Theoretical study of IR spectra of paraphenylenediamine

Vibrational Spectroscopy 22 Ž2000. 3–10 www.elsevier.comrlocatervibspec

Theoretical study of IR spectra of paraphenylenediamine Elif Akalin, Sevim Akyuz ¨

)

Department of Physics, Faculty of Science, UniÕersity of Istanbul, Veznecieler, 34459, Istanbul, Turkey Received 23 April 1999; received in revised form 10 August 1999; accepted 23 August 1999

Abstract Normal coordinate analysis of the paraphenylenediamine Ž1,4-diaminobenzene, PPD. molecule has been carried out and complete interpretation of the vibrational spectrum is given for both trans and cis isomers. The reliable force field and electro-optical parameters of PPD have been determined by refinement in order to fit the experimental wavenumbers and intensities of PPD molecule. The initial force field parameters of PPD were refined from the corresponding parameters of aniline molecule. The initial values of bond dipole moments of the molecule were calculated by MINDOr3 method. The combination of the calculated IR spectra of trans and cis isomers of PPD is found to reproduce the experimental IR spectrum of solid PPD, satisfactorily, indicating that PPD exists as a mixture of both conformations. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Normal coordinate analysis; Paraphenylenediamine; Diaminobenzene; IR spectrum; Force field and electro-optical parameters

1. Introduction Paraphenylenediamine Ž1,4-diaminobenzene., as an aromatic amine, is an important class of molecules having photochemical interest. It is a well-known electron donor in CT complexes. It has been noted that IR and Raman data on paraphenylenediamine Žabbreviated here as PPD. are not plentiful in literature. Ernsbrunner et al. w1x reported IR and Raman data of PPD on the aid of normal coordinate analysis, but, a number of fundamental modes remained unassigned. Tzeng and Narayanan w2x recently performed an ab initio molecular orbital study on PPD. But some contradictions still exist particularly on the

) Corresponding author. Tel.: q90-212-5118480 ext 1323; fax: q90-212-5190834; e-mail: [email protected]

amino groups vibrations. Although the results for the ring modes, from ab initio calculations on PPD w2x and aniline w3x, agree well with the reported experimental values, those for amino rocking, wagging and twisting motions show some deviations. This is thought to occur due to imperfection of the present levels of calculations to account for the effect of molecular interactions on some substituent relatedmodes w2x. Better results from ab initio computations cannot be expected, until improved wave functions are available. After having reported on calculation and analysis of IR spectra of aniline earlier w4x, we discuss now the vibrational spectra of PPD, in order to clarify the contradiction on the vibrational assignment of this molecule and to show the transferability of the force field of aniline to PPD, which contains two amino group on the aromatic ring. In this study, the harmonic force field and electro-optical parame-

0924-2031r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 2 0 3 1 Ž 9 9 . 0 0 0 5 7 - 0

E. Akalin, S. Akyuz ¨ r Vibrational Spectroscopy 22 (2000) 3–10

4

Table 1 Numbering of the internal coordinates of PPD and the geometry parameters Type

˚ Value ArDeg

Atoms

Type

˚ Value ArDeg

Atoms

1 bond 2 bond 3 bond 4 bond 5 bond 6 bond 7 bond 8 bond 9 bond 10 bond 11 bond 12 bond 13 bond 14 bond 15 bond 16 bond 17 angle 18 angle 19 angle 20 angle 21 angle 22 angle 23 angle 24 angle 25 angle 26 angle 27 angle 28 angle

1.402 1.386 1.412 1.412 1.386 1.412 1.404 1.000 1.000 1.404 1.100 1.100 0.998 0.998 0.998 0.998 120.887 118.225 120.825 119.960 120.887 119.153 119.153 118.225 119.960 120.825 120.887 120.825

C1–C2 C2–C3 C3–C4 C4–C5 C5–C6 C6–C1 C1–N7 C2–H8 C3–H9 C4–N10 C5–H11 C6–H12 N7–H13 N7–H14 N10–H15 N10–H16 C1–C2–C3 C2–C1–C6 C2–C1–N7 C1–C2–H8 C2–C3–C4 C3–C2–H8 C2–C3–H9 C3–C4–C5 C4–C3–H9 C3–C4–N10 C4–C5–C6 C5–C4–N10

29 angle 30 angle 31 angle 32 angle 33 angle 34 angle 35 angle 36 angle 37 angle 38 angle 39 angle 40 angle 41 o.p.bend 42 o.p.bend 43 o.p.bend 44 o.p.bend 45 o.p.bend 46 o.p.bend 47 torsion 48 torsion 49 torsion 50 torsion 51 torsion 52 torsion 53 torsion 54 torsion 55 torsion 56 torsion

119.960 120.887 119.153 119.153 120.825 119.960 113.808 113.808 113.808 113.808 112.415 112.415 y3.460 0.222 0.222 y3.460 0.222 0.222 179.507 0.512 0.499 0.498 179.493 179.507 26.789 26.789 26.789 26.789

C4–C5–H11 C5–C6–C1 C6–C5–H11 C5–C6–H12 C6–C1–N7 C1–C6–H12 C1–N7–H13 C1–N7–H14 C4–N10–H15 C4–N10–H16 H13–N7–H14 H15–N10–H16 C6–C1–C2–N7 C3–C2–C1–H8 C2–C3–C4–H9 C5–C4–C3–N10 C4–C5–C6–H11 C1–C6–C5–H12 C6–C1–C2–C3 C1–C2–C3–C4 C2–C3–C4–C5 C3–C4–C5–C6 C4–C5–C6–C1 C5–C6–C1–C2 C2–C1–N7–H13 C6–C1–N7–H14 C3–C4–N10–H15 C5–C4–N10–H16

ters of free PPD have been determined by refinement in order to fit the experimental wavenumbers and intensities of free molecule. Our main interest is to investigate the IR spectrum and coupling peculiarities of the metal–ligand vibrational modes of PPD in transition metalŽII. complexes. Now we start with the investigation of ligand molecule, PPD. Our future publication will be devoted to this subject.

dures for vibrational frequencies and IR intensities calculation, force field and electro-optical parameters ŽEOP. refinement, quantum chemistry Žsemiemprical. molecular geometry optimization and calculation of force constants with respect to the natural internal coordinates and many other features. The methods implemented in the program were described in de-

2. Method of calculation The theoretical calculations were performed by using LEV 1 program. The program includes proce1

LEV ŽLight Elucidations of Vibrations. program for personal computers designed at Laboratory of Molecular Modelling, V.I. Vernadsky Institute of Geo- and Analytical Chemistry, Moscow, Russia.

Fig. 1. Atom numbering and the directions of the bond vectors of PPD molecule.

E. Akalin, S. Akyuz ¨ r Vibrational Spectroscopy 22 (2000) 3–10

5

Table 2 ˚ . and PED Ž%. of trans-PPD together with the calculated wavenumbers Žcmy1 . Calculated wavenumbers Žcmy1 ., itensities ŽDrAMU= A Ž . of ND 2 2 -PPD and cis-PPD molecules Assignment

a nexp

trans-PPD

ncal NH 2 stretching Ž n NH2 . Ž A u . NH 2 stretching Ž n NH2 . Ž Bg . NH 2 stretching Ž n NH2 . ŽA g . NH 2 stretching Ž n NH2 . ŽB u . CH stretching Ž n CH . ŽB g . CH stretching Ž n CH . ŽA g . CH stretching Ž n CH . ŽB u . CH stretching Ž n CH . ŽA u . NH 2 Scissoring Ž d NH2 . ŽA g .

3410 s w3409xw 3398 w w3376x w 3374 s w3384xvw 3384 s w3061x vs 3058 vw w3044x vs 3045 vw 3028 w 3008 w w1638x sh

cis-PPD ŽI.

ŽND 2 . 2

ncal

PED Ž%. for trans-PPD ŽI.

3498 3498

Ž2.9. Ž0.0.

2600 2600

3504 3504

Ž3.2. Ž0.0.

n NH 2 Ž100. n NH 2 Ž100.

3422

Ž0.0.

2487

3416

Ž1.0.

n NH 2 Ž100.

3422

Ž5.2.

2487

3416

Ž3.8.

3031

Ž0.0.

3031

3031

Ž0.0.

n NH 2 Ž100. n CH Ž99.

3044

Ž0.0.

3044

3044

Ž0.0.

n CH Ž99.

3034 3042 1619

Ž0.6. Ž1.4. Ž0.0.

3034 3042 1171

3034 3042 1657

Ž0.6. Ž1.4. Ž2.9.

n CH Ž99. n CH Ž99. d NH 2 Ž35. q n CCŽ34. qd CNHŽ15. q d CHŽ6. d NH 2 Ž68. q d CNHŽ28. n CCŽ38. q d NH 2 Ž25. qn CNŽ10. q d CNHŽ10. qd CHŽ6. n CCŽ72. q d ringŽ9. qd CHŽ9. n CCŽ36. q d CHŽ35. qn CNŽ20. q d ringŽ6. n CCŽ52. q d CHŽ27. qd CNŽ9. q d CNHŽ7. n CCŽ86. q d CNHŽ7. d CHŽ86. q d CNHŽ8. n CNŽ59. q n CCŽ37. n CNŽ38. q d CHŽ25. qd ringŽ29. d CHŽ89. q n CCŽ7.

NH 2 Scissoring Ž d NH2 . ŽB u . CC stretching Ž n CC . ŽA g .

1630 s w1618x m 1610 w

1611 1600

Ž5.4. Ž0.0.

1183 1608

1655 1607

Ž1.2. Ž0.6.

CC stretching Ž n CC . ŽB g .

1563 vw

1571

Ž0.0.

1564

1572

Ž0.0.

CC stretching Ž n CC . ŽB u .

1517 vs

1525

Ž15.5.

1524

1525

Ž14.2.

CC stretching Ž n CC . ŽA u .

1454 m

1436

Ž3.6.

1427

1437

Ž3.6.

CC stretching Ž n CC . ŽA u . CH i.p. bending Ž d CH . ŽB g . C–NH 2 stretching Ž n C – NH2 . ŽA g . C–NH 2 stretching Ž n C – NH2 . ŽB u .

1311 m w1278x sh w1268x s 1264 vs

1311 1297 1266 1280

Ž2.8. Ž0.0. Ž0.0. Ž11.1.

1295 1288 1284 1291

1315 1298 1266 1279

Ž3.0. Ž0.0. Ž0.7. Ž10.2.

CH i.p. bending Ž d CH . ŽA g .

w1179x m 1179 w 1128 m w1050xvw 1066 w 1041 w 1014 vw

1154

Ž0.0.

1154

1153

Ž0.0.

1135

Ž4.1.

1130

1135

Ž4.0.

1066 1033 1020

Ž0.0. Ž0.8. Ž0.1.

828 801 1020

1077 1041 1020

Ž0.0. Ž1.0. Ž0.1.

936

Ž0.0.

935

935

Ž0.0.

CH o.p. bending Žg CH . ŽA u . CH o.p. bending Žg CH . ŽB g . Ring breathing ŽA g .

w933x vw 933 vw 947 vw w900x vw w849x vs

943 896 866

Ž0.1. Ž0.0. Ž0.0.

943 896 855

941 895 863

Ž0.1. Ž0.0. Ž0.6.

CH o.p. bending Žg CH . ŽB u . n C – NH2 q n CC q d ring ŽB u .

825 vs 799 s

828 793

Ž11.7. Ž6.5.

828 756

828 781

Ž11.1. Ž0.4.

CH i.p. bending Ž d CH . ŽA u . NH 2 twist ŽB g . NH 2 twist ŽA u . CH i.p. bending Ž d CH . ŽB u . CH o.p. bending Žg CH . ŽA g .

d CHŽ70. q n CCŽ22. qd CNHŽ6. NH 2 twistŽ84. q n CCŽ7. NH 2 twistŽ66. q n CCŽ29. d CHŽ29. q n CCŽ36. qd ringŽ26. g CHŽ86. q g ringŽ10. g CHŽ95. g CHŽ100. n CCŽ60. q n CNŽ18. qd ringŽ8. g CHŽ96. n CNŽ34. q d CNHŽ21. qd ringŽ19. q d NH 2 Ž9. qn CCŽ7.

6

E. Akalin, S. Akyuz ¨ r Vibrational Spectroscopy 22 (2000) 3–10

Table 2 Žcontinued. Assignment

a nexp

trans-PPD

ncal

ŽI.

cis-PPD ŽND 2 . 2

ncal

PED Ž%. for trans-PPD ŽI.

NH 2 Wagging ŽA g .

w761x w

746

Ž0.0.

582

695

Ž0.8.

NH 2 Wagging ŽB u .

717 vs

721

Ž11.2.

569

689

Ž18.9.

o.p. ring deform. Žgring . ŽA g .

w651x m

648

Ž0.0.

660

636

Ž0.1.

Ring deformation Ž dring . ŽB g .

w643x sh 643 w 514 s w473x m 472 vw 423 m

645

Ž0.0.

641

645

Ž0.0.

513 461

Ž8.4. Ž0.0.

510 435

511 461

Ž10.3. Ž0.1.

426 388 294 295

Ž1.7.

425 359 262 284

427 387 293 295

Ž2.1.

185 172 166

249 248 180

o.p. ring deform. Žgring . ŽB u . Ring deformation Ž dring . ŽA g . o.p. ring deform. Žgring . ŽA u . C–NH 2 i.p. bending Ž d CN . ŽB g . C–NH 2 i.p. bending Ž d CN . ŽA u . CN o.p. bending Žg CN . ŽA g . NH 2 Rocking ŽB g . NH 2 Rocking ŽA u . CN o.p. bending Žg CN . ŽB u .

253 237 179

NH 2 wag Ž62. q d NH 2 Ž18. qNH 2 rock Ž11. NH 2 wag Ž54. q d NH 2 Ž15. qNH 2 rock Ž8. q n CNŽ8. g ringŽ68. q g CNŽ15. qg CHŽ13. d ringŽ68. q n CCŽ16. qd CHŽ15. g ringŽ47. q g CNŽ41. q g CHŽ10. d ringŽ56. q n CNŽ15. qn CCŽ13. q d CNŽ9. g ringŽ60. q g CHŽ38. d CN Ž89. d CNŽ65. q NH 2 rock Ž29. g CNŽ78. q g ringŽ12. qNH 2 rock Ž5. NH 2 rock Ž98. NH 2 rock Ž81. q d CNŽ12. g CNŽ80. q g ringŽ10. qNH 2 rock Ž6.

a

Raman bands are designated in brackets w..x, whereas IR bands are given without brackets. The computed intensities are given in paranthesis Ž....

tails in Refs. w5–7x. The quantum optimized geometry was used for spectral calculations. The geometry optimization of PPD was performed by AM1 method by using LEV 1. As the initial geometrical parameters of PPD, we have used corresponding parameters of the experimental geometry of aniline w8x and assumed the molecule as in trans position ŽC 2h symmetry.. In a recent study on ab initio molecular orbital study of PPD, Tzeng and Narayanan w2x proposed that the molecule has two conformational isomers; trans and cis forms, in the ground electronic state. Therefore, we first did all our calculations for the trans isomer ŽC 2h symmetry. and then repeated them for the cis form without altering the force constants and electro-optical parameters found for the trans form. The geometric parameters of the cis isomer is taken as the same as those of the trans isomer except the relative orientations of the NH 2 groups, which are altered to give C 2v symmetry. The optimized geometry results of the PPD molecule for the trans form is given in Table 1. Atom numbering and the directions of bond vectors are shown in Fig. 1. PPD force constants were obtained by the refine-

ment of corresponding force constants of aniline molecule w4x. The resultant force field is checked by the deuteration process. As the last step, the intensities in the IR spectrum of PPD were calculated. Although very important information about the normal vibrations and molecular chemical properties can be extracted from the IR intensities w9,10x, in most theoretical studies, unfortunately, this subject is omitted. Program LEV has incorporated an approach for calculation of the IR absolute intensities on the basis of valance optical theory described in Refs. w5,6x. The initial EOP and bond dipole moments of the aniline were calculated with the help of MINDOr3 program incorporated in LEV. The obtained EOP were then refined until they produce a close IR absorption spectrum to that of experimental spectrum of PPD.

3. Results and discussion Atom numbering and the directions of bond vectors in PPD molecules are shown in Fig. 1. The

E. Akalin, S. Akyuz ¨ r Vibrational Spectroscopy 22 (2000) 3–10

optimized geometry of PPD in trans form and the definition of natural coordinates are given in Table 1. Approximate analysis of normal vibrations of PPD molecule and their intensities are given in Table 2, together with the experimental IR and Raman vibrational wavenumbers. The assignments were established by the analysis of the PED. The refined force constants are given in Table 3. The electrooptical parameters and bond dipole moments are given in Tables 4 and 5, respectively. The theoretical calculation indicates the presence of significant mixing of all ring and CH modes.

7

Deuteration of PPD amino groups enables a ready distinction to be made between the amino group or X-sensitive modes and the ring modes of the molecule. As seen in Table 2, the deuteration of the PPD ŽND 2 . 2 –PPD.4 causes substantial shifts in NH 2 modes. The n Drn H ratio for the calculated values of ŽND 2 . 2 –PPD and PPD vibrational modes falls within the 0.72 to 0.79 for NH 2 group vibrations and 0.95 to 1.00 for ring and CH modes. The values of the n D rn H ratio falls within the expected ranges w4x. The H-bonding interaction through the NH 2 groups of PPD molecules is expected to occur in

Table 3 Calculated force field of trans and cis-PPD molecules Type

Force constantsa

Type

Force constants

1–1, 3–3, 4–4, 6–6 2–2, 5–5 7–7, 10–10 8–8, 9–9, 11–11, 12–12 13–13, 14–14, 15–15, 16–16 17–17, 21–21, 27–27, 30–30 18–18, 24–24 19–19, 26–26, 28–28, 33–33 20–20, 25–25, 29–29, 34–34 22–22, 23–23, 31–31, 32–32 35–35, 36–36, 37–37, 38–38 39–39, 40–40 41–41, 44–44 42–42,43–43, 45–45, 46–46 47–47, 49–49, 50–50, 52–52 48–48, 51–51 53–53, 54–54, 55–55, 56–56 1–2, 2–3, 4–5, 5–6 1–6, 3–4 1–7, 3–10, 4–10, 6–7 7–13, 7–14, 10–15, 10–16 13–14, 15–16 1–3, 1–5, 2–4, 2–6, 3–5, 4–6 1–4, 2–5, 3–6 1–17, 3–21, 4–27, 6–30 1–18, 3–24, 4–24, 6–18 1–19, 3–26, 4–28, 6–33, 7–19 7–33, 10–26, 10–28 1–20, 3–25, 4–29, 6–34 1–22, 3–23, 4–31, 6–32 1–33, 3–28, 4–26, 6–19 2–17, 2–21, 5–27, 5–30 2–20, 2–25, 5–29, 5–34 2–22, 2–23, 5–31, 5–32 7–18, 10–24

6.424 6.935 5.675 5.407 7.134 1.138 1.261 0.609 0.509 0.503 0.606 0.542 0.443 0.533 0.353 0.203 0.048 0.994 0.699 0.641 0.063 0.037 y0.288 0.160 0.158 0.038

7–35, 7–36, 10–37, 10–38 7–39, 10–40 8–17, 9–21, 11–27, 12–30 8–20, 9–25, 11–29, 12–34 8–22, 9–23, 11–31, 12–32 13–35, 14–36, 15–37, 16–38 13–36, 14–35, 15–38, 16–37 13–39, 14–39, 15–40, 16–40 17–18, 18–30, 21–24, 24–27 17–21, 27–30 19–33, 26–28 20–22, 23–25, 29–31, 32–34 35–36, 37–38 35–39, 36–39, 37–40, 38–40 41–42, 41–46, 43–44, 44–45 41–43, 41–45, 42–44, 44–46 41–47, 41–52, 44–49, 44–50 41–48, 41–51, 44–48, 44–51 41–49, 41–50, 44–47, 44–52 42–43, 45–46 42–46, 43–45 42–47, 43–49, 45–50, 46–52 42–48, 43–48, 45–51, 46–51 42–49, 43–47, 45–52, 46–50 42–50, 43–52, 45–47, 46–49 42–51, 43–51, 45–48, 46–48

0.190 y0.092 y0.110 0.063 0.063 0.060 y0.060 0.060 y0.014 y0.033 y0.038 y0.033 y0.038 y0.030 0.059 0.050 0.235 0.072 y0.002 0.050 0.002 0.190 0.173 0.081 y0.047 y0.047

0.159 0.094 y0.115 y0.101 0.048 y0.138 0.140 y0.222

42–52, 43–50, 45–49, 46–47 47–48, 48–49, 50–51, 51–52 47–49, 50–52 47–50, 49–52 47–51, 48–50, 48–52, 49–51 47–52, 49–50 48–51 53–54, 55–56

0.084 0.115 0.007 y0.085 0.009 0.152 y0.090 0.015

a

˚ y1 , mdyn and mdyn A, ˚ recpectively. Bond–bond, bond–angle and angle–angle force constants are in mdyne A

8

E. Akalin, S. Akyuz ¨ r Vibrational Spectroscopy 22 (2000) 3–10

Table 4 Calculated electrooptic parameters of trans and cis-PPD i–j

˚2. m i, j ŽDrA

i–j

˚2. m i, j ŽDrA

1–1, 3–3, 4–4, 6–6 6–1, 4–3, 3–4, 1–6 7–1, 10–3, 10–4, 7–6 8–1, 9–3, 11–4, 12–6 1–2, 3–2, 4–5, 6–5 8–2, 9–2, 11–5, 12–5 1–7, 6–7, 3–10, 4–10 7–7, 10–10 13–7, 14–7, 15–10, 16–10 1–8, 3–9, 4–11, 6–12 8–8, 9–9, 11–11, 12–12 7–13, 7–14, 10–15, 10–16 13–13, 14–14, 15–15, 16–16 14–13, 13–14, 16–15, 15–16 1–17, 3–21, 4–27, 6–30 3–17, 1–21, 6–27, 4–30 6–17, 4–21, 3–27, 1–30 7–17, 10–21, 10–27, 7–30 8–17, 9–21, 11–27, 12–30 9–17, 8–21, 12–27, 11–30 1–18, 6–18, 3–24, 4–24 7–18, 10–24 8–18, 12–18, 9–24, 11–24 1–19, 3–26, 4–28, 6–33 6–19, 4–26, 3–28, 1–33 7–19, 10–26, 10–28, 7–33 8–19, 9–26, 11–28, 12–33 13–19, 15–26, 16–28, 14–33 14–19, 16–26, 15–28, 13–33 1–20, 3–25, 4–29, 6–34 6–20, 4–25, 3–29, 1–34 7–20, 10–25, 10–29, 7–34 8–20, 9–25, 11–29, 12–34 1–22, 3–25, 4–29, 6–34 3–22, 1–25, 6–29, 4–34

0.104 y0.232 0.387 0.023 y0.184 0.023 0.002 y0.997 0.434 0.082 y0.140 0.321 y0.303 y0.033 y0.033 0.021 0.021 y0.074 y0.092 y0.074 y0.071 y0.517 y0.074 y0.136 0.162 0.221 0.051 y0.088 y0.088 0.003 0.061 –0.173 0.017 0.043 0.059

8–22, 9–25, 11–29, 12–34 9–22, 8–25, 12–29, 11–34 1–35, 6–36, 3–37, 4–38 6–35, 1–36, 4–37, 3–38 7–35, 7–36, 10–37, 10–38 13–35, 14–36, 15–37, 16–38 14–35, 13–36, 16–37, 15–38 7–39, 10–40 13–39, 14–39, 15–40, 16–40 1–41, 6–41, 3–44, 4–44 7–41, 10–44 8–41, 12–41, 9–44, 11–44 13–41, 14–41, 15–44, 16–44 1–42, 3–43, 4–45, 6–46 3–42, 1–43, 6–45, 4–46 7–42, 10–43, 10–45, 7–46 8–42, 9–43, 11–45, 12–46 9–42, 8–43, 12–45, 11–46 1–47, 3–49, 4–50, 6–52 3–47, 1–49, 6–50, 4–52 6–47, 4–49, 3–50, 1–52 7–47, 10–49, 10–50, 7–52 8–47, 9–49, 11–50, 12–52 9–47, 8–49, 12–50, 11–52 12–47, 11–49, 9–50, 8–52 1–48, 3–48, 4–51, 6–51 4–48, 6–48, 1–51, 3–51 7–48, 10–48, 7–51, 10–51 8–48, 9–48, 11–51, 12–51 1–53, 6–54, 3–55, 4–56 6–53, 1–54, 4–55, 3–56 7–53, 7–54, 10–55, 10–56 8–53, 12–54, 9–55, 11–56 13–53, 14–54, 15–55, 16–56 14–53, 13–54, 16–55, 15–56

0.017 0.286 0.045 y0.041 y0.433 0.204 y0.019 y0.474 0.612 0.115 y0.271 y0.047 0.079 0.072 y0.049 0.358 0.042 0.017 0.016 y0.002 y0.077 0.144 0.302 y0.047 y0.076 0.131 y0.076 y0.027 0.031 0.095 y0.057 0.036 y0.057 0.036 y0.057

crystalline PPD. Therefore, calculated wavenumbers of n NH 2 stretching vibrations of free PPD should be

Table 5 Bond dipole moments of trans and cis-PPD in Debye Bonds

Refined values

MINDOr3

C 1 –C 2 , C 4 –C 5 C 2 –C 3 , C 5 –C 6 C 3 –C 4 , C 6 –C 1 C 1 –N7 , C 4 –N10 C 2 –H 8 , C 6 –H 12 , C 3 –H 9 , C 5 –H 11 N7 –H 13 , N7 –H 14 , N10 –H 15 , N10 –H 16

y0.100 0.000 0.100 y1.000 y0.080 0.600

y0.302 0.000 0.302 y0.194 y0.035 0.350

found to be higher, whereas, the NH 2 bending vibrational wavenumber of free PPD should be found to be lower values than the corresponding modes observed in the IR and Raman spectra of the crystalline PPD. This is what we found Žsee Table 2.. Our vibrational assignments which are established based on PED agree with those proposed with Tzeng and Narayanan w2x except NH 2 wagging, twisting and rocking modes. But as concluded by the authors w2x, the calculated NH 2 wagging, twisting and rocking vibrational wavenumbers were found to show some deviation with the experimental values and thought to occur due to imperfection of the present levels of calculation to account for the effect of molecular

E. Akalin, S. Akyuz ¨ r Vibrational Spectroscopy 22 (2000) 3–10

interactions on some substituent related modes w2x. The NH 2 group vibrational wavenumbers of our results are found to be in agreement with that of

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aniline w4,11x, benzidine w12x and experimental results of PPD w1,13x. A glance at Table 2 shows that the wavenumbers of ring and CH modes the trans

Fig. 2. The calculated IR spectra of cis-PPD Ža., trans PPD Žb. and the experimental IR spectrum of crystalline PPD as a KBr disc Žc..

10

E. Akalin, S. Akyuz ¨ r Vibrational Spectroscopy 22 (2000) 3–10

and cis isomers are almost the same except for NH 2 group vibrations. The IR intensity calculation serves as additional check for the force field correctness. As can be seen in Table 2, there is a qualitative agreement between the calculated and experimental IR absorption band intensities. In Fig. 2, theoretical IR spectra of cis and trans isomers of PPD are given together with the experimental IR spectrum of crystalline PPD Žas KBr disc.. As seen in Fig. 2, the combination of the calculated IR spectra of trans and cis isomers of PPD is found to reproduce the experimental IR spectrum of solid PPD, satisfactorily, indicating that PPD exists as a mixture of both conformations.

4. Conclusion A complete vibrational assignment of the fundamentals has been proposed for PPD. The force constants for PPD calculated through the normal coordinate analysis agree well with the corresponding ones in aniline and other related aromatic amines. The intensity calculation served an additional check for the force field correctness. The combination of the calculated IR spectra of trans and cis isomers of PPD is shown to establish well the experimental IR spectrum of crystalline PPD, satisfactorily, indicating that PPD exists as a mixture of both conformations. Reliable force field and electro-optical parameters of PPD were determined for further use PPD complexes. Our future study will be devoted to force field and electro-optic parameters calculations of PPD complexes.

Acknowledgements This work was supported by the Research Fund of The University of Istanbul. Project Number B81r020399.

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