Article pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Hydrogen Abstraction Reaction H2Se + OH → H2O + SeH: Comparison with the Analogous Hydrogen Sulfide and Water Reactions Mei Tang,† Xiangrong Chen,*,† Yaoming Xie,‡ and Henry F. Schaefer*,‡ †
College of Physical Science and Technology, Sichuan University, Chengdu 610065, China Center for Computational Quantum Chemistry, University of Georgia, Athens, Georgia 30602, United States
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‡
S Supporting Information *
ABSTRACT: The mechanism for the reaction of hydrogen selenide (H2Se) with the OH radical is examined using the “gold standard” CCSD(T) method along with the correlation consistent basis sets up to aug-cc-pV5Z. For the Se atom, the corresponding basis sets are combined with the core relativistic pseudopotential, designated as aug-cc-pVnZ-PP (n = D, T, Q, and 5). The predicted geometries and vibrational frequencies for reactants and products agree well with the available experimental results. For the entrance complex, the two-center three-electron hemibonded H2Se··· OH structure (RC-B) is predicted to be the lowest. This structure lies 5.1 kcal/mol below the separated reactants H2Se + OH without zero-point energy correction. For the analogous H2S and H2O reactions, the entrance complexes lie 3.3 and 5.7 kcal/mol, respectively, below the reactants. The relative energies of the transition states are −1.7 (H2Se), +0.1 (H2S), and +9.5 (H2O) kcal/mol with respect to the appropriate reactants. The three reaction exoergicities are 40.8 (H2Se), 29.5 (H2S), and 0.0 (H2O) kcal/mol. Compared to the separated products XH + H2O (X = O, S, and Se), the exit well depths are 2.1 (Se), 2.9 (S), and 5.7 (O) kcal/mol. Intrinsic reaction coordinate analysis indicates that all stationary points were correctly identified. Several other stationary points were identified and analyzed. Finally, 29 density functional theory methods were tested systematically to explore their ability in describing the potential energy surface of the H2Se + OH reaction. The hemibonded H2Se···OH entrance complex should be observable, initially via matrix isolation spectroscopy.
1. INTRODUCTION The OH radical has been found in many situations to be the most common oxidant. It reacts with most biomolecules in living cells, such as nucleic acids and amino acids,1 leading to several neurological diseases.2 On the other hand, the reactive OH radical is found to be a major atmosphere scavenger3 because it reacts with many types of pollutants such as NO2, CO, H2S, and RSH in the Earth’s atmosphere.4 Recently, the reaction of OH radical with hydrogen sulfide (H2S) was studied using the “gold standard” CCSD(T) method.5 This reaction was also explored earlier with the MCQCISD method and several density functional theory (DFT) methods by Truhlar et al.6 The potential energy surface (PES) for reaction H2S + OH → SH + H2O has features different from the analogous reaction OH + H2O → H2O + OH.7 Because the hydrogen bonding for S···H is much weaker than that for O···H, the binding energies of the entrance complexes and exit complexes for the H2S + OH reaction are smaller than those for the H2O + OH reaction. Accordingly, the energy barrier of the transition state for the H2S + OH reaction is lower than that for the H2O + OH reaction. It is important that there exist © XXXX American Chemical Society
energetically low-lying complexes between H2S and OH with two-center-three-electron (2c-3e) hemibonds,8 such as complexes H2S···OH and H2O···SH, while such complexes are not observed between H2O and OH. The above finding stimulates us to further explore the analogous reaction for the next chalcogen element, i.e., the H2Se + OH → SeH + H2O reaction. In fact, selenium is an environmentally and biologically essential element,9 and hydrogen selenide (H2Se) is found to be an important metabolite of Se compounds in living cells.10,11 H2Se is known to display chemical markers different from those of hydrogen sulfide (H2S). Especially, H2Se plays a significant role related to the nanomaterials, such as in the synthesis of CdSe and CuSe nanocrystals, by directly reacting with metal ions.12−15 A recent study reported that the adsorption of H2Se on the AlN and AlP nanocone sheets is favorable.16 H2Se can be also adsorbed on the ZnO, TiO2, and Zn2TiO4 surfaces to facilitate its dehydrogenation.17,18 The reactions of H2Se with laser-ablated actinide Received: November 7, 2018
A
DOI: 10.1021/acs.inorgchem.8b03140 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 1. Profile of the CCSD(T) potential energy surface without ZPVE corrections for the H2Se + OH → H2O + SeH reaction. For the 5Z basis set, only single-point energies were computed at the optimized QZ geometries. pVnZ-PP (for Se) basis sets for brevity. The geometries were optimized with basis sets up to QZ, and more accurate single-point energies were evaluated using the larger 5Z basis sets. In the CCSD(T) study, the 1slike molecular orbital (MO) is frozen for oxygen, whereas the 3s3p3dlike MOs are frozen for selenium atom. To characterize the nature of these stationary points, harmonic vibrational frequencies were computed at their CCSD(T) equilibrium geometries with up to QZ basis sets. All coupled cluster computations were carried out using the CFOUR program.29 To estimate the pseudopotential effect, the results of the all-electron aug-cc-pVnZ (n = D, T) basis set for Se atom is also reported for comparison.30 With the all-electron basis set, the 1s2s2p3s3p3d-like MOs for Se atom are frozen. DFT methods are very popularly used in computational chemistry because correlation effects are in some sense considered, and the computational costs are relatively low. However, the performances of the currently proposed functionals are quite different, depending on the quality of their approximate exchange and correlation functional terms. In this study, a series of DFT methods were tested, and we chose 29 popular functionals to study the H2Se + OH reaction. The unrestricted DFT method was used for the open-shell systems, and the ultrafine (99 590) grid was used for the DFT numerical integrations. Among these DFT functionals, the MPW1K method31 has been successfully utilized to study both H2O + F reaction32 and H2S + OH reaction.5 In addition, the M06-2X functional of Truhlar et al. generally performs well in main group thermochemistry and noncovalent interactions.33 Thus, the results from these and other DFT methods are compared with those from the more reliable CCSD(T). All intrinsic reaction coordinated (IRC) analyses in the present study are performed with the DFT methods. The DFT computations were carried out with the Gaussian09 program.34
metal atoms produce H2AnSe (An = Th, U), which have the unusual highly polarized actinide−selenium triple bond (Andrews, Dixon, and coworkers).19 Mechanisms and reaction rates for some H2Se reactions have been reported, such as the reactions of H2Se with oxygen atom,20 Al atom,21 and ozone.22 Nevertheless, to our knowledge, no studies have been made of the prototypical reaction of H2Se with the OH radical. Thus, following high-level ab initio studies of the reactions of OH radical with H2O and H2S, we predict a reliable potential energy surface in the present research for the H2Se + OH → SeH + H2O reaction. The present research includes structures, energetics, and vibrational frequencies for stationary points on the H3SeO PES, and we compare these results with the analogous reactions of the OH radical with H2O and H2S.
2. THEORETICAL METHODS The CCSD(T) method is often considered as a gold standard method for systems dominated for a single configuration.23−25 In the present research, we adopted CCSD(T) to study the potential energy surface for the H2Se + OH reaction. The UHF reference was used for the unrestricted CCSD(T) study for the open-shell systems. The CCSD(T) abbreviation represents the coupled cluster single and double excitations with perturbative triple excitations. Dunning’s correlation-consistent polarized valence basis sets with diffuse functions aug-cc-pVnZ (n = D, T, Q, and 5) were used in this research for hydrogen and oxygen atoms.26,27 For the selenium atom, we utilized the MCDHF (multiconfiguration Dirac−Hartree−Fock adjusted) small-core relativistic pseudopotential (PP) in combination with the corresponding correlation-consistent aug-cc-pVnZ-PP (n = D, T, Q, and 5) basis sets of Peterson et al.28 With the pseudopotential for the Se atom, 10 core electrons (1s22s22p6) are embodied in the effective core. In the text below, we will simply use DZ, TZ, QZ, and 5Z to denote the aug-cc-pVnZ (n = D, T, Q, and 5, for H and O) and aug-cc-
3. RESULTS AND DISCUSSION 3.1. Structures and Energies of Stationary Points. The potential energy surface for the H2Se + OH → SeH + H2O reaction is illustrated in Figure 1. The geometries and relative B
DOI: 10.1021/acs.inorgchem.8b03140 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry energies for the five stationary points on the potential surface are discussed below. 3.1.1. Reactants (H2Se and OH). With regard to the reactants (H2Se and OH), the geometric parameters obtained with the CCSD(T) method are convergent with the size of basis set. The CCSD(T) Se−H distances in H2Se are predicted to be 1.476, 1.467, and 1.465 Å, and the H−Se−H bond angle is 91.0°, 90.8°, and 90.9° with the DZ, TZ, and QZ basis sets, respectively (Figure 2). The predicted results are in good agreement with the
hemibonded isomer and two hydrogen-bonded isomers. We name these isomers RC-A, RC-B, and RC-C as was done for the H2S + OH reaction. However, the energetic order of these structures for the H2Se + OH reaction (Table 1) is different from that for the H2S + OH reaction. The hydrogen-bonded isomer H2Se···HO (RC-A) in its 2A′ electronic state has a hydrogen bond between the Se atom in H2Se and the H atom in the OH radical (Figure 3). The dissociation energy of RC-A with respect to the reactants (H2Se and OH) is predicted to be ∼3.0 kcal/mol. For comparison, the dissociation energy for the corresponding complex H2S···HO is ∼3.3 kcal/mol),5 and that for complex H2O···HO is much greater, 5.7 kcal/mol.7 The smaller dissociation energy for H2Se···HO (RC-A) correlates with the electronegativity of Se (2.4) being smaller than those of S (2.5) and O (3.5), and thus, the hydrogen bonding between Se and H is weaker. Although the corresponding complex (RC-A) in the H2S + OH reaction and that (CP1) for the H2O + OH reaction are global minima,5,7 the complex RC-A in this study (H2Se···HO) is not, and more specifically, it lies above RC-B by 2.1 kcal/mol (5Z). For the H2Se + OH reaction, the other entrance complex, the hemibonded isomer RC-B (2A′ ground state), is the global minimum, found to be a 2c-3e hemibonded H2Se···OH complex (Figure 3). Namely, the Se atom and O atom share 3 electrons, with the net Se···O bond order being 0.5. The H2Se···OH distance in RC-B is 2.418, 2.361, and 2.327 Å with the DZ, TZ, and QZ basis sets, respectively. The dissociation energy with respect to the reactants (H2Se and OH) is 4.30, 4.66, 5.04, and 5.14 kcal/mol with DZ, TZ, QZ, and 5Z basis sets, respectively. Analogous cases have been considered in the study by Joshi et al. of the bonding in neutral and cationic dimers of H2Se with H2A (A = O, S, Se).37 While the H-bond is the most favored for the neutral H2Se···HOH/HSH/HSeH dimers, the 2c-3e hemibonded (H2Se···SeH2)+ and (H2Se···SH2)+ structures are lowest in energy for the cationic dimers.37 This Joshi result was
Figure 2. Optimized geometries of the reactants (H2Se and OH) and products (H2O and SeH). All bond distances are in Å. The experimental geometries are listed at the top.
experimental results of re = 1.459 Å and θe = 91.0°.35 The bond distances for the OH radical are reported to be 0.980, 0.973, and 0.971 Å using the DZ, TZ, and QZ basis sets, respectively (Figure 2), and are well converged toward the experimental measurement of 0.9697 Å.36 3.1.2. Entrance (Reactant) Complex. Analogous to the H2S + OH → SH + H2O reaction,5 there exist three stationary points associated with the entrance (reactant) complex for the H2Se + OH → SeH + H2O reaction. Figure 3 and Table S1 show the optimized geometries of the three structures, including one
Figure 3. Optimized geometries of the reactant complexes and the product complexes with the CCSD(T) method. All bond distances are in Å. C
DOI: 10.1021/acs.inorgchem.8b03140 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry Table 1. Relative Energies (kcal/mol) for the Isomers of the Reactant Complexes and Product Complexes methods
RC-A
RC-B
RC-C
PC-A
PC-B
PC-C
CCSD(T)/aug-cc-pVDZ-PP CCSD(T)/aug-cc-pVTZ-PP CCSD(T)/aug-cc-pVQZ-PP CCSD(T)/aug-cc-pV5Z-PP
0.94 1.44 1.94 2.09
0.00 0.00 0.00 0.00
2.71 3.19 3.65 3.79
0.42 0.27 0.18 0.17
0.84 0.55 0.46 0.41
0.00 0.00 0.00 0.00
Figure 4. CCSD(T) optimized geometries of the transition states TS1 and TS2 for the H2Se + OH → H2O + SeH reaction. All bond distances are in Å.
Figure 5. Two possible pathways without ZPVE corrections for the H2Se + OH reaction at the CCSD(T)/aug-cc-pV5Z-PP single point level of theory with the QZ optimized geometries.
3.1.3. Transition State. An early transition state structure TS1 for the title reaction was located using the CCSD(T) method with the DZ, TZ, and QZ basis sets (Figure 4 and Table S2). This transition state is geometrically related to that for the H2S + OH reaction.5 With the increase of basis set size, this TS1 structure sees the hydrogen bond distance (HSeH···OH) vary from 1.646 to 1.666 Å and the ∠H···O−H bond angle from 103.5° to 103.2°. The ∠Se−H···O bond angle for this species is predicted to be 128.8°, slightly smaller than the ∠S···H···O angle of 137.5° for the analogous transition state for the H2S + OH reaction.5 Structure TS1 is confirmed by the IRC analysis to connect the two minima RC-B (entrance complex) and PC-B (exit
then supported by the recently developed projected Hartree− Fock study of Scuseria and coworkers,38 which also predicted the hemibond isomer to be the lowest energy isomer for the (H2Se)2+ radical cation system. Another hydrogen-bonded isomer HSeH···OH, RC-C in its 2 A′′ state has a hydrogen bond between the H atom in H2Se and O atom in OH radical (Figure 3). This configuration with Cs symmetry is comparable to the complexes HSH···OH (RC-C in ref 5) and HOH···OH (CP2 in ref 6). Structure RC-C is higher than the lowest-energy isomer RC-B by 2.7, 3.2, 3.6, and 3.8 kcal/mol with DZ, TZ, QZ, and 5Z basis sets, respectively. In general, the energy differences among the three isomers are of the order of a hydrogen bond (within 4 kcal/mol). D
DOI: 10.1021/acs.inorgchem.8b03140 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Table 2. Harmonic Vibrational Frequencies (cm−1) and ZPVEs (kcal/mol) for the Stationary Points of the H2Se + OH Reaction from the CCSD(T) Method with the aug-cc-pVnZ-PP (n = D, T, Q) Basis Sets with aug-cc-pVTZ Basis Sets for Comparisona ZPVE
ΔZPVE
ΔE
H2Se + OH RC-B TS1 PC-B H2O + SeH
13.72 16.10 14.61 17.60 16.77
0.00 2.38 0.89 3.88 3.05
0.00 −4.30 −1.40 −41.20 −39.08
0.00 −1.92 −0.51 −37.32 −36.04
H2Se + OH RC-B TS1 PC-B H2O + SeH
13.79 16.32 14.76 17.70 16.84
0.00 2.53 0.97 3.91 3.05
0.00 −4.66 −1.66 −42.37 −40.25
0.00 −2.13 −0.69 −38.46 −37.19
H2Se + OH RC-B TS1 PC-B H2O + SeH
13.83 16.43 14.85 17.76 16.92
0.00 2.60 1.01 3.93 3.09
0.00 −5.04 −1.76 −42.82 −40.72
0.00 −2.44 −0.75 −38.89 −37.63
H2Seb H2Sec OHd H2Ob SeHd H2Se + OH RC-B TS1 PC-B H2O + SeH
vibrational frequencies ω
ΔEZPVE CCSD(T)/aug-cc-pVDZ-PP 2432 2418 1060 3708 2441 2427 3688 2424 1956 3898 3779 2413 3905 3787 1638 CCSD(T)/aug-cc-pVTZ-PP 2438 2424 1062 3743 2450 2436 3718 2431 2007 3912 3803 2421 3920 3811 1646 CCSD(T)/aug-cc-pVQZ-PP 2445 2431 1060 3764 2457 2442 3737 2438 2038 3932 3822 2428 3941 3831 1650 experimental results 2454 2439 2358 2345 1034
(H2Se) 1050 1007 1635 (H2O)
708 605 190
3684 367 252 134 2398
(OH) 356 170 130 (SeH)
155 118 67
52 569i 66
(H2Se) 1052 1014 1643 (H2O)
739 597 208
3718 385 256 131 2405
(OH) 383 185 131 (SeH)
164 116 68
63 560i 65
(H2Se) 1048 1014 1647 (H2O)
757 596 210
3739 400 257 130 2413
(OH) 400 186 126 (SeH)
178 118 68
47 553i 61
(OH) 397 192 130 (SeH)
164 119 68
78 573i 68
3738 3943
3832
1649 2400
13.81 16.42 14.77 17.73 16.86
0.00 2.61 0.96 3.91 3.04
0.00 −4.75 −1.56 −41.88 −39.72
CCSD(T)/aug-cc-pVTZ (all-electron basis sets) 0.00 2448 2436 1059 (H2Se) −2.14 3744 2461 2447 1045 −0.61 3718 2442 1989 1009 −37.97 3912 3803 2432 1644 −36.67 3920 3811 1646 (H2O)
751 605 212
3718 398 257 131 2415
a Relative energies are given with and without ZPVE corrections (in kcal/mol). bHarmonic frequencies in ref 41. cFundamental frequencies in ref 40. dHarmonic frequencies in ref 36.
3.1.4. Exit (Product) Complex. Like the entrance complex, there are three stationary points associated with the exit (product) complex (Figure 3 and Table S3). The relative energies of these structures are reported in Table 1. The CCSD(T) method predicts that the hydrogen-bonded structure HOH···SeH (PC-C) in its 2A″ electronic state has the lowest energy, but the energy differences among the three isomers are small (
