Theoretical prediction of the structure and ...

Communication: Theoretical prediction of the structure and spectroscopic properties of the and states of hydroxymethyl peroxy (HOCH2OO) radical Mickael G. Delcey, Roland Lindh, Roberto Linguerri, Majdi Hochlaf, and Joseph S. Francisco Citation: J. Chem. Phys. 138, 021105 (2013); doi: 10.1063/1.4775782 View online: http://dx.doi.org/10.1063/1.4775782 View Table of Contents: http://jcp.aip.org/resource/1/JCPSA6/v138/i2 Published by the AIP Publishing LLC.

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THE JOURNAL OF CHEMICAL PHYSICS 138, 021105 (2013)

Communication: Theoretical prediction of the structure and ˜ and A ˜ states of hydroxymethyl spectroscopic properties of the X peroxy (HOCH2 OO) radical Mickael G. Delcey,1 Roland Lindh,1 Roberto Linguerri,2 Majdi Hochlaf,2 and Joseph S. Francisco3 1 Department of Chemistry – Ångström, The Theoretical Chemistry Program, Uppsala University, P.O. Box 518, SE-751 20 Uppsala, Sweden 2 Laboratoire Modélisation et Simulation Multi Echelle, Université Paris-Est, MSME UMR 8208 CNRS, 5 bd Descartes, 77454 Marne-la-Vallée, France 3 Department of Chemistry and Department of Earth and Atmospheric Science, Purdue University, West Lafayette, Indiana 47907, USA

(Received 14 November 2012; accepted 28 December 2012; published online 11 January 2013) The hydroxymethyl peroxy (HMOO) radical is a radical product from the oxidation of non-methane hydrocarbons. The present study provides theoretical prediction of critical spectroscopic features of this radical that should aid in its experimental characterization. Structure, rotational constants, and harmonic frequencies are presented for the ground and first excited electronic states of HMOO. ˜ ←X ˜ process is 7360 cm−1 , suggesting that this transition, The adiabatic transition energy for the A occurring in the mid to near infrared, is the most promising candidate for observing the radical ˜ ←X ˜ transition of HMOO is calibrated and benchmarked spectroscopically. The band origin of the A with the corresponding state of the HOO radical, which is experimentally and theoretically well characterized. © 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4775782] I. INTRODUCTION

In the oxidation of non-methane hydrocarbons, a major class of species resulting from the oxidation both under atmospheric conditions and in combustions are alkyl hydroperoxides (ROOH).1–6 The photochemistry and oxidation of these species are resulting in the production of α-hydroxyalkyl peroxy radicals (see Fig. 1). The simplest α-hydroxyalkyl peroxy radical is the hydroxymethyl peroxy (HMOO) radical. This radical can also result from the atmospheric oxidation of hydroxymethyl hydroperoxide (HOCH2 OOH), often referred to as HMHP. There has been considerable attention given to the atmospheric chemistry of HMHP,7–18 with studies focusing on the photochemistry of HMHP by both dissociation through overtone states15 and by UV photolysis.16 Both experimental15 and theoretical studies18 have examined the removal of HMHP by OH radicals in the atmosphere. Atmospheric oxidation studies show that the two major pathways by which HMHP is removed is by hydrogen abstraction of either the HO-group hydrogen HO + HOCH2 OOH → H2 O + ·OCH2 OOH

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This suggests that the reaction of HO2 radical with carbonyl compounds will be an important sink for carbonyl compounds in the upper atmosphere. In the literature, the decomposition of HOCH2 OO was theoretically and experimentally investigated;19–22 however, because of the lack of accurate spectroscopic data for the species, the rate constant for this reaction is known only within ±50%.19 ˜ absorption Most of organic peroxides have the B˜ − X band in the UV spectral region.23 But the B˜ state is generally characterized as a repulsive potential energy surface (PES), and as such this absorption band is generally featureless, making it almost impossible to distinguish different radicals.24 On ˜ −X ˜ transition is weak comthe other hand, although the A ˜ ˜ the A state is bound and thus the transipared to the B˜ − X, tion presents sharp vibrational and rotational structure.25 It is therefore a better alternative to probe especially in a mixture. In this Communication, we present the main structural ˜ states of the HMOO radical ˜ and A characteristics of the X and the first high-level ab initio determination of the verti˜ ←X ˜ transition energies. This will aid the cal and adiabatic A experimental characterization of the HMOO radical.

or the HOO-group hydrogen HO + HOCH2 OOH → H2 O + HOCH2 OO·

II. COMPUTATIONAL DETAILS

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resulting in the formation of the HMOO radical. Theoretical studies19 of the reaction of HO2 radicals with formaldehyde suggest that the HMOO radical will be a major byproduct of this reaction via HO2 + CH2 O → HOCH2 OO· . 0021-9606/2013/138(2)/021105/4/$30.00

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The doublet state of HOCH2 OO was studied as follows, structures (conformers I-III and some associated TSs) and harmonic frequencies were derived at the restricted explicitlycorrelated coupled-cluster singles and doubles with perturbative triples (RCCSD(T)-F12) level of theory. Additionally, stationary structures at the ground and lowest excited state were optimized with the multiconfigurational reference

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J. Chem. Phys. 138, 021105 (2013) TABLE I. Optimized geometry and rotational constants at equilibrium for HOO and HOCH2 OO. Bond distances in Ångströms, angles in degrees, and the rotational constants in MHz.

Species

FIG. 1. The complete oxidation mechanism for non-methane hydrocarbons. The α-hydroxyalkyl peroxy radical is represented in red. The simplest member, hydroxymethyl peroxy radical, is found for Q = CH2 .

section second order perturbation theory method. Using these structures RCCSD(T) energetics was compiled. The complete active space self-consistent field (CASSF) active space was designed to contain the σ /σ * orbitals of the R–O and O–O bonds, and the two out-of-plane and one inplane oxygen lone pair orbitals, resulting in 9 electrons in 7 orbitals. It is expected that the radical electron will occupy one of the non-bonding lone-pair orbitals and that single excitations within this manifold will produce at least one low electronically excited state. For a complete and detailed description of the computational details, see the supplementary material.26

CASPT2/ Coordinate RCCSD(T)-F1235 ANO-RCC-VTZP Expt31, 32

HO2 ˜ state ) (X

r(OH) r(OO) θ (HOO) Ae Be Ce

0.971 1.327 104.5 620 676 33 829 32 080

0.974 1.336 104.1 613 708 33 403 31 678

0.9707 1.3305 104.3 610 273 33 518 31 668

HO2 ˜ state ) (A

r(OH) r(OO) θ (HOO) Ae Be Ce

0.969 1.396 102.0 606 713 30 778 29 292

0.971 1.405 101.5 601 154 30 409 28 945

0.9647 1.3930 102.69 614 151 30 619 29 020

1.456 1.318 108.5 19 289 6361 5285

1.458 1.318 108.6 19 471 6332 5281

HOCH2 OO r(CH2 –OO) ˜ state ) (X r(OO) θ (COO) Ae Be Ce HOCH2 OO r(CH2 –OO) ˜ state ) (A r(OO) θ (COO) Ae Be Ce

1.426 1.398 105.9 19 527 5977 5124

III. RESULTS AND DISCUSSION

As in HOO, HOCH2 OO presents two low-lying states: ˜ with the singly occupied orthe ground state, labelled X, bital being perpendicular to the COO plane and the first ex˜ where the radical electron occupy an cited state, labelled A, in-plane orbital (see Fig. 2). This study initially located the critical points of the HOCH2 OO PES and characterized the structure and rotational constants of the global minimum at ˜ state and the A– ˜ X ˜ equilibrium. Then, the structure of the A transition energy was established. Details from these calculations are presented below, however, explicit structural data and harmonic frequencies are found in Tables I–III in the supplementary material.26 Before we proceed we like to point out that hydroxymethyl peroxy radical has planar chirality around

FIG. 2. The SA-CASSCF orbitals of conformer I corresponding to the singly ˜ state of HOCH2 OO, respectively. ˜ and (b) the A occupied orbitals of (a) the X

the O–CH2 –O plane (see Figure I in the supplementary material). This is the origin of an interesting optical spectrum which will be the subject of future investigations. In the rest of the text, we will make no explicit difference between the Sp and Rp enantiomers if not called for. ˜ state A. HOCH2 OO ground (X)

As shown in Fig. 3, the HOCH2 OO system has a shallow but well-defined equilibrium structure. Associated with

FIG. 3. Relative energies of the ground (blue) and first excited (green) state of the CASPT2/ANO-RCC-VTZP MEP around the O–O–C–O torsional angle. Energies in kcal/mol and angles in degrees.

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this structure is a hindered internal rotation around the O–C–O–O torsion angle. Along this internal rotation the bond distances and angles exercise minute changes as compared to the equilibrium structure. No significant change in the electronic structure is observed between the different conformers along the internal rotation. Rather, the presence of the subtle minima of the internal rotation is due to favorable Coulombic interaction between the terminal oxygen and the three hydrogens, while the transition state (TS) is a high symmetry case. A simplified exploration of the HOCH2 OO potential energy surface along this reaction coordinate (see Fig. 3) shows that there are three conformers and one distinct transition state. This is in-line with previous studies performed at lower level of theory.19, 21, 22 The global minimum, conformer I, is 1.67 kcal mol−1 below conformer II and 1.90 kcal mol−1 below conformer III. The energetic ordering is in accord with that of Hermans et al.19 We also note that conformer III is found in a more shallow minimum as compared to the two other conformers. This is manifested by the lowest vibrational mode which is found to be 134, 124, and 81 cm−1 for conformers I, II, and III, respectively. Furthermore, a well-developed transition state connecting conformers I and II is characterized and the rotational barrier is estimated to be 3.29 kcal mol−1 (uncorrected for zero-point energy). Two more TSs are expected to be present but to be of no significance due to the low energy barrier they represent. The main structural features and rotational constants of conformer I optimized at the RCCSD(T)-F12/VDZ-F12 and CASPT2/ANO-RCC-VTZP are tabulated in Table I. As a benchmark, the corresponding geometrical parameters for the ˜ states of HOO radical were also computed and com˜ and A X pared to available experimental data. Excellent agreement is found for both states at the CASPT2 and RCCSD(T) levels with error on bond distances smaller than 1 pm. Both methods can thus be expected to provide a reasonable estimate of the geometry for HOCH2 OO. The rotational constants at the equilibrium state for HOCH2 OO, conformer I, show that HOCH2 OO is an asymmetrical top molecule. It is a near prolate symmetrical top since rotational constant Ae > Be ≈ Ce . Interestingly, the rotational constants of HOCH2 OO and HOO are radically different. For example, the rotational constants for HOO ground state at the RCCSD(T)-F12 level of theory are Ae = 620 676; Be = 33 829, and Ce = 32 080 MHz compared to constants for HOCH2 OO at the same level of theory; Ae = 19 289; Be = 6361, and Ce = 5285 MHz. Therefore, there should be no ambiguity in assigning the microwave spectra of HOCH2 OO or HOO. ˜ state of HOCH2 OO B. A

˜ The main structural characteristics of the optimized A state are collected in Table I. The geometry is essentially sim˜ state, except for a significantly elongated O–O ilar to the X bond and a slightly shorter C–O bond, which is very similar to what was found for the HOO radical.27–29 A direct experimen˜ state of HOCH2 OO tal access to the geometry change in the A can be found in the rotational constants. The computed rotational constants corresponding to the optimized geometry of ˜ state are given in Table I. the A

J. Chem. Phys. 138, 021105 (2013) TABLE II. Vertical and adiabatic energies (in cm−1 ) for HOO and ˜ −X ˜ transition calculated with the ANO-RCC-VTZP HOCH2 OO for the A basis set. Method

Tv (cm−1 )

T0 (cm−1 )

HOO

CASPT2 RCCSD(T) Expt23, 32

8316 7611

7383 7000 7030

CH3 OO

CASPT2 RCCSD(T) Expt33, 34

8781 8190

8039 7278 7383

HOCH2 OO

CASPT2 RCCSD(T)

9101 8355

7844 7360

Species

˜ −X ˜ transition energies for Vertical and adiabatic A HOCH2 OO computed at the CASPT2 and RCCSD(T) level of theory are tabulated in Table II. The values for HOO and CH3 OO are also included and benchmarked against experimental values. An excellent agreement with experiment is obtained for the RCCSD(T) method. The CASPT2 results, as anticipated, are not in such an accurate agreement. The error, however, is always below 1000 cm−1 and in par with the empirically established accuracy of the CASPT2 method of 0.1– 0.2 eV. The quantitative accuracy of the RCCSD(T) values for HOO and CH3 OO allows us to predict that the band origin of ˜ −X ˜ transition for HOCH2 OO is about 7360 cm−1 . This the A compares well to 7362 and 7389 cm−1 , which are the transition energies for the isoelectronic ethyl peroxy (CH3 CH2 OO) and HOCH2 CH2 OO29, 30 radical, respectively. This is consistent with the fact that the excitation is very local and hence, the effect of the substituent is limited. IV. CONCLUSION

The main spectroscopic features for the HOCH2 OO radi˜ state, have been presented cal, with a special focus on its A in order to facilitate its experimental characterization. Results from this study confirmed the existence of 3 principle conformers, two well-developed and several low lying transitions states on the ground state potential energy surface, all the structures being within 5 kcal/mol in energy. However, the global minimum, conformer I, is lower in energy than any other structure by almost 2 kcal/mol. RCCSD(T) and CASPT2 structural parameters and rotational constants for the HOCH2 OO and HOO radicals contrast the structural differences between the two radicals that should highlight the spectroscopic distinction between the species. Finally, verti˜ −X ˜ transition energies computed, both cal and adiabatic A with RCCSD(T) and CASPT2 levels of theory, are shown to give accurate transition energies for the experimentally known transitions of the HOO and CH3 OO radicals, and en˜ −X ˜ able the reliable prediction of the band origin of the A transition for HOCH2 OO to be 7360 cm−1 . ACKNOWLEDGMENTS

We like to thank the Swedish Research Council (VR) for financing.

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