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The theoretical investigation on gas-phase chemistry of YNH+ with propene
Journal of Molecular Structure: THEOCHEM 866 (2008) 5–10
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The theoretical investigation on gas-phase chemistry of YNH+ with propene Hui-Zhen Li, Yong-Cheng Wang *, Zhi-Yuan Geng, Qing-Li Zhang, Qing-Yun Wang, Yu-Bing Si Gansu Key Laboratory of Polymer Materials, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou, Gansu 730070, PR China
a r t i c l e
i n f o
Article history: Received 18 April 2008 Received in revised form 17 June 2008 Accepted 17 June 2008 Available online 6 July 2008 Keywords: Dehydrogenation Demethanation Density functional theory (DFT) Natural bond orbital (NBO) analysis
a b s t r a c t Calculations based on density functional theory (DFT) have been carried out to investigate geometries and bonding characteristics of all the stationary points associated with the gas-phase ion/molecule reacP tions of YNH+ (1 +) with propene. The potential energy surface (PES), including six reaction pathways (four dehydrogenation and two demethanation processes), has been explored and analyzed. By contrast, dehydrogenation reaction channels that are associated with two consecutive hydrogen transfers from C atoms of propene fragment to Y atom of YNH+ moiety and then elimination of H2, are energetically favorable, which is in good agreement with the experimental observation. Ó 2008 Published by Elsevier B.V.
1. Introduction
experimentally proposed involve the formation of YC3H5N+ and YC2H3N+ as the ionic products:
The gas-phase chemistry of transition-metal ions has developed into an active and interesting area of research due to their importance in several fields [1,2]. During the past few years, numerous ionic metal-ligand species have been studied in the gas phase [3– 9]. Such studies not only examine the intrinsic effect of the ligand on the reactivity of the metal center but also provide important mechanistic information and models for analogous reactions. Moreover, important types of hydrocarbon activation by d-block elements have been rapidly developed over the past several years in the gas phase [10–15,7,16–22]. Generally, in these processes CAH activation is initiated by electron-rich metal center via oxidative insertion pathway which is predominant. In contrast, low-valence d0 systems preclude the oxidative insertion of the metal center into the CAH bond as the initial step [23] and, hence, must undergo different mechanisms. Early studies [3,24–26] mainly concerning the reactivity of transition-metal-imido species MNH+ have revealed that their reactivity toward small molecules containing prototype bonds (e.g. CAC, CAH, NAH, HAH) in the gas phase depends on the electronic environment provided by M+. YNH+ is just right a species with low-valence electron metal center. Therefore, we focus our attention on the activation of propene with YNH+ in search of alternative mechanisms besides oxidative insertion. The interaction between YNH+ with propene has been investigated by Freiser and co-workers through Fourier transformation ion cyclotron resonance mass spectrometer (FTICRMS) [27]. The corresponding mechanisms
YNHþ þ C3 H6 ! YC3 H5 Nþ þ H2
* Corresponding author. Tel.: + 86 0931 7970237; fax: + 86 0931 7971989. E-mail address: [email protected] (Y.-C. Wang). 0166-1280/$ - see front matter Ó 2008 Published by Elsevier B.V. doi:10.1016/j.theochem.2008.06.026
YNHþ þ C3 H6 ! YC2 H3 Nþ þ CH4
92% 8%
However, the difficulties of an experimental determination of their geometries, together with the lack of reliable thermochemical data, limit a complete understanding of the chemical reactivity involving the ion metal-ligand species and reaction mechanisms. Therefore, in this work, we employ DFT to investigate the reaction mechanisms of YNH+ with propene. This includes a complete illustration of all possible reaction pathways and analyses of PESs and the dominant product channels.
2. Computational methods Geometry optimization and vibrational frequencies for all the stationary points considered here have been determined with the UB3LYP [28,29] method. Previous investigations [30,31] on transition metal compound have indicated it is reliable that numerous properties such as binding energies, geometries and frequencies were obtained at the B3LYP DF. The basis set used consists of the relativistic effective core potential (ECP) of Stuttgart on Y, and the 4d and 5s in Y are treated explicitly by a (8s7p6d) Gaussian basis set contracted to [6s5p3d] [32]. For N, C and H, we use 6311+G(2d,2p) basis set [33]. For each optimized stationary point, vibrational frequency analysis obtained at the same theoretical level was carried out to determine its character as either minima (the number of imaginary frequencies NIMAG = 0) or transition state structures (NIMAG = 1) and to evaluate the zero-point energy
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(ZPE) corrections, which are included in all relative energies. The intrinsic reaction coordinate (IRC) analyses have been used to confirm the connectivity of transition state with the corresponding reactant and product. Finally, the natural population analysis (NPA) has been made with the natural bond orbital (NBO) method [34]. All the calculations reported here have been carried out with GAUSSIAN98 [35] program package. 3. Results and discussion For transition metal containing systems, the possible crossing between surfaces corresponding to different spins should be taken into account. We have probed the first exited state – the triplet state – for several important reaction paths accompanying the YNH+ + propene reaction and found that this surface lies systematically higher than the singlet one, suggesting that indeed no crossing takes place between both the high- and low-spin PESs. Thus we only focus our study on the ground-state (singlet) surface. P 3.1. Electronic structure of YNH+ (1 +) P Our calculations indicate the structure of YNH+ (1 +) is linear and the bond strength of D0(Y+ANH) = 126.0 kcal/mol is consistent with the experimental value of D0(Y+ANH) > 101 kcal/mol [36]. In addition, NBO analysis of the electronic structure of YNH+ is summarized in Table 1 and the results show the YAN r bond is formed from a Y sd15.35 hybrid orbital and an N sp hybrid orbital and the two YAN p bonds are formed from the Y 4d p orbitals and the N 2p p orbitals. The NAH r bond is formed primarily from the other N sp hybrid orbital and the H 1s orbital. Obviously, in YNH+, multiple bond character develops by donation from the N 2p p electron pairs into the formally empty Y 4d p orbitals and accounts for the linear structure and the higher bond strength of YAN bond. P 3.2. The gas-phase reaction mechanisms of YNH+ (1 +) with propene In the following sections, we will examine the title reaction in detail, including geometries of partial stationary points and PES profiles for all possible product channels. Selected geometrical parameters of stationary points are reported in Fig. 1 and the reaction PES corresponding to paths 1–6 is depicted in Fig. 2. The results of the NPA for stationary points on PES are listed in Table 2. Our calculations indicate there are two ways of adding propene to YNH+, which is in good agreement with the experimental prediction. The addition of YNH+ to CAC double bond of propene is the starting point of the barrier less reaction that forms the initial encounter complexes 1and 2, when they approach each other in
Table 1 P Natural bond orbital analysis of YNH+ (1 +)a NBO type Natural orbitals TAN r bond YAN p bond YAN p bond NAH r bond
Natural population analysis Y N H a b
Occupancy 1.98 2.00 2.00 1.99
Hybrid compositionb 13.8% 19.8% 19.8% 70.5%
Y sd15.35 + 86.2% Nsp Y d + 80.2% Np Y d + 80.2% N p N sp + 29.5% H s
Charge
Electronic configuration
1.94527 1.33621 0.39094
[core]5S(0.02)4d(1.09) [core]2S(1.68)2p(4.65) 1S(0.61)
The natural lewis structure represents 99.8% of the total electronic density. Hybrid contributions listed if greater than 0.03.
two distinct modes. The intermediates 1 and 2, which imply the formation of a four-membered ring between Y and N atoms of the YNH+ moiety and the two carbon atoms of the ethylenic fragment of propene, are more stable than the reactants asymptote by 25.3 and 24.9 kcal/mol, respectively. In the intermediate 1, YAN bond is elongated to 2.024 Å compared with 1.846 Å in free YNH+, with the YANAH bond angle diminishing from 180° to 151°and the CAC double bond is also elongated to 1.556 Å compared with 1.328 Å in free propene, with the three CAH bonds and the CAC singlet bond bending away from the Y and N, allowing better overlap of Y, N with p electrons of propene. The situation of two is virtually similar to that of 1. Moreover, the NBO analysis shows that in 1 and 2, both the NAC and YAC bonding orbitals are doubly occupied. Thus, it is obvious that the p bonds in both YNH+ and propene are effectively broken. From separated reactants to the initial complexes 1 and 2, due to the affluence of electrons on the nitrogen center, the NBO analysis indicates there is a net electronic transfer (0.29 a.u. for 1 and 0.25 a.u. for 2, see Table 2) from YNH+ moiety to the propene fragment. Therefore, the formation of 1 and 2 is a nucleophilic addition of the YNH+ fragment to the CAC double bond of propene in terms of a [2 + 2] cycloaddition mechanism. It can be seen from Fig. 2 that starting from reactants (YNH+ + propene), the reaction bifurcates into two equivalent downhill pathways to reach complexes 1 and 2, respectively. Thereafter, six reaction pathways are found for the interaction between YNH+ and propene. Paths 1–4 correspond to the elimination of H2 to yield YC3H5N+, while paths 5 and 6 correspond to the elimination of CH4 to yield YC2H3N+. Path 1. Starting from complex 1, there is a hydrogen migration from C(2) of propene to Y of YNH+. The corresponding transition structure TS1/3 presents one imaginary frequency of 827i cm1 and the H(2) (attached to the C(2) atom) being transferred carries a positive charge of 0.24 a.u. From complex 1 it is necessary to surmount an activation barrier of 15.2 kcal/mol to reach TS1/3 and then go down to 3, which is 34.8 kcal/mol more stable than the separated reactants and is the global minimum on the whole dehydrogenation PES. The following step comprises the second hydrogen transfer from C(1) to Y via TS3/4 to obtain the intermediate 4. The four-membered ring TS3/4 has one imaginary frequency of 1074i cm1 which corresponds mainly to the expected movement of H(1’) detaching from C(1) and moving toward Y, while the hydrogen being transferred is 0.12 a.u. positively charged. Finally, the products (5 + H2) can be directly obtained from the minimum 4 by surmounting barrier height of 4.0 kcal/mol. In the consecutive steps 1?TS1/3?3 and 3?TS3/4?4, the charge on Y changes from 2.00 to 1.96 and then to 1.86 a.u., while on C(1) it varies from 0.93 to 0.84 and then to 0.82 a.u. Note that in this case the YAC(1) bond is further strengthened, with the negative charge transfer from C(2) to C(1) and from H(2) to Y. Therefore, on going from 1 to 4, the bond distance between Y and C(1) was shortened from 2.264 to 2.129 Å, which is indicative of increased bond order between Y and C(1) atoms. Another four channels (paths 2–5) start from complex 2, so it can be considered as a branching point. Thereafter, this reaction will occur via TS2/6 along path 2 and proceed via TS2/9 to reach the intermediate 9. TS2/9 corresponds to the H (10 ) migration from C(1) to Y and lies 4.5 kcal/mol below the reactants asymptote. In the step 2?TS2/9 ?9, the positive charge on the transferred hydrogen H(10 ) disappears and at 9, it is negatively charged (0.47). As can be seen from Table 2 that on going from 2 to 9, the charge on C(1) changes from 0.22 to 0.06 a.u. and on C(2) it changes from 0.66 to 0.50 a.u., with the negative charge transfer from C(2) to C(1) and then to H(10 ). Additionally, in this step the positive charge on Y slightly increases from 1.94 to 1.96 a.u. Obviously, all the variations of charges on every atom are caused by the breaking of
H.-Z. Li et al. / Journal of Molecular Structure: THEOCHEM 866 (2008) 5–10
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Fig. 1. Geometrical parameters of minima and transition states on the ground-state PES for the reaction of YNH+/propene. Bond lengths are in Angstroms and angles in degrees.
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P Fig. 2. Potential energy profiles (including zero-point energy) for the reaction YNH+ (1 +)/propene. Energies are in kcal/mol and relative to the ground-state reactants.
C(1)AH(10 ) bond and the formation of YAH(10 ) bond in this step. Starting from the minimum 9, the reaction bifurcates into three uphill pathways corresponding to paths 3–5. Therefore, the intermediate 9 can be considered as another branching point. Path 2. From complex 2, the first step is associated with a hydrogen transfer process between C(3) and Y, to yield the minimum 6, via TS2/6, characterized by a low imaginary frequency of 602i cm1 and situated 10.3 kcal/mol below the reactants asymptote. At TS2/6, the positive charge on H(3) (attached to the C(3) of propene) being transferred disappears and it is negatively charged (0.31). In the step 2?TS2/6?6, the charge on Y varies from 1.94 to 1.97 and then to 1.98 a.u.; on C(3) it fluctuates from 0.60 to 0.33 and then to 0.50 a.u.; on H(3) it changes from 0.24 to 0.31 and then to 0.50 a.u.. Obviously, from the charge–transfer point of view, the C(3)AH(3) bond cleavage and the YAH(3) bond formation successively occur in this stage. Then, the reaction from 6 to the product
complex 7 occurs via five-membered ring TS6/7, associated with another hydrogen H(3’) migration from C(3) to Y. Finally, from 7, the separated products 8 + H2, can be directly obtained surmounting barrier height of 4.2 kcal/mol. In the two consecutive steps 2?TS2/6?6 and 6?TS6/7?7, the bond distance between C(2) and C(3) changes from 1.520 to 1.346 and then to 1.350 Å. Note that in the first step the hybridization state of C(3) varies from sp3 to sp2, due to the removal of H(3) from C(3). From TS6/7 to 7, the bond distance of C(2)AC(3) bond is slightly elongated due to the formation of C(3)AY bond, with the rearrangement of the corresponding structure from a four-membered ring to a fivemembered one. Path 3. Starting at the intermediate 9, the H(300 ) attached to the C(3) migrates to Y, to yield 10, via five-membered ring TS9/10, characterized by one imaginary frequency of 1298i cm1 and situated 10.0 kcal/mol below the reactants asymptote. The reaction channel
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H.-Z. Li et al. / Journal of Molecular Structure: THEOCHEM 866 (2008) 5–10 Table 2 Natural population analysis for the stationary points on the singlet state PES
+
TNH C3H6 1 TS1/3 3 TS3/4 4 2 TS2/6 6 TS6/7 7 TS2/9 9 TS9/10 10 TS9/12 12 TS9/14 14 TS1/16 16 TS16/14
Y
N
H
1.95
1.34
0.39
2.00 1.80 1.96 1.88 1.86 1.94 1.97 1.98 1.91 1.97 1.80 1.95 l.88 l.88 1.86 l.88 1.83 1.92 1.98 2.11 1.99
1.11 0.99 0.94 0.97 1.00 1.09 1.11 1.11 1.09 1.10 0.99 0.97 1.01 1.00 0.96 0.99 0.91 1.03 1.02 0.95 0.99
0.40 0.39 0.40 0.40 0.41 0.40 0.40 0.41 0.40 0.39 0.39 0.40 0.40 0.41 0.41 0.41 0.41 0.41 0.39 0.40 0.41
C(l)
C(2)
C(3)
H(l)
H(l0 )
H(2)
H(3)
H(30 )
H(3”)
0.39 0.93 0.09 0.84 0.83 0.82 0.22 0.21 0.24 0.25 0.24 0.09 0.06 0.01 0.06 0.04 0.01 0.04 0.83 0.85 0.11 0.82
0.17 0.09 0.55 0.30 0.24 0.19 0.66 0.51 0.15 0.08 0.12 0.55 0.46 0.33 0.34 0.58 0.57 0.72 0.01 0.04 0.82 0.05
0.61 0.58 0.59 0.64 0.62 0.62 0.60 0.33 0.50 0.72 O.SO 0.59 0.64 0.77 0.S5 0.61 0.62 0.58 0.90 0.86 1.28. l.08
0.18 0.23 0.25 0.25 0.25 0.23 0.19 0.20 0.20 0.23 0.22 0.25 0.22 0.22 0.22 0.22 0.22 0.22 0.23 0.22 0.25 0.24
0.19 0.23 0.08 0.25 0.12 0.04 0.20 0.21 0.23 0.22 0.22 0.08 0.49 0.26 0.01 0.18 0.01 0.1S 0.22 0.24 0.21 0.25
0.18 0.20 0.24 0.46 0.18 0.00 0.24 0.23 0.23 0.22 0.21 0.24 0.25 0.24 0.24 0.12 0.04 0.23 0.24 0.24 0.25 0.22
0.20 0.20 0.11 0.24 0.24 0.23 0.24 0.31 0.50 0.20 0.00 0.11 0.24 0.23 0.23 0.22 0.21 0.23 0.29 0.21 0.23 0.21
0.21 0.21 0.26 0.22 0.23 0.24 0.24 0.23 0.21 0.13 0.04 0.26 0.28 0.27 0.25 0.23 0.22 0.22 0.23 0.20 0.25 0.24
0.21 0.22 0.25 0.26 0.25 0.24 0.19 0.23 0.23 0.24 0.22 0.25 0.15 0.12 0.03 0.22 0.22 0.21 0.21 0.28 0.23 0.28
The calculated charge (a.u.) on the indicated atoms, for the corresponding stationary points, is reported at the UB3LYP level.
from 9 to 10 is associated with the C(3)AH(300 ) bond breaking and the H(300 )AY bond formation, respectively. The final minimum, 10, can be viewed as a product complex and connected with the separated products. On going from 9 to 10, via TS9/10, the negative charge on C(3) remarkably increases (from 0.64 to 0.85 a.u.), while on C(2) it decreases (from 0.46 to 0.34 a.u.) and the positive charge on Y atom slightly decreases (from 1.95 to 1.88 a.u.), due to the formation of the YAC(3) bond. Path 4. From the minimum 9, the following step of the reaction is associated with another hydrogen transfer process between C(2) and Y to yield the minimum 12. Therefore, the transition structure, TS9/12, presents a high value of imaginary frequency (1106i cm1) and the charge on the transferred hydrogen is 0.12 a.u. positively charged. During this step, the activation barrier is 25.7 kcal/mol and the formation of intermediate 12 is exothermic, 7.5 kcal/mol. Finally, the separated products, 13 + H2 can be directly obtained surmounting barrier height of 4.0 kcal/mol. In the step 9?TS9/12 ?12, the distance between Y and C(2) is shortened from 2.531 Å to 2.214 Å and then to 2.151 Å. The NBO analyses show in this stage the negative charge on C(2) slightly increases from 0.46 to 0.57 a.u., which accompanies the negative charge transfer from H(2) (0.25?0.12?0.04 a.u.) to C(2), while the positive charge on Y decreases from 1.95 to 1.86 and then to 1.85 a.u., which couples with the negative charge transfer from H(1’) (0.49?0.18? 0.01 a.u.) to Y. Thus, the increment of electronic density between Y and C(2) facilitates the shortening of YAC(2) bond, which is indicative of increased bond order between the Y and C(2) atoms. Path 5. From the minimum 9, it is possible to continue the reaction with a step associated with another hydrogen migration process between Y and C(3) and methyl moving away from C(2) atom to yield the minimum 14, via TS9/14. The transition structure, TS9/14, standing 33.7 kcal/mol above the reactants asymptote, presents one imaginary vibrational frequency, 1065i cm1 and the hydrogen being transferred, H(1’), is 0.18 a.u. negatively charged. Finally, the product complex 14, directly decomposes into the separated products (CH4 and YC2H3N+) via heat absorption, 9.5 kcal/mol. It is worth noting that on going from 9 to 14, the breaking of C(2) AC(3) and YAH(1’) bonds occurs simultaneously, so the higher activation barrier prevents the reaction smoothly moving forward.
As shown in Fig. 2(b), the path 5 is allowed thermodynamically, but it is rather unfavorable dynamically. In other words, this reaction channel is very formidable to proceed at low energy, due to the higher activation barrier (64.5 kcal/mol) of the rate-determining step (from 9 to 14). Path 6. Another demethanation channel was found and depicted in Fig. 2(b). Starting from the initial encounter complex 1, the first step is associated with methyl detaching from C(2) and moving to Y. The corresponding transition state TS1/16 presents one imaginary vibrational frequency (428i cm1). From complex 1 it is necessary to surmount an activation barrier of 21.0 kcal/mol to reach TS1/16 and then go down to the minimum 16, which is 40.3 kcal/mol more stable than the separated reactants. From 16, the next step is connected with a hydrogen transfer between C(1) and C(3) (the carbon atom of methyl) to yield the minimum14, via TS16/14, which is 9.5 kcal/mol below the reactants asymptote. Finally, the separated products (15 + CH4) can be directly obtained surmounting energy barrier of 9.5 kcal/mol. In the first step (1?TS1/16?16), the removal of methyl from C(2), which leads to the hybridization state of C(2) changing from sp3 to sp2, is accompanied by transfer of electron density to C(2) along the four-membered ring (see table 2). As a consequence, the bond distances of YAC(1) and YAN are elongated (see Fig. 1). The NBO analysis indicates the carbon atom in migratory methyl carries a negative charge of 0.86 a.u. to bond with electron-deficient Y. At the minimum 16, the negative charge on C(3) increases to 1.28 a.u., while the positive charge on Y increases to 2.11 a.u., due to the formation of YAC(3) bond. In the following step (16?TS16/14?14), the transition vector for the imaginary frequency (1510i cm1), clearly indicates the breaking of C(1) AH(1) bond and formation of C(3) AH(1) bond. The resulting CH4AYC2H3N+ complex (14) is best described as a complex between YC2H3N+ and molecular CH4 (see Fig. 1). The intermediate 14 is very product-like, for the YC2H3N unit of the complex is virtually identical to the YC2H3N+ described below. The YAC(3) distance is long (2.759 Å) and the C(3) AH(1) distance (1.098 Å) is nearly that of the calculated equilibrium CAH bond length of molecular methane, 1.088 Å. Therefore, the product-like complex 14 directly dissociates to YC2H3N+ + CH4 surmounting energy barrier of 9.5 kcal/mol. As clearly displayed in Fig. 2(b), this channel
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to Y atom and then elimination of H2. According to their energies, the order of stability can be determined as follows: 11 > 7 > 13 > 8. Thus, the exothermicity of the reaction corresponding to paths 1–4 follows the same order as their stability.
is feasible thermodynamically and dynamically, which is in line with the experimental report [27]. 4. Conclusion P To gain further insight into reaction mechanism of YNH+ (1 +) with propene in gas phase, a detailed theoretical investigation on the complicated singlet PES has been performed with DFT calculation at UB3LYP level. The chemical reactivity patterns have been characterized by geometrical and natural population analyses of the stationary points on the reaction PES. The theoretical data obtained may provide a helpful tool for the interpretation of the experimental observation and a useful guide for understanding the mechanism of other analogous reactions. The conclusions of the present study can be summarized as follows: (i) Six reaction pathways have been found: paths 1–4 correspond to the elimination of H2 to yield YC3H5N+ while paths 5 and 6 lead to the elimination of CH4 to yield YC2H3N+. All the reaction paths proceed in a two-step manner via two multi-centered transition states, which is different from CAH activation by electronic-rich metal center. (ii) All the reaction pathways proceed on the singlet PES and no spin crossing occurs between the ground state (singlet) and the first excited state (triplet) PESs. (iii) The energy profiles of paths 1–6 are downhill toward products direction, but it is worth noting that for paths 1–4 and 6 the energies of all the involved minima and transition states are located below the entrance channel and for path 5 one (TS9/14) of the transition states stands (33.7 kcal/mol) higher above the reactants asymptote, which prohibits the activation of CAC bond in this channel at low energy. (iv) Dehydrogenation and demethanation processes are parallel reactions, but b-CH3 transfers are more difficult than the bH transfers (see Fig. 2), which limits the reaction efficiency and influences the branch ratio of corresponding products. Therefore, for the title reaction, dehydrogenation is the predominant process and demethanation is a minor one, which is in good agreement with the experimental observation [27]. (v) Along paths 1–4, the dehydrogenation products (7, 8, 11 and 13) are isomers, which are yielded by two consecutive hydrogen transfers from different carbon atoms of propene
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Journal of Molecular Structure: THEOCHEM journal homepage: www.elsevier.com/locate/theochem
The theoretical investigation on gas-phase chemistry of YNH+ with propene Hui-Zhen Li, Yong-Cheng Wang *, Zhi-Yuan Geng, Qing-Li Zhang, Qing-Yun Wang, Yu-Bing Si Gansu Key Laboratory of Polymer Materials, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou, Gansu 730070, PR China
a r t i c l e
i n f o
Article history: Received 18 April 2008 Received in revised form 17 June 2008 Accepted 17 June 2008 Available online 6 July 2008 Keywords: Dehydrogenation Demethanation Density functional theory (DFT) Natural bond orbital (NBO) analysis
a b s t r a c t Calculations based on density functional theory (DFT) have been carried out to investigate geometries and bonding characteristics of all the stationary points associated with the gas-phase ion/molecule reacP tions of YNH+ (1 +) with propene. The potential energy surface (PES), including six reaction pathways (four dehydrogenation and two demethanation processes), has been explored and analyzed. By contrast, dehydrogenation reaction channels that are associated with two consecutive hydrogen transfers from C atoms of propene fragment to Y atom of YNH+ moiety and then elimination of H2, are energetically favorable, which is in good agreement with the experimental observation. Ó 2008 Published by Elsevier B.V.
1. Introduction
experimentally proposed involve the formation of YC3H5N+ and YC2H3N+ as the ionic products:
The gas-phase chemistry of transition-metal ions has developed into an active and interesting area of research due to their importance in several fields [1,2]. During the past few years, numerous ionic metal-ligand species have been studied in the gas phase [3– 9]. Such studies not only examine the intrinsic effect of the ligand on the reactivity of the metal center but also provide important mechanistic information and models for analogous reactions. Moreover, important types of hydrocarbon activation by d-block elements have been rapidly developed over the past several years in the gas phase [10–15,7,16–22]. Generally, in these processes CAH activation is initiated by electron-rich metal center via oxidative insertion pathway which is predominant. In contrast, low-valence d0 systems preclude the oxidative insertion of the metal center into the CAH bond as the initial step [23] and, hence, must undergo different mechanisms. Early studies [3,24–26] mainly concerning the reactivity of transition-metal-imido species MNH+ have revealed that their reactivity toward small molecules containing prototype bonds (e.g. CAC, CAH, NAH, HAH) in the gas phase depends on the electronic environment provided by M+. YNH+ is just right a species with low-valence electron metal center. Therefore, we focus our attention on the activation of propene with YNH+ in search of alternative mechanisms besides oxidative insertion. The interaction between YNH+ with propene has been investigated by Freiser and co-workers through Fourier transformation ion cyclotron resonance mass spectrometer (FTICRMS) [27]. The corresponding mechanisms
YNHþ þ C3 H6 ! YC3 H5 Nþ þ H2
* Corresponding author. Tel.: + 86 0931 7970237; fax: + 86 0931 7971989. E-mail address: [email protected] (Y.-C. Wang). 0166-1280/$ - see front matter Ó 2008 Published by Elsevier B.V. doi:10.1016/j.theochem.2008.06.026
YNHþ þ C3 H6 ! YC2 H3 Nþ þ CH4
92% 8%
However, the difficulties of an experimental determination of their geometries, together with the lack of reliable thermochemical data, limit a complete understanding of the chemical reactivity involving the ion metal-ligand species and reaction mechanisms. Therefore, in this work, we employ DFT to investigate the reaction mechanisms of YNH+ with propene. This includes a complete illustration of all possible reaction pathways and analyses of PESs and the dominant product channels.
2. Computational methods Geometry optimization and vibrational frequencies for all the stationary points considered here have been determined with the UB3LYP [28,29] method. Previous investigations [30,31] on transition metal compound have indicated it is reliable that numerous properties such as binding energies, geometries and frequencies were obtained at the B3LYP DF. The basis set used consists of the relativistic effective core potential (ECP) of Stuttgart on Y, and the 4d and 5s in Y are treated explicitly by a (8s7p6d) Gaussian basis set contracted to [6s5p3d] [32]. For N, C and H, we use 6311+G(2d,2p) basis set [33]. For each optimized stationary point, vibrational frequency analysis obtained at the same theoretical level was carried out to determine its character as either minima (the number of imaginary frequencies NIMAG = 0) or transition state structures (NIMAG = 1) and to evaluate the zero-point energy
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(ZPE) corrections, which are included in all relative energies. The intrinsic reaction coordinate (IRC) analyses have been used to confirm the connectivity of transition state with the corresponding reactant and product. Finally, the natural population analysis (NPA) has been made with the natural bond orbital (NBO) method [34]. All the calculations reported here have been carried out with GAUSSIAN98 [35] program package. 3. Results and discussion For transition metal containing systems, the possible crossing between surfaces corresponding to different spins should be taken into account. We have probed the first exited state – the triplet state – for several important reaction paths accompanying the YNH+ + propene reaction and found that this surface lies systematically higher than the singlet one, suggesting that indeed no crossing takes place between both the high- and low-spin PESs. Thus we only focus our study on the ground-state (singlet) surface. P 3.1. Electronic structure of YNH+ (1 +) P Our calculations indicate the structure of YNH+ (1 +) is linear and the bond strength of D0(Y+ANH) = 126.0 kcal/mol is consistent with the experimental value of D0(Y+ANH) > 101 kcal/mol [36]. In addition, NBO analysis of the electronic structure of YNH+ is summarized in Table 1 and the results show the YAN r bond is formed from a Y sd15.35 hybrid orbital and an N sp hybrid orbital and the two YAN p bonds are formed from the Y 4d p orbitals and the N 2p p orbitals. The NAH r bond is formed primarily from the other N sp hybrid orbital and the H 1s orbital. Obviously, in YNH+, multiple bond character develops by donation from the N 2p p electron pairs into the formally empty Y 4d p orbitals and accounts for the linear structure and the higher bond strength of YAN bond. P 3.2. The gas-phase reaction mechanisms of YNH+ (1 +) with propene In the following sections, we will examine the title reaction in detail, including geometries of partial stationary points and PES profiles for all possible product channels. Selected geometrical parameters of stationary points are reported in Fig. 1 and the reaction PES corresponding to paths 1–6 is depicted in Fig. 2. The results of the NPA for stationary points on PES are listed in Table 2. Our calculations indicate there are two ways of adding propene to YNH+, which is in good agreement with the experimental prediction. The addition of YNH+ to CAC double bond of propene is the starting point of the barrier less reaction that forms the initial encounter complexes 1and 2, when they approach each other in
Table 1 P Natural bond orbital analysis of YNH+ (1 +)a NBO type Natural orbitals TAN r bond YAN p bond YAN p bond NAH r bond
Natural population analysis Y N H a b
Occupancy 1.98 2.00 2.00 1.99
Hybrid compositionb 13.8% 19.8% 19.8% 70.5%
Y sd15.35 + 86.2% Nsp Y d + 80.2% Np Y d + 80.2% N p N sp + 29.5% H s
Charge
Electronic configuration
1.94527 1.33621 0.39094
[core]5S(0.02)4d(1.09) [core]2S(1.68)2p(4.65) 1S(0.61)
The natural lewis structure represents 99.8% of the total electronic density. Hybrid contributions listed if greater than 0.03.
two distinct modes. The intermediates 1 and 2, which imply the formation of a four-membered ring between Y and N atoms of the YNH+ moiety and the two carbon atoms of the ethylenic fragment of propene, are more stable than the reactants asymptote by 25.3 and 24.9 kcal/mol, respectively. In the intermediate 1, YAN bond is elongated to 2.024 Å compared with 1.846 Å in free YNH+, with the YANAH bond angle diminishing from 180° to 151°and the CAC double bond is also elongated to 1.556 Å compared with 1.328 Å in free propene, with the three CAH bonds and the CAC singlet bond bending away from the Y and N, allowing better overlap of Y, N with p electrons of propene. The situation of two is virtually similar to that of 1. Moreover, the NBO analysis shows that in 1 and 2, both the NAC and YAC bonding orbitals are doubly occupied. Thus, it is obvious that the p bonds in both YNH+ and propene are effectively broken. From separated reactants to the initial complexes 1 and 2, due to the affluence of electrons on the nitrogen center, the NBO analysis indicates there is a net electronic transfer (0.29 a.u. for 1 and 0.25 a.u. for 2, see Table 2) from YNH+ moiety to the propene fragment. Therefore, the formation of 1 and 2 is a nucleophilic addition of the YNH+ fragment to the CAC double bond of propene in terms of a [2 + 2] cycloaddition mechanism. It can be seen from Fig. 2 that starting from reactants (YNH+ + propene), the reaction bifurcates into two equivalent downhill pathways to reach complexes 1 and 2, respectively. Thereafter, six reaction pathways are found for the interaction between YNH+ and propene. Paths 1–4 correspond to the elimination of H2 to yield YC3H5N+, while paths 5 and 6 correspond to the elimination of CH4 to yield YC2H3N+. Path 1. Starting from complex 1, there is a hydrogen migration from C(2) of propene to Y of YNH+. The corresponding transition structure TS1/3 presents one imaginary frequency of 827i cm1 and the H(2) (attached to the C(2) atom) being transferred carries a positive charge of 0.24 a.u. From complex 1 it is necessary to surmount an activation barrier of 15.2 kcal/mol to reach TS1/3 and then go down to 3, which is 34.8 kcal/mol more stable than the separated reactants and is the global minimum on the whole dehydrogenation PES. The following step comprises the second hydrogen transfer from C(1) to Y via TS3/4 to obtain the intermediate 4. The four-membered ring TS3/4 has one imaginary frequency of 1074i cm1 which corresponds mainly to the expected movement of H(1’) detaching from C(1) and moving toward Y, while the hydrogen being transferred is 0.12 a.u. positively charged. Finally, the products (5 + H2) can be directly obtained from the minimum 4 by surmounting barrier height of 4.0 kcal/mol. In the consecutive steps 1?TS1/3?3 and 3?TS3/4?4, the charge on Y changes from 2.00 to 1.96 and then to 1.86 a.u., while on C(1) it varies from 0.93 to 0.84 and then to 0.82 a.u. Note that in this case the YAC(1) bond is further strengthened, with the negative charge transfer from C(2) to C(1) and from H(2) to Y. Therefore, on going from 1 to 4, the bond distance between Y and C(1) was shortened from 2.264 to 2.129 Å, which is indicative of increased bond order between Y and C(1) atoms. Another four channels (paths 2–5) start from complex 2, so it can be considered as a branching point. Thereafter, this reaction will occur via TS2/6 along path 2 and proceed via TS2/9 to reach the intermediate 9. TS2/9 corresponds to the H (10 ) migration from C(1) to Y and lies 4.5 kcal/mol below the reactants asymptote. In the step 2?TS2/9 ?9, the positive charge on the transferred hydrogen H(10 ) disappears and at 9, it is negatively charged (0.47). As can be seen from Table 2 that on going from 2 to 9, the charge on C(1) changes from 0.22 to 0.06 a.u. and on C(2) it changes from 0.66 to 0.50 a.u., with the negative charge transfer from C(2) to C(1) and then to H(10 ). Additionally, in this step the positive charge on Y slightly increases from 1.94 to 1.96 a.u. Obviously, all the variations of charges on every atom are caused by the breaking of
H.-Z. Li et al. / Journal of Molecular Structure: THEOCHEM 866 (2008) 5–10
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Fig. 1. Geometrical parameters of minima and transition states on the ground-state PES for the reaction of YNH+/propene. Bond lengths are in Angstroms and angles in degrees.
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H.-Z. Li et al. / Journal of Molecular Structure: THEOCHEM 866 (2008) 5–10
P Fig. 2. Potential energy profiles (including zero-point energy) for the reaction YNH+ (1 +)/propene. Energies are in kcal/mol and relative to the ground-state reactants.
C(1)AH(10 ) bond and the formation of YAH(10 ) bond in this step. Starting from the minimum 9, the reaction bifurcates into three uphill pathways corresponding to paths 3–5. Therefore, the intermediate 9 can be considered as another branching point. Path 2. From complex 2, the first step is associated with a hydrogen transfer process between C(3) and Y, to yield the minimum 6, via TS2/6, characterized by a low imaginary frequency of 602i cm1 and situated 10.3 kcal/mol below the reactants asymptote. At TS2/6, the positive charge on H(3) (attached to the C(3) of propene) being transferred disappears and it is negatively charged (0.31). In the step 2?TS2/6?6, the charge on Y varies from 1.94 to 1.97 and then to 1.98 a.u.; on C(3) it fluctuates from 0.60 to 0.33 and then to 0.50 a.u.; on H(3) it changes from 0.24 to 0.31 and then to 0.50 a.u.. Obviously, from the charge–transfer point of view, the C(3)AH(3) bond cleavage and the YAH(3) bond formation successively occur in this stage. Then, the reaction from 6 to the product
complex 7 occurs via five-membered ring TS6/7, associated with another hydrogen H(3’) migration from C(3) to Y. Finally, from 7, the separated products 8 + H2, can be directly obtained surmounting barrier height of 4.2 kcal/mol. In the two consecutive steps 2?TS2/6?6 and 6?TS6/7?7, the bond distance between C(2) and C(3) changes from 1.520 to 1.346 and then to 1.350 Å. Note that in the first step the hybridization state of C(3) varies from sp3 to sp2, due to the removal of H(3) from C(3). From TS6/7 to 7, the bond distance of C(2)AC(3) bond is slightly elongated due to the formation of C(3)AY bond, with the rearrangement of the corresponding structure from a four-membered ring to a fivemembered one. Path 3. Starting at the intermediate 9, the H(300 ) attached to the C(3) migrates to Y, to yield 10, via five-membered ring TS9/10, characterized by one imaginary frequency of 1298i cm1 and situated 10.0 kcal/mol below the reactants asymptote. The reaction channel
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H.-Z. Li et al. / Journal of Molecular Structure: THEOCHEM 866 (2008) 5–10 Table 2 Natural population analysis for the stationary points on the singlet state PES
+
TNH C3H6 1 TS1/3 3 TS3/4 4 2 TS2/6 6 TS6/7 7 TS2/9 9 TS9/10 10 TS9/12 12 TS9/14 14 TS1/16 16 TS16/14
Y
N
H
1.95
1.34
0.39
2.00 1.80 1.96 1.88 1.86 1.94 1.97 1.98 1.91 1.97 1.80 1.95 l.88 l.88 1.86 l.88 1.83 1.92 1.98 2.11 1.99
1.11 0.99 0.94 0.97 1.00 1.09 1.11 1.11 1.09 1.10 0.99 0.97 1.01 1.00 0.96 0.99 0.91 1.03 1.02 0.95 0.99
0.40 0.39 0.40 0.40 0.41 0.40 0.40 0.41 0.40 0.39 0.39 0.40 0.40 0.41 0.41 0.41 0.41 0.41 0.39 0.40 0.41
C(l)
C(2)
C(3)
H(l)
H(l0 )
H(2)
H(3)
H(30 )
H(3”)
0.39 0.93 0.09 0.84 0.83 0.82 0.22 0.21 0.24 0.25 0.24 0.09 0.06 0.01 0.06 0.04 0.01 0.04 0.83 0.85 0.11 0.82
0.17 0.09 0.55 0.30 0.24 0.19 0.66 0.51 0.15 0.08 0.12 0.55 0.46 0.33 0.34 0.58 0.57 0.72 0.01 0.04 0.82 0.05
0.61 0.58 0.59 0.64 0.62 0.62 0.60 0.33 0.50 0.72 O.SO 0.59 0.64 0.77 0.S5 0.61 0.62 0.58 0.90 0.86 1.28. l.08
0.18 0.23 0.25 0.25 0.25 0.23 0.19 0.20 0.20 0.23 0.22 0.25 0.22 0.22 0.22 0.22 0.22 0.22 0.23 0.22 0.25 0.24
0.19 0.23 0.08 0.25 0.12 0.04 0.20 0.21 0.23 0.22 0.22 0.08 0.49 0.26 0.01 0.18 0.01 0.1S 0.22 0.24 0.21 0.25
0.18 0.20 0.24 0.46 0.18 0.00 0.24 0.23 0.23 0.22 0.21 0.24 0.25 0.24 0.24 0.12 0.04 0.23 0.24 0.24 0.25 0.22
0.20 0.20 0.11 0.24 0.24 0.23 0.24 0.31 0.50 0.20 0.00 0.11 0.24 0.23 0.23 0.22 0.21 0.23 0.29 0.21 0.23 0.21
0.21 0.21 0.26 0.22 0.23 0.24 0.24 0.23 0.21 0.13 0.04 0.26 0.28 0.27 0.25 0.23 0.22 0.22 0.23 0.20 0.25 0.24
0.21 0.22 0.25 0.26 0.25 0.24 0.19 0.23 0.23 0.24 0.22 0.25 0.15 0.12 0.03 0.22 0.22 0.21 0.21 0.28 0.23 0.28
The calculated charge (a.u.) on the indicated atoms, for the corresponding stationary points, is reported at the UB3LYP level.
from 9 to 10 is associated with the C(3)AH(300 ) bond breaking and the H(300 )AY bond formation, respectively. The final minimum, 10, can be viewed as a product complex and connected with the separated products. On going from 9 to 10, via TS9/10, the negative charge on C(3) remarkably increases (from 0.64 to 0.85 a.u.), while on C(2) it decreases (from 0.46 to 0.34 a.u.) and the positive charge on Y atom slightly decreases (from 1.95 to 1.88 a.u.), due to the formation of the YAC(3) bond. Path 4. From the minimum 9, the following step of the reaction is associated with another hydrogen transfer process between C(2) and Y to yield the minimum 12. Therefore, the transition structure, TS9/12, presents a high value of imaginary frequency (1106i cm1) and the charge on the transferred hydrogen is 0.12 a.u. positively charged. During this step, the activation barrier is 25.7 kcal/mol and the formation of intermediate 12 is exothermic, 7.5 kcal/mol. Finally, the separated products, 13 + H2 can be directly obtained surmounting barrier height of 4.0 kcal/mol. In the step 9?TS9/12 ?12, the distance between Y and C(2) is shortened from 2.531 Å to 2.214 Å and then to 2.151 Å. The NBO analyses show in this stage the negative charge on C(2) slightly increases from 0.46 to 0.57 a.u., which accompanies the negative charge transfer from H(2) (0.25?0.12?0.04 a.u.) to C(2), while the positive charge on Y decreases from 1.95 to 1.86 and then to 1.85 a.u., which couples with the negative charge transfer from H(1’) (0.49?0.18? 0.01 a.u.) to Y. Thus, the increment of electronic density between Y and C(2) facilitates the shortening of YAC(2) bond, which is indicative of increased bond order between the Y and C(2) atoms. Path 5. From the minimum 9, it is possible to continue the reaction with a step associated with another hydrogen migration process between Y and C(3) and methyl moving away from C(2) atom to yield the minimum 14, via TS9/14. The transition structure, TS9/14, standing 33.7 kcal/mol above the reactants asymptote, presents one imaginary vibrational frequency, 1065i cm1 and the hydrogen being transferred, H(1’), is 0.18 a.u. negatively charged. Finally, the product complex 14, directly decomposes into the separated products (CH4 and YC2H3N+) via heat absorption, 9.5 kcal/mol. It is worth noting that on going from 9 to 14, the breaking of C(2) AC(3) and YAH(1’) bonds occurs simultaneously, so the higher activation barrier prevents the reaction smoothly moving forward.
As shown in Fig. 2(b), the path 5 is allowed thermodynamically, but it is rather unfavorable dynamically. In other words, this reaction channel is very formidable to proceed at low energy, due to the higher activation barrier (64.5 kcal/mol) of the rate-determining step (from 9 to 14). Path 6. Another demethanation channel was found and depicted in Fig. 2(b). Starting from the initial encounter complex 1, the first step is associated with methyl detaching from C(2) and moving to Y. The corresponding transition state TS1/16 presents one imaginary vibrational frequency (428i cm1). From complex 1 it is necessary to surmount an activation barrier of 21.0 kcal/mol to reach TS1/16 and then go down to the minimum 16, which is 40.3 kcal/mol more stable than the separated reactants. From 16, the next step is connected with a hydrogen transfer between C(1) and C(3) (the carbon atom of methyl) to yield the minimum14, via TS16/14, which is 9.5 kcal/mol below the reactants asymptote. Finally, the separated products (15 + CH4) can be directly obtained surmounting energy barrier of 9.5 kcal/mol. In the first step (1?TS1/16?16), the removal of methyl from C(2), which leads to the hybridization state of C(2) changing from sp3 to sp2, is accompanied by transfer of electron density to C(2) along the four-membered ring (see table 2). As a consequence, the bond distances of YAC(1) and YAN are elongated (see Fig. 1). The NBO analysis indicates the carbon atom in migratory methyl carries a negative charge of 0.86 a.u. to bond with electron-deficient Y. At the minimum 16, the negative charge on C(3) increases to 1.28 a.u., while the positive charge on Y increases to 2.11 a.u., due to the formation of YAC(3) bond. In the following step (16?TS16/14?14), the transition vector for the imaginary frequency (1510i cm1), clearly indicates the breaking of C(1) AH(1) bond and formation of C(3) AH(1) bond. The resulting CH4AYC2H3N+ complex (14) is best described as a complex between YC2H3N+ and molecular CH4 (see Fig. 1). The intermediate 14 is very product-like, for the YC2H3N unit of the complex is virtually identical to the YC2H3N+ described below. The YAC(3) distance is long (2.759 Å) and the C(3) AH(1) distance (1.098 Å) is nearly that of the calculated equilibrium CAH bond length of molecular methane, 1.088 Å. Therefore, the product-like complex 14 directly dissociates to YC2H3N+ + CH4 surmounting energy barrier of 9.5 kcal/mol. As clearly displayed in Fig. 2(b), this channel
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to Y atom and then elimination of H2. According to their energies, the order of stability can be determined as follows: 11 > 7 > 13 > 8. Thus, the exothermicity of the reaction corresponding to paths 1–4 follows the same order as their stability.
is feasible thermodynamically and dynamically, which is in line with the experimental report [27]. 4. Conclusion P To gain further insight into reaction mechanism of YNH+ (1 +) with propene in gas phase, a detailed theoretical investigation on the complicated singlet PES has been performed with DFT calculation at UB3LYP level. The chemical reactivity patterns have been characterized by geometrical and natural population analyses of the stationary points on the reaction PES. The theoretical data obtained may provide a helpful tool for the interpretation of the experimental observation and a useful guide for understanding the mechanism of other analogous reactions. The conclusions of the present study can be summarized as follows: (i) Six reaction pathways have been found: paths 1–4 correspond to the elimination of H2 to yield YC3H5N+ while paths 5 and 6 lead to the elimination of CH4 to yield YC2H3N+. All the reaction paths proceed in a two-step manner via two multi-centered transition states, which is different from CAH activation by electronic-rich metal center. (ii) All the reaction pathways proceed on the singlet PES and no spin crossing occurs between the ground state (singlet) and the first excited state (triplet) PESs. (iii) The energy profiles of paths 1–6 are downhill toward products direction, but it is worth noting that for paths 1–4 and 6 the energies of all the involved minima and transition states are located below the entrance channel and for path 5 one (TS9/14) of the transition states stands (33.7 kcal/mol) higher above the reactants asymptote, which prohibits the activation of CAC bond in this channel at low energy. (iv) Dehydrogenation and demethanation processes are parallel reactions, but b-CH3 transfers are more difficult than the bH transfers (see Fig. 2), which limits the reaction efficiency and influences the branch ratio of corresponding products. Therefore, for the title reaction, dehydrogenation is the predominant process and demethanation is a minor one, which is in good agreement with the experimental observation [27]. (v) Along paths 1–4, the dehydrogenation products (7, 8, 11 and 13) are isomers, which are yielded by two consecutive hydrogen transfers from different carbon atoms of propene
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