Density functional theory study of the Fourier transform infrared and Raman spectra of Cu(II) bis-acetylacetone

Spectrochimica Acta Part A 62 (2005) 343–352

Density functional theory study of the Fourier transform infrared and Raman spectra of Cu(II) bis-acetylacetone Haidar Raissi a, ∗ , AliReza Nowroozi b , Farzaneh Farzad a , Mohammad Saeid Hosseini Bojd a b

a Chemistry Department, Birjand University, Birjand, Iran Chemistry Department, Sistan and Balouchestan University, Zahedan, Iran

Received 22 November 2004; received in revised form 25 December 2004; accepted 11 January 2005

Abstract Fourier transform infrared and Fourier transform Raman spectra of Cu(II) bis-acetylacetone have been obtained. The geometry, frequency and intensity of the vibrational bands of this compound and its 1,5-13 C2 , 3-13 C, 1,3,5-13 C3 , 2,4-13 C2 , 18 O2 and 2,4-13 C2 –18 O2 derivatives were obtained by the density functional theory (DFT) with the B3LYP functional and using the 6-31G* and 3-21G* basis sets. The calculated frequencies are compared with the solid infrared and Raman spectra. All the measured infrared and Raman bands were interpreted in terms of the calculated vibrational modes. The percentage of deviation of the bond lengths and bond angles gives a good picture of the normal modes, and serves as a basis for the assignment of the wavenumbers. Most computed bands are predicted to be at higher wavenumbers than the experimental bands. The calculated geometrical parameters show slight differences compared with the experimental results. These differences can be explained by the different physical state of Cu(II) bis-acetylacetone. The DFT-B3LYP calculations assumed a free molecule in the gas phase. Analysis of the vibrational spectra indicates a strong coupling between the chelated ring modes. © 2005 Elsevier B.V. All rights reserved. Keywords: DFT calculations; Fourier transform IR and Raman spectra; Cu(II) bis-acetylacetone

1. Introduction The metal complexes of ␤-diketones and especially those of acetylacetone are well known and have been extensively studied [1–15]. Previous studies on metal acetylacetonate complexes have been reported for quite some time [1–5] and Thornton’s excellent review [6] summarizes the information available on acetylacetonate complexes up to 1990. Sch¨onherr et al. [7] presented a short review on the works reported up to 1993, concerning the normal coordinate analysis and applied the Wilson–El’yashevich method [8–11] to obtain the normal vibrations of M(acac)Cl4 , M = Sn(IV), Sb(IV). T´ellez and coworkers [12–14] carried out a vibrational study of Uranyl 2,4-pentanedionate and dichlorobis(2,4-pentanedionate) tin(IV) reporting the Fourier transform infrared and Raman spectra. Recently, Tayyari et ∗

Corresponding author. Tel.: +98 561 2224803; fax: +98 561 8438032. E-mail addresses: [email protected], [email protected] (H. Raissi). 1386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2005.01.001

al. [15] analyzed the infrared and Raman spectra of aluminum(III) tris-acetylacetone by the aid of ab initio calculations at Hartree–Fock (HF) and density functional theory levels. Among the metal complexes of acetylacetone, a good deal of attention has been given to the vibrational spectra of Cu(II) bis-acetylacetone (hereafter, Cu(AA)2 ). Nakamoto and Martell [1] based on normal coordinate analysis, interpreted the vibrational spectra of this complex. Junge and Musso [5] in an extensive work studied the infrared spectra of Cu(AA)2 and some metal acetylacetonates in the 1600–400 cm−1 region by considering the 18 O, 13 C and 2 H labeling of the ligand atoms. These results are undoubtedly very useful for interpretation of the vibrational spectra of this complex. Yet, there are many discrepancies between Nakamoto’s and Junge and Musso’s works. Furthermore, there is no ab initio calculation reported for this complex. Cu(AA)2 is unique among the metal complexes of acetylacetonate so far investigated. The bivalent metal complexes

344

H. Raissi et al. / Spectrochimica Acta Part A 62 (2005) 343–352

with M = Ca, Mn, Fe, Co, Ni, Zn are polymeric with general formula [M(AA)2 ]n except for the square-planar monomeric copper(II) complex [16]. The aim of the present paper is to predict the structure and vibrational spectra (harmonic wavenumbers, and relative intensities for Raman and IR spectra) of Cu(AA)2 by means of density functional theory (DFT) level. The calculated geometrical parameters are compared with the X-ray diffraction results [17]. The calculated harmonic force constants of Cu(AA)2 were used for predicting the Raman and IR spectra of 1,5-13 C2 , 3-13 C, 1,3,5-13 C3 , 2,4-13 C2 , 18 O2 and 2,4-13 C2 –18 O2 substituted species. The vibrational frequencies are compared with those observed experimentally. The calculated band assignment at DFT level will be compared with that given by Nakamoto and Martell [1], based on normal coordinate analysis, and Junge and Musso [5], based on isotopic substitutions.

Table 1 Theoretical geometry of Cu(AA)2 in comparison with experimental geometry Theoretical

X-ray (experimental)a

Bond lengths Cu O C O C C C C(Me) C H10 O· · ·O

1.931b , 1.926c 1.277, 1.301 1.404, 1.397 1.514, 1.517 1.083, 1.080 2.789, 2.759

1.9, 1.96 1.24, 1.29 1.45, 1.50 1.54, 1.55 1.08 2.669

Bond angles O6 CuO7 O6 CuO9 C2 O6 Cu C 3 C2 O6 C2 C3 C4 C1 C2 O6 C1 C2 C3

92.5, 91.5 87.5, 88.5 126.6, 127.6 125.4, 125.2 123.5, 123.0 114.9, 115.4 119.8, 119.4

87.5

a b

2. Experimental

c

122, 134 119 113, 130 113, 118

Data from [17]. Calculated at B3LYP/6-31G* . Calculated at B3LYP/3-21G* .

The Gaussian 98 W program suite [19] was used for all quantum chemical computations. The geometry of the complex was fully optimized, except for D2h symmetry restriction, at the hybrid density functional B3LYP method [20,21] with the 6-31G* and 3-21G* basis sets with extra basis on Cu atom. The vibrational frequencies were calculated at the same level numerically. This level of theory is an often used compromise between reliability [22] and economy of calculation. The assignment of the experimental frequencies are based on the observed band frequencies and intensities in the infrared and Raman spectra confirmed by establishing one to one correlation between observed and theoretically calculated frequencies.

in Table 1. The different bond distances were reported for two C C and C O in the same chelate ring. It seems, as Holm and Cotton [23] pointed out, the apparent discrepancies of the bond distances found in the Cu(AA)2 compound might be ascribed to incomplete refinements in the calculations. As it is obvious from Table 1 the calculated geometrical parameters are not very sensitive to the choice of basis set at DFT level. The agreement between the geometrical parameters calculated at the DFT level and the experimental is satisfactory except for C C and C CH3 bond lengths, which are significantly different. The reason of discrepancies was discussed in previous work [15]. Furthermore, the theoretical structural parameters correspond to a single free molecule in gas phase, the X-ray experimental results were obtained from diffraction data of a solid sample. From the Xray study [17] the structure of Cu(AA)2 was determined to ˚ is be square-planar. The metal–oxygen distance about 1.93 A ˚ ˚ considerably shorter than 2.01 A for [Ni(AA)2 ]3 [24], 2.02 A ˚ for Ni(AA)2 ·2H2 O [25], 2.03 A for Zn(AA)2 ·H2 O [25] and ˚ for Co(AA)2 ·2H2 O [26]. This contraction of about 2.05 A 0.1 is attributed to the smaller repulsion between the copper and oxygen atoms in the four-coordinate square-planar than in the six-coordinate octahedron. ˚ The average of C C distance in Cu(AA)2 is about 1.47 A ˚ which is about 0.07 A longer than C C bond length in acetylacetone [27]. The longer bond length of the acetylacetone ring carbons in Cu(AA)2 is due to the expected more delocalization of the ␲-electrons.

4. Molecular geometry

5. Vibrational analysis

The optimized geometry parameters of Cu(AA)2 are summarized in Table 1 and its geometry is shown in Fig. 1. For comparison, the X-ray diffraction results [17] are also given

The Raman spectrum of Cu(AA)2 is given in Fig. 2. Lorentzian function is utilized for deconvolution of the infrared spectrum of Cu(AA)2 in the 1800–400 cm−1 region

Cu(AA)2 was prepared and purified according to the method described in the literature [18]. The Infrared spectra were taken on a Nicolet 800 spectrometer. The Raman spectra were collected employing a 180◦ back scattering geometry and a Nicolet 910 Fourier transform Raman spectrometer. The Raman spectrometer was equipped with a ZnSe beam splitter and a liquid N2 cooled germanium detector. Rayleigh filtering was afforded by a set of two holographic technology filters. Laser power at the sample was 40 mW.

3. Method of analysis

H. Raissi et al. / Spectrochimica Acta Part A 62 (2005) 343–352

345

Fig. 1. View of Cu(AA)2 showing the atomic numbering scheme used in the text.

as shown in Figs. 3 and 4. The calculated frequencies, Raman and IR intensities at the B3LYP/6-31G* level along with the experimental results and their assignments are given in Table 2. No negative frequency is obtained, which confirms the convergence of calculations. The corresponding normal modes of the chelate ring are shown in Fig. 5. The calculated frequencies and isotopic shifts are slightly higher than the observed values for the majority of the normal modes. Two factors may be responsible for the discrepancy between the experimental and computed spectra of Cu(AA)2 . First, as is mentioned in the previous section, is caused by the

Fig. 2. Raman spectrum of Cu(AA)2 .

environment. The second reason for this discrepancy is the fact that the experimental value is an anharmonic frequency while the calculated value is a harmonic frequency. 5.1. Vibrational irreducible representation According to D2h symmetry for the Cu(AA)2 complex, the 3N − 6 = 81 vibrational modes can be classified among the symmetry species: Γvib = 13ag (R) + 12b1g (R) + 8b2g (R) + 6b3g (R) + 7au + 9b1u (IR) + 13b2u (IR) + 13b3u (IR)

Fig. 3. Deconvoluted infrared spectrum of Cu(AA)2 in 1800–1000 cm−1 region.

346

H. Raissi et al. / Spectrochimica Acta Part A 62 (2005) 343–352

same complex, the band at 2925 cm−1 was assigned to an E species of CH stretching mode. For Cu(AA)2 we could not observe all the 12 stretching modes. In the infrared spectrum we observe only three bands, which can be correlated with the calculated DFT-B3LYP values. In the Raman spectrum we also observed three bands. The calculated values show that three foursomes of bands have practically the same wavenumber value, these finding agree with the experimental observation: bands must be overlapped. According to the theoretical calculations, the weak infrared and Raman bands at about 3000 and 2970 cm−1 are assigned to asymmetric CH3 stretching modes. The Raman spectrum of Cu(AA)2 shows a strong band at about 2910 cm−1 . By considering the calculated Raman intensities and comparing with the results for acetylacetone and related compounds [22,28–30] this band is assigned to symmetric CH3 stretching. For the infrared spectrum, no new CH stretching band was found by analyzing the band deconvolution. 6.2. CH methine stretching Fig. 4. Deconvoluted infrared spectrum of Cu(AA)2 in 1000–400 cm−1 region.

5.2. Band assignments The band assignments presented here obey the following criteria: (a) comparison in some cases with reported vibrational assignment of characteristic molecular groups and bonds found in different molecules; (b) the visual 3D computerized representation of the normal modes; (c) the Cartesian representation of the normal modes, in the absence of the potential energy distribution (PED), permits in the quantum mechanic ab initio DFT method, to study the distorted geometry of each mode [14].

6. CH stretching region 6.1. Methyl groups stretching The stretching CH vibrational modes can be divided in to 2ag (R) + 2b1g (R) + b2g (R) + b3g (R) + b1u (IR) + 2b2u (IR) + 2b3u (IR) + au symmetry types. In spite of the assignments of these vibrational modes being straightforward, we compare our observed infrared and Raman bands with other 2,4-pentanedionate metal complexes. For uranyl 2,4-pentanedionate [12,13], T´ellez and G´omez assigned infrared bands at 2958 and 2925 cm−1 to the b1u and b2u CH stretching modes. The band at 2855 cm−1 was attributed by the same authors to the b3u CH stretching mode. In a previous work on aluminum(III) tris-acetylacetone [15], the observed wavenumbers at 3001 and 2969 cm−1 were assigned to the A2 and E species of CH stretching, and in the

As we have pointed out in Refs [15,28,29] it is expected that the CH stretching vibrational wavenumber of the methine groups occurs at higher frequency than those of the CH methyl groups. Theoretical calculation suggest that the observed bands at 3064 (ag ) and at 3060 cm−1 (b3u ) can be assigned as the ν(CH) of methine group. 6.3. 1700–1000 cm−1 region In this region, in addition to the CH3 deformation and rocking and also the CH in plane bending modes, we expect to observe four bands due to the C O and C C stretching modes. As we have pointed out in the discussion of the infrared and Raman spectra of Al(AA)3 [15], for the νC O and νC C infrared stretching modes assuming a “benzoid resonance” structure of the ligand by coordination with the metal, we can consider equivalent internal co-ordinate for the C O and C C bonds. In the infrared spectrum we observe only three bands at 1578, 1554 and 1534 cm−1 . The deconvolution analysis also gives three absorption bands confirming the observed values. The Raman spectrum shows two bands at 1586 and 1566 cm−1 , which are inactive in the infrared spectrum. This result is quite in agreement with the calculation, which attributes the former band in the infrared and Raman spectrum to the ag and b3u species of the symmetric C O stretching mode, respectively. For this band, the agreement between the results of experimental and theoretical frequency shift upon isotopic substitution are excellent. The assignment for the Raman band, as far as we know, has not bee reported, but the infrared band is assigned by Nakamoto and Martell [1] to νC C and by Junge and Musso [5] to ␯C O. According to description of the normal modes in Cartesian coordinates, the infrared band at 1554 cm−1 and the Raman band at 1566 cm−1 are mainly due to asymmetric (C2

H. Raissi et al. / Spectrochimica Acta Part A 62 (2005) 343–352

347

Table 2 Fundamental band assignment of Cu(AA)2 a Theoreticalb c

Frequency

Isotopic shifts IR × I

13

Experimental 13

13

13

13

18

Assignments

RI

3- C

1,5- C2

2,4- C2

1,3,5- C2

18

31

−10c −10 0 0 0 0 0

0c 0 −11 −11 −11 −11 −11

0c 0 0 0 0 0 0

−10c −10 −11 −11 −11 −11 −11

0c 0 0 0 0 0 0

0c 0 0 0 0 0 0

0.1

0

−11

0

−11

0

0

2968 (1)

53

0

−11

0

−11

0

0

2968 (1)

0

−11

0

−11

0

0

0 0 0 0 −3 −3, −3f −24, −22

−4 −4 −4 −4 0 0, 0f −1, −1

0 0 0 0 −43 −43, −30f −18, −7

−4 −4 −4 −4 −3 −3, −3f −26, −23

0 0 0 0 −20 −20, −26f −3, −5

0 0 0 0 −63 −63, −64f −21, −13

−24

−1

−18

−25

−3

−21

0

−1

−11

−1

−6

−13

0.5

0

−1

−7

−1

−3

−8

8

0 0 0, 0 0 0 0 −5, −3

−3 −3 −20 −2 −2 −2 −20

−2 −3 0, 0 0 0 0 −28, −19

−3 −3 −10 −1 −1 −1 −7, −3

−1 −2 0, 0 0 0 0 −12, −9

−4 −6 0, −1 0 0 0 −44, −30

2

−5

−2

−26

−7

−12

−27

1420 (3)

9

0 0 0, 0 0 −2

−11 −11 −1, −4 −1 −3

−1 −1 0, −2 0 −38

−12 −12 −11, −4 −11 −5

0 0 0, 0 0 −5

−1 −1 −1 −19 −43

1363 (sh) 1356 (31)

−1, −1

−3, −1

−38, −34

−4, −2

−5, −4

−43, −38

1275 (28)

0, 0 0 0 0 −1 −1 −2

−1, −1 −1 −9 −8 −7 −7 −9

−8, −5 −8 −9 −9 −9 −9 −4

−1, −1 −1 −9 −9 −8 −8 −11

−9, −8 −9 −1 0 0 0 −12

−18 −19 −10 −9 −9 −10 −15

1190 (11)

4

−2, −5

−9, −3

−4, −7

−11, −1

−12, −2

−14, −9

1020 (26)

1

−2

−9

0

−12

−2

−2

1011 (3)

0.1

−2

−9

0

−12

−2

−2

1013 (4)

0.3

−19

−2

−1

−21

−10

−10

942 (sh)

1

−19, −17

−2, −4

−1, −4

−21, −16

−10, −9

−10, −8

937 (16)

2

−7

−11

−8

−19

−10

−17

923 (4)

−7

−11

−8

−19

−9

−17

1 2 3 4 5 6 7

3226ag d 3226b3u 3152b2u 3152ag 3152b1g 3152b3u 3121b1u

8

3121b2g

9

3121b3g

10

3121au

11 12 13 14 15 16 17

3060ag 3060b3u 3060b2u 3060b1g 1656ag 1637b3u 1572b2u

18

1570b1g

19

1526b2u

20

1520b1g

21 22 23 24 25 26 27

1507ag 1506b3u 1502b1u 1502b2g 1501b3g 1501au 1472b2u

28

1451b1g

29 30 31 32 33

1426ag 1426b3u 1424b2u 1424b1g 1310ag

34

1307b3u

8

35 36 37 38 39 40 41

1232b2u 1229b1g 1076b3g 1075au 1061b1u 1061b2g 1058ag

3

42

1055b3u

43

1051b2u

44

1049b1g

45

973ag

46

971b3u

47

953b2u

48

948b1g

2 0.1 40 0.2 7 3

100 1 3 3 8 100 67 0.5 4

13 2 11 3 46

0.1 2 0.1 6

3 ∼0 4 0.1 0.3

3

O2

2,4- C2 – O2

Infrared

Raman 3064 (8)

3060 (2) 3002 (1) 2998 (1) 2998 (1) 3002 (1) 2970 (1)

2909 (30) 2912 (5) 2912 (5) 2909 (30) 1586 (1) 1578 (100) 1554 (32) 1566 (4) 1534 (68) 1529 (1) 1513 (1) 1503 (13) 1497 (4) 1499 (1) 1499 (1) 1415 (43)

1356 (9) 1270 (13)

1179 (23) 1062 (1) 1045 (7) 1041 (1)

928 (20)

νCH␣ (98%)e νCH␣ (98%) νa CH3 (in plane) (93%) νa CH3 (in plane) (91%) νa CH3 (in plane) (93%) νa CH3 (in plane) (91%) νa CH3 (out of plane) (91%) νa CH3 (out of plane) (91%) νa CH3 (out of plane) (91%) νa CH3 (out of plane) (91%) νs CH3 (96%) νs CH3 (96%) νs CH3 (96%) νs CH3 (96%) νs C O (65%) νs C O (65%) νa (C2 C3 ,C3 C4 ) (49%) + ␦CH␣ (27%) νa (C2 C3 ,C3 C4 ) (49%) + ␦CH␣ (27%) δa CH3 (53%) + δCH␣ (17%) δa CH3 (58%) + δCH␣ (15%) δa CH3 (78%) δa CH3 (78%) δa CH3 (86%) δa CH3 (86%) δa CH3 (86%) δa CH3 (86%) νa C O (57%) + ␦a CH3 (21%) νa C O (45%) + ␦a CH3 (19%) δs CH3 (75%) δs CH3 (75%) δs CH3 (77%) δs CH3 (77%) νs (C2 C3 ,C3 C4 ) (56%) + ␦s CH3 (9%) νs (C2 C3 ,C3 C4 ) (56%) + ␦s CH3 (9%) δCH␣ (73%) δCH␣ (73%) πCH3 (78%) πCH3 (78%) πCH3 (77%) πCH3 (77%) ρCH3 (59%) + ␦C C O (12%) ρCH3 (59%) + ␦C C O (12%) ρCH3 (57%) + ␦CH␣ (21%) ρCH3 (57%) + ␦CH␣ (21%) νC CH3 (31%) + ␳CH3 (26%) + ␦CH␣ (21%) νC CH3 (31%) + ␳CH3 (26%) + ␦CH␣ (21%) νa C CH3 (38%) + ␳CH3 (23%) + ␦C C C (12%) νa C CH3 (38%) + ␳CH3 (23%) + ␦C C C (12%)

H. Raissi et al. / Spectrochimica Acta Part A 62 (2005) 343–352

348 Table 2(Continued ) Theoreticalb Frequencyc

Isotopic shifts IR × I

RI

Experimental

3-13 C

1,5-13 C2

2,4-13 C2

1,3,5-13 C2

18

−2, −1 −2 −5, −5 −5 0, 0

0, 0 0 0, −1 0 −8, −4

−10 0 −11, −6 −12 −10

−3, −1 −2 −6, −6 −6 −9, −4

−1

−8

−2

−1, −1

−2, −2

0 0 −1

2,4-13 C2 –18 O2

Infrared

−10 −1 −30 −3 −13

−10 −1 −15, −15 −15 −15, −11

782 (36)

−10

−13, −1

−16

−1, −1

−3, −2

−20, −16

−22, −17

0 0 −2

−4 −4 −1

0 0 −4

−3 −4 −19

−17 −17 −20

0, 0

−5, −3

−2, −1

−6, −3

−9, −7

−11

0

−5

−3

−6

−13

−15

0, 0

−3, −2

−4, −2

−3, −3

−11, −9

−15

0

−3

−4

−4

−11

−14

O2

49 50 51 52 53

799b1u 798b2g 681b1u 680b2g 679b3u

54

678ag

55

613b2u

56 57 58

570au 570b3g 569b1g

59

460b3u

60

448ag

61

437b2u

62

412b1g

63

332b2u

∼0

0

−3

−2

−4

0

−2

n.m.

64

296b3u

4

0

−1

0

−2

−2

−2

n.m.

65

261ag

−1

−4

−1

−5

−4

−5

n.m.

66

255b3u

0.1

−1

−4

−1

−5

−4

−5

n.m.

67

231b1u

2

0

−1

−1

−1

−6

−6

n.m.

68

207b1g

2

0

−2

−1

−3

−4

−5

n.m.

194 (80)

69

194ag

4

0

−2

−1

−3

−3

−4

n.m.

180 (sh)

70

163b2g

0.3

−2

−1

0

−4

0

0

n.m.

154 (13)

71

162b1u

−2

−1

0

−3

0

0

n.m.

72

147au

0

−1

0

−1

−6

−6

n.m.

73

140b3g

∼0

0

−1

0

−1

−5

−5

n.m.

136 (7)

74

109b2g

0.5

0

0

0

0

−4

−4

n.m.

93 (8)

75 76 77

94b3g 93au 86b2u

0.1

0 0 0

0 0 −1

0 0 0

0 0 −1

−1 0 0

−1 0 −1

n.m. n.m. n.m.

79 (20)

78 79 80

82b1u 79b2g 37b1u

0 0 0

0 0 −1

0 0 0

0 0 −1

0 0 −1

0 0 1

n.m. n.m. n.m.

67 (23) n.m.

81

20au

0

−1

0

−1

0

1

n.m.

n.m.

3 0.3 2 0.1 2 1 3

1 0.01 8 8 1 0.2

0.03

∼0

0.2 0.4 0.02 0.4

Assignments

Raman 769 (7)

685 (10) 676 (14) 651 (9) 643 (1) 613 (13)

550 (20)

455 (35) 437 (100) 429 (7) 396 (10)

238 (1)

γCH␣ (94%) γCH␣ (94%) Γ (71%) Γ (71%) ∆(51%) + ␦C CH3 (25%) ∆(51%) + ␦C CH3 (25%) ∆(61%) + ␦C CH3 (19%) Γ (67%) Γ (67%) ∆(59%) + ␦C CH3 (18%) νs CuO (49%) + ␦C (23%) νs CuO (49%) + ␦C (23%) νa CuO (48%) + ␦C (18%) νa CuO (48%) + ␦C (18%) ∆(61%) + ␦C CH3 (17%) ∆(64%) + ␦C CH3 (15%) ∆(51%) + ␦C CH3 (21%) ∆(51%) + ␦C CH3 (21%) Γ (70%) + ␦C CH3 (10%) ∆(54%) + ␦C CH3 (14%) ∆(54%) + ␦C CH3 (14%) Γ (46%) + ␦C CH3 (19%) Γ (46%) + ␦C CH3 (19%) Γ (53%) + ␦C CH3 (15%) Γ (57%) + ␦C CH3 (18%) Γ (57%) + ␦C CH3 (18%) τCH3 (83%) τCH3 (83%) ∆(38%) + ␦C CH3 (26%) τCH3 (76%) τCH3 (76%) τ(OCuOC) (31%) + τ(CCOCu) (23%) ␶(OCCC) (39%) + τ(CuOCC) (19%)

CH3 CH3 CH3 CH3

a Frequencies are in cm−1 and intensities are relative. IR, infrared; R, Raman; I, calculated relative intensity; ν, stretching; δ, in plane bending; γ, out of plane bending; ρ, rocking in plane; π, rocking out of plane; ∆, in plane ring deformation; Γ , out of plane ring deformation; sh, shoulder; τ, torsion; n.m., not measured; relative intensities are given in parentheses. b Unscaled frequencies. c Calculated at B3LYP/6-31G* level except for Raman intensities, which are calculated at B3LYP/3-31G* level. d Symmetry species. e For the DFT assignment means the percentage of deviation of the bond stretching and bond angle. f Experimental, data from [5].

H. Raissi et al. / Spectrochimica Acta Part A 62 (2005) 343–352

C3 , C3 C4 ) stretching (49%), which are coupled to CH␣ in plane bending mode (27%). The frequency shift upon isotopic substitution confirms this assignment. The weakness of the Raman band at 1566 cm−1 has been considered by

349

Tayyari et al. [31,32] as a measure of delocalization of ␲system of the chelated ring. Infrared spectrum of Cu(AA)2 shows a medium band at 1415 cm−1 . Theoretical calculations attribute this band to b2u species of asymmetric C O stretch-

Fig. 5. Selected normal modes of Cu(AA)2 .

350

H. Raissi et al. / Spectrochimica Acta Part A 62 (2005) 343–352

Fig. 5. (Continued ).

ing (57%) which is coupled to asymmetric CH3 deformation mode (21%). The asymmetric C O stretching in the Raman spectrum appears at 1420 and belongs to b1g symmetry species. Theoretical calculation predicts that this band coupled to δa CH3 (19%). The infrared band at about 1415 cm−1 has been assigned by Nakamoto and Martell [1] to νCO + νCH and by Junge and Musso [5] to δa CH3 . The Raman spectrum shows a strong band at 1270 cm−1 . According to our theoretical calculations, this band is mainly due to the symmetric (C2 C3 , C3 C4 ) stretching (56%), which is slightly coupled to symmetric CH3 deformation mode (9%). This band is absent in the infrared spectrum and

therefore, it is not considered by other investigators [1,5]. Theoretical calculations attributes the strong infrared band at 1275 cm−1 to b3u species of νs (C2 C3 , C3 C4 ) which is coupled to δs CH3 . Junge and Musso assigned this band to νs C C C but Nakamoto and Martell attributed this band to νC C + νC CH3 . The agreement between experimental and theoretical frequencies shift upon isotopic substitutions are excellent and confirm this assignment. The infrared and Raman spectra show two relatively strong to strong bands at about 1190 and 1180 cm−1 region, respectively. These results are quite in agreement with the calculation, which attributes the former band to the b2u and the latter to the b1g species of

H. Raissi et al. / Spectrochimica Acta Part A 62 (2005) 343–352

CH␣ in plane bending modes. The assignment for the latter band has not been reported up to now, but Nakamoto and Martell assigned the former band correctly to δCH␣ . Four methyl groups give 24 bond angle bending, which by means of them we can describe, the asymmetric and symmetric CH3 deformation and CH3 rocking modes. In the infrared spectrum of Cu(AA)2 only two ␦CH3 vibrational modes could be identified at 1534 and 1356 cm−1 . The deconvolution analysis shows only two new bands at 1503 and 1497 cm−1 , which can be assigned to δCH3 . Theoretical calculation predicts that the infrared band at 1534 cm−1 is mainly due to b2u species δa CH3 (53%), which is coupled to δCH␣ (17%). Two other bands at 1503 and 1497 cm−1 are assigned to δa CH3 and the last band at 1356 cm−1 to δs CH3 . In the Raman spectrum we found six bands between 1550 and 1350 cm−1 region and similarly assigned them to asymmetric and symmetric CH3 deformation modes. The Raman spectrum shows two bands at 1062 and 1041 cm−1 . By considering the calculations, we assigned these bands to b3g and b2g species of CH3 rocking mode, respectively. Since these two bands are not infrared active, they are left out of Nakamoto and Martell and Junge and Musso assignments. In this region, infrared spectrum shows to bands at 1045 and 1020 cm−1 . Furthermore, the deconvolution analysis shows another band at 1011 cm−1 . According to theoretical calculations, the band at 1045 cm−1 is due to b1u species of πCH3 and two other bands belong mainly to ρCH3 , which are slightly coupled to δC C O (12%) and δCH␣ (21%), respectively. 6.4. Below 1000 cm−1 In this region one expects to observe C CH3 and Cu O stretching, CH out of plane bending and in plane and out of plane ring deformation modes. Previous worker [5,33] have assigned an infrared band at about 935 cm−1 to νC CH3 . Behnke and Nakamoto [34] based on a normal coordinate analysis pointed out that a strong coupling between the central νC C and the stretching νC CH3 mode exist. According to the potential energy distribution, these authors found that the observed predominant 1350 cm−1 band νC C was strongly coupled with νC CH3 . For UO2 (AA)2 Nakamoto et al. [3] reported that the band at 925 cm−1 should be the C CH3 (b2u ) stretching mode. In the case of Al(AA)3 [15] we assigned the observed Raman shifts at 949 and 933 cm−1 to A1 and E species of νC CH3 . Continuing with the analysis, for Cu(AA)2 complex, we observe two Raman bands at 942 and 928 cm−1 . In the infrared spectrum we observed a band at 937 cm−1 . On the other hand, by deconvolution analysis of the infrared spectrum another band appears at 923 cm−1 . According to theoretical calculation, we assigned these bands to C CH3 stretching, which are coupled to other modes (see Table 2). A good agreement exists between theoretical and experimental frequency shift upon isotopic substitution and confirm these assignments. Nakamoto and Martell [1] believe that the infrared band at about 935 cm−1 is due to νC CH3 , which is coupled to νCO. The contribution of νCO

351

to this mode seems doubtful because the νCO is expected to be observed at higher frequencies. The infrared spectrum of Cu(AA)2 indicates only a medium band at 782 cm−1 . The deconvolution analysis confirms only the observed band. Theoretical calculations predict this band belong to b1u species of the CH␣ out of plane bending mode. Similarly, the Raman band at 769 cm−1 is assigned to b2g species of γCH␣ . Other investigators [1,5] considered the infrared band at 782 cm−1 as CH out of plane bending mode, which is in agreement with our results. The Raman spectrum indicates two medium to strong bands at 676 and 550 cm−1 . According to theoretical calculations, we assigned these bands to b2g and b3g species of out of plane ring deformation modes, respectively. The b1u species of Γ appears at 685 cm−1 in the infrared spectrum of Cu(AA)2 as medium band and quite in agreement with experimental and theoretical frequency shift upon isotopic substitution. Nakamoto et al. assigned this band to ring deformation + νCuO. According to theoretical calculation, the infrared band at 651 cm−1 is mainly due to in plane ring deformation (51%), which is coupled to δC CH3 (25%). This band is assigned by Nakamoto et al. to δC CH3 + νCuO and by Junge and Musso to γC C C. The Raman spectrum of Cu(AA)2 shows two bands at 437 and 396 cm−1 , which appear as very strong and medium bands, respectively. In this region, the IR spectrum shows two bands at 455 and 429 cm−1 , which are Raman inactive. These results are in excellent agreement with the calculations, which attributes the infrared and Raman band at 455 and 437 cm−1 to the b3u and ag species of νs Cu O, respectively, which coupled to δC CH3 . The assignment for the latter is not appeared in the literature yet, but the former is assigned by Nakamoto and Martell [1] to the pure νCuO. Similarly, the infrared and Raman band at 429 and 396 cm−1 are assigned to b2u and b1g species of νa Cu O, respectively, which coupled to νC CH3 . Pinchas and coworkers [35,36] believed that M O stretching of metal complexes of acetylacetone appears in the 500–600 cm−1 region. Their results are based on the isotopic shifts due to the 18 O substitution. However, the results of an isotopic substitution on the γ atom of the ligand (atom directly bonded to metal) such as that made by Pinchas and coworkers [35,36] must be interpreted with caution since the M O stretching vibrations as well as ligand vibrations involving the motion of γ-atom (oxygen) are affected. It is, therefore, not possible to assign uniquely the metal–ligand stretching vibration by this method. The Raman spectrum of Cu(AA)2 shows two bands at 238 and 194 cm−1 , which the former appears as a weak and the latter is very strong. Theoretical calculations suggest that these two bands belong to ag and b1g species of ∆, respectively, which coupled to δC CH3 . According to theoretical calculation, we assign the band at 154 and 136 cm−1 to b2g and b3g species of out of plane ring deformation, respectively, which are coupled to γC CH3 . By considering the calculation results, the ag species of νM O in metal acetylacetonate complexes could be used as a measure of complex stability,

352

H. Raissi et al. / Spectrochimica Acta Part A 62 (2005) 343–352

since in this vibration mode metal is not moving, therefore, its frequency is solely depends on M O bond strength and not on the metal ion weight.

7. Conclusion The structural parameters, vibrational frequencies, IR and Raman intensities of the vibrational bands were calculated at DFT level of theory using B3LYP functional with 6-31G* and 3-21G* basis sets. The predicted frequencies were compared with the experimental data in the solid state. A satisfactory reproduction of the experimental frequencies is obtained. The short M O distance in Cu(AA)2 suggests a strong metal–ligand bond. This fact is confirmed by other experimental results, such as C C bond length and Cu O stretching frequencies. Analysis of the vibrational spectra indicates a strong coupling between the chelated ring modes. Four bands in the frequency range 500–390 cm−1 are assigned to vibrations of metal–ligand bonds. The Cu O symmetry stretching was observed at 437 cm−1 in the Raman spectrum of Cu(AA)2 and considered as a measure of complex stability. This band has not been considered up to now.

References [1] K. Nakamoto, A.E. Martell, J. Chem. Phys. 32 (1960) 588. [2] K. Nakamoto, P.G. McCarthy, A. Ruby, A.E. Martell, J. Am. Chem. Soc. 83 (1961) 1066. [3] K. Nakamoto, Y. Morimoto, A.E. Martell, J. Am. Chem. Soc. 83 (1961) 4533. [4] M. Mikami, I. Nakagawa, T. Shimanouchi, Spectrochim. Acta 23A (1967) 1037. [5] H. Junge, H. Musso, Spectrochim. Acta 24A (1968) 1219. [6] D.A. Thornton, Coord. Chem. Rev. 104 (1990) 173. ¨ [7] T. SchOnherr, U. Rosellen, H.H. Schmidtke, Spectrochim. Acta 49A (1993) 357. [8] M.A. El’yashevich, Dokl. AN SSSR 28 (1940) 605. [9] M.A. El’yashevich, Zh. Fiz. Khim. 14 (1940) 1381. [10] E.B. Wilson, J. Decius, P.C. Cross, Molecular Vibrations, the Theory of Infrared and Raman Vibrational Spectra, McGraw-Hill, New York, 1955.

[11] D.E. Mann, T. Shimanouchi, J.H. Meal, L. Fano, J. Chem. Phys. 27 (1957) 213. [12] S.C.A. T´ellez, J. G´omez Lara, Spectrochim. Acta 51A (1995) 395. [13] S.C.A. T´ellez, J. G´omez Lara, Spectrosc. Lett. 27 (1994) 209. [14] S.C.A. T´ellez, E. Hollauer, M.I. Pais da Silva, M.A. Mondrag´on, I. Haiduc, M. Curtui, Spectrochim. Acta 57A (2001) 1149. [15] S.F. Tayyari, H. Raissi, Z. Ahmadabadi, Spectrochim. Acta 58 (2002) 2669. [16] J.P. Fackler, Prog. Inorg. Chem. 7 (1966) 361. [17] H. Koyama, Y. Saito, K. Kuroya, J. Inst. Polytech. Osaka City Univ. C4 (1953) 43. [18] T. Moeller, Inorganic Syntheses, vol. 5, McGraw-Hill, Book Co. Inc., New York, 1957, p. 108. [19] M.J. Frisch, G.W. Trucks, H.B. Schlegel, F.E. Scuseria, M.A. Robb, J.R. Cheeseman, V.G. Zakrzewski, J.A.G. 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, 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, in: Revision A.3, Gaussian Inc., Pittsburgh, PA, 1998. [20] A.D. Becke, J. Chem. Phys. 98 (1993) 5648. [21] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 875. [22] S.F. Tayyari, H. Raissi, F. Milani-Nejad, I.S. Butler, Vib. Spectrosc. 26 (2001) 187. [23] R.H. Holm, F.A. Cotton, J. Am. Chem. Soc. 80 (1958) 5658. [24] F.A. Cotton, J.J. Wise, Inorg. Chem. 5 (1966) 1200. [25] H. Montgomery, E.C. Lingafelter, Acta Crystallogr. 17 (1964) 1481. [26] L.M. Shkolnikova, E.A. Shugam, Zh. Strukt. Khim. 2 (1961) 72. [27] A. Camerman, D. Mastropalo, N. Camerman, J. Am. Chem. Soc. 105 (1983) 1584. [28] S.F. Tayyari, F. Milani-Nejad, Spectrochim. Acta 56 (2000) 2679. [29] S.F. Tayyari, H. Raissi, F. Tayyari, Spectrochim. Acta 58A (2002) 1681. [30] H. Raissi, S.F. Tayyari, F. Tayyari, J. Mol. Struct. 613 (2002) 195. [31] S.F. Tayyari, Th. Zeegers-Huyskens, J.L. Wood, Spectrochim. Acta 35A (1979) 1265. [32] S.F. Tayyari, Th. Zeegers-Huyskens, J.L. Wood, Spectrochim. Acta 35A (1979) 1289. [33] R. Mecke, F. Funck, Z. Elektrochem. 60 (1956) 1124. [34] G.T. Behnke, K. Nakamoto, Inorg. Chem. 6 (1967) 433. [35] S. Pinchas, B.L. Silver, I. Laulicht, J. Chem. Phys. 46 (1967) 1506. [36] S. Pinchas, J. Shamir, J. Chem. Soc., Perkin Trans. 2 (1974) 1098.