Steric as well as n→π* Interaction Controls the Conformational Preferences of Phenyl Acetate: Gas‐phase Spectroscopy and Quantum Chemical Calculations

N···C═O n → π* Interaction: Gas-Phase Electronic and Vibrational Spectroscopy Combined with Quantum Chemistry Calculations

Herein, we have used gas-phase electronic and vibrational spectroscopic techniques for the first time to study the N···C═O n → π* interaction in ethyl 2-(2-(dimethylamino) phenyl) acetate (NMe2-Ph-EA). We have measured the electronic spectra of NMe2-Ph-EA in the mass channels of its two distinct fragments of m/z = 15 and 192 using a resonant two-photon ionization technique as there was extensive photofragmentation of NMe2-Ph-EA. Identical electronic spectra obtained in the mass channels of both fragments confirm the dissociation of NMe2-Ph-EA in the ionic state, and hence, the electronic spectrum of the fragment represents that of NMe2-Ph-EA only. UV-UV hole-burning spectroscopy proved the presence of a single conformer of NMe2-Ph-EA in the experiment. Detailed quantum chemistry calculations reveal the existence of a N···C═O n → π* interaction in all six low-energy conformers of NMe2-Ph-EA. A comparison of the IR spectrum of NMe2-Ph-EA acquired from the gas-phase experiment with those obtained from theoretical calculations indicates that the experimentally observed conformer has a N···C═O n → π* interaction. The present finding might be further valuable in drug design and their recognition based on the N···C═O n → π* interaction.

Boosting organic phosphorescence in adaptive host-guest materials by hyperconjugation

Phosphorescence is ubiquitous in heavy atom-containing organic phosphors, which attracts considerable attention in optoelectronics and bioelectronics. However, heavy atom-free organic materials with efficient phosphorescence are rare under ambient conditions. Herein, we report a series of adaptive host-guest materials derived from dibenzo-heterocyclic analogues, showing host-dependent color-tunable phosphorescence with phosphorescence efficiency of up to 98.9%. The adaptive structural deformation of the guests arises from the hyperconjugation, namely the n→π* interaction, enabling them to inhabit the cavity of host crystals in synergy with steric effects. Consequently, a perfect conformation match between host and guest molecules facilitates the suppression of triplet exciton dissipation, thereby boosting the phosphorescence of these adaptive materials. Moreover, we extend this strategy to a ternary host-guest system, yielding both excitation- and time-dependent phosphorescence with a phosphorescence efficiency of 92.0%. This principle provides a concise way for obtaining efficient and color-tunable phosphorescence, making a major step toward potential applications in optoelectronics.

Modulation of n → π* Interaction in the Complexes of p-Substituted Pyridines with Aldehydes: A Theoretical Study

n → π* interaction is analogous to the hydrogen bond in terms of the delocalization of the electron density between the two orbitals. Studies on the intermolecular complexes stabilized by the n → π* interaction are scarce in the literature. Herein, we have studied intermolecular N···C═O n → π* interactions in the complexes of p-substituted pyridines (p-R-Py) with formaldehyde (HCHO), formyl chloride (HCOCl), and acetaldehyde (CH₃CHO) using quantum chemistry calculations. We have shown that the strength of the n → π* interaction can be modulated by varying the electronic substituents at the donor and acceptor sites in the complexes. Variation of the substituents at the para position of the pyridine ring from the electron-withdrawing groups (EWGs) to the electron-donating groups (EDGs) results in a systematic increase in the strength of the n → π* interaction. The strength of this interaction is also modulated by tuning the electron density toward the carbonyl bond by substituting the hydrogen atom of HCHO with the methyl and chloro groups. The modulation of this interaction due to the electronic substitutions at the n → π* donor and acceptor sites in the complexes is monitored by probing the relevant geometrical parameters, binding energies, C═O frequency redshift, NBO energies, and electron density for this interaction derived from QTAIM and NCI index analyses. Energy decomposition analysis reveals that the electrostatic interaction dominates the binding energies of these complexes, while the charge transfer interaction, which is representative of the n → π* interaction, also has a significant contribution to these.

Effect of Substituents on the Intramolecular n→π* Interaction in 3-[2-(Dimethylamino) phenyl] propanal: A Computational Study

n→π* non-covalent interaction (NCI) and hydrogen bond have similarity in terms of delocalization of the electron density between the two orbitals involved in the interaction. Hydrogen bond (X-H···Y) involves delocalization of the lone pair electrons (n) on the Y atom into the σ* orbital of the X-H bond. In contrast, the n→π* interaction deals with delocalizing the lone pair electrons (n) on the N, O, or S atom into the π* orbital of a C═O group or aromatic ring. Herein, we have shown a resemblance of this weak n→π* interaction with the relatively stronger hydrogen bond in terms of folding the side chains in flexible molecules. This work reports the study of folding of the flexible side-chain in 3-[2-(dimethylamino) phenyl] propanal (DMAPhP) through a N···C═O n→π* interaction using various computational approaches such as NBO, QTAIM, and NCI analyses. The folding of the molecule by the n→π* interaction observed in this study is found to be similar to that present in the secondary structures of peptides or proteins through hydrogen bonding interactions. Interestingly, the stabilization of the global minimum conformer of DMAPhP by the n→π* interaction demonstrates the importance of this NCI in providing conformational preferences in molecular systems. Another important finding of this study is that the theoretical redshift obtained in the C═O stretching frequency of the most stable conformer of the DMAPhP is contributed mostly by the n→π* interaction as the C═O group is not involved in hyperconjugation with any neighboring heteroatom, which is a common phenomenon in any ester or amide. We have also demonstrated here that the strength of the intramolecular n→π* interaction can be modulated by varying the electronic substituents at the para position of the donor group involved in the interaction.

Microhydration of Phenyl Formate: Gas-Phase Laser Spectroscopy, Microwave Spectroscopy, and Quantum Chemistry Calculations

Herein, we have investigated the structure of phenyl formate⋅⋅⋅water (PhOF⋅⋅⋅H₂ O) dimer and various non-covalent interactions present there using gas-phase laser spectroscopy and microwave spectroscopy combined with quantum chemistry calculations. Two conformers of PhOF⋅⋅⋅H₂ O (C1 and T1), built on the two cis/trans conformers of the bare molecule, have been observed in the experiment. In cis-PhOF, there is an n_CO → π A r * ${{{\rm \pi }}_{{\rm A}{\rm r}}^{{\rm {^\ast}}}}$ interaction between the lone-pair orbital of the carbonyl oxygen atom and the π* orbital of the phenyl ring, which persists in the monohydrated C1 conformer of PhOF⋅⋅⋅H₂ O according to the NBO and NCI analyses. On the other hand, this interaction is absent in the trans-PhOF conformer as the C=O group is away from the phenyl ring. The C1 conformer is primarily stabilized by an interplay between O-H⋅⋅⋅O=C hydrogen bond and O-H⋅⋅⋅π interactions, while the stability of the T1 conformer is primarily governed by the O-H⋅⋅⋅O=C hydrogen bond. The most important finding of the present work is that the conformational preference of the PhOF monomer is retained in its monohydrated complex.

Understanding the n → π* non-covalent interaction using different experimental and theoretical approaches

Herein, a perspective on the recent understanding of weak n → π* interaction obtained using different experimental and theoretical approaches is presented. This interaction is purely an orbital interaction that involves the delocalization of the lone pair electrons (n) on nitrogen, oxygen, and sulfur to the π* orbitals of CO, CN, and aromatic rings. The n → π* interaction has been found to profoundly influence the stabilization of peptides, proteins, drugs, and various small molecules. Although the functional properties of this non-covalent interaction are still quite underestimated, there are recent demonstrations of applying this interaction to the regulation of synthetic chemistry, catalysis, and molecular recognition. However, the identification and quantification of the n → π* interaction remain a demanding task as this interaction is quite weak and based on the electron delocalization between the two orbitals, while hyperconjugation interactions between neighboring atoms and the group involved in the n → π* interaction are simultaneously present. This review provides a comprehensive picture of understanding the n → π* interaction using different experimental approaches such as the X-ray diffraction technique, and electronic, NMR, microwave, and IR spectroscopy, in addition to quantum chemistry calculations. A detailed understanding of the n → π* interaction can help in modulating the strength of this interaction, which will be further helpful in designing efficient drugs, synthetic peptides, peptidomimetics, etc.

Spectroscopic evidence of n → π* interactions involving carbonyl groups

n → π* has emerged as an important noncovalent interaction that can affect the conformations of both small- and macromolecules including peptides and proteins. Carbonyl-carbonyl (COCO) n → π* interactions involving CO groups are well studied. Recent studies have shown that the COCO n → π* interactions are the most abundant secondary interactions in proteins with a frequency of 33 interactions per 100 residues and, among the various secondary interactions, n → π* interactions are expected to provide the highest enthalpic contributions to the conformational stability of globular proteins. However, n → π* interactions are relatively weak and provide an average stabilization of about 0.25 kcal mol-1 per interaction in proteins. The strongest n → π* interaction could be as strong as a moderate hydrogen bond. Therefore, it is challenging to detect and quantify these weak interactions, especially in solution in the presence of perturbation from other intermolecular interactions. Accordingly, spectroscopic investigations that can provide direct evidence of n → π* interaction are limited, and the majority of papers published in this area have relied on X-ray crystallography and/or theoretical calculations to establish the presence of this interaction. The aim of this perspective is to discuss the studies where a spectroscopic signature in support of n → π* interaction was observed. As the "n → π* interaction" is a relatively new terminology, there remains the possibility of there being earlier studies where spectroscopic evidence for n → π* interactions was obtained but it was not discussed in light of the n → π* terminology. We noticed several such studies and, as can be expected, these studies were often overlooked in the discussion of n → π* interactions in the recent literature. In this perspective, we have also discussed these studies and provided computational support for the presence of n → π* interaction.