G. Naresh Patwari

Charge transfer aided selective sensing and capture of picric acid by triphenylbenzenes

A fluorescent chemo-sensor, 1,3,5-tris(4′-(N,N-dimethylamino)phenyl)benzene was synthesized by substituting the N–H protons of 1,3,5-tris(4′-aminophenyl)benzene with methyl groups. The chemo-sensor shows highly selective and remarkable fluorescence quenching in the presence of picric acid with a detection limit of 1.5 ppm. The origin of the selectivity was investigated using absorption, fluorescence emission and 1H NMR spectroscopic techniques. The solid state structure of 1,3,5-tris(4′-(N,N-dimethylamino)phenyl)benzene and its picric acid complex reveals multiple hydrogen bonds (N–H⋯O and C–H⋯O), π–π interactions and electrostatic interactions between 1,3,5-tris(4′-(N,N-dimethylamino)phenyl)benzene and picric acid. The proton transfer process from picric acid to 1,3,5-tris(4′-(N,N-dimethylamino)phenyl)benzene results in the formation of picrate anions and the triply protonated 1,3,5-tris(4′-(N,N-dimethylamino)phenyl)benzene species containing dimethylammonium (–NHMe2+) groups.

Hydrogen-Bonded Complexes of Phenylacetylene with Water, Methanol, Ammonia, and Methylamine. The Origin of Methyl Group-Induced Hydrogen Bond Switching

The infrared spectra in the acetylenic C-H stretching region for the complexes of phenylacetylene with water, methanol, ammonia, and methylamine are indicative of change in the intermolecular structure upon substitution with a methyl group. High-level ab initio calculations at CCSD(T)/aug-cc-pVDZ level indicate that the observed complexes of water and ammonia are energetically the most favored structures, and electrostatics play a dominant role in stabilizing these structures. The ability of the pi electron density of the benzene ring to offer a larger cross-section for the interaction and the increased polarizability of the O-H and N-H groups in methanol and methylamine favor the formation of pi hydrogen-bonded complexes, in which dispersion is the dominant force. Further, the observed phenylacetylene-methylamine complex can be tentatively assigned to a kinetically trapped higher energy structure. The observed methyl group-induced hydrogen bond switching in the phenylacetylene complexes can be attributed to the switching of the dominant interaction from electrostatic to dispersion.

Structure of the phenylacetylene-water complex as revealed by infrared-ultraviolet double resonance spectroscopy.

The structure of the phenylacetylene-water complex has been elucidated based on spectral shifts in electronic and vibrational transitions. Phenylacetylene forms a cyclic complex with water incorporating C-H...O and O-H...pi hydrogen bonds, which is different from both the benzene-water and acetylene-water complexes, even though phenylacetylene combines the features of both benzene and acetylene. Formation of such a complex can be rationalized on the basis of cooperativity between the two sets of hydrogen bonds.

Phenylacetylene: A Hydrogen Bonding Chameleon

Molecules with multiple hydrogen bonding sites offer the opportunity to investigate competitive hydrogen bonding. Such an investigation can become quite interesting, particularly when the molecule of interest has neither lone-pair electrons nor strongly acidic/basic groups. Phenylacetylene is one such molecule with three hydrogen bonding sites that cannot be ranked into any known hierarchical pattern. Herein we review the structures of several binary complexes of phenylacetylene investigated using infrared optical double-resonance spectroscopy in combination with high-level ab initio methods. The diversity of intermolecular structures formed by phenylacetylene with various reagents is remarkable. The nature of intermolecular interaction with various reagents is the result of a subtle balance between various configurations and competition between the electrostatic and dispersion energy terms, while trying to maximize the total interaction strength.

IR-UV Double Resonance Spectroscopic Investigation of Phenylacetylene-Alcohol Complexes. Alkyl Group Induced Hydrogen Bond Switching

The electronic transitions of phenylacetylene complexes with water and trifluoroethanol are shifted to the blue, while the corresponding transitions for methanol and ethanol complexes are shifted to the red relative to the phenylacetylene monomer. Fluorescence dip infrared (FDIR) spectra in the O-H stretching region indicate that, in all the cases, phenylacetylene is acting as a hydrogen bond acceptor to the alcohols. The FDIR spectrum in the acetylenic C-H stretching region shows Fermi resonance bands for the bare phenylacetylene, which act as a sensitive tool to probe the intermolecular structures. The FDIR spectra reveal that water and trifluoroethanol interact with the pi electron density of the acetylene C-C triple bond, while methanol and ethanol interact with the pi electron density of the benzene ring. It can be inferred that the hydrogen bonding acceptor site on phenylacetylene switches from the acetylene pi to the benzene pi with lowering in the partial charge on the hydrogen atom of the OH group. The most significant finding is that the intermolecular structures of water and methanol complexes are notably distinct, which, to the best of our knowledge, this is first such observation in the case of complexes of substituted benzenes.

Binary complexes of tertiary amines with phenylacetylene. Dispersion wins over electrostatics

The structures of the binary complexes between phenylacetylene and several tertiary amines viz., triethylamine, 1-ethylpiperidine, 1-ethylpiperazine, 1-azabicyclo[2.2.2]octane, and 1,4-diazabicyclo[2.2.2]octane were inferred using infrared-optical double resonance spectroscopy. The IR spectra in the acetylenic C-H stretching region clearly rule out the formation of electrostatic dominated C-HN hydrogen bonded complexes. The IR spectra also point to the fact that all the five tertiary amines interact with the extended pi electron density of the phenylacetylene moiety, leading to the formation of multidentate C-Hpi hydrogen bonded complexes. Additionally a very weak electrostatic C-HN hydrogen bond enhances the stability of the complex marginally. The multidentate C-Hpi hydrogen bonded complexes are stabilized by a substantial contribution from the dispersion energy.

Hydrogen Bonding to Multifunctional Molecules: Spectroscopic and ab Initio Investigation of Water Complexes of Fluorophenylacetylenes

The water complexes of 4-fluorophenylacetylene and 2-fluorophenylacetylene were investigated using IR-UV double resonance spectroscopy. Both 4-fluoro- and 2-fluorophenylacetylenes form a cyclic complex with water incorporating C-H...O and O-H...pi hydrogen bonds. These structures are similar to the phenylacetylene-water complex, implying that the fluorine substitution on phenylacetylene does not alter the intermolecular structure. Further, the presence of fluorine enhances the interaction of water with the acetylenic pi electron density. This behavior of fluorophenylacetylenes is dramatically different from that of fluorobenzene and fluorostyrene. A second water complex was also observed in the case of 2-fluorophenylacetylene in which water interacts with fluorine atom and acetylenic C-C triple bond in a double-donor fashion. Additionally, two distinct 2-fluorophenylacetylene-(water)(2) complexes were also observed. The first is a cyclic complex in which two water molecules bridge the hydrogen bond donor and acceptor sites present in 2-fluorophenylacetylene. The second is a kinetically trapped higher energy structure in which one water molecule acts as a double-acceptor.

Octanuclear Zinc Phosphates with Hitherto Unknown Cluster Architectures: Ancillary Ligand and Solvent Assisted Structural Transformations Thereof

Structural variations in zinc phosphate cluster chemistry have been achieved through a careful selection of phosphate ligand, ancillary ligand, and solvent medium. The use of 4-haloaryl phosphates (X-dippH2) as phosphate source in conjunction with 2-hydroxypyridine (hpy) ancillary ligand in acetonitrile solvent resulted in the isolation of the first examples of octameric zinc phosphates [Zn8(X-dipp)8(hpy)4(CH3CN)2(H2O)2]·4H2O (X = Cl 2, Br 3) and not the expected tetranuclear D4R cubane clusters. Use of 2,3-dihydroxypyridine (dhpy) as ancillary ligand, under otherwise similar reaction conditions with the same set of phosphate ligands and solvent, resulted in isolation of another type of octanuclear zinc phosphate clusters {[(Zn8(X-dipp)4(X-dippH)4(dhpyH)4(dhpyH2)2(H2O)2]·2solvent} (X = Cl, solvent = MeCN 4; Br, solvent = H2O 5), as the only isolated products. X-ray crystal diffraction studies reveal that 2 and 3 are octanuclear clusters that are essentially formed by edge fusion of two D4R zinc phosphates. Although 4 and 5 are also octanuclear clusters, they exhibit a completely different cluster architecture and have been presumably formed by the ability of 2,3-dihydroxypyridine to bridge zinc centers in addition to the X-dipp ligands. Dissolution of both types of octanuclear clusters in DMSO followed by crystallization yields D4R cubanes [Zn(X-dipp)(DMSO)]4 (X = Cl 6, Br 7), in which the ancillary ligands such as hpy, H2O, and CH3CN originally present on the zinc centers of 2-5 have been replaced by DMSO. DFT calculations carried out to understand the preference of Zn8 versus Zn4 clusters in different solvent media reveal that use of CH3CN as solvent favors the formation of fused cubanes of the type 2 and 3, whereas use of DMSO as the solvent medium promotes the formation of D4R structures of the type 6 and 7. The calculations also reveal that the vacant exocluster coordination sites on the zinc centers at the bridgehead positions prefer coordination by water to hpy or CH3CN. Interestingly, the initially inaccessible D4R cubanes [Zn(X-dipp)(hpy)]4·2MeCN (X = Cl 8, Br 9) could be isolated as the sole products from the corresponding DMSO-decorated cubanes 6 and 7 by combining them with hpy in CH3CN.

Electronic and Vibrational Spectroscopic Investigation of Phenylacetylene-Amine Complexes. Evidence for the Diversity in the Intermolecular Structures

Shifts in the electronic transitions for the complexes of phenylacetylene with ammonia, methylamine, and triethylamine clearly indicate the variation in the intermolecular structures of the three complexes. The infrared spectrum of phenylacetylene in the acetylenic C-H stretching region shows Fermi resonance bands, which act as a sensitive tool to probe the intermolecular structures. The IR-UV double resonance spectra of the three complexes are disparate and signify the formation of distinct structures. The formation of C-H...N hydrogen-bonded complex with ammonia and two distinct types of pi complexes with methylamine and triethylamine can be inferred from the analysis of electronic and vibrational spectra in combination with ab initio calculations. These complexes clearly point out the fact that marginal changes in the interacting partner can significantly alter the intermolecular structure.

A Combined Spectroscopic and ab Initio Investigation of Phenylacetylene−Methylamine Complex. Observation of σ and π Type Hydrogen-Bonded Configurations and Fluorescence Quenching by Weak C−H···N Hydrogen Bonding†

Two distinct isomers for the binary complex between phenylacetylene and methylamine were observed. The first complex is characterized by the presence of a C-H···N hydrogen bond between the acetylenic C-H group and the N atom of methylamine. In the second complex the N-H group of methylamine interacts with the π electron density of the benzene ring accompanied by a peripheral interaction between the methyl C-H group and the π electron density of the C≡C bond. Stabilization energies and Gibbs free energies at the complete basis set (CBS) limit of the coupled cluster theory with single, double, and perturbative triple excitations [CCSD(T)] suggest that while the C-H···N hydrogen bonded complex is the global minimum, the N-H···π hydrogen bonded complex is a high energy local minimum. The formation of the N-H···π complex could be related to kinetic trapping or higher accessibility. Comparison of the laser induced fluorescence (LIF) excitation and the one-color-resonant two-photon ionization (1C-R2PI) spectra suggests that formation of C-H···N hydrogen bonding leads to fluorescence quenching in phenylacetylene, most probably due to dipolar coupling in the excited state. The binary complex between the phenylacetylene and methylamine shows interesting isomer-dependent fluorescent properties.