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DFT study of structure, IR and Raman spectra of the first generation dendrimer built from cyclotriphosphazene core with terminal pyrazine groups
Vibrational Spectroscopy 92 (2017) 54–61
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DFT study of structure, IR and Raman spectra of the first generation dendrimer built from cyclotriphosphazene core with terminal pyrazine groups V.L. Furera,* , A.E. Vandyukovb , V. Tripathic, J.P. Majoralc , A.M. Caminadec, V.I. Kovalenkob,** a
Kazan State Architect and Civil Engineering University, Zelenaya, 1, Kazan 420043, Russia A.E. Arbuzov Institute of Organic and Physical Chemistry, Russian Academy of Science, Arbuzov Str., 8, Kazan 420088, Russia c Laboratorie de Chimie de Coordination, CNRS,205 route de Narbonne, 31077 ToulouseCedex 4, France b
A R T I C L E I N F O
Article history: Received 21 March 2017 Received in revised form 21 April 2017 Accepted 10 May 2017 Available online 15 May 2017 Keywords: Dendrimers Raman spectra IR spectra Normal vibrations DFT
A B S T R A C T
A phosphorus-containing dendrimer with pyrazine end groups that can be used for biological purposes has been synthesized. The vibrational spectra of the first generation dendrimer G1 constructed from the OC6H4–CH¼NN(CH3)–P(S)< and twelve 4cyclotriphosphazene core, six repeating units O C6H4–(CH2)2–NHCO C4N2H3 were registered. Analyoxyphenethylamidopyrazine end groups sis of the IR spectra of G1 shows that the amide groups form an intermolecular hydrogen bond. Optimization of the structure and analysis of normal vibrations were carried out for dendrimer G1 using the density functional theory (DFT). The six repeating units are arranged in a symmetric manner about the cyclophosphazene ring; each side of the ring contains three repeating units. It turned out that dendrimer G1 is a double bowl with almost flat repeating units which represent the concave surface and a core as a bottom of the bowl. The assignment of bands in the vibrational spectra was carried out by analyzing the distribution of the potential energy. Amide groups show bands at 3393, 1675 cm1 in the IR spectrum of G1. The line at 1577 cm1 in the Raman spectrum is characteristic of repeating units. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Dendrimers are highly branched monodisperse macromolecular compounds [1–3]. The shape and properties of dendrimers can be controlled [1–3]. Fragments of the dendrimer molecule: core, repeating units and end groups can be changed in a given direction [1–3]. Dendrimers are multivalent systems due to their numerous end groups [4]. Phosphorus-containing dendrimers play a specific role in interaction with biological systems from the DNA and the cells to the delivery of drugs [4]. They are a key component of nanomedicine [4]. Pyrazine and its derivatives were used to synthesize various nanomaterials based on dendrimers [5–8]. Vibrational spectroscopy makes it possible to monitor the synthesis of dendrimers to determine the type of terminal groups and the character of intra- and intermolecular interactions [9–16].
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (V.L. Furer), [email protected] (V.I. Kovalenko). http://dx.doi.org/10.1016/j.vibspec.2017.05.003 0924-2031/© 2017 Elsevier B.V. All rights reserved.
In this article we report on the investigation of the vibrational spectra together with DFT calculations of the first generation dendrimer constructed from the cyclotriphosphazene core, six repeating units O C6H4–CH¼N N(CH3)–P(S)< with twelve 4-oxyphenethylamidopyrazine O C6H4–(CH2)2– NHCO C4N2H3 terminal groups (Fig. 1). Such a dendrimer was chosen because a related compound with amino-bismethylene phosphonic groups can be used to activate monocytes [17]. The synthesized dendrimer has valuable biological properties and is a promising drug for the treatment of a number of diseases [18]. The calculated bond lengths of the molecule G1 are in agreement with the experimental data [19]. So the main purpose of this work was to obtain the characteristic spectral features of various structural parts of the dendrimer: cyclotriphosphazene core, repeating units and end groups on the basis of quantum chemical calculations. The interpretation of the vibrational spectra of the dendrimer in combination with the DFT calculation is important for investigating their interactions with various guest molecules. The calculation of the spatial distribution of the electron density revealed the existence of places which in the appropriate mediums will attract ions or metal atoms. It is important to clarify how the structure of
V.L. Furer et al. / Vibrational Spectroscopy 92 (2017) 54–61
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Fig. 1. Structure of dendrimer molecule G1.
dendrimers is manifested in their vibrational spectra. The results obtained from such an analysis contribute to an understanding of the dynamics and properties of dendrimers. 2. Experimental The general scheme for obtaining phosphorus dendrimers has been described in sufficient details [19,20]. The most commonly used core is hexachlorocyclotriphosphazene (N3P3Cl6). It the first stage, a dendrimer with terminal amine groups was obtained. Then the first generation dendrimer with terminal amine groups (0.1094 mmol), cesium carbonate (4.157 mmol), diisopropyleneamine (1.422 mmol) and phenol derivative (1.422 mmol) were stirred for 48 h in tetrahydrofuran at room temperature up to completion. After completion cesium salts were removed by centrifugation and the solvent was evaporated. The residue was extracted using dichloromethane and brine solution. The organic layer was dried over sodium sulfate and concentrated in rotary evaporator. It was further purified by column chromatography using silica as the stationary phase and methanol and chloroform (0.5:9.5) mixture as mobile phase. The molecule G1 consists of a cyclotriphosphazene core (NP)3, six repeating units O C6H4–CH¼N N(CH3)–P(S) < , and twelve end 4-oxyphenethylamino groups OC6H4–(CH2)2– NH2 (Fig. 1). The studied dendrimer is an amorphous solid compound. IR spectra were recorded in the region 4000–400 cm1 using Bruker Vector-22 FTIR-spectrophotometer with a resolution of 4 cm1. The samples were placed between the KBr plates. Sixtyfour scans were added for each spectrum. The Raman spectra were excited by Nd: YAG laser line 1064 nm with a power of 50 mW in the sample in the region 3500–200 cm1 and recorded using RAMII with Bruker Vertex70 FT-Raman module with a resolution of 4 cm1.
3. Computational method Since the total molecule G1 contains 522 atoms, then for the calculation a model was chosen in which only one branch was left and five branches were replaced by methoxy groups (Fig. 2). DFT calculations were performed with the PBE density functional [21] using the TZ2P basis set [22] and the program PRIRODA [23]. For a better agreement with the experiment, the calculated the wavenumbers in the range from 3500 to 2700 cm1 were scaled by a factor 0.967. The numeric value for the scaling factor was obtained by minimizing the mean square deviation of calculated and experimental wavenumbers [24]. The relative intensities of the lines in the Raman spectrum were calculated from the Raman activity Si, obtained using the program f ðn0 ni Þ Si PRIRODA according to the equation: Ii ¼ n ½1exp ;where n0 ðhcni =kT Þ i 1 being the exciting wavenumber in cm , ni the vibrational wavenumber of i-th normal mode, h, c and k universal constants and f normalization factor. Theoretical spectral curves were constructed using the Lorentzian band shape and half-width of bands equal to 10 cm1. The assignment of bands was performed by calculating the distribution of potential energy (PED) and using the animation option of the program GaussView 4.1 [25]. To calculate PED the software package SHRINK was used [24]. Calculations of the natural bonding orbitals (NBO) [26] for G1 were performed using the Gaussian 09 program package [25]. The electronic chemical potential, the chemical hardness, softness, and global electrophilicity index were obtained from the expressions m (IE + EA)/2, h (IE EA), S = 1/h, and v = m2/2h in terms of the first vertical ionization energy IE and electron affinity EA, respectively [27]. þ The Fukui functions f k ðrÞ ¼ ½qk ðN þ 1Þ qk ðNÞ for nucleophilic attack, and f k ðrÞ ¼ ½qk ðNÞ qk ðN 1Þ for electrophilic attack, 4
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Fig. 2. Optimized geometry and atom numbering for G1 (only one branch is shown for the clarity).
Table 1 Experimental and calculated bond distances (Å) and bond angles ( ) of G1. Exp. [14,20] Bond distances P(1)–N(4) P(1)–N(6) P(1)–O(7) P(2)–N(4) P(2)–N(5) P(3)–N(5) P(3)–N(6) O(7)–C(8) C(14)–C(18) C(18)–N(20) N(20)–N(21) N(21)–C(22) Angles P(1)–N(4)–P(2) P(1)–N(6)–P(3) P(1)–O(7)–C(8) P(2)–N(5)–P(3) N(4)–P(1)–N(6) N(4)–P(2)–N(5) N(5)–P(3)–N(6) O(7)–C(8)–C(10) C(8)–C(10)–C(15) C(18)–N(20)–N(21) Dihedral angles P(1)–N(4)–P(2)–N(5) P(1)–N(6)–P(3)–N(5) P(1)–O(7)–C(8)–C(10) N(4)–P(1)–O(7)–C(8) O(7)–C(8)–C(10)–C(15) C(14)–C(18)–N(20)–N(21) C(18)–N(20)–N(21)–P(26) N(20)–N(21)–P(26)–S(27) S(27)–P(26)–O(25)–C(30)
Calc.
Exp. [14,20]
Calc.
1.578 1.576 1.585 1.572 1.573 1.574 1.575 1.401 1.465 1.263 1.471 1.459
1.606 1.613 1.645 1.623 1.608 1.606 1.625 1.386 1.462 1.292 1.364 1.461
N(21)–P(26) O(25)–P(26) P(26)–S(27) C(37)–C(50) C(50)–C(53) C(53)–N(64) N(64)–C(66) C(66)–O(67) C(66)–C(79) C(70)–C(71) C(71)–N(75)
1.624 1.634 1.899 1.514 1.514 1.454 1.340 1.234 1.505 1.390 1.337
1.713 1.648 1.929 1.510 1.547 1.452 1.360 1.227 1.516 1.400 1.343
122.0 121.3 128.7 122.4 117.3 117.6 116.6 118.8 118.8 119.4
119.6 120.6 127.0 120.8 119.0 118.9 118.9 115.4 120.2 119.9
N(20)–N(21)–C(22) N(20)–N(21)–P(26) N(21)–P(26)–O(25) N(21)–P(26)–S(27) C(37)–C(50)–C(53) C(50)–C(53)–N(64) C(53)–N(64)–C(66) N(64)–C(66)–C(79) C(70)–C(71)–N(72) C(70)–N(75)–C(74)
121.7 105.2 110.8 115.6 111.3 113.3 121.5 115.2 121.9 116.2
122.7 114.5 104.8 114.8 111.5 113.0 122.1 112.6 122.2 116.2
8.7 9.9 172.8 174.7 174.1 179.5 178.1 179.9
17.3 0.2 163.2 174.0 178.8 179.0 174.1 173.8 52.4
C(33)–C(37)–C(50)–C(53) C(37)–C(50)–C(53)–N(64) C(50)–C(53)–N(64)–C(66) C(53)–N(64)–C(66)–C(79) N(64)–C(66)–C(79)–N(84) C(66)–C(79)–C(80)–N(81) C(79)–C(80)–N(81)–C(82) C(80)–N(81)–C(82)–C(83)
174.0 83.2 179.2 2.4 179.9 2.0 0.4
95.7 178.3 87.2 177.7 2.2 179.9 0.1 0.0
V.L. Furer et al. / Vibrational Spectroscopy 92 (2017) 54–61
where qk is the electronic population of atom k in the molecule, N the number of electrons, were calculated. The local softness is obtained by projecting the global quantity onto any atomic center k þ in the molecule by using the Fukui function: sþ ¼ Sf k ; s k ¼ Sf k . The k Fukui function and local softness for each reactive atom were calculated using Natural atomic charges population analysis [26] 4. Results and discussion The dendrimer G1 is an amorphous compound and its structural parameters are unknown, but we can use the parameters of related molecules [28,29]. At the first stage, the conformational search using the semiempirical method PM6 was fulfilled, and then the low-energy conformer was optimized by the DFT method. A satisfactory agreement is found between the theoretical calculations of G1 and the experimental parameters for hexaphenoxycyclotriphosphazene [28], hexakis(4-N’(-dichloro(thio) phosphonyl)-N’-methyl-diazobenzene)cyclotriphosphazene [19] and N-(2-chloroethyl)pyrazine-2-carboxamide [24] (Table 1, Fig. 2). The theoretical dihedral angles of the cyclotriphosphazene ring are less than 20 in complete agreement with the literature data [30]. The calculations show that the fragment O C6H4–CH¼N N (CH3)–P(S)< is flat. The scanning of the potential energy has shown that the molecule G1 exists predominantly in one of the most stable conformation with dihedral angles N(4)–P(1)–O(7)–C(8) and P(1)– O(7)–C(8)–C(10) equal to 173.2 and 161.9 (Fig. 2, Table 1). The experimental dihedral angles are equal to 174.7 and 172.8 . The fragment O C6H4–(CH2)2–NH COC4N2H3 contains flat regions comprising aromatic and pyrazine rings and an amide group. Optimization shows that the conformer with dihedral angles C(37)–C(50)–C(53)–N(64) and N(64)–C(66)–C(79)–N(84) equal to 178.0 and 0.0 (Fig. 2, Table 1) is the predominant. The calculated bond lengths and valence angles in the molecule G1 are in agreement with the experimental values (Table 1). Our study reveals that the cyclotriphosphazene ring is slightly nonplanar. The six repeating units are arranged in a symmetric manner about the cyclophosphazene ring; each side of the ring contains three repeating units. The overall arrangement is an approximate cyclic array of the pyrazine groups about the cyclophosphazene framework. It turned out that dendrimer G1 is a double bowl with almost flat repeating units which represent the concave surface and a core as a bottom of the bowl. These trends are consistent with the experimental X-ray data for substituted cyclophosphazenes [16,31]. The NBO theory can explain the structural characteristics of dendrimer G1 [26]. The flat structure of the pyrazine terminal groups is realized due to interactions s2(C70–N75) ! s*2(C71– N72) and s2(C70–N75) ! n(LP1C74) with energies 19.82 and 56.42 kcal/mol (Table 2). The flat structure of the O C6H4– CH¼N N(CH3)–fragment is maintained by p electron delocalization around C8–C10, C14–C15 distributed to p* antibonding orbitals of C11–C13, C14–C15 and C18–N20 with a stabilizing energy of about 18.91, 22.12, 16.70 kcal/mol (Table 2). The other interaction energy in this molecule is p electron donating from s2(C18–N20) ! s*2(C14–C15), s1(P21–S27) ! s*1(P21–O25) resulting a stabilizing energy of about 7.56, 5.55 kcal/mol. As follows from Table 2, in the dendrimer molecule G1, a hyper conjugation occurs between the lone electron pair of sulfur S27 and the antibonding orbital s*1(N21–P26) with a stabilization energy 12.81 kcal/mol. Important interactions in the molecule G1 include lone pairs of phosphorus, sulfur, oxygen, and nitrogen atoms. The calculation of the spatial structure of the molecule G1 shows that there are no steric hindrances in it and the end groups are able to enter into subsequent reactions.
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Table 2 Second order perturbation theory analysis of Fock matrix of G1 using NBO analysis. Donor (i) Acceptor (j) interaction
E(2)a (kcal mol-1)
E(j) E(i)b (a.u.)
F(i,j)c (a.u.)
(LP2N4) ! n*(LP1P1) (LP2N4) ! n*(LP1P2) (LP2N5) ! n*(LP1P2) (LP2N5) ! n*(LP1P3) s1(O7–C8) ! n*(LP1P1) s2(C8–C10) ! s*2(C11–C13) s2(C8–C10) ! s*2(C14–C15) s2(C14–C15) ! s*2(C18–N20) s2(C18–N20) ! s*2(C14–C15) s1(P26–S27) ! s*1(P26–O28) s2(C40–C41) ! s*2(C42–C45) s2(C40–C41) ! s*2(C43–C47) s2(C42–C45) ! s*2(C40–C41) s2(C42–C45) ! s*2(C43–C47) s2(C70–N75) ! n(LP1C74) s2(C70–N75) ! s*2(C71–N72) s2(C70–N75) ! s*2(C68–O69) s2(C71–N72) ! s*2(C70–N75) n (LP1O7) ! s*2(C8–C10) n (LP1N20) ! s*1(C18–H19) n (LP1N21) ! n*(LP1P26) n (LP1N21) ! s*2(C18–N20) n (LP2S27) ! n*(LP1P26) n (LP2S27) ! s*1(N21–P26) n (LP2S27) ! n*(LP1P26) n (LP3S27) ! n*(LP1P26) n (LP3S27) ! s*1(P26–O28) n (LP2O28) ! n*(LP1P27) n (LP1N62) ! s*2(C68–O69) n (LP2O69) ! s*1(N62–C68) n (LP2O69) ! s*1(C68–C70) n (LP1C74) ! s*2(C70–N75)
60.87 27.84 38.95 50.50 14.39 18.91 22.12 16.70 7.56 5.55 19.66 19.03 21.40 20.23 56.42 19.82 9.70 18.70 17.56 11.08 20.86 30.02 44.27 12.81 44.27 30.94 18.07 14.53 51.86 25.24 22.23 70.89
0.20 0.20 0.19 0.19 0.87 0.29 0.29 0.25 0.37 0.85 0.30 0.30 0.27 0.29 0.18 0.30 0.36 0.31 0.38 0.81 0.19 0.29 0.13 0.40 0.13 0.12 0.37 0.27 0.31 0.71 0.64 0.13
0.105 0.071 0.083 0.083 0.116 0.066 0.072 0.061 0.052 0.065 0.068 0.067 0.069 0.068 0.106 0.069 0.054 0.069 0.078 0.086 0.062 0.084 0.074 0.064 0.074 0.061 0.074 0.064 0.114 0.122 0.108 0.104
n n n n
LP Lone pair. a Stabilization (delocalization) energy (interactions with stabilization energy greater than 5.0 kcal mol1 are listed). b Energy difference between i(donor) and j(acceptor) NBO orbitals. c Fock matrix element i and j NBO orbitals.
Reactivity descriptors are presented in Table 3 and 4. Compressed Fukui functions fk+ and fk, which are the most commonly used reactivity descriptors, control nucleophilic and electrophilic attack. Table 4 shows the reactivity order for electrophilic attack S27 > N21 > N20 > O7 > N72 > O67 > N75. The most electron rich sites at O25, O7, N62 and O67 are suitable for protonation due to the formation O HN hydrogen bonding. The local softness condensed to an atom location sk+, sk and local electrophilicity indices vk+, vk were also calculated to illustrate the reactivity of atoms in the G1 molecule. The most electrophilic site in molecule has the maximum values of sk+ and vk+, while maximum value of sk and vk corresponds to the nucleophilic site in G1. Table 4 demonstrates that the most electrophilic sites in the molecule G1 are N72 and N75 whereas the most nucleophilic sites are N21 and S27. The reactivity order for local electrophilicity index vk+ is N75 > N72 > N20 > O67 > S27 and for vk S27 > N21 > N20 > O7 > N72. The illustration of the frontier molecular orbitals of the molecule G1 is shown in Fig. 3. The HOMO of the molecule G1 is distributed over two repeating units while the LUMO is localized at the one of end groups. The examination of molecular orbitals shows that in the frontier molecular orbitals HOMO and LUMO are mainly composed of p atomic orbitals and in the phosphoruscontaining dendrimer under study, conjugation is realized. The ratios of the principal quantities of the shape of the gyration tensor I1/I3 and I2/I3 depend on the shape of the molecules. For spherical molecules these ratios are 1. For the dendrimer studied, the calculated values of I1/I3 and I2/I3 are 0.39 and 0.70. Therefore, the molecules G1 have an asymmetric shape which determines their ability to self-order in various structures [32].
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Table 3 Global reactivity descriptors of G1. Ionization energy, eV
Electron affinity, eV
Chemical potential, eV
Softness, eV
Electrophilicity index, eV
7.006
0.693
3.850
0.158
1.173
Table 4 The partial charges of the atoms determined by natural population analysis (NPA) and the local reactivity properties of denrimer G1. Atom
qn
qn-1
qn+1
fk +
fk
sk+
sk
vk+
vk
P1 N4 O7 N20 N21 O25 P26 S27 N62 H63 O67 N72 N75
2.507 1.504 0.806 0.264 0.613 0.816 2.016 0.567 0.662 0.423 0.594 0.413 0.383
2.497 1.495 0.770 0.226 0.526 0.808 1.997 0.476 0.658 0.430 0.577 0.393 0.367
2.509 1.505 0.810 0.296 0.617 0.816 2.018 0.585 0.674 0.420 0.621 0.478 0.453
0.002 0.001 0.004 0.032 0.004 0.000 0.002 0.018 0.012 0.003 0.027 0.065 0.070
0.010 0.009 0.036 0.038 0.087 0.008 0.019 0.091 0.004 0.007 0.017 0.020 0.016
0.000 0.000 0.001 0.005 0.001 0.000 0.000 0.003 0.002 0.000 0.004 0.010 0.011
0.002 0.001 0.006 0.006 0.014 0.001 0.003 0.014 0.000 0.001 0.003 0.003 0.003
0.002 0.001 0.005 0.038 0.005 0.000 0.002 0.021 0.014 0.004 0.032 0.076 0.082
0.012 0.011 0.042 0.045 0.102 0.009 0.022 0.107 0.002 0.008 0.020 0.023 0.019
Fig. 3. Molecular orbital surfaces of G1.
The molecule G1 has a sufficient large cavity for the accommodation of guest molecules. The distance between the P = S bonds of repeating units is equal to 12 Å. Electrostatic interactions determine the structure of host – guest systems. To describe these interactions natural population analysis (NPA) [21] data of G1 were used to determine the spatial distribution of the electron density for various parts of the dendrimer. It turned out that the G1 contains polar bonds in the core, repeating units, and end groups and has active sites in various parts of the molecule. The dipole moment is an important characteristic of the electrical properties of molecules. The calculations show that the molecule G1 has an appreciable dipole moment which is equal to 7.35 D and can be explained by an asymmetric distribution of the lone electron pairs. The branches of the dendrimer induce the isolation and polarity of the core. Experimental and theoretical vibrational spectra of G1 are presented in Figs. 4–7 and Table 5. There is a satisfactory agreement between the calculated and experimental spectra. (Figs. 4 and 5). Thus, DFT calculations can be used to interpret the vibrational spectra of dendrimers. The assignment of bands was carried out by calculating the distribution of potential energy (PED) (Table 5). The NH stretching modes of the amide groups in peptides appear near 3405 cm1, which is typical frequency for a free amide group [33]. The value of the maximum of the absorption band of the NH groups in the IR spectrum of G1 at 3393 cm1 shows that these groups participate in the formation of an intermolecular hydrogen bond with the carbonyl oxygen of the amide group (Fig. 7). It is known that when an H-bond is formed, this band shifts to low frequencies. The actual theoretical value of the frequency n(NH) at 3432 cm1 calculated for a free molecule is much higher than the experimental value. The band of stretching vibrations of
Fig. 4. Theoretical (1) and experimental (2) IR spectra of G1.
V.L. Furer et al. / Vibrational Spectroscopy 92 (2017) 54–61
Fig. 5. Theoretical (1) and experimental (2) Raman spectra of G1.
the carbonyl group at 1675 cm1 in the IR spectrum of G1 is also higher than the theoretical value. Four CH stretching modes can be expected in IR spectrum for para-substituted benzene derivatives. The bands at 3054, 3041 and 2999 cm1 related to the CH stretching of aromatic groups are observed in this region of the IR spectrum of G1 (Fig. 6). In the Raman spectrum of G1 bands at 3067 and 3003 cm1 belonging to this type of oscillations are observed (Fig. 6). The bands at 2933 and
Fig. 6. Experimental Raman (1) and IR (2) spectra of G1 in the region 500–200 cm1.
59
2867 cm1 in the IR spectrum of G1 can be referred to as antisymmetric and symmetric CH2 stretch vibrations of the methylene groups. The corresponding bands at 2930 and 2867 cm1 are seen in the Raman spectra of G1. The intense bands at 1605 and 1504 cm1 in the IR spectrum of G1 refer to the stretching and bending vibrations of the aromatic ring. The bands at 1604 and 1505 cm1 find themselves in this region of the Raman spectrum of G1. A line of medium intensity in the Raman spectrum at 1577 cm1 and weak shoulder 1580 cm1 in the IR spectrum of G1 can be attributed to C¼N stretching vibrations of hydrazone fragment mixed with stretching vibrations of the aromatic ring. The rather weak bands at 1467 and 1446 cm1 in the IR spectrum of G1 are related to CN stretching and CCH bending vibrations. The corresponding band at 1468 cm1 is seen in the Raman spectrum of G1. The bands at 1440, 1420, 1409 cm1 in the Raman spectrum of G1 are due to HCH and OCH deformation vibrations. The weak band at 1401 cm1 in the IR spectrum of G1 is caused by the CC stretching vibrations and the CCH deformations of aromatic rings. The rather weak line at 1369 cm1 in the Raman spectrum of G1 is assigned to NCH and CCH deformation vibrations. The appropriate band at 1366 cm1 was detected in the IR spectrum of G1. The group of intense bands in the IR spectrum of G1 at 1162, 1179, 1193 cm1 is caused by stretching vibrations of the C O, P–N and C C bonds. The Raman spectrum of G1 in this region shows line at 1165 cm1. The band at 1105 cm1 in the IR spectrum of G1 is due to the PN stretching vibrations. The strong bands at 944, 928 and 889 cm1 in the IR spectrum of G1 were attributed to the stretching vibrations of the PO, PN and NN bonds. The weak band at 837 cm1 in the IR spectrum of G1 refers to the CC stretch. The band at 639 cm1 in the IR and Raman spectra of G1 includes contribution of CCC, CCH and OCC bend.
Fig. 7. Experimental Raman (1) and IR (2) spectra of G1 in the region 3700– 2600 cm1.
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Table 5 Experimental and corresponded calculated frequencies n (cm1), intensity I (km/mol) of bands in the IR spectra and relative intensity J (a.u.) of lines in the Raman spectra of G1 in the region 3400–200 cm1. Experiment IR
n
3393 w 3054 w 3041 w 2999 vw 2933 w 2867 w 1675 s 1605 w 1580 w 1530 m 1504 s 1467 w 1446 w
Calculation Raman
n
3067 m 3003 vw 2930 w 2867 w 1670 vw 1604 s 1577 s 1529 w 1505 vw 1468 vw 1440 vw 1409 vw
1401 w 1369 vw 1366 w 1304 vw 1292 vw 1268 vw 1243 vw 1215 sh 1193 s 1179 w 1162 s 1105 w 1052 w 1020 m
1337 vw 1306 w 1293 w 1246 w 1219 w
1165 w 1150 sh 1103 w 1051 w 1021 w 984 vw 967 w
944 s 928 s 889 m 865 sh
914 w 888 vw 865 vw 852 vw
837 w 822 w 778 m 803 w 799 vw 783 w 778 w 752 w 735 w
777 vw
723 vw 716 vw 704 vw 683 vw 669 vw 639 vw 625 w 599 vw 583 vw 577 vw 553 w 544 vw 534 vw 512 vw 499 vw 493 vw
708 w
640 w 620 w 600 w
556 vw
491 vw 468 vw 465 vw 448 w 412 w 397 w 389 w 371 vw
n 3432 3043 3041 2987 2931 2874 1737 1609 1594 1559 1524 1489 1473 1447 1441 1415 1404 1380 1353 1343 1301 1300 1266 1233 1218 1215 1178 1161 1143 1116 1038 1024 963 963 948 930 907 896 861 851 840 824 811 803 782 771 771 752 743 720 718 702 693 669 637 618 609 587 574 561 536 526 518 499 494 491 479 456 444 420 402 394 364
I 21.9 0.9 4.8 35.3 1.2 48.7 134.5 119.8 2.9 3.9 3.8 142.6 245.3 74.9 4.0 10.4 1.2 25.8 57.6 0.4 40.1 41.6 1.8 38.1 91.3 725.0 263.7 171.3 305.5 63.2 498.6 462.4 0.1 0.1 216.8 355.1 98.3 343.7 267.5 10.3 42.3 80.3 12.1 5.6 223.9 6.4 13.4 39.9 204.5 22.4 22.3 17.3 42.1 4.6 0.8 91.7 3.3 23.8 9.6 19.3 80.7 14.5 21.1 89.8 52.7 25.0 74.1 22.6 18.3 0.3 33.6 15.3 15.5
J 0.4 0.3 1.5 0.8 0.3 1.5 2.8 3.2 100.0 23.4 4.5 2.3 1.5 0.2 0.1 0.7 0.2 0.2 5.8 8.6 1.8 1.0 1.4 2.4 0.6 0.2 0.6 0.2 15.5 0.2 0.1 0.4 0.1 0.1 11.5 2.1 1.0 4.7 1.4 1.1 2.4 0.4 0.8 0.6 1.6 16.5 1.3 0.4 1.6 0.4 0.3 0.6 0.6 7.1 0.2 6.9 0.2 0.8 5.0 1.6 0.9 0.3 1.3 0.5 0.4 1.7 0.2 0.5 0.7 1.6 1.4 12.9 1.2
Assignment 100 n(N62H63) 97 n(E32H36) 100 n(E11H12) 100 n(E83H87) 99 n(E56H57) 100 n(E50H51) 82 n(C66O67), 8 n(C66N64), 3 n(C66C79) 48 n(C8C10), 24 n(C18N20), 16 d(C8C10H9) 43 n(C18N20), 22 n(C8C10), 10 n(C14C18) 58 n(C8C10), 15 n(C18N20), 12 d(C8C10H9) 76 n(C71N72), 9 d(C70C71H76), 8 d(C72C71N76) 47 d(C30C31H34), 38 n(C30C31), 7 n(C30O25) 37 n(C68N62), 36 d(C68N62H63), 8 n(C70C68) 27 n(C59N62), 24 d(C47C56H57), 23 d(N62C59H60) 75 d(H23C22H24) 28 x(C56E59), 27 d(H57C56H58), 14 x(C59N62) 49 n(C40C41), 36 d(C40C41H44), 8 d(C40C41C43) 39 d(N20C18H19), 24 d(C14C18H19), 21 n(C14C18) 26 d(N20C18H19), 26 n(C13C14), 14 d(C14C18H19) 92 n(C40C11), 3 d(C40C41H44) 40 d(C56C59H60), 16 d(N62C59H60), 8 n(C41C43) 40 d(C37C50H51), 16 d(N64C53H52), 8 n(C50C53) 37 n(P1N4), 17 n(C1O7), 11 n(C8C10) 29 n(C68C70), 14 n(C71N72), 10 d(C72N71H76) 27 n(C8O7), 17 n(P1N4), 14 d(C8C11H12) 88 n(P1N4), 2 d(N4P1N6) 36 n(C30O25), 32 d(C30C32H26), 13 n(C30C31) 42 n(C22N21), 17 d(N21C22H23), 14 d(C8C10H9) 58 d(C40C42H46), 14 n(C40C42), 11 n(C30O25) 93 n(P1N4) 69 n(C8O7), 9 n(P1O7) 73 n(C8O7), 17 n(P1O7) 91 r(C71H76) 91 r(C80H85) 33 n(N20N21), 20 n(P26N21), 17 d(N21C22H23) 37 n(P1N4), 23 n(P1O7), 8 n(C8O7) 45 n(P1N4), 19 n(P1O7), 20 n(C8O7) 45 n(P26O25), 12 n(C30O25), 9 n(P26N21) 57 n(P1O7), 15 n(C7O8), 14 n(P1N4) 30 r(C71H76), 10 n(C70C71), 9 d(N72C71E70) 19 n(C40C41), 17 r(C42H46), 16 n(C47C56) 59 r(C10H9), 18 n(P1O7), 6 n(C7O8) 45 r(C31H34), 12 n(P26O25), 6 n(C37C50) 65 r(C41H44), 7 n(C47C56), 4 n(C40C41) 25 n(P26S27), 15 n(P26O25), 10 n(P26N21) 32 r(C73H77), 15 n(P1O7), 13 x(C71N72) 52 r(C80H85), 21 x(C80N81), 17 x(C79C80) 24 r(C71H76), 23 d(C70E71N72), 13 d(C70C71H76) 22 n(P1O7), 14 n(P1N4), 12 r(C42H46) 24 x(C68C70), 16 n(P1O7), 10 r(C71H76) 35 x(C71N72), 13 r(C71H76), 7 n(P26S27) 44 r(C71H76), 13 d(C70C71N72), 11 d(C70C71H76) 28 n(P26O25), 12 d(C31C30C32), 8 n(C30O25) 47 n(P1N4), 19 d(P1N4P2), 11 d(N4P1N6) 60 d(C30C31C33), 17 d(C30C31H34), 6 n(C30C31) 23 n(P26S27), 16 d(C30C31C33), 15 n(P26O25) 53 d(C71N72C73), 24 d(C70C71N72) 21 d(P26O25C30), 16 d(C40C41C43), 9 n(P26O25) 20 d(P26O25C30), 13 d(C40C41C43), 11 n(C30O25) 22 d(P1O7C8), 17 d(C10C8C11), 8 d(O7C8C10) 20 d(P1O7C8), 9 x(P1N4), 8 n(P1O7) 18 d(C14C18N20), 14 d(O7C8C10), 11 r(C10H9) 23 r(C10H9), 20 x(C8C10), 10 x(C18N20) 13 d(C10C8C11), 11 r(C10H9), 9 d(C18N20N21) 26 d(P1O7C8), 24 x(P1N4), 10 d(N4P1O7) 15 d(C31C30C32), 10 d(C50C53N64), 10 r(C31H34) 14 d(C10C8C11), 11 d(P1O7C8), 10 n(P26S27) 20 d(P1O7C8), 8 d(N4P1O7), 7 x(P1N4) 26 d(P1O7C8), 10 d(N4P1O7), 9 x(P1N4) 82 x(C8C10), 6 x(C18N20) 17 x(C18N20), 15 x(C8C10), 15 x(P1O7) 57 x(C18N20), 6 d(P1N4P2), 5 d(P1O7C8) 47 x(C71N72), 8 d(C70C71N72), 5 d(C68C70C71)
V.L. Furer et al. / Vibrational Spectroscopy 92 (2017) 54–61
61
Table 5 (Continued) Experiment 357 vw 313 vw
Calculation 307 vw 281 vw
356 304 279
35.8 13.5 1.4
Thus, the core of the dendrimer manifests itself as a band at 1268 cm1 in the IR spectra of G1 related to PN stretch. The 4oxyphenethylamidopyrazine end groups show bands at 3393 and 1675 cm1 assigned to NH and C¼O stretching vibrations. Pyrazine fragments cause a band at 1530 cm1 in the IR and Raman spectra of dendrimer. Stretching vibrations of the C¼N bonds of the repeating units are responsible for the appearance of the line at 1577 cm1 in the Raman spectrum. 5. Summary The vibrational spectra of the dendrimer G1 constructed from the cyclotriphosphazene core with 4-oxyphenethylamidopyrazine end groups were registered. The full geometry optimization and normal mode analysis were performed using the method of DFT. A satisfactory agreement is observed between the theoretical and experimental spectra. The analysis of the gyration tensor shows that the molecule G1 has an asymmetric shape. Our study reveals that the cyclotriphosphazene ring is slightly nonplanar. The six repeating units are arranged in a symmetric manner about the cyclophosphazene ring; each side of the ring contains three repeating units. The overall arrangement is an approximate cyclic array of the pyrazine groups about the cyclophosphazene framework. It turned out that dendrimer G1 is a double bowl with almost flat repeating units which represent the concave surface and a core as a bottom of the bowl. Vibrational spectroscopy in combination with quantum chemical calculations makes it possible to obtain a sufficiently complete description of the structure of biologically important materials. Acknowledgement Vishwa Tripathi thankfully acknowledges the financial support from the CEFIPRA (project 5303-2). References [1] G.R. Newkome, C.N. Moorefield, F. Vogtle, Dendrimers and Dendrons: Concepts, Syntheses, Applications, VCH, Weinheim, 2001. [2] J.M.J. Frechet, D.A. Tomalia, Dendrimers and Other Dendritic Polymers, Wiley, New York, 2002. [3] G.R. Newkome, V. Shreiner, Polymer 49 (2008) 1.
0.1 0.1 1.8
20 x(C71N72), 19 x(C8O7), 17 d(P1O7C8) 22 x(C8O7), 13 d(N4P1O7), 13 x(P1O7) 23 d(P1O7C8), 14 x(C68N62), 10 d(P1N4P2)
[4] A.M. Caminade, C.O. Turrin, J.P. Majoral, New J. Chem. 34 (2010) 1512. [5] S. Serroni, S. Camagna, F. Puntoriero, C. Di Pietro, N.D. McClenaghan, F. Loiseau, Chem. Soc. Rev. 30 (2001) 367. [6] S. Serroni, G. Denti, S. Camagna, A. Juris, M. Ciano, V. Balzani, Angew. Chem. Int. Ed. Engl. 31 (1992) 1493. [7] D. Astruc, Nature Chem. 4 (2012) 255. [8] Q. Li, J. Li, Y. Duan, Z. Gao, D. Liu, Chin. J. Chem. 29 (2011) 2655. [9] A.P. Davis, G. Ma, H.C. Allen, Anal. Chim. Acta. 496 (2003) 117. [10] M.Y. El-Sayed, M.S. Refat, J. Mol. Struct. 604 (2002) 45. [11] K. Gardikis, S. Hatziantoniou, K. Viras, M. Wagner, C.A. Demetzos, Int J. Pharm 318 (2006) 118. [12] Y. He, L. Yang, Q. Chen, Surf. Coat. Technol. 228 (2013) 137. [13] I. Kulszewicz-Bajer, G. Louran, D. Djurado, L. Skorka, M. Szymanski, J.Y. Mevellec, S. Rols, A. Pron, J. Phys. Chem. B 118 (2014) 5278. [14] V.L. Furer, I.I. Vandyukova, A.E. Vandyukov, J.P. Majoral, A.M. Caminade, V.I. Kovalenko, Chem. Phys. 358 (2009) 177. [15] V.L. Furer, I.I. Vandyukova, A.E. Vandyukov, J.P. Majoral, A.M. Caminade, V.I. Kovalenko, J. Mol. Struct. 875 (2008) 587. [16] V.L. Furer, A.E. Vandyukov, J.P. Majoral, A.M. Caminade, V.I. Kovalenko, Chem. Phys. 326 (2006) 417. [17] A.M. Caminade, A. Maraval, J.P. Majoral, Eur. J. Inorg. Chem. 2006 (2006) 887. [18] A.M. Caminade, C.O. Turrin, R. Laurent, A. Ouli, B. Delavaux-Nicot, Dendrimer: Towards Catalytic Material and Biomedical Uses, Wiley, Chichester, 2011. [19] A. Maraval, R. Laurent, B. Donnadieu, M. Mauzac, A.M. Caminade, J.P. Majoral, J. Am. Chem. Soc. 122 (2000) 2499. [20] A.M. Caminade, A. Hameau, J.P. Majoral, Dalton. Trans. 45 (2016) 1810. [21] J.P. Perdew, K. Burke, M. Erznzerhof, Phys. Rev. Lett. 77 (1996) 3865. [22] T.H. Dunning Jr., J. Chem. Phys. 55 (1971) 716. [23] D.N. Laikov, Yu.A. Ustynyuk, Russ. Chem. Bull Int. Ed. 54 (2005) 821. [24] V.A. Sipachev, J. Mol. Struct. (Theochem). 121 (1985) 143. [25] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.A. Montgomery, J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, O. Farkas, J.B. Foresman, J.V. Ortiz, J. Cioslowski, D.J. Fox, Gaussian 09 Revision C.01, Gaussian Inc., Wallingford CT, 2010. [26] E.D. Glendening, C.R. Landis, F. Weinhold, Comput. Mol. Sci. 2 (2012) 1. [27] R.G. Parr, R.G. Pearson, J. Am. Chem. Soc. 105 (1983) 7512. [28] W.C. Marsh, J. Trotter, J. Chem. Soc. A: Inorg. Phys. Theor. (1971) 169. [29] C.H. da Silva Lima, M.V.N. de Souza, S.M.S.V. Wardell, J.L. Wardell, E.R.T. Tiekink, Acta Cryst. E. 66 (2010) o2886. [30] A.B. Chaplin, J.A. Harrison, P.J. Dyson, Inorg. Chem. 44 (2005) 8407. [31] M. Ujihara, T. Imae, Polym. Int. 59 (2010) 137. [32] V. Chandrasekhar, G.T.S. Andavan, S. Nagendran, V. Krishnan, R. Azhakar, R.J. Butcher, Organometallics 22 (2003) 976. [33] A.M. Abo-Riziq, J.E. Bushnell, B. Crews, M.P. Callahan, L. Grace, M.S. De Vries, Int. J. Quant. Chem. 105 (2005) 437.
Contents lists available at ScienceDirect
Vibrational Spectroscopy journal homepage: www.elsevier.com/locate/vibspec
DFT study of structure, IR and Raman spectra of the first generation dendrimer built from cyclotriphosphazene core with terminal pyrazine groups V.L. Furera,* , A.E. Vandyukovb , V. Tripathic, J.P. Majoralc , A.M. Caminadec, V.I. Kovalenkob,** a
Kazan State Architect and Civil Engineering University, Zelenaya, 1, Kazan 420043, Russia A.E. Arbuzov Institute of Organic and Physical Chemistry, Russian Academy of Science, Arbuzov Str., 8, Kazan 420088, Russia c Laboratorie de Chimie de Coordination, CNRS,205 route de Narbonne, 31077 ToulouseCedex 4, France b
A R T I C L E I N F O
Article history: Received 21 March 2017 Received in revised form 21 April 2017 Accepted 10 May 2017 Available online 15 May 2017 Keywords: Dendrimers Raman spectra IR spectra Normal vibrations DFT
A B S T R A C T
A phosphorus-containing dendrimer with pyrazine end groups that can be used for biological purposes has been synthesized. The vibrational spectra of the first generation dendrimer G1 constructed from the OC6H4–CH¼NN(CH3)–P(S)< and twelve 4cyclotriphosphazene core, six repeating units O C6H4–(CH2)2–NHCO C4N2H3 were registered. Analyoxyphenethylamidopyrazine end groups sis of the IR spectra of G1 shows that the amide groups form an intermolecular hydrogen bond. Optimization of the structure and analysis of normal vibrations were carried out for dendrimer G1 using the density functional theory (DFT). The six repeating units are arranged in a symmetric manner about the cyclophosphazene ring; each side of the ring contains three repeating units. It turned out that dendrimer G1 is a double bowl with almost flat repeating units which represent the concave surface and a core as a bottom of the bowl. The assignment of bands in the vibrational spectra was carried out by analyzing the distribution of the potential energy. Amide groups show bands at 3393, 1675 cm1 in the IR spectrum of G1. The line at 1577 cm1 in the Raman spectrum is characteristic of repeating units. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Dendrimers are highly branched monodisperse macromolecular compounds [1–3]. The shape and properties of dendrimers can be controlled [1–3]. Fragments of the dendrimer molecule: core, repeating units and end groups can be changed in a given direction [1–3]. Dendrimers are multivalent systems due to their numerous end groups [4]. Phosphorus-containing dendrimers play a specific role in interaction with biological systems from the DNA and the cells to the delivery of drugs [4]. They are a key component of nanomedicine [4]. Pyrazine and its derivatives were used to synthesize various nanomaterials based on dendrimers [5–8]. Vibrational spectroscopy makes it possible to monitor the synthesis of dendrimers to determine the type of terminal groups and the character of intra- and intermolecular interactions [9–16].
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (V.L. Furer), [email protected] (V.I. Kovalenko). http://dx.doi.org/10.1016/j.vibspec.2017.05.003 0924-2031/© 2017 Elsevier B.V. All rights reserved.
In this article we report on the investigation of the vibrational spectra together with DFT calculations of the first generation dendrimer constructed from the cyclotriphosphazene core, six repeating units O C6H4–CH¼N N(CH3)–P(S)< with twelve 4-oxyphenethylamidopyrazine O C6H4–(CH2)2– NHCO C4N2H3 terminal groups (Fig. 1). Such a dendrimer was chosen because a related compound with amino-bismethylene phosphonic groups can be used to activate monocytes [17]. The synthesized dendrimer has valuable biological properties and is a promising drug for the treatment of a number of diseases [18]. The calculated bond lengths of the molecule G1 are in agreement with the experimental data [19]. So the main purpose of this work was to obtain the characteristic spectral features of various structural parts of the dendrimer: cyclotriphosphazene core, repeating units and end groups on the basis of quantum chemical calculations. The interpretation of the vibrational spectra of the dendrimer in combination with the DFT calculation is important for investigating their interactions with various guest molecules. The calculation of the spatial distribution of the electron density revealed the existence of places which in the appropriate mediums will attract ions or metal atoms. It is important to clarify how the structure of
V.L. Furer et al. / Vibrational Spectroscopy 92 (2017) 54–61
55
Fig. 1. Structure of dendrimer molecule G1.
dendrimers is manifested in their vibrational spectra. The results obtained from such an analysis contribute to an understanding of the dynamics and properties of dendrimers. 2. Experimental The general scheme for obtaining phosphorus dendrimers has been described in sufficient details [19,20]. The most commonly used core is hexachlorocyclotriphosphazene (N3P3Cl6). It the first stage, a dendrimer with terminal amine groups was obtained. Then the first generation dendrimer with terminal amine groups (0.1094 mmol), cesium carbonate (4.157 mmol), diisopropyleneamine (1.422 mmol) and phenol derivative (1.422 mmol) were stirred for 48 h in tetrahydrofuran at room temperature up to completion. After completion cesium salts were removed by centrifugation and the solvent was evaporated. The residue was extracted using dichloromethane and brine solution. The organic layer was dried over sodium sulfate and concentrated in rotary evaporator. It was further purified by column chromatography using silica as the stationary phase and methanol and chloroform (0.5:9.5) mixture as mobile phase. The molecule G1 consists of a cyclotriphosphazene core (NP)3, six repeating units O C6H4–CH¼N N(CH3)–P(S) < , and twelve end 4-oxyphenethylamino groups OC6H4–(CH2)2– NH2 (Fig. 1). The studied dendrimer is an amorphous solid compound. IR spectra were recorded in the region 4000–400 cm1 using Bruker Vector-22 FTIR-spectrophotometer with a resolution of 4 cm1. The samples were placed between the KBr plates. Sixtyfour scans were added for each spectrum. The Raman spectra were excited by Nd: YAG laser line 1064 nm with a power of 50 mW in the sample in the region 3500–200 cm1 and recorded using RAMII with Bruker Vertex70 FT-Raman module with a resolution of 4 cm1.
3. Computational method Since the total molecule G1 contains 522 atoms, then for the calculation a model was chosen in which only one branch was left and five branches were replaced by methoxy groups (Fig. 2). DFT calculations were performed with the PBE density functional [21] using the TZ2P basis set [22] and the program PRIRODA [23]. For a better agreement with the experiment, the calculated the wavenumbers in the range from 3500 to 2700 cm1 were scaled by a factor 0.967. The numeric value for the scaling factor was obtained by minimizing the mean square deviation of calculated and experimental wavenumbers [24]. The relative intensities of the lines in the Raman spectrum were calculated from the Raman activity Si, obtained using the program f ðn0 ni Þ Si PRIRODA according to the equation: Ii ¼ n ½1exp ;where n0 ðhcni =kT Þ i 1 being the exciting wavenumber in cm , ni the vibrational wavenumber of i-th normal mode, h, c and k universal constants and f normalization factor. Theoretical spectral curves were constructed using the Lorentzian band shape and half-width of bands equal to 10 cm1. The assignment of bands was performed by calculating the distribution of potential energy (PED) and using the animation option of the program GaussView 4.1 [25]. To calculate PED the software package SHRINK was used [24]. Calculations of the natural bonding orbitals (NBO) [26] for G1 were performed using the Gaussian 09 program package [25]. The electronic chemical potential, the chemical hardness, softness, and global electrophilicity index were obtained from the expressions m (IE + EA)/2, h (IE EA), S = 1/h, and v = m2/2h in terms of the first vertical ionization energy IE and electron affinity EA, respectively [27]. þ The Fukui functions f k ðrÞ ¼ ½qk ðN þ 1Þ qk ðNÞ for nucleophilic attack, and f k ðrÞ ¼ ½qk ðNÞ qk ðN 1Þ for electrophilic attack, 4
56
V.L. Furer et al. / Vibrational Spectroscopy 92 (2017) 54–61
Fig. 2. Optimized geometry and atom numbering for G1 (only one branch is shown for the clarity).
Table 1 Experimental and calculated bond distances (Å) and bond angles ( ) of G1. Exp. [14,20] Bond distances P(1)–N(4) P(1)–N(6) P(1)–O(7) P(2)–N(4) P(2)–N(5) P(3)–N(5) P(3)–N(6) O(7)–C(8) C(14)–C(18) C(18)–N(20) N(20)–N(21) N(21)–C(22) Angles P(1)–N(4)–P(2) P(1)–N(6)–P(3) P(1)–O(7)–C(8) P(2)–N(5)–P(3) N(4)–P(1)–N(6) N(4)–P(2)–N(5) N(5)–P(3)–N(6) O(7)–C(8)–C(10) C(8)–C(10)–C(15) C(18)–N(20)–N(21) Dihedral angles P(1)–N(4)–P(2)–N(5) P(1)–N(6)–P(3)–N(5) P(1)–O(7)–C(8)–C(10) N(4)–P(1)–O(7)–C(8) O(7)–C(8)–C(10)–C(15) C(14)–C(18)–N(20)–N(21) C(18)–N(20)–N(21)–P(26) N(20)–N(21)–P(26)–S(27) S(27)–P(26)–O(25)–C(30)
Calc.
Exp. [14,20]
Calc.
1.578 1.576 1.585 1.572 1.573 1.574 1.575 1.401 1.465 1.263 1.471 1.459
1.606 1.613 1.645 1.623 1.608 1.606 1.625 1.386 1.462 1.292 1.364 1.461
N(21)–P(26) O(25)–P(26) P(26)–S(27) C(37)–C(50) C(50)–C(53) C(53)–N(64) N(64)–C(66) C(66)–O(67) C(66)–C(79) C(70)–C(71) C(71)–N(75)
1.624 1.634 1.899 1.514 1.514 1.454 1.340 1.234 1.505 1.390 1.337
1.713 1.648 1.929 1.510 1.547 1.452 1.360 1.227 1.516 1.400 1.343
122.0 121.3 128.7 122.4 117.3 117.6 116.6 118.8 118.8 119.4
119.6 120.6 127.0 120.8 119.0 118.9 118.9 115.4 120.2 119.9
N(20)–N(21)–C(22) N(20)–N(21)–P(26) N(21)–P(26)–O(25) N(21)–P(26)–S(27) C(37)–C(50)–C(53) C(50)–C(53)–N(64) C(53)–N(64)–C(66) N(64)–C(66)–C(79) C(70)–C(71)–N(72) C(70)–N(75)–C(74)
121.7 105.2 110.8 115.6 111.3 113.3 121.5 115.2 121.9 116.2
122.7 114.5 104.8 114.8 111.5 113.0 122.1 112.6 122.2 116.2
8.7 9.9 172.8 174.7 174.1 179.5 178.1 179.9
17.3 0.2 163.2 174.0 178.8 179.0 174.1 173.8 52.4
C(33)–C(37)–C(50)–C(53) C(37)–C(50)–C(53)–N(64) C(50)–C(53)–N(64)–C(66) C(53)–N(64)–C(66)–C(79) N(64)–C(66)–C(79)–N(84) C(66)–C(79)–C(80)–N(81) C(79)–C(80)–N(81)–C(82) C(80)–N(81)–C(82)–C(83)
174.0 83.2 179.2 2.4 179.9 2.0 0.4
95.7 178.3 87.2 177.7 2.2 179.9 0.1 0.0
V.L. Furer et al. / Vibrational Spectroscopy 92 (2017) 54–61
where qk is the electronic population of atom k in the molecule, N the number of electrons, were calculated. The local softness is obtained by projecting the global quantity onto any atomic center k þ in the molecule by using the Fukui function: sþ ¼ Sf k ; s k ¼ Sf k . The k Fukui function and local softness for each reactive atom were calculated using Natural atomic charges population analysis [26] 4. Results and discussion The dendrimer G1 is an amorphous compound and its structural parameters are unknown, but we can use the parameters of related molecules [28,29]. At the first stage, the conformational search using the semiempirical method PM6 was fulfilled, and then the low-energy conformer was optimized by the DFT method. A satisfactory agreement is found between the theoretical calculations of G1 and the experimental parameters for hexaphenoxycyclotriphosphazene [28], hexakis(4-N’(-dichloro(thio) phosphonyl)-N’-methyl-diazobenzene)cyclotriphosphazene [19] and N-(2-chloroethyl)pyrazine-2-carboxamide [24] (Table 1, Fig. 2). The theoretical dihedral angles of the cyclotriphosphazene ring are less than 20 in complete agreement with the literature data [30]. The calculations show that the fragment O C6H4–CH¼N N (CH3)–P(S)< is flat. The scanning of the potential energy has shown that the molecule G1 exists predominantly in one of the most stable conformation with dihedral angles N(4)–P(1)–O(7)–C(8) and P(1)– O(7)–C(8)–C(10) equal to 173.2 and 161.9 (Fig. 2, Table 1). The experimental dihedral angles are equal to 174.7 and 172.8 . The fragment O C6H4–(CH2)2–NH COC4N2H3 contains flat regions comprising aromatic and pyrazine rings and an amide group. Optimization shows that the conformer with dihedral angles C(37)–C(50)–C(53)–N(64) and N(64)–C(66)–C(79)–N(84) equal to 178.0 and 0.0 (Fig. 2, Table 1) is the predominant. The calculated bond lengths and valence angles in the molecule G1 are in agreement with the experimental values (Table 1). Our study reveals that the cyclotriphosphazene ring is slightly nonplanar. The six repeating units are arranged in a symmetric manner about the cyclophosphazene ring; each side of the ring contains three repeating units. The overall arrangement is an approximate cyclic array of the pyrazine groups about the cyclophosphazene framework. It turned out that dendrimer G1 is a double bowl with almost flat repeating units which represent the concave surface and a core as a bottom of the bowl. These trends are consistent with the experimental X-ray data for substituted cyclophosphazenes [16,31]. The NBO theory can explain the structural characteristics of dendrimer G1 [26]. The flat structure of the pyrazine terminal groups is realized due to interactions s2(C70–N75) ! s*2(C71– N72) and s2(C70–N75) ! n(LP1C74) with energies 19.82 and 56.42 kcal/mol (Table 2). The flat structure of the O C6H4– CH¼N N(CH3)–fragment is maintained by p electron delocalization around C8–C10, C14–C15 distributed to p* antibonding orbitals of C11–C13, C14–C15 and C18–N20 with a stabilizing energy of about 18.91, 22.12, 16.70 kcal/mol (Table 2). The other interaction energy in this molecule is p electron donating from s2(C18–N20) ! s*2(C14–C15), s1(P21–S27) ! s*1(P21–O25) resulting a stabilizing energy of about 7.56, 5.55 kcal/mol. As follows from Table 2, in the dendrimer molecule G1, a hyper conjugation occurs between the lone electron pair of sulfur S27 and the antibonding orbital s*1(N21–P26) with a stabilization energy 12.81 kcal/mol. Important interactions in the molecule G1 include lone pairs of phosphorus, sulfur, oxygen, and nitrogen atoms. The calculation of the spatial structure of the molecule G1 shows that there are no steric hindrances in it and the end groups are able to enter into subsequent reactions.
57
Table 2 Second order perturbation theory analysis of Fock matrix of G1 using NBO analysis. Donor (i) Acceptor (j) interaction
E(2)a (kcal mol-1)
E(j) E(i)b (a.u.)
F(i,j)c (a.u.)
(LP2N4) ! n*(LP1P1) (LP2N4) ! n*(LP1P2) (LP2N5) ! n*(LP1P2) (LP2N5) ! n*(LP1P3) s1(O7–C8) ! n*(LP1P1) s2(C8–C10) ! s*2(C11–C13) s2(C8–C10) ! s*2(C14–C15) s2(C14–C15) ! s*2(C18–N20) s2(C18–N20) ! s*2(C14–C15) s1(P26–S27) ! s*1(P26–O28) s2(C40–C41) ! s*2(C42–C45) s2(C40–C41) ! s*2(C43–C47) s2(C42–C45) ! s*2(C40–C41) s2(C42–C45) ! s*2(C43–C47) s2(C70–N75) ! n(LP1C74) s2(C70–N75) ! s*2(C71–N72) s2(C70–N75) ! s*2(C68–O69) s2(C71–N72) ! s*2(C70–N75) n (LP1O7) ! s*2(C8–C10) n (LP1N20) ! s*1(C18–H19) n (LP1N21) ! n*(LP1P26) n (LP1N21) ! s*2(C18–N20) n (LP2S27) ! n*(LP1P26) n (LP2S27) ! s*1(N21–P26) n (LP2S27) ! n*(LP1P26) n (LP3S27) ! n*(LP1P26) n (LP3S27) ! s*1(P26–O28) n (LP2O28) ! n*(LP1P27) n (LP1N62) ! s*2(C68–O69) n (LP2O69) ! s*1(N62–C68) n (LP2O69) ! s*1(C68–C70) n (LP1C74) ! s*2(C70–N75)
60.87 27.84 38.95 50.50 14.39 18.91 22.12 16.70 7.56 5.55 19.66 19.03 21.40 20.23 56.42 19.82 9.70 18.70 17.56 11.08 20.86 30.02 44.27 12.81 44.27 30.94 18.07 14.53 51.86 25.24 22.23 70.89
0.20 0.20 0.19 0.19 0.87 0.29 0.29 0.25 0.37 0.85 0.30 0.30 0.27 0.29 0.18 0.30 0.36 0.31 0.38 0.81 0.19 0.29 0.13 0.40 0.13 0.12 0.37 0.27 0.31 0.71 0.64 0.13
0.105 0.071 0.083 0.083 0.116 0.066 0.072 0.061 0.052 0.065 0.068 0.067 0.069 0.068 0.106 0.069 0.054 0.069 0.078 0.086 0.062 0.084 0.074 0.064 0.074 0.061 0.074 0.064 0.114 0.122 0.108 0.104
n n n n
LP Lone pair. a Stabilization (delocalization) energy (interactions with stabilization energy greater than 5.0 kcal mol1 are listed). b Energy difference between i(donor) and j(acceptor) NBO orbitals. c Fock matrix element i and j NBO orbitals.
Reactivity descriptors are presented in Table 3 and 4. Compressed Fukui functions fk+ and fk, which are the most commonly used reactivity descriptors, control nucleophilic and electrophilic attack. Table 4 shows the reactivity order for electrophilic attack S27 > N21 > N20 > O7 > N72 > O67 > N75. The most electron rich sites at O25, O7, N62 and O67 are suitable for protonation due to the formation O HN hydrogen bonding. The local softness condensed to an atom location sk+, sk and local electrophilicity indices vk+, vk were also calculated to illustrate the reactivity of atoms in the G1 molecule. The most electrophilic site in molecule has the maximum values of sk+ and vk+, while maximum value of sk and vk corresponds to the nucleophilic site in G1. Table 4 demonstrates that the most electrophilic sites in the molecule G1 are N72 and N75 whereas the most nucleophilic sites are N21 and S27. The reactivity order for local electrophilicity index vk+ is N75 > N72 > N20 > O67 > S27 and for vk S27 > N21 > N20 > O7 > N72. The illustration of the frontier molecular orbitals of the molecule G1 is shown in Fig. 3. The HOMO of the molecule G1 is distributed over two repeating units while the LUMO is localized at the one of end groups. The examination of molecular orbitals shows that in the frontier molecular orbitals HOMO and LUMO are mainly composed of p atomic orbitals and in the phosphoruscontaining dendrimer under study, conjugation is realized. The ratios of the principal quantities of the shape of the gyration tensor I1/I3 and I2/I3 depend on the shape of the molecules. For spherical molecules these ratios are 1. For the dendrimer studied, the calculated values of I1/I3 and I2/I3 are 0.39 and 0.70. Therefore, the molecules G1 have an asymmetric shape which determines their ability to self-order in various structures [32].
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Table 3 Global reactivity descriptors of G1. Ionization energy, eV
Electron affinity, eV
Chemical potential, eV
Softness, eV
Electrophilicity index, eV
7.006
0.693
3.850
0.158
1.173
Table 4 The partial charges of the atoms determined by natural population analysis (NPA) and the local reactivity properties of denrimer G1. Atom
qn
qn-1
qn+1
fk +
fk
sk+
sk
vk+
vk
P1 N4 O7 N20 N21 O25 P26 S27 N62 H63 O67 N72 N75
2.507 1.504 0.806 0.264 0.613 0.816 2.016 0.567 0.662 0.423 0.594 0.413 0.383
2.497 1.495 0.770 0.226 0.526 0.808 1.997 0.476 0.658 0.430 0.577 0.393 0.367
2.509 1.505 0.810 0.296 0.617 0.816 2.018 0.585 0.674 0.420 0.621 0.478 0.453
0.002 0.001 0.004 0.032 0.004 0.000 0.002 0.018 0.012 0.003 0.027 0.065 0.070
0.010 0.009 0.036 0.038 0.087 0.008 0.019 0.091 0.004 0.007 0.017 0.020 0.016
0.000 0.000 0.001 0.005 0.001 0.000 0.000 0.003 0.002 0.000 0.004 0.010 0.011
0.002 0.001 0.006 0.006 0.014 0.001 0.003 0.014 0.000 0.001 0.003 0.003 0.003
0.002 0.001 0.005 0.038 0.005 0.000 0.002 0.021 0.014 0.004 0.032 0.076 0.082
0.012 0.011 0.042 0.045 0.102 0.009 0.022 0.107 0.002 0.008 0.020 0.023 0.019
Fig. 3. Molecular orbital surfaces of G1.
The molecule G1 has a sufficient large cavity for the accommodation of guest molecules. The distance between the P = S bonds of repeating units is equal to 12 Å. Electrostatic interactions determine the structure of host – guest systems. To describe these interactions natural population analysis (NPA) [21] data of G1 were used to determine the spatial distribution of the electron density for various parts of the dendrimer. It turned out that the G1 contains polar bonds in the core, repeating units, and end groups and has active sites in various parts of the molecule. The dipole moment is an important characteristic of the electrical properties of molecules. The calculations show that the molecule G1 has an appreciable dipole moment which is equal to 7.35 D and can be explained by an asymmetric distribution of the lone electron pairs. The branches of the dendrimer induce the isolation and polarity of the core. Experimental and theoretical vibrational spectra of G1 are presented in Figs. 4–7 and Table 5. There is a satisfactory agreement between the calculated and experimental spectra. (Figs. 4 and 5). Thus, DFT calculations can be used to interpret the vibrational spectra of dendrimers. The assignment of bands was carried out by calculating the distribution of potential energy (PED) (Table 5). The NH stretching modes of the amide groups in peptides appear near 3405 cm1, which is typical frequency for a free amide group [33]. The value of the maximum of the absorption band of the NH groups in the IR spectrum of G1 at 3393 cm1 shows that these groups participate in the formation of an intermolecular hydrogen bond with the carbonyl oxygen of the amide group (Fig. 7). It is known that when an H-bond is formed, this band shifts to low frequencies. The actual theoretical value of the frequency n(NH) at 3432 cm1 calculated for a free molecule is much higher than the experimental value. The band of stretching vibrations of
Fig. 4. Theoretical (1) and experimental (2) IR spectra of G1.
V.L. Furer et al. / Vibrational Spectroscopy 92 (2017) 54–61
Fig. 5. Theoretical (1) and experimental (2) Raman spectra of G1.
the carbonyl group at 1675 cm1 in the IR spectrum of G1 is also higher than the theoretical value. Four CH stretching modes can be expected in IR spectrum for para-substituted benzene derivatives. The bands at 3054, 3041 and 2999 cm1 related to the CH stretching of aromatic groups are observed in this region of the IR spectrum of G1 (Fig. 6). In the Raman spectrum of G1 bands at 3067 and 3003 cm1 belonging to this type of oscillations are observed (Fig. 6). The bands at 2933 and
Fig. 6. Experimental Raman (1) and IR (2) spectra of G1 in the region 500–200 cm1.
59
2867 cm1 in the IR spectrum of G1 can be referred to as antisymmetric and symmetric CH2 stretch vibrations of the methylene groups. The corresponding bands at 2930 and 2867 cm1 are seen in the Raman spectra of G1. The intense bands at 1605 and 1504 cm1 in the IR spectrum of G1 refer to the stretching and bending vibrations of the aromatic ring. The bands at 1604 and 1505 cm1 find themselves in this region of the Raman spectrum of G1. A line of medium intensity in the Raman spectrum at 1577 cm1 and weak shoulder 1580 cm1 in the IR spectrum of G1 can be attributed to C¼N stretching vibrations of hydrazone fragment mixed with stretching vibrations of the aromatic ring. The rather weak bands at 1467 and 1446 cm1 in the IR spectrum of G1 are related to CN stretching and CCH bending vibrations. The corresponding band at 1468 cm1 is seen in the Raman spectrum of G1. The bands at 1440, 1420, 1409 cm1 in the Raman spectrum of G1 are due to HCH and OCH deformation vibrations. The weak band at 1401 cm1 in the IR spectrum of G1 is caused by the CC stretching vibrations and the CCH deformations of aromatic rings. The rather weak line at 1369 cm1 in the Raman spectrum of G1 is assigned to NCH and CCH deformation vibrations. The appropriate band at 1366 cm1 was detected in the IR spectrum of G1. The group of intense bands in the IR spectrum of G1 at 1162, 1179, 1193 cm1 is caused by stretching vibrations of the C O, P–N and C C bonds. The Raman spectrum of G1 in this region shows line at 1165 cm1. The band at 1105 cm1 in the IR spectrum of G1 is due to the PN stretching vibrations. The strong bands at 944, 928 and 889 cm1 in the IR spectrum of G1 were attributed to the stretching vibrations of the PO, PN and NN bonds. The weak band at 837 cm1 in the IR spectrum of G1 refers to the CC stretch. The band at 639 cm1 in the IR and Raman spectra of G1 includes contribution of CCC, CCH and OCC bend.
Fig. 7. Experimental Raman (1) and IR (2) spectra of G1 in the region 3700– 2600 cm1.
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Table 5 Experimental and corresponded calculated frequencies n (cm1), intensity I (km/mol) of bands in the IR spectra and relative intensity J (a.u.) of lines in the Raman spectra of G1 in the region 3400–200 cm1. Experiment IR
n
3393 w 3054 w 3041 w 2999 vw 2933 w 2867 w 1675 s 1605 w 1580 w 1530 m 1504 s 1467 w 1446 w
Calculation Raman
n
3067 m 3003 vw 2930 w 2867 w 1670 vw 1604 s 1577 s 1529 w 1505 vw 1468 vw 1440 vw 1409 vw
1401 w 1369 vw 1366 w 1304 vw 1292 vw 1268 vw 1243 vw 1215 sh 1193 s 1179 w 1162 s 1105 w 1052 w 1020 m
1337 vw 1306 w 1293 w 1246 w 1219 w
1165 w 1150 sh 1103 w 1051 w 1021 w 984 vw 967 w
944 s 928 s 889 m 865 sh
914 w 888 vw 865 vw 852 vw
837 w 822 w 778 m 803 w 799 vw 783 w 778 w 752 w 735 w
777 vw
723 vw 716 vw 704 vw 683 vw 669 vw 639 vw 625 w 599 vw 583 vw 577 vw 553 w 544 vw 534 vw 512 vw 499 vw 493 vw
708 w
640 w 620 w 600 w
556 vw
491 vw 468 vw 465 vw 448 w 412 w 397 w 389 w 371 vw
n 3432 3043 3041 2987 2931 2874 1737 1609 1594 1559 1524 1489 1473 1447 1441 1415 1404 1380 1353 1343 1301 1300 1266 1233 1218 1215 1178 1161 1143 1116 1038 1024 963 963 948 930 907 896 861 851 840 824 811 803 782 771 771 752 743 720 718 702 693 669 637 618 609 587 574 561 536 526 518 499 494 491 479 456 444 420 402 394 364
I 21.9 0.9 4.8 35.3 1.2 48.7 134.5 119.8 2.9 3.9 3.8 142.6 245.3 74.9 4.0 10.4 1.2 25.8 57.6 0.4 40.1 41.6 1.8 38.1 91.3 725.0 263.7 171.3 305.5 63.2 498.6 462.4 0.1 0.1 216.8 355.1 98.3 343.7 267.5 10.3 42.3 80.3 12.1 5.6 223.9 6.4 13.4 39.9 204.5 22.4 22.3 17.3 42.1 4.6 0.8 91.7 3.3 23.8 9.6 19.3 80.7 14.5 21.1 89.8 52.7 25.0 74.1 22.6 18.3 0.3 33.6 15.3 15.5
J 0.4 0.3 1.5 0.8 0.3 1.5 2.8 3.2 100.0 23.4 4.5 2.3 1.5 0.2 0.1 0.7 0.2 0.2 5.8 8.6 1.8 1.0 1.4 2.4 0.6 0.2 0.6 0.2 15.5 0.2 0.1 0.4 0.1 0.1 11.5 2.1 1.0 4.7 1.4 1.1 2.4 0.4 0.8 0.6 1.6 16.5 1.3 0.4 1.6 0.4 0.3 0.6 0.6 7.1 0.2 6.9 0.2 0.8 5.0 1.6 0.9 0.3 1.3 0.5 0.4 1.7 0.2 0.5 0.7 1.6 1.4 12.9 1.2
Assignment 100 n(N62H63) 97 n(E32H36) 100 n(E11H12) 100 n(E83H87) 99 n(E56H57) 100 n(E50H51) 82 n(C66O67), 8 n(C66N64), 3 n(C66C79) 48 n(C8C10), 24 n(C18N20), 16 d(C8C10H9) 43 n(C18N20), 22 n(C8C10), 10 n(C14C18) 58 n(C8C10), 15 n(C18N20), 12 d(C8C10H9) 76 n(C71N72), 9 d(C70C71H76), 8 d(C72C71N76) 47 d(C30C31H34), 38 n(C30C31), 7 n(C30O25) 37 n(C68N62), 36 d(C68N62H63), 8 n(C70C68) 27 n(C59N62), 24 d(C47C56H57), 23 d(N62C59H60) 75 d(H23C22H24) 28 x(C56E59), 27 d(H57C56H58), 14 x(C59N62) 49 n(C40C41), 36 d(C40C41H44), 8 d(C40C41C43) 39 d(N20C18H19), 24 d(C14C18H19), 21 n(C14C18) 26 d(N20C18H19), 26 n(C13C14), 14 d(C14C18H19) 92 n(C40C11), 3 d(C40C41H44) 40 d(C56C59H60), 16 d(N62C59H60), 8 n(C41C43) 40 d(C37C50H51), 16 d(N64C53H52), 8 n(C50C53) 37 n(P1N4), 17 n(C1O7), 11 n(C8C10) 29 n(C68C70), 14 n(C71N72), 10 d(C72N71H76) 27 n(C8O7), 17 n(P1N4), 14 d(C8C11H12) 88 n(P1N4), 2 d(N4P1N6) 36 n(C30O25), 32 d(C30C32H26), 13 n(C30C31) 42 n(C22N21), 17 d(N21C22H23), 14 d(C8C10H9) 58 d(C40C42H46), 14 n(C40C42), 11 n(C30O25) 93 n(P1N4) 69 n(C8O7), 9 n(P1O7) 73 n(C8O7), 17 n(P1O7) 91 r(C71H76) 91 r(C80H85) 33 n(N20N21), 20 n(P26N21), 17 d(N21C22H23) 37 n(P1N4), 23 n(P1O7), 8 n(C8O7) 45 n(P1N4), 19 n(P1O7), 20 n(C8O7) 45 n(P26O25), 12 n(C30O25), 9 n(P26N21) 57 n(P1O7), 15 n(C7O8), 14 n(P1N4) 30 r(C71H76), 10 n(C70C71), 9 d(N72C71E70) 19 n(C40C41), 17 r(C42H46), 16 n(C47C56) 59 r(C10H9), 18 n(P1O7), 6 n(C7O8) 45 r(C31H34), 12 n(P26O25), 6 n(C37C50) 65 r(C41H44), 7 n(C47C56), 4 n(C40C41) 25 n(P26S27), 15 n(P26O25), 10 n(P26N21) 32 r(C73H77), 15 n(P1O7), 13 x(C71N72) 52 r(C80H85), 21 x(C80N81), 17 x(C79C80) 24 r(C71H76), 23 d(C70E71N72), 13 d(C70C71H76) 22 n(P1O7), 14 n(P1N4), 12 r(C42H46) 24 x(C68C70), 16 n(P1O7), 10 r(C71H76) 35 x(C71N72), 13 r(C71H76), 7 n(P26S27) 44 r(C71H76), 13 d(C70C71N72), 11 d(C70C71H76) 28 n(P26O25), 12 d(C31C30C32), 8 n(C30O25) 47 n(P1N4), 19 d(P1N4P2), 11 d(N4P1N6) 60 d(C30C31C33), 17 d(C30C31H34), 6 n(C30C31) 23 n(P26S27), 16 d(C30C31C33), 15 n(P26O25) 53 d(C71N72C73), 24 d(C70C71N72) 21 d(P26O25C30), 16 d(C40C41C43), 9 n(P26O25) 20 d(P26O25C30), 13 d(C40C41C43), 11 n(C30O25) 22 d(P1O7C8), 17 d(C10C8C11), 8 d(O7C8C10) 20 d(P1O7C8), 9 x(P1N4), 8 n(P1O7) 18 d(C14C18N20), 14 d(O7C8C10), 11 r(C10H9) 23 r(C10H9), 20 x(C8C10), 10 x(C18N20) 13 d(C10C8C11), 11 r(C10H9), 9 d(C18N20N21) 26 d(P1O7C8), 24 x(P1N4), 10 d(N4P1O7) 15 d(C31C30C32), 10 d(C50C53N64), 10 r(C31H34) 14 d(C10C8C11), 11 d(P1O7C8), 10 n(P26S27) 20 d(P1O7C8), 8 d(N4P1O7), 7 x(P1N4) 26 d(P1O7C8), 10 d(N4P1O7), 9 x(P1N4) 82 x(C8C10), 6 x(C18N20) 17 x(C18N20), 15 x(C8C10), 15 x(P1O7) 57 x(C18N20), 6 d(P1N4P2), 5 d(P1O7C8) 47 x(C71N72), 8 d(C70C71N72), 5 d(C68C70C71)
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61
Table 5 (Continued) Experiment 357 vw 313 vw
Calculation 307 vw 281 vw
356 304 279
35.8 13.5 1.4
Thus, the core of the dendrimer manifests itself as a band at 1268 cm1 in the IR spectra of G1 related to PN stretch. The 4oxyphenethylamidopyrazine end groups show bands at 3393 and 1675 cm1 assigned to NH and C¼O stretching vibrations. Pyrazine fragments cause a band at 1530 cm1 in the IR and Raman spectra of dendrimer. Stretching vibrations of the C¼N bonds of the repeating units are responsible for the appearance of the line at 1577 cm1 in the Raman spectrum. 5. Summary The vibrational spectra of the dendrimer G1 constructed from the cyclotriphosphazene core with 4-oxyphenethylamidopyrazine end groups were registered. The full geometry optimization and normal mode analysis were performed using the method of DFT. A satisfactory agreement is observed between the theoretical and experimental spectra. The analysis of the gyration tensor shows that the molecule G1 has an asymmetric shape. Our study reveals that the cyclotriphosphazene ring is slightly nonplanar. The six repeating units are arranged in a symmetric manner about the cyclophosphazene ring; each side of the ring contains three repeating units. The overall arrangement is an approximate cyclic array of the pyrazine groups about the cyclophosphazene framework. It turned out that dendrimer G1 is a double bowl with almost flat repeating units which represent the concave surface and a core as a bottom of the bowl. Vibrational spectroscopy in combination with quantum chemical calculations makes it possible to obtain a sufficiently complete description of the structure of biologically important materials. Acknowledgement Vishwa Tripathi thankfully acknowledges the financial support from the CEFIPRA (project 5303-2). References [1] G.R. Newkome, C.N. Moorefield, F. Vogtle, Dendrimers and Dendrons: Concepts, Syntheses, Applications, VCH, Weinheim, 2001. [2] J.M.J. Frechet, D.A. Tomalia, Dendrimers and Other Dendritic Polymers, Wiley, New York, 2002. [3] G.R. Newkome, V. Shreiner, Polymer 49 (2008) 1.
0.1 0.1 1.8
20 x(C71N72), 19 x(C8O7), 17 d(P1O7C8) 22 x(C8O7), 13 d(N4P1O7), 13 x(P1O7) 23 d(P1O7C8), 14 x(C68N62), 10 d(P1N4P2)
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