The Prominence of Facilitator π-Holes: The Classic N←N Pnictogen Bonding in Nitrobenzene-Ammonia Dimer with its Structural Elucidation and Experimental Characterization at Low Temperatures.

The nitrogen of nitro group is a paradigmatic pnictogen due the presence of a π-hole and a number of studies have been performed recently on prototypical nitromethane (NM). Homodimers and heterodimers of NM are sustained by π-hole driven pnictogen bonds hosted by nitrogen. To understand the effect of substitution on this π-hole and thus the pnictogen bond, heterodimers of nitrobenzene (NB; phenyl substitution in place of methyl) with ammonia (AM) have been probed, as a test case, using matrix isolation infrared spectroscopy and ab initio computations. Of the four structures optimized on the potential energy surface the energetically dominant global minimum, stabilized by π-hole driven O=N←N pnictogen bonding with co-operative N-H←O hydrogen bonding, was experimentally identified at low temperatures. A comparison of the pnictogen bonding of NB-AM dimers with NM counterpart (NM-AM dimers) divulged the dominance of electrostatic origin of pnictogen bonding in both the class of dimers. The reduced strength of pnictogen bonding in NB-AM dimers in comparison to NM-AM dimers was discerned, which has been established to be a consequence of the reduced electrostatic potential at the π-hole of NB relative to that in NM. The strength of π-hole driven pnictogen bond was directly correlated with the binding energy and the infrared shifts in the signature vibrational bands of the NB, NM and AM submolecules due to dimerization under matrix isolated conditions at low temperatures.

Interplay of unique N⋯π pnicogen and H⋯π/H⋯O hydrogen bonding interactions in the heterodimers of nitromethane with acetylene and benzene as π-electron donors: experimental characterization at low temperatures under isolated conditions with computational corroboration.

The existence of unique ON⋯π pnicogen bonding in combination with C-H⋯π/C-H⋯O hydrogen bonding interactions has been experimentally affirmed in the heterodimers of nitromethane with aliphatic (acetylene; NMAc) and aromatic (benzene; NMBz) π-electron donors at low temperatures under isolated conditions. The potential energy surfaces of NMAc and NMBz dimers have been probed for stationary points using ab initio and DFT computational methods. Three unique geometries for NMAc dimers were optimized, which were stabilized through mixed ON⋯π pnicogen bonding and C-H⋯π/C-H⋯O hydrogen bonding interactions with varying strength. Within the Ar matrix at 12 K, only the C-H⋯π and C-H⋯O bound NMAc dimer was generated, while in N2, two geometries, one stabilized by pnicogen bonded ON⋯π and C-H⋯π interactions and the other by C-H⋯π and C-H⋯O interactions, were produced. The NMBz dimer has only one structure stabilized by both ON⋯π pnicogen and C-H⋯π hydrogen bonding interactions that appears to be generated preferentially. The binding energies of both the dimers have the greatest contribution from electrostatics in both classes of NMAc and NMBz dimers, which is closely followed by dispersion forces in the case of NMBz. The increased proton affinity of benzene over acetylene appears to enhance the strength of C-H⋯π hydrogen bonds in NMBz while the ON⋯π pnicogen bonds remain quite similar in strength in both NMBz and NMAc dimers.

Enhanced luminescence of tris(carboxylato)uranyl(VI) complexes and energy transfer to Eu(III): a combined spectroscopic and theoretical investigation.

Complex formation between uranyl and carboxylate ligands (benzoate, nicotinate and isonicotinate) has been studied extensively by absorption and luminescence spectroscopy in acetonitrile medium. Experimental data had indicated the existence of stable and enhanced luminescent tris(carboxylato) uranyl(VI) complexes i.e. [UO2(L)3]- with D3h symmetry. The high luminescence of these complexes was due to the sensitization of the Oyl → U ligand to metal charge transfer (LMCT) emission by extremely intense equatorial (carboxylate ligands) LMCT bands. The variation in the experimentally observed parameters such as intensity of equatorial LMCT bands, luminescence lifetimes, quantum yields and structural parameters among tris(carboxylato) uranyl(VI) complexes are affirmed by quantum chemical calculations using density functional theory and the computational results are found to be in good agreement with experimental findings. Interestingly, in a very dilute mixture of [UO2(L)3]- and Eu(III), energy transfer from uranyl to Eu(III) is observed and it leads to the detection of europium at trace levels. This is an intriguing observation as none of the previous studies have reported such a low level of detection limit of Eu(III) by means of energy transfer from any metal.

Nd(III) hypersensitive peak as an optical absorption probe for determining nitric acid in aqueous solution: An application to aqueous raffinate solutions in nuclear reprocessing.

A new method using Nd(III) absorption peak as a probe is described for the measurement of nitric acid concentration in aqueous solution. The hypersensitive peak of Nd(III) at 575.1 nm shows a substantial enhancement in the absorbance in comparison to other absorption peaks with increasing nitric acid concentration. The integrated area and absorbance of this hypersensitive peak show a linear dependency over a large dynamic range of 0.5-15.5 M of nitric acid. A methodology for the correction of spectral interference to the probing absorption peak of Nd(III) is also reported. The method is applied for the measurement of nitric acid in synthetic high level liquid waste solution and shown to be comparable to that obtained by titrimetric method. The present method can be easily adopted for the measurement of nitric acid concentration in aqueous raffinate solutions of nuclear reprocessing streams.

Nitrogen as a pnicogen?: evidence for π-hole driven novel pnicogen bonding interactions in nitromethane-ammonia aggregates using matrix isolation infrared spectroscopy and ab initio computations.

The role of nitrogen, the first member of the pnicogen group, as an electron donor in hypervalent non-covalent interactions has been established long ago, while observation of its electron accepting capability is still elusive experimentally, and remains quite intriguing, conceptually. In the light of minimal computational exploration of this novel class of pnicogen bonding so far, the present work provides experimental proof with unprecedented clarity, for the existence of N(acceptor)N(donor) interaction using the model nitromethane (NM) molecule with ammonia (AM) as a Lewis base in NM-AM aggregates. The NM-AM dimer, in which the nitrogen atom of NM (as a unique pnicogen) accepts electrons from AM (the traditional electron donor), was synthesized at low temperatures under isolated conditions within inert gas matrixes and was characterized using infrared spectroscopy. The experimental generation of the NM-AM dimer stabilized via NN interaction has strong corroboration from ab initio calculations. Furthermore, confirmation regarding the directional prevalence of this NN interaction over C-HN and N-HO hydrogen bonding is elucidated quantitatively by quantum theory of atoms in molecules (QTAIM), electrostatic potential mapping (ESP), natural bond orbital (NBO), non-covalent interaction (NCI) and energy decomposition (ED) analyses. The present study also allows the extension of σ-hole/π-hole driven interactions to the atoms of the second period, in spite of their low polarizability.

Zinc finger protein-440 promotes cartilage degenerative mechanisms in human facet and knee osteoarthritis chondrocytes.

OBJECTIVES: To investigate the role of zinc finger protein 440 (ZNF440) in the pathophysiology of cartilage degeneration during facet joint (FJ) and knee osteoarthritis (OA). METHODS: Expression of ZNF440 in FJ and knee cartilage was determined by immunohistochemistry, quantitative (q)PCR, and Western blotting (WB). Human chondrocytes isolated from FJ and knee OA cartilage were cultured and transduced with ZNF440 or control plasmid, or transfected with ZNF440 or control small interfering RNA (siRNA), with/without interleukin (IL)-1ß. Gene and protein levels of catabolic, anabolic and apoptosis markers were determined by qPCR or WB, respectively. In silico analyses were performed to determine compounds with potential to inhibit expression of ZNF440. RESULTS: ZNF440 expression was increased in both FJ and knee OA cartilage compared to control cartilage. In vitro, overexpression of ZNF440 significantly increased expression of MMP13 and PARP p85, and decreased expression of COL2A1. Knockdown of ZNF440 with siRNA partially reversed the catabolic and cell death phenotype of human knee and FJ OA chondrocytes stimulated with IL-1ß. In silico analysis followed by validation assays identified scriptaid as a compound with potential to downregulate the expression of ZNF440. Validation experiments showed that scriptaid reduced the expression of ZNF440 in OA chondrocytes and concomitantly reduced the expression of MMP13 and PARP p85 in human knee OA chondrocytes overexpressing ZNF440. CONCLUSIONS: The expression of ZNF440 is significantly increased in human FJ and knee OA cartilage and may regulate cartilage degenerative mechanisms. Furthermore, scriptaid reduces the expression of ZNF440 and inhibits its destructive effects in OA chondrocytes.

Unusual blue to red shifting of C-H stretching frequency of CHCl3 in co-operatively P⋯Cl phosphorus bonded POCl3-CHCl3 heterodimers at low temperature inert matrixes.

Heterodimers of POCl3-CHCl3 were generated in Ne, Ar, and Kr matrixes at low temperatures and were studied using infrared spectroscopy. The remarkable role of co-operative pentavalent phosphorus bonding in the stabilization of the structure dictated by hydrogen bonding is deciphered. The complete potential energy surface of the heterodimer was scanned by ab initio and density functional theory computational methodologies. The hydrogen bond between the phosphoryl oxygen of POCl3 and C-H group of CHCl3 in heterodimers induces a blue-shift in the C-H stretching frequency within the Ne matrix. However, in Ar and Kr matrixes, the C-H stretching frequency is exceptionally red-shifted in stark contrast with Ne. The plausibility of the Fermi resonance by the C-H stretching vibrational mode with higher order modes in the heterodimers has been eliminated as a possible cause within Ar and Kr matrixes by isotopic substitution (CDCl3) experiments. To evaluate the influence of matrixes as a possible cause of red-shift, self-consistent Iso-density polarized continuum reaction field model was applied. This conveyed the important role of the dielectric matrixes in inducing the fascinating vibrational shift from blue (Ne) to red (Ar and Kr) due to the matrix specific transmutation of the POCl3-CHCl3 structure. The heterodimer produced in the Ne matrix possesses a cyclic structure stabilized by hydrogen bonding with co-operative phosphorus bonding, while in Ar and Kr the generation of an acyclic open structure stabilized solely by hydrogen bonding is promoted. Compelling justification regarding the dispersion force based influence of matrix environments in addition to the well-known dielectric influence is presented.

Conformational topography of tris(2-methylbutyl) phosphate and the influence of methyl branching at the non-hyperconjugative carbon on the conformational landscape: insights from matrix isolation infrared spectroscopy and DFT computations.

The branching of a methyl group in a linear chain has a profound influence on the conformational morphology as it wields a strong control in reducing a large number of conformations. To unravel the effect of branching on the second non-hyperconjugative carbon atom on the conformational landscape, the conformations of tris(2-methylbutyl)phosphate (T2MBP) were studied using Density Functional Theory (DFT) computations and matrix isolation infrared spectroscopy. Experimentally, T2MBP along with N2/Ar/Kr/Xe gases was effusively expanded and deposited at a low temperature of 12 K, which was subsequently probed using infrared spectroscopy. The computations of all the conformations were accomplished using the B3LYP level of theory with the 6-311++G(d,p) basis set. A dimethyl(2-methylbutyl) phosphate (DM2MBP) prototype, a molecule containing a single 2-methylbutyl moiety, was examined for its conformations. Computations predicted 18 and 9 conformations each for the 'gauche' and 'trans' families, respectively, in which the third branched carbon completely influences the orientation of the fourth carbon, which simplifies the conformational problem of DM2MBP. Of the 18 and 9 bunches each in the 'gauche' and 'trans' families, only 7 and 3 conformations, respectively, became energetically important, which when extrapolated to T2MBP resulted in 343 and 147 conformational possibilities. The factor of degeneracy further reduced these numbers and a total of 168 conformations effectively contribute to the conformational composition of T2MBP in the gas phase. The role of stereo electronic and steric factors prevalent in the conformational clusters of T2MBP was unravelled respectively using natural bond orbital and non-covalent interaction analyses.

Dominance of unique Pπ phosphorus bonding with π donors: evidence using matrix isolation infrared spectroscopy and computational methodology.

Albeit the first account of hypervalentπ interactions has been reported with halogenπ interactions, the feasibility of their extension to other hypervalent atoms as possible Lewis acids is still open. In this work, the role of phosphorus as an acceptor from the π electron cloud (Pπ pnicogen or phosphorus bonding) in PCl3-C2H2 and PCl3-C2H4 heterodimers is explored, by combining matrix isolation infrared spectroscopy with ab initio and DFT computational methodologies. The respective potential energy surfaces of the PCl3-C2H2 and PCl3-C2H4 heterodimers reveal unique minima stabilized by a concert of reasonably strong to weak interactions, of which Pπ phosphorus bonding was energetically dominant. Heterodimers, trimers and tetramers bound primarily by this unique phosphorus bond were generated at low temperatures. The dominance of phosphorus bonding in the PCl3-C2H2 and PCl3-C2H4 heterodimers over other interactions (such as Hπ, HCl, HP, Clπ and lone pair-π interactions) was confirmed and substantiated using extended quantum theory of atoms in molecules, natural bond orbital, electrostatic potential mapping and energy decomposition analyses. The following inferences in correlation with results from non-covalent-interaction analysis offer a complete understanding of the nature of the Pπ phosphorus bonding interactions. The significance of electrostatic forces kinetically favoring the formation of phosphorus bonded heterodimers, in addition to thermodynamic stabilization, is demonstrated experimentally.

Prototypical cyclohexane dimers: spectroscopic evidence for σ stacking at low temperatures.

Owing to the fact that σ stacking is as important as π stacking in determining the structural motifs of aliphatic saturated cyclic hydrocarbons, in this work we have provided the first unambiguous spectroscopic evidence for the existence of σ stacking interactions in cyclohexane dimers at low temperatures. Molecular beam experiments performed using effusive nozzle and supersonic jet sources on cyclohexane in an N2 matrix generated cyclohexane dimers stabilized through σ stacking and the dimers were characterized by infrared spectroscopy. The ab initio computations carried out on cyclohexane dimers identified eclipsed (face-to-face), parallel displaced and T-shaped structures, which are predominantly stabilized by σ stacking interactions. While natural bond orbital analysis substantiated a significant amount of σ → σ* interactions involved in the stabilization, the Atoms in Molecules analysis indicated that the stacking is induced by a plausible 'dihydrogen bonding' interaction. Energy decomposition analysis disclosed that a large measure of dispersion interactions effectively contributes for the overall stability of cyclohexane dimers.

Elusive hypervalent phosphorusπ interactions: evidence for paradigm transformation from hydrogen to phosphorus bonding at low temperatures.

The π electron systems are the conventional electron donors to the hydrogen acceptors in hydrogen bonding. Apart from the hydrogen atom, halogens, chalcogens, pnicogens and triel/tetrel atoms can also be envisaged as electron acceptors involving π clouds. Markedly, in pnicogenπ interactions, the bonding of the hypervalent (predominantly pentavalent) state of the phosphorus atom with π electron donors is elusive and can be thought of as an intuitive extension to trivalent phosphorusπ interactions. In this work, on the one hand, POCl3 was taken as a prototypical molecule to explore these pentavalent phosphorus interactions and on the other hand, acetylene (C2H2), ethylene (C2H4) and benzene (C6H6), in which phosphorusπ bonding can be expected to compete with hydrogen and halogen bonding interactions, were taken as π electron donors. All three POCl3-C2H2, POCl3-C2H4 and POCl3-C6H6 heterodimers were experimentally generated at low temperatures in Ar and N2 matrices and were characterized by both infrared spectroscopy and state-of-the-art quantum chemical computations. Though hydrogen bonding dominates in POCl3-C2H2 and POCl3-C2H4 heterodimers, phosphorus bonding plays a definite and non-trivial role in their overall stabilization. An interesting paradigm transformation was noticed in the POCl3-C6H6 system, where pentavalent phosphorusπ bonding was observed to completely influence the hydrogen bonding interaction. To further shed light on these Pπ systems, the interaction characteristics were analyzed with the help of electrostatic potential mapping, natural bond orbital and energy decomposition analyses.

Conformations of diethyl ether and its interaction with pyrrole at low temperatures.

Conformations of diethyl ether (DEE) were studied at low temperatures in N2 and Ar matrixes. Computations performed at B3LYP/aug-cc-pVDZ level of theory yielded three minima corresponding to tt, tg± and g±g± conformers of DEE. Of the three, the tt and tg± conformers of DEE were experimentally identified in N2 and Ar matrixes. Furthermore, hydrogen bonded complexes of pyrrole (py) with DEE have been investigated using Density Functional Theory (DFT) and matrix isolation infrared spectroscopy. Computations performed at B3LYP level of theory using aug-cc-pVDZ basis set on pyrrole with tt and tg± conformers of DEE gave py-DEE-tt and py-DEE-tg± complexes, both characterized by NH⋯O interaction. Experimental evidence for the formation of py-DEE-tt and py-DEE-tg± complexes was affirmed from the shifts in the NH stretching, NH bending regions of pyrrole and COC and CH stretching regions of DEE. NBO analysis was carried out to understand the charge-transfer delocalization interactions in the conformers of DEE and its hydrogen bonded complexes.

Experimental Evidence of Synergistic Interactions in Pyrrole-Phenol Complexes at Low Temperatures under Isolated Conditions.

The simultaneous possession of π-electron clouds and acidic hydrogen atoms in pyrrole (C4H5N) and phenol (C6H5OH) framework opens the potentiality in exploring the synergistic interactions in their weakly bonded complexes. In this work, the synergistic hydrogen bonding in C4H5N-C6H5OH complexes is therefore investigated using FTIR spectroscopy under isolated conditions at low temperatures. Computations performed at DFT, DFT-GD3, M06, and MP2 level of theories employing aug-cc-pVDZ basis set yielded three minima on the potential energy surface for the 1:1 complex of C4H5N-C6H5OH. All three optimized structures showed synergistic interactions, where both C6H5OH and C4H5N simultaneously act as a proton donor and acceptor at MP2/aug-cc-pVDZ level of theory. In the global minimum complex A, the hydroxyl proton and the C-H group of C6H5OH interact with the π-cloud of C4H5N. The first local minimum corresponds to complex B, where the N-H and π-electrons of C4H5N interact with π-electrons of C6H5OH. In complex C, the N-H and C-H groups of C4H5N interact with O-H and π-cloud of C6H5OH, respectively. Complex A was the lowest energy structure at all levels of theory, whereas the stabilization energies of complexes B and C varied depending upon the levels of theory used. Interestingly, the stabilization energies as predicted by the DFT method are in accordance with Etter's and Legon-Millen rules; however, a deviation in the Legon-Millen rule was discerned with empirical (DFT-GD3, M06) and dispersion corrected (MP2) methods. On comparing the experimental vibrational wavenumber shifts in the N-H stretching and bending modes of C4H5N and O-H stretching mode of C6H5OH submolecules with the computed shifts, all three complexes were identified in the N2 matrix. Natural Bond Orbital and Energy Decomposition analyses were performed to characterize the nature of the synergistic interaction in these complexes.

Influence of Branching on the Conformational Space: Case Study of Tri- sec-butyl Phosphate Using Matrix Isolation Infrared Spectroscopy and DFT Computations.

The conformational analysis of long chain phosphates poses a serious challenge due to the presence of rotationally flexible multiple alkyl groups. Tri- sec-butyl phosphate (TsBP) is an interesting example, in which branching can be expected to influence the conformational landscape. To solve the conformational problem of TsBP systematically, the conformations of model dimethyl- sec-butyl phosphate (DMsBP), a molecule possessing a single secondary butyl strand, were analyzed. On the basis of the analysis of the energy profile of DMsBP, a few conformational bunches were eliminated. The presence of branched methyl group appears to completely influence the conformational space of TsBP and as a result, the number of conformations is drastically reduced in comparison to its structural isomer, tri- n-butyl phosphate (TBP). B3LYP level of theory in association with 6-311++G(d,p) basis set was used for computing all the conformer geometries. Experimentally, the conformations of TsBP were studied using infrared spectroscopy by trapping the molecule in N2 and Ar matrixes at low temperatures, which were correlated well with the computational results.

Development of an experimental set-up for low-temperature spectroscopic studies of matrix-isolated molecules and molecular ices using synchrotron radiation.

An experimental set-up for studying photophysics and photochemistry of molecules in an inert gas medium (matrix-isolated) and in the ice phase at low temperatures has been developed and commissioned at the Photophysics beamline, Indus-1 synchrotron radiation source. This end-station uses an in-house-developed closed-cycle cryostat for achieving cryo-temperatures (∼10 K). Synchrotron radiation from the Photophysics beamline is used as the source of UV-VUV photons and the system is equipped with a Fourier transform infrared spectrometer for characterization of the molecular species formed at low temperature. Various individual components of the end-station like closed-cycle cryostat, experimental chamber, gas mixing and deposition systems are tested to ascertain that the desired performance criteria are satisfied. The performance of the composite system after integration with the Photophysics beamline is evaluated by recording IR and UV-VUV photoabsorption spectra of sulfur dioxide at low temperatures (10 K), both in the ice phase as well as isolated in argon matrices. Results obtained are in good agreement with earlier literature, thus validating the satisfactory performance of the system. As an off-shoot of the study, the VUV absorption spectrum of matrix-isolated SO2 in argon matrix up to 10.2 eV is reported here for the first time. This experimental end-station will provide new opportunities to study photon-induced reactions in molecules of environmental, astrochemical and industrial importance. Details of the design, development and initial experimental results obtained are presented.

Pentavalent phosphorus as a unique phosphorus donor in POCl3 homodimer and POCl3-H2O heterodimer: matrix isolation infrared spectroscopic and computational studies.

Phosphorus, an important element among the pnicogen group, opens up avenues for experimental and computational explorations of its interaction in a variety of compounds. Although experimental proof of trivalent phosphorus bonding is limited and is growing with time, phosphorus bonding with pentavalent phosphorus has been a long sought after interaction both computationally and experimentally. In the present work, for the first time, we have provided unambiguous experimental evidence for the pentavalent phosphorus bonding interaction by exploiting a phosphoryl chloride (POCl3) prototype under isolated conditions at low temperatures. The POCl3 dimer and higher aggregates can be set as a unique example possessing pentavalent phosphorus bonding with a competing halogen bonding interaction. The POCl3-H2O heterodimer is another interesting system, stabilized through multiple phosphorus and hydrogen bonded interactions. Using matrix isolation infrared spectroscopy, the POCl3 homodimer and POCl3-H2O heterodimer were characterized and the structures were elucidated by employing ab initio and DFT methods. The multifaceted interactions in the POCl3 paradigm were investigated by using Natural Bond Orbital, Energy Decomposition and Electrostatic Potential Mapping analyses.

Effect of Methyl Substitution on the N-H···O Interaction in Complexes of Pyrrole with Water, Methanol, and Dimethyl Ether: Matrix Isolation Infrared Spectroscopy and ab Initio Computational Studies.

Hydrogen-bonded interactions of pyrrole with water and methanol have been studied using matrix isolation infrared spectroscopy and compared with the calculation performed on dimethyl ether. Computations carried out at MP2/aug-cc-pVDZ level of theory yielded two minima for the pyrrole-water and pyrrole-methanol complexes. The global and local minima correspond to the N-H···O and O-H···π complexes, respectively, where the N-H group of pyrrole interacts with oxygen of water/methanol and O-H of water and methanol interacts with the π cloud of pyrrole. Computations performed on the pyrrole-dimethyl ether gave only N-H···O type complex. From the experimental vibrational wavenumber shifts in the N-H stretching and N-H bending modes of pyrrole, as well as in the O-H stretching modes of water and methanol, the 1:1 N-H···O complexes were discerned. The strength of the N-H···O hydrogen bond and the corresponding shift in the N-H stretching vibrational wavenumbers increases in the order pyrrole-water < pyrrole-methanol < pyrrole-dimethyl ether, where a proton is successively replaced by a methyl group. Apart from the 1:1 complexes, higher clusters of 2:1 and 1:2 pyrrole-water and pyrrole-methanol complexes were also generated in N2 matrix. Atoms in molecules and natural bond orbital analyses were carried out at the MP2/aug-cc-pVDZ level to understand the nature of interaction in the 1:1 pyrrole-water, pyrrole-methanol and pyrrole-dimethyl ether complexes.

Uranyl tris nitrato as a luminescent probe for trace water detection in acetonitrile.

Uranyl tris nitrato i.e. [UO2 (NO3 )3 ]- was formed by adding tetramethylammonium nitrate to uranyl nitrate in acetonitrile medium. The luminescence features of this complex in acetonitrile are very sensitive to water content, which could lead to the use of it as a luminescent probe for water present in acetonitrile. The luminescence intensity ratio of 507 to 467 nm peak of uranyl tris nitrato showed a linear response in the range 0-5% (v/v) water content in acetonitrile. The present method was applied for three synthetic samples of acetonitrile for water detection and the results obtained were compared using Karl Fischer titration. There was a good agreement in the values obtained by both the methods.

Nitrogen: A New Class of π-Bonding Partner in Hetero π-Stacking Interaction.

Spectroscopy under isolated conditions at low temperatures is an excellent tool to characterize the aggregates stabilized through weak interactions. Within the framework of weak interactions, the π-stacking interactions are considered unconventional with the limited experimental proofs, wherein the bonding associates are either aromatic and heterocyclic compounds or their combinations. Besides aromatic compounds, π-stacking networks can even be realized with molecules possessing electron rich π-clouds. In this work, the N2 molecule as a possible π-bonding partner is explored for the first time in which hetero π-stacking was achieved between pyrrole and N2 precursors. The matrix isolation experiments performed by seeding pyrrole and N2 mixtures in an Ar matrix at low temperatures with subsequent infrared spectral characterization revealed the generation of adducts stabilized through a π(pyrrole)···π(N2) interaction. Under identical conditions with the likelihood of two competing π-stacking and hydrogen-bonding interactions in pyrrole-N2 associates, π-stacking dominates energetically over hydrogen-bonding interaction.

Conformational Landscape of Tri-n-butyl Phosphate: Matrix Isolation Infrared Spectroscopy and Systematic Computational Analysis.

The conformations of tri-n-butyl phosphate (TBP) were studied using matrix isolation infrared spectroscopy and density functional theory (DFT) calculations. TBP was trapped in a N2 matrix using both effusive and supersonic sources, and its infrared spectra were recorded. The computational exploration of TBP is a very demanding problem to confront, due to the presence of a large multitude of conformations in TBP. To simplify the problem, computations were done on model compounds, dimethyl butyl phosphate (DMBP) and dibutyl methyl phosphate (DBMP), to systematically arrive at the conformations of TBP that are expected to contribute to its chemistry at room temperature. Some predictive rules seem to simplify this complex conformational landscape problem. The predictive rules that were formulated enabled us to search the relevant portion of the conformational topography of this molecule. The computations were performed at the B3LYP level of theory using the 6-31++G(d,p) basis set. Vibrational wavenumber calculations were also performed for the various conformers to assign the infrared features of TBP, trapped in solid N2 matrix.