuous by its absence from the literature. We have developed a molecular tailoring approach (MTA)-based method enabling a direct and reliable estimation of the energy of intra- as well as intermolecular interactions. This method offers a direct and reliable estimation of these interactions, in particular of the hydrogen bonds (HB) in molecules/weakly bound clusters along with the respective cooperativity contribution. In Section 2, the basis of our method is discussed, along with some illustrative examples. The application of this method to a variety of molecules and clusters, with a special emphasis on estimating the HB energy along with the energy of other NCIs is presented in Section 3. Section 4 discusses some computational strategies for applying our method to large molecular clusters. The last Section provides a summary and a discussion on future developments.

Quantification of hydrogen bond energy based on equations using spectroscopic, structural, QTAIM-based, and NBO-based descriptors which calibrated by the molecular tailoring approach

CONTEXT

Hydrogen bonds critically influence the structure and properties of both organic molecules and biomolecules, as well as supramolecular assemblies. For this reason, the development and elaboration of methods for quantitative assessment of hydrogen bond energy is an urgent challenge. In this study, using a large series of hydroxycarbonyl aliphatic compounds with the O‒H···O = C intramolecular hydrogen bond, a bank of hydrogen bond descriptors was created, including spectroscopic, structural, QTAIM-based, and NBO-based parameters. It was shown that the O‒H vibration frequency, OH chemical shift as the spectroscopic descriptors, the O···H hydrogen bond length, O···O distance, and O‒H covalent bond length as the structural descriptors, the electron density and its Laplacian, electron potential energy density in the hydrogen bond critical point, the electron density at the ring critical point as the QTAIM-based descriptors change in a correlated manner. The same correlation is found in change of the charge transfer energy through a hydrogen bond, the occupancy of the O‒H bond antibonding orbital, the Wiberg indices of the O···H hydrogen bond, and the O‒H covalent bond, as well as the polarization of the O‒H bond, which are the NBO-based descriptors. It was also recognized that the specified descriptors from the spectroscopic, structural, QTAIM-based, and NBO-based categories are functionally related to the values of intramolecular hydrogen bond energy, quantified via the molecular tailoring approach. This allowed one to obtain a system of equations for quantitative estimation of intramolecular hydrogen bond energy based on the spectroscopic, structural, QTAIM, and NBO descriptors, which makes such quantification more dependable and reliable.

METHODS

To obtain the spectroscopic descriptors, the vibrational spectra and shielding constants were calculated using the GIAO method. Structural descriptors were obtained for the equilibrium geometry of molecules, calculated at the MP2(FC)/6-311 +  + (2d,2p) level using the Gaussian 09 program. The QTAIM-based descriptors were calculated using the AIMAll program within the framework of the quantum theory "Atoms in Molecules." The NBO-based descriptors were calculated using the NBO 3.1 program implemented into Gaussian 09. To quantify the energy of intramolecular hydrogen bonds, molecular fragmentation was used within the molecular tailoring approach.

Revealing the Reasons for Degeneration of Resonance-Assisted Hydrogen Bond on the Aromatic Platform: Calculations of Ortho-, Meta-, Para-Disubstituted Benzenes, and (Z)-(E)-Olefins

The energies of the O-H∙∙∙O=C intramolecular hydrogen bonds were compared quantitatively for the series of ortho-disubstituted benzenes and Z-isomers of olefins via a molecular tailoring approach. It was established that the hydrogen bond energy in the former series is significantly less than that in the latter one. The reason for lowering the hydrogen bond energy in the ortho-disubstituted benzenes compared to the Z-isomers of olefins is the decrease in the π-contribution to the total energy of the complex interaction, in which the hydrogen bond per se is enhanced by the resonance effect. By the example of the para- and meta-disubstituted benzenes, as well as E-isomers of olefins, it was explicitly shown that the aromatic ring is a much poorer conductor of the resonance effect compared to the double bond. The hydrogen bond in the ortho-disubstituted benzenes has a lower energy than a typical resonance-assisted hydrogen bond because the aromatic moiety cannot properly assist the hydrogen bond with a resonance effect. Thus, a hydrogen bond on an aromatic platform should fall into a special category, namely an aromaticity-assisted hydrogen bond, which is closer by nature to a simple hydrogen bond rather than to a resonance-assisted one.

Structural characterization and bioimaging of Zn2+ using meta-benziporphodimethene analogue

"Prevention is better than cure, especially when something has no cure." Cancer, in most patients is detected at the stage beyond which it becomes non-curative. Thus, the early detection of cancer cells can play a crucial role in enhancing the chances of a patient's survival. In this light, we present a non-fluorescent receptor employed for the detection of Zn²⁺ ion in MDA-MB-231 carcinoma cells that exhibits fluorescence turn-on behaviour upon binding with the metal ion. In this work, the synthesis of 11,16-bis(2,6-difluorobenzene)-6,6,21,21-tetramethyl-meta-benziporpho-6,21-dimethene and its Zn²⁺ chloride complex have been reported. The compounds were fully characterized using UV-Visible, NMR, IR and mass spectrometry. Furthermore, the X-ray polymorphs of meta-benziporphodimethene analogue have been added. The study of its bioimaging applications in MDA-MB-231 breast cancer cells for the detection of Zn²⁺ ions have been reported.

Guide to tuning the chalcone molecular properties based on the push-pull effect energy scale created via the molecular tailoring approach

Using the molecular tailoring approach, a total energy scale for the push-pull effect in the range from -40 to 100 kcal/mol is created for the wide series of neutral, charged and doubly charged compounds on the chalcone platform. Taking into account similar energy scale for hydrogen bonds, the strength of the push-pull effect is ranked in the seven categories, ranging from negative (anti-push-pull) to very weak and very strong push-pull effect. It is demonstrated that the molecular properties of chalcone can be tuned prior synthesis due to the created energy scale for the push-pull effect. The single bonds of the π-spacer in the chalcones are shortened, the double ones are lengthened, and the C=O bond vibrations are red shifted when the push-pull effect is enhanced along the energy scale. The HOMO and LUMO energies change systematically while the HOMO-LUMO energy gap narrows as the strength of the push-pull effect increases.

Molecular Tailoring Approach for the Estimation of Intramolecular Hydrogen Bond Energy

Hydrogen bonds (HBs) play a crucial role in many physicochemical and biological processes. Theoretical methods can reliably estimate the intermolecular HB energies. However, the methods for the quantification of intramolecular HB (IHB) energy available in the literature are mostly empirical or indirect and limited only to evaluating the energy of a single HB. During the past decade, the authors have developed a direct procedure for the IHB energy estimation based on the molecular tailoring approach (MTA), a fragmentation method. This MTA-based method can yield a reliable estimate of individual IHB energy in a system containing multiple H-bonds. After explaining and illustrating the methodology of MTA, we present its use for the IHB energy estimation in molecules and clusters. We also discuss the use of this method by other researchers as a standard, state-of-the-art method for estimating IHB energy as well as those of other noncovalent interactions.

Direct and Reliable Method for Estimating the Hydrogen Bond Energies and Cooperativity in Water Clusters, Wn, n = 3 to 8

No direct method for estimating the individual O-H···O hydrogen bond (H-bond) energies in water clusters (W_n) exists in the literature. In this work, we propose such a direct method based on the molecular tailoring approach, which also enables the estimation of the cooperativity contributions. The calculated H-bond energies at MP2(full)/aug-cc-pVTZ and CCSD(T)/aug-cc-pVDZ levels for W_n, n = 3 to 8, agree well with one another and fall between 0.3 and 11.6 kcal mol^-1 with the cooperativity contributions in the range of -1.2 and 7.0 kcal mol^-1. For gauging the accuracy of our H-bond energies for a cluster, the H-bond energy sum is added to the sum of monomer energies, and the results are compared with the respective total energy. These two values agree with each other to within 8.3 mH (∼5 kcal mol^-1), testifying the accuracy of our estimated H-bond energies. Further, these H-bond strengths show a good correlation with the respective O-H stretching frequencies and the molecular electron density values at the (3, -1) O-H···O H-bond critical point.

A molecular tailoring approach – a new guide to quantify the energy of push–pull effects: a case study on (E)-3-(1H-pyrrol-2-yl)prop-2-enones

We report a new approach to quantify the push–pull effect in molecules with internal charge transfer.