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Geometric and electronic structure of the diphenylamine radical cation: an EPR, ENDOR and MO study
Journal of Molecular Structure 733 (2005) 13–17 www.elsevier.com/locate/molstruc
Geometric and electronic structure of the diphenylamine radical cation: an EPR, ENDOR and MO study Wei Liua,*, Anders Lundb a Department of Applied Chemistry, Faculty of Engineering, Oita University, Dannoharu 700, Oita 870-1192, Japan Chemical Physics Laboratory, Department of Physics and Measurement Technology, Linko¨ping University, S-581 83 Linko¨ping, Sweden
b
Received 28 April 2004; revised 17 July 2004; accepted 22 July 2004 Available online 2 September 2004
Abstract The geometric and electronic structure of the diphenylamine radical cation Ph2(H)N%C obtained by X-irradiation of diphenylamine in CFCl3 at low temperature have been studied by the methods of EPR and ENDOR combined with theoretical calculations. The 110 K ENDOR spectrum of Ph2(H)N%C is mainly attributed to the ortho and meta phenyl ring protons. The accurate assignment of spectroscopic parameters is based on spectral simulations and theoretical calculations. Close agreement was obtained between the experimental and calculated hyperfine tensors of ortho and meta protons. q 2004 Elsevier B.V. All rights reserved. Keywords: Diphenylamine radical cation; EPR; ENDOR
1. Introduction Diphenylamine (Ph2NH) finds wide industrial applications as a useful intermediate for the synthesis of rubber chemicals, medicine, dyes, etc. Besides, diphenylamine, having a sufficiently low ionization potential (IPS Z 7:25G0:03 eV) [1] and suitable dimensions to enter the zeolite channels, has been used for probing acid sites on the internal surface of zeolites by the method of spontaneous generation of radical cations from organic molecules upon adsorption of organic molecules inside the constrained environment of zeolites [2–7]. Zeolites act as electron acceptors due to the presence of Lewis or Bro¨nsted acid sites while Ph2NH is an electron donor. The formed radical Ph2(H)N%C showed a broad EPR spectrum (gZ2.002) [6,7]. Compared with the liquid EPR spectrum of Ph2(H)N%C in CF3COOH [8], the powder EPR spectral line shape of Ph2(H)N%C was not well resolved. Therefore, no detailed information of the geometric and electronic structure was obtained for Ph2(H)N%C in solids. * Corresponding author. Tel.: C81-97-554-7894; fax: C81-97-5547979. E-mail address: [email protected] (W. Liu). 0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2004.07.032
Matrix isolation technique in combination with radiolysis provide us with an appropriate and uncomplicated method to extend the lifetime of radical cation Ph2(H)N%C, then the radical can be characterized by EPR spectroscopy. As well known, the most widely used matrices include Freon or other related halogenated solvents and zeolites. Among the halocarbon matrices, CFCl3 having ionization potential of 11.8 eV [1] is typical and was used. Another kind of alternative matrices, zeolites, has attracted much attention in the recent decades because of well-defined and controllable voids. In the present work, we aim to obtain the detailed information of geometric and electronic structure of Ph2(H)N%C in solid matrices including CFCl3 and HZSM-5 by the methods of EPR, Electron Nuclear Double Resonance (ENDOR) and theoretical calculations.
2. Experimental Diphenylamine (Ph2NH, 99C%, A.C.S Reagent) and CFCl3 purchased from Aldrich were used as received without further purification. Samples of Ph2NH in CFCl3 (Ph2NH/CFCl3) were prepared using standard techniques. Small quantities of diphenylamine were added into a quartz
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W. Liu, A. Lund / Journal of Molecular Structure 733 (2005) 13–17
tube, then the tube was connected to a vacuum line and pumped to 10K5 Torr. Degassed CFCl3 was transferred into the quartz tube through the vacuum line using liquid nitrogen to freeze the bottom of the sample tube, at last the sample was sealed. Less than 1 mol% solute mixture was prepared for EPR and ENDOR measurement. Samples of Ph2NH in HZSM-5 (Ph2NH/HZSM-5) were prepared using the method described in the literature [6]. X-ray irradiation of samples Ph2NH/CFCl3 and Ph2NH/HZSM-5 was performed at liquid nitrogen temperature for 40 min (about 10 kGy) utilizing a Philips X-ray tube with a tungsten anode operating at 70 kV and 20 mA. X-band EPR spectra were recorded with a Bruker ER200DSRC spectrometer. The ENDOR data for the irradiated Ph2NH/CFCl3 and Ph2NH/HZSM-5 were obtained with a Bruker ER250 ENDOR accessory and an ENI 3100LA RF amplifier. The radio frequency (rf) modulation depth for ENDOR measurement was 200 kHz. Sample temperatures above 77 K were regulated by a Bruker VT4111 temperature controller. ENDOR spectral simulation was performed on the program described by Lund et al. [9,10]. All theoretical calculations were carried out using the GAUSSIAN 98 program suite [11], and density functional theory (DFT). The geometry of the Ph2(H)N%C radical cation was optimized by the B3LYP/6-31GC(d,p) method. The B3LYP/EPR-II method was employed to calculate the isotropic and anisotropic hyperfine coupling constants (HFCC) of Ph2(H)N%C.
3. Results and discussion 3.1. EPR and ENDOR spectra The EPR spectrum of X-ray irradiated Ph2NH/CFCl3 sample at 110 K is shown as the solid curve in Fig. 1(a). The sharp peak at the center is ascribed to the signal of quartz due to irradiation. The spectrum gives a main peak with quite badly resolved smaller hyperfine (hf ) splittings. No more useful information was obtained from the EPR spectrum. Because the sample Ph2NH/CFCl3 is a simple and typical system for X-irradiation, it is reasonable to attribute the EPR spectrum to the Ph2(H)N%C radical cation in consideration of the relative ionization potential of Ph2NH and CFCl3. The features of Ph2(H)N%C are not resolved and identified in the EPR spectrum (Fig. 1(a)). The reason maybe that the EPR lines are inhomogeneously broadened by the large number of nuclei which interact with the unpaired electron. As a consequence, ligand hf couplings are often not resolved. For the purpose of overcoming the disadvantage of EPR spectroscopy, the ENDOR technique [12,13], in which the spin system is simultaneously irradiated by a microwave (MW) and a rf field, is an
Fig. 1. (a) Experimental EPR spectrum of X-ray irradiated Ph2NH/CFCl3 sample at 110 K. The arrow indicates the field position for the ENDOR spectrum. (b) Experimental (—) and simulated (/) powder ENDOR spectra of Ph2NH/CFCl3 (about 1 mol%) at 110 K. The experimental ENDOR spectrum consists of 50 time-averaged scans. The RF modulation depth was 200 kHz. The ENDOR spectral simulation was performed using parameters listed in Table 1.
alternative to resolve smaller hf couplings which are not accessible in the EPR spectra. ENDOR measurement were made by observing the central line at different field positions in order to resolve the hf splittings which are hiding under the main peak. Similar ENDOR spectra were obtained at all fields. The best resolved ENDOR spectrum of X-irradiated Ph2NH/CFCl3 sample is presented as the solid line in Fig. 1(b), obtained by setting the field of ENDOR spectrum at the position marked with an arrow in the figure and sweeping the rf in the range 1–27 MHz. The ENDOR lines mainly appeared between 7 and 22 MHz together with several weak lines in the low frequency region. The region 12.5–17 MHz is complicated due to the overlapping lines from the Freon matrix. The sharp band at 13.75 MHz is attributed to 19F matrix signal that is marked in the figure. The main sharp lines are symmetrically placed around the proton Larmor frequency (nHZ14.43 MHz). These line pairs can be separated into two groups, namely the 15.3–17.6 and 18.5–22.7 MHz lines in the high frequency region and the matching group of 11.4–12.8, 6.9–9.4 MHz lines in the low frequency region. The atoms of Ph2(H)N%C are labeled as shown in Fig. 2 (the atoms of the left phenyl ring are symmetry related to those at the right) together with the molecular coordinate system which adopts the standard orientation obtained from the geometry optimization. It was reported that Ph2(H)N%C in CF3COOH gave relatively larger isotropic hf splittings for the N atom (a(N)Z9.17 G), the a-proton ðaðHa ÞZ 11:14 GÞ and the two para protons (a(H 3)Z4.88 G) [8]. Accordingly, in the case of
W. Liu, A. Lund / Journal of Molecular Structure 733 (2005) 13–17
15
Fig. 2. The atom labeling and coordinate system used for radical cation Ph2(H)N%C. The coordinate system adopts the standard orientation obtained from the ˚. geometry optimization of Ph2(H)N%C by the B3LYP/6-31CG(d,p) method. The unit of bond length is A
Ph2(H)N%C in CFCl3, the hf tensors of the N atom, the a-proton ðHa Þ and the two para protons (H3) are comparatively large and anisotropic due to strong confinement of unpaired electrons to the 2pz orbital on the central N and the p-conjugation of the phenyl ring, and this is supported by the theoretical calculation of Ph2(H)N%C (see details in Table 2). In addition, the numbers of these atoms are small in comparison with those of the other phenyl ring protons. Thus, the ENDOR absorption from N, Ha and H3 can therefore be neglected in consideration of the weak absorption intensity and the line broadening caused by their larger anisotropic hf couplings. We conclude that the ENDOR spectral line shape is mainly due to the ring protons in ortho (H1,H5) and meta (H2,H4) position. The group of 18.5–22.7 MHz lines (matching lines 6.9–9.4 MHz) is attributed to the ortho protons because the hf tensors of ortho protons are relatively larger. The meta protons mainly contribute the lines pair of 15.3–17.6 and 11.4–12.8 MHz. The intensity of lines in the low frequency region 4.0–6.8 MHz are too weak to identify, but contributions of the N atom or para protons are considered. The fact that the intensity of lines below nH are lower than those of the matching lines is due to the hf enhancement effect for powder ENDOR in solids [9]. The powder ENDOR spectral line shape of Ph2(H)N%C is quite complex to interpret because of line broadening due to the anisotropy of the hf and, possibly, 14N quadrupolar interactions in comparison to solution or single crystal spectra. In addition, the powder ENDOR spectrum is further complicated by orientational selectivity and is a sum of contributions from a range of orientations [9]. Accurate assignment of spectroscopic parameters therefore has to rely on computer simulation of the spectrum. The theory of the ENDOR simulation was described by Lund et al. [9]. The 110 K ENDOR spectrum was
tentatively simulated and the best-fit simulation spectrum is shown as the dotted line in Fig. 1(b) using the parameters listed in Table 1. Because both phenyl rings are equivalent, only one set of ortho (H1,H5) and meta (H2,H4) protons of the right phenyl ring were treated in the spectral simulation. Matrix fluorine, N atom, a-proton and para protons were excluded in the simulation in order to simplify the calculation. A Gaussian line shape with an EPR line width of 3 G (8.4 MHz) and an NMR line width of 0.2 MHz was employed. The simulated ENDOR spectra were very sensitive to the principal values of the ortho and meta proton hf tensors and the best fit obtained therefore permitted determining the values to within G0.2 MHz. The spectral simulation confirms the assignment of the ENDOR spectrum. The obtained isotropic hf couplings of the ortho and meta protons (Table 2) agree well with the reported results (a(Ho)Z3.51 G, a(Hm)Z1.36 G, 1 GZ 2.8 MHz [8]). Furthermore, the inaccurate relative intensity distribution is acceptable in consideration of the disturbing absorption due to matrix fluorine, N atom, a-proton or para proton hf interactions. For the Ph2NH/HZSM-5 sample, no clear observation was made for the spontaneous generation of Ph2(H)N%C. A X-irradiated sample was therefore studied by EPR and ENDOR. The EPR spectrum observed at 110 K shows a broad line and is similar to the reported spectra of Ph2(H)N%C in silica–alumina as well as in Y and HZSM-5 zeolites [6,7]. Unfortunately, no ENDOR absorption was observed. The generation of Ph2(H)N%C was considered in the case of X-ray irradiation of sample Ph2NH/HZSM-5 at low temperature, in agreement with our past research of X- or g-irradiated amine/zeolite system showing that radical formation proceeded by transferring holes to the incorporated organic molecules from the solid matrix, i.e. g-irradiated Et3N/AlPO4-5 and Pr3N/AlPO4-5 [14].
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W. Liu, A. Lund / Journal of Molecular Structure 733 (2005) 13–17
Table 1 The hyperfine tensors obtained by the spectral simulation of the ENDOR spectrum of Ph2(H)N%C in CFCl3 at 110 K Tensorsa
Principal component
g
A (H2)
A (H4)
A (H5)
Direction cosinesc
2.0021 2.0021 2.0030 K12.8 K11.6 K4.0 1.8 4.1 5.3 1.7 3.4 4.4 K14.7 K11.8 K4.7
gx gy gz Aa Ab Ac Aa 0 Ab 0 Ac 0 Ap Aq Ar Ap 0 Aq 0 Ar 0
A (H1)
Principal valueb
x
y
z
1 0 0 K0.2637 0.8429 0.4690 0.8694 0.4055 0.2824 0.9160 0.3015 0.2645 0.2384 0.9392 0.2471
0 1 0 0.7689 K0.1099 0.6298 K0.1043 K0.4079 0.9071 K0.1519 0.8712 K0.4669 0.9596 K0.2670 0.0889
0 0 1 K0.5824 K0.5267 0.6191 K0.4830 0.8180 0.3123 K0.3712 0.3875 0.8438 K0.1495 K0.2159 0.9649
The g tensor was estimated from the EPR spectrum. a Tensors A are the hyperfine tensors of H1, H2, H4, H5 labeled in Fig. 2. b Hyperfine tensor values in MHz. c Direction cosines of the hyperfine tensors of H1, H2, H4, H5 are given in the molecular coordinate system as defined in Fig. 2 and are obtained from the optimized geometric structure of Ph2(H)N%C.
3.2. Theoretical calculations All theoretical calculations of the Ph2(H)N%C radical cation were performed with DFT because of the very
accurate resulted HFCC and great computational advantages. The geometry for Ph2(H)N%C was optimized by Becke’s three parameter hybrid method where the non-local correlation functional is that of Lee, Yang, and Parr [15a–d]
Table 2 Comparison of the observed and calculated hyperfine tensorsa for radical cation Ph2(H)N%C Atom
Calculated spin density
Principal dipolar HFCCb
Observed isotropic HFCCb
Calculated isotropic HFCCb
Observed value
Calculated value
Direction cosinesc
0.367
– – –
K18.26 K17.93 36.19
K0.0116 0.0000 0.9999
0.9999 0.0000 0.0116
0.0000 1.0000 0.0000
–
21.49
ðHa Þ
K0.018
– – –
K20.71 K6.73 27.44
K0.0968 0.9953 0.0000
0.9953 0.0968 0.0000
0.0000 0.0000 1.0000
–
K35.87
H1
K0.008
H2
0.004
H3
K0.011
H4
0.004
H5
K0.008
K3.3 K2.2 5.5 K1.9 0.4 1.5 – – – K1.5 0.2 1.3 K4.3 K1.4 5.7
N
a b c d
K3.51 K2.66 6.17 K1.73 K0.25 1.97 K8.54 K0.93 9.46 K1.67 K0.3 1.97 K4.13 K2.46 6.59
–d –d –d –d –d –d
–d –d –d –d –d –d
–d –d –d –d –d –d
0.3454 0.9384 K0.0030 –d –d –d –d –d –d
0.3370 K0.1211 0.9337 –d –d –d –d –d –d
0.8759 K0.3235 K0.3581 –d –d –d –d –d –d
K9.5
K10.41
3.7
5.49
–
K14.67
3.2
5.01
K10.4
K11.07
Calculated hyperfine tensors by the method of B3LYP/EPR-II//B3LYP/6-31CG(d,p). Unit of HFCC is MHz. Direction cosines are obtained from the optimized geometric structure of Ph2(H)N%C with molecular coordinate system as defined in Fig. 2. Same values as Table 1.
W. Liu, A. Lund / Journal of Molecular Structure 733 (2005) 13–17
in conjunction with Pople’s 6-31CG(d,p) basis set (B3LYP/ 6-31GC(d,p)). The isotropic and anisotropic HFCC of Ph2(H)N%C radical were obtained by the B3LYP/EPR-II method. The molecular EPR hf splittings, which consist of an isotropic part and an anisotropic part, are caused by the magnetic interaction between the nuclear spin and the electronic magnetic moments. The isotropic coupling (Aiso) depends on the unpaired spin density, whereas the anisotropic coupling is related to the interactions between magnetic dipoles. The full coupling tensor of the nucleus in the principal axes is calculated by the addition of the isotropic HFCC to each component of the dipolar hf tensor (Tii), AiiZAisoCTii [16]. The optimized geometric structure of Ph2(H)N%C belongs to the C2 point group and is shown in Fig. 2 with standard coordination. Direction cosines of hf tensors are obtained from the calculation results of geometric structure optimization of Ph2(H)N%C following the molecular coordinate system as defined in Table 2. There is a dihedral angle of 22.48 between both phenyl ring planes and the Ha –N–Ca –Ca plane. The Ha –N–Ca –Ca planar structure of Ph2(H)N%C gives a singly occupied molecular orbital (SOMO) that should be strongly confined to the Px orbital on the nitrogen atom. The calculated result that the spin density on the N atom is only 36.7% suggests that the p-delocalisation of unpaired electrons on the N atom to the phenyl rings plays an important role for the distribution of spin density. The calculated full hf tensors as well as the experimentally determined tensors are listed in Table 2 together for comparison. As shown in the table, the observed hf tensors of protons H1, H2, H4 and H5 are in close agreement with the calculated results. The calculated hf tenors of the N atom, the a-proton ðHa Þ and the two para protons (H3) are demonstrated without the corresponding experimental ones. Because the ENDOR intensity is usually about 1% of the ESR intensity, it is difficult to observe all spectral features of Ph2(H)N%C, especially those corresponding to the couplings with larger anisotropic couplings. Experimental hf tenors of N, Ha and H3 were accordingly not obtained from the ENDOR measurements.
4. Conclusion The geometric and electronic structures of Ph2(H)N%C were studied by EPR, ENDOR and theoretical calculation
17
methods. Radical cation Ph2(H)N%C is a C2 structure. The ortho and meta ring protons which have smaller hf coupling are mainly responsible for the line shape of the 110 K ENDOR spectrum. The ENDOR spectral simulation was successfully employed in obtaining relatively accurate experimental hf tenors and verifying the assignment of the ENDOR lines. Good agreement was achieved for the experimental and calculated hf tensors of Ph2(H)N%C.
References [1] R.C. Weast (Ed.), Handbook of Chemistry and Physics, 51st ed., The Chemical Rubber Publishing Company, 1970–1971. [2] D.N. Stamires, J. Turkevich, J. Am. Chem. Soc. 86 (1964) 749. [3] F.R. Chen, J.J. Fripiat, J. Phys. Chem. 96 (1992) 819. [4] M. Bonora, M. Brustolon, L. Storaro, M. Casagrande, D. Biglino, Y. Itagaki, U. Segre, M. Lenarda, Phys. Chem. Chem. Phys. 2 (2000) 4823. [5] V. Martı´, L. Ferna´ndez, V. Forne´s, H. Garcı´a, H.D. Roth, J. Chem. Soc. Perkin Trans. 2. (1999); 145. [6] H. Garcı´a, V. Martı´, I. Casades, V. Forne´s, H.D. Roth, Phys. Chem. Chem. Phys. 3 (2001) 2955. [7] B.D. Flockhart, L. McLoughlin, R.C. Pink, J. Catal. 25 (1972) 305. [8] V.A.F. Neugebauer, S. Bamberger, Angew. Chem. 83 (1971) 48. [9] R. Erickson, A. Lund, M. Lindgren, Chem. Phys. 193 (1995) 89. [10] R. Erickson, Chem. Phys. 202 (1996) 263. [11] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, V.G. Zakrzewski, J.A. Montgomery, Jr., R.E. Stratmann, J.C. Burant, S. Dapprich, J.M. Millam, A.D. Daniels, K.N. Kudin, M.C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G.A. Petersson, P.Y. Ayala, Q. Cui, K. Morokuma, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J. Cioslowski, J.V. Ortiz, A.G. Baboul, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, J.L. Andres, C. Gonzalez, M. Head-Gordon, E.S. Replogle, J.A. Pople, GAUSSIAN 98, Revision A.7, Gaussian, Inc., Pittsburgh, PA, 1998. [12] L. Kevan, L.D. Kispert, Electron Spin Double Resonance Spectroscopy, Wiley, New York, 1976. [13] A. Schweiger, Electron Nuclear Double Resonance of Transition Metal Complexes with Organic Ligands, Springer, Berlin, 1982. [14] W. Liu, S. Yamanaka, M. Shiotani, J. Michalik, A. Lund, Phys. Chem. Chem. Phys. 3 (2001) 1611. [15] (a) A.D. Becke, Phys. Rev. A38 (1988) 3098; (b) A.D. Becke, J. Chem. Phys. 98 (1993) 1372; (c) A.D. Becke, J. Chem. Phys. 98 (1993) 5648; (d) C. Lee, W. Yang, R.G. Parr, Phys. Rev. 37 (1988) 785. [16] L.A. Eriksson, Encyclopedia of Computational Chemistry, Wiley, New York, 1998 pp. 925–958.
Geometric and electronic structure of the diphenylamine radical cation: an EPR, ENDOR and MO study Wei Liua,*, Anders Lundb a Department of Applied Chemistry, Faculty of Engineering, Oita University, Dannoharu 700, Oita 870-1192, Japan Chemical Physics Laboratory, Department of Physics and Measurement Technology, Linko¨ping University, S-581 83 Linko¨ping, Sweden
b
Received 28 April 2004; revised 17 July 2004; accepted 22 July 2004 Available online 2 September 2004
Abstract The geometric and electronic structure of the diphenylamine radical cation Ph2(H)N%C obtained by X-irradiation of diphenylamine in CFCl3 at low temperature have been studied by the methods of EPR and ENDOR combined with theoretical calculations. The 110 K ENDOR spectrum of Ph2(H)N%C is mainly attributed to the ortho and meta phenyl ring protons. The accurate assignment of spectroscopic parameters is based on spectral simulations and theoretical calculations. Close agreement was obtained between the experimental and calculated hyperfine tensors of ortho and meta protons. q 2004 Elsevier B.V. All rights reserved. Keywords: Diphenylamine radical cation; EPR; ENDOR
1. Introduction Diphenylamine (Ph2NH) finds wide industrial applications as a useful intermediate for the synthesis of rubber chemicals, medicine, dyes, etc. Besides, diphenylamine, having a sufficiently low ionization potential (IPS Z 7:25G0:03 eV) [1] and suitable dimensions to enter the zeolite channels, has been used for probing acid sites on the internal surface of zeolites by the method of spontaneous generation of radical cations from organic molecules upon adsorption of organic molecules inside the constrained environment of zeolites [2–7]. Zeolites act as electron acceptors due to the presence of Lewis or Bro¨nsted acid sites while Ph2NH is an electron donor. The formed radical Ph2(H)N%C showed a broad EPR spectrum (gZ2.002) [6,7]. Compared with the liquid EPR spectrum of Ph2(H)N%C in CF3COOH [8], the powder EPR spectral line shape of Ph2(H)N%C was not well resolved. Therefore, no detailed information of the geometric and electronic structure was obtained for Ph2(H)N%C in solids. * Corresponding author. Tel.: C81-97-554-7894; fax: C81-97-5547979. E-mail address: [email protected] (W. Liu). 0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2004.07.032
Matrix isolation technique in combination with radiolysis provide us with an appropriate and uncomplicated method to extend the lifetime of radical cation Ph2(H)N%C, then the radical can be characterized by EPR spectroscopy. As well known, the most widely used matrices include Freon or other related halogenated solvents and zeolites. Among the halocarbon matrices, CFCl3 having ionization potential of 11.8 eV [1] is typical and was used. Another kind of alternative matrices, zeolites, has attracted much attention in the recent decades because of well-defined and controllable voids. In the present work, we aim to obtain the detailed information of geometric and electronic structure of Ph2(H)N%C in solid matrices including CFCl3 and HZSM-5 by the methods of EPR, Electron Nuclear Double Resonance (ENDOR) and theoretical calculations.
2. Experimental Diphenylamine (Ph2NH, 99C%, A.C.S Reagent) and CFCl3 purchased from Aldrich were used as received without further purification. Samples of Ph2NH in CFCl3 (Ph2NH/CFCl3) were prepared using standard techniques. Small quantities of diphenylamine were added into a quartz
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W. Liu, A. Lund / Journal of Molecular Structure 733 (2005) 13–17
tube, then the tube was connected to a vacuum line and pumped to 10K5 Torr. Degassed CFCl3 was transferred into the quartz tube through the vacuum line using liquid nitrogen to freeze the bottom of the sample tube, at last the sample was sealed. Less than 1 mol% solute mixture was prepared for EPR and ENDOR measurement. Samples of Ph2NH in HZSM-5 (Ph2NH/HZSM-5) were prepared using the method described in the literature [6]. X-ray irradiation of samples Ph2NH/CFCl3 and Ph2NH/HZSM-5 was performed at liquid nitrogen temperature for 40 min (about 10 kGy) utilizing a Philips X-ray tube with a tungsten anode operating at 70 kV and 20 mA. X-band EPR spectra were recorded with a Bruker ER200DSRC spectrometer. The ENDOR data for the irradiated Ph2NH/CFCl3 and Ph2NH/HZSM-5 were obtained with a Bruker ER250 ENDOR accessory and an ENI 3100LA RF amplifier. The radio frequency (rf) modulation depth for ENDOR measurement was 200 kHz. Sample temperatures above 77 K were regulated by a Bruker VT4111 temperature controller. ENDOR spectral simulation was performed on the program described by Lund et al. [9,10]. All theoretical calculations were carried out using the GAUSSIAN 98 program suite [11], and density functional theory (DFT). The geometry of the Ph2(H)N%C radical cation was optimized by the B3LYP/6-31GC(d,p) method. The B3LYP/EPR-II method was employed to calculate the isotropic and anisotropic hyperfine coupling constants (HFCC) of Ph2(H)N%C.
3. Results and discussion 3.1. EPR and ENDOR spectra The EPR spectrum of X-ray irradiated Ph2NH/CFCl3 sample at 110 K is shown as the solid curve in Fig. 1(a). The sharp peak at the center is ascribed to the signal of quartz due to irradiation. The spectrum gives a main peak with quite badly resolved smaller hyperfine (hf ) splittings. No more useful information was obtained from the EPR spectrum. Because the sample Ph2NH/CFCl3 is a simple and typical system for X-irradiation, it is reasonable to attribute the EPR spectrum to the Ph2(H)N%C radical cation in consideration of the relative ionization potential of Ph2NH and CFCl3. The features of Ph2(H)N%C are not resolved and identified in the EPR spectrum (Fig. 1(a)). The reason maybe that the EPR lines are inhomogeneously broadened by the large number of nuclei which interact with the unpaired electron. As a consequence, ligand hf couplings are often not resolved. For the purpose of overcoming the disadvantage of EPR spectroscopy, the ENDOR technique [12,13], in which the spin system is simultaneously irradiated by a microwave (MW) and a rf field, is an
Fig. 1. (a) Experimental EPR spectrum of X-ray irradiated Ph2NH/CFCl3 sample at 110 K. The arrow indicates the field position for the ENDOR spectrum. (b) Experimental (—) and simulated (/) powder ENDOR spectra of Ph2NH/CFCl3 (about 1 mol%) at 110 K. The experimental ENDOR spectrum consists of 50 time-averaged scans. The RF modulation depth was 200 kHz. The ENDOR spectral simulation was performed using parameters listed in Table 1.
alternative to resolve smaller hf couplings which are not accessible in the EPR spectra. ENDOR measurement were made by observing the central line at different field positions in order to resolve the hf splittings which are hiding under the main peak. Similar ENDOR spectra were obtained at all fields. The best resolved ENDOR spectrum of X-irradiated Ph2NH/CFCl3 sample is presented as the solid line in Fig. 1(b), obtained by setting the field of ENDOR spectrum at the position marked with an arrow in the figure and sweeping the rf in the range 1–27 MHz. The ENDOR lines mainly appeared between 7 and 22 MHz together with several weak lines in the low frequency region. The region 12.5–17 MHz is complicated due to the overlapping lines from the Freon matrix. The sharp band at 13.75 MHz is attributed to 19F matrix signal that is marked in the figure. The main sharp lines are symmetrically placed around the proton Larmor frequency (nHZ14.43 MHz). These line pairs can be separated into two groups, namely the 15.3–17.6 and 18.5–22.7 MHz lines in the high frequency region and the matching group of 11.4–12.8, 6.9–9.4 MHz lines in the low frequency region. The atoms of Ph2(H)N%C are labeled as shown in Fig. 2 (the atoms of the left phenyl ring are symmetry related to those at the right) together with the molecular coordinate system which adopts the standard orientation obtained from the geometry optimization. It was reported that Ph2(H)N%C in CF3COOH gave relatively larger isotropic hf splittings for the N atom (a(N)Z9.17 G), the a-proton ðaðHa ÞZ 11:14 GÞ and the two para protons (a(H 3)Z4.88 G) [8]. Accordingly, in the case of
W. Liu, A. Lund / Journal of Molecular Structure 733 (2005) 13–17
15
Fig. 2. The atom labeling and coordinate system used for radical cation Ph2(H)N%C. The coordinate system adopts the standard orientation obtained from the ˚. geometry optimization of Ph2(H)N%C by the B3LYP/6-31CG(d,p) method. The unit of bond length is A
Ph2(H)N%C in CFCl3, the hf tensors of the N atom, the a-proton ðHa Þ and the two para protons (H3) are comparatively large and anisotropic due to strong confinement of unpaired electrons to the 2pz orbital on the central N and the p-conjugation of the phenyl ring, and this is supported by the theoretical calculation of Ph2(H)N%C (see details in Table 2). In addition, the numbers of these atoms are small in comparison with those of the other phenyl ring protons. Thus, the ENDOR absorption from N, Ha and H3 can therefore be neglected in consideration of the weak absorption intensity and the line broadening caused by their larger anisotropic hf couplings. We conclude that the ENDOR spectral line shape is mainly due to the ring protons in ortho (H1,H5) and meta (H2,H4) position. The group of 18.5–22.7 MHz lines (matching lines 6.9–9.4 MHz) is attributed to the ortho protons because the hf tensors of ortho protons are relatively larger. The meta protons mainly contribute the lines pair of 15.3–17.6 and 11.4–12.8 MHz. The intensity of lines in the low frequency region 4.0–6.8 MHz are too weak to identify, but contributions of the N atom or para protons are considered. The fact that the intensity of lines below nH are lower than those of the matching lines is due to the hf enhancement effect for powder ENDOR in solids [9]. The powder ENDOR spectral line shape of Ph2(H)N%C is quite complex to interpret because of line broadening due to the anisotropy of the hf and, possibly, 14N quadrupolar interactions in comparison to solution or single crystal spectra. In addition, the powder ENDOR spectrum is further complicated by orientational selectivity and is a sum of contributions from a range of orientations [9]. Accurate assignment of spectroscopic parameters therefore has to rely on computer simulation of the spectrum. The theory of the ENDOR simulation was described by Lund et al. [9]. The 110 K ENDOR spectrum was
tentatively simulated and the best-fit simulation spectrum is shown as the dotted line in Fig. 1(b) using the parameters listed in Table 1. Because both phenyl rings are equivalent, only one set of ortho (H1,H5) and meta (H2,H4) protons of the right phenyl ring were treated in the spectral simulation. Matrix fluorine, N atom, a-proton and para protons were excluded in the simulation in order to simplify the calculation. A Gaussian line shape with an EPR line width of 3 G (8.4 MHz) and an NMR line width of 0.2 MHz was employed. The simulated ENDOR spectra were very sensitive to the principal values of the ortho and meta proton hf tensors and the best fit obtained therefore permitted determining the values to within G0.2 MHz. The spectral simulation confirms the assignment of the ENDOR spectrum. The obtained isotropic hf couplings of the ortho and meta protons (Table 2) agree well with the reported results (a(Ho)Z3.51 G, a(Hm)Z1.36 G, 1 GZ 2.8 MHz [8]). Furthermore, the inaccurate relative intensity distribution is acceptable in consideration of the disturbing absorption due to matrix fluorine, N atom, a-proton or para proton hf interactions. For the Ph2NH/HZSM-5 sample, no clear observation was made for the spontaneous generation of Ph2(H)N%C. A X-irradiated sample was therefore studied by EPR and ENDOR. The EPR spectrum observed at 110 K shows a broad line and is similar to the reported spectra of Ph2(H)N%C in silica–alumina as well as in Y and HZSM-5 zeolites [6,7]. Unfortunately, no ENDOR absorption was observed. The generation of Ph2(H)N%C was considered in the case of X-ray irradiation of sample Ph2NH/HZSM-5 at low temperature, in agreement with our past research of X- or g-irradiated amine/zeolite system showing that radical formation proceeded by transferring holes to the incorporated organic molecules from the solid matrix, i.e. g-irradiated Et3N/AlPO4-5 and Pr3N/AlPO4-5 [14].
16
W. Liu, A. Lund / Journal of Molecular Structure 733 (2005) 13–17
Table 1 The hyperfine tensors obtained by the spectral simulation of the ENDOR spectrum of Ph2(H)N%C in CFCl3 at 110 K Tensorsa
Principal component
g
A (H2)
A (H4)
A (H5)
Direction cosinesc
2.0021 2.0021 2.0030 K12.8 K11.6 K4.0 1.8 4.1 5.3 1.7 3.4 4.4 K14.7 K11.8 K4.7
gx gy gz Aa Ab Ac Aa 0 Ab 0 Ac 0 Ap Aq Ar Ap 0 Aq 0 Ar 0
A (H1)
Principal valueb
x
y
z
1 0 0 K0.2637 0.8429 0.4690 0.8694 0.4055 0.2824 0.9160 0.3015 0.2645 0.2384 0.9392 0.2471
0 1 0 0.7689 K0.1099 0.6298 K0.1043 K0.4079 0.9071 K0.1519 0.8712 K0.4669 0.9596 K0.2670 0.0889
0 0 1 K0.5824 K0.5267 0.6191 K0.4830 0.8180 0.3123 K0.3712 0.3875 0.8438 K0.1495 K0.2159 0.9649
The g tensor was estimated from the EPR spectrum. a Tensors A are the hyperfine tensors of H1, H2, H4, H5 labeled in Fig. 2. b Hyperfine tensor values in MHz. c Direction cosines of the hyperfine tensors of H1, H2, H4, H5 are given in the molecular coordinate system as defined in Fig. 2 and are obtained from the optimized geometric structure of Ph2(H)N%C.
3.2. Theoretical calculations All theoretical calculations of the Ph2(H)N%C radical cation were performed with DFT because of the very
accurate resulted HFCC and great computational advantages. The geometry for Ph2(H)N%C was optimized by Becke’s three parameter hybrid method where the non-local correlation functional is that of Lee, Yang, and Parr [15a–d]
Table 2 Comparison of the observed and calculated hyperfine tensorsa for radical cation Ph2(H)N%C Atom
Calculated spin density
Principal dipolar HFCCb
Observed isotropic HFCCb
Calculated isotropic HFCCb
Observed value
Calculated value
Direction cosinesc
0.367
– – –
K18.26 K17.93 36.19
K0.0116 0.0000 0.9999
0.9999 0.0000 0.0116
0.0000 1.0000 0.0000
–
21.49
ðHa Þ
K0.018
– – –
K20.71 K6.73 27.44
K0.0968 0.9953 0.0000
0.9953 0.0968 0.0000
0.0000 0.0000 1.0000
–
K35.87
H1
K0.008
H2
0.004
H3
K0.011
H4
0.004
H5
K0.008
K3.3 K2.2 5.5 K1.9 0.4 1.5 – – – K1.5 0.2 1.3 K4.3 K1.4 5.7
N
a b c d
K3.51 K2.66 6.17 K1.73 K0.25 1.97 K8.54 K0.93 9.46 K1.67 K0.3 1.97 K4.13 K2.46 6.59
–d –d –d –d –d –d
–d –d –d –d –d –d
–d –d –d –d –d –d
0.3454 0.9384 K0.0030 –d –d –d –d –d –d
0.3370 K0.1211 0.9337 –d –d –d –d –d –d
0.8759 K0.3235 K0.3581 –d –d –d –d –d –d
K9.5
K10.41
3.7
5.49
–
K14.67
3.2
5.01
K10.4
K11.07
Calculated hyperfine tensors by the method of B3LYP/EPR-II//B3LYP/6-31CG(d,p). Unit of HFCC is MHz. Direction cosines are obtained from the optimized geometric structure of Ph2(H)N%C with molecular coordinate system as defined in Fig. 2. Same values as Table 1.
W. Liu, A. Lund / Journal of Molecular Structure 733 (2005) 13–17
in conjunction with Pople’s 6-31CG(d,p) basis set (B3LYP/ 6-31GC(d,p)). The isotropic and anisotropic HFCC of Ph2(H)N%C radical were obtained by the B3LYP/EPR-II method. The molecular EPR hf splittings, which consist of an isotropic part and an anisotropic part, are caused by the magnetic interaction between the nuclear spin and the electronic magnetic moments. The isotropic coupling (Aiso) depends on the unpaired spin density, whereas the anisotropic coupling is related to the interactions between magnetic dipoles. The full coupling tensor of the nucleus in the principal axes is calculated by the addition of the isotropic HFCC to each component of the dipolar hf tensor (Tii), AiiZAisoCTii [16]. The optimized geometric structure of Ph2(H)N%C belongs to the C2 point group and is shown in Fig. 2 with standard coordination. Direction cosines of hf tensors are obtained from the calculation results of geometric structure optimization of Ph2(H)N%C following the molecular coordinate system as defined in Table 2. There is a dihedral angle of 22.48 between both phenyl ring planes and the Ha –N–Ca –Ca plane. The Ha –N–Ca –Ca planar structure of Ph2(H)N%C gives a singly occupied molecular orbital (SOMO) that should be strongly confined to the Px orbital on the nitrogen atom. The calculated result that the spin density on the N atom is only 36.7% suggests that the p-delocalisation of unpaired electrons on the N atom to the phenyl rings plays an important role for the distribution of spin density. The calculated full hf tensors as well as the experimentally determined tensors are listed in Table 2 together for comparison. As shown in the table, the observed hf tensors of protons H1, H2, H4 and H5 are in close agreement with the calculated results. The calculated hf tenors of the N atom, the a-proton ðHa Þ and the two para protons (H3) are demonstrated without the corresponding experimental ones. Because the ENDOR intensity is usually about 1% of the ESR intensity, it is difficult to observe all spectral features of Ph2(H)N%C, especially those corresponding to the couplings with larger anisotropic couplings. Experimental hf tenors of N, Ha and H3 were accordingly not obtained from the ENDOR measurements.
4. Conclusion The geometric and electronic structures of Ph2(H)N%C were studied by EPR, ENDOR and theoretical calculation
17
methods. Radical cation Ph2(H)N%C is a C2 structure. The ortho and meta ring protons which have smaller hf coupling are mainly responsible for the line shape of the 110 K ENDOR spectrum. The ENDOR spectral simulation was successfully employed in obtaining relatively accurate experimental hf tenors and verifying the assignment of the ENDOR lines. Good agreement was achieved for the experimental and calculated hf tensors of Ph2(H)N%C.
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