Nearby Dark States Research Articles

Experimental and theoretical studies of the doubly substituted methyl-ethyl Criegee intermediate: Infrared action spectroscopy and unimolecular decay to OH radical products.

The infrared (IR) action spectrum of the doubly substituted methyl-ethyl Criegee intermediate (MECI) is observed in the CH stretch overtone region with detection of OH products. The MECI exhibits four conformers, all of which undergo unimolecular decay via a 1,4 H-atom transfer mechanism, followed by the rapid release of OH products. Conformers with different orientations of the carbonyl oxide group with respect to the methyl and ethyl substituents (i.e., anti and syn) decay via distinct transition state barriers (16.1 kcal mol-1 and 15.4 kcal mol-1, respectively). The observed IR action spectrum is in good agreement with the predicted anharmonic IR absorption spectrum, but exhibits significant congestion, which is attributed to couplings between spectroscopic bright states and nearby dark states. Energy-dependent OH appearance rates are measured upon IR excitation of the strongest features in the IR action spectrum and are found to be on the order of 106-107 s-1. The experimental rates are in good agreement with computed Rice-Ramsperger-Kassel-Marcus rates for the unimolecular decay of MECI at these energies, which incorporate quantum mechanical tunneling and sophisticated hindered rotor treatments, as well as high-level theoretical calculations of the TS barrier heights, rovibrational properties, and torsional barriers associated with the MECI conformers. Master equation modeling is used to predict thermal rates for the unimolecular decay of anti- and syn-MECI of 473 s-1 and 660 s-1, respectively. Comparison with other previously studied Criegee intermediate systems provides insights into substituent effects on unimolecular decay under both energy-dependent and thermal conditions.

CH Stretch Activation of CH3CHOO: Deep Tunneling to Hydroxyl Radical Products.

Alkene ozonolysis, an important source of hydroxyl (OH) radicals in the Earth's troposphere, proceeds through unimolecular decay of Criegee intermediates. In this work, infrared activation of the methyl-substituted Criegee intermediate, syn-CH3CHOO, in the CH stretch fundamental region (2850-3150 cm-1) is shown to result in unimolecular decay to OH radical products. These excitation energies correspond to only half of the transition state barrier height, and thus the resultant 1,4 H atom transfer that leads to OH products occurs exclusively by quantum mechanical tunneling. Infrared action spectra recorded with UV laser-induced fluorescence detection of the OH products reveal the four CH stretch fundamentals and CO stretch overtone predicted to have strong transition strength. The vibrational band origins, relative intensities, and transition types derived from rotational band contour analyses are in good accord with theory. Distinctly different Lorentzian line broadening of the observed features is attributed to mode-specific anharmonic couplings predicted theoretically between spectroscopically bright and nearby dark states. The measured OH product state distribution shows a strong λ-doublet preference arising from pπ orbital alignment, which is indicative of the vinyl hydroperoxide intermediate along the reaction pathway. The unimolecular decay of syn-CH3CHOO at ca. 3000 cm-1 is predicted to be quite slow (ca. 105 s-1) using statistical Rice-Ramsperger-Kassel-Marcus theory with tunneling and much slower than observed at higher energies.

First detection and analysis of the 3ν1 + ν2 + ν3 band of NO2 by CRDS near 6156 cm−1

The high-sensitivity absorption spectrum of the NO2 molecule is recorded for the first time by cavity ring down spectroscopy between 6100 and 6200cm−1 with noise equivalent absorption αmin≈1×10−10cm−1. The spectrum is formed by weak rovibrational transitions belonging to the (311)-(000) band and the very weak (023)-(000) band at 6155.85 and 6183.92cm−1, respectively. In total, about 1881 rovibrational transitions are assigned. Upper energy levels with rotational quantum numbers N and Ka up to 49 and 8, and 30 and 3 are derived for the (311) and (023) vibrational states, respectively. Experimental line positions are modeled within an rms deviation of 0.0027cm−1 using the effective Hamiltonian approach which takes explicitly into account the spin-rotation interaction. Interpolyad resonance coupling between the (311) and (023) states as well as interactions involving the (311) state and two nearby dark states – (330) and (042) – were taken into account. The main parameters in the transition moment series are determined from a fitting of experimental intensities for the (311)-(000) and (023)-(000) bands.

High sensitivity Cavity Ring Down Spectroscopy of the 2ν1 + 3ν2 + ν3 band of NO2 near 1.57 µm

The weak 2ν1+3ν2+ν3absorption band of the nitrogen dioxide main isotopologue (14N16O2) is investigated near 6350cm−1. The absorption spectrum was recorded by high sensitivity Cavity Ring Down Spectroscopy with a noise equivalent absorption of αmin ≈ 1 × 10−10cm−1. In total, 1069 lines of the 2ν1 +3ν2 +ν3 band were assigned with rotational quantum numbers N and Ka up to 48 and 8, respectively, that corresponds to 1276 vibration-rotation transitions. The derived set of line positions is reproduced with an effective Hamiltonian with an rms of 2.35 × 10−3cm−1 for the (obs.-calc.) deviations. The effective Hamiltonian takes explicitly into account the Coriolis interactions between the spin rotational levels of the (2,3,1) upper vibrational state and those of two nearby dark states - (2,5,0) and (2,1,2) - together with the electron spin-rotation interactions. Using the corresponding eigenfunctions the principal effective dipole moment parameter responsible for the line intensities of the 2ν1 +3ν2 +ν3 band is fitted to selected measured line intensities which are reproduced with an rms deviation of 6.2%.

High sensitivity cavity ring down spectroscopy of the 3ν1+3ν2+ν3 band of NO2 near 7587 cm−1

The very weak 3ν1+3ν2+ν3 absorption band of the main isotopologue of nitrogen dioxide, 14N16O2, is investigated for the first time near 7587cm−1. The absorption spectrum was recorded by high sensitivity Continuous Wave-Cavity Ring Down Spectroscopy with a noise equivalent absorption of αmin≈1×10−10cm−1. 414 lines of the 3ν1+3ν2+ν3 band were assigned with rotational quantum numbers N and Ka as high as 32 and 6, respectively, what corresponds to 518 rotation–vibration transitions. The overall set of spin–rotation energy levels was modeled in the frame of the effective Hamiltonian approach and reproduced with an RMS of 6×10−3cm−1 for the (obs.–calc.) deviations. The effective Hamiltonian includes interactions with three nearby dark states – (350), (062) and (312) – in Coriolis interaction with the (331) bright state. Using a selected set of experimental line intensities and the fitted values of the vibration–rotation Hamiltonian parameters, the principal parameter in the dipole moment operator expansion is determined for the 3ν1+3ν2+ν3 band. With maximum line intensity on the order of 2.5×10−27cm/molecule at 296K, the 3ν1+3ν2+ν3 band is the weakest band of the NO2 molecule rovibrationnally assigned so far.

High-Resolution Rovibrational Spectroscopy of Jet-Cooled Phenyl Radical: The ν19 Out-of-Phase Symmetric CH Stretch

Phenyl radical has been studied via sub-Doppler infrared spectroscopy in a slit supersonic discharge expansion source, with assignments for the highest frequency b2 out-of-phase C-H symmetric stretch vibration (ν19) unambiguously confirmed by ≤6 MHz (0.0002 cm(-1)) agreement with microwave ground state combination differences of McMahon et al. [Astrophys. J. 2003, 590, L61-64]. Least squares analysis of over 100 resolved rovibrational peaks in the sub-Doppler spectrum to a Watson Hamiltonian yields precision excited-state rotational constants and a vibrational band origin (ν0 = 3071.8915(4) cm(-1)) consistent with a surprisingly small red-shift (0.9 cm(-1)) with respect to Ar matrix isolation studies of Ellison and co-workers [J. Am. Chem. Soc. 2001, 123, 1977]. Nuclear spin weights and inertial defects confirm the vibrationally averaged planarity and (2)A1 rovibronic symmetry of phenyl radical, with analysis of the rotational constants consistent with a modest C2v distortion of the carbon backbone frame due to partial sp rehybridization of the σ C radical-center. Most importantly, despite the number of atoms (N = 11) and vibrational modes (3N - 6 = 27), phenyl radical exhibits a remarkably clean jet cooled high-resolution IR spectrum that shows no evidence of intramolecular vibrational relaxation (IVR) phenomena such as local or nonlocal perturbations due to strongly coupled nearby dark states. This provides strong support for the feasibility of high-resolution infrared spectroscopy in other aromatic hydrocarbon radical systems.

Ab initio quartic force field of stannane and rotational analysis of the ν1 infrared band of H120SnD3

Stannane, SnH(4), has been studied both theoretically, using high-level ab initio methods, and experimentally, using high-resolution spectroscopy to analyze the Sn-H stretching fundamental band of the H(120)SnD(3) isotopologue. The geometry and the anharmonic force field of the molecule have been calculated ab initio, using the coupled-cluster with single, double, and perturbative triple excitations level of theory. H(120)SnD(3), present as minor isotopologue in (120)SnD(4), has been studied by Fourier transform spectroscopy at an effective resolution of ca. 0.005 cm(-1) near 1900 cm(-1) and the ν(1) band was identified. About 360 ro-vibration transitions with J(') up to 18 and K up to 10 have been assigned. Since the spectrum evidenced the existence of some perturbations, the transitions were analyzed either neglecting, or including in the model A(1)∕E Coriolis-type interactions with nearby dark states. The standard deviation of the fits, ca. 1.5 × 10(-3) cm(-1), is about one order of magnitude larger than the estimated experimental precision, and is only slightly dependent on the adopted model. The spectroscopic parameters obtained from this and from previous analyses of stannane isotopologues have been compared with the theoretical results. The theoretical force field is also used to obtain semi-experimental harmonic frequencies and the equilibrium geometry.

The 2ν1 overtone band of H28SiD3

The 2ν1 (A1) Si−H stretching overtone band of HSiD3 has been recorded at a resolution of ca. 0.009cm−1 between 4200 and 4400cm−1. About 790 ro-vibration transitions of the H28SiD3 isotopologue have been assigned, with J′ up to 24 and K up to 21. The spectrum evidences the existence of several perturbations. The assigned transitions have been analyzed either neglecting or including in the model A1/E Coriolis-type interactions between v1=2 and nearby dark states. The standard deviation of all the fits is, however, more than one order of magnitude larger than the estimated experimental precision and is independent of the adopted model.

The stretching fundamental bands ν1, ν2 and ν4 of H 28SiD 3

The ν 1(A 1), Si–H stretching, ν 2(A 1) and ν 4(E), Si–D stretchings, fundamental bands of HSiD 3 have been recorded at an effective resolution of ca. 0.003 cm −1 between 2080 and 2280 cm −1 and between 1480 and 1720 cm −1, respectively. Ro-vibrational transitions of the H 28SiD 3 isotopologue have been assigned in the two spectral ranges, about 700 belonging to ν 1, with J′ up to 25 and K up to 21, and about 1600 to the ν 2/ ν 4 dyad, with J′ up to 24 and K′ up to 19. The spectra of all the bands evidence the existence of several perturbations. The transitions of ν 1 have been analyzed either neglecting or including in the model A 1/E Coriolis-type interactions with nearby dark states. The υ 2 = 1 and υ 4 = 1 states have been fitted simultaneously taking into account several ro-vibrational interactions between them and, in addition, with the υ 5 = 2, l = 0 component, and with few other close dark states. The standard deviation of the fit for both ν 1 and the ν 2/ ν 4 dyad is, however, more than one order of magnitude larger than the estimated experimental precision and is independent on the adopted model.

First High-Resolution Infrared Observation of the Symmetry-Forbidden ν5 Band of 10B2H6

Spectra of 10B monoisotopic diborane, B2H6, have been recorded at high resolution (2–3 × 10−3 cm−1) by means of Fourier transform spectroscopy in the region 700–1300 cm−1. A thorough analysis of the ν18a-type, ν14c-type, and ν5 symmetry-forbidden band has been performed. Of particular interest are the results concerning the ν5 symmetry-forbidden band, which is observed only because it borrows intensity through an a-type Coriolis interaction with the very strong ν18 infrared band located approximately 350 cm−1 higher in wavenumber. The ν5 band has been observed around 833 cm−1 and consists of a well-resolved Q branch accompanied by weaker P- and R-branch lines. Very anomalous line intensities are seen, with the low Ka transitions being vanishingly weak, and Raman-like selection rules observed. The determination of the upper state Hamiltonian constants proved to be difficult since the corresponding energy levels of each of the bands are strongly perturbed by nearby dark states. To account for these strong localized resonances, it was necessary to introduce the relevant interacting terms in the Hamiltonian. As a result the upper state energy levels were calculated satisfactorily, and precise vibrational energies and rotational and coupling constants were determined. In particular the following band centers were derived: ν0 (ν5) = 832.8496(70) cm−1, ν0 (ν14) = 977.57843(70) cm−1, and ν0 (ν18) = 1178.6346(40) cm−1. (Type A standard uncertainties (1ς) are given in parentheses.)

State selected unimolecular dissociation of HOCl near threshold: The 6νOH vibrational state

The spectroscopy and unimolecular dissociation dynamics of HOCl are examined by accessing rotational resonances of the 6νOH vibrational level over the Ka=0–5 manifolds using overtone–overtone double resonance. The spectroscopic analysis indicates that state mixing between the zeroth-order “bright” O–H stretching overtone state, 6 0 0, and “dark” background vibrational levels is incomplete as the bright state couples to only a fraction of the available states. The coupling of 6 0 0 to a set of nearby dark states is mediated primarily by anharmonic coupling with the fourth-order vibrational resonance k1,223 playing a particularly important role through its ability to couple the 6 0 0 state directly to the 5 2 1 vibration and indirectly to the 4 4 2 vibration. The measured state-specific unimolecular dissociation rates for 6 0 0 show large fluctuations with J and Ka and are substantially slower than that expected on the basis of statistical theory. The rate fluctuations are interpreted on the basis of spectroscopic data which suggest that the fluctuations arise as a result of variation in state mixing as different dark vibrational states come in and out of resonance with the bright state for different values of J and Ka.

High-Resolution Infrared Study of the ν14, ν17, and ν18Bands of11B2H6and10B11BH6

Using high-resolution Fourier transform spectra, a thorough analysis of the ν14c-type, ν17a-type, and ν18a-type bands of both11B2H6and10B11BH6has been carried in the 10.3-, 6.2-, and 8.5-μm spectral regions, respectively. From this analysis a large set of precise ground state combination differences withJvalues up to 36 (31) andKavalues extending to 18 (18) was derived for11B2H6(10B11BH6). These data were fitted using a Watson-type Hamiltonian leading to accurate ground state rotational constants. Anrsvalue for the B–B distance has been determined to be 1.7645(10) Å. The determination of upper state Hamiltonian constants proved to be much more difficult since the corresponding rotational levels of each of the bands are strongly perturbed by nearby dark states. To account for these strong localized resonances, it was necessary to introduce the relevant interacting terms in the Hamiltonian matrix. As a result it was possible to calculate the upper state energy levels quite satisfactorily. From these fits, estimates of the band centers and a few of the rotational constants of the resonating dark states were obtained.

Vibrational predissociation of ArCl2: Toward the determination of the potential energy surface of the B state

Accurate quantum mechanical calculations are carried out to test the sensitivity of the spectroscopy and dynamics of the B state of ArCl2 to the steepness of the Morse term, α, of an atom–atom potential. It is discovered that the predissociation dynamics for this molecule are very complicated even in the Δv=−1 regime due to resonances in the continuum manifold of states. In both the Δv=−1 regime and the Δv=−2 regime the rate of vibrational predissociation and the product rotational distribution are extremely sensitive to the value chosen for α, but not in a regular way. For the Δv=−2 regime the variations can be attributed to spacings between resonances and the overlaps of the bright state wave functions with nearby dark states as expected from the intramolecular vibrational relaxation model. In the Δv=−1 regime, the variations are shown to originate from resonances in the v−1 continuum set of states. Although this makes it difficult to determine the value for α, a value of 1.8 Å−1 is probably close to the true value. The most useful new data to determine the potential would be measurements of the lifetimes for as many vibrational levels as possible and rotational distributions for excitation to low vibrational levels of the B state.

Quasistable extreme motion vibrational states of HFCO above its dissociation threshold

The anharmonic mixing of highly excited vibrational states of HFCO above its dissociation threshold is studied by stimulated emission pumping spectroscopy. At 0.05-cm−1 resolution, individual molecular eigenstates were resolved and state mixings observed by the distribution of oscillator strength to nearby dark states of the background. Most of the zero-order vibrational levels observed in the energy range between 13 000 and 23 000 cm−1 are assigned to long Franck–Condon-allowed progressions of extremely high overtones of the C–H out-of-plane bending mode (ν6) in combination with the C■O stretching mode (ν2). The extent of state mixing of highly excited vibrational states is strongly mode dependent. For vibrational states with almost the same total vibrational energy, states with the most quanta in ν6 show the least coupling with other states. More strikingly, as the total energy is increased by adding ν6 quanta, the state mixing becomes weaker. For the vibrational states observed above 18 000 cm−1, those that have all of the energy in ν6 or at most one quantum of excitation in ν2 are extremely stable against state mixing. These appear to be the regular states or quasiperiodic trajectories predicted in some theoretical studies of two-dimensional systems. Extreme motion of the C–H out-of-plane bending mode seems to localize and prevent coupling with other modes.