Ethyl + O2 in Helium Nanodroplets: Infrared Spectroscopy of the Ethylperoxy Radical.

Helium-solvated ethylperoxy radicals (CH3CH2OO•) are formed via the in situ reaction between 2A' ethyl radical and 3Σg- dioxygen. The reactants are captured sequentially through the droplet pick-up technique. Helium droplets are doped with ethyl radical via pyrolysis of di- tert-amyl peroxide or n-propylnitrite in an effusive, low-pressure source. An infrared spectrum of ethylperoxy, in the CH stretching region, is recorded with species-selective droplet beam depletion spectroscopy. Spectral assignments are made via comparisons to second-order vibrational perturbation theory with resonances (VPT2 + K) based on coupled-cluster full quartic force fields. Cubic and quartic force constants, evaluated using a small basis set, are transformed into the normal coordinate system of the higher level quadratic force constants. This transformation procedure eliminates the mismatch between normal modes, which is a source of error whenever normal coordinate force constants from different levels of theory are combined. The spectrum shows signatures of both the C1 gauche and C s trans rotamers in an approximate 2:1 ratio; this is despite the prediction that the gauche rotamer lies 44 cm-1 lower on the zero-Kelvin enthalpic potential surface for torsional interconversion. Helium droplets are 0.4 K at equilibrium; therefore, in situ ethylperoxy production is highly nonthermal.

Infrared laser spectroscopy of the n-propyl and i-propyl radicals: Stretch-bend Fermi coupling in the alkyl CH stretch region.

The n-propyl and i-propyl radicals were generated in the gas phase via pyrolysis of n-butyl nitrite [CH3(CH2)3ONO] and i-butyl nitrite [(CH3)2CHCH2ONO], respectively. Nascent radicals were promptly solvated by a beam of He nanodroplets, and the infrared spectra of the radicals were recorded in the CH stretching region. Several previously unreported bands are observed between 2800 and 3150 cm-1. The CH stretching modes observed above 3000 cm-1 are in excellent agreement with CCSD(T) anharmonic frequencies computed using second-order vibrational perturbation theory. However, between 2800 and 3000 cm-1, the spectra of n- and i-propyl radicals become congested and difficult to assign due to the presence of multiple anharmonic resonance polyads. To model the spectrally congested region, Fermi and Darling-Dennison resonances are treated explicitly using "dressed" Hamiltonians and CCSD(T) quartic force fields in the normal mode representation, and the agreement with experiment is less than satisfactory. Computations employing local mode effective Hamiltonians reveal the origin of the spectral congestion to be strong coupling between the high frequency CH stretching modes and the lower frequency CHn bending/scissoring motions. The most significant coupling is between stretches and bends localized on the same CH2/CH3 group. Spectral simulations using the local mode approach are in excellent agreement with experiment.

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 Laser Spectroscopy of the L-Shaped Cl-HCl Complex Formed in Superfluid (4)He Nanodroplets.

Chlorine atoms, generated through the thermal decomposition of Cl2, are solvated in superfluid helium nanodroplets and clustered with HCl molecules. The H-Cl stretching modes of these clusters are probed via infrared laser spectroscopy. A band centered at ∼2880.8 cm(-1) is assigned to the binary Cl-HCl complex on the basis of HCl pressure dependence and difference mass spectra. The band lies in the "free" HCl stretching region, implying that the complex is not hydrogen bound. Furthermore, the breadth of the band (∼2 cm(-1) fwhm) is consistent with an assignment to a predominantly b-type component of the H-Cl stretch, as the dominant b-type selection rules and A rotational constant allow for high energy rotational excitations that efficiently couple to droplet excitations, resulting in fast rotational deactivation. Despite the lack of rotational fine structure, which would verify the assignment, the observed band is consistent with the stabilization of a weakly bound complex having an approximately L-shaped geometry. Frequency computations for a rigid, L-shaped complex reveal that the transition dipole moment vector points almost entirely along the b inertial axis; indeed, the signal-to-noise ratio in our experiment precluded the observation of an a-type component of the HCl stretching band for the complex. No bands were observed that could be assigned to a linear H-bonded Cl-HCl complex. Additionally, we located bands that are consistent with the formation of Cl2-HCl, Cl2-(HCl)2, and Cl-(HCl)2. Two vibrations of the Cl-(HCl)2 complex were found, and harmonic frequencies and intensities computed for a cyclic structure are consistent with the observations.

Reactive intermediates in (4)He nanodroplets: infrared laser Stark spectroscopy of dihydroxycarbene.

Singlet dihydroxycarbene (HOC̈OH) is produced via pyrolytic decomposition of oxalic acid, captured by helium nanodroplets, and probed with infrared laser Stark spectroscopy. Rovibrational bands in the OH stretch region are assigned to either trans,trans- or trans,cis-rotamers on the basis of symmetry type, nuclear spin statistical weights, and comparisons to electronic structure theory calculations. Stark spectroscopy provides the inertial components of the permanent electric dipole moments for these rotamers. The dipole components for trans, trans- and trans, cis-rotamers are (µa, µb) = (0.00, 0.68(6)) and (1.63(3), 1.50(5)), respectively. The infrared spectra lack evidence for the higher energy cis,cis-rotamer, which is consistent with a previously proposed pyrolytic decomposition mechanism of oxalic acid and computations of HOC̈OH torsional interconversion and tautomerization barriers.

Communication: Helium nanodroplet isolation and rovibrational spectroscopy of hydroxymethylene.

Hydroxymethylene (HCOH) and its d1-isotopologue (HCOD) are isolated in low temperature helium nanodroplets following pyrolysis of glyoxylic acid. Transitions identified in the infrared spectrum are assigned exclusively to the trans-conformation based on previously reported anharmonic frequency computations [P. R. Schreiner, H. P. Reisenauer, F. C. Pickard, A. C. Simmonett, W. D. Allen, E. Mátyus, and A. G. Császár, Nature 453, 906 (2008); L. Koziol, Y. M. Wang, B. J. Braams, J. M. Bowman, and A. I. Krylov, J. Chem. Phys. 128, 204310 (2008)]. For the OH(D) and CH stretches, a- and b-type transitions are observed, and when taken in conjunction with CCSD(T)/cc-pVTZ computations, lower limits to the vibrational band origins are determined. The relative intensities of the a- and b-type transitions provide the orientation of the transition dipole moment in the inertial frame. The He nanodroplet data are in excellent agreement with anharmonic frequency computations reported here and elsewhere, confirming an appreciable Ar-matrix shift of the OH and OD stretches and strong anharmonic resonance interactions in the high-frequency stretch regions of the mid-infrared.

Infrared laser spectroscopy of the helium-solvated allyl and allyl peroxy radicals.

Infrared spectra in the C-H stretch region are reported for the allyl (CH2CHCH2) and allyl peroxy (CH2=CH-CH2OO·) radicals solvated in superfluid helium nanodroplets. Nine bands in the spectrum of the allyl radical have resolved rotational substructure. We have assigned three of these to the ν1 (a1), ν3 (a1), and ν13 (b2) C-H stretch bands and four others to the ν14/(ν15+2ν11) (b2) and ν2/(ν4+2ν11) (a1) Fermi dyads, and an unassigned resonant polyad is observed in the vicinity of the ν1 band. Experimental coupling constants associated with Fermi dyads are consistent with quartic force constants obtained from density functional theory computations. The peroxy radical was formed within the He droplet via the reaction between allyl and O2 following the sequential pick-up of the reactants. Five stable conformers are predicted for the allyl peroxy radical, and a computed two-dimensional potential surface for rotation about the CC-OO and CC-CO bonds reveals multiple isomerization barriers greater than ≈300 cm(-1). Nevertheless, the C-H stretch infrared spectrum is consistent with the presence of a single conformer following the allyl + O2 reaction within helium droplets.

Propargyl + O2 reaction in helium droplets: entrance channel barrier or not?

A combination of liquid He droplet experiments and multireference electronic structure calculations is used to probe the potential energy surface for the reaction between the propargyl radical and O2. Infrared laser spectroscopy is used to probe the outcome of the low temperature, liquid He-mediated reaction. Bands in the spectrum are assigned to the acetylenic CH stretch (ν1), the symmetric CH2 stretch (ν2), and the antisymmetric CH2 stretch (ν13) of the trans-acetylenic propargyl peroxy radical ((•)OO-CH2-C≡CH). The observed band origins are in excellent agreement with previously reported anharmonic frequency computations for this species [Jochnowitz, E. B.; Zhang, X.; Nimlos, M. R.; Flowers, B. A.; Stanton, J. F.; Ellison, G. B. J. Phys. Chem. A 2010, 114, 1498]. The Stark spectrum of the ν1 band provides further evidence that the reaction leads only to the trans-acetylenic species. There are no other bands in the CH2 stretching region that can be attributed to any of the other three propargyl peroxy isomers/conformers that are predicted to be minimum energy structures (gauche-acetylenic, cis-allenic, and trans-allenic). There is also no evidence for the kinetic stabilization of a van der Waals complex between propargyl and O2. A combination of multireference and coupled-cluster electronic structure calculations is used to probe the potential energy surface in the neighborhood of the transition state connecting reactants with the acetylenic adduct. The multireference based evaluation of the doublet-quartet splitting added to the coupled-cluster calculated quartet state energies yields what are likely the most accurate predictions for the doublet potential curve. This calculation suggests that there is no saddle point for the addition process, in agreement with the experimental observations. Other calculations suggest the possible presence of a small submerged barrier.

Helium nanodroplet isolation and infrared spectroscopy of the isolated ion-pair 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide.

The ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide was vaporized at 420 K, and the ion-pair constituents were entrained in a beam of liquid He nanodroplets and cooled to 0.4 K. The vapor pressure was optimized such that each He droplet picked up a single ion-pair from the gas phase. Infrared spectroscopy in the CH stretch region reveals bands that are assigned to intact ion-pairs on the basis of comparisons to ab initio harmonic frequency computations of 23 low energy isomers. The He droplet spectrum is consistent with a weighted sum of the computed harmonic spectra, in which the weights are determined from ab initio computations of the relative free energies at 420 K. Anharmonic resonance polyads in the CH stretch region are treated explicitly, which improves the agreement between the experiment and computed spectra for ion-pairs. For isomers having a strong cation···anion hydrogen bonding interaction, the imidazolium C(2)-H stretch fundamental is shifted to lower energy and into resonance with the overtones and combination bands of the imidazolium ring stretching modes, resulting in a spectral complexity in the CH stretch region that is fully resolved in the He droplet spectrum. The assignment of the infrared spectrum to ion-pairs is confirmed through polarization spectroscopy measurements that reveal the permanent electric dipole moment of the He-solvated species to be 11 ± 2 D. The computed permanent electric dipole moments for the low energy isomers of the [emim(+)][Tf2N(-)] ion-pairs fall in the range 9-13 D, whereas the computed dipole moments of decomposition products of the ionic liquid are less than 4.3 D.