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A theoretical study on the mechanism of the oxidation of hydroxylamine by VO2+
Journal of Molecular Structure: THEOCHEM 805 (2007) 143–152 www.elsevier.com/locate/theochem
A theoretical study on the mechanism of the oxidation of hydroxylamine by VO2þ Dianyong Tang, Liangfang Zhu, Song Qin, Zhishan Su, Changwei Hu
*
Key Laboratory of Green Chemistry and Technology, MOE, College of Chemistry, Sichuan University, Chengdu, Sichuan 610064, PR China Received 4 October 2006; received in revised form 6 November 2006; accepted 7 November 2006 Available online 14 November 2006
Abstract The oxidation of hydroxylamine by VO2 þ to the primary product HNO in gas phase were investigated with density functional theory (DFT) at the B3LYP/6-311++G(d,p) level. Calculations including geometry optimization and vibrational analysis for the stationary points on the ground and the first excited states were performed. Two possible mechanisms were investigated. The fist one is the oneelectron mechanism through firstly the formation of NH2O and NHOH radical complexes ([VO(OH)(NH2O)]+ and [VO(OH)(NHOH)]+) from NH2OH and VO2 þ , and then the oxidation of the stable intermediate NH2O by VO2 þ to produce the products HNO and VO(OH)+. The one-electron mechanism is predicted to be spin-conserved and the rate-limiting step is the cleavage of the O–H bond with 27.92 kcal/mol energy barrier. The other one is the two-electron mechanism in which the first half is also the formation of NH2O and NHOH radical complexes and the second half is the intra-molecular hydrogen transfer of NH2O and NHOH radical complexes to the product HNO together with the reduction of VIV–VIII. The crossing point between singlet and triplet potential energy surfaces (PESs) results in the stable product triplet VðOHÞ2 þ in the two-electron mechanism. The two-electron mechanism may be kinetically competitive with the one-electron mechanism only if the spin inversion between singlet and triplet PESs occurs easily, while, the one-electron mechanism is energetically more favorable than the two-electron mechanism. Therefore, the one-electron mechanism is predominant. 2006 Elsevier B.V. All rights reserved. Keywords: Hydroxylamine; Oxidation, VO2 þ ; Density functional theory; Mechanism
1. Introduction Hydroxylamine plays an important role in modern industrial chemistry due to its wide range of chemical properties. One of the most attractive aspects is its reactivity with metal oxides, especially the transition metal oxides. Hydroxylamine was also used as reductant to manufacture low-valent transition metal oxides [1,2]. The oxidation of NH2OH is complicated because it can function as one or two-electron donor and form a variety of products such as N2, N2O, NO+, etc., depending on the oxidant, the pH, and the medium. Many reactions concerning the metal ions and metal oxides with hydroxylamine had been studied by various experimental methods *
Corresponding author. Tel./fax: +86 28 85411105. E-mail addresses: [email protected], [email protected] (C. Hu). 0166-1280/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2006.11.008
[3–6]. Johnson et al. investigated the kinetics and mechanism of the ferrate oxidation of hydroxylamine and suggested that this reaction proceeded through a twoelectron oxidation [4]. Kumasaki reported the reactions between hydroxylamine and Cr3+, Cr6+, Mn7+, Co3+, Cu2+ via UV–vis technique and made it clear that these reactions were strongly exothermic.5 Alluisetti et al. investigated the reaction mechanism of NH2OH with [FeIII(CN)5H2O]2 at different conditions and proposed a radical mechanism [6]. Fattah et al. made kinetic and mechanistic investigations on the reaction between VV and NH2OH in aqueous solution and suggested a one-electron mechanism [3]. The proposed one- and two-electron pathways are shown in Scheme 1 [4]. Although so many experimental studies on the reactions of transition metal and transition metal oxides with hydroxylamine had been performed, the detailed mechanism for the oxidation of hydroxylamine is still unclear.
144
A
D. Tang et al. / Journal of Molecular Structure: THEOCHEM 805 (2007) 143–152 The first stage of both one- and two-electron mechanisms
VO 2+
B
[VO(OH)] + NH2O
[VOOH(NHOH)]+
+ [VO(OH)] + NHOH (2)
(1)
The second stage of the one-electron mechanism +
[VO(OH)]+ + HNO
+
+
VO 2 + NHOH
D
[VOOH(NH2O)]+ + NH2OH
VO 2 + NH2O
C
+
[VO(OH)] + HNO
(3) (4)
The second stage of the two-electron mechanism [VOOH(NH2O)]+
[VO(H2O)(HNO)]+
[VOOH(NHOH)]+
[VO(OH)2(HNO)]+
VO ++H2O+HNO
(5)
+
[V(OH)2] +HNO (6)
[22,23] calculations were performed in both directions to connect these corresponding intermediates at the above level. A step of 0.1 amu1/2 Bohr was used in the IRC procedure. Natural charges were calculated by the natural population analysis at the same level as the one used for geometry optimization [24,25]. Unless otherwise specified, the natural charges obtained by natural population analysis (NPA) were used in the following discussions. Wave function stability calculations were performed to confirm that the calculated wave functions corresponded to the ground state [26–28]. All the calculations reported in the present work were carried out with Gaussian 03 package [29].
The formation of the final products 2NH2O
N2 + 2H2O (7)
2HNO
N2 O + H2O (8)
Scheme 1. The proposed mechanism of NH2OH oxidized by VO2 þ .
A combination of experimental and computational studies of the reaction of metal oxides with hydroxylamine in the gas phase can enhance our understanding of the elementary processes occurring in real conditions. As stated elsewhere, the vanadium existed as VO2 þ cation with the pH around 1.0 [7–9]. Therefore, the theoretical studies based on density functional theory on the hydroxylamine oxidized by VO2 þ in the gas phase were performed for the previously suggested one- and two-electron mechanisms to clarify the detailed mechanism (Scheme 1) [4]. Both the ground and the first excited states were investigated because the reaction of transition metal oxides may involve low-lying excited electronic states [10–12]. All the pathways are terminated at the primary product HNO because the reaction mechanism of the dimerization of the primary product HNO to form final product N2O had been well studied at MP2/6-31G(d,p) level [13]. The present computational study is, to the best of our knowledge, the first detailed comprehensive theoretical mechanistic investigation of the complete reaction mechanism for hydroxylamine oxidized by transition metal oxides. The primary focus of the present study is to understand the mechanism of the hydroxylamine oxidized by VO2 þ in the gas phase and furthermore some hints on the condensed reaction.
2. Computational details Geometry optimizations as well as frequency calculations for all the stationary points considered here were performed at the density functional level of theory, employing the hybrid B3LYP functional, [14–16] together with the 6-311++G(d,p) basis set [17–21]. For each optimized stationary point, vibrational analysis was performed to determine its character (minimum or saddle point) and to obtain the zero-point vibrational energy (ZPVE) correction. For each transition state, intrinsic reaction coordinates (IRC)
3. Results and discussions In the present paper, only the oxidation of hydroxylamine leading to the primary product (HNO) was investigated. As shown in Scheme 1, the first stage of both one- and two-electron mechanisms is the activation of the O–H or N–H bond in hydroxylamine leading to NH2O or NHOH radical complexes [VO(OH)(NH2O)]+ or [VO(OH)(NHOH)]+. The second half of the one-electron mechanism is the oxidation of NH2O radical by VO2 þ to give rise to products HNO and [VO(OH)]+. The second stage of the two-electron mechanism is the intramolecular oxidation–reduction of [VO(OH)(NH2O)]+ or [VO(OH)(NHOH)]+ through hydrogen transfer. In the following sections, firstly, the formation of NH2O or NHOH radical complexes is demonstrated. Secondly, the oxidation of NH2O radical by VO2 þ and the intra-molecular oxidation–reduction of [VO(OH)(NH2O)]+ or [VO(OH) (NHOH)]+ are discussed. Lastly, the comparison of oneand two-electron mechanisms is presented. The prefixes ‘s’,‘d’, ‘t’, and ‘q’ are used to denote the structures in the singlet, doublet, triplet, and quartet electronic states, respectively. 3.1. The formation of NH2O and NHOH radical complexes from NH2OH and VO2 þ The optimized structures of various species on the singlet and triplet states potential energy profiles are depicted in Fig. 1 and the relative energies of various species and energy diagram along singlet and triplet reaction pathways at B3LYP/6-311++G(d,p) level are shown in Fig. 2. As shown in Fig. 1, the first step of both singlet and triplet states is the coordination of hydroxylamine to vanadium center of VO2 þ to generate the g2-N, O intermediates s-1, and t-1 without energy barrier. The s-1 stands below t-1 about 35.18 kcal/mol. The geometric parameters of the hydroxylamine moiety in s-1 and t-1 indicate that the main interaction between hydroxylamine and VO2 þ moieties is electrostatic. A partial positive charge transfer, 0.30 au for both s-1 and t-1, takes places from the VO2 þ fragment to the hydroxylamine fragment. The geometric parameters of s-1 and t-1 are very similar except the V–O
D. Tang et al. / Journal of Molecular Structure: THEOCHEM 805 (2007) 143–152
145
Fig. 1. The optimized structures and selected parameters involved in the activation of the O–H and N–H bonds in both singlet and triplet states. Bond lengths are in angstrom and bond angles are in degree.
bond length and the O–V–O bond angle in the VO2 þ fragment (Fig. 1). The N–H and O–H bonds in s-1 and t-1 are longer than those in the free hydroxylamine, indicative of their activation. This activation is in favor of the subsequent cleavage of the O–H and N–H bonds. Interestingly, the N–O bond lengths in s-1 and t-1 are slightly shorter than that in the free hydroxylamine while the Wiberg bond indexes of the N–O bond are slightly decreased. Thus the activation of the N–O bond is not obvious and the subsequent cleavage of the N–O bond is not feasible. Subsequently, a hydrogen transfer from O3 or N to O1 in s-1 and t-1 occurs (Fig. 1). The O–H bond activation via s-TS1/2 and t-TS1/2 in s-1 and t-1 produces intermediates s/t-2, respectively. Meanwhile, the s-1 and t-1 leads to s-3 and t-3 and s-4 through the activation of the N-H bond with the four-center transition states s-TS1/3, t-TS1/3 and
s-TS1/4, respectively. The imaginary frequencies for these TSs are between 1750i and 1810i cm1 except 1275i cm1 for t-TS1/2. These high frequencies are a direct consequence of the O–H and N–H bonds cleavage as well as the O–H bond formation, as the eigenvector’s coordinates of those imaginary frequencies suggest. Simultaneously, the related bond distances of these transition states are reasonably responsible for the cleavage of the O–H and N–H bonds and the formation of the O–H bond (Fig. 1). All of the five reaction channels are strongly exothermic. The energy barriers of triplet reaction channels are 20.16 (t-TS1/2) and 24.96 (t-TS1/3) kcal/mol and those of singlet reaction channels are 27.92 (s-TS1/2), 38.27(s-TS1/3), and 40.21(s-TS1/4) kcal/mol. It suggests that the triplet reaction channels are more favorable than the singlet reaction channels kinetically. The values of the relative energies of
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Er(kcal/mol) +
t-VO2 +d-NH2O 33.40
Er (kcal/mol) +
t-VO2 +NH2OH +
33.40
q-VOOH +HNO 24.60
+
s-VO2 +d-NH2O 0.00
q-TS1/2 -10.89
s-VO2++NH2OH t-TS1/3 -9.91 t-TS1/2 -14.71
0.00
+
VOOH +NHOH -14.82 + VOOH +NH2O
s-TS1/4 t-1 -34.87
-25.00
-29.84 s-TS1/3 -31.78
d-1 -42.76 d-3 -63.17
s-TS1/2 -42.13 s-1
q-1 -27.59
t-3 -67.53
d-TS1/2 -21.30
d-TS3/4 -41.28
+
d-VOOH +HNO -26.66
q-2 -52.52 d-4 -64.13 d-2 -77.89
-70.05 s-4 -70.85 s-3 -75.08 t-2 -81.24 s-2 -95.90
Fig. 2. The total energy profile of the activation of the O–H and N–H bonds in singlet and triplet states, relative to the reactants at the singlet state. The total energy for the separated reactants is 1225.901903 Hartrees at the B3LYP/6-311++G(d,p) level.
the highest TSs for all singlet reaction channels are smaller than those of the triplet reaction channels. Therefore, the singlet pathways are energetically favorable. So the singlet
Fig. 4. The total energy profile of the oxidation of NH2O radical by VO2+ in doublet and quartet states, relative to the reactants at the doublet state. The total energy for the separated reactants is 1225.288115 Hartrees at the B3LYP/6-311++G(d,p) level.
PES is exclusively relevant to this step. The activation of the O–H bond is kinetically and energetically easier than that of the N–H bond in both singlet and triplet states (Fig. 2), which is in line with the bond energy order of the O–H and N–H bonds in hydroxylamine in the previous study (75.5 and 84.0 kcal/mol for the O–H and N–H bonds, respectively) [30]. The natural population analysis (NPA) indicates that all these hydrogen transfers can be considered as proton transfer because the positive charges on the transferred hydrogen atoms are about 0.41 0.50 au. The structural parameters of t-VO2 þ and
Fig. 3. The optimized structures and selected parameters involved in the oxidation of NH2O radical by VO2+ in both doublet and quartet states. Bond lengths are in angstrom and bond angles are in degree.
D. Tang et al. / Journal of Molecular Structure: THEOCHEM 805 (2007) 143–152
the VO2 þ fragment in t-1 indicate that the triplet VO2 þ is actually [O@VIV–O]+. The hydrogen transfer step involves clearly the reduction of VV(O@V@O) to VIV (O@VAOH) by hydroxylamine on the singlet PES. While the hydrogen transfer on the triplet PES can be viewed as a diradical process which involves the electron transfer from oxygen atom of [O@VIVAO]+ fragment to oxygen and nitrogen atoms of NH2OH fragment. Namely, the triplet PES does not touch upon to the reduction of VV to VIV. The predicted spin density distributions strongly support this statement (0.98 and 1.00 au on V and O1 atoms in t1, respectively, while 1.12, 0.64, and 0.39 au on V, N, and O3 atoms in t-2, respectively). These differences result in the kinetic favorability of the triplet pathway. However, the high excitation energies between singlet and triplet species s-VO2 þ , t-VO2 þ , s/-1, and t-1) reflect that the singlet pathway is energetically more favorable than the triplet pathways. Finally, the intermediates s-2, s-3, s-4, t-2, and t-3 may decompose into [VO(OH)]+ and NH2O or NHOH radi-
147
cals. These processes are endothermic significantly. In other words, the NH2O and NHOH radicals are tightly bonded with [VO(OH)]+. The NH2O radical is more stable than NHOH about 10.18 kcal/mol and this is close to the previous values obtained at various levels [30–32]. So, the main product of hydroxylamine oxidation in this process is NH2O radical. Comparing the relative energies and energy barriers discussed above, it is seen that the most feasible pathway for the primary oxidation of hydroxylamine is the activation of the O–H bond via s-TS1/2 and t-TS1/2 in singlet and triplet states, respectively. The activation of the O–H bond can be viewed as a proton transfer process. The singlet state PES is exclusively relevant to this process. The most stable intermediate is s-2 and t-2 for singlet and triplet states, respectively, and they can decompose into [VO(OH)]+ and NH2O radical. The NH2O and NHOH radicals are tightly bonded with [VO(OH)]+. The NH2O radical is predicted to be the main product in this process.
Fig. 5. The optimized structures and selected parameters involved in the intra-molecular oxidation–reduction of [VO(OH)(NH2O)]+ in singlet state. Bond lengths are in angstrom and bond angles are in degree.
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3.2. The oxidation of NH2O radical by VO2 þ As presented above, the most feasible pathway gives birth to intermediate NH2O radical. So, only those pathways involving in the one-electron mechanism using NH2O radical as reactant were investigated. Due to the electron pairing between t-VO2 þ and NH2O radical, the quartet reactants may lead to the doublet intermediates. The optimized structures of various species on the doublet and quartet potential energy profiles are shown in Fig. 3. The relative energies of various species and energy diagrams along doublet and quartet reaction pathways at B3LYP/6-311++G(d,p) level are shown in Fig. 4. Initially, the coordination of NH2O radical to VO2 þ leads to g1-O (d-3 and q-1) or g2-N, O (d-1) intermediates. Any attempt to locate the quartet g2-N, O complex was failed, which may be caused by the Pauli’s repulsion between the lone pair electron of nitrogen atom and the single electron on V center. The formation of d-1, d-3, and q-1 are quite exothermic. Interestingly, the single electron localizes on oxygen atom of the VO2 þ moiety in d-1. Therefore, d-1 may be viewed as an adduct of spin-pairing t-VO2 þ and NH2O. The geometric parameters of d-3 and q-1 are very similar except the V–O1 bond length. The N–O3 bond obviously elongates with the formation of d1 (Fig. 3), which may be caused by the electron transfer from the lone-pair electron on N and the single electron on O3 to V center. In d-3 and q-1, the geometric parameters
of NH2O moiety are analogous to those of the free NH2O radical. The N–H bonds are slightly activated in the coordination process. The d-3 is more stable than d-2 about 20.39 kcal/mol. Then, intermediate d-1 gives rise to g2-N,O intermediate d-2 through four-center transition state d-TS1/2, while intermediates d-3 and q-1 transform to g1-O intermediate d-4 and q-2 via five-center d-TS3/4 and q-TS1/2. Intermediates d-TS3/4 and d-4 are similar to intermediates q-TS1/2 and q-2 with respect to the geometric parameters, respectively. The transferred hydrogen atoms are also positively charged, in the range of 0.41–0.46 au, therefore, high imaginary frequencies (1902i, 1396i and 1952i cm1 for d-TS1/2, d-TS3/4 and q-TS1/2) are obtained. Intermediates d-1, d-3 and q-1 must surmount 21.46, 22.09 and 16.70 kcal/mol energy barriers to reach d-TS1/2, d-TS3/4 and q-TS1/2 to complete this reaction. These proton transfer processes are predicted to be quite exothermic besides d-3 fi d-4 (Fig. 4). In virtue of the same reason resulting from the difference between the s-VO2 þ and t-VO2 þ , the quartet PES is more favorable than the doublet PES kinetically while the doublet PES is more feasible than the quartet PES energetically because the stationary points on the quartet PES are always higher than those on the doublet PESs. So the doublet PES is exclusively involved in this cleavage of the N–H bond. Lastly, products HNO and d-[VO(OH)]+ and q-[VO(OH)]+ are obtained directly from intermediates d-2, d-4 and q-2. These processes are exceedingly endothermic. The products
Fig. 6. The optimized structures and selected parameters involved in the intra-molecular oxidation–reduction of [VO(OH)(NH2O)]+ in triplet state. Bond lengths are in angstrom and bond angles are in degree.
D. Tang et al. / Journal of Molecular Structure: THEOCHEM 805 (2007) 143–152
of doublet and quartet states stand below their corresponding reactants. So the whole reactions are feasible for doublet and quartet states. Based on the above discussions, only the doublet PES is relevant to the oxidation process of NH2O radical by VO2 þ . The most feasible pathway is s-VOþ 2 þ d-NH2 O ! d-3 ! d-TS3=4 ! d-4 ! d-½VOðOHÞþ þ HNO. Combined Sections 3.1 and 3.2, the one-electron mechanism mostly proceeds through the following pathway: s-VOþ 2 þ þ NH2 OH ! s-1 ! s-TS1=2 ! s-2 ! d-VOðOHÞ þ d-NH2 O and then s-VO2 þ d-NH2 O ! d-3 ! d-TS3=4 ! d-4 ! þ d-VOðOHÞ þ HNO. The whole one-electron mechanism, relative to the reactants VO2 þ þ NH2 OH, is exothermic by 51.66 kcal/mol. The rate-limiting step for the one-electron mechanism is the first hydrogen transfer step with 27.92 kcal/ mol energy barrier. The reaction easily occurs because all the stationary points on the PESs are below the separated reactants (Figs. 2 and 4). 3.3. The intra-molecular oxidation–reduction of [VO(OH)(NH2O)]+ The second half of the two-electron mechanism is that the intermediates s-2, s-3, s-4, t-2, and t-3 give rise to the final product HNO and s/t-VO+ or s/t-[V(OH)2]+ via intra-molecular hydrogen transfer. Similar to the one-elec-
149
tron mechanism, the following discussions will focus on those pathways originating from NH2O complexes s-2 and t-2. The optimized structures of various species on the singlet and triplet states potential energy profiles deriving from s-2 and t-2 are shown in Figs. 5 (singlet) and 6 (triplet), and the relative energies of various species and energy diagram along singlet and triplet reaction pathways at B3LYP/6-311++G(d,p) level are shown in Fig. 7. Those pathways deriving from s-3, s-4 and t-3 will not be discussed. While the optimized structures of various species on the singlet and triplet potential energy profiles deriving from s-3, s-4 and t-3 are supplied in supporting information (Fig. S1–S3) and the relative energies of various species and energy diagrams along singlet and triplet reaction pathways at B3LYP/6-311++G(d,p) level are given in Fig. S4 [33]. From the minimum s-2, it is possible to continue the two-electron mechanism with a step associated with the cleavage of the V–N bond via s-TS2/5 to render the minimum s-5. Along s-2 fi s-TS2/5 fi s-5, the V–N bond inchmeal lengthens while the V–O3 and N–O3 bonds gradually shorten. The vibrational mode of the sole imaginary frequency (332i cm1) and IRC calculation for sTS2/5 indicates that the transition state actually connects s-2 and s-5. The energy barrier is predicted to be 22.68 kcal/mol and the formation of s-5 is endothermic
s-TSinter+HNO 36.89 s-VO ++H2O+HNO 36.06
Er(kcal/mol) t-VO2+ +NH2OH 33.40
t-TSinter+HNO 13.72 t-VO+ +H 2O+HNO 11.74
s-VO2+ +NH2OH 0.00
s-VO(H 2O) ++HNO s-TS6/7 -12.54 t-TS4/5 -27.46 s-TS8/9 -34.61
t-TS1/2 -14.71 t-1 -34.87
VOOH++NH 2O
s-TS1/2
-25.00 s-TS5/7 -46.36 s-TS2/9 -48.71
-42.13
s-TS5/6 -49.40 s-TS2/8 -56.69 t-TS2/5 -57.83 t-TS2/4 -64.83 s-1 s-TS2/5 -73.22
-70.05
CP1 s-5 -80.70 t-2 -81.24
-7.60
-12.18 s-14+H O 2 t-VO(H 2O)+ +HNO -29.76
s-V(OH) 2++HNO -17.29 t-V(OH) 2+ +HNO -28.56
s-6 -52.40
s-8 -63.75 t-4 -64.61
s-7 -58.57
t-5 -75.46 s-9 -77.95
s-2 -95.90
Fig. 7. The total energy profile deriving from s-2 and t-2 of two-electron mechanism in singlet and triplet states, relative to the reactants (s-VO2 þ s-NH2 OH) at the singlet state. The total energy for the separated reactant is 1225.901903 Hartrees at the B3LYP/6-311++G(d,p) level.
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D. Tang et al. / Journal of Molecular Structure: THEOCHEM 805 (2007) 143–152
by 15.20 kcal/mol. The geometric parameters of s-5 are very similar to those of t-2. And the t-2 stands below s-5 about 0.54 kcal/mol. So there may exist a PES crossing point (CP1) between singlet and triplet states. The existence of CP1 opens the possibility for an intersystem crossing to take place. Comparing the relative energies and geometric parameters, we can conclude that the structure of CP1 may be very close to s-5 and t-2 [34]. The CP1 locates in the exit channel so it is not kinetically relevant. A hydrogen transfer via five-center transition states s-TS5/6 and s-TS5/7 forms the intermediates s-6 and s-7 from s-5. While the triplet state intermediates t-4 and t-5 are generated from t-2 through hydrogen transfer transition states, t-TS2/4 and t-TS2/5, between N and O atoms (O1 or O2). Additionally, the s-2 can also generate g2-N, O intermediates s-8 and s-9 passing through four-center hydrogen transfer transition states s-TS2/8 and s-TS2/9, respectively. All these hydrogen transfer processes can also be viewed as proton transfer because all the transferred hydrogen atoms in transition states are positively charged in the range of 0.46–0.49 au. As expected, the high imaginary frequencies in the range of 920i– 1630i cm1 are obtained. The transition states t-TS2/4 and t-TS2/5 on the triplet PES are more stable than its corresponding transition states s-TS5/6 and s-TS5/7 on the singlet PES, and also more stable than s-TS2/8 and s-TS2/9. The energy barriers are 31.3, 34.34, 39.21, 47.19, 16.41, and 23.41 kcal/mol for s-TS5/6, s-TS5/7, s-TS2/8, s-TS2/9, t-TS2/4, and t-TS2/5, respectively. Therefore, the triplet state reaction pathways are kinetically and energetically favorable. As shown in Fig. 7, the dihydroxyl intermediates s-7, s-9, and t-5 stand below their corresponding intermediates s-6, s-8, and t-4, respectively. In addition, intermediates s-6, s-8, and t-4 transform to intermediates s-7, s-9, and t-5 through s-TS6/7, s-TS8/9, and t-TS4/5 with 39.86, 29.14, and 37.15 kcal/ mol energy barriers, respectively. All the three transition states are characterized by positively charged transferred hydrogen (around 0.52 au) and high imaginary frequency in the range of 1800–1870 cm1. The geometry of water in s-6, s-8 and t-4 is almost undisturbed compared to the free water. It indicates that the interaction between H2O and [VO(HNO)]+ moieties is predominantly electrostatic and this is in agreement with the previous study [35]. Finally, various products such as s-VO+ and t-VO+, HNO, H2O, etc. are formed from intermediates s-6, s-8 and t-4 (Figs. 5 and 6). The products s-[V(OH)2]+, t-[V(OH)2]+ and HNO are obtained from intermediates s-7, s-9, and t-5. The relative energies of the separated products s-VO+, t-VO+, HNO, and H2O are higher than those of the separated singlet reactants while the separated products s-[V(OH)2]+, t-[V(OH)2]+, and HNO lie below the separated singlet reactants. Moreover, the separated triplet products are more stable than the corresponding singlet products. As depicted in Fig. 7, s-[VO(H2O)]+ and t-[VO(H2O)]+ can be converted to s-VðOHÞ2 þ and t-VðOHÞ2 þ via s-TSinter and t-TSinter with 44.49 and
43.48 kcal/mol energy barriers and this is in agreement with the literatures [36,37]. Let us now consider the difference between the singlet and triplet states of the intra-molecular hydrogen migration. The singlet pathways proceed through the reduction of the close-shell VIV species [VIVO(OH)(NH2O)+, d1 state] to close-shell VIII species (d2). While the triplet pathways involve the transform of the open-shell VIV species to the open-shell VIII species associating with the single electron on NH2O moiety transferring to d orbital of vanadium center. The variation of the spin densities of vanadium clearly indicates that the single electron on vanadium is little relevant to the t-2 fi t-4/5 process. Additionally, the triplet pathways are obviously more feasible than the singlet pathways because the high-spin (HS) d2 state of vanadium is more stable than the low-spin (LS) d2 state in the gas phase. Till now, two reaction channels that connect the most stable reactants s-VO2 þ þ NH2 OH with products s[V(OH)2]+ + HNO (no spin inversion) and t-[V(OH)2]+ + HNO (spin inversion), through s-1 fi s-TS1/2 fi s2 fi s-TS2/6 fi s-TS6/7 fi s-7 and s-1 fi s-TS1/2 fi s-2 fi s-TS2/5 fi t-2 fi t-TS2/4 fi t-4 fi t-TS4/5 fi t-5, respectively, are obtained. For the first pathway, the ratelimiting step is the cleavage of the N–H bond in s-2 in the second hydrogen transfer step with an energy barrier of 39.29 kcal/mol. The rate-limiting step is the breakage of the O–H bond in the first hydrogen transfer process for the spin inversion pathway. All the stationary points on the two reaction channels lie below the separated reactants. So these reactions proceed easily. The spin-inversion pathway can be chemically expressed as the LS-VV (d0, s-1) fi LS-VIV (d1, s-2) fi HS-VIV (d1, t-2) fi HS-VIII (d2, t-4) process. The single electron on vanadium is rarely interrelated for the bond cleavage and formation processes in the triplet PES. 3.4. The comparison of one- and two-electron mechanisms As presented above, the one- and two-electron mechanisms are initiated from the activation of the O–H or N– H bond in hydroxylamine to form the NH2O and NHOH radical complexes. The singlet PES is exclusively relevant to the activation of the O–H and N–H bonds. The activation of the O–H bond is easier than that of the N–H bond both energetically and kinetically. The s-2, namely, [VO(OH)(NH2O)]+, is the global minimum on the singlet PES. Thus NH2O radical is the main intermediate product. For both the two mechanisms, the stationary points on the PESs lie below the separated reactants and so these reactions occur easily. The one-electron mechanism mainly proceeds on the ground state (singlet for the formation of NH2O radical and doublet for the oxidation of NH2O radical). The two-electron mechanism may take place at hypersurface of singlet and triplet PESs via spin inversion because the most stable products and reactants reside on triplet and
D. Tang et al. / Journal of Molecular Structure: THEOCHEM 805 (2007) 143–152
singlet PESs, respectively. From the kinetic viewpoint, the one- and two-electron mechanisms are competitive in the gas phase only if the spin inversion occurs easily. The NH2O radical oxidized by VO2 þ to products HNO and d-[VO(OH)]+ is quite downhill (Figs. 2 and 4). However, the PESs for s-2 and t-2 toward the products HNO and s-VO2 þ are uphill in all cases and must overcome high energy barriers (Fig. 7). So the one-electron mechanism is predicted to be predominant, that is, the VIV species should be the most stable. Simultaneously, previous studies [38– 41] considered the VIV species as the most stable state in aqueous hydroxylamine. So, the reaction between VO2 þ with NH2OH in aqueous solution may proceed also via the one-electron mechanism. 4. Conclusions The reaction PESs for hydroxylamine oxidized by VO2 þ had been computed and analyzed. Both low and high-spin PESs of one- and two-electron mechanisms had been characterized in detail at B3LYP/6-311++G(d,p) level of theory. Two possible mechanisms were investigated. The first one is the one-electron mechanism through firstly the formation of NH2O and NHOH radical complexes, and then the oxidation of the stable intermediate NH2O by VO2 þ leading to the final products HNO and VO(OH)+. The one-electron mechanism is predicted to be spin-conserved and the rate-limiting step is the cleavage of the O–H bond by VO2 þ with 27.92 kcal/mol energy barrier. The other one is the two-electron mechanism in which the first half is also the formation of NH2O and NHOH radical complexes and the second half is the intra-molecular hydrogen transfer of NH2O and NHOH radical complexes to form the product HNO with VIV reduced to VIII. The crossing point between singlet and triplet PESs results in the stable product triplet VðOHÞ2 þ in the two-electron mechanism. The two-electron mechanism can be chemically expressed as the LS–VV[VO2(NH2OH)+, d0] fi LS–VIV [VO(OH) (NH2O)+, d1] fi HS–VIV[VO(OH)(NH2O)+, d1] fi HS– VIII [V(OH)2(HNO)+, d2] process in the gas phase. The two-electron mechanism may be competitive with one-electron mechanism kinetically only if the spin inversion between singlet and triplet states PESs occurs easily, while, the one-electron mechanism is energetically more favorable than the two-electron mechanism. Therefore, the one-electron mechanism is predicted to be predominant. Simultaneously, these results may imply that the reaction between VO2 þ with NH2OH in aqueous solution proceeds via the one-electron mechanism. Acknowledgements The authors are grateful for the financial support of the National Natural Science Foundation of China (Nos. 200720024 and 20502017) and the Teaching and Research Award Program for Outstanding Young Teachers in Higher Education Institutions of MOE, P.R.C. (2002).
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Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.theochem.2006.11.008. References [1] V. Manivannan, J.B. Goodenough, Inorg. Chem. 37 (1998) 3448. [2] V. Manivannan, J.B. Goodenough, Mater. Res. Bull. 33 (1998) 1353. [3] A. Fattah, J. Mazer, C.F. Wells, J. Chem. Soc. Dalton Trans. (1980) 1532. [4] M. Johnson, B. Hornstein, Inorg. Chem. 42 (2003) 6923. [5] M.J. Kumasaki, Hazar. Mater. 115 (2004) 57. [6] G.E. Alluisetti, A.E. Almaraz, V.T. Amorebieta, F. Doctorovich, J.A. Olabe, J. Am. Chem. Soc. 126 (2004) 13432. [7] M. Jian, L. Zhu, J. Wang, J. Zhang, G. Li, C. Hu, J. Mol. Catal. A: Chem. 253 (2006) 1. [8] S.E. O’Donnell, T.P. Michael, J. Chem. Soc. Dalton Trans. (1976) 2290. [9] M.T. Pope, B.W. Dale, Quart. Rev. 22 (1968) 527. [10] D. Schro¨der, S. Shaik, H. Schwarz, Acc. Chem. Res. 33 (2000) 139. [11] P.B. Armentrout, J.L. Beauchamp, Acc. Chem. Res. 22 (1989) 315. [12] P.B. Armentrout, Science 251 (1991) 175. [13] K. Ruud, T. Helgaker, E. Uggerud, J. Mol. Struc. (THEOCHEM) 393 (1997) 59. [14] A.D. Becke, J. Chem. Phys. 98 (1993) 5648. [15] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1998) 785. [16] B. Miehlich, A. Savin, H. Stoll, H. Preuss, Chem. Phys. Lett. 157 (1989) 200. [17] R. Krishnan, J.S. Binkley, R. Seeger, J.A. Pople, J. Chem. Phys. 72 (1980) 650. [18] A.D. McLean, G.S. Chandler, J. Chem. Phys. 72 (1980) 5639. [19] A.J.H. Wachters, J. Chem. Phys. 52 (1970) 1033. [20] P.J. Hay, J. Chem. Phys. 66 (1977) 4377. [21] K. Raghavachari, G.W. Trucks, J. Chem. Phys. 91 (1989) 1062. [22] C. Gonzalez, H.B. Schlegel, J. Chem. Phys. 90 (1989) 2154. [23] C. Gonzalez, H.B. Schlegel, J. Phys. Chem. 94 (1990) 5523. [24] A.E. Reed, R.B. Weinstock, F. Weinhold, J. Chem. Phys. 83 (1985) 735. [25] E.D. Glendening, A.E. Reed, J.E. Carpenter, Weinhold, F. NBO Ver. 3.1, 1997. [26] R. Seeger, J.A. Pople, J. Chem. Phys. 66 (1977) 3045. [27] R. Bauernschmitt, R. Ahlrichs, J. Chem. Phys. 104 (1996) 9047. [28] H.B. Schlegel, J. McDouall, in: C. Ogretir, I.G. Kluwer Csizmadia (Eds.), Computational Advances in Organic Chemistry, Academic, The Netherlands, 1991, pp. 167–185. [29] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, J.A. Montgomery, Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. AlLaham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople, Gaussian 03, Reversion B.05; Gaussian Inc., Pittsburgh, PA, 2003. [30] D.A. Dixon, J.S. Francisco, Y. Alexeev, J. Phys. Chem. A 110 (2006) 185. [31] M.R. Soto, M. Page, M.L. McKee, Chem. Phys. Lett. 187 (1991) 335.
152
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[32] R. Sumathi, D. Sengupta, M.T. Nguyen, J. Phys. Chem. A 102 (1998) 3175. [33] The energetic pattern of those pathways originating from species s-3, s-4 and t-3 (Fig. S4) are similar to that of s-2 and t-2. However, CP2 and CP3 locating on PES open the possibility for an intersystem crossing to take place. CP2 and CP3 locate in the entrance channel so it is kinetically relevant. [34] The energy gap between t-2 and s-5 is smaller than the error of B3LYP functional. Therefore, the crossing point may locate in the domain (Fig. 7) that is the following hydrogen transfer step.
[35] L. Gracia, J. Andres, V.S. Safont, A. Beltran, Organometallics 23 (2004) 730. [36] G.K. Koyanagi, D.K. Bohme, I. Kretzschmar, D. Schro¨der, H. Schwarz, J. Phys. Chem. A 105 (2001) 4259. [37] A. Ricca, C.W. Bauschlicher, J. Phys. Chem. A 101 (1997) 8949. [38] G. Bengtsson, Acta Chem. Scan. (1947–1973) 27 (1973) 3053. [39] A.T. Pilipenko, O.I. Karpova, V.V. Lukachina, V.V. Trachevskii, Zhurnal Analiticheskoi Khimii 27 (1972) 78. [40] R. Benes, J. Novak, Collect. Czechoslovak Chem. Comm. 5 (1970) 3788. [41] G. Ramababu, P.V.S. Rao, K. Ramakrishna, P. Syamala, A. Satyanarayana, J. Ind. Chem. Soc. 78 (2001) 237.
A theoretical study on the mechanism of the oxidation of hydroxylamine by VO2þ Dianyong Tang, Liangfang Zhu, Song Qin, Zhishan Su, Changwei Hu
*
Key Laboratory of Green Chemistry and Technology, MOE, College of Chemistry, Sichuan University, Chengdu, Sichuan 610064, PR China Received 4 October 2006; received in revised form 6 November 2006; accepted 7 November 2006 Available online 14 November 2006
Abstract The oxidation of hydroxylamine by VO2 þ to the primary product HNO in gas phase were investigated with density functional theory (DFT) at the B3LYP/6-311++G(d,p) level. Calculations including geometry optimization and vibrational analysis for the stationary points on the ground and the first excited states were performed. Two possible mechanisms were investigated. The fist one is the oneelectron mechanism through firstly the formation of NH2O and NHOH radical complexes ([VO(OH)(NH2O)]+ and [VO(OH)(NHOH)]+) from NH2OH and VO2 þ , and then the oxidation of the stable intermediate NH2O by VO2 þ to produce the products HNO and VO(OH)+. The one-electron mechanism is predicted to be spin-conserved and the rate-limiting step is the cleavage of the O–H bond with 27.92 kcal/mol energy barrier. The other one is the two-electron mechanism in which the first half is also the formation of NH2O and NHOH radical complexes and the second half is the intra-molecular hydrogen transfer of NH2O and NHOH radical complexes to the product HNO together with the reduction of VIV–VIII. The crossing point between singlet and triplet potential energy surfaces (PESs) results in the stable product triplet VðOHÞ2 þ in the two-electron mechanism. The two-electron mechanism may be kinetically competitive with the one-electron mechanism only if the spin inversion between singlet and triplet PESs occurs easily, while, the one-electron mechanism is energetically more favorable than the two-electron mechanism. Therefore, the one-electron mechanism is predominant. 2006 Elsevier B.V. All rights reserved. Keywords: Hydroxylamine; Oxidation, VO2 þ ; Density functional theory; Mechanism
1. Introduction Hydroxylamine plays an important role in modern industrial chemistry due to its wide range of chemical properties. One of the most attractive aspects is its reactivity with metal oxides, especially the transition metal oxides. Hydroxylamine was also used as reductant to manufacture low-valent transition metal oxides [1,2]. The oxidation of NH2OH is complicated because it can function as one or two-electron donor and form a variety of products such as N2, N2O, NO+, etc., depending on the oxidant, the pH, and the medium. Many reactions concerning the metal ions and metal oxides with hydroxylamine had been studied by various experimental methods *
Corresponding author. Tel./fax: +86 28 85411105. E-mail addresses: [email protected], [email protected] (C. Hu). 0166-1280/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2006.11.008
[3–6]. Johnson et al. investigated the kinetics and mechanism of the ferrate oxidation of hydroxylamine and suggested that this reaction proceeded through a twoelectron oxidation [4]. Kumasaki reported the reactions between hydroxylamine and Cr3+, Cr6+, Mn7+, Co3+, Cu2+ via UV–vis technique and made it clear that these reactions were strongly exothermic.5 Alluisetti et al. investigated the reaction mechanism of NH2OH with [FeIII(CN)5H2O]2 at different conditions and proposed a radical mechanism [6]. Fattah et al. made kinetic and mechanistic investigations on the reaction between VV and NH2OH in aqueous solution and suggested a one-electron mechanism [3]. The proposed one- and two-electron pathways are shown in Scheme 1 [4]. Although so many experimental studies on the reactions of transition metal and transition metal oxides with hydroxylamine had been performed, the detailed mechanism for the oxidation of hydroxylamine is still unclear.
144
A
D. Tang et al. / Journal of Molecular Structure: THEOCHEM 805 (2007) 143–152 The first stage of both one- and two-electron mechanisms
VO 2+
B
[VO(OH)] + NH2O
[VOOH(NHOH)]+
+ [VO(OH)] + NHOH (2)
(1)
The second stage of the one-electron mechanism +
[VO(OH)]+ + HNO
+
+
VO 2 + NHOH
D
[VOOH(NH2O)]+ + NH2OH
VO 2 + NH2O
C
+
[VO(OH)] + HNO
(3) (4)
The second stage of the two-electron mechanism [VOOH(NH2O)]+
[VO(H2O)(HNO)]+
[VOOH(NHOH)]+
[VO(OH)2(HNO)]+
VO ++H2O+HNO
(5)
+
[V(OH)2] +HNO (6)
[22,23] calculations were performed in both directions to connect these corresponding intermediates at the above level. A step of 0.1 amu1/2 Bohr was used in the IRC procedure. Natural charges were calculated by the natural population analysis at the same level as the one used for geometry optimization [24,25]. Unless otherwise specified, the natural charges obtained by natural population analysis (NPA) were used in the following discussions. Wave function stability calculations were performed to confirm that the calculated wave functions corresponded to the ground state [26–28]. All the calculations reported in the present work were carried out with Gaussian 03 package [29].
The formation of the final products 2NH2O
N2 + 2H2O (7)
2HNO
N2 O + H2O (8)
Scheme 1. The proposed mechanism of NH2OH oxidized by VO2 þ .
A combination of experimental and computational studies of the reaction of metal oxides with hydroxylamine in the gas phase can enhance our understanding of the elementary processes occurring in real conditions. As stated elsewhere, the vanadium existed as VO2 þ cation with the pH around 1.0 [7–9]. Therefore, the theoretical studies based on density functional theory on the hydroxylamine oxidized by VO2 þ in the gas phase were performed for the previously suggested one- and two-electron mechanisms to clarify the detailed mechanism (Scheme 1) [4]. Both the ground and the first excited states were investigated because the reaction of transition metal oxides may involve low-lying excited electronic states [10–12]. All the pathways are terminated at the primary product HNO because the reaction mechanism of the dimerization of the primary product HNO to form final product N2O had been well studied at MP2/6-31G(d,p) level [13]. The present computational study is, to the best of our knowledge, the first detailed comprehensive theoretical mechanistic investigation of the complete reaction mechanism for hydroxylamine oxidized by transition metal oxides. The primary focus of the present study is to understand the mechanism of the hydroxylamine oxidized by VO2 þ in the gas phase and furthermore some hints on the condensed reaction.
2. Computational details Geometry optimizations as well as frequency calculations for all the stationary points considered here were performed at the density functional level of theory, employing the hybrid B3LYP functional, [14–16] together with the 6-311++G(d,p) basis set [17–21]. For each optimized stationary point, vibrational analysis was performed to determine its character (minimum or saddle point) and to obtain the zero-point vibrational energy (ZPVE) correction. For each transition state, intrinsic reaction coordinates (IRC)
3. Results and discussions In the present paper, only the oxidation of hydroxylamine leading to the primary product (HNO) was investigated. As shown in Scheme 1, the first stage of both one- and two-electron mechanisms is the activation of the O–H or N–H bond in hydroxylamine leading to NH2O or NHOH radical complexes [VO(OH)(NH2O)]+ or [VO(OH)(NHOH)]+. The second half of the one-electron mechanism is the oxidation of NH2O radical by VO2 þ to give rise to products HNO and [VO(OH)]+. The second stage of the two-electron mechanism is the intramolecular oxidation–reduction of [VO(OH)(NH2O)]+ or [VO(OH)(NHOH)]+ through hydrogen transfer. In the following sections, firstly, the formation of NH2O or NHOH radical complexes is demonstrated. Secondly, the oxidation of NH2O radical by VO2 þ and the intra-molecular oxidation–reduction of [VO(OH)(NH2O)]+ or [VO(OH) (NHOH)]+ are discussed. Lastly, the comparison of oneand two-electron mechanisms is presented. The prefixes ‘s’,‘d’, ‘t’, and ‘q’ are used to denote the structures in the singlet, doublet, triplet, and quartet electronic states, respectively. 3.1. The formation of NH2O and NHOH radical complexes from NH2OH and VO2 þ The optimized structures of various species on the singlet and triplet states potential energy profiles are depicted in Fig. 1 and the relative energies of various species and energy diagram along singlet and triplet reaction pathways at B3LYP/6-311++G(d,p) level are shown in Fig. 2. As shown in Fig. 1, the first step of both singlet and triplet states is the coordination of hydroxylamine to vanadium center of VO2 þ to generate the g2-N, O intermediates s-1, and t-1 without energy barrier. The s-1 stands below t-1 about 35.18 kcal/mol. The geometric parameters of the hydroxylamine moiety in s-1 and t-1 indicate that the main interaction between hydroxylamine and VO2 þ moieties is electrostatic. A partial positive charge transfer, 0.30 au for both s-1 and t-1, takes places from the VO2 þ fragment to the hydroxylamine fragment. The geometric parameters of s-1 and t-1 are very similar except the V–O
D. Tang et al. / Journal of Molecular Structure: THEOCHEM 805 (2007) 143–152
145
Fig. 1. The optimized structures and selected parameters involved in the activation of the O–H and N–H bonds in both singlet and triplet states. Bond lengths are in angstrom and bond angles are in degree.
bond length and the O–V–O bond angle in the VO2 þ fragment (Fig. 1). The N–H and O–H bonds in s-1 and t-1 are longer than those in the free hydroxylamine, indicative of their activation. This activation is in favor of the subsequent cleavage of the O–H and N–H bonds. Interestingly, the N–O bond lengths in s-1 and t-1 are slightly shorter than that in the free hydroxylamine while the Wiberg bond indexes of the N–O bond are slightly decreased. Thus the activation of the N–O bond is not obvious and the subsequent cleavage of the N–O bond is not feasible. Subsequently, a hydrogen transfer from O3 or N to O1 in s-1 and t-1 occurs (Fig. 1). The O–H bond activation via s-TS1/2 and t-TS1/2 in s-1 and t-1 produces intermediates s/t-2, respectively. Meanwhile, the s-1 and t-1 leads to s-3 and t-3 and s-4 through the activation of the N-H bond with the four-center transition states s-TS1/3, t-TS1/3 and
s-TS1/4, respectively. The imaginary frequencies for these TSs are between 1750i and 1810i cm1 except 1275i cm1 for t-TS1/2. These high frequencies are a direct consequence of the O–H and N–H bonds cleavage as well as the O–H bond formation, as the eigenvector’s coordinates of those imaginary frequencies suggest. Simultaneously, the related bond distances of these transition states are reasonably responsible for the cleavage of the O–H and N–H bonds and the formation of the O–H bond (Fig. 1). All of the five reaction channels are strongly exothermic. The energy barriers of triplet reaction channels are 20.16 (t-TS1/2) and 24.96 (t-TS1/3) kcal/mol and those of singlet reaction channels are 27.92 (s-TS1/2), 38.27(s-TS1/3), and 40.21(s-TS1/4) kcal/mol. It suggests that the triplet reaction channels are more favorable than the singlet reaction channels kinetically. The values of the relative energies of
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D. Tang et al. / Journal of Molecular Structure: THEOCHEM 805 (2007) 143–152
Er(kcal/mol) +
t-VO2 +d-NH2O 33.40
Er (kcal/mol) +
t-VO2 +NH2OH +
33.40
q-VOOH +HNO 24.60
+
s-VO2 +d-NH2O 0.00
q-TS1/2 -10.89
s-VO2++NH2OH t-TS1/3 -9.91 t-TS1/2 -14.71
0.00
+
VOOH +NHOH -14.82 + VOOH +NH2O
s-TS1/4 t-1 -34.87
-25.00
-29.84 s-TS1/3 -31.78
d-1 -42.76 d-3 -63.17
s-TS1/2 -42.13 s-1
q-1 -27.59
t-3 -67.53
d-TS1/2 -21.30
d-TS3/4 -41.28
+
d-VOOH +HNO -26.66
q-2 -52.52 d-4 -64.13 d-2 -77.89
-70.05 s-4 -70.85 s-3 -75.08 t-2 -81.24 s-2 -95.90
Fig. 2. The total energy profile of the activation of the O–H and N–H bonds in singlet and triplet states, relative to the reactants at the singlet state. The total energy for the separated reactants is 1225.901903 Hartrees at the B3LYP/6-311++G(d,p) level.
the highest TSs for all singlet reaction channels are smaller than those of the triplet reaction channels. Therefore, the singlet pathways are energetically favorable. So the singlet
Fig. 4. The total energy profile of the oxidation of NH2O radical by VO2+ in doublet and quartet states, relative to the reactants at the doublet state. The total energy for the separated reactants is 1225.288115 Hartrees at the B3LYP/6-311++G(d,p) level.
PES is exclusively relevant to this step. The activation of the O–H bond is kinetically and energetically easier than that of the N–H bond in both singlet and triplet states (Fig. 2), which is in line with the bond energy order of the O–H and N–H bonds in hydroxylamine in the previous study (75.5 and 84.0 kcal/mol for the O–H and N–H bonds, respectively) [30]. The natural population analysis (NPA) indicates that all these hydrogen transfers can be considered as proton transfer because the positive charges on the transferred hydrogen atoms are about 0.41 0.50 au. The structural parameters of t-VO2 þ and
Fig. 3. The optimized structures and selected parameters involved in the oxidation of NH2O radical by VO2+ in both doublet and quartet states. Bond lengths are in angstrom and bond angles are in degree.
D. Tang et al. / Journal of Molecular Structure: THEOCHEM 805 (2007) 143–152
the VO2 þ fragment in t-1 indicate that the triplet VO2 þ is actually [O@VIV–O]+. The hydrogen transfer step involves clearly the reduction of VV(O@V@O) to VIV (O@VAOH) by hydroxylamine on the singlet PES. While the hydrogen transfer on the triplet PES can be viewed as a diradical process which involves the electron transfer from oxygen atom of [O@VIVAO]+ fragment to oxygen and nitrogen atoms of NH2OH fragment. Namely, the triplet PES does not touch upon to the reduction of VV to VIV. The predicted spin density distributions strongly support this statement (0.98 and 1.00 au on V and O1 atoms in t1, respectively, while 1.12, 0.64, and 0.39 au on V, N, and O3 atoms in t-2, respectively). These differences result in the kinetic favorability of the triplet pathway. However, the high excitation energies between singlet and triplet species s-VO2 þ , t-VO2 þ , s/-1, and t-1) reflect that the singlet pathway is energetically more favorable than the triplet pathways. Finally, the intermediates s-2, s-3, s-4, t-2, and t-3 may decompose into [VO(OH)]+ and NH2O or NHOH radi-
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cals. These processes are endothermic significantly. In other words, the NH2O and NHOH radicals are tightly bonded with [VO(OH)]+. The NH2O radical is more stable than NHOH about 10.18 kcal/mol and this is close to the previous values obtained at various levels [30–32]. So, the main product of hydroxylamine oxidation in this process is NH2O radical. Comparing the relative energies and energy barriers discussed above, it is seen that the most feasible pathway for the primary oxidation of hydroxylamine is the activation of the O–H bond via s-TS1/2 and t-TS1/2 in singlet and triplet states, respectively. The activation of the O–H bond can be viewed as a proton transfer process. The singlet state PES is exclusively relevant to this process. The most stable intermediate is s-2 and t-2 for singlet and triplet states, respectively, and they can decompose into [VO(OH)]+ and NH2O radical. The NH2O and NHOH radicals are tightly bonded with [VO(OH)]+. The NH2O radical is predicted to be the main product in this process.
Fig. 5. The optimized structures and selected parameters involved in the intra-molecular oxidation–reduction of [VO(OH)(NH2O)]+ in singlet state. Bond lengths are in angstrom and bond angles are in degree.
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3.2. The oxidation of NH2O radical by VO2 þ As presented above, the most feasible pathway gives birth to intermediate NH2O radical. So, only those pathways involving in the one-electron mechanism using NH2O radical as reactant were investigated. Due to the electron pairing between t-VO2 þ and NH2O radical, the quartet reactants may lead to the doublet intermediates. The optimized structures of various species on the doublet and quartet potential energy profiles are shown in Fig. 3. The relative energies of various species and energy diagrams along doublet and quartet reaction pathways at B3LYP/6-311++G(d,p) level are shown in Fig. 4. Initially, the coordination of NH2O radical to VO2 þ leads to g1-O (d-3 and q-1) or g2-N, O (d-1) intermediates. Any attempt to locate the quartet g2-N, O complex was failed, which may be caused by the Pauli’s repulsion between the lone pair electron of nitrogen atom and the single electron on V center. The formation of d-1, d-3, and q-1 are quite exothermic. Interestingly, the single electron localizes on oxygen atom of the VO2 þ moiety in d-1. Therefore, d-1 may be viewed as an adduct of spin-pairing t-VO2 þ and NH2O. The geometric parameters of d-3 and q-1 are very similar except the V–O1 bond length. The N–O3 bond obviously elongates with the formation of d1 (Fig. 3), which may be caused by the electron transfer from the lone-pair electron on N and the single electron on O3 to V center. In d-3 and q-1, the geometric parameters
of NH2O moiety are analogous to those of the free NH2O radical. The N–H bonds are slightly activated in the coordination process. The d-3 is more stable than d-2 about 20.39 kcal/mol. Then, intermediate d-1 gives rise to g2-N,O intermediate d-2 through four-center transition state d-TS1/2, while intermediates d-3 and q-1 transform to g1-O intermediate d-4 and q-2 via five-center d-TS3/4 and q-TS1/2. Intermediates d-TS3/4 and d-4 are similar to intermediates q-TS1/2 and q-2 with respect to the geometric parameters, respectively. The transferred hydrogen atoms are also positively charged, in the range of 0.41–0.46 au, therefore, high imaginary frequencies (1902i, 1396i and 1952i cm1 for d-TS1/2, d-TS3/4 and q-TS1/2) are obtained. Intermediates d-1, d-3 and q-1 must surmount 21.46, 22.09 and 16.70 kcal/mol energy barriers to reach d-TS1/2, d-TS3/4 and q-TS1/2 to complete this reaction. These proton transfer processes are predicted to be quite exothermic besides d-3 fi d-4 (Fig. 4). In virtue of the same reason resulting from the difference between the s-VO2 þ and t-VO2 þ , the quartet PES is more favorable than the doublet PES kinetically while the doublet PES is more feasible than the quartet PES energetically because the stationary points on the quartet PES are always higher than those on the doublet PESs. So the doublet PES is exclusively involved in this cleavage of the N–H bond. Lastly, products HNO and d-[VO(OH)]+ and q-[VO(OH)]+ are obtained directly from intermediates d-2, d-4 and q-2. These processes are exceedingly endothermic. The products
Fig. 6. The optimized structures and selected parameters involved in the intra-molecular oxidation–reduction of [VO(OH)(NH2O)]+ in triplet state. Bond lengths are in angstrom and bond angles are in degree.
D. Tang et al. / Journal of Molecular Structure: THEOCHEM 805 (2007) 143–152
of doublet and quartet states stand below their corresponding reactants. So the whole reactions are feasible for doublet and quartet states. Based on the above discussions, only the doublet PES is relevant to the oxidation process of NH2O radical by VO2 þ . The most feasible pathway is s-VOþ 2 þ d-NH2 O ! d-3 ! d-TS3=4 ! d-4 ! d-½VOðOHÞþ þ HNO. Combined Sections 3.1 and 3.2, the one-electron mechanism mostly proceeds through the following pathway: s-VOþ 2 þ þ NH2 OH ! s-1 ! s-TS1=2 ! s-2 ! d-VOðOHÞ þ d-NH2 O and then s-VO2 þ d-NH2 O ! d-3 ! d-TS3=4 ! d-4 ! þ d-VOðOHÞ þ HNO. The whole one-electron mechanism, relative to the reactants VO2 þ þ NH2 OH, is exothermic by 51.66 kcal/mol. The rate-limiting step for the one-electron mechanism is the first hydrogen transfer step with 27.92 kcal/ mol energy barrier. The reaction easily occurs because all the stationary points on the PESs are below the separated reactants (Figs. 2 and 4). 3.3. The intra-molecular oxidation–reduction of [VO(OH)(NH2O)]+ The second half of the two-electron mechanism is that the intermediates s-2, s-3, s-4, t-2, and t-3 give rise to the final product HNO and s/t-VO+ or s/t-[V(OH)2]+ via intra-molecular hydrogen transfer. Similar to the one-elec-
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tron mechanism, the following discussions will focus on those pathways originating from NH2O complexes s-2 and t-2. The optimized structures of various species on the singlet and triplet states potential energy profiles deriving from s-2 and t-2 are shown in Figs. 5 (singlet) and 6 (triplet), and the relative energies of various species and energy diagram along singlet and triplet reaction pathways at B3LYP/6-311++G(d,p) level are shown in Fig. 7. Those pathways deriving from s-3, s-4 and t-3 will not be discussed. While the optimized structures of various species on the singlet and triplet potential energy profiles deriving from s-3, s-4 and t-3 are supplied in supporting information (Fig. S1–S3) and the relative energies of various species and energy diagrams along singlet and triplet reaction pathways at B3LYP/6-311++G(d,p) level are given in Fig. S4 [33]. From the minimum s-2, it is possible to continue the two-electron mechanism with a step associated with the cleavage of the V–N bond via s-TS2/5 to render the minimum s-5. Along s-2 fi s-TS2/5 fi s-5, the V–N bond inchmeal lengthens while the V–O3 and N–O3 bonds gradually shorten. The vibrational mode of the sole imaginary frequency (332i cm1) and IRC calculation for sTS2/5 indicates that the transition state actually connects s-2 and s-5. The energy barrier is predicted to be 22.68 kcal/mol and the formation of s-5 is endothermic
s-TSinter+HNO 36.89 s-VO ++H2O+HNO 36.06
Er(kcal/mol) t-VO2+ +NH2OH 33.40
t-TSinter+HNO 13.72 t-VO+ +H 2O+HNO 11.74
s-VO2+ +NH2OH 0.00
s-VO(H 2O) ++HNO s-TS6/7 -12.54 t-TS4/5 -27.46 s-TS8/9 -34.61
t-TS1/2 -14.71 t-1 -34.87
VOOH++NH 2O
s-TS1/2
-25.00 s-TS5/7 -46.36 s-TS2/9 -48.71
-42.13
s-TS5/6 -49.40 s-TS2/8 -56.69 t-TS2/5 -57.83 t-TS2/4 -64.83 s-1 s-TS2/5 -73.22
-70.05
CP1 s-5 -80.70 t-2 -81.24
-7.60
-12.18 s-14+H O 2 t-VO(H 2O)+ +HNO -29.76
s-V(OH) 2++HNO -17.29 t-V(OH) 2+ +HNO -28.56
s-6 -52.40
s-8 -63.75 t-4 -64.61
s-7 -58.57
t-5 -75.46 s-9 -77.95
s-2 -95.90
Fig. 7. The total energy profile deriving from s-2 and t-2 of two-electron mechanism in singlet and triplet states, relative to the reactants (s-VO2 þ s-NH2 OH) at the singlet state. The total energy for the separated reactant is 1225.901903 Hartrees at the B3LYP/6-311++G(d,p) level.
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by 15.20 kcal/mol. The geometric parameters of s-5 are very similar to those of t-2. And the t-2 stands below s-5 about 0.54 kcal/mol. So there may exist a PES crossing point (CP1) between singlet and triplet states. The existence of CP1 opens the possibility for an intersystem crossing to take place. Comparing the relative energies and geometric parameters, we can conclude that the structure of CP1 may be very close to s-5 and t-2 [34]. The CP1 locates in the exit channel so it is not kinetically relevant. A hydrogen transfer via five-center transition states s-TS5/6 and s-TS5/7 forms the intermediates s-6 and s-7 from s-5. While the triplet state intermediates t-4 and t-5 are generated from t-2 through hydrogen transfer transition states, t-TS2/4 and t-TS2/5, between N and O atoms (O1 or O2). Additionally, the s-2 can also generate g2-N, O intermediates s-8 and s-9 passing through four-center hydrogen transfer transition states s-TS2/8 and s-TS2/9, respectively. All these hydrogen transfer processes can also be viewed as proton transfer because all the transferred hydrogen atoms in transition states are positively charged in the range of 0.46–0.49 au. As expected, the high imaginary frequencies in the range of 920i– 1630i cm1 are obtained. The transition states t-TS2/4 and t-TS2/5 on the triplet PES are more stable than its corresponding transition states s-TS5/6 and s-TS5/7 on the singlet PES, and also more stable than s-TS2/8 and s-TS2/9. The energy barriers are 31.3, 34.34, 39.21, 47.19, 16.41, and 23.41 kcal/mol for s-TS5/6, s-TS5/7, s-TS2/8, s-TS2/9, t-TS2/4, and t-TS2/5, respectively. Therefore, the triplet state reaction pathways are kinetically and energetically favorable. As shown in Fig. 7, the dihydroxyl intermediates s-7, s-9, and t-5 stand below their corresponding intermediates s-6, s-8, and t-4, respectively. In addition, intermediates s-6, s-8, and t-4 transform to intermediates s-7, s-9, and t-5 through s-TS6/7, s-TS8/9, and t-TS4/5 with 39.86, 29.14, and 37.15 kcal/ mol energy barriers, respectively. All the three transition states are characterized by positively charged transferred hydrogen (around 0.52 au) and high imaginary frequency in the range of 1800–1870 cm1. The geometry of water in s-6, s-8 and t-4 is almost undisturbed compared to the free water. It indicates that the interaction between H2O and [VO(HNO)]+ moieties is predominantly electrostatic and this is in agreement with the previous study [35]. Finally, various products such as s-VO+ and t-VO+, HNO, H2O, etc. are formed from intermediates s-6, s-8 and t-4 (Figs. 5 and 6). The products s-[V(OH)2]+, t-[V(OH)2]+ and HNO are obtained from intermediates s-7, s-9, and t-5. The relative energies of the separated products s-VO+, t-VO+, HNO, and H2O are higher than those of the separated singlet reactants while the separated products s-[V(OH)2]+, t-[V(OH)2]+, and HNO lie below the separated singlet reactants. Moreover, the separated triplet products are more stable than the corresponding singlet products. As depicted in Fig. 7, s-[VO(H2O)]+ and t-[VO(H2O)]+ can be converted to s-VðOHÞ2 þ and t-VðOHÞ2 þ via s-TSinter and t-TSinter with 44.49 and
43.48 kcal/mol energy barriers and this is in agreement with the literatures [36,37]. Let us now consider the difference between the singlet and triplet states of the intra-molecular hydrogen migration. The singlet pathways proceed through the reduction of the close-shell VIV species [VIVO(OH)(NH2O)+, d1 state] to close-shell VIII species (d2). While the triplet pathways involve the transform of the open-shell VIV species to the open-shell VIII species associating with the single electron on NH2O moiety transferring to d orbital of vanadium center. The variation of the spin densities of vanadium clearly indicates that the single electron on vanadium is little relevant to the t-2 fi t-4/5 process. Additionally, the triplet pathways are obviously more feasible than the singlet pathways because the high-spin (HS) d2 state of vanadium is more stable than the low-spin (LS) d2 state in the gas phase. Till now, two reaction channels that connect the most stable reactants s-VO2 þ þ NH2 OH with products s[V(OH)2]+ + HNO (no spin inversion) and t-[V(OH)2]+ + HNO (spin inversion), through s-1 fi s-TS1/2 fi s2 fi s-TS2/6 fi s-TS6/7 fi s-7 and s-1 fi s-TS1/2 fi s-2 fi s-TS2/5 fi t-2 fi t-TS2/4 fi t-4 fi t-TS4/5 fi t-5, respectively, are obtained. For the first pathway, the ratelimiting step is the cleavage of the N–H bond in s-2 in the second hydrogen transfer step with an energy barrier of 39.29 kcal/mol. The rate-limiting step is the breakage of the O–H bond in the first hydrogen transfer process for the spin inversion pathway. All the stationary points on the two reaction channels lie below the separated reactants. So these reactions proceed easily. The spin-inversion pathway can be chemically expressed as the LS-VV (d0, s-1) fi LS-VIV (d1, s-2) fi HS-VIV (d1, t-2) fi HS-VIII (d2, t-4) process. The single electron on vanadium is rarely interrelated for the bond cleavage and formation processes in the triplet PES. 3.4. The comparison of one- and two-electron mechanisms As presented above, the one- and two-electron mechanisms are initiated from the activation of the O–H or N– H bond in hydroxylamine to form the NH2O and NHOH radical complexes. The singlet PES is exclusively relevant to the activation of the O–H and N–H bonds. The activation of the O–H bond is easier than that of the N–H bond both energetically and kinetically. The s-2, namely, [VO(OH)(NH2O)]+, is the global minimum on the singlet PES. Thus NH2O radical is the main intermediate product. For both the two mechanisms, the stationary points on the PESs lie below the separated reactants and so these reactions occur easily. The one-electron mechanism mainly proceeds on the ground state (singlet for the formation of NH2O radical and doublet for the oxidation of NH2O radical). The two-electron mechanism may take place at hypersurface of singlet and triplet PESs via spin inversion because the most stable products and reactants reside on triplet and
D. Tang et al. / Journal of Molecular Structure: THEOCHEM 805 (2007) 143–152
singlet PESs, respectively. From the kinetic viewpoint, the one- and two-electron mechanisms are competitive in the gas phase only if the spin inversion occurs easily. The NH2O radical oxidized by VO2 þ to products HNO and d-[VO(OH)]+ is quite downhill (Figs. 2 and 4). However, the PESs for s-2 and t-2 toward the products HNO and s-VO2 þ are uphill in all cases and must overcome high energy barriers (Fig. 7). So the one-electron mechanism is predicted to be predominant, that is, the VIV species should be the most stable. Simultaneously, previous studies [38– 41] considered the VIV species as the most stable state in aqueous hydroxylamine. So, the reaction between VO2 þ with NH2OH in aqueous solution may proceed also via the one-electron mechanism. 4. Conclusions The reaction PESs for hydroxylamine oxidized by VO2 þ had been computed and analyzed. Both low and high-spin PESs of one- and two-electron mechanisms had been characterized in detail at B3LYP/6-311++G(d,p) level of theory. Two possible mechanisms were investigated. The first one is the one-electron mechanism through firstly the formation of NH2O and NHOH radical complexes, and then the oxidation of the stable intermediate NH2O by VO2 þ leading to the final products HNO and VO(OH)+. The one-electron mechanism is predicted to be spin-conserved and the rate-limiting step is the cleavage of the O–H bond by VO2 þ with 27.92 kcal/mol energy barrier. The other one is the two-electron mechanism in which the first half is also the formation of NH2O and NHOH radical complexes and the second half is the intra-molecular hydrogen transfer of NH2O and NHOH radical complexes to form the product HNO with VIV reduced to VIII. The crossing point between singlet and triplet PESs results in the stable product triplet VðOHÞ2 þ in the two-electron mechanism. The two-electron mechanism can be chemically expressed as the LS–VV[VO2(NH2OH)+, d0] fi LS–VIV [VO(OH) (NH2O)+, d1] fi HS–VIV[VO(OH)(NH2O)+, d1] fi HS– VIII [V(OH)2(HNO)+, d2] process in the gas phase. The two-electron mechanism may be competitive with one-electron mechanism kinetically only if the spin inversion between singlet and triplet states PESs occurs easily, while, the one-electron mechanism is energetically more favorable than the two-electron mechanism. Therefore, the one-electron mechanism is predicted to be predominant. Simultaneously, these results may imply that the reaction between VO2 þ with NH2OH in aqueous solution proceeds via the one-electron mechanism. Acknowledgements The authors are grateful for the financial support of the National Natural Science Foundation of China (Nos. 200720024 and 20502017) and the Teaching and Research Award Program for Outstanding Young Teachers in Higher Education Institutions of MOE, P.R.C. (2002).
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Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.theochem.2006.11.008. References [1] V. Manivannan, J.B. Goodenough, Inorg. Chem. 37 (1998) 3448. [2] V. Manivannan, J.B. Goodenough, Mater. Res. Bull. 33 (1998) 1353. [3] A. Fattah, J. Mazer, C.F. Wells, J. Chem. Soc. Dalton Trans. (1980) 1532. [4] M. Johnson, B. Hornstein, Inorg. Chem. 42 (2003) 6923. [5] M.J. Kumasaki, Hazar. Mater. 115 (2004) 57. [6] G.E. Alluisetti, A.E. Almaraz, V.T. Amorebieta, F. Doctorovich, J.A. Olabe, J. Am. Chem. Soc. 126 (2004) 13432. [7] M. Jian, L. Zhu, J. Wang, J. Zhang, G. Li, C. Hu, J. Mol. Catal. A: Chem. 253 (2006) 1. [8] S.E. O’Donnell, T.P. Michael, J. Chem. Soc. Dalton Trans. (1976) 2290. [9] M.T. Pope, B.W. Dale, Quart. Rev. 22 (1968) 527. [10] D. Schro¨der, S. Shaik, H. Schwarz, Acc. Chem. Res. 33 (2000) 139. [11] P.B. Armentrout, J.L. Beauchamp, Acc. Chem. Res. 22 (1989) 315. [12] P.B. Armentrout, Science 251 (1991) 175. [13] K. Ruud, T. Helgaker, E. Uggerud, J. Mol. Struc. (THEOCHEM) 393 (1997) 59. [14] A.D. Becke, J. Chem. Phys. 98 (1993) 5648. [15] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1998) 785. [16] B. Miehlich, A. Savin, H. Stoll, H. Preuss, Chem. Phys. Lett. 157 (1989) 200. [17] R. Krishnan, J.S. Binkley, R. Seeger, J.A. Pople, J. Chem. Phys. 72 (1980) 650. [18] A.D. McLean, G.S. Chandler, J. Chem. Phys. 72 (1980) 5639. [19] A.J.H. Wachters, J. Chem. Phys. 52 (1970) 1033. [20] P.J. Hay, J. Chem. Phys. 66 (1977) 4377. [21] K. Raghavachari, G.W. Trucks, J. Chem. Phys. 91 (1989) 1062. [22] C. Gonzalez, H.B. Schlegel, J. Chem. Phys. 90 (1989) 2154. [23] C. Gonzalez, H.B. Schlegel, J. Phys. Chem. 94 (1990) 5523. [24] A.E. Reed, R.B. Weinstock, F. Weinhold, J. Chem. Phys. 83 (1985) 735. [25] E.D. Glendening, A.E. Reed, J.E. Carpenter, Weinhold, F. NBO Ver. 3.1, 1997. [26] R. Seeger, J.A. Pople, J. Chem. Phys. 66 (1977) 3045. [27] R. Bauernschmitt, R. Ahlrichs, J. Chem. Phys. 104 (1996) 9047. [28] H.B. Schlegel, J. McDouall, in: C. Ogretir, I.G. Kluwer Csizmadia (Eds.), Computational Advances in Organic Chemistry, Academic, The Netherlands, 1991, pp. 167–185. [29] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, J.A. Montgomery, Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. AlLaham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople, Gaussian 03, Reversion B.05; Gaussian Inc., Pittsburgh, PA, 2003. [30] D.A. Dixon, J.S. Francisco, Y. Alexeev, J. Phys. Chem. A 110 (2006) 185. [31] M.R. Soto, M. Page, M.L. McKee, Chem. Phys. Lett. 187 (1991) 335.
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[32] R. Sumathi, D. Sengupta, M.T. Nguyen, J. Phys. Chem. A 102 (1998) 3175. [33] The energetic pattern of those pathways originating from species s-3, s-4 and t-3 (Fig. S4) are similar to that of s-2 and t-2. However, CP2 and CP3 locating on PES open the possibility for an intersystem crossing to take place. CP2 and CP3 locate in the entrance channel so it is kinetically relevant. [34] The energy gap between t-2 and s-5 is smaller than the error of B3LYP functional. Therefore, the crossing point may locate in the domain (Fig. 7) that is the following hydrogen transfer step.
[35] L. Gracia, J. Andres, V.S. Safont, A. Beltran, Organometallics 23 (2004) 730. [36] G.K. Koyanagi, D.K. Bohme, I. Kretzschmar, D. Schro¨der, H. Schwarz, J. Phys. Chem. A 105 (2001) 4259. [37] A. Ricca, C.W. Bauschlicher, J. Phys. Chem. A 101 (1997) 8949. [38] G. Bengtsson, Acta Chem. Scan. (1947–1973) 27 (1973) 3053. [39] A.T. Pilipenko, O.I. Karpova, V.V. Lukachina, V.V. Trachevskii, Zhurnal Analiticheskoi Khimii 27 (1972) 78. [40] R. Benes, J. Novak, Collect. Czechoslovak Chem. Comm. 5 (1970) 3788. [41] G. Ramababu, P.V.S. Rao, K. Ramakrishna, P. Syamala, A. Satyanarayana, J. Ind. Chem. Soc. 78 (2001) 237.