Excited state tautomerization of 7-azaindole in a 1:1 complex with δ-valerolactam: a comparative study with the homodimer.

A comparative analysis for relative stability between normal and tautomeric forms in the excited electronic states of 7-azaindole···Î´-valerolactam 1:1 complex and 7-azaindole homodimer has been presented. The tautomeric configuration of the complex is estimated to be ~6 kcal/mol more stable than normal form, and the same for homodimer appears to be ~10 kcal/mol. Consistent with these estimates both the complex and homodimer undergo facile double proton transfer tautomerization upon UV excitation in hydrocarbon solutions (Chou; et al. J. Am. Chem. Soc. 1995, 117, 7259). However, we notice that such similarity in photophysical behavior of the two hydrogen-bonded systems is lost completely in a cold supersonic jet expansion. The jet-cooled homodimer emits only the tautomer fluorescence in the visible spectral region, but the complex emits exclusively from the locally excited state in ultraviolet. We have interpreted this contrast by arguing that the effective barrier for excited state double proton exchange tautomerization of the complex is larger compared to that of the homodimer, and the difference originates because of asymmetric nature of the two hydrogen bonds of the complex.

UV photolysis of α-cyclohexanedione in the gas phase.

Ultraviolet absorption spectrum of α-cyclohexanedione (α-CHD) vapor in the wavelength range of 220-320 nm has been recorded in a 1 m long path gas cell at room temperature. With the aid of theoretical calculation, the band has been assigned to the S(2) ← S(0) transition of largely ππ* type. The absorption cross section at the band maximum (∼258 nm) is nearly 3 orders of magnitude larger compared to that for the S(2) ← S(0) transition of a linear α-diketo prototype, 2,3-pentanedione. The photolysis was performed by exciting the sample vapor near this band maximum, using the 253.7 nm line of a mercury vapor lamp, and the products were analyzed by mass spectrometry as well as by infrared spectroscopy. The identified products are cyclopentanone, carbon monoxide, ketene, ethylene, and 4-pentenal. Geometry optimization at the CIS/6-311++G** level predicts that the carbonyl group is pyramidally distorted in the excited S(1) and S(2) states, but the α-CHD ring does not show dissociative character. Potential energy curves with respect to a ring rupture coordinate (C-C bond between two carbonyl groups) for S(0), S(1), S(2), T(1), T(2), and T(3) states have been generated by partially optimizing the ground state geometry at DFT/B3LYP/6-311++G** level and calculating the vertical transition energies to the excited states by TDDFT method. Our analysis reveals that the reactions can take place at higher vibrational levels of S(0) as well as T(1) states.

Inhibition of light-induced tautomerization of 7-azaindole by phenol: indications of proton-coupled electron/energy transfer quenching.

The photophysical behavior of a 1:1 complex between phenol and 7-azaindole (7AI) has been investigated in methylcyclohexane solutions at temperatures in the range of 27 to -50 °C. A linear Benesi-Hildebrand plot associated with changes in absorbance of the complex with phenol concentration in the solutions ensures 1:1 stoichiometry of the produced complex. Our estimate for the value of the association constant (K(a)) of the complex is ~120 M⁻¹ at 27 °C, and it is nearly twice compared to that for 1:1 complex between 7AI and ethanol measured under the same condition. The complexation results in dramatic quenching of the normal fluorescence of 7AI and the process is accelerated upon lowering of temperature. The measured spectra show no indication that phenol promotes tautomerization of 7AI in the excited state. We have argued that the hydrogen bonding between pyridinic N and phenolic O-H (N···O-H) is a vital structural factor responsible for quenching of 7AI fluorescence, and this idea has been corroborated by showing that under same condition the fluorescence of 7AI is enhanced in the presence of anisole. As a plausible mechanism of quenching, we have invoked a proton-coupled electron transfer (PCET) process between phenol and excited 7AI, which outweighs the competing tautomerization process. An analysis in terms of Remm-Weller model reveals that the PCET process involving phenol and excited 7AI could be energetically favorable (ΔG(ET)(0) < 0). An alternative mechanism, where quenching can occur via electronic energy transfer from the excited protonated 7AI to phenoxide ion, following a proton transfer along the N···O-H hydrogen bond, is also discussed.

Blue shifting C-H...O hydrogen bonded complexes between chloroform and small cyclic ketones: ring-size effects on stability and spectral shifts.

Blue-shifting C-H...O hydrogen bonded complexes between chloroform and three small cyclic ketones (cyclohexanone, cyclopentanone, and cyclobutanone) have been identified by use of FTIR spectroscopy in CCl(4) solution at room temperature. The shifts of the C-H stretching fundamental of chloroform (nu(C-H)) in the said three complexes are +1, +2, and +5 cm(-1), respectively, and the complexation results in enhancement of the nu(C-H) transition intensity in all three cases. The 1:1 stoichiometry of the complexes is suggested by identifying distinct isosbestic points between the carbonyl stretching (nu(C=O)) fundamentals of the monomers and corresponding complexes for spectra measured with different chloroform to ketone concentrations. The nu(C=O) bands in the three complexes are red-shifted by 8, 19, and 6 cm(-1), and apparently have no correlation with the respective blue shifts of the nu(C-H) bands. Spectral analysis reveals that the complex with cyclohexanone is most stable, and the stability decreases with the ring size of the cyclic ketones. A qualitative explanation of the relative stabilities of the complexes is presented by correlating the hydrogen bond acceptor abilities of the carbonyl groups with the ring size of the cyclic ketones. Quantum mechanical calculations at the DFT/B3LYP/6-311++G(d,p) and MP2/6-31+G(d) levels were performed for predictions of the shapes of the complexes, electronic structure parameters of C-H (donor) and C=O (acceptor) groups, intermolecular interaction energies, spectral shifts, and evolution of those properties when the hydrogen bond distance between the donor-acceptor moieties is scanned. The results show that the binding energies of the complexes are correlated with the dipole moments, proton affinity, and n(O) --> sigma*(C-H) hyperconjugative charge transfer abilities of the three ketones. NBO analysis reveals that the blue shifting of the nu(C-H) transition in a complex is the net effect of hyperconjugation and repolarization/rehybridization of the bond under the influence of the electric field of carbonyl oxygen.