Vibrational spectroscopy and theoretical studies on 2,4-dinitrophenylhydrazine

Journal of Molecular Structure 744–747 (2005) 363–368 www.elsevier.com/locate/molstruc

Vibrational spectroscopy and theoretical studies on 2,4-dinitrophenylhydrazine V. Chis¸a, S. Filipb, V. Micla˘us¸c, A. Pıˆrna˘ua, C. Ta˘na˘seliaa, V. Alma˘s¸and, M. Vasilescua a Faculty of Physics, Babes¸-Bolyai University, 1 Koga˘lniceanu, RO-400084 Cluj, Romania Faculty of Science, University of Oradea, Str Armatei Romane 5, RO-410087 Oradea, Romania c Faculty of Chemistry and Chemical Engineering, Babes¸-Bolyai University, 11 Arany Janos, RO-400028 Cluj, Romania d INCDTIM Cluj-Napoca, 71-103 Donath, RO-400331 Cluj, Romania b

Received 9 September 2004; revised 6 November 2004; accepted 6 November 2004 Available online 19 February 2005

Abstract In this work, we will report a combined experimental and theoretical study on molecular and vibrational structure of 2,4dinitrophenylhydrazine. FT-IR, FT-IR/ATR and Raman spectra of normal and deuterated DNPH have been recorded and analyzed in order to get new insights into molecular structure and properties of this molecule, with particular emphasize on its intra- and intermolecular hydrogen bonds (HB’s). For computational purposes we used density functional theory (DFT) methods, with B3LYP and BLYP exchange-correlation functionals, in conjunction with 6-31G(d) basis set. All experimental vibrational bands have been discussed and assigned to normal modes on the basis of DFT calculations and isotopic shifts and by comparison to other dinitro- substituted compounds [V. Chis¸, Chem. Phys., 300 (2004) 1]. To aid in mode assignments, we based on the direct comparison between experimental and calculated spectra by considering both the frequency sequence and the intensity pattern of the experimental and computed vibrational bands. It is also shown that semiempirical AM1 method predicts geometrical parameters and vibrational frequencies related to the HB in a pleasant agreement with experiment, being surprisingly accurate from this perspective. q 2005 Elsevier B.V. All rights reserved. Keywords: Vibrational spectroscopy; 2,4-Dinitrophenylhydrazine; Semiempirical AM1 method

1. Introduction A number of hydrazine derivatives have been widely used in the medical and industrial fields. However, their modes of action have not yet been established and in addition, they have been shown to be potentially carcinogenic in animals. Among these, 2,4-dinitrophenylhydrazine (Fig. 1) is a potential DNA damaging and mutagenic agent. Its molecular and crystal structure was reported quite recently by Okabe et al. [2]. However, there is neither experimental nor theoretical investigation on the vibrational properties of DNPH. In addition, a rather strong intramolecular hydrogen bond is realized in this compound. The hydrogen bond received considerable attention due to its importance in biological

E-mail address: [email protected] (S. Filip). 0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2004.11.097

phenomena and in the relations between gas and condensed phase. Hydrogen bonding is important in biological processes due to the occurrence of electronegative atoms, especially N and O, in biomolecules such as amino acids and nucleotides [3]. Particularly, hydrogen bonding between biomolecules and water plays an important role in determining the energetics and dynamics of bioprocesses. These interactions have been intensively studied both from experimental and theoretical perspective and substantial progress has been achieved in the elucidation of their mechanisms. However, the ab initio approaches becomes prohibitively expensive in terms of computer resources and CPU time when dealing with biomolecules. Therefore, it is worth to investigate the applicability of semiempirical methods for the confident prediction of the parameters characterizing the intra- and intermolecular hydrogen bonds. For non-empirical calculations we used DFT methods which evolved to a powerful quantum chemical

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Fig. 1. Possible conformers of 2,4-dinitrophenylhydrazine and atom numbering scheme.

tool for the determination of the electronic structure of molecules. B3LYP combination of exchange and correlation functionals [4,5] is the most used since it proved its ability in reproducing various molecular properties, including vibrational spectra. The combined use of B3LYP functional and standard split valence basis set 6-31G(d) has been previously shown to provide an excellent compromise between accuracy and computational efficiency of vibrational spectra of molecules. On the other hand, the B exchange functional has the advantage of standard frequency scaling factor very close to unity so that the B-based procedures can often be used without scaling. For this reason we calculated the vibrational spectra both by B3LYP and BLYP methods.

2. Experimental DNPH was purchased from Sigma and used without further purification. FT-IR spectra for DNPH powder sample were recorded at room temperature on a conventional Equinox 55 FT-IR spectrometer equipped with an InGaAs detector and by using KBr (Merck UVASOL) tablet samples. FT-IR-Attenuated Total Reflectance (FT-IR/ATR) spectra were recorded on the above-mentioned spectrometer, coupled with a Bruker Miracle ATR sampling device. The FT-Raman spectra were recorded in a backscattering geometry with a Bruker FRA 106/S Raman accessory attached to the FT-IR spectrometer. The 1064 nm Nd:YAg laser was used as excitation source and the laser power was set to 400 mW. All spectra were recorded with a resolution of 4 cmK1 by co-adding 32 scans.

3. Computational details The molecular geometry optimizations and vibrational frequencies calculations were performed with the GAUSSIAN 98W software package [6] by using DFT methods with B3LYP [4,5] and BLYP [5,7] functionals. The split-valence 6-31G(d) basis set of the Pople’s group [8] has been used in conjunction with non-empirical methods. All the calculations have been carried out with the restricted closed-shell formalism. The geometries were fully optimized without any constraint with the help of analytical gradient procedure implemented within GAUSSIAN 98W program. The force constants were calculated by analytical differentiation algorithms, for each completely optimized geometry, in order to check that they are minima on the potential energy surface. Prior to compare the calculated vibrational frequencies with the experimental counterparts the former have been scaled by appropriate scaling factors recommended by Scott and Radom [9]. For n(CH) frequencies we used a scaling factor of 0.951, obtained previously for 2,4-dinitrophenol [1]. Vibrational mode assignments were made by visual inspection of modes animated by using the Molekel program [10].

4. Results and discussion Four possible conformers of DNPH (Fig. 1) have been considered for this investigation. Their geometries were fully optimized using B3LYP and BLYP methods, in conjunction with 6-31G(d) basis set. The geometry optimizations were made without any constraint and were then followed by a calculation of the harmonic vibrational wave number values at the same level of theory.

Table 1 Total and relative energies of the conformers of 2,4-dinitrophenylhydrazine calculated at B3LYP/6-31G(d) level of theory

Total energy (a.u.)a DE (kcal/mol)a,b a b

C1

C2

C3

C4

K751.9248643 K751.7932246 0.00 0.00

K751.9228496 K751.7853577 1.26 4.92

K751.9075139 K751.7706163 10.88 14.19

K751.9048778 K751.7676722 12.53 16.03

First row: not ZPE corrected; second row: ZPE values scaled with uniform scaling factor 0.9806 [9]. Energy differences relative to the CI conformer.

V. Chis¸ et al. / Journal of Molecular Structure 744–747 (2005) 363–368

According to calculations, the most energetically favorable conformers are those in which a NH/O intramolecular hydrogen bond is realized (C1 and C2 conformers in Fig. 1). Table 1 summarizes the total and relative energies of the four conformers, calculated by B3LYP/6-31G(d) method. The computed total energy for each conformer includes the zero point vibrational energy correction with a scaling factor of 0.9806 [9]. As seen in Table 1, the hydrogen bonded conformers C1 and C2 are very close in energy and at the same time they are distinctly apart from the other two, non-HB conformers C3 and C4. This energetic difference between the HB and non-HB conformers is comparable to intra-molecular hydrogen bond energies for other nitro compounds [1,11]. Table 2 compares the calculated bond lengths and angles for DNPH with those experimentally available from X-ray diffraction data [2]. As shown, there is a very good agreement between the experimental and the calculated B3LYP/6-31G(d) geometry. The major discrepancies are noted for CH and NH bond lengths. The calculated dihedral angles suggest a planar molecular skeleton, while the experimental values indicate small deviations from the planarity for the NO2 groups. As easily can be seen, the BLYP method provides systematic longer bonds with respect to B3LYP method. As reported by other works [9, 12,13] this is the reason why this method yields a better agreement between the computed and experimental vibrational frequencies. The B3LYP method has been previously shown to provide very good HB parameters [1,14–17], so we have chosen it as a standard procedure for our investigation. In addition, we tested the BLYP method, which is known as a very good combination for calculating the infrared spectra and the semiempirical AM1 method, which gave surprisingly good theoretical hydrogen bonding parameters for 2,4-dinitrophenol [1]. For the HB conformers C1 and C2 but also for the C4 conformer, the N2O2AO2B nitro group is essentially coplanar with the aromatic ring and the other nitro group while for the C3 conformer this group is twisted 38.68 outside of the ring plane. Theoretical data for C1, C2 and C4 conformers are consistent with experimental data, which indicate also a quasi-planar skeleton of DNPH molecule, the NO2 group being only 7.78 out of the ring plane. Theoretical results at B3LYP/6-31G(d) level of theory give an energetic difference between the two HB and nonHB conformers of about 14–16 kcal/mol. This difference is equivalent to the intramolecular bond enthalpy reported for a series of 2-substituted phenols, calculated as the difference between the fully optimized conformers [11]. The calculated HB parameters for DNPH are qualitatively in good agreement with those corresponding to the experimental counterparts [2]. The major discrepancy is noted for NH bond length, which is predicted by B3LYP/ ˚ larger than the experimental 6-31G(d) method 0.143 A value. The computed B3LYP/6-31G(d) N–H, H/O, N/O

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Table 2 Experimental and theoretical geometrical parameters for 2,4-dinitrophenylhydrazine Geometrical parameter Bond lengths C1–C2 C2–C3 C3–C4 C4–C5 C5–C6 C1–C6 C1–N1A N1A–N1B C2–N2 N2–O2A N2–O2B C4–N4 N4–O4A N4–O4B C3–H3 C5–H5 C6–H6 N1A–H1A N1B–H1B N1B–H1C H1A/O2B N1A/O2B Bond angles C2–C1–C6 C1–C2–C3 C2–C3–C4 C3–C4–C5 C4–C5–C6 C1–C6–C5 C2–C1–N1A C1–N1A–N1B N1A–C1–C6 C1–C2–N2 C2–N2–O2A O2A–N2–O2B C3–C4–N4 C4–N4–O4A O4A–N4–O4B C2–C3–H3 C4–C5–H5 C5–C6–H6 C1–N1A–H1A N1A–N1B–H1B N1A–N1B–H1C H1B–N1B–H1C N1A–H1A–O2B N2–O2B–H1A Dihedral angles C2–C1–N1A–N1B C2–C1–N1A–H1A C1–N1A–N1B–H1B C1–N1A–N1B–H1C N1A–C1–C2–N2 C1–C2–N2–O2B C1–C2–N2–O2A C3–C4–N4–O4A C3–C4–N4–O4B

Experimentala

1.423 1.391 1.364 1.383 1.359 1. 415 1.347 1.405 1. 442 1.222 1.237 1. 460 1.226 1.223 0.920 0.917 0.884 0.872 0.908 0.887 1.941 2.621

Calculated B3LYP/ 6-31G(d) 1.432 1.396 1.381 1.405 1.376 1.423 1.356 1.403 1.453 1.228 1.247 1.459 1.232 1.233 1.081 1.083 1.082 1.015 1.019 1.019 1.853 2.619

BLYP/ 6-31G(d) 1.445 1.406 1.392 1.416 1.386 1.432 1.366 1.423 1.466 1.248 1.272 1.475 1.252 1.254 1.088 1.090 1.089 1.027 1.029 1.029 1.837 2.633

116.4 121.1 119.6 121.0 120.3 121.6 123.3 119.8 120.3 122.5 119.7 122.1 119.8 118.5 123.7 121.7 119.8 122.5 115.2 103.9 112.2 100.8 133.7 105.7

116.8 121.3 119.6 120.9 119.9 121.6 122.7 120.7 120.5 122.4 118.7 122.7 119.4 117.9 124.7 119.6 119.1 121.1 117.1 109.0 109.0 108.2 129.6 109.6

117.0 121.1 119.6 121.0 119.9 121.5 122.1 120.7 120.9 122.5 118.8 122.4 119.4 117.9 124.7 119.6 119.0 121.2 116.4 108.0 108.0 107.2 131.5 108.8

176.6 K2.1 K118.8 133.1 1.9 7.7 172.8 K9.0 171.4

180.0 0.0 K121.0 121.0 0.0 0.0 180.0 0.0 180.0

180.0 0.0 K122.2 122.2 0.0 0.0 180.0 0.0 180.0

˚ and angles in degrees. Bond lengths in A a Ref. [2].

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Table 3 Selected experimental and calculated harmonic frequencies (cmK1), infrared intensities and Raman activities for 2,4-dinitrophenylhydrazne Experimental IR

Calculated

%I

20.3 68.3

3103 3087 1646 1609

%I

18.3 21.1 64.3 51.4

3346 3322 3153 3102 3087 1644 1607

31.4 94.7 23.2 47.2 56.1 66.9 65.5

1588 1529 1512 1495 1423 1412 1371 1330 1320 1286 1274 1224 1147 1132 1110 1062 981 931 924

46.4 37 49.1 49.2 36.3 57.7 29.8 80.2 100 74.1 70.7 57 34.9 45.3 45.7 48.3 36.1 18 22

1585 1529 1507 1490 1420 1409 1370 1331 1315 1282 1267 1222 1147 1129 1108 1060 976 931 924

72.6 42.1 76 100 46.2 56.9 45.4 59.7 92.1 86.7 95.2 88.4 64.4 77 79.7 72.1 64.3 55 52.9

832

22.8

827

766 744 708 692 669

6.6 24.1 34.6 14.8 6.5

630

21.4

532 509

17.5 10.5

434

2.3

Raman

%A

3346 3329

2.3 6.4

3105 3091 1643 1603

2.7 3.3 3.6 10.1

1524 1509 1492 1422

13.4 11.5 7.1 15.2

1368 1330 1314 1297 1271 1220

16.5 100 84.2 48.9 40.7 21.9

1132 1104 1062 981

14.3 9.3 17 6

925

5.2

61.8

834

31.2

766 743 707 691

21.3 49.9 32 33.9

766 747 711

1.7 1.7 4.9

656

2.3

657 633

3 2.9

534

3.5

448 435

5.9 5.4

B3LYP/ 6-31G(d) Scaled 3407 3391 3325 3117 3100 3088 1663 1609

I%

A%

BLYP/ 6-31G(d)

I%

A%

0.6 20.8 0.1 3.4 0.1 0.2 17.9 44.5

43.7 42.3 99.8 8.7 29.3 12.9 15.8 12.4

3404 3352 3323 3194 3172 3161 1678 1603

0.1 20.3 0.2 3 0 0.2 10.2 40.4

36.5 42 84 6.8 24.4 9.4 12.1 6.9

1592 1565 1540 1504 1436 1412 1349 1335 1309 1286 1267 1209 1134 1118 1096 1042 969 932 915 907 835 813 791 736 723 693 675 644 616 581 516 508 431 423

53.1 6.1 6.1 20.1 12.8 3.3 3.3 100 31.9 0.1 37.3 5.9 2.8 16.3 1.4 13.6 0 1.8 18.9 1.8 3.7 2 0.8 0.3 4.1 7.2 1.7 0.1 1.5 15 1.4 0.4 0.1 0.2

0 7.5 13.6 11.2 9 3.7 42.9 100 24.4 1.9 39.3 5.4 3.9 10.3 1.2 1.7 0.1 0.6 3.5 2.9 0.9 8.5 0.8 0.4 0.3 0.9 0.2 0.4 0.9 0.2 1 0 0.1 1.5

1571 1527 1497 1484 1431 1408 1353 1313 1298 1278 1234 1204 1134 1108 1097 1041 965 929 915 894 837 802 790 723 712 688 669 646 616 591 514 511 434 421

18.3 11.1 33.7 7.6 4.7 17.8 2.4 9.4 0.1 100 35.9 33.3 8.2 15.4 0.1 13.8 0 2 13.9 3.3 3.4 0.9 1.5 0.3 1.8 5 2.3 0.1 1.2 15.8 1.2 0.5 0.2 0.2

2.3 4 13 3.8 5.2 2 10.1 1.9 1.8 100 16.1 12.2 5.2 4 0.4 1 0.1 0.4 2.7 1.7 0.8 8.1 0.7 0.1 0.2 1.1 0.2 0.2 1 0.1 0.9 0.1 0 1.1

nas(NH() n(N1H) ns(NH() n(C3H) n(C6H) n(C6H)Kn(C5H) d(NH2)Cd(NH) n(C2C3)Cn(C5C6)Cnas(2NO2)Cd(CH ring)C d(NH2)Cd(NH) n(C1C2)Cn(C4C5)Cnas(N4O2)Cd(NH)Cd(NH2) d(NH)Cnas(N2O2)Cd(NH2)Cn(C5C6) nas(2NO2)Cd(NH)Cn(C1C2)Kn(C3C4) n(CN)Cn(C4C5)Cd(NH)Cd(NH2)Cd(CH ring) d(NH)Cd(CH ring)Cn(C1C6)Kn(C3C4) n(C2C3)Kn(C5C6) d(C1C2C3)Cd(CH ring)Cns(2NO2) ns(N4O2)Cy(C1C6)Cd(C3H) Ring def. Cd(NH)Cd(C5H)Cd(C3H)Cys(2NO2) t(NH2) ys(N2O2)Cy(C2N2)Cd(NH)Cd(CH ring) d(C6H)Cd(C3H)Cd(NH) y(NN)Cd(NH)Cd(C5H)Kd(C6H) d(C1C2C3)Cd(C3C4C5)Cd(C5H)Kd(C6H) y(NN)Cd(C3H)Kd(C6H)Cd(C5H) d(C1C2C3)Cd(CH)Kd(C5H)Kd(C6H) g(C5H)Kg(C6H) g(C3H) w(NH2) y(C2N)Ky(C4N)Cd(C4C5C6)Cw(NH2) g(C6H)Cg(C5H) d(2NO2)Cd(C1C6C5)Cw(NH2) d(C3C4C5)Cd(N4O2)Cw(NH2) g(C2N2)Kg(C4N4)Cg(C6H)Cg(C5H) g(C4N4)Kg(C2N2)Cg(C3H)Cg(C5H) d(C1C2C3)Cd(N2O2)Kd(N4O2) g(C6C1)Cg(C6C5)Kg(C3C2)Kg(C3C4) d(C2C1C6)Cd(C1N1AN1B) d(C1C6C5)Cd(C2C3C4) g(NH)Cr(NH2) r(N4O2)Cr(N2O2) g(C4C3)Cg(C4C5)Kg(C1C2)Kg(C1C6) g(C2C3)Cg(C5C6) r(N2O2)Cd(C1N1AN1B)

V. Chis¸ et al. / Journal of Molecular Structure 744–747 (2005) 363–368

3346 3326

ATR

Assignment

15 9.1

10.5

219 174

136

Atom numbering shown in Fig. 1. n, stretching; d, in-plane bending; g, out-of-plane bending; r, rocking; t, twisting; w, wagging; s, symmetric; as, asymmetric.

4.4 2.1 3.9 3.6 362 340 317 307

375 337 306 298 256 229 162 160 120 74 61 55

0 0.4 0.1 0.7 10.4 2 3 0.1 0.9 0.3 0.2 0

1.7 0.6 1 0.2 1 1 0.2 0.1 0.2 0 0.2 0.2

378 336 307 298 233 229 161 160 116 69 63 57

0 0.4 0.1 0.3 9.1 2.3 3.5 0.1 1.1 0.4 0.6 0.1

1.7 0.7 1 0.1 1 1 0.1 0.1 0.1 0 0.1 0.2

d(C2C1N1A)Cd(N4O4B) d(C2C1N1A)Cd(C2C3C4) r(ring)Cr(2NO2) r(NH2)Cg(C6C5)Cg(C3C4) r(NH2)Cg(NH) d(C1N1AN1B)Kd(C1C2N2) g(C1C2)Cg(C5C4)Cr(NH2) r(N4O2)Kr(N2O2) g(N1AN1B) r(NH2)Cg(C4N4) t(2NO2) t(2NO2)

V. Chis¸ et al. / Journal of Molecular Structure 744–747 (2005) 363–368

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˚ distances and NHO angle, are equal to 1.015, 1.853, 2.619 A and 129.68, respectively. Compared to experimental values, NH bond is predicted significantly longer, H/O distance is ˚ shorter, but the N/O distance is very well 0.088 A reproduced. The NHO bond angle is predicted slightly narrowed due to a minor twist of the H1A atom away from the C2 atom about the CN bond, leading to a rather strong N/O contact in DNPH. Our values are in line with other calculated intramolecular H-bond parameters for NH/O hydrogen bonds [3,18,19]. We tested also the accuracy of the AM1 method in predicting the HB parameters for DNPH molecule. Thus, our AM1 calculated r(NH) and r(CN) geometrical parameters are in good qualitative agreement with the high level theoretical data, while the H1A/O2B and N1A/O2B distances are predicted substantially longer than their experimental counterparts. It is worth to note that contrary to theoretical results, experimental data show a location of the H1A atom significantly closer to the nitrogen atom from NH group. Another characteristic of an intramolecular XH/Y hydrogen bond is the n(XH) stretching frequency. In a hydrogen bond ensemble, the XH stretching frequency is reduced whereas the X–H bending frequency is increased. For DNPH, the scaled value of 3391 cmK1 for n(NH), calculated at B3LYP/6-31G(d) level is in good qualitative agreement with experiment. As expected, the unscaled BLYP value (3352 cmK1) is even closer to the experimental value. A pleasant agreement with experiment is given by the semiempirical AM1 method which gives a n(NH) wave number of 3318 cmK1 (unscaled value). As seen in Tables 2 and 3, the overall H-bonding picture in DNPH is the most reliable reproduced by B3 exchange functional, while the B exchange functional slightly overestimate the r(NH) bond length, having as a result an important lowering in n(NH) frequency. The tile compound, 2,4-dinitrophenylhydrazine has C1 symmetry and contains 20 atoms. Therefore, its 54 normal modes are active both in infrared absorption and Raman scattering. The selected experimental and calculated normal modes of DNPH along with their IR and Raman intensities are summarized in Table 3. The experimental and calculated intensities are expressed as percentage of the most intense experimental or calculated intensity, respectively. Although the relative intensities are not well predicted for all IR or Raman bands, they still provide useful help for the assignment of the normal modes in the experimental spectra. The last column contains the motions that contribute the most to the different normal modes. Our tentative band assignments are based on known group frequencies [20] and supported by the normal mode calculations at B3LYP/6-31G(d) level of theory. As seen, almost all vibrations are complex and involve strongly coupled motions. Prior to compare the B3LYP/6-31G(d) calculated wavenumbers with the experimental counterparts we used

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the appropriate frequency scaling factor (0.9614) derived by Scott and Radom [9]. To aid in mode assignments, we based on the direct comparison between experimental and calculated spectra by considering both the frequency sequence and the intensity pattern. The maximum deviations in reproducing the whole FTIR spectrum of DNPH are 45 and 74 cmK1 for B3LYP/631G(d) and BLYP/6-31G(d) methods, respectively. The largest discrepancies are noted for n(NH) frequency in the case of B3 functional and for n(CH) frequencies for B exchange functional. Of particular importance are NH stretching vibration which has been discussed previously and the vibration of the amino group. The experimental band attributed to the asymmetric stretch of the NH2 is seen neither in IR nor in Raman spectrum. Our theoretical values are in very good agreement with those reported for other amino groups in a hydrogen bond free state [21,22]. For the dimer we have a calculated the band at 3411 cmK1 which is readily assigned to the nas(NH2) vibration, with NH2 group not involved in a hydrogen bond [21]. ns(NH2) vibration gives a band at 3326 cmK1, with the calculated corresponding value at 3325 cmK1. In-plane bending vibration of the amino group gives rise to the band at 1646 cmK1, very close to the corresponding value for isoniazid (1640 cmK1) and isonicotinamide (1622 cmK1) [23]. For a proper assignment of the vibrational bands discussed above, we recorded the FT-IR/ATR and Raman spectra of deuterated DNPH at amino group. nas(ND2) and ns(ND2) experimental frequencies are 2534 and 2421 cmK1, respectively, in very good agreement with B3LYP/6-31G(d) calculated values: 2516 and 2400 cmK1. The twisting vibration t(NH2) with 1286 cmK1 IR wavenumber shifts to 992 cmK1 for ND2 group, the calculated value being 962 cmK1. Also, the wagging w(NH2) normal mode at 924 cmK1 in FT-IR spectrum is lowered to 715 cmK1 for deuterated DNPH, with calculated corresponding value at 737 cmK1. Finally, the rocking mode r(NH2) seen for normal DNPH at 532 cmK1 is observed at 282 cmK1 for the deuterated sample, also in very good match with theoretical value of 297 cmK1. The very good agreement between our calculated isotopic shifts and those observed experimentally allows us to confirm the assignment of these bands.

The vibrational wave-numbers, IR intensities and Raman activities are calculated in very good agreement with experiment at B3LYP/6-31G(d) level of theory. Using this method, and a standard and uniform scaling factor recommended by Scot and Radom, the mean deviation in predicting the whole vibrational spectrum of DNPH is 13 cmK1. n(CH) vibrations have been almost perfect reproduced by using a scaling factor of 0.951. The semiempirical AM1 method predicts geometrical parameters and vibrational frequencies related to the HB in a pleasant agreement with experiment, being surprisingly accurate from this perspective.

5. Conclusions

[17]

The vibrational properties of 2,4-dinitrophenylhydrazine have been investigated by FT-IR and FT-Raman spectroscopies and by different quantum chemical methods. Special emphasize has been put on the intramolecular hydrogen bonding parameters of this molecule. According to calculations, the two intramolecular hydrogen bonded conformers are about 12 kcal/mol more stable than the non-HB conformers.

References [1] [2] [3] [4] [5] [6]

[7] [8] [9] [10]

[11] [12] [13] [14] [15] [16]

[18] [19] [20]

[21] [22] [23]

V. Chis¸, Chem. Phys. 300 (2004) 1. N. Okabe, T. Nakamura, H. Fukuda, Acta Crystallogr. C49 (1993) 1678. M.S. Gordon, J.H. Jensen, Acc. Chem. Res. 29 (1996) 536. A.D. Becke, J. Chem. Phys. 98 (1993) 5648. C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785. M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, V.G. Zakrzewski, J.A. Montgomery Jr., R.E. Stratmann, J.C. Burant, S. Dapprich, J.M. Millam, A.D. Daniels, K.N. Kudin, M.C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G.A. Petersson, P.Y. Ayala, Q. Cui, K. Morokuma, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J. Cioslowski, J.V. Ortiz, A.G. Baboul, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, 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, and J.A. Pople, GAUSSIAN 98, Revision A.7, Gaussian, Inc., Pittsburgh PA, 1998. A.D. Becke, Phys. Rev. 38 (1998) 3098. W.J. Hehre, L. Radom, P.v.R. Schleyer, J.A. Pople, ‘Ab Initio’ Molecular Orbital Theory, Wiley, New York, 1986. A.P. Scott, L. Radom, J. Phys. Chem. 100 (1996) 16502. (a) MOLEKEL 4.2 P.Flu˝kiger, H.P. Lu˝hti, S. Portmann, J. Weber, Swiss Center for Scientific Computing, Manno (Switzerland), 2000–2002; (b) S. Portmann, H.P. Lu˝hti, MOLEKEL: an interactive molecular graphics tool, Chimia 54 (2000) 766. H.G. Korth, M.I. de Heer, P. Mulder, J. Phys. Chem. 106 (2002) 8779. B.G. Johnson, P.M.W. Gill, J.A. Pople, J. Chem. Phys. 98 (1993) 5612. G. Rauhut, P. Pulay, J. Phys. Chem. 99 (1995) 3093. M.J. Frisch, A.C. Schneider, H.F. Schaefer, J. Chem. Phys. 82 (1985) 4194. J.M.L. Martin, C. van Alsenoy, J. Phys. Chem. 100 (1996) 697. F. Sim, A. St-Amant, I. Papai, D. Salahub, J. Am. Chem. Soc. 114 (1992) 4391. G. Gilli, F. Bellucci, V. Ferretti, V. Bertolosi, J. Am. Chem. Soc. 111 (1989) 1023. A. Mu¨ller, M. Losada, S. Leutwyler, J. Phys. Chem. A 108 (2004) 157. F. Tu¨recek, E.A. Syrstad, J. Am. Chem. Soc. 125 (2003) 3353. D. Lin-Vien, N.B. Colthup, W.G. Fately, J.G. Grasselli, The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules, Academic Press, London, 1991. B. Ishimoto, K. Tonan, S. Ikawa, Spectroschim. Acta, Part A 55 (1999) 2321. K. Tonan, S. Ikawa, Spectrochim. Acta, Part A 53 (1997) 2695. Y. Wang, Y.-S. Li, J. Wu, Z. Zhang, D. An, Spectrochim. Acta, Part A 56 (2000) 2637.