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Oxygen-18 Isotopic Studies of HOOO and DOOO Lou Barreau, Oscar Martinez, Kyle N. Crabtree, Caroline Womack, John F Stanton, and Michael C. McCarthy J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b05380 • Publication Date (Web): 28 Jul 2017 Downloaded from http://pubs.acs.org on July 31, 2017
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Oxygen-18 Isotopic Studies of HOOO and DOOO Lou Barreau,†,‡ Oscar Martinez, Jr.,†,¶ Kyle N. Crabtree,†,§ Caroline C. Womack,k,⊥ John F. Stanton,#,@ and Michael C. McCarthy∗,† †Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA and School of Engineering and Applied Sciences, Harvard University, 29 Oxford Street, Cambridge, MA 02138, USA ‡Present Address: LIDYL, CEA, CNRS, Universit´e Paris-Saclay, CEA-Saclay 91191, Gif-sur-Yvette, France ¶Permanent Address: Present address: Air Force Research Laboratory, Space Vehicles Directorate, Kirtland AFB, New Mexico 87117, USA §Permanent Address: Department of Chemistry, University of California-Davis, Davis, CA 95616, USA kDepartment of Chemistry, MIT, Cambridge, MA 02138, USA ⊥Present Address: Chemical Sciences Division, NOAA ESRL, Boulder, CO 80305, USA #Department of Chemistry & Biochemistry, The University of Texas at Austin, 1 University Station A5300, Austin, TX 78712-0165, USA @Permanent Address: Department of Chemistry, University of Florida, Gainesville, FL 32611, USA E-mail:
[email protected] 1
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Abstract Owing to questions which still persist regarding the length of the O–H and central O–O bond, and large-amplitude torsional motion of trans hydridotrioxygen HOOO, a weakly-bound complex between OH and O2 , new 18 O isotopic measurements of HOOO and DOOO have been undertaken using Fourier transform microwave and microwavemillimeter-wave double resonance techniques. Rotational lines from three new
18 O
species of DOOO (D18 OOO, DO18 O18 O, and D18 O18 O18 O) have now been detected, along with the two singly-substituted
18 O
isotopic species of HOOO (HO18 OO and
HOO18 O) that were not measured in the previous isotopic investigation. From a leastsquares fit, the leading spectroscopic constants, including the three rotational constants, were precisely determined for all five species. The inertial defect of DOOO and its
18 O
species is uniformly negative: of order −0.04 amu ˚ A2 , regardless of the number
or location of the
18 O
atoms, in contrast to that found for HOOO or its
18 O
isotopic
species. A re-analysis of the molecular structure has been performed using either normal HOOO and its four singly-substituted isotopic species, the new DOOO data, or all the isotopic species (10 in total). The differences between the purely experimental (r0 ) structures are generally quite small, of order ±0.01 ˚ A for the bond lengths and ±1◦ for the bond angle. The length of the O–H bond remains unrealistically short compared to free OH, and the central O–O bond length is consistently very close to 1.68 ˚ A. On the basis of the effective O–H bond length derived from the experimental structure, the average displacement of the large amplitude torsional motion from planarity is estimated to be ∼ 22◦ .
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Introduction The hydridotrioxygen radical (HOOO) has been one of the most extensively studied small molecules during the past several years, owing to its possible importance as a temporary reservoir of atmospheric OH, 1–4 and because questions still persist concerning both the stability of its very low-lying cis isomer and the precise geometrical structure of trans-HOOO, the most stable conformer on the potential energy surface. HOOO was first detected at high resolution in the gas phase using infrared action spectroscopy in 2007, 1 and many experimental and theoretical studies have since followed. A large number have focused on its binding energy, with recent theoretical calculations and kinetic studies concluding that D0 is probably very close to 12 kJ mol−1 , 5,6 and that the reaction of HO + O2 → HOOO is almost certainly barrierless. 7,8 Although an unstructured band in the infrared action spectra of HOOO was tentatively assigned to the cis isomer, 4 only trans-HOOO has been detected by rotational spectroscopy. 9,10 Recent infrared studies of HOOO and DOOO in 4 He nanodroplets 8 also found no spectroscopic evidence for the formation of cis-HOOO, suggesting instead that the feature in the gas-phase infrared spectrum might arise from HOOO–(O2 )n clusters. Although
18
O isotopic studies and coupled cluster calculations 10,11 have now been un-
dertaken on HOOO, the experimental and theoretical structures of trans-HOOO remain at odds. For example, the experimental (r0 ) structure differs from the equilibrium (re ) structure calculated at the CCSD(T)/CBS level 11 in several significant respects, including the HO1 length (using the notation HO1 O2 O3 ) and the HO1 O2 angle, but the most egregious difference between the two is the central O–O bond (O1 O2 ): the r0 length is 1.684 ˚ A — substantially longer (by 0.05-0.10 ˚ A) than the re value obtained from coupler-cluster theory. 10,11 Correcting the experimental rotational constants for the effects of zero-point vibration using vibration-rotation constants (αi ) calculated at the CCSD(T) level yield an semi-experimental (re emp ) structure which gives only somewhat better agreement with the theoretical re structure. However, the inertial defects of the various isotopic species deviate considerably from 3
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zero and from each other by this method, indicating that second-order perturbation theory is inadequate to properly describe large-amplitude, low-frequency motion in HOOO. The force field and geometrical structure of HOOO has recently been examined by Suma et al. 12 using Davidson-corrected multi-reference configuration interaction level of theory. Differences between the re and r0 structures were also found, and attributed to contributions of the long-range van der Waals complex from the OH–O2 local minimum. In one of the most recent studies of HOOO, Liang et al. 13 performed elegant Stark measurements on trans-HOOO solvated in superfluid helium. They found the experimental and computed dipole moments could only be brought into agreement by shifting the potential along the central O–O bond coordinate by ∼0.08 ˚ A. An optimized geometry was also computed for this radical at the composite all-electron CCSDT(Q)/CBS level of theory, and found to increase the O1 –O2 bond length by 0.07 ˚ A relative to that calculated at the CCSD(T)/CBS level, suggesting that inclusion of full triples and a perturbative treatment of quadruple excitations is important. This structure, however, can not be directly compared to either the experimental or the semi-experimental ones because no correction for zero-point vibrational motion has been made; it is not surprising therefore that the CCSDT(Q)/CBS structure does not reproduce either the HO1 length or the HO1 O2 angle derived from experiment. To further investigate the molecular structure of trans-HOOO, additional
18
O isotopic
spectroscopy has now been undertaken on HOOO and DOOO using rotational spectroscopy. The motivation for this work is two-fold: (1) to examine the effect that the heavier mass of deuterium has on the large amplitude, light (hydrogen) atom motion, since DOOO should have smaller zero-point vibrational corrections to the equilibrium rotational constants compared to HOOO. As noted in the original study of Suma et al., the inertial defect of transDOOO (∆ = −0.043 amu ˚ A2 ) differs in sign and magnitude relative to that of the normal isotopic species (0.0123 amu ˚ A2 ). On the assumption that the geometry is strictly planar, this difference implies torsional motion of the hydrogen atom: (2) to derive a structure for
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HOOO which is based on a complete set of singly-substituted isotopic species. Deriving a structure in this manner minimizes any possible correlations between structural parameters which might have existed in the original determination — one that relied on data for one singly-substituted species, but several multiply-substituted species. In this paper, the rotational spectra of three new isotopic species of DOOO have been precisely characterized using FT microwave spectroscopy and microwave-millimeter-wave double resonance techniques. In addition, rotational spectra of the two singly-substituted isotopic species of HOOO (HO18 OO and HOO18 O) that were not studied in the original 18 O isotopic investigation of HOOO 10 have been detected by the same means. The structural parameters derived using different combinations of the isotopic data are generally quite close to one another and those found in the original study, including the anomalously short OH bond length. On the assumption that the OH bond length in HOOO is comparable to that of free OH (0.9697 ˚ A; Ref. 14), the average displacement from planarity is estimated to be ∼22◦ .
Experimental The new
18
O isotopic species of DOOO and HOOO were produced and detected in a very
similar manner to that used in the original structural study of HOOO. 10 Details of the FT microwave spectrometer 15,16 and the double resonance techniques 17 have been described in detail elsewhere. Both radicals were produced in the throat of an small electrical discharge source that has been used with good success to produce many highly-reactive molecules, including a number of species of the form HOXO (X = C, S, N, O, etc.) 18 The discharge source consists of a pinhole expansion followed by a series of cylindrical teflon insulators and copper electrodes, in which the orifice diameter systematically increases from 1 mm to 3 mm, and eventually to 10 mm. 19 It is attached to the back of one of the two large aluminum mirrors that compose the Fabry-Perot cavity, and gas expands into the large vacuum chamber, along
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the axis of the cavity, via a small hole in the center of this mirror. The discharge source is electrically and thermally isolated from the mirror, and has been designed in such a way that the position of the last electrode is very close to the front face of the mirror. Reactive molecules are produced by applying a low-current dc discharge across the copper electrodes as a gas mixture, normally consisting of one or more precursors heavily diluted in an inert buffer gas, passes through the nozzle source, prior to supersonic expansion to about Mach 2. Because HOOO and DOOO are radicals, care was taken to minimize the Earth’s field in the center of the Fabry-Perot cavity by using three sets of mutually perpendicular Helmholtz coils. However, even when the ambient field (∼ 400 mG) was reduced to ∼10 mG over the large active volume of our spectrometer, this residual field is still large enough to broaden some hyperfine components beyond the intrinsic instrumental linewidth of ∼2 kHz that arises from time-of-flight of our highly divergent supersonic jet 20 and to split each line into several closely-spaced Zeeman sub-components. Nevertheless, since the Zeeman sub-components are frequently observed as a pair of ∆MJ = ±1 transitions, it is still possible to derive rest frequencies to an accuracy of ∼5 kHz. In such cases, the rest frequency was determined by the arithmetic average of both the Zeeman and Doppler components. For each new
18
O species, the two lowest a-type rotational lines from the Ka =0 level
were first detected between 5 and 40 GHz with a FT microwave spectrometer, and then two strong, low-J b-type lines were detected above 40 GHz by microwave-millimeter-wave double resonance spectroscopy. Measurements of both a- and b-type transitions are required to determine the three rotational constants of each isotopic species to high precision. Two lines originating from the Ka = ±1 levels (21,2 → 11,1 and 21,2 → 11,0 ), lying only 2.5 K above ground, also fall within the range of our microwave spectrometer, but, owing to the very low rotational temperature in our supersonic jet (Trot ∼ 1 K), these lines are very faint for both HOOO and DOOO, and consequently no attempt was made to detect them for
18
O isotopic
species sought here. Following detection of the two a-type lines, a search for 11,1 → 00,0 and the 11,0 → 10,1 6
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b-type lines was undertaken at higher frequency. In each of these measurements, the intensity of a single hyperfine component was monitored with the FT spectrometer as the millimeter source was tuned in frequency. Because the dominant process is coherence rather than population, 21,22 particularly when the two linked transitions share a common lower level, double resonance features are normally observed as a depletion in the line intensity; line profiles are then fit to a Gaussian profile in which the centroid is typically determined to an uncertainty of
