COMMENT pubs.acs.org/JPCA
Reply to “Comment on ‘Electronic Structures, Vibrational and Thermochemical Properties of Neutral and Charged Niobium Clusters Nbn, n = 7 12’” Pham Vu Nhat,†,‡ Vu Thi Ngan,† Truong Ba Tai,† and Minh Tho Nguyen*,†,§ †
Department of Chemistry, Katholieke Universiteit Leuven, B-3001 Leuven, Belgium Department of Chemistry, Can Tho University, Can Tho, Vietnam § Institute for Computational Science and Technology, Thu Duc, HoChiMinh City, Vietnam ‡
n recent papers,1,2 we reported the results of a theoretical study on the geometric and electronic structures, vibrational properties, and relative stabilities of a series of small niobium clusters in both neutral and singly charged states Nbn0/( with n = 1 61 and 7 12.2 Different quantum chemical techniques were employed including density functional theory using both BPW91 and M06 functionals and coupled-cluster theory CCSD(T) method. A comparison with available experimental far-IR spectra of clusters smaller than Nb10 allowed us to draw a conclusion that the BPW91 functional in conjunction with the correlation consistent cc-pVTZ-PP basis set is relatively reliable for determining vibrational spectroscopic information of pure niobium clusters. It has also been found that in the series of Nbn (n = 2 9) clusters considered, the lowest-energy isomer is in each case consistently the main carrier of the observed FIR spectrum. In addition, we predicted the lowest-energy structures and their IR spectra for the larger clusters from Nb10 to Nb12, whose experimental IR spectra had not been available yet. In a subsequent comment on our paper,2 Fielicke and Meijer3 reported their experimental far-IR spectra of clusters containing from 10 to 12 Nb atoms in both neutral and cationic states. These authors also calculated the IR spectra for these systems using DFT with the TPSS functional.4 In general, their results agreed well with those previously predicted by us.2 The shapes of the optimal geometries of the other clusters, namely, Nb100/+ and Nb110/+, have already been found in our work.2 Their calculated vibrational spectra are actually manifested in and can be assigned to the experimentally observed spectra reported in ref 3. The main difference concerns the neutral Nb12 cluster. For this size, an encapsulated structure, denoted as isomer B, which was not reported in our paper, has been assigned as the carrier for the observed FIR spectrum of Nb12, even though it is not the lowestlying structure.3 Fielicke and Meijer3 insisted that the band pattern on the computed IR spectrum of isomer B convincingly agrees with experiment, whereas the spectra of other isomers, including the most stable isomer A, do not match the experimental findings at all. Therefore, we now consider again the structural and spectral features of Nb12 and Nb12+. For Nb12, we predicted its lowest-energy structure to be a distorted icosahedron, denoted as isomer A in ref 3. As stated above, Fielicke and Meijer3 pointed out that the computed IR spectrum of this isomer A does not match the experimental FIR findings. Instead, the second lowest-energy isomer B was assigned to be responsible for the recorded spectrum even though its relative band intensities do not match well with experiment.
I
r 2011 American Chemical Society
Figure 1. Theoretical IR spectra of the encapsulated isomer B along with the experimental spectrum of Nb12 taken from ref 3.
In this context, we again carried out quantum chemical calculations for these isomers using several functionals including the BPW91, B3P86, M06, and TPSS. The results displayed in Figures 1 and 2 show that the predicted spectra of the less stable isomer B are, as a matter of fact, in better agreement with experimental band pattern than those of the most stable isomer A. Although the band pattern of the isomer B (Figure 1) predicted by BPW91 and TPSS functionals is in line with experiment, the corresponding relative band intensities do not match quite well (Figure 1). For isomer A, only the picks centered at 225 and 260 cm 1 are seen in the calculated IR spectrum obtained by the BPW91 functional (Figure 2). Received: October 18, 2011 Published: November 02, 2011 14127
dx.doi.org/10.1021/jp210011a | J. Phys. Chem. A 2011, 115, 14127–14128
The Journal of Physical Chemistry A
Figure 2. Theoretical IR spectra of the distorted icosahedron isomer A, along with the experimental spectrum of Nb12 taken from ref 3.
Concerning the relative energies, the authors of the Comment3 found from their computations using the TPSS functional that isomer B is 0.26 eV higher in energy than isomer A. Using the cc-pVTZ-PP basis set, we found that the former lies from 0.33 (TPSS), to 0.40 (B3P86 and M06), to 0.6 eV (BPW91) above the latter. Accordingly, the point of interest is as to whether the most stable form A was present in the IR experiment. As far as calculated results are concerned, the results displayed in both Figures 1 and 2 tend to suggest that the experimental FIR spectrum of Nb12 likely arises from vibrations of isomer B rather than from isomer A, even though the presence of different structural isomers for Nb12 was reported in a previous study on N2 and D2 reaction kinetics.5 For the cation Nb12+, these authors3 also suggested the existence of a small amount of a similar isomer B+ in experiment based on some bands centered above 300 cm 1. However, we find that such isomer is much higher in energy than isomer A+, that is, from 1.00 (TPSS) to 1.30 eV (BPW91). In addition, the intensities of the bands at above 300 cm 1 on the observed spectrum are too low to be safely assigned to the peaks at 310 and 340 cm 1 on the calculated one. (See Figure 3.) In summary, we agree with the Comment of Fielicke and Meijer3 about the existence of the second lowest-energy isomer B for the neutral Nb12 and also confirm the previous spectrum of the cation Nb12+. Our results support the view that the experimental FIR spectrum of Nb12 recorded by these authors arises from a high-energy isomer. In this case, the lowest-energy structure A of Nb12 was apparently absent under experimental conditions, due to some subtle kinetics that we cannot usefully comment on. The relative energies between cluster isomers are known to be strongly method-dependent;6 a question of interest is as to whether the relative energy ordering between both isomers
COMMENT
Figure 3. Theoretical IR spectra of the distorted icosahedron isomer A+ (at 0.0 eV) and B+ (at 1.3 eV) in the cation Nb12+, along with the experimental spectrum taken from ref 3.
A and B could be reserved when higher level methods could be employed.2 This issue needs to be adequately addressed in following quantum chemical studies.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We thank Dr. Andre Fieliecke from Fritz-Haber Institute, Berlin for giving us the original plots of their experimental spectra. We are indebted to the KULeuven Research Council (GOA, IDO, IUAP, and PDM programs) for support. P.V.N. thanks the Vietnamese Government for a doctoral scholarship. ’ REFERENCES (1) Nhat, P. V.; Ngan, V. T.; Nguyen, M. T. J. Phys. Chem. A 2010, 114, 13210. (2) Nhat, P. V.; Ngan, V. T.; Tai, T. B.; Nguyen, M. T. J. Phys. Chem. A 2011, 115, 3523. (3) Fielicke, A.; Meijer, G. J. Phys. Chem. A 2011, 115, 7869. (4) Tao, J.; Perdew, J. P.; Staroverov, V. N.; Scuseria, G. E. Phys. Rev. Lett. 2003, 91, 146401. (5) Hamrick, Y.; Taylor, S.; Lemire, G. W.; Fu, Z. W.; Shui, J. C.; Morse, M. D. J. Chem. Phys. 1988, 88, 4095. (6) Ngan, V. T.; Gruene, P.; Claes, P.; Janssens, E.; Fielicke, A.; Nguyen, M. T.; Lievens, P. J. Am. Chem. Soc. 2010, 132, 15589.
14128
dx.doi.org/10.1021/jp210011a |J. Phys. Chem. A 2011, 115, 14127–14128
