Towards clarifying the NM vibrational nature of metallo-phthalocyanines

Spectrochimica Acta Part A 60 (2004) 2195–2200

Towards clarifying the N–M vibrational nature of metallo-phthalocyanines Infrared spectrum of phthalocyanine magnesium complex: density functional calculations Xianxi Zhang, Yuexing Zhang, Jianzhuang Jiang∗ Department of Chemistry, Shandong University, 27 ShanDa South Road, Jinan 250100, PR China Received 15 October 2003; accepted 21 November 2003

Abstract Infrared frequencies and intensities for the magnesium phthalocyanine complex MgPc have been calculated at density functional B3LYP level using the 6-31G(d) basis set. Detailed assignments of the metal–nitrogen (N–M) vibrational bands in the IR spectrum have been made on the basis of comparison of the calculated data of MgPc with the experimental result and also with that of H2 Pc. The empirical controversial assignment of the characteristic band at 886–919 cm−1 for metallo-phthalocyanines is also clearly interpreted. Nevertheless, the previous assignments of N–H stretchings, in-plane bending (IPB) and out-of-plane bending (OPB) modes made based on the comparative calculation of H2 Pc and D2 Pc are confirmed again by the present research result. © 2003 Elsevier B.V. All rights reserved. Keywords: Phthalocyanine; DFT method; IR spectra

1. Introduction IR and Raman spectroscopy have proved versatile techniques for studying the intrinsic properties of phthalocyanine compounds. However, assignments of the phthalocyanine IR bands had been made mainly from empirical comparison of experimental results with or between related compounds [1,2]. For instance, by simply comparing the IR spectrum of metal free phthalocyanine H2 Pc with some metal phthalocyanine complexes MPc, the most intense band at 1005 cm−1 in the IR spectrum of H2 Pc had been wrongly assigned to the N–H related vibrations based on such a fact that this band disappears after the replacement of central hydrogen atoms with metal ion in MPc because corresponding researchers considered that the peaks locate at similar positions for H2 Pc and MPc should have similar nature [1–5]. However, Sammes [6] found that the band at 1005 cm−1 for H2 Pc was not affected either in frequency or in intensity upon deuteration of the central protons into D2 Pc, which suggests that the band at 1005 cm−1 for H2 Pc is not a N–H related vibration. The other unreliable empirical assignment depending ∗ Corresponding author. Tel.: +86-531-856-4088; fax: +86-531-856-5211. E-mail address: [email protected] (J. Jiang).

1386-1425/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2003.11.015

on the simple comparison method is for the metal–nitrogen (M–N) vibration in metal phthalocyanine complexes MPc. The band at 888–919 cm−1 in the IR spectra of MPc was assigned to the metal–nitrogen vibration by Kobayashi et al. [3] and Kenn and Malerbi [7]. However, the other researchers did not agree with this point because of the observation of a similar peak at 874 cm−1 for H2 Pc [2,4]. They preferred to attribute the band at 810–830 cm−1 to the M–N vibrations for MPc, which is absent in the IR spectrum of H2 Pc. To clarify this kind of problems further studies are thus necessary with assistance of quantum chemistry calculations. Actually, trials on the theoretical studies over the vibrational spectra of phthalocyanine derivatives have been started quite recently. In 1998, Day et al. [8] calculated the vibrational frequencies of H2 Pc at the Hartree–Fock level using the 3-21G basis set and compared the calculated frequencies below 700 cm−1 with experimental data. More recently, Gong et al. [9,10]also tried to simulate the infrared spectra of H2 Pc and tert-butyl substituted phthalocyanine using DFT method at the B3LYP/3-21G and B3LYP/6-31G(d) levels but gave no assignment . In order to understand the nature of N–H vibrations of metal free phthalocyanine, calculation results on the isotope effect in the IR spectra of H2 Pc and D2 Pc at B3LYP, RBLYP and SVWN levels using 6-31G(d) basis set have just been reported by our group [11].

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dicted in the following frequency calculations, indicating that the energy-minimized structure is a true energy minimum. Corresponding structural parameters have been reported elsewhere [17]. Normal coordinate analyses have been performed based on these minimum-energy geometries. 3.2. M–N related vibrations in the infrared spectrum

Fig. 1. Structure of MgPc.

It is worth noting that theoretical study on the vibrational spectrum of phthalocyanine metal complexes seems to be limited to that of ZnPc based on semi-empirical PM3 [12] and density functional RBLYP/6-31G(d,p) calculations [13]. In the present paper, infrared frequencies and intensities of MgPc were calculated using the 6-31G(d) basis set at the density functional B3LYP level. Detailed assignments of N–M vibrational modes are reached through comparison of the calculated and experimental results of MgPc and also with the help of previous calculations of isotope effects on metal free phthalocyanine [11].

2. Computational method The structure of MgPc is shown in Fig. 1, according to which the input structure is deduced. The 6-31G(d) basis set was used at density functional B3LYP level for both geometry optimization and frequency/intensity calculations. The Berny algorithm using redundant internal coordinates [14] was employed in energy minimization and tight convergence criteria were used throughout. D4h symmetry for MgPc in the input structure was detected and then enforced by the program. Using the energy-minimized structure generated in the previous step, normal coordinate analysis was carried out. The IR frequencies and intensities were calculated using the same method and basis set. All calculations were carried out using the Gaussian98W program [15]. The calculated vibrational frequencies were scaled by the factor 0.9613 that was recommended in previous research works [9,16]. Detailed assignments of the N–M vibrational modes were made according to the comparison of theoretical data for MgPc with that for H2 Pc conducted in the previous work [11]. The normal mode descriptions were obtained with assistance of animated pictures produced based on the normal coordinates.

3. Results and discussion 3.1. Energy-minimized structure The energy-minimized structure of MgPc calculated at this level has D4h symmetry. No imaginary vibration is pre-

MgPc molecule contains eight nitrogen atoms, 32 carbon atoms, 16 hydrogen atoms and one magnesium atom, which has 165 normal vibrational modes. The vibrational representation for this compound found from the calculation results is given below: Γvib (MgPc) = 8A2u (IR) + 28Eu (IR) + 14A1g (Ra) + 14B1g (Ra) + 6A1u + 13A2g + 14B2g + 7B1u + 7B2u + 13Eg IR and Ra represent infrared active and Raman active modes, respectively. A2u and Eu modes are infrared active for MgPc. According to these analyses, there are 64 infrared vibrations for MgPc molecule. According to the previous work, for the H2 Pc molecule, the B1u type vibration modes and corresponding dipole moments generated mainly occur along the central N–H–H–N axis. The B2u type vibrations of H2 Pc are very similar with corresponding B1u vibrations, except that these vibrations occur perpendicular to the N–H–H–N axis in the phthalocyanine plane [11]. However, for the MgPc compound, the difference between these two directions completely disappears. The B1u and B2u vibrations for H2 Pc change to be doubly-degenerate Eu vibrations for MgPc. Among the fifteen B3u vibrations of H2 Pc, one typical N–H out-of-plane bending (OPB) vibration corresponds to the A2u vibration for MgPc. In the remaining fourteen B3u vibrations of H2 Pc, seven vibrations in which the atoms in the four isoindole units bend to the same side of the Pc plane correspond to seven A2u vibrations and maintain to be infrared active for MgPc. The other seven B3u vibrations of H2 Pc, in which the atoms in the opposite two isoindole units bend to one side of the Pc plane while atoms in the other two isoindole units the other side of the Pc plane, become symmetrical for MgPc, and therefore, are infrared in-active. To clarify the nature of N–M vibrations in metallo-phthalocyanine complexes, the full set of calculated vibrational modes for MgPc are compared with previous results for H2 Pc mode by mode assisted with animated pictures [11]. Only those very similar vibrational modes are considered to be corresponding each other. Corresponding vibrational modes are summarized in Table 1. The intensities of the calculated frequencies are also included in parenthesis. Since the reported experimental data and assignments of the IR spectrum for MgPc are not complete [5], related data and assignments of other complexes are also used as proof in the present work [1–5,7,18,19].

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Table 1 Calculated together with some experimental infrared active vibrational modes for MgPc and H2 Pca Experimentalb

Symmetry

436

502

H2 Pcc

MgPc Frequency

Assignment

2, A2u 7, Eu 8, Eu

30.29 (0.311) 117.0 (3.708) 117.0 (3.708)

Skeletal OPB Isoindole IPB Isoindole IPB

13, A2u

150.7 (25.16)

Skeletal M–N OPB

23, Eu 24, Eu 28, Eu

278.7 (6.101) 278.7 (6.101) 412.0 (0.0539)

Isoindole IPB, M–N IPB Isoindole IPB, M–N IPB Isoindole M–N def.

29, Eu 22, A2u

412.0 (0.0538) 260.2 (10.90)

Isoindole M–N def. C–N, M–N benzene OPB

27, A2u 38, Eu

340.4 (0.6647) 494.7 (9.277)

C–N, M–N, C–H OPB Pyrrole M–N IPB

39, 43, 44, 50, 51,

494.7 565.4 565.4 636.8 636.8

Pyrrole M–N IPB Isoindole str. def., Isoindole str. def., Isoindole def. str., Isoindole def. str.,

Eu Eu Eu Eu Eu

(9.277) (11.20) (11.20) (1.961) (1.961)

M–N M–N M–N M–N

str. str. str. str.

Symmetry

Frequency

Assignment

1, B3u 2, B3u 8, B2u 9, B1u 13, B3u 17, B3u 21, B3u 23, B2u 24, B1u

19.74 37.50 116.5 117.7 133.3 214.6 251.5 265.4 271.7

(0.002) (1.312) (3.245) (4.096) (0.3115) (4.148) (2.220) (0.3135) (8.093)

Skeletal OPB Skeletal OPB Isoindole (N–H) IPB Isoindole (no N–H) IPB Skeletal OPB Skeletal OPB Skeletal OPB Isoindole (N–H) IPB Isoindole (no N–H) IPB

26, 29, 31, 33,

B3u B3u B3u B2u

328.2 418.2 431.2 480.3

(2.384) (0.0584) (7.383) (3.065)

C–N C–H C–H N–H

34, 39, 40, 45, 46, 53, 56, 58,

B1u B2u B1u B1u B2u B3u B3u B1u

482.8 539.3 545.3 606.9 607.2 671.4 711.8 717.2

(7.477) (0.0307) (3.253) (27.87) (3.687) (5.735) (98.41) (69.39)

Pyrrole (no N–H) IPB N–H IPB, isoindole str. def. Isoindole str. def. Isoindole (no N–H) def. str. Isoindole (N–H) def. str. C–H OPB C–H, N–H OPB (doming) Isoindole str. def.

benzene OPB benzene OPB benzene OPB IPB, pyrrole (N–H) IPB

34, A2u 60, Eu

434.0 (4.611) 740.3 (54.62)

C–H OPB Isoindole str. def., M–N str.

752

61, Eu

740.3 (54.62)

Isoindole str. def., M–N str.

59, B2u 61, B3u

722.4 (67.56) 755.4 (2.687)

N–H IPB, isoindole str. def. C–H, N–H OPB

728

58, A2u 71, Eu 72, Eu

720.8 (195.6) 792.2 (3.085) 792.2 (3.085)

C–H OPB, M–N OPB Isoindole bre. def., M–N str. Isoindole bre. def., M–N str.

63, B3u 69, B1u 70, B2u

757.3 (140.0) 771.0 (3.691) 775.3 (0.8252)

C–H, N–H OPB Isoindole (N–H) bre. def. Isoindole (no N–H) bre. def.

781

66, A2u 79, Eu

763.2 (37.49) 873.2 (45.79)

C–H OPB, M–N OPB Isoindole def., M–N str. IPB

72, B3u 74, B2u

788.2 (106.0) 822.4 (0.0242)

N–H OPB N–H IPB, isoindole def.

888

80, Eu

873.2 (45.79)

Isoindole def., M–N str. IPB

85, A2u 91, Eu

926.1 (3.186) 999.0 (5.162)

C–H OPB Benzene bre.

75, 81, 84, 89,

857.7 922.5 928.8 997.9

Isoindole def. C–H OPB C–H OPB Benzene (N–H) bre.

956

92, Eu 95, Eu

999.0 (5.162) 1044 (92.71)

Benzene bre. M–N str., Isoindole def.

91, B2u 94, B1u

998.4 (5.594) 1023 (544.3)

Benzene (no N–H) bre. C–N (pyrrole) IPB, isoindole def.

1060

96, Eu 98, Eu

1044 (92.71) 1084 (165.2)

M–N str., Isoindole def. C–H IPB, isoindole str. def.

95, B2u 96, B1u

1031 (6.880) 1049 (53.81)

N–H IPB, isoindole def. C–H IPB, isoindole str. def.

1084

99, Eu 101, Eu

1084 (165.2) 1102 (136.4)

C–H IPB, isoindole str. def. C–H IPB, benzene bre., M–N str.

98, B2u 100, B1u

1082 (161.3) 1096 (62.95)

N–H, C–H IPB; isoindole str. def. C–H IPB, benzene bre.

1114

102, Eu 107, Eu

1102 (136.4) 1153 (49.31)

C–H IPB, benzene bre., M–N str. C–H IPB

103, B2u 105, B2u

1104 (140.5) 1146 (11.18)

C–H, N–H IPB; benzene bre. C–H IPB

1163

108, Eu 110, Eu

1153 (49.31) 1172 (3.481)

C–H IPB C–H IPB, isoindole def.

107, B1u 109, B1u

1148 (28.98) 1171 (0.6632)

C–H IPB C–H IPB, isoindole def.

1171

111, Eu

1172 (3.481)

C–H IPB, isoindole def.

114, Eu

1273 (36.81)

C–H IPB, C–N str.

111, B2u 114, B2u 115, B1u

1177 (39.12) 1251 (22.32) 1266 (38.17)

C–H IPB, isoindole def. N–H IPB; C–N (pyrrole) str. C–H (no N–H) IPB

1283

115, 119, 120, 121,

1273 1310 1310 1330

C–H IPB, C–N str. C–N, M–N str. C–N, M–N str. Isoindole str. def.

117, 120, 141, 121,

1288 1304 1533 1308

C–H (N–H) IPB C–H IPB, C–N (aza) str. N–H IPB, C–N (aza) str. Isoindole str. def.

1332

122, Eu 126, Eu

Isoindole str. def. Isoindole str. def., C–H IPB

122, B1u 124, B2u

Eu Eu Eu Eu

(36.81) (10.87) (10.87) (265.2)

1330 (265.2) 1393 (61.78)

B1u B3u B3u B1u

B2u B1u B2u B2u

(41.15) (1.601) (1.953) (0.3008)

(18.51) (13.13) (1.929) (125.3)

1329 (202.8) 1341 (125.0)

Isoindole str. def. Isoindole str. def., C–H IPB

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Table 1 (Continued ) Experimentalb

H2 Pcc

MgPc Symmetry

Frequency

Assignment

Symmetry

Frequency

Assignment

1408

127, Eu 132, Eu

1393 (61.78) 1447 (42.26)

Isoindole str. def., C–H IPB C–H IPB, benzene aza str.

127, B1u 130, B2u

1391 (35.93) 1428 (107.1)

Isoindole str. def., C–H IPB C–H N–H IPB, benzene str.

1454

133, 135, 136, 138,

1447 1463 1463 1464

C–H IPB, benzene aza str. C–H IPB C–H IPB Pyrrole bre.

132, 134, 135, 137,

1449 1460 1468 1491

C–H IPB, benzene str. C–H (no N–H) IPB C–H (N–H) IPB Pyrrole (no N–H) bre.

1480

139, Eu 143, Eu 144, Eu

1464 (71.23) 1572 (6.889) 1572 (6.889)

Pyrrole bre. C–C (benzene) str. C–C (benzene) str.

138, B1u 143, B1u 145, B2u

1494 (100.4) 1569 (4.300) 1584 (12.40)

Pyrrole (N–H) bre. C–C (benzene, N–H) str. C–C (benzene, no N–H) str.

1586

147, 148, 151, 152, 155, 156, 159, 160, 163, 164,

1598 1598 3065 3065 3079 3079 3093 3093 3096 3096

Isoindole def. Isoindole def. C–H str. C–H str. C–H str. C–H str. C–H str. C–H str. C–H str. C–H str.

148, 149, 151, 153, 155, 157, 160, 161, 164, 165, 167,

1596 1603 3064 3068 3078 3082 3092 3095 3097 3100 3423

Isoindole (no N–H) def. Isoindole (N–H) def. C–H str. C–H str. C–H str. C–H str. C–H str. C–H str. C–H str. C–H str. N–H str.

Eu Eu Eu Eu

Eu Eu Eu Eu Eu Eu Eu Eu Eu Eu

(42.26) (8.100) (8.100) (71.23)

(11.21) (11.21) (7.894) (7.894) (54.36) (54.36) (17.03) (17.03) (70.86) (70.86)

B1u B1u B2u B2u

B1u B2u B1u B2u B2u B1u B1u B2u B2u B1u B1u

(63.41) (0.1313) (8.965) (27.21)

(10.98) (15.52) (7.203) (8.820) (58.83) (50.79) (20.67) (70.41) (15.23) (58.06) (117.9)

a

OPB, out-of-plane bending; IPB, in-plane bending, def., deformation, str., stretching; bre., breathing; N–H (in parentheses), vibration occur in the isoindole units which have N–H bonds; no N–H (in parentheses), vibration occur in the isoindole units which have no N–H bond. b Cited from reference [5]. c Cited from reference [11].

The IR spectrum was simulated from the calculated frequency and intensity data by adding a Lorentzian lineshape with a half-width at half-maximum of 6 cm−1 . The simulated IR spectrum obtained from B3LYP/6-31G(d) method for MgPc is shown in Fig. 2. It is obvious that the number of peaks in the simulated spectrum of MgPc is much smaller than that of H2 Pc due to the increasing molecular symmetry from D2h to D4h [11]. As can be seen in Table 1, no vibrational mode over 3100 cm−1 for MgPc is found according to our calculation except the C–H stretching vibrations predicted at 3065–3096 cm−1 region, which agrees well with the exper-

imental result [1–5,7,18]. This gives further proof to our previous assignment that the absorption at 3273 cm−1 found in the IR spectrum of ␤-H2 Pc [6] is corresponding to the asymmetrical N–H stretching mode predicted at 3423 cm−1 for H2 Pc [11]. The pyrrole N–H in-plane bending (IPB) coupled with C–N (pyrrole) stretching mode predicted at 1251 cm−1 for H2 Pc is found to disappear in this region according to the present calculation for MgPc. Two new degenerate vibrations are obtained at 412.0 cm−1 for MgPc, which is considered to correspond to the 1251 and 3423 cm−1 modes of H2 Pc. These result agrees well with the previous

Fig. 2. Simulated IR spectrum for MgPc.

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assignments based on the calculation of H2 Pc and D2 Pc that the bands predicted at 1251 and 3423 cm−1 for H2 Pc are corresponding to the N–H IPB and N–H stretching vibrations [11]. In the eight out-of-plane A2u vibrations of MgPc, only one typical OPB vibration corresponds to one similar vibration mode in the fifteen B3u vibration modes of H2 Pc as described above. The remaining A2u vibrations for MgPc all correspond to two similar B3u vibration modes of H2 Pc each. Assisted with animated pictures, only the similar vibration mode at 788.2 cm−1 for H2 Pc is found to correspond with the 763.2 cm−1 mode of MgPc. This confirms the reasonability of our previous assignment to attribute the band at 788.2 cm−1 to typical N–H OPB vibration [11]. As pointed out before, the most intense band at 1005 cm−1 observed in the IR spectrum of H2 Pc has been empirically, and therefore, wrongly assigned to N–H in-plane bending or out-of-plane bending mode [11]. Calculations on the infrared spectra of H2 Pc and D2 Pc revealed that the C–N (pyrrole) in-plane bending vibration at the two pyrrole rings which do not contain N–H bonds predicted at 1023 cm−1 contributes to the strong absorption band at 1005 cm−1 of H2 Pc. The present calculation result indicates that the vibration modes at 1044 cm−1 for MgPc, which are doubly-degenerate in nature derive from the C–N (pyrrole) in-plane bending mode predicted at 1023 cm−1 and the N–H in-plane bending mode predicted at 1031 cm−1 for H2 Pc. This clearly explains the disappearance of the band at 1005 cm−1 in the IR spectra of metallo-phthalocyanines [11]. The bands observed at 1586, 1480, 1454, 1408, 1283, 1060 and 888 cm−1 for MgPc have been attributed to metal dependent peaks [5]. According to our present calculation, the vibrational modes predicted for these bands contain Mg–N, isoindole, pyrrole or C–N (pyrrole and/or aza) vibrations, which agrees with the experimental results [5]. Kobayashi et al. [3] attributed a singlet peak in the range 888–919 cm−1 for a series of MPc (M = Fe, Co, Ni, Cu, Zn, Pd and Pt) to metal–ligand vibrations. Kenn and Malerbi [7]also attributed the similar band at 918 cm−1 for PtPc and PdPcCl to metal–ligand vibration. Janczak and Clarisse observed similar vibrations at 886 cm−1 for LuPc2 and InPc2 [2,4]. However, the observation of a band at 874 cm−1 for H2 Pc makes the assignment to attribute these band to metal–ligand vibrations controversial. Another band at 810–830 cm−1 for metallo-phthalocyanines, which is absent in metal free phthalocyanine, was more preferred to be assigned as metal–ligand vibration [2,4]. According to the present work, two degenerate vibrations predicted at 792.2 cm−1 which are considered to correspond to the band at 810–830 cm−1 for MPc are found to include M–N stretching and isoindole breathing and deformation. Corresponding vibrational modes are predicted at 771.0 and 775.3 cm−1 for H2 Pc [11]. However, the degenerate vibrational modes predicted at 873.2 cm−1 which are considered corresponding to the peak at 886–919 cm−1 for MPc have also been found to include M–N stretching and in-plane

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bending vibrations and isoindole deformation. These two vibrational modes are found to be derived from the H2 Pc mode predicted at 857.7 cm−1 , which is considered corresponding to the peak observed at 874 cm−1 for H2 Pc, and another mode predicted at 822.4 cm−1 [11]. The intensity of 857.7 cm−1 mode is much higher than that at 771.0 cm−1 , thus a band can be observed at 874 cm−1 for H2 Pc but not near 810 cm−1 . That is to say, both the bands at 888–919 and 810–830 cm−1 include M–N vibration, but vibration of the isoindole units has contribution to the appearance of the 874 cm−1 band of H2 Pc. These results clearly interpret the previous controversial assignments of vibrations in this region as detailed above [2–4,7]. In our recent research over IR spectroscopic data for a series of bis(phthalocyaninato) rare earth complexes [18], a similar band at 876–887 cm−1 were also found to be metal size dependent. This gives additional experimental support over the above-mentioned assignment for this vibrational mode. Similar trends were also observed for vibrations at 1439–1454, 1110–1116 and 1063–1072 cm−1 of the bis(phthalocyaninto) rare earth complexes, which correspond to the vibrational modes predicted at 1447, 1102 and 1044 cm−1 containing Mg–N, isoindole or aza vibrations [18]. Kobayashi et al. [3]found that three peaks observed at 637–648, 574–581 and 509–521 cm−1 for Fe–, Co–, Ni–, Cu–, Zn–, Pd– and Pt–phthalocyanines appeared as the doublets for H2 Pc and thus attributed these peaks to be metal–nitrogen related vibrations. According to the present calculation results, the three doubly-degenerate vibrational modes predicted at 636.8, 565.4 and 494.7 cm−1 for MgPc seem to correspond with these peaks. Significant metal–nitrogen vibrations are observed in these vibrational modes assisted with animated pictures, which agrees well with the experimental results [3]. In the far infrared region, two bands observed at 258–320 and 352–380 cm−1 for Fe–, Co–, Ni–, Cu–, Zn–, Pd– and Pt–phthalocyanines are assigned to be metal dependent and isoindole ring related vibrations [19], which corresponds well with the predicted vibrational modes at 278.7 and 340.4 cm−1 of the present work. The bands appearing with high intensities at 150–200 cm−1 for Fe–, Co–, Ni– and Cu–phthalocyanines and 100–150 cm−1 for Zn–, Pd– and Pt–phthalocyanines were assigned to the metal–ligand vibrations because these strong bands were not observed in the spectrum of metal-free phthalocyanine [19]. According to our present calculation, the vibrational mode predicted at 150.7 cm−1 for MgPc contains significant M–N vibration, which agrees well with the experimental results described above [19]. Cross shifts are also observed from H2 Pc to MgPc. For example, the vibrational mode predicted at 757.3 cm−1 for H2 Pc was found shifted to 720.8 cm−1 for MgPc. While the vibrational modes predicted at 717.2 and 722.4 cm−1 for H2 Pc were found to shift to 740.3 cm−1 for MgPc. The former is an out-of-plane vibration while the later in-plane

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vibration. The vibrational mode predicted at 788.2 cm−1 for H2 Pc was found to be red-shifted to 763.2 cm−1 for MgPc. While another two vibrational modes predicted at 771.0 and 775.3 cm−1 for H2 Pc were found to blue-shift to 792.2 cm−1 for MgPc. The former is an out-of-plane vibration and the later in-plane vibration. It is clear that the natures of the peaks observed in this region change significantly, which tells us the fact that the traditional empirical assignments, according to comparison of IR spectra between free-base phthalocyanine and metallo-phthalocyanines, are not completely reliable. 4. Conclusions IR frequencies and intensities of MgPc have been calculated using 6-31G(d) basis set at density functional B3LYP level. Detailed assignments of the N–M vibrational bands in the IR spectrum have been tried on the basis of comparison of the calculated data with experimental data of MgPc. The empirical controversial assignments of the characteristic band at 886–919 cm−1 for metallo-phthalocyanines are clearly interpreted. Since cross shifts are also observed in the IR spectrum of MgPc in comparison with H2 Pc, quantum chemistry calculations assisted with animated pictures of the vibrational modes, are therefore, proved necessary for exact assignments of corresponding vibrational modes of phthalocyanine derivatives and related analogues. Acknowledgements The authors thank the National Natural Science Foundation of China (Grant no. 20171028), Ministry of Science and Technology of China (Grant no. 2001CB6105-04), Ministry of Education of China, Natural Science Foundation of Shandong Province (Grant no. Z99B03) and Shandong University for financial support.

References [1] H.F. Shurvell, L. Pinzuti, Can. J. Chem. 44 (1966) 125–136. [2] J. Janczak, Pol. J. Chem. 72 (1998) 1871–1878. [3] T. Kobayashi, F. Kurokawa, N. Uyeda, E. Suito, Spectrochim. Acta 26A (1970) 1305–1311. [4] C. Clarisse, M.T. Riou, Inorg. Chim. Acta 130 (1987) 139–144. [5] B. Stymne, F.X. Sauvage, G. Wettermark, Spectrochim. Acta 35A (1979) 1195–1201. [6] M.P. Sammes, J. Chem. Soc. Perkin II 2 (1972) 160–162. [7] I.M. Kenn, B.W. Malerbi, J. Inorg. Nucl. Chem. 27 (1965) 1311. [8] P.N. Day, Z.Q. Wang, R. Pachter, THEOCHEM 455 (1998) 33– 50. [9] X.D. Gong, H.M. Xiao, H. Tian, Int. J. Quantum Chem. 86 (2002) 531–540. [10] X.D. Gong, H.M. Xiao, H. Tian, THEOCHEM 593 (2002) 93– 100. [11] X.X. Zhang, Y.X. Zhang, J.Z. Jiang, Vib. Spectrosc. 33 (2003) 153–161. [12] H.M. Ding, S.Y. Wang, S.Q. Xi, J. Molec. Struct. 475 (1999) 175– 180. [13] D.R. Tackley, G. Dent, W.E. Smith, Phys. Chem. Chem. Phys. 2 (2000) 3949–3955. [14] C. Peng, P.Y. Ayala, H.B. Schlegel, M.J. Frisch, J. Comput. Chem. 17 (1996) 49. [15] 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, 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.9, Gaussian, Inc., Pittsburgh, PA, 1998. [16] M.W. Wong, Chem. Phys. Lett. 256 (1996) 391–399. [17] X.X. Zhang, Y.X. Zhang, J.Z. Jiang, J. Molec. Struct. THEOCHEM, in press. [18] F.L. Lu, M. Bao, C.Q. Ma, X.X. Zhang, D.P. Arnold, J.Z. Jiang, Spectrochim. Acta Part A 59 (2003) 3273–3286. [19] T. Kobayashi, Spectrochim. Acta 26A (1970) 1313–1322.