Theoretical and experimental studies on selected 1,3-diazolium salts

Vibrational Spectroscopy 42 (2006) 317–324 www.elsevier.com/locate/vibspec

Theoretical and experimental studies on selected 1,3-diazolium salts Kamilla Malek a,*, Małgorzata Skubel a, Grzegorz Schroeder b, Oles P. Shvaika c, Leonard M. Proniewicz a,d,** a Faculty of Chemistry, Chemical Physics Department, Jagiellonian University, R. Ingardena 3, 30-060 Krakow, Poland Faculty of Chemistry, Organic Chemistry Department, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland c Institute of Physical Organic and Coal Chemistry, National Academy of Sciences of Ukraine, R. Luxemburg 70, 83114 Donetsk, Ukraine d Regional Laboratory of Physicochemical Analysis and Structural Research, Jagiellonian University, R. Ingardena 3, 30-060 Krakow, Poland b

Available online 19 June 2006

Abstract An analysis of the molecular structures of the benzyl and adamantyl derivatives of 1,3-diazolium salts in solid state was carried out based on the detailed interpretation of their vibrational spectra. The description of IR and Raman spectra was supported by DFT (B3LYP) calculations of the unsubstituted and substituted imidazolium cations as models. These results allowed finding markers bands originated mainly from the stretching, rocking and wagging vibrations of the C2–H bond. Additionally, calculations of the heteroaromaticity (HOMA index) showed that cation formation causes dearomaticity of the imidazole ring. However, the substitution of the cationic species by the benzyl and adamantyl groups increases aromatic nature of the heterocyclic moiety. Moreover, the electron distribution performed using the GAPT method indicated the positive charge delocalization in the ring.

# 2006 Elsevier B.V. All rights reserved. Keywords: Imidazolium salts; IR and Raman spectra; DFT calculation; Aromaticity; Atomic charges

1. Introduction

* Corresponding author. Tel.: +48 12 663 22 53/20 64; fax: +48 12 634 05 15. ** Corresponding author. E-mail addresses: [email protected] (K. Malek), [email protected] (L.M. Proniewicz). 0924-2031/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2006.05.017

The 1,3-diazolium salts are widely used in many chemical processes and technologies. For example, they serve as ionic liquids in organic reactions, catalytic processes, and separation technologies [1]. Due to their biological activity group of them is applied as herbicides [2]. The most famous are the azolium derivatives that have been synthesized predominantly as

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far-infrared (FT-FIR, 512 scans) spectra were run in KBr and low molecular weight polyethylene discs, respectively. Resolution was set at 4 cm1 (MIR) and 2 cm1 (FIR). FT-Raman spectra were recorded on a Bio-Rad step-scan spectrometer (FTS 6000) combined with a Bio-Rad Raman Accessory (FTS 40). Excitation at 1064 nm was made by a Spectra-Physics Topaz T10-106c cw Nd:YAG laser. FT-IR spectra were measured on Bruker (IFS 48) and Bio-Rad (FTS 60 V) spectrometers in the mid and far IR regions, respectively. The accuracy of the readings was 1 cm1. Scheme 1. Compounds studied.

3. Computational methods precursors of N-heterocyclic carbens in organometallic chemistry, ruthenium-mediated olefin metathesis and palladium cross-coupling chemistry [3]. Until now, studies on such molecular systems have been focused on synthesis of ligands and their metallocomplexes followed by crystallographic determination of their structures [4]. Only few papers of diazolium salts have reported some general remarks on conformational analysis, cycloaddition reactions or aromaticity of the unsubstituted azolium cations [5]. According to our best knowledge, there has been no explanation of experimental data, including spectroscopic analysis, by using quantum-chemical calculations [6]. Thus, as the subject of this work the following imidazolium salts were chosen: 1,3-dibenzylimidazolium chlorate(VII) [Im(CH2C6H5)2]ClO4 and 1,3-di-1-adamantylimidazolium chlorate(VII) [Im(Ad)2]ClO4 (see Scheme 1). These compounds possess different chemically and geometrically substituents. To study the influence of the steric and electronic factors on the imidazolium cation and to afford a direct comparison between the different derivatives, we carried out the DFT calculations. The cationic forms were applied as a model structures for the unsubstituted diazolium ring ([ImH2]+) as well as the mentioned-above derivatives ([Im(CH2C6H5)2]+ and [Im(Ad)2]+). We performed simulation of vibrational spectra and calculated geometrical parameters, aromaticity and atomic charges. Theoretical results were compared to the available experimental data. It allowed us to determine molecular structure of both compounds and discuss the substituent effect on imidazolium cation. 2. Experimental The synthesis of the compounds studied was performed as described elsewhere [7]. The elemental analysis for [Im(CH2C6H5)2]ClO4 is: C, 58.5%; H, 4.8%; N, 7.9%, found; and C, 58.5%; H, 4.9%; N, 8.0%, calculated. For [Im(Ad)2]ClO4 is: C, 63.6%; H, 8.1%; N, 6.9%, found, and C, 63.2%; H, 7.6%; N, 6.4%, calculated. For FT-Raman measurements, a few milligrams of the compound was placed in capillary tube and measured directly; 200 scans were collected (with a resolution of 4 cm1) for both compounds. Fourier transform midinfrared (FT-MIR, 256 scans) and Fourier transform

All calculations were performed using the Gaussian 03 set of programs [8]. The geometries and harmonic frequencies with their IR and Raman intensities were obtained by using the DFT method employing the hybrid B3LYP functional [9]. No imaginary frequencies for all structures proved that the applied models are at their potential energy minima. The 6-31G basis set was used in computations, with one additional set of polarization as well as diffusion function on the heavy atoms [6-31+G(d)]. To adjust for anharmonicity effects, the computed frequencies were scaled by the factor of 0.96 for the stretching modes of C–H and N–H bonds [10]. The modes in the 0–2000 cm1 region of the IR and Raman spectra were scaled by the factor of 0.98. Since only Raman scattering activities are obtained from the Gaussian outputs, the following expression [11] was used to determine the theoretical Raman intensities: I i ¼ ð1012 ðn0  ni Þ4 Þ= ðni RAðiÞ Þ; where: Ii—Raman intensity, RA(i)—Raman scattering activities, ni—wavenumber of the normal modes; n0— wavenumber of the excitation laser. Program Gar2ped [12] was used for calculation of potential energy distribution (PED) of normal modes in terms of natural internal coordinates evaluated by Pulay and Fogarasi [13]. The PED calculations were carried out for [ImH2]+ and [Im(CH2C6H5)2]+, only. Due to limitation of the internal coordinates for fused rings present in the adamantane molecule, the description of the vibrational spectra of [Im(Ad)2]+ was based on visualization (Molekel program [14]). Atomic charges were obtained using Generalized Atomic Polar Tensors (GAPT) elaborated by Cioslowski [15]. This method, similarly to the other quantum methods, distributes the electron density among atoms in a molecule. Furthermore, the GAPT method sums charges up to the total charge of a molecule and reflects its symmetry. The most popular in aromaticity studies is the Harmonic Oscillator Model of Aromaticity (HOMA) index [16]. It is computed equation:   following h according to the i P 2 HOMA ¼ 1  aðRopt Rave Þ  a = n ðRave  Ri Þ2 ¼ 1  EN  GEO; where a is an empirical factor and Ropt, Ri and Rave are optimal bond lengths, bond lengths in a real system and average bong lengths, respectively, and n is the number of bonds. Two effects can decrease aromaticity of a molecule, e.g. bond length alternation (GEO factor) and bond length elongation (EN factor). For the aromaticity determination,

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Fig. 1. Atom numbering scheme of the imidazolium ring for the theoretical models.

experimental as well as theoretical bond lengths can be used. In the case of the diazolium salts studied here, crystallographic data are not available. Therefore, the calculated geometrical parameters were applied to evaluate their HOMA index. 4. Results and discussion 4.1. Geometries The elemental analysis did not show the presence of the other species than the diazolium and chlorate(VII) ions. For this reason and because of the steric bulk of the adamantyl and benzyl substituents, the presence of strong electrostatic interaction (H-bonding, p-stacking) between cation and anion can be excluded. Moreover, the available crystallographic data for other diazolium salts indicate that the distance cation–anion ˚ [17–19]. Therefore, the interactions is in the range of 3–4 A between ions are neglected and cationic species are used as models in calculations (Fig. 1). Symmetry Cs is imposed for [ImH2]+ and [Im(CH2C6H5)2]+ and C2 for [Im(Ad)2]+ during the optimization process. The values of bond lengths and valence angles of the imidazolium cation are given in Table 1 and the atom numbering in Fig. 1. Calculated structural parameters show that introduction of two different chemically substituent groups on the imidazolium cation causes insignificant changes in bond distances ˚ ) as well as in angles (1–38). It indicates that (0.001–0.005 A only steric effect of the benzyl and adamantyl groups on the imidazolium ring may play role in any changes of the cation structure.

4.2. GAPT charge distribution Table 2 shows the atomic charges of the imidazolium cation (the complete data for all atoms are given in Table I in Supplementary Materials). The C2 atom, which is a potential site of the carbene center or metal coordination, shows a strong positive charge (+0.225 a.u. for [ImH2]+) that increases by 0.8 a.u. in the substituted derivatives. Similarly to the discussed geometry, the same increase of the charge on this atom does not indicate an effect of the chemical character of the substituents. On the other hand, the charge of the hydrogen atom adjacent to C2 is close to charges of the other hydrogen atoms of the heterocyclic ring. The charge values on C4 and C5 are typical for organic molecules (close to zero). The negative charge on the nitrogen atoms (0.239 a.u.) causes the relatively high decrease of the electron distribution on their hydrogen atoms (+0.325). After substitution, the nitrogen charge increases significantly to 0.425 a.u. for the benzyl, and Table 2 Computed atomic charges (in a.u.) of the imidazolium cation in [ImH2]+, [Im(CH2C6H5)2]+ and [Im(Ad)2]+ Atoms

[ImH2]+

[Im(CH2C6H5)2]+

[Im(Ad)2]+

N1/N3 C2 C5/C4 H(N1(3))/C(N1(3)) H(C2) H(C4(5))

0.239 +0.225 +0.051 +0.325 +0.177 +0.162

0.425 +0.307 0.004 +0.583 +0.136 +0.137

0.432 +0.309 +0.004 +0.449 +0.114 +0.119

Table 1 ˚ ) and angles (in 8) of the imidazolium derivatives of the studied compounds Computed bond lenghts (in A Bonds

[ImH2]+

[Im(CH2C6H5)2]+

[Im(Ad)2]+

Angles

[ImH2]+

[Im(CH2C6H5)2]+

[Im(Ad)2]+

C2–H C2–N1 C2–N3 N1–H/N1–Cbenz/Ad N3–H/N3–Cbenz/Ad N1–C5 N3–C4 C5–C4 C5–H C4–H

1.080 1.337 1.337 1.015 1.015 1.385 1.385 1.364 1.080 1.080

1.080 1.340 1.340 1.495 1.495 1.382 1.382 1.365 1.079 1.079

1.075 1.341 1.341 1.507 1.507 1.385 1.385 1.363 1.078 1.078

N1–C2–N3 N3–C2–H N1–C2–H N3–C4–C5 N3–C4–H C5–C4–H C2–N3–C4 C2–N3–H(C) C4–N3–H(C) C4–C5–N1 C4–C5–H N1–C5–H C2–N1–H(C) C5–N1–H(C) C2–N1–C5

107.1 126.4 126.4 106.5 122.4 131.1 110.0 124.5 125.5 106.5 131.1 122.4 124.5 125.5 110.0

109.1 125.4 125.4 107.2 121.8 131.0 108.3 125.4 126.3 107.2 131.0 121.8 125.4 126.3 108.3

109.8 125.1 125.1 107.5 122.4 130.1 107.6 126.8 125.5 107.5 130.1 122.4 126.8 125.5 107.6

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Table 3 Aromaticity indexes for rings of imidazole [Im], imidazolium cation [ImH2]+ and its derivatives studied here [Im(CH2C6H5)2]+ and [Im(Ad)]+)

HOMA GEO EN

Imidazolium ring in [Im] exp.

Imidazolium ring in [ImH2]+ exp.

Imidazolium ring in [ImH2]+ calc.

Imidazolium ring in [Im(CH2C6H5)2]+ calc.

Phenyl ring in [Im(CH2C6H5)2]+ calc.

Imidazolium ring in [Im(Ad)2]+ calc.

0.888 0.106 0.005

0.797 0.185 0.017

0.872 0.110 0.018

0.884 0.097 0.019

0.968 0.002 0.030

0.868 0.111 0.021

0.432 a.u. for adamantyl derivatives. Simultaneously, the electron density on adjacent carbon atoms of the substituents falls down to ca. +0.58 and +0.45 a.u. from 0.1 a.u., for Im(CH2C6H5)2]+ and [Im(Ad)2]+, respectively. Despite those atoms bonded to the ring nitrogen atoms, the electron density on the other atoms of the substituent molecules remains unchanged. Moreover, the electron distribution indicates delocalization of the positive charge in the diazolium cations between C2 and the carbon atoms of the substituents attached to the ring nitrogen atoms. 4.3. HOMA aromaticity As mentioned above, the aromaticity of the heterocyclic and aromatic rings were determined by using the HOMA index and can be calculated using the available experimental as well as theoretical bond distances. Table 3 shows the values of HOMA, GEO and EN terms for the compounds studied here. To compare, the experimental data are used to compute the heteroaromaticity of the unsubstituted imidazole [18] and imidazolium cation [19]. These results indicate dearomaticity (by 0.09) of the heterocyclic ring to cation formation. On the other hand, the discrepancy (0.07 unit) between the HOMA values of the ‘‘neat’’ cation obtained by using computed and experimental geometries [19] is due to the fact that the latter refers to the imidazolium cation surrounded by the H-bonded

sulfate anions and the first refers to a single cation molecule. Upon substitution, aromaticity of compounds studied here increases by factor of 0.01 for the benzyl derivative and remains almost unchanged for the adamantyl one. These changes are caused mainly by decrease of the bond length alternation (GEO factor). The larger change of aromaticity in [Im(CH2C6H5)2]+ in comparison to [Im(Ad)2]+ can also result from the close vicinity of phenyl p-electrons to the imidazolium ring. The HOMA index was calculated also for the phenyl rings in [Im(CH2C6H5)2]+. Their aromaticity decrease slightly by 0.03 unit comparing to the benzene molecule (HOMA = 1). 4.4. IR and Raman spectra Infrared and Raman spectra of Im(CH2C6H5)2]ClO4 and [Im(Ad)2]ClO4 are presented in Figs. 2 and 3, respectively. The theoretical spectra (B3LYP/6-31+G(d)) for the models are also given for comparison. The analysis of the most characteristic normal modes of the studied compounds is presented in Table 4 (the full description of the spectra with the internal coordinates’ definitions is given in the Supplementary Materials, Tables II–V). The match between the theoretical and experimental spectra for both compounds is fairly good. It shows that the applied model structure neglecting the cation–anion interaction can be

Fig. 2. IR and Raman spectra of [Im(CH2C6H5)2]ClO4: (A) Theoretical Raman spectrum; (B) Experimental Raman spectrum; (C) Experimental IR spectrum; (D) Theoretical IR spectrum. (*) Denotes ClO4 vibrations.

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Fig. 3. IR and Raman spectra of [Im(Ad]ClO4: (A) Theoretical Raman spectrum; (B) Experimental Raman spectrum; (C) Experimental IR spectrum; (D) Theoretical IR spectrum. (*) Denotes ClO4 vibrations; (**) band originates from water in KBr.

used for the cationic organic compounds. The analysis of the normal modes allows proposing marker bands that can be useful in the further examination of the diazolium salts. 4.4.1. The C2–H vibrations The most interesting modes are those related with the C2–H bond (see the numbering in Fig. 1) which should vanish in vibrational spectra of diazolium carbens and metallocomplexes. Therefore, the stretching [n(C2–H)], rocking [r(C2–H)] and wagging [v(C2–H)] are discussed below. The calculation for the imidazolium cation [Im(H)2]+ clearly indicates that n(C2–H) is an isolated vibration (97% PED) expected at 3179 cm1 as a weak and a medium intensity band in IR and

Raman spectra, respectively. The DFT results predict that the substitution of the benzyl group shifts this mode slightly by 8 cm1 to the lower wavenumber range, while the effect of the adamantyl group is more significant (the blue-shift by 47 cm1). However, the IR and Raman intensities of n(C2– H) for both compounds decrease substantially preventing from their unambiguous identification in the experimental spectra, especially for [Im(Ad)2]ClO4. Thus, n(C2–H) is assigned only to FT-IR and FT-Raman bands of Im(CH2C6H5)2]ClO4 at 3142 cm1. As can be seen from Table 4, the rocking [r(C2–H)] and wagging [v(C2–H)] vibrations of the derivatives studied here are also shifted in comparison to ‘‘the neat’’ diazolium ring. Among them, only r(C2–H) of [Im(Ad)2]ClO4 occurs as a

Table 4 Selected normal modes (in cm1) of the imidazolium cation in [ImH2]+, [Im(CH2C6H5)2]+, and [Im(Ad)2]+ [H2Im]+

[Im(Ad)2]+

Im(CH2C6H5)2+ n¯ calc ð%PEDÞ

IR Irel

R Irel

nIR exp

nR exp

35.2 4.4 vw vw 1.9 vw

3171(98) 1274(28) 801(48) 736(32) 1564(54) 1560(52)

8.7 5.5 19.7 5.7 5.3 41.4

6.6 2.5 8.9 vw 5.2 vw

3143br,m 1316vw 820br,w 752m 1558vs 1558vs

3142vw 1315vw 826br,m – 1560w 1560w

7.4

6.2

1128(18)

100

6.9

1149vs

11.1 vw vw vw 14.7

vw 1.3 2.1 vw vw

1012(32) 1012(38) 1011(54) 625(49) 612(61)

vw vw vw 4.9 5.4

50.8 50.8 8.6 4.8 17.2

– –

n¯ calc ð%PEDÞ

IR Irel

R Irel

3179(97) 1178(49) 833(59) 716(30) 1603(38) 1537(60)

14.9 vw vw 43.0 15.7 2.7

1110(36) 1049(48) 919(83) 902(89) 612(80) 611(70)

640m 624s

Assignment

n¯ calc ð%PEDÞ

IR Irel

R Irel

nIR exp

nR exp

3226 1122 802

6.2 100 7.2

1.5 vw vw

3028br,m 1155br,vs 814br,m

3040br,m – –

n(C2–H) r(C2–H) v(C2–H)

1564 1541

3.3 51.2

7.9 vw

1561vw 1539br,m

1561w –

–

1122

100

vw

1155s

–

n(C4 C5) n(C2–N1), n(C2–N3) n(C4–N3), n(C5–N1)

1028s – 1028s – 621br,m

1054 992 983 637 633

3.0 vw 1.7 2.7 0.0

vw 3.6 vw vw vw

1088br,m –

1079w 1003w

634m –

646br,w

IR R Irel and Irel indicate the relative intensity (in %) of theoretical bands in respect with the strongest one.

b1(R5) b2(R5) g1(R5) g2(R5)

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very intense IR band (1155 cm1) what is consistent with our calculation (1122 cm1). On the other hand, v(C2–H) is assigned to the medium-intense IR band at ca. 820 cm1 for both imidazolium derivatives. Again, calculations predict this frequency very well (801 and 802 cm1 for [Im(CH2C6H5)2]+ and [Im(Ad)2]+, respectively). 4.4.2. The imidazolium ring vibrations According to the Pulay’s and Fogarasi’s definition of the internal coordinates, the motions of the 5-membered ring are as follows: the stretching modes of its bonds (n), two deformations in-plane [b1(R5), b2(R5)] and two deformations out-of-plane [g1(R5), g2(R5)]. The characteristic vibration of the heterocyclic ring is stretching mode of the C4 C5 bond [n(C4 C5)]. For both studied compounds, n(C4 C5) is observed at the same wavenumber (1560 cm1) in IR and Raman spectra. It is found to be in excellent agreement with DFT calculations that predict the same frequency and intensity of this mode (1564 cm1) for both compounds. Upon substitution, this vibration is red-shifted by ca. 40 cm1 with reduction in intensity. For the benzyl derivative, the outof-phase stretching of the N1-C2-N3 fragment (PED: 50%) is assigned to the same bands as n(C4 C5) (FT-IR: 1558 cm1; FT-Raman: 1560 cm1). It is consistent with our calculations that predict frequency of n(C2–N1) and n(C2–N3) red-shifted by 4 cm1 comparing to n(C4 C5). However, for Im(Ad)2]ClO4, the out-of-phase n(C2–N1) and n(C2–N3) are present as a single IR medium band at 1539 cm1 (calc. 1541 cm1). Calculation for [ImH2]+ suggests that these modes should be found at 1537 cm1. This slight shift can imply an increase of the ‘‘double’’ character of C2–N bonds after the substitution. Smaller contribution (15%) of n(C2–N1,3) is also observed for several experimental bands in the range 1340–1460 cm1. The strong-intensity bands at 1149 and 1155 cm1 in the experimental spectra of [Im(CH2C6H5)2]ClO4 and [Im(Ad)2]ClO4, respectively, are assigned to the out-of-phase stretches of the C2–N1 and C2–N3 bonds (calc. 1128 and 1122, respectively). Their contribution to these normal modes is small (20% PED). It is worth noting that DFT calculations predict these bands as the most intense in the theoretical IR spectra and it is consistent with the experimental spectra. On the other hand, the in-phase counterparts are observed in Raman spectra as strong and weak-intensity bands at 1028 ([Im(CH2C6H5)2]ClO4) and 1079 cm1 ([Im(Ad)2]ClO4). The next characteristic modes of the imidazolium ring are those representing its bending motions. Our calculations indicate that all of them [b1(R5), b2(R5)], g1(R5), g2(R5)] should be observed in vibrational spectra as very weak bands. Those with the greatest contribution in potential energy of the normal modes are predicted at: 919 [b1(R5), 83%], 902 [b2(R5), 89%], 612 [g1(R5), 80%], and 611 cm1 [g2(R5), 70%], for [ImH2]+. Magnitude and direction of the wavenumber shifts of those vibrations is similar for both compounds in respect with the ‘‘neat’’ imidazolium cation (see Table 4). The greatest change in the positions of bands is observed rather for the in-plane deformation of the ring than for its out-of-plane bend.

4.4.3. The substituents’ vibrations In the studied [Im(CH2C6H5)2]ClO4, the phenyl rings are bonded to the imidazolium cation through the methylene groups. According to DFT calculations, the stretching modes of the CH2 group (asymmetric, symmetric, in-phase and out-of phase) are observed as the broad IR band with maximum at 2924 cm1 and as weak- and medium-intensity Raman bands at 3003 and 2968 cm1. On the other hand, the methylene scissoring vibrations [d(CH2)] are present in IR and Raman spectra at 1496 cm1 [calc. 1484 (in-phase) and 1480 cm1 (out-of phase)]. Typical range for the stretches of the phenyl C–H bonds is 3100–3000 cm1. As calculated, these vibrations are responsible for the strong and broad Raman band with maximum at 3067 cm1 and the weak IR band at 3084 cm1. Moreover, the wagging mode [v(C–H)R6] is sensitive to substitution of benzene. For the mono-substituted benzene, v(C–H)R6 is expected to be at ca. 750 and 700 cm1. Calculations show that this vibration is observed as IR bands at 697 and 752 cm1 (calculated: 703 cm1, 36% PED and 736 cm1, 36% PED, respectively). Still, the agreement is considered to be fairly good with the relative differences in experimental and calculated frequencies by ca. 2%. On the basis of the theoretical data, the stretches of the C–C bonds appears clearly as bands in Raman spectrum of [Im(CH2C6H5)2]ClO4 at 1605 and 1587 cm1 (DFT: 1621 and 1606 cm1, respectively). The former corresponds to the symmetric stretching of C2–C3 and C5–C6 bonds (numbering as follows: C1 is a carbon atom of the phenyl ring substituted by the CH2 group). According to PED, the band at 1605 cm1 originates from two normal modes with the same frequency but different phase motion (see Table IV in Supplementary Materials). The mode at 1587 cm1 is represented by the asymmetric stretching of the other C–C bonds than C2–C3 and C5–C6 (68% PED). Similarly, the breathing vibration of the phenyl ring is predicted as two normal modes with the same energy but different phase of movement. It is present as an isolated mode (over 60% PED) at 1003 cm1 in Raman spectrum (calc. 997 cm1). The out-ofplane deformations of the ring (ca. 30% PED) are observed mainly in IR lower-frequency range (483–413 cm1) and are assigned to the experimental weak-intensity bands at 476 and 460 cm1. As was mentioned above, it is impossible to define internal coordinates of [Im(Ad)2]+. Moreover, the adamantyl group is a complex polycyclic hydrocarbon for which 72 normal modes are expected. To identify vibrations of this substituent in [Im(Ad)2]ClO4, the B3LYP calculation (6-31+G(d) basis set) was performed for the adamantane molecule itself. Theoretical spectra with assignment of calculated bands are shown in Fig. 4. Based on this, experimental strong-intensity and broad bands with maxima at 2913 (IR) and 2926 cm1 (Raman) are assigned to the asymmetric –CH2– stretches [nas(CH2)Ad] and >CH– stretches [n(CH)Ad] (calc. range: 2963–2944 cm1). ns(CH2)Ad is observed as IR and R medium bands at 2853 cm1 (calc. range: 2921–2902 cm1). Comparison of calculation for [Im(Ad)2]+ and adamantane indicates that the stretching C–H vibrations are shifted to the lower frequencies by ca. 40 and

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changes of characteristic modes frequencies of the imidazolium ring upon substitution. Our studies indicate also that the most useful range is the mid-infrared region of the spectra (presence of marker bands). Comparison of the calculated vibrational spectra of [Im(CH2C6H5)2]+ and [Im(Ad)2]+ to the results obtained for the unsubstituted imidazolium cation indicate significant shifts of the ring vibrations. For instance: 23 cm1 for n(C4 C5) and ca. 40 cm1 for the out-of-phase stretching of the N1–C2–N3 fragment. Moreover, these shifts of the ring mode frequencies are very similar for both compounds, except n(C2–H) (55 cm1 of difference between [Im(CH2C6H5)2]+ and [Im(Ad)2]+). Geometrical parameters show only small effect of the benzyl and adamantyl substituents on structure of the imidazolium cation. The bonds in N1–C2–N3 moiety are equal to each other and shorter than N1–C5 and N3–C4. It is consistent with proposed electron delocalization over the imidazolium moiety. According to the HOMA calculations, the aromaticity of the imidazolium ring is 0.87 and 0.88 units for the adamantyl and benzyl derivatives, respectively. It shows that this heterocyclic ring belongs to aromatic compounds. The larger increase of the heteroaromaticity for [Im(CH2C6H5)2]+ is due to presence of p-electron system (phenyl ring) bonded to the imidazolium ring. In the case of atomic charges, the influence of the substituted groups is negligible. Our GAPT calculations indicate also that positive charge of the imidazolium ring is delocalized between the C2 and carbon atoms of the substituents attached to the imidazolium nitrogen atoms. Fig. 4. Theoretical Raman (A) and IR (B) spectra of adamantane molecule.

Acknowledgement

20 cm1 in the [Im(Ad)2]+ spectra. Bands observed at 1454 (Raman) and 1439 cm1 (IR) are attributed to in-phase scissoring mode of the CH2 groups (calculated: 1486 and 1473 cm1, respectively). They are insensitive to the substitution of adamantane for that DFT calculation predicts these vibrations at 1488 and 1474 cm1. Vibrations of –CH2– groups appear also in range 1391–1271 cm1. In this region of vibrational spectra, the rocking [r(CH2)] and wagging v[(CH2)] modes of the adamantyl group are also expected. Our calculations indicate that they couple with r and v of the imidazolium ring. The most characteristic mode of the adamantyl moiety is skeletal stretching vibration [n(C–C)Ad] found in the theoretical Raman spectrum at 739 and 752 cm1, for adamantane and [Im(Ad)2]+, respectively. In the experimental spectrum of [Im(Ad)2]ClO4, this mode is observed at 770 cm1. Several weak intensity bands below 684 cm1 (Raman spectrum) include chiefly skeletal deformation of the adamantyl rings [b(C–C–C)Ad].

The authors would like to express appreciation to the Academic Computer Center Cyfronet in Krakow (Poland) for computer time (Grant no. MNiI/SGI2800/UJ/002/2005).

5. Summary B3LYP method is showed to be an effective tool for interpreting and predicting IR and Raman spectra of such large molecular systems as studied here. Our DFT-based analysis reproduces the experimental vibrational spectra of the imidazolium derivatives with the error less than 5%. Moreover, we present the detailed description of these spectra and discuss

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