Search

Stark spectral shifts of a molecular probe are commonly used to estimate the local electric field in condensed media. The very large fields reported, typically in the 0.1-10 GV m -1 range, are, however, difficult to reproduce in a controlled manner, limiting the calibration of these molecular probes to ranges below 0.1 GV m -1 . In this context, we investigated gas-phase, isolated, molecular ion pairs, where a phenyl ring is immersed in the electric field produced by the nearby ionic groups. The intensity of the electric field is chemically tuned in the 1 GV m -1 range by changing the nature of the cations, and the phenyl ring response is monitored by UV spectroscopy. A quadratic Stark effect is observed, demonstrating the possibility to characterize molecular probes in a solvent-free environment and in the very large field range they typically meet in condensed media such as biological environments.

Chirped-pulse Fourier transform microwave spectroscopy (CP-FTMW) is combined with a flash pyrolysis (hyperthermal) microreactor as a novel method to investigate the molecular structure of cyclopentadienone (C5H4═O), a key reactive intermediate in biomass decomposition and aromatic oxidation. Samples of C5H4═O were generated cleanly from the pyrolysis of o-phenylene sulfite and cooled in a supersonic expansion. The (13)C isotopic species were observed in natural abundance in both C5H4═O and in C5D4═O samples, allowing precise measurement of the heavy atom positions in C5H4═O. The eight isotopomers include: C5H4═O, C5D4═O, and the singly (13)C isotopomers with (13)C substitution at the C1, C2, and C3 positions. Microwave spectra were interpreted by CCSD(T) ab initio electronic structure calculations and an re molecular structure for C5H4═O was found. Comparisons of the structure of this "anti-aromatic" molecule are made with those of comparable organic molecules, and it is concluded that the disfavoring of the "anti-aromatic" zwitterionic resonance structure is consistent with a more pronounced C═C/C-C bond alternation.