A tunable ultrafast spectrometer based on hollow waveguides: Demonstration in violet–blue

′ -hydroxy-5^′-methylphenyl)-benzotriazole. Received: 2 June 1998/Revised version: 31 July 1998     

The O-H stretching mode of a prototypical photoacid as a local dielectric probe

We investigate the OH stretch vibrational frequency shifts of a prototype photoacid, 2-naphthol (2N), when dissolved in solvents of low polarity. We combine femtosecond mid-infrared spectroscopy and a theoretical model based on the Pullin-van der Zwan-Hynes perturbative approach to explore vibrational solvatochromic effects in the ground S(0) and the first electronically excited (1)L(b) states. The model is parametrized using density functional theory (DFT), at the B3LYP/TZVP and TD-B3LYP/TZVP levels for the 2N chromophore in the S(0) and (1)L(b) states, respectively. From the agreement between experiment and theory we conclude that vibrational solvatochromic effects are dominated by the instantaneous dielectric response of the solvent, while time-dependent nuclear rearrangements are of secondary importance.     

New insight into the mechanisms of reactions between some anionic nucleophiles and phenyl acetate in the gas phase

Experiments carried out in an ion cyclotron resonance (ICR) drif cell, an ICR trapped ion cell and a flowing afterglow (FA) system show that in the gas phase phenyl acetate reacts with a variety of clustered and unclustered nucleophiles to yield mainly the product ion C₆H₅O^?.     

Artifact formation due to ethyl thio-incorporation into silylated steroid structures as determined in doping analysis

Trimethylsilylation of target substances in a mixture of N-methyl-N-trimethylsilyltrifluoroacetamide (MSTFA), ammonium iodide and ethanethiol is frequently applied for the application of gas chromatography-mass spectrometry (GC-MS) in steroid analysis. However, artifacts were formed when using this mixture to silylate the steroids androsterone and etiocholanolone obtained from a urine matrix. The artifacts were identified as ethyl thio-containing products of the respective trimethylsilyl derivatives. The conversion of the studied products increased slowly as a function of time, was dependent on the presence of the urine matrix and was significantly accelerated by adding diethyl disulfide to the reagent before incubation. Also ethyl thio-incorporation into testosterone and epitestosterone was established. A mechanism for ethyl thio-incorporation is proposed. The conversion achieved after 120-h sample storage at room temperature was insufficient to significantly influence the analysis of androsterone and etiocholanolone under the studied conditions. However, the results provide fundamental insight into the mechanism of silylation and the occurring side-reactions. Moreover, when investigating the formation of new metabolites, the ethyl thio-incorporation can lead to misinterpretation.     

A computational study of the double hydrogen migration in the molecular ions of endo tricyclo[5.2.1.0(2,6)]decene

Previous field ionization kinetic experiments have supported strongly that the double hydrogen migration prior to the elimination of C5H8 from the molecular ions of the endo isomers of 8,9-disubstituted tricyclo[5.2.1.0(2,6)]decenes, which is not observed for the exo isomers, proceeds in a concerted, i.e., dyotropic way. This paper describes the results of ab initio calculations at the ROHF/4-31G and DFT level of theory, performed on the double hydrogen migration for the ionized endo isomer of unsubstituted tricyclo[5.2.1.0(2,6)]decene. An intrinsic reaction coordinate calculation at the DFT level has shown indeed a direct connection between the structures of the molecular ions of the endo isomer before and after the double hydrogen migration, thus corroborating the earlier suggested concerted, i.e., dyotropic pathway of this double hydrogen rearrangement.     

Gas-phase deprotonation of arylalkylamines. A collision-induced dissociation study

The collision-induced dissociation (CID) of deprotonated arylalkylamines of general formula R(1)C(6)H(4)CHR(2)CH(2)NR(3)(2) (where R(1) = H, OH, F or NO(2); R(2) = H or OH; R(3) = H or CH(3)) generated by negative chemical ionization with H(2)O and D(2)O as ionizing reagents, is discussed. The negative chemical ionization mass spectra show that, in the absence of a hydroxy group in the aromatic ring, deprotonation takes place at the benzylic position whereas the proton is lost from the OH group when present. The nitro compound forms only M(-.) ions. The CID spectra of the deprotonated molecules show that fragmentations are strongly dependent on the structural features of the molecules, namely the presence or absence of substituents in the aromatic ring or aliphatic chain. Copyright 1999 John Wiley & Sons, Ltd.     

On the stabilization of carbanions by adjacent phenyl,cyano, methoxy-carbonyl,and nitro groups in the gas phase

Institute of Mass Spectrometry, University of Amsterdam, Amsterdam, The Netherlands By using the method of Fourier transform ion cyclotron resonance mass spectrometry, substituent stabilization energies of homologous series of cycloalkyl carbanions, Ξ-c-C_nH_2n?2 (n = 3, 4, 5, 6, 7) with π-accepting substituents (Ξ = Ph, CN, COOMe, NO₂) have been determined experimentally in the gas phase as the difference between the proton affinity of the substituted and corresponding unsubstituted (Ξ = H) cycloalkyl carbanions. The stabilization energy data have been analyzed in terms of Taft’s parametrization of polarizability, field/inductive, and resonance effects. The linear regression analyses show excellent correlations within the ΞCH₂ ^? Ξ-c-C_nH_2n?2 ^? (n = 4, 5, 6, 7), and Ξ-c-C₃H₄ ^? carbanion series, from which it appears that the contributions of polarizability effects are independent of the above type of carbanions and only depend on the nature of the substituent. Further, it follows that inductive stabilization is more effective in the substituted methyl, ΞCH₂ ^?, than in the substituted cycloalkyl, Ξ-c-C_nH_2n?2 ^? (n = 4, 5, 6, 7) carbanions. This result suggests that inductive stabilization is counteracted by the electron releasing effect of alkyl groups. Resonance stabilization is significantly more effective in the substituted cycloalkyl, Ξ-c-C_nH_2n?2 ^? (n = 4, 5, 6, 7), than in the substituted methyl, ΞCH₂ ^?, carbanions, which suggests that m contrast to inductive stabilization, resonance stabilization is assisted by the electron releasing effect of alkyl groups. Finally, it appears that substitutent stabilization in the geometrically restricted substituted cyclopropyl carbanions, Ξ-c-C₃H₄ ^?, is dramatically less effective than in the corresponding geometrically unrestricted larger substituted cycloalkyl carbanions, Ξ-c-C_nH_2n?2 ^? (n = 4, 5, 6, 7). The linear regression analyses of the substituted cycloalkyl carbanions indicate that reduction of the stabilization energy is caused not exclusively by a geometrically hindered resonance stabilization, but also to a smaller extent by a less efficient inductive stabilization in the substituted cyclopropyl carbanions.     

Femtosecond ultraviolet pulses generated using noncollinear optical parametric amplification and sum frequency mixing

We demonstrate the generation of ultraviolet 33-fs pulses with a shot-to-shot energy fluctuation of less than 3% using sum frequency mixing of visible pulses of a noncollinear optical parametric amplifier with sub-40-fs pulses of a 1-kHz Ti:sapphire-amplified system. The pulses are transform-limited (ΔνΔτ=0.36) and tunable in the range from 315 nm to 355 nm with energy above 1 μJ (2.6 μJ at 330 nm). Received: 21 July 2000 / Published online: 8 November 2000     

Polyhedral shaped gold nanoparticles with outstanding near-infrared light absorption

A time-resolved X-ray absorption study of the structural dynamics of liquid water on a picosecond timescale is presented. We apply femtosecond midinfrared pulses to resonantly excite the intramolecular O–H stretching band of liquid water and monitor the transient response in the oxygen K-edge absorption spectrum with picosecond X-ray pulses. In this way, structural changes in the hydrogen bond network of liquid water upon an ultrafast temperature jump of approximately 20 K are investigated. The changes of the X-ray absorption as induced by such a temperature jump are about 3.2%. This demonstrates that our method serves as a sensitive probe of transient structural changes in liquid water and that combined infrared-laser–synchrotron experiments with substantially shorter X-ray pulses, such as generated with a femtosecond slicing scheme, are possible.