Vibrational study of tolazoline hydrochloride by using FTIR-Raman and DFT calculations

Spectrochimica Acta Part A 79 (2011) 1710–1714

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Vibrational study of tolazoline hydrochloride by using FTIR-Raman and DFT calculations C.D. Contreras a , A.E. Ledesma a , J. Zinczuk b,1 , S.A. Brandán c,∗ a Cátedra de Fisicoquímica I, Facultad de Bioquímica, Química y Farmacia, Universidad Nacional de Tucumán, San Lorenzo 456, T 4000 CAN, San Miguel de Tucumán, Tucumán, Argentina b Instituto de Química Rosario (CONICET-UNR), Facultad de Ciencias Bioquímicas y Farmacéuticas, Suipacha 531, 2000 Rosario, Santa Fé, Argentina c Cátedra de Química General Instituto de Química Inorgánica, Facultad de Bioquímica, Química y Farmacia, Universidad Nacional de Tucumán, Ayacucho 471, 4000 S. M. de Tucumán, Argentina

a r t i c l e

i n f o

Article history: Received 21 February 2011 Received in revised form 24 April 2011 Accepted 16 May 2011 Keywords: Tolazoline hydrochloride Vibrational spectra Molecular structure Force field DFT calculations

a b s t r a c t Quantum mechanical (QM) calculations have been carried out in order to study the tolazoline hydrochloride theoretical structure and vibrational properties. This compound was characterized by infrared and Raman spectroscopies in the solid phase. For a complete assignment of the IR and Raman spectra, the density functional theory (DFT) calculations were combined with Pulay’s Scaled Quantum Mechanics Force Field (SQMFF) methodology in order to fit the theoretical frequency values to the experimental ones. An agreement between theoretical and available experimental results was found. Three intense bands in the infrared spectrum characteristic of the protonated species of the compound were detected. Also, the possible charge-transfer and the topological properties for both benzyl and imidazoline rings were studied by means of Natural Bond Orbital (NBO) and Atoms in Molecules theory (AIM) investigation. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The study of heterocyclic compounds of great pharmacological interest is important for our research [1–6]. The tolazoline hydrochloride compound is a non-selective competitive ␣adrenergic receptor antagonist and a vasodilator being used in hypertensive therapy [7]. The crystal and molecular structure of tolazoline hydrochloride has been determined by means of X-ray diffraction methods [8]. These results have shown that the protonated form is the existent structure in the solid state where the imidazol group is involved in a protonation process and the positive charge is dispersed over both nitrogen atoms of the imidazoline ring. Nowadays, there are few data in the literature about the molecular and vibrationals properties of this compound; so far, there are no published high-level theoretical studies on the geometries and spectroscopic parameters of this compound. The knowledge of these properties is important for the design and synthesis of new and better ␣-adrenergic imidazoline receptor agonist and antagonistic drugs. The aim of this paper is to study tolazoline hydrochloride from a structural and spectroscopic point of view, not only theoretically but also experimentally. For this purpose, we optimized the geometries of the protonated species of the

compound in the gas phase by using B3LYP/6-31G* and B3LYP/6311++G** methods. Later, a complete assignment of all observed bands in the IR and Raman spectra in the solid state was performed by using DFT calculations combined with Pulay’s SQMFF methodology [9–11]. Moreover, NBO [12–14] and topological properties of both rings of tolazoline hydrochloride in the gas phase by means of AIM [15,16] calculations were used to analyze the possible chargetransfer and the intramolecular bond path. 2. Experimental methods A pure MP Biomedicals commercial sample of tolazoline hydrochloride (T) was used. The IR spectrum of the solid substance in KBr pellets was recorded in the wavenumbers range from 4000 to 400 cm−1 with an FT-IR Perkin Elmer spectrometer provided with a Globar source and a DGTS detector. Raman spectrum of the compound in solid state was recorded between 4000 and 100 cm−1 with a Bruker RF100/S spectrometer provided with a Nd:YAG laser (excitation line of 1064 nm, 800 mW of laser power) and a Ge detector cooled at liquid nitrogen temperature. The spectra were recorded with a resolution of 1 cm−1 and 200 scans. 3. Computational details

∗ Corresponding author Tel.: +54 381 4247752x7073; fax: +54 381 4248169. E-mail address: [email protected] (S.A. Brandán). 1 Member of the Carrera de Investigador Científico, CONICET, Argentina. 1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2011.05.041

The protonated structure of (T) and labelling of the atoms can be seen in Fig. 1. The structure was fully optimized by using the hybrid

C.D. Contreras et al. / Spectrochimica Acta Part A 79 (2011) 1710–1714

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Table 1 Comparison of calculated geometrical parameters for tolazoline hydrochloride with the corresponding experimental values. Parameter

B3LYPa 6-31G*

Bond lengths (Å) C5 N4 C3 N4 C5 N1 C2 N1 C2 C3 C5 C11 C14 C11 C14 C15 C15 C16 C16 C17 C17 C18 C18 C19 C19 C14 RMSD

Fig. 1. Theoretical structure and atoms numbering for the protonated form of tolazoline hydrochloride in gas phase.

B3LYP method [17,18] employing the 6-31G* and 6-311++G** basis sets, as implemented in the GAUSSIAN 03 program [19]. The electronic charge density topological analysis was performed by using the AIM methodology [15], by the AIM200 program [16] while the NBO calculation was performed by using the NBO 3.1 [20] program, as implemented in the GAUSSIAN 03 package [19]. The harmonic wavenumbers and the valence force field in Cartesian coordinates were calculated at the same approximation levels. The resulting force fields were transformed into “natural” internal coordinates by using the MOLVIB program [21,22]. The natural internal coordinates have been defined according to those reported in the literature [10,11,23] and are listed in Table S1 (Supporting Material). Following the SQMFF procedure [9–11], the harmonic force field was evaluated at B3LYP/6-31G* level and the potential energy distribution components (PEDs) higher than or equal to 10% were subsequently calculated with the resulting SQM. The dimer species, with two units of (T) linked through a Cl atom (Fig. 2) was optimized at B3LYP/6-31G* calculation and then, at this same level of theory, a vibrational analysis was performed. The nature of all the vibration modes was carried out by means of the GaussView program [24]. The total energy for the dimer species by using the 6-31G* basis set was corrected for Basis Set Superposition Error (BSSE) by the standard Boys–Bernardi counterpoise method [25]. 4. Results and discussion 4.1. Geometry optimization The optimized structure for the protonated form of (T) has C1 symmetry. Table 1 shows a comparison of the calculated geometrical parameters with the corresponding ones observed from X-ray diffraction [8] by means of the root mean of square deviations (rmsd) values. Note that both basis sets reproduce reasonably ˚ The calculawell the theoretical bond lengths (0.023–0.022 A). tion predicts that the C5 N4 and C5 N1 distances are lower than the C3 N4 distance in accordance with the X-ray diffraction results. These observations are justified because the two N atoms of the imidazoline ring are protonated, as it was experimentally observed. The atomic charges derived from the ESPs (MK) for (T) [26] by using 6-31G* basis set and the natural charge values are given in Table S2. As expected, the results show that the N1 and N4 atoms have approximately the same values. The bond orders, expressed by Wiberg’s index, are given in Table S3. Again, the bond order value of the N1 atom is approximately similar to the value of the N4 atom. On the other hand, the second order perturbation energies E(2) (donor → acceptor) that involve the

Bond angles (◦ ) C11 C5 N1 C11 C5 N4 C5 C11 C14 C3 N4 C5 C2 N1 C5 N1 C5 N4 N1 C2 C3 N4 C3 C2 RMSD Dihedral angles (◦ ) C15 C14 C11 C5

1.326 1.477 1.319 1.474 1.558 1.504 1.517 1.401 1.395 1.396 1.396 1.396 1.402 0.023

Exp.b 6-311++G** 1.325 1.478 1.317 1.476 1.555 1.501 1.515 1.399 1.393 1.394 1.393 1.394 1.399 0.022

1.289 (8) 1.491 (8) 1.352 (8) 1.472 (8) 1.532 (10) 1.509 (9) 1.481 (9) 1.415 (10) 1.383 (11) 1.360 (12) 1.421 (10) 1.392 (10) 1.405 (9)

124.2 124.8 113.6 112.2 112.5 110.8 102.1 102.0 2.5

124.4 124.6 113.8 112.2 112.4 110.9 102.1 102.0 2.5

120.5 (6) 123.7 (6) 115.1 (6) 109.3 (5) 107.2 (5) 115.8 (6) 104.6 (6) 102.8 (5)

80.4

89.4

89.3 (2)

a

This work. b Ref [8].

most important delocalization were analyzed by means of NBO calculations [12–14] (Table S4). We found that the contributions of the stabilization energies for the ET → * charge transfers of the benzyl ring are the most important delocalizations while, the topological analysis [15] shows that the electron density, () and the Laplacian values, O2 (r) for the Ring Critical Point (RCP) of the imidazoline ring have higher values than those corresponding to the benzyl ring. Both results might be explained since there is a gradation of the positive charge distribution from one N atom to the other N atom in the same region of the imidazoline ring [8]. In addition, the five bond critical points (BCPs), calculated by means of the AIM analysis for the dimer species, clearly reveal (Table S5) the halogen bonds between both structures such as, Cl26· · ·H12 (() = 0.0077 a.u.), Cl26· · ·H20 (() = 0.0073 a.u.), Cl26· · ·H25 (() = 0.0365 a.u.), Cl26· · ·H27 (() = 0.0272 a.u.) and Cl26· · ·H39 (() = 0.0076 a.u.). Here, we observed that the bonds between Cl26 and H12, H20 and H39 atoms show ellipticity values larger than 1.1, indicating that those bonds tend to distort to a more stable form, whereas the values for the bonds Cl26· · ·H25 and Cl26· · ·H27 suggest stable bonds. On the other hand, the NBO study only corroborate the energies and occupancies of the two main contributions LP(3)Cl26 → *N28-H27 (76.99 kJ/mol) and LP(4)Cl26 → *N4-H25 (135.64 kJ/mol), as expected, because they have higher density values, while the remaining delocalization energy values ranges from 2.05 to 3.97 kJ/mol. 5. Vibrational analysis The recorded infrared and Raman’s spectra for the compound in the solid phase can be seen in Fig. 3. The protonated species of (T) has 69 normal vibration modes and all modes are active in both spectra. The experimental and SQM wavenumbers for the expected normal vibration modes by using 6-31G* basis set together with

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Fig. 2. Theoretical structure and atoms numbering for the tolazoline hydrochloride dimer. The halogen bonding is indicated by dashed lines.

the complete assignment are shown in Table 2, while the corresponding PED contribution is shown in Table S6. The vibrational assignment of the experimental bands to the normal vibration modes is based on the comparison with related molecules [1–6,27] and with the results of the calculations performed here. The best results are obtained with a B3LYP/6-31G* calculation, as expected because the Pulay’s scaling factors are defined for this basis set. The SQM force fields for this compound can be obtained at request. The discussion of assignment for the compound is presented as follows. 5.1. Band assignments 5.1.1. NH modes The broad band observed in the IR spectra of the compound in the solid phase at 3440 cm−1 is easily assigned to the two N H stretching vibrations according to the values reported for the 2-(2 furyl)-1H-imidazole molecule [1] at 3440 cm−1 and at 3434 cm−1 in similar compounds [2–6]. The corresponding in-plane deformation modes could be associated to the strong bands observed in the infrared spectrum at 1286 and 1203 cm−1 according to similar compounds [1–6] and to the theoretical calculations. The out-of-plane deformation modes are assigned to the weak IR band at 614 cm−1 and to the shoulder in the IR spectrum at 438 cm−1 . In a similar way as observed in the molecular packing of the 2-(2 -furyl)-4,51H-dihydroimidazole molecule [5], the broad band between 2500 and 1900 cm−1 can be attributed to the N H· · ·N hydrogen bond-

Fig. 3. Experimental infrared (upper) and Raman (bottom) spectra of tolazoline hydrochloride compound in solid state.

ing formed by the spatial arrangement of molecules in the lattice crystal [8]. 5.1.2. CH modes The group of bands in the 3100–3020 cm−1 region in both spectra of the solid substance can be easily assigned, due to their position, to the C H stretching modes, as shown in Table 2. The in-plane deformation modes are clearly predicted in the 1490–1145 cm−1 region [6], hence, the IR bands at 1451, 1334, 1180 and 1160 cm−1 are assigned to those modes. The modes corresponding to out-of-plane deformations were assigned taking into account the results of the theoretical calculations and the assignments for similar molecules [5,6]; thus, the IR bands at 1000, 925, 855 and 698 cm−1 and the shoulder at 974 cm−1 are assigned to those modes. 5.1.3. CH2 modes The group of bands between 2990 and 2925 cm−1 can be clearly assigned to the antisymmetric and symmetric stretching modes of these groups, as indicated in Table 2, the symmetric modes being more intense in Raman as expected. The scissoring modes are assigned, in agreement with heterocyclic compounds containing the imidazoline ring [5], to the shoulder at 1482 cm−1 , to the strong IR band at 1426 cm−1 and to the Raman band at 1473 cm−1 . As predicted by the calculation, the shoulders at 1350 and 1270 cm−1 and the IR band of medium intensity at 1170 cm−1 are easily assigned to the wagging modes, while the expected rocking modes are assigned according to the PED contribution to the band and shoulders at 1360, 1246 and 1213 cm−1 , respectively. The twisting modes are predicted by the calculations between 1337 and 786 cm−1 ; thus, these modes are associated with the bands at 1030, 863 and 778 cm−1 . 5.1.4. Skeletal modes Here, the description of the skeletal stretching modes appears strongly mixed among them as can be seen in Table S7. The very strong band at 1621 cm−1 and the strong band at 1585 cm−1 are mainly associated with the C C stretching modes in accordance with similar compounds [5,6,27] and with our theoretical results. The strong bands at 1599 and 1585 cm−1 are associated with the

C.D. Contreras et al. / Spectrochimica Acta Part A 79 (2011) 1710–1714 Table 2 Observed and calculated wavenumbers (cm−1 ) and assignments for tolazoline hydrochloride. IRa solid 3440 w, br 3440 w, br 3110 w br 3100 w 3070 sh 3055 br 3047 br 3044 br

2982 sh 2970 w br 2967 w 2958 w 2938 w 2925 sh 2885 w 1621 vs 1599 s 1585 s 1585 s 1497 s 1482 sh

Ramana solid 3200 vw 3160 sh 3101 sh 3082 sh

3047 (100) 3035 (48) 3026 (30) 2996 (23) 2982 (39) 2970 (45) 2958 (47) 2924 (48) 2885 (28) 1615 (15) 1600 (26) 1583 (16) 1583 (16) 1497 (4) 1473 (19)

1468 sh 1451 m 1426 s 1360 w 1350 sh 1334 w 1300 vs 1286 s 1270 sh 1246 sh 1213 sh 1203 s 1189 sh 1180 w 1170 m 1160 w 1148 w 1077 w 1046 w 1030 w 1015 sh 1000 vw 995 sh 986 vw 974 sh 941 w 925 w 888 vw 863 vw 855 vw 822 m 810 m 778 m 778 m 757 sh 746 vs 698 m 693 m 681 w 643 m 636 sh 614 w 528 m 520 m 463 s 438 sh 419 vvw 409 vvw

1451 (18) 1425 (23) 1362 (10) 1349 sh 1335 (5) 1301 (10) 1287 (8) 1271 sh 1245 (4) 1212 (7) 1204 sh 1188 (25)

1154 (12) 1077 (3) 1050 (3) 1030 (27) 1015 (10) 1000 (81)

SQMb

Assignmenta

3492 3456

N4 H25 N1 H6 aCH2 imid dimer C17 H22 C16 H21 C18 H23 C15 H20 sCH2 dimer sCH2 dimer C19 H24 aCH2 imid aCH2 imid sCH2 imid N Hdimer sCH2 imid aCH2 sCH2 , sN Hdimer N H· · ·N (see text) C15 C16 C5 N1 C16 C17 C5 N4 ˇC16 H21 ıCH2 imid ıCH2 imid ˇN Hopdimer ˇC17 H21 ıCH2 CH2 wagCH2 imid ˇC15 H20, ˇC19 H24 C14 C15 ˇN1 H6 wagCH2 imid CH2 imid CH2 imid ˇN4 H25 C11 C14 ˇC16 H21 wagCH2 ˇC18 H23 C18 C19 ˇR1 imid ˇR1 benc CH2 imid C1 N2 C17 H22 C17 C18 C Hdimer C18 H23 C3 N4 C19 H24 (C2 C3)imid CH2 C15 H20 N Hdimer (C11 C14) CH2 imid R1 benc ˇRimid dimer ˇR2 imid C16 H21 C Hdimer N Hdimer C11 C5 ˇR2 benc N1 H6 ˇR3 benc (C11 C5) N Hdimer (C11 C14), R3 benc , R2 benc N4 H25 Rbenc dimer R2 benc ˇC Cdimer

3093 3083 3075 3053

3049 3027 3011 2975 2972 2968 2926 1606 1595 1589 1571 1498 1486 1471 1456 1415 1383 1337 1335 1309 1295 1284 1254 1209 1197 1188 1182 1170 1166 1079 1025 1024 1014 1012 1010 998

988 (5)

924 (33) 890 (2) 858 (2) 825 (10) 812 (12)

764 (5) 744 (4) 698 (2) 685 (1) 644 (14) 611 (14) 589 (1) 520 (7) 464 (3)

409 (2) 331 (4)

971 961 933 888 863 853 815 786 771 745 702

656 629 617 586 529 469 435 402

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Table 2 (Continued) IRa solid

Ramana solid 327 303 (23) 241 (4) 206 (16) 139 112

SQMb 311 275 204 187

43 34 24 19

Assignmenta ˇ(C11 C14) ı(C4 C11 C5) R3 benc R2 imid. w CH2 imid .dimer w CH2 imid .dimer R1 imid (C11 C4) ␶(C11 C5) ˇ(C11 C5)

, stretching; ı, angle deformation; wag, wagging; , rocking; ˇ, in plane deformation; , out plane deformation; , torsion; benc., bencene; imid., imidazoline; a, antisymmetry; s, symmetry; R, ring; s, strong; m, medium; w, weak; v, very; sh, shoulder; br, broad. a This work. b From scaled quantum mechanics force field by using B3LYP/6-31G*.

C N stretching modes, while the remaining C C stretching modes are strongly coupled with other modes and are associated to the bands at 1300, 1148, 888 and 643 cm−1 and to the shoulders at 1189 and 995 cm−1 . The C N stretching modes, as predicted by the calculations, are associated with the shoulders in the IR spectrum at 1015 and 941 cm−1 , as can be seen in Table 2. According to the calculated PED contribution and to similar compounds [6,27], the benzyl ring deformations (ˇR ) are clearly assigned to the IR bands at 1046 and 636 cm−1 and to the Raman band at 528 cm−1 while the three benzyl ring torsions ( R ) are calculated strongly coupled with others modes; for this reason, the IR bands at 778 and 409 cm−1 and the Raman band at 241 cm−1 are associated with these modes. The deformations and torsion ring modes corresponding to the imidazoline ring are clearly predicted by the calculations, thus, the (ˇR1 ) and (ˇR2 ) deformations modes are assigned respectively to the weak IR bands at 1077 and 746 cm−1 while only the ( R2 ) mode can be assigned to the Raman band at 206 cm−1 . 5.1.5. Dimer modes The optimized dimer (T) structure by using the B3LYP/6-31G* level has C1 symmetry and 150 normal vibration modes, all active in the infrared and Raman spectra. The assignment of the experimental bands to the 150 expected normal vibration modes was made by means of GaussView program [24] by analyzing the nature of the vibrations. In this case, we presented in Table 2 only the bands not assigned to the (T) monomer. Thus, the weak and broad band at 3110 cm−1 is associated with a antisymmetric CH2 stretching mode of the imidazoline ring while the broad IR bands at 3047 and 3044 cm−1 are associated with symmetric CH2 stretching modes because both bands are intense in the Raman spectrum. The weak IR band and the shoulder respectively at 2967 and 2925 cm−1 are associated with N H stretching modes while the shoulder at 1468 cm−1 is associated with an out-of-phase N H in-plane deformation mode. The IR bands at 986 and 693 cm−1 are associated with the C H out-of-plane deformation modes while the bands at 822, 681 and 520 cm−1 are attributed to the out-of-plane deformation modes of the N H group. The very weak IR band at 419 cm−1 ca be assigned to a benzyl ring deformations (ˇR ) mode, while the Raman bands at 331, 139 and 112 cm−1 are assigned, as predicted by calculations, to the in-plane C C deformation and to the twisting modes of the imidazoline rings, respectively. 6. Force field For the protonated form of (T), the corresponding force constants were estimated by using Pulay et al. [9–11] scaling procedure as mentioned before. The force constants expressed in terms of simple valence internal coordinates were calculated from the corre-

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sponding scaled force fields by using the MOLVIB program [21,22]. The force constants calculated at the B3LYP/6-31G* level were compared with the ones obtained by B3LYP/6-311++G** calculation, both results being shown in Table S7. The calculated force constants values are in accordance with the reported values for similar molecules [1–6,27]. The discrepancy observed in the deformations and torsion force constants values between imidazoline and benzyl rings are attributed to the corresponding topological properties of both rings. Thus, those values are higher for the imidazoline ring in relation to the benzyl ring and, in general, the force constants decrease when the basis set size increases. 7. Conclusions - The substance was characterized by infrared and Raman spectroscopic techniques in the solid state. The presence of tolazoline hydrochloride protonated form was detected in the IR spectrum by means of three very intense characteristic bands at 1621, 1300 and 746 cm−1 of which the first two are attributed to the C C stretching modes of the benzyl ring while the other one is assigned to a deformation mode of the imidazoline ring, respectively. - The theoretical molecular structures of tolazoline hydrochloride protonated form were determined by the B3LYP/6-31G* and B3LYP/6-311++G** methods, while calculations suggest the existence of the protonated form in the solid state, as it was experimentally observed. - A complete assignment of the 69 normal vibration modes for the protonated form of tolazoline hydrochloride was performed. - The stability of the protonated form of tolazoline hydrochloride was justified by means of NBO and AIM analyses. - The SQM force fields were obtained for the protonated form of tolazoline hydrochloride after adjusting the theoretically obtained force constants in order to minimize the difference between observed and calculated wavenumbers. Acknowledgements This work was subsidized with grants from CIUNT (Consejo de Investigaciones, Universidad Nacional de Tucumán), and CONICET (Consejo Nacional de Investigaciones Científicas y Técnicas, R. Argentina). The authors thank Prof. Tom Sundius for his permission to use MOLVIB. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.saa.2011.05.041.

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