DFT/TDDFT study on the excited-state hydrogen bonding dynamics of hydrogen-bonded complex formed by methyl cyanide and methanol

Role of Solvent Polarity and Hydrogen-Bonding on Excited-State Fluorescence of 3-[(E)-{4-[Dimethylamino]benzylidene}amino]-2-naphthoic Acid (DMAMN): Isomerization vs Rotomerization

The present experimental and theoretical study on a new chromophore DMAMN of the type push-π-pull (push = dimethylaniline, π = imine, pull = 2-naphthoic acid), allows understanding of the mechanism by which the molecular conformational undergoes isomerization/rotomerization following electronic excitation. The steady-state fluorescence spectra of this compound, carried out in solvents of different polarities and proticities, showed significant changes in both the shape and peak positions. The wavelength and intensity change depend on the polarity and hydrogen-bonding environment. In highly polar solvents, the emission is weak and red-shifted compared to that for cyclohexane, but it is more red-shifted in moderate aprotic polar solvents. In hydroxyl solvents, a new weak low-energy emission band appears at ∼525 nm, attributed to the intermolecularly H-bonded open conformer. On the basis of the generated potential energy landscapes of the ground state and low-lying excited state in the gas phase and solution, we found that selective photon absorption, brings this molecule to a "bright" state, from which N═C isomerization Z → E, takes place. This isomerization in gas-phase and low-polarity solvents leads to two minima with a barrier, whereas in highly polar-protic media, it gives one minimum on the S₁ surface with low ΔE_S1/T1 (0.17 eV), facilitating deactivation via ISC.

Time-dependent density functional theory study on the excited-state hydrogen-bonding characteristics of polyaniline in aqueous environment

A theoretical study was carried out to study the excited-state of hydrogen-bonding characteristics of polyaniline (PANI) in aqueous environment. The hydrogen-bonded PANI-H₂O complexes were studied using first-principles calculations based on density functional theory (DFT). The electronic excitation energies and the corresponding oscillator strengths of the low-lying electronically excited states for hydrogen-bonded complexes were calculated by time-dependent density functional theory (TDDFT). The ground-state geometric structures were optimized, and it is observed that the intermolecular hydrogen bonds CN⋯HO and NH⋯OH were formed in PANI-H₂O complexes. The formed hydrogen bonds influenced the bond lengths, the charge distribution, as well as the spectral characters of the groups involved. It was concluded that all the hydrogen-bonded PANI-H₂O complexes were primarily excited to the S1 states with the largest oscillator strength. In addition, the orbital transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) involved intramolecular charge redistribution resulting to increase the electron density of the quinonoid rings.

A TDDFT/EFP1 study on hydrogen bonding dynamics of coumarin 151 in water

Change in energy of hydrogen bonds (HBs) upon excitation, plays an important role on the spectra of chemical and biological molecules. Effective fragment potential (EFP) method of explicit water molecules embedded in polarizable continuum medium (PCM) is used for the solvation of 7-Amino-4-(trifluoromethyl)coumarin (C151). Time dependent density functional theory (TDDFT) calculations combined with EFP/PCM had been carried out to study the electronic structure and the exited state properties of C151 with five water molecules (C151-(H2O)5 complex). S0 state and S1 state geometries were optimized using DFT/TDDFT with PBE0 functional combined with cc-pVDZ basis set, the transition energies are computed with same basis set and functional. Change in HB energy is calculated using the procedure proposed by T. Nagata et al. to calculate solute-solvent interaction energy in Nagata et al. (2011). Upon photoexcitation of C151-(H2O)5 complex, A type (N⋯H-O) HB is weakened with decrease of energy by 4.37 kJ/mol, whereas B and C type (C=O⋯H-O and N-H⋯O) HBs are strengthened with increase of 5.62 and 10.21 kJ/mol energy, respectively. This study again confirmed that the intermolecular hydrogen bonds between C151 chromophore and aqueous solvents are strengthened, not cleaved upon electronic excitation.

Molecular modeling in dioxane methanol interaction

Molecular interaction between dioxane and methanol involves certain polar and nonpolar bonding to form a one to one complex. Interatomic distances between hydrogen and oxygen within 3 Å have been considered as hydrogen bonding. Optimizations of the structures of dioxane-methanol complexes were carried out considering any spatial orientation of a methanol molecule around a chair/boat/twisted-boat conformation of dioxane. From 45 different orientations of dioxane and water, 23 different structures with different local minima were obtained and the structural characteristics like interatomic distances, bond angles, dihedral angles, dipole moment of each complex were discussed. The most stable structure, i.e., with minimum heat of formation is found to have a chair form dioxane, one O-H…O, and two C-H…O hydrogen bonds. In general, the O-H…O hydrogen bonds have an average distance of 1.8 Å while C-H…O bonds have 2.6 Å. The binding energy of the dioxane-methanol complex is found to be a linear function of number of O-H…O and C-H…O bonds, and hydrogen bond length.

A DFT/TD-DFT study of thiazolidinedione derivative in dimethylformamide: cooperative roles of hydrogen bondings, electronic and vibrational spectra

The time-dependent density functional theory (TDDFT) method has been applied to investigate the thiazolidinedione (TZD) derivative A and its hydrogen-bonded complexes with dimethylformamide (DMF) (A-DMF and A-2DMF). The calculation results showed that the excited-state hydrogen bondings of O-H⋯O=C and N-H⋯O=C are strengthened and weakened in the hydrogen-bonded trimer A-2DMF, and their cooperation effect caused a blue shift in the electronic spectrum of A-2DMF. This modulation mechanism of the hydrogen-bond strengthening and weakening and its role in influencing the spectroscopy property of the TZD derivative A in DMF have been analyzed and showed in details.

On the nature of non-covalent interactions in isomers of 2,5-dichloro-1,4-benzoquinone dimers – ground- and excited-state properties

Impact of intermolecular NCIs (C–H⋯O, C–H⋯Cl, C–Cl⋯O, C–Cl⋯Cl–C, C–O⋯C, C–Cl⋯C and C–O⋯π) on chirality arrangement in the isomers of the 2,5-dichloro-1,4-benzoquinone (DCBQ) dimer.

MOLECULAR STRUCTURE, VIBRATIONAL ASSIGNMENTS, CONFORMATIONAL STABILITY, GROUND AND EXCITED STATE HYDROGEN-BONDING ANALYSIS OF 2-NITROSO VINYL AMINE

In the present work, a detailed conformational study is performed using several computational methods, including density functional theory (DFT) (B3LYP), MP2 and G2MP2 on 2-Nitroso vinyl amine (NVA) in order to determine the stability order of conformers and the various possibilities of intramolecular hydrogen bonding (HB) formation. Four conformers exhibit HB . This feature, although not being the dominant factor in energetic terms, appears to be of foremost importance to define the geometry of the molecule. According to our theoretical results, oximimine conformers are more stable than the corresponding nitrosoamine and nitrosoimine analogues. Theoretical calculations show the following order for intramolecular HB strength in the conformers of title compound: [Formula: see text]

The nature of intramolecular HB has been investigated by means of the Bader theory of atoms in molecules (AIM) and natural bond orbital (NBO) analysis. Also, Harmonic Oscillator Model of Aromaticity (HOMA) index as a geometrical indicator of a local aromaticity are investigated. The influence of the solvent on the stability order of conformers and the strength of intramolecular HB is considered using the Onsager reaction field model. The calculated highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies confirm that charge transfer occur within the molecule. Further verification of the obtained transition state structures were implemented via intrinsic reaction coordinate (IRC) analysis. Calculations of the 1 H NMR chemical shift at GIAO/B3LYP/6–311++G** levels of theory are also presented. The excited-state properties of intramolecular HB in H -bonded systems have been investigated theoretically using the time-dependent density functional theory (TD-DFT) method. The complete vibrational assignment for three H -bonded conformers has been made on the basis of the calculated potential energy distribution (PED).

Hydrogen bonding in the electronic excited state

Because of its fundamental importance in many branches of science, hydrogen bonding is a subject of intense contemporary research interest. The physical and chemical properties of hydrogen bonds in the ground state have been widely studied both experimentally and theoretically by chemists, physicists, and biologists. However, hydrogen bonding in the electronic excited state, which plays an important role in many photophysical processes and photochemical reactions, has scarcely been investigated. Upon electronic excitation of hydrogen-bonded systems by light, the hydrogen donor and acceptor molecules must reorganize in the electronic excited state because of the significant charge distribution difference between the different electronic states. The electronic excited-state hydrogen-bonding dynamics, which are predominantly determined by the vibrational motions of the hydrogen donor and acceptor groups, generally occur on ultrafast time scales of hundreds of femtoseconds. As a result, state-of-the-art femtosecond time-resolved vibrational spectroscopy is used to directly monitor the ultrafast dynamical behavior of hydrogen bonds in the electronic excited state. It is important to note that the excited-state hydrogen-bonding dynamics are coupled to the electronic excitation. Fortunately, the combination of femtosecond time-resolved spectroscopy and accurate quantum chemistry calculations of excited states resolves this issue in laser experiments. Through a comparison of the hydrogen-bonded complex to the separated hydrogen donor or acceptor in ground and electronic excited states, the excited-state hydrogen-bonding structure and dynamics have been obtained. Moreover, we have also demonstrated the importance of hydrogen bonding in many photophysical processes and photochemical reactions. In this Account, we review our recent advances in electronic excited-state hydrogen-bonding dynamics and the significant role of electronic excited-state hydrogen bonding on internal conversion (IC), electronic spectral shifts (ESS), photoinduced electron transfer (PET), fluorescence quenching (FQ), intramolecular charge transfer (ICT), and metal-to-ligand charge transfer (MLCT). The combination of various spectroscopic experiments with theoretical calculations has led to tremendous progress in excited-state hydrogen-bonding research. We first demonstrated that the intermolecular hydrogen bond in the electronic excited state is greatly strengthened for coumarin chromophores and weakened for thiocarbonyl chromophores. We have also clarified that the intermolecular hydrogen-bond strengthening and weakening correspond to red-shifts and blue-shifts, respectively, in the electronic spectra. Moreover, radiationless deactivations (via IC, PET, ICT, MLCT, and so on) can be dramatically influenced through the regulation of electronic states by hydrogen-bonding interactions. Consequently, the fluorescence of chromophores in hydrogen-bonded surroundings is quenched or enhanced by hydrogen bonds. Our research expands our understanding of the nature of hydrogen bonding by delineating the interaction between hydrogen bonds and photons, thereby providing a basis for excited-state hydrogen bonding studies in photophysics, photochemistry, and photobiology.