ARTICLE pubs.acs.org/JPCC
Theoretical Investigation of a TitaniumAniline Complex with and without an Alkyl Chain Takeshi Iwasa,†,‡ Kazuki Horiuchi,‡ Masaya Shikishima,‡ Yuji Noguchi,‡ Shuhei Nagaoka,‡ and Atsushi Nakajima†,‡,* † ‡
JST, ERATO, Nakajima Designer Nanocluster Assembly Project, 3-2-1 Sakado, Takatsu-ku, Kawasaki, 213-0012, Japan Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan ABSTRACT: Density functional theory computations are employed to investigate the geometric, electronic, and vibrational properties of a neutral Ti(aniline) experimentally soft-landed onto an alkanethiol selfassembled monolayer (CH-SAM) matrix. Five optimized structures of a bare Ti(aniline) are obtained: Three of the five optimized structures show Tiphenyl ring bonds with singlet, triplet, and quintet spin states and the other two show TiN bonds with triplet and quintet states. For their calculated IR spectra in the range 02000 cm1, the peak position and intensity of the NH2 scissor mode (∼1600 cm1) are insensitive to the Ti binding and spin states, whereas those of the NH2C6H5 intergroup stretching (∼1200 cm1) and NH2 inversion (5001000 cm1) modes are sensitive to these factors. To study Ti(aniline) in a CH-SAM matrix, geometric structures of a modeled system of Ti(aniline)C3H8 are optimized and their infrared reflection absorption spectra are directly calculated by projecting the normal mode derivatives of the dipole moment of the system onto the surface normal. C3H8 stabilizes Ti(aniline) by about 810 kcal/mol but causes little change in the IR spectrum in the range 13001700 cm1, except for the quintet state. From a comparison of the predicted IRAS spectra with the experiment, Ti(aniline) in CH-SAM is concluded to be in a triplet state with a Ti-phenyl ring bond.
1. INTRODUCTION Functionalized surfaces decorated by chemical species have been intensively investigated because of their promising applications as nanodevices1,2 or biosensors.3 The chemical species used to functionalize a surface varies from small molecules to large macromolecules. Among them, transition metalbenzene sandwich clusters have attracted a wide range of interests because of their unique geometry,4,5 magnetic properties,6,7 and the potential use for hydrogen storage.8,9 As for the application of these clusters to advanced functional nanomaterials, it is desirable to immobilize and properly align the organometallic clusters on a surface to construct cluster assembly in a definite way while retaining their functionality. For this purpose, as well as to characterize these clusters geometrically and electronically, the soft-landing technique, in which gas-phase synthesized clusters are size-selectively deposited into a substrate covered with alkanethiol self-assembled monolayer (CH-SAM), has been extensively developed.714 For practical applications, robustness of the functionalized surface is mandatory. Our previous studies have shown that soft-landed transition metalbenzene sandwich clusters could be trapped via physisorption inside the ordered surroundings of the alkyl chains of a CH-SAM matrix above room temperature.10,11 One promising way to improve their robustness is the use of benzene derivatives and a terminal substituted alkanethiol SAM that can make a chemical bond. This technique is sometimes called reactive-landing,12 as opposed to soft-landing. r 2011 American Chemical Society
If a metalaniline cluster is landed onto a COOH-terminated SAM (COOH-SAM), an amide bond could form. In addition, decoration with chemically reactive species such as an opensandwich of metalaniline will open up a new avenue to functionalize a substrate as a reactive designer surface. This reactivity, however, increases the complexity of the geometry and electronic states of a cluster, especially in a SAM matrix. The experimental characterization of species on a surface is also more complicated than in the gas phase. Hence, it is desirable to perform theoretical computations for characterizing the geometry and electronic state of a newly synthesized organometallic complex in a step by step approach. In this paper, we theoretically focus on the open sandwich 1:1 complex of titaniumaniline, Ti(aniline), which is thought to be a potential hydrogen storage material,8,9 which we have recently synthesized in the gas phase and then soft- and reactive-landed onto CH-SAM and COOH-SAM matrices, respectively.13 As a first step toward preparing a reactive designer surface, this paper aims at characterizing Ti(aniline) itself and soft-landed Ti(aniline) as well. The geometric, electronic, and vibrational properties of a bare Ti(aniline) and that interacting with an alkanethiol are investigated using density functional theory (DFT) computations, which generally provide good predictions Received: May 25, 2011 Revised: July 15, 2011 Published: July 18, 2011 16574
dx.doi.org/10.1021/jp204881b | J. Phys. Chem. C 2011, 115, 16574–16582
The Journal of Physical Chemistry C of the geometry and spin states of these complexes with a relatively reasonable computational cost.59 It should be noted that exhaustive studies15,16 show that the lowest energy state given by the DFT computations of transition metalaniline (or other benzene derivatives) open sandwich complexes depends on the functionals and basis sets. On the other hand, infrared (IR) spectroscopy is an informative tool for geometry determination because its peak positions and intensities vary in a diagnostic manner, depending on the structures and electronic spin states. We therefore rely on a comparison of theoretical vibrational spectra with the experiments to characterize the geometric and electronic states of the Ti(aniline) in a CHSAM matrix. Infrared reflection absorption spectroscopy (IRAS) is useful to obtain information on the adsorbed state of these soft-landed complexes in their neutral form. In IRAS measurements, as explained elsewhere,14 molecules on a surface mainly absorb the electromagnetic field that polarizes in the surface normal direction because of the interference between the incident and reflected electromagnetic fields. Analysis of IRAS spectra reveals the orientation of molecules adsorbed because vibrational modes whose dynamic dipole moments lie along the surface normal direction are selectively excited. Because the introduction of a functional group into a benzene ligand lowers the geometric symmetry of the complex, the directions of the dynamic dipole moment of the complex are as not apparent as in the case for metalbenzene complexes. In the present study, we develop a method to directly simulate IRAS spectra utilizing the individual Cartesian components of the dynamic dipole moment, i.e., the normal mode derivatives of the dipole moment, whose squares correspond to the IR intensity.15,16 The structure of this paper is as follows. Section 2 describes the computational details and the formulations for predicting the IRAS spectrum. In section 3A, the geometric and electronic structures of a bare Ti(aniline) are presented, followed by its IR spectrum in section 3B. Then for a better understanding of the complex-SAM interaction, the geometric and electronic structures of a hybrid system of Ti(aniline)C3H8 are presented in section 3C, where C3H8 models the SAM alkyl chain to describe the local surroundings. The IR and IRAS spectra as well as comparisons with experimental results are given in section 3D to determine a plausible geometry, including the electron spin state and the orientation of the cluster in the CH-SAM of n-octadecanethiol (C18); C18-SAM. Concluding remarks are presented in section 4.
2. COMPUTATIONAL METHODS All the electronic structure calculations are performed with TURBOMOLE 6.1 and 6.217,18 with an SCF energy convergence criterion of 107 Eh. The geometry of Ti(aniline) is optimized starting from the initial guess structure mimicking the π- and n-states of the Cr+(aniline) complex19,20 at the level of the KohnSham density functional theory (KS-DFT) employing the Becke three-parameter hybrid exchange functional with the LeeYangParr correlation functional (B3LYP).21,22 The triple-ζ valence-quality plus polarization basis, def2TZVP,23 from the TURBOMOLE basis set library was used in all the calculations. The geometric optimizations are continued until the harmonic frequency analysis performed by aoforce, a part of the TURBOMOLE program package, gives no imaginary frequencies.
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To compute IR spectra for our purpose given below, a slightly modified version of SNF 4.024 is used. Briefly, in this program the kth IR peak, Ik, is computed using the following formulation. 2 !2 ∂μ 3N ∂μR ~ ck, i ð1Þ Ik ∼ ¼ ∂Qk ∂bi R ¼ x, y, z i ¼ 1
∑
∑
where μB is the dipole moment of the molecule, Qk is the kth normal mode, and ck,i is the transformation matrix from the Cartesian atomic displacements bi = (i = x, y, z) to the kth normal mode Qk. In all of the calculations using SNF 4.0, the absolute value of bi is taken to be 0.01 au. To better enable IR peak analysis in terms of a functional group, the “population” of a functional group A in a peak intensity of a kth normal mode, Ik(A), is defined as follows: ! ! 3N ∂μR ∂μR ck, i ck, j Ik ðAÞ ð2Þ ∂bi ∂bj R ¼ x, y, z i∈A j¼1
∑
∑
∑
This kind of partition scheme is widely used in determining atomic charge by the Mulliken population analysis.25 To compare the computed IRAS spectra with the experimental ones, the scaling factor of 0.977 is used for frequencies throughout this paper. This value is chosen so that the IR spectrum of aniline in the gas phase taken from the NIST Chemistry webbook26 is reproduced in the range of 13001700 cm1 at the B3LYP/ def2-TZVP level of theory (Figure 3b). Then, we shift our focus to the interaction between the Ti(aniline) complex and CH-SAM matrix to discuss the stability and IR peak positions and intensities of Ti(aniline) together with the orientation of Ti(aniline) in the matrix. To that end, further DFT computations are performed for a model system of Ti(aniline)C3H8 with possible geometric configurations that are derived from the optimized bare Ti(aniline). It may safely be assumed that the local environment of an alkyl chain of CH-SAM is reasonably represented by C3H8 because C3H8 contains both a methylene and methyl groups. The orientation of Ti(aniline) in the CH-SAM matrix is determined by fitting the C3H8 group in the geometrically optimized model system to an alkyl group in an alkanethiol that constitutes the CH-SAM matrix, assuming that the alkanethiols tilt from the surface normal by 15°, as has been reported in previous studies.10,11 The IRAS spectra are computed for those configurations using the next formula: 2 ∂μ ~ Ik ∼ ð3Þ n ∂Qk 3 B where B n represents the unit vector in the surface normal direction. Before moving on to Results and Discussion, the accuracy of the present DFT calculations with the B3LYP functional should be mentioned. The total energies and IR spectra are compared to those computed at the B3LYP and PBE027,28/def2-TZVP level of theory using TURBOMOLE and at the B3PW9122,29 and MPW1PW9130/TZVP31 level of theory using the Gaussian 03 program package32 for π geometries with singlet, triplet, and quintet states. The prominent differences in these calculations are as follows. The most stable spin state is quintet with the B3PW91/TZVP calculation. The relative total energies calculated with MPW1PW91/TZVP between different spin states are smaller by an order of magnitude than the results with the other functionals and basis sets. The differences in the peak positions 16575
dx.doi.org/10.1021/jp204881b |J. Phys. Chem. C 2011, 115, 16574–16582
The Journal of Physical Chemistry C
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are up to 100 cm1. Despite these differences, the number of peaks and the peak intensity ratios in the range 13001700 cm1 are rather insensitive to the different functionals and basis sets. In determining the geometric structure and electron spin state, we rely on comparisons of computed IR spectra with experiments rather than the total energy, because the total energy differences between the different spin states are small and depend on the functional/basis sets, as discussed by Oomens et al.19,20
3. RESULTS AND DISCUSSION We first present the optimized structures and infrared spectra for a bare Ti(aniline) and then those for a modeled system of Ti(aniline)C3H8 to assess the influence of the CH-SAM matrix on both the geometric and electronic structures and IR spectrum, as well as to determine the orientation of Ti(aniline) in the matrix for the purpose of comparing the IRAS spectrum with that from experiment. A. Geometric and Electronic Structures of Ti(aniline). Figure 1 displays the optimized structures of Ti(aniline) along with the relative total energies measured from the most stable π3, whose total energy is on average 0.55 eV lower than the others. The inset in Figure 1 assigns numbers to each carbon atom in aniline and defines the x and y axes for later use. The structures shown in Figure 1 are labeled as π and n to denote the binding
Figure 1. Side views of the optimized structures of Ti(aniline) and the relative total energies. Inset shows the atomic labels of aniline and the xy axes. TiN and TiC bonds are drawn if these interatomic distances are shorter than 2.233 and 2.254 Å, respectively.
sites of Ti to aniline following the notation Moore et al.19 and the subscripts show the spin multiplicities. Thus, πn1, π3, and π5 are the singlet, triplet, and quintet spin states with Tiphenyl ring bonds, and n3, and n5 are the triplet and quintet spin states with TiN bonds, respectively. All of the complexes have Cs symmetry except for n3, which is of C1 symmetry (but almost Cs). A geometry optimization for a singlet state starting from n1 form, in which the Ti atom is placed near the amino group, gives the same structure as πn1. In the πn1 form, the planar geometry of aniline is broken so that its amino group approaches to the Ti atom. This suggests a strong interaction between the Ti and N atoms. Hence, in the πn1 form, the Ti atom interacts not only with the π-ring but also with the N atom of the amino group. The latter interaction between Ti and N atoms is strong compared to that in the n3 and n5 forms, in which Ti mainly interacts with N and C1 atoms. Indeed, as shown in Table 1, when the nearest neighbor interatomic distances and shared electron numbers are compared for TiN, NC1, and TiC1, the TiN distance and the corresponding shared electron number of πn1 are comparable to those of the n3 and n5 forms. In the π3 form, the phenyl ring bends slightly, which makes the TiC2,3,5, and 6 bond distances shorter than TiC1 and 4 by about 0.1 Å. The Tianiline bond distances generally elongate as their spin multiplicities increase. The geometric structures are related to the KohnSham (KS) orbitals. In Figure 2, the KS orbitals from HOMO to HOMO3 of the optimized structures are shown together with the corresponding KS energy diagrams in the same order as the KS orbitals at the top of each column, where the up and down arrows show the up and down electron spins, respectively. The πn1 form has two degenerate dδ bonding orbitals,33 consisting of Ti(3dx2y2) and Ti(3dxy) and two unoccupied π-orbitals of aniline (π*), which correspond to the doubly degenerate unoccupied πorbitals of benzene having two nodes. We hereafter will refer to the dδ bonding orbitals consisting of Ti(3dx2y2) as dδx2y2 and Ti(3dxy) as dδxy. The frontier orbitals of π3 are one dδx2y2-like bonding orbital, two doubly occupied dδxy bonding orbitals, and one dz2 nonbonding orbital. This unbalance in the occupation of the two dδ bonding orbitals causes the difference in bond length between TiC2,3,5, and 6 and TiC1 and 4. Furthermore, the bond
Table 1. Bond Lengths for Ti(aniline) (Å) and Shared Electron Numbers (SEN) for TiN, NC1, and TiC1 Bondingsa TiN
TiC1
NC1
aniline
(a) Bond Lengths TiC2 TiC3
Ti--C4
1.39
C1C2
C2C3
C3C4
1.40
1.39
1.39
πn1
2.25
1.47
2.02
2.11
2.21
2.31
1.46
1.44
1.42
π3
3.40
1.40
2.36
2.21
2.22
2.34
1.41
1.47
1.41
π5
3.58
1.39
2.59
2.50
2.47
2.48
1.41
1.42
1.40
n3
2.32
1.42
2.29
2.64
3.37
3.73
1.41
1.40
1.39
n5
2.31
1.43
2.41
2.97
3.86
4.25
1.41
1.39
1.39
(b) Shared Electron Number TiN aniline
a
NC1
TiC1
1.36
πn1
0.11
1.23
π3
