Optical Spectroscopy of Matrix Isolated Organic Radicals

Most matrix-isolated radicals are generally prepared by photodissociation of an appropriate precursor (usually by a laser). However we produce target radicals, such as propargyl (HCCCH₂ ˜X ²B₁), by thermally dissociation of a suitable precursor molecule in a hyperthermal nozzle (see Figure 1), which expands through a supersonic jet. [Rev. Sci. Instrum. 74, 3077-3086 (2003)]. Pyrolysis of propargyl bromide affords intense beams of propargyl:
HC≡CCH₂Br + Δ(1000 K)→ HCCCH₂ + Br

Production of radicals in a hyperthermal nozzle

Figure 1: Production of radicals by thermal dissociation of molecules in a hyperthermal nozzle

To optimize radical production, the dosing nozzle is interfaced with a photoionization mass spectrometer (PIMS). The skimmed output of the nozzle is crossed with 118.2 nm (10.487 eV) light from the 9^th harmonic of a YAG laser. Molecules with an ionization potential less than 10.5 eV are ionized and analyzed by a reflectron time-of-flight spectrometer. Mass spectra resulting from each precursor are measured as a function of pyrolysis temperature.

We have deployed this device to study a number of fundamental organic radicals: C₆H₅ [phenyl radical, J. Am. Chem. Soc., 123, 1977-1988 (2001)], CH₂CHCH₂ [allyl radical, J. Phys. Chem. A, 105, 7514-7524 (2001)], CH₃OO [methylperoxyl, J. Phys. Chem. A, 106, 7547-7556 (2002)], and HCCCH₂ [propargyl radical, J. Phys. Chem. A. in press (2005)]. The propargyl radical, HCCCH₂, is a nice example of the power of these techniques.

The propargyl radical has twelve fundamental vibrational modes, Γ_vib(HCCCH₂) = 5a₁ + 3b₁ + 4b₁, and nine have been detected in a cryogenic matrix. Ab initio coupled-cluster anharmonic force field calculations were used to help guide some of the assignments. The experimental HCCCH₂ matrix frequencies (cm^-1) and polarizations are:
  a₁ modes — 3308.5 ± 0.5, 3028.3 ± 0.6, 1935.4 ± 0.4, 1440.4 ± 0.5, 1061.6 ± 0.8;
  b₁ modes — 686.6 ± 0.4, 483.6 ± 0.5;
  b₂ modes — 1016.7 ± 0.4, 620 ± 2.
We recommend a complete set of gas-phase vibrational frequencies for the propargyl radical, HCCCH₂ ˜X ²B₁. The infrared absorption spectrum in the fingerprint region is shown bellow in Figure 2.

Figure 2: Fringerprint region of infrared absorption spectrum of propargyl radical

An IR depletion spectrum and the linear dichroism (LD) spectrum for propargyl are shown in Figure 3. A matrix of propargyl radicals was depleted by irradiation at 248 nm, exciting the dissociative ˜B ²B₁ ← ˜X ²B₁ transition. Excitation of HCCCH₂ radicals with a polarized laser beam at 248 nm depletes roughly 75 % of the radicals, and consequently photo-orients the matrix. By exciting to the ˜B ²B₁ dissociative state, radicals with a₁ transition moments [< ˜B ²B₁ ‌ µ ‌ ˜X ²B₁ >] aligned with the laser are destroyed. This depleting laser light is horizontally polarized with respect to the laboratory frame, I_Ζ. Any molecule that has a significant projection of its transition dipole moment, µ_a1(˜B → ˜X), parallel to the depleting laser light will be selectively depleted. The remaining HCCCH₂ molecules will be oriented with their transition dipole moments perpendicular to the laboratory frame. Consequently, if we measure the infrared linear dichroism spectrum (I_Ζ - I_γ) of the matrix, the a₁ modes will be depleted. That is, I_Ζ - I_γ < 0 (exhibit negative LD) for levels of a₁ symmetry. Correspondingly, absorption intensities for b₁ and b₂ modes will be greater using horizontally oriented (Z direction) IR light for measurement; that is, I_Ζ - I_γ > 0 (exhibit positive LD) for modes of b₁ or b₂ symmetry. Use of the observed and calculated frequencies, and the polarization spectra, enable assignment of the HCCCH₂ vibrations.

Figure 3: IR depletion spectrum and the linear dichroism (LD) spectrum for propargyl radical