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The hydrogen-bonded docking preferences of small solvent molecules on 2-(2′-pyridyl)-benzimidazole (PBI) were studied experimentally aided by computational findings. The PBI–S1,2 complexes (S = H2O, CH3OH, and NH3) were produced in a supersonically jet-cooled molecular beam and probed using resonant two-photon ionization and laser-induced fluorescence spectroscopy, with multiple isomers confirmed by UV–UV hole-burning spectroscopy. Two distinct isomers of PBI–H2O and PBI–(H2O)2 complexes were identified, while PBI–CH3OH and PBI–NH3 each formed a single 1 : 1 and 1 : 2 complex. Computational results with experimental findings revealed PBI–S-a as the most stable structure, with a solvent molecule forming a hydrogen-bonded bridge between imidazolyl-NH (NIH) and pyridyl-N (NP) at site-a. The site-a isomers exhibit higher S1 state stability compared to the S0 state, resulting in red-shifted S0 → S1 band origin for PBI–S-a and a further red-shift for the PBI–(S)2-aa isomers. In contrast, the PBI–S-b isomer, with a hydrogen bond between imidazolyl-N (NI) and pyridyl-CH (CPH) at site-b opposite to site-a, showed a blue-shifted band origin transition. A unique PBI–(H2O)2-ab isomer was detected with solvent molecules bound at both sites a and b, displaying a smaller red-shift in the band origin transition than the aa-isomer. The energy barrier for solvent-to-chromophore proton transfer varies with isomeric configuration. PBI–H2O-b isomers show significantly higher barriers (>800 cm−1), while PBI–(H2O)-aa has a slightly increased barrier (>436 cm−1) compared to the PBI–H2O-a (420 ± 10 cm−1) isomer. This study explores the potential landscape of PBI, enhancing our understanding of stabilization effects, spectral shifts, and their impact on chromophore excited-state dynamics in various environments. [ABSTRACT FROM AUTHOR]

We report the hyper-Raman (HR) spectrum of trifluoroethanol, excited with 532 nm light, in an aqueous solution at basic pH. The HR spectrum exhibits a distinct spectral pattern that diverges entirely from the infrared and Raman spectra of trifluoroethanol. This observed unique HR signal was attributed to the products of photoinduced radical reactions in the aqueous solution. This result exemplifies the exceptional capabilities of HR spectroscopy based on resonance conditions. [ABSTRACT FROM AUTHOR]

We report the first gas phase comprehensive study of the electronic spectroscopy of 2,7-diazaindole molecule in the ground and excited states. Single vibronic level fluorescence spectroscopy (SVLF) was performed to determine the ground state vibrations of the molecule, which depicted a large Franck–Condon activity beyond 2600 cm−1. For the excited state characterization, laser-induced fluorescence (LIF) and two-color resonant two-photon ionization spectroscopy (2C-R2PI) were performed. The band origin (0 00) for S1 ← S0 transition appeared at 33910 ± 1 cm−1 which was red shifted by 718 cm−1 and 1322 cm−1 compared to that of 7-azaindole and indole respectively. The Franck–Condon active vibrational modes in the spectra were seen till the (0 00) + 1600 cm−1 region. The IR-UV hole burning spectroscopy confirmed the absence of any other isomeric species in the molecular beam. The ionization energy (IE) of the molecule was measured as 8.921 ± 0.001 eV, recorded using photoionization efficiency spectroscopy. The above IE value was significantly higher than that of the related indole derivatives, suggesting the higher photostability of the 27DAI molecule due to N(2) insertion. The ground and excited state N–H stretching frequencies of the molecule were determined using fluorescence-dip infrared spectroscopy (FDIR) and resonant ion-dip infrared spectroscopy (IDIR), and the values are 3523 and 3467 cm−1, respectively. The lower value of νNH in the electronic excited state implied the increased photoacidity of the group. A comparative analysis of the experimental LIF/2C-R2PI spectra was done against Franck–Condon simulated spectra at three different levels of theory. The vibrational frequencies calculated at B3LYP-D4/def2-TZVPP showed the most accurate prediction in comparison with the experimentally detected symmetric modes in the ground state. However, in the excited state, the lower energy asymmetric modes simulated at the B3LYP/def-SVP level of theory provided the best agreement with the experiment. This is most probably due to the distortion observed at the pyrazolyl ring leading to the appearance of asymmetric vibrational modes. The above study highlights the possibility to appropriately tune the excitation wavelengths as well as alter the photostability of the organic chromophores via additional N-insertion in the molecular systems. [ABSTRACT FROM AUTHOR]

We present experimental determination of the energies associated with the gradual desorption of Ar atoms from the aromatic molecular surface. Non-covalently bound 2,2′-pyridylbenzimidazole-Ar1–3 complexes were produced in the gas phase and characterized using resonant two-photon ionization (R2PI) spectroscopy. The single Ar desorption from the PBI–Ar, PBI–Ar2 and PBI–Ar3 complexes were measured as 581 ± 18, 656 ± 30 and 537 ± 31 cm−1, respectively. The energies were bracketed between the last observed band in the respective R2PI spectra and the disappeared intramolecular modes of PBI. The Arn dissociation energies in the S1 state were measured as 581 ± 18, 1237 ± 48 and 1774 ± 79 cm−1, respectively, for n = 1, 2 and 3. The calculated dissociation energies of the respective complexes, obtained using three computational methods, show excellent agreement with the experimental data. The ground state dissociation energies were estimated by subtracting the Δν shift of the origin band, and the respective values are 541 ± 18, 1160 ± 48 and 1634 ± 79 cm−1. Overall, the calculated values resulted in scaling factors ranging from 0.956 to 1.017, which depict the predictive power of the methods to determine dispersion energies. The current investigation describes a unique methodology to accurately determine the dissociation and desorption energies of Ar atoms from the surfaces of bio-relevant aromatic molecules. [ABSTRACT FROM AUTHOR]

Using the stimulated-emission-pumping/resonant 2-photon ionization (SEP-R2PI) method, we have determined accurate intermolecular dissociation energies D0 of supersonic jet-cooled intermolecular complexes of 1-naphthol (1NpOH) with alkanes, 1NpOH·S, with S = methane, ethane, propane, and n-butane. Experimentally, the smaller alkanes form a single minimum-energy structure, while 1-naphthol·n-butane forms three different isomers. The ground-state dissociation energies D0(S0) for the complexes with propane and n-butane (isomers A and B) were bracketed within ±0.5%, being 16.71 ± 0.08 kJ/mol for S = propane and 20.5 ± 0.1 kJ/mol for isomer A and 20.2 ± 0.1 kJ/mol for isomer B of n-butane. All 1NpOH·S complexes measured previously exhibit a clear dissociation threshold in their hot-band detected SEP-R2PI spectra, but weak SEP-R2PI bands are observed above the putative dissociation onset for the methane and ethane complexes. We attribute these bands to long-lived complexes that retain energy in rotation-type intermolecular vibrations, which couple only weakly to the dissociation coordinates. Accounting for this, we find dissociation energies of D0(S0) = 7.98 ± 0.55 kJ/mol (±7%) for S = methane and 14.5 ± 0.28 kJ/mol (±2%) for S = ethane. The D0 values increase by only 1% upon S0 → S1 excitation of 1-naphthol. The dispersion-corrected density functional theory methods B97-D3, B3LYP-D3, and ωB97X-D predict that the n-alkanes bind dispersively to the naphthalene “Face.” The assignment of the complexes to Face structures is supported by the small spectral shifts of the S0 → S1 electronic origins, which range from +0.5 to −15 cm−1. Agreement with the calculated dissociation energies D0(S0) is quite uneven, the B97-D3 values agree within 5% for propane and n-butane, but differ by up to 20% for methane and ethane. The ωB97X-D method shows good agreement for methane and ethane but overestimates the D0(S0) values for the larger n-alkanes by up to 20%. The agreement of the B3LYP-D3 D0 values is intermediate between the other two methods. [ABSTRACT FROM AUTHOR]

The 1-naphthol·cyclopropane intermolecular complex is formed in a supersonic jet and investigated by resonant two-photon ionization (R2PI) spectroscopy, UV holeburning, and stimulated emission pumping (SEP)-R2PI spectroscopy. Two very different structure types are inferred from the vibronic spectra and calculations. In the "edge" isomer, the OH group of 1-naphthol is directed towards a C--C bond of cyclopropane, the two ring planes are perpendicular. In the "face" isomer, the cyclopropane is adsorbed on one of the π-aromatic faces of the 1-naphthol moiety, the ring planes are nearly parallel. Accurate ground-state intermolecular dissociation energies D0 were measured with the SEP-R2PI technique. The D0(S0) of the edge isomer is bracketed as 15.35 « 0.03 kJ/mol, while that of the face isomer is 16.96 « 0.12 kJ/mol. The corresponding excited-state dissociation energies D0(S1) were evaluated using the respective electronic spectral shifts. Despite the D0(S0) difference of 1.6 kJ/mol, both isomers are observed in the jet in similar concentrations, so they must be separated by substantial potential energy barriers. Intermolecular binding energies, De, and dissociation energies, D0, calculated with correlated wave function methods and two dispersion-corrected density-functional methods are evaluated in the context of these results. The density functional calculations suggest that the face isomer is bound solely by dispersion interactions. Binding of the edge isomer is also dominated by dispersion, which makes up two thirds of the total binding energy. [ABSTRACT FROM AUTHOR]

The 1³Σu- ← X³Σg- transition of linear HC5H (A) has been observed in a neon matrix and gas phase. The assignment is based on mass-selective experiments, extrapolation of previous results of the longer H C 2 B + 1 H homologues, and density functional and multi-state CASPT2 theoretical methods. Another band system starting at 303 nm in neon is assigned as the 1¹A1 ← X¹A1 transition of the cumulene carbene pentatetraenylidene H2C5 (B). [ABSTRACT FROM AUTHOR]