Article pubs.acs.org/JPCA
Collision-Induced Dissociation and Infrared Photodissociation Studies of Methane Adsorption on V5O12+ and V5O13+ Clusters Bo Xu,†,‡ Yan-Xia Zhao,† Xun-Lei Ding,† Qing-Yu Liu,†,‡ and Sheng-Gui He*,† †
Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *
ABSTRACT: V5O12CH4+ and V5O13CH4+ clusters are generated from interactions of pregenerated V5O12+ and V5O13+ with CH4 in a fast flow reactor, respectively. The two adsorption complexes are then characterized by collision-induced dissociation (CID) and infrared photodissociation (IRPD) methods. The CID studies indicate that CH4 is molecularly adsorbed on V5O12+ and V5O13+. Each of the IRPD spectra of V5O12CH4+ and V5O13CH4+ has a broad red band located around 2770 cm−1 and a narrow blue band located around 2990 cm−1. The red and blue bands have large and small red shifts with respect to the symmetric and antisymmetric C−H stretch vibrations of free CH4, respectively. Density functional theory calculations are carried out for the structures and vibrational frequencies of V5O12+ and V5O12CH4+. The computed results suggest that the anharmonicity including Fermi resonance should be taken into account to interpret the observed IRPD spectrum. In V5O12CH4+, the CH4 unit adsorbs on the 3-fold coordinated V5+ site with an η2 configuration. The stretch of the two C−H bonds close to the V5+ ion is associated with the red band and the stretch of the other two C−H bonds is associated with the blue band. This study may shed light on the nature of methane adsorption onto vanadium pentoxide surfaces.
1. INTRODUCTION Methane, the principal component of natural gas, can be converted to diverse value-added products such as ethylene and liquid fuels.1−5 Due to the thermodynamical stability of methane, these conversions are often accomplished under high temperature in the presence of catalysts such as transition metal oxides (TMOs) and undesirable byproducts are always produced. Motivated by substantial economic benefits and scientific challenges, the activation of methane at low temperature has been explored during recent decades.6−8 To design promising catalysts for the low-temperature activation of methane, the fundamental understanding of the underlying mechanisms involving the catalytic processes at a molecular level is very important.9,10 One appealing approach to gain insight into the molecular-level details of methane activation is to investigate the interactions as well as reactions of methane with atomic clusters such as TMO clusters11−23 under isolated and well controlled conditions. In the reactions of methane with cationic TMO clusters (MxOy+), the methane adsorption products (MxOyCH4+) were often observed.13−15,17,23 The study of MxOyCH4+ can provide useful information on the adsorption and activation of CH4 on the TMO clusters, which can be used to understand the nature of CH4 interaction with TMO surfaces at a molecular level. In this work, collision-induced dissociation (CID) and infrared photodissociation (IRPD, including infrared multi© 2013 American Chemical Society
photon dissociation) methods are used to characterize the CH4 adsorption on the cationic clusters of vanadium oxide (note that the bulk vanadium oxides are widely used as catalysts for hydrocarbon conversions24−27 and the vanadium oxide clusters are being actively studied to interpret the mechanisms12,15,28−35). The CID method had been used in our previous studies to characterize the adsorption of small molecules including CO, CO2, and H2O on the TMO clusters.36,37 The IRPD spectroscopy paired with quantum chemistry can provide reliable structural information for neutral and charged species in the gas phase.38,39 The IRPD method has been successfully used to study many gas-phase species including polycyclic aromatic hydrocarbons,40−42 biomolecules,43 metal ion−molecule complexes,44−50 protonated water clusters,51,52 and so on. The IRPD investigations on many TMO clusters were also performed,53−59 but the IRPD characterization on the interactions between small molecules and TMO clusters60,61 such as the vanadium oxide clusters has rarely been reported.62−64 To study CH4 adsorption on vanadium oxide clusters, we couple a crossed helium beam and an IR laser system with a tandem time-of-flight mass spectrometer (TOF-MS) that has a Received: February 1, 2013 Revised: March 15, 2013 Published: March 18, 2013 2961
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Figure 1. Diagram of the tandem TOF-MS coupled with a cluster source, a crossed helium beam, and an IR laser system. P1−P3, P10, and P11: electrodes for accelerating ions. P4 and P12: deflectors. P5, P6, and P15: einzel lens. P7−P9: mass gate. P13 and P14: reflectors. P16: flight tube. Typical potentials (U): U (P1) = 1450 V (positive pulse, rise time ∼100 ns); U (P2) = 1350 V (positive pulse, rise time ∼100 ns); U (P3, P7, P9, and P10) = 0 V; U (P8) = 1650 V (negative pulse, fall and rise time ∼100 ns, pulse width ∼800 ns); U (P11) = 0 V (for A1) or −1900 V (for A2).
laser ablation and fast flow reaction cluster source. In our experiments, vanadium oxide cluster cations V5O12+ and V5O13+ are pregenerated and reacted with CH4 in a fast flow reactor to produce V5O12CH4+ and V5O13CH4+. The adsorption complexes are then characterized by the CID and IRPD methods. Because the valence state of V ions in V5O12+ and V5O13+ is close to five, this study may shed light on the adsorption and activation of CH4 on the surface of bulk vanadium pentoxide (V2O5).
an approaching velocity of 1 km/s. Because the length (60 mm) of the reactor is much longer than 1 mm, the intracluster vibrations are likely equilibrated (cooled or heated, depending on the vibrational temperature after exiting cluster formation channel with a supersonic expansion) to close to the bath gas temperature before reacting with the CH4 molecules. Our recent experiments indicated that the cluster vibrational temperature in the reactor can be close to 300 K.36 After reacting in the fast flow reactor, the reactant and product ions are skimmed (3 mm diameter) into the vacuum system of the tandem TOF-MS for analysis including CID and IRPD characterizations. The primary TOF-MS including the mass gate (P1−P9 in Figure 1) is the same as before,36 and two new assemblies (A1 and A2) for the secondary TOF-MS are designed for the CID and IRPD experiments that are described below. In the CID measurements, the clusters of interest, for example, V5O12CH4+ and V5O13CH4+, are mass selected with a mass gate (P7−P9) and then enter into a field free region between P9 and P10, where the cluster ions are collided with a crossed He beam formed by a third pulsed valve. The daughter (fragment) and the parent ions pass through two identical reflectors (P13 and P14) with a Z-shaped configuration. The direction of the outgoing ions from each reflector is at 174° with respect to that of the incoming ions to the reflector. The depth that the ions can pass through in each reflector is about 22 cm. The mass resolution of the secondary TOF-MS is significantly improved by using the reflector assembly (ReTOF-MS) compared with that of the previous (linear) TOFMS,36 in which the direction of the flying ions is at a right angle with respect to the ion direction in the primary TOF-MS. In the IRPD measurements, the He beam is switched off and a tunable IR beam is switched on to interact with the massselected ions. The IRPD spectrum of the cluster ion is obtained by monitoring the yield of the fragment ion of interest as a
2. METHODS 2.1. Experimental Methods. A schematic diagram of the experimental apparatus is shown in Figure 1. The experimental details for cluster generation and reaction, mass analysis and selection, and CID and IRPD characterization are described below. The system (“Cluster source” in Figure 1) for cluster generation and reaction is the same as that in our previous works.36,65−67 Briefly, the VxOy+ clusters are generated by pulsed by laser ablation of a rotating and translating vanadium disk in the presence of about 0.5% O2 seeded in a He carrier gas (99.999%) with a backing pressure of 4.5 atm. The gas is controlled by a pulsed valve (General Valve, Series 9; the same for the second and third valves below). A 532 nm (second harmonic of Nd3+:yttrium aluminum garnet-YAG) laser with an energy of 5−8 mJ/pulse and a repetition rate of 10 Hz is used. The clusters formed in a gas channel (2 mm diameter × 25 mm length) are expanded and reacted with pure or helium-diluted CH4 (CD4) that is managed with a second pulsed valve in a fast flow reactor (6 mm diameter × 60 mm length). By using the method in ref 68, the instantaneous total gas pressure in the fast flow reactor is estimated to be around 490 Pa at T = 300 K. The number of collisions that a cluster (radius = 0.5 nm) experiences with the bath gas (radius = 0.05 nm, T = 300 K, P = 490 Pa) in the reactor is about 110 per 1 mm of forward motion. This corresponds to a collision rate of 1.1 × 108 s−1 for 2962
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stretching of free CH4 molecule with the experimental value of 2917 cm−1.81 Moreover, the anharmonic frequencies for the vibrational modes of CH4 unit in V5O12CH4+ are selected to be computed as well using analytic third derivatives methods.82−84
function of the IR wavelength and normalizing to parent ion signal. The IR source is a Nd3+-YAG pumped optical parametric oscillator/amplifier system (OPO/A, LaserVision) that is tunable from 2.1 to 5 μm, producing ∼10 mJ/pulse near 2800 cm−1 and ∼20 mJ/pulse near 3600 cm−1 with a bandwidth of ∼3 cm−1. The IR laser is loosely focused in the field free region (between P9 and P10 in Figure 1) by a CaF2 lens to interact with the mass-selected ions and then reflected back to interact with the ions for a second time. The system with the Re-TOF-MS (A1) has been tested to successfully record the IRPD spectrum of V(H2O)5+ clusters69 generated using the same cluster source in Figure 1 (replacing O2/He with H2O/He and removing the fast flow reactor). However, a significant amount of ions are lost in the Re-TOFMS, and this system is not sensitive enough to record good IRPD spectra for V5O12,13CH4+ that are produced by interactions of pregenerated V5O12,13+ with CH4 in the reactor. To improve the sensitivity in the IRPD measurements, the ReTOF-MS (A1) is switched to a collinear TOF-MS (A2) running at a floating-ground mode36 in our IRPD study reported below. To record IRPD spectra, a LabView based program has been written to communicate with the OPO/A system and a digital oscilloscope (LeCroy WaveSurfer 62Xs) that collects the ion signals from a dual microchannel plate (MCP) detector. Typical spectra are obtained at 2.0 cm−1 steps and averaged over 80 laser shots per step. The wavelength (λmid‑IR) of the middle IR laser used to interact with the cluster ions is determined with eq 1. 1/λP = 1/λnear‐IR + 1/λmid‐IR
3. RESULTS 3.1. Experimental Results. 3.1.1. Reactions of V5O12+ and V5O13+ with CH4. The TOF mass spectrum for the distribution of V5Oy+ (y = 11−15) clusters is plotted in Figure 2a. The V5O12+ and V5O13+ clusters are produced with relatively
(1)
in which λP is the wavelength of the pump laser (fundamental of Nd3+-YAG) and λnear‑IR is the wavelength of the near IR beam in the OPO/A system. The λP/2 and λnear‑IR are measured using a grating spectrograph (Acton SpectroaPro 500I) calibrated with atomic spectral lines from a mercury−argon lamp. The air to vacuum conversion for the wavelengths is made with IAU standard.70 The λP/2 obtained using the above method is 532.283 ± 0.045 nm. The accuracy of wavenumbers reported below is ±5 cm−1. 2.2. Computational Methods. The density functional theory (DFT) computations are carried out with the Gaussian 09 program.71 The hybrid B3LYP exchange−correlation functional72−74 and TZVP basis sets75 are used. A genetic algorithm based program76 is applied to find the stable structures of V5O12+. Geometry optimizations for V5O12CH4+ are performed starting from many possible structures with full relaxation. Note that the geometric and electronic structures of V5O13+ are much more complex (open shell system with O−O unit)77 than those of V5O12+, so reliable theoretical results for V5O13CH4+ may be obtained in the future. The transition states (TSs) through which the isomeric structures transfer to each other are optimized using the Berny algorithm method.78 The initial guess structure of the TS species is obtained generally through relaxed potential energy surface (PES) scans using an appropriate internal coordinate. The zero-point vibration corrected energies (ΔH0K) are reported in this work. To assign the spectrum of V5O12CH4+, the harmonic vibrational frequencies are obtained from second analytic derivatives.79 Note that B3LYP vibrational frequencies are overestimated,80 so the calculated harmonic frequencies of V5O12CH4+ are scaled by 0.962, the factor required to reconcile the calculated frequency of the totally symmetric C−H
Figure 2. TOF mass spectra for (a) distribution of V5Oy+ and for reactions of V5Oy+ with (b) 1.8 Pa CH4 and (c) 1.8 Pa CD4. Numbers 5, y denote V5Oy+.
high abundances whereas V5O11+, V5O14+, and V5O15+ have very low abundances. Figure 2b presents the mass spectrum for the interactions of V5Oy+ with CH4. After the reactions, the peak intensity of V5O12+ decreases significantly while the signals at the positions of 463 and 479 amu increase. The signal increase of the peak at the position of 463 amu can be due to the generation of V5O12CH4+ (overlapped with V5O13+), and that at the position of 479 amu may be due to the generation of V5O12(CH4)2+ and (or) V5O13CH4+. The isotopic labeling experiment with CD4 (Figure 2c) yields two new peaks that can be assigned as V5O12CD4+ and V5O13CD4+, and no peak can be assigned as V5O12(CD4)2+. This suggests that both V5O12+ and V5O13+ can adsorb one unit of CH4, as shown in eq 2. V5Oy+ + CH4 → V5Oy CH4 +
V5O12+,
(y = 12 and 13)
(2)
3.1.2. CID of V5O12CH4 , and V5O13CH4+. Figure 3a plots the typical mass spectrum for the CID of mass selected V5O12+. For collision of a cluster with the collision gas, the center-of-mass kinetic energy (Ec) can be calculated by using eq 3. Ec = U × m /M
V5O13+,
+
(3)
in which U is the average potential applied to P1 and P2 in Figure 1, M is the mass of the cluster, and m is the reduced mass of the cluster with He. The Ec for the collisions of V5O12+ with He in our experiment is 12.4 eV; however, almost no fragment ions are observable in the CID of V5O12+ with He. It 2963
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Figure 4. Experimental IRPD spectra of (a) V5O12CH4+ and (b) V5O13CH4+. Frequencies for the v1 (a1) and v3 (t2) bands of the free CH4 are indicated by the dashed lines.
(3019 cm−1) C−H stretching vibrations of the free CH4, respectively. The spectrum of V5O13CH4+ is similar to that of V5O12CH4+: it also has a broad red band (2769 cm−1) and a narrow blue band (2990 cm−1). The similarity of the two spectra implies that the nature of methane adsorption on V5O12+ and V5O13+ can be similar. 3.2. Computational Results. 3.2.1. Structures. Three most stable isomers determined by the DFT calculations for V5O12+ are plotted in Figure 5a. Each isomer has Cs symmetry and closed shell electronic structure. In the lowest-energy isomer I01, all of the five V atoms (V5+ ions) are 4-fold coordinated and the oxygen atoms (O2− ions) have three different coordination numbers. One of the oxygen atoms is 3-fold coordinated (denoted as O3f). The bond lengths of Vi−O3f (i = 1−3) are much longer than those of other V−O bonds. The elongations of V1−O3f and V3−O3f distances of I01 can generate the second (I02) and the third (I03) isomers that contain a 3-fold coordinated V5+ ion in each system, respectively. The conversion barrier from I01 to I02 is computed to be 0.60 eV, as shown in Figure 5c. The energy of I03 is high with respect to I01 and I02, so I03 may not be abundant in the cluster source. Note that these structures of V5O12+ are very different from the bulk crystal structure of V2O5. However, they may akin to the structures of the steps, ledges, or corners at the crystal surface. The V5O12CH4+ cluster ions in the experiment are produced for the interaction of pregenerated V5O12+ with CH4 and the CID studies suggest that CH4 is molecularly adsorbed on V5O12+, so the structural computations of V5O12CH4+ are only carried out for association products of I01−I03 with CH4. Figure 5b shows the lowest-lying adsorption complex for each system. In these adsorption complexes, the methane units coordinate to V1 (for I04) or 3-fold coordinated V ions (for I05 and I06) with η2 configurations. In addition, the η1 and η3 configurations have also been investigated for methane adsorption on V5O12+. The isomers of V5O12CH4+ with CH4 coordinated to the above-mentioned V ions in η1 and η3 configurations are not stable and can be optimized to the isomers given in Figure 5b. In contrast, with η1 and η3 configurations, CH4 can coordinate to other sites, such as the terminal O and O3f ions. However, these isomers are higher in energy than I04 by about 0.14 eV, which means that the binding energies between V5O12+ and CH4 are very small
Figure 3. TOF mass spectra for collision of the crossed He beam with mass-selected (a) V5O12+, (b) V5O13+, (c) V5O13+ and V5O12CH4+, (d) V5O12CD4+, and (e) V5O14+ and V5O13CH4+.
indicates that the CID with the crossed He beam is a relatively soft collision method and it does not lead to severe cluster fragmentation, as discussed in our previous study.36 Figure 3b shows that the loss of O2 is observed for the collisions of the oxygen-rich cluster V5O13+ with He. Note that O2 loss between the two reflectors (P13 and P14 in Figure 1) due to collisions with the background He gas results in the unlabeled weak peak in Figure 3b. For the collisions of the overlapped components of V5O12CH4+ and V5O13+ with He (Figure 3c), in addition to the loss of O2, another daughter ion peak is apparently observed. However, it is hard to determine whether the appearance of the peak is due to the generation of V5O12+ or V5O12H+ because the two ion peaks are somewhat overlapped. In addition, the peak may also be assigned as V5O11CH4+. To determine the daughter ion, The CID of the isotopic-labeling V5O12CD4+ (Figure 3d) is conducted. At the resolution of our instrument, the peaks of V5O12+ and V5O12D+ can be well resolved. The CID experiment unambiguously suggests that the fragment ion is V5O12+ rather than V5O12D+ or V5O11CD4+. The CID of V5O13CH4+ (with some contribution of V5O14+) with He is also conducted and the loss of CH4 and O2 as well as both of them can be identified (Figure 3e). The CID of the isotopic-labeling V5O13CD4+ is not conducted because of its low intensity. The easy loss of CH4 from V5O12CH4+ and V5O13CH4+ upon the CID indicates that CH4 can be molecularly adsorbed on the vanadium oxide cluster cations. 3.1.3. IRPD of V5O12CH4+ and V5O13CH4+. Parts a and b of Figure 4 show the IRPD spectra of V5O12CH4+ and V5O13CH4+ by monitoring the loss of methane in the range 2600−3100 cm−1, respectively. The frequencies of the symmetric (v1) and antisymmetric (v3) stretches of the free CH4 are indicated by the dashed lines.81 The spectrum of V5O12CH4+ is dominated by two bands. The red band is centered at 2767 cm−1 and with full width at half-maximum (fwhm) of 63 cm−1 whereas the blue band is centered at 2989 cm−1 and with fwhm of 20 cm−1. The red and blue bands have large and small red shifts with respect to the symmetric (2917 cm−1) and antisymmetric 2964
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Figure 5. DFT optimized structures of (a) V5O12+ and (b) the corresponding CH4 adsorption complexes V5O12CH4+. The calculated potentialenergy profiles for the isomer transfers between (c) I01 and I02 for V5O12+ and (d) the corresponding CH4 adsorption complexes I04 and I05 for V5O12CH4+. In and TSn denote isomers and transition states, respectively. The symmetry, electronic state, and energy (eV) with respect to the most stable isomer are given below each isomer. The binding energy (BE) between each V5O12+ isomer and CH4 is given below V5O12CH4+ isomers. Some of the bond lengths (pm) and the zero point vibration corrected energy (ΔH0K) are given.
Table 1. Calculated Harmonic, Anharmonic, and Experimental Frequencies (cm−1) for free CH4 and Adsorbed CH4 Unit in I05 of V5O12CH4+ (Infrared Absorption Intensities (km/mol) in Parentheses) harmonic CH4 V5O12CH4+
ν3 (t2) v1 (a1) v1 v2 v3 v4 v5 v6 v7 v8 v9 v6 + v8
3136 3031 3137 3086 2988 2891 1533 1522 1386 1331 1212 2797
(76) (0) (34) (60) (7) (126) (13) (12) (30) (20) (108)
2965
scaled harmonic
anharmonic
3018 2917 3019 2970 2875 2782 1475 1465 1334 1281 1166 2692
2997 2907 2974 2958 2859 2753 1504 1500 1351 1310 1187 2804
experimental 3019 2917 2989 not observed 2767
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(∼0.00 eV), so these isomers can not be generated in our experiments. The binding energies of I04 and I05 (I01 and I02 with CH4) are 0.14 and 0.91 eV, respectively. This indicates that the binding of CH4 to 3-fold coordinated V5+ is much stronger than that to the 4-fold coordinated V5+. The binding energy of I04 is very small so that it may dissociate back into I01 and CH 4 . The Rice−Ramsberger−Kassel−Marcus (RRKM) theory with a modification to variational transition state theory (VTST)85 is used to estimate the dissociation rate. By using a procedure described in ref 68, the rate of I04 → I01 + CH4 at Tvib = 300 K is 3.7 × 1012 s−1, which is much larger than the rate for cluster collision with the He carrier gas in the fast flow reactor (kcollision ≈ 1.1 × 108 s−1). As a result, the collisions by the carrier gas can not stabilize complex I04 and it will quickly dissociate back into I01 and CH4 or transfer to other structures with lower energies. Figure 5d indicates that I04 can convert to I05 through a small energy barrier (0.06 eV) and the RRKM estimated conversion rate is fast (3.8 × 1011 s−1). In contrast, the rate of the back-conversion (I05 → I04) is only 7.2 × 104 s−1, which is significantly smaller than the bath gas collision rate. So the complex I05 can be produced by collisions of I01 with CH4 in our experiment. Note that one I05 may be generated with about ten collisions of I01 with CH4 according to the estimated RRKM rates of I04 → I01 + CH4 versus I04 → I05. The much lower transfer barrier of TS02 (Figure 5d) than that of TS01 (Figure 5c) indicates that the adsorption of CH4 can cause structural change of V5O12+ very easily. In I05, the two C−H bonds pointing to the V5+ are lengthened to 111 pm while the lengths of the other two C−H bonds are almost identical in comparison with the bond lengths (109 pm) of the free CH4. 3.2.2. Harmonic and Anharmonic Frequencies. The harmonic and anharmonic frequencies (for the adsorbed CH4 moiety) of I04 and I05 are computed, and the results of I05 are listed in Table 1. The stretching vibrational frequencies of free CH4 are also listed to illustrate the accuracy of the computational method. The computed stick spectra are convoluted with a Lorentzian function with a fwhm of 20 cm−1. Figure 6 plots the experimental and computed spectra for V5O12CH4+. The detailed features of these spectra are discussed below.
Figure 6. (a) Experimental IRPD spectrum of V5O12CH4+. Simulated IR absorption spectra of the I05 isomer obtained from (b) anharmonic frequencies and (c) scaled harmonic frequencies. Simulated IR absorption spectra of the I04 isomer obtained from (d) anharmonic frequencies and (e) scaled harmonic frequencies. Peak centers are labeled.
in the harmonic frequency calculations, we calculated the anharmonic frequencies of CH4 adsorbed on V5O12+ with the Gaussian 09 program at the B3LYP/TZVP level. It is noteworthy that the Fermi resonances that usually occur in molecules such as CH4 are also automatically included in the anharmonic frequency calculations. Table 1 indicates that such a method predicts that the symmetric and antisymmetric stretches of the free CH4 are at 2907 and 2997 cm−1 that are 10 and 22 cm−1 smaller than the experimental values, respectively. This result is reasonable considering that no predefined parameters (expect those in the DFT functionals) are used in the frequency calculations. The simulated spectrum of V5O12CH4+ generated using anharmonic frequencies is plotted in Figure 6b. The v4 band is at the position of 2753 cm−1, which is very close to the experimental value of 2767 cm−1. The output of the Gaussian program indicates that there is significant Fermi resonance between the v6 + v8 combinational and the v4 fundamental vibrations. Through diagonalization of the Hamiltonian matrix that accounts for the Fermi resonance between v6 + v8 and v4, the vibrational wave function mixing and the intensities of v6 + v8 and v4 bands after the Fermi resonance can be determined. It turns out that the v6 + v8 band (2804 cm−1 in Figure 6b) can borrow significant intensity (14%) from the v4 fundamental band by the calculations. This qualitatively interprets the broad band centered at 2767 cm−1 observed in the experiment (Figure 6a). The v1 and v2 bands predicted by the anharmonic frequency calculations are located at 2974 and 2958 cm−1, respectively. This v1−v2 frequency difference is decreased from 49 cm−1 (Figure 6c) to only 16 cm−1 upon the anharmonic corrections. After convolution with a Lorentzian function (fwhm = 20 cm−1), the v1 and v2 bands in Figure 6b nearly merge into one band. The fwhm for the band at 2989 cm−1 (Figure 6a) is 20
4. DISCUSSION 4.1. Assignment of the V5O12CH4+ Spectrum. As discussed in section 3.2.1, isomer I05 of V5O12CH4+ is most likely generated in the experiment. The simulated IR adsorption spectrum of I05 obtained from the scaled harmonic frequencies is plotted in Figure 6c. In I05, the IR inactive symmetric stretch in free CH4 now correlates to an IR active symmetric stretch (v4 at 2782 cm−1) of the two C−H bonds pointing toward V5+. The triply degenerated IR active antisymmetric stretch is split into three modes. The weak band at 2875 cm−1 (v3) is due to antisymmetric stretch of the two C−H bonds pointing toward V5+. The two bands at 2970 and 3019 cm−1 are associated with the symmetric stretch (v2) and antisymmetric stretch (v1) of the two C−H bonds pointing away from V5+, respectively. However, this spectrum does not agree with the measured IRPD spectrum (Figure 6a), even if we suppose that the band at 2875 cm−1 is not observed due to its low absorption intensity. Because the anharmonicities for different vibrational modes of a polyatomic molecule are usually different and use of a single scale factor may not satisfactorily account for neglected anharmonicities for all vibrational modes 2966
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cm−1 whereas the bandwidth in our experiments can be even smaller. For example, we can observe a narrower band (fwhm = 17 cm−1) around 2775 cm−1 for V(CH4)4+ in the experiments. As a result, the experimental band centered at 2989 cm−1 (Figure 6a) can be assigned to the nearly overlapped v1 and v2 bands of V5O12CH4+ (I05). So the simulated spectrum of I05 is well consistent with the IRPD spectrum of V5O12CH4+ if the anharmonicity including the Fermi resonance is taken into account. It is noteworthy that in addition to the Fermi resonance, there may be other effects contributing to the fact that the red band is much broader than the blue band. Note that the vibrational temperature of the ions generated in our experiments is about 300 K, which means that some transitions may originate from vibrationally excited states rather than from the ground vibrational state. The anharmonic coupling between the vibrational modes of the V5O12 moiety and C−H stretch modes will cause the broadening of the two bands, especially the red band, because the coupling of the vibrational modes of V5O12 moiety to v4 stretch mode (associated with the two C−H bonds bound to the V5+ site) is stronger than the coupling to the v1 and v2 stretch modes (associated with the other two C− H bonds). Another possibility for the assignment of the IRPD spectrum of V5O12CH4+ has also been considered. Parts d and e of Figures 6 indicate that both the harmonic and anharmonic spectra of I04 (v1−v3 bands are all very weak) differ largely from that of the experimental spectrum (Figure 6a). The energetics of Figure 5d indicates that I04 is also very unstable. As a result, one can completely discard the possibility of I04 in the experiment. It is important to point out that I04 is the direct association product of CH4 with the lowest-lying isomer of V5O12+ (I01 in Figure 5a). However, this product does not exist in the experiment due to the structural rearrangement (I04 → I05 in Figure 5d) for the V5O12 cluster moiety upon the CH4 adsorption. This rearrangement is very similar to the structural change of the gas-phase V3O7+ cluster ion after addition of Ar atom as confirmed by recent IRPD experiments paired with theoretical calculations.86 The structural change upon adsorption of small molecules is also well-known in the condensedphase studies.87 For the dissociation of I05, it will transfer to I04 first and then dissociate back to CH4 and V5O12+. To reach the dissociation threshold, the energy of 0.66 eV is needed. The vibrational energy of V5O12CH4+ at 300 K is 0.55 eV, which means that one-photon absorption can lead to the dissociation of I05 in the range 2600−3100 cm−1 (0.32−0.38 eV). However, to reach a quick dissociation (
