Matin Kaufmann

IR-spectroscopic study of the allyl + NO reaction in helium nanodroplets

The IR-spectrum of the allyl-NO adduct (CH2-CH-CH2-NO) in helium nanodroplets has been recorded in the frequency region 2850-3120 cm(-1). CH2-CH-CH2-NO has been investigated as a prototype of the product of radical-radical reactions at 0.37 K. The product of the reaction, 3-nitroso-1-propene, was formed via the reaction of allyl and NO within the helium droplets. For an assignment we have predicted the conformers of the CH2-CH-CH2-NO using density functional theory (DFT) with a BLYP functional and a TZVPP basis set. By comparison with the experimental spectrum we can show that all three conformers are stabilized in superfluid helium nanodroplets.

From the tunneling dimer to the onset of microsolvation: Infrared spectroscopy of allyl radical water aggregates in helium nanodroplets

The infrared spectrum of allyl:water clusters embedded in helium nanodroplets was recorded. Allyl radicals were produced by flash vacuum pyrolysis and trapped in helium droplets. Deuterated water was added to the doped droplets, and the infrared spectrum of the radical water aggregates was recorded in the frequency range 2570-2820 cm-1. Several absorption bands are observed and assigned to 1:1 and 1:2 allyl:D2O clusters, based on pressure dependent measurements and accompanying quantum chemical calculations. The analysis of the 1:1 cluster spectrum revealed a tunneling splitting as well as a combination band. For the 1:2 cluster, we observe a water dimer-like motif that is bound by one π-hydrogen bond to the allyl radical.

Helium droplet infrared spectroscopy of glycine and glycine–water aggregates

Infrared absorption spectra of glycine and glycine-water aggregates embedded in superfluid helium nanodroplets were recorded in the frequency range 1000-1450 cm-1. For glycine monomer, absorption bands were observed at 1106 cm-1, 1134 cm-1, and 1389 cm-1. These bands were assigned to the C-OH stretch mode of the glycine conformers I, III and II, respectively. For glycine-water aggregates, we observed two bands at 1209 cm-1 and 1410 cm-1 which we assign to distinct conformers of glycine-H2O. In all cases, the water is found to preferentially bind to the carboxyl group of the glycine.

Two-center three-electron bonding in ClNH3 revealed via helium droplet infrared laser Stark spectroscopy: Entrance channel complex along the Cl + NH3 → ClNH2 + H reaction

Pyrolytic dissociation of Cl2 is employed to dope helium droplets with single Cl atoms. Sequential addition of NH3 to Cl-doped droplets leads to the formation of a complex residing in the entry valley to the substitution reaction Cl + NH3 → ClNH2 + H. Infrared Stark spectroscopy in the NH stretching region reveals symmetric and antisymmetric vibrations of a C3v symmetric top. Frequency shifts from NH3 and dipole moment measurements are consistent with a ClNH3 complex containing a relatively strong two-center three-electron (2c-3e) bond. The nature of the 2c-3e bonding in ClNH3 is explored computationally and found to be consistent with the complexation-induced blue shifts observed experimentally. Computations of interconversion pathways reveal nearly barrierless routes to the formation of this complex, consistent with the absence in experimental spectra of two other complexes, NH3Cl and Cl-HNH2, which are predicted in the entry valley to the hydrogen abstraction reaction Cl + NH3 → HCl + NH2.

Infrared Spectroscopy of the Tropyl Radical in Helium Droplets

The infrared spectrum of the X2E2″ tropyl radical has been recorded in the range of the CH-stretch vibrational modes using the helium droplet isolation technique. Two bands are observed at 3053 and 3058 cm–1. The electronic degeneracy of the ground state results in a Jahn–Teller interaction for two of the CH-stretch modes, i.e., first-order interaction for E3′ symmetry modes and second-order interaction for E2′ symmetry modes. The experimentally observed bands are assigned to the E1′ and E3′ CH-stretch modes. The E1′ mode is infrared-active, whereas the E3′ mode is inactive in the absence of the Jahn–Teller interaction. The transition to the upper component of the Jahn–Teller split E3′ mode gains intensity via vibronic coupling, giving rise to the second experimentally observed band.

Understanding the microsolvation of radicals: Infrared spectroscopy of benzyl radical water clusters

The IR spectrum of benzyl radical:water clusters was recorded. Benzyl radicals were produced by vacuum flash pyrolysis and trapped in superfluid helium nanodroplets. The infrared spectrum of benzyl radical water aggregates in the range 2585-2820 cm-1 was recorded by subsequent addition of deuterated water. A total of seven peaks are observed at 2766, 2750, 2656, 2638, 2633, 2598, and 2593 cm-1. Based on pressure dependent measurements and comparison to accompanying ab initio calculations, five of these bands are assigned to distinct O-D stretch vibrations of benzyl:water clusters with one and two water molecules. In line with previous experiments on benzene:water clusters, we observe the formation of a water dimer-like motif that is attached to one face of the benzyl radical.